1.Mechanism of nano-alumina in Refractory

The application of nanotechnology in Refractory is mainly the application of nano-alumina. Because of its small size and large proportion of surface atoms, nano-alumina has high surface energy, high activity, instability and easy to combine with other atoms. Nano-alumina is mainly used in amorphous Refractory and some special Refractory. Adding nano-alumina to the amorphous Refractory in the form of binders and additives, using the surface and interface effects of nano-alumina to reduce the amount of cement used, thereby reducing the amount of water added and reducing harmful ingredients, improve some mechanical properties of castable. In the Refractory, nano-alumina is introduced as a micro-additive, which can improve the sinterability and microstructure of the product, thus affecting the mechanical properties of the product. When nano-alumina is added into the aggregate particles, the nano-powder is distributed among or within the particles, which causes the lattice of the grains to be distorted and promotes the sintering, and at the same time, many times interfaces are formed in the grains, it can cause crack deflection or crack pinning, improve fracture strength and toughness, improve high temperature resistance, and improve thermal shock resistance and high temperature creep resistance.

2. Effect on mechanical properties

Nano-alumina material has the following excellent properties: small particle size, large specific surface area and high chemical activity, can improve the sintering densification degree, Save Energy; Densification and homogenization of the Refractory structure can improve the performance and reliability of the materials, and the composition and structure of the materials can be controlled from the structural level of the nano-materials (1-100 mm) , it is beneficial to give full play to the potential properties of the material. In addition, since the particle size of the Refractory determines the microstructure and macro-properties of the material, the smaller the particle size, the smaller the defect size, if the particles are stacked evenly, the sintering shrinkage is consistent, and the grains are uniformly grown, the strength of the prepared material is correspondingly higher, which may present some unique properties that large granular materials do not possess. Adding a certain amount of nano-alumina to the Refractory significantly improves the strength and toughness of the Refractory, as well as other properties. It is generally considered that nano-powders have the following effects on the mechanical properties of Refractory.

(1) grain refinement factor. Adding nanomaterials to the Refractory can inhibit the growth of the matrix grains, homogenize the microstructure and improve the mechanical properties of the materials.

(2) microstructural factors. In the Micron system, the second phase particles at the Micron scale are distributed at the grain boundary of the matrix. In micron-nanocomposite materials, most of the nanoparticles form an inner crystalline structure in the matrix except for a certain amount of nanoparticles which are still at the matrix grain boundary, as shown in Fig. 1. The formation of internal crystal structure has the following effects on the mechanical properties of the material.

1) the crack deflection or pinning caused by residual stress can increase the fracture work and improve the material toughness.

2) potential nanocrystallization of micron grains. It is one of the main reasons to improve the strength of the material that the formation of“Inner Crystal” structure causes a lot of sub-grain boundaries and potential micro-crack sub-grain boundaries in the matrix.

3) the nanocrystalline effect is beneficial to the induction of transgranular fracture. On the one hand, transgranular fracture is induced by pinning of nano-particles in crystal, which strengthens the main grain boundary of matrix, on the other hand, it is caused by nano-particles in crystal. The main grain boundary is strengthened, the main crack does not propagate along the grain boundary of the micro-matrix, but propagates along the matrix grain. The residual stress field near the nano-particles in the grain makes the crack deflect and pin, which makes the crack propagation path very complicated and blocked in many places. Therefore, it is considered that outburst fracture is an important factor to strengthen and toughen the material.