Theoretical study on electronic structure and thermoelectric properties of PbS_xTe_(1-x)(x = 0.25, 0.5, and 0.75) solid solution

The electronic structure and thermoelectric（TE） properties of PbS_xTe_（1-x）（x = 0.25, 0.5, and 0.75） solid solution have been studied by combining the first-principles calculations and semi-classical Boltzmann theory. The special quasirandom structure（SQS） method is used to model the solid solutions of PbS_xTe_（1-x）, which can produce reasonable electronic structures with respect to experimental results. The maximum zT value can reach 1.67 for p-type PbS0.75Te0.25 and 1.30 for PbS0.5Te0.5 at 800 K, respectively. The performance of p-type PbS_xTe_（1-x） is superior to the n-type ones, mainly attributed to the higher effective mass of the carriers. The z T values for PbS_xTe_（1-x） solid solutions are higher than that of pure Pb Te and Pb S, in which the combination of low thermal conductivity and high power factor play important roles.

Electronic and thermoelectric properties of Mg2GexSn1-x(x=0.25,0.50,0.75) solid solutions by first-principles calculations

The electronic structure and thermoelectric（TE） properties of Mg2GexSn1-x（x = 0.25, 0.50, 0.75） solid solutions are investigated by first-principles calculations and semi-classical Boltzmann theory. The special quasi-random structure（SQS） is used to model the solid solutions, which can produce reasonable band gaps with respect to experimental results.The n-type solid solutions have an excellent thermoelectric performance with maximum zT values exceeding 2.0, where the combination of low lattice thermal conductivity and high power factor（PF） plays an important role. These values are higher than those of pure Mg2Sn and Mg2Ge. The p-type solid solutions are inferior to the n-type ones, mainly due to the much lower PF. The maximum zT value of 0.62 is predicted for p-type Mg2Ge（0.25）Sn（0.75） at 800K. The results suggest that the n-type Mg2GexSn1-x solid solutions are promising mid-temperature TE materials.

Enhanced thermoelectric properties in two-dimensional monolayer Si2BN by adsorbing halogen atoms

Using the first principles calculation and Boltzmann transport theory, we study the thermoelectric properties of Si2BNadsorbing halogen atoms (Si2BN-4X, X = F, Cl, Br, and I). The results show that the adsorption of halogen atoms cansignificantly regulate the energy band structure and lattice thermal conductivity of Si2BN. Among them, Si2BN-4I has thebest thermoelectric performance, the figure of merit can reach 0.50 K at 300 K, which is about 16 times greater than that ofSi2BN. This is because the adsorption of iodine atoms not only significantly increases the Seebeck coefficient due to banddegeneracy, but also rapidly reduces the phonon thermal conductivity by enhancing phonon scattering. Our work proves theapplication potential of Si2BN-based crystals in the field of thermoelectricity and the effective method for metal crystals toopen bandgaps by adsorbing halogens.

ZrC1-xNx固溶体的价电子结构与性能

为了预测ZrC1-xNx固溶体的性能,根据固体与分子经验电子理论(EET),对该固溶体相进行了价电子结构分析;采用键距差(BLD)方法,计算了固溶体相中的最强键共价电子数、共价键电子数百分比、最强键键能、熔点等,并分别与ZrC、ZrN基体的价电子结构进行比较。结果表明,ZrC与ZrN相互溶入后,非金属原子的杂阶没有变化,但ZrC1-xNx固溶体中Zr原子的杂阶、最强键上共价电子数、共价键电子数百分比、最强键键能以及熔点均随x值变化而变化。随着x值的增加,Zr原子的杂阶从B11上升到B13,最强键上共价键数、最强键键能、熔点逐渐下降,而共价键数百分比逐渐上升,表明固溶体硬度、韧性、结合能、熔点、导电性能逐渐减小,而强度却逐渐增加。

HfC_(1-x)N_x固溶体的价电子结构与性能

为了预测HfC1-xNx固溶体的性能,根据固体与分子经验电子理论(EET),对这类固溶体相进行了价电子结构分析,并分别与HfC,HfN基体进行比较。结果表明,HfC与HfN相互溶入后,非金属原子的杂阶保持不变,而Hf原子的杂阶从B11逐渐上升到B16;随着x值的增加,最强键上共价电子数、最强键键能、熔点都是逐渐下降,而共价电子总数百分比逐渐上升,表明固溶体的硬度、韧性、结合能、熔点逐渐减小,而强度却增加。

HfC+4TaC固溶体的价电子结构与性能

HfC与TaC在热压过程中易生成HfC+4TaC固溶体,为了预报该固溶体的宏观性能,采用固体与分子经验电子理论(EET),对其价电子结构进行分析,并与TaC、HfC基体进行比较。结果表明,在4TaC+HfC固溶体中,Ta的杂阶保持不变,而Hf、C原子的杂阶略有变化;在4HfC+TaC固溶体中,Hf、C原子的杂阶保持不变,而Ta原子的杂阶剧烈变化。2种固溶体的价电子结构与宏观性能均在HfC与TaC基体之间,并分别接近对应的基体。4种材料的硬度、强度、结合能、熔点排序从低到高依次为HfC、4HfC+TaC、4TaC+HfC、TaC。从能量与熔点角度分析,HfC与TaC接触时容易生成4TaC+HfC固溶体,而4HfC+TaC固溶体就难以生成。

