sol-gel derived tio2-sio2 fibres

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Journal of Sol-Gel Science and Technology KL383-09-Murlidharan January 4, 1997 13:23

Journal of Sol-Gel Science and Technology 9, 85–93 (1997)c© 1997 Kluwer Academic Publishers. Manufactured in The Netherlands.

Sol-Gel Derived TiO2-SiO2 Fibres

B.G. MURALIDHARAN∗AND D.C. AGRAWALMaterials Science Programme, I.I.T. Kanpur 208016, India

[email protected]

Received August 29, 1995; Accepted February 12, 1996

Abstract. TiO2-SiO2 fibres with 0, 5, 10 and 20 volume % SiO2 have been prepared by drawing from a gelfollowed by sintering at different temperatures. Nearly one meter long fibres can be drawn easily in conditions ofabout 50% relative humidity. Addition of SiO2 inhibits the crystallisation of TiO2 and also the anatase→ rutiletransformation and improves the strength of the fibres. While the pure TiO2 fibres are brittle, those with 5, 10 and20 volume % SiO2 are flexible and strong. Tensile strength values as high as 3 GPa have been achieved in the 10volume % SiO2-TiO2 fibres. Fibres heated above 900◦C are brittle. The shape of the cross section of the fibres isfound to depend on their diameters.

Keywords: fibres, TiO2, SiO2, strength

1. Introduction

Ceramic fibres are of interest because of their potentialfor use in metal and ceramic matrix composites, hightemperature filters, catalyst supports in petroleum, foodand plastics industries and numerous other applications[1, 2]. Methods such as the ‘relic process’ (where anorganic fibre or woven form is soaked in an inorganicsalt solution followed by drying and calcination to pro-duce a ‘relic’ of the original fibre or cloth) and extrusionof ‘slips’ (where a suspension is made of a refractoryoxide along with rheology aids is extruded, dried andcalcined) have been employed to obtain oxide fibres [3].By comparison, the sol-gel process offers a simple andpowerful method to produce fibres of various ceramics.Fibres of ZrO2, TiO2, SiO2, SiO2-ZrO2, Na2O-SiO2-ZrO2, SiO2-TiO2 and cordierite have been made by thistechnique in recent years [5–10].

For many applications, particularly as reinforce-ments in composites, the most important considerationis the strength of the fibres. However the data on themechanical strength of such fibres is meagre. Marshallet al. reported high strength values between 1.5 to

∗Now at Indira Gandhi Centre for Atomic Research, Kalpakkam,India.

2.6 GPa for Y2O3 doped ZrO2 fibres [10]. The precur-sors were zirconium acetate and yttrium nitrate. Therehave been only a few reports describing mechanicalproperties of sol-gel derived fibres from alkoxide so-lutions. Tensile strength measurements have been car-ried out in the case of SiO2 fibres heated up to 1000◦C[11] and SiO2/ZrO2 fibres heated up to 800◦C [12].More recently mechanical properties were determinedfor the fibres of molar composition 80SiO2/20ZrO2 af-ter heating them at 900◦C and 1200◦C which causedcrystallization of zirconia [13].

Fibres of TiO2 and SiO2-TiO2 (upto 70 wt.% TiO2)

starting from alkoxide precursor have been prepared byKamiya [5] and Sakka [14] respectively. However nostrength measurements have been reported.

In this paper we report synthesis of TiO2-SiO2 fibreswith 0, 5, 10 and 20 volume % SiO2 and the studies oftheir phases, morphology and strength after heat treat-ments at various temperatures. It is found that the ad-dition of SiO2 has a dramatic influence on the strengthand phase transformation of the fibres.

2. Experimental

TiO2 fibres with 0, 5, 10 and 20 volume % SiO2 wereprepared starting from alkoxides of Ti and Si. Tetra



86 Muralidharan and Agrawal

ethyl ortho silicate (TEOS) and titanium tetra but-oxide (TTB) (Alfa products, Denvers, MA, USA)were first mixed in isopropanol. Molar ratios of theingredients used were TTB : TEOS :i -PrOH : H2O :HCl :: 1 :x : 10 : 0.5 : 0.1. The value ofx was 0, 0.04,0.09 or 0.205 to give approximately 0, 5, 10 or 20 vol%SiO2 in TiO2 respectively. The mixture was stirredfor 30 minutes after which water and hydrochloricacid mixed with an equal amount of isopropanol wereadded. The mixing was continued further for half anhour. The solution was then kept at 60◦C in an oven.The solutions turned fibrizable after 24 to 28 hours in

