Of course, there is not any kind of power source that is never-ending. But the nuclear batteries are the great candidates for the long-lasting energy sources for our daily energy consumption. Think of a battery that almost never ends. Here's what I'm going to tell you about the nuclear batteries that can be used in many different areas. The so-called 'betavoltaic battery' means that these batteries will start to be used as a very effective option in the solution of energy problems.
It is good to talk about some photovoltaic batteries before we can make sense of what we will be talking about. Photovoltaics, which can obtain electricity from solar energy, is now known as solar energy systems (solar panels). Solar systems are systems that can be used effectively in a wide range of areas ranging from traffic signaling systems to irrigation systems. Generally, silicon mono- or poly-crystalline solar cells are produced and sold in rectangular or square shape panels. They are framed by connecting them in series or parallel and in a suitable panel.
Solar panels or, in other words, photovoltaic batteries do this by a physics principle called Einstein's 'photo-electric effect'. As is known, a semiconductor material is neither fully conductive nor fully insulator. When we look at the energy diagrams which are called the energy band, we see that there is a banned (due to quantum) energy gap between the conduction band and the valence band. Now let me try to explain what all these are by using a slightly simpler language. The conduction band shows the level of the energies of charge carriers in the semiconductor (ie, the particles who makes the conduction, which is mostly electrons). Meaning that it is the region (in terms of energy) of particles of the conduction in the semiconductor material. On the other hand, most of us know that not all the current carriers (mostly electrons) make a contribution to the current. This case is even valid for metals which are the best conductors. Metal is a conductive material that conducts even the smallest portion of electrical energy. Even for the metals, not all the electrons are for conducting the electricity. However, there are electrons that travel in the orbits of the atoms, and they are not responsible for the conduction. These are called valence electrons, and the energy band formed by them is called the valence band. Now that we summarize here, the conduction band is the region in which the conduction is in question, the valence band is the region in which the conduction is not in question. OK, we understand. There are electrons in the conduction band that contribute to the conduction, and there are also other electrons who do not contribute to conduction which dwell in the valence band. So for a semiconductor, what's between the valence band and the conduction band?
Here's a banned energy band gap as you can see in the above image. What do we mean by banned bands? No electron (or hole) in this energy zone can be involved in the conduction region or in an atomic orbit. What's the importance of this place for us? It is really very important. The most characteristic feature of various semiconductor materials, from diodes to transistors, depends on the size of this forbidden bandgap here. This is where it is decided to be a silicon, a germanium or other types of semiconductor material for photovoltaic battery applications. The diode is a semiconductor device formed by a combination of two different types of semiconductor (n and p-type) materials. Photovoltaic batteries are also diodes. At this point, I know perhaps you are going to ask that ' what this p and n-type?'. Calm down I'm going to touch on it very briefly.
If you expose any kind of suitable material at a certain temperature under suitable process conditions to a semiconductor material, then your semiconductor material becomes a doped semiconductor material. If you ask 'So what is the sense of this doping?' And I would answer this 'that is what determines a semiconductor as p or n-type'. By doping the material this way, you can obtain a semiconductor material with either a high negative (-) current carrier density (that is, electrons) or a high positive (+) current carrier density. Materials with high negative carrier density are n-type semiconductors. Their Fermi Energy levels are close to their conduction bands. A little homework for you: What is Fermi Energy? Please do a little research and learn it. On the other hand, if the positive charge carrier density is higher, then this material is a p-type semiconductor. But, at this point, please pay attention to that; I already defined that the negative charge carriers are electrons. However, I didn’t say what positive charge carriers are. They are called ‘holes’ with no electrons that must be located in their locations. For example, if electrons move from a point to b point in a wire, we say that the current flow in this wire is from b to a. So how should we consider those holes if we think of holes as the current carriers? This time we shouldn’t forget that the real physical particles of current conduction are electrons. When those electrons go from one side to the other, they leave their holes behind them, so you can think of these holes as if they were moving. It can also be thought of as another type of conduction model. But as you can see, it is again the electron that actually does the real physical work. The hole is like something that seems to be doing it here! For this reason, holes are considered quasi-particles in physics. Anyway, enough of this part I think ...
In a p-n diode as in the above figure, the region where the p and n semiconductors are connected is called a p-n contact. This contact includes an electric field and thus has a potential difference depending on this region’s characteristics, ie a barrier that the electrons can simply run over them. The essence of a diode is hidden here at this point. By changing the voltage you apply to the metal contacts on a 'p' and an 'n', you can obtain a current flow in the diode. This current usually becomes a reverse bias. The point we have to be aware of here is the potential barrier in the p-n contact which I just mentioned. This barrier, which is formed of two different types of semiconductors, causes a change of the banned gap between the energy bands that I've described in the previous paragraphs. An electron which makes the conductance from the n-type semiconductor must overcome that barrier in that region when passing to the p-type semiconductor. You can do this by applying a voltage difference on that electron, ie by increasing its energy. But in the case of photovoltaic applications, the energy from the outside is light. Yeah, it's light. As I mentioned at the beginning of this article, this physical phenomenon is called the photoelectric effect.
If you ask a physicist 'what is light?' he will answer 'according to which theory?'. Don’t ask "What does that mean, huh?"! Indeed, light is a two-way thing. That is to say either by electromagnetic theory or by quantum mechanics. The photoelectric effect is a phenomenon that explains the light based on quantum mechanics. Not based on electromagnetic theory. According to the electromagnetic theory, light is a wave that oscillates by time and has magnetic and electric field components that are perpendicular to each other. But if you try to describe it with quantum mechanics, you should leave this explanation aside. According to quantum physics, light is the energy package that is depended on its frequency. So, in fact, it is, in a sense, a particle. Okay, okay, think of it as a ball. This energy pack (the particle) is called a photon. This photon, which falls on the material with a certain energy, ejects electrons from that material (semiconductor) depending on the size of its energy and throws these electrons to the potential barrier in the p-n junction. Here, a DC (direct current) electric energy is generated and we get this energy from the back of the solar panel.
