Tailored Nanostructured Zirconia Polymorphs Are Prepared from ...

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which were used to prepare zirconia differed from one another in their chemical and phase compositions. X ray diffraction patterns for these samples are shown.
ISSN 00360236, Russian Journal of Inorganic Chemistry, 2014, Vol. 59, No. 4, pp. 286–290. © Pleiades Publishing, Ltd., 2014. Original Russian Text © L.M. Rudkovskaya, T.V. Pavlenko, R.N. Pshenichny, A.A. Omel’chuk, A.A. Vishnevsky, 2014, published in Zhurnal Neorganicheskoi Khimii, 2014, Vol. 59, No. 4, pp. 439–444.

SYNTHESIS AND PROPERTIES OF INORGANIC COMPOUNDS

Tailored Nanostructured Zirconia Polymorphs Are Prepared from Zircon Concentrate Decomposition Products L. M. Rudkovskayaa, T. V. Pavlenkoa, R. N. Pshenichnya, A. A. Omel’chuka, and A. A. Vishnevskyb a Vernadsky b

Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine Institute of Geology, Mineralogy, and Ore Formation, National Academy of Sciences of Ukraine, Kiev, Ukraine email: [email protected] Received December 25, 2012

Abstract—We report on the hydrothermal synthesis of nanodisperse zirconia powders from the products of zircon concentrate decomposition by alkali solutions of various concentrations in the presence of calcium fluoride. Either tetragonal or monoclinic zirconia, or mixtures of both can be prepared depending on the chemical and phase composition of the zircon concentrate decomposition products and on the conditions under which amorphous zirconium hydroxide gel was prepared. The tetragonal phase is stabilized by calcium ions contained in the initial solutions. DOI: 10.1134/S0036023614040160

Zirconiabased materials are widely used, thanks to the unique properties of zirconia, as catalyst sup ports, as oxygen sensors, in protective coatings for optical mirrors, in microelectronics, in ceramic biom aterials, and in other fields [1]. One popular method to prepare nanodisperse functional oxide materials is a hydrothermal process, which allows purposefully alter the morphology and structural properties of the disperse material via vary ing process parameters. Varying the process tempera ture, pressure, and duration and the concentration and acidity of the solution to be treated offers a means for preparing chemically pure and homogeneous nanocrystalline powders. Numerous literature sources concerned with the application of the hydrothermal process for preparing nanocrystalline ZrO2 powders describe the prepara tion of zirconia by treating amorphous precipitates obtained from pure salts: zirconium oxochloride ZrOCl2 or zirconium oxonitrate ZrO(NO3)2 [2–7]. Here, we report the results of our study into the hydrothermal synthesis of different nanostructured zirconia polymorphs from the decomposition prod ucts of zircon concentrate. We demonstrate the effect of the chemical and phase composition of initial prod ucts on the preparation of amorphous zirconium hydroxide gel and the phase composition of the result ing zirconia. EXPERIMENTAL We studied a zircon concentrate from the Vol nogorsk Mining and Processing Plant (Dnepropetrovsk oblast, Ukraine) that had the following composition

(wt %): ZrO2, 67.0; SiO2, 31.4; and HfO2, 1.5. The concentrate was decomposed under hydrothermal conditions (T = 320°C, τ = 6 h) by sodium hydroxide solutions containing NaOH in concentrations from 230 to 880 g/L in the presence of calcium fluoride [8]. Depending on the alkali solution concentration, the processes occurring in autoclaves can be described by the following reaction equations: 1) decomposition of zircon by highconcentration solutions (400–800 g/L NaOH) ZrSiO4 + 2NaOH + 3CaF2 +H2O → Na2ZrF6 + CaSiO3 + 2Ca(OH)2; 2) decomposition of zircon by lowconcentration solutions (200–350 g/L NaOH) ZrSiO4 + 2NaOH + CaF2 → ZrO2 + CaSiO3+ 2NaF +H2O. The thusobtained zircon concentrate decomposi tion products had the following composition (wt %): ZrO2, 21–25; CaO, 19–20; SiO2 – 12–15, Na2O, 14– 17; F, 4–20; and loss on ignition (LOI), 7–10. The phase compositions of the decomposition products are shown in the table as dependent on the concentration of the initial alkali solution. The zircon concentrate decomposition products contain various calcium silicates in addition to sodium fluorozirconates or zirconia. Samples 1, 3, and 5 which were used to prepare zirconia differed from one another in their chemical and phase compositions. Xray diffraction patterns for these samples are shown in Fig. 1. All zircon concentrate decomposition products are well soluble in mineral acids. The acid used in this

