Microwave Dielectric Properties Of Materials Research Articles

Exploring the Ln–O bonding nature and charge characteristics in monazite in relation to microwave dielectric properties

AbstractThe microstructure design is important for regulating the microwave dielectric properties of materials. However, in‐depth studies on the frequency temperature stability and related micromechanism remain poorly understood. The work investigates the correlation among the sintering behavior, crystal structure, bonding nature, and microwave dielectric properties of LnPO4 (Ln = Eu, Pr) ceramics by combining first‐principles calculations and experimental perspective. The high density (ρ > 97%) and large grains associated with lattice expansion benefit the optimum dielectric properties: εr = 11.24, Q×f = 61,138 GHz @ 13.311 GHz, and τf = −30.3 ppm/°C for EuPO4 sintered at 1500°C (εr = 11.35, Q×f = 63,496 GHz @ 13.042 GHz and τf = −39.5 ppm/°C for PrPO4 sintered at 1525°C). Bond valence analysis shows a rattling effect in [EuO9] due to a smaller ionic radius. The effect induces an abnormally large polarization, effectively shifting the negative τf toward near‐zero values. The electron localization functions, charge transfer, and bonding nature are discussed by density functional theory calculations, which illustrate stronger charge transfer and ionicity between Eu and O. This observation effectively predicts and validates the nonharmonic lattice vibrations and abnormally large polarization obtained from Raman spectrums and Rietveld refinement. These findings systematically clarify the optimized effect and micromechanism of lanthanides on the dielectric properties of monazite ceramics.

Microstructure and crystal structure dependence of microwave dielectric properties of non-stoichiometric (Sr0.7Ca0.3)z(Zr0.95Ti0.05)O3 perovskite ceramics

Solid-state reaction process has been used to fabricate (Sr0.7Ca0.3)z(Zr0.95Ti0.05)O3 (0.98 ≤ z ≤ 1.04) ceramics, the effects of non-stoichiometric ratio on sintering behavior, microstructure, structural properties and microwave dielectric properties were investigated. The ceramics contain the secondary phase ZrO2 with Fm-3m and P42 space structure as z ≤ 1.01. Electron back scatter diffraction analysis indicates that the secondary phase is concentrated at the grain boundary, which hinders the grain growth. Based on Rietveld refinement and Raman spectra, the secondary phase content and structural characteristics, including cell parameters, polarizability and lattice vibration anharmonicity, are obtained to establish the correlation between structure and microwave dielectric properties. The relative permittivity depends on the polarizability, and the variation of Q × f values is determined by the combined effects of grain size, secondary phase and lattice intrinsic loss, and is mainly driven by lattice vibration anharmonicity. In addition, the τf values are affected by lattice distortion. Particularly, (Sr0.7Ca0.3)1.02(Zr0.95Ti0.05)O3 (z = 1.02) ceramics have the optimized microwave dielectric properties: εr = 35.9, Q × f = 26,000 GHz, τf = −13 ppm/°C. This work provides a reference for optimizing microwave dielectric properties of materials.

Evolution and Analysis of Dielectric Properties of Typical Materials Under Strong Microwave Field

The complex dielectric constant is the intrinsic parameter of microwave material, and the accurate acquisition of its value is critical for its application. Generally, the dielectric properties of microwave materials are obtained under weak electromagnetic field. However, with the continuous development of high power microwave technology, the microwave field intensity around materials gradually increases, and the test results of the dielectric properties of materials under the weak electromagnetic field will not be able to characterize the test results under the high power environment. Therefore, a novel testing technology based on the resonator method is proposed, which provides an effective solution for the measurement and characterization of microwave dielectric properties of materials under strong microwave field. Based on the constructed system, the evolution rules of the microwave characteristics of four typical materials are obtained. In the experiment, the temperature of the samples is precisely monitored, and the evolution rules of the temperature characteristics of the dielectric properties of the samples under strong and weak microwave fields are compared. The comparison results demonstrate that the dielectric properties of materials under strong fields are related not only to the microwave heating effect but also to the mechanism of the non-thermal microwave effect.

