浸塑成型中冷却时间的优化

采用数学建模的方法对浸塑成型中制件的冷却过程进行了模拟。通过对浸塑件冷却过程中的传热作合理简化,建立传热过程的扩散方程,并由边界条件的齐次化等数学方法,推导出确定浸塑件冷却时间的计算方程。将方程应用于聚氯乙烯(PVC)浸塑件冷却时间的计算与确定,并对比经验公式做了讨论与分析。结果表明,通过公式计算确定的冷却时间可以减少生产中脱模困难的问题,降低制件的后收缩率,达到优化浸塑工艺和改善制件性能的目的。     

Numerical simulation of mass transfer during the osmotic dehydration of biological tissues

In this paper a mathematical model based on mass transfer in plant tissues is developed. The model takes into account the diffusion and convection of each constituent within the tissue. The driving force for the convection is assumed to be the gradient of hydrostatic pressure. The mass balance equation for the transport of each constituent is established separately for intracellular and extracellular volumes but taking into account the mass exchange across the cell membrane between the intracellular and extracellular volumes. The mass transfer results in not only the change of intracellular and extracellular volumes but also the shrinkage of whole tissue. The model allows us to quantitatively simulate the time evolution of intracellular and extracellular volumes, which was observed in histological sections under the microscope.     

球铁件三维缩孔预测的实验研究及其在计算机上的实现

以正交实验为基础，从球铁件缩孔形成机制出发，建立了动态体积改变积聚地及结合临界固相率来预测二次缩孔的数学模型，提出了采用了数据结构中广度优先搜索的方法对整个铸件进行遍历，来寻找孤立区的大小及孤立区内各单元的具体位置，实现了对二次缩孔定量，定位的预测。     

Modeling of drying shrinkage of concrete specimens at the meso-level

In this paper, an existing mesomechanical model for cementitious materials is extended to the domain of diffusion-driven phenomena. The model is based on the Finite Element Method, and uses zero-thickness interface elements equipped with a fracture-based constitutive formulation to represent cracks. The new developments presented in this paper consist of the application of the model to the hygro-mechanical coupled analysis of drying shrinkage in concrete specimens, explicitly taking into account the influence of (micro) cracks on the diffusion of moisture. In a first part of the paper, the model is presented in some detail, especially the new aspects regarding moisture diffusion including effects of cracks, and H-M coupling. The model predictions are then quantitatively compared with classical drying shrinkage experiments on concrete specimens. The consideration of different assumptions for the relation linking shrinkage strains and weight losses is discussed in some detail. Finally, the effect of size and volume fraction of the main heterogeneities of concrete on the drying process and drying-induced microcracking is also addressed.     

NiFe2O4陶瓷U型套管的注浆成型和无压烧结

采用注浆成型和无压烧结技术制备铝电解测温热电偶NiFe₂O₄陶瓷U型保护套管, 对浆料稳定性、生坯烧结行为和组装热电偶的测温性能进行了研究。结果表明: 添加10wt% NiFe₂O₄纳米粉和5wt%纳米粘结剂能有效提高浆料体系的Zeta电位, pH在10.0~11.0之间时浆料体系的稳定性较好。升温至1100 ℃以上生坯开始大量收缩, 随烧结温度的升高线收缩和线收缩速率逐渐增大, 在1300 ℃时线收缩速率达到最大值; 烧结初期活化能为295.11 kJ·mol ^-1, 受体积扩散和晶界扩散共同控制。测温结果表明组装热电偶在升降温过程中温度响应及时, 具备良好的抗熔盐腐蚀性能和抗热震性能, 满足工业铝电解测温热电偶套管的要求, 具备工业应用前景。     

Enhanced tensile properties in a Mg-6Gd-3Y-0.5Zr alloy due to hot isostatic pressing (HIP)

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.     

Recent void shrinkage models and their applicability to diffusion bonding

Abstract

Recent void shrinkage models are reviewed and refined. The role of surface selfdiffusion is discussed for modelling void shrinkage by selfdiffusion along the void surface and bond interface. The refinement of the existing models makes it possible to discuss the effect of void crushing by power law creep, the validity of combining models of diffusion and creep, and the applicability of the existing models. The discrepancies between the various models are demonstrated by reference to the stress dependence of void shrinkage rate (log–log plots). The void crushing rate due to power law creep of the surrounding matrix is highly dependent on the degree of bulk deformation, i.e. the severity of bulk constraint. The applicability of the combined models of creep and diffusion are therefore limited by this condition.

