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Transparent La2−xGdxZr2O7 ceramics obtained by combustion method and vacuum sintering
Journal of Alloys and Compounds 585 (2014) 497–502
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Transparent La2xGdxZr2O7 ceramics obtained by combustion method and vacuum sintering Zhengjuan Wang a,b, Guohong Zhou a,⇑, Xianpeng Qin a, Yan Yang a, Guangjun Zhang c, Yvonne Menke d, Shiwei Wang a,⇑ a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China SCHOTT Glass Technologies (Suzhou) Co. Ltd., Suzhou 215009, China d Schott AG, Corporate Research and Technology Development, Hattenbergstrasse 10, 55122 Mainz, Germany b c
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 24 September 2013 Accepted 27 September 2013 Available online 10 October 2013 Keywords: La2xGdxZr2O7 Transparent ceramics Combustion method Vacuum sintering
a b s t r a c t Transparent La2xGdxZr2O7 (x = 0–2.0) ceramics were prepared via vacuum sintering from nanometric powders synthesized by a simple combustion method. The changes of phase composition, morphology and in-line transmittance of the resulting ceramics with Gd3+ content’s variation were investigated. With the increase of Gd3+ content, the samples keep the pyrochlore structure, but the X-ray diffraction peaks of the powders and ceramics shift to higher angle as the lattice parameters become smaller. All the ceramics are transparent with high in-line transmittance and high refractive index (2.08 @ 632.8 nm, x = 0.4–1.6). These results indicate that La2xGdxZr2O7 ceramics might be used as optical lens. Moreover, with the increase of Gd3+ content, the effective atomic number and density of the ceramics increase, therefore making them promising host candidates for scintillators. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth zirconates (RE2Zr2O7) have increasingly become the focus of recent investigations for their various applications such as thermal barrier coating [1,2], solid electrolytes for solid oxide fuel cells (SOFCs) [3,4], catalysts [5,6], matrices for immobilization of actinides in nuclear waste [7,8], host materials for scintillators [9] and so on. RE2Zr2O7 belongs to A2B2O7 system with a pyrochlore structure or a defective fluorite structure depending on the ionic radii ratios (rA/rB) [10] or can transform from one phase to another at a transition temperature [11]. The pyrochlore structure is closely related to that of fluorite and can be considered as a BO2 fluorite in which half of the B4+ cations are replaced by A3+ cations. Charge compensation takes place by introducing oxygen vacancies into the lattice, resulting in a change in space group from fluorite Fm3m to pyrochlore Fd3m. In pyrochlore structure, the cations and oxygen vacancies show long-range order, and it can be considered as a superstructure with a lattice constant which is twice that of fluorite-type structure [11]. The A cation adopts a distorted cubic coordination with oxygen (coordination number = 8), while B is octahedrally coordinated (coordination number = 6) [12]. ⇑ Corresponding authors. Tel.: +86 021 52414320; fax: +86 021 52415263. E-mail addresses: [email protected] (G. Zhou), [email protected]. ac.cn (S. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.187
As RE2Zr2O7 single crystals are difficult to grow because of the high melting point (>2000 °C), fabricating polycrystalline transparent ceramics becomes a good solution. In 2011, La2Zr2O7 transparent ceramics were fabricated by reactive spark plasma sintering [13], but the transmittance was lower than 25% (1 mm thick) in the visible spectral region. Recently, Zou et al. fabricated Y2Zr2O7 transparent ceramics by combustion method and vacuum sintering, and the in-line transmittance was up to 68% (1.0 mm thick) in the visible spectral region [14]. These investigations suggested that La2xGdxZr2O7 transparent ceramics could be fabricated. For host materials of scintillators, higher density and effective atomic number are essential to obtain higher stopping power for X-ray or c-ray [15]. In the present work, the doping of Gd with higher atomic number into La site can improve the density and effective atomic number of La2Zr2O7, therefore making it more possible to be used as host materials for scintillators. Theoretically, the ionic radius of Gd3+ (1.053 Å) is quite close to that of La3+ (1.160 Å) [16], which means it is easier for Gd3+ to substitute La3+. RE2Zr2O7 powders have been prepared by many methods, such as solid-state reaction [12], sol–gel [17], coprecipitation–calcination [18], hydrothermal synthesis [19]. Among them, solid-state reaction is the simplest, but it needs much higher sintering temperature to achieve full densification because of the poor sinterability of the powder. For the other methods, the processes are complicated and time-consuming. In this research, a rapid and simple combustion method was utilized to synthesize
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Z. Wang et al. / Journal of Alloys and Compounds 585 (2014) 497–502
La2xGdxZr2O7 (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) powders. After compaction and vacuum sintering, transparent La2xGdxZr2O7 (x = 0– 2.0) ceramics were obtained. The effects of Gd3+ content (x value) on phase composition, morphology and in-line transmittance of the final ceramics were investigated.
