| Ion exchange techniques are often used in the fabrication of devices for photonic applications, such as micro-optics and waveguides. However, device performance depends on the ability to predict and control the exchange to achieve the desired concentration profile. In many glass systems, the diffusion coefficient is strongly dependent on the dopant concentration. This is a manifestation of the Mixed Mobile Ion Effect (MMIE). Although the MMIE may be viewed mostly as an unwanted complication for waveguide fabrication, its presence is required for fabrication of micro-optics, where a concentration dependence is needed to achieve the necessary profiles. However, to date it has most often been characterized only empirically--improved methods of describing, predicting and controlling ion exchanges for micro-optics are needed. In this work, the relationship between this concentration dependence and the chemical structure of the glass is examined in the commercially-important silver-for-sodium ion exchange pair.;The research utilized three approaches to investigate diffusion structure/properties relationships: structural investigations, modeling of the concentration dependence and studies measuring the diffusion coefficient. X-ray Absorption Fine Structure (XAFS) studies were performed on the environments of the mobile cations in silicate glasses. These studies are used to help formulate a structural picture of ion exchange in which the interactions between the mobile cations affect ion transport rates. This structural picture is related to the diffusion rate through the use of the Modified Quasi-Chemical (MQC) Diffusion Coefficient, a new formulation that models the diffusion coefficient based on cation mobilities, the number of nearest cation neighbors and cation-cation interaction energies. The MQC expression successfully models the strong concentration dependence observed in real glass systems. This model is applied to ion exchanges performed into endpoint glasses of a sodium aluminosilicate series. It is found that the non-bridging oxygen (NBO) content of the glass has a major impact on both the chermical structure of the glass and the concentration dependence of the diffusion coefficient. Specifically, the NBO-rich glass has the highest excess interaction energy and exhibits the greatest MMIE, consistent with the structural picture. |
