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Preparation of La2Zr2O7 ceramic from Egyptian black sand
Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
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Journal of Radiation Research and Applied Sciences journal homepage: http://www.elsevier.com/locate/jrras
Preparation of La2Zr2O7 ceramic from Egyptian black sand Mohammed Y. Elkady a, Ashraf A. Mohmed a, A.M.E. Daher b, Wafaa H. Saleh b, S. Negm b, H. Mashaal b, * a b
Ain Shams University, Faculty of Science, Cairo, Egypt Nuclear Material Authority, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2017 Received in revised form 14 October 2017 Accepted 25 October 2017 Available online xxx
Lanthanum zirconates has been suggested as a thermal barrier coating for many high temperature applications. This work is focusing on the possibility of La2Zr2O7 from economic natural resources using Egyptian monazite and zircon. For this purpose, the prepared product of monazite REEs concentrate was subjected to Ce separation by its oxidation and precipitation as Ce (IV) at pH 3 with contact time 15 min and 30% excess amount of KMnO4. The REE-cake almost free from Ce (IV) was passing through Dowex50X8 cation exchange resin for the separation of pure product of La2O3. A homogeneous single phase compound of La2Zr2O7 has been formed at 3 h sintering time with sintering temperature 1100 C and ZrO2/La2O3 ratio 50% this confirmed with XRD (X-ray diffraction), Raman and EDX analysis techniques. © 2017 The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).
Keywords: La2Zr2O7 Black sand Lanthanum zirconate Egyptian monazite
1. Introduction Recently, pyrochlore oxides have received great attention mainly due to their interesting magnetic properties, but also due to some other unique physical properties such as thermal stability and low thermal conductivity. Pyrochlore oxides showed spin ice, spin glass, and spin liquid or long-range magnetic ordered states (Gaulin, Raimers, Masont, Greedan, & Tun, 1992, Harris, Bramwell, Mcmorrow, Zeiske, & Godfrey, 1997; Ramirez et al., 1999). Rare earth pyrochlores of the type A2B2O7 (where A “Rare Earth, B ”Transition Metal), are frustrated magnets. The frustration arises from the arrangement of the A and B ions in the lattice. Zirconate pyrochlore series, A2Zr2O7 has shown strong quantum fluctuations and spin ice correlations at finite temperature. (Gingras & McClarty, 2014). The attention to the rare earths zirconate pyrochlore has boosted recently after reports that showed Pr2Zr2O7 may display quantum effects. However, the interesting magnetic properties are not all what someone could get from rare earths zirconate pyrochlore. The rare earths zirconates were also suggested as replacement of yttria stabilised zirconia for applications that required thermal stability
* Corresponding author. E-mail address: [email protected] (H. Mashaal). Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications.
and low thermal conductivity, such as thermal barrier coatings. (Feng, Xiao, Zhao, & Pan, 2013) Amorphous lanthanum zirconate showed great potential in the electronic industry due to the high dielectric constants, and low leakage current densities. (Woods, Chiang, Plassemeyer, & Kast, 2017) Lanthanum zirconate was also proposed as a buffer layer for high-temperature superconducting coated conductors because of its chemical and structural compatibility with many traditional bulk superconductors; such as NiW and YBCO (Wang, Li, Feng, Yu, & Jin, 2016). Lanthanum zirconate, although has very a low conductivity, can exhibit high proton conductivity when doped with lower valence state ions, such as Ca, into La and Zr-site (Omata & Otsuka-Yao-Matsuo, 2001). This high proton conductivity opens the door for using rare earths zirconate pyrochlore as solid electrolyte for fuel cells and also for some sensing applications. Conventionally, LZ is prepared through solid-state reaction between pure oxides of zirconium and lanthanum. Oher methods that have been used to prepare LZ include: co precipitation, (Chen, Gao, Liu, & Luo, 2009) sol gel, (Rao, Banu, Vithal, & Kumar, 2002) Stearic acid combustion, and salt assisted combustion. (Tong et al., 2008) The solid state reaction is usually conducted by heating a mixture of pure lanthanum and zirconium oxides at high temperature (up to 1500 C) under a flow of argon for 10 h (Bolech, Vanmiltenburg, Cordfunke, & Laan, 1997) The most common method to prepare pure La2O3 is by dissolving La2(CO3)3$8H2O in a hot acid (usually concentrated nitric acid), followed by a precipitation step using
https://doi.org/10.1016/j.jrras.2017.10.005 1687-8507/© 2017 The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
ammonia solution. The precipitate is subsequently dried in air and then sintered in oxygen at ~900 C to decompose the nitrogen containing compound to pure La2O3 (Sedmidubský, Benes, & Konings, 2005). Pure zirconia is usually obtained by precipitating from the oxychloride solution (ZrOCl2$8H2O) using either ammonia or sodium hydroxide solution. The precipitate is then dried in oven and then heated up at ~1000 C to remove the chemically bonded water (or ammonia) and obtain pure ZrO2 Powder (Abdelkader, Elkashef, & Daher, 2008). However, to obtain precursors of pure lanthanum nitrate hexahydrate (La(NO3)3$6H2O) and oxychloride solution (ZrOCl2$8H2O), several purification steps are required; starting by impure minerals. These purifications steps are usually very expensive and involve using harsh chemistry and generating large amount of waste. It is therefore equally important to optimise the purification steps as well as the solid state reaction conditions. Also, preparing the LZ from domestic ore would certainly offer economic benefits and fits in the Egyptian plan of the sustainable use of resources. The present work is instigating the production of LZ starting form Egyptian ores, zircon for ZrO2 and monazite for the La2O3. The solid state reactions between the two pure oxides are also investigated and a portfolio on the preparing flowchart is built up. 2. Experimental work 2.1. Preparation of pure La2O3 from Egyptian monazite The preparation of La2O3 from Egyptian monazite involved three main steps including: 2.1.1. Preparation of RE concentrate from Egyptian monazite Monazite concentrate was obtained from the physical processing of Rossitta black sand, Egypt. The material has in average 95% of monazite estimated using ICP technique. A 200 gm sample portion of monazite concentrate was ground to <300 mesh size followed by stirring with 50 wt % NaOH solutions for 3 h at 170 C using solid/ liquid ratio of 1/1.5. The sample was then washed with distilled water and derided at 80 C before it was treated with concentrate sulfuric acid (70% concentration) for dissolution the elements of interest e.g. REEs, Th and U. Thorium and REEs were then separated from U as their insoluble oxalates. Thorium with almost associated heavy REEs especially yttrium were selectively separated from the prepared Th-REEs oxalate cake via their selective dissolution in ammonium carbonate solution. The REEs cake free from thorium with small amount of yttrium represented the started material of our experimental work. 2.1.2. Separation of Ce from REE concentrates through selective oxidation and precipitation The separation of Ce (IV) from the REs cake free from Th was performed through its oxidation and precipitation as Ce (IV). All the experiments for Ce recovery were carried out in beakers under magnetic agitation at room temperature. A volume of 20 ml REEs chloride solutions was used. The oxidation of Cerous to ceric and its precipitation Ce (IV) was taken place through the addition of KMnO4 while Na2CO3 was added to pH adjustment. After precipitation, the precipitate was filter at through filter paper and washed with dist. H2O and dried at 120 C in addition the filter at solution was subjected to ICP chemical analysis for calculations the recovery and purity Ce at all experiments. For this purpose, different experiments were performed including studying the added amount of oxidant (KMnO4), pH and time of precipitation was illustrated in the table below (see Table 1).