2. Materials and methods The raw materials were La(NO3)36H2O (99.99%), Gd2O3 (99.99%), Zr(NO3)43H2O (A.R.), and glycine (A.R.). Firstly, Gd2O3 (99.99%) was dissolved in excess nitric acid solution to prepare gadolinium nitrate solution. Then, stoichiometric amounts of the three nitrates with different Gd3+ content (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) and glycine were mixed respectively and stirred thoroughly till the solutions were clear. After that ammonia was added to adjust pH value of the solutions to 4. Then the solutions were transferred into different quartz crucibles, respectively. On a hot plate, the solutions were heated. With the evaporation of water, the solutions became sol and gel. Finally the combustion reaction took place within a few minutes to form the primary fluffy powders. The combustion reaction can be described as follows:
ð2 xÞ LaðNO3 Þ3 þ x GdðNO3 Þ3 þ 2 ZrðNO3 Þ4 þ 70=9 C2 H5 NO2 ! La2x Gdx Zr2 O7 þ 140=9 CO2 þ 98=9 N2 þ 175=9 H2 O
Fig. 1. XRD patterns of La2xGdxZr2O7 (x = 0–2.0) powders calcined at 1200 °C for 2 h.
ð1Þ
The as-synthesized powders were calcined at 1200 °C for 2 h. After ball milling for 20 h, the powders were cold isostatically pressed into pellets (Ø20 2.5 mm) at 200 MPa for 2 min. The green pellets were sintered at 1850 °C for 6 h in vacuum with 10–3 Pa vacuum level and then annealed at 1500 °C for 5 h in air. Finally, the resultant ceramics were double-sided polished for test. The phase compositions of the powders calcined at 1200 °C and the resultant ceramics were analyzed by X-ray diffraction (XRD) using a Germany Bruker D8 Focus diffractometer with Cu Ka radiation (k = 0.15418 nm) in the range of 2h = 10–80°. Morphology of the powders was observed with Field Emission Scanning Electron Microscopy (FE-SEM, JSM-6700F, JEOL, Tokyo, Japan). Thermally etched surfaces and fracture surfaces of the ceramics were observed with Scanning Electron Microscopy (SEM, JSM-6390, JEOL, Tokyo, Japan). The in-line transmittance was measured using a spectrophotometer (Cary 5000, Varian Inc., USA) in the range of wavelength between 200 nm and 1100 nm. The refractive indices were measured on a prism coupler (Metricon Model 2010, Schott, Suzhou) at the wavelength of 632.8 nm. The density was measured by the Archimedes method in distilled water.
Fig. 2. Morphologies of the La2xGdxZr2O7 powders ball milled for 20 h (a–f): x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0.
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3. Results and discussion 3.1. Powder preparation Fig. 1 shows XRD patterns of La2xGdxZr2O7 (x = 0–2.0) powders calcined at 1200 °C for 2 h. All the powders possess the similar crystal structure. In Fig. 1, the JCPDS card PDF#71-2363 and PDF#80-0471 correspond to La2Zr2O7 (pyrochlore structure) and Gd2Zr2O7 (defective fluorite structure). Due to the low calcination temperature, all the diffraction peaks are not sharp and it shows insufficient crystallinity degree. But the crystal structure can be distinguished compared to the JCPDS card. With x (Gd3+ content) increased, the two weak peaks corresponding to (3 3 1) and (5 1 1) reflections of pyrochlore structure [4] become weaker and cannot be observed after x = 1.2, indicating the ordered pyrochlore structure begin to become disordered. The order–disorder transition temperature from pyrochlore to defective fluorite structure is 1530 °C [20] or 1575 °C [21]. So the x = 1.2–2.0 powders calcined at 1200 °C are supposed to be pyrochlore structure with low degree of ordering. Powders derived from combustion synthesis were severely agglomerated with porous network structure. The ball milling process was carried out to eliminate the agglomerates and porous network structures [14]. Morphologies of the powders ball milled for 20 h are shown in Fig. 2. It can be seen that all the particle size distribution are relatively uniform for different Gd3+ content and the average particle size is smaller than 100 nm, which implies higher sintering activity of the powders.
Fig. 4. XRD patterns of La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air.
3.2. Fabrication of La2xGdxZr2O7 transparent ceramics Fig. 3 is the photo of the mirror-polished La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and then annealed at 1500 °C for 5 h in air. All of the ceramics exhibit high optical transparency in the visible spectral region. And with the increase of x value from 0.4 to 2.0, the colors of the ceramics become lighter. The post-annealing process was performed to eliminate the excess oxygen vacancies derived from vacuum sintering, as the oxygen vacancies can produce strong light absorption through the formation of color centers, which would lead to the coloration of the ceramics. Fig. 4 shows the XRD patterns of La2-xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air. After sintered, the obtained ceramics were much better crystallized compared to the calcined powders (Fig. 2). All the La2xGdxZr2O7 (x = 0–2.0) ceramics exhibit pyrochlore structure with two weak peaks corresponding to (33 1) and (51 1) reflections, which characterize the superstructure of pyrochlore phase and is used to distinguish between pyrochlore and fluorite structure [4]. In contrast to the powders, the detectable (3 3 1) and (5 1 1) reflections for x = 1.2–2.0 shows the ordering of the pyrochlore structure increases because of higher crystalline degree. For x = 2.0, the generation of obvious cracks (Fig. 3) could be the result of volume change produced in the phase transition process. Furthermore, according to Subramanian et al. [10], the range of pyrochlore stability of A2B2O7 exists from ionic radii ratios rA/rB = 1.46 to
Fig. 3. Photo of La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air (1.0 mm thick).
