Microstructure design and synthesis of transparent zinc oxide functional materials

[Truncated] This PhD thesis reports a study on achieving visible light transparency for dispersedparticle systems that consist of micro-sized porous ZnO particles dispersed in acontinuous transparent medium, with the aim for their application as UV light absorbingproducts. The scientific argument of the thesis may be abstracted as following. For UVprotection, adequate UV absorbing dispersant particle loading in the system is required.For aesthetic reasons the product is required to be transparent. Whereas visible lighttransparency of dispersed particle systems can be achieved by using nano-sizeddispersants, nano-sized dispersants cause health and safety concerns as skin-contactproducts.

The aims of this study are:

To establish the theoretical background for visible light transparency ofdispersed particle systems with large particle dispersants;

To design and to synthesize porous ZnO micro-particles as suitable dispersant intransparent dispersed particle systems;

To characterize the visible light transparency and UV absorbing characteristicsof prototype systems as validation of the theory and the materials created.

The achievement of these aims has the potential to provide a formulation for transparentsunscreen products to meet health and safety requirement as well as the aesthetics andUV protection requirements by the sunscreen market, and other potential applicationsrequiring UV protection and high visible light transparency. The specific outcomes ofthis study are summarized as following:

(1) Visible light transparency of dispersed particle systems containing large particles

The fundamental theories of materials transparency and particle scattering were studied,and the theoretical background of achieving visible light transparency in dispersedparticle systems containing large-particle dispersant is established. The maindiscoveries are: (i) In a dispersed particle system where the particles and the mediumare non-absorbers of visible light, particle scattering is the main reason of lowtransparency of the system; (ii) Light scattering of dispersed particle systems isinfluenced by the ratio of refractive index of the particles to that of the medium; (iii)The visible light transmittance of a dispersed particle system can be enhanced bymaking the refractive index ratio between the dispersant and the medium close to unity microstructure, the measured visible light transmittance can be compared with theparticle loading that achieves zero UV transmittance; (ii) For non-porous particlesystems, visible light transmittance decreases with increasing particle size; (iii) Systemscontaining ribbon shape porous ZnO particles of ~ 20% open porosity achieved ~ 63%visible light transmittance and close to zero UV transmittance, compared to ~ 52% and~ 27% visible light transmittance reached by systems containing rice grain shape ZnOparticle and spherical porous ZnO particles, respectively.