Inhibitory effect of the Al2O3–SiO2 mixed additives on the anatase–rutile phase transformation

Inhibitory effect of the Al2O3–SiO2 mixed additives on the anatase–rutile phase transformation

August 1998 Materials Letters 36 Ž1998. 320–324 Inhibitory effect of the Al 2 O 3 –SiO 2 mixed additives on the anatase–rutile phase transformation ...

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August 1998

Materials Letters 36 Ž1998. 320–324

Inhibitory effect of the Al 2 O 3 –SiO 2 mixed additives on the anatase–rutile phase transformation J. Yang ) , J.M.F. Ferreira Department of Ceramics Engineering, UniÕersity of AÕeiro, 3810 AÕeiro, Portugal Received 8 January 1998; accepted 15 January 1998

Abstract The inhibitory effect of the mixed alumina and silica additives with molar ratios of wAlxrwTix s 0.05 and wSixrwTix s 0.05 on the phase transformation of anatase to rutile was investigated. The suppressing effect on the anatase–rutile transformation by the mixed additives was observed to be much stronger than that of the single additive. The formation of the anatase solid solution with both alumina and silica was probably responsible for the stronger inhibitory effect. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Titania; Phase transformation; Solid solution

1. Introduction The anatase–rutile ŽA ™ R. phase transformation process has been extensively studied over the past several decades w1–3x. The transformation takes place at around 7808C for the pure titania w4x. The temperatures for the transformation can vary from 400 to 12008C, depending on Ža. the type and amount of additives, Žb. powder preparation method, and Žc. atmosphere. These processing variables would significantly change the transformation rate and activation energy and thus produce a transition temperature either higher or lower than that of pure titania. The mechanism by which the additives either inhibit or

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promote A ™ R transformation has been related to the defect structure of the titania, i.e., the concentration of oxygen vacancies or Ti interstitials. It was suggested that w1x the additives which can increase the concentration of oxygen vacancies would also tend to promote the transformation, while some additives would retard the transformation by increasing the lattice defect concentration of Ti interstitials in titania. However, many exceptions were observed; for instance, the addition of niobium oxide ŽNb 2 O5 . showed an inhibitory effect on the A ™ R transformation, although it suppressed rather than promoted the formation of lattice defects of either oxygen vacancies or Ti interstitials w5x. This indicates that the mechanism dominating in the A ™ R transformation process is complex and not fully understood yet. It is known that both Al 2 O 3 and SiO 2 as single additives exert inhibitory effects on the transforma-

00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 4 2 - 1

J. Yang, J.M.F. Ferreirar Materials Letters 36 (1998) 320–324

tion of anatase to rutile w3,6x. Our previous work w4x has shown that a solid solution of Al 2 O 3 in anatase was formed, corroborating the findings of Yamaguchi and Mukaida w7x. However, these authors did not observe any inhibitory effect by alumina. Actually in their work the exothermic reaction due to A–R phase transformation appeared at slightly decreasing temperatures with increasing amounts of alumina. An opposite tendency was observed in our work w4x, where the transformation temperature was delayed to around 850–10608C, depending on the amount of Al 2 O 3 added. The inhibiting effect on A ™ R transformation by the SiO 2 additive was reported by Suyama and Kato w6x. The transition temperature was displaced to 980 ; 10308C, depending on the amount of SiO 2 added and on the homogeneity of SiO 2 in TiO 2 . The reduction in the anataseranatase contact points by the adjacent silica particles, which would suppress the nucleation of rutile, was proposed to account for the observed retarding effect. Note that even though both the SiO 2 and Al 2 O 3 additives showed inhibiting effects on the transformation of anatase to rutile, the degree of inhibition by either Al 2 O 3 or SiO 2 as the single additive is limited Žgenerally below 11008C.. When the amount of additives reached to a critical level Žsaturation., the inhibiting effect by the SiO 2 or Al 2 O 3 additive would not be further increased. In the present letter, we reported that the mixed Al 2 O 3rSiO 2 additives can move the A ™ R transformation temperature to 12008C.

