Aluminium titanate (Al2TiO5) is a refractory compound made from one mole of alumina (Al2O3) and one mole of titania (TiO2). This polycrystalline ceramic material is normally prepared by reactive sintering of alumina and titania powders, forming a solid solution of stoichiometric proportions. Due to its good chemical resistance, low thermal conductivity, and high thermal shock resistance (as a result of a low thermal expansion coefficient), aluminium titanate can be an appropriate material for various technological applications such as foundry parts (nozzles, crucibles, pouring spouts), converters for motor vehicles, and moulds for glass industries .
Properties of aluminium titanate
High resistance to heat
The melting point of aluminium titanate is 1860°C, which makes it a good option for high-temperature monolithic supports used in catalytic converters, catalytic reactors, vehicle emissions control, and diesel particulate filters .
Thermal shock resistance
Aluminium titanate in combination with refractory oxide phases, such as mullite and celsian, produces an excellent thermal shock resistant ceramic. The presence of celsian reduces the melting temperature to less than 1600°C and also lowers the overall thermal expansion of the ceramic, with the celsian-mullite-iron aluminium titanate composition being the ceramic that has the lowest thermal expansion of the micro-cracked ceramics .
Low thermal expansion coefficient
A large number of micro-cracks exist inside aluminium titanate. It is important to control the micro-crack size and density as these characteristics dictate the main properties and the thermal expansion coefficient of the material. The lower the micro-crack size and density, the higher the thermal expansion and mechanical properties, and vice versa. The main way to overcome those cracks and defects is by introducing additives to aluminium titanate, such as SiO2, MgO, and ZrO2, which results in an increase in the mechanical properties of the material. Sintering temperature also seems to have an inversely proportional relationship with the mechanical properties and thermal expansion coefficient, in which an increase in the former causes a reduction in the latter .
Other properties include :
- Density: 3.1-3.35 g/cm3
- Elastic modulus: 10-20 GPa
- Vickers Hardness: 5 GPa
- Bending strength: 4-20 MPa
- Thermal conductivity (k): 1.5-2.5 W/mK
How to produce aluminium titanate
Aluminium titanate is produced by heating a combination of alumina and titania to a temperature above 1350°C at atmospheric pressure. However, this ceramic presents two major problems. Firstly, pure aluminium titanate is thermally unstable in a temperature range between 750°C and 1280°C. It tends to decompose between these temperatures during cooling, which would render the material useless for industrial applications. This can be controlled or delayed by doping with oxide additives such as magnesium oxide (MgO), titanium dioxide (TiO2), ferric oxide (Fe2O3), zirconium oxide (ZrO2) or silicon dioxide (SiO2) to stabilise the solid solution structure .
Secondly, the crystal structure anisotropy, which promotes the low coefficient of thermal expansion, results in micro-cracking. This micro-cracking lends the compound a poor mechanical resistance, which – together with the low thermal stability below 1280°C – limits its technical use. This can be addressed by preparing composite materials such as aluminium titanate-mullite-zirconium dioxide and aluminium titanate-mullite .
An important note in mixing additives is that they should not significantly decrease the aluminium titanate properties. Small increments (≤ 5% by weight) are added when making solid solutions. Taking all of these issues into account will significantly influence the production process and the main properties of the final product .
What are the applications of aluminium titanate?
Aluminium titanate is classified as a specialised material commonly used in extreme temperature conditions and found mostly in the non-ferrous molten metal industry. Some applications are [5,6]:
- Nozzles as casting mould inserts or mould joints
- Flow rods and retainer rings
- Riser tubes, feeders, and stalk for low-pressure die casting
- Sliding plates for flow control of molten masses
- Break rings for continuous casting of brass
- Dosing tubes for dosing furnaces
- Customised special components such as crucibles, casting tips, spacers, port liners, and casting moulds
- Bushes as casting mould inserts or mould joints
- Gate bushes, connectors, and cups
- Stoppers for flow control of molten masses
- Insulating rings
- Spouts and closing plates for flow control of molten masses
Porous aluminium titanate can also be a good candidate for use as a substrate in catalytic converters for motors and as a high-temperature flue gas filtration supporter . Other applications of aluminium titanate include spacing rings of catalytic converters, soot particulate filters in diesel engines, thermal insulation liners, launders, thermocouples, and master moulds in glass industries .