Zirconates formed in the systems Ln2O3‒ZrO2 (where Ln is a rare earth) have been extensively used in recent years due to a combination of their valuable properties necessary for the preparation of thermal barrier coatings [1], electrolytes in solid oxide fuel cells [2], materials for radioactive waste immobilization [3], gas sensors [4], photocatalysts [5], and luminescence matrices [6]. The system Yb2O3‒ZrO2 includes solid solutions based on monoclinic and cubic ZrO2, cubic Yb2O3, and one stoichiometric compound, Yb4Zr3O12 (δ-phase) with rhombohedral structure (space group R-3) [7, 8]. At 1630°C, the rhombohedral structure of Yb4Zr3O12 is transformed into defective fluorite structure (solid solution based on cubic ZrO2) [7]. Thermodynamic modeling has been performed, and thermodynamic characteristics of phases in the system Yb2O3‒ZrO2 have been determined [9–12].

Wet chemistry methods and solid state synthesis have been applied to obtain Yb4Zr3O12 (Table 1). Because of the low reaction rate, solid state syntheses are carried out at high temperatures (>1500°C) for a long time (up to several days) [13–16]. Wet chemistry methods based on calcination of precipitated hydroxides [12, 17, 18] or of the residue obtained after thermal decomposition of zirconium and ytterbium nitrates [8] make it possible to reduce the temperature of the process; however, the synthesis at 1000‒1100°C lasts 4 months and longer. Combustion of the precursor prepared from a nitrate solution in the presence of citric acid provided milder conditions of the subsequent thermal treatment of the residue, but the synthesis of δ-Yb4Zr3O12 was incomplete, and pelleting and annealing at 1525°C were necessary to complete the process [19]. The synthesis from oxides using a microwave plasma generator was characterized by a high temperature (up to 1910°C) and short time (tens of minutes), but additional calcination in a furnace at 1550°C for 24 h was required to obtain the ordered δ-phase [15]. The above considered methods are disadvantageous since the high temperature and/or prolonged thermal treatment favor increase of the size of Yb4Zr3O12 crystallites, hamper formation of the product in the nanocrystalline state, and make it impossible to obtain nanosized ceramics with enhanced performance characteristics in comparison to microcrystalline analogs [20, 21].

Table 1. Methods of synthesis of δ-Yb4Zr3O12
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An efficient way to accelerate solid state reactions consists of preliminary mechanical activation of the initial reactants in energy-intensive mills. Mechanical activation not only makes the mixture of reactants more homogeneous and increases their contact area but also gives rise to various structural defects [22–24], which significantly intensifies the solid state synthesis upon further heating.

In the synthesis of Yb4Zr3O12, the initial oxide mixture was mechanically activated in a Retsch PM 100 planetary ball mill using ceramic balls made of zirconium dioxide doped with yttrium oxide and grinding vials lined with the same ceramics [25]. After mechanical activation for 30 h, Yb4Zr3O12 with defective fluorite structure was obtained. The ordered δ-phase was formed as a result of thermal treatment of the mechanically activated oxide mixture at 1500°C for 6 h. The size of the obtained δ-Yb4Zr3O12 crystallites was not given, but the high temperature and long duration of sintering suggests that nanocrystalline compound was not formed.

An AGO-2 mill is one of the most efficient domestic and foreign planetary mills used for mechanical activation [26]. In comparison to the traditional solid state method, the temperature of the synthesis La2Zr2O7 [27] and Gd2Zr2O7 [28] can be reduced by 300‒500°C due to preliminary mechanical activation of an oxide mixture in AGO-2.

We studied how mechanical activation of a stoichiometric mixture of Zr and Yb oxide in an AGO-2 planetary mill affects the solid state synthesis of nanocrystalline δ-Yb4Zr3O12. According to the X-ray powder diffraction (XRD) data, mechanical activation over a period of 10 min leads to significant reduction of the peaks of ytterbium and zirconium oxides and their broadening, and the most part of low-intense peaks typical of the initial mixture disappears (Fig. 1). These findings suggest considerable structural defects and/or reduction in the size of ZrO2 and Yb2O3 crystallites as a result of intense mechanical treatment in a planetary mill.

Fig. 1.
figure 1

X-Ray powder patterns of a mixture of zirconium and ytterbium oxides: (1) after mechanical activation, (2) initial mixture, (3) initial mixture after calcination at 1200°C for 3 h.

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As in the case of the compositions La2O3/ZrO2 [27] and Gd2O3/ZrO2 [28], mechanical activation of a Yb2O3/ZrO2 mixture involves hydration and carbonization of lanthanide oxide via reaction of Yb2O3 with atmospheric moisture and carbon dioxide. This follows from the appearance in the IR spectrum of the mechanically activated mixture of an O–H stretching band at 3440 cm–1 and a doublet band at 1507/1400 cm–1 due to CO32‒ stretchings (see Supplementary Materials).

Figures 2 and 3 show the differential scanning calorimetry (DSC) and thermogravimetric (TG) curves of the initial oxide mixtures before and after mechanical activation, respectively. The total weight loss of the initial mixture (0.89 wt %) is likely to result from dehydration and decarbonization. The DSC curve of the initial mixture lacks clearly defined endo- or exothermic effects (Fig. 2). This consistent with the XRD data (Fig. 1), according to which calcination of the initial Yb2O3/ZrO2 mixture at 1200°C for 3 h was not accompanied by appearance of new phases.

