Research Progress on Preparation Methods of Skutterudites

Research Progress on Preparation Methods of Skutterudites: Comparison

Please note this is a comparison between Version 1 by Zhiyuan Liu and Version 2 by Lindsay Dong.

Thermoelectric material is a new energy material that can realize direct conversion of thermal energy and electric energy. It has important and wide applications in the fields of the recycling of industrial waste heat and automobile exhaust, efficient refrigeration of the next generation of integrated circuits and full spectrum solar power generation. Skutterudites thermoelectric material has attracted much attention because of their excellent electrical transport performance in the medium temperature region. In order to obtain skutterudites thermoelectric materials with excellent properties, it is indispensable to choose an appropriate preparation method. This review summarizes some traditional and advanced preparation methods of skutterudites thermoelectric material in recent years. The basic principles of these preparation methods are briefly introduced. Single-phase skutterudite thermoelectric materials can be successfully obtained by these preparation methods. The study of these preparation methods also provides technical support for the rapid, low-cost and large-scale preparation of high-performance thermoelectric materials.

thermoelectric materials
skutterudites
preparation methods

1. Introduction

The global consumption of nonrenewable energy is increasing rapidly and the total amount of energy consumed is more than half of the known reserves. If pweople do not carry out planned exploitation and develop new energy, the nonrenewable resources on the earth will be gradually exhausted [1]. With the overexploitation and use of underground energy, the carbon balance on the earth’s surface has been broken, resulting in a series of environmental problems such as the greenhouse effect. Therefore, it is urgent to develop renewable and sustainable new energy. In addition, the maximum energy efficiency of engines is about 41% [2]. Most of the remaining energy is consumed in other forms and cannot be effectively utilized. Thermoelectric (TE) material is a kind of new energy functional material, which can directly convert electric energy and heat energy into each other using the Seebeck effect and the Peltier effect ^[3][4][3,4]. Therefore, TE materials can convert lost energy into electric energy for human use through the Seebeck effect. The TE conversion efficiency of devices made of TE materials is relatively low. To improve the conversion efficiency of devices, the key is to improve the TE figure of merit ZT of materials ^[5][6][7][8][5,6,7,8].

In the 1990s, Slack was the first to put forward the ideal concept of “phonon glass electronic crystal” [9]. Slack pointed out that high-performance TE materials should have the same low thermal conductivity as glass and the same high conductivity as crystals. Since then, people have successively discovered clathrates with this feature [9]. Skutterudites are materials with this typical cage structure ^{[10][11][12][13][14]}[10,11,12,13,14]. Filling in the icosahedral gap consisting of 12 Sb atoms with other impurity atoms can achieve independent electron and phonon synergistic modulation and significantly enhance the TE properties of skutterudites. In recent years, skutterudites have been dramatically improved by doping the framework atoms or filling the icosahedral voids and introducing the nano inclusions, with ZT values increasing from 1.0 to about 2.0 ^{[15][16][17][18][19][20][21][22][23][24][25][26]}[15,16,17,18,19,20,21,22,23,24,25,26]. Wang et al. [15] prepared Yb_xCo₄Sb₁₂-filled skutterudite that could reach ZT values of about 1.5. The ZT value of polyatomic filled skutterudite (R, Ba, Yb)_yCo₄Sb₁₂ (R = Sr, La, Mm, DD, SrMm, SrDD) prepared by Rogl et al. [16] can even reach about 2.0. In addition to enhancing their properties by means of optimized doping and filling, these skutterudite materials have been prepared using some suitable preparation methods to obtain rich microstructures with independent electronic and phonon synergistic regulation, resulting in high ZT values. Therefore, the preparation methods are crucial to obtain high-performance skutterudites. Figure 1In this review, we summarize some conventional and advanced preparation methods for skutterudite materials in recent years (Figure 1). Using these preparation methods, better performance skutterudites were successfully obtained. The study of these preparation methods also provides technical support for the rapid and low-cost large-scale preparation of high-performance TE materials.

Figure 1. Schematic diagram of some preparation methods for skutterudite materials ^[27][28][27,28]. (MA: mechanical alloying; MS: melt spinning; SHS: self-propagating high-temperature synthesis; HTHP: high temperature and high pressure; SPS: spark plasma sintering).

