Showing papers on "Grain size published in 1993"

Surface-based Fractional Transport Rates: Mobilization Thresholds and Partial Transport of a Sand-gravel Sediment

Abstract: Twenty-eight coupled observations of flow, transport, and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted, bimodal grain size distribution. These observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface. The key to our observations is the use of a sediment in which each size fraction has been painted a different color, which permits reliable, repeatable, and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface. At a given flow, the fractional transport rates may be divided into two parts: a finer-grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate, and a coarser-grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run. We define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength. Complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction. Zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution, although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed. Critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude, a result strongly in contrast to many earlier observations, but consistent with our observations of incipient motion in sediments with bimodal grain size distributions.

Role of grain size and particle velocity distribution in secondary electron emission in space plasmas

Abstract: By virtue of being generally immersed in a plasma environment, cosmic dust is necessarily electrically charged. The fact that secondary emission plays an important role in determining the equilibrium grain potential has long been recognized, but the fact that the grain size plays a crucial role in this equilibrium potential, when secondary emission is important, has not been widely appreciated. Using both conducting and insulating spherical grains of various sizes and also both Maxwellian and generalized Lorentzian plasmas (which are believed to represent certain space plasmas), we have made a detailed study of this problem. In general, we find that the secondary emission yield delta increases with decreasing size and becomes very large for grains whose dimensions are comparable to the primary electron penetration depth, such as in the case of the very small grains observed at comet Halley and inferred in the interstellar medium. Moreover, we observed that delta is larger for insulators and equilibrium potentials are generally more positive when the plasma has a broad non-Maxwellian tail. Interestingly, we find that for thermal energies that are expected in several cosmic regions, grains of different sizes can have opposite charge, the smaller ones being positive while the larger ones are negative. This may have important consequences for grain accretion in polydisperse dusty space plasmas.

Nanocrystalline soft magnetic materials

Abstract: Nanocrystalline structures offer a new opportunity for tailoring soft magnetic materials The most prominent example are devitrified glassy FeCuNbSiB alloys which reveal a homogeneous ultrafine grain structure of bcc-FeSi with grain sizes of typically 10-15 nm and random texture Owing to the small grain size the local magneto-crystalline anisotropy is randomly averaged out by exchange interaction so that there is only a small anisotropy net-effect on the magnetization process Moreover the structural phases present lead to low or vanishing saturation magnetostriction which minimizes magneto-elastic anisotropies Both the suppressed magnetocrystalline anisotropy and the low magnetostriction provide the basis for the superior soft magnetic properties comparable to those of permalloys or near zero-magnetostrictive Co-base amorphous alloys but at a higher saturation induction Like in other soft magnetic material the hysteresis loop can be tailored by uniaxial anisotropies induced by magnetic field annealing

Hall-Petch strengthening in nanocrystalline metals

Abstract: The tensile behavior and microhardness of nanocrystalline Cu and Pd were measured as a function of grain size In the case of Cu, an increase in strength with grain refinement continues down to the finest-grain material tested, whereas Pd shows little dependence of strength on grain size in the nanocrystalline regime Both nanocrystalline Cu and Pd are significantly stronger than conventional grain size material A literature search and experiments show that negative Hall-Petch slopes at ultrafine grain sizes observed in some studies are not associated with room temperature creep but rather with the use of a single sample annealed repeatedly to change the grain size

Relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks

Abstract: New microstructural data on experimentally deformed “wet” and “dry” natural olivine rocks (Anita Bay and Aheim dunite), together with the other reliable experimental data, indicate that the experimental stress-recrystallized grain size relationship in olivine-rocks is largely independent of water content and temperature, and is only slightly dependent on the flow properties of the material. The experimental data cover a stress range of 30–300 MPa, water contents from <30 ppm to 300 ppm, and temperatures in the range 1100–1650°C. Local melt contents of up to 10 volume% cannot be demonstrated to have a significant effect on the stress—grain size relationship.

