Dielectric coefficient and density of subcooled liquid oxygen

The results of high precision dielectric coefficient measurements of subcooled liquid oxygen in the temperature range from 56 to 91 K and under pressures up to 1 MPa are reported. The measurements are performed using a three-terminal flat plate capacitor and a single-frequency, ultra-precision capacitance bridge. Measurement results are combined with the previously published data in different pressure and/or temperature ranges to express the dielectric coefficient and the Clausius-Mossotti relation in an equation form. This new expression covers both the liquid and the gaseous state, and predicts the density of oxygen to better than 0.1% over the entire range of interest. The results reported in this paper are expected to be especially useful for capacitance-based measurements involving fluid oxygen such as liquid level and void fraction measurements of two phase flow.     

Capacitive level meter for liquid hydrogen

A capacitive level meter working at low temperatures was made to use in magnetic refrigerator for hydrogen liquefaction. The liquid level was measured from the capacitance between parallel electrodes immersed in the liquid. The meter was tested for liquid nitrogen, hydrogen, and helium. The operation was successful using an AC capacitance bridge. The estimated sensitivity of the meter is better than 0.2 mm for liquid hydrogen. The meter also worked with pressurized hydrogen.     

SrTiO3为基的介质陶瓷的结构与介电性质

本文主要研究0.91(Sr_(0.84)Pb_(0.16))TiO_(?) 0.09(Bi_2O_3·3.5TiO_2)介质陶瓷的结构与介电性质。用扫描电镜和 X 射线衍射分析表明,此种材料在室温下为顺电立方相,具有晶粒小且均匀、致密度高等特点。介电测量表明,在10kHz～90MHz 范围内样品的介电常数ε′基本上保持不变,而介电常数ε″随频率的增加而平坦地增大。     

Behaviour of photosensitive soft materials: Thermo-optical,dielectric and elastic constant studies on azo-dye doped nematic liquid crystals

Optical absorption, thermo-optical and dielectric studies have been performed on three guest–host nematic mixtures featuring photo-sensitized flexoelectric polarization. The liquid crystal 4-butyl-cyclohexane carboxylic acid 4-pentyloxy-phenyl ester, a room temperature nematic with a negative dielectric anisotropy served as the host material. Three different azo-dye compounds, also exhibiting liquid crystallinity were employed as the low-concentration guest component in the mixtures. In each case illumination of the sample with actinic (UV) light leads to strong photoisomerization driven effects. Correlations among the shift of the isotropic–nematic phase transition temperature, population of the cis isomers, and the change in the length of the azo-dyes upon UV illumination, are discussed. The UV-induced changes in the dielectric constant values have been shown to be dependent on the nature of the photoactive dopant. The article also describes the first observation of the lowering of the bend elastic constant upon photoisomerization, a feature ascribed to the formation of the bent-shaped cis isomers.     

Effect of frequency on dielectric properties of liquid crystal doped with side-chain liquid crystalline polymer

The frequency and applied bias voltage dependence of the dielectric properties and dielectric anisotropy of liquid crystal (LC) doped with side-chain liquid crystalline polymer (SLCP) mixture have been investigated using the admittance spectroscopy method (C–V and G/ω–V) in the frequency range of 10 kHz to 10 MHz at room temperature. The liquid crystal used in this experiment is E63. The doping material used in this study is SLCP and its concentration is ensured 1 wt % in E63. Dielectric constant (?′), dielectric loss (?), dielectric loss tangent (tan δ) and real and imaginary parts of electrical modulus (M′ and M″) of the E63/SLCP mixture was also calculated. Moreover, dielectric anisotropy (Δ?) as a function of frequency was obtained. Results show that the values of the all dielectric parameters are strong functions of frequency and applied bias voltage. After a critical frequency, dielectric anisotropy has negative values according to p/n type changing.     

