The name "mixed metal oxide
anodes" is justifiably confusing to many who hear the term for the first time.
One may begin the understanding mechanism by dropping back to the 1960's. During
this period, investigators began to focus attention away from platinum coated
anodes to the oxides of the platinum group metals. The Periodic Table of Elements
found in chemistry text books identify the platinum - type metals within Groups
IV and VIII. One of the unique properties of many of the oxides of these metals
is their ability to show near - metallic conductivities. As research progressed
it was found that mixtures of these metal oxides showed even more unique properties
with regard to electrochemical requirements such as potential and low consumption
rate. The term mixed metal oxide soon became a by word when referring to this
technology.
Physically, the mixed metal oxides referred to above have little strength or structural
integrity of their own. In order to become useful, they must reside on or be placed
upon a suitable metal substrate. Once again, history was re-visited by way of
the platinum coated anode technology. This earlier technology had shown "film
forming metals" such as titanium to be an ideal candidate as a base for the mixed
metal oxides. Pure titanium is naturally protected by a thin, tightly adherent
oxide film which is acid tolerant and resistant to the passage of current in the
anodic direction. Expressing this differently, in order for titanium to behave
as an anode, considerable applied voltage is required on its surface in order
for it to pass a significant amount of current. This property has earned titanium
the title as one of the "electrochemical valve metals."
The mixed metal oxides now come into play once more. Some of the mixed oxides
(ruthenium, iridium tantalum, etc.) have the ability to form a solid solution
with the oxide film on titanium and render it conductive. Now, a composite anode
has been formed which consists of an inert conductive oxide coating on a "valve
metal" titanium base. The electrochemical characteristics of this anode rests
within the mixed metal oxide coating; the anode strength resides within the titanium
substrate.
An advantage of titanium as a coating base for the mixed metal oxides is its "self
healing" property. If there is a defect or holiday in the conductive coating,
the valve metal immediately forms its protective oxide over the exposed area.
Current from the anode continues to come from the surface covered by the mixed
metal coating.
The mixed oxides are formed by spraying aqueous salts of the metals on to the
titanium substrate and then heating the titanium to temperature of several hundred
degrees Celsius.