Showing papers on "Crystallization published in 1970"

Properties and structure of vitreous silica. I

Abstract: This review is concerned with the properties and structure of silica glass. The following topics are treated: Types of silica glasses; The vitreous state of silica glasses: thermodynamical approach, atomistic approach; Optical properties; absorption and fluorescence, refractive index and homogeneity; Mechanical and thermal properties: specific volume, volume relaxation, volume and pressure, elastic and internal friction behaviour, heat capacity and heat conduction, strength, crystallization.

Magnetic Properties of Barium Ferrite Formed by Crystallization of a Glass

Abstract: A glass with composition 0.265 B2O3-0.405 BaO-0.33 Fe2O3 (mole ratio) was prepared by a fast-quenching technique. When it is heat-treated, this glass exsolves up to ∼45 wt% BaFe12O19 as the only magnetic phase. Magnetic measurements of glasses heated at various temperatures show that superparamagnetic, single-domain, or multidomain magnetic behavior is present, depending on the thermal history. The volume of a typical superparamagnetic particle (calculated from the magnetic data) is equivalent to that of a sphere 47 A in diameter. The intrinsic coercive forces of two heat-treated glasses were independent of temperature at high levels of Hci (2600 and 2900 Oe) from 77° to 300°K. Another heat-treated glass has an Hci of 5350 Oe at 300°K. Apparently, the coherent rotation model of Stoner and Wohlfarth describes the magnetic behavior of BaFe12O19 very well. The single-domain critical size for BaFe12O19 was ∼0.5 μm. An attractive feature of this system is that the BaFe12O19 powder can be recovered from the barium-borate-rich matrix by leaching with a weak acid.

Hydrodynamically induced crystallization of polymers from solution

Abstract: A detailed description of several phenomena observed during crystallization in a stirred solution of linear polyethylene is presented. It is found that simple shear flow is incapable of accelerating the formation of primary nuclei at elevated temperatures in 5% xylene solution of linear polyethylene (Marlex). The onset of fibrillar crystal formation is noted when the stirrer speed of the inner cylinder of aCouette instrument exceeds a minimum value which corresponds to the onset ofTaylor vortices. Closed loops of fibrillar crystals occurring periodically around the stirrer, revolve in orbits which coincide with the centers of theTaylor vortices. Turbulent flow seems to be most effective in inducing fibrillar crystallization and a maximum crystallization temperature of 112°C was achieved with the 5% xylene solution of Marlex. A mechanism is proposed for the formation of primary nuclei of bundlelike type which is based upon orientation and deformation of aggregates in regions of elongational flow between the vortices. Solutions of high molecular weight polyethylene (M w=1.5×106) exhibit a pronouncedWeissenberg effect prior to the appearance of the first fibers. Normal stresses lower the rate of stirring inCouette flow at which vortices occur. This may account for the observed fibrillar crystallization of the high molecular weight polyethylene at stirrer speeds which in case ofNewtonian fluids give rise to laminar flow according to theTaylor criterion. Settlement of the fibrous network on the stirrer results from secondary flow.

Concentration and Behavior of Carbon in Semiconductor Silicon

Abstract: The concentration of carbon in semiconductor silicon and the behavior of carbon in the fusion and crystallization of silicon have been studied by the use of charged particle activation analysis. The solubility of carbon in solid silicon has been found to be or slightly less at the melting point, and the equilibrium distribution coefficient of carbon between solid and liquid silicon has been determined to be . Carbon content over the above solubility value has seldom been observed in silicon single crystals produced by modern industrial techniques. The content appears to depend more on the growth conditions of the single crystal than on the chemical purification method. The phase diagram of the C‐Si system in the extremely low carbon concentration range is given, together with a discussion of the kinematics of the carbon behavior in zone melting. Also, a new technique for studying the evaporation of carbon from the silicon melt is shown.

Thermochemical studies on amorphous calcium phosphate.

