The Study Of Enhanced Electrocaloric Effect In BaTiO₃Thin Films

With the constant progress of science and technology, devices develop towards tominiaturization. Traditional way of cooling will not be able to realize the refrigerationof micro device, and solid refrigeration as a new way of refrigeration can meet thedemand of refrigeration of micro device. Solid state refrigeration mainly includesmagnetocaloric effect refrigeration, semiconductor refrigeration and electrocaloriceffect refrigeration. Magnetocaloric effect refrigeration had the disadvantage of highcost, low working temperature; The shortcoming of Semiconductor refrigeration islow cooling efficiency, big energy loss; And electrocaloric effect refrigeration had theadvantages of high efficiency, and low cost. So the study of electrocaloric effectrefrigeration is very meaningful. Problems existing in the electrocaloric effectrefrigeration include that the maximum value of adiabatic temperature changesappeared in the region of high temperature and adiabatic temperature changes in thevalue of a narrow temperature range corresponding to the maximum. Therefore, weshould study how to improve the heating effect. In this thesis, we want to enhance theelectrocaloric effect of BaTiO3thin films by adjusting and controlling phase transitionand preparing PbTiO3/BaTiO3composite thin film. The main contents and results arelisted as follows:1. The study of enhancement of room-temperature electrocaloric effect ofBaTiO3thin films by adjusting and controlling phase transitionSol-Gel method is used to prepare BaTiO3thin films with thickness of about300nm on the substrate of Pt/Ti/SiO2/Si. We can assure the crystal structure ispseudo-cubic by analyzing the X-ray diffraction result and ferroelectric measurementresult. From the temperature dependent dielectric constant of BaTiO3thin film, wefind that the dielectric constant of BaTiO3thin film decreases with the increase oftemperature in the temperature range from300K to320K which indicates there is aphase transition in BaTiO3thin film below300K. There is no obvious change fordielectric constant of BaTiO3thin film in the temperature range from320K to390Kand dielectric constant of BaTiO3thin film begin to decrease beyond the temperatureof390K. During this process, there is a diffusive phase transition from pseudo-cubicto cubic in BaTiO3thin film. It is caused by BaTiO3of pseudo-cubic phase due to theshort time annealing process for BaTiO3thin film. The result of temperaturedependent curve of adiabatic temperature change (T) of BaTiO3thin film is in agreement with the result of its temperature dependent curve of dielectric constant.The T of BaTiO3thin film decreases with the increase of temperature in thetemperature range from300K to320K and this phenomenon has never been reportedby other papers. The value of T of BaTiO3thin film is0.46K at the temperature of300K with the external electrical field of216.7kV cm1. It is larger than roomtemperature T of BaTiO3thin film reported in other papers, which means adjustingand controlling phase transition can enhance the room temperature electrocaloriceffect of BaTiO3thin film.2. The study of value enhancement of adiabatic temperature change (T) ofBaTiO3thin films by preparing PbTiO3/BaTiO3composite thin filmsPbTiO3/BaTiO3composite thin films with thickness of about300nm wereprepared by Sol-Gel method on the substrate of Pt/Ti/SiO2/Si. From the temperaturedependent curve of adiabatic temperature change (T) of PbTiO3/BaTiO3compositethin films we can find T increases quickly in the temperature range from375K to400K and the value of T is1.3K at the temperature of400K. However, themaximum of T of BaTiO3thin films is0.73K. Therefore, we can findPbTiO3/BaTiO3composite thin films have obvious enhancement effect to the value ofT of BaTiO3thin films.