I think we're good so far without being bored! From now on I would like to talk about nuclear batteries or namely the betavoltaic battery. When we try to understand the operation of a betavoltaic battery, we clearly see that it resembles photovoltaics in a similar way as described above. It's a diode. It may be a 'p-n' or 'n-p' or 'p-i-n' one. Let me guess that now you say that 'Okay we knew, p-n and n-p, but what the heck is p-i-n bro!'. Here, 'i' denotes a semiconductor material which is an intrinsic one. I have already stated that the majority of the carriers of a p-type semiconductor are the 'holes' and the majority of the carriers of an n-type semiconductor are 'electrons'. This 'intrinsic' means that the number of both electrons and holes is equal to each other. Well, why do you need something like an 'intrinsic region'? Because: If you send a beta particle on a betavoltaic cell just like the light in the photovoltaic, this particle will enter into this material. Then, as a result of collisions in the material, it will transfer its energy to atoms. The electrons that are ejected by means of the energy of these beta particle transfers are sent to the junction. And as a result, it provides us power with a certain 'conversion efficiency'. The more the particle collides with the atoms, the more we will expect the current to rise. In this context, 'i' is added directly to the p-n junction in the p-i-n diode to make the path that the beta particle can travel longer.
Here, we should mention a little bit of 'beta' ... The question of how the materials become radioactive' is important at this point. Because, as is known, every material (here, the source) is not radioactive. We know the fact that radioactive materials are the materials that scare us and emit radiation. What we call radiation is actually the light with a very high frequency according to electromagnetic theory. According to QM, this is a photon-like particle. These particle types are divided into varieties according to their energies. Alpha particles are the ones that have a relatively low range to emit. Then there are beta and gamma rays, respectively. We can easily state that gamma is the most dangerous one and has the highest energy. The alpha has a very short range that it cannot pass through a thick sheet of paper or a thin iron plate. To be honest, we need to think of these particles as nuclei of atoms that are stripped of their electrons. In fact, we should mention that the word 'nuclear' comes from this atomic nucleus. We're not talking about freaky particles when we say 'high-energy particles'. We are talking about the atomic nucleus particles that are thrown out at a certain speed (with energy), just like a bullet from the radioactive material. We know that the atomic nucleus is formed by neutrons and protons.
How is it defined as an atom to be radioactive? No atom in nature wants to be unstable. In no way. The number of protons in the atomic nucleus is equal to the number of electrons in the orbit of the atom. In this way, atoms with the number of protons equal to the number of neutrons are stable elements. And if you don't force them into anything, they’ll just stop forever. If you eject an electron or something like that, then they want to get their electrons from another atom because they are chemically unstable. But if you do this job in the form of decreasing or increasing one or more protons from the nucleus of the atom (which is not so easy) then that atom goes crazy and starts to shoot out high energy particles very insanely. It goes on emission until it reaches half of its mass. This may take thousands of years. Meanwhile, The'atomic bomb' concept is also originating from sending something to the nuclei of the atoms. For our betavoltaic battery, the event in the material is like this: A proton in the atom suddenly turns into a neutron. As a result of this maniac indecision, the atom living its inner delusions never stops. It immediately throws a particle equal to an electron mass. Here it becomes our beta particle.
If you've noticed that I did not mention any kind of nuclear reaction, fission or fusion about these nuclear batteries. Therefore, these devices are not subject to any explosion, uncontrollable reaction, or horrendous radiation rains. Although the betavoltaic batteries may sound like a sort of atomic bomb type, it does not represent a risk or danger in this context, as there is no fission or fusion reaction. The source of beta particles, which provides radioactivity we need, is usually radio-isotopes. They can be found in nature or can be obtained manually. The Ni63 source is now used as the most common source of choice for betavoltaic batteries. A betavoltaic battery is connected to PCBs with bonded pins from its contacts after microfabrication. Below is a picture of a betavoltaic battery that I fabricated. A Ni63 source is placed about 1 mm away from the sample mounted on a PCB with an adhesive. This source makes the beta emission of particular energy (eg 17keV) directly to the diodes on the sample. As a result, we can take the current we expect from the sample.
There is no doubt that Betavoltaic batteries will be the energy generators of the future. We only deal with development, optimization, and better productivity. As far as I know, there are even commercial ones in the market. I'm talking about a battery that will provide you with energy for around 30 years, so you don't need to charge it anymore. If another type of source is developed and implemented instead of the Ni63, perhaps we will have batteries that will be able to provide energy for thousands of years. Normal batteries provide us with electrical energy as a result of a series of chemical reactions. The result of these chemical reactions is actually 'warming'. But there is no such problem in betavoltaics. For this reason, instead of using conventional lithium-ion batteries, their use means a much more effective source of energy for laptops and mobile phones. You can check out some other further reading and reference links, and you can get more information about this.
References and Further Reading
- This nuclear battery could power your smartphone forever – as long as you don’t value your life or sperm count too highly
- Prototype nuclear battery packs 10 times more power
- A 25-Year Battery
- Betavoltaic device
- Betavoltaic Devices
- Beta Decay
- What is Photovoltaics?
- How A PV System Works
- Understanding the PN Junction
- 6mm nuclear battery lasts for 100 years
