Prospects of microwave processing: An overview

Microwave processing has been emerging as an innovative sintering method for many traditional ceramics, advanced ceramics, specialty ceramics and ceramic composites as well as polymer and polymer composites. Development of functionally gradient materials, joining, melting, fibre drawing, reaction synthesis of ceramics, synthesis of ceramic powder, phosphor materials, whiskers, microtubes and nanotubes, sintering of zinc oxide varistors, glazing of coating surface and coating development have been performed using microwave heating. In addition, microwave energy is being explored for the sintering of metal powders also. Ceramic and metal nanopowders have been sintered in microwave. Furthermore, initiatives have been taken to process the amorphous materials (e.g. glass) by microwave heating. Besides this, an attempt has been made to study the heating behaviour of materials in the electric and magnetic fields at microwave frequencies. The research is now focused on the use of microwave processing for industrial applications.     

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.     

模基水声信号处理技术的研究现状和展望

常规信号处理方法的前提条件相对简单,不能满足水声信号处理中海洋环境非平稳和极低信噪比等实际情况。基于模型的信号处理方法(模基处理技术)能够将物理模型引入到信号处理算法中,并且能够有效利用先验信息提高信号处理的性能,是一种有潜力的信号处理技术。结合多年的模基水声信号处理研究基础,首先介绍了模基处理的基本概念,然后介绍了模基处理器的基本框架和其主要优势,进而对模基技术在水声信号处理各个应用领域的研究现状做了全面的归纳和简要的评述,在此基础上,分析了该领域进一步的研究方向。     

Enhanced microwave processing of epoxy nanocomposites using carbon black powders

The conversion of electromagnetic energy into heat depends largely on the dielectric properties of the material being treated. Therefore, the knowledge of dielectric properties of the test specimen is required in order to understand the extent of curing using microwaves. In this study, a detailed investigation is carried out by considering a number of carbon black (CB) samples having particulate sizes in the range of 15–65 nm. The dielectric properties of the synthesized CB/epoxy nanocomposites, before and after microwave curing, are measured using the advanced cavity perturbation method (CPM). It is observed that the CB/epoxy nanocomposite having smallest particulate size i.e., 15 nm attains the maximum value of dielectric constant (ε_r′) and loss tangent (tan δ) of 10.79 and 0.05, respectively. These results indicate that the epoxy reinforced with the CB having least particulate size would interact more effectively with microwaves, which are confirmed by the experimental data showing that the nanocomposite with smallest CB particle size of 15 nm requires the minimum curing time. The dielectric properties especially the loss factors of fully cured samples are found to decrease after curing indicating that the dielectric properties of post cured samples can provide an idea about the extent of curing. At last, thermal, mechanical and morphological analyses are also performed on all the microwave cured epoxy samples.     

高性能钛酸钡/聚合物复合材料的研究进展

具有高介电常数(k)的钛酸钡/聚合物复合材料,兼有钛酸钡陶瓷和聚合物的各自优势,是一种有广泛应用前景的电子材料,因而备受关注。综合给出了近5年来高性能钛酸钡/聚合物复合材料的研究进展,分析指出了原材料选择、制备工艺及其对复合材料介电性能的影响,概括介绍了这类复合材料的主要应用,预测展望了其未来的发展趋势。     

O-3型压电陶瓷/聚合物复合材料的制备工艺新进展

0－3型压电陶瓷／聚合物复合材料具有单相压电陶瓷或聚合物所不具备的良好的综合性能,因此引起了人们广泛的兴趣和研究。本文综述了0－3型压电复合材料的制备工艺及相应复合材料的压电性能,重点介绍了水解－聚合法、凝聚－胶体法、溶液聚合法3种新型制备工艺,简要分析各种制备工艺的优缺点,为压电陶瓷／聚合物复合材料（甚至是纳米级压电复合材料）的进一步研究、开发和应用提供依据。     

Study on microwave absorption properties of metal-containing foam glass

Materials with the properties of electromagnetic (EM) wave absorption are attractive topics. In this work, we report that EM wave absorption composites, consisting of foam glass, zinc and zinc oxide, were prepared by sintering mixture of foam glass raw material and zinc powder. Microwave reflection loss of composite was calculated based on the permittivity in the range of 8.2-12.4 GHz. The results show that zinc-containing foam glass absorbs efficiently microwaves. The sample with zinc filler to foam glass mass ratio of 3/18 had a reflection loss below −10 dB in the range of 11.3-12.4 GHz, and the minimum reflectivity was −15.6 dB at both 12.0 and 12.4 GHz. Microwave absorption performances of specimens can be controlled by changing the ratio between zinc powder and foam glass mass. The detailed mechanism of the control was investigated through X-ray diffraction (XRD) analysis and scanning electrical microscopy (SEM) observations.     

