Enhancement of the diffusional creep of polycrystalline Al2O3 by simultaneous doping with manganese and titanium

The effect of simultaneous doping with manganese and titanium on diffusional creep was studied in dense, polycrystalline alumina over a range of grain sizes (4–80m) and temperatures (1175–1250° C). At a total dopant concentration of 0.32–0.37 cation %, diffusional creep rates were enhanced considerably such that the temperature at which cation mass transport was significant was suppressed by at least 200° C compared to that observed in undoped material. The Mn-Ti (and Cu-Ti) dopant couple was far more effective in enhancing creep rates and suppressing sintering temperatures than the Fe-Ti couple. The enhanced mass transport kinetics are believed to be caused by significant increases in both aluminium lattice and grain-boundary diffusion. When aluminium grain-boundary diffusion is enhanced by increasing the concentration of divalent impurity (Mn²⁺, Fe²⁺) or by creep testing at low temperatures, creep deformation is Newtonian viscous.     

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al₂O₃ were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 m. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.     

Improvement of creep resistance in polycrystalline Al2O3 by Lu-doping

High-temperature creep resistance in high-purity Al₂O₃ (nominal purity of 99.99%) and Al₂O₃ doped with 0.05 mol% Lu₂O₃ was examined by uniaxial compression testing in air at a constant load at temperatures between 1150 and 1350°C and the applied stress range of 10∼200 MPa. The creep resistance was found to highly improve in this temperature range by doping of Lu₂O₃ into polycrystalline Al₂O₃ even at the level of 0.05 mol%. The stress exponent was about 2 in the two materials, but the activation energy for creep was different; 410 kJ/mol for undoped Al₂O₃ and 780 kJ/mol for Lu₂O₃-doped Al₂O₃. Lutetium ions were found to segregate in Al₂O₃ grain boundaries without forming amorphous phase or second-phase particles. The improved creep resistance in polycrystalline Al₂O₃ due to Lu₂O₃ doping was attributed to the segregation of Lu ions in Al₂O₃ grain boundaries probably due to the suppression of grain boundary diffusion. The change of chemical bonding state in grain boundaries with the segregation of Lu ions is supposed by HREM–EELS analysis.     

Newtonian flow process in polycrystalline silicon carbides: diffusional creep or Harper-Dorn creep?

Some previous studies on hot-pressed and sintered SiC polycrystalline materials have been reexamined. Mechanical data and microstructures strongly suggest that the Newtonian creep behaviour observed in these SiC materials was induced by a dislocation process operating in Harper-Dorn creep, rather than by diffusional creep as concluded before. The supporting evidence for this suggestion includes extensive development of dislocation substructures, no dependence of creep rate upon grain size, and the measured creep rates being far faster than those predicted by the model of diffusion creep, but consistent with those estimated by the model of Harper-Dorn creep.     

High oxygen ion conduction in some Bi2O3-Y2O3 (Er2O3) solid solutions

Highly densified bodies (~ 98 % theoretical density) of the Bi₂O₃-Y₂O₃(Er₂O₃) systems containing 30 moles % Y₂O₃ and 20 moles % Er₂O₃ respectively were prepared from both mixed oxides and coprecipitated oxalates. DC ionic conductivity and impedance complex plane analysis on sintered samples were performed. Under oxygen partial pressure ranging from 1 to 10 Pa was found that, samples containing 30 moles % yttria showed a pure ionic conductivity up to an oxygen partial pressure of 10⁻²⁰ Pa. Samples containing 20 moles % erbia, extended its ionic conductivity up to an oxygen partial pressure of 10⁻²² Pa. The impedance analysis shows the presence of only one semicircle at low tempertures and it is attributed to bulk conductivity contribution. The conductivity was higher for the Bi₂O₃-Er₂O₃ sintered solid electrolytes, in such a case, a conductivity as high as 1.38 ohm cm at 700ºC was obtained. The activation enthalpy for the conduction process was calculated in the temperature range from 200º to 700ºC in all the cases. Microstructural development in the sintered sample was also studied.     

Crystallization behavior of amorphous solid solutions and phase sepration in the Cr2O3-Fe2O3 system

Fine particles of the amorphous Cr₂O₃-Fe₂O₂ solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe₂O₃ content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe₂O₃ content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe₂O₃ content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles.     

