Identical Temperature Levels Research Articles

Crack-healing ability and strength recovery of different hot-pressed TZ3Y20A–SiC ceramics by heat treatment in air

Abstract Crack-healing ability and strength recovery of different ZrO2(Y2O3)–Al2O3–SiC (TZ3Y20A–SiC) ceramics have been investigated by tailoring heat treatment temperature, heat treatment time, SiC content and crack size. Heat treatment in air enhances distinctly the flexural strength of TZ3Y20A–SiC ceramics. A complete crack-healing of TZ3Y20A–10 vol%SiC ceramic can be realized by optional heat treatments at 1073 K for 30 h, 1273 K for 10 h or 1373 K for 5 h, respectively. With increasing the content of nanosize SiC particles from 10 to 20 vol%, the crack-healing process of TZ3Y20A–SiC ceramics speeds up at identical temperature levels. The pre-existing cracks with a width up to 0.45 μm are healed completely under the optimum heat treatment conditions, and the flexural strengths of crack-healed TZ3Y20A–SiC ceramics are completely recovered to the value of smooth specimens. Crack closure and rebonding of crack wall due to the formation of SiO2 glassy phase by oxidation reaction between SiC and the oxidizing atmosphere are dominant healing mechanisms in TZ3Y20A–SiC ceramics.

Sintering and electrical conductivity of the GdSmZr2O7 ceramic with and without ZnO sintering aid

The effects of zinc oxide (ZnO) additive on the crystal structure, densification, and electrical conductivity of the GdSmZr2O7 ceramic have been examined. The GdSmZr2O7 ceramic with and without the addition of 1 wt.% ZnO sintered at temperatures between 1773 and 1973 K exhibits a single cubic pyrochlore-type phase. ZnO is very effective as a sintering aid for the GdSmZr2O7 ceramic, and it reduces the sintering temperature by about 200 K with a positive effect on the total conductivity. The total conductivity of the ZnO-modified GdSmZr2O7, as measured by A.C. impedance spectroscopy under an air atmosphere, is higher than that of the unmodified GdSmZr2O7 at identical temperature levels. At 1173 K, the highest total conductivity of the ZnO-modified GdSmZr2O7 is about fifteen percent higher than that of the unmodified GdSmZr2O7. The 1 wt.% ZnO-modified GdSmZr2O7 is one of good oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10−4 to 1.0 atm at different test temperatures.

Preparation, Structural Characterization, and Enhanced Electrical Conductivity of Pyrochlore‐type (Sm1–xEux)2Zr2O7 Ceramics

Abstract(Sm1–xEux)2Zr2O7 (0 ≤ x ≤ 1.0) samples are prepared by solid state reaction method using Sm2O3, Eu2O3, and ZrO2 as starting materials. The phase composition and microstructure of (Sm1–xEux)2Zr2O7 ceramics are investigated by X‐ray diffraction (XRD), scanning electron microscopy, high‐resolution transmission electron microscopy (HRTEM) coupled with selected area electron diffraction and Raman spectroscopy. XRD and TEM show that all the samples exhibit a single pyrochlore‐type structure. HRTEM observation indicates that the whole grain interior of Sm2Zr2O7 ceramic is a perfect crystal free of any dislocation. Raman spectroscopy reveals that the degree of structural disorder of (Sm1–xEux)2Zr2O7 ceramics increases gradually with increasing Eu content. The electrical conductivity of (Sm1–xEux)2Zr2O7 ceramics is investigated by impedance spectroscopy in the air and hydrogen atmospheres, respectively. The electrical conductivity of (Sm1–xEux)2Zr2O7 ceramics increases with increasing Eu content at identical temperature levels. Both the activation energy Eg and the pre‐exponential factor σ0g for the grain conductivity gradually increase with increasing Eu content. As the ionic conductivity shows no obvious change in both air and hydrogen atmospheres, the conduction of (Sm1–xEux)2Zr2O7 is purely ionic with negligible electronic conduction.

Preparation, structure and electrical conductivity of pyrochlore-type samarium–lanthanum zirconate ceramics

A series of zirconate compounds with the general formula Sm 2– x La x Zr 2O 7 (0 ⩽ x ⩽ 1.0) were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity of Sm 2– x La x Zr 2O 7 ceramics were investigated by the Archimedes method, X-ray diffraction and impedance spectroscopy measurements. Sm 2– x La x Zr 2O 7 (0 ⩽ x ⩽ 1.0) ceramics exhibit a pyrochlore-type structure. The measured electrical conductivity of Sm 2– x La x Zr 2O 7 ceramics obeys the Arrhenius relation and gradually increases with increasing temperature from 673 to 1173 K. Sm 2– x La x Zr 2O 7 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10 −4 to 1.0 atm at all test temperature levels. The electrical conductivity of Sm 2– x La x Zr 2O 7 ceramics decreases with increasing lanthanum content at identical temperature levels.

