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Mechanical Properties of Metal Oxide Aerogels Albrecht Benad, Florian Jürries, Birgit Vetter, Benjamin Klemmed, René Hübner, Christoph Leyens, and Alexander Eychmüller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03911 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 9, 2017

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Chemistry of Materials

Mechanical Properties of Metal Oxide Aerogels Albrecht Benad, Florian Jürries†, Birgit Vetter†, Benjamin Klemmed, René Hübner††, Christoph Leyens†, Alexander Eychmüller* Physical Chemistry, Technische Universität Dresden, Bergstraße 66b, 01062 Dresden, Germany † Materials Technology, Technische Universität Dresden, Helmholtzstraße 7, 01062 Dresden, Germany †† Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden ABSTRACT: In this study we report on mechanical properties of moulded, single component, Al2O3, Ga2O3, Fe2O3, ZrO2 as well as mixed aerogels, made from YSZ, YAG and ZAS. Initially all aerogels were produced equally in moulded bodies by a facile epoxy method and were annealed afterwards at 300 °C. Then we performed uniaxial pressure tests on cylindrical aerogel monoliths to gain Young’s modulus which depends on composition, density and post-treatment. Already pure aerogels like ZrO2 show well-promising Young’s modulus of 10.7 MPa, whereas most popular SiO2 materials display modulus between 2 and 3 MPa at comparable densities. Moreover we focused on Al2O3 aerogels which exhibit high stability and interesting densification behaviour depending on the annealing temperature. Based on this observation, we combined the toughness of the Al2O3 scaffold with the extraordinary hardness of ZrO2, by adding up to 20 at% Zr, to increase the specific Young’s modulus. For the mixed material with a Zr content of 20 at% we reach a record value for compressible aerogels of 125 MPa mL g-1.

orthosilcate (TEOS) or related compounds allowed a Introduction better control of the hydrolysis and condensation Aerogels are non-ordered, porous structures with high processes and contributed to obtain aerogels of high specific surface area and low density.1–3 For this reason quality.9 However, in comparison to bulk silica a lot of they are attractive for catalysis,4–7 as membranes8 or other materials show higher Young’s modulus such as γsensors.9 Nevertheless, mechanical properties like alumina (corundum) or zirconia, for which aerogels hardness, tensile modulus, and compressive strength (CS) consisting of these should be attractive alternatives. play a crucial role concerning commercial usage. Such Hence, groups, especially Poco et al. and Zu et al., work materials, which merge low density and high mechanical on alumina, zirconia and titania aerogels.10,12,29,30 In strength, can be deployed in the form of lightweight respect of specific Young’s modulus aerogels based on components as (acoustic) dampener, insulator1,10–12 or alumina with 63 MPa mL g-1 (181 mg mL-1)30 surpasses kinetic energy absorber like the cosmic STARDUST silica with 21 MPa mL g-1 (158 mg mL-1).15 Unfortunately, collector.13,14 Respectively, the potential application is producing aerogels with alkoxide precursors is expensive depending on the thermal and chemical stability of the and seems to be difficult because of their instability or aerogel. This is closely linked to the composition, insolubility in adequate solvents like water or ethanol. whereby inorganic components, especially metal oxides, Itoh et al. and Tillotson et al. established a new, facile exhibit a higher thermal stability than organic ones. In approach based on alkoxide free synthesis using halide that account we focused on inorganic aerogels. First, salts as precursor and propylene oxide as gelating agent in Fricke et al. characterised silica aerogels by static pressure 1993 and 1994.31,32 Furthermore, it was shown that mixing 15 and ultrasonic experiments in 1988. Further studies were of various halide salts lead to mixed metal oxide aerogels. performed which included a variation of the density and Later, Gash et al. and Baumann et al. recognized the high synthesis parameters of the materials.16–21 Latest research potential of the so called epoxy approach (EA) and from Wong et al. provided a comprehensive investigation produced various oxide aerogels consisting of iron of compressive behaviour regarding the density, oxide,33–35 alumina,36 zirconia as well as other materials especially at high strains.22 Furthermore, silica composites preferred with a valency up to three.34 Additionally, an were formed by incorporating fibres,23–25 crosslinking explanation of the formation mechanism was given. In agents26 or filling materials27 to improve the mechanical first step the oxygen of epoxy is protonated, followed by a properties. Generally, silica is attractive to apply, ring opening of the epoxy. The occurring negative charge attributable to well-understood and manifold approaches is compensated by the counter ion. Due to the controlled and a long history.28 The usage of new alkoxide precursors proton consumption the hydrolysis as well as the like tetramethyl orthosilicate (TMOS), tetraethyl ACS Paragon Plus Environment

