Oxides
These include, magnesia, alumina, zirconia, beryllia and thoria. Other oxides which are uytilized sparingly due to high cost include yttria, ceria and hafnia. Variety of shapes in oxide refractories may be obtained by process of slip casting. This is a procedure in which fine particles of oxides are mixed in temporary organic binders such like latex, wax starch or polyvinyl alcohol. Then the mixture is moulded into desired shape built in plaster of paris mould. After the mould has dried sufficiently so that it might be handled it is fired at a temperature of approx 1800oC. At this temperature the fine particles sinter rapidly, facilitating the solid surface reaction among fine particles. The resulting material is self bonded refractory of coherent structure whose refractoriness approaches that of the material itself.
Other methods of forming refractories comprise pressing, isostatic pressing & hot pressing.
In simple pressing the raw particulates are pressed dry, or wet in a die of desired shape. In isostatic pressing the particulate material is pressed in all of the direction by hydraulic pressure against flexible walls of shaped container. The part obtained is fired to obtained desired strength. In hot pressing the compacting and sintering occurs simultaneously. The force might be applied axially or it might be isostatic.
While silica, alumina and magnesia have been utilized to produce variety of shapes, thoria and beryllia are limited to small sizes due to cost and difficulty of shaping. Alumina is hardest oxide and utilized where resistance to wear is of prime importance at high temperatures. It is utilized as pyrometer tubes, spark plug insulation & crucible for melting metals and glasses. It has markedly low porosity & high strength. This generates a porosity free microstructure. Alunina is also utilized for high quality electrical applications where low dielectric loss and high resistivity are needed.
Beryllia has high melting point, high resistance to thermal shock and is inert to carbon and carbon monoxide up to 2000oC. However, this is much costlier than alumina and toxic in nature. Magnesia is characaterised by extremely high melting point of 2800oC. Its high vapour pressure, however, restricts its use to 2200oC in an oxidising atmosphere and to 1700oC in reducing atmosphere. Magnesia contains higher thermal conductivity, greater refractoriness and better resistance to basic slags. It is utilized against corrosion at high temperatures and for electrical purposes as an insulator. Thoria also has extremely high melting point and is utilized for such applications as melting of platinum. Zirconia also has very high melting point of 2700oC but may not be utilized at high temperature as it losses its resistance to thermal shock at temperature of about 1100oC to 1300oC. This loss in shock resistance occurs because of polymorphic transformation in which monoclinic crystals of zirconia are formed from denser tetragonal from resulting into volume expansion and tendency to crack. Though, the structure of zirconia is stabilised into cubic form by addition of calcium oxide and/or magnesium oxide. Therefore, transformation generates much smaller volume changes. Adding 9% MgO with special heat treatment, a partially stabilised zirconia (PSZ) is achieved. This material develops much in fracture toughness. Truly stable zirconia shall be much stronger (above 400 MPa) and shall have high refractoriness. Such product is under development and shall have much wider applications. Zirconia products are utilized for oxygen probes, small crucible & dishes etc. and for heating elements in air at high temperature.