Key role of milling in the optimization of TiO 2 nanoinks
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Nanostructured films of TiO2 are becoming more and more attractive as a consequence of their improved sensing properties. Screen printing represents an important low-cost alternative for the production of high-performance devices for the automotive industry. However, to obtain films with superior properties, the composition and each step of the ink production must be carefully controlled. Milling strongly influences the rheological properties of the ink and, as a consequence, the quality of the deposited film. The as-prepared ink was homogenized in a four steps-process with a three-roll mill, and the rheological properties at each intermediate stage were measured. The results showed the dramatic effect of the milling on the flow properties of the nanoink and suggested the importance of a careful control of this step. The rheological behavior of the inks was explained using the basic idea of the transient network theory (TNT) for physically cross-linked networks of polymer solutions. Only an optimized cycle of milling can assure the necessary reproducibility of the ink properties as well as their time stability. I. INTRODUCTION
It is widely accepted that a nanostructured morphology enhances the sensing performances of gas sensors.1,2 Among the materials suitable for this application, titania (TiO2) is widely used for oxygen detection3 as well as in many other applications.4 Thick films of this material can be conveniently deposited by screen printing. This simple and low-cost technique allows layers to be obtained, ranging from a few to tens of microns and provides a high degree of integrability into the microelectronics production system.5 The process consists in forcing the inks through a screen by a tangential motion of a squeegee blade. The screen comes in contact with the substrate in correspondence with the squeegee tip, and a film of ink is deposited. After the squeegee stroke, the screen snaps off from the substrate. It is very important to know the viscosity of the ink throughout the process because it influences the efficiency of the deposition process and the film’s quality. During the squeegee stroke and the snap off of the screen from the substrate, the viscosity should remain sufficiently low to allow an easy flow of the ink through the meshes and the filling of the unevenness from the screen wires. Once on the substrate, after a short period of time at the very slow shear rate due to gravity, the viscosity should increase rapidly to a very high value to prevent the film from
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0188 J. Mater. Res., Vol. 21, No. 6, Jun 2006
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bleeding out. Trease et al.6 calculated the shear stresses corresponding to these two steps of the process: 100 s−1 when the ink passes through the screen and 0.1–0.01 s−1 during its slow movement (combination of leveling and edge sagging) on the substrate. As a consequence, the flow behavior of the ink is generally analyzed in the
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