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condensation proceeds well-defined and enables producing monolithic gel bodies which are necessary for mechanical experiments. Regarding that studies about aerogels using EA are comparatively rare. So far, Baumann et al. reach for annealed, pure alumina 3.2 MPa using an ultrasonic technique.36 Cao et al. work on incorporating attapulgite in an alumina network via an organic solvent sublimation drying approach to enhance compressive strength.37 However, mechanical tests on supercritically dried aerogels are still pending. On that account we focused on producing various pure aerogels consisting of alumina,36 gallia,34 zirconia34,38 and iron oxide33,35 as well as mixed ones like yttrium aluminum garnet (YAG),39 yttrium stabilised zirconia (YSZ),40,41 zinc aluminum spinel (ZAS)42 and zirconia doped alumina (AZ) aerogels with defined geometries and tested these in uniaxial pressure experiments.

Experimental Section Chemicals: All chemicals used were analytical grade or higher. AlCl3 6 H2O (Alfa Aesar, 99 %), GaCl3 (SigmaAldrich, ultra dry 99.999 %), FeCl3 6 H2O (p.a., Merck), YCl3 (ultra dry, 99.99 %, Alfa Aesar), ZnCl2 (99 %,Grüssing), ZrCl4 (Alfa Aesar, sublimed grade, 99.95 %), ethanol (EtOH, 99 %, 1 % petroleum ether, Berkel AHK), water (Milli-Q, >18.25 MΩ), (±)-propylene oxide (PrO, >99 %, Sigma-Aldrich) were used. All metal salts were diluted in EtOH to 0.5 M solutions or in case of YCl3 to a 0.25 M solution. Synthesis of moulded bodies: In order to warrant comparability between different materials, a general method was created to obtain moulded gel bodies. All syntheses were carried out via EA which was established by Itoh et al. and Tillotson et al.31,32 First, 10 mL of metal precursor were given in a beaker and 3 mL of water was added under vigorous stirring and at 0 °C. In the case of mixed metal oxide aerogels, precursors were mixed according to their molar ratio. Subsequently, 5 mL PrO were added under vigorous stirred for 10 s. In the next step 1.840 mL of the solution was casted in each cylindrical vial (PE, inner diameter 12.5 mm, Kartell) and completely gelated overnight at room temperature (RT). Furthermore, alumina moulded bodies for the annealing series were cast in larger vials (PE, inner diameter 22.5 mm, Kartell). Stock solutions of 10 mL precursor, 12 mL of EtOH and 3 mL of water were stirred at 0 °C. After adding 5 mL of PrO and stirring for 10 s, 11.4 mL of solution was given in the vials and entirely gelated overnight at room temperature. Varying the syntheses is necessary because of distinct shrinkage induced by annealing and in order to ensure a sufficient size for mechanical experiments. Finally, the solvent of the resulting solvogels was exchanged eight times with acetone and monolithic metal oxide aerogels were transferred in the autoclave. After five times flushing with liquid CO2 at 10 °C and 50 bar within 24 h, the CO2 was heated up to 37 °C and a pressure of

