Mechanical Properties of Metal Oxide Aerogels - Chemistry of

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Cite This: Chem. Mater. 2018, 30, 145−152

Mechanical Properties of Metal Oxide Aerogels Albrecht Benad,† Florian Jürries,‡ Birgit Vetter,‡ Benjamin Klemmed,† René Hübner,§ Christoph Leyens,‡ and 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, Germany ‡

S Supporting Information *

ABSTRACT: In this study we report on mechanical properties of molded, single component Al2O3, Ga2O3, Fe2O3, and ZrO2 as well as mixed aerogels, made from yttrium stabilized zirconia, yttrium aluminum garnet, and zinc aluminum spinel. Initially all aerogels were produced equally in molded bodies by a facile epoxy method and were annealed afterward at 300 °C. Then we performed uniaxial pressure tests on cylindrical aerogel monoliths to gain Young’s modulus which depends on composition, density, and posttreatment. Already pure aerogels like ZrO2 show wellpromising Young’s modulus of 10.7 MPa, whereas most popular SiO2 materials display a modulus between 2 and 3 MPa at comparable densities. Moreover we focused on Al2O3 aerogels which exhibit high stability and interesting densification behavior depending on the annealing temperature. On the basis of this observation, we combined the toughness of the Al2O3 scaffold with the extraordinary hardness of ZrO2, by adding up to 20 atom % Zr, to increase the specific Young’s modulus. For the mixed material with a Zr content of 20 atom %, we reach a record value for compressible aerogels of 125 MPa mL g−1.

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long history.28 The usage of new alkoxide precursors like tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), or related compounds allowed a better control of the hydrolysis and condensation processes and contributed to obtaining aerogels of high quality.9 However, in comparison to bulk silica a lot of other materials show higher Young’s modulus such as γ-alumina (corundum) or zirconia, for which aerogels consisting of these should be attractive alternatives. Hence, groups, especially Poco et al. and Zu et al., work on alumina, zirconia, and titania aerogels.10,12,29,30 With respect to specific Young’s modulus aerogels based on alumina with 63 MPa mL g−1 (181 mg mL−1),30 they surpass silica with 21 MPa mL g−1 (158 mg mL−1).15 Unfortunately, producing aerogels with alkoxide precursors is expensive and seems to be difficult because of their instability or insolubility in adequate solvents like water or ethanol. Itoh et al. and Tillotson et al. established a new, facile approach based on alkoxide free synthesis using halide salts as precursor and propylene oxide as gelating agent in 1993 and 1994.31,32 Furthermore, it was shown that mixing of various halide salts leads to mixed metal oxide aerogels. Later, Gash et al. and Baumann et al. recognized the high potential of the so-called epoxy approach (EA) and produced

INTRODUCTION Aerogels are nonordered, porous structures with high specific surface area and low density.1−3 For this reason they are attractive for catalysis4−7 or as membranes8 or sensors.9 Nevertheless, mechanical properties like hardness, tensile modulus, and compressive strength (CS) play a crucial role concerning commercial usage. Such materials, which merge low density and high mechanical strength, can be deployed in the form of lightweight components as (acoustic) dampener, insulator1,10−12 or kinetic energy absorber like the cosmic STARDUST collector.13,14 Respectively, the potential application depends on the thermal and chemical stability of the aerogel. This is closely linked to the composition, whereby inorganic components, especially metal oxides, exhibit a higher thermal stability than organic ones. In that account we focused on inorganic aerogels. First, Fricke et al. characterized silica aerogels by static pressure and ultrasonic experiments in 1988.15 Further studies were performed which included a variation of the density and synthesis parameters of the materials.16−21 The latest research from Wong et al. provided a comprehensive investigation of compressive behavior regarding the density, especially at high strains.22 Furthermore, silica composites were formed by incorporating fibers,23−25 crosslinking agents,26 or filling materials27 to improve the mechanical properties. Generally, silica is attractive to apply, attributable to well-understood and manifold approaches and a © 2017 American Chemical Society

Received: September 15, 2017 Revised: December 7, 2017 Published: December 8, 2017 145

DOI: 10.1021/acs.chemmater.7b03911 Chem. Mater. 2018, 30, 145−152

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Chemistry of Materials

Figure 1. (a) All alumina aerogels show sheet-like morphology, here exemplarily seen after heat treatment at 300 °C applying SEM. (b) After annealing, the specific surface area decreases from an original 750 to a constant value of about 480 m2 g−1. (c) The temperature behavior was observed with DTA-TG. The mass loss until 550 °C was caused by oxidizing organics and structural changes associated with endothermic DTA signals. (d) XRD patterns display a phase transition from (pseudo)boehmite to a manifold mix of aluminum oxide between 300 and 550 °C.

various oxide aerogels consisting of iron oxide,33−35 alumina,36 and zirconia as well as other materials preferred with a valency up to three.34 Additionally, an explanation of the formation mechanism was given. In the first step the oxygen of the epoxy is protonated, followed by a ring opening of the epoxy. The occurring negative charge is compensated by the counterion. Due to the controlled proton consumption, the hydrolysis as well as the 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. reached for annealed, pure alumina 3.2 MPa using an ultrasonic technique.36 Cao et al. worked 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 zirconia,34,38 and iron oxide33,35 as well as mixed ones like yttrium aluminum garnet (YAG),39 yttrium stabilized 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.

