Chung Kwei Lin

Formation of NiAl–Al2O3 intermetallic-matrix composite powders by mechanical alloying technique

This study investigated the feasibility of preparing intermetallic-matrix composite powders (NiAl/Al2O3) by mechanical alloying of Ni, Al and Al2O3 powder mixtures with various compositions of (NiAl)x(Al2O3)100–x. The as-milled powders were examined by X-ray diffraction, scanning electron microscopy, and differential thermal analysis. The formation of NiAl phase was noticed after 5 h of milling. Intermetallic-matrix composite powders (NiAl/Al2O3) were prepared successfully at the end of milling for (NiAl)x(Al2O3)100–x (x=79, 66, and 49), but no alumina phase was detected for (NiAl)95(Al2O3)5. It is suspected that the additions of alumina hampered the cold welding and fracturing process. The thermal analysis of (NiAl)x(Al2O3)100–x powders after 1 h of milling revealed that the transition temperature of NiAl phase increased with increasing amount of Al2O3 additions.

Investigation on electromagnetic and microwave absorbing properties of La0.7Sr0.3MnO3−δ/carbon nanotube composites

In this study, the influence of the addition of carbon nanotube (CNT) (0, 0.5, 1.0, and 2.0 wt %) on complex permittivity, complex permeability, and reflection loss for La0.7Sr0.3MnO3−δ (LSMO)/polymer composites was investigated. Microwave absorbing composites were prepared by molding and curing a mixture of LSMO powders, CNTs, and epoxy resin. The complex permittivity and complex permeability for LSMO/CNT composites were examined by a cavity perturbation technique. The experimental results showed that the complex permittivity increased with increasing CNT addition and the imaginary part of the permeability decreased with rising frequency. In the present study, a microwave absorbing composite filled with 80 wt % LSOM powders and 2 wt % CNTs exhibited the optimum performance, that is, the microwave absorbing peak was −22.85 dB at 9.5 GHz and the −10 dB absorbing bandwidth reached 3.3 GHz for the sample with a matching thickness of 3 mm.

Complex permittivity and permeability of iron-based composite absorbers measured by cavity perturbation method in X-band frequency range

A cavity perturbation resonance technique is used to determine the complex permittivity and permeability of iron-based composite absorbers prepared by mixing various weight percentages of carbonyl iron powder with epoxy as the matrix. The complex permittivity (e′-je″) and permeability (μ′-jμ″) are measured around X-band (7–12.5 GHz for permittivity and 7.5–14 GHz for permeability) by the cavity perturbation resonance technique and the results are compared with those obtained by the transmission/reflection method. Experimental results show that the average differences of real parts of permittivity (e′) and real and imaginary parts of permeability (μ′ and μ″) measured by these two techniques are less than 10%. The differences in e″, however, is apparently higher than the others. As a general trend, the difference increases with increasing weight percentage of carbonyl iron.

Frequency-dependent complex permittivity and permeability of iron-based powders in 2–18 GHz

A novel approach is presented to investigate the complex permittivity (e′−je″) and permeability (μ′−jμ″) of iron-based powders in the frequency range of 2-18 GHz. The effective permeability and permittivity of carbonyl iron-epoxy composites for various volume fractions are measured by using the transmission/reflection method. A genetic algorithm is implemented with four nonempirical effective medium theories to obtain the complex permittivity and permeability of the carbonyl iron powders in the corresponding frequency range. The use of the extracted permittivity and permeability of the carbonyl powders with Looyenga’s formula fit the experimental data well not only over a wide range of frequencies but also various concentrations of carbonyl iron.

Fabrication and optical properties of Ti-doped W18O49 nanorods using a modified plasma-arc gas-condensation techniquea)

In this study, Ti-doped W18O49 nanorods were prepared without any templates or catalysts using a modified plasma-arc gas-condensation technique. Structure investigations were conducted and the optical properties of the as-prepared nanorods were characterized by field emission gun scanning electron microscope, high-resolution transmission electron microscope, x-ray diffractometer, x-ray photoelectron spectroscope, micro-Raman, and ultraviolet (UV)-visible spectrometers. The experimental results showed that the diameter and the length of the as-prepared nanorods were approximately 20nm and 3μm, respectively. The as-prepared nanorods exhibited W18O49 structures and characteristic Raman peaks could be observed. UV-visible absorption investigations revealed that Ti-doped W18O49 nanorods exhibited a redshift phenomenon when compared with the pure TiO2 and WO3.

