Showing papers on "Crystallization published in 1983"

Preparation of ‘‘amorphous’’ Ni60Nb40 by mechanical alloying

Abstract: ‘‘Amorphous’’ Ni60Nb40 has been prepared by mechanical alloying of elemental nickel and niobium powders in a laboratory ball mill in controlled environments. X‐ray diffraction was used to follow the progress of the mechanical alloying which eventually produced ‘‘amorphous’’ diffraction patterns similar to those for liquid quenched amorphous Ni60Nb40. Crystallization behavior was measured by differential scanning calorimetry for the mechanically alloyed and liquid quenched material. The differences that were observed in this behavior, and in the products of crystallization, may be attributed to impurities (especially oxygen) introduced during mechanical alloying.

Applications of thermoanalytical techniques to the study of crystallization kinetics in glass-forming liquids, part I: Theory☆

Abstract: A critical review is presented of mathematical methods advanced over the last 20 years for the analysis of data obtained from non-isothermal thermoanalytical studies of crystallization of glass-forming liquids. Methods proposed by Ozawa, Piloyan and Borchardt, Coats, Redfern and Sestak, Ozawa and Chen, Takhor, Kissinger, and Augis and Bennett are examined in detail. All these methods are based on the Avrami treatment of transformation kinetics and define an effective crystallization rate coefficient having an Arrhenian temperature dependence. Several different ways of mathematically treating the data have been proposed. Most are shown to be based on an incorrect neglect of the temperature dependence of the rate coefficient. By taking proper account of the temperature dependence of the crystallization rate coefficient, all the methods are shown to lead to similar conclusions. In detail, it is shown that the effective activation energy of the overall crystallization process can be calculated from the slope of the line obtained by plotting 1n[ Q /( T P − T 0 )] versus 1/ T P is the temperature of maximum crystallization rate, T O is an initial temperature and Q is the heating rate employed in the experiment. It is further argued that in general the overall crystallization rate coefficient is not Arrhenian in character. Thus, in general, non-isothermal transformation cannot be treated analytically. A detailed description of nonisothermal transformation can, however, be obtained by numerical methods. Such a method is described and its uses in obtaining kinetic data from thermoanalytical studies are demonstrated.

Heat of crystallization and melting point of amorphous silicon

Abstract: Thin layers of amorphous silicon (a‐Si) were produced by noble gas ion implantation of (100) substrates held at 77 K. Rutherford backscattering and channeling, and differential scanning calorimetry were used to measure the heat of crystallization, ΔHac, to be 11.9±0.7 kJ/mol, substantially less than the value predicted by scaling ΔHac of a‐Ge. The crystal growth velocity is found to have the form v=v0 exp(−2.24 eV/kT). We obtain a new estimate, 1420 K, for the melting temperature of a‐Si.

Electron microscopy of frozen biological suspensions

Abstract: The methodology for preparing specimens in the frozen, hydrated state has been assessed using crystals and T4 bacteriophages. The methods have also been demonstrated with lambda bacteriophages, purple membrane of Halobacterium halobium and fibres of DNA. For particles dispersed in an aqueous environment, it is shown that optimum structural preservation is obtained from a thin, quench-frozen film with the bulk aqueous medium in the vitreous state. Crystallization of the bulk water may result in solute segregation and expulsion of the specimen from the film. Contrast measurements can be used to follow directly the state of hydration of a specimen during transition from the fully hydrated to the freeze-dried state and permit direct measurement of the water content of the specimen. By changing the concentration and composition of the aqueous medium the contrast of particles in a vitreous film can be controlled and any state of negative, positive or zero contrast may be obtained. At 100 K, frozen-hydrated, freeze-dried or sugar embedded crystals can withstand a three- to four-fold increase in electron exposure for the same damage when compared with similar sugar-embedded or freeze-dried samples at room temperature.