Dielectric and pyroelectric properties of Ba-modified lead lanthanum zirconate stannate titanate ceramics

(Pb_0.97−xLa_0.02Ba_x)(Zr_0.75Sn_0.12Ti_0.13)O₃ ceramics in the composition range 0.1 ≤ x ≤ 0.16 were prepared by conventional solid state reaction process. On increasing Ba content from 0.1 to 0.16 mol, the specimens underwent phase transition from the first order to the second order and the Curie temperature decreased from 85 to 35 °C. With x = 0.16, the specimen showed good pyroelectric properties for practical applications. When a 500 V/mm dc bias field was applied, the specimen showed the maximum pyroelectric coefficient of 5800 μC/m² K and figure of merit of 58 × 10⁻⁵ Pa^−0.5 at Curie temperature.     

电容式密度传感器低密度液氢校准实验研究

设计、搭建了一套可调节密度式多点电容式密度传感器校准装置,并进行了电容式密度传感器低密度液氢的校准试验。实现了对液氢在一定低密度范围内(50—70kg/m3)的调节,突破了传统的常压单一密度点校准技术,实现了液氢多密度点的校准,提高了校准结果的可靠性。对电容式密度传感器校准装置各因素引入的不确定度分量进行了评定,分析得到了该装置的不确定度,试验结果表明采用给液氢增压的方式可以对液氢低密度进行准确控制,该调节装置的不确定度满足电容式密度传感器校准设计要求,并且能够对电容式密度传感器在多种低温流体介质中进行较宽密度范围校准。     

Dielectric and modulus studies of LiNiPO4

Olivine-structured LiNiPO₄ has been prepared by a Pechini-type polymerizable precursor method. The structure and the morphology of the compounds are studied by the Rietveld refined X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman Spectroscopy techniques, respectively. Brunauer, Emmett and Teller surface area (BET) of the compound also studied. LiNiPO₄ has orthorhombic structure whose dielectric and modulus properties are studied over the frequency range of 1 MHz to 10 Hz at different temperatures (150–300 °C) using impedance spectroscopy. The frequency and temperature dependence of dielectric permittivity, dielectric loss and electric modulus of LiNiPO₄ are studied. The frequency dependent plot of modulus show that the conductivity relaxation is of non-Debye type.     

Dielectric and magnetic properties of ferrite/poly(vinylidene fluoride) nanocomposites

Particulate composite films of poly(vinylidene fluoride) and CoFe₂O₄ and NiFe₂O₄ were prepared by solvent casting and melt processing. The well-dispersed ferrite nanoparticles nucleate the piezoelectric β-phase of the polymer, but the different ferrites nucleate the whole polymer crystalline phase at different filler concentrations. The macroscopic magnetic and dielectric response of the composites demonstrates a strong dependence on the volume fraction of ferrite nanoparticles, with both magnetization and dielectric constant increasing for increasing filler content. The β-relaxation in the composite samples is similar to the one observed for β-PVDF obtained by stretching. A superparamagnetic behavior was observed for NiFe₂O₄/PVDF composites, whereas CoFe₂O₄/PVDF samples developed a hysteresis cycle with coercivity of 0.3 T.     

纳米ZnO/低密度聚乙烯复合材料的介电特性

为探讨纳米ZnO/低密度聚乙烯（LDPE）复合材料的介电特性,首先,采用硅烷偶联剂和钛酸酯偶联剂对纳米ZnO进行改性,并利用两步法制备了不同纳米ZnO质量分数、不同纳米ZnO粒径、不同纳米ZnO表面修饰方式和不同冷却方式的纳米ZnO/LDPE复合材料;然后,通过FTIR、SEM、DSC和热激电流（TSC）测试了纳米ZnO在基体中的分散情况、复合材料的等温结晶过程参数变化及陷阱密度;最后,在不同实验温度下分别进行了交流击穿、绝缘电导率、介电常数和空间电荷实验。结果表明:纳米ZnO的加入使纳米ZnO/LDPE复合材料内部陷阱深度和密度均有所增加;当纳米ZnO的粒径为40 nm且质量分数为3%时,复合材料的结晶速度最快,纳米ZnO在基体中的分散性较好,击穿场强达到最高值133.3 kV/mm,电导率及介电常数也相对较低,加压时复合材料内部空间电荷少,短路时释放电荷速度快,介电性能较好;由于纳米粒子增加了材料内部的热传导速率,降低了复合材料随着温度升高而降解的速度,因而相对于纯LDPE,随着实验温度的提高,纳米ZnO/LDPE复合材料的击穿场强下降幅度及电导率上升幅度均较小。      