2. Traditional Preparation Methods

2.1. Melt Growth Method

For the melt growth method, there is no destructive phase transition. Homo-component melted compounds or high-purity monomers with low vapor pressure/dissociation pressure are ideal materials for melt growth to obtain high-quality single crystals. Its growth process is accomplished by the movement of the solid–liquid interface, which is a directional solidification process under controlled conditions. Its quality is difficult to control because a finite solid solution is formed during its growth. In experiments, the required elements are often added in calculated proportions to obtain the target material after melting. This preparation method is also used to prepare skutterudite materials ^[28][29][30][28,29,30]. Pillaca et al. [28] successfully grew impurity-free single crystals of CoSb₃ using the tilted rotating Bridgman method (Figure 2a,b), whose single crystal growth was achieved in the high-temperature liquid phase with a high concentration of Sb. The single-phase CoSb₃ single crystals were finally prepared by first sealing the required raw materials in ampoules by mixing them thoroughly, followed by tilting the ampoules in a Bridgman-type crystal growth apparatus at an angle of 15° with respect to the horizon (Figure 2c), and finally by ramping up the cooling rate according to a constant rotation speed and temperature. Caillat et al. [29] prepared single crystals of the skutterudite phase using a melting vertical gradient cooling technique. The raw materials were sealed in a vacuum quartz tube in a certain ratio, melted and cooled by a melting furnace with a temperature difference, and finally, single-crystal CoSb₃ and RhSb₃ compounds were obtained.

Figure 2. (a) Optical micrograph of the lapped surface of CoSb₃ ingot grown by the Inclined Rotary Bridgman method; (b) observed and calculated X−ray powder diffraction patterns of CoSb₃; (c) schematic of the apparatus used for the inclined Rotary Bridgman experiments [28].

2.2. Solvothermal Method

The solvothermal method is also known as the hydrothermal method. As the name implies, the chemical reaction is carried out under moist conditions. The specific implementation involves placing a certain proportion of high-purity raw materials together with the selected solvent in a reaction kettle. When the kettle is heated at the appropriate temperature, high temperature and pressure will be formed inside the kettle, which in turn will produce the desired target material. Since the ratio of reactants and the external environment can be controlled artificially, the advantages of this reaction are: the hydrothermal method facilitates the control of the reaction kinetics and is more conducive to adjusting the shape and structure of the products; the solvothermal method generates materials with better crystallinity, faster reaction rate, and lower reaction temperature, which allows the synthesis of low-temperature isomers more easily. Therefore, this method is also used to prepare skutterudites ^{[13][31][32][33]}[13,31,32,33]. One drawback of the solvent thermal method is that the yields of the prepared target materials are generally low, and further optimization of the conditions is needed to enhance the yields of the target products.

2.3. Solid Phase Reaction Method

The solid phase reaction method, also called melt annealing method, is one of the traditional methods for the preparation of skutterudite materials ^{[34][35][36][37][38][39]}[35,36,37,38,39,40]. In this method, pure monomers or compounds are weighed, mixed, pressed into shape, vacuum sealed according to the reaction ratio and then subjected to a long solid phase reaction at high temperatures. The method is simple to operate and the temperature is easily controlled. The preparation time of the material is long and the cost is high. Su et al. ^[39][40] synthesized single-phase CoSb_2.75Ge_0.25−xTe_x (x = 0.125~0.20) skutterudite compounds using melt quenching, annealing and spark plasma sintering (SPS) methods. The doping of Te and Ge led to the in situ generation of special nanostructures inside the material (about 30 nm) (Figure 34a), which, combined with the strain fluctuations caused by Te and Ge doping, led to a significant suppression of heat transfer phonons inside the material and a significant reduction of thermal conductivity (Figure 34b). In addition, the doping of Te also optimizes the mobility, significantly enhancing the electrical conductivity (Figure 34c) and TE power factor of the material. As a result, the ZT value of the CoSb_2.75Ge_0.05Te_0.20 sample can exceed 1.1 at 527 °C, which is higher than the performance of some single-filled n-type skutterudite compounds. In conclusion, the thermoelectric materials prepared by solid state reaction are compact in structure, uniform in components and stable in performance, making it a good method for the preparation of skutterudite materials.

