Equilibrium Grain Research Articles

Effect of Hot Isostatic Pressing on the Elastic and Optical Properties of Polycrystalline CVD ZnS

Hot isostatic pressing (HIP) is shown to increase the Young's modulus and transmittance of polycrystalline zinc sulfide produced by chemical vapor deposition. Both effects are associated with the decrease in the relative volume of nonequilibrium crystallite boundaries containing dangling bonds, which are responsible for the formation of bound surface charges, leading to interfacial polarization. The secondary recrystallization induced by HIP leads to the formation of thin, equilibrium grain boundaries.

Nano-scale grain growth inhibited by reducing grain boundary energy through solute segregation

Combining Gibbs adsorption equation and McLean's grain boundary segregation model derives an empirical relation for isothermal grain growth. This relation predicts that driving grain boundary energy to zero is possible in alloy with high segregation energy. Then the grain boundary is in a thermodynamic equilibrium in the presence of solute atoms and, therefore, grain growth is stopped. On this basis reducing grain boundary energy through solute segregation inhibits nano-scale grain growth, and then metastable equilibrium can be reached with saturated grain boundaries. Exact values for solute excess of grain boundary available for segregation at saturation, Γ b0, Gibbs free energy change of grain boundary segregation pre mole solute, Δ G seg, and/or equilibrium grain diameter, D *, can be achieved by fits of this model to the experimental results.

Faceting behavior of an asymmetric SrTiO3 Σ5 [001] tilt grain boundary close to its defaceting transition

Using high-resolution transmission electron microscopy (HRTEM), an asymmetric Σ5 [001] grain boundary in SrTiO3 with an inclination of 2.5° away from the symmetric (310) plane was studied. A defaceting transition is observed between 1500 and 1600 °C. From 1520 to 1570 °C, the grain boundary is faceted into the symmetric (310) and asymmetric (100)//(430) planes. At 1590 °C, the grain boundary is observed to be defaceted, which implies that the defaceting occurs between 1570 and 1590 °C. The faceted structure observed between 1500 and 1590 °C is interpreted in terms of the step free energy of the asymmetric (100)//(430) facet orientation. The faceting-defaceting transition is schematically represented as the temperature evolution of the equilibrium grain shape.

Grain-boundary diffusion in nanocrystals with a time-dependent diffusion coefficient

A solution to the equation of grain-boundary diffusion is obtained under conditions where migration of the diffusant from the boundaries into the grains is absent and the diffusion coefficient decreases with time from an increased value to a value characteristic of equilibrium grain boundaries. The specific features of the grain-boundary diffusion in nanocrystals are qualitatively analyzed in terms of this solution.

Grain boundary segregation in Cu–Sb alloys

Equilibrium grain boundary (GB) segregation of Sb in Cu–Sb alloys was studied by the methods of Auger (AES) and X-ray photoelectron spectroscopy (XPS). On the basis of Langmuir–McLean isotherm, the atomic fraction of Sb ( X b 0) at the first layer was calculated provided that all available sites at the first layer were filled and equilibrium between the first layer and the volume was reached. The values of ( X b 0) do not exceed atomic concentration of Sb in the nearest chemical compound in equilibrium with solid solution according to the phase diagram. The segregation capacity of grain boundaries in Cu-based alloys averages between 0.38 and 0.65 of Sb monolayer. The enthalpy of Sb segregation is equal to −8.9 kJ mol −1.

