Fluorinated nanoporous SiO2 films with ultra-low dielectric constant

Fluorinated nanoporous silica (denoted as SiO₂:F) thin films with low dielectric constant were prepared by a sol-gel method and spin coating technique. The leakage current densities of the SiO₂:F thin films were 10⁻⁸ and 3 × 10⁻⁶ A/cm² respectively for the as-deposited films and for those subjected to annealing at a temperature of 450 °C. These currents are more than one order of magnitude lower than those of the common SiO₂ films. Photoluminescent results showed strong blue-light emission and a small blue shift in the SiO₂:F films that were related to the increment of the porosity. The dielectric properties were also characterized and the k value of the annealed SiO₂:F film was found to be about 1.67. The hole size in the films is small and the size distribution is uniform for the annealed SiO₂:F samples due to the effects of fluorination. The underlying mechanism for fluorination is discussed in this paper.     

Glass viscosity as a function of temperature and composition: A model based on Adam-Gibbs equation

This paper shows that a generalized Adam-Gibbs relationship reasonably approximates the real behavior of glasses with four temperature-independent parameters of which two are linear functions of the composition vector. The equation is subjected to two constraints, one requiring that the viscosity-temperature relationship approaches the Arrhenius function at high temperatures with a composition-independent pre-exponential factor and the other that the viscosity value is independent of composition at the glass-transition temperature. Several sets of constant coefficients were obtained by fitting the generalized Adam-Gibbs equation to data of two glass families: float glass and Hanford waste glass. Other equations (the Vogel-Fulcher-Tammann equation, original and modified, the Avramov equation, and the Douglass-Doremus equation) were fitted to a float-glass data series and compared with the Adam-Gibbs equation, showing that Adam-Gibbs glass appears an excellent approximation of real glasses even as compared with other candidate constitutive relations.     

Model-free method for analysis of non-isothermal kinetics of a bulk sample of selenium

In this work, the activation energies of crystallization of amorphous Se were studied under non-isothermal conditions using a differential scanning calorimetric (DSC) technique. The Johnson-Mehl-Avrami and isoconversional models were used to describe the DSC crystallization data. The isoconversional methods of Friedman, Kissinger-Akahira-Sunose (KAS) and Vyazovkin were used to determine the variation of the activation energy for crystallization with temperature, E_α (T). The KAS and Vyazovkin methods gave identical values, and the range of E_α (T) was found to vary in the range 110.4-44.2 kJ mol⁻¹, while the Friedman method gave lower values with E_α (T) in the range 100.5-28.3 kJ mol⁻¹. The effects of annealing were revealed by studying the morphology of the samples with a scanning electron microscope.     

Development of optical nonlinearity, high dielectric constant and ferromagnetic behavior in a silicate glass nanocomposite by suitable heat treatment

We have explored the development of multifunctionalities viz, optical nonlinearity, high dielectric constant and ferromagnetic behavior in a nanostructured silica based glass of 14.0Na₂O, 26.0BaO, 26.0TiO₂, 16.0B₂O₃, 17.0SiO₂, 1.0NiO (mol%) composition. A heat treatment at 863 K for 4 h led to nonlinear refractive index and absorption coefficients at wavelength 800 nm of 0.11 × 10⁻¹⁹ m²/W and 1.15 × 10⁻³ cm/GW, respectively. A heat treatment at 1073 K for 2 h followed by 1113 K for 3 h increased the dielectric constant from 11 to 50, apparently due to the formation of nanocrystals of BaTiO₃ within the glass medium. Glass samples reduced at 923 K for 1 h exhibited ferromagnetic behavior due to the presence of nickel nanoparticles.     

