Preparation of single crystals by zone heating method.
Among many other methods belonging to the same family as the Czochralski method, We should be interested in one more zone melting method. This method is used for two different purposes: growing single crystals and purifying single crystals from harmful impurities. It is one of the youngest methods - developed and used to purify metals only thirty years ago, in year 1952, by the American W.. G. Pfanna, and only much later adapted to the cultivation of single crystals.
Polycrystalline, material consisting of many tiny crystals, from which we want to get a single crystal, is placed in a special crucible, a boat-shaped vessel, made of a material with a high melting point, non-reactive with the material of the single crystal being grown, and subjected to heating.
However, this is very special heating. According to the name of the method, only a very narrow zone of the material is heated until it melts. If we heat, we start from only one end of the material, and we will move the source of high temperature towards the other end, then we bring it to a melt along its entire length. However, each time the heat source moves, only one will melt, narrow layer of material. It will be cooler in front of and behind her, the temperature rises in front of the heat source - the material is melted, and behind the traveling heat source, the temperature goes down – the previously melted material solidifies and crystallizes. When the temperature difference between the liquid layer and the solidification layer and the rate of movement of the molten zone will be correspondingly small, the single crystal will grow during solidification.
Mainly organic crystals are grown by zone melting, but also precious stones, and among them the already known synthetic yttrium-aluminum garnet. Moreover, this method facilitates the preparation of YAG single crystals, because it does not require prior melting of ingredients. It is enough to mix very finely divided yttrium and aluminum oxides, heat to a low temperature under a certain pressure and form into the desired shape by sintering, e.g.. fight. This roller is inserted into a cylinder made of quartz glass, we subject the process to zone melting and we obtain a finished single crystal.
In the same way, we can also obtain a single crystal… water, thus an ice roller with a perfectly ordered internal structure. The difference between ice and pomegranate crystallization is only one temperature of the heat source, which causes zone melting. In the zone melting process, induction heating is used in the same way as for surface hardening of steel, and during ice crystallization… freezing.
The process of purifying single crystals by the zone melting method is identical, like the single crystal cultivation process and is inseparably connected with it. When crystallizing downstream of the liquid zone, all impurities are repelled from the face of the growing crystal, which is a natural feature of any crystallization process. The only one, as we know, the condition for obtaining very pure crystals is that they grow very slowly. In the process of crystallization, the growing crystal attracts the ions it needs, atoms or molecules, and it repels foreign elements. So if the zone melting process is slow enough, all the impurities will remain in the molten zone and will move with this zone to one end of the crystal. When we care about very pure crystals, this process is repeated several times, and the end of a single crystal, in which all the debris has gathered, we cut.
However, it does happen sometimes, especially in the case of gemstones, that we consciously introduce contaminants into single crystals. We find them useful. After all, the color of the precious stones depends on the pollution. It happens, that we want to evenly distribute impurities throughout the crystal volume. This applies, for example, to. single crystals used in the construction of crystal lasers, thus also a silvery-colored neodymium YAG crystal. Should the zone melting method be abandoned in such cases?? On the contrary. Using this method, we can even increase the homogeneity of the contamination distribution. There is one condition. Instead of moving the melted zone several times in one direction, as in the zonal cleaning process, e.g.. from the left to the right end of the crystal, we melt the single crystal several times, but moving the heat source alternately once to the left, one time to the right. We will obtain a homogeneous single crystal, perfect for building a laser.
The zonal melting method also has several different variations.
In addition to the horizontal methods, there are vertical crucible methods. There are also vertical non-crucible methods, in which the molten layer of material does not rest against the walls of the crucible, a does not spread sideways due to the surface tension of the liquid. The same, which keeps the steel needle delicately placed on the surface of the water.