Table 1 Showing the studding parameter of Ce separation. KMnO4, X of stoichiometric amount
PH
Agitation time, (min.)
10 15 20 30 40
2 2.5 3 3.5 3
10 15 20 30 15
2.1.3. Selective recovery of pure La from rare earth concentrate almost free from cerium oxide The cation exchange resin Does 50 W-X8 of 150 mesh size was used to achieve the separation of Ln from almost Ce-free rare earth cake. To realize this separation, two columns of the same internal diameter (2 cm) have been packed in the working resin. However the first adsorption column the resin was packed in its hydrogen form and was used for the preparation of the saturated rare earth bead. This resin column was then coupled with the second retaining columns in which the resin volumes in the second was double that of the first column besides having been converted to its Cu (II) through passing CuSO4 solution till saturation. The purpose to increase the separation factor between the individual rare earth elements. A 0.015 M EDTA solution (in its NHþ 4 form and at the pH 8.5) was then used as eluent for the REEs from the first column through the second and third column with a flow rate of 1 ml/min (calculated as equivalent to a contact time of 1 min). The obtained eluate fractions of individually separated REEs were successfully collected every 50 ml after complete elution of Cu (II) to avoid the expected mixing of the eluted individual REEs fraction. These fractions were then subjected for determination the individually separated REEs through ICP chemical; analysis.
2.2. Preparation of pure ZrO2 from Egyptian zircon The sample of zirconia (ZrSiO4) in this work was obtained from the physical processing of Rossita black sand, Egypt and specified using XRF. The present work used this eutectic melt concept to breakdown the bond between the silica and zirconia in the zircon mineral. Zircon concentrate was heated with KOH-NaOH equimolar mixture at temperature5500C for 2 h. After removing silica from zircon frit, the impurities, especially U and Th must be removed. The most conventional way for that is by converting zirconium content in the residual solid after water leach to zirconium oxychloride by the action of hydrochloric acid. Zirconia obtained by the above optimum conditions through oxychloride treatment was subjected to XRF to determine the impurities level.
2.3. Preparation of La2Zr2 O7 ceramic A 5 gm sample of pure lanthanum oxide which produced from Egyptian monazite in step 1 and 5 gm of zirconium oxide produced from Egyptian Zirconia in the second step, both of two materials were mixed and grinded inside disc mill for two hours in dry atmosphere, then the sample was dried inside oven to a temperature at 100 C, the dried sample was mixed with bindery materials paraffin wax with a weight percentage 1.5%; estimated as function of the total weight of the sample, then the sample was mounted inside a die with a dimension 23 m diameter and 12 mm height, the pressing applied load estimated to be 15 ton, then sintering in electric furnace at 1100 C for 3 h, XRD, SEM, and Raman were done.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
3. Results and discussion 3.1. Preparation of RE concentrate from Egyptian monazite The preparation of pure La2O3 from Egyptian monazite involving the characterization of monazite mineral as well as processing of Egyptian monazite with separation of La2O3 from the prepared rare earth cake after separation of U, Th and Ce (IV). Complete chemical analysis of the studied monazite sample (Table 2) revealed that RE2O3 is major content in the monazite concentrate where its assay is 59.53% while P2O5 comprising 27.2% reflects the relatively high rare earth phosphate mineral content. In this context, uranium was separated from REEs-Th oxalate cake as soluble uranyl. Oxalate solution depends on the fact the uranium is soluble in oxalic acid solution. (The obtained oxalate cake of rare earth elements and thorium after washing and drying at 110 C was subjected to carbonate solution for selective dissolution of thorium from the rare earth cake as soluble thorium carbonate solution while rear earth elements cerium group) form insoluble carbonate solution. In addition yttrium group are relatively soluble in carbonate media (Amer, Abdella, Abdelwahab, & Elshikh, 2013, 2016). 3.1.2. Separation of pure cerium (IV) from the prepared monazite rare earth concentrates The recovery of cerium from monazite rare earth cake was investigated through two main steps namely; Oxidation of Ce (IIII) to Ce (IV) by KMnO4 and precipitation of cerium hydroxide via the addition of Na2CO3 solution. The separation of cerium from REEs can be take place by selective dissolution of trivalent RE(OH)3 by keeping Ce (IV) hydroxide in its insoluble form through its selective precipitation from the acid solution in the other case the Ce separation take place given the solubility difference between the Ce (IV) hydroxide (KSp ¼ 1054) and the trivalent REEs hydroxide (KSp ¼ 1022). Oxidation using permanganate solution led to simultaneous precipitation of Ce(OH)4 and manganese oxide. The following equation (1) illustrate the mechanism of oxidation process þ þ4 þ MnO2(S) þ 2H2O 3Ceþ3 þ MnO1 4 þ 4H / 3Ce
obtained data summarized in Fig. 1 It was found that the cerium recovery increased with increasing pH value while its purity decreases due to the precipitation of other associated rare earth elements. In addition the pH 3 represented the optimum value for the recovery of Ce (IV) with possible purity.
