Effect of Ce4+-modified amorphous SiO2 on phase transformation towards α-cordierite

Effect of Ce4+-modified amorphous SiO2 on phase transformation towards α-cordierite

December 2002 Materials Letters 57 (2002) 409 – 413 www.elsevier.com/locate/matlet Effect of Ce4+-modified amorphous SiO2 on phase transformation to...

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December 2002

Materials Letters 57 (2002) 409 – 413 www.elsevier.com/locate/matlet

Effect of Ce4+-modified amorphous SiO2 on phase transformation towards a-cordierite Z.M. Shi a,b,*, F. Pan b, D.Y. Liu c, K.M. Liang b, S.R. Gu b a Department of Basic Sciences, Inner Mongolia Polytechnic University, 010062 Hohhot, China Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, 100084 Beijing, China c Department of Materials Engineering, Inner Mongolia Polytechnic University, 010062 Hohhot, China

b

Received 31 August 2001; received in revised form 19 March 2002; accepted 21 March 2002

Abstract The sintering temperature of cordierite ceramic is dependent on the transformation temperature to a-cordierite. In the present work, the effect of Ce4+-modified amorphous SiO2 on the transformation behavior of the ceramics were studied using the techniques such as X-ray Diffraction, Differential Thermal Analysis (DTA) and Infrared Spectrometry so as to decrease the sintering temperature. Experimental results show that, Ce4+-free amorphous SiO2 has no effect on the transformation, while Ce4+-modified amorphous SiO2 can obviously decrease the onset temperature. This is correlated to that Ce4+ addition, which makes the structure of tetragonal SiO2 loosened and decreases the transformation temperature of SiO2 to tetragonal and so, improves the condition of solid solution of Al3+ and Mg2+ into tetragonal SiO2. Moreover, Ce4+ addition has little effect on the conversion rate, because it cannot effectively promote the diffusion of Al3+ and Mg2+ into tetragonal SiO2. D 2002 Elsevier Science B.V. All rights reserved. PACS: 64.70.Kb; 83.80Pc Keywords: Cordierite; Phase transformation; Cerium; Amorphous

1. Introduction Cordierite ceramic (2MgO2Al2O35SiO2) is a good candidate for filters, catalytic supports and substrates of integrated circuits, owing to its low thermal expansion coefficient and dielectric constant [1– 5]. Low-temperature sintering of the ceramics is important to the cost reduction and the preparation of the sub-

* Corresponding author. Department of Basic Sciences, Inner Mongolia Polytechnic University, 010062 Hohhot, China. Tel.: +86471-6576143; fax: +86-471-6503298. E-mail address: [email protected] (Z.M. Shi).

strates, however, sintering temperature is dependent on the transformation temperature towards a-cordierite. A lot of works have been concerned with the methods of lowering the transformation temperature. In oxide-powder sintering process, the onset temperature of transformation to a-cordierite was decreased from 1280 to 1240 jC by adding 2 wt.% of CeO2 into the mixture of SiO2, MgO and Al2O3 [6,7]. Generally, the crystallization of amorphous powders with stoichiometric composition occurs at about 1200 jC in sol – gel method and glass-crystallization process. While adding CeO2, TiO2 and B2O3 to the glass powders, and adding 4 wt.% of CeO2 in the form of Ce4+ to the sol – gel-derived amorphous powders, the

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 8 0 1 - 7

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crystallization temperatures decreased to below 1000 jC, respectively [8,9]. The reason has also been analyzed in literature [10]. However, the oxide-powder sintering is authorized to be more economical than the sol –gel method and glass-crystallization route. It was found that the cordierite transformation was largely dependent on the transformation of SiO2, and cordierite was a solid solution of SiO2 saturated by Mg2+ and Al3+ in the oxide-powder sintering [6]. On the other hand, Ce4+ intensively affected the phase transformation to cordierite as mentioned above. Therefore, the present work aims to study the effect of amorphous SiO2 modified with none or definite amount of Ce4+ on the cordierite transformation in order to further decrease the sintering temperature of the ceramics.

Universal V2.5H), using a-Al2O3 powder as reference sample, at a heating rate of 10 jC/min. The phases occurred in the samples were identified by an X-ray Powder Diffractometer (XRD, Rigaku D/max2400X), with copper Ka radiation (40 kV, 120 mA) and at a scanning speed of 4j/min. The samples used for XRD were sintered at different temperatures in air for 2 h. An Infrared Spectrometer (Perkin Elmer GX/ FT-IR) with a resolution of 4 cm1 was employed to examine the behavior of Ce4+ in the microstructure of the amorphous SiO2. The powders of the samples were mixed with KBr and then compressed into slices.

