Structure, energy, and structural transformations of graphene grain boundaries from atomistic simulations

Domain/grain boundaries are often introduced into graphene during chemical vapor deposition growth processes. Here, we performed a series of hybrid molecular dynamics simulations to study the structures, energies, and structural transformations of symmetric tilt grain boundaries of graphene. The grain boundary comprises an array of edge dislocations, with the dislocation density increasing upon increasing the grain boundary misorientation angle. The dislocation in the zigzag-oriented grain boundary contains an edge-sharing pentagon/heptagon defect, whereas the dislocation in the armchair-oriented grain boundary contains two paired pentagon/heptagon defects. In some grain boundaries, out-of-plane buckling exists due to the presence of dislocations. In the transition region (the region between the zigzag- and armchair-oriented grain boundaries), the grain boundary structures feature complex mixtures of both zigzag and armchair grain boundaries. We also discuss the grain boundary transformations and migrations that occur upon adding or removing carbon atoms at the grain boundaries for all of our investigated types of grain boundaries.     

Influence of grain boundary and grain size on the mechanical properties of polycrystalline ceramics: Grain-scale simulations

The Orowan-Petch relation is a famous model to describe the strength of polycrystalline ceramics covering a wide range of grain sizes. However, it becomes difficult to explain the strength trend when the grain size decreases to the sub-microscale or nanoscale. This is because some microstructural parameters (such as grain size, grain boundary fracture energy, and grain boundary defects) vary with different processing technologies, and their coupling effects on mechanical properties are still unclear. In this study, a finite element method (FEM) was applied to investigate the dependence of mechanical properties, such as strength and damage resistance, on the abovementioned microstructural parameters on example of alumina. The numerical results show that the grain boundary energy is weakly coupled with the grain size and grain boundary defects. The grain size and grain boundary are intercoupling, which affects mechanical properties. The mechanical properties could be improved by increasing the grain boundary fracture energy and decreasing the grain size and the grain boundary defect density.     

不定形耐火材料颗粒级配的优化

针对生产实际情况 ,结合紧密堆积理论 ,建立了在给定的条件下如何选取最优配比的优化模型 ,并求解最优配比。通过将所配出坯料的粒度分布和按照经验配比给出的坯料的粒度分布与紧密堆积时 (Dinger-Funk方程 )的粒度分布进行比较 ,说明按最优配比配出的坯料 ,其粒度分布与紧密堆积时的粒度分布接近程度较好     

Effects of Nb Doping and Segregation on the Grain Boundary Plane Distribution in TiO2

Orientation imaging microscopy has been used to investigate the effects of Nb doping on the grain boundary plane distributions in TiO₂ (rutile). By comparing the grain boundary plane distribution of the undoped material with theoretical estimates of grain boundary energy anisotropy of pure rutile, it is possible to show that the frequency of grain boundary planes is inversely correlated with grain boundary energy. In the case of Nb-doped material, this inverse correlation can also be demonstrated by a comparison of grain boundary plane frequency with measurements of surface energy anisotropy. In addition, by comparing the grain boundary plane distribution of doped and undoped materials with previous measurements of Nb grain boundary segregation in TiO₂ as a function of grain boundary plane orientation, it is possible to explain the changes in grain boundary plane distribution which result from doping.     

Analysis of abnormal grain growth behavior during hot-press sintering of boron carbide

Dense boron carbide bearings with a central through hole were fabricated by hot-press sintering. When the sintering temperature was above the critical grain growth temperature, an increase in the applied pressure at 2100 °C caused a distinct abnormal grain size region. Two grain growth mechanisms were explored; normal grain growth (NGG) is controlled by a grain boundary diffusion mechanism whereas the abnormal grain growth (AGG) pattern abides by the ‘Bose–Einstein’ grain growth law.     

Role of Grain Size in Strength Variability of Alumina

Relationships between grain size and strength variability in alumina were investigated. Results are presented for an Ultra-high-purity alumina with a narrow grain size distribution and an equiaxed grain morphology at three grain sizes ranging from 5 to 27 μ.m. Vickers indentations are used to introduce controlled flaws into the specimens. It is found that increasing the grain size leads to enhanced flaw tolerance owing to a rising R -curve. However, contrary to recent theoretical predictions, no reduction in strength variability is found with increasing grain size. It is proposed that increasing variability in local fracture toughness with grain size offsets any improvement in reliability. This suggests that an upper bound to the reliability of grain bridging materials may exist.     

