Showing papers on "Crystal published in 1988"

Giant flux creep and irreversibility in an Y-Ba-Cu-O crystal: An alternative to the superconducting-glass model.

Abstract: We report strong, anisotropic magnetic relaxation of the field-cooled and zero-field-cooled magnetization along the principal axes of an Y-Ba-Cu-O single crystal and interpret it with a thermally activated flux-creep model. A simple scaling argument shows that high thermal activation causes magnetic irreversibilities and critical currents to drop below the threshold of detectability at a reduced temperature difference $1\ensuremath{-}t$ proportional to ${H}^{\frac{2}{3}}$, a power frequently observed in experiment and in particular in our crystal.

Starch Crystal Transformations and Their Industrial Importance

Abstract: The structures and transformations of the starch crystal forms A, B, C and V are reviewed from the viewpoint of their industrial importance. Reviewed also is the non-crystalline or amorphous state of starch and its role in determining the physical properties of native and gelled starches.

Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization II: Makaopuhi lava lake

Abstract: Crystal size distribution (CSD) theory has been applied to drill core samples from Makaopuhi lava lake, Kilauea Volcano, Hawaii. Plagioclase and Fe-Ti oxide size distribution spectra were measured and population densities (n)were calculated and analyzed using a steady state crystal population balance equation: n=n0 exp(-L/Gτ). Slopes on ln(n) versus crystal size (L) plots determine the parameter Gτ, a. product of average crystal growth rate (G) and average crystal growth time (τ). The intercept is J/G where J is nucleation rate. Known temperature-depth distributions for the lava lake provide an estimate of effective growth time (τ), allowing nucleation and growth rates to be determined that are independent of any kinetic model. Plagioclase growth rates decrease with increasing crystallinity (9.9−5.4×10−11 cm/s), as do plagioclase nucleation rates (33.9−1.6×10−3/cm3 s). Ilmenite growth and nucleation rates also decrease with increasing crystallinity (4.9−3.4 ×10−10 cm/s and 15−2.2×10−3/cm3 s, respectively). Magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus (G =2.9×10−10 cm/s, J=7.6×10−2/cm3 s). Moments of the population density function were used to examine the change in crystallization rates with time. Preliminary results suggest that total crystal volume increases approximately linearly with time after ∼50% crystallization; a more complete set of samples is needed for material with <50% crystals to define the entire crystallization history. Comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling. This interpretation is consistent with proposed thermal models of magmatic cooling, where heat loss is balanced by latent heat production to maintain equilibrium cooling.

Crystals in gels and Liesegang rings

Abstract: Providing the first comprehensive overview of the method of crystal growth in gels, Professor Henisch reviews the field, covering the underlying physics as well as the empirical experience of growth techniques accumulated over the past century. In addition, the book discusses the phenomenon of periodic precipitation, which often governs the distribution of crystal in laboratory growth systems. For the first time, computer techniques are brought to bear on the subject, the diffusion equations being solved numerically, in association with the conditions governing precipitations and crystal growth.

New Data and a Revised Structural Model for Ferrihydrite

Abstract: Synthetic 2-line and 6-line ferrihydrite samples prepared from ferric nitrate solutions have the bulk compositions Fe4(O,OH,H2O)12 and Fe4,6(O,OH,H2O)12, respectively. The composition depends on crystal size, which averages 20 A for 2-line and 35 A for 6-line ferrihydrite. X-ray absorption edge spectra indicate the presence of tetrahedral Fe3+, a conclusion supported by heating experiments which show the development of maghemite after heating to 300°C in the presence of N2, followed by the formation of hematite at higher temperatures. These two reactions are recorded on differential thermal analysis traces by exotherms at 350° and 450°C. Transmission electron microscopy shows that 2-line ferrihydrite has no Z-axis regularity but does show hexagonal 2.54-A lattice fringes. Six-line ferrihydrite forms faceted crystals having 9.4-A c-parameter only detectable in dark field. In bright field, 2.54-A lattice fringes indicate greater atomic regularity than in 2-line ferrihydrite. Analysis of the X-ray powder diffraction pattern of 6-line ferrihydrite suggests a structure based on double-hexagonal close-packed oxygens, containing 36% Fe in tetrahedral sites. Selective chemical dissolution, surface area measurements, and magnetic susceptibility are consistent with the recorded properties of ferrihydrite.

Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization

Abstract: Crystal size distributions (CSDs) measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (ΔT) of reactions during metamorphism. CSDs are described through use of a population density function n=dN/dL, where N is the cumulative number of crystals per unit volume and L is a linear crystal size. Plots of ln (n) vs. L for olivine+pyroxene and magnetite in high-temperature (1000° C) basalt hornfelses from the Isle of Skye define linear arrays, indicating continuous nucleation and growth of crystals during metamorphism. Using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at ΔT<10° C, and nucleation rates between 10−4 and 10−1/cm3/s. Garnet and magnetite in regionally metamorphosed pelitic schists from south-central Maine have CSDs which are bell-shaped. We interpret this form to be the result of two processes: 1) initial continuous nucleation and growth of crystals, and 2) later loss of small crystals due to annealing. The large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth. The extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known. Finally, the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs. contact metamorphosed rocks: because contact metamorphism involves high temperatures for short durations, resulting CSDs are linear and unaffected by annealing, similar to those produced by crystallization from a melt; because regional metamorphism involves prolonged cooling from high temperatures, primary linear CSDs are later modified by annealing to bell shapes.

Formation of highly orientated graphite from polyacrylonitrile by using a two-dimensional space between montmorillonite lamellae

Abstract: Carbon materials have many attractive physical and chemical properties, because of a wide variety of chemical bonding, crystal structure and microtexture. Carbon properties can be created by selecting a preparation procedure and a raw material. For example, many new carbon materials such as exfoliated graphite, benzenederived fibre and diamond film have come into use recently. The possibility of creating a new carbon material by a new method still remains. Here we report a novel method of preparing a highly orientated graphite from polyacrylonitrile (PAN) by making use of the interlamellar opening of montmorillonite (MONT) as a two-dimensional space for carbonization. A MONT-PAN intercalation compound was prepared and heat-treated at 700 °C to produce carbon from PAN between MONT lamellae. The carbon was then released from MONT by acid treatment and subjected to further heat treatment at various temperatures up to 2,800°C. The interplanar spacing (d002) of the carbon treated at 2,800 °C was very close to that of ideal graphite crystal and the crystallite size, Lc and La, were ~40 nm and >1μm, respectively. The formation of such a highly orientated carbon is probably a consequence of the orientation of the two-dimensional carbon precursor produced between the lamellae of MONT. When PAN itself is heat-treated, it produces a three-dimensional carbon with a network of intertwined ribbons of stacked graphitic sheets.

High‐Resolution Solid‐State 13C‐NMR Spectroscopy of Polymers [New Analytical Methods (37)]

Abstract: Until a few years ago, solid-state nuclear resonance yielded spectra containing broad lines only. Meanwhile, CP/MAS-NMR spectroscopy has provided a method which gives narrow nuclear resonance lines from a solid-state specimen as well. Using this technique, it is now possible to produce spectra of “rare” nuclei (13C, 29Si, 15N etc.) which are resolved in terms of chemical structure. The analytical capabilities of NMR spectroscopy can be applied to the solid state: it may be that it is necessary to identify compounds in the solid state because, for example, a solvent would alter the coordination sphere, or that it is desired to monitor chemical reactions in the solid state, for example the baking of an enamel. Where a substance in the solid state is concerned, high-resolution 13C-NMR spectroscopy provides not only information about the chemical structure, but also about the solid state itself. To mention just a few examples, information on the conformation, crystal structure and molecular dynamics, as well as molecular miscibility is given. This opens up a broad spectrum of applications, from a statement concerning the crystal modification of an active substance in ready-to-use pharmaceutical preparations, e.g. tablets, to the question of whether two polymers are miscible with one another at a molecular level.

