Changes in surface stress, morphology and chemical composition of silica and silicon nitride surfaces during the etching by gaseous HF acid

HF acid attack of SiO₂ and Si₃N₄ substrates is analyzed to improve the sensitivity of a sensor based on microcantilever. Ex situ analysis of the etching using XPS, SIMS and AFM show significant changes in the anisotropy and the rate of the etching of the oxides on SiO₂ and Si₃N₄ surface. Those differences influence the kinetic evolution of the plastic bending deflection of the cantilever coated with SiO₂ and Si₃N₄ layer, respectively. The linear dependence between the HF concentration and the Si₃N₄ cantilever bending corresponds to a deep attack of the layer whereas the non-linear behavior observed for SiO₂ layer can be explained by a combination of deep and lateral etching. The cantilever bending is discussed in terms of free surface energy, layer thickness and grain size.     

Influence of V2O5 on the magnetic properties of nickel–zinc–copper ferrites

Additions of V₂O₅ were used to decrease the sintering temperature and to enhance the densification of NiZnCu ferrite. With a small amount of V₂O₅ (0.6–1.2 wt%) as a sintering aid, densification at low sintering temperature was observed. EDS analyses showed the presence of vanadium inside the ferrite grains. Microstructure investigation revealed heterogeneous grain size. Magnetic properties were deteriorated when increasing the V₂O₅ content. As a consequence, the permeability decreased and the core losses increased, which can be explained by the pinning of the domain walls to the grain defects.     

Software Engineering and complexity in effective Algebraic Geometry

One may represent polynomials not only by their coefficients but also by arithmetic circuits which evaluate them. This idea allowed in the past fifteen years considerable complexity progress in effective polynomial equation solving. We present a circuit based computation model which captures all known symbolic elimination algorithms in effective Algebraic Geometry and exhibit a class of simple elimination problems which require exponential size circuits to be solved in this model. This implies that the known, circuit based elimination algorithms are already optimal.     

Pairwise-substitution effects and intramolecular hydrogen bonds in nitrophenols and methylnitrophenols. Thermochemical measurements and ab initio calculations

The standard molar enthalpies of formation in the gaseous state of a series of nitrophenols, 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 5-methyl-2-nitrophenol, 2-methyl-5-nitrophenol, and 3-methyl-4-nitrophenol, have been obtained from combustion calorimetry and results from the temperature dependence of the vapor pressure measured by the transpiration method. To verify the experimental data, ab initio calculations of all compounds have been performed using MP, DFT, and G3 methods. Enthalpies of formation derived from the G3 methods are in a good agreement with the experimental results. The quantitative analysis of ortho, meta, and para pairwise-substituent effects in nitrophenols has been performed, and the strength of intramolecular hydrogen bonding in o-nitrophenol has been derived from thermochemical results and compared with those obtained from spectroscopic experiments and ab initio calculations. The new results help to resolve uncertainties in the available thermochemical data on extended series of nitrophenols.     

Comprehensive experimental and theoretical study of chemical equilibria in the reacting system of the tert-amyl methyl ether synthesis

The chemical equilibrium of the reactive system (methanol+isoamylenes<-->methyl tert-amyl ether) was studied in the temperature range 298-393 K in the liquid phase using the method of sealed ampoules as well as in the gaseous phase using a tubular flow reactor in the temperature range 355-378 K. In both cases, a cation exchanger Amberlist-15 was used as a heterogeneous catalyst. The reactive system of the methyl tert-amyl ether synthesis exhibits a strong nonideal behavior of the mixture compounds in the liquid phase. The knowledge of the activity coefficients is required in order to obtain the thermodynamic equilibrium constants Ka. Two well-established procedures, UNIFAC and COSMO-RS, have been used to assess activity coefficients of the reaction participants in the liquid phase. Thermodynamic equilibrium constants KP measured in the gaseous phase together with the vapor pressures of the pure compounds have been used to obtain Ka in the liquid phase on a consistent experimental basis in order to check the results obtained from the UNIFAC and COSMO-RS methods. Enthalpies of reactions DeltarH degrees of the methyl tert-amyl ether synthesis reaction in the gaseous and in the liquid phase were obtained from temperature dependences of the corresponding thermodynamic equilibrium constants. Consistency of the experimental data of DeltarH degrees was verified with help of enthalpies of formation and enthalpies of vaporization of methyl tert-amyl ether, methanol, and methyl-butenes, available from the literature. For the sake of comparison, high-level ab initio calculations of the reaction participants have been performed using the Gaussian-03 program package. Absolute electronic energy values, normal frequencies (harmonic approximation), and moments of inertia of the molecules have been obtained using G2(MP2), G3(MP2), and G3 levels. Using these results, calculated equilibrium constants and the enthalpy of reaction of the methyl tert-amyl ether synthesis in the gaseous phase based on the principles of statistical thermodynamics are found to be in acceptable agreement with the data obtained from the thermochemical measurements.     

Limiting diffusion coefficients of ionic liquids in water and methanol: a combined experimental and molecular dynamics study

Mutual diffusion coefficients D(12) of the ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(2)MIM][NTf(2)]) and [C(4)MIM][NTf(2)] in highly diluted solutions of water and methanol have been measured at different temperatures between 288 K and 313 K using the Taylor dispersion technique. Tracer diffusion coefficients of the two cations [C(2)MIM](+) and [C(4)MIM](+) as well as the anion [NTf(2)](-) in these solutions have been obtained by molecular dynamics (MD) simulations. For our simulations we used well established force fields for the solvents water and methanol and a recently developed force field for imidazolium-based ionic liquid [C(n)MIM][NTf(2)]. Mutual diffusion coefficients D(12) have been calculated from the tracer diffusion coefficients using the Nernst-Hartley equation strictly valid only at low ionic concentration. The agreement between the diffusion coefficients reported in the literature, the experimental data obtained in this work and the MD results is excellent.     

Effect of porosity on the thermal conductivity of copper processed by powder metallurgy

Powder metallurgy is a preferred method of processing copper–carbon composites due to the non-wetting nature of these materials. Porosities are inherently introduced in these material systems, and adversely affect the thermal conductivity of the composite material, among other factors including interfaces and reinforcement distribution. In this work, we focus on the matrix material of pure copper and systematically analyzed the effect of volume fraction of porosities on the thermal conductivity. Spherical and dendritic copper powder materials were processed and it was found that the surface chemistry and morphology of particles affected the thermal conductivity apart from the porosity values. In order to study the effect of porosities alone, dentritic powder was used in the study. The thermal conductivity vs. porosity behavior showed three distinct domains. In all the domains the thermal conductivity decreases as volume fraction of porosities increases; however, in domain II, the decrease was much steeper than the other two. We are able to explain the variation based on the presence of interconnected and open pores in domain III to closed pores in domain I, and the transition occurring in domain II. None of the existing models capture the overall behavior. However, if we specifically account for the variation of number of grain boundaries with sintering, then the modified EMT model can match the experimental data.