Showing papers on "Grain size published in 1981"

Modeling and optimization of monolithic polycrystalline silicon resistors

Abstract: The processing parameters of monolithic polycrystalline silicon resistors are examined, and the effect of grain size on the sensitivity of polysilicon resistivity versus doping concentration is studied theoretically and experimentally. Because existing models for polysilicon do not accurately predict resistivity dependence on doping concentration as grain size increases above 600 A, a modified trapping model for polysilicon with different grain sizes and under various applied biases is introduced. Good agreement between theory and experiments demonstrates that an increase in grain size from 230 to 1220 A drastically reduces the sensitivity of polysilicon resistivity to doping levels by two orders of magnitude. Such an increase is achieved by modifications of the integrated-circuit processes. Design criteria for the optimization of monolithic polysilicon resistors have also been established based on resistivity control, thermal properties, and device geometry.

Grain boundary effects on ionic conductivity in ceramic GdxZr1–xO2–(x/2) solid solutions

Abstract: Complex admittance measurements are performed on high-purity ceramics prepared by means of the alkoxide synthesis and on less pure ceramics obtained from the citrate synthesis. The results on ceramic materials with grain sizes ranging from 0.4 to 20 µm are compared with those from a single crystal. The activation enthalpy for grain boundary conductivity Hgb = (118 ± 2) kJ/mol for the samples studied, is independent of composition, grain size, and preparation method. Grain boundary conductivity values and consequently the relevant pre-exponential factors are an order of magnitude smaller for the alkoxide materials than for the citrate materials. The ratio of grain bulk and grain boundary conductivity (Qb/Qgb) for alkoxide materials with grain-sizes 0.4 to 0.8 µm varies from 8.5 to 1.0 in the temperature range 500 to 700 °C.

Grain refinement in 7075 aluminum by thermomechanical processing

Abstract: A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.

Phonon scattering at grain boundaries in heavily doped fine-grained silicon-germanium alloys

Abstract: Thermocouples made from heavily doped n - and p-type silicon-germanium alloys are used in the nuclear powered thermoelectric generators which provides onboard power to the American Voyager spacecraft. The isotopic heat source is expensive and any improvement in the generators' heat energy to electrical energy conversion efficiency would save both money and weight. The possibility of reducing the thermal conductivity of thermoelectric semiconductor alloys by compacting fine grained material has been considered and various estimates have been made of the reduction in thermal conductivity with grain size. The electrical properties of hot pressed compacts of heavily doped silicon germanium alloys which possess density values dose to that of single crystal are unaffected by grain boundary scattering effects. Consequently, this method of preparation holds out a possibility of improving the ‘figure of merit’ of the thermocouple material and hence the conversion efficiency of the generator. Here the first measurements of the thermal diffusivity of fine grained hot pressed compacts of heavily doped n-type silicon germanium alloy are reported. The compacts investigated possessed grain sizes (L) in the range 10

Grain‐Size Dependence of Fracture Energy in Ceramics: I, Experiment

Abstract: Evaluation of literature data combined with new experimental measurements shows that crystal structure is a basic factor in the grain-size dependence of the critical fracture energy of ceramics. Notch beam (NB), double cantilever beam (DCB), and work-of-fracture (WOF) tests of materials having a cubic crystal structure generally agree and show, at most, a limited variation of fracture energy with grain size. However, maxima of the order of 25% in γ at intermediate grain size and 25% decreases in γ at larger grain size may occur. Materials of noncubic crystal structure show broader variations in γ with G. Both DCB and WOF tests show γ passing through maxima that are typically 100 to >400% of values at fine or large grain size. Limited DT data are consistent with this trend but various results of NB tests are mixed. Some NB test data agree with the DCB and WOF trends, whereas others taken from the same materials show no change or only a limited continuous decrease of γ as G increases.

