Ki-Woong Chae

Millimeter-Wave Dielectric Properties of Cordierite/Indialite Glass Ceramics

Millimeter-wave wireless communications with high data transfer and radar system for Pre-Crash Safety System have been developed recently. Cordierite (Mg2Al4Si5O18) is one of the silicates with good millimeter-wave dielectric properties. Cordierite has two polymorphs: cordierite and indialite. Cordierite is of low symmetry form with ordered structure, and indialite is of high symmetry form with disordered structure. In this study, indialite/cordierite glass ceramics are fabricated with good millimeter-wave properties. The Qf values are dependent on the amount of indialite phase and crystallization conditions. Indialite with high symmetry showed higher Qf values than cordierite with low symmetry. This result clearly shows that high symmetry results in high Qf when the samples are prepared under the same crystallization conditions. However, these glass ceramics are very difficult to make under the same conditions because of non-uniform crystallization. These dielectric glass ceramics show high Qf values of >200,000 GHz, low dielectric constant er = 4.7, and TCf= -27 ppm/°C.

Diamond thick film deposition in wafer scale using single-cathode direct current plasma assisted chemical vapour deposition

Deposition behavior of CVD diamond thick film by direct current (DC) plasma assisted chemical vapour deposition method was investigated on a wafer scale of 4 inches (10 cm) in diameter. A diode configuration with a cathode connected to the DC power supply and a grounded substrate was used, on which diamond was deposited. Plasma of methane and hydrogen mixed gas was generated at 150 Torr with the power up to 50 kW. Typically wafers showed a thickness gradient in a radial direction. A wafer bowing was also observed due to residual stress in the film. Both the thickness gradient and the bow could be minimized by process control. The physical properties such as Raman characteristic and thermal conductivity were very uniform. The average growth rate increased with methane concentration up to 19 μm/h, which could be increased further by increasing methane concentration and input power.

Generation of pulsed direct-current plasma above 100 torr for large area diamond deposition

A high-temperature, large-size plasma was successfully generated at a pressure above 100 torr using a pulsed direct current (DC) electric power for the application of wafer-scale diamond thick film growth. A diode type electrode configuration was used, with a Mo disk of 13 cm in diameter as a cathode and a W disk of 10 cm in diameter as a substrate. By adjusting the pulse condition and maintaining the cathode temperature between 1000°C and 1100°C, the arc between the electrodes was suppressed completely and the plasma was maintained very stably for over 100 h. Above and below this cathode temperature range, solid carbon was deposited on the cathode, which induced either the arc or non-uniformity of plasma state. A diamond wafer with a thickness up to 1 mm was successfully grown using this plasma of methane and hydrogen mixed gas. The plasma characteristics were analyzed using an optical emission spectroscopy.

The pronounced grain size refinement at the edge position of the diamond-coated WC-Co inserts under microwave plasma with negative bias

Abstract The effect of negative direct current (DC) bias on the deposition of diamond on WC–Co insert was investigated. Diamond was deposited by microwave plasma assisted chemical vapor deposition (MW PACVD) on the SPGN type of WC–Co insert at 30 Torr with H2-2%CH4 mixed gas. During the deposition process, DC bias was applied to the insert with the range between 0 and −120 V. The grain size of diamond film deposited on WC–Co insert was observed to decrease with the negative bias voltage. Furthermore, its tendency was more pronounced near the edge than at the center of the insert. The ion impact on the growth surface during deposition was considered to induce surface defects, which resulted in the renucleation of diamond crystals and grain size refinement.

Dependence of the diamond coating adhesion on the microstructure of SiC-based substrates

Abstract Diamond thin films were deposited on the SiC–30TiC–10Cr 3 C 2 substrates by using the microwave plasma CVD method and the effect of microstructural morphology of substrate was examined on the diamond–substrate adhesion strength. Two types of substrate were prepared by hot-pressing, one with small and equiaxed β -SiC grains and the other with large and elongated α -SiC grains. The SiC–30TiC–10Cr 3 C 2 substrate surfaces were chemically etched to remove (Ti,Cr)C matrix phase by Murakami solution. Better diamond–substrate bonding was obtained on the substrate composed of large, elongated grains. Vickers indentation results indicated that mechanical interlocking between elongated and protruded grains on the etched surface and diamond thin film resulted in an increase in adhesion strength.

