Showing papers on "Grain size published in 1991"

Grain size effects on gas sensitivity of porous SnO2-based elements

Abstract: Effects of grain size on gas sensitivity (S) are investigated by using porous sintered SnO2 elements fabricated with pure SnO2, foreign oxide-stabilized SnO2, or impurity-doped SnO2. When the SnO2 crystallite size (D) is controlled to a size in the range 5–32 nm, S for H2, CO and i-C4H10 is found to increase steeply as D decreases to be comparable with or less than 2L (≈ 6 nm) is both pure and stabilized elements, where L is the depth of the space-charge layer. However, S for ethyl alcohol gas is found to be also affected by surface acid-base properties, being greatly promoted by basic oxides. It is found that the control of L by doping impurities (Al3+ or Sb5+) into the SnO2 lattice results in great changes in S even when D is the same. Thus Al-doped SnO2 shows high sensitivity with increasing L even at D above 20 nm, while Sb-doped SnO2 is insensitive in the whole D region. A model for the grain-size effects is proposed, in which the transducer function is operated by a mechanism of grain control, neck control or grain-boundary control, depending on D.

Mechanical behavior of nanocrystalline Cu and Pd

Abstract: This report gives results of a study of the bulk mechanical properties of samples of nanocrystalline Cu and Pd consolidated from powders prepared by inert gas condensation. Fourier analysis x-ray diffraction techniques, used to determine average grain size and mean lattice strains of the as-consolidated samples, show grain sizes in the range of 3–50 nm and lattice strains ranging from 0.02–3%. Sample densities range from 97–72% of the density of a coarse-grained standard. Microhardness of the nanocrystalline samples exceeds that of annealed, coarse-grained samples by a factor of 2–5, despite indications that sample porosity reduces hardness values below the ultimate value. Uniaxial tensile strength of the nanocrystalline samples is similarly elevated above the value of the coarse-grained standard samples. Restrictions on dislocation generation and mobility imposed by ultrafine grain size are believed to be the dominant factor in raising strength. Residual stress may also play a role. Room temperature diffusional creep, predicted to be appreciable in nanocrystalline samples, was not found. Instead, samples appear to show logarithmic creep that is much smaller than the predicted Coble creep.

The dependence of grain size on stress during dynamic recrystallisation

Abstract: The steady state grain size during dynamic recrystallisation by grain boundary migration is shown to be simply related to deformation stress for a number of metals and minerals. The data plotted on a scale of grain size (D) divided by Burger's vector (b) against stress (σ) divided by shear modulus (μ) fills in a remarkably narrow range bounded by loci of form σ μ ( D b 2/3 = K with K = 1 and 10. Models of the dynamic recrystallisation process are developed to show that such a relation can be predicted by considering a dynamic balance between the rate of formation of the deformation substructure and the mean velocity of recrystallising grain boundaries. Hence providing a physical basis for the empirical relation derived from the normalised plot of experimental results.

Physical and chemical factors influencing transport of microorganisms through porous media.

Abstract: Resting-cell suspensions of bacteria isolated from groundwater were added as a pulse to the tops of columns of clean quartz sand. An artificial groundwater solution (AGW) was pumped through the columns, and bacterial breakthrough curves were established and compared to test the effects of ionic strength of the AGW, cell size (by using strains of similar cell surface hydrophobicity but different size), mineral grain size, and presence of heterogeneities within the porous media on transport of the bacteria. The proportion of cells recovered in the effluent ranged from nearly 90% for AGW of a higher ionic strength (I = 0.0089 versus 0.00089 m), small cells (0.75-micron-diameter spheres versus 0.75 by 1.8-micron rods), and coarse-grained sand (1.0 versus 0.33 mm) to less than 1% for AGW of lower ionic strength, large cells, and fine-grained sand. Differences in the widths of peaks (an indicator of dispersion) were significant only for the cell size treatment. For treatments containing heterogeneities (a vein of coarse sand in the center of a bed of fine sand), doubly peaked breakthrough curves were obtained. The first peak represents movement of bacteria through the transmissive coarse-grained vein. The second peak is thought to be dominated by cells which have moved (due to dispersion) from the fine-grained matrix to the coarse-grained vein near the top of the column and thus had been retarded, but not retained, by the column. Strength of effects tests indicated that grain size was the most important factor controlling transport of bacteria over the range of values tested for all of the factors examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Cataclasis and processes of particle size reduction