Mechanically driven alloying and grain size changes in nanocrystalline Fe-Cu powders

Abstract: Highly supersaturated nanocrystalline FexCu100−x alloys (10≤x≤95) have been prepared by mechanical alloying of elemental crystalline powders The development of the microstructure is investigated by x‐ray diffraction, differential scanning calorimetry, and transmission electron microscopy The results are compared with data for ball‐milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films The deformation during milling reduces the grain size of the alloys to 6–20 nm The final grain size of the powders depends on the composition of the material Single‐phase fcc alloys with x≤60 and single‐phase bcc alloys with x≥80 are formed even though the Fe‐Cu system exhibits vanishingly small solid solubilities under equilibrium conditions For 60≤x≤80, fcc and bcc solid solutions coexist The alloy formation is discussed with respect to the thermodynamic conditions of the material The role of the large volume fraction of grain boundaries between the nanometer‐sized cryst

Critical Shear Stress of Natural Sediments

Abstract: The critical shear stress of individual fractions τci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture. For strongly bimodal sediments, τci increases with grain size, an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes τci to deviate from size independence in the direction of unisize (Shields) values. A quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of τci is observed and to predict the variation of τci with grain size in bimodal sediments. Because properties of sediment grain‐size distributions other than bimodality appear to have little influence on τci the trends presented here for 14 unimodal and bimodal sediments may be quite general.

Relationship between plagioclase crystallization and cooling rate in basaltic melts

Abstract: Rock textures commonly preserve a record of the near-surface crystallization history of volcanic rocks. Under conditions of simple cooling without convection or mixing, textures will reflect sample cooling rate, the temperature at which crystallization was initiated, and the distribution of mineral phase precipitation across the crystallization interval. Compilation of plagioclase size and number density data on natural (dike, sill and lava lake) and experimental samples suggests that (1) growth and nucleation rates of plagioclase in natural basaltic samples are a predictable function of cooling rate, and (2) the observed crystallization rate dependence on cooling rate is similar to that observed in experiments initiated at subliquidus temperatures. Comparison of natural and experimental samples thus suggests that most basalts crystallize under conditions of heterogeneous nucleation, with the number density of preexisting nucleii partially controlling textural responses to cooling rate changes. Time scales of crystallization and cooling in magmatic systems are intimately linked through a balance between heat removal from the system and heat evolved through crystallization. Evaluation of textural data in the context of recent numerical models of crystallization in simple (one- and two-component systems) provides new insight into regularities in the crystallization behavior of basaltic magmas. For example, the rate of change in crystal size (and number density, as dictated by mass balance) has been used as a measure of the relative importance of time scales of crystallization and cooling in numerical models of crystallizing systems. In natural samples, plagioclase size scales with the length scale of cooling such that a logarithmic plot of grain size as a function of normalized distance across the dike has a slope that appears approximately independent of dike width (solidification time). Comparison with available textural data for other phenocryst phases suggests that the same may be true for pyroxene and magnetite crystallization, with each phase having a characteristic slope probably controlled by the thermodynamic properties of the crystallizing phase. Measured crystal size distributions are unimodal and show maximum frequencies in the smaller size classes; distributions broaden and the grain size at peak frequency increases with increasing crystallization times (decreasing cooling rates). In contrast, partially crystallized Makaopuhi lava lake samples have crystal size distributions that decrease exponentially with increasing crystal size. Measured size distributions in dikes can be explained by late stage modification of Makaopuhi-type distributions through loss of small crystals, possibly the consequence of growth without nucleation. Finally, this compilation of the textural response of basaltic magmas to changes in cooling rate suggests that empirical calibrations of crystallization rate dependence on cooling rate from natural samples provide a reasonable model for plagioclase crystallization in near-surface basaltic systems. Predicted growth rates will be slow and relatively constant (10-10–10-11 cm/s) for crystallization times expected in most shallow volcanic systems (<1000 years).

