Hierarchical structural silica-fiber-woven/mullite-whisker material prepared by surface etching and gas-phase reaction

Abstract

Hierarchical structural silica-fiber-woven/mullite-whisker material was prepared by surface etching and gas-phase reaction. In the material, the silica-fiber-woven served as the substrate, and a mass of mullite whiskers grew on the silica fibers by fluorine-catalyzed gas-phase reactions (mixed AlF₃–SiO₂ powders were used as the raw material). The silica-fiber-woven was first etched by fluorine-rich gas, which was produced in the gas-phase reaction. The etching pits on the silica fibers offered nucleate sites for the mullite whiskers' growth. The volume density, tensile strength and thermal conductivity of the hierarchical structural silica-fiber-woven/mullite-whisker material were 0.572 g cm⁻³, 0.441 MPa and 0.1233 W m⁻¹ K⁻¹, respectively, indicating the suitability of the material for use as a heat-sealing/insulation gasket at high temperatures.

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Heat-sealing/insulation materials have been of significant concern in the fields of aerospace engineering, industrial production and civil use.¹ They could be used between the components in the machines, which were applied at high temperatures. In order to satisfy the usage requirements, the heat-sealing/insulation materials should have low volume density, high thermal resistance, low thermal-expansion coefficient, low thermal conductivity and proper mechanical strength.^2–4 Fibers with excellent high-temperature properties were always used for preparing the heat-sealing/insulation materials. For example, Z. G. Hou et al.² prepared mullite-fiber/silica composite fibrous frameworks by the TBA (tert-butyl alcohol)-based gel-casting method. X. Dong et al.³ also reported mullite-fiber/silica–boron-glass composite frameworks, which were prepared by the infiltration method. In addition, the fibers could be added in the aerogel to enhance the mechanical performance of the material, and some related work had been reported by J. J. Zhao et al.⁴ and J. J. Shi et al.⁵ Furthermore, for filling some narrow gaps between the high-temperature components, the preparation of thin heat-sealing/insulation gaskets became much more significant. However, the methods used for preparing the bulk heat-sealing/insulation material, which were reported in the mentioned ref. 2–5, were considered to be unaccommodating for preparing thin heat-sealing/insulation gaskets.

It is well-known that the silica fibers were usually applied in high-temperature circumstances, by possessing a high thermal-melting point, low thermal conductivity, low thermal-expansion coefficient, high thermal-shock resistance and good mechanical properties.^6,7 Silica-fiber woven, which was prepared by continuous silica fibers through various knitting methods, could be applied as high-temperature structural materials or as reinforcement in ceramic matrix composite materials (CMCs).^8–10 Thus, in our present work, the silica-fiber-woven with a thickness of ∼1 mm was used as a matrix for preparing a thin heat-sealing/insulation gasket. However, the very thin silica-fiber-woven was too soft/flexible, which made it handing difficult during its usage. For making the material more applicable, it was necessary to introduce some suitable rigid material into the silica-fiber-woven to balance the flexibility and rigidity. In addition, it would be desirable if the introduced rigid material could modify the properties of the silica-fiber-woven. It was known that mullite whiskers, as a kind of rigid material, had excellent high-temperature properties.^11–13 Therefore, we introduced the mullite whiskers into the silica-fiber-woven in our work for modifying the applicability and properties of the silica-fiber-woven.

In this work, hierarchical structural silica-fiber-woven/mullite-whisker material was prepared through surface etching and gas-phase reaction. In the preparation procedure, the silica-fiber-woven was placed in an airtight alumina crucible, and the mixed AlF₃–SiO₂ powders {[(mixed AlF₃–SiO₂ powders)/silica-fiber-woven]_mass = 1/1, (AlF₃/SiO₂)_mole = 3/1} were placed surrounding the silica-fiber-woven in the crucible. After being heat treated at 1000 °C for 2 h, the hierarchical structural silica-fiber-woven/mullite-whisker material was formed (the detailed information of the preparation process and the test methods of the hierarchical structural silica-fiber-woven/mullite-whisker material are given in Part 1 of the ESI†).

