Showing papers on "Crystallization published in 1985"

The synthesis of amorphous NiTi alloy powders by mechanical alloying

Abstract: Crystalline powders of Ni and Ti were mechanically alloyed by high-energy ball milling in an inert atmosphere at T < 240 K. The alloyed NixTi1−x powders were characterized by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. For 0.28 < x < 0.72, the powder is amorphous. This composition range is in good agreement with that deduced from a free energy diagram for the crystalline terminal solutions and the amorphous phase. Outside this regime, the powder is a two-phase mixture of an amorphous phase and the crystalline terminal solution of the major element. After ball milling, the solubility limit of Ti in FCC Ni is approximately 28%, which is significantly larger than that in annealed Ni. Atomic pair distribution functions, G(r), were calculated from the X-ray patterns for x = 0.32, 0.4, 0.6, and 0.7. For x = 0.4, the G(r) of the mechanically alloyed powder is almost identical with that of rapidly quenched amorphous Ni40Ti60. For Ni33Ti67, the crystallization temperature, the crystallization enthalpy, and the apparent activation energy for crystallization are 712 K, 0.74 kcal/g at., and 65.8 kcal/g at., respectively. These values are within 10% of those measured by Buschow for rapidly quenched Ni32Ti68. The amorphization by mechanical alloying is attributed to mechanical mixing and to a solid state interdiffusion reaction taking at or near clean boundaries between polycrystalline Ni and Ti.

Calorimetric studies of crystallization and relaxation of amorphous Si and Ge prepared by ion implantation

Abstract: Amorphous Si and Ge layers, produced by noble gas (Ar or Xe) implantation of single crystal substrates, have been crystallized in a differential scanning calorimeter (DSC). The MeV implantation energies resulted in amorphous layers of micron thickness whose areal densities were determined using the Rutherford backscattering and channeling of 1‐MeV protons. These techniques allow determination of the amorphous‐crystal interface velocity (which is proportional to the rate of heat evolution ΔHac) and the total enthalpy of crystallization ΔHac. Amorphous Ge was found to relax continuously to an amorphous state of lower free energy, with a total enthalpy of relaxation of 6.0 kJ/mol before the onset of rapid crystallization. The interface velocity for crystallization on (100) substrates, was found to have an Arrhenius form with an activation energy of 2.17 eV. The value of ΔHac was found to be 11.6±0.7 kJ/mol, the same as for samples prepared by deposition. For Si, ΔHac was determined to be 11.9±0.7 kJ/mol wit...

The> kinetics of industrial crystallization

Abstract: This is the most comprehensive collection of data on the kinetics of crystallization from solution. This feature alone makes it a valuable addition to the literature on industrial crystallization. It provides a link between crystallization mechanisms and crystallization kinetics. The authors have put much effort into unifying different trends in measurement, derivation and interpretation of crystallization kinetics, and to provide cohesion between published results from different sources.

Polymer decoration: The orientation of polymer folds as revealed by the crystallization of polymer vapors

Abstract: A new technique of decoration of the fold surface of polymer crystals is described. It is similar in methodology to gold decoration, but makes use of vapors of crystallizable polymers, principally polyethylene, as decorating material. Upon condensation and crystallization, the highly anisometric decorating molecules become oriented parallel to the fold direction. They build up small crystalline lamellae which, seen edge-on, appear as elongated rods and can be easily observed by conventional transmission electron microscopy. This decoration technique reveals the sectorization of polymer single crystals grown from solution. It can be used to determine the local fold orientation (with a resolution of ca. 10 nm) of polymers crystallized under a wide range of conditions, including crystallization from the bulk. The technique reveals that under all crystallization conditions so far investigated the outermost part of the polymer fold surface is relatively ordered, and the folds and/or loops are, as a rule, oriented nearly parallel to the macroscopic growth front.

Effect of silicon on the crystallization and adsorption properties of ferric oxides.

Abstract: Influence de silicate dissous sur les proprietes globales et superficielles d'oxyhydroxydes de fer vieillis

X-ray scattering from poly(thiophene): crystallinity and crystallographic structure

Abstract: : X-ray scattering has been used to investigate the crystallinity and crystal structure of chemically coupled polythiophene. Heat treatment at elevated temperatures leads to significant increases in crystallinity (from approx. 35% as-synthesized up to approx. 56% after annealing at 380 C for 30 minutes) and coherence length indicative of chain growth and extension. Chemical analysis of the chain extended PT shows a major reduction in residual iodine content consistent with growth of the polymer chains to approximately 1200 thiophene rings. An initial model of the crystal structure of polythiophene is presented. (Author)

