ÁREAS QUE COMPONEN LA CÁMARA DE COMERCIO DE CARTAGO

For chemical stability of a material, it can be categorized into thermal dynamic

stability and kinetic stability. Thermal dynamic stability is the intrinsic chemical stability

property for a material, which is related with the thermal dynamic behaviors of the metal

oxides. The HSC Chemistry software can be used to calculate the thermal dynamic

equations for many common reactions by providing the database for Enthalpy (H),

Entropy (S) and Specific Heat Capacity (C) [97].

For some of the materials, although it is thermal dynamically not stabile, it could

be kinetic stable. For example, in our study [80], for BaCeO3 based proton conductors, it

is thermal dynamically not stable below 450oC. However in the presence of water vapor,

after water reacting with a few surface monolayers of BaCeO3, since fewer water

molecules penetrate into the grain boundaries compared with those in liquid water

environment, the dissolution of Ba(OH)2 in water vapor would be very slow, and the

subsequent reaction rate for the decomposition of BaCeO3 would be expected to be much

slower than that in liquid water.

For the search of chemical stable proton conducting materials, the ultimate goal is

to find materials that are thermal dynamically stable. While sometimes, it is difficult to

find a proton conductor which possesses both high conductivity and excellent thermal

stable in specific situations and conditions; one of such example is the

BaZr0.1Ce0.7Y0.1Yb0.1O3-δ proton conductor, which shows remarkable chemical stability

under certain conditions [59].

4.9 SUMMARY

We measure the electrical conductivity property, including the bulk and grain

boundary conductivities, the chemical stabilities towards water and/or CO2, as well as the

thermal expansion properties of selected proton conductors.

For Ba1-xSrxCe0.8Y0.2O3-δ system prepared from modified Pechini method, the

conductivity tests indicated that Ba1-xSrxCe0.8Y0.2O3-δ possessed the electrical

conductivity between BaCe0.8Y0.2O3-δ and SrCe0.8Y0.2O3-δ. The conductivity decreased

and the activation energy increased with the increase in Sr content in Ba1-xSrxCe0.8Y0.2O3-

δ. The stability tests indicated that the resistance to boiling water for Ba1-xSrxCe0.8Y0.2O3-δ

was between that of BaCe0.8Y0.2O3-δ and SrCe0.8Y0.2O3-δ. Contrary to the reported data,

Ba1-xSrxCe0.8Y0.2O3-δ was less stable than BaCe0.7Zr0.2Y0.1O3-δ when exposed to boiling

water. Due to the feasibility of the reaction with CO2 for both BaCe0.8Y0.2O3-δ and

SrCe0.8Y0.2O3-δ, it was not surprising that Ba1-xSrxCe0.8Y0.2O3-δ was also not stable in CO2

containing atmospheres.

The conductivity as a function of sintering temperature for

BaCe0.7Zr0.1Y0.1Yb0.1O3-δ proton conductor has been investigated. An optimum sintering

temperature of 1400oC has been obtained, resulting in a compromise between

densification of samples which will improve the conductivity, and segregation of

temperatures. The thermal expansion coefficient values for BaCe0.7Zr0.1Y0.1Yb0.1O3-δ are

9.1-9.8×10-6 K-1 from 25 to 1200oC, which is favorably matched to that of Pr based

cathode materials such as Pr0.8Sr0.2MnO3-δ for SOFC applications. The conductivity

property of co-precipitated BaCe0.7Zr0.1Y0.1Yb0.1O3-δ powders sintered via two-step

sintering method has also been investigated. Since there are trace amount of impurities in

the as prepared powders, compared with conventional sintering methods, improved

conductivity was obtained for the two-step sintered sample pellet, which was largely

attributed to the improved grain boundary conductivity as a result from fewer amounts of

impurity defects in the grain boundaries.

The effects of Ce ions substituted with either Ca or Nb for complex perovskite

Ba3Ca1.18Nb1.82O9-δ on the conductivity have also been investigated. Ce substituted with

Nb ions enhances electrical conductivity, especially the grain boundary conductivity. The

introduction of Ce ions into Ca and/or Nb sites does not show any detrimental effect on

the chemical stability for BCN18 system, demonstrating very satisfactory chemical

stability compared with that of cerate based simple perovskite proton conductor systems.

The thermal expansion coefficient value is larger than conventional electrolyte materials,

showing good compatibility with typical intermediate temperature cathode materials in

SOFC area.

We further investigated the Ba3Ca1.18Nb1.82-xYxO9-δ system. The conductivity

increased with decreased activation energy when Nb was partially substituted by Y. The

total conductivity increased before x reaches 0.3 and then decreased with x=0.5. The

introduction of Y into the BCN18 system improved the bulk proton conductivity, as well

composition, Ba3Ca1.18Nb1.52Y0.3O9-δ shows predominantly proton conduction below

600oC. The improved proton conductivity, coupled with high chemical stability, makes

Ba3Ca1.18Nb1.52Y0.3O9-δ a promising electrolyte material for intermediate-temperature