Showing papers on "Crystallization published in 1987"

Recrystallization of amorphized polycrystalline silicon films on SiO2: Temperature dependence of the crystallization parameters

Abstract: This paper presents a theoretical and experimental study of the recrystallization behavior of polycrystalline silicon films amorphized by self‐implantation. The crystallization behavior was found to be similar to the crystallization behavior of films deposited in the amorphous state, as reported in the literature; however, a transient time was observed, during which negligible crystallization occurs. The films were prepared by low‐pressure chemical vapor deposition onto thermally oxidized silicon wafers and amorphized by implantation of silicon ions. The transient time, nucleation rate, and characteristic crystallization time were determined from the crystalline fraction and density of grains in partially recrystallized samples for anneal temperatures from 580 to 640 °C. The growth velocity was calculated from the nucleation rate and crystallization time and is lower than values in the literature for films deposited in the amorphous state. The final grain size, as calculated from the crystallization param...

Crystallization of amorphous silicon during thin-film gold reaction

Abstract: The crystallization of a‐Si in a‐Si (50‐nm) and Au (5‐nm) thin‐film bilayers has been investigated during heat treatment in a transmission electron microscope. When crystallization of a‐Si first begins at 130 °C, the Au‐Si alloy (Au and a precursor phase) reflections observed at lower temperatures vanish, and several new reflections from metastable Au‐Si compounds occur. Dendritically growing islands of poly‐Si are observed after heating at 175 °C. If the samples are held at a constant temperature of 175 °C for 10 min, the poly‐Si islands coalesce. The formation of poly‐Si depends on the diffusion of Au into a‐Si and the formation of metastable Au‐Si compounds, which act as transport phases for both Si and Au. After crystallization Au segregates to the front and back surfaces of the poly‐Si film. The result of this work and earlier diffraction investigations are interpreted in terms of superlattices based on a sublattice. A fundamental body‐centered‐cubic structure with a=5.52 A and composition Au4Si is s...

Sorbitol coated comestible and method of preparation

Abstract: A process for producing a sorbitol coated comestible is disclosed. The process comprises applying to a substantially anhydrous edible core at least two coating solutions comprising sorbitol to coat the edible core, wherein the first coating solution comprises: (a) about 77 to about 81 wt % sorbitol solution comprising about 65 to about 75 wt % sorbitol; (b) about 9.5 to about 12.5 wt % crystalline sorbitol powder; (c) about 0.25 to about 1.5 wt % of at least one film-forming agent; and (d) about 0.1 to about 5.0 wt % of at least one crystallization retardant; and wherein the second coating solution comprises: (a) about 82 to about 92 wt % sorbitol solution comprising about 65 to about 75 wt % sorbitol; (b) about 1.0 to about 2.5 wt % crystalline sorbitol powder; (c) about 0.05 to about 2.0 wt % of at least one film-forming agent; and (d) about 0.1 to about 0.3 wt % of at least one crystallization retardant; and after application of each coating solution, the solution is dried to prepare a final product which is a smooth, hard, and crunchy comestible.

The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses

Abstract: The role of oxygen and other impurities on the crystallization characteristics of Ni‐Zr glasses near the composition NiZr2, as well as for FeZr2, CoZr2, and NiHf2, has been investigated. For NiZr2 glasses with 1 at. % oxygen, the first crystallization product is the metastable E93 structure with a =1.227 nm instead of the equilibrium C16 structure. A similar effect is found for samples containing ≳3 at. % B. For FeZr2, CoZr2, and NiHf2 the first crystallization product is also E93 structure, even with very small levels of oxygen (≤0.2 at. %). The formation of the E93 structure is always accompanied by an increase in the electrical resistivity, an increase which transmission electron microscopy shows is intrinsic to the phase and unrelated to crystallite size. For Ni36.5Zr63.5 and Ni42Zr58 the crystallization is also accompanied by an increase in electrical resistance and the evolution of a crystal structure similar to the E93 structure in the size of the unit cell and packing fraction but with a different...

Ion-beam-induced crystallization and amorphization of silicon

Abstract: Thin amorphous silicon layers can be produced in crystalline silicon substrates by ion-implantation. Subsequent ion-irradiation at elevated temperatures can induce such layers to either crystallize epitaxially or increase in thickness, layer by layer. This paper examines these processes and their dependence on substrate temperature and ion-irradiation parameters. It is shown that both processes, epitaxial crystallization and layer-by-layer amorphization, are controlled by ion-beam induced defect production at, or near, the crystalline/amorphous interface. The competition between defect production (determined by the ion flux and rate of nuclear energy deposition) and dynamic defect annealing (determined by the substrate temperature) is shown to play an important role in determining whether the layer crystallizes or amorphizes. Possible models for the observed behavior are discussed.

Raman spectroscopic study of the synthesis of zeolite Y

Abstract: The formation of zeolite Y from colloidal silica and soluble silicate species was investigated by Raman spectroscopy. The role of aging of the reactant mixture was studied. During the nucleation period, the solid amorphous phase consists of predominantly six-membered aluminosilicate rings, which act as building blocks for the formation of zeolite Y. It is essential to have polymeric, highly condensed silicate units as a reactant if zeolite Y crystallization is to take place.

