High thermal conductivity of chain-oriented amorphous polythiophene

Journal name:
Nature Nanotechnology
Volume:
9,
Pages:
384–390
Year published:
DOI:
doi:10.1038/nnano.2014.44
Received
Accepted
Published online
Corrected online

Abstract

Polymers are usually considered thermal insulators, because the amorphous arrangement of the molecular chains reduces the mean free path of heat-conducting phonons. The most common method to increase thermal conductivity is to draw polymeric fibres, which increases chain alignment and crystallinity, but creates a material that currently has limited thermal applications. Here we show that pure polythiophene nanofibres can have a thermal conductivity up to ~4.4 W m–1 K–1 (more than 20 times higher than the bulk polymer value) while remaining amorphous. This enhancement results from significant molecular chain orientation along the fibre axis that is obtained during electropolymerization using nanoscale templates. Thermal conductivity data suggest that, unlike in drawn crystalline fibres, in our fibres the dominant phonon-scattering process at room temperature is still related to structural disorder. Using vertically aligned arrays of nanofibres, we demonstrate effective heat transfer at critical contacts in electronic devices operating under high-power conditions at 200 °C over numerous cycles.

At a glance

Figures

  1. Microstructure of polythiophene nanofibres.
    Figure 1: Microstructure of polythiophene nanofibres.

    a, Chain orientation morphology in drawn semicrystalline polymer. The folded chains are crystallites or crystalline domains surrounded by amorphous regions. b, Chain orientation morphology in amorphous polymer: chain orientation without folded crystalline domains. The direction of heat transfer is horizontal in a and b. c, Scanning electron microscopy image of vertical polythiophene nanofibre arrays on a metal substrate. The arrays contained either solid fibres or mostly tubes depending primarily on pore diameter. Template pore channels of varying diameter (200, 100, 55 and 18 nm) control the fibre diameter, although template irregularities caused fluctuations about these nominal diameters (for example, the ranges of polythiophene nanofibre diameters from 200 nm and 100 nm templates were 145–300 nm and 70–120 nm, respectively). The smaller-diameter fibres (18 and 55 nm templates) did not remain vertically aligned after removing the template due to their reduced stiffness, making them more difficult to apply as heat transfer materials. d, TEM image of a polythiophene nanofibre from a 200 nm template. Inset: Selected-area electron diffraction analysis consistent with amorphous material. e, High-resolution TEM image of a polythiophene nanotube wall, showing amorphous material.

  2. Thermal conductivity measurements of single fibres and vertically aligned arrays.
    Figure 2: Thermal conductivity measurements of single fibres and vertically aligned arrays.

    a, Single-fibre thermal conductivity at room temperature as a function of fibre diameter. Inset: Scanning electron microscopy image of a polythiophene nanofibre on a suspended microbridge for thermal conductivity measurement. b, Representative single-fibre thermal conductivity measurements on the microbridge as a function of temperature (the horizontal error bars are approximately the width of the data marker in all cases). The error bars in a and b are explained in the section ‘Microbridge technique’ in the Supplementary Information. Data of amorphous polythiophene (a-PT) fibres with different diameters (specified in the plot) are from this work. Crystalline polyethylene (c-PE) and crystalline polybenzobisoxazole (c-PBO) data are measurements reported in ref. 4. Black dashed line represents the predicted minimum thermal conductivity, κmin,21 for polythiophene. c, Photoacoustic cell used to measure array thermal conductivity. d, Effective polythiophene nanotube (~200 nm diameter) array thermal conductivity as a function of height for vertically aligned arrays (60% fill fraction assuming solid fibres). The values for bulk polythiophene in b and d were obtained from photoacoustic measurements on electrodeposited films. Explanation of the error bars is given in the section ‘Photoacoustic technique’ in the Supplementary Information.

