Search

Fractal theory and regression analysis were employed for the first time to investigate the effect of pore size and pore distribution on high-temperature mechanical properties of porous alumina ceramics (PAC). In the present work, PAC with the comparable porosity, different pore sizes and pore distributions were prepared using carbon black as the pore-forming agent. Particular emphasis in this study was placed on the establishment of correlation between the thermal shock resistance and pore properties. The relationship between fractal dimension (D f) andthermal shock resistance parameter (R st) in specimens presented the negative power function, indicating that low D f could benefit the improvement of thermal shock resistance in specimens. The results showed that the increase of pore size and pore sphericity leads to a reduced D f, the enhanced hot modulus of rupture (HMOR) and. The decrease of proportion of micro-pores below 2 μm, the increase of mean pore size and pore sphericity could result in the decrease of D f, and then improve R st and HMOR of specimens. Based on the correlation between R st and pore characteristics, PAC with improved thermal shock resistance could be achieved when their pore structure meets the above features.

Low-cost porous ceramic microspheres from waste gangue were prepared by simple spray drying and subsequent calcination. Effects of calcination temperature on phase and microstructure evolution, specific surface area, pore structure, and dye adsorption mechanism of the microspheres were investigated systematically. Results showed that the microspheres were spherical, with some mesopores both on the surface and inside the spheres. The phase kept kaolinite after calcined at 800 and 900°C and transformed into mullite at 1000°C. The microspheres calcined at 800°C showed larger adsorption capacity and removal efficiency than those calcined at higher temperatures. Methylene blue (MB) and basic fuchsin (BF) removal efficiency reached 100% and 99.9% with the microsphere dosage of 20 g/L, respectively, which was comparable to that of other low-cost waste adsorbents used to remove dyes in the literature. Adsorption kinetics data followed the pseudo-second-order kinetic model, and the isotherm data fit the Langmuir isotherm model. The adsorption process was attributed to multiple adsorption mechanisms including physical adsorption, hydrogen bonding, and electrostatic interactions between dyes and gangue microspheres. The low-cost porous microspheres with excellent cyclic regeneration properties are promising absorbent for dyes in wastewater filtration and adsorption treatment.

A novel temperature induced gelation method for alumina suspension using oleic acid as dispersant is reported. Non–aqueous suspension with high solid loading and low viscosity is prepared using normal octane as solvent. Influence of oleic acid on the dispersion of suspension was investigated. There was a well disperse alumina suspension with 1.3 wt% oleic acid. Influence of gelation temperature on the coagulation process and properties of green body was investigated. The sufficiently high viscosity to coagulate the suspension was achieved at −20 °C. The gelation temperature was controlled between the melting point of dispersant and solvent. The gelation mechanism is proposed that alumina suspension is destabilized by dispersant separating out from the solvent and removing from the alumina particles surface. The alumina green body with wet compressive strength of 1.07 MPa can be demolded without deformation by treating 53 vol% alumina suspension at −20 °C for 12 h. After being sintered at 1550 °C for 3 h, dense alumina ceramics with relative density of 98.62% and flexural strength of 371±25 MPa have been obtained by this method.

The present work studied a simple direct foaming method for preparation of porous alumina ceramics by expansion of a ceramic suspension based on polyurethane (PU) foam system. The effects of polyurethane formulas including catalyst composition, blowing agent content, NCO index and solid content on the samples properties were investigated. The results showed that the homogeneity, porosity and mechanical properties are various for different formulas. The dried green bodies showed diametrical compressive strength in the range of 0.39–1.25 MPa and were amenable to machining operations such as milling, drilling and lathing. Meanwhile, PU formulas play an important role in the microstructures and mechanical properties of green bodies and sintered ceramic foams. Pyrolytic removal of polyurethane skeleton followed by sintering at 1550 °C produced alumina bodies with open cell porosity 54–75% and diametrical compressive strength 1.39–28.47 MPa. Microstructure showed both large (200–300 μm) and small (50–100 μm) pores all with various sizes of windows. Based on the optimization of polyurethane formulation, the porous alumina foam with porosity of 64% and compressive strength of 25.26 MPa was prepared. This polyurethane foam system is easily available and low-cost, which could be widely applied in preparation of other porous ceramics, such as ZrO2, SiO2, etc.

