Large Number Of Grains Research Articles

Properties of spray-deposited CdIn2Se4 thin films for photovoltaic applications

Thin films of cadmium indium selenide have been deposited onto amorphous glass substrates using a simple and low-cost spray pyrolysis technique. The aqueous solutions containing precursors of Cd, In and Se have been used to obtain good quality deposits at optimized substrate temperature. The preparative parameters such as concentration, substrate temperature, etc. have been optimized by photoelectrochemical technique. The films have been characterized by techniques such as X-ray diffraction, scanning electron microscope, EDAX and electrical characterization. The photoelectrochemical characterization shows that both short-circuit current (I sc ) and open-circuit voltage (V oc ) are at their optimum values at the optimized substrate temperature of 280C and concentration (0.0125 M). The XRD patterns show that the films are polycrystalline with cubic structure. The SEM studies reveal the compact morphology with large number of grains. Compositional analysis reveals that the material formed is nearly stoichiometric at the optimized preparative parameters. The electrical resistivity shows that the films are semiconducting and the resistivity is found to be minimum at optimized substrate temperature. The thermoelectric power (TEP) measurement reveals that the TEP is relatively maximum at same substrate temperature of 80°C. Photovoltaic output characteristics study reveals that the fill factor and efficiency; to be 69% and 3%, respectively.

Crystal plasticity simulation of the hydrostatic bulge test

In this article the finite–strain deformation of a membrane under hydrostatic pressure is investigated using a rate-dependent polycrystal plasticity formulation based on Taylor assumptions, where each material point in the sheet is considered to be a polycrystalline aggregate of a large number of grains. A finite element program is developed for arbitrary die aperture geometries and numerical results are generated for a circular specimen using parallel computing features. The plane stress assumption is invoked at the macro level, since the sheet thickness is small compared to the surface dimensions of the specimen. The deformation gradient is computed with the requirement that the two shearing strains along the sheet normal are explicitly enforced to be zero. The effect of deformation induced anisotropy is examined for a single crystal as well as for different polycrystalline materials, with and without initial orthotropy. Using the maximum stress criterion for the rate-dependent material model employed, the onset of instability for the membrane is examined for succesive localization modes. First, the onset and development of biaxial straining localized around the pole, and then the onset of an instability at this region involving localized necking (similar to the forming limit diagrams), are demonstrated and discussed from numerical examples.

Bounds on fatigue damage: relation to microstructure dependent crack growth – I. Constant amplitude cyclic damage

The paper attempts to account for the complex micro-processes at a crack tip through a phenomenological model of damage accumulation based on macro damage curves. With the S-N curve as a basis, bounds are established that bracket the mean cyclic damage in reversed constant amplitude cycling. Proceeding from single slip and opening mode damage curves, two-mode damage curves are derived that reflect the micro process of crack tip advance by the presence on a macro scale of both damage modes in each cycle. Discrete microstructure barriers are identified that help divide the fatigue process on a macro scale into three stages, the analogues of the three creep stages under constant stress whereby the microstructure short crack (MSC), physically small crack (PSC) and macro crack regimes correspond to the `primary', `steady' and `tertiary' creep macro stages. The fatigue process is found to consist at each cyclic load level of a macro slip stage followed by an opening mode stage with an abrupt damage rate change at the transition, in line with experimental evidence (Socie, 1993). The barriers divide the entire region behind the S-N curve into distinct cyclic strain (stress) domains dominated on a macro scale by the opening mode in regions nearest the fatigue limit and by the slip mode in the uppermost region while an intermediate region is dominated by both. In the region nearest the fatigue limit the opening mode dominates virtually from first load reversal, in line with a view (Miller, 1985, 1987) that relates damage to surface imperfections. Macro damage accumulation involving a discontinuous damage rate transition is treated on the basis of discrete barriers while a continuous spectrum of barriers is introduced in an attempt to reflect the crack tip process on a macro scale. A theoretical model of microstructure resistance is introduced that describes crack arrest by the barriers and leads to a distinct set of macro-damage bounds in the interval between barriers, hence in each region of the S-N curve. The entire range of fatigue lifetimes below and above the fatigue limit is examined showing that only the slip mode carries over to the region below the fatigue limit resulting in non-propagating damage. Assuming that macro damage can be defined as a ratio of crack lengths, two scales are introduced that lead to bounds on mean crack length: a macro scale connecting two disparate levels of the fatigue process defines a macro volume element enclosing a sufficiently large number of grains to be representative of the actual medium as a whole and a micro scale of grain size order with a corresponding role. Bounds on mean crack growth rate are found to reduce in the limit of long cracks to the known Paris rate relation. Two distinct sets of bounds on mean damage and damage rate are found: one for unloading from a high to low cyclic strain another for loading from a low to high strain (stress), congruent with the distinct loading/unloading paths in a hysteresis loop. A simple test method is proposed to locate the fatigue limit indirectly. The model is applied to cumulative damage analysis in part II where residual lifetime mean and variance in two-stage fatigue cycling are predicted.

