【作者】 王善禹；

【导师】 张清杰；

【作者基本信息】 武汉理工大学 ， 光电子及信息材料， 2010， 硕士

【摘要】 热电材料利用材料本身的物理效应来实现电热之间的转换,既可以利用Seebeck效应将热能转化为电能,也可以利用Peltier效应用于制冷领域。碲化铋合金及其固溶体是最早被发现也是现今最重要的热电材料体系之一,其主要用于热电制冷领域。作为现今唯一得到商业化应用的热电材料,因区熔技术具有工艺简单、容易产业化、制备过程无相变和区熔材料的热电优值较高等诸多优点,使之成为现今商用碲化铋合金主要制备技术。但采用区熔材料制备的器件热电转换效率仍然偏低,使得其大规模应用受到了限制,而且区熔材料的机械性能较差,不利于材料的加工和器件的制备,因此必须通过优化材料的制备工艺来提高材料的热电性能和机械性能。本论文以n型碲化铋基区熔材料为起始原料,采用自主开发的材料制备技术—熔体旋甩(MS)结合放电等离子技术(SPS)来制备具有高热电优值和高机械性能的n型碲化铋基块体热电材料,系统地研究了熔体旋甩工艺和放电等离子烧结工艺条件对材料的微结构和热电性能的影响规律,从而确定了一套最佳的MS+SPS工艺。主要的研究内容和研究结果如下。探索了MS+SPS技术制备高性能纳米结构n型碲化铋化合物的可能性,从而发展了MS+SPS技术制备单相块体碲化铋材料的新方法。薄带与铜辊接触的表面结晶较差,无明显的晶界,而未接触铜辊的自由面为枝状晶,枝状晶的尺度约为几百纳米。将薄带样品研磨后经SPS烧结得到致密的块体材料,相比区熔材料,MS+SPS样品的晶粒得到了显著细化,晶粒排列无明显择优取向,而且存在大量尺寸在10-100nm的纳米层状结构。区熔样品的电热传输性能存在明显的取向性,沿基面方向最大的热电优值为ZTmax=0.72@420K；将区熔样品研磨SPS后,样品的取向性得到了降低,但其热电性能有一定程度的降低；原料经MS+SPS处理后,样品的热电性能为各向同性的,最大的ZTmax=0.90@360K,室温ZT值相比区熔材料提高了50%。对上述三种不同工艺得到样品进行抗压强度测试,结果表明：区熔样品沿基面容易解理,强度低,仅为40MPa；研磨SPS后,强度增加可达到110 MPa；而MS+SPS样品的强度得到了大幅提高,可达200 MPa,相比区熔材料提高了近400%。在确定的MS喷气压力和管口直径,通过改变铜辊转速来改变冷却速率,探索不同冷却速率对材料的微结构和热电性能的影响。研究表明：随着铜辊转速的提高,薄带样品自由面的枝晶和接触面上的晶粒尺寸都逐渐减小,同时存在大量结构更为精细的纳米结构；铜辊转速为40m/s的薄带样品的HRTEM分析可知样品中存在大量尺寸在10-50nm的纳米晶粒,在纳米晶周围为非晶成分。对块体样品的FESEM观察也可以看出随着铜辊转速的增加,样品的晶粒尺寸有减小的趋势,但是变化较小,样品中同样存在大量层状结构,HRTEM观察也可以看到样品中存在大量尺寸5-20nm的纳米晶,很好地继承了薄带的结构。对烧结块体样品的热电性能测试表明：随着铜辊转速的提高,样品的电导率逐渐降低,Seebeck系数逐渐提高,且热导率逐渐降低；铜辊转速为30m/s的样品在360K最大ZT可以达到0.93,室温ZT值相比区熔原料提高了47%。在确定的MS工艺的条件下,研究不同SPS温度和压力对烧结块体样品的微结构和热电性能影响。SPS温度对材料的微结构和热电性能有着显著影响：低温下烧结的样品显著地保留了原始粉体和MS带子的形貌,且块体致密度较低；当烧结温度为高于400℃,样品烧结致密,无带子痕迹。随烧结温度增加,样品的晶粒逐渐长大,500℃烧结样品的晶粒长大较为显著。热电性能测试结果表明：随着烧结温度的增加,样品的电导率和Seebeck逐渐增加,且不同温度烧结样品的热导率变化较小；随着SPS烧结温度的增加,样品的ZT值逐渐升高,450℃烧结的样品具有最高的ZT,在360K可达到0.96,而且其室温ZT值相比原料ZM样品提高了53%,500℃烧结样品因成分挥发出现反常。在确定SPS温度为450℃的条件下,调节SPS压力,可以看出压力对样品的微结构和热电性能影响较小,可以认为SPS压力对烧结块体的微结构和热电性能没有影响。综上可知,以商业区熔材料为原料,最佳的MS+SPS工艺条件为：MS转速为3000 rpm,喷气压力为0.02 MPa; SPS温度为450℃,压力为20MPa。更多还原