Fig. 5. In-line transmittance curves of the mirror-polished La2xGdxZr2O7 ceramics (1.0 mm thick).
rA/rB = 1.78. For smaller ionic radii ratios, rA/rB < 1.46, anion-deficient fluorite is the stable structure, whilst the monoclinic structure is stable for ionic radii ratios rA/rB > 1.78. In present work, 3þ 4þ r3þ Gd ¼ 1:053 Å, r La ¼ 1:160 Å and r Zr ¼ 0:720 Å respectively, the ionic radii ratios (rA/rB) of La2xGdxZr2O7 (A: La3+ and Gd3+, B: Zr4+) range from 1.61 (x = 0) to 1.46 (x = 2.0), which are all in the range of pyrochlore structure and further verified the XRD results of Fig. 4. The x = 2.0 sample (rA/rB = 1.46) is at the critical point between defective fluorite structure and pyrochlore structure, which is also an evidence of the instability of the ceramic. Moreover, with x (Gd3+ content) increase, the diffraction peaks shift to higher angle, indicating that the lattice parameter decreases on 3þ account that r3þ Gd < r La . The lattice parameters were calculated using the XRD results by Bragg equation. As Gd3+ content increases from 0 to 2.0, the lattice parameters are linearly decreased and they are 10.814 Å, 10.772 Å, 10.717 Å, 10.665 Å, 10.598 Å, and 10.524 Å, respectively, which is agreement with Vegard’s rule and also confirm the above analysis. Fig. 5 shows the in-line transmittance curves of the mirror-polished La2xGdxZr2O7 ceramics (1.0 mm thick). All the ceramics have high in-line transmittance, and the highest can reach to 73.6% at 1100 nm when x = 0.4. As x value increased from 0.4 to 2.0 (except for x = 0), the absorption edges of the in-line
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transmittance curves shifted to ultra-violet region. The absorption edge is the intrinsic property determined by band structures which
is related to crystal structure. As the ionic radius of Gd3+ is smaller than that of La3+, the substitution of La3+ by Gd3+ could shorten the
Fig. 6. Thermally etched surfaces and fracture surfaces of La2xGdxZr2O7 ceramics: (a and b): x = 0; (c and d): x = 0.4; (e and f): x = 0.8; (g and h): x = 1.2; (i and j): x = 1.6; and (k and l): x = 2.0.
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oxgen-metal band, which leads to broader energy band (Eg). Thus, the absorption edge shifts to UV region (k = hc/Eg). The in-line transmittance is somehow lower for x = 2.0 sample, implying there may be more pores in x = 2.0 ceramic. Furthermore, the microcracks (Fig. 3) in x = 2.0 sample can also result in scattering and reduce the in-line transmittance. The refractive indices of La2xGdxZr2O7 (x = 0.4–1.6) ceramics are all about 2.08 at the wavelength of 632.8 nm, which are high in transparent ceramics. It makes La2xGdxZr2O7 ceramics applicable as optical lens in imaging devices like cameras to realize the miniaturization of these devices [22]. The theoretical transmittance of the ceramics can be calculated from the following equations [23]:
T ¼ I=I0 ¼ ð1 RÞ2 expðatÞ
ð2Þ
where R = (n 1)2/(n + 1)2, in which n is the refractive index of the ceramics, a is the optical attenuation coefficient, and t is the thickness of the transparent ceramics. Assuming that the spectral loss can be ignored, that is to say, optical attenuation coefficient a is 0. Then, Eq. (2) can be simplified to T = (1 R)2. When the wavelength is 632.8 nm, the refractive index n of La2xGdxZr2O7 ceramics is 2.08 and the calculated theoretical transmittance is 76.9%. In this work, the in-line transmittance of La2xGdxZr2O7 (x = 0.4) reaches to the highest (70.9%) at 633 nm, about 92% of the theoretical value. Fig. 6 shows the thermally etched surfaces and fracture surfaces of the obtained ceramics. The average grain sizes vary with Gd3+ content. In this study, the average grain sizes of the final ceramics are determined by the average linear intercept length multiplied by a statistical factor 1.56 [24]. The calculated average grain sizes of x = 0 and x = 2.0 samples are 21 lm and 50 lm, respectively. For x = 0.4–1.2, the average grain sizes are almost the same, about 12 lm. The x = 1.6 sample has the smallest average grain size and is about 9 lm. It is evident that Gd is the sintering aid for La2Zr2O7 and the same for La to Gd2Zr2O7. The doping of La into Gd2Zr2O7 can inhibit grain growth and reduce the grain size greatly just as the case of x = 1.6 sample, of which the average grain size reduced from about 50 lm (x = 2.0) to 9 lm (x = 1.6). Similarly, the doping of Gd into La2Zr2O7 can also inhibit the grain growth as the case of x = 0.4, but the effect of Gd doping into La2Zr2O7 is not as obvious as that of La into Gd2Zr2O7. Few residual micropores were observed in x = 0–1.6 samples and more pores in x = 2.0 sample. As in this study all the ceramics are single cubic pyrochlore structure, the main scattering source is the residual pores which would result in the decrease of the transparency of the ceramics. The more pores in x = 2.0 sample lead to a significant reduction in the in-line transmittance as shown in Fig. 5. Fig. 6 also shows the different fracture modes. The main fracture mode is transgranular but in some areas is intergranular (Fig. 6b). The densities of ceramics measured by the Archimedes method are plotted in Fig. 7. As Gd3+ content increased, the density of ceramics linearly increases which is attributed to the higher atomic number of Gd. Based on the theoretical density of La2xGdxZr2O7 ceramics (x = 0, 6.057 g/cm3; x = 2.0, 6.932 g/cm3), the calculated relative densities of the ceramics were 99.1%, 99.2%, 98.6%, 98.7%, 99.4%, and 96.9%, respectively. The relative densities of x = 0.8 and 1.2 samples are slightly lower, which accounts for the lower in-line transmittance of the two samples (Fig. 5). The lower densities of x = 0.8 and 1.2 samples implies that there are more pores remained in the two samples. However, the differences are so slight that there is not much difference on the microstructures. For x = 2.0 sample, the lowest density was achieved owing to the microcracks and pores in the ceramic. In addition, with higher atomic number and density, the obtained transparent ceramics are promising candidates as host materials of scintillators which can be used in X-ray or c-ray detectors [15].
