Low temperature microwave characterisation of lithium fluoride at different frequencies

Precise knowledge of dielectric properties of materials is required to implement the material in high frequency devices and circuits. At microwave frequencies complex permittivity (dielectric constant and loss tangent) are the two mandatory parameters prior to any design. We have identified Lithium Fluoride as a potential candidate, which can be used in conjunction with superconducting and non-superconducting parts of several microwave communication devices. Even though dielectric constant of LiF is known at room temperature there only limited data presented at cryogenic temperatures. We have used a dielectric post resonator for the microwave characterisation of the rod shaped LiF crystal. In this paper, we have reported the dielectric constant (perpendicular component of the real part of complex permittivity) and loss tangent of two LiF crystals as a function of temperature (15–290 K) at frequencies of 8 and 16.5 GHz. We have also studied and reported the temperature coefficient of frequency and permittivity. The concept of using temperature coefficient of frequency as a standard is proved to be wrong in this paper. Microwave properties of other Fluorides are also compared with the LiF crystal.     

Cavity perturbation technique for the measurement of permittivitytensor of uniaxially anisotropic dielectrics

Based on microwave resonant perturbation theory, a bimodal TE₁₁₂ cylindrical cavity is developed for the measurement of permittivity tensor of uniaxially anisotropic dielectrics. The two perpendicular TE₁₁₂ degenerate modes in the cavity are used to measure the two complex components in the permittivity tensor of a uniaxial sample, respectively. In the measurement of each component, only the corresponding mode resonates in the bimodal cavity, and the real and imaginary parts of the complex component are deduced from the changes of the resonant frequency and the quality factor of the cavity due to sample insertion. In the present technique, the quality factors of the cavity before and after sample insertion are measured at the same frequency, so the uncertainties caused by the variation of the total stored energy in the cavity due to sample insertion are eliminated. Experimental results show that the present technique is accurate and reliable for the measurement of the permittivity tensor of uniaxial media. As an example, the dielectric properties of α-SiO₂ samples with different crystal orientations are characterized using the present technique     

新型微波介质陶瓷材料与元件的研制

报告本课题组承担的863计划项目“新型微波介质陶瓷材料与元件的研制”，在实验工作中不仅采用传统的固相反应法而且利用共沉淀和水热合成等溶液反应技术，还采用凝胶浇注成型复杂形状的微波陶瓷元件，并利用流延成型技术制备大尺寸微波集成陶瓷基板等；研制成功具有高电容率和品质因子并接近于零的谐振频率温度系数的系列化微波介质陶瓷；开发成功多种微波介质元器件，包括：介质谐振器、介质滤波器、GPS片式天线、微波电容器、通信电缆滤波接头和微波集成基板等。     

Spectroscopy of one-dimensionally inhomogeneous linear absorbing media with arbitrary frequency dispersion

This study focuses on a one-dimensionally inhomogeneous absorbing plane-parallel plate with dielectric properties characterized by a diagonal tensor of dielectric permittivity. The research has found that based on the reflection and transmission coefficients of p- and s-polarized plane monochromatic waves with frequency ω known for a certain range of incidence angles the coordinate dependence of all the components of the dielectric permittivity tensor of the plate material at frequency ω is determined in a unique manner according to the proposed reconstitution algorithm.     

Synthesis and properties of bismuth ferrite multiferroic flowers

Novel, flowerlike bismuth ferrite BiFeO₃ (BFO) multiferroic structures were prepared for the first time by means of microwave assisted hydrothermal synthesis. The flowers are composed of numerous petals formed by BFO nanocrystals and some amount of amorphous phase. The growth of the flowers begins from the central part of calyx composed of only few petals toward which subsequent petals are successively attached. The flowers exhibit enhanced magnetization due to size effect and lack of spin compensation in the spin cycloid. The dielectric properties of the flowers are influenced by BFO amorphous phase resulting in a broad dielectric permittivity maximum at 200–300 K and also by Polomska transition due to anomalous surface magnon damping above the temperature of 450 K. Possible applications of BFO flowerlike structures assume optoelectronic devices, excellent field emitters, effective solar cells, or catalyst supports.     

Three-Nucleon Forces and Triplet Pairing in Neutron Matter

We report an experimental study of the electrical properties of liquid helium-4 in the temperature range 1.2–3 K. The experiment is carried out in the millimeter wave range using a whispering gallery mode dielectric resonator, and the complex permittivity of liquid helium is extracted from the data using the resonant perturbation method. The results for the temperature dependence of the dielectric constant are consistent with the previous studies. In addition, we find strong enhancement of the loss tangent around the superfluid transition temperature.     

