Influence of Bi 3+ ions in enhancing the magnitude of positive temperature coefficients of resistance in n-BaTiO3 ceramics

Bi3+ ions substituting at Ba-sites in a limited concentration range with another donor dopant occupying the Ti-sites in polycrystalline BaTiO3 enhanced the positive temperature coefficient of resistance (PTCR) by over seven orders of magnitude. These ceramics did not require normal post sinter annealing or a change to an oxygen atmosphere during annealing. These ceramics had low porosities coupled with better stabilities to large applied electric fields and chemically reducing atmospheres. Bi3+ ions limited the grain growth to less than 8 mum in size, they enhanced the concentration of acceptor-type trap centres at the grain-boundary-layer regions and maintained complete tetragonality at low grain sizes in BaTiO3 ceramics.

Temperature-programmed desorption and surface reaction of thiophene over co-mo/.gamma.-Al2O3 hydrodesulfurization catalysts

The temperature-programmed desorption (TPD) and temperature-programmed surface reaction (TPSR) of thiophene over a series of Co-Mo/gamma-Al2O3, hydrodesulfurization (HDS) catalysts with varying Co to Mo ratios have been studied with the objective of understanding the promotional role of Co in the HDS reaction. As part of the study, the desorptions (TPD) and hydrogenations (TPSR) of butane, butene, and butadiene over these catalysts have also been investigated. The TPD of the hydrocarbons over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site, with higher heats of desorption, without significantly affecting desorption from the original site. The TPSR measurements showed that the two sites had separate and independent activity for the hydrogenation of the C-4 hydrocarbons. The TPD of thiophene over catalysts with varying Co to Mo ratios showed a single desorption profile with identical heats of desorption, implying that Co does not affect or influence the adsorption sites for thiophene. The TPSR of the HDS of thiophene, however, showed that, although the products of the HDS reaction-butane, butene, and H2S-are the same irrespective of the Co content, the temperature profiles and the activation barriers for the formation of these species show considerable change with the Co/Co+Mo ratio. The results are discussed in light of the existing models for the promotional role of Co in the HDS reaction.

Temperature programmed desorption studies on hydrodesulfurization catalysts

The temperature programmed-desorption (TPD) of butane, butene, butadiene and thiophene over a series of Co-MO/gamma-Al2O3 catalysts with varying Co to Mo ratio has been investigated. The TPD of butane, butene and butadiene over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site without significantly affecting desorption from the original site. The TPD of thiophene over a series of catalysts with varying Co content showed identical desorption temperature as well as heat of desorption. It was concluded that thiophene was adsorbed on the ''Mo-S'' component of the catalyst and was unaffected by the presence of Co.

Robust/Optimal Temperature Profile Control of a High-Speed Aerospace Vehicle Using Neural Networks

An approximate dynamic programming (ADP)-based suboptimal neurocontroller to obtain desired temperature for a high-speed aerospace vehicle is synthesized in this paper. A I-D distributed parameter model of a fin is developed from basic thermal physics principles. "Snapshot" solutions of the dynamics are generated with a simple dynamic inversion-based feedback controller. Empirical basis functions are designed using the "proper orthogonal decomposition" (POD) technique and the snapshot solutions. A low-order nonlinear lumped parameter system to characterize the infinite dimensional system is obtained by carrying out a Galerkin projection. An ADP-based neurocontroller with a dual heuristic programming (DHP) formulation is obtained with a single-network-adaptive-critic (SNAC) controller for this approximate nonlinear model. Actual control in the original domain is calculated with the same POD basis functions through a reverse mapping. Further contribution of this paper includes development of an online robust neurocontroller to account for unmodeled dynamics and parametric uncertainties inherent in such a complex dynamic system. A neural network (NN) weight update rule that guarantees boundedness of the weights and relaxes the need for persistence of excitation (PE) condition is presented. Simulation studies show that in a fairly extensive but compact domain, any desired temperature profile can be achieved starting from any initial temperature profile. Therefore, the ADP and NN-based controllers appear to have the potential to become controller synthesis tools for nonlinear distributed parameter systems.

An aqueous-solution based low-temperature pathway to synthesize giant dielectric CaCu3Ti4O12-Highly porous ceramic matrix and submicron sized powder

A simple, cost-effective and environment-friendly pathway for preparing highly porous matrix of giant dielectric material CaCu3Ti4O12 (CCTO) through combustion of a completely aqueous precursor solution is presented. The pathway yields phase-pure and impurity-less CCTO ceramic at an ultra-low temperature (700 degrees C) and is better than traditional solid-state reaction schemes which fail to produce pure phase at as high temperature as 1000 degrees C (Li, Schwartz, Phys. Rev. B 75, 012104). The porous ceramic matrix on grinding produced CCTO powder having particle size in submicron order with an average size 300 nm. On sintering at 1050 degrees C for 5 h the powder shows high dielectric constants (>10(4) at all frequencies from 100 Hz to 100 kHz) and low loss (with 0.05 as the lowest value) which is suitable for device applications. The reaction pathway is expected to be extended to prepare other multifunctional complex perovskite materials. (C) 2010 Elsevier B.V. All rights reserved.

Bloch-Nordsieck thermometers: One-loop exponentiation in finite temperature QED

We study the scattering of hard external particles in a heat bath in a real-time formalism for finite temperature QED. We investigate the distribution of the 4-momentum difference of initial and final hard particles in a fully covariant manner when the scale of the process, Q, is much larger than the temperature, T. Our computations are valid for all T subject to this constraint. We exponentiate the leading infra-red term at one-loop order through a resummation of soft (thermal) photon emissions and absorptions. For T > 0, we find that tensor structures arise which are not present at T = 0. These carry thermal signatures. As a result, external particles can serve as thermometers introduced into the heat bath. We investigate the phase space origin of log (Q/M) and log (Q/T) terms.

