Domande aperte Science and Technology of ceramics

∆G ∆S. ∆G

interaction, the value of depends only on the value of If the latter is negative,

becomes positive and the suspension is stabilized. In the entropic theory of steric stabilization it

is assumed that the adsorbed layers are impenetrable to each other. The approach to a distance

less than twice the thickness of the layers therefore involves their compression and a

consequent reduction in the number of possible configurations for the segments of the polymer

chains, with a reduction in entropy. The resulting increase in free energy leads to a repulsive

effect. The most suitable polymers for achieving steric stabilization are 2-block amphiphilic

polymers, which are characterized by one block that is very similar to the particle surface and

one that is similar to the dispersant medium. This favors extended polymer configurations in

the dispersing medium. Steric stabilization is relatively insensitive to the presence of

electrolytes; it is effective in both aqueous and non-aqueous media, both with high and low

solid content; flocculation is reversible by acting on the concentration of the dispersant media.

It offers, therefore, advantages over electrostatic stabilization.

The electrostatic and steric effects are often cooperative, and we then speak of electrosteric

stabilization.

10) Describe in what ways and according to what principles a suspension of powders in a liquid

medium can be stabilized.

We can have the electrostatic stabilization, described before and the steric stabilization. Very

effective is the steric stabilization, obtained by adsorption, on the surface of the particles, of

macromolecules. These act as particle spacers. Among the theories proposed to explain the

effect of steric stabilization, the best known is the entropic theory. When suspended particles

come into contact with a polymer dispersed in liquid they adsorb it in ways that may differ

depending on the nature of the polymer and the dispersing medium, as for example depicted in

in the case of a polymer that is soluble or insoluble in the dispersing medium (left and right,

respectively).

As the particles approach each other, the adsorbed polymers come into contact first and begin

= ∆ ∆S)

to interact. The variation of free energy of interaction (∆ - can be negative, in

which case the interaction is favored and flocculation or coagulation occurs, or positive and

then stabilization occurs. Under isothermal conditions, and assuming negligible enthalpy

∆G ∆S. ∆G

interaction, the value of depends only on the value of If the latter is negative,

becomes positive and the suspension is stabilized. In the entropic theory of steric stabilization it

is assumed that the adsorbed layers are impenetrable to each other. The approach to a distance

less than twice the thickness of the layers therefore involves their compression and a

consequent reduction in the number of possible configurations for the segments of the polymer

chains, with a reduction in entropy. The resulting increase in free energy leads to a repulsive

effect. The most suitable polymers for achieving steric stabilization are 2-block amphiphilic

polymers, which are characterized by one block that is very similar to the particle surface and

one that is similar to the dispersant medium. This favors extended polymer configurations in

the dispersing medium. Steric stabilization is relatively insensitive to the presence of

electrolytes; it is effective in both aqueous and non-aqueous media, both with high and low

solid content; flocculation is reversible by acting on the concentration of the dispersant media.

It offers, therefore, advantages over electrostatic stabilization.

The electrostatic and steric effects are often cooperative, and we then speak of electrosteric

stabilization.

11) Describe the process of dry press forming. (da pag 89-98)

Dry pressing consists of feeding a granulated mixture into a mold where it is pressed and

immediately ejected. Fir of all there is the mixing step. Mixtures used in dry forming processes

normally consist of a ceramic powder, a binder, a plasticize and a lubricant. Later of we proceed

with the granulation step. In order to facilitate the flow of the mixtures and to achieve a more

uniform and regular filling of the molds, the powders with the appropriate additives are often

granulated in a controlled manner. The granulation can be carried out directly by pressing, by

extrusion or by spraying the liquid on the powders kept in agitation. In the latter process

granule formation is by agglomeration of the powders into primary cores which subsequently

grow by addition of other powders (layer by layer) or other cores. The forces responsible for

aggregation are capillary forces and those exerted by binder adsorption. The size of the

granules increases with the amount of liquid sprayed. In addition, the size of the granules can

be controlled by controlling the size of the liquid droplets generated by the sprayers. Small

droplets generate many cores, which in turn will grow in a limited way resulting in small

granules. An indirect method of granulation is spray drying, which involves producing a

dispersion of fine droplets of the mixture within a chamber. These droplets, falling, meet (in

counter-current or co- current) a flow of hot air that causes the evaporation of most of the

solvent and the consequent formation of granules. The starting mixture contains 50 to 60

percent liquid, which is reduced to 10 percent or less in the granules. This process is used for

the production of both traditional ceramics (eg, tiles) and specialty ceramics. The characteristics

of the granules vary greatly depending on the type of product. Then we have the die filling step.

