Papers by Keyword: Tetragonal Zirconia

Abstract: The preparation of tetragonal zirconia nanopowders by sol–gel method using zirconium oxychloride as raw material, ammonia water and sodium hydroxide solution as precipitant, and calcium oxide or magnesium oxide powders as stabilizing agents is described. After suction filtration, drying, and calcination, tetragonal zirconia nanopowders with different particle size and tetragonal phase content were obtained. The particle size and phase composition of the powders are characterized by using a laser particle size analyzer and an X-ray diffractometer, and the tetragonal phase content and grain size are calculated from the crystal plane formula and Scherrer formula. The analysis of the relationship between the tetragonal phase content and the particle size of the zirconia nanopowders stabilized by calcium oxide and magnesium oxide at room temperature reveals the inhibitory effect of the stabilizing agents on the growth of zirconia grains. The stabilized zirconia nanopowder is finer than unstabilized zirconia nanopowder, and the particle distribution is more uniform in the former. The stabilizing effect of calcium oxide is superior to that of magnesium oxide; the critical transformation size of the zirconia grains stabilized by calcium oxide is the largest, and that of unstabilized zirconia is the smallest. The critical transformation size of calcium oxide-stabilized, magnesium oxide-stabilized, and unstabilized zirconia nanopowders is 18–22.6 nm, 24–28 nm, and 26–33.6 nm, respectively. Under the same calcination condition, the calcium oxide-stabilized zirconia nanopowder retains the highest tetragonal phase content at room temperature.

Abstract: Using zirconium oxychloride hydrate ( ZrOCl₂•8H₂O) and ammonia water (NH₃•H₂O) as raw materials, and ammonium dihydrogen phosphate (NH₄H₂PO₄) as additives, tetragonal zirconia (t-ZrO₂) with size range of 8–12 nm were prepared by coprecipitation method under hydrothermal conditions. The influence factors on phase transformation and the particle size such as phosphor loading, hydrothermal temperature and calcination temperature were studied by X-ray diffraction (XRD), Fourier transform Roman spectra (FT-Roman), the Brunauer-Emmett-Teller (BET) method and X-ray photoelectron spectroscopy (XPS) techniques etc. Research results show that a small amount of phosphor has been incorporated into the framework of ZrO₂ crystals, producing a certain amount of oxygen vacancies. Phosphor can effectively restrain crystal particles growth and improve the thermal stability of metastable t-ZrO₂. The phosphor doped t-ZrO₂ had a high surface area (244.2 m²/g). In contrast to the pure ZrO₂ particles readily aggregating, the phosphor species deposited on the framework of ZrO₂ crystals prevented the agglomeration of the primary particles during calcinations.

Abstract: In tetragonal zirconia, possibility is investigated of densification with finer grain sizes under the combination of doping and sintering in air. The materials used are CIP'ed compacts of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) doped with a small amount of cations. For a given sintering temperature and initial density of the compacts, while the doped cations enhances densification in the latest stage of sintering, the effect is different in grain growth during densification: a doped cation tended to enhance grain growth, whereas the other cations tended to suppress grain growth. As a result, the doping of the latter cations brings about a grain size finer than that of the undoped 3Y-TZP for a given relative density.

Abstract: Industrial applications of partially stabilized zirconia (PSZ) has increased substantially recently, considering its excellent thermal stability, strength and ionic conductibility. Its main application includes oxygen sensors and fuel cells. In this work zirconia ceramic powder is produced by Pechini process, mixing citric acid and ethylene glycol, adding yttrium and zirconium precursors. All the process was carried out with complete agitation. The obtained powder after this procedure is heat treated at 650°C during two hours. Characterization was performed by infra-red spectroscopy, x-ray diffractometry and quantitative analysis by Rietveld Method. The results show tetragonal and monoclinic phases, with nanometric crystallite.

Abstract: The citrate gel method, similar to the polymerized complex method, was used to synthesize homogenous tetragonal zirconia at 800oC and 1000oC. Nanocrystalline tetragonal single phase has been fully stabilized with 3, 7, 10 mol% CaO and 10, 15 mol% MgO at 800oC, respectively. In addition, the XRD analysis showed the absence of monoclinic phase after addition of 7 and 10 mol% CaO into zirconia-based solid solutions, which have been fully stabilized both 800oC and 1000oC. The crystallite sizes of the t-ZrO2 with 3, 7 and 10 mol% CaO at 1000oC were 32, 28 and 29nm, respectively. For ZrO2- x mol% MgO (x=3, 10, 15) solid solution, the crystallite sizes of samples at 800oC were less than 29nm, however it was increased up to 69nm at 1000oC. The prepared gel and subsequent heat-treated powders were characterized by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM) to get detail information regarding to differentiation of polymorphs of zirconia as well as formation of powders.

Abstract: The effect of nanocrystalization on superplastic flow was examined in ZrO2-30vol%spinel composite. The nanocrystalization can increase the strain rate by one order of magnitude or lower the deforming temperature by about 100 K. Irrespective of the lowered flow stress, however, the tensile elongation to failure of nanocrystalline composite is lower than that of submicrom-grain composite. The limited tensile elongation in nanocrystalline composite can be ascribed mainly to accelerating cavity damage accumulation.

Abstract: The fracture strength σf of ZrO2-based ceramics tends to increase with decreasing grain size d. As compared with the data for submicrometer-grain-sized material with d ≈ 350 nm, nano-crystallization of ZrO2 ceramics less than d ≈ 90 nm can improve σf by a factor of 2.0-2.5. The maximum strength reaches σf ≈ 2500 MPa, which is classed as the highest σf of oxide ceramic materials. The high σf can be associated with an increase in the critical t →m transformation stress and a decrease in the flaw size due to nano-crystallization.

Abstract: Multi-wall carbon nanotubes (MWCNTs) with a diameter of 20-30 nm were dispersed as a conductive phase into yttria stabilized tetragonal zirconia polycrystalline (3Y-TZP) to add electrical conductivity. The 3Y-TZP/MWCNT nanocomposites were fabricated by pressureless sintering under inert atmosphere. Electrical conductive function was successfully introduced by small amount of CNT addition. Critical volume fraction of the conductive phase for the percolation was analyzed and was found to be 0.390 vol% of CNT, which was much smaller than that for nano-sized carbon black dispersed 3Y-TZP (2.55 vol%). Microstructural investigation revealed that dispersed CNTs formed continuous 3-dimensional nano-network within the 3Y-TZP matrix, that contributed to the excellent conductive properties. Fracture strength was not improved much, while fracture toughness was increased by the CNT addition, due mainly to its crack bridging and/or pull-out mechanisms. It was considered that the use of anisotropic nano-sized conductive phase is advantageous to obtain electrically functionalized nanocomposite ceramics.

Abstract: Multi Wall Carbon Nanotubes (MWCNTs) with a diameter of 20-30 nm were used as a conductive phase to add electric conductivity to yttria stabilized tetragonal zirconia (3Y-TZP). Almost fully dense 3Y-TZP/MWCNTs nanocomposite was obtained by pressureless sintering under inert atmosphere and Hot Isostatic Pressing (HIP) treatment. The conductivity of the nanocomposites increased with increasing content of MWCNTs. Moreover, the fracture toughness increment of the composite was confirmed at 0.5 wt% addition. Scanning electron microscopy and transmission electron microscopy observation of the microstructures showed that MWCNTs were fairly homogeneously dispersed in the 3Y-TZP matrix.