Cu-Zr-Ti Bulk Metallic Glass Composites Produced by Mechanical Alloying and Vacuum Hot-Pressing

In the present study, WC/Cu60Zr30Ti10 metallic glass composite powders were prepared by mechanical alloying of pure Cu, Zr, Ti, and WC powder mixtures. Cu60Zr30Ti10 metallic glass composite powders were obtained after 5 h of milling as confirmed by X-ray diffraction and differential scanning calorimetry. The metallic glass composites powders were found to exhibit a supercooled liquid region before crystallization. Bulk metallic glass (BMG) composites were synthesized by vacuum hot pressing the as-milled Cu60Zr30Ti10 metallic glass composite powders at 723 K in the pressure range of 0.72-1.20 GPa. BMG composite with submicron WC particles homogeneously embedded in a highly dense anocrystalline/amorphous matrix was successfully prepared under applied pressure of 1.20 GPa. It was found that the pressure could enhance the thermal stability and promotes nocrystallization of WC/Cu60Zr30Ti10 BMG composites.

Experimental characterization of an inductively coupled acetylene/hydrogen plasma for carbon nanofiber synthesis

A plasma-enhanced chemical-vapor deposition process was employed to synthesize carbon nanofibers (CNFs) on glass substrates patterned with Ni catalytic films. At the gas pressure of 20mTorr and the substrate temperature (surface) of ∼500°C, the isolated and vertically aligned carbon nanofibers have been successfully synthesized. This paper reports experimental investigation of plasma properties characterized by the optical emission spectroscopy of the spectral line intensities of the various species such as hydrogen, C2, and CH, as well as the rf characteristics at the biased substrate stage measured by an impedance meter. The measurement results reveal that the C2 density increases with the acetylene/hydrogen flow ratio and the inductively coupled plasma (ICP) source power, as expected. The atomic hydrogen density, however, decreases with the flow ratio but increases with the ICP power. The resulting growth rate of CNFs increases with the C2 density if atomic hydrogen density also increases accordingly, ...

Magnetic and structural properties of nanophase AgxFe1-x solid solution particles

Abstract In this study, silver and iron elements are evaporated simultaneously to form nanocrystalline solid solution particles and then quench to liquid nitrogen temperature. The average composition of the nanocrystalline Ag-Fe system analyzed by scanning electron microscopy with energy-dispersive spectroscopy is close to the gross composition of the raw materials. X-ray diffraction patterns indicate only Ag peaks in those of the nanocrystalline Ag-Fe solid solutions. Transmission electron microscopy images of the Ag-Fe system indicates a mean particle of about 10 nm for these nanocrystalline solid solutions. The magnetic properties of Ag-Fe systems depend on the mean particles sizes and the concentration of solid solutions. The temperature dependence of the magnetic properties is analyzed and related to the microstructural changes induced by the thermal treatments. Nanocrystalline Ag-Fe system with a Curie temperature of Fe occurs at about 580 °C. The magnetization and remanence of as prepared or after heat-treatment nanocrystalline Ag-Fe solid solutions increase with increasing atomic percentage of iron.

Improvement in the characteristics of GaN-based light-emitting diodes by inserting AlGaN-GaN short-period superlattices in GaN underlayers

We report the influence of short-period superlattice (SPSL)-inserted structures in the underlying undoped GaN on the characteristics of GaN-based light-emitting diodes (LEDs). The measurements of current-voltage (I-V) curves indicate that GaN-based LEDs having pseudomorphic Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures exhibit improvements in device characteristics with the best LED being inserted with two sets of five-pair Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL structure. Based upon the results of etch pit counts, double-crystal X-ray diffraction measurements and transmission electron microscopic observations of the GaN-based LEDs, it was found that the Al/sub 0.3/Ga/sub 0.7/N(2 nm)-GaN(2 nm) SPSL-inserted structures tended to serve as threading dislocation filters in the LEDs so that the improved I-V characteristics were achieved.

Amorphization of transition metal-Si alloy powders by mechanical alloying

Amorphization of TM‐Si (TM: trasition metal) alloy powders by mechanical alloying has been systematically investigated. Pure element powder mixtures were mechanically alloyed for ten hours with a high energy ball mill. The results show that amorphous metallic silicide powders were successfully synthesized in Ti‐Si, Zr‐Si, Nb‐Si, and Ta‐Si systems. Prediction of the amorphization range of mechanically alloyed TM‐Si powders by the Miedema model is inappropriate. The possible criteria for the amorphization of TM3Si powders after mechanical alloying 10 h in a shaker ball mill are (a) the free energy of amorphous state of TM3Si powders is less than −31.95 kJ/mole, and (b) the silicon to trasition metal atomic radius ratio is less than 0.985.