Synthesis and structure of zeolite ZSM-5: a Raman spectroscopic study

Abstract: The Raman spectra of the solution and solid phases present during various stages of ZSM-5 synthesis were examined by monitoring the vibrations of the tetrapropylammonium ion. It was found that this cation is trapped into the amorphous solid phase at the earliest stages of the synthesis in the all-trans configuration. Upon crystallization of the zeolite, there is a forced change in the conformation of the trapped tetrapropylammonium cation, such that it can fit into the zig-zag zeolite channels. The Raman spectra of the aluminosilicate zeolite framework is also distinct from other zeolites, such as A, X, and Y, and exhibits a band at 385 cm/sup -1/ characteristic of the five-membered building units of this zeolite.

Application of the Miedema model to formation enthalpies and crystallisation temperatures of amorphous alloys

Abstract: The function fBA-which defines the degree by which an A atom is surrounded by neighbours of B atoms in Miedema's semi-empirical model and is used for the calculation of the formation enthalpy of crystalline alloys-is modified for amorphous alloys. For the five alloy systems considered, the fBA function turns out to be well described by fBA=cBs(1+5(cAscBs)2). This fBA also reflects the existence of chemical short-range order. In addition, Buschow's prediction for the crystallisation temperature of amorphous alloys is modified using this fBA, resulting in the relation Tx=5 Delta Hhs+275, where Delta Hhs represents the formation enthalpy of a hole of the same size as the smaller atom.

Crystallization of bisphenol a polycarbonate induced by supercritical carbon dioxide

Abstract: Supercritical carbon dioxide readily induces crystallization in bisphenol A polycarbonate. Crystallization begins within one h of exposure to the CO2 at temperatures and pressures as low as 75°C and 100 atm. The degree of crystallinity increases sharply as the CO2 pressure is raised from 100 to 300 atm but levels off thereafter. This behavior is likely due to a minimum in the Tg of the polycarbonate/CO2 mixture owing to the opposite effects of the pressure on the Tg of the polymer and on the equilibrium weight fraction of CO2 absorbed. Percent crystallinities of over 20%, comparable to that achieved using acetone or other organic liquids, have been obtained after 2 h exposure to supercritical CO2. Since polycarbonate degasses quickly and quantitatively at ambient temperature and pressure, the high Tg of bisphenol A polycarbonate can be regained in the crystallized material without further vacuum treatment.

Crystallization and Morphology of Poly(β-hydroxybutyrate) and Its Copolymer

Abstract: The morphology and structure of the bacterial plastics of poly(β-hydroxy-butyrate-β-hydroxyvalerate) P(HB-HV) were studied as well as the reference homopolymer, poly(β-hydroxybutyrate) (PHB). Because of bacterial origins they are exceptionally pure. Crystallization behavior of them is expected to be almost ideal, so that we can easily infer how the minor component (HV) acts within the major component (HB). The solution-grown single crystals of PHB and P(HB-HV) showed linear increase in melting points and hyperbolic increase in long spacings as the crystallization temperature increased. The HV component in the polymer chain was inclined to be excluded outwards as much as possible from the crystal of HB component during the crystallization. A small part of the HV component included in the HB component crystal acts as a defect, by which small expansion of the a-parameter of the unit cell is mainly caused with increasing HV content. The severe effect of the HV component was observed on crystallization characteristics and morphological changes.

The kinetics of nucleation and crystal growth and scaling laws for magmatic crystallization

Abstract: Magmatic crystallization depends on the kinetics of nucleation and crystal growth It occurs over a region of finite thickness called the crystallization interval, which moves into uncrystallized magma We present a dimensional analysis which allows a simple understanding of the crystallization characteristics We use scales for the rates of nucleation and crystal growth, denoted by I m and Y m respectively The crystallization time-scale τ c and length-scale d c are given by (Y 3 /I m )−1/4 and (κ·τ) m 1/2 respectively, where κ is thermal diffusivity The thickness of the crystallization interval is proportional to this length-scale The scale for crystal sizes is given by (Y m /I m )1/4 We use numerical calculations to derive dimensionless relationships between all the parameters of interest: position of the crystallization front versus time, thickness of the crystallization interval versus time, crystal size versus distance to the margin, temperature versus time We assess the sensitivity of the results to the form of the kinetic functions The form of the growth function has little influence on the crystallization behaviour, contrary to that of the nucleation function This shows that nucleation is the critical process In natural cases, magmatic crystallization proceeds in continously evolving conditions Local scaling laws apply, with time and size given by τ =(Y 3/I)−1/4 and R=(Y/I)1/4, where Y and I are the rates at which crystal are grown and nucleated locally τ is the time to achieve crystallization and R the mean crystal size We use these laws together with petrological observations to infer the in-situ values of the rates of nucleation and growth Two crystallization regimes are defined In the highly transient conditions prevailing at the margins of basaltic intrusions, undercoolings are high and the peak nucleation and growth rates must be close to 1cm−3· −1 and 10−7cm/s, in good agreement with laboratory measurements In quasi-equilibrium conditions prevailing in the interior of large intrusions, undercoolings are small We find ranges of 10−7 to 10−3 cm−3 s−1 and of 10−10 to 10−8cm/s for the local rates of nucleation and growth respectively

