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Low temperature preparation of nanocrystalline solid solution of strontium–barium–niobate by chemical process
January 2002
Materials Letters 52 Ž2002. 180–186 www.elsevier.comrlocatermatlet
Low temperature preparation of nanocrystalline solid solution of strontium–barium–niobate by chemical process Asit B. Panda, Amita Pathak, Panchanan Pramanik ) Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal-721 302, India Received 20 March 2001; accepted 4 April 2001
Abstract Nanocrystalline powders of strontium–barium–niobate ŽSBN. with the composition Sr x Ba 1yx Nb 2 O6 Žwith x s 0.4, 0.5 and 0.6. have been prepared using a single step chemical synthesis process starting from a precursor solution constituting of triethanolamine ŽTEA., niobium–tartarate and EDTA complexes of strontium and barium ions. The complete dehydration of the TEA-soluble metal ion complex precursor solution through heating yield in a fluffy, carbonaceous precursor material, which on heat-treatment at 750 8Cr2 h resulted in the single phase SBN powders. The precursor and heat-treated powders have been characterized by thermal analysis and X-ray diffraction ŽXRD. studies. The crystallite size and average particle size as measured from X-ray broadening and transmission electron microscope were found to be 15 and 20 nm, respectively. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline powders; SBN; EDTA
1. Introduction The growth of strontium–barium–niobate ŽSr x Ba 1y x Nb 2 O6 . single crystals, of varying compositions, and studies on their different properties w1–7x are widely reported in literature because of their significant technological importance. They find applications in pyroelectric w2x, piezoelectric w3x, and photo-refractive w4x devices and in optical phase conjugation w8,9x. But the high cost and fabrication difficulties of the strontium–barium–niobate ŽSBN.
)
Corresponding author. Fax: q91-3222-55303.
single crystals have often laid restrictions to their applications. In recent years, the SBN ceramics Žwith 0.25 F x G 0.75. have gained immense attention as their possible substitutes in many of their technological applications because of its low cost, and easy fabrication in to larger sizes and more complex shapes. The expansive research reports since 1980 testifies the increasing trend of research and development occurring on SBN ceramics solid-solutions w10–14x. Potential applications of SBN ceramics, in both electrical and optical fields, generally demand a uniform, fine-grained microstructure with nearly theoretical density in the material, which however has been found difficult to achieve with the help of simple pressure less sintering because of abnormal
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 8 9 - 5
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
grain growth w12,15x. The difficulty might be attributed to the high calcination temperatures ŽF 1150 8C. that are requisites for the obtaining the desired SBN phase when synthesized through conventional solid-state reaction of the reactants. The solid-state reactions are known to start and propagate within the solid phase through diffusion of the atoms, where atoms need to pass through the contact interface among the phases to meet the other kinds of atoms. In conventional solid-state method Žinvolving oxidesrcarbonates etc. as the reactants., the microscopic homogeneity in the starting material being low, the atoms here have to diffuse through longer distances to meet up with the other atoms. In which case, the realization of the desired phase in final product is achieved only by raising the temperatures and prolonging the reaction time. The use of solution-based chemical synthesis methods ensures atomic level mixing of the reactants and hence can circumvent the high calcination temperature requirements associated with the conventional solid-state methods. Unfortunately, the preparation of dense
181
SBN ceramics through chemical synthesis methods gets complicated w16x by the moisture sensitivity and easy hydrolysis of the niobium sources w17–20x Žsuch as niobium pentachloride and niobium ethoxide, etc.. during the reaction period. In our laboratory, we have overcome many such problems associated with the preparation of nanocrystalline powders of PZT, PLZT and PMN by the use of simple metallo-organic complex chemistry w21–23x. In the present study, we report a novel chemical route for the preparation of single-phase, nanocrystalline Sr x Ba 1yx Nb 2 O6 Ž x s 0.4, 0.5 and 0.6. powders starting from an aqueous precursor solution of the respective metal-complexes and triethanolamine ŽTEA.. In this method, we have used niobium– tartarate complex as the source of niobium which eliminates the problem of moisture sensitivity. The route involved the desiccation of the aqueous precursor solution through heating to obtain a carbonaceous precursor mass which was dried, ground to powders and then calcined to obtain the desired SBN powders.
Fig. 1. Schematic representation of the precursors solution method for the synthesis of Sr x Ba 1yx Nb 2 O6 ŽSBN. powders.