Electrical Conductivity Improvement of Nd2Ce2O7 Ceramic Co‐doped with Gd2O3 and ZrO2

Abstract(Nd1–xGdx)2(Ce1–xZrx)2O7 (0 ≤ x ≤ 0.5) ceramic powders synthesised by the chemical‐coprecipitation and calcination method were pressureless‐sintered at 1,973 K for 10 h in air. The structure and electrical conductivity of (Nd1–xGdx)2(Ce1–xZrx)2O7 ceramics were investigated by the X‐ray diffraction and impedance spectroscopy measurements. (Nd1–xGdx)2(Ce1–xZrx)2O7 (0 ≤ x ≤ 0.5) ceramics exhibit a single phase of defect fluorite‐type structure. The electrical conductivity of (Nd1–xGdx)2(Ce1–xZrx)2O7 ceramics increases with temperature in the range 623–1,173 K following an Arrhenius law. At identical temperature levels, the measured electrical conductivity of (Nd1–xGdx)2(Ce1–xZrx)2O7 ceramics varies with doping different Gd2O3 and ZrO2 contents and exhibits a maximum at x = 0.1.

The effect of co-doping with MgO and Yb2O3 on the structure and electrical conductivity of the Sm2Zr2O7 pyrochlore

SmYb1−xMgxZr2O7−x/2 (0≤x≤0.15) ceramics are pressureless-sintered at 1973K for 10h in air. The structure and electrical conductivity of SmYb1−xMgxZr2O7−x/2 ceramics are investigated by the X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb1−xMgxZr2O7−x/2 ceramics exhibit a defect fluorite-type structure. The measured electrical conductivities of SmYb1−xMgxZr2O7−x/2 ceramics obey the Arrhenius relation, and electrical conductivity of each composition increases with increasing temperature from 673 to 1173K. At identical temperature levels, the electrical conductivity of SmYb1−xMgxZr2O7−x/2 ceramics gradually increases with increasing magnesia content. SmYb1−xMgxZr2O7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0×10−4 to 1.0atm at all test temperature levels. The electrical conductivity obtained in SmYb1−xMgxZr2O7−x/2 ceramics reaches the highest value of 2.72×10−3Scm−1 at 1173K for the SmYb0.85Mg0.15Zr2O6.925 ceramic.

Electrical conductivity of 5 mol.% Yb 2O 3 and 5 mol.% Gd 2O 3 co-doped Sm 2Zr 2O 7

Sm 2Zr 2O 7 co-doped with and without 5 mol.% Yb 2O 3 and 5 mol.% Gd 2O 3 were prepared by a pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectra measurements. Both Sm 2Zr 2O 7 and (Sm 0.9Gd 0.05Yb 0.05) 2Zr 2O 7 ceramics exhibit a single phase of pyrochlore-type structure. The grain conductivity, grain-boundary conductivity and total conductivity obey the Arrhenius relation, respectively, and gradually increase with increasing temperature from 723 to 1173 K. (Sm 0.9Gd 0.05Yb 0.05) 2Zr 2O 7 ceramic is the oxide-ion conductor in an oxygen partial pressure range of 1.0 × 10 −4 to 1.0 atm at all test temperature levels. The grain conductivity, grain-boundary conductivity and total conductivity of (Sm 0.9Gd 0.05Yb 0.05) 2Zr 2O 7 with dual Yb 3+ + Gd 3+ doping are higher than those of undoped Sm 2Zr 2O 7 at identical temperature levels.

