Cathodic precipitation of ceramic precursor materials

An electrochemical technique has been developed for the production of precursors to ceramic films on hydrogen sorbing metal substrates. It involves the electrolysis of aqueous metal salt solutions which yields hydrogen at the cathode, resulting in local generation of base (hydroxide ions) around this electrode. Such conditions promote the precipitation of metallic hydroxides from a suitable electrolyte. If the local alkaline environment is not disrupted by convective or other forces, then a solid phase accumulates near the cathode, and forms an adherent gel-like structure on its surface. In order to maintain deposition, it is essential that gaseous hydrogen evolution is minimised, and preferably eliminated. This can be achieved by use of a hydrogen sorbing cathode material, such as palladium. The electrode, and adherent film (or, in appropriate circumstances, the deposit alone) can then undergo a subsequent calcination treatment to yield the ceramic layer. It is possible to generate both porous and compact structures by this method, depending on the potential programme employed during deposition. Research has been conducted into the understanding of mechanisms involved in porosity control of films deposited during different potential regimes, with view to establishing routes to layers of predetermined physical structure. In-situ optical methods were employed to complement the electrochemical techniques, providing valuable insight into the initial mechanisms of film formation and the subsequent thickening processes. The utility of the precipitation process was illustrated by the fabrication of films which demonstrated a variable conductivity over a range of humidities appropriate to sensing application. Investigation into the use of a bipolar palladium electrode as an aid to generating thick film deposits was carried out. The device comprised a palladium plate, operated as a bipolar electrode in aqueous electrolyte. Under suitable conditions, the negative face of this electrode can be made to generate and absorb hydrogen, whilst simultaneously, the positive face oxidises hydrogen transported across the bipolar substrate by diffusion. Thus the cathode face is a non-gassing electrode on which thick deposits of metal hydroxide can be grown. This line of research lead to the realisation of a self-feeding hydrogen anode at the electrode's positive face. Further research was undertaken to assess the electrochemical properties of this anode. The effective operating window for hydrogen oxidation was investigated, and the effect of prolonged potential cycling, elevated temperature and bipolar plate thickness on this region was also considered.