【作者】 刘洪权；

【导师】 王福平；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2007， 博士

【摘要】 采用高分子网络凝胶法合成Ca₃Co₄O_9+δ前驱粉体,借助放电等离子烧结（SPS）技术制备高致密掺杂Ca₃Co₄O_9+δ陶瓷。采用XRD、SEM、TEM等分析手段对其组织结构进行表征。测试了钆掺杂Ca₃Co₄O_9+δ、钇掺杂Ca₃Co₄O_9+δ、钆钇复合掺杂Ca₃Co₄O_9+δ三个系列试样的电阻率、热电系数及热导率随温度的变化规律;并研究了其高温输运机制。采用XPS光电子谱,分析材料掺杂后XPS特征峰的偏移及材料中离子价态的变化。利用热处理调整晶粒大小,并研究其对电输运性能的影响。采用电场力显微镜观察材料的微观形貌,并将其与传导性质联系起来。通过不同氧气氛下的电阻率测试,分析材料电阻率的变化原因。首先利用高分子网络凝胶法制备系列钆掺杂Ca₃Co₄O_9+δ,系列钇掺杂Ca₃Co₄O_9+δ,系列钆钇复合掺杂Ca₃Co₄O_9+δ三个体系的前驱粉体,经粉体的XRD和透射电镜观察证实,此方法成功获得纯相、晶粒较小的粉体。通过实验研究,确定最佳工艺参数为,丙烯酰胺为单体,N,N’亚甲基双丙烯酰胺为交联剂,单体与交联剂质量比为5:1,单体相对溶液加入量为6g/100ml,pH值在6左右,在348K以上形成均匀的凝胶。金属离子浓度对晶粒大小影响不大。DTA的分析结果表明Ca₃Co₄O_9+δ的合成温度远低于固相法的合成温度。对SPS烧结的高致密陶瓷进行扫描电镜观察,晶粒呈片层状、大小1²μm,并具有一定的取向性;经透射电镜照片观察,可以看出晶粒间结合紧密,没有观察到晶间相和非晶层,表明界面结合良好。电阻率的测试表明,Ca₃Co₄O_9+δ试样电阻率在500K以下呈现正温度系数特征,在500K以上呈现负温度系数特征;钆掺杂Ca₃Co₄O_9+δ、钇掺杂Ca₃Co₄O_9+δ、钆钇复合掺杂Ca₃Co₄O_9+δ试样在测试温度范围均呈现负温度系数特征。通过对材料高温区的传导机制分析,可知材料在650K以上属于小极化子跳跃传导机制;并且随着掺杂量增加跳跃激活能逐渐增大。由于自旋散射的影响,钆掺杂Ca₃Co₄O_9+δ试样的激活能明显高于钇掺杂Ca₃Co₄O_9+δ试样,钆钇复合掺杂Ca₃Co₄O_9+δ试样的激活能介于两者之间。Ca₃Co₄O_9+δ、10mol%钆掺杂Ca₃Co₄O_9+δ、10 mol %钇掺杂Ca₃Co₄O_9+δ、10mol%钆钇复合掺杂Ca₃Co₄O_9+δ试样（钆、钇各掺杂5mol%）ZT值分别是0.136、0.249、0.223、0.263（973K）。经不同退火温度处理试样,可以适当调整晶粒尺寸,973K处理后晶粒尺寸大约1²μm,1123K处理后,晶粒尺寸大约3⁵μm,对不同晶粒大小的试样进行电阻率测试,两者差别不大;说明晶粒大小不是影响陶瓷材料电阻率的主要因素。XPS分析结果表明,掺杂后Co²⁺的含量增加,并且在掺杂位和其附近离子的XPS特征峰都向高结合能方向偏移,说明在掺杂位置存在弱束缚电子。掺杂使得价带谱的峰加强,载流子浓度减少。原子力形貌相及电场力相位图形象地将晶粒特征与材料性能联系起来,显示了材料电阻率的各向异性。同一试样在不同氧气氛下的电阻率测试结果表明,富氧条件下测得的电阻率更低。通过高温线性拟合结果表明,在富氧气氛下,跳跃激活能变小。这主要因为氧压力增大,对材料氧化能力增强,材料中Co⁴⁺的浓度增大,Co⁴⁺和Co³⁺之间的有效跳跃距离缩短,载流子在其间跳跃变得容易,所以激活能变小。钆掺杂Ca₃Co₄O_9+δ试样在不同氧气氛下测试结果差别不大,说明钆掺杂具有稳定材料热电性能的作用。更多还原