Interface reactions between machinable bioactive glass‐ceramics and bone

Abstract: A new biomaterial for bone substitution, a "machinable bioactive glass-ceramic" has been developed. The material contains two main crystal phases, mica and apatite, and is therefore machinable and bioactive. It has the advantage to be workable by the surgeon, if necessary, during operation. The preparation method of this glass-ceramic is described. Different types of the material can be produced in dependence of the composition, nucleation, and crystallization of the basic glass. In vivo and in vitro investigations showed a characteristic solubility of the material. A Ca-phosphate-rich interface layer with apatite crystals (from the basic glass-ceramic) and a thickness of about 5-10 micrometers grows as solid-state reaction between glass-ceramic and bone. This interface reaction is interpreted as a chemical process which includes a slight solubility of the glass-ceramic and a solid state reaction between the stable apatite crystals in the glass-ceramic and the bone.

New types of phase separation behavior during the crystallization process in polymer blends with phase diagram.

Abstract: There is a possibility that the phase separation process is coupled with other ordering processes such as crystallization. New types of phase separation behavior during the crystallization process in polymer blends with phase diagram are presented probably for the first time. Two different types of nonequilibrium phenomena take place simultaneously and new structures and new dynamics of pattern formation are observed as the result of the competition between these two phenomena.

The nature and origin of titaniferous magnetite-rich layers in the upper zone of the Bushveld Complex; a review and synthesis

Abstract: The occurrence and geologic relationships of Ti magnetite layers indicate that their genesis is intimately related to fractional crystallization processes that were responsible for the formation of their silicate host rocks. The exact mechanism by which these oxide-rich layers form is, however, still debated. Most recent models recognize that precipitation of copious quantities of Ti magnetite is triggered by episodic increases in f (sub O 2 ) , but the process whereby this occurs is not well understood. The nature and occurrence of Ti magnetite-rich layers in the Bushveld Complex is reviewed and the requirements of any satisfactory genetic model are outlined. The formation and subsequent textural evolution of Ti magnetite layers is believed to have taken place as follows:1. Favorable conditions for the precipitation of large quantities of Ti magnetite were created by a lengthy period of fractional crystallization that resulted in the concentration of large amounts of Fe, Ti, and V in the residual magma.2. Large-scale in situ bottom crystallization of plagioclase resulted in a marked increase in the total Fe content and density of the immediately surrounding melt which collected as a dense layer on the bottom of the magma chamber. This dense Fe-Ti-(V)-enriched liquid did not mix with the overlying magma and formed a stagnant layer from which copious amounts of Ti magnetite may have crystallized.3. Crystallization of Ti magnetite is controlled by the Fe 2 O 3 /FeO ratio of the liquid and is a function of f (sub O 2 ) , temperature, and the f (sub H 2 O) /f (sub H 2 ) ratio. The Fe 2 O 3 content of the liquid will also increase during the crystallization of ilmenite and Fe (super +2) -bearing silicates (clinopy-roxene, pigeonitc, and fayalitic olivine). The H 2 O content of the residual liquid will also be increased by the fractionation of anhydrous silicate phases.4. A complex interplay of these factors resulted in the precipitation of relatively large amounts of Ti magnetite required for the development of ore-rich layers. The bulk of the Ti magnetite formed by in situ bottom crystallization. The nucleation and growth of magnetite within the stagnant layer and gravitative settling of these crystals over a short distance (on the order of meters) may have augmented the growth of the layers.5. Variations in the relative proportions of coexisting oxide and silicate phases may be related to differences in concentrations and diffusion rates of ions in the liquid immediately above the crystallizing layer. This resulted in differences in the rates of nucleation and growth of individual phases.6. Precipitation of abundant magnetite lowered the density of the stagnant layer until it reached that of the overlying magma. The two liquids then mixed, thus terminating the cycle of magnetite layer formation and marking a return to silicate-dominated fractionation.7. The ore-rich layers originally consisted of Ti magnetite crystals and variable amounts of silicate crystals which contained a certain amount of interstitial residual liquid. Densification of these layers and expulsion of significant amounts of this liquid were accomplished by annealing at high subsolidus temperatures, possibly augmented by the addition of suitable material from interprecipitate liquids.8. Variations in f (sub O 2 ) during subsolidus cooling resulted in the development of a wide range of ulvoespinel and ilmenite microintergrowths.

