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Electrochemical control of thermal conductivity in thin films David G. Cahill, Jiung Cho, and Paul V. Braun Department of Materials Science and Engineering, Materials Research Laboratory, University of Illinois at Urbana-Champaign International Institute for Carbon Neutral Energy Research, Kyushu U., Fukuoka, Japan Supported by AFOSR
Outline • Thermal conductivity and measurement by timedomain thermoreflectance (TDTR) • Big picture goals of our work: – Understand and push the limits of thermal conductivity in various classes of materials – enhance thermal function in materials, e.g., abrupt changes in conductivity, actively controlled conduction, more efficient heat pumping. • Electrochemical modulation of the thermal conductivity of LixCoO2 – Materials science and phenomenology – Materials physics
Thermal conductivities of dense solids span a range of 40,000 at room temperature
Adapted from Goodson, Science (2007)
Long-pass optical filter
Short-pass optical filter
Clone built at Fraunhofer Institute for Physical Measurement, Jan. 7-8 2008
psec acoustics and time-domain thermoreflectance • Optical constants and reflectivity depend on strain and temperature • Strain echoes give acoustic properties or film thickness • Thermoreflectance dR/dT gives thermal properties
Time-domain Thermoreflectance (TDTR) data for TiN/SiO2/Si TiN SiO2
• reflectivity of a metal depends on temperature
Costescu et al., PRB (2003)
• one free parameter: the “effective” thermal conductivity of the thermally grown SiO2 layer (interfaces not modeled separately)
TDTR: validation experiments
Costescu et al., PRB (2003)
TDTR: Flexible, convenient, and accurate Λl (W m-1 K-1)
h (nm) Radiation damage High resolution mapping
TDTR is all optical method: adaptable to “extreme” environments such as high pressure Thermal conductivity of PMMA is independent of thickness and agrees well with the predicted scaling with (C11)1/2 Thermal Conductivity (W m-1 K-1 )
Diamond anvil cell
Hsieh et al., PRB (2011)
Andersson et al.
6 nm 13 nm
High throughput measurements of polymer fibers by time-domain thermoreflectance 30
Thermal conductivity (W m-1K-1)
10 8 6
4 2 0 100
10X 1000 Time delay (ps)
ZylonHM 20 Dyneema
ZylonAS PBT Spectra2000 5 μm M5AS Spectra900
30 Kevlar μm 2
Tensile modulus (GPa)
Wang et al., Macromolecules (2013)
Electrochemical modulation of thermal conductivity of LixCoO2 • Polycrystalline thin film prepared by sputter deposition and annealing • Real-time measurement by TDTR and picosecond acoustics. Thermal conductivity 3.65.4 W m-1 K-1 Elastic modulus 220300 GPa Ex-situ thermal conductivity contrast as large as a factor of 2.7 Cho et al., Nat. Commun. (2014)
Sputter deposit LixCo2 and anneal in air
500 nm LixCoO2; 0.3C rate • TDTR works best with Al transducer. — Limit annealing temperature of samples for in-situ studies to 500°C
Characterize microstructure by electron microscopy • After annealing at 500C in air • Nanocrystalline, dense microstructure
Characterize microstructure by electron diffraction • No strong texture; would eventually like to study textured films
In-situ measurements of thermal conductivity and elastic constants • Full delay time scans of Li0.5CoO2 and LiCoO2
Continuous real-time measurements during electrochemical cycling • With delay time set to a fixed value, ratio can be measured continuously and converted to thermal conductivity.
• Position of acoustic echo requires a scan over a limited range of delay times. Peak volume change is only 1.3% so changes in thickness are negligible.
Continuous real-time measurements during electrochemical cycling • Convert time-axis to composition. (We assume irreversible capacity loss occurs only during the lithiation cycle.) • Thermal conductivity is not a linear function of x; plateau for 0.5
Ex-situ measurements of film annealed at 700°C shows higher conductivity in fully lithiated state.
• Not yet sure of the mechanism. • Different texture? • Larger grain size? • Fewer point defects?
Do Li vacancies scatter phonons? • Classic example of point defect scattering is mass disorder created by isoelectronic substitution, e.g., SiGe alloy dilute SiGe alloys Change in thermal resistivity (Reciprocal of thermal conductivity)
• Unlikely that random Li vacancies alone can explain the dependence of thermal conductivity on x.
Dimensionless mass disorder
Mixture of Li rich and Li poor nanoscale phases? • Evidence in the literature (Reimer et al., JES (1992)) for a two-phase region 0.75
Li content has a strong influence on stiffness of bonds in the CoO2 sheets • Our samples are not textured so the change in longitudinal modulus is most due to C11 (stretch/compress along a-b plane)
• Higher Li content greater electron density in the CoO2 sheets increased bond strengths (?)
Summary • Time-domain thermoreflectance and picosecond acoustics enable real-time measurements of thermal conductivity and elastic constants of electrode materials. • Contrast between low and high thermal conductivity states of LixCoO2 up to a factor of 2.7. • Working on getting full set of elastic constants: by experiment (surface-acoustic waves; orientation dependence) and theory (DFT by Prof. Elif Ertekin). • Changes in longitudinal elastic modulus are linear in x; i.e., virtual crystal or effective medium seems to apply. • Changes in thermal conductivity are not linear in x and show a plateau for 0.5 < x< 0.8. — Speculate that this is caused by changing mixture of phases.