Gels in the CaO-A12O3-SiO2 (CAS) system were successfully converted into glass in a single mode, tuneable, cylindrical microwave applicator, operating at 2.45 GHz in the TM012 mode. Transparent glasses were formed as a result of the direct microwave heating of homogeneous CAS gels to well above their melting temperature (1170°C). The effect of processing parameters, such as incident power level, sample mass and location, and thermal insulation has been investigated. The excellent coupling of the CAS gel with microwave can be understood from the structural evolution during sol-gel processing.

A single-mode cavity microwave furnace, operating in the TE103 mode at 2.45 GHz is being used to investigate sintering of pure and doped nanocrystalline alumina. The purpose of these experiments is to determine the effect of additives on the sintering process in the nanocrystalline regime. Using the sol-gel method, high purity Al2O3 nanocrystalline powders were synthesized. These powders were calcined at 700°C and then CIP'ed to 414 MPa, producing 0.4 in. diameter, 0.25 in. high cylindrical compacts. The compacts were heated in the microwave furnace to temperatures between 1100°C to approximately 1800°C and were then brought back to room temperature using a triangular heating profile of about 30 minutes duration. A two-color IR pyrometer was used to monitor the surface temperature of the workpiece. The additives tested in this work lowered the temperature needed for densification but this effect was offset by increased grain growth. Initial grain growth from < 5 nm to ∼ 50 nm was closely correlated with the γ to α-alumina phase transition.

Metal Organic Deposition (MOD) technique, which represents a flexible method for obtaining both platelets and films of YBCO, is based on baking, pyrolyzing and annealing stages of neodecanoates solutions. The annealing step is generally very long. The whole annealing stage can be simply substituted by a heat treatment (about 1 h) in a commercial microwave (MW) oven, followed by a 15 min. annealing at 900°C in a resistive furnace without oxygenation. With this process, YBCO platelets, films and wires were obtained starting from powders prepared by MO (Metal-Oxide). XRD patterns are indicative of a strong texturing by the evidence of (003) and (005) lines. By the same technique, YBCO films were prepared by spray deposition on A12O 3 for a total thickness of about 5 μm, each spray step, of 1 gm, was followed by 5 min pyrolysis at 500 °C and 1 h MW annealing.

The MW technique was also used for preparation of quartz and ceramic fibers covered by YBCO.

Preliminary results on microwave sintering of seeded silicon nitride show that a well defined bi-modal grain size distribution is attainable in Si3N4-Y2O3-Al2O 3-MgO sintered bodies by microwave sintering at 28 GHz of materials seeded with ß-Si3N4 particles (2 vol. %). A positive effect on the mechanical performance is anticipated for these microstructurally controlled silicon nitride ceramics

The phase transformation and densification behavior under high power millimeter-wave (mm-wave) radiation of a 30 GHz gyrotron and during fast conventional sintering of nanocrystalline γ-A12O3 powder have been investigated and compared. The powder used for compacts was synthesized from aluminum metal by application of the exploding wire technique in an oxidizing atmosphere. The particle size distribution of this powder has a maximum at about 20 nm. Magnetic pulse technique was applied for the compression of samples up to 80% of the theoretical density (TD). Both mm-wave sintering and fast firing in a conventional electrical resistance furnace enable the densification and a complete phase transformation into α-A12O3 already at a temperature of approximately 1150 %C. The average grain size of the sintered ceramic is in the range of 50 to 100 nm. With mm-waves densification starts at about 50 °C lower temperatures compared to conventional techniques and higher final densities were obtained already at 150°C lower temperatures.

The GCM (Microwave Heating Group) within the Communications Department at the Technical University of Valencia is dedicated to the study of microwaves and their use in the current industrial processes in the Valencian Community and in Spain. To this end, a microwave heating laboratory has been developed and the benefits of incorporating microwave technologies into current industrial processes have been demonstrated. In this paper some of the industrial applications which are being investigated are presented.

The use of microwave energy for the sintering and annealing of high-Tc YBCO superconductors has been investigated with a view to taking advantage of the opportunities presented by this heating technique. It has been found to offer the possibility of sintering 'from the inside out' due to the nature of the temperature profile developed and as a result bulk YBCO bodies measuring 35 mm in diameter by 5 mm thick have been produced with completely uniform and high oxygen content (x = 7 in Yba2Cu3Ox) and densities of ≈ 98% of theoretical. These properties were achieved using processing times approximately one sixth of those required conventionally. A model for the microwave heating process is proposed.

Microwave reaction sintering of tungsten- carbon-, cobalt mixtures is described as a method that combines the liquid phase reaction sintering of tungsten carbide cobalt hardmetals with the enhanced densification behavior of hardmetals in the microwave field. Starting with green parts composed of W-, Co- powder, soot as carbon source and with varying amount of additives a dense and extremely fine grained hardmetal can be obtained in one pressureless microwave reaction sintering step.

By this method hardmetals with improved mechanical properties can be obtained in a drastically simplified processing cycle, without time consuming steps such as carburizing and milling.

Recently the heating of a thin ceramic cylinder in a single mode applicator was modeled and analyzed assuming a small Biot number and a known uniform electric field through out the sample. The resulting simplified mathematical equations explained the mechanism for the generation and growth of localized regions of high temperature. The results predicted that a hot-spot, once formed, will grow until it consumes the entire sample. Most experimental observations show that the hot-spot stabilizes and moves no further.

A new model is proposed which incorporates the effect of the cavity and the nlonuiniform character of the electric field along the axis of the sample. The resulting simplified mathematical equations indicate that these effects stabilize the growth of hot-spots.