Composition and Structure of Ceramic Fibers Prepared from Polymer Precursors

Abstract: Ceramics fibers in the system Si-C-N-O prepared by pyrolysis of melt-spun, cured, amorphous organosilicon polymers have a predominantly amorphous chemical structure, consisting of carbon, nitrogen, and oxygen bonds to silicon. These amorphous silicon oxycarbonitrides have chemical bonding approach a random distribution: i.e. individual silicon atoms are simultaneously bonded to C, N, and O in a random fashion. Such structures have not been characterized previously. Excess C, present in all compositions studied, appears to be present as very small polyaromatic crystallites ({approx} 4 nm domain size in the aromatic plane) resembling pyrolytic carbon in structure. This carbon is highly resistant to oxidation. The amorphous, random structure of these ceramics is a consequence of small-scale elemental homogeneity in the amorphous polymer precursor and of insufficient thermal energy during pyrolysis to promote crystallization.

Differential scanning calorimetry study of solid‐state amorphization in multilayer thin‐film Ni/Zr

Abstract: Differential scanning calorimetry (DSC) has been used to study solid‐state amorphization and subsequent crystallization in sputtered multilayer Ni/Zr thin films Initial results provide quantitative information about the thermodynamics and kinetics of these processes An analysis of DSC data enables the activation energy and pre‐exponential factor for interdiffusion of Ni and Zr in a‐NiZr to be found

On the crystal texture of linear polyaryls (PEEK, PEK and PPS)

Abstract: Three linear ppolyaryls, polyetheretherketone (PEEK) polyetherketone (PEK) and polyphenylenesulphide (PPS) have been examined regarding crystal morphologies as obtained from solutions and molecular orientation in melt grown spherulites, the latter involving also experiments on the carbon fibre containing polymer. In addition to the individual features observed in solution grown crystals agreeing with those being reported concurrently from elsewhere, the present coordinated results on three polymers underline the common crystallization behaviour and texture for this family of polymers. These common features comprise the existence of lamellae as symptomatic of chain folding, acicular shapes indicative of uniaxial growth (b being the growth direction in all three cases), and irregular crystal edges. The sheaf development of these crystals then leads readily to a postulated radiolb-axis orientation in spherulites, confirmed directly in melt crystallized samples possessing transcrystalline textures induced by nucleating carbon fibres. Observations on the effect of carbon fibres drew our attention to the combined influence of the crystal nucleating effect and separation distance of these fibres on the resulting crystal texture of the polymer matrix: varying the relative magnitude of the two effects, can even reverse the overall crystal orientation in the sample. Such considerations should be pertinent to crystal texture development in composites with crystaallizable thermoplastic matrices in general.

Controlled Crystallization of the Amorphous Phase in Silicon Nitride Ceramics

Abstract: The controlled crystallization of amorphous second phases in SiAlON is demonstrated in two systems. In a magnesia-containing SiAlON, cordierite crystallized on annealing after hot-pressing. Similarly, garnet crystallized in yttria-contain-ing SiAlON. The resulting microstructures are characterized by TEM and qualitatively related to changes on room-temperature toughness and high-temperature deformation.

Structure and tensile behavior of irradiation-and peroxide-crosslinked polyethylenes

Abstract: Three grades of polyethylene differing mainly in their molecular weight were irradiation and peroxide crosslinked. Their gel content, degree of swelling, melting temperature, degree of crystallinity, and tensile properties were determined. The irradiation and peroxide crosslinking of the same polymers makes it possible to properly compare the effects of the two crosslinking methods. Upon irradiation the competition between crosslinking and chain scission reactions determines the level of the critical dose required to form the first gel and the magnitude of maximum attainable gel content. Crosslinking causes trapping of entanglements which then contribute to the effective network density determined by solvent swelling. crystalline polyethylene enhances the degree of crystallinity and crystal perfection while subsequent crystallization from the melt is hindered by the presence of crosslinks. The homogeneity of the crosslinked network, or distribution of crosslinks, depends on the crosslinking metho...

Crystallization studies of poly(ε‐caprolactone).I. Morphology and kinetics

Abstract: The crystallization behavior of three molecular weight samples of poly(e-caprolactone) has been studied as a function of temperature. Crystallization begins in the form of axialities and changes to spherulite growth as time progresses, presumably owing to the molecular weight distribution. Determinations of equilibrium melting point and analyses of growth kinetics are complicated by a major lamellar thickening process occurring at the crystallization temperature. Secondary nucleation analyses of spherulitic growth rates, carried out assuming a similar growth face to that of polyethylene, result in values of σσe. Use of the Thomas–Stavely relation to calculate a value of σ results in values of fold-surface free energy, σe, similar to that of polyethylene.