Showing papers on "Crystallization published in 1995"

Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-Liquid-Solid Growth

Abstract: Until now, micrometer-scale or larger crystals of the III-V semiconductors have not been grown at low temperatures for lack of suitable crystallization mechanisms for highly covalent nonmolecular solids. A solution-liquid-solid mechanism for the growth of InP, InAs, and GaAs is described that uses simple, low-temperature (≤203°C), solution-phase reactions. The materials are produced as polycrystalline fibers or near-single-crystal whiskers having widths of 10 to 150 nanometers and lengths of up to several micrometers. This mechanism shows that processes analogous to vapor-liquid-solid growth can operate at low temperatures; similar synthesis routes for other covalent solids may be possible.

Formation of Ti–Zr–Cu–Ni bulk metallic glasses

Abstract: Formation of bulk metallic glass in quaternary Ti–Zr–Cu–Ni alloys by relatively slow cooling from the melt is reported. Thick strips of metallic glass were obtained by the method of metal mold casting. The glass forming ability of the quaternary alloys exceeds that of binary or ternary alloys containing the same elements due to the complexity of the system. The best glass forming alloys such as Ti34Zr11Cu47Ni8 can be cast to at least 4-mm-thick amorphous strips. The critical cooling rate for glass formation is of the order of 250 K/s or less, at least two orders of magnitude lower than that of the best ternary alloys. The glass transition, crystallization, and melting behavior of the alloys were studied by differential scanning calorimetry. The amorphous alloys exhibit a significant undercooled liquid region between the glass transition and first crystallization event. The glass forming ability of these alloys, as determined by the critical cooling rate, exceeds what is expected based on the reduced glass transition temperature. It is also found that the glass forming ability for alloys of similar reduced glass transition temperature can differ by two orders of magnitude as defined by critical cooling rates. The origins of the difference in glass forming ability of the alloys are discussed. It is found that when large composition redistribution accompanies crystallization, glass formation is enhanced. The excellent glass forming ability of alloys such as Ti34Zr11Cu47Ni8 is a result of simultaneously minimizing the nucleation rate of the competing crystalline phases. The ternary/quaternary Laves phase (MgZn2 type) shows the greatest ease of nucleation and plays a key role in determining the optimum compositions for glass formation.

Calcium-induced conformational transition revealed by the solution structure of apo calmodulin.

Abstract: The solution structure of Ca(2+)-free calmodulin has been determined by NMR spectroscopy, and is compared to the previously reported structure of the Ca(2+)-saturated form. The removal of Ca2+ causes the interhelical angles of four EF-hand motifs to increase by 36 degrees-44 degrees. This leads to major changes in surface properties, including the closure of the deep hydrophobic cavity essential for target protein recognition. Concerted movements of helices A and D with respect to B and C, and of helices E and H with respect to F and G are likely responsible for the cooperative Ca(2+)-binding property observed between two adjacent EF-hand sites in the amino- and carboxy-terminal domains.

Rheology of crystal-bearing silicate melts : an experimental study at high viscosities

Abstract: The viscosity of partially crystallized Mg3Al2Si3O12 melts has been measured under uniaxial compression in the interval 1010 - 1013 poise as a function of the volume fraction of crystals. These inclusions are well-rounded spherulites of aluminous enstatite, having the same composition as the melt, and whose growth rate is negligible at the temperature of the measurements. The viscosity increases by less than 1 order of magnitude for crystal fractions ϕ of 40 vol % and remains Newtonian up to the maximum stress exerted, namely 1 kbar. The Einstein-Roscoe equation, η = η0 (1 - ϕ/ϕm)−n, provides very good fits to the measurements only if either the ϕm or n parameter is allowed to depend on temperature. For modeling of magmatic processes, however, the widely recommended constant values ϕm = 0.6 and n = 2.5 should be adequate. The rheology changes abruptly when the clustered spherulites begin to oppose shear deformation, at a crystal fraction of about 40 vol %. The viscosity becomes non-Newtonian, with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise. As long as the crystal fraction remains lower than 70 vol %, the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt. The deformation becomes again regular at low stresses with lower melt fractions, but samples undergo extensive fracturation along the direction of uniaxial stress. Similar rheology changes have been observed during the isothermal crystallization of Li2Si2O5 melts, which produces small ellipsoidal inclusions. These results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction, even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions.