低温烧结陶瓷BiMg2VO6的微波介电性能

通过传统的固相反应方法制备了低温烧结微波介质陶瓷BiMg₂VO₆, 研究了该陶瓷与银的化学兼容性、物相、形貌及在720~840℃内的密度和微波介电性质, 并测试了陶瓷的红外反射光谱。结果表明: 陶瓷在780℃条件下与银共烧不发生反应, 相对密度大于93.8%。在780℃条件下烧结2 h得到的陶瓷具有最好的微波介电性能: 介电常数为13.4, Q×f值为15610 GHz (f = 8.775 GHz), 温度系数为-87.2×10^-6/℃。红外反射谱数据处理显示, BiMg₂VO₆的光频介电常数ε_∞ = 3.4, 微波频段的外推值为13.5。BiMg₂VO₆陶瓷好的微波介电性能和低的烧结温度, 使其有望用作新的低温共烧陶瓷。     

Temperature dependent complex permittivities of graphitized carbon blacks at microwave frequencies between 0.2 and 26 GHz

Many carbonaceous materials are amenable to dielectric heating at microwave frequencies. The primary indicator of microwave susceptibility is the complex permittivity (^*), of which, the real component correlates with polarization, and the imaginary term represents dielectric loss. For a given material, the complex permittivity is dependent upon both frequency and temperature. Here we report the complex permittivities over the frequency range from 0.2 to 26 GHz of four laboratory grade graphitized carbon blacks which are used as adsorbents and chromatographic media in a variety of separation, purification, and analytical applications. Measurements were made using a vector network analyzer based system across the temperature region between 23°C and 191°C. Dielectric polarization-relaxation phenomena are also characterized.     

The microwave characterization of single crystal lithium and calcium fluoride at cryogenic temperatures

The whispering-gallery mode method is used for very accurate permittivity and dielectric loss measurements of single crystal lithium fluoride (LiF) and calcium fluoride (CaF2) over the temperature range of 4.5 K to 300 K. The absolute uncertainty in the real part of permittivity was estimated to be less than 0.1%, and it was limited principally by uncertainty in dimensions of the samples. Dielectric losses were measured with uncertainties of about 10% limited by the accuracy of Q-factor measurements. The measured materials exhibited dielectric losses between 2-4 x 10(-7) near 5 K. The relative permittivity was evaluated as 6.502 (4.9 K) to 6.844 (296 K) at 17.5 GHz for CaF2 and 8.534 (4.6 K) to 9.063 (300 K) at 13.5 GHz for LiF.     

Effect of La and K on the microstructure and dielectric properties of Bi0.5Na0.5TiO3-PbTiO3

Small amounts of lanthanum and potassium dopants could modify the microstructure and dielectric properties of 0.90Bi_0.5Na_0.5TiO₃-0.10PbTiO₃ and 0.88Bi_0.5Na_0.5TiO₃-0.12PbTiO₃ solid solutions. La lowered both phase transition temperatures of ferroelectric to antiferroelectric and antiferroelectric to paraelectric. It also decreased the maximum value of relative dielectric permittivity of the composition. In contrast, K shifted the first phase transition to the lower temperature but insignificantly affected the Curie temperature and raised the maximum dielectric permittivity. Furthermore, K influenced the microstructure in the way to enhance the long grains of this solid solution but La inhibited this effect.     

微波烧结LiCo1/3Ni1/3Mn1/3O2电磁场与温度场的仿真模拟

分析了微波烧结的原理和特点,利用COMSOL Multi-physics模拟软件对矩形微波炉进行了仿真模拟,研究了微波烧结正极材料LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2时电磁场与温度场的分布,测量了粉末样品LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2的介电常数,并与模拟结果相对照.研究表明:微波在烧结LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2样品时,炉腔内电磁场的分布受到影响,微波炉内表面的电场强度减弱;材料内部温度场的分布不均匀,材料的下半部分温度较高;同时,在仿真模拟计算过程中,求得LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2在微波中烧结到不同温度时的能量损耗,根据李赫德涅凯法则计算出其对应的相对介电常数,发现在20~620℃,相对介电常数随温度的上升而变大.根据实验测得的复合介电常数,求出对应温度点LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2的相对介电常数.利用Origin对仿真模拟计算和实验求得的两组相对介电常数数据进行拟合对比,发现实验求得的LiCo_(1/3)Ni_(1/3)Mn_(1/3)O_2的相对介电常数与仿真模拟计算所得数据趋势吻合.     

