T.S. Zhang

Sintering and grain growth of CoO-doped CeO2 ceramics

Abstract Co-doped CeO 2 powders with atomic ratios equal to 0.25, 1 and 3% were synthesized by the conventional mixed-oxide method. No binary compounds were detected in the CeO 2 –CoO system, and the Co element exists as the state of Co 2+ in the samples sintered above 1000°C. A small amount of Co doping reduces sintering temperatures and promotes grain boundary mobility dramatically. Over 99.0% of relative density (R.D.) can be obtained for 0.25% Co-doped sample sintered at 1300°C for 2 h, compared with ∼ 96% of relative density for pure CeO 2 sintered at 1525°C for 2 h. The results from grain growth kinetics study indicate that grain growth exponent, n , and activation energy, Q , are 3 and 697 ± 37 kJ/mol for pure CeO 2 , 4 and 572 ± 57 kJ/mol for 0.25% Co-doped CeO 2 , respectively.

Effect of alumina addition on the electrical and mechanical properties of Ce0.8Gd0.2O2−δ ceramics

Abstract (1− x ) Ce 0.8 Gd 0.2 O 2− δ + x Al 2 O 3 ceramics with compositions x =0 to 0.2 were prepared by the conventional mixed-oxide method from high purity (>99.9%) of commercial powders, and sintered at 1550 °C for 5 h in air. Sintered density, crystal phase, and microstructure were characterized. The ionic conductivity was measured over 200 to 800 °C in air using an impedance analyzer in the frequency range of 0.001 to 100 Hz. Microhardness and indentation fracture toughness were also determined. A new phase (i.e., GdAlO 3 ) has been detected in the samples with Al content greater than 5% AlO 1.5 . Ce 0.8 Gd 0.2 O 2− δ ceramic has a Vickers hardness of ∼9.23±0.15 GPa, and an indentation fracture toughness of ∼1.47±0.2 MPa m 1/2 . A remarkable improvement in both fracture toughness and microhardness can be observed for the samples with over 10% AlO 1.5 . However, the addition of Al 2 O 3 exhibits a detrimental effect on the conductivities, especially on grain boundary conductivity, which is possibly related to the formation of GdAlO 3 .

Sinterability and ionic conductivity of coprecipitated Ce0.8Gd0.2O2−δ powders treated via a high-energy ball-milling process

Abstract Ceria-based solid solutions are promising electrolytes for intermediate-temperature, solid oxide fuel cells. The effect of a dry, high-energy, ball-milling process on the sintering and densification behaviour of coprecipitated ceria-based powders is investigated by means of X-ray diffraction, Brunauer–Emmett–Teller (BET) surface-area measurements, density measurements, and electron microscopy. The dry ball-milling process leads to (i) a larger specific surface-area with weak agglomeration; (ii) rearrangement of grains into dense granules; (iii) a higher green density. These effects significantly reduce sintering temperatures and promote densification of ceria-based ceramics. Moreover, a comparison is made of the sintering behaviour and ionic conductivity of the milled samples with and without cobalt oxide doping. Cobalt oxide is a very effective sintering aid, but usually results in an enlarged grain-boundary effect for Si-containing samples. Thus, since SiO 2 is a ubiquitous background impurity in both raw materials and ceramic processing, the dry ball-milling process is a more feasible method for improving the sinterability of coprecipitated ceria-based powders.

Densification, microstructure and grain growth in the CeO2–Fe2O3 system (0⩽Fe/Ce⩽20%)

Mixtures of CeO2 and Fe2O3 with Fe/Ce atomic ratios ranging from 0 to 0.2 were prepared by the conventional mixed-oxide technique. Small amount of Fe doping (Fe/Ce⩽1%) significantly promotes the densification and grain growth of CeO2 ceramic. The results from the dilatometric measurement and SEM (scanning electronic microscopy) observation reveal that 0.5% Fe doping reduces the sintering temperatures by at least 200°C. For the samples with a large amount of Fe2O3 (Fe/Ce⩾1%), however, above 1400°C the densification behavior deteriorates remarkably; the density decreases with increasing sintering temperatures due to appearance of a lot of microcracks along grain boundaries. The so-called pining effect of second phase starts to take effect in the samples with Fe content greater than 5%. Fe2O3 grains grow more quickly at a lower sintering temperature (⩽1050°C), compared with those of CeO2.

Anisotropic grain growth of mullite in high-energy ball milled powders doped with transition metal oxides

Abstract Dense mullite ceramics with anisotropic grains were derived from the high-energy ball milled mixtures of Al 2 O 3 and amorphous silica with the presence of transition metal oxides (FeO 1.5 , CoO and NiO). The mullitization and grain growth behavior of the unmilled mixture without the addition of the transition metal oxides and the undoped system of Al 2 O 3 and amorphous silica with and without milling were also investigated and compared. The mullitization temperature was about 1200xa0°C in the milled systems, 100xa0°C lower than that required by the conventional solid-state reaction process. The lowered mullitization temperature, as well as the anisotropic grain growth, was attributed to the refined structure of the oxide powders, as a result of the high-energy ball milling. The experimental results have been explained by a dissolution-precipitation mechanism.

