Category Pressure and Temperature Sensitive Paints

4.1. Radiometric Notation

Luminescent radiation from a luminescent paint (PSP or TSP) on a surface involves two major transport processes of radiative energy. The first process is absorption of an excitation light through a paint layer and the second process is luminescent radiation that is an absorbing-emitting process in the paint layer. These processes can be described by the transport equations of radiative energy (Modest 1993; Pomraning 1973). The luminescent intensity emitted from a paint layer in plane geometry can be analytically determined by solving the transport equations. Thus, the corresponding photodetector output can be derived for an analysis of measurement system performance and uncertainty. Before doing a detailed analysis, it is necessary to discuss the radiometric notation. In the literature of PSP and TSP, the term ‘luminescent intensity’ or ‘fluorescent intensity’, which is usually denoted by the capital English letter ‘I’, has been widely used. In a strict radiometric sense, the luminescent intensity I is the luminescent radiance defined as the radiant energy flux (power) per unit solid angle and per unit projected area of an elemental surface of PSP or TSP (units:

,, -2 -1 -1 -2 – K

watt-m – sr or J-s – m – sr ).

The radiance is a function of both position and direction, which is graphically represented by a cone of a solid angle element in radiometry as shown in Fig. 4.1. The direction of the radiance (incident or emitting radiance) is given by the polar angle в (measured from the surface normal) and the azimuthal angle ф (measured between an arbitrary axis on the surface and the elemental solid angle on the surface) in a local coordinate system. In radiometry, the radiance is conventionally denoted by the captical English letter ‘L’. The term ‘intensity’ is sometimes confusing because its definition is different in a number of different disciplines. In radiometry, the radiant intensity (units: watt-sr-1), denoted by the letter T, is the radiant flux per unit solid angle, which is different from the radiance (McCluney 1994; Wolfe 1998). However, in the literature of radiative heat transfer, the radiative intensity, denoted by the same letter 7’, is essentially equivalent to the radiance in radiometry (Modest 1993). In order to avoid confusion in notation, we specifically define the luminescent intensity T as the luminescent radiance from PSP or TSP, which is consistent with the notation and terminology commonly used in the literature of PSP and TSP. In a general case, we still use the traditional radiometric notation ‘L to denote the radiance in other

radiometric measurements and modeling. The spectral radiance such as Ix and L} at a wavelength Я (units: watt-m-2-sr-1-nm-1) is usually denoted by a subscript

Я ; the radiance I (or L) is the integration of the spectral radiance Ix (or Lx ) over

a certain range of the radiation wavelengths. Since the radiation from a luminescent molecule is isotropic, a plausible assumption is that the luminescent radiance from PSP or TSP is independent of the azimuthal angle ф. Under this

assumption, an analysis of transport of the luminescent radiative energy in PSP or TSP is considerably simplified. The following analysis is given for PSP, but it is also valid for TSP that is treated as a special case of PSP when the oxygen quenching vanishes.

As pointed out before, PSP or TSP is prepared by dissolving a luminescent dye and a polymer binder in a solvent solution; the resulting mixture is then applied on a surface by spraying, brushing or dipping. After the solvent evaporates, a thin coating of the paint remains on the surface, in which the luminescent molecules are immobilized in the polymer matrix. The polymer binder is an important ingredient of the paint adhering to the surface of interest. In some cases, the polymer matrix is only a passive anchor; in other cases, the polymer may significantly affect the photophysical behavior of the paint through complicated interaction between the luminescent molecule and the macromolecule of the polymer. Since how the polymer affects the photophysical processes in the paint is not well understood, it is basically a trial and error process to find an optimal combination of a luminophore and a polymer. A good paint (PSP or TSP) for aerodynamic applications should have certain required physical and chemical properties. The following discussion is focused on the required properties of PSP while some requirements are generally applicable to TSP. A general strategy for the development of improved PSP formulations was proposed by Benne et al. (2002).

Pressure Response

The Stern-Volmer coefficients of PSP should be chosen to match the pressure range on a tested article and the performance requirements of a photodetector (e. g. CCD camera) used in particular measurements. A large Stern-Volmer coefficient B(T) generally indicates a good pressure response. However, for aerodynamic experiments at high pressures, a large Stern-Volmer coefficient B(T) of PSP may cause unwanted severe oxygen quenching in the ambient reference conditions, considerably reducing the luminescent emission from the paint and therefore the signal-to-noise ratio (SNR) of a photodetector.

