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|Title:||Extrinsic dielectric relaxation of colossal dielectric constant material-CaCu₃Ti₄O₁₂(CCTO)||Authors:||Cao, Mingxiang||Keywords:||Hong Kong Polytechnic University -- Dissertations
Ceramic materials -- Electric properties
|Issue Date:||2010||Publisher:||The Hong Kong Polytechnic University||Abstract:||The main objective of this project is to determine the origins underlying the unusual dielectric properties of the colossal dielectric constant material: CaCu₃Ti₄O₁₂ (CCTO). The A₂/₃Cu₃Ti₄O₁₂ (A=trivalent rare earth cation) family of compounds has been known since 1967. This family was expanded and accurate structures were determined in 1979. CaCu₃Ti₄O₁₂, one of the family members, was reported to hold a colossal dielectric constant (CDC) (~10⁵) at room temperature under various frequencies and show small temperature dependence around this temperature range. Both properties are very important for dielectric device development. Extensive studies have been conducted in recent years in order to understand the underlying mechanism of these "unusual" dielectric properties. Although it has been reported that the huge dielectric constant might arise from local dipole moments associated with off-center displacement of Ti ions in the Ti-O octahedra, extrinsic effects such as lattice defects (oxygen vacancies), grain boundaries in polycrystalline materials and twin boundaries in single crystals actually play a more important role and form the main sources of CDC of CCTO. In the present work, we mainly focus on the effects of oxygen vacancies and grain boundaries in CCTO ceramics. For pure CCTO ceramics, the effect of post-sintering annealing on the dielectric properties and internal frictions (IF) of CCTO ceramics was studied. The dielectric properties were investigated in the temperature range of 150 to 500 K. A broad dielectric peak was observed in ε'(T) curve between 300 and 500 K. IF and Young's modulus of CCTO ceramics were measured by using the reed vibration method in the temperature range of 100 to 400 K with a measuring frequency of around 1.6 KHz. Three IF peaks were found and each peak can be well fitted by a single Debye relaxation. Below room temperature, two IF peaks appeared at 167 and 225 K, with corresponding activation energies of 0.394 and 0.263 eV, respectively. These activation energies agree well with those calculated from the temperature dependence of dielectric loss. Above room temperature, a third IF peak appeared at around 330 K, which was proven to be related to oxygen vacancies with an activation energy of 1.156 eV. A series of annealing processes were also used to find out the effect of oxygen vacancies on the dielectric properties and IF of CCTO ceramics. Our results showed that the dielectric properties and IF of CCTO above room temperature could be dramatically affected by annealing in either oxidizing (oxygen) or reducing (nitrogen) atmosphere, which suggests that oxygen vacancies play an important role in the CDC properties of CCTO and the IF peak at 330K is induced by oxygen vacancies.
To further study the effect of grain boundaries on the dielectric properties of CCTO ceramics, different percentages of MnO₂ were doped into CCTO samples. Our results showed that the doping of MnO₂ could dramatically change the dielectric properties. After doping with MnO₂, the broad dielectric peak above room temperature was greatly suppressed and shifted to higher temperatures. With increasing concentration of MnO₂, the broad dielectric peak was further suppressed and even disappeared at 3% of MnO₂. The dielectric constant and loss of CCTO were also dramatically reduced with increasing MnO₂ concentration. A great decrease in dielectric constant (for example, from 30600 to 2700 under 1 kHz at 200 K) was obtained when the amount of MnO₂ additive is 1%. The corresponding dielectric loss was also reduced. All of these results suggested that grain boundaries of CCTO make great contribution to its dielectric properties since the dopant of MnO₂ mainly segregated at the grain boundaries. The dielectric loss maximum was shifted to higher temperatures with increasing MnO₂ concentration. For example, for 1% MnO₂, the peak was at 198K, and it shifted to 203, 218 and 248K for 2, 3, and 5% MnO₂, respectively. The polarization strength was decreased with increasing MnO₂ concentration, i.e., from 2717.5 for 1% MnO₂ to 1659 for 5% MnO₂.
|Description:||1 v. (various pagings) : ill. ; 30 cm.
PolyU Library Call No.: [THS] LG51 .H577M AP 2010 Cao
|URI:||http://hdl.handle.net/10397/4259||Rights:||All rights reserved.|
|Appears in Collections:||Thesis|
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