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Precipitation synthesis and sintering of yttria nanopowders
Materials Letters 58 (2004) 2137 – 2142 www.elsevier.com/locate/matlet
Precipitation synthesis and sintering of yttria nanopowders Zhenguo Huang a,*, Xudong Sun a, Zhimeng Xiu a, Shaowei Chen b, Chi-Tay Tsai b a
School of Materials and Metallurgy, Northeastern University, Shenyang, China b Department of Mechanical Engineering, Florida Atlantic University, USA
Received 24 October 2003; received in revised form 30 January 2004; accepted 5 February 2004 Available online 18 March 2004
Abstract Yttria nanopowders were synthesized using a chemical precipitation method. pH value at the end of the precipitation process has a significant effect on the size and morphology of the precursor and the yttria powders. Under the same calcination condition, the yttria powders made from the precursor obtained at a pH of 8 are smaller in mean particle size and narrower in size distribution than those made from the precursor obtained at a pH of 10. It was found that the optimum calcination temperature is 1000 jC, and the yttria powder obtained is fine (30 nm) and well dispersed. Using the yttria powder, transparent yttria ceramics was produced by vacuum sintering at 1700 jC for 4 h without any additives. D 2004 Elsevier B.V. All rights reserved. Keywords: Yttria; Nanopowder; Precipitation; Sintering; Ceramics
1. Introduction In addition to the well-known use in zirconia stabilization and as sintering additives for silicon nitrite, yttria has also received considerable attention for many other applications because of its intrinsic properties [1– 6]. The Nd3 + or Eu3 + doped transparent yttria has been used as crystals for solidstate lasers. The conventional method for fabricating transparent yttria ceramics requires chemically derived high purity ultrafine yttria powders, the addition of certain additives (such as La2O3 [7], LiF [8] and ThO2 [9]), and the employment of hot pressing [10] or high temperature (>2000 jC) sintering [11] process. The use of ultrafine, monosized, low-agglomerated and spherical powders is essential to the fabrication of transparent yttria ceramics [12,13]. Many methods have been studied to fabricate ultrafine yttria powders, such as combustion [2,3], precipitation [12 – 15], hydrothermal synthesis [16], electrospray pyrolysis [17], and sol –gel [18]. In order to improve dispersion and sinterability of the yttria powder, certain ions * Corresponding author. Tel.: +86-24-83687787; fax: +86-2423906316. E-mail address: [email protected] (Z. Huang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.008
(such as sulfate) are introduced during the precipitation process [4] or seed crystals are added in the hydrothermal process [16]. Most of these methods involve complicated steps, expensive raw materials and additives to fabricate transparent yttria ceramics. Precipitation using inorganic salts has been verified its feasibility for fabricating yttria powders with favorable powder characteristics. Furthermore, this method includes fewer steps and lower cost raw materials. There are mainly two kinds of precursor precipitates of yttrium using inorganic starting materials: hydroxide and carbonate. Carbonate-derived powders have better sinterability than those derived from hydroxide. Saito et al. [14] indicated that carbonate has a favorable morphology for the creation of deagglomerable oxide powders. Moreover, the yttria powders obtained from carbonate precursors are usually nanosized, which is beneficial to the sintering and transparency of the yttria ceramics. In this paper, yttria nanopowders with a narrow size distribution are synthesized using a precipitation method. The effect of pH at the end of the precipitation process and calcination temperature on morphology, size, distribution and agglomeration of the yttria powders are investigated. Sintering behavior of the powders and factors affecting transparency are also discussed.