Linking atomistic and mesoscale simulations of nanocrystalline materials: quantitative validation for the case of grain growth

Using grain growth in nanocrystalline palladium as a simple case study, we demonstrate how a novel mesoscale approach for simulating microstructural evolution in polycrystalline materials can be validated directly against atomic-level simulations of the same system. We first describe molecular dynamics simulations of grain growth in a columnar model microstructure. The atomic-level insights into the grain-growth mechanism gained from these simulations, particularly in the role of grain rotations, are captured theoretically for incorporation into the mesoscale approach, in which the objects evolving in space and time are the grain boundaries and grain junctions rather than the atoms. With all the input parameters to the mesoscale being physically well defined and obtained directly from the atomic-level simulations, the mesoscale simulations are fully prescribed. We find that the morphology of the mesoscale system evolves in an almost identical manner with that of the molecular dynamics simulation, demonstrating that the length- and time-scale linking has been performed correctly. When applied to systems containing large numbers of grains, the now validated mesoscale simulation approach allows the growth topology and long-time growth kinetics to be determined. As an outlook, we describe how the effects of applied stress can be incorporated.

Orientation rotation behavior during in situ tensile deformation of polycrystalline 1050 aluminum alloy

Microstructural evolutions and orientation rotation behavior taking place during uniaxial tensile deformation at room temperature are studied in polycrystalline 1050 aluminum alloy. Specimens are elongated in an in situ deformation stage equipped in a high-resolution electron backscattered diffraction (HR-EBSD) system. Variations in grain boundary characteristics, such as misorientation and grain boundary distribution, during in situ tensile deformation are analyzed statistically using HR-EBSD. Orientation rotation behavior in individual grains is traced during the whole deformation process. A large number of grains in polycrystalline 1050 aluminum alloy rotate during deformation and are divided into subgrains due to multiple slip. The plane normal directions of individual grains show no remarkable tendency of the orientation rotation during in situ tensile deformation, while the orientation rotation of the tensile direction is found to be strongly dependent on the initial orientation of each grain prior to deformation.

On the mechanics of the grinding process – Part I. Stochastic nature of the grinding process

Grinding of metals is a complex material removal operation involving cutting, ploughing, and rubbing depending on the extent of interaction between the abrasive grains and the workmaterial under the conditions of grinding. It is also a stochastic process in that a large number of abrasive grains of unknown geometry, whose geometry varies with time, participate in the process and remove material from the workpiece. Also, the number of grains passing through the grinding zone per unit time is extremely large. To address such a complex problem, it is necessary to analyze the mechanics of the grinding process using probability statistics, which is the subject of this investigation. Such an analysis is applicable to both form and finish grinding (FFG), such as surface grinding and stock removal grinding (SRG), such as cut-off operation. In this investigation, various parameters of the process including the number of abrasive grains in actual contact, the number of actual cutting grains per unit area for a given depth of wheel indentation, the minimum diameter of the contacting and cutting grains, and the volume of the chip removed per unit time were determined analytically and compared with the experimental results reported in the literature. Such an analysis enables the use of actual number of contacting and cutting grains in the grinding wheel for thermal and wheel wear analyses. It can also enable comparison of analytical work with the experimental results and contribute towards a better understanding of the grinding process. The analysis is applied to some typical cases of fine grinding and cut-off operations reported in the literature. It is found that out of a large number of grains on the surface of the wheel passing over the workpiece per second (~million or more per second), only a very small fraction of the grains merely rub or plough into the workmaterial (∼3.8% for FFG and ∼18% for SRG) and even a smaller fraction (∼0.14% for FFG and ∼1.8% for SRG) of that participate in actual cutting, thus validating Hahn’s rubbing grain hypothesis.