85 bar was applied reaching the supercritical state. Finally, the pressure was released slowly (2 h) to atmospheric pressure and the resulting aerogels were removed from the autoclave. Characterization: After supercritical drying in the autoclave (13200J0AB from Spi Supplies) samples were annealed in the furnace CSF1100 from Carbolite at different temperatures with ramp of 5 K min-1 under air and holding at that temperature for 180 min. Structure and composition analysis of all samples were performed on a scanning electron microscope (SEM) Hitachi SU8020. Specific surface areas (SSA) and total pore volume (TPV) at p/p0 = 0.95 of aerogels were measured via nitrogen physisorption at standard temperature (77 K) and pressure (1 atm) applying a Nova3000e from Quantachrome. Crystallinity and phases were determined on a powder X-ray diffractometer (XRD) Bruker Phaser 2D (Cu Kα = 1.5406 Ǻ), after the samples were dispersed in ethanol and dropped onto silicon wafers. Furthermore, differential thermal analysis (DTA) and thermogravimetry (TG) were carried out on a DSC/TGA Stare 1 from Mettler-Toledo between 25 and 1000 °C, heating rate 5 K min-1 and under air flow. For mechanical experiments, the density was calculated by mass and sample geometry which was measured on a digital microscope from Keyence type VHX-700F. Subsequently, uniaxial tests were performed on a universal testing machine from Zwick Z005 mounted with 2.5 kN force transducer and a testing rate of 1 mm min-1. Generally three samples were deployed for each test which were prepared by polishing their menisci carefully with sandpaper. Young’s moduli were determined in the linear range between 0.5 and up to 10 % strain depending on specific properties and behaviour of the materials. To characterize the chemical composition of the synthesized aerogels, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and spectrum imaging based on energydispersive X-ray spectroscopy (EDXS) were performed at 200 kV with a Talos F200X microscope equipped with an X-FEG electron source and a Super-X EDXS detector system (FEI). Prior to STEM analysis, the specimen mounted in a high-visibility low-background holder was placed for 2 s into a Model 1020 Plasma Cleaner (Fischione) to remove contamination.

Results and Discussion Annealed alumina aerogels: Alumina aerogels are well-promising materials due to the high Young’s modulus of the respective solid, particularly in respect of the corundum phase. Baumann et al. already presented alumina aerogel monoliths with high specific surface area and open-cell structure using a facile EA.36 Moreover they observed a sheet structure and a phase transition to γalumina after heat treatment at 800 °C. Both features make this material attractive for mechanical experiments. Therefore, we produced alumina moulded bodies which were untreated (RT) as well as annealed at 300, 550 and 700 °C. While the shrinkage is increased after treatment

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Chemistry of Materials at higher temperatures (see table S1), the microscopic structure seems to be almost unaltered (see figure 1a). This is in good agreement with the constant surface area of about 480 m2 g-1 (see figure 1b) after each heat treatment and reveals excellent thermal stability of the scaffold. With respect to crystallinity, the phase changed between 300 and 550 °C from (pseudo-)boehmite to a manifold mix of aluminum oxides related to an on-set of a strong endothermic signal seen in the DTA-TG measurement(see figure 1c and S2). Furthermore, the XRD peaks are broadened which indicates a nano crystalline structure (see figure 1d). Also shrinkage was kept in mind as an important factor for the material properties. Principally, the moulded body size decreased about 41 % in the synthesis vessel during solvent exchange (see table S1). As a consequence this leads to internal tension and potential cracking. Employing a stepwise exchange this side effect was slowed down and hindered. The difference between untreated and 700 °C annealed samples is merely 13 % (in total 54 %) and is closely related to sintering of the structure and to the loss of organics.

Subsequently, uniaxial pressure tests were performed, whereby the mechanical properties are strongly correlated with the annealing temperature. Generally, the first section ensues a linear elastic behaviour as described by the Hookian law enabling to gain Young’s modulus (see figure 2). The untreated sample exhibits a modulus of 4.4 MPa, and an increasing trend is given to higher annealing temperature up to 12.2 MPa for the 700 °C treatment. Considering the influence of shrinkage, crystallization and mass loss, the change of densities were regarded applying specific Young’s modulus (E/rho), the quotient from Young’s modulus and density. Initially, the specific Young’s modulus of 75 MPa m2 g-1 (RT) increases to 110 MPa mL g-1 (300 °C) induced by a mass loss of approximately 20 wt%. At higher annealing temperatures the progressing phase transition and crystallization leads to a structural consolidation of alumina aerogel up to 142 MPa mL g-1. Meaning this material is in comparison to other complete inorganic aerogels (see table S2) the hardest one with a Young’s modulus of 12.2 MPa and a density below 200 mg mL-1.

Figure 2: a) Uniaxial pressure experiments after each annealing step. Slopes rise with increasing temperature. Until 550 °C the alumina aerogels are compressible at strain above 50 %. b) Alumina aerogels plotted with Young’s moduli, density and consequently the specific Young’s moduli. All values increase with rising temperature and show a bench mark at 700 °C with 12.2 MPa, respectively, -1 142 MPa mL g .

