Research on microelectromechanical systems and nanotechnology has made tremendous progress in the past 20 years, and researchers have developed various types of micron and nanoscale devices. However, it is difficult to miniaturize the energy supply device to the corresponding size. Traditional batteries or energy supply devices are still used in micron and nanometer devices, which leads to an increase in the volume of the entire system, frequent charging, or difficulty in arranging battery unit groups. Therefore, researchers Since the 1990s, we have begun to turn our attention to developing various micro-battery technologies. Among them, micro energy generation devices and micro fuel cells based on turbine combustion aim to convert mechanical energy, thermal energy and chemical energy into electrical energy. These technologies all require external microfluidic structures and external energy sources to drive the engine and supply fuel to the working chamber, or to promote chemical reactions to achieve energy conversion. Micro-Ceng batteries are also under research, but these batteries have low energy density and short lifespan. One of the hot spots in research is micro-solar cell arrays, which have the disadvantage of requiring light as the original energy source. Radioactive energy can be used in many different areas such as industry, agriculture and medical services. Energy generation is its most important application area. This is because nuclear energy is in many cases a more efficient method of energy generation than conventional forms of energy generation.
In 1999, researchers at the University of Wisconsin-Madison, with funding from the U.S. Department of Energy, were the first to propose a combination of microelectromechanical systems technology and nuclear energy science and technology to develop micro-nuclear batteries or radioisotopes. Research on the battery subsequently continued at Cornell University in the United States with funding from the U.S. Department of Defense. Many research groups at home and abroad, including the Sabendong Microelectromechanical Research Center of Xiamen University, have also begun to work on this research. Compared with other technologies, micronuclear batteries have application prospects in many fields, especially in applications that require long-term functionality, such as implantable biomedical microdevices and microsensors or sensor networks for environmental monitoring. Energy from radioisotopes The density is twice as high as the energy density of fossil or chemical fuels, and if appropriate radioactive isotopes are selected, long-life micro-nuclear batteries can be achieved.
Space research agencies such as NASA have long recognized the potential of radioactive materials to generate electricity. NASA has adopted the method in a series of space missions starting in the 1960s, such as the Voyager probe and the recently launched Cassini probe currently orbiting Saturn. Radioisotope Thermoelectric Generators (RTG). These space probes are too far from the sun to be powered by solar arrays.
RTG converts thermal energy into electrical energy through the thermoelectric effect (also known as the Seebeck-effect). The so-called Seebeck effect means that when one end of a metal rod (made of two metals or semiconductor materials butted together - Translator's Note) is heated, the electrons at the heated end gain more kinetic energy and flow to the other end. A voltage is generated across both ends. Most of the RTGs used by NASA are about the size of a washing machine and use the high-energy rays of plutonium-238 to generate huge heat energy.
But RTG cannot significantly reduce the size. For microdevices like MEMS, the ratio of their surface area to their volume is very large. The large relative surface area makes the heat loss problem difficult to solve, and to maintain the normal operation of the RTG, a certain temperature must be maintained. So we have to find other ways to convert nuclear energy into electricity.
In early 2003, a micro-battery was developed that can directly convert high-energy particles emitted by radioactive materials into electric current. In this battery, a small amount of nickel-63 is placed near a regular silicon p-n junction (basically a diode). Nickel-63 emits beta particles when it decays. Beta particles are high-energy electrons emitted spontaneously from the unstable nuclei of radioactive isotopes. In a battery, beta particles ionize the atoms of the diode, creating electron-hole pairs. These electrons and holes are divided on both sides of the p-n junction interface. These separated electrons and holes flow in the direction away from the p-n junction, forming an electric current.
Ni-63 is ideal for this application because the beta particles it emits can travel up to 21 μm in the silicon material before decaying.
If a particle has more kinetic energy, it will travel a longer distance and thus radiate outside the cell. In our nuclear battery, each milliCi of nickel-63 produces 3 nanometers (10-9) watts of power. Although the power is not high, it can already power nanomemories and simple microprocessors used in environmental sensors and battlefield sensors being developed by other institutions. The selection of radioisotopes is the most important aspect of realizing micron nuclear batteries, mainly based on radiation type, safety, energy, relative radioactivity, price and half-life. The most important consideration when using radioactive isotopes is always safety. Gamma rays have a strong penetrating ability and require considerable external shielding to reduce the radiation dose ratio. Alpha particles can be used to create electron-hole pairs in semiconductors, but they can cause severe lattice defects. Pure beta ray generators are the best choice for micro nuclear batteries. Table 1 gives the pure Beta radioactive sources considered for micronuclear batteries in our study. Nickel-63 has a radiation life of over 100 years and was chosen as the first choice in our study. The particles or electrons emitted from nickel-63 have an average energy and a maximum energy, which is lower than the 200~250KeV threshold energy that causes permanent damage to the silicon crystal structure
. On the other hand, electrons with the highest motion energy of 67KeV cannot penetrate the outer layer of human skin, which ensures the safety of the operator. The first type of micronuclear battery developed is based on the Beta radial voltaic effect, the flow of positive charges due to electron-hole pairs (EHPs), which create a potential difference. As shown in Figure 1, when EHPs diffuse into the depletion region of the semiconductor pn junction, under the action of the built-in electric field of the pn junction, the separation of electron-hole pairs is achieved, that is, the electrons move to the n region and the holes move to the p region. , produces a current output.
Although the radiative voltaic effect is similar to the photovoltaic effect, the development of micro-nuclear cells is much more difficult than the development of solar cells. The main reason is that the electron flux density in nuclear batteries is lower than the photon flux density in solar cells. For microbatteries, the electron flux density is also reduced due to the use of very low emitting intensity isotopes. The energy distribution of electrons emitted from Beta radioisotopes usually has a very wide spectral range. Electrons with different energies will stay at different depths in semiconductor pn junction devices. Therefore, the spatial distribution of the produced EHPs is different. In order to obtain higher energy output, it is necessary to optimize the design of pn junction devices and adopt micro-fabrication processes to collect EHPs into the depletion layer as much as possible. In fact, most microelectromechanical and nanodevices, as well as low-energy electronic devices, consume energy in the milliwatt range. In order to increase the energy output of micro-nuclear batteries, if allowed, high-energy emitters should be selected with higher radiation intensity. Although the half-life of central radioisotopes is only 2.6 years, their average energy is 62KeV and the maximum energy is 250KeV, which is the highest in silicon. It is allowed in base pn junction devices. As shown in Figure 5, a Beta-type micro-battery using promethium-147 radioactive isotope as the original energy source was designed and produced. The area of ??the planar pn junction device as a battery is 10mm*100mm, and about 200mCi of Ju-147 is used. The measured open-loop voltage is 0.29V, and the short-circuit current is 0.033mA. The maximum output energy is 5.7uW. The next step is to use stacking or chip array connection methods to increase the output voltage of the micro-battery.
Two micro-nuclear batteries used in micro-electromechanical systems and nanodevices, and a Beta-type micro-nuclear battery that uses promethium-147 radioactive isotope to achieve an output of milliwatts is presented.