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2. Experimental procedure An yttria powder with a size of 1 Am and a 65% nitric acid solution were used to prepare the 0.295 mol/ l yttrium nitrate solution. Carbonate yttria precursors were prepared by adding a mixture of aqueous ammonia and ammonium hydro-carbonate to a 0.3-l yttrium nitrate solution at a rate of 2 ml/l. The molar ratio of ammonia to ammonium hydro-carbonate was 1:1.1. The pH of the solution was monitored and the final pH of the system was 8. For comparison, a precipitation system with a final pH of 10 was also prepared. After a 10-h aging, the precipitates were washed by deionized water for three times to remove the byproducts, and then was washed by alcohol for two times to disperse the precursor powders. During the precipitation and aging process, the system was stirred with a magnetic stirrer. After washing, the precipitates were filtered and then dried in an oven at 60 jC for 8 h. The precursor powders were then calcined at 600, 700, 800, 900, 1000, and 1100 jC, respectively, for 4 h in oxygen flow (3 l/min). After milling, the powder was uniaxially compacted into pellets in a 13-mm-diameter steel die at 35 MPa. The pellets were then pressed isostatically at 200 MPa, and heat treated at 1000 jC to remove any organic materials and absorbed gases. The pellets were sintered under vacuum of 1.3 10 3 Pa in a furnace (VSF-7, Shenyang Vacuum Institute, China) with molybdenum heating elements. Green densities of the compacts were determined from geometric measurement. Phase identification of the calcined powders was performed using an X-ray diffractometer (Model Dmax-RB, Japan). Morphology and size of the powder were observed using a Transmission Electron Microscope (TEM, Model Philips EM420). Mean particle size and size distribution of the powder were measured using an Image Analyzer (Model IAS-4, software provided by Cambridge University) on TEM photographs containing at least 150 distinguishable particles. Densities of the specimens were measured by the Archimedes’ method with water as the immersion medium. Relative densities were calculated using a theoretical density of 5.031 g/cm3 [10]. The sintered specimens were polished and etched in a boiling 20% hydrochloric acid solution for about 3 min to reveal grain boundaries. Microstructure of the specimens was observed using a scanning electron microscope (SEM, SHIMADZU SSX550, Japan).
anions. During the precipitation, the following chemical reactions occur: NH4 OH þ NH4 HCO3 fðNH4 Þ2 CO3 þ H2 O
ð1Þ
2YðNO3 Þ3 þ 3ðNH4 Þ2 CO3 fY2 ðCO3 Þ3 þ 6NH4 NO3
ð2Þ
2YðNO3 Þ3 þ 6NH4 HCO3 þ n H2 OfY2 ðCO3 Þ3 nH2 O þ 6NH4 NO3 þ 3CO2 þ 3H2 O
ð3Þ
Y2 ðCO3 Þ3 þ 2OH f2YðOHÞCO3 þ CO2 3
ð4Þ
It was found that in a system with the co-existence of OH and CO32 , Y2(CO3)3 normally forms if Y3 + is introduced [19]. However, if the pH value of the system increased to a critical value, Y(OH)CO3 forms, as shown in Eq. (4). According to Li et al. [20] and Saito et al. [14], at an approximate pH value of 8, the precipitate is still Y2(CO3)3. Thus, in our experiment, Y2 (CO3)3 should be the product of precipitation when the pH value is 8. As the pH value increases to 10, some of the Y2(CO3)3 may transform to Y(OH)CO3. X-ray diffraction pattern of the precursor precipitates shows that the precursors are amorphous (Fig. 1). Fig. 2 shows the morphology of the two precursors. At a pH value of 8, the precursor particles are irregular in shape, while at a pH value of 10, the precursor particles are flakeslike and bigger in size, which may be an indication of the formation of Y(OH)CO3 during the precipitation. 3.2. Calcination of the precursor The precursor obtained at a pH of 8 was calcined at various temperatures to produce yttria powders. XRD ana-
3. Results and discussions 3.1. Precipitation of the yttria precursor The composition of the precursor precipitates is determined by the competition between hydroxyls and carbonate
Fig. 1. X-ray diffraction profiles of the precursor and Y2O3 powders calcined at various temperatures (pH = 8).
Particle Diameter/nm
16
Diameter
40
14
35
12
30
10
25
8
20
6 4
15
2
10 600
lysis shows that the precursor transforms completely to cubic yttria at 600 jC, and no other phases can be detected (Fig. 1). With the increase of calcination temperature, the peaks become higher and sharper, implying particle growth of the yttria powders. TEM observation shows that the yttria powders produced at various calcination temperatures are spherical in shape (Fig. 3). Statistic distribution of mean particle diameter and root-mean-square deviation at different calcination temperatures are shown in Fig. 4. Both the particle diameter and the root-mean-square deviation increase with increasing calcination temperature, identical with the XRD analysis. At 600 jC mean particle diameter of the yttria powder is 12 nm, and at 1100 jC the particle has grown to a size of 43 nm. The root-mean-square deviation also increases from 1.7 to 13.2 nm, indicative of a broadened particle size distribution at higher calcination temperature. It can be seen in Fig. 4 that there is an evident increase of slope at 900 jC, implying a turning point at which rapid particle growth begin to occur.
18
Deviation
45
Fig. 2. TEM photograph of the precursors: (a) pH = 8, (b) pH = 10.
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Calcination Temperature/ °C Fig. 4. Particle diameter and root-mean-square deviation versus calcination temperatures from precursor at pH = 8.
