Papers by Keyword: Microwave Dielectric Properties

Abstract: The Y₃Al_5-xNb_xO₁₂ (0 ≤ x ≤ 0.06) garnet ceramics were prepared by solid-state reaction method. The effects of Nb⁵⁺ substitution on the phase composition, microstructure and microwave dielectric properties were investigated. All samples retained the yttrium-aluminum garnet solid solution form until x up to 0.06 at which YNbO₄ secondary phase precipitated. The doping of Nb₂O₅ improved the grain development and enhanced the physical properties of the ceramics under the sintering condition of 8 h at 1700 °C. Specifically, with the substitution of Nb⁵⁺, the degree of micromorphology improvement and quality factor first increased and then decreased, with a maximum value at x = 0.015. Finally, the best combination of microwave dielectric properties was obtained, i.e., ε_r = 10.79, Q×f = 165,478 GHz, τ_f = -31.7 ppm/°C. This study provides a theoretical basis for the further expansion and application of yttrium aluminum garnet in the microwave field. Keywords: Yttrium Aluminum Garnet ceramics, Nb₂O₅, microwave dielectric properties.

Abstract: In this work, a new microwave dielectric material for LTCC applications, xAl₂O₃+ (1-x) BaO-Al₂O₃-B₂O₃-SiO₂-ZnO (x=0.3, 0.4, 0.5 and 0.6) glass-ceramics, was synthesized. The effect of Al₂O₃ on the microstructure, mechanical properties and electrical properties of BaO-Al₂O₃-ZnO-B₂O₃-SiO₂ glass-ceramics was studied in this paper. The crystallinity was studied by jade6.5 software using the whole pattern fitting method. The XRD analysis showed that the content of BaAl₂Si₂O₈ increased first and then decreased with the increase of Al₂O₃ content. It is found that the formation of phase BaAl₂Si₂O₈ contributes to strong bending strength. The xAl₂O₃+ (1-x) BaO-Al₂O₃-SiO₂-ZnO-B₂O₃ glass-ceramics sintered 2h at 920°C has the excellent microwave dielectric properties at x=0.4, ε_r=7.4, tanσ=1.49×10^-3 at 13.5GHz, τ_f=-42.67ppm/°C, flexural strength= 184MPa.

Abstract: (Ca_0.2Sr_0.8)₃(Zr_xTi_1-x)₂O₇ (x = 0.02-0.1) ceramic prepared by the solid state method was investigated for its microstructure and microwave dielectric properties. The correlation between the microstructure and microwave dielectric properties was also investigated. By increasing x from 0.01 to 0.06, the dielectric constant and Q×f value of the specimen could be increased from 72 to a maximum of 74, and from 11000 GHz to a maximum of 18000 GHz, respectively. The ε_r value of 74, the Q×f value of 18,000 GHz, and the τ_f value of 418 ppm/^oC were obtained for (Ca_0.2Sr_0.8)₃(Zr_0.06Ti_0.94)₂O₇ ceramics sintering at 1520^oC for 4h, and (Ca_0.2Sr_0.8)₃(Zr_xTi_1-x)₂O₇ (x = 0.02-0.1) is proposed as a suitable material candidate for application in microwave ceramic temperature sensing antenna.

Abstract: 0.67CaTiO₃-0.33LaAlO₃ ceramics doped with Nd₂O₃ were prepared using a conventional two-step solid-state reaction, and the effect of Nd₂O₃ doping on the microwave dielectric properties were studied. A single phase of perovskite structure was confirmed with the Nd₂O₃ doping. The results showed that the addition of Nd₂O₃ obviously improved the density and Q×f values of CTLA ceramics. The dielectric constant were almost not affected and the τ_f values were close to zero with the increase of the doping amount. CTLA ceramics with 0.2 wt% Nd₂O₃ doping sintered at 1380 °C for 4 h exhibited the optimal microwave dielectric properties of ε_r = 45.36, Q×f = 37,890 GHz, τ_f = 7.80 ppm/°C.

Abstract: Dielectric ceramics find application as dielectric resonators (DRs) in communications systems operating at microwave frequencies. RDs for this application require a unique set of properties: high value of the dielectric constant, low dielectric loss and high frequency stability. This paper presents an investigation of the correlation between the dielectric properties, the characteristics of microstructure and the crystalline phases of Ga₂O₃-doped ZnO-Nb₂O₅-TiO₂ ceramic system. The ceramics sintered at 1200 °C were characterized as for density, crystalline phases, microstructure and microwave dielectric properties. The results showed that these dielectric ceramics, obtained from the TiO₂ anatase crystalline structure, present dielectric constant and quality factor (Q) values appropriate for their use as dielectric resonators in microwave circuits. According to the experiments, as the gallium doping has raised, the dielectric constant increased, the Q factor decreased and the temperature coefficient had a tendency to decrease to a certain extent.

