Microwave processing

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Introduction

Microwave processing is not a new idea, however has not been implemented openly by industry until recent years. Since the 1940's microwaves have been implemented in food processing sectors. The technology required to heat materials via microwaes can be seen in almost every kitchen across north america. However, microwave processing is only starting to gain strength in the materials processing sector. Improvements in the understanding of microwave absorption properties, and new techniques for improving absorption are opening up new fields of application for a relatively old technology. 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 capacity to efficiently absorb set energy. As shown by the standard kitchen microwave oven, this is a quite possible feat for a range of materials, and is quite fast comparatively to other heating methods such as convection oven. Application of such benefits for high temperature heating process such as powder metallurgy, drying and surface treatments.

In comparison to conventional heating methods, microwave processing has an inherit benefit of increased efficiency. By transferring energy via electromagnetic waves, heat transfer is not limited to only particles on the surface of a material, improving the rate of energy transfer in comparison to conventional heating. As energy is transferred using electromagnetic waves ability to penetrate surface layers, a new temperature profile exist for microwave processing, As heating is no longer dependent upon surface area, and is now dependent upon volume. In contrast to conventional heating, microwave processing has an inverse heating scheme. As electromagnetic penetrate the material, heating is modeled relative to the volume of material absorbing energy.

Dielectric Properties

The interaction of electromagnetic radiation with materials, causing heat transfer is understood to work via the mechanisms of conduction and polarization. The electromagnetic wave causes movement, or rotation of dipoles, adding energy to a material.

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 permissibility. 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. Factor effecting absorbency are out lined below:

Frequency - 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. Material testing tends to be around < 2.4 GHz, roughly the wavelength of a consumer microwave oven. High frequencies up to the range of 300 GH have shown increases in non-absorbent materials, and use for applications with the need for high rates of heating.

Temperature – During the heating of materials it is seen 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 dieletric properties, depth affects where the heat is transferred to. In a convection oven, heat is transferred by infared 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.

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.

Applications

Powder Metallurgy

The application of microwave processing to powder metallurgy looks to be extremely advantageous in processing. Through use of this technology, process time has been seen to decrease, through fast, uniform heating. A significant decrease in energy requirements during processing, testing has shown over ten fold decreases in specific cases. Final products have shown advantageous over old methods including smaller, finer grain size and increasingly uniform micro structures.

Application currently is applied only to ceramic and semi metals products, as metallic materials tend to be highly reflective and not absorb heat. Low penetration therefore negates the majority of the benefits. The reflectivity issue can be avoided through the use of alloys (semi metals). In the last decade the field of powdered metals has emerged, as powdered unsintered metals tend to absorb heat very well. 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.

Polymer Curing

Studies have looked at processing of polymers such as polyesters, polyimides, polyurethanes, and epoxies, presenting reductions in curing time, along with property enhancements. Mechanical properties were affected by affecting chemical interactions during polymerization.

Post Fabrication Processing

Given a focused electromagnetic wave on the surface of the appropriate material at an appropriate frequency as to impose heating at the surface, the micro structure of the surface of the material is alterable. Uing microwave processing high temperature surface processing is thought believable.

References

1. Clark D, and Sutton H 1996 Microwave Processing of Materials Annu. Rev. Mater. Sci. 26: 299-331 2. Bykov Y V et al 2001 High temperature microwave processing Journal of Physics D: Applied Physics 3. Agrawal D 1998 Microwave processing of ceramics , Curr. Opin. Solid-State Mater Sci. 3

Optimization Techniques

Use of a Resonator – explained above under reflection Frequency invisible Layer – 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. [] 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. [] Solutes [] – objective pending solute may be added to materials in order to increase efficiency. Use with of high gain solutes in low gain materials, may alter the temperature absorbance profile. [] Application - Proper choices of frequency, Field strength, temperature control, and insulation in the furnace will control the final efficiency.

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