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.

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

Microwave heating is rapidly emerging as an effective and efficient tool in various technological and scientific fields. A comprehensive understanding of the fundamentals of microwave-matter interactions is the precondition for better utilization of microwave technology. However, microwave heating is usually only known as dielectric heating, and the contribution of the magnetic field component of microwaves is often ignored, which, in fact, contributes greatly to microwave heating of some aqueous electrolyte solutions, magnetic dielectric materials and certain conductive powder materials, etc. This paper focuses on this point and presents a careful review of microwave heating mechanisms in a comprehensive manner. Moreover, in addition to the acknowledged conventional microwave heating mechanisms, the special interaction mechanisms between microwave and metal-based materials are attracting increasing interest for a variety of metallurgical, plasma and discharge applications, and therefore are reviewed particularly regarding the aspects of the reflection, heating and discharge effects. Finally, several distinct strategies to improve microwave energy utilization efficiencies are proposed and discussed with the aim of tackling the energy-efficiency-related issues arising from the application of microwave heating. This work can present a strategic guideline for the developed understanding and utilization of the microwave heating technology.

Review on Microwave-Matter Interaction Fundamentals and Efficient Microwave-Associated Heating Strategies

Oxidation of Metals and Alloys By Dr. O. Kubaschewski and B. E. Hopkins. Pp. xv + 239. (London: Butterworths Scientific Publications, Ltd.; New York: Academic Press, Inc., 1953.) 35s. net.

The Oxidation of Metals

Enhanced mass transport during the high temperature processing of ceramics by microwave heating, generally known as the 'microwave effect', has been observed by many researchers and some hypotheses have been proposed to explain it. However it is still a controversial issue, centring on temperature measurement accuracy. The primary goal for the present work was to achieve definitive evidence of the microwave effect via the microwave hybrid processing of ceramics using accurate temperature measurement.


Three temperature measurement techniques, viz. thermocouple, pyrometer and optical fibre thermometer, have been investigated to determine their accuracy during the hybrid microwave heating of ceramic materials. The experimental results, obtained from melting point measurement on vanadium pentoxide and the heating of three other materials with different loss factors and thermal conductivities, showed that the optical fibre thermometer was the best choice. An accuracy of ±2oC was obtained,
provided the probe had been calibrated and was protected from stray radiation from conventional heating elements.


Direct evidence for the microwave effect was sought from experiments involving the sintering and annealing of ceramics at different microwave power levels but using an identical temperature I time profile in each case. Sample temperature was monitored using optical fibre thermometry and sample size, composition and precursor powder size were all varied. Specifically, four main ceramics with different loss factors were investigated, viz. zinc oxide (very lossy), yttria stabilised zirconia (intermediate loss) and alumina and silica (low loss).


The results from the experiments showed that an increase in final density was observed for the samples that saw the highest microwave power level compared to


I
 
those sintered using pure conventional heating, nearly 25% in the largest case for zinc oxide with the highest loss factor among the four investigated materials. This was equivalent to an effective temperature increase of approximately 1OOoC. For the low
loss alumina samples, the increase was extremely small, with the yttria stabilised zirconia intermediate between the two. In addition, during annealing of fully dense ZnO ceramic pellets, enhanced grain growth was observed during hybrid heating compared to pure conventional heating. Temperature gradients within the samples, a major potential cause of the effect, were assessed using two different approaches and found to be

Evidence for the microwave effect during hybrid sintering and annealing of ceramics

Microwave material processing is a novel energy efficient technology with improved mechanical properties, minimized defects and economical and environmental advantages making it a convenient application for various types of materials. Although, microwave interaction with matter has been largely investigated and published in food processing, ceramics and chemistry, no particular work has been involved in collecting the interactions of microwaves with metals and placing a special emphasis on their interaction with metals and metal-based formulations, and here resides the aim of the review: consolidating the fundamentals of microwave heating applications as a time and energy saving application and addressing its various applications and mechanisms with metal interactions seeking a more sustainable environment. This review reports the latest literature findings on microwave processing fundamentals and highlights the advanced technological improvement applied on metals in this field. It focuses on the relevant industrial applications related to the development of microwave technology on metals and its possible future processing in this specific scope of investigation.

Microwave dielectric heating: Applications on metals processing

Glaze in the CaO–MgO–Al2O3–SiO2 system was heated at 950–1190 °C for 2 h and characterized. X-ray diffraction showed that only trace amount of mullite was formed in the glass-ceramic glaze heated at 950 °C. Both mullite and α-cordierite were formed in the glass-ceramic glaze heated at 1050 °C as primary and secondary phases. Glass-ceramic glazes heated at 1120 °C and 1190 °C contained α-cordierite and mullite as major and minor phases. Rietveld analysis revealed that the amount of α-cordierite increased and mullite decreased with increasing heating temperature. Field emission scanning electron microscopy showed presence of mullite crystals dispersed within residual glassy phase in the glass-ceramic glazes heated at 950 °C and 1050 °C. In the microstructures of glass-ceramic glazes heated at 1120 °C and 1190 °C α-cordierite crystals were mainly appeared. Energy Dispersive X-ray analysis corroborated X-ray diffraction results. Vickers microhardness measurement demonstrated highest hardness (8.38 ± 0.07 GPa) of the glass-ceramic glaze heated at 1190 °C.

Glass-ceramic glazes for future generation floor tiles

Characterization of alumina–alumina/graphite/monel superalloy brazed joints

Characterization of microwave processed aluminium powder

Alumina was joined with alumina using microwave-assisted and conventional brazing methods at 960
 ∘
 C for 15 min using TiCuSil (68.8Ag–26.7Cu–4.5Ti in wt.%) as the brazing alloy. The brazed joints were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, Vickers microhardness evaluation, brazing strength measurement and helium leak test. X-ray diffraction analysis confirmed the formation of Ti-based compounds at the substrate-filler alloy interfaces of the microwave and conventionally brazed joints. The elemental compositions at the joint cross-section were determined by energy dispersive X-ray analysis. Vickers microhardness measurement indicated reliable joint performance for the microwave-assisted brazed joints during actual application in an electron tube. Brazing strength measurement and helium leak test provided the evidence for good alumina-alumina joint formation.

/pdf/microwave-assisted-brazing-of-alumina-ceramics-for-electron-h8voh5brif.pdf

Microwave-assisted brazing of alumina ceramics for electron tube applications

The present investigation demonstrated the comparative studies carried out on conventional, microwave and vacuum sintering of alumina added yttria stabilized zirconia (YSZ). The conventional, microwave and vacuum sintered specimens were characterized by density measurement, XRD, SEM with EDX analysis and hardness evaluation. Microwave sintering was proved to be the best efficient sintering technique with respect to energy and time savings. Enhanced densification was observed for the microwave and vacuum sintered specimens at lower temperatures compared to the conventionally sintered ones. Further, it was observed that the particle size had significant influence on the enhancement of densification. The microwave sintered specimen showed the highest hardness compared to conventional and vacuum sintered specimens.

/pdf/comparative-study-on-conventional-sintering-with-microwave-j6qr65i54z.pdf

Sumana Ghosh

Papers

Glass-ceramic glazes for future generation floor tiles

Characterization of alumina–alumina/graphite/monel superalloy brazed joints

Characterization of microwave processed aluminium powder

Microwave-assisted brazing of alumina ceramics for electron tube applications

Comparative Study on Conventional Sintering with Microwave Sintering and Vacuum Sintering of Y2O3-Al2O3-ZrO2 Ceramics