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Synthesis of nanocrystalline yttria by sol–gel method
May 2001
Materials Letters 48 Ž2001. 342–346 www.elsevier.comrlocatermatlet
Synthesis of nanocrystalline yttria by sol–gel method R. Subramanian, P. Shankar ) , S. Kavithaa, S.S. Ramakrishnan, P.C. Angelo, H. Venkataraman P.S.G. College of Technology, Coimbatore 641004, India Received 4 August 2000; accepted 5 September 2000
Abstract A method for the synthesis of nanocrystalline yttria by sol–gel method has been developed and is described in the paper. X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM. investigations confirm the grain sizes to be in the range of 20–40 nm. EDAX analysis from the yttria particles in TEM further confirmed the high chemical purity of the powders. Development of such an inexpensive and easy technique for synthesis of high purity nanocrystalline yttria can be of commercial significance. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel; Yttria; Nanocrystalline; Transmission electron microscopy
1. Introduction Need for high performance materials for advanced applications have led to the development of new concepts in materials design, processing and their fabrication. The development of nanocrystalline materials with improved and novel properties is an important turning point in materials research w1–3x. Nanophase ceramics, in particular, have excellent potential in many engineering and medical applications w4x. Conventionally brittle ceramics have now shown superplastic behavior by synthesizing them in nanocrystalline form, thus opening newer venues for their engineering applications. )
Corresponding author. Physical Metallurgy Section, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Fax: q91-4114-40360. E-mail address: [email protected] ŽP. Shankar..
Yttria finds many high technology applications due to its excellent high temperature stability, high affinity for oxygen and sulfur and well-defined crystal structures, whose properties can be tailored by incorporating different types of ions into the structure w5,6x. Yttria is used as a host matrix for phosphors used in televisions and also in synthesis of yttria–thoria type translucent ceramics for lasers. Yttria is also being tried as a mould coating for precision Ti-6Al-4V castings. One of the most widespread application of yttria has of course been as stabilizer of cubic zirconia. Further, by virtue of being one of the most stable oxides, yttria is gaining importance in the production of Oxide Dispersion Strengthened ŽODS. materials w7,8x. The production of ODS alloys for enhanced high temperature strength, oxidation and corrosion resistance has resulted in a major breakthrough in high temperature materials research. These ODS alloys include iron
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 2 4 - 4
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
and nickel base alloys containing fine dispersions of thoria or yttria particles. Yttria is being used in the present day ODS alloys in preference to thoria. The properties of the ODS alloys are found to be improved if a uniform dispersion of nanocrystalline yttria particles in the ferrite matrix is achieved. The commercially available yttria is therefore of little use. A number of techniques for producing nanocrystalline yttria have been reported w9,10x. These include inert gas atomization, chemical vapor deposition, forced hydrolysis and emulsion route. Inert gas atomization and chemical vapor deposition are energy intensive and the high melting point of yttria makes these processes economically unviable. Forced hydrolysis and emulsion routes involve close control of process parameters Žsuch as pH, hydrophilerlipophile balance.. The objective of the present work was to develop an alternative simpler and economical chemical route for synthesis of nanosized yttria particles by sol–gel method. The other inherent advantages of producing ceramic powders by sol–gel technique includes that it results in a free flowing, spherical, strain free particle over a narrow size distribution. Apart from producing powder particles, the unique advantage of sol–gel method over inert gas atomization technique is that it can also be used to give coatings, monoliths and fibers on various substrate materials for possible future applications w11x.
2. Experimental The experimental procedures for the production of yttria from its metal salts were undertaken. This involves three distinct steps namely, Ži. solution preparation, Žii. filtration and drying and Žiii. calcination. These steps are described in detail below. 2.1. Solution preparation Ten grams of yttrium nitrate of 99.99% purity was dissolved in 100 ml of distilled water. Solution
343
was stirred till all the salt was dissolved and a clear solution was obtained. This concentration was maintained constant in all experiments. Precipitation of the powder was done using aqueous ammonium hydroxide as the hydrolyzing agent added at constant rate of 20 dropsrmin. The hydroxide solution was added till the precipitation of the salt was complete. During the precipitation, the solution was continuously stirred using a mechanical stirrer. 2.2. Filtration and drying The precipitated solution was washed thoroughly with distilled water and dried in an oven at 373 K for 3 h. After drying, the powder was obtained as agglomerates, which was subsequently ground carefully using a pestle and mortar. 2.3. Calcination The ground yttrium hydroxide powders were converted into yttrium oxide through calcination. The powders were calcined in a silica crucible at 1123 K for 3 h. After calcination, the powders were cooled in the furnace to room temperature.