Fig. 7. Densities of La2xGdxZr2O7 ceramics.
4. Conclusions Transparent La2xGdxZr2O7 (x = 0–2.0) ceramics were successfully obtained by simple combustion method and vacuum sintering. All the samples are pyrochlore structure, and the lattice parameters decrease with the increase of Gd3+ content. All the ceramics are transparent and the in-line transmittance can reach to 73.6% at 1100 nm when x = 0.4. Meanwhile, with the increase of Gd3+ content, the effective atomic number and density increased. Integrated with the high refractive index (n = 2.08 at 632.8 nm) and optical transparency, La2xGdxZr2O7 transparent ceramics are promising optical lens material and host candidates for scintillators.
Acknowledgments The authors gratefully acknowledge financial supports from National Natural Science Foundation of China (No. 51172258) and partly from SCHOTT Glass Technologies (Suzhou) Co. Ltd.
References [1] Robert Vassen, Xueqiang Cao, Frank Tietz, Debabrata Basu, Detlev Stöver, J. Am. Ceram. Soc. 83 (8) (2000) 2023–2028. [2] David R. Clarke, Simon R. Phillpot, Mater. Today (June) (2005) 22–29. [3] M. Kumar, I. Arul Raj, R. Pattabiraman, Mater. Chem. Phys. 108 (2008) 102– 108. [4] B.J. Wuensch, K.W. Eberman, C. Heremans, E.M. Ku, P. Onnerud, E.M.E. Yeo, S.M. Haile, J.K. Stalick, J.D. Jorgensen, Solid State Ionics 129 (2000) 111–133. [5] Stefan J. Korf, Harry J.A. Koopmans, Bernard C. Lippens, Anthonie J. Burggraaf, Paul J. Gellings, Chem. Soc. Faraday Trans. 1 83 (1987) 1485–1491. [6] Masayoshi Uno, Atsuko Kosuga, Mihoko Okui, Kentarou Horisaka, Hiroaki Muta, Ken Kurosaki, Shinsuke Yamanaka, J. Alloys Comp. 420 (2006) 291–297. [7] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann, Science 289 (2000) 748–751. [8] Rodney C. Ewing, William J. Weber, Jie Lian, J. Appl. Phys. 95 (2004) 5949– 5972. [9] A. Chaudhry, A. Canning, R. Boutchko, M.J. Weber, N. Grnbech-Jensen, S.E. Derenzo, J. Appl. Phys. 109 (2011) 083708. [10] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55–143. [11] M.J.D. Rushton, Robin W. Grimes, J. Mater. Res. 19 (6) (2004) 1603–1604. [12] R. Karl, J. Solid State Chem. 182 (2009) 442–450. [13] Liqiong An, Akiniko Ito, Takashi Goto, Key Eng. Mater. 484 (2011) 135–138. [14] Xiaoqing Zou, Guohong Zhou, Hailan Yi, Guangjun Zhang, Shiwei Wang, J. Am. Ceram. Soc. 94 (4) (2011) 1002–1004. [15] C. Greskovich, S. Duclos, Annu. Rev. Mater. Sci. 27 (1997) 69–88. [16] R.D. Shannon, Acta Cryst. A 32 (1976) 751–767. [17] H. Kido, S. Komarneni, R. Roy, J. Am. Ceram. Soc. 74 (2) (1991) 422–424.
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[18] C. Wan, W. Pan, Q. Xu, Y. Qin, J. Wang, Z. Qu, M. Fang, Phys. Rev. B 74 (2006) 144109. [19] L. Gao, Y. An, H. Zhu, L. Wang, J. Chen, N. Wang, G. Ou, J. Mater. Sci. 46 (2010) 1337–1340. [20] Bernhardt J. Wuensch, Kevin W. Eberman, JOM 52 (2000) 19–21. [21] Chunlei Wan, Qu Zhixue, Du Aibing, Wei Pan, J. Am. Ceram. Soc. 94 (2) (2011) 592–596.
