Ying-Chung Chen

The effect of extrusion temperature on the development of deformation microstructures in 5052 aluminium alloy processed by equal channel angular extrusion

Commercial 5052 aluminium alloy was used to study the effect of extrusion temperature on the development of deformation microstructures processed by equal channel angular extrusion (ECAE). The extrusion temperatures used were between 50 and 300 °C, and the cumulative equivalent strain was 5.6. Transmission electron microscopy was adopted to characterize quantitatively the metallographical parameters, namely grain size, grain aspect ratio, and boundary disorientation. Raising extrusion temperature causes grain size to increase, grain shape to become more equiaxed-like, and a dramatic increase of low angle boundaries. Both grain and subgrain boundaries with low disorientations are formed at elevated temperatures. Comments on disorientation measuring techniques in transmission electron microscope are also given.

Improvements in the sintering and microwave properties of BiNbO4 microwave ceramics by V2O5 addition

Abstract The effects of V2O5 on the phase formation and the microwave dielectric properties of BiNbO4 ceramics were investigated as a function of the amount of V2O5. As the amount of V2O5 addition increased from 0.125 to 1 wt%, the densification temperatures of BiNbO4 ceramics decreased from 960 to 900°C. For densified BiNbO4 ceramics, as the amount of V2O5 increased from 0.125 to 1 wt% V2O5 added, the dielectric constants decreased from 43.7 to 43.4 and the temperature coefficients (τf) increased from +2.8 to +19.5 ppm/°C. The quality values (Q) of V2O5-doped BiNbO4 ceramics first increased, reached a maximum at 0.5 wt% V2O5, and then decreased with the further increase amount of V2O5 addition. The 0.5 wt%-V2O5-doped BiNbO4 ceramics sintered at 960°C had the optimum microwave dielectric properties: er=43.6, τf=+13.8 ppm/°C, and Q=3410.

Properties of LiTaO3 Thin Films Derived by a Diol-Based Sol–Gel Process

Lithium tantalite (LiTaO3) thin films have been successfully deposited on Pt(111)/SiO2/Si(100) substrates by means of sol–gel spin-coating technology. The XRD results show that the LiTaO3 thin films heated at temperatures higher than 700°C exhibit randomly oriented polycrystalline. The relative dielectric constant (er) increases from 35 to 300 at temperature ranging from 30 to 610°C, then decreases as the temperature increases above 610°C. The dielectric loss factor (tan δ) is as low as 0.006 at temperatures ranging from 30 to 770°C. For the first time, the ferroelectric properties of LiTaO3 thin films (~0.5 µm) deposited on Pt(111)/SiO2/Si(100) substrates have been investigated. The sample shows a phase transition at 610°C, a coercive field (Ec) of 22.3 kV/cm, a remnant polarization (Pr) of 2.4 µC/cm2 and a maximum pyroelectric coefficient (γ) of 4.0×10-8 C/cm2K at 62°C. The high figures of merit (FV and Fm of 2.1×10-10 Ccm/J and 2.4 ×10-8 Ccm/J, respectively) of LiTaO3 films reveal that a good infrared response of this LiTaO3 thin film can be utilized in its application to detectors.

Sintering and microwave dielectric characteristics of MCAS glass-added 0.84Al2O3-0.16TiO2 ceramics

Abstract Different amounts of MgO–CaO–SiO 2 –Al 2 O 3 (MCAS) glass content are used as the sintering aid of 0.84(mol%)Al 2 O 3 –0.16(mol%)TiO 2 composition. The sintering characteristics, the crystal structures, and the microwave characteristics of MCAS glass-added 0.84Al 2 O 3 –0.16TiO 2 ceramics have been investigated under the sintering temperatures of 1200–1350 °C. During the sintering process, the TiO 2 and Al 2 O 3 will form the Al 2 TiO 5 phase. The crystal intensity of Al 2 TiO 5 phase increases and that of TiO 2 phase decreases with the increase of sintering temperature. In this study, the MCAS glass content and sintering temperature have large effects on the sintering and microwave dielectric characteristics of MCAS glass-added 0.84Al 2 O 3 –0.16TiO 2 ceramics.

Synthesis and surface acoustic wave properties of AlN films deposited on LiNbO/sub 3/ substrates

The c-axis-oriented aluminum nitride (AlN) films were deposited on z-cut lithium niobate (LiNbO/sub 3/) substrates by reactive RF magnetron sputtering. The crystalline orientation of the AlN film determined by x-ray diffraction (XRD) was found to be dependent on the deposition conditions such as substrate temperature, N/sub 2/ concentration, and sputtering pressure. Highly c-axis-oriented AlN films to fabricate the AlN/LiNbO/sub 3/-based surface acoustic wave (SAW) devices were obtained under a sputtering pressure of 3.5 mTorr, N/sub 2/ concentration of 60%, RF power of 165 W, and substrate temperature of 400/spl deg/C. A dense pebble-like surface texture of c-axis-oriented AlN film was obtained by scanning electron microscopy (SEM). The phase velocity and the electromechanical coupling coefficient (K/sup 2/) of SAW were measured to be about 4200 m/s and 1.5%, respectively. The temperature coefficient of frequency (TCF) of SAW was calculated to be about -66 ppm//spl deg/C.

