IMPORTANCE OF THE DIELECTRIC PROPERTIES OF MATERIALS FOR MICROWAVE HEATING

In this paper, we show the importance of having a good knowledge of the dielectric properties of a material when it is intended to heat or dry this material by microwaves. We study the frequency, temperature and composition dependence of the dielectric permittivity in order to see how these dependences can affect the microwave heating process. We also study the non-uniform absorption of energy in a sample due to dielectric or conductive losses.     

MICROWAVE THAWING OF LOSSY DIELECTRIC MATERIALS

Microwave thawing of lossy dielectric materials was examined theoretically. A “temperature” approach was used to model the microwave thawing of frozen slabs composed of beef or water and multilayer beef/water slabs. The heat conduction equation and Stefan equation were solved using a finite element front-tracking method and Newton iteration. The microwave power deposition in the slab was determined by an analytical solution to Maxwell's equations. Solutions to the governing equations provided transient temperature and power deposition profiles, and the position of the phase-change interface with respect to time. Dirichlet (essential) and Robin (convection) boundary conditions were investigated and thawing times were calculated for incident power levels up to 2·5 × 10⁴ Wm^-2 and slab thicknesses of 5, 10 and 15  cm. The beef samples thawed more quickly and for all situations there was less of a change in the thawing time as the power increased above 5000 Wm^-2. The thawing time versus sample thickness was found to follow a power law relationship. Thus, given a material's thawing time at a specified incident power, it is possible to estimate the thawing time for different sample thicknesses. In the absence of microwaves, the power law exponent is 2 in the case of the Dirichlet boundary condition. This value decreases below 2 upon microwave illumination.     

微波介质陶瓷的界面特性及其对介电性能的影响

详细地总结了界面的偏析、扩散和润湿性对微波介电性能的影响机理。并评述了粉末的初始状态、烧结工艺、添加剂(掺杂)和烧结方法等因素对材料界面特性的影响，进而影响到材料介电性能的研究进展。最后指出了在微波介质陶瓷界面研究领域面临的问题及今后的发展方向。     

钨酸盐类陶瓷微波介电性能

研究了钨酸盐AWO4(A=Ca,Sr,Ba,Zn,Mg)系列陶瓷材料的微结构、烧结性能以及微波介电性能.AWO4具有黑钨矿和白钨矿两种结构.无论白钨矿还是黑钨矿,其相对介电常数εr和介电品质因素Q×f值都随着阳离子A2 半径的增大而减小,谐振频率温度系数τf绝对值增大.白钨矿结构的CaWO4具有较好的微波性能εr=10.0,Q×f=50.842 THz,τf=-50× 10-6/℃.     

Bi基微波介质材料研究进展

低温烧结微波介质陶瓷是近年来电介质材料方面的一个重要研究方向,也是发展片式多层微波器件的基础材料.Bi基材料具有较低的烧结温度和优良的介电性能,因而受到了广泛的关注.据此,对不同介电常数Bi基微波介质材料体系的研究进展及应用作了综合介绍,并分析了低熔点氧化物掺杂、离子取代对不同体系微波介质材料结构、介电性能的影响.     

原材料和制备工艺对硅酸锌陶瓷微波介电性能的影响

为了提高硅酸锌介质陶瓷的性能,研究了添加物、原材料和制备工艺对其微波介电性能的影响。采用固相法、并以去离子水代替乙醇作分散剂制备陶瓷粉料,闭腔法测量其无载Q值和频率温度系数。研究结果表明原材料的粒度、球磨工艺和烧结温度对Q.f值影响大,添加物TiO2不仅调节频率温度系数(τf),而且促进陶瓷烧结。当TiO2(wt%)12%,1240℃烧结时,获得优良的微波介电性能:介电常数(εr)为10.2,Q.f=91640GHz,τf=-5.78ppm/℃。并用该组成的材料制作了中心频率f0=5.4GHz,带宽Δf=96MHz,插损小于1.3dB的两级片式介质带通滤波器,可以用于通信系统。     

