Research ArticleMATERIALS SCIENCE

Tailor-made temperature-dependent thermal conductivity via interparticle constriction

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Science Advances  17 Nov 2017:
Vol. 3, no. 11, eaao5238
DOI: 10.1126/sciadv.aao5238
  • Fig. 1 Key aspects for heat management devices and their realization based on constriction-controlled thermal transport in colloidal assembly structures.

    (A) By exceeding Tg, the thermal conductivity irreversibly increases based on the enlargements of contact points during particle sintering. (B) The transition temperature can be tailored by assembling the crystal from particles having different Tg. (C) The random coassembly of equal-sized particles but different Tg results in a broad transition. (D) Multiple transition steps can be introduced by a discrete layer-by-layer assembly. (E) The height of the transition steps is controllable by the thickness of the respective layer.

  • Fig. 2 Thermal conductivity of polymer colloidal crystals having different Tg.

    (A) Optical and SEM images of the split edges of the assembled crystals. The strong opalescence indicates a long-range crystalline order within the freestanding monoliths. The high crystallinity is confirmed by the corresponding SEM images. (B) Specific heat capacity of the synthesized copolymer particles. With increasing MMA content, the Tg of the polymer is shifted to higher temperatures. (C) Temperature-dependent thermal conductivity of polymer colloidal crystals from particles having different Tg (heating and cooling cycle). By adjusting the copolymer composition, it is possible to tailor the transition temperature systematically. Error bars represent the SD derived from three individual measurements. Closed symbols represent the heating cycle; open symbols represent the cooling cycle.

  • Fig. 3 Temperature-dependent thermal conductivity of coassembled polymer colloidal assemblies.

    (A) Schematic illustration of the composition of a coassembled colloidal crystal. The coassembly leads to structurally homogeneous colloidal crystals due to the comparable particle size. (B) Optical micrographs of the split edges of pure colloidal crystals (mixing ratios, 0 and 100% of Tg = 103°C particles) in comparison to a coassembled binary crystal (mixing ratio, 50%:50%). (C) Temperature-dependent thermal conductivity of the 50%:50% colloidal crystal compared to its pure counterparts. The binary colloidal crystal shows a broad transition, ranging between the glass transition temperatures (dashed lines) of the pure copolymer particles. Error bars represent the SD derived from three individual measurements. Thermal diffusivity data can be found in fig. S3A. Closed symbols represent the heating cycle; open symbols represent the cooling cycle.

  • Fig. 4 Introduction of multiple-step transitions.

    (A) Schematic illustration of the structure of a colloidal monolith consisting of one, two, and three particle layers where each layer has a different Tg (blue, green, and red). (B) Temperature-dependent thermal conductivity of colloidal monoliths consisting of one, two, and three particle layers. On the basis of the discrete layer assembly, multiple step-like increases (dashed red lines) at the specific Tg of the copolymer particle are observed. Error bars represent the SD derived from three individual measurements. Thermal diffusivity data can be found in fig. S3 (A and B). Closed symbols represent the heating cycle; open symbols represent the cooling cycle.

  • Fig. 5 Tuning the transition height.

    (A) Schematic illustration of the structure of a colloidal monolith with varying layer thickness. The thickness of the red particle layer increases from left to right. (B) Temperature-dependent thermal conductivity of different colloidal assemblies with varying particle layer thickness. The thickness of the higher Tg layer is increased from left to right, leading to an increasing transition height at the second Tg (red arrow). Error bars represent the SD derived from three individual measurements. Thermal diffusivity data can be found in fig. S3 (D and E). Closed symbols represent the heating cycle; open symbols represent the cooling cycle.

  • Fig. 6 Combining a broad and a step-like transition.

    Temperature-dependent thermal conductivity of a two-layer colloidal assembly. Whereas the bottom layer is fabricated by evaporation-induced self-assembly of two particles having different Tg (90 volume % MMA-2, Tg = 103°C; 70 volume % MMA-2, Tg = 61°C), the upper layer consists of only one particle type (100 volume % MMA-1, Tg = 127°C). Thermal diffusivity data can be found in fig. S3F. Closed symbols represent the heating cycle; open symbols represent the cooling cycle.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/11/eaao5238/DC1

    Description of XFA evaluation

    table S1. Hydrodynamic diameter dh, polydispersity index (PDI), hard sphere diameter d(sem), and glass transition temperature Tg of particles used in this study.

    fig. S1. Temperature-dependent specific heat capacity of the investigated samples.

    fig. S2. Temperature-dependent thickness and density of co-assembled colloidal crystals.

    fig. S3. Temperature-dependent and thickness-corrected thermal diffusivity of the measured colloidal specimens.

    fig. S4. Temperature-dependent thermal conductivity of coassembled colloidal crystals from two particles having a Tg of ~61° and ~103°C.

    fig. S5. Optical micrographs of a two-layer colloidal monolith made by filtration, and a two-layer monolith fabricated by a combination of evaporation-induced self-assembly and filtration.

    fig. S6. SEM cross section images of the gradual film formation of crystalline binary assemblies.

  • Supplementary Materials

    This PDF file includes:

    • Description of XFA evaluation
    • table S1. Hydrodynamic diameter dh, polydispersity index (PDI), hard sphere diameter d(sem), and glass transition temperature Tg of particles used in this study.
    • fig. S1. Temperature-dependent specific heat capacity of the investigated samples.
    • fig. S2. Temperature-dependent thickness and density of co-assembled colloidal crystals.
    • fig. S3. Temperature-dependent and thickness-corrected thermal diffusivity of the measured colloidal specimens.
    • fig. S4. Temperature-dependent thermal conductivity of coassembled colloidal crystals from two particles having a Tg of ~61° and ~103°C.
    • fig. S5. Optical micrographs of a two-layer colloidal monolith made by filtration, and a two-layer monolith fabricated by a combination of evaporation-induced self-assembly and filtration.
    • fig. S6. SEM cross section images of the gradual film formation of crystalline binary assemblies.

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