Microstructural Mechanics Model of Anisotropic-Thermal-Expansion-Induced Microcracking

Thermal-expansion-induced microcracking in single-phase ceramics has been simulated using a simple mechanics model based upon a regular lattice of brittle, elastic springs. Microcracks preferentially form at grain boundaries and propagate either into the bulk or along grain boundaries, depending on the toughness of the boundaries relative to the grain interiors. The present results show that anisotropic-thermal-expansion-induced microcracking can be more severe for either large or small grain size samples depending on the damage measure employed. At very small misfit strains, the large grain microstructure develops microcracks before the small grain microstructure. However, over most of the misfit strain regime examined, the total length/area of all cracks in a sample is larger when the grain size is small. This is manifested in a larger decrement of the elastic modulus in small grain size samples as compared with large grain size samples at the same misfit (Δ T ). However, large grain sizes are more detrimental with regard to fracture properties. This is because the fracture stress scales as inversely with the crack length and large grain samples exhibit larger microcracks than small grain samples. Unlike in the unconstrained samples, when a sample is constrained during a temperature excursion, the stress created by the overall thermal expansion can directly lead to fracture of the entire sample.     

Bimodal grain structure effect on the static and dynamic mechanical properties of transparent polycrystalline magnesium aluminate (spinel)

Transparent polycrystalline magnesium aluminate (spinel) with bimodal and unimodal grain structures were prepared. The influence of grain size distribution on static and dynamic mechanical properties were systematically investigated. The results showed that bimodal grain structure spinel has larger flexural strength (236.31 MPa) compared to unimodal grain structure spinel (221.38 MPa). Whereas, their values of hardness are very similar (15.1 vs 14.7 GPa) and fracture toughness remains unchanged (1.1 MPa∙m^1/2 for both spinel). Although static compression strength of bimodal grain structure spinel (1236 MPa) is higher than that of unimodal one (1078 MPa) due to a smaller average grain size in the former, the negative effect of bimodal grain structure reduced the spinel strength compared to theoretically predicted value. Bimodal grain structure spinel shows slightly lower increment (49%) in compression strength from static to dynamic loading compared to that of unimodal one (57%) due to a decreased strain-rate sensitivity ascribed to bimodal grain structure. A brittle mode in inelastic deformation at Hugoniot elastic limit was demonstrated in both bimodal and unimodal grain structures. Bimodal grain structure has an influence on the Hall-Petch-like relation of yield strength under planar impact loading.     

Poling behaviour at the grain boundaries of ferroelectric PZT thin films investigated by electric scanning force microscopy

The poling behaviour of PZT(53/47) films was investigated separately at the grain boundaries and inside the grain volumes by ESFM. The films were prepared by sol-gel processing on Pt-metallized polycrystalline Al₂O₃, substrates and consisted of columnar grains with mean diameter of 220 nm. It was found that the polarization was decreased at simple grain boundaries in a border zone of 20 nm to 40 nm width, with up to 42% related to the grain volumes. At triple points, the polarization was decreased by the same amount in a border zone of 40 nm to 80 nm width with respect to the grain volumes. An asymmetric poling behaviour of the saturation polarization was found at the grain boundaries. The switchable polarization of the grain boundaries was determined to be 74% of that of the grain volumes. The coercive field was clearly increased at the grain boundaries.     

Thermal conductivity of pressed powder compacts: tin oxide and alumina

Three aspects, which significantly reduce heat transfer through a polycrystalline material, are considered in this paper: porosity, grain boundary thermal resistance and the state of the grain–grain contacts. Tin oxide and alumina were chosen as model systems. Tin oxide, without a sintering additive, does not densify during thermal treatment but grain growth is not inhibited and consequently the microstructure can be varied. In alumina, variation of the thermal treatment conditions varies both grain size and porosity. Thermal conductivity measurements, using the laser-flash technique, reveal that the thermal resistance of a pressed powder compact is almost independent of temperature and at least a factor of 2.5 greater than a consolidated material with similar pore volume fraction and grain size. The reduced contact area of the grain–grain interfaces in the green body can explain this as demonstrated by numerical simulation. We also show that larger grain size increases the thermal conductivity of the porous ceramic.     

