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Preparation of barium hexa-aluminate through a hydrothermal precipitation–calcination route and characterization of intermediate and final products
November 2002
Materials Letters 56 (2002) 873 – 879 www.elsevier.com/locate/matlet
Preparation of barium hexa-aluminate through a hydrothermal precipitation–calcination route and characterization of intermediate and final products D. Mishra a, S. Anand a,*, R.K. Panda b, R.P. Das a a
Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India b Berhampur University, Berhampur, India
Received 6 September 2001; received in revised form 7 February 2002; accepted 8 February 2002
Abstract A high surface area (f 22 m2/g) micro-sized (0.3 – 2 A) barium hexa-aluminate powder was prepared by the calcination (at 1400 jC for 2 h) of hydrothermally prepared precursors of orthorhombic barium carbonate and boehmite. The precursors were obtained by a hydrothermal treatment (at 180 jC, for 1 h) of aqueous solutions of barium and aluminum nitrates of appropriate compositions in the presence of aqueous urea as the neutralizing precipitant. Results of XRD, TG-DTA and SEM measurements suggest the most probable reaction sequence for the preparation of barium hexa-alumnate ceramic powder as: (i) the hydrothermal precipitation of orthorhombic BaCO3 and boehmite (g-AlOOH) precursors from the aqueous Ba and Al nitrates in the presence of urea, (ii) the formation of an interim mixture of g-Al2O3, BaCO3 and Ba – h1-alumina in the temperature range of 800 – 1200 jC, and finally (iii) the formation of a ceramic powder corresponding to the composition BaO6Al2O3 having halumina type structure at f 1400 jC. The final material did not show presence of any other crystalline phases like alumina, baria, barium mono-aluminate and tri-barium mono-aluminate phases. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal; Calcination; Ba – h-alumina; Barium hexa-aluminate
1. Introduction It is well known that metal aluminates find extensive applications as refractories, high alumina cements useful for nuclear constructions, pigment and glazes, combustion catalysts, display phosphors, to cite a few [1– 6]. Barium is known to form several types of well studied stoichiometric aluminates with compositions as BaOAl2O3 (barium mono-aluminate), 3BaOAl2O3
*
Corresponding author. E-mail address: [email protected] (S. Anand).
(tri-barium mono-aluminate), BaO6Al2O3 (barium hexa-aluminate) as well as a large number of illdefined nonstoichiometric Ba –O – Al complex aluminates. The various routes employed for preparation of these aluminates are: ceramic, sol – gel followed by calcination, spray ICP and spray drying. The initial compositions of barium and aluminum are chosen in the desired ratio to obtain different types of aluminates. Barium hexa-aluminate has been prepared by ceramic route [7 –10], spray-ICP technique [11], spray drying technique [12], sol –gel syntheses followed by calcination [13 –16] and heat treatment of the hydrolysis product(s) of metal-alkoxides obtained through
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 6 3 0 - 4
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D. Mishra et al. / Materials Letters 56 (2002) 873–879
sol –gel routes [17,18]. A recent work has been reported describing preparation and properties of alumina –baria derived nano-composites, in which the preparation of high surface area barium aluminates were obtained by a combination of sol – gel/microwave/hydrothermal-calcination route [19]. Usually, the barium hexa-aluminate or trivalent metal substituted barium hexa-aluminates are a combination of barium deficient composition Ba1 yMxIIIAl12 xO19 y (known as h1 phase), and a barium-rich composition Ba1 + y MxIIIAl12 xO19 + y ( known as hII phase), where x and y are the fractions and MIII could be a trivalent substituent [20 – 24] absent or present in the structure. We have reported recently [25] the preparation and characterization of high surface area nanosized hexagonal powders of BaAl2O4 through a combination of hydrothermal precipitation – calcination route. The present work describes a convenient synthesis of barium hexa-aluminate powders of high surface area from barium and aluminum nitrate salts through a similar hydrothermal precipitation– calcination route using urea as a neutralizing agent.
2. Experimental Reagent grade Ba(NO 3)2 and Al(NO3)39H2O (BDH India) were dissolved in distilled water to obtain a 0.1- and 1.0-M solution, respectively. One hundred milliliters of the barium nitrate and 120 ml of the aluminum nitrate solutions were thoroughly mixed and the volume was made up to 1000 ml by dilution with distilled water. To this solution a weighed amount of urea was added and the contents were transferred to a Parr autoclave (Model 4542) of 2-L capacity for hydrothermal precipitation at 180 jC. After 1 h at 180 jC, the autoclave was quickly cooled to room temperature, the products were filtered and washed thoroughly with distilled water to remove washable impurities and dried overnight inside an air oven maintained at 95– 100 jC. These washed and air oven dried powder samples were subjected to calcination at the desired temperatures inside a furnace for 2 h. These samples were also used for thermal analysis and other characterization studies. X-ray diffraction (XRD) patterns of the samples were obtained with a Philips Powder Diffractometer model PW 1710 in a range of 10 – 70j (2h) at a
scanning rate of 2j/min, using a Ni-filtered Cu target. A LEO-435VP Model scanning electron microscope was used to study the morphology. TG-DTA traces were obtained by a Stanton Thermal Analyzer (STA740 Model) and measurements were carried out from room temperature to 1400 jC. Surface area measurements were done using a Monosorb Quantachrome instrument following the BET method. The washed and air oven dried (but uncalcined) precursor powder sample P was coded, for convenience, according to its hydrothermal treatment temperature [T, jC] and duration (n, in h): P-H[T(n)]. Similarly, the calcined samples C were coded as CH[calcination temperature T, jC], and calcination time (n, h): C-H[T(n)].
