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Dilatometric studies on nano-crystalline ceria: Powder properties-sintering correlation
Journal of Alloys and Compounds 459 (2008) 477–480
Dilatometric studies on nano-crystalline ceria: Powder properties-sintering correlation V. Bedekar a , S.V. Chavan a , M. Goswami b , G.P. Kothiyal b , A.K. Tyagi a,∗ a
b
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received 28 March 2007; received in revised form 26 April 2007; accepted 26 April 2007 Available online 29 April 2007
Abstract Nano-crystalline ceria was synthesized by different routes to give powders with different properties and morphologies. These powders were characterized by X-ray diffraction (XRD), surface area and light scattering studies. The linear shrinkage and the sintering behavior of all these powders were in situ studied by a dilatometer up to 1200 ◦ C for hold period of 1 h. The sintered microstructures were investigated by scanning electron microscope (SEM). It was observed that the powder morphologies are a function of the preparative method. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-crystalline materials; Ceramics; Rare earth; Sintering
1. Introduction Cerium oxide belongs to the technologically important family of rare-earth oxides with a wide range of applications such as a glass-polishing material, ultraviolet absorbent, catalysts for automotive exhaust treatment [1], oxygen sensors [2], electrode materials for solid oxide fuel cells (SOFC) [3], etc. High bulk density pellets of ceria are desirable for many of these applications and also for studies such as bulk thermal expansion, ionic conductivity measurements, etc. However, commercially available ceria powders are difficult to sinter to high densities due to the highly refractive nature and undesirable characteristics, mainly large crystallite size and severe agglomeration [4]. Also, ceria exhibits a strong tendency to undergo reduction at high temperatures 4CeO2 → 2Ce2 O3 + O2 (g) and the oxygen thus released during this process tends to retard the densification behavior [5]. Thus, the synthesis of ceria powder with controlled powder characteristics is of practical importance to get dense sintered product at lower sintering ∗ Corresponding author. Tel.: +91 22 2559 5330; fax: +91 22 2550 5151/2551 9613. E-mail addresses: [email protected], [email protected] (A.K. Tyagi).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.290
temperatures. There are several reports describing solution processing techniques to yield ceria powders which are sinterable at lower temperatures [6–10]. The major driving force for sintering in these cases is the higher surface area associated with the nano-crystalline nature of the powders and lower agglomeration, which enhances the sintering [11]. In the present study it was first attempted to prepare ceria powders with a range of varying characteristics and then to investigate the role of these powder properties on the sintering behavior. Thus, a number of solution routes such as decomposition, precipitation, combustion, were used to prepare the ceria powders in the nano-crystalline form. Each of these routes resulted in powders with different characteristics in terms of particle size, surface area, agglomeration, etc., which in turn were expected to affect the sinterability of these powders. The sintering behavior of all these powders was then in situ monitored by a dilatometer. Herein, we have made an effort to correlate and compare the powder characteristics viz., crystallite size, surface area, agglomerate size with the sintering behavior and the sintered microstructures of ceria powders prepared by different routes. 2. Experimental Ceria powders were prepared starting from cerium nitrate [Ce(NO3 )3 ·6H2 O] and using each of the following reagents individually viz., glycine, citric acid,
478
V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
hydrazine, ammonium hydroxide and oxalic acid as the fuels/precursors. Also ceria powder was prepared by decomposition of cerium nitrate itself. The details of all these processes which are categorized as combustion, precipitation, decomposition are given below. In the gel combustion process, cerium nitrate was used as the oxidant and glycine and citric acid were used as the fuels. The oxidant and fuel were taken in the ratio of 1:1.0 in both the cases. These solutions after thermal dehydration (at ≈80 ◦ C on a hot plate to remove the excess solvent) gave highly viscous liquid termed as gel. On increasing the temperature to about ≈300 ◦ C, these gels swelled and auto-ignited, with a rapid evolution of large volume of gases to produce voluminous ceria powders. The