3.1.2.2. Effect of reaction time. The effect of reaction time upon the recovery of Ce (IV) was studied between 10 and 30 min where the added amount of KMnO4 was kept content at 20% excess amount and the pH value of precipitation was 3. The recorded time when all content of the oxidant (KMnO4) was added and the pH of precipitation arrived to its optimal value (3). The results illustrated in Fig. 2 Show that 15 min was sufficient o led to possible cerium recovery (79.8%) with suitable purity (97.1%).
3.1.2.3. Effect of added amount of KMnO4. The present work, the effect of excess amount of KMnO4 upon the cerium recovery and its purity was studied in the range from 10 to 40, were the other factors were fixed at 15 min reaction time and pH value 3. From the obtained data showing in Fig. 3 it was found that, the excess amount of KMnO4 has significant effect upon the recovery of Ce (IV) where its recovery increased from 53.2 up to 94.9% when excess amount of KMnO4 increased from 10 up to 30% in addition more increase in the added amount of oxidant (40% excess) has no significant effect upon the recovery of Ce (IV). On the otherwise, the excess amount of KMnO4 hasn't recorded any effect upon the purity of Ce (IV). From the above studied selective Ce (IV) oxidation/precipitation factors, it can be concluded that the optimum conditions for recovery of about 95% of Ce (IV) with purity of 97.5% from the prepared monazite rare earth chloride solution would be summarized as follow: Reaction time: 15 min, Final pH of precipitation: 3 and Added excess KMnO4: 30%.
(1)
The effect of initial and final pH of precipitation, the added amount of oxidant as well as the reaction time of cerium oxidation precipitation process were studied. 3.1.2.1. Effect of pH factor. The effect of pH factor upon the cerium oxidation/precipitation was investigated. The initial and the final pH values were tested. In the range between 2 up to 3.5, where the other experimental factors were kept constant at 10 min reaction time and 20% excess amount of KMnO4. In this context, its important to mention here in that, the pH value less than 1 led to more KMno4 consumption due to the oxidation of chloride ion. From the
Fig. 1. Showing the effect of pH on the recovery and purity of Ce.
Table 2 Shows the chemical composition of the working Egyptian Rossita monazite concentrate (95%). Element
%
P2O5 ThO2 U3O8 Fe2O3 TiO2 SiO2 P RE2O3
27.2 6.25 0.32 2.1 2.21 2.19 59.2
3
Fig. 2. Showing the effect of time on the recovery and purity of Ce.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6 Table 3 Showing XRF analysis of chemical composition of Egyptian zircon.
Fig. 3. Showing the effect of added amount of KMnO4 on the recovery and purity of Ce.
3.1.3. Preparation of pure La2O3 from Egyptian monazite For the selective recovery of the individual REE especially the almost Ce-free RECl3 solution (600 ml and it contains 13.1 g/l RE2O3) was allowed to pass through the prepared adsorption resin column of Dowex 50 W-X8 in its Hþ form until its saturation. This was followed by washing the resin bed with distilled water to remove any excess chloride. The practical capacity for the prepared adsorption resin bed of 76 ml wsr attained about 7.4 g RE2O3; a value that matches with the theoretical capacity of the working resin (1.7 meq/ml) and which was calculated using a mean molecular weight of 140 for the mixed light REE cake free from cerium. Elution was then carried out by allowing EDTA solution (0.015 M and pH 8.5) to pass through the first column to elute the REE into the second column (retaining column and twice of the first) with a contact time 20 min till complete elution of Cu (II). After the latter, the eluted REE-EDTA sample fractions were collected every 100 ml to avoid the expect interference between the eluted individual REEs fractions. The separated individual (or mixed) REEs in each fraction were then subjected to ICP chemical analysis. The obtained results indicate that La was separated in the final 5 fractions with its high purity 99.1 due to its high stability constant with EDTA complex. Fig. 4 Showing EDX examination of the prepared La2O3 product. 3.2. Preparation of pure ZrO2 from Egyptian zircon The preparation of pure ZrO2 from zircon involving the characterization of zircon mineral as well as processing of Egyptian zircon with separation of ZrO2 from zirconium silicate. Complete chemical analysis of the studied zircon sample using XRF (Table 3) revealed that ZrO2 is major content in the zircon concentrate where its assay is 61.9% while SiO2 comprising 30.6% reflects the relatively high zirconium silicate mineral content. In the same time the ThO2
Fig. 4. Showing EDX pattern for pure La2O3 obtained from Egyptian monazite.
Element
Content %
ZrO2 SiO2 TiO2 Fe2O3 HfO2 ThO2 Na2O MgO Al2O3 Cr2O3 U3O8
61.9 30.6 3.3 1.17 1.2 0.56 0.12 0.18 0.87 0.043 0.039
content is about 0.56% and U3O8 is 0.039% this reflects that the concentrate characterized by relatively low gangue mineral content eg., TiO2 and Fe2O3 reaching about 3.3, 1.17 respectively where HfO2 is 1.2%. After fusion of zircon with equimolar KOHeNaOH at 550 C for 2 h and removing impurities by zirconium oxychlorid method, then the product is dried and calcined at 1000 C for 2 h to obtained high pure ZrO2 showing in Table 4. 3.3. Preparation of La2Zr2 O7 ceramic One aim of the current work is to prepare LZ ceramics from cheap precursor extracted from domestic ore. First, pure lanthanum oxide was isolated from its monazite ore through the process described in section 1. Second, Zirconia was prepared through the alkali fusion of the zircon ore, followed by the oxychlorid treatment of the produced zirconium silicate frit, and finally calcination in open air., we have used the traditional solid state
Table 4 XRF Analyses for the obtained zircon. Oxide
Concentration (%)
Fe2O3 TiO2 MgO CaO Na2O U3O8 ThO2 SiO2 ZrO2þ HfO2,%
0.1 0.05 0.08 0.04 0.01 17 PPm 10 PPm 0.12 99.59
Fig. 5. XRD trace of the LZ prepared from the Egyptian ore.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
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Fig. 6. Raman spectra of zirconia, lanthanum oxide and lanthanum zirconates prepared from the Egyptian ore.
