摘要
随着全球能源危机的加剧,半导体温差发电作为一种新兴能源技术在废热利用等方面的应用再次引起人们的关注,而在温差发电技术及其器件的研究中,高温电极的设计及制备是其重点也是其难点之一。基于上述背景,本研究进行了有关余热温差发电器件高温电极的理论、设计及制备工艺方面的研究。
本文首先简要综述了氧化物热电材料的研究现状,介绍了影响材料热电性能的主要因素及提高材料热电性能的途径；并对温差发电器件的基本原理、金属半导体接触理论及金属半导体焊接工艺也进行了论述。讨论了ZnO基体材料制备的工艺过程和相关性能的测试原理及测试方法。在此基础上进行了N型Zn0电极基体制备的研究,提出了一种新的电极基体制备工艺—复合烧结工艺(热压烧结+高温烧结)；并通过实验研究确定了合适的复合烧结工艺参数。实验结果表明,复合烧结工艺是可行的,可成功地制备出具有一定结构强度和良好热电性能的块体材料,有利于降低电极基体的热导率,从而提高其热电性能。
文中进行了有关高温电极设计及制备方面的理论分析及工艺研究。以Ag基合金浆料为电极材料,采用烧渗法,初步制备出N型ZnO基高温电极,所制备的电极具有良好的电学性能和良好的结合力,并形成了良好的欧姆接触。通过实验研究烧结工艺和退火处理对电极性能的影响,确定了合适的高温电极制备工艺。
在第五章中,本文首先简要综述了NaCo2O4电子结构第一性原理分析的理论模型及CASTEP软件的特点及功能,搭建了理想及掺杂(掺Ca及掺Ni)NaCo2O4晶体结构的理论模型,采用GGA交换关联近似,先优化晶体结构模型,再在此基础之上计算了Na占位(1)时和Na占位(2)时的理想NaCo2O4晶体的能带结构、总态密度及分波态密度及掺杂(掺Ca及掺Ni)NaCo2O4晶体的能带结构、总态密度及分波态密度。结果显示,NaCo2O4电子态结构具有典型的半导体特征,具有较低的能隙,其导电性主要来源于Co-3d和O-2p轨道,导带主要由Co-3d和O-2p构成,价带主要由Co-3d构成。Co-3d的高的态密度主要集中在费米能级附近,不仅使载流子浓度增加,而且也使声子增多,因此,通过掺杂等手段改变Co原子的占据态,改变其在费米能级附近的态密度,可以增大NaCo2O4的热电系数。分别用Ca替位Na和用Ni替位Co。Ca替位Na后,与Na的态密度分布基本保持一致,对导带和价带的贡献不大,分布在很宽的区域内,不会带来导电性能的提升。Ni替位Co后,电子结构仍保持半导体结构特征,Ni主要在费米能级附近原子贡献能量,使材料中的载流子浓度增加,有助于提高材料的导电性,因此应主要通过掺杂改变Co原子的占据态,改变其在费米能级附近的态密度的方式来增大NaCo2O4的热电系数。这一结论对于NaCo2O4的掺杂改性研究提供依据具有一定的指导作用。
With the increasing of energy crisis and Global Warming, The application of thermoelectric power generation in waste heat utilization has caused for c oncern again, the design and preparation of high temperature electrode is one of the difficulties and focus in research of Temperature power generation tec hnology. On this basis,Theory、design and preparation of High-temperature electrode of waste heat thermoelectric power generation was researched.
This paper briefly reviews the status of Oxide Thermoelectric Materials. T he main factors affect thermoelectric properties and ways to improve the ther moelectric properties of materials introduced; basic principles of thermal powe r generation devices, metal semiconductor contact theory and the welding pro cess discussed too.
The preparation of ZnO substrate、Principles and Methods of performance test discussed. On this basis above, a new preparation process of N- type Zn O substrate:combined sintering process (hot-pressing+sintering) was offered.
The appropriate complex sintering process parameters identified by experimen t. Experimental results show that the composite sintering process is feasible a nd the ZnO bulk materials with a certain structural strength and good thermal properties can be successfully prepared by composite sintering process, it hel p to reduce the thermal conductivity of the materials, thereby enhancing the t hermoelectric properties.
Theoretical Analysis and preparation Technology of high-temperature electr odes were researched in the paper. N-type ZnO-based high-temperature electro de with Ag alloy as the electrode material successfully prepared by burn-osm osis, The prepared electrode with good electrical properties and good adhesi on and well ohm contact. The appropriate preparation of high-temperature ele ctrode was determine by Sintering and annealing experiment.
In the fifth chapter, the paper briefly summarizes theoretical model for the analysis of first principles of NaCo2O4 electronic structure and reasons for s elect the CASTEP software package as computational tools.theoretical model of ideal and doped (Ca-doped and doped Ni) NaCo2O4 was Build. By GGA exchange-correlation approximation used, to Optimization of the crystal struct ure model firstly, then on this basis the ideal and doped (Ca-doped and dope d Ni) NaCo2O4 crystal band structure, total density of states and partial densi ty of states were calculated. The results showed that NaCo2O4 has electronic state structure of a typical semiconductor characteristic, has a lower energy g ap. Its conductivity comes mainly from Co-3dand O-2p orbital, Conduction ba nd mainly composed by Co-3d and O-2p, the valence band mainly constituted by the Co-3d.The high Co-3d density of states concentrated near the Fermi 1 evel, not only to increase the carrier concentration, but also to increased phon on. Thus, by means of doping change the occupy state of Co atoms, change it's density of state near the Fermi level; the pyroelectric coefficient of NaCo 2O4 can be increased. Na were replaced by Ca, Ni substituted Co-bit, After Na substituted by Ca density of states Remained the same as Na, has small contribution in Conduction band and valence band, Distributed over a wide ar ea, will not bring improvement of conductivity. Ni substituted Co, electronic structure remains semiconductor structure, Ni mainly contribute atomic energy near the Fermi level, increase material's carrier concentration, Help to impro ve the conductivity of materials. Should be primarily through the the way of change the occupy state of Co atoms, change density of states near fermi le vel to increase the pyroelectric coefficient of NaCo2O4.This conclusion provid es has a guiding role in NaCo2O4’s modification of the doping.
引文
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