Evolution of dust properties in an interstellar filament

We present submillimetre observations obtained using the balloon-borne experiment PRONAOS/SPM, from 200 to 600 μm with an angular resolution of 2-3.5', of a quiescent dense filament (typically A V ∼ 4) in the Taurus molecular complex. This filament, like many other molecular clouds, presents a deficit in its IRAS I 6 0 μ m /I 1 0 0 μ m flux ratio in comparison with the diffuse interstellar medium. We show, from the combination of the PRONAOS/SPM and IRAS data, that, inside the filament, there is no evidence for emission from the transiently heated small particles responsible for the 60 μm emission, and that the temperature of large grains in thermal equilibrium with the radiation field is reduced in the inner parts of the filament. The temperature is as low as 12.1 + 0 . 2 - 0 . 1 K with β = 1.9 ′ 0.2 (or 12.0 + 0 . 2 - 0 . 1 K using β = 2) toward the filament centre. These phenomena are responsible for the IRAS colour ratio observed toward the filament. In order to explain this cold temperature, we have developed a model for the emission from the filament using star counts from the 2MASS catalog as an independent tracer of the total column density and a radiative transfer code. We first use the optical properties of the dust from the standard model of Desert et al. (1990). The computed brightness profiles fail to reproduce the data inside the filament, showing that the dust properties change inside the filament. An agreement between data and model can be found by removing all the transiently heated particles from the densest parts of the filament, and multiplying the submillimetre emissivity by a significant factor, 3.4 + 0 . 3 - 0 . 7 (for typically n H > 3 ′ 1 × 10 3 cm - 3 , A V > 2.1 ′ 0.5), We show that grain-grain coagulation into fluffy aggregates may occur inside the filament, explaining both the deficit of small grain abundance and the submillimetre emissivity enhancement of the large grains.

Equilibrium grain boundary segregation of calcium and equilibrium grain boundary depletion of silicon in Ni3Al

Equilibrium grain boundary segregation of calcium and equilibrium grain boundary depletion of silicon in Ni3Al

Low temperature superplasticity of a fine-grained ZK60 magnesium alloy processed by equal-channel-angular extrusion

Fine-grained ZK60 magnesium alloy with the grain size of 1.4 μm was processed by equal-channel-angular extrusion. The material exhibited low temperature superplasticity. The normalized plot suggested that the present material had equilibrium grain boundaries at the superplastic temperature in contrast to typical materials processed by severe plastic deformation.

Analysis of the evolution of the grain size distribution in WC–Co sintered materials with random set models

Coarsening of WC grains in equilibrium with a Co-based liquid is characterised by a gradual evolution of the mean grain size, while a population of large grains can form under certain conditions. Coarsening phenomena in such cases cannot be analysed simply in terms of average grain size. In this paper, grain size distribution is followed using a Boolean model. WC grains are represented by triangular prisms, with a log-normal grain size distribution. Model parameters are estimated from image analysis measurements. Variation of size distribution with sintering time is analysed. Accuracy of the method is discussed and its efficiency in characterising the abnormal character of grain growth is compared with more conventional measurements of intercept length distributions.

Phlogopite from the Serre ultramafic rocks, Central Calabria, Southern Italy

A 6–7 meter thick phlogopite-bearing peridotite “body” interlayered within a 200 m thick sequence of layered metagabbro and pyroxene-bearing felsic granulite outcrops in the Serre Massif, southern sector of the Calabrian Peloritan Arc (southern Italy). Phlogopite occurs throughout the whole peridotitic body, but in different modal amounts and in various textural sites. It occurs as Type-I, isolated and oriented grains in textural equilibrium with the other phases, either in the peridotite or in included orthopyroxenite veins; Type-II, in a kind of melange, as millimetric smearings of flakes which separate anastomosing phlogopite-rich and orthopyroxene-rich peridotitic portions; Type-III, in centimetric phlogopitite layers alternating with phlogopite-rich layers. The phlogopite has mg values [mg = Mg/(Mg+Fe)] in the range 0.90 to 0.94. It also shows appreciable variations of TiO 2 , Cr 2 O 3 and BaO contents, i.e., 1.26–2.44, 0.21–1.35, 0.15–2.02 wt%, respectively. The primitive mantle-normalized REE pattern of Type-I phlogopite shows a steep negative slope from La to Eu, followed by a slight Gd positive anomaly and a moderately positive slope from Tb to Lu. The REE patterns of Types-II and -III show negative slopes from La to Lu, but the Type-II pattern displays a marked Eu negative anomaly, and Type-III phlogopite has lower M- and HREE contents. The trace element spider diagrams show a generally decreasing trend in elemental amounts, in accordance with decreasing incompatibility, with evident K, Pb, Mo, P, Sn and Ti positive anomalies. The occurrence of phlogopite in peridotite from the Serre Massif probably indicates that the rocks underwent a severe metasomatic event in upper mantle and/or lower crust-mantle boundary conditions.

