【作者】 张文浩；

【导师】 唐新峰；

【作者基本信息】 武汉理工大学 ， 材料学， 2010， 硕士

【摘要】 AgSbTe2化合物作为(PbTe)x(AgSbTe2)1-x (LAST)和(GeTe)x(AgSbTe2)1-x (TAGS)两类高性能热电材料的共有组分,其低热导率及可预期的优良热电传输性能吸引了各国学者的关注与研究。目前该化合物的研究包括其晶体结构类型分析、能带结构的理论计算、制备工艺以及热电性能等方面,而且报道的性能结果也相对较高。因此,优化制备方法进一步提升该化合物的热电性能成为当前热电领域一大研究热点。本文系统研究了机械合金化制备AgSbTe2化合物各工艺参数对产物相组成、粉体形貌与颗粒尺寸的影响,同时采用放电等离子烧结(SPS)技术对所得机械合金化粉体进行了烧结致密化处理,研究了球磨、退火等工艺参数对最终块体微结构及热电性能的影响,得到以下主要结论：采用机械合金化制备了AgSbTe2单相化合物,研究了球磨主要工艺参数(球磨转速、球磨时间、球料比)对产物粉体相组成、微结构及颗粒尺寸的影响。结果表明,球料比固定于20：1,球磨转速为400rpm、600rpm和800rpm时,得到AgSbTe2化合物单相所需最短时间分别为15h、12h和6h。这说明增大球磨转速可以提高球磨效率,缩短耗时。在相同球料比和球磨时间下,粉体平均颗粒尺寸随球磨转速增加而减小,其中的小颗粒数目也增多。固定球料比20：1和球磨转速600rpm,增加球磨时间(12h-48h),产物相组成仍保持为单相AgSbTe2,但平均颗粒尺寸减小,粉体颗粒均匀性提高,小尺寸颗粒明显增多。其中球磨48h后粉体平均颗粒尺寸为370nm,其中小尺寸的一次颗粒仅为30～100nm。改变球料比(10：1,20：1,30：1),发现球料比30：1时,相组成不为单相,其中有Ag5-xTe3杂相出现。这说明球料比的增加会因为改变磨球在球磨罐中的运动轨迹、有效碰撞而影响球磨效率。采用SPS技术对典型工艺粉体(600rpm,20:1,24h)进行烧结探索,确定一最佳工艺后对不同球磨条件下粉体进行了烧结,并研究了球磨工艺参数以及退火等工艺对所得块体热电性能及微结构的影响。结果表明,SPS400℃对于球料比为20：1,球磨转速为600rpm,球磨24h制备的AgSbTe2粉体烧结最适宜,且得到的块体在498K时ZT值达到1.1。采用此烧结工艺对其余粉体烧结块体的研究表明,随着球磨转速增加,由于粉体颗粒尺寸显著细化,粉体吸附气体量增加导致烧结过程中发生喷样、分层等现象,导致烧结难度增大。故而800rpm样品因难于烧结而采用二次烧结,出现性能劣化,最终600rpm样品获得最大ZT值1.0。不同球料比对AgSbTe2块体性能影响结果表明,球料比10：1样品比20：1样品的电导率要高,但Seebeck系数变化不大,最终由于前者的高温热导率下降更多而热电性能更好,在550K时ZT值达到1.25。随着球磨时间增加(12h-36h),所对应样品的Seebeck系数减小,电导率增加,热导率也随之有所增加, ZT值总体上随球磨时间增加而增大,36h样品获得最大ZT值1.03。48h球磨粉体由于颗粒细小难于SPS烧结成功,采用高温高压(450℃,2.5／3.5GPa)方式获得块体,其中压力2.5GPa所得样品的热电性能随温度增加变化较快,于500K时ZT值即达到1.14,而3.5GPa样品在575K时达到1.03。对粉体进行预退火后再进行SPS烧结的研究表明,预退火确实有利于烧结过程中块体致密度的提升,但是也会导致AgSbTe2分相,最终使其性能劣化。为了使AgSbTe2制备工艺简化,缩短制备周期,减少合成能耗,进行了单质元素结合SPS技术一步合成烧结制备AgSbTe2的探索,最终发现这种方法可以获得AgSbTe2化合物。同时对不同SPS温度工艺研究对比表明,460℃烧结最优。对420℃、440℃和460℃烧结样品的对比研究表明,随着烧结温度增加,三者的Seebeck系数变化不大,但电导率随之增大,且热导率相差不大,最终使得460℃样品的ZT值最高,在470K左右达到0.97。虽然与机械合金化后烧结块体最佳性能相比有一定的差距,但是其过程的简化,样品致密度的提升,以及进一步优化可能带来的更好结果,都说明这是一种有潜力的合成AgSbTe2的方法。更多还原

【Abstract】 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.更多还原