**1. Introduction**

336 Polyurethane

Czech)

Scucka, J., Soucek, K. (2007). *Architecture and Properties of Geocomposite Materials with Polyurethane Binders*, Institute of Geonics of ASCR, ISBN 978-80-86407-15-9, Ostrava (in

*Trade mark GEOPUR, no. 148 179, 1999 Czech Republic*, Arrangement et Protocole de Madrid

no. 864 266 EU, GEOPUR, no. M0002030 , 2000 Hungary

The use of cellular materials in general in the automotive industry, and polymeric foams in particular, has been increasing significantly for the last few decades. These materials are used within a particular vehicle for many different purposes, among which are, for example, sound and thermal insulation, vibration damping, fire protection and, of course, crashworthiness. Thus, crashworthiness, safety and protection parameters are strongly influenced by the materials used and, as a consequence, polymeric foams play a major role in the vehicle's crashworthiness levels. In absolute terms, the energy absorption capability of this class of materials can lead to significant improvements on the vehicle's passive safety, better protecting the passengers from aggressive impacts, by absorbing impact energy in a gradual and controlled manner. In addition, design limitations due to environmental constraints are growing steeply as are safety concerns. Whilst the former often leads to a reduction in the weight of the vehicle, the latter will most probably lead to the opposite. Therefore, the combination of properties such as low density, low cost and design flexibility with a great energy absorption capability, is what makes cellular materials so attractive for the automotive industry.

Presently, vehicle structures with high levels of crashworthiness protection are almost always light-weight and must deform in such a way as to dissipate the largest amount of impact energy possible. Several distinct mechanisms may contribute to this, such as, for example, plastic deformation, wrinkling, heat generation, etc. [20, 35]. One way to achieve these effects is to fill tubular or hollow metallic or composite structures with cellular materials, such as foams. During the last few decades, many researchers have been working on these issues [22, 28, 30–32].

©2012 Paulino and Teixeira-Dias, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ©2012 Paulino and Teixeira-Dias, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### 2 Will-be-set-by-IN-TECH 338 Polyurethane On the Use of Polyurethane Foam Paddings to Improve Passive Safety in Crashworthiness Applications 3

Polyurethane foam is nowadays being widely used in many energy absorption engineering applications such as cushioning and packaging [4, 15, 50]. Its use in automotive industry as an energy absorbing material in passive safety mechanisms goes beyond the protection functionality since it also provides more comfort, insulation and sound absorption. Thus, the role of this class of materials in vehicles is of special interest from both the consumer and the manufacturer points of view.

consists of a foam like impact padding confined inside the lateral doors of the vehicle. In order to be efficient, this foam padding must be mounted aligned with the occupant's pelvis, protecting one of the most critical areas in this type of collision. From the experiments and analyses made it is expected that this padding may absorb a significant part of the impact energy, and thus minimise both the forces transmitted to the body of the occupants and, most importantly, the magnitudes of the decelerations experienced, consequently reducing possible

On the Use of Polyurethane Foam Paddings to Improve Passive Safety in Crashworthiness Applications 339

The design of structures and the choice of materials for crashworthiness and protection systems within a vehicle are also of major importance and relevance for the overall safety of the driver and passengers. These are two of the research fields where there is still quite a large margin for improvement [16, 17, 24, 41, 46]. In the present work, the authors will present, discuss and compare the applicability of several distinct types of cellular materials for impact

The behaviour of three polymer based structural foams under compressive impact loading polypropylene, polyamide and rigid polyurethane foam — has been investigated by Avalle *et al.* [4]. As a conclusion, these materials are indicated as players of a very important role in passive safety systems. These authors obtained stress-strain curves in both static and impact loading (dynamic) conditions for the materials examined for different densities at room temperature. They analysed in detail, using energy-absorption and efficiency diagrams, the energy absorption characteristics of each material. Among the materials tested, these authors observed that polyurethane foam is the less sensitive to strain-rate and the one that presents the longest intermediate plateau stage. These facts distinguish this material from the remaining foams studied. The authors also concluded that the rigid polyurethane foam exhibited one of the highest efficiency levels, however, it lost its integrity during compression. The energy absorption behaviour properties of polyurethane foam were also investigated by Anindya and Shivakumar [2]. The authors evaluated the energy absorption attributes of polyurethane foam in various forms — flexible high resilience, flexible viscoelastic and semi-rigid — as a function of the overall foam density, based on the load-displacement

Taher *et al.* [47] investigated the use of polyurethane foam with a density of 47 kg/m3 as a core filler of a composite keel beam as a way of preventing global buckling and improving crashworthiness performance in aeroplanes and helicopters. The results obtained by these authors revealed that the energy absorbing mechanism can meet the requirements for the

Likewise, the behaviour of polyurethane foam filled thin-wall structures was investigated by Ghamarian *et al.* [13] in terms of crashworthiness improvement for the aerospace industry. The quasi-static crushing behaviour and efficiency of empty and foam-filled structures was investigated experimental and numerically and the efficiency and the authors were able to demonstrate that the filled tubes presented higher energy absorption capabilities than that of

injury levels of the occupants of the vehicle.

