ZrO2 SiO2 Aerogels Research Articles

A three-dimensional network modifier (dimethyldiethoxysilane) makes ZrO2-SiO2 aerogel with excellent thermal insulation performance and high-temperature stability

Dimethyl-diethoxysilane-modified ZrO2-SiO2 aerogels (DDS/ZSAs) have been prepared by using diethoxydimethylsilane as a co-precursor in the sol-gel process. Dimethyldiethoxysilane acts as a modifier for the three-dimensional structure and composition of aerogels. The pore volume increased from 1.65 cm3/g to 3.21 cm3/g, and the thermal conductivity decreased from 0.03013 Wm−1K−1 to 0.02332 Wm−1K−1 at room temperature. The number of hydroxyl groups on the aerogel surface is effectively reduced, and the particle growth caused by hydroxyl condensation was inhibited during firing. The increase in the number of Si-O-Zr bonds inhibited the growth of t-ZrO2, the phase transition temperature of the aerogel was significantly improved, and the crystallinity and crystal plane types were significantly reduced. Therefore, the porous microstructure of the modified aerogel remains good at 1000 °C. After heat treatment at 1000 °C, it still has a high specific surface area (259.6 m2/g), a high pore volume (1.51 cm3/g), and a low thermal conductivity (0.04206 Wm−1K−1). The thermal insulation performance and temperature resistance of ZrO2-SiO2 aerogels were double improved. In addition, DDS/ZSAs have high hydrophobicity and can be used in harsh wet and high-temperature environments. The research work is instructive for the design of superthermal insulation materials with high thermal stability and low thermal conductivity.

Dependence of the refractive index of transparent ZrO2–SiO2 aerogels on the density and zirconium content

In this work ZrO2–SiO2 aerogels with ZrO2 content of 3, 6 and 10 mol. % in the form of transparent blocks were synthesized using the sol-gel method. The possibility of introducing ZrO2 into a SiO2 aerogel without considerable decline of the optical characteristics of the material was shown for the first time. Aerogel samples were studied by several techniques including UV–Vis, ATR-FTIR spectroscopy and nitrogen adsorption, the refractive index was determined using beam tests (“Extracted beams” of the VEPP-4M complex). It was shown that the addition of ZrO2 to SiO2 aerogel increases the refractive index without increasing of the aerogel density. For the first time, the possibility of obtaining highly transparent ZrO2–SiO2 aerogel blocks with acceptable optical characteristics for radiator in Cherenkov detectors has been demonstrated. This result makes it possible to achieve high refractive indices in the production of aerogel blocks for Cherenkov detectors to increase their performance.

High thermal stability of SiO2–ZrO2 aerogels using solvent-thermal aging

The ZrO2–SiO2 aerogels (ZSA) with varied particle size were synthesized by solvent-thermal aging (STA) method. The effects of STA temperature (60, 110, 170 and 210 ​°C) on microstructure and thermal stability of the ZSA were investigated by SEM, TEM, FT-IR and XRD. The ZSA-210 presents the minimum shrinkage (9.3%), and it has the largest retention values of specific surface area (59.2%) and pore volume (90.8%) after 1000 ​°C heat treatment for 1 ​h. Compared with the aerogels prepared by traditional aging, the ZSA prepared by STA can maintain a low thermal conductivity (0.0285–0.0328 ​W ​m−1 ​K−1) while improving the compression strength (~1.28 ​MPa, 10% strain). STA can restrain viscous flow between ZSA particles by particles growth and skeleton coarsening; and STA can restrain the growth of t-ZrO2 by forming more Si–O–Zr bonds in the system. The work will be meaningful for the design of super thermal insulation materials with high thermal stability and low thermal conductivity.

