Dense Solid Electrolyte and Oxide Membranes

Small-size starting particles with an appropriate size distribution that lead to a maximum packing density and final fired density are essential in the sintering step to form dense membranes. The required sintering temperatures are lower because of the active state of the Hne particulate materials used. For example, when particles of an average size of 5 nm made by the alkoxide approach are used to make stabilized zirconia, 1450X instead of the usual 2(X)0X is all that is necessary to produce fully dense material [Mazdiyasni et al., 1%7]. The amount of additives such as the stabilizers affects densification of the final solid electrolyte membranes as well. Generally there is an optimum amount of stabilizer for maximum densification. Excess addition actually can lead to lower densification. 

Other methods such as glass fabrication techniques and different deposition methods are also used. 

For pressing as well as extrusion, the solid electrolyte precursor particles (e.g., zirconia) are often mixed or reacted with an inorganic cementing substance. It is preferred that such adhesive materials also have ion permselective properties as the precursor particles. Phosphates of zirconium, titanium and zinc are examples of such cements although other materials such as calcium aluminate and calcium aluminosilicates are candidates as well [Arrance et al., 1969]. For these cementing materials to be effective, the metal oxides must be only partially hydrated so that they are reactive with the bonding compounds. 

The subsequent sintering step for the pressed parts is similar to that for the extrusion method and the considerations for achieving a maximum density are therefore for the most part applicable here as well 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

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