**3. Processing and sintering of transparent ceramic materials**

Although many transparent ceramics are single crystal materials, transparent polycrystalline ceramics have different advantages such as low price, ease of manufacture, mass-production and more versatility in properties. Furthermore, the capability of producing complex shapes could broaden the fields of application.

The classical issues of ceramic processing are related with powder quality, purity, defect free processing and elimination of minor defects and pores. When the aim is to reach transparency, requirements will be similar from a qualitative point of view but noticeably more restrictive from a quantitative point of view. In addition, microstructural features such as maximum pore and grain size (the latter for birefringent materials) are critical in scattering process. The requirements for these parameters depend on the wavelength at which transparency is desired. In general, as it will be discussed in section 4, it is assumed that defect size must be < λ/10 in order to obtain transparent materials.

Then, in this chapter the processing and sintering techniques most widely used in ceramic manufacturing will be revised, describing the key parameters to be controlled and their effect on the optical properties of the final material.

Sintering to Transparency of Polycrystalline Ceramic Materials 535

Stability of colloidal suspensions depends on particle size and their surface properties. The particles must not be too large otherwise gravity will produce rapid sedimentation. On the other hand, if the attractive force between the particles is large enough, the particles will

Flocculation will therefore occur unless some process is promoted to produce repulsion between the particles which is sufficiently strong to overcome the attractive force. There are

1. Electrostatic stabilization in which the repulsion between the particles is based on

2. Steric stabilization in which the repulsion is produced by uncharged polymer chains

3. Electrosteric stabilization, consisting of a combination of electrostatic and steric repulsion, achieved by the adsorption of charged polymers (polyelectrolytes) onto the

Rheological measurements are widely used to characterize the properties of colloidal suspensions. They can be used as a method of analysis as, for example, in determining the optimum concentration of dispersant required to stabilize a suspension by measuring the

Once the stable colloidal suspension is prepared, two alternative processes can be distinguished; direct consolidation in order to form a green body or drying under controlled

In the first case, the processes are known as colloidal forming techniques and their interest is due to the complete deagglomeration of starting powders, reducing the generation of defects. However, other limitations such as difficulties for obtaining larger or complex parts with simultaneously thin and thick cross sections due to density gradients in the green body or problems like the differential sedimentation due to the particle size distribution in the starting powders are found. Slip-casting will be described as an example of colloidal

Slip casting, with or without pressure, constitutes an ideal combination of dewatering and shaping. As much of the slurry liquid at or near the mould surface is absorbed in the pores in the mould, a layer of solid is formed by the interlocking solid particles in the region near the mould surface. As the process continues, this solid layer increases so long as the mould

The biggest advantage of this process is its versatility in terms of shape, size and materials applicability. It can also accommodate a range of particle sizes, working with typical particle sizes from a fraction of a micron (green densities of ~ 40-50%) to several microns. It can also work fairly well with finer, nanoscale particles. A comparison between slip casting and uniaxial pressing of yttria ceramics show how the minimization of density gradients in green bodies prepared by slip casting allows obtaining more homogeneous materials in

viscosity of the suspension as a function of the concentration of dispersant added.

collide and stick together, leading to rapid sedimentation of particle clusters.

several ways for achieving this, but the most commonly used are:

electrostatic charges on the particles

adsorbed onto to the particle surfaces

conditions in order to produce a ready to press powder.

pores continue to absorb the liquid of the slurry.

terms of translucency and microstructure (J. Mouzon et al., 2008).

particle surfaces.

forming technique.

**Slip-Casting** 

Ceramic fabrication methods can be classified in different categories depending on the starting materials involved (gas, liquid or solid phase). Polycrystalline ceramics are usually manufactured by compacting powder to a body which is then sintered at high temperatures. The geometry, production volume and characteristic requirements for the component govern the choice of manufacturing process. Alternatively, ceramic materials can be simultaneously formed and sintered when pressure assisted sintering techniques are used (see Fig. 3.1).

Fig. 3.1. Flow chart for the production of polycrystalline ceramics.

#### **Polycrystalline ceramic processing**

The processing steps involved in conventional fabrication of ceramics can be divided in two main parts; formation of green body and firing, and both of them must be carefully controlled in order to avoid residual porosity in the final material.