Improvement of Thermal Stability of ZrO<sub>2</sub>-SiO<sub>2</sub> Aerogel Modified by Ca(II) Cations

Ca (II) modified ZrO2-SiO2 aerogel (CaZSA) with excellent thermal stability at 1000 °C was prepared by aging the ZrO2-SiO2 wet gel in calcium nitrate (Ca (NO3)2) ethanol solution followed with alcohol supercritical fluid drying method. The reaction between surface hydroxyl groups on the aerogel and Ca (II) ions played an important role in reducing the high temperature activity and inhibiting the particle growth caused by the condensation of hydroxyl groups of aerogel. Moreover, tiny secondary-phase particles, Ca (II) ions, introduced along grain boundaries serve as the pinning particles to inhibit the crystallization of ZrO2-SiO2 aerogel (ZSA). Thus the high-temperature stability of ZSA was significantly improved by surface modification with Ca (II). The specific surface area of CaZSA still maintained 223 m2·g-1 after 1000 °C calcination, which was 52.7% higher than that of ZSA at the same treatment condition.

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity

Novel self-reinforcing ZrO2–SiO2 aerogels with high mechanical strength and ultralow thermal conductivity are fabricated by impregnating hydrolyzed ZrO2–SiO2 sol into wet gel matrix and drying. The ZrO2–SiO2 sol fills the macropores and defects of ZrO2–SiO2 aerogel matrix generating during the gelation process, which contributes to the improvement of the mechanical properties of the ZrO2–SiO2 aerogel matrix. The mechanical and thermal properties of the as-prepared ZrO2–SiO2 aerogel are investigated and discussed. The results show that the mechanical strength of the self-reinforcing aerogels obviously increases from 0.51 to 3.11 MPa with the increase of impregnation times, while the thermal conductivity of the aerogels slightly increases from 0.0235 to 0.0306 W m−1 K−1. The novel self-reinforcing ZrO2–SiO2 aerogel could have interesting applications in aerospace and energy because of its outstanding mechanical and thermal properties.

Large-scale and ultra-low thermal conductivity of ZrO2 fibrofelt/ZrO2-SiO2 aerogels composites for thermal insulation

The large-scale fibrous/aerogels composites are prepared by using zirconia fibrofelt (ZFF) as skeleton to give high strength and ZrO2-SiO2 aerogels (ZSA) as filler to give excellent thermal insulation through vacuum impregnation. The ZFF/ZSA with a low density of 0.302 g/cm3 and a high porosity (89%) exhibits large size of 180 mm in length, 180 mm in width and 25 mm in height which is larger than other fibrous aerogels. Meanwhile, the ZFF/ZSA exhibits high compressive strength of up to 0.17 MPa which is approximately six times higher than that of ZFF (0.028 MPa). The ZFF/ZSA shows a much lower thermal conductivity of 0.0341 W m−1 K−1 at room temperature and 0.0460–0.096 m−1 K−1 during 500 °C and 1100 °C which are lower than that of conventional fibrous materials, indicating its excellent thermal insulation property in a wide temperature range, and the thermal insulation mechanism is analyzed. Thus, the large-scale, low density, high strength, and low thermal conductivity of ZFF/ZSA composites show enormous potential application in the fields of architecture, engineering pipes and aerospace for thermal insulation and protection.

A novel aerogels/porous Si3N4 ceramics composite with high strength and improved thermal insulation property

A novel ZrO2-SiO2 aerogels/porous Si3N4 ceramics composite with high strength, low density, good dielectric properties and low thermal conductivity was synthesized by filling ZrO2-SiO2 aerogels into the porous Si3N4 ceramics through vacuum sol-impregnating. The effects of aerogels on the microstructure and properties of composite were discussed. The results show that aerogels could form a mesoporous structure and significantly decrease the thermal conductivity from 9.8 to 7.3Wm−1K−1. Meanwhile, the density, mechanical and dielectric properties of the porous Si3N4 ceramics could not be affected after introducing ZrO2-SiO2 aerogels. The composite exhibits high porosity (62.6%), high flexural strength (53.86MPa) and low dielectric constant (2.86). The ZrO2-SiO2 aerogels/porous Si3N4 ceramics composite shows great potential as radome materials applied in the fields of aerospace.