Phonon radiative heat transfer and surface scattering.

Abstract: We have performed thermal-conductance measurements on polished single crystals of pure silicon in the low-temperature boundary-scattering regime. Our data show that the thermal conductance depends not only on the sample size, but also on the size and spacing of the thermometers used to measure the temperature gradient along the crystal. We have analyzed the heat transport in terms of phonon blackbody radiation subject to surface scattering from the attached thermometers and from the free surfaces of the crystal. We have made this analysis quantitative by using Monte Carlo techniques, and have calculated the magnitude of both surface scattering effects. We show that thermal-conductance measurements provide a very sensitive and precise technique for studying diffuse phonon scattering at crystal surfaces, and find that our highly polished and clean silicon surfaces will specularly reflect more than 99% of the incident phonons below 1 K, temperatures which correspond to dominant phonon frequencies less than 90 GHz.

Transient boron diffusion in ion-implanted crystalline and amorphous silicon

Abstract: Boron diffusion in ion‐implanted and annealed single‐crystal and amorphized Si is compared to determine the effect of amorphization on the initial transient boron motion reported for single crystal. The boron was implanted at 20 keV and at doses of 1×1015 and 3×1015cm−2. The Si was either preamorphized or postamorphized to a depth of 320 nm by implantation of Si ions at three different energies. In the amorphized samples the entire boron profile was always contained within this distance. The samples were annealed by furnace or rapid thermal annealing to 900–1100 °C with or without a preanneal at 600 °C. The initial rapid diffusion transient in the tail region of the boron profile was observed in all the crystal samples. This transient was totally absent in the amorphized samples. This is manifest by careful comparison of boron concentration profiles determined by secondary ion mass spectrometry of single‐crystal and amorphized samples after annealing. For anneals where significant motion occurs, the profi...

A defect model for ion-induced crystallization and amorphization

Abstract: Extensive experimental investigations have been reported on the ion-induced motion of the interface between the crystalline and amorphous phases of silicon. The crystal grows into the amorphous phase at low ion fluxes and high temperatures. The amorphous phase grows into the crystal at high ion fluxes and low temperatures. The experimental observations are shown to fit a model based on a single defect. The concentration of this defect decays by binary recombination, this is, two of the defects annihilate one another. The model accounts for the linear relationship between interface motion and reciprocal temperature, for the Arrhenius temperature dependence of the flux at which no interface motion occurs, and for the temperature independence of the crossover frequency observed in beam pulsing experiments. The defect on which this model is based has a motion energy of 1.2 eV. Assuming that the same defect is also responsible for thermal recrystallization of the amorphous phase gives a formation energy of 1.5 eV for the defect. The defect is believed to be a dangling bond in the amorphous phase.

Growth of superconducting single crystals in the Bi-Sr-Ca-Cu-O system from alkali chloride fluxes

Abstract: The discovery of superconductivity at temperatures above the boiling point of nitrogen has led to intense interest in the physics and chemistry of materials exhibiting this behaviour. Although some experiments can be performed on sintered polycrystalline samples, large single crystals are important for many measurements of physical properties, including investigations of the relationship between structural parameters and superconducting properties. Like its predecessors YB2Cu3O7 (refs 1–4) and La2–xSrxCuO4 (ref. 5), the recently discovered 84-K bismuth–strontium–calcium cuprate superconductor6–9 melts incon-gruently, and crystals have been grown from eutectic melts10. This technique yields crystals large enough for some experiments, but is generally difficult to control, placing limits on the size of crystals that can be grown. In addition, the crystals must be mechanically separated from the melt. Here we report the growth of single crystals of Bi2.2Sr2Ca0.8Cu2O8+δ from alkali chloride fluxes. The crystals are superconductors, showing large Meissner effects and zero-resistance transitions above 80 K. Flux growth is a standard high-temperature solution growth technique, and should allow crystal size and quality to be controlled by parameters such as equilibration temperature, cooling rate and melt composition. The fluxes reported here are easily washed from the crystals.

