Papers by Keyword: Millimeter-Wave

Abstract: Millimeter-wave sintering of ceramic laser host materials has been under investigation for high-energy laser (HEL) applications. Advantages of polycrystalline, compared to single-crystal, laser host materials include lower processing temperature, higher gain from higher dopant concentration, cheaper fabrication, and larger devices. We are currently investigating the solid-state reactive sintering of neodymium-doped yttrium aluminum garnet (Nd:YAG) using a high power millimeter-wave beam as the heat source. The 83 GHz beam is generated in the Naval Research Laboratory (NRL) High Frequency Materials Processing Facility that is powered by a 15 kW, CW, 83 GHz GYCOM gyrotron. The starting powder is a mixture of commercially available alumina, yttria, and neodymia powders. Near transparency and over 99% theoretical density have been achieved with grain sizes of 5 to 10 µm. The fluorescence lifetime of the Nd+3 1.06 µm lasing transition was measured to be about 200 µs, in good agreement with other work. SEM studies of the sintered microstructure show residual porosity caused by trapped pores that must be eliminated to produce fully transparent material.

Abstract: By using 24 GHz millimeter-wave irradiation, full densification of AlN added with Y2O3 was attained at 1700 °C, which is comparable to 28GHz-wave irradiation. Thermal conductivity of a resultant sample exceeded 200 W/mK, which is the highest value in AlN fabricated in non-reducing atmosphere.

Abstract: The diamond photonic crystals with the periodic arrangement of high dielectric constant (ε=100) were fabricated, and photonic band gap properties in the millimeter waveguides were investigated. Acrylic diamond lattice structures with TiO2 dispersion at 40 vol. % were fabricated by Micro-stereolithography. The forming accuracy was 10m. After sintering process, TiO2 diamond lattice structures are obtained. The relative density reached 96%. The millimeter wave transmittance properties were measured with network analyzer and W-band millimeter waveguide. The band gap was measured between 90 GHz and 110 GHz in the Γ-X <100> direction, which was well agreed with the results calculated by the plane wave expansion method and simulated by the Transmission Line Modeling method.

Abstract: Characteristics of heating processing based on millimeter-wave or pulsed high current are discussed from the standpoint of the interaction between electromagnetic energy and solid materials. Capabilities of the electromagnetic processing are indicated by exemplifying several successful results such as millimeter-wave sintering of AlN, millimeter-wave post-annealing of aerosol-deposited PZT films and synthesis of single-phase nano-structured anatase by SPS (or pulsed high current heating). It is shown in these examples that well-characterized properties such as high thermal conductivity and preferential orientation are created by the inherent effect due to the electromagnetic field, which is called microwave or SPS effect in millimeter-wave or SPS processing.

Abstract: Menger-sponge is a three dimensional fractal structure with self-similar patterns. We fabricated the Menger-sponge structure composed of epoxy with titania-based ceramic particles dispersion by using a stereolithography CAD/CAM system. It has a cubic body of 27 mm in edge size with square through holes of 1, 3 and 9 mm. The structure is characterized with a fractal dimension D = 2.73 and a fractal stage 3. The electromagnetic wave response of the Menger-sponge was measured by using a network analyzer. Both reflection and transmission amplitudes of incident waves showed remarkable attenuations to -50 dB at 8 GHz simultaneously. The electric field intensity in the center holes in the Menger-sponge was measured by using a mono-pole antenna. The electromagnetic energy was localized in the central air cavity by forming the strong localization mode. The localized mode frequency can be controlled by changing the structure size, number of stage, and the effective dielectric constant. We call such fractal structures as the photonic fractal.

Abstract: We present results on microwave and millimeter-wave processing of materials. The research is primarily based on two systems– a 2.45 GHz, 6 kW S-band system and an 83 GHz, 15 kW gyrotron based quasi-optical system. These systems have been used for a wide range of material processing experiments. We describe the capabilities of these systems and discuss some of the results, including nanophase material production, rapid sintering, coating removal and joining of high temperature ceramics.