Abstract: Previous studies have indicated that synthetically-precipitated amorphous calcium phosphate, even though not a periodically regular structure, possesses a chemically definable local unit of structure. The present paper is a report of thermochemical studies performed to more clearly define this local chemical unit. Upon ignition in the absence of water, freezedried amorphous calcium phosphate preparations converted into crystalline α-and/or β-tricalcium phosphates. At temperatures just below the crystallization point, the amorphous calcium phosphate became almost completely dehydrated. Additionally, pyrophosphate production at these subcrystallization temperatures was very low. From these data, it appears that water in amorphous calcium phosphate does not exist in combination with the P2O5 component as acid phosphate but retains its molecular identity, suggesting, together with previously reported chemical data, that amorphous calcium phosphate when formed under specific chemical conditions is a hydrated tricalcium phosphate.

Crystallization of sheared polymer melts

Abstract: The Crystallization kinetics of molten polyethylene subjected to a constant shear stress were investigated theoretically and experimentally. The rate of crystallization depends on the difference of entropy (Δs) between the crystalline and molten state. The constant shear stress in the melt decreases Δs causing an abrupt increase in the rate of crystallization and a decrease of the thickness of the folded-chain lamellar crystals.

An equation for trace element distribution during magmatic crystallization

Abstract: Unlike previous models for the equilibrium distribution of elements between liquid and solid during crystallization, the general equation developed here can take account of variations in mineral proportions and in distribution coefficients. Several exampies demonstrate the use of the equation and illustrate the magnitude of the errors involved in the assumptions of constant distribution coemcients and constant mineral proportions. The equation is expected to be of particular use to the petrologist wishing to relate individual samples of a suite to the degree of crystallization of the magma IwrnorucuoN Several discussions of the distribution of elements between crystals and liquid during crystallization difierentiation of a magma have appeared in the l iterature, (Neumann, 1948; Holland and Kulp, 1949; Neumann et al., 1954). These have been reviewed and discussed by Mclntire (1963). More recently, Anderson and Greenland (1970) have shown that the variation of phosphorus in some rock suites may be directly related to the fraction of liquid remaining at any point, and Gast (1968) has used partition coefficients in a consideration of partial melting processes. Previous models for the distribution of trace elements during crystallization are limited in their usefulness by two assumptions required in the mathematical derivation: (1) the various minerals occur in constant proportions throughout crystallization, (2) the distribution coefficient of an element between liquid and crystals is a constant. Neither of these assumptions can be regarded as geologically realistic. The purpose of the present paper is to derive a general equation giving the composition of liquid or solid as a function of the fraction of liquid crystallized (i.e., degree of crystallization) which does not require these assumptions. The model described here is the \"homogeneous crystallization\" case of Mclntire (1963); that is, as the l iquid crystall izes the solids are effectively removed from the liquid and no reaction with subsequent liquids occurs. For this case, only Henry's law and the equations for conservation of mass need be considered. Thermodynamics and kinetics are excluded, since this is not a theoretical attempt to predict element distributions; it is a general analysis of mass balance as required to deduce crystallization histories from the chemistry of rocks. I Publication authorized by the Director, U. S. C\".f.gi.al Survey. 455 456 L. PAUL GREENLAND Tnnonrrrcar, The svmbols used in the following derivation are defined in table 1. We start with the mass balance equation. dx \"+dx1 : 6 which, with the definit ions of table 1, may be written dX\" + CrM\"dF I MiC'r : O ( 1 )

Factors influencing the formation of silver-rich solid solutions in rare- earth--silver alloy systems.

Abstract: The solid solubilities of all of the naturally occurring rare-earth metals in silver have been determined by using the X-ray parametric method. The solubilities range from 0.01 to 0.02 at. pct for Eu to 10.5 at. pct for Sc. The stoichiometries of the silver-rich compounds and some of their crystal structures were determined. The silver-rich compounds found in this study are: RAg5(La, Ce, Pr, Eu, and Yb), RAg4(Lu and Sc), R14Ag5l(La to Sm, Gd to Er, Yb, and Y) and TmAg3. The silver/silver-rich compound eutectic temperatures were also determined. Analysis of these data indicates that several factors in addition to the Hume-Rothery criteria influence the formation and extent of solid solutions. These include the mutual adjustment of size and electronegativity of the solvent and solute if the pure metal size differences are less than a critical value of 22 to 25 pct, the composition of the first solvent-rich compound, and the lattice rigidity of the solvent.