Characterization and significance of shocked quartz from the Woodleigh mpact structure, Western Australia

Abstract— We re‐examined the buried Woodleigh structure in Western Australia, which has been inferred to be a multi‐ringed, 120 km diameter impact crater, because the proposed size and possible synchronicity with one of the pre‐Mesozoic mass extinction events has attracted controversy. We undertook a detailed study of the petrology and mineralogy of a number of samples of core from the Woodleigh‐1 borehole that was drilled into the central uplift of the structure. Crystalline Proterozoic basement rocks comprising granites and gneisses in the Woodleigh‐1 core contain minor brecciation in discrete veins and reveal clear evidence of shock metamorphism over the full extent of the core. Imaging of laboratory‐etched quartz showed that a large number of grains contain shock deformation lamellae. Microstructural and crystallographic analysis of these lamellae by TEM showed that they are planar deformation features (PDFs) that have subsequently undergone annealing and water assisted recrystallization. The available geological, petrographic, and mineralogical evidence suggest that Woodleigh is an eroded impact crater that is nearer to 60 km than 120 km in diameter. Future drilling projects should better constrain the level of erosion, and may reveal any preserved impact lithologies.

Micromagnetic modeling of first-order reversal curve (FORC) diagrams for single-domain and pseudo-single-domain magnetite

First-order reversal curve (FORC) diagrams have been experimentally shown to be a better way of discriminating domain states in a sample compared to the straightforward use of major hysteresis loops. In order to better understand the fundamental behavior of assemblages of single-domain (SD) grains, we used a micromagnetic model with a conjugate gradient algorithm to calculate FORC diagrams for isolated grains of magnetite as well as for arrays of grains. In the case of individual elongated grains, we found that the FORC diagram consists of a single peak centered on the coercive force Hc if the grain is SD. For a pseudo-single-domain (PSD) grain with vortex structure, we observe multiple peaks on the FORC diagram. The modeling of arrays of elongated SD particles reveals two distinct types of patterns depending on the spacing between particles. In a 2×2×2 array of particles, a secondary branch on the reversal curves appears if the spacing between particles is less than about twice the particle length. This feature translates into the appearance of one negative and three positive peaks on the FORC diagram. In the case of a 3×3×3 array of particles, we again observe several secondary branches when the spacing between grains is less than about twice the particle length, leading to the appearance of multiple peaks on the FORC diagram. Splitting of the central peak on the Hu axis when particles interact could explain the vertical spread of FORC distributions of natural interacting samples as an effect of superposition of multiple peaks caused by the random orientation and distributions of particle spacing and switching fields of a large number of grains. The presence of symmetric peaks on a FORC diagram can be an indicator of the presence of either small PSD grains or magnetic interactions in an ensemble of grains.

Construction of an averaged stress tensor for a granular medium

The aim of this paper is to construct an averaged stress tensor for a granular media, valid in static and in dynamics, which takes into account the contact reactions and the body forces acting at the grain level. The construction, based on a simple formula of linear algebra, is inspired from the Cauchy processes. General balance principles ensure the fundamental properties of the averaged stress tensor so defined: it is automatically symmetric and invariant by translation (independent from the origin chosen). In the second part, a simple analytical example shows that taking account of the inertia forces is essential to insure the symmetry of the averaged stress tensor in dynamics. Finally, numerical simulations illustrating this definition of the averaged stress tensor are presented for granular media containing a large number of grains.

Optical dating of dune sand from Blombos Cave, South Africa: II--single grain data.

A sequence of optically stimulated luminescence measurements was made on each of 8,961 grains from three sand samples from Blombos on the southern Cape coast. One sand unit overlay Middle Stone Age deposits in Blombos Cave. The measurement sequence, the single aliquot regenerative dose protocol, was used to obtain values for the total effective radiation dose to which each grain had been exposed since burial. A series of checks was carried out on each grain to ensure that the luminescence signals were reproducible, and that they were derived from quartz. This led to acceptance of less than 5% of the grains. An estimate of the radiation dose for the sand unit was obtained by combining the values using the central age model. In order to use a larger number of grains that might be representative of the sand unit, the radiation dose was also estimated by using the signal from the above grains, combined with the signals from those grains that had lower signals, but nonetheless contributed to the total light sum; this utilised between 9 and 18% of the grains. This enables us to obtain estimates of the ages as 67.3+/-3.8 ka, 65.6+/-2.8 ka and 68.8+/-3.0 ka for the three samples. These values agree with ages obtained using the single aliquot regenerative dose protocol for aliquots composed of several hundred grains.

