Fabrication of Mg2Si thermoelectric materials by mechanical alloying and spark-plasma sintering process

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.     

Nanostructuring of thermoelectric Mg(2) Si via a nonequilibrium intermediate state

A new route to self-assembled nanocomposite thermoelectric materials is proposed. High-energy mechanical alloying brings materials into a nonequilibrium intermediate state, such as a solid solution with an extended solubility. The large driving force for the transformation to the equilibrium state leads to nanometer-scale microstructure formation, which is ideal for reducing lattice thermal conductivity.     

Spark plasma sintering and hot compression behaviour of AZ91 Mg alloy

Abstract

In the present investigation, the microstructural characterisation of the AZ91 Mg alloy produced by spark plasma sintering (SPS), as well as the evaluation of its hot compression behaviour, has been performed. Based on the differential scanning calorimetry analyses of the starting powders, three SPS cycles are investigated, using temperatures of 400 and 450°C, and at 450°C with previous solubilisation soaking at 420°C. Despite different microstructural and hardness characteristics, the three alloys display similar hot compression behaviour. At 200°C, the formation of an unstable crack, which propagates at 45° with respect to the loading axis, is observed after the occurrence of the peak load. At higher testing temperatures, after reaching the peak stress, the flow stress decreases slowly with increased strain of ～0·51. Such behaviour corresponds with the observation of an accelerated cracking due to the propagation of decohesions at the interparticle regions. Ultimately, SPS allowed for attainment of high relative density; however, the sintering degree of the materials was quite low.     

Fabrication parameters for optimized thermoelectric Mg2Si

Here, we report on the effect of sintering temperature, particle size, and sintering pressure on the thermoelectric properties of Sb-doped Mg₂Si samples. We find a strong dependence of thermoelectric performance on sintering temperature with the best properties achieved at 900 °C. We furthermore show a strong influence of the particle size of the employed powder, with coarser powder giving better properties. The difference stems mainly from the electrical conductivity; Seebeck coefficient and thermal conductivity are affected to a lesser extent. With respect to sintering pressure, a small positive influence of increasing pressure is found. We obtain a peak thermoelectric figure of merit of 0.75 ± 0.18 at 820 K employing optimized fabrication parameters.     

Formaton of nanocrystalline Mg2Si and Mg2Si dispersion strengthened Mg-Al alloy by mechanical alloying

Formation of Mg₂Si via mechanical alloying of elemental Mg and Si powders has been investigated. The formation of Mg₂Si occurs after 10 hours of mechanical alloying. Nanocrystalline structure of Mg₂Si with grain size of 22 nm obtained after 50 hours of milling was found to be stable upon heating to about 390 °C. Sudden increase in crystalline size to 157 nm after annealing at 520 °C was observed. Although the reaction between Mg and Si could be completed after about 50 hours of mechanical alloying, thermal assisted reaction starting at as low as 190 °C could promote the formation of Mg₂Si at a short milling duration and hence reduce Fe contamination. Mg-Al alloy reinforced by Mg₂Si was prepared by milling Mg, Si and Al powders. Intermediate phase of Al₁₂Mg₁₇ has been detected after 5 hours of mechanical alloying. This intermediate phase was observed to disappear to form equilibrium solid solution of Mg-Al alloy after annealing at 300 °C.     

Thermal stability and thermoelectric properties of Mg2Si0.4Sn0.6 and Mg2Si0.6Sn0.4

Compounds of Mg₂Si_1?x Sn _x are environmentally friendly, inexpensive and high-efficiency thermoelectric materials for energy conversion in the temperature range 300–550 °C. In this study, the thermal stability is investigated of fine powders and sintered pellets of the compounds Mg₂Si_0.4Sn_0.6 and Mg₂Si_0.6Sn_0.4 by heating the samples from room temperature to ~400 °C in air, while measuring powder X-ray diffraction patterns. The diffractograms of the pellets show no significant changes upon heating for several hours, while the powder samples show increasing emergence of a Mg₂Sn-rich, Mg₂Si_1?x Sn _x phase, and other impurities upon heating for only several minutes. This is attributed to the larger amount of surface area in the powder samples. The appearance of the Mg₂Sn-rich phase is most pronounced for the Sn-rich composition. In addition, the thermal expansion coefficients were extracted from the powder diffraction patterns. All materials have been synthesized by induction-melting followed by ball milling and spark plasma sintering. The thermal conductivity, Seebeck coefficient, electrical resistivity and Hall carrier concentrations have been measured from room temperature to 400 °C on the pellets.     