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2. Experimental The raw materials that were used for the preparation of the ceramic SBNs were SrŽNO 3 . 2 , BaŽNO 3 . 2 , EDTA, TEA, and niobium–tartarate. The solution of the niobium–tartarate complex was prepared in the laboratory from its hydrated oxide ŽNb 2 O5 nH 2 O. and the details of the preparation process is discussed elsewhere w16x. In the preparation of varying compositions of Sr x Ba 1yx Nb 2 O6 Ži.e., with x s 0.4, 0.5 and 0.6., aqueous solutions of SrŽNO 3 . 2 , BaŽNO 3 . 2 , and ammonium EDTA Žof 1 M each. were freshly prepared and stocked. Stoichiometric amounts of these solutions were taken and mixed together with continuous stirring to obtain a clear solution of the barium- and strontium-EDTA complexes, while the overall EDTA to metal ion mole ratio in the solution was maintained at unity. Stoichiometric amount of the prepared niobium–tartarate complex solution was then introduced into the solution mixture which was followed by the addition of optimum amounts of TEA solution mixture Ž; 8–10 mol, with respect to the total moles of the metal ions.. The solution mixture of the cationic complexes was constantly stirred to result in a red colored, homogeneous solution. The resultant precursor solution was heated over a hot plate Žat ; 200 8C. for evaporation. On complete dehydration of the precursor solution, the TEA and the metal-complexes decomposed with the evolution of dense fumes and resulted in a voluminous, fluffy, black organic-based mass. The fluffy precursor material was ground to powders and then calcined at 750 8C for 2 h to produce the desired single phase of SBN powders. The entire preparative process has been illustrated in Fig. 1.
powders were carried out on a transmission electron microscope ŽTM-300, Phillips, Holland..
4. Results and discussions 4.1. Thermal analysis The DTA curves revealed an exothermic thermal affect for all the SBN precursor samples, with their respective peak around 540 " 10 8C. The exotherm could be assigned to the ixodation of carbonaceous remains of the decomposed metal-complexes and TEA. The entire thermal effect was accompanied by the evolution of various gases, such as CO, CO 2 , NH 3 , water vapor, etc., which are manifested by a single step weight loss in the TG curve. Above 750 8C, there was no significant thermal effect observed in DTA curves and the corresponding TG curves showed no weight loss, implying the formation of the oxides phase at this temperature. The DTArTG curves for the Sr0.5 Ba 0.5 Nb 2 O6 Ždenoted as SBN-5. precursors are depicted in Fig. 2 as a typical representative. 4.2. X-ray diffraction studies The virgin precursors of varying SBN compositions were heat-treatment at various temperatures ranging between 400 and 900 8C for 2 h. The room temperature X-ray diffraction ŽXRD. studies revealed that the virgin precursor powder of the all the SBN compositions were X-ray amorphous. The broad humps in the diffractograms that characterized the
3. Characterization of materials Simultaneously recorded thermogravimetric and differential thermal analysis ŽShimadzu DT-40, Japan. ŽTGrDTA. of the precursor powders were carried out in air at a heating rate of 5 8Crmin. The X-ray diffraction ŽPhilips P.W. 1710, Holland. ŽXRD. pattern of the precursor and heat-treated powders were studied using CuK a radiation. The microstructure and particle size studies in the final
Fig. 2. Thermal studies of the Sr0.5 Ba 0.5 Nb 2 O6 Ždenoted as SBN-5. precursors.
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
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samples at relatively low external heat-treatment temperatures indicated the presence of small atomic clusters of appropriate chemical homogeneity in the amorphous precursors, which facilitated the crystallization process. The X-ray diffractograms of the virgin SBN precursor and their respective heat-treated powders with Sr0.5 Ba 0.5 Nb 0.2 O6 Ži.e., ISBN-5. composition are depicted in Fig. 3 as typical examples. The minimum heat-treatment temperatures required to get the carbon free powders and to initiate the crystallization of the pure SBN phase in all the studied SBN compositions are tabulated in Table 1. The crystallite size in the heat-treated powders of varying SBN compositions were calculated from Xray line broadening studies using Scherrer’s equation w25x. The crystallite sizes in all the SBN compositions were observed to increase with the rise in the heat-treated temperatures. The average crystallite sizes calculated using the Scherrer’s formula applied to the different d hkl lines that were obtained after heat-treatment of the various SBN precursors at their respective crystallization temperatures are tabulated in Table 2. 4.3. Microstructure studies
Fig. 3. The X-ray diffractograms Žusing CuK a radiation. of the SBN-5 precursors on heat-treatment for 2 h at: Ža. 500 8C, Žb. 600 8C and Žc. 750 8C.
X-ray amorphous nature of the samples persisted up to the heat-treatment temperatures of 500 8C and they gradually gave way to discrete lines on heattreatment of their respective virgin precursors at ; 600 8C. The emergence of characteristic diffraction lines at 600 8C and their gradual sharpening on heat-treatment at ; 750 8C and above inferred the onset and then their eventual crystallization into the pure SBN phase in the samples at the two respective temperatures. For all the SBN compositions, crystalline SBN phase was realization on direct heat-treatment of their respective virgin samples without passing through any intermediate w24x metal-oxide phasesr pre-phase compounds. The direct crystallization of the pure SBN phase from their respective virgin
The bright field transmission electron micrographs for the respresentative Sr0.5 Ba 0.5 Nb 2 O6 Ži.e., SBN-5. composition, after heat-treatment of the precursor powders at their crystallization temperatures Ži.e., at 750 8Cr2 h., is illustrated as a typical example in Fig. 4Ža.. The bright field TEM micro-
Table 1 Summary of the minimum heat-treatments required for obtaining carbon free powders, for the initiation of the crystalline phase in the various SBN compositions SBN composition
Carbon free temperaturea Ž8C.
