RBaCo₂O_{5+ δ}(R=Y, Dy, Gd, Pr, Nd, Sm, and Eu) ceramic samples were prepared through the solid-state reaction method. Stoichiometric amounts of R₂O₃, BaCO₃, and Co₃O₄ raw materials were mixed thoroughly in an agate mortar. The mixed powder was slowly heated up to 1000℃, kept at this temperature for 10 h in air, then cooled to room temperature slowly. After regrinding, the power was pressed into bar-shape, and sintered at 1100℃ in air for 20 h again. X-ray diffraction (XRD, χ’pert Pro system using CuKα radiation) analysis was carried out on the sample powders to check the phase structures of the samples. The standard four-probe method was used to measure the change of electrical resistivity with temperatures or surrounding oxygen pressures of R112 samples. The samples for electrical measurements are cuboid bars with 15.0 mm×5.0 mm×2.0 mm.

Figure 1 shows the XRD patterns for RBaCo₂O_{5+ δ} (R=Y, Dy, Gd, Pr, Nd, Sm, and Eu). It can be seen that all of them have similar double perovskite structure, and little impurity phase. Their lattice parameters are listed in Table 1. Summarily, the lattices parameters decrease with the decreasing R³⁺ ion size. The results mean that R112 samples can be prepared in air successfully, which are consistent with the earlier results^{[ 19, 20, 21, 22]}.

Fig. 1

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	Fig. 1 XRD patterns of RBaCo2O5+ δ (R=Y, Dy, Gd, Pr, Nd, Sm and Eu) ceramics

Table 1

Table 1

Table 1 Lattice parameters of R112 ceramics Parameters a/nm b/nm c/nm V/nm3 Y112 0.3866 0.3871 0.7485 0.11200 Dy112 0.3882 0.3883 0.7511 0.11322 Gd112 0.3894 0.3885 0.7527 0.11387 Eu112 0.3911 0.3887 0.7548 0.11475 Sm112 0.3919 0.3887 0.7573 0.11535 Nd112 0.3930 0.3900 0.7602 0.11648 Pr112 0.3915 0.3912 0.7634 0.11690

Table 1 Lattice parameters of R112 ceramics

Figure 2 shows the dependence of resistivity on temperature from room temperature to 600℃ measured in air. Firstly, the resistivities of all samples decrease with the rising temperature, and show typical semiconducting behavior. When the temperature rises to a certain temperature (it is different for R112 samples, such as, Y112 300℃, Dy112 190℃, Gd112 120℃, Nd112 110℃, Eu112 105℃, Pr112 102℃, and Sm112 100℃), however, the resistivities of all R112 begin to increase slowly with the increasing temperature, and exhibit semimetal conducting behavior. These phenomena are attributed to a consequence of competing two factors: i.e., intrinsic semiconductive behavior and resistance increase upon oxygen desorption. For p-type semiconductor, oxygen desorption from the lattice results in a drop in the carrier concentration, and thus a rise of the resistivity. Zhang, et al^{[ 14]} ever reported the similar resistivity transition behavior of Y112. At high temperature, the resistivities of R112 decrease with the decrease of the size of R³⁺ ions, which was ever reported by Maignan, et al^{[ 5]}.

Fig. 2

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	Fig. 2 Temperature dependence of electrical resistivity of R112 ceramics in air

Figure 3 illustrates the relative resistivity change ([( ρ( t)-ρ(0))/ ρ(0)]×100) of the R112 samples with the time during the atmosphere switching cycle (oxygen → nitrogen → oxygen) at 600℃. It can be seen that when the atmosphere is shifted from nitrogen to oxygen, the resistivities drops drastically and reaches its balance value in about 30 s. This means that the resistivities of R112 are sensitive to the increasing surrounding oxygen pressure induced by the oxygen adsorption of R112. However, when oxygen atmosphere is shifted to nitrogen, the resistivities increase slowly and need a long time to recover. The reason is that the oxygen adsorption rates of R112 are faster than their oxygen desorption at each temperature^{[ 17]}. Obviously, the total responding rates are mainly determined by the resistivities increasing time depending on the oxygen adsorption rates. For the seven R112 ceramic samples, the total responding rates are Y112 > Dy112 > Gd112 > Pr112 > Nd112 > Sm112> Eu112. It seems that the response speed of the R112 sample series is not systematic with respect to the size of R³⁺ ions.

Fig. 3

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	Fig. 3 Dependence of resistance on oxygen pressure of R112 ceramics at 600℃

Figure 4 shows the resistivity change of the Y112 with the time during the atmosphere switching cycle at 500℃, 600℃, and 700℃, respectively. Obviously, when the atmosphere is shifted from oxygen to nitrogen the responding rate of Y112 rises with increasing temperature. On the contrary, the resistivity change magnitude (meaning sensitivity) declines with increasing temperature. If magnifying locally of resistivity transition sections, it can be seen that the response time is within 90 s at 700℃. While, when nitrogen atmosphere is shifted to oxygen, the recovery time has less change, and maintains about 30 s.

Fig. 4

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	Fig. 4 Dependence of resistance on oxygen pressure of YBaCo2O5+ δ at 500℃, 600℃ and 700℃

The potential application of the double peroviskite structure compounds R112 as oxygen resistance sensors was investigated. The resistivities of R112 samples are sensitive to the environmental oxygen pressure, and their changes are more rapid with increasing oxygen pressure than decreasing oxygen pressure. Among them, the Y112 has the fastest responding rate, and its responding rates rise with the increasing temperature. The responding rate is not enough to apply directly. However, it can be expected that if the ceramic samples are replaced by films the responding properties would be promoted greatly because the oxygen adsorption/desorption rate at the surface layer is faster than the oxygen diffusion in ceramic inner part.