Grain boundary segregation of phosphorus, manganese, and carbon in boiler shell weld material

Submerged arc weld materials have been employed in a study of the effects of manganese and carbon on phosphorus segregation and intergranular embrittlement. The equilibrium grain boundary segregation behaviour of these elements during aging has been studied in two different boiler shell weld materials, which differ mainly in the manganese concentration and operating temperature during service. The materials have seen temperatures above 300C during operation of the boiler in service, at which temperature thermally induced segregation and embrittlement occurs. A new co-segregation model has been compared with the existing site competition model. The microstructure shows fine and coarse grained regions. The effects of manganese and carbon on the grain boundary segregation of phosphorus have been examined. Thermally induced grain boundary segregation during full service life up to 50 years as a function of temperature is described. To evaluate the free matrix concentration of a given element, equilibrium thermodynamic software was used to allow for the tendency to form precipitates within the alloy matrix. The predicted results reveal the dependence of the grain boundary concentration on temperature and show that manganese and carbon decrease the phosphorus segregation by site competition. The final segregation consists of non-equilibrium and equilibrium segregation, which occur during quenching after welding, post-weld heat treatment, and service. The microstructure has been investigated by optical microscopy and transmission electron microscopy to show carbide formation at the grain boundaries and intergranular precipitation of MnS. Preliminary analysis of the grain boundary has been made and the results compared with theoretical segregation predictions.

Electrodynamic balance for studies of cosmic dust particles

Knowledge of the formation and distribution of interstellar, interplanetary, and planetary dust grains, and their physical, chemical and optical characteristics provide valuable information about many issues dealing with the origin and formation of the solar system bodies, interplanetary and interstellar environments as well as various industrial processes. Understanding the microphysics of individual grains and their interaction with the surrounding environment is a key to properly model various conditions and interpret existing data. The theory and models of individual dust grains are well developed for environments that vary from dense planetary atmospheres to dusty plasmas to diffuse environments such as interplanetary space. However, experimental investigations of individual dust grains in equilibrium are less common, perhaps due to the difficulty of these experiments. Laboratory measurements of dust grains have primarily measured ensemble properties or transient properties of single grains.A technique developed in the 1950's for ion spectroscopy, generally referred to as a quadrupole trap has recently been employed as an electrodynamic balance to investigate single micron-sized dust grains and for atmospheric aerosol research. A description of the theoretical basis and the experimental setup of the electrodynamic balance being developed in our laboratory are given. This laboratory technique lends itself to many applications that relate to planetary atmospheres, heliospheric environments, pre-stellar and pre-planetary conditions, and industrial settings. We present results from some recent experiments carried out to investigate the equilibrium potential of dust grains exposed to far ultraviolet light or to an electron beam. Some future experiments using an electrodynamic balance to investigate the optical characteristics, and condensation process involving dust grains in various astrophysical environments are discussed.

Synthesis and Sintering of Nanocrystalline Barium Titanate Powder under Non-Isothermal Conditions. Part 6. Structure, Grain Boundaries, and Dielectric Properties of Barium Titanate Obtained by Various Sintering Methods

The grain boundary structure of barium titanate obtained by controlled-rate sintering and high-pressure sintering (HPS), and the dependence of dielectric properties on grain size and consolidation method were studied. It was shown that sintering without the application of pressure leads to a diffusion-controlled formation of equilibrium grain boundaries with minimal impedance factor, which minimally decrease the dielectric constant of the ceramics. HPS results in the formation of non-equilibrium grain boundaries which have a large free volume, and which substantially decrease the dielectric constant. The Curie-Weiss constant was analyzed from the viewpoint of a matrix structural model, and a «brick-wall» model.