Fabrication and characterization of metallic glasses with a specific microstructure for micro-electro-mechanical system applications

Metallic glass microstructures with high aspect ratios for micro-electro-mechanical system applications have been fabricated by micro-electro-discharge machining and selective electrochemical dissolution methods. Micro-holes and three-dimensional microstructures machined on the La₆₂Al₁₄Ni₁₂Cu₁₂, Zr₅₅Al₁₀Ni₅Cu₃₀ and Cu₄₆Zr₄₄Al₇Y₃ bulk metallic glasses by micro-electro-discharge machining are evaluated by using X-ray diffraction, scanning electron microscopy, and nanoindentation. The experimental results demonstrate that the machined samples kept their amorphous structure without devitrification, and their machining characteristics are related to the thermo-physical properties of the alloys and the electrode diameters. Porous, single-pore and thin-walled Zr-based metallic glass tubes with micro-pore structures can be prepared by selective electrochemical dissolution method. The high aspect ratio microstructures fabricated by the two methods have the potential applications as micro-nozzles, polymer micro-injection molding tools, micro-channels or micro-flow meters in micro-electro-mechanical system devices.     

Monte Carlo simulation of a cluster model for the chirality conversion of crystals with grinding

By means of Monte Carlo simulation of a cluster model, we study statistical aspect of the chirality conversion of crystals. The chirality conversion is known to occur in a saturated solution by grinding crystals, and the enantiomeric excess increases exponentially. In the simulation, with crystallization and dissolution of monomers alone, the behavior of the enantiomeric excess observed in experiment cannot be reproduced although the homochirality is realized as a result of statistical fluctuation. By allowing crystallization and dissolution of dimers, the exponential amplification of enantiomeric excess can be realized.     

Study of 1,6-bis-O-(N-propionyl-L-alanyl)-1,6-hexanediol as a model compound of poly(ester amide)s derived from L-alanine

The conformation of 1,6-bis-O-(N-propionyl-L-alanyl)-1,6-hexanediol (PrAHAPr) has been investigated in the crystalline state by X-ray analysis. PrAHAPr crystallizes in the monoclinic system, space group P2₁ with a = 4.861(4), b = 26.24 (1), c = 8.716 (3) Å and = 105.25 (1)°. In the crystal, the molecules are hydrogen bonded in a single direction that runs parallel to the a crystallographic axis. In spite of its chemical symmetry the molecule adopts an asymmetric conformation, where one of the alanine residues takes a folded conformation. PrAHAPr belongs to a series of model compounds for poly(ester amide)s derived from amino acids that we are presently investigating.     

Sulfur behavior in silicate glasses and melts: Implications for sulfate incorporation in nuclear waste glasses as a function of alkali cation and V2O5 content

The presence of sulfur in radioactive waste to be incorporated in borosilicate glasses entails difficulties mainly due to the relatively low solubility of sulfates in the vitreous phase. In this work a study is presented on the effects of the ratio R = [Na₂O]/[B₂O₃], the type of sulfate added and the addition of V₂O₅ on the incorporation of sulfates in borosilicate glasses. Glass samples were prepared at the laboratory scale (up to 50-100 g) by melting oxide and sulfate powders under air in Pt/Au crucibles. XRF and ICP/AES chemical analysis, SEM/EDS, microprobe WDS and Raman spectroscopy were employed to characterize the fabricated samples. The main experimental results confirm that the incorporation of sulfates in borosilicate glasses is favored by the network depolymerization, which evolves with the ratio R. The addition of V₂O₅ seems to accelerate the kinetics of sulfur incorporation in the glass and, probably, increase the sulfate solubility by modifying the borate network and fostering the formation of voids of shape and size compatible with the sulfur coordination polyhedron in the glassy network. The kinetics of X₂SO₄ incorporation in the glass seems to be slower when X = Cs.     