3. Results and discussion 3.1. XRD Results The X-ray diffraction (XRD) patterns of the hydrothermally obtained precursor sample P-H[180(1)] and the calcined samples C-H[800(2)], C-H[1000(2)], CH[1200(2)] and C-H[1400(2)], obtained after calcination (for 2 h) of the said precursor at 800, 1000, 1200 and 1400 jC are shown in Fig. 1a, b, c, d and e, respectively. The XRD pattern of the P-H[180(1)] powder in Fig. 1a exhibits (i) four broad peaks at 2h = 14.2, 28.0, 38.0 and 49.0j corresponding to the presence of crystallites of orthorhombic boehmite [26a], and (ii) relatively sharper and weaker peaks at 2h = 23.80, 24.07 and 27.75j corresponding to the presence of crystalline (orthorhombic) a-BaCO3 [26b]. Similar observations were made earlier in the case of barium mono-aluminate [25]. The XRD pattern of the calcined sample C-H[800(2)] (Fig. 1b) shows the presence of crystalline BaCO3 and g-Al2O3 [26c], the two prominent peaks corresponding to the latter phase appeared at 2h = 67.2 and 44.8j with relative intensities (R.I.) of 30% and 33%, respectively. However, in the XRD pattern of the calcined sample C-H[1000(2)] (Fig. 1c) 100% R.I. g-Al2O3 peak at 2h = 67j was not present even as a very low intensity peak, hence, it is inferred that the aluminum oxide is no longer crystalline. Fig. 1c shows all the reflections due to crystalline orthorhombic BaCO3. In case of BaAl2O4 [25], such transformation occurred at a lower temperature of
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about 500 jC. The XRD pattern of the calcined sample C-H[1200(2)] also does not show peak at 2h = 67j (Fig. 1d) but exhibits reflections due to the crystalline barium carbonate and distinct peaks for a developing Ba– O – Al complex oxide corresponding to a composition Ba –h1 – Al2O3 (26d). Calcination at a still higher temperature of 1400 jC produces a powder sample CH[1400(2)], the XRD of which (Fig. 1e) shows reflections indicating formation of Ba –h1 – Al2O3, Ba – hII – Al2O3 (26e) and BaAl12O19 (h1hII) (26f) phases of barium hexa-aluminate ( Table 1). The barium hexaaluminate prepared by the present route has Ba – hAlumina type structure. 3.2. TG-DTA results
Fig. 1. The XRD pattern of (a) P-H[180(1)], (b) C-H[800(2)], (c) CH[1000(2)], (d) C-H[1200(2)] and (e) C-H[1400(2)]. (4) gAlOOH, ( ) a-BaCO3, (q) g-Al2O3, (z) Ba – hI-Al2O3, (E) Ba – hII-Al2O3, (o) Ba – hIhII-Al2O3, (?) not found in the JCPDS file for h-alumina.
The TG-DTA results of the hydrothermally prepared sample P-H[180(1)] is shown in Fig. 2. The TGA trace shows three regions of weight loss: (i) around 4% weight loss in the 100– 250 jC region (which is most probably due to the loss of free and bound water molecules) [27,28], (ii) around 12% weight loss in the 250– 550 jC region (which could be due to the dehydroxylation/dehydration of boehmites) [29,30] and (iii) about 4% continuous weight loss in the f 850– 1250 jC associated with the decomposition of BaCO3 [31,32]. The latter two weight losses are in agreement with the calculated weight loss expected for dehydroxylation/dehydration of boehmite and decomposition of BaCO3 from the stoichiometric mixture of BaCO312AlOOH to the composition BaO6Al2O3. The slow decomposition of BaCO3 in the present case occurs at a wide temperature range up to f 1250 jC beyond which there is no weight loss. Further the weight loss from the precursor in the 100– 250 jC region suggests that the boehmites precipitated in the present case might contain a slight excess of water than that accounted for in the stoichiometric composition Al2O3H2O only. This is evidenced from our earlier observation on the hydrothermally precipitated boehmites that the excess water associated with boehmite/ pseudoboehmite is lost within 250 jC [33]. Interestingly, the DTA trace of the uncalcined precursor PH[180(1)] shows only one endothermic peak centered at f 500 jC, which corresponds to the dehydroxylation/dehydration of boehmite; no peaks either at f 800 or at f 1000 – 1300 jC region are apparent, which might have corresponded to the phase transformation
