Freeze Drying.ppt

8/10/2019 Freeze Drying.ppt

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Freeze Drying A solution of metal salt is broken up by an atomizer into fine droplets,

which are then frozen rapidly by being sprayed into a cold bath of

immiscible liquid such as hexane and dry ice or directly into liquid nitrogen

The frozen droplets are then placed in a cooled vacuum chamber and the

solvent is removed, under the action of a vacuum, by sublimation without

any melting.

The technique produces spherical agglomerates of fine primary particles

with the agglomerate size being the same size as that of the frozen

droplets.

The size of the primary particles (in the range of 10500 nm) depends on

the processing parameters such as the rate of freezing, the concentration

of metal salt in the solution, and the chemical composition of the salt.

After drying, the salt is decomposed at elevated temperatures to produce

an oxide.


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Gel Routes

The formation of the gel or resin, mixing of the constituents occurs on the

atomic scale by a polymerization process.

Provided none of the constituents are volatilized during the

decomposition and calcination steps, then the cation composition of the

powder can be identical to that of the original solution.

A drawback is that the decomposition product is commonly not in the

form of a powder but consists of charred lumps.

These lumps have to be ground and calcined to achieve the desired

powder characteristics.

Gel routes to ceramic powders are currently used mainly at the laboratoryscale.


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SolGel Processing

1. The Pechini Method

The method has since been applied to many complex oxide

compositions

Metal ions from starting materials such as carbonates, nitrates, and

alkoxides are complexed in an aqueous solution with -carboxylic acidssuch as citric acid.

When heated with a polyhydroxy alcohol, such as ethylene glycol,

polyesterification occurs, and on removal of the excess liquid, a

transparent resin is formed.

The resin is then heated to decompose the organic constituents,ground, and calcined to produce the powder.


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2. The Citrate Gel Method

The citrate gel method was developed by Marcilly et al. and can be

illustrated by the synthesis of the ceramic superconductor YBa2Cu3O7-x.

Nitrate solu- tions of Y, Ba, and Cu were added to citric acid solution, and

the pH was kept at 6 to prevent precipitation of barium nitrate.Heating the solution at 75C in air produced a viscous liquid containing

polybasic chelates.

Further heating at 85C in a vacuum produced an amorphous solid that was

pyrolyzed in air at 900C to produce a crystalline powder.

3. The Glycine Nitrate Process

The glycine nitrate process is one of a general class of combustion

methods for the preparation of ceramic powders.

A highly viscous mass formed by evaporation of a solution of metal

nitrates and glycine is ignited to produce the powder.

Glycine, an amino acid, forms complexes with the metal ions in solution

which increases the solubility and prevents the precipitation of the metal

ions as the water is evaporated.

Glycine also serves another important function: it provides a fuel for the

ignition step of the process since it is oxidized by the nitrate ions.


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The reactions occurring during ignition are highly explosive, and extreme caremust be exercised during this step.Normally, only small quantities should be

ignited at a time.

Under well controlled conditions, a loose mass of very fine, crystalline powder

(particle size less than a tens of nanometers) is obtained after ignition.

When compared to the Pechini method, grinding and calcination of the

product are not required.The very fine size and crystalline nature of the powder is believed to be a direct

result of the short exposure to high temperatures during the ignition step.

With adequate process control, the glycine nitrate process offers a relatively

inexpensive route to the preparation of very fine, chemically homogeneous

powders.

It has been used for the preparation of simple oxides as well as

complexoxides(e.g.,manganites,chromites,ferrites,andoxidesuperconductors).


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Nonaqueous Liquid Reaction Reactions involving nonaqueous liquids have been used for the synthesis of

Si3N4 and other nonoxide powders.

The advantage of these methods is the higher purity and finer particle size

of the powder compared with methods that involve grinding of a solid

product.

The reaction between liquid SiCl4 and liquid NH3 has been used on an

industrial scale by UBE Industries (Japan) to produce Si3N4 powder.

SiCl4 + NH3 Si(NH)2 + NH4Cl

A more involved sequence of reactions occurs, involving the formation of

poly- meric silicon diimide and ammonium chloride triammoniate,

NH4Cl3NH3. Silicon diimide decomposes according to the overall reaction

3Si(NH)2 Si3N4 + N2 + 3H2

In the UBE process, the products formed by the interfacial reaction

between the SiCl4 and NH3 liquids are collected and washed with liquid

NH3 and calcined at 1000 C to produce an amorphous Si3N4 powder.

Subsequent calcination at 1550 C in N 2 yields a crystalline powder with a

particle size of 0.2 mm.

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Freeze Drying.ppt