A model compound for poly(ester amide)s: diethyl-3,9-diaza-4,8-dioxoundecanedioate

The compound diethyl-3,9-diaza-4,8-dioxoundecanedioate (EtGGGEt) has crystallized in the space group P2₁ with cell parameters a = 8.267(2), b = 4.853(4), c = 20.361(6) Å, = 97.96(2)°. The crystal structure has been solved by direct methods and refined using the full matrix least squares methodology. There is one molecule in the asymmetric unit, in spite of its symmetric constitution. The central part of the molecule, the glutaramide moiety, has a partial folded conformation (T TTST), which differs from the all trans conformation found in related moieties but with an even number of carbon atoms. The two flanking glycine residues have a different conformation one from each other; the first one adopts the standard conformation ( = –156.5°, = 73°), while the second one has an unusual conformation. In this glycine the oxygen atom has a big temperature factor and it is disordered in two positions. Both facts, conformation and disorder of the second glycine, seem to be due to improvement of contacts with neighboring molecules. The molecules are hydrogen bonded along the b axis, forming infinite rows with the same sense of orientation, so the crystal has a polar structure.     

Efficient solution of a batch crystallization model with fines dissolution

In this paper, an efficient and accurate numerical method is proposed for solving a batch crystallization model with fines dissolution. The dissolution of small crystals (fines dissolution) is useful for improving the quality of a product. This effectively shifts the crystal size distribution (CSD) towards larger crystal sizes and often makes the distribution narrower. The growth rate can be size-dependent and a time-delay in the dissolution unit is also incorporated in the model. The proposed method has two parts. In the first part, a coupled system of ordinary differential equations (ODEs) for moments and solute mass is numerically solved in the time domain of interest. These discrete values are used to get growth and nucleation rates in the same time domain. In the second part, the discrete growth and nucleation rates along with the initial CSD are used to construct the final CSD. The analytical expression for CSD is obtained by applying the method of characteristics and Duhamel's principle on the given population balance model (PBM). A Gaussian quadrature method, based on orthogonal polynomials, is used for approximating integrals in the ODE-system of moments and solute mass. The efficiency and accuracy of the proposed numerical method is validated by a numerical test problem.     

ASL model for prediction of CSD in a continuous crystallizer

A Monte Carlo simulation scheme is proposed for crystal size distribution (CSD) in a continuous crystallizer for size dependent growth rate. Crystal growth rates are described by Abegg, Stevens, and Larson (ASL) model. The proposed model is used to predict CSD from potassium carbonate crystallizer. The agreement between theory and available data confirms the validity of the model.     

Validation of a 3D multi-physics model for unidirectional silicon solidification

A model for transient movements of solidification fronts has been added to X-stream, an existing multi-physics simulation program for high temperature processes with flow and chemical reactions. The implementation uses an enthalpy formulation and works on fixed grids. First we show the results of a 2D tin solidification benchmark case, which allows a comparison of X-stream to two other codes and to measurements. Second, a complete 3D solar silicon Heat Exchange Method (HEM) furnace, as built by PVA TePla is modeled. Here, it was necessary to model the complete geometry including the quartz crucible, radiative heaters, bottom cooling, inert flushing gas, etc. For one specific recipe of the transient heater power steering, PVA TePla conducted dip-rod measurements of the silicon solidification front position as function of time. This yields a validation of the model when applied to a real life industrial crystallization process. The results indicate that melt convection does influence the energy distribution up to the start of crystallization at the crucible bottom. But from that point on, the release of latent heat seems to dominate the solidification process, and convection in the melt does not significantly influence the transient front shape.     

Purification and crystal growth of TlBr for application as a radiation detector

Thallium bromide is a semiconductor compound with high atomic number and density. It has a CsCl‐type simple cubic crystal structure and it is non‐hygroscopics. The TlBr crystals are relatively soft with a knoop hardness number of 12. In this work, the TlBr commercial powder was purified by zone refining and the purest material section was used for crystal growth by Bridgman method. Efforts have been concentrated on the purification of the TlBr. The purification efficiency has been evaluated (NAA and ICP‐MS) by impurities reduction results after zone refining passes. The crystalline quality was evaluated by X‐ray diffraction. The characterized TlBr crystal as a detector has shown good response to gamma radiation. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Renormalized perturbation theory for a toy model of fluctuating nonlinear hydrodynamics of supercooled liquids

