resistance of 1 Â°C/W, the total resistance is 2 Â°C/W. A common engineering design problem involves the selection of an appropriate sized heat sink for a given heat source. Working in units of thermal resistance greatly simplifies the design calcula
May 13, 2015 - cases, respectively. Our numerical method quantifies the role of electronic band structure and carrier scattering mechanisms. We have successfully demonstrated bipolar thermal conductivity reduction in doped semiconductors via electron
average of at least six measurements generally from between measurements five and ten. The average thermal conductivity values reported for each sample have typical standard devia- tions from 0.01 to 0.30 W mÐ1 KÐ1, with most less than. 0.10 W mÐ1
Jan 15, 2009 - Rawat, Vijay; Koh, Yee Kan; Cahill, David G.; and Sands, Timothy D., "Thermal conductivity of (Zr,W)N/ScN metal/semiconductor ... to the lattice component of the thermal conductivity of a Zr0.65Sc0.35N alloy film (5 W/m K). Alloying th
Measurement of thermal conductivity. Part A: P t Bar : â¢ Time domain ... thermal conductance from sidewalls to ambient. Modern application to suspended ... Schmidt et al., RSI 2008. â¢ Heat supplied by modulated pump beam (fundamental. Fourier com
between predictions from molecular dynamics simulations (a method valid at temperatures above the Debye temperature) ... A key feature of ionic materials is the presence of the long-range electrostatic interactions whose influence ... ranged and repr
May 2, 2011 - Qavg=50 mW, and has Tavg=7.6 K. Caution should be taken to minimize Tavg for low thermal conductivity samples by using low pump laser power or low duty-cycle periodic modulation that result in low Qavg. For the room temperature validati
Dec 31, 2016 - oils) are usually used as heat transfer fluids: in thermal engines (coolants ... alumina/water and copper oxide/water nanofluids are performed and ... Thus, we try in this work to answer the following fundamental ... In this case, the
Abstract. In recent years, many studies have been conducted regarding polymer nanocomposites, namely polymers doped with various forms of graphene. The impact toughness, tensile strength, and thermal conductivity of Polypropylene/ Graphene and. Polys
Feb 19, 2016 - To date, there is no experimental characterization of thermal conductivity of semiconductor polymeric individual nanowires embedded in a matrix. This work reports on scanning thermal microscopy measurements in a 3u configuration to det
Dec 14, 2010 - This thesis, âIn-Plane Thermal Conductivity Modeling of Carbon Filled Liquid. Crystal Polymer Based Resins,â is hereby approved in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in the field of Chemical
For example, figure 1 shows a set of 1340 measurements for the conductivity of expanded polystyrene (EPS) at 10ÂºC, and dry and aged material. The key independent variable is density. Dispersion around the average conductivity is clearly seen in this
May 4, 2016 - for thermal measurements was fabricated by dry-transfer method, whose sample quality, due to less polymer residues ... A PMMA-free technique was used to fabricate suspended ... 1Center for Phononics and Thermal Energy Science, School of
NANOFLUID THERMAL CONDUCTIVITY-A REVIEW. Ravi Sankar.B. 1. , Nageswara Rao. D. 2. ,Srinivasa Rao.Ch. 3. 1. Lecturer, Mechanical Engg. Deptt., R.V.R&J.C. College of Engg., Guntur, A.P, India. 2. Vice-Chancellor, Centurion University, Odisa, India. 3.
Oct 21, 2016 - host lattice of MoS2 undergoes a phase transition from 2H to 1T phase19. The voltage dip at the initial stage of the discharge curve for bulk MoS2 sample is caused by the mass transport limitation of lithium ions. The voltage gradually
applications ranging from microelectronic devices to energy storage and energy conversion devices. Here, we report ultralow lattice thermal conductivities of solution-synthesized, single- crystalline all-inorganic halide perovskite nanowires composed
Revised Manuscript Received November 30, 2000. A new series of highly conducting polymer ... (5) Killis, A.; LeNest, J. F.; Cheradame, H.; Gandimi, A. Makromol. Chem. 1982, 183, 2835. (6) Druger, S. D. ..... Second International Meeting on Polymer El
Sep 15, 1972 - values of high temperature gases is evident, all but a in their long (8 ft) and narrow (1.22-in. diam) test ..... Eqs. (4) and (5) yield. (6). TABLE I. Estimate of random errors (percent). Temp. range. Temp. 800-1200 1200-1600 1600-200
Downloaded 03 Apr 2012 to 184.108.40.206. Redistribution .... Sections 2.1 and 3.1 review steady-state techniques, and. Sections ... A "free-standing bridge" consists of a layer without a substrate or a layer- substrate composite for which Eq (3) was
Aug 25, 2011 - modulation frequency to measure MFPs, but this technique is limited by the modulation frequency . Here, we introduce a thermal conductivity spectroscopy technique which can measure MFP distributions over a wide range of length scal
May 23, 2013 - The TEG efficiency can be broken into two terms: the limit- ing Carnot efficiency and the efficiency relative to Carnot called the reduced device efficiency (gr,d)6 g Â¼. DTTE. Th gr,d. (3). In the constant property model (CPM), a, q,
Jun 15, 2012 - strated the use of TDTR for measuring the heat flow in non- conformal, porous solids as well as liquids,8â10 we show both experimentally, computationally, and theoretically the advantage of TDTR for measuring highly porous films due
May 29, 2013 - temperature thermal conductivity of Si/Ge cross-section modulated nanowires is almost three orders of magnitude lower than that of bulk Si.
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.