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Table 1 Comparison of XRD data for the experimentally obtained BaO6Al2O3 powder with the literature (JCPDS) values (important reflections are listed) Ba – hI-Al2O3, Ref. [26d]
Ba – hII-Al2O3, Ref. [26e]
BaAl12O19, Ref. [26f ]
C-H [1400(2)], present work
d (Aj)
R.I. (%)
d (Aj)
R.I. (%)
d (Aj)
R.I. (%)
2h (j)
d (Aj)
R.I. (%)
11.40 5.72 4.83 4.74 4.45 4.07 – 3.68 3.31 – 2.79 2.71 2.69 2.62 – 2.50 2.30 2.27 2.05 2.04 1.93 1.90 1.80 1.76 1.74 1.62 – 1.57 1.39
40 10 60 80 80 60 – 40 60 – 90 60 90 10 – 100 40 60 60 80 10 20 60 60 60 60 – 60 90
11.40 5.69 4.84 4.73 4.48 4.09 – 3.70 3.33 3.27 2.79 2.71 – 2.62 – 2.51 2.31 2.29 – – 1.93 1.90 1.81 1.75 – 1.62 – 1.57 1.40
60 60 60 40 100 10 – 60 80 20 100 100 – 10 – 100 20 20 – – 10 10 20 10 – 20 – 20 90
– – 4.86 4.75 4.47 4.10 – 3.70 3.30 – 2.80 2.71 – 2.63 2.52 – 2.31 2.29 2.07 2.05 1.93 1.91 1.81 1.75 – 1.62 – 1.57 1.40
– – 10 60 60 20 – 35 50 – 75 75 – 88 100 – 10 10 20 35 5 5 5 20 – 10 – 15 40
– – – 19.65 19.84 20.31 20.75 24.19 26.92 – 32.20 33.14 33.46 34.41 35.13 36.04 39.11 39.72 43.34 44.42 46.95 47.83 50.67 51.82 52.63 57.57 57.85 58.79 67.01
– – – 4.76 4.47 4.37 4.08 3.68 3.31 – 2.77 2.70 2.68 2.61 2.55 2.49 2.30 2.27 2.08 2.04 1.93 1.90 1.80 1.76 1.74 1.60 1.59 1.57 1.39
– – – 36.0 33.1 36.0 6.2 49.0 17.0 – 37.5 100 90 25.0 45.6 70.10 20.3 27.6 33.1 64.0 19.1 19.1 11.4 19.1 37.5 74.4 81.1 58.1 76.6
of alumina and/or decomposition of BaCO3, although the presence of BaCO3 is indicated from XRD. Similarly, no exothermic peak is apparent in the 1000 – 1300 jC region which could specifically indicate the formation of a barium hexa-aluminate phase. In a recent work also [19], similar observations were made during the formation of barium hexa-aluminate. A plausible explanation for the above two observations could be that the slow decomposition of BaCO3 in the temperature range of 900– 1300 jC [19,34] probably downplays the peaks expected for the formation of the barium hexa-aluminate. 3.3. Surface area results The surface area values of the calcined samples CH[800(2)], C-H[1000(2)], C-H[1200(2)] and C-
H[1400(2)] (derived from the hydrothermally treated precursor P-H[180(1)] after calcination at the specified temperature) drawn against the calcination temperature are shown in Fig. 3. The surface area of the hydrothermally treated sample P-H[180(1)] was f 108 m2/g. It is observed that by increasing the calcination temperature to 1000 jC, the surface area increases to 178.5 m2/g and then decreases with increase in the calcination temperature, the decrease being more prominent when the calcination temperature increases from 1200 to 1400 jC. The marked increase in the surface area of the low temperature calcined sample (up to 1000 jC) vis a` vis that of the uncalcined sample is difficult to explain but similar observations have been made during preparation of barium mono-aluminate where a maximum in the surface area was observed at 500 jC [25]. Further
D. Mishra et al. / Materials Letters 56 (2002) 873–879
877
Fig. 2. The TG-DTA of hydrothermally prepared precursor P-H[180(1)].
work is required to clearly assign reasons for the trend observed.
agglomerated form. However, the particle diameter value calculated from this micrograph is estimated as about 1 Am.
3.4. SEM results 3.5. The sequence of reactions The SEM micrograph of the calcined sample CH[1400(2)] is shown in Fig. 4. Irregular block shaped particles having layered structures can be seen from the micrograph. This type of layered structure is associated with barium h/hexa-aluminates [35]. As can be seen from the micrograph the particles show a large variation in their sizes as most of them are in an
From the XRD and TG-DTA studies the most probable sequence of reactions and phase transformations taking place for the formation of ‘‘barium hexaaluminates’’ can be suggested by Eqs. (1) – (9). It may be noted here that the chemical analysis of the samples confirms to the compositions almost quantitatively.
Fig. 3. Variation of surface are with calcination temperature of hydrothermally prepared precursor of barium hexa-aluminate P-H[180(1)].
Fig. 4. The SEM micrograph of the calcined sample C-H[1400(2)].
