A new combustion process for nanosized YBa2ZrO5.5 powders

NanoStructured Materials, Vol. 11, No. 5, pp. 623– 629, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter

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A NEW COMBUSTION PROCESS FOR NANOSIZED YBa2ZrO5.5 POWDERS R. Jose, J. James, Asha M. John, D. Sundararaman1, R. Divakar1 and J. Koshy* Regional Research Laboratory (CSIR), Trivandrum, India 695 019 Indira Gandhi Center for Atomic Research (DOE), Kalpakkam, India 603 102

1

(Received April 5, 1999) (Accepted July 2, 1999) Abstract—A new single step process for the synthesis of nanoparticles of YBa2ZrO5.5, a complex perovskite ceramic oxide, is reported for the first time. In the present modified combustion method no calcination step was needed to obtain phase pure powder of YBa2ZrO5.5. The YBa2ZrO5.5 powder, obtained by self sustained combustion of a precursor complex of the respective metal ions, is characterised by High Resolution Transmission Electron Microscopy, Powder X-ray Diffraction, Differential Thermal Analysis, Thermo-gravimetric Analysis, FT-IR Spectroscopy, BET surface area Analysis and Agglomerate size analysis. The particle size of the as prepared YBa2ZrO5.5 was in the range 5 nm to 50 nm. The lattice image of the nanocrystalline particle showed very little distortion and the intercrystalline boundary was quite sharp without any second phase. ©1999 Acta Metallurgica Inc.

Introduction Synthesis of nanoparticles of ceramic oxides is one of the major challenges in the development of advanced ceramics and other specialty materials. Nanosized particles, because of its high surface energy and driving force, can be densified at much lower temperature as compared to large grained powders and the final product will exhibit unique mechanical, optical, magnetic and electrical properties (1–3). By using conventional solid state routes it is not possible to obtain a nanosized powder because the initial sizes of the reactants themselves are much larger. Recently we have made attempts to synthesize a new material YBa2ZrO5.5 using conventional solid state route for their possible applications as the substrate for high Tc superconductors (4,5). It was not possible to get a phase pure YBa2ZrO5.5 even after repeated calcination at 1500°C for prolonged duration. However, it was found that the addition of a small amount of CuO to the reaction mixture enhanced the phase formation of YBa2ZrO5.5 considerably (4). But the addition of CuO adversely affected the electric and dielectric properties of YBa2ZrO5.5 for substrate application at microwave frequencies. In order to improve the performance of this material, especially its sinterability and phase purity, attempts were made to synthesize YBa2ZrO5.5 as nanosized particles by conventional chemical methods such as co-precipitation, sol gel and spray pyrolysis and combustion synthesis (6 –7). Our attempts were not successful in obtaining nano sized particles of YBa2ZrO5.5 using any of the above methods. We have now developed a new single step method for the preparation of nanosized particles of YBa2ZrO5.5 and in the present paper we report the details of the preparation of the powder and their characterisation for the first time.

*Corresponding author, e-mail: [email protected]; Fax: ⫹91-471-490186/491712

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Experimental Aqueous solutions containing ions of Y, Ba and Zr were prepared from Y2O3, Ba(NO3)2 and ZrOCl2 .8H2O. Since Y2O3 is insoluble in water, it was dissolved in dilute nitric acid. To get the precursor complex, citric acid was added to the solution containing the metal ions maintaining the citric acid to cation ratio at unity. The oxidant fuel ratio of the system was adjusted by using nitric acid and ammonium hydroxide. The solution containing the complex precursor at neutral pH was then heated on a hot plate at ⬃250°C. On heating, the solution boils and undergoes dehydration followed by decomposition leading to smooth deflation producing a foam. The foam then ignites giving voluminous and fluffy product of combustion which is subsequently characterised as phase pure YBa2ZrO5.5 powder. In our present modified combustion method, we have used citric acid as the complexing agent instead of poly vinyl alcohol (PVA) and replaced urea with ammonia for preparation of nanosized particles of YBa2ZrO5.5 complex perovskite oxide. By replacing the complexing agent and the oxidant we were able to obtain the phase pure nanosized particles of the YBa2ZrO5.5 in a single step and the usual calcination for prolonged duration at high temperature was not required. The as-prepared YBa2ZrO5.5 ceramic powder was pressed at a pressure of 600 MPa in the form of circular discs of diameter ⬃13 mm and thickness ⬃2 mm and sintered at the temperature of 1500°C for 4h in air. The structure of the as prepared and sintered product was examined by X-ray powder diffraction (XRD) technique using a computerized Rigaku, Dmax (Japan) X-ray diffractometer with Nickel filtered CuK␣ radiation. In order to see whether there is any phase transition, Differential Thermal Analysis (DTA) of the reaction product was carried out in the temperature range 30 –1100°C at a heating rate of 10°C/min. Thermogravimetric Analysis (TGA) was done on a Schimadzu TGA-50H in the temperature range 30 –1100°C at a heating rate of 10°C/min. The FT-IR spectrum of the combustion product was recorded on the Nicolet I 400 D FT-IR spectrometer by using KBr pellet method. The BET surface area of the sample was obtained using a Micomeritics surface area analyser model 2360. The agglomerate size distribution of the as-prepared powder was studied using a Micromeritics 5100 sedigraph. The YBa2ZrO5.5 powders obtained after combustion was examined using a JEOL 2000 EX II High Resolution Transmission Electron Microscope (HRTEM) with a top entry stage at 200 kV. The powder particles were supported on a carbon film coated on a 3 mm diameter fine mesh copper grid. A suspension in methanol was agitated with ultrasound and two drops from the topmost layers were dropped on the support film.