Crystallization temperatureb Ž8C.
Sr0.4 Ba 0.6 Nb 2 O6 Sr0.5 Ba 0.5 Nb 2 O6 Sr0.6 Ba 0.4 Nb 2 O6
650 600 600
725 750 750
a
The minimum heat-treatments required for obtaining carbon free powders. b The minimum heat-treatments required for the initiation of the respective crystalline phase.
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
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Table 2 Summary of the crystallite sizes and the average TEM particle sizes for the various SBN compositions SBN composition
Crystallite size a Žnm.
Average particle size b Ž"5 nm.
Sr0.4 Ba 0.6 Nb 2 O6 Sr0.5 Ba 0.5 Nb 2 O6 Sr0.6 Ba 0.4 Nb 2 O6
13.6 16.2 16.8
15 18 20
a
The average crystallite sizes calculated using the Scherrer’s formula applied to the various d hkl lines of the precursor powders that were heat-treated at their respective crystallization temperatures. b Average diameters of the smallest visible isolated particler crystallite agglomerate as observed from TEM studies for the precursor powders heat-treated at their respective crystallization temperatures.
graphs represented the basic powder morphology in the samples, where the smallest visible isolated spot can be identified with particlercrystallite agglomerates. From the TEM study of the SBN powders, it was observed that the particles are almost spherical with average particle diameters lying between 15 and 20 nm. The corresponding selected area electron diffraction pattern of the same sample Ži.e., SBN-5. showed distinct rings, characteristic of an assembly of nanocrystallites ŽFig. 4Žb... Average diameters of the smallest visible isolated particlercrystallite agglomerate, as observed from TEM studies, for the various SBN precursor powders after being heattreated at their respective crystallization temperatures are given in Table 2. The values of the crystallite size and the average TEM particle size were observed to be the lowest for the Sr x Ba 1yx Nb 2 O6 composition with x s 0.4 while they were almost the same for the compositions with x s 0.5 and x s 0.6. The discussed chemical process is based on the homogeneous and atomistical distribution of the metal ions in a precursor solution that was constituted of an aqueous mixture of soluble niobium– tartarate complex Žas the niobium source., EDTA complexes of Sr 2q and BA2q ions, and TEA. The amount of TEA was always kept in excess to the total cations present in the in the reaction mixture, which was around 8–10 mol, with respect to the total moles of the metal ions. TEA helped the metal ions to remain in solution and provided sufficient
flexibility to the system to exist homogeneously throughout the evaporation process without undergoing any precipitation and segregation. Complete evaporation of the precursor solution resulted in a weightless, fluffy black decomposed mass, which in essence is a carbonaceous material. The evolution of various gases Žsuch as CO, CO 2 , NH 3 , NO 2 and water vapor. accompanying the decomposition of the metal-complexes and TEA made the structure highly porous and fluffy. The BET surface area Žcarried out by adsorption of nitrogen gas at liquid nitrogen temperatures using the Micromeritics high-speed area analyzer. of the generated carbon was found to range between 160 and 200
Fig. 4. Ža. The bright field transmission electron micrographs of the SBN-5 samples after heat-treatment of the precursors powders at 750 8Cr2 h. Žb. The corresponding selected area electron diffraction pattern of the SBN-5 powders after heat-treatment of the precursors powders at 750 8Cr2 h.
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
m2rg. This led us to infer that the precursor material was essentially a matrix of mesoporous carbon. Based on the experimental findings, it can be presumed that the complete evaporation of the precursor solution and consequent decomposition of the metal-EDTA complexes generated, in-situ, a matrix of polar mesoporous carbon enriched with oxygen atoms. The decomposition of TEA along with the metal-complexes during the complete evaporation process was expected to impart a higher polarity to the mesoporous carbonaceous material due to induction of nitrogen into the matrix. And these highly polar carbonaceous mesoporous precursor structure probably favored the accomdation of the metal ions in its matrix. The volatilization of the residual mesoporous carbon through aerial oxidation of this highly polar precursor material, on calcination at temperatures lower than 500 8C, presumably led to the formation of nascent metal oxides in the material, which were basically small atomic clusters of proper chemical homogeneity. The heat generated by the combustion of the residual mesoporous carbon on heat-treatment of this glassy phase material at external temperatures ranging between 700 and 750 8C facilitated the rearrangement of these nascent metal oxides and the eventual direct crystallization into the nanocrystals of the respective SBN phase. Moreover, the evolution of large amounts of gases Žsuch as: CO, CO 2 , water vapor and some NO 2 . during the oxidative decomposition of carbonaceous material not only helped to disintegrate the agglomerated particles but also helped to inhibit the sintering of nanosized particles. It has been an empirical observation that fluffier was the precursor powders, finer were the SBN obtained on heat-treatment and narrower was their grain-size distribution.
