Goran Štefanić

The influence of thermal treatment on phase development in ZrO2-Fe2O3 and HfO2-Fe2O3 systems

Abstract The effects of thermal treatment (500, 600, 800 and 1100°C) of the amorphous precursors of ZrO 2 –Fe 2 O 3 and HfO 2 –Fe 2 O 3 systems, coprecipitated from aqueous solutions of the corresponding salts, were studied at room temperature (RT) by X-ray powder diffraction (XRD) and, in some cases, by laser Raman spectroscopy. The incorporation of Fe 3+ ions partially stabilized the high temperature polymorphs of zirconia (ZrO 2 ) and hafnia (HfO 2 ). The stabilization of the cubic polymorphs of ZrO 2 and HfO 2 occurred in the solid solutions with an Fe 2 O 3 content of ≥10 mol%. The presence of Fe 3+ ions above their solid solubility limit caused the transition of metastable polymorphs of ZrO 2 and HfO 2 to monoclinic polymorphs. The terminal solid solubility limit of Fe 2 O 3 in ZrO 2 , as estimated at RT, was ∼33 mol% after calcination at 600°C, ∼9 mol% after calcination at 800°C and ∼2 mol% after calcination at 1100°C. After the same thermal treatment, the terminal solid solubility limits of Fe 2 O 3 in HfO 2 proved to be very similar only a little smaller. The unit-cell volume of the cubic polymorphs of ZrO 2 and HfO 2 , determined by powder-pattern-fitting methods, decreased linearly with the increase in iron content, but the rate of decrease was greater in the ZrO 2 polymorph. The thermal behavior of the amorphous precursors was also followed by differential thermal analysis and thermogravimetric analysis. The crystallization temperature of both systems increased with an increase in the Fe 2 O 3 content, from 405 to 730°C in the ZrO 2 –Fe 2 O 3 system, and from 520 to 720°C in the HfO 2 –Fe 2 O 3 system. The results indicated that the amorphous precursors with more than 30 mol% of Fe 2 O 3 were single co-gels. The status of the Fe 3+ ions in the ZrO 2 -type lattice and the process of their segregation into the α-Fe 2 O 3 -phase were examined by 57 Fe Mossbauer spectroscopy. Significantly lower values of the hyperfine magnetic fields compared to that of α-Fe 2 O 3 indicated the incorporation of Zr 4+ ions into the α-Fe 2 O 3 lattice, the amount of which decreased with the increasing temperature of treatment.

Influence of pH on the hydrothermal crystallization kinetics and crystal structure of ZrO2

Abstract Differential scanning calorimetry was used to measure the heat evolved as zirconia samples are heated at constant rate. The data are used to deduce the kinetics of zirconia crystallization during hydrothermal treatments at pH 2, 7, 9.5 and 13. Hydrothermal crystallization was found to proceed much more slowly in the neutral pH medium than in the acidic or alkaline medium. The analogy between the rate of the hydrothermal crystallization of ZrO 2 and solubility of zirconium hydroxide was discussed. The results of phase analysis showed that metastable cubic zirconia, c -ZrO 2 , could be produced by hydrothermal crystallization in the presence of NaOH as a stabilizing agent. The results of X-ray diffraction showed that, even at low pH, monoclinic zirconia, m -ZrO 2 , was not the only product of the dissolution/precipitation mechanism of hydrothermal crystallization. It is concluded that in situ crystallization of amorphous zirconium hydroxide, if it occurs, is not a topotactic process.

Influence of Cr2O3 on the stability of low temperature t-ZrO2

Abstract Aqueous solutions containing different molar fractions of Cr(NO 3 ) 3 ·9H 2 O and ZrO(NO 3 ) 2 ·2H 2 O salts were dried at 110°C for 24 h and calcinated up to 1100°C. Precise X-ray powder diffraction measurements of the obtained samples, covering the whole concentration range in the system ZrO 2 –Cr 2 O 3 , were performed. It was found that the solubility of Cr 2 O 3 in ZrO 2 , as well as the solubility of ZrO 2 in Cr 2 O 3 , was negligible. Except for the end samples, identified as pure m -ZrO 2 and Cr 2 O 3 , other samples contained the phases Cr 2 O 3 , m -ZrO 2 and metastable t -ZrO 2 . The fraction of the t -ZrO 2 phase in the total content of ZrO 2 ( m -ZrO 2 + t -ZrO 2 ) increased with an increase of the initial content of Cr 2 O 3 but this relation was not linear. The observed stabilization of metastable t -ZrO 2 could not be attributed to the formation of a solid solution. It was suggested that the stabilization of t -ZrO 2 at room temperature resulted from a very strong surface interaction between chromium oxide and zirconia. This interaction prevented the diffusion of oxygen from the atmosphere into the ZrO 2 lattice, which otherwise triggers the transition t -ZrO 2 → m -ZrO 2 on cooling.