An ultralight silica-modified ZrO2–SiO2 aerogel composite with ultra-low thermal conductivity and enhanced mechanical strength

A novel, ultralight, ZrO2-fiber-reinforced ZrO2–SiO2 aerogel composite with superior thermal and mechanical properties is fabricated via a rapid gelation method. Pure ZrO2-SiO2 aerogel is modified by silica to effectively strengthen its nanostructure. The results demonstrate that the fabricated fiber-reinforced aerogel composites with ZrO2-fiber contents ranging from 0 to 15wt% exhibit ultra-low densities (0.16–0.33g/cm3), low thermal conductivities (0.0235–0.0296Wm−1K−1), and high compressive strengths (0.36–0.82MPa). The present study provides an efficient method to prepare light aerogel composites with low thermal conductivity and high mechanical strength, which can be used as superior heat-insulators in aerospace applications.

Improvement of thermal stability of ZrO2–SiO2 aerogels by an inorganic–organic synergetic surface modification

The thermal stability of ZrO2–SiO2 aerogels was significantly improved by inorganic–organic synergetic surface modifications: inorganic ions [Fe(III)] surface modification and hexamethyldisilazane gas phase modification. The replacement of Hs from surface hydroxyl groups on the aerogel by Fe(III) ions and silyl groups played a critical role in isolating the hydrous particles of ZrO2–SiO2 aerogels. So the particle growth caused by the condensation of hydroxyl groups upon firing was inhibited. Meanwhile, the decomposition of the silyl groups upon heat treatment produced SiO2 particles, which could serve as pining particle to inhibit the crystallization of ZrO2. Hence, the porous microstructure of the modified aerogels was still well preserved up to 1000 °C, with a high specific surface area of 203.5 m2/g, and a considerable pore volume of 0.721 cc/g. These characteristics of the modified aerogels suggest that it has great potential on ultrahigh-temperature applications in the fields of thermal insulation, catalysis, and catalyst support, etc.

The Effect of Aging Temperature and pH Value of Aging Solution on the Microstructure of ZrO<sub>2</sub>-SiO<sub>2</sub> Aerogels

ZrO2-SiO2 gels were prepared by prehydrolysis method with sol-gel process. Then,the wet gels were aged at different temperatures (60,110,170 °C) in ethanol or alkaline solution (pH=8). Finally, the monolithic ZrO2-SiO2 aerogels were obtained by supercritical fluid drying. N2 adsorption-desorption, scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the microstructure of aerogels. The aim of this research is to study the effect of aging temperature and pH value of aging solution on the microstructure of ZrO2-SiO2 aerogels. The results show that the specific surface area and pore volume of ZrO2-SiO2 aerogels aging in alkaline solution (pH=8) is lower than that of aging in the ethanol. And there is a shift of the pore size distribution towards larger values. This is because the cross-linking reaction in alkaline solution enlarges the particle size. As the aging temperature increases, the specific surface area and pore volume decrease and the particle size distribution of ZrO2-SiO2 aerogels is more uniform.

Super-hydrophobic hexamethyl-disilazane modified ZrO2–SiO2 aerogels with excellent thermal stability

Hexamethyl-disilazane (HMDS) is introduced to replace the hydroxyl groups of ZrO2–SiO2 aerogels (ZSAs) to form inert methyl siloxy surface groups and produce SiO2 particles as the “pinning” particles in air at elevated temperatures. Thus, HMDS/ZSAs exhibit an excellent thermal stability and super-hydrophobic properties with a contact angle of 154°.