OSL ages and possible bioturbation signals at the Upper Paleolithic site of Lagoa do Bordoal, Algarve, Portugal

Three sediment samples were collected from a soil profile at the Lagoa do Bordoal site, Algarve, Portugal. Quartz extracted from soil horizons within the profile, was optically dated using the single-aliquot regenerative-dose (SAR) protocol outlined by Murray and Wintle (Radiation Measurements 5 (2000) 523). The optically stimulated luminescence (OSL) emitted by quartz within the 90–150 μm size range was measured using three aliquot sizes. The equivalent doses ( D E) that were obtained show an asymmetric distribution. As the numbers of grains in each aliquot are reduced, the distribution of doses is clearly shifted to lower D E values including zero doses on single grain aliquots. This shift is accompanied by an increase in the asymmetry of the distribution. These patterns indicate that as the aliquot size decreases, the distributions are increasingly characterized by aliquots with lower D E's. With respect to the present day modern surface, this shift is most evident in the sample collected from a depth of 37 cm, within an ancient B-horizon. It is least evident in the sample located only 10 cm below it, within a sand unit. An asymmetric distribution of D E's can be a result of a large number of grains that were well bleached at burial being mixed with a small number of grains that were partially bleached at that time (Radiation Measurements 30 (1999) 207). However, the shift in D E's that is seen in the Lagoa East samples is most likely due to the postburial downward movement of fully and partially bleached grains from the surface, with possible contributions from the downward movement of grains that received low doses while buried in overlying horizons. The simplest explanation for our results is that grains carrying zero or small D E's have been moved downward during bioturbation events. These results suggest that downward bioturbation in ancient soil profiles can be identified based on the OSL characteristics of quartz grains. We are also able to provide a minimum age of 24.4±3.2 ka for the Upper Palaeolithic lithic assemblage at the site.

Concurrent finite element analysis of periodic boundary value problems

A parallel finite element approach for analyzing micromechanical problems with periodic unit cells is discussed. The method uses a direct solution strategy so that general periodic boundary conditions can be treated using a two-step domain decomposition strategy. The speedup results show a good performance of the method on coarse-grained problems, i.e. for cases where the computational work done on the substructures that are treated in parallel is relatively large compared to the total amount of computational work. Application examples using crystal-plasticity on an array of planar crystals and a metal matrix composite are used to show that the overall response of these materials is rather strongly dependent on the constraint imposed on the unit cell so that a correct treatment of the periodic boundary conditions is required to accurately predict the macroscopic response of a periodic material even though a unit cell with a large number of grains or fibers is used.

Contact dynamics simulations of compacting cohesive granular systems

The modelling of cohesion on the particle scale in granular matter [Vermeer et al., 2001] [1] has recently received increasing attention, because various important applications call for a microscopic foundation for the existing phenomenological continuum theories. Specifically for understanding the different behaviour of nano-powders and ordinary granular matter cohesion has to be taken into account. An example in this context is the so-called sinter-forging of nano-powders, i.e. pressure sintering at relatively low temperatures [Skandan et al., 1994] [2]. We model the corresponding compaction by the rearrangement of a large number of grains by means of contact-dynamics. For this purpose we introduce an attractive force (cohesion) as well as rolling friction between the (round) grains. The latter turns out to be crucial for the stabilization of force chains.

