Showing papers by "Missouri University of Science and Technology published in 1991"

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

Abstract: DTA is presently used to characterize the nucleation and crystallization processes of the Na2O-2CaO-3SiO2 glass. A nucleation rate-temperaturelike curve is obtained by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, or the height of the crystallization peak, as a function of nucleation temperature. The nucleation-temperature range for this glass composition, 550-650 C, and the maximum nucleation temperature of 600 + or - 5 C, are found to be in excellent agreement with those associated with the classical nucleation technique, followed by isothermal crystallization. It is noted that when most of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate the crystallization energy.

Crystallization Kinetics of a Lithia–Silica Glass: Effect of Sample Characteristics and Thermal Analysis Measurement Techniques

Abstract: DTA and both isothermal and nonisothermal DSC techniques are presently used to investigate the crystallization kinetics of a 40 (mol) percent Li2O-60 percent SiO2 glass as a function of glass powder particle size, the use of either alumina or Pt as the crucible material, the use of N, O, or Ar atmospheres, and surface pretreatments of the glass powder with deionized water, HCl, or HF. Neither the furnace atmosphere nor the crucible material had a significant effect on activation energy, frequency factor, or Avrami exponent. Washings of the glass with the three different fluids decreased the crystallization temperature by 25 to 30 C.

Transport Anomalies in the High-Temperature Hopping Conductivity and Thermopower of Sr-doped La(Cr,Mn)O, 3

Abstract: A minimum exists in the electrical conductivity of the perovskite-type ceramic ${\mathrm{LaCr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$${\mathrm{O}}_{3}$ as a function of Mn content near x=0.05. This minimum has been explained in terms of a crossover from multiple trapping to percolation among energetically lower Mn sites. In this paper electrical conductivity and Seebeck measurements are presented on a similar series in which 10 mol % Sr was substituted for La in order to increase the small polaron concentration through the compensation of Sr ions according to the Verway mechanism. The data suggests that there is an apparent suppression of the Verway compensation mechanism in all Mn-doped samples. The hopping crossover observed in the Sr-free series is retained with Sr doping, although the position and depth of the electrical-conductivity minimum are altered. Difficulties in the present understanding and interpretation of the electrical conductivity and Seebeck measurements as a function of Mn and Sr content in these materials are discussed. An electronic structure is suggested, which seems to resolve many of these problems.

Evaluation of a chiral crown ether LC column for the separation of racemic amines

Abstract: Enantiomeric resolution of more than fifty racemic primary amines can be achieved on a column that utilizes a crown ether as a chiral selector. the racemic solute is solubilized in an acidic solvent, forming an ammonium ion from the primary amine functional group. an interaction between the lone pair electrons on the oxygens of the crown ether and the positive charge of the ammonium group leads to the formation of an inclusion complex. Due to the chirality of the crown ether there is stereoselective interaction resulting in enantiomeric separation. Excellent resolution is possible for amino acids, amino alcohols, amino esters and amines. Compounds are separated that were poorly resolved by conventional ligand exchange columns and by other means.

Effect of Green Density on Densification and Creep During Sintering

Abstract: The effect of green density on both the densification rate and the creep rate was measured simultaneously during sintering by loading dilatometry. The experiments were performed on zinc oxide powder compacts with five different green densities covering a range of 0.39 to 0.73 of theoretical. The samples were heated at a constant rate of 4°C/min up to 1100°C in air. The densification rate at any temperature increases significantly with decreasing green density. The data for the densification rate and creep rate as a function of density show two quite distinct regimes of behavior; the rates were strongly dependent on density below 0.80, while above this value they were weakly dependent on density. The ratio of the densification rate to the creep rate was almost independent of temperature but increased almost linearly with increasing green density. The representation of the data in terms of models for sintering and creep is discussed.

Evaluation of the Enantiomeric Separation of Dipeptides Using a Chiral Crown Ether LC Column

Abstract: The direct optical resolution of six dipeptides into four stereoisomers each was achieved on an enantioselective crown ether column. An inclusion complex is formed between the stationary phase and the solute when using an acidic mobile phase. The acidic mobile phase serves to protonate the requisite primary amine of the dipeptide thereby allowing an attractive interaction between the ammonium functional group and the oxygens of the crown ether. Due to the differences in stability of the complexes formed, the four optical isomers elute at different times allowing the stereoisomeric separation. One of the factors affecting enantioselectivity is the distance between the primary amine functional group and the stereogenic center of the chiral moiety. Dipeptides are particularly useful molecules for the studying this “distance effect” since the bonding order of the two amino acids can be reversed. In addition to the enantiomeric separations of dipeptides possessing two stereogenic centers, the behavior...

