- a change from the liquid phase to the solid occurs by crystallization from a melt or a solution;
- a change from the gaseous phase to the solid crystallization occurs by sublimation; and
- a change from one solid phase to another is accompanied by alteration in the shape of the crystal structure; this phenomenon is called re-crystallization.
The above three cases of formation of crystalline matter are not equally widespread; the first ease is much more common than the second or the third. In crystal growth, nucleation and growth kinetics are both important.
The nucleation of new crystals is an important aspect of crystal growing because one is always interested in getting a single crystal of maximum size and perfection, rather than a multiplicity of smaller crystals.
One of the best ways to avoid the formation of more than one nucleus is to utilize a tiny crystal of a “seed” on which growth of the crystal takes place under such conditions which discourage the growth of many crystals. The crystal may grow in a number of ways:
- Crystals may grow by layers.
- Crystals may also grow dendritically.
- Crystals may also grow to yield a cellular or honeycomb or interfacial structure.
The capacity of forming large crystals varies with substance and with the condition of crystallization. According to Hofer, the size (as measured by the mass M) of the crystals separating from in aqueous solution is related to the solubility, S by the equation:
M = constant x S2
Crystals can grow in opposition to considerable forces. The disintegration of rocks by freezing of water to ice and the bursting of thick cast iron globes containing water on freezing are well known examples of crystalline forces.
Ramberg computed the force of crystallization per mm change of vapor pressure P caused by the hydrostatic pressure, p by means of Poynting’s equation:
F = dP/dp = (RT/P)VePV/RT
A substance usually has a definite crystal habit e.g., its crystal may tend to grow more rapidly in one direction or have a prismatic habit, it may grow in two directions by a tabular habit forming plates or tablets etc. the habit often depends upon the speed of crystallization.
The presence of foreign bodies in solution often modifies the crystal form. A well known case is NaCl which grows from pure water solution as cubes but from 15% aqueous urea solution as octahedral. It is believed that urea is preferentially adsorbed on the octahedral faces preventing deposition of Na+ and ions and therefore, causing these faces to develop rapidly in urea.
Crystal Growth Controlling Factors
Factors having the greatest effect on the shape of the growing crystal are given below:
Concentration Currents: The concentration currents strongly influence the shape of growing crystal; if the growing crystal is at the bottom of a vessel, the concentration currents tend to make it flat; if the growing crystal is suspended, the concentration currents tend to elongate it.
The effect of the concentration currents can be eliminated by rotating the crystal about a horizontal axis or by shaking the crystalliser flask.
Impurities in the Solution: Impurities often strongly affect the crystal shape. A classical example is the introduction of carbamide into a solution of sodium chloride. Without this addition, sodium chloride crystallizes in cubes, with it in octahedra.
Effect of Temperature: The variations in temperature often cause faces to appear which would never be formed under normal conditions of growth.
Viscosity of the Solution: If the viscosity is high enough, it will prevent the formation of concentration currents. Then, the crystal will be able to grow only through the diffusion of supersaturation which has a peculiar effect on the shape of the growing crystal.
Solidification and Crystallization
In order to understand the crystalline state and its difference from the amorphous state, it is important to consider the process of solidification. Solidification is the transformation of materials from liquid to the solid state on cooling.
When the liquid solidifies, the energy of each atom is reduced and this energy is given out as latent heat during the solidification process, which for a pure metal occurs at a fixed temperature, Ts.
During solidification, the disordered structure of the liquid (constituents of material in liquid state have more velocity, more collisions and hence have random position) transforms to the orderly arrangement depending upon the time of solidification.
If the growth process is slow, the constituents take definite positions during growth. There is a tendency for the constituents to settle down in positions where the potential energy of the configuration is minimum.
This leads to an ordered arrangement of the constituents which are arranged in a pattern that repeats itself in all the three dimensions. Under these conditions a long range order exists in the solid and it is this order that characterizes the crystalline state. Various stages in the solidification of a poly-crystalline specimen are represented schematically in Figure.
However, in certain extreme cases when the growth process or the phase change takes place rather quickly and the constituents do not have sufficient mobility, the constituents do not get sufficient time to obtain the configuration of minimum energy. Consequently, a long range order, where a perfect periodicity, is maintained over much larger distances as compared to lattice periodicity, is not achieved.
However, a short range order persists in local regions is still present. Solids characterized by this short range order represent the amorphous state or noncrystalline state. Sometimes such materials are called super cooled liquids in as much as their atomic structure resembles that of a liquid.
Whether a crystalline or amorphous solid forms depends on the ease with which a random atomic structure in the liquid can transform to an ordered state during solidification. Amorphous materials, therefore, are characterized by atomic or are molecular structures that relatively complex and become ordered only with some difficulty. Furthermore, rapidly cooling through the freezing temperature favors the formation of a noncrystalline solid, since little time is allowed for the ordering process.
Metals normally form crystalline solids; but some ceramic materials are crystalline, whereas, the inorganic glasses are amorphous. Polymers may be completely crystalline, entirely noncrystalline, or a mixture of the two.
Thanks for reading about “Crystal growth in materials”.
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- Crystal Growth in Materials | Solidification | Crystallization
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