Processing of Advanced Materials with a High Frequency, Millimeter- Wave Beam Source and Other Microwave Systems

Materials Science and Component Technology Directorate, Naval Research Laboratory,4555 Overlook Avenue S.W., Washington DC 20375-5343, U. S. A.'E-mail: imam@anvil.nr.navy.milKeywords: microwave processing, millimeter-wave processing, ceramics, materials, joining,coating, coating removal, rapid sinteringAbstract. An 83 GHz gyrotron-based, millimeter-wave beam system is being used in materialprocessing: in rapid sintering of oxides, ferrites, and metal-ceramic composites to retain very finegrain structure in the product for improved mechanical and electromagnetic properties; in joining ofceramic and ceramic composite materials (with unique advantages- localization of heating, permit-ting inexpensive fixturing and instrumentation and minimizing thermal damage to components; de-positing energy specifically in a narrow joint region through a guided wave effect), and in coatingdensification and coating removal where the high frequency and short wavelength permit significantenergy deposition in relatively thin coatings. In a related effort, we are using a low frequency mi-crowave system for low cost continuous production of nanophase metals. This process should pro-duce a range of nanophase metals, metal oxides and mixtures of these, in sizeable quantities and atlow cost. The results of various experiments in these areas will be discussed, as will the potential oflow and high frequency microwave processing of advanced materials.IntroductionThe authors have been investigating the use of microwave and millimeter-wave systems inmaterial processing. Such systems can have unique advantages or capabilities vs. conventionalmaterial processing. We have investigated the use of 2.45 GHz and 35 GHz cavity systems in rapidthermal processing of ceramics [1-6]. We are currently using two particular systems: an S-Band,2.45 GHz system for continuous production of nanophase oxides and metals, and an 83 GHz, mil-limeter-wave beam system for sintering, joining, coating and coating removal [7-13]. Presently, theS-Band system uses a waveguide or a resonant cavity, while the millimeter-wave system operates ina quasi-optical mode, with the millimeter-wave beam controlled by reflective optics.Typically, microwave processing is promoted on the basis of thermal efficiency or rapidheating via in-depth energy deposition. We have been exploring the use of the millimeter-wavesystem in very rapid sintering of nanophase materials, particularly ferrites. Here, the in-depthheating permits very rapid processing (<10 minute cycles) intended to preserve very fine grainstructure in the product. For small components, cycle times of less than 1 minute are possible.Theory suggests that the nature of the magnetic behavior of these ferrite materials is expected tochange dramatically at grain sizes comparable to the magnetic domain. In the case of other materi-als, the fine grain size obtained leads to excellent mechanical properties and the possibility of su-perplastic net shape forming- a possible processing route for ceramics is rapid sintering of powdercompacts to nanophase green parts, then consolidation to full density and simultaneous shaping viausing superplastic forming.We are also investigating the use of the millimeter-wave beam system in joining of ceram-ics, applying coatings to ceramics and metals, and removal of coatings from metals and composites.Here, the millimeter-wave beam system provides unique advantages. The heating effects can beconfined to an area as small as a square centimeter, permitting very localized thermal processing.