Luminescent Output

The luminescent emission of a luminophore is characterized by the quantum yield (or efficiency); it is generally desirable to have as high a luminescent output as possible to maximize the SNR of a photodetector. The luminescent intensity is proportional to the concentration of the probe molecules over a certain range. However, it cannot be increased indefinitely by increasing the dye concentration; if the concentration is too high, self-quenching of luminescence occurs. Similarly, the luminescent intensity is no longer linearly proportional to the excitation light intensity at a very high excitation level, and eventually it saturates when the excitation light intensity increases further.

Paint Stability

Ideally, the luminescent intensity of PSP should not change with time under excitation. Usually, the luminescent intensity decreases with time due to photodegradation of a luminophore (Egami and Asai 2002). A decrease in the luminescent intensity could also be due to the presence of certain chemicals other than oxygen that can quench the luminescence. The polymer binder undergoes aging, which can change its characteristics with respect to the oxygen solubility and diffusivity. As a result, the Stern-Volmer coefficients of PSP may be altered.

Response Time

The response time of PSP is mainly determined by oxygen diffusion through a paint layer when the luminescent lifetime is much shorter than the diffusion timescale. The high porosity of the paint will increase the time response. The need for fast time response depends on a particular application; a short response time of PSP is required for unsteady aerodynamic measurements. For steady-state measurements, however, the use of a fast-responding PSP does not necessarily offer an advantage. For a highly oxygen-permeable PSP with a short response time, the Stern-Volmer coefficient B(T) is usually large, and thus weak luminescence of PSP in the ambient conditions may lead to a low SNR.

Temperature Sensitivity

A good PSP should have a weak temperature effect. The temperature sensitivity arises from two sources: the intrinsic temperature dependency of a luminophore and the temperature dependency of the solubility and diffusivity of oxygen in a polymer matrix. The latter is a major contributor to the temperature sensitivity of PSP.

Physical Characteristics

The physical properties of a polymer binder, such as adhesion, hardness, coating smoothness and thickness, should be considered prior to a test. Adhesion should be strong enough to sustain skin friction on a surface particularly in high-speed flows, which is related to surface tension, solvent softening and chemical bonding. Hardness primarily depends on the type of polymers, the molecular weight and the degree of cross-linking. For example, silicone rubbers (or RTVs) are generally soft, whereas Acrylates and methacrylates are generally hard. Smoothness depends primarily on paint itself and application techniques; for most paints uniform leveling of the paint is essential to a smooth finish. The coating thickness is very dependent on application techniques for both the basecoat and PSP topcoat. It is generally desirable to minimize the coating roughness and thickness to avoid any effect on the aerodynamic characteristics of a model. Typically, the maximum rms roughness of a coating should be less than 0.25 |jm, and the coating thickness ranges from 20 to 40 |jm.

Chemical Characteristics

Toxicity of paint is a major concern of safety; toxic solvents such as chlorinated solvents should be avoided. Painter must be protected against contact with paint spray through the use of fresh air breathing equipment and adequate ventilation. The paint must be easily sprayed, leveled, and cured to give the specified physical characteristics of coating under different environmental conditions in wind tunnels. The solvent evaporation rate must be controlled under different conditions of temperature and humidity. Since the wind tunnel time is expensive, application of the paint should be as fast as possible. The curing temperature must be reasonable (less than 100oC); if the curing temperature is too high, it is difficult to achieve uniform curing over different metal materials. In addition, paint removal and reapplication on a model is a practical issue in wind tunnel testing.

Some paints, particularly those designed for good and robust adhesion, are difficult to remove and generally require an aggressive paint stripper like methylene chloride. This introduces problems with toxicity and insuring adequate ventilation.

A perfect PSP should be completely temperature independent. According to Eq. (2.23), a temperature insensitive PSP should have such small activity energy ED for the oxygen diffusion process that the Stern-Volmer coefficient Bpoiymer is a

weak function of temperature over a certain range of temperature. However, since the excited-state decay rates of a luminophore are intrinsically temperature dependent, it is unlikely to develop an absolutely temperature-insensitive PSP whose the Stern-Volmer coefficients Apoiymer and Bpoiymer are constants. Instead,

researchers seek a so-called ‘ideal’ PSP exhibiting invariant temperature dependency at different pressures over a certain range of temperatures (Puklin et al. 1998; Coyle et al. 1999; Bencic 1999; Ji et al. 2000). Note that the term ‘ideal PSP’ does not accurately describe the invariant property of this special paint. Nevertheless, since this term has been used in the PSP community, we adopt it here and discuss its true meaning below.

