Mark A. Janney

Gelcasting : From laboratory development toward industrial production

Gelcasting, a ceramic forming process, was developed to overcome some of the limitations of other complex-shape forming techniques such as injection molding and slip casting. In gelcasting, a concentrated slurry of ceramic powder in a solution of organic monomers is poured into a mold and then polymerized in situ to form a green body in the shape of the mold cavity. Thus, it is a combination of polymer chemistry with slip processing and represents minimal departure from standard ceramic processing. The simplicity of the process has attracted industrial partners and by collaboration between them and the developers, the process is being advanced from the laboratory toward industrial production.

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.

Gelcast Tooling: Net Shape Casting and Green Machining

Abstract Gelcasting is an advanced powder forming process. It is most commonly used to form ceramic or metal powders into complex, near-net shapes. Turbine rotors, gears, nozzles, and crucibles have been successfully gelcast in silicon nitride, alumina, nickel-based superalloy, and several steels. Gelcasting can also be used to make blanks that can be green machined to near-net shape and then high fired. Green machining has been successfully applied to both ceramic and metal gelcast blanks. Recently, we have used gelcasting to make toolinj; for metal casting applications. Most of the work has centered on H13 tool steel. We have demonstrated an ability to gelcast and sinter H13 to near net shape for metal casting tooling. We have also been successful in green machining gelcast blanks using a three-axis CNC milling machine.

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.

Gelcasting Ceramic Defense Parts CRADA FINAL REPORT ORNL95-0397

The goal of this work was to demonstrate the ability to gelcast and fire a particular defense-related part to net shape and to hold the required tolerances on size and shape with minimal or no post firing machining. The precision required represents a huge increase in the precision currently achievable by standard ceramic processing.