*5.2.5. Experimental results*

The ADC with the proposed parallel reference network was implemented in 0.25 µm SiGe BiCMOS technology. The Chip micrograph of the ADC is depicted in Fig. 42.

**Figure 42.** ADC's chip micrograph.

**Static measurements:** For measuring static errors of the ADCs, a low frequency 50 MHz sine signal was applied to the input of the converter at 5 GS/s sample rate. A deviation of a transition from the mean value, the differential nonlinearity (DNL), was calculated for each step. A cumulative sum of differential errors represents the integral nonlinearity (INL). The results are graphically presented in Fig. 43.

**Figure 43.** DNL and INL from histogram testing of ADC.

414 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

**Figure 41.** Master-slave comparator, full schematic diagram.

*5.2.5. Experimental results*

**Figure 42.** ADC's chip micrograph.

latch.

The master latch has an auxiliary current source *Iaux.* This current source prevents the crosscoupled differential pair from being completely switched off, thus keeping the base-emitter capacitance charged. The time to charge this capacitance is decreased, and as a result the overall speed of the latch is increased. The *Iaux* has to be sufficiently small because it adds hysteresis which decreases the sensitivity of the comparator. Setting the value of *Iaux* equal to 10 % of *IEE2* is a good compromise between speed and sensitivity. In the slave latch, there is no auxiliary current source because the input signal of the slave latch is relatively large, and an auxiliary current source does not have a strong influence as in the case of the master

The ADC with the proposed parallel reference network was implemented in 0.25 µm SiGe

BiCMOS technology. The Chip micrograph of the ADC is depicted in Fig. 42.

**Dynamic measurements**: Signal-to-noise and distortion ratio (SINAD) of the test circuit was measured over the frequency range up to 6 GHz at a constant sample rate of 15.01 GS/s. The small frequency offset of 10 MHz was made to accumulate quantization errors over the whole dynamic range. The measurement results are presented in Fig. 44, which shows SINAD of the converter up to the input frequency of 6 GHz. The dashed line shows a level where SINAD drops 3 dB below its value at low frequency. The frequency where SINAD crosses the 3 dB line indicates the effective resolution bandwidth of the converter, which in this case is greater than 6 GHz.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 417

The functional diagram of the predictor is depicted in Fig. 45. The predictor consists of the memory where the data from the ADC are accumulated; an averaging block, which makes averaging of the accumulated data; and a block where the output DAC value is calculated. An algorithm to calculate the DAC value is a modified version of the successive

The described functionality is coded using VHDL language and implemented using ECL library available in IHP BiCMOS Technology. For speed purposes, the predictor was divided into several sub-blocks which were implemented separately. This method decreases the complexity of the separate sub-block, and achieves a higher operational speed. The block diagram of the predictor is depicted in Fig. 46. The predictor consists of a demultiplexer, a bank of predicting blocks and a multiplexer. A predicting block carries out three functions: accumulation, averaging, and prediction. The demultiplexer deserializes the M-Sequence and commutates M-Sequence parts (chips) to the individual predicting blocks so that each has to work with only one defined chip. The multiplexer reverses the parallel processing

approximation algorithm with a constant ±LSB step.

and serializes the predicted values which finally fed the DAC.

**5.4. High-speed digital-to-analog converter with off-chip calibration**

The digital-to-analog converter transforms a digitally predicted value into the analog domain. To prevent information loss, the accuracy of the DAC should correspond to the accuracy of the whole capturing device. Simultaneously, the DAC should work at 10 GS/s. To satisfy both requirements, the converter is implemented using a segmented current steering architecture. The block diagram of the converter is depicted in Fig. 47. It consists of the two segments: A unary sub-converter and an R-2R sub-converter. The current sources of the both sub-converters are connected to the summing node. As will be seen later from measurements relying only on technology, component matching would give insufficient accuracy, which in this particular case is 10 times lower than required. Therefore, an additional calibration of the current sources is implemented. The current sources are

**Figure 46.** Structure diagram of the predictor.

**Figure 44.** Measured SINAD over a frequency range up to 6 GHz.

### **5.3. High-speed predictor**

The main function of the predictor of predicting the part of the received value was described above. The predictor also carries out two additional functions:

**Figure 45.** Functional diagram of the predictor.

The functional diagram of the predictor is depicted in Fig. 45. The predictor consists of the memory where the data from the ADC are accumulated; an averaging block, which makes averaging of the accumulated data; and a block where the output DAC value is calculated. An algorithm to calculate the DAC value is a modified version of the successive approximation algorithm with a constant ±LSB step.

The described functionality is coded using VHDL language and implemented using ECL library available in IHP BiCMOS Technology. For speed purposes, the predictor was divided into several sub-blocks which were implemented separately. This method decreases the complexity of the separate sub-block, and achieves a higher operational speed. The block diagram of the predictor is depicted in Fig. 46. The predictor consists of a demultiplexer, a bank of predicting blocks and a multiplexer. A predicting block carries out three functions: accumulation, averaging, and prediction. The demultiplexer deserializes the M-Sequence and commutates M-Sequence parts (chips) to the individual predicting blocks so that each has to work with only one defined chip. The multiplexer reverses the parallel processing and serializes the predicted values which finally fed the DAC.

**Figure 46.** Structure diagram of the predictor.

416 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

**Figure 44.** Measured SINAD over a frequency range up to 6 GHz.

0

5

10

15

SINAD, dB

20

25

30

above. The predictor also carries out two additional functions:

Decreasing the data throughput for further data processing.

The main function of the predictor of predicting the part of the received value was described

1 2 3 4 5 6

SINAD

3 dB decay of SINAD

Input frequency, GHz

Making subsequent averaging of the digitized values, increasing signal-to-noise ratio of

*Navg*

**5.3. High-speed predictor**

the measured signal.

**Figure 45.** Functional diagram of the predictor.

### **5.4. High-speed digital-to-analog converter with off-chip calibration**

The digital-to-analog converter transforms a digitally predicted value into the analog domain. To prevent information loss, the accuracy of the DAC should correspond to the accuracy of the whole capturing device. Simultaneously, the DAC should work at 10 GS/s. To satisfy both requirements, the converter is implemented using a segmented current steering architecture. The block diagram of the converter is depicted in Fig. 47. It consists of the two segments: A unary sub-converter and an R-2R sub-converter. The current sources of the both sub-converters are connected to the summing node. As will be seen later from measurements relying only on technology, component matching would give insufficient accuracy, which in this particular case is 10 times lower than required. Therefore, an additional calibration of the current sources is implemented. The current sources are 418 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

realized as voltage controlled current sources. The controlling voltages are produced by auxiliary low-power µDACs which are externally controlled via SPI interface.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 419

uncalibrated calibrated

uncalibrated calibrated

**Static measurements**: The DAC test chip was implemented in SiGe 0.25 µm BiCMOS IHP technology. The chip was mounted on a test board and connected to the FPGA board. The results of static tests are given in Fig. 48 where both DNL and INL values before and after calibration are given. INL errors were also recalculated in percent of the input range. To

5 10 15 20 25 30

5 10 15 20 25 30

5 10 15 20 25 30

code

code

code

**Figure 48.** DNL and INL of uncalibrated and calibrated DAC output.

INL, LSB

0 0.5

1

DNL, LSB

**Figure 49.** Static accuracy of the calibrated DAC.

0

0.05

0.1

0.15

error, %

0.2

0.25

*5.4.1. Experimental results*

The calibration algorithm could be characterized as successive approximation of the DAC output to the reference value. The detailed calibration flow of the each current source is as follows:

A set-up to implement the proposed calibration scheme is depicted in Fig. 47. The calibration algorithm is implemented on a Spartan-3AN Starter Kit board.

**Figure 47.** Off-chip calibration of the DAC.

### *5.4.1. Experimental results*

418 Ultra-Wideband Radio Technologies for Communications, Localization and Sensor Applications

auxiliary low-power µDACs which are externally controlled via SPI interface.

For this purpose, the corresponding digital input is applied to the DAC.

measurement is performed with a 14-bit ADC on an FPGA board.

4. According to a binary search algorithm, the MSB of µDAC is set to "1".

calibration algorithm is implemented on a Spartan-3AN Starter Kit board.

3. The CSUC is connected to the summing node.

value" and the measured value is calculated.

8. Steps 1-7 are repeated for each current source.

**Figure 47.** Off-chip calibration of the DAC.

7. Steps 4-6 are repeated for the remaining 9 bits of µDAC.

follows:

µDAC is made.

realized as voltage controlled current sources. The controlling voltages are produced by

The calibration algorithm could be characterized as successive approximation of the DAC output to the reference value. The detailed calibration flow of the each current source is as

1. The current source under calibration (CSUC) is disconnected from the summing node.

2. The analog output of the DAC is measured and stored in memory as "zero-value". The

5. The output of ADC is measured again, and the difference between the stored "zero-

6. Depending on this difference, the decision concerning the value of the MSB of the

A set-up to implement the proposed calibration scheme is depicted in Fig. 47. The

**Static measurements**: The DAC test chip was implemented in SiGe 0.25 µm BiCMOS IHP technology. The chip was mounted on a test board and connected to the FPGA board. The results of static tests are given in Fig. 48 where both DNL and INL values before and after calibration are given. INL errors were also recalculated in percent of the input range. To

**Figure 48.** DNL and INL of uncalibrated and calibrated DAC output.

**Figure 49.** Static accuracy of the calibrated DAC.

achieve 9-bit overall resolution, the DAC should have an error below 0.2 %. The measured error after calibration is depicted in Fig. 49. The error is below 0.15 %.

