High-thermal-conductivity polymers are very sought after for applications in various thermal management systems. Although improving crystallinity is a common way for increasing the thermal conductivity ( k ) of polymers, it has very limited capacity when the crystallinity is already high. In this work, by heat-stretching a highly crystalline microfiber, a significant k enhancement is observed. More interestingly, it coincides with a reduction in crystallinity. The sample is a Spectra S-900 ultrahigh-molecular-weight polyethylene (UHMW-PE) microfiber of 92% crystallinity and high degree of orientation. The optimum stretching condition is 131.5 °C, with a strain rate of 0.0129 s –1 to a low strain ratio (∼6.6) followed by air quenching. The k enhancement is from 21 to 51 W/(m·K), the highest value for UHMW-PE microfibers reported to date. X-ray diffraction study finds that the crystallinity reduces to 83% after stretching, whereas the crystallite size and crystallite orientation are not changed. Cryogenic thermal characterization shows a reduced level of phonon-defect scattering near 30 K. Polarization Raman spectroscopy finds enhanced alignment of amorphous chains, which could be the main reason for the k enhancement. A possible relocation of amorphous phase is also discussed and indirectly supported by a bending test.

Conventional polymer composites normally suffer from undesired thermal conductivity enhancement which has hampered the development of modern electronics as they face a stricter heat dissipating requirement. It is still challenging to achieve satisfactory thermal conductivity enhancement with reasonable mechanical properties. Herein, we present a three-dimensional (3D), lightweight, and mechanically strong boron nitride (BN)-silicon carbide (SiC) skeleton with aligned thermal pathways via the combination of ice-templated assembly and hightemperature sintering. The sintering has introduced atomic-level coupling at the BN-SiC junction which contributes to efficient phonon transport via the newly formed borosilicate glass BC x N 3−x (0 ≤ x ≤ 3) and SiC x N 4−x (0 ≤ x ≤ 4) phases, leading to much lower interfacial thermal resistance. Thus, the obtained BN-SiC skeleton shows satisfactory thermal performance. The prepared 3D BN-SiC/polydimethylsiloxane (PDMS) composites exhibit a maximum through-plane thermal conductivity of 3.87 W•m −1 •K −1 at a filler loading of only 8.35 vol %. The thermal conductivity enhancement efficiency reaches 220% per 1 vol % filler when compared to pure PDMS matrix, superior to other reported BN skeleton-based composites. The feature of our strategy is to allow the oriented three-dimensional skeleton to be strongly bonded by a sintered ceramic phase instead of polymer-like adhesive, namely, to improve the intrinsic thermal conductivity of the skeleton to the greatest extent. This strategy can be applied to develop novel thermal management materials that are lightweight and mechanically tough that rapidly transfer heat. It represents a new avenue to addressing the heat challenges in traditional electronic products.

This work uncovers that free-standing partly reduced graphene aerogel (PRGA) films in vacuum exhibit extraordinarily bolometric responses. This high performance is mainly attributed to four structure characteristics: extremely low thermal conductivity (6.0−0.6 mW•m −1 •K −1 from 295 to 10 K), high porosity, ultralow density (4 mg• cm −3 ), and abundant functional groups (resulting in tunable band gap). Under infrared radiation (peaked at 5.8−9.7 μm), the PRGA film can detect a temperature change of 0.2, 1.0, and 3.0 K of a target at 3, 25, and 54 cm distance. Even through a quartz window (transmissivity of ∼0.98 in the range of 2−4 μm), it can still successfully detect a temperature change of 0.6 and 5.8 K of a target at 3 and 28 cm distance. At room temperature, a laser power as low as 7.5 μW from a 405 nm laser and 5.9 μW from a 1550 nm laser can be detected. The detecting sensitivity to the 1550 nm laser is further increased by 3-fold when the sensor temperature was reduced from 295 K to 12 K. PRGA films are demonstrated to be a promising ultrasensitive bolometric detector, especially at low temperatures.

Small molecules with functional groups can show different electron affinity and binding behavior on nanocrystal surface, which in principle could be used to alternate the electrical transport in self-assembled nanocrystal thin films. These small molecules can also serve for scattering the phonons to reduce the thermal conductivity. Here, we present our research on the thermoelectric characteristic of self-assembled silver telluride (Ag2Te) nanocrystal thin films that are fabricated by a layer-by-layer (LBL) dip-coating process. We perform investigations on the electrical conductivity and Seebeck coefficient on the Ag2Te nanocrystal thin films containing hydrazine, 1,2-ethanedithiol, and ethylenediamine between 300 and 400 K. We also use photothermal (PT) technique to obtain the thermal conductivity of the films at room temperature and estimate the thermoelectric figure of merit (ZT). The experimental results suggest that the surface-bound small molecules could serve as a beneficial component to build nanocrystal-based thermoelectric devices operating at low temperature.