Molecular resolution images of amino acid crystals with the atomic force microscope

Abstract: The atomic force microscope has been used to image arrays of molecules at the surface of DL-leucine crystals. Lattice spacings are consistent with X-ray diffraction data. In contrast to metals and semiconductors, the surface of these amino acid crystals seems to be a simple termination of the bulk; there is no evidence of a surface reconstruction for this molecular crystal. This initial success in imaging amino acid molecules points to the potential usefulness of atomic force microscopy for imaging molecules of biological importance.

Crystallization of fcc (111) and (100) crystal-melt interfaces: A comparison by molecular dynamics for the Lennard-Jones system

Abstract: The crystal growth rates of Lennard‐Jones fcc (face‐centered cubic) (111) and (100) faces into the melt have been studied as a function of undercooling by molecular dynamics. The (100) grows without activation energy barrier at rates determined by the difference in the free energies of the crystal and melt phases, and the arrival rate of atoms across a plane determined from the kinetic theory of gases. The maximum velocity occurs at approximately half the melting point and represents 80 m/s for argon. The (111), on the other hand, grows at rates two to three times lower than this; the exact rate being size dependent. The growth kinetics are now activated and resemble Wilson–Frenkel behavior. However, the step responsible for such activation is not the simple liquid diffusion of Wilson–Frenkel theory, but rather the concerted motion of atoms at the interface selecting either all fcc or all hcp (hexagonal close packed) triangular lattice sites before a layer can grow.

Origin of the 110-K superconducting transition in the Bi-Sr-Ca-Cu-O system.

Abstract: Superconducting critical transitions with an onset at 112 K and zero resistance at 107 K are obtained within the Bi-Sr-Ca-Cu-O system. The synthesis and formation of the 110-K superconducting phase using the 85-K material as a precursor is explained. The 110-K phase grows from the 85-K phase such that the resulting faceted crystal (a pseudomorph) can contain some of the 85-K phase in the core. With such a microstructure our magnetic data can be simply explained. A major structural difference between the 85- and 110-K materials is that the 85-K material can grow (relatively) large single crystals having long-range order whereas the 110-K material has only intermediate-range order (cryptocrystalline) of about 100--200 A.

The elastic interaction of high-spin and low-spin complex molecules in spin-crossover compounds

Abstract: Several transition metal compounds show a transition from the low-spin (LS) to the high-spin (HS) electronic state with increasing temperature. The cooperative nature of the transition is usually parametrised by an interaction constant Gamma , the origin of which is still under discussion. In the frame of the lattice expansion mode, the interaction Gamma is attributed to the elastic interaction between the spin-changing ions as a result of the deformation of the crystal accompanying the transition. The authors calculate the complete elastic energy originating from the so-called image pressure in closed form by considering the crystal as an isotropic homogeneous elastic medium with the spin-changing ions as incompressible inclusions described by the full elastic dipole tensors PHS and PLS, respectively. The calculated values of Gamma based on X-ray data and reasonable estimates of the elastic constants of the compounds (Fe(2-pic)3)Cl2.Sol (2-pic=2-aminomethylpyridine, Sol=MeOH,EtOH) and (Fe(2-pic-ND2)3)Cl2.EtOD are compared with the experimental values of Gamma .

Observation of individual organic molecules at a crystal surface with use of a scanning tunneling microscope.

Abstract: A scanning-tunneling-microscopy (STM) study of the surface of a TTF-TCNQ (tetrathiafulvalene tetracyanoquinodimethane) crystal is described. The surface of this conducting organic molecular crystal is well ordered, with flat terraces separated by steps of a single molecular layer and with periodicity that agrees with the bulk crystal structure. Individual organic molecules at the surface are resolved in the STM images. The principal features of the experimental STM images are reproduced in simulations in which contours of constant electron density are calculated from the molecular orbitals that are involved in conduction.

Theoretical studies of the structure and molecular dynamics of a peptide crystal.

