Showing papers on "Crystal published in 1993"

Parameterization of the Radiative Properties of Cirrus Clouds

Abstract: A new approach for parameterization of the broadband solar and infrared radiative properties of ice clouds has been developed. This parameterization scheme integrates in a coherent manner the δ-four-stream approximation for radiative transfer, the correlated k-distribution method for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. A mean effective size is used, representing an area-weighted mean crystal width, to account for the ice crystal size distribution with respect to radiative calculation. Based on physical principles, the basic single-scattering properties of ice crystals, including the extinction coefficient divided by ice water content single-scattering albedo, and expansion coefficients of the phase function, can be parameterized using third-degree polynomials in terms of the mean effective size. In the development of this parameterization the results computed from a light scattering program that includes a Geometric ray-tracing progr...

Thermal conductivity of isotopically modified single crystal diamond.

Abstract: We present new experimental results on the thermal conductivity of isotopically enriched $^{12}\mathrm{C}$ diamond crystals at low temperatures. To our knowledge, the measured value for a 99.9% $^{12}\mathrm{C}$ crystal at 104 K, 410 W/cm K, is the highest measured thermal conducitivity for a solid above liquid nitrogen temperature. Our measured temperature dependent conductivities for the isotopically enriched diamond and natural abundance diamond specimens are well described by Callaway's theoretical model. We predict that the thermal conductivity of a 99.999% $^{12}\mathrm{C}$ diamond crystal should exceed 2000 W/cm K at \ensuremath{\sim}80 K.

Biological Control of Crystal Texture: A Widespread Strategy for Adapting Crystal Properties to Function

Abstract: Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for sea urchin and mollusk calcite crystals, whereas none was found for the foraminifer, Patellina, and the control calcite crystals. These results show that the manipulation of crystal texture in different organisms is under biological control and that crystal textures in some tissues are adapted to function. A better understanding of this apparently widespread biological phenomenon may provide new insights for improving synthetic crystal-containing materials.

TYPIX Standardized Data and Crystal Chemical Characterization of Inorganic Structure Types

Abstract: This major reference contains condensed crystal chemical information about individual structure types as well as an extensive section on the crystal chemistry of particular structure families. It clarifies and classifies published data on intermetallic and other inorganic structures.

Continuous crystal growth of YBa2Cu3O7−x by the modified top-seeded crystal pulling method

Abstract: Continuous growth of YBa 2 Cu 3 O 7− x (Y123) crystals was achieved by a pulling method using a BaO-CuO solution with the solid Y 2 BaCuO 5 (Y211) as a solute. A suitable crucible was also investigated, and an yttria crucible was found to be the best selection for this method. The growth was performed in an ambient atmosphere at about 1000°C. When a textured Sm- Ba 2 Cu 3 O 7− x (Sm123) rod was used as a seed crystal, the Y123 crystal grew epitaxially from the seed. The grown crystal contained no Y211 particles; however, it had a twin structure near the surface, which had been formed during the in situ cooling process. The temperature dependence of the resistance was metallic in both directions, along the c -axis and in the ab -plane, after appropriate oxygen annealing.

Prediction of possible crystal structures for C-, H-, N-, O-, and F-containing organic compounds

Abstract: A procedure is reported for the prediction of dense crystal structures of C-, H-, N-, O-, and F-containing organic compounds in the primitive triclinic, monoclinic, and orthorhombic space groups with Z ≤ 4. The crystal environments of molecules in 242 crystal structures have been analyzed to determine the common coordination sphere pattens. This led to the development of the MOLPAK (MOLecular PAcKing) program, which uses a rigid-body molecular structure probe to build packing arrangements (possible crystal structures) in the various space groups. A MOLPAK search, which involves the investigation of all unique orientations of a central molecule and the construction of the appropriate coordination patterns about the central molecule, provides a 3-D map of minimum unit cell volume as a function of the orientation of the central molecule. MOLPAK uses a repulsion-only potential and a preset threshold to place molecules in contact with each other. The 5–10 smallest volume packing arrangements from a search are subjected to a lattice energy minimization refinement with the WMIN program to yield possible crystal structures. The results are described from the analyses of several known compounds starting with the crystal molecular structures as the MOLPAK search probes in the P1, P21, P21/c, and P212121 space groups. In addition, several examples are given in which the search probes were created by AM1 geometry optimization of preliminary molecular models. More extensive data are given in supplementary tables. © 1993 John Wiley & Sons, Inc.

