[en] Short-period (Si_m/Ge_n)_N superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step-graded SSLs as buffer layers, 2000 Aa uniform Si_0.75Ge_0.25 alloy layers are grown on the same substrates. The growth temperature of the SSLs and uniform layers is 500 degree C. In the SSLs layers, m and n are the number of monolayers of Si and Ge, respectively. N is the period of (Si_m/Ge_n) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a function of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 Aa]. A dramatic decrease in roughness is observed in the uniform Si_0.75Ge_0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticeable increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation shows that the rms surface roughness behavior of the SSLs is reflected to their corresponding top alloy layers. The residual strains in alloy layers are considerably lower, with a maximum relaxation rate of about 80% for the sample with a seven-step SSL buffer. Cross-sectional TEM images show that strained SSL buffer layers effectively deflect threading dislocations in the substrate or confine the dislocations in the SSL buffer layers. [copyright] 2001 American Institute of Physics

[en] This study focuses on improvement of mechanical and physical properties of polypropylene (PP) composite using filler, modifier and jute fiber as reinforcement. Rice husk ash (RHA), low-density polyethylene (LDPE) and jute fiber were used as filler, modifier, and reinforcement in the PP matrix, respectively. A series of test specimens were using various compositions of PP, fillers, modifiers, and reinforcement. Materials were mixed using two roll mixing machine, and specimens were prepared using injection molding machine. The specimens were evaluated on the basis of mechanical (tensile strength, elongation at break) and Physical (water absorption) properties. Effect of RHA was assessed on the basis of mechanical properties. Surface property of the PP composites was studied using Scanning Electron Microscope (SEM). The water absorption property of composites with various compositions was investigated. Further attempts had been made to optimize the three process parameters (RHA, LDPE, jute fiber) in the PP composite with respect to its tensile strength. The optimum parameters for the PP composite were found to be 10 wt% RHA, 10 wt% LDPE and 25 wt% jute reinforcement, respectively in the PP matrix. (paper)

[en] Electron-acoustic shock waves (EASWs) in an unmagnetized electron-positron-ion plasma system (consisting of a cold mobile viscous electron fluid, hot electrons and positrons following the q-nonextensive distribution, and immobile positive ions) are studied analytically. The Burgers equation is derived by using the well-known reductive perturbation method. The basic features (viz. polarity, amplitude, width, phase speed, etc.) of EASWs are briefly addressed. The basic features of EASWs are found to be significantly modified by the effects of nonextensivity of the hot electrons and positrons, the relative number density and temperature ratios, and the kinematic viscosity of the cold electrons. The present investigation can be useful in understanding the fundamental characteristics of EASWs in various space plasmas.

[en] Nitrogen-doped hydrogenated amorphous carbon films have been deposited on silicon substrates by radio-frequency plasma-enhanced chemical vapor deposition using different N₂/CH₄ gas ratios from 0 to 3. The real and imaginary parts, n and k, of the complex index of refraction of these films have been determined for wavelengths between 300 and 830 nm by spectroscopic ellipsometry. Excellent agreement has been found between measured and modeled spectra, in which an empirical dielectric function based on classical Lorentz oscillator and Tauc joint density of states, and a linear void distribution along the thickness of the films have been assumed. Decrease in the optical energy gap and increase in the extinction coefficient, k, with increase in nitrogen concentration have been observed. Refractive index, n, increases rapidly with increase in nitrogen concentration up to 6.8 at.% (∼7.0 at.%) and then increases slowly with further increase in nitrogen concentration. For all the samples, n is found to be highest at the film-substrate interface which gradually decreases towards the film surface. [copyright] 2001 American Institute of Physics

[en] Nitrogen doped hydrogenated amorphous carbon thin films have been deposited by rf plasma-enhanced chemical vapor deposition using CH₄ as the source of carbon and with different nitrogen flow rates (N₂/CH₄ gas ratios between 0 and 3), at 300 K. The dependence modifications of the optical and the structural properties on nitrogen incorporation were investigated using different spectroscopic techniques, such as, Raman spectroscopy, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, ultraviolet-visible (UV-VIS) spectroscopy, electron spin resonance (ESR), photoluminescence (PL) and spectroscopic ellipsometry (SE). Raman spectroscopy and IR absorption reveal an increase in sp²-bonded carbon or a change in sp² domain size with increasing nitrogen flow rate. It is found that the configuration of nitrogen atoms incorporated into an amorphous carbon network gradually changes from nitrogen atoms surrounded by three (σ bonded) to two (π bonded) neighboring carbons with increasing nitrogen flow rate. Tauc optical gap is reduced from 2.6 to 2.0 eV, and the ESR spin density and the peak-to-peak linewidth increase sharply with increasing nitrogen flow rate. Excellent agreement has been found between the measured SE data and modeled spectra, in which an empirical dielectric function of amorphous materials and a linear void distribution along the thickness have been assumed. The influence of nitrogen on the electronic density of states is explained based on the optical properties measured by UV-VIS and PL including nitrogen lone pair band. [copyright] 2001 American Institute of Physics

