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Formation of celsian from mechanically activated BaCO3–Al2O3–SiO2 mixtures
Journal of Alloys and Compounds 290 (1999) 230–235
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Formation of celsian from mechanically activated BaCO 3 –Al 2 O 3 –SiO 2 mixtures ˇ ´ a , *, D« . Kosanovic´ b , Dj. Bahloul-Hourlier c , P. Thomas c , S.J. Kiss a S. Boskovic a
ˇ , Material Science Laboratory, POB 522, 11001 Belgrade, Yugoslavia Institute of Nuclear Sciences – Vinca b Refractory Institute Magnohrom–Kraljevo, Kraljevo, Yugoslavia c Laboratory for Ceramic Materials and Surface Treatment, University of Limoges, Limoges, France Received 26 November 1998; received in revised form 10 February 1999
Abstract A ternary mixture BaCO 3 –Al 2 O 3 –SiO 2 was mechanically activated for different lengths of time. Chemical composition of the mixture corresponded to BaAl 2 Si 2 O 8 –BAS. As a function of activation time, reaction course was followed in the temperature range 750–12008C. Reaction of celsian formation was followed using thermogravimetry as well as conventional and high-temperature X-ray diffractometry. The obtained data show that reaction rate increases with prolonged activation time, under the same conditions of thermal treatment. Formation of hexacelsian via a series of solid state reactions involving Ba-silicates, was favoured with increasing activation time. Direct formation of monoclinic celsian was retarded, however, with prolonged activation. 1999 Elsevier Science S.A. All rights reserved. Keywords: Mechanical activation; Celsian
1. Introduction Ba-aluminium silicate has attracted much attention lately because of properties which make it suitable for numerous applications. BAS is known for its low CTE (coefficient of thermal expansion) [1], excellent oxidation and reduction resistance [2], resistance to slag attack, stable dielectric properties [3] etc. Because of these properties, BAS has already been used in electroporcelains, as dielectric glassceramics in thick film technology [4], refractory glassceramics, corrosion resistant refractory materials, etc. It is also expected that BAS will be applicable for radar dome windows [5] and for high temperature ceramic matrix composites [6]. The properties which make it suitable for use in ceramic matrix composites are its high melting temperature, low density, thermodynamic stability and low CTE. BAS is therefore used as a matrix for alumina, SiC and C fibers, and Si 3 N 4 . BaAl 2 Si 2 O 8 has three polymorphic forms: monoclinic (celsian), most suitable for the abovementioned applications, hexagonal and orthorhombic which appeared during cooling of hexacelsian at 3008C. Independent of the preparation procedure, whether by glass crystallization or *Corresponding author.
solid state reaction [7], hexacelsian although metastable at these temperatures is the first to form. Its conversion to the monoclinic form, which is thermodynamically stable up to 15908C, is very sluggish. Reaction paths to BAS from BaCO 3 –Al 2 O 3 –SiO 2 mixture was studied by several authors [6,7]. Formation of BaAl 2 Si 2 O 8 was observed at 900–11508C via Ba-silicate – path (1) BaSi 2 O 5 1 Al 2 O 3 5 BaAl 2 Si 2 O 8
(1)
or via Ba-aluminate – path (2) BaAl 2 O 4 1 2SiO 2 5 BaAl 2 Si 2 O 8
(2)
whereby the activation energy for the path (2) to BAS is much higher [8] in comparison with activation energy for reaction (1). There are numerous examples in the literature on the influence of mechanical activation on both reaction rate [9,10] and phase transformation rate [11,12]. The published data [9–12] point to the increased rate of solid state reactions and phase transformations due to introduced mechanical energy in the system during activation. This method is applied in our work with the aim to enhance the reaction of BAS formation from BaCO 3 , a-Al 2 O 3 and
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00091-2
ˇ ´ et al. / Journal of Alloys and Compounds 290 (1999) 230 – 235 S. Boskovic
SiO 2 (a-quartz). On the other hand the changes of the physical conditions of the reactants including steric factors caused by mechanical activation, are expected to affect the reaction path to BAS.
2. Experimental work ˇ Mixture of BaCO 3 (‘Zorka’-Sabac), SiO 2 (a-quartz ‘Merck’), and a-Al 2 O 3 (‘Alcoa’, A-16), the composition of which corresponded to BaAl 2 Si 2 O 8 , was mechanically activated in a vibratory mill made of WC-based hard metal (Fritsch-Pulverisset-9). Twenty grams of mixture was activated for 2 and 4 h, respectively. The non-activated mixture was, however, milled for only 2 min. The changes developed during milling were followed by X-ray diffraction, specific surface area measurements by modifying BET method and thermogravimetry, performed in air at 78 min 21 heating rate. The reaction course was followed by X-ray diffraction after heat treatment of mixtures at 750 and 11508C, 30 min, as well as by high temperature X-ray diffraction in the temperature range 800–12008C (1 h annealing time, nitrogen) at a heating rate 408 min 21 .
