Showing papers on "Crystallization published in 1969"

Under what conditions can a glass be formed

Abstract: Summary Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, - T, the liquid volume, v], and the seed density, ps and upon a set of materials constants: the reduced crystal–liquid interfacial tension, α the fraction, f, of acceptor sites in the crystal surface, and the reduced glass temperature, Trg . The glass-forming tendency will be greater the larger are - T and Trg and the smaller are v]. ps, and f. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass temperature. T...

Experimental studies of pegmatite genesis; l, A model for the derivation and crystallization of granitic pegmatites

Abstract: The genesis of granitic igneous pegmatites is here considered in terms of a model conceived from results of field and laboratory studies and subsequently tested by means of experimental investigations. This model emphasizes the roles of water (and/or other relatively volatile substances), both as a dissolved constituent in granitic magmas and as the dominant constituent of a separate fluid phase that is in the supercritical state under most conditions of pegmatite formation. Pegmatite magma, as distinguished by a content of dissolved water that is high relative to the limit of solubility under existing confining pressure, can be formed either through partial melting of crustal materials or as rest-liquid in a cooling igneous body yielding dominantly anhydrous crystalline phases. Such granitic magma can be expected to consolidate according to the following three-fold sequence: 1. Crystallization from hydrous silicate melt, yielding anhydrous solid phases with or without OH-bearing phases. The product is characterized by normal phaneritic textures that generally are coarse grained. It has been termed pegmatite in some occurrences, and granite in others. 2. Crystallization concomitantly from silicate melt and from a coexisting exsolved aqueous fluid of considerably lower viscosity, yielding giant-textured pegmatite along with much finer-grained, even aplitic, mineral aggregates. Segregation of these products can vary enormously in scale and degree. Partitioning of constituents between melt and aqueous fluid, rapid diffusion of constituents through the aqueous phase, and gravitational rising of this fluid through the system contribute to formation of pods, zones, and other rock units of unusual composition and texture. 3. Crystallization in the absence of silicate melt, yielding a wide variety of late-stage products. These include so-called "pocket minerals" and numerous mineral aggregates formed through exchanges of material among aqueous fluid and earlier-formed crystal-line phases. Development of pegmatite bodies can begin with either Step 1 or Step 2, but it is suggested that the processes involved in Step 2 are essential to the formation of all true pegmatites of igneous origin. The appearance of a second fluid phase, in general a supercritical aqueous fluid derived from the crystallizing melt, is regarded as the decisive event; it is promptly followed by fundamental changes in distribution and texture of the solid phases being formed.The processes can operate effectively in a fully closed system, and they also can modify the surrounding rocks if the system is open at any stage. Step 1 can include reactions between magma and earlier-formed crystals, but far more rapid and extensive exchanges of materials are subsequently effected by processes included in Steps 2 and 3; indeed, such exchanges also can account satisfactorily for pegmatites of metamorphic origin.Crystallization of most granitic magmas in the absence of a separate aqueous phase probably would begin within the temperature range 1,300 degrees -650 degrees C, the specific liquidus temperature depending mainly upon the amounts of volatile constituents held in solution at the time. This compositional factor also would be important in controlling the stage of crystallization\---|late, intermediate, or early\---|at which a separate aqueous fluid would make its appearance. Depending upon confining pressure as dictated by geologic conditions for a given system, the stage in crystallization represented by the presence of both silicate melt and aqueous fluid could begin within about the same temperature range of 1,300 degrees -650 degrees C. Exhaustion of the melt could occur within range extending downward to temperatures of 600 degrees C or even somewhat lower. Textural and structural features appear to be the most reliable indicators of the stages and fundamental processes involved in crystallization of both natural and synthetic pegmatites. The contrasting processes of crystallization from one fluid and from more than one fluid can operate over such broad P-T-X ranges that simple genetic pegmatite classifications based largely upon "key minerals," presumed temperature or pressure intervals, or the presence or absence of supercritical conditions appear to be somewhat unrealistic.