Structural difference rule for amorphous alloy formation by ion mixing

Abstract: We formulate a rule which establishes a sufficient condition that an amorphous binary alloy will be formed by ion mixing of multilayered samples when the two constituent metals are of different crystalline structure, regardless of their atomic sizes and electronegativities. The rule is supported by the experimental results we have obtained on six selected binary metal systems, as well as by the previous data reported in the literature. The amorphization mechanism is discussed in terms of the competition between two different structures resulting in frustration of the crystallization process.

Crystallization characteristics of Ni‐Zr metallic glasses from Ni20Zr80 to Ni70Zr30

Abstract: We present the results of a crystallization study in glassy Ni‐Zr. The alloys were prepared by melt spinning over the continuous range Ni20Zr80 to Ni70Zr30 including for the first time the region between Ni40Zr60 and Ni60Zr40. As expected we find that the glasses are most stable at and around the eutectic compositions, except that at the extreme Zr‐rich eutectic the glasses are destabilized by the premature precipitation of ω‐Zr. The products of crystallization indicate that the current crystalline phase diagram is incomplete: a new peritectoid phase must be added at the composition Ni2Zr.

Vanadium pentoxide gels

Abstract: Vanadium pentoxide gels consist of entangled polymeric fibers. Electron diffraction experiments have shown that these fibers are actually arranged as flat ribbons about 103 A long, 102 A wide, and 10 A thick. A two-dimensional order is observed along these ribbons which does not vary upon swelling. The two-dimensional cell parameters a = 27.0 A and b = 3.6A suggest that the organization within the fibers is closely related to the lamellar structure of orthorhombic V2O5. The fiber is made of basic blocks containing 10 vanadium atoms along the a direction. These blocks seem to be linked together by water molecules which are strongly bonded to the structure and cannot be removed without crystallization of the xerogel into orthorhombic V2O5.

Kinetics of Crystallization of ZrF4‐Ba2‐LaF3 Glass by Differential Scanning Calorimetry

Abstract: The kinetics of devitrification in ZrF4-BaF2-LaF3 (62–33–5 mol %) glass were studied by isothermal and nonisothermal methods using differential scanning calorimetry. The crystallization reaction followed the Avrami equation, x=1-exp[—(kt)], where x is the fraction crystallized after time t. The rate con stant k obeyed an Arrhenius equation, k(s−1)=6.69 × 1021 exp The value of the Avrami exponent, n, bas 3.2×0.2, indicating three-dimensional crystal growth at a constant number of nucleation sites. Values of kinetics parame ters obtained from isothermal and nonisothermal techniques were in excellent agreement. The heat of crystallization measured from isothermal peaks was computed to be 38.5×3.5 J/g.

The influence of morphology and molecular weight on ductile-brittle transitions in linear polyethylene

Abstract: The tensile behaviour of linear polyethylene was examined over a wide range of temperatures. Samples were prepared from low and medium molecular weight polymer with different morphologies, by varying the initial crystallization conditions. It was found that the temperature of the ductile-brittle transition was markedly different for different samples. In particular, low molecular weight polymer crystallized at a low degree of of supercooling, showed brittle behaviour over most of the temperature range, with a ductile-brittle transition near to room temperature. Rapidly quenched material, where the degree of supercooling is high, showed a very low ductile-brittle transition temperature, especially in high molecular weight polymer. The reasons for these differences in behaviour are discussed both at a phenomenological level and in terms of known structural differences between the different materials examined.

Epitaxial crystallization of polymers onto benzoic acid: Polyethylene and paraffins, aliphatic polyesters, and polyamides

Abstract: Epitaxial crystallization of polyethylene, n-paraffins, aliphatic polyesters, and various polyamides has been achieved on benzoic acid crystal substrates, specifically on their (001) crystals faces, which are made up of the aromatic rings. The epitaxial relationship is very similar for all polymers investigated: it is characterized by a unique chain orientation (parallel to the b axis of the substrate) and a plane of contact which is either the b-c, plane of polyethylene, or crystallographically similar ones for polyesters, or the plane of the hydrogen-bonded sheet for polyamides. Since benzoic acid is structurally similar to its alkali-metal salts, it is inferred that the nucleating efficiency of the latter toward chemically different but structurally similar polymers may well rest on an epitaxial relationship. Finally, the highly oriented morphology made it possible, for some polyesters with a monoclinic cell, to determine the angle between the chain axis and the Z axis of the optical indicatrix, a quantity not measured so far in polymers.