High-Temperature Wetting of Sapphire by Aluminum

Abstract: Sessile drop studies of molten aluminum on single-crystal sapphire substrates were conducted to investigate the effects of atmosphere on contact angle, substrate reactions, and interfacial crystal growth. Unlike previous investigations performed briefly in a vacuum environment in a temperature range within 600°C of the aluminum melting point, these experiments were conducted at higher temperatures (1200° to 1600°C) and at 1-atm total pressure over longer experimental times to more closely approach equilibrium conditions. A continuously flowing buffered gas system utilizing high-purity metered mixtures of hydrogen and helium in combination with a thoria ceramic electrolyte sensor were employed to achieve variations of the oxygen partial pressure from 10−19 to 10−15 atm while continuously maintaining the total pressure at 1 atm. At constant temperature, it was found that neither the oxygen partial pressure nor the crystallographic orientation of the sapphire substrate had a significant effect on the observed contact angles. A continuous decrease of acute contact angles and a single reaction ring characterized the 8-h experiments without the alternating spreading and contracting behavior repeatedly reported in the literature. This phenomenon can be attributed to the lower rate of metal evaporation and interfacial reaction at the higher total gas pressure and yet extremely low oxygen partial pressure of these experiments. Profilometric analysis of sapphire substrates subsequent to the removal of the quenched sessile drops indicates a reduction in metal–solid interaction due to the closer approach to equilibrium than in previous studies. An epitaxial orientation with respect to the substrate was observed in α-alumina crystallite formation at the metal–ceramic interface. Experimental evidence suggests that it was formed by a nucleation and growth process during the cooling period.

Evaluation of D-amino acid levels in human urine and in commercial L-amino acid samples.

Abstract: Detectable levels of four free D-amino acids (phenylalanine, tyrosine, tryptophan and leucine) were found in human urine. This was accomplished using a liquid chromatographic coupled column (i.e., achiral-chiral) method that was developed for the rapid and sensitive analysis of these compounds. The technique was tested on a series of commercial L-amino acids. Care was taken to minimize racemization. Trace to percent levels of contaminating D-enantiomers were found in all commercial samples. In urine the D-amino acids ranged from several hundredths of a percent up to percent levels of the corresponding excreted natural L-amino acids. There were no apparent correlations between the amount of amino acids excreted and the relative amount of D-enantiomers present. Currently this study is being expanded to include additional amino acids and a variety of other physiological fluids.

Mineralogical Characteristics of Electric Arc Furnace Dusts

Abstract: Reflected light microscopy can contribute important information regarding the mineralogy, mineral abundance, internal textures, sizes and shapes of particles in electric arc furnace (EAF) dusts. Scanning electron microscopy-energy dispersive spectroscopy and electron microprobe analysis are useful to determine the chemical compositions of the specific mineral grains in the dust particles. Furthermore, the mineralogical reactions that have taken place during the pyro-metallurgical treatment of EAF dusts and the mineralogy and textural character of those treated dust samples can be directly observed by reflected light microscopy. Such studies are useful in monitoring the efficiency of experimental pyrometallurgical treatment of EAF dusts which are designed to render them nonhazardous.

Enhanced Mechanical Properties of Alumina by Dispersed Titanium Diboride Particulate Inclusions

Abstract: The mechanical properties of composite ceramics composed of 0 to 20 vol% of titanium diboride particles dispersed in an α-alumina matrix were investigated. The alumina–titanium diboride composite powder was hot-pressed at 1470°C for 20 min to achieve over 98.8% of the theoretical composite density. The strength and fracture toughness of the twophase, hot-pressed composite were both significantly improved compared to the single-phase alumina. Results from different methods of measuring the stress intensity factor, (KIc) are compared and discussed.

Hydrolysis, adsorption, and dynamics of silane coupling agents on silica surfaces

Abstract: The hydrolysis of alkoxysilane coupling agents has been followed using proton NMR. The disappearance of the silane ester and the appearance of the alkoxy group were observed to follow pseudo-first-order kinetics. The apparent rate constant was found to be dependent on the concentration of the silane coupling agent and the pH. The adsorption of the coupling agents onto Cab-O-Sil silica was quantified using FT-IR spectroscopy. It was found that monolayer coverage was obtained with about 0.65 mmol/100 m2. Solid-state deuterium NMR was used to probe the behavior of the coupling agent on the silica. The coupling agents adsorbed on the surface from deuterated aminopropyltriethoxysilane (DAPES) and deuterated aminobutyltriethoxysilane (DABES) showed similar motional characteristics. In contrast, the mobility of condensed polymers from DABES was much less restricted than that from DAPES. The amount of hydrolysis in acetone-water mixtures increased the amount of surface-adsorbed material and lessened surface mobil...

