Showing papers on "Crystallization published in 1991"

Sparse matrix sampling: a screening method for crystallization of proteins

Abstract: A set of screening conditions for initial experiments in protein crystallization has been developed, tested, and is herein presented. These solution and precipitant conditions are empirically derived based on known or published crystallization conditions of various proteins in the past, so as to sample as large a range of buffer, pH, additive and precipitant variables as possible, using small amounts of proteins. The 50 crystallization conditions have been tested on 15 previously crystallized proteins, all of which were also crystallized in at least one form by this screen. This method is also shown to be highly successful in the crystallization of proteins which had not previously been crystallized.

Sonochemical synthesis of amorphous iron

Abstract: AMORPHOUS metallic alloys ('metallic glasses') lack long-range crystalline order and have unique electronic, magnetic and corrosion-resistant properties1–3. Their applications include use in power-transformer cores, magnetic storage media, cryothermometry and corrosion-resistant coatings. The production of metallic glasses is made difficult, however, by the extremely rapid cooling from the melt that is necessary to prevent crystallization. Cooling rates of about 105 to 107 K s−1 are generally required; for comparison, plunging red-hot steel into water produces cooling rates of only about 2,500 K s−1. Metallic glasses can be formed by splattering molten metal on a cold surface using techniques such as gun, roller or splat quenching4,5. Acoustic cavitation is known to induce extreme local heating in otherwise cold liquids, and to provide very rapid cooling rates6–11. Here we describe the synthesis of metallic-glass powders using the microscopically extreme (yet macroscopically mild) conditions induced by high-intensity ultrasound. The sonolysis of iron pentacarbonyl, a volatile organometallic compound, produces nearly pure amorphous iron. This amorphous iron powder is a highly active catalyst for the Fischer–Tropsch hydrogenation of carbon monoxide and for hydrogenolysis and dehydrogenation of saturated hydrocarbons.

Plasticizing Effect of Water on Thermal Behavior and Crystallization of Amorphous Food Models

Abstract: Dehydrated sugar solutions were used as models of thermal behavior of amorphous foods, and of the effect of temperature, moisture content and time on physical state of such foods. The transition temperatures determined were glass transition (Tg), crystallization (Tcr) and melting (Tm) which all decreased with increasing moisture. Tg of a sucrose/ fructose model had a slightly lower value than the empirical “sticky point,” at all moisture contents studied. Crystallization of sucrose was delayed by addition of fructose or starch. Crystallization above Tg was time-dependent, and the relaxation time of this process followed the WLF equation.

Magnetization process in nanocrystalline ferromagnets

Abstract: Annealing of initially amorphous FeCuNbSiB alloys at temperatures above their crystallization temperature leads to an ultrafine grain structure of α-FeSi with average grain sizes down to D ≈ 10 nm and random texture. Owing to the nanocrystalline structure the magneto-crystalline anisotropy is randomly averaged out by exchange interaction. Moreover magnetostriction decreases on crystallization owing to the formation of α-FeSi. As a consequence the material reveals excellent soft magnetic properties.

The Science of Crystallization: Microscopic Interfacial Phenomena

Abstract: This book, together with its companion volume The Science of Crystallization: Macroscopic Phenomena and Defect Generation, make up a complete course that will teach an advanced student how to understand and analyse scientifically any of the phenomena that are observed during natural or technological crystallization from any medium and via any technique of crystallization. It is an advanced text that goes into considerable detail concerning the many elements of knowledge needed to understand quantitatively a crystallization event. This particular volume deals with the important atomistic-level processes occurring at the interface between a crystal and its nutrient. It also provides the necessary scientific background of both thermodynamics and kinetics needed for the understanding of crystallization for both bulk crystals and thin film formation.

Dynamic light-scattering study of the glass transition in a colloidal suspension.

Abstract: This paper describes a light-scattering study of the glass transition in nonaqueous suspensions of sterically stabilized colloidal spheres. The observed phase behavior, fluid, crystal, and glass, is consistent with an essentially hard-sphere interaction between the particles. Metastable fluid states were obtained upon shear melting the crystalline phases by tumbling the samples. Their intermediate scattering functions, measured by dynamic light scattering, showed the emergence of a nondecaying component, implying structural arrest, at essentially the same concentration as that at which homogeneously nucleated crystallization was no longer observed. The overall forms of the intermediate scattering functions are consistent with the predictions of mode-coupling theories for the glass transition. Supplementary studies of the static structure factors indicated only short-ranged spatial order for particle concentrations ranging from the equilibrium fluid through the metastable fluid to the glass.

