Ralph W. Bruce

A study of millimeter-wave sintering of fine-grained alumina compacts

A number of high-frequency microwave sintering studies of alumina have reported that sintering proceeds much faster in microwave furnaces when compared to conventional furnaces, and that densification can occur at lower temperatures. These differences have motivated the search for a nonthermal microwave enhancement effect such as the time-averaged microwave field-induced mass transport effect proposed by Rybakov and Semenov (1994). To assess the difference between microwave and conventional sintering, and the presence of a nonthermal effect in microwave sintering, a study of millimeter-wave (mm-wave) (35 GHz) sintering has been conducted at the Naval Research Laboratory (NRL) using a well-studied fine-grained (submicron) commercial alumina with reproducibly manufactured properties, Sumitomo AKP-50. This paper reports our results, which generally indicate no large differences in the required temperatures for densification of conventionally and microwave sintered compacts, or between the resulting microstructures. The nonthermal effect proposed by Ryakov and Semenov was evaluated for fine-grained alumina and found to be small compared to the surface energy driving force for sintering.

Effects of Proton-Induced Displacement Damage on Gallium Nitride HEMTs in RF Power Amplifier Applications

The effects of proton-induced displacement damage in GaN HEMTs on circuit-level RF power amplifier parameters such as circuit gain, stability, and RF output power are presented. The results are explained based on the device-level degradation. Commercial-off-the-shelf GaN HEMTs from two manufacturers were compared. Differences are observed in both device and circuit level responses. Suggestions to mitigate the negative effects of displacement damage on GaN based amplifiers are also provided.

Joining of ceramic tubes using a high-power 83-GHz millimeter-wave beam

High purity, high density alumina tubes have been successfully joined using a high-power millimeter-wave beam. This technique exploits the use of the beam-forming capability of an 83-GHz gyrotron-based system allowing the deposition of energy into a narrow region surrounding the joint area with minimal heating (<100/spl deg/C) of the metal fixturing (a modified microlathe). The power deposition and heating was modeled using a closed form analytical approach that has been compared with experimental results. The modeling results indicated areas of improvement that were implemented to make the process more effective. Conjoined tubes resulting from this technology meet the requirements for the dielectric-loaded accelerator (DLA) being developed by the Argonne National Laboratory.

New Frontiers in the Use of Microwave Energy: Power and Metrology

The Microwave/Radio Frequency portion of the electromagnetic spectrum has been used principally for the transmission and reception of information in communications and radar. With the advent of low cost commercial and consumer items (satellite downlinks and microwave ovens) has also come a resurgence of interest in using the energy and properties of these frequencies for other important tasks. The purpose of this paper is to give a short review of the basics of electromagnetic heating and an overview of new trends in this use of microwave/RF energy. These trends can be divided into two broad categories: power and metrology.

Millimeter-Wave Driven Polyol Processing of Nanocrystalline Metals

Nanocrystalline metallic powders and coatings have been synthesized using a millimeter wave driven polyol process. We have been able to prepare powders of single elements, alloys, metastable alloys, composites and coatings. Examples of a few of the metals processed in this study include Fe, Co, Ni, Cu, Ru, Rh, Pt, Au, FePt, Fe x Co 100−x , NiAg and Cu-Ni. The polyol experiment was set up in the millimeter wave processing chamber, the beam was directed into the center of the solution and it was brought to reflux, using the millimeter wave beam as a heat source. Varying the power input easily controlled the rate of reflux.