Fig. 1 shows the microstructure of the silica-fiber-woven and the samples that formed at 800–1000 °C. From Fig. 1a, it can be seen that the silica-fiber-woven was uniformly knitted, and the surface of the silica fibers was smooth. From Fig. 1b, it can be seen that a mass of etching pits formed on the silica fibers. In addition, some rod-like grains grew from the etching pits. It is shown in Fig. 1(b1) that the etching pits were silica phase (SiO₂), and according to Fig. 1(b2), the rod-like grains were determined to be topaz (Al₂SiO₄F₂). When the heating temperature was raised to 900 °C, the rod-like topaz grains grew densely on the silica fibers (Fig. 1c and c1). From Fig. 1d and d1, it can be seen that the topaz grains on the silica fibers all transformed into mullite whiskers. This microstructure-changing process could also be clearly observed in the SEM images of the cross section of the silica fibers in the samples, which were heat treated at 800–1000 °C (Fig. S1†).

Fig. 1 SEM images and EDS results: (a) silica-fiber-woven as substrate, (b) the sample formed at 800 °C, (c) the sample formed at 900 °C, (d) the hierarchical structural of silica-fiber-woven/mullite-whisker material formed at 1000 °C, (b1) EDS result of the point marked with “●” in (b), (b2) EDS result of the point marked with “▲” in (b), (c1) EDS result of the rod-like grains in (c), and (d1) EDS result of the whiskers in (d).

Fig. 2 shows the XRD patterns of the silica-fiber-woven and the hierarchical structural silica-fiber-woven/mullite-whisker material that formed at 1000 °C. A large diffraction peak of silicon oxide (SiO₂, PDF#29-0085) existed in the XRD pattern of the silica-fiber-woven (Fig. 2a). From Fig. 2b, mullite (Al₆Si₂O₁₃, PDF#15-0776) was detected in the hierarchical structural silica-fiber-woven/mullite-whisker material. This was in accordance with the results shown in Fig. 1(d1). However, even if the hierarchical structural material was prepared at 1000 °C, the crystallinity of silicon oxide in the silica-fiber-woven still remained unchanged.

Fig. 2 XRD patterns of the silica-fiber-woven (a), the hierarchical structural silica-fiber-woven/mullite-whiskers material (b), and the standard pattern of mullite phase (PDF#15-0776) (c).

Fig. 3 shows the TEM images and reciprocal lattice pattern (by fast Fourier transform) of the mullite whiskers with a diameter of about 0.1–0.8 μm (Fig. 3a), which were peeled off from the hierarchical structural material. Fig. 3b showed a good crystallinity of the mullite whiskers, and the interplanar spacing measured in the TEM image was in accordance with the standard value in the PDF card (PDF#15-0776). In addition, it was detected from the reciprocal lattice pattern (Fig. 3c) that the mullite whiskers elongated along the [001] direction. This was because of the crystal growth's intrinsic character of the mullite. It had been reported that the stable mullite's orthorhombic structure consisted of edge-shared AlO₆ octahedral chains, which aligned in the c-direction and were cross-linked by corner-shared (Si, Al)O₄ tetrahedrals. Therefore, the mullite grains might grow faster in the direction, which was parallel to the c-axis.¹⁴

Fig. 3 TEM images (a and b) of the mullite whiskers, which were peeled off from the hierarchical structural silica-fiber-woven/mullite-whisker material, and (c) the mullite lattice's reciprocal lattice pattern by fast Fourier transform (FFT).