We study the field theoretic renormalized perturbation theory for a toy model of fluctuating nonlinear hydrodynamics (FNH) of compressible liquids. The toy model contains a density-like and a momentum-like variable without any spatial dependence. We present a detailed derivation of a set of coupled equations among correlation and response functions for these variables. In particular, we focus on how the static limit of the correlation and response functions can be achieved in the renormalized perturbation theory. Numerical methods of solving these equations at a given order of the loop expansion are explained and the results for the one-loop theory are given in detail. The simple nature of the toy model enables us to compare the static limit obtained from the exact solution with that of the one-loop order. This shows explicitly the range of validity of the one-loop theory in the field theoretic formulation.     

Conformation and segmental mobility of a diluted single polymer chain simulated with bond fluctuation model

The conformation of a single long polymer chain has been modelled using the Bond Fluctuation Model. The interaction between non-bonded segments has been introduced in the model by means of a Lennard-Jones potential while two energy potentials depending on bond length and bond angle took into account intramolecular interactions. The effect on chain configuration of varying bond angle potential, which tends to extend the polymer chain, was studied. The chain was allowed to equilibrate at high temperature, adopting a random coil conformation. When the system was subjected to a cooling ramp, its energy and dynamically accessible volume decreased but the system maintained the liquid conditions up to a temperature range in which the glass transition yielded a glassy coil with very restricted but not null segmental mobility. The remaining mobility in the glassy state, as well as its conformation, characterised through correlation functions, also depended significantly on the strength of bond angle potential. Further isothermal annealing yielded very compact structures, always amorphous with particular shapes depending on annealing temperature.     

Aggregation model for the gelation of a sol starting from the processing conditions

A stochastic computational model for the gelation of a sol is explained and tested for the case of neutral silica aerogels. The computational model produces the final structure of the sol after gelation, using two of the several physical phenomena occurring during gelation of sols. Diffusion, represented by Brownian motion, is modeled by a random walk, and chemical reactions are incorporated through a stochastic aggregation model using a probability function; the latter determined in terms of the processing conditions based on the knowledge of the cluster formation energies. The two phenomena are coupled by a Monte Carlo simulation. The analysis of the connected structure and its functionality is demonstrated for neutral silica aerogels. It is shown how the gelation process can be controlled to obtain different structures for different application requirements. The only parameters required by the model are the density and the processing conditions. The results of the model show that those parameters strongly affect the structure of the generated samples. Therefore, processing conditions could be selected to produce aerogels with structures tailored to specific applications, which would constitute a major achievement in aerogel fabrication.     

Optical properties of oxides and fluorides with a rutile-type structure in terms of the point-dipole model

The polarizabilities of ions in the MgF₂, ZnF₂, TiO₂, and SnO₂ compounds have been calculated based on the point-dipole model. It is shown that cation polarizabilities produce a stronger effect on the bire-fringence of AX ₂(X = F, O) compounds than anion polarizabilities.     

Abnormal frequency dispersion of the admittance associated with a chromium/plasma deposited a-SiNx:H/p-Si structure

Admittance (Y_m) versus applied gate bias (V_G) on MIS structure (Cr/a-SiNx:H/p-Si) was measured as a function of frequency (mHz-MHz)/temperature (77-400 K) as parameters to investigate minority carrier behavior. Strong frequency dispersion in measured capacitance at inverting gate bias (positive biases for p-type silicon substrate) and low frequency behavior in capacitance-voltage (C-V) curves under high measuring frequencies (above kHz) at 300-400 K temperature interval are reported. This phenomenon is interpreted via lateral hopping conductivity of self-inversion charges beyond the gate inside a-SiNx:H film near interface as generation mechanism of minorities with a lower activation energy (0.11 eV) rather than prevailing mechanisms of much higher activation energies (namely, generation-recombination and diffusion).