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(a) During hydrothermal precipitation at 180 jC: (i) Hydrolysis of urea at 180 jC NH2 CO NH2 þ H2 O ! 2NH4 OH þ CO2 ðgÞ
ð1Þ
(iii) At the 800 – 1000 jC region (from XRD, TG-DTA) 6Al2 O3 ðcÞ þ BaCO3 ðcÞ ! X BaO ðby partÞ þ 6Al2 O3 ðams: sÞ þ X CO2 ðgÞ þ ð1 X ÞBaCO3 ðcÞ
(ii) Reaction with barium nitrate
ð8Þ
(iv) At 1200 jC (from XRD and TG-DTA), BaðNO3 Þ2 þ CO2 ðgÞ þ H2 O ! BaCO3 ðcÞ þ 2HNO3
ð2Þ
2HNO3 þ 2NH4 OH ! 2NH4 NO3 þ H2 O
ð3Þ
with the overall reaction of formation of BaCO3 as: BaðNO3 Þ2 þ NH2 CO NH2 þ H2 O ! BaCO3 ðcÞ þ 2NH4 NO3
ð4Þ
(iii) The formation of boehmite 2AlðNO3 Þ3 þ 7NH2 CO NH2 þ ð14 þ 2xÞH2 O ! 2AlOOH xH2 OðcÞ þ 6NH4 NO3 þ 4ðNH4 Þ2 CO3 þ 3CO2 ðgÞ
! 12AlOOH xH2 OðcÞ þ 36NH4 NO3 ð5bÞ
(b) Calcination at different temperature regions: (i) In the 100 –250 jC (from TG-DTA)
! 12AlOOHðcÞ þ BaCO3 ðcÞ þ xH2 O
ð9Þ
(v) At 1400 jC (from XRD) Mixture of Ba – hI-Al2O3(c), BaCO3(c, t) and Al2O3(ams. s) when calcined to 1400 jC gives Ba – hI-Al2O3, Ba –hII-Al2O3 Ba – hI-hIIAl2O3 (the exact percentage of each phase in the final product cannot be ascertained) (amp.: amorphous, s: solid, c: crystalline or pollycrystalline, g: gaseous, t: trace and all other are aqueous species).
ð5aÞ
12AlðNO3 Þ3 þ 42NH2 CO NH2 þ ð84 þ 12xÞH2 O
BaCO3 ðcÞ þ 12AlOOH xH2 OðcÞ
! Mixture of Ba hI Al2 O3 ðcÞ, BaCO3 ðc, tÞ andAl2 O3 ðams: sÞ
4. Conclusions
or
þ 24ðNH4 Þ2 CO3 þ 18CO2 ðgÞ
X BaO ðby partÞ þ 6Al2 O3 ðams: sÞ þ ð1 X ÞBaCO3 ðcÞ ðunreactedÞ
The present study describes a hydrothermal precipitation –calcination route wherein the hydrothermally produced precursors of orthorhombic BaCO3 and crystalline boehmite after calcinations at 1400 jC for 2 h produced micron-sized high surface area ceramic powders of barium hexa-aluminate having Ba – h-Alumina type structure. The ceramic powder finally obtained was found to be free from any contaminants such as g-/ u-/y-/a-Al2O3, BaO, BaCO3, BaAl2O4 and Ba3Al2O6. The increasing and decreasing trends in surface area values of intermediates obtained at different calcination temperatures are difficult to explain, perhaps associated with the presence of intermediates.
ð6Þ
(ii) In the 250– 550 jC region (from TG-DTA)
Acknowledgements
BaCO3 ðcÞ þ 12AlOOHðcÞ ! 6Al2 O3 ðcÞ þ 6H2 O þ BaCO3 ðcÞ
The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar, for his kind permission to publish this paper, and
ð7Þ
D. Mishra et al. / Materials Letters 56 (2002) 873–879
wish to thank Dr. S.C. Das, Head, Department of Hydrometallurgy.
References [1] W.H. Gitzen, Alumina as a Ceramic Material, Am. Ceram. Society, Columbus, OH, 1970, p. 141. [2] R. Prasad, L.A. Kennedy, E. Ruckenstein, Catal. Rev.-Sci. Eng. 26 (1984) 10. [3] D.L. Trimn, Appl. Catal. 7 (1983) 249. [4] J.J. Kingsley, K. Suresh, K.C. Patil, J. Mater. Sci. 25 (1990) 1305. [5] A. Neville, High Alumina Cement, Wiley, New York, 1975, Part 1. [6] A.N.L. Stevels, J. Electrochem. Soc. 125 (4) (1978) 588. [7] J.C. Debsikdar, O.S. Sowemimo, J. Mater. Sci. Lett. 11 (10) (1992) 637. [8] O. Masaaki, N. Kazumi, K. Katsuhide, Patent no. 93/280, 861, November 1993. [9] O. Masaaki, N. Kazumi, K. Katsuhide, Patent no. 93/280, 862, November 1993. [10] Z.R. Kadyrova, N.A. Sirazhiddinov, S.Kh. Tuganova, Uzb. Khim. Zn. 16 – 18 (3) (1995) 36B. [11] A. Douy, Key Eng. Mater., Part II 132 – 136 (1994) 101. [12] M. Uzawa, M. Kayawa, Y. Syono, Mater. Lett. 17 (1993) 187. [13] T.J. Davies, W.A. Al-Douri, Q.G. Chen, H.G. Emblem, J. Mater. Sci. Lett. 16 (20) (1997) 1673. [14] D. Chen, L. Yang, Xiamen Daxue Xuebao, Ziran Kexueban 36 (2) (1997) 225. [15] G. Groppi, C. Cristiani, P. Forzatti, M. Belloto, J. Mater. Sci. 29 (1994) 3441. [16] T.J. Davies, W.A. Al-Douri, M. Biedermann, Q.G. Chen, H.G. Emblem, J. Mater. Sci. Lett. 15 (6) (1996) 482. [17] M. Machida, K. Egguchi, H. Arai, J. Catal. 103 (1987) 385. [18] M. Machida, K. Egguchi, H. Arai, Bull. Chem. Soc. Jpn. 61 (1998) 3659. [19] B. Djuricic, S. Pickering, D. Mcgarry, J. Mater. Sci. 34 (1999) 2685. [20] N. Iyi, Z. Inoue, S. takekawa, S. Kimura, J. Solid State Chem. 51 (1984) 66.