Fig. 7. SEM images of the produced LZ from the Egyptian ore.
preparation method, with the sintering conditions temperature 1100 C for 3 h. Fig. 5 showed the XRD pattern of the obtained LZ from the Egyptian ore. Clearly, the XRD trace is very similar to that recorded for the LZ prepared from pure commercially available oxides and there are no new peaks of any new contamination. This suggesting the successful preparation of LZ ceramic from Egyptian ore with the same crystallinity of that prepared form expensive commercial oxides sources. The grain size calculated from Scherrer equation is in the range of 500e800 nm, very similar to that otaned from the pure comercial oxides. The Raman analysis of the sintered samples confirms the formation of well crystalline lanthanum zirconates. The Raman spectra of zirconia and lanthanum oxides obtained from the Egyptian ore as well as LZ are presented in Fig. 6. From the Raman spectra one can conclude that all the peaks are roughly in the position of the zirconium and lanthanum ions peaks. No peaks that can be associated with monoclinic zirconia, tetragonal zirconia or any other impurities can be detected, confirming the purity of the LZ powder. Fig. 7 shows the scanning electron microscopic images of the LZ prepared from the Egyptian ore. AS shown, the microstructures of the grains are rather homogeneous and no secondary phases can be detected, which is in agreement with the XRD and Raman analysis findings. The particles size are also ranging between 500 nm to I micron, with the particles end to aggregate into bigger clusters. The grains are therefore slightly bigger than that obtained from pure
commercial oxides, but still agree roughly with the particle size calculated from the XRD pattern. 4. Conclusion Lanthanum zirconate ceramic can be obtained with minimum impurities with cheap precursor extracted from Egyptian black sand (monazite and zircon) by using the traditional solid state preparation method, with the sintering conditions temperature 1100 C for 3 h and La2O3/ZrO2 ratio. References Abdelkader, El-kashef, E., & Daher, A. (2008). Novel decomposition method for zircon. Journal of Alloys and Compounds, 460(1), 577e580. Amer, T. E., Abdella, W. M., Abdelwahab, G. M., & Elshahat, M. F. (2016). Selective separation of yittrium and cerium (IV) from the prepared Abu Hamata lanthanides cake. Chimical and Biomolecular Engineering, 1(3), 26e31. Amer, T. E., Abdella, W. M., Abdelwahab, G. M., & Elshikh, E. M. (2013). A suggested alternative procedure for processing of monazite mineral concentrate. International of Mineral Processing, 125, 106e111. Bolech, M., Vanmiltenburg, J. C., Cordfunke, E. H. P., & Laan, V. D. (1997). The heat capacity and derived thermodynamic functions of La2Zr2O7 and Ce2Zr2O7 from 4 to 1000 k. Journal of Physics and Chemistry of Solids, 58(3), 433e439. Chen, H., Gao, Y., Liu, y., & Luo, H. (2009). Coprecipitation synthesis and thermal conductivity of La2Zr2O7. Journal of Alloys and Compounds, 480(2), 843e848. Feng, J., Xiao, B., Zhao, R., & Pan, W. (2013). Thermal conductivity of rare earth zirconate pyrochlore from first principles. Scripta Materialia, 68(9), 727e730. Gaulin, B., Raimers, J. N., Masont, E., Greedan, J. E., & Tun, Z. (1992). Spin freezing in the geometrically frustrated pyrochlore antiferromagnet Tb2Mo2O7. Physical Review Letters, 69(22), 3244.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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Gingras, M. J., & McClarty, P. A. (2014). Quantum spin ice: A search for gapless quantum spin liquids in pyrochlore magnets. Reports on Progress in Physics, 77(5), 056501. Harris, M., Bramwell, S. T., Mcmorrow, D. F., Zeiske, T., & Godfrey, K. W. (1997). Geometrical frustration in the ferromagnetic pyrochlore Ho2Ti2O7. Physical Review Letters, 79(13), 2554. Omata, T., & Otsuka-Yao-Matsuo, S. (2001). Electrical properties of protonconducting Ca2þ-doped La2Zr2O7 with a pyrochlore-type structure. Journal of the Electrochemical Society, 148(6), E252eE261. Ramirez, A. P., Hayashi, A., Cava, R. J., Cava, R. J., Siddarthan, R., & Shastry, B. S. (1999). Zero-point entropy in'spin ice'. Nature, 399(6734), 333. Rao, K. K., Banu, T., Vithal, M., & Kumar, K. R. (2002). Preparation and characterization of bulk and nano particles of La2Zr2O7 and Nd2Zr2O7 by solegel method. Materials Letters, 54(2), 205e210.