(a)

Figure 1. X-ray diffractograms from (a) TiO2 fibres (0 volume % SiO2) and (b) 20 volume % SiO2-TiO2 fibres after subjecting to heattreatments at different temperatures; (R-rutile, B-brookite, A-anatase). (Continued on next page.)

all the cases. The fibres were hand drawn by touching astainless steel wire to the sol and pulling it away. Fibredrawing could be carried out for only about 5 min-utes before the sol became too viscous for the fibredrawing to continue. As the hydrolysis and the con-densation rates in TTB and TEOS differ considerably[16], steps are usually taken to retard the hydrolysisrate of the former or prehydrolyze the latter. In orderto achieve this there are reports in the literature aboutthe use of chelating agents like acetylacetone or aceticacid. Though the chelating agents simplify the prepa-ration and allow longer fibres to be drawn [17–19] their



Sol-Gel Derived TiO2-SiO2 Fibres 87

(b)

Figure 1. (Continued).

use can adversely affect the mechanical properties asit becomes difficult to remove the organics during heattreatment. In the present work, no chelating agent wasused nor any effort made to control the relative hydrol-ysis rates. Despite this the TiO2-SiO2 fibre could bedrawn easily and heat treated to high strengths. Fibresas long as 1 metre could be drawn under humid con-ditions (about 50% R.H) whereas it was very difficultto draw fibres in rather dry conditions (less than 35%R.H). The reason for this appears to be relatively slowcondensation reaction in the fibre during drawing in thedry atmosphere due to which sufficient strength in thefibre is not developed to allow the fibre drawing.

The fibres were dried in an oven at 110◦C for 12hours and then held for 1 hour at different temperatures(750◦C, 800◦C, 900◦C, 1000◦C, 1100◦C and 1250◦C).The heating rate was 1◦C min−1 upto 800◦C and 5◦Cmin−1 after that.

X-ray diffractograms were obtained using a diffrac-tometer (Rich Seifert Iso Debyeflex 2002) with Cu-Kαradiation. Tensile testing was carried out in a universal

testing machine (Instron 1195) using a cross head speedof 0.02 cm/min and a gauge length of 1.5 cm. The ten-sile strength was calculated after measuring the fibre di-ameter at the fracture point in an optical microscope. Atleast 10 samples were tested for each diameter and foreach composition and heat treatment temperature. Thesurface morphology and microstructure were examinedin a Scanning Electron Microscope (JEOL JSM 840 A).

3. Results

3.1. Phases

The X-ray diffractograms obtained from pure TiO2 fi-bres after heat treatment at different temperatures areshown in Fig. 1(a). Also shown in Fig. 1(b) are the X-ray diffraction results for 20 volume % SiO2 fibres heattreated at various temperatures. Other compositionsyielded similar results. The phases present at each ofthe sintering temperature for all the compositions (0, 5,




Table 1. Phases present after heat treatment at different temperatures.

Heat treatment temperatures (◦C)Volume%SiO2 750 800 900 1000 1100 1250

0 Anatase Anatase Anatase Rutile Rutile Rutile+ + +

Rutile Rutile Rutile

5 Anatase Anatase Anatase Anatase Rutile Rutile+

Rutile

10 Anatase Anatase Anatase Anatase Anatase Rutile+ +

Rutile Rutile

20 Amorphous Anatase Anatase Anatase Anatase Rutile+ +

Rutile Rutile

Table 2. Weibull parametersm andσ0 from the strength data of the fibres for bulk and surface analysis.

Volume %SiO2 5 10 20

Heat treatmenttemp (◦C) 1 hr 750 800 900 750 800 900 750 800 900

σ0(s) 682 1141 948 1321 1295 1001 584 1013 693

ms 1.25 1.01 1.06 1.08 1.44 1.28 1.37 1.33 1.11

Correlationcoeff. (rs) 0.965 0.965 0.967 0.938 0.959 0.975 0.969 0.954 0.942

σ0(v) 1059 1875 2042 1906 1784 1367 839 1460 1175

mv 1.52 1.24 1.40 1.38 1.68 1.70 1.68 1.58 1.29

Correlationcoeff. (rv) 0.916 0.927 0.913 0.900 0.925 0.947 0.932 0.904 0.879

10 and 20 vol% SiO2) are listed in Table 1 using thedata in Ref. [15] to identify the phases.