HaLoS – Integrated RF-Hardware Components for Ultra-Wideband Localization and Sensing 421

predictor to comply with different system parameters, such as the M-sequence length or

An off-chip calibration was implemented for the high-speed digital-to-analog converter. The calibration is implemented on an FPGA-board. After having been modified slightly, it could be integrated into the DAC. The static errors of the DAC after calibration are lower than 0.15 % which allows the use of a converter in the data capturing device with a target resolution

While previous sections were aimed to discuss specific sub-components such as individual semi-conductor chips of an UWB-sensor, we would like to consider some aspects of the whole sensor electronics here. For that purpose, several M-Sequence devices were implemented at different integration levels, and some Ukolos-partners (*ultraMedis*, *CoLoR*) were provided with demonstrator devices for their own use. In order to have a running sensor system, the device implementation has to cover the whole manufacturing cycle from chip-design and manufacture, chip housing, RF-PCB-design and assembly, design and implementation of the digital components (ADC, FPGA, interfaces etc) up to the programming of sensor internal pre-processing, the data transfer to the host PC and application-specific software for data evaluation and visualization. Furthermore, device specific test and evaluation methods and routines had to be developed and implemented in

In what follows, we will first introduce an experimental device which is aimed to evaluate new concepts or modifications under real conditions. Secondly, we refer to a device configuration which implements the principle depicted in Fig. 4 for the practical use by other Ukolos-projects and finally, there will be some discussions toward single chip

The aim of an experimental demonstrator device is to investigate the impact of individual sub-components on the performance of the whole device, as well as to have the opportunity to flexibly perform device modifications without the need of redesigning complex RF-PCBs. The device is organized in a modular concept as symbolically depicted in Fig. 51. Fig. 52

The individual sub-components as e.g. shift register for stimulus generation, T&H-circuits, RF-power distribution, RF-synchronization etc. are organized as plug-ins. Hence, one can simply replace a device component by a new one if improved circuits, better IC-housing or

order to perform high-resolution device characterization (e.g. [48])

shows an example of a demonstrator implementation of such kind.

**6.2. Experimental demonstrator device**

*6.2.1. Device concept and aim*

averaging factor.

**6. M-sequence devices**

**6.1. Introduction**

of 9 bits.

solutions.

**Dynamic measurements**: Dynamic characteristics of the DAC were measured together with the 4-bit ADC under the assumption that LSB usually works faster than MSB. The direct measurement of the spurious free dynamic range (SFDR) has no practical sense since the ADC limits the overall performance. For estimating the performance, an envelope test was applied [45]. Proper work of the converters assumes the presence of the all transition steps at the frequency of interest. Fig. 50 shows DAC outputs at 5 GHz and 5.5 GHz. Both converters (ADC and DAC) have all 16 transition levels up to 5.5 GHz input. Only the amplitude at 5.5 GHz starts to decay.

**Figure 50.** Envelope test of ADC-DAC at **a)** 5 GHz and **b)** 5.5 GHz.


**1. Introduction**

The energy demand of the world is continuously increasing due to new industrial developments and population growth [1]. Internal combustion engines with different operation principles will remain the main form of propulsion for terrestrial, air, and maritime transportation in the next 20–30 years. Accordingly, hydrocarbons as engine fuels for internal combustion engines will continue to play an important role as demonstrated in Figures 1 and 2a,b [2]. The proportion of gasoline will decrease slightly, but will still account for a one-third share in 2040. This decrease is due to the better fuel economy and the spread of electric vehicles [3] (Figure 3). However, contrary to forecasts, electric vehicles cannot spread as rapidly as expected, due to reasons such as availability of raw materials, recycling, sustainability, etc.

**Figure 1.** Light-duty vehicles by fleet type.

**Figure 2.** (**a**) Global energy mix by engine fuels. (**b**) Energy demand by type of transportation.

**Figure 3.** Expected gasoline demand taking into account to better fuel economy and the spread of electric vehicles.

Sustainable production of alternative engine fuels is a big challenge. Accordingly, different biofuels presently play a significant role for many countries in the world. The foreseen decrease of fossil reserves, global protection of the environment, and sustainability of mobility are the main driving forces for the research into renewable alternative energy sources [4]. The latest directive of the European Union requires the production of fuels from non-edible, renewable, or waste feedstocks [5]. Currently, the main bio-component of gasoline is bioethanol [6], which is also a molecular constituent in ethyl-tert-butyl-eter (bio-ETBE), an important gasoline blending component [7]. Despite the numerous advantages of ethanol, it has also many disadvantages, such as high solubility in water, increase of vapor pressure in gasoline, lower energy content, corrosion effects, high ozone producing potential, and high atmospheric reactivity [8]. Moreover, it is mainly produced from food-based plants, e.g., corn. As a result of persistent research and development in the last few years, the construction of plants using lignocellulose as feedstock has already begun [9]. However, their technological efficiency and reliability need to be proven [10].

The conversion processes of alternative feedstocks, such as biomass and waste, to different fuels result in the formation of a significant amount of light hydrocarbons (e.g., C5-C7 fractions), mainly as by-products in the boiling range of gasoline. The following processes can result in light hydrocarbons from alternative sources:

• The simplest reaction pathway for the production of C5-C7 paraffins is the special hydrocracking of natural/waste fatty acids as a by-product and/or natural/waste fatty acid esters, e.g., waste cooking oil [23] or waste lards [24] from protein processing industry [25].

Based on the results of the presented publications it was concluded that by-products containing mainly C5-C7 paraffins can be obtained from different alternative sources. These by-products have very low octane numbers (<55–65) and contain undesirable components, such as olefins, aromatics, oxygenates, and other corrosive compounds; therefore, they are not suitable as gasoline blending components ("drop in fuel"). Isomerization and dehydrocyclization (aromatization) are the main chemical processes for the quality improvement of n-paraffins to enhance their octane number. Via isomerization the products are iso-paraffins, and via the dehydrocyclization process the obtained products are aromatics. Aromatics, including benzene, have very high octane numbers, e.g., the research octane number (RON) of benzene is 101. However, they are carcinogenic, therefore their concentration in gasoline must be reduced. Gasoline standards strictly limit the concentration of benzene, e.g., in California, USA, the limit is 0.62 v/v%, and in the EU this value is 1.0 v/v% [4].

No experimental results were provided in the publications about quality improvement, e.g., increasing the octane number of light gasoline fraction obtained from alternative sources. Based on the aforesaid reasons, the main research target of this paper was to investigate the quality improvement of benzene containing light hydrocarbons (rich in n-paraffins), from alternative sources via hydroisomerization. The main goal was also to convert the n-paraffins to branched paraffins with a higher octane number via hydrogenation, i.e., saturation of benzene to cyclo-hexane and its isomerization to methyl-cyclo-pentane.


*3.1.1 Experiment*

The friction and wear life of journal bearings with bonded MoS2 film lubricant applied to some substrates were investigated in vacuum using the test equipment shown in **Figure 4** [8]. **Figure 5** illustrates the friction measurement part of the equipment set in the vacuum chamber. The shaft was oscillated by AC servo motor through reduction gear and feedthrough. The radial load was applied by an air cylinder outside the vacuum chamber through a bellows. The bore and width of the bearing were 10 mm and 7 mm, respectively. The lubricant was applied both to the bearing and the shaft. The frictional torque was measured by the load cell above the test bearing.

The commercially available bonded MoS2 film lubricant, including about 25 wt. % MoS2 and phenolic resin binder, was spray coated and heat cured at 150°C for 1 h. The film thickness was about 10 micro-meters and the diameter clearance between the bearing and the shaft with lubricant film was 10-20 micrometers.

**Table 1** [1] shows the substrate materials and pretreatment for the bearings and the shafts. **Table 2** [1] indicates the bearing and the shaft combinations. The circled numbers in the table correspond to those in **Table 1**. **Table 3** [1] shows the test conditions.

#### *3.1.2 Experimental results*

**Figure 6** [1] illustrated the typical measured friction evolution. The measured friction drew the rectangular wave due to the oscillation motion and half of the total

**Figure 4.** *In-vacuum journal bearing test equipment [7].*

#### *Wear Life of Bonded MoS2 Film Lubricant DOI: http://dx.doi.org/10.5772/intechopen.99802*

#### **Table 1.**

*Substrate materials and pretreatment for bearings and shafts [1].*

#### **Table 2.**

*Bearing and shaft combinations [1].*

#### **Table 3.**

*Test conditions [1].*

#### **Figure 6.**

*Typical measured friction evolution [1].*

amplitude was used as the frictional force. The friction gradually decreased with the number of oscillations and suddenly increased. This sudden increase point was used as the wear life of the lubricant.

