【作者】 刘晓虎；

【导师】 赵新兵；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2003， 硕士

【摘要】 β-FeSi₂和MnSi_1.75基半导体热电材料，由于它们在利用工业废热余热和其他设施热损失发电方面有很大的潜力，而且它们还具有成本低，无毒，无污染，热和化学稳定性好的优点，所以它们备受关注。 本文采用悬浮熔炼、快速凝固和单轴热压等制备工艺，制备了P型高锰硅和N型β-FeSi₂热电材料。采用XRD、SEM以及材料热电性能测试分析手段，系统的研究了材料微观结构特征及其对材料热电性能的影响。另外进行了以N型β-FeSi₂和P型HMS热电材料为基体的温差电池模型的理论计算和实际测量。 本文首次采用快速凝固热压技术制备高锰硅热电材料。研究表明快速凝固热压技术是制备高锰硅的有效方法之一。对快速凝固HMS热电材料的微观组织研究表明在Mn₄Si₇半导体相基体中，存在小区域平行分布的薄片状MnSi金属相，其形成机制是在快速凝固时的准定向凝固。研究表明不同Si含量对MnSi_1.75-x的热电性能有显著的影响。电导率随Si含量的增大而下降，Seebeck系数和热导率均随Si含量的增大而上升。综合各项测试的结果发现，在整个测试温度范围内，MnSi_1.75的ZT值最高。在500℃时MnSi_1.75的ZT值有最大值为0.42。 研究发现在800℃退火8h后的MnSi_1.75-x试样的电学性能得到了提高。这是由于退火后的MnSi_1.75-x中MnSi金属相有所减少的缘故。 研究表明掺Cr是提高HMS热电材料电学性能的另一种有效方法。掺杂Cr的量对Mn_1-xCr_xSi_1.73材料的电导率和Seebeck系数有显著的影响。几乎所有掺Cr的HMS热电材料的电导率随掺杂Cr量的增加而增加，Seebeck系数随掺杂Cr量的增加而减小。掺杂量x=0.02的试样的功率因子是所有掺杂材料中最优的，并且在500℃比未掺杂的快凝热压试样MnSi_1.73的功率因子要高260μWm^-1K^-10。 Fe_1-xCo_xSi（1-y）Al_y样品经悬浮熔炼，快速凝固，在975℃下氮气保护下热压30min，最后在800℃下真空退火20h后基本都转变成β-FeSi₂。快凝热压退火后的样品中，掺杂量为x=0.05的样品的电学性能为最优。在500℃下其功率因子为1072.75μWm^-1K^-1。 计算得到了理想温差电偶模型和实际温差电偶模型的最大输出功率。实验测得的温差电偶的最大输出功率远低于理论计算得到的最大输出功率。研究表明要正确估算温差电偶的最大输出功率必须考虑接触电阻和接触热阻的影响。更多还原

【Abstract】 Semiconducting p-FeSi2 and MnSi 1.75 based alloys are of great interests due to their potential applications in thermoelectric generators used to recover waste energy from exhaust gas and other infrastructure heat losses in industrial processing plants, and also due to their attractive characteristics such as low cost, environmental friendship, chemical stability at high temperatures.In the present work, P-type higher manganese silicides (HMS) MnSi1.75_x and N-type p-FeSi2 thermoelectric alloys were prepared with levitation melting, rapid solidification (RS) and hot uniaxial pressing (HUP). The microstructures of the materials were analysed with XRD and SEM, and the transport properties were measured. Both theoretical estimation and experimental measurements were done for the thermoelectric power generator module using p-FeSi2 and HMS as the P- and N-leg.State-of-the-art techniques of RS and HUP were used firstly to prepare HMS in the present work. It was shown that the microstructures of the MnSius-x prepared by RS and HUP consist of locally parallelly distributed MnSi thin flakes in the semiconductor MrLjSi7 matrix due to the gwos/’-directional solidification during RS. It was found that with the increase of the silicon content, the electrical conductivities of the materials decrease and both Seebeck coefficients and thermal conductivities increase. The ZT value of MnSi 1 75 is the highest in the MnSii.75-x in the range from room temperature to 600℃. The maximal ZT value of MnSi1.75 is 0.42 at 500℃.It was found that the electrical properties of the MnSi1 75-x samples are improved with the annealing at 800℃ for 8h, because the amount of metal phase MnSi decreases when the MnSi1 75-x are annealed.It was found that doping with Cr was an effective method to improve the electrical properties of MnSius.x. The doping content of Cr has a significant effect to the electrical conductivities and the Seebeck coefficients of the Cr-doped Mn1-xCrxSi1.73. The electrical conductivities increase and the Seebeck coefficients decrease with the increasing doping amount of Cr. The highest power factor of the Mn1xCrjSi1.73 was found for x = 0.02, which is 260tiWnf ’K’1 higher than that of the undoped MnSi].73 at 500℃.The p-phase transformation are almost completed after 20 hours annealing at 800℃ for ail Fei.jCOjSii-yAlj, samples prepared by RS and HUP. The highest power factor of about 1070 K"1 at 500℃ was measured for the sample doped with 5at% Co.The maximum power outputs were calculated and measured for a thermoelectric module. It was show that the power outputs from experiments were much lower than that calculated. Itwas suggested that both electric and thermal resistances at the contact interfaces should be taken into account for the estimation of the power output of a thermoelectric generator module.更多还原