Besides the excellent high-temperature mechanical properties, Si3N4 and SiC based ceramics containing insulating or electrically conductive phase are attractive for their tunable dielectric propert...

Electromagnetic properties of Si–C–N based ceramics and composites

The dielectric loss (tan δ) of titanium dioxide (TiO2) disks has been measured at a frequency of 3 GHz High-purity TiO2 sintered to almost-full density exhibits a very high tan δ, which is interpreted to be due to oxygen deficiency To counter this, doping with stable divalent and trivalent cations, such as Mg and Al, leads to a low tan δ, probably by preventing Ti4+ reduction The tan δ of polycrystalline TiO2 doped with divalent and trivalent ions with ionic radii in the range of 05–095 A at 3 GHz can be very low: 6 × 10−5 (Q∼ 17 000) at a temperature of 300 K The tan δ of undoped pure TiO2 disks increases when the disks are cooled from 300 K to ∼100 K At temperatures <100 K, the tan δ decreases rapidly, which is interpreted as carrier freeze-out The tan δ for all the high-Q doped TiO2 polycrystalline samples smoothly decrease to ∼5 × 10−6 (Q∼ 200 000) at 15 K, comparable to that of single crystals

Microwave Dielectric Loss of Titanium Oxide

Li2ATi3O8 (A=Mg, Zn) ceramics are prepared by the conventional solid-state ceramic route, and the phase purity, microstructure, and microwave dielectric properties are investigated. Li2MgTi3O8 ceramics show ɛr=27.20, Qu×f=42 000 GHz, and τf=(+) 3.2 ppm/°C, and Li2ZnTi3O8 ceramics have ɛr=25.6, Qu×f=72 000 GHz, and τf=(−) 11.2 ppm/°C, respectively, when sintered at 1075°C/4 h.

Synthesis and Microwave Dielectric Properties of Novel Temperature Stable High Q, Li2ATi3O8 (A=Mg, Zn) Ceramics

This paper reviews the basic physical notions underlying microwave sintering and the theoretical and numerical models of the microwave sintering process. The propagation and absorption of electromagnetic waves in materials, and the distribution of electromagnetic field in cavity resonators that serve as applicators for microwave processing are discussed and the electromagnetic modeling of such applicators is reviewed. The microwave absorption properties of ceramic and metal powder materials and the methods of their description are addressed. Self-consistent electromagnetic and thermal models that are capable of predicting the temperature distribution in the microwave-heated materials and dynamic effects such as thermal runaway instabilities are reviewed. The multiphysics simulations that combine electromagnetic, thermal, and sintering models and result in predicting densification, shrinkage, and grain structure evolution are discussed in detail. The significance of microwave nonthermal effects in sintering is demonstrated based on the experimental results, and the models of such effects are reviewed.

Microwave Sintering: Fundamentals and Modeling

The microwave dielectric properties of scheelite (A = Ca, Sr, Ba) and wolframite (A = Mg, Mn, Zn) AMoO4 compounds and their relations with structure have been examined using a network analyzer and X-ray powder diffraction. The Mo ion polarizability has been also investigated from AMoO4 compounds using a least square refinement technique in conjunction with the Clausius–Mosotti equation. It was found that dielectric properties such as dielectric constant, temperature coefficient of resonant frequency, and quality factor were found to be correlated with the size of A-cations and the structure of compounds. The well sintered AMoO4 samples (>95% of theoretical density) exhibited dielectric constant of 7–11, quality factor of 37,000–90,000 GHz and temperature coefficient of resonant frequency of −57 to −87 ppm/°C, respectively. These investigations showed that AMoO4 ceramic could be selected as a possible candidate for microwave dielectric ceramics because of its low dielectric constant and high quality factor.

Microwave dielectric properties of scheelite (A = Ca, Sr, Ba) and wolframite (A = Mg, Zn, Mn) AMoO4 compounds

The real part of the permittivity (E') and the tan δ of sintered alumina (Al 2 O 3 ) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan δ is found to depend very strongly on the pore volume, purity, and grain size. e' is far less sensitive to impurities and grain size. The dependence of e' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and e' of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near-theoretical density ranges between 9.1 x 10 -5 and 2.4 x 10 -5 depending on grain size.

Meishing Xu

Papers

Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina