Showing papers on "Crystallization published in 1989"

Geochemical consequences of in situ crystallization

Abstract: In situ crystallization of a magma chamber in a transition zone between solidus and liquidus may cause the magma to evolve along a chemical path that differs markedly from that produced by fractional crystallization in closed or periodically replenished magma chambers. The chemical evolution expected from in situ crystallization is consistent with the data from the Kiglapait layered intrusion and may account for some enigmatic features of the chemistry of volcanic and plutonic rocks from diverse tectonic environments.

Molecular Beam Epitaxy: Fundamentals and Current Status

Abstract: This study describes a technique in wide-spread use for the production of high-quality semiconductor devices. It discusses the most important aspects of the MBE apparatus, the physics and chemistry of the crystallization of various materials and device structures, and the characterization methods that relate the structural parameters of the grown (or growing) film or structure to the technologically relevant procedure. In this edition two new fields have been added: crystallization of as-grown low-dimensional heterostructures, mainly quantum wires and quantum dots and in-growth control of the MBE crystallization process of strained-layer structures.

Crystallized Si films by low‐temperature rapid thermal annealing of amorphous silicon

Abstract: Low‐temperature rapid thermal annealing has been used to crystallize both undoped and doped amorphous silicon (a‐Si) films deposited at low temperatures. The polycrystalline films produced are completely crystallized with time temperature budgets such as 4 min at 700 °C. Unlike deposited polycrystalline Si films, the grain size in these crystallized films is not limited by film thickness. In the case of undoped a‐Si films crystallized by this approach, the resulting conductivity is comparable to that achieved in undoped polycrystalline Si films produced by much higher processing temperatures. In the case of doped a‐Si films, the resulting crystallized film yields a conductivity of 160 S/cm, a value which is comparable to the highest reported for doped polycrystalline and microcrystalline silicon. These doped films are found to have mobility values of ∼13 cm2/V s.

Selective area crystallization of amorphous silicon films by low-temperature rapid thermal annealing

Abstract: We report the first demonstration of selective area crystallization of amorphous silicon films using low‐temperature rapid thermal annealing. Crystallization temperatures as low as 500 °C were achieved with the help of a thermally evaporated ultrathin metal layer. The selective area crystallization was accomplished by using this ultrathin metal layer to define the region to be crystallized. The edge between two regions, that which has been crystallized and that which has not, is found to be very sharp.

Preparation of transparent poly(vinyl alcohol) hydrogel

Abstract: A transparent hydrated gel is prepared from a poly(vinyl alcohol)(PVA) solution in a mixed solvent consisting of water and dimethyl sulfoxide(DMSO). Upon cooling the PVA solution below the room temperature, a gel is formed as a consequence of crystallization of PVA molecules. Exchange of DMSO in the formed gel with water gives the hydrated PVA gel which is high in tensile strength, water content, and light transmittance.

Calorimetric evidence for structural relaxation in amorphous silicon.

Abstract: Differential scanning calorimetry of amorphous silicon (a-Si) prepared by ion implantation shows a one-time low-temperature heat release, equal to one-third of the heat of crystallization. This heat release is direct evidence for structural relaxation of a-Si. It is in agreement with predictions made on the basis of Raman spectrometry.

The significance of intergranular diffusion to the mechanisms and kinetics of porphyroblast crystallization

Abstract: The spatial disposition, compositional zoning profiles, and size distributions of garnet crystals in 11 specimens of pelitic schist from the Picuris Range of New Mexico (USA) demonstrate that the kinetics of intergranular diffusion controlled the nucleation and growth mechanisms of porphyroblasts in these rocks. An ordered disposition of garnet centers and a significant correlation between crystal radius and near-neighbor distances manifest suppressed nucleation of new crystals in diffusionally depleted zones surrounding pre-existing crystals. Compositional zoning profiles require diffusionally controlled growth, the rate of which increases exponentially as temperature increases with time; an acceleration factor for growth rate can be estimated from a comparison of compositional profiles for crystals of different sizes in each specimen. Crystal size distributions are interpreted as the result of nucleation rates that accelerate exponentially with increasing temperature early in the crystallization process, but decline in the later stages because of suppression effects in the vicinity of earlier-formed nuclei. Simulations of porphyroblast crystallization, based upon thermally accelerated diffusionally influenced nucleation kinetics and diffusionally controlled growth kinetics, quantitatively replicate textural relations in the rocks. The simulations employ only two variable parameters, which are evaluated by fitting of crystal size distributions. Both have physical significance. The first is an acceleration factor for nucleation, with a magnitude reflecting the prograde increase during the nucleation interval of the chemical affinity for the reaction in undepleted regions of the rock. The second is a measure of the relative sizes of the porphyroblast and the diffusionally depleted zone surrounding it. Crystal size distributions for the Picuris Range garnets correspond very closely to those in the literature from a variety of other localities for garnet and other minerals. The same kinetic model accounts quantitatively for crystal size distributions of porphyroblastic garnet, phlogopite, sphene, and pyroxene in rocks from both regional and contact metamorphic occurrences. These commonalities indicate that intergranular diffusion may be the dominant kinetic factor in the crystallization of porphyroblasts in a wide variety of metamorphic environments.

Disorder-to-Order Transition in Settling Suspensions of Colloidal Silica: X-ray Measurements.

Abstract: Dispersions of colloidal particles exhibit thermodynamic properties similar to those of molecular systems, including a hard sphere disorder-to-order transition. In experiments with organophilic silica in cyclohexane, gravity settling was used to concentrate the particles. With small particles the slow sedimentation permits rearrangement into the iridescent ordered phase, but larger particles form amorphous sediments instead. Scanning electron microscopy of the crystalline sediment indicates hexagonally closepacked layers. X-ray attenuation measurements reveal a discontinuity coincident with the observed boundary between iridescent and opaque regions. Sediments accumulating faster than the maximum rate of crystallization produce a glass, in accord with the classical theory for crystal growth.

Protein crystal growth in microgravity

Abstract: The crystals of most proteins or other biological macromolecules are poorly ordered and diffract to lower resolutions than those observed for most crystals of simple organic and inorganic compounds. Crystallization in the microgravity environment of space may improve crystal quality by eliminating convection effects near growing crystal surfaces. A series of 11 different protein crystal growth experiments was performed on U.S. space shuttle flight STS-26 in September 1988. The microgravity-grown crystals of gamma-interferon D1, porcine elastase, and isocitrate lyase are larger, display more uniform morphologies, and yield diffraction data to significantly higher resolutions than the best crystals of these proteins grown on Earth.

Crystal Engineering: A Textbook

Abstract: Crystal Engineering Intermolecular Interactions Crystal Design Strategies Crystallization and Crystal Growth Polymorphism Multi-component Crystals Coordination Polymers.

High‐Temperature Passive Oxidation of Chemically Vapor Deposited Silicon Carbide

Abstract: The oxidation behavior of chemically vapor deposited (CVD) SiC at high temperature was investigated using a thermogravimetric technique in the temperatures range of 1823 to 1948 K. The specimens were prepared by chemical vapor deposition using SiCl4, C3H8, and H2 as source gases. The oxidation behavior of the CVD-SiC indicated “passive” oxidation and a two-step parabolic oxidation kinetics over the entire temperature range. The crystallization of the SiO2 film formed may have caused this two-step parabolic behavior. The parabolic oxidation rate constant (Kp) varied with the square root of the oxygen partial pressure (P1/2O2). The activation energy for the oxidation was determined to be 345 and 387 kJ · mol−1. These values suggest that the diffusion process of the oxygen ion which passes through the SiO2 film is rate-controlling.

Formation of quasicrystals by mechanical alloying

Abstract: Quasi‐crystalline Al65Cu20Mn15 powder has been produced by mechanical alloying from crystalline elemental powders. The alloying process has been monitored by x‐ray diffraction, and the resulting product has been characterized by transmission electron microscopy. The quasi‐crystalline phase forms after about 90 h of milling. The crystallization temperature and enthalpy have been determined by differential scanning calorimetry. The results are compared with data for melt‐spun material.

Low temperature selective crystallization of amorphous silicon

Abstract: We have used rapid thermal annealing to crystallize PECVD a-Si:H films deposited on glass at low thermal budgets of 700°C/4 min. The a-Si:H films can be selectively crystallized using thermal budgets lower than 700°C/4min. by selectively depositing an ultrathin Pd layer on the silicon surface. We have investigated the electrical and the structural properties of these selectively crystallized films.

Local phase separation at the growth front of a polymer spherulite during crystallization and nonlinear spherulitic growth in a polymer mixture with a phase diagram.

Abstract: Local phase separation at the growth front of a spherulite has been found during crystallization in a polymer mixture of poly(\ensuremath{\epsilon}-caprolactone) and polystyrene. The phenomenon can be qualitatively explained by the coupling between the preferential rejection of impurities (polystyrene) during crystallization and the existence of a phase diagram. This unusual phase separation clearly shows that there is an impurity-rich layer on the growth front of the spherulite. A nonlinear growth of spherulite and a shape instability have also been found in this system. They are discussed on the basis of the formation of an impurity-rich layer at the growing interface. We also discuss the relation between our results on spherulitic growth and the current theories on dense branching morphology in diffusion-controlled growth.

Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone

Abstract: Summary Complex processes of magma mixing are revealed by the crystallization histories of several magnesian, porphyritic abyssal tholeiites dredged from the Siqueiros Fracture Zone at about 8°30′ N near its intersection with the East Pacific Rise. The compositions of chromian spinels, olivines, plagioclases and glass inclusions in these rocks indicate that polybaric crystallization and mixing occurred at high temperatures involving a range of refractory (low-TiO2, low-Na2O) melts. These later coalesced with cooler much less refractory components to produce the bulk rocks. The crystallization histories and comparisons with experimental data indicate that mantle sources of these basalts were lithologically heterogeneous, ranging from fertile (lherzolitic) to refractory (in the extreme, harzburgitic) with respect to a basaltic melt fraction. The separate magma strains going into a typical magnesian abyssal tholeiite were produced by simultaneous partial melting of the different lithofacies, after which they coalesced and mixed. Such mixing is more likely than batch melting to explain the ranges in CaO/Al2O3, and abundances of Na2O and TiO2, within the fairly small range and low (non-picritic) magnesium numbers (0.75–0.64) exhibited by all liquids including those inferred from mineral compositions. The early crystallization of minerals from refractory magma strains suggests that melting domains beneath spreading ridges consist of cores of refractory peridotite surrounded by cooler, more fertile peridotite. Sequential mixing of refractory and fertile magma strains is an unavoidable consequence of the ascent of magma batches from the interiors of complex melting regimes (diapirs) in the mantle. The diapirs may have been triggered into ascent by the buoyancy of low-density (low-iron) refractory peridotite. Several of the Siqueiros magnesian abyssal tholeiites are among the most depleted basalts ever obtained from the eastern Pacific, and yet alkalic basalt was also dredged from the fracture zone a few kilometres away. The tholeiites are even more depleted than typical, more fractionated, summit eruptives of the East Pacific Rise. The contrast may arise because of the homogenizing effects of axial magma chambers, within which truly depleted tholeiites are mixed with enriched alkalic magmas instead of erupting separately, as they do in the fracture zone. Mantle in this region thus appears to be segregated into strongly contrasting depleted and enriched peridotites, with depleted peridotite also ranging from refractory to fertile lithofacies. Primary lithological variability in the mantle thus was probably imposed by one or more ancient melting events that left distinctively enriched and depleted reservoirs in their wake.

