Structure and properties of grain boundary interfaces plays an important role in determining the thermal, electrical and mechanical properties of a polycrystalline material. In this work, we studied various grain boundary(GB) structures in silicon and their heat transport properties using classical molecular dynamics simulations. A systematic study of symmetric tilt GBs with and misorientation axis has been conducted using Stillinger-weber and a new modified Tersoff interatomic potential. The stable grain boundary structure with minimum energy is identified using conjugate gradient energy relaxation method followed by an annealing. Further, we studied the thermal resistance of a few low-angle and high-angle grain boundaries using non-equilibrium molecular dynamics simulations. Thermal heat flux is induced in the bicrystal system by maintaining high and low temperatures at regions to the left and right of the GB interface. A finite temperature drop is then observed at the GB interface once the steady state conditions are established. This temperature drop is a measure of thermal resistance(Kapitza resistance) offered by the GB to the flow of heat through it. We expect to see a strong correlation between the excess GB energy and Kapitza resistance and explain the results by examining the vibrational density of states. To get insights into the mechanism of heat transport behaviors at GB interfaces, phonon wave-packet simulation across the simple two-dimensional grain boundaries modeled using LJ potential is attempted. The transmission energy coefficient of phonons with different frequencies is calculated for different GBs.
Keywords:Molcular Dynamics, Grain Boundaries, Wave propogation, Atomistic simulations, Materials Modelling
The quest to develop new materials for thermal applications such as jet engine isolation and the need to eliminate the excess heat generated in the micro-electronic semiconductor devices is highly motivating material scientists and researchers for many years. Electrical/electronic behaviour of the functional materials like thin film transistors and solar cells depends on the properties of the polycrystalline silicon that is being used. It is a well known fact today that most of the electrical, mechanical and thermal properties stems form the structure and properties of the grain boundary interfaces. Therefore, having a thorough understanding of the structure and properties of these grain boundaries at atomic level is essential to manipulate the material properties thereby influencing the performance of the functional materials. Various, both experimental and theoretical studies have been made in the past, trying to understand the structure of the GB interfaces and their influencing properties on materials. Most of the experimental work is carried out in polycrystalline Si, which fails to reveal the characteristics of individual GB interfaces. Even with the advancement of new crystal growing techniques and sophisticated EBSD/OIM, it’s very challenging to grow the bicrystal samples and study them. On the other hand, modelling the GBs using computers and studying the atomic level interactions of atoms to interpret the properties has given a plethora of opportunities to researchers world wide. Numerical simulation techniques such molecular dynamics give us deeper insights into the material behaviour and properties. Accuracy of these models in predicting the properties depends on the inter-atomic force fields defined in the simulation as different potentials are derived from different theories. Molecular dynamics is such powerful tool to investigate energetics and thermal properties of the GB interfaces. Si is one among the widely studied materials in the literature and is also an excellent candidate to study the thermal properties using molecular dynamics. When the energy carrier such as phonon or electrons tries to traverse across the interface they get scattered at the interface offering a resistance to the flow of thermal energy. This is referred to as interfacial thermal resistance or kapitza resistance. Since there are no electron-electron interactions involved in Si unlike metals, modeling GBs using empirical potentials gives better approximation of the scattering behaviour at the interface. Experimental studies on five parameter character distribution of grain boundaries in polycrystalline Si shows that distribution of grains is non-random[\cite]. Based on the multiple random distribution of the grain boundaries we choose to study a few GB interfaces with <001> misorientation axis. So far no dedicated study has been made to understand the kapitza resistance at the interface. In this work we use Stillinger-Weber potential to calculate the kapitza thermal resistance as it captures the thermal properties better than Tersoff empirical potential. In this work, we systematically study the kapitza resistance of 10 Si GB models with <001> misorientation angle. We geometrically construct the bicrystal samples with desired GB interface with the periodic boundary conditions and study using molecular dynamics simulations. The grain boundary energy of these interfaces is also calculated to examine its correlation with the kapitza resistance.
Last update:10-June-2020
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