Immiscibility and Crystallization in A12O3‐SiO2 Glasses

Abstract: Metastable glass-in-glass separation was observed on rapid quenching of A12O3-SiO2 melts containing from 10 to 50 mol% A12O3. Nucleation and subsequent crystallization of mullite within the high-alumina-dispersed glass phase may occur either during cooling from the melt or on reheating. The metastable binary two-liquid region is compositionally defined, and a structural interpretation of the phase separation and the effects of small oxide additions on it is offered. Evidence for both classical and possible spinodal nucleation mechanisms during liquid segregation in this system is presented.

An electron microscope study of the formation of the nacreous layer in the shell of certain bivalve molluscs.

Abstract: An electron-microscopic study was made of nacreous shell growth in several species of marine molluscs. Studies of sections of mantle-shell preparations show that the first step in crystal formation is the “polymerization” of part of the pallial fluid to form lamellae parallel to the surface of the epithelium. These lamellae form compartments enclosing a modified apallial fluid. Initiation of crystals occurs in these compartments in contact with a crystal in an adjacent layer. During crystal growth the organic matrix present in the compartment is displaced by the growing surface of the crystal. When growth is complete the crystal is entirely enveloped by a delicate organic sheath. These studies show that the pallial fluid with its organic constituents is responsible for supplying a matrix or substrate for crystal initiation and growth. It serves as a regulatory device for guiding the orderly growth and arrangement of crystals and, further, it may participate in the induction of new crystals. The formation of compartments during shell growth accounts for the uniform thickness, preferred exhibited orientation and mineralogy of the crystals as well as other features exhibited by the mature nacre.