**Figure 7** [1] shows the appearance of the tested shaft. A part of the film lubricant was removed and the metal substrate was exposed and scratched. **Figure 8** [1] shows

**Figure 7.** *Appearance of the tested shaft [1].*

**Figure 8.**

*Lubricant film profiles: (a) about 60% of wear life, (b) after the friction increase [1].*

the lubricant film profiles of Set No.3 specimen in **Table 2** at the points (a) and (b) in the wear life shown in the right diagram. Most of the film thickness remained near the end of the wear life, and the scratched part was observed at the end of the test.

**Figure 9** [1] indicated the wear life and friction coefficient of the tested combinations. There was no significant difference in the friction coefficients, but the wear life of the SUS304 was much longer than the others.

#### *3.1.3 Discussion*

It was observed that the film thickness of the lubricant gradually decreased, but it seems that the wear life suddenly came with most of the film thickness remaining. This form of wear has been observed in several previous studies [9, 10]. This means that specific wear rates cannot be used to predict wear life. In some studies, specific wear rates has been proposed for the wear life estimation of the bonded MoS2 film lubricant [9, 11]. It may be used for relatively large sliding speed and low load conditions, but cannot be used for small sliding speed and high load conditions like this case.

#### **Figure 9.**

*Wear life and friction coefficient of the tested combinations [1].*

The sudden decrease of the film is probably due to the de-bonding of the film from the substrate, possibly due to fatigue. The adhesion strength between the substrate and the film must affect the fatigue strength, that is, the wear life, and is strongly dependent on the anchoring effect by the surface morphology of the pretreated substrate.

The surface morphology of the pretreated substrate was investigated using "pretreated surface specimen" shown in **Figure 10** [1]. A portion of one side of a

**Figure 10.** *Pretreated surface specimen [1].*

#### *Tribology of Machine Elements - Fundamentals and Applications*

rectangular metal specimen was pretreated and the bonded MoS2 film lubricant was applied to half of the pretreated area. **Figure 11** [1] shows the surface profiles of the specimens. As shown in (a) and (c), the pretreated areas of the aluminum alloy substrate and the SUS304 stainless steel substrate had a roughness that went up and down across the original surface, while that of the SUS440C stainless steel substrate

**Figure 11.** *Surface profiles of the pretreated surface specimens, (a) aluminum alloy, (b) SUS440C, (c) SUS 304 [1].*

#### *Wear Life of Bonded MoS2 Film Lubricant DOI: http://dx.doi.org/10.5772/intechopen.99802*

was below the original surface. This means that ductile metal surfaces such as aluminum alloy and SUS304 stainless steel (austenitic stainless steel) were deformed plastically by the blasting and that of brittle metal such as SUS440C (martensitic stainless steel) seems to have had its surface layer taken away by the blasting. This resulted in a characteristic surface morphology.

**Figure 12** [1] shows the cross- sections of the pretreated surface specimens. The SUS304 specimen indicated the intricate surface morphology, while the SUS440C specimen had a monotonous wavy surface morphology. Probably these morphologies brought the strong adhesion between the substrate and the film, that is, the long wear life, to the SUS304 substrate and short wear life to the 440C substrate. The work hardening would also have contributed to the long wear life of the SUS304 substrate. The short wear life of aluminum alloy substrate could be attributable to the deformation of surface morphology by the load due to the lack of the hardness.

#### **3.2 Effects of atmosphere**

#### *3.2.1 Experiment and results*

Friction and wear life characteristics were investigated under various loads, sliding speeds in air and vacuum atmospheres using the test equipment used in Section 3.1. Test materials and test conditions are shown in **Table 4** [8] and **Table 5** [8], respectively. SUS630 is a precipitation hardened stainless steel with high strength, and was chosen for the shaft specimen in consideration of actual applications.

**Figure 13** [8] shows a typical change in the friction coefficient with the number of oscillations in air and in vacuum. The friction coefficient was several times larger and the wear life was several ten times shorter in air than in vacuum. The relationship between the friction coefficient and the load is shown in **Figure 14** [8]. Friction coefficient used was in the steady state as shown in **Figure 13**. Friction coefficient was about 0.2 in air and about 0.05 in vacuum regardless of test conditions.

**Figure 15** [8] shows the relationship between the bearing pressure and the wear life. The wear life refers to the number of oscillations when friction increased sharply. There are two groups, one in vacuum and one in air, with differences in wear life of hundreds of thousands of oscillations. It seems that there is no relationship between the groups.

#### *3.2.2 Discussion*

**Figure 16** [8] shows the lubricant film profile at about 70% wear life in air, obtained under the same test conditions as No. 5 in **Table 5**. Most of the film thickness remained, as in the in-vacuum test shown in **Figure 8**. This suggests that the wear life in air, as well as in vacuum, is due to the de-bonding of the film from the substrate, and that the fatigue strength of the film-substrate interface may determine the wear life.

Fatigue strength of some metals are known to be affected by atmosphere and be larger in vacuum than in air (e.g. [12]). However, it has been shown that the propagation rate of fatigue cracks in epoxy resins is almost the same in both

#### *Tribology of Machine Elements - Fundamentals and Applications*

#### **Table 5.** *Test conditions [7].*

**Figure 13.** *Typical change in friction coefficient in air and in vacuum [7].*

**Figure 14.** *Relationship between the friction coefficient and load [7]. The numbers correspond to those in Table 5.*

**Figure 15.** *Relationship between bearing pressure and wear life [7]. The numbers correspond to those in Table 5.*

**Figure 16.**

*Lubricant film profile at about 70% wear life in air [7].*

vacuum and air [13], and in general, the fatigue of resins, which are the binders of the films, is considered to be less affected by the atmosphere. Therefore, the short wear life due to fatigue of film lubricants in air is not considered to be due to their reaction to the environmental atmosphere. The factor that differs between vacuum and air and is considered to affect fatigue is the friction coefficient.

Stress analysis was performed to investigate the effects of the friction coefficient on the stress in the film [8]. The analysis was performed as a plane strain perfect elasticity problem. Young's modulus of the film was measured in dry air to be about 10 GPa. Calculation conditions are shown in **Table 6** [8].

**Figure 17** [8] shows examples of the calculated stresses in the film. Since the film thickness is small compared to the contact length, the stress is almost constant in the depth direction of the film.

**Figure 18** [8] shows the relationship between the maximum shear stress at the interface between the film and the substrate in the direction parallel to the interface and wear life. All points are on a straight line, whether they are in vacuum

#### *Tribology of Machine Elements - Fundamentals and Applications*

#### **Table 6.**

*Calculation conditions [7].*

#### **Figure 17.** *Examples of calculated stresses in the film [7].*

or in air. This is a typical S-N curve for fatigue phenomenon. Therefore, the wear life of the bonded MoS2 film lubricant can be attributed to fatigue due to shear stress at the interface. Since the contact width is much larger than the film thickness, the shear stress at the interface is almost the same as the shear stress at the film surface, i.e., the product of the friction coefficient and the contact pressure,

**Figure 18.** *Relationship between the maximum shear stress at the interface between the film and the substrate and wear life [7].*

**Figure 19.**

*Repeated vertical loading machine [13].*

as shown in **Figure 17(b)** and **(d)**. Since the maximum von Mises stress, which contains a large component of vertical loading, did not show the same relationship as the shear stress, damage inside the film is not considered to be the cause of wear life in this case. Thus, the difference in wear life between vacuum and air is due to the friction coefficient between vacuum and air.

The effect of repeated vertical loading was investigated separately. **Figure 19** shows the "repeated vertical loading machine" [14], in which the bonded MoS2 film lubricant on the flat surface was subjected to the repeated vertical pressure by a steel ball with 5/16 in. (~7.9 mm) diameter. A sinusoidal load of 0.98 N to 4.4 N was applied at a frequency of 1000 cpm.

Only dents with a few micrometer depth were observed on the tested lubricant films after more than 107 times loading, as shown in **Figure 20** [14], and no debonding was observed. Hence, the repeated shear stress, i.e. the friction, rather than the repeated vertical load causes the de-bonding of the film lubricant.


*Article* **Layered-Fabric Materiality Fibre Reinforced Polymers (L-FMFRP): Hysteretic Behavior in Architectured FRP Material**

**Arielle Blonder \* and Maurizio Brocato**

Laboratoire GSA, Ecole National Supérieure d'Architecture Paris-Malaquais, Université PSL, 75006 Paris, France<EMAIL_ADDRESS>
**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** L\_FMFRP is an architectural fiber composite surface element with an airy internal structure and variable section. This architectured material is the product of an alternative design and fabrication process that integrates *fabric materiality*, suggesting moldless shaping of the material through pleating and layering. Initial study of the mechanical properties of the element showed a structural behavior that would satisfy the requirement for schematic architectural cladding configurations, indicating a unique hysteretic behavior of the material. This paper further investigates the hysteretic capacities of L-FMFRP, examining the behavior under repeated loading and the effect of its internal material architecture. Parallels to entangled materials are suggested for a deeper understanding of the phenomenon, and the potential future application as an energy-absorbent material for façade cladding is outlined.