[en] This paper presents the characteristic model of the wishbone suspension system using the quarter car model approach. Suspension system in an automobile provides vehicle control and passenger comfort by providing isolation from road disturbances. This makes it essential that the detailed behavior of suspension should be known to optimize the performance. A kinetic study is performed using multi body system (MBS) analysis. The dirt road profile is considered as an applied loading. The spring constant, damping coefficient and sprung mass are studied on the performance of the suspension system. It can be observed that the spring constant is inversely related with time required to return to initial position and the amount of deformations. The damping ratio affects the suppression of spring oscillations, beyond a certain limit damping ration has the negligible effect. Sprung mass effected the equilibrium position of the suspension system with a small effect on its oscillation behavior. It is shown that the spring constant, damping ratio and sprung mass are significant parameters to design the suspension system. This study is essential for complete understanding of working of the suspension system and a future study with real geometries.

[en] The photoconductivity of TlGaSe₂ layered single crystals are investigated in the temperature range 78-300 K. Both the ac-photoconductivity (ac-PC) and the spectral distribution of the photocurrent were studied at different values of light intensity, applied voltage and temperature. Dependences of carrier lifetime on light intensity, applied voltage and temperature have been investigated as results of the ac-PC and dc-photoconductivty (dc-PC) measurements. The temperature dependence of the energy gap width was described as a result of studying the dc-PC

[en] Burning of fossil fuels is one of the largest sources of greenhouse gas emissions in the USA as well as other parts of the World. Increased greenhouse emissions not only cause the global warming but also pose threat to public health. This has led to increased interest in the production of renewable energy to reduce greenhouse gas emissions. Biodiesel is an alternative fuel source that is obtained from renewable biomass, such as canola, soybean, corn, palm, sunflower, and algae. The green alga Botryococcus braunii shows great potential for biodiesel production due to its ability to produce high amounts of hydrocarbon. In this study, we investigated culture media (starter mix, miracle-Gro, and commercial fertilizer) and light sources (yellow, white LED, and pink/blue) effects on the growth of B. braunii for the production of biomass and hydrocarbons. The test results indicated that starter mix media under pink/blue light source with the light intensity of 24.3 μmol/s and photoperiod of 12 h light: 12 h dark at room temperature (22–25 °C) were found to be best condition. The highest optical density (OD₆₈₀) value (1.853) was obtained on day 23 of the incubation period. Variation of the dry biomass expectedly showed similar characteristics to the optical density.

[en] In this paper, the parametric study of a two shafts gas turbine cycle model of the power plant was proposed. The power output, compression work, specific fuel consumption and thermal efficiency are evaluated with respect to the cycle temperature and compression ratio for a typical set of operating conditions. Two shafts gas turbine cycle with realistic parameters is modeled. The computational model was developed utilizing the MATLAB codes. Turbine work found to be decreases as ambient temperature increases as well as the thermal efficiency decreases. It can be seen that the thermal efficiency and power output increases linearly with increases of compression ratio while decreases of ambient temperature. The power of the simulated two shafts gas turbine reach to 135MW, which is higher than the simple gas-turbine cycle (Baiji gas turbine power plant, power < 131MW). The specific fuel consumption increases with increases of ambient temperature as well as the lower turbine inlet temperature. Even though at the lower turbine inlet temperature is decrement the thermal efficiency dramatically and the power output increases linearly with increases of compression ratio and decreases the ambient temperature.

[en] Obliquely propagating electron-acoustic solitary waves (EASWs) in a magnetized electron−positron−ion plasma (containing nonextensive hot electrons and positrons, inertial cold electrons, and immobile positive ions) are precisely investigated by deriving the Zakharov–Kuznetsov equation. It is found that the basic features (viz. polarity, amplitude, width, phase speed, etc.) of the EASWs are significantly modified by the effects of the external magnetic field, obliqueness of the system, nonextensivity of hot positrons and electrons, ratio of the hot electron temperature to the hot positron temperature, and ratio of the cold electron number density to the hot positron number density. The findings of our results can be employed in understanding the localized electrostatic structures and the characteristics of EASWs in various astrophysical plasmas.