Low temperature perovskite formation of lead zirconate titanate thin films by a seeding process

Abstract: A two-step seeding process has been developed to lower the transformation temperature and modify the grain structure of ferroelectric lead zirconate titanate (PZT) thin films with high Zr/Ti ratio. Previous study has shown that nucleation is the rate-limiting step for the perovskite formation. Therefore, any process that enhances the kinetics of nucleation is likely to decrease the transformation temperature. In this process, a very thin (45 nm) seeding layer of PbTiO3, which has a low effective activation energy for perovskite formation, was used to provide nucleation sites needed for the low temperature perovskite formation. In this study, we have shown that the pyrochlore-to-perovskite phase transformation temperature of PbZrxTi1−xO3 films of high Zr/Ti ratio (e.g., x = 53/47) can be lowered by as much as 100 °C. The grain size of these films can also be substantially modified by this two-step approach.

Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys

Abstract: X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.

The yield stress of polycrystalline thin films

Abstract: In recent experiments it has been shown that the yield stress of polycrystalline thin films depends separately on the film thickness and the grain size. It was also shown that the grain size dependence varies as the reciprocal of the grain size. In this paper an analysis is presented which leads to these results and provides a more detailed understanding of the origins of the observed behavior.

On graphite and the 2175 Å extinction profile

Abstract: Classical electromagnetic theory is used to study the constraints placed on interstellar graphite grains by the observations of Fitzpatrick & Massa (who found large variations in FWHM γ of the bump with minimal, and uncorrelated, variations in the central wavelength λ 0 ). Accurate calculations using the discrete dipole approximation are used to test the accuracy of the commonly used 1/3-2/3 approximation for graphite spheres. We show that the 1/3-2/3 approximation is sufficiently accurate for use in studying variations in the extinction profile due to changes in graphite grain size or coatings on the grains. We investigate the effect on the 2175 A extinction profile of (1) changes in size distribution, (2) changes in grain shape, and (3) coatings of ice or other material

Hydrothermal synthesis and sintering of ultrafine CeO2 powders

Abstract: Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of almost-equal-to 10-15 nm, were prepared under hydrothermal conditions of 10 MPa at 300-degrees-C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1-10-degrees-C/min up to 1350-degrees-C. The undoped CeO2 started to sinter at almost-equal-to 800-900-degrees-C and reached a maximum density of 0.95 of the theoretical at 1200-degrees-C, after which the density decreased slightly. Isothermal sintering at 1150-degrees-C produced a sample with a relative density of almost-equal-to 0.98 and an average grain size of almost-equal-to 100 nm. The samples sintered above 1200-degrees-C exhibited microcracking. The decrease in density and the microcracking above 1200-degrees-C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution Of O2 gas. Doping with Y2O3 produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO2. Sintered samples of the doped CeO2 showed no microcracks. The CeO2 doped with up to 3 mol % Y2O3 was sintered to almost full density and with a grain size of almost-equal-to 200 nm at 1400-degrees-C.

An investigation of ductility and microstructural evolution in an Al−3% Mg alloy with submicron grain size

Abstract: A submicrometer-grained (SMG) Al−3% Mg solid solution alloy, with an initial grain size of ∼0.2 μm, was produced by intense plastic straining. Experiments show that tensile specimens of the SMG alloy exhibit high elongations to failure at low testing strain rates at the relatively low temperature of 403 K. The stress exponent is high (∼7–8) and calculations show deformation is within the region of power-law breakdown. The initial microstructure of the alloy consists of diffuse boundaries between highly deformed grains. At strain rates of ∼10−4 s−1 and lower, plastic deformation leads to dynamic recrystallization and the formation of highly nonequilibrium grain boundaries that gradually evolve into a more equilibrated configuration.