[22] U. Peuchert, Y. Okano, Y. Menke, S. Reichel, A. Ikesue, J. Eur. Ceram. Soc. 29 (2009) 283–291. [23] D. Zhou, Y. Shi, J. Xie, Y. Ren, P. Yun, J. Am. Ceram. Soc. 92 (2009) 2182–2187. [24] Mel I. Mendalson, J. Am. Ceram. Soc. 52 (8) (1969) 443–446.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Transparent La2xGdxZr2O7 ceramics obtained by combustion method and vacuum sintering Zhengjuan Wang a,b, Guohong Zhou a,⇑, Xianpeng Qin a, Yan Yang a, Guangjun Zhang c, Yvonne Menke d, Shiwei Wang a,⇑ a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China SCHOTT Glass Technologies (Suzhou) Co. Ltd., Suzhou 215009, China d Schott AG, Corporate Research and Technology Development, Hattenbergstrasse 10, 55122 Mainz, Germany b c
a r t i c l e
i n f o
Article history: Received 13 August 2013 Received in revised form 24 September 2013 Accepted 27 September 2013 Available online 10 October 2013 Keywords: La2xGdxZr2O7 Transparent ceramics Combustion method Vacuum sintering
a b s t r a c t Transparent La2xGdxZr2O7 (x = 0–2.0) ceramics were prepared via vacuum sintering from nanometric powders synthesized by a simple combustion method. The changes of phase composition, morphology and in-line transmittance of the resulting ceramics with Gd3+ content’s variation were investigated. With the increase of Gd3+ content, the samples keep the pyrochlore structure, but the X-ray diffraction peaks of the powders and ceramics shift to higher angle as the lattice parameters become smaller. All the ceramics are transparent with high in-line transmittance and high refractive index (2.08 @ 632.8 nm, x = 0.4–1.6). These results indicate that La2xGdxZr2O7 ceramics might be used as optical lens. Moreover, with the increase of Gd3+ content, the effective atomic number and density of the ceramics increase, therefore making them promising host candidates for scintillators. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth zirconates (RE2Zr2O7) have increasingly become the focus of recent investigations for their various applications such as thermal barrier coating [1,2], solid electrolytes for solid oxide fuel cells (SOFCs) [3,4], catalysts [5,6], matrices for immobilization of actinides in nuclear waste [7,8], host materials for scintillators [9] and so on. RE2Zr2O7 belongs to A2B2O7 system with a pyrochlore structure or a defective fluorite structure depending on the ionic radii ratios (rA/rB) [10] or can transform from one phase to another at a transition temperature [11]. The pyrochlore structure is closely related to that of fluorite and can be considered as a BO2 fluorite in which half of the B4+ cations are replaced by A3+ cations. Charge compensation takes place by introducing oxygen vacancies into the lattice, resulting in a change in space group from fluorite Fm3m to pyrochlore Fd3m. In pyrochlore structure, the cations and oxygen vacancies show long-range order, and it can be considered as a superstructure with a lattice constant which is twice that of fluorite-type structure [11]. The A cation adopts a distorted cubic coordination with oxygen (coordination number = 8), while B is octahedrally coordinated (coordination number = 6) [12]. ⇑ Corresponding authors. Tel.: +86 021 52414320; fax: +86 021 52415263. E-mail addresses: [email protected] (G. Zhou), [email protected]. ac.cn (S. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.187
As RE2Zr2O7 single crystals are difficult to grow because of the high melting point (>2000 °C), fabricating polycrystalline transparent ceramics becomes a good solution. In 2011, La2Zr2O7 transparent ceramics were fabricated by reactive spark plasma sintering [13], but the transmittance was lower than 25% (1 mm thick) in the visible spectral region. Recently, Zou et al. fabricated Y2Zr2O7 transparent ceramics by combustion method and vacuum sintering, and the in-line transmittance was up to 68% (1.0 mm thick) in the visible spectral region [14]. These investigations suggested that La2xGdxZr2O7 transparent ceramics could be fabricated. For host materials of scintillators, higher density and effective atomic number are essential to obtain higher stopping power for X-ray or c-ray [15]. In the present work, the doping of Gd with higher atomic number into La site can improve the density and effective atomic number of La2Zr2O7, therefore making it more possible to be used as host materials for scintillators. Theoretically, the ionic radius of Gd3+ (1.053 Å) is quite close to that of La3+ (1.160 Å) [16], which means it is easier for Gd3+ to substitute La3+. RE2Zr2O7 powders have been prepared by many methods, such as solid-state reaction [12], sol–gel [17], coprecipitation–calcination [18], hydrothermal synthesis [19]. Among them, solid-state reaction is the simplest, but it needs much higher sintering temperature to achieve full densification because of the poor sinterability of the powder. For the other methods, the processes are complicated and time-consuming. In this research, a rapid and simple combustion method was utilized to synthesize
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Z. Wang et al. / Journal of Alloys and Compounds 585 (2014) 497–502
La2xGdxZr2O7 (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) powders. After compaction and vacuum sintering, transparent La2xGdxZr2O7 (x = 0– 2.0) ceramics were obtained. The effects of Gd3+ content (x value) on phase composition, morphology and in-line transmittance of the final ceramics were investigated.
2. Materials and methods The raw materials were La(NO3)36H2O (99.99%), Gd2O3 (99.99%), Zr(NO3)43H2O (A.R.), and glycine (A.R.). Firstly, Gd2O3 (99.99%) was dissolved in excess nitric acid solution to prepare gadolinium nitrate solution. Then, stoichiometric amounts of the three nitrates with different Gd3+ content (x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0) and glycine were mixed respectively and stirred thoroughly till the solutions were clear. After that ammonia was added to adjust pH value of the solutions to 4. Then the solutions were transferred into different quartz crucibles, respectively. On a hot plate, the solutions were heated. With the evaporation of water, the solutions became sol and gel. Finally the combustion reaction took place within a few minutes to form the primary fluffy powders. The combustion reaction can be described as follows:
ð2 xÞ LaðNO3 Þ3 þ x GdðNO3 Þ3 þ 2 ZrðNO3 Þ4 þ 70=9 C2 H5 NO2 ! La2x Gdx Zr2 O7 þ 140=9 CO2 þ 98=9 N2 þ 175=9 H2 O
Fig. 1. XRD patterns of La2xGdxZr2O7 (x = 0–2.0) powders calcined at 1200 °C for 2 h.