The heat capacity and glass transition of hyperquenched glassy water

Abstract: The glass transition and heat capacity of hyperquenched glassy water have been studied by differential scanning calorimetry and by isothermal measurements from 103 K to a temperature where its crystallization to cubic ice is complete Glassy water shows a thermally reversible glass-liquid transition and has a Tg, of 136 ± 1 K The activation energy of structural relaxation in the transition range is ∼55kjmol−1 and is a reflection of the energy required to break two hydrogen bonds before a rotational-translational diffusion of a water molecule in the H-bonded network can occur The temperature width of the transition is ∼12°, and the increase in the heat capacity is 16±01JK−1 mol−1 Liquid water formed on heating the glassy water to 146 K is more stable against crystallization than that which exists near 232 K The hyperquenched glassy form of water can be thermodynamically continuous with liquid water, but whether or not it has the same structure as water above 273 K or supercooled water near t

Semiorganics: A New Class of Nlo Materials

Abstract: Recent results indicate that certain organic molecules whose electronic structures are characterized by extended pi-molecular orbitals can exhibit significant second and third order nonlinear optical (NLO) effects [1]. Unfortunately, this same arrangement which leads to the NLO effects, can also result in essentially one-dimensional bonding coordination. This in turn means that crystals grown from these materials do not readily form good three-dimensional optical-quality crystals, but rather tend to form needles. In addition, pure organic crystals are usually bonded by weak van der Waals forces, often resulting in poor mechanical properties. Indeed, organic impurities are frequently incorporated into these systems during crystallization resulting in poor crystallinity, spurious absorptions, and low damage thresholds. This is particularly true in the case of polymeric NLO materials, where impurities result from the polymerization steps and/or starting materials.

Homogeneous and interfacial heat releases in amorphous silicon

Abstract: Amorphous Si layers prepared by MeV Si implantation of (100) wafers have been studied by scanning and isothermal calorimetry. A homogeneous heat release of 5.1±1.2 kJ/mole and an interfacial heat release, due to crystallization, of 13.4±0.7 kJ/mole have been measured. Isothermal measurements unambiguously demonstrate the occurrence of the homogeneous release. The heat released isothermally at each temperature is between 6% and 8.5% of the total homogeneous release, and the time constants are only very weakly temperature dependent.

The conformation of the DNA double helix in the crystal is dependent on its environment

Abstract: Studies of the crystal structures of more than 30 synthetic DNA fragments have provided structural information about three basic forms of the double helix: A-, B- and Z-form DNA. These studies have demonstrated that the DNA double helix adopts a highly variable structure which is related to its base sequence. The extent to which such observed structures are influenced by the crystalline environment can be found by studying the same molecule in different crystalline forms. We have recently crystallized one particular oligomer in various crystal forms. Here we report the results of structural analyses of the different crystal structures and demonstrate that the DNA double helix can adopt a range of conformations in the crystalline state depending on hydration, molecular packing and temperature. These results have implications on our understanding of the influence of the environment on DNA structure, and on the modes of DNA recognition by proteins.

β-Modification of Polypropylene and Its Two-Component Systems

Abstract: Preparation methods and thermal preconditions of formation of the β-modification of isotactic polypropylene (β-IPP) are rewieved. The existence of theoretical upper and lower limit temperature (T(βα) and T(αβ) respectively) of β-IPP formation is demonstrated. Essential characteristics of crystallization, melting and recrystallization of β-IPP are also summarized. In this respect, the attention is called to some new and peculiar phenomena, including the βα- and αβ-bifurcation of growth on the growing crystal fronts as well as the melting and annealing memory effects. Filled systems (composites) and polymer blends based on β-IPP are also covered. An interpretation to polymorphism of IPP is attempted on the basis of enantiotropy.