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[21] P.E.D. Morgan, T.M. Shaw, Mater. Res. Bull. 18 (1983) 539. [22] J.G. Park, A. Cormack, J. Solid State Chem. 121 (1996) 278. [23] N. Iyi, S. Takekawa, Y. bando, S. Kimura, J. Solid State Chem. 47 (1983) 34. [24] G. Groppi, F. Asandri, M. Belloto, C. Cristiani, P. Forzatti, J. Solid State Chem. 114 (1995) 326. [25] D. Mishra, S. Anand, R.K. Panda, R.P. Das, J. Am. Ceram. Soc., in press. [26a] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Duffraction Data, Swathmore, PA, File nos. 5 – 378, 1994. [26b] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Duffraction Data, Swathmore, PA, File nos. 21 – 1307, 1994. [26c] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Duffraction Data, Swathmore, PA, File nos. 17 – 306, 1994. [26d] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Duffraction Data, Swathmore, PA, File nos. 30 – 128, 1994. [26e] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Duffraction Data, Swathmore, PA, File nos. 33 – 129, 1994. [26f] Joint Committee On Powder Diffraction Standard (JCPDS), International Center For Diffraction Data, Swathmore, PA, File nos. 14 – 107, 1994. [27] W.J. Smothers, I. Chiang, Differential Thermal Analysis Theory and Practice, Chemical Publishing, New York, 1958. [28] D. Mishra, S. Anand, R.K. Panda, R.P. Das, Mater. Lett. 42 (1 – 2) (2000) 38. [29] P.A. Buining, C. Pathamamanoharan, J.B.H. Jansen, H.N.W. Kekkerkerker, J. Am. Ceram. Soc. 73 (1990) 2385. [30] T. Tsukada, H. Segawa, A. Yasumori, K. Okada, J. Mater. Chem. 9 (1999) 549. [31] R.B. Fahime, M.I. Jaki, G.A.M. Hussien, Powder Technol. 33 (1982) 162. [32] M.I. Zaki, G.A.M. Hussien, R.B. Fahim, J. Mater. Sci. Lett., (4) (1985) 517. [33] D. Mishra, S. Anand, R.K. Panda, R.P. Das, unpublished results. [34] M. Nayak, T.N.R. Kutty, Mater. Res. Bull. 31 (2) (1996) 227. [35] A.F. Wells, Structural Inorganic Chemistry, Oxford University Press/Clarendon, 1975.
Materials Letters 56 (2002) 873 – 879 www.elsevier.com/locate/matlet
Preparation of barium hexa-aluminate through a hydrothermal precipitation–calcination route and characterization of intermediate and final products D. Mishra a, S. Anand a,*, R.K. Panda b, R.P. Das a a
Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India b Berhampur University, Berhampur, India
Received 6 September 2001; received in revised form 7 February 2002; accepted 8 February 2002
Abstract A high surface area (f 22 m2/g) micro-sized (0.3 – 2 A) barium hexa-aluminate powder was prepared by the calcination (at 1400 jC for 2 h) of hydrothermally prepared precursors of orthorhombic barium carbonate and boehmite. The precursors were obtained by a hydrothermal treatment (at 180 jC, for 1 h) of aqueous solutions of barium and aluminum nitrates of appropriate compositions in the presence of aqueous urea as the neutralizing precipitant. Results of XRD, TG-DTA and SEM measurements suggest the most probable reaction sequence for the preparation of barium hexa-alumnate ceramic powder as: (i) the hydrothermal precipitation of orthorhombic BaCO3 and boehmite (g-AlOOH) precursors from the aqueous Ba and Al nitrates in the presence of urea, (ii) the formation of an interim mixture of g-Al2O3, BaCO3 and Ba – h1-alumina in the temperature range of 800 – 1200 jC, and finally (iii) the formation of a ceramic powder corresponding to the composition BaO6Al2O3 having halumina type structure at f 1400 jC. The final material did not show presence of any other crystalline phases like alumina, baria, barium mono-aluminate and tri-barium mono-aluminate phases. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrothermal; Calcination; Ba – h-alumina; Barium hexa-aluminate
1. Introduction It is well known that metal aluminates find extensive applications as refractories, high alumina cements useful for nuclear constructions, pigment and glazes, combustion catalysts, display phosphors, to cite a few [1– 6]. Barium is known to form several types of well studied stoichiometric aluminates with compositions as BaOAl2O3 (barium mono-aluminate), 3BaOAl2O3
*
Corresponding author. E-mail address: [email protected] (S. Anand).
