Preparation of nano-sized 6MgO–2Y₂O₃–ZrO₂ powders by a combined co-precipitation and high energy ball milling process

Abstract

Nanostructured ceramic materials doped with stabilizers have superior mechanical, chemical, and electrical properties. In this study, tetragonal zirconia stabilized by 6 mol% MgO and 2 mol% Y₂O₃ (t-6MgO–2Y₂O₃–ZrO₂) nanopowders with quasi-spherical morphology, uniform particle size, and narrower grain size distributions were prepared by a combination process including two steps: namely co-precipitation and high-energy ball milling. The effect of ball milling time on ZrO₂ crystal particles was investigated by characterizations including XRD, Raman, FT-IR, FEESM, BET, and TEM. With the increase of ball milling time, the average grain size of the powder showed a gradual decrease tendency, the particle size distribution changed from wide to narrow, the particle morphology tended to be spherical, and the specific surface area gradually increased. Under the optimized conditions (ball milling for 8 h, calcination temperature of 800 °C, and holding time of 2 h), the highly dispersed spherical nanopowders with a minimum particle size of 18.47 nm and an average particle size of 29.02 nm were obtained. These zirconium oxide nanopowders are suitable for the preparation of inorganic coatings, biomedical materials, catalyst materials, and other types of functional materials.

Introduction

Zirconia is an excellent structural and functional material with outstanding thermal, mechanical, and electrical properties. Zirconia also has good corrosion resistance and wears resistance, and thus is widely applied as refractory materials, ceramic pigments, bioceramics, and electronic ceramics [[1], [2], [3]]. However, the crystalline structure of pure zirconia undergoes martensitic phase transformation during the heating or cooling process [4], resulting in that zirconium oxide exists as monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic (c-ZrO₂) forms at different temperatures [5,6]. The martensitic phase transformation is usually accompanied by shear strain and volume changes, leading to a reduction in the mechanical properties of zirconia, and thus limiting its applications [7].

To further expand the range of applications of zirconia in structural and functional materials, many scientific researchers have investigated the microstructure and properties of zirconia. The addition of stabilizers can inhibit the martensitic phase transformation [[8], [9], [10], [11]]. The addition of metal oxides such as MgO, CaO, Y₂O_3, or CeO₂ to ZrO₂ can form a solid solution with ZrO₂, thus the stabilizers can increase the radius of ZrO₂ cations, making the radius ratio of anions and cations closer to the stable eight coordination, and eventually stabilized in the form of t-ZrO₂ or c-ZrO₂ [12]. Yuan [13] et al. prepared MgO–ZrO₂ and MgO-6YSZ ceramic fibers with MgO as additive by the electrostatic spinning method. The polycrystalline stability and microstructural evolution of the zirconia fibers were characterized. The influences of MgO on the phase stability and grain size of zirconia fibers were obtained. At 800 °C, the phase stability of zirconia fibers is mainly attributed to the excess of MgO on the surface, while the excess of MgO on the surface improves the grain size stability of zirconia fibers. Heuer [14] et al. studied the thermal decomposition behaviour of zirconia and its phase evolution at high temperatures. A pure tetragonal zirconia material was obtained at a heating rate of 10 °C/min and a calcination temperature of 1100 °C, with 3 mol% yttrium oxide and 10 mol% magnesium oxide as stabilizing additives. This material can be used as a reinforcing material for sub-stable austenitic steel matrix composites. In addition, Wen [15] et al. also successfully improved the ionic conductivity and compressive strength of electrolyte ceramic materials by doping Y₂O₃ into Mg-PSZ. Consequently, yttrium oxide and magnesium oxide are excellent co-doping stabilizers that can effectively modulate the phase composition stability, microstructure, ionic conductivity, and thermal shock resistance of ZrO₂ materials.