Figure 1: a) All alumina aerogels show sheet-like morphology, here exemplary seen after heat treatment at 300 °C applying SEM. b) After annealing the specific surface area decreases 2 -1 from an original 750 to a constant value of about 480 m g . c) The temperature behaviour was observed with DTA-TG. The mass loss until 550 °C was caused by oxidizing organics and structural changes associated to endothermic DTA signals. d) XRD patterns display a phase transition from (pseudo-)boehmite to a manifold mix of aluminium oxide between 300 and 550 °C.

Additionally, the untreated samples, those annealed at 300 °C and some of those annealed at 550 °C exhibited a similar elastic behaviour which can be subdivided into three sections. First, the typical linear, elastic area at strain up to 15 % is observed. This is followed by a plateau and at high strain of a so called densification (see figure 2a). Above an annealing at 700 °C all samples are brittle and crushed at strain higher than 10 %. Nevertheless, robust samples with a densification area attain higher compressive strengths (see table S1) than the brittle ones. In view of this, the strain reached values higher than 50 % and made this material more versatile and valuable and hinders an easy breaking. Such behaviour is often observed for polymeric foams and aerogels, however, rarely for inorganic gels. Herein,



we firstly report on other pure inorganic aerogels than silica. Models for such open-cell structures are given by Gibson and Ashby.43 Describing compressive strength by its function is inaccurate because of various characteristics like composition, wide pore size distribution from small mesopores to macropores and a sheet structure accompanied by appearing and disappearing interaction between pore walls.44,45 Hence, we use the compressive strength at a strain of 50 % (CS50) to compare the differently treated materials (see table S1). For an evaluation we used only two samples per annealing step because of the sensitivity of the moulded bodies which break quite easily through unintended environmental impact. In accordance with the Young’s moduli the CS50 values also increase at higher annealing temperatures. At 300 °C the internal tension is reduced which appears through the strong shrinkage during the gelation and the sintering of the grain boundaries. The alumina aerogels become augmented against the mechanical deformation to a CS50 of 1.4 MPa. Annealing at 550 °C doubled the CS50 compared to that at RT from 1.0 to 2.1 MPa, but only three of ten samples show this densification behaviour. Obviously, a transition exists between the compressible and the brittle behaviour associated with the third strong mass loss (see figure 1c) through a phase transition from boehmite to aluminumoxide (see figure 1d). Due to the partial brittleness, we focused on the 300 °C annealed samples.

was found, that the surface sheets of alumina assembled nicely but only few layers. During the compression small mesopores were lost, but in general, the pore structure remained intact and larger areas were densified and kept accessible for nitrogen physisorption, accordingly. This is reflected in the specific surface area which is reduced from 564 to 504 m² g-1, meaning a 11 % loss with a 59 % strain (see figure 3 and S3). Single compound aerogels: Taking into account the variety of materials and potential applications of aerogels, we extended our studies on exploring other oxide aerogels than the principally used silica. Respectively, after small modification (without additional EtOH) of the first alumina synthesis, a universal synthesis for alumina, gallia, iron oxide and zirconia was developed to receive similar moulded bodies as above. This approach provides the basis of a comparison concerning chemical and physical features. Moreover, all samples were annealed at 300 °C, whereby the stability was enhanced by reducing internal tensions and sintering particle edges and tips.

For a better insight of the densification behaviour we used an Al2O3 sample after a strain of 59 %.

Figure 4: a) SEM images of sponge-like gallia, iron oxide and zirconia aerogel which b) exhibit high specific surface areas 2 -1 of above 300 m g . c) The mechanical properties differ tremendously. Brittle iron oxide and zirconia reach high Young’s moduli of 8.1 and 10.7 MPa. Respectively, the specific Young’s modulus of alumina achieves the highest value due -1 to the lowest density of 50 mg mL .

Figure 3: a) Photos of transparent moulded bodies before and after the pressure test. b) and c) Larger areas were compressed and d) only a few of the sheets were assembled on top. e) Small mesopores were compressed. Nevertheless, the pore structure remains intact with high surface area.

