Nucleation Tendency Research Articles

Surface nucleation in cordierite glass

Kinetics and surface nucleation sites are discussed for surface nucleation of μ-cordierite crystals growing from differently prepared cordierite (2MgO2Al 2O 35SiO 2) glass surfaces. Surface nucleation of μ-cordierite starts from a limited number of nucleation sites which are rapidly consumed, causing a constant surface nucleation density of μ-cordierite, N, with respect to the thermal nucleation treatment. A distribution of crystal sizes occurs which allows a calculation of the consumption rate of these nucleation sites. This rate increases with temperatures where crystal growth occurs simultaneously. Full saturation is thus attained even for one-step thermal treatments at high temperatures. N varies from 10 −1 to 10 −8 μ −2 due to different surface preparations and ambient conditions. Pristine glass surfaces did not show nucleation tendency ( N ≈ 10 −8 μm −2). Mechanical damage increases N most efficiently ( Nup to ≈ 10 −1 μm −2). Convex surface tips or edges seem to be the most favored nucleation sites due to the easier elastic response of the melt to the formation of nuclei at these points. Dust particles can increase N causing a strong scatter of N on smooth surfaces (Δ N up to seven orders of magnitude). The nucleating efficiency of solid particles dusted on fractured glass surfaces seem to be correlated to their decomposition or melting temperature.

Some studies on thermal treatments of polyester fibers: Structural changes

The development of the changes of structure during thermal treatment of polyester fiber is studied. Critical dissolution time (CDT) was evaluated to trace the changes in crystallinity and crystal morphology of polyester fibers. CDT increased as the time of thermal treatment was increased. The change in CDT was less effective for samples thermally treated at temperatures up to 160°C, which may indicate a nucleation tendency of tiny crystals. CDT of polyester treated at 200°C in air was found to be relatively high, while silicon oil medium can affect both amorphous and crystalline regions, increasing the structure disorder. The effect of treatment under constant length revealed smaller CDT values than those attained by samples treated in free conditions. An indication to crystal size increase was traced by a decrease in peak width in X-ray diffraction pattern. Thermal treatment increased the crystallinity of polyester fibers up to 160°C. Dry air medium was more effective in increasing the disorder in amorphous regions than silicon oil medium.

Development of Surface Impurity Segregation during Dissolution of Aluminum

Caustic dissolution, when used as a pretreatment for etching of aluminum in chloride solutions, is observed to increase the rate of pit nucleation. Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy were used to measure the composition in the near‐surface region of 99.98% purity aluminum after dissolution in 1 N NaOH at room temperature. During dissolution, concentrations of impurities such as Fe, Cu, and Ga were found to accumulate continuously within a layer less than about 10 nm thick adjacent to the surface, because they dissolved more slowly than did aluminum. Impurity concentrations on the order of 1 atom percent (a/o) in this layer, much higher than equilbrium values, were found after 40 min dissolution. It is argued that the large impurity concentrations are consistent with a highly defective region in the metal near the metal/oxide interface, which has been detected using positron annihilation measurements. Dissolution produced a scalloped surface topography with typically 30 nm high ridges separated by 130 nm. A simulation of RBS measurements based on scattering from spherical particles was developed to test for the possibility of preferential impurity segregation to ridges. No such segregation was detected, suggesting that this is not the reason for the strong tendency for pit nucleation to occur on ridges, as has been observed previously.

Effects of urinary macromolecules on the nucleation of calcium oxalate in idiopathic stone formers and healthy controls

Urinary macromolecules have attracted great interest because of their possible role as both promoters and inhibitors of calcium oxalate (CaOx) crystallization and it remains unclear whether there is any difference, in their nucleating activity, between stone formers and controls. We selected 9 male idiopathic CaOx stone formers whose 24-h urines presented no evidence of common urinary stone risk factors such as hypercalciuria, hyperoxaluria, hyperuricosuria, hypocitraturia, hypomagnesiuria or low glycosaminoglycans excretion and 12 male controls (matched for age and body weight) whose 24-h urines did not differ from those of stone formers. The study of urinary CaOx nucleation was made in freshly voided overnight urines whose biochemical composition was almost identical in the two groups. In filtered (0.22 μm) and ultrafiltered (10 kDa) urine we performed an oxalate tolerance test to determine the permissible increment of oxalate, the oxalate level for nucleation and the permissible increment of CaOx relative supersaturation (CaOx RS). In filtered urine from stone formers the permissible increment of oxalate was lower than controls (30 ± 10.2 vs. 46.7 ± 9.7 mg/l, P = 0.001), the oxalate level for nucleation was lower (64.4 ± 14.2 vs. 79.5 ± 15.6 mg/l, P = 0.035) and the permissible increment of CaOx RS was also lower (9.71 ± 2.59 vs. 13.39 ± 3.62, P = 0.018). In ultrafiltered urine these differences disappeared because the removal of macromolecules in stone formers significantly enhanced the oxalate-tolerance values. The difference between the change of the oxalate permissible increment of filtered and ultrafiltered urine allowed a distinction to be made between stone formers and controls that was not feasible in other ways (7.6 ± 5.3 vs. 3.3 ± 5.9 mg/l, P < 0.0001). The study suggests that, in idiopathic CaOx stone formers free from common urinary risk factors of CaOx crystallization, there is an increased tendency for CaOx nucleation in urine, which is mediated by macromolecular components.