With the development of nanostructured ceramic materials, nano-zirconium dioxide materials with ultra-fine particle size, uniform particle size distribution, and high dispersion are of increasing interest. Compared to conventionally prepared micron-sized YSZ and MgO-PSZ products, nanoscale ceramic powders have a narrow particle size distribution and a spherical or equiaxed particle shape [16,17]. Nanoscale ceramic powders with fine and homogeneous particle size and good sintering activity result in high-density green bodies and sintered bodies, which are conducive to the production of structural ceramic materials with excellent mechanical properties [[18], [19], [20]]. At present, zirconium dioxide nanopowders have been widely used in the preparation of ceramics, fibers, inorganic coatings, bio-films, fuel cells, sensors, catalysts, and chromatography-filled columns [[21], [22], [23]]. Since the properties of zirconia powders have a significant influence on the subsequent forming and sintering, especially on the final microstructure and mechanical properties of the ceramics. The desired powder properties for the preparation of structural ceramics or advanced nanoceramics should therefore be: the nano scale average size, narrower grain size distributions, spherical or equiaxed particle shape, no agglomeration, or only soft agglomeration, high powder purity without impurities, and monophase and monodisperse systems. There are various methods to prepare ZrO₂ nanoparticles [[24], [25], [26], [27]], such as hydrothermal method, co-precipitation method, sol-gel method, microemulsion method. Compared to other liquid phase methods, the co-precipitation method is now widely used for the preparation of various oxide ceramic powders, because of its simple and practical process, lower production costs, and the ability to prepare high purity, ultra-fine, homogeneous composition, and good sintering properties of the raw material powder [27]. The co-precipitation method involves adding a precipitating agent to a mixed metal salt solution to obtain a mixed precipitate of uniform chemical composition, which is washed, dried, and calcined to obtain a composite oxide [28]. This chemical control method can also be adopted in combination with physical control methods to achieve better preparation results.

In the preparation of nanosized materials, high-energy ball milling is an effective physical method for the reduction of particle size [[29], [30], [31]]. This method uses the strong motion energy of high-speed rotation balls to grind and homogenize powder samples. In addition, this process increases the mechanical activation energy on the surface of the samples, thus inducing physical and chemical reactions of the powder, resulting in prepared nanoscale precursor powders [32,33]. The specific surface area and the dispersion condition can be improved if the shape of the nanopowders trends to be more regular with fewer angles [34,35].

With a more intense alignment of the powders, the final sintered product will also have better properties [36,37]. Ball milling-assisted hydrothermal experiments were proceeded by Duran [38] et al. to investigate the effect of mechanical forces on phase formation, crystallinity, and particle size distribution. A comparison of the formed monoclinic crystalline ZrO₂ between grinding and non-grinding processes was carried out. Average particle size of 94 nm was found for the ground powder, and for the non-ground powder, the corresponding value was 117 nm. Goyal [39] et al. synthesized nano-zirconia powders from industrial micron-sized zirconia powders using a high-energy ball milling method. The powders were characterized by SEM, XRD, and laser particle size analyzer, and the results suggested that the prepared powders were almost spherical after a grinding time of 15 h. The grain sizes were reduced to 50 nm, 34 nm, and 29 nm after a grinding duration of 10 h, 15 h, and 20 h, respectively. The final ZrO₂ powders showed a bimodal particle size distribution with an average particle size of 11 nm and 39 nm. Sari [40] et al. prepared 3 mol% yttrium stabilized zirconia nanopowders from ZrO₂ and Y₂O₃ using a high-energy ball milling method. The ball-milled products were investigated using X-ray diffraction, Rietveld refinement, and Mössbauer spectroscopy. The tetragonal crystal Zr_1–xY_xO_1.982(x = 0.037) was formed within 30 min of ball milling. After ball milling for 30 h, the grain size was reduced to 13 nm and remained constant, while the peak intensity of the monoclinic phase decreased until it disappeared with increasing ball-milling time. As discussed above, ball milling can reduce the grain size and average particle size of nanopowders, and restrain the agglomeration of the prepared material thus controlling the particle size distribution. However, few detailed studies have been reported on the use of high-energy ball milling for the preparation of co-doped tetragonal zirconia nanopowders.

In this paper, the tetragonal zirconia stabilized by 6 mol% MgO and 2 mol% Y₂O₃ (t-6MgO–2Y₂O₃–ZrO₂) nanopowders with good sphericity, uniform particle size, and narrower grain size distributions were prepared by a combination of two processes, co-precipitation, and high-energy ball milling. The experimental procedure and parameters were adjusted to prepare zirconia nanopowders with different particle sizes. X-ray diffraction analyzer, Raman spectrometer, and Fourier transform infrared spectrometer were used to analyze the composition of the prepared nanopowders. Scanning electron microscopy and transmission electron microscopy were used to observe the microstructure of the powders, while BET-specific surface area tests were carried out on the powders. The effects of ball milling time on the phase composition, particle size distribution, dispersion, and morphology of the final t-6MgO–2Y₂O₃–ZrO₂ were investigated. The process is dedicated to simplifying existing methods for the preparation of zirconia nanomaterials and enriching experimental data for the industrial preparation of nanomaterials.