However, the cylindrical shape was retained in the compressed aerogel and it remained transparent. For a better comprehension, HR-SEM was performed. There it

This is also accompanied by shrinkage of the structure (see table S3), with iron oxide and zirconia experiencing a stronger shrinkage (45 and 42 %, respectively) than alumina (29 %) or gallia (23 %) owing to particle sintering. On the other hand a large mass loss was observed in the DTA-TG measurements (see figure S3) due to oxidation of organics. This opposing effect to the shrinkage finally leads to a reduction of density. Treatments at higher temperatures can cause stiffer structures, but it was already shown that some aerogels disintegrated to powder, like zirconia or iron oxide.35,41


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Chemistry of Materials Furthermore, the annealed moulded aerogels exhibit a high specific surface area of above 300 m2 g-1 which coincides well with literature. The values varied between 327 and 357 m2 g-1 which is attributed to similar sponge morphology (see figures 4a and b), while alumina displays the sheet-like morphology with a specific surface area of 709 m2 g-1. In view of crystallinity we observed a peak broadening due to the nano crystalline structure. Although the samples possess similar surface area, crystallinity and morphology, their mechanical properties are distinguished crucially from each other. Alumina aerogels reached a Young’s modulus of 2.9 MPa while brittle gallia displays the lowest modulus of 0.36 MPa which is one order of magnitude less. Notably, alumina aerogels from the first synthesis are stronger than from the uniform synthesis. Only additional EtOH is missing, but this small variation influences mechanical parameters significantly. In contrast, iron oxide and zirconia are hard and show high Young’s moduli of 8.1 MPa and 10.7 MPa, respectively, but these materials are brittle and break at strain below 6 %. Alumina aerogels possess a remarkable value of 58.1 MPa mL g-1 which benefits from its low density of 50 mg mL-1. Mixed aerogels: So far we obtained single compound aerogels, but in view of the field of potential applications it is preferred to also produce and study composites. For this reason, rather popular compositions were synthesized, like yttrium stabilized zirconia (YSZ),40,41 zinc aluminum spinel (ZAS)42 and yttrium alumina garnet (YAG)39 which were derived from single compound aerogels and are relevant in daily life as sensors,46 as supporting material in catalysis47 or in laser technology.48 Related to all former discussed materials, for this composites the mechanical tests are due, respectively, for the proximity and importance of potential applications. Certainly, materials differ completely from each other, but their aerogel properties are still similar as a result of the same EA (see figure S6). All gels gain a large surface area above 350 m²/g and show the typical sponge like morphology (see figure 5a). Furthermore, the samples show amorphous behaviour in XRD. In case of YAG and ZAS we observe an absence of the boehmite phase. Because of this we suggest a mixing of cations during the gel formation and exclude a formation of a co-network. Nevertheless, the peaks of YSZ3 (3 at% Y) and YSZ6 (6 at% Y) slightly shifted to lower 2θ angles by lattice contraction reflecting the influence of the yttrium doping (see figure S8).

Figure 5: a) According to the expectations the SSAs of mixed 2 -1 aerogels are high (above 350 m g ). b) In contrast to pure alumina, ZAS becomes brittle at high zinc concentration, but the Young’s modulus of 1.2 MPa is moderate. YAG stays compressible, but only accompanied by a large loss of toughness resulting in 0.07 MPa.

With respect to the mechanical properties an increasing content of yttrium leads to a loss of stability and to a lowering of the Young’s modulus, but YSZ stays as brittle as pure zirconia. In the case of ZAS and YAG we observe completely different behaviour than the pure alumina. ZAS gains a moderate Young’s modulus of 1.2 MPa, but becomes brittle at a strain of about 6 %. In contrast, YAG is compressible which is accompanied by softness with a Young’s modulus of 0.07 MPa. Both materials reflect extraordinary influence of intermixing cations at high concentrations. Moreover, the large potential of EA to synthesize straightforward mixed aerogels is demonstrated. Enhanced mechanical properties of alumina zirconia aerogels: Finally, we combined our experience to receive a high performance aerogel. Referring to ceramics49 and our evaluated data we choose to produce aerogels which merge the compressibility of alumina and the hardness of zirconia in one material. Hence, the low density alumina gels were doped with up to 20 at% zirconium. Interestingly, the gels seem to form a conetwork of the sheet-like alumina and the sponge-like zirconia as seen in SEM (see figure S9). This stands in a clear difference to the aforementioned mixed metal oxide aerogels which show a uniform network. Furthermore, a trend to a more sponge-like structure can be noticed with increasing zirconium content. Using HAADF-STEM imaging and spectrum imaging based on EDXS, the conetwork of AZ20 shows sheets of pure alumina and filaments of a mixed aluminum-zirconium oxide (see figure 6 and S10).



