The Naval Research Laboratory has been investigating the use of several different microwave and millimeter-wave systems in material processing. There are a number of instances in which such systems have unique advantages over conventional processing, or provide unique capabilities. In the past we have investigated the use of 2.45 and 35 GHz resonant and non-resonant cavity systems in rapid thermal processing of ceramics and continuous production of nanophase oxides and metals. We are currently exploring the use of two particular systems: an S-Band, 2.45 GHz system for sintering and melting of metal and alloys powders, and an 83 GHz, millimeter-wave beam system for a variety of material processing including sintering and joining of ceramics, coating and coating removal. Both systems have advantages for certain types of materials processing. Presently, the processing in the S-band system is done in a waveguide or a resonant cavity, while the millimeter-wave system operates in a quasi-optical mode, with the millimeter-wave beam controlled by reflective optics.
Typically, microwave processing is promoted on the basis of thermal efficiency or rapid heating via in-depth energy deposition. We have been exploring the use of the millimeter-wave system in very rapid sintering of nanophase materials, particularly ferrites. Here, the in-depth heating permits very rapid processing (cycle times of less than 10 minutes) that is intended to preserve a very fine grain structure in the final product. For small components, cycle times of less than 1 minute can be achieved. Theory suggests that the nature of the magnetic behavior of these ferrites is expected to change dramatically if the grain size can be reduced to the size of a magnetic domain, i.e., grains contain only a single domain. In the case of rapid sintering of other materials, the fine grain size obtained can lead to excellent mechanical properties, and to the possibility of superplastic forming of such materials to net shape components. One possible processing route for ceramic processing is rapid sintering of powder compacts to produce nanophase green parts, which in turn are consolidated to full density and shaped simultaneously using superplastic forming techniques such as hot forging.
We have also been investigating the use of the millimeter-wave beam system in joining of ceramics, applying coatings to ceramics and metals, and removal of coatings from metals and composites. Here, the characteristics of the millimeter-wave beam system provide unique advantages. In the beam system, the heating effects can be confined to an area as small as a square centimeter, if desired, permitting very localized thermal processing. In addition, through a guided wave effect, the millimeter-wave energy can be confined to a joint region as small as 50-100 microns for even more localized effects. The short wavelength of the millimeter-wave energy, 3.6 mm in air, about 0.3-2 mm in most ceramics, permits deposition of significant amounts of energy in even relatively thin polymeric and ceramic coatings and films. The localization of heating here has several advantages. One is the ability to do rapid, localized high temperature processing, e.g., heat treating of joints in ceramics or densification of ceramic coatings, while minimizing damage to thermally sensitive components or substrates. The same advantage pertains in the case of removal of coatings from thermally sensitive substrates, e.g., aluminum, fiber reinforced plastic. In the case of the beam system, the localization of heating, and the lack of coupling of the millimeter-wave to metals, permits the use of inexpensive, base metal fixturing and instrumentation. This can be a major advantage in the case of joining of ceramics, where it is generally desirable to provide for alignment and pressure on the joint during processing. In a conventional system, where everything in the processing chamber is heated, such fixturing and instrumentation must survive the same thermal conditions to which the joint is subjected. In the case of joining of high temperature ceramics, this necessitates the use of either superalloy or ceramic fixturing, at great cost and requiring long lead times for procurement.
We patented the use of an S-band system in continuous production of nanophase metals and metal oxides. Here the interest has been in providing an economical, well-controlled continuous analog to the polyol process used extensively for production of nanophase metal powders. In conventional polyol processing, and batch microwave polyol processing, the amounts of materials produced are exceedingly small (g/day) and the resultant material costs are far beyond what most applications would support. We have developed a continuous microwave process that should be capable of producing much larger quantities (kg/day) at much lower cost and potentially of much higher quality.
Recently microwave processing experiments are focused to study reactive sintering of neodymium doped yttria aluminum garnet (YAG) using an 83 GHz beam to obtain near transparent laser host material, and the sintering of complex shape titanium compacts in a multimode 2.45 GHz furnace to reduce the cost of processing titanium parts needed for wide range of applications.