**2. Polyurethane foam as an energy absorbing material**

behaviour of the material under compressive loads.

purpose desired together with substantial savings.

the combined effect of the empty structured and the foam.

and energy absorbing paddings.

On a microscopical level, most cellular materials, including polyurethane foams, have the ability to absorb energy while deforming due to the mechanics of cell crushing. In the process of absorbing impact energy, cell walls deform plastically and get damaged (e.g. fractured) [7, 18].

Vehicle-to-vehicle side-impacts and vehicle rollover are presently among the most common types of car accidents and collisions. Additionally, these are also frequently the most serious accidents in terms of occupant injuries [6, 8, 39, 48]. Among these, frontal and side impact are the most severe. As a consequence, quite a large effort has been widely focused in improving passive and active safety mechanisms for frontal impact situations for the last decades. However, more recently, the number of serious injuries resulting from side-impacts has brought the attention of many researchers to the importance of developing similar or adapted mechanisms for such collisions [12, 33, 53]. In this type of collisions, the risk and/or the severity of the resulting injuries is frequently a direct consequence of the contact between the occupants and the lateral structure of the vehicle, given the reduced space between the occupant and the door [21]. Pelvic and chest areas have been reported by many authors as the the two areas most affected in this type of car-to-car collision [29, 43, 44].

In the late 1990s Morris *et al.* [37] observed, through a series of numerical simulations of side-impact collisions, that the space available between the structure of the vehicle and the passengers is one of the most important parameters with direct influence on the levels of occupant's injuries. This statement was also supported by many other researches, as can be seen, for example, from the works of Tencer *et al.* [49] and Schiff *et al.* [44]. Morris *et al.* state that the space available not only has influence on the impact velocity but also on the point of the velocity profile at which the door initiates contact with the occupant. These authors also evaluated the benefits of the use of paddings of different sizes in the door interior and of lateral airbags. Lim *et al.* [26] also studied, numerically, the inclusion of padding material for protection of the occupant pelvic area and concluded that it significantly reduced the severity of the resulting injuries. Additionally, Majumder *et al.* [29] studied the dynamic response of the pelvis and established fracture limits in side-impact collisions. These authors supported their conclusions with the results from numerical simulations using finite element modelling software. One of the most important conclusion these researchers derived from their work was that with a more appropriate design of the lateral door and the inclusion of padding material on the level of the pelvic area, the risk and/or severity of occupants' injury could be significantly reduced.

Based on the previous considerations, the authors propose the use of cellular materials, among which polyurethane foams, within an energy absorbing system specifically designed in such a way as to significantly improve passive safety on the event of side-impacts. This system consists of a foam like impact padding confined inside the lateral doors of the vehicle. In order to be efficient, this foam padding must be mounted aligned with the occupant's pelvis, protecting one of the most critical areas in this type of collision. From the experiments and analyses made it is expected that this padding may absorb a significant part of the impact energy, and thus minimise both the forces transmitted to the body of the occupants and, most importantly, the magnitudes of the decelerations experienced, consequently reducing possible injury levels of the occupants of the vehicle.

The design of structures and the choice of materials for crashworthiness and protection systems within a vehicle are also of major importance and relevance for the overall safety of the driver and passengers. These are two of the research fields where there is still quite a large margin for improvement [16, 17, 24, 41, 46]. In the present work, the authors will present, discuss and compare the applicability of several distinct types of cellular materials for impact and energy absorbing paddings.


2.3 Electrospinning

The previous prepared solution was injected in a 5 mL syringe with a 20-gauge needle for electrospinning. The syringe was set to the syringe pump and voltage was applied between the needle and the collector. The collector was a non-stick cookie sheet giving good electrospinning, reusability and easy recovery of the nanofibres. The electrospinning parameters were based on results obtained previously [26]. The conditions were a flow rate of 0.1 mL/h with an applied DC voltage of 15 kV. The collector was placed 20 cm away from the tip of the needle. The temperature was kept at 22°C and relative humidity maintained between 10 and 40%. A razor blade was used to recover the nanofibrous mat from the collector. All experiments were conducted in a customized electrospinning box. The electrospun nanofibre mat was then kept overnight in a laboratory oven at 80°C for drying and stabilization purposes.

#### 2.4 Nanofibre stabilization

Due to electrospun nanofibres' high solubility in water, AL:PVA nanofibres mats were stabilized using two consecutive techniques. Both techniques are based on previous works [26]. The first method used the glass transition temperature of polymers to raise their crystallinity and hence their water resistance. Therefore, nanofibres were heated in a laboratory oven at 160°C for 3 h. Next, the membranes were immersed in a 0.5 M sodium citrate buffer pH 4.5 for a period of 3 h. This process aims to protonate AL's phenol groups which were previously deprotonated during the preparation of the electrospinning solution in a method similar to the extraction methods of black liquor [15]. During this step, the morphology of the membrane changes drastically due to the dissolution of a part of the PVA. The dissolution causes a rise in the concentration of AL (the membranes become browner) and cross-linking of the nanofibres. After exposure to the buffer, the membranes were washed several times with purified water, stretched and dried on a metallic surface. Finally, the nanofibrous mats were recovered using a razor blade.

