Showing papers on "Crystallization published in 1982"
TL;DR: In this article, the authors used the Avrami exponent of 3 to calculate the number of nuclei per unit volume, N, in annealing Fe80B20 glass, assuming an isokinetic reaction with a constant number of quenched-in spherulites.
330 citations
TL;DR: In this article, Molten spheroids of up to 0.53 cm minor diameter were slowly cooled (1.4 K/s) on a fused silica surface under 10 to the -6 torr vacuum to a form which was entirely glassy except for some superficial crystallinity comprising less than 0.5% of the volume.
Abstract: Molten spheroids of Pd40Ni40P20, of up to 0.53-cm minor diameter, were slowly cooled (1.4 K/s) on a fused silica surface under 10 to the -6 torr vacuum to a form which was entirely glassy except for some superficial crystallinity comprising less than 0.5% of the volume. The occurrence of crystallization was eliminated by subjecting the specimens to surface etching followed by a succession of heating and cooling cycles. The absence of crystallization in bulk was confirmed by X-ray diffraction, transmission electron microscopy, and calorimetry. Using the last technique, the heat of crystallization of the glass was measured to be 5.3 + or - 0.3 kJ/g atom.
304 citations
TL;DR: In this paper, it was shown that the crystallization rates of a Lennard-Jones liquid are not limited by the mobility of atoms in the bulk liquid; measurable rates are even observed below the glass transition temperature.
Abstract: The crystallization of a Lennard-Jones liquid has been studied with molecular-dynamics techniques. In particular, the motion of the fcc (100) interface has been measured for a range of temperatures below the melting point. The crystallization rates are not limited by the mobility of atoms in the bulk liquid; measurable rates are even observed below the glass transition temperature. This result suggests the existence of a class of materials which is qualitatively different from conventional glass-forming materials.
296 citations
TL;DR: In this paper, the concentration of the major electron trap (0.82 eV below the conduction band) in GaAs (Bridgman grown) was found to increase with increasing As pressure during growth.
Abstract: The concentration of the major electron trap (0.82 eV below the conduction band) in GaAs (Bridgman grown) was found to increase with increasing As pressure during growth. It was further found that (for a given As pressure) the concentration of this trap decreased with increasing concentration of shallow donor dopants (Si, Se, and Te). Donor concentrations above a threshold of about 1017 cm−3 led to the rapid elimination of the trap. On the basis of these findings, the 0.82‐eV trap was attributed to the antisite defect AsGa formed during the postgrowth cooling of the crystals.
243 citations
TL;DR: Spherical crystallization could eliminate the usual separate agglomeration step after crystallization and may be adaptable to other pharmaceutical and chemical systems.
Abstract: Direct spherical agglomeration of salicylic acid crystals during crystallization is described. The needle-like salicylic acid crystals simultaneously form and agglomerate in a mixture of three partially miscible liquids, such as water, ethanol, and chloroform, with agitation. The agglomerates can be made directly into tablets because of their excellent flowability. Spherical crystallization could eliminate the usual separate agglomeration step after crystallization and may be adaptable to other pharmaceutical and chemical systems.
242 citations
TL;DR: An atomistic model for the transformation of amorphous (α) to crystalline silicon films while in contact with a crystalline substrate is presented in this paper, where the atomic structure of the {100, {110, and {111} surfaces is examined and related to the observed interface migration rates.
Abstract: An atomistic model for the transformation of amorphous (α) to crystalline silicon films while in contact with a crystalline substrate is presented. The atomic structure of the {100}, {110}, and {111} surfaces is examined and related to the observed interface migration rates. The assumption that for an atom to attach successfully to the crystal it must complete at least two undistorted bonds, leads to the prediction that the {100} amorphous/crystalline interface should advance fastest and the {111} slowest. The origin of crystal defects is discussed in terms of the atomistic recrystallization mechanism. Microtwins are found to be a logical consequence of crystallization on the {111} surfaces but are not expected to form on any other interface. Once microtwins are formed they can increase the recrystallization rate of a {111} surface. This phenomenon is both described in the model and experimentally observed.
231 citations
TL;DR: The results of these experiments have been integrated with available phase equilibrium data in the system CaO-MgO-Al-2O_3-SiO-2TiO_2 and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse-grained Ca-Alrich inclusions has been developed as discussed by the authors.
225 citations
TL;DR: The structure of 52/48 mol % copolymer of vinylidene fluoride and trifluoroethylene has been investigated at various temperatures by X-ray diffraction.