The effect of phonon‐grain boundary scattering on the lattice thermal conductivity and thermoelectric conversion efficiency of heavily doped fine‐grained, hot‐pressed silicon germanium alloy

Abstract: The preparation of fine‐grained high‐density compacts of heavily doped n‐type Si63.5Ge36.5 alloy and the measurement of their thermal diffusivity over the temperature range 300 to 1150 K are reported. The compacts investigated possessed grain sizes (L) in the range 10

Grain‐Size Dependence of Fracture Energy in Ceramics: II, A Model for Noncubic Materials

Abstract: A mathematical model of the grain-size dependence of fracture energy of noncubic ceramics is developed and discussed. This model attributes the maxima in fracture energy of noncubic materials to countertrends of increasing numbers of microcracks and decreasing energy absorption per microcrack as grain size increases. The model predicts results similar to those experimentally observed. Agreement between calculated and measured fracture energy values is generally good, except for materials with extreme anisotropy. Sources of these differences and methods of improving calculations are discussed.

Measurements and analysis of setting velocities of natural quartz sand grains

Abstract: Settling velocities in water were measured for natural quartz sand grains in the sieve-size range -0.75 to 1.50 (0.35-1.68 mm). Obtained from beach sand samples, the grains were selected so as to provide a wide range of roundness from very angular to very-well rounded (r = 0.6-5.1). In a first set of experiments only two axial diameters were measured under a microscope, the longest axial diameter, D1, and the intermediate diameter, Di. In a second data set the smallest axial diameter, Ds, was measured as well. Settling velocities were measured in a 6-meter long settling tube. It is found that the intermediate grain diameter is on average equal to the nominal diameter computed as Dn = (DsDiD1)1/3. This permits a simpler analysis of grain settling through microscopic measurements of Di alone but does not allow for corrections of grain shape effects. The measured settling velocity, wm, is then compared with the settling velocity of a sphere, ws, calculated using Di as the sphere diameter, and the relationship wm = 0.977w0.913s is found to be consistent with the data. An alternative approach is provided where ws is calculated directly from the sieve diameter, and then corrected to the actual settling velocity wm for that mean grain size. The results thus provide graphs and empirical relationships which permit the evaluation of actual grain-settling rates (rather than settling velocities of spheres) from the sieve diameter or from microscopic measurements of Di for individual grains. These procedures provide a mean settling velocity that takes into consideration the average degree of grain irregularities found in natural sands. A more detailed analysis is then conducted to examine the effects of roundness and sphericity on the settling of individual grains. It is found that grain roundness has no measurable effect on the settling rate. The settling velocity is shown to be a function of grain sphericity. either expressed as the Corey Shape Factor or E Shape Factor. Grain asymmetries ust also be important, accounting for much of the scatter in the results.

Grain refinement in magnetically stirred GTA welds of aluminum alloys

Abstract: The mechanisms of grain refinement have been examined for magnetically stirred gas tungsten arc (GTA) welds completely penetrating thin sheets of several aluminum alloys. Grain refinement in unstirred welds may be brought about by adding sufficient titanium to produce heterogeneous nucleation by Ti-rich particles. In some alloys magnetic stirring is shown to extend the range of welding conditions which produce a partially equiaxed structure, and to widen the equiaxed fraction of partially equiaxed welds. This is attributed to magnetic stirring lowering the temperature gradient, allowing nucleation and growth of Al-rich grains further ahead of the columnar interface growing in from the fusion boundaries. In alloys with low Ti levels, magnetic stirring may cause refinement by sweeping grains from the partially molten zone ahead of the advancing solidification interface. This mechanism requires that the partially molten zone be sufficiently wide, and that the grain size in this zone remain small.

The origin of acoustic emission during deformation of aluminium and an aluminium–magnesium alloy

Abstract: The acoustic emission power during deformation of pure aluminium single crystals and polycrystals, and specimens of A1 1·3wt% Mg, has been measured. In pure aluminium the acoustic emission activity increased with grain size to approach the levels of activity from single crystals, whereas the emission activity of the Al 1·3wt% Mg solid solution exhibited a peak at a grain size of ∼80 μm, and samples with large grain size generated almost no detectable emission. A model is presented for acoustic emission by a dislocation, and is used to explain the observed emission dependence upon grain size in both metals.

Creep mechanism in Mg2GeO4: Effects of a phase transition

Abstract: We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs-type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by = 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and (s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is = 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation-induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.