#### 2.5 Adsorption tests

In this section, three types of tests were performed: adsorption of a single contaminant on AL:PVA membranes as well as commercial adsorbents, and adsorption of multiple contaminants on AL:PVA membranes. All adsorption tests were conducted in batches by adding a defined amount of adsorbent to a stirred solution containing a specific concentration of contaminants. All tested solutions were composed of purified water with 5% of methanol and contaminants adjusted at the targeted concentration. The organic solvent's purpose was to ensure that pharmaceuticals were solubilized in water. Separate 2500 ppm standard solutions of FLX, VEN, CAR and IBU were prepared by dissolving the corresponding stock solutions in methanol. Those solutions were then diluted for adsorption tests. Before, during and after the adsorption test, aliquots of 500 μL of the contaminated water were sampled, diluted with 500 μL of mobile phase, vortexed and injected in HPLC-DAD to determine the concentration of contaminants in solution. For tests using one contaminant on AL:PVA membranes, 50 ppm FLX solution was used as a model contaminated water. For tests with commercial adsorbents, adjustments were made to compensate for the size difference between adsorbents. Therefore, solutions of 250 ppm FLX in 10 mL were prepared to keep the same contaminant to Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

adsorbent mass ratio. For tests with multiple contaminants, 12.5 ppm of FLX, IBU, CAR and VEN were added to water to simulate contaminated water.

The adsorption tests were initialized by the addition of 25 mg of adsorbent (nanofibres or commercial adsorbent) to the solution. The tests were conducted over a period of 150 min to ensure that equilibrium was reached. Using a calibration curve and the area under the peaks on the chromatograms, the remaining concentration of the solution was calculated. From this value, the adsorption capacity at time t (Qt) was calculated using the following equation:

$$Q\_t = \frac{(C\_0 - C\_t)}{m} \times V \tag{1}$$

where Ct is the concentration of the contaminant (ppm) at time t (min), C0 is the initial concentration of contaminants (ppm), V is the volume of the solution (L), and m is the mass of adsorbents (g).

For samples containing one contaminant, samples injected in HPLC-DAD were eluted using a mobile phase composed of acetonitrile and a 0.1% solution of phosphoric acid (60:40% v/v ratio). The flow rate was adjusted at 0.45 mL/min for 3.75 min with a detection at 230 nm. For samples containing more than one contaminant, the mobile phase was composed of acetonitrile and 0.1% phosphoric acid with a ratio of 40:60% v/v. The flow rate was adjusted to 0.5 mL/min for a period of 20 min with a detection still at 230 nm. In all cases, 10 μL of the samples were injected using an autosampler. For all contaminants, a 10-point (0.5–100 ppm) calibration curve was established to determine the concentration. All tests were performed in triplicate.

#### 2.6 Kinetic studies

1 month. Before use, the refrigerated solution was brought to room temperature in a

The previous prepared solution was injected in a 5 mL syringe with a 20-gauge needle for electrospinning. The syringe was set to the syringe pump and voltage was applied between the needle and the collector. The collector was a non-stick cookie sheet giving good electrospinning, reusability and easy recovery of the nanofibres. The electrospinning parameters were based on results obtained previously [26]. The conditions were a flow rate of 0.1 mL/h with an applied DC voltage of 15 kV. The collector was placed 20 cm away from the tip of the needle. The temperature was kept at 22°C and relative humidity maintained between 10 and 40%. A razor blade was used to recover the nanofibrous mat from the collector. All experiments were conducted in a customized electrospinning box. The electrospun nanofibre mat was then kept overnight in a laboratory oven at 80°C for drying and stabilization purposes.

Due to electrospun nanofibres' high solubility in water, AL:PVA nanofibres mats were stabilized using two consecutive techniques. Both techniques are based on previous works [26]. The first method used the glass transition temperature of polymers to raise their crystallinity and hence their water resistance. Therefore, nanofibres were heated in a laboratory oven at 160°C for 3 h. Next, the membranes were immersed in a 0.5 M sodium citrate buffer pH 4.5 for a period of 3 h. This process aims to protonate AL's phenol groups which were previously deprotonated during the preparation of the electrospinning solution in a method similar to the extraction methods of black liquor [15]. During this step, the morphology of the membrane changes drastically due to the dissolution of a part of the PVA. The dissolution causes a rise in the concentration of AL (the membranes become browner) and cross-linking of the nanofibres. After exposure to the buffer, the membranes were washed several times with purified water, stretched and dried on a metallic surface. Finally, the nanofibrous mats were recovered using a razor blade.