Characterization of stoichiometric LiNbO3 grown from melts containing K2O

Abstract: The growth of LiNbO3 single crystals from a melt with the Li/Nb ratio of 0.946, to which 6 wt.% K2O has been added, leads to stoichiometric specimens, essentially free of potassium, with (50±0.15) mol% Li2O in the crystal. This is established by studying the composition dependence of the following properties: linewidths of the electron paramagnetic resonance (EPR) of Fe3+, energy of the fundamental absorption edge, Raman linewidths of phonon modes, and dispersion of the optical birefringence. Comparison of the results with relevant calibration scales leads to the above composition. In all cases the Li2O content was found to be closer to 50% than that of a LiNbO3 crystal vapor-phase equilibrated to 49.9mol% Li2O. The photorefractive effect at light intensities I≥107 W/m2 is suppressed in this stoichiometric material. The features of the ternary system K2O-Li2O-Nb2O5, which are possibly responsible for the unexpected growth of stoichiometric LiNbO3 from the indicated melts, are discussed.

Models for breakdown-resistant dielectric and ferroelectric ceramics

Abstract: Models for dielectric breakdown are proposed and analysed, with emphasis on concepts leading to breakdown-resistant materials. The Griffith energy balance is extended to cracks under combined electrical and mechanical loading, and to conductive tubular channels. Breakdown strength for a perfect crystal is estimated by an analogue of the Frenkel model. In a crystal subjected to an electric field the equilibrium displacement of the electron clouds is described by a curve with periodicity of the lattice constant. A theory of breakdown-resistant laminates is proposed on the basis of charge relocation, facilitated by breakdown of the weak layers and the interfaces. A process by which a conducting path grows like a crack in ferroelectric ceramics is discussed, followed by an outline of fields around conducting cracks in piezoelectric ceramics.

Method of fabricating a gallium nitride based semiconductor device with an aluminum and nitrogen containing intermediate layer

Abstract: Disclosed are a gallium nitride type semiconductor device that has a single crystal of (Ga 1-x Al x ) 1-y In y N, which suppresses the occurrence of crystal defects and thus has very high crystallization and considerably excellent flatness, and a method of fabricating the same. The gallium nitride type semiconductor device comprises a silicon substrate, an intermediate layer consisting of a compound containing at least aluminum and nitrogen and formed on the silicon substrate, and a crystal layer of (Ga 1-x Al x ) 1-y In y N (0≦x≧1, 0≦y≦1, excluding the case of x=1 and y=0). According to the method of fabricating a gallium nitride base semiconductor device, a silicon single crystal substrate is kept at a temperature of 400° to 1300° C. and is held in the atmosphere where a metaloganic compound containing at least aluminum and a nitrogen-containing compound are present to form a thin intermediate layer containing at least aluminum and nitrogen on a part or the entirety of the surface of the single crystal substrate, and then at least one layer or multiple layers of a single crystal of (Ga 1-x Al x ).sub. 1-y In y N are formed on the intermediate layer.

Kinetics of chiral symmetry breaking in crystallization

Abstract: Results of our study of the kinetics of stirred crystallization that produces large (greater than 99%) asymmetry are presented. The change of concentration with time as the crystallization progresses is obtained for stirred and unstirred crystallizations. A sharp difference in the two concentration vs time curves, due to secondary nucleation in the stirred system, can be observed. Experimental results are compared with the predictions of a set of stochastic kinetic equations. It is found that the proposed kinetics can quite accurately reproduce the experimental data if it is assumed that secondary nuclei are produced in stirred systems only when the parent crystal reaches a minimum size

Osteoblast attachment monitored with a quartz crystal microbalance

Abstract: A quartz crystal microbalance is used in aqueous solutions to monitor the rate of attachment of osteoblasts, bone-forming cells, to the surface of the crystal. Changes in resonant frequency of the crystal are measured for various surface coverages by osteoblasts. Crystal surface coverages are determined by digital image processing of scanning electron micrographs. A linear relationship is established between the surface coverages and the changes in resonant frequency of the crystal. The osteoblasts are observed to behave viscoelastically. Hence, the Sauerbrey equation can not be used to describe the relationship between the change in mass of osteoblasts on the surface and the change in resonant frequency of the crystal. Apparent viscosities at 5.0 MHz are also determined for osteoblasts.

