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Grain growth behavior in fully dense compacts of MgO of very high purity was studied, and the results compared with other similar behaving materials. The activation energy for the intrinsic self-diffusion of Mg(2minus) is discussed along with the grain boundary diffusion of O(2minus). Grain boundary diffusion of O(2minus) is proposed as the controlling mechanism for grain growth.
It is shown that the compliance of an orthogonal grain-grain contact is so insensitive to the grain geometry in the contact region that this geometry is not at present an important parameter in theories of the speed of propagation of elastic waves in granular media, such as occur in the earth and in the moon.
Interstellar grain temperatures, considering radiant energy distribution and optical properties effects
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Questions of interstellar extinction are discussed together with the spatial distribution of dust, the scattering of light by grains, diffuse interstellar absorption lines, and questions of interstellar polarization. Theoretical extinction curves are considered, giving attention to ice grains, graphite grains, polycyclic hydrocarbons, silicate grains, core-mantle grains, mixtures of grains, and the total amount of dust. Aspects of grain temperature and grain potential are examined along with processes on grain surfaces, the scattering of X rays by interstellar grains, the origin and destruction of grains, and the dust in the HII regions.
This paper presents a sample calculation of the effects of temperature fluctuations on the far-IR emission by very small grains and on the total radiation emitted by the combination of normal and small grains in interstellar space. The bimodal size distribution of interstellar grains is discussed, steady-state grain temperatures are computed for core-mantle and bare particles, and temperature fluctuations of small bare particles are analyzed. Far-IR emission properties are calculated for spherical core-mantle and bare grains. The results obtained indicate that the use of a fluctuating temperature distribution for small grains shifts the radiation maximum for emission by all grains from about 170 to about 250 microns and that the total IR radiation emitted by small grains is substantially larger than the total emitted by classical-sized grains. It is also found that the temperature ratio between small and classical grains is higher in H II regions than in dark clouds.
The welding process is a potential way of repairing a damaged metal component, especially cavity damage caused by a harsh environment like the lunar environment, which is characterized by large temperature differences and reduced gravity. The adjustment of welding parameter (e.g., hatch spacing) can improve production efficiency in the repair process. Seen from the microstructural level, hatch spacing sensitivity affects the metallic grain evolution and morphology in the welding process, which can further influence a repaired part’s mechanical properties; however, the study of hatch spacing’s effect on microstructure is challenging. Traditional experimental procedures are costly and time-consuming, and any change in hatch spacing value needs roll-back of experimental procedure. A modeling study can address the above challenges in experimental observation. In this research, a modeling approach based on the Kinetic Monte Carlo (KMC) Potts theory was used to simulate grain evolution and morphology with three hatch spacings. Through quantifying and analyzing the predicted grain morphologies, the effect of hatch spacing on microstructure in a welding-fabricated part was investigated. The predicted grain morphologies were validated with an EBSD image of welding microstructure, which has been published before. The primary grain morphologies were columnar grains with a small amount of fine equiaxed grains formed in the scanning path centerline. When increasing the hatch spacing, the columnar grains become larger and more lengthy, while the effect of hatch spacing on the equiaxed grains is not obvious.
The aeolian mechanics of saltating grains on Mars is studied using both an experimental investigation in an atmospheric wind tunnel and numerical solutions to the equations of motion of a single grain under Martian surface conditions. Numerical solutions for terrestrial conditions are empirically correlated with experimental data for Mars, and the modified equations are solved to estimate Martian grain motion. The calculations demonstrate the importance of a lifting force in initiating grain motion on both planets. It is found that Martian grain-trajectory length scales are longer than those on earth and fall with a higher terminal grain velocity. The wind-tunnel experiments show that saltating grains have an enhanced erosive capability in crater wakes. It is suggested that this enhanced capability may well explain the streaks associated with craters in certain regions on the Martian surface.
Mariner 9 spacecraft images showing evidence of variable surface features and surface erosion resulting from atmospheric wind on Mars have caused a renewed interest in the eolian mechanics of saltating grains. To study this phenomenon, both experimental investigation in an atmospheric wind tunnel and numerical solutions of the equations of motion of a single grain under Martian surface conditions were conducted. The numerical solutions for earth were used and empirically adjusted to correlate with existing experimental data for Mars. These modified equations were then solved to estimate grain motion for Mars. These calculations show the importance of a lifting force on the grain to initiate motion in both earth and Mars calculations. Major findings include a comparison of earth and Mars grain trajectories that show Mars length scales to be longer and to fall with a higher terminal grain velocity. The grains in the Mars calculation also made a smaller collision angle with the surface on rebound.
It has recently been suggested that extinct I-129 and Pu-244 were trapped in primitive-solar-nebula ('presolar') grains and decayed into radiogenic Xe-129 and fission Xe before the grains were incorporated into meteorite bodies. This idea is reconsidered in light of the thermal and metamorphic history of meteorites. The criteria that parent and daughter species should never separate and that minerals or grains containing the anomalous xenon should not be subjected to temperatures exceeding 500 C are applied to iron meteorites, achondrites, and chondrites to determine whether presolar grains could be the carriers of rare-gas anomalies to meteorites. The results strongly indicate that the xenon anomaly could not have originated in presolar grains. Other difficulties with the presolar-grain model are discussed, including insufficiently small grain sizes, large variations in Xe-129/I-127 ratios in various meteorites, and apparently unrealistic meteorite formation times and locations.
