Composites are important engineering materials being adopted by numerous applications. Once the type of the materials and content of each component are selected, mechanical behaviors of a composite are highly dependent on the geometric arrangement of the components. This dissertation takes a zirconia nanoparticle (NP) reinforced resin-based composite, which is widely utilized in dental, optical and many other industries, as study object to investigate the effect of geometric arrangement of ceramic NPs on the elastic behavior of the composite. Once aggregated NPs demonstrates superior performance over randomly scattered NPs, a new method to assemble ceramic NPs is proposed.

Firstly, the effect of geometric arrangement of spherical zirconia NPs on the elastic behavior of a resin-based composite is investigated by using finite element analysis (FEA) in Chapter 3. NPs are arranged in body centered cubic (BCC) pattern or face centered cubic (FCC) pattern in resin-based composite unit cell. Degree of anisotropy, which is denoted by Zener ratio (ZR) in this work, along with elastic properties such as bulk modulus, Young’s modulus, shear modulus and Poisson’s ratio, are evaluated. For NP content ranging from 10 to 40 volume percentage (vol%), ZR highly depends on the geometric arrangement of the NPs. By tuning geometry parameter, two isotropic unit cells can be constructed at each vol% for BCC structures; depending on vol%, one or two isotropic unit cells can be constructed for FCC structures. Young’s modulus, Poisson’s ratio and shear modulus are found to be significantly dependent on the geometric arrangement as well. By contrast, bulk modulus is least affected by geometry arrangement at given NP vol%. Bulk modulus is therefore able to be utilized as the characteristic parameter to represent linear elastic behavior of composite containing isolated particles in Chapter 4. In addition, substituting NPs by voids, elastic behaviors of porous metamaterial are studied as well. In general, geometric arrangement of the voids shows similar effect on the elastic behavior of the metamaterial as that of the NPs on composite. In summary, this chapter identifies isotropic and anisotropic designs and studies their elastic mechanical properties. The investigation may provide guidance to utilize such designs in structural and biomedical applications.

Secondly, effect of primary NP aggregates, in which NPs are fused together by chemical bonds, on bulk modulus and stress concentration status of a resin-based composite is investigated with FEA in Chapter 4. A novel representative volume element (RVE) of resin-based composite reinforced by primary aggregates formed by zirconia NPs are constructed. Primary aggregates are generated by a diffusion limited aggregation algorithm. FEA results suggests that, given the same NP aggregate vol%, both bulk modulus of the composite and stress concentration factor of the resin correlate with the size of aggregates positively, although the former shows a stronger dependency. Hence, balance between modulus/stiffness and stress concentration factors shall be considered in the design of aggregates reinforced composites. The strategy shall be enhancing modulus/stiffness while maintaining fracture resistance of the matrix. Furthermore, since bulk modulus is demonstrated, in the 3rd chapter of this dissertation, not dependent on anisotropic status, its dependency on the size of aggregates has little to do with anisotropy but mainly from the size of the aggregates.

Finally, suggested by the 4th chapter of this dissertation, primary NP aggregates may enhance elastic properties of the composite. To realize primary NP aggregates, ultrasonication was reported as an effective and economic friendly experimental method by literature. However, proper explanation on the phenomenon is still in short. This work investigates aggregation mechanism of zirconia NP with the help of molecular dynamics (MD) simulation and identifies it as melting and subsequent quenching of the local atoms around impact facets between colliding NPs. Value of the critical velocity is one-third of that estimated by complete-melting mechanism, which suggested that NPs were too small to be aggregated by ultrasonication, in literature. The partial melting mechanism explains experimental phenomena observed by literature successfully, and therefore is validated as a more realistic mechanism to predict NP aggregation. Furthermore, the work proves that ceramic NPs, despite of their low diffusivity, could be aggregated through ultrasonication, which proposes a convenient manufacturing method to assemble ceramic nanoparticles.