High-powered 83-GHz millimeter-wave beam sintering and heat treating of engineered ceramics in controlled atmospheres

Summary form only given. An experimental facility based on an 83-GHz, 15 kW CW industrial gyrotron has been set up at the Naval Research Laboratory to investigate novel mm-wave-beam-based approaches to processing ceramic materials, including sintering, coating, and joining applications. Current experimental and theoretical studies are investigating the sintering and heat-treating of advanced engineered ceramics such as ferrites, rare-earth magnet materials and laser hosts. Sintering and heat-treating of these materials requires stringent atmospheric and environmental control, including high vacuum, inert or reducing atmospheres or precise levels of oxygen partial pressure. Past work has described initial results using a large, 1 m/sup 3/ chamber. In order to more adequately control the processing conditions, a small, specialized chamber has been devised to fit within the large chamber. This specialized chamber has a variety of gas and electrical feedthroughs as well as a tuned window for RF transmission and visual monitoring. In this paper, the results of current experiments will be described. In particular, the results from the processing of nanopowder derived ferrites; rare-earth magnet materials and laser host materials will be presented.

Consolidation of Blended Titanium/Magnesium Powders by Microwave Processing

Mg-Ti alloys are attractive for structural applications because of low density and improved corrosion resistance by selective oxidation including hydrogen storage and switchable mirror applications. Titanium has a melting point (1670°C) that greatly exceeds the boiling point of magnesium (1090°C) and therefore, alloying of Mg and Ti by conventional methods is extremely difficult. Secondly, the solubility of Ti in liquid Mg is very low and it is difficult to extend solubility by rapid solidification. Physical vapor deposition by electron beam deposition and magnetron co-sputtering has been used to extend the solubility of Ti in Mg. Mechanical alloying and anvil-cell processing at extreme temperatures and pressures have also used to enforce alloying of Mg with Ti. The present paper deals with the consolidation of blended magnesium-titanium powders by microwave heating, an approach that appears highly cost effective.

Consolidation of Polycrystalline Yttria Powder By Millimeter-Wave Sintering for Laser Host Applications

Summary form only given. We report recent results of an investigation of millimeter-wave processing of yttria (Y2O3) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for high energy laser (HEL) applications. The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000degC and the high melting temperature which is over 2400degC. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single-crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.

Modeling the millimeter-wave beam joining of ceramic tubes

Summary form only given. The high power beams that can be generated by CW gyrotrons represent a promising energy source for high-temperature processing of materials. To effectively use these sources, modeling of the beam interaction with candidate materials, e.g., ceramics, can give significant insight into the field and power distribution within the material. From these field assessments, heat generation with the material can be determined and coupled with thermal transfer analysis, heating and heat flow can be assessed. In this paper, a method of evaluating the time-dependent temperature profile of ceramic cylinders was discussed. Both solid cylinders and tubes have been analyzed. For the tubes, the region is treated as a composite system comprised of cylinders of dielectric material having different dielectric and thermal properties. Theoretical results was compared to qualitative results from current research into the joining of ceramic cylinders that use the 83 GHz gyrotron based material processing system at NRL.

Sintering of ceramic compacts in a 35 GHz gyrotron-powered furnace

Summary form only given, as follows. The development of powerful gyrotrons has opened up the millimeter-wave regime (/spl ges/28 GHz) for processing ceramic materials. Millimeter-waves couple more strongly than pure oxides, eliminating the need for auxiliary heating at low temperatures, and highly uniform fields intensities can be achieved in compact overmoded cavity applicators. A number of low and high frequency microwave sintering studies have generally indicated that sintering proceeds much faster in microwave furnaces than in conventional furnaces, and that densification occurs at lower temperatures. Lower sintering temperatures are desirable for minimizing grain growth which usually has a negative effect on mechanical properties of ceramics. To assess the potential of high frequency microwave sintering, and to investigate the possibility of a specific microwave mechanism, the Naval Research Laboratory (NRL) has recently undertaken a systematic study focused on alumina because of its industrial importance, large data base, and challenging microwave properties. This paper presents 35 GHz sintering data obtained at NRL, using a gyrotron-powered furnace, for fine grain (submicron) alumina compacts and compares the data with results from other high frequency microwave and conventional sintering studies. The results indicate that the microwave sintering process appears to densify more quickly at a given temperature than in a conventional furnace.