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Crystals Growing K-12 Experiments
Crystal Background 
A crystal (most likely from a Greek word which means "ice") is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. Generally, crystals form when they undergo a process of solidification. Under ideal conditions, the result may be a single crystal, where all of the atoms in the solid fit into the same crystal structure. However, generally, many crystals form simultaneously during solidification, leading to a polycrystalline solid. For example, most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins. Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. The process of forming a crystalline structure is often referred to as crystallization. 
While the cooling process usually results in the generation of a crystalline material, under certain conditions, the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between solids and glasses: most notably, the process of forming a glass does not release the latent heat of fusion. For this reason, many scientists consider glassy materials to be viscous liquids rather than solids, although this is a controversial topic; see the entry on glass for more details. 
Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or when it condenses from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevents complete crystallization. Weak Van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite. Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can have a profound effect on the properties of the materials. 
While the term "crystal" has a precise meaning within materials science and solid-state physics, colloquially "crystal" refers to solid objects that exhibit well-defined and often pleasing geometric shapes. Various shapes of such crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and common salt are common examples of crystals. Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect or the piezoelectric effect. The behaviour of light in crystals is described by crystal optics. In periodic dielectric structures a range of unique optical properties can be expected as described in photonic crystals. Crystallography is the scientific study of crystals and crystal formation. Historical and mythical usesAccording to Rebbenu Bachya, the word "Achlmah" in the verse Exodus 28:19 means "Crystal" and was the stone on the Ephod representing the tribe of Gad. Crystallization ProcessCrystallization is the process of formation of solid crystals from a homogeneous solution. The natural process are also know as Crystal growth. Artificial Crystallization process is a solid-liquid separation technique. For crystallization to occur the solution at hand ought to be supersaturated. Put simply, the solution should contain more solute molecules than it would under ordinary conditions. This can be achieved by various methods -- solvent evaporation, cooling, chemical reaction, 'drowning' being the most common ones used in industrial practice. The picture of the snow crystallization is an example of crystallization. 
To make things clear we can use a simple example. We take a bowl of water to which we add sugar crystals. We keep adding sugar to it until we reach a stage when no more crystals can be dissolved. This solution so obtained is a saturated one. One can dissolve further crystals into this saturated solution, by heating it: solubility of solutes generally increases with temperature. When the temperature of the solution is allowed to attain equilibrium with the surroundings, the solubility of the solute decreases (because the temperature of the solution has decreased) and the 'excess' sugar so added crystallizes out. This process illustrates the simplest of supersaturation techniques. 'Drowning' is the addition of a nonsolvent in the solution that decreases the solubility of the solid. Alternatively, chemical reactions can also be used to decrease the solubility of the solid in the solvent, thus working towards supersaturation. Crystallization can be divided into stages - primary nucleation is the first. Simply defined, it's the growth of a new crystal, which in turn causes secondary nucleation - the final stage (if removal of the crystals is not an issue). Secondary nucleation requires existing crystals to perpetuate crystal growth. In our sugar example, we had obtained such nuclei when the 'excess' sugar had just about crystallized out assisting further crystal formation. Secondary nucleation is the main stage in crystallization for this is what causes the 'mass production' of crystals. Crystal Structure
In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a unit cell, a set of atoms arranged in a particular way; which is periodically repeated in three dimensions on a lattice. The spacing between unit cells in various directions is called its lattice parameters. The symmetry properties of the crystal are embodied in its space group. A crystal's structure and symmetry play a role in determining many of its properties, such as cleavage, electronic band structure, and optical properties. Unit cellA unit cell is a spatial arrangement of atoms which is tiled in three-dimensional space to describe the crystal. The positions of the atoms inside the unit cell are described by the symmetric unit or basis, the set of atomic positions (xi,yi,zi) measured from a lattice point. For each crystal structure there is a conventional unit cell, usually chosen to make the resulting lattice as symmetric as possible. However, the conventional unit cell is not always the smallest possible choice. A primitive unit cell of a particular crystal structure is the smallest possible unit cell one can construct such that, when tiled, it completely fills space. A Wigner-Seitz cell is a particular kind of primitive cell which has the same symmetry as the lattice. Crystal systemThe crystal systems are a grouping of crystal structures according to their basic symmetry characteristics, and each crystal system can be described by a set of three axes in a particular geometrical arrangement. There are seven unique crystal systems. The simplest and most symmetric, the cubic (or isometric) system, has the symmetry of a cube. The other six systems, in order of decreasing symmetry, are hexagonal, tetragonal, rhombohedral (also known as trigonal), orthorhombic, monoclinic and triclinic. Some crystallographers consider the hexagonal crystal system not to be its own crystal system, but instead a part of the trigonal crystal system. Classification of lattices