ceria powders were also prepared by precipitation, where hydrazine and ammonium hydroxide were added to a solution of cerium nitrate, till complete precipitation occurred. The precipitates were then washed thoroughly with warm water and decomposed by heating at about 300 ◦ C to give pure nano-crystalline ceria powders. Another aqueous solution of cerium nitrate was precipitated using a sufficient quantity of oxalic acid, then washed for unwanted nitrates and excess oxalic acid using de-ionized water and dried to synthesize nano-crystalline ceria via the oxalate route. Finally, ceria powder was also obtained by decomposing cerium nitrate alone again at about 300 ◦ C. The ceria powders obtained in all the cases were further calcined at 500 ◦ C for 30 min to maintain uniformity and phase homogeneity. X-ray diffraction (XRD) was carried out on these ceria powders, for phase identification and crystallite size estimation, using Cu K␣ radiation on a Philips X-ray diffractometer (Model PW 1710). Silicon was used as an external standard. The surface area analysis was carried out by the standard BET technique with N2 adsorption using a Quantachrome autosorb-1 analyzer. The nature and extent of agglomeration was studied by Horiba, Model LA-500 (Japan), particle size analyzer based on laser diffraction, which covers the particle size range of 0.20–200 m. A non-ionic dispersant (Triton X 100) was also added initially for facilitating the dispersion of the powders. The in situ sintering behavior of these powder samples was studied on cold pressed pellets of 8 mm diameter. The sintered density of the pellets was measured after the dilatometric experiments by Archimedes’ principle. The sintered pellets were gold coated and their microstructures were recorded using Tescan VEGA MV2300T/40 Electron Microscope. The dilatometric measurements, from 25 to 1200 ◦ C, were carried out on a dilatometer (model TMA/92 Setaram, France) using an alumina probe, with a heating rate of 10 ◦ C/min in argon atmosphere.
3. Results and discussion Combustion synthesis involves a redox reaction between an oxidizer (nitrates) and a fuel. Glycine is one of the cheapest amino acids. It is known to act as a complexing agent for a number of metal ions as it has a zwitter ionic character due to a carboxylic group at one end and an amino group at the other end [12]. Citric acid is a good complexing agent due to its multi-denticity. It has three carboxylic and one hydroxyl group for coordinating with the metal ions, which facilitate the formation of viscous gel [13]. In the present case, nano-crystalline ceria powders have been prepared by glycine-nitrate and citratenitrate combustion in the ratio 1:1.0.
Fig. 1. Agglomerate size distribution of powder prepared using hydrazine.
The properties of the ceria powders synthesized by all the above-mentioned different routes are tabulated in Table 1. It can be seen that the crystallite size as calculated by X-ray line broadening using Schererr’s formula, was found to range in between 10 and 17 nm. The surface area of the powder obtained from glycine combustion was as low as 18 m2 /g while the oxalate precipitated product showed largest surface area of 70 m2 /g. It is usually observed that smaller the crystallite size, higher is the surface area and sintered density. However, in this case, no such trend is observed for the ceria powders. An important aspect of the nano-powders which is generally overlooked is their agglomeration. In the case of agglomerated particles, the packing density tends to decrease which can be correlated with increasing attraction between particles with decreasing particle size due to Vander-Waals interaction [14]. In order to understand the role of agglomerates, their size was investigated by dynamic light scattering experiments. The particle size analyzer is based on the laser diffraction. Although, this technique is not capable of giving the true size of the nano-particles but it is a good tool to find out the size and nature of the agglomerates. The agglomerate size of the ceria nano-powders after ultrasonication for about 10 min is given in Table 1. It is seen that the product obtained using hydrazine as the fuel was highly agglomerated with an average agglomerate size of about 6.18 m (Fig. 1). The ceria nano-powders synthesized using glycine gave the smallest agglomerates with average particle size of 1.43 m. The green pellets were sintered in situ during dilatometric studies at 1200 ◦ C for 1 h. It was observed that although the
Table 1 Powder properties of ceria powders prepared from different routes Process
Fuel/precursor
Crystallite size (nm)
Surface area (m2 /g)
Agglomerate size (m)
Density (% th.d.)