On the accurate measurement of structure-factor amplitudes and phases by electron diffraction

Abstract: A review is given of research into the measurement of crystal structure-factor amplitudes and phases by transmission electron diffraction. Accuracies for amplitudes are commonly a fraction of a percent (after conversion to X-ray structure factors) while phases may now be measured in favorable cases using three-beam electron diffraction to an accuracy of much better than 1°. Following a brief review of theory, the main techniques are outlined. These include quantitative convergent-beam electron diffraction, the critical-voltage method, the intersecting Kikuchi- and HOLZ-Iine methods and methods based on weak high-order reflections in wide-angle patterns. Enantiomorphs and polarity are discussed. Summaries are given of measurements of the mean inner potential and of structure factors generally. A brief review of the implications of this work for studies of crystal bonding and ordering in alloys is given, and its use to test ab initio computations of charge density. The mean inner potential is found to be the quantity most sensitive to the bonding distribution of valence electrons and sensitively constrains accurate X-ray structure-factor measurements. Electron diffraction techniques are to be preferred for studies of individual microcrystals or artificially formed structures and for the very accurate measurement of structure-factor phases in acentric crystals with small unit cells.

Transition regions and surface melting in partially crystalline polyethylene : a Raman spectroscopic study

Abstract: Structures of semi crystalline high-molecular weight polyethylene and their changes with temperature were studied by analysis of Raman spectra. Decomposition of spectra into contributions of an orthorhombic crystalline phase, a meltlike phase, and two kinds of transition regions yields the respective mass fractions. The results of temperature dependent measurements is indicative of a largely reversible surface melting process, the transition regions being located at the crystal surfaces. As-polymerized samples show a large fraction of the intermediate phase. Annealing leads to a reduction of this phase. © 1993 John Wiley & Sons, Inc.

Nucleation, growth and inhibition of barium sulfate-controlled modification with organic and inorganic additives

Abstract: The crystal habit of barium sulfate formed by the rapid mixing of Ba2+ and SO2–4 containing brines at 95 °C has been investigated and the effects of coprecipitated K+, Mg2+, Ca2+ and Sr2+ ions, supersaturation ratio and barium-to-sulfate mixing ratio on the crystal morphology studied. The most marked crystal morphological changes were induced by varying the supersaturation and mixing ratios. The foreign ions produced relatively subtle effects. At high supersaturations (within the homogeneous nucleation regime), eight-pointed star-like crystals were formed, whilst at low supersaturations, the equilibrium rhombohedral crystal form was recovered. All precipitates were single crystals.In the presence of crystal growth modifiers, marked morphological changes could be induced. For example, certain polymers induced the formation of millimetre long bundles of needles at pH ≈ 6 and fractal-like hollow cones at pH ≈ 5. In contrast, a range of phosphonate-based molecules, designed to act as barium sulfate scale inhibitors, produced oblate spheroids and very distorted star-like crystals 15–20 times smaller than the unmodified crystals. These particles were found to be porous on the nanometer scale. All the precipitates described were single crystals.We have found that the control and prevention of barium sulfate precipitation by phosphonate-based materials at high Ba2+ concentration and low pH (ca. 4.5) are complicated. In the absence of Ca2+ ions in solution, little or no inhibition occurs and evidence suggests that calcium phosphonate complexes are the active inhibitors. In addition, although the inhibitors may act by the classical mechanism of blocking crystal growth sites, they also act as nucleation promoters. This provides an additional mechanism for scale inhibition involving depletion of scaling ions from solution. As part of this work, using dynamic light scattering, we have for the first time demonstrated the presence of 1–10 nm microcrystallites in fully and partially inhibited barium sulfate scaling systems.