Abstract: : The structure of 52/48 mol % copolymer of vinylidene fluoride and trifluoroethylene has been investigated at various temperatures by X-ray diffraction. Melt-solidified samples consist of a mixture of two disordered crystalline phases, one trans-planar, the other 3/1-helical. Samples may be transformed to either phase by appropriate means to reveal a hexagonal (or pseudo-hexagonal) molecular packing. The all-trans phase may be obtained by drawing or poling at low temperatures; both treatments cause a transformation of the disordered mixture of phases into a well-ordered planar zig-zag phase. Isolation of the disordered 3/1-helical phase is achieved by heating to high temperatures, where-upon all samples, irrespective of orientation or polarization, undergo transformation to a poorly ordered helical structure analogous to that of trifluoroethylene homopolymer; upon cooling, the original, disordered mixture of phases is recovered. (Author)
192 citations
ARCO1
TL;DR: In this article, the crystallization, the morphology and the thermal behaviour of thin films of isotactic polypropylene (iPP) blended with elastomers such as random ethylene-propylene copolymers (EPM) with different ethylene content and polyisobutylene (PiB) were investigated by means of optical microscopy, differential scanning calorimetry and wide angle X-ray diffractometry.
190 citations
TL;DR: In this article, it was shown that the equiatomic CuZr does not exist as a single phase in the crystalline state and observed parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which they interpret through changes in the d density of states.
Abstract: The crystallization characteristics of glassy Cu‐Zr has been studied by differential scanning calorimetry, x‐ray diffraction, and measurement of electrical resistance and magnetic susceptibility. Our results allow us to remove many discrepancies between previously published data; they also suggest that the equiatomic Cu‐Zr does not exist as a single phase in the crystalline state. We observe parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which we interpret through changes in the d density of states.
187 citations
TL;DR: In this article, a number of electrolyte crystals in aqueous solutions, α, determined from crystallization data are collected form literature sources, and correlations of α with solubility and/or heat of crystallization are verified; the former being reliable for sparingly soluble substances and the latter for soluble salts.
TL;DR: In this article, it was shown that a film with low resistivity (5×10−4 Ω cm) and high optical transmission (>85% between 400 and 800 nm) can be produced on low-temperature substrates with a relatively high deposition rate.
Abstract: Highly conductive films of zinc oxide have been prepared by rf magnetron sputtering deposition on a substrate suspended perpendicular to the target under an applied external magnetic field in pure argon gas. It is shown that a film with low resistivity (5×10−4 Ω cm) and high optical transmission (>85% between 400 and 800 nm) can be produced on low‐temperature substrates with a relatively high deposition rate. The sheet resistance and Hall mobility of the film are 10 Ω/⧠ and 120 cm2/Vs respectively. The high conductivity is achieved by an increase in Hall mobility due to improved crystallization.
TL;DR: Schwertmann et al. as mentioned in this paper determined the point of zero charge (pzc) of Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe2O3.9H2O.
Abstract: The point of zero charge (pzc) of synthetic Fe-oxides is well documented and usually ranges between pH 7 and 9 (Parks, 1965; Schwertmann & Taylor, 1977). In contrast, the pzc of natural Fe-oxides has only rarely been determined. Using electrophoretic mobility, Van Schuylenborgh & Arens (1950) found that a natural goethite had a much lower pzc (∼3) than synthetic goethites. They attributed this to better crystallinity of the natural goethite caused by slower crystallization. Soils dominated by Fe- (or Al-) oxides rarely have pzc values as high as those of pure oxides. This is usually attributed to the presence of negatively charged impurities such as clay silicates and organic matter (Parfitt, 1981). Ferrihydrite, a natural, poorly-crystalline Fe-oxide mineral of bulk composition 5Fe2O3.9H2O, occurs in hydromorphic soils (Schwertmann et al., 1982) and is the main component in ochrous precipitates formed when Fe-bearing fresh waters come in contact with air (Schwertmann & Fischer, 1973; Carlson & Schwertmann, 1981). Under these conditions the ferrihydrite is reasonably free of other charge-active minerals. The aim of this study was to find out if the pzc of these natural ferrihydrites differed from those of synthetic samples.
TL;DR: In this paper, the authors have studied the kinetics of Fe•B•Si metallic glasses, as characterized by the effective activation energy Ec and the Avrami exponent n have been studied using Differential Scanning Calorimetry.
Abstract: Crystallization kinetics of Fe‐B‐Si metallic glasses, as characterized by the effective activation energy Ec and the Avrami exponent n have been studied using Differential Scanning Calorimetry. The data show that in the hypoeutectic (Fe‐rich) region, the glasses crystallize into α‐(Fe,Si) and Fe3B in that order. The growth of crystallites is diffusion controlled, and Ec falls off rapidly away from the eutectic trough. In the hypereutectic region, both crystalline phases are obtained simultaneously. Thermal stability is greatly enhanced by addition of Si(∼3 at. %) and the growth process is no longer purely diffusion controlled. The behavior of Ec and n versus composition is discussed. Effects on Ec and n of additions of small amounts of Ni and Mo are also reported. Such additions have direct relevance to the design of new glasses for applications at high frequencies.