Ultra-low thermal conductivity and high strength of aerogels/fibrous ceramic composites

Novel aerogels/fibrous ceramic composite, inspired by the bird’s nest structure in nature, is synthesized using mullite fibers as matrix and ZrO2–SiO2 aerogels as filler through vacuum impregnation. Thus, the macropore of mullite fiber perform are filled by aerogels to form similar bird’s nest structure with high porosity (85%) with a large amount of mesoporous. The as-prepared aerogels/fibrous ceramic composite exhibits high compressive strength of up to 1.05MPa which is approximately two times as much as that of mullite fiber perform and ten times higher than that of pure aerogels, and the compressive failure mechanism is analyzed. Compared to conventional fibrous materials, the aerogels/fibrous ceramic composite shows a much lower thermal conductivity of 0.0524Wm−1K−1 at room temperature and 0.082–0.182Wm−1K−1 during 500°C and 1200°C indicating its excellent thermal insulation property in a wide temperature range. Therefore, this ultra-low thermal conductivity aerogels/fibrous ceramic composite with high strength is an excellent heat-insulation material applied in the fields of aerospace.

High-strength mullite fibers reinforced ZrO2–SiO2 aerogels fabricated by rapid gel method

A rapid gelation process is adopted to fabricate mullite fibers-reinforced ZrO2–SiO2 (M/ZrO2–SiO2) aerogels. The short-cut mullite fibers are introduced into ZrO2–SiO2 sol via aging and supercritical drying, and the epoxides are used as gelation accelerators. The as-prepared M/ZrO2–SiO2 aerogels have a three-dimensional reticulated porous structure similar to those of pure ZrO2–SiO2 aerogels observed by scanning electron microscopy, which indicates that the addition of fibers does not obviously affect the morphology of aerogels. It is observed that the mullite fibers disperse in the aerogels homogeneously, and fibers combine well with aerogels. M/ZrO2–SiO2 aerogel composites exhibit high compressive strengths up to 0.438 MPa, which indicates that this structure benefits the loading transfer and thus enhances their mechanical properties. Moreover, the thermal conductivity of M/ZrO2–SiO2 aerogel composites is as low as that of the pure ZrO2–SiO2 aerogels (~0.0270 W m−1 K−1).

Thermal stability of ZrO2–SiO2 aerogel modified by Fe(III) ion

ZrO2–SiO2 aerogel modified by Fe(III) ion was prepared and the stability of the samples under high temperature was investigated. The structure and properties of modified aerogels were characterized by N2 adsorption–desorption, FT-IR, XRD and TEM. The samples still contain a specific surface area about 228 m2/g after 1,000 °C 0.5 h calcinations. The inhibition of ZrO2 particle growth is attributed to the Fe(III) ion modified aerogel surface, which strongly retards the ZrO2 tetragonal phase transformation as well.

Study on the Equivalent Thermal Conductivity of Superalloy Honeycomb Core Structures

The equivalent thermal conductivity of superalloy honeycomb core structures have been studied by experimental method. Test results show that the decrease in honeycomb core height and the increase in honeycomb core diameter would lead to a lower equivalent thermal conductivity for superalloy honeycomb cores. Filling adiabatic materials such as ZrO2 fibers and SiO2 aerogels into core cells would significantly reduce the equivalent thermal conductivity of honeycomb cores with a minor penalty in structure weight.

ZrO2-SiO2 AEROGEL SUPPORTED COBALT CATALYSTS FOR THE SYNTHESIS OF LONG-CHAIN HYDROCARBON FROM SYNGAS

ABSTRACT The effect of support and preparation method on the texture, surface area, extent of reduction, H2 desorption and Fischer-Tropsch synthesis(FTS) reaction properties of ZrO2-SiO2 aerogel supported cobalt-based catalysts was investigated. The results indicate that under the conditions that favor the formation of long-chain hydrocarbons, cobalt catalyzed FTS reaction appears to be structure sensitive. Support effects significantly influence the catalytic behavior. Cobalt catalyst supported on zirconia-coated silica aerogel leads to heavy products from syngas, in which case C5+ yield could reach 150g/Nm3(CO+H2) under the optimal conditions (T=293K, P=2.0MPa, GHSV=500h−1); cobalt catalyst supported on ZrO2-SiO2 mixed aerogel, however, was shown to produce middle distillate and the yield of C5-C20 products for this catalyst is about 120g/NM3 (CO+H2)