ð1Þ
The as-synthesized powders were calcined at 1200 °C for 2 h. After ball milling for 20 h, the powders were cold isostatically pressed into pellets (Ø20 2.5 mm) at 200 MPa for 2 min. The green pellets were sintered at 1850 °C for 6 h in vacuum with 10–3 Pa vacuum level and then annealed at 1500 °C for 5 h in air. Finally, the resultant ceramics were double-sided polished for test. The phase compositions of the powders calcined at 1200 °C and the resultant ceramics were analyzed by X-ray diffraction (XRD) using a Germany Bruker D8 Focus diffractometer with Cu Ka radiation (k = 0.15418 nm) in the range of 2h = 10–80°. Morphology of the powders was observed with Field Emission Scanning Electron Microscopy (FE-SEM, JSM-6700F, JEOL, Tokyo, Japan). Thermally etched surfaces and fracture surfaces of the ceramics were observed with Scanning Electron Microscopy (SEM, JSM-6390, JEOL, Tokyo, Japan). The in-line transmittance was measured using a spectrophotometer (Cary 5000, Varian Inc., USA) in the range of wavelength between 200 nm and 1100 nm. The refractive indices were measured on a prism coupler (Metricon Model 2010, Schott, Suzhou) at the wavelength of 632.8 nm. The density was measured by the Archimedes method in distilled water.
Fig. 2. Morphologies of the La2xGdxZr2O7 powders ball milled for 20 h (a–f): x = 0, 0.4, 0.8, 1.2, 1.6 and 2.0.
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3. Results and discussion 3.1. Powder preparation Fig. 1 shows XRD patterns of La2xGdxZr2O7 (x = 0–2.0) powders calcined at 1200 °C for 2 h. All the powders possess the similar crystal structure. In Fig. 1, the JCPDS card PDF#71-2363 and PDF#80-0471 correspond to La2Zr2O7 (pyrochlore structure) and Gd2Zr2O7 (defective fluorite structure). Due to the low calcination temperature, all the diffraction peaks are not sharp and it shows insufficient crystallinity degree. But the crystal structure can be distinguished compared to the JCPDS card. With x (Gd3+ content) increased, the two weak peaks corresponding to (3 3 1) and (5 1 1) reflections of pyrochlore structure [4] become weaker and cannot be observed after x = 1.2, indicating the ordered pyrochlore structure begin to become disordered. The order–disorder transition temperature from pyrochlore to defective fluorite structure is 1530 °C [20] or 1575 °C [21]. So the x = 1.2–2.0 powders calcined at 1200 °C are supposed to be pyrochlore structure with low degree of ordering. Powders derived from combustion synthesis were severely agglomerated with porous network structure. The ball milling process was carried out to eliminate the agglomerates and porous network structures [14]. Morphologies of the powders ball milled for 20 h are shown in Fig. 2. It can be seen that all the particle size distribution are relatively uniform for different Gd3+ content and the average particle size is smaller than 100 nm, which implies higher sintering activity of the powders.
Fig. 4. XRD patterns of La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air.
3.2. Fabrication of La2xGdxZr2O7 transparent ceramics Fig. 3 is the photo of the mirror-polished La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and then annealed at 1500 °C for 5 h in air. All of the ceramics exhibit high optical transparency in the visible spectral region. And with the increase of x value from 0.4 to 2.0, the colors of the ceramics become lighter. The post-annealing process was performed to eliminate the excess oxygen vacancies derived from vacuum sintering, as the oxygen vacancies can produce strong light absorption through the formation of color centers, which would lead to the coloration of the ceramics. Fig. 4 shows the XRD patterns of La2-xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air. After sintered, the obtained ceramics were much better crystallized compared to the calcined powders (Fig. 2). All the La2xGdxZr2O7 (x = 0–2.0) ceramics exhibit pyrochlore structure with two weak peaks corresponding to (33 1) and (51 1) reflections, which characterize the superstructure of pyrochlore phase and is used to distinguish between pyrochlore and fluorite structure [4]. In contrast to the powders, the detectable (3 3 1) and (5 1 1) reflections for x = 1.2–2.0 shows the ordering of the pyrochlore structure increases because of higher crystalline degree. For x = 2.0, the generation of obvious cracks (Fig. 3) could be the result of volume change produced in the phase transition process. Furthermore, according to Subramanian et al. [10], the range of pyrochlore stability of A2B2O7 exists from ionic radii ratios rA/rB = 1.46 to
Fig. 3. Photo of La2xGdxZr2O7 ceramics vacuum sintered at 1850 °C for 6 h and annealed at 1500 °C for 5 h in air (1.0 mm thick).