Consider the Stern-Volmer relation in the following form

where 10(T) = I(p = 0,T) is the luminescent intensity at zero pressure (vacuum) and KSV(T) is related to the coefficients Apoiymer(T) and Bpoiymer(T) by

KSV(T ) = [ Bpolymer(T )/Apolymer(T )] ^ Pref. (3.2)

If the coefficients Apoiymer(T) and Bpoiymer(T) have the same temperature dependency, the Stern-Volmer coefficient KSV(T) becomes temperature independent. According to Eq. (2.23), this situation may occur under the conditions Ed ~ Enr and n ~ 1 over a certain range of temperatures. Therefore, for an ‘ideal’ PSP, the Stern-Volmer coefficient KSV(T) in Eq. (3.1), rather than the coefficients Apolymer(T) and Bpoiymer(T), is temperature independent.

Consequently, the Stern-Volmer relation in the form for aerodynamic application can be written as

and the coefficients Apolymer, ref and Bpolymer, ref are temperature independent given

the reference conditions. For an ‘ideal’ PSP, the Stern-Volmer relation Eq. (3.3) enjoys such similarity that it is invariant at different temperatures for the variable g( T )Iref /I. The temperature effect of PSP is concentrated in a single scaling

factor g(T); this similarity simplifies the temperature correction procedure for PSP.

Ji et al. (2000) developed a bichromophic molecule Ru-Pyrene for an ‘ideal’ PSP, which, as shown in Fig. 3.25, consisted of a covalently linked assembly of a Ruthenium (II) polypyridyl complex and Pyrene. They also synthesized the MPP acrylate polymer binder for Ru-Pyrene. Figure 3.26(a) shows the Stern-Volmer plots for the Ru-pyrene/MPP PSP at pressures ranging from 0.005 to 1 atm and temperatures from 25 to 55oC. The Stern-Volmer plots at different temperatures are collapsed onto a single curve. Figure 3.26(b) illustrates the temperature dependency of the Ru-Pyrene/MPP PSP at 0.005, 0.14, 0.55 and 1 atm, indicating that the temperature dependency of this PSP is independent of pressure.

The intensity-based method for PSP and TSP requires a ratio between the wind-on and wind-off images of a painted model. When a model moves in a non­homogenous illumination filed during a test, the image-ratio method inevitably causes inaccuracy in determining pressure and temperature. A multiple – luminophore paint is designed to eliminate the need for a wind-off reference image. Generally, a two-luminophore PSP consists of a pressure-sensitive luminophore and a pressure-insensitive reference luminophore; similarly, a two – luminophore TSP combines a temperature-sensitive luminophore with a temperature-insensitive reference luminophore. The probe and reference luminophores can be excited by the same illumination light. Ideally, there is no overlap between the emission spectra of the probe and reference luminophores such that the luminescent emissions from the two components can be completely separated using optical filters. Theoretically, a ratio Ix /1^ between the probe and reference images could able to eliminate the effects of spatial non-uniform illumination, paint thickness and luminophore concentration, where Ix and I Xi

are the luminescent intensities at the emission wavelengths X1 and X2, respectively. However, McLean (1998) pointed out that since two luminophores cannot be perfectly mixed, the simple two-color intensity ratio I x /1^ cannot completely compensate the effect of non-homogenous dye concentration. In this case, a ratio of ratios (I Xj /1^ )/(Ix /1^ )0 should be used to correct the effects

of non-homogenous dye concentration and paint thickness variation, where the subscript 0 denotes the wind-off condition.

Besides the above combinations of luminophores, a temperature-sensitive luminophore, which cannot be quenched by oxygen, can be combined with an oxygen-sensitive luminophore. This two-luminophore temperature/pressure paint can be used for correcting the temperature effect of PSP. In particular, when the temperature dependencies of the two luminophores are close, a two-color intensity ratio between the two luminophores exhibits a very weak temperature dependency (Engler et al. 2001a). Figure 3.24 shows a ratio of ratios of a two-luminophore PSP (PtTFPP in FIB with a proprietary reference luminophore) as a function of pressure at different temperatures (Crafton et al. 2002). Clearly, the data at different temperatures overlap, and a ratio of ratios of this PSP is almost temperature insensitive in a range of 5-45oC. Furthermore, a multiple-

luminophore PSP can be developed to correct the temperature effect as well as the effect of non-uniform illumination simultaneously.

1.5

Oglesby et al. (1995b) used PtOEP or PtTFPP as a pressure probe luminophore and Fluorol Green Gold 084 (3,9-perylenedicarboxylic acid, bis(2- methylpropyl)ester) as a reference luminophore in the GP-197 polymer. Harris and Gouterman (1995, 1998) used PtTFPP as a pressure-sensitive luminophore and incorporated a solid-state phosphor BaMg2Al16O27:Eu2+ (BaMgAl) as a reference luminophore in an Acrylic copolymer. Since BaMgAl is insoluble, the reference luminophore was