To capitilize on the special advantages of gyrotron radiation for fast and continuous materials processing, surface and interface layers are ‘alloyed’ with lossy chemicals and fine-disperse powders. The principles and practical uses are described herein.

To make meaningful comparisons between conventional and microwave processing of materials, one must conduct experiments that are as similar as possible in the two environments. Particular attention must be given to thermal conditions, sample parameters, and furnace environment. Under thermal conditions, one must consider temperature measurement (pyrometer or thermocouple, sheath type, and arcing of thermocouples), thermal history (heating and cooling rates, thermal gradients), and exothermic reactions. Regarding sample parameters, one must. consider sample size, and packing powders and insulation systems. With respect to furnaces, one must consider differences in atmosphere, impurities, and uniformity of heating. Examples will be drawn from diffusion, grain growth, sintering, nitridation, and drying experiments conducted at the Oak Ridge National Laboratory (ORNL) over the past six years.

We have used 2.0-μsec microwave pulses at a frequency of 2.856 GHz to rapidly heat thin amorphous yttrium-barium-copper-oxide (YBCO) films deposited onto silicon substrates. The samples were irradiated inside a WR-284 waveguide by single-pass TE10 pulses in a traveling wave geometry. X-ray diffractometry studies show that an amorphous-to-crystalline phase transition occurs for incident pulse powers exceeding about 6 MW, in which case the amorphous YBCO layer is converted to Y2BaCuO5. Microscopy of the irradiated film reveals that the phase transition is brought about by melting of the YBCO precursor film and crystallization of the molten layer upon solidification. Time-resolved in situ experiments of the microwave reflectivity (R) and transmissivity (T) show that there is an abrupt change in R for microwave pulse powers exceeding the melt threshold, so that measurements of R and T can be used to monitor the onset of surface melting.

A method for microstructural modelling of ceramic materials utilising precursor compounds and heating by microwave radiation to introduce a second phase as well as to control grain growth is described. The precursor compounds are introduced into the ceramic material by infiltration of a porous compact or by impregnating a classical powder with the precursor prior to compaction. Microwave radiation is used for decomposition of the precursor and for sintering. Decomposition rate controlled pyrolysis of the precursor is only possible by microwave heating. The resulting microwave processed ceramics display a finer and in some cases more bimodal microstructure as compared to conventionally processed samples with the same density.

A novel approach to synthetic ceramic powders based on microwave heating of metallorganic ceramic precursor compounds or mixtures of such compounds with a appropriate matrix material is described.

Numerous oxides as well as nonoxide powders with high specific surface area were obtained. Pyrolysis as well as enhanced reactivity is observed with microwave heating at lower temperatures than in conventional heating.

Preliminary results are decribed on synthesis of ceramic-metal composites with a functional gradient material character utilizing infiltration of a porous metallic matrix with ceramic precursors and thermal processing by MW-heating. Infiltration of the metal matrix with a ceramic precursor facilitates bonding between metal and ceramic. By repeated infiltration and decomposition, complete filling of the pores with a ceramic phase is achieved.

Using SiC as MW absorbing material in a sandwich-like configuration, heating of the metal-ceramic composite in a strong MW field is possible. Temperatures of apr. 1000°C were achieved with this method.

Microwave heating was used to reaction sinter Al2O3 + SiO2 and Al2O3 + TiO2 powder mixtures to prepare mullite and aluminum titanate ceramics, respectively. In comparison with conventional heating, microwave heating leads to a decrease in the temperatures of treatment of 50 to 100°C.

Porous tantalum anodes were sintered at temperatures from 1600 to 1900°C using a conventional high-vacuum furnace as well as both 2.45 GHz fixed-frequency and 4-8 GHz variable-frequency microwave furnaces. Various insulation and casketing techniques were used to couple the microwave power to the tantalum compacts. Several types of tantalum powder were used to assess the effect of microwave processing on sintered surface area and impurity levels.

