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L2.3: Bulk MOS Online simulations via nanoHUB: Nanoscale thermal transport via MD In this tutorial: • Non-equilibrium MD simulations of thermal transport • Explore size effects Keng-hua Lin and Ale Strachan [email protected] School of Materials Engineering & Birck Nanotechnology Center Purdue University West Lafayette, Indiana USA © Alejandro Strachan – Online simulations: Getting Started 1 STEP 1: • From All Tools find: nanoMATERIALS nanoscale heat transport • Launch tool by clicking on: © Alejandro Strachan – Online simulations: getting started 2 STEP 2: setup the atomistic simulation cell From the Input Model tab of the tool • Select prebuilt structures • Or create your own structure by checking the box We will create our own © Alejandro Strachan – Online simulations: getting started 3 How the simulation cells are defined • Along the transport direction the simulation contains: • 2 Heat baths (blue) • The middle bin defines the hot and cold regions • Interior material (red) • This can be defined to be a superlattice • Periodic boundary conditions are applied along the transport direction Material (5 bins) Heat bath (3 bins) Cold bin Hot bin F. Müller-Plathe, J. Chem. Phys. 106, 6082 (1997) Keng-Hua Lin and A. Strachan, Physical Review B, 87, 115302 (2013). © Alejandro Strachan – Online simulations: getting started 3 STEP 2: setup the atomistic simulation cell • We would like to create a Si supercell by replicating the cubic Si unit cell • The tool allows users to build superlattices (laminates) so it is a bit convoluted Along the transport the system will be divided in bins Each bin is one unit cell long Cross-sectional area of the simulation cell (5x5 unit cells) Lattice parameter (5.43) Along the transport direction the “supercell” will have 5 Si bins (1 bin = 1 unit cell) and zero Ge (just Si) The number of Si bins will be varied to explore size effects The total simulation length in this case is: (3+5) x 2 x 5.43A = 86.88A The effective size for transport is the separation between heat baths (1/2 of the simulation cell length) Heat baths are 3 bins long The “supercell” will be repeated only once © Alejandro Strachan – Online simulations: getting started 4 STEP 2: setup the atomistic simulation cell If you select to build your own structure In the Selections for Material Shape tab Choose to simulate the material as a bulk (superlattice thin film) or a superlattice square nanowire 3-D periodic boundary condition for the simulation cell Periodic boundary condition along the z direction and free boundary condition along the x and y directions We will use 3D periodic boundary conditions © Alejandro Strachan – Online simulations: getting started 7 STEP 3: setup the Simulation Details From the Driver Specification tab of the tool Thermalize the system For 1000 MD steps (2 ps) This will equilibrate the system to the desired temperature before starting the thermal transport calculation © Alejandro Strachan – Online simulations: getting started 9 STEP 3: setup the Driver Specification From the Driver Specification tab of the tool This sets up the non-equilibrium simulation Timestep 2 fs MD steps 120,000 Swap atomic velocities every 100 MD steps Atomistic snapshots for visualization Output temperature profile every this many steps for analysis Ignore the first 20,000 steps when computing averages © Alejandro Strachan – Online simulations: getting started 10 STEP 4: explore the results interactively Temperature profiles at different times Average temperature profile Remember simulation cell was ~86A © Alejandro Strachan – Online simulations: getting started 13 STEP 4: explore the results interactively Analyze output: calculated thermal conductivity Material thermal conductivity computer from temperature gradient inside the material (away from contacts) Use this value © Alejandro Strachan – Online simulations: getting started 13 .
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