Original Article
High-strength thermal insulating mullite nanofibrous porous ceramics

https://doi.org/10.1016/j.jeurceramsoc.2020.01.011Get rights and content

Abstract

Mullite fibrous porous ceramics is one of the most commonly used high temperature insulation materials. However, how to improve the strength of the mullite fibrous porous ceramics dramatically under the premise of no sacrificing the low sample density has always been a difficult scientific problem. In this study, the strategy of using mullite nanofibers to replace the mullite micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics by the gel-casting method. Results show that mullite nanofibrous porous ceramics present a much higher compressive strength (0.837 MPa) than that of mullite micron-fibrous porous ceramics (0.515 MPa) even when the density of the mullite nanofibrous porous ceramics (0.202 g/cm3) is only around three quarters of that of the mullite micron-fibrous porous ceramics (0.266 g/cm3). The obtained materials that present the best combination of mechanical and thermal properties can be regarded as potential high-temperature thermal insulators in various thermal protection systems.

Introduction

Mullite fibrous porous ceramics is one of the most commonly used high temperature (>1300 °C) insulation materials in various thermal protection systems owing to their low density, high porosity, excellent thermal shock resistance and low thermal conductivity [[1], [2], [3], [4]]. The mullite fibrous porous ceramics are mainly composed of the interlocked micron-sized mullite fibers bound by a high-temperature binder [5]. This three-dimensional porous skeleton structure not only endows the material with the characteristics of low density and low thermal conductivity, but also ensures that the three-dimensional pores existing in the material don’t collapse at high temperatures [[6], [7], [8], [9]].

Density, compressive strength and thermal conductivity are the most important properties of the mullite fibrous porous ceramics (MFCs). Generally, the thermal conductivity of the MFCs is mainly determined by the solid-phase heat conduction and the gas-phase heat conduction due to its highly porous structure. Owing to the fact that the thermal conductivity of the air is much lower than that of the mullite fiber as the solid phase, the most effective way to reduce the thermal conductivity of the MFCs is to reduce the content of the solid phase, namely to decrease the sample density [10]. However, the decrease of density will inevitably lead to a decrease of the material strength, which in turn limits the further application of the mullite fibrous ceramics. Therefore, how to improve the strength of the mullite fibrous porous ceramics under the premise of no sacrificing the low sample density has always been a difficult scientific problem. Many researchers have tried to change the microstructure of the MFCs, including the distribution of the binder and the arrangement of the mullite fiber, in order to increase the sample strength [11,12]. However, results show that these methods only increase the sample strength in a limited range because they did not change the fundamental unite (mullite fibers) of the mullite fibrous ceramics.

The fracture of mullite fibrous porous ceramics is mainly caused by the fracture of the mullite fiber or the binder [13]. Therefore, one effective method to increase the sample strength is increasing the strength of mullite fibers and binders. Based on the above-mentioned, it can be assumed that the properties of MFCs can be greatly improved if the size of the mullite fiber as the matrix of MFCs reduces to the nanoscale. Firstly, with the reduction of the mullite fiber diameter from the micron scale to the nanometer scale, the strength of the fibers will substantially increase due to the decrease of the internal defects existing in mullite fibers [14]. Secondly, the size of the pores lapped by the fibers would reduce dramatically due to the decrease of fiber diameter, bringing about the increase of the pore wall content and the bonding points, finally resulting in the increase of the sample strength.

The forming methods of mullite micron-fibrous porous ceramics include vacuum filtration forming method and pressure assisted molding method, and both methods will exert a longitudinal pressure on the materials [[15], [16], [17]]. However, as the fiber diameter reduced to the nanometer scale, the longitudinal pressure would destroy of the 3D skeleton structure during the forming process, and consequently lead to the increase of the sample density. Therefore, the best forming method to fabricate the mullite nanofibrous porous ceramics should be an in-situ forming method to prevent the structural collapse. Freeze casting which could obtain samples with small linear shrinkage and high porosity was proposed to prepare nanofibrous porous ceramics by many researchers [18,19]. However, the mullite nanofibrous porous ceramics prepared by the freeze casting usually showed a multilevel pore structure with small pores formed by the overlapped nanofibers and large pores caused by the sublimation of the ice crystal, which was not beneficial to the sample strength [20]. Different from the freeze casting method, the gel casting method as another quite common in-situ forming method is able to fabricate samples with uniform microstructure and high strength [21,22]. However, the water-based gel casting often leads to the break of samples with ultralow solid content during the drying process. Considering that the tertbutyl alcohol (TBA) has higher saturated vapor pressure and lower surface tension relative to water, and can volatilize rapidly at 50 °C, the TBA-based gel-casting is a promising method for the fabrication of mullite nanofibrous porous ceramics [23,24].

In this work, in order to further increase the strength of the mullite fibrous ceramics, the strategy of using mullite nanofibers to replace the mullite micron-fibers was propose to fabricate the mullite nanofibrous porous ceramics. The fabrication process of mullite nanofibrous porous ceramics was analyzed. Moreover, the effects of the mullite fiber content and the fiber aspect ratio on the sample microstructures and properties were investigated.

Section snippets

Materials

The precursor of mullite nanofibers was prepared using polyhydromethylsiloxane (Alfa Aesar Chemistry Co., Ltd., China) as the silicon source, aluminum-tri-sec-butoxide (Sigma-Aldrich Trading Co., Ltd., China) as the aluminum source, polyvinylpyrrolidone (PVP-K90, Mw = 1300000, Sigma-Aldrich Trading Co., Ltd., China) as the spinning aid, respectively. The silica sol which was used as the high temperature binder for 3D skeleton structure was prepared with tetraethyl orthosilicate (TEOS),

Fabrication of mullite nanofibrous porous ceramics

It is a challenge to improve the strength of the MFCs dramatically without sacrificing its low sample density. In this paper, the strategy of using mullite nanofibers to replace the micron-fibers was proposed to fabricate the mullite nanofibrous porous ceramics with low density and high strength. The fabrication flow chart of the mullite nanofibrous porous ceramics is shown in Fig. 2a. The matrix in MNFCs was mullite nanofibers with the alumina/silica molar ratio of 3:1 prepared by the

Conclusions

In summary, mullite nanofibrous porous ceramics were successfully prepared by the TBA-based gel casting, and the effects of fiber content and fiber aspect ratio on the microstructure and properties of the samples were studied. The strength of the MNFCs increased from 0.114 MPa to 0.158 MPa with the solid content of long nanofibers increasing from 1.5 wt% to 2.5 wt%. In addition, compared with the S-MNFCs, the L-MNFCs showed an ultralow density (0.061 g/cm3) and low thermal conductivity (0.0597

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is supported by the National Natural Science Foundation of China (Project No. 51502196 and No. 51872194).

References (25)

Cited by (58)

View all citing articles on Scopus
View full text