a. Combustion b. Decomposition c. Precipitation d. Combustion e. Precipitation f. Precipitation
Citric acid Nitrate Ammonium hydroxide Glycine Oxalic acid Hydrazine
10 13 15 12 17 10
63 49 41 18 70 39
2.15 3.63 1.84 1.43 5.43 6.18
96 93 85 88 86 70
V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
Fig. 2. SEM micrograph of sintered sample prepared using citric acid.
powders synthesized using citric acid and hydrazine showed similar crystallite size (10 nm), their sintered densities showed a remarkable difference. The citrate-nitrate combustion resulted in highly sinter-active nano-powders, the sintered density being 96% of theoretical density (th.d.) of ceria. The ceria powder prepared by hydrazine route when sintered under the identical conditions gave a density of 70% of th.d. The sintered densities of the powders obtained using other fuels essentially range in between these two extremes. For example, ceria obtained from glycine combustion gave nano-powder with average crystallite size of 12 nm and a sintered density of 88% of th.d. The sintered microstructures were investigated by SEM. Figs. 2 and 3 show the micrographs of the fractured surface of the sintered pellets prepared using citric acid and glycine as fuels, respectively. In the case of citric acid, the grains are much larger with an average grain size of about 1–1.5 m. Thus, the ultrafine nature and high surface area of the starting powder seems to be responsible for achieving high sintered density at a low temperature in shorter duration. The microstructure of the sintered pellet obtained from glycine-nitrate combustion
Fig. 3. SEM micrograph of sintered sample prepared using glycine.
479
Fig. 4. Linear shrinkage as a function of temperature.
is shown in Fig. 3. Here all the grains were of fairly regular shape with most of the grains being smaller than 1.0 m. It can be seen that the microstructure is not fully developed. The presence of small voids confirms the low sintered density of 88%. The linear shrinkage as a function of temperature for the different samples is shown in Fig. 4. It can be seen that ceria obtained by citrate-nitrate combustion (sample a) showed maximum shrinkage. Another striking observation is that maximum shrinkage occurred in the narrowest temperature range (∼800–900 ◦ C) for this sample. In contrast, the ceria prepared by hydrazine route (sample f) showed minimum shrinkage which was diffused over a wide temperature range. The ceria obtained from cerium nitrate decomposition (sample b) also showed a high shrinkage. Other three samples showed the shrinkage, which was in between these two extremes (samples a and f). This kind of sintering behavior of the ceria samples could be related to their powder characteristics such as crystallite size, surface area, sintered density and agglomerate size. It can be seen that the sample prepared by citrate-nitrate combustion consists of finest crystallites and relatively high surface area, which is the driving force for maximum shrinkage of the green pellet over a narrow temperature range; hence, a sintered density as high as 96% of th.d. could be obtained for this sample. However, the ceria powder prepared by hydrazine route resulted in only 70% of th.d. This poor density can be attributed to the presence of large agglomerates. It is seen from Table 1 that although this nano-powder had small crystallite size (10 nm), its agglomerate size was largest (6.18 m). Also the surface area of this powder was not so large (39 m2 /g) compared to the powder prepared by citrate-nitrate combustion (63 m2 /g). Thus, it can be said conclusively that apart from the crystallite size and surface area, the agglomerate size also plays a vital role in governing the sintering behavior. Large agglomerates are not favorable for achieving high sintered density. All other samples showed sintered densities in between these two samples (prepared by citrate-nitrate combustion and hydrazine method, respectively). The powder prepared by glycine-nitrate combustion had a small surface area
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V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
(18 m2 /g) however it showed a reasonably high sintered density (88%). In can be noticed that the agglomerate size of this sample is the smallest (1.43 m), which is responsible for getting a somewhat high sintered density. It is inferred that the final sintered density is an interplay between three powder properties namely crystallite size, surface area and agglomerate size. In fact, all these three powder characteristics are interdependent. The finer crystallite size need not always result in highest sintered density. One needs to optimize the agglomerate size also. It also appears from Table 1 that the surface area and agglomerate size have predominant influence on the sintering behavior as compared to the crystallite size. Another important conclusion which can be made from the present investigation is that much better sintering can be obtained if the sintering occurs over a narrow range of temperature. A diffuse sintering step (e.g. in sample f) is highly detrimental for attaining high sintered densities. A close look at the linear shrinkage data clearly reveals that the sintering is poor even up to 1200 ◦ C in the case of powder f. On the other hand the shrinkage behavior for the powders a and b had almost attained saturation at 1200 ◦ C. In order to achieve the comparable sintered densities for powders e and f, one would need much higher temperatures, as seen by the slope of the linear shrinkage curve for these samples. The combustion reaction performed using citric acid as the fuel was found to be sluggish. During this process, there is evolution of gaseous products which dissipates the heat of combustion and limits the rise of temperature, thus reducing the possibility of premature local partial sintering among the primary particles. Hence, the ceria powder obtained by citrate-nitrate combustion showed excellent powder properties and a sharp and maximum shrinkage during the sintering process.