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Sanbornite gives, finally, BAS according to reaction path (1). In the presence of quartz, however, the reaction was slower, because some of BaCO 3 did not react with SiO 2 but it reacted with Al 2 O 3 instead to give BaAl 2 O 4 at higher temperatures [7]. This reaction leads to BAS formation via reaction path (2). Our results on the effect of mechanical activation time on the structure of BaCO 3 –Al 2 O 3 –SiO 2 mixture, nonactivated (A), as well as activated mixtures B and C are given in Fig. 1. With increasing activation time, the relative intensities of BaCO 3 , SiO 2 and Al 2 O 3 diffraction lines decrease, and the broadening of the diffraction lines is evident. New compounds were not detected. During the activation procedure, particles get micronized, which is
3. Results and discussion Formation of BaAl 2 Si 2 O 8 is a complex process involving a series of intermediate reaction steps. Quander et al. [6] found that starting with BaCO 3 , Al 2 O 3 and SiO 2 (amorphous), BaSiO 3 was the first intermediate reaction product. The reaction thereafter proceeded via a series of reactions giving Ba-silicates ever richer in SiO 2 , until BaSi 2 O 5 (sanbornite) was formed. Sanbornite can directly react with alumina to give the BAS as a final reaction product, according to reaction (1). Planz ¨ and Muhler-Hesse [7] showed that starting from BaCO 3 , g-Al 2 O 3 and SiO 2 (amorphous or quartz), BaSiO 3 and Ba 2 SiO 4 formed first at 6508C. The reaction thereafter, proceeds via several reaction steps [8], i.e.: 2BaCO 3 1 SiO 2 5 Ba 2 SiO 4 1 2CO 2( g)
(3)
Ba 2 SiO 4 1 SiO 2 5 2BaSiO 3
(4)
BaCO 3 1 SiO 2 5 BaSiO 3 1 CO 2( g)
(4a)
2BaSiO 3 1 SiO 2 5 Ba 2 Si 3 O 8
(5)
5Ba 2 Si 3 O 8 1 SiO 2 5 2Ba 5 Si 8 O 21
(6)
3Ba 5 Si 8 O 21 1 SiO 2 5 5Ba 3 Si 5 O 13
(7)
Ba 3 Si 5 O 13 1 SiO 2 5 3BaSi 2 O 5
(8)
Fig. 1. X-ray patterns of non-activated (A) and activated B and C samples.
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Table 1 Specific surface area of activated and non-activated mixtures Sample
Specific surface area (m 2 g 21 )
A – non-activated B – 2 h activated C – 4 h activated
2.4 3.3 1.1
also seen from Table 1, according to specific surface area measurement. Decreased value of specific surface area for sample activated for 4 h, can be a consequence of formation of new bondings (between particles) which do not take part in the absorption of nitrogen during specific surface area measurements, as well as of the increased ability for agglomeration of very fine particles obtained after long time activation. Besides micronization, amorphization and the distortion of the crystal lattice is brought about during mechanical activation [10,11], which is recognized from the shape of the diffraction lines (Fig. 1). Thermogravimetric analysis of the three mixtures (Fig. 2), revealed that with prolonged activation time the onset of weight loss was shifted towards lower temperatures. Moreover, the reaction course of weight loss appeared to be different for different activation time. While in nonactivated mixture decomposition starts at 10008C, and proceeds in a single step, with prolonged activation time, two-step process gradually becomes evident. At the same time, the onset of weight loss was shifted below 5008C. Total weight loss decreased with activation time, indicating that BaCO 3 got partially decomposed during mechanical treatment. This was proved by separate experiments on TG balance with vibrated and non-vibrated pure BaCO 3 . Total weight loss was 23 and 25% for activated and non-activated BaCO 3 , respectively. However, BaO was not detected in vibrated mixtures, possibly because of the small quantity and fine particle size. Mentioned reaction steps are more clearly seen from Fig. 3 in which the first derivative of weight loss vs. time is presented, at a constant heating rate (78 min 21 ). The maximum of the curve for non-activated sample is placed at 11608C, while
Fig. 2. Weight loss of non-activated (A) and activated B and C samples.
Fig. 3. dw / dt (w5Dm /m) vs. time, for activated and non-activated samples.
the maxima for the activated mixtures appeared at ¯7508C and 11608C for first (I) and second (II) reaction steps, respectively. In order to obtain the information on how far the reaction advanced in the range of the first and second maxima in Fig. 3, samples were heat treated at 750 and 11608C for 30 min. X-ray analysis of samples heated at 7508C (Fig. 4), shows that the relative intensities of the diffraction lines of all the reactants decrease with increasing activation time. The advanced reaction in activated mixtures in comparison with non-activated one, is easily recognized on the basis of the type of Ba-silicate formed under the same conditions of thermal treatment (see reactions 2–8). Namely, in the early stages of the BAS formation Ba 2 SiO 4 is formed in non-activated sample, while sanbornite is formed in both activated samples (reaction 8), which means that in these samples the convenient conditions are created for immediate development of the reaction (1). In all samples A, B and C heated at 11608C for 30 min (reaction step II, Fig. 3), the reaction advanced (Fig. 5) in comparison with data for 7508C (Fig. 4). In non-activated sample A, beside celsian, Ba 2 SiO 4 , Ba 2 Si 3 O 8 and BaAl 2 O 4 are detected. Sample B (2 h activated) contains hexacelsian (Fig. 5, middle), which is the dominant phase. Besides, celsian, alumina and Ba-silicates are detected. Hexacelsian is also the dominant phase in the 4-h activated sample C (Fig. 5, bottom), while celsian content is very low. In addition, alumina and Ba-silicates were detected. Also the results in Fig. 6 show that celsian is present in non-activated mixture A, both hexacelsian and celsian are present in 2-h activated mixture B, while single phase hexacelsian was detected in 4-h activated sample. It is very unusual for celsian to be formed during the reaction, without previous appearance of hexacelsian, although there are such examples in the literature [13]. It is a common feature of the reaction that hexacelsian forms first, and during the heat treatment it gets converted into monoclinic form. On the other hand it was shown [14] that celsian is
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Fig. 4. X-ray patterns of samples A, B and C heated at 7508C, 30 min.