Correlation of Kauzman temperature with odd-even effect in n-alkanes

A unique correlation has been established between Kauzmann temperature (Tk1) and the odd–even effect in n‐alkanes. The derived new parameter, i.e., Tm/Tk1 obtained from entropy conservation at Tk1, when plotted against chain length, provides a much sharper odd–even contrast than entropy of fusion plot reported earlier. © 1996 American Institute of Physics.

Linear electro-optic effect and temperature coefficient of birefringence in 4-nitro-4script-methylbenzylidene aniline single crystals

A study of the linear electro?optic effect in single crystals of the organic compound, 4?nitro�4??methylbenzylidene aniline is reported. The reduced half?wave voltages have been found to have values 2.8, 1.3, and 1.1 kV at 632.8, 514.5, and 488.0 nm, respectively and the corresponding values of the largest linear electro?optic coefficient have been calculated. The thermal variation of the birefringence has also been investigated and the temperature variation of the refractive index difference is found to have the value, d?n/dT = 15.8 × 10?5 K?1.

Low temperature synthesis and hcaracterization of Ag2S1-x Tez(O

A low-temperature route for the synthesis of Ag2S,Ag2Te and their solid solutions Ag2S1-xTex(0 less-than-or-equal-to x less-than-or-equal-to 1) is reported. Ag2S is prepared by the direct addition of silver nitrate solution to thiourea, while Ag2Te is prepared by reacting silver nitrate solution with tellurium in nitric acid and subsequently reducing it with hydrazine hydrate. The solid solutions of Ag2S and Ag2Te are obtained by the addition of nitrate solutions of silver and tellurium to thiourea followed by its reduction with hydrazine hydrate. The method enables the synthesis of low-temperature crystalline phase of Ag2S1-xTex solid solutions. The powder X-ray diffraction studies suggest that the solid solutions of compositions x < 0.3 have a phase akin to alpha-Ag2S and those with compositions x > 0.6 are similar to alpha-Ag2Te. In the intermediate range of compositions (x = 0.4 and 0.5), the solid solutions are found to be mixtures of alpha-Ag2S and alpha-Ag2Te phases which transform totally to alpha-Ag2S phase on prolonged annealing at about 473 K.

Monte Carlo simulation of low temperature phase diagrams of YBa2Cu3O6+x

We report the results of Monte Carlo simulation of oxygen ordering in the oxygen deficient portion (x<0.5) of YBa2Cu3O6+x at low temperatures. We find qualitative agreement among cluster - variation, Monte Carlo and transfer matrix methods. However, low temperature and ground state simulations clearly indicate the presence of a tetragonal phase. There is also evidence for two second order phase transition lines separating the tetragonal and the �double cell� ortho II phase. The effect of decreasing the inter-chain repulsion on oxygen ordering has also been investigated.

Low-temperature structure of two copper-based precursors for MOCVD: Aquabis(tert-butyl acetoacetato)copper(II) and Bis(dipivaloylmethanido)copper(II)

The Cu atoms in aquabis(tert-butyl acetoacetato)copper(II),[Cu(C8H13O3)(2)(H2O)], and bis(dipivaloylmethanido)copper(II), [Cu(C11H19O2)(2)], adopt square-pyramidal and planar conformations, respectively, with average Cu--O distances of 1.933 Angstrom in the former (not including the water ligand) and 1.892 Angstrom in the latter. It is interesting to note that the lability of the tert-butyl and methyl groups in both structures, which renders even the location of the terminal C atoms difficult, is reduced at T = 130 K, enabling location of all the protons in the difference Fourier map.

Low-temperature synthesis of bismuth cuprates

A low temperature aqueous solution preparation under strong alkaline medium is reported for the synthesis of bismuth cuprates. Highly crystalline products were obtained at temperatures around 90 degrees C. Tetragonal Bi2CuO4 appears to be the only stable phase formed in the Bi-Cu-O system under these conditions.

Methanol-to-gasoline (MTG) conversion over ZSM-5. A temperature programmed surface reaction study

The conversion of methanol to gasoline over zeolite ZSM-5 has been studied by temperature programmed surface reaction (TPSR). The technique is able to monitor the two steps in the process: the dehydration of methanol to dimethyl ether and the subsequent conversion of dimethyl ether to hydrocarbons. The activation barriers associated with each step were evaluated from the TPSR profiles and are 25.7 and 46.5 kcal/mol respectively. The methanol desorption profile shows considerable change with the amount of methanol molecules adsorbed per Bronsted site of the zeolite. The energy associated with the desorption process, (CH3OHH+-ZSM5 --> (CH3OHH+-ZSM5 + CH3OH, shows a spectrum of values depending on n.

Temperature-dependent photoluminescence of GaN grown on beta-Si3N4/Si (111) by plasma-assisted MBE

Photoluminescence (PL) of high quality GaN epitaxial layer grown on beta-Si3N4/Si (1 1 1) substrate using nitridation-annealing-nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5-300 K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an `S-like' shape of free exciton (FX) emission peaks. Distant shallow donor-acceptor pair (DAP) line peak at approximately 3.285 eV was also observed at 5 K, followed by LO replica sidebands separated by 91 meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be similar to 27.8 +/- 0.7 meV from the temperature-dependent PL studies. Low carrier concentrations were observed similar to 4.5 +/- 2 x 10(17) Cm-3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2 eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy. (C) 2010 Elsevier B.V. All rights reserved.