Mid and Far IR properties of late-type galaxies in the Coma and A1367 clusters: ISOCAM and ISOPHOT observations

We present Mid (MIR) and Far (FIR) Infrared observations of 18 spiral/irregular galaxies belonging to the Coma and A1367 clusters, carried out with the CAM and PHOT instruments on board the ISO satellite. Complementary photometry from the UV to the Near Infrared (NIR) together with Hα imaging, HI and ^(12)CO line measurements allow us to study the relationships between the IR emission and the star formation properties of these galaxies. Most of the resolved galaxies show extended MIR emission throughout their disks even where no Hα emission is present. This suggests that the Aromatic carriers can be excited by the general interstellar radiation field (ISRF), i.e. by visible photons. Only close to HII regions the UV photons are the principal sources of Aromatic carrier excitation. However, when the UV radiation field becomes intense enough these carriers can be destroyed. The average integrated 15/6.75µ_m ratio of the observed galaxies is ∼1, i.e. the typical value for the photodissociation regions (PDRs). This suggests that, despite the high star formation rate (SFR) and the very luminous HII regions of these galaxies, their integrated MIR emission is dominated by PDR-like regions rather than HII-like regions. A cold dust component with average temperature ∼22 K exists in most of the target galaxies, probably arising from big dust grains (BGs) in thermal equilibrium with the ISRF. The contribution to the BGs heating from the ionizing stars decreases with increasing wavelength. A warmer dust component whose emission dominates the spectrum between 20 and 100µ_m is likely to exist. This is probably due to both Very Small Grains (VSGs) and warm BGs emission. The dust to gas ratio of the target galaxies is comparable to that of the solar neighborhood. There is a weak trend between the dust total mass and both the atomic and molecular gas content. The MIR and FIR properties of the analyzed galaxies do not seem to be affected by the environment despite the fact that most of the targets are interacting with the Intra-Cluster-Medium.

Infrared Properties of Molecular Cirrus. II. Cloud‐to‐Cloud Variations in Graphite and Polycyclic Aromatic Hydrocarbon Content

We have conducted a photometric survey of translucent molecular clouds in the four IRAS wave bands. We find notable cloud-to-cloud variations in mid-IR emission, with the ratio I12/I25 varying by up to 1 order of magnitude and often above unity. Because the clouds in our sample are nearby and translucent, the cloud heating is well constrained, so that the observed differences in infrared emission must represent differences in grain content. We show from first principles that a dust model containing only two components, Mathis-Rumpl-Nordsieck (MRN) grains in thermal equilibrium and very small grains undergoing stochastic heating, can never produce I12/I25 ≥ 1 under the conditions that prevail in the cloudy interstellar medium. We fit the clouds in this sample with a three-component model which contains continuum emission from both MRN grains and very small graphite grains, plus emission features from a mixture of polycyclic aromatic hydrocarbon (PAH). The cloud-to-cloud variations in mid-IR emission require significant differences in their composition of small grains and PAH molecules. Models which describe the formation and evolution of these particles in the interstellar medium must explain the presence of these variations in a cloud sample which is located in the solar neighborhood.

Low-frequency electrostatic dust-modes in a strongly coupled dusty plasma with dust charge fluctuations

Low-frequency electrostatic dust-modes are theoretically investigated accounting for dust grain charge fluctuation and for equilibrium grain charge inhomogeneity in a strongly coupled dusty plasma. A new stable extremely low-frequency mode, which is due to the inhomogeneity in the equilibrium dust grain charge, is found to exist in such a dusty plasma. It is also found that the dust-acoustic mode becomes unstable due to the effect of this equilibrium dust grain charge inhomogeneity. It is observed here that the influence of strong correlations in the dust fluid significantly modify the dispersion properties of this new mode as well as of the existing dust-acoustic mode. The implications of our results to recent experimental observations and to some space and astrophysical situations are briefly discussed.

Equilibrium Point Defect and Electronic Carrier Distributions near Interfaces in Acceptor‐Doped Strontium Titanate

A mathematical model based on Frenkel's theory for charged defect segregation at interfaces is used to calculate the equilibrium grain boundary depletion layer widths and conductivity profiles in acceptor‐doped SrTiO3 ceramics. The calculations examine the effect of oxygen vacancy equilibration during annealing at moderate temperatures (∼1000 K) on the development of interfacial charge that influences grain boundary electrical properties at lower temperatures. Good agreement is demonstrated between the model predictions and experimental results reported in the literature.