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.     

Laser processing of polymers: an overview

A review of the author’s work on laser processing of polymers and others’ work on photo-etching in polymers is presented. Processes based on laser-induced volatile decomposition like cutting, engraving, etc. are investigated in thermoset and thermopolymers using a continuous wave CO₂ laser. Dependence of geometrical parameters and gross thermal effects on parameters of the laser beam for these processes are discussed. It is shown that a highly combustible polymer like cellulose nitrate can be processed with a CO₂ laser in air because of the faster kinetics of the volatile decomposition than that of the combustion. It is found that irreversible surface softening can be achieved in a thermoset polymer of allyl diglycol carbonate by treating it with a CO₂ laser. Basic parametric relations of this phenomenon are established. Mechanism of laser softening in the surface of a thermoset polymer is discussed. Finally, the process of fine etching based on photodecomposition in polymers is presented and various theories proposed to explain this phenomenon are discussed in brief.     

Acoustic Properties of Aerogels: Current Status and Prospects

Noise reduction remains an important priority in the modern society, in particular, for urban areas and highly populated cities. Insulation of buildings and transport systems such as cars, trains, and airplanes has accelerated the need to develop advanced materials. Various porous materials, such as commercially available foams and granular and fibrous materials, are commonly used for sound mitigating applications. In this review, a special class of advanced porous materials, aerogels, is examined, and an overview of the current experimental and theoretical status of their acoustic properties is provided. Aerogels can be composed of inorganic matter, synthetic or natural polymers, as well as organic/inorganic composites and hybrids. Aerogels are highly porous nanostructured materials with a large number of meso- and small macropores; the mechanisms of sound absorption partly differ from those of traditional porous absorbers possessing large macropores. The understanding of the acoustic properties of aerogels is far from being complete, and experimental results remain scattered. It is demonstrated that the structure of the aerogel provides a complex three-dimensional architecture ideally suited for promising high-performance materials for acoustic mitigation systems. This is in addition to the numerous other desirable properties that include low density, low thermal conductivity, and low refractive index.     

Prospects of solid state lasers for material processing

Since the invention of LASER in 1960, lasers have made a great impact in a wide range of scientific and technological applications. The first half of this paper discusses the basic differences between lasers and conventional heat sources and the second half is devoted to solid state lasers with specific reference to ‘high average power’ solid state lasers used in material processing. The various physical processes that influence their operation and the role of focusing optics are also discussed.     

Factors affecting the suitability of glass jars for heating in microwave ovens. Comparison with plastic jars and paper board tubs

The suitability of glass jars for heating in microwave ovens was studied. The aim of the study was to determine the effects of the type of foodstuff, the shape and the volume of the jars and the heating conditions (e.g. the type of microwave oven and the initial temperature) on the functionality and durability of glass jars heated in microwave ovens and on the heating result. Moreover, the behaviour of glass jars was compared with that of paperboard and plastic containers. A total of 243 glass jars were heated during the the tests. None of the jars were broken, even when the contents were initially frozen or when all the jars were placed on a cold surface immediately after heating. However, splashing of the contents and/or jumping of the jar with a resulting loud clatter represented a major problem. Jumping of the jars and splashing of the contents depended on both the recipe and the consistency of the food and the shape of the jar, but less on the packaging material, the initial temperature or the heating time. The plastic and board containers also jumped and food splashed out of them. If glass jars or cylindrical/jar-like packages are to be used for the packaging of microwaveable foods, both the recipe of the foodstuffs and the shape of the jars need to be developed further.     

Development of novel metallic glass/polymer composite materials by microwave heating in a separated H-field

Using a single mode 915 MHz applicator, we fabricate novel Cu₅₀Zr₄₅Al₅ metallic glass/polyphenylene sulfide (PPS) composites with high relative densities by microwave processing the constituents in a separated H-field with an applied pressure of about 5 MPa. The heating behaviors and structural changes of the composites have been investigated. A good bonding state between metallic glassy and PPS particles is found. The gradient structure is also induced by microwave heating of the composites with a high fraction of PPS phase.     

Advanced ceramics—an overview

In sharp contrast to conventional ceramics, advanced ceramics use extensively processed starting materials of controlled purity and particle size, and novel fabrication techniques such as tape casting, chemical vapour deposition,in situ oxidation, injection moulding, followed by fast, often low temperature, sintering. Advanced ceramics exhibit unusual behaviour at times and quantum jumps in properties. Improved fracture toughness of structural ceramics makes them suitable candidates for stringent engineering applications. Composites involving special ceramics display some outstanding properties, e.g. piezoelectrics. Examples of dielectrics, super conductors, optical fibres and ionic conductors are cited.     

