Microwave processing

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Introduction

Microwave ovens were introduced as a food processing appliance in the 1940s. The technology required to heat materials via microwaves can be found commonly in kitchens across North America. Despite its ubiquity in our lives, microwave processing is only recently starting to develop in the materials processing sector. Improvements in the understanding of microwave absorption properties have led to the introduction of new techniques for improving absorption. In turn, new applications for this relatively old technology are being realized. Recent advances have presented the world with continuous microwave systems for commercialization, the ability to sinter powdered metals, and transparent ceramics.

Thermo Physical Properties of Microwave Heating

Heating of a material using electromagnetic energy is based on a material's capacity to efficiently absorb set energy. As shown by the standard kitchen microwave oven, it is quite possible for a range of materials and is quick in comparison to conventional heating methods.

In comparison to conventional heating methods, microwave processing has the possibility of increased materials and energy efficiency. By transferring energy via electromagnetic waves, heat transfer is not limited to only particles on the surface of a material, but is transferable to all particles, allowing for increased rates of heat transfer. . As energy is transferred using ^W, having the ability to penetrate surface layers, a new temperature profile exists for microwave processing. As heating is no longer dependent upon surface area, now dependent upon volume, an inverse heating profile is present. Efficiency of conversion of electromagnetic radiation to heat is however dependent upon a number of factors.

^W Properties

Interaction of electromagnetic radiation with materials producing heat works via the mechanisms of conduction and ^W. Electromagnetic waves enable a dipolar reorganization, through movement, and rotation of dipoles, thereby adding energy to a material^[1]. Conversion of energy tends to be around 50% efficient, material depending. Losses result from both ionic conduction, and polarization.

Dielectric properties of materials vary in accordance to there molecular structure, atomic bond strength and type. Research into dielectric properties is still lacking, ,and corresponding relationship's have failed to accurately predict the precise properties of material's dielectric ^W. Although, the interaction between microwaves and materials is understood, this is thought to be relative to factors,such as structure and bond type having significant effects upon the permissibility . The dielectric properties, effect the absorbency and reflection coefficients of materials, causing a variety of problems, from the low penetration of electromagnetic waves to frequency transparency.

Absorption

Depending upon dielectric properties, the absorption coefficient of a material such as a ceramic may vary greatly. Varying with time, temperature, field power and volume, the efficiency of absorbency should be viewed as a changing value. Material's with low absorption coefficients, are viewed as transparent, where as penetration is complete.

Factors affecting absorbency are out lined below:

• Frequency Dependence- A frequency which may be ideal for heating one ceramic, may be invisible to another ceramic, depending upon, dielectric or mechanical properties such as packing factor composition and temperature. High frequencies up to the range of 300 GHz have shown increases in non-absorbent materials, and use for applications with the need for high rates of heating.

• Temperature Dependence– During the heating of materials it is observed the rate of absorption increases with temperature. It is noted that this property leads to an acceleration in heat transfer, in high temperature heating, causing processing issues, such as runaway, where temperature changes faster then initially planed for a process.

• Penetration Depth - Dependent upon frequency and dielectric properties, depth affects where the heat is transferred to. In a convection oven, heat is transferred by infrared frequency radiation, and conduction occurs across the surface area.

Reflection

When a wave is incident, to a surface plane, i.e. passing from air to a solid. A portion of the incident wave will be reflected by the surface.

Application to ^W

Processing Benefits

The application of microwave processing to powder metallurgy is advantageous in processing. Associated benefits appear to be the result of the inverse temperature profile, and atomic excitement during sintering.

• Increased Production
• selective heating
• Uniform heating
• Precise control of heating ^[1]
• Grain Structure – marginally smaller grain growth is experienced using microwave processing rather than conventional heating. Most commonly smaller grains are found around the edges of sintered materials. Theoretically this is explained by by the inverse heating profile associated with processing and the direction of energy flow. Also, the reduction in sintering time reduces the amount grian growth associated with sintering.
• Density – Sintering of ceramics, has shown a minor increase in density over conventional heating. ^[2].
• Mechanical Properties – As result of microwave processing enhanced mechanical properties of hardness, and strength are obtained. The decrease in surface grain size and increase in density are seen as responsible.

Ceramics

oxides

non-oxides

Alloys

Metals

Low penetration therefore negates the majority of the benefits. The reflectivity issue can be avoided through the use of alloys (semi metals). Testing can produce highly sinter bodies in very short periods of time, with mechanical properties of modulus or rupture and hardness high then that of conventionally prepared samples.

Optimization of Energy Efficiency=

For many material's initial absorption levels are very low, techniques discussed are ways to circumvent this issue.

Resonating Cavity

Using the principal of reflection efficiency of a microwave heating can be increased. Through the use of a reflective metal cavity, the bouncing of electromagnetic radiation through a material can greatly increase absorption. A given wave will effectively pass through a sample multiple times increasing the electromagnetic field strength. Constructive interference between electromagnetic waves, will produces greater field strengths, using a frequency near the resignation frequency of the resonator is there fore ideal. Use of resonating arrangement is applicable to both high and low loss materials. Single moded, depending upon application single or multi-moded cavities are preferred. A single moded cavity produces a single stannding wave pattern, whereas a muti-moded contains multiple standing wave patterns ^[2]Single moded cavities require set volume, to produce the desired standing wave, detracting from there implementation in industry as there is a limited processing volume. Whereas, cavities with volumes greater than the wavelength used are preferential, due to over lapping modes causing more uniform heating^[3].

Layers, Coatings and Shells

Use of an insulating frequency invisible coating, or shell, allowing energy to pass through at high frequencies while slowing energy leaving via convection. Frequency visible – In cases of low gain materials (low conduction constants), use of a highly absorbent material around the outside will force heat to conduct in to the desired subject material.

Hybrid Ovens

Preheating via other techniques – hybridization of traditional convection heating systems with modern materials processing systems is commonly found in industry. As in many materials, the absorption coefficient increases relative to temperature. In many instances with low gain materials, it is common for a preheating stage to occur using conventional heating, or for the two to be used in conjunction^[4].

Addition of Solutes

• Solutes – objective pending solute may be added to materials in order to increase efficiency. Use with of high gain solutes in low gain materials is to alter the temperature absorption profile.

Technical Specifications

Proper choices of frequency, Field strength, temperature control, and insulation in the furnace will control the final efficiency of any process. However, even though microwaves consisting of 1 to 300 Ghz spectrum could be used. However, much of the microwave frequency range is currently in reserved for communication and radar technology. Only two specific frequencies are of general use for industrial and scientific, applications, 915 MHz, 2.45 GHz. Field strength limits are set at 10 mV/m at 1600 m for industrial heater. ^[2]

Economics

Expected sintering costs using microwave processing has estimated as costing up to $0.40^[2] and as low as $0.155^[5]. Variation raises from differences in modeling, models differed, mainly relating to facets of scale, price of electricity, and material processed. The higher price eing for alumina, the lower for an Average ceramic.^[2] The power conversion efficiency of using microwave heating efficiency is thought to be approximately 80-90%. However, considering that converting fossil fuels to electricity is 30%-40% efficient and conversion of electricity to microwaves is approximately 50% efficient. An over all efficiency of 12%, is obtained. Whereas, conventional burning of fossil fuels, directly producing heat around 40% efficient. However application of heat directly to sintering process is roughly 40% efficient. The use of other power sources is possible, the use of microwaves rather then an electrical oven has been estimated as 90% ^[6] more efficient.

References

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