Effect of the addition of acetic acid on spinning sol and mullite fibers properties

The preparation of mullite fibers by sol-gel method is the preferred method in industrial production, where the stability, rheological properties and microstructure of spinning sols are essential for the preparation of continuous, low-defect fibers. In this study, aluminum carboxylate solutions (ACs) were prepared using aluminum powder, formic acid and acetic acid as raw materials, silica sol was added, and spinnable sol was obtained by concentration and aging. After dry spinning and calcination at 1200 °C, continuous mullite fibers with mullite as the main crystal phases were obtained. The effect of acetic acid addition on the stability of Al-Si mixed sol was studied, and the polymerization process of spinning sol was studied on its rheological properties and microstructure. The cross-section of mullite fibers obtained after calcination at 1200 °C was “dumbbell” type, and the cause of this cross-section was discussed, which laid a foundation for subsequent optimization research.


Introduction
Mullite fibers are a type of alumina-based ceramic fibers, and since mullite is the only compound that exists stably in the Al2O3-SiO2 binary system, mullite fibers have excellent properties of traditional alumina fibers, such as high strength and high modulus, high temperature resistance to oxidation, electrical insulation, and chemical corrosion resistance, in addition to good resistance to high temperature creep and thermal stability [1][2][3] .There are many methods for the preparation of continuous mullite fibers, and it has become the preferred method in the industrial production of mullite fibers because of the advantages of high purity, good homogeneity, and low reaction temperature using the sol-gel method [4][5] .There are two main types of raw materials: metal alcohol salts, which are difficult to apply in industrial production because of high production costs and difficult process control [6][7] ; and inorganic aluminum salts to get aluminum chloride, aluminum nitrate or aluminum carboxylate [8][9][10] .Aluminum carboxylate solutions (ACs) are usually produced by reacting aluminum powder with carboxylic acids [11][12] , and these raw materials are not only easy to obtain, but also have no other anionic impurities, do not produce polluting gases during the heat treatment, are more economical and environmentally friendly, and are suitable for industrial production.
The preparation of alumina fibers using ACs is less reported.Richards et al [13] synthesized ACs using aluminum powder, formic acid, and acetic acid to prepare mullite fibers by adding appropriate silica sols.Among them, the molar ratio of aluminum powder, formic acid and acetic acid was 1:1:1.Their study focused on the effect of additives on the microstructure and composition of mullite fibers.Chen Lifu`s group [14] made ACs from aluminum powder, formic acid and acetic acid with the ratio 1:4:3.Ma Yunzhu's group [10,[15][16] prepared ACs from aluminum powder, formic acid and acetic acid by sol-gel method, and obtained α-Al2O3 ceramic fibers by dry spinning and sintering at 1200 ℃.It was shown that the aluminum powder had the highest solubility at 95 ℃, and a colorless and transparent alumina sol with the highest solid content was obtained; the spinning sol was best spinnable when the molar ratio of formic acid to acetic acid was 1.15:0.85.Tan Hongbin`s group [12,17- 18] used tartaric acid, malic acid and lactic acid separately to react with aluminum nitrate and get ACs.When the mass ratio of aluminum nitrate: organic acid was 10:3, the sol spinning performance was the best.
In the sol-gel process, the rheological properties as well as the microstructure of the spinning sol are essential for the preparation of continuous low-defect gel fibers, and the stability of the spinning sol determines the process window of the spinning process; therefore, the preparation of a sol with good spinnability and stability is necessary to obtain continuous mullite fibers.In this study, ACs were prepared from aluminum powder, formic acid and acetic acid, added to silica sols, and spinning sols were obtained by concentration and aging.The effect of pH on the stability of Al-Si mixed sols was investigated, and the effect of polymerization process of spinning sols on their rheological properties as well as microstructure was studied.The effect of pH on the stability of the Al-Si mixed sol was investigated, and the influence of the polymerization process on the rheological properties and microstructure of the spun sol was studied.

