Sintering Nanodisperse Zirconium Powders with Various Stabilizing Additives

Effect of various stabilizing additives on sintering kinetics of nanodisperse powders was studied by thermomechanical analysis. Temperature ranges of the most intense shrinking, characteristic points of shrinking rate changes were established. Peaks characterizing the most intense shrinking of nanodisperse zirconium powder samples were shown to allow to arrange the stabilizing additives as follows: Y2O3→CeO2→TiO2.


Introduction
High-quality powders are necessary to obtain up-to-date ceramic materials.The Scientific Centre (SC) of Power Materials Science conducts research in synthesis of nanodisperse zirconium powders stabilized with various oxides.Zirconium powders which are produced commercially in Russia at the time, include a limited number of stabilizing additives and may not be exactly classified among nanodisperse powders [1][2][3].
Meanwhile, zirconia (mainly as stabilized forms) is considered by many researchers [4][5][6] as an extremely promising base for obtaining a new generation of filtering and catalytically active materials with specific surface properties.Stabilized zirconia is of interest due to the possibility of formation of solid solutions with high lattice oxygen mobility, high oxygen capacity, mechanical strength and thermal stability [7].SC of Powder Materials Science has developed techniques for obtaining tetragonal zirconium doped with individual and mixed yttrium, ceria, titanium.
The work deals with the effect of various stabilizing additives on sintering kinetics of nanodisperse zirconium powders.

Experimental
Stabilized zirconium powders were obtained in laboratory by precipitating from aqueous-ethanol solutions of zirconium hydroxychloride and required nitrates or chlorides by ammonia solution [8].Cerium was also introduced as nitric acid solution of rare-earth carbonate concentration containing mainly cerium carbonate by Solikamsky Magnievy Zavod OJSC (Perm Territory, Russia).The powders were sintered at 520 to 550 °C.Conditioning was carried out at temperatures of characteristic points at the differential thermal analysis curves for the sediments.
The specific surface area of sintered powders was determined by thermal nitrogen desorption using Sorbi 4.1 and average particle size was calculated.Average size and size distribution of particles were studied immediately using DC-24000 particle size analyzer (CPS Instruments).
The phase composition was determined using XRD-6000, Shimadzu diffractometer in Cu k α -radiation.The diffraction patterns were processed using Shimadzu XRD-6000/7000 V5.21 software package for data collection and processing.Raman spectra were obtained using «SENTERRA» multifunctional Raman spectrometer (Bruker) at radiating laser wavelength of 532 nm and radiation intensity of 5 mV.
Sintering kinetics of the synthesized powders were examined by means of "SENTSYS Evolution 24" thermomechanical analyzer/dilatometer (Setaram), on samples obtained by uniaxial compaction at 200 MPa.All measurements were performed in argon medium at temperature of up to 1500 °C, heating rate of 10 to 15 °C/min, cooling rate of 15 to 20 °C/min.

Results and Discussion
Tab, I gives the marking, composition and average particle size of the powders examined.The additive content is in wt %.Cerium added as rare-earth concentrate is given as CeO 2 and designated as CeO 2 (k).

Tab. I. Particle Sizes of Zirconium Powders with Various Stabilizing Additives
Thermal nitrogen desorption method (BET) Close values of average particle sizes obtained by various methods indicate that agglomerates of particles formed while obtaining the powder, are broken relatively easily under mechanical actions.The exception is ZrO 2 -13,7CeO 2 -3,2TiO 2 , the largest of the powders obtained.Individual powder particles are close to spherical shape.

Nos
Fig. 1 gives distribution diagrams of total particle percent versus their size for powders Nos. 4 and 5 based on data obtained using DC-24000 particle analyzer.
In ZrO 2 −18% CeO 2 (k) powder, 90% of particles are smaller than 40 nm.In ZrO 2 -13,7CeO 2 -3,2TiO 2 powder, 60% of particles are smaller than 80 nm.The diagrams for powders Nos. 1 to 3 essentially coincide with distribution diagram in Fig. 1a.Thus, the powders obtained have a narrow particle size distribution in nanoscale range.Essential differences in particle size distribution of ZrO 2 -13,7CeO 2 -3,2TiO 2 powder whose synthesis conditions were the same as powders Nos. 1 to 4 appear to be due to the fact that yttrium and cerium were added as ethanol nitrate solutions while titanium came from titanium chloride solution with high content of hydrochloric acid.All powders examined had the same phase composition.According to X-ray diffraction analysis and Raman spectroscopy, the powders consist of tetragonal zirconium.Fig. 2 represents Raman spectra of as-sintered materials.Tab.II gives the characteristics of Raman spectra.As-fired powders have similar composition but the intensity of the spectra depends considerably on a stabilizing additive..17Note: * -powder with cerium concentrate has an intense peak at 85 cm −1 which could not be identified and probably pertains to impurities in cerium concentrate Various sources attribute 4 to 5 peaks to tetragonal zirconium [9,10] because the peak at 315-316 cm −1 is not isolated in some cases.This peak was fixed in the powders examined but the intensity ratios I 260 /I 315 differ significantly (see Tab. 2).The peak has the minimum relative value in ZrO 2 −8%Y 2 O 3 , the powder closest to cubic zirconium.Fig. 3 and Tab.III represent the examination results for shrinking rate of zirconium with various stabilizing additives.3 peaks were fixed on shrinking rate curves for powders containing 5 and 8 wt % of yttrium which is associated in modern literature with presence of agglomerated particles in the powder.The first, the most intense peak at heat treatment temperature of 1000 to 1050 °C does not depend on yttrium content and heating rate.The second peak (1200 °C) shifts towards temperature increase as yttrium increases and towards temperature decrease as heating rate decreases.The position of the third peak (1400 °C) does not depend on yttrium content and heating rate.This peak is the most intense in commercial super dispersed partially stabilized zirconium powders (ДЦИ5, ДЦИ1).
Increase in powder particle size results in shift of maximum shrinking rate.Thus, commercial powder ZrO 2 -5 wt.% Y 2 O 3 (ДЦИ5) with as-supplied specific surface area of 3.5 to 6.5 m 2 /g and <500 nm particle content of 27% (optical microscopy data) has maximum shrinking rate at 1400 °C, i. e. 300 deg higher than in nanodisperse powder.

Conclusions
Zirconium powders with various stabilizing additives obtained by precipitation from aqueous-ethanolic solutions of corresponding salts were examined.Agglomerates of particles formed while obtaining powder were shown to be broken relatively easily under mechanical actions.According to various methods, the powders obtained may be classified as nanodisperse.
Effect of various stabilizing additives on sintering kinetics of nanodisperse zirconium powders was studied by thermomechanical analysis.Temperature ranges of the most intense shrinking, characteristic points of shrinking rate changes were established.
The peaks characteristic of the most intense shrinking of nanodisperse stabilized zirconium powder samples were shown to allow to arrange the stabilizing additives as follows: Y 2 O 3 →CeO 2 →TiO 2 .
When optimizing the sintering mode, intense shrinking processes at temperatures of less than 1200 °C are of particular interest.
Tab. III.Effect of Temperature and Stabilizing Additives on Shrinking Kinetics of Samples