Synthesis, characterization and sintering of fluorite and pyrochlore-type compounds: Pr2Zr2O7, Sm2Zr2O7 and PrSmZr2O7

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ScienceDirect Materials Today: Proceedings 16 (2019) 156–162

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Synthesis, characterization and sintering of fluorite and pyrochloretype compounds: Pr2Zr2O7, Sm2Zr2O7 and PrSmZr2O7 Branko Matovića,*, Jelena Maletaškića,b, Katsumi Yoshidab, Toyohiko Yanob, a

Centre of Excellence-CextremeLab Vinca, Institute of Nuclear Sciences Vinča, University of Belgrade, Mike Petrovića-Alasa 12-14, 11000 Belgrade, Serbia b Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8550 Tokyo, Japan

Abstract Fluorite and pyrochlore-type compounds (Pr2Zr2O7, Sm2Zr2O7 and PrSmZr2O7) powders were prepared by glycine nitrate process (GNP) starting from metal (Sm, Pr) nitrate, zirconium chloride and glycine as a fuel. The GNP process at room temperature initially yielded amorphous powders, which crystallized after subsequent calcination to form crystalline ceramics. The formation of well crystalline compounds took place at temperature as low as 950C. The phase evolution with thermal treatment as well as powder properties such as crystallite size, lattice strain and lattice parameter were studied by X-ray powder diffraction (XRPD) at room temperature. High-density ceramic pellets free of any additives were obtained after compaction of the obtained powders and subsequent sintering at 1600C for 4 h in air. Sintering behaviour of the synthesized pyrochlore phases was followed with a scanning electron microscope. Hardness of the sintered samples was found to be in range of 8.9 to 9.9 GPa depending of chemical composition. © 2019 Published by Elsevier Ltd. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics. Keywords: Pyrochlore, Synthesis, Microstructure, Sintering

1. Introduction Pyrochlores are ternary compounds with the general chemical formula (A2B2O7), where A represents the site occupied by larger trivalent 8-coordinated cation, typically rare earth elements, whereas B represents the smaller

* Corresponding author. Tel.: +381 648505080 E-mail address: [email protected] 2214-7853 © 2019 Published by Elsevier Ltd. Selection and/or Peer-review under responsibility of 6th International Symposium on Advanced Ceramics.

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tetravalent 6-coordinted cations of transition metals 1. The properties of this huge group such as ionic conductivity 2,3, geometrically frustrated magnetism 4, neutron absorption 5, nuclear waste storage capacity 6 and thermal conductivity 7-9 can be easily controlled by changing the constituent ions. The crystalline structure and properties of pyrochlores are highly dependent on composition and the ratio of radii of A-site cation (rA3+) and B-site cation (rB4+). The compound can exist in two closely related structures known as ordered pyrochlore structure (space group 227) and disordered fluorite structure (space group 225) 10. The pyrochlore-type structure is normally observed for radius ratio rA3+/rB4+ = 1.46-1.78. Several methods have been used to fabricate pyrochlore compounds. Solid state reaction method [11, 12], sol-gel method [13], combination of sol–gel processing and complex precipitation [14], mechanical milling [15], molten salt reaction [16, 17], precipitation method [18], hydrothermal route [19] to name a few. An important technique for the synthesis and processing of advanced ceramics is combustion synthesis (CS). The CS is based on exothermicity of the redox reaction between fuels and oxidizers. One the best CS method is glycine nitrate process (GNP) owing to its high efficiency, simplicity, low energy consumption and uniform morphology of the obtained powders. The aim of this work was to obtain a high-temperature compounds (Sm2Zr2O7, Pr2Zr2O7 and their solid solution SmPrZr2O7) by combustion reaction between metal nitrides and/or chlorides and glycine as a fuel. This reaction was selected as it provides excellent mixing of constituent elements in atomic state which allows formation of final, crystalline, product after thermal treatment at relatively low temperature. The phase evolution during the thermal treatment as well as sinterability of the obtained powder were investigated along with crystallinity of final phase in the sintered body. 2. Experimental methods 2.1. Powder preparation and synthesis Starting chemicals used for the synthesis of powders were aminoacetic acid-glycine, (Fischer Scientific, USA), and nitrates (Pr, Sm), produced by Aldrich, USA. As precursor for Zr was Zr-chloride. Synthesis was carried out in a stainless steel reactor in which all reactants dissolved in distilled water were added according to previously calculated composition of the final powder. We used nitrates in the form of solutions, and Zr chloride and glycine was also added in the as received form. The reactants were heated on a hot plate up to about 540°C, until the evolution of the smoke terminated. The obtained ashes were afterwards calcined at temperatures 950 - 1550°C, for 2 h in air. We proposed the reaction according to equations: 2Pr(NO3)36H2O + 2ZrCl4 + 4NH2CH2COOH + 4O2 = Pr2Zr2O7 + 22H2O + 8CO2 +10N2 + 4Cl2 2Sm(NO3)36H2O + 2ZrCl4 + 4NH2CH2COOH + O2 = Sm2Zr2O7 + 22H2O + 8CO2 + 10N2 + 4Cl2 Pr(NO3)36H2O + Sm(NO3)36H2O + 2ZrCl4 + 4NH2CH2COOH + O2 = PrSmZr2O7 + 22H2O + 8CO2 + 10N2 + 8Cl2