Novel Series of Low‐Firing Microwave Dielectric Ceramics: (1−x)Li3Bi2P3O12–xTiO2

A new thermally stable microwave dielectric ceramic with low‐firing temperature in (1−x)Li3Bi2P3O12–xTiO2 [(1−x)LBP–xT, 0 ≤ x ≤ 0.6] ceramics system was obtained using conventional solid‐state reaction method. The phase transformation, microstructure, and microwave dielectric properties of materials were investigated using X‐ray powder diffraction (XRD), scanning electron microscopy (SEM) , and a network analyzer. The LBP ceramic (x = 0) sintered at 725°C has optimum microwave dielectric properties with relative permittivity (εr) of 15.8, Q × f of 26 600 GHz and temperature coefficients of resonant frequency (τf) of −130 ppm/°C. The εr and τf of (1−x)LBP–xT ceramics were tailored with increasing x values. At x = 0.55, the ceramic exhibits improved microwave dielectric properties with εr = 22.1, Q × f = 14 900 GHz and τf = −2.6 ppm/°C.

Measurement and Analysis of Microwave Dielectric Properties of BaTiO3-Based Ceramics

A new measurement technique using micro planar electrode and electromagnetic field analysis was developed to measure the microwave dielectric properties of materials with high dielectric constant and high loss factor. Dielectric dispersions of (Ba0.6Sr0.4)TiO3 and Ba(Zr0.25Ti0.75)O3 were measured up to 3 GHz as a function of temperature, and were well explained by the stretched exponential model of the decay function in dielectric relaxation. The phase transition in these ceramics was investigated by dielectric relaxation analysis, X-ray diffraction analysis and specific heat measurement. (Ba0.6Sr0.4)TiO3 showed a diffused phase transition and its relaxation frequency was much high than Ba(Zr0.25Ti0.75)O3, while Ba(Zr0.25Ti0.75)O3 showed typical relaxors behavior.

Effect of milling process on the microwave dielectric properties of Ba2Ti9O20 materials

The effects of the milling process on the characteristics of Ba2Ti9O20 materials were investigated. The chemical analyses using transmission electron microscopy (EDAX in TEM) revealed that the SiO2 species incorporated into the Ba2Ti9O20 materials were expelled by the Ba2Ti9O20 grains. It induced the dissociation of the Ba2Ti9O20 materials near the grain boundaries and degraded the microwave dielectric properties of the materials. The same phenomenon was assumed to be the procedure by which the high-energy-milling (HEM) process using Si3N4 grinding media (Si3N4-HEM) deleteriously influenced the microwave dielectric properties for the Ba2Ti9O20 materials. Utilizing the three-dimensional-milling (3DM) process in place of the Si3N4-HEM one markedly improved the characteristics of the Ba2Ti9O20 materials. The 3DM-processed samples own the same crystallinity as the HEM-processed ones but possess a pronouncedly more uniform microstructure and, therefore, exhibit a superior quality factor [(Q × f)3DM = 28 500 GHz and (Q × f)HEM = 21,900 GHz] with the same large dielectric constant (K = 38–39), when sintered at the same conditions (1350 °C/4 h). Such a phenomenon is ascribed to the fact that the 3DM process can pulverize the powders efficiently but induce no SiO2-contamination.

Microstructural defects in Ba(Mg 1/3Ta 2/3)O 3 microwave dielectric materials

Microstructures of Ba(Mg 1/3Ta 2/3)O 3 (BMT) materials were examined using high resolution and conventional transmission electron microscopy (TEM) as well as electron diffraction and microstructural characteristics were correlated with microwave dielectric properties of materials. TEM investigations exhibit different types of defects observed at large grains in over-sintered specimens, including dislocations, planar defects and three-dimensional defects. Trace analysis and displacement vector determination imply that stacking faults lying on {1 1 1} planes with a displacement vector of [1 1 0] type may form in the materials. The stacking fault is probably induced by diffusion discrepancy during ordering development of BMT materials which exhibit a concentration wave composition fluctuation at the initial stage. The defects may account for the degradation of over-fired BMT materials.