Figure 7: AZ aerogels show a) decreasing SSA and b) reflection intensities of alumina with increasing zirconium concentration. Furthermore, c) and d) a significantly increased slope of Young’s modulus and of compression strength is obtained by enhancing of the alumina scaffold through zirconia addition.

Figure 6: AZ aerogels show a co-network structure. Using HAADF-STEM-EDXS imaging combined with spectrum imaging based on EDXS, sheets of pure alumina and filaments of a mixed aluminum-zirconium oxide are observed for the Az20 aerogel.

Doping the alumina aerogel with Zr, the specific surface area was reduced from 709 m2 g-1 for pure Al2O3 to 509 m2 g-1 for AZ20 (see figure 7a). As a consequence of the Zr doping, the boehmite reflections reduced in intensity while the zirconia reflections grow in. Due to the nanometer-sized dimensions of the aerogel building blocks, all Bragg reflections show the typical peak broadening (see figure 7b). The shrinkage of moulded bodies depends also on the zirconium content, which reaches at 15 at% Zr a maximum of 40 %. At 20 at% Zr shrinkage dropped down to 34 % (see table S5). In accordance with this, the density slope obtained at 15 at% Zr a maximum value of 95 mg mL-1. At a content of 20 at% Zr a density of 87 mg mL-1 was determined which is less than the half of that of pure zirconia with 202 mg mL-1 (see figure 7d).

The Young’s moduli show continues slopes of up to 10.8 MPa due to the increasing zirconium content (see figure 7c and d). As a side effect, the aerogels become more brittle at large amounts of zirconium, which is the reason why we pursued aerogels up to 20 at% Zr only. Regarding the density and the specific Young’s moduli the growth and the enforcement of the structures work already with small amounts. Due to the drop of density at 20 at% Zr, the highest specific value was achieved with 125 MPa mL°g-1. At comparable density this AZ20 exceeds silica aerogels (see in “introduction” and table S2), by an order of magnitude. Additionally, the aerogels stay compressible and a CS50 of 3.5 MPa was obtained.

Conclusions In summary, we report on a rather large number of aerogels based on alumina, gallia, iron oxide and zirconia as well mixed ones like yttrium stabilized zirconia, zinc aluminum spinel and yttrium aluminium garnet aerogels and their characteristics. Particularly, the rarely examined mechanical properties are in the focus of our attention performing uniaxial pressure experiments. For producing the needed cylindrical moulded bodies we employ uniform, facile EA for all materials and annealed them at 300 °C to reduce the internal tension and density. All aerogels show the expected porous structure, less crystallinity and large surface areas around 350 m2 g-1. However, the mechanical properties deviate tremendously. Single compound aerogels like brittle iron oxide and zirconia attained large Young’s moduli of 8.1 and 10.7 MPa. Alumina shows a moderate value of 2.9 MPa, but reaches a remarkable specific Young’s modulus of 58 MPa mL°g-1 due to the low density of only 50 mg mL-1. Moreover, alumina shows an exceptional densification ability for all-inorganic aerogels which allows a strain up to 50 % without cracking. Aerogels remain compressible after heat treatments below 550 °C, whereas annealing at 700 °C leads to an embrittlement conditioned by phase transitions and crystallization. Though, these effects are responsible for a continuous increasing of Young’s moduli up to 12.2 MPa one of the highest. After all we combine the toughness of alumina and the hardness of zirconia to create a high performance aerogel. An increase of the Young’s module was obtained with an increase of the concentration of zirconia and reached a maximum at 10.8 MPa for 20 at% Zr. Meanwhile, the aerogels retain their compressibility which opens a wide field of applications as supporting materials, dampener, insulators and collectors.

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Chemistry of Materials This material is available free of charge via the Internet at http://pubs.acs.org. Tables contain all relevant values which were shown in the diagrams. HR-SEM images, EDXS data and DTA-TG of annealed alumina, aerogels, single compound aerogels, mixed aerogels and alumina zirconia aerogels

AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the DFG Project EY 16/14-3 and the ERC AdG AEROCAT. We also thank the Department of Inorganic Chemistry for sharing HR-SEM Hitachi SU8020 and Tamara Friedrich for performing pressure measurements. Furthermore, the use of HZDR Ion Beam Center TEM facilities and the funding of TEM Talos by the German Federal Ministry of Education of Research (BMBF), Grant No. 03SF0451 in the framework of HEMCP are acknowledged.

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