Nano-Scale Thermal Property Prediction by Molecular Dynamics Simulation with Experimental Validation

Author

Kyle S. Horne, Utah State University

Date of Award:

5-2014

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Mechanical and Aerospace Engineering

Committee Chair(s)

Heng Ban

Committee

Heng Ban

Committee

Byard Wood

Committee

Robert E. Spall

Committee

Nick Roberts

Committee

Stephen Bialkowski

Abstract

A new type of laser diode, called a quantum cascade laser (QCL), was developed in the mid 1990's. These new lasers have many applications including industrial emissions analysis and explosives detection. Like many solid-state devices, they work better at cooler temperatures, but operating the device generates heat; this results in a cooling problem if the device is to operate continuously at high-power. To improve this situation, a better understanding of how heat leaves the laser diode is needed. The thermal conductivity of a material is a measure of how quickly heat will leave it. In this work, two approaches are used to better understand thermal conduction in laser diodes. First, the motions of the atoms in the diode are simulated using a tool called molecular dynamics (MD). Statistical analysis of the atoms' motions is used to compute the thermal conductivity of the diode. Second, an experimental method called photothermal radiometry (PTR) is used to obtain the thermal conductivity through measuring thermal "echoes" from heat put into the diode.

The molecular dynamics results conclude that the thermal conductivity of the QCL across the layers is less than the conductivity of either of the materials that are used to make the laser, which demonstrates that heat transfer is being restricted at the layer interfaces. In the direction which runs along with the layers, the thermal conductivity is found to be between the values for the two materials that make up the QCL.

The experimental results are not strongly conclusive because the models used in the PTR measurement process do not represent all the heat transfer that is occurring in the samples. Without improved models, the results from the PTR measurements are qualitative at best, but they do demonstrate the presence of the QCL film on the InP wafer. Additional work must be done to better-quantify the results.

While the experimental results are not entirely conclusive, the simulations and experiments corroborate each other sufficiently to warrant further investigation using these techniques. Additionally, the simulations present sufficient internal consistency so as to be useful for thermal property investigation independent of the PTR results.

Checksum

7db518cba5b9afc138b58bad4bfecf76

Recommended Citation

Horne, Kyle S., "Nano-Scale Thermal Property Prediction by Molecular Dynamics Simulation with Experimental Validation" (2014). All Graduate Theses and Dissertations, Spring 1920 to Summer 2023. 3089.
https://digitalcommons.usu.edu/etd/3089