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Starch consolidation casting (SCC) technique was successfully employed to produce both porous alumina and graded porous alumina ceramics. The solid content in the alumina suspension was maintained at 40 vol.%, with potato starch varying from 5 to 15%. Structures of the porous alumina (monolithic) samples obtained by SCC and uniaxial pressing were compared. In addition, the influence of the SCC consolidation temperature and the starch content were evaluated in the monolithic samples, while the consolidation temperature and the number of layers numbers were evaluated in the graded samples. The lower SCC consolidation temperature resulted in lower linear shrinkage and a slight increase in total porosity due to the increased pore size. The compressive strength values for the monolithic samples ranged from 60 to 200MPa, which can be considered high when compared to previous works. The graded samples exhibited porosity variations across layers and interfaces were free of cracks and imperfections. Linear shrinkage was the same for the adopted consolidation temperature and the porosity was slightly higher for the 3-layer samples. They achieved strength of 60MPa with fracture mode parallel to the applied load.

Composites of Al2O3/ZrO2 (containing 25, 50, and 75 vol% ZrO2) were prepared by mixing Al2O3 and ZrO2 suspensions. The microstructural control via two‐step sintering (TSS) was the main objective of this work. For this purpose, different sintering curves were constructed, aiming to achieve the best temperature combination for the sintering steps that provides higher density and finer microstructure. The results were compared with single‐step sintering (SSS). Furthermore, microhardness and fracture toughness were measured for the best TSS specimens under each composition. The results showed that the high densities were obtained, and the reduction of grain size was greater than 40% for two‐step sintered specimens, compared to SSS ones. Consequently, microhardness values increased. However, fracture toughness values remained unchanged. [ABSTRACT FROM AUTHOR]

This work presents the initial results of the dry‐sliding wear behavior of 3 mol% yttria‐stabilized zirconia reinforced with 5 vol% alumina‐niobium carbide (3Y‐TZP/5 vol% Al2O3‐NbC) nanocomposites sintered by conventional sintering and spark plasma sintering methods in the temperature range of 1350‐1450°C. The reinforcement of 3Y‐TZP matrix with hard nanoparticles aimed to improve wear strength of the composites. Wear tests were performed by the ball‐on‐disc method using alumina (Al2O3) and tungsten carbide with 6 wt% cobalt cermet (WC‐6%Co) balls as counter‐materials, a load of 15 N, a sliding distance of 2000 m, and a sliding speed of 0.1 m/s. Wear behavior was evaluated in terms of wear rate and FE‐SEM micrograph analysis of the wear tracks. The nanocomposite sintered at 1450°C by conventional sintering exhibited the least wear when tested with the WC‐6%Co ball. Generally, the wear mechanism showed evidence of severe wear regime with both counter‐materials. [ABSTRACT FROM AUTHOR]

A complete solid solution forms between the perovskite proton conductor BaZr0.7Ce0.2Y0.1O3−δ (BZCY72) and BaPr0.9Y0.1O3−δ (BPY) on synthesis by the Pechini method and high-temperature annealing. Phase fields of selected members of the Ba(Zr0.7Ce0.2)1−(x/0.9)PrxY0.1O3−δ series were studied as a function of composition and temperature by high-resolution neutron powder diffraction revealing symmetry changes in the sequence Pnma→Imma→UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/→Pm3̅m. Higher symmetry is favoured for low Pr contents and high temperatures, as consideration of tolerance factor suggests. A volume contraction, ascribed to dehydration, is observed by synchrotron X-ray diffraction on heating in air for lower x. Magnetic measurements and structural data support the presence of Pr in the IV valence state on the perovskite B site. Thermogravimetric analysis in CO2 to ∼1253 K indicates better chemical stability for x≤ 0.445, whereas decomposition occurred for higher x. Electrical conductivity increases by over two orders of magnitude in dry air at lower temperature from x = 0.225 to 0.675; total conductivity reaches a value of 0.4 S cm−1 at 1173 K for x = 0.675. The series exhibits electron–hole transport with a positive pO2 dependence which increases with temperature, consistent with participation of oxygen vacancies in charge compensation of the Y3+ acceptor dopant. The activation energy for thermally activated hole hopping in air in the range 523–773 K decreases from ∼1 eV for BZCY72 to ∼0.4 eV for x = 0.675. Conductivity is generally lower in humidified N2 and air (pH2O ≈ 0.023 atm) than the corresponding dry atmospheres, consistent with consumption of holes by less mobile protonic species; however for x≤ 0.225 the lower concentration of electron holes concomitant with higher oxygen-vacancy content in N2 results in slightly higher conductivity in wet conditions due to hydration of vacancies. [ABSTRACT FROM AUTHOR]

A solid solution forms for Sr3NdNb3−xTixO12−δ with approximate limits 0 ≤x≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R3̅, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550–800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550–700 °C, as confirmed by the H+/D+ isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10−6 S cm−1 for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a −¼ slope for both doped and undoped samples in the range 10−19≤pO2≤ 10−8 atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of pO2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron–hole contribution. The dielectric constant at RT of nominally stoichiometric Sr3NdNb3O12 is εr∼ 37, with a moderately high quality factor of Q×f∼ 16 400 GHz at fr∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τf∼ 12 ppm °C−1, which lowers to −3 ppm °C−1 for the Ti-doped phase x = 0.06. [ABSTRACT FROM AUTHOR]

A solid solution forms for Sr3NdNb(3-x)Ti(x)O(12-δ) with approximate limits 0 ≤ x ≤ 0.06. The system crystallizes with a 12R-type hexagonal perovskite structure in the space group R3, as determined by neutron diffraction and selected area electron diffraction. The electrical properties of the end members have been investigated by impedance spectroscopy in the temperature range 550-800 °C under various gas atmospheres and as a function of oxygen and water-vapour partial pressure. Proton transport dominates under wet oxidising conditions in the temperature range 550-700 °C, as confirmed by the H(+)/D(+) isotope effect. Acceptor doping considerably enhances proton conductivity with a value of 3.3 × 10(-6) S cm(-1) for the bulk response of x = 0.06 at 700 °C in moistened air. The presence of a -¼ slope for both doped and undoped samples in the range 10(-19) ≤ pO2 ≤ 10(-8) atm at 900 °C indicates n-type transport under reducing conditions following the extrinsic model attributable to acceptor centres. The conductivity is essentially independent of pO2 at 600 °C under dry oxidising conditions, consistent with oxide-ion transport; a positive power-law dependence at higher temperature indicates extrinsic behaviour and a significant electron-hole contribution. The dielectric constant at RT of nominally stoichiometric Sr3NdNb3O12 is εr ∼ 37, with a moderately high quality factor of Q × f ∼ 16,400 GHz at fr ∼ 6.4 GHz. The temperature coefficient of resonant frequency of x = 0 is τf ∼ 12 ppm °C(-1), which lowers to -3 ppm °C(-1) for the Ti-doped phase x = 0.06.