[ RadSafe ] Silicon Solution Could Lead to a Truly Long-life Battery (using tritium)

[John Jacobus]

I saw this and thought it would be of interest.  The
original is at
http://www.nsf.gov/news/news_summ.jsp?cntn_id=104140

National Science Foundations
Press Release 05-075
Silicon Solution Could Lead to a Truly Long-life
Battery

New devices may provide power for decades

May 10, 2005

Using some of the same manufacturing techniques that
produce microchips, researchers have created a
porous-silicon diode that may lead to improved
betavoltaics. Such devices convert low levels of
radiation into electricity and can have useful lives
spanning several decades.

While producing as little as one-thousandth of the
power of conventional chemical batteries, the new
"BetaBattery" concept is more efficient and
potentially less expensive than similar designs and
should be easier to manufacture. If the new diode
proves successful when incorporated into a finished
battery, it could help power such hard-to-service,
long-life systems as structural sensors on bridges,
climate monitoring equipment and satellites.

The battery's staying power is tied to the enduring
nature of its fuel, tritium, a hydrogen isotope that
releases electrons in a process called beta decay. The
porous-silicon semiconductors generate electricity by
absorbing the electrons, just as a solar cell
generates electricity by absorbing energy from
incoming photons of light.

Supported by grants from the NSF Small Business
Innovation Research (SBIR) program, a
multi-disciplinary team of researchers from the
University of Rochester, the University of Toronto,
Rochester Institute of Technology and BetaBatt, Inc.
of Houston, Texas, describe their new diode in the May
13 issue of Advanced Materials.

Researchers have been attempting to convert various
types of radiation into electricity since the
development of the transistor more than 50 years ago.
Mastering the junctions between relatively
electron-rich and electron-poor regions of
semiconductor material (p-n junctions) led to many
modern electronic products.

Yet, while engineers have been successful at capturing
electromagnetic radiation with solar cells, the flat,
thin devices have been unable to collect enough
beta-decay electrons to yield a viable betavoltaic
device.

The BetaBatt will not be the first battery to harness
a radioactive source, or even the first to use
tritium, but the new cell will have a unique advantage
- the half-millimeter-thick silicon wafer into which
researchers have etched a network of deep pores. This
structure vastly increases the exposed surface area,
creating a device that is 10 times more efficient than
planar designs.

"The 3-D porous silicon configuration is excellent for
absorbing essentially all the kinetic energy of the
source electrons," says co-author Nazir Kherani of the
University of Toronto. Instead of generating current
by absorbing electrons at the outermost layer of a
thin sheet, surfaces deep within these porous silicon
wafers accommodate a much larger amount of incoming
radiation. In early tests, nearly all electrons
emitted during the tritium's beta decay were absorbed.

There were a number of practical reasons for selecting
tritium as the source of energy, says co-author Larry
Gadeken of BetaBatt - particularly safety and
containment.

"Tritium emits only low energy beta particles
(electrons) that can be shielded by very thin
materials, such as a sheet of paper," says Gadeken.
"The hermetically-sealed, metallic BetaBattery cases
will encapsulate the entire radioactive energy source,
just like a normal battery contains its chemical
source so it cannot escape."

Even if the hermetic case were to be breached, adds
Gadeken, the source material the team is developing
will be a hard plastic that incorporates tritium into
its chemical structure. Unlike a chemical paste, the
plastic cannot not leak out or leach into the
surrounding environment.

Researchers and manufacturers have been producing
porous silicon for decades, and it is commonly used
for antireflective coatings, light emitting devices,
and photon filters for fiber optics. However, the
current research is the first patented betavoltaic
application for porous silicon and the first time that
3-D p-n diodes have been created with standard
semiconductor industry techniques.

"The betavoltaic and photovoltaic applications of 3-D
porous silicon diodes will result in an exciting arena
of additional uses for this versatile material," says
co-author Philippe Fauchet of the University of
Rochester.

"This is the first time that uniform p-n junctions
have been made in porous silicon, which is exciting
from the point of view of materials science," says
Fauchet. For example, because of its characteristics
and photon sensitivity, each diode pore could serve as
an individual detector, potentially creating an
extremely high-resolution image sensor.

"The ease of using standard semiconductor processing
technology to fabricate 3-D p-n junctions was
surprising," adds co-author Karl Hirschman of the
Rochester Institute of Technology. That manufacturing
ease is an important breakthrough for increasing
production and lowering costs, and it makes the device
scalable and versatile for a range of applications.

"The initial applications will be for remote or
inaccessible sensors and devices where the
availability of long-life power is critical," says
Gadeken.

The BetaBattery may prove better suited to certain
tasks than chemical batteries when power needs are
limited. The structures are robust--tolerant to motion
and shock, and functional from -148° Fahrenheit (-100°
Celsius) to 302° F (150°C)--and may never have to be
changed for the lifetime of the device.

-NSF-


+++++++++++++++++++
"Embarrassed, obscure and feeble sentences are generally, if not always, the result of embarrassed, obscure and feeble thought."
Hugh Blair, 1783

-- John
John Jacobus, MS
Certified Health Physicist
e-mail:  crispy_bird at yahoo.com


	
		
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