Showing papers on "Crystallization published in 1974"

The Crystallization of Spinel from Basaltic Liquid as a Function of Oxygen Fugacity

Abstract: The effect of changing oxygen fugacity on the equilibrium crystallization of two basaltic melts has been determined at 1 atm total pressure. Liquidus curves for spinel (chromite-titaniferous magnetite), hexagonal oxide, olivine, pyroxene, and plagioclase are represented on temperature versus $$log f_{O_{2}}$$ diagrams covering the $$log f_{O_{2}}$$ range -0.68 (air) to -14.0 atm and a temperature range of l,100°C-1,325°C. Particular emphasis has been placed on the role of oxygen fugacity as applied to the relationship between chromite and titaniferous magnetite and the ferrous-ferric ratio. Complete solid solution between chromite and titaniferous magnetite is demonstrated at basaltic liquidus temperatures, however at oxygen fugacities below about $$10^{-8} atm$$ the crystallization of early chromite is interrupted by the crystallization of clinopyroxene. At lower temperatures, a spinel again crystallizes as titaniferous magnetite together with pyroxene and other silicates. This supports the suggestion of...

A high‐pressure phase of polyethylene and chain‐extended growth

Abstract: Optical and x‐ray observations of polyethylene have been made at high pressures and temperatures using a gasketed diamond‐anvil cell. The experiments confirm the existence of the high‐pressure phase previously postulated by Bassett and Turner. The new phase is hexagonal, with orthohexagonal lattice parameters of a = 8.46 A and b = 4.88 A. Comparison with the previously measured volume change indicates that there is a decrease in the c dimension to 2.45 A per ethylene unit in transforming from orthorhombic to hexagonal structures. The likely implication is that the molecules in the hexagonal phase do not have an all‐trans conformation. Chain‐extended growth is the result of crystallization from the melt into the hexagonal phase, whereas chain‐folded growth is the familiar process of melt crystallization.into the orthorhombic phase. Chain‐extended lamellae are observed to grow outwards behind a growing edge with a permanent narrowed profile, showing that the lamellar thickness is determined in a region extending several microns behind the growth front.

Pressure-induced semiconductor-metal transitions in amorphous Si and Ge

Abstract: A pressure-induced semiconductor-metal transition accompanied by a sharp resistance drop with a factor of approximately 10–6 has been found for amorphous Si at 100 kbar and amorphous Ge at 60 kbar. The transition in Si is a reversible process between amorphous semiconductor and amorphous metal, while that in Ge is an irreversible process with crystallization into a diamond-type structure. The pressure-induced metallic character in amorphous Si and Ge has been demonstrated by superconductivity at low temperatures. The metallic conduction is believed to be due to structural modifications which remain amorphous.

Crystallization of Ge and Si in metal films. I

Abstract: The behavior of amorphous Si in contact with Ag films and Ge in contact with Al films has been studied at temperatures well below those at which any liquid phase is present. MeV 4He+ ion backscattering techniques, transmission electron diffraction, scanning electron microscopy, and electron microprobe analysis have been used. We find that of the many possible reactions which carry the amorphous Si or Ge into their crystalline forms the reaction predominating under our experimental conditions consists of dissolution, diffusion, and crystal growth. During isothermal heat treatment, the semiconductor film is dissolved into the metal film where it diffuses and precipitates as crystalline Si or Ge. These processes are solid-solid reactions, since this behavior is observed over temperatures of 300°C to as low as 100°C for Ge/Al, compared to the 424°C eutectic in this system. In Si/Ag, this behavior was observed from 700 to 400°C, compared with the 840°C eutectic.

The Growth of Hydrous Oxide Films on Aluminum

Abstract: The weight gains of Al specimens immersed for short times in water at 50°–70°C were measured. An analysis of these data and published results at other temperatures indicated that the kinetics of pseudoboehmite film growth in the temperature range 50°–100°C is determined initially by the nucleation and growth of hydrolysis sites on the amorphous oxide surface, and subsequently by solid‐state diffusion through the pseudoboehmite layer. At 40° C, pseudoboehmite and bayerite were observed to grow simultaneously at first, with bayerite crystallization eventually becoming the dominant process. The two oxide phases occupied completely separate layers.

Theoretical analysis of oriented and non-isothermal crystallization

Abstract: Phenomenological analysis of the effects of molecular orientation and deformation on crystallization kinetics is given. It was shown that nucleation- and growth rates can be represented by functions of six invariants of the molecular displacement gradient tensor and related tensors. For some simple geometries of deformation (including simple shear and uniaxial extension), crystallization rate functions can be also expressed as functions of orientation factorsf (for simple shear) orf* (for uniaxial extension).