玄武岩纤维及其复合材料的微波介电性能

考察了玄武岩纤维及玄武岩纤维织物在2~18GHz频率范围的微波介电性能,结果表明玄武岩纤维的介电常数及介电损耗小,玄武岩纤维三轴向布和玄武岩纤维毡的反射损失均小于5d B。采用真空灌注成型法制备了玄武岩纤维-环氧树脂复合材料,采用弓形法测试其在2~18GHz频率范围的反射损耗,结果表明其在整个频段的反射损失均小于10d B,透波性能良好。     

AN INTEGRAL MODEL FOR DRYING OF HYGROSCOPIC AND NONHYGROSCOPIC MATERIALS WITH DIELECTRIC HEATING

Heat and mass transport phenomena in drying assisted by microwave or radio-frequency dielectric heating are analyzed. When drying at temperatures near boiling point or with high temperature gradients, the effect of the gas phase pressure gradient on moisture transfer within the solid can be important. The governing heat and mass transfer equations, including consideration of internal heat generation and the effect of the gas phase pressure gradient, are derived and solved in a one-dimensional system using an integral method. The integral model has been used to simulate dielectrically-enhanced convective drying of beds of polymer pellets, glass beads and alumina spheres with flow over the bed surface. Model predictions of drying rates and temperatures agree well with experimental data for these cases.

The model provides a relatively fast and efficient way to simulate drying behavior with dielectric heating, and may be useful in design and optimization of dielectrically-enhanced convective drying processes.     

AN INTEGRAL MODEL FOR DRYING OF HYGROSCOPIC AND NONHYGROSCOPIC MATERIALS WITH DIELECTRIC HEATING

Heat and mass transport phenomena in drying assisted by microwave or radio-frequency dielectric heating are analyzed. When drying at temperatures near boiling point or with high temperature gradients, the effect of the gas phase pressure gradient on moisture transfer within the solid can be important. The governing heat and mass transfer equations, including consideration of internal heat generation and the effect of the gas phase pressure gradient, are derived and solved in a one-dimensional system using an integral method. The integral model has been used to simulate dielectrically-enhanced convective drying of beds of polymer pellets, glass beads and alumina spheres with flow over the bed surface. Model predictions of drying rates and temperatures agree well with experimental data for these cases.

The model provides a relatively fast and efficient way to simulate drying behavior with dielectric heating, and may be useful in design and optimization of dielectrically-enhanced convective drying processes.     

DIELECTRIC PROPERTIES OF MATERIALS AND MEASUREMENT TECHNIQUES

The dielectric properties of materials are defined, and the nature of their dependence on moisture content, frequency of the applied electric field, temperature of the material, and density of particulate materials is discussed. Dielectric properties of liquid water are summarized. The influence of moisture content, frequency, temperature, and density on the values of the dielectric constant and loss factor, real and imaginary parts of the relative complex permittivity, respectively, are illustrated for several different kinds of materials. Examples include dielectric properties for the cereal grains corn and wheat, pecans, and pulverized coal. Techniques for the measurement of dielectric properties of materials at high frequencies and microwave frequencies useful for dielectric heating and drying applications are discussed briefly, and numerous publications describing these methods in greater detail are cited for reference.     

DIELECTRIC PROPERTIES OF MATERIALS AND MEASUREMENT TECHNIQUES

The dielectric properties of materials are defined, and the nature of their dependence on moisture content, frequency of the applied electric field, temperature of the material, and density of particulate materials is discussed. Dielectric properties of liquid water are summarized. The influence of moisture content, frequency, temperature, and density on the values of the dielectric constant and loss factor, real and imaginary parts of the relative complex permittivity, respectively, are illustrated for several different kinds of materials. Examples include dielectric properties for the cereal grains corn and wheat, pecans, and pulverized coal. Techniques for the measurement of dielectric properties of materials at high frequencies and microwave frequencies useful for dielectric heating and drying applications are discussed briefly, and numerous publications describing these methods in greater detail are cited for reference.     