Sedmidubský, D., Benes, O., & Konings, J. M. (2005). High temperature heat capacity of Nd2Zr2O7 and La2Zr2O7 pyrochlores. The Journal of Chemical Thermodynamics, 37(10), 1098e1103. Tong, Y., Wang, Z., Yu, Z., Wang, X., Yang, X., & Lu, L. (2008). Preparation and characterization of pyrochlore La2Zr2O7 nanocrystals by stearic acid method. Materials Letters, 62(6), 889e891. Wang, Y., Li, C. S., Feng, J. Q., Yu, Z. M., & Jin, L. (2016). Improved morphological and barrier properties of lanthanum zirconium oxide buffer layers obtained by chemical solution deposition for coated conductors. Journal of Materials Science: Materials in Electronics, 27(5), 4336e4343. Woods, K. N., Chiang, T. H., Plassemeyer, P. N., & Kast, M. G. (2017). High-k lanthanum zirconium oxide thin film dielectrics from aqueous solution precursors. ACS Applied Materials & Interfaces, 9(12), 10897e10903.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
Contents lists available at ScienceDirect
H O S T E D BY
Journal of Radiation Research and Applied Sciences journal homepage: http://www.elsevier.com/locate/jrras
Preparation of La2Zr2O7 ceramic from Egyptian black sand Mohammed Y. Elkady a, Ashraf A. Mohmed a, A.M.E. Daher b, Wafaa H. Saleh b, S. Negm b, H. Mashaal b, * a b
Ain Shams University, Faculty of Science, Cairo, Egypt Nuclear Material Authority, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2017 Received in revised form 14 October 2017 Accepted 25 October 2017 Available online xxx
Lanthanum zirconates has been suggested as a thermal barrier coating for many high temperature applications. This work is focusing on the possibility of La2Zr2O7 from economic natural resources using Egyptian monazite and zircon. For this purpose, the prepared product of monazite REEs concentrate was subjected to Ce separation by its oxidation and precipitation as Ce (IV) at pH 3 with contact time 15 min and 30% excess amount of KMnO4. The REE-cake almost free from Ce (IV) was passing through Dowex50X8 cation exchange resin for the separation of pure product of La2O3. A homogeneous single phase compound of La2Zr2O7 has been formed at 3 h sintering time with sintering temperature 1100 C and ZrO2/La2O3 ratio 50% this confirmed with XRD (X-ray diffraction), Raman and EDX analysis techniques. © 2017 The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).
Keywords: La2Zr2O7 Black sand Lanthanum zirconate Egyptian monazite
1. Introduction Recently, pyrochlore oxides have received great attention mainly due to their interesting magnetic properties, but also due to some other unique physical properties such as thermal stability and low thermal conductivity. Pyrochlore oxides showed spin ice, spin glass, and spin liquid or long-range magnetic ordered states (Gaulin, Raimers, Masont, Greedan, & Tun, 1992, Harris, Bramwell, Mcmorrow, Zeiske, & Godfrey, 1997; Ramirez et al., 1999). Rare earth pyrochlores of the type A2B2O7 (where A “Rare Earth, B ”Transition Metal), are frustrated magnets. The frustration arises from the arrangement of the A and B ions in the lattice. Zirconate pyrochlore series, A2Zr2O7 has shown strong quantum fluctuations and spin ice correlations at finite temperature. (Gingras & McClarty, 2014). The attention to the rare earths zirconate pyrochlore has boosted recently after reports that showed Pr2Zr2O7 may display quantum effects. However, the interesting magnetic properties are not all what someone could get from rare earths zirconate pyrochlore. The rare earths zirconates were also suggested as replacement of yttria stabilised zirconia for applications that required thermal stability
* Corresponding author. E-mail address: [email protected] (H. Mashaal). Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications.
and low thermal conductivity, such as thermal barrier coatings. (Feng, Xiao, Zhao, & Pan, 2013) Amorphous lanthanum zirconate showed great potential in the electronic industry due to the high dielectric constants, and low leakage current densities. (Woods, Chiang, Plassemeyer, & Kast, 2017) Lanthanum zirconate was also proposed as a buffer layer for high-temperature superconducting coated conductors because of its chemical and structural compatibility with many traditional bulk superconductors; such as NiW and YBCO (Wang, Li, Feng, Yu, & Jin, 2016). Lanthanum zirconate, although has very a low conductivity, can exhibit high proton conductivity when doped with lower valence state ions, such as Ca, into La and Zr-site (Omata & Otsuka-Yao-Matsuo, 2001). This high proton conductivity opens the door for using rare earths zirconate pyrochlore as solid electrolyte for fuel cells and also for some sensing applications. Conventionally, LZ is prepared through solid-state reaction between pure oxides of zirconium and lanthanum. Oher methods that have been used to prepare LZ include: co precipitation, (Chen, Gao, Liu, & Luo, 2009) sol gel, (Rao, Banu, Vithal, & Kumar, 2002) Stearic acid combustion, and salt assisted combustion. (Tong et al., 2008) The solid state reaction is usually conducted by heating a mixture of pure lanthanum and zirconium oxides at high temperature (up to 1500 C) under a flow of argon for 10 h (Bolech, Vanmiltenburg, Cordfunke, & Laan, 1997) The most common method to prepare pure La2O3 is by dissolving La2(CO3)3$8H2O in a hot acid (usually concentrated nitric acid), followed by a precipitation step using
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Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
ammonia solution. The precipitate is subsequently dried in air and then sintered in oxygen at ~900 C to decompose the nitrogen containing compound to pure La2O3 (Sedmidubský, Benes, & Konings, 2005). Pure zirconia is usually obtained by precipitating from the oxychloride solution (ZrOCl2$8H2O) using either ammonia or sodium hydroxide solution. The precipitate is then dried in oven and then heated up at ~1000 C to remove the chemically bonded water (or ammonia) and obtain pure ZrO2 Powder (Abdelkader, Elkashef, & Daher, 2008). However, to obtain precursors of pure lanthanum nitrate hexahydrate (La(NO3)3$6H2O) and oxychloride solution (ZrOCl2$8H2O), several purification steps are required; starting by impure minerals. These purifications steps are usually very expensive and involve using harsh chemistry and generating large amount of waste. It is therefore equally important to optimise the purification steps as well as the solid state reaction conditions. Also, preparing the LZ from domestic ore would certainly offer economic benefits and fits in the Egyptian plan of the sustainable use of resources. The present work is instigating the production of LZ starting form Egyptian ores, zircon for ZrO2 and monazite for the La2O3. The solid state reactions between the two pure oxides are also investigated and a portfolio on the preparing flowchart is built up. 2. Experimental work 2.1. Preparation of pure La2O3 from Egyptian monazite The preparation of La2O3 from Egyptian monazite involved three main steps including: 2.1.1. Preparation of RE concentrate from Egyptian monazite Monazite concentrate was obtained from the physical processing of Rossitta black sand, Egypt. The material has in average 95% of monazite estimated using ICP technique. A 200 gm sample portion of monazite concentrate was ground to <300 mesh size followed by stirring with 50 wt % NaOH solutions for 3 h at 170 C using solid/ liquid ratio of 1/1.5. The sample was then washed with distilled water and derided at 80 C before it was treated with concentrate sulfuric acid (70% concentration) for dissolution the elements of interest e.g. REEs, Th and U. Thorium and REEs were then separated from U as their insoluble oxalates. Thorium with almost associated heavy REEs especially yttrium were selectively separated from the prepared Th-REEs oxalate cake via their selective dissolution in ammonium carbonate solution. The REEs cake free from thorium with small amount of yttrium represented the started material of our experimental work. 2.1.2. Separation of Ce from REE concentrates through selective oxidation and precipitation The separation of Ce (IV) from the REs cake free from Th was performed through its oxidation and precipitation as Ce (IV). All the experiments for Ce recovery were carried out in beakers under magnetic agitation at room temperature. A volume of 20 ml REEs chloride solutions was used. The oxidation of Cerous to ceric and its precipitation Ce (IV) was taken place through the addition of KMnO4 while Na2CO3 was added to pH adjustment. After precipitation, the precipitate was filter at through filter paper and washed with dist. H2O and dried at 120 C in addition the filter at solution was subjected to ICP chemical analysis for calculations the recovery and purity Ce at all experiments. For this purpose, different experiments were performed including studying the added amount of oxidant (KMnO4), pH and time of precipitation was illustrated in the table below (see Table 1).