TL;DR: The method described by Strobl and Hagedorn to analyze the Raman spectrum internal modes of semi-crystalline polyethylene has been applied to a set of selected polymethylene samples crystallized under controlled conditions as discussed by the authors.
Abstract: The method described by Strobl and Hagedorn to analyze the Raman spectrum internal modes of semi-crystalline polyethylene has been applied to a set of selected polyethylene samples crystallized under controlled conditions The crystallite structure can be described in terms of the relative amounts of the crystalline orthorhombic phase, the liquid-like amorphous phase and the interfacial region The dependence of the level of crystallinity on molecular weight and crystallization conditions is very similar to that found by other methods However, this method allows for the quantitative determination of the interfacial content which becomes significant for molecular weights greater than about 1×105 for linear polyethylene fractions, and for all the branched samples and copolymers The degree of crystallinity determined from density measurements is equal to the sum of the crystallinity and interfacial content obtained from the Raman analysis while enthalpy of fusion measurements yield values which are equal to just the crystallinity content The difference between the level of crystallinity obtained from density and enthalpy of fusion is thus found to be primarily due to interfacial contributions
TL;DR: In this paper, a crystalline to amorphous transition was observed in shape memory NiTi alloos by in-situ electron irradiation in a high voltage electron microscope (1-2MeV).
TL;DR: In this paper, cubic ZrO2 particles coagulated in a highly oriented fashion and appeared to crystallize via collapse of the ZRO2-nH2O structure and subsequent slight rearrangement of the lattice.
Abstract: During constant-rate heating to 350°C in concentrated NaOH solutions, cubic ZrO2 crystallized at ≈120°C from hydrated amorphous ZrO2; these fine cubic ZrO2 particles abruptly changed into needlelike monoclinic ZrO2 single crystals at 300°C. Crystallization and phase transformation were studied by XRD, TEM, and EPMA. Cubic ZrO2 appears to crystallize via collapse of the ZrO2-nH2O structure and subsequent slight rearrangement of the lattice. The abrupt formation of monoclinic ZrO2 was considered to result when the very fine cubic ZrO2 particles coagulated in a highly oriented fashion.
TL;DR: A Tektronix-31 programmable calculator interfaced to a Perkin Elmer differential scanning calorimeter, model 2, substantially improves the accuracy of measuring the time-dependent development of the degree of crystallinity (× 10) and hence improves the quality of the rate data as mentioned in this paper.
TL;DR: In this paper, the structure-property relationship of a series of segmented polyurethanes was investigated segment was 4,4′-diphenylmethane diisocyanate (MDI) extended with 1,4-butanediol, and the soft segment was 2000 mw poly (tetramethylene oxide) ether.
TL;DR: In this article, a branched poly(3-hydroxybenzoate) was obtained by condensation of 3-(trimethylsiloxy) benzoyl chloride and 3,5-(bistrimethylsiloxide) benzoylel chloride.
15 Apr 1982
TL;DR: In this paper, the crystal structures and phase transitions in fluoranthenyl radical cation salts are reported. Butler et al. used cyclic voltammetry and chronoamperometry to study the electrochemical production of these cation radical crystals of the type (A2), where A was either fluoranthene or perylene and X is PF6.
Abstract: Crystal structures and phase transitions in fluoranthenyl radical cation salts, a new family of organic conductors are reported. The metal to semiconductor transition which is observed seems to be related to a distortion of the stacks of aromatic molecules perpendicular to the stacking direction. Cyclic voltammetry and chronoamperometry were employed to study the electrochemical production of these cation radical crystals of the type (A2)⊚⊕X⊖, where A was either fluoranthene or perylene and X is PF6. It appears that following the initial oxidation of A at the electrode a dimer radical cation is formed. This species then complexes with the anion to give the species of the proper stoichiometry from which the crystals grow. The crystals growing on the anode function directly as electrodes.
TL;DR: Mossbauer spectroscopy has been used to investigate the precipitates formed by hydrolysis of 0.1 M solutions of Fe(NO 3 ) 3, FeCl 3, Fe 2 SO 4 ) 3, and NH 4 Fe(SO) 2 at 90°C as discussed by the authors.
TL;DR: In this paper, the authors investigated the kinetics of crystallization of solvent-free poly(vinyl alcohol) by isothermal crystallization at temperatures of 142-192°C, for up to 18 min.
