Co-precipitation of nano Mg–Y/ZrO₂ ternary oxide eutectic system: Effects of calcination temperature

Abstract

In the family of inorganic nanomaterials, zirconia is a highly promising functional ceramic with a high refractive index, hardness, and dielectric constant, as well as excellent chemical inertness and thermal stability. These properties are enhanced in nano-zirconia ceramics, because nanopowders have a small particle size, good morphology, and uniform and dispersive distribution. In this study, a co-precipitation process was proposed to synthesise highly dispersed MgO–Y₂O₃ co-stabilized ZrO₂ nanopowders. The effects of different calcination temperatures on the crystallisation degree and particle dispersion of zirconia nanopowders were characterised by X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption using the Brunauer–Emmett–Teller (BET) theory, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). The optimum synthesis conditions were obtained as follows: 6 h of high-energy planetary grinding and calcination at 800 °C in an electric furnace. Under these optimum conditions, the average particle size of the prepared powder was 28.7 nm. This process enriches the literature on the controllable preparation of Mg–Y/ZrO₂ nanopowders obtained by the co-precipitation method.

Introduction

Zirconia is an important inorganic nonmetallic material with applications in ceramics, catalysts, refractories, optics, aerospace, biology, and chemistry [[1], [2], [3], [4]]. However, pure zirconia materials exhibit volume change effects during the reversible martensitic transformation between the monoclinic phase and tetragonal crystals, leading to a decline in their mechanical and thermal shock properties, which limits their application range [[5], [6], [7], [8]].

To resolve this issue, studies have reported that the phase stability of pure zirconia materials may be improved by co-doping zirconia with stabilisers, thus enhancing their mechanical properties and thermal shock resistance [[9], [10], [11], [12]]. Common stabilisers include oxides such as Y₂O₃, CaO, Al₂O₃, MgO, Sc₂O, and CeO₂ [[13], [14], [15]]. The stable states of the zirconia materials generally include: tetragonal zirconia polycrystalline (TZP), fully stabilized zirconia (FSZ), and partially stabilized zirconia (PSZ) [16]. Wen et al. [17] investigated the improvement in the phase composition, packing density, microstructure, and compressive strength of MgO-PSZ ceramics by doping with a Y₂O₃ stabiliser. The good performance of zirconia ceramics was reported to be owing to the ultrafine particle diameter and a reasonable proportion of stabilisers. Abed et al. [18] explored the effect of the grain size of Y₂O₃–ZrO₂–MgAl₂O₄ powders on the properties of ceramic sintered bodies. The grain size greatly influenced the properties of zirconia ceramics, in addition to the effect of stabiliser content. Therefore, the grain size and dispersion of the precursor material are considered key factors in the preparation of high-toughness and high-strength zirconia materials.

Currently, the study of ultrafine nanoscale zirconia materials is attracting increasing attention [19,20]. Nanoparticles are solid particles with sizes in the range of 0.1–100 nm, between bulk materials and atoms and molecules, and serve as the raw materials in nanotechnology [21]. Nanosystems have properties, such as the small size, dielectric limit, and quantum tunnelling effects, which are not present in macroscopic systems (with objects visible to the human eye as the lower limit) or mesoscopic systems (submicron level, 0.1–1 μm). Therefore, nanosystems exhibit high catalytic activity and selectivity, high adsorption and diffusion, and excellent optical properties (such as high transparency and magnetic properties), enhanced toughness, lubricity, and other physical and chemical properties, which creates great potential for the application of nanomaterials [[22], [23], [24]]. However, owing to the high surface energy of nanoparticles and van der Waals forces between particles, nanoparticles are highly prone to agglomeration during preparation, separation, storage, and application, which affects the advantages gained by the small size of these materials. Therefore, maintaining good precursor dispersion and inhibiting nanoparticle agglomeration are important research topics in nanotechnology [25,26].

Many methods are available for the preparation of nanoscale zirconia. Currently, wet chemical preparation methods, such as hydrothermal [27], precipitation [28], sol-gel [29], microemulsion [30], electrochemical synthesis [31], and solvent evaporation [32], are used. During chemical precipitation, a precipitant is added to a mixed solution to produce insoluble precipitates, such as hydroxides or sulfates. The precipitates serve as the raw precursors. After the subsequent washing-filtering-heating process, precursors decompose into powder products. Chemical precipitation [33] is often selected for the preparation of powder products at the laboratory scale because of its low reactant count and equipment requirements, ease of operation, and high product purity. To prepare nanopowders, including multiple oxides, the chemical method of co-precipitation is widely used [34,35].

Co-precipitation has been successfully applied to the preparation of nanoscale zirconia. Santoyo et al. [36] used this method to obtain nanoscale tetragonal-phase zirconia. Yttrium oxide (3 mol%) partially stabilized zirconia particles were dissolved in an Al(NO₃)–9H₂O solution and subsequently co-precipitated with ammonia to obtain 3 mol% YSZ-Al₂O₃ particles (the target precursor). The target precursor was washed with ultrapure water and anhydrous ethanol, followed by drying in an oven at 90 °C. Subsequently, the precursor sample was calcined at 300 °C for 20 h to obtain Al₂O₃ and Y₂O₃ stabilized zirconium dioxide nanopowders. The cold pressed nanopowders were sintered in air at 1100 °C for 6 h to obtain YSZ-Al₂O₃ ceramics. Hsu et al. [37] obtained tetragonal-phase ZrO₂ with a size of 21.3 nm using the co-precipitation method.

In this study, a co-precipitation process is proposed to prepare highly dispersed yttrium oxide and magnesium oxide-stabilized zirconium oxide nanopowders. ZrOCl₂⋅8H₂O is used as the zirconium source mixed with Y(NO₃)₃⋅6H₂O and MgCl₂⋅6H₂O. The mixture is co-precipitated by titration with NH₃⋅H₂O under high-speed stirring, and nanoscale zirconium oxide powder is produced after extraction and subsequent washing, ball milling, drying, and calcination. The obtained samples are characterised by thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption using the Brunauer–Emmett–Teller (BET) theory, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). The effects of the calcination temperature on the degree of crystallisation, physical phase composition, and particle dispersion of the final tetragonal-phase Mg–Y/ZrO₂ (t-Mg-Y/ZrO₂) are investigated.