Modeling Chemical Vapor Deposition of Silicon with Local Equilibrium Consideration at the Substrate

The purpose of this article is to present a general purpose mathematical model for describing the transport phenomena and the resulting rate of deposition in chemical vapor deposition (CVD) reactors. The model employs a finite difference scheme to solve the governing partial differential equations in order to predict the velocity field, temperature distribution, and concentration profiles of various gas species in a CVD reactor. The model predicts the rate of deposition for CVD reaction whose rate is mass transport controlled, for example, silicon deposition by the reaction of silicon tetrachloride and hydrogen at temperatures above 1050 °C. Silicon deposition by reaction of SiCl₄ and H₂ in a vertical CVD reactor with a rotating substrate is studied to compare the predicted and the experimental deposition rates. The effects of inlet gas composition, reactor pressure, and speed of substrate rotation on the rate of silicon deposition have been studied to verify the model's predictions. Local equilibrium and flux balance concepts have been used to determine the partial pressures of species at the substrate as well as the consequent deposition rate of silicon. The effects of thermal diffusion, grid size, and etching by HC1 on the deposition rate have also been studied. The results show that implementation of the local equilibrium concept and the flux balance principle, use of sufficiently small grid size above the substrate, thermal diffusion of SiCl₄ away from the substrate, and etching of deposited silicon by HC1 are all very important in evaluating the rate of silicon deposition in the mass transport-controlled regime. With all of the above-mentioned effects taken into consideration, model predictions agree well with the experimental data over a wide range of operating conditions for the system considered. Finally, use of the optimized design of a flow control device (FCD) in the reactor shows that once properly validated, the model can be used as a tool in computer-aided process optimization.     

Cross-entropy optimisation of multiple-input multiple-output capacity by transmit antenna selection

Radio channel capacity can be increased dramatically using a multiple-input multiple-output (MIMO) scheme, but at the expense of hardware complexity. An efficient approach for complexity reduction is antenna subset selection at the transmitter and/or receiver. A novel transmit antenna selection algorithm is presented using the cross-entropy optimisation method to maximise channel capacity. In contrast with the existing work, the proposed algorithm guarantees a result to within 99% of the true optimum (i.e. the maximal capacity with selected transmit antennas) with substantially low complexity. The simulation results indicate that the proposed algorithm is independent of the relationship between the selected transmit array size and receive array size. The proposed scheme has the potential to make practical MIMO systems with high performance simpler to implement.     

The Tube to Tube Through Transmission Technique for Inspecting Finned Heat Exchanger Tubes

Finned tubes are used in chemical processing and air conditioner heat exchangers. Conventional electromagnetic methods cannot be used to inspect the finned tubes for flaws because of the interference caused by the fins. We describe here tests of a recently developed method, the Tube to Tube Through Transmission (T4) technique which uses coplanar exciter and detector coils in neighboring tubes. As the method is new there is, as yet, little theoretical or experimental explanation for it. We report first, therefore, studies of the magnetic fields and the induced eddy currents. Since the coupling path between exciter and detector coils is primarily diffusive, skin effect equations can be applied to give useful approximations for signal analysis. Signals from machined full circumferential grooves in finned heat exchanger tubes were analyzed using these equations, and it was found that the depth of the grooves or general thinning could be estimated to within ±10%.     

Effect of pressure on mechanical and optical properties of ThO2 and PuO2

We have used DFT + U with spin–orbit coupling to understand the effect of pressure on the mechanical and optical properties of ThO₂ and PuO₂. Both the compounds are mechanically stable in a cubic and an orthorhombic structure. The cubic AnO₂ has higher elastic constants and Bader charges on each atom than the orthorhombic AnO₂. With an increase in pressure on cubic AnO₂, elastic modulus increases for both the structures. Bader charge on AnO₂ and magnetic moment of PuO₂ decrease with an increase in pressure for cubic structure. The static refractive index and static dielectric function are higher for PuO₂ in both the cubic and orthorhombic structures as compared to that in ThO₂. The mechanical and optical properties drawn comply with the experimental outcomes.     