(Sub)millimetre emission from NGC 1569: An abundance of very small grains

We present new data of the dwarf galaxy NGC 1569 at 450 $\mu$m, 850 $\mu$m and 1200$\mu$m taken with SCUBA at the JCMT and the bolometer array at the IRAM 30m telescope. After including data from IRAS at 12, 25, 60 and 100 $\mu$m, we have successfully fitted the dust grain population model of D\'esert et al. (1990) to the observed midinfrared-to-millimeter spectrum. The fit requires a combination of both large and very small grains exposed to a strong radiation field as well as an enhancement of the number of very small grains relative to the number of large grains. We interpret this as the consequence of large grain destruction due to shocks in the turbulent interstellar medium of NGC 1569. The contribution of polyaromatic hydrocarbons (PAH's) is found to be negligible. Comparison of the dust emission maps with an HI map of similar resolution shows that both dust and molecular gas distributions peak close to the radio continuum maximum and at a minimum in the HI distribution. From a comparison of these three maps and assuming that the gas-to-dust mass ratio is the same everywhere, we estimate the ratio of molecular hydrogen column density to integrated CO intensity to be about 25 -- 30 times the local Galactic value. The gas-to-dust ratio is 1500 -- 2900, about an order of magnitude higher than in the Solar Neighbourhood.

Analysis of the milli-forming of single-crystal and poly-crystal bar

This paper is concerned with the analysis of the milli-forming process, which forms milli-size products. Since the size of the workpiece in a milli-forming process ranges from a few hundred micrometers to a few millimeters, microstructural changes such as the growth of micro-voids and the development of preferred orientation in a grain become crucial factors for the success of milli-forming. In the proposed finite element model, anisotropy from deformation texture and the deterioration of mechanical properties due to the growth of micro-voids are coupled in the virtual work expressions for momentum and mass balance. Two extreme cases of a milli-bar, one made up of a single crystal and the other of poly-crystals or a large number of grains, are chosen for the simulations of extrusion/drawing. The predicted results are carefully examined in terms of extrusion/drawing forces, preferred orientations, and damage evolution in order to understand the deformation characteristics of a milli-bar.

Grain Yield Components of Winter Wheat New Cultivars in Correlation with Sowing Rate

During the process of plant breeding and selection of new winter wheat cultivars a lot of attention is paid to the improvement of the main components of grain yield-number of ears/m2, number of grains per ear and 1000 seed weight. Production of cultivars that achieve a large number of fertile ears per unit area, a large number of grains per ear and a heavy 1000 seed weight, even with a lower density, would mean that a high grain yield is achieved with lower costs. It is of great importance for wheat production in Croatia. Along with the research on sowing rate influence (400, 500, 600 and 700 germinable seeds/m2) on winter wheat grain yield, other trials were done on the grain yield components (number of ears per unit area, number of grains per ear and 1000 seed weight) during periods 1996–1997 and 1997–1998 both on three AG cultivars of winter wheat Lara, Kruna and Lenta and on the check cultivar Ẑitarka. The trial was done according to the split-plot method in 4 replications. Statistically speaking, various sowing rates had a very significant influence on the number of ears per unit area. The largest number was achieved with the sowing rate of 700 germinable seeds/m2 (783,38 ears/m2). This number of ears is statistically more significant (P<0,05) in relation to the sowing rate of 600 germinable seeds/m2, and statistically very significant (P<0,01) in relation to the sowing rates of 400 and 500 germinable seeds/m2. On the grounds of the results obtained by a statistical analysis it was established that the sowing rate did not have a statistically significant influence on the other two grain yield components — number of grains per ear and 1000 seed weight. The investigated cultivars had a statistically very significant influence (P<0,01) on the number of grains per ear, and a statistically significant influence on 1000 seed weight, whereas there were no statistically significant differences between the investigated cultivars in relation to the number of ears per unit area. The largest grain number was achieved with the cultivar Lenta (46,36 grains) and the heaviest 1000 seed weight with the cultivar Lara (45,72 g). The year of research had a very significant influence on the height of all investigated yield components in terms of statistics; the first year of research is characterized by better results, unlike the second. On the grounds of the obtained results it may be concluded that production of the investigated cultivars with lower density is possible, despite the fact that two out of three main yield components were not under influence of the sowing rate.