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study was concentrated (d = 1.19) or 10% hydrochlo ric acid (chemically pure grade). The initial solutions to be used in produce amor phous zirconium hydroxide were prepared as follows. A weighed (1g) portion of decomposition product sample 1 or 3 was dissolved under heating in 50 mL hydrochloric acid (10%), the undecomposed residue was filtered off, and a zirconium hydroxide gel was precipitated from the filtrate by slowly adding (1 : 1) aqueous ammonia (chemically pure grade) under con tinuous stirring. pH was monitored with a pH meter. Precipitation was carried out at 20°С. The solution with a zirconium hydroxide precipitate was vigorously stirred and then filtered through a paper filter. The thusobtained precipitate was washed with dilute aqueous ammonia and distilled water until the test for chloride ion (silver nitrate test) was negative. The freshly precipitated amorphous gel was transferred to a beaker; the slurry was added with NaOH solution (0.5 M) to adjust pH to 9–11 (0.5 М) and then trans ferred to autoclaves to be used in hydrothermal syn thesis. The zirconium hydroxide slurry was hydrother mally treated in 70mL autoclaves equipped with nickel inserts. The autoclave fillage was 60–70%. The synthesis parameters were as follows: temperature was 250°С and duration was 6 h. When an experiment was over, autoclaves were cooled in air; the resulting pre cipitates were separated by centrifugation, washed sev eral times with distilled water, and then dried at 100°С. Powder Xray diffraction patterns were recorded on a DRON3M diffractometer (СuКα radiation; angle range: 10°–90°). The JCPDS file was used to identify the patterns. Xray and Match computer programs were used to process the diffraction patterns. Crystal lite sizes (coherent scattering lengths) were calculated using the Scherrer relationship. The reference used in the calculations was potassium chloride. Scanning electron microscopy (SEM) on a JEOL JSM6700F was used to study the microstructure of samples. RESULTS AND DISCUSSION When the zircon concentrate decomposition prod ucts are dissolved in mineral acids, zirconiumcon taining compounds and calcium silicates contained in the precipitate are both transferred into solution. The dissolution of the zircon concentrate decom position products (for samples 1–3) follows the scheme below: Na2ZrF6 + 4HCl + H2O = ZrOCl2 + 2NaCl + 6HF, CaSiO3 + 2HCl = CaCl2 + H2SiO3. The provision of Ca2+ and SiO32− ions in the dis solved decomposition products requires that special RUSSIAN JOURNAL OF INORGANIC CHEMISTRY

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Effect of NaOH concentration on the composition of the zirconium compounds formed upon hydrothermal decom position of the concentrate. Decomposition temperature: 320°C Sam ple no.

Initial alkali solution Zirconiumcontaining concentration, phases in zircon concentrate g/L NaOH decomposition products