**Keywords:** fiber composites; FRP; architecture; architectured materials; hysteresis

#### **1. Introduction**

Fiber-reinforced polymers (FRPs) are the composite outcome of advanced fibers and polymer resins, making a family of high-performance materials that offers high strengthto-weight ratio, durability and versatility. In the past decades, numerous types of FRPs have been developed and introduced into extensive use across industries, from aeronautics and space, to infrastructure, automotive and consumer products. In the construction industry, FRPs are widely used for civil infrastructure [1], rehabilitation and reinforcement, light and moveable constructions, footbridges, profiles and decks, under various forms and fabrication processes [2]. Their architectural use is re-emerging [3], expanding from experimental pavilions and academic research [4] to wider commercial applications by world-leading architects [5].

Elements of FRPs are traditionally manufactured based on rigid molds. A compact laminate is made by pressing a number of fiber layers, mainly under the form of textile fabrics, over the mold. The composite piece is shaped by the mold's morphology, assuring conformity to the requested designed shape as well as material homogeneity [6]. While being substantial to applications in fields such as the automotive, security or aeronautics sectors, the dependence on rigid molds can be restrictive in the case of architectural applications [7]. The typical size-of-element and its one-off nature, together with contemporary architectural practices that promote complex morphologies and high variability, stand in contradiction to the practice and economy of mold-based manufacturing. Standard industrial composite-forming processes therefore represent a significant factor in the barrier to the wider application of the material in the architectural field [8]. This barrier is reflected in numerous contemporary research projects, seeking for alternative, adaptive, reconfigurable moldless forming processes and material systems in architecture [9–11]. In fiber composites, a major approach to alternative shaping is tackled at the fiber-bundle level, through robotic filament winding fabrication [12,13].

**Citation:** Blonder, A.; Brocato, M. Layered-Fabric Materiality Fibre Reinforced Polymers (L-FMFRP): Hysteretic Behavior in Architectured FRP Material. *Polymers* **2022**, *14*, 1141. https://doi.org/10.3390/ polym14061141

Academic Editor: Rajesh Mishra

Received: 26 January 2022 Accepted: 2 March 2022 Published: 12 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

This research tackles the alternative shaping of FRPs on the fabric level, enhancing the textile qualities of the fiber constituent of the composite. The term "*fabric materiality* FRP" (FM-FRP) was coined to represent the integration of textile-related practices, techniques, design paradigms and material qualities into the world of fiber composites, as novel FRP material systems [14]. FM-FRP typically would inherit key assets from the world of textiles, such as parametric variability, self-organization and resilience, resulting in novel architectural material systems. Fabric manipulations and self-organization capacities substitute the extensive use of molds, thus suggesting alternative architectural outcomes. The FM-FRP material system that was developed based on the layering of pleated surfaces resulted in an airy element with a variable and intricate section; blurring the boundaries between structure and matter, it resembles a thick panel [14] (Figure 1). The combination of matter and voids as an internal material structure and the creation of porous systems with enhanced capacities for multifunctional applications [15] is currently of high interest, developing *architectured materials*, or *metamaterials* [16]. The voids can be distributed periodically (such as in lattices [17]) or in relative disorder (such as in entangled materials [18]).

**Figure 1.** Layered FM-FRP material system (L-FMFRP) resulting in architectured material shaped as a thick panel.

In the field of composites, the integration of voids in the material architecture is mainly applied as tubular hollow composites, based on techniques such as braiding, knitting, or spacer [19]. Such hollow-structured elements can also be incorporated as nested inserts in more complex sandwich structures when seeking to further optimize performance and light weight [20]. Out of the various applications of tubular composites, from sports equipment and printing rollers to rocket structures and helicopter landing gears, various applications make use of the crashworthiness and energy absorption properties of hollow composites, as energy-absorbent composite structures (EACS). Composites can absorb a substantial amount of energy per unit mass in comparison to metals. Other advantages of EACS include higher strength, lower weight, higher specific-stiffness, better potential in terms of vibration control and noise reduction [21]. Due to the brittle nature of composites, the energy is absorbed in FRP structures mostly through the conversion of kinetic energy to a form of deformation absorbed energy—a complex fracture mechanism of cracking, delamination and fiber breakage [21]. The level of energy absorption is affected by a variety of parameters of geometrical and material nature, depending on the fiber architecture as well as the resin and matrix material characteristics [22]. The unique energyabsorbing qualities of composite structures gradually make them a preferred choice for crashworthiness applications, along with increasing research interest and technological progress in the field [21].

The preliminary testing of L-FMFRP material demonstrated its general suitability for service as an architectural façade element [23]. In particular, compression tests revealed a capacity of the panel for large quasi-elastic deformation and a good recovery of the material after extensive compression, indicating the potential energy-absorption qualities of the material.

In the past two decades, there has been a growing concern for safety and security in the built environment. From the scale of urban planning to material design, efforts are being invested in the creation of resilient built environments that would actively contribute toward facing the challenges of terrorism and of natural disasters [24]. The growing threat of terrorist attacks in city centers urges the development of blast-resistant façade systems [25] and energy-absorbent material systems [26]. In parallel, climate change accentuates the occurrence of natural disasters such as cyclones, hurricanes and typhoons, causing tremendous casualties and physical damage. Studies have shown that the windborne debris of storms, generated either by unsecured items or by the progressive failure of the built environment, plays a major part in causing damage. Failure of the façade elements prove to be hazardous for the surroundings, the structure itself and its occupants, and can ultimately lead to the general failure of the structure by changing the internal–external balance of air pressure [27]. Aiming to improve performance with regard to both natural disasters and terrorist attacks, façade materials are re-evaluated with regard to updated codes and regulations [28], and structural concepts such as sacrificial façade systems are developed [29]. Fiber composites can potentially answer the need for cladding systems that are lightweight and perform as efficient energy absorbers [26]. The typical plastic deformation of extensive micro-cracking, rather than general buckling failure, together with the high ratio of density to flexural stiffness make FRP materials a potentially suitable domain for the development of such solutions [30].

Following the initial indications for the potential energy-absorption capacities and interesting properties of L-FMFRP under compression, three issues were defined for further investigation through compressive tests:

Two experimental campaigns were carried out for the investigation of the above three issues. The main results of these tests are the observation of a phenomenon of hysteresis and some correlations between the shape of the loading–unloading paths and the mesoarchitecture of the specimens.

#### **2. Materials and Methods**

#### *2.1. Materials and Sample Preparation*

Testing was realized on L-FMFRP specimens, all made of fiberglass–epoxy prepreg 0/90 satin weave of 300 g/m2: Prepreg E-glass 7781: fabric thickness 0.23 mm, weave pattern 8HS; resin content 30% +/−3%, Tg 124◦.

The samples were fabricated according to a protocol that was developed in the framework of previous research [23], which consists of the pleating of single sheets and their stacking with light peripheral constraint, in order to form the equivalent of a laminate, a volumetric element (Figure 2):

**Figure 2.** L-FMFRP fabrication process: (**a**) introducing metal rods in pre-cut holes in the fabric; (**b**) contracting the fabric at points along the rod; (**c**) assuring the local contractions with a temporary string; (**d**) super-imposing the manipulated fabric; (**e**) contracting the layered assembly with a jig for oven curing.

Pleating of the single sheet:

All samples were composed of two prepreg layers, 600/200 mm each. The stacked assembly was oven-cured at a temperature of 125 ◦C for 2 h.

#### 2.1.1. First Experimental Campaign

The testing was carried over three samples of identical pattern and similar internal structure (Figure 3a). The pleating pattern is composed of three rows, with metal rods aligning five pleats (gathering points) along its central row. As the product is not molded, and the manipulated layers are just lightly compressed in the curing jig, the different samples exhibit variations; the final dimensions of the panel were averagely 300–320 L/190 W/40–55 H (mm).

**Figure 3.** Samples of two experimental campaigns. (**a**) First campaign: typical sample top (left), and bottom view (middle), pleating pattern for both layers (right). (**b**) Second campaign: four tested samples, views and pleating patterns of top and bottom layers.

#### 2.1.2. Second Experimental Campaign

Testing was carried over four samples, each with a different pleating pattern. Two pleat types were used: a 'simple pleat' pinched along two rods (top and bottom) and a 'full pleat' pinched along three rods (top, center, bottom) (Figure 3b). Each sample comprised two layers of similar pleating type and a variable number of pleats. The final dimensions of the samples varied according to the pleat types (Table 1).

**Table 1.** Pleating types–second experimental campaign: samples' data.

#### *2.2. Mechanical Testing*

#### 2.2.1. First Experimental Campaign

The testing was realized in an electromechanical tension–compression MTS testing machine of 100 kN capacity, with the samples placed between two steel plates (20 mm thickness) for levelling and load distribution over the surface. An average of 20 loading cycles were realized per sample, at constant displacement rate (between 8 and 40 mm/min, varying between test sets). The maximal load was averagely 3000 N (excluding the first cycles of the setting and the last cycles of the rupture).