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value of 1.5 by 3 N HNO 3 . Hydrolysis was carried out by adding distilled water ŽwH 2 Oxrw M x s 2, w M x, the metal molar concentration of the alkoxides used. to the mixed solutions. The final concentration of Ti precursor was fixed at 0.33 M for all cases. The obtained gels were placed at room temperature for 2 days, and then dried in an oven at 508C and 1208C for one day each. The as-prepared xerogels were ground and then calcined at 9008C and 12008C for 1 h at a heating rate of 58Crmin. Differential thermal analysis ŽDTA. was used to characterise the obtained powder precursors at a heating rate of 108Crmin. The crystalline phases of the calcined powders were analysed by a X-ray diffractometer ŽDrMAX-C, Rigaku, Japan.. The lattice parameters of anatase were determined by analysing the XRD patterns obtained by using a 2 u scanning step of 18rmin and a scanning range of 20–1408.

3. Results and discussion Fig. 1 shows the DTA curves of the obtained three powder precursors. The exothermic reaction due to the A–R phase transformation occurs at 10638C for the sample with wSixrwTix s 0.10 and at 9738C for the sample with wAlxrwTix s 0.10. These temperatures are higher than that observed for the pure titania sample Ž; 7808C. w4x, indicating the retarding effect by the alumina or silica additive. Within the experimental temperature range no

2. Experimental Titania powders doped with silica ŽwSixrwTix s 0.10, molar ratio., alumina ŽwAlxrwTix s 0.10. and the mixed additives of silica and alumina ŽwAlxrwTix s 0.05 and wSixrwTix s 0.05. were prepared by a sol – gel processing. Titanium isopropoxide ŽTiŽOPr i .4 . ŽAldrich, A.R. Grade., alumina nitrate nonahydrate ŽAlŽNO 3 . 3 P 9H 2 O. ŽMerck, A.R. Grade. and silicon ethoxide ŽAldrich, A.R. Grade. were separately dissolved into absolute alcohol ŽAldrich, A.R. Grade.. The as-prepared solutions were then mixed according to the required proportions. The mixed solutions were then adjusted to a constant pH

Fig. 1. DTA curves of the titania powder precursors with various additives.

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J. Yang, J.M.F. Ferreirar Materials Letters 36 (1998) 320–324

exothermic peak corresponding to the A–R transformation is observed for the sample with the mixed Al 2 O 3rSiO 2 additives. This suggests that the A ™ R transformation in the powder with the mixed additives occurs at a higher temperature than those of the samples doped separately with alumina or silica. The variation in transformation temperature seemingly suggests that the sequence of the retarding effect on the A ™ R transformation by the additives is: Al 2 O 3rSiO 2 ) SiO 2 ) Al 2 O 3 . The XRD results, as shown in Fig. 2, also reflect such a trend. After calcination at 9008C for 1 h, the anatase phase in the sample doped with alumina partially transforms into rutile, while no evident transformation is observed in the powders with the SiO 2 or mixed Al 2 O 3rSiO 2 additives. With further increasing the calcination temperature to 12008C, the transformation reaction is entirely completed in the samples doped with alumina or silica, while a substantial amount of anatase still remains in the powder with the mixed Al 2 O 3rSiO 2 additives. This again indicates that the retarding effect by the mixed Al 2 O 3rSiO 2 additives is stronger than those observed for the samples doped with Al 2 O 3 or SiO 2 as the single additive. Inhibitory effects by SiO 2 or Al 2 O 3 as single additives on the transformation of anatase to rutile have been widely observed. The formation of an anatase solid solution with alumina was proposed to account for the observed retarding effect w4x. Yamaguchi and Mukaida w7x also reported the formation of an anatase solid solution. However, contrary to our observations, they registered exothermic effects due to the A–R transformation at temperatures that slightly decrease with increasing amounts of alumina. These differences might be due to different raw materials and preparation methods used. As far as we know, the anatase–rutile transformation behaviour is very sensitive to the raw materials, atmospheres, and impurities. Al 2 O 3 does not form a solid solution with rutile and when the anatase solid solution transforms to rutile a-Al 2 O 3 is also formed, as seen in Fig. 2. Explanations for the inhibiting effect by the SiO 2 additive appear also controversial w6,8x. Suyama and Kata w6x stated that the presence of SiO 2 would prevent the nucleation of rutile by interfering the mutual contact of TiO 2 particles. This was proposed

