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  • Other Sources  (12)
  • 1
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    The undercooling of liquids (1984)
    In:  CASI
    Details
    Publication Date: 2006-06-08
    Description: The formation by melt quenching of such metastable structures as glassy or microcrystalline solids and highly supersaturated solutions is made possible by the extreme resistance of most melts to homophase crystal nucleation at deep undercooling. This nucleation resistance contrasts sharply with the very low kinetic resistance to the movement of crystal-melt interfaces, once formed, in metals and other fluid systems at even minute undercooling. The methods of nucleation study which have proven especially effective in bypassing nucleation by heterophase impurities thereby exposing the high resistance of melts to homophase nucleation may be summarized as follows: observation of the crystallization behavior of dispersed small droplets; drop tube experiments in which liquid drops solidify, under containerless conditions, during their fall in the tube; and observation of the crystallization of bulk specimens immersed in fluxes chosen to dissolve or otherwise deactivate (e.g., by wetting) heterophase nucleants. This method has proven to be remarkably effective in deactivating such nucleants in certain pure metals.
    Keywords: METALLIC MATERIALS
    Type: NASA. Lewis Research Center Fundamentals of Alloy Solidification Appl. to Industrial Processes; p 11-14
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  • 2
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    Dependence of growth rate of quartz in fused silica on pressure and impurity content (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The effects of pressure, temperature, and some variations in impurity content on the growth rate u of quartz into fused silica were measured. Under all conditions the growth rate was interface controlled and increased exponentially with pressure with an activation volume averaging -21.2 cu cm/mole. The activation enthalpy for all specimens is extrapolated to a zero pressure value of 64 kcal/mole, within the experimental uncertainty. At a given stoichiometry the effect of hydroxyl content on growth rate is described entirely by a linear term C(OH) in the prefactor of the equation for the growth rate. The effect of chlorine impurity can be described similarly. Also u is increased as the ideal stoichiometry is approached from the partially reduced state.
    Keywords: SOLID-STATE PHYSICS
    Type: Journal of Applied Physics; 51; Sept
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  • 3
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    Crystal nucleation in glass-forming alloy and pure metal melts under containerless and vibrationless conditions (1982)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The undercooling behavior of large spheroids of Pd40Ni40P40 was investigated. By surface etching, supporting the specimens on a fused silica substrate, and successive heating and cooling, crystallization can be eliminated, presumable due to the removal of surface heterogeneities. By this method samples up to 3.2g with a 0.53 mm minor diameter, were made entirely glassy, except for some superficial crystals comprising less than 0.5% of the volume. These experiments show that a cooling rate of approximately 1 K/sec is adequate to avoid copious homogeneous nucleation in the alloy, and that by eliminating or reducing the effectiveness of heterogeneous nucleation sites, it is possible to form bulk samples of this metallic glass with virtually unlimited dimensions.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-162075 , NAS 1.26:162075
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  • 4
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    The mechanism of growth of quartz crystals into fused silica (1980)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is proposed that the growth of quartz crystals into fused silica is effected by a mechanism involving the breaking of an Si-O bond and its association with an OH group, followed by cooperative motion of the nonbridging oxygen and the hydroxyl group which results in the crystallization of a row of several molecules along a crystalline-amorphous interfacial ledge. This mechanism explains, at least qualitatively, all the results of the earlier experimental study of the dependence of quartz crystal growth upon applied pressure: large negative activation volume; single activation enthalpy below Si-O bond energy; growth velocity constant in time, proportional to the hydroxyl and chlorine content, decreasing with increasing degree of reduction, and enhanced by nonhydrostatic stresses; lower pre-exponential for the synthetic than for the natural silica.
    Keywords: SOLID-STATE PHYSICS
    Type: Journal of Applied Physics; 51; Dec. 198
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  • 5
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    Undercooling of molten silicon (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Droplets of uncoated molten Si(0.4-0.8 mm diameter) have been undercooled 250 C. Ge droplets of similar size have been undercooled 280 C in a B2O3 flux. The observed nucleation onset temperatures of both Si and Ge droplets are at or below the predicted amorphous phase melting temperatures T(al). The solidified structures were polycrystalline. The nucleation frequency I, calculated from the Si data (20,000/cu cm s at 240 + or - 20 C undercooling), should be an upper limit of the homogeneous nucleation frequency of the crystal phase, since it has not been established that nucleation was homogeneous in these experiments. However, this limiting I for Si indicates that homogeneous nucleation of crystal would not become appreciable in laser pulsing experiments until the liquid Si is undercooled to well below T(al).
    Keywords: SOLID-STATE PHYSICS
    Type: Applied Physics Letters (ISSN 0003-6951); 46; 844
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  • 6
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    Crystal nucleation in glass-forming alloy and pure metal melts under containerless and vibrationless conditions (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Crystal nucleation behavior in metallic alloys known to form glasses in melt quenching was characterized and from this characterization the possibility that massive amounts of certain alloys could be slow cooled to the glass state was assessed. Crystal nucleation behavior of pure liquid metals was examined experimentally, under containerless conditions, and theoretically.
    Keywords: METALLIC MATERIALS
    Type: NASA-CR-161267
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  • 7
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    Crystal nucleation in Pd-Si alloys (1982)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd(84.5)Si(15.5) eutectic composition is reported. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. It is found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with a previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.
    Keywords: ASTRONAUTICS (GENERAL)
    Type: Materials processing in the reduced gravity environment of space; Annual Meeting; Nov 16, 1981 - Nov 18, 1981; Boston, MA
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  • 8
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    Negative pressures and melting point depression in oxide-coated liquid metal droplets (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: Negative pressures and melting point depression in oxide-coated liquid metal droplets are studied. The calculation presented show the existence of large negative pressures when the oxide coating is thick enough. The change in the melting point caused by these negative pressures should be considered in studies of homogeneous crystal nucleation. Furthermore, since the negative pressure raises the entropy of the melt, it increases the entropy loss at the crystal-melt interface; the resulting increase of the surface tension could have a large effect on the homogeneous nucleation frequency.
    Keywords: CHEMISTRY AND MATERIALS (GENERAL)
    Type: Scripta Metallurgica; 13; Feb. 197
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  • 9
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    Formation of bulk metallic glass by fluxing (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Bulk specimens (0.4-4 g mass) of the alloy Pd40Ni40P20 have been undercooled consistently to the glass state, with no detectable superficial crystallinity, in a molten flux of dehydrated boron oxide. The minimum dimension of the most massive glass specimen, so formed, was 1.0 cm. The absence of crystallinity in the specimens was confirmed by X-ray diffraction, scanning electron microscopy, and calorimetry.
    Keywords: SOLID-STATE PHYSICS
    Type: Applied Physics Letters (ISSN 0003-6951); 45; 615
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  • 10
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    Bulk formation of a metallic glass - Pd40Ni40P20 (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Molten spheroids of Pd40Ni40P20, of up to 0.53-cm minor diameter, were slowly cooled (1.4 K/s) on a fused silica surface under 10 to the -6 torr vacuum to a form which was entirely glassy except for some superficial crystallinity comprising less than 0.5% of the volume. The occurrence of crystallization was eliminated by subjecting the specimens to surface etching followed by a succession of heating and cooling cycles. The absence of crystallization in bulk was confirmed by X-ray diffraction, transmission electron microscopy, and calorimetry. Using the last technique, the heat of crystallization of the glass was measured to be 5.3 + or - 0.3 kJ/g atom.
    Keywords: SOLID-STATE PHYSICS
    Type: Applied Physics Letters; 41; Oct. 15
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