5. Conclusion A simple chemical process has been developed for the preparation of nanocrystalline powders of various Sr x Ba 1yx Nb 2 O6 compositions through evaporation of a homogeneous aqueous precursor solution of soluble niobium–tartarate complex, EDTA complexes of Sr 2q and Ba2q ions, and TEA. The use of soluble niobium–tartarate complex as the niobium source was found to be superior alternatives
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for the preparation of any niobium-based oxides since it can overcome the problems of moisture sensitivity and hydrolysis that are usually associated with the other reported sources of niobium. The calcination of the precursors, which where obtained from the complete dehydration of the precursor solutions, at external temperatures as low as 750 8C directly resulted in the nanocrystals of the SBN phase without passing through any intermediate phase. The direct crystallization of the SBN phase from the precursor mass can be attributed to the formation of nascent metal-oxides embedded in the highly polar matrix of the generated mesophorouscarbon. And the heat generated due to the combustion of the carbonaceous residues obtained from the decomposition of TEA and metal-EDTA complexes reduced the external temperatures required for the formation of the SBN phase. Thus, it can be concluded that the developed single step chemical method is a simple and minimal thermal budge process that is suitable for the preparation of any niobium-based nanocrystalline, ceramic-oxides powders.
Acknowledgements The authors are grateful to the Council for Scientific and Industrial Research, New Delhi, India for the financial grant offered in support of this work.
References w1x P.V. Lenzo, E.G. Spencer, A.A. Ballman, Appl. Phys. Lett. 11 Ž1967. 23–24. w2x A.M. Glass, J. Appl. Phys. 40 Ž1969. 4699–4713. w3x R.R. Neugaonkar, M.H. Kalisher, T.C. Lim, E.J. Staples, K.L. Leester, Mater. Res. Bull. 15 Ž1980. 1535–4150. w4x J.B. Thaxter, Appl. Phys. Lett. 15 Ž7. Ž1969. 210–212. w5x M.H. Francombe, Acta Crystallogr. 13 Ž1960. 131–140. w6x A.A. Ballman, H. Bown, J. Cryst. Growth 1 Ž1967. 311–314. w7x R.R. Neurgaonkar, L.E. cross, Mater. Res. Bull. 21 Ž1986. 893. w8x B. Fischer, M. Cronin-Golomb, J.O. White, A. Yariv, R.R. Neurgaonkar, Appl. Phys. Lett. 40 Ž1982. 863–865. w9x M.D. Ewbank, R.R. Neurgaonkar, W.K. Cory, J. Feinberg, Appl. Phys. Lett. 62 Ž1987. 374–380. w10x K. Nagata, Y. Yamamoto, H. Igarashi, K. Okazaki, Ferroelectrics 38 Ž1–4. Ž1981. 853–856.
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w11x S. Kuroda, K. Kubota, J. Phys. Chem. Solids 42 Ž7. Ž1981. 573–577. w12x N.S. VanDamme, A.E. Sutherland, L. Jones, K. Bridger, S.R. Winzer, J. Am. Ceram. Soc. 74 Ž8. Ž1981. 1785–1792. w13x S.B. Despande, H.S. Potdar, P.D. Godbole, S.K. Date, J. Am. Ceram. Soc. 75 Ž9. Ž1992. 2581–2585. w14x T.T. Fang, N.T. Wu, J. Mater. Sci. 30 Ž1995. 3376–3382. w15x W.J. Lee, T.T. Fang, J. Am. Ceram. Soc. 81 Ž4. Ž1998. 1019–1024. w16x R.N. Das, P. Pramanik, Mater. Lett. 46 Ž2000. 7–14. w17x F. Chabut, J.P. Boilot, M. Lejeune, R. Papiernik, L.J. Houbert-pfalzgraf, J. Am. Ceram. Soc. 78 Ž1989. 1355. w18x J.H. Choi, J.S. Woo, S.G. Hong, D.J. Kim, Mater. Res. Bull. 25 Ž1990. 283.
w19x Y. Narender, G.L. Messing, J. Am. Ceram. Soc. 80 Ž1997. 915. w20x M.M.A. Sekar, A. Halliyal, J. Am. Ceram. Soc. 81 Ž1998. 380. w21x R.N. Das, P. Pathak, P. Pramanik, J. Am. Ceram. Soc. 81 Ž12. Ž1998. 3357. w22x R.N. Das, P. Pramanik, Nanostruct. Mater. 11 Ž3. Ž1999. 325. w23x R.N. Das, P. Pramanik, J. Mater. Res. Ž2000. Žin press.. w24x T.T. Fang, N.T. Wu, F.S. Shaiau, J. Mater. Sci. Lett. 13 Ž1994. 1746. w25x P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd edn., Wiley, New York, 1974.