#### 2.2.2. Second Experimental Campaign

The testing was realized in an electromechanical tension–compression Instron testing machine of 10 kN capacity, with the samples placed between two steel plates (20 mm thickness) for levelling and load distribution over the surface (Figure 4).

**Figure 4.** Experiment setup: sample positioned between two steel levelling plates.

#### **3. Results**

#### *3.1. Preliminary Testing*

A preliminary study of L-FMFRP's structural characteristics was carried out in the framework of previous research, testing the surface element under tension, compression and bending. The results showed a structural behavior that would satisfy the requirements for different theoretical schematic architectural cladding applications [23]. The compression test set (distributed load perpendicular to the panel surface) indicated a capacity of the panel for large quasi-elastic deformations. The generation of large displacement under loading (compressing the system to less than half of its initial thickness) did not lead to failure, but for the appearance of a few minor localized cracks. Furthermore, the initial thickness of the panel was almost entirely recovered after unloading (Figure 5). The observed behavior indicated the potential properties of hysteresis, and the capacity of energy absorption of the system, which led to the following two experimental campaigns.

#### *3.2. First Experimental Campaign: Cyclic Loading*

The results of the first experimental campaign showed a significant difference between the loading and the unloading curves. When plotting the force against deformation, the two curves form a loop with an area enclosed between the curves (Figure 6). The loading curve shows positive hardening (a greater force required as the deformation increases) and the difference between the curves indicates a different path taken by the material upon loading/unloading. This typical curve, resembling exponential progression, is different from the curves that were plotted in the initial compression tests (operated on samples of various architectures (Figure 5)), which showed a more linear progression of force as the deformation advances. Looking at the first cycles of compression in the repeated loading of the current experiments, we see an evolution of the curve, from a quasi-linear slope in the first compression cycle, which resembles the previous initial compression tests, to the exponential type of curve by the third compression cycle. The evolution between the test sets of a single sample shows a settling of the material during the first loading cycles, and as the cycles are repeated, the response gradually stabilizes (Figure 6).

Increasing the applied force beyond the previously achieved threshold generates an irreversible plastic deformation, visible in the dented curve section. The repeated loading thereafter within the newly achieved threshold keeps a reversible elastic behavior with smooth curves. A small shift in the zero-state at each loading cycle is noticeable (within a similar threshold), showing an accumulated minimal irreversible deformation (about 6% overall). However, as the number of cycles increases under the same threshold, the accumulated irreversible deformation decreases (Figure 7).

**Figure 6.** Typical loading/unloading curve: evolution along test sets. Sections in detail below: (**a**) settling, (**b**) first hysteretic cycles, (**c**) steady repeated cycles (Experimental campaign1\_sample01).

A similar loop curve appears among all three samples, despite their relative differences due to manual fabrication and material self-organization. A superposition of loops of different samples under identical loading thresholds strongly resembles the curves and the enclosed area between the loading–unloading (Figure 8).

**Figure 8.** Similarity of loading–unloading loops between two samples of identical pleating pattern. (**a**) Repeated loading of sample 01 (red) and sample 03 (blue). (**b**) Comparison of loops between samples: enclosed area of sample 03 (blue stain) superimposed over loop curve of sample 01 (red).

#### *3.3. Second Experimental Campaign: Pleating Pattern*

The results of the second experimental campaign show the variation of the hysteretic loop between the samples of different pleating patterns. General similarity remains between the samples, as all demonstrate the hysteretic loop. However, the enclosed area of the loops as well as the overall steepness of the curves vary between samples. The plotting force against the normalized displacement of all samples combined shows that the samples of the full pleats (red, purple) go through larger relative deformation compared to the simple pleats (blue, green). Accordingly, the enclosed area of their loops is larger, showing a stronger difference between the loading and unloading paths (Figure 9). A comparative look at two samples of a single pleat in the two configurations (simple/full) shows the steeper curves of the single pleat compared to the full one, as well as a larger area enclosed in the loading–unloading loop.

**Figure 9.** Effect of pleat types force plotted against normalized deformation and grouped by color shades. (**a**) All samples combined: one simple pleat (blue); one full pleat (red/yellow); three simple pleats (green); three full pleats (purple). (**b**) Comparison between simple and full pleat types: simple pleat (blue), showing steeper curves and smaller enclosed area in path; full pleat (red), showing larger enclosed area and shallower curves.

The comparison between single and triple pleats of the same kind shows a similar behavior for both simple and full pleats (Figure 10). The higher displacement values for a larger number of pleats indicate lower stiffness. The enclosed area of the hysteresis loops rises with the number of pleats, indicating higher energy absorption.

**Figure 10.** Effect of pleat number—increasing the number of pleats reduces stiffness and increases energy absorption. (**a**) Full pleats: single (purple) and triple (red). (**b**) Simple pleats: single (blue) and triple (green).

#### **4. Discussion**

The experiments show a loop of a defined deformation cycle (loading and unloading curves) enclosing an area, which indicates a loss of elastic energy in the process, allegedly attributed to friction. Such behavior is known as hysteresis, occurring in a variety of materials, structures and systems, where the dependence of a physical system upon its history is expressed as a non-linear behavior with a retardation of the effect when external forces acting upon the system are changed. This phenomenon is typically associated with materials of visco-elastic behavior such as elastomers [31], and is not particularly typical of FRP, where no intrinsic dissipation mechanisms exist in statics before failure [32]. On the contrary, when in dynamics, FRP may have damping capacities by dissipating some elastic energy through the visco-elastic behavior of their resin, or by various fracture mechanisms [20].

The similarity of load–unload loops between the different samples clearly demonstrates that the hysteretic behavior is a typical property of the L-FMFRP architectured material. While the samples show significant variations in their specific material configuration, due to manual manufacturing and material self-organization, the strong similarity of hysteretic loops between the samples indicates that this property is not sensitive to specific internal configuration (Figure 8). Rather, it seems to be dictated by the overall material architecture, i.e., the pleating pattern and the connections between the layers. A parallel can be made to other architectured materials with randomly disordered structure at the meso scale, such as entangled materials [33] or the jamming of aggregate systems [18]. Such systems demonstrate hysteretic behaviors that depend on their internal architectural parameters. Parameters such as the number of connections between the discrete components of the system (such as fibers or granules), their orientation or aspect ratio determine the level of energy dissipation of the system [34].

The effect of different parameters of the material architectures of L-FMFRP and its behavior can be initially observed through the comparison between different pleating patterns (campaign 02). Although the isolation and modelling of the effect of each parameter requires further study, the difference between the full and simple pleats is noticeable in the typical loading–unloading paths (Figure 9). The two different patterns generate surfaces of different geometrical complexity; the manipulation of the simple pleat develops into a barrel-like curved surface, and the full pleat develops into conical-type formations with a higher degree of geometrical complexity. The elements generated by patterns that are more geometrically complex show reduced stiffness (indicated by the reduced overall steepness of the loading curve). The connection between the folding complexity of the surface and its reduced stiffness could be compared to the behavior of knitted hollow composites, where the elastic stage of the compression curve corresponds to the flattening and ovalization of the hollow channels, and is correlated with the channel's radius of curvature [24]. Whether the model describing the compression curve of hollow composites could be suitable for our case should be further investigated as well (transitioning from elastic flattening of the structure to deformation at the joining points, and finally the densification of fibers and load transfer to matrix). In parallel to reduced stiffness, a larger area is enclosed in the hysteretic loop of the load–unload curves, with increased surface complexity. This is visible both in the comparison between simple and full pleats, as well as between surfaces of single or multiple pleats. This could be explained by the role friction plays in the hysteretic process, determined by parameters such as the number of contact points, the tangency of surfaces and areas of contact, which are generated by the pleating pattern and will be further investigated. A parallel can be drawn with knitted tubular lattice composites, where energy absorption improves with a higher number of cells to be deformed [35].

The role of the connection between the layers is to be further studied. The contact points between the pleated layers of the laminate are essential for the material to perform as a constrained unit under load and prevent immediate sliding of the layers. However, the displacement of the layers during compression shows a relative sliding of the layers. This sliding is within the elastic range of the material and its contact points, since the meso-scale architecture of the material is not destroyed through the extensive loading, and the layers within each sample remain connected.

Looking at the analysis of entangled materials, the connection between the components' contact length and permanence of contact seems to be a critical architectural parameter in the material performance [34]. A fundamental difference was found between entangled materials with initial permanent connections between components (epoxy cross-links between fibers) and without rigid connections (loose fibers). The type of stress–strain curves of the two materials is different and indicates an evolution of connection points along loading; the unlinked material starts with low initial stiffness, which increases with deformation as new contact points are created by compression and friction augments. This typically exponential stress–strain curve resembles the curve of L-FMFRP, where stiffness increases with deformation. The material of permanent links goes through a three-step process, starting with elastic behavior, going through a plateau where permanent connections are destroyed and new contact points are constructed, and ending with pure densification of the contact points [34]. This type of curve resembles the curve of the first loading cycle (Figure 5). The evolution of the stress–strain curve in cyclic loading, from linear-like curve to exponentially growing curve, indicates the evolution of the contact points and friction level of the material during increased compression. Contact points between the pleated layers of L-FMFRP are permanent and persist during compression, indicating that these contacts perform within the elastic range of the resin incorporating them. However, we have no indication of the number of contacts and the extension of the contact areas before and after the loading, and it is probable that a certain number of connections are at least partly destroyed during the first cycle of compression. This might explain the resemblance between the curve of the first compression cycles and the typical curves of the permanently linked entangled systems, with a three-step evolution of contact points.