(tri-barium mono-aluminate), BaO6Al2O3 (barium hexa-aluminate) as well as a large number of illdefined nonstoichiometric Ba –O – Al complex aluminates. The various routes employed for preparation of these aluminates are: ceramic, sol – gel followed by calcination, spray ICP and spray drying. The initial compositions of barium and aluminum are chosen in the desired ratio to obtain different types of aluminates. Barium hexa-aluminate has been prepared by ceramic route [7 –10], spray-ICP technique [11], spray drying technique [12], sol –gel syntheses followed by calcination [13 –16] and heat treatment of the hydrolysis product(s) of metal-alkoxides obtained through
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 6 3 0 - 4
874
D. Mishra et al. / Materials Letters 56 (2002) 873–879
sol –gel routes [17,18]. A recent work has been reported describing preparation and properties of alumina –baria derived nano-composites, in which the preparation of high surface area barium aluminates were obtained by a combination of sol – gel/microwave/hydrothermal-calcination route [19]. Usually, the barium hexa-aluminate or trivalent metal substituted barium hexa-aluminates are a combination of barium deficient composition Ba1 yMxIIIAl12 xO19 y (known as h1 phase), and a barium-rich composition Ba1 + y MxIIIAl12 xO19 + y ( known as hII phase), where x and y are the fractions and MIII could be a trivalent substituent [20 – 24] absent or present in the structure. We have reported recently [25] the preparation and characterization of high surface area nanosized hexagonal powders of BaAl2O4 through a combination of hydrothermal precipitation – calcination route. The present work describes a convenient synthesis of barium hexa-aluminate powders of high surface area from barium and aluminum nitrate salts through a similar hydrothermal precipitation– calcination route using urea as a neutralizing agent.
2. Experimental Reagent grade Ba(NO 3)2 and Al(NO3)39H2O (BDH India) were dissolved in distilled water to obtain a 0.1- and 1.0-M solution, respectively. One hundred milliliters of the barium nitrate and 120 ml of the aluminum nitrate solutions were thoroughly mixed and the volume was made up to 1000 ml by dilution with distilled water. To this solution a weighed amount of urea was added and the contents were transferred to a Parr autoclave (Model 4542) of 2-L capacity for hydrothermal precipitation at 180 jC. After 1 h at 180 jC, the autoclave was quickly cooled to room temperature, the products were filtered and washed thoroughly with distilled water to remove washable impurities and dried overnight inside an air oven maintained at 95– 100 jC. These washed and air oven dried powder samples were subjected to calcination at the desired temperatures inside a furnace for 2 h. These samples were also used for thermal analysis and other characterization studies. X-ray diffraction (XRD) patterns of the samples were obtained with a Philips Powder Diffractometer model PW 1710 in a range of 10 – 70j (2h) at a
scanning rate of 2j/min, using a Ni-filtered Cu target. A LEO-435VP Model scanning electron microscope was used to study the morphology. TG-DTA traces were obtained by a Stanton Thermal Analyzer (STA740 Model) and measurements were carried out from room temperature to 1400 jC. Surface area measurements were done using a Monosorb Quantachrome instrument following the BET method. The washed and air oven dried (but uncalcined) precursor powder sample P was coded, for convenience, according to its hydrothermal treatment temperature [T, jC] and duration (n, in h): P-H[T(n)]. Similarly, the calcined samples C were coded as CH[calcination temperature T, jC], and calcination time (n, h): C-H[T(n)].
3. Results and discussion 3.1. XRD Results The X-ray diffraction (XRD) patterns of the hydrothermally obtained precursor sample P-H[180(1)] and the calcined samples C-H[800(2)], C-H[1000(2)], CH[1200(2)] and C-H[1400(2)], obtained after calcination (for 2 h) of the said precursor at 800, 1000, 1200 and 1400 jC are shown in Fig. 1a, b, c, d and e, respectively. The XRD pattern of the P-H[180(1)] powder in Fig. 1a exhibits (i) four broad peaks at 2h = 14.2, 28.0, 38.0 and 49.0j corresponding to the presence of crystallites of orthorhombic boehmite [26a], and (ii) relatively sharper and weaker peaks at 2h = 23.80, 24.07 and 27.75j corresponding to the presence of crystalline (orthorhombic) a-BaCO3 [26b]. Similar observations were made earlier in the case of barium mono-aluminate [25]. The XRD pattern of the calcined sample C-H[800(2)] (Fig. 1b) shows the presence of crystalline BaCO3 and g-Al2O3 [26c], the two prominent peaks corresponding to the latter phase appeared at 2h = 67.2 and 44.8j with relative intensities (R.I.) of 30% and 33%, respectively. However, in the XRD pattern of the calcined sample C-H[1000(2)] (Fig. 1c) 100% R.I. g-Al2O3 peak at 2h = 67j was not present even as a very low intensity peak, hence, it is inferred that the aluminum oxide is no longer crystalline. Fig. 1c shows all the reflections due to crystalline orthorhombic BaCO3. In case of BaAl2O4 [25], such transformation occurred at a lower temperature of
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about 500 jC. The XRD pattern of the calcined sample C-H[1200(2)] also does not show peak at 2h = 67j (Fig. 1d) but exhibits reflections due to the crystalline barium carbonate and distinct peaks for a developing Ba– O – Al complex oxide corresponding to a composition Ba –h1 – Al2O3 (26d). Calcination at a still higher temperature of 1400 jC produces a powder sample CH[1400(2)], the XRD of which (Fig. 1e) shows reflections indicating formation of Ba –h1 – Al2O3, Ba – hII – Al2O3 (26e) and BaAl12O19 (h1hII) (26f) phases of barium hexa-aluminate ( Table 1). The barium hexaaluminate prepared by the present route has Ba – hAlumina type structure. 3.2. TG-DTA results
Fig. 1. The XRD pattern of (a) P-H[180(1)], (b) C-H[800(2)], (c) CH[1000(2)], (d) C-H[1200(2)] and (e) C-H[1400(2)]. (4) gAlOOH, ( ) a-BaCO3, (q) g-Al2O3, (z) Ba – hI-Al2O3, (E) Ba – hII-Al2O3, (o) Ba – hIhII-Al2O3, (?) not found in the JCPDS file for h-alumina.