Selective Epitaxial Growth of Silicon by the A.C. Technique: I . Nonimplanted Substrate/Oxide Surfaces

Alternating cyclic, A.C., selective area epitaxial growth of silicon in the Si‐H‐Cl and Si‐H‐Cl‐Ar systems was carried out in a hot wall, low pressure epitaxial reactor, using patterned thermal oxide masks. The A.C. process is based on the existence of an embedded disproportionation reaction within the overall deposition chemistry, which provides an effective mechanism for preventing the formation of nuclei in the areas where deposition is not desired. This disproportionation chemistry is made dominant cyclically, by pulsing the hydrogen off and on periodically, in order to eliminate incipient nucleation. Experiments were conducted over a portion of the available parameter space, as determined by extensive thermodynamic analyses, using a reference non‐A.C. deposition process as a control, and comparing it with different A.C. frequencies. Thus, the temperature was varied from 750 to 1000°C, the pressure from 1.5 to 10 Torr, the ratio from 12.5/1 to 100/1, the ratio from 0 to 11. Total system flow rate was varied from 2.04 slpm to 24.04 slpm, with the argon flow rate accounting for 0 to 22 slpm of these total values. Oxide coverage was varied from 100% (worst case situation) to 0% (best case situation). Epitaxial thickness varied between 0.1 and 3.5 μm. It was found that the substrate topology, and various experimental conditions influenced, to varying degrees, the tendency for spurious nucleation in the masked areas. However, under all conditions, the A.C. technique prevented formation of spurious nuclei, guaranteeing essentially 100% selectivity control.

Selective Epitaxial Growth of Silicon by the A.C. Technique: II . Ion‐Implanted Substrate/Oxide Surfaces

Alternating cyclic, A.C., selective area epitaxial growth of silicon on thermal oxide masked, ion‐implanted substrates was studied in a hot wall, low pressure epitaxial system. Ion‐implanted surfaces are notoriously bad insofar as obtaining selective growth is concerned. Using the Si‐H‐Cl‐Ar system, the A.C. process employs an embedded disproportionation chemistry within the overall deposition process, which provides an effective mechanism for the prevention of the formation of spurious nuclei on dielectric masks. The disproportionation chemistry is made dominant cyclically, by pulsing the hydrogen off and on periodically during the deposition process, while maintaining the flow of the reducible source gas, , in order to prevent the formation of incipient nuclei. Experiments were conducted at 2.5 Torr, 950°C, , , with , , and , which were found in an earlier study to represent worst case conditions in terms of spurious nucleation. It was found that the heavier ion‐implantation dose, as well as increased deposition thickness, enhanced dramatically the tendency for spurious nucleation to occur on the masked regions. However, in all instances, the A.C. technique was found capable of preventing the formation of spurious nuclei on masked areas, guaranteeing essentially 100% selectivity control, even for epitaxial films as thick as 2.4 μm (which were the thickest grown).

Crystallization of poly(ethylene oxide) in binary blends containing poly( p-vinyl phenol)

Blends of poly(ethylene oxide) (PEO) with an amorphous polymer, poly( p-vinyl phenol) (PVPh), showed a single, composition-dependent, glass transition temperature as a consequence of the miscibility between components. X-ray diffraction and differential scanning calorimetry (d.s.c.) results indicated a strong reduction of blend crystallinity with increasing PVPh content. Observation of melting point depression allowed the determination of the interaction energy density, B, between the two polymers in the melt. To obtain equilibrium melting point data, the Hoffman—Weeks procedure was employed to take into account the influence of crystal morphology. The values of B obtained by d.s.c. and by thermal optical microscopy were −7.1 and −8.8 cal cm −3, respectively. Analysis of the isothermal crystallization by means of the Avrami equation led to average values of the Avrami index of 2.5 for both pure PEO and a PEO/PVPh 90/10 blend, and 3.1 for the remaining compositions analysed. The temperature and composition dependence of the growth rates were analysed using an expression incorporating both the diffusion mechanism and the nucleation tendency. A discontinuity in the temperature coefficient of the growth rate curve was found for PEO/PVPh blends with 90 and 80 wt% PEO. The experimental results indicate that the rate of crystallization, the degree of crystallinity and the equilibrium melting temperature are strongly dependent on the composition of the blends.