The role of friction in the hysteresis of entangled material was studied previously [36,37], indicating its direct relation to the hysteresis in stretching and bending energy; it was found to be determined by the friction coefficient and average number of contact points between

the elements. As L-FMFRP only partially resembles entangled materials, the relevance of its friction model to describe the behavior of our system requires further study.

#### **5. Conclusions**

The L-FMFRP material system product resembles a thick panel with highly articulated surfaces of extreme low density. It stands in contrast to typical FRP panels: its section is not uniform and dense, but variable and airy; it has an intricate internal configuration set by the pleats and the partial adherence between the layers. This, in turn, affects and determines the characteristics of the overall resulting element, its stiffness and strength. L-FMFRP blurs the boundaries between material and structure, and thus can be considered an 'architectured material', for which the overall macro-scale behavior is related to two underlying scales: the micro-scale, where the density and the direction of the fibers play a principal role in defining the textile's properties; and the meso-scale, where a complex geometry and a set of internal bonds give rise to a particular, airy, structure.

The finite deformation properties of L-FMFRP and the hysteresis phenomena demonstrated in the experiments indicate a potential capacity of the material system for energy absorbance. The experiments show that this property is inherent to the material's architecture, and is affected by parameters such as pleat type and number. Such a capacity for energy absorbance, if further investigated and developed, could prove to be beneficial for architectural cladding applications, as retrofit or for new structures. Other energy absorbent composite structures designed for crashworthiness applications could serve as a reference [26]. While these are mainly implemented in moving structures (transportation), here, another application in architectural structures is suggested. Possible implementation could be a development of L-FMFRP, either as part of a composite sandwich construction or as a single "thick skin" with cellular characteristics.

An initial estimation is to be carried out to assess the capacities of the L-FMFRP system to withstand the range of loads typically considered for windborne debris, blast mitigation by sacrificial façade systems and general energy dissipation for fluctuating air pressure. Referring to its crashworthiness properties, its characterization should refer to indicators of performance such as the specific energy absorption (SEA), energy absorption (EA) capacity, crush force efficiency (CFE), mean crushing force (MCF) and sound transmission loss (STL) [26]. With that, a better understanding is required as to the capacity of the material system to be enhanced by variation of the fabrication parameters or the association of additional elements to the system.

The further investigation of L-FMFRP calls for a deeper understanding of the internal physical phenomena of energy dissipation in the system and its determining parameters. Wishing to augment its capacities for energy absorbance, the relation between friction, structure and geometrical pattern is to be evidenced. Monitoring the heat loss and the sound emitted throughout the full loading cycle could indicate the fractional energy loss that takes place. The role of contact points, their evolution along compression cycles, their effect on friction, their relation to the folding pattern and the effect of those on the hysteresis level should be further investigated in order to optimize the energy absorbance of the system. Through further physical experimentations and mathematical modelling, control of the system's performance could be obtained through the variation of the geometrical folding pattern and raw material properties.

**Author Contributions:** Conceptualization, A.B.; Data curation, A.B.; Formal analysis, M.B.; Investigation, A.B.; Supervision, M.B.; Visualization, A.B.; Writing—original draft, A.B.; Writing—review & editing, M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** The data that support the findings of this study are openly available in the Figshare repository: https://doi.org/10.6084/m9.figshare.18515129 (accessed on 25 January 2022).

**Acknowledgments:** The research was realized in the framework of postdoctoral scholarship by the Israeli Council for Higher Education, PBC (Vatat). The mechanical tests were realized at the Laboratoire Navier at Université Paris-Est, Ecole des Ponts ParisTech. Special thanks to Gilles Foret for his assistance and guidance.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


3.2.3 Battery bank technical data

sector. This information must be validated each time the optimization model is used

fuel curve parameters. This table was built using information supplied by the Colombian Regulation Commission of Energy and Gas (CREG—Comisión de Regulación de Energía y Gas) in [23]. The cost per kW presented in Table 5 includes the direct and indirect costs related to the installation of a Diesel plant in

> Derate factors of the initial capital cost invested [%]

 2724.09 31.83 1.4 2.6 0.020 0.240 1697.26 32.43 3.4 6.05 0.037 0.265 1540.12 31.63 3.6 6.4 0.032 0.224 1934.44 23.00 6.8 10.96 0.088 0.277 1654.09 23.71 8.69 15.12 0.056 0.321 1434.92 25.12 9.825 16.63 0.060 0.272 1343.75 25.26 10.96 18.14 0.063 0.239 1788.83 18.13 11.43 19.77 0.044 0.238 1686.08 18.56 11.9 21.4 0.030 0.237 1723.40 17.24 12.85 23.06 0.026 0.204 1587.11 17.92 18.9 34.4 0.027 0.248 1572.63 17.55 22.3 41.2 0.022 0.252 1373.73 19.32 29.11 54.43 0.019 0.253

Battery bank input data Symbol Description Value Vdcbc Battery voltage [V] 2 Vdcsist DC system voltage [V] 48 Crate Capacity rate [h] 5 ηbat\_c Charge efficiency 0.9 ηbat\_d Discharge efficiency 1 σ Self-discharge rate 0.000083 Lbat Lifecycle [years] 10 δbat Factor of the initial capital cost invested for the battery bank 0.7 ρbat Fixed OM factor as ratio of the battery bank initial investment 0.02 DODmax Maximum depth of discharge 0.5

Table 5 shows a database of diesel generation units with the cost per kW and the

1/2 load 1 hour in liters

Full-load 1 hour in liters

f0 [L/ kWh]

f1 [L/ kWh]

since it can vary depending on the studied case.

Wind Solar Hybrid Renewable Energy System

non-interconnected zones.

Cost per kW installed [USD/ kW]

DG power [kW]

Table 5.

Table 6.

198

Battery bank technical inputs.

Diesel genset unit database.

In this chapter book, vented lead-acid battery banks only are considered. This kind of battery cells are often selected for large energy storage banks due the low cost, low maintenance, and high cycle stability. Table 6 shows the input data required by the battery bank. The battery bank charge and discharge efficiency and the self-discharge ratio is taken from [24]. The maximum depth of discharge is set in 0.5 since the battery bank can accomplish 3000 cycles during its life service

#### Table 7.

Battery cell database.

Table 8. System input parameters.

#### Table 9.

PSO input parameters.

according the datasheet. Other values as maintenance cost, ρbat, and the fraction of reposition cost, δbat, are set according to the recommendation of experts in the energy sector.

The main characteristics and price of the battery cells of the reference used in this work are presented in Table 7. The information was obtained from inquiries to local companies.

5. Conclusions

Results of the case study.

Table 10.

energy storage.

fiscal incentive factor.

DG units are commonly underestimated.

resolution.

201

In this work, an optimization methodology was developed and described in detail to help sizing HRSE integrated by photovoltaic and diesel generation with

Component Design Unit Indicator Value Unit Npv 13 Units CCpv 7800.00 USD Ppvstc 3.9 [kWp] CCDG 48257.99 USD wDG 25 [kW] CCbat 3864.00 USD NDG 2 Units O&Mpv 78.00 USD/year PDG 50 [kW] O&MDGf 4825.80 USD/year Nbp 1 Units O&MDGv 26884.74 USD/year Nbs 24 Units O&MDG 31710.54 USD/year Nbat 24 Units O&Mbat 77.28 USD/year Ebcell, nom 1.04 [kWh] RCDG 7019.48 USD Ebat, n 24.96 [kWh] RCbat 1243.60 USD

Methodology for Sizing Hybrid Battery-Backed Power Generation Systems in Off-Grid Areas

DOI: http://dx.doi.org/10.5772/intechopen.88830

FC 38406.77 [l] ACSadj 38737.05 USD/year LPSP 1.25 % COEadj 0.26 USD/kWh ACloss 475.03 USD/year LPVG 0.00 % Cost 0.21 USD/kWh

The main features of the sizing methodology developed were as follows: (a) it allows the simulation of hybrid renewable systems and the evaluation of its economic and reliability integrated by diesel and photovoltaic generation with energy storage, (b) the dispatch strategy developed prioritize the use of renewable energy among other energy sources, and (c) fiscal incentives granted by the Act 1715 of 2014 in Colombia were considered on the calculation of the cost of energy using the

The reliability of the system was included in the objective function of the PSO algorithm through the annual cost of the energy not supplied. Also a fiscal incentive factor was used to include the financial benefits granted by the Act 1715 of 2014 in Colombia to non-conventional renewable source of energy. The results were obtained after simulating the energy flow of the system for 1 year with 1-hour

Dispatch strategy was described in detail, prioritizing the use of renewable resource over diesel generation to supply the load. Also diesel generation cannot be used to charge the battery bank. This condition was based on the fact that, in offgrid areas, the complications associated to supply the fuel and the maintenance of

### 3.2.4 System inputs

The system input parameters are shown in Table 8. The cost of energy lost is assumed in 0.2 USD/kWh. This value depends on the necessities and characteristics of the users of the select location. The interest rate considered in this work is 8.08% taken in [25].

