Cathodoluminescence and Field-Emission Properties of β-Ga2O3 Nanobelts

β-Ga₂O₃ nanobelts were synthesized using a vapor transport process in a controlled ambient. Structural characterization revealed that the as-synthesized samples consisted of monoclinic β-Ga₂O₃ nanobelts, and the presence of gallium-associated defects was verified using cathodoluminescence (CL). The formation of gallium-associated defects was explained by the insufficiency of the supply of cations, generating gallium vacancies on the (010) facet during growth. Furthermore, field-emission measurements indicated that β-Ga₂O₃ nanobelts exhibited defect-related electron emission. The turn-on fields of β-Ga₂O₃ nanobelts increased significantly with the degree of structural defects. For a sample prepared under 15% ambient oxygen, Fowler–Nordheim (F–N) analysis revealed two distinct field-enhancement factors of 1194 and 276, respectively. A correlation between field emission and structural defects was proposed. The experimental results demonstrate the presence of gallium-associated defects, which behave as electron traps, degrading the electron field-emission properties of β-Ga₂O₃ nanobelts.     

Design of linear phase FIR filters using fractional derivative constraints

In this paper, the designs of linear phase FIR filters using fractional derivative constraints are investigated. First, the definition of fractional derivative is reviewed briefly. Then, the linear phase FIR filters are designed by minimizing integral squares error under the constraint that the ideal response and actual response have several same fractional derivatives at the prescribed frequency point. Next, the fractional maximally flat FIR filters are designed by letting the number of fractional derivative constraints be equal to the number of filter coefficients. Finally, numerical examples are demonstrated to show that the proposed method has larger design flexibility than the conventional integer derivative constrained methods.     

InGaAs-Capped InAs–GaAs Quantum-Dot Infrared Photodetectors Operating in the Long-Wavelength Infrared Range

A ten-period InAs-GaAs quantum-dot infrared photodetector (QDIP) with 8-nm In_0.15Ga_0.85 As capping layer grown after quantum-dot (QD) deposition is investigated. With reduced InAs QD coverage down to 2.0 mono-layers, responses at 10.4 and 8.4 mum are observed for the device under positive and negative biases, respectively. The phenomenon is attributed to the large Stark effect resulted from the asymmetric band diagrams of the device under different voltage polarities. The demonstration of long-wavelength infrared detections with the simple structures of the InGaAs-capped QDIP is advantageous for the development of multicolor QDIP focal-plane arrays.     

Fabrication of High-Curie-Point Barium-Lead Titanate PTCR Ceramics

High-Curie-point semiconducting barium-lead titanate positive temperature coefficient of resistivity (PTCR) ceramics of composition Ba_0.897Pb_0.1La_0.003TiO₃ and Ba_0.5Pb_0.5La_0.003TiO₃ were prepared. The starting powders were synthesized by reacting commercial BaTiO₃, PbO, and TiO₂. To avoid the nonstoichiometry due to the volatilization of Pb during the sintering process, a lead atmosphere sintering approach with PbTiO₃ as packing powder was used. The samples being fabricated by this method show a PTCR effect of 3 to 4.5 orders of magnitude above the Curie point. The curie points were about 180°C for Ba_0.897Pb_0.1La_0.003TiO₃ and about 360°C for Ba_0.497Pb_0.5La_0.003TiO₃.     

The structural and optical properties of ZnO nanowire arrays prepared by hydrothermal synthesis method

ZnO nanowire arrays have been grown on the ZnO film-coated silicon (100) substrates by hydrothermal method, and the deposited nanowires are found to have a uniform size distribution with sharp hexagonal-shaped tips. The structural and optical properties of the nanowires were investigated using atomic force microscopy (AFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and cathodoluminescence (CL) techniques. The XRD and SEM results demonstrate that the well-aligned ZnO nanowires are single crystalline structure formed along the c-axis orientation. TEM analysis further confirms that the ZnO nanowires are highly preferred grown along the (002) crystal plane. The spacing between adjacent (002) lattice planes is estimated as 0.52 nm. The optical properties of the nanowires were measured using CL after annealing in oxygen and nitrogen atmospheres at 550 °C for various times. The CL spectra in the visible spectrum exhibit two weak deep-level emission bands that may be attributed to the intrinsic or extrinsic defects. It can be observed that the ZnO nanowires show different optical behaviors after various annealing times. The dependence of the optical properties on the annealing conditions is also discussed.     

Predicting shrinkage and warpage of fiber-reinforced composite parts

One major problem that arises in the design of plastic parts, especially those that are fiber reinforced, is the change of shape and dimension as a result of shrinkage and warpage. These material inhomogeneities are caused by flowinduced fiber orientation, curing, poor thermal mold lay-out, and other processing conditions. This paper presents a simulation that predicts shirnkage and warpage of 3-D compression molded fiber reinforced composite parts. The simulation represents the structure with the 3-noded shell elements used in mold filling simulations. The calculated results indicate that fiber orientation strongly affect the final properties, which vary with different chage locations, have a significant effect on warpage. Unsymmetric curing, caused by uneven mold temperatures, could lead to a thermal moment that could possibly help reduce warpage.     

Modelling complex manufacturing systems using discrete event simulation

This paper presents an approach in simulating complex manufacturing systems. The approach is founded on developing several general purpose simulation generators for an assembly station, a manufacturing cell, and an inventory transfer function. These simulation generators can then be linked together to create a model of a complex manufacturing system. A typical manufacturing system is modelled using these simulation generators and the results summarized.     

Resistance projection welding for TO-Can style package

The effects of the process parameters on the joint quality of TO-Can using resistance projection welding were investigated.A capacitor discharge stored energy welding machine was used for joining caps and headers both coated with nickel SPCC steel.The electrode materials used in this study were brass and W-Cu alloy.Experimen-tal results indicated that the peak current of the W-Cu electrode was higher than that of the brass electrode,irrespective of the operating pressure variation.Increased operating pressu...     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Study of SO2 adsorption and thermal regeneration over activated carbon-supported copper oxide catalysts

The mechanisms of SO₂ adsorption and regeneration over activated carbon-supported copper oxide sorbent/catalysts were analyzed. Studies were carried out in a fixed-bed reactor equipped with a non-dispersive infrared gas analyzer to detect the reaction products and by using X-ray powder diffraction (XRPD) and temperature-programmed desorption (TPD) experiments to characterize the nature of the sulfate species and surface oxygen complexes. The results indicate that SO₂ was catalytically oxidized to SO₃ over a copper phase in the presence of gaseous oxygen, and then reacted with a copper site to form a sulfate linked to copper without desorption into the gas phase. The activated carbon support did not participate in this sulfation reaction. After the adsorption of SO₂, the exhausted sorbent/catalysts could be regenerated by direct heat treatment in inert gas at temperatures between 260 and 480 °C, while the neighboring surface oxygen complexes on the carbon surface were acting as the reducing agents to reduce CuSO₄ to Cu. During the subsequent adsorption process, the copper is rapidly oxidized by oxygen in the flue gas.