Chemically driven carbon-nanotube-guided thermopower waves

Journal name:
Nature Materials
Year published:
Published online


Theoretical calculations predict that by coupling an exothermic chemical reaction with a nanotube or nanowire possessing a high axial thermal conductivity, a self-propagating reactive wave can be driven along its length. Herein, such waves are realized using a 7-nm cyclotrimethylene trinitramine annular shell around a multiwalled carbon nanotube and are amplified by more than 104 times the bulk value, propagating faster than 2ms−1, with an effective thermal conductivity of 1.28±0.2kWm−1K−1 at 2,860K. This wave produces a concomitant electrical pulse of disproportionately high specific power, as large as 7kWkg−1, which we identify as a thermopower wave. Thermally excited carriers flow in the direction of the propagating reaction with a specific power that scales inversely with system size. The reaction also evolves an anisotropic pressure wave of high total impulse per mass (300Nskg−1). Such waves of high power density may find uses as unique energy sources.

At a glance


  1. Schematic of an exothermic chemical reaction guided along a thermally conducting CNT and experimental realization of a MWNT encapsulated by TNA.
    Figure 1: Schematic of an exothermic chemical reaction guided along a thermally conducting CNT and experimental realization of a MWNT encapsulated by TNA.

    a, CNT wrapped with TNA and mechanism of chemical reaction. Schematic (top left) and chemical structure (top right) of TNA–CNT. The TNA physically contacts the MWNT sidewall. TNA and MWNT structures are both preseved in the composite TNA–CNT. Mechanism of reaction (bottom). Ignition at one end of a TNA–CNT results in an exothermic reaction and heat transfer along the length of the CNT, with feedback creating an anisotropic reaction wave of amplified velocity. b, Transmission electron microscopy image of a TNA–MWNT synthesized by wet impregnation. The dashed line indicates the boundary between the MWNT and TNA layers. c, X-ray diffraction showing a TNA–MWNT after synthesis (black), MWNT alone (blue) and TNA alone (red).

  2. An accelerated, anisotropic reaction wave of TNA confined to an annular region around a nanotube thermal waveguide.
    Figure 2: An accelerated, anisotropic reaction wave of TNA confined to an annular region around a nanotube thermal waveguide.

    a, Reaction propagation along an aligned MWNT array (average diameter 22nm) after ignition by electrical discharge (no preheating). The height of the TNA–MWNT array is ~2mm, and its cross-section is about 0.1mm2 (frame rate=3.33kHz). b, The reaction velocity differs between samples preheated parallel and orthogonal to the aligned direction (average MWNT diameter 13nm) as measured with an optical fibre array along the array length (see Supplementary Fig. S5). The x axis shows the preheating temperature of the TNA–MWNT array (Supplementary Table S1). c, Predicted reaction velocity from the Fourier model (equations (1)–(3)) as a function of β, the dimensionless inverse adiabatic temperature of the annular material (10.6 for TNA, Supplementary Table S2) versus CNT thermal diffusivity (m2s−1). Note that the reaction velocity increases with increasing CNT thermal conductance. Experimentally observed reaction velocities for both 13- and 22-nm MWNT are plotted for comparison.

  3. A chemically driven thermopower wave.
    Figure 3: A chemically driven thermopower wave.

    a, Illustration of the experimental set-up used for measuring the corresponding thermopower waves that result from reaction wave propagation. Laser ignition (or electric arc discharge) at one end results in a thermopower wave in the same direction of reaction propagation; either positive or negative voltage generation is observed. b, The d.c. voltage generated by exothermic reaction of TNA is observed immediately after laser ignition with a single-polarity peak voltage in this experiment of 30–35mV (maximum observed=210mV) in both positive and negative directions for a total system mass of 0.8mg with a TNA/MWNT ratio of 9. c, These thermopower waves are distinct from conventional, static thermopower generation mechanisms. If the system mass is increased, the reaction wave moves slowly and multiple peaks are observed instead, showing a reversal of polarity and an inflection point. Similar behaviour is seen if initiation occurs at the centre of the sample. d, The specific peak power plotted as a function of system mass for three different TNA/MWNT mass ratios (9, 4.5 and 2.8) and two different MWNT diameters (13 and 22nm) shows an inverse scaling, highlighting that the thermopower wave is enhanced at the micro–nanoscale, and can produce power densities that far exceed conventional energy storage devices. The green line is equation (4) based on the conventional thermoelectric effect and the temperature gradient from reaction (300–2,800K).

  4. Anisotropic reaction propagation.
    Figure 4: Anisotropic reaction propagation.

    a, For larger samples, there remains a large axial component to the reaction wave, but also a portion that is orthogonal. The reaction velocities along the aligned (0°) and orthogonal (90°) directions show a discernible peak along the axial direction. (Reactions were initiated at the base.) Parenthetical numbers are the mass ratio and mass of TNA. b, The thermopower wave evolves a thrust force of commensurate duration. The transient force response is larger for the five samples (5×5×6mm, 5–20mg) aligned perpendicular to the sensor compared with the control aligned parallel to the surface. c, An illustration of directed thrust: moving as a free body, the TNA–MWNT array (0.4×0.3×2mm) proceeds in the direction of its orientation without external confinement. d, The total impulse and specific impulse per total mass of TNA–MWNT is significantly higher than other microthruster systems35, 36, 37, 38, 39, 40 because of the lack of a need for external containment, in contrast to electrokinetic37, ferroelectric plasma39 and laser ablation jet36 microthruster approaches.


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Author information


  1. Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

    • Wonjoon Choi,
    • Joel T. Abrahamson,
    • Jae-Hee Han,
    • Changsik Song,
    • Nitish Nair &
    • Michael S. Strano
  2. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA

    • Wonjoon Choi
  3. SKKU Advanced Institute of Nanotechnology, Department of Energy Science and School of Mechanical Engineering, Sungkyunkwan University, Gyeonggi, 440-746, Korea

    • Seunghyun Hong &
    • Seunghyun Baik


W.J.C., M.S.S. and J-H.H. developed the concept. W.J.C. and S.H. carried out experiments. W.J.C. and J.T.A. conducted modelling and simulations. All authors contributed to data analysis and scientific discussion.

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