DEVELOPMENT OF PRESSURE SENSITIVE PAINT BASED ON SILICON NANOSTRUCTURED POWDERS A. Castaldo *, E. Massera, L. Quercia, G. Di Francia ENEA, Centro Ricerche di Portici 80055 (NA), Italy Abstract: A new pressure sensitive paint family, employing porous silicon nanostructured powders, has been developed for application in stationary wind tunnel test and advanced turbomachinery purposes. A library of powders, obtained with a combined ball milling – chemical etching process, is synthesized, so that our paint formulation can be tuned on some different applications of the pressure measurement. Keywords: Pressure Sensitive Paint (PSP); silicon powders; oxygen sensing.
INTRODUCTION Pressure Sensitive Paints (PSP) are polymeric formulations consisting of an oxygen permeable polymer matrix into which a photoluminescent material, strongly reactive to molecular oxygen, is either dispersed or dissolved. As a result, paint photoluminescence quenches with molecular oxygen and using the Stern-Volmer model the air pressure can be determined. By converse, determination of the PL quenching can be used to measure pressure [1]. Detailed explanation of the principle of PSP can be find in several papers [2] and several software programs have been developed to correlate quenching photoluminescence measurements on a surface coated with such paints to the pressure map [3]. From the early developments of the pressure sensitive paint technique for external aerodynamics, initiated at the Central Aero-Hydrodynamics Institute (TsAGI) in Moscow and simoultaneously in the United States at NASA Ames Research Center, interest in PSP has steadily increased, due to attractiveness of its high spatial resolution and the feasibility of low-cost. At present this technique is used in stationary wind tunnel test [4], but potentially extensible to advanced turbomachinery applications.
Fig.1 – Pressure-sensitive paint image data mapped onto model surface of an airplane. From J.H.Bell et al (NASA), see Ref 2.
* Corresponding author:
Fig.1 reports, for example, the pressure map of a Pioneer Rocketplane obtained by the means of the PSP tecnique. Various paints formulations have been conceived based on organometallic complexes, dissolved in polymeric matrices [5]. Testing of the paints, however, has led to discovery of a number of problems and limitation, e.g. photodegradation of the complexes, response time and temperature dependency [6]. Most of the problems results from the use of molecular based formulation. Here, we propose an innovative formulation, in which the sensible luminophore is nanostructured silicon powder, while the binder is a polysilsesquioxane containing nanosized cages, capable to host the luminophore. In order to solve O2 permeability problems of the polymer matrix and to accelerate response time to variations of its concentration, i.e. pressure, a process to disperse the nanopowder on the binder surface has been investigated. The ultimate aim of this work will be to have a library of powders with different O2 sensing properties and to cover with them various potential aerodynamics applications.
EXPERIMENTAL Single-crystal p-type silicon wafers with various resistivity have been used in the current study. They are fragmented and milled by ball milling technique [5], performed in various conditions, up to a dimension of few hundreds of nanometers. Nanostructured powder samples have finally obtained by chemical etching, using an activated HF- HNO3 solution (50:1, wt %). The etching time has been adjusted between 60s to 300s. J.H.Bell, After the etching, samples are rinsed and stored in pentane. Poly [(propylmethacrylheptaisobutyl-POSS)-co-(n-butylmethacrylate)] is solubilized in toluene and cast on the test
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pressure. This correlation enables us to the utilization of such paint in the PSP tecnique. 80
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surface, where the selected powder is dispersed by means of spraying. PL quenching measurements are performed in a pressure chamber equipped with mass flow controller, turbomolecular vacuum pump and with an accurate pressure sensitive measurement system . Through a quartz window, PL is excited by He-Cd Laser radiation (442 nm). Emission spectra are collected and registred on a CCD spectrometer by an optical fiber.
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Fig. 3 – Typical calibration curve of PSP: photoluminescence intensity decreases at the increasing pressure.
