Shan Liu,
Jiachen Liu,
Haiyan Du,
Feng Hou*,
Sue Ren and
Haitao Geng
Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, School of Materials Science, Engineering of Tianjin University, Tianjin 300072, China. E-mail: houf@tju.edu.cn; Fax: +86-22-27402102
First published on 24th January 2014
Mullite fibres/whiskers frameworks (MF/MW frameworks) were prepared by mixed-slurry-filtration and heat-treating. Hierarchical fibres/whiskers structure formed in the frameworks through fluorine-catalyzed gas-phase reaction. Topaz rods as a transient product first formed in the frameworks, and then gradually transformed into mullite whiskers. Intersected mullite whiskers served as lap-jointing points of the fibres. The volume density, apparent porosity and thermal conductivity of the samples heat-treated at 1100–1500 °C were 0.402–0.541 g cm−3, 81.2–85% and 0.1191–0.1647 W m−1 k−1, respectively. Cyclic compression–resilience performance (maximum stress = 0.4 MPa) of the samples was tested at room temperature. The compression ratio, permanent deformation ratio and resilience ratio of the samples in the first circle compression–resilience test were 2.031–3.833%, 0.125–0.042% and 93.85–98.28%, respectively. The excellent thermal/mechanical properties indicated that the MF/MW frameworks were suitable for being applied as high temperature heat-insulation/sealing materials.
Mullite had been extensively used in high-temperature field, due to its high mechanical strength, low thermal conductivity, high temperature stability, good thermal shock resistance and creep resistance, etc.3–7 Short mullite fibres, which possessed all the properties of mullite, were always introduced in the ceramics as toughening phase,8 or to form frameworks as the main structure of the materials.9,10 Short mullite fibres' frameworks used for heat-insulation/sealing had been reported in some literatures. X. Dong et al.11 reported a kind of bird's nest-structural fibrous mullite ceramic which was prepared through infiltration process. In their samples, the cross-linked mullite fibres were bonded by silica–boron binder. Besides, Z.G. Hou et al.2 had prepared analogous fibrous mullite ceramic through TBA (tert-butyl alcohol) based gel-casting method, and the fibres were bonded by SiO2-sintered compact in their material. However, for these two kinds of materials, the binders in the ceramics had disadvantageous effects on keeping the heat-resistance of the whole composite material. Moreover, the lap-jointing points of the fibres were immovable “soldered dots” which would produce inner stress in thermal/mechanical cyclic circumstance, and this would result in a relatively low thermal shock resistance and low fatigue resistance. Therefore, modifying the lap-jointing points of the fibres was extremely necessary for improving the thermal/mechanical properties of these fibrous ceramics and made them more usable and durable at high temperature.
In our former work,12 mullite fibres/whiskers frameworks (MF/MW frameworks) applied for heat-insulation/sealing had been prepared through mixed-slurry filtration and heat-treating method. In the frameworks, hierarchical fibres/whiskers structure formed through fluorine-catalyzed gas-phase reaction. Lap-jointing points of the fibres were served by intersected mullite whiskers. These unfixed lap-jointing points were benefit for obtaining good thermal/mechanical properties. In this paper, we focused on the details of the phase transition and structure formation of the MF/MW frameworks. Besides, the heat-insulation and cyclic compression–resilience properties of the MF/MW frameworks formed at different temperatures were also carefully discussed.
According to Fig. 2, phase transition showed a clearly gas-phase reaction process in the samples. The complex reaction equations of the whole process were inferred as follow, which was carefully analyzed according to the XRD patterns in our work (Fig. 2) and some other related literatures.14–18
AlF3 + H2O → AlOF + 2HF | (1) |
SiO2 + 4HF → SiF4 + 2H2O | (2) |
2AlOF + SiF4 + (4 − x)H2O → Al2SiO4Fx(OH)2−x(topaz, x = 0 ∼ 1) + (6 − x)HF | (3) |
3Al2SiO4Fx(OH)2−x + (4 − 3x)HF → Al6Si2O13 + SiF4 + (5 − 3x)H2O | (4) |
At the first stage (∼800 °C), all of the AlF3 reacted with H2O and produced gaseous AlOF14 and catalysis atmosphere HF15,16 [reaction (1)]. A part of SiO2 transformed into gaseous SiF4 under the catalysis of HF14 [reaction (2)]. The two gaseous products, AlOF and SiF4, rapidly reached their saturated vapour pressures in the airtight crucible and then reacted to form transient phase topaz17 [reaction (3)]. Plentiful of HF gas was also released in this stage. Then, topaz transformed into mullite at higher temperature (900–1000 °C)18 [reaction (4)]. In the whole reaction process, the equilibrium of reaction (2) gradually shifted to the positive direction due to the continuous consumption of the SiF4. At 1100–1400 °C, the generation rate of topaz [reaction (3)] was almost equal to its transformation rate [reaction (4)]. In other words, once the topaz formed, it would swiftly transformed into mullite. Hence, the topaz phase could not be detected in Fig. 2i–j. The gas-phase reaction was almost finished at 1500 °C.