A Bravais lattice is a set of points constructed by translating a single point in discrete steps by a set of basis vectors. In three dimensions, there are 14 unique Bravais lattices (distinct from one another in that they have different space groups) in three dimensions. All crystalline materials recognized until now (not including quasicrystals) fit in one of these arrangements. The fourteen three-dimensional lattices, classified by crystal system, are shown to the right. The crystal structure consists of the same group of atoms positioned around each and every lattice point. This group of atoms therefore repeats indefinitely in three dimensions according to the arrangement of one of the 14 Bravais lattices. The symmetry characteristics of the group of atoms, or unit cell, is described by its point group. Point and space groupsThe crystallographic point group or crystal class is the set of non-translational symmetry operations that leave the appearance of the crystal structure unchanged. These symmetry operations can include mirror planes, which reflect the structure across a central plane, rotation axes, which rotate the structure a specified number of degrees, and a center of symmetry or inversion point which inverts the structure through a central point. There are 32 possible crystal classes. Each one can be classified into one of the seven crystal systems. The space group of the crystal structure is composed of the translational symmetry operations in addition to the operations of the point group. These include pure translations which move a point along a vector, screw axes, which rotate a point around an axis while translating parallel to the axis, and glide planes, which reflect a point through a plane while translating it parallel to the plane. There are 230 distinct space groups. Defects in crystalsReal crystals feature defects or irregularities in the ideal arrangements described above and it is these defects that critically determine many of the electrical and mechanical properties of real materials. In particular dislocations in the crystal lattice allow shear at much lower stress than that needed for a perfect crystal structure. Crystal symmetryCrystal structures can be divided into 32 classes, or point groups, according to the number of rotational axes and reflection planes they exhibit that leave the crystal structure unchanged. Twenty of the 32 crystal classes are piezoelectric. All 20 piezoelectric classes lack a center of symmetry. Any material develops a dielectric polarization when an electric field is applied, but a substance which has such a natural charge separation even in the absence of a field is called a polar material. Whether or not a material is polar is determined solely by its crystal structure. Only 10 of the 32 point groups are polar. All polar crystals are pyroelectric, so the 10 polar crystal classes are sometimes referred to as the pyroelectric classes. There are a few crystal structures, notably the perovskite structure, which exhibit ferroelectric behaviour. This is analogous to ferromagnetism, in that, in the absence of an electric field during production, the ferroelectric crystal does not exhibit a polarisation. Upon the application of an electric field of sufficient magnitude, the crystal becomes permanently polarised. This polarisation can be reversed by a sufficiently large counter-charge, in the same way that a ferromagnet can be reversed. However, it is important to note that, although they are called ferroelectrics, the effect is due to the crystal structure, not the presence of a ferrous metal. Incommensurate crystals have period-varying translational symmetry. The period between nodes of symmetry is constant in most crystals. The distance between nodes in an incommensurate crystal is dependent on the number of nodes between it and the base node. Crystal GrowthCrystal growth occurs from the addition of new atoms, ions, or polymer strings into the characteristic arrangement, or lattice, of a crystal. This happens in two stages: nucleation and growth. In the first stage, a small nucleus containing the newly forming crystal is created. Nucleation occurs relatively slowly as the initial crystal components must "bump" into each other in the correct orientation and placement for them to adhere and form the crystal. After crystal nucleation, the second stage, growth, rapidly ensues. Crystal growth spreads outwards from the nucleating site. In this faster process, the elements which form the motif add to the growing crystal in a prearranged system, the crystal lattice, started in crystal nucleation. Nucleation can be either homogeneous, without the influence of foreign particles, or heterogeneous, with the influence of foreign particles. Generally, heterogeneous nucleation takes place more quickly since the foreign particles act as a scaffold for the crystal to grow on. Heterogeneous nucleation can take place by several methods. Some of the most typical are small inclusions, or cuts, in the container the crystal is being grown on. This includes scratches on the sides and bottom of glassware. Other nucleating sites can be the dust, dandruff, and random other particles which are found in air. A common practice in crystal growing is to add a foreign substance, such as a string or a rock, to the solution, thereby providing a nucleating site for the project and speeding up the time it will take to grow a crystal. The number of nucleating sites can also be controlled in this manner. If a brand-new piece of glassware or a plastic container is used, crystals may not form because the container surface is too smooth to allow heterogeneous nucleation. On the other hand, a badly scratched container will result in many lines of small crystals. To achieve a moderate number of medium sized crystals, a container which has a few scratches works best. Likewise, adding small previously made crystals, or seed crystals, to a crystal growing project will provide a nucleating sites to the solution. The addition of only one seed crystal should result in a larger single crystal. Some important features during growth are the arrangement, the origin of growth, the interface form (important for the driving force) and the final size. When origin of growth is only in one direction for all the crystals, this can have as result that the material becomes very anisotropic (different properties in different directions). The interface form determines the additional free energy for each volume of crystal growth. Lattice arrangement in metals often takes the structure of body centered cubic, face centered cubic, or hexagonal close packed. The final size of the crystal is important for mechanical properties of materials (in metals it is widely acknowledged that large crystals can stretch further due to the longer deformation path and thus lower internal stresses). See also
External links
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Crystal" |
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