Solution crystallization of polyethylene at high temperatures

Abstract: Single crystals of polyethylene have been grown from solution in various solvents at temperatures between 70 and 120° C. This represents an overlap in crystallization conditions with those used for melt growth, where substantial isothermal thickening is known to occur during growth. The crystal thicknesses have been measured by Raman spectroscopy. Values of the equilibrium dissolution temperature and fold surface free energy are calculated for each solvent and the results analysed using the kinetic theory. Variations in crystal properties with time of crystallization are also investigated. A specific dependence of fold length on supercooling has been found to apply over the whole temperature range, consistent with predictions by the kinetic theories of crystallization in spite of changes in morphology which are incompatible with assumptions underlying the theoretical model. No evidence for isothermal thickening has been observed, except possibly for a small marginal effect at the highest temperature of 120° C investigated, over the same temperature range where melt crystallized material shows the effect prominently. Crystals grown at all temperatures displayed a rise in dissolution temperature with time which could be associated with an increase in surface perfection. All these findings have wider implications for our picture of polymer crystallization and crystal structure which are discussed here. A further, explicit, correlation with melt crystallization is deferred to a subsequent publication.

The Relation between Structure and Properties of Crystalline Polymers

Abstract: The influence of both structure and crystallization conditions on a variety of properties characteristic of semi-crystalline polymers are discussed. A careful distinction needs to be made between molecular structure and the independent structural variables which describe the crystalline state. This set of independent variables includes the degree of crystallinity; the structure of the non-crystalline regions; the crystallite thickness distribution; the structure and relative amount of interface; the crystallite structure and the supermolecular structure. The dependence of these variables on molecular structure and crystallization conditions are examined in detail as is their influence on properties. By following this procedure the influence of molecular structure on properties is deduced. A variety of thermodynamic, spectral and mechanical properties are analyzed by these methods.

Stability of amorphous Cu/Ta and Cu/W alloys

Abstract: Although both the Cu/W and Cu/Ta alloy systems are known to be immiscible, the response of coevaporated amorphous films of a‐CuxW1−x (x=0.45–0.72) and a‐CuxTa1−x (x=0.10–0.55) alloys to thermal annealing and ion irradiation is very different. Alloys of a‐CuxTa1−x crystallize between 600 and 800 °C, with the crystallization temperature increasing with Ta concentration, while a‐CuxW1−x alloys crystallize below 200 °C. Likewise in Xe‐ and Ne‐ion irradiation studies, a‐Cu/Ta can still be observed after ∼13 displacements per atom (DPA), whereas a‐Cu/W alloys completely transform to solid solutions after ∼0.08 DPA. The results of this study, in combination with previous investigations of amorphous alloys of immiscible systems, show that the crystallization temperature Tc is not predicted equally well for systems with a positive or negative heat of formation. It appears that the kinetic model of crystallization only applies if the transformation proceeds through long‐range diffusion. Amorphous alloys which have ...

Polymer crystallization induced by sorption of CO2 gas

Abstract: Previous work has shown that sorption of CO2 at relatively high pressures by glassy polymers reduces their glass transition temperatures and may convert the glass into a rubber under certain conditions. It is shown here that this plasticization by a gas can induce crystallization just as sorption of vapors or liquids is known to do. This point is extensively explored for miscible blends of poly(vinylidene fluoride) and poly(methyl methacrylate) and to a lesser extent for poly(ethylene terephthalate). In some cases, this secondary crystallization process results in small crystals whose melting endotherms are just above the glass transition and are very similar to peaks resulting from heat capacity overshoots, or enthalpic relaxation, caused by sub-Tg annealing; however, by appropriate techniques peaks arising from these two separate mechanisms can be distinguished. For oriented materials, evidence is shown which demonstrates that the additional crystals formed on CO2 sorption have the same preferential orientation as the original material.

Laboratory simulation of rock weathering by salt crystallization and hydration processes in hot, arid environments

Abstract: The results of a series of experiments are reported in this paper which were designed to differentiate between the forces of crystal growth and hydration in salt weathering, using a single salt-hydrate system (sodium sulphate), five contrasting rock types, and several diurnal temperature-relative humidity cycles which permitted or inhibited these processes and simulated ground surface climates in hot, arid environments. It was shown that hydration of sodium sulphate is an effective mechanism of rock disintegration but that it is significantly less destructive than crystal growth pressure. Crystallization of thenardite (Na2SO4) is, in turn, more effective in rock weathering than the crystal growth of mirabilite (Na2SO4.10H2O). In general, rates of disintegration were most rapid where the diurnal temperature range was extreme and relative humidity lowest.

Crystallization kinetics of polyphenylene sulfide

Abstract: The crystallization kinetics of unfilled and glass-reinforced grades of polyphenylene sulfide has been investigated by using differential scanning calorimetry. The maximum rate of crystallization is observed at about 170°C. From the crystallization data, it is recommended that the molding parameters should be so specified that the polymer spends 10–15 s over the temperature range of 155–190°C during cooling, before demolding, in order to ensure stable morphology of the molded part. The glass fibers have an accelerating influence on crystallization resulting in a 15–25% reduction in crystallization time. The kinetic data have been interpreted by using Avrami analysis followed by a discussion of the possible crystallization mechanisms.