Figure 34. (a) Nanostructure consisting of circular shapes produced in situ inside the material; (b) temperature dependence of thermal conductivity and (c) electrical conductivity for CoSb_2.75Ge_0.25−xTe_x (x = 0.125~0.20) ^[39][40].

2.4. Mechanical Alloying Method

The mechanical alloying method is a preparation technique in which several monolithic powder particles are put into a high-energy planetary ball mill in a specific ratio, after which the powder particles are impressed, squeezed, and ground for a long time to cause the diffusion of atoms among the powder particles to obtain nanoscale alloyed powders ^[40][41]. Due to the extremely high purity requirements of the mechanical alloying on the monolithic powder, it is theoretically possible to achieve real interatomic bonding and the formation of homogeneous compounds in a sufficiently long-time state. In fact, the obtained material only reaches or tends to the atomic level in some states, forming compounds of homogeneous composition. This method is also often used to prepare the skutterudite materials ^{[41][42][43][44][45]}[42,43,44,45,46]. Ur et al. ^[41][42] synthesized Fe_xCo_4-xSb₁₂ (0 ≤ x ≤ 2.5), Fe-doped skutterudite by mechanical alloying using a high-purity monolithic powder as a starting material. It was found that single-phase skutterudite with a nanostructure could be successfully prepared by introducing Fe doping when x ≤ 1.5; when x ≥ 2, the material forms a second phase. x ≤ 1.5 samples have lower lattice thermal conductivity due to the introduction of Fe to increase the nanostructure, which causes strong phonon scattering and thus improves the TE properties. x = 1.5 samples have a ZT value reaching 0.3 at 600 K.

3. New Preparation Methods

3.1. Melt Spinning

Melt spinning is one of the new methods for the preparation of TE materials at present ^{[46][47][48][49][50][51]}[47,48,49,50,51,52]. This method is performed by weighing and mixing a certain stoichiometric amount of high-purity single elements in a vacuum quartz tube, heating it above the melting point of the material for a certain time, and then quenching it. Finally, the sample is subjected to melt-spin at a certain speed, after which the finished product can be annealed and sintered to obtain the bulk material. A p-type Ce-filled skutterudite material Ce_0.9Fe₃CoSb₁₂ was prepared by Jie et al. ^[48][49] using both equilibrium (conventional solid phase method) and non-equilibrium (melt-spin) methods. By studying the fracture surface scanning electron microscopy (FESEM) image of the material (Figure 46a), it was found that the fracture direction of the material tends to propagate more along the grain boundaries (which may have good fracture strength), and the grain size (nano size) is much smaller than that prepared by the conventional method. Compared with the Ce_0.9Fe₃CoSb₁₂ material prepared by the conventional solid-phase reaction, the material prepared by the melt spinning has abundant nano-grain boundaries that can significantly scatter phonons as well as a large number of defects that significantly reduce the thermal conductivity (Figure 46b). At the same time, the quantum-limited domain effect generated by the low-dimensional nanostructure causes an increase in the density of states near the Fermi surface of the Ce_0.9Fe₃CoSb₁₂ material, which can effectively increase the Seebeck coefficient and thus the power factor of the material (Figure 46c).

Figure 46. (a) SEM image of fracture surface of Ce_0.9Fe₃CoSb₁₂ material; temperature dependence of (b) power factor and (c) thermal conductivity for the Ce_0.9Fe₃CoSb₁₂ material ^[48][49]; (d) SEM image of Yb_0.9Fe₃CoSb₁₂ sample; temperature dependence of (e) thermal conductivity and (f) ZT values for Yb_0.9Fe₃CoSb₁₂ material prepared under different preparation conditions. The inset shows the temperature dependence of lattice thermal conductivity ^[49][50].