Results and Discussion The XRD pattern of the as-prepared YBa2ZrO5.5 is shown in Figure 1. All the peaks are indexed for a complex cubic perovskite structure corresponding to YBa2ZrO5.5. The lattice constant value calculated from the XRD patterns of the as-prepared powders of YBa2ZrO5.5 are in good agreement with that reported earlier (4). There was no indication in the XRD pattern of any additional phase in the as-prepared powder. These observations clearly show that the phase formation was complete during the combustion process itself, without the need for a calcination step. The broad nature of the peaks in Fig. 1 indicate the ultrafine nature of the crystallites. The crystallite size calculated from FWHM using Scherrer formula for (220) plane is 18 nm. It may be noted here that YBa2ZrO5.5 synthesised through solid state route, did not yield a single phase material even after calcination at 1500°C for 10h with intermediate grindings (4). To the best of our knowledge the present method is the only one which has effectively produced a phase pure YBa2ZrO5.5 without the use of CuO as an additive. Fig. 2 shows the XRD pattern taken on sintered YBa2ZrO5.5 for 2␪ values between 5° and 90° and the XRD data comprising 2␪ values, d values, line intensities and (hkl) values are given in Table 1.

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Figure 1. X-ray diffraction pattern of as synthesised YBa2ZrO5.5.

Figure 2. X-ray diffraction patterns of sintered YBa2ZrO5.5.

TABLE 1 X-Ray Diffraction Data of YBa2ZrO5.5 2␪ (deg.) 18.278 21.136 30.060 35.413 37.050 42.910 48.422 53.930 56.914 62.496 70.897 78.888 86.667

d(Å)

I/Io

(hkl)

4.849 4.200 2.970 2.532 2.424 2.106 1.878 1.715 1.616 1.484 1.328 1.212 1.122

4 6 100 6 6 33 8 27 6 10 11 4 8

111 200 220 311 222 400 420 422 511 440 620 444 642

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Figure 3. DTA curve of the YBa2ZrO5.5 combustion product.