Yttria powders derived from precursor obtained at a pH of 10 (Fig. 5) have larger particle size and broader size distribution under the same calcination condition. For example, when calcinated at 1000 jC, the mean particle diameter and root-mean-square deviation of the yttria powders produced using the precursor obtained at a pH of 10 is 100 and 13.6 nm, respectively, higher than those (30 and 5.9 nm) of the powder derived from the precursor obtained at a pH of 8. This may be due to the existence of Y(OH)CO3 in the precursor which decomposes at a different temperature than that of the yttrium carbonate, resulting in a broadened size distribution due to the asynchronous particle nucleation and growth.
Fig. 3. TEM morphology of the Y2O3 powders produced by calcining the precursor (pH = 8) at (a) 1100 jC, (b) 1000 jC, (c) 900 jC, (d) 800 jC, (e) 700 jC, and (f) 600 jC.
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Fig. 5. Morphology of the Y2O3 powders produced by calcining the precursor obtained at pH = 10: (a) calcined at 1000 jC, (b) calcined at 1100 jC.
Fig. 6. Effect of the calcination temperature of Y2O3 powders on relative density of the greenbody and the sintered Y2O3 ceramics (pH = 8).
3.3. Agglomeration of the yttria powder The oxides derived from carbonates were usually lowagglomerated [14]. The yttria powder obtained from the precursor at pH = 8 is soft agglomerated and can be crushed easily using a pestle. Whereas, the yttria powder derived from the precursor at pH = 10 is difficult to pulverize, indicating the formation of hard agglomerates. The decomposition process of Y2(CO3)3 and Y(OH)CO3 are shown by Eqs. (5) –(8). Both water and carbon dioxide are released from the decomposition of the hydroxycarbonate precursor during calcination, and only carbonate dioxide is given out from the carbonate precursor. It was confirmed that the oxobridging is the main cause of aggregation of zirconia and alumina powders derived from hydroxide precursors [21,22]. Therefore, the hydroxyls existing in the hydroxycarbonate precursor is likely to be responsible for the formation of hard agglomerates of the yttria powder. This is demonstrated by the formation of sintering necks between yttria particles obtained from the hydroxycarbonate precursor (Fig. 5). For the yttria powder produced by the YðOHÞCO3 ! Y2 O2 CO3 þ H2 O þ CO2
ð5Þ
Y2 ðCO3 Þ3 ! Y2 OðCO3 Þ2 þ CO2
ð6Þ
Y2 OðCO3 Þ2 ! Y2 O2 CO3 þ CO2
ð7Þ
Y2 O2 CO3 ! Y2 O3 þ CO2
ð8Þ
green density [23]. Linear shrinkage of the sintered yttria ceramics decreases with increasing calcination temperature (Fig. 7). This is a result of the increase of green density with increasing calcination temperature. Relative density of the sintered compacts increases with increasing calcination temperature at first, reaches maximum at 1000 jC, and then decreases a little. Under certain sintering conditions, density of a sintered ceramic compact is mainly determined by the green density, green homogeneity, particle size, distribution and agglomeration of the powder. At a low calcination temperature, although the powder obtained is fine, narrowly distributed in size and less agglomerated, the green density is low, resulting in a comparatively low sintered density after sintering. If the calcination temperature is too high, the powder obtained has bigger particle size and broader size distribution, as well as more severe agglomeration due to the formation of particle necking, which is detrimental for the sintering of the green compact even though the green
carbonate precursor, the carbon dioxide released during calcination may contribute to the dispersion of particles. 3.4. Sintering of the yttria powders For the yttria powders obtained from the precursor of pH = 8, green density increases with increasing calcination temperature (Fig. 6). The finer the powder, the bigger the friction force between particles during compaction due to the larger surface areas of the powder, and thus the lower the
Fig. 7. Linear shrinkage of the compacts after sintering at 1700 jC as a function of calcination temperature of the Y2O3 powders (pH = 8).
Z. Huang et al. / Materials Letters 58 (2004) 2137–2142
Fig. 8. Photographs of the transparent Y2O3 ceramics made from the Y2O3 powders produced at (a) pH = 10, and 1000 jC, (b) pH = 8, and 1100 jC and (c) pH = 8, and 1000 jC.