4. Conclusions Each of the ceria powders synthesized were essentially nanocrystalline in nature and showed reasonably high surface areas. The variation in the properties such as surface area and agglomerate size appears to be characteristic of the complex combustion or the decomposition processes occurring during each synthesis. The in situ sintering studies revealed that the sintering behavior was a cumulative function of the various powder morphologies and in particular strongly influenced by the agglomeration behavior. It may be suggested that in order to enhance the sinterability and the microstructure, one has to optimize the initial powder characteristics by means of selecting suitable preparatory methods. References [1] A. Trovarelli, F. Zamar, J. Llorca, C. Leitenburg, G. Dolcetti, J.T. Kiss, J. Catal. 169 (1997) 490. [2] P. Jasinski, T. Suzuki, H.A. Anderson, Sens. Actuators B 95 (2003) 73. [3] D.S. Bae, B. Lim, B.I. Kim, K.S. Han, Mater. Lett. 56 (2002) 610. [4] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, Acta Mater. 49 (2001) 419. [5] Y.C. Zhou, M.N. Rahaman, Acta Mater. 45 (1997) 3635. [6] T. Mokkelbost, I. Kaus, T. Grande, M. Einarsrud, Chem. Mater. 16 (2004) 5489. [7] M. Hirano, Y. Fukuda, H. Iwata, Y. Hotta, M. Inagaki, J. Am. Ceram. Soc. 83 (2000) 1287. [8] D-Si. Bae, B. Lim, B.-I. Kim, K.-S. Han, Mater. Lett. 56 (2002) 610. [9] H.S. Kang, Y.C. Kang, H.Y. Koo, S.H. Ju, D.Y. Kim, S.K. Hong, J.R. Sohn, K.Y. Jung, S.B. Park, Mater. Sci. Eng. B 127 (2006) 99. [10] J.-S. Lee, S.-C. Choi, Mater. Lett. 58 (2004) 390. [11] R.D. Purohit, S. Saha, A.K. Tyagi, Ceram. Int. 32 (2005) 143. [12] R.D. Purohit, B.P. Sharma, K.T. Pillai, A.K. Tyagi, Mater. Res. Bull. 36 (2001) 2711. [13] J. Tsay, T. Fang, J. Am. Chem. Soc. 82 (1999) 1409. [14] H. Ferkel, R.J. Hellmig, Nanostruct. Mater. 11 (1999) 617.
