Borohydride synthesis and stabilization of flake-like tetragonal zirconia nanocrystallites

Flake-like particles of tetragonal zirconia (t-ZrO₂) were synthesized using ZrOCl₂·8H₂O as zirconium precursor, and sodium borohydride (NaBH₄) and cetyltrimethylammonium bromide (CTAB) as precipitating agent and surfactant, respectively. Small-sized nuclei, which formed during the borohydride synthesis, play an important role in the formation of small sized crystallites (5 nm) even at 600 °C and stabilization of t-ZrO₂ in both the samples synthesized with and without surfactant. In the sample synthesized without CTAB, a minor m-ZrO₂ phase (5 vol.%) along with the major t-ZrO₂ phase having a crystallite size of 20 nm, was observed at 700 °C. However, the use of surfactant leads to the formation of stabilized t-ZrO₂ nanoparticles with a smaller crystallite size of 15 nm. SEM micrographs of t-ZrO₂ show elliptical as well as elongated shaped flake-like morphology. The formation of small sized crystallites and slow growth of nucleation embryo play an important role in stabilizing the t-ZrO₂ up to as high temperature as 700 °C.     

Zr surface diffusion in tetragonal yttria stabilized zirconia

The value for surface diffusivity of Zr tetragonal ZrO₂–3mol% Y₂O₃ has been calculated from measurements of surface area reduction and pore growth in powder compacts during sintering. The surface diffusivity thereby obtained can be described by D =5.52×10⁵ exp[–531(kJ mol^-1)/RT] m²/s, which is in reasonable agreement with values calculated by prior researchers from direct TEM observation of neck growth between touching particles.     

Stabilization of metastable tetragonal zirconia nanocrystallites by surface modification

Metastable tetragonal zirconia nanocrystallites were studied in humid air and in water at room temperature (RT). A stabilizing effect of different surfactants on the tetragonal phase was observed. Furthermore, the phase stability of silanized metastable tetragonal zirconia nanocrystallites was tested by prolonged boiling in water. The samples were analyzed with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Changes in the monoclinic volume fraction in the samples were calculated. A number of surfactants were screened for their ability to stabilize the tetragonal phase upon exposure to humidity. Only silanes and phosphate esters of these were able to stabilize the tetragonal phase in water. Even as small amounts of silanes as 0.25 silane molecule per nm² are able to stabilize the tetragonal phase in water at RT. Aminopropyl trimethoxy silane and γ-methacryloxypropyl trimethoxy silane were even capable of preventing phase transformation during boiling for 48 h in water.     

Nanosize stabilization of cubic and tetragonal phases in reactive plasma synthesized zirconia powders

Pure zirconium oxide powders with particle size 2–33 nm are synthesized by reactive plasma processing. Transmission electron microscopy investigation of these particles revealed size dependent behavior for their phase stabilization. The monoclinic phase is found to be stable when particle size is ≥20 nm; Tetragonal is found to be stabilized in the range of 7–20 nm and as the particle size decreases to 6 nm and less, the cubic phase is stabilized.     

四方相纳米氧化锆低温稳定机制的研究现状

介绍了近几十年t-ZrO2纳米晶在室温下稳定机制的研究现状,并且重点分析表面能理论和氧空位理论等主要机制,在此基础上对镀膜、水蒸气、杂质离子、形核位置以及结构相似对其稳定性的影响进行进一步的讨论.     

Fracture of metastable tetragonal zirconia crystals

Crystals of single-phased metastable tetragonal zirconia (MTZ) were prepared from skullmelting at the composition ZrO₂-3 mol % M₂O₃ (M = Y, Yb or Gd). The fracture of the tetragonal crystals occurred differently than for the cubic stabilized zirconia ones. The toughness was much higher for the tetragonal specimens and the cleavage planes were not the same as for the fluorite crystals. The results were interpreted by considering the domain microstructure induced by the cubic → tetragonal phase transformation undergone by the crystals.     

Electrical conductivity of tetragonal stabilized zirconia

The electrical conductivity change on annealing for tetragonal stabilized zirconia (TZP) was studied with the help of a.c. impedance dispersion analysis techniques. The dependences of the conductivity on annealing time at 1000 ° C and on temperature cycling between room temperature and 1000 ° C were investigated. A decrease in conductivity of about 30% at 1000 ° C of TZP with 3 mol% Y₂O₃ was observed during the first 200 h of annealing at 1000 ° C, and no change was observed during further annealing. A similar result was observed for TZP with 2.9 mol% Sc₂O₃. For TZP with 3.0mol% Yb₂O₃, the conductivity decreased gradually during an annealing time of over 2000 h. The impedance dispersion analysis at lower temperature suggested that the decrease in electrical conductivity by annealing at 1000 ° C could be attributed to the increases of both grain boundary and intragrain resistance. No monoclinic phase was observed for the samples annealed at 1000 ° C for 2000 h. On the other hand, a trace of a monoclinic phase was found for TZP with 3mol% Y₂O₃ after the 50th temperature cycling, but no significant decrease in conductivity was observed with the cycling.     

