Boron Nitride helps to build Hall Effect Thruster
1. Travel in space needs high efficient thrusters
When the satellite is in operation in space, it will gradually reduce its speed due to the resistance of a small amount of gas in orbit. As a result, there will be a decrease in height and thus more air resistance. So, it requires additional power to supplement the kinetic energy of the satellite or other space. This task does not require a high thrust level, but extremely high thrust production efficiency.
The advantage of chemical rockets is that they can deliver enormous amounts of energy in a short period of time. However, they are incredibly inefficient. A typical chemical rocket only has about 35% energy efficiency. Also, as the speed of propellent throw behind is limited, these chemical rockets used too much matter to obtain the thrust. Then, the electromagnetic propulsion system shows its advantages
2. How ion engines work
As an example, hall effect thrusters combine electric field and magnetic field to ionize a gas stream (usually xenon) and then produce plasma. Next, the electric field in the circular channel accelerates the ions in the plasma to a very high exhaust velocity. Although the thrust would be very "gentle", the velocity of ion can be very fast. So, the efficiency of propellent is very high, compared with traditional chemical rockets, and the spacecraft could build up a high speed over time.
On the very basic level, the hall effect thrusters work by accelerating charged particles, or in other words ions. Firstly, it starts with a circular channel. Between the internal and outer channels, there are magnetic coils that generate a magnetic field. At the bottom of the channel, there is an electrically charged plate called an anode, and it creates a strong electron field. Outside the channel, there is a cathode to emit electrons.
When the thruster is powered up, the cathode starts releasing electron. The electron are attracted to the anode so they flying into the channel. There, they are caught by the magnetic field and start to zooming in circles around the channel. Then the thruster will pump out a bit of propellant into the channel, usually a neutral gas like xenon (Xe). The gas-particle gets hit by the incoming electrons and that hit knocks off some of its electrons. When an electron was lost, the gas-particle becomes a positively charged ion. At that moment, the electron field in the channel pushes those gas ions out of the channel at a very high speed and generates the thrust to move the spacecraft forward.
As the ions and electrons hit each other, the hall effect thruster emits light when it is working. And the color of light could vary with the different working matters.
3. Why it's better in deepspace
In space travel far away from earth, as the spacecraft is moving forwards in a vacuum, it is extremely hard to get a supply of matter, while energy could be supplied by solar panels or nuclear reactors. According to Newton's 3rd law, it is not hard to know that if you have no matter to throw, you have no way to accelerate. As a result, the faster the thruster could push back the propellant, the higher the efficiency of matter it carried. The most efficient chemical rocket can throw hot gases out the back at about 5km/s, while ion engines can eject particles at about 90km/s. This high velocity gives the spacecraft much more efficiency in using propellent to obtain acceleration. So, hall effect thrusters and other ion engines will lead the way for human beings to travel into deep space.
4. Materials to build a hall effect thruster
There are many different types of materials involved in building a hall effect thruster (HET). For example, as the plasma in the circular channel of HET is highly reactive with a lot of material, it needs to be made of ceramic materials with a high dielectric constant. Boron nitride (BN) is an ideal material for this task. Fortunately, boron nitride ceramic is machinable, so it could be easily made into complicated shapes to fulfill the requirement of the ion engine. Large pieces of material can be sintered by the hot pressing furnace, and then the required shape can be obtained by machining. The only problem with this is that its size is limited by the sintering furnace and generally cannot exceed 500mm diameter. At present, the size of most Hall effect thrusters is still very small. However, with the continuous progress of large spacecraft, the demand for material manufacturing will also increase.