Low-Temperature Synthesis of ZrO2-8 mol.% Y2O3 Nanopowder with High Sinterability

The possibility of low-temperature synthesis of yttrium-stabilized (8 mol.%) zirconium dioxide nanopowder from a mixture of hydrated zirconium oxynitrate and yttrium carbonate mechanically activated in a continuous-type mill is examined. It is demonstrated that the powder formed at as low temperature as 600°C and then subjected to a disaggregation procedure can be easily compacted by dry pressing and achieves nearly full density on sintering.


Introduction
Zirconium dioxide belongs to refractory, chemically resistant material and has wide application in the commercial production of technical ceramics.To eliminate cracking due to the phase transition from the tetragonal modification of zirconium dioxide into the monoclinic one during cooling, stabilization of the material with metal oxides (Y 3+ , Ca 2+ , Mg 2+ , Fe 3+ etc.) is a common practice.The introduction of stabilizing cations into zirconium oxide lattice occurs due to diffusion processes at rather high temperatures.However, homogenization of the system may be achieved also under mild conditions using the co-precipitation from solution method.Widely used yttrium stabilized zirconium dioxide is obtained, for example, by co-precipitation of zirconium and yttrium hydroxides resulted from salt hydrolysis with ammonia according to reaction Zr(OH) 3 Cl + YCl 3 + 4NH 4 OH = Zr(OH) 4 + Y(OH) 3 + 4NH 4 Cl (1) The precipitate is aged, then filtered out, dried and calcined at a temperature below 1000°С [1].However, this method involves large volume of solutions, the necessity to filter them, inevitable losses of reagents, and the use of ammonia requiring special precautions.
The mechanochemical method is considered today as a significant alternative to ubiquitous thermal synthesis owing to the possibility to decrease energy consumption for the synthesis of materials.With the help of the mechanochemical synthesis, it is possible to accelerate chemical interactions and also to activate materials for sintering, which is the final stage in the industrial production of ceramic hardware for various applications.A promising route for obtaining zirconium dioxide is the so called soft mechanochemical synthesis based upon the use of such starting reagents as mixtures of solid hydroxides and acids, hydrated oxides, basic and acidic salts, crystal hydrates, to be charged into the mechanochemical activator.This allows one to decrease the level of mechanical load substantially and to use less powerful grinding activators than those required for processing metal oxides.It was reported that the mechanical treatment of a mixture of reagents corresponding to equation (2) may result in obtaining solid solution ZrO 2 -Y 2 O 3 during successive heating, beginning at little more than 450ºC [2].Nevertheless, the application of these results seems to be of any promise only with productive continuous operating mills.This issue can be investigated using a recently designed centrifugal mill of continuous action in the vertical version providing the input of the necessary amount of mechanical energy per unit mass of the substance under treatment in the continuous mode [3].We have made such an attempt in the present work.The goal was to study the application of the mentioned above centrifugal planetary mill for activation of the salt mixture followed by heating to yield the solid solution with the composition ZrO 2 -8 mol.%Y 2 O 3 , and to examine the possibility to use the resulting nanopowder for low-temperature sintering to dense high-quality ceramics.

Experimental
A longitudinal section of the centrifugal mill of continuous action is shown schematically in Fig. 1.The mill casing 1 is composed of coaxially connected cylinders; shaft 2 passes through the axis of the cylinders.Sections containing the upper and lower pressure plates 3 and 4 are fixed on shaft 2. The shafts for ring-like milling bodies are fixed on the pressure plates at an equal distance from the central shaft.Milling bodies in the form of round discs 5 with holes in the centre and milling bodies with the holes displaced from the centre are put on the shafts.The size of holes in the discs is much (2-3 times) more than the shaft diameter.Milling bodies (discs) are united into stacks separated with partitions in which the milling bodies with centered holes are interchanged with the milling bodies having off-centre holes.Pressure plates have holes for the central shaft and grooves for the rigid connection with the shaft which has a ridge.The sections are replaceable to provide the possibility of changing the milling bodies by disassembling.The casing has a pipe for loading the initial material and the pipe for unloading the ground material, and is placed into the housing.There is the possibility to feed a cooling liquid into the housing.
The mill operates the in following manner.The material to be ground enters the inner cylinder of the mill through the pipe in the upper part.Shaft 2 drags into rotation the sections and stacks with milling bodies on shafts.When the material gets onto the upper pressure plate of each section, it goes to the wall of the inner cylinder under the action of centrifugal forces and get exposed to the action of milling bodies that either roll under the action of centrifugal forces (milling bodies with a hole in the centre) or strike (milling bodies with off-centre holes) the wall.During the rotation of the central shaft, the discs that have the centered holes interact with the shafts only at the moment of starting run; quite contrary, the discs having off-centre holes interact with the shafts at all times thus providing a strike and dragging of the disc over the casing wall.The productivity of the apparatus can be varied by varying the frequency of rotation of the main shaft.Mills that have been manufactured till present allow one to process up to 35 kg of the material per hour.In the present work, the joint treatment of a mixture of zirconyl nitrate and yttrium carbonate was carried out in the laboratory version of the centrifugal mill with the productivity of 600 g/h.To prevent possible contamination of the mixture with the wear material, the discs and the inner cylinder of the laboratory mill are made of metal zirconium.
After passing the salt mixture corresponding to the composition 92% ZrO 2 and 8% Y 2 O 3 through the mill, powders were subjected to thermal treatment for 3 hours at a temperature of 600, 800 and 1000ºC.X-ray phase analysis of the powders and sintered materials was carried out with the help of X-ray diffractometer DRON-4 with CuKα radiation and graphite monochromator and Bruker D8 Advance diffractometer.Determination of the size of crystallites in powders was carried out using the PowderCell 2.4 software with compulsory introduction of the parameters of standard samples determined under the identical recording conditions.
The specific surface of powders was determined with the help of sorption meter Katakon by means of the thermal desorption of nitrogen; micrographs of sintered ceramics were taken with the help of Hitachi TM-1000 microscope.