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Fig. 3 SEM images of the green body (a) and the MF/MW frameworks that were heat-treated at 800 °C (b), 900 °C (c) and 1000 °C (d). |
Fig. 4 showed the microstructure of the MF/MW frameworks that formed at 1100–1500 °C, and the TEM images of the mullite whiskers were showed in Fig. S1.† At 1100 °C, the framework structure became more clear. The mullite whiskers in the framework had relatively small size, and the diameter of them was about 0.15–0.25 μm (Fig. S1a†). As the heating temperature rising (1100–1400 °C, Fig. 4a–d), the aspect ratio of the mullite whiskers enlarged obviously, and the frameworks were observed to be more denser. When the heating temperature was 1400 °C, the mullite whiskers' aspect ratio increased significantly, and the diameter of them was approximately 0.1–0.3 μm (Fig. S1b†). However, for the MF/MW framework forming at 1500 °C (Fig. 4e), the aspect ratio of the mullite whiskers became decreased obviously and the whiskers' diameter was about 0.25–0.4 μm (Fig. S1c†). The decreased aspect ratio indicating coarsening process of the mullite whiskers. It had been reported that, at a higher heating temperature, the growth rate along every growth orientation tended to be the same, and this ultimately led to coarsening of the mullite whiskers.19 Moreover, in the frameworks, intersected mullite whiskers served as lap-jointing points of the mullite fibres, which was showed in Fig. 4f as example.
In the MF/MW frameworks, the mullite fibres served as curved substrates for the mullite whiskers' growth, and the fibres' surfaces that were full of small mullite grains acted as the seeding layers.12 The schematic diagram of MF/MW framework's forming procedure was showed in Fig. 5. Firstly, AlF3 and SiO2 as raw material transformed into gaseous AlOF and SiF4, and these two products deposited on the fibres' surfaces and reacted to form topaz rod-like grains. Meanwhile, a part of topaz in-situ formed at the pores of the framework. Then the topaz which grew on the fibres transformed into mullite whiskers, and also the in situ topaz grains transformed into in situ mullite whiskers at the pores of the framework. As the heating temperature rising and under the catalysis of HF, the in situ mullite whiskers decomposed into AlOF and SiF4 again [reaction (5)], and these two products subsequently deposited and reacted on the mullite whiskers which had grown on the fibres, according to the evaporation-condensation mechanism. Therefore, the hierarchical mullite fibres/whiskers structure was formed. At the lap-jointing points of the fibres, mullite whiskers intersected with each other to keep the whole framework stable.
Al6Si2O13 + 14HF → 6AlOF4 + 7H2O | (5) |
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Fig. 6 Volume density (triangle symbol), apparent porosity (circle symbol) and thermal conductivity (square symbol) curves of the MF/MW frameworks that formed at 1100–1500 °C. |
According to Fig. 7, the compression ratio, permanent deformation ratio and resilience ratio of the 1100 °C-sample' first circle compression–resilience test were 2.031%, 0.125% and 93.85%, separately. The mullite whiskers had relatively small scale (Fig. 4a), which made the whiskers' deeper intersection and sliding more easily to be realized, and the temporary deformation forming in these two stages was smaller. Besides, the weaker whiskers were damaged in the compression process, and this resulted in higher permanent deformation ratio and lower resilience ratio. For the 1200 °C-sample, the compression ratio, permanent deformation ratio and resilience ratio of the sample's first circle compression–resilience test were 2.188%, 0.120% and 94.5%, respectively (Fig. 7). In the 1200 °C-sample, the mullite whiskers had larger scale (Fig. 4b). In the compression process, deeper intersection of the whiskers could result in a larger deformation, which directly led to increased compression ratio. Moreover, the lower permanent deformation ratio and higher resilience ratio of the 1200 °C-sample was on account of the increased strength of the intersected mullite whiskers. For the 1300 °C-sample, the compression ratio, permanent deformation ratio and resilience ratio of the sample's first circle compression–resilience test were 3.833%, 0.096% and 97.50%, respectively (Fig. 7). The mullite whiskers grew more larger in this sample, and this resulted in higher compression deformation which formed through the mullite whiskers' deeper intersection. Also, the decreased permanent deformation and increased resilience ratio were due to the heightened strength of the intersected mullite whiskers. For the 1400 °C-sample, the compression ratio, permanent deformation ratio and resilience ratio of the sample's first circle compression–resilience test were 3.250%, 0.076% and 97.82%, respectively (Fig. 7). The mullite whiskers in this sample became more larger and denser, and also larger resistance was produced between the whiskers. Therefore, the deeper intersection was relatively difficult to the whiskers, and this led to decreased compression ratio of the sample. Besides, the decreased permanent deformation ratio and increased resilience ratio also indicated the increased stiffness of the sample. For the 1500 °C-sample, the compression ratio, permanent deformation ratio and resilience ratio were 2.438%, 0.042% and 98.28%, respectively (Fig. 7). The decreased compression ratio, lower permanent deformation ratio and higher resilience ratio were both owing to the increased density and stiffness of the sample. In the samples, the permanent deformation was mainly due to the structure damage of the mullite whiskers, such as obscission or collapse which formed in the compression–resilience process. The example of the mullite whiskers' structure damage was showed in Fig. 8. After one circle compression–resilience test, the samples still had stable structure which could bear a new cycle compression–resilience test under the same conditions. For each sample, the consecutive compression–resilience curves were almost parallel with each other (Fig. S2†), indicating that the MF/MW frameworks could be cycle used under proper conditions (including proper maximum compression stress, proper loading rate, etc).
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Fig. 8 SEM images, example of the structural damage of mullite whiskers after being employed in the compression–resilience test. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47394e |
This journal is © The Royal Society of Chemistry 2014 |