基于印刷工艺的介质层绝缘特性研究

绝缘介质层是三极场致发射显示器的重要组成部分,本文通过丝网印刷技术与一次烧结和重复烧结两种烧结工艺制备不同膜厚的介质层,利用台阶仪和扫描电子显微镜分别对介质层膜厚和形貌进行表征,并对介质层的绝缘耐压性能进行了测试。结果表明,在重复烧结工艺下制备的膜厚约为34μm的介质层的耐压特性较好,电场强度达0.15 MV/cm,可耐压421 V,漏电流小,且重复性好,基本上可以满足后栅型FED栅极调控电压对介质层耐压性能的要求。同时在电场较弱时,介质层漏电流的输运符合欧姆输运机制;随着电场的升高,介质层漏电流的输运以Schottky发射机制为主,Frenkel-Poole发射机制对漏电流的影响很小。     

新型四元系铋基焦绿石陶瓷的介电性能

采用固相反应烧结工艺制备了(Bi1.5Sr0.5)(M1.5Nb0.5)O7(BSMN,M=Sn4+、Zr4+)新型铋基焦绿石陶瓷。X射线衍射分析表明,BSSN和BSZN陶瓷均形成了立方焦绿石单相结构,无杂相产生。不同于BSSN,BSZN陶瓷材料在1MHz具有显著的正介电常数温度系数特性。四价阳离子Sn4+、Zr4+的分别引入,使得两种材料的介电常数及其温度系数有很大的差别。由于立方焦绿石的主干骨架为顶角相连的BO6八面体[1-2],而氧八面体之间的关联程度与介电性能尤其是介电常数有本质联系。进一步讨论了四价取代阳离子对材料的介电性能的作用规律。     

Dielectric properties and morphologies of composites filled with whisker and nanosized zinc oxide

Two kinds of zinc oxide (ZnO) powders, which are ZnO whisker (w-ZnO) and nanosized ZnO (n-ZnO), were used to prepare the composites with a matrix of low-density polyethylene (LDPE). Dielectric constants and losses of the composites were measured in a frequency range of 1-10 MHz. The dielectric constants and losses increase with increasing w-ZnO content in composites. Modified w-ZnO has a remarkable effect on dielectric properties of the composites. Dielectric constants and losses of the composites containing n-ZnO are larger than those containing w-ZnO under the same content. The results can be explained by nanoscale effects. Some theories of mixing laws are also referred to the dielectric properties of composites in this paper.     

Dielectric modeling of multiphase composites

In this paper effective medium theory based on Clausius–Mossoti relation was used to predict dielectric properties of multiphase composite system. The composite consisted of E-glass fibers (plain weave S-glass, J.B. Martin) embedded in bisphenol A diglycidylether epoxy matrix (D.E.R. 324, D.O.W. chemicals) with hollow ceramic spherical inclusions (SF 14, P.Q. Corp.) at different volume fractions. In many engineering applications, materials with designed dielectric properties are often sought. An important question in engineering design of a composite material is how the overall properties of the composite depend on those of the individual constituents. Mixing the epoxy resin with hollow ceramic inclusions can effectively reduce dielectric constant of the resin, which is often desirable, rendering composites as good candidates for many applications, especially in telecommunications. Compensation for degradation of mechanical properties of matrix (due to inclusion insertion) is obtained by embedding fibers into the mixture. Measurements of dielectric constant and loss tangent for this multiphase composite system were conducted in the region between 0.1 and 100 kHz range using DEA 290 (T.A. Instruments) dielectric analyzer and experimental results are presented. Good correlation between analytical model and experimental results was observed throughout all frequency range of investigation.     