Materials Letters 52 Ž2002. 180–186 www.elsevier.comrlocatermatlet
Low temperature preparation of nanocrystalline solid solution of strontium–barium–niobate by chemical process Asit B. Panda, Amita Pathak, Panchanan Pramanik ) Department of Chemistry, Indian Institute of Technology, Kharagpur, West Bengal-721 302, India Received 20 March 2001; accepted 4 April 2001
Abstract Nanocrystalline powders of strontium–barium–niobate ŽSBN. with the composition Sr x Ba 1yx Nb 2 O6 Žwith x s 0.4, 0.5 and 0.6. have been prepared using a single step chemical synthesis process starting from a precursor solution constituting of triethanolamine ŽTEA., niobium–tartarate and EDTA complexes of strontium and barium ions. The complete dehydration of the TEA-soluble metal ion complex precursor solution through heating yield in a fluffy, carbonaceous precursor material, which on heat-treatment at 750 8Cr2 h resulted in the single phase SBN powders. The precursor and heat-treated powders have been characterized by thermal analysis and X-ray diffraction ŽXRD. studies. The crystallite size and average particle size as measured from X-ray broadening and transmission electron microscope were found to be 15 and 20 nm, respectively. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline powders; SBN; EDTA
1. Introduction The growth of strontium–barium–niobate ŽSr x Ba 1y x Nb 2 O6 . single crystals, of varying compositions, and studies on their different properties w1–7x are widely reported in literature because of their significant technological importance. They find applications in pyroelectric w2x, piezoelectric w3x, and photo-refractive w4x devices and in optical phase conjugation w8,9x. But the high cost and fabrication difficulties of the strontium–barium–niobate ŽSBN.
)
Corresponding author. Fax: q91-3222-55303.
single crystals have often laid restrictions to their applications. In recent years, the SBN ceramics Žwith 0.25 F x G 0.75. have gained immense attention as their possible substitutes in many of their technological applications because of its low cost, and easy fabrication in to larger sizes and more complex shapes. The expansive research reports since 1980 testifies the increasing trend of research and development occurring on SBN ceramics solid-solutions w10–14x. Potential applications of SBN ceramics, in both electrical and optical fields, generally demand a uniform, fine-grained microstructure with nearly theoretical density in the material, which however has been found difficult to achieve with the help of simple pressure less sintering because of abnormal
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 8 9 - 5
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
grain growth w12,15x. The difficulty might be attributed to the high calcination temperatures ŽF 1150 8C. that are requisites for the obtaining the desired SBN phase when synthesized through conventional solid-state reaction of the reactants. The solid-state reactions are known to start and propagate within the solid phase through diffusion of the atoms, where atoms need to pass through the contact interface among the phases to meet the other kinds of atoms. In conventional solid-state method Žinvolving oxidesrcarbonates etc. as the reactants., the microscopic homogeneity in the starting material being low, the atoms here have to diffuse through longer distances to meet up with the other atoms. In which case, the realization of the desired phase in final product is achieved only by raising the temperatures and prolonging the reaction time. The use of solution-based chemical synthesis methods ensures atomic level mixing of the reactants and hence can circumvent the high calcination temperature requirements associated with the conventional solid-state methods. Unfortunately, the preparation of dense
181
SBN ceramics through chemical synthesis methods gets complicated w16x by the moisture sensitivity and easy hydrolysis of the niobium sources w17–20x Žsuch as niobium pentachloride and niobium ethoxide, etc.. during the reaction period. In our laboratory, we have overcome many such problems associated with the preparation of nanocrystalline powders of PZT, PLZT and PMN by the use of simple metallo-organic complex chemistry w21–23x. In the present study, we report a novel chemical route for the preparation of single-phase, nanocrystalline Sr x Ba 1yx Nb 2 O6 Ž x s 0.4, 0.5 and 0.6. powders starting from an aqueous precursor solution of the respective metal-complexes and triethanolamine ŽTEA.. In this method, we have used niobium– tartarate complex as the source of niobium which eliminates the problem of moisture sensitivity. The route involved the desiccation of the aqueous precursor solution through heating to obtain a carbonaceous precursor mass which was dried, ground to powders and then calcined to obtain the desired SBN powders.
Fig. 1. Schematic representation of the precursors solution method for the synthesis of Sr x Ba 1yx Nb 2 O6 ŽSBN. powders.