Fig. 5. X-ray patterns of samples A, B and C, heated at 11608C, 30 min.
easily nucleated in the bulk while hexacelsian is nucleated at the surface of the particles. Taking this into account it may be clear why in our case only the celsian phase was formed in coarse grained, non-activated sample A. In activated samples, which are composed of very fine particles with developed, freshly created surface, the conditions for nucleation of hexagonal celsian are created. The hexagonal phase is the dominating phase becoming also a single phase for sample C at 12008C (Fig. 6). These data (Figs. 4 and 5) show not only that the reaction to BAS is enhanced by mechanical activation, but also that differ-
ent polymorphs of BaAl 2 Si 2 O 8 are formed depending on activation time. In the case of the studied ternary mixture, under nonisothermal conditions (Figs. 2 and 3), reaction takes place at higher temperature in non-activated mixture, in a single step, and is followed by weight loss. It should be added that the reaction was at the very beginning at 7508C in samples A, according to the results in Fig. 4. Because of high temperature which favors BaAl 2 O 4 formation, and because of less homogeneous distribution of constituents particles, BaCO 3 obviously reacts both with SiO 2 and
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Al 2 O 3 . As a result Ba 2 SiO 4 and BaAl 2 O 4 are formed simultaneously (BaAl 2 O 4 is lower in quantity), by reactions (3) and (9), BaCO 3 1 Al 2 O 3 5 BaAl 2 O 4 1 CO 2(g)
(9)
which is thermodynamically possible below 10508C [8]. Under the same conditions, the reaction is shifted towards lower temperatures in activated mixtures due to increased contact area, fine particles size, better homogenization, distorted lattice, all provoked by mechanical activation [15]. In these samples (B and C), BaAl 2 O 4 was not detected. It seems very likely that in the first reaction step (Figs. 2 and 4) very fine, active particles of BaCO 3 and SiO 2 react forming silicates rich in Ba, at the very beginning of the reaction. These Ba rich silicates form through series of the reactions (3–8) sanbornite prior to BAS formation. Weight loss during this reaction step comes up to 65% of the total weight loss. During the second step (Figs. 2 and 5), the rest of the BaCO 3 which may have been blocked by the reaction product layer, reacts further with SiO 2 (followed by 35% of weight loss) to give Ba-silicates. At 11608C, the reaction is incomplete in each of the samples, but very much close to an end in activated samples (without weight change). On the basis of above-presented data, the change of the intermediate reaction products as a consequence of the activation, may be connected with the granulometry of the reactants. Since staring alumina particles are much finer (Alcoa A-16, d av ¯0.5 mm) in comparison with silica particles (Merck, d av ,50 mm), in non-activated mixtures formation of BaAl 2 O 4 is favoured due to steric factors (e.g. number of contacts of reacting phases). In activated samples, however, considerable particle reduction of quartz takes place, which brings about the increasing of the number of particle contacts of reacting phases. Thereby the formation of Ba-silicates becomes easier and even becomes the dominant process in the formation of the intermediate products. On the basis of these results it is obvious that even the reaction mechanism of the ternary compound formation is changed by applying mechanical activation. Experimental data obtained under isothermal conditions after 1 h of annealing in high-temperature X-ray device (Fig. 6) indicate that some amount of BaAl 2 O 4 may be formed by reaction (9) [8] in sample A. However, the fact that the quantity of BaAl 2 O 4 increases, while the content of Ba 2 SiO 4 decreases simultaneously (Fig. 6A), at higher temperatures at which BaCO 3 had already been consumed points to the development of another reaction (10) which is thermodynamically possible at these temperatures [8], i.e. Ba 2 SiO 4 1 Al 2 O 3 5 BaAl 2 O 4 1 BaSiO 3
Fig. 6. High temperature X-ray patterns of samples A, B and C, 1 h annealing. (^) BaAl 2 O 4 , (s) Ba 2 SiO 4 , (s) Al 2 O 3 , (h) hexacelsian, (c) celsian, (*) Pt sample holder.
(10)
by which Ba-aluminate is formed. In activated samples, BaAl 2 O 4 first appears after BaCO 3 had been consumed (Fig. 6) which indicates that BaAl 2 O 4 is also formed in these samples by reaction (10).
ˇ ´ et al. / Journal of Alloys and Compounds 290 (1999) 230 – 235 S. Boskovic
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been formed, before BaAl 2 O 4 appeared, a smaller amount of BAS is formed via the reaction path involving BaAl 2 O 4 . The reaction path via BaAl 2 O 4 plays a more considerable role in non-activated mixture, in comparison with activated samples. Thus, mechanical activation affects the mechanism of BAS formation. With prolonged time of mechanical activation the relative content of monoclinic celsian decreases, while the relative amount of hexacelsian increases at the same time. The conditions for hexacelsian nucleation in activated samples may be connected with increased particles surface which favours the nucleation of hexacelsian.