In the first part, the preparation of shaped green body from ceramic powders, three operations can be identified: mixing, consolidation and debinding. Mixing process includes the preparation of stable slurries from ceramic powders by addition of dispersants or pH controllers and the incorporation of binders and other additives. Consolidation can be done directly from wet slurries or after preparing conditioned powders. Finally, a debinding process for removing the additives used in previous actions before sintering is required.

Although the properties of the starting ceramic powders also play an important role in their behaviour during shaping and sintering, in this review the different ceramic powder synthesis methods will not be described. The description will be focused on the influence of the different processing steps starting from commercial raw materials.

Wet processing of ceramic powders is usually done in order to avoid the formation of undesired powder aggregates that could be responsible of microstructural defects in the final material. As it has been previously mentioned, when the aim is to obtain a transparent material, the critical flaw size above which an important loss of transmittance is observed is very small.

In the processing of ceramics, colloidal suspensions, consisting of a dispersion of solid particles in a liquid, are of particular interest. They are being used increasingly in the consolidation of ceramic powders to produce the green body. Compared to powder consolidation in the dry state, colloidal methods can lead to better packing uniformity in the green body which, in turn, leads to a better microstructural control during firing. Moreover, colloidal suspensions are usually prepared in order to obtain conditioned powders to be consolidated by dye pressing.

Stability of colloidal suspensions depends on particle size and their surface properties. The particles must not be too large otherwise gravity will produce rapid sedimentation. On the other hand, if the attractive force between the particles is large enough, the particles will collide and stick together, leading to rapid sedimentation of particle clusters.

Flocculation will therefore occur unless some process is promoted to produce repulsion between the particles which is sufficiently strong to overcome the attractive force. There are several ways for achieving this, but the most commonly used are:


Rheological measurements are widely used to characterize the properties of colloidal suspensions. They can be used as a method of analysis as, for example, in determining the optimum concentration of dispersant required to stabilize a suspension by measuring the viscosity of the suspension as a function of the concentration of dispersant added.

Once the stable colloidal suspension is prepared, two alternative processes can be distinguished; direct consolidation in order to form a green body or drying under controlled conditions in order to produce a ready to press powder.

In the first case, the processes are known as colloidal forming techniques and their interest is due to the complete deagglomeration of starting powders, reducing the generation of defects. However, other limitations such as difficulties for obtaining larger or complex parts with simultaneously thin and thick cross sections due to density gradients in the green body or problems like the differential sedimentation due to the particle size distribution in the starting powders are found. Slip-casting will be described as an example of colloidal forming technique.

### **Slip-Casting**

534 Sintering of Ceramics – New Emerging Techniques

Ceramic fabrication methods can be classified in different categories depending on the starting materials involved (gas, liquid or solid phase). Polycrystalline ceramics are usually manufactured by compacting powder to a body which is then sintered at high temperatures. The geometry, production volume and characteristic requirements for the component govern the choice of manufacturing process. Alternatively, ceramic materials can be simultaneously formed and sintered when pressure assisted sintering techniques are used

The processing steps involved in conventional fabrication of ceramics can be divided in two main parts; formation of green body and firing, and both of them must be carefully

Consolidation Debinding Firing

Pressure assisted sintering techniques

Green Body

Polycrystalline product

In the first part, the preparation of shaped green body from ceramic powders, three operations can be identified: mixing, consolidation and debinding. Mixing process includes the preparation of stable slurries from ceramic powders by addition of dispersants or pH controllers and the incorporation of binders and other additives. Consolidation can be done directly from wet slurries or after preparing conditioned powders. Finally, a debinding process for removing the additives used in previous actions before sintering is required.

Although the properties of the starting ceramic powders also play an important role in their behaviour during shaping and sintering, in this review the different ceramic powder synthesis methods will not be described. The description will be focused on the influence of

Wet processing of ceramic powders is usually done in order to avoid the formation of undesired powder aggregates that could be responsible of microstructural defects in the final material. As it has been previously mentioned, when the aim is to obtain a transparent material, the critical flaw size above which an important loss of transmittance is observed is

In the processing of ceramics, colloidal suspensions, consisting of a dispersion of solid particles in a liquid, are of particular interest. They are being used increasingly in the consolidation of ceramic powders to produce the green body. Compared to powder consolidation in the dry state, colloidal methods can lead to better packing uniformity in the green body which, in turn, leads to a better microstructural control during firing. Moreover, colloidal suspensions are usually prepared in order to obtain conditioned powders to be

Fig. 3.1. Flow chart for the production of polycrystalline ceramics.