Effect of Crystal Structural Environment of Pr 3+ on Photoluminescence Characteristics of Double Tungstates

In this article, the effect of the crystal structural environment of Pr³? ions on the photoluminescence (PL) characteristics of double tungstates, such as A(M 1-x Pr x )W₂O? (A=Li, Cs, M = In, Y, Sc, La; 0.007≤x≤0.1) and La 1.96 Pr 0.04 W₃O₁₂ are characterized. By varying the ion radius in A and M sites, the structural environment of Pr³? ions were modified. The structural criteria, that is, the point charge electrostatic potentials V around the Pr³? activator, were calculated using the crystal structural parameters. The point charge potential V can be a valid criterion for ³P? quenching in various double tungstates. When the calculated V values are large (＞ 6.0), the luminescence from the ³P? level becomes dominant. When the calculated V values are about 3.8, the ¹D₂ line appears weakly but ³P?-level luminescence is absent. When the calculated V values are small (＜ 2.0), the luminescence from the ¹D₂ level becomes dominant and ³P?-level luminescence is absent. At 2.0＜V＜4.0, the ³P? quenching to ¹D₂ level occurs substantially in accordance with the structural criterion of the point charge potential model.

Transparent and highly luminescent Eu-oxide thin film phosphors on sapphire substrates

Transparent and highly luminescent thin film phosphors were prepared by the thermal evaporation of Euoxide and its subsequent deposition on (006) sapphire substrates in a vacuum in a strongly reducing atmosphere at 1200°C–1300°C. The resulting phosphor films, which were 1–3 µm thick, strongly adhered to the substrate and exhibited photoluminescence spectra similar to those of Eu-doped β″-alumina, with an emission peak centered at about 525 nm. The films consisted of Eu2O3 and β″-alumina phases with preferential growth planes. The transparency of the thin film phosphors was measured over the UV-visible range (370–720 nm). The average transparency was 26%–55% of the total incident light. The integrated photoluminescence intensities of the phosphor films were compared to that of a commercial powder phosphor with green-light emission.

Al₂O₃:xCr₂O₃ 고용상의 발광특성과 적색형광체의 연색성 향상을 위한 첨가제로의 응용

In this article photoluminescence of the Al₂O₃:xCr₂O₃ solid solutions prepared by solid state reaction method are represented. The effect of Cr₂O₃-activator concentration and heat treatment time on the PL characteristics have been discussed in conjunction with microstructure of phosphor samples. The Al₂O₃:xCr₂O₃ phosphors show the highest PL intensity at x=0.003 mole when the samples are reacted at 1600℃ for 5 h. The PL emission and absorption spectra show the maximum peaks at 698 ㎚ and at 398 ㎚ respectively. The CIE color coordinate is (x=0.646, y=0.316) at 0.003 mole Cr₂O₃, which value is very close to the NTSC coordinate of red color. This characteristic feature of Al₂O₃:xCr₂O₃ has been applied for an additive to improve the color characteristic of other red phosphor LiEuW₂O? which has a relatively poor color purity with an emission peak centered at 615 ㎚ and with a CIE coordinate (x=0.530, y=0.280). The Al2O3:0.003Cr₂O₃ phosphor has been mixed with the LiEuW₂O? phosphor powder and the PL characteristics and CIE color coordinates are characterized. The Al₂O₃:xCr₂O₃ phosphor was found effective for improving the CRI (color rendering index) of LiEuW₂O? phosphor.

Phase Analysis of Na1−xLixNbO3 Ceramics by Neutron and Its Electric Property

Abstract Crystal structures of the Na1−xLixNbO3 (x = 0.04, 0.1) ceramic pellets were analyzed using neutron diffraction. The pellets sintered at 1220°C consisted of an orthorhombic (Pb21 m) and rhombohedral (R3c) phases. Morphotrophic phase boundary started at x << 0.1. The R3c (4 mass%) coexists with the Pb21 m (96 mass%) in the x = 0.04. Neutron analysis results were compared to that of the XRD. The measured P-E characteristics conformed to the analyzed crystal structures. Remanent polarization and coercive field were strongly dependent on sintering temperature (1220∼1260°C) and Li-concentration (x = 0.04∼0.15) respectively.

Selective Dissolution of ZnO Crystal by a Two-step Thermal Aging in Aqueous Solution

ZnO hexagonal rods grown in aqueous solution can be changed into a tubular shape by two-step aging in the course of the growing process. In the first step, hexagonal ZnO rods is grown by aging at 90℃ under a highly supersaturated aqueous solution giving rise to a fast precipitation rate. Meanwhile, during the second step aging at 60℃ in the same aqueous solution, the hexagonal polar face (001) having higher surface energy than (010) side planes dissolves to minimize surface energy. Hence the flat (001) face changes to a craterlike face and the hexagonal rod length of ZnO decreases at an initial-stage of this step aging. The formation of the (101) wedge-type faces is ascribed to the resultant of competitive reactions between the dissolution of polar face minimizing the surface energy which is a dominant reaction at the initial stage and the precipitation reaction dissipating supersaturation. At a later stage of the second-step the reaction rates of these two processes in the aqueous solution become similar and the overall reaction is terminated.