Applications envisioned for medicine employ ultra sensitive instruments
that can analyze brain currents without surgery, providing a more
accurate diagnosis of disease and injury while reducing trauma to
the patient. Superconductors can greatly increase the capacity of
cellular telecommunications by increasing the number of available
channels in the available bandwidth. Most of these exciting applications
for high temperature superconductors await improvements in these
new materials, a focus area for the MURR Center research.
Neutron irradiation is used to introduce well-characterized
defects into semiconductors and high temperature superconductors.
In the case of semiconductors, there are two basic
types of experiments. First, thermal neutrons are used to produce
transmutations.
In favorable circumstances, the transmutations introduce desirable
electronic doping that often cannot be achieved by other doping
methods. Transmutation doping of silicon is done on a commercial
scale to provide the starting material for high power electronic
devices. In the case of zinc selenide (ZnSe, used in phosphors and
infrared optics), the transmutation doping is used to investigate
the fundamental process of self-compensation that prevents effective
p-type doping of this material. Second, fast neutrons are utilized
to introduce defects that reduce the minority-carrier lifetime.
The radiation induced defects can decrease switching times significantly,
an important feature in high speed switching devices. In the case
of high temperature superconductors, fast neutron irradiation induced
defects act as pinning centers for quantized magnetic fields called
fluxoids. Pinning these fluxiods is a critical issue, because the
fluxoid motion introduces resistance, thus defeating the purpose
of the superconductor to conduct current without loss. Therefore,
increased fluxoid pinning leads to greater critical currents, making
many of the exciting applications possible. A significant effort
in this research area is involved in understanding the fundament
process of fluxoid pinning and in finding methods to optimize this
pinning, thus producing material that can support the highest possible
currents. At present, critical currents in high temperature superconductors
limit many of the possible applications of these exciting materials.
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University of Missouri-Columbia