The schematic diagram of the hierarchical structural silica-fiber-woven/mullite-whisker material is shown in Fig. 4. At lower temperatures, AlF₃ as raw-material powders reacted with O₂ to form gaseous AlOF and fluorine-rich gas.¹⁵ The fluorine-rich gas diffused in the silica-fiber-woven and etched the surface of the silica fibers (Fig. 1b) (the completely etched sample, which was composed of mullite whiskers, is shown in Part 2 of the ESI,† and it could prove the etching process of the silica-fiber-woven in the heating procedure). The etching pits could offer nucleate sites for the growth of the secondary structures.¹⁶ Moreover, the SiO₂ powders transformed into gaseous SiF₄ under the catalysis of fluorine-rich gas.¹⁵ At higher temperatures (800–900 °C), gaseous reactants (AlOF and SiF₄), which diffused among the silica fibers deposited at the etching pits and reacted to form rod-like topaz grains. As the heating temperature increased to 1000 °C, all the topaz grains were transformed into mullite whiskers; thus, the hierarchical structural of silica-fiber-woven/mullite-whiskers material was formed.

Fig. 4 Schematic diagram and structure-forming process of the hierarchical structural silica-fiber-woven/mullite-whisker material.

The volume density, tensile strength and thermal conductivity of the silica-fiber-woven and the hierarchical structural silica-fiber-woven/mullite-whisker material are listed in Table 1. The volume density of the hierarchical structural material was 0.572 g cm⁻³, which was obviously decreased compared with that of the silica-fiber-woven (0.774 g cm⁻³). This result was closely related with the surface etching of the silica fibers. However, the etching process also resulted in a lower tensile strength (0.441 MPa) compared with that of the silica-fiber-woven (9.399 MPa). In addition, the growth of the rigid mullite whiskers increased the brittle character of the fibrous woven to some extent, which also led to relatively lower tensile strength. Moreover, the thermal conductivity of the hierarchical structural material was 0.1233 W m⁻¹ K⁻¹ at room temperature, which was obviously lower compared with that of the silica-fiber-woven (0.1475 W m⁻¹ K⁻¹). This was because of the mullite whiskers, which were introduced into the silica-fiber-woven and helped to decrease the thermal conductivity of the material by lengthening the heat-transfer route in the material.¹⁷ It had been reported that the increased heat-transfer route could reduce the heat-transfer rate in the material, thereby decreasing the thermal conductivity of the material.^4,5 In addition, as the mass ratio of the raw-material powders (AlF₃ + SiO₂) increased to some extent, the mullite whiskers, which grew on the silica fibers became larger, and the volume density/thermal conductivity/tensile strength were determined to be decreased in some extent (the detailed information is provided in Part 3 of the ESI†).

Table 1 Volume density, tensile strength and thermal conductivity of the silica-fiber-woven and the hierarchical structural silica-fiber-woven/mullite-whisker material

Sample	Volume density (g cm^−3)	Tensile strength (MPa)	Thermal conductivity (W m^−1 K^−1)
Silica-fiber-woven	0.774	9.399	0.1475
Hierarchical structural silica-fiber-woven/mullite-whisker material	0.572	0.441	0.1233

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In summary, hierarchical structural silica-fiber-woven/mullite-whisker material was prepared by surface etching and gas-phase reaction in this study. Mixed AlF₃–SiO₂ powders were used as raw-material powders. The silica fibers were first etched by fluorine-rich gas that was produced in the gas-phase reaction, and a mass of etching pits formed on the silica fibers. The etching pits offered nucleate sites for the growth of the secondary structures in the gas-phase reaction. Topaz rod-like grains grew at the etching pits at 800–900 °C, and then transformed into mullite whiskers at 1000 °C. The mullite whiskers, which were introduced into the silica-fiber-woven decreased the volume density and the thermal conductivity of the material in some extent. Moreover, the tensile strength of the hierarchical structural material was tested to be 0.441 MPa, which could satisfy the usage requirements. Therefore, the hierarchical structural silica-fiber-woven/mullite-whisker material, which possessed high heat resistance, low volume density, low thermal conductivity and proper tensile strength was deemed to be suitable for use as a heat-sealing/insulation gasket at high temperatures.

Acknowledgements

The authors would like to acknowledge the National Natural Science Foundation of China (Project no. 51172156 and Project no. 51272171) for financial support.

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01196a

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