Void nucleation in spheroidized carbon steels Part 2: Model

Abstract: A double criterion for void nucleation at particle/matrix interfaces is proposed. It is based on a combined critical normal stress condition and critical elastic energy release requirement. The theory of nucleation of a spherical void cap of limited size incorporates the effects of grain boundaries and plastic incompatibility of the particle-grain-boundary-matrix system. The theoretical model predicts the appropriate form for the void-nucleation rate in spheroidized steels as a function of plastic strain, flow stress, stress triaxiality, particle size, and grain size in qualitative agreement with experimental observations.

The use of beam shaping to achieve large-grain cw laser-recrystallized polysilicon on amorphous substrates

Abstract: Laser beam shaping has been used to increase the grain size of laser‐recrystallized polycrystalline silicon on amorphous substrates. Using a slanted liquid‐solid interface, rectangular grains as large as 45×50 μm have been grown in 0.5‐μm‐thick chemical vapor deposited polysilicon on quartz substrates. Improved surface smoothness of recrystallized material has also been achieved using this procedure.

Optical properties and transport in microcrystalline silicon prepared at temperatures below 400 °C

Abstract: Microcrystalline silicon (grain size ∼100 A) films have been prepared at temperatures below 400 °C through an H2‐plasma assisted transport process. The films contain an average of ∼2 at. % of hydrogen which is predominantly situated in the grain boundary regions in the form of Si‐H2. The optical absorption coefficient of these films exceeds that of crystalline Si by up to two orders of magnitude below 3.0 eV. The films can be doped by adding PH3 to the hydrogen plasma. The highest room‐temperature conductivity so achieved was ∼1 (Ω−1 cm−1). From an analysis of the activation energies vs dopant concentration we deduce that the high‐temperature transport (T≳330 °K) is barrier limited. The upper limit for the density of trapping states in the grain boundaries is 1×1013 cm−2.

Coarsening of cobalt grains dispersed in liquid copper matrix

Abstract: Coarsening of spherical Co-rich grains dispersed in Cu-rich liquid matrix is investigated. The specimen compositions are 50 pct Co-50 pct Cu, 40 pct Co-60 pct Cu, and 30 pct Co-70 pct Cu by weight. The annealing temperatures have been varied between 1150 and 1300 °C. The specimens have been prepared by usual powder metallurgy technique from fine Co and Cu powders. Due to the equal density of Co and Cu, the Co-rich grains remain uniformly dispersed in the liquid matrix. The increase of average grain size with annealing time,t, follows closely thet 1/3 law predicted for diffusion controlled mechanism by Lifshitz, Slyozov, Wagner (LSW) and Ardell. The observed increase of the growth rate with increasing solid grain fraction is a clear evidence for diffusion controlled mechanism. The linear intercept distribution of the grains agrees closely with the predictions for reaction controlled growth in LSW theory, but the result is also consistent with Ardell’s prediction that when the grains grow with small inter-grain distance under diffusion control, the grain size distribution is almost identical to that of the reaction controlled growth in LSW theory. The estimated values of the diffusion constant and its activation energy agree in order of magnitude with the typical values for diffusion in liquid metal.

Effect of Grain anisotropy on limit strains in biaxial stretching: part i. influence of sheet thickness and grain size in weakly textured sheets

Abstract: Processes of inhomogeneous deformation which lead to localized necking in biaxial stretching have been investigated in sheets of copper, Cu-30 pct Zn brass and low-carbon steels of good commercial quality without strongly developed preferred orientations. With sheet thickness,to, in the range 0.4 to 1.2 mm, it was found that limit strains in biaxial stretching decreased with decreasingto/do, wheredo is average grain diameter. It was concluded that, whento/do was less than about 20, plastic anistropy of individual grains of the primary phase was the dominant source of the strain inhomogeneities which developed to cause eventual necking failure. Measurement of the rate at which surface roughness developed with increasing strain,dR’/d- ge, indicated that, in the early stages of stretching, the growth of thickness inhomogeneity was close to being proportional todo and -ge and it was insensitive toto and the applied strain ratio, p, but, at an applied strain -gem which depended onto anddo, dR /d- ge started to increase progressively. In this latter phase of the process strain localization developed on a macroscopic scale. It is concluded that the dependence of -gem onto underlies the effects of sheet thickness on biaxial limit strains, also that the influence of p on the rate of growth of thickness inhomogeneities can change progressively during the evolution of strain localization through the microstructural and macroscopic phases.

