Dielectric Dispersion of Polycrystalline Aluminum Nitride at Microwave Frequencies

Complex permittivity of polycrystalline AlN was measured over the frequency range 100 MHz to 13 GHz using reflection coefficient and cavity resonator methods. Dielectric relaxation peaks of loss tangent were observed in the frequency range 200 MHz to 3 GHz. The maximum value of loss tangent was ∼200 × 10⁻⁴. Relative permittivities decreased by ∼4% with increased frequency at the region of relaxation peaks. It was shown that these dispersion frequencies were inversely proportional to the grain sizes of AlN and independent of purities and processes. Model calculations for elastic vibration of a single-crystal grain of AlN were performed to investigate the origin of the dielectric dispersion due to piezoelectric effect. The calculated results of elastic sphere and 14-faced-polyhedron models showed a good quantitative agreement with the observed dispersion frequencies, which suggested that the dispersion was due to a piezoelectric vibration of a single grain of AlN ceramics at microwave frequencies.     

A New BaO-TiO2 Compound with Temperature-Stable High Permittivity and Low Microwave Loss

The dielectric properties of ceramics in the TiO₂-rich region of the BaO-TiO₂ system were investigated. In the composition range between BaTi₄O₉ and TiO₂, another compound, Ba₂Ti₉O₂₀, can be obtained when calcining and sintering conditions are controlled carefully. When dense and single-phase, this ceramic has excellent dielectric and physical properties. At 4 GHz, the dielectric K = 39.8, Q = 8000, and τ _K (temperature coefficient of dielectric constant) =−24 ± 2 ppm/°C.     

Novel Low‐Firing Microwave Dielectric Ceramic LiCa3MgV3O12 with Low Dielectric Loss

Novel low temperature firing microwave dielectric ceramic LiCa₃MgV₃O₁₂ (LCMV) with garnet structure was fabricated by the conventional solid‐state reaction method. The phase purity, microstructure, and microwave dielectric properties were investigated. The densification temperature for the LCMV ceramic is 900°C. LCMV ceramic possessed ε_r = 10.5, Q_u × f = 74 700 GHz, and τ_f = ?61 ppm/°C. Furthermore, 0.90LiCa₃MgV₃O₁₂–0.10CaTiO₃ ceramic sintered at 925°C for 4 h exhibited good properties of ε_r = 12.4, Q_u × f = 57 600 GHz, and τ_f = 2.7 ppm/°C. The LCMV ceramic could be compatible with Ag electrode, which makes it a promising ceramic for LTCC technology application.     

Ultra Low Loss Ceramic Dielectrics

The object of this research was to develop ultra low loss ceramic dielectrics. The possible causes of the dielectric loss of ceramic bodies were first analyzed theoretically. Experimental bodies, designed on the basis of this analysis, provide for ultra low dielectric loss.     

Microwave Dielectric Loss of Titanium Oxide

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 0.5–0.95 Å 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.     

固有烧结温度低的微波介质陶瓷

微波介质陶瓷是现代通讯技术中的关键基础材料。在制备多层微波介质片时,为了使用Cu、Ni等低熔点电极,必须降低微波介质陶瓷的烧结温度。开发固有烧结温度低(<1060℃)同时具有良好性能的微波介质材料成为必然的选择。本文简要介绍了7类固有烧结温度低的微波介质陶瓷的烧结特性与微波介电性能,同时也指出了研究中存在的一些问题。     

High-Permittivity Temperature-Stable Ceramic Dielectrics with Low Microwave Loss

The compounds SrZrO₃ and CaZrO₃ combine high permittivity with low loss and positive temperature coefficient of permittivity. Temperature-stable materials were produced by forming solid solutions between these zirconates and zirconates or titanates having negative coefficients. The properties of temperature-stable compositions in the systems Ca(Zr,Ti)O₃ and (Ba,Sr)(Zr,Ti)O₃ were investigated. At 4 GHz the relative permittivities were from 29 to 35 and the loss tangents from 3 to 11×10⁻⁴. Both permittivity and microwave loss increased as the principal alkaline-earth ion was changed from Ca to Sr to Ba, but the higher losses could be reduced by adding ∼1 mol% Nb or Ta. Dense fine-grained ceramics with the properties required for microwave applications were prepared.     

