Certain doped-oxide ceramics resist Ohm's Law

Sep 21, 2010

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 to current and resistance. Applying or removing a voltage caused a gradual change in the materials' . 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 that are attached to 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 , 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.

Explore further: Interfaces within materials can be patterned as a means of controlling the properties of composites

More information: The article, "Field enhanced bulk conductivity of acceptor-doped BaTi1-xCaxO3-x ceramics" by Nahum Maso, Marta Prades, Hector Beltran, Eloisa Cordoncillo, Derek C. Sinclair, and Anthony R. West appears in the journal Applied Physics Letters. See: link.aip.org/link/applab/v97/i6/p062907/s1

Provided by American Institute of Physics

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Parsec
3 / 5 (4) Sep 21, 2010
Ohm's law is a real law and cannot be overcome.

While it is normally written with the resistance as a fixed value, if the resistance is a function of current, voltage, or temperature, or anything else, the instantaneous values of voltage, current, and resistance have a fixed relationship. Note that resistance that is also reactive also must follow Ohm's law.. Reactive resistance changes depending on how fast the voltage is changing instant to instant, indeed the history of the voltage changes. But at any instant, the current is determined by the voltage and instantaneous value of the total resistance.
TDK
2.5 / 5 (2) Sep 21, 2010
Maybe it's a sort of memristor effect. Many varistors have quite nonlinear characteristics, too.
holoman
not rated yet Sep 21, 2010
Ceramic is ferroelectric and possibly multiferroic
MorituriMax
not rated yet Sep 21, 2010
Parsec, maybe it ISN'T a real law in every situation. Don't we get no resistance to current flow in materials if they can be modified to become superconductive? Hmmm?

Perhaps we need to adjust the "law" as we discover that there are situations where the "law" breaks down. Just hiding our heads in the sand and ignoring new science isn't very productive.
MaxwellsDemon
5 / 5 (4) Sep 22, 2010
Ohm's Law isn't a fundamental physical law like Conservation of Energy: it simply describes the property of some conductive materials that they have a constant resistance at a range of currents: http://maxwell.by...00000000

Lots of materials are non-ohmic, such as semiconductors: http://science.jr...ors.html

And superconductors don't really violate Ohm's Law, because they have a constant zero resistance over a range of currents as long as they're superconducting.

The hubbub here seems to be over this behavior in Ca-doped BaTiO3 ceramic conductors, and the action mechanism, which may (or may not) someday reveal something useful about superconductivity.