Though microwaves have been in use for many applications for over 60 years, their application in ceramic processing involving synthesis, sintering, melting, joining, surface modifications, etc, has developed only in the last twenty years. Microwave materials processing is recognized for many advantages, namely, substantial reduction in cycle time resulting in large energy savings, selective and volumetric heating, providing fine microstructures, improved mechanical properties, and eco-friendliness. In the last two decades, various researchers have worked with a variety of traditional and advanced ceramics such as alumina, zirconia. hydroxyapatites, transparent ceramics, electroceramics, ceramic superconductors, glass-ceramics, non-oxide ceramics including Si₃N₄, AIN, SiC and WC/Co, etc. In all of these materials, substantial improvements in their properties over conventional products, as well as enhancements in the diffusion and reaction kinetics, were reported. Recently, microwave melting of glasses has also been achieved with many advantages over conventional melting. It was generally believed that metals reflect microwaves, and hence cannot be processed in a microwave field like ceramics. However, recently it has been discovered that if the metals are in powder form, they will also absorb microwaves and will get heated very effectively. Almost all metals, including refractory metals (W, Re, Mo, etc), alloys, steels, have now been sintered successfully in 5-15 min. The selective heating feature of microwaves has led to effective brazing and joining of metal parts. Even bulk metals can be heated and melted in a microwave field and the melt can be cast into useful products.

A unique cavity perturbation method has been used to study the dielectric behavior of chromium oxide $\left({\mathrm{Cr}}_2{\mathrm{O}}_3\right)$ in the microwave region at $\sim 3.5\mathrm{GHz}$ and temperatures up to $\sim 1100°\mathrm{C}$ . The specimen in the form of pellet was heated by microwave power at $2.45\mathrm{GHz}$ in e-field of a single mode cavity. In the temperature range of measurement, the dielectric constant is found to increase within 10% and loss factor within one and a half times the room temperature values. Results are discussed in the light of agitated ionic motion in the material.

Nature 399, 668–670 (1999) In the penultimate paragraph of this Letter, there are two instances where “electric field” has been used when “magnetic field” is meant: one is in the fourth sentence and the other in the last.

In the present work, nano Cu (0, 5, 10, 15, 20, 25 wt.%) was added to W, and W–Cu composites were fabricated using the spark plasma sintering (S.P.S.) technique. The densification, microstructural evolution, tensile strength, micro-hardness, and electrical conductivity of the W–Cu composite samples were evaluated. It was observed that increasing the copper content resulted in increasing the relative sintered density, with the highest being 82.26% in the W75% + Cu25% composite. The XRD phase analysis indicated that there was no evidence of intermetallic phases. The highest ultimate (tensile) strength, micro-hardness, and electrical conductivity obtained was 415 MPa, 341.44 HV_0.1, and 28.2% IACS, respectively, for a sample containing 25 wt.% nano-copper. Fractography of the tensile tested samples revealed a mixed-mode of fracture. As anticipated, increasing the nano-copper content in the samples resulted in increased electrical conductivity.

A microwave power-combining technique based on magnetrons has been widely researched to solve the urgent demands for high-power, low-cost microwave sources in microwave industrial applications. To achieve high resultant efficiency, conventionally, injection locking with static frequency is utilized to drag the output frequency of each magnetron source to the exact same frequency, and a phase shifter is required to adjust the output phase of each combining source. In this article, we propose a novel phase shifterless microwave power system with two magnetrons utilizing a frequency-searching injection-locking technique. Coaxial cables with different physical lengths are used to realize phase adjustment under different injection frequencies. By sweeping the injection frequency in a small band, the phase difference between combining signals can be adjusted. Thus, high resultant efficiency can be obtained. When the injection frequency is swept, the phase difference between the output and injection signals has been analyzed theoretically, the phase difference between the different-length coaxial injection cables has also been analyzed. Furthermore, the relationship between the sweeping frequency bandwidth and requirements of the minimum length difference of the coaxial injection cables has been derived and analyzed. Experimental results show that a high resultant efficiency of 94.6% can be obtained with the proposed phase shifterless system. The system could maintain high resultant efficiency even when the free-running frequency and power of one magnetron are changed by adjusting its anode current. The experiments reveal that the proposed method works for different combining sources.