The magnetic dipole moments and internal magnetic fields due to the spin of electrically charged elongated nonmagnetic interstellar grains in kinetic equilibrium with their surroundings are computed for the grain-size range from 0.01 to 1.0 micron. It is shown that the induced magnetic moments and internal magnetic fields of charged spinning nonmagnetic grains of arbitrary composition and prolate spheroidal shape can be appreciable, possibly even exceeding 0.01 emu/cu cm for 0.01-micron grains. The results indicate that virtually all grains smaller than 0.1 micron in mean diameter, and all elongated grains smaller than about 1 micron in length, are immersed in local magnetic fields due to spin that are much larger than the ambient galactic field. Some implications of this effect are discussed in relation to the polarization of starlight by aligned dust grains and the primordial remanent magnetization found in primitive carbonaceous chondrites.
Various color excess ratios, the ratio (R) of total to selective extinction, and values of the wavelength of maximum interstellar linear polarization (lambda max) are computed and displayed as functions of mean interstellar grain size and absorption coefficients. E(V-K)/E(B-V)-E(u-b)/E(b-y) is shown to be highly sensitive to changes of mean grain size. The shape of the R versus lambda max curve is shown to depend on the amount of absorption initially present in the grains, and on the absorptivity of the material added to the grains. While no simple model of grain growth has been found to fit all the observations, the two stars with the largest values of lambda max (HD 147889 and W67) support the concept of dielectric mantles growing on dielectric grains. It is further shown that more accurate uby and JKL photometry of heavily reddened stars would permit better discrimination between grain growth theories.
Isotopic analyses of meteorites suggest the possibility that some interaction between supernova ejecta and grains occurred in the solar nebula. In particular, the dynamics of grain motions in the solar nebula can explain the observed mixing of nucleosynthetic components. The effect of a shock wave on the motions of grains is examined. On the basis of calculations, it is estimated that if grains carried the isotopic anomalies investigated by Lee, Papanastassoiu, and Wasserburg (1978), then those grains could be no bigger than 0.0002 cm in size. A scenario is suggested in which the sluggishness of grains provides a natural way to concentrate and mix the nucleosynthetic components carried by grains in the ejecta and in the solar nebula.
The grain size distribution of six Luna 24 samples has been determined. These samples are characterized by a bimodal distribution which is indicative of soils formed primarily by mixing rather than by reworking. Although agglutinate content decreases with depth, it is not likely that the Luna 24 soils have undergone appreciable in situ reworking. Particle types and abundances in each of four size fractions have been determined petrographically. Mineral fragments are very abundant in all analyzed size fractions. Pyroxene and plagioclase increase slightly in abundance at finer grain sizes, but olivine decreases significantly. Compared with typical mare soils, the Luna 24 trends are anomalous. They are compatible with the hypothesis than many, if not most, of the mineral grains in the 20-250-micron fractions come from coarse-grain rocks having average mineral grain sizes greater than 250 microns. The mineralogy and chemistry of the coarse-grained rocks has not been well characterized, but there is evidence that at least some of them are higher in MgO than the analyzed finer-grained basalts.
The physics of magnetic diffusion in dense molecular clouds is examined, with particular attention given to the role of charged grains in controlling the process. The ionization fraction of dense molecular clouds in the presence of grains is determined from considerations of charge exchange, dissociative recombination, radiative recombination and collisions between grains and charged species, and it is found that the inclusion of grains tends to lower the ionization fraction for a given cosmic-ray ionization rate and metal depletion. The kinematics of grain motion is discussed and it is shown that at temperatures less than 30 K, each grain alternates rapidly in charge between -1 and 0 and thus executes periodic motion in a self-gravitating cloud containing a magnetic field. The full kinematics of magnetic diffusion including the motions of ions and electrons are then examined, taking into account the additional viscous force from charged grains, and numerical calculations of the diffusion time scales of uniform, magnetically supported clouds or cloud cores are presented.
Characteristics of the coarse grain transformation occurring in TD-nickel 1 in. bar under certain conditions of deformation and annealing were examined. The transformation exhibits Avrami-type kinetics, with an activation energy of 250 kcal per mole. Characteristics of untransformed regions are like those of the as-received state. The transformed grain size increases with increasing deformation and decreasing annealing temperature. The coarse grain transformation is significantly different from primary recrystallization in pure nickel. Its characteristics cannot be rationalized in terms of primary recrystallization concepts, but may be explained in terms of an abnormal grain growth description. The coarse grain transformation in TD-nickel is abnormal grain growth rather than primary recrystallization. The analysis suggests an explanation for the effect of thermomechanical history on the deformation and annealing behavior of TD-nickel.
Application of the Kramers-Kronig relations to grains in the interstellar medium. It is found that the recent UV, visual, and infrared extinction curves provide marginal evidence against silicates as the major cause of this extinction. In the far infrared, grains contribute little to the extinction, but can be observed in thermal emission. It is shown that the far infrared emission from grains in a given bandwidth sets a lower limit to the grain temperature. From observed intensities from the galactic plane at 100 microns and from Orion at 350 microns, minimum grain temperatures are found which are larger than the observed brightness temperature. The emission and extinction data can be used together to constrain the allowable range of parameters for the grains.