Final-stage sintering behavior of Fe-doped CeO2

The samples with Fe/Ce atomic ratios ranging from 0 to 3% were prepared via a ball-milling process from commercial powders (i.e. Fe2O3 and CeO2). A small amount of Fe doping (<1%) promotes the densification rate of ceria ceramics and increases the grain growth remarkably. Over 99.0% of relative density (RD) can be obtained for the 0.5% Fe-doped CeO2 sintered at 1300xa0°C in 1 h, while pure CeO2 has only 82% RD under the same sintering condition. The results from grain–growth kinetics study indicate that the grain growth exponent, n, and activation energy, Q, are 3 and 731±61 kJ mol−1 for pure CeO2, 4 and 590±50 kJ mol−1 for the 0.5% Fe-doped CeO2, respectively. Based on the present and previous work, an explanation on why the Fe doping results in the enhanced densification rate and rapid grain growth of ceria based ceramics is presented and discussed.

Sintering study on commercial CeO2 powder with small amount of MnO2 doping

Abstract Mn-doped samples with atomic ratios equal to 0.5%, 1% and 3% were prepared by the conventional mixed-oxide method from commercial cerium oxide (CeO2) and manganese oxide (MnO2) powders. The effect of Mn doping on the densification behavior of CeO2 was investigated by means of dilatometer and scanning electron microscopy (SEM). It was found that Mn doping reduced sintering temperature and promoted the grain growth dramatically. The sintering kinetics study indicated that under the isothermal sintering condition, the grain growth activation energy, Q, decreased from 731±61 kJ/mol for pure CeO2 to 593±53 kJ/mol for 1% Mn-doped CeO2.

Effect of transition metal oxides on mullite whisker formation from mechanochemically activated powders

The effect of transition metal oxides, including FeO1.5, CoO and NiO, on the phase formation and morphology development of mullite (3Al2O3·2SiO2) whiskers from oxide mixtures activated by the high-energy ball milling process, were investigated. With the addition of FeO1.5, the mullite formation temperature was almost the same as that required for the system of Al2O3 and SiO2 without doping, while the presence of CoO and NiO inhibited the mullitization. The effect of the transition metal oxides on the mullite phase formation could be explained in terms of the mullitization mechanism. The size of the whiskers in the CoO-doped samples was larger than that in the samples doped with FeO1.5 and NiO. The present result showed that the dimension of the whiskers could be altered through addition transition metal oxides.

Zinc niobate derived from mechanochemically activated oxides

Abstract ZnO and Nb 2 O 5 mixtures partially reacted to form ZnNb 2 O 6 during a high-energy ball milling process after milling for 5–15 h. The milled powders were extremely active and almost single phase ZnNb 2 O 6 could be achieved at a temperature as low as 500xa0°C. The effect of post-annealing on the grain growth and microstructure of the milled powders was investigated and discussed. Fully dense ZnNb 2 O 6 ceramics with an average grain size of 5, 3.5 and 2.4 μm can be obtained at 1100xa0°C from the powders milled for 5, 10 and 15 h, respectively.

Improvements in Sintering Behavior and Grain-Boundary Conductivity of Ceria-Based Electrolytes by a Small Addition of Fe2 O 3

The small addition of FeO 1.5 [e.g., 0.5 atom % (atom percent)] reduced sintering temperature (by ∼200°C), promoted the densification rate and grain growth for the ceria-based powders. The measurements of lattice parameter indicated that the FeO 1.5 addition could also enhance the dissolution of Gd 2 O 3 in CeO 2 at lower sintering temperatures. The 0.5 atom % FeO 1.5 -doped Ce 0.8 Gd 0.2 O 2-δ sintered at 1450°C for 5 h had a maximum total conductivity, as compared to the undoped one sintered at 1600°C for 5 h. Moreover, a remarkable improvement in grain boundary conductivity was achieved in the Ce 0.8 Gd 0.2 O 2-δ ceramics with small additions of FeO 1.5 (<3 atom %). The two possible mechanisms, that is, SiO 2 attracted to the Fe 2 O 3 interface or/and a change in the viscosity and wetting nature of SiO 2 due to the dissolution of Fe 2 O 3 , have been considered to be responsible for the scavenging effect of Fe 2 O 3 on SiO 2 impurity. It was also found that the optimum scavenging was achieved in the Ce 0.8 Gd 0.2 O 2-δ ceramics with 0.5 atom % FeO 1.5 , and sintered at 1400 to 1500°C, with a negligible effect on the grain interior conductivity. At 550°C, the total conductivity of Ce 0.8 Gd 0.2 O 2-δ ceramics was increased by more than 22% by adding 0.5 atom % FeO 1.5 , when sintered at 1450°C for 5 h. Therefore, the addition of Fe 2 O 5 is a feasible and promising way for improving the densification behavior and grain boundary conductivity of ceria-based electrolytes.