Nanostructured materials for microwave receptors

Microwave heating promises numerous benefits over conventional heating including rapid thermal ramps, energy transfer rather than heat transfer, material selectivity, and improved automation and safety. This set of advantages has led to growing application in industrial processes. Currently, use of microwave heating is restricted because many materials of interest have poor dielectric loss properties and therefore respond poorly to microwave radiation. For this reason, nanostructured materials with high dielectric loss constants that can absorb microwave energy and convert it to heat are desired. Combination of the nanoscale receptors with base materials offers the opportunity to create composites with a high dielectric loss factor. This review covers the development of nanostructured microwave receptors and their applications. The structure of microwave receptors and their compatibility with the base material have a significant effect on the final dielectric properties. Therefore, various nanostructured microwave receptors, their surface modification, and the effect of the interface between the nanostructured receptors and the base materials are reviewed. Fundamental aspects of dielectric materials and their role in dielectric performance are discussed. Finally, key challenges, directions for further studies, and some promising nanostructured microwave receptors are suggested.     

Microwave glazing of alumina–titania ceramic composite coatings

The application envelop of atmospheric plasma sprayed ceramic composites can be widened considerably by reducing/eliminating inherent surface defects by treating them through techniques like microwave irradiation. In microwave processing, microwave energy is directly applied to the material. High-frequency microwaves (>1 GHz usually) penetrate into the bulk of the material and the volumetric interaction of the electromagnetic fields with the material results in dielectric (volumetric) heating. This leads to higher heating efficiency with faster processing. Ceramics are transparent to microwaves at low temperatures, however, start absorbing microwaves at higher temperatures resulting change in microstructure and material characteristics. This paper presents microwave processing of atmospheric plasma sprayed alumina–titania ceramic composite coatings in conventional microwave heating system and evaluation of the processed materials through XRD, SEM, microhardness survey and surface finish with illustrations. Results indicate microwave irradiation induces densification of the material and possible flow of dominant gamma-alumina phase that leads to glazing of coated surface. Glazed surfaces exhibit enhanced microhardness as well as surface finish.     

Microwave–material interaction phenomena: Heating mechanisms,challenges and opportunities in material processing

Efforts to use microwaves in material processing are gradually increasing. However, the phenomena associated with the processing are less understood; popular mechanisms such as dipolar heating and conduction heating have been mostly explored. The current paper reviews most of the significant phenomena that cause heating during microwave–material interaction and heat transfer during microwave energy absorption in materials. Mechanisms involved during interaction of microwave with characteristically different materials – metals, non-metals and composites (metal matrix composites, ceramic matrix composites and polymer matrix composites) have been discussed using suitable illustrations. It was observed that while microwave heating of metal based materials is due to the magnetic field based loss effects, dipolar loss and conduction loss are the phenomena associated with the electric field effects in microwave heating of non-metals. Challenges in processing of advanced materials, particularly composites have been identified from the available literature; further research directions with possible benefits have been highlighted.     

微波加热制备特种陶瓷材料研究进展

特种陶瓷广泛应用于航天航空、电子信息、新能源、机械、化工等新兴工业领域,其高温制备过程仍以传统燃气窑炉和电加热炉为主;碳排放高、能耗大,节能减排形势严峻。当前,我国面临实现“双碳”目标的巨大压力,研究推广清洁高效的加热技术迫在眉睫。微波加热是利用材料自身对微波进行吸收,将电磁能转化为热能,能量的转移发生在分子水平上,通过这种方式,加热在整个材料内外同时产生,整个材料体系中的温度梯度非常低。除体积加热外,选择性加热、功率再分配、热剧变以及微波等离子效应等也是微波烧结的显著特征。微波加热具有节能环保、改善制品性能、减少燃烧碳排放等优点,国内外有许多关于微波合成各种氧化物、碳化物、氮化物陶瓷粉体和微波烧结陶瓷复合材料的报道。本文首先对微波和微波混合烧结的基本理论进行综述,然后介绍了微波加热制备陶瓷粉体与微波烧结制备陶瓷材料的最新研究进展,最后总结了微波加热在陶瓷工程制品烧结中的一些研究成果,体现出微波烧结的优越性,并提出了微波烧结制备特种陶瓷的关键问题和今后的发展方向。     