3. Results and discussions The powder samples were characterized using XRD, TEM and EDAX for ascertaining the purity, morphology and size distribution of the particles. XRD characterization was performed using a Philips X-ray diffractometer using Cu-K a radiation. The result of the XRD analysis is shown in Fig. 1. From the XRD pattern, it is observed that only peaks corresponding to cubic yttria were present. No other impurity phases could be detected by XRD, indicating the high phase purity of the yttria powders produced by this method. The grain sizes of the yttria particles were calculated using the Debye– Scherrer formula, from the FWHM of the peaks. Since the sol–gel method is expected to result in nearly spherical, strain free particles, it is reasonable to assume the entire peak broadening to result from the fine grains size of the powders. This is further justified by the fact that the peaks were found to be nearly symmetrical, thus ruling out the contribution
344
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
Fig. 1. XRD chart from yttria powders showing the presence of only cubic phase.
of defects like dislocations and stacking faults. The instrumental broadening of the XRD used in the present study was 0.088 and its contribution to the peak broadening was corrected using the standard Warren–Averbach formalism. The average grain size was estimated to be about 50 nm, indicating the ultra fine grain size of the yttria. Samples for TEM investigations were prepared by suspending the yttria powders on a carbon film supported by a copper grid. The samples were then analyzed using a Philips CM 200 TEM. A low
magnification micrograph showing the distribution of yttria particles is shown in Fig. 2. A bimodal size distribution of yttria particles was observed, having large number of particles in the 50–100 nm and a few particles of about 500–1000 nm in size. The dark field image of yttria particles is shown in Fig. 3. The grain sizes are found to be in the range of 20–40 nm, as can be seen from the micrograph. This is in close agreement with the grain size estimates from XRD analysis. Fig. 4 shows a typical EDAX output
Fig. 2. A low magnification TEM micrograph showing the distribution of yttria particles.
Fig. 3. Dark field TEM micrograph showing the NC sizes of yttria.
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
345
Fig. 4. EDAX pattern from a yttria particle showing the high chemical purity. The small copper peak is from the supporting TEM grid.
from the particle. Only yttrium peaks can be seen. The relatively small peak from copper is from the supporting grid. This observation further supports the high purity of yttria synthesized by this technique. For commercial applications, both high phase and chemical purity is of prime importance. The investigations showed that the yttria prepared by sol–gel method in the present study meet these requirements and hence can be of commercial industrial applications.
4. Conclusions
1. The chemical route is a simple and efficient method for synthesis of high purity yttria from its molecular precursors. 2. EDAX and XRD investigations have confirmed the high chemical and phase purity of the yttria produced. 3. TEM observations confirm the nanocrystalline size of the yttria powders, with a bimodal size distribution of particles. The grain sizes of the
particles were found to be about 20–40 nm by dark field imaging in TEM.
Acknowledgements The authors are highly indebted to Dr P. Rodriguez ŽDirector, IGCAR. and Dr Baldev Raj ŽDirector, MMG, IGCAR. for their boundless motivation, inspiration and support. They also thank Dr V.S. Raghunathan ŽAssociate Director, MCG, IGCAR. for many useful discussions and suggestions. The authors would also like to record their thanks to Dr P. Radhakrishnan Nair ŽPrincipal, PSG College of Technology. for encouraging words and useful support. References w1x w2x w3x w4x w5x w6x
H. Gleiter, Prog. Mater. Sci. 33 Ž1989. 223. H. Gleiter, Nanostruct. Mater. 1 Ž1992. 1. R.W. Siegel, Nature ŽDecember 1996. 42. J. Livage, F. Beteille, Acta Mater. 46 Ž1998. 743. Mc N. Alford, J. Mater. Sci. 23 Ž1988. 761. M.B. Beaver ŽEd.., Encyclopaedia of Materials Science and Engineering, vol. 7, 1986.
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R. Subramanian et al.r Materials Letters 48 (2001) 342–346
w7x S. Nomura, T. Okuda, in: A.H. Clauer, J.J. de Barbadillo ŽEds.., Proc. of the Inter. Conf. on Solid State Powder Processing, Indianapolis, USA, The Minerals, Metals & Materials Society ŽTMS., USA, 1990, 195. w8x S. Ukai, M. Hirada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, K.A. Sabe, T. Nishida, M. Fujiwara, J. Nucl. Mater. 204 Ž1993. 65.
w9x G. Skandan, H. Hahn, J.C. Parker, Scr. Metall. Mater. 25 Ž1991. 2389. w10x A. Sagayaraj, MS Graduate ŽClemson University., private communication. w11x J. Livage, F. Beteille, C. Roux, M. Chatry, P. Davidson, Acta Mater. 46 Ž3. Ž1998. 743.