Transient Effects of Nuclear Radiation on the Dielectric Properties of Refractory Low-Loss Ceramics at Microwave Frequencies

Selected ceramic materials were exposed to an intense burst of nuclear radiation to determine the transient effects of such an environment on their dielectric properties at frequencies in the L, S, and X bands. The radiation source was a pulse reactor of the swimming pool type which delivers a maximum dose of 2 × 10¹⁴/cm² and 5 × 10⁵ rd gamma during a 13 ms pulse. The dielectric constant and loss of the materials under test were measured before, during, and after irradiation using a resonant cavity technique successfully adapted to the unusual conditions of the experiment. The time resolution in the dielectric measurements was 10 ms. Results are reported for polycrystalline alumina, beryllia, magnesia, boron nitride, clear and opaque fused silica, and, for comparison, four organic dielectrics. Among the materials which exhibited radiation effects, changes in dielectric constant ranged from 0.001 to 0.008 and in tan δ from a factor of 2 to several orders of magnitude.     

A New Microwave Dielectric Ceramic for LTCC Applications

A new low-sintering temperature microwave dielectric ceramic, the Li₂TiO₃ solid solution, was found and investigated in the Li₂O–Nb₂O₅–TiO₂ system. The compound with the composition of Li_2.081Ti_0.676Nb_0.243O₃ crystallizes as a monoclinic structure. This new microwave dielectric ceramic shows a relatively low permittivity (∼20), high Q × f values up to 50 000 (7.8 GHz), and near-zero temperature coefficients (13 ppm/°C), which were obtained via sintering at 1100°C. The addition of ≤2 wt% B₂O₃ was very effective in lowering the sintering temperature ( T _s), and dense ceramics could be obtained at T _s≤900°C. The addition of B₂O₃ does not induce apparent degradation in the microwave properties but lowers the τ_f value to near zero. It is obvious that the ceramics could be promising candidates for multilayer low-temperature co-fired ceramics applications.     

BMI Based Composites With Low Dielectric Loss

Summary O,O’-diallylbisphenol A (BA), allyl epoxy resins and epoxy acrylate resins are adopted to copolymerize with 4,4’-bismaleimidodiphenyl methane (BDM) resins and modify mechanical properties and processing charicteristics. The new modified BMI resin systems have more than two times improved impact strength without a great decrease in excellent dielectric properties or thermal and hot–wet resistance of neat BDM resin. Composites based on modified BMI resins and reinforced by glass fibre and quartz fibre possess excellent mechanical properties. The fracture surfaces of the composites are examined by scanning electron microscopy (SEM). It is indicted that modified BMI resin matrix composites put up typical toughness rupture and the adhesion efficiency in interface of composites is fine. When the test frequency scope is from 1 GHz to 20 GHz, the dielectric constant and dielectric loss of composites almost hold the line. After 100 h in boil water, mechanical and dielectric properties of composites are higher than 85% retention of their original values.     

Ba(Zn1/3Ta2/3)O3 Ceramics with Low Dielectric Loss at Microwave Frequencies

The dielectric properties at microwave frequencies of Ba(Zn_1/3Ta_2/3)O₃ ceramics prepared by sintering were investigated. These ceramics had lower density but higher loss quality than ceramics hot-pressed at 1400°C. Loss quality was greatly improved by prolonged sintering. The Q of the ceramics measured by the dielectric resonator method was 14 000 at 12 GHz. The ceramics were investigated by X-ray diffraction analysis. It was found that Q improvement corresponds with increased Zn and Ta ordered structures in the ceramics.     

A Novel Low‐Firing and Low Loss Microwave Dielectric Ceramic Li2Mg2W2O9 with Corundum Structure

The crystal structure and microwave dielectric properties of a novel low‐firing compound Li₂Mg₂W₂O₉ were investigated in this study. The phase purity and crystal structure were investigated using X‐ray diffraction analyses and Rietveld refinement. The best microwave dielectric properties of the ceramic with a low permittivity (ε_r) ~11.5, a quality factor (Q × f) ~31 900 GHz (at 10.76 GHz) and a temperature coefficient of the resonant frequency (τ_f) ~ ?66.0 ppm/°C were obtained at the optimum sintering temperature (920°C). CaTiO₃ was added into the Li₂Mg₂W₂O₉ ceramic to obtain a near zero τ_f, and 0.93Li₂Mg₂W₂O₉–0.07CaTiO₃ ceramic exhibited improved microwave dielectric properties with a near‐zero τ_f ~ ?1.3 ppm/°C, a ε_r ~21.6, a high Q_u × f value ~20 657 GHz. The low sintering temperature and favorable microwave dielectric properties make it a promising candidate for LTCC applications.     

高介电常数微波介质陶瓷

近年来，微波介质陶瓷作为现代通信技术中关键的核心材料，广泛应用于微波谐振器、滤波器、介质基板、移相器等微波元器件，极大的促进了现化通信事业的飞速发展。本文综述了高介电常数微波介质陶瓷近几十年来的研究进展，并对高介电常数微波介质陶瓷的发展趋势进行了讨论和展望。     

Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides?