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Aesar), ZnCl2 (99%, Grüssing), ZrCl4 (sublimed grade, 99.95%, Alfa Aesar), ethanol (EtOH, 99%, 1% petroleum ether, Berkel AHK), water (Milli-Q, >18.25 MΩ), and (±)-propylene oxide (PrO, > 99%, Sigma-Aldrich) were used. All metal salts were diluted in EtOH to 0.5 M solutions or in the case of YCl3 to a 0.25 M solution. Synthesis of Molded Bodies. To warrant comparability between different materials, a general method was created to obtain molded 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 was placed in a beaker, and 3 mL of water was added under vigorous stirring at 0 °C. In the case of mixed metal oxide aerogels, precursors were mixed according to their molar ratio. Subsequently, 5 mL of PrO was added under vigorous stirring 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 molded bodies for the annealing series were cast in larger vials (PE, inner diameter 22.5 mm, Kartell). Stock solutions of 10 mL of 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 placed 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 flushing five times with liquid CO2 at 10 °C and 50 bar within 24 h, the CO2 was heated to 37 °C, and a pressure of 85 bar was applied, reaching the supercritical state. Finally,

EXPERIMENTAL SECTION

Chemicals. All chemicals used were analytical grade or higher. AlCl3·6H2O (99%, Alfa Aesar), GaCl3 (ultradry 99.999%, SigmaAldrich), FeCl3·6 H2O (p.a., Merck), YCl3 (ultra dry, 99.99%, Alfa 146


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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, 142 MPa mL g−1. 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 was 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 Å), 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 behavior of the materials. To characterize the chemical composition of the synthesized aerogels, high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) imaging and spectrum imaging based on energy-dispersive 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.

for mechanical experiments. Therefore, we produced alumina molded bodies which were untreated (RT) as well as annealed at 300, 550, and 700 °C. While the shrinkage is increased after treatment 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 onset of a strong endothermic signal seen in the DTA-TG measurement (see Figures 1c and S2). Furthermore, the XRD peaks are broadened, which indicates a nanocrystalline structure (see Figure 1d). Also shrinkage was kept in mind as an important factor for the material properties. Principally, the molded 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 behavior 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 was regarded when applying a 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 that this material compares to other complete inorganic aerogels (see Table S2), the hardest

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RESULTS AND DISCUSSION Annealed Alumina Aerogels. Alumina aerogels are promising materials due to the high Young’s modulus of the respective solid, particularly with respect to 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 γ-alumina after heat treatment at 800 °C. Both features make this material attractive 147


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Chemistry of Materials one with a Young’s modulus of 12.2 MPa and a density below 200 mg mL−1. Additionally, the untreated samples, those annealed at 300 °C and some of those annealed at 550 °C, exhibited a similar elastic behavior 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%, made this material more versatile and valuable, and hinders an easy breaking. Such behavior is often observed for polymeric foams and aerogels but rarely for inorganic gels. Herein, we first report on pure inorganic aerogels other 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 molded 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 3 of 10 samples show this densification behavior. Obviously, a transition exists between the compressible and the brittle behavior 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. For a better insight of the densification behavior we used an Al2O3 sample after a strain of 59%. However, the cylindrical shape was retained in the compressed aerogel, and it remained transparent. For a better comprehension, HR-SEM was performed. There it was found that the surface sheets of alumina assembled nicely but only in a 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 an 11% loss with a 59% strain (see Figures 3 and S3). Single Compound Aerogels. Taking into account the variety of materials and potential applications of aerogels, we extended our studies on exploring oxide aerogels other 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 molded bodies as above. This approach provides the basis of a comparison concerning chemical and physical features. Moreover, all samples were

Figure 3. (a) Photos of transparent molded 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.

annealed at 300 °C, whereby the stability was enhanced by reducing internal tensions and sintering particle edges and tips. 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 Furthermore, the annealed molded 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 Figure 4a,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 nanocrystalline 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 1 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 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 148


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Figure 4. (a) SEM images of sponge-like gallia, iron oxide, and zirconia aerogel which (b) exhibit high specific surface areas of above 300 m2 g−1. (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 to the lowest density of 50 mg mL−1.

Figure 5. (a) According to the expectations the SSAs of mixed aerogels are high (above 350 m2 g−1). (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.

YSZ6 (6 atom % Y) slightly shifted to lower 2θ angles by lattice contraction reflecting the influence of the yttrium doping (see Figure S8). 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 behavior 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

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 catalysis,47 or in laser technology.48 Related to all former discussed materials, for these 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 behavior in XRD. In the cases 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 conetwork. Nevertheless, the peaks of YSZ3 (3 atom % Y) and 149


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Chemistry of Materials 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 atom % 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 Figures 6 and S10).

Particularly, the rarely examined mechanical properties are in the focus of our attention performing uniaxial pressure experiments. For producing the needed cylindrical molded 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 atom % Zr. Meanwhile, the aerogels retain their compressibility, which opens a wide field of applications as supporting materials, dampeners, insulators, and collectors.

Figure 6. AZ aerogels show a conetwork structure. Using HAADFSTEM-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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03911. Tables containing all relevant values that were shown in the diagrams, HR-SEM images, EDXS data, and DTATG of annealed alumina, aerogels, single compound aerogels, mixed aerogels, and alumina zirconia aerogels (PDF)

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 molded bodies depends also on the zirconium content, which reaches at 15 atom % Zr a maximum of 40%. At 20 atom % Zr shrinkage dropped down to 34% (see Table S5). In accordance with this, the density slope obtained at 15 atom % Zr a maximum value of 95 mg mL−1. At a content of 20 atom % Zr a density of 87 mg mL−1 was determined which is less than half of that of the 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,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 atom % 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 atom % 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.

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AUTHOR INFORMATION

Corresponding Author

*(A.E.) E-mail: [email protected]. ORCID

Alexander Eychmüller: 0000-0001-9926-6279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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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 aluminum garnet aerogels and their characteristics.

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