Table 1 Showing the studding parameter of Ce separation. KMnO4, X of stoichiometric amount
PH
Agitation time, (min.)
10 15 20 30 40
2 2.5 3 3.5 3
10 15 20 30 15
2.1.3. Selective recovery of pure La from rare earth concentrate almost free from cerium oxide The cation exchange resin Does 50 W-X8 of 150 mesh size was used to achieve the separation of Ln from almost Ce-free rare earth cake. To realize this separation, two columns of the same internal diameter (2 cm) have been packed in the working resin. However the first adsorption column the resin was packed in its hydrogen form and was used for the preparation of the saturated rare earth bead. This resin column was then coupled with the second retaining columns in which the resin volumes in the second was double that of the first column besides having been converted to its Cu (II) through passing CuSO4 solution till saturation. The purpose to increase the separation factor between the individual rare earth elements. A 0.015 M EDTA solution (in its NHþ 4 form and at the pH 8.5) was then used as eluent for the REEs from the first column through the second and third column with a flow rate of 1 ml/min (calculated as equivalent to a contact time of 1 min). The obtained eluate fractions of individually separated REEs were successfully collected every 50 ml after complete elution of Cu (II) to avoid the expected mixing of the eluted individual REEs fraction. These fractions were then subjected for determination the individually separated REEs through ICP chemical; analysis.
2.2. Preparation of pure ZrO2 from Egyptian zircon The sample of zirconia (ZrSiO4) in this work was obtained from the physical processing of Rossita black sand, Egypt and specified using XRF. The present work used this eutectic melt concept to breakdown the bond between the silica and zirconia in the zircon mineral. Zircon concentrate was heated with KOH-NaOH equimolar mixture at temperature5500C for 2 h. After removing silica from zircon frit, the impurities, especially U and Th must be removed. The most conventional way for that is by converting zirconium content in the residual solid after water leach to zirconium oxychloride by the action of hydrochloric acid. Zirconia obtained by the above optimum conditions through oxychloride treatment was subjected to XRF to determine the impurities level.
2.3. Preparation of La2Zr2 O7 ceramic A 5 gm sample of pure lanthanum oxide which produced from Egyptian monazite in step 1 and 5 gm of zirconium oxide produced from Egyptian Zirconia in the second step, both of two materials were mixed and grinded inside disc mill for two hours in dry atmosphere, then the sample was dried inside oven to a temperature at 100 C, the dried sample was mixed with bindery materials paraffin wax with a weight percentage 1.5%; estimated as function of the total weight of the sample, then the sample was mounted inside a die with a dimension 23 m diameter and 12 mm height, the pressing applied load estimated to be 15 ton, then sintering in electric furnace at 1100 C for 3 h, XRD, SEM, and Raman were done.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6
3. Results and discussion 3.1. Preparation of RE concentrate from Egyptian monazite The preparation of pure La2O3 from Egyptian monazite involving the characterization of monazite mineral as well as processing of Egyptian monazite with separation of La2O3 from the prepared rare earth cake after separation of U, Th and Ce (IV). Complete chemical analysis of the studied monazite sample (Table 2) revealed that RE2O3 is major content in the monazite concentrate where its assay is 59.53% while P2O5 comprising 27.2% reflects the relatively high rare earth phosphate mineral content. In this context, uranium was separated from REEs-Th oxalate cake as soluble uranyl. Oxalate solution depends on the fact the uranium is soluble in oxalic acid solution. (The obtained oxalate cake of rare earth elements and thorium after washing and drying at 110 C was subjected to carbonate solution for selective dissolution of thorium from the rare earth cake as soluble thorium carbonate solution while rear earth elements cerium group) form insoluble carbonate solution. In addition yttrium group are relatively soluble in carbonate media (Amer, Abdella, Abdelwahab, & Elshikh, 2013, 2016). 3.1.2. Separation of pure cerium (IV) from the prepared monazite rare earth concentrates The recovery of cerium from monazite rare earth cake was investigated through two main steps namely; Oxidation of Ce (IIII) to Ce (IV) by KMnO4 and precipitation of cerium hydroxide via the addition of Na2CO3 solution. The separation of cerium from REEs can be take place by selective dissolution of trivalent RE(OH)3 by keeping Ce (IV) hydroxide in its insoluble form through its selective precipitation from the acid solution in the other case the Ce separation take place given the solubility difference between the Ce (IV) hydroxide (KSp ¼ 1054) and the trivalent REEs hydroxide (KSp ¼ 1022). Oxidation using permanganate solution led to simultaneous precipitation of Ce(OH)4 and manganese oxide. The following equation (1) illustrate the mechanism of oxidation process þ þ4 þ MnO2(S) þ 2H2O 3Ceþ3 þ MnO1 4 þ 4H / 3Ce
obtained data summarized in Fig. 1 It was found that the cerium recovery increased with increasing pH value while its purity decreases due to the precipitation of other associated rare earth elements. In addition the pH 3 represented the optimum value for the recovery of Ce (IV) with possible purity.