Thermal behaviour of carbohydrates studied by heat flow calorimetry

Abstract: The technique of heat flow calorimetry was used to study the thermal behaviour of different carbohydrates between 20°C and 270°C. The samples were analyzed by heating in sealed cells. The temperature range in which exothermic reactions, due to thermal decomposition, occurred varied widely depending on the type of carbohydrate investigated. Reaction enthalpies of 44 sugars and polysaccharides are given. Endothermic phenomena, such as fusion or vaporization of crystallized water, were also observed: fusion temperatures and enthalpies of 34 sugars and sugar alcohols are listed. Calorimetric curves showing crystallization of amorphous sucrose, cellobiose and lactose are also presented.

Kinetic study of isothermal and continuous heating crystallization in GeSe2GeTeSb2Te3 alloy glasses

Abstract: Calorimetric studies of chalogenide alloy glasses, (GeSe2)60(GeTe)40 − x(Sb2Te3)x obtained by water quenching at room temperature, have been performed by isothermal and continuous heating in a Perkin-Elmer DSC-2 calorimeter. The samples show a simple peaked crystallization exotherm when x = 0, 10 and two stage crystallization for x = 20. The results obtained in both iso- and non-iso-thermal experiments have been successfully interpreted with the same set of kinetic parameters. Among the several kinetic models which we analysed, the best fit to our experimental data has been obtained in the case of the Johnson-Mehl-Avrami-Erofe'ev equation, with values of the kinetic exponent dependent only on composition. The effective activation energy decreases slowly from E = (229 ± 6)kJ/mol at x = 0 to E = (184 ± 5)kJ/mol at x = 20 (first crystallization peak).

Influence of nucleating agents on crystallization of polypropylene

Abstract: The effect of talc as an artificial nucleating agent in different concentrations on the crystallization of polypropylene (PP) has been studied. It is considered that the induction time should be taken into account in the Avrami evaluation of isothermal crystallization. From a study of nucleated PP samples prepared in different ways, it has been proved that the nucleating effect of talc slightly decreases with increasing time spent by the sample in the state of the polymer melt. It has been shown that crystallization of non-nucleated polypropylene strongly depends on the material of the sample pan. It has been established that dilatometry cannot be used to study the isothermal crystallization of nucleated polypropylene, since microcracks appear in the sample, partly compensating the volume decrease due to the crystallization process.

Isothermal and nonisothermal crystallization of polyethylene

Abstract: Differential scanning calorimetry is employed to study the crystallization kinetics of two commercial injection-molding high-density polyethylene resins. Also, polarized light microscopy has been used to elucidate the morphological structures formed at various crystallization conditions. Corrections and operating procedures are recommended in order to correct for temperature lag in scanning, transient response during isothermal crystallization and for thermal gradients within the polymer sample. As a result of these studies, a modified Avrami equation has been proposed in order to obtain a more extensive and reliable characterization of isothermal crystallization kinetics. Moreover, a procedure is recommended and employed to predict nonisothermal crystallization behavior on the basis of isothermal crystallization kinetics.

