**1. Introduction**

With excellent electronic characteristics such as a large band gap, high critical breakdown strength, high electronic saturation rate, high thermal conductivity, and high irradiation resistance, silicon carbide (SiC) has become an outstanding representative of the third generation of semiconductor materials and has been increasingly widely applied in a variety of fields, including computer, aviation, power, and nuclear energy development [1–4]. In addition, it is an attractive option to use SiC to produce space mirrors and large ground-based reflectors thanks to its remarkable advantages of large sti ffness, small thermal deformation coe fficient, and good thermal stability [5]. However, it is di fficult to obtain SiC parts with high forming accuracy and ideal machined surface quality due to its characteristics of high hardness (with a Mohs hardness between 9.0 and 9.5) and brittleness [6,7].

The traditional cutting force models tend to ignore the e ffect of elastic recovery on the macroprocessing of ductile materials. However, numerous studies [8–10] have shown that the role of elastic recovery is significant in the micro–nano-machining of brittle materials. Wasmer et al. [11] proposed a typical scratch pattern for brittle materials using an increasing load along a scratch path that is divided into five stages: elastic regime, ductile regime, subsurface crack regime, surface crack regime, and micro-abrasive regime. Elastic and ductile deformations were observed before the ductile regime, and brittle fractures appeared and began to dominate the latter deformation with increasing cutting force. Therefore, there is a cutting depth, referred to as the critical depth of cut, where the ductile region transitions to the brittle region. Lawn et al. [12] established a microfracture model under the point indentation of brittle materials consisting of the following processes: (1) the sharp indenter induces a zone of elastic and ductile deformation around the contact point; (2) a median crack suddenly initiates below the contact point; (3) the median crack stably extends with increasing indenter load; (4) the median crack begins to close during the initial unloading process; (5) lateral cracks begin to appear due to residual stress; and (6) lateral cracks continue to extend and cause chipping.

Methods such as grinding and chemical mechanical polishing are used in traditional processing and are characterized by low production e fficiency, high production cost, and, particularly, surface damage caused by the contamination of the polishing slurry [13]. In 1951, researchers found that hard and brittle materials show the characteristics of ductile removal under certain processing conditions [14]. Since the 1990s, researchers have conducted many studies into removal mechanisms in silicon carbide ductile regime processing, such as ductile regime grinding [7,15], ductile regime laser-assisted processing [16], ductile regime diamond cutting [17], ductile regime diamond wiresaw [18], and ductile domain ultrasonic-assisted processing [19]. These studies indicated that in the course of ductile regime processing, the chip is removed by ductile deformation, causing no damage or cracks to the machined surface of the workpiece, and the surface processing quality can be maintained [20–22]. The critical depth of cut at the ductile–brittle transition, the maximum cutting depth where no cracks appeared on the surface or subsurface of the sample, is a fundamental parameter of all methods of ductile regime processing. A formula was obtained by Bifano according to Gri ffith's principle through a quasi-static scratch test of several kinds of common brittle materials [23]:

$$d\_{\mathbb{C}} = a \left( \frac{E}{H} \right) \left( \frac{K\_{\mathbb{C}}}{H} \right)^2 \tag{1}$$

where α is a constant, *E* is the elasticity modulus, *H* is the hardness, and *K*c is the fracture toughness. This formula was amended by later scholars; for example, Gaobo's study on the critical depth of cut of 6H-silicon carbide indicated that the experimental results were not in line with the calculation of Formula (1) and the amended constant α [24].

Based on the above, most research has focused on the critical depth of cut using Gri ffith's principle and experimental method, which is based on cracks in the ductile extension which would have an influence on the performance of devices. With the development of nanotechnology, particularly the scanning electron microscope (SEM) and nanomechanical testing technology, a number of powerful tools has been provided to investigate the properties of silicon carbide at the nanoscale. This paper proposes a method considering the elastic recovery of the workpiece surface in nanoscratching in order to obtain the critical depth of cut for SiC using scratching stress and cleavage strength.


