Towards an Electrically Conductive Ceramic

1.1 INTRODUCTION

It may on first consideration seem implausible that monolithic titanium dioxide ceramic, well-known as an electrical insulator/dielectric material, could ever be transformed into an electrically conductive material, with a potential application as a commercial electrode to rank alongside existing materials such as graphite, magnetite, silicon iron and the family of noble metal/oxide-coated titanium anodes. Yet this is now the case, and what follows is a first-hand account of its inception in an industrial metallurgical laboratory, and the successes and failures, as those involved, guided largely by intuition, backed with empirical trials, felt their way towards a useful and totally novel material.

The work described, was carried out in the Central Research & Development Laboratories of Messrs IMI plc, at Witton, Birmingham, and began in 1981. As it became apparent that a novel, electrically conductive and relatively corrosion-resistant material was within reach, there were some who felt it might provide an alternative substrate to titanium as a basis for precious metal (or oxide) coated electrodes to be used in aggressive electrolytes, such as the strong sulphuric acid used in many metal electro winning processes. However the commercial incentive for change from the widely-used lead-silver alloy anodes was weak, and it seemed to some, too big a step to introduce a brittle ceramic into an industry where large and robust components were handled with abandon .. Others contemplated an opportunity for the electrode material in reversing-polarity swimming pool electrochlorinators, a vast market. Yet others, from a manufacturing background, perceived improved methods of manufacture, leading to a much wider range of possible applications, including lower weight lead-acid-type batteries than hitherto possible. These and other applications are discussed below.

The material, for which the registered Trademark "Ebonex"™, was secured, because of its lustrous dark grey or black appearance, has been commercially available for some years now, and has been used in a range of applications, principally cathodic protection (Fig 1-1). Originally registered to Messrs Marston Palmer Ltd (a wholly-owned subsidiary of IMI Ltd), the Trademark is now the property of Messrs Atraverda Ltd, and covers a range of electrically-conducting sub-oxides of titanium. Many other applications remain to be explored. Some are contingent on further improvement in manufacturing technique, while others, as is so often the case, have to surmount the barrier posed by existing investment in long-established plant and processes. Significant capital investment is called for before new markets can be addressed and such ventures are not without risk.

1.2 THE PATH TO A NEW MATERIAL

A description of the events leading up to the identification of Ti4 O7, as an electrically conductive ceramic and a possible electrode material here begins with a series of paragraphs that might seem disjointed and non-sequitur, but which – taken together – describe how the concept of the all-titanium oxide electrode came about.

The 1970's and 1980's were a time when many of the graphite-based anodes used in the chlor-alkali industry (including production of chlorine, chlorate and hypochlorite) were replaced by low overpotential, noble metal oxide-type, coated titanium electrodes. The incentives for phasing out graphite anodes were powerful. Most chlor-alkali processes are operated at high current density meaning that quite small savings in cell voltage (typically 0.3V or so) result in substantial long-term energy savings. So rapid and rewarding was the introduction of the new coated anodes that, over and above electrode re-coating/replacement market, the electrode industry set its sights on related markets, that is on the introduction of titanium based electrodes in other processes, notably electrowinning of metals, where huge quantities of electricity were used. The anode manufacturers were confident that in these industries too, impressive cost-savings could be won. The most obvious target industries were those involved in metal winning, principally zinc, copper and manganese.

Electrowinning is, for the most part, carried out at much lower current densities than those used in chlor-alkali manufacture, and to this extent, energy savings might be expected to be smaller. The electrolyte used, however is usually strong sulphuric acid, often containing significant concentrations of hydrofluoric acid. While titanium can broadly be considered to be highly corrosion-resistant to most chlor-alkali electrolytes, the same is not true in strong sulphuric acid, even when anodically polarized. The presence of hydrofluoric acid in these electrolytes further increases corrosion rates. The result of such attack is a tendency to undermine the applied noble metal oxide electrocatalyst coatings, thereby shortening the electrode life.

The substrate attack described above, posed a dilemma for those seeking to move into these huge new markets by developing replacement anodes for the lead-silver alloys widely used in electrowinning. (For completeness, it should be noted that there have been extensive studies of other binary, ternary and quaternary lead-based alloys to replace the conventional lead silver. Some did show an apparent superiority, but for various reasons do not appear to have been widely introduced on an industrial scale). The obvious approach was to deposit coatings which would be so impervious to electrolyte as effectively to hide the substrate titanium for most of the electrode useable life. Another approach related to...