Thermal and Magnetic Properties of Bulk Fe-Based Glassy Alloys Prepared by Copper Mold Casting

Abstract: Bulk glassy Fe 73 Al 5 Ga 2 P 11 C 5 B 4 alloys in cylindrical form with diameters of 0.5 and 1.0 mm were found to form by a copper mold casting method. The further increase in diameter causes the formation of coexistent glassy, Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 1.5 mm ? sample and coexistent Fe 3 (B, C), Fe 2 B and Fe 3 P phases for the 2.0 mm ? sample. It is to be noticed that the maximum thickness for glass formation is about 10 times larger than the largest thickness for Fe-based glassy alloys reported up to date. The glass transition temperature (T g ), crystallization temperature (T x ) and heat of crystallization of the 1.0 mm ? glassy alloy are 732 K, 785 K and 3.76 kJ/mol, respectively. No appreciable difference in the thermal stability and magnetic properties is seen between the bulk glassy alloys and the melt-spun ribbon. The 1.0 mm ? glassy alloy has ferromagnetism with a Curie temperature of 606 K and exhibits 1.26 T for saturation magnetization (B s ), 82 A/m for coercivity (H c ) and 0.38 for the ratio of residual magnetization to B s at room temperature. The large ΔT x ( = T x - T g ) and large glass-forming ability can be obtained for the Fe-based alloy containing simultaneously the five solute elements. The effectiveness of the multiplication is presumably due to the combination of the following three effects ; (1) the suppression of crystalline nuclei due to the increase in dense random packing density for the glassy structure containing P, C and B with significantly different atomic sizes, (2) the difficulty of atomic rearrangements for the precipitation of the Fe-metalloid compounds caused by the generation of Al-metalloid pairs with strongly attractive bonding nature, and (3) the decrease in the preferential precipitation tendency of Fe-B and Fe-C compounds by the dissolution of Ga which is immiscible to B and C and soluble to Fe.

Preparation of Zeolite ZSM-5 Membranes by In-Situ Crystallization on Porous α-Al_2O_3

Abstract: Zeolite ZSM-5 membranes were grown on porous α-Al_2O_3 disks by in-situ hydrothermal synthesis at 175 "C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. Extensive experimentation was carried out to find compositions that gave continuous polycrystalline films. The films grown with the optimized composition were about 10 µm thick and consisted of well-intergrown crystals of about 2 µm size. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185 °C, respectively.

Understanding and control of nucleation, growth, habit, dissolution and structure of two‐ and three‐dimensional crystals using `tailor‐made' auxiliaries

Abstract: Tailor-made auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes, which include morphological engineering and etching, reduction of crystal symmetry, assignment of absolute structure of chiral molecules and polar crystals, elucidation of the effect of solvent on crystal growth, and crystallization of a desired polymorph. As for crystal growth promoters, monolayers of amphiphilic molecules on water have been used to induce the growth of a variety of three-dimensional crystals at the monolayer-solution interface by means of structural match, molecular complementarity or electrostatic interaction. A particular focus is made on the induced nucleation of ice by monolayers of water-insoluble aliphatic alcohols. The two-dimensional crystalline structures of such monolayers have been studied by grazing incidence X-ray diffraction. It has become possible to monitor, by this method, the growth, dissolution and structure of self-aggregated crystalline monolayers, and indeed multilayers, affected by the interaction of solvent molecules in the aqueous subphase with the amphiphilic headgroups, and by the use of tailor-made amphiphilic additives

An experimental study of the effects of melt composition on plagioclase - melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

Abstract: An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H2O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H2O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H2O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H2O content can notably increase the An content of plagioclase, up to 10 mol% for H2O-undersaturated melts, and 20 mol% for H2O-saturated melts. Exceptionally calcic plagioclases (up to An100) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H2O (up to 3 wt%). Rather, they must be due to the involvement of extremely refractory (CaO/Na2O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na2O=12–15, or from melts with exceptionally high Al2O3(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H2O contents (∼6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An95 in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na2O values (>8) may account for the presence of highly anorthitic plagioclases in these rocks.

Structural Characterization and High‐Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol‐Gel Precursors Containing Si‐H Bonds

Abstract: Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)3, and methyl-diethoxysilane, HMeSi(OEt)2. The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.