Einfluß der thermischen vorgeschichte auf die eigenschaften von polyvinylchlorid

Abstract: Der Einflus einer Temperung auf den Zustand von technischem Polyvinylchlorid wird zunachst durch kalorimetrische Messungen untersucht. Eine Temperung unterhalb der Glastemperatur fuhrt infolge von Enthalpie-Relaxation zu Zustandsanderungen, die nicht eine Erhohung der Ordnung der amorphen Bereiche im Sinne zunehmender Kristallinitat bedeuten, sondern durch eine Verringerung der Leerstellen-Konzentration bedingt sind. Die Enthalpie-Relaxation ausert sich bei kalorimetrischen Untersuchungen darin, das je nach dem Verhaltnis von Aufheiz- und Abkuhlgeschwindigkeit der cp-Stufe bei der Glastemperatur ein endothermes Maximum („Uberhitzungs-Maximum”) unmittelbar uberlagert ist oder das bei scharf abgeschreckten Proben am unteren Ende des Glasubergangs-Bereiches der Stufe ein Maximum oder Minimum vorgelagert ist. Die Uberhitzungsenthalpie, die ein quantitatives Mas fur die bei der Temperung abgelaufenen Zustandsanderungen ist, ist stark von der angewandten Aufheizgeschwindigkeit abhangig. Der Glas-Ubergang beginnt bei um so tieferen Temperaturen, je scharfer Polyvinylchlorid von oberhalb der Glastemperatur abgeschreckt wird. Den gleichen Befund erhalt man aus dynamisch-mechanischen Untersuchungen. Das untersuchte Polyvinylchlorid ist partiell kristallin. Die beim langsamen Abkuhlen aus der Schmelze gebildeten Kristalle schmelzen beim Aufheizen im Temperaturbereich zwischen 115 und 205°C. Die Schmelzwarme betragt ca. 2 cal/g. Durch scharfes Abschrecken last sich die Kristallinitat verringern. Beim nachfolgenden Aufheizen findet in diesem Fall eine Nachkristallisation statt. Eine nachtragliche Temperung oberhalb der Glastemperatur fuhrt auch bei langsam aus der Schmelze kristallisiertem Polyvinylchlorid zu einer Nachkristallisation. Die bei dieser Nachkristallisation gebildeten Kristalle ergeben beim Aufheizen ein endothermes Schmelzmaximum, dessen Flache (Schmelzwarme) von der Heizgeschwindigkeit unabhangig ist und das immer oberhalb der Glastemperatur-Stufe, und zwar ca. 25°C oberhalb der jeweiligen Tempertemperatur (Temperzeit 1–2 Stdn.), liegt. Es steht in keinem ursachlichen Zusammenhang mit dem durch Temperung unterhalb der Glastemperatur hervorgerufenen Uberhitzungs-Maximum. Die Diffusion von Flussigkeiten und die Permeation von Gasen hangen stark von der thermischen Vorgeschichte ab. Durch Temperung sowohl oberhalb als auch unterhalb der Glastemperatur steigt die im Zugversuch ermittelte Streck-Spannung an, und die Bruchdehnung fallt stark ab. Die oberhalb und unterhalb der Glastemperatur erzielten Veranderungen beruhen jedoch auf zwei verschiedenen Ursachen. The influence of annealing upon the physical state of polyvinylchloride is investigated by calorimetric measurements. As a consequence of enthalpy relaxation an annealing below the glass transition temperature results in changes of state, which can not be interpreted by an increase of crystallinity or order, but by a decrease of the hole concentration of the amorphous regions. Depending upon the relation between heating and cooling rate either an endothermal peak („super-heating peak”) is superimposed on the cp-step at the glass transition temperature, or a peak or minimum is found below Tg. The super-heating enthalpy, which quantitatively measures the changes of state during annealing strongly depends on the heating rate. With increasing quenching rate the beginning of the glass transition range is shifted to lower temperature. The same result is obtained by dynamic mechanical measurements. The polyvinylchloride samples are partially crystalline. The crystals, grown during cooling, melt within a broad temperature range between 115 and 205°C. The heat of fusion is about 2 cal/g. By sharp quenching the crystallinity is reduced. An exothermal crystallisation process is observed if the quenched samples are re-heated in the calorimeter. During annealing even the slowly cooled samples undergo a further crystallization, if the annealing is performed above the glass transition temperature. The melting of the crystals grown during annealing yields an endothermal melting peak above Tg and that about 25°C above the annealing temperature. The corresponding heat of fusion is independent of heating rate. There is no fundamental relation between the super-heating peaks and these melting peaks. The diffusion of liquids and the permeation of gases are found t o be strongly dependent on thermal history. The relations between thermal history and the ultimate properties are discussed. Annealing above and below the glass transition results in increasing tensile yield strength and strongly decreasing ultimate elongation. The changes above and below Tg are due to different mechanisms.

Crystal Growth and Properties of Mica‐Like Potassium Niobates

Abstract: Large single crystals of and can be grown by Czochralski pulling from the melt. In the case of a small amount of excess is used to avoid incongruent crystallization. Both materials show pronounced cleavage very similar to that of mica. The electrical properties of are also very similar to those of mica, making this material a possible mica substitute as well as being an interesting new dielectric material in its own right. is orthorhombic Pmmm, with , not the previously reported perovskite structure. Other characteristics determined include optical, mechanical, and chemical properties. When exposed to atmospheres between about 25 and 85% relative humidity, becomes a trihydrate, with higher hydration above about 85% relative humidity. Optical, electrical, and structural properties of all three forms of are described.

Effect of storage temperature on the ageing of concentrated wheat starch gels

Abstract: Flour Milling and Baking Research Association, Chorleywood, Rickmansworth, Herts. Using differential thermal analysis (d.t.a.) the progress of ageing of concentrated wheat starch gels stored at temperatures from−1° to 43° has been investigated. A very close relationship has been found between the ageing of starch gels as measured by d.t.a. and the staling of bread as measured by crumb firmness at storage temperatures of−1°, 10° and 21° but some differences have been found at 32° and 43°. The results at −1°, 10° and 21 ° provide very strong confirmatory evidence that starch crystallisation is the chief factor in the firming of bread. At elevated storage temperatures (32° and 43°) the role of starch crystallisation in the firming of bread apparently gradually diminishes. Analysis of the results indicates that the mechanism of crystallisation of the starch, instantaneous nucleation followed by rod-like growth of crystals, is the same over the whole range of storage temperatures −1° to 43°. Evidence is also presented to show that there is a possibility that at higher storage temperatures a more symmetrically perfect crystal structure is being formed.