复合钙钛矿陶瓷的结构与微波介电性能

介绍了复合钙钛矿化合物结构的本质特征，讨论了钙钛矿结构对微波介质陶瓷材料的介电性能（介电常数、介质损耗、频率温度系数）的各种影响因素。     

A型沸石分子筛膜微波合成及渗透汽化性能

引　言沸石分子筛具有均匀的分子尺寸微孔结构和良好的热稳定性、机械强度、催化作用 ,是当前无机膜材料研究的热点之一 .沸石分子筛膜合成方法主要有原位水热合成法和汽相合成法 ,应用这些方法已成功合成出Ａ型[1] 、Ｙ型[2 ] 、Ｐ型[3 ] 、ＭＦＩ等[4 ] 等沸石分子筛膜 .Ａ型沸石分子筛由于具有0 .3～ 0 .5ｎｍ的有效孔径和三维孔道结构 ,亲水性强 ,有可能在小分子气体如低碳烃类分离及有机物脱水等方面得到应用 .Ｊａｎｓｅｎ等[5] 认为晶体粒径愈大 ,产生堆积孔径愈大 ,所合成的沸石分子筛膜存在缺陷可能性愈大 .但从文献报道看 ,沸石分…     

低介电常数微波介质陶瓷的研究进展

介绍了四种典型的相对介电常数在10-15之间的低介微波介质陶瓷系列(Al2O3系、Mg4Nb2O9系、Mg5(Nb,Ta)4O15系、R2BacuO5系)的晶体结构和微波介电性能,并指出其目前存在的普遍问题和解决的方法.     

微波介质陶瓷的研究进展

微波介质陶瓷是现代通信技术中谐振器、滤波器、振荡器等重要元件的基础材料,它的研究越来越受到人们的重视。介绍了低介电常数、中介电常数、高介电常数三类微波介质陶瓷的研究现状,存在的问题及未来的发展趋势。     

氧化铈复合钛酸镧钙介质陶瓷的微波性能

以1%(质量分数)CuO为烧结助剂,采用固相合成法制备了(1-x)Ca0.6La0.27TiO3 xCeO2单纯和复合微波介质陶瓷材料.研究了此体系陶瓷的微观结构和微波介电性能.研究表明:当x从0增加到1时,体系的相对介电常数εr从111.2线性降低到22.5;谐振频率温度系数τf从x=0时的正值 297.8×10-6/℃线性变化为x=1的负值-54.2×10-6/℃;而无载品质因数与谐振频率乘积Qf值则呈非线性增加.当x=0.85,且在1 400 ℃烧结,可获得εr=32.2,Qf=6682GHz,τf= 5×10-6/℃的新型微波介质陶瓷.     

ZnO粉体的固相法合成及微波介电性能

采用固相反应法制备了ZnO前驱体,在不同煅烧温度和保温时间分解得到ZnO粉体.用扫描电镜和X射线衍射仪表征合成粉体.采用矢量网络分析仪测试ZnO粉体在8.2～12.4 GHz的微波介电性能.结果表明:在不同煅烧温度和保温时间,前驱体均分解生成ZnO粉体.低温时,得到的样品颗粒大小均匀一致;随着温度的升高,颗粒逐渐长大,但均匀性变差.保温时间对样品颗粒大小的影响不明显.样品介电常数的实部(ε')和虚部(ε')随温度的升高先增大后减小,700℃时,达到最大;而随保温时间的增加,ε'和ε'的变化不显著.对产生介电损耗的机理进行了讨论,并提出改善介电性能的途径.     

陶瓷微波加热过程模型研究

建立了ＴＦ１０Ｎ单模微波烧结腔中陶瓷同波烧结加热过程的有限元数学模型，并对小尺寸圆柱形和长方柱形氧化铝陶瓷试体分别用二维轴对和二维模型进行了加热过程的数值模拟，所得结果能基本准确地反映实际过程，文中还讨论了材料物性参数，介电损耗因子、热导率等对加热过程的影响作用。     

钡改性PLZT系材料的介电性能及微观结构

研究了PLZT系统中处在反铁电-铁电相界并在反铁电相一侧的组成用Ba改性后的介电性能和微观结构,获得了介电系数大于5000,介质损耗小于1×10~(-2)和电阻系数为10~(11)Ω·Cm的介质材料。研究结果表明:用Ba置换Pb有助于微观结构中晶粒均匀皮和化学组成均一性的提高。文章对此进行了讨论。