Hydrous zirconium dioxide: interfacial properties, the formation of monodisperse spherical particles, and its crystallization at high temperatures

Abstract: The characteristics of hydrous zirconia gels obtained by hydrolysis of highly acidic ZrOCl2 · 8H2O solutions at ∼100‡C are described. Under adequate conditions, monodisperse spherical hydrous zirconia is obtained. The role of sulphate and chloride ions is described. The gel has a large water content, indicating only modest crosslinking. Crystallization upon heating takes place with the formation of both monoclinic and tetragonal zirconia polymorphs; X-ray line widening studies do not indicate a particle size-crystal structure correlation. On the basis of this observation and using Stranki's rule, the crystallization sequence upon heating is rationalized. Surface properties of hydrous zirconia are discussed on the basis of electrophoretic measurements, and compared with those of baddeleyte. The site binding model is not very adequate to describe these systems, and the existence of a very thin gel-like region at the baddeleyte-water interface is postulated.

Formation And Characterization of Amorphous Erbium-Based Alloys Prepared By Near-Isothermal Cold-Rolling of Elemental Composites

Abstract: We report the formation of bulk single‐phase amorphous Cu‐Er and Ni‐Er alloys by extensive cold‐rolling of elemental foils. The reaction is driven by the negative enthalpies of mixing of the constitutent elements and occurs near ambient temperature. The crystallization behavior of the alloys obtained was studied by means of differential scanning calorimetry and found to agree closely with that of the corresponding sputtered and liquid‐quenched alloys. Radial distribution functions were measured for sputtered and rolled Cu72Er28 and were found to be in good agreement.

The supercooling dependence of the initial fold length of polyethylene crystallized from the melt: unification of melt and solution crystallization

Abstract: Following the previous recognition [1], reached with the aid of real time low angle X-ray diffraction (using a synchrotron X-ray source) that in melt crystallized polyethylene the initial (primary) lamellar thickness is much smaller than hitherto envisaged, we have proceeded to construct the full relationship between primary fold length (Ig*) and supercooling (ΔT) covering a wide range of crystallization temperature (Tc). The principal result of this work is the identification of supercooling as the sole factor which determinesIg*. Comparison with crystallization from solution [2–4] has revealed that theIg* against ΔT curves are completely superposable thus removing the gap which has existed up to the present between melt and solution crystallization, bringing about a welcome unification of these two separate (at least as far as fold length was concerned) aspects of polymer crystallization. Further, we show that while ΔT determinesIg*, subsequent thickening is determined by the absolute temperature. Isothermal thickening in particular proceeds first by a large discontinuous step followed by a continuous logarithmic increase with time. The importance of these findings and in particular the affirmation of the unique role of supercooling for chain folding and lamellar crystallization in general is emphasized.

Crystallization and transformation mechanisms of α, β- and γ-polymorphs of ultra-pure oleic acid

Abstract: Crystallization and transformation mechanisms of ultrapure (99.999%) oleic acid were examined by Differential Scanning Calorimetry (DSC) and X-ray diffraction. X-ray diffraction spectra revealed three different polymorphs newly named α, β and γ, which differ from each other most significantly in the short spacing spectra. α and β were found to be equivalent to the previous data which Lutton named low and high melting polymorphs, whereas γ was newly identified in the present study. DSC studies have clarified the thermodynamic stability of the three polymorphs in a range of temperature from -20 to 16.2 C. β is always most stable, whereas α and γ are metastable, undergoing a reversible first-order solid-state transformation at -2.2 C (on heating). DSC also showed that the crystallization behaviors are strongly dependent on the polymorphs; α crystallizes at a much higher rate than β; despite the fact that they have close melting points (α, 13.3 C; β, 16.2 C). It was demonstrated for the first time that the above peculiar polymorphic behaviors of oleic acid are quite different from those of stearic acid, a saturated fatty acid with the same carbon chain length.

Nucleation and crystallization in glass-forming melts: Olds problems and new questions

Abstract: A review is given of basic problems and results in the field of nucleation and crystallization in glass-forming melts. The temperature dependence of the driving force for crystallization and of the free energy of the crystal/melt interface, as well as the connection between the melt viscosity and entropy are considered in some detail. The influence of various factors on the rates of nucleation, crystal growth and overall crystallization is discussed and existing evidence for nonstationary nucleation is surveyed. A generalized kinetic criterion for vitrification is formulated accounting for the effect of nucleation nonstationarity. Finally, an approximate expression is obtained for the minimum cooling rate for glass formation.