Showing papers by "Missouri University of Science and Technology published in 1992"

Electron production in proton collisions with atoms and molecules: energy distributions

Abstract: All known data on the energy distribution of secondary electrons from collisions of protons with atoms and molecules have been reviewed and differential cross sections have been collected. The two experimental methods used to obtain the data are discussed and possible sources of error pointed out. Theoretical treatments are reviewed and several methods of checking the consistency of the data are discussed. Two semiempirical models have been chosen to represent the differential cross sections, and parameters for these models are given which fit the average of the experimental data, subject to known constraints. Recommended values of differential cross sections are given for ten target gases by means of these models.

Modified resin–intermediate processing of perovskite powders: Part I. Optimization of polymeric precursors

Abstract: The formation of a polyester between citric acid (CA) and ethylene glycol (EG) was found to be a decisive factor for the foaming of resin intermediates in a Pechini-type powder process. This process was modified by changing the organic mass ratio of CA/EG which results in ceramic powders with different morphologies. The most porous resin intermediate (with or without chelated cations) was prepared using a polymeric gel made of equimolar citric acid and ethylene glycol. It was also found that a premixing of organic components, prior to adding constituent nitrate solutions, makes the whole process more controllable.

Early diagenetic recrystallization of Holocene (<3000 years old) peritidal dolomites, Ambergris Cay, Belize

Abstract: Three peritidal carbonate crusts and associated intercrust sediments (total thickness of ∼30cm; aged <3000 years BP) on Ambergris Cay, Belize, contain 32–100% calcian dolomite (δx=72·5% dolomite) ranging in composition from 40 to 46 mol% MgCO3 (δx=43·3). Dolomite replaced high Mg calcite foraminiferal muds penecontemporaneously with sedimentation, forming partially dolomitized sediments and lithified crusts. Dolomitization probably occurred in normal to moderately evaporated seawater and is apparently continuing at the present. Detailed scanning electron microscope analysis shows a linear increase in mean dolomite crystal size with depth; 0·4 μm near the top of the section to 1·0 μm near the base of the dolomitized section. This size increase is not accompanied by any significant decrease in porosity. Crystal size distributions appear to be log-normal and become increasingly broad and flat with depth. Rietveld X-ray pattern-fitting structure refinements indicate increasing Ca and Mg concentrations on their respective sites (cation ordering) as a function of increasing depth. Most of the ordering occurs within the first 15 cm of the surface. Stoichiometry does not increase with depth indicating no relationship between the Ca/Mg ratio and cation ordering. Strong geochemical trends were observed down-section in the dolomite, including: (1) increasing Mn content (44 to 274 ppm), and (2) decreasing δ13C values (−0·9 to −5·5‰ PDB). Oxygen isotope values range from δ18O = 1·3‰ PDB in the upper part of the section to 2·6‰ PDB in the lower part of the section and are interpreted to represent two distinct groups of values rather than a continuous trend. Down-section dolomite crystal size increase and shapes of crystal size distributions are consistent with recrystallization via a surface energy-driven dissolution-reprecipitation process (Ostwald ripening). The observed trends in carbon isotopes and Mn content probably result from geochemical re-equilibration during recrystallization and reflect reducing conditions and an isotopically light, organically derived, carbon source. Oxygen isotope compositions probably reflect relict original dolomite values and are a result of decreasing evaporation due to rising sea level.

A new approach for the direct resolution of racemic beta adrenergic blocking agents by HPLC

Abstract: Although beta-adrenergic blocking agents are a relatively new group of drugs, they have proven to be very useful in medical pharmacology. Since the successful introduction of propranolol (Inderal) a variety of analogous compounds have been developed. Most beta-blockers are racemic modifications and it is known that their enantiomers have different potencies and pharmacological effects. Hence, there has been considerable impetus for the production and study of the pure enantiomers. The chromatographic separation of these compounds has been dominated by the protein-based chiral stationary phases. Although selective, protein columns have limited capacity and stability. An efficient, alternative method has been found that does not suffer from the limitations of the protein phases. By using an unusual mobile phase consisting of a mixture of polar organic solvents in conjunction with the original native cyclodextrin bonded phase, the following compounds were resolved: propranolol, metoprolol, timolol, ...

