There are two main groups of superconductor, type I and type II. For the type I superconductors, they normally require a lower temperature to gain superconductivity while having a smaller critical current and magnetic field. Scientists later discovered BCS theory that explained how these superconductors work.
When one electron enters a crystal lattice, it attracts and distorts the lattice formed by positively charged atoms. The distortion of the lattice makes the positive charge condenses, thus attracting another incoming electron due to its negative charge. This phenomenon can be considered as two electrons are bonded by a weak force, and we call these two electrons as a Cooper Pair. For normal substances, based on the Exclusion Principle, their electrons are staying in different energy levels because they have different quantum numbers. When a current starts (electrons begin to move), the electrons with different energy gain different momentum’s, thus colliding with each other and losing energy. This is the why resistance occurs. However, for Cooper Pairs, which are not limited by the Exclusion Principle (two electrons have opposite spin quantum numbers so the overall spin is zero, not considered in the principle), they can travel in a same energy level. When a current starts, all pairs move in a same momentum, without colliding and losing energy, and contributing to the superconductivity. Nevertheless, type I superconductors’ uses are significantly limited by its required low temperature, and BCS theory predicts that their critical temperatures cannot exceed 30-40K.
In 1986, the discovery of copper-oxide-based compounds, or cuprates, completely changed the field of superconductor. These cuprates, like YBCO, can achieve a critical temperature of 93K, going much further than the predicted limit by BCS theory. Although scientists haven’t yet fully understood the mechanism of these type II superconductors, we do know that they have a special layered microstructure, where conducting Cu-O layers are separated by insulating layers consisted of other elements like yttrium.
With type II superconductors, we are never so close to a future with zero energy loss in energy transition, flying trains and cars, and more. Now, let’s make a superconductor together.
Material & Instrument
Anhydrous Citric Acid
Several Tall Beakers (>300mL)
One Magnetic Stirrer & Heater (>320ºC)
One Vacuum Dryer & Its Vacuum Pump
One pH meter
One Tube Furnace
One Oxygen Generator
One Hydraulic Pellet Press (>15T)
One Mortar & Pestle
One Pellet Press Die Set (<20mm diameter)
One Electrical Balance (able to measure <0.1mg)
To make YBCO, yttrium nitrate is required. However, this chemical is generally impure and unavailable in the market, so we need to first convert Y2O3 into Y(NO3)3. This involves mixing Y2O3 with concentrated nitric acid.
Firstly, I mix 20g of Y2O3 with 100ml H2O + 200ml conc. HNO3, heating the solution with a 120ºC hot plate while magnetically stirring it. The amount of chemicals here is not critical, and the temperature is set as higher than the boiling point since the following reaction it endothermic and the water will not boil.
Then, when the solution is hot enough, the reaction happens, clearing up the solution as insoluble Y2O3 turns into soluble Y(NO3)3. Here, if the solution is transparent, add more Y2O3 to consume the excess HNO3, otherwise add more HNO3 to consume the Y2O3 until the amount of product is satisfied.
Then, the solution is filtered as fast as possible to make it remain hot while getting rid of any insoluble impurities. The hot, saturated solution will precipitate white crystals of pure Y(NO3)3 as it colds down. Here I use a glass rod as the nucleation site to accelerate the process.
The Y(NO3)3 crystal is vacuum-dried. I did this by connecting a glass container to my vacuum chamber used for vacuum distillation. The crystal dries very slow, taking me a whole week of pumping to finish the process.
The fully dried product is white and not transparent (picture below) while the wet crystals are transparent like the kitchen salt.
After preparing Y(NO3)3, we can officially begin to produce YBCO. This begins by mixing Y(NO3)3, Ba(NO3)2, Cu(NO3)2·3H2O, with a molar ratio Y:Ba:Cu = 1:2:3. The chemical is weighted to 0.5mg using an electronic balance with 0.1mg smallest division.
To mix the chemicals, water is used to dissolve the three compounds. The mass ratio of water:Y(NO3)3:Ba(NO3)2:Cu(NO3)2 is 225:38.3:52.27:72.48 according to NileRed (I mixed about four times the chemicals he used). After adding Cu(NO3)2, it is common that the solution is not clear due to impurities, and this can be solved by adding some conc. HNO3; and it might need a long heating time for all the Ba(NO3)2 to dissolve, but one can begin to add Y(NO3)3 to the solution and proceed even when there is some remaining Ba(NO3)2, because the following reaction will consume the dissolved ions and allow more Ba2+ to dissolve.
Although there is some solid remaining, it is ok to begin to add aqueous ammonia to the solution, which turn the solution from blue to dark purple. Remember that here you need a lot NH3 (about 1L for my experience). Also, it is critical to control a pH precisely, as this will largely affect the burning process afterward. According to my research and trials, the optimum pH should be around 6.6-6.8.
After that, the solution is heat to its boiling point to evaporate the water. This process should stop once the solution becomes a bit sticky. Remember to wear a mask or heat it outdoor as ammonia gas is produced. The sticky product is added in a small amount to a tall beaker, which is then heated to 320ºC. The Pyrolysis reaction begins by a self-burn. The solution will grow into a brown tree-shaped thing made up of porous powder of Y123 (YBCO where Y:B:C = 1:2:3). The powder is then collected. Remember to wear a mask to prevent smoking in the powder and heat the solution outdoor as a large amount of ammonia gas is procured.
The powder would need to be baked under an oxygen flow. Because YBCO will only be superconductive when it obtains enough oxygen. This works by transforming the insulating orthorhombic lattice to the tetragonal lattice with the possibility of superconducting as oxygen is absorbed between 550ºC to 600ºC.
Benzi, P., Bottizzo, E., & Rizzi, N. (2004). Oxygen determination from cell dimensions in YBCO superconductors. Journal of Crystal Growth, 269, 625-629.
The baking is done with a tube furnace and a medical oxygen generator. The temperature curve is: heat from 20ºC to 950ºC with 3h, keep at 950ºC for 18h, cold from 950ºC to 580ºC with 3h, stay at 580ºC for 15h, turn off the furnace and cold to the room temperature. The 580ºC plateau is added to NileRed’s method to allow sufficient time for the lattice structure shift, which is later proved successful.
The resulting product is much denser, but it also posted an issue that every time even I fill the whole furnace, only less than 10g of the final powder can be produced due to the extreme decrease in the volume.
Then, the powder is compressed into disks using a hydraulic press and an appropriate die set. The disk’s diameter should not be more than 25mm, otherwise it will be very difficult to make a disk thick enough (the powder is still loose and the die set can only hold a small volume of it, so a thick, big disk is hard to make).
The first disk is not very nice, with many cracks. This is mainly due to the low density of the powder. But this can be solved later. This disk is not the final product.
The disk is broken into powder again using a mortar and pestle. The resulting powder is repressed into a disk. This disk is compressed using the dense powder, so it is less likely to have cracks.
This final disk is placed in the furnace once again, undergoing the same thermal treatment as before with the next batch of rough power to enough optimum oxygen absorption and thus superconductor performance.
Here is the final product:
This is a disk only undergone one thermal treatment cycle. And the later version with a recomposes procedure and two thermal cycles allows the levitation height to increase by one fold.