In this section, three types of tests were performed: adsorption of a single contaminant on AL:PVA membranes as well as commercial adsorbents, and adsorption of multiple contaminants on AL:PVA membranes. All adsorption tests were conducted in batches by adding a defined amount of adsorbent to a stirred solution containing a specific concentration of contaminants. All tested solutions were composed of purified water with 5% of methanol and contaminants adjusted at the targeted concentration. The organic solvent's purpose was to ensure that pharmaceuticals were solubilized in water. Separate 2500 ppm standard solutions of FLX, VEN, CAR and IBU were prepared by dissolving the corresponding stock solutions in methanol. Those solutions were then diluted for adsorption tests. Before, during and after the adsorption test, aliquots of 500 μL of the contaminated water were sampled, diluted with 500 μL of mobile phase, vortexed and injected in HPLC-DAD to determine the concentration of contaminants in solution. For tests using one contaminant on AL:PVA membranes, 50 ppm FLX solution was used as a model contaminated water. For tests with commercial adsorbents, adjustments were made to compensate for the size difference between adsorbents. Therefore, solutions of 250 ppm FLX in 10 mL were prepared to keep the same contaminant to

hot water bath for an hour.

2.4 Nanofibre stabilization

2.5 Adsorption tests

28

2.3 Electrospinning

Sorption in 2020s

Kinetic curves were obtained by sampling at intervals during the adsorption process. Samples were collected at 0, 5, 10, 15, 20, 30, 40, 50, 60, 75, 90, 120 and 150 min after the addition of the adsorbent. The equilibrium time of 150 min was determined by an initial kinetic test. The same sampling and injection processes as traditional adsorption tests were conducted for kinetic studies. By calculating the adsorption capacity through time, it is possible to obtain a kinetic curve which can be compared to adsorption kinetic models. Adsorption kinetic models give crucial information on the adsorption parameters and the limiting processes occurring during the adsorption. Typically, three steps occur during the adsorption: transfer of the adsorbate to the external surface of the adsorbent, internal diffusion of the adsorbate to active sites and sorption reaction with the adsorbent [27, 28]. In this study, three models were compared: pseudo-first order, pseudo-second order and Elovich. In all cases, the kinetic constants were calculated using Matlab's curve fitting app. To determine the best fitting model, determination coefficients and root of mean square errors (RMSE) were compared. The pseudo-first order model is represented by Eq. (2):

$$Q\_t = Q\_\varepsilon \left(1 - e^{-k\_1 t}\right) \tag{2}$$

In Eq. (2), Q e corresponds to the adsorption capacity at equilibrium (mg/g), Qt the adsorption capacity (mg/g) at time t (min) and k1 the pseudo-first order kinetic adsorption constant (min�1 ) [10, 29]. The pseudo-second order model is represented by Eq. (3):

$$Q\_t = \frac{k\_2 Q\_\epsilon \,^2 t}{1 + k\_2 Q\_\epsilon t} \tag{3}$$

where k2 is associated to the pseudo-second order kinetic adsorption constant (g mg�1 min�1 ) [10, 29]. The Elovich model is represented by Eq. (4):

$$Q\_t = \frac{\ln\left(a\beta\right) + \ln t}{\beta} \tag{4}$$

where ΔS° is the standard entropy (J mol�1 K�1

DOI: http://dx.doi.org/10.5772/intechopen.88621

salts, and then dried in a vacuum desiccator.

3.1 Electrospinning and nanofibre stabilization

3. Results and discussion

31

where ΔG° is the standard Gibbs's free energy (J mol�1

give information on the amount of thermal energy produced, the energy of the bonds, the spontaneity of the reaction and the favourability of an adsorption

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical…

The capacity of an adsorbent to be desorbed is also an important characteristic

Mixed solutions of AL and PVA were prepared for the production of membranes for adsorption tests. Using the specified electrospinning parameters, it was possible to

obtain steady nanofibre formation for periods of a few hours to produce thin nanofibrous mats. Those were thermally stabilized giving the nanofibres a brownish colour and more rigidity. Their immersion in a sodium citrate buffer finalized the stabilization process to provide fibres stability at various pHs enabling their use for adsorption. As shown in Figure 2, the stabilization process had a slight impact on the visual aspects of the membranes. However, the impact is more obvious when seen by scanning electron microscopy. Figure 2b shows that nanofibres of 183 � 5 nm in diameter were obtained by the electrospinning with a low number of beads or defects. This size does not technically correspond to nanofibres (0–100 nm), but the adsorption properties shall be akin to real nanofibres considering the small difference. Figure 2d shows the nanofibres after a thermal process. This image shows a similar nanofibrous aspect with small variations of the nanofibre diameter (156 � 5 nm). However, nanofibres seem to be closer to each other with slight cross-linking giving it

value since it can have a significant contribution on economics and life cycle assessment of the process. Therefore, the reusability of the AL:PVA membranes was evaluated by desorption. For this purpose, multiple conditions were tested to recover the contaminant safely. Hence, the nanofibres were exposed to solutions of methanol (to create an environment in which the contaminant is highly soluble), purified water (to verify the risk of desorption due to equilibrium), heated solutions (might be able to revert the sorption reaction), salts (ion exchange and/or competition) and combined techniques (except heated methanol solutions). In all cases, the membranes were immersed in 50 mL of solution for 4 h. For temperature effect, solutions were heated to 60°C to verify desorption. The salt used for desorption was sodium chloride since it is a simple and non-toxic substance, largely found in typical wastewater. Concentrations of sodium chloride of 1, 2 and 3 M were tested. Initial and final samples were injected in HPLC-DAD to determine the concentration of FLX recovered. The desorption solution showing the best desorption efficiency was used for repeated adsorption/desorption cycles to evaluate the reusability. Between each test, the membranes were recovered and dried in a vacuum desiccator to prevent humidity from interfering with the measured masses. For membranes exposed to salts, these were washed several times with purified water to dissolve

enthalpy (J mol�1

2.9 Desorption tests

reaction.

