Raytheon Technology Today 2011 Issue 1

to the patient. Although in use for a number
of years, this application was replaced
several years ago when improvements in
pacemaker technology reduced energy
requirements to the point where lithium
battery technology became viable.
Betavoltaics
Betavoltaics, another form of atomic battery,
are the little brothers to RTGs; the
difference is that this energy source is not
based on the heat generated, but on its
ability to generate sufficient quantities
of material-ionizing beta particles. While
betavoltaics are similar in concept to photovoltaic
cells, there is a notable difference.
Where photovoltaic cells harvest energy
from interacting photons, betavoltaics function
by capturing and converting the kinetic
energy of energetic electrons, emitted from
decaying radioactive isotopes, into large
amounts of secondary electrons.
Betavoltaics-powered devices may be
engineered to be extremely robust. Since
the source of power is electrons emitted
from the isolated atomic nucleus, electron
emission rates are immune from effects of
stressful, harsh environmental conditions.
Since this technology is based on feature
sizes on the scale of an atom, betavoltaics
show potential improvement in both
energy density and total energy content,
compared with conventional power sources
such as AA batteries. This large energy
density is attributed to the huge number
of radioisotope atoms contained in a small
amount of material (recall Avogadro’s number),
and each atom is primed to unload its
Type
Lithium AA
Battery
Betavoltaic
1 cm 2
Power
(mW)
~1
(1.5 V)
~0.3
(2 V)
Total
Energy
(mWh)
energy-generating beta particle emission at
a rate that is only dependent, in a statistical
manner, upon the particular isotope’s
half life. This advantage in energy density is
indicated in Table 1, which shows a relative
comparison of capabilities for a notional
betavoltaic battery design with those of a
typical lithium AA battery.
Two betavoltaic manifestations are
possible: the so-called direct conversion
category, where secondary electron-hole
pairs are generated in P-N semiconductor
diodes, or the vibrating cantilever concept
that converts mechanical energy to electrical
energy using a piezoelectric-driven, energyscavenging
mechanical converter. Miniature,
low-powered technology devices, based
on either of these two general operational
classes, hold the potential for the development
and integration of tiny smart sensors
that will never need their power supplies
replaced. Specific designs based on
atomic batteries are customized for their
intended applications; some of the basics
that help dictate the design are briefly
discussed below.
Direct Conversion Betavoltaics
One unique rendering of betavoltaics is
the direct conversion approach based on
a P-N semiconductor diode such as gallium
nitride (GaN) placed in direct contact
with a source of beta particles. Figure 2
is a notional design for the P-N junction
method. In the figure, the source adjacent
to the semiconductor is a thin plated film
layer of a beta particle-emitting isotope. A
typical useful source for these applications
Volume
(cm 3 )
Weight
(g)
Total
Energy
Density
(mWh/g)
4,350 7.9 14.5 300
10,512 0.025 0.08 131,400
Table 1. Comparison of a lithium AA battery with conceptual betavoltaic power source.
Source: M.V.S Chandrashekhar, et al., “Design and Fabrication of a 4H SiC Betavoltaic Cell,”
Cornell University.
I GEN
+
−
Feature
Incident beta radiation
p-semiconductor
n-semiconductor
Built-in
electric
field
Figure 2. Schematic betavoltaics P-N junction
power source. One betavoltaic conceptual
design configuration is based on “direct
conversion” that derives small currents from
electron-hole pairs produced by impinging
beta rays in P-N junction depletion zones.
is a 5-micron layer of the pure beta particleemitting
isotope Ni-63, providing an activity
of roughly 0.25 milliCuries, that emit beta
particles over a wide range of energies, with
an average energy of 17 kiloelectronvolts
(keV) and peaking at 67 keV. On average,
half of all emitted beta particles are transported
toward the semiconductor P-layer
where, upon interacting with the material,
some beta particles are backscattered from
the interface and do not penetrate into the
semiconductor.
Those beta particles that make it into the
semiconductor begin losing energy quickly,
primarily through ionization, generating
electron-hole pairs that are captured once
all their energy is dissipated. Beta-particle
path lengths depend on initial beta-particle
energy and the material through which it
is transported; in general, they are in the
range of a few tens of micrometers. For this
energy transfer to be effective as a power
source, beta particles should be able to
reach deep enough into the semiconductor
to deposit most of their energy, through
ionization, in the P-N junction depletion
region. Those electron-hole pairs generated
in the depletion region — where the
number of pairs depends on material band
gap and beta energy — are swept across
the junction by the generated electric field
and are converted into useful electricity to
power an attached load (Figure 2).
Continued on page 14
RAYTHEON TECHNOLOGY TODAY 2011 ISSUE 1 13