Certain doped-oxide ceramics resist Ohm's Law
For months, Anthony West could hardly believe what he and his colleagues were seeing in the lab -- or the only explanation for the unexpected phenomena that seemed to make sense.
Several of the slightly doped high-purity barium titanate (BT) ceramics his research group was investigating were not following the venerable Ohm's Law, which relates electrical voltage to current and resistance. Applying or removing a voltage caused a gradual change in the materials' electrical resistance. The new effect was seen consistently regardless of the temperature or whether the experiments were conducted in vacuum, air, or in an oxygen atmosphere. The time to stabilize and the final, steady-state resistance were, however, both temperature-dependent.
"I was not immediately convinced myself about the non-Ohm's Law behavior," said West, Professor of Electroceramics and Solid State Chemistry at the University of Sheffield in England. "Interfacial effects are well known for their non-Ohmic behavior. We needed to be really convinced that our results were not influenced in some way by interfacial effects."
West's proposed mechanism for the non-Ohm behavior is also unconventional: the ionization of only one of the two extra electrons from oxygen atoms that are attached to dopant atoms. This process leaves behind a positively charged "hole" that can move fairly readily in what is called a hole current. West and his colleagues at Sheffield and the Universidat Jaume 1 in Castellon, Spain, described their latest experiments with calcium-doped BT in the journal Applied Physics Letters, which is published by the American Institute of Physics. Similar results with zinc and magnesium dopants were published earlier this year in other technical journals. Calcium, zinc and magnesium are known as "acceptor" dopants, which can promote hole currents.
Undoped BT and "donor"-doped materials did not exhibit this unusual behavior. West believes that these results may ultimately lead to a better understanding of how ceramics used in electrical circuits degrade and may possibly even stimulate new insights into high-temperature superconductivity mechanisms in oxide ceramics.