Processing nanocrystalline ceramics for applications in superplasticity

Abstract: The production of nanocrystalline ceramics for subsequent use in superplastic forming operations requires that the ceramics be made in large quantities, with high densities, and under stringent grain growth control. To make large amounts of nanocrystalline starting powders, two popular wet chemical techniques (precipitation from salt solutions and alkoxide hydrolysis) can be used and are described in this paper. Unfortunately, pressureless sintering of these powders does not typically lead to the high densities and ultrafine grain sizes desired in the final product. Sintering data suggest that pore shrinkage occurs only when grains reach a critical size with respect to the pore size; thus, if the ceramic contains large pores, densification can require significant grain growth. Separation of large pores from grain boundaries may also occur and lead to incomplete densification, even at extremely large grain sizes. In all cases the pressureless sintering behavior of the nanocrystalline ceramics appears to adhere to well established theories used to explain the sintering of conventional, larger-grained ceramics. During both pressureless sintering and sinter-forging experiments, the grain size of a nanocrystalline ceramic is identical to the average spacing between open pores in the sample. Pressureless sintering results in the closure of these pinning pores by about 90% density and thus3leads to a substantial grain growth at densities greater than 90%. Sinter-forging, however, often allows one to maintain a stable population of small open pores (for pinning purposes) throughtout sintering, while preferentially eliminating the large pores which detract from the sample density. The deformation regime in which sinter-forging is performed has a decided effect on whether large pores or small pores are eliminated preferentially and, consequently, on whether a high density and fine grain size combination is achieved or not.

Effects of Austenite Grain Size on ε Martensitic Transformation in Fe-15mass%Mn Alloy

Abstract: The effect of austenite (γ) grain size on the morphology of e martensite (e) and the transformation from γ to e has been investigated by means of optical microscopy, transmission electron microscopy and X-ray analysis in Fe-15 mass% Mn alloy, whose γ grain size was controlled between 1 and 130 μm by the reversion treatment of deformation induced bcc martensite to γ. With refining γ grain size, the formation of e tends to be suppressed and the starting temperature of γ-e transformation is also lowered. In the grain size range below 30 μm, the transformation is markedly suppressed. In small γ grains below 30 μm, one variant of e plates go through a γ grain from one grain boundary to the other of the opposite side

Sintering kinetics and microstructure development of nanoscale Y-TZP ceramics

Abstract: Zirconia samples doped with 3 mol% yttria were prepared by gel precipitation from a metal chloride solution and their sintering behaviour compared with that of a commercial powder. Dense (relative density 97%) nanoscale ceramics with a mean grain size of 60 nm are obtained after sintering at 1050°C for 7h. Important densification mechanisms in the initial sintering stage are grain boundary sliding and grain boundary diffusion. Grain growth in the final sintering stage seems to be impurity drag controlled. Extremely low activation energies are obtained for both densification and grain growth in the initial sintering stages. Special attention has been paid to the effect of aggregate size of the precursor powder on the final grain size.

Statistical Properties of Residual Stresses and Intergranular Fracture in Ceramic Materials

Abstract: The problem addressed in this paper concerns the statistical characterization of the state of residual stress generated in polycrystalline ceramics during cooling from the fabrication temperature. Detailed finite element simulations are carried out for an ensemble of large numbers of randomly oriented, planar hexagonal grains with elastic and thermal expansion anisotropy, and brittle grain interfaces. The calculations show that the distribution of normal and shear tractions induced by thermal contraction mismatch among grains is gaussian and that these tractions are statistically independent random variables. Although the gaussian nature of the distributions remains unaffected by the introduction of elastic anisotropy, the results indicate that elastic anisotropy has a significant effect on the residual stresses for finite departures from isotropy. When the hexagonal grains are randomly distorted, the magnitude and distribution of residual stresses are found to be insignificantly altered. Spontaneous microfracture due to the generation of internal stresses is also simulated in the analysis by allowing for the nucleation and growth of intergranular microcracks when the fracture energy along the grain facets exceeds a certain critical value. When such microcracking is incorporated into the computation, the levels of residual stress are markedly reduced as a consequence of stress dissipation. The dependence of intergranular microcracking on grain size and temperature variation is examined and the predicted trends on material degradation or the complete suppression of microfracture are discussed in the light of available experimental results.