On the phenomenology of chain-extended crystallization in polyethylene

Abstract: The melting and crystallization of polyethylene have been examined in the pressure range 0 to 6 kbar. Volumetric and thermal measurements show that fractionated polymer melts and crystallizes in two clearly resolved stages above 4 kbar. It is suggested that these are due to the formation of a new, high pressure phase of polyethylene with a triple point at ∼ 3 kbar. This phase is intermediate in specific volume and entropy between orthorhombic solid and melt, but closer to the latter, and is either liquid-crystalline or solid. It is shown further that the formation of chain-extended polyethylene correlates with the occurrence of two exothermic processes (unresolved below 4 kbar) during crystallization. This leads to the hypothesis that whereas chain folded crystallization occurs by direct transformation from melt to orthorhombic solid, chain-extended polyethylene results when the intermediate phase intervenes in crystallization. This hypothesis appears capable of accounting for most of the known f...

Kinetics of polymer crystallization from the melt (calorimetric approach)

Abstract: Crystallization kinetics for 12 polymers including polyolefins, polyesters, polyurethanes, polysiloxanes was measured by the evolution of heat in a modified Calvet-type calorimeter over wide temperature ranges. The results are analyzed in terms of the Avrami equation and a comparison between calorimetric and dilatometric results is carried out. It is concluded that, although in the majority of cases experimental results do not obey the Avrami equation, for some polymers the agreement is rather good. The Avrami parameter obtained, however, depends on the experimental technique. Possible reasons for this disagreement are discussed. Analysis of the calorimetric crystallization rate in the vicinity of the melting point by using the kinetic theory of crystallization shows that the growth is controlled by surface (two-dimentional) nucleation. Energy parameters for the crystallites were determined and it is shown that the surface energy of the crystallites depends on the molecular structure of the polymer. Temperature dependence of the calorimetric crystallization rate of the polymers for which crystallization rates could be determined above and below the maximum rate are analyzed using a kinetic equation with common approximations for the transport term. The influence of melting conditions on the crystallization rate was studied. The results indicate heterogeneous nucleation in the polymer melt. It is concluded that this may be due both to impurities and to high regularity of macromolecules in the polymer melt.

Constant temperature device

Abstract: A flexible container having a substantially planar configuration encloses a first chemical having characteristics for being supercooled to maintain a liquid state even at temperatures below its melting point. A second chemical can be mixed with the first chemical in the container to initiate the crystallization of the first chemical at the substantially constant temperature of the melting point of the first chemical. The container can be enclosed in an insulation envelope to moderate the temperature of the device. The first chemical may include sodium thiosulfate pentahydrate which provides a desirable constant temperature of 48° C. The second chemical may include sodium borate pentahydrate or sodium sulfite which provides the first chemical with a desirable sandy configuration during crystallization in the α pentahydrate form. The sodium borate pentahydrate or sodium sulfite goes into solution or chemically complexes with the sodium thiosulfate in a manner such that subsequent supercooling can be achieved. Recycling of the device can then be accomplished by releasing another portion of the second chemical into the container. The resulting constant temperature device is particularly effective for use as a warm baby mattress, a warm blanket, or a hot water bottle substitute.

Injection laser writing on chalcogenide films.

Abstract: The crystallization and amorphization of Te-As-Ge films with GaAs injection lasers has been investigated. Power density measurements for crystallization and amorphization are presented, and a reverse-mode write-read-erase cycle is demonstrated. The results show that direct bit storage is feasible with an injection laser-chalcogenide film system.

Crystallization of poly(ethylene terephthalate) induced by inorganic compounds. I. Crystallization behavior from the glassy state in a low‐temperature region

Abstract: The kinetics of isothermal crystallization from the glassy state at low temperatures and the morphology of poly(ethylene terephthalate) (PET) filled with additives are reported. Talc, kaolin, silicon oxide, and titanium oxide have been used as fillers; they act as effective nucleating agents for PET. The overall rate of crystallization depends on the volume concentration, the size distribution, and the nucleating ability of the additives. An electron microscopic study reveals a transcrystalline morphology at the surface of the filler particles. The occurrence of transcrystallinity is attributed to extensive heterogeneous nucleation induced at the filler surface. From the shape of the crystallization isotherms, it can be concluded that the crystallization mechanism depends on the type of filler.

Crystallization of amorphous selenium films. II. Photo and impurity effects

Abstract: Superposition of illumination with white light on thermal treatments of amorphous films, formed by deposition of high‐purity Se on mica, had the following effects on crystallization: (i) interface cylindrites thickened much more rapidly but their radial growth rates were not detectably changed; (ii) the number density of interface cylindrites was sharply increased—this result may signify that the transition of single to cylindritic crystals, and not necessarily primary nucleation, is photoenhanced; (iii) surface cylindrites grew at much higher radial rates and developed a concentric ``hill‐and‐valley'' surface topology. The radial growth rates of interface cylindrites were reduced markedly by small amounts of certain impurities (e.g., Te or As) in the films and they are photoenchaned but not to the rates in the highest‐purity films without illumination. In general, our results suggest that the primary effect of photoillumination on the crystal growth rates was to reduce the restraining action of impuritie...

