Pressureless sintered beta prime-Si3N4 solid solution: Fabrication, microstructure, and strength

Si3N4, AlN, and Al2O3 were used as basic constituents in a study of the pressureless sintering of beta prime-Si3N4 solid solution as a function of temperature. Y2O3-SiO2 additions were used to promote liquid-phase sintering. The sintered specimens were characterized with respect to density, microstructure, strength, oxidation, and thermal shock resistance. Density greater than 98 percent of theoretical was achieved by pressureless sintering at 1750 C. The microstructure consisted essentially of fine-grained beta prime-Si3N4 solid solution as the major phase. Modulus of rupture strengths up to 483 MPa were achieved at moderate temperature (1000 C), but decreased to 228 MPa at 1380 C. This substantial strength loss was attributed to a glassy grain boundary phase formed during cooling from the sintering temperature. The best oxidation resistance was exhibited by a composition containing 3 mol % Y2O3-SiO2 additives. Water quench thermal shock resistance was equivalent to that of reaction sintered silicon nitride but lower than hot-pressed silicon nitride

Effect of dimerization on dynamics of spin-charge separation in Pariser-Parr-Pople model: A time-dependent density matrix renormalization group study

We investigate the effect of static electron-phonon coupling, on real-time dynamics of spin and charge transport in $\pi$-conjugated polyene chains. The polyene chain is modeled by the Pariser-Parr-Pople Hamiltonian with dimerized nearest-neighbor parameter $t_{0}(1+\delta)$ for short bonds and $t_{0}(1-\delta)$ for long bonds, and long-range electron-electron interactions. We follow the time evolution of the spin and charge using time-dependent density matrix renormalization group technique, when a hole is injected at one end of the chain in its ground state. We find that spin and charge dynamics followed through spin and charge velocities, depend both on chain length and extent of dimerization, $\delta$. Analysis of the results requires focusing on physical quantities such as average spin and charge polarizations, particularly in the large dimerization limit. In the dimerization range 0.0 $\le$ $\delta$ $\le$ 0.15, spin-charge dynamics is found to have a well defined behavior, with spin-charge separation (measured as the ratio of charge velocity to spin velocity) as well as, the total amount of charge and spin transported in a given time, along the chain, decreasing as dimerization increases. However, in the range 0.3 $\le$ $\delta$ $\le$ 0.5, it is observed that the dynamics of spin and charge transport becomes complicated. It is observed that for large $\delta$ values, spin-charge separation is suppressed and the injected hole fails to travel the entire length of the chain.Comment: Published in Phys. Rev. B; preprint format of published versio

Length-scales of Dynamic Heterogeneity in a Driven Binary Colloid

Here we study characteristic length scales in an aqueous suspension of symmetric oppositely charged colloid subject to a uniform electric field by Brownian Dynamics simulations. We consider a sufficiently strong electric field where the like charges in the system form macroscopic lanes. We construct spatial correlation functions characterizing structural order and that of particles of different mobilities in-plane transverse to the electric field at a given time. We call these functions as equal time density correlation function (ETDCF). The ETDCF between particles of different charges, irrespective of mobilities, are called structural ETDCFs, while those between particles of different mobilities are called the dynamic ETDCF. We extract the characteristic length of correlation by fitting the envelopes of the ETDCFs to exponential dependence. We find that structural ETDCF and the dynamical-ETDCFs of the slow particles increase with time. This suggests that the slow particles undergo microphase separation in the background of the fast particles which drive the structural pattern in the plane transverse to the lanes. The ETDCFs can be measured for colloidal systems directly following particle motion by video-microscopy and may be useful to understand patterns out of equilibrium