Role of convective heat removal and electromagnetic field structure in the microwave heating of materials

The temperature distributions arising in a low-loss dielectric sample in the process of microwave heating have been studied by means of numerical simulation. The convective heat removal has been demonstrated to play the determining role in the energy balance of the sample. The obtained temperature distributions have been compared with the results of a purposely designed microwave heating experiment with multi-channel temperature measurement. The temperature on the surface of the sample agreed well to the simulation accounting for convective heat removal. The temperature measured inside the sample was higher than predicted by simulation. The electrodynamic calculations have shown that due to diffraction effects the electromagnetic field inside the sample is inhomogeneous even if the sample is irradiated by microwaves isotropically. Thus, it is concluded that in order to simulate the microwave heating process accurately, it is necessary to account for air convection and calculate the structure of electromagnetic field inside the sample.     

Role of particle size in microwave processing of metallic material systems

Processing of metallic materials using microwave energy has been a challenge. However, exploiting the size factor of particles in these candidate materials opened up further opportunities. The amount of heat evolved inside a particle per unit volume primarily depends upon its size, shape, electromagnetic and thermal properties. Finer particles can potentially absorb more microwave energy than coarser units and get heated rapidly with higher uniformity. It results in better properties in the processed part and energy economy. This work reviews the relevant literature and summarises fundamentals of microwave heating of metallic materials. Roles of particle size in processing these materials have been discussed. Challenges in microwave processing of finer metallic particles have been identified; opportunities for future research are outlined.     

The heating of polymer composites by electromagnetic induction – A review

Since the late 1980s a small number of research groups have been attracted with the idea of using induction heating technology for the processing of fibre reinforced polymer composites. Induction technology is suitable for the processing of thermoplastic and thermoset polymer materials but requires special susceptor additives (conductive materials) either in the form of structured fibres and fabric or particulate that can transform the electromagnetic energy into heat. This paper aims to summarize the principles of induction heating with respect to polymer composites processing taking a look first at material and equipment based process influences. State of the art applications and research activities are then reviewed, from thermoplastic composite welding, thermoset curing, selective material heating and fast mould heating technologies. Current simulation possibilities and available software tools have also been covered. Finally, some new ideas and possibilities for future developments in the field of polymer composites processing have been discussed.     

An overview on microwave processing of material: A special emphasis on glass melting

Material processing adopting microwave heating has emerged as an alternative tool owing to faster processing, a cleaner environment, and several other advantages. This review provides a summary of recent reports of microwave synthesis of materials. This study reviews the use of microwave energy for application in several material processing technologies apart from food processing. A special emphasis has been made in the processing of glass adopting microwave energy. Melting of glass comprising SiO₂, P₂O₅, B₂O₃ as the main building block has been discussed. It has been revealed that silica, a microwave transparent material as reported earlier, can be heated under microwave heating directly. Microwave absorption of raw materials and different glass system has been discussed. Dielectric properties, particularly loss tangent or loss factor, are presented for some glass composition. Less evaporation of ingredient and low contamination from the crucible wall are noticed during glass melting using microwave heating. Enhanced iron redox ratio (Fe⁺²/∑Fe) in microwave processing may be considered an advantage in the preparation of heat absorbing filter glass. Small-scale glass melting using the microwave heating has a significant impact on energy and time saving. However, the challenges associated with the upscaling glass melting with microwave heating and future scope have been talked about.     

