MICROSTRUCTURE

1.5.1 POLYCRYSTALLINE MATERIALS

Crystal orientation has a significant effect on the physical properties o f crystalline solids. Factors like electrical conductivity, thermal expansion, elastic modulus etc. are greater in some direction through the crystal than in others. Such factors can affect the material and its application including gas se n sin g .H o w e v e r , in practice, many materials consist o f a very large number o f randomly oriented crystals deployed in a polycrystalline form and joined together via grain boundaries. The overall properties o f the material generally take the value averaged over the crystal orientations. If the crystallite size in a polycrystalline material is very much smaller than the component dimension, then this material generally has an isotropic distribution o f physical properties. There are exceptional circumstances where there is a marked anisotropy o f the single crystal and there is a preferred orientation o f the individual crystallites.

1.5.2 CONTROL OF MICROSTRUCTURE

As noted in section 1.3, microstructure o f the solid plays a major role in determining the response o f a gas sensor o f the type considered here. The more porous the solid is or the smaller the grains are in size then there is more surface area for the gas to interact with the solid.

Microstructure may be altered by altering the particle size in the preparation, changing the sintering temperature, or introducing substances which will generate a small amount o f a liquid phase at the chosen processing temperature.

Generally there is a gradual increase in the average size o f a particle as the firing temperature increases. Small grains get bigger and agglomerate together. The incentive for grain growth is provided by the reduction in relative surface area o f a body composed o f a few large grains relative to a body that has the same amount o f material but present as a large number o f small grains. Grain growth is essential for controlling grain size and hence the porosity o f the solid."^^^"^^

To achieve grain growth the solid is sintered. This is done by starting with a finely powdered material and taking the temperature up close to, but below the solidus. Taking the temperature above the solidus causes partial melting to occur and then matter is transported from one grain to another by means o f the liquid phase, which is acting as a transport medium.

At sub-solidus temperatures while sintering at the initial stages, an increase in the area o f inter-particle contact occurs with time. The formation o f ‘necks’ between the grains, which grow thicker with time and temperature, have the effect o f pulling the grains closer together. This will lead to increasing the density o f the body, which will eventually lead to the shrinkage o f the body, with the pores between the particles becoming smaller and losing their connectivity.

If the size o f these pores can be shrunk to zero or be ‘swept out’ to the surface o f the body by grain growth, then the bulk density o f the body approaches the tluoretical crystal density (figure 1.10).

( a )

Figure 1.10. Progressive stages o f sintering (a) loosely compacted powder (b) onset o f contact between grains (c) formation o f a porous 3-D network o f linked particles (d) formation o f a solid."^^ A. West, “Solid State Chemistry and its Applications”, John W iley & Sons, 1996, chapter 20, page 656.

Often, sintering o f a solid ceramic is controlled by the presence of small amounts o f liquid phase, present as a consequence o f deliberate or adventitious additions o f other phases. Then, liquid and crystal surface tension properties have a significant bearing on sintering kinetics and on

succeeding slag attack. The dihedral angle, the angle in the liquid phase between two crystalline grains, is dependent on grain growth and ultimately the texture o f the body (figure 1.11). A low dihedral angle gives a large amount o f liquid penetration between the grains and a small amount o f grain-to-grain contact (figure 1.11b). A small dihedral angle generally gives a more rapid grain growth and a larger final grain size. The initial grain size plays a major role in the kinetics o f grain growth, as a fine-grained body sinters more rapidly than a coarse grained one. In practice when sintering a powder compact, considerable attention must be paid to the fine grained starting materials, with a consequent high surface area.

The dihedral angle is important in determining the strength o f the material and the degree o f the slag attack as well as influencing grain growth kinetics. Therefore for a solid grain to grain contact, a large dihedral angle is required (figure 1.11c).

iquid (a)

d ih e d ra l an g le

(b) (c)

Figure 1.11. The dihedral angle and its effect on the amount o f grain-to- grain contact."^° A. West, “Solid State Chemistry and its Applications”, John Wiley & Sons, 1996, chapter 20, page 657.