Showing papers on "Crystallization published in 1988"

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

Abstract: Crystal-size in crystalline rocks is a fundamental measure of growth rate and age. And if nucleation spawns crystals over a span of time, a broad range of crystal sizes is possible during crystallization. A population balance based on the number density of crystals of each size generally predicts a log-linear distribution with increasing size. The negative slope of such a distribution is a measure of the product of overall population growth rate and mean age and the zero size intercept is nucleation density. Crystal size distributions (CSDs) observed for many lavas are smooth and regular, if not actually linear, when so plotted and can be interpreted using the theory of CSDs developed in chemical engineering by Randolph and Larson (1971). Nucleation density, nucleation and growth rates, and orders of kinetic reactions can be estimated from such data, and physical processes affecting the CSD (e.g. crystal fractionation and accumulation, mixing of populations, annealing in metamorphic and plutonic rocks, and nuclei destruction) can be gauged through analytical modeling. CSD theory provides a formalism for the macroscopic study of kinetic and physical processes affecting crystallization, within which the explicit affect of chemical and physical processes on the CSD can be analytically tested. It is a means by which petrographic information can be quantitatively linked to the kinetics of crystallization, and on these grounds CSDs furnish essential information supplemental to laboratory kinetic studies. In this three part series of papers, Part I provides the general CSD theory in a geological context, while applications to igneous and metamorphic rocks are given, respectively, in Parts II and III.

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.

Controlled crystallization of CaCO 3 under stearic acid monolayers

Abstract: A fundamental concept in the study of biomineralization concerns the molecular recognition of inorganic materials at organized organic macromolecular substrates1. Here we investigate this concept through the use of stearic acid monolayers in the controlled crystallization of CaCO3 from supersaturated solutions. Whereas crystallization in the absence of a monolayer results in rhombohedral calcite crystals, the presence of an organized monolayer gives rise to oriented vaterite formation. The vaterite nuclei are aligned with their (0001) face parallel to the plane of the organic substrate and develop initially in the form of disk-shaped single crystals. The degree of compression of the monolayer dictates the homogeneity of vaterite nucleation. In particular, partially compressed films are optimal for controlled crystallization, suggesting that the mobility of organic surfaces may be of general importance. Our results can be explained by electrostatic and stereochemical interactions at the inorganic–organic interface and these observations support current theories of biomineralization, as well as being of potential significance in the crystal engineering of microscopic inorganic assemblies2.

Crystallization and polymorphism of fats and fatty acids

Abstract: Deals with the physical and chemical characteristics of fats and fatty acids, coordinating two approaches the microscopic analysis of polymorphic structures, and macroscopic technical control of production. Topics include fundamentals of crystallization and polymorphism, crystal structure, polymorph

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C.

Abstract: We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

A New Process for Silica Coating

Abstract: Silica film was deposited on the surface of soda lime silicate glass by immersing it in hydrofluosilicic acid solution supersaturated with silica gel at low temperature. The composition and the structure of these films were evaluated using ESCA, IRRS, fluoric acid etching test, and sodium leaching test, and they were compared with those of various silica films prepared by other methods. The results showed that these films made at low temperature had a dense structure and a great alkali barrier effect. By heat‐treatment, the structure of this film became denser, getting close to that of fused silica. It was also found that the degree of supersaturation and the solution temperature had a great influence on the deposition rate.

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.

Glass-forming range in mechanically alloyed Ni-Zr and the influence of the milling intensity

Abstract: Amorphous Ni‐Zr powders have been prepared by mechanical alloying from crystalline elemental powders. The glass‐forming range has been determined by x‐ray diffraction, differential scanning calorimetry and saturation magnetization measurements. From 27 to 83 at. % Ni the powders become amorphous. This shows that deep eutectics do not play any role, contrary to amorphization by melt spinning. Crystallization temperatures, crystallization enthalpies, and wave numbers Qp, obtained from x‐ray diffraction investigations, are compared with the data received for rapidly quenched samples. In addition, the effect of the milling intensity on the glass formation has been studied for the first time. If the intensity is too high, crystalline intermetallic phases are formed. On the other hand, the powder needs an extended milling time to become completely amorphous if the milling intensity is too low. Conclusions on the actual temperature of the individual particle during mechanical alloying and on the glass‐forming process are drawn from these results.

Crystal structure and morphology of syndiotactic polypropylene single crystals

Abstract: In the past several years there have been an increased interest in the crystal structure and morphology of s-PP due to the new development of homogeneous metallocene catalysts which can produce s-PP having a high stereoregularity. In this research, the crystal structure and morphology of s-PP single crystals grown from the melt were investigated. A series of ten fractions of s-PP was studied with different molecular weights ranging from 10,300 to 234,000 (g/mol). These fractions all possess narrow molecular weight distributions (around 1.1-1.2) and high syndiotacticities ([r]{approximately}95%). The main techniques employed including transmission electron microscopy (TEM), atomic force microscopy (AFM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS).