density is higher. Thus, there exists an optimum calcination temperature of the powder (1000 jC for the present work) to obtain the highest sintered density of the yttria ceramic. Fig. 8 shows the transparent yttria ceramic samples 1 mm thick. It can be seen that pH value at the end of the precipitation process has great effect on the transparency of the yttria ceramics. The yttria ceramic made from the precursor of pH = 10 is poor in transparency (Fig. 8a) and low in density (98.67%) compared with the ceramic produced from the precursor of pH = 8 (density = 99.92%, Fig. 8c). SEM observation of the polished and etched surfaces of the sample reveals the existence of pores both in the grain and at the grain boundaries (Fig. 9a). This is due to the comparatively large particle size, broad size distribution, and the possible existence of hard agglomerates in the yttria powder, which leads to the unevenness of the greenbody after compaction. The existence of hard agglomerates in the greenbody is the cause of differential sintering, which results in the existence of large pores between agglomerates that will be difficult be removed by the sintering process. Pores embraced in the sample are major factors that affect transparency [24]. Calcination temperature also affects significantly the microstructure and transparency of the yttria ceramics. The yttria ceramics fabricated using the powder calcined at a temperature of 900 jC or lower are opaque and some of
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the samples are cracked. This is due to the low green density of the samples as a result of the too small particle size of the yttria powders. 1000 jC is a suitable calcination temperature, and the yttria ceramic fabricated using the powder has the best transparency (Fig. 8c) and highest density (99.92%). Pores were not observed under SEM (Fig. 9c). Further increase of the calcination temperature to 1100 jC results in a decrease of density and transparency of the sintered sample. Pores can be observed in grains and at grain boundaries (Fig. 9b). If pores are entrapped in grains, they are very difficult to be removed by sintering due to the smaller lattice diffusion rate of Y3 + than grain boundary diffusion. The yttria ceramic fabricated using the powder calcined at 1000 jC (pH = 8) has a larger mean grain size, about 30 Am compared with 10 and 15 Am for the samples (a) and (b) shown in Fig. 8. This indicates the pinning effect of grain boundaries by the pores.
4. Conclusion A chemical precipitation process was optimized to fabricate well-dispersed yttria nanopowders. It was found that pH value at the end of the precipitation process influences the composition and morphology of the precursors, which in turn affects significantly the size, size distribution and agglomeration of the yttria powder. Yttria powders produced from the precursor of pH = 8 is fine (in the nanometer scale), narrow in size distribution and loosely agglomerated, while calcination of the precursor of pH = 10 results in yttria powders with bigger primary particle size, severe agglomeration and broad size distribution. Density of the sintered yttria ceramics is mainly influenced by the particle size, size distribution and agglomeration of the powder. If the calcination temperature is V 900 jC, the yttria powder is very fine ( < 30 nm), resulting in a
Fig. 9. SEM micrograph of etched yttria ceramics made from the Y2O3 powders produced at (a) pH = 10, and 1000 jC, (b) pH = 8, and 1100 jC, and (c) pH = 8, and 1000 jC.
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low green density after compaction, which is detrimental to densification of the yttria ceramics during sintering. If the calcination temperature is z 1100 jC, the yttria powder obtained is comparatively large in particle size and broad in size distribution, and powder agglomeration is comparatively severe. Although the green density is high, the existence of hard agglomerates leads to the formation of large pores during sintering. The optimum calcination temperature of the powder is 1000 jC, and transparent yttria ceramics can be produced by vacuum sintering at 1700 jC using the yttria powder.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50172010) and the Trans-Century Training Program Foundation for the Talents by the Ministry of Education of China.
References [1] Y.N. Xu, Z.Q. Gu, W.Y. Ching, Phys. Rev., B 56 (23) (1997) 14993. [2] W.J. Kim, J.Y. Park, S.J. Oh, Y.S. Kim, G.W. Hong, I.H. Kuk, J. Mater. Sci. Lett. 18 (1999) 411. [3] S. Roy, W. Sigmund, F. Aldinger, J. Mater. Res. 14 (4) (1999) 1524. [4] T. Ikegami, J.G. Li, T. Mori, J. Am. Ceram. Soc. 85 (7) (2002) 1725.