Temperature dependences of the optical properties of aluminum nanoparticles

The spectral dependences of the optical properties of aluminum at various temperatures have been analyzed. We have proposed and tested the approximation expressions for the imaginary part of the dielectric permittivity. The reflection coefficients at normal incidence of light and the optical properties of nanoparticles with a radius of 150 nm in a temperature range of 298–933 K have been calculated for practically important wavelengths. The optical properties of aluminum are found to be atypical, with the absorption coefficient decreasing with increasing temperature. This is due to the contribution of interband transitions to the optical properties of aluminum.     

碳/氧化铝/二氧化硅涂层的介电和吸波性能研究

研究了炭黑或碳纤维填充氧化铝/二氧化硅吸波涂层在X波段范围的介电和吸波性能. 结果表明: 吸波涂层的复介电常数随着炭黑或碳纤维含量的增加而增大. 当吸收剂含量相同时, 填充碳纤维的吸波涂层比填充炭黑的吸波涂层具有更大的复介电常数. 当吸收剂含量大于5wt%时, 吸波涂层的介电常数在低频急剧增加, 且随频率增大而减少, 出现频散效应. 反射率测试结果表明: 吸波涂层的最大吸收峰随涂层厚度的增大向低频移动, 当涂层中炭黑含量为2wt%、厚度为1.8 mm时, 吸波涂层在9.2~12.4 GHz范围内反射率小于-10 dB, 具有较好的吸波效果.     

Dielectric Properties of (C60 + C70)—Ferroelectric Liquid Crystal Composite

A stable composite of fullerene and ferroelectric liquid crystal (FLC) has been obtained. Analysis of the frequency dependence of imaginary part of complex dielectric permittivity showed that orientation of the liquid crystal (LC) in the composite is close to homeotropic. At temperatures above 329 K, the composite conductance is determined by the conductance of fullerene. At 344 K, the composite conductance is 3.2 × 10^-8 Ω^-1 m^-1, and the activation energy for the temperature dependence of conductance is 0.99 eV. At temperatures below 329 K, the composite properties are determined by the LC, and the frequency dependences of the components of complex dielectric permittivity showed dispersion due to rotation of molecular dipoles around the helicoid (Goldstone mode) axis. The relaxation time is close to 1 msec and almost equal to the value obtained for homeotropically oriented LC.     

Effect of thermal treatment conditions on the properties of onion-like carbon based polymer composite

Dielectric properties of onion-like carbon (OLC) and polyurethane composite prepared using different procedures were investigated in the frequency range up to 1 MHz. We show that broadband dielectric spectroscopy is powerful tool to determine technological fingerprints in the studied materials. It is demonstrated that cured samples annealed at temperature close to the melting point (450 K) exhibit substantially higher dielectric permittivity and electrical conductivity in comparison with untreated samples. With the increase of temperature of an untreated sample, its dielectric permittivity, electric conductivity and critical frequency increase, while Maxwell–Wagner mean relaxation time and aggregate sizes of OLC decreases. Annealing of the composite at temperature close to the melting temperature produce sample with more homogeneous distribution of OLC. The temperature dependence of conductivity of the homogeneous sample is mainly caused by a weak positive temperature coefficient effect.     

低介电常数微波介质陶瓷研究进展

综述了国内外低介电常数微波介质陶瓷近10年来的研究进展,总结了低介电常数微波介质陶瓷的晶体结构、成分体系和烧结工艺等相互之间的制约关系,以及对介电性能影响的规律;归纳了研究中存在的问题,并对其它的低介电常数微波介质陶瓷也进行了简要介绍.最后展望了低介电常数微波介质陶瓷今后的发展方向.     

