Henryk Tomaszewski

Yttria-stabilized zirconia thin films grown by reactive r.f. magnetron sputtering

Abstract Zirconia thin films were deposited on different substrates by reactive r.f. magnetron sputtering. It was found that zirconia layers grew preferentially in the (200) direction with a columnar microstructure that is independent of the oxygen flow, argon to oxygen flow ratios and sputtering pressure. On the other hand the substrate roughness clearly affects the orientation of zirconia. Distinct changes were observed in the case of heated substrates. The heating of the substrates does not change the form of the zirconia, but strongly influences its orientation. When the substrate temperature increases, the crystallographic orientation gets more and more random, typically for the polycrystalline state. A decrease in the optical transmittance and an increase in the colour intensity of the layers with an increase in the substrate temperature was also found. As is shown, the oxygen non-stoichiometry of the zirconia is responsible for these changes.

TiO2 films prepared by DC magnetron sputtering from ceramic targets

Abstract The deposition of stoichiometric TiO 2 films for V 2 O 5 /TiO 2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. The target behaviour upon sputtering was followed by means of the target voltage. The layers were prepared with different oxygen concentrations in the plasma and the influence upon electronic and optical properties of the deposited layers was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti 3+ species are increasingly present. This departure from stoichiometry is ascribed to active resputtering of the deposited layer by O − ions. Due to the higher sputter yield of oxygen as compared to titanium, this resputtering resulted in the observed reduction. From optical transmission measurements, the refractive index was found to decrease with increasing oxygen mole fraction, while the band gap increased considerably. The variation of the optical absorption with the oxygen concentration in the plasma was found to confirm the XPS analysis regarding the stoichiometry of the layers. Polycrystalline anatase layers were obtained at elevated substrate temperatures.

Influence of oxygen partial pressure on the metastability of undoped zirconia dispersed in alumina matrix

Abstract The influence of oxygen content in the sintering atmosphere on zirconia phases in alumina-zirconia ceramics was studied. As was demonstrated, in the case of samples sintered in an atmosphere with oxygen partial pressure between 2.80 ± 10 −5 Pa and 2.80 × 10 −2 Pa, 100% of zirconia was found to be in the cubic form. The appearance of transformable cubic zirconia is a result of oxygen vacancy concentration in zirconia. The increase of oxygen partial pressure and decrease of oxygen vacancy concentration leads to a breakdown of the cubic phase which transforms to the t- and/or m-phases. Critical values of oxygen vacancy concentration responsible for the metastability of cubic and tetragonal zirconia were estimated.

Electronic and optical characterisation of TiO2 films deposited from ceramic targets

Abstract The deposition of stoichiometric TiO2 films for abrasion resistant V2O5/TiO2 anatase catalysts was investigated. DC magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as deposition technique. Smaller amounts of oxygen gas proved necessary to achieve stoichiometry in the deposited layers, than during deposition processes involving metal targets. TiO2 layers were prepared with different oxygen mole fractions and the influence upon electronic and optical properties was investigated. Surface XPS measurements showed that stoichiometry is obtained for low oxygen mole fractions, while for higher oxygen contributions, Ti3+ species are increasingly present. This departure from stoichiometry was ascribed to active resputtering of the deposited layer by O− ions. Due to the higher sputter yield of oxygen as compared to titanium, this resputtering resulted in the observed reduction. From optical transmission measurements, the refractive index was found to decrease with increasing oxygen mole fraction, while the band gap increased considerably. Post-deposition annealing in oxygen improved the crystallinity of the layers. The refractive index and optical band gap were both reduced by this treatment and the oxygen deficiency of the layers was in general seen to decrease.

The role of residual stresses in layered composites of Y-ZrO2 and Al2O3

Abstract Laminar composites, containing layers of Y–TZP and either Al 2 O 3 or a mixture of Al 2 O 3 and Y–ZrO 2 have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr +3 dopants in alumina.