Reducing loss in microwave dielectrics is critical to improving performance in wireless communications systems. Grain boundaries in polycrystalline microwave dielectric ceramics have long been suspected of increasing dielectric loss. They are often cited as the main contributor to the observed difference in dielectric losses between single crystals and polycrystalline ceramics. The exact configuration of grain boundaries is problematic to quantify in practice and their influence on the dielectric loss difficult to distinguish from other defects such as porosity, oxygen vacancies, impurities, and dislocations. Here we measure the sensitivity of a single grain boundary in a magnesium oxide bi-crystal to the polarization of an applied microwave field as a function of temperature.     

Low Dielectric Loss PTFE/CeO2 Ceramic Composites for Microwave Substrate Applications

Cerium oxide (CeO₂) filled polytetrafluoroethylene (PTFE) composites prepared by powder processing technique for microwave substrate application is presented in this paper. The PTFE is used as the matrix and the dispersion of CeO₂ in the composite is varied up to 0.6 by volume fraction, and the dielectric properties were studied at 1 MHz and microwave frequencies. The relative permittivity and dielectric loss increased with increase in CeO₂ content. For 0.6 volume fraction loading of the ceramic, the composite has ɛ_r of 5 and tan δ of 0.0064 at 7 GHz. Different theoretical approaches have been employed to predict the effective permittivity of composite systems and the results were compared with that of experimental data. The serial mixing model shows good correlation with the experimental results.     

低介电损耗环氧树脂复合材料的研究进展

物联网（IoT）和第五代移动通信技术（5G技术）的飞速发展,对封装材料和元器件基材的介电性能提出了更高的要求,开发具备低介电常数以及低介电损耗的环氧树脂（EP）成为重要的研究方向。本文从原料、固化条件、填料、共混共聚和层状复合等几个方面对制备低介电损耗环氧复合材料的方法进行了综述,同时简要分析了各种研究方案获得低介电损耗的主要原因,最后例举了低介电环氧树脂目前仍存在的一些问题,并对未来的研究方向进行了展望。     

几种典型的低介硅酸盐微波陶瓷材料的研究现状

概述了MgO-SiO2,Zn2SiO4,CaSiO3和Re2O3-SiO2(Re=稀土元素)等低介电常数硅酸盐微波陶瓷材料体系的研究进展.讨论了微波介质陶瓷的结构、微波介电性能、降温方法、机理及其存在的问题,指出了微波陶瓷材料今后的研究方向.     

Method for Measuring the Dielectric Constant of Ferroelectric Ceramics at S-Band Frequencies

A method is presented for measuring the complex dielectric constant of ferroelectric ceramics at microwave frequencies. The method employs a disk-shaped sample in a radial-line configuration at the end of a coaxial-line structure. An equivalent circuit, established by admittance measurements, is used to relate the radial admittance at the sample face to the measured admittance on the slotted line preceding the coaxial sample holder. The nonuniform variation of electric field over the face of the sample is taken into account by means of a continued fraction expansion of x J _t( x )/ J ₀( x ) in terms of x = k'a , where k' is the wave number in the ferroelectric sample and a is its radius. Representative data on the variation of the complex dielectric constant with temperature at 3 kmc. are given for two samples of BaTiO₃ fired under atmospheric pressure and 5000 lb. per sq. in. The frequency spectrum of hot-pressed Cd₂Nb₂O₇ in the range 1.8 to 4.0 kmc. is also presented.     

高Qm,低损耗压电陶瓷材料的研究

本文对ＰｂＯ３四元素和以ＣｅＯ２掺杂的ＰｂａＳｒ１－ａＯ３三元系压电陶瓷进行了研究。该材料具有高机械品质因数、低介电损耗、高居里点、高机械强度、烧结范围宽、工艺重复性好等特点。这两种材料在驻波超马达中得到了应用。     

Humidity Dependence of Apparent Dielectric Constant for DSP Cement Materials at High Frequencies

The relationship between humidity and dielectric constant for cement densified with small particles (DSP) has been studied in the relative humidity range 0%-93% and the frequency range 1 MHz to 1 GHz. The calculated dielectric constant appears to increase with increasing humidity as a linear relation at fixed frequency. According to experimental data and basic principles for dielectrics, two experimental expressions are suggested for heterogeneous dielectric materials to describe the observed behavior of the dielectric constant. The expressions fit the experimental data well in the frequency range studied. Apparent dielectric constant decreases with increasing frequency. Polarization of DSP cement is also discussed.