Temperature-compensated sapphire resonator for ultra-stableoscillator capability at temperatures above 77 K

We report on the design and test of a whispering gallery sapphire resonator for which the dominant (WGH_n11) microwave mode family shows frequency-stable, compensated operation for temperatures above 77 K. The resonator makes possible a new ultra-stable oscillator (USO) capability that promises performance improvements over the best available crystal quartz oscillators in a compact cryogenic package. A mechanical compensation mechanism, enabled by the difference between copper and sapphire expansion coefficients, tunes the resonator to cancel the temperature variation of sapphire's dielectric constant. In experimental tests, the WGH₈₁₁ mode showed a frequency turnover temperature of 87 K in agreement with finite element calculations. Preliminary tests of oscillator operation show an Allan Deviation of frequency variation of 1.4-6×10^-12 for measuring times 1 s ⩽τ⩽100 s with unstabilized resonator housing temperature and a mode Q of 2×10⁶. We project a frequency stability 10^-14 for this resonator with stabilized housing temperature and with a mode Q of 10⁷     

Dielectric characterisation of Barium Fluoride at cryogenic temperatures using TE011 and quasi TE0mn mode dielectric resonators

Barium Fluoride (BaF₂), mainly used in optical applications, has similar properties to those of CaF₂ but better stability under environmental conditions. In this paper, we report measurements of loss tangent and the real part of the relative permittivity ε_r of single crystal BaF₂ in the temperature range from 14 to 290 K at frequencies of 8, 10.4, 12.1, 17.6, 21.1 and 24.4 GHz. Microwave properties of BaF₂ were determined by measurements of the resonance frequency and the unloaded Q-factor of TE₀₁₁ and quasi TE_0mn modes cylindrical cavities containing the sample under test. Two techniques namely Hakki-Coleman Dielectric Resonator and dielectric post resonators have been used for characterizing the dielectric material. Losses due to the uncalibrated cables and adaptors inside the cryocooler are accounted in the calculation of unloaded Q-factor using the Transmission Mode Q-Factor data processing technique. The permittivity of BaF₂ exhibited a 5% increase from 6.9 (14 K) to 7.35 (290 K). The loss tangent increased from 1 × 10⁻⁵ to 1.1 × 10⁻⁴ over the measured temperature range, 14-290 K at a frequency of 10.4 GHz. The measured microwave properties show that BaF₂ can be used in many microwave devices.     

Measuring frequency- and temperature-dependent permittivities of food materials

The permittivities or dielectric properties of food materials are of vital importance in understanding the behavior of these materials when they are exposed to electromagnetic fields in the process of microwave cooking or in other processes involving RF or microwave dielectric heating. Understanding these properties is also important in quality sensing by RF and microwave instruments. The most prominent example is instruments designed for rapidly sensing or measuring the moisture content of cereal grains and other food materials. An open-ended coaxial-line probe was used with sample temperature control equipment, designed for use with the probe, to measure permittivities of some liquid, semisolid, and pulverized food materials as a function of frequency and temperature. Graphical data for the dielectric constant and loss factor of homogenized macaroni and cheese, ground whole-wheat flour, and apple juice illustrate the diverse frequency- and temperature-dependent behavior of food materials, and the need for measurements when reliable permittivity data, are required. The materials were selected because interest had been expressed by others in their dielectric properties.     

The effect of growth temperature on the nanostructure and dielectric response of BaTiO3 ferroelectric films

BaTiO₃ ferroelectric films were grown on Si/SiO₂/Ti/Pt/Au/Pt templates at different temperatures in the range 560-680 °C by pulsed laser deposition. Cross section scanning electron microscopy images and atomic force microscopy surface morphology analysis reveal films with columnar structure and in-plane grain size distribution, in the range 10-60 nm, depending on growth temperature. Low-field dielectric measurements were performed as functions of temperature in the range 40-500 K and external dc field up to 400 kV/cm. The apparent permittivity of ferroelectric films grown at 680 °C shows Curie-Weiss behavior above 400 K with Curie temperature and Curie-Weiss constant 240 K and 1 · 10⁵ K, respectively. The films grown at lower temperatures reveal a decrease of Curie temperature down to − 80 K, reduced values of apparent permittivity and loss tangent, and broadening of maximum of temperature dependence of apparent permittivity. The film grown at 590 °C demonstrates state of the art combination of temperature stability (temperature coefficient of apparent permittivity 300 ppm/K in the range 50-350 K), high tunability of apparent permittivity (up to 60% at room temperature), and relatively low loss tangent (less than 0.05 in the frequency range up to 10 GHz). The change in apparent permittivity and its temperature dependence, with variation of growth temperature are analyzed using two different composite models. The first model assumes the film to be a composite with vertical inclusions of low permittivity dielectric material associated with grain boundaries. This model may explain the observed decrease of permittivity with decreasing growth temperature, but not the shift of Curie temperature. The second model assumes a layered type of composite with low permittivity material associated with the film/electrode interfaces, and allows explanation of the Curie temperature shift.