微波辐射溶胶-凝胶自蔓延法制备磁性ZnFe_2O_4纳米粉体(英文)

采用微波辐射溶胶-凝胶自蔓延法合成了铁酸锌(ZnFe2O4)纳米粉体,通过差示扫描量热仪、X射线衍射仪、扫描电镜和超导量子干涉仪表征粉体的相结构、微观形貌和磁性能。结果表明:随微波辐射时间由10min增加至15min,微波加热由原来的介电损耗转变为磁滞损耗,使得ZnFe2O4晶粒尺寸增大,结晶度提高。随着微波辐射时间的增加,饱和磁化强度和矫顽力均增加。     

Electromagnetic field effects in the microwave sintering of electrically conductive powders

Rapid sintering of compacted electrically conductive powders under heating by intense microwave radiation is investigated within a one-dimensional simulation model. The model accounts for the effective dielectric and magnetic properties of the powder material, electromagnetic field distribution, heat losses by thermal radiation, and densification. It is demonstrated that metal powders with insulating boundary layers on particles have low dielectric and magnetic losses, which results in a resonant character of microwave absorption. The influence of the interparticle contacts is investigated by introducing an effective electrical conductivity of the boundary layers, which can depend on the density. It is demonstrated that the losses in the boundary layers can stabilize the microwave heating process, facilitating rapid densification.     

Microwave-Absorbing Ferrite-Dielectric Composites

Two-phase composites of spinel ferrites and lithium aluminosilicates were prepared and investigated as microwave-absorbing materials. The magnetic and dielectric loss parameters which determine the heating characteristics were examined. Low-thermal-expansion composites were developed which were effective absorbers of microwave radiation. Some compositions investigated exhibited a temperature maximum limited by the Curie temperature of the ferrite in the composite.     

Catalytic decomposition of TCE under microwave

The decomposition of trichloroethylene (TCE) by hydrogen over Co/γ-Al₂O₃ and Ni/γ-Al₂O₃ catalysts under microwave heating was studied. The comparison between the catalytic activity in the microwave heating mode and that in the conventional thermal mode demonstrated that the microwave heating could greatly reduce the reaction temperature, accelerate the TCE decomposition speed and improve the TCE decomposition ratio. The results suggest that the microwave heating has a novel effect in the decomposition of TCE.     

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption-desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.     

Dielectric properties and electromagnetic wave transmission performance of polycrystalline mullite fiberboard at 2.45 GHz

Refractory lining is an indispensable part of high temperature microwave heating equipment, and its wave transmission performance exerts an important impact on the mode and efficiency of microwave heating, while the complex dielectric constant (dielectric constant and dielectric loss) of the material is the decisive factor in determining the wave transmission performance of the material. In this work, we measured the complex dielectric constant of polycrystalline mullite fiber board (PMF) in the temperature range of 25–1000 °C, and the effect of temperature on dielectric constant and dielectric loss was analyzed; The wave-transmission properties of the material were calculated according to the theory of electromagnetic wave transmission line, and the effects of temperature, material thickness, polarization modes of electromagnetic wave and incident angle on the wave transmission performance were analyzed. The results reveal that the dielectric constant of PMF does not change much with the increase of temperature, which is about 1.6; The dielectric loss does not change much within 200 °C, but when the temperature is higher than 200 °C, the change presents approximately exponential increase with the rise of temperature. The wave transmission performance fluctuates with the increase of the thickness, and there are maximum value and minimum value, and the overall wave transmission performance decreases with the increase of the material thickness. In a transverse electric (TE) field, the overall wave transmission performance decreases with the increase of the incident angle, and better wave transmission performance can be obtained by priority selection of vertical incidence of electromagnetic wave. In a transverse magnetic (TM) field, with the increase in the incident angle, the wave transmission performance firstly climbs up then declines, and there is an optimal incident angle where total transmission can occur. Finally, this work selected the thickness corresponding to different temperature as the preferred thickness. This work is of important theoretical significance for understanding the mechanism of the dynamic change of the wave transmission performance of the thermal insulation materials in microwave heating, and provides important practical guidance for the design and optimization of microwave heating equipment.     