【Abstract】 Thermoelectric (TE) materials can realize the dreactly convertion of electricity and thermal by the physical effect of the material, which is either used for power generations grounding on Seebeck coefficient or for cooling by Peltier effect. As one of earliest found and most important thermoelectric material, Bismuth telluride (Bi2Te3) and its alloys are mainly used for thermoelectric cooling. Owning to its many advantages such as simple procedure, easy industrialization, no phase transition and prepared material with high TE’s performance, zone melting technique has become the demonating technique to prepare commercial applied bismuth telluride material. Compared with tranditional mechanical refrigeration, the mouduls prepared by zone melted material still display much lower efficience because of lower TE’s performance; at the same time, the bad mechnicle properties of zone melted material generate some difficulties in material process and device preparation.In this research, starting with the commercialized zone melted n-type Bi2Te3 based material, we employ melt spinning (MS) subsequently combined with spark plasma sintering (SPS) technique to obtain densified bulk material with nanostructures, and systematically explore the effect of MS and SPS technique parameters on the microstructures and TE’s properties, which is in order to obtain optimal MS+SPS technique parameters.We studied the possibility to prepare high performance n-type Bi2Te3 based material with nanostructures and developed a novel synthesis technique that is MS+SPS to prepare high performance n-type Bi2Te3 based bulk material. The microstructures analysis of ribbon samples indicate that the contact surface exhibits unconspicuous crystalline and without distinct interfaces, whereas the free surface (which is opposite side of cintact surface) distributes dendritic structures having widths of several hundreds nanometers. The ribbons were hand ground into powder and then sintered into bulk pellet by SPS technique. Compared with zone melted (ZM) sample, the size of crystalline grains of sinterd samples are dramatically decreased after MS+SPS process, and the crystalline grains don’t display preferred orientation, furthermore, lots of refined layer structures with the size of 10-100 nm are obseaved. The thermoelectric properties of ZM sample exist apparantlly preferred orentetion, and the ZTmax along the base plane can reach 0.72 at 420 K; afer hand grinding and SPS, the preferred orentetion of thermoelectric properties decreases but the ZT value has somewhat reduction; the thermoelectric properties of MS+SPS sample are anisotopic and maximum ZT value can reach 0.90 at 360 K, whose room temperature ZT increase 50% compared with ZM sample. The measurements of pressive strength for three samples prepared by different techniques indicate that:due to its great crystalline and slippage of base planes, ZM sample shows lowest strength and only 40 MPa; after SPS process, the strength can reach 110 MPa, but the strength of MS+SPS sample increases dramantically and reaches 200 MPa, which is about 400% improvement compared with ZM sample.On condition that the injection pressure and nozzle size are fixed, the effects of cooling rate which is altered by variation of rotating speed of copper roller on the microstructures and TE’s properties of MS+SPS samples were investigated. With the increasing of cooling rate, the crystalline sizes of both surfaces for ribbons decrease, and some more refined microstructures are also obseaved. The HRTEM analysis of 40 m/s ribbon indicate there are lots of nanodots with the size of 10-30 nm in the contact surface and also amounts of amphous structures distribute around the nanodots. The FESEM photos of MS+SPS bulk samples show the grain size has the tendency of reduction with the cooling rate increasing, and lots of layer structure can also be observed which is consistent with previous results. From the HRTEM photos, lots of nanocrystals with the size of 5-20 nm can be observed which inherits very well the morphology of ribbons. The effects of cooling rate on thermoelectric transport properties were investigated:with the cooling rate increasing, electrical and thermal conductivity decrease gradually but Seebeck coefficient increase, and the ZTmax of 30 m/s sample can reaches 0.93 at 360 K, whose room temperature ZT is about 47% improvement compared with ZM sample.Based on the fixed MS technique conditions, the effects of sintering temperature and pressures of SPS on the microstructures and thermoelectric properties were systematically studied. The SPS temperature has great impacts on the microstructures and thermoelectric properties:there are heaps of ribbons randomly stacked throughout the SPS-300 and SPS-350 samples. The micro-morphology of the ribbons is largely preserved, as is evident in the SPS-400 sample. Significant changes in the micro-morphology occurred when the SPS temperature is increased to 450℃. No trace of a ribbon was found; conversely, lots of closely packed microsized crystals are observed. The crystalline size grows bigger with the sinter temperature increasing and the crystalline size of SPS-500 grows remarkably. The electrical conductivity and Seebeck coefficient increase but no distinct change is observed for thermal conductivity with the sinter temperature increasing and the resulted figure of merit ZT increase synchronously. Corrspondingly, the ZTmax of SPS-450 sample can reaches 0.96 at 360 K, whose room temperature ZT is about 53% improvement compared with ZM sample. Sample SPS-500 exhibits anomalousness may becacaue of element volatilization. Moreover, the effects of sinter pressure were also investigated when the sinter temperature was fixed in 450℃, and the research indicated that the sinter pressure has negligiable impacts on the microstructures and thermoelectric properties of MS+SPS samples.In conclusion, starting with commercial ZM ingots, the opitimal MS+SPS technique conditions are as follows:MS rotating speed is 3000 rpm and injected pressure is 0.02 MPa; SPS temperature is 450℃and pressure is 20 MPa.更多还原