3.2. Strength of Fibres

While the pure TiO2 fibres were very fragile, and couldnot be tested for strength those with 5, 10 and 20 volume% SiO2 and heat treated at temperatures upto 900◦Cwere quite strong. Figure 2(a) shows the plots of ten-sile strength (MPa) vs fibre diameter (µm) for 10 vol%SiO2 fibres treated at 800◦C/1 hr. Other compositionsand treatments showed similar behaviour. The strengthdata of all these fibres is discussed below.

The strength data was analysed using the 3 parameterWeibull distribution:

P(σ f ) = 1− exp[−V/V0(σ f /σ0)m]

Here P(σ f ) is the probability that the strength is≤ σ f , (V/V0) is the normalized volume of the fibreunder test andm andσ0 are the parameters of the dis-tribution. The data was also analysed by replacing(V/V0) in the above relation by(S/S0), the normal-ized surface area, to determine whether the strengthcontrolling flaws are predominantly on the surface orin the bulk. Results of the analysis, normalised to a di-ameter of 10µm for all the fibres, are given in Table 2.The σ0 (surface) parameter, which is an indicator ofthe strength of the fibre, is plotted as a function of theheat treatment temperature for different SiO2 contentsin Fig. 2(b).

3.3. Microstructure

Figures 3(a) and (b) show the end views of some of thefibres. Note that while the cross section of the fibres is




(a)

(b)

Figure 2. (a) Tensile strength vs. fibre diameter for 10 volume % SiO2-TiO2 fibres heat treated at 800◦C/1 hr. (b) Weibull parameterσ0

(surface) for fibres containing 5, 10 and 20 vol% SiO2 treated at different temperatures.

circular in Fig. 3(a), in Fig. 3(b), this is not so for allthe fibres. Figures 4(a) and (b) show the surfaces ofthe fibres. The surface is featureless at these magnifi-cations for fibres treated below 1250◦C (Fig. 4(a)). Infibres treated at 1250◦C (Fig. 4(b)), grains of'1 µmcan be seen on the surface.

4. Discussion

4.1. Phases

TiO2 exists in three polymorphic forms, anatase (tetra-gonal), brookite (orthorhombic) and rutile (tetragonal).




(a)

(b)

Figure 3. (a) and (b) Scanning Electron Micrographs showing the end views of some fibres.

Rutile structure is known to be thermodynamicallymost stable amongst these three and both anatase andbrookite can be transformed to rutile by thermal oreven by mechanical methods [20]. However suchtransformation temperature varies with various meth-ods of preparation as well as with the presence of im-purities. Thus the TiO2 prepared from TiCl4 changesfrom anatase to rutile at 650–900◦C; this temperatureis significantly higher than that for TiO2 prepared usingalkoxides [21]. The X-ray diffraction studies carriedout by us on TiO2-SiO2 fibres have shown that theaddition of SiO2 inhibits crystallization of TiO2 aswell as the subsequent anatase to rutile transforma-tion. This is clear from Fig. 1 and also from Table 1.

The pure TiO2 fibres crystallise to anatase at 300◦Cwhile the 20% SiO2-TiO2 fibres remain amorphouseven at 750◦C. Moreover, while in pure TiO2, therutile phase is well developed at 800◦C, in the 20%SiO2-TiO2 fibres a single rutile peak just appears at1000◦C. The rutile peaks are much more sharper andwell developed in the former as compared to the lattercase.

These findings are consistent with those reportedin literature. Suyama et al. reported that the anatase-rutile transition rate of TiO2 in the TiO2-SiO2 powdersobtained by chemical vapour deposition and in vari-ous mechanical mixtures of TiO2 and SiO2 decreasedas SiO2 covered TiO2 particles more densely [22]. Also




(a)

(b)

Figure 4. Scanning Electron Micrographs of the surface of fibres after heat treating at (a) 800◦C/1 hr and (b) 1250◦C/1 hr.

it was noted that the ‘c’ axis of the anatase phase in-creased in length with increasing reaction temperatureand SiO2 concentration, indicating that SiO2 dissolvedin the anatase phase but not in the rutile phase [23].Gunji et al. also found that in TiO2-SiO2 fibres, SiO2

inhibited crystallization of TiO2 [24]. As remarked bythem, it appears that the rearrangement of Si-O-Ti link-age in the fibres into Si-O-Si and Ti-O-Ti is needed forthe crystallization of TiO2. With increasing amountof SiO2 this rearrangement becomes difficult such thatthe compositions having a high % of SiO2 (20 vol-ume %) remain amorphous even after treating at hightemperatures (750◦C/1 hour).