Crystallization of yttria under hydrothermal conditions

Abstract The effect of precursor type and hydrothermal crystallization conditions on the morphology of yttria grains was studied. Yttrium nitrate and yttrium chloride were used as yttrium salts soluble in water. Yttrium hydroxide obtained by the reaction of the chosen salt with ammonia was subjected to hydrothermal conditions (200–270 °C, 0–120 min) in an autoclave. The shape of the yttria grains was strongly dependent on the crystallization temperature and the type of precursor used. With yttrium chloride as a precursor and at crystallization temperatures up to 498K, yttria grains are isometric in shape. Above this temperature yttria grains get elongated with a needle-like shape. The size of the needles depends on the crystallization time. For the case of the yttrium nitrate precursor, the critical temperature of crystallization, above which extended yttria grains appear, is about 25 °C higher. From the results a model mechanism for the hydrothermal crystallization of yttria is proposed.

Effect of Substrate Sodium Content on Crystallization and Photocatalytic Activity of TiO2 Films Prepared by DC Magnetron Sputtering

The effect of sodium content of the glass support on the crystallinity of sputtered TiO2 films and photocatalytic breakdown of ethanol has been studied. It was found that the activity of the as-deposited (amorphous) films does not depend on the type of support used. The chemical composition of the glass support does influence the activity of annealed films. When using soda-lime glass support sodium diffuses into the film upon annealing, suppressing anatase crystallization and decreasing its photocatalytic activity. To decrease the influence of sodium, soda-lime glass coated with an e-beam evaporated SiO2 barrier layer was used with good result. A reduced sodium concentration in the film leads to well crystallized anatase after annealing. An increased photocatalytic activity was observed for these films.

Yttria-stabilized zirconia thin films grown by r.f. magnetron sputtering from an oxide target

Abstract Yttria-stabilized zirconia thin films with cubic crystallographic structure were deposited onto glass substrates by r.f. magnetron sputtering from an oxide target. It was found that zirconia growth is strongly dependent on the sputtering power and pressure. At low power and high pressure, zirconia grows preferentially in the (200) direction with columnar microstructurc. In contrast, high power and low sputtering pressures promote the growth of randomly oriented polycrystallinc zirconia. Increasing the argon flow at constant power and sputtering pressure again favours preferential growth of zirconia layers, however, not in the (200) direction as before, but in the (111) direction.

Residual stresses in layered ceramic composites

Abstract Laminar composites containing layers of Y-TZP or Ce-TZP and either A1 2 O 3 or a mixture of alumina and zirconia have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection was observed in alumina layers. In the case of layers made of an oxide mixture, crack deflection was not found independently on layer thickness. The observed changes have been correlated with distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature.

Effect of grain size on R-curve behaviour of alumina ceramics

Abstract Alumina samples with average grain sizes ranging from 2.6 to 67.4 μm were prepared by sintering at 1500–1900°C for 2–20 h in high vacuum. Crystallographic and thermal expansion mismatch between adjacent grains during cooling involved residual stresses in these ceramics. The effect of these stresses on fracture behaviour of alumina ceramics was investigated by testing controlled crack growth during three point bending of single-edge-notched samples. After initiation, the crack grew slowly by repeated loading and unloading. The crack length c , was measured and registered in situ by means of a CCD camera coupled to an appropriate microscope, which was fitted to the test equipment by a system of elevator stages driven by stepping motors. The force P , necessary to produce an increasing crack length was computer controlled. The stress intensity factor K I , was calculated from values of the crack length c , and force P . The data of K I =f(c) obtained in the range of crack lengths studied were fitted by a linear function y=ax+b . As a result, the slope was used as a parameter describing R-curve behaviour of ceramics. The tests showed that R-curve behaviour of alumina ceramics strongly increases with the increase of Al 2 O 3 grain size. This phenomenon was explained by analysis of microstructures and residual stresses found in ceramics by piezospectroscopic measurements. In several samples the crack growth tests were performed without unloading. The time dependent displacement d of the sample was measured and recorded together with values of force P . The stress intensity factor K I , maximal stress intensity factor K Imax , resistance to crack initiation K Ii , and work-of-fracture γ F , were inferred from measured data.