Boron carbide has been sintered to 95% of theoretical density in less than 12 minutes without the use of sintering aides by heating to 2000°C with 2.45 GHz microwave radiation. An average grain size of about 20 μm resulted. Twins and microcracking were present in the microstructure. Energy usage for microwave sintering was found to be 18% less than for inductive hot-pressing.

Microwave sintering of Y2O3-2wt.% ZrO2 has produced microstructures which yield mechanical and thermal properties that differ from those observed in commercially sintered Y2O3-2wt.% ZrO2. This paper discusses mechanical and thermal property data obtained on samples of Y2O3-2wt.% ZrO2 heated using 2.45 GHz electromagnetic radiation.

Mullite, Al6Si2O13 composition gels were made by the single phase and diphasic routes. The wet mullite gels were heated in the insulated cavity of an ordinary kitchen microwave oven (2.45 GHz; 700 watts). Crystalline mullite powders were obtained from single phase gels while the diphasic gels yielded α-Al2O3 and mullite in a matter of 20–25 minutes of microwave heating. Trans- mission electron microscopy showed that the mullite particle size varied between 50 nm to 1 μm. The results of this study suggest that fine powders of mullite can be obtained from single phase mullite gels by optimizing the microwave processing parameters.

Superconducting ceramics have been successfully prepared employing a novel internal heating mechanism which uses microwave energy. Calcining with microwaves requires further investigation to improve the superconducting properties. Sintering and annealing in microwave energy shows refined microstructure and improved oxygen content in the YBa2Cu3O7-x phase. This is attributed to the coupling of CuO with microwave energy. The onset of the superconductive transition occurs at 93°K for the conventionally calcined/microwave annealed samples. The conventionally processed samples have an onset transition at 90°K and exhibit weaker diamagnetism when compared with the microwave annealed sample.

The effect of power level on the microwave heating characteristics of a variety of reagent-grade chemicals and minerals has been determined in a Bureau of Mines study. Heating rates of the powdered samples are presented for incident power ranging from 500 to 2,000 W at 2.45 GHz. The apparatus consisted of a WR 975 waveguide-applicator mounted to WR 284 waveguide sections and connected to a 3-kW power source. Tests were conducted in an alumina crucible enclosed in a fused-quartz beaker that was fitted with a Teflon lid to allow for a controlled inert atmosphere and thermocouple insertion. In general, heating rates increased as input power increased. Exceptions to this were some very high-lossy (microwave receptive) and very low-lossy materials that showed negligible changes with increased power. Microwave data collected should provide insight as to possible chemical and mineral processing applications as well as to assist in predictions of processing parameters.

This paper presents a process developed at Quest Research Corporation for using microwave energy to join ceramics. It begins with a brief overview of the concept and a recent history of microwave ceramic processing. A mathematical treatment of the microwave heating process then is presented followed by a description of the joining technique, and a summary of the experimental results and joint characterizations. For one set of ceramic samples used, electron microscopy and fracture toughness and hardness measurements were made at the Naval Research Laboratory. Another set of sample material was provided by Coors and photomicrographic evaluations of joints produced by Quest using these samples were performed by Coors.

A new butt joining method for ceramics by microwave heating was developed. Ceramics were heated in a rectangular cavity. A klystron of maximum 3 kW at 6 GHz was used as the power amplifier. The heating system can control the iris, plunger and microwave power to keep a power efficiency up to 90% and a accuracy within ±10°C at 1800°C. Microwave ceramic-ceramic joining was tried by using this system. A bending strength of the joined alumina rod (92% purity) was 420 MPa without adhesive. This value was equal to the original strength. Silicon nitride ceramics were joined with adhesive, which was a sintered ceramic sheet having lower purity and larger dielectric loss factor than the base ceramics. The microwave energy was concentrated on the sheet, so that only the joining area was heated. The strengths of joined specimens were in excess of 70% of original strengths. The joined boundary line was not detected in microscopic observation, and there was little difference in microstructure between before and after joining. These results suggest that sintering aids in grain boundary phases were preferentially heated and melted or diffused, resulting in sound joining of ceramics.

Microwave heating in the fabrication process of high quality quartz fibers for telecommunication is described. In this process tubes and rods of ultrapure quartz are heated to temperatures above 2000 °C.

High quality fibers, referring to both optical attenuation and geometrical properties, have been manufactured. The homogeneous nature of microwave heating results in a significantly decreased production time per fiberkilometer.

Measurements of the most important materials parameter, the dielecric loss factor of quartz as function of temperature and frequency, are presented.

Microwave processing techniques were established for all of the thermal treatment steps necessary to prepare high density PZT and PLZT. The microstructure and ferroelectric properties of the microwave processed materials were compared to conventionally sintered and fast fired PZT. Unique high density, small grain size, PZT and PLZT were obtained by microwave sintering either conventionally or fully microwave processed powders. It was estimated that microwave sintering would use only five percent of the energy used by conventional sintering.

The suitability of microwave processing for the production of two modifications (Sr and La) of lead zirconate - lead titanate (PZT) ceramics was investigated in this program. A commercial microwave oven was used to evaluate drying, calcining, binder burnoff, and sintering. These materials were then compared to those produced by fast-firing and conventional sintering methods.

