Low-density ceramic materials, ceramic composites and fibres are widely used for their refractory properties in many high-temperature applications. The materials provide low thermal conductivity and high resistance to chemical attack.
Typical Refractory Components (courtesy of Christy Refractories)
The high melting point and excellent chemical properties of zirconia would suggest its use as a refractory. However, the tetragonal-monoclinic phase transformation and the associated volume change preclude the use of unstabilised zirconia in the bulk form.
It is, however, fairly common practice to use zirconia in a finely divided dispersed form to enhance the thermal shock resistance of other ceramic materials. It is added to alumina for use in the sliding gate valves which are an integral part of the continuous casting process for steel and also for nozzles and stoppers of transfer and holding ladles, where the combined effect of thermal shock and erosion would lead to the rapid failure of other materials.
The superior properties of zirconia-toughened refractories are thought to be due to microcracks generated in the vicinity of the zirconia particles, which help to arrest any cracks propagating as a result of thermal stresses. Thus a refractory, although not of great strength, would have a superior crack stopping ability and would find applications where integrity of the structure is all important.
Zirconia Refractory Production
Zirconia for use as a refractory is made by two processes. A stabilising oxide, e.g. MgO, is added in the desired amount, typically 6 mol%, briquetted and fired to a temperature above 1600°C. This converts the main body of the zirconia to the cubic phase.
The fired briquettes are then ground in a ball mill to <10 mm, using steel grinding media. The powder is then washed in dilute HC1, to remove impurities and used to form a casting slip (pH@ 3). Drying produces a shrinkage of 2-4% and following firing at typically 1900°C, a further contraction of ~ 15% occurs.
Fused stabilised zirconia can also be produced using an electric arc process, the product containing more than 90% of the cubic phase. It is thus able to withstand repeated thermal cycling and a maximum temperature of 2400°C under oxidising conditions.
In the loose granulated form this material is often used as a thermal insulation refractory, in ultra-high-temperature furnaces. The granules may also be milled and sintered to form dense bulk refractories.
At high temperatures although amphoteric and comparatively stable to both acid and basic slags and glasses, stabilised zirconia can be destabilised on prolonged contact with siliceous and alumina silicate refractories above 1400°C. The best backing material for the ultra-high-temperature applications in refractory walls is thus a porous insulating stabilised zirconia brick.
Recently, low density, low thermal conductivity insulating material has become available in the form of fibre, paper, felt, board, and shaped articles. The zirconia is a cubic solid solution stabilised with additions of yttria and has in the purest form a maximum usable temperature of >2100°C.
Alternative material such as zirconia bonded with zircon is available, but has a lower maximum operating temperature of 1650-1700°C.
The manufacturing process is proprietary although certain details have been made available which outline the general fabrication route.
The innovative fabrication technique involves the use of an organic precursor fibre, rayon, as an internal former. The organic fibre is impregnated with an aqueous solution of zirconium chloride and yttrium chloride.
On drying the metallic salts are deposited within the organic fibre which can then be burnt off by controlled oxidation; the fibres are then fired at sufficiently high temperatures (800-1300°C) to first induce crystallisation and finally the sintering of the particles of oxide to develop a ceramic bond.
The material can be produced in a wide range of conventional forms, as monofilaments or as rigid boards and shapes. The material offers outstanding properties; notable stability to its melting point ~ 2600°C, corrosion resistance to hot alkalis and many chemicals, and it is not wetted by a range of molten metals.
Its uses are many and varied, ranging from highly efficient thermal insulation to separators in aerospace batteries, hot gas filters and electrolysis diaphragms. A particularly topical application under test is its use as a chemical and thermal barrier to be placed beneath the core of a nuclear reactor, where it would act as both a chemical and thermal shroud in the event of a meltdown.