http://www.ibmc.up.pt/projectos/FJMonteiro/Bonamidi%20Project/References%20Bonamidi/Denry%20-%20State%20of%20the%20art%20zirconia%20dental%20applications.pdf

State of the art of zirconia for dental applications

The recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress-activated tetragonal (t) to monoclinic (m) transformation in ZrO2-based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader-ranging research into the toughening mechanisms available to enhance the fracture properties of brittle-matrix materials. ZrO2-based systems have remained a major focal point for research as developments in understanding of the crystallography of the t→m transformation have led to more-complete descriptions of the origins of transformation toughening and definition of the features required of a transformation-toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t→m transformation in ZrO2 and its application in the modeling of transformation-toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO2-toughened ceramics.

Transformation Toughening in Zirconia‐Containing Ceramics

TiO2 thin films were prepared on fused quartz by the liquid-phase deposition (LPD) method from a (NH4)2TiF6 aqueous solution upon addition of boric acid (H3BO3) and calcined at various temperatures. The as-prepared films were characterized with thermogravimetry (TG), Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), UV−Visible spectrophotometry (UV−Vis), scanning electron microscopy (SEM), photoluminescence spectra (PL), and X-ray photoelectron spectroscopy (XPS), respectively. The photocatalytic activity of the samples was evaluated by photocatalytic decolorization of methyl orange aqueous solution. It was found that the as-prepared TiO2 thin films contained not only Ti and O elements, but also a small amount of F, N, and Si elements. The F and N came from the precursor solution, and the amount of F decreased with increasing calcination temperature. Two sources of Si were identified. One was from the SiF62- ions, which were formed by a reaction between the treatment solution and quartz ...

The Effect of Calcination Temperature on the Surface Microstructure and Photocatalytic Activity of TiO2 Thin Films Prepared by Liquid Phase Deposition

Local atomic structures of Zr and dopant cations in zirconia solid solutions with Fe2O3, Ga2O3, Y2O3, and Gd2O3 have been determined. The Zr ions in both partially stabilized tetragonal and fully stabilized cubic zirconia have their own characteristic structures which are dopant-independent. The dopant cations substitute for Zr ions despite severe local distortions necessitated by the large difference in dopant–O distance ana Zr─O distance. Dopant ionic size determines the preferred locations of oxygen vacancies. Vacancies introduced by oversized dopants (Y and Gd) are located as nearest neighbors to Zr atoms, leaving 8-fold oxygen coordination to dopant cations. Undersized dopants (Fe and Ga) compete with Zr ions for the oxygen vacancies in zirconia, resulting in 6-fold oxygen coordination and a large disturbance to the surrounding next nearest neighbors. Since oxygen vacancies associated with Zr can provide stability for tetragonal and cubic zirconia, these results suggest an explanation for the observation that oversized trivalent dopants are more effective than undersized trivalent dopants in stabilizing cubic and tetragonal phases.

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Effect of Dopants on Zirconia Stabilization—An X‐ray Absorption Study: I, Trivalent Dopants

The light harvesting efficiency of dye-sensitized photoelectrodes was enhanced by coupling a TiO2 photonic crystal layer to a conventional film of TiO2 nanoparticles. In addition to acting as a dielectric mirror, the inverse opal photonic crystal caused a significant change in dye absorbance which depended on the position of the stop band. Absorbance was suppressed at wavelengths shorter than the stop band maximum and was enhanced at longer wavelengths. This effect arises from the slow group velocity of light in the vicinity of the stop band, and the consequent localization of light intensity in the voids (to the blue) or in the dye-sensitized TiO2 (to the red) portions of the photonic crystal. By coupling a photonic crystal to a film of TiO2 nanoparticles, the short circuit photocurrent efficiency across the visible spectrum (400−750 nm) could be increased by about 26%, relative to an ordinary dye-sensitized nanocrystalline TiO2 photoelectrode.

Standing wave enhancement of red absorbance and photocurrent in dye-sensitized titanium dioxide photoelectrodes coupled to photonic crystals.

Dense Ce-TZP ceramics containing about 7 to 16 mol% CeO2 were fabricated using fine powders prepared by the hydrolysis technique. The mechanical properties of these ceramics were evaluated. The bending strength of sintered bodies with 10 to 12 mol% CeO2 content and small grain-size was about 800 MPa. Fracture toughness was measured by two different methods; a micro-indentation technique and the chevron notched beam technique. A high fracture toughness was obtained for sintered bodies with 7 to 10% CeO2 content and large grain-size. Fracture toughness and hardness were dependent on CeO2 content and grain-size. These mechanical properties are discussed on the basis of the stability of the metastable tetragonal phase depending on CeO2 content and grain-size.

Strength, fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP)

Low-temperature synthesis of anatase thin films on glass and organic substrates by direct deposition from aqueous solutions

The addition of Al2O3 to Y2O3-partially stabilized ZrO2 submicrometer powders consolidated by isostatic hot pressing was noted to yield a substantial increase in bending strength. Average bending strength for a 20-percent Al2O3 Y2O3-partially stabilized ZrO2 composite was 2400 MPa, with a fracture toughness of 17 MN m to the -3/2. Microindentation and chevron-notched beam test method results are presented. 9 references.

Strength and Fracture Toughness of Isostatically Hot‐Pressed Composites of Al2O3 and Y2O3‐Partially‐Stabilized ZrO2

The thin film of tin oxide was formed in the solution containing 0.005–0.3 mol/l SnF2. The procedure of film formation was very simple; the solution, in which a substrate is immersed, is maintained above 40°C for tens of hours. In this method, the hydrolysis product of SnF2 deposited as the film on a substrate. As-deposition film included 6–16 mol% fluorine. The chemical component was deduced as SnO2−05xFx, where 0.17<×<0.5. The film was modified to pure SnO2 by heating above 300°C. The electrical conductivity was improved to 1.4×10−2 Ω cm by heating at 500°C. The model of liquid phase deposition was proposed to extend another oxide film.

Liquid phase deposition film of tin oxide

Transparent MgAl 2 O 4 spinel ceramics were fabricated by hot isostatic pressing of the sintered materials composed of fine grains. Since the fine-grained sintered materials were difficult to obtain from pure powders at low temperatures, the additives reducing sintering temperatures were examined. As a result, a small addition of boron oxides (0.05-0.5 mass %-B 2 O 3 ) was found to be very effective. The influence of boron oxides on the microstructure was investigated. The restriction of anomalous grain growth was important for transparency. The samples added with 0.15 mass%-B 2 O 3 showed high optical transmittance (80% with 1 mm thick).

Koji Tsukuma

Papers

Strength, fracture toughness and Vickers hardness of CeO2-stabilized tetragonal ZrO2 polycrystals (Ce-TZP)

Low-temperature synthesis of anatase thin films on glass and organic substrates by direct deposition from aqueous solutions

Strength and Fracture Toughness of Isostatically Hot‐Pressed Composites of Al2O3 and Y2O3‐Partially‐Stabilized ZrO2

Liquid phase deposition film of tin oxide

Transparent MgAl2O4 spinel ceramics produced by HIP post-sintering