3. Results and discussion 3.1. XRD analysis

2. Experimental procedures The chemical reagents such as Tetraethyl silicate (TEOS) and cerium nitrate (Ce(NO3)36H2O) were used as precursor materials to prepare the amorphous SiO2 by sol – gel method. The hydrolyzed TEOS were mixed with the nitrate solution and strongly stirred at 60 jC for 3 h, followed by the addition of catalysts NH4OH so as to obtain gelatins, which then convert into transparent amorphous material by calcinations at 600 jC. The mixtures composed of the amorphous SiO2, crystal Al2O3 and MgO (AR) were ground through ball-milling for 3 h. Finally, the mixed powders with the granularity of 74 – 63 Am were selected by two screeners of 200# and 230# (ASTM E11-58T) for use, so as to exclude the effect of the granularity on the transformation temperature. The batch sheet of the samples is shown in Table 1. The crystallization sequences of the samples were determined by a Differential Thermal Analyzer (DTA,

Table 1 Batch composition of samples (wt.%) Sample

SiO2(A)

SOz SOC

51.3

SiO2(B)

Al2O3

MgO

51.3

34.9 34.9

13.8 13.8

z: stoichiometric composition of cordierite; SiO2(A): amorphous SiO2; SiO2(B): amorphous SiO2 with a molecular ratio of Ce4+/ Si4+=0.008.

Fig. 1 represents the phases existed in the samples sintered at different temperatures. The a-cordierite clearly appears at 1200 jC in sample SOC, which is lower than that (1250 jC) in sample SO. Along with the increase of sintering temperature, the peaks of spinel (MgAl2O4) and tetragonal SiO2 decrease and cordierite increases in both of samples. When sintered at 1350 jC, SOC contains less spinel and tetragonal SiO2 than SO does. These prove that Ce4+ addition facilitates the transformation towards a-cordierite. Besides, in sample SOC, there is none of the diffraction peaks of CeO2 and the scattered peak of glass phase, indicating that the samples consist of cordierite, spinel and tetragonal SiO2, and Ce4+ remained in the tetragonal SiO2. Actually, sample SOC is approximately the same as the cordierite ceramic with 2 wt.% of CeO2 in the previous work [6] in term of the nominal composition. The difference is, Ce4+ mainly exists in amorphous SiO2 for sample SOC, and throughout the boundaries of the oxide powders. Obviously, Ce4+ in the amorphous SiO2 results in a stronger decrease in the onset temperature of transformation to cordierite than CeO2 mixed with the oxide powders does. 3.2. DTA analysis As shown in Fig. 2, the peaks at 870– 890 jC are correlated with the exothermic reactions of amorphous SiO2 to cristobalite with cubic structure, while the

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Fig. 2. DTA curves of samples.

Fig. 1. XRD patterns of samples sintered at different temperatures for 2 h (c: a-cordierite; q: tetragonal SiO2; s: spinel). (a) Sample SO. (b) Sample SOC.

endothermic reactions at about 1210 jC resulted from the transformation of cubic SiO2 to tetragonal, accompanying an expansion of the lattice of SiO2. Both of the phase transformation temperatures in sample SOC are lower than those in sample SO, respectively. This indicates that Ce4+ addition facilitates both of the conversions of SiO2 from amorphous to cubic and from cubic to tetragonal. But, the peaks describing the transformation to cordierite cannot be determined in the present tests. As is mentioned above, before the cordierite phase occurs, there is a transformation of SiO2 to tetragonal. The transformation temperatures of SiO2 in the samples and in the previous work [6] are shown in Table. 2. It can be found that, the Cubic!Tetragonal transformation in sample SOC occurs at the lowest temperature (1198 jC), and the temperature of sample SO is

lower than that of Hexagonal!Tetragonal in CeO2free sample, close to that of the sample with 2 wt.% of CeO2. Accordingly, the amorphous SiO2 is easier to transform into tetragonal than hexagonal, and the solid solution of Ce4+ in amorphous SiO2 results in a further decrease in the temperature, which, in turn, decreases the transformation temperature to cordierite. 3.3. Analysis to structure and phase transformation of amorphous SiO2 Fig. 3 shows the infrared spectrographs of the amorphous SiO2. The absorption band at about 460

Table 2 Comparison on the transformation temperature of SiO2 to tetragonal Sample

Form

Temperature (jC)

