Papers by Keyword: Microwave Ceramic

Abstract: 0.22CaTiO₃-0.78(Li_0.5Sm_0.5)TiO₃ + x wt% MgO (0.25 ≤ x ≤ 2.0) ceramics with pure perovskite structure were prepared by a conventional two-step solid-state reaction process. The effect of different content of MgO additive on the microwave dielectric properties of the 0.22CaTiO₃-0.78(Li_0.5Sm_0.5)TiO₃ ceramics was investigated in detail. The results of microwave dielectric properties showed that the permittivity (ε_r) and temperature coefficient of resonant frequency (τ_f) decreased gradually, while quality factor (Q×f) values increased when the x value increased. Raman spectra and infrared reflectivity spectra were used to study the relationship between vibrational modes and microwave dielectric properties. The harmonic oscillator model was used to fit infrared reflectivity spectra, and the obtained complex dielectric response was extrapolated down into the microwave region. It was found that the complex dielectric spectra were in good agreement with the measured microwave permittivity and dielectric loss. The 0.22CaTiO₃-0.78(Li_0.5Sm_0.5)TiO₃ + 0.50 wt% MgO ceramic sintered at 1240°C for 4 h exhibited good microwave dielectric properties with ε_r = 102.6, Q×f = 5084 GHz (at 3.8 GHz), and τ_f = -16.3 ppm/°C.

Abstract: xBaZn₂Ti₄O₁₁–(1-x)BaNd₂Ti₄O₁₂ (x = 0.18–0.30) ceramics were prepared by solid-state reaction method and their microwave dielectric properties were investigated with the purpose of finding a microwave ceramics with high dielectric constant (ε_r), high quality factor (Q×f ) and zero temperature coefficient of resonant frequency (τ_f).The two phase system BaZn₂Ti₄O₁₁–BaNd₂Ti₄O₁₂ affected the unit cell volume and the microstructure, the microwave dielectric properties (ε_r, Q×f, τ_f). As increasing x from 0.18 to 0.30, the main phase composition was BaNd₂Ti₄O₁₂. Therefore, the ε_r decreased from 64.9 to 60.3 and the Q×f values raised from 11,350GHz to 13,210GHz, and the τ_f values decreased from 7ppm/° to -5ppm/°.The 0.22BaZn₂Ti₄O₁₁–0.78BaNd₂Ti₄O₁₂ ceramics sintered at 1250°C for 4 h displayed the best dielectric constant ε_r,Q×f and τ_f, as 63.9, 12380 GHz and-0.1 ppm/° respectively.

Abstract: BaTi₄O₉(BT4), has dielectric constant (εr)≈36 and unloaded Q-factor (Qf) >35,000GHz, but has temperature coefficient of frequency (τf~16-20ppm/°C). Ba₂Ti₉O₂₀(B2T9), very near to BT4 in the BaO-TiO₂ phase diagram, has εr~38, Qf~35,000GHz and τf<4ppm/°C. Though this phase is selected for most of the applications, there are many challenges like slow formation, decomposition, reaction with substrate etc. But, BT4 formed quickly and densified easily, but only high τf precluded it from many practical applications. Most of the literature on the modification of BT4 decreased the εr≤35. Here, the preparation and properties of ZnO/ZnTiO₃ added BaTi₄O₉ ceramics are reported. The ceramics have been prepared by the solid state method. Raw materials have been homogenized and calcined at 1125°C. Calcined powders are ball-milled, compacted and sintered in 1275−1350°C to get dense ceramics with ρ=4.38-4.44 g/cc (TD>97.5%). XRD analysis has confirmed the BaTi₄O₉ phase formation along with secondary phases. FeSEM microstructures show liquid phase assisted sintering leading to fused grains and exaggerated grain growth. The dielectric properties have been measured near 4GHz by standard rod resonator methods using cylindrical specimens. The ceramics have εr36.5−38, τf<11ppm/°C and Qf>17THz. It is possible to further improve the Qf of these ceramics by controlling the processing.

Abstract: The single phase of ZnTiNb2O8 ceramics was obtained at 1050 °C via traditional solid-state method. The formation of the ZnTiNb2O8 phase in the calcined powders has been investigated as a function of calcination conditions by XRD technique. Morphology and particle sizes have been determined via SEM technique. The microwave dielectric ceramics ZnTiNb2O8 which sintered at 1150°C exibit the best microwave properties: high permittivity (εr =34.2), high quality factor (Q*f = 28696 GHz), but very negative temperature coefficient of resonant frequency (τf = -75.841ppm/°C).

