Ordinary batteries powering clocks, flashlights, toys, and other
electrical devices use the energy of so-called redox chemical reactions in
which electrons are transferred from one electrode to another via an
electrolyte. This gives rise to a potential difference between the electrodes.
If the two battery terminals are then connected by a
conductor, electrons start flowing to remove the potential difference,
generating an electric current. Chemical batteries, also known as galvanic
cells, are characterized by a high power
density—that is, the ratio between the power of
the generated current and the volume of the battery. However, chemical cells
discharge in a relatively short time, limiting their applications in autonomous
devices. Some of these batteries, called accumulators, are rechargeable, but
even they need to be replaced for charging. This may be dangerous, as in the
case of a cardiac pacemaker, or even impossible, if the battery is powering a
spacecraft.
Fortunately, chemical reactions are just one of the possible sources of
electric power. In 1913, Henry Moseley invented the first power generator based
on radioactive decay. His nuclear battery consisted of a glass sphere
silvered on the inside with a radium emitter mounted at the center on an
isolated electrode. Electrons resulting from the beta decay of
radium caused a potential difference between the silver film and the central
electrode. However, the idle voltage of the device was way too high—tens of
kilovolts—and the current was too low for practical applications.
The nuclear battery prototype consisted of 200 diamond converters
interlaid with nickel-63 and stable nickel foil layers. The amount
of power generated by the converter depends on the thickness of the nickel foil
and the converter itself, because both affect how many beta particles are
absorbed. Currently available prototypes of nuclear batteries are poorly
optimized, since they have excessive volume. If the beta radiation source is
too thick, the electrons it emits cannot escape it. This effect is known as
self-absorption. However, as the source is made thinner, the number of atoms
undergoing beta decay per unit time is proportionally reduced. Similar
reasoning applies to the thickness of the converter.
The goal of the researchers was to maximize the power density of their
nickel-63 battery. To do this, they numerically simulated the passage of
electrons through the beta source and the converters. It turned out that the
nickel-63 source is at its most effective when it is 2 micrometers thick, and
the optimal thickness of the converter based on Schottky barrier diamond diodes
is around 10 micrometers.
Manufacturing technology
The main technological challenge was the fabrication of a large number
of diamond conversion cells with complex internal structure. Each converter was
merely tens of micrometers thick, like a plastic bag in a supermarket.
Conventional mechanical and ionic techniques of diamond thinning were not
suitable for this task. The researchers from TISNCM and MIPT developed a unique
technology for synthesizing thin diamond plates on a diamond substrate and
splitting them off to mass-produce ultrathin converters.
The team used 20 thick boron-doped diamond crystal plates as the
substrate. They were grown using the temperature gradient technique under high
pressure. Ion implantation was used to create a 100-nanometer-thick defective,
"damaged" layer in the substrate at the depth of about 700
nanometers. A boron-doped diamond film 15 micrometers thick was grown on top of
this layer using chemical vapor deposition. The substrate then underwent
high-temperature annealing to induce graphitization of the buried defective
layer and recover the top diamond layer. Electrochemical etching was used to
remove the damaged layer. Following the separation of the defective layer by
etching, the semi-finished converter was fitted with ohmic and Schottky
contacts.
As the operations were repeated, the loss of substrate thickness
amounted to no more than 1 micrometer per cycle. A total of 200 converters were
grown on 20 substrates. This new technology is important from an economic
standpoint, because high-quality diamond substrates are very expensive and
therefore mass-production of converters by substrate thinning is not feasible.
All converters were connected in parallel in a stack. The technology for rolling 2-micrometer-thick nickel foil was developed at
the Research Institute and Scientific Industrial Association LUCH. The battery
was sealed with epoxy.
The prototype battery is characterized by the current-voltage curve
shown in figure 3a. The open-circuit voltage and the short-circuit current are
1.02 volts and 1.27 microamperes, respectively. The maximum output power of
0.93 microwatts is obtained at 0.92 volts. This power output corresponds to a
specific power of about 3,300 milliwatt-hours per gramm, which is 10 times more
than in commercial chemical cells or the previous nickel-63 nuclear battery
designed at TISNCM.
The main setback in commercializing nuclear batteries in Russia is the
lack of nickel-63 production and enrichment facilities. However, there are
plans to launch nickel-63 production on an industrial scale by mid-2020s.
There is an alternative radioisotope for use in nuclear batteries:
Dimond converters could be made using radioactive carbon-14, which has an
extremely long half-life of 5,700 years. Work on such generators was earlier
reported by physicists from the University of Bristol.
Nuclear batteries: Prospects
The work reported in this story has prospects for medical applications.
Most state-of-the-art cardiac pacemakers are over 10 cubic centimeters in size
and require about 10 microwatts of power. This means that the new nuclear
battery could be used to power these devices without any significant changes to
their design and size. "Perpetual pacemakers" whose batteries need
not be replaced or serviced would improve the quality of life of patients.
The space industry would also greatly benefit from compact nuclear
batteries. In particular, there is a demand for autonomous wireless external
sensors and memory chips with integrated power supply systems for spacecraft.
Diamond is one of the most radiation-proof semiconductors. Since it also has a
large bandgap, it can operate in a wide range of temperatures, making it the
ideal material for nuclear batteries powering spacecraft.
The researchers are planning to continue their work on nuclear
batteries. They have identified several lines of inquiry that should be
pursued. Firstly, enriching nickel-63 in the radiation source would
proportionally increase battery power. Secondly, developing a diamond p-i-n
structure with a controlled doping profile would boost voltage and therefore
could increase the power output of the battery at least by a factor of three.
Thirdly, enhancing the surface area of the converter would increase the number
of nickel-63 atoms on each converter.
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