(1) (2) (3)

2.2. Characterization All of the samples were characterized at room temperature by X-ray powder diffraction (XRPD) using Ultima IV Rigaku diffractometer (Japan), equipped with CuKα1,2 radiations, using a generator voltage (40.0 kV) and a generator current (40.0 mA). The range of 10 - 80° 2θ was used for all powders in a continuous scan mode with a scanning step size of 0.02° and at a scan rate of 2 °/min. Phase analysis was done by using the PDXL2 software (version 2.0.3.0) [20], with reference to the patterns of the International Centre for Diffraction Data database (ICDD) [21], version 2012. The average crystallite size (D) was calculated on the basis of the full-width at half-maximum intensity (FWHM) of the main reflections by applying Scherrer’s formula: Dhkl  Kλ / (·cos)

(4)

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where K is a Scherrer’s constant (~0.9), λ is the wavelengths of the X-ray used, θ is diffraction angle and β is corrected half-width for instrumental broadening given as β  (βm - βs) where βm and βs are observed half-width and half-width of the standard monoclinic sphene sample, respectively. Microstructure and grain size were investigated with Au/Pd coating using a field-emission scanning electron microscope (FE-SEM). The density of sintered samples was determined by employing Archimedes’ principle using water as the immersion liquid. Hardness of the sample was evaluated by Vickers hardness test at room temperature. Loadingunloading test was conducted. Vickers hardness (HV [GPa]) was calculated using Eq. (5) from Vickers indentations; HV=1.854×10-9·P/d2

(5)

where P is an indentation load (9.8 N) and d [m] is an average length of the diagonals of the Vickers indentation. 3. Results and Discussion

(622)

55

56

57

58

59

2 Degree

(662) (840)

Intensity (a.u)

(622)

(444)

(511) (440)

(400) (331)

(311)

(222)

PrSmZr2O7

(111)

Intensity (a.u)

331 420

Sm2Zr2O7

222

220

200

Intensity (a.u.)

111

Typical XRD patterns of synthesized powders that calcined at 950C are shown in Fig. 1. The XRD analysis reveals that pure disorder-defective fluorite structure was obtained after heat treatment at 950C for 2 h. All resolved peaks correspond to the fundamental fluorite reflections: F{111}, F{200}, F{220}, F{311} and F{222}, which represent a disorder-defective fluorite structure, belonging to the 225 space group. As mentioned, the stability of pyrochlore structure strongly depends on the ratio of the ionic radii of the A- and B-type cations and it is experimentally observed for rA3+/rB4+ = 1.46 - 1.78 22. The ratio of Shannon radii of A cation (Sm3+) and (Pr3+) in VIII-fold coordination and B cation (Zr4+) in VI-fold coordination is 1.498 and 1.563, respectively. It is important to note that the compositions with more similar cation radii tend to form disordered fluorite structure rather than ordered pyrochlore. Thus, it is expected that the material obtained in this study possesses pyrochlore structure. This is supported by the results of XRD measurement of powders obtained at 1250C (Fig. 2) showing the presence of typical superstructure diffraction peaks at the 2 values of 14° {111}, 28° {311}, 37° {331}, 45° {511}, and 51° {531}, belonging to the 227 space group [23].

Pr2Zr2O7 PrSmZr2O7

Pr2Zr2O7

Sm2Zr2O7 10

20

30

40

50

60

2Degree

(a)

70

80

90

10

20

30

40

50

60

70

80

90

2 Degree

(b)

Fig. 1. X-ray diffraction patterns of pyrochlore phases annealed at 950 (a) and 1250C (b) for 2 h in the air: Pr2Zr2O7, PrSmZr2O7 and Sm2Zr2O7. Insert in Fig. 1(b) the peak shift of the reflection {622}.