A nondestructive approach to correlate the interfacial defects induced during processing of MMCs and CMCs with the consolidation parameters

An ultrasonic nondestructive methodology for evaluating the consolidation and microstructure of advanced fiber-reinforced composites has been developed to aid in their design and fabrication. The use of this nondestructive evaluation (NDE) technique can enable optimization of the processing parameters to obtain complete densification around the fibers. In addition, the methodology can be used to ensure that the composite panels are devoid of any global problems such as fiber swimming, ply delamination, embedded manufacturing anomalies such as voids, etc. Such a post processing NDE is also essential before any interfacial characterization is performed. The technique described in this paper, being generic, is applicable to both metal matrix and ceramic matrix composites.This work was supported by and performed on-site in the Materials Directorate, Wright Laboratory, Materiel Command, Wright-Patterson Air Force Base, Ohio, USA. Contract nos. F33615-94-C-5213 (T. E. Matikas and P. Karpur), F33615-91-C-5663 (S. Krishnamurthy).     

Material processing of hydroxyapatite and titanium alloy (HA/Ti) composite as implant materials using powder metallurgy: A review

The bio-active and biodegradable properties of hydroxyapatite (HA) make this material a preferred candidate for implants such as bone replacement in replacing natural tissues damaged by diseases and accidents. However, the low mechanical strength of HA hinders its application. Combining HA with a biocompatible material with a higher mechanical strength, such as a titanium (Ti) alloy, to form a composite has been of interest to researchers. A HA/Ti composite would possess characteristics essential to modern implant materials, such as bio-inertness, a low Young’s modulus, and high biocompatibility. However, there are issues in the material processing, such as the rheological behavior, stress-shielding, diffusion mechanism and compatibility between the two phases. This paper reviews the HA and Ti alloy interactions under various conditions, in vitro and in vivo tests for HA/Ti composites, and common powder metallurgy processes for HA/Ti composites (e.g., pressing and sintering, isostatic pressing, plasma spraying, and metal injection molding).     

Temperature stable high-Q microwave dielectric ceramics in (1 − x)BaTi4O9-xBaZn2Ti4O11 system

The microwave dielectric properties of (1 − x)BaTi₄O₉-xBaZn₂Ti₄O₁₁ ceramics were investigated by solid-state reaction technique for obtaining high-Q dielectric ceramics in BaO-ZnO-TiO₂ system. And they were strongly determined by the chemical composition. As x was increased from 0.05 to 0.50, BaZn₂Ti₄O₁₁ phase formed more and more. Therefore, the ε_r decreased from 37.3 to 32.8 and the Q × f values first raised from 45,300 GHz to 60,600 GHz (x = 0.30) and then started to decline to 58,700 GHz (x = 0.40), and the τ_f values varied gradually from 12 ppm/°C to − 13 ppm/°C. 0.7BaTi₄O₉-0.3BaZn₂Ti₄O₁₁ ceramics sintered at 1240 °C for 3 h had excellent comprehensive microwave dielectric properties: ε_r = 34.2, Q × f = 60,600 GHz and τ_f = − 2 ppm/°C.     

Microwave processing: A potential technique for preparing NiO-YSZ composite and Ni-YSZ cermet

In the present study, microwave energy (2·45 GHz) has been used to prepare nickel oxide-yttria stabilized zirconia (NiO-YSZ) composites of composition, mNiO-(1 − m) Zr_0·9Y_0·1O_1·95 (m = 0·2, 0·3, 0·4, 0·5 and 0·6), from a precursor obtained by mixing NiO, Y₂O₃ and monoclinic ZrO₂ in their stoichiometric ratio. The composites have been prepared by conventional processing also to compare the products with those of microwave processed products. During comparison, it was observed that NiO-YSZ composites of each composition obtained by microwave processing had cubic phase of YSZ while in the conventionally prepared composites of compositions, m = 0·2 and 0·3, monoclinic, tetragonal and cubic phases of zirconia existed instead of its pure cubic phase. The composites were reduced to yield Ni-YSZ.     

喷射成形颗粒增强金属基复合材料研究进展

喷射成形是将熔融金属雾化和沉积过程相结合,直接由熔融金属制备具有快速凝固组织特征坯体的新型金属材料成形工艺。被引入到非连续增强体金属基复合材料制备领域后,先后发展出了预混喷射成形、反应喷射成形和喷射沉积等工艺方法。介绍了上述颗粒增强金属基复合材料喷射成形工艺方法的基本原理、特点及研究现状。通过对比分析发现喷射沉积技术由于其成形条件限制较少,适用范围广等特点得到了更为广泛的发展,最后结合自身研究对传统喷射沉积技术进行了一定的改进,采用气体-增强颗粒两相流作为雾化介质以期获得提高雾化效率和改善增强颗粒分布均匀性的双重效果。