Stability of Remanent Magnetization of Igneous Rocks

Abstract: Summary Most tests of the magnetic stability of a rock provide only a crude evaluation. A better estimation could be obtained by measuring the spectrum of coercivities possessed by a rock; a simple means of obtaining the coercivity spectrum is presented. It is also shown that a relation between the distribution of grain size and the spectrum of coercivities exists, not only for prepared samples, but for rocks as well. An evaluation of the sizes of the opaque grains effectively controlling the magnetic properties must not only take account of initial crystallization processes but must include consideration of post-consolidation, high- and low-temperature oxidation modifications. The development of effective grain size is discussed. Results of this study have provided additional insight into the relation of the shape of a Js-T curve and the opaque mineralogy.

Effects of shear stress on the crystallization of linear polyethylene and polybutene‐1

Abstract: The effects of shear stress on the crystallization kinetics and morphology of linear polyethylene and polybutene-1 were studied with the aid of a specially designed apparatus. With this equipment, it was possible to heat a thin polymer sample between glass slides to a melt temperature, quench the sample to a crystallization temperature, and then deform the sample in shear by applying a constant load to one of the glass slides. During the deformation, the crystallization process was observed and photographed under a polarizing microscope. Also, the displacement of the glass slide was simultaneously recorded which made possible a determination of the shear strain as a function of time. The results demonstrate that two phenomena may occur in the initially supercooled polymer samples in response to the applied shear stress. In one case, the sample deformed until it fractured, generally exhibiting no evidene of crystallization; in the other, the sample deformed until an inflection point was reached after which the sample became rigid. This latter phenomenon was attributed to crystallization. At low shear stresses, the inflection point was associated with the growth of spherulites which simply became large enough to bridge the glass slides and prevent further deformation of the sample. This generally occurred prior to the completion of the radial growth of the lamellae. At high shear stresses, however, no evidence of crystallization was seen in the microscope until the inflection point was reached. At this point, birefringence was observed in the sample. The resulting structure generally could not be resolved in the microscope, thereby indicating very profuse nucleation. The results obtained clearly demonstrate that the application of a sufficiently high shear stress to an initially supercooled melt has a substantial effect on the rates of crystallization of both polyethylene and polybutene-1. This was shown most dramatically at temperatures close to the melting point, e.g., both polyethylene at 130°C and polybutene-1 at 113°C, which require over 104 sec to crystallize under quiescent conditions, crystallized at approximately 0.05 seconds. The application of a shear stress to a polymer melt is envisaged as resulting in molecular orientation. In accord with the theories of Flory, and Krigbaum and Roe, the associated decrease in entropy of the melt may be considered to increase the supercooling. Under high stresses at which large increases in supercooling result, crystallization occurs more rapidy at the high temperatures and with polymers of lower molecular weight. At low shear stresses, the influence of temperature and molecular weight on the crystallization kinetics is essentially the same as that obseved under quiescent conditions. Observations through the microscope have shown that the application of a shear stress to a polymer melt leads to large increases in the number of crystalline structures formed and to the formation of oriented morphologies. This latter phenomenon arises due to nucleation lines formed by impurities and spherulites in the deforming melt. The impurities and spherulites apparently cause a disturbance which is thought to result in a local increase in stress of the melt and, hence, a local increase in supercooling. Lamellae then nucleate on these lines and grow out radially.