4.2. Strength of the Fibres

The strength of the fibres increases as the fibre diameterbecomes smaller (Fig. 2(a)). The strength of brittlesolids is determined by the size of the most severe flawaccording to the relation

KIC = Yσ (πa)1/2

whereKIC is the fracture toughness,Y is a parameterwhich depends on the specimen and loading geometry,σ is the strength and ‘a’ is the length of the flaw atthe surface (2a in the bulk) causing the fracture. To




determine whether the flaws are on the surface or in thebulk, the strength data was analyzed using the Weibulldistribution for the two cases. A significantly highercorrelation coefficient for the fit is obtained for the caseof surface flaws, indicating that the fracture occurs dueto the flaws on the surface (Table 2). The size of thelargest flaw is expected to increase due to increase inthe surface area with increasing fibre diameter leadingto a decrease in strength, as observed (Fig. 2(a)).

The fibre strength is also found to depend onthe SiO2 content and the heat treatment temperature(Fig. 3). Fibres containing 10% SiO2 possess the max-imum strength while heat treatment at 800◦C/1 hr leadsto maximum strength for all SiO2 contents (Fig. 2(b))(for the 10% SiO2, 750◦ and 800◦ treatments pro-duce nearly equal strengths). X-ray results show thatthe degradation in strength coincides with the anatase-rutile transformation. This transformation involves acollapse of the relatively open anatase structure byabout 8% [25]. The flaws generated due to this vol-ume change could be the reason for this deteriorationin strength.

Fibres with 10 volume % SiO2 heated at 750◦C and800◦C possess the best values of strength. The strengthvalues of fibres of 10µm diameter in this compositionvaried between 1000 MPa to 3000 MPa which nearlymatches the strength values of commercially availableSumitomo alumina fibre of a nominal composition 85%Al2O3-15% SiO2 [26].

4.3. Surface Morphology and Microstructure

Figures 3(a) and (b) show the cross sectional shape ofsome fibres. The fibres shown in Fig. 3(a) are all smallin diameter and are circular in cross section. In thefibres in Fig. 3(b), which were drawn from the samebatch of sol, smaller(<30 µm) fibres are circular incross section (fibres marked 1, 2 and 3) while the largerfibres (5 and 6) have an elliptical cross section.

The cross sectional shape of the sol-gel derived fi-bres has been reported to depend on the alcohol andwater content of the sol. As noted by Sakka [6] andLaCourse [27], increased water or decreased alcoholcontents result in a more circular cross-section. In thepresent work it is found that cross sectional shape ofthe fibres drawn from the same sol also depends on thediameter of the fibres. This could be due to the radialshrinkage during drying becoming more nonuniformas the fibre diameter increases.

The fibres possess a very fine microstructure whichis difficult to reveal by scanning electron microscopy.Only after heating at 1250◦C for 1 hour, the 10 vol-ume % SiO2-TiO2 fibres show a dense microstructurecomprising of fine grains of the size of about 0.5µm(Fig. 4(b)). The fibres are featureless at heat treatingtemperatures less than 1250◦C (Fig. 4(a)).

5. Conclusions

Titania fibres with 0, 5, 10 and 20 volume % SiO2

can be drawn from sols of titanium tetra butoxide andtetraethylorthosilicate. Fibre drawing is possible whenthe relative humidity is about 50% while under drierconditions it is difficult to draw the fibres. Smalldiameter fibres (<30 µm) have a circular cross sec-tion which changes to elliptical when the diameterincreases.

Pure TiO2 fibres are very weak and fragile. Additionof SiO2 improves the strength dramatically. Maximumstrength, between 1000 and 3000 MPa, is obtained forthe 10% SiO2 composition. Maximum in strength forall the compositions is achieved when the fibres are heattreated at 800◦C for 1 hr. Heating at higher temper-atures degrades the strength, probably because of theanatase-rutile transformation which is accompanied byconsiderable volume shrinkage.

Addition of SiO2 inhibits the amorphous-anatasetransformation as well as the subsequent anatase-rutiletransformation. The crystallized fibres have very smallgrain size (<0.1µm).

Acknowledgment

This work was supported partly by a grant from De-partment of Atomic energy, Govt. of India.

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sol-gel derived tio2-sio2 fibres

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