Dielectric properties of liquid refrigerants: Facts and trends

In this paper we analyse the situation for the dielectric properties of the refrigerants, from the experimental and theoretical point of view, based on more than one decade of research of our group. Examples of the predictive power of simple models will illustrate the existing tools for electrical permittivity and dipole moment prediction and correlation. The relation of Báron and Buep was analysed and extended to include the effects of pressure variation. Analogies between the Vedam equation and rough hard-sphere theory of transport properties were encountered and discussed.     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

Stress development in plasma-deposited silicon dioxide thin-films due to hydrogen evolution

The kinetics of stress change in plasma-enhanced chemical vapor deposited (PECVD) silicon dioxide dielectric films, during annealing, is studied. When silicon dioxide is deposited in excess nitrous oxide, a fraction of the oxide is in the form SiO_3/2OH, instead of SiO₂, which on annealing condenses to SiO₂ and H₂O. The condensed water subsequently diffuses out of the film inducing a stress change (increase in tension or decrease in compression) in the film. A theoretical model relating the hydrogen (or water) concentration to the stress change is developed. Secondary ion mass spectrometry analysis revealed a relatively flat hydrogen concentration profile along the thickness of the film, indicating that the stress change is not diffusion limited. The model thus accounts for first order kinetics for bond breaking accompanied by fast diffusion with a fraction of the atoms or molecules undergoing a trap and release process. From the theoretical fit for the experimental data, activation energy of 1.1 eV for bond breaking is obtained. The model allows predicting the time dependent stress evolution in the PECVD film subjected to a prescribed temperature history.     

Dielectric properties of on-chip-cured polyimide films

Spin-coated polyimide thin films were locally cured using microelectromechanical system microhotplates as an alternative approach to the conventional wafer-level curing process. The on-chip cured polyimide films were characterized Fourier transform infrared spectroscopy. The polyimide was found to be fully cured at a temperature over 350 °C for 1 h. The dielectric properties of 350 °C-cured polyimide films were measured over broad frequency, temperature, and humidity ranges. As is expected for most polymer materials, the dielectric constant decreased with increasing frequency due to dielectric relaxation, which may be due to the fact that multiple relaxation times (i.e., several types of polarization), rather than a single one are involved. The dissipation factor was very low and nearly flat for excitation frequencies less than 1 MHz. Both the dielectric constant and the dissipation factor increased with increasing temperature from 10 °C to 90 °C, and relative humidity (RH) from 30%RH to 95%RH, with a steeper slope at lower frequencies. These results indicate that on-chip-cured polyimide films have weak low frequency dispersion due to the aging effect at high temperature and humidity.     

EDU liquid acquisition device outflow tests in liquid hydrogen: Experiments and analytical modeling

This paper presents experiments and modeling of the most recent set of liquid acquisition device (LAD) vertical outflow tests conducted in liquid hydrogen. The Engineering Development Unit (EDU) was a relatively large tank (4.25 m³) used to mimic a storage tank for a cryogenic storage and transfer flight demonstration test. Six 1-g propellant tank outflow tests were conducted with a standard 325 × 2300 rectangular cross-section curved LAD channel conformal to the tank walls over a range of tank pressure (158–221 kPa), ullage temperature (22–39 K), and mass flow rate (0.0103–0.0187 kg/s) per arm. An analytical LAD channel solver, an exact solution to the Navier-Stokes equations, is used to model propellant outflow for the LAD channel. Results shows that the breakdown height of the LAD is dominated by liquid and ullage gas temperatures, with a secondary effect of flow rate. The best performance is always obtained by exposing the channel to cold pressurant gas and low flow rates, consistent with the cryogenic bubble point model. The model tracks the trends in the data and shows that the contribution of flow-through-screen pressure drop is minimized for bottom outflow in 1-g, versus the standard inverted outflow.     

Dielectric properties and heating effect of multiferroic BiFeO3 suspension

Bismuth ferrite (BiFeO₃) ceramic particles of micro-meter size were prepared by a solid-state route. This study revealed an appropriate method to measure the dielectric properties of BiFeO₃ particle suspension using a system designed by ourselves. The heating effect of the BiFeO₃ suspension with applied AC voltage was confirmed. Application of this material in hyperthermia treatment of biological tissues with the goal of tumor therapy may be possible.