Linhagens diaplóides de trigo obtidas via cultura de antera

Realizaram-se quatro experimentos de campo no Estado de São Paulo, tanto em condição de sequeiro como de irrigação por aspersão, empregando o delineamento experimental de blocos casualizados, visando a comparação de 20 genótipos de trigo, sendo 18 linhagens diaplóides obtidas e dois cultivares, IAC-24 e Anahuac, quanto à produção de grãos, características agronômicas, resistência à ferrugem-da-folha e outros componentes da produção. Também estudou-se a tolerância ao alumínio em soluções nutritivas em condição de laboratório. As linhagens diaplóides 11, 12, 14, 17 e 18, originárias do cruzamento IAS-63/ALDAN "S"//GLEN/3/IAC-24, de porte baixo, com resistência ao acamamento, com ciclo precoce da emergência ao florescimento e da emergência à maturação e tolerância à toxicidade de alumínio, destacaram-se quanto à produção de grãos, considerando a média dos quatro experimentos. A linhagem 8 mostrou ser fonte genética de espiga comprida; a 15, de maior número de espiguetas por espiga; o cultivar Anahuac de maior número de grãos por espiga e por espigueta, e as linhagens 10, 11, 13, 15, 16 e 18 de grãos mais pesados. Todos os genótipos foram tolerantes à toxicidade de Al3+, exceto a linhagem 5, sendo que o cultivar Anahuac exibiu elevada sensibilidade. A técnica de obtenção de linhagens diaplóides via cultura de anteras de plantas em geração F1 de cruzamentos de trigo foi eficiente originando genótipos produtivos, com características agronômicas desejáveis e com tolerância à toxicidade de alumínio, num menor período de tempo em relação ao método convencional de melhoramento genético do Instituto Agronômico.

Crack-tip constraint effects on creep fracture

A three-term expansion describing the crack tip fields in steady power-law creeping solids is used in a small-scale damage microstructural approach to study the initial stages of creep fracture under plane strain mode I loading conditions. These fields contain three important parameters. The first one is C ∗, which sets the loading level, and the remaining, A ∗ 2 and σ ∞, account for the constraint effect imposed by the specific geometry and loading configuration. The microstructural model incorporates a process window around the tip of the crack, which contains a large number of grains that are represented discretely by so-called grain elements. The grain boundaries are described by interface elements that incorporate the principal damage mechanisms, including cavity nucleation, diffusive-creeping cavity growth and grain boundary sliding. The process window is surrounded by a standard creeping continuum which is subject to remote boundary conditions corresponding to the crack tip fields in a steadily creeping material. Numerical results of the model are presented for a range of constraints using values of A ∗ 2 and σ ∞ that correspond to typical test specimens. The effect of crack-tip constraints is demonstrated and explained for material parameter sets that give rise to either ductile or brittle creep fracture processes.

The Linkage Between Microscopic Cavitation Damage and Macroscopic Crack Growth

This paper is concerned with a recent microstructural approach to model creep crack growth. The model spans three different length scales, from the scale of individual cavities, through the grain scale up to the macroscopic scale of cracks in components and test specimens. In order to study the initial stages of creep crack growth, we consider a near-tip process window in which a large number of grains are represented discretely. This window is surrounded by a standard continuum. Macroscopic specimen dimensions and loading configuration are communicated to this near-tip region by applying boundary conditions in accordance with the asymptotic stress fields for power-law creeping materials. The paper presents some novel results of this type of modeling obtained using remote higher-order crack-tip fields. Specific attention is focused on the effect of random nucleation and grain deformation on nonsymmetric crack growth from either initially sharp or blunt cracks. [S0094-4289(00)00703-9]

Comparison of the Semisolid Shear Behaviour of Al-7Si-0.35Mg Alloys Produced by Two Casting Methods

Al-7%Si-0.35%Mg alloys, with and without grain refiner addition, were produced by casting into cylindrical moulds. Their microstructure and shear behaviour in the semisolid state were compared with a standard thixotropic 356 alloy that was produced by electromagnetic stirring (EMS). The as-cast microstructures of the cast materials consisted of equiaxed grains with rosette-dendritic morphology, while the EMS material was made up of very fine and rosette-like grains but with a significant number of large grains or agglomerates. After partial remelting and isothermal holding, the cast materials lost their dendritic nature and became globular. The EMS material continued to contain large globular particles after isothermal holding. The shear behaviour of the semisolid materials was measured by a direct shear test. The shear resistance was high if the material had a dendritic microstructure, and was reduced after a globular microstructure developed during isothermal holding. The shear strength for the three alloy conditions studied varied with isothermal holding time and this was related to microstructural differences between the alloys.