Abstract
Mullite fibres were prepared using aluminium carboxylates (ACs) and tetraethylorthosilicate by sol–gel process. ACs were synthesised from dissolution aluminium in a mixture of formic acid and glacial acetic acid using aluminium chloride hexahydrate as catalyst. The optimum condition for obtaining ACs is as follows: the molar ratio of aluminium, formic acid and acetic acid was 1∶3∶2·26 and aluminium chloride hexahydrate was 10 wt-%. All the Al and Si components were mixed at the molecular level and linear molecules were formed in the precursor sol. The dried gel fibres completely transformed to mullite fibres at 1200°C and the calcinated fibres had a smooth surface and uniform diameter.
Introduction
Mullite (3Al2O3.2SiO2) is only crystalline phase in the Al2O3–SiO2 system. 1 The mullite structure can be described as edges shared AlO6 octahedron chains parallel to c axis bounded by aluminium and/or silicon tetrahedron. 2 Mullite has been recognised as an outstanding ceramic material, owing to its high temperature strength, creep resistance, thermal and chemical stability, low thermal expansion coefficient and good dielectric properties. An important potential application of mullite is as fibres reinforcement in metals, ceramics and resins.
There are mainly two processes for the manufacturing of ceramic fibres. 3 They are melt spinning processes and sol–gel spinning processes. Conventionally, melt spinning methods are adopted for the synthesis of low melting point ceramic fibres. However, it is difficult to prepare mullite fibres because of the higher melting points. In order to overcome the difficulties, the sol–gel technique is employed to synthesise mullite fibres.4–6
Many successful processes have been reported in the preparation of mullite fibres by the sol–gel method. In most of starting materials, isopropoxide (AIP) was selected as Al source, because its polymerisation was responsible for the appropriate spinning viscosity. However, the AIP is expensive in the synthesis process of mullite fibres, so the process is limited to its wide spread applications, although the mullite fibres with smooth surface and dense microstructure. 3
Zhang et al. fabricated mullite fibres using aluminium nitrate [Al(NO3)3.9H2O, AN], tetraethylorthosilicate (TEOS) with commercial grade polyvinyl butyral as binder. However, the fibres were prepared using absolute ethyl alcohol as a solvent. It is desirable to fabricate the mulite fibres using the Al source with low cost and distilled water as solvent, but obtaining high fibres quality.
In the present work, mullite fibres were prepared by sol–gel method using aluminium carboxylates (ACs) and TEOS, but ACs were synthesised from dissolution aluminium in a mixture of formic acid and glacial acetic acid using aluminium chloride hexahydrate as a catalyst. Some organic acids could be evaporated using ACs as an aluminium source, while the mullite precursor sols were concentrated. Therefore, high-density fibre would be obtained.
Experimental
Starting materials used were aluminium powder (Chemically grade, Shanghai chemistry Co. Ltd, Shanghai, China), aluminium chloride hexahydrate (Chemically grade, Xi'an reagent factory, Xi'an, China), formic acid (Chemically grade, Tianjin Dengfeng chemistry Co. Ltd, Tianjin, China), glacial acetic acid (Chemically grade, Tianjin Yaohua chemistry Co. Ltd, Tianjin, China) and TEOS (Chemically grade, Tianjin Yaohua chemistry Co. Ltd, China).
The solutions with different compositions were prepared according to Table 1 for obtained AC solution. The processing steps are shown in Fig. 1a. The aqueous solution was prepared as follows: aluminium powder, aluminium chloride hexahydrate, formic acid and glacial acetic acid are added into a flask. Subsequently, distilled water was added (30∶1 molar ratio of H2O and Al). Finally, the mixture was heated and stirred using magnetic stirring under reflux at 80°C.
Schematic view of production route for a AC solution and b mullite fibres
Effect of dissolution condition on solubility of aluminium in mixed solvent of formic and acetic acids*
*Aluminium chloride hexahydrate by weight based on aluminium was added as a catalyst. A molar ratio of Al powder and distilled water was 1∶30. FA was formic acid and HAc was acetic acid.
The mullite fibres processing steps are shown in Fig. 1b. The transparent solution (2) was condensed to obtain viscous sol under atmosphere in water bath (50°C). The viscous sol was then dissolved in the mixing solution of distilled water and absolute alcohol (1∶1 volume ratio of H2O and EtOH). TEOS was then added in the mixture solution to obtain a solution with stoichiometric mullite composition. After that, the mullite precursor solution was kept open and concentrated to obtain spinning sol in water bath (50°C). When the spinning sol was obtained, the sol fibres were prepared by immersing a thin glass rod into the condensed sol and pulling it out slowly by hand at ambient temperature (∼25°C).