YBa2ZrO5.5 has a cubic perovskite structure with four formula units per unit cell and belongs to Fm3m space group. It is also found to be isostructural with other rare earth cubic perovskites with the general formula A2(BB’)O6 such as EuBa2NbO6 and YBa2NbO6 reported in the JCPDS file in which the doubling of the basic perovskite unit cell is observed. The doubling of the basic perovskite unit cell in these material is due to the ordering of B and B’ atoms on the octahedral sites. In a substitutional solid solution there is random arrangement of B and B’ atoms in equivalent positions in the crystal structure. If, upon suitable heat treatment, the random solid solution rearranges into a structure in which the B and B’ occupy the same set of positions but in a regular way, the structure is described as a superstructure (8). The presence of superstructural lines in the XRD patterns shown in Fig. 2 indicates the ordering of the basic ABO3 perovskite unit cell in the YBa2ZrO5.5 material. Though YBa2ZrO5.5 material has the A2(BB’)O6 structure, taking into account the valency of Zr at 4⫹, the chemical formula of the compound is written as YBa2ZrO5.5. The XRD peaks, including the minor ones of YBa2ZrO5.5, are now indexed for a cubic perovskite structure. Theoretical density of YBa2ZrO5.5 was calculated from lattice constant value, and sintered density was measured by Archimede’s method. The nanostructured powder was pressed in the form of pellets with dimensions ⬃13 mm diameter, ⬃2 mm thickness and a single phase sinterd sample with density ⬎96% of the theoretical density was obtained by sintering the pellets at 1500°C for 4 h. A typical DTA curve of an as-prepared YBa2ZrO5.5 powder, directly after combustion is shown in Fig. 3. The endotherms at ⬃100°C may be explained as the removal of loosely bound adsorbed water. The endotherms at ⬃150°C may be due to the removal of chemically bonded water. These assignments are also consistent with the weight changes observed in TGA. The overall weight change in the sample was only 5% in two stages, ⬃100°C and ⬃150°C and thereafter the weight of the sample remained constant until 1100°C as shown by TGA. The most important aspect evident from the thermal studies is the absence of enthalpy changes at high temperatures, which implies that the decomposition is complete and no organic matter is present in the sample. There is no evidence of a phase transition taking place in the sample up to a temperature of 1100°C. To examine whether the decomposition is complete and to detect the presence of organic impurities left in the sample after combustion, FT-IR studies have been carried out in the frequency range 400-4000 cm⫺1. Fig. 4 shows the FT-IR spectrum of a typical as prepared YBa2ZrO5.5 powder. There is no evidence for the presence of any organic matter in the sample. Except for the peaks due to water or adsorbed moisture (3447 cm⫺1 and 848 cm⫺1) all the other peaks are of the characteristic vibrations

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Figure 4. FT-IR spectrum of the as-synthesised YBa2ZrO5.5.

of YBa2ZrO5.5. This observation further supports the DTA and XRD results that no organic matter remains in the sample and that the decomposition is complete. The BET surface area of the as-prepared YBa2ZrO5.5 powder is 21 m2/g and the average particle size calculated from the surface area is 48 nm. The agglomerate size distribution of YBa2ZrO5.5 powders directly after combustion is shown in Fig. 5. The maximum size of the agglomerates as seen from the agglomerate size distribution curve was 20 ␮m. The average agglomerate size of the samples are less than 800 nm. The TEM studies of the YBa2ZrO5.5 obtained by combustion synthesis showed the powder particles to be submicron sized aggregates of nanocrystallites. Fig. 6(a) is a typical bright field image of a powder particles and Fig. 6(b) corresponding selected area diffraction pattern. Aggregate sizes ranged from about 300 nm to more than 1000 nm while the crystallite sizes, as measured in dark field images, ranged from 5 nm to 50 nm. Fig. 6(c) is a representative powder particle, imaged in the dark field mode using the (220) beams. The electron diffraction pattern obtained for the nanocrystalline particle has been

Figure 5. Agglomerate size distribution of the as-synthesised YBa2ZrO5.5.

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(a)

(b)

(c)

Figure 6. (a) Bright field micrograph of a powder particle of the as-synthesised YBa2ZrO5.5 (b) corresponding selected area diffraction pattern, and (c) dark field (220) image of a particle of the as-synthesized YBa2ZrO5.5.

indexed and is in agreement with the XRD results. The HRTEM lattice imaging of the nano crystalline powder showed very little distortion or strain in the lattice and the interface is quite sharp and seems to be without any second phases. Conclusions Phase pure nanoparticles of YBa2ZrO5.5 has been synthesised through a modified combustion route. The X-ray diffraction and thermal studies revealed that YBa2ZrO5.5 was obtained in a single step

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process by directly subjecting a precursor complex of the respective metal ions to self sustained combustion and no calcination procedure was needed to obtain a phase pure powder. The particle size of the as-synthesised powder was in the range 5–50 nm. The nanostructured YBa2ZrO5.5 can be sintered to high density (⬎96%) at a relatively low temperature of 1500°C. To the best of our knowledge the present modified combustion route is the only successful method to obtain a high density phase pure sintered sample of YBa2ZrO5.5 and this could be attributed to the extremely small particle size of 5–50 nm of the powder. The lattice image of the nanocrystalline particle showed very little distortion and this intercrystalline boundary was quite sharp without any second phase. Acknowledgments The authors R. Jose and Asha M. John are thankful to CSIR for financial support. References 1. 2. 3. 4. 5. 6. 7. 8.

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