The microwave heating of a ceramic laminate composed of three layers is modeled arid analyzed. Two materials with widely disparate effective electrical conductivity's comprise the laminate. The ratio of the these conductivities is exploited as a small parameter in the development of an asymptotic theory. Two physically distinct situations are considered. In the first, a low-loss ceramic is surrounded by lossy material. In the second situation, a lossy material is surrounded by a low-loss material. The results are physically interpreted to attain insight into the dynamics and parameter dependence of the microwave heating of ceramic laminates.

Adhesive bonding/joining through microwave radiation curing has been evaluated as an alternative processing technology. This technique significantly reduces the required curing time for the adhesive while maintaining equivalent physical characteristics as the adhesive material is polymerized (crosslinked). This results in an improvement in the economics of the process. Testing of samples cured via microwave radiation for evaluation of mechanical properties indicated that the obtained values from the single lap-shear test are in the range of the conventionally cured samples. In general, the ultimate tensile strength, OB, for the microwave processed samples subjected to this single lap-shear test was slightly higher than for conventionally cured samples. This technology shows promise for being applicable to a wide range of high volume, consumer goods industries, where plastics and polymer composites will be processed.

In this paper, a ridge rectangular waveguide is designed, and its cutoff frequency, impedance and electric field intensity are given by formulas or curves. A few ceramic samples are sintered in it by microwave energy. It is concluded that the device can be as a satisfactory microwave sintering cavity.

High critical temperature superconducting (HTcS) bulk ceramics with the high values of critical current densities (Jc) necessary for practical applications, are only achieved through careful control of the material microstructure. Consequently, many stages in the production of HTcSs are extremely time consuming; often taking a number of days to proceed to completion, and usually requiring precise control of sample temperature.

The volumetric nature of microwave heating will clearly have an impact on the processing of large quantities of these materials, particularly when it is balanced with the simultaneous application of radiant heating. In this way, the production of thermal gradients can be avoided, even when relatively fast heating rates (for these materials) are used.

Perhaps more interesting, is the possibility of non-thermal microwave effects (due to the presence of an electric field) enhancing the processing times and even the material properties of HTcSs.

This work assesses the impact of microwave-assisted processing on a number of stages involved in the production of bulk pieces of Bi-2212 and Y-123 based superconductors, and demonstrates quite clearly that non-thermal as well as thermal microwave effects are relevant to these materials.

The electronics market is constantly pushing the state of the art in design to reduce cost, size, weight, and power consumption. New designs are emerging causing rapid changes in technology driving high design turnover. This drives the need for component and material measurements that will reduced design cycles and time to market. In the design and measurement of linear devices, error corrected S-parameters are traditionally measured with a network analyzer. the network analyzer combines magnitude with phase measurements for improved accuracy. Time domain techniques are used to get a better physical understanding of the device characteristics. Error correction procedures have been improved to provide high accuracy and ease of use. New methods to measure impedance results in high precision capacitor and inductor models necessary for both surface mount and integrated circuit applications. Note: In the following paper the relevant text follows each slide.

Dielectric measurements for frequencies in the tens of gigahertz are discussed. Interpretation of measurements obtained for mixtures are presented based on some simple analytical models and finite-element analysis of the wave structure in the measurement apparatus.

A metallized ceramic probe has been designed for high temperature broadband dielectric properties measurements. The probe was fabricated out of an alumina tube and rod as the outer and inner conductors respectively. The alumina was metallized with a 3 mil layer of moly-manganese and then covered with a 0.5 mil protective layer of nickel plating. The probe has been used to make complex dielectric properties measurements over the complete frequency band from 500 MHz to 3 GHz, and for temperatures as high as 1000 °C.

The use of a circular waveguide radiating into a multi-layered media allows the characterization of heterogeneous and fluid subtances. Many microwave measurement devices, based on reflection coefficient measurements, are subjected to air gap problems that introduce some inaccuracy in the determination of the unknown complex permittivity of the materials. Our purpose is to try and take the air gap into account in these measurements.

Most of the algorithms for cavity dielectric measurements use a full network frequency scan for the perturbation method. At high temperature the measurement points in one frequency scan will have different temperatures due to the fast cooling of the sample inside the cold cavity. This paper presents a new approach for the perturbation method to measure dielectric constants at high temperatures with a two port, two piston cylindrical cavity and external furnace. Special features of this cavity are the possibility to work with different modes and different relative positions of the coupling aperture.

The dielectric properties are extracted from the full sets of S-parameters for two successive frequency points corresponding to approximately the same temperatures. An extraction algorithm is developed and programmed.

A rigorous and accurate method for the experimental determination of the complex permeability of demagnetized ferrites at microwave frequencies is presented. The measurement uses low- loss dielectric ring resonators, is nondestructive, and allows complex permeability characterization of a single ferrite sample to be performed at frequencies from 2 GHz to 25 GHz. A wide variety of ceramic microwave ferrites having various compositions and differing saturation magnetizations were measured in the demagnetized state. Generally, at any frequency greater than gyromagnetic resonance, the real part of the complex permeability increases as saturation magnetization increases.For the same frequency magnetic losses increases as saturation magnetization increases. The real permeability results are compared with magnetostatic theoretical predictions. Measurement data show excellent agreement with theoretical predictions, but only when 2πγMs/ω < 0.75, where γ is the gyromagnetic ratio, Ms is saturation magnetization, and ω is the radian rf frequency.

In this paper, a resonant cavity method is developed and some experimental results for measuring dielectric constants of ceramic samples (e. g. Al2O3) under different sintering temperatures are reported. The experiments show that this method has higher precision and good prospects of in—line monitoring the high temperature dielectric constant in the process of raising the temperature of the samples. These results provide some scientific experimental basis for physical research of ceramic materials.