Creep Behavior of a Model Refractory System MgO-CaMgSiO4

The effects of the presence of a silicate boundary phase on the high-temperature creep behavior of a model refractory system MgO-CaMgSiO₄ (monticellite, CMS) were studied at 1200° to 1450°C. A change in the dominant mechanism of deformation was determined with increasing temperature and decreasing applied stress. It was concluded that, at 1200°C, deformation is controlled by a dislocation mechanism in the MgO framework, whereas at higher temperatures creep is the result of simultaneous mechanisms but dominated by viscous deformation of the silicate boundary region.     

Study of Ordering in High-Quartz Solid Solutions by Substitutions Affecting Superlattice Reflections

Ordering of the interstitial and network atoms in stuffed high-quartz solid-solution ( ss ) phases was studied by correlating suitable substitutions in the structure with the resultant changes in the superlattice reflections arising from these phases. In an MgO·Al₂O₃·3SiO₂ high-quartz ss , the special ordering of the interstitial Mg atoms, which produces satellite reflections around superlattice positions, was disturbed by replacement of ∼20 mol% of the MgO by Li₂O or ZnO. The interstitial atoms were further disordered by larger substitutions. The streaked superlattice reflections arising from an Li₂O·Al₂O₃·2SiO₂ high-quartz ss disappeared on replacement of ∼40 mol% of the Li₂O by MgO. This result implies that the interstitial Li atoms are ordered and were disordered by the substitution. The superlattice reflections were much intensified when ∼25 mol% of the SiO₂ was replaced by GeO₂. However, these reflections were intensified to a lesser degree when presumably both SiO₂ and Al₂O₃ were partially replaced by GeO₂ and Ga₂O₃, respectively. These observations strongly suggest an ordering of the network Si and Al atoms, which, together with the ordering of the interstitial Li atoms, contributed to the original streaked superlattice reflections.     

Air entrainment on a moving continuous web

The air boundary layer on a moving continuous web was experimentally investigated in a laboratory coating equipment. The boundary layer thickness was obtained by smoke visualization and measured by image analysis in the range 2.4×10⁴<Re<3×10⁵. According to the web speed, three laminar flow mechanisms were observed, for which the air boundary layer thickness was found to vary from 0.74 to . A survey of the published boundary layer solutions for a moving flat surface was also carried out and their applicability to the present boundary layer problem was studied. It was shown that the Blasius solution is the most suitable to represent the air flow in the case of a moving web.     

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.     

Finite difference solution for a transient temperature distribution in an advanced polymer fiber

A model is presented for the calculation of transient combined radiative and conductive heat transfer in a semitransparent layer of advanced fiber polymer. This model is based on optical material properties. Different boundary conditions were examined. Each side of the layer was exposed to hot or cold radiative surroundings, whereas each boundary was heated or cooled by convection. Emission within the layer and internal reflections depended on the layer refractive index. The reflected energy and heat conduction distributed energy across the layer and partially equalized the transient temperature distributions. The numerical method is an implicit finite difference procedure with nonuniform space and time increments that has been expanded to include external convection and incident radiation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100:4181–4189, 2006     

氧化物弥散强化Pt/Pt-Rh合金机理研究

运用位错理论,综述氧化物弥散强化Pt／Pt-Rh合金的机理。一方面,基体中的位错被高度弥散分布的第二相颗粒所钉扎,使位错运动受到阻碍：另一方面,分布在晶界的第二相颗粒阻碍晶界迁移和晶粒长大,从而提高基体的再结晶温度。由以上因素共同作用达到强化材料的目的。对氧化物弥散强化机理的深入分析,可为开发更优质的Pt／Pt-Rh合金弥散强化材料提供理论依据。     

Influence of Iron Solute on the Structure of Small-Angle [001] Twist Boundaries in Magnesium Oxide