Dilatometric studies on nano-crystalline ceria: Powder properties-sintering correlation V. Bedekar a , S.V. Chavan a , M. Goswami b , G.P. Kothiyal b , A.K. Tyagi a,∗ a
b
Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, India Received 28 March 2007; received in revised form 26 April 2007; accepted 26 April 2007 Available online 29 April 2007
Abstract Nano-crystalline ceria was synthesized by different routes to give powders with different properties and morphologies. These powders were characterized by X-ray diffraction (XRD), surface area and light scattering studies. The linear shrinkage and the sintering behavior of all these powders were in situ studied by a dilatometer up to 1200 ◦ C for hold period of 1 h. The sintered microstructures were investigated by scanning electron microscope (SEM). It was observed that the powder morphologies are a function of the preparative method. © 2007 Elsevier B.V. All rights reserved. Keywords: Nano-crystalline materials; Ceramics; Rare earth; Sintering
1. Introduction Cerium oxide belongs to the technologically important family of rare-earth oxides with a wide range of applications such as a glass-polishing material, ultraviolet absorbent, catalysts for automotive exhaust treatment [1], oxygen sensors [2], electrode materials for solid oxide fuel cells (SOFC) [3], etc. High bulk density pellets of ceria are desirable for many of these applications and also for studies such as bulk thermal expansion, ionic conductivity measurements, etc. However, commercially available ceria powders are difficult to sinter to high densities due to the highly refractive nature and undesirable characteristics, mainly large crystallite size and severe agglomeration [4]. Also, ceria exhibits a strong tendency to undergo reduction at high temperatures 4CeO2 → 2Ce2 O3 + O2 (g) and the oxygen thus released during this process tends to retard the densification behavior [5]. Thus, the synthesis of ceria powder with controlled powder characteristics is of practical importance to get dense sintered product at lower sintering ∗ Corresponding author. Tel.: +91 22 2559 5330; fax: +91 22 2550 5151/2551 9613. E-mail addresses: [email protected], [email protected] (A.K. Tyagi).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.290
temperatures. There are several reports describing solution processing techniques to yield ceria powders which are sinterable at lower temperatures [6–10]. The major driving force for sintering in these cases is the higher surface area associated with the nano-crystalline nature of the powders and lower agglomeration, which enhances the sintering [11]. In the present study it was first attempted to prepare ceria powders with a range of varying characteristics and then to investigate the role of these powder properties on the sintering behavior. Thus, a number of solution routes such as decomposition, precipitation, combustion, were used to prepare the ceria powders in the nano-crystalline form. Each of these routes resulted in powders with different characteristics in terms of particle size, surface area, agglomeration, etc., which in turn were expected to affect the sinterability of these powders. The sintering behavior of all these powders was then in situ monitored by a dilatometer. Herein, we have made an effort to correlate and compare the powder characteristics viz., crystallite size, surface area, agglomerate size with the sintering behavior and the sintered microstructures of ceria powders prepared by different routes. 2. Experimental Ceria powders were prepared starting from cerium nitrate [Ce(NO3 )3 ·6H2 O] and using each of the following reagents individually viz., glycine, citric acid,
478
V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
hydrazine, ammonium hydroxide and oxalic acid as the fuels/precursors. Also ceria powder was prepared by decomposition of cerium nitrate itself. The details of all these processes which are categorized as combustion, precipitation, decomposition are given below. In the gel combustion process, cerium nitrate was used as the oxidant and glycine and citric acid were used as the fuels. The oxidant and fuel were taken in the ratio of 1:1.0 in both the cases. These solutions after thermal dehydration (at ≈80 ◦ C on a hot plate to remove the excess solvent) gave highly viscous liquid termed as gel. On increasing the temperature to about ≈300 ◦ C, these gels swelled and auto-ignited, with a rapid evolution of large volume of gases to produce voluminous ceria powders. The ceria powders were also prepared by precipitation, where hydrazine and ammonium hydroxide were added to a solution of cerium nitrate, till complete precipitation occurred. The precipitates were then washed thoroughly with warm water and decomposed by heating at about 300 ◦ C to give pure nano-crystalline ceria powders. Another aqueous solution of cerium nitrate was precipitated using a sufficient quantity of oxalic acid, then washed for unwanted nitrates and excess oxalic acid using de-ionized water and dried to synthesize nano-crystalline ceria via the oxalate route. Finally, ceria powder was also obtained by decomposing cerium nitrate alone again at about 300 ◦ C. The ceria powders obtained in all the cases were further calcined at 500 ◦ C for 30 min to maintain uniformity and phase homogeneity. X-ray diffraction (XRD) was carried out on these ceria powders, for phase identification and crystallite size estimation, using Cu K␣ radiation on a Philips X-ray diffractometer (Model PW 1710). Silicon was used as an external standard. The surface area analysis was carried out by the standard BET technique with N2 adsorption using a Quantachrome autosorb-1 analyzer. The nature and extent of agglomeration was studied by Horiba, Model LA-500 (Japan), particle size analyzer based on laser diffraction, which covers the particle size range of 0.20–200 m. A non-ionic dispersant (Triton X 100) was also added initially for facilitating the dispersion of the powders. The in situ sintering behavior of these powder samples was studied on cold pressed pellets of 8 mm diameter. The sintered density of the pellets was measured after the dilatometric experiments by Archimedes’ principle. The sintered pellets were gold coated and their microstructures were recorded using Tescan VEGA MV2300T/40 Electron Microscope. The dilatometric measurements, from 25 to 1200 ◦ C, were carried out on a dilatometer (model TMA/92 Setaram, France) using an alumina probe, with a heating rate of 10 ◦ C/min in argon atmosphere.