Abstract: The kinetics of crystallization of solvent-free poly(vinyl alcohol) were investigated by isothermal crystallization at temperatures of 142–192°C, for up to 18 min. The crystallization isotherms were analyzed by Avrami's theory. The magnitude of the exponent n was almost constant (0.67–0.71) in the range of 142–182°C, increasing to 1.53 for crystallization at 192°C. Based on thermodynamic analysis of the isotherms and the crystallite growth rate, there are strong indications that crystallization of PVA is one-dimensional. In the absence of water or other swelling agents, kinetic hindrances predominate due to the interactions of the hydroxyl groups. Therefore, the maximum attainable weight fraction of crystallized PVA is considerably lower than that of hydrated PVA samples. Additional parameters affecting the growth rate are discussed, including the degree of undercooling and the average chain length, as controlled by crosslinking.
TL;DR: In this paper, the authors used enhanced self-nucleation to increase the rate of polyethylene crystallization by an order of magnitude using a technique which they have called enhanced self nucleation, and they found that spherulites crystallize uniformly over the field of view, and not, as is usual, by a radial growth process.
Abstract: This article is part of the general project laid out in Part I (ref. 9) and is concerned with obtaining information on primary (unthickened) crystals of polyethylene formed at low supercoolings. For this, a technique had to be devised by which crystallization could be speeded up so as to eliminate or at least reduce lamellar thickening. Indeed we were able to increase the rate of crystallization by an order of magnitude using a technique which we have called enhanced self-nucleation. Using this technique we find that when viewed under an optical microscope, spherulites crystallize uniformly over the field of view, and not, as is usual, by a radial growth process. Isothermal crystallization in bulk linear polyethylene has been studied by means of the enhanced self-nucleation technique as a function of crystallization time by using Raman LAM and melting points to assess variations of fold length Data have been obtained at very much shorter times than before. At short times, we find a constant fold length; at longer times the crystals thicken linearly with the logarithm of time. Values of the initial fold length for crystallization temperatures between 118 and 130°C are presented. Associated with the thickening at short times we find an induction time which increases with temperature.
TL;DR: In this paper, a simple method based on the measurements of two peak temperatures on a single derivative differential thermal analysis (DDTA) curve for determining the activation energy for bulk or surface crystallization in glasses is described.
Abstract: A simple method, based on the measurements of two peak temperatures on a single derivative differential thermal analysis (DDTA) curve for determining the activation energy for bulk or surface crystallization in glasses is described. The devitrification of Li2O-2SiO2 glass has been investigated; the experimental results agree well with those obtained by alternative techniques and with the activation energy of viscous flow in the crystallization-range temperatures.
TL;DR: In this article, it was shown that the crystallization was strongly influenced by the amount of alkali oxides present in the gel and the effect of additives such as boric anhydride.
Abstract: Silica gels were prepared by two different methods: (1) destabilization of a silica hydrosol (gel 1); (2) hydrolysis and polycondensation of a tetra-methoxysilane (gel 2). The crystallization of the gels was then studied as a function of the temperature by means of X-ray diffraction. It was evident that the crystallization was strongly influenced by the amount of alkali oxides present in the gel. It is effectively the concentration of akali impurities which could explain the lower temperature of crystallization necessary for gel 1 compared with that of gel 2. During the crystallization of the gel containing Na2O the crystalline phase of silica which appears first is the cristobalite; with Li2O it is quartz. The effect of additives such as boric anhydride was studied. This oxide was found to reduce the tendency of the gels to crystallize. The glasses of the system SiO2B2O3 obtained by the hot-pressing of the gels confirmed this phenomenon. Above 10 mol% B2O3 it was impossible to crystallize the gels and the glasses of this system under two hours.
TL;DR: In this article, the effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effect of using different titanium compounds on the formation of silica-titania gels and glasses were investigated.
Abstract: Clear monolithic samples of silica and silica-titania glasses were prepared by the sol-gel process from alkoxides as starting materials. The effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effects of using different titanium compounds on the formation of silica-titania gels and glasses were investigated. DTA and TGA revealed losses of water and organic volatiles during heat treatment of the gels at lower temperatures (up to 400°C) and the glass transformation and crystallization behaviour at higher temperatures (up to 1500°C). The effects of using atmospheres with varying oxygen contents on the DTA peaks caused by oxidation reactions were also studied.
Structural changes occurring during heat treatment were monitored by infra-red spectroscopy which indicated that the water contents of the glasses after heat treatment to 900°C were about 1000 ppm. Transmission electron microscopy of ion beam thinned foils of a 80 SiO220TiO2 composition showed a microstructure of extremely fine pores for heat treatments up to 1000°C. However, after extended heat treatments above 950–1000°C, the porosity appeared to decrease and a high concentration of fine crystallites of anatase (approximately 100 A in diameter) embedded in a silica-rich glass matrix were obtained.
TL;DR: In this article, a differential scanning calorimetry (d.s.c) approach was used to assess the morphology of polypropylene (PP) and thermoplastic rubber (TR) blends, such as the type of the dispersion (phase continuity) and the degree of the particle size.