Fig. 5. In-line transmittance curves of the mirror-polished La2xGdxZr2O7 ceramics (1.0 mm thick).
rA/rB = 1.78. For smaller ionic radii ratios, rA/rB < 1.46, anion-deficient fluorite is the stable structure, whilst the monoclinic structure is stable for ionic radii ratios rA/rB > 1.78. In present work, 3þ 4þ r3þ Gd ¼ 1:053 Å, r La ¼ 1:160 Å and r Zr ¼ 0:720 Å respectively, the ionic radii ratios (rA/rB) of La2xGdxZr2O7 (A: La3+ and Gd3+, B: Zr4+) range from 1.61 (x = 0) to 1.46 (x = 2.0), which are all in the range of pyrochlore structure and further verified the XRD results of Fig. 4. The x = 2.0 sample (rA/rB = 1.46) is at the critical point between defective fluorite structure and pyrochlore structure, which is also an evidence of the instability of the ceramic. Moreover, with x (Gd3+ content) increase, the diffraction peaks shift to higher angle, indicating that the lattice parameter decreases on 3þ account that r3þ Gd < r La . The lattice parameters were calculated using the XRD results by Bragg equation. As Gd3+ content increases from 0 to 2.0, the lattice parameters are linearly decreased and they are 10.814 Å, 10.772 Å, 10.717 Å, 10.665 Å, 10.598 Å, and 10.524 Å, respectively, which is agreement with Vegard’s rule and also confirm the above analysis. Fig. 5 shows the in-line transmittance curves of the mirror-polished La2xGdxZr2O7 ceramics (1.0 mm thick). All the ceramics have high in-line transmittance, and the highest can reach to 73.6% at 1100 nm when x = 0.4. As x value increased from 0.4 to 2.0 (except for x = 0), the absorption edges of the in-line
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transmittance curves shifted to ultra-violet region. The absorption edge is the intrinsic property determined by band structures which
is related to crystal structure. As the ionic radius of Gd3+ is smaller than that of La3+, the substitution of La3+ by Gd3+ could shorten the
Fig. 6. Thermally etched surfaces and fracture surfaces of La2xGdxZr2O7 ceramics: (a and b): x = 0; (c and d): x = 0.4; (e and f): x = 0.8; (g and h): x = 1.2; (i and j): x = 1.6; and (k and l): x = 2.0.
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oxgen-metal band, which leads to broader energy band (Eg). Thus, the absorption edge shifts to UV region (k = hc/Eg). The in-line transmittance is somehow lower for x = 2.0 sample, implying there may be more pores in x = 2.0 ceramic. Furthermore, the microcracks (Fig. 3) in x = 2.0 sample can also result in scattering and reduce the in-line transmittance. The refractive indices of La2xGdxZr2O7 (x = 0.4–1.6) ceramics are all about 2.08 at the wavelength of 632.8 nm, which are high in transparent ceramics. It makes La2xGdxZr2O7 ceramics applicable as optical lens in imaging devices like cameras to realize the miniaturization of these devices [22]. The theoretical transmittance of the ceramics can be calculated from the following equations [23]:
T ¼ I=I0 ¼ ð1 RÞ2 expðatÞ
ð2Þ
where R = (n 1)2/(n + 1)2, in which n is the refractive index of the ceramics, a is the optical attenuation coefficient, and t is the thickness of the transparent ceramics. Assuming that the spectral loss can be ignored, that is to say, optical attenuation coefficient a is 0. Then, Eq. (2) can be simplified to T = (1 R)2. When the wavelength is 632.8 nm, the refractive index n of La2xGdxZr2O7 ceramics is 2.08 and the calculated theoretical transmittance is 76.9%. In this work, the in-line transmittance of La2xGdxZr2O7 (x = 0.4) reaches to the highest (70.9%) at 633 nm, about 92% of the theoretical value. Fig. 6 shows the thermally etched surfaces and fracture surfaces of the obtained ceramics. The average grain sizes vary with Gd3+ content. In this study, the average grain sizes of the final ceramics are determined by the average linear intercept length multiplied by a statistical factor 1.56 [24]. The calculated average grain sizes of x = 0 and x = 2.0 samples are 21 lm and 50 lm, respectively. For x = 0.4–1.2, the average grain sizes are almost the same, about 12 lm. The x = 1.6 sample has the smallest average grain size and is about 9 lm. It is evident that Gd is the sintering aid for La2Zr2O7 and the same for La to Gd2Zr2O7. The doping of La into Gd2Zr2O7 can inhibit grain growth and reduce the grain size greatly just as the case of x = 1.6 sample, of which the average grain size reduced from about 50 lm (x = 2.0) to 9 lm (x = 1.6). Similarly, the doping of Gd into La2Zr2O7 can also inhibit the grain growth as the case of x = 0.4, but the effect of Gd doping into La2Zr2O7 is not as obvious as that of La into Gd2Zr2O7. Few residual micropores were observed in x = 0–1.6 samples and more pores in x = 2.0 sample. As in this study all the ceramics are single cubic pyrochlore structure, the main scattering source is the residual pores which would result in the decrease of the transparency of the ceramics. The more pores in x = 2.0 sample lead to a significant reduction in the in-line transmittance as shown in Fig. 5. Fig. 6 also shows the different fracture modes. The main fracture mode is transgranular but in some areas is intergranular (Fig. 6b). The densities of ceramics measured by the Archimedes method are plotted in Fig. 7. As Gd3+ content increased, the density of ceramics linearly increases which is attributed to the higher atomic number of Gd. Based on the theoretical density of La2xGdxZr2O7 ceramics (x = 0, 6.057 g/cm3; x = 2.0, 6.932 g/cm3), the calculated relative densities of the ceramics were 99.1%, 99.2%, 98.6%, 98.7%, 99.4%, and 96.9%, respectively. The relative densities of x = 0.8 and 1.2 samples are slightly lower, which accounts for the lower in-line transmittance of the two samples (Fig. 5). The lower densities of x = 0.8 and 1.2 samples implies that there are more pores remained in the two samples. However, the differences are so slight that there is not much difference on the microstructures. For x = 2.0 sample, the lowest density was achieved owing to the microcracks and pores in the ceramic. In addition, with higher atomic number and density, the obtained transparent ceramics are promising candidates as host materials of scintillators which can be used in X-ray or c-ray detectors [15].
