Adsorptive pressure-sensitive coatings on porous anodized aluminium

Abstract

A novel pressure-sensitive luminescent coating on porous anodized aluminium is developed. A method of making the coating is described in detail. The coating is a thin anodized aluminium layer, which is formed onto the surface of aluminium by an electro-chemical process. The luminophore is adsorbed directly onto the surface of the layer via chemical or physical adsorption. This coating is suitable for measuring unsteady pressure fields due to its fast-responding characteristics. The time response of the present coating is evaluated theoretically and experimentally. Four kinds of luminophore, tris(4,7-diphenylphenanthroline)ruthenium(II) ([Ru(dpp)3]2+), tetrakis(4-carboxyphenyl)porphyrin (TCPP), platinum tetrakis(4-carboxyphenyl)porphyrin (PtTCPP), and pyrene butylic acid (PBA), have been tested on their response to a step change in pressure. A pressure jump apparatus and a shock tube were utilized to generate a pressure discontinuity. Some static characteristics were also tested. The theoretical analysis shows that the present coating should have a time response in the order of microseconds due to its porous structure. The time response depends not only on luminescence lifetime, which imposes an ultimate limit on the time response, but also on the thickness of the anodized aluminium layer, because oxygen permeation to the pores existing on the anodized aluminium layer can be described as a diffusive phenomenon. The effective diffusion coefficient is estimated to be approximately 5 × 10−6 m2 s−1. Experimental results show that all the tested coatings except the PtTCPP coating have a response time of less than 1 ms. Only the PBA coating shows a substantial photodegradation. The response time of the [Ru(dpp)3]2+ coating is longer than 20 µs, and depends on the thickness of the anodized aluminium layer. The response time of the TCPP coating, on the other hand, is less than 10 µs, and is independent of the thickness of the layer. This independence suggests that the arrangement of the luminophore on the surface of the anodized aluminium layer affects the time response.