Fig. 4 reports calibration characteristics of our paint in comparison to two paints obtained at NASA Ames Research Center from Mc Lachlan and colleagues. In our case, calibration is not linear, probably because our system is inhomogeneous. It is noticeable that we synthesize, at the moment, our paint far from test chamber, with unavoidable loss of sensing activity during 1 0.9 0.8 0.7
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We fabricated a set of powders with different properties. Some of these properties determine different application fields. In general the light emission range of our powders, when pumped by the 442 line of the He-Cd laser is from 500 to 800 nm. Spectral data are typical of porous silicon samples. In the following we describe a formulation for static application, in which we choose a micrometric p-type silicon powder with a specific area of 3.18 m2/g. The powder is deposited on a POSS coating, bounded to the surface to analyze. In Fig. 2 we report quenching of the photoluminescence intensity of our paint vs time. Initially the system is in nitrogen and its photoluminescence is stable. At time t=200s oxygen is introduced in test chamber and the paint emission is rapidly quenched. From another experiment, in which our paint is placed in vacuum and then in nitrogen, a response time of only 5 seconds could be estimated.
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Fig.4 Calibration characteristic of our PSP in comparison to PSP1-PSP2 from a study of McLachlan.
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Fig.2- Photoluminescence intensity decay in presence of oxygen. Initially system is in nitrogen, at t=200s oxygen is introduced, determining a rapid PL quenching.
For static applications this is a very good result in comparison to 10 s of the PSP reported by McLachlan (NASA Ames Res. Center) [1]. In fig 3 we report the correlation between photoluminescence intensity and pressure: PL intensity decreases with an increase of
transportation. Clearly we will tend to ameliorate this aspect, for example with the aid of gloves box and appropriate storage media. As far as the operating mechanism is concerned, our experiments show that powder surface oxidation affects paint response time and quenching strength. In other terms, fresh samples are more rapid and sensitive than the oxidized ones. By converse this major sensitivity and minor response time is counterbalanced by a strong dependence of the PL quenching on the surrounding atmosphere. The role of the environment in the sensing performance of fresh samples used in our paints needs further investigations.
In Fig.5 PL of fresh powder based samples is reported vs time upon introducing in test chamber different gases. Dry air results in a reversible PL quenching correspondent to a pressure increase. With humid air the PL quenching is more stronger than dry air but not entirely reversible. Moreover a little quenching is observed under nitrogen.
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Fig. 5 – PL quenching of fresh powder based samples in different atmosphere: in nitrogen (quadrat), in dry air (triangle), in humid air (circle).
Since our samples are silicon nanocrystallites covered by hydrogen, the appearance of other nonradiative recombination paths (e.g. Si-H vibrations on the surface or mechanical stresses), independent from oxygen concentration, cannot be ruled out. Work is in progress to value this aspect and to propose the best trade-off from response time for dynamic application (e.g. turbomachinery) and reliability retention.
CONCLUSIONS We have fabricated pressure sensitive paints based on silicon nanostructured powders. The formulation we propose is very simple, covers a wide range of applications, responding in few seconds to variations of oxygen concentration. We are studying how to deposit an optimal polymeric shield over the paint in order to face the problem of preserving performance before measurements. Moreover, we are valuing how different atmospheres influence sensing properties of the paints.
REFERENCES [1] B. G McLachlan, J.H Bell ; Experimental Thermal and Fluid Science 10 ( 1995) pp 470-485.
[2] J.H Bell, E.T. Schairer, L.A. Hand, R. Metha; Annual Review of Fluid Mechanics 33 (2001) pp155-206.
[3] M. Gouterman, J Callis, D. Burns, J Kavandi, J Gallery et al; Proc.ONR/NASA Workshop, Purdue University, W.Lafayette I. [4] V Mosharov., M Kuzmin., A Orlow., V.Radchenko, N Sadovskii, I Troyanovsky; EP 0558771 A1 (f.d. 01.03.92). [5] W. Xu, R.C. McDonough III, B.Langsdorf, J.N. Demas, B. A. DeGraff; Anal.chem 66 (1994) pp 4133-4141. [6] L.M. Coyle, M. Goutermann Sensors and Actuators B 61 (1999) pp 92-99. [7] Y. Sakamura, T. Suzuki, M.Matsumoto, G. Masuya, Y. Ikeda Meas. Sci. Technol. 13 (2002) pp 1591-1598.