References Fig. 7. High temperature X-ray patterns of A, B and C samples at 12008C, 1 h. (h) Hexacelsian, (c) celsian, (*) Pt sample holder.
In the final reaction step of BAS formation, BaAl 2 O 4 reacts with SiO 2 . However, SiO 2 was not detected in our samples above 9008C. This can be due to a small amount of unreacted SiO 2 and at higher temperatures, due to liquid phase formation. Namely, according to BaO–Al 2 O 3 –SiO 2 , phase diagram [16] eutectic liquid appears at 11248C, between celsian, sanbornite and tridymite. BaAl 2 O 4 was detected also in mixture BaCO 3 –gAl 2 O 3 –SiO 2 (amorphous) [7], above 8508C which is consistent with our data. The presence of BaAl 2 O 4 which appears also in activated samples only at longer heating times, as pointed out already, indicates the development of reaction path (2), at higher temperatures (Fig. 6) in all the investigated samples to a small extent. According to the results in Fig. 5, the contribution of reaction path (2) to overall reaction of BAS formation decreases with increasing activation time, which is also seen from Figs. 6 and 7 (the intensities of BaAl 2 O 4 diffraction lines in activated samples are rather low). Figs. 5 and 7 prove that at 12008C, 1 h BaAl 2 O 4 content decreases with increasing activation time, which means that the reaction is very close to an end at longer activation times and is practically complete for sample C (Fig. 7).
4. Conclusion Mechanical activation enhances the reaction of BaAl 2 Si 2 O 8 formation, the appearance of which is observed at 8008C in activated samples. The reaction path to BAS is dominantly developing via a series of intermediate reaction products – Ba-silicates. At higher temperatures or longer heating times, the reaction of BaAl 2 O 4 formation takes place. Since the great majority of BAS has already
[1] S.M. Corral, A.G. Verduch, The solid solution of celsian in silica, Trans. J. Br.Ceram. Soc. 77 (2) (1978) 40–44. [2] M. Starcewski, Resistance of celsian to some corrosive agents, Proc. Inst. Hutn. 14 (1962) 69–74. [3] J.A. Zaykoski, I.G. Talmy, Gallium and geranium substitutions in celsian, Ceram. Eng. Sci. Proc. 15 (9–10) (1994) 779–786. [4] C.H. Drummond, W.E. Lee, N.P. Bansal, M.J. Hyatt, Crystallization of a barium-aluminosilicate glass, Ceram. Eng. Sci. Proc. 10 (9–19) (1989) 1485–1502. [5] I.G. Talmy, D.A. Haught, Celsian Based (BaO–Al 2 O 3 –2SiO 2 ) Ceramics as Candidate for Randoms, in: 14th Conf. on Composite Materials and Structures, NASA Conf. Publ.3097, 1990, pp. 227– 2398. [6] S.W. Quander, A. Bandyopadhyay, P.B. Aswath, Synthesis and properties of in situ Si 3 N 4 -reinforced BaO–Al 2 O 3 –SiO 2 ceramic matrix composites, J. Mater. Sci. 32 (1997) 2021–2029. ¨ ¨ ¨ [7] J.E. Planz, H. Muller-Hesse, Untersuchungen uber Festkorperreak¨ tionen im System BaO–Al 2 O 3 –SiO 2 Teil II: Uber die Celsianbil¨ dung durch Festkorperreaktionen, Ber. Detsch. Keram. Ges. 40 (H3) (1963) 191–200. [8] S.M. Allameh, K.H. Sandhage, Synthesis of celsian (BaAl 2 Si 2 O 8 from solid Ba–Al–Al 2 O 3 –SiO 2 precursors. I. X-ray and SEM / EDX analyses of phase evolution, J. Am. Ceram. Soc. 80 (12) (1997) 3109–3126. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ Decrease of MgAl 2 O 4 formation [9] E. Kostic, temperature, Powder Technol. 92 (1997) 271–274. ˇ T. Havlik, J. Brinacin, ˇ [10] P. Balaz, R. Kammel, Non-stoichiometric niobium sulfide synthesized by mechanochemical route, J. Mater. Sci. Lett. 15 (1986) 1161–1163. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ S. Zec, Mechanical activation of the [11] E. Kostic, gamma to alpha transition on Al 2 O 3 , Powder Technol. 91 (1997) 49–54. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ S. Zec, Mechanically activated [12] E. Kostic, transition of anatase to rutile, Am. Ceram. Soc. Bull. 76 (1997) 60–64. [13] C.H. Drummond, N.P. Bansel, Crystallization behaviour and properties of BaO?Al 2 O 3 ?2SiO 2 glass matrices, Ceram. Eng. Sci. Proc. 11 (7–8) (1990) 1072–1086. [14] D. Bahat, Kinetic study on the hexacelsian–celsian phase transformation, J. Mater. Sci. 5 (1970) 803–810. [15] T. Ikeya, M. Senna, J. Mater. Sci. 22 (1987) 2497–2502. [16] G. Oehlschlegeland, W. Ohnmacht, Glastech. Ber. 48 (11) (1975) 232–236.