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A.B. Panda et al.r Materials Letters 52 (2002) 180–186
2. Experimental The raw materials that were used for the preparation of the ceramic SBNs were SrŽNO 3 . 2 , BaŽNO 3 . 2 , EDTA, TEA, and niobium–tartarate. The solution of the niobium–tartarate complex was prepared in the laboratory from its hydrated oxide ŽNb 2 O5 nH 2 O. and the details of the preparation process is discussed elsewhere w16x. In the preparation of varying compositions of Sr x Ba 1yx Nb 2 O6 Ži.e., with x s 0.4, 0.5 and 0.6., aqueous solutions of SrŽNO 3 . 2 , BaŽNO 3 . 2 , and ammonium EDTA Žof 1 M each. were freshly prepared and stocked. Stoichiometric amounts of these solutions were taken and mixed together with continuous stirring to obtain a clear solution of the barium- and strontium-EDTA complexes, while the overall EDTA to metal ion mole ratio in the solution was maintained at unity. Stoichiometric amount of the prepared niobium–tartarate complex solution was then introduced into the solution mixture which was followed by the addition of optimum amounts of TEA solution mixture Ž; 8–10 mol, with respect to the total moles of the metal ions.. The solution mixture of the cationic complexes was constantly stirred to result in a red colored, homogeneous solution. The resultant precursor solution was heated over a hot plate Žat ; 200 8C. for evaporation. On complete dehydration of the precursor solution, the TEA and the metal-complexes decomposed with the evolution of dense fumes and resulted in a voluminous, fluffy, black organic-based mass. The fluffy precursor material was ground to powders and then calcined at 750 8C for 2 h to produce the desired single phase of SBN powders. The entire preparative process has been illustrated in Fig. 1.
powders were carried out on a transmission electron microscope ŽTM-300, Phillips, Holland..
4. Results and discussions 4.1. Thermal analysis The DTA curves revealed an exothermic thermal affect for all the SBN precursor samples, with their respective peak around 540 " 10 8C. The exotherm could be assigned to the ixodation of carbonaceous remains of the decomposed metal-complexes and TEA. The entire thermal effect was accompanied by the evolution of various gases, such as CO, CO 2 , NH 3 , water vapor, etc., which are manifested by a single step weight loss in the TG curve. Above 750 8C, there was no significant thermal effect observed in DTA curves and the corresponding TG curves showed no weight loss, implying the formation of the oxides phase at this temperature. The DTArTG curves for the Sr0.5 Ba 0.5 Nb 2 O6 Ždenoted as SBN-5. precursors are depicted in Fig. 2 as a typical representative. 4.2. X-ray diffraction studies The virgin precursors of varying SBN compositions were heat-treatment at various temperatures ranging between 400 and 900 8C for 2 h. The room temperature X-ray diffraction ŽXRD. studies revealed that the virgin precursor powder of the all the SBN compositions were X-ray amorphous. The broad humps in the diffractograms that characterized the
3. Characterization of materials Simultaneously recorded thermogravimetric and differential thermal analysis ŽShimadzu DT-40, Japan. ŽTGrDTA. of the precursor powders were carried out in air at a heating rate of 5 8Crmin. The X-ray diffraction ŽPhilips P.W. 1710, Holland. ŽXRD. pattern of the precursor and heat-treated powders were studied using CuK a radiation. The microstructure and particle size studies in the final
Fig. 2. Thermal studies of the Sr0.5 Ba 0.5 Nb 2 O6 Ždenoted as SBN-5. precursors.
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
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samples at relatively low external heat-treatment temperatures indicated the presence of small atomic clusters of appropriate chemical homogeneity in the amorphous precursors, which facilitated the crystallization process. The X-ray diffractograms of the virgin SBN precursor and their respective heat-treated powders with Sr0.5 Ba 0.5 Nb 0.2 O6 Ži.e., ISBN-5. composition are depicted in Fig. 3 as typical examples. The minimum heat-treatment temperatures required to get the carbon free powders and to initiate the crystallization of the pure SBN phase in all the studied SBN compositions are tabulated in Table 1. The crystallite size in the heat-treated powders of varying SBN compositions were calculated from Xray line broadening studies using Scherrer’s equation w25x. The crystallite sizes in all the SBN compositions were observed to increase with the rise in the heat-treated temperatures. The average crystallite sizes calculated using the Scherrer’s formula applied to the different d hkl lines that were obtained after heat-treatment of the various SBN precursors at their respective crystallization temperatures are tabulated in Table 2. 4.3. Microstructure studies
Fig. 3. The X-ray diffractograms Žusing CuK a radiation. of the SBN-5 precursors on heat-treatment for 2 h at: Ža. 500 8C, Žb. 600 8C and Žc. 750 8C.
X-ray amorphous nature of the samples persisted up to the heat-treatment temperatures of 500 8C and they gradually gave way to discrete lines on heattreatment of their respective virgin precursors at ; 600 8C. The emergence of characteristic diffraction lines at 600 8C and their gradual sharpening on heat-treatment at ; 750 8C and above inferred the onset and then their eventual crystallization into the pure SBN phase in the samples at the two respective temperatures. For all the SBN compositions, crystalline SBN phase was realization on direct heat-treatment of their respective virgin samples without passing through any intermediate w24x metal-oxide phasesr pre-phase compounds. The direct crystallization of the pure SBN phase from their respective virgin
The bright field transmission electron micrographs for the respresentative Sr0.5 Ba 0.5 Nb 2 O6 Ži.e., SBN-5. composition, after heat-treatment of the precursor powders at their crystallization temperatures Ži.e., at 750 8Cr2 h., is illustrated as a typical example in Fig. 4Ža.. The bright field TEM micro-
Table 1 Summary of the minimum heat-treatments required for obtaining carbon free powders, for the initiation of the crystalline phase in the various SBN compositions SBN composition
Carbon free temperaturea Ž8C.
