基于谐振法测试微波介质材料的介电参数

介电常数是介质材料的主要性能参数之一,该文讨论了微波介质的介电参数的理论计算,分析了谐振法的模型,研制了基于谐振法测量微波介质材料介电参数的测量系统.通过对多个样品进行实测,结果表明与参考值比较吻合,评价了该测量系统测量结果不确定度,从而建立了微波频段内微波介质材料介电常数测量平台.     

基于VC++的介质复介电常数带状线法自动测试系统的研究

随着微波技术的迅猛发展,对微波介质材料提出了新的要求,对介质材料的各种性能的测量有了更多的需求,复介电常数是介质的一个主要参数,本文就微波介质复介电常数测量的带状线法进行了一个简单的介绍,并重点就基于VC++所编制的自动测量程序做了相关的论述。自动化测量可以大大提高我们的工作效率。     

手性材料手性参数的圆波导测量方法

本文提出了用普通微波圆波导测量系统测量了手性材料复手性参数的方法。该方法还能同时测量手性材料复介电常数和复磁导率。为了得到手性参数,首先需要测量短路和开路时的复反射系数,及电磁波通过手性材料后的偏转角和轴比。测量结果与自由空间法的测量结果一致。     

SrCO_3复合改性FeSiAlCr微波吸收剂研究

采用气体雾化和高能球磨处理工艺并结合SrCO3复合改性技术制备FeSiAlCr薄片状微波吸收剂材料;利用X射线衍射仪和扫描电镜分析观测试样的微观结构和形貌;使用矢量网络分析仪测量系统测量样品在2～18GHz频率范围的复介电常数和复磁导率;计算模拟了FeSiAlCr吸收剂和经纳米SrCO3复合改性的SrCO3/FeSiAlCr吸收剂构成的单层吸波材料的吸波性能。结果表明,气体雾化工艺结合高能球磨处理技术可以获得纳米晶结构和薄片状形貌的Fe-SiAlCr材料,通过纳米SrCO3复合改性处理还能有效降低材料的介电常数,改善微波电磁参量的匹配性能;SrCO3/FeSiAlCr复合改性材料具有良好的吸波性能,厚度为1.2mm的吸波涂层在2～6GHz范围的反射系数均低于-5dB。     

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.     

玻璃包覆磁性微丝的制备及微波电磁性能

采用Taylor-Ulitovsky方法制备了Fe基和Co基非晶态玻璃包覆微丝,测试并分析了玻璃包覆微丝的微结构、微观形貌、静磁性能及其微波复磁导率和复介电常数.研究结果表明,通过改变拉丝速度,可以制备出直径6.3～23.5μm的非晶态玻璃包覆微丝;制备时通过水冷或气氛保护可以控制样品的物相组成.典型样品的各项异性等效场高达7.96×103A/m.Fe基样品磁导率在6.3GHz具有自然共振峰,Co基样品在所测频率范围没有共振峰,此测量结果和根据理论计算的自然铁磁共振频率基本一致.典型样品的相对微波复介电常数均低于25且随频率升高而降低.     

六端口反射计测量复介电常数的改进方法

提出了六端口反射计测量介质材料复介电常数的一种改进方法,新的算法使测量面与参考面分离,消除了传统测量方法中测量面与参考面无法完全重合引入的误差.给出超越方程的简单准确的数值解法,得到了相对介电常数多值间隔与测量频率和样品长度的关系.实际测量结果表明本文方法的有效性和实用性.     

Bi系铜氧化物射频介电性质的研究

测量了不同离子掺杂Bi2Sr2CaCu2Oy多晶样品的射频介电常数,发现样品的介电常数存在明显差别,分析认为这是由于离子掺杂改变了材料中的载流子浓度,导致材料的空间电荷极化差异所引起的.基于这种机制,成功解释了不掺杂样品在1700MHz附近的弛豫现象和不同样品损耗性质的差异.     

微波介质介电常数和磁导率测试方法

讨论了微波介质介电常数和磁导率测试方法的物理模型,并对该模型进行了理论分析。给出了电磁波透过被测样品材料的波长数计算式,解决了求解介电常数和磁导率的模糊性问题。研制了实验装置、实验系统和系统校准件,讨论了TRL校准方法和提高测试准确度的时域门技术。利用该实验系统对多种介质材料进行了实测,实验表明,理论分析是正确的,测试结果是可靠的。     

微扰和标定技术测量薄膜微波介电常数

提出了一种测量介质薄膜微波段介电常数的方法.该方法基于金属空腔谐振器微扰理论,用已知介电常数的基片作为标样,标定测量系统的有关参数,然后分别测量空腔、基片插入空腔、镀有介质薄膜的基片插入空腔三种情况下的谐振腔谐振频率,计算出镀覆于基片上介质薄膜的微波复介电常数.本文SiO2和MgTiO3-CaTiO3(MCT)介质陶瓷薄膜作了实验测量验证,结果表明该方法具有良好的测量精度(小于6%).     