The higher temperature of thermal tretment allowed the better ordering structure and therefore transition from disorder-defective fluorite structure into ordered Pyrochlore structure. Insert in Fig. 1(b) revealed the peak shift due

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(662) (840)

(440) (531) (620) (622) (444)

(511)

(400) (331)

Intensity (a.u.)

(111)

(311)

(222)

to different cation size of (Sm3+) and (Pr3+) in pyrochlore compounds. The powders treated at highest temperature showed that all compositions possess the pyrochlore structure with the 227 space group. It is clear that the intensity of the super-structure peaks increased with temperature indicating the stabilization of pyrochlore structure (Fig. 2).

Sm2Zr2O7

PrSmZr2O7 Pr2Zr2O7 10

20

30

40

50

60

70

80

90

2 Degree Fig. 2. XRD patterns of pyrochlore phases annealed at 1550C for 2 h in the air: Pr2Zr2O7, PrSmZr2O7 and Sm2Zr2O7. The pyrochlore superlattice reflections are indicated by the arrows; space group Fm3 m (227).

The synthesized pyrochlore powders was found to have a good sinterability. The density, open porosity and micro-hardness are shown in Table 1. The all samples have a very high density, indicating relatively fast densification process during sintering at 1600C for 4 h. It is important to stress that no sintering additive was used. The high density and therefore low porosity of the sintered samples was accompanied by relatively high hardness of 8.9-9.9 GPa. This hardness values are sufficiently high to consider obtained materials as promising material for high temperature application such as thermal barriers coatings. Fig. 3 showed densely compacted grains in the sintered sample. The majority of grain have a diameter in the range of 3-5 m, whereas some grains can be larger than 10 m in diameter, revealing the tendency of pyrochlore particles to coarsen during sintering process. Table 1. Density, open porosity and micro hardness of pyrochlore phases; Pr2Zr2O7, PrSmZr2O7 and Sm2Zr2O7 at 1600C for 4 h in air. Sample Sm2Zr2O7 PrSmZr2O7 Pr2Zr2O7

Theoretical density (%) 98 99 99

Open porosity (%) 1.38 0.79 0.07

Hardness (GPa) 9.9 9.8 8.9

Fig. 3 also shows that polyhedral grain boundaries as well as triple grain junctions are clean, without the presence of secondary phase which is an advantage of this material when it comes to high-temperature applications such as thermal barrier coatings.

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(a)

(b)

(c) Fig. 3. FE-SEM images of sintered Sm2Zr2O7 (a), PrSmZr2O7 (b) and Pr2Zr2O7 (c) samples at 1600C for 4 h in air.

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4. Conclusion Three different powders with composition Pr2Zr2O7, Sm2Zr2O7 and PrSmZr2O7 were successfully synthesized by glycine nitrate process (GNP) starting from metal (Sm, Pr) nitrate and zirconium chloride and glycine as a fuel. It was found that the powder samples annealed at temperatures 950C are that pure and have disorder-defective fluorite structure. Thermal treatment at higher temperature 1250 and 1550C showed the phase transition from defect fluorite cubic structure to ordered pyrochlore structure. The obtained, pure powders showed a good sinterability. The relative density of samples sintered at 1600C for 4 h without sintering additive was higher 98-99 % of theoretical density. The microstructure consisted of faceted grains with diameter in range 3-5 m. Relatively high hardness of 8.9-9.9 GPa which were achieved in sintered samples making pyrochlore compounds promising material for high-temperature application. Acknowledgements This work was supported by The Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the framework of Innovative Nuclear Research and Development Program. Financial support from the Serbian Education and Science Ministry in the Framework of project No. 45012 is gratefully acknowledged. One of the authors Jelena Maletaskic, as well as Branko Matovic, gratefully acknowledge the financial support from the Tokyo Institute of Technology, Laboratory for Advanced Nuclear Energy, Institute of Innovative Research, as visiting professors. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