The TG-DTA results of the hydrothermally prepared sample P-H[180(1)] is shown in Fig. 2. The TGA trace shows three regions of weight loss: (i) around 4% weight loss in the 100– 250 jC region (which is most probably due to the loss of free and bound water molecules) [27,28], (ii) around 12% weight loss in the 250– 550 jC region (which could be due to the dehydroxylation/dehydration of boehmites) [29,30] and (iii) about 4% continuous weight loss in the f 850– 1250 jC associated with the decomposition of BaCO3 [31,32]. The latter two weight losses are in agreement with the calculated weight loss expected for dehydroxylation/dehydration of boehmite and decomposition of BaCO3 from the stoichiometric mixture of BaCO312AlOOH to the composition BaO6Al2O3. The slow decomposition of BaCO3 in the present case occurs at a wide temperature range up to f 1250 jC beyond which there is no weight loss. Further the weight loss from the precursor in the 100– 250 jC region suggests that the boehmites precipitated in the present case might contain a slight excess of water than that accounted for in the stoichiometric composition Al2O3H2O only. This is evidenced from our earlier observation on the hydrothermally precipitated boehmites that the excess water associated with boehmite/ pseudoboehmite is lost within 250 jC [33]. Interestingly, the DTA trace of the uncalcined precursor PH[180(1)] shows only one endothermic peak centered at f 500 jC, which corresponds to the dehydroxylation/dehydration of boehmite; no peaks either at f 800 or at f 1000 – 1300 jC region are apparent, which might have corresponded to the phase transformation
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Table 1 Comparison of XRD data for the experimentally obtained BaO6Al2O3 powder with the literature (JCPDS) values (important reflections are listed) Ba – hI-Al2O3, Ref. [26d]
Ba – hII-Al2O3, Ref. [26e]
BaAl12O19, Ref. [26f ]
C-H [1400(2)], present work
d (Aj)
R.I. (%)
d (Aj)
R.I. (%)
d (Aj)
R.I. (%)
2h (j)
d (Aj)
R.I. (%)
11.40 5.72 4.83 4.74 4.45 4.07 – 3.68 3.31 – 2.79 2.71 2.69 2.62 – 2.50 2.30 2.27 2.05 2.04 1.93 1.90 1.80 1.76 1.74 1.62 – 1.57 1.39
40 10 60 80 80 60 – 40 60 – 90 60 90 10 – 100 40 60 60 80 10 20 60 60 60 60 – 60 90
11.40 5.69 4.84 4.73 4.48 4.09 – 3.70 3.33 3.27 2.79 2.71 – 2.62 – 2.51 2.31 2.29 – – 1.93 1.90 1.81 1.75 – 1.62 – 1.57 1.40
60 60 60 40 100 10 – 60 80 20 100 100 – 10 – 100 20 20 – – 10 10 20 10 – 20 – 20 90
– – 4.86 4.75 4.47 4.10 – 3.70 3.30 – 2.80 2.71 – 2.63 2.52 – 2.31 2.29 2.07 2.05 1.93 1.91 1.81 1.75 – 1.62 – 1.57 1.40
– – 10 60 60 20 – 35 50 – 75 75 – 88 100 – 10 10 20 35 5 5 5 20 – 10 – 15 40
– – – 19.65 19.84 20.31 20.75 24.19 26.92 – 32.20 33.14 33.46 34.41 35.13 36.04 39.11 39.72 43.34 44.42 46.95 47.83 50.67 51.82 52.63 57.57 57.85 58.79 67.01
– – – 4.76 4.47 4.37 4.08 3.68 3.31 – 2.77 2.70 2.68 2.61 2.55 2.49 2.30 2.27 2.08 2.04 1.93 1.90 1.80 1.76 1.74 1.60 1.59 1.57 1.39
– – – 36.0 33.1 36.0 6.2 49.0 17.0 – 37.5 100 90 25.0 45.6 70.10 20.3 27.6 33.1 64.0 19.1 19.1 11.4 19.1 37.5 74.4 81.1 58.1 76.6
of alumina and/or decomposition of BaCO3, although the presence of BaCO3 is indicated from XRD. Similarly, no exothermic peak is apparent in the 1000 – 1300 jC region which could specifically indicate the formation of a barium hexa-aluminate phase. In a recent work also [19], similar observations were made during the formation of barium hexa-aluminate. A plausible explanation for the above two observations could be that the slow decomposition of BaCO3 in the temperature range of 900– 1300 jC [19,34] probably downplays the peaks expected for the formation of the barium hexa-aluminate. 3.3. Surface area results The surface area values of the calcined samples CH[800(2)], C-H[1000(2)], C-H[1200(2)] and C-
H[1400(2)] (derived from the hydrothermally treated precursor P-H[180(1)] after calcination at the specified temperature) drawn against the calcination temperature are shown in Fig. 3. The surface area of the hydrothermally treated sample P-H[180(1)] was f 108 m2/g. It is observed that by increasing the calcination temperature to 1000 jC, the surface area increases to 178.5 m2/g and then decreases with increase in the calcination temperature, the decrease being more prominent when the calcination temperature increases from 1200 to 1400 jC. The marked increase in the surface area of the low temperature calcined sample (up to 1000 jC) vis a` vis that of the uncalcined sample is difficult to explain but similar observations have been made during preparation of barium mono-aluminate where a maximum in the surface area was observed at 500 jC [25]. Further
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Fig. 2. The TG-DTA of hydrothermally prepared precursor P-H[180(1)].
work is required to clearly assign reasons for the trend observed.
agglomerated form. However, the particle diameter value calculated from this micrograph is estimated as about 1 Am.