Some microwave sintered anodes have an unusual surface rippling not seen on conventionally fired parts. The rippling suggests that a microscopic arcing or plasma might have been generated. Two important effects could be exploited if this phenomenon can be controlled. First, the effective tantalum surface area could be increased, yielding higher capacitance per volume. Second, surface impurities might be cleaned away, allowing the formation of a better dielectric film during the anodization process and, ultimately, higher working voltage.

Multilayer ceramic capacitors of several compositions (both Z5U and NPO types) were sintered in air in both conventional and microwave furnaces. Several casketing and insulation techniques were used to improve temperature uniformity and minimize dopant losses. Zirconia and yttria are suitable packing materials as either powders or fibers. After sintering, the packing material remains friable and can be recycled. The presence of metallization layers did not interfere with the microwave process and no arcing was observed at surfaces where the internal electrodes emerged. In some runs, however, local thermal runaway was observed, which could severely decrease process yields and must be eliminated in a commercial process.

Various hydroxyapatite (HAp) ceramics were fabricated by microwave processing. By carefully selecting the starting material and controlling the processing conditions, regular HAp ceramics with density up to 97% of the theoretical density, porous HAp ceramics with porosity up to 76%, and fully densified transparent HAp ceramics were obtained by using a 2.45 GHz, 500W microwave oven. These ceramics were characterized for density, microstructure, strength, and phase composition, and the results were compared with the conventionally processed ceramics.

The activities of EA Technology in microwave processing of oxide ceramics are reviewed. Comparative sintering rate measurement techniques have been used to demonstrate microwave enhanced densification of oxide ceramics. Temperature distributions within microwave and conventionally heated ceramics are simulated. Based on these simulations a method for firing large components has been developed which achieves uniform microstructures and low thermal stresses by careful control of temperature distribution.

As part of the development of generic microwave processes for spray-drying of homogeneous complex metal oxide powders and for inorganic fiber processing, we have investigated the use of 2.45 GHz microwaves in a high-Q single-mode TM010 cavity coupled directly to aerosols and fibers. Aqueous and ethanol aerosols of ferric nitrate solutions have been successfully dried at 1.8 kW of cavity power for a loaded Q greater than 6000 in flowing nitrogen gas. Similarly, we have observed extremely rapid heating rates in the TM0l0 cavity for small-diameter confined cylinders of water and lossy inorganic fibers. These observations suggest using 2.45 GHz microwave power for drying, calcining, and sintering extruded ceramic filaments. Droplet modeling indicates that the large dielectric shielding for spherical droplets can significantly limit the coupling of 2.45 GHz microwave with spherical aerosols, but not with fibers. Experimental observations on the microwave interactions with ferric nitrate aerosols and with ceramic filaments in the TM010 cavity are described.

The drying of electrical porcelain using a combination of convection and microwave heating has been experimentally studied. To date, a total of 58 individual tests have been conducted to evaluate the effect of a range of microwave and convection conditions on the drying behavior of both solid and cavity core bodies. Preliminary indications are that order of magnitude reduction in drying time may be achieved. Furthermore, improved control over quality problems appears to be possible because of the ability of microwave heating to minimize the amount of convective surface heating.

The heating of a ceramic slab under TEM illumination is modeled and analyzed in the small Biot number regime. The temperature distribution is almost spatially uniform in this limit and its evolution in time is governed by a first order nonlinear amplitude equation. This equation admits a time independent solution which is a multivalued function of the microwave power. The dynamics of the heating process are deduced, from the amplitude equation and the multivalued response, and are dependent upon the microwave power and initial conditions. The results of this analysis give a plausible explanation of certain difficulties arising in sintering experiments, such as thermal runaway. A simple control process is presented and analyzed which mitigates against these deleterious effects. Abbreviated parameter studies are performed showing trends in the controlled heating process.