The structure of small-angle [001] twist boundaries in pure and Fe-doped MgO was examined by electron imaging techniques in order to study the influence of solute on grain-boundary structure. In addition, the effect of the oxidation state of the Fe on the boundary structure was also examined. The [001] twist boundaries were produced in the form of bicrystals, by the hot-pressing of cleaved single crystals of pure MgO and MgO containing Fe. In pure MgO the [001] twist boundaries produced with small plastic strains are planar and contain a square array of screw dislocations with Burgers vectors of the type a /2<110>. In the presence of solute the original interface becomes wavy and dissociates into subboundaries. Distorted hexagonal dislocation networks as well as square arrays of screw dislocations are often observed in Fe-doped MgO. These observations are evidence that [001] twist boundaries in MgO whose production was accompanied by small amounts of plastic deformation undergo a structural transformation induced by the presence of Fe solute. The influence of the Fe oxidation state on boundary structure is less well understood. It appears that when the [001] twist boundaries are produced using crystals containing a substantial amount of Fe³⁺ ions, the resulting boundary structure is more likely to contain distorted hexagonal dislocation networks than when the boundary is produced from crystals containing predominantly Fe²⁺ ions.     

Mechanism of superplastic deformed transparent hydroxyapatite

Superplastic deformed hydroxyapatite (HA) samples were examined by TEM to elucidate the deformation mechanism in the HA. The present paper was to study the main mechanism of the superplastic behaviour of nanocrystalline transparent HA with a mean grain size of 170 nm fabricated by spark plasma sintering, which has exhibited superplasticity at temperatures ranging from 950 to 1050°C with a maximum elongation of 486% at 1000°C at an initial strain rate of 1·0×10^?4 s^?1. The post-deformation microstructure of HA indicated that activated dislocation motion assisted grain boundary sliding was the predominant mechanism of superplastic flow with the values of the stress exponent, 4 to 5, according to a theoretical model of interface reaction controlled creep. Unexpectedly, no dislocation was observed by TEM.     

High temperature deformation of a 5 wt.% zirconia-spinel composite: influence of a threshold stress

Creep experiments performed on a 5 wt.% zirconia- MgAl₂O₄ spinel material, in the stress and temperature ranges 8–200 MPa and 1350–1410°C, have shown the importance of grain boundaries in deformation of this material. Deformation can be analysed as the result of two sequential contributions. At low stress, an increase in the apparent stress exponent and the occurrence of a threshold stress, whose value roughly varies inversely proportional to spinel grain size, were observed. At high stress, grain boundary diffusion is the most likely mechanism that controls the grain boundary sliding. These observations are consistent with previous experiments showing that sliding of spinel/spinel boundaries is more difficult than sliding of spinel/zirconia boundaries in the low stress range. The plastic flow is analysed by means of grain boundary dislocations whose density increases with stress. At low stress, when the density of boundary dislocations is low, creep rates are interface-controlled while at high stress, when the boundary dislocation density is large, rates are limited by the long-range diffusion process.     

Dislocation analysis of a complex sub-grain boundary in UO2 ceramic using accurate electron channelling contrast imaging in a scanning electron microscope

In this work, accurate electron channelling contrast imaging (A-ECCI) assisted by high resolution selected area channelling patterns (HR-SACP) was used to characterize the structure of a complex low sub-grain boundary in a creep deformed uranium dioxide (UO₂) ceramic. The dislocations were characterized using TEM-style g·b = 0 and g·b × u = 0 contrast criteria. Misorientations across the boundary were measured using HR-SACPs with 0.04° precision and high accuracy EBSD. The boundary was determined to be asymmetric and mixed in nature, composed of two distinct regions with different dislocation morphologies and a misorientation below 0.5°. The A-ECCI, HR-SACP, and HR-EBSD results are consistent, confirming A-ECCI as a powerful tool for characterizing even complex dislocations structures using scanning electron microscopy. This is particularly true for UO₂, since this material is very difficult to thin, which makes TEM examination of sub-boundaries over the scale of several micrometers difficult. Furthermore, in this study, the change in dislocations arrangement along the breath of the complex low angle sub-grain boundary is related to the misorientation across the boundary.     