  3. Application of vertically aligned polythiophene nanofibres as a TIM.
    Figure 3: Application of vertically aligned polythiophene nanofibres as a TIM.

    a, TIM illustration with component thermal resistances. The total thermal resistance of a TIM is a function of the layer thermal conductivity κ, layer thickness L and contact resistance Rc1, Rc2 on each side of the material, and is given by Rtotal = Rc1 + Rlayer + Rc2 where Rlayer = L/κ. b, Total thermal resistance measurements of polythiophene-nanotube TIMs with the photoacoustic technique. Post-bake data were obtained after the sample was heated in air for 100 h at 200 °C. Re-work + post-bake data were obtained after the same sample was wetted, removed from the quartz, then rewetted and dried on the quartz. Error bars represent one standard deviation of four to six measurements on each TIM. c, Comparison of total resistance values associated with a number of commercial TIM technologies. Values for carbon nanotube tubes (CNT) were obtained from refs 33 and 34, for thermal grease from refs 11 and 30, and for all others from ref. 30.

  4. Device demonstration of polythiophene-nanofibre TIM at high temperature.
    Figure 4: Device demonstration of polythiophene-nanofibre TIM at high temperature.

    a, Polythiophene nanofibre array grown on a Cu heatsink and dried in contact with a SiC RF device simulator. b, Device operated while cycling in air between 5 °C and 200 °C for 16 h (80 cycles with 5 min dwell times at each temperature). c, Total thermal resistance Rtotal of the polythiophene nanofibre TIM measured as a function of power density before baking at 130 °C for 308 h (pre-bake), after baking (post-bake) and after thermal cycling (post-cycle) (as in b). Explanation of the error bars is given in the Supplementary Section ‘Device testing’. d, Cross-sectional scanning electron microscopy image of the device after testing. The magnified image reveals a void in the Cu heatsink that prevented the polythiophene nanofibre TIM from making good thermal contact.

Change history

Corrected online 17 June 2014
In the version of this Article originally published, in the section 'Thermal conductivity of individual fibres', the second sentence should have read "The measured thermal conductivity of the several nanofibre samples increases with decreasing diameter..." This error has now been corrected in the online versions of the Article.

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Author information

  1. These authors contributed equally to this work

    • Virendra Singh &
    • Thomas L. Bougher

Affiliations

  1. George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta, Georgia 30332, USA

    • Virendra Singh,
    • Thomas L. Bougher,
    • Wei Lv,
    • Asegun Henry &
    • Baratunde A. Cola
  2. Department of Mechanical Engineering, The University of Texas at Austin, 204 East Dean Keeton Street, Austin, Texas 78712, USA

    • Annie Weathers,
    • Kedong Bi,
    • Michael T. Pettes,
    • Sally A. McMenamin &
    • Li Shi
  3. School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, J. Erskine Love Building, Atlanta, Georgia 30332, USA

    • Ye Cai,
    • Kenneth H. Sandhage,
    • Asegun Henry &
    • Baratunde A. Cola
  4. School of Mechanical Engineering, Southeast University, Nanjing, 211189, China

    • Kedong Bi
  5. Raytheon Company, Sudbury, Massachusetts 01776, USA

    • Daniel P. Resler,
    • Todd R. Gattuso &
    • David H. Altman

Contributions

V.S., T.L.B. and B.A.C. conceived and designed the experiments. V.S. prepared the samples and performed the material spectroscopy and adhesion tests. T.L.B. performed the photoacoustic measurements. A.W., K.B., M.T.P., S.A.M. and L.S. performed the microbridge measurements. D.P.R., T.R.G. and D.H.A. performed the SiC chip tests. Y.C. and K.H.S. provided the TEM images and crystallinity characterization. W.L. and A.H. provided the single chain simulations. V.S., T.L.B. and B.A.C. analysed and discussed the data. V.S., T.L.B. and B.A.C. co-wrote the manuscript. All authors commented on the manuscript.

Competing financial interests

Georgia Tech has applied for a patent, application no. PCT/US 61/484,937, related to the design methods and materials produced in this work. Nanostructured composite polymer thermal/electrical interface material and method for making the same, B.A. Cola, K. Kalaitzidou, H.T. Santoso, V. Singh, US 2012/0285673 A1, November 15, 2012.

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