Impact of Aluminum Oxide Nanopowder on the Mechanical and Sintering Properties of Refractory Materials? The Influence of Ultra-fine Nanometer Aluminum Oxide on Mechanical Properties
Nano materials possess excellent properties such as small particle size, large specific surface area, and high chemical activity, which can significantly enhance the densification degree of materials during sintering and save energy. Adding a certain amount of nano powder to refractory materials can significantly improve their strength and toughness, and other properties of the refractory materials will also be greatly improved. It is generally believed that the influencing factors of nano powder on the mechanical properties of refractory materials are as follows:
(1) Grain Refinement Factor. Adding nano-powders to refractory materials can inhibit the growth of matrix particles, resulting in a uniform microstructure and enhancing the material's mechanical properties. Ultra-fine nano-alumina (2) Microstructure Factor. In micron-sized systems, bimodal particles of micron size are distributed at the grain boundaries of the matrix. In micro-nano composite materials, in addition to a certain amount of nano-particles still at the grain boundaries of the matrix, most nano-particles form an intragranular structure within the matrix. The formation of intragranular structures has the following effects on the material's mechanical properties: ① Residual stresses cause crack deflection or pinning to increase the material's fracture work, thereby enhancing its toughness; ② Potential nanocrystallization of micron particles. The formation of "intragranular" structures leads to a large number of sub-grain boundaries and potential micro-cracks within the matrix, with the formation of sub-grain boundaries making the matrix finer, which is one of the main reasons for the further increase in material strength. (3) Nano-effects are conducive to the induction of transgranular fracture. On one hand, transgranular fracture is induced by the pinning effect of nano-particles within the crystal, which strengthens the primary grain boundaries of the matrix; on the other hand, the nano-effect of matrix particles caused by intragranular nano-particles. The result of these effects is the strengthening of primary grain boundaries, with main cracks extending along the matrix grains rather than the micron matrix grain boundaries. The residual stress field near nano-particles within the grains causes crack deflection and pinning, making the crack propagation path extremely tortuous and complex, and blocked in many places. Therefore, it is considered that the induction of transgranular fracture is an important factor in the toughening of materials. Although the application of nano-powders in refractory materials is an extension of the application of ultra-fine powders in the field of refractory materials, there are few reports on this aspect, and further research is needed. For amorphous refractory materials, the focus should be on the agglomeration, size, shape, and rheological properties of nano-powders. The surface activity and size effects of nano-powders on the sinterability and mechanical properties of shaped refractory materials should be studied.
2. Influence on Sintering Performance
The enormous specific surface area of nanoscale powders results in a sharp increase in surface energy as a driving force for powder sintering, leading to increased diffusion rates and reduced diffusion paths. During the sintering process involving chemical reactions, the increased contact area between particles enhances the likelihood of reactions and accelerates reaction rates. This all contributes to a lower activation energy for sintering, faster overall sintering speed, reduced sintering temperatures, and shorter sintering times. However, the entire sintering process, including grain growth during recrystallization, is also accelerated. The decrease in sintering temperature and the reduction in sintering time, however, slow down the recrystallization process. It is necessary to re-examine and study the effects of these synergistic and inhibitory factors to establish a kinetics suitable for nanoscale particle sintering.
Nanoparticles have significantly lower melting points, sintering temperatures, and crystallization temperatures compared to conventional powders. The small size of nanoparticle volumes results in high surface free energy and a higher atomic surface area. These surface atoms have incomplete coordination, high activity, and are much smaller in volume than bulk materials, leading to a minimal increase in internal energy required for melting and a sharp decrease in melting points. During the sintering process, high interfaces can act as a driving force for atomic movement, benefiting the shrinkage of interface voids and the proliferation of vacancy clusters. Therefore, low-temperature sintering can achieve densification, i.e., lower sintering temperatures.