On Oxygen Diffusion in tetragonal zirconia

Oxygen diffusion in dense scales of tetragonal zirconia at 1100–1300°C has been studied by the so-called interruption kinetic method (or the Rosenberg method). Assuming that oxygen vacancies are the predominant defects in zirconia (ZrO₂), studies by this method provide values of the oxygen self diffusion coefficient in this oxide at its lower limit of stability, D, and the value in tetragonal zirconia can be expressed as D =2.2 · 10^?3 exp (?140(kJ/mole)/RT). The studies furthermore show that the maximum non-stoichiometry in tetragonal zirconia, X in ZrO_2?x, is small and has a value about x* ～ 0.03 at 1000–1300°C.     

Stabilization of tetragonal phase in polycrystalline zirconia

It is shown that the tetragonal phase can be stabilized in the sintered body of a partially stabilized zirconia (PSZ) containing low concentrations of yttria. Such sintered body containing the metastable phase undergoes stress-induced phase transformation by the absorption of thermal or mechanical stress and exhibits strengths in excess of 690 MPa (100ksi).     

Phase-transformation study of metastable tetragonal zirconia powder

The rate of the transformation from the metastable tetragonal to the monoclinic phase of ZrO2 was measured in air from 850°C to 1000°C by neutron diffraction. This rate was found to be temperature dependent, and its measured values were considerably lower then those reported previously. The kinetics of this phase transformation is discussed in terms of a modified crystallite growth-martensitic transformation model' that includes the distribution of crystallite sizes.     

Sintering of ultra-fine tetragonal yttria-stabilized zirconia ceramics

High-frequency induction heated sintering (HFIHS) is utilized to consolidate ultra-fine grain tetragonal zirconia stabilized with 3 mol%Y₂O₃ (3Y-SZ) ceramics. Densification to near theoretical density in a relatively short time can be accomplished using this method. Samples of 3Y-SZ with a relative density of up to 99.5% and an average grain size of about 170 nm could be obtained by sintering at 950 °C for 5 min under a pressure of 100 MPa pressure. The influence of sintering temperature and mechanical pressure on the final density and grain size of the sintered products was investigated. The sintered materials had fracture toughness and hardness values of 4.4 MPa m^1/2 and 10.7 GPa, respectively.     

Synthesis and structural characterization of calcia-stabilized tetragonal zirconia

Synthesis of calcia-stabilized tetragonal zirconia by the reaction of precipitated zirconium hydroxide with calcium oxide at ∼1400 K is described. The unit cell parameters of the oxidea ₀=b ₀=0.3644 nm,c ₀=0.5092 nm andZ=2 are in good agreement with the high-temperature tetragonal crystallographic modification of pure zirconia.     

Characterization and metastability of alkoxy-derived tetragonal zirconia powder

The zirconia powders were synthesized through the hydrolysis of zirconiumn-butoxides with air moisture as well as with aqueous solutions of different pH values to investigate the metastability of tetragonal (t)-ZrO₂. The SEM and TEM observations reveal that powders prepared with air moisture hydrolysis consist of spherical particles of 0.5–3.5 m diameter, while gel-like powders composed of finer particles were obtained with aqueous solution hydrolysis. The samples obtained show different crystallization and tetragonal/monoclinic transformation temperatures on differential thermal analysis. The metastability of t-ZrO₂, investigated in terms of relative content with X-ray diffraction, was not explicable through the crystallite size effect. Instead, the existence of unpaired electrons, detected using electron paramagnetic resonance, and the strain within the powders, were found to be probably influential in affecting the metastability of t-ZrO₂.     

Mechanical and magnetic properties of nickel-dispersed tetragonal zirconia nanocomposites

Effects of Ni dispersions on microstructure and mechanical properties have been studied for Y2O3-stabilized tetragonal zirconia (Y-TZP)/Ni nanocomposites with Ni dispersion up to 10 vol%. Composites were successfully fabricated by reducing and hot-pressing Y-TZP/NiO powder mixtures. Fracture strength was significantly improved from 1.5 GPa for monolithic Y-TZP to 1.9 GPa for nanocomposites with a small addition of Ni (1-2 vol%). Magnetic properties of Y-TZP/Ni nanocomposites were also investigated. Magnetization curves of Y-TZP/Ni nanocomposites showed typical hysteresis loops of soft magnetic materials, whereas coercivity was much larger than that of pure Ni metal. A new function arising from magnetomechanical effects of metallic Ni is also discussed for the present nanocomposites.     

Superplastic flow in nanocrystalline and sub-microcrystalline yttria-stabilized tetragonal zirconia

Tensile deformation of nanocrystalline ZrO₂ + 5 mol% Y₂O₃ at temperatures in the range of 1283–1403 K is described. It is demonstrated (a) that steady state flow is possible at temperatures of the order of 0.42 T _m, where T _m is the absolute melting point, (b) that 70% engineering strain could be obtained at 1403 K (0.46 T _m), and (c) that significant grain boundary sliding was present during deformation. Static and dynamic grain growth as also a decrease in the relative density of the specimen with deformation could be observed. The present results as well as those of Owen and Chokshi concerning superplastic flow in sub-microcrystalline materials taken from literature could be accounted for quantitatively using the grain boundary sliding controlled flow model of Padmanabhan and Schlipf, originally proposed for microcrystalline superplastic alloys.