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Formation of celsian from mechanically activated BaCO 3 –Al 2 O 3 –SiO 2 mixtures ˇ ´ a , *, D« . Kosanovic´ b , Dj. Bahloul-Hourlier c , P. Thomas c , S.J. Kiss a S. Boskovic a
ˇ , Material Science Laboratory, POB 522, 11001 Belgrade, Yugoslavia Institute of Nuclear Sciences – Vinca b Refractory Institute Magnohrom–Kraljevo, Kraljevo, Yugoslavia c Laboratory for Ceramic Materials and Surface Treatment, University of Limoges, Limoges, France Received 26 November 1998; received in revised form 10 February 1999
Abstract A ternary mixture BaCO 3 –Al 2 O 3 –SiO 2 was mechanically activated for different lengths of time. Chemical composition of the mixture corresponded to BaAl 2 Si 2 O 8 –BAS. As a function of activation time, reaction course was followed in the temperature range 750–12008C. Reaction of celsian formation was followed using thermogravimetry as well as conventional and high-temperature X-ray diffractometry. The obtained data show that reaction rate increases with prolonged activation time, under the same conditions of thermal treatment. Formation of hexacelsian via a series of solid state reactions involving Ba-silicates, was favoured with increasing activation time. Direct formation of monoclinic celsian was retarded, however, with prolonged activation. 1999 Elsevier Science S.A. All rights reserved. Keywords: Mechanical activation; Celsian
1. Introduction Ba-aluminium silicate has attracted much attention lately because of properties which make it suitable for numerous applications. BAS is known for its low CTE (coefficient of thermal expansion) [1], excellent oxidation and reduction resistance [2], resistance to slag attack, stable dielectric properties [3] etc. Because of these properties, BAS has already been used in electroporcelains, as dielectric glassceramics in thick film technology [4], refractory glassceramics, corrosion resistant refractory materials, etc. It is also expected that BAS will be applicable for radar dome windows [5] and for high temperature ceramic matrix composites [6]. The properties which make it suitable for use in ceramic matrix composites are its high melting temperature, low density, thermodynamic stability and low CTE. BAS is therefore used as a matrix for alumina, SiC and C fibers, and Si 3 N 4 . BaAl 2 Si 2 O 8 has three polymorphic forms: monoclinic (celsian), most suitable for the abovementioned applications, hexagonal and orthorhombic which appeared during cooling of hexacelsian at 3008C. Independent of the preparation procedure, whether by glass crystallization or *Corresponding author.
solid state reaction [7], hexacelsian although metastable at these temperatures is the first to form. Its conversion to the monoclinic form, which is thermodynamically stable up to 15908C, is very sluggish. Reaction paths to BAS from BaCO 3 –Al 2 O 3 –SiO 2 mixture was studied by several authors [6,7]. Formation of BaAl 2 Si 2 O 8 was observed at 900–11508C via Ba-silicate – path (1) BaSi 2 O 5 1 Al 2 O 3 5 BaAl 2 Si 2 O 8
(1)
or via Ba-aluminate – path (2) BaAl 2 O 4 1 2SiO 2 5 BaAl 2 Si 2 O 8
(2)
whereby the activation energy for the path (2) to BAS is much higher [8] in comparison with activation energy for reaction (1). There are numerous examples in the literature on the influence of mechanical activation on both reaction rate [9,10] and phase transformation rate [11,12]. The published data [9–12] point to the increased rate of solid state reactions and phase transformations due to introduced mechanical energy in the system during activation. This method is applied in our work with the aim to enhance the reaction of BAS formation from BaCO 3 , a-Al 2 O 3 and
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00091-2
ˇ ´ et al. / Journal of Alloys and Compounds 290 (1999) 230 – 235 S. Boskovic
SiO 2 (a-quartz). On the other hand the changes of the physical conditions of the reactants including steric factors caused by mechanical activation, are expected to affect the reaction path to BAS.
2. Experimental work ˇ Mixture of BaCO 3 (‘Zorka’-Sabac), SiO 2 (a-quartz ‘Merck’), and a-Al 2 O 3 (‘Alcoa’, A-16), the composition of which corresponded to BaAl 2 Si 2 O 8 , was mechanically activated in a vibratory mill made of WC-based hard metal (Fritsch-Pulverisset-9). Twenty grams of mixture was activated for 2 and 4 h, respectively. The non-activated mixture was, however, milled for only 2 min. The changes developed during milling were followed by X-ray diffraction, specific surface area measurements by modifying BET method and thermogravimetry, performed in air at 78 min 21 heating rate. The reaction course was followed by X-ray diffraction after heat treatment of mixtures at 750 and 11508C, 30 min, as well as by high temperature X-ray diffraction in the temperature range 800–12008C (1 h annealing time, nitrogen) at a heating rate 408 min 21 .