Hard magnetic alloys of a transition metal and lanthanide

Abstract: A hard magnetic alloy comprises iron, boron, lanthanum, and a lanthanide is prepared by heating the corresponding amorphous alloy to a temperature from about 850 to 1200 K. in an inert atmosphere until a polycrystalline multiphase alloy with an average grain size not exceeding 400 A is formed.

Microstructures developed during thermomechanical treatment of HSLA steels

Abstract: One purpose of thermomechanical treatment of steels, for example the control rolling of plate, is to produce the finest uniform microstructure in the product to optimize its strength and toughness. To achieve this end requires control of the structual changes that occur during reheating, high-temperature (roughing) deformation, lower temperature (finishing) deformation, and austenite transformation. A study has been made of the effects of the deformation processing variables on the microstructural changes that occur in high-strength low-alloy (HSLA) steels in the temperature ranges in which complete, partial, or no recrystallization occurs. The experimental technique comprised a sequence of plane-strain compressions of specimens being cooled at rates controlled to simulate the rate of cooling of slabs being rolled to plates. The results show that in the complete recrystallization range a fixed pass schedule refines the initial grain sizes in steels of a wide range of compositions and initial grain sizes to about the same final size; the final recrystallized grain size decreases with deformation temperature (within the complete recrystallization range), increasing strain rate, and increasing draft,i.e., with any deformation parameter that increases flow stress; the solute content and initial grain size, which are fixed by reheat temperature, control the temperature at which complete recrystallization stops during hot rolling. Deformation in the “partial recrystallization” range causes duplexing in the austenite that cannot be removed by subsequent rolling, consequently rolling in this range should be avoided. During finishing, the height of the flattened austenite grains decreases with increasing finishing reduction, but at a rate less than the rate of decrease of specimen height, indicating that some recovery is occuring. The grain diameter of the ferrite formed from fine, elongated austenite is about half the austenite grain-boundary separation (measured in the throughthickness direction) indicating that the elongated boundaries are the primary nucleation sites for ferrite.

Modelling and minimization of membrane penetration effects in tests on granular soils

Abstract: In experimental research into the constitutive properties of granular materials the phenomenon of membrane penetration is one of the major sources of error. For instance, in drained isotropic loading and unloading tests on medium loose fine sand the errors in the volume change due to membrane penetration can be of the order of 20% and 40% respectively. In the first stage of an undrained triaxial compression test on the same sand, when the effective isotropic stress decreases, the deviator stress can become 50% greater owing to membrane penetration; because of this a tendency towards lique faction might remain obscure. It is shown that such errors can be reduced by a factor of 5 if the proper membranes are used and if the remaining membrane effect of restraint is accounted for properly. To this end the membrane should consist of much stiffer material than latex rubber, the membrane thickness should be of the order of the mean grain size and the ratio of the mean grain size and the sample diameter should no...

Low-temperature process to increase the grain size in polysilicon films

Abstract: The letter describes a novel process for increasing the grain size in polycrystalline silicon films at temperatures near 500°C. This process, once perfected, could lead to mono-crystalline or large-grain, uniformly oriented polysilicon films on amorphous surfaces.

Method of increasing the grain size of polycrystalline materials by directed energy-beams

Abstract: Crystal grain size in a material is increased by scanning the material with an appropriately directed energy beam. Short-term oscillation in the scan, and a particular temperature gradient configuration in the wake of the scan, results in growth of large-grain crystallites.

Electrodeposition of Silicon at Temperatures above Its Melting Point

Abstract: The electrowinning of molten silicon has been achieved for the first time using a melt. The silicon formed roughly spherical or ellipsoidal droplets up to 15 mm in diameter in the small experimental system used, and these solidified with a grain size of up to 1 mm. Material of 99.97% purity can be produced from 99.5% pure starting material without additional purification or pre‐electrolysis.