Direct evidence for distinctive, stress-induced nucleus crystals in the crystallization of oriented polymer melts

Abstract: A previous paper proposed a model for polymer crystallization under stress [1]. This paper suggested that there could be distinctive crystals which formed under stress at unusually high temperatures to act later as nuclei for the bulk of the crystals. The present paper details definitive evidence for these nucleating crystals. Electron microscope observations are fully consistent with the expected two-phase structure for crystallization under stress, although they are not conclusive. However, X-ray diffraction patterns recorded while the samples crystallized under stress, at the appropriate elevated temperature, conclusively demonstrated the two-stage nature of the crystallization. First, a small number of crystals (type I) form which are highly c-axis oriented, followed by a second group of crystals (type II) having a more complex stress dependence of texture patterns. This is in complete agreement with the model [1] that c-axis-oriented fibers induce transversely growing lamellar crystals. Type...

Specific Heat and Heat of Crystallization of Amorphous Germanium

Abstract: : The specific heat and heat of crystallization of vapor- and electro-deposited amorphous germanium were measured with a differential scanning calorimeter. The excess specific heat of the amorphous germanium over that of crystalline phase increased linearly with temperature over the temperature range from 215 to 460K. The heat of crystallization of the amorphous films was found to be about 2.75 k cal/gm-atom. The interpretation of the results in terms of the structure is discussed. The specific heat of crystalline germanium was also measured for the temperature range from 215 to 700K. (Author)

Crystallization and Melting Kinetics of Cristobalite

Abstract: By measuring internal crystallization in vitreous silica, the kinetics of crystallization and melting of cristobalite were determined near the melting point. Both processes followed linear growth kinetics and could be completely represented by the equation μ=AΔT/η. Analyses of the crystallization data and observations of the crystal morphology indicate that crystallization occurs by a continuous growth mechanism. Melting was heterogeneous, occurring only at the glass-crystal interface and along grain boundaries.

EDTA and urea peroxide for root canal preparation

Abstract: Early combinations of EDTA and urea peroxide led to discoloration and crystallization. A new formula, using a stable carbowax base can have clinical value in permitting medication to permeate the root structure more completely and destroy remaining microorganisms.

Polymorphism in isotactic polypropylene

Abstract: The conditions under which a given form of any of the three distinct polymorphic crystalline phases of isotactic polypropylene can be obtained have been investigated. The relationship between crystallization conditions and the formation of a given crystalline phase is discussed. Evidence is cited in support of the hypothesis that the β-form of isotactic polypropylene can be produced only when appropriate nucleating materials are present. It is shown that under particular high-pressure crystallization conditions a triclinic γ-phase can be produced that does, not convert to the monoclinic α-phase either by action of heat or mechanical stress. Based on considerations of stability, density, and structure, it is suggested that the equilibrium crystal form of the polymer is the γ-form rather than the α-form which is ordinarily observed.

Graphite Formation from Low Temperature Pyrolysis of Methane over some Transition Metal Surfaces

Abstract: THE transition between the “random layer order”1 of amorphous carbons and the three-dimensional layer structure of graphite, first defined by the classical work of Franklin2, has stimulated interest in the preparation of perfectly crystalline graphites, especially by the thermal decomposition of gases. These carbonaceous products, however, begin to show a significant degree of three-dimensional layer ordering only at deposition temperatures of 2,000° C to 2,500° C (refs. 3–5). For pyrolytic carbons to possess structural properties comparable with those of natural graphite, it is necessary to carry out “stress recrystallization” at about 3,000° C under 400 atmospheres pressure, followed by annealing at 3,400° C (see ref. 6).

The Metastable Liquidus and Its Effect on the Crystallization of Glass

Abstract: There is a range of compositions and temperatures below the monotectic where the equilibrium solid cannot form directly from an undercooled single-phase liquid. Thermodynamics requires either a prior liquid-liquid Phase separation or a simultaneous formation of the second liquid if the solid is to form. The implications for controlled crystallization of glass are discussed.

Extended‐chain crystals. I. General crystallization conditions and review of pressure crystallization of polyethylene

Abstract: This is the first paper of a series of reports concerning extended-chain crystals of flexible, linear high polymers. The general conditions for crystal growth are discussed. Polymer crystallization is described as a two-step process: nucleation of each crystallizing molecule to a folded-chain conformation, followed by an increase in fold length in a solid-state reorganization step. This reorganization step is enhanced in the case of polyethylene by crystallization at high temperature under elevated pressure. Mechanical deformation during crystallization is also able to produce extended-chain crystals. The most promising method, however, is crystallization during polymerization. Previous work on crystallization of polyethylene under elevated pressure is critically reviewed.