L. Cai, J. C. Nino, Complex ceramic structures, Acta Cryst. (2009). B65, 269-290 H. Yamamura, H. Nishino, K. Kakinuma, K. Nomura, Electrical conductivity anomaly around fluorite-pyrochlore phase boundary, Solid State Ionics 158 (2003) 359-365. J.A. Diaz-Gullen, M.R. Diaz-Gullen, K.P. Padmasree, A.F. Fuentes, J. Santamaria, C. Leon, High ionic conduxtivity in the pyrochloretype Gd2-yLayZr2O7 solid solution (0  y ≤ 1), Solid State Ionics, 179 (2008) 2160-2164. J. Gardner, M. Gingras, J. Greedan, Magnetic pyrochlore oxides, Rev. Mod. Phys. 82 (2010) 53-107. V. Risovany, A. Zakharov, E. Muraleva, V. Kosenkov, R. Latypov, Dysprosium hafnate as absorbing material for control rods, J. Nucl. Mater. 335 (2006) 163-170. R. Ewing, W. Weber, J. Lian, Nuclear waste disposal - pyrochlore (A2B2O7): nuclear waste form for the immobilization of plutonium and “minor” actinides, J. Appl. Phys. 95 (2004), 5949-5071. X.Q. Cao, R. Vassen, D. Stoever, Ceramic materials for barrier coatings, J. Eur. Ceram. Soc., 24 (2004) 1-10. W. Pan, S. Phillpot, C. Wan, A. Chernatynskiy, Z. Qu, Low thermal conductivity oxides, Mater. Res. Bull. 32 (2012) 917-922. M. Winter, D. Clarke, Oxide materials with low thermal conductivity, J. Amer. Ceram. Soc. 90 (2007) 533-540. M. A. Subramanian, G. Aravamudan, G. V. Subba Rao, Oxide Pyrochlores - A Review, Prog. Solid St. Chem. 15 (1983) 55-143. M.A. Subramanian, A. Clearfield, A.M. Umarji, G.K. Shenoy, G.V. Subba Rao, Syntehsis and solid state studies on Mn2Sb2O7 and (Mn1-xCdx)2Sb2O7 pyrochlore, J. Solid State Chem., 52 (1984) 124-129. Z. Wang, G. Zhou, X. Qin, F. Zhang, J. Ai, P. Liu, S. Wang, Fabrication and phase transition of La2-xLuxZr2O7 transparent ceramics, J. Eur. Ceram. Soc., 34 (2014) 3951-3958. S. Wang, W. Li, S. Wang, Z. Chen, Synthesis of nanostructured La2Zr2O7 by a non-alkoxide sol-gel method: From gel to crystalline powders, J. Eur. Ceram. Soc., 35 (2015) 105-112. L. Kong, I. Karatchevtseva, D.J. Gregg, M.G. Blackford, R. Holmes, G. Triani, A Novel Chemical Route to Prepare La2Ze2O7 Pyrochlore, J. Am. Ceram. Soc., 96 (2013) 935-941. A.F. Fuentes, K. Boulahya, M. Maczka, J. Hanuza, U. Amador, Synthesis of disorder pyrochlore A2Ti2O7 (A = Y, Gd and Dy), by mechanical milling of constituent oxides, Solid State Sci. 7 (2005) 343-353. M.L. Hand, M.C. Stennet, N. C. Hyatt, Rapid low temperature synthesis of a titanete pyrochlore by molten salt mediated reaction, J. Eur. Ceram. Soc., 32 (2012) 3211-3219 V.G. Sevastyanov, E.P. Simonenko, N.P. Simonenko, K.A. Sakharov, N.T. Kuznetsov, Synthesis of fine dispersed oxides La2Zr2O7, La2Hf2O7, Gd2Zr2O7, Gd2Hf2O7, 15th European Confeerence on composite materials, 24-28 June, 2012, Venice, Italy, 1-6. L. Kong, I. Karatchevtseva, D.J. Gregg, M.G. Blackford, R. Holmes, G. Triani, Gd2Zr2O9 and Nd2Zr2O7 pyrochlore prepared by aqueous chemical synthesis, J. Eur. Ceram. Soc., 33 (2013) 3273-3285.

162 [19] [20] [21] [22] [23]

B. Matović et al. / Materials Today: Proceedings 16 (2019) 156–162 J. Zeng, H. Wang, Y.C. Zhang, M.K. Zhu, H. Yan, Hydrothermal Synthesis and Photocatalytic Properties of Pyrochlore La2Sn2O7 Nanocubes, J. Phys. Chem. C, 111 (2007) 11879–11887. PDXL Version 2.0.3.0 Integrated X-ray Powder Diffraction Software, Rigaku Corporation, Tokyo, Japan (2011) 196–8666. Powder Diffraction File, PDF-2 Database, announcement of new data base release 2012, International Centre for Diffraction Data (ICDD). H. Yamamura, H. Nishino, K. Kakinuma, K. Nomura, J. Ceram Soc, Jpn. 111 (2003) 902. H. Yamamura, H. Nishino, K. Kakinuma, J. Ceram Soc, Jpn. 112 (2004) 553.