231
Sanbornite gives, finally, BAS according to reaction path (1). In the presence of quartz, however, the reaction was slower, because some of BaCO 3 did not react with SiO 2 but it reacted with Al 2 O 3 instead to give BaAl 2 O 4 at higher temperatures [7]. This reaction leads to BAS formation via reaction path (2). Our results on the effect of mechanical activation time on the structure of BaCO 3 –Al 2 O 3 –SiO 2 mixture, nonactivated (A), as well as activated mixtures B and C are given in Fig. 1. With increasing activation time, the relative intensities of BaCO 3 , SiO 2 and Al 2 O 3 diffraction lines decrease, and the broadening of the diffraction lines is evident. New compounds were not detected. During the activation procedure, particles get micronized, which is
3. Results and discussion Formation of BaAl 2 Si 2 O 8 is a complex process involving a series of intermediate reaction steps. Quander et al. [6] found that starting with BaCO 3 , Al 2 O 3 and SiO 2 (amorphous), BaSiO 3 was the first intermediate reaction product. The reaction thereafter proceeded via a series of reactions giving Ba-silicates ever richer in SiO 2 , until BaSi 2 O 5 (sanbornite) was formed. Sanbornite can directly react with alumina to give the BAS as a final reaction product, according to reaction (1). Planz ¨ and Muhler-Hesse [7] showed that starting from BaCO 3 , g-Al 2 O 3 and SiO 2 (amorphous or quartz), BaSiO 3 and Ba 2 SiO 4 formed first at 6508C. The reaction thereafter, proceeds via several reaction steps [8], i.e.: 2BaCO 3 1 SiO 2 5 Ba 2 SiO 4 1 2CO 2( g)
(3)
Ba 2 SiO 4 1 SiO 2 5 2BaSiO 3
(4)
BaCO 3 1 SiO 2 5 BaSiO 3 1 CO 2( g)
(4a)
2BaSiO 3 1 SiO 2 5 Ba 2 Si 3 O 8
(5)
5Ba 2 Si 3 O 8 1 SiO 2 5 2Ba 5 Si 8 O 21
(6)
3Ba 5 Si 8 O 21 1 SiO 2 5 5Ba 3 Si 5 O 13
(7)
Ba 3 Si 5 O 13 1 SiO 2 5 3BaSi 2 O 5
(8)
Fig. 1. X-ray patterns of non-activated (A) and activated B and C samples.
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Table 1 Specific surface area of activated and non-activated mixtures Sample
Specific surface area (m 2 g 21 )
A – non-activated B – 2 h activated C – 4 h activated
2.4 3.3 1.1
also seen from Table 1, according to specific surface area measurement. Decreased value of specific surface area for sample activated for 4 h, can be a consequence of formation of new bondings (between particles) which do not take part in the absorption of nitrogen during specific surface area measurements, as well as of the increased ability for agglomeration of very fine particles obtained after long time activation. Besides micronization, amorphization and the distortion of the crystal lattice is brought about during mechanical activation [10,11], which is recognized from the shape of the diffraction lines (Fig. 1). Thermogravimetric analysis of the three mixtures (Fig. 2), revealed that with prolonged activation time the onset of weight loss was shifted towards lower temperatures. Moreover, the reaction course of weight loss appeared to be different for different activation time. While in nonactivated mixture decomposition starts at 10008C, and proceeds in a single step, with prolonged activation time, two-step process gradually becomes evident. At the same time, the onset of weight loss was shifted below 5008C. Total weight loss decreased with activation time, indicating that BaCO 3 got partially decomposed during mechanical treatment. This was proved by separate experiments on TG balance with vibrated and non-vibrated pure BaCO 3 . Total weight loss was 23 and 25% for activated and non-activated BaCO 3 , respectively. However, BaO was not detected in vibrated mixtures, possibly because of the small quantity and fine particle size. Mentioned reaction steps are more clearly seen from Fig. 3 in which the first derivative of weight loss vs. time is presented, at a constant heating rate (78 min 21 ). The maximum of the curve for non-activated sample is placed at 11608C, while
Fig. 2. Weight loss of non-activated (A) and activated B and C samples.
Fig. 3. dw / dt (w5Dm /m) vs. time, for activated and non-activated samples.
the maxima for the activated mixtures appeared at ¯7508C and 11608C for first (I) and second (II) reaction steps, respectively. In order to obtain the information on how far the reaction advanced in the range of the first and second maxima in Fig. 3, samples were heat treated at 750 and 11608C for 30 min. X-ray analysis of samples heated at 7508C (Fig. 4), shows that the relative intensities of the diffraction lines of all the reactants decrease with increasing activation time. The advanced reaction in activated mixtures in comparison with non-activated one, is easily recognized on the basis of the type of Ba-silicate formed under the same conditions of thermal treatment (see reactions 2–8). Namely, in the early stages of the BAS formation Ba 2 SiO 4 is formed in non-activated sample, while sanbornite is formed in both activated samples (reaction 8), which means that in these samples the convenient conditions are created for immediate development of the reaction (1). In all samples A, B and C heated at 11608C for 30 min (reaction step II, Fig. 3), the reaction advanced (Fig. 5) in comparison with data for 7508C (Fig. 4). In non-activated sample A, beside celsian, Ba 2 SiO 4 , Ba 2 Si 3 O 8 and BaAl 2 O 4 are detected. Sample B (2 h activated) contains hexacelsian (Fig. 5, middle), which is the dominant phase. Besides, celsian, alumina and Ba-silicates are detected. Hexacelsian is also the dominant phase in the 4-h activated sample C (Fig. 5, bottom), while celsian content is very low. In addition, alumina and Ba-silicates were detected. Also the results in Fig. 6 show that celsian is present in non-activated mixture A, both hexacelsian and celsian are present in 2-h activated mixture B, while single phase hexacelsian was detected in 4-h activated sample. It is very unusual for celsian to be formed during the reaction, without previous appearance of hexacelsian, although there are such examples in the literature [13]. It is a common feature of the reaction that hexacelsian forms first, and during the heat treatment it gets converted into monoclinic form. On the other hand it was shown [14] that celsian is
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Fig. 4. X-ray patterns of samples A, B and C heated at 7508C, 30 min.
