AFOSR Research

AFOSR (Air Force Office of Scientific Research) Project at the Nanoscale Heat Transfer Lab

Modeling Interfacial Thermal Boundary Conductance of Engineered Interfaces

Dr. Pamela M. Norris, in collaboration with Dr. Leonid N. Zhigilei, University of Virginia

Summary

As device dimensions shrink, it becomes more likely that energy carriers will scatter at an interface between two adjacent materials than in the materials that form the interface. As a result, heat transport around active regions in these devices is restricted by the presence of interfaces between the materials and the structures surrounding them. Thus, critical to thermal management of next generation nanodevices is the interfacial thermal transport. Thermal applications affected include thermoelectric power generation and cooling, heat dissipation from electronics, phase change memory, quantum cascade lasers, and thermal barrier coatings.

scattering

The objective of this research is to investigate how the physical aspects of the interfacial region affect the thermal conductance across material interfaces. Historically, models for hBD largely neglected the physical structure and composition of the interface, resulting in a discrepancy between theoretical predictions and volumes of experimental data.  Understanding the role of interface morphology in thermal boundary conductance not only improves agreement between theory and experiment, but also enables the active tuning of interfacial transport for device applications. This research project is divided into three mutually supporting tasks leading towards development of more realistic models that account for the physical aspects of the interfacial region:

  • measurement of the effects of the interfacial mixing layer on hBD through atomistic calculation
  • measurement of the effects of an added interstitial layer on hBD
  • experimental verification of atomistic calculations

Selected accomplishments to date:

Modeling:

Whereas previous models only included consideration of the bulk materials comprising the interface and energy transmission via Debye acoustic phonons, we developed extensions to the diffuse mismatch model in order to account for realistic phonon dispersion [1], optical phonons [2], and inelastic phonon-phonon coupling [3] across the interface.  We found that these effects significantly contribute to the predicted thermal boundary conductance, and are thus necessary to include when modeling a variety of interfaces.

Modeling

Figure from Ref. 2. Optical phonons contribute up to 40% to the
thermal boundary conductance for a wide variety of common interfaces.
Copyright 2010 AIP, personal use only.

Simulations:

Computational modeling of interfacial mixing and an added interstitial layer have revealed that these methods may offer an effective method for tuning thermal boundary conductance.  In particular, the introduction of a bridging material at the interface with vibrational properties intermediate to those of the bulk materials offers an innovative method of enhancing the thermal boundary conductance of a given interface [4].  Recently, we developed computational tools that will facilitate the study of individual phonon transport at interfaces and individual phonon transport through nanostructures [5, 6], enabling a better understanding of the physics underlying transport across non-ideal interfaces.

Figure adapted from Ref. 4. Copyright 2012 APS, personal use only.

Figure adapted from Ref. 4. Engineering of the interface using an
inserted film and moderate mixing enables enhancement of the thermal boundary conductance by about 53%.
Copyright 2012 APS, personal use only.

Experiment:

Experiments have confirmed the drastic effect of local interface structure on thermal boundary conductance.  Using the time domain thermoreflectance method, we observed that the thermal boundary conductance is influenced by the surface preparation during interface creation.  For example, surface adatoms from as-cleaved samples acted to bridge the vibrational gap between mismatched materials while ion bombardment prior to material deposition caused a large degree of disorder about the interface, reducing the thermal boundary conductance [7].

Figure from Ref. 7. Copyright 2012 ASME, personal use only.Figure from Ref. 7. Local interface composition and structure can tune the
thermal boundary conductance by about a factor of four.
Copyright 2012 ASME, personal use only.

Selected publications:

[1] J. C. Duda et al., “Role of dispersion on phononic thermal boundary conductance.” J. Appl. Phys. 108, 073515 (2010).

[2] T. Beechem et al., “Contribution of optical phonons to thermal boundary conductance.” Appl. Phys. Lett. 97, 061907 (2010).

[3] P. E. Hopkins et al., “Anharmonic Phonon Interactions at Interfaces and Contributions to Thermal Boundary Conductance.” J. Heat Transfer 133, 062401 (2011).

[4] T. S. English et al., “Enhancing and tuning phonon transport at vibrationally mismatched solid–solid interfaces.” Phys. Rev. B 85, 035438 (2012).

[5] C. H. Baker et al., “Application of the wavelet transform to nanoscale thermal transport.” Phys. Rev. B 86, 104306 (2012).

[6] N. Q. Le et al., “Strategies for tuning phonon transport in multilayered structures using a mismatch-based particle model.” J. Appl. Phys. 111, 084310 (2012).

[7] P.M. Norris et al., “Prediction and Measurement of Thermal Transport Across Interfaces Between Isotropic Solids and Graphitic Materials.” J. Heat Transfer 134, 020910 (2012).