1

880

Na7Zr6F31

2

524

Na7Zr6F31 + Na2ZrF6

3

380

Na2ZrF6 + trace ZrO2

4

320

Na2ZrF6 + ZrO2

5

234

ZrO2

approaches be used in preparation to the synthesis of nanocrystalline zirconia powders. ZrO2 synthesis from the amorphous zirconium hydroxide gel that was prepared by precipitation with aqueous ammonia (1 : 1) from aqueous solutions was found to yield a mixture of phases consisting of nano disperse tetragonal ZrO2 and wellcrystallized zircon. Figure 2 shows an Xray diffraction pattern for a sample prepared from such solutions. This pattern fea tures distinct diffraction lines that are characteristic of the zircon crystal lattice (experimentally determined interplanar spacings are d = 4.47, 3.34, 2.53, 2.08, and others) and less strong lines from tetragonal zirconia. When zirconium hydroxide is precipitated from solutions containing SiO32− ions, it is coprecipitated with silicic acid. As a result, polymeric aggregates are formed such that contain zirconium and silicon ions linked via olic bridges. These aggregates are not homo geneous and have no definite composition in terms of zirconium and silicon. Zirconia starts to crystallize in regions that are free of silicon ions, since zirconium ions in these regions are linked by oxo bridges directly to one another. This leads to the formation of tetrago nal zirconia nanocrystals. The growth of crystals and their transition into the monoclinic phase are inhib ited by amorphous SiO2 isolating the crystals from one another. Newly formed ZrO2 crystals appear tightly surrounded by silica, and this promotes zircon forma tion [9]. The simplest way to remove the effect of silicic acid is to separate it at the step of tretreating the zircon con centrate decomposition products prior to subsequent precipitation of amorphous zirconium hydroxide gel. The pretreatment was accomplished as follows: a weighed portion of a sample was dissolved in concen trated hydrochloric acid and the solution was concen trated to wet salts in order to separate silicic acid. Fol Vol. 59

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I/I0, % 100

I/I0, % 100

(а)

80

80

60 60 40 40 20 20 0 (b) 100

10

20

30

40 50 2θ, deg

60

70

80 Fig. 2. Xray diffraction pattern for a sample prepared from amorphous gel without preseparating silicic acid: 䊉 tet ragonal ZrO2 and 䉬 zircon.

60 40 20

sample 3 yielded mixtures of phases consisting of tet ragonal and monoclinic ZrO2 phases. This result may be due to the special role played in nanodisperse zirco nia crystallization by calcium ions that were present in the dissolved zircon concentrate decomposition prod ucts. Group II or Group III metal ions are known to be necessary for zirconia to be stabilized in the tetrag onal phase [10]. In the case at hand, calcium ions have the decisive effect on the structure of subsequently formed nanocrystals as a factor that can alter the struc ture of the precursor zirconium hydroxide. Calcium ions contained in the solution play the role of a stabi lizer to enhance the crystallization of tetragonal ZrO2 during the synthesis of nanocrystalline zirconia from zircon concentrate decomposition products.

* * *

** **

*

*

0 (c) 100 80 60 40 * *

20 *

0 10

20

*

30

*

*

*

*

40 50 2θ, deg

* * **

60

70

80

Fig. 1. Xray diffraction patterns for zircon concentrate decomposition products: (a) sample 1 (䉬 sodium fluo rozirconate Na7Zr6F31); (b) sample 3 (䉬 sodium fluo rozirconate Na2ZrF6 and * monoclinic ZrO2); and (c) sample 5 (䊉 tetragonal ZrO2 and * monoclinic ZrO2).

lowing this, the precipitate was dissolved in hot water, filtered, and then washed. The desilicized filtrate was used to precipitate a zirconium hydroxide gel. In order for the precipitates to be lowhydrated, wellfilterable, they were purified by double precipitation. Xray powder diffraction of the products prepared from desilicized solutions shows that sample 1 and

Zirconium hydroxide precipitation from acid solu tions of zircon concentrate decomposition products is carried out at pH ≤ 5 in order to avoid the precipitation of impurity cations contained in the solutions. Acid solu tions of the decomposition products contain 0.20 g/L CaO on the average. After zirconium hydroxide is pre cipitated, 0.18–0.19 g/L CaO is determined in the fil trate. This proves that the nascent amorphous zirco nium hydroxide gel sorbs some amount of calcium ions from the solution. The penetration of these ions into the gel becomes more difficult with time because of their mutual repulsion. Sorbate calcium ions appear insufficient for stabilizing the entire zirconium hydroxide precipitate. It is for this reason that mix tures of monoclinic and tetragonal zirconia phases crystallize during hydrothermal synthesis from such the precipitates. Emission spectral analysis shows cal cium (