Low cost composites for vacuum insulation core material Boyce Chang, Landi Zhong, Mufit Akinc * Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA article info Article history: Received 12 January 2016 Received in revised form 25 May 2016 Accepted 27 May 2016 Available online 28 May 2016 Keywords: Vacuum insulation Fumed silica VIP Diatomaceous earth abstract Vacuum insulation technology provides unprecedented performance and its applications have expanded, particularly in the form of vacuum insulation panels (VIP). One of the challenges of using VIP include greater cost compared to traditional insulation because fumed silica is commonly used as the core material. The demand for low cost material with comparable service life to fumed silica constantly grows. Diatomaceous earth (DE) and glass bubbles (GB) represent alternative materials that have potential for cost effective VIP applications. The pore size distribution of the DE was determined quantitatively by mercury porosimetry and nitrogen sorption. The majority of pores in DE are ranged around 1 mm. The average pore size of GB was estimated to be 50 mm via scanning electron microscopy. Subsequently, the relationship between pore size and gaseous thermal conductivity was established using the Kaganer relation. Thermal conductivity measurements were made using the transient plane source technique, which produces higher values compared to steady state methods such as guarded hot plate. Therein, fumed silica was used as a baseline for all measurements performed in this study. DE composites showed very promising results, with its thermal conductivity only 26% higher than pure fumed silica below 10 4 Pa. © 2016 Elsevier Ltd. All rights reserved. 1. Introduction The advent of vacuum insulation occurred more than a 100 years ago and it was then widely used in cryogenic containers known as Dewars, named after the founder of this technology [1]. Dewars utilized steel and glass barriers to enclose a hollow evac- uated region because of the structural strength required to with- stand pressure difference. Vacuum insulation has since made significant progress, its applications expanded to various sectors including building, transportation and appliances in the form of vacuum insulation panels (VIP). Thermal conductivity as low as 4 mW/m.K at the center of the panel have been reported, approx- imately 10 times lower compared to traditional insulation materials such as expanded polystyrene, which have thermal conductivity values ranging from 30 to 40 mW/m.K [2,3]. VIPs consist of a porous core material that withstands compression due to pressure differ- ence and a barrier laminate, which provides the vacuum seal for the underlying core material. At high vacuum, heat flow is limited to the porous solid, which entails very large interfacial thermal resistance and hence, resulting in a low thermal conductivity. The introduction of a core material permits the application of thin metallized polymer foils for barrier laminate. However, VIPs have a limited service life because vacuum is lost over a period of time as air permeates through the thin metallized polymer foil. The rate of vacuum degradation in the panel dictates its lifetime and this is especially critical for building applications, which typically require efficient insulation for an extended period of time (30e50 years). Rate of vacuum degradation can be reduced by changing the type of barrier laminate used [4], however the sensitivity of thermal con- ductivity to air intake depends on the core material. Silica aerogels are by far the best materials for vacuum insu- lation [5]. However the cost of production of aerogels are far too high and they lack the mechanical strength for VIPs applications [6]. Fumed silica (FS) instead became a popular core material used for VIPs because it is similar to silica aerogel in terms of insulating ability. Although less expensive than silica aerogel, fumed silica is still a costly material (at z$4/kg) [7]. This creates a challenge for the widespread application of VIPs in appliance and large scale structures due to its cost relative to traditional insulation materials. Thus, alternative core materials were introduced into the market. Glass fiber, for example, are now commonly used for VIPs because of its lower cost compared to fumed silica [8]. Glass fibers have low thermal conductivity (~30 mW/m.K) even at atmospheric pressure and have been reported to show slight improvement in terms of * Corresponding author. E-mail address: makinc@iastate.edu (M. Akinc). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum http://dx.doi.org/10.1016/j.vacuum.2016.05.027 0042-207X/© 2016 Elsevier Ltd. All rights reserved. Vacuum 131 (2016) 120e126