Showing papers on "Nucleation published in 1985"

Controlled Transformation and Sintering of a Boehmite Sol-Gel by α-Alumina Seeding

Abstract: The transformation, microstructural development, and densification of an α-alumina-seeded boehmite sol-gel was studied. α-Alumina particles are shown to act as nuclei for the trans- formation of θ- to α-alumina and to result in an increase in the transformation kinetics and lowering of the transformation temperature by as much as 170°C. By increasing the seed concentration (i.e., nucleation frequency), a submicrometer aggregate-free microstructure develops, rather than the vermicular microstructure that usually characterizes the α-alumina transformation. As a result, the transformed α-alumina sinters to full density with a submicrometer grain size at 1200°C. It is believed that seeding may represent a unique method for microstructure control in the many ceramic systems that transform by nucleation and growth.

Phase Stability Under Irradiation

Abstract: Irradiation of metals and alloys with neutrons, electrons, heavy ions, or γ-rays may introduce up to 108 J/mol of energy in the form of atomic displacements. This energy, which is in the form of vacancies, self-interstitials, and cores of displacement cascades is then available to produce a range of phase changes and microstructural alterations which are not observed under thermal conditions. There exist numerous mechanisms to convert part of this displacement energy into microstructural change, including irradiation-induced solute segregation, Frenkel pair recombination at the particle: matrix interface, irradiation disordering or amorphization, and recoil resolution of atoms from precipitates. In addition, the cores of displacement cascades may act as precipitate nucleation sites and Frenkel pair recombination may trigger spinodal-like instabilities. The theory of these mechanisms is developed in some detail, and is followed by a systematic review of experimentals observations of irradiation-altered phase stability. The observations include enhanced nucleation on displacement cascades, precipitation induced by solute segregation to defect sinks, nucleation of wrong phases, disordering and amorphization, Frenkel pair recombination driven precipitate, and inverse Ostwald ripening.

Kinetics of crystal nucleation in silicate glasses

Abstract: The kinetics of volume nucleation are analysed in a number of “simple” systems in which the crystallising phase has the same composition as the parent glass. The compositions considered are Li2O·2SiO2, Na2O·2CaO·3SiO2, BaO·2SiO2, 2Na2O·CaO·3SiO2, Na2O·SiO2, 3BaO·5SiO2 and CaO·SiO2. In the first three cases classical homogeneous nucleation theory provides a satisfactory description of both the temperature dependence and magnitude of the nucleation rate if the crystal-liquid interfacial energy σ is temperature dependent. All seven compositions showed striking similarities. The values of TMAX/Tm, where TMAX is the temperature of maximum nucleation and Tm the melting point (or effective melting point) were all in the range 0.54 to 0.59. TMAX was always at, or somewhat above, Tg. Values of Td/Tm where Td is the “just detectable” nucleation temperature (I = 106 m−3 s−1) were 0.62, 0.64 and 0.66 in the first three systems. These represent somewhat higher undercoolings for the onset of homogeneous nucleation than observed in “droplet” nucleation studies of non-silicate systems. However, values of the Turnbull α parameter calculated from the σ results were in general accord with droplet studies for non-metals. Values of W ∗ /kT (W ∗ is thermodynamic barrier to nucleation) in the various systems also showed close similarities being ∼ 30 armatTMAX remarkably consistent pattern of the results suggests that the nucleation observed is predominantly homogeneous. Reasons for the failure to observe homogeneous nucleation in other “simple” compositions are discussed.

The Homogeneous Nucleation Limits of Liquids

Abstract: This work provides a critical compilation of the homogeneous nucleation limits of liquids. Data for 90 pure substances and 28 mixtures have been compiled over a range of pressures, nucleation rates, and compositions. Detailed descriptions of the experimental methods used to obtain the included data are given to assess the accuracy of measured values. Criteria used to select the measurements included in the final listing are discussed.