用三维网络模型模拟多孔介质的吸着回路

在简单立方网络框架的基础上构造了具有规整和随机特性的三维网络模型,并引入了周期边界条件.采用Weibull分布作为孔尺寸参数的分布函数,模拟了不同配位数和孔尺寸范围下氮在多孔介质中的等温吸附和脱附过程.着重考察了多孔介质网络模型的几何性质和拓扑性质对吸着等温回路的影响.结果发现,影响吸着等温线回路形状的主要因素是孔喉的半径和网络的配位数.     

Dislocation Structures in Creep-Deformed Polycrystalline MgO

Secondary creep of polycrystalline MgO with grain sizes of 100 and 190 μm was investigated at 1300° to 1460°C under compressive loads of 2.5 to 5.5 kgf/mm². The dependence of creep rate on load follows a power law with an exponent of 3.2±0.3. The process is thermally activated, with an activation energy of 76 ± 12 kcal/mol. The creep rate is independent of grain size. The dislocation structure was investigated by transmission electron microscopy. The total dislocation density follows the relation, σ= bG √ρ, commonly found for metals. The dislocations form a 3-dimensional network in which many dislocation segments lie in their slip or climb planes. On the basis of this structure, a model is proposed in which glide is the principal cause of deformation but the rate-limiting process, i.e. annealing of the network, is diffusion-controlled. Theoretical estimates and experimental results agree within 1 order of magnitude.     

Effects of dislocation densities and distributions on graphene grain boundary failure strengths from atomistic simulations

The failure strengths of graphene grain boundaries, over a range of misorientation angles from 0° to 60°, are investigated by molecular dynamics simulations. A correlation between the level of bond pre-strain and bond rupture is revealed. It is shown that the distribution of grain boundary dislocations is, in addition to the misorientation angle, also an important factor in determining the failure strength. For grain boundaries featuring a uniform dislocation distribution, a higher misorientation angle will yield a higher failure strength as a result of overlapping and mutual cancelation of the strain fields of neighboring dislocations. For grain boundaries with a non-uniform dislocation distribution, however, local structural inhomogeneity introduces large local tensile stresses and the failure strength decreases significantly. Therefore, a complicated interplay exists between the dislocation density and distribution, and the failure strength of graphene grain boundaries.     

Nano-fabrication of double gyroid network structure via ozonolysis of matrix phase of polyisoprene in poly(2-vinylpyridine)-block-polyisoprene films

The film specimens having the double gyroid network structure with space group symmetry were prepared by solution casting of poly(2-vinylpyridine)-block-polyisoprene mixed with a small but varying amounts of polyisoprene with chloroform as a solvent. The network phase composed of poly(2-vinylpyridine) was first crosslinked with 1,4-diiodobutane and then the matrix phase composed of polyisoprene was degraded by ozonolysis to obtain “free-standing” double gyroid network texture in which the two sets of networks are interlaced without contacts with one another in vacant space via their grain boundary connections. The network structures before and after ozonolysis were investigated, respectively, by transmission electron microscopy and field-emission scanning electron microscopy with the aid of numerically calculated three-dimensional computer graphics (3d CG) of the structure. The results clarified a success of the nano-fabrication on the free-standing double gyroid structure. We propose that the 3d CG-SEM, as discussed in the text, on the morphology of the freeze-fractured surface is particularly useful for the identification of the structure.     

Diffusiophoresis of a colloidal sphere in nonelectrolyte gradients in a circular cylindrical pore

