1. Crystal Habit Modification And Its Industrial Importance Presented By: Kishor K. Prajapati Guided By: Dr. Bina R. Sengupta

2. Introduction Crystallization : Crystallization is the spontaneous arrangement of the particles repetitative orderly, i.e.,regular geometric patterns. Crystal : A Crystal can be defined as a solid particle, which is formed by solidification (crystallization) process (under suitable environment) in which structural units are arranged by a fixed geometric pattern or lattice. Crystal Habit : Crystal Habit is a description of the shapes and aggregates that a certain mineral is likely to form. Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

3. Different Shapes of Crystal Crystals can be classified into seven crystal systems : Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

4. Crystal Habit The relative sizes of the faces of a particular crystal can vary considerably. This variation is called a modification of habit. The crystals may grow more rapidly, or be stunted, in one direction; thus an elonngated growth of the prismatic habit gives a needle shape crystal (acicular habit) and a stunted growth gives a flat plate like crystal (tubular, platy or flaky habit). Nearly all manufactured and natural crystal are distorted to some degree,and this frequently leads to a misunderstanging of the term symmetry. Perfect geometric symmetry is rarely observed in crystals, but crystallographic symmetry is readily detected by means of a goniometer. The relative growths of the faces of a crystal can be altered, and often controlled, by a number of factors. Rapid crystallization, such as that produced by the sudden cooling or seeding of a supersaturated solution, may result in the formation of needle crystals; impurities in the crystallizing solution can stunt the growth of a crystal in certain directions; and crystallization from solutions of the given substance in different solvents generally results in a change of habit. The degree of supersaturation or supercooling of a solution or melt often exerts a considerable influence on the crystal habit, and so can the state of agitation of the system. Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

5. A stunted growth in the vertical direction (or elongated growth in the directions of the other axes) results in a tabular crystal (a); excessively flattened crystals are usually called plates or flakes. This combination-form crystal is terminated by hexagonal pyramids and two flat faces perpendicular to the vertical axis; these flat parallel faces cutting one axis are called pinacoids. An elongated growth in the vertical direction yields a needle or acicular crystal (c); flattened needle crystals are often called blades. Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

6. Crystal habit of Potassium sulphate & Sodium chloride Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

7. Habit Changes in ammonium sulphate and Sodium chloride crystals caused by traces of impurity Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506

8. Industrial Importance For most commercial purposes a granular or prismatic habit is usually desired, but there are specific occasions when other morphologies, such as plates or needles, may be wanted. In nearly every industrial crystallization some form of habit modification procedure is necessary to control the type of crystal produced. This may be done by controlling the rate of crystallization, e.g. the rate of cooling or evaporation, the degree of supersaturation or the temperature, by choosing a particular solvent, adjusting the solution pH, deliberately adding an impurity that acts as a habit modifier, or even removing or deactivating some impurity that already exists in the solution. Surface active agents (surfactants) are frequently used to change crystal habits. Common anionic surfactants include the alkyl sulphates, alkane sulphonates and aryl alkyl sulphonates. Polymeric substances such as polyvinylalcohol, polyacrylates, polyglutamates, polystyrene sulphonates, alginates, polyacrylamides, etc., have also found application, as have long-chain and proteinaceous Mullin. J.W., (1961) Crystallization (4th Edition Reprinted) Butterworth-Heinemann, ISBN O 7506 materials like sodium carboxymethylcellulose, gelatin and

9. Crystal Habit Modification For crystal-habit modification, crystals are grown in the presence of naturally occurring soluble additives, which usually adsorb or bind to the crystal faces and influence the crystal growth or morphology. The crystal-habit modifiers may be of a very diverse nature, such as multivalent cations, complexes, surface active agents, soluble polymers, biologically active macromolecules, fine particles of sparingly soluble salts, and so on. These crystal modifiers often adsorb selectively on to different crystal faces and retard their growth rates, thereby influencing the final morphology of the crystals. Using water-soluble polymers as crystal modifiers for controlled crystallization is widely expanding and becoming a benign route for controlling and designing the architectures of inorganic materials. Investigators have used polyvinylalcohol (PVA), agar-, gelatin-, and pectin-based gel matrices to control the morphology of inorganic crystals such as PbI2,AgI, Ag2Cr2O7,PbSO4 ,PbCl2 ,and so forth. Among the reported common gel matrices used as crystal-habit modifiers, PVA is a water soluble synthetic polymer with excellent film-forming and emulsifying Edition Reprinted) Butterworth-Heinemann, ISBN O 7506 Mullin. J.W., (1961) Crystallization (4th properties.

10. Crystal Habit modification by polymers of Copper chromite nanomaterials Amorphous and monodispersed copper chromite nanoparticles were prepared by aqueous thermolysis method using PVA (Polyvinyl alcohol) and different ratios of urea-PVA as fuel in air. Morphology and size of nanoparticles were measured by SEM (Scanning electron microscopy) and TEM (Transmission electron microscopy) analysis. Copper chromite (CuCr2O4) is a tetragonally distorted normal spinel; this distortion is due to Jahn Teller effect of Cu+2 ions in tetrahedral sites. It is a p-type semiconductor which is widely used as a catalyst for the oxidation of CO, hydrocarbons ,alcohols and as a burn rate catalyst in composite solid propellants, Well resolved square bipyramidal morphology was seen in all copper chromite samples using PVA alone. The habit modification of copper chromite was observed due to presence of urea. The urea molecule is planar in the crystal structure, but the geometry around the nitrogen is pyramidal in the gas-phase minimum-energy structure. In solid urea, the oxygen center is engaged in two N-H-O hydrogen bonds. The resulting dense and energetically hydrogen-bond network probably changes the morphology after combustion process. http://www.intechopen.com/download/pdf/26216

11. Polymer is adsorbed and acting as bridge between particles. The linear chains of PVA can be cross linked in aqueous medium . The cross linking between the chains may provide small cages wherein the sol of the reactant mixture gets trapped. During combustion, the sol trapped in the cages may get converted to ultrafine particles of copper chromite. Thus cages formed by the cross linking may offer resistance to the agglomeration of the particles and particle growth. Perfect square bipyramidal morphology is seen in PVA capped and orthorhombic in annealed samples. Sharpness of edges decreases http://www.intechopen.com/download/pdf/26216 gradually with increase in urea content

12. Conclusions: In most of the habit modification and morphological changes of some inorganic materials in microsize and nanosize studies polymers play multiple roles as a fuel in combustion synthesis, encapsulating agent and as a habit modifier in other synthesis method applied. We have observed that the size, shape, morphology of the synthesized material depends on various factors like nature of polymer, its degree of polymerization, molecular weight, reaction time, synthetic method applied and also on heat of reaction. In the methods applied at high temperature, rapid nucleation time gives rise to short burst of nuclei which might react with intermediate species and the reactions are more kinetically controlled. When the synthesis was carried out at low temperatures, nucleation process is slow and thermodynamically driven process. With aging, growth process becomes more favorable. Final morphology of the material depends on equilibrium conditions related to minimum surface energy, rate of nucleation and growth. Thank You