Fig. 7. Densities of La2xGdxZr2O7 ceramics.
4. Conclusions Transparent La2xGdxZr2O7 (x = 0–2.0) ceramics were successfully obtained by simple combustion method and vacuum sintering. All the samples are pyrochlore structure, and the lattice parameters decrease with the increase of Gd3+ content. All the ceramics are transparent and the in-line transmittance can reach to 73.6% at 1100 nm when x = 0.4. Meanwhile, with the increase of Gd3+ content, the effective atomic number and density increased. Integrated with the high refractive index (n = 2.08 at 632.8 nm) and optical transparency, La2xGdxZr2O7 transparent ceramics are promising optical lens material and host candidates for scintillators.
Acknowledgments The authors gratefully acknowledge financial supports from National Natural Science Foundation of China (No. 51172258) and partly from SCHOTT Glass Technologies (Suzhou) Co. Ltd.
References [1] Robert Vassen, Xueqiang Cao, Frank Tietz, Debabrata Basu, Detlev Stöver, J. Am. Ceram. Soc. 83 (8) (2000) 2023–2028. [2] David R. Clarke, Simon R. Phillpot, Mater. Today (June) (2005) 22–29. [3] M. Kumar, I. Arul Raj, R. Pattabiraman, Mater. Chem. Phys. 108 (2008) 102– 108. [4] B.J. Wuensch, K.W. Eberman, C. Heremans, E.M. Ku, P. Onnerud, E.M.E. Yeo, S.M. Haile, J.K. Stalick, J.D. Jorgensen, Solid State Ionics 129 (2000) 111–133. [5] Stefan J. Korf, Harry J.A. Koopmans, Bernard C. Lippens, Anthonie J. Burggraaf, Paul J. Gellings, Chem. Soc. Faraday Trans. 1 83 (1987) 1485–1491. [6] Masayoshi Uno, Atsuko Kosuga, Mihoko Okui, Kentarou Horisaka, Hiroaki Muta, Ken Kurosaki, Shinsuke Yamanaka, J. Alloys Comp. 420 (2006) 291–297. [7] K.E. Sickafus, L. Minervini, R.W. Grimes, J.A. Valdez, M. Ishimaru, F. Li, K.J. McClellan, T. Hartmann, Science 289 (2000) 748–751. [8] Rodney C. Ewing, William J. Weber, Jie Lian, J. Appl. Phys. 95 (2004) 5949– 5972. [9] A. Chaudhry, A. Canning, R. Boutchko, M.J. Weber, N. Grnbech-Jensen, S.E. Derenzo, J. Appl. Phys. 109 (2011) 083708. [10] M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55–143. [11] M.J.D. Rushton, Robin W. Grimes, J. Mater. Res. 19 (6) (2004) 1603–1604. [12] R. Karl, J. Solid State Chem. 182 (2009) 442–450. [13] Liqiong An, Akiniko Ito, Takashi Goto, Key Eng. Mater. 484 (2011) 135–138. [14] Xiaoqing Zou, Guohong Zhou, Hailan Yi, Guangjun Zhang, Shiwei Wang, J. Am. Ceram. Soc. 94 (4) (2011) 1002–1004. [15] C. Greskovich, S. Duclos, Annu. Rev. Mater. Sci. 27 (1997) 69–88. [16] R.D. Shannon, Acta Cryst. A 32 (1976) 751–767. [17] H. Kido, S. Komarneni, R. Roy, J. Am. Ceram. Soc. 74 (2) (1991) 422–424.
502
Z. Wang et al. / Journal of Alloys and Compounds 585 (2014) 497–502
[18] C. Wan, W. Pan, Q. Xu, Y. Qin, J. Wang, Z. Qu, M. Fang, Phys. Rev. B 74 (2006) 144109. [19] L. Gao, Y. An, H. Zhu, L. Wang, J. Chen, N. Wang, G. Ou, J. Mater. Sci. 46 (2010) 1337–1340. [20] Bernhardt J. Wuensch, Kevin W. Eberman, JOM 52 (2000) 19–21. [21] Chunlei Wan, Qu Zhixue, Du Aibing, Wei Pan, J. Am. Ceram. Soc. 94 (2) (2011) 592–596.
[22] U. Peuchert, Y. Okano, Y. Menke, S. Reichel, A. Ikesue, J. Eur. Ceram. Soc. 29 (2009) 283–291. [23] D. Zhou, Y. Shi, J. Xie, Y. Ren, P. Yun, J. Am. Ceram. Soc. 92 (2009) 2182–2187. [24] Mel I. Mendalson, J. Am. Ceram. Soc. 52 (8) (1969) 443–446.