Calculated X-ray Diffraction Data for Diamond Polytypes

Abstract: Calculated X-ray diffraction pattern data for diamond polytypes are presented, which provides the required parameters for the characterization of the proposed diamond polytypes. The literature on diamond polytypes contains some small but significant errors with respect to the details of the crystal structure, including space groups, the atom positions in the unit cell, and the atomic layer stacking. In this paper, the crystal structures of four different hexagonal diamond and two different rhombohedral diamond polytypes are theoretically calculated to provide correct crystallographic information with which the diamond polytypes in a given sample can be characterized.

Modified resin--intermediate processing of perovskite powders:Part II. Processing for fine, nonagglomerated Sr-doped lanthanum chromite powders

Abstract: Nonagglomerated ceramic precursor for Sr-doped lanthanum chromite powder was prepared by a modified resin–intermediate process using equimolar citric acid, ethylene glycol, and metallic nitrates. Pure and well-crystallized submicron powders with a narrow particle-size distribution were calcined from an optimally pulverized resin at a temperature of 750 °C for 2 h in flowing air. The material cost for organics used in the investigated process was considerably lower than the original Pechini process.

Knowledge Maps and the Presentation of Related Information Domains

Abstract: An experiment was conducted to assess the relative effectiveness of multiple-relationship knowledge maps and traditional text for the presentation of related information domains. A secondary purpose was to investigate comparative versus sequential presentations. Knowledge maps were found to be superior to traditional text in acquisition and affect associated with studying for one type of material. Moreover, students in the map groups reported gaining more knowledge about their information processing and study strategies that would help them in future learning than did those in the text groups. However, these results were not replicated with respect to a second sample of text on a different topic. Subject matter and structural parameters that might account for the inability to replicate the knowledge map effect across passages are identified.

Empirical Procedure That Uses Molecular Structure To Predict Enantioselectivity of Chiral Stationary Phases

Abstract: A total of 121 racemic compounds were separated in the normal-phase mode on a (S)-(1-naphthylethyl)carbamoylated beta-cyclodextrin (S-NEC-beta-CD) bonded phase and 74 on the R equivalent (R-NEC) chiral stationary phase (CSP) All compounds are of the type that have four substituents on a stereogenic center, rather than an "axis of chirality" It is shown that the binary solvent pair used as the mobile phase has a significant influence on chiral recognition However, the proportions of the components of a specific pair have little effect From the results, the individual contributions to chiral recognition by these CSPs were estimated for 81 different substituents of the stereogenic center Varying the arrangement of these 81 substituents could produce over 16 million compounds Hydrogen was chosen as the reference substituent and was assigned a 0 cal/mol free energy The chiral recognition increased when sp2-hybridized carbons were connected to the stereogenic center Conversely, sp3-hybridized carbons decreased the enantioselectivity Amido groups increased the chiral recognition, especially when associated with pi-acid (3,5-dinitrobenzoyl) or pi-basic (naphthyl) groups This approach does not allow one to know which enantiomer elutes first However, the "substituent energy" list for chiral compounds can be used to obtain an estimated value for the enantioselectivity of a compound by adding the energy contributions of the four substituents connected to the stereogenic center In this way one can predict a priori whether or not a compound will separate on a CSP and estimate its separation factor (alpha) Theoretically, this approach can be used for most CSPs, provided a sufficient data base is generated on them

Fluid-Inclusion Studies of Regionally Extensive Epigenetic Dolomites, Bonneterre Dolomite (Cambrian), Southeast Missouri: Evidence of Multiple Fluids during Dolomitization and Lead-Zinc Mineralization

Abstract: Sources of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri historically have been a subject of debate. This study presents microthermometric data for fluid inclusions in the regionally extensive epigenetic dolomite at the base of the ore-hosting Bonneterre Dolomite and in gangue dolomite of the Viburnum Trend Pb-Zn district. Samples of epigenetic dolomite cover an area of more than 25,000 km 2 west of the St. Francois Mountains and permit determination of regional variations of temperature and composition of the mineralizing fluids and possible fluid interactions. Homogenization temperature-ice-melting relationships (temperature-salinity) among these inclusions document at least two end-member fluid components: a warmer, less saline fluid (120 to 187 °C; 5 wt% equiv. NaCl) and a cooler, more saline fluid (60 to 80 °C; >30 wt% equiv. NaCl). Intermediate temperatures and fluid compositions indicate that the end-member fluids likely mixed as they flowed through the region. Mixing was not confined to a stationary front but occurred throughout the study area. Comparison of homogenization temperatures to distance from possible basinal fluid sources indicates no discernible temperature gradient over the >25,000 km 2 study area. The data are interpreted to indicate multiple basinal-fluid interactions coeval with dolomitization and associated Pb-Zn ore formation.

