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      Novel Polyethylene Fibers of Very High Thermal Conductivity Enabled by Amorphous Restructuring

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          Abstract

          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.

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            Thermal conductivity of individual silicon nanowires

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              Polyethylene nanofibres with very high thermal conductivities.

              Bulk polymers are generally regarded as thermal insulators, and typically have thermal conductivities on the order of 0.1 W m(-1) K(-1). However, recent work suggests that individual chains of polyethylene--the simplest and most widely used polymer--can have extremely high thermal conductivity. Practical applications of these polymers may also require that the individual chains form fibres or films. Here, we report the fabrication of high-quality ultra-drawn polyethylene nanofibres with diameters of 50-500 nm and lengths up to tens of millimetres. The thermal conductivity of the nanofibres was found to be as high as approximately 104 W m(-1) K(-1), which is larger than the conductivities of about half of the pure metals. The high thermal conductivity is attributed to the restructuring of the polymer chains by stretching, which improves the fibre quality toward an 'ideal' single crystalline fibre. Such thermally conductive polymers are potentially useful as heat spreaders and could supplement conventional metallic heat-transfer materials, which are used in applications such as solar hot-water collectors, heat exchangers and electronic packaging.
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                Author and article information

                Journal
                ACS Omega
                ACS Omega
                ao
                acsodf
                ACS Omega
                American Chemical Society
                2470-1343
                26 July 2017
                31 July 2017
                : 2
                : 7
                : 3931-3944
                Affiliations
                []Department of Mechanical Engineering, Iowa State University , Ames, Iowa 50011, United States
                []Nebraska Center for Materials and Nanoscience, University of Nebraska at Lincoln , Lincoln, Nebraska 68588, United States
                [§ ]Automotive Engineering College, Shanghai University of Engineering Science , 333 Longteng Road, 201620 Shanghai, P. R. China
                Author notes
                [* ]E-mail: shxu16@ 123456sues.edu.cn . Tel: 001-86-138-164-26219 (S.X.).
                [* ]E-mail: xwang3@ 123456iastate.edu . Tel: 001-515-294-8023 (X.W.).
                Article
                10.1021/acsomega.7b00563
                6641735
                Copyright © 2017 American Chemical Society

                This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

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                ao-2017-00563y

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