Monolithic zirconia aerogel from polyacetylacetonatozirconium precursor and ammonia hydroxide gel initiator: formation mechanism, mechanical strength and thermal properties

Abstract

Zirconia (ZrO₂) aerogels are potential candidates for use at temperatures higher than those attainable with silica aerogels. However, fabricating a robust ZrO₂ aerogel with a high thermal stability is still a challenge. The extreme electronegativity of Zr makes the hydrolysis and polycondensation of zirconium precursors difficult to control, leading to poor structural integrity and unsatisfactory physical properties. In the present research, we prepared a ZrO₂ aerogel by using a synthetic zirconium precursor, namely polyacetylacetonatozirconium (PAZ), and ammonia hydroxide as the gel initiator. The ammonia hydroxide catalyzes the cross-linking of PAZ via promotion of the dehydration between hydroxyls in PAZ and the acetylacetonate group in PAZ binds the zirconium ion firmly upon the addition of ammonia hydroxide to avoid a gel precipitate. A monolithic ZrO₂ aerogel with a large diameter size of 4.4 cm and high optical transmittance was achieved after drying. The surface area and pore volume of the as-dried ZrO₂ aerogel were as high as 630.72 m² g⁻¹ and 5.12 cm³ g⁻¹, respectively. They decreased to 188.62 m² g⁻¹ and 0.93 cm³ g⁻¹ after being heat-treated at 1000 °C for 2 h. The best mechanical performances of the ZrO₂ aerogels showed a compressive strength of 0.21 ± 0.05 MPa and a modulus of 1.9 ± 0.3 MPa with a density of 0.161 ± 0.008 g cm⁻³. Both pore structures and mechanical performances varied according to the ammonia hydroxide gel initiator used. The thermal insulating properties of the ZrO₂ aerogel performed better than a silica aerogel blanket with a thermal conductivity of 0.020 W (m⁻¹ K⁻¹).