Tantalum‐based diffusion barriers in Si/Cu VLSI metallizations

Abstract: We have studied sputter-deposited Ta, Ta36Si14, and Ta36Si14N50 thin films as diffusion barriers between Cu overlayers and Si substrates. Electrical measurements on Si n + p shallow junction diodes demonstrate that a 180-nm-thick Ta film is not an effective diffusion barrier. For the standard test of 30-min annealing in vacuum applied in the present study, the Ta barrier fails after annealing at 500 °C. An amorphous Ta74Si26 thin film improves the performance by raising the failure temperature of a /Ta74Si26(100 nm)/Cu(500 nm) metallization to 650 °C. Unparalled results are obtained with an amorphous ternary Ta36Si14N50 thin film in the Si/Ta36Si14N50 (120 nm)/Cu(500 nm) and in the Si/TiSi2(30 nm)/Ta36SiN50 (80 nm)/Cu(500 nm) metallization that break down only after annealing at 900 °C. The failure is induced by a premature crystallization of the Ta36Si14N50 alloy (whose crystallization temperature exceeds 1000 °C) when in contact with copper.

Al induced crystallization of a‐Si

Abstract: The crystallization of amorphous Si induced by Al during heat treatment has been investigated by cross section and plan view transmission electron microscopy. The lowest temperature of Al induced crystallization of amorphous Si was found to be 440 K. The crystallization temperature, however, depends on the thickness of Al layers in layered structures and on the concentration of Al in co‐deposited layers below 1‐nm‐layer thickness and 15 at.% of Al concentration, respectively. Al‐induced crystallization in layered structures starts at the Al/amorphous Si interfaces and is located close to them. The amount of crystallized Si depends on the quantity of Al and on the temperature and increases with them. The mechanism of crystallization involves intermixing of Al with Si and the formation of an alloy of high metal concentration in the amorphous/crystalline interface. When the formation of this alloy is not assured due to low Al concentration, then crystallization does not start or the process of crystallization stops. In Al induced crystallization the nucleation of polycrystalline Si grains rather than their crystal growth is affected.

Spontaneous Resolution by Stirred Crystallization

Abstract: It does not take long for a beginning chemist to develop a naive trust in predictions based on probability. Unlike a physician, who deals with a handful of cases, or an epidemiologist, who deals with hundreds of thousands, a chemist has the luxury of dealing with samples that contain more than lo2’ molecules. For such large samples, predictions based on probability verge on certainty. Chemists are not disturbed that quantum mechanics precludes strict determinism in the behavior of an individual molecule, because it supplies reliable probabilities, and for macroscopic chemical samples molecular probability amounts to collective determinism. A chemist who has difficulty repeating a literature experiment may attribute the problem to unobserved or unreported differences in procedure or materials, or to error in measurement, but not to sampling error. The precise reproducibility from experiment to experiment that is the hallmark of good chemistry depends on having samples with enormous numbers of molecules. Because of their training, chemists are particularly intrigued by phenomena that seem improbable. Several such cases involve chirality. A familiar and profound example is the occurrence in nature of only one enantiomer of complex biological molecules. How did living systems become resolved? Does biological handedness result from some weak interaction that provided a tiny evolutionary bias in favor of one set of enantiomers, or from one single molecular event that occurred at random but was destined to become a sort of molecular “Adam” by being multiplied into the whole kingdom of living things? Crystallization provides a more homely example of curious statistics involving chirality. Although growth of a macroscopic crystal involves a large number of molecular events, formation of the original nucleus occurs but once per crystal. It is not uncommon for a solution to deposit only one crystal. Even if 100 or 1000 crystals grow, the number of nucleation events is small by chemical standards, and they differ from most chemical phenomena in that they can often be counted individually as well as measured collectively. Recently, Dilip Kondepudi et al. of Wake Forest University (USA) have reported results from counting chiral crystals that provide new insights into the mechanism of nucleation.“] Their interest in the role of competition and autocatalysis in breaking chiral symmetry in the biosphere,L2’ led Kondepudi et al. to repeat 19th Century work by Kipping and Pope on the crystallization of NaCIO, . I3] This salt is particularly well suited for such experimentation because aqueous solutions of its achiral ions produce beautiful rectangular prisms in the chiral cubic space group P2,3. Since the individual ions are achiral, they can join either enantiomeric

Crystallization method to improve crystal structure and size

Abstract: Impinging fluid jet streams are used in a continuous crystallization process to achieve high intensity micromixing of fluids so as to form a homogeneous composition prior to the start of nucleation. This process permits direct crystallization of high surface area particles of high purity and stability.