Abstract: The particle size distribution (P.S.D.) of fragmented geological materials is affected by the fragmentation process, initial size distribution, number of fracturing events, energy input, strain, and confining pressure. A summary of literature shows that the fractal dimension (D) of the P.S.D. is increased by the number of fracturing events, energy input, strain, and confining pressure. Cenozoic cataclasis of granite, granodiorites, gneisses and arkose seen in cores from the Cajon Pass dritlhole, southern California, produced P.S.D.s with values of D that varied from 1.88 to 3.08. Each rock type has a characteristic and more limited range of D. Areas of dilatant texture and mode I fracture-fillings have low average values (2.32 and 2.37) compared to an average value of 2.67 in shear fracture-fillings. D has a good inverse correlation with average particle size. Data from fault rocks in the San Gabriel fault zone, southern California (ANDERSON et al., 1983) have been reanalyzed to show that values of D are higher (2.10 5.52) and average particle size is lower than the Cajon Pass samples, but the ranges of values overlap, and the inverse correlation between D and average particle size is extended. Microstructural observations combined with these results suggest that three processes contributed to particle size reduction during cataclasis. The first process of feldspar alteration, which leads to low values of D, has not been previously recognized. The second process is probably constrained comminution (SAMMIS et al., 1987), since the average D in shear fracture-fillings is close to the value of 2.58 predicted by this theory. A further stage of particle size reduction is demonstrated by an increase of D with cataclasis. This third process is selective fracture of larger particles, which may also operate during localization and the cataclastic flow-to-faulting transition as observed in experiments. A transition from constrained comminution to selective fracture of large particles, and increasing D values with cataclastic evolution and grain size reduction, may be general features of experimental and natural cataclasis.

Grain-size strengthening in terms of dislocation density measured by resistivity

Abstract: The effect of grain size on flow stress has been investigated in terms of dislocation density. The measurement of dislocation density was made for nickel having a high stacking fault energy, by means of electrical resistivity with which the dislocation density can be measured up to larger strains compared with transmission electron microscopy. It was found that the dislocation density for a given strain in specimens deformed in tension at 77 and 295 K increases in a linear manner with the reciprocal of grain size. It was also ascertained that the flow stress is proportional to the square root of dislocation density, irrespective of grain size, deformation temperature and the amount of plastic strain (ϵ). From the above two relationships, an equation between flow stress and grain size was obtained in a general form, which gives the Hall-Petch relation as the limited case at yield point or at small strains.

The particle size of martian aeolian dunes

Abstract: The effective particle size of unconsolidated materials on the Martian surface can be determined from thermal inertia, due to a pore size dependence of thermal conductivity at Martian atmospheric pressures. Because dunes consist of a narrow range of well-sorted, unconsolidated particles, they provide for a test of the relationship between particle size and thermal inertia calculated from midinfrared emission data for the Martian surface. Two independent approaches are used. First, thermal inertia data indicate that Martian dunes have an average particle size of about 500 +/-100 microns, or medium to coarse sand. Second, expected dune particle sizes are determined from grain trajectory calculations and the particle size transition from suspension to saltation. On earth, the transition occurs for a grain when the ratio of the terminal fall velocity to the wind friction speed, u*(t) is near unity; for grains at u*(t) this occurs at about 52 microns. Terrrestrial dune sands have a mean of 250 microns and are composed entirely of grains greater than 52 microns. The corresponding Martian transition grain size is about 210 microns, suggesting that Martian dunes should be significantly coarser than terrestrial dunes. Grain saltation path length as a function of particle size also shows that, under Martian conditions, larger grains than on earth will become suspended. Both approaches indicate that Martian dune sand should be coarser than terrestrial dune sand. These results closely match the grain sizes determined from thermal inertia models, providing the first direct test of the validity of these models for actual Martian surface materials.

