Properties of SiC Particulate Preforms Based on Different Pore-forming Agents and Bonding Methods for Making SiC/Al Composites

The preforms with high SiC volume fraction （〉50%） were successfully fabricated by two bonding methods. Moreover, the dimensional change, compressive strength, and microstructure of SiC preforms were investigated, and the bonding mechanism among SiC particulates in preforms was also discussed. Results show that, after heating to 1 100 ~C and holding for 2 h, a uniform and interconnected structure in the SiC preforms can be obtained by using starch, stearic acid, and graphite respectively as the pore-forming agents, which benefits the subsequent infiltration by the molten metals. More neck-like-jointing among SiC particulate by using graphite as the pore-forming agent improves the dimensional accuracy and compressive strength of the preform. Besides, the properties of the preforms by the binder bonding are better than those by the oxidation bonding, which is mainly because the mixed neck-like-jointing and binder at high temperature provide effective bonding together.

DEM investigation of weathered rocks using a novel bond contact model

The distinct element method（DEM） incorporated with a novel bond contact model was applied in this paper to shed light on the microscopic physical origin of macroscopic behaviors of weathered rock, and to achieve the changing laws of microscopic parameters from observed decaying properties of rocks during weathering. The changing laws of macroscopic mechanical properties of typical rocks were summarized based on the existing research achievements. Parametric simulations were then conducted to analyze the relationships between macroscopic and microscopic parameters, and to derive the changing laws of microscopic parameters for the DEM model. Equipped with the microscopic weathering laws, a series of DEM simulations of basic laboratory tests on weathered rock samples was performed in comparison with analytical solutions. The results reveal that the relationships between macroscopic and microscopic parameters of rocks against the weathering period can be successfully attained by parametric simulations. In addition, weathering has a significant impact on both stressestrain relationship and failure pattern of rocks.

Quantum chemical calculations of bond dissociation energies for COOH scission and electronic structure in some acids

Quantum chemical calculations are performed to investigate the equilibrium C-COOH bond distances and the bond dissociation energies（BDEs） for 15 acids.These compounds are studied by utilizing the hybrid density functional theory（DFT）（B3LYP,B3PW91,B3P86,PBE1PBE） and the complete basis set（CBS-Q） method in conjunction with the 6311G^＊＊ basis as DFT methods have been found to have low basis sets sensitivity for small and medium molecules in our previous work.Comparisons between the computational results and the experimental values reveal that CBS-Q method,which can produce reasonable BDEs for some systems in our previous work,seems unable to predict accurate BDEs here.However,the B3P86 calculated results accord very well with the experimental values,within an average absolute error of 2.3 kcal/mol.Thus,B3P86 method is suitable for computing the reliable BDEs of C-COOH bond for carboxylic acid compounds.In addition,the energy gaps between the highest occupied molecular orbital（HOMO） and the lowest unoccupied molecular orbital（LUMO） of studied compounds are estimated,based on which the relative thermal stabilities of the studied acids are also discussed.

Complementary method to locate atomic coordinates by combined searching method of structure-sensitive indexes based on bond valence method

Bond valence method illustrates the relation between valence and length of a particular bond type. This theory has been used to predict structure information, but the effect is very limited. In this paper, two indexes, i.e., global instability index（GII） and bond strain index（BSI）, are adopted as a judgment of a search-match program for prediction. The results show that with GII and BSI combined as judgment, the predicted atom positions are very close to real ones. The mechanism and validity of this searching program are also discussed. The GII ＆ BSI distribution contour map reveals that the predicted function is a reflection of exponential feature of bond valence formula. This combined searching method may be integrated with other structure-determination method, and may be helpful in refining and testifying light atom positions.

Theoretical study on non-sequential double ionization of carbon disulfide with different bond lengths in linearly polarizedlaser fields

By using a two-dimensional Monte-Carlo classical ensemble method, we investigate the double ionization（DI） process of the CS_2 molecule with different bond lengths in an 800-nm intense laser field. The double ionization probability presents a ＂knee＂ structure with equilibrium internuclear distance R = 2.9245 a.u.（a.u. is short for atomic unit）. As the bond length of CS increases, the DI probability is enhanced and the ＂knee＂ structure becomes less obvious. In addition,the momentum distribution of double ionized electrons is also investigated, which shows the momentum mostly distributed in the first and third quadrants with equilibrium internuclear distance R = 2.9245 a.u. As the bond length of CS increases,the electron momentum becomes evenly distributed in the four quadrants. Furthermore, the energy distributions and the corresponding trajectories of the double-ionized electrons versus time are also demonstrated, which show that the bond length of CS in the CS_2 molecule plays a key role in the DI process.

Revisiting the ionic diffusion mechanism in Li_(3)PS_(4) via the joint usage of geometrical analysis and bond valence method

Inorganic solid electrolytes have obvious advantages on safety and electrochemical stability compared to organic liquid electrolytes,but the advance on high ionic conductivity of typical electrolytes is still undergoing.Although the first-principles calculation in the ion migration simulation is an important strategy to develop high-performance solid electrolyte,the process is very time-consuming.Here,we propose an effective method by combining the geometrical analysis and bond valance sum calculation to obtain an approximate minimum energy path preliminarily,in parallel to pave the way for the interoperability of low-precision and high-precision ion transport calculation.Taking a promising electrolyte Li_(3)PS_(4) as an example,we revisit its Li-ionic transport behavior.Our calculated Li-ion pathways and the activation energies(the corresponding values:1.09 eV vs.0.88 eV vs.0.86 eV)in γ-,β- and α-Li_(3)PS_(4) are consistent with the ones obtained from the first-principles calculations.The variations of the position of P-ions lead the rearrangement of the host PS_(4) tetrahedron,affecting the diffusion positions of Li-ions and further enabling high Li^(+) conductivity in β-Li_(3)PS_(4).

DEM Analysis of Single-Particle Crushing Considering the Inhomogeneity of Material Properties

Crushing characteristics of single particles are the basis of granular material simulation with discrete element method(DEM).To improve the universality and precision of crushable DEM model,inhomogeneous stiffness and strength properties are introduced into the bonded particle method,with which the Weibull distribution and size effect of particle strength can be reproduced without deleting elementary balls.The issues of particle strength and carrying capacity under complex contact conditions are investigated in this work by symmetric loading tests,asymmetric loading tests,and ball-ball loading tests.Results of numerical experiments indicate that particle carrying capacity is significantly influenced by coordination numbers,the symmetry of contact points,as well as the relative size of its neighbors.Contact conditions also show impact on single-particle crushing categories and the origin position of inner particle cracks.The existing stress indexes and assumptions of particle crushing criterion are proved to be inappropriate for general loading cases.Both the inherent inhomogeneity and contact conditions of particles should be taken into consideration in the simulation of granular materials.