This article reviews the physical aspects of a cross-disciplinary science and technology field: the microwave processing of materials. High-temperature microwave processing has a clear industrial perspective in such areas as the production of advanced ceramics, the deposition of thermal barrier coatings, the remediation of hazardous wastes etc. This review starts with the relevant fundamental notions regarding the absorption of electromagnetic waves, heat transfer and the electrodynamics of single- and multimode microwave cavities. Useful formulae, estimates, and interrelations between process variables are presented. This is followed by a review of process examples illustrating the specific features of microwave processing: reduction in energy consumption and process duration, rapid and controllable heating, peculiar temperature distribution, and selectivity of energy deposition. Much attention is given to the advantages of higher-frequency millimetre-wave processing, which include the enhanced absorption in many materials of industrial interest, improved uniformity of electromagnetic energy and temperature, and the possibility of surface treatment. The phenomenon of microwave process rate enhancement is addressed in connection with the problem of the non-thermal microwave effect on mass transport in solids. Both experimental and theoretical approaches to the identification of the mechanism responsible for this effect are illustrated. Finally, the physical and technical factors influencing microwave technology scaleup and transfer to industry are discussed.

http://iopscience.iop.org/article/10.1088/0022-3727/34/13/201/pdf

High-temperature microwave processing of materials

The author reviews surface flashover (i.e. voltage breakdown along the surface of insulators), primarily in vacuum. He discusses theories and models relating to surface flashover and pertinent experimental results. Surface flashover of insulators in vacuum generally is initiated by the emission of electrons from the cathode triple junction (the region where the electrode, insulator, and vacuum meet). These electrons usually then multiply as they traverse the insulator surface, either as a surface secondary-electron-emission avalanche or as an electron cascade in a thin surface layer, causing desorption of gas which had been adsorbed on the insulator surface. This desorbed gas is then ionized, which leads to surface flashover of the insulator. The theory and modeling of this phenomena and experimental studies of surface charging, the applied voltage waveform, prestressing, conditioning, discharge delay and speed, insulator geometry AMD material, surface treatment, surface gases, temperature, and pressure are reviewed. Some suggestions are made regarding how to choose the material geometry and processing when selecting an insulator for a particular application. >

Surface flashover of insulators

Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

This work is focused on the comparative analysis of electrochemical and transport properties in the major families of cathode and anode compositions for intermediate-temperature solid oxide fuel cells (SOFCs) and materials science-related factors affecting electrode performance. The first part presents a brief overview of the electrochemical and chemical reactions in SOFCs, specific rate-determining steps of the electrode processes, solid oxide electrolyte ceramics, and effects of partial oxygen ionic and electronic conductivities in the SOFC components. The aspects associated with materials compatibility, thermal expansion, stability, and electrocatalytic behavior are also briefly discussed. Primary attention is centered on the experimental data and approaches reported during the last 10–15 years, reflecting the main challenges in the field of materials development for the ceramic fuel cells.

Electrode materials and reaction mechanisms in solid oxide fuel cells: a brief review

The development of the method of electric explosion of wires and research results concerning preparation of nanopowders by this method has been reviewed. The method is highly productive (up to 200g/h), provides powders with an average particle size of 20–100nm, and requires an energy consumption of about 25kWh/kg. Several characteristics of the nanopowders will be given and their applications will be exemplified too.

Electric Explosion of Wires as a Method for Preparation of Nanopowders

YSZ, YSZ + Al 2 O 3 , Ce 0.78 Gd 0.22 O 2-δ , and 5NdY 2 O 3 nanopowders are obtained using target evaporation with a repetitively pulsed CO 2 laser and subsequent vapor condensation in the flow of a carrier gas. The design of the laser complex for producing the nanopowder and the block diagram and the characteristics of the repetitively pulsed CO 2 laser pumped by a combined discharge are presented. The size distribution of the nanoparticles is studied and the x-ray data are reported. It is demonstrated that a nanopowder output rate of 15-75 g/h linearly increases with the mean laser power. Under equal conditions, the size distribution of the particles is weakly affected by the type of the target material. The results obtained are interpreted.

Laser synthesis of nanopowders

This overview covers investigations and designs that demonstrate the opportunity to use the phenomenon of the electrical explosion of wire to produce weakly aggregated nanopowders of metals, alloys, mixtures, and oxides. Some general regularities that facilitate the choice of the circuit parameters and dimensions of the exploding wire necessary for producing particles of the required size are described. The limitations to synthesizing the nanopowders of some metals and oxides are indicated. The method is environmentally clean, offers sufficiently high output capacity, and is energy efficient.

The electrical explosion of wire: A method for the synthesis of weakly aggregated nanopowders

Synthesis and dynamic compaction of ceramic nano powders by techniques based on electric pulsed power

A novel nanosecond semiconductor opening switch (SOS) has been developed which has a pulsed power of the GW range and voltage levels of up to a few 100's of kV. The SOS is based on high-voltage solid state rectifiers and is designed for pulsed power generators with inductive energy storage. A 30 115 opening phase duration. a 45 k A interrupted current. and a 450 kV opened SOS voltage have been attained with the use of a three-stage, 2 kJ. 150 kV open circuit Marx generator as the SOS driver. On the basis of the experimental results obtained, we have developed and tested repetitive high-current generators and accelerators with a 0.5 MV output voltage and a 15 to 100 ns pulse width. The ideology is presented of constructing high-power megavolt pulsed generators with an all-solid-state switching system. A description is provided of the setups developed on this principle. We discuss features peculiar to the setups developed and prospects of developing these further .

Yu. A. Kotov

Papers

Electric Explosion of Wires as a Method for Preparation of Nanopowders

Laser synthesis of nanopowders

The electrical explosion of wire: A method for the synthesis of weakly aggregated nanopowders

Synthesis and dynamic compaction of ceramic nano powders by techniques based on electric pulsed power

A novel nanosecond semiconductor opening switch for megavolt repetitive pulsed power technology: exp