吸湿多孔介质冷冻干燥传质传热模型

本研究在作者提出的吸附—解吸平衡关系的基础上,建立了一个全新的考虑吸湿效应的多孔介质冷冻干燥数学模型。模型用有限差分法进行求解,并带有一个移动边界,以模拟介电材料辅助的微波冷冻干燥过程。介电材料选用碳化硅（SiC）,原料液为脱脂奶。模拟结果表明：介电材料能够有效强化微波冷冻干燥过程。在典型操作条件下,介电材料辅助的微波冷冻干燥所用的时间比普通微波冷冻干燥减少33.1%。当料液中固体含量较低或者固体产品的损耗因子较小时,介电材料对微波加热的效果不明显。基于冰饱和度、温度和水蒸气浓度的分布,本文分析了干燥过程中的传质传热机理,并对干燥速率控制因素进行了讨论。     

The generation of microwave-induced plasma in granular active carbons under fluidised bed conditions

Some of the phenomena associated with the interaction of microwave (MW) radiation with a granular activated carbon (GAC) fluidised bed were studied in a single mode microwave apparatus. The sample heating section incorporated ports for a light measurement device (light dependent resistor, LDR) and camera, and a fluoroptic probe was used for sample temperature measurement. During fluidisation under various MW power regimes, the discharge activity and plasma generation within the carbon bed were quantified using the LDR and images captured. The MW dielectric heating of the carbons was measured with the fluoroptic probe and its relation to the fundamental dielectric parameter, the loss factor, ε″, studied. Factors found to affect the MW heating and plasma generation within the fluidised carbon were the thermal history of the sample and presence of trace amounts of metals, especially potassium. These factors influenced the degree to which a tendency to thermal runaway was observed during MW plasma generation.     

Separation of polyaromatic hydrocarbons from contaminated soils using microwave heating

The microwave treatment of soils contaminated with heavy- and light-hydrocarbons was investigated. The soils were characterised to determine the total organic liquid content and PAH contents, and the dielectric properties of the soils were measured across a range of temperatures. The heavy- and light-contaminated soils behaved very differently in a microwave environment, with bulk soil temperatures limited to 100 °C for the light-contaminated soil. Microwave treatment is shown to remove PAHs from both the heavy- and light-contaminated soils, and it is demonstrated that 95%+ PAH removal can be achieved under moderate processing conditions. Complete remediation of the soils is possible at high microwave powers or long residence times. It is shown that PAH removal can take place at bulk temperatures well below the boiling point of those compounds and a number of explanations are proposed for this behaviour. The mechanisms of PAH removal are investigated for both the heavy- and light-contaminated soils and thermal desorption, selective heating and entrainment mechanisms can all be exploited.This is the first step in the development of a continuous microwave treatment process for the removal of PAHs from contaminated soil on an industrial scale.     

BST/MgO系铁电移相材料微波电性能的研究

采用正交设计实验方法分析了SnO2、MnCO3、B i2O3和A l2O3添加量对BST/MgO系铁电移相材料微波电性能影响。实验结果表明:SnO2的引入促进样品烧结,有助于降低低频和微波损耗,但过多的加入会产生第二相,会使微波损耗上升。Mn受主取代Ti产生空穴等缺陷,不利于微波损耗的降低,而等价取代类似SnO2的情况,有助于降低微波损耗,但过多的加入会产生第二相,也会使微波损耗上升。B i2O3对材料微波电性能有显著的影响,从离子半径匹配角度,B i2O3一般出现在晶粒间,形成玻璃相,一方面促进烧结,使致密化提高,低频损耗变小;另一方面低熔点的玻璃相引起微波损耗急剧上升。A l2O3对材料电性能影响不显著。     

Basic ideas of microwave processing of polymers

The objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, ε″, loss tangent, tan δ, and oscillator strength, (ε_S ?- ε_∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.     

In-situ measurements of high-temperature dielectric properties of municipal solid waste incinerator bottom ash

Microwave heating is a potential green technology demonstrating many advantages over conventional heating methods. Prior to designing an industrial microwave process, however, a fundamental knowledge of the dielectric properties of the material to be thermally treated is imperative, as these properties determine the response of the material to an applied electromagnetic field. In this study, the fundamental interactions between microwave energy and municipal solid waste incinerator (MSWI) bottom ash (BA) are investigated through in-situ complex permittivity measurements. Using an enhanced version of the cavity perturbation method, the dielectric properties were determined from room temperature up to 1100 °C at a frequency close to the industrial 2.45 GHz. The results demonstrated that BA is a low-loss microwave absorber up to 320 °C, above which microwave flash pyrolysis of the organic matter abruptly enhances the dielectric loss of BA, resulting in a thermal runaway. The addition of water and graphite to BA induces a higher dielectric constant and loss factor. The evolution of the dielectric properties as a function of temperature is correlated to changes in the material as determined by Simultaneous Differential Scanning Calorimetry, Thermogravimetric Analysis and High Temperature X-ray Diffraction. The reported results form a baseline for the assessment of the MSWI BA response under microwave irradiation.     