Synthesis of C-Axis-Oriented Aluminum Nitride Films by Reactive RF Magnetron Sputtering for Surface Acoustic Wave

C-axis-oriented aluminum nitride (AlN) films were deposited on SiO2-coated Si substrates by reactive rf magnetron sputtering. The crystallization of the AlN films, identified by X-ray diffraction (XRD) , was dependent on the deposition conditions. Highly c-axis-oriented AlN films, for fabricating AlN/SiO2/Si-based surface acoustic wave (SAW) devices, were obtained under an rf power of 300 W, substrate temperature of 350°C, sputtering pressure of 7.5 mTorr and N2 concentration of 75%. A dense pebblelike surface texture of the c-axis-oriented AlN films with an average grain size of about 100 nm was observed by scanning electron microscopy (SEM). The phase velocity showed a tendency to decrease with increasing kh, where k=2π/λ is the wavenumber and h is the AlN film thickness. In partucular, the phase velocity and the electromechanical coupling coefficient of the sample at kh=0.4 were calculated to be about 6080 m/s and 1.1%, respectively.

Annealing effect on the thermoelectric properties of Bi 2 Te 3 thin films prepared by thermal evaporation method

Bismuth telluride-based compounds are known to be the best thermoelectric materials within room temperature region, which exhibit potential applications in cooler or power generation. In this paper, thermal evaporation processes were adopted to fabricate the n-type Bi2Te3 thin films on SiO2/Si substrates. The influence of thermal annealing on the microstructures and thermoelectric properties of Bi2Te3 thin films was investigated in temperature range 100-250°C. The crystalline structures and morphologies were characterized by X-ray diffraction and field emission scanning electron microscope analyses. The Seebeck coefficients, electrical conductivity, and power factor were measured at room temperature. The experimental results showed that both the Seebeck coefficient and power factor were enhanced as the annealing temperature increased. When the annealing temperature increased to 250°C for 30 min, the Seebeck coefficient and power factor of n-type Bi2Te3-based thin films were found to be about -132.02 µV/K and 6.05 µW/cmċK2, respectively.

Influence of surface roughness of Bragg reflectors on resonance characteristics of solidly-mounted resonators

The solidly mounted resonator (SMR) is fabricated using planar processes from a piezoelectric layer sandwiched between two electrodes upon Bragg reflectors, which then are attached to a substrate. To transform the effective acoustic impedance of the substrate to a near zero value, the Bragg reflectors are composed of alternating high and low acoustic impedance layers of quarter-wavelength thickness. This paper presents the influence of Bragg reflector surface roughness on the resonance characteristics of a SMR. Originally, an AlN/Al multilayer is used as the Bragg reflector. The poor surface roughness of this Bragg reflector results in a poor SMR frequency response. To improve the surface roughness of Bragg reflectors, a molybdenum (Mo)/titanium (Ti) multilayer with a similar coefficient of thermal expansion is adopted. By controlling deposition parameters, the surface roughness of the Bragg reflector is improved, and better resonance characteristics of SMR are obtained

Suppression of Superconductivity in Y1-xPrxBa2Cu4O8 Prepared by Nitrite Pyrolysis Method

X-ray diffraction patterns show that samples contain the nearly single 124 phase for x<0.7. Lattice parameters a and b increase with Pr concentration. Thermal stability is reduced with increasing Pr concentration. Zero resistance temperature Tco decreases monotonically fiom 80 K at x=0 to 12 K at x=0.65. Room-temperature resistivity changes linearly to x=0.7 and increases abruptly at x=0.75. Thus the critical concentration xcr is estimated to be 0.7. Effective magnetic moments of Y1-xPrxBa2Cu4O8 are 3.63, 3.35 and 3.23µB for x=0.2, 0.4 and 0.6, respectively. In addition, the similarities and differences between Y1-xPrxBa2Cu4O8 and Y1-xPrxBa2Cu3O7-y are discussed.

Grain Growth and Electrical Properties in ZnO Varistors with Various Valence States of Additions

Grain growth and electrical properties in ZnO system with various valence states of manganese and cobalt were studied. The results were discussed by means of defects produced by the additions. The grain growth was analyzed from the kinetic grain growth equation: Gn=Dtexp (-E/RT). In this work, the grain growth kinetic exponent n was 6 and activation energy was 230±21 kJ/mol. The grain size increased with the valence states of manganese and cobalt. The compositions with lower trap density have higher nonlinear coefficient, higher voltage ratio and lower leakage current. The varistor properties were improved by increasing the sintering temperature and time because of the more obvious tunneling effect.