【作者】 李宁；

【导师】 郑金成；

【作者基本信息】 厦门大学 ， 凝聚态物理， 2014， 硕士

【摘要】 缺陷是影响晶体热导过程的重要物理机制。论文首先对热电材料的研究背景和现状进行了简要的介绍。为了研究氧缺陷对钛酸锶热传导过程的影响,利用高温高真空条件下对钛酸锶进行退火。先用高温型差示扫描量热仪测出单晶钛酸锶退火前后钛酸锶的比热,结合德拜模型比较比热随温度出现变化的原因。接着利用激光导热仪测出它们的热扩散系数,计算出它们的热导率,比较退火前后它们之间的差异。之后引入Callaway模型对这一实验结果进行拟合。最后利用X射线氧吸收谱对钛酸锶做了进一步的研究。研究结果显示,钛酸锶的比热整体随着温度的上升而变化,在温度较低时,比热快速增加,当温度接近钛酸锶的德拜温度时,比热增加速度变缓,在高温附近时,比热趋于不变,实验测量的结果符合德拜模型比热。钛酸锶退火后的比热与退火前相比,比热减小了,这是缘于钛酸锶在高温真空条件下退火后产生了氧缺陷。比热实验测量结果符合杜隆-柏替定律。钛酸锶的热导率随着温度升高逐渐降低。钛酸锶晶体导热过程中主要依靠声子来导热。温度升高时,声子振动能量加大,声子间的碰撞增多,声子间碰撞几率增大使得声子的平均自由程减小,热导率也就愈低。退火后,钛酸锶的热导大幅降低,这是由于钛酸锶退火后产生了氧缺陷,氧缺陷的存在对声子进行散射,有效的降低了声子的平均自由程,从而减小钛酸锶的热导。用不同温度对钛酸锶进行退火,退火温度越高,钛酸锶的热导率越低,用Callaway模型对这一实验结果进行拟合,得出原因是退火温度越高时晶体内会产生更多的氧缺陷,更有效的对声子进行散射,进而大幅降低热导。钛酸锶样品退火前后的X射线氧K边吸收谱表明钛酸锶在退火后确实产生了大量氧缺陷。进一步通过钛酸锶X射线Ti边吸收谱得出退火温度较高时,钛酸锶会产生更多的氧缺陷。这些结果对了解晶体热传导过程有所帮助,以利于下一步提高热电材料效率的研究。更多还原

【Abstract】 Defects are important physical mechanism in the thermal conductivity process of crystal. In this Paper, the research background and present situation of thermoelectric materials were briefly reviewed firstly. In order to study the influence of oxygen vacancies in SrTiO3on its thermal conductivity, SrTiO3single crystals were annealed in high temperature under vacuum. The thermal conductivities of the as-received and annealed samples were determined by measuring their specific heat Cp and their thermal diffusivities. By using experimental data and combining with Callaway model, fitting curves were obtained. The near edge X-ray absorption fine structure were used to study the influence of different annealing temperature on the oxygen vacancies in SrTiO3. Our results imply specific heat of sample increases along with temperature. In low temperature, specific heat increase rapidly, near the high temperature, specific heat tends to be the same, which conformed to the Debye model. According to Dulong and Peti’s law, the specific heat is proportional to the number of atoms. So the decreases of specific heat of annealed sample should be attributed to the produce of oxygen vacancies after annealing. After annealing, thermal conductivity of SrTiO3greatly decreased, this is due to oxygen vacancies. The oxygen vacancies act as strong phonon scattering centers, and decrease the mean free path of the phonons, effectively decreases the thermal conductivity of SrTiO3. The Callaway models show that the physical mechanism of vacancy defects affecting the thermal conductivity is the phonon scattering of oxygen vacancy, and the annealing temperature get higher it will produce more oxygen vacancies. The O-K edge X-ray absorption fine structure for the annealed and as-received sample indicate that the oxygen vacancies exist in the annealed SrTiO3. Further, in order to shed light on the relationship between annealing temperature and oxygen vacancies, the samples were characterized by NEXAFS of Ti L-edge. These results help in understanding thermal conductivity process, and facilitate the next research for improving the efficiency of the thermoelectric materials.更多还原