Mechanical properties of thermoelectric Mg2Si using molecular dynamics simulations

In this paper, the mechanical properties of thermoelectric Mg₂Si are investigated using molecular dynamics (MD) method, with Mg₂Si in the forms of bulk, nanofilm, and nanowire, respectively. The effects of the Mg vacancy on the mechanical properties of Mg₂Si in these three forms are studied in details. First of all, the equilibrium state of Mg₂Si is simulated after choosing the proper potential function, boundary conditions, and the speed algorithm. Nondimensionalization is also implemented during the simulations. This part of simulation aims to verify the correctness of the crystal model established via the use of the molecular dynamics analysis. Next, the models of Mg₂Si in the forms of bulk, nanofilm, and nanowire are established with different Mg vacancy proportions, and then the mechanical properties of each model are studied via the uniaxial tensile test. Finally, the stress–strain curve and subsequently the ultimate tensile strength are obtained for each model. Simulation results indicate that the ultimate tensile strength of Mg₂Si in each model is decreased with the increase of the Mg vacancy proportion. Moreover, through the comparison of the ultimate tensile strengths of Mg₂Si bulk, nanofilm, and nanowire, it is found that low-dimensionalization significantly reduces the ultimate tensile strength of thermoelectric Mg₂Si. Results obtained in this paper can provide valuable guidance to the future applications of thermoelectric devices.     

Spark plasma sintering of ultra refractory compounds

Spark plasma sintering experiments were conducted on Zr- and Hf-based borides and carbides with the addition of 1, 3, and 9 vol% MoSi₂ as sintering aid. For comparison, as-received ZrC, HfC, ZrB₂, HfB₂ powders were also sintered. The microstructural features were investigated by means of scanning electron microscop–energy dispersive spectroscopy technique. Silicon carbide was detected in all the doped compositions along with significant amounts of oxide species (Hf/ZrO₂, and SiO₂). The effect of the MoSi₂ content on densification, microstructure, and mechanical properties is analyzed.     

Microstructure and thermoelectric performance evaluation of p-type (Bi,Sb)2Te3 materials synthesized using mechanical alloying and spark plasma sintering process

Journal of Materials Science: Materials in Electronics - In this paper, three p-type thermoelectric compounds, namely Bi0.5Sb1.5Te3, Bi0.3Sb1.7Te3, and Bi0.2Sb1.8Te3 were manufactured by mechanical...     

Spark plasma synthesis and densification of TaB2 by pulsed electric current sintering

The synthesis and consolidation in one processing step of single phase tantalum diboride by Spark Plasma Sintering (SPS) is proposed. TaB₂ formation from Ta and B elemental powders takes place at about 800 °C through a solid-state combustion reaction while product densification requires higher temperatures. Consolidation is significantly improved (~ 96% density) when increasing the applied pressure from 20 to 60 MPa immediately after the synthesis reaction occurs. Simplicity, short duration, milder temperatures and pressure conditions, in comparison with other routes utilized so far for obtaining bulk TaB₂ are, along with the good mechanical properties of the obtained product, the main benefits of the adopted approach.     

Spark plasma sintering of silicon nitride using nanocomposite particles

The spark plasma sintering (SPS) of silicon nitride (Si₃N₄) was investigated using nanocomposite particles composed of submicron-size α-Si₃N₄ and nano-size sintering aids of 5 wt% Y₂O₃ and 2 wt% MgO prepared through a mechanical treatment. As a result of the SPS, Si₃N₄ ceramics with a higher density were obtained using the nanocomposite particles compared with a powder mixture prepared using conventional wet ball-milling. The shrinkage curve of the powder compact prepared using the mechanical treatment was also different from that prepared using the ball-milling, because the formation of the secondary phase identified by the X-ray diffraction (XRD) method and liquid phase was influenced by the presence of the sintering aids in the powder compact. Scanning electron microscopy (SEM) observations showed that elongated grain structure in the Si₃N₄ ceramics with the nanocomposite particles was more developed than that using the powder mixture and ball-milling because of the enhancement of the densification and α-β phase transformation. The fracture toughness was improved by the development of the microstructure using the nanocomposite particles as the raw material. Consequently, it was shown that the powder design of the Si₃N₄ and sintering aids is important to fabricate denser Si₃N₄ ceramics with better mechanical properties using SPS.     