Frequency dependent electrical properties of polycrystalline olivine compacts

Abstract: The complex electrical properties of poly crystalline San Carlos olivine compacts were measured over the range of frequency l0−4–104Hz from 800° to 1400°C under controlled oxygen fugacity. The impedance data display a strong frequency dependence that is evidenced most clearly when the results are displayed in the complex impedance plane. A parameterized model of the frequency dependent electrical response using equivalent electrical circuits is presented. Two distinct conduction mechanisms of the sample are observed: grain interior and grain boundary conduction. Each occurs over a different range of frequency. The resistance of each mechanism adds in series resulting in a lower total DC conductivity for polycrystalline olivine than for either mechanism separately. The total DC conductivity is dominated by the grain interior conductivity above 1200°C, whereas the grain boundary conductivity has the strongest influence below 1000° C. Impedance spectra of natural dunite samples exhibit a similar type of frequency dependence. The grain interior conductivity displays a change in slope at 1344°C and has activation energies of 1.45 eV (800°–1344°C) and 4.87 eV (>1344°C). The grain boundary conductivity has an activation energy of 2.47 eV. In these cases, the ƒO2 for each experimental run was controlled at that of the wustite-magnetite oxygen buffer. Experiments on samples with different grain sizes reveal no dependence of DC conductivity on grain size for either mechanism, although the relaxation time and real relative permittivity of the grain boundary mechanism are dependent on grain size. Because of the electrical response observed at low frequencies, care must be taken in the inversion of electromagnetic field observations using laboratory measurements made in the kilohertz range since they may not be the same as DC measurements. Impedance measurements must be performed over a range of relatively low frequencies to assess the role of grain boundaries on the overall electrical response of polycrystalline materials.

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in pure O2

Abstract: Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The isothermal and cyclic oxidation resistance of the alloys were studied at 900°C in pure O2 to elucidate the role of alloy microstructure and Si content on oxidation properties. The conventionally-processed, large-grained alloy that contained no silicon formed Fe-rich nodules during oxidation. The nodule formation was effectively eliminated by either reducing the alloy grain size by rapid solidification or by adding Si to the alloy. The lowest weight gains were achieved when a continuous silica layer formed between the alloy and the external chromia scale. The formation of the continuous silica layer required a ombination of fine alloy grain size and high Si content. The presence of S in the alloy was found to be detrimental to oxide scale adherence when the silica layer was continuous.

Recrystallization of amorphous silicon films deposited by low‐pressure chemical vapor deposition from Si2H6 gas

Abstract: This paper investigated the recrystallization of low‐pressure chemical vapor deposition amorphous silicon (a‐Si) films deposited using Si2H6 gas at various substrate temperatures. The grain size of recrystallized films formed from Si2H6 is larger than that formed from SiH4. The maximum grain size is obtained at the substrate temperature of 460 °C, where the nucleation rate is minimum due to the maximum structural disorder of the Si network. The structural disorder is increased not only by lowering the substrate temperature but also by increasing the deposition rate. The field effect mobility of thin‐film transistors (TFTs) using the recrystallized films reaches 120 cm2 V−1 s−1, even though the highest temperature during the TFT fabrication process is only 600 °C.

Compositional changes of minerals associated with dynamic recrystallizatin

Abstract: The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.

