Perovskite barium-based

State-of-the-art proton conductors comprise acceptor-substituted perovskites, such as the barium-based ones (BaCe03, BaZr03, etc.) which exhibit proton conductivities in excess of 0.01 S cm i [51-55] and strontium-based ones (SrCe03) with somewhat lower conductivities. Both BaCe03 and BaZr03 are almost pure ionic conductors, and the electronic conductivity would, as such, rate limit the H2 flux across membranes of these materials [56]. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

The compositions of most dielectric materials used for ceramic capacitors are based on ferroelectric barium titanate. As discussed in detail in Pragraph 1.3 the permittivity of ferroelectric perovskites shows marked changes with temperature, particularly close to the phase transition. From the device point of view a high dielectric permittivity with stable properties over a wide temperature range is required. There are various specifications which have to be fulfilled (e.g. X7R AC/C(T = 25°C) < 0.15 in a range between -55°C and 125°C). 

Bismuth oxide forms a number of complex mixed-metal phases with the divalent metal oxides of calcium, strontium, barium, lead, and cadmium, and these show a wide variety in composition. With transition metal oxides, mixed-metal oxide phases have been observed which are based upon a Perovskite-type lattice (10) containing layers of Bi202. It is notable that the high Tc superconducting materials which include bismuth also have this Perovskite-type of lattice with layers of copper oxide interleaved with bismuth oxide layers. 

A new method was developed, based on the original citrate route, which allows to synthesize perovskites with different incorporated metallic cations. Various catalysts based on the barium zirconate were prepared by this method. Perovskites were obtained containing either noble (Pd, Rh) or other metals (K, Cu, Mn). These systems were tested for several catalytic reactions, giving promising results. 

The perovskite materials which are considered suitable for the electron-conducting phase are based around the generic composition (Ln,Ae)Tm03, where Ln is a lanthanide, most commonly lanthanum, Ae is an alkaline earth, most commonly calcium, strontium or barium, and Tm is a transition metal, most commonly Ti,... 

Calcium, strontium and barium each have slightly different impacts on the properties of the perovskite. This is due, in part, to the difference in ionic radii of the ions. Calcium has a radius of 1.36 A, whereas the radii of strontium and barium are 1.44 and 1.65, respectively. Thus barium-containing perovskites have a more open crystal lattice and thus have a higher ionic conductivity compared to strontium- and calcium-based materials. However, barium is more reactive towards C02-containing gas mixtures and forms barium carbonate very readily. 

An example of a complex material used in syngas applications is Lag jSro gFeo g. Coo.iCro.203 [26]. This composition contains many of the features previously discussed. It is predominately an iron-based perovskite, providing both electronic and ionic conductivity. In order to improve the chemical stability and resistance to chemical expansion, the B-site is doped with chromium. A small amount of cobalt is added in order to improve the electronic conductivity, and cobalt wiU also increase the ionic conductivity slightly. Strontium is used as the A-site dopant in order to avoid the problems associated with calcium, and particularly with barium, in regard to reaction with CO2. In addition, the material has been made substantially A-site deficient to improve its stability. 

By examining the solubility of the different salts of barium, lanthanum and cobalt it appeared at for the three, the nitrate salts are very soluble as opposed to the carbonate salts which are insoluble in water. For tungsten, no salt was found to be very soluble in water, or any inorganic base except ammonia. However cobalt is known to form a large number of ammoniacal complexes, thus this base caimot be used therefore no co-precipitation pathway was found to prepare tungsten containing perovskites. 

In some metal oxides such as barium cerate-based perovskites, water can be incorporated at oxygen vacancies as well as oxygen atoms, leading to the formation of singly charged protonic defects ... 

Moving to strontium-based perovskites, we find the same trend as for the barium members. However, the conductivity is smaller, peaking between 10 and 10 S/cm for the best ones (SrZrOs and SrCeOs). SrCe03 is one of the best studied proton conductors, but the tolerance factor is low and the material is on the verge of decomposition into the binary oxides. It is therefore very vulnerable to reaction with CO2, for example. 

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