Epitaxial protein inclusion in sinapic acid crystals

Abstract: The authors report on the inclusion of protein molecules in growing crystals of trans-3,5-dimethoxy-4-hydroxy-cinnamic acid (trans-sinapic acid). This material is important as the 'matrix' compound in matrix-assisted lasar desorption, a promising new ion source for heavy molecule mass spectrometry. The crystal structure of trans-sinapic acid was determined. It belongs to the space group P21/n and has a structure in which the planar trans-sinapic acid molecules are hydrogen bonded into extended sheets. Dyes were used to follow protein incorporation into growing trans-sinapic crystals. The staining pattern obtained demonstrated that protein molecules were included only on crystal faces parallel to these extended sheets. This result is the first demonstration that proteins can be incorporated into a growing crystal by a selective interaction with one of the crystal faces. The structure of the crystal plane that interacts with the protein was consistent with a 'hydrophobic' bonding of the protein to the crystal face.

High resolution neutron powder diffraction: a case study of the structure of C60

Abstract: High resolution time-of-flight neutron powder diffraction has been used to determine the detailed structure of C$\_{60}$ as a function of temperature. Rapid data collection coupled with high resolution has enabled subtle aspects of the 86 K orientational glass transition and precursor effects of the 260 K order-disorder transition to be observed. This surveying capability complements traditional single crystal methods. The power of the Rietveld method of profile refinement is demonstrated in the elucidation of the detailed crystal structure of the orientationally-ordered low temperature phase and in the evaluation of the departure from isotropic scattering of the C$\_{60}$ molecule in the disordered high temperature phase. The counter-intuitive success in obtaining high-order cubic-harmonic coefficients, albeit to poorer precision than single crystal X-ray measurements, confirms the efficacy of the Rietveld profile refinement method. The collapse of three dimensions of diffraction information on to the one dimension of a high resolution powder diffraction pattern can still lead to an impressive amount of structural information that substantiates the assertion made by W. H. Bragg `the second method [powder diffraction], first used independently by Debye and Hull, can be used when the crystal is in powder, and can, therefore be employed when no single crystal can be obtained of sufficient size. All the spectra of the different planes are thrown together on the same diagram or photograph, and must be disentangled. This is not as difficult as it may seem...'.

Simultaneous crystallization of diamond and cubic boron nitride from the graphite relative boron carbide nitride (BC2N) under high pressure/high temperature conditions

Abstract: Graphite BC[sub 2]N has been compressed with Co metal at a pressure of 5.5 GPa and temperatures of 1400-1600[degrees]C. The principal resulting products were crystals (average dimension 3 [mu]m) with cubiclike facets. The powder X-ray diffraction pattern revealed two kinds of cubic phase, in approximately equal amounts, which were identified as diamond and cBN on the basis of their lattice parameters. Microelemental analyses on individual crystal fragments by K-edge electron energy-loss spectroscopy confirmed this disproportionating crystallization scheme: half of the grains were composed of carbon-only signals of which gave fine-structure characteristic of sp[sup 3] bonding and the other half gave spectra characteristic of sp[sup 3] boron and nitrogen. The crystallization of cBN as well as diamond in the catalytic solvent of pure Co metal is observed here for the first time and is of relevance to the mechanism of the accepted catalytic action of cobalt on the hexagonal/cubic transformation. 26 refs., 4 figs., 2 tabs.

Andradite crystal chemistry, dynamic X-site disorder and structural strain in silicate garnets

Abstract: Andradite, Ca 3 Fe 2 (SiO 4 ) 3 , single crystals up to 0.4 mm were synthesized in a piston cylinder device from a glass at 1473 K and 20 kbar. The samples were characterized optically, by single-crystal infrared and by 57 Fe Mossbauer spectroscopy. Precise single-crystal X-ray diffraction data between 100 and 500 K were collected from a synthetic crystal and a natural nearly end-member andradite with a low OH content. Both samples yielded similar oxygen coordinates (within 5 esd's) and anisotropic displacement parameters (within 5 esd's). In addition, a 500 K single-crystal X-ray data set on synthetic pyrope, Mg 3 Al 2 (SiO 4 ) 3 , was measured for crystal chemical comparisons. A slight temperature dependent out-of-centre displacement was observed for Ca in andradite, in spite of the Ca overbonding on the dodecahedral X-site. This X-site disorder is less than the stronger'rattling effects' of Mg and Fe 2+ in pyrope and almandine, respectively, but still measurable. Anisotropic displacement parameters of oxygen were successfully modelled by rigid tetrahedral librations and translations. Structural strain in silicate garnets is discussed on the basis of the bond valence model. There is no difference in strain between Ca garnets refined in space group Ia3d and I1. It is suggested that strain relaxation in Ca-rich garnets may be accomplished by a small pyralspite or hydrogarnet substitution