) and ΔH° is the standard

) [31]. These values will

) [31]. The second thermodynamic law equation corresponds to:

ΔG° ¼ ΔH° � TΔS° (6)

where α is the initial adsorption rate constant (mg g�1 min�1 ) and β is the initial desorption rate constant (g mg�1 ) [10, 29].

#### 2.7 Isotherms

The adsorption isotherms were performed for AL:PVA membranes to obtain information on the adsorption sites and the type of reaction occurring. For these tests, 50 ppm solutions of FLX were prepared as typical adsorption tests. Samples were collected at 0 and 180 min (equilibrium). From those samples, the concentration (Ce) and adsorption capacity (Q e) at equilibrium were calculated. Isotherms are obtained by varying the mass of adsorbents (resulting in a varying adsorption capacity and concentration at equilibrium) at fixed temperatures. For our tests, adsorbent masses of 5, 10, 15, 20, 25, 30 and 35 mg were tested and temperatures of 25, 40 and 60°C were compared. The curves obtained by plotting the Ce versus Q e are then compared to isotherm models (Freundlich, Langmuir, Sips, Redlich-Peterson) to gain important information. Table 1 shows the different equations for the models studied.

Here, Q e is the adsorption capacity at equilibrium (mg/g), Ce is the concentration in solution at equilibrium (ppm), kF is the Freundlich isotherm constant (mg/g [L/mg]1/n), n is the heterogeneity factor (dimensionless), Q max is the maximum adsorption capacity (mg/g), kL is the Langmuir isotherm constant (L/mg), kS is the Sips isotherm constant ([L/mg]1/n), kR is the Redlich-Peterson isotherm constant (L/g), aR being the Redlich-Peterson isotherm constant ([L/g]bR) and bR is the Redlich-Peterson model exponent (dimensionless).

#### 2.8 Thermodynamic study

The thermodynamic parameters (enthalpy, entropy and Gibbs's free energy) are calculated through a thermodynamic study. These parameters are obtained by using the Ce and Qe recovered from isotherms adsorption tests and the Van't Hoff and 2nd thermodynamic law equations. The Van't Hoff equation corresponds to:

$$\ln\frac{Q\_{\epsilon}}{C\_{\epsilon}} \ast 1000 \text{ g} \frac{\text{g}}{L} = \frac{\Delta S^{\circ}}{R} - \frac{\Delta H^{\circ}}{RT} \tag{5}$$

Table 1.

Isotherm models non-linear equations [10, 30].

Sorption Capacities of a Lignin-Based Electrospun Nanofibrous Material for Pharmaceutical… DOI: http://dx.doi.org/10.5772/intechopen.88621

where ΔS° is the standard entropy (J mol�1 K�1 ) and ΔH° is the standard enthalpy (J mol�1 ) [31]. The second thermodynamic law equation corresponds to:

$$
\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ \tag{6}
$$

where ΔG° is the standard Gibbs's free energy (J mol�1 ) [31]. These values will give information on the amount of thermal energy produced, the energy of the bonds, the spontaneity of the reaction and the favourability of an adsorption reaction.

## 2.9 Desorption tests

where k2 is associated to the pseudo-second order kinetic adsorption constant

Qt ¼ ln ð Þþ αβ ln t

The adsorption isotherms were performed for AL:PVA membranes to obtain information on the adsorption sites and the type of reaction occurring. For these tests, 50 ppm solutions of FLX were prepared as typical adsorption tests. Samples were collected at 0 and 180 min (equilibrium). From those samples, the concentration (Ce) and adsorption capacity (Q e) at equilibrium were calculated. Isotherms are obtained by varying the mass of adsorbents (resulting in a varying adsorption capacity and concentration at equilibrium) at fixed temperatures. For our tests, adsorbent masses of 5, 10, 15, 20, 25, 30 and 35 mg were tested and temperatures of 25, 40 and 60°C were compared. The curves obtained by plotting the Ce versus Q e are then compared to isotherm models (Freundlich, Langmuir, Sips, Redlich-Peterson) to gain important information. Table 1 shows the different equations for

Here, Q e is the adsorption capacity at equilibrium (mg/g), Ce is the concentration in solution at equilibrium (ppm), kF is the Freundlich isotherm constant (mg/g [L/mg]1/n), n is the heterogeneity factor (dimensionless), Q max is the maximum adsorption capacity (mg/g), kL is the Langmuir isotherm constant (L/mg), kS is the Sips isotherm constant ([L/mg]1/n), kR is the Redlich-Peterson isotherm constant (L/g), aR being the Redlich-Peterson isotherm constant ([L/g]bR) and bR is the