Abstract: A new type of porous ceramics, Mg2SiO4, for microwave application with low permittivity and high quality factors were prepared via gel-casting processing in this paper. Microstructure of the ceramics was observed and crystal structure of the samples was also identified. Moreover, emphasis was paid on the characterization of the microwave ceramics. Dielectric properties of the ceramics were measured by an automatic system of microwave measurement, resulting in dielectric constant of 3.51 and quality factor of 11,774 for the ceramic sample at frequency about 10GHz. Mechanical performance of the samples were also evaluated with maximal bending strength of 140MPa.

Abstract: The microwave properties and microstructures of (ZnMg)TiO3-based dielectric prepared by conventional solid-state method were investigated as functions of CaTiO3 and CaO-B2O3-SiO2 additions. The effects of CaTiO3 on the crystal phase and the evolution of microstructure of (Zn0.65Mg0.35)TiO3 were studied. The result indicated that CaTiO3 secondary phase coexists with (ZnMg)TiO3 main phase in the ZMT-CT ceramics, which confirmed by EDS analysis. Because of CaTiO3 with large τf value (τf = 800 ppm/°C), the temperature coefficient of resonant frequency (τf) of ZMT-CT with biphasic structure was adjusted to near zero value. The microwave properties of (Zn0.65Mg0.35)TiO3 ceramics doped with 5wt% CaTiO3 sintered at 1150°C were ε ≈ 24, τf ≈ ±10 ppm/°C, Q×f > 45,000 GHz. Further, it was found that the CaO-B2O3-SiO2 additive could successfully reduce the sintering temperature of (Zn0.65Mg0.35)TiO3–CaTiO3 ceramics from 1150 to 950°C, and significantly improve the densification of this system, which were densified below 1000°C. This was due to the formation of liquid phases during the sintering observed by SEM. The (Zn0.65Mg0.35)TiO3–0.05CaTiO3 dielectrics with 1 wt% CaO-B2O3-SiO2 sintered at 950~1000°C exhibited the optimum microwave properties: ε ≈ 22, Q×f ≈ 20,000 GHz and τf ≈ ±10 ppm/°C.

Abstract: Automatic measurement system with multimode for microwave ceramic materials is developed. Based on traditional parallel-plate resonance method, the whole system is composed of a parallel-plate cavity, a vector network analyzer (VNA), an interface card for GPIB to USB, and a PC with measurement programs. Besides the fundamental mode, higher order resonant modes are also measured at the same time. Based on a platform of VEE, a set of analysis software is programmed with the functions of not only automatically calibrating and searching resonant frequencies but also automatically computing and displaying the dielectric parameters of microwave materials with high reliability and efficiency in a wide frequency range.

Abstract: Microwave dielectric ceramics 0.4(Ca0.61Nd0.26)TiO3-0.6(Li0.5Sm0.5)TiO3 (CNLST) were prepared by doping CuO as the liquid-phase sintering aid. The effects of CuO on the microstructures and dielectric properties were investigated. Due to the liquid-phase effect, the sintering temperature of CNLST with 0.5wt.% CuO addition has be effectively reduced to 1100°C, while good microwave dielectric properties of εr=83.4, Q•f=3880GHz, τf =2.1ppm/°C.

Abstract: In this paper, several kinds of synthesis techniques were adopted; not only conventional solidstate reaction method but also solution synthesis techniques, including co-precipitation and hydrothermal synthesis, in addition to the gel-casting for complex shape of ceramic components and tape-casting of large scale thin plate for microwave IC. Different kinds of microwave ceramics were prepared, such as materials with low permittivity and high quality factor, moderate permittivity and good quality factor, and, high permittivity and reasonable quality factor, in addition to near zero of temperature coefficient of resonance frequency. Series of microwave devices were developed, for examples, dielectric resonators, dielectric filters, GPS antennas, communication connectors, and thin substrates for microwave IC.

Abstract: Ba5(Nb1-xVx)4O15 (0≤x≤0.1) ceramics doped with H3BO3 were prepared by solid-state reaction. The effects of H3BO3 additives on the sinterability and microwave dielectric properties of the ceramics were investigated. The formation of a single phase was identified with the X-ray powder diffraction patterns of Ba5(Nb1-xVx)4O15 ceramics in the composition x from 0 to 0.025. Minor phases including Ba3(VO4)2 were formed as the secondary phase for the compositions with x>0.025. The dielectric constants (εr) of the ceramics increased firstly from 38.7 to 39.69 with x increasing from 0 to 0.01, and, then decreased to 36.91 with x=0.1, meanwhile, the unloaded quality factor and temperature coefficient of resonance frequency (τf) decreased as the x value increased from 0 to 0.1. Microwave dielectric properties, εr=36.91, Q×f =16,167GHz and τf =35ppm/°C, were obtained for Ba5(Nb0.9V0.1)4O15 ceramics with 1wt% H3BO3 sintered at 900°C for 2h.