Phase composition and defect substructure of the zirconia nanopowder modified by the powerful ultrasonic action

Phase composition and defect substructure of the zirconia nanopowder modified by the powerful ultrasonic action

Khasanov Oleg^a, Ivanov Yury^b, Dvilis Edgar^c and Tolkachev Oleg^d

Nano-Centre of Tomsk Polytechnic University, Tomsk, 634050, Russia

E-mails: ^akhasanov@tpu.ru, ^byufi55@mail.ru, ^cdvilis@tpu.ru, ^dole.ts@mail.ru

Keywords: Zirconium dioxide, сommercial nanopowder, powerful ultrasonic action, XRD, SEM, TEM analysis.

Abstract. Commercial nanopowder of the partially yttrium-stabilized zirconia (ZrO₂+3 mol.% Y₂O₃) was treated by the non-cavitational powerful ultrasonic action (PUA). Influence of PUA on the microstructure and phase composition of the powder has been investigated by scanning and transmission electron microscopy. Investigated powder before and after PUA has polycrystalline structure. It was found that TZ-3YS particles after PU-treatment have tetragonal and cubic zirconia phases; some of the particles consist of a mixture of monoclinic, tetragonal and cubic phases, but these particles have substructure with random orientation of fragments.

Introduction

The promising approach for manufacturing the functional and structural ceramics is application of method of uniaxial compaction of dry nanopowders at simultaneous powerful ultrasound action (PUA) [1]. It was shown that pressing of dry powders under non-cavitational ultrasonic action leads to decreasing the die-wall and interparticle friction that provides more dense and uniform packing of the powder particles and so higher density of green compacts of different shapes [2]. However it was shown that non-cavitational powerful ultrasound action on the initial nanopowders just before pressing also influences their particle size distribution, lattice structure and phase composition [3-5].

The aim of this work was investigation of the structure and phase composition, analysis of effects of structure modification and phase composition of zirconia nanopowders by non-cavitational powerful ultrasound action.

Experiments and results

Commercial nanopowder (NP) of partially yttrium-stabilized zirconium dioxide (ZrO₂ + 3 mol. %Y₂O₃), TZ-3YS, produced by TOSOH (Japan) has been investigated. Specific surface area of the granulated powder was (7 ± 3) m²/g. Size of the powder aglomerates was about 200-300 nm.

Powerful ultrasonic treatment of the dry nanopowder (without any liquid additives, i.e non-cavitational processing) was carried out in the cavity inside of special acoustic waveguide by technique described in [3]. Output power of ultrasonic generator was 3 kW, treatment duration was 10 min.

XRD analysis (XRD-7000S Shimadzu) of powder before ultrasonic action showed the presence of two zirconia crystal lattices: 64% of tetragonal phase and 36% of monoclinic one. After PUA, according XRD, the phase composition of the powder was slightly changed: the tetragonal phase percentage was increased up to 65%. The size of the coherent scattering area was ~32 nm for crystallites of monoclinic phase and ~144 nm for crystallites of tetragonal phase.

SEM analysis (JSM-7500F JEOL) showed fracture of NP agglomerates after PUA (Fig. 1).

Phase composition and substructure of the nanopowder have been investigated by transmission electron microscopy using JEM 2100 JEOL (Fig. 2, Fig. 3).

TEM method unlike to XRD allows to study the phase composition of a single particle.

Fig. 1. SEM image of agglomerated initial TZ-3YS nanopowder before (a) and after (b) PUA

Fig. 2. TEM images of TZ-3YS powder structure before PUA; a, b - bright-field images; c-f electron diffraction patterns with areas 1-4

Discussion

Analyses of a set of TEM images as for initial TZ-3YS powder, as for TZ-3YS NP after PUA had following results:

1. Initial TZ-3YS powder consists of a mixture of monoclinic, tetragonal and cubic zirconia phases. Some particles have increased reflexes of monoclinic phase. It means that the powder has non uniform distribution of yttria which stabilizes the tetragonal and cubic phases in the zirconia lattice structure.
2. Non-cavitational powerful ultrasonic treatment of the TZ-3YS powder does not change the particle morphology but leads to the fracture (fragmentation) of crystallites: in the crystallites of particles the regions having low angles of the random orientation. Size of the crystallite fragments is from 10 to 30 nm which as a whole corresponds to results obtained by XRD and SEM. The angle of random orientation reached 6.5 degree.
3. Fragmentation of crystallites and formation substructure with random orientation are confirmed by the peculiarities of electron diffraction patterns of PU-treated particles: the radial widening of the pattern reflexes as well as twinning reflexes were found.
4. PU-treated TZ-3YS particles have tetragonal and cubic zirconia phases. Some of the particles consist of a mixture of monoclinic, tetragonal and cubic phases, but these particles have substructure with random orientation of fragments.

Fig. 3. TEM images of TZ-3YS powder structure after PUA; a, b - bright-field images; c - electron diffraction pattern

Conclusions

Non-cavitational powerful ultrasonic treatment of the TZ-3YS powder does not change the particle morphology but leads to the fracture (fragmentation) of crystallites. Size of the crystallite fragments is from 10 to 30 nm.

Non-cavitational powerful ultrasonic treatment results in fragmentation of crystallites and formation substructure with random orientation. PU-treated TZ-3YS particles have tetragonal and cubic zirconia phases. Some of the particles consist of a mixture of monoclinic, tetragonal and cubic phases, but these particles have substructure with random orientation of fragments.

Acknowledgments

The work has been supported by the Russian Ministry of Education and Science (State assignment "Science", State Contracts #14.518.11.7017; #14.513.11.0039). Authors are thankful to Ph.D. S. Filimonov for treatment of investigated powders and Ph.D.-student A. Kachaev for XRD analysis.

References

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