The sol fibres were dried around 24 h at 60°C using an oven. The dried gel fibres were then calcined at 1000 and 1200°C at heating rate of 1°C min−1. The holding time was 1 h. The fibres were then cooled in the furnace.
The pH of the solution was measured using a PHS-25 pH-meter (Shanghai precision instruments Co. Ltd, Shanghai, China). The viscosity measurement of sol was carried out at room temperature by using a NDJ-1 viscometer (Shanghai Balance Tech. Co., Ltd, Shanghai, China) at different shear rates (6, 12, 30 and 60 rev min−1). The thermal behaviours of the gels were investigated at a heating rate of 10°C min−1 in flowing air condition by SDT Q600 thermogravimetric/differential scanning calorimetry (TG/DSC) instruments (TA, American). The Fourier transform infrared (FTIR) of the dried gel samples was recorded on 6700 Infrared Spectrometer (Nicolet Magna, American) with the samples as KBr pellets. X-ray diffraction test was performed with the DX-2500 X-ray diffraction spectroscopy (Dandong Fangyuan, Dandong, China) using Cu Kα radiation, a step width of 0·05° s−1. Microstructure features was observed under a JSM-6390LV scanning electron microscope instrument (JEOL, Japan). All characterisations were tested at room temperature. 7
Results and discussion
Aluminium powder was dissolved during the stirring and heating in the mixed solution of formic acid and acetic acid and its main chemical reactions between aluminium and acids can be simplified in the following equation (1), though the actual reactions were complexity The compound was stable in an acidic aqueous solution and the excess acids were necessary to ensure aluminium dissolution. It was required that the molar ratio of formic acid and glacial acetic acid was more than 1 to prevent the formation of insoluble acetates. On the other hand, if the ratio of formic acid to acetic acid was too high, and the concentration of aluminium triformate increased in the solution, then precipitates were present.
As shown in Table 1, the solution (1) and (2) were transparent but solution (3) was translucent. The homogeneous ACs had been synthesised in solution (1) and (2). The solution (3) was translucent, which it could be explained that the aluminium did not be dissoluble because it lacked acid. The solution (4) was opaque because aluminium hydroxide precipitates were present. The further research about these factors is carrying on.
The solution (2) was suitable for fibres preparation since the organic acids content in the solution (2) was less than the solution (1). The transparent solution (2) was condensed under atmosphere in order to evaporate unreacted organic acids until the remained solution was so viscous that it flowed very slowly at room temperature while the flask was turned upside down. It will produce precipitation if the quantity of leavings organic acids is too high, when the TEOS hydrolysis.
The viscosity and rheological behaviour of the AC sol are shown in Fig. 2. As can be seen, the viscosity was about 3260 mPa s, and no change in the viscosity as a function of the shear rate, meaning that the sol behaves as a Newtonian fluid. In addition, the sol showed spinnability because ACs hydrolysised and condensation polymerisation took place while solution was condensed, which was attributed to the formation of linear molecules. The mainly reactions can be simplified in the following equations (2) and (3), though the actual reactions were complexity
Viscosity–shear rate relationship of AC sols and MP sol at room temperature
The AC sol was dissolved in the mixing solution of distilled water and absolute alcohol, and TEOS was then added in the solution to obtain a mullite precursor solution. After that, the precursor solution was kept open and concentrated to obtain a sol in water bath. The viscosity and rheological behaviour of the mullite precursor (MP) sol are shown in Fig. 2. As can be seen, the viscosity was about 3080 mPa s, and there is no change in the viscosity as a function of the shear rate, meaning that the sol behaves as a Newtonian fluid near the gelling point. TEOS was also hydrolysised under acidic condition, and condensation polymerisation took place. The main reactions can be simplified in the following equations (4) and (5), though the actual reactions also were complexity
The sol fibres were prepared by immersing a thin glass rod into the mullite precursor sol and pulling it out slowly by hand. The diameter of these sol fibres ranges from 10 to 30 μm. The length and diameter of fibres were influenced by viscosity and surface tension of spinning sol, speed of hand drawing and so on. The further research about these factors is carrying on. Scanning electron micrographs of the dried gel fibres are shown in Fig. 3. The dried gel fibres have a smooth surface, uniform diameter and transparency property.