MST/1502     

Constant heating rate analysis of simultaneous sintering mechanisms in alumina

Constant heating rate sintering experiments were conducted on a submicron alumina powder during the initial stage. Shrinkage was measured by precision dilatometry and surface area reduction was monitored with gas adsorption measurements. Furthermore, grain size and pore size results were collected using X-ray line broadening and mercury porosimetry. Analysis of the shrinkage and surface area reduction data showed excellent correlation with a computer simulation based on simultaneous surface diffusion and grain boundary diffusion mechanisms. A comparison of the simulated and the experimental sintering paths on a plot of surface area reduction versus shrinkage indicated the combination of mechanisms and activations energies which best describe this sintering behaviour. From this analysis the estimated activation energies for grain boundary and surface diffusion are 440 and 508 kJ mol^–1, respectively.     

Shrinkage stress in concrete under dry–wet cycles: an example with concrete column

This paper focuses on the simulation of shrinkage stress in concrete structures under dry–wet environments. In the modeling, an integrative model for autogenous and drying shrinkage predictions of concrete under dry–wet cycles is introduced first. Second, a model taking both cement hydration and moisture diffusion into account synchronously is used to calculate the distribution of interior humidity in concrete. Using the above two models, the distributions of shrinkage strain and stress in concrete columns made by normal and high strength concrete respectively under dry–wet cycles are calculated. The model results show that shrinkage gradient along the radial direction of the column from the center to outer surface increases with age as the outer circumference suffers to dry. The maximum and minimum shrinkage occur at the outer surface and the center of the column, respectively, under drying condition. As wetting starts, the shrinkage strain decreases with increase of interior humidity. The closer to the wetting face, the higher the humidity and the lower the shrinkage strain, as well as the lower the shrinkage stress. As results of the dry–wet cycles acting on the outer circumference of the column, cyclic stress status is developed within the area close to the outer surface of the column. The depth of the influencing zone of dry–wet cyclic action is influenced by concrete strength and dry–wet regime. For low strength concrete, relatively deeper influencing zone is expected compared with that of high strength concrete. The models are verified by concrete-steel composite ring tests and a good agreement between model and test results is found.     

A study of diffusion mechanisms in sintering by shrinkage measurements

Shrinkage measurement is important in the study of sintering to ascertain the operative mechanism. Surface area reduction is related to shrinkage with the help of which operative mechanisms at different temperatures are inferred. Densification during sintering is mainly related to volume diffusion, grain boundary diffusion and plastic flow.     

An investigation of co-fired varistor-NiZn ferrite multilayers

The co-firing characteristics of ZnO varistor and NiZn ferrite thick films were investigated, the objective being to create an integrated passive device. Bismuth oxide additions were used to increase the shrinkage of the ferrite during sintering. Dilatometry analysis proved that well-matched shrinkage characteristics could be obtained for bismuth oxide-modified ferrite compositions and the starting varistor material. Cross-boundary diffusion across the co-fired varistor-ferrite interface occurred during sintering, but application of a barrier layer of pure zinc oxide reduced this phenomenon considerably. By tailoring the shrinkage, strong, crack-free co-fired layers with no evidence of cracking or delamination were obtained. No deleterious effects were observed due to electroding of the varistor and ferrite materials.     

钢纤维对超高性能混凝土干燥收缩的影响

研究了在(20±2)℃、相对湿度为(50±5)%的环境中钢纤维体积掺量为0%、1%、2%和3%的超高性能混凝土(UHPC)的干燥收缩。结果表明:UHPC在前7d的干燥收缩发展速率较快,7d后发展速率逐渐减缓;但当钢纤维掺量超过2%后,钢纤维对干燥收缩的改善作用明显降低,相比钢纤维掺量为2%的UHPC,3%掺量UHPC的干燥收缩仅仅降低了1.5%。钢纤维高弹模及它与基体的界面粘结有效降低了混凝土的干燥收缩,但钢纤维掺量过多可导致多孔薄弱的界面区增加,从而使其对混凝土的收缩抑制作用减小。粉煤灰对超高性能混凝土干燥收缩的抑制作用大于矿粉。提出的新的数学拟合指数公式相比于文献中常用的ACI和王铁梦公式与实测结果吻合度更好。     

Phase-field simulation of irradiated metals: Part II: Gas bubble kinetics

The phase-field model developed in Part I of this work is expanded to include fission gas generation, diffusion, and segregation within bubbles nucleated both homogeneously and heterogeneously along grain boundaries. Illustrative results are presented that characterize bubble growth and shrinkage, as well as the bubble density, size and nucleation rate as a function of the irradiation conditions. Finally, intergranular bubble characteristics such as shape, pinning energy and bubble density are investigated.     

The physics and chemistry of the sintering of silicon

Silicon is a model material for studying the sintering of covalently bonded non-oxide ceramics. The sintering of silicon has direct applications as well to polycrystalline photovoltaics and the reaction sintering of silicon nitride. Surface diffusion is found to be the dominant mass transport path for pure silicon of all particle sizes of interest. Both boron and oxygen are surface-active and effective in inhibiting surface transport, thereby allowing shrinkage to occur by either grain-boundary or lattice diffusion.     