The thermal and metallurgical state of steel strip during hot rolling: Part III. Microstructural evolution

Abstract: A mathematical model has been developed to compute the changes in the austenite grain size during rolling in a hot-strip mill. The heat-transfer model described in the first of this series of papers has been employed to calculate the temperature distribution through the thickness which serves as a basis for the microstructure model. Single-and double-hit compression tests have been conducted at temperatures of 900 °C, 850°C, 950 °C, and 875 °C on 0.34 and 0.05 pct carbon steels to determine the degree of recrystallization by metallographic evaluation of quenched samples and by measuring the magnitude of fractional softening. The Institut de Recherches de la Siderurgie Francaise, (IRSID) Saint Germain-en-Laye, France equation has been found to yield the best characterization of the observed recrystallization kinetics. The equations representing static recrystallization kinetics, recrystallized grain size, and grain growth kinetics have been incorporated in the model. The principle of additivity has been invoked to permit application of the isothermal recrystallization data to the nonisothermal cooling conditions. The model has been validated by comparing predicted austenite grain sizes with measurements made on samples quenched after one to four passes of rolling on the CANMET pilot mill. The austenite grain size evolution during rolling of a 0.34 pct carbon steel on Stelco’s Lake Erie Works (LEW) hot-strip mill has been computed with the aid of the model. The grain size decreased from an initial value of 180μm to 35μm in the first pass due to the high reduction of 46 pct. The changes in austenite grain size in subsequent passes were found to be small in comparison because of the lower per pass reductions. It has been shown that the equation employed to represent grain growth kinetics in the interstand region has a significant influence on the computed final grain size. Altering the rolling schedule had a negligible influence on the final grain size for a given finished gage. A 200°C increase in entry temperature to the mill resulted in a 20μm increase in final grain size, which is significant. This can be attributed to increased grain growth at the higher temperature.

Microstructure and strength of nanocrystalline copper alloy prepared by mechanical alloying

Abstract: Polycrystalline materials having an ultrafine grain size may be prepared by mechanical alloying. Such a material has been prepared here with a copper matrix and a uniform dispersion of particles which stabilises the fine microstructure. It is shown that the grain size of the copper matrix may be explained in terms of the conventional models of boundary pinning by particles, even for grain sizes below 40 nm. For grain sizes larger than about 100 nm, material strength may be explained by dislocation-particle interactions as illustrated by TEM observations. For grain sizes below this limit, however, strengthening is not as great as dislocation theory would predict based on the distribution of particles in the material; in addition TEM observations show no indication of the presence of dislocations. A different deformation mechanism seems to control strengthening for these materials of nano-scale grain size.

Nanocrystalline materials by crystallization of metal-metalloid glasses

Abstract: The formation of ultrafine microstructures by crystallization of metal-metalloid glasses was investigated by means of electron microscopy as well as in situ time-resolved X-ray diffraction. The results can be understood on the basis of nucleation and growth theories, taking into account the effect of recalescence during massive crystallization and the differences in the mode of crystallization and the diffusivity. In a polymorphic crystallizing Fe66Ni10B24 glass the finest microstructure can be achieved by annealing at temperatures significantly below the “nose” of the TTT diagram; the finest grain size can be calculated and observed to be in the range of about 0.1 μm. In glassy Fe73.4Cu1Nb3.1Si13.4B9.1 (FINEMENT) the combination of a reduced growth rate due to the niobium content as well as with increasing size of the primary crystals and an accelerated nucleation rate due to the copper additions allows the formation of extremely fine-grained microstructures in primary crystallizing metal-metalloid glasses at temperatures above the glass transition.

Experimental compaction of quartz sand at low effective stress and temperature conditions

Abstract: Experiments have been carried out on dry and fluid-saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress-induced solution transfer (9pressure solution9) as a compaction mechanism. The experiments were performed at temperatures ( T ) in the range 150–350°C, applied effective stresses (σ e ) up to 20.7 MPa and pore fluid pressures (P 1 ) of 12.5 and 15.5 MPa, using material with a grain size ( d ) in the range 20–100 μm and an initial porosity of 45–52%. Dry quartz sands underwent significant compac­tion during the loading stage, but showed very little compaction creep once full load was achieved (i.e. essentially time-independent compaction). In contrast, fluid-saturated material at constant applied effective stress showed substantial time-dependent compaction (i.e. creep). With increasing temperature, there is a decreasing number of grains in the wet-compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains. In addition, there is a decreasing dependence of the volumetric strain rate, β on σ e , with both increasing volumetric strain (e v ) and increasing σ e and a decreasing dependence of β on e v with increasing temperature. These observations suggest that, for wet quartz sand, a gradual change might occur from compaction creep controlled by time-dependent microcracking, at T = 250–300 °C, to compaction creep controlled by stress-induced intergranular solution transfer at T = 300–350 °C.

