fabricated using polypyrrole (PPy) as electrodes and solidified polyvinyl alcohol (PVA) as electrolyte.
The lateral dimensions of the capacitor were 2 2 mm and the thickness was much less than 1 mm. The
capacitor structure could be deformed, rolled up, etc., without changes in performance. The structure
had a capacitance of several mF and was tested at a maximum voltage of 0.6 V. In principle, such an all-
solid polymer capacitor could have better scalability due to relaxed encapsulation constraints.
It should also be note d that embedded miniature supercapacitors would find many applications in
semiconductor microelectronics. For example, in order to respond to the power integrity problem in
microprocessors, integrated thin-film high capacitances are needed [29]. Currently existing super-
capacitors are not available in a planar format suitable for on-chip integration. The potential for
integrated miniature supercapacitors for microprocessors was recently discussed in [29].
In conclusion, embedded miniature supercapacitors appear to be a potentially feasible power
source, whose estimated upper bound for power lies between 0.1 and 1 W.
2.3 ENERGY FROM RADIOISOTOPES
2.3.1 Radioisotope energy sources
Electrochemical sources described in previous sections have energy output ~1 eV/atom, whichis related to
the energy of inter-atomic bonds. Thus, the maximum energy stored in, e.g., a 10
m
m-sized box is
~10
–5
J. In principle, the ‘intra-atomic’ energy (i.e. of nuclear bonds) is much higher and therefore its
utilization seems very attractive for embedded micro-power sources in size-constrained systems [35–37].
The energy of radionuc lides is released in energetic particles, typically a- (He ions),
b
- (electrons),
and
g
- (electromagnetic radiation) particles. a- and
b
-emission can in principle be utilized in energy
sources. Several examples of a- and
b
-radioisotopes are given in Table 2.3.1.
The energy release by radionuclides can be calculated using the radioactive decay formula:
EðtÞ¼E
at
,NðtÞ¼E
at
N
0
exp
t
s
(2.3.1)
where N
0
is the initial number of the atoms, N(t) is the number of atoms that have not released an
energetic particle by the time t,
s
is the ‘mean life time’ of a radioactive atom, and
e
is the average
energy of the particle released by a radio active atom. For a-emission (discrete energy spectrum,
Fig. 2.3.1a),
3
z E
max
, while for
b
-emission (continuous energy spectrum, Fig. 2.3.1b), the average
energy of electrons is approximately 1/3 of the maximum energy E
max
[39].
Another common characteristic time of radioactive reactions is the radionuclide half-life t
1/2
,
which is related to the mean life time
s
as
t
1=2
¼ s ln 2 (2.3.2)
The total energy released by the radioac tive sources is
E ¼ 3N
0
(2.3.3)
The average power delivery by a single-atom radioactive reaction is
e
/t and the power of N radioactive
atoms is
28 CHAPTER 2 Energy in the small: Integrated micro-scale energy sources
Table 2.3.1 Characteristic parameters of several radioisotopes
Radioisotope
(1)
E
max
,
eV
(2)
s
(s)
(3)
L
(E
max
),
m
m [5,6]
(4)
J/cm
3 (5)
W/cm
3
Comment
(a)
210
Po 5.4 10
6
1.7 10
7
w26 2.3 10
10
1.4 10
3
Has been
investigated as
a light-weight
energy source
for space
applications
(a)
238
Pu 5.5 10
6
4.0 10
9
w27 4.4 10
10
11 Early use in
pacemakers
(Medtronic,
Numec Corp.
[38])
(
b
)
3
H 1.86 10
4
5.6 10
8
w4 4.9 10
4
8.8 10
5
Used to
produce light in
self-illuminating
watches
(
b
)
63
Ni 6.69 10
4
4.6 10
9
w34 2.3 10
8
5.1 10
2
Demonstrated
[35,45–47]
(
b
)
147
Pm 2.24 10
5
3.3 10
5
w300 3.5 10
8
1.1 10
3
Early use in
Betacell 400
pacemakers [4]
Used in QynCell
batteries
(1)
Maximum energy of an energetic particle.
(2)
Mean life time of a radioactive atom.
(3)
Stopping range in silicon.
(4)
Total energy stored in the radioactive source.
(5)
Total power released by the radioactive source.
Kinetic energy
Intensity
ε
(a)
Kinetic energy
Intensity
E
max
max
3
1
E≈
ε
(b)
FIGURE 2.3.1
Energy spectra of: (a) a-particles (immediately after decay) and (b)
b
-decay electrons
2.3 Energy from radioisotopes 29
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