## LSU Historical Dissertations and Theses

1997

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

Rajiv K. Kalia

#### Abstract

Properties and processes in silicon nitride and graphite are investigated using molecular-dynamics (MD) simulations. Scalable and portable multiresolution algorithms are developed and implemented on parallel architectures to simulate systems containing 10$\sp6$ atoms interacting via realistic potentials. Structural correlations, mechanical properties, and thermal transport are studied in microporous silicon nitride as a function of density. The formation of pores is observed when the density is reduced to 2.6 g/cc, and the percolation occurs at a density of 2.0 g/cc. The density variation of the thermal conductivity and the Young's modulus are well described by power laws with scaling exponents of 1.5 and 3.6, respectively. Dynamic fracture in a single graphite sheet is investigated. For certain crystalline orientations, the crack becomes unstable with respect to branching at a critical speed of $\sim$60% of the Rayleigh velocity. The origin of the branching instability is investigated by calculating local-stress distributions. The branched fracture profile is characterized by a roughness exponent, $\alpha\sim0.7,$ above a crossover length of 50A. For smaller length scales and within the same branch, $\alpha\sim0.4.$. Crack propagation is studied in nanophase silicon nitride prepared by sintering nanoclusters of size 60A. The system consists of crystalline cluster interiors, amorphous intercluster regions, and isolated pores. These microstructures cause crack branching and meandering, and the clusters undergo significant rearrangement due to plastic deformation of interfacial regions. As a result, the system can withstand enormous deformation (30%). In contrast, a crystalline sample in the same geometry cleaves under an applied strain of only 3%.

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