Physical and functional repetition in a bacterial ice nucleation gene

Abstract: Nucleation of a physical process is distinct from catalysis, and as the function of a protein it is highly unusual. The ability to nucleate ice formation in supercooled water is a property of some members of the bacterial genera Erwinia, Pseudomonas and Xanthomonas1–3. This property is implicated in the ability of bacteria to cause frost injury to plants. Orser et al.4 have demonstrated that a single small region of DNA encodes this phenotype, and there is evidence that phosphatidylinositol is involved5. Biological ice nuclei are also found in the haemolymph of certain freeze-tolerant insects6. It is thought that an ice nucleation site must bind water molecules in an orderly array resembling an ice crystal. We have now determined the sequence of an ice nucleation gene from Pseudomonas syringae. It predicts a translation product whose structure is consistent with the role of template, containing 122 imperfect repeats of the consensus octapeptide Ala-Gly-Tyr-Gly-Ser-Thr-Leu-Thr. All the repeats are contiguous and higher-order periodicities are superimposed on the octapeptide pattern throughout. Deletion analysis showed that most regions of the repeating structure are not essential for function. Up to 68 octapeptides could be deleted without abolishing ice nucleation, but all deletions showed reduced activity. This indicates that the repeated peptides contribute individually to the nucleation process, as expected if they act as individual units of a water-binding array.

Metastability of Tetragonal Zirconia Powders

Abstract: The formation of metastable t-ZrO/sub 2/ by thermal treatment was studied in a systematic way on two different samples: a gel (G) and zirconyl acetate (A). The results show that initially nucleation of t-ZrO/sub 2/ is favored by creation of anionic vacancies with trapped electrons. At higher temperatures the electronic defects disappear, the crystallites grow, and the m-phase can nucleate. This evolution is observed perfectly on sample A because of its special morphological characteristics.

Experimental tests of the classical nucleation theory for glasses

Abstract: New data for crystal nucleation rates and viscosities were obtained for Li2O·2SiO2 and BaO·2SiO2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln (Iη/T) vs. 1/ΔG2T plots yielding straight lines over a wide range of temperatures (ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.

Simulation of Aerosol Size Distribution Evolution in Systems with Simultaneous Nucleation, Condensation, and Coagulation

Abstract: A sectional model is presented that will simulate formation, growth, and coagulation processes for an aerosol formed by gas-to-particle conversion. Test cases have simulated a system with a source of condensable vapor, showing a burst of nucleation that is quenched by condensation onto the freshly generated and rapidly growing fine aerosol. The influence of preexisting aerosol on the size distribution evolution and on the rate of nucleation and criteria for inhibition of nucleation by initial aerosol are presented.

Swelling of Hot-Pressed A12O3

Abstract: Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po2= 10-7 Pa). The reaction followed the relation p - po = -K log t, where po and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO2 and SO2 gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.

Molecular beam epitaxial growth of GaAs on Si(211)

Abstract: The molecular beam epitaxial growth of GaAs on Si(211) has been investigated. Theoretical considerations had suggested the (211) orientation to be particularly suitable for the nucleation and growth of a zincblende‐type compound semiconductor on a diamond‐type elemental one. The experimental results support the theoretical prediction. Morphologies of thin (≤0.1 μm) (211) layers are substantially better than for (100) layers, which nucleate poorly and require large layer thicknesses (≂1μm) to yield good morphologies. When the (211) layer growth is initiated with a thin (GaAs/Al, Ga)As superlattice buffer (0.1 μm), consisting of 10 periods of 5+5 nm, the (211) morphology rivals that of GaAs(100) homoepitaxial growth. Chemical etching studies as well as transmission electron microscope investigations show the layers to have the (211)B orientation and to be free of antiphase domains, both as predicted. The (211) layers show strong photoluminescence at 4 K. Not intentionally doped layers are n type, with elect...

Mechanisms of amorphization and recrystallization in ion implanted III–V compound semiconductors

Abstract: This review attempts to cover the mechanisms of the crystalline to amorphous transformation in ion implanted III–V compound semiconductors and silicon. Existing models for such a transformation are reviewed and validity of these models has been tested in view of the recent high resolution transmission electron microscopy results from damage or amorphous/crystalline interfaces. Drastic reduction in amorphization due to in situ annealing during ion implantation has been demonstrated. In the latter part of this review the mechanisms of atom migrations across the amorphous/crystalline interface, origin or the nucleation of dislocations, stacking faults, microtwins, precipitates, etc. on annealing of amorphous or damage regions in III–Vs and Si have been covered. Finally, correlations among the reported structural and electrical results have been discussed.

Nucleation of cavitons in strong langmuir turbulence.

Abstract: Plasmon wave packets are shown to be nucleated in narrow density holes which are the remnants of previous generations of ``burnt out'' cavitons. The proximity of the nucleation length scales to those of dissipation precludes the possibility of a self-similar inertial range for wide range of parameters. The nucleation mechanism may also arise in other contexts.

Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles

Abstract: Particle size effects on magnetic properties of BaFe12−2xTixCoxO19 fine particles were investigated. Saturation magnetization of these particles decreased with decreasing particle size. This result can be explained with the existence of a nonmagnetic thin layer, several angstroms in thickness, on the surface of the particles. Particle size dependence of coercivity for these particles, below 1000 A in size, follows the superparamagnetism theory. For the decrease in coercivity above 2000 A, multidomain nucleation is inferred as one of the possible mechanisms. Particle shape dependence of coercivity is well explained with the coherent rotation model, considering shape anisotropy.

Compaction and Sintering Phenomena in Titanium—Nickel Shape Memory Alloys

Abstract: Ti–Ni specimens of the equiatomic composition have been processed by conventional powder metallurgy techniques. The effects of compaction pressure, sintering temperature, and powder particle size on dimensional changes of sintered compacts are reported. During sintering, anisotropy of dimensional change occurs, with expansion in the radial and contraction in the axial direction. Densities decrease during sintering. From metallographic evidence it is suggested that these observations are connected with the difference in interdiffusion rates of Ti and Ni and the segregation of powder particles in the green compacts. The sintered compacts demonstrate a well defined shape memory behaviour, with martensite nucleation being enhanced by the presence of pores. PM/0358

Nucleation in emulsified supercooled water.

Abstract: A calorimetric technique was used to measure the nucleation rate of ice in supercooled ${\mathrm{H}}_{2}$O and ${\mathrm{D}}_{2}$O droplets in oil emulsions. The temperature dependence of the nucleation rate was determined for a wide range of temperatures and droplet volumes with use of emulsions stabilized by two different surfactants. The results are used to test quantitatively the theory of homogeneous nucleation for emulsified samples of water. The nucleation-rate measurements are also discussed in the context of theories which predict singularities in the thermodynamic functions of supercooled water.

Ice nucleation temperature of individual leaves in relation to population sizes of ice nucleation active bacteria and frost injury.

Abstract: Ice nucleation temperatures of individual leaves were determined by a tube nucleation test With this assay, a direct quantitative relationship was obtained between the temperatures at which ice nucleation occurred on individual oat (Avena sativa L) leaves and the population sizes of ice nucleation active (INA) bacteria present on those leaves In the absence of INA bacteria, nucleation of supercooled growth-chamber grown oat leaves did not occur until temperatures were below approximately -5 degrees C Both nucleation temperature and population size of INA bacteria were determined on the same individual, field-grown oat leaves Leaves with higher ice nucleation temperatures harbored larger populations of INA bacteria than did leaves with lower nucleation temperatures Log(10) mean populations of INA bacteria per leaf were 514 and 351 for leaves with nucleation temperatures of -25 degrees C and -30 degrees C, respectively Nucleation frequencies (the ratio of ice nuclei to viable cells) of INA bacteria on leaves were lognormally distributed Strains from two very different collections of Pseudomonas syringae and one of Erwinia herbicola were cultured on nutrient glycerol agar and tested for nucleation frequency at -5 degrees C Nucleation frequencies of these bacterial strains were also lognormally distributed within each of the three sets The tube nucleation test was used to determine the frequency with which individual leaves in an oat canopy harbored large populations of INA bacteria throughout the growing season This test also predicted relative frost hazard to tomato (Lycopersicon esculentum Mill) plants

Microstructure and magnetic properties of CoCr thin films formed on Ge layer

Abstract: To improve the c-axis oriented columnar growth, the nucleation of CoCr crystals on various underlayers which are formed on substrates prior to CoCr alloy deposition is studied. Microstructures of vacuum deposited CoCr alloy films are examined by transmission electron microscopy (TEM). It is found that an amorphous-like Ge is a suitable underlayer material to prepare highly oriented CoCr films. Cross-sectional TEM study indicates that the CoCr film formed on Ge layer consists of pillarlike crystals grown vertically throughout the film thickness. The CoCr film formed on Ge layer has a large perpendicular magnetic anisotropy. The read-write (R/W) characteristics have been markedly improved using the highly oriented CoCr film and a very high recording density of D 50 =230 kFCI is achieved, The role of the Ge layer on nucleation of CoCr crystal is discussed.