Mechanical Alloying Synthesis And Thermoelectric Properties Of AgSbTe₂ Compound

AgSbTe2 compound as the mutual composition of the two kinds of high performance thermoelectric materials, LAST and TAGS, which possesses extremely low thermal conductivity and foreseeable good thermoelectric transport properties, has been investigated extensively. At present, the research of this compound is mainly focused on its crystal structural analysis, theoretical calculation on its band structure, preparation techniques and its thermoelectric properties, which has been reported relatively high. Hence, optimizing the processing methods so as to promote the thermoelectric properties of AgSbTe2 further becomes a hot spot.AgSbTe2 compound has been synthesized by mechanical alloying method and the impacts of the mechanical alloying parameters on the phase composition, powder morphology and particle size have been investigated systematically. At the same time, spark plasma sintering (SPS) technique has been used to get bulk materials from the as-got ball milling powders, and the influence of ball milling technical parameters and annealing process on the microstructure and thermoelectric properties of the bulk has been studied and summarized as follows:Single phase AgSbTe2 compound has been prepared via mechanical alloying, and the effects of the main ball milling parameters, such as ball milling rotation speed, milling time, and charge ratio, on phase composition, microstructure, and particle size of the resulting powders have been investigated. The results suggested that the shortest time to get single phase AgSbTe2 compound was 15 hours,12 hours and 6 hours, respectively, when charge ratio was 20:1, and the rotation speed was 400rpm, 600rpm, and 800rpm, accordingly. This suggests that increasing the ball milling rotation speed can promote the alloying efficiency and decrease the consumption of time. When charge ratio and milling time were constant, the average particle size of the powder decreased with the increase of rotation speed, while the amount of small size particles increased. When charge ratio was 20:1, rotation speed was 600rpm, and the milling time as a variable changed from 12 hours to 48 hours, the phase composition of the powders still retained as single phase AgSbTe2, but their average particle size decreased, the homogeneity of particles rose, as well as the small size particles increased. The average particle size of the powder after 48hs'milling was 370nm, and the small size of the primary particles ranged from 30nm to 100nm. When the charge ratio varied from 10:1 to 20:1, then 30:1, and other conditions kept constant, it was only to find that the phase composition became impure with Ag5-xTe3. It suggests that the increase of charge ratio causes the change of the milling balls' movement trail and collision efficiency, thereby reduces the ball milling efficiency.SPS technique has been explored to get bulk materials from the typical processing powder (20:1,600rpm,24h), and an optimized technique was ascertained from the sintering experiment above, then the microstructures and thermoelectric properties of the bulks from the powders obtained by different ball milling conditions have been investigated. It shows that the 400℃SPS technique has been proved to be the most suitable, and the ZT value of the bulk from the typical processing powder achieves 1.1 at 498K. The SPS technique mentioned above has been used for the sintering of other powders, and the results suggest that the higher the rotation speed is, the more difficult the sintering of the according powder is, for the case that the particle size of the powders decreases with the increase of the rotation speed, and the amount of the air absorbtion by the powders increases, which results in the spray or delamination of the sample during SPS process. If the same sintering process was used repeatedly so as to get the bulk, there would be only to find that the performance of the bulk degraded. So compared with 400rpm and 600rpm samples, the 800rpm one haven't got the highest ZT for the sake of twice sintering. The study about the effects of different charge ratio on the properties of AgSbTe2 bulks suggests the 10:1 sample got a higher ZT of 1.25 at 550K, because it possessed higher electrical conductivity and a steep reduce of thermal conductivity during the high temperature range, compared with the 20:1 one. With the ball milling time increasing from 12h to 36h, the according bulk samples have such properties as the decrease of Seebeck coefficient, the increase of electrical conductivity, and a low increase of thermal conductivity, which finally makes the 36h sample get the highest ZT value of 1.03. As the powder obtained by 48hs'ball milling is hard to be sintered by SPS, a high temperature high press(450℃,2.5/3.5GPa) approach was applied to get it into bulk material. The properties which the samples exhibit show that the thermoelectric properties of the 2.5GPa sample varies with the increase of temperature quickly, and gets the ZT value of 1.14 at 500K, while the 3.5GPa sample changes gently, and has the ZT value of 1.03 at 575K. The annealing process towards the mechanical alloying powders makes the sintering process easier, but it also causes the phase decomposition and the performance degradation of the bulk from the powders after annealing.In order to simplify the preparation process of AgSbTe2, cut down the preparation period, and reduce the reaction energy consumption, a directly SPS synthesis after a simple blend of the elementary powders (Ag, Sb, Te) approach was explored to get AgSbTe2 bulk material, and finally it works well. In this research, it is proved that SPS 460℃is the best sintering temperature. And according to comparison of the samples from the three different sintering temperature 420℃,440℃,460℃, it suggests that with the increase of sintering temperature, the Seebeck coefficient and thermal conductivity of them didn't vary so much, but the electrical conductivity rose. This makes the ZT value of the 460℃sample be the highest, and achieve 0.973 at 470K. Although its property still is lower, compared with the samples obtained by the mechanical alloying powders after SPS, the simplification of process, the promotion of the relative density of the samples, and a probable better result after further optimization, all suggest that it is a potential and competitive way to synthesize AgSbTe2 compound.