Crystallization of polycarbonate from the glassy state. part I. Thin films cast from solution

Abstract: The morphology of glassy amorphous thin polycarbonate film cast from solution is affected by thermal treatments. Annealing above 80° C and below Tg results in an increase in the size of the ordered regions, nodules, up to several hundred Angstroms. The crystallization process from the glass, taking place at 145° C, is divided into three major steps. At first the nodules merge into patches which aggregate to form lamellar planar structures. In some cases the planar structures are well-formed single crystals. Following this, spherulitic arms develop from the planar structures as centers. These arms at first consist of aggregates of large nodules which recrystallize to form lamellae; the final morphology is spherulitic in nature. The effect of film thickness and of several substrates on the morphology has been observed. Applying stress at room temperature to the crystalline film results in a breaking up of the lamellae into small blocks.

Crystal nucleation in lithium silicate glasses

Abstract: The development of the microstructure of simple glass-ceramics, of molar compositions 70SiO2·30Li2O and 69SiO2·30Li2O·1P2O5, has been studied by the techniques of transmission electron microscopy, optical microscopy, X-ray powder diffraction and electrical loss measurements. Surface nucleation of lithium disilicate crystals occurred in all samples, and internal nucleation occurred in samples of the first glass treated for 1 h at 450, 475 or 500°C before crystallization at 750°C. Internal nucleation always occurred in samples of the glass containing P2O5 which were crystallized at 750°C, but the grain size of the crystalline product depended strongly on the temperature of the previous lower temperature heat treatment (the “nucleation stage”). The optimum nucleation range was close to 500°C for 1 h heat treatments. The results suggest that glass-in-glass phase separation, which was observed in both glasses, did not promote crystal nucleation directly, but effects due to an influence of phase separation on crystal growth rates are suggested. Possible reasons for the action of P2O5 as a nucleation catalyst are discussed, but firm conclusions cannot be reached on the present evidence.

Morphology and deformation behavior of “row-nucleated” polyoxymethylene film

Abstract: Crystallization from a stressed polymer melt produces not the familiar randomly nucleated spherulitic structures, but instead a highly oriented “row-nucleated” morphology. We have crystallized films of polyoxymethylene from stressed melts; just as polyethylene, the surfaces are covered with protruding lamellar edges highly oriented in the extrusion direction. There is no evidence for the more familiar spherulitic morphology. Electron microscopy directly revealed for the first time that the row nuclei are fibers, only about 200 to 300 A in diameter, extending for distances up to 10 mU. Although comprising a negligibly small amount of total sample volume, they are of prime importance in influencing how the sample will crystallize. Their presence determines, more than any other single factor, whether the sample will crystallize spherulitically or into a row-nucleated morphology. When deformed in the extrusion direction, the twisted lamellar bundles open up to form a fishnetlike structure. Simultaneo...

The polymorphism of poly(vinylidene fluoride) IV. The structure of high-pressure-crystallized poly(vinylidene fluoride)

Abstract: A detailed study of high-pressure-crystallized poly(vinylidene fluoride) has indicated that a mixture of low-melting phase II and high-melting phase I is present, rather than a new crystalline phase (phase III) as originally suggested. The relative amounts of phase I and phase II resulting from crystallization under pressure are a function of pressure and the degree of supercooling. Pressure crystallization at 285°C and 5500 atm results in samples which were pure phase I with an increased melting point of 187°C.

