Werkstoffe der technischen Keramik

3.4.2.5 Aluminium Titanate

Aluminium titanate (ATI) ceramic is the stoichiometric solid solution of aluminium oxide and titanium dioxide (Al2O3 . TiO2), and is distinguished by:

• low modulus of elasticity
  
• low thermal conductivity,
  
• very low coefficients of thermal expansion, leading to excellent thermal shock resistance and to
  
• low wettability by molten non-ferrous metals

These properties result from a high a residual porosity and from micro-cracks resulting from highly anisotropic thermal expansion in the individual ATI crystals. These crystals display very different coefficients of thermal expansion along their three primary axes. In two directions it is positive, i.e. there is an expansion as the temperature rises, but along the third axis it is negative, generating a contraction. As a result of this, during manufacture, and in particular during cooling, tensions develop on a microscopic scale, the consequence of which is the formation of micro-cracks in the microstructure.
In terms of the macroscopic expansion, due to the anisotropic thermal expansion of the individual crystals, the material yields a very low thermal expansion, since this initially involves the micro-cracks; in other words, the cracks close.

Figure 18: Microstructure of an aluminium titanate

The outstanding thermal shock resistance of ATI results from the low coefficient of thermal expansion on the macroscopic scale, in combination with the low modulus of elasticity. The dependency of the strength and of the modulus of elasticity on temperature also have what for ceramics are unusual curves. Both values increase as the temperature rises, effects that are again due to the closing of the micro-cracks at higher temperatures. The low mechanical strength specific to this material can be compensated for through composite structures.

The material is used, for instance, in portliners, cylinder liners and vehicle engines, in the spacing rings of catalytic converters, as kiln furniture and in foundry engineering. Here it is particularly found in aluminium smelting works where it is used for rising pipes and proportioning pipes, nozzles, crucibles, flow regulators and so forth.

	ATI