3.2. High-Temperature and High-Pressure Method

The high-temperature and high-pressure (HTHP) method is one of the effective methods to prepare high-performance skutterudites ^{[52][53][54][55][56][57]}[53,54,55,56,57,58]. Generally, the experimental raw materials are weighed in a fixed proportion, fully ground under Ar atmosphere (to prevent the material from being oxidized), and later placed in a vessel for sintering at a certain temperature and pressure. This method is convenient for controlling the external temperature and pressure conditions, while it can greatly reduce the experimental time and has important practical significance in large-scale production. Han et al. ^[54][55] prepared Te-doped filled skutterudites under different pressure conditions using a high temperature and high-pressure method and investigated the synergistic relationship between Te doping and pressure regulation. It was found that Te doping could effectively optimize the electrical transport properties of the samples, while some defects appeared in the crystals at high pressure. This further reduced the lattice thermal conductivity of the materials, and the lattice thermal conductivity of the In_0.05Ba_0.15Co₄Sb_11.5Te_0.5 samples prepared at 2.0 GPa was only 1.02 W·m⁻¹·K⁻¹ with a maximum ZT value of 1.23.

3.3. Pulsed Laser Deposition

Pulsed laser deposition (PLD), also known as pulsed laser ablation (PLA), is the use of laser light to bombard a target material so that the bombarded plasma is deposited on a specific substrate to form a thin film. At present, with the continuous development of laser technology, pulsed laser technology is gradually being applied in many material preparation fields ^[58][59]. In recent years, pulsed laser deposition has also been applied to the preparation of skutterudite TE films ^[59][60][61][60,61,62]. This technique has the advantages of a relatively short preparation time, homogeneous film material composition, and no special requirements for the target type. Sarath et al. ^[59][60] prepared In and Yb doped CoSb₃ thin films using pulsed laser deposition. During the preparation, the process window for the growth of single-phase skutterudite thin films was very narrow. It was found that the information and the increase in surface roughness of CoSb₃ after heating in an argon environment may lead to irreversible changes in film resistivity and Seebeck coefficient at 207 °C. The highest power factor of 0.68 W·m⁻¹·K⁻¹ could be obtained for this film at 427 °C, which is five times lower compared to most of the blocks, probably due to the high resistivity of the film material.

3.4. Magnetron Sputtering

Magnetron sputtering is one of the types of physical vapor deposition (PVD). With the advantages of magnetron sputtering coming to the fore, it has gradually gained wide application ^[62][63]. The specific principle is that when the accelerated electrons hit the argon atoms, the resulting argon ions then collide with the target material, causing the bombarded target atoms to be deposited on the substrate to form a thin film. This technique has the characteristics of fast low temperature, large deposition rate, and can be made into a large area of thin film. In recent years, this technique has also been applied to the preparation of skutterudite TE films ^[63][64][65][64,65,66]. Fan et al. ^[63][64] used magnetron sputtering to grow Ag-doped CoSb₃ films directly on heated substrates. It was found that the doped films had a single-phase CoSb₃ crystal structure and good crystallinity, and the CoSb₃ films with high electrical transport properties could be obtained with the appropriate amount of doping. The films had a maximum power factor of 2.97 × 10⁻⁴ W·m⁻¹·K⁻² at 0.3% Ag doping.

3.5. Molecular Beam Epitaxy (MBE)

MBE is a novel method for the epitaxial preparation of thin film materials and has also been used in recent years for the preparation of skutterudite TE thin film materials ^{[66][67][68][69][70]}[67,68,69,70,71]. MBE is a novel process for coating on substrates under ultra-high vacuum. The advantages of this preparation method are: (1) the thickness of the film can be precisely controlled at a slower growth rate; (2) the preparation method is a physical process without considering intermediate chemical processes, which can interrupt the progress of the experiment at any time; and (3) the substrate temperature of this method does not need to be too high, which reduces various adverse effects caused by thermal expansion, etc. Daniel et al. ^[68][69] deposited CoSb₃ thin films with a thickness of 30 nm at different substrate temperatures using the MBE method. It was found that the deposition method and the temperature of the substrate used for the deposition process had a significant effect on the grain size of the CoSb₃ films, and the higher the temperature, the smaller the grain size. After deposition at room temperature, annealing is required to crystallize them, and they can crystallize into phases quickly when deposited at high temperatures. The annealed film has a very smooth surface, less roughness, and possesses a larger single-phase component. In addition, the smaller grain size of the films prepared at higher substrate temperatures allows for lower thermal conductivity.