Mixing Shaped

controlled in order to avoid residual porosity in the final material.

the different processing steps starting from commercial raw materials.

**Polycrystalline ceramic processing** 

(see Fig. 3.1).

Starting Powder

very small.

consolidated by dye pressing.

Slip casting, with or without pressure, constitutes an ideal combination of dewatering and shaping. As much of the slurry liquid at or near the mould surface is absorbed in the pores in the mould, a layer of solid is formed by the interlocking solid particles in the region near the mould surface. As the process continues, this solid layer increases so long as the mould pores continue to absorb the liquid of the slurry.

The biggest advantage of this process is its versatility in terms of shape, size and materials applicability. It can also accommodate a range of particle sizes, working with typical particle sizes from a fraction of a micron (green densities of ~ 40-50%) to several microns. It can also work fairly well with finer, nanoscale particles. A comparison between slip casting and uniaxial pressing of yttria ceramics show how the minimization of density gradients in green bodies prepared by slip casting allows obtaining more homogeneous materials in terms of translucency and microstructure (J. Mouzon et al., 2008).

Sintering to Transparency of Polycrystalline Ceramic Materials 537

The sprays are produced by either rotary (wheel) or nozzle atomizers. Evaporation of moisture from the droplets and formation of dry particles proceed under controlled temperature and airflow conditions. Powder is discharged continuously from the drying chamber. Operating conditions and dryer design are selected according to the drying

Spray formation is usually coupled to freeze drying process. This technique is named spray

1. Atomization of liquid solutions or suspension using ultrasound, one-or two fluid

3. Ice sublimation at low temperature and pressure or alternatively atmospheric freeze-

The advantage of using conditioned powder for obtaining transparent ceramic materials is known (I. Amato et al., 1976). The enhanced behaviour during compaction of spray dried or freeze dried powders leads to a more homogeneous distribution of particles in the green body and finally a reduction in the residual porosity of the material. Nevertheless, the incorporation of additives during slurry preparation or binders for favouring soft granulation of powders makes an additional process before firing necessary. This process is

Binders, which are used in the slip casting process or in pressing, give the green body a sufficient strength by gluing together particles at their boundary surfaces. Usually those binders are based on polyvinyl alcohols (PVA), polyacrylate or cellulose. High-polymeric compounds such as cellulose and polysaccharides work as plastification agents. They make

The thermal treatment of the debinding process destroys the polymers by oxidation or combustion in oxygen containing atmosphere. Very often it is an uncontrolled reaction of

Fig. 3.2. Spray-drying and freeze–drying processes.

characteristics of the product and powder specification.

nozzles or vibrating orifice droplet generators

drying using a cold desiccant gas stream

the flow of ceramic masses during extruding possible.

2. Freezing of the droplets in a cryogenic liquid or cryogenic vapour

freeze drying. This process consists of

named debinding.

However, slip casting has limitations, which, from an operational point of view, are primarily its slow casting rates, with thickness proportional to the square root of casting time, and hence increased cost of casting and of drying to avoid cracking, as well as costs for preparing and maintaining a large mould inventory and facilities for mould storage and drying. Using pressure to accelerate the dewatering process improves the productivity of the process. The main difference in comparison with slip casting is that the water is not removed by capillary suction (negative pressure in the plaster mould) but by pressurisation (positive slip pressure). Control of the filtration process is based on four parameters; the pressure differential on the body, the liquid-medium viscosity, the specific surface area of the slip's solids content and the body porosity (body formation is dependent on the permeability of the layer of body that has already formed from filtered material).

In order to solve those problems a great diversity of advanced forming techniques has been developed. Amongst them, aqueous injection moulding, centrifugal slip casting, direct coagulation casting, electrophoretic casting, gelcasting, hydrolysis assisted solidifications, etc can be mentioned. In all cases, the objective is to obtain a very homogenous green body in order to facilitate the preparation of defect free materials.

However, in other cases, stable suspensions of ceramic powders are prepared in order to produce dry powders especially conditioned for forming by pressing. In this case, the most critical issue is to avoid the formation of hard agglomerates that could lead to formation of defects in the final material which could not be removed during firing. Two drying techniques are especially suitable for this purpose; spray drying and freeze-drying.
