AVS 58th Annual International Symposium and Exhibition
    Advanced Surface Engineering Division Tuesday Sessions
       Session SE-TuP

Paper SE-TuP4
Crystalline Thin Film Materials with Ultra-Low Thermal Conductivity

Tuesday, November 1, 2011, 6:00 pm, Room East Exhibit Hall

Session: Advanced Surface Engineering Poster Session
Presenter: Christopher Muratore, Air Force Research Laboratory
Authors: C. Muratore, Air Force Research Laboratory
V. Varshney, Air Force Research Laboratory
A. Reed, Air Force Research Laboratory
J. Hu, Air Force Research Laboratory
J. Bultman, Air Force Research Laboratory
T. Smith, Air Force Research Laboratory
A.A. Voevodin, Air Force Research Laboratory
Correspondent: Click to Email
Transition metal dichalcogenide (TMD) crystals are characterized by their distinct layered atomic structures, with strong covalent bonds comprising each layer, but weak van der Waals forces holding the layers together. The relationship between chemical bonding in a material and its thermal conductivity (k) is well-known, however the thermal properties of TMD thin films with such highly anisotropic chemical bonds have only recently been investigated with remarkable results, such as ultra-low kz. Materials with very low thermal conductivity in the z-axis, but higher kx and ky have potential as next-generation thermal barrier or heat spreading materials. Molecular dynamics (MD) simulations predicted kx=ky=4kz for perfect TMD crystals (MoS2 in this case). Experiments to determine kx,y and kz were conducted by developing processes to grow crystalline TMD thin film materials with strong (002) (basal planes parallel to surface) or (100) (perpendicular basal planes) preferred orientation. Initially, no correlation between structure and thermal conductivity was apparent, as water intercalation and reactivity to ambient air resulted in a thermal “short-circuit” across basal planes, such that the time between deposition and k measurement had a stronger impact on thermal conductivity than film orientation. Experiments to measure intrinsic thermal conductivity of MoS2 revealed values approximately one order of magnitude lower than those predicted using MD simulations, however, measurement of kx=ky=4kz was consistent with simulation results. Simulations to evaluate the dependence of thermal conductivity on grain size were evaluated, which correlated well to measured values. Comparison of measured k values for MoS2, WS2, WSe2 and other materials with analogous crystal structures are discussed in the context of the Slack Law, which accounts for intrinsic physical properties of the crystal, but not film microstructure. Alternatives to TMDs, with less environmental sensitivity, will also be illustrated.