Spark plasma sintering of BiFeO3

Dense BiFeO₃ ceramics were prepared by a novel spark plasma sintering (SPS) technique. The sintering was conducted at temperatures ranging from 675 to 750 °C under 70 MPa pressure. A bulk density value up to 96% of theoretical density was achieved in the process. This contrast to around 90% of the theoretical density achieved by conventional sintering at around 830 °C. It was found that the tendency to form unwanted Bi₂Fe₄O₉ phase is higher at a high sintering temperature for SPS. The dielectric and ferroelectric properties also improved (with respect to conventionally sintered sample) for spark plasma-sintered samples.     

Characteristics evaluation of SiC/Si nanocomposites produced by spark plasma sintering

SiC–Si composites are widely used either as a bulk material or as a matrix for fibre reinforced ceramics. In the current research, nanocomposites of SiC–Si with different volume fractions of Si were sintered by spark plasma sintering (SPS) for the first time. The effect of Si content and different sintering parameters on relative density, microstructure, hardness and fracture toughness of the sintered materials have been investigated. The relative density increased from about 83 to 99% by increasing the sintering temperature to 1700°C, sintering time to 10?min, and pressure to 70?MPa for composites containing >20?vol.-% Si. The results revealed that the full dense SiC–20?vol.-%Si composite can be obtained by SPS at 1700°C, 10?min and 70?MPa. Moreover, in this condition, the hardness and toughness of the composites reached the optimum values.     

Spark plasma sintering of Co-WC cubic boron nitride composites

25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC-6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC-6Co-cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed.     

Skutterudite化合物FexCO4-xSb12的放电等离子烧结原位合成及热电性能

采用放电等离子烧结(SPS)技术在 800 ~1000K温度范围内原位反应合成了富 Co基 Skutteru dite化合物FexCo4-xSb12(x=0~1.0),并对化合物的结构、微观形貌及热电性能进行了研究。结果表明化合物的晶格常数随Fe含量的增加而线性增加,电导率随着Fe置换量的增加而增加,Seebeck系数的峰值随着Fe置换量的增加而向高温方向移动,热导率随 Fe置换量的增加而进一步降低。对于富 Co基 Skutteru dite化合物FexCo4-xSb12,x=1.0 的 Fe1.0 Co3.0 Sb12化合物具有较高的热电性能指数,在 673K取得最大 ZT值约0.3。     

Electronic properties of the Mg2Si thermoelectric material investigated by linear-response density-functional theory

This paper presents Density-Functional Perturbation Theory (DFPT) calculations on the electronic, vibrational, and electron–phonon (EP) coupling properties of the Mg₂Si thermoelectric compound. The DFPT yields very satisfactory results for the electronic and vibrational properties when compared to experiment. Regarding the EP interactions, as far as we know, they have never been reported so far. We show that the EP interactions in Mg₂Si mainly involve the silicon atom. This result explains the improvement of the thermoelectric properties of Mg₂Si using a solid solution Mg₂Si_1?xA_x, where A is a heavier atom than Si. By guiding the choice of the substitution site, the study of the EP coupling properties could be used in the search of new thermoelectric materials based on solid solutions.     

放电等离子烧结制备Ca2Co2O5热电材料及其性能研究

以Co(NO3)2·6H2O、Ca(NO3)2·4H2O为原料,NaOH为沉淀剂,采用化学共沉淀方法制备了前驱物.该前驱物预烧后,利用SPS进行烧结获得纯相Ca2Co2O5块体.分别采用DTA-TG、XRD和SEM对前驱体的热分解过程、Ca2Co2O5的形成过程进行了表征,并对其热导率进行了测试.结果表明在Ph=13.20获得所需化学计量比的前驱物.该前驱物在700～800℃预烧2h后,采用SPS烧结.烧结温度为800℃,压力30MPa,保温时间5min,可以获得纯相Ca2Co2O5块体.断口形貌为层状,晶粒长约1.4μm.热导率随温度的升高而降低.     

A comparative study of Spark Plasma Sintering (SPS), Hot Isostatic Pressing (HIP) and microwaves sintering techniques on p-type Bi2Te3 thermoelectric properties

The sintering of a synthesized p-type Bi₂Te₃ nano-powder has been investigated by three different techniques. Sintering techniques such as Hot Isostatic Pressing (HIP), microwave sintering and Spark Plasma Sintering (SPS) also known as electric field-assisted sintering technique (FAST) have been compared in terms of sintering parameters i.e. temperature, pressure, and power, microstructure and thermoelectric properties of the prepared ceramics. This study demonstrates that the highest figure of merit ZT has been obtained using microwaves or SPS techniques. Ceramics observations reveal differences in microstructure as well as the presence of intra-granular precipitates in the pellets sintered by the three techniques. We finally conclude about the relationship between properties and microstructure to get optimum thermoelectric materials.