The superconductivity community has been agog for the last 6 weeks,
digesting and expanding on the results of research into magnesium diboride
(MgB2). The story had been embargoed until 4pm today, and I just returned
from the press conference (AIP/APS March meeting in Seattle [1]).
Here's an overview article from USAToday:
http://www.usatoday.com/news/science/2001-03-09-superconductor.htm
However, that doesn't include details from the press conference. Here are
some juicy tidbits I picked up...
* First, the discoverers, who were described as "scientists in Japan" by
that USAToday article, are actually Jun Akimitsu and his four students.
The new issue of Nature's cover describes Dr. Akimitsu's discovery as
a "serendipitous superconductor", but he says it wasn't serendipitous at
all, but the result of much hard work :-) Akimitsu was really an
outsider to the superconducting community represented by Cava and others
on the panel, but everyone was very respectful to him and appreciative of
his discovery, which the rest of the community then confirmed and
extended.
* Tc is 39 K. That's twice the old temperature (23 K) for intermetallic
superconductors. Magnesium diboride gives new life to the classic
BCS theory, at higher temperatures than before.
* Unlike the ceramic-style superconductors from the late 80s, MgB2 is very
easy to work with. Small wires and films have already been made (two
weeks ago), and there are no known physical reasons (as opposed to
engineering reasons) why kilometer-long wires couldn't be created "soon".
* MgB2 is very cheap. It costs about $5 per pound, and is available off
the shelf in powder form from chemical supply companies. (Chuck, this
could be another basement project for you.)
* As speakers noted today, it was a great superconductivity candidate back
in the 50s, but researchers skipped over it; they were convinced you had
to have 3.8 electrons per atom to be a good superconductor. Theory
blinded them.
* MgB2 wire can be made by taking commercial boron fibers and exposing
them to magnesium vapor at about 950 K. Where do you get boron fibers?
>From Textron, who makes them in up to kilometer length or longer. (Textron
weaves the fibers into a high-tensile strength fabric used by the military
and in very high-end sports equipment(!) However, in the last month the
wires produced have been more like 1/8th inch long. That is because it's
hard to contain a lot of magnesium vapor at 950 K, and because the fiber
likes to curl up during the process. But the panel sounded confident they
could soon improve on that length.
* Right above the transition temperature, resistivity is comparable to
copper wire (instead of other materials that are 20 times worse when not
superconducting).
* At the temperature of 39 K, MgB2 can be cooled with "single stage"
refrigerators, rather than with messy liquid cryogens. This convenience
comes from having a Tc greater than 25 K.
* It can currently carry roughly 2x10^5 amps/cm^2
* Akimitsu says some people think you have to have boron in there to be
superconducting, but he disagrees. Magnesium diberyllium also can
superconduct. In fact, he showed a graph of the resistivity as you
replace beryllium with boron, and the lowest resistance is at
MgBe0.65B0.35.
* Some people have mentioned MRI scanners as a good application, but Paul
Grant said that most MRI machines these days use conventional permanent
(iron) magnets. They started with superconductors in the 60s because the
electronic sensors at the time weren't sensitive enough to work with the
weaker fields from permanent magnets. Nowadays, the electronics are good
enough for clinical work with iron magnets (but not so true in research).
* There are 77 presenters at a special session tonight (8pm). Each
presenter will get two minutes(!) to speak.
-Matt Jensen
NewsBlip.com
Seattle
[1] http://www.aps.org/meet/MAR01/
This archive was generated by hypermail 2b29 : Fri Apr 27 2001 - 23:14:07 PDT