Sugar crystallization in food products

Abstract: A review of the recent literature on crystallization of the commercial sugars (fructose, glucose, lactose, and sucrose) is presented. Topics include: NUCLEATION--The formation of the crystalline phase from supersaturated solutions can occur by either a spontaneous or a forced nucleation mechanism. Recent work on the mechanisms, kinetics, and impact of both heterogeneous and secondary (contact) nucleation is discussed. GROWTH--Recent studies on the mechanisms and kinetics of crystal growth will be reviewed. This discussion includes work on the growth rate dispersion exhibited by these sugars. EFFECTS OF IMPURITIES AND ADDITIVES--The presence of impurities and additives (including mixed sugar systems) affects both the nucleation and growth steps. A discussion of the recent work in this area is included. Emphasis is placed on the relationship between these crystallization phenomena and the solution structure for comparison purposes.

A new method for synthesizing nanocrystalline alloys

Abstract: A new method to prepare nanocrystalline alloys was developed by means of crystallization from amorphous alloys. By using this method, a Ni-P alloy with 9 nm crystallites was synthesized. The structure and grain sizes of the alloy were examined by means of x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy. Specific heat capacity and thermal expansion coefficient of the nanocrystalline Ni-P alloy prepared by this method were found to be greater than those of the coarse-grained crystalline alloy by 12.3% and 56.2%, respectively. A new micromechanism for nanometer-sized crystallites formation is discussed.

Crystallization kinetics of random ethylene copolymers

Abstract: The influence on the crystallization of molecular weight at a fixed co-unit content as well as that of co-unit content at a fixed molecular weight was studied for hydrogenated polybutadienes anf for an ethylene-hexene copolymer. The most general features of the crystallization process are found to be very similar to those of homopolymers. Some important exceptions are found: the isotherms do not superpose one with the other; deviations from the Avrami equation occur at low levels of crystallinity; relatively low levels of crystallinity can be attained after long-time crystallization.

Role of mobile phases in the crystallization of polyethylene. Part 1. Metastability and lateral growth

Abstract: As part of a more comprehensive investigation of following crystallization of polyethylene isobarically and isothermally at preselected portions of the pressure (P ) and temperature (T ) phase diagram within the P range of 2-5 kbar and supercoolings (AT ) up to 10 "C, the present work is centered on formation, lateral growth (including measurement of growth rates), and melting of crystals. In the course of it the salient observation was made that, within the above specified P and AT range at least, all crystal growth occurs in the hexagonal phase, and only in this phase, irrespective of whether in the hexagonal (h) or orthorhombic (0) stability regime of the P-T phase diagram. In the latter case the h crystals represent a metastable form and transform into the stable o phase at some stage of growth, when, as now observed, all growth stops. The observed lower melting temperature of the h phase in the appropriate portion of the P-T phase diagram ("below" the triplet point) is consistent with all the above and introduces a newly recognized "nongrowth" region in the phase diagram. The lateral growth measurement of h crystal (the only crystals which are seen to grow) is readily interpretable by an activated growth mechanism when referred to the supercooling in the h form (as opposed to the o form, even when the latter is the stable one). Here the barrier attributable to nucleation is largely uneffected by P, but the preexponential (including transport) is retarded by increasing pressure. The implications of these findings for polymer crystallization are discussed, at this stage, in a provisional manner.

Epitactic Nucleation of Spinel in Aluminosilicate Gels and Its Effect on Mullite Crystallization

Abstract: Control over the structure of hybrid (colloidal + molecular) aluminosilicate gels was utilized to demonstrate that precursor chemistry has a direct and controllable effect on the ∼1000°C crystallization of spinel and mullite in molecular precursor systems. Synthesis or preparation conditions leading to the development of a cubic, transition alumina result in the epitactic nucleation of spinel at ∼1000°C in gels that otherwise crystallize directly to mullite at ∼1000°C. Thus, the preference for spinel nucleation in gels derived from solution precursor systems whose chemistries promote formation of transition alumina readily explains the reported inability to obtain substantial mullite yields at ∼1000°C. Isothermal transformation kinetics of colloidal and hybrid gels show that in the absence of direct mullite formation at ∼1000°C, the release of alumina from the spinel-type crystal structure becomes the rate-controlling step in the transformation. This necessitates higher temperatures for mullite formation and limits the kinetic enhancement possible with extrinsic increases in mullite nucleation frequency.