Fiscal incentive factor is calculated applying an effective corporate tax income rate of 33%. The resulting incentive factor is 0.938.

The parameters for the PSO algorithm and the boundaries for each decision variable are shown in Table 9.

## 4. Results of the case study

Table 10 summarized the obtained results after applying the proposed sizing methodology. The best cost achieved was 0.2090 USD/kWh being the lowest obtained. The optimization results deliver no only the design (number of components) but also economic and reliability indicators.

Methodology for Sizing Hybrid Battery-Backed Power Generation Systems in Off-Grid Areas DOI: http://dx.doi.org/10.5772/intechopen.88830

Table 10. Results of the case study.

according the datasheet. Other values as maintenance cost, ρbat, and the fraction of reposition cost, δbat, are set according to the recommendation of experts in the

PSO input parameters Symbol Description Value NPVl Lower bound number of PV modules 0 wDGl Lower bound nominal power of diesel 0 NBp l Lower bound number of battery cell in parallel 0 Ebcell, noml Lower bound nominal capacity of battery cell [kWh] 0 NPVu Upper bound number of PV modules 20,000 wDGu Upper bound nominal power of diesel unit in [kW] 200 NBp u Upper bound number of battery cell in parallel 10 Ebcell, nomu Upper bound nominal capacity of battery cell [kWh] 9.40 Maxit Maximum number of iterations 50 nPop Population size 200 w Inertia coefficient 1 wmax Inertia coefficient max 0.9 wmin Inertia coefficient min 0.5 c1 Personal acceleration coefficient 2.5 c2 Social acceleration coefficient 1.5

Wind Solar Hybrid Renewable Energy System

The main characteristics and price of the battery cells of the reference used in this work are presented in Table 7. The information was obtained from inquiries to

The system input parameters are shown in Table 8. The cost of energy lost is assumed in 0.2 USD/kWh. This value depends on the necessities and characteristics of the users of the select location. The interest rate considered in this work is 8.08%

Fiscal incentive factor is calculated applying an effective corporate tax income

The parameters for the PSO algorithm and the boundaries for each decision

Table 10 summarized the obtained results after applying the proposed sizing methodology. The best cost achieved was 0.2090 USD/kWh being the lowest obtained. The optimization results deliver no only the design (number of compo-

rate of 33%. The resulting incentive factor is 0.938.

nents) but also economic and reliability indicators.

energy sector.

PSO input parameters.

Table 9.

local companies.

3.2.4 System inputs

taken in [25].

200

variable are shown in Table 9.

4. Results of the case study


**2. Surface energy in Froth flotation**

The concept of surface energy provides a satisfactory basis for explaining a wide range of "capillary phenomena", many of which are pertinent to the flotation process.

#### **Figure 3.**

*Surface and Interfacial tensions for the molecules in bulk liquid.*

According to molecular theory, different crystal structures have different surface energy [9–11]. Interface is the boundary between two or more phases exist together. The properties of the molecules forming the interface are different from those in the bulk that these molecules are forming an interfacial phase. Several types of interface can exist depending on whether the two adjacent phases are in solid, liquid or gaseous state.

### **2.1 Interfacial tension**

In the liquid state, the cohesive forces between adjacent molecules are well developed. There are various forces present between surface and interfaces molecules presented in **Figure 3.**

Cohesive forces are present with the other molecules which are situated below or adjacent to them. Adhesive forces are being developed with the molecules of other phases in the interface. Surface tension (γ) is the force per unit length that must be applied parallel to the surface so as to counter balance the net inward pull. Interfacial tension is the force per unit length existing at the interface between two different phases.


**Simultaneous Genome Sequencing of** *Prosthecochloris ethylica* **and** *Desulfuromonas acetoxidans* **within a Syntrophic Mixture Reveals Unique Pili and Protein Interactions**

#### **John A. Kyndt 1,\*, Jozef J. Van Beeumen 2 and Terry E. Meyer 3,**†

Received: 1 November 2020; Accepted: 4 December 2020; Published: 7 December 2020

**Abstract:** Strains of *Chloropseudomonas ethylica*, 2-K, N2, and N3 are known to be composed of a syntrophic mixture of a green sulfur bacterium and a sulfur-reducing colorless component. Upon sequence analysis, the green sulfur photosynthetic bacterial component of strain N3 was dominant and was readily sequenced, but the less abundant sulfur-reducing bacterial component was apparent only when analyzed by metagenomic binning. Whole-genome comparison showed that the green bacterium belonged to the genus *Prosthecochloris* and apparently was a species for which there was no genome sequence on file. For comparison, we also sequenced the genome of *Prosthecochloris* sp. DSM 1685, which had previously been isolated from the 2-K mixture in pure culture and have shown that all three *Prosthecochloris* genomes belong to a new species, which we propose to be named *Prosthecochloris ethylica* comb. nov. Whole genomes were also sequenced for the isolated *Desulfuromonas* strains DSM 1675 (from strain 2-K) and DSM 1676 (from strain N2) and shown to be nearly identical to the genome found in the N3 mixture. The genome of the green sulfur bacterium contains large genes for agglutination proteins, similar to the ones proposed to be involved in larger photosynthetic consortia of *Chlorochromatium aggregatum*. In addition, we also identified several unique "tight adhesion (tad)" pili genes that are presumably involved in the formation of cell–cell interactions. The colorless component, on the other hand, contained a unique large multiheme cytochrome C and unique genes for e-pili (geopilin) formation, genetically clustered with a conserved ferredoxin gene, which are all expected to play an electron transfer role in the closed sulfur cycle in the syntrophic mixture. The findings from the simultaneous genome sequencing of the components of *Cp. ethylica* have implications for the phenomenon of direct interspecies interactions and coupled electron transfer in photosynthetic symbionts. The mechanisms for such interactions appear to be more common in the environment than originally anticipated.

**Keywords:** green sulfur bacteria; syntrophy; e-pili; adhesion protein; photosynthetic symbionts; large multiheme cytochrome; metagenomic binning


*3.5. Tape-Stripping Drug Penetration Studies*

The stratum corneum layers depth in porcine skin was determined by infrared densitometry using a standardized procedure [66]. The total thickness obtained was around

20 µm, according with the standard values between 17–28 µm, previously reported by Jacobi et al. [99]. Tape-stripping studies were performed using the most promising formulations according to the afore-presented data: L1, L2, T1d, T2d, and E1. As incubation times, 2 h, 4 h, and 6 h were selected, covering the longest time a topical formulation would be maintained onto the skin. Besides, longer incubation times lead to overhydration of the skin and removal of the whole stratum corneum in a single strip [83]. The B12 solution 0.5% *w/v* (S) was used as a reference since it represents the free diffusion, and no enhancing absorption effects are expected from the vehicle. B12 has been delivered through the skin using chemical and physical methods by other authors. Recently, Ramöller et al. achieved an effective delivery of B12 to plasma in a rat model. Rapidly dissolving microneedle allowed the permeation of 60% of dose after 2 h post-insertion, achieving 0.4 μg/mL plasma levels [28]. Yang et al. studied in vitro the effects of chemical enhancers and physical methods in topical administration of B12. Chemical enhancers (ethanol, oleic acid, and propylene glycol) allowed an effective permeability of B12 in comparison with passive diffusion. The permeability enhancement of-

*Pharmaceutics* **2021**, *13*, x FOR PEER REVIEW 17 of 23

Figure 8 shows the progressive B12 penetration in the different layers of the stratum corneum at different incubation periods. In general, the amounts of B12 detected in the deeper layers of the skin increased as a function of the incubation time, as expected. As shown, B12 solution presented the highest amount of B12 extracted in the first strip regardless of the incubation time. Two reasons can explain this fact: immediate availability of B12 on the skin leading to a quick distribution over the first stratum corneum layer, where it is retained; and a deficient cleaning of the skin surface. Concerning the latter, many authors discard this strip because the inaccuracy of the data is typically high and considered as an experimental artifact [100]. However, the rest of B12 from the solution did not penetrate the stratum corneum nor the deeper skin layers, and only low quantifiable concentrations were detected up to the third strip, which corresponds to 6 µm depth, after 6 h. These findings confirm the inability of B12 to diffuse through intact skin. The ethosome samples presented the lowest quantifiable amounts among all the vesicles tested, probably due to the low B12 load and the gradual release. Samples could have easily been under the detection/quantification limits. fered by iontophoresis was around 2-fold in comparison with a combination of all enhancers (50% ethanol, 10% oleic acid, and 40% propylene glycol) [27]. Although these methods have proven to effectively delivery B12, they present several inconveniences in B12 delivery for atopic dermatitis. For example, they were not able to localize their effects in the epidermis and dermis leading to an absorption of B12 to systemic circulation, which is unnecessary for psoriatic and atopic dermatitis patients. Additionally, local skin reactions and poor effectiveness in combination with hydrophilic molecules has been reported for chemical enhancers [102]. Moreover, ultraflexible lipid vesicles are technically easier to produce and a low cost in comparison to microneedles and iontophoresis devices [102]. From the results presented here, it can be concluded that the developed formulations are able to efficiently deliver B12 to the deeper skin layers after 6 h, as desired. We also demonstrate that B12 does not diffuse through the stratum corneum if formulated in water media. Further studies should be performed to formulate these vesicles in adequate dosage forms for an effective topical application of B12 that can be transferred to the clinic.