Sample preparation
The deionized water, formic acid and acetic acid were added in a three-necked flask, stirred well with magnetic force and heated to 95 ℃.Aluminum powder was added, condensed and refluxed at 95 ℃ for 10 h.The clear solution was taken after filtration, and the ACs were obtained, where the molar ratio of aluminum powder to mixed carboxylic acid was 1:2, and the molar ratio of formic acid to acetic acid was 1:1.
The silica sol was added to the ACs, stirred well, and subsequently different amounts of acetic acid were added to adjust the pH value of the mixed Al-Si sol; the spinning sol was obtained by concentrating and aging at 60 ℃.Among them, the silica sol was added according to the ratio of 3:2 of Al2O3 to SiO2 molar ratio in the spunlace sol, and the amount of acetic acid added was calculated according to the amount of Al 3+ to CH3COOH molar ratio of 1:0, 1:0.5, 1:1, 1:2, 1:3.The spinnable spinning sol was spun dry to produce continuous gel fibers, and the gel fibers were kept in an oven at 80 ℃ for 10 h and then calcined at 1200 ℃ for 1 h under an air atmosphere (the heating rate was all 5 ℃/min) to obtain continuous mullite fibers.

Characterization
The pH value of mixed sols was measured by a pH meter (Shanghai Yidian, PHS-3C), the phase composition of mixed sols and precipitates was analyzed by infrared spectroscopy (FT-IR, Nicolet 6700, USA), the rheometer (Rheometer, HAAKE MARS40, Germany) was used to test the rheological properties of spinning sol, and the microscopic morphology of spinning sol was characterized by transmission electron microscopy (TEM, JEM-2100F, Japan).The rheometer (Rheometer, HAAKE MARS40, Germany) was used to test the rheological properties of the spinning sols, the transmission electron microscope (TEM, JEM-2100F, Japan) was used to characterize the microscopic morphology of the spinning sols, the X-ray diffractometer (XRD, Bruker D2 PHASER, Germany) was used to characterize the fiber structure and phase composition, and the cold field emission scanning electron microscope (SEM, Hitachi SU8010, Japan) was used to observe the surface and cross-sectional morphology of the calcined fiber.

Effect of acetic acid addition on the stability of Al-Si mixed sol
In the preparation of ACs, the raw material ration is 1:1:1 molar ratio of aluminum powder, formic acid and acetic acid, and the reaction theoretically produces The pH value of Al-Si mixed sol was 4.10 after adding silica sol to ACs and stirring well.The stability of silica sol is directly related to the pH value, when the pH value is 5~7, the negative charge on the particle surface decreases, the zeta potential decreases, and the particles easily collide to produce gelling; when the pH value decreases to 2~4, there is a substable region, and the sol particles change from originally negatively charged to positively charged.At this time, the sol is acidic and has better stability [19] .In the case of aluminum salt solutions, when the pH is below 3, only the hydrated ion [Al(H2O)6] 3+ is present, and as the pH increases, the hydroxide [Al(OH)n(H2O)6-n] (3+z)+ occurs.At higher solution concentrations, condensation reactions will occur between ions to form linear or nonlinear shaped molecular chains such as Al-O-Al through bridging oxygen and hydroxyl groups, which are essential for spinning sol to have spinnability.Acetic acid was chosen as a pH adjusting agent to provide an acidic environment for the solution and also to improve the spinnability of the spinning sol [20] .Therefore, the pH of the mixed sol should theoretically be adjusted to 3-4.
Table 1 shows the changes of the properties of the sol after adding different amount of acetic acid to the Al-Si mixed sol.When no acetic acid was added, the pH of the sol was 4.10, and it could be stored stably for 2 days, and then the precipitation formed (Figure 1(b): 1#), the spinning sol could be stretched for meters long ((Figure 2: 1#).When the amount of acetic acid added was 1 mol per molar Al 3+ , the pH of the sol was 4.00, it could be stored stably for 2 weeks without precipitation (Figure 1(d): 3#), the spinning sol could be stretched for meters long (Figure 2: 3#).When the amount of acetic acid increased to 2 mol per molar Al 3+ and above, the mixed sol could still be stored stably (Figure 1    Figure 3 shows the FT-IR spectra of the precipitations of sample 1#, 2# and the spinning sol of sample 3#.The absorption peak at 3150 cm -1 corresponds to the -OH expansion vibration in free water, structured water and carboxylic acid.The absorption peak at 1700 cm -1 corresponds to the -OH bending vibration and C=O vibration.The absorption peaks between 1500 and 1200 cm -1 correspond to the characteristic vibration peaks of C-H, CH3 and CH2.The absorption peaks between 1200 and 800 cm -1 correspond to the vibration peaks of Al-O and Si-O.It can be seen that the precipitations are consistent with the same composition of the spinning sol.

