Functional materials for thermal regulation

Zheng, Qiye

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https://hdl.handle.net/2142/99353 Copy

Description

Title

Functional materials for thermal regulation

Author(s)

Zheng, Qiye

Issue Date

2017-12-04

Director of Research (if dissertation) or Advisor (if thesis)

• Cahill, David G.
• Braun, Paul V.

Doctoral Committee Chair(s)

• Cahill, David G.
• Braun, Paul V.

Committee Member(s)

• Shoemaker, Daniel P.
• Ertekin, Elif

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Materials physics
• Nanoscale thermal transport
• Time-domain thermoreflectance
• Raman spectroscopy
• Thermal conductivity
• 2D materials
• Electron and phonon
• Phase transition
• Martensitic transition

Abstract

Developing thermal management materials with greater functionality for the control of heat dissipation and regulation is a fundamental step to improve the performance in many engineering systems. This dissertation presents experimental studies of heat conduction in distinctive thermal functional materials with an emphasis on advancing understanding of the correlation between thermal conductivity Λ and specific microscopic mechanisms that are closely related to their applications. The primary tools for the studies presented in this dissertation are time-domain thermoreflectance (TDTR) for measurements of thermal conductivity of thin films and bulk materials and thermal conductance of interfaces. The thermal conductivity of two dimensional materials is of interest for many applications, including energy storage, nanoelectronics and heat dissipation. I studied thermal conductivity of graphite thin films grown by chemical vapor deposition (CVD) on single crystal Ni (111) at temperatures between 825 and 900 C. The cross-plane thermal resistance of Al/graphite/Ni structures is a linear function of graphite film thickness in the thickness range 20-140 nm corresponding to a thermal conductivity of ≈3.3 W m−1 K−1, ≈50% of the cross-plane thermal conductivity of highly-oriented pyrolytic graphite (HOPG). The in-plane thermal conductivity varies with deposition temperature between 650 and 1000 W m−1 K−1, and is 30-50% of the in-plane thermal conductivity of HOPG. The reduced thermal conductivity in comparison to HOPG is attributed to a combination of grain boundaries, structural disorder and size effects. I described a figure-of-merit for flexible heat-spreaders and found that this figure-of-merit for CVD graphite outperform that of Au by a factor of 2. In another layered material of MoS2, the anisotropic thermal conductivity can be modified by electrochemical intercalation differently in thin films with vertically-aligned basal planes and natural bulk crystals. The change of thermal conductivity as a function of the degree of lithiation correlates with the stacking order of the layered structure and the lithiation-dependent semiconductor to metal phase transition as supported by Raman spectroscopy. Further, the ratio of the in-plane to through-plane thermal conductivity is enhanced by the disorder in bulk crystals. These results suggest that stacking disorder and mixture of phases is an effective mechanism to modify the anisotropic thermal conductivity of two-dimensional materials, opening a promising way for thermal management in two-dimensional materials based electronics. In comparison to the above materials where electron is ignorable in heat transport, transition metal nitrides provide a model system where electron dominates and electron-phonon interaction is strong. I studied thermal conductivities of VNx/MgO(001) (0.76 ≤ x ≤ 1.00) epitaxial layers, grown by reactive magnetron sputter deposition, in the temperature range 300 < T < 1000. Data for the total thermal conductivity are compared to the electronic contribution to the thermal conductivity calculated from the measured electrical conductivity, the Wiedemann-Franz law, and an estimate of the temperature dependence of the Lorenz number L(T). The total thermal conductivity is dominated by electron contribution and varies between 13 W m-1 K-1 at x = 0.76 and 20 W m-1 K-1 at x = 1.00 for T = 300 K and between 25 and 35 W m-1 K-1 for T = 1000 K. The lattice thermal conductivity vs. x ranges from 5 to 7 W m-1 K-1 at 300 K and decreases by 20% at 500 K. The low magnitude and weak temperature dependence of the lattice thermal conductivity are attributed to strong electron-phonon coupling in VN. Finally, in search of materials with variable thermal conductivity, I studied temperature dependent thermal conductivity of Ni-Mn-In and MnxMGe (M = Ni, Co) alloys across their magnetic and structural martensitic phase transitions. A sharp change in thermal conductivity is observed through the martensitic transition but not through the magnetic transition in Ni-Mn-In and MnxMGe alloys. In Ni-Mn-In, Λ changes from 8 W m-1 K-1 300 K to 14 W m-1 K-1 at 400 K. The average rate of the thermal conductivity change in Ni-Mn-In and MnxNiGe in the 50 K range of their transition temperatures is faster than common metals and alloys including the conventional shape memory alloy of NiTi. The austenite phase electrical conductivity of Ni-Mn-In is around 100 µΩ-cm at 400 to 500 K, which is comparable with stainless steel. Both Λ and electrical conductivity are more than twice as large as the metal-insulator transition material of VO2 thin film in its metallic state. By comparing the result with the electronic thermal conductivity calculated from the Wiedemann-Franz law I attribute the rapid change through the phase transition in the Λ to the in the electronic contribution in Ni-Mn-In. My work suggests that Ni-Mn-In and MnNiGe based alloys may serve as functional materials for thermal management applications that require a high-contrast variable thermal and electrical conductivity as well as large high-end conductivities.

Graduation Semester

2017-12

Type of Resource

text

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http://hdl.handle.net/2142/99353

Copyright and License Information

Copyright 2017 Qiye Zheng

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