Simulation of Recirculating Circular Grain Dryer with Tempering Stage

The diiffusion model describing internal diiffusion of moisture within a grain kernel during drying and tempering stages was incorporated in the cross-flow drying model to simulate the recirculating circular grain dryer with drying and tempering stages. Experiments were conducted on an experimental prototype recirculating circular grain dryer for wheat and rough rice drying. The simulated grain temperature and moisture content were compared with the experimental data of drying wheat and rough rice, the maximum deviation of the outlet grain temperature was 5°C and the maximum deviation ofthe final grain moisture content was 0.3% w.b. The simulating program for recirculating circular grain dryer was used for analyzing the effects of structure parameters and hot air parameters on the dryer performance. Recommendations for design of the recirculating circular grain dryers are drawn from the experiments and simulation.     

Grain Growth in Very Porous Al2O3 Compacts

Extensive grain growth was observed by scanning electron microscopy in very porous Al₂O₃ compacts, even at densities <40% of theoretical. After ∼7% shrinkage at 1700°C, the grain size increased from ∼0.3 to 0.51 μm in a compact having a relative green density of 0.31. During grain growth in highly porous compacts, the grains appear initially to be chainlike, then to be oblong, and finally to be equiaxed. The proposed mechanism of initial grain growth involves the filling of necks between adjacent grains followed by the movement of the grain boundary through the smaller grain. Although grain growth in very porous compacts is quite different from coalescence and ordinary grain growth, the kinetics are similar.     

Effect of cerium on yttrium-doped barium zirconate with a ZnO sintering aid: Grain and grain boundary protonic conduction

The grain and grain boundary protonic transport of yttrium-doped barium zirconate has been investigated for various Ce-substituted concentrations, and the origins of blocking on the grain and grain boundary resistances are considered. Ba(Zr_1-xCe_x)_0.85Y_0.15O_3-δ samples (BZCY, x = 0, 0.05, 0.1, and 0.2) with a ZnO sintering aid have been prepared via conventional ceramic processing, and their grain and grain boundary resistances have been experimentally measured as a function of temperature under humid conditions using 2-probe AC impedance measurements. In the grain, a high-Ce-substituted sample with a large lattice volume is more conductive than a low-substituted specimen. The high-Ce-substituted sample also exhibits low resistance to the grain boundary. In the investigation of the origin of current blocking on the grain boundary, it is found that the space charge significantly affects grain boundary transport in the low-Ce-substituted sample. However, other mechanisms such as the charge-neutral effect (structural effect) of the grain boundary transport in the high-Ce-substituted samples are considered. In terms of the total conductivity (the summation of the grain and grain boundary components) of BZCY, it is found that the highest Ce-substituted barium zirconate (x = 0.2) exhibited the highest conductivity, demonstrating that it can potentially be utilized as a solid electrolyte in electrochemical devices such as solid oxide fuel cells.     

Engineering grain boundary anisotropy to elucidate grain growth behavior in alumina

Current grain growth models have evolved to account for the relationship between grain boundary energy/mobility anisotropy and the five degrees of grain boundary character. However, the role of grain boundary networks on overall growth kinetics remains poorly understood. To experimentally investigate this problem, a highly textured Al₂O₃ was fabricated by colloidal casting in a strong magnetic field to engineer a unique spatial distribution of grain boundary character. Microstructural evolution was quantified and compared to an untextured sample. From this comparison, a prevalence of (0001)/(0001) terminated grain boundaries with anisotropic networks were identified in the textured sample. These boundaries and their networks were found to be driving grain growth at a faster rate than predicted by models. These findings will allow better modelling of grain growth in real systems by experimentally exploring the impact thereon of grain boundary plane anisotropy and relative energy/mobility differences between neighboring boundaries.     

Understanding the self-sharpening characteristics of polycrystalline cubic boron nitride super-abrasive in high-speed grinding of Inconel 718

This study aims to understand the self-sharpening characteristics of the polycrystalline cubic boron nitride (PCBN) super-abrasive grains in high-speed grinding of Inconel 718. Comparative single grain grinding operations by separately using the brazed PCBN and conventional CBN abrasive grains are carried out. An in-depth analysis on the grain fracture morphologies and the workpiece scratch profiles during grinding are provided. The cutting mechanism and the material removal difference induced by the PCBN and conventional CBN grain fracture behaviour during grinding are revealed. The obtained results indicate that, the PCBN abrasive grain tends to intergranular fracture during grinding, which not only leads to the grain micro-fracture behaviour but also provides plenty of micro cutting-edges of microcrystalline CBN particles to participate in the grain cutting process. The surface roughness of the workpiece scratch grooves produced by the PCBN abrasive grain is 0.30 μm in average, which is 0.10 μm higher than that produced by the conventional CBN abrasive grain. This is mainly attributed to the additional cutting effects of the outcropped microcrystalline CBN particles on the PCBN grain fracture surface in the actual grinding process, and the additional grain cutting effects on the workpiece materials contribute to the self-sharpening characteristics of the PCBN. Finally, a mechanical analysis model of the single abrasive grain in grinding is also developed to provide a more comprehensive understanding on the self-sharpening mechanism of PCBN, and the established mechanical model can be correctly demonstrated by the calculated grinding force ratios in the single grain grinding operations.     