**Figure 8.** *Cont.*

**Figure 8.** Penetration profiles of B12 delivered from liposomes, transferosomes, ethosomes, and solution after 2 h (**a**), 4 h (**b**), and 6 h (**c**). All results are expressed as mean ± SD (*n* = 3). **Figure 8.** Penetration profiles of B12 delivered from liposomes, transferosomes, ethosomes, and solution after 2 h (**a**), 4 h (**b**), and 6 h (**c**). All results are expressed as mean ± SD (*n* = 3).

**4. Conclusions**  Enhanced penetration of B12 through the skin is possible using the lipid vesicle formulations (L1, T2d, and T1d) designed in this work, which opens the possibility to improve the clinical results obtained in previous studies and to investigate its utility for topical treatment of atopic dermatitis and psoriasis. Several particle properties such as size, stability, and purification method were revealed as key parameters to achieve a suitable and efficient production of lipid vesicles. Skin permeability studies should be further investigated to elucidate if these nanosystems are able not only to increase the penetration in the skin but also to allow the passage of B12 to the systemic circulation, thus broadening the possible applications of the systems for the treatment of other pathologies related with The best penetration results were obtained using liposomes and transferosomes. L2 carries less B12 amounts than the other prototypes, consequently showing a considerably lower B12 penetration amounts, only until approximately 15 µm depth (Figure 8c). Nevertheless, L1, T1d, and T2d vesicles allowed the B12 to reach the dermis (>25 µm). L1 and T2d contained similar B12 doses, but after 4 and 6 h of incubation, the transferosomes showed higher permeation rates up to the deepest layers (Figure 8b,c). This means that the stratum corneum is saturated in B12 at that time point, and a diffusion gradient to the deeper layers starts. T1d showed the highest permeation profiles. Our penetration results suggest that transferosome enhancing effect was much higher than the one of liposomes and are in agreement with Abd et al. [101], who compared the penetration of a hydrophilic drug (caffeine) delivered from liposomes, transferosomes, and solution.

B12 deficiencies. **Supplementary Materials:** The following are available online at www.mdpi.com/xxx/s1, Figure S1: Release parameters (10 and 72 h) for release kinetic models: Higuchi, Korsmeyer–Peppas, Kim, Peppas–Sahlin, zero order, and first order; Figure S2: *R*2 values of Korsmeyer–Peppas and first order models for the B12 vesicles release data (10 and 72 h). B12 has been delivered through the skin using chemical and physical methods by other authors. Recently, Ramöller et al. achieved an effective delivery of B12 to plasma in a rat model. Rapidly dissolving microneedle allowed the permeation of 60% of dose after 2 h post-insertion, achieving 0.4 µg/mL plasma levels [28]. Yang et al. studied in vitro the effects of chemical enhancers and physical methods in topical administration of B12. Chemical enhancers (ethanol, oleic acid, and propylene glycol) allowed an effective permeability of B12 in comparison with passive diffusion. The permeability enhancement

offered by iontophoresis was around 2-fold in comparison with a combination of all enhancers (50% ethanol, 10% oleic acid, and 40% propylene glycol) [27]. Although these methods have proven to effectively delivery B12, they present several inconveniences in B12 delivery for atopic dermatitis. For example, they were not able to localize their effects in the epidermis and dermis leading to an absorption of B12 to systemic circulation, which is unnecessary for psoriatic and atopic dermatitis patients. Additionally, local skin reactions and poor effectiveness in combination with hydrophilic molecules has been reported for chemical enhancers [102]. Moreover, ultraflexible lipid vesicles are technically easier to produce and a low cost in comparison to microneedles and iontophoresis devices [102].

From the results presented here, it can be concluded that the developed formulations are able to efficiently deliver B12 to the deeper skin layers after 6 h, as desired. We also demonstrate that B12 does not diffuse through the stratum corneum if formulated in water media. Further studies should be performed to formulate these vesicles in adequate dosage forms for an effective topical application of B12 that can be transferred to the clinic.


**2.2 Knoevenagel condensation reaction**

An efficient green one-pot synthetic method for the synthesis of 3-substituted coumarin derivatives **21/22** has been observed by Knoevenagel condensation of various *o*-hydroxybenzaldehydes **18/19** with 1,3-dicarbonyl compounds **20** using

#### *Phytochemicals in Human Health*

nano-ZnO catalyst under microwave or thermal conditions, which affords moderate to good yield of the products (**Figure 6**) [81]. Reactions under microwave-irradiation conditions are found to be more convenient than thermal conditions.

Various coumarin-3-carboxylic acid derivatives **25/26** have been synthesized in good yields using catalytic amounts of SnCl2.2H2O under solvent-free condition (**Figure 7**) [82].

Ultrasound irradiation technique is also useful to synthesize 3-aryl coumarin derivatives. Treatment of *o*-hydroxybenzaldehydes **18** with aryl substituted acetyl chloride **27** in the presence of K2CO3 as a catalyst in tetrahydrofuran (THF) using ultrasound irradiation leads to the formation of 3-aryl coumarin derivatives **28** in moderate to high yields (**Figure 8**) [83]. This green method appears to be a convenient and simple pathway than that of conventional heating.

Coumarin-substituted benzimidazole or benzoxazole derivatives **32** that are known as coumarin dyes have been synthesized in good yields from 4-diethylamino-2-hydroxybenzaldehyde **29**, ethyl cyanoacetate **30**, and ortho-phenylenediamine/phenylenehydroxyamine derivatives **31** in the presence of reusable green solid acid like HZSM-5 zeolite, heteropoly acids, e.g., tungstophosphoric acid (H3PW12O40), and/or tungstosilicic acid (H4O40SiW12) in *n*-pentanol or water and even solvent-free conditions (**Figure 9**) [84].

Cellulose sulfonic acid (CSA) is an efficient catalyst for the synthesis of 3 substituted coumarin via Knoevenagel condensation reaction. Thus, 3-acetyl coumarin **34** is obtained in 88% yield in the reaction between salicylaldehyde **33** and ethyl acetoacetate **7** in the presence of CSA under solvent-free conditions (**Figure 10**) [85].

**113**

**Figure 10.**

*Synthesis of 3-acetyl coumarin.*

**Figure 8.**

**Figure 9.**

*Synthesis of 3-aryl coumarin derivatives.*

*Synthesis of coumarin-substituted benzimidazoles/benzoxazoles.*

*One-Pot Synthesis of Coumarin Derivatives DOI: http://dx.doi.org/10.5772/intechopen.89013*

media and catalyst as well.

under heating condition (**Figure 11**, Condition C) [87].

Shaabani et al. [86] have described the synthesis of 3-substituted coumarins **21** in good yields via Knoevenagel condensation of 2-hydroxybenzaldehydes **18** with β-dicarbonyl compounds **35** in the presence of a recyclable ionic liquid 1,1,3,3-*N*,*N*,*N*′,*N*′-tetramethylguanidinium trifluoroacetate (TMGT) under thermal heating (**Figure 11**, Condition A) and/or microwave irradiation conditions (**Figure 11**, Condition B). 3-Substituted coumarins **21** are also synthesized from similar starting precursors using the 1,3-dimethylimidazolium methyl sulfate [MMIm][MSO4] ionic liquid in the presence of L-proline as an additional promoter

Imidazolium based phosphinite ionic liquid (IL-OPPh2) catalyzed synthesis of 3-substituted coumarin derivatives has been reported in literature; when *o*-hydroxy benzaldehydes **18** are treated with active methylene containing compounds **35** in the presence of IL-OPPh2 catalyst at 60°C, 3-substituted coumarin derivatives are obtained in moderate to good yields (**Figure 12**) [88]. TSIL plays both the reaction

Reactions of *o*-hydroxybenzaldehydes **18** with activated methylene compounds **35** catalyzed by Bronsted acid ionic liquid (BAIL) and 1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogen sulfate [(CH2)4SO3HMIM][HSO4] in water lead to

Synthesis of substituted coumarins via Knoevenagel condensation using various organic catalysts such as piperidine, ammonia, L-lysine, L-proline, benzoic acid,

3-substituted coumarin derivatives in good yields (**Figure 13**) [89].

etc. has been reported in literature and some are summarized in **Table 2**.