Polysilicon thin-film transistors with channel length and width comparable to or smaller than the grain size of the thin film

Abstract: Poly-Si thin-film transistors (TFTs) with channel dimensions (width W, and length L) comparable to or smaller than the grain size of the poly-Si film were fabricated and characterized. The grain size of the poly-Si film was enhanced by Si ion implantation followed by a low-temperature anneal and was typically 1 to 3 mu m in diameter. A remarkable improvement was observed in the device characteristics as the channel dimensions decreased to W=L=2 mu m. On the other hand, TFTs with submicrometer channel dimensions were characterized by an extremely abrupt switching in their I/sub D/ versus V/sub GS/ characteristics. The improvement was attributed to a reduction in the effect of the grain boundaries and to the effect of the device's floating body. >

Grain Size Dependence of the Fracture Toughness of Silicon Nitride Ceramics

Abstract: Improved KIC values of monolithic Si3N4 ceramics will be expected by growing-in a matrix of microcrystals of β-phase Si3N4-an appreciable amount of extensively large, rod-shape crystals of the same phase. It is anticipated that the large grains should play as crack impediments like whiskers mixed in composite ceramics. The test materials prepared by changing firing conditions had such well-grown crystals of different sizes, ranging from 2 to 10 microns in diameter. The fracture toughness (measured by the SEPB method) increases up to 11.3MPa·m1/2 with the increase in the grain size. Prominent crack bridging along the fracture surface behind the propagating crack tip is found to be the possible toughening mechanism.

Nucleation and Concurrent Anomalous Grain Growth of α‐Al2O3 During γ ‐ α Phase Transformation

Abstract: Nanocrystalline Al203 thin films (50 nm in thickness) have been synthesized by rf reactive sputtering deposition and subjected to annealing at temperatures ranging from 800° to 1200°C. TEM analysis indicated that the as-deposited alumina films contained both amorphous phase and metastable γ phase. Structural texture evolved in the films annealed at 800°C for 24 h; the texture had a [00l] preferred orientation and occurred along the {400} and {440} planes of γ Al203, . In the films annealed a t 1200°C for 2h, nucleation and concurrent anomalous grain growth of α-Al203took place in a fine-grained, polycrystalline γ-Al203 matrix. The anomalously grown γ-AL2O3 grains were primarily [0001]-oriented single crystals with grain sizes varying from 3 to 15 pm, while the γ-Al2O3 matrix had an average grain size of 50 nm. The γ-Al2O3 matrix was also strongly textured along the [001] axis and exhibited a heavily faulted, layered micro-structure. Most of these layers were oriented along the {220} crystallographic planes. Periodic superstructure was identified in the layered γ- Al2O3. The formation of layered structure in γ- Al2O3 is attributable to the change of stacking sequence of atomic layers along the {220} orientations. An atomic model is presented to explain the formation of layered structure in γ- Al2O3. The nucleation of α- Al2O3 appears to occur along the {220} crystallographic planes of γ- Al2O3. The explosive grain growth of α- Al2O3 during the γαphase transformation is explained by a mechanism involving interface boundary migration and lattice epitaxy. The orientation relationships between γ- and α- Al2O3 are determined.