Conductivity of reduced solid solutions in the system Na2OAl2O3TiO2

Solid solutions of the “titanium bronze” phase found in the system Na₂OAl₂O₃TiO₂ were made conductive either by H₂ reduction or the use of metallic Ti. Conductivity measurements by pulse and D.C. methods (blocking electrodes) show semiconducting behavior with activation energies decreasing with the extent of reduction, from 101 kJ/mole in “unreduced” samples to 39.8 kJ/mole in samples reduced with Ti metal. At about 600°C, all samples have conductivities about 7×10^?3 ohm^?1cm^?1, indicating that reduction occurs by oxygen loss in the Ar atmosphere. The occurrence of semiconductivity rather than metallic conductivity is explained by the blocking of conduction paths by the Al ions. Absorption spectra show no discrete spectrum for Ti³⁺, but a broad absorption band indicates the presence of delocalized electrons.     

Synthesis and dielectric properties of pyrochlore solid solutions in the Bi2O3-ZnO-Nb2O5-TiO2 system

Phase studies of solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ revealed extended regions of pyrochlore formation in the Bi₂O₃-ZnO-TiO₂-Nb₂O₅ system. At room temperature and 1 MHz (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ has a high permittivity ( = 200), low dielectric loss (tan<1·10^–4) and a temperature coefficient of the permittivity, , = –1300ppm/K. Pyrochlore solid solutions based on (Bi_1.5Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be formed with (Bi_2–xZn_x)(Ti_{2– x} Nb_x)O₇ (0.35x1.0) and with (Bi_1.5 Zn_0.5–y/3Ti_1.5+yNb_0.5–2y/3) O₇ (–1.5y0.75). Investigations of the dielectric characteristics showed that the high temperature dependence of the permittivity of (Bi_1.5 Zn_0.5)(Ti_1.5Nb_0.5)O₇ can be significantly modified by changing the composition of the pyrochlore within these regions of solid solubility. Below room temperature several of these compositions also exhibit a diffuse frequency dependent maximum in their permittivity characteristic of a transition to a relaxor type ferroelectric state. A third region of high permittivity pyrochlores with (Bi_1.5+2zZn_0.25–z Ti_2.25–z)O₇ (0.0z<0.15) was also identified in the Bi₂O₃-ZnO-TiO₂ sub system.     

An X-ray diffraction and Mossbauer study of nano-crystalline Fe2O3–Cr2O3 solid solutions

A series of gels with nominal composition Fe_2−xCr_xO₃ (x=0–2) was prepared at room temperature by an inorganic sol–gel route and studied by X-ray diffraction and Mossbauer spectroscopy. The gels dried at 105°C were found to be X-ray amorphous, but Mossbauer studies revealed the gels to be nano-crystalline solid-solution particles of the composition above, with super-paramagnetic properties. Further heating to 600°C gave crystalline X-ray patterns which allowed lattice parameter and crystallite size calculations to be made. It was found that lattice parameters and crystallite sizes decreased with increasing chromia content, and that at the higher chromia/iron ratios, a partially collapsed Mossbauer pattern results, indicating reductions in crystallite size and hyperfine field with increasing chromia content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Tensile failure of unflawed polycrystalline Al2O3

Scanning electron microscopy and acoustic emission are used to investigate the initial stages of tensile failure in unflawed polycrystalline alumina. It is found that deformation twinning plays an important role in crack initiation even at low homologous temperatures, and that the temperature-dependent strength behaviour between 23 to 410° C is controlled by twinning.     

Zr-Hf interdiffusion in polycrystalline Y2O3-(Zr+Hf)O2

Lattice and grain-boundary interdiffusion coefficients were calculated from the concentration distributions determined for Zr-Hf interdiffusion in polycrystalline 16Y₂O₃·84(Zr_1–x Hf _x )O₂ withx=0.020 and 0.100. The lattice interdiffusion coefficients were described byD=0.031 exp [–391 (kJ mol^–1)/RT] cm² sec^–1 and the grain-boundary diffusion parameters byD=1.5×10^–6exp [–309(kJ mol^–1)/RT] cm³ sec^–1 in the temperature range 1584–2116° C. Comparison of the results with those for the systems CaO-(Zr+Hf)O₂ and MgO-(Zr+Hf)O₂ indicated that the Zr self-diffusion coefficient was insensitive to the dopants in the fluorite-cubic ZrO₂ solid solutions.