Monitoring Wall Paintings Affected by Soluble Salts

Abstract: The general features of the evolution of salt systems in walls is studied first, starting with the genesis of the ions. The behavior of the salt concentration is examined next under the following headings: transport, accumulation, solute concentration, precipitation, fractionation, and local concentration. How crystallization and hydration pressures build up and burst stone and plaster structures is investigated next. This is shown as a four-phase process, where a model is used that is applicable to all moderately porous materials. A separate aspect of decay consists in the crystallization of salt phases from a mixture of different solutes controlled by relative humidity. Here, observation, determination, and documentation of decay forms are reported as conducted on six churches in Switzerland, in a project known as N.R.P. 16, some heated continuously, some intermittently, and some unheated. The findings stress that only when accurate climate measurements are coordinated with in situ observations can a relationship be established and monitored among the variables of room climate, i.e. relative humidity and temperature, the crystallization of distinct salts, and the decay they produce. The last section is based on a 1986 integrated study of the decay and environmental conditions in the Convent Church at Mustair. It presents a methodological approach to monitoring wall paintings affected by salt weathering, and presents a program of concerted actions in short and reversible steps taken to reduce the frequency of salt crystallization.

Dynamic mechanical and calorimetric analysis of compression-molded PLLA of different molecular weights : effect of thermal treatments

Abstract: The effect of thermal treatment on compression-molded poly(L-lactic acid) (PLLA) has been investigated by means of viscosimetric molecular weight () determination, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). Starting from two initial molecular weight PLLAs, amorphous samples with different have been obtained due to degradation occurring during the molding. The crystallization capability of the materials after different thermal treatments has been determined as a function of the molecular weight, and their dynamic mechanical properties have been measured. Initially fully amorphous PLLA matrices attained very high degrees of crystallinity (up to 90%) following different annealing processes. Concomitantly, PLLA degrades due to thermal cleavage of the chains. This is an unavoidable effect that must be taken into consideration when defining the material processing and annealing conditions. Crystallization phenomena occurring in the material during the treatment are clearly documented by DMTA.

Dynamic mechanical measurement of crystallization-induced gelation in thermoplastic elastomeric poly(propylene)

Abstract: The thermoplastic elastomeric poly(propy1ene) (TPE-PP) of this study has a linear structure comprised of multiple blocks of stereoregular and stereoirregular sequences. Above a melting temperature of about 65 "C, the TPE-PP is a liquid. In our experiments, the samples were first melted by heating to a prescribed temperature Tp (75,100, or 150 "C) and then cooled to 40 "C and held there for observation of the crystallization. Upon this cooling into a supercooled state, TPE-PP undergoes a gradual liquid-solid transition as the stereoregular sequences crystallize and hence form physical network junctions. At the gel point of this physical gelation process, the TPE-PP exhibits the typical scaling behavior expressed in power law relaxation, G(t) = St-" with a prefactor S of about 2.0 MPa s" and a relaxation exponent n close to 0.13. S is constant, while the value of n decreases slightly with the degree of crystallinity at the gel point. The degree of crystallinity at the gel point depends on the thermal history. If T,, is too low (e.g., 75 "C), some of the microphase-separated solid-state structure persists into the melted state and the subsequent gelation is characterized by the absence of scaling behavior

Temperature dependence of structure and morphology of syndiotactic polypropylene and epitaxial relationships with isotactic polypropylene

Abstract: It is shown that the structure and morphology of syndiotactic polypropylene depend very sensitively and in a unique wanner on the temperature of crystallization. Moreover, as the temperature of crystallization is lowered, edge-on lamellae of isotactic polypropylene grow at the lateral sides of the syndiotactic laths through a specific epitaxial relationship

Nucleation and Crystallization of Na2O · 2CaO · 3SiO2 Glass by Differential Thermal Analysis

Abstract: DTA is presently used to characterize the nucleation and crystallization processes of the Na2O-2CaO-3SiO2 glass. A nucleation rate-temperaturelike curve is obtained by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, or the height of the crystallization peak, as a function of nucleation temperature. The nucleation-temperature range for this glass composition, 550-650 C, and the maximum nucleation temperature of 600 + or - 5 C, are found to be in excellent agreement with those associated with the classical nucleation technique, followed by isothermal crystallization. It is noted that when most of the nucleation occurs during the DTA measurements, a modified Kissinger equation must be used to calculate the crystallization energy.