Room Temperature Composition Research Articles

Monomer Conversion of Pre-heated Composite

The potential for maximizing conversion of room-temperature, photoactivated resin composite in the oral environment is limited. Pre-heating composite prior to light-curing is hypothesized to increase monomer conversion and reduce the duration of light exposure. Composite temperature was controlled at between 3 degrees C and 60 degrees C prior to exposure with a conventional quartz-tungsten-halogen curing unit: 5, 10, 20, or 40 sec. Monomer conversion was calculated from infrared spectra at 0 mm (top) and 2-mm-deep surfaces 5 min after light initiation. A strong, positive correlation existed between temperature and monomer conversion: top r(2) = 0.999, 2 mm r(2) = 0.998. Conversion ranged from 31.6% (3 degrees C) to 67.3% (60 degrees C). The duration of light exposure, reduced by 50 to 75% with pre-heated composite, yielded the same or significantly higher conversion (p = 0.001) than with control (22 degrees C, 20 sec). Both hypotheses were accepted: Pre-heating composite prior to photoactivation provides greater conversion requiring reduced light exposure than with room-temperature composite.

Investigation of benzene-hexafluorobenzene dynamics in liquid binary mixtures

The structure and microscopic dynamics of liquid mixtures of benzene and hexafluorobenzene at room temperature and several compositions have been studied by molecular-dynamics simulations. In this implementation we have rescaled the intermolecular H-F cross potential parameters obtained from the Lorentz-Berthelot combining rules, in order to avoid the substantial overestimation of the energy of mixing predicted by the model when the usual rules are employed. We found that a reduction in the strength of cross H-F interactions by 50% relative to the geometric mean is required in order to get a good agreement with experiments. Radial-angular pair-correlation functions between like and unlike species have been computed and analyzed, by comparing them with the correlations in the corresponding neat liquids. We have also studied the microscopic intermolecular momentum transfer, by computing the time correlation function between the initial velocity of a central molecule and later velocities of neighboring molecules. Structural and dynamical information extracted from the mentioned functions seem to be consistent with the picture of relatively long-lived benzene-hexafluorobenzene (Bz-Hf) complexes present in the mixtures, which would be responsible for the considerable perturbation of the structure in the first shell of like species, and would be moving within the liquid in a parallel face-to-face configuration. Using the tools developed originally to estimate hydrogen-bond lifetimes in liquids, we have computed the lifetimes of the Bz-Hf complexes as a function of the mixture composition, by two different methods: the direct time-averaging scheme and from the autocorrelation function of bond occupation numbers. The obtained lifetimes are strongly dependent on the scheme chosen to compute the characteristic times. We have obtained for the Bz-Hf dimer in solution, at room temperature, lifetimes in the range of 30-40 ps from averaging schemes and around 60-120 ps from autocorrelation function methods. In the latter case, the longest times correspond to the equimolar mixture.

Surface structure and composition of Pt 50Rh 50( [formula omitted]): room temperature analysis of the (1 × 3) missing-row reconstruction

The room temperature structure and composition of the clean Pt 50Rh 50(1 1 0) surface is investigated by low energy ion scattering (LEIS), scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and grazing incidence X-ray diffraction (GIXRD). While Pt 25Rh 75(1 1 0) reconstructs with a (1 × 2) missing-row structure, Pt 50Rh 50(1 1 0) exhibits a (1 × 3) structure in analogy with Pt 80Fe 20(1 1 0) and Pt 90Co 10(1 1 0). Three missing rows lead to the formation of (1 1 1) facets in which all atomic sites are enriched with platinum. Similarly all sites directly underneath are enriched with Rh leading to oscillation of the Pt concentrations similar to that of (1 1 1) surfaces. These composition changes are accompanied by a marked inwards relaxation of the top row (−10%) and large buckling in layers 3–5. Additionally the atomic positions of the facets are shifted laterally towards the valleys formed by the missing rows. Consistent pictures are derived from LEED, GIXRD, STM and LEIS concerning the composition while some discrepancy––similar to that recorded for pure Pt––are found concerning the interlayer distances between LEED and X-rays.

Characterization of rotameric mixtures in o- and m-substituted benzaldehydes by matrix isolation IR spectroscopy.