The thermodynamic parameters (enthalpy, entropy and Gibbs's free energy) are calculated through a thermodynamic study. These parameters are obtained by using the Ce and Qe recovered from isotherms adsorption tests and the Van't Hoff and 2nd

L ¼ ΔS°

1þkLCe

1þaRCe bR

1=n 1þkSCe 1=n

Models Non-linear equation Equation

R � ΔH°

RT (5)

(6)

(7)

(8)

1=n (5)

thermodynamic law equations. The Van't Hoff equation corresponds to:

∗ 1000 g

ln Qe Ce

Freundlich Qe ¼ kFCe

Langmuir Qe ¼ Qmax kLCe

Sips Qe ¼ Qmax kSCe

Redlich-Peterson Qe ¼ kRCe

Isotherm models non-linear equations [10, 30].

where α is the initial adsorption rate constant (mg g�1 min�1

) [10, 29].

β (4)

) and β is the initial

) [10, 29]. The Elovich model is represented by Eq. (4):

(g mg�1 min�1

Sorption in 2020s

2.7 Isotherms

the models studied.

2.8 Thermodynamic study

Table 1.

30

Redlich-Peterson model exponent (dimensionless).

desorption rate constant (g mg�1

The capacity of an adsorbent to be desorbed is also an important characteristic value since it can have a significant contribution on economics and life cycle assessment of the process. Therefore, the reusability of the AL:PVA membranes was evaluated by desorption. For this purpose, multiple conditions were tested to recover the contaminant safely. Hence, the nanofibres were exposed to solutions of methanol (to create an environment in which the contaminant is highly soluble), purified water (to verify the risk of desorption due to equilibrium), heated solutions (might be able to revert the sorption reaction), salts (ion exchange and/or competition) and combined techniques (except heated methanol solutions). In all cases, the membranes were immersed in 50 mL of solution for 4 h. For temperature effect, solutions were heated to 60°C to verify desorption. The salt used for desorption was sodium chloride since it is a simple and non-toxic substance, largely found in typical wastewater. Concentrations of sodium chloride of 1, 2 and 3 M were tested. Initial and final samples were injected in HPLC-DAD to determine the concentration of FLX recovered. The desorption solution showing the best desorption efficiency was used for repeated adsorption/desorption cycles to evaluate the reusability. Between each test, the membranes were recovered and dried in a vacuum desiccator to prevent humidity from interfering with the measured masses. For membranes exposed to salts, these were washed several times with purified water to dissolve salts, and then dried in a vacuum desiccator.


Perpendicular Magnetic Insulator Films for Spintronics

*Laith Alahmed and Peng Li*

## **Abstract**

The recent progress in spintronics opens up new directions for novel device concepts and fundamental understandings. This is possible because of magnetic insulators (MIs), which have paved the way toward pure spin current-based spintronics. MIs with perpendicular anisotropy expand the horizon further, enabling new functionalities such as low-power spin-orbit torque switching, highspeed domain-wall motion, high-frequency spin-orbit torque oscillation, etc. In this chapter, we review recent progress in spintronic experiments using barium hexagonal ferrite BaFe12O19—a magnetic insulator with perpendicular anisotropy. These results lay the foundation for using MIs with perpendicular anisotropy as a medium to develop new energy-efficient pure spin current-based electronics.

**Keywords:** magnetic insulator, perpendicular anisotropy, spin current, spin-orbit torque, spintronics

### **1. Introduction**

Spintronics, also known as spin electronics, is a newly emerging field of research that focuses on the spin degree of freedom of electrons rather than their charge. Charge current is a flow of electrons from one point to another under the influence of an electric field. In spintronics, spin current can propagate within the material. A pure spin current can be generated through effects such as the spin Hall effect (SHE), spin pumping, spin-wave propagation, etc. The pure spin currents consume much less energy than charge currents. This is because of the absence of charge flow that eliminates the power consumption needed for the electric field required to drive charge flow [1–3].

In spintronics, magnetization switching is of both fundamental interest and technological significance. One way to switch the magnetization of a ferromagnetic film is through the spin filtering effect. In this case, a spin-polarized electrical current will be generated. As the polarized electrons flow through the ferromagnetic film, they transfer angular momentum to the film and produce a spin-transfer torque to switch the film. This torque is called spin-transfer torque (STT). Magnetic random-access memory based on STT has already been commercialized in recent years.

The above-mentioned spin-torque switching, however, has a limit. The angular momentum transferred per unit charge in the applied current usually cannot exceed a quantum of spin (ℏ/2). Recent work demonstrates that one can exceed this limit by the use of spin-orbit torque (SOT). The demonstration generally takes a nonmagnetic heavy metal (HM)/ferromagnetic metal (FM) bilayered structure and makes use of the spin-orbit coupling-produced SHE in the HM film to convert an in-plane charge current to a pure spin current that flows across the HM thickness. This produces spin accumulation at the HM/FM interface and therefore exerts a SOT on FM. In this case, each electron in the applied current can undergo multiple spin-flip scattering at the interface, therefore enabling more efficient switching than in the conventional spin-transfer torque case.

topological insulator/BaM heterostructure for magnetization switching. Finally, Section 3.5 provides an outlook in the field of BaM materials and devices.