[5] S.N. Mukherjee, C.R. Aita, J. Vac. Sci. Technol., A 10 (5) (1992) 3356. [6] A.L. Micheli, D.F. Dungan, J.V. Mantese, J. Am. Ceram. Soc. 75 (3) (1992) 709. [7] W.H. Rhodes, J. Am. Ceram. Soc. 64 (1) (1981) 13. [8] R.A. Lefever, J. Matsho, Mater. Res. Bull. 2 (9) (1967) 865. [9] T. Reetz, I. Haase, H. Ullmann, H.J. Lang, Solid State Ionics 36 (1989) 193. [10] O. Yeheskel, O. Tevet, J. Am. Ceram. Soc. 82 (1) (1999) 136. [11] Y. Tsukuda, A. Muta, J. Ceram. Soc. Jpn., Int. Ed. 84 (12) (1976) 585. [12] D.J. Sordelet, M. Akinc, J. Am. Ceram. Soc. 71 (12) (1988) 1148. [13] T. Ikegami, T. Mori, Y. Yajima, S. Takenouchi, T. Misawa, Y. Moriyoshi, J. Ceram. Soc. Jpn., Int. Ed. 107 (3) (1999) 297. [14] N. Saito, S. Matsuda, T. Ikegami, J. Am. Ceram. Soc. 81 (8) (1998) 2023. [15] C.J.J. Tool, E.H.P. Cordfunke, Solid State Ionics 32/33 (1989) 691. [16] P.K. Sharma, M.H. Jilavi, R. Nab, H. Schmidt, J. Mater. Sci. Lett. 17 (1998) 823. [17] A.J. Rulison, R.C. Flagan, J. Am. Ceram. Soc. 77 (12) (1994) 3244. [18] R. Subramanian, P. Shankar, S. Kavithaa, S.S. Ramakrishnan, P.C. Angelo, H. Venkataraman, Mater. Lett. 48 (2001) 342. [19] D. Sordelet, M. Akinc, J. Colloid Interface Sci. 122 (1) (1988) 47. [20] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, Y. Yajima, J. Eur. Ceram. Soc. 20 (2000) 2395. [21] J.L. Shi, J.H. Gao, Z.X. Lin, D.S. Yan, J. Mater. Sci. 28 (1993) 342. [22] S. Braun, L.G. Appel, L.B. Zinner, M. Schmal, Br. Ceram. Trans. 98 (2) (1999) 81. [23] J.M. Zheng, J.S. Reed, J. Am. Ceram. Soc. 72 (5) (1989) 810. [24] A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, J. Am. Ceram. Soc. 78 (4) (1995) 1033.
Precipitation synthesis and sintering of yttria nanopowders Zhenguo Huang a,*, Xudong Sun a, Zhimeng Xiu a, Shaowei Chen b, Chi-Tay Tsai b a
School of Materials and Metallurgy, Northeastern University, Shenyang, China b Department of Mechanical Engineering, Florida Atlantic University, USA
Received 24 October 2003; received in revised form 30 January 2004; accepted 5 February 2004 Available online 18 March 2004
Abstract Yttria nanopowders were synthesized using a chemical precipitation method. pH value at the end of the precipitation process has a significant effect on the size and morphology of the precursor and the yttria powders. Under the same calcination condition, the yttria powders made from the precursor obtained at a pH of 8 are smaller in mean particle size and narrower in size distribution than those made from the precursor obtained at a pH of 10. It was found that the optimum calcination temperature is 1000 jC, and the yttria powder obtained is fine (30 nm) and well dispersed. Using the yttria powder, transparent yttria ceramics was produced by vacuum sintering at 1700 jC for 4 h without any additives. D 2004 Elsevier B.V. All rights reserved. Keywords: Yttria; Nanopowder; Precipitation; Sintering; Ceramics
1. Introduction In addition to the well-known use in zirconia stabilization and as sintering additives for silicon nitrite, yttria has also received considerable attention for many other applications because of its intrinsic properties [1– 6]. The Nd3 + or Eu3 + doped transparent yttria has been used as crystals for solidstate lasers. The conventional method for fabricating transparent yttria ceramics requires chemically derived high purity ultrafine yttria powders, the addition of certain additives (such as La2O3 [7], LiF [8] and ThO2 [9]), and the employment of hot pressing [10] or high temperature (>2000 jC) sintering [11] process. The use of ultrafine, monosized, low-agglomerated and spherical powders is essential to the fabrication of transparent yttria ceramics [12,13]. Many methods have been studied to fabricate ultrafine yttria powders, such as combustion [2,3], precipitation [12 – 15], hydrothermal synthesis [16], electrospray pyrolysis [17], and sol –gel [18]. In order to improve dispersion and sinterability of the yttria powder, certain ions * Corresponding author. Tel.: +86-24-83687787; fax: +86-2423906316. E-mail address: [email protected] (Z. Huang). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.02.008
(such as sulfate) are introduced during the precipitation process [4] or seed crystals are added in the hydrothermal process [16]. Most of these methods involve complicated steps, expensive raw materials and additives to fabricate transparent yttria ceramics. Precipitation using inorganic salts has been verified its feasibility for fabricating yttria powders with favorable powder characteristics. Furthermore, this method includes fewer steps and lower cost raw materials. There are mainly two kinds of precursor precipitates of yttrium using inorganic starting materials: hydroxide and carbonate. Carbonate-derived powders have better sinterability than those derived from hydroxide. Saito et al. [14] indicated that carbonate has a favorable morphology for the creation of deagglomerable oxide powders. Moreover, the yttria powders obtained from carbonate precursors are usually nanosized, which is beneficial to the sintering and transparency of the yttria ceramics. In this paper, yttria nanopowders with a narrow size distribution are synthesized using a precipitation method. The effect of pH at the end of the precipitation process and calcination temperature on morphology, size, distribution and agglomeration of the yttria powders are investigated. Sintering behavior of the powders and factors affecting transparency are also discussed.
