TABLE 4-1 Some Key Characteristics and Features of Microwave Processing



Benefits (over conventional heating)


1. Penetrating radiation, direct bulk heating

• Materials heat internally

• Reversed thermal gradients (ΔT)

• Lower surface temperatures

• Instantaneous power/temperature response

• Low thermal mass

• Applicator can be remote from power source

• Potential to heat large sections uniformly

• ΔT favors chemical vapor infiltration; matrix infiltration

• Reduced skin effect on drying

• Removal of binders & gases without cracking

• Improved product quality and yields

• Materials & composite synthesis

• Automation, precise temp. control

• Rapid response to power level; pulsed power

• Heat in clean environment

• Materials synthesis

• See differential coupling

• M/W transparent materials difficult to heat

• Hot spots, cracking

• Large ΔT in low thermal conductivity materials, and nonuniform heating

• Controlling internal temperature

• Arcing, plasmas

• Require new equipment designs special reaction vessels

2. Field distributions can be controlled

• High energy concentration

• Optimize power level versus time

• mm-waves can be focused or defocused, rastered as desired

• Precise heating of selected regions (brazing, welding, plasma generation, fiber drawing)

• Process automation, flexibility, energy saving

• Synthesis of materials, composites, powders, coatings

• Equipment more costly and complex

• Requires specialized equipment

3. Dielectric losses accelerate rapidly above Tcnt

• Very rapid heating

• Rapid processing (2-1000x factor)

• Heat materials > 2000 ºC

• Capable to heat M/W transparent materials > Tcnt

• Hot spots, arcing

• Nonuniform temperature

• Control of thermal runaway

4. Differential coupling of materials

• Selective heating of internal or surface phases, additives or constituents

• Heating of M/W transparent, materials via additives, fugitive phases, etc.

• Hybrid heating (active containers)

• Materials synthesis

• Selective zone heating (joining, brazing, sealing)

• Controlled chemical reactions, oxidation, reduction; use of M/W transparent containers

• Drying, curing, annealing; matrix infiltration

• Reactions with unwanted impurities

• Contamination with insulation or other phases

5. Self-limiting

• Selective heating ceases (self regulating) after certain processes have been completed

• Below critical temperature, drying & curing are self-regulating

• Completion of certain phase changes is self-regulating

• Undesired decoupling during heating in certain products

• Difficult to maintain temp.

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