3.1.2.2. Effect of reaction time. The effect of reaction time upon the recovery of Ce (IV) was studied between 10 and 30 min where the added amount of KMnO4 was kept content at 20% excess amount and the pH value of precipitation was 3. The recorded time when all content of the oxidant (KMnO4) was added and the pH of precipitation arrived to its optimal value (3). The results illustrated in Fig. 2 Show that 15 min was sufficient o led to possible cerium recovery (79.8%) with suitable purity (97.1%).
3.1.2.3. Effect of added amount of KMnO4. The present work, the effect of excess amount of KMnO4 upon the cerium recovery and its purity was studied in the range from 10 to 40, were the other factors were fixed at 15 min reaction time and pH value 3. From the obtained data showing in Fig. 3 it was found that, the excess amount of KMnO4 has significant effect upon the recovery of Ce (IV) where its recovery increased from 53.2 up to 94.9% when excess amount of KMnO4 increased from 10 up to 30% in addition more increase in the added amount of oxidant (40% excess) has no significant effect upon the recovery of Ce (IV). On the otherwise, the excess amount of KMnO4 hasn't recorded any effect upon the purity of Ce (IV). From the above studied selective Ce (IV) oxidation/precipitation factors, it can be concluded that the optimum conditions for recovery of about 95% of Ce (IV) with purity of 97.5% from the prepared monazite rare earth chloride solution would be summarized as follow: Reaction time: 15 min, Final pH of precipitation: 3 and Added excess KMnO4: 30%.
(1)
The effect of initial and final pH of precipitation, the added amount of oxidant as well as the reaction time of cerium oxidation precipitation process were studied. 3.1.2.1. Effect of pH factor. The effect of pH factor upon the cerium oxidation/precipitation was investigated. The initial and the final pH values were tested. In the range between 2 up to 3.5, where the other experimental factors were kept constant at 10 min reaction time and 20% excess amount of KMnO4. In this context, its important to mention here in that, the pH value less than 1 led to more KMno4 consumption due to the oxidation of chloride ion. From the
Fig. 1. Showing the effect of pH on the recovery and purity of Ce.
Table 2 Shows the chemical composition of the working Egyptian Rossita monazite concentrate (95%). Element
%
P2O5 ThO2 U3O8 Fe2O3 TiO2 SiO2 P RE2O3
27.2 6.25 0.32 2.1 2.21 2.19 59.2
3
Fig. 2. Showing the effect of time on the recovery and purity of Ce.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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M.Y. Elkady et al. / Journal of Radiation Research and Applied Sciences xxx (2017) 1e6 Table 3 Showing XRF analysis of chemical composition of Egyptian zircon.
Fig. 3. Showing the effect of added amount of KMnO4 on the recovery and purity of Ce.
3.1.3. Preparation of pure La2O3 from Egyptian monazite For the selective recovery of the individual REE especially the almost Ce-free RECl3 solution (600 ml and it contains 13.1 g/l RE2O3) was allowed to pass through the prepared adsorption resin column of Dowex 50 W-X8 in its Hþ form until its saturation. This was followed by washing the resin bed with distilled water to remove any excess chloride. The practical capacity for the prepared adsorption resin bed of 76 ml wsr attained about 7.4 g RE2O3; a value that matches with the theoretical capacity of the working resin (1.7 meq/ml) and which was calculated using a mean molecular weight of 140 for the mixed light REE cake free from cerium. Elution was then carried out by allowing EDTA solution (0.015 M and pH 8.5) to pass through the first column to elute the REE into the second column (retaining column and twice of the first) with a contact time 20 min till complete elution of Cu (II). After the latter, the eluted REE-EDTA sample fractions were collected every 100 ml to avoid the expect interference between the eluted individual REEs fractions. The separated individual (or mixed) REEs in each fraction were then subjected to ICP chemical analysis. The obtained results indicate that La was separated in the final 5 fractions with its high purity 99.1 due to its high stability constant with EDTA complex. Fig. 4 Showing EDX examination of the prepared La2O3 product. 3.2. Preparation of pure ZrO2 from Egyptian zircon The preparation of pure ZrO2 from zircon involving the characterization of zircon mineral as well as processing of Egyptian zircon with separation of ZrO2 from zirconium silicate. Complete chemical analysis of the studied zircon sample using XRF (Table 3) revealed that ZrO2 is major content in the zircon concentrate where its assay is 61.9% while SiO2 comprising 30.6% reflects the relatively high zirconium silicate mineral content. In the same time the ThO2
Fig. 4. Showing EDX pattern for pure La2O3 obtained from Egyptian monazite.
Element
Content %
ZrO2 SiO2 TiO2 Fe2O3 HfO2 ThO2 Na2O MgO Al2O3 Cr2O3 U3O8
61.9 30.6 3.3 1.17 1.2 0.56 0.12 0.18 0.87 0.043 0.039
content is about 0.56% and U3O8 is 0.039% this reflects that the concentrate characterized by relatively low gangue mineral content eg., TiO2 and Fe2O3 reaching about 3.3, 1.17 respectively where HfO2 is 1.2%. After fusion of zircon with equimolar KOHeNaOH at 550 C for 2 h and removing impurities by zirconium oxychlorid method, then the product is dried and calcined at 1000 C for 2 h to obtained high pure ZrO2 showing in Table 4. 3.3. Preparation of La2Zr2 O7 ceramic One aim of the current work is to prepare LZ ceramics from cheap precursor extracted from domestic ore. First, pure lanthanum oxide was isolated from its monazite ore through the process described in section 1. Second, Zirconia was prepared through the alkali fusion of the zircon ore, followed by the oxychlorid treatment of the produced zirconium silicate frit, and finally calcination in open air., we have used the traditional solid state
Table 4 XRF Analyses for the obtained zircon. Oxide
Concentration (%)
Fe2O3 TiO2 MgO CaO Na2O U3O8 ThO2 SiO2 ZrO2þ HfO2,%
0.1 0.05 0.08 0.04 0.01 17 PPm 10 PPm 0.12 99.59
Fig. 5. XRD trace of the LZ prepared from the Egyptian ore.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005
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Fig. 6. Raman spectra of zirconia, lanthanum oxide and lanthanum zirconates prepared from the Egyptian ore.