The mechanical behavior of a bimaterial strip subjected to simple shear is investigated by using elastic micromorphic model as analytical and numerical treatments respectively. First, the analytical solutions are carried out under micro-clamping and couple traction free boundary conditions, which indicate that the boundary layer effects are triggered by the microscopic constraint or material non-uniformity. Then the effects of length scale, material modulus and thickness ratio on the macroscopic performance are discussed. The results show that the strain jump at the bimaterial interface increases with the increasing thickness ratio. Furthermore, the finite element implementation is developed for the micromorphic model and the simulation results agree well with the analytical solutions. The analysis shows that the coupling factor acts as a penalty parameter, so the numerical method could be used to solve the problem of the second gradient theory with such internal constraint.

A new numerical dislocation climb model based on incorporating the pipe diffusion theory (PDT) of vacancies with 3D discrete dislocation dynamics (DDD) is developed. In this model we hold that the climb rate of dislocations is determined by the gradient of the vacancy concentration on the segment, but not by the mechanical climb force as traditionally believed. The nodal forces on discrete dislocation segments in DDD simulation are transferred to PDT to calculate the vacancy concentration gradient. This transfer establishes a bridge connecting the DDD and PDT. The model is highly efficient and accurate. As verifications, two typical climb-involved examples are predicted, e.g. the activation of a Bardeen–Herring source as well as the shrinkage and annihilation of prismatic loops. Finally, the model is applied to study the breakup process of an infinite edge dislocation dipole into prismatic loops. This coupling methodology provides us a useful tool to intensively study the evolution of dislocation microstructures at high temperatures.

In this article, we use the small strain micromorphic plasticity (MP) to study the wedge indentation of a thin film on a substrate and find qualitative agreement with experiments. A two-dimensional plane strain finite element formulation of the entire MP theory framework is outlined. The generalization of the radial return method for modeling the elasto-plastic deformation is presented. The numerical results show that the MP theory is capable of describing the initial fall in hardness at small depth of indentation and then the rise at larger depth for a soft film on a hard substrate. The indentation force and hardness increase with decreasing film thickness for a given depth. It is also shown that the hardness falls monotonically as the indentation depth increases and never approaches a constant for a hard film on a soft substrate. Contrary to the soft film/hard substrate system, the force and hardness diminish with decreasing film thickness for a given depth. Besides, the influences of internal length scale and hardening modulus of the film on hardness predictions are investigated.

A higher-order crystal plasticity model based on the continuum description of dislocation dynamics is developed to investigate the confined thin-film plasticity at micro-scale. In this model the “back stress” and the “slip resistance” for each slip system are incorporated into a standard diffusion equation for crystal slip, which accounts for the motion of dislocations in a continuum level. Furthermore, a surface energy based interfacial model is introduced here to take account of the interaction between dislocations and the interface. It can provide a more comprehensive study of the interface effect on the confined crystal plastic behavior rather than the two extreme boundary models used in other higher-order crystal plasticity models in which the dislocations can freely or hardly pass through the crystal interface. Then by implementing these models into finite element code the tensions of single-crystal/polycrystal thin Al films with passivation layers are numerically investigated. Two hardening factors associated respectively with the “back stress” and “slip resistance” are qualitatively studied, and it can be concluded from present study that the “back stress” hardening may dominate the strengthening of flow stress in confined thin-film plasticity at sub-micro scale. The interfacial model is applied to successfully model the interactions of dislocation with the film-passivation interfaces.

In this paper, a small strain micromorphic elasto-plastic model with isotropic/kinematic hardening is presented for modeling the size effect and Bauschinger effect in material with microstructure. A nonlinear kinematic hardening model is embedded into the micromorphic framework by employing a backstress, a micro-backstress and a micro-couple-backstress in a physical way. The material intrinsic length scale is introduced in the constitutive law, leading to the presence of higher order stress. The present model is further implemented into a 2D plane strain finite element frame with a fully implicit stress integration scheme. The generalized consistent tangent modulus is derived to achieve the parabolic convergence of the global nodal force equilibrium equation. Two numerical examples, including a thin film and a plate with underlying structures subjected to cyclic loading, are analyzed to verify the theoretical developments and numerical formulations. Plastic behaviors in micromorphic continuum, such as size effect, Bauschinger effect, ratcheting effect and plastic shakedown phenomenon, are investigated.

A shell theory established from the interatomic potential for carbon nanotubes is compared with the atomistic simulations. This shell theory is implemented in the finite element program ABAQUS via its user-material subroutine UGENS for shells. The numerical results for the representative loadings of tension, torsion and bending agree well with the atomistic simulations, which provide direct validation of this atomistic-based shell theory for carbon nanotubes.