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2. Experimental procedure An yttria powder with a size of 1 Am and a 65% nitric acid solution were used to prepare the 0.295 mol/ l yttrium nitrate solution. Carbonate yttria precursors were prepared by adding a mixture of aqueous ammonia and ammonium hydro-carbonate to a 0.3-l yttrium nitrate solution at a rate of 2 ml/l. The molar ratio of ammonia to ammonium hydro-carbonate was 1:1.1. The pH of the solution was monitored and the final pH of the system was 8. For comparison, a precipitation system with a final pH of 10 was also prepared. After a 10-h aging, the precipitates were washed by deionized water for three times to remove the byproducts, and then was washed by alcohol for two times to disperse the precursor powders. During the precipitation and aging process, the system was stirred with a magnetic stirrer. After washing, the precipitates were filtered and then dried in an oven at 60 jC for 8 h. The precursor powders were then calcined at 600, 700, 800, 900, 1000, and 1100 jC, respectively, for 4 h in oxygen flow (3 l/min). After milling, the powder was uniaxially compacted into pellets in a 13-mm-diameter steel die at 35 MPa. The pellets were then pressed isostatically at 200 MPa, and heat treated at 1000 jC to remove any organic materials and absorbed gases. The pellets were sintered under vacuum of 1.3 10 3 Pa in a furnace (VSF-7, Shenyang Vacuum Institute, China) with molybdenum heating elements. Green densities of the compacts were determined from geometric measurement. Phase identification of the calcined powders was performed using an X-ray diffractometer (Model Dmax-RB, Japan). Morphology and size of the powder were observed using a Transmission Electron Microscope (TEM, Model Philips EM420). Mean particle size and size distribution of the powder were measured using an Image Analyzer (Model IAS-4, software provided by Cambridge University) on TEM photographs containing at least 150 distinguishable particles. Densities of the specimens were measured by the Archimedes’ method with water as the immersion medium. Relative densities were calculated using a theoretical density of 5.031 g/cm3 [10]. The sintered specimens were polished and etched in a boiling 20% hydrochloric acid solution for about 3 min to reveal grain boundaries. Microstructure of the specimens was observed using a scanning electron microscope (SEM, SHIMADZU SSX550, Japan).
anions. During the precipitation, the following chemical reactions occur: NH4 OH þ NH4 HCO3 fðNH4 Þ2 CO3 þ H2 O
ð1Þ
2YðNO3 Þ3 þ 3ðNH4 Þ2 CO3 fY2 ðCO3 Þ3 þ 6NH4 NO3
ð2Þ
2YðNO3 Þ3 þ 6NH4 HCO3 þ n H2 OfY2 ðCO3 Þ3 nH2 O þ 6NH4 NO3 þ 3CO2 þ 3H2 O
ð3Þ
Y2 ðCO3 Þ3 þ 2OH f2YðOHÞCO3 þ CO2 3
ð4Þ
It was found that in a system with the co-existence of OH and CO32 , Y2(CO3)3 normally forms if Y3 + is introduced [19]. However, if the pH value of the system increased to a critical value, Y(OH)CO3 forms, as shown in Eq. (4). According to Li et al. [20] and Saito et al. [14], at an approximate pH value of 8, the precipitate is still Y2(CO3)3. Thus, in our experiment, Y2 (CO3)3 should be the product of precipitation when the pH value is 8. As the pH value increases to 10, some of the Y2(CO3)3 may transform to Y(OH)CO3. X-ray diffraction pattern of the precursor precipitates shows that the precursors are amorphous (Fig. 1). Fig. 2 shows the morphology of the two precursors. At a pH value of 8, the precursor particles are irregular in shape, while at a pH value of 10, the precursor particles are flakeslike and bigger in size, which may be an indication of the formation of Y(OH)CO3 during the precipitation. 3.2. Calcination of the precursor The precursor obtained at a pH of 8 was calcined at various temperatures to produce yttria powders. XRD ana-
3. Results and discussions 3.1. Precipitation of the yttria precursor The composition of the precursor precipitates is determined by the competition between hydroxyls and carbonate
Fig. 1. X-ray diffraction profiles of the precursor and Y2O3 powders calcined at various temperatures (pH = 8).
