Production and properties of ceramics prepared from dioxide zirconium nanopowder by SPS method
INTRODUCTION Different properties of the nanocrystalline and coarse-grained materials depend not only on small size grains of nanocrystalline material but also on the special state of their surface or grain boundaries .
Manufacturing of small grain size ceramics made of nanoscale powder is one of the ways to develop nanotechnologies. It is assumed that such nanoceramics will have some special properties (e.g., superplasticity ) as compared with the ceramics made of coarse-grained material.
It is well known that the smaller the ceramics grains are and the more developed their grain structure is, the harder and more solid the ceramics is. At the same time the stable and hard-to-break agglomerates of nanoparticles used for ceramics manufacturing  are present in nanopowders, and it is necessary to apply non-standard methods for their compaction (for example, the hot-pressing method).
The current state of research in nanoceramics made of different nanopowders is sufficiently well described in [4–6] and in the author’s works [7–16].
In this paper we present the research in ceramics prepared of zirconium dioxide nanopowder. Zirconium dioxide is used in the composites, ceramics compound, glasses, emanels, abrasives and polishing and high-temperature electrodes (in power plants). It plays an important role in optics as a photocatalyst and lens coating that retains ultraviolet radiation. It usually is used as the substrates and dielectric layers in electronic devices, protective coatings, optical reflective coatings, wear-resistant coatings of instrument parts, refractory material for aircraft and rocket engines, as a component of multilayer coatings of laser mirrors and beam separators, feedstock and an intermediate product in the production of Zr or its compounds. This substance is also used as ceramic pigments and insulating materials, in functional radio ceramics, in the production of catalysts in petrochemistry and organic synthesis, in jewelry, in piezoelectric elements. It is used in impurity doping Al-Mg alloys to obtain structural materials in the aerospace, aviation and defense industries. Zirconium dioxide has a unique combination of dissimilar properties: high strength, crack, wear and heat resistance, biological compatibility, etc.
This work is aimed at production of dense and solid ceramics with fine grain (about 1µ) structure made of nanodisperse zirconium dioxide powder by SPS method.
RESEARCH METHODS For this research chosen was the zirconium dioxide (ZrO2) nanopowder produced by "PlazmoTherm", a Russian company (they synthesize powders in the thermal plasma flow generated in electrical discharge). Fig.1 shows an image of zirconium dioxide nanopowder obtained by electron microscopy.
The average size of particles equals d ≈ 40–70 nm, and their specific surface is S ≈ 15–25 m2/g. The powder has had following properties: CAS number – 1314-23-4, purity – 99,5%, white colour, spherical form of particles, polydisperse powder. Distribution of particles by sizes is close to logarithmically normal function.
Agglomeration of the powder was accomplished with the aid of Labox "Sinter Land" installation in Lavrentyev Institute of Hydrodynamics of SB RAS using the SPS method (hot-pressing together with the spark plasma sintering) when electrical pulses were applied to the previously hot-pressed powder (in these experiments strength of electrical current reached 2 kA at applied voltage of 3–4 V).
The SPS method differs from the traditional pressing (when pressing and sintering are carried out sequentially) by applying pulsed electric current directly to a sample in order to heat the powder rapidly and preserve, to a considerable extent, its micro-structural parameters in the consolidated material. Hot-pressing was accomplished at the maximum temperature of 1,500 ⁰С and 40 MPa pressure. The heat rate was equal to 100 ⁰С/min without any exposure at the maximum temperature. Ceramic chippings were studied with ZEISS EVO-50WDS-XVP-BU electron scanning microscope in the Khristianovich Institute of Theoretical and Applied Mechanics, SB RAS, after spraying a gold layer on them. The micro-hardness of all ceramic samples was studied with PMT-3 microhardness tester.
RESULTS The diameter and thickness of the obtained samples are 9.6 mm and 3.2 mm, correspondingly. Electron scanning microscopy of the ceramic chippings shows that grain sizes of the obtained ceramics is about 1µ, in other words, the SPS method enabled to obtain the fine-grain solid ceramics.
The micro-hardness of the obtained ceramics appears to be very high (Hv=24 GPa). For the sake of comparison: the microhardness of the ceramics prepared by the traditional method  (by sequential pressing and sintering) from the nanodispersed ZrO2 powder with particle size of 20 nm when sintering at maximum temperature of 1 500 ⁰С was 14 GPa while the microhardness of coarse ceramics prepared by the traditional method from coarse ZrO2 powder (particle size exceeds 4000nm) was equal to 4,8 GPa only.
Thus, the fine-grained (grain size is about 1µ) dense and solid ceramics with microhardness of 24 GPa made of nanoscale zirconium dioxide (ZrO2) was prepared by SPS method. ■
The author is grateful to A.G.Anisimov, V.I.Mali and V.A.Emelkin for their assistance in the presented work.