3.6. Self-Spreading High-Temperature Synthetic (SHS)

The self-spreading high-temperature synthetic process is also called combustion synthesis technology (SHS) ^[71][72][73][72,73,74]. This technology uses external energy to initiate chemical reactions. Then, the exothermic reaction is used to initiate new chemical reactions. Thus, the chemical reaction will spread to the whole reactor. Finally, the target product can be obtained. Su et al. ^[74][75] proposed for the first time the use of SHS for the rapid preparation of TE materials. This Aarticle also reported a high-performance Cu₂Se TE material is also reported. Because this technology has the characteristics of high purity of products, low energy consumption, simple equipment and short reaction time, TE researchers have rapidly prepared high-performance TE materials of different systems through SHS ^{[27][75][76][77]}[27,76,77,78]. Liang et al. [27] applied SHS to the synthesis of CoSb₃ TE materials for the first time. In this experiment, the single-phase CoSb₃ material was quickly synthesized by igniting the powder of Co and Sb using the characteristics of heat released by chemical reaction. Then, CoSb_3−xTe_x bulk materials were prepared by plasma-activated sintering (PAS). The bulk materials prepared by SHS-PAS have rich nanostructures. Combined with Te-doping to control the carrier concentration of the material, the electrical conductivity of the material is improved. As a result, the maximum ZT value of this sample at 547 °C is 0.98, which is the CoSb_2.85Te_0.15 sample prepared by SHS-PAS.

3.7. Microwave Sintering

At present, 300~3 × 10⁵ MHz is generally defined as the microwave frequency band. Its wavelength is 1~1 × 10³ mm. In practice, the microwave frequency band used in microwave sintering is 2.45 × 10³ MHz. Because microwaves can be absorbed by materials, it changes from electromagnetic energy to thermal energy in the material, which makes the material temperature rise rapidly and realizes the purpose of sintering. Compared with traditional sintering, microwave sintering has the characteristics of short sintering time, selective sintering and energy saving. Thus, the sintering process has also been used in the preparation of TE materials in recent years ^{[78][79][80][81][82][83]}[79,80,81,82,83,84]. Biswas et al. ^[78][79] used a microwave synthesis device to synthesize In_0.2Co₄Sb₁₂ skutterudite powder in a short time (2 min), which is much shorter than the traditional preparation method (3 days). After sintering, the ZT value of the powder sample synthesized by microwave is equivalent to that of the bulk sample obtained by the traditional preparation method.

3.8. High Pressure Torsion (HPT)

Severe plastic deformation (SPD) of materials can be formed via high-pressure torsion (HPT). The ultra-fine grains in the sub-micrometer or nanometer range can be obtained by SPD via HPT ^[84][85]. At the same time, a large number of dislocations will be produced in the material. Due to the huge changes in the grain size and dislocation density of the materials, these changes will significantly improve the performance of the TE materials ^[85][86]. It enables the production of samples in large quantities (50 g) by this advanced preparation technology, and is therefore usable for industrial production ^[86][87][88][87,88,89]. A high ZT value of p- and n-type skutterudites can be obtained through this advanced preparation technology ^[86][87][87,88].

4. Conclusions

Skutterudite is a kind of TE material with excellent performance in the middle temperature region, which is expected to have a good application and development prospects in the field of power generation. Due to the special icosahedral cage structure, there are many ways to improve the ZT value of skutterudite material. In order to obtain skutterudite with excellent properties, it is very important to select appropriate preparation methods. This review systematically summarized some traditional and advanced preparation methods of skutterudites in recent years, and the principles of these preparation methods were briefly introduced. The traditional preparation methods of TE materials (such as the solid-state reaction method) are time-consuming and use large amounts of energy, but the prepared materials have high density, are of uniform composition, and have good mechanical properties and stable TE properties. The new preparation method has a shorter preparation time, lower energy consumption and higher properties. The rapidly prepared TE materials usually have different types of defect structures (such as dislocation, pores, superlattice and nano-grain boundaries). These defects help to scatter multi-scale phonons and significantly reduce the thermal conductivity of the material. Therefore, TE materials can have high TE properties. The approach is relatively poor in terms of densification and mechanical properties compared with traditional preparation methods. Therefore, there are still some problems in the preparation process of TE materials, which need to be further studied.