A frictional-wear mechanism for fatigue-crack growth in grain bridging ceramics

Abstract: A frictional-wear mechanism to account for cyclic fatigue degradation in polycrystalline ceramics exhibiting grain bridging as the principal toughening mechanism is contemplated. The mechanism is based on repetitive sliding wear degradation of frictional grain bridges which reduces the toughening capacity from bridging under cyclic loads. A micromechanical model is developed to relate the reduction in the frictional pullout stress of bridging grains to material removed by wear processes at the grain/matrix interface. The model provides an insight into the effects of salient mechanical and microstructural variables such as wear rate at the grain/matrix interface, initial residual stress of bridging grains, and the grain size, on fatigue-crack growth behavior. Results of the analysis are compared to experimentally observed fatigue-crack growth rates in coarse grained alumina and silicon nitride and implications of the predicted crack-growth behavior discussed.

Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method

Abstract: Dramatic lateral grain growth of nondoped poly-Si films (maximum grain size: ~4.5 µm, film thickness: 500 A) with strong crystallographic (111) orientation on glass substrates has been achieved using an excimer laser annealing method, namely at low temperature below 400°C and in a processing time shorter than a second, for the first time. The surface morphology of these poly-Si films was very smooth and the crystallinity was excellent with minimal internal defects. These poly-Si films have monomodally distributed grain sizes, with an average grain size of around 1.5 µm. As a result of experimental study, we speculate that the basic driving force behind this lateral grain growth was surface free energy anisotropy, as the same mechanism was observed in high-temperature furnace annealing of doped poly-Si thin films.

Effects of Pore Morphology and Grain Size on the Dielectric Properties and Tetragonal–Cubic Phase Transition of High‐Purity Barium Titanate

Abstract: The effects of pore morphology and grain size on the dielectric behavior of high-purity stoichiometric BaTiO3 have been intensively investigated. It was found that the dielectric constant was influenced not only by grain size but also by pore morphology. Dielectric constants below the Curie temperature could be evaluated by the Maxwell relationship for specimens with fractional density >90%ρt and be estimated by the modified Niesel's equation, but depolarization might be involved for specimens with fractional density 6100 at 25°C and 1 kHz) and a low dissipation factor (<0.025) could be achieved.

Raman scattering from PbTiO3 thin films prepared on silicon substrates by radio frequency sputtering and thermal treatment

Abstract: Raman scattering has been carried out on PbTiO3 thin films prepared on platinum‐coated (100) silicon by radio‐frequency (rf)‐magnetron sputtering without substrate heating and a post‐deposition thermal treatment. The Raman spectra obtained from the thin film are characteristic of powder Raman spectra: In comparison with the single crystal spectra, the intensity of the background is relatively high at low frequencies and the Raman lines are broad. The lattice phonon modes corresponding to the observed lines are identified by comparison with the data on single crystals and powder. The Raman frequencies for the thin film remarkably shift to low frequencies compared with single‐crystal data. It is shown that the phenomenon of the frequency shifts is similar with the hydrostatic pressure effect on single crystals of PbTiO3. The result indicates that the thin films are composed of grains that are stressed depending on the grain size by neighboring grains of different orientations when they are split up into ferroelectric domains at the paraelectric‐to‐ferroelectric transition. This stress effect is significant even for a grain size of ∼0.5 μm. It is found that the lowest frequency E transverse optical (TO) mode in the thin film shows softening with increasing temperature as was reported in previous studies on single crystals.

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

Abstract: The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr0.2Ba0.8)TiO3 materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al2O3–SiO2–TiO2 (AST). The ultimate PTCR properties with ρht/ρrt as great as 105.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO3 is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.