Order–disorder transformation and enhanced oxide-ionic conductivity of (Sm 1− xDy x) 2Zr 2O 7 ceramics

(Sm 1− x Dy x ) 2Zr 2O 7 (0 ≤ x ≤ 1) ceramics are prepared by a solid state reaction process at 1973 K for 10 h in air. (Sm 1− x Dy x ) 2Zr 2O 7 (0 ≤ x ≤ 0.3) ceramics exhibit a single phase of pyrochlore-type structure, while (Sm 1− x Dy x ) 2Zr 2O 7 (0.5 ≤ x ≤ 1.0) possess a defective fluorite-type structure. The full width at half-maxima in the Raman spectra increases with increasing Dy content, which indicates that the degree of structural disorder increases as the Dy content increases. The ionic conductivity of (Sm 1− x Dy x ) 2Zr 2O 7 ceramics is investigated by impedance spectroscopy over a frequency range of 0.2 Hz to 8 MHz in the temperature range of 873–1173 K in air and hydrogen atmospheres, respectively. The ionic conductivity has a maximum near the phase boundary between the pyrochlore- and the defective fluorite-type phases under identical temperature levels. The ionic conductivity is determined by the degree of structural disorder or unit cell free volume, which is depending on the Dy content. As the ionic conductivity in the hydrogen atmosphere is almost the same as that obtained in air, the conduction of (Sm 1− x Dy x ) 2Zr 2O 7 is purely ionic with negligible electronic conduction.

Effect of Gd and Yb co-doping on structure and electrical conductivity of the Sm 2Zr 2O 7 pyrochlore

SmYb 1− x Gd x Zr 2O 7 (0 ≤ x ≤ 1.0) ceramics were pressureless-sintered at 1973 K for 10 h in air. The relative density, structure and electrical conductivity of SmYb 1− x Gd x Zr 2O 7 ceramics were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb 1− x Gd x Zr 2O 7 (0 ≤ x ≤ 0.5) ceramics exhibit a defect fluorite-type structure, while SmYb 1− x Gd x Zr 2O 7 (0.7 ≤ x ≤ 1.0) ceramics have a pyrochlore-type structure. The measured values of the electrical conductivities obey the Arrhenius relation. The grain conductivity of each composition in SmYb 1− x Gd x Zr 2O 7 ceramics increases with increasing temperature from 723 to 1173 K. The grain conductivity of SmYb 1− x Gd x Zr 2O 7 ceramics gradually increases with increasing gadolinium content at identical temperature levels. An increase of about one order of magnitude in grain conductivity is found at all temperature levels when the gadolinium content increases from 0.5 to 0.7. SmYb 1− x Gd x Zr 2O 7 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10 −4 to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 2.69 × 10 −2 S cm −1 at 1173 K for SmGdZr 2O 7 ceramic.

Influence of dysprosium oxide doping on thermophysical properties of LaMgAl 11O 19 ceramics

This paper deals with the influence of dysprosium oxide doping on thermophysical properties of LaMgAl 11O 19 ceramics. LaMgAl 11O 19 ceramic powders doped with different contents of dysprosium oxide were pressureless-sintered at 1700 °C for 10 h in air to fabricate dense bulk ceramics. La 1− x Dy x MgAl 11O 19 ( x = 0, 0.1, 0.2, 0.3) ceramics have a relative density of 90.7–96.0%, and exhibit a single phase of magnetoplumbite structure. Thermal diffusivity and thermal expansion coefficients of La 1− x Dy x MgAl 11O 19 ceramics were measured with a laser flash method and a high-temperature dilatometer. Thermal diffusivity of La 1− x Dy x MgAl 11O 19 ceramics decreases with increasing Dy 2O 3 content at identical temperature levels. The measured thermal conductivity of La 1− x Dy x MgAl 11O 19 ceramics is located in the range of 2.52–2.89 W m −1 K −1 at 1200 °C. Thermal expansion coefficient of La 0.8Dy 0.2MgAl 11O 19 ceramic is slightly higher than that of undoped LaMgAl 11O 19 ceramic at identical temperature levels.

Electrical conductivity and thermal expansion of neodymium–ytterbium zirconate ceramics

(Nd 1− x Yb x ) 2Zr 2O 7 (0 ≤ x ≤ 1) ceramics were prepared by pressureless-sintering to obtain dense bulk materials. The electrical conductivity of (Nd 1− x Yb x ) 2Zr 2O 7 was investigated by complex impedance spectroscopy over a frequency range of 20 Hz to 2 MHz from 723 to 1173 K in air. A high-temperature dilatometer was used to analyze thermal expansion coefficient of (Nd 1− x Yb x ) 2Zr 2O 7 in the temperature range of 373–1523 K. The measured electrical conductivity obeys the Arrhenius relation. The grain conductivity of each composition in (Nd 1− x Yb x ) 2Zr 2O 7 gradually increases with increasing temperature. A decrease of about one order of magnitude in grain conductivity is found at all temperature levels when the Yb content increases from x = 0.3 to x = 0.5. The highest electrical conductivity value obtained in this work is 9.32 × 10 −3 S cm −1 at 1173 K for (Nd 0.7Yb 0.3) 2Zr 2O 7 ceramic. (Nd 1− x Yb x ) 2Zr 2O 7 ceramics are oxide-ion conductors in the oxygen partial pressure range from 1.0 × 10 −4 to 1.0 atm at all test temperature levels. Thermal expansion coefficients of (Nd 1− x Yb x ) 2Zr 2O 7 gradually decrease with increasing ytterbium content at identical temperature levels.

