Glass-Oxide Nanocomposites as Effective Thermal Insulation Materials
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Glass-Oxide Nanocomposites as Effective Thermal Insulation Materials Qing Hao1, Minqing Li1, Garrett Joseph Coleman2, Qiang Li1, Pierre Lucas2 1 Aerospace & Mechanical Engineering, University of Arizona, 1130 N Mountain Ave, Tucson, AZ 85721, U.S.A. 2 Materials Science & Engineering, University of Arizona, 1235 E. James E. Rogers Way, Tucson, AZ 85721, U.S.A. ABSTRACT With extremely disordered atomic structures, a glass possesses a thermal conductivity k that approaches the theoretical minimum of its composition, known as the Einstein’s limit.1 Depending on the material composition and the extent of disorder, the thermal conductivity of some glasses can be down to 0.1-0.3 W/m∙K at room temperature,2,3 representing some of the lowest k values among existing solids. Such a low k can be further reduced by the interfacial phonon scattering within a nanocomposite that can be used for thermal insulation applications. In this work, nanocomposites hot pressed from the mixture of glass nanopowder (GeSe4 or Ge20Te70Se10) and commercial SiO2 nanoparticles, or pure glass nanopowder, are investigated for the potential k reduction. It is found that adding SiO2 nanoparticles will instead increase k if the measured k values for usually porous nanocomposites are converted into those for the corresponding solid (kSolid) with Eucken’s formula. In contrast, pure glass nano-samples always show kSolid data significantly reduced from that for the starting glass. For a pure GeSe4 nanosample, kSolid would beat the Einstein’s limit for its composition. INTRODUCTION To help solve the emerging energy challenges, developing technologies for effective energy conservation and storage can be as important as searching for renewable energy resources. In this aspect, thermal insulation materials (TIMs) are critical to the large-scale thermal energy conservation of buildings and thermal storage systems for later usage. To better serve many existing or emerging applications, next-generation TIMs are not only required to have a thermal conductivity k as low as possible but also need to maintain such a low k over a long period of time and under perforation by external objects. These requirements cannot be easily satisfied by conventional TIMs based on a highly porous structure (e.g. polyurethane foam, mineral wool, expanded polystyrene, and extruded polystyrene), for which the mechanical strength is sacrificed and durability becomes paramount in harsh environments. Instead of pushing the porosity to extremes, there has been a continuous effort on effectively reducing the thermal conductivity of fully dense materials. In a typical dielectric material for TIMs, heat is carried out by phonons and the thermal conductivity (with lattice contribution k L only) is given by k L = Cv / 3 ,4 in which C, v, and represent the volumetric specific heat of phonons, averaged phonon group velocity, and averaged phonon mean free path (MFP), respectively. In theory, the lowest possible k L value of a solid can be achieved when decreases to half of the phonon wavelengt
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