Hal D. Kimrey

Diffusion-Controlled Processes in Microwave-Fired Oxide Ceramics

Processing oxide-based ceramics using microwave heating leads to a number of unexpected results, which can only be interpreted in terms of enhanced diffusion. Enhanced sintering has been observed in alumina and zirconia. Accelerated grain growth in dense, hot-pressed alumina has been demonstrated. Increased diffusion coefficients have been observed for diffusion of oxygen in sapphire. As yet, a satisfactory theory to account for these phenomena has not been developed. This paper reviews the experimental work conducted at the Oak Ridge National Laboratory during the past four years on the processing of oxides in both 2.45 and 28 GHz microwave furnaces. 18 refs., 10 figs.

Enhanced diffusion in sapphire during microwave heating

The diffusion of oxygen in sapphire was accelerated by heating in a 28 GHz microwave furnace as compared with heating in a conventional furnace. Tracer diffusion experiments were conducted using 18O. Single crystal sapphire wafers with a (1 0 1 2) rhombohedral planar orientation were used as the substrate. Concentration depth profiling was done by proton activation analysis using a 5 MeV Van de Graaff accelerator. The diffusion of 18O was greatly enhanced by microwave heating as compared with conventional heating in the 1500–1800°C range. The apparent activation energy for 18O bulk diffusion was determined to be 390 kJ mol-1 with microwave heating and 650 kJ mol-1 with conventional heating.

Design principles for high-frequency microwave cavities

The characteristics of untuned cavities are determined for microwave processing of alumina. Alumina is a low-loss ceramic that is difficult to heat in the lower microwave region. System efficiency is determined as a function of cavity and workpiece size, frequency, and temperature. With operation at 28 GHz, a cavity applicator can be designed which is capable of generating uniform fields at high efficiency and with a reasonable size. 8 refs., 5 figs.

Microwave Processing of Ceramics: Guidelines Used at the Oak Ridge National Laboratory

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.

Microwave Sintering of Zirconia-Toughened Alumina Composites

Microwave sintering possesses unique attributes and has the potential to be developed asa new technique for controlling microstructure to improve the properties of advanced ceramics. 1–6 Because microwave radiation penetrates most ceramics, uniform volumetric heating is possible. Thermal gradients, which are produced during conventional sintering because of conductive and radiative heat transfer to and within the part, can be minimized. By eliminating temperature gradients, it is possible to reduce internal stresses, which contribute to cracking of parts during sintering, and to create a more uniform microstructure, which may lead to improved mechanical properties and reliability. With uniform, volumetric temperatures, the generation of nonuniform particle/grain growth due to temperature gradients and associated sintering gradients can be regulated.

Finite-Difference Time-Domain Simulation of Microwave Sintering in A Variable-Frequency Multimode Cavity

There have been recent indications that variable-frequency microwave sintering of ceramics provides several advantages over single-frequency sintering, including more uniform heating, particularly for larger samples. The Finite-Difference Time-Domain (FDTD) code at the University of Utah was modified and used to simulate microwave sintering using variable frequencies and was coupled with a heat-transfer code to provide a dynamic simulation of this new microwave sintering process. This paper summarizes results from the FDTD simulations of sintering in a variable-frequency cavity. FDTD simulations were run in 100-MHz steps to account for the frequency variation in the electromagnetic fields in the multimode cavity. It is shown that a variable-frequency system does improve the heating uniformity when the proper frequency range is chosen. Specifically, for a single ceramic sample (4 × 4 × 6 cm 3 ), and for a variable-frequency range from f = 2.5 GHz to f = 3.2 GHz, the temperature distribution pattern was much more uniform than the heating pattern achieved when using a single-frequency sintering system at f = 2.45 GHz.

FDTD simulation of microwave sintering in multimode cavities

An FDTD (finite-difference time-domain) code was developed to simulate realistic microwave sintering experiments in multimode microwave cavities. Results showing the effect of various geometrical and physical parameters on the microwave sintering process are presented. After an extensive number of simulation runs, observations were made and guidelines were developed towards optimization of the sintering process. One of the more successful microwave sintering experiments involves the use of SiC rods in a picket-fence arrangement to help stimulate the microwave heating process, particularly at lower temperatures. In some sintering experiments the arrangement of these SiC rods is critical to the success of the experiment. The developed FDTD code was used to examine various aspects of the use of SiC rods to stimulate sintering of ceramics samples.<<ETX>>