Microwave‐absorbing properties of linear low‐density polyethylene/ethylene–octene copolymer/carbonyl iron powder composites

Using linear low‐density polyethylene (LLDPE)/ethylene–octene copolymer (POE) as a polymer matrix and carbonyl iron powders (CIPs) as filler, we prepared polymer matrix composites with microwave‐absorbing properties by means of melt blending. Scanning electron microscopy and transmission electron microscopy were used to characterize the samples. The absorbing properties of the composites were measured with the arch method in the range of frequency 2.0–18.0 GHz. The results indicate that the absorbing peaks moved to low frequency as the CIP content in composites increased and that there was an appropriate CIP content in LLDPE/POE/CIP composites to achieve the best absorbing effectiveness. The electromagnetic parameters of the composites were determined with the transmission/reflection method in the range 2.6–17.8 GHz. The experimental results show that there were both dielectric loss and magnetic loss in the LLDPE/POE/CIP composites. Therefore, the microwave absorption of the LLDPE/POE/CIP composites was attributed to the combining contributions of the dielectric loss and magnetic loss. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Microwave power absorption profile of detergent surfactant agglomerates during microwave heating

The microwave energy absorption behavior of the linear alkylbenzene sulfonate (LAS) surfactant detergent agglomerates was studied while considering changes in the physical properties of the samples. Microwave heating was used to change the internal structure of the agglomerates to make a reduced density (fluffy) product. The absorption of energy within LAS samples indicated fluctuating trends as microwave heating progressed. This was associated with the dielectric properties of the material which are strongly dependent upon the nature (“free” or “bound”) and quantity of water present in them at any instant which changes during heating. Therefore, complete profiles of the energy absorbed by the samples have been recorded to determine their actual power absorption behavior/total energy consumption over time. The bulk density of the agglomerates decreased significantly when exposed to microwave fields. Hot air drying can be combined with microwave heating to reduce the total heating time. It has been observed that the pre-heating of the samples reduces the total heating time and microwave energy requirement. This is due to the temperature-dependent decomposition of hydrates releasing more “free” water.     

Dielectric Loss Spectrum of Ceramic MgTiO3 Investigated by AC Impedance and Microwave Resonator Measurements

MgTiO₃ ceramic was selected as a model material of the microwave dielectric ceramics for the investigation of the dielectric loss spectrum. Conductivity spectra in the low-frequency range 1–10⁶ Hz at elevated temperatures and the dielectric loss spectrum in the microwave range 6–14 GHz at room temperature were characterized by AC impedance spectroscopy and the microwave resonator method, respectively. The experimental data were analyzed by using Jonscher's universal power law, the Arrhenius equation as well as the classic dispersion theory. Consequently, a dielectric loss spectrum (tg δ∼ f plot) at room temperature in a wide frequency range was obtained. Based on the loss spectrum, the dielectric loss can be classified into three mechanisms in MgTiO₃ ceramic (each dominates the loss in different frequency ranges): DC conductance (in DC field), AC conductance (<10⁷ Hz), and anharmonicity of lattice vibration (in GHz range). The influence of defects on the microwave dielectric loss possibly is because of atom defects via damage of the perfection of lattice and modification of the anharmonic force in lattice, while the electronic defects trapped on the atomic defects affect the DC and AC losses.     

Materials processed by indirect microwave heating in a single-mode cavity

We report in this paper the synthesis, the sintering and the properties of La_0.8Sr_0.2MnO₃ (LSMO) and BaZrO₃ (BZ) compound using microwave heating. The starting raw material was prepared by solid state reaction. Synthesis and sintering were carried out using an indirect microwave heating process. SiC susceptor tube was utilized to produce the indirect heating in a microwave symmetrical single mode cavity TE10p. The LSMO and BZ phases have been successfully sintered in a microwave cavity in a very short time. Scanning electron microscopy (SEM), X-ray diffraction (XRD), magnetic or dielectric and electrical properties were carried out for both processing conditions, i.e., using classical and microwave heating. The results and advantages of this microwave process to synthesize many materials with various physicals properties are discussed.     

吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型

为了研究吸波材料辅助微波加热对传统冷冻干燥过程的强化作用,建立了多孔介质温度、浓度和电磁场耦合的多相传递模型;以烧结的碳化硅(SiC)为吸波材料、以甘露醇水溶液为待干料液进行了微波冷冻干燥实验,并测定了甘露醇固体的介电特性。模拟和实验结果均表明,吸波材料对初始非饱和多孔物料微波冷冻干燥具有显著的强化作用。初始非饱和样品微波冷冻干燥时间比传统冷冻干燥缩短了18%,比常规饱和样品传统冷冻干燥缩短了30%。模拟结果与实验数据吻合良好。这表明提出的新型干燥方法确实能够实现过程传热传质的同时强化。通过考察样品内部温度、饱和度和电场强度的实时分布,分析了微波冷冻干燥过程的传热传质和电磁波传播与耗散机理。在微波冷冻干燥过程中,初始非饱和样品累计吸收的辐射能和微波能的总和与传统冷冻干燥相当。这说明,该干燥方法只是提高了能量效率,从而大幅缩短了冷冻干燥时间。     

Effects of BaTiO3 and FeAlSi as fillers on the magnetic,dielectric and microwave absorption characteristics of the epoxy-based composites

A systematic study was carried out on the magnetic, dielectric and microwave absorption properties of the two-phase FeAlSi/epoxy and three-phase FeAlSi/BaTiO₃/epoxy composites through experimental and simulation method. A percolation effect was observed in the dielectric, but not in the magnetic behavior of the FeAlSi/epoxy composites when the FeAlSi content was high. The 2D modeling shows that the high electric energy density is responsible for the high permittivity near the percolation threshold (49 vol%). On the other hand, the permeability of the composites matches well with the effective medium theory model (EMT), indicating that the permeability of FeAlSi/epoxy composite is more associated with the filler size and shape. Theoretical calculations show that the increment of the FeAlSi and BaTiO₃ loading, as well as the thickness will cause the absorption peak position to shift to low frequencies. The microwave absorption of these composites can be mainly attributed to the dielectric loss and magnetic loss.     

Microwave radiation as a tool for process intensification in exhaust gas treatment

The present work deals with the technology transfer from basic research on the microwave-assisted heterogeneous gas-phase catalysis into pilot plant scale. This enables a direct comparison of the novel microwave-assisted catalysis technology with conventional heating systems by means of ohmic losses. The work is focused on the technology and not on the catalyst material, which represents only one possible alternative. The electromagnetic radiation of 2.45 GHz gives the opportunity to transfer energy highly efficient by volumetric dielectromagnetic heating. This results in higher heating rates and lower energy losses compared to conventionally heated systems or other heating methods. The heat flow density in microwave systems can reach up to 35,000 W m⁻² [1]. Operating under those conditions the utilized catalyst has to have a high catalytic activity as well as large dielectric and magnetic loss factors. The highest potential for those properties exhibit materials based on mixed metal oxides such as perovskites and spinels. Selected compounds of these substance classes have been prepared by various synthetic methods and were characterized with respect to their catalytic activity, complex permittivity, and complex permeability [2]. The experimentally determined values built the foundation for calculation and simulation of an optimized reactor and microwave absorber design. The constructed pilot plant includes both heating systems, magnetrons for microwave-assisted and resistance heaters for conventional gas-phase catalysis, and was used for a comparison of the heating methods.     

Microwave Characteristics of Polyaniline Based Short Fiber Reinforced Chloroprene Rubber Composites

This report presents the microwave characteristics of conducting polymer composites (CPCs) based on chloroprene rubber with special reference to dielectric properties. CPCs based on polyaniline (PANI), polyaniline-coated short nylon fiber (PANI-N) and chloroprene rubber (CR) were prepared by mechanical mixing. The important properties like dielectric permittivity, loss tangent, conductivity, and dielectric heating coefficient were evaluated and compared. It was found that PANI/CR composites had very good dielectric properties in the microwave range. The addition of PANI-N improved the mechanical properties of the composites with reasonably good dielectric properties. The CPCs were also found to have good microwave absorption.