Fig. 2. X-ray diffraction patterns of the titania powders with various additives after being calcined at Ža. 9008C and Žb. 12008C for 1 h ŽI—anatase, B—rutile, )— a-Al 2 O 3 ..

as the reason for the observed retarding effect on the A ™ R transformation. Recently Yoshinaka et al. w8x reported the formation of an anatase solid solution containing SiO 2 up to ; 15 mol% SiO 2 , which

J. Yang, J.M.F. Ferreirar Materials Letters 36 (1998) 320–324

Fig. 3. The lattice parameters of anatase in various samples after being calcined at 6008C for 1 h.

would be responsible for the observed inhibiting effect on the transformation. In order to understand the enhanced inhibiting effect dominating in the A ™ R transformation process by the Al 2 O 3rSiO 2 mixed additives, the lattice parameters of anatase in various calcined samples were measured, as shown in Fig. 3. It can be seen that, in comparison to the pure anatase, the SiO 2-doped sample shows a reduction in lattice parameters, clearly suggesting the formation of an anatase solid solution with SiO 2 since the ionic radium of Si 4q Ž0.040 nm. is smaller than

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that of Ti 4q Ž0.061 nm. w9x. The lattice parameters of anatase in the sample with the Al 2 O 3rSiO 2 mixed additives are between the values of the Al 2 O 3-doped and SiO 2-doped samples. This indicates that both alumina and silica probably enter the anatase lattice to form an anatase solid solution, even though Al 3q and Si 4q substituting for Ti 4q or existing as interstitials in anatase lattice is not known. If so, when the calcination temperature was raised to a level Ž12008C. allowing the A ™ R reaction to proceed, silica probably first exsolutes from the anatase solid solution. The exsoluted silica would exist in amorphous state which can not be detected by XRD. When the calcination temperature was further increased to 12258C, a-Al 2 O 3 would precipitate as the A ™ R transformation proceeded. The as-formed a-Al 2 O 3 then reacted with rutile to form a new compound: aluminium titanate ŽAl 2TiO5 ., as seen in Fig. 4. Note that a considerable amount of anatase still remains at this temperature stage and a higher temperature is required to complete the A ™ R transformation.

4. Conclusions The inhibitory effect of the alumina, silica and a mixture of the two oxides on the transformation of anatase to rutile in the sol–gel-derived titania powders was investigated. The aluminarsilica mixed additives showed an enhanced retarding effect over alumina or silica as single additives. The formation of an anatase solid solution with both alumina and silica was probably the reason for the enhanced inhibiting effect on the A ™ R transformation.

Acknowledgements The first author is grateful to Foundation Oriental and JNICT of Portugal for the grants.

References Fig. 4. X-ray diffraction pattern of the powder with the mixed additives after being calcined at 12258C for 1 h ŽI—anatase, B— rutile, )— a-Al 2 O 3 ,—Al 2TiO5 ..

w1x R.D. Shannon, J.A. Pask, J. Am. Ceram. Soc. 48 Ž1965. 391. w2x R.A. Eppler, J. Am. Ceram. Soc. 70 Ž1987. C–64.

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w3x X.Z. Ding, X.H. Liu, Y.Z. He, J. Mater. Sci. Lett. 13 Ž1994. 462. w4x J. Yang, Y.X. Huang, J.M.F. Ferreira, J. Mater. Sci. Lett. 16 Ž1997. 1977. w5x S. Hishita, M. Takata, H. Yanagida, Yogyo-Kyokai-Shi 86 Ž1978. 631. w6x Y. Suyama, A. Kata, Yogyo-Kyokai-Shi 86 Ž1978. 119.

w7x O. Yamaguchi, Y. Mukaida, J. Am. Ceram. Soc. 72 Ž1989. 330. w8x M. Yoshinaka, K. Hirota, O. Yamaguchi, J. Am. Ceram. Soc. 80 Ž1997. 2749. w9x W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd edn., Wiley, New York, 1976, p. 58.