【A10】A facile method to fabricate monolithic alumina–silica aerogels with high surface aeras and good mechanical property .pdf

Contents lists available at ScienceDirectJournal of the European Ceramic Societyjournal homepage: articleA facile method to fabricate monolithic aluminasilica aerogels with highsurface areas and good mechanical propertiesFei Peng, Yonggang Jiang*, Junzong Feng, Liangjun Li, Huafei Cai, Jian Feng*Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Technology, National University of Defense Technology,Changsha 410073, PR ChinaA R T I C L E I N F OKeywords:Aluminasilica aerogelSolvent exchangeHigh temperatureSinteringMechanical propertiesA B S T R A C TIn this study, monolithic aluminasilica aerogels with high surface areas and good mechanical properties weresynthesized via a facile solgel method without solvent exchange. Furthermore, surface areas, microstructures(up to 1300 C), and mechanical properties of the prepared aluminasilica aerogels were investigated. Thesintering and phase transformations of metastable alumina nanoparticles are suppressed owing to the uniformlydistributed Si in the aluminasilica aerogels; therefore, the aluminasilica aerogels can maintain much higherspecific surface areas after being calcined at 800 C (575.5 m2/g), 1000 C (443.2 m2/g), and 1200 C (120.6 m2/g) compared to pristine alumina aerogels. In addition, the prepared high surface area aluminasilica aerogelsshow considerably higher strengths than those obtained in previous works. The compressive stress (3 % strain)and elastic modulus of the aluminasilica aerogels reached 1.78 and 65.6 MPa, respectively. The reported alu-minasilica aerogels in this study can be good candidates as high-temperature thermal insulators and catalysts.1. IntroductionAlumina aerogels have been considered as promising thermal in-sulators and catalyst supports at high temperatures (up to 1000 C)owing to their high heat resistance 110, low thermal conductivity1,3,8, and high catalytic activity. Monolithic alumina aerogels withhigh porosity and specific surface areas (SSAs) can be prepared usingaluminum alkoxides, inorganic aluminum salts, or boehmites via thesolgelandsupercriticalfluiddrying(SCFD)technologies1,2,5,6,915.AluminaaerogelsexperienceadistinctlossinSSA57,911,1418 at temperatures above 1000C due to sintering andphase transformations of the alumina nanoparticles. Dopants such as Si,La, Y, Ba, Zr, and P have been introduced to suppress the sintering andphase transition, thereby improving the thermal stability of aluminaaerogels 7,1927. In comparison with pristine aerogels, aluminaaerogels doped with Si show a higher SSA maintenance even after beingheated at 1100 C 1300 C 7,15,1822,2832, probably owing to thephase transformation-inhibiting effect of the Si distributed in the alu-mina lattices 19,32,33. For high-temperature thermal insulators orcatalysts, larger SSAs often mean a lower thermal conductivity orhigher catalytic activity at elevated temperatures. SSAs of a few alu-minasilica aerogels reported are below 400 and 100m2/g at 1000 Cand 1200C 7,21,29,32, respectively, which can be further improved.During the preparation of aluminasilica aerogels, it is necessary toremove the water and byproducts from the pores of the gels for main-taining the porosity and monolithic characteristics of the aerogels;therefore, solvent exchange is required for several days; the most fre-quently used solvents include ethanol 19,20,22, methanol 29,34,and isopropanol 7,30. This repeated and lengthy solvent exchangeprocess is complex, costly, and involves risks of cracking of the gels.Therefore, a method without solvent exchange is preferred. In addition,although monolithic aluminasilica aerogels could be prepared, theyare found difficult to handle and use, mainly owing to their fragility andpoor strength. Zu et al. 11 reported a core-shell structured silica-modified alumina aerogel with elastic modulus of 6.7 MPa via an alk-oxide chemical liquid deposition technology, which involved complexethanol solvent exchange and colloid solution soaking. He et al. ob-tained an aluminasilica aerogel with compressive modulus of 9.3MPavia ice-template freeze drying and vacuum impregnation process 35.However, the SSA (231.5 m2/g) of the aerogel is relatively low.Therefore, synthesizing strong aluminasilica aerogels with high sur-face area is challenging.This study aims to determine a simple method to fabricate mono-lithic aluminasilica aerogels with high surface areas and good me-chanical strengths. The alumina and silica precursors were simplymixed to form the aluminasilica sols and gels. The gels were aged for afew days without solvent exchange and dried in the supercriticalhttps:/doi.org/10.1016/j.jeurceramsoc.2020.01.058Received 27 November 2019; Received in revised form 21 January 2020; Accepted 22 January 2020Corresponding authors.E-mail addresses: (Y. Jiang), (J. Feng).Journal of the European Ceramic Society 40 (2020) 24802488Available online 23 January 20200955-2219/ 2020 Elsevier Ltd. All rights reserved.Tetha