Fig. 7. SEM images of the produced LZ from the Egyptian ore.
preparation method, with the sintering conditions temperature 1100 C for 3 h. Fig. 5 showed the XRD pattern of the obtained LZ from the Egyptian ore. Clearly, the XRD trace is very similar to that recorded for the LZ prepared from pure commercially available oxides and there are no new peaks of any new contamination. This suggesting the successful preparation of LZ ceramic from Egyptian ore with the same crystallinity of that prepared form expensive commercial oxides sources. The grain size calculated from Scherrer equation is in the range of 500e800 nm, very similar to that otaned from the pure comercial oxides. The Raman analysis of the sintered samples confirms the formation of well crystalline lanthanum zirconates. The Raman spectra of zirconia and lanthanum oxides obtained from the Egyptian ore as well as LZ are presented in Fig. 6. From the Raman spectra one can conclude that all the peaks are roughly in the position of the zirconium and lanthanum ions peaks. No peaks that can be associated with monoclinic zirconia, tetragonal zirconia or any other impurities can be detected, confirming the purity of the LZ powder. Fig. 7 shows the scanning electron microscopic images of the LZ prepared from the Egyptian ore. AS shown, the microstructures of the grains are rather homogeneous and no secondary phases can be detected, which is in agreement with the XRD and Raman analysis findings. The particles size are also ranging between 500 nm to I micron, with the particles end to aggregate into bigger clusters. The grains are therefore slightly bigger than that obtained from pure
commercial oxides, but still agree roughly with the particle size calculated from the XRD pattern. 4. Conclusion Lanthanum zirconate ceramic can be obtained with minimum impurities with cheap precursor extracted from Egyptian black sand (monazite and zircon) by using the traditional solid state preparation method, with the sintering conditions temperature 1100 C for 3 h and La2O3/ZrO2 ratio. References Abdelkader, El-kashef, E., & Daher, A. (2008). Novel decomposition method for zircon. Journal of Alloys and Compounds, 460(1), 577e580. Amer, T. E., Abdella, W. M., Abdelwahab, G. M., & Elshahat, M. F. (2016). Selective separation of yittrium and cerium (IV) from the prepared Abu Hamata lanthanides cake. Chimical and Biomolecular Engineering, 1(3), 26e31. Amer, T. E., Abdella, W. M., Abdelwahab, G. M., & Elshikh, E. M. (2013). A suggested alternative procedure for processing of monazite mineral concentrate. International of Mineral Processing, 125, 106e111. Bolech, M., Vanmiltenburg, J. C., Cordfunke, E. H. P., & Laan, V. D. (1997). The heat capacity and derived thermodynamic functions of La2Zr2O7 and Ce2Zr2O7 from 4 to 1000 k. Journal of Physics and Chemistry of Solids, 58(3), 433e439. Chen, H., Gao, Y., Liu, y., & Luo, H. (2009). Coprecipitation synthesis and thermal conductivity of La2Zr2O7. Journal of Alloys and Compounds, 480(2), 843e848. Feng, J., Xiao, B., Zhao, R., & Pan, W. (2013). Thermal conductivity of rare earth zirconate pyrochlore from first principles. Scripta Materialia, 68(9), 727e730. Gaulin, B., Raimers, J. N., Masont, E., Greedan, J. E., & Tun, Z. (1992). Spin freezing in the geometrically frustrated pyrochlore antiferromagnet Tb2Mo2O7. Physical Review Letters, 69(22), 3244.
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Gingras, M. J., & McClarty, P. A. (2014). Quantum spin ice: A search for gapless quantum spin liquids in pyrochlore magnets. Reports on Progress in Physics, 77(5), 056501. Harris, M., Bramwell, S. T., Mcmorrow, D. F., Zeiske, T., & Godfrey, K. W. (1997). Geometrical frustration in the ferromagnetic pyrochlore Ho2Ti2O7. Physical Review Letters, 79(13), 2554. Omata, T., & Otsuka-Yao-Matsuo, S. (2001). Electrical properties of protonconducting Ca2þ-doped La2Zr2O7 with a pyrochlore-type structure. Journal of the Electrochemical Society, 148(6), E252eE261. Ramirez, A. P., Hayashi, A., Cava, R. J., Cava, R. J., Siddarthan, R., & Shastry, B. S. (1999). Zero-point entropy in'spin ice'. Nature, 399(6734), 333. Rao, K. K., Banu, T., Vithal, M., & Kumar, K. R. (2002). Preparation and characterization of bulk and nano particles of La2Zr2O7 and Nd2Zr2O7 by solegel method. Materials Letters, 54(2), 205e210.
Sedmidubský, D., Benes, O., & Konings, J. M. (2005). High temperature heat capacity of Nd2Zr2O7 and La2Zr2O7 pyrochlores. The Journal of Chemical Thermodynamics, 37(10), 1098e1103. Tong, Y., Wang, Z., Yu, Z., Wang, X., Yang, X., & Lu, L. (2008). Preparation and characterization of pyrochlore La2Zr2O7 nanocrystals by stearic acid method. Materials Letters, 62(6), 889e891. Wang, Y., Li, C. S., Feng, J. Q., Yu, Z. M., & Jin, L. (2016). Improved morphological and barrier properties of lanthanum zirconium oxide buffer layers obtained by chemical solution deposition for coated conductors. Journal of Materials Science: Materials in Electronics, 27(5), 4336e4343. Woods, K. N., Chiang, T. H., Plassemeyer, P. N., & Kast, M. G. (2017). High-k lanthanum zirconium oxide thin film dielectrics from aqueous solution precursors. ACS Applied Materials & Interfaces, 9(12), 10897e10903.
Please cite this article in press as: Elkady, M. Y., et al., Preparation of La2Zr2O7 ceramic from Egyptian black sand, Journal of Radiation Research and Applied Sciences (2017), https://doi.org/10.1016/j.jrras.2017.10.005