Phosphide from metorites: barringerite, a new iron-nickel mineral.

Abstract: Schreibersite, (Fe,Ni)3P, and a higher phosphide, barringerite, occur in the Ollague pallasite. The composition of the higher phosphide, determined by electron probe microanalysis, is (Fe0.58Ni0.42Co0.003)1.95P. It is hexagonal, with space group P62m and a = 5.87 ± 0.07 angstroms and c = 3.44 ± 0.04 angstroms. If we assume a primary origin, the new mineral indicates that troilite and schreibersite crystallized at high temperatures. The occurrence of a higher phosphide in a pallasite indicates local nonequilibria within a group of meteorites that are remarkable for their overall degree of equilibrium crystallization.

Dielectric relaxation of high polymers in the solid state

Abstract: Dielectric relaxation processes observed in solid polar polymers seem to be generally classified into three kinds. Amorphous polymers with flexible polar side groups usually show two kinds of relaxation process. The high-temperature process is attributed to the large scale conformational rearrangements of the main chains, while the low-temperature one results from the motion of side groups. We shall call the former αa relaxation and the latter β relaxation, where the subscript refers to the amorphous phase. Even in amorphous polymers without flexible side groups, two processes are observed. The molecular mechanism of high temperature one is the same as the αa relaxation. The low-temperature one is due to the “local relaxation mode” of the main chains. We shall call it also β relaxation because of the similarity of the observed characteristics. Semicrystalline polymers show two relaxation processes classified as αa and β, although their behavior is modified by crystallization. When the crystallinity is greatly increased, a third relaxation process with associated with crystalline phase appears. We shall call it αc relaxation, where the subscript means the crystalline phase. The effects of chemical structure, stereoregularity, pressure, and crystallization on the dielectric behavior can be explained consistently based on the above molecular mechanisms.

Segmented Polyurethanes: Properties as a Function of Segment Size and Distribution

Abstract: Segmented polyurethanes represented by formula I are block copolymers composed of alternating soft and hard segments. They exhibit properties characteristic of crosslinked elastomers over a wide temperature range but, at higher temperatures, melt and can be processed by techniques used for plastics and fibers(1). This behavior is explained by the fact that the polymers possess a three-dimensional network built up by intermolecular association (crystallization) of the hard segments. It was therefore of interest to study the effect upon polymer properties of changes in hard segment size, distribution, and spacing along the polymer chain.

The Effect of Environment Temperature of Plants on the Physicochemical Properties of Their Starches

Abstract: Two aspects of experiments, in vivo and in vitro, are described in this review . In in vitro experiments, the crystallization of starch was studied on some model systems using amylodextrin. The results suggested that the crystalline structure of a starch granule was dependent on external temperature and microenvironment of cellular location developing a starch granule and it was in keeping with results of the follwing in vivo experiments. In in vivo experiments, the temperature of soybean seedlings, potato tubers, sweet potato roots and rice ears was controlled at constant during starch accumulation period using some devices. The following changes were noted in respective starches by elevation of environment temperature. (1) Pasting temperature decreased considerably in all the test series. (2) Maximum viscosity (amylogram) decreased in potato and sweet potato, increased in rice and was approximately constant in soybean seedlings (cotyledon) . (3) a-Amylase susceptibility of raw granules and (4) content of bound phosphate were decreased. (5) Amylose content was decreased appreciably in rice and slightly in soybean seedling hypocotyls and was constant in potato and soybean seedling cotyledons. (6) Crystalline type shifted from B or C type toward A type in soybean seedlings and sweet potato and was constant in potato (B type) and rice (A type). From these results, a new day-night rhythm structure in a native starch granule was suggested and relations of the structure to the property were discussed .