BaM is a hexagonal ferrite, which consists of close-packed layers of oxygen ions. **Figure 1** shows a unit cell of BaM. The Ba2+ ion is large, as is the O2 ion, and the barium always replaces oxygen somewhere in the oxygen lattice. The

, R\*

, R, R\*

, and T\* blocks are 180°

, T, and T\*

,

close-packed layers form six fundamental blocks, namely, S, S\*

[5–7]. The S block consists of close-packed oxygen layers stacking in an ABCABC … sequence. It has a cubic spinel arrangement with the <111 > axis along the vertical direction. There are two units of Fe3O4 without any barium ions in each S block. The R block comprises close-packed oxygen layers stacking in an ABAB … sequence. It has a hexagonal closest packed structure along the vertical axis. Each R block has a unit formula of BaFe6O11. The T block is made of four oxygen layers, with a barium ion replacing an oxygen ion in the middle two layers,

rotations around the c-axis from the S, R, and T blocks. BaM is built from the

Trivalent Fe3+ ions occupy tetrahedral and octahedral sites as well as one trigonal bipyramidal site. Different sites account for different spin orientations and Bohr magnetons (*μB*). For example, a tetrahedral site contributes 2*μB*, while an octahedral site contributes 4*μB* with opposite spin orientations in the S block. In the end, S, S\*

R, and R\* blocks contribute 2*μB* each, leading to a moment of 40*μB* for each unit cell. This gives a saturation magnetization of 4700 G in bulk BaM. BaM has a strong

*Crystalline structure of M-type barium ferrite. Blue ball, Ba2+. Yellow ball, Fe3+. Red ball, O2.*

**2. Properties of barium ferrite thin films**

*Perpendicular Magnetic Insulator Films for Spintronics DOI: http://dx.doi.org/10.5772/intechopen.92277*

which gives a unit formula of Ba2Fe8O14. The S\*

, and R\* blocks.

stacking of S, R, S\*

**Figure 1.**

**39**

**2.1 Atomic structure of BaM thin films**

The ferromagnetic films used in most of the SOT studies were all conductive. A direct consequence is the severe shunting current in the ferromagnet layer, which not only limits the switching efficiency but also causes parasitic effects. For example, previous works have shown that interfacing a TI with a conductive FM film can result in a significant modification or even complete suppression of the topological surface states (TSSs) in the TI layer. In a TI/FM heterostructure, the TSSs may have been largely spoiled by the FM electrons. This means that many large spin-orbit torques observed in TI/FM structures may not be due to TSS. In this context, the use of MIs in an HM/MI heterostructure can effectively avoid the shunting current. Moreover, the TSSs in a TI/MI structure can be preserved except for the opening of a small gap at the Dirac point when strong coupling exists at the interface. This will enable the magnetization switching due to *bona fide* TSSs.

Magnetic insulators include a large class of materials, including spinels, garnets, and ferrites. They have a general chemical formula of M(Fe*x*O*y*), where M is representing non-iron metallic elements. MIs have several advantages over magnetic metals for SOT device applications. First, in a heavy metal/MI heterostructure, the charge current only flows in the HM layer but not in the MI layer. In contrast, in an HM/magnetic metal structure, the charge current also flows in the FM, resulting in certain parasitic effects. When the HM layer is replaced by a topological insulator with high resistivity, the advantage of zero shunting currents in the MI film becomes particularly important. Moreover, interfacing a topological insulator (TI) with a conductive FM can result in a significant modification or even complete suppression of the topological surface states (TSSs) in the TI layer. The use of a magnetic insulator can effectively avoid the shunting current; TSSs in a TI/magnetic insulator (MI) structure can also be well preserved.

In the ferrite family, hexagonal ferrites have strong magnetocrystalline anisotropy. For example, M-type barium ferrite (BaFe12O19, noted as BaM) has an anisotropy field of 17 kOe. The perpendicular anisotropy in MI films originates from bulk intrinsic anisotropy rather than interfacial anisotropy [4]. This means that, when being used for actual devices, the BaM film has no constrains on the thickness. This is in strong contrast with the ferromagnetic metal counterpart (e.g., CoFeB/MgO) that often has to be very thin to realize interfacial perpendicular anisotropy. In addition, the magnetic damping is usually significantly lower in MIs than in FMs. For example, the intrinsic Gilbert damping constant in BaM materials is 7 104 , which is at least 10 times smaller than the value in permalloy [5]. This advantage is significant for spin-torque oscillator applications, where the current threshold for self-oscillations decreases with the damping, as well as for logic device applications that require low-damping, insulating spin channels.