3. Results and discussion Combustion synthesis involves a redox reaction between an oxidizer (nitrates) and a fuel. Glycine is one of the cheapest amino acids. It is known to act as a complexing agent for a number of metal ions as it has a zwitter ionic character due to a carboxylic group at one end and an amino group at the other end [12]. Citric acid is a good complexing agent due to its multi-denticity. It has three carboxylic and one hydroxyl group for coordinating with the metal ions, which facilitate the formation of viscous gel [13]. In the present case, nano-crystalline ceria powders have been prepared by glycine-nitrate and citratenitrate combustion in the ratio 1:1.0.
Fig. 1. Agglomerate size distribution of powder prepared using hydrazine.
The properties of the ceria powders synthesized by all the above-mentioned different routes are tabulated in Table 1. It can be seen that the crystallite size as calculated by X-ray line broadening using Schererr’s formula, was found to range in between 10 and 17 nm. The surface area of the powder obtained from glycine combustion was as low as 18 m2 /g while the oxalate precipitated product showed largest surface area of 70 m2 /g. It is usually observed that smaller the crystallite size, higher is the surface area and sintered density. However, in this case, no such trend is observed for the ceria powders. An important aspect of the nano-powders which is generally overlooked is their agglomeration. In the case of agglomerated particles, the packing density tends to decrease which can be correlated with increasing attraction between particles with decreasing particle size due to Vander-Waals interaction [14]. In order to understand the role of agglomerates, their size was investigated by dynamic light scattering experiments. The particle size analyzer is based on the laser diffraction. Although, this technique is not capable of giving the true size of the nano-particles but it is a good tool to find out the size and nature of the agglomerates. The agglomerate size of the ceria nano-powders after ultrasonication for about 10 min is given in Table 1. It is seen that the product obtained using hydrazine as the fuel was highly agglomerated with an average agglomerate size of about 6.18 m (Fig. 1). The ceria nano-powders synthesized using glycine gave the smallest agglomerates with average particle size of 1.43 m. The green pellets were sintered in situ during dilatometric studies at 1200 ◦ C for 1 h. It was observed that although the
Table 1 Powder properties of ceria powders prepared from different routes Process
Fuel/precursor
Crystallite size (nm)
Surface area (m2 /g)
Agglomerate size (m)
Density (% th.d.)
a. Combustion b. Decomposition c. Precipitation d. Combustion e. Precipitation f. Precipitation
Citric acid Nitrate Ammonium hydroxide Glycine Oxalic acid Hydrazine
10 13 15 12 17 10
63 49 41 18 70 39
2.15 3.63 1.84 1.43 5.43 6.18
96 93 85 88 86 70
V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
Fig. 2. SEM micrograph of sintered sample prepared using citric acid.
powders synthesized using citric acid and hydrazine showed similar crystallite size (10 nm), their sintered densities showed a remarkable difference. The citrate-nitrate combustion resulted in highly sinter-active nano-powders, the sintered density being 96% of theoretical density (th.d.) of ceria. The ceria powder prepared by hydrazine route when sintered under the identical conditions gave a density of 70% of th.d. The sintered densities of the powders obtained using other fuels essentially range in between these two extremes. For example, ceria obtained from glycine combustion gave nano-powder with average crystallite size of 12 nm and a sintered density of 88% of th.d. The sintered microstructures were investigated by SEM. Figs. 2 and 3 show the micrographs of the fractured surface of the sintered pellets prepared using citric acid and glycine as fuels, respectively. In the case of citric acid, the grains are much larger with an average grain size of about 1–1.5 m. Thus, the ultrafine nature and high surface area of the starting powder seems to be responsible for achieving high sintered density at a low temperature in shorter duration. The microstructure of the sintered pellet obtained from glycine-nitrate combustion
Fig. 3. SEM micrograph of sintered sample prepared using glycine.