Abstract: Silicone rubber composites filled with fused silica were prepared through sigma mixing followed by hot pressing. Filling fraction of fused silica in the silicone rubber matrix was varied from 0-0.51 volume fraction (V_f) and its effects on dielectric properties at different frequencies, thermal properties and moisture absorption were investigated. The results indicate that with the increase of filler volume fraction the relative permittivity increases and dielectric loss decreases. The coefficient of thermal expansion decreased and the moisture absorption increased marginallyas the filler loading increased.

Abstract: Li₂ZnTi₃O₈ ceramics were prepared by reaction-sintering process (calcination free). The crystal phase and microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). A pure phase of Li₂ZnTi₃O₈ ceramics sintered at 1075 °C-1150 °C with cubic spinel structure was confirmed by XRD. The microwave dielectric properties (ε_r, Qxf) of Li₂ZnTi₃O₈ ceramics were strongly dependent on the densification and grain size. The τ_f of Li₂ZnTi₃O₈ ceramics was almost independent with the sintering temperatures. In particular, Li₂ZnTi₃O₈ ceramics by reaction-sintering method sintered at 1125 °C for 5 h exhibited good combination microwave dielectric properties of ε_r=21.7, Q×f=70 500 GHz (at 7.5 GHz) and τ_f=-13 ppm/°C.

Abstract: The dielectric resonator (DR) is a ceramic component used in electronic circuits that can operate in microwave frequency range, where it plays the role of resonant element and enables the construction of high selective filters and oscillators. The dielectric properties of a ceramic resonator are influenced by their microstructure characteristics as pores amount. This work shows a study of the influence of pores amount (porosity) on the dielectric constant values of ceramics from ZnO-Nb₂O₅-TiO₂ system. Mixtures of Nb₂O₅, ZnO, and TiO₂ powders were compacted by uniaxial (100 MPa) and isostatic (300 MPa) pressing and sintered at 1100, 1200, and 1250°C. The experimental dielectric constant values of the sintered ceramics were corrected to eliminate the porosity influence. The results showed that the porosity, the type and quantities of crystalline chemical compounds in the ceramics influence the dielectric constant values.

Abstract: CaCu_(3+x)Ti₄O₁₂ (CCTO) ceramics with different Cu-excess (x = 0 – 0.6) were prepared by conventional solid-state reaction method. Characterization of the prepared ceramics with XRD and FESEM showed that lattice parameter and grain size are slightly increased, indicating Cu-excess to have the big impact on the both phase structure and microstructure. The XRD profiles indicated that the secondary phase (CuO or Cu₂O) existed at edge/corner of CCTO grain, which promoted inhibited grain growth behavior. The CCTO ceramics exhibited two trends of dielectric constant related to frequency, which showed a flatter curve about ~50 in 1 – 25 GHz regions, and it’s dropped rapidly to ~35 in 25 – 50 GHz region. With Cu-excess, the dielectric constant of the ceramics was increased for an average of a quarter-order of magnitude, while the tangent loss also increased up to triple times than x = 0, for the same frequency range. Despite enormous increase of dielectric constant related to varying Cu-excess, the tangent loss also increased.

Abstract: The effect of LnAlO₃ (Ln = Nd, Sm) additives on the densification, phase evolution, microstructure and dielectric properties of BaO-Nd₂O₃-5TiO₂ (BNT5) ceramics was investigated. The LnAlO₃ (Ln = Nd, Sm) had greatly influence on the densification behavior of the BNT5 ceramics and the grain sizes of BNT5 ceramics could be effectively refined by LnAlO₃ additive. The microstructure of BNT5 ceramics was transformed from quasi-rectangular grain to columnar grain while the content of LnAlO₃ was increasing. The dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) were reduced with the increase of LnAlO₃ content, whereas the Q×f values showed a non-linear behaviour with LnAlO₃ content increasing. The BNT5 ceramics doped with 12.5 wt% SmAlO₃, sintered at 1320°C for 4 h, had the excellent dielectric properties of ε_r = 62.78, Q×f = 11108 GHz and τ_f = -1.55 ppm/°.