Simulation of shrinkage distress and creep relief in concrete repair

This paper addresses the problem of stress buildup in the repair layer of a concrete patch repair system resulting from moisture diffusion. As moisture evaporates from the repair layer into the surrounding ambience of known relative humidity, the hardened concrete substrate restrains free shrinkage movement of the repair layer. As a consequence, primary tensile stresses are set up in the repair layer together with shear and peeling stresses at the interface of the repair layer-concrete substrate. The repair layer under non-uniformly increasing tensile shrinkage stresses undergoes restrained creep in tension, which results in the development of secondary stresses in the system. The secondary stresses due to restrained creep being of opposite sign to that of restrained shrinkage serve to relieve the primary shrinkage stress field and the net or combined stress buildup as a result is reduced.A finite element based computer program used for computing the time dependent moisture loss profile in the repair system is interfaced with a finite element based 2-D stress analysis program for computing the time dependent restrained shrinkage and creep stresses.Variation of normal and shear stresses across depth and width at critical locations in the patch repair and temporal variation of these stresses are presented. Influence of ultimate free shrinkage strain ε_sh^∞ and the buildup of tensile stresses versus the evolution of tensile strength capacity f′_t of the repair is highlighted. Also, possible zones of failure are identified in the repair layer and at the interface of the patch repair system.     

Fabrication of a graded-index polymer optical fiber preform without a cavity by inclusion of an additional monomer under a centrifugal force field

When two or more monomers with different densities and refractive indices are polymerized under a centrifugal force field, a radially varying refractive index is generated owing to the difference in density of the monomers. After the polymerization is completed, a cavity is generated about the rotational axis as a result of inherent volume shrinkage during bulk radical polymerization. Therefore it is necessary to feed an additional monomer into the cavity to compensate for the undesirable volume shrinkage. We have successfully fabricated a preform with graded indices for polymer optical fiber without a cavity by adding another monomer during rotation of the reactor. One can control the overall refractive-index profile by changing the rotation speed. Furthermore, the refractive-index profile can be predicted as a function of rotating speed by use of a simple mathematical model.     

Simulation of coarsening of inter-lath cementite in a Q&T steel during tempering

A mathematical model, using the finite difference approach, was established to consider tempering in a low carbon low alloy quenched and tempered steel based on experimental observations, treating the coarsening of a larger particle and dissolution of a smaller particle as a continuous and simultaneous process for coupled inter-lath cementite systems. The diffusion of Mn was simplified as a 1D diffusion and occurred between the interface elements and their adjacent elements in the model. The mathematical model predicted the shortest dissolution times for smaller particles in coupled inter-lath cementite systems, which agreed well with experimental observations on tempering from 2 to 4?h. However, the larger particle coarsening was under predicted due to the simplification of considering two particle arrangements.     

Sintering kinetics of pure and doped chromium oxide

The mechanism of sintering in chromium oxide in the presence of varying amounts of magnesium oxide in a firing condition that simulates a controlled reducing atmosphere has been investigated. The investigation is based on isothermal shrinkage measurements at different temperatures. The data suggest grain boundary diffusion to be the main process with evidence of vapour transport.     

Isothermal Dilatometric Study of Sintering in Kaolin

Solid-state sintering for kaolin samples was studied by dilatometric measurements in the isothermal regime in the temperature range from 600 °C to 1100 °C. The relative expansion was measured for a period of 10h. For the temperatures up to 850 °C, we observed only a small shrinkage (less than 0.5 %), most of which took place within the first 3h of the measurements. For the temperatures above 850 °C, a significant shrinkage occurred for the whole measured time interval and reached up to 2.7 %. Anomalous behavior—a decrease in the shrinkage with the temperature—was observed in the range from 700 °C to 850 °C. The dilatometric measurements are supplemented by porosity distribution measurements. The standard spherical-grains microscopic model was applied to determine that for the initial stages of the sintering process, grain boundary diffusion was the dominant mechanism at lower temperatures (600 °C to 850 °C), whereas lattice diffusion was dominant at higher temperatures (900 °C, 1050 °C, and 1100 °C).     

Numerical Simulation of Microporosity Evolution of Aluminum Alloy Castings

A mathematical model to calculate the size and distribution of microporosities was studied and coupled with a stochastic microstructure evolution model. The model incorporates various solidification phenomena such as grain structure evolution, solidification shrinkage, interdendritic fluid flow and formation and growth of pores during solidification processes. The nucleation and growth of grains were modeled with a cellular automaton method that utilizes the results from a macro scale modeling of the solidification process. Experiments were made to validate the proposed models. The calculated results of aluminum alloy castings agreed with the experimental measurements.