The characteristics of the powder affect mold filling, which must be complete, uniform and

reproducible. This can be achieved by good flowability of powders or granules. Flowability is

measured by measuring the amount of powder flowing through an orifice in a unit of time.

Alternatively the angle of repose of the powder cone formed by pouring the powder onto a flat

surface can be measured. Flowability depends on the surface roughness of the powder, the

presence of viscous liquids on the surface, and the average particle size. The roughness is less if

the granules are very fine particles and there is no aggregation between coarse and fine

granules. The granules become sticky if the binder is present on the surface and is at a

temperature above the glass transition temperature, or if it is water soluble and is in a humid

environment. In the latter case, the fluidity decreases with increasing relative humidity. The

m.

average particle or grain size should preferably be greater than 20 Sometimes 5 % finer

particles are sufficient to block the flow.

Later on we proceed with the pressing step. Once in the mold, the mixture is compressed and

compacted. Compaction behavior is complex and it is worth briefly describing what happens to

the system. The compression of an unsaturated system of particles (ie., in which the porosity is

not saturated by the liquid) is sustained by the stress at the contact points between the

particles and by the pressure, P , sustained by the fluid present in the pores.

f

Denoting by P , the applied pressure, the effective stress acting on the solid is:

a = −

This stress must not, however, be confused with the contact stress, , which acts on the

particles in the areas where they touch, and which is given, if we neglect the electrostatic

interaction forces, by the relation:

= + ≅ − +

( )

where A is the average contact area and A is the section of the stressed body. In the early

C T

stages of the compaction process, the contact area is only a small fraction of the section on

which the applied pressure acts; the contact stress is therefore very high. Assuming, for

example, A /A =0.01, an applied pressure of 10 MPa and a P 3 MPa, the contact stress is 703

c T f

MPa. In the absence of fluid, the tension would rise to 1000 MPa. The high value of can

cause plastic deformation and/or fractures, which lead the particles to move and change shape,

to more efficiently occupy the available volume, increasing the A /A ratio and decreasing the

C T

contact stress. The presence of a fluid between the particles or surfactants adsorbed on their

surface changes the behavior of the system. In the presence of pressure gradients, the fluid

tends to migrate. This flow is not instantaneous, but depends on time. It is assumed that during

the phase of increasing pressure there is no drainage of the liquid. Initially, compaction leads to

a decrease in porosity and, since the liquid remains all in the pores, this leads to saturation and

a rapid increase in the pressure sustained by the liquid. The effective stress, given by the

difference between the applied pressure and the pressure in the pores, initially increases and

then decreases. As the liquid begins to drain, the pore pressure decreases and the effective

stress increases. Porosity decreases less and less in the null drainage phase as both the effective

stress and, consequently, the contact stress are reduced. As the effective stress increases again

and the pores begin to empty due to liquid drainage, the contact stress increases rapidly

leading to further compaction. The degree of compaction increases as the pressure increases,

but this increase becomes insignificant above about 50 MPa. At these pressures, however, mold

wear becomes important. Industrial presses for tile forming reach 40 MPa, while for technical

ceramics they reach 100 MPa.

A typical compaction curve (guarda grafici pag 94). The curve in logarithmic scale highlights the

presence of three distinct stages in the compaction process:

st

- 1 stage, in which the granules, under the action of pressure, move and rearrange themselves

nd

- 2 stage, in which most of the densification takes place, mainly due to the plastic deformation

of the granules, at the end of this stage the intergranular voids are almost completely

eliminated, thus approaching the density value of the granules

rd

- 3 stage in which the densification of the granules predominates, with the elimination of the

intragranular spaces.

Once the pressing cycle has been completed, the object must be extracted from the mold.

Extraction is favored if the elastic return of the object is greater than that of the mold, because

this favors the detachment of the object from the walls. A considerable reduction in the force

required for extraction is given by the presence of lubricants in the mixture and on the walls of

the mold, which also reduce the wear of the mold and the superficial defects of the compacts.

The main defects of the compacts obtained by pressing are laminations and cracks. They are

generated, almost always, by the differential elastic behavior at the time of extraction, which is

in turn due to the pressure gradients in the compact, to the unevennes