Laser sintering of screen-printed TiO2 nanoparticles for improvement of mechanical and electrical properties

Abstract

Conducted research presents a rapid and cost-effective approach to technological processing of screen-printed films with anatase TiO2 nanoparticles, by utilizing the high fluence laser radiation. The influence of laser sintering on the screen-printed films was characterized with optical and scanning electron microscopy, energy-dispersive X-ray (EDX) spectroscopy, Raman spectroscopy, nanoindenter measurements and current vs voltage measurements. Investigation of surface morphology of screen-printed films revealed that higher laser fluences caused significant decrease in film thickness, trough evaporation of organic additives used in the paste matrix. EDX mapping of carbon content in untreated and laser sintered surface confirmed removal of organic additives. Laser sintering stimulated breaking of large agglomerates into much finer nano-sized particles and promoted formation of necking between individual grains. Crystal structure and vibrational properties of anatase TiO2 nanoparticles was monitored with Raman spectroscopy before and after laser sintering. Obtained results point out that anatase polymorph was preserved during the sintering process, without appearance of other phases. From observation of the behavior of the most intense Eg Raman active mode it was deduced that laser sintering provoked a formation of structural defects i.e. oxygen vacancies in TiO2 nanoparticles, whose concentration increased in the samples treated with higher laser fluences. Mechanical properties of untreated and laser sintered samples were investigated with nanoindenter measurements using several load forces, in order to carefully probe the Young modulus and mechanical hardness. From the analysis of collected data, we established that overall improvement of the mechanical properties with laser sintering originates from formation of very dense ceramic layer with enhanced interconnectivity between individual TiO2 nanoparticles. Measurements of current vs voltage characteristics clearly demonstrated that increase in laser fluence leads to drastic increase in current values and improvement of electric conductivity.