Gram-Schmidt orthogonalization by Gauss elimination

Abstract: (1991). Gram-Schmidt Orthogonalization by Gauss Elimination. The American Mathematical Monthly: Vol. 98, No. 6, pp. 544-549.

Phase separation in glasses

Abstract: The detection, acceptance and further investigation of glass-glass phase separation was closely connected with the development of methods making detection possible, such as electron microscope technique pioneered and exploited by Werner Vogel, to whom this paper is dedicated on the occasion of his 65th birthday. Structures are not always found in agreement with rules derived from thermodynamic principles, and also depend on thermal history, as well as mode of preparation. The phenomenon is exemplified in many glass-forming systems. Among more recent applications are the control of crystallization by preceding phase separation as in glass-ceramics and semiconductor-doped glasses for non-linear optics.

Modified Exponential Series Approximation for the Theodorsen Function

Abstract: An improved method is developed for the approximation of unsteady aerodynamics in the time domain by a series of decaying exponentials. The new method is different from the previous procedures in that it consistently accounts for the case when the optimum values of the lag parameters in the exponential series are close to one another. This is achieved by introducing a time-weighted exponential series for the repeated pole case. The method uses a nongradient optimizing procedure. Approximations are presented for Theodorsen's lift deficiency function and results are compared with those of a gradient-based method that was published recently. HE representation of the general motion of an aeroelastic structure requires the availability pf the unsteady aerody- namic forces in the time domain. An important feature of these forces is the lag associated with the circulatory wake. Theodorsen1 employed a lift deficiency function in the re- duced frequency domain to represent this effect for the oscilla- tory flow over an airfoil. Jones2 used a two-term series of decaying exponentials in the time domain to approximate the effect of circulation for the transient aeroelastic motion and solved for the linear coefficients of the series by using the Fourier transform to convert the transfer function into the frequency domain where it must equal Theodorsen's circula- tion function. This idea was extended by Do well3 who used an exponential series in the time domain to represent the unsteady aerodynamic transfer function and then transformed it into a rational function in the Laplace domain. In contrast with this approach is the conventional least-squares method4 that be- gins with a suitable approximation in the Laplace domain and then transforms it into the time domain using the inverse Laplace transform. It is to be noted that, although certain effects, such as the aerodynamic damping and the aerody- namic inertia, can be included in the rational function approx- imation in the Laplace domain, they are essentially left out in the exponential series approximation in the time domain. However, this difference between the two methods is not present when only the circulatory effect is being approxi- mated, such as the lift deficiency function of Theodorsen. The accuracy of an exponential series approximation de- pends crucially on the values of the nonlinear lag parameters that occur in the exponentials. Recently, Peter son and Craw- ley5 have shown .that, when these parameters are optimized to give a minimum squared error between the exact and the approximate values of the transfer function, the accuracy of the approximation is greatly enhanced. However, the objec- tive-function minima obtained by Peterson and Crawley5 may not be unique because the gradient-based optimizer is unable to escape the local minima. Also, the frequently encountered cases when the optimum values of two or more lag parameters are nearly the same are mistaken to indicate that the same fit accuracy can be achieved by reducing the number of lag states. Moreover, their inclusion of the numerator as well as the denominator coefficients of the transfer function in the Laplace domain as the free parameters of optimization ap- pears to be a less efficient procedure than the one in which the numerator coefficients are determined by a least-squares fit. The latter option is utilized in the present method, which uses a simplex nongradient optimizer to locate the absolute minima of the objective function. The most significant improvement is achieved by using a consistent optimizing procedure, intro- duced earlier by the authors,6 which correctly accounts for the case of repeated lag parameters by employing a new approxi- mation that contains time-weighted exponentials. The time- weighting functions are polynomials in time of one degree less than the multiplicity of the corresponding lag parameters. By using this series, the repeated pole case not only becomes meaningful but also allows an improvement in the fit accuracy with the frequency domain data. Gradient-Based Method as Compared to the Present Method The present example compares the gradient-based opti- mization scheme of Peterson and Crawley,5 who used the exponential time-series approximation of Dowell3 as the basic transfer function, with the nongradient optimized scheme of the present method employing the same transfer function. For the purpose of comparison, both methods approximate the Theodorsen circulation function.