Materials with a low thermal expansion coefficient are those wherein the volume variations associated to temperature changes are very small.
Temperature-driven changes in the volume of materials is normally evaluated using the thermal expansion coefficient (TEC), which is defined as the variation in volume of a material with increasing temperature and must always refer to the temperature range wherein said variation was observed.
Materials with a low TEC are of great interest for a wide range of applications in very diverse fields wherein the dimensional stability of precision components must be ensured. The design of composites with a required (and homogeneous) TEC is therefore a challenge that may impact sectors such as aerospace seeking for increasing precision components.
The lithium aluminosilicate (LAS) family of ceramics and vitroceramics is frequently used for this purpose in many fields of application, from vitroceramics for kitchens to mirrors for satellites. Some mineral phases of this family have a negative TEC, which allows their use in composites with a controlled and customised TEC. Materials with a negative TEC often have low fracture resistance, as their negativity is due to a strong anisotropy between the different crystallographic orientations, wherein one of these usually exhibits negative behaviour and the other two positive behaviour. This anisotropy usually causes micro-fissures, resulting in low values in the mechanical properties of these materials.
The traditional manufacturing method of lithium aluminosilicate materials is based on the formation of glasses to produce vitroceramics. This method comprises the manufacture of molten material that is subsequently shaped and subjected to a thermal treatment for obtaining a partial crystallisation. The products obtained are thus frequently heterogeneous and with a reduced rigidity and resistance.
CINN researchers have overcome the before mentioned problems and have patented a new process for obtaining lithium aluminosilicate-based ceramic materials with a near-zero and negative thermal expansion coefficient within the temperature range (-150ºC. to 450º C). These materials lack of glassy phases and therefore feature enhanced mechanical properties and will allow the design of a new generation of nanocomposite materials with a “customised” TEC.