On chain-extended and chainfolded crystallization of polyethylene

Abstract: Various polyethylenes have been crystallized from the melt at pressures between 0 and 6 kbar. Their atmospheric melting points fall, in each case, into two non-overlapping regions, with a sharp changeover at a pressure in the region of 3 kbar which is a decreasing function of molecular weight. This separation is used in conjunction with differences in spherulitic texture to distinguish chainfolded polymer (which predominates at lower pressures) from chain-extended polyethylene produced at higher pressures. The thickness of chain-extended lamellae increases with crystallization temperature at a given pressure, with crystallization pressure at a constant super-cooling and with molecular weight (between 2·104 and 5·104) for crystallization at the same temperature and pressure. It is then shown from differential thermal analysis of crystallization as it occurs that chain-folded and chain-extended polyethylene result from two distinct processes and that when both occur at the same pressure and tempera...

Epitaxial crystallization of polyethylene

Abstract: Thin epitaxial layers of polyethylene, formed on the surface of alkali halide crystals by isothermal solution crystallization, were studied via electron microscopy and electron diffraction as a function of molecular weight, solution concentration, and degree of supercooling. Growth of the monoclinic form of polyethylene was found to increasingly predominate over the usual orthorhombic form as the thickness of the deposit decreased in the case of epitaxy on NaCl. This phenomenon was postulated to have occurred because of a favorable packing scheme for interfacial monoclinic layers and interactions between the polymer and the specific Van der Waals forces of the surface. Although this pseudomorphic growth pattern was relatively independent of molecular weight, initial growth rate perpendicular to the surface had a large molecular weight dependence. Concentration and degree of supercooling also were observed to have significant effects on growth rate. Lattice match between polymer crystallite and al...

Crystallization of polyethylene oxide under shear

Abstract: A concentric cyclinder dilatometer was designed and built to study the influence of shear on the crystallization kinetics of polymers. This instrument allows crystallization to be followed at both constant temperature and shear rate. Several samples of polyethylene oxide (Carbowax 4000, Carbowax 20-M, and WSR-205) were used. A low molecular weight fraction of the Carbowax 20-M, as well as the unfractionated material, was crystallized under shear. The WSR-205 was studied only in a mixture with Carbowax 4000. It was shown that the kinetics of crystallization of uncrosslinked melts of polyethylene oxide are altered by shear. The induction times for the appearance of crystallinity are shorter in the sheared systems than in the quiescent melts. The Avrami exponents are also higher for crystallization in sheared melts than in quiescent systems and increase with decreasing supercooling. The high values of the Avrami exponent are attributed to the disruption of crystalline aggregates into particles larger than the critical sized nucleus. These particles will persist in the melt and continue to grow spontaneously. A continuous infusion of growing particles into the melt occurs. At constant temperature and shear rate, the induction time of the crystallization curve is influenced by polymer molecular weight. In moderate to high molecular weight samples, the effect of shear becomes saturated at very low shear rates. Decreasing the molecular weight separates the crystallization curves. The curves from the higher shear rates appear at the shorter induction times. However, decreasing the molecular weight below that at the critical entanglement molecular weight allows the nucleation rate, strongly dependent upon the supercooling, to influence the relative positions of the sheared crystallization curves.

The pyrethrins and related compounds. XIX Geometrical and optical isomers of 2,2‐dimethyl‐3‐(2,2‐dichlorovinyl)‐cyclopropanecarboxylic acid and insecticidal esters with 5‐benzyi‐3‐furylmethyi and 3‐phenoxybenzyl alcohols

Abstract: Esters of 2,2-dimethyl-3-(2,2-dichlorovinyl)-cyclopropanecarboxylic acid with appropriate alcohols are more active insecticides than the corresponding 3-isobutenyl compounds (chrysanthemates). (±)-Cis and (±)-trans forms of the dichloro acid are obtained by fractional crystallisation of the mixed acids or by hydrolysis of the ethyl esters separated by fractional distillation. The (±)-cis and (±)-trans acids are resolved into their (+)- and (-)-forms with α-methyl-benzylamine and threo-l-p-nitrophenyl-2-N,N-dimethyiamino-propane-1,3-diol respectively. As for the corresponding chrysanthemates, the (+)-cis and (+)-trans acids give esters more active as insecticides than their enantiomers.