微波对物料微结构的影响

本文介绍了微波加热在陶瓷烧结、木材干燥、食品膨化、超细粉体干燥等过程中对物料微结构的影响。指出，微波加热具有抑制微结构垮塌、膨化食品微结构、抑制纳米粒子团聚等作用。认为微波加热可为纳米粉体干燥和纳米材料制备提供一种潜在处理方法。应当充分研究微细粒子传热传质特性和物料微细粒子介电行为，从而深入认识微波加热对物料微结构的影响。     

微波与无机非金属介质的相互作用

本文从麦克斯韦电磁理论出发,理论上分析了微波与物质相互作用机理,指出介质吸收微波源于介质对微波的电导损耗和极化损耗,高温下,电导损耗将占主要地位．利用微波加热物质,物质的损耗有一定的限制,对干2．45GHz的微波,物质电导率应在1Ω^-1cm^－1以下,根据现有的无机非金属材料的电导率数据,对某些无机非金属材料的微波加热特性进行了简单的评估;在一些简单的假设下,对微波加热样品时,样品内部温度分布情况作了简要描述,对微波场中非均匀固体的行为进行了简单的分析和预测．     

Microwave heating of materials with impurities

The microwave heating of materials is important in many industrial processes. For example, it is used for the smelting of metals and the sintering of ceramics. Hot-spots (localised areas of high temperature) can develop in the material being heated or in the microwave oven itself, with disastrous consequences. Impurities in the material or in a component of the microwave oven can have different electromagnetic and thermal properties to the surrounding material. Different rates of heating occur at these sites, which gives rise to differential heating, which can lead to the generation of hot-spots. The generation of hot-spots by this mechanism is considered for a finite one-dimensional slab with a single impurity at its centre. A fixed-temperature boundary condition is applied at both ends of the slab and one end of the slab is irradiated by microwaves of constant amplitude. The heat absorption at the impurity is assumed to have a power-law dependence on temperature (hence hot-spot generation can occur via thermal runaway). Depending on the electrical and thermal properties of the material there are two possibilities; either a hot-spot occurs or a steady-state solution occurs due to a balance between heat absorption in the material and heat loss through the boundaries. These steady-state solutions are found for both linear and non-linear thermal absorptivity and constant and decaying electric-field amplitude. If possible the region of parameter space in which they occur (in the rest of the parameter space hot-spots occur) is also found. In addition, numerical solutions are developed to verify the steady-state solutions and to investigate cases where analytical solutions are difficult to derive, such as for materials with multiple impurities.     

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.     

Measurement of the temperature field distribution of materials in microwave-devices based on slow-wave systems

The results of a calculation and measurements of the temperature field of sheet dielectric materials, obtained when they are heat treated in microwave devices, using different slow-wave structures as the heating elements, are presented. __________ Translated from Metrologiya, No. 2, pp. 24–31, February, 2008.     

石墨烯基吸波材料的研究进展

诸如铁氧体、磁性金属粒子及其合金等传统吸波材料,密度大、环境稳定性差、对电磁波的吸收弱以及吸收频带窄的缺陷限制了其在吸波领域的应用,而石墨烯因其较高的机械强度、较小的密度以及优异的介电性能受到了吸波材料领域众多学者的关注;但由于石墨烯的阻抗匹配性能较差,损耗机制比较单一,导致其吸波性能较差,因此,研究人员通常将石墨烯与其他介电损耗型或者磁损耗型材料复合来增强其吸波性能,此外对吸波剂的结构进行合理的设计也是增强其吸波性能的有效途径。结合国内外的发展状况,对石墨烯基吸波材料的制备以及性能研究做了综述性介绍,并展望了未来石墨烯基吸波材料的发展方向。     

Microwave dielectric heating to disassemble a modified cementitious joint

According to existing theory on the deterioration of cement and concrete, a cement hardener undergoes a hydrate disassembly reaction at high temperatures of 300 °C or above; this increases the internal void volume and decreases the residual strength. This weakness mechanism has already been applied to manufacturing recycled aggregates and has reached the stage of practical application for removing adhered mortar from the aggregate surface. However, heating using an external heat source consumes a considerable amount of energy, and greenhouse gases are emitted during the recycling process because of low energy efficiency. In this study, a susceptor with outstanding microwave heating efficiency was selected, and the temperature elevation characteristics from the microwave heating of cementitious material containing the susceptor were analyzed. Through measurement, the bond strength of a cementitious joint including a modifier with the susceptor was found to weaken after microwave heating; thus, a new cementitious joint that can be reused or recycled as construction material and members is proposed. If the susceptor can selectively make cement material vulnerable in a short time by absorbing microwaves at the cementitious joint, the combined waste from the cementitious joint can be separated into single materials.     