A quasi-three-dimensional problem modeling the heating process of a thin cylindrical sample in a waveguide applicator is also presented. For certain excitations the physical phenomenon deduced from this model and the required control process are the same as those obtained for the slab. For other excitations there is a strong spatial structure along the axis of the sample. For a certain choice of parameters the middle portion of the sample is at an elevated temperature while the remaining portion is at a much lower temperature. This phenomenon may be useful in joining applications.

A temperature control system is designed and simulated for microwave joining of ceramics. The objective is to develop a control system for reliable, high quality joints to be produced on a variety of ceramics. The described procedure permits the derivation of a closed-loop configuration for controlling temperature. The ceramics (alumina) heating model equation is linearized around an operating point and represented as a transfer function. Conditions for closed-loop stability with any linearized model are presented and mathematically proven. A computer simulation is implemented for the closed-loop control and numerical integration is used for solving the non-linear heating equations. Results are very good and show that the temperature can be regulated to a set point value and thermal runaway in the ceramics can be prevented.

Microwave joining of sintered silicon carbide, both to itself and reaction bonded silicon carbide, has been accomplished in a single mode rectangular resonant cavity. Several approaches using different interlayer materials were employed to join sintered silicon carbide. Effective joining of reaction bonded silicon carbide to itself and sintered silicon carbide was accomplished without the use of any interlayer material in the single mode resonant cavity as well as in a multi-mode oven. Specimens cut from 0.95 cm diameter rods were joined in the single mode cavity, whereas a variety of arbitrary shapes and larger specimens (of reaction bonded silicon carbide) were joined in the multi-mode oven.

The use of microwave heating for the processing of reaction-bonded silicon nitride offers at least two advantages over conventional heating methods. First, the reaction can be made to proceed preferentially in the interior of the reacting compact by maximizing the temperature gradient resulting from microwave heating. This helps maintain the flow of nitrogen into the compact during the later stages of the reaction, when the overall porosity is reduced. Second, the reaction rate can be controlled by changing the power, which gives better control over the processing than can be obtained using conventional heating.

Sintered reaction-bonded silicon nitride (SRBSN) tiles were fabricated using microwave and conventional heating. Materials from both processes were analyzed at various stages in their fabrication. Microwave processing resulted in a SRBSN material of higher density and strength than the conventionally processed material.

A method for sintering silicon nitride using microwave energy at 2.45 GHz is described. Sintering takes place in air, in times of between 30 and 120 minutes and has been scaled up to give isothermal conditions over a batch size of 150 mm diameter by 200 mm in height and weighs approximately 1.0 kg. Additions of 5% alumina and 5% yttria result in a sintered product density of approximately 97% of theoretical, with a density variation better than:±0.5% throughout the batch.

It has been estimated that a load of 7.0 kg can be conventionally sintered using a 12 hour cycle and an energy consumption of 19.7 kWh/kg. In contrast with microwave energy, a batch of 540 g can be sintered in 120 minutes with an energy consumption of approximately 3.1 kWh/kg. This results in a possible energy savings of up to 78% for microwave heating.

Microwave energy at 2.45 GHz has been used to sinter silicon nitride in times of between 30 and 120 minutes to form a dense product with an average grain size of 0.35 μm and substantially improved uniformity of properties over all commercial silicon nitrides and sialons investigated.

A comparison between microwave and conventionally sintered parts of the same density showed the average mechanical properties to be the same for both sintering techniques, but revealed an improvement in the uniformity of mechanical properties of the microwave sintered products.

Silicon nitride components with 5% Al2O3 and 5% Y2O3 as sintering aids, has been sintered to a density of approximately 97% of theoretical. Hipping of these components at 1800 °C for 60 minutes, resulted in the complete removal of residual porosity. Dense components were evaluated in machining tests against commercial ceramic cutting tools. The results show a reduction in wear rate of batter than 10% for microwave sintered Sl3N4 in turning and milling operations on cast iron at a cutting speed of 2000 surface feet per minute.