Shaped crystal growth

Abstract: Preface. Introduction. 1. Dynamic Stability of Crystallization - the Basis of Shaped Crystal Growth. 2. The Czochralski Technique. 3. Techniques of Pulling from Shaper. 4. The Verneuil Technique. 5. The Floating-Zone Technique. 6. Radial Instability of Vapor Whiskers. 7. Stability of Cylindrical Pore Growth. 8. Two Shaping Elements Technique. 9. Variable and Local Shaping Techniques. 10. Crystal Cross-Section Shapes. 11. Shaped Crystal Growth by TPS. 12. Crystallization under Microgravity Conditions. Appendix: Calculation of the Growth Angle from the Shape of a Crystallization Drop. References.

Exhaust gas filter and method for making the same

Abstract: An exhaust gas filter for internal combustion engine and of honeycomb structure comprising a ceramic fiber and inorganic binder, the ceramic fiber consists mainly of Al₂O₃ and SiO₂, the inorganic binder including Al₂O₃ and SiO₂ to bind the said ceramic fiber, and the inorganic binder is of single glass phase or of mixed phase of a glass phase and a crystal phase keeping the same crystal structure in the temperature range of 20 to 1200°C, whereby, in the inorganic binder, any crystal such as quartz which produces abnormal expansion due to crystal phase transition is excluded, and occurrance of crack is prevented..

Surface Free Energy Model of Silicon Anisotropic Etching

Abstract: Silicon micromachining is used extensively for the microfabrication of integrated circuits, microsensors and microactuator devices. We address both calculations of the surface free energy of the crystal planes of silicon and the results of etching single‐crystal silicon spheres in and under controlled conditions. The silicon spheres were prepared by the cup‐grinding method. They were etched in a constant temperature bath at 50 and 75°C and inspected under the SEM. An optical reflectance technique was used to determine the angle of inclination of the etch facets. The order of etch rates for the crystal planes was {311}, . The fastest etching planes in were {320} and in were {110}. Also, with etching, the high index plane {432} was observed. The surface free energy of the crystal planes was calculated based upon the number of bonds on the surface. A minimum in surface free energy occurred for all the low index planes i.e., {100}, {110}, and {111}. There was also a minimum near the {522} planes in agreement with the experimental observations that these planes show anisotropic etching behavior. The {110} and {522} planes had a significant number of in‐plane bonds. When the in‐plane bond density was added to the surface bond density a good fit with the observed hierarchy of etch rates was obtained for the slower etching planes: {311}, .

Appearance of Single-Crystalline Properties in Fine-Patterned Si Thin Film Transistors (TFTs) by Solid Phase Crystallization (SPC)

Abstract: Uniformity of thin film transistor (TFT) characteristics was evaluated by varying the channel size As channel length decreased and width fell below 1 µm, uniformity was degraded drastically A few samples showed high-performance characteristics such as sharp gate voltage swing below 100 mV/dec and high mobility as high as 100 cm2/Vs near room temperature Upon evaluation of their temperature dependence, the conduction mechanism showed not polycrystalline but single-crystal-like properties in the lattice scattering The improved TFTs are thought to have been formed in grain boundary-free crystal grains

In‐plane epitaxial alignment of YBa2Cu3O7−x films grown on silver crystals and buffer layers

Abstract: Superconducting YBa2Cu3O7−x (YBCO) films were grown by laser ablation on Ag(001), Ag(110), and Ag(111) single‐crystal surfaces. X‐ray diffraction measurements showed that, in all cases, the films were aligned with specific in‐plane epitaxial orientation with respect to the crystallographic axes of the substrate. The observed orientations were consistent with predictions of near‐coincident site lattice models of the YBCO‐Ag interface. This technique for achieving three‐dimensional film alignment was extended to include a Ag epitaxial buffer layer on mica in place of a bulk Ag crystal. In‐plane epitaxial alignment on metal substrates and buffer layers has important consequences in practical applications for inhibiting weak‐link behavior caused by high‐angle grain boundaries.