Protein-protein interaction at crystal contacts

Abstract: Packing contacts are crystal artifacts, yet they make use of the same forces that govern specific recognition in protein-protein complexes and oligomeric proteins. They provide examples of a nonspecific protein-protein interaction which can be compared to biologically relevant ones. We evaluate the number and size of pairwise interfaces in 152 crystal forms where the asymmetric unit contains a monomeric protein. In those crystal forms that have no element of 2-fold symmetry, we find that molecules form 8 to 10 pairwise interfaces. The total area of the surface buried on each molecule is large, up to 4400 A2. Pairwise interfaces bury 200-1200 A2, like interfaces generated at random in a computer simulation, and less than interfaces in protease-inhibitor or antigen-antibody complexes, which bury 1500 A2 or more. Thus, specific contacts occurring in such complexes extend over a larger surface than nonspecific ones. In crystal forms with 2-fold symmetry, pairwise interfaces are fewer and larger on average than in the absence of 2-fold symmetry. Some bury 1500-2500 A2, like interfaces in oligomeric proteins, and create "crystal oligomers" which may have formed in the solution before crystallizing.

Stereocomplexation and morphology of polylactides

Abstract: Blends of isotactic polylactides of opposite configurations lead to the formation of stereocomplexes, provided the enantiomeric excess of the two homopolymers in contact is high enough. In this study, the stereocomplex formation between a poly(L-lactide) (100L) having an enantiomeric excess of 100% and a poly(D-lactide) having an enantiomeric excess of 80% (80D) has been investigated using differential scanning calorimetry and optical microscopy. These results have been compared to those obtained between two polylactides having both an enantiomeric excess of 100% (100L/100D blend). The melting temperatures of blends 100L/100D and 100L/80D are 230 and 208 °C, respectively. In both cases, the stereocomplex formation is preferred to the homopolymer crystallization and the stereocomplex controls the morphology of the blends over a wide range of concentrations. However, this control is more rigorous when the D component is 100% isotactic. This difference leads to a more complete crystallization of homopolymer 100L and to a greater influence of crystallization conditions in 100L/80D blends in comparison with 100L/100D blends. An epitaxial crystallization between the homopolymer 100L and the stereocomplex 100L/80D has also been observed at certain compositions.

Pd induced lateral crystallization of amorphous si thin films

Abstract: A thin palladium layer (∼40 A) was selectively formed on top of amorphous silicon films before annealing and the effects of palladium layer on the crystallization behavior of the amorphous silicon films were investigated. It was observed that the amorphous silicon right under the Pd layer could be crystallized to grain sizes of several hundred angstroms by annealing at 500 °C. In addition, the area between the Pd thin pads, patterned by lithography, was found to be crystallized to grain sizes of a few tens of microns in length by the same annealing. Such lateral crystallization was found to reach more than 100 μm in some cases. The lateral crystallization phenomenon might be useful for the fabrication of low temperature polycrystalline‐Si thin film transistors, providing large‐grained Si films.

Structure Development in Semicrystalline Diblock Copolymers Crystallizing from the Ordered Melt

Abstract: Time-resolved simultaneous synchrotron small-angle and wide-angle X-ray scattering (SAXS and WAXS) and DSC experiments have been performed on poly(ethylene)-poly(ethylethylene) and poly(ethylene)-poly(ethylene-propylene) diblock copolymers quenched from melts with lamellar and hexagonal-packed cylinder structures. We find that the original microphase-separated morphologies are destroyed due to poly(ethylene) (PE) chain folding upon crystallization. Below the melting temperature, a sample with a volume fraction f PE =0.49 forms a lamellar structure distinct from that in the melt. On quenching, a hexagonal f PE =0.25 sample forms a lamellar structure similar to that of the f PE =0.49 sample, while the hexagonal-packed cylinder structure of an f PE =0.75 sample is also destroyed by crystallization but does not form well-ordered lamellae. The SAXS profiles fromcrystallized materials are shown to correspond to the sum of scattering from block copolymer lamellae, with up to four orders of reflection, plus a broad peak arising from semicrystalline PE. Analysis of scattering density correlation functions calculated using the SAXS data shows that the PE lamellar thickness is (45±5) A for all samples, similar to that observed for PE homopolymer. The WAXS data reveal that PE crystallizes in its usual orthorhombic form in all samples. The relative degree of crystallinity as a function of time after a quench, determined from the SAXS invariant, is fitted by Avrami equations for spherulitic crystallite growth. The Avrami exponent is found to be n=(3.0±0.1) for all samples, consistent with a nucleation and growth process