Rate of crystallization of an amorphous FePC alloy

Abstract: An alloy containing iron, phosphorus and carbon in the atomic concentrations corresponding to Fe75P15C10 can be obtained in the amorphous state by rapid quenching from the liquid state. The transformation of this alloy from the amorphous to the equilibrium crystalline state has been studied by thermal analysis, X-ray and electrical resistivity. At rates of heating above approximately 320°C/min, the alloy transforms very rapidly into a microcrystalline structure with an average crystal size of about 300A. At slightly lower rates (about 100°C/min) it is possible to stop the massive crystallization and microcrystalline clusters of metastable phases imbedded in an amorphous matrix are present. At a relatively low rate of heating of 1°C/min, crystallization starts around 375°C, but has a sudden increase in the range of 420 to 440°C. This increase in crystallinity is reflected in the electrical resistivity which also shows a sharp drop within the same temperature range. Before this massive crystallization takes place, the electrical resistivity increases slowly with temperature. This behavior has been observed previously in a number of amorphous alloys. In the amorphous FePC alloy, this increase in resistivity is not a reversible phenomenon, and the results of the present investigation suggest that it is due to additional scattering of the conduction electrons by very small clusters of crystalline phases, about 50A in size.

A Petrologic Model for the Moon Based on Petrogenesis, Experimental Petrology, and Physical Properties

Abstract: Crystallization experiments under highly reducing conditions of synthetic material of mean Apollo 11 rock composition yielded: ilmenite in at 1,150° C; clinopyroxene in at 1,130° C, plagioclase in at 1,120° C. The same sequence of crystallization for the A pollo 11 vesicular ferrobasalts was interpreted from the mineral textures. The low liquidus temperature, narrow crystal-liquid interval, and delayed appearance of plagioclase are interpreted by crystallization from a magma formed by remelting of a mixture of accumulated pyroxene and ilmenite crystals in a liquid which has undergone advanced fractional crystallization. Under the Sea of Tranquillity, a residual ferrobasaltic liquid was formed by fractional crystallization of ultrabasic magma deeper in the moon. Heavy ilmenite and pyroxene crystals sank in the liquid, while light plagioclase floated to augment a crust. Meteorite impact blasted away the plagioclase-rich crust and released a ferro-basaltic magma formed by melting of ilmenite and pyroxene into the differentiated liquid. Fractional crystallization of a molten moon of modified chondritic composition yielded a small metallic core surrounded by pressure-stable, Mg-rich olivine and pyroxene. The fractionated liquid became Fe-and Ti-rich. Primitive ultrabasic crust in the highlands was augmented by dominant plagioclase-rich cumulates. The preferential occurrence of large irregular seas on the earth side of the moon is explained by differential tidal attraction of the late liquid fraction, and release by meteorite impact. The model can satisfy the density and moment of inertia of the moon if the olivine is Mg-rich. Early removal of radioactive material from the center by fractional crystallization, and enhancement of radiative heat transfer in volatile-free silicates would ease the problem of cooling the center of the moon. The low volatile content of A pollo 11 rocks may be a feature of the entire moon, permitting high rigidity and refractoriness. Incorporation of metal-seeking elements into a metal core, loss of volatiles from a hot surface, and crystal-liquid fractionation can explain some element concentrations of A pollo 11 rocks. Nevertheless the low density of the moon indicates a low Fe content of the primary material of the moon. Although there are no'meteorites which match the Apollo 11 rocks in all chemical and textural respects, several types have one or more features in common.

Low-angle X-ray diffraction and physical properties of carbon fibres

Abstract: A number of low-angle parameters have been evaluated for a high-modulus carbon fibre, the most useful is considered to be lp, Porod's `distance of heterogeneity'. When I−½ is linearly related to θ2, lp=2a, where a is the correlation length obtained from the (I−½, θ2) plot. The parameter lp and the internal surface area have been evaluated for carbon fibres heat treated in the range 1000-2800°c. There is good correlation between the low-angle parameters and strength, but a marked discontinuity at about 1900°c. This discontinuity is related to a change from highly cross-linked graphite with many small pores and small crystallite size, to graphite with little crosslinking, a lower internal surface, but larger individual pores and crystallites where crystallite perfection is an important factor. High strength in stress-graphitized carbon fibres would appear to be a result of strain-induced crystallization with the removal of lattice defects.