Page 44 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

History 25

Figure 2.1 shows one of the many designs of the Nernst lamp [Zl]. When the
lamp was switched on, the voltage was applied to the Nernst rod, h, and to
the parallel heating resistor, i. Both these components were incorporated in a
glass envelope containing air. After sufficient preheating, the current started
flowing through the Nernst rod h and through the winding k of an electromagnet b.
At a specified electric current the magnet switched off the heater by opening the
contacts between w1 and I and then the Nernst rod carried all the current and
emitted light due to resistive heat generation.

Figure2.l ArrangementofaNernstglowerinalamp(DRP114241,filed 9April1899).

The light efficiency of the Nernst lamp exceeded that of the carbon filament
lamp by nearly 80%. However, there were problems. It was difficult to fabricate
reliable contacts to the glower, and the platinum leads and heater made the lamp
expensive. The glowers had to be prevented from melting with the aid of special
series resistors. It was necessary to wait in darkness for half a minute after
switching on the lamp until the light appeared. In view of these and other
disadvantages, interest in the Nernst lamp, although considerable for a few
years, soon disappeared with the introduction of the first tungsten filament
lamps, which were much simpler and permitted a substantial increase in the
light efficiency by raising the filament temperature.
The Nernst zirconia rods were similar to metallic conductors in that
decomposition did not occur with the passage of direct current. Nevertheless,
Nernst was convinced that his filaments were ionic conductors, and he assumed
that, e.g. in yttria-stabilised zirconia (YSZ), the yttria provided the necessary
charge carriers [ 161. He observed evidence of oxygen transport, but believed that
metal cations were also deposited by the direct current, later oxidising and
diffusing back into the filament.
It was not until 1943 that Wagner [22] (in memory of WaIther Nernst who
died on 18 November 194 1) recognised the existence of vacancies in the anion
sublattice of mixed oxide solid solutions and thus explained the conduction
mechanism of the Nernst glowers. We now know that Nernst lamp filaments