Direct measurements of latent heat during crystallization and melting of a ugandite and an olivine basalt

Step-scanning calorimetric measurements using a Setaram HT1500 calorimeter were performed between 800 and 1400°C on two natural samples: a ugandite from the East African rift and an olivine basalt from the western Mexican arc. Our measurements provide the first in-situ quantitative assessment of enthalpy during melting of initially crystalline natural samples. The distribution of latent heat across the liquidus-solidus intervals of the two samples is distinctly different, reflecting significant variation in the sequence and abundance of mineral phases during melting (clinopyroxene and leucite in the ugandite; olivine, clinopyroxene, and plagioclase in the basalt). Our data further indicate that the common assumption of a uniform distribution of latent heat across the liquidus-solidus interval of a magma is a reasonable approximation for the olivine basalt, but is grossly in error for the ugandite. This is due to cotectic precipitation of leucite and clinopyroxene, leading to a large, disproportionate release of latent heat early in the crystallization sequence. The implication for the thermal history of a crystallizing ugandite magma is that therate of heat loss during conductive cooling will unitially be more rapid than the average rate. The net result will be to produce lower magmatic temperatures after a given cooling interval relative to models assuming a uniform release of latent heat. An additional series of scanning calorimetric experiments were performed at variable rates (1,2 and 3°/min) to evaluate the role of kinetics on the distribution of enthalpy during both melting and crystallization of the ugandite and olivine basalt. The results indicate that clinopyroxene is the most important mineral phase in controlling the shapes of the enthalpy profiles during cooling; this is due to its large enthalpy of fusion and its tendency for sluggish nucleation, followed by rapid crystallization at temperatures that vary with cooling rate. The resolution of the calorimeter (in terms of heat detected per unit time) is also important in determining the shapes of theobserved enthalpy profiles during these rapid scans. Estimates based on the observed calorimetric signal associated with melting of olivine, and the lack of a calorimetric signal during melting of leucite, combined with known enthalpies of fusion for the two phases, indicate detection limits of approximately 0.6–1.2 kJ per 5 min increments.

The formation of aerosols from SO 2 and NH 3 in humid air

The formation of ammonium sulphate aerosols from SO2 and NH3 in humid air is a source of atmospheric aerosol production. In particular, the visible aerosol ('blue smoke') formed in the stack plumes of power plants with ammonia scrubbers has been ascribed to this reaction (see e.g. Moore, 1977). In the present study preliminary measurements of the formation of aerosols under conditions similar to those of flue gas ammonia scrubbers are presented. Rather surprisingly, they show that the tendency for homogeneous nucleation of aerosols in this system depends primarily upon an exposure of the gas to light in the near ultra-violet range. Several reactions in the gas may lead to the formation of solids. NH3 and SO2 alone can react to form NH3SO2 and (NH3)2SO2 which have been proposed as the primarily formed seeds of aerosol (Hartley and Matteson, 1975). However, the formation of these compounds is not possible at the conditions of the present study due to the chemical equilibria. In the presence of H20 the following reactions are relevant (Moore, 1977):

Rapid solidification of a droplet-processed stainless steel

Individual powder particles of a droplet-processed and rapidly solidified 303 stainless steel are characterized in terms of microstructure and composition variations within the solidification structure using scanning transmission electron microscopy (STEM). Fcc is found to be the crystallization phase in powder particles larger than about 70 micron diameter, and bcc is the crystallization phase in the smaller powder particles. An important difference in partitioning behavior between these two crystal structures of this alloy is found in that solute elements are more completely trapped in the bcc structures. Massive solidification of bcc structures is found to produce supersaturated solid solutions which are retained to ambient temperatures in the smallest powder particles. Calculated liquid-to-crystal nucleation temperatures for fcc and bcc show a tendency for bcc nucleation at the large liquid supercoolings which are likely to occur in smaller droplets. The importance of small droplet sizes in rapid solidification processes is stressed.

A Comparison of Parameters for the Machining of Gray Cast Iron

Abstract This investigation was designed to compare several types of data as parameters for predicting cutting speeds for gray iron. A series of eleven irons, representing ranges of alloys, ferrite, graphite, and mechanical properties was used in the study. Cutting forces, power, and energy were determined from drilling and milling tests, and cutting speed - tool life lines were determined for each of the irons. Finally, correlations were attempted between the cutting speed (V20) for a 20-min tool life, and all other available types of information. The net result of this approach was a comparison of the relative reliability of the different types of data. This comparison showed that combined carbon, microhardness, and the nucleation or graphitizing tendency hold the greatest promise of developing into reliable parameters, while Brinell hardness, tensile strength, drilling forces, and milling power show little promise of providing useful information. This study was preliminary in nature and was intended only as a screening process for narrowing down the field for future investigations. Consequently, the correlations between cutting speed and combined carbon, microhardness, and nucleation tendency are undeveloped, and applications should not be attempted without further investigation.