Magnetic properties and microstructures of the Nd‐Fe‐B magnet powder produced by hydrogen treatment

Abstract: The hydrogen treatment of the Nd‐Fe‐B alloy ingots was found to produce magnet powders with good magnetic properties. Typical magnetic properties of these powders are as follows; 4πIs = 9.5 kG, Br = 7.7 kG, iHc = 9.4 kOe, and (BH)max = 12.2 MGOe. Microstructural studies of these powders showed that they are made of fine crystalline grains of ∼0.3 μm diameter and that these crystalline grains in the individual magnet powder are not necessarily enclosed with boundary phase(s), which is quite different from previously known Nd‐Fe‐B magnets, i.e., the sintered magnet (the nucleation type magnet) or the amorphous ribbon magnet (the pinning type magnet). It is also noted that the size of these crystalline grains is comparable to that of the single magnetic domain of the tetragonal Nd2Fe14B intermetallic compound and the coercive force of these powders appears to be related to their fine crystalline grain size.

Effect of Alumina Addition on the Tetragonal‐to‐Monoclinic Phase Transformation in Zirconia–3 mol% Yttria

Abstract: The tetragonal-to-monoclinic martensitic phase transformation in ZrO–3 mol% Y2O3 (PSZ) containing 0 to 12 wt% Al2O3 was investigated by dilatometry, XRD, and SEM-EDS methods. The propagation of the transformation into the specimen interiors was suppressed by the addition of Al2O3. The grain size was independent of the addition of Al2O3. Both Y2O3 and Al2O3 segregated at grain boundaries. From this segregation behavior, it was suggested that a certain compound or phase of Y2O3–Al2O3 could be formed at grain boundaries, which would presumably prevent the propagation of the transformation into interiors of PSZ-containing Al2O3.

Structural characterization of nanometer-sized crystalline Pd by x-ray-diffraction techniques.

Abstract: Quantitative x-ray-diffraction measurements of ultrafine-grained (nanocrystalline) Pd samples and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The intensity profiles of the Bragg reflections from the nanocrystalline samples were considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of intensity from the Bragg peaks was found in the tails of the reflections. The remaining intensity differed only slightly for different grain-sized materials, therefore, atomic relaxations in the vicinity of grain boundaries in nanocrystalline Pd must be small in magnitude and/or extremely localized. The results of the present work do not support the previously proposed existence of either a ``gaslike'' grain boundary phase, or large quantities of vacancies or voids within the grains of nanocrystalline Pd, which produce broadly distributed diffuse scattering. The broadening of the Bragg reflections was related to the small particle size of nanocrystalline Pd, and strain located in the grains and/or interfacial regions. Evidence was seen for anisotropic grain shapes preferentially elongated along the [111] direction. The Debye-Waller parameter of nanocrystalline Pd was observed to be larger than the literature value for coarse-grained Pd, which suggests larger displacements of the atoms from their ideal lattice locations in the nanocrystalline material than in the coarse-grained material.

Domain observation on nanocrystalline material

Abstract: Bulk polycrystalline material with a grain size in the nanometer range can be prepared by controlled crystallization of certain metallic glass alloys. The material shows excellent soft magnetic properties which are attributed to an effective averaging of the local crystal anisotropy over the scale of a Bloch wall width. The general appearance of observed domain patterns resembles those of metallic glasses. Wide in‐plane domains as well as the characteristic stress patterns are observed. On annealing beyond the optimum temperature the coercivity rises which manifests itself by an irregular domain appearance. An interesting behavior is found if the samples are observed at temperatures beyond ∼320 °C. At this temperature the amorphous ferromagnetic grain boundary phase which couples the iron‐rich crystallites becomes paramagnetic. The domains reversibly show a similar character as the overannealed samples. This observation indicates that overannealing leads in this material not only to grain growth beyond th...

A model for the effect of line width and mechanical strength on electromigration failure of interconnects with “near-bamboo” grain structures

Abstract: A simple analytical model for the effect of mechanical strength and line width (for the case of narrow lines) on the electromigration failure of metallic interconnects is presented. Because the line width/grain size ratio and the diffusivity enter differently in the model, application of the resulting failure time equation to published data can provide insight into the mechanisms of enhancement of electromigration resistance by grain structure optimization and alloying.