Particle Diameter/nm
16
Diameter
40
14
35
12
30
10
25
8
20
6 4
15
2
10 600
lysis shows that the precursor transforms completely to cubic yttria at 600 jC, and no other phases can be detected (Fig. 1). With the increase of calcination temperature, the peaks become higher and sharper, implying particle growth of the yttria powders. TEM observation shows that the yttria powders produced at various calcination temperatures are spherical in shape (Fig. 3). Statistic distribution of mean particle diameter and root-mean-square deviation at different calcination temperatures are shown in Fig. 4. Both the particle diameter and the root-mean-square deviation increase with increasing calcination temperature, identical with the XRD analysis. At 600 jC mean particle diameter of the yttria powder is 12 nm, and at 1100 jC the particle has grown to a size of 43 nm. The root-mean-square deviation also increases from 1.7 to 13.2 nm, indicative of a broadened particle size distribution at higher calcination temperature. It can be seen in Fig. 4 that there is an evident increase of slope at 900 jC, implying a turning point at which rapid particle growth begin to occur.
18
Deviation
45
Fig. 2. TEM photograph of the precursors: (a) pH = 8, (b) pH = 10.
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Calcination Temperature/ °C Fig. 4. Particle diameter and root-mean-square deviation versus calcination temperatures from precursor at pH = 8.
Yttria powders derived from precursor obtained at a pH of 10 (Fig. 5) have larger particle size and broader size distribution under the same calcination condition. For example, when calcinated at 1000 jC, the mean particle diameter and root-mean-square deviation of the yttria powders produced using the precursor obtained at a pH of 10 is 100 and 13.6 nm, respectively, higher than those (30 and 5.9 nm) of the powder derived from the precursor obtained at a pH of 8. This may be due to the existence of Y(OH)CO3 in the precursor which decomposes at a different temperature than that of the yttrium carbonate, resulting in a broadened size distribution due to the asynchronous particle nucleation and growth.
Fig. 3. TEM morphology of the Y2O3 powders produced by calcining the precursor (pH = 8) at (a) 1100 jC, (b) 1000 jC, (c) 900 jC, (d) 800 jC, (e) 700 jC, and (f) 600 jC.
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Fig. 5. Morphology of the Y2O3 powders produced by calcining the precursor obtained at pH = 10: (a) calcined at 1000 jC, (b) calcined at 1100 jC.
Fig. 6. Effect of the calcination temperature of Y2O3 powders on relative density of the greenbody and the sintered Y2O3 ceramics (pH = 8).
3.3. Agglomeration of the yttria powder The oxides derived from carbonates were usually lowagglomerated [14]. The yttria powder obtained from the precursor at pH = 8 is soft agglomerated and can be crushed easily using a pestle. Whereas, the yttria powder derived from the precursor at pH = 10 is difficult to pulverize, indicating the formation of hard agglomerates. The decomposition process of Y2(CO3)3 and Y(OH)CO3 are shown by Eqs. (5) –(8). Both water and carbon dioxide are released from the decomposition of the hydroxycarbonate precursor during calcination, and only carbonate dioxide is given out from the carbonate precursor. It was confirmed that the oxobridging is the main cause of aggregation of zirconia and alumina powders derived from hydroxide precursors [21,22]. Therefore, the hydroxyls existing in the hydroxycarbonate precursor is likely to be responsible for the formation of hard agglomerates of the yttria powder. This is demonstrated by the formation of sintering necks between yttria particles obtained from the hydroxycarbonate precursor (Fig. 5). For the yttria powder produced by the YðOHÞCO3 ! Y2 O2 CO3 þ H2 O þ CO2
ð5Þ
Y2 ðCO3 Þ3 ! Y2 OðCO3 Þ2 þ CO2
ð6Þ
Y2 OðCO3 Þ2 ! Y2 O2 CO3 þ CO2
ð7Þ
Y2 O2 CO3 ! Y2 O3 þ CO2
ð8Þ
green density [23]. Linear shrinkage of the sintered yttria ceramics decreases with increasing calcination temperature (Fig. 7). This is a result of the increase of green density with increasing calcination temperature. Relative density of the sintered compacts increases with increasing calcination temperature at first, reaches maximum at 1000 jC, and then decreases a little. Under certain sintering conditions, density of a sintered ceramic compact is mainly determined by the green density, green homogeneity, particle size, distribution and agglomeration of the powder. At a low calcination temperature, although the powder obtained is fine, narrowly distributed in size and less agglomerated, the green density is low, resulting in a comparatively low sintered density after sintering. If the calcination temperature is too high, the powder obtained has bigger particle size and broader size distribution, as well as more severe agglomeration due to the formation of particle necking, which is detrimental for the sintering of the green compact even though the green
carbonate precursor, the carbon dioxide released during calcination may contribute to the dispersion of particles. 3.4. Sintering of the yttria powders For the yttria powders obtained from the precursor of pH = 8, green density increases with increasing calcination temperature (Fig. 6). The finer the powder, the bigger the friction force between particles during compaction due to the larger surface areas of the powder, and thus the lower the
Fig. 7. Linear shrinkage of the compacts after sintering at 1700 jC as a function of calcination temperature of the Y2O3 powders (pH = 8).