Results and discussion
The X-ray diffraction patterns of the products obtained after mechanical activation and thermal treatment of the salt mixture are presented in Fig. 2. The diffraction pattern presents a set of reflections corresponding to the initial reagents and intermediate products of their interaction (Fig. 2a).The most intense peak in Fig. 2a (10 degrees) corresponds to the 100% peak of ZrO(NO 3 )•3H 2 O.After thermal annealing at 600°C, the only identifiable product is the cubic modification of zirconium oxide (Fig. 2b), with crystallite size 15-17 nm (see Tab. I) and lattice parameter equal to 5.135Å.With this crystallite size, even in the absence of stabilizing cations, thermodynamically equilibrium modifications of zirconium oxide are tetragonal or cubic ones [4,5], so one cannot make conclusions concerning the completeness of reaction ( 2) under the accepted conditions on the basis of the data obtained in X-ray phase analysis.Annealing of the product of mechanical activation at higher temperatures points to the appearance of the reflections of monoclinic modification along with an increase in the crystallite size at 1000°C (Fig. 2c) and return to the single phase solid solution at 1300°C.So, we may conclude that the mechanical treatment of the mixture of reagents followed by thermal treatment at 1000°C causes insertion of yttrium cations into the zirconium dioxide lattice, however, the degree of conversion does not reach 100%.Some ZrO 2 grains do not get sufficient amount of yttrium cations to achieve stabilization of the cubic modification; so as soon as the size of the growing grain exceeds the critical value, transition into the monoclinic modification occurs.At higher temperatures, the increased efficiency of diffusional mass transfer brings the insertion of stabilizing cations to completion.The transition to the monoclinic modification is accompanied by a substantial increase in the volume of zirconium dioxide, therefore, the expanding grains cause distortions of particle packing in the sample formed by pressing, the latter being clearly seen in the dilatometric sintering curve (Fig. 3a).Nanocrystalline ~ 15 nm powder comprising the pressed sample exhibits high sinteractivity.Intense shrinkage of the sample starts at a temperature of 820 o C, which is extremely low for zirconium oxide, however, then the grains undergoing the transformation into the monoclinic phase cause sample swelling.Sintering resumes only after the transformation of all the particles of the powder into the cubic phase.It is evident that the damage to particle arrangement in pressed specimen during sintering has a negative effect on the final density of sintered bodies, which does not exceed 75% of the theoretically possible value even at 1550°C (see Tab. I).An additional adverse influence on the final density was undoubtedly brought about by particle aggregation caused by thermal annealing at a temperature of 600-800°С [6,7].This is evidenced, for example, by the disagreement of the average crystallite size obtained from the X-ray diffraction data and particle size estimated on the basis of equation D=6/(S•ρ), where ρ is the density of zirconium oxide, and S is its specific surface.Even for 600 o C, the particles size (which are likely to be dense aggregates in our case) turns out to be 6.5 times that of crystallites.After the soft-mode disaggregation of the powder being annealed at 600 o C, the dilatometric curve of the pressed sample (Fig. 3 b) drastically changes its behavior.The onset of intense shrinkage was observed at the same temperature but the region of sample expansion is absent.The destruction of aggregates is likely to result in further homogenization of the powder.Now the size of the areas rich in yttrium and depleted ones does not exceed the size of crystallites that are uniformly distributed over the volume.On heating the pressed sample, equalization of the concentration of stabilizing cation between nanocrystallites takes place even before the start of grain growth, thus it becomes possible to avoid the expansion region on the dilatometric curve.As a consequence, the densities of the sintered material proved to be much higher reaching 90% of the theoretical value already at 1300°C, 97% at 1450 o C, and >99% at 1600 o C. The microstructure of the dense sample composed of grains 10-20 μm in size is shown in Fig. 4; it indicates almost complete absence of pores.Well-faced black crystals distributed over the whole surface of the photograph are, according to EDS, aluminium oxide; the nature of their appearance remains not clear yet.The crystals are observed over the whole sample depth and may be the result of dissolution of aluminium from the ceramic substrate of the dilatometer at sintering temperature >1500 o C [8,9]; during cooling it gets segregated mainly along the grain boundaries [10].Some characteristics of the mechanically activated powder mixture after annealing at different temperatures and densities of sintered specimens are given in Tab.I.

Conclusions
Using mechanical activation of the mixtures of hydrated zirconium and yttrium salts in the flow-type centrifugal mill operating in continuous mode, low-temperature synthesis of nanopowder with the composition ZrO 2 -8mol.%Y 2 O 3 was realized.The powder is easily compacter by dry uniaxial pressing and sintered in the air to the density higher than 99% of the theoretical value.
. I. Parameters of mechanically activated and annealed salt mixtures and sintered bodies.