Reference

SO SOC CeO2 free 2 wt.% 4 wt.% 10 wt.%

C!T

1220 1198 1230 1219 1212 1207

Present work

H!T

Ref. [6]; adding different amount of CeO2

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cm1 denotes the deviation vibration of Si– O bond, while the bands at 800 –1100 cm1 are related to the stretching vibrations of Si –O bond; and the bands at 1632 cm1 and about 3420 cm1 are the result of the stretching vibrations of hydroxyl – OH. The addition of Ce4+ (sample SOC) makes these bands shift towards the direction of low wave number, which is probably caused by the occurrence of Si– O – Ce bonds. Because Ce4+ is a modifier for glass network [11], and has a larger radius than Si4+ [12], the bond of Si –O –Ce may weaken the strength of Si– O bond and loosen the microstructures of the glass networks and crystal lattice of SiO2. Consequently, both of the conversions can be accelerated from amorphous SiO2 to cubic and from cubic to tetragonal. This can be confirmed by the DTA curves of single amorphous SiO2 shown in Fig. 4. An endothermic peak, which corresponds the transformation to tetragonal, appears at 1231 jC for the Ce4+-modified sample SOC. On the contrary, no evidence of the transformation presents up to 1500 jC for sample SO (the exothermic peak at 600 jC is a disturbance, rather

Fig. 4. DTA curves of amorphous SiO2.

than the crystallization of amorphous SiO2). Therefore, Ce4+ addition is favorable of the transformation of SiO2 towards tetragonal. Comparing Fig. 2 with Fig. 4, one can see that in the polynary system of MgO – Al2O3 – SiO2, amorphous SiO2 present an interphase of cristobalite before the tetragonal structure occurs, while in the unitary system of SiO2, only tetragonal structure is in the possibility. This suggests that MgO, Al2O3 can induce the metastable phase of SiO2. The hydroxyl –OH resulted from the imperfect deprivation of the organic components, which has been found to contribute much to the crystallization of amorphous phase [13,14].

4. Summaries

Fig. 3. Infrared spectrograph of amorphous SiO2.

In the oxide-powder sintering process, there exist two forms of cordierite transformations: (1) by solid diffusion, corresponding to the onset temperature, and (2) the crystallization from liquid phase at relatively high temperature. Moreover, prior to the cordierite transformation there appears a reaction of spinel phase

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by MgO and Al2O3, and a conversion from hexagonal SiO2 to tetragonal [6]. As a comparison, the crystal SiO2 was substituted by the Ce4+-modified amorphous SiO2 in the present work. The advantage is that, the onset temperature of forming cordierite decreases to 1200 jC for sample SOC, lower than those in the oxide-powder sintering process, approaching that of crystallization of the glass powders and the sol – gel derived amorphous powder, with stoichiometric composition (Mg2Al4Si5O18) [8,9]. This benefits from the decrease of the transformation temperature from cubic SiO2 to tetragonal. Essentially, because of the formation of CeOSi bond and the relatively large radius of Ce4+, it results in expansions in the glass networks and crystal lattice of SiO2. It is the expansion that brings about a decrease in the transformation temperatures from amorphous SiO2 to cubic and from cubic to tetragonal. Although Ce4+ addition decreases the temperature of the cordierite transformation, it cannot effectively accelerate the transformation rate. This is because the transformation is under the control of the diffusions of Mg2+ and Al3+ into tetragonal SiO2. In the situation of little liquid-phase sintering occurring, spinel and SiO2 inevitably remained in the samples sintered at 1350 jC for 2 h.

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Acknowledgements The present work is supported by the emphasis project of Science and Technology Research Foundation, Ministry of Education, China.

References [1] A.K. Nandi, J. Am. Ceram. Soc. 82 (1999) 789. [2] K. Sumi, Y. Kobayashi, E. Kato, J. Am. Ceram. Soc. 82 (1999) 783. [3] J.R. Gonzalez-Velasco, M.A. Gutierrez-Ortiz, R. Ferret, A. Aranzabal, J.A. Botas, J. Mater. Sci. 34 (1999) 1999. [4] J. Ma, K. Liao, P. Hing, J. Mater. Sci. 35 (2000) 4137. [5] S. Mei, J. Yan, J.M.F. Ferreira, J. Eur. Ceram. Soc. 20 (2000) 2191. [6] Z.M. Shi, K.M. Liang, S.R. Gu, Mater. Lett. 50 (2001) 68. [7] Z.M. Shi, K.M. Liang, S.R. Gu, J. Tsinghua Univ. 41 (2001) 1. [8] B.H. Kim, K.H. Lee, J. Mater. Sci. 29 (1994) 6592. [9] Z.M. Shi, K.M. Liang, Q. Zhang, S.R. Gu, J. Mater. Sci. 36 (2001) 5227. [10] Z.M. Shi, K.M. Liang, S.R. Gu, J. Chin. Rare Earth Soc. 18 (2000) 79, special issue. [11] A. Paul, Chemistry of Glasses, 2nd edn., Chapman & Hall, London, 1990. [12] R.D. Shannon, Acta Crystallogr., A 32 (1976) 751. [13] C.J.R. Gonzalez-Oliver, P.S. Johnson, P.F. James, J. Mater. Sci. 14 (1979) 1159. [14] C.J.R. Gonzalez-Oliver, P.F. James, Glass Technol. 20 (1979) 204.