Analysis of rheological properties and microstructure of spinning sols
The gelation time of Al-Si mixed sol is not an inherent characteristic of the sol during the concentration aging process, but varies with various parameters.to go from sol to gel gradually decreases.Also, gelation is accelerated under vacuum conditions.According to Sakka, the degree of gelation can be expressed in terms of RT and the relationship is [21][22] : RT=t/tg (3) where RT is the relative polymerization time, t is the actual aging time, and tg is the time used for complete gelation.
The following reactions take place during the concentrated aging of the spinning sol: (5) (HO)Al(HCOO)(CH 3 COO) + S i (OH) 4 → (Al O S i ) n + nH 2 O (6) Figure 4a and Figure 4b show the curves of the variation of shear stress and viscosity with shear rate for the sample 3# spinning sols with different relative polymerization times RT, respectively.Due to the large amount of solvent, when RT ≤ 0.75, the viscosity of spinning sol was very low and behaved as a Newtonian fluid.With the increase of RT, the change of spinning sol viscosity was low until RT ≥ 0.75, the spinning sol gradually appeared as shear thinning behavior.When RT ≥ 0.9, the viscosity of spinning sol rose sharply with the increase of RT and started to become spinnable.Figure 4c shows the variation curve of spinning sol viscosity with temperature with different RT.When RT ≥ 0.9, the spinning sol was spinnable.At this time the viscosity was very sensitive to temperature and decreased significantly with the increase of temperature.Therefore, in the spinning process, the temperature should be adjusted to achieve the purpose of controlling the viscosity and fluidity of the spinning sols.
Figure 5 shows the TEM photos of the spinning sol for sample 3# with different RT.In Figure 5a when RT < 0.75, the colloidal particles in the sol were close to spherical shape and basically without cross-linking, so the viscosity of the sol was very low and did not have spinnability.In Figure 5b, the spinning sol RT = 0.85, the obvious chain structure could be observed, which was formed by the polymerization and cross-linking of colloidal particles, and the sol had certain viscosity and spinning performance at this time.In Figure 5c when RT = 0.96, the sol particles were mainly chain-like structure, and there was also a certain degree of connection between chains, this structure was the basis for the sol to have spinnability.In Figure 5d when RT ≥ 1, the precursor was gelated without spinnability any more.

Effect of acetic acid addition on the properties of mullite fibers
The spinning sol was spun through dry spinning to obtain continuous gel fibers.Then the gel fibers were dried in an oven at 80 ℃ for 10 h, after which they were heated to 1200 ℃ at a rate of 5 ℃/min under air atmosphere and calcined for 1 h to obtain continuous mullite fibers.
Figure 6 shows the XRD patterns of continuous mullite fibers from sample 1# and 3#.Both samples were sintered to 1200 ℃ for 1h.Characteristic diffraction peaks of mullite phase were identified as the main phase for both fibers labeled in the graph [23 ], such as 2θ at 26 °, 35 °, 41 °ect.Figure 7 shows the SEM pictures of continuous mullite fibers, and it can be seen that the fibers have a dense structure with no obvious cracks or pore defects after calcination at 1200 ℃.The cross-section of the fibers obtained for sample 1# is round, with a diameter of about 10 μm The cross-section of the fiber obtained from sample #3 is "dumbbell" shaped, with the long axis about 20μm and the short axis about 10μm.The explanation of why the cross-section shapes and diameters are different is in the next section (3.4 Figure 8).

Analysis of the formation of fiber cross-sectional shape
The cross-sectional shape is one of the important structural features of the fiber, and the degree of deviation from circularity is mainly related to the curing conditions and also affects the physical properties of the fiber [24] .A simple model of fiber cross-section formation is shown in Figure 8a.When the flux of solvent outward is less than the flux of coagulant inward (js/jn < 1), the filaments dissolve and the fiber cross-section is circular.When the rate of solvent leaving the filament is higher than the rate of non-solvent entering the filament (js/jn > 1), the shape of the cross-section depends on the mechanical behavior of the cured layer.The soft, deformable surface layer in Figure 8b shrinks to form a round cross-section, and when the hard "skin" layer in Figure 8c is present, the cross-section collapses resulting in a non-circular "dumbbell" shape.Figure 8. Illustration of the cross-section structure formed during curing [24] : (a) soft, deformable skin ( js/jn < 1), (b) soft, deformable skin ( js/jn > 1), (c) hard skin ( js/jn > 1).

Figure 2 .
Figure 2. Digital photos of gel fibers stretched from spinning sols.

Table 1
Effect of acetic acid adding amount on the stability and spinnability of spinning sol