Seed crystal

Seed crystals are small single crystals ; they are used as starting crystals for growing larger crystals .

Silicon seed crystal for the Czochralski process, the left part is dipped into the melt and then pulled out, the notch on the right is used for mechanical support

Manufacturing

Seed crystals are generated under laboratory conditions, with the same crystal orientation (e.g. [111]) that the fully grown crystal should have.

example

Consider e.g. B. the following reaction equation:

${\displaystyle \mathrm {Alaun(fest)\leftrightarrow Alaun(gel{\ddot {o}}st)} }$

${\displaystyle \mathrm {KCr(SO_{4})_{2}\leftrightarrow K^{+}+Cr^{3+}+2SO_{4}^{2-}} }$

According to Arrhenius, the rate constant of the forward reaction is temperature-dependent: ${\displaystyle k_{1}}$

${\displaystyle k_{1}=A\cdot \exp \left(-{\frac {E_{\mathrm {akt} }}{R\cdot T}}\right)}$,

where A is a substance-specific constant, the molar activation energy , the universal gas constant and the temperature. ${\displaystyle E_{\mathrm {akt} }}$ ${\displaystyle R}$ ${\displaystyle T}$

With the given constants it can be seen that the rate constant increases at high temperatures. So the forward reaction has become faster, more ions go into solution, but a solid still remains. This is then filtered off, and the saturated solution is obtained as the filtrate. On cooling, the rate constant decreases again, the forward reaction is no longer promoted and the reverse reaction occurs: alum precipitates as a solid. The fact that this adsorbs on the seed crystal and the crystal growth begins is both a so-called cooperative effect and a consequence of the reduction in surface energy on the seed crystal surface, caused by lowered galvanic potentials. ${\displaystyle k_{1}}$

use

With the help of seed crystals, supersaturated solutions or melts can crystallize . The seed crystals serve as crystallization nuclei on which substance is deposited from the solution or melt. This process is called seeding . When crystallizing proteins, a distinction is made between micro seeding and macro seeding , depending on the size of the seed crystals . The practical procedure is described in the Czochralski method .

For the growth of single crystals it is necessary that the seed crystal is crystal-chemically identical to the pure substance to be extracted. In the case of seed alloys, however, it is sufficient if chemically closely related crystals are used for seeding. Due to the crystal structure of the crystal orientation, the shape and orientation of the seed crystals is decisive for the appearance of the larger crystals grown.

Most of the time, seed crystals are used in the semiconductor industry to grow single crystals for wafers , which are used, for example, as carriers for photovoltaic products, integrated circuits or transistors .