【Abstract】 Gd-doped Ca₃Co₄O_9+δ,Y-doped Ca₃Co₄O_9+δ and Y,Gd-doped Ca₃Co₄O_9+δ were synthesized by the polyacrylamide gel method, and their powders were sintered by Spark Plasma Sintering（SPS）. The microstructures of samples were characterized by X-ray diffraction （XRD）, scanning electron microscope （SEM） and transmission electron microscope （TEM）. The resistivity, Seebeck coefficient and thermal conductivity of the three series samples were tested at different temperatures. The conduct mechanism of samples was studied at temperatures above 650K. The hopping conduct mechanism of small polarons was determined for the materials. The XPS peak shift and changes of ion value were studied. Grain sizes of samples were controlled by heat treatment and the effects of grain sizes on resistivity were studied. Micro-topography was characterized by electric force microscopy （EFM） and relationships between micro-topography and resistivity were built up.The results of XRD and TEM indicated that the powders synthesized by the polyacrylamide gel method were fine and single phase. The parameters of optimal process were defined: the proportion between Acrylamide（monomer） and N,N’-methylene-bisacrylamide（crosslinked agent） is 5:1. The value of pH was controlled at about 6. The uniform gel was gained above 348K. Effects of metal ions concentrations on grain sizes are not obvious. DTA analysis showed that the synthesized temperature of Ca₃Co₄O_9+δ was lower than that of other methods.Samples sintered by SPS process showed c-Axis aligned at some degree. TEM observation of the Samples indicated that there were no other phases and non-crystal layers among grains. The temperature dependence on resistivity of Ca₃Co₄O_9+δ sample showed positive temperature coefficient below about 500K and negative temperature coefficient above about 500K. But that of doped Ca₃Co₄O_9+δ samples only showed negative temperature coefficient. According to the analysis of high temperature conduct mechanism, the materials have the hopping conduct mechanism of small polarons. With increasing of doping, the hopping activation energy increased. In addition, the hopping activation energy of samples doping with Gd was higher than that of samples doping with Y due to the effects of spin scatter. At 973K, ZT values of 10% Y-doped and 10% Gd-doped samples reached 0.223 and 0.249 respectively. The ZT value of 10% double-doped sample reached 0.263, which was the highest in the three material systems.The grain size was adjusted by heat treatment. After treated at 973K, the size of grain was about 1²μm; after treated at 1123K, the size of grain was about 3⁵μm. But the resistivities of two samples were almost same. So the grain size was not the main factor on the resistivity.XPS results showed that Co²⁺ increases after doping. The peaks of elements in doped sites shifted to high bind energy, which indicated there were weak bound charges in doped sites. The intensity of peaks increased in value band spectra of doped samples, which showed the decreasing of carriers. The electric force microscopy phase image connected resistivity with grain shape, and showed anisotropic properties of materials.The effect of oxygen nonstoichiometry on resistivity was obvious. The resistivity tested in full oxygen condition was lower than that tested in air condition due to oxided cobalt ions in materials. The concentration of Co⁴⁺ increases, and effective distance between Co⁴⁺ and Co³⁺ decreases; the hoping between two sites is easier, so hopping activation energy becomes smaller. For samples doping with Gd, there is no obvious difference for the resistivity tested in different conditions, which showed that the doped Gd element could stabilize thermoelectric properties of materials.更多还原