Fig. 2.
figure 2

(1) TG and (2) DSC curves of an initial mixture of zirconium and ytterbium oxides.

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Fig. 3.
figure 3

(1) TG and (2) DSC curves of a mixture of zirconium and ytterbium oxides after mechanical activation.

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The total weight loss of the mechanically activated mixture was appreciably larger (2.25 wt % according to the TG data; Fig. 3), which is related to dehydration and decarbonization processes, in keeping with the IR data (see Supplementary Materials). The DSC curve shape in the temperature range 1000‒1300°C is likely to be determined by superposition of endothermic effects due to decomposition of carbonate groups and exothermic effects due to crystallization of amorphous Zr and Yb oxides, as well as of the exothermic effect due to start of formation of ytterbium zirconate. Presumably, the distinct exothermic DSC peak at 1236°C corresponds to crystallization of Yb4Zr3O12, which (as follows from the TG data; Fig. 3) is accompanied by thermal decomposition of the residual carbonate groups. According to the IR data, complete decarbonization of the mechanically activated mixture is achieved after isothermal calcination at 1100°C for 3 h (see Supplementary Materials).

The X-ray powder patterns of the mechanically activated mixture after calcination at different temperatures are shown in Fig. 4. The δ-phase of Yb4Zr3O12 crystallizes at a temperature of 1100°C and above. This follows from the appearance of additional reflections marked with arrows in Fig. 4. Elemental analysis showed that the mechanically activated mixture contains 0.9% of iron originating from abrasion of the mill balls and cylinder. The subsequent thermal treatment produces YbFeO3 as an impurity phase (Fig. 4). To prevent contamination, it is advisable to carry out mechanical activation with the use of balls made of zirconium metal and zirconium-lined steel cylinders. In this case, the abraded material will be oxidized to one of the reactants (ZrO2) during thermal treatment.

Fig. 4.
figure 4

X-Ray powder patterns of mechanically activated mixtures of zirconium and ytterbium oxides after calcination at different temperatures for 3 h.

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The effective size D of Yb4Zr3O12 crystallites was estimated from the broadening of the major diffraction peak at 2θ = 29.89° using the Scherrer formula (1) [29].

$$D = {\lambda \over {B\cos \theta }}.$$
((1))

Here, λ is the radiation wavelength, B is the integral reflection width (in 2θ), and θ is the diffraction angle. The calculated size of ytterbium zirconate crystallites was 12, 17, 27, and 41 nm for samples obtained by sintering of the mechanically activated mixtures at 900, 1000, 1100, and 1200°C, respectively.

The SEM image of Yb4Zr3O12 obtained by calcination of the mechanically activated mixture at 1100°C (see Supplementary Materials) shows polydisperse character of the powder and a tendency for particle aggregation. Large particles with a size of several micrometers are formed as a result of aggregation of primary particles most of which have a submicron size. The specific surface area of that sample was 1.48 m2/g. The average size DS (nm) of Yb4Zr3O12 particles was estimated from the calculated density (7.933 g/cm3; PDF 01-072-0816) using formula (2). The DS value was 511 nm, which is consistent with the SEM data.

$${D_S} = {\rm{ }}6000/(\rho \cdot {S_{{\rm{sp}}}}).$$
((2))

where ρ is the density (g/cm3), and Ssp is the specific surface area (m2/g).

Thus, mechanical activation of a mixture of Yb2O3 and ZrO2 in an AGO-2 centrifugal planetary mill for 10 min substantially accelerates the formation of Yb4Zr3O12 upon subsequent sintering. Calcination of the mechanically activated mixture at 1100‒1200°C for 3 h afforded nanocrystalline ytterbium zirconate. To the best of our knowledge, the nanocrystalline δ-phase of Yb4Zr3O12 was not synthesized previously by the solid state method.

EXPERIMENTAL

Zirconium oxide (ZrO2) or pure grade was calcined at 600°C for 5 h, and ytterbium oxide (Yb2O3) of ultrapure grade (99.8%) was calcined at 900°C for 5 h. The specific surface areas of the initial oxides were determined by low-temperature nitrogen adsorption using a Flow-Sorb II 2300 analyzer (Micromeritics) and were 21.9 (ZrO2) and 1.52 m2/g (Yb2O3).

A stoichiometric mixture of ZrO2 and Yb2O3 was subjected to mechanical activation in an AGO-2 centrifugal planetary mill [30] in an air environment at a centrifugal factor of 40 g using steel vials and steel balls 8 mm in diameter. A grinding vial was charged with 10 g of an initial oxide mixture and 200 g of balls. The duration of mechanical activation was 10 min. To achieve macro-homogeneity of the resulting powders, the mill was turned off every 1 min, and the mixture was stirred with a metallic spatula. The initial and mechanically activated oxide mixtures were calcined in the temperature range from 600 to 1200°C for 3 h in a SNOL 6,7/1300 electrical furnace on exposure to air. The X-ray powder diffraction analysis was performed on a Shimadzu XRD 6000 diffractometer (CuKα radiation); the X-ray powder patterns were recorded with a step of 0.02° (2θ), and the signal accumulation time in each point was 1 s. Thermal analysis was carried out on a Netzsch STA 409 PC/PG. The oxide mixtures were heated in corundum crucibles to 1250°C under argon at a rate of 10 deg/min. Scanning electron microscopy was performed with a LEO-1450 microscope. The IR spectra were recorded on a Nicolet 6700 FT-IR Fourier spectrometer from samples prepared as KBr disks.