3.4. SEM results 3.5. The sequence of reactions The SEM micrograph of the calcined sample CH[1400(2)] is shown in Fig. 4. Irregular block shaped particles having layered structures can be seen from the micrograph. This type of layered structure is associated with barium h/hexa-aluminates [35]. As can be seen from the micrograph the particles show a large variation in their sizes as most of them are in an
From the XRD and TG-DTA studies the most probable sequence of reactions and phase transformations taking place for the formation of ‘‘barium hexaaluminates’’ can be suggested by Eqs. (1) – (9). It may be noted here that the chemical analysis of the samples confirms to the compositions almost quantitatively.
Fig. 3. Variation of surface are with calcination temperature of hydrothermally prepared precursor of barium hexa-aluminate P-H[180(1)].
Fig. 4. The SEM micrograph of the calcined sample C-H[1400(2)].
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(a) During hydrothermal precipitation at 180 jC: (i) Hydrolysis of urea at 180 jC NH2 CO NH2 þ H2 O ! 2NH4 OH þ CO2 ðgÞ
ð1Þ
(iii) At the 800 – 1000 jC region (from XRD, TG-DTA) 6Al2 O3 ðcÞ þ BaCO3 ðcÞ ! X BaO ðby partÞ þ 6Al2 O3 ðams: sÞ þ X CO2 ðgÞ þ ð1 X ÞBaCO3 ðcÞ
(ii) Reaction with barium nitrate
ð8Þ
(iv) At 1200 jC (from XRD and TG-DTA), BaðNO3 Þ2 þ CO2 ðgÞ þ H2 O ! BaCO3 ðcÞ þ 2HNO3
ð2Þ
2HNO3 þ 2NH4 OH ! 2NH4 NO3 þ H2 O
ð3Þ
with the overall reaction of formation of BaCO3 as: BaðNO3 Þ2 þ NH2 CO NH2 þ H2 O ! BaCO3 ðcÞ þ 2NH4 NO3
ð4Þ
(iii) The formation of boehmite 2AlðNO3 Þ3 þ 7NH2 CO NH2 þ ð14 þ 2xÞH2 O ! 2AlOOH xH2 OðcÞ þ 6NH4 NO3 þ 4ðNH4 Þ2 CO3 þ 3CO2 ðgÞ
! 12AlOOH xH2 OðcÞ þ 36NH4 NO3 ð5bÞ
(b) Calcination at different temperature regions: (i) In the 100 –250 jC (from TG-DTA)
! 12AlOOHðcÞ þ BaCO3 ðcÞ þ xH2 O
ð9Þ
(v) At 1400 jC (from XRD) Mixture of Ba – hI-Al2O3(c), BaCO3(c, t) and Al2O3(ams. s) when calcined to 1400 jC gives Ba – hI-Al2O3, Ba –hII-Al2O3 Ba – hI-hIIAl2O3 (the exact percentage of each phase in the final product cannot be ascertained) (amp.: amorphous, s: solid, c: crystalline or pollycrystalline, g: gaseous, t: trace and all other are aqueous species).
ð5aÞ
12AlðNO3 Þ3 þ 42NH2 CO NH2 þ ð84 þ 12xÞH2 O
BaCO3 ðcÞ þ 12AlOOH xH2 OðcÞ
! Mixture of Ba hI Al2 O3 ðcÞ, BaCO3 ðc, tÞ andAl2 O3 ðams: sÞ
4. Conclusions
or
þ 24ðNH4 Þ2 CO3 þ 18CO2 ðgÞ
X BaO ðby partÞ þ 6Al2 O3 ðams: sÞ þ ð1 X ÞBaCO3 ðcÞ ðunreactedÞ
The present study describes a hydrothermal precipitation –calcination route wherein the hydrothermally produced precursors of orthorhombic BaCO3 and crystalline boehmite after calcinations at 1400 jC for 2 h produced micron-sized high surface area ceramic powders of barium hexa-aluminate having Ba – h-Alumina type structure. The ceramic powder finally obtained was found to be free from any contaminants such as g-/ u-/y-/a-Al2O3, BaO, BaCO3, BaAl2O4 and Ba3Al2O6. The increasing and decreasing trends in surface area values of intermediates obtained at different calcination temperatures are difficult to explain, perhaps associated with the presence of intermediates.
ð6Þ
(ii) In the 250– 550 jC region (from TG-DTA)
Acknowledgements
BaCO3 ðcÞ þ 12AlOOHðcÞ ! 6Al2 O3 ðcÞ þ 6H2 O þ BaCO3 ðcÞ
The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar, for his kind permission to publish this paper, and
ð7Þ
D. Mishra et al. / Materials Letters 56 (2002) 873–879
wish to thank Dr. S.C. Das, Head, Department of Hydrometallurgy.
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