Shielded open-circuited sample holder for dielectric measurementsof solids and liquids

This paper presents a technique for permittivity measurements that can be used for either liquids or solid coaxial samples from near dc to gigahertz frequencies. The method uses both a full-mode model of a coaxial open-circuited termination and a capacitive method for permittivity determination. The method requires the use of an impedance bridge and a network analyzer. Measurements indicate good accuracy for the permittivity over a very wide band of frequencies. For high permittivity materials the sample surfaces must be metallized. An uncertainty analysis is also presented     

The permittivity and dielectric loss of reaction-bonded silicon nitride

Using recently developed coaxial line methods values of permittivity and dielectric loss have been determined over the frequency range 0.5 to 7 GHz for a series of reaction-bonded silicon nitride specimens in which the degree of nitridation has been varied. For fully nitrided material (having a weight gain of 62% and a volume porosity of 19%) the measured permittivity was 4.60 and was almost independent of frequency; fitting both the permittivity and loss data to the Universal Law of dielectric response confirmed that the limiting condition of lattice loss applied withn=0.98±0.02. Reduction of the degree of nitridation caused progressive increases in permittivity and loss, both of which closely approached the quoted values for pure silicon at weight gains below about 40%.     

An improved technique for permittivity measurements using a coaxialprobe

An enhanced model for an open-ended coaxial probe used for making permittivity measurements is presented. A permittivity measurement system consisting of the coaxial probe and a network analyzer is described including details of the error correction and curve fitting techniques. Determination of the percent dissolved solids in fructose solutions from permittivity measurements is presented as example of the usefulness of the coaxial probe/network analyzer measurement system     

Analysis of an open-ended coaxial probe with lift-off fornondestructive testing

The open-ended coaxial probe with lift-off is studied using a full-wave analysis, and an uncertainty analysis is presented. The field equations for the following terminations are worked out: (1) the sample extends to ∞ in the positive axial direction, (2) the sample is backed by a well-characterized material, and (3) the sample is backed by a short-circuit termination. The equations are valid for both dielectric and magnetic materials. The model allows the study of the open-ended coaxial probe as a nondestructive testing tool. The analysis allows a study of the effects of air gaps on probe measurements. The reflection coefficient and phase are studied as a function of lift-off, coaxial line size, permittivity, permeability, and frequency. Numerical results indicate that the probe is very sensitive to lift-off. For medium to high permittivity values and electrically small probes, gaps on the order of fractions of a millimeter strongly influence the reflection coefficient. In order for the field to penetrate through the air gap, larger size coaxial line or higher frequencies need to be used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included     

Finite element models of the application of stress-relieving dielectrics

The addition of ferroelectric fillers to a polymer can produce a material with electric field-dependent permittivity properties, which can be used for the purposes of electrical stress relief for applications under AC fields. The paper illustrates the electrical stress-relieving properties of these composite polymers through finite element models. The electric field-dependent permittivity characteristics of the stress-relieving dielectric have been based on a barium titanate filler in an acrylic resin, for a range of filler levels. Two sets of models are considered. The first set of models considers the case of a needle-point plane geometry and the electrical stress distribution in the stress-relieving dielectric for needles of different geometries. The second set of models considers the stress distribution in a coaxial cable using the stress-relieving dielectric. The effectiveness of the stress-relieving dielectric in each of these geometries is clearly illustrated     

Determination of electromagnetic properties of materials usingflanged open-ended coaxial probe-full-wave analysis

A full-wave analysis is applied to the scheme of measuring electromagnetic (EM) properties of materials over the frequency range of 0.3-4 GHz using a flanged open-ended coaxial probe. The excitation of complex waves, which include surface waves, radiative waves and radial guided waves in layered material media, is considered. With an accurate theory, this scheme may be used to measure both complex permittivity and permeability of the material simultaneously     

A New Method for Measuring the Electrical Properties of Sea Water and Wet Earth at Microwave Frequencies

By replacing the outer conductor of a section of 50-? coaxial line with a conductive material, the complex permittivity (dielectric constant and conductivity) of the material is determined from calculations based upon attenuation and phase shift measurements. The method is most accurate for materials with loss tangent greater than about 0.1 and conductivity around 1 mho/m or greater, such as sea water and highly conductive earth. Measurements made using NaCl solutions are in good agreement with published data.     

Open-Ended Coaxial Probe: Model Limitations

We propose a procedure to determine the validity range of any empirical model of the open-ended coaxial probe transition that is used to measure the permittivity of materials. The procedure is illustrated by an application of the method to a standard coaxial cable probe, a permittivity range of interest in the food industry and, as an empirical model, the well-known “lumped capacitor model.”      

Broad-band dielectric spectroscopy of low-permittivity fluids usingone measurement cell

Dielectric spectroscopy is a relevant method for quality characterization of petrochemical products, provided that the permittivity spectra are measured with high precision over a broad frequency range. This paper describes how the permittivity of low-loss liquids such as crude oils can be measured with high precision using a single measurement cell in the broad frequency range 1 kHz to 6 GHz. The use of a single cell ensures that the whole dielectric spectrum is measured under the same conditions, which is an advantage for both on-line and laboratory applications. The permittivity spectrum is obtained by combining impedance and S-parameter measurements of a coaxial transmission/reflection cell with suitable permittivity calculation methods. The high sensitivity of the system is confirmed by measurements on crude oils and known low-loss liquids     

Complex permittivity and moisture measurements of oil palm fruits using an open-ended coaxial sensor

An open-ended coaxial sensor for the determination of complex permittivity and moisture content of oil palm fruits is presented in this paper. The measurement system consisting of the sensor and a PC-controlled vector network analyzer have been tested successfully on a range of oil palm fruits of various degrees of ripeness. The initial values of the complex permittivity were estimated using the admittance model of the sensor. The amount of moisture content was found by matching the values of permittivity from the quasistatic model with the permittivity of a dielectric mixture model using the moisture content values obtained from the standard oven drying method.