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Fig. 8. Photographs of the transparent Y2O3 ceramics made from the Y2O3 powders produced at (a) pH = 10, and 1000 jC, (b) pH = 8, and 1100 jC and (c) pH = 8, and 1000 jC.
density is higher. Thus, there exists an optimum calcination temperature of the powder (1000 jC for the present work) to obtain the highest sintered density of the yttria ceramic. Fig. 8 shows the transparent yttria ceramic samples 1 mm thick. It can be seen that pH value at the end of the precipitation process has great effect on the transparency of the yttria ceramics. The yttria ceramic made from the precursor of pH = 10 is poor in transparency (Fig. 8a) and low in density (98.67%) compared with the ceramic produced from the precursor of pH = 8 (density = 99.92%, Fig. 8c). SEM observation of the polished and etched surfaces of the sample reveals the existence of pores both in the grain and at the grain boundaries (Fig. 9a). This is due to the comparatively large particle size, broad size distribution, and the possible existence of hard agglomerates in the yttria powder, which leads to the unevenness of the greenbody after compaction. The existence of hard agglomerates in the greenbody is the cause of differential sintering, which results in the existence of large pores between agglomerates that will be difficult be removed by the sintering process. Pores embraced in the sample are major factors that affect transparency [24]. Calcination temperature also affects significantly the microstructure and transparency of the yttria ceramics. The yttria ceramics fabricated using the powder calcined at a temperature of 900 jC or lower are opaque and some of
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the samples are cracked. This is due to the low green density of the samples as a result of the too small particle size of the yttria powders. 1000 jC is a suitable calcination temperature, and the yttria ceramic fabricated using the powder has the best transparency (Fig. 8c) and highest density (99.92%). Pores were not observed under SEM (Fig. 9c). Further increase of the calcination temperature to 1100 jC results in a decrease of density and transparency of the sintered sample. Pores can be observed in grains and at grain boundaries (Fig. 9b). If pores are entrapped in grains, they are very difficult to be removed by sintering due to the smaller lattice diffusion rate of Y3 + than grain boundary diffusion. The yttria ceramic fabricated using the powder calcined at 1000 jC (pH = 8) has a larger mean grain size, about 30 Am compared with 10 and 15 Am for the samples (a) and (b) shown in Fig. 8. This indicates the pinning effect of grain boundaries by the pores.
4. Conclusion A chemical precipitation process was optimized to fabricate well-dispersed yttria nanopowders. It was found that pH value at the end of the precipitation process influences the composition and morphology of the precursors, which in turn affects significantly the size, size distribution and agglomeration of the yttria powder. Yttria powders produced from the precursor of pH = 8 is fine (in the nanometer scale), narrow in size distribution and loosely agglomerated, while calcination of the precursor of pH = 10 results in yttria powders with bigger primary particle size, severe agglomeration and broad size distribution. Density of the sintered yttria ceramics is mainly influenced by the particle size, size distribution and agglomeration of the powder. If the calcination temperature is V 900 jC, the yttria powder is very fine ( < 30 nm), resulting in a
Fig. 9. SEM micrograph of etched yttria ceramics made from the Y2O3 powders produced at (a) pH = 10, and 1000 jC, (b) pH = 8, and 1100 jC, and (c) pH = 8, and 1000 jC.
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low green density after compaction, which is detrimental to densification of the yttria ceramics during sintering. If the calcination temperature is z 1100 jC, the yttria powder obtained is comparatively large in particle size and broad in size distribution, and powder agglomeration is comparatively severe. Although the green density is high, the existence of hard agglomerates leads to the formation of large pores during sintering. The optimum calcination temperature of the powder is 1000 jC, and transparent yttria ceramics can be produced by vacuum sintering at 1700 jC using the yttria powder.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 50172010) and the Trans-Century Training Program Foundation for the Talents by the Ministry of Education of China.
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