Crystallization temperatureb Ž8C.
Sr0.4 Ba 0.6 Nb 2 O6 Sr0.5 Ba 0.5 Nb 2 O6 Sr0.6 Ba 0.4 Nb 2 O6
650 600 600
725 750 750
a
The minimum heat-treatments required for obtaining carbon free powders. b The minimum heat-treatments required for the initiation of the respective crystalline phase.
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
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Table 2 Summary of the crystallite sizes and the average TEM particle sizes for the various SBN compositions SBN composition
Crystallite size a Žnm.
Average particle size b Ž"5 nm.
Sr0.4 Ba 0.6 Nb 2 O6 Sr0.5 Ba 0.5 Nb 2 O6 Sr0.6 Ba 0.4 Nb 2 O6
13.6 16.2 16.8
15 18 20
a
The average crystallite sizes calculated using the Scherrer’s formula applied to the various d hkl lines of the precursor powders that were heat-treated at their respective crystallization temperatures. b Average diameters of the smallest visible isolated particler crystallite agglomerate as observed from TEM studies for the precursor powders heat-treated at their respective crystallization temperatures.
graphs represented the basic powder morphology in the samples, where the smallest visible isolated spot can be identified with particlercrystallite agglomerates. From the TEM study of the SBN powders, it was observed that the particles are almost spherical with average particle diameters lying between 15 and 20 nm. The corresponding selected area electron diffraction pattern of the same sample Ži.e., SBN-5. showed distinct rings, characteristic of an assembly of nanocrystallites ŽFig. 4Žb... Average diameters of the smallest visible isolated particlercrystallite agglomerate, as observed from TEM studies, for the various SBN precursor powders after being heattreated at their respective crystallization temperatures are given in Table 2. The values of the crystallite size and the average TEM particle size were observed to be the lowest for the Sr x Ba 1yx Nb 2 O6 composition with x s 0.4 while they were almost the same for the compositions with x s 0.5 and x s 0.6. The discussed chemical process is based on the homogeneous and atomistical distribution of the metal ions in a precursor solution that was constituted of an aqueous mixture of soluble niobium– tartarate complex Žas the niobium source., EDTA complexes of Sr 2q and BA2q ions, and TEA. The amount of TEA was always kept in excess to the total cations present in the in the reaction mixture, which was around 8–10 mol, with respect to the total moles of the metal ions. TEA helped the metal ions to remain in solution and provided sufficient
flexibility to the system to exist homogeneously throughout the evaporation process without undergoing any precipitation and segregation. Complete evaporation of the precursor solution resulted in a weightless, fluffy black decomposed mass, which in essence is a carbonaceous material. The evolution of various gases Žsuch as CO, CO 2 , NH 3 , NO 2 and water vapor. accompanying the decomposition of the metal-complexes and TEA made the structure highly porous and fluffy. The BET surface area Žcarried out by adsorption of nitrogen gas at liquid nitrogen temperatures using the Micromeritics high-speed area analyzer. of the generated carbon was found to range between 160 and 200
Fig. 4. Ža. The bright field transmission electron micrographs of the SBN-5 samples after heat-treatment of the precursors powders at 750 8Cr2 h. Žb. The corresponding selected area electron diffraction pattern of the SBN-5 powders after heat-treatment of the precursors powders at 750 8Cr2 h.
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m2rg. This led us to infer that the precursor material was essentially a matrix of mesoporous carbon. Based on the experimental findings, it can be presumed that the complete evaporation of the precursor solution and consequent decomposition of the metal-EDTA complexes generated, in-situ, a matrix of polar mesoporous carbon enriched with oxygen atoms. The decomposition of TEA along with the metal-complexes during the complete evaporation process was expected to impart a higher polarity to the mesoporous carbonaceous material due to induction of nitrogen into the matrix. And these highly polar carbonaceous mesoporous precursor structure probably favored the accomdation of the metal ions in its matrix. The volatilization of the residual mesoporous carbon through aerial oxidation of this highly polar precursor material, on calcination at temperatures lower than 500 8C, presumably led to the formation of nascent metal oxides in the material, which were basically small atomic clusters of proper chemical homogeneity. The heat generated by the combustion of the residual mesoporous carbon on heat-treatment of this glassy phase material at external temperatures ranging between 700 and 750 8C facilitated the rearrangement of these nascent metal oxides and the eventual direct crystallization into the nanocrystals of the respective SBN phase. Moreover, the evolution of large amounts of gases Žsuch as: CO, CO 2 , water vapor and some NO 2 . during the oxidative decomposition of carbonaceous material not only helped to disintegrate the agglomerated particles but also helped to inhibit the sintering of nanosized particles. It has been an empirical observation that fluffier was the precursor powders, finer were the SBN obtained on heat-treatment and narrower was their grain-size distribution.
