DIELECTRIC AND TUNABLE BEHAVIOR OF LEAD STRONTIUM CERAMICS AND COMPOSITES
Open Access
- Author:
- Somiya, Yoshitaka
- Graduate Program:
- Materials
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 03, 2004
- Committee Members:
- Leslie Eric Cross, Committee Chair/Co-Chair
Amar S Bhalla, Committee Chair/Co-Chair
Dr R Guo, Committee Member
Sridhar Komarneni, Committee Member
Michael T Lanagan, Committee Member - Keywords:
- Curie-Weiss
dielectric
lead strontium titanate
composite
tunable - Abstract:
- The needs of materials for microwave applications have been increasing due to the demands of mobile communication systems and it is preferable for most applications to be manufactured inexpensively using the least amount of space possible. One of the ideas presented to achieve this goal is to reduce the number of components. By changing certain properties under a specific electric and/or magnetic field condition, a component is able to have more than one function, which decreases the number of components necessary. Although microelectromechanical systems (MEMS), semiconductors, and ferrite based devices are available for tuning applications, ferroelectrics in the paraelectric regions and incipient ferroelectric materials are the most promising for low cost and miniaturized products over a wide frequency range of 1-10 GHz and higher frequencies. Barium strontium titanate ((Ba, Sr)TiO3 (BST)) is a considerably studied field dependent ferroelectric material. However, BST requires special techniques to prepare samples which show good reproducibility because the conventional mixed oxide method is not expected to offer high homogeneity due to low reactivity among the raw oxide chemicals of BST. On the contrary, lead strontium titanate ((Sr, Pb)TiO3 (SPT)) permits much simpler processing due to the high reactivity of lead oxide, a raw oxide chemical, towards the other component oxides. Therefore, the SPT system has been selected as a potential candidate for the frequency agile ferroelectrics for electronics (FAME) applications. Selected compositions, for example, Sr-Pb (0.7:0.3 by mole and 0.8:0.2 by mole) in the pure SPT system show the following properties: i) high relative permittivity, ƒÕr, ii) low tangent ƒÔ in the paraelectric states, iii) moderate DC bias dependence of ƒÕr far above the transition temperatures, and iv) high DC bias dependence of ƒÕr close to the transition temperatures. In addition, the SPT system does not show a significant composition dependence of the properties. Because microwave applications need low capacitance materials in order to match the impedance for a device, the relative permittivity of the composition used should not be too high. Thus the ƒÕr of the SPT system is tailored by the addition of non-ferroelectric low ƒÕr and low tangent £_ oxides such as magnesium oxide (MgO). In order to achieve low tangent ƒÔ in SPT based systems around room temperature, the composition, Sr-Pb (0.8:0.2 by mole), has been chosen as the SPT source of the composites. The SPT-MgO composites show ƒÕr ~ 100, tangent ƒÔ less than 0.001 at room temperature and a frequency of 10 kHz using an LCR meter, and Q „e frequency ~ 700 GHz in a microwave frequency range based on the Hakki-Coleman method at room temperature. The tunability is ~ 0.02 with a DC field of 20 kV/cm (the reference field to compare various materials) and ~ 0.30 with a DC field of 175 kV/cm at room temperature and at 10 kHz. Mixing rules are analyzed for the best estimate relative permittivity of the composites. The ƒÕr of the MgO based composites at room temperature and a frequency of 10 kHz show a reasonable agreement with the mixing model proposed by Wakino et al. Besides the MgO based composites, other composite systems have been explored. Composites prepared from SPT and aluminum oxide (Al2O3) show unusually low temperature coefficients of dielectric permittivity and modest but relatively constant values of dielectric tunability over a wide temperature range. Composites prepared from SPT and zirconium oxide (ZrO2) show the possibility to achieve higher Q „e frequency values than most MgO based composites, but it would be difficult to achieve high density samples. In the pure SPT system, detailed studies are focused upon a departure from the Curie-Weiss law above the Curie point, Tc. The studies reveal that the Tc is unexpectedly below the Curie-Weiss temperature, theta. In addition, unusually high values of weak AC field relative permittivity are observed at the transition temperatures. These behaviors have been traced to the inhomogeneity in the solid solutions, but further work is necessary to establish both the nano-scale and the amplitude of the composition fluctuations, which are enhancing the relative permittivity maximum at the Tc. A more precise phase diagram has been established for the (Sr1-xPbx)TiO3 system in the important range of x (0.05 - 0.30) for room temperature applications.