Melt-vapor solubilities and elemental partitioning in peraluminous granite-pegmatite systems: experimental results with Macusani glass at 200 MPa

Abstract: Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H2O and silicate melt components. The H2O content of melt at saturation (11.5+-0.5 wt.%) is almost twice that predicted by existing melt speciation models. The corresponding solubility of melt components in vapor decreases from 15 wt.% dissolved solids (750°–775° C) to 9 wt.% at 600° C. With regard to major and most minor components, macusanite melt dissolves congruently in vapor. Among the elements studied (B, P, F, Li, Rb, Cs, Be, Sr, Ba, Nb, Zr, Hf, Y, Pb, Th, U, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), only boron has a vapor/melt partition coefficient (D[B]) consistently ≥1 at superliquidus temperatures (>645° C). Phosphorus and fluorine behave similarly, with D[P] and D[F]<0.5. Little or no significant vapor/melt fractionation is evident among most periodic groups (alkalis, alkaline earths, Zr/Hf, or the REE). The temperature dependence of vapor/melt partition coefficients is generally greatest for cations with charge ≥ +3 (except Nb and U); most vapor/melt partition coefficients for trace elements increase with decreasing temperature to the liquidus. Crystallization proceeds by condensation of crystalline phases from vapor; most coexisting melts are aphyric. Changes in the major element content of melt are dominated by the mineral assemblage crystallized from vapor, which includes subequal proportions of white mica, quartz, albite, and orthoclase. The volumetric proportion of (mica + or-thoclase)/albite increases slightly with decreasing T, creating a sodic, alkaline vapor. Vapor deposition of topaz (T≤500° C), which consumes F from melt, returns K/Na ratios of melt to near unity with the vapor-deposition of albite. The abundances of most trace elements in residual melt change little with the crystallization of major phases, but in some cases are strongly controlled by the deposition of accessory phases including apatite (T≤550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.

Thermal analysis of poly(butylene terephthalate) for heat capacity, rigid-amorphous content, and transition behavior

Abstract: Thermal analysis was performed on poly(butylene terephthalate), PBT, between 210 and 560 K. By combination of experimental heat capacities with computations with an approximate frequency spectrum of 65 group and 19 skeletal vibrations, preliminary recommended ATHAS (1988) heat capacities are proposed for the solid state from 0 to 600 K. The Tarasov parameters used for the computation of the skeletal vibrations were θ1 = 542 K and θ3 = 80 K for crystalline PBT and θ3 = 40 K for amorphous PBT. The glass transition temperature of amorphous PBT was found on efficiently quenched samples to be 248 K, much lower than for semicrystalline PBT where a 310–325 K glass transition temperature is typical. The increase in heat capacity calculated for 100% amorphous samples is 107 J/(K · mol) at 248 K and 77 J/(K · mol) at 320 K. The equilibrium melting temperature is estimated to be 518 K. The unique existence of rigid-amorphous fractions of the semicrystalline polymers is discussed with quantitative data for samples crystallized from the glass and from the melt between 275 and 490 K. The rigidamorphous fraction varies between above 0,9 for cold-crystallized samples to 0,3 for samples crystallized at 490 K. The crystallinity varied from below 0,1 to 0,5. The crystallinity could be separated into four parts, melting at high, medium, and low temperatures, and a part crystallized on cooling after isothermal crystallization. The sequence of crystallization of differently melting crystals was established.

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.

Non – Isothermal Crystallization Crystallization of Polymers

Abstract: The kinetics of non-isothermal crystallization, taking into account both formation and growth of nuclei, is formulated in terms of rate equations. Depending on whether Avrami's or Tobin's models of impingement are applied, sets of, respectively, (m + 1) and (m + 2) first-order differential equations are obtained, where m is the number of spatial dimensions of growth. The number of rate equations reduces to one if the activation of nuclei is much faster than their growth. Avrami's well-known formula is re-obtained from isothermal limiting cases.

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.

Formation of LiNbO3 films by hydrolysis of metal alkoxides

Abstract: Homogeneous, crack-free, thin films of crystalline LiNbO3 were synthesized above 250°C on Si(100) substrates by the dip-coating method using a double alkoxide solution. The coating solution, which was prepared by the controlled partial hydrolysis of the double alkoxide, gave stoichiometric LiNbO3 crystalline films at temperatures as low as 250°C. The concentration of the alkoxide solution influenced both of thickness and quality of films. Crystallinity of thin film top-coated directly on the substrate affected the crystallization state of films coated on the film remarkably. Films crystallized on α-Al2O3(0001) showed preferred orientation along the c-axis, while the preferred orientation could not be observed on Si(100) substrates.

Crystallization process of Sb‐Te alloy films for optical storage

Abstract: Sb‐Te alloy films are developed as rewritable optical recording materials based on amorphous crystalline phase transformations. The crystallization process of Sb‐Te sputtered films is systematically studied through measurement of recording characteristics to solve the trade‐off problem between data (amorphous) stability and erasing sensitivity. Sb2Te3 is shown to be the best practical phase change medium, having room‐temperature stability in amorphous states, short erasing times, and potentially good reversibility. The carrier‐to‐noise ratio of 50 dB in writing and a decrease in the carrier level of over 30 dB in erasing are achieved in dynamic measurement with a single beam optical head. These favorable properties are attributable to the wide composition margin for a single phase formation in the Sb‐Te alloy system.