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.
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