Decomposition and primary crystallization in undercooled zr41.2ti13.8cu12.5ni10.0be22.5 melts

Abstract: Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glasses were prepared by cooling the melt with a rate of about 10 K/s and investigated with respect to their chemical and structural homogeneity by atom probe field ion microscopy and transmission electron microscopy. The measurements on these slowly cooled samples reveal that the alloy exhibits phase separation in the undercooled liquid state. Significant composition fluctuations are found in the Be and Zr concentration but not in the Ti, Cu, and Ni concentration. The decomposed microstructure is compared with the microstructure obtained upon primary crystallization, suggesting that the nucleation during primary crystallization of this bulk glass former is triggered by the preceding diffusion controlled decomposition in the undercooled liquid state.

The structure and property characteristics of amorphous/nanocrystalline silicon produced by ball milling

Abstract: The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline Si has been produced by ball milling of polycrystalline elemental Si. The nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660 °C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

Laser‐induced crystallization phenomena in GeTe‐based alloys. I. Characterization of nucleation and growth

Abstract: The laser‐induced crystallization behavior of GeTe‐based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal‐growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as‐deposited and melt‐quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as‐deposited layer can differ by over an order of magnitude from that of the melt‐quenched amorphous layer. The as‐deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of...

Method for optimal crystallization to obtain high electrical performance from chalcogenides

Abstract: A method of fabricating a chalcogenide memory cell wherein a layer of chalcogenide material is deposited in an amorphous state. The layer of amorphous chalcogenide material is then etched to its final geometry while maintaining its amorphous structure. The final geometry of the chalcogenide material is then annealed thereby transforming it to a crystalline form.

Zigzag Morphology of a Poly(styrene-b-hexyl isocyanate) Rod-Coil Block Copolymer

Abstract: The solid state morphology of an anionically synthesized P(S-b-HIC) rod-coil block copolymer was studied using a number of techniques and casting solvents. Liquid crystalline ordering was seen in concentrated solutions using optical microscopy (OM). Bulk and thin film samples cast from solutions in toluene and studied using transmission electron microscopy (TEM) revealed a new zigzag morphology. Electron diffraction (ED) experiments were able to show that the PHIC rods were tilted with respect to the interface separating the PS and PHIC domains. In addition, the PHIC rods were found to be highly crystalline, having an 8 3 or 8 5 helical conformation and packing in a two chain monoclinic or triclinic (pseudohexagonal) unit cell with a = b = 15.1 A, c = 15.6 A, γ = 120°, and a crystal density of 1.10 g/cm 3 . A model for the zigzag morphology which allows interdigitation of the rods is consistent with TEM and ED results as well as domain spacing predictions based on molecular weight information. The formation of such a morphology is also consistent with thermodynamic arguments based on a theory developed by Halperin for rod-coil block copolymers if, in addition, quantization of the allowed tilt angle by crystallization is taken into account. Solvent quality was found to profoundly affect the morphology formed from solution cast samples. In addition to the zigzag morphology, morphologies consisting of fragmented PS zigzags and micelle-like regions were also observed. The choice of solvent most likely determines what phases macrophase separate from the isotropic solution before microphase separation of the rod-coil and crystallization of the PHIC take place and also whether chain stretching or interfacial energy is more dominant in the thermodynamics of microphase separation.

Prediction of Crystal-Growth Morphology Based on Structural-Analysis of the Solid-Fluid Interface

Abstract: PREDICTING the shape of growing crystals is important for industrial crystallization processes. The equilibrium form of a crystal can be determined unambiguously from a consideration of the surface free energies of the various crystallographic faces {hkl}1, but the growth morphology is determined by kinetic factors which are harder to predict. This morphology depends on the relative growth rates R rel hkl of the crystal faces. Several theories have been advanced2,3 to relate R rel hkl to geometric or energetic characteristics of the surfaces {hkl}, but these have met with limited success in predicting the crystal morphologies observed. Here we present a theoretical approach to the problem in which R rel hkl is determined by quantities that are accessible either from kinetic models or from computer simulations of the solid-fluid interface. The important parameters controlling the growth rate are the energy required to create a step at the crystal surface and the free-energy barrier for an adsorbed solute molecule to be incorporated into the crystal. Both can be related to the mole fraction of adsorbed solute molecules in dynamic equilibrium with those in the crystal surface. When this approach is applied to the case of urea crystals grown from aqueous solution, we predict a needle-like shape which is consistent with experimental observations.