HfC_x缺位陶瓷的价电子结构与性能

HfC在制备或服役过程中易出现C的缺位生成HfCx缺位固溶体,为了预报该固溶体的宏观性能,采用固体与分子经验电子理论(EET),对缺位固溶体的价电子结构进行分析,并与HfC基体进行比较。结果表明,随着x值的减小,即C原子缺位的增加,HfCx固溶体的晶胞常数逐渐缩小,最强共价键数、最强共价键键能、共价键数百分比逐渐减小,表明硬度、强度、结合能、熔点都逐渐下降;金属性逐渐增加,固溶体的韧性、导电性能、烧结性能逐渐改善。

Strongly Correlated Effect in TiS2

The thermoelectric compound TiS2 is studied by using the full-potential linearized augmented plane-wave method on the density functional theory with the generalized gradient approximation （GGA） as well as the on-site Coulomb interaction correction （＋U）. The Seebeck coefficient of TiS2 is calculated based on the electronic structure obtained within the GGA under the consideration of the on-site Coulomb interaction. The calculated Seebeck coefficient at 300K shows that Coulomb interaction U in the range of 4.97-5.42eV is important to reproduce the experimental data. The obtained energy gap Eg around 0.05 eV indicates that TiS2 is an indirect narrow-gap semiconductor.

三位置掺杂对BiCuSeO功能陶瓷热电性能的影响

对BiCuSeO功能陶瓷进行Bi/Cu/Se三位置掺杂,采用机械合金化和放电等离子烧结工艺制备Bi_(1-x)Ba_(x/2)Pb_(x/2)Cu_(1-x)Ni_(x)Se_(1-x)Te_(x)O(x=0,0.02,0.04,0.06,0.08,0.10,摩尔分数)陶瓷,通过掺杂前后的物相组成、组织结构、电传输参数、热传输参数等表征,研究三位置掺杂对Bi Cu Se O功能陶瓷热电性能的影响和强化机理。结果表明,三位置掺杂可杂糅几种元素的增益效果,使Bi Cu Se O功能陶瓷保持较高Seebeck系数的前提下,电导率和功率因子显著提高。最佳掺杂量x为0.10,所得Bi_(0.90)Ba_(0.05)Pb_(0.05)Cu_(0.90)Ni_(0.10)Se_(0.90)Te_(0.10)O陶瓷在873 K温度下获得最高功率因子0.71 mW/(m·K^(2))和最大热电优值1.06,分别约为未掺杂陶瓷的2.5倍和2倍。

Improvement of mechanical properties and investigation of strengthening mechanisms on the Ti_(3)AlC_(2) ceramic with nanosized WC addition

Ti_(3)AlC_(2) (TAC) has great potential for use as an ablation material in aerospace applications due to its great oxidation/ablation resistance, but its high-temperature strength and thermal shock resistance still have much room for simultaneous improvement under fast temperature variation conditions. Herein, we used Ti_(3)AlC_(2) and WC powders as raw materials and successfully fabricated textured (Ti,W)_(3)AlC_(2) ceramic with small amounts of TiC and Al_(2)O_(3), and room temperature mechanical properties such as flexural strength (1146±46.9 MPa), fracture toughness (11.78±0.44 MPa·m^(1/2)), and hardness (5.81±0.11 GPa) at 5 wt% WC addition were achieved. The high-temperature strength of the ceramic was significantly improved, and better thermal shock resistance from 298 to 1173 K was simultaneously acquired together with the regulation of the elastic modulus, thermal conductivity, and thermal expansion coefficient, providing (Ti,W)_(3)AlC_(2) with more possibilities for fast-temperature variation applications. Strengthening and toughening mechanisms were proposed. Scanning transmission electron microscopy high-angle annular dark-field imaging (STEM-HADDF) showed that W randomly replaced the Ti1 and Ti2 sites of Ti_(3)AlC_(2), providing a good reference for establishing crystal models, and further density functional theory (DFT) calculations based on these models indicated a higher fracture energy of (Ti,W)_(3)AlC_(2) along different crystal planes, providing superior resistance to transgranular fracture;a lower mismatch degree of (Ti,W)_(3)AlC_(2)/Al_(2)O_(3) resulted in stronger interface bonding, resulting in greater resistance to intergranular fracture as well as more balanced stress distributions at different interfaces.

Nano-scale compositional oscillation and phase intergrowth in Cu_(2)S_(0.5)Se_(0.5) and their role in thermal transport

Solid solution alloying is a promising strategy to establish high performance thermoelectrics.By alloying different elements,phase structures and phase compositions may vary accompanied by appearance of variety of interesting microstructures including mass fluctuation,lattice strain,nano-scale defects and spinodal decomposition,all of which may greatly influence the electrical and specifically the thermal transport of the material.In the present study,atomic structures of Cu_(2)S_(0.5)Se_(0.5) solid solution have been examined by using atom-resolved electron microscopy in order to investigate the structure-correlated physical insights for the abnormal thermal transport in this solid solution.Then the exceptional intergrowth nanostructures were observed.The solid solution consists of two high symmetrical phases,i.e.the hexagonal and cubic phase,which alternately intergrow to form highly oriented ultra-thin lamellae of nano or even,unit cell scales.The compositional oscillation in Se/S atomic ratio during alloying is responsible for the phase stability and intergrowth nanostructures.The unique binary phase intergrowth nanostructures make great contribution to the ultra-low lattice thermal conductivity comparable to glass and extremely short phonon mean free path of only 1.04Å,peculiar continuous hexagonal-to-cubic structural transformation without a critical transition temperature and its corresponding abnormal changes of thermal characters with temperatures.The present study further evokes the unlimited possibilities and potentials for tailoring nanostructures by alloying for improved thermoelectric performance.