The cis and trans conformers of benzaldehydes substituted in the o- or m-positions by Cl or CN can be differentiated by IR spectroscopy in Argon matrices. Partial photochemical rotamerization allows assignment of the IR bands and establishment of their relative intensities in pairs of rotamers, which permits, in turn, quantitation of equilibrium compositions prior to photolysis. The observed spectra and equilibrium compositions are in very good agreement with the predictions of vibrational spectra and free energy differences from B3LYP/6-31G calculations. The present work represents the first attempt to quantitate the small contributions that the cis rotamers make to the room-temperature equilibrium compositions of the two o-substituted benzaldehydes. The results for the m-substituted derivatives are compared to earlier estimates based on other methods.

SANS study of the distribution of water within starch granules

This study describes contrast variation small angle neutron scattering (SANS) experiments which focus on the role which the intra-granular room temperature distribution of water and carbohydrate plays in determining the native structure and subsequent functionality of starch. It is shown that variations in botanical origin and amylose content do not correlate with significant differences in room temperature composition of A-type starch granules. In turn, variations in the gelatinisation behaviour of A-type starches do not correlate with variations in room temperature water distribution. In contrast, the room temperature water content is found to differ significantly between granules of potato (B-type) and a range of A-type starch cultivars. A correlation is found between these compositional differences and variations in crystal structure, which has implications for biological growth conditions and gelatinisation behaviour.

New method for visualizing heterogeneous reactions

There are already methods for visual observation of heterogeneous reaction rates. A new method for visualizing low intensity heterogeneous reactions is presented. The method is based on a previously unknown “chemography” effect when a surface undergoing oxidation creates its own hidden image on a photoemulsion layer situated close to the surface. The image arises due to the flow of active molecules which are desorbed spontaneously by the oxidizing solid surface in the environment. The effect is observed for the oxidation of some metals and semiconductors. The run of the low intensity Si-oxidation reactions at room temperature versus environment composition has been investigated by this method. The influence of a glow discharge atmosphere on the “chemography” effect of Si was also studied. The possible use of this effect for recording molecular and ion flows was demonstrated. The authors propose to use the “chemography” effect for the investigation of kinetics of the heterogeneous reactions and heterogeneous catalysis.

The Faber-Ziman resistivity of amorphous MgxZn1−x

Abstract We present the results of a resistivity calculation for amorphous Mg70Zn30 using the Faber-Ziman theory of electrical transport, together with experimental X-ray structure factor data. Percus-Yevick hard sphere partial structure factors are scaled to be consistent with experimental X-ray structure factor data and are then used in the theory. A Debye model gives the temperature variation of the partial structure factors. Calculations are done for both Ashcroft empty-core pseudopotentials and T-matrices. The correct magnitude for the resistivity and sign for its temperature coefficient are found. Use of the Ashcroft pseudopotential gives the correct room temperature composition dependence of the resistivity. Use of the t-matrix elements gives the correct temperature dependence of the resistivity. Neither is capable of explaining both the temperature and composition dependence.

Incorporation of neutral molecules into alkali halides (salting). Room-temperature composites from normally immiscible components and room-temperature matrix isolation

Incorporation of neutral molecules into alkali halides (salting). Room-temperature composites from normally immiscible components and room-temperature matrix isolation

Quantitative Auger electron spectroscopy and X-ray fluorescence analysis of CoSm alloy thin films deposited by co-evaporation

High energy electron excitation (up to 50 keV) of Auger spectra with high signal-to-background ratios and a low and flat background slope was used to analyse CoSm alloy thin films. The experiments were performed in an ultrahigh vacuum high energy electron diffraction chamber with in situ co-evaporation of the components onto molybdenum substrates at room temperature and at 550°C. The intensity distributions of the secondary electrons were analysed with a cylindrical mirror analyser and detected via a phosphor-multiplier combination using counting techniques and a multichannel analyser for recording. The analysis of overlapping Auger peaks in the alloy spectra was accomplished by computer-aided subtraction of normalized spectra of the single components. The Auger electron spectroscopy (AES) results were compared with quantitative X-ray fluorescence analysis (XFA) of the films. For deposition of the components at constant rates and the substrate at room temperature, the Auger and X-ray compositions agreed fairly well. However, for deposition at substrate temperatures higher than 500°C, the samarium concentrations were substantially lower than that in the vapour beam, but a relative enrichment of samarium near the film surface was found by comparison of the AES and XFA results. The concentration profile of the films is discussed in terms of an initially low samarium condensation coefficient which increases with the codeposition of cobalt and with time.