Materials Letters 48 Ž2001. 342–346 www.elsevier.comrlocatermatlet
Synthesis of nanocrystalline yttria by sol–gel method R. Subramanian, P. Shankar ) , S. Kavithaa, S.S. Ramakrishnan, P.C. Angelo, H. Venkataraman P.S.G. College of Technology, Coimbatore 641004, India Received 4 August 2000; accepted 5 September 2000
Abstract A method for the synthesis of nanocrystalline yttria by sol–gel method has been developed and is described in the paper. X-ray diffraction ŽXRD. and transmission electron microscopy ŽTEM. investigations confirm the grain sizes to be in the range of 20–40 nm. EDAX analysis from the yttria particles in TEM further confirmed the high chemical purity of the powders. Development of such an inexpensive and easy technique for synthesis of high purity nanocrystalline yttria can be of commercial significance. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Sol–gel; Yttria; Nanocrystalline; Transmission electron microscopy
1. Introduction Need for high performance materials for advanced applications have led to the development of new concepts in materials design, processing and their fabrication. The development of nanocrystalline materials with improved and novel properties is an important turning point in materials research w1–3x. Nanophase ceramics, in particular, have excellent potential in many engineering and medical applications w4x. Conventionally brittle ceramics have now shown superplastic behavior by synthesizing them in nanocrystalline form, thus opening newer venues for their engineering applications. )
Corresponding author. Physical Metallurgy Section, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Fax: q91-4114-40360. E-mail address: [email protected] ŽP. Shankar..
Yttria finds many high technology applications due to its excellent high temperature stability, high affinity for oxygen and sulfur and well-defined crystal structures, whose properties can be tailored by incorporating different types of ions into the structure w5,6x. Yttria is used as a host matrix for phosphors used in televisions and also in synthesis of yttria–thoria type translucent ceramics for lasers. Yttria is also being tried as a mould coating for precision Ti-6Al-4V castings. One of the most widespread application of yttria has of course been as stabilizer of cubic zirconia. Further, by virtue of being one of the most stable oxides, yttria is gaining importance in the production of Oxide Dispersion Strengthened ŽODS. materials w7,8x. The production of ODS alloys for enhanced high temperature strength, oxidation and corrosion resistance has resulted in a major breakthrough in high temperature materials research. These ODS alloys include iron
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 2 4 - 4
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
and nickel base alloys containing fine dispersions of thoria or yttria particles. Yttria is being used in the present day ODS alloys in preference to thoria. The properties of the ODS alloys are found to be improved if a uniform dispersion of nanocrystalline yttria particles in the ferrite matrix is achieved. The commercially available yttria is therefore of little use. A number of techniques for producing nanocrystalline yttria have been reported w9,10x. These include inert gas atomization, chemical vapor deposition, forced hydrolysis and emulsion route. Inert gas atomization and chemical vapor deposition are energy intensive and the high melting point of yttria makes these processes economically unviable. Forced hydrolysis and emulsion routes involve close control of process parameters Žsuch as pH, hydrophilerlipophile balance.. The objective of the present work was to develop an alternative simpler and economical chemical route for synthesis of nanosized yttria particles by sol–gel method. The other inherent advantages of producing ceramic powders by sol–gel technique includes that it results in a free flowing, spherical, strain free particle over a narrow size distribution. Apart from producing powder particles, the unique advantage of sol–gel method over inert gas atomization technique is that it can also be used to give coatings, monoliths and fibers on various substrate materials for possible future applications w11x.
2. Experimental The experimental procedures for the production of yttria from its metal salts were undertaken. This involves three distinct steps namely, Ži. solution preparation, Žii. filtration and drying and Žiii. calcination. These steps are described in detail below. 2.1. Solution preparation Ten grams of yttrium nitrate of 99.99% purity was dissolved in 100 ml of distilled water. Solution
343
was stirred till all the salt was dissolved and a clear solution was obtained. This concentration was maintained constant in all experiments. Precipitation of the powder was done using aqueous ammonium hydroxide as the hydrolyzing agent added at constant rate of 20 dropsrmin. The hydroxide solution was added till the precipitation of the salt was complete. During the precipitation, the solution was continuously stirred using a mechanical stirrer. 2.2. Filtration and drying The precipitated solution was washed thoroughly with distilled water and dried in an oven at 373 K for 3 h. After drying, the powder was obtained as agglomerates, which was subsequently ground carefully using a pestle and mortar. 2.3. Calcination The ground yttrium hydroxide powders were converted into yttrium oxide through calcination. The powders were calcined in a silica crucible at 1123 K for 3 h. After calcination, the powders were cooled in the furnace to room temperature.