Image (SEM) of microstructures of different diameter gel fibres
The TG/DSC curves of the gel fibres are shown in Fig. 4. The DSC curves of the dried gel fibres show two endothermic peaks at about 96 and 326°C and two exothermic peaks at about 438 and 978°C. Two endothermic peaks are assigned to dehydration of the residual water and decomposition of dydroxides in the dried gel fibres, whereas two exothermic peaks are assigned to decomposition of organic component and crystallisation of mullite respectively. The TG curves of the dried gel fibres show the weight loss is around 61 wt-% up to 1000°C, but it is almost completed at 600°C. In the above thermal analysis, the most interesting thing is the intensity of the DSC exothermic peaks at 978°C. 8 The crystallisation of monphasic gel and diphasic gel occurs ∼980°C and above 1250°C respectively. 9 Sharp exothermic peak at 978°C indicates better homogeneity in the degree of mixing of the Al and Si components on a molecular scale. Thus, the homogeneity in the mixing of Al and Si components in the gels is deemed to be a necessary condition for obtaining mullite phase at lower temperature.
Thermogravimetric and differential scanning calorimetry curves of gel fibres
The FTIR spectra of the gel fibres are shown in Fig. 5. The bands at 3464 and 1105 cm−1 are assigned to the OH stretching modes and hydroxides bending modes as well as alcoholic OH respectively. These bands are extremely broad with width more than 150 cm−1. The weak band at 2862 cm−1 along with the shoulder around 3053 cm−1 is assigned to the stretching vibrations of CH2 and CH3 groups respectively. The band at 1070 cm−1 has a shoulder around 1125 cm−1. This shoulder and the band at 475 cm−1 are assigned to the stretching and bending modes of Si–O–Si of the network. The bending modes of these groups appeared as the most intense band at 1396 cm−1. The band at 857 cm−1 is assigned to Si–OH stretching mode. 10 The stretching modes of Al–O–Al linkages are observed at 630−1 and 787 cm−1. 11 The strong band located at 694 cm−1 may be assigned to the (Si, Al)–O–(Si, Al) linkages bending modes, because the band assigned to the (Si, Al)–O–(Si, Al) bending modes of the tetrahedral network. 12 This assignment is in agreement with the proposed T–O–T bending mode for the band at 737 cm−1 in mullite.
Fourier transform infrared spectra of gel fibres
The X-ray diffraction patterns of calcined fibres are shown in Fig. 6 at 1000 and 1200°C. The main phase was mullite phase in the samples sintered at 1000°C, although some Al–Si spinel remained. 3 It was implied that the mullite and Al–Si spinel phase formed simultaneously as a result of the reaction of amorphous SiO2 and Al2O3, according to the DSC/TG curve in Fig. 2. The quantification of Al–Si spinel content was about 45·6 wt-% according to calculation of the diffraction peak intensity.
X-ray diffraction pattern of fibres heated at a 1000 and b 1200°C for 1 h
The complete transformation to mullite was also observed at 1200°C. The mullitisation temperature was considered to be an important criterion in the assessment of the mixing scale of the Al and Si components in the gels. 4 Crystallisation temperatures in the range of 1600–1700°C are required to achieve complete mullitisation when alumina and silica particles are mixed in the micrometre size range. 4 If the mixing scale is at molecular level, complete mullitisation temperatures of 1000–1100°C can be achieved. Such a low mullitisation temperature indicates that all the Al and Si components in the sol are mixed at the molecular level.
Scanning electron micrographs of mullite fibres calcined at 1200°C are shown in Fig. 7. The fibres have a smooth surface and uniform diameter. The excellent transparency of the precursor fibre remains unchanged after heat treatment.
Images (SEM) of microstructures of mullite fibres heated at 1200°C for 1 h
Conclusion
The optimum condition for obtaining ACs is as follows: the molar ratio of aluminium, formic acid and acetic acid is 1∶3∶2·26 and aluminium chloride hexahydrate by weight based on aluminium was 10%. All the Al and Si components are mixed at the molecular level, the sol behaves as a Newtonian fluid near the gelling point and the (Si, Al)–O–(Si, Al) molecular chains was formed in the mullite precursor sol. The main phase was mullite phase at 1000°C, although some Al–Si spinel remained. The completely mullitisation was 1200°C. The calcinated fibres had a smooth surface and uniform diameter.