轻质核壳结构吸波材料的研究进展

随着现代科技的发展,电磁波辐射对人类的影响越来越大,在电子电路中释放的电磁波会破坏其他设备的性能并且损害人体健康,因此吸波材料的研究显得尤为重要。此外,具有优良电磁性能的复合吸波材料还可以用于制备飞行器隐身材料。这是因为高强度的微波吸收材料具有良好的介电损耗和磁性损耗,同时具有优越的阻抗匹配,而核壳结构的吸波材料是复合吸波材料中较为理想的材料。本文详细介绍了核壳结构吸波材料的合成方法,并根据核壳结构材料的分类及具体应用,阐述了近年来国内外核壳结构吸波材料的最新研究进展。     

Impregnated carbon as a susceptor material for low loss oxides in dielectric heating

Dielectric heating of materials may have distinct advantages over conventional heating in view of the fact that it is a bulk heating technique, and does not rely on conduction or convection, the driving force of which are temperature gradients. Dielectric heating is currently predominantly practised in food and ceramic processing. Research is conducted also in the areas of chemistry and in solid catalysed reactions. Base materials used in these applications, like food containers, catalysts and green ceramic products, may not demonstrate sufficiently high dielectric loss factors to allow dielectric heating to be feasible. In such cases microwave susceptor materials may be added. A whole variety of such susceptor materials are known, such as zirconia, silicon carbide and carbon. In this work, the impregnation with carbon was investigated as a way to enhance the dielectric loss factor for low loss porous oxidic materials. To this end a variety of porous oxides was impregnated with poly furfuryl alcohol which was subsequently carbonised. Impregnation conditions, dielectric heating behaviour, calculations on dielectric properties and oxidation resistance will be reported.     

高分子磁体/ TiO2复合缩波材料的电磁性能

研究了在100 MHz～1800 MHz 的高频、微波下, 高分子磁体/ TiO₂( 简称OPM/ TiO₂ ) 复合缩波材料的电磁参数变化。研究表明: 丙烯酸改性的OPM/ TiO₂ 复合缩波材料有效地提高其介电常数, 降低介电损耗和磁损耗,而60Co-r 辐照的OPM/ TiO₂ 材料却显著地提高其磁导率及介电常数, 但材料的磁损耗和介电损耗基本不变, 因此,OPM/ TiO₂ 作为一种用于缩小电子器件及天线几何尺寸的复合缩波材料, 对高频, 微波通信、宇航和计算机有重要的理论和应用价值。      

微波对褐煤介电特性及干燥的影响

为研究微波干燥褐煤的可行性,选取云南昭通褐煤进行试验研究,采用微波介电特性变温测试系统研究褐煤介电特性及微波穿透深度随表观密度的变化。结果表明,褐煤介电特性与表观密度成正相关,穿透深度与表观密度成负相关,同时拟合得到褐煤表观密度与介电特性、微波穿透深度的关系式。采用微波干燥系统对不同功率和质量下的褐煤升温特性进行研究。结果表明,微波可以在1min内将褐煤升温至100℃,最大干燥速率为0.198(g/g db)·min-1,微波加热过程中,温度变化表现出3个阶段:快速升温阶段、恒温阶段、减速升温阶段。褐煤升温至100℃前,物料的质量和功率对升温速率影响不大,100℃之后,褐煤的升温速率随着功率的增加而增大,随物料质量增加而减小;通过微波干燥与常规干燥对比,发现当褐煤完全干燥时,微波干燥用时17min,常规干燥用时320min,微波干燥明显优于常规干燥。