Fig. 5. X-ray patterns of samples A, B and C, heated at 11608C, 30 min.
easily nucleated in the bulk while hexacelsian is nucleated at the surface of the particles. Taking this into account it may be clear why in our case only the celsian phase was formed in coarse grained, non-activated sample A. In activated samples, which are composed of very fine particles with developed, freshly created surface, the conditions for nucleation of hexagonal celsian are created. The hexagonal phase is the dominating phase becoming also a single phase for sample C at 12008C (Fig. 6). These data (Figs. 4 and 5) show not only that the reaction to BAS is enhanced by mechanical activation, but also that differ-
ent polymorphs of BaAl 2 Si 2 O 8 are formed depending on activation time. In the case of the studied ternary mixture, under nonisothermal conditions (Figs. 2 and 3), reaction takes place at higher temperature in non-activated mixture, in a single step, and is followed by weight loss. It should be added that the reaction was at the very beginning at 7508C in samples A, according to the results in Fig. 4. Because of high temperature which favors BaAl 2 O 4 formation, and because of less homogeneous distribution of constituents particles, BaCO 3 obviously reacts both with SiO 2 and
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Al 2 O 3 . As a result Ba 2 SiO 4 and BaAl 2 O 4 are formed simultaneously (BaAl 2 O 4 is lower in quantity), by reactions (3) and (9), BaCO 3 1 Al 2 O 3 5 BaAl 2 O 4 1 CO 2(g)
(9)
which is thermodynamically possible below 10508C [8]. Under the same conditions, the reaction is shifted towards lower temperatures in activated mixtures due to increased contact area, fine particles size, better homogenization, distorted lattice, all provoked by mechanical activation [15]. In these samples (B and C), BaAl 2 O 4 was not detected. It seems very likely that in the first reaction step (Figs. 2 and 4) very fine, active particles of BaCO 3 and SiO 2 react forming silicates rich in Ba, at the very beginning of the reaction. These Ba rich silicates form through series of the reactions (3–8) sanbornite prior to BAS formation. Weight loss during this reaction step comes up to 65% of the total weight loss. During the second step (Figs. 2 and 5), the rest of the BaCO 3 which may have been blocked by the reaction product layer, reacts further with SiO 2 (followed by 35% of weight loss) to give Ba-silicates. At 11608C, the reaction is incomplete in each of the samples, but very much close to an end in activated samples (without weight change). On the basis of above-presented data, the change of the intermediate reaction products as a consequence of the activation, may be connected with the granulometry of the reactants. Since staring alumina particles are much finer (Alcoa A-16, d av ¯0.5 mm) in comparison with silica particles (Merck, d av ,50 mm), in non-activated mixtures formation of BaAl 2 O 4 is favoured due to steric factors (e.g. number of contacts of reacting phases). In activated samples, however, considerable particle reduction of quartz takes place, which brings about the increasing of the number of particle contacts of reacting phases. Thereby the formation of Ba-silicates becomes easier and even becomes the dominant process in the formation of the intermediate products. On the basis of these results it is obvious that even the reaction mechanism of the ternary compound formation is changed by applying mechanical activation. Experimental data obtained under isothermal conditions after 1 h of annealing in high-temperature X-ray device (Fig. 6) indicate that some amount of BaAl 2 O 4 may be formed by reaction (9) [8] in sample A. However, the fact that the quantity of BaAl 2 O 4 increases, while the content of Ba 2 SiO 4 decreases simultaneously (Fig. 6A), at higher temperatures at which BaCO 3 had already been consumed points to the development of another reaction (10) which is thermodynamically possible at these temperatures [8], i.e. Ba 2 SiO 4 1 Al 2 O 3 5 BaAl 2 O 4 1 BaSiO 3
Fig. 6. High temperature X-ray patterns of samples A, B and C, 1 h annealing. (^) BaAl 2 O 4 , (s) Ba 2 SiO 4 , (s) Al 2 O 3 , (h) hexacelsian, (c) celsian, (*) Pt sample holder.
(10)
by which Ba-aluminate is formed. In activated samples, BaAl 2 O 4 first appears after BaCO 3 had been consumed (Fig. 6) which indicates that BaAl 2 O 4 is also formed in these samples by reaction (10).
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been formed, before BaAl 2 O 4 appeared, a smaller amount of BAS is formed via the reaction path involving BaAl 2 O 4 . The reaction path via BaAl 2 O 4 plays a more considerable role in non-activated mixture, in comparison with activated samples. Thus, mechanical activation affects the mechanism of BAS formation. With prolonged time of mechanical activation the relative content of monoclinic celsian decreases, while the relative amount of hexacelsian increases at the same time. The conditions for hexacelsian nucleation in activated samples may be connected with increased particles surface which favours the nucleation of hexacelsian.
