Electromechanical and electrothermal behaviours of carbon whisker reinforced elastomer composites

The electromechanical and electrothermal properties of conducting carbon whisker reinforced thermoplastic elastomer (TPE) composites were investigated. The carbon whiskers were derived by a catalytic chemical vapour Deposition (CCVD) process and the TPE was a styrene-ethylene-butylene-styrene (S-EB-S) block copolymer. The electrical resistivity (ϱ) of the composites can be varied either by uniaxial deformation (10¹–10⁸ Ω cm) or by temperature (10¹–10⁵ Ω cm). The temperature-resistivity studies indicated that the resistivity of these composites was influenced by the glass transition temperature (T _g) of the TPE. The ϱ versus 1/T curves exhibited two distinct regimes each with a different negative slope which intersected at the T _g of the elastomer. This was correlated to the T _gof the EB segments in the S-EB-S block copolymer (∼ -50°C) by the dynamic mechanical thermal analysis. Further, uniaxial deformation studies at room temperature (20 °C) demonstrated that the resistivity increased exponentially with the deformation. Processing technique considerations and electron micrographs of the morphology of the composites indicated the formation of polymeric film on the carbon whiskers. Thus, the electrical conduction between carbon whiskers in these highly loaded (33 and 52 vol % fraction) composites occurred through the elastomeric film by electron tunnelling. This is explained on the basis of Mott's electron hopping theory, for conduction through several carbon-polymer-carbon (C-P-C) junctions. Further studies by scanning electron microscopy, dielectric thermal analysis and voltage-current characteristics confirmed this observation. Mechanical and electrical properties of the composites indicated that CCVD carbon whiskers can be used to improve the strength and electrical conductivity of TPEs. The change in resistivity (up to five orders of magnitude) of the composites with respect to the deformation or temperature can find use in electromechanical and electrothermal device applications.     

Precoded random spreading multiple access system in AWGN channels

In this paper, we compare the asymptotic performance of the decorrelator approaches to multiuser detection in code-division multiple-access (CDMA). Precoding is the decorrelation which is performed at a common transmitter such as the base station or satellite that allows single-user detection at the handset receiver. Receiver-based decorrelator decorrelates multiple user signals at the receiver side. We analyze the performance of the aforementioned decorrelation approaches with time-varying random spreading sequences in additive white Gaussian noise channels. The results demonstrate that the performance of precoding and that of receiver-based decorrelator are asymptotically equivalent.     

Low‐Complexity MPEG‐4 Shape Encoding towards Realtime Object‐Based Applications

Although frame‐based MPEG‐4 video services have been successfully deployed since 2000, MPEG‐4 video coding is now facing great competition in becoming a dominant player in the market. Object‐based coding is one of the key functionalities of MPEG‐4 video coding. Realtime object‐based video encoding is also important for multimedia broadcasting for the near future. Object‐based video services using MPEG‐4 have not yet made a successful debut due to several reasons. One of the critical problems is the coding complexity of object‐based video coding over frame‐based video coding. Since a video object is described with an arbitrary shape, the bitstream contains not only motion and texture data but also shape data. This has introduced additional complexity to the decoder side as well as to the encoder side. In this paper, we have analyzed the current MPEG‐4 video encoding tools and proposed efficient coding technologies that reduce the complexity of the encoder. Using the proposed coding schemes, we have obtained a 56 percent reduction in shape‐coding complexity over the MPEG‐4 video reference software (Microsoft version, 2000 edition).     

The contraction of lattice constant and the reduction of growth rate in p-InGaAs grown by organometallic vapor phase epitaxy

We have investigated the effect of diethylzinc (DEZn) on the lattice constant and the growth rate of InGaAs. Introducing DEZn for p-type doping induces the contraction of lattice constant and the reduction of growth rate compared to undoped InGaAs. Depletion of indium is responsible for these effects. These effects are reduced at lower growth temperatures or at lower growth pressures. From the observed effects of the growth temperatures and the growth pressures on the contraction of the lattice constant, it is concluded that depletion of indium occurs in the gas phase.     

Polystyrene/bisphenol a polycarbonate (PS/PC) molecular composite by in situ polymerization. II. Effect of different molecular size of PC on the blending behavior and properties

The influences of different molecular sizes and contents of bisphenol A polycarbonate (PC) on the miscibility, morphology, and physical properties of a polystyrene/PC (PS/PC) molecular composite are examined. The polymerization of the matrix monomer, styrene, which is completely miscible with PC, affords an apparently homogeneous mixture that remains stable on heating to above the glass transition temperature (T_g) of the pure matrix and solubilizes in common organic solvents. Both the molecular and material characterizations for the purified products are examined in full detail by various instrumental analyses and microscopy. An enhanced miscibility with higher molar mass of PC was confirmed by both the unambiguous scanning electron microscopy (SEM) images, showing subinclusions of PS/PC microdomains in the matrix, and the characteristic size exclusion chromatograms (SEC) of the fractionated sample, exhibiting homogeneous molecular mixing. With increased molecular size of PC and its content, the T_g increases proportionally and shows a shift to higher temperatures than that of pure PC, indicating the reduced segmental motion of PC chains due to the intimate chain entanglement with PS chains. Except for the case of the lowest molar mass of PC, both the thermal and tensile properties of the composite are also elevated. The conformational similarity between the two long flexible‐chain polymers has clearly accentuated the tendency for miscibility. This effect is due mainly to the increase in entropy gained by mixing, which in turn leads to reinforcement of n–π complex formation between the carbonate and benzene mixtures. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1610–1618, 2003     

Filtered fast neutron irradiation system using Texas A&M University Nuclear Science Center Reactor

A heavily filtered fast neutron irradiation system (FNIS) was developed for a variety of applications, including the study of long-term health effects of fast neutrons by evaluating the biological mechanisms of damage in cultured cells and living animals such as rats or mice. This irradiation system includes an exposure cave made with a lead–bismuth alloy, a cave positioning system, a gamma and neutron monitoring system, a sample transfer system, and interchangeable filters. This system was installed in the irradiation cell of the Texas A&M University Nuclear Science Center Reactor (NSCR). For a realistic modeling of the NSCR, the irradiation cell, and the FNIS, this study used the Monte Carlo N-Particle (MCNP) code and a set of high-temperature ENDF/B-VI continuous neutron cross-section data. Sensitivity analysis was performed to find the characteristics of the FNIS as a function of the thickness of the lead–bismuth alloy. A paired ion chamber system was constructed with a tissue-equivalent plastic (A-150) and propane gas for total dose monitoring and with graphite and argon for gamma dose monitoring. This study, in addition, tested the Monte Carlo modeling of the FNIS system, as well as the performance of the system by comparing the calculated results with experimental measurements using activation foils and paired ion chambers.     

DC and AC characteristics ofAL0.25Ga0.75As/GaAs quantum-well delta-dopedchannel FET grown by LP-MOCVD

Al_0.25Ga_0.75As/GaAs quantum well delta-doped channel FETs (QWDFETs) have been successfully fabricated by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). The FETs with a gate dimension of 1.8 μm×100 μm had a maximum extrinsic transconductance of 190 mS/mm and a maximum current density of 425 mA/mm. The device showed extremely broad transconductances around its peak. The S-parameter measurements indicated that the current gain and power gain cutoff frequencies of the device were 7 and 15 GHz, respectively. The transconductance versus gate voltage profiles showed a plateau region through a range of 1.7 V supporting spatial confinement of the electrons. These values are among the best reported for delta-doped GaAs-based FETs with a similar device geometry