3. Results and discussions The powder samples were characterized using XRD, TEM and EDAX for ascertaining the purity, morphology and size distribution of the particles. XRD characterization was performed using a Philips X-ray diffractometer using Cu-K a radiation. The result of the XRD analysis is shown in Fig. 1. From the XRD pattern, it is observed that only peaks corresponding to cubic yttria were present. No other impurity phases could be detected by XRD, indicating the high phase purity of the yttria powders produced by this method. The grain sizes of the yttria particles were calculated using the Debye– Scherrer formula, from the FWHM of the peaks. Since the sol–gel method is expected to result in nearly spherical, strain free particles, it is reasonable to assume the entire peak broadening to result from the fine grains size of the powders. This is further justified by the fact that the peaks were found to be nearly symmetrical, thus ruling out the contribution
344
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
Fig. 1. XRD chart from yttria powders showing the presence of only cubic phase.
of defects like dislocations and stacking faults. The instrumental broadening of the XRD used in the present study was 0.088 and its contribution to the peak broadening was corrected using the standard Warren–Averbach formalism. The average grain size was estimated to be about 50 nm, indicating the ultra fine grain size of the yttria. Samples for TEM investigations were prepared by suspending the yttria powders on a carbon film supported by a copper grid. The samples were then analyzed using a Philips CM 200 TEM. A low
magnification micrograph showing the distribution of yttria particles is shown in Fig. 2. A bimodal size distribution of yttria particles was observed, having large number of particles in the 50–100 nm and a few particles of about 500–1000 nm in size. The dark field image of yttria particles is shown in Fig. 3. The grain sizes are found to be in the range of 20–40 nm, as can be seen from the micrograph. This is in close agreement with the grain size estimates from XRD analysis. Fig. 4 shows a typical EDAX output
Fig. 2. A low magnification TEM micrograph showing the distribution of yttria particles.
Fig. 3. Dark field TEM micrograph showing the NC sizes of yttria.
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
345
Fig. 4. EDAX pattern from a yttria particle showing the high chemical purity. The small copper peak is from the supporting TEM grid.
from the particle. Only yttrium peaks can be seen. The relatively small peak from copper is from the supporting grid. This observation further supports the high purity of yttria synthesized by this technique. For commercial applications, both high phase and chemical purity is of prime importance. The investigations showed that the yttria prepared by sol–gel method in the present study meet these requirements and hence can be of commercial industrial applications.
4. Conclusions
1. The chemical route is a simple and efficient method for synthesis of high purity yttria from its molecular precursors. 2. EDAX and XRD investigations have confirmed the high chemical and phase purity of the yttria produced. 3. TEM observations confirm the nanocrystalline size of the yttria powders, with a bimodal size distribution of particles. The grain sizes of the
particles were found to be about 20–40 nm by dark field imaging in TEM.
Acknowledgements The authors are highly indebted to Dr P. Rodriguez ŽDirector, IGCAR. and Dr Baldev Raj ŽDirector, MMG, IGCAR. for their boundless motivation, inspiration and support. They also thank Dr V.S. Raghunathan ŽAssociate Director, MCG, IGCAR. for many useful discussions and suggestions. The authors would also like to record their thanks to Dr P. Radhakrishnan Nair ŽPrincipal, PSG College of Technology. for encouraging words and useful support. References w1x w2x w3x w4x w5x w6x
H. Gleiter, Prog. Mater. Sci. 33 Ž1989. 223. H. Gleiter, Nanostruct. Mater. 1 Ž1992. 1. R.W. Siegel, Nature ŽDecember 1996. 42. J. Livage, F. Beteille, Acta Mater. 46 Ž1998. 743. Mc N. Alford, J. Mater. Sci. 23 Ž1988. 761. M.B. Beaver ŽEd.., Encyclopaedia of Materials Science and Engineering, vol. 7, 1986.
346
R. Subramanian et al.r Materials Letters 48 (2001) 342–346
w7x S. Nomura, T. Okuda, in: A.H. Clauer, J.J. de Barbadillo ŽEds.., Proc. of the Inter. Conf. on Solid State Powder Processing, Indianapolis, USA, The Minerals, Metals & Materials Society ŽTMS., USA, 1990, 195. w8x S. Ukai, M. Hirada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, K.A. Sabe, T. Nishida, M. Fujiwara, J. Nucl. Mater. 204 Ž1993. 65.
w9x G. Skandan, H. Hahn, J.C. Parker, Scr. Metall. Mater. 25 Ž1991. 2389. w10x A. Sagayaraj, MS Graduate ŽClemson University., private communication. w11x J. Livage, F. Beteille, C. Roux, M. Chatry, P. Davidson, Acta Mater. 46 Ž3. Ž1998. 743.