Low pressure mineral-melt equilibria in natural anhydrous mafic systems

Abstract: Low pressure phase equilibria for natural mafic systems may be calculated utilizing empirically derived single component distribution coefficients and the constraints placed upon mineral-melt equilibria by stoichiometry and mass balance, without any knowledge of thermochemical properties of melts (Langmuir and Hanson 1981). Variations in distribution coefficients caused by differences in melt composition may be largely eliminated by the application of a two-lattice melt model based upon the models of Nielsen and Drake (1979), Drake (1976b), and Bottinga and Weill (1972). In the two-lattice melt model, the melt is assumed to be made up of two independent quasi-lattices, the network formers, composed of the components SiO2, NaAlO2, and KAlO2, and the network modifiers, composed of the free oxides of Ca, Mg, Fe, Al, Ti, and Cr. Compositionally independent mineral-melt distribution coefficients were calculated for all the major and some minor components in olivine, plagioclase, high-Ca pyroxene, low-Ca pyroxene, spinel, and ilmenite. Regression constants were calculated from data from 1 atmosphere, anhydrous, equilibrium experiments on natural and synthetic mafic compositions including data obtained from the literature and from new synthetic spinel-melt experiments in the Fo-Ab-An system doped with Ti, Mn, Ni, and Fe. The distribution coefficients are internally consistent within 3 mole % (1σ) for all components. The effects of variable oxygen fugacity on mafic mineral-melt equilibria were calculated utilizing the relations of Sack et al. (1980) for the determination of the Fe+3/Fe+2 ratio, and the results of Schreiber and Haskin (1976) for Cr+3/Cr+2. A computer program, EQUIL.FOR, incorporating the derived mineral-melt distribution coefficients, calculates the sequence of mineral and melt compositions, and liquidus temperatures for mafic melt compositions undergoing equilibrium or fractional crystallization. This program reproduces, within 15° C, 52 experimentally determined crystallization sequences collected from the literature. These published crystallization sequences are for mafic systems not represented in the data set used to calculate the mineral-melt distribution coefficients.

6 – Shell Formation

Abstract: I. Summary and Perspective The formation of shell can be described in terms of two major phases: (1) cellular processes of ion transport, protein synthesis, and secretion and (2) a series of physicochemical processes in which crystals of CaCO3 are nucleated, oriented, and grow in intimate association with a secreted organic matrix. In providing the mineral of shell, Ca2+ and HCO3− are first transported across epithelia at the body surface and the mantle epithelium facing the inner shell surface. Movement of these ions is incompletely understood but almost certainly involves active transport of Ca2+. The physicochemical phase takes place in fluid between the outer mantle epithelium and the inner shell surface and on this shell surface. For crystals to be deposited, the following conditions are required (1) concentrations of Ca2+ and CO32- exceeding the solubility product, (2) conditions favoring crystal nucleation, and (3) the elimination of H+ resulting from CaCO3 formation. Following crystal nucleation oriented crystal growth leads to a shell constructed of a variety of crystal patterns. In the process protein secreted by the mantle surrounds the individual crystals and becomes the cement which binds them together as a shell. Our understanding of the processes of the crystal depositional phase of shell formation is fragmentary. However, we can expect that through analyses of the fluid at the site of crystallization and study of effects of organic fractions of the fluid and shell on crystallization, the conditions governing the initiation and control of crystal growth will be further defined. Substances that initiate crystallization have been proposed but have not been tested. Preliminary studies indicate that the compounds at the crystallization site include inhibitors of CaCO3 deposition. A major aspect of shell construction of great interest yet to receive experimental study is the relation of particular segments of the mantle epithelium to the structure and orientation of strikingly different types of crystal patterns. A promising approach to these crystallographic problems appears to be the application of in vitro methods of physical biochemistry to crystal development and orientation in association with fractions of the organic phase of shell. Studies indicating involvement of the nervous system and hormonal and neurosecretory changes associated with shell formation will almost certainly be expanded and contribute to the understanding of mechanisms of mineralization and their control.

High quality polysilicon by amorphous low pressure chemical vapor deposition

Abstract: Undoped and in situ phosphorus‐doped low pressure chemical vapor deposited polysilicon films have been studied by various structural analysis and optical techniques as a function of deposition temperature. Polysilicon films of high quality can only be obtained by deposition in the amorphous phase and subsequent crystallization.