References Fig. 7. High temperature X-ray patterns of A, B and C samples at 12008C, 1 h. (h) Hexacelsian, (c) celsian, (*) Pt sample holder.
In the final reaction step of BAS formation, BaAl 2 O 4 reacts with SiO 2 . However, SiO 2 was not detected in our samples above 9008C. This can be due to a small amount of unreacted SiO 2 and at higher temperatures, due to liquid phase formation. Namely, according to BaO–Al 2 O 3 –SiO 2 , phase diagram [16] eutectic liquid appears at 11248C, between celsian, sanbornite and tridymite. BaAl 2 O 4 was detected also in mixture BaCO 3 –gAl 2 O 3 –SiO 2 (amorphous) [7], above 8508C which is consistent with our data. The presence of BaAl 2 O 4 which appears also in activated samples only at longer heating times, as pointed out already, indicates the development of reaction path (2), at higher temperatures (Fig. 6) in all the investigated samples to a small extent. According to the results in Fig. 5, the contribution of reaction path (2) to overall reaction of BAS formation decreases with increasing activation time, which is also seen from Figs. 6 and 7 (the intensities of BaAl 2 O 4 diffraction lines in activated samples are rather low). Figs. 5 and 7 prove that at 12008C, 1 h BaAl 2 O 4 content decreases with increasing activation time, which means that the reaction is very close to an end at longer activation times and is practically complete for sample C (Fig. 7).
4. Conclusion Mechanical activation enhances the reaction of BaAl 2 Si 2 O 8 formation, the appearance of which is observed at 8008C in activated samples. The reaction path to BAS is dominantly developing via a series of intermediate reaction products – Ba-silicates. At higher temperatures or longer heating times, the reaction of BaAl 2 O 4 formation takes place. Since the great majority of BAS has already
[1] S.M. Corral, A.G. Verduch, The solid solution of celsian in silica, Trans. J. Br.Ceram. Soc. 77 (2) (1978) 40–44. [2] M. Starcewski, Resistance of celsian to some corrosive agents, Proc. Inst. Hutn. 14 (1962) 69–74. [3] J.A. Zaykoski, I.G. Talmy, Gallium and geranium substitutions in celsian, Ceram. Eng. Sci. Proc. 15 (9–10) (1994) 779–786. [4] C.H. Drummond, W.E. Lee, N.P. Bansal, M.J. Hyatt, Crystallization of a barium-aluminosilicate glass, Ceram. Eng. Sci. Proc. 10 (9–19) (1989) 1485–1502. [5] I.G. Talmy, D.A. Haught, Celsian Based (BaO–Al 2 O 3 –2SiO 2 ) Ceramics as Candidate for Randoms, in: 14th Conf. on Composite Materials and Structures, NASA Conf. Publ.3097, 1990, pp. 227– 2398. [6] S.W. Quander, A. Bandyopadhyay, P.B. Aswath, Synthesis and properties of in situ Si 3 N 4 -reinforced BaO–Al 2 O 3 –SiO 2 ceramic matrix composites, J. Mater. Sci. 32 (1997) 2021–2029. ¨ ¨ ¨ [7] J.E. Planz, H. Muller-Hesse, Untersuchungen uber Festkorperreak¨ tionen im System BaO–Al 2 O 3 –SiO 2 Teil II: Uber die Celsianbil¨ dung durch Festkorperreaktionen, Ber. Detsch. Keram. Ges. 40 (H3) (1963) 191–200. [8] S.M. Allameh, K.H. Sandhage, Synthesis of celsian (BaAl 2 Si 2 O 8 from solid Ba–Al–Al 2 O 3 –SiO 2 precursors. I. X-ray and SEM / EDX analyses of phase evolution, J. Am. Ceram. Soc. 80 (12) (1997) 3109–3126. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ Decrease of MgAl 2 O 4 formation [9] E. Kostic, temperature, Powder Technol. 92 (1997) 271–274. ˇ T. Havlik, J. Brinacin, ˇ [10] P. Balaz, R. Kammel, Non-stoichiometric niobium sulfide synthesized by mechanochemical route, J. Mater. Sci. Lett. 15 (1986) 1161–1163. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ S. Zec, Mechanical activation of the [11] E. Kostic, gamma to alpha transition on Al 2 O 3 , Powder Technol. 91 (1997) 49–54. ˇ Kiss, S. Boskovic, ´ S. ˇ ´ S. Zec, Mechanically activated [12] E. Kostic, transition of anatase to rutile, Am. Ceram. Soc. Bull. 76 (1997) 60–64. [13] C.H. Drummond, N.P. Bansel, Crystallization behaviour and properties of BaO?Al 2 O 3 ?2SiO 2 glass matrices, Ceram. Eng. Sci. Proc. 11 (7–8) (1990) 1072–1086. [14] D. Bahat, Kinetic study on the hexacelsian–celsian phase transformation, J. Mater. Sci. 5 (1970) 803–810. [15] T. Ikeya, M. Senna, J. Mater. Sci. 22 (1987) 2497–2502. [16] G. Oehlschlegeland, W. Ohnmacht, Glastech. Ber. 48 (11) (1975) 232–236.