新钴基层状化合物AxCoO2的制备与热电性能研究

Oxide thermoelectric materials are the promising candidates for thermoelectric materials application at high temperatures. Layered cobaltites NaxCoO2 was discovered to exhibit good thermoelectric properties such as a large Seebeck coefficient and a low resistivity and thermal conductivity. However, the deliquescence and high vapor pressure of sodium make it not very suitable to be used for thermoelectric devices. It's reported that CoO2 layers with the CdI2 type as well as the alternately layered structure play a very important role in the thermoelectric properties of the layered cobaltites. As a result, in this study, we try to synthesize new layered cobaltites AxCoO2 with the same structure of NaxCoO2 by the ion exchange technique from NaxCoO2 precursor, because layered cobaltites NaxCoO2 has a remarkable property, which is that the sodium ions between the CoO2 layers can be exchanged with the other cation ions. According to the results of my study about the ion exchange resultants CaxCoO2 and SrxCoO2, we will choose some cation ions such as Ba2+ and Pb2+ with the radius larger than Sr2+ to react with NaxCoO2, and prepare novel layered cobaltites AxCoO2 by the ion exchange technique, and try to search for one or two new layered cobaltites with good thermoelectric properties and high reliability for high temperature thermoelectric devices.

氧化物热电材料是热电材料高温应用中十分重要的候选材料，作为氧化物热电材料杰出代表的NaxCoO2，虽然具有优异的热电性能，但是由于含有钠而存在易潮解以及高温时钠易挥发等缺点，极大地影响了其推广和应用。大量研究表明，含有具有CdI2型晶体结构的CoO2亚层以及层状交替结构是这类钴基层状化合物具有优异热电性能的关键因素。本研究拟利用NaxCoO2能与其它阳离子盐发生离子交换反应生成新的化合物AxCoO2，而本身结构只进行局部调整即仍含有CoO2亚层且仍具有层状交替结构这一特点，结合本人前期关于离子替代产物CaxCoO2和SrxCoO2的研究结果，选择比Sr2+半径大的二价阳离子如Ba2+和Pb2+等的硝酸盐与NaxCoO2进行离子交换反应，制备新的钴基层状化合物AxCoO2，并对其热电性能进行研究，最终开发出一至两种性能优异且可靠性高的氧化物热电材料，为热电材料的高温应用提供新的重要候选材料。

氧化物热电材料是热电材料高温应用中十分重要的候选材料。大量研究表明，含有具有CdI2型晶体结构的CoO2亚层以及层状交替结构是这类钴基层状化合物具有优异热电性能的关键因素。本研究利用NaxCoO2能与其它阳离子盐发生离子交换反应生成新的化合物AxCoO2，而本身结构只进行局部调整即仍含有CoO2亚层且仍具有层状交替结构这一特点。以层状化合物NaxCoO2为前驱体，在一定条件下分别与硝酸钡、硝酸钾的熔体以及硝酸铅、硝酸钾的熔体进行反应，生成与前驱体具有相同晶体结构的新钴基层状氧化物热电材料BaxCoO2和PbxCoO2，丰富了钴基层状氧化物热电材料的种类。通过调节前驱体中钠的含量，制备出不同钡含量的BaxCoO2(x = 0.19、0.28、0.30、0.33)，并对其磁性、比热和热电性能进行了研究。BaxCoO2的磁化率随着样品中钡含量的升高而降低，50 K以上时磁化率符合居里-外斯特性，低温时BaxCoO2的比热来自于电子比热和晶格比热的贡献，且电子比热系数随着样品中钡含量的升高而增大。BaxCoO2的低温电阻率对样品中钡的含量很敏感，电阻率随着样品中钡含量的升高而下降，且随着样品中钡含量的升高，BaxCoO2的导电特性从半导体导电特性转变为金属导电特性。随着温度的升高，BaxCoO2电阻率之间的差距逐渐减小，最大电阻率与最小电阻率从2.5 K时的54300倍，缩小到300 K时的8倍，再到800 K时的0.68倍。样品的赛贝克系数对成分的变化不敏感，热导率随着样品中钡含量的升高而增大。800 K时Ba0.19CoO2样品的ZT值最高，为0.21，高于相同温度时纯Ca3Co4O90.12的ZT值。