Influence of pH on the stability of low temperature t-ZrO2

The influence of the conditions of the chemical synthesis on stability of metastable t-ZrO2 was studied. Zirconia samples were crystallized from zirconium hydroxide precipitated at pH 13, 7 and 2.5. In all the samples, t-ZrO2 was the dominant phase but the m-ZrO2 phase was also present as a minor component. The samples obtained were subjected to the influence of temperature (500 and 700 °C), pressure (500, 1000 and 1350 MPa) and γ-irradiation (10 MGy). The results of phase analysis showed that the change in the pH of zirconium hydroxide precipitation significantly changed the sensitivity of the metastable t-ZrO2 to temperature and pressure. The metastable t-ZrO2 was the most stable in the sample obtained from zirconium hydroxide precipitated at pH 2.5 (a decrease of 2% of the starting t-ZrO2 phase after heating at 700 °C and a decrease of 35% after the application of a pressure of 1350 MPa) and the most unstable in the sample obtained from zirconium hydroxide precipitated at pH 7 (a decrease of 97% of the starting t-ZrO2 phase after heating at 700 °C, and a decrease of 65% after application of a pressure of 1350 MPa). The effects of the factors mentioned were monitored by X-ray powder diffraction and laser Raman spectroscopy. The results of laser Raman spectroscopy indicated that, without regard to the precipitation pH, γ-irradiation had very little influence on the metastable t-ZrO2.

FT-IR AND LASER RAMAN SPECTROSCOPIC INVESTIGATION OF THE FORMATION AND STABILITY OF LOW TEMPERATURE T-ZRO2

Abstract The influence of the preparation chemistry on the formation and the stability of metastable t -ZrO 2 was studied. γ-Irradiation has very small (if any) influence on the t -ZrO 2 → m -ZrO 2 transition, while increased temperature and pressure caused gradual transition of pure t -ZrO 2 to m -ZrO 2 . Metastable t -ZrO 2 containing SO 2− 4 impurities proved to be more stable, and its transition was not uniform. The mentioned effects were monitored by FT-IR and laser Raman spectroscopy. X-ray diffraction was used as a complementary technique.

The high impact of a milling atmosphere on steel contamination

High-energy ball-milling in an oxidative atmosphere caused gradual transition of pure zincite into zinc ferrite due to the oxidation of steel contamination. The rate of contamination increased dramatically (>3×) in an inert atmosphere due to the abrasion of milling tools by the steel chips coming from it.

The effect of mechanical treatment of zirconium(IV) hydroxide on its thermal behaviour

Abstract Precipitated zirconium(IV) hydroxide was ball-milled at room temperature for 1, 3, 9, 35 and 60 h. The samples were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD). DSC curves of the samples, obtained by ball-milling for 1, 3 and 9 h, showed two exothermic peaks due to crystallization of tetragonal ZrO 2 as determined by XRD and FT-IR. These two exothermic peaks were ascribed to the heterogeneity of the zirconium(IV) hydroxide structure caused by ball-milling. The samples, obtained by ball-milling for 35 and 60 h, showed only one exothermic peak. Three important phenomena, as a consequence of the ball-milling of zirconium hydroxide, were observed: (a) dehydration, (b) an increase in the activation energy of the crystallization and (c) a decrease in the enthalpy of crystallization of t -ZrO 2 from ball-milled zirconium hydroxide. The present study indicated that the process of crystallization of t -ZrO 2 from zirconium(IV) hydroxide is more complex than a simple structural (topotactic) rearrangement.

A search for solid solutions in the α-Al2O3-m-ZrO2 system by XRD

Abstract A search for solid solutions in the system α-Al 2 O 3 - m -ZrO 2 was undertaken using precise X-ray powder diffraction measurements. The samples covering the whole concentration range were prepared using chemical coprecipitation and ceramic sintering. Maximum heating temperature was 1100 °C. Except for the very ends of the concentration range, the samples contained two phases, structurally closely related to m -ZrO 2 (monoclinic) and α-Al 2 O 3 , respectively. Most samples also contained a small fraction of t -ZrO 2 (tetragonal), which was estimated to be less than 1 mol%. It was found that the solubility of α-Al 2 O 3 in m -ZrO 2 was small, amounting to 0.7 ± 0.3 mol%, while the solubility of m -ZrO 2 in α-Al 2 O 3 was negligible. These solid solubility limits were discussed in terms of ionic radii of A1 3+ and Zr 4+ cations.

Thermal Investigation of Copper-Doped-Zirconia Nanoparticles

Detailed thermal studies were carried out to determine the nano-structural changes in zirconia (ZrO2) ceramics upon incorporation of different amounts of the Cu2+ cations. In order to calculate the mean particle size and particle size distribution of ZrO2-CuO (ZC) system low frequency Raman spectroscopy (LFRS) was used. The results show that the particle growth is insignificant up to the temperature of 450°C for all the samples due to annealing of inner defects. With further increase of the temperature the nanoparticles grow rapidly with discontinuity at 540°C which is connected with the phase transition from tetragonal to monoclinic phase.