The potential for the microwave processing of high alumina castables was investigated*. Originally a study was conducted on the removal of the physical water from cast shapes. However, it was observed that the refractory castable material (alumina plus calcium aluminate phases) would also interact with the 2.45 GHz M/W radiation, and with proper thermal insulation it could be heated to well above 1427°C (2600°F). Full size production shapes, weighing up to 62 kg (136 lb) were heated in a 13.5 kW oven to these high temperatures. Over 1360 kg (3000 lb) of production shapes were successfully fired using M/W energy. The results of these studies are discussed along with several of the benefits which were made possible by microwave processing.

The Bureau of Mines, U.S. Department of the Interior, has conducted studies to utilize rapid microwave heating to stress fracture ore samples. Iron ores containing hematite, magnetite, and goethite were subjected to microwave energy in batch operations at 3 kW and heated to average temperatures between 840° and 940° C. Scanning electron microscope (SEM) photomicrographs verified fracturing along grain boundaries and throughout the gangue matrix. Standard Bond grindability tests showed that microwave heating reduced the work index of iron ores by 9.9 to 23.7 pct. Preliminary studies using a continuous feed belt in a microwave applicator indicated that samples heat more uniformly and with better temperature control than in batch operations.

Ontario Hydro has established a Process Applications Laboratory dedicated to helping industry apply electrotechnologies for a variety of processes. Demonstration and development of industrial microwave applications are being conducted to provide an important interface between the industrial user and microwave equipment manufacturer. Curing, heating, tempering, sintering, calcining and drying are among the operations that have been successfully demonstrated on a wide range of products including polymers, ceramics, composites, paper and wood products, food, pharmaceuticals and textiles. The benefits of microwave heating are illustrated with examples of feasibility studies performed for manufacturers and a R&D project being conducted in-house.

The presentation deals with Hydro-Québec implementation in the industrial sectors with special emphasis on microwave amd radiofrequency applications.

A rapid electromagnetic induction bonding system has been developed that joins composites, plastics, metals, and combinations of these materials. The equipment is self-contained, low power (100–400 watts) and portable. Gangs of bonders have been used as a seem welder to rapidly join plastic reinforced composite and sheet metal. Structures can be bonded in a factory or in the field. Bonding times for laboratory specimens have been cut by a factor of 10 to 100 times compared to standard press or autoclave bonding. This type of equipment allows for applying heat directly to the bond lines and/or the adherends without heating the entire structure, supports, and fixtures of a bonding assembly.

The bonder's portability and low power requirements allow for its use on site in any type of climate with power supplied by a variety of portable sources.

The bonding of composites, thermosets, and thermoplastics via microwave energy is not only technically feasible but economically viable compared to conventional technologies.

In recent years the growth in the use of composites, thermosets and thermoplastics has demonstrated the ability of these materials to fulfill specific performance needs in the automotive sector. In a contrary fashion the increased demand has served to focus attention on those factors which mitigate against further growth. In many instances this becomes a discussion of cycle times in primary or secondary operations.

A comprehensive program of research into dielectricallyassisted heating and drying of industrial products includes studies of the heat and mass transport processes and the spatial distribution of microwave (MW) and radio frequency (RF) power deposition in the objects of interest. Such studies enable the design of product specific drying machinery.

Dielectric heating has clearly demonstrated its production benefits in many materials processing applications. It is, however, a relatively expensive technology and its use vis a vis less expensive alternatives must be justified by a thorough economic analysis of all factors relevant to the given application. These include not only direct production costs, but also less easily quantified “implicit” factors such as product quality, reduced impact on the external environment, and improved workplace environment.

An economic model which includes both production cost considerations and implicit factors in dielectric heating applications is under development. This paper will focus on the capital and operating costs of dielectric heating equipment and the factors which influence these costs. Capital and operating cost information was obtained from radio-frequency and microwave suppliers and consultants in the United States. Mail-out questionnaires were followed up by extensive telephone discussions to correlate and clarify data. The intent was to identify the major cost components of dielectric heating equipment and to summarize the qualitative and quantitative factors which affect them. Since microwave and radio-frequency equipment are distinctly different in design and application, they are treated separately.

Modern applications of microwave energy have imposed new requirements upon microwave processing systems. Interest in energy efficiency, processing uniformity and control of process cycles has placed new design conditions upon microwave power oscillators, microwave systems and microwave applicator design. One approach of meeting new application requirements is the use of single-mode or controlled multimode applicators. The use of a single-mode applicator for plasma generation and materials processing will be presented. Descriptions of actual applicator designs for heating, curing and processing of solid materials and the generations of high and low pressure discharges will be given. The impact of these applicators on the total microwave system including the microwave power source will be described. Specific examples of applicator and associated microwave systems will be detailed for the applications of (1) plasma thin film deposition and (2) the precision processing and diagnosis of materials. Methods of process control and diagnosis, control of process uniformity and process scale up are discussed.

This paper describes a device used for the introduction of microwave energy into a plasma chamber over a wide area with high transfer efficiency and good energy uniformity.

The applicator design was based on a slotted waveguide combined with a quartz window. The impedance properties were calculated for slots in the broad wall of WR430 waveguide. Low power simulating tests were conducted to determine the optimum geometry. A simple test system was used to measure and plot the three-dimensional spatial energy distribution

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.