High temperature protonic-conducting oxides based on the perovskite structure have been studied extensively since their discovery in 1981. 1 The hydrogen and deuterium isotope conductivities in these oxides have been evaluated 2,3 recently, and their potential application as hydrogen separation membranes has been discussed. 4 However, there are no reported studies on the tritium ion-conduction in these perovskites. In this paper, we report the hydrogen (H), deuterium (D), and tritium (T) conductivities of three protonic conducting perovskites, viz. SrZr 0.9 Yb 0.1 O 2.95 , BaCe 0.9 Yb 0.1 O 2.95 , and SrCe 0.95 Yb 0.05 O 2.975 . The conductivity of tritium ions through these perovskites not only gives fundamental information regarding the hydrogen transport in these perovskites, but also allows for the exciting possibility of tritium separation using these oxides as an electrochemical membrane.The proton-conducting compounds that were investigated in this study were selected for the following reasons. (1) The strontium zirconates are known to have the best chemical stability in CO 2 containing atmospheres (stable in 20 vol % CO 2 at 700ЊC). 5 (2) The barium cerates have one of the highest magnitudes of protonic conductivity. 6 (3) The strontium cerates, having a high protonic transference number, are the most extensively studied proton-conducting perovskites. [7][8][9] Although a H/D isotope effect has been shown in proton-conducting perovskites, the data in the literature is neither extensive nor consistent enough to clearly establish a model for protonic conduction in these perovskites. 2 The measured difference in activation energies for the H-and D-ion conduction has eliminated the possibility of proton hopping governed by a classical theory and has given support to a semiclassical approach. This approach takes into consideration the difference in zero point energies between the ground state of H and D, given by Eq. 1 10where E H and E D are the activation energies for the H-and D-ion conduction, respectively, and H and D are the OH and OD stretching frequencies. However the pre-exponential factors presented in the literature do not obey the predictions of the semiclassical theory given by Eq. 2, c eff -concentration of mobile protons, and v ϭ unit cell volume.In this paper, we present the H-, D-, and T-ion conductivities of SrZr 0.9 Yb 0.1 O 2.95 and BaCe 0.9 Yb 0.1 O 2.95 , and attempt to explain the various factors that need to be addressed in order to unambiguously determine the conduction mechanism in these perovskites. ExperimentalSingle-phase perovskite samples of SrZr 0.9 Yb 0.1 O 2.95 , BaCe 0.9 Yb 0.1 O 2.95 , and SrCe 0.95 Yb 0.05 O 2.975 were prepared using standard solid-state methods by heating appropriate stoichiometric mixtures of BaCO 3 , SrCO 3 , CeO 2 , ZrO 2 , and Yb 2 O 3 at 1773 K for 20 h with two intermediate ballmilling steps. The formation of a single-phase orthorhombic perovskite was confirmed by powder X-ray diffraction using a Siemens D 5000 diffractometer. Dense ceramic samples (...
a b s t r a c tAccelerated thermal ageing studies on foamed condensation cured polysiloxane materials have been performed in support of life assessment and material replacement programmes. Two different types of filled hydrogen-blown and condensation cured polysiloxane foams were tested; commercial (RTV S5370), and an in-house formulated polysiloxane elastomer (Silfoam). Compression set properties were investigated using Thermomechanical (TMA) studies and compared against two separate longer term ageing trials carried out in air and in dry inert gas atmospheres using compression jigs. Isotherms measured from these studies were assessed using time-temperature (T/t) superposition. Acceleration factors were determined and fitted to Arrhenius kinetics. For both materials, the thermo-mechanical results were found to closely follow the longer term accelerated ageing trials. Comparison of the accelerated ageing data in dry nitrogen atmospheres against field trial results showed the accelerated ageing trends over predict, however the comparison is difficult as the field data suffer from significant component to component variability. Of the long term ageing trials reported here, those carried out in air deviate more significantly from field trials data compared to those carried out in dry nitrogen atmospheres. For field return samples, there is evidence for residual post-curing reactions influencing mechanical performance, which would accelerate compression set. Multiple quantum-NMR studies suggest that compression set is not associated with significant changes in net crosslink density, but that some degree of network rearrangement has occurred due to viscoelastic relaxation as well as bond breaking and forming processes, with possible post-curing reactions at early times.Crown
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