Electrical-conductivity and seebeck coefficient of (la, ca) (cr, co)o3

Abstract: Electrical conductivity and Seebeck coefficients of (La, Ca) (Cr, Co)O3 were measured as a function of temperature. The electrical conductivity as measured in air from 100 to 1100 °C increased with increasing Co and Ca content. The Seebeck coefficients were positive, indicating p-type conductivity. The substitution of Co for Cr significantly decreased the Seebeck coefficients, indicating that the substitution resulted in an increase in site occupancy, associated with the Co. The additional Ca substitution for La resulted in further decrease in the Seebeck coefficients, then exhibited a temperature-independent behaviour, indicating that the carrier mobility, rather than carrier concentration, was thermally activated. The activation energies were 0.18 and 0.25 eV for LaCrO3 and LaCoO3, respectively, and increased to about 0.50 eV with substitution of 10 mol% Co for Cr and then linearly decreased as Co content increased.

An adaptive mesh refinement technique for the analysis of shear bands in plane strain compression of a thermoviscoplastic solid

Abstract: We have developed an adaptive mesh refinement technique that generates elements such that the integral of the second invariant of the deviatoric strain-rate tensor over an element is nearly the same for all elements in the mesh. It is shown that the finite element meshes so generated are effective in resolving shear bands, which are narrow regions of intense plastic deformation that form in high strain-rate deformation of thermally softening viscoplastic materials. Here we assume that the body is deformed in plane strain compression at a nominal strain-rate of 5000 sec-1, and model a material defect by introducing a temperature perturbation at the center of the block.

Fabrication and Mechanical Properties of an Optically Transparent Glass Fiber/polymer Matrix Composite

Abstract: A composite with substantial optical transparency was produced by rein forcing poly(methyl methacrylate) (PMMA) with unidirectional borosilicate glass fibers. Optical transparency of the composite was achieved by matching the refractive index (n D) of the glass fiber and polymer matrix to within ±0.002. The optical transmission of the composite decreased with increasing volume percentage of glass fibers. Composites con taining 30 volume percent of unidirectional 12 μm diameter fibers had a transmission at 589.3 nm of 20 percent measured through a 10 mm thickness. Composites containing 30 volume percent of 56 μm diameter fibers had an optical transmission of 45 percent. Type written text could be clearly read through the composite when it was laid directly on a printed page. Composites with 40 volume percent fiber had a flexural strength of 600 MPa which is six times greater than that of pure PMMA. The flexural modulus for the same composite was 18 MPa which is nine times greater than the 2 MPa measured fo...

The scaling of nucleation rates

Abstract: The homogeneous nucleation rate,J, forT → T c can be cast into a “corresponding states” form by exploiting scaled expressions for the vapor pressure and for the surface tension, δ. In the vapor-to-liquid case with δ= δ0[T c -T], the classical cluster energy of formation /kT = [16π/3] • Ώ3 [T c -1]3/(lnS)2 = [x0/x]2, where Ώ ≡ δ0/[k n2/3] and n is liquid number density. [1] The Ώ≈ 2 for normal liquids. (A similar approach can be applied to homogeneous liquid to solid nucleation and to heterogeneous nucleation formalisms using appropriate modifications ofσ and Ώ.[2]) The above [x0/x]2 is sufficiently tenable that in some cases, one can use it to extract approximate critical temperatures from experimental data.[3,4] In this work, we point out that expansion cloud chamber data (for nonane, toluene, and water) are in excellent agreement with lnJ ≈ const. -[x0/x]2 [centimeter-gram-second (cgs) units], and that the constant term is well approximated by ln (Γc), whereT c is the inverse thermal wavelength cubed per second atT =T c . The ln (Γc) is ≈ 60 in cgs units (74 in SI units) for most materials. A physical basis for the latter form, which includes the behavior at smalln, the discrete integer behavior ofn, and a configurational entropy term, τ ln (n), is presented.