479
Fig. 4. Linear shrinkage as a function of temperature.
is shown in Fig. 3. Here all the grains were of fairly regular shape with most of the grains being smaller than 1.0 m. It can be seen that the microstructure is not fully developed. The presence of small voids confirms the low sintered density of 88%. The linear shrinkage as a function of temperature for the different samples is shown in Fig. 4. It can be seen that ceria obtained by citrate-nitrate combustion (sample a) showed maximum shrinkage. Another striking observation is that maximum shrinkage occurred in the narrowest temperature range (∼800–900 ◦ C) for this sample. In contrast, the ceria prepared by hydrazine route (sample f) showed minimum shrinkage which was diffused over a wide temperature range. The ceria obtained from cerium nitrate decomposition (sample b) also showed a high shrinkage. Other three samples showed the shrinkage, which was in between these two extremes (samples a and f). This kind of sintering behavior of the ceria samples could be related to their powder characteristics such as crystallite size, surface area, sintered density and agglomerate size. It can be seen that the sample prepared by citrate-nitrate combustion consists of finest crystallites and relatively high surface area, which is the driving force for maximum shrinkage of the green pellet over a narrow temperature range; hence, a sintered density as high as 96% of th.d. could be obtained for this sample. However, the ceria powder prepared by hydrazine route resulted in only 70% of th.d. This poor density can be attributed to the presence of large agglomerates. It is seen from Table 1 that although this nano-powder had small crystallite size (10 nm), its agglomerate size was largest (6.18 m). Also the surface area of this powder was not so large (39 m2 /g) compared to the powder prepared by citrate-nitrate combustion (63 m2 /g). Thus, it can be said conclusively that apart from the crystallite size and surface area, the agglomerate size also plays a vital role in governing the sintering behavior. Large agglomerates are not favorable for achieving high sintered density. All other samples showed sintered densities in between these two samples (prepared by citrate-nitrate combustion and hydrazine method, respectively). The powder prepared by glycine-nitrate combustion had a small surface area
480
V. Bedekar et al. / Journal of Alloys and Compounds 459 (2008) 477–480
(18 m2 /g) however it showed a reasonably high sintered density (88%). In can be noticed that the agglomerate size of this sample is the smallest (1.43 m), which is responsible for getting a somewhat high sintered density. It is inferred that the final sintered density is an interplay between three powder properties namely crystallite size, surface area and agglomerate size. In fact, all these three powder characteristics are interdependent. The finer crystallite size need not always result in highest sintered density. One needs to optimize the agglomerate size also. It also appears from Table 1 that the surface area and agglomerate size have predominant influence on the sintering behavior as compared to the crystallite size. Another important conclusion which can be made from the present investigation is that much better sintering can be obtained if the sintering occurs over a narrow range of temperature. A diffuse sintering step (e.g. in sample f) is highly detrimental for attaining high sintered densities. A close look at the linear shrinkage data clearly reveals that the sintering is poor even up to 1200 ◦ C in the case of powder f. On the other hand the shrinkage behavior for the powders a and b had almost attained saturation at 1200 ◦ C. In order to achieve the comparable sintered densities for powders e and f, one would need much higher temperatures, as seen by the slope of the linear shrinkage curve for these samples. The combustion reaction performed using citric acid as the fuel was found to be sluggish. During this process, there is evolution of gaseous products which dissipates the heat of combustion and limits the rise of temperature, thus reducing the possibility of premature local partial sintering among the primary particles. Hence, the ceria powder obtained by citrate-nitrate combustion showed excellent powder properties and a sharp and maximum shrinkage during the sintering process.
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