Spinnablity and structure characterization of mullite fibers via sol–gel-ceramic route

https://doi.org/10.1016/j.jnoncrysol.2009.06.030Get rights and content

Abstract

Viscosity and rheology behaviors of the mullite sols prepared from aqueous solution of aluminum nitrate, aluminum isopropoxide and tetraethylorthosilicate has been investigated. Rheological measurement suggested that mullite sols exhibited good spinniabilty when the evaporation of the solvent is allowed during sol–gel process. Spinnable sols showed shear-thinning flow behavior with high viscosity to the time of gelation. By adjusting temperature, the gelation degree and viscosity of the sol could be stabilized at a certain value and the sol–gel transition could be transferred to the spinning line of a laboratory dry-spinning apparatus. Continuous mullite fibers were obtained from such sols using sol gel dry-spinning method. The final ceramic fibers had smooth surfaces with an average diameter of 50 μm. Structure evolution of mullite ceramic fibers were characterized by MAS NMR and specific surface area analyses.

Introduction

The sol–gel process is a versatile solution process for making ceramic and glass materials. Fiber formation from metal alkoxide solutions in the sol–gel process is an attractive technique for making glass and ceramic fibers [1], [2], [3], [4], [5], [6], [7]. This method provides oxide glass or ceramic fibers which are difficult to prepare by the conventional fiber formation technique such as melt spinning, wet spinning or dry-spinning. However, most reports are confined to the ‘hand drawing’ or ‘extruding’ method in laboratory. Sol–gel derived mullite fibers have been greatly improved in the past few years [8], [9], [10], [11]. It is well know that the composition of the raw alkoxide solution including the kinds of catalyst, solvent, presence or absence of any additive, and the reaction conditions which affect the rate of hydrolysis and condensation, the shape of polymers or particles prepared in the solution and the state of particle aggregation [12]. The flow properties of the sols are very important in obtaining information on the structure of the sol as well as in fiber formation.

In this paper, mullite sols were prepared from an aqueous solution of aluminum isopropoxide (AIP), aluminum nitrate (AN) and tetraethylorthosilicate (TEOS), the gel fibers could be formed successfully from spinnable sols after an appropriate concentration process before gelling. The rheological properties such as viscosity change with share rate and solid content of sols have been discussed. In addition, the rheological conditions for continuous spinnability and the different factors that control the rheological behavior of the sol were determined. Dry spinning of mullite fibers was realized based on the investigation of the flow behavior. Sintering of the precursor fibers at 1100 °C yields crack-free mullite ceramic fibers.

The microstructure of mullite gel and ceramic fibers were principally studied by using 27Al and 29Si MAS NMR spectroscopy. The densification of mullite fibers during sintering process was investigated by specific surface area analyses.

Section snippets

Sample preparation

All chemicals used in this work were chemical reagent grade (>99%). The processing steps are shown in Fig. 1. An aqueous solution was prepared from AN with deionized H2O, and then AIP and TEOS were added to this solution successively and stirred vigorously at room temperature for 20 h till AIP dissolved completely to give a clear solution. The mole ratio of [AIP]:[AN]:[TEOS]:[H2O] was 9:3:4:20. The solution was heated to 90 °C using a water bath under refluxing using a condenser to produce a sol.

Results and discussion

The rheological behavior of the mullite sol as a function of solid content was studied. In addition, the rheological conditions for continuous spinning were also determined. In order to make the experimental result closer to the real spinning formation, the rheological behavior of three mullite sols with different solid content were investigated using capillary rheometer. A variety of viscosity ranges were attained by evaporating solvent during the condensation process. Fig. 2(a–c) shows the

Conclusion

A sol–gel-ceramic route was developed to produce continuous mullite fibers. The conditions to achieve continuous spinnability and the factors that affect the rheological behavior of mullite sol were determined. Sol viscosity increased significantly with solid content and concentrating process. Good spinnablity was achieved when the sol flow behavior was slightly shear-thinning by adjusting the solid content at 52% and spinning temperature at 45 °C. The 27Al and 29Si MAS NMR spectroscopy were

Acknowledgments

The authors acknowledge the financial and technique assistance from Professor Gang Sun’s research group in University of California, Davis (CA, USA).

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