This chapter reviews the main advances made in spintronic experiments with BaM over the past several years. Section 2 gives a brief introduction to BaM and discusses its crystalline structure, magnetic properties, and thin film growth techniques. This section serves to provide a background for the discussions in the following sections. Section 3 reviews the advances of spintronic experiments with BaM. Section 3.1 provides an overview of the related spintronic experiments. Section 3.2 discusses the generation of pure spin currents through the spin Seebeck effect and photo-spin-voltaic effect in the Pt/BaM structure. Section 3.3 discusses the spin-orbit torque-assisted switching in BaM. Section 3.4 discusses the use of

topological insulator/BaM heterostructure for magnetization switching. Finally, Section 3.5 provides an outlook in the field of BaM materials and devices.


**2.1 The techniques and protocols of hypnosis**

The following is a brief synopsis of the techniques and protocols of hypnosis with corroboration of these statements taken from the literature:

*Hypnosis enhances the effectiveness of therapy and creates the belief of self-efficacy* [12]. Evidence from the literature informs us that perceived self-efficacy not only creates a sense of hope but also affects the treatment [13], and that expectation of self-efficacy is central to all forms of therapeutic change [14].

*Hypnosis adds leverage to treatment and shortens treatment time* [15]. When patients are fully relaxed, positive subjective experiences occur replacing negative cognitions, which appear to bring great comfort and relief [16]. Rapid changes are attributed to the brisk and profound behavioural, emotional, cognitive and physiological changes brought about by hypnosis [17].

*Hypnosis breaks resistance when indirect hypnotic suggestions are applied in therapy* [18]. Oppositional statements given by the therapist are used to obtain compliance. An example of this would be 'and the more you try to open your eyes the more they remain tightly shut'. The word 'try' pre-disposes failure, so it is used in therapy whenever the opposite is required [18].

*Hypnosis facilitates rapid transference, which reinforces the suggestions given in therapy*. *During hypnosis, there is greater access to fantasies, memories and emotions, allowing the rapid occurrence of full-blown transference manifestations* [19]. During the hypnotic state, the critical faculty of the mind is bypassed, enabling the processing of thoughts in the unconscious mind, which are then transferred to the conscious in the waking state [4].

*Hypnosis induces deep relaxation, and in this state suggestions can be made that are effective for the reduction of anxiety* [10]. A trance is a special psychological state in which patients can re-associate and reorganise their inner psychological complexities. It is argued that during therapy an inner resynthesis of the patient's behaviour is achieved by the patient [4], and further argued that suggestion and post-hypnotic suggestions during therapy emphasise the innate tendency of the mind to heal itself [20].

*Hypnosis strengthens the ego by enhancing self-confidence and self-worth* [21]. Ego strengthening is a belief that when positive suggestions are repeated sufficiently they become embedded in the unconscious mind to be acted upon in the conscious state [22].

*Hypnosis facilitates divergent thinking, maximising awareness, attentional focus and concentration. It minimises distraction and interference from other sources and stimuli increasing the potential for learning alternatives* [23]. Corroboration from the literature reports that breaking through the limitations of conscious attitudes frees the unconscious potential for problem-solving [4].

*Hypnosis allows engagement of the non-dominant hemisphere in the brain. It provides direct entry into the cognitive processing of the right cerebral hemisphere (in right-handed subjects), which accesses and organises emotional and experiential information. It can be utilised to teach restructuring of cognitive and emotional processes that are influenced by the non-dominant cerebral hemisphere* [22]. Many of the sensoryperceptual languages of the mind (visual, auditory and kinaesthetic information) are encoded like a map over the cerebral cortex of the brain [24, 25].

*Hypnosis facilitates imagery conditioning. In this state, imagery and cognitive restructuring are intensified. The use of the word 'hypnosis' and the application of various hypnotic techniques appear to augment the power of the suggestion* [26]*.* Evidence from the literature suggests that when the patient is hypnotised, the power of imagination is increased and that possibly hypnosis, imagery and affect are all mediated by the same right cerebral hemisphere [27].

*Hypnosis uses post-hypnotic suggestions and is an important part of therapy and is used to shape desired future behaviour; it can be powerful in altering problem behaviours, dysfunctional cognitions and negative emotions* [28]. It is reported that posthypnotic suggestions function as positive ideas for desired future behaviour, and are regarded as a necessary part of the therapeutic process, enabling the patient to act upon the suggestions in future experiences [16].

*Hypnosis enhances training in positive self-hypnosis, which provides a strategy for counteracting negative cognitions* [29]. It is argued that negative thinking can lead to negative affect, biased thoughts, impaired motivation, concentration and cognition [30]. Positive techniques can be practised during self-hypnosis, thus reinforcing and strengthening hypnotherapy.

*Hypnotic techniques are easily exported and can be easily assimilated with many forms of therapy. When hypnosis is used as an adjunct to a particular form of therapy whether behavioural, cognitive or cognitive behavioural therapy, the effects can enhance the treatment* outcome [12].

The above techniques and protocols suggest that by adopting hypnosis as an adjunct to therapy addressing the unconscious mind, change can be implemented quickly, is more profound and therapy outcome is enhanced. By adopting this therapeutic procedure, it follows that therapy outcome should be more effective than CBT alone. An overview of the background of CBT is now given together with the rationale for the integration of CBT with hypnosis.