The problem of the diffusiophoretic motion of a spherical particle in a fluid solution of a nonionic solute along the centerline of a circular cylindrical pore is studied theoretically in the quasisteady limit of negligible Reynolds and Peclet numbers. The imposed solute concentration gradient is uniform and parallel to the pore wall, which may be either impermeable to the solute molecules or prescribed with the far-field concentration distribution. The particle-solute interaction layer at the particle surface is assumed to be thin relative to the particle radius and to the particle-wall gap width, but the polarization effect of the diffuse solute in the thin interfacial layer caused by the strong adsorption of the solute is incorporated. The presence of the pore wall causes two basic effects on the particle velocity: first, the local solute concentration gradients on the particle surface are altered by the wall, thereby speeding up or slowing down the particle; secondly, the wall increases viscous retardation of the moving particle. To solve the equations of conservation of mass and momentum, the general solutions are constructed from the fundamental solutions in both cylindrical and spherical coordinates. The boundary conditions are enforced first at the pore wall by the Fourier transforms and then on the particle surface by a collocation technique. Numerical results for the diffusiophoretic velocity of the particle relative to that under identical conditions in an unbounded fluid solution are presented for various values of the relaxation parameter of the particle as well as the relative separation distance between the particle and the pore wall. The collocation results agree well with the approximate analytical solution obtained by using a method of reflections. The wall-corrected particle velocity depends on the surface properties of the particle, the ratio of particle-to-pore radii, and the solutal boundary condition at the wall. In general, the boundary effect on diffusiophoresis is quite significant and complicated.     

Microstructure of AlN with two-domain structure on (001) diamond substrate grown by metal-organic vapor phase epitaxy

We investigate the microstructures of domain boundaries in an AlN layer grown on a (001) diamond substrate by metal-organic vapor phase epitaxy. The AlN layer has a two-domain structure with crystal orientation along either <112?0> AlN [named by AlN^I domain] or <101?0> AlN [named by AlN^II domain] parallel to [110] direction of diamond. The AlN^I and AlN^II domains are not atomically bonded at two-domain boundary from initial to final step of growth, while an edge-type dislocation is generated at single-domain boundary (SDB). In addition, an inversion AlN^I domain [named by AlN^I?] is randomly-ordered at the initial stage of the coalescence between the AlN^I domains. The AlN^I? is easily terminated with increasing the thickness of AlN^I domain. The inversion domain boundary changes to the edge-type dislocation at the SDB with further growth, which reduces the defect density in the AlN^I domains.     

Effect of Grain Boundaries on Plastic Deformation of Sodium Chloride

Sodium chloride single crystals were joined at a melt zone with a hot-wire technique to produce bicrystals of controlled crystallographic orientation. The plastic deformation properties of the bicrystals were measured in bending while the birefringence was observed with a petrographic microscope. The birefringence at the boundary was interpreted to indicate that significant deformation across the grain boundaries occurred. The stress required for deformation across the boundary was found to increase with increased angle between crystallographic axes. Twist boundaries were found to be more resistant than tilt boundaries. Behavior was consistent with the dislocation model for grain boundaries.     

Electrical and Structural Characterization of a Low-Angle Tilt Grain Boundary in Iron-Doped Strontium Titanate

The atomistic structure and electrical properties of a symmetrical 5.4° [001] tilt grain boundary in Fe-doped SrTiO₃ have been investigated, respectively, by means of various transmission electron microscopy (TEM) techniques and impedance spectroscopy. In weak-beam dark-field images, the grain boundary is revealed to consist of a periodic array of dislocations; high-resolution TEM images show that the dislocation cores are separated by regions of strained lattice. The impedance response of the bicrystal has been measured in the frequency range 20 Hz ≤ f ≤ 10⁶ Hz as a function of temperature and oxygen partial pressure. The transport of charge across the array of dislocations that form the grain boundary is strongly hindered. Analysis of the impedance data in terms of a double-Schottky-barrier model yields a space-charge potential that exhibits a weak dependence on temperature and oxygen partial pressure and is ∼0.55 V in the investigated regime.     

Wide-angle X-ray scattering studies on polyethylene at low temperatures

Summary Linear polyethylene (Lupolen 6041 D) and a polyethylene fraction (PE 2000) were investigated by WAXS at temperatures 8 K相似文献   

Role of boundary conditions in four-wave mixing in photorefractive materials

The exact solution to the boundary value problem for a simple model of photorefractive phase conjugate mirror is presented. The slowly varying envelope approximation (SVEA) is not made. In this way in the derivation of the formula describing reflection of a plane incident wave reflections from the boundaries of the conjugator are taken into account.