Abstract: Energy minimizations and molecular dynamics simulations have been performed on the cyclic peptide cyclo-(Ala-Pro-D-Phe)2 in both the isolated and crystal states. The results of these calculations have been analyzed, both to investigate our ability to reproduce experimental data (structure and vibrational and NMR spectra) and to investigate the effects of environment on the energy, structure, and dynamics of peptides. Comparison of the minimized and time-averaged crystal systems with the experimental peptide structure shows that the calculations have closely reproduced the experimental structure. Molecular dynamics of the isolated molecule has led to a new conformation, which is approximately equal to 8.5 kcal/mol more stable than the conformation that exists in the crystal, the latter conformation being stabilized by intermolecular (packing) forces. This illustrates the considerable effect that environment can have on the conformation of peptides. The crystal environment has also been shown to significantly reduce the dynamic conformational fluctuations seen for the isolated molecule. The behavior of the peptide during the isolated simulation also supports the experimental NMR observation of a symmetric structure that differs from the asymmetric, instantaneous structures which characterize the molecule during the dynamics. Calculations of vibrational frequencies of the peptide in the crystal and isolated states show the expected shifts in bond-stretching frequencies due to intermolecular interactions. Finally, we have calculated NMR coupling constants from the dynamics simulation of the isolated peptide and have compared these with the experimental values. This has led to a possible reinterpretation of the experimental data.

Hardness and fracture toughness of semiconducting materials studied by indentation and erosion techniques

Abstract: In recent years, the growing field of semiconductor micromechanics has created an increasing demand for strength data on semiconductors and for adequate tests and evaluations of their mechanical properties. In a recently published paper, the present authors have demonstrated that the solid particle erosion rate can be taken as a simple and highly reproducible statistical measure of the susceptibility of silicon and GaAs to contact damage in the micron range. In the present work the scope is broadened to include several new crystal orientations (and one new doping level), as well as three other materials: germanium, InP and InAs, for which the hardness and fracture toughness K Ic values are determined by means of the indentation technique. K Ic values are also derived from erosion data by means of a recently reported brittle fracture model, based on non-lateral spalling in single-crystal semiconductors. These values are compared with results obtained by the indentation technique and conventional test methods reported in the literature. Fracture surface energies are deduced from the experimental K Ic results. The materials tested are ranked with respect to elastic properties, microhardnesses, fracture toughnesses, and sensitivities to contact damages in general. The influence of crystallographic orientation on room temperature microfracture properties is clearly established, but the corresponding influence on microhardness is found to be rather limited. The influence of doping on the room temperature mechanical properties is noticeable but small.

Crystallographic study of tetragonal, superconducting YBa2(Cu0.93Fe0.05)3O7

Abstract: Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (T c ≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 A in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.

Gypsum nucleation and crystal morphology in analog saline terrestrial environments

Abstract: Gypsum crystals were grown in experimental conditions analogous to saline terrestrial environments within bentonite clay gels by diffusion control at three different temperatures, four brine salinities, and four tannic acid (a model terrestrial humic substance) concentrations. The resulting crystals correspond to natural gypsum formed in terrestrial environments. Prismatic gypsum typically grew at both high and low temperatures in the absence of the organic additive. With increasing organic acid concentrations, the prismatic crystals progressively became flattened perpendicular to [001], and two temperature-dependent trends developed. At low temperatures, a hemi-bipyramidal habit dominated by 1111 faces developed, whereas the lenticular e103 dominated habit forme at higher temperatures. With progressively greater concentrations of organic material a(100) penetration twinning developed, secondary complex nucleation occurred near the twin interfaces, and finally, rosette and rosette-like aggregates formed. Higher temperatures generally favored better-formed and larger rosettes. The presence of 5% and 15% NaCl greatly decreased nucleation density and resulted in larger single crystals and crystal aggregates. The a(100) penetation twins appear to be diagnostic of gypsum growth in natural terrestrial sediments at a pH greater than 7.5.