Zn-spinel in the metamorphosed Zn-Pb-Cu sulphide deposit at Mamandur, southern India

Abstract At Mamandur, southern India, zincian spinel is associated with Zn-Pb-Cu sulphide ores, metamorphosed to the granulite facies. The Zn-spinel (XZn = 0.44–0.82) occurs in assemblages of: (a) cordierite + biotite + sillimanite + sphalerite + pyrite and/or pyrrhotite; (b) garnet + biotite + sphalerite + pyrite and/or pyrrhotite; (c) hornblende + biotite + sphalerite + pyrite and/or pyrrhotite; and (d) it also occurs in highly sheared quartz veins with sphalerite + pyrite/pyrrhotite. Two texturally and compositionally distinct modes of occurrence of Zn-spinel have been recorded from the metamorphic assemblages: one shows a polygonal, granular (equilibrium) fabric with Zn-spinel grains in textural equilibrium with cordierite in the granulite facies assemblage; the other which is involved in the formation of coronas in garnet- and hornblende-bearing assemblage of a younger paragenesis (presumably belonging to a late retrograde stage), shows distinct compositional zoning with depletion of Zn and enrichment in Fe and Mg from the rim inwards. A third mode of occurrence is as perfectly euhedral grains embedded in a quartzose matrix in quartz veins, often with sphalerite moulded over the Zn-spinel grains. From textural evidence and consideration of stoichiometric balance, Zn-spinel occurring in association with cordierite is suggested to have formed by the prograde reactions: 0.46 biotite (Mg-rich) + 1.80 sillimanite + 3.01 quartz + 0.11 sphalerite = cordierite + 0.20 Zn-spinel + 0.80 K-feldspar + 0.06 pyrite + 4(OH,F) or 0.48 biotite (Mg-rich) + 2.08 sillimanite + 2.72 quartz + 0.26 sphalerite = cordierite + 0.50 Zn-spinel + 0.83 K-feldspar + 0.13 S2 + 4(OH,F) and, the Zn-spinels occurring in association with garnet and hornblende by the retrograde reactions: garnet + 4.53 Al2SiO5 + 4.17 sphalerite = 5.38 Zn (-Fe)-spinel + 0.22 Ca-feldspar + 0.92 pyrite + 7.06 quartz + 1.23 S2 and hornblende + 26.93 Al2SiO5 + 20.17 sphalerite + 0.88 pyrite = 25.93 Zn-spinel + 2 Ca-feldspar + 56.81 quartz + 11.32 S2 + 2(OH,F) Zn-spinel in the sheared quartz vein may be of (meta-) hydrothermal origin. Formation of sphalerite through sulphurization of gahnite is ruled out on the evidence of the perfectly euhedral nature of the spinel and the absence of any breakdown product from the spinel.

Thermal stability of cobalt silicide stripes on Si (001)

The effect of lateral scaling on the thermal stability of cobalt disilicide has been investigated on Si (001) substrates. Resistance measurements on four terminal resistors, with linewidth dimensions ranging from 1.3 to 0.5 μm, have been performed after annealing in the range between 900 and 1000 °C. The resistance increases faster in the stripes than in blanket regions. This degradation process has been correlated to the morphological change in the strip cross section, as analyzed by transmission electron microscopy and atomic force microscopy. Grain growth, degradation processes at the line edge, and surface and interface roughness have been observed. These analyses show that the lateral constraints of the silicide lines are mainly responsible for the lowering of the silicide thermal stability compared with blanket regions. Moreover, from measurements of agglomerated silicide grains in equilibrium with the substrate, the silicide surface free energy is 2.3 times the free energy of the CoSi2/Si interface.

Influence of pressing speed on microstructural development in equal-channel angular pressing

The influence of pressing speed in equal-channel angular (ECA) pressing was investigated using samples of pure Al and an Al-1 pct Mg alloy and a range of pressing speeds from ∼10−2 to ∼10 mm s−1. The results show that the speed of pressing has no significant influence on the equilibrium grain size, at least over the range used in these experiments. Thus, the equilibrium grain sizes were ∼1.2 µm for pure Al and ∼0.5 µm for the Al-1 pct Mg alloy for all pressing conditions. However, it is shown that the nature of the microstructure is dependent on the pressing speed, because recovery occurs more easily at the slower speeds, so that the microstructure is then more equilibrated. There is also indirect evidence for the advent of frictional effects when the cross-sectional dimensions of the samples are at or below ∼5 mm.