Diagnosing sulfur deficiency in rice by grain analysis

The effect of varying the sulfur (S) and nitrogen (N) supply on theyield and composition of brown rice (Oryza sativa L.)grainwas studied to determine whether grain analysis could be used for the diagnosisof S deficiency in this crop. Plants were grown to maturity in an S deficientyellow podzolic soil, under flooded and upland conditions in a glasshouse. Inthe first experiment there were six application rates of S combined with threeapplication rates of N. The amount of grain per plant varied from 0.69 to 7.62g, depending on the level of S and N supplied. Rice grown underflooded conditions produced approximately twice the grain yield of upland ricegrown at field capacity. Grain from the flooded series generally had a lower Sconcentration, and apart from the grain from the low N treatment, had lower Nconcentrations than the grain from the upland series. The grain S concentrationvaried from 0.069% to 0.154% and the N concentration ranged from 1.06% to 2.14%with changing S and N supply. Strong positive relationships were obtainedbetween grain yield and grain S concentration, and negative relationshipsbetween grain yield and the N:S ratios in the grain. The critical Sconcentration determined for 90% maximum yield in plants well supplied with Nwas not sufficient to distinguish between S responsive and unresponsive plantswhen N was limiting; both grain S concentration and N:S ratio are necessary todetermine the S status of the rice crop. Deficiency was indicated when the Scontent of the grain was less than 0.1% and the N:S ratio was wider than 14:1,and the same criteria applied to rice grain from flooded and upland treatments.Note that these values were derived in a glasshouse experiment using one ricecultivar on one soil and are therefore preliminary and require confirmationbefore practical application in the field. The second experiment examined theeffect of supplemental S applied after anthesis on grain composition. Latesupplemental S had no effect on grain yield, or on the composition of grainfromplants adequately supplied with S. In marked contrast, S concentration in grainof previously S deficient plants increased from 0.08% to 0.2%, well above thehighest level achieved by applying S at sowing. It is concluded that grainanalysis can be used to diagnose retrospectively S status for yield, provided Ssupply does not increase between the stage when grain yields are beingdetermined and the subsequent grain filling stage. An increase in S supplybetween these two stages will change the relationship between grain Scomposition and yield and complicate interpretation of grain analysis fordiagnosis. The advantages of using grain analysis for retrospective diagnosisofS deficiency are discussed, and the preliminary results suggest that theconcepts warrant further testing in the field.     

Average Grain Size in Polycrystalline Ceramics

A model is proposed which realistically characterizes the grain structure of polycrystalline ceramics. The average grain size of a log-normal distribution of grain sizes with tetrakaidecahedral (truncated octahedral) shape is related to the average intercept size by a proportionality constant. This result can be used to determine the average grain size of a sintered powder compact composed of nontextured grains which shows no discontinuous grain growth.     

Research on Modeling and Simulation of Mixed Flow Grain Dryer

Mathematical modeling and computer simulation of grain drying are the major emphasis of grain drying research in CAU (China Agricultural University). Since 1988, research on simulation of concurrent flow, counterflow, cross-flow, and mixed-flow grain dryers has been accomplished. The developed mixed-flow grain drying software has been used in analysis of dryer performance, design, and optimization of new grain dryers and management of existing ones. Effect of size, shape, number of rows, and arrangement of air duct on the performance of a mixed-flow grain dryer has been studied.     

Evolution of the Grain Size Distribution during the Sintering of Alumina at 1350°C

The objectives of this study are to provide some rare unfolded grain size distribution data for the sintering of alumina and to test for time invariance of the normalized grain size distribution as required by normal grain growth. The results show that ln ς doubled during the sintering times studied. The changes in the grain size distribution may be due to an increase in the number of relatively large grains combined with a reduction in the number of grain annihilation events compared with that required for time invariance of the normalized grain size distribution.