High-Performance Ceramics IV

Abstract: Based on the relation between the stress intensity factor Ki and the crack velocity V, lifetime prediction and control were discussed and studied for machinable Ce-ZrO2/CePO4 ceramics. The relationship among lifetime, starting cracks and grain sizes were investigated by a set of designed simulation setting with a definite actual environmental conditions. It was concluded that crack started from larger weak interfaces of two phases and linking of cracking weak interfaces each other, and the lifetime of Ce-ZrO2/CePO4 composites could be controlled and predicted by adjusting of grain sizes of CePO4, which was depended on design of composites.

Abstract: The electronic structures of anatase titanium oxides (TiO2) substitutional doping with N, F, C, P and S for O have been studied by first-principles method based on the density functional theory. The lattice distortion and densities of states of nonmetal-doped anatase TiO2 as well as photocatalytic activity were discussed. Comparing the effects of these five nonmetal ions (N, F, C, P and S) in the anatase TiO2, the substitutional doping of N is the most effective to get better visible-light activity because of its least lattice distortion and a large band-gap narrowing effect and the suitable relative position of the impurity states in band gap.

Abstract: The full potential linearized augmented plane wave method within the generalized gradient approximation was used to calculate electronic structure of nanocrystalline BaTiO3 ceramics. We calculated the total and partial density of states of 50 nm BaTiO3 ceramics. The results show that the atoms distribution of nanograin BaTiO3 ceramics is different from those of coarse BaTiO3 ceramics. It is also revealed that the hybridization between Ti 3d and O 2p is very strong, which is very important to the ferroelectric stability of nanocrystalline BaTiO3 ceramics.

Abstract: A micromechanical approach is developed to investigate the behavior of composite materials, which undergo interfacial delamination. The main objective of this approach is to build a bridge between the intricate theories and the engineering applications. On the basis of the spring-layer model, which is useful to treat the interfacial debonding and sliding, the present paper proposes a convenient method to assess the effects of delamination on the overall properties of composites. By applying the Equivalent Inclusion Method (EIM), two fundamental tensors are derived in the present model, the modified Eshelby tensor, and the compliance tensor (or stiffness tensor) of the weakened inclusions. Both of them are the fundamental tensors for constructing the overall constitutive law of composite materials. By simply substituting these tensors into an existing constitutive model, for instance, the Mori-Tanaka model, one can easily evaluate the effects of interfacial delamination on the overall properties of composite materials. Therefore, the present method offers a pretty convenient tool. Some numerical results are carried out in order to demonstrate the performance of this model.

Abstract: First-principles calculations have been widely used to describe the ground state properties of materials over almost 20 years. Recently, a great progress was made in the first-principle calculations. Thermodynamic properties can also be gotten by calculations of the phonon densities of states (phonon DOS) and phonon dispersions of materials, which show widely potential applications in material researches. In the present work, the energetics and bonding properties of interfaces between ZrO2 and Ni metal were given by first-principles calculations. The results show that alloy element impurities (Al, Cr and Y) influence remarkably the adhesion of the ceramic and metal. On the other hand, the phonon densities of states and phonon dispersions of ZrO2 were calculated with density functional perturbation theory. From the phonon DOS, the thermodynamic properties were derived and the phase transformation of ZrO2 was discussed. By this method, the thermodynamic properties of material can be gotten from atom and electron levels without any experiment data. It is a new approach to design and study the thermodynamic properties in new material system.

Abstract: The adsorption of Au on clean Si(001) surface is investigated by the local density approximation using first-principles pseudopotentials. We found that the adsorption energy of Au on ideal Si(001)-(1×1) surface is lower than that on reconstructed Si(001)-(2×1) surface, suggesting that adsorbed Au atoms chemically react with the surface Si atoms and break Si-Si dimer bonds of the substrate. Furthermore, the intermixing of Au and Si is also considered and the calculation suggests that intermixing will not take place at low temperature. But due to the small energy barrier for Au atoms to diffuse into Si substrate, we can conclude that the Au-Si alloy is easily formed at relatively high temperature. This result should be one of the reasons of the lack of consensus on the issue of intermixing of Au and Si.

Abstract: Based on the observation that the ratio of the perimeter, P, to the square root of the area, A0.5, of the grains for a given material is nearly constant, it is suggested that the grain shape may be treated as a regular polygon with a non-integral side number. Examining the variation of P/A0.5-ratio with sintering holding time may provide useful information for sintering dynamics. Further analysis suggests that the P/A0.5-ratio may be a potential parameter for studying the grain-boundary effect in polycrystalline materials.

Abstract: The composite ceramics that contains nano-fibers and transformation particles, fabricated through SHS process, is performed with high fracture toughness and high plasticity. The matrix of composite ceramics was mainly composed of fiber eutectics with nano-fibers. The transformation particles were distributed along boundaries of the fiber eutectic structures. First, Mori-Tanaka method was used to predict the stiffness of the fiber eutectic. The fiber eutectic is transverse isotropy and has five independent elastic constants. Then considering random orientation of the fiber eutectic, the Young’s modulus and Poisson’s ratio of the matrix is determined by even strain. The matrix is isotropy. Finely, assuming the transformation particles as spheres distributed in the matrix, the effective stiffness for composite ceramics was computed. When the volume fraction of fibers and particles increase, the Young’s modulus of composite ceramics decrease and are little smaller than the volume average value, the Poisson’s ratio of composite ceramics decrease and are little bigger than the volume average value.

Abstract: The SEM observation on a conch’s shell shows that the shell is a kind of laminated bioceramic composite composed of aragonite layers and organic matrix. Each aragonite layer is parallel with the surface of the shell and consists of many thin aragonite sheets. These aragonite sheets are perpendicular to the layer where they are located. The observation also shows that the orientations of the sheets in different layers are different and these aragonite sheets compose various layups. A kind of lambdoidal layup is found. The maximum pullout force of the lambdoidal layup is analyzed based on its representative model. The result shows that the lambdoidal layup can markedly increase the pullout force of the layup and improve the fracture toughness of the shell.

Abstract: An improved serial computation method is proposed for simulating ceramic grain growth at an atomic scale. The data structure of a ternary tree is used to store orderly the atomic information and speed up the editing of data. Combining with the idea of space partition, an index for space searching is established to reduce the computation time. Simulation results demonstrate that the speed of the serial computation is increased significantly and simulated images are in good agreement with micrographs of practical ceramics. It is illustrated that kinetic exponents and fractal dimensions during the simulation of grain growth are also reasonable in compare with quantitative analyses.