5. Conclusion A simple chemical process has been developed for the preparation of nanocrystalline powders of various Sr x Ba 1yx Nb 2 O6 compositions through evaporation of a homogeneous aqueous precursor solution of soluble niobium–tartarate complex, EDTA complexes of Sr 2q and Ba2q ions, and TEA. The use of soluble niobium–tartarate complex as the niobium source was found to be superior alternatives
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for the preparation of any niobium-based oxides since it can overcome the problems of moisture sensitivity and hydrolysis that are usually associated with the other reported sources of niobium. The calcination of the precursors, which where obtained from the complete dehydration of the precursor solutions, at external temperatures as low as 750 8C directly resulted in the nanocrystals of the SBN phase without passing through any intermediate phase. The direct crystallization of the SBN phase from the precursor mass can be attributed to the formation of nascent metal-oxides embedded in the highly polar matrix of the generated mesophorouscarbon. And the heat generated due to the combustion of the carbonaceous residues obtained from the decomposition of TEA and metal-EDTA complexes reduced the external temperatures required for the formation of the SBN phase. Thus, it can be concluded that the developed single step chemical method is a simple and minimal thermal budge process that is suitable for the preparation of any niobium-based nanocrystalline, ceramic-oxides powders.
Acknowledgements The authors are grateful to the Council for Scientific and Industrial Research, New Delhi, India for the financial grant offered in support of this work.
References w1x P.V. Lenzo, E.G. Spencer, A.A. Ballman, Appl. Phys. Lett. 11 Ž1967. 23–24. w2x A.M. Glass, J. Appl. Phys. 40 Ž1969. 4699–4713. w3x R.R. Neugaonkar, M.H. Kalisher, T.C. Lim, E.J. Staples, K.L. Leester, Mater. Res. Bull. 15 Ž1980. 1535–4150. w4x J.B. Thaxter, Appl. Phys. Lett. 15 Ž7. Ž1969. 210–212. w5x M.H. Francombe, Acta Crystallogr. 13 Ž1960. 131–140. w6x A.A. Ballman, H. Bown, J. Cryst. Growth 1 Ž1967. 311–314. w7x R.R. Neurgaonkar, L.E. cross, Mater. Res. Bull. 21 Ž1986. 893. w8x B. Fischer, M. Cronin-Golomb, J.O. White, A. Yariv, R.R. Neurgaonkar, Appl. Phys. Lett. 40 Ž1982. 863–865. w9x M.D. Ewbank, R.R. Neurgaonkar, W.K. Cory, J. Feinberg, Appl. Phys. Lett. 62 Ž1987. 374–380. w10x K. Nagata, Y. Yamamoto, H. Igarashi, K. Okazaki, Ferroelectrics 38 Ž1–4. Ž1981. 853–856.
186
A.B. Panda et al.r Materials Letters 52 (2002) 180–186
w11x S. Kuroda, K. Kubota, J. Phys. Chem. Solids 42 Ž7. Ž1981. 573–577. w12x N.S. VanDamme, A.E. Sutherland, L. Jones, K. Bridger, S.R. Winzer, J. Am. Ceram. Soc. 74 Ž8. Ž1981. 1785–1792. w13x S.B. Despande, H.S. Potdar, P.D. Godbole, S.K. Date, J. Am. Ceram. Soc. 75 Ž9. Ž1992. 2581–2585. w14x T.T. Fang, N.T. Wu, J. Mater. Sci. 30 Ž1995. 3376–3382. w15x W.J. Lee, T.T. Fang, J. Am. Ceram. Soc. 81 Ž4. Ž1998. 1019–1024. w16x R.N. Das, P. Pramanik, Mater. Lett. 46 Ž2000. 7–14. w17x F. Chabut, J.P. Boilot, M. Lejeune, R. Papiernik, L.J. Houbert-pfalzgraf, J. Am. Ceram. Soc. 78 Ž1989. 1355. w18x J.H. Choi, J.S. Woo, S.G. Hong, D.J. Kim, Mater. Res. Bull. 25 Ž1990. 283.
w19x Y. Narender, G.L. Messing, J. Am. Ceram. Soc. 80 Ž1997. 915. w20x M.M.A. Sekar, A. Halliyal, J. Am. Ceram. Soc. 81 Ž1998. 380. w21x R.N. Das, P. Pathak, P. Pramanik, J. Am. Ceram. Soc. 81 Ž12. Ž1998. 3357. w22x R.N. Das, P. Pramanik, Nanostruct. Mater. 11 Ž3. Ž1999. 325. w23x R.N. Das, P. Pramanik, J. Mater. Res. Ž2000. Žin press.. w24x T.T. Fang, N.T. Wu, F.S. Shaiau, J. Mater. Sci. Lett. 13 Ž1994. 1746. w25x P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd edn., Wiley, New York, 1974.