Structural and Electrical Properties of Ni-Co Nanoferrites Prepared by Co-precipitation Route

A series of Ni_1?x Co _x Fe₂O₄ (x=0.1, 0.2, 0.3, 0.4, 0.5) spinel ferrites have been synthesized successfully using the chemical co-precipitation route. The materials were characterized by X-rays powder diffractometry (XRD) and the electrical properties. The obtained crystallite size variation was within 15 to 33 nm using the Scherrer formula. The dc electrical resistivity was measured as a function of temperature. It is noticed that ?? _dc increases with a rise in temperature. The dielectric measurements were carried out at room temperature as a function of frequency and composition (x). The dielectric constant (????) and dielectric loss tangent (tan???) showed a decreasing trend with increasing field frequency. The ac electrical conductivity is calculated from the dielectric measurements; it increases with the rise in frequency.     

Nanostructured titanium/diamond-like carbon multilayer films: deposition, characterization, and applications

Titanium/diamond-like carbon multilayer (TDML) films were deposited using a hybrid system combining radio frequency (RF)-sputtering and RF-plasma enhanced chemical vapor deposition (PECVD) techniques under a varied number of Ti/diamond-like carbon (DLC) bilayers from 1 to 4, at high base pressure of 1 × 10(-3) Torr. The multilayer approach was used to create unique structures such as nanospheres and nanorods in TDML films, which is confirmed by scanning electron microscopy (SEM) analysis and explained by a hypothetical model. Surface composition was evaluated by X-ray photoelectron spectroscopy (XPS), whereas energy dispersive X-ray analysis (EDAX) and time-of-flight secondary ion mass spectrometer (ToF-SIMS) measurements were performed to investigate the bulk composition. X-ray diffraction (XRD) was used to evaluate the phase and crystallinity of the deposited TDML films. Residual stress in these films was found to be significantly low. These TDML films were found to have excellent nanomechanical properties with maximum hardness of 41.2 GPa. In addition, various nanomechanical parameters were calculated and correlated with each other. Owing to metallic interfacial layer of Ti in multilayer films, the optical properties, electrical properties, and photoluminescence were improved significantly. Due to versatile nanomechanical properties and biocompatibility of DLC and DLC based films, these TDML films may also find applications in biomedical science.     

Role of dopant concentration, crystal phase and particle size on microbial inactivation of Cu-doped TiO2 nanoparticles

The properties of Cu-doped TiO(2) nanoparticles (NPs) were independently controlled in a flame aerosol reactor by varying the molar feed ratios of the precursors, and by optimizing temperature and time history in the flame. The effect of the physico-chemical properties (dopant concentration, crystal phase and particle size) of Cu-doped TiO(2) nanoparticles on inactivation of Mycobacterium smegmatis (a model pathogenic bacterium) was investigated under three light conditions (complete dark, fluorescent light and UV light). The survival rate of M. smegmatis (in a minimal salt medium for 2 h) exposed to the NPs varied depending on the light irradiation conditions as well as the dopant concentrations. In dark conditions, pristine TiO(2) showed insignificant microbial inactivation, but inactivation increased with increasing dopant concentration. Under fluorescent light illumination, no significant effect was observed for TiO(2). However, when TiO(2) was doped with copper, inactivation increased with dopant concentration, reaching more than 90% (>3 wt% dopant). Enhanced microbial inactivation by TiO(2) NPs was observed only under UV light. When TiO(2) NPs were doped with copper, their inactivation potential was promoted and the UV-resistant cells were reduced by over 99%. In addition, the microbial inactivation potential of NPs was also crystal-phase-and size-dependent under all three light conditions. A lower ratio of anatase phase and smaller sizes of Cu-doped TiO(2) NPs resulted in decreased bacterial survival. The increased inactivation potential of doped TiO(2) NPs is possibly due to both enhanced photocatalytic reactions and leached copper ions.     

An artificial neural network based adaptive power system stabilizer

An artificial neural network (ANN)-based power system stabilizer (PSS) and its application to power systems are presented. The ANN-based PSS combines the advantages of self-optimizing pole shifting adaptive control strategy and the quick response of ANN to introduce a new generation PSS. A popular type of ANN, the multilayer perceptron with error backpropagation training method, is used in this PSS. The ANN was trained by the training data group generated by the adaptive power system stabilizer (APSS). During the training, the ANN was required to memorize and simulate the control strategy of APSS until the differences were within the specified criteria. Results show that the proposed ANN-based PSS can provide good damping of the power system over a wide operating range and significantly improve the dynamic performance of the system