*3.4.2 Recent developments in CFT*

Researchers are innovating Cold Finger designs by integrating nanoparticles to enhance nucleation and combat supercooling, providing more effective nucleation sites. Focus has also been placed on refining temperature control within these setups to improve PCM efficacy. Customizing Cold Finger configurations to match the specific needs of different PCMs is key to optimizing phase change behavior. Hybrid approaches combining the Cold Finger Technique with other methods aim to offer comprehensive solutions. Efforts to adapt and optimize Cold Finger Techniques for thermal energy storage, incorporating renewable energy and ensuring grid stability, are underway. Advances in materials for Cold Finger components seek to improve durability and heat transfer, while developments in control and monitoring systems enhance understanding and optimization of PCM performance [81].


**5.2. Software setup**

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feedback

TCP/IP TCP/IP GPS PPS PPS NMEA NMEA

Fig. 3: Network structure of four-multi test-bed.

V. KTH FOUR-MULTI TEST-BED SETUP

and movable multi-antenna nodes. A software framework accompanies the hardware setup of the test-

*KTH four-multi* is a USRP-based wireless communication test-bed consisting of several stationary and movable multi-antenna nodes. A software framework accompanies the hardware setup of the test-bed which facilitates the rapid testing of multi-antenna schemes (see http://fourmulti.sourceforge.net/). In the following, the hardware and software structure

bed which facilitates the rapid testing of multi-antenna schemes (see http://fourmulti.sourceforge.net/).

The current version of the test-bed consists of six nodes where three of them are fixed and take the role of transmitting sources while the other three are movable receiving destinations. All the nodes are equipped with two vertically polarized dipole antennas spaced 20 cm apart which is 1.6 times of the carrier's wavelength. Twelve Ettus Research USRP N210 (see www.ettus.com) are used to govern the twelve antennas in the network. The source USRPs are equipped with the standard Ettus XCVR2450 RF dautherboards while the destination USRPs use custom boards to achieve sufficient noise figure and dynamic range. The output signal of each source USRP is amplified by a ZRL-2400LN power amplifier. Two Linux

The current version of the test-bed consists of six nodes where three of them are fixed and take the

role of transmitting sources while the other three are movable receiving destinations. All the nodes

are equipped with two vertically polarized dipole antennas spaced 20 cm apart which is 1.6 times

of the carrier's wavelength. Twelve Ettus Research USRP N210 (see www.ettus.com) are used to

govern the twelve antennas in the network. The source USRPs are equipped with the standard Ettus

XCVR2450 RF dautherboards while the destination USRPs use custom boards to achieve sufficient

noise figure and dynamic range. The output signal of each source USRP is amplified by a ZRL-

2400LN power amplifier. Two Linux computers control all the USRPs in the network. The network

The network is designed to work at 2.49 GHz center frequency with 12 MHz bandwidth. Syn-

chronization of the network is performed in three levels, namely time, frequency and transmit-receive

synchronizations. The time and transmit-receive synchronizations are done by means of a pulse-

per-second (PPS) signal (0-5 V, 1 Hz square wave) and a national marine electronics association

(NMEA) signal (an ASCII protocol that provides hour-minute-second time), respectively. Both signals

October 14, 2014 DRAFT

In the following, the hardware and software structure of the test-bed is described.

*KTH four-multi* is a USRP-based wireless communication test-bed consisting of several stationary

D3

CLK

CLK

computer

CLK

D2

D1

S3

S2

S1

CLK

backhaul

62 Contemporary Issues in Wireless Communications

computer

**Figure 3.** Network structure of four-multi test-bed.

**5. KTH four-multi test-bed setup**

of the test-bed is described.

**5.1. Hardware setup**

structure of four-multi test-bed is illustrated in Fig. 3.

*A. Hardware Setup*

master node

The four-multi software framework has been developed in C++(see http://fourmulti.source‐ forge. net/). It runs on two Linux computers separately. One of the computers controls the three source nodes while the other one controls the three destination nodes connected to them via Ethernet connections. The sources' computer generates the transmitted frames and feeds them to the source nodes while the destinations' computer process the received frames at the destination nodes. A TCP/IP connection between the source and the destination computers provides the feedback links. Backhaul communication among the source nodes is also implemented by the help of TCP/IP connections between the source computer and the source nodes.

The framework contains a toolbox for coding and modulation (AMC and OFDM1) which was used in the implementations of the next two sections. The KTH four-multi Modula‐ tion and coding toolbox includes an LDPC channel encoder/decoder, a QAM modulator/ demodulator and an OFDM modulator/demodulator. The specification of these built-in functions is summarized in Table 1.

**Table 1.** Modulation and coding toolbox specifications.