Electrical conductivity of defect fluorite-type (Gd1−xYbx)2Zr2O7 solid solutions

Abstract (Gd 1− x Yb x ) 2 Zr 2 O 7 (0 ≤ x ≤ 1) solid solutions were prepared by pressureless-sintering process at 1973 K to investigate the substitution effect of Yb for Gd on their electrical properties. Electrical conductivity of (Gd 1− x Yb x ) 2 Zr 2 O 7 solid solutions were measured using complex impedance spectroscopy from 673 to 1173 K over a frequency range of 2 Hz to 2 MHz in air. (Gd 1− x Yb x ) 2 Zr 2 O 7 are oxide-ion conductor in the oxygen partial pressure range from 1.0 × 10 −4 to 1.0 atm at all test temperature levels. The measured values of grain conductivity obey the Arrhenius relation. The grain conductivity of (Gd 1− x Yb x ) 2 Zr 2 O 7 increases with increasing temperature. However, the grain conductivity of (Gd 1− x Yb x ) 2 Zr 2 O 7 gradually decreases with increasing ytterbium content at identical temperature levels.

Preparation and microstructural characterization of (Nd 1− xGd x) 2(Ce 1− xZr x) 2O 7 solid solutions

(Nd 1− x Gd x ) 2(Ce 1− x Zr x ) 2O 7 (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd 1− x Gd x ) 2(Ce 1− x Zr x ) 2O 7 powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd 1− x Gd x ) 2(Ce 1− x Zr x ) 2O 7 solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd 1− x Gd x ) 2(Ce 1− x Zr x ) 2O 7 solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd 1− x Gd x ) 2(Ce 1− x Zr x ) 2O 7 solid solutions agree well with the Vegard's rule.

Effect of Ti substitution for Zr on the thermal expansion property of fluorite-type Gd 2Zr 2O 7

Gd 2(Zr 1− x Ti x ) 2O 7 (0 ⩽ x ⩽ 1.00) solid solutions were synthesized by solid state reaction at 1550–1650 °C for 10 h in air. The relative density, microstructure and thermal expansion property of Gd 2(Zr 1− x Ti x ) 2O 7 were investigated by the Archimedes method, X-ray diffraction, laser Raman spectroscopy, scanning electron microscopy and high-temperature dilatometry. Gd 2Zr 2O 7 exhibits a defect fluorite-type structure; however, Gd 2(Zr 1− x Ti x ) 2O 7 (0.25 ⩽ x ⩽ 1.00) solid solutions have an ordered pyrochlore-type structure. Gd 2Zr 2O 7 and Gd 2Ti 2O 7 are completely soluble. The degree of structural order for Gd 2(Zr 1− x Ti x ) 2O 7 solid solutions increases with increasing Ti content. The thermal expansion coefficients of Gd 2(Zr 1− x Ti x ) 2O 7 increase with increasing temperature from 30 to 1250 °C. The thermal expansion coefficients of Gd 2(Zr 1− x Ti x ) 2O 7 gradually decrease with increasing Ti content at identical temperature levels.

Heat capacities and derived thermodynamic functions of neodymium–gadolinium zirconates from 298.15 to 1050 K

X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize Nd x Gd 2 − x Zr 2O 7 ( x = 0, 0.6, 1.0, 1.4, and 2.0) solid solutions pressureless-sintered by using ceramic powders acquired by chemical-coprecipitation and calcination method. Heat capacity measurements were performed with a heat flux-type differential scanning calorimetry in the temperature range of 298.15–1050 K. The heat capacity of Nd x Gd 2 − x Zr 2O 7 solid solutions increases with increasing the concentration x of neodymium cations from x = 0 to x = 2.0 at identical temperature levels. At 298.15 K, the heat capacities of Nd 0.6Gd 1.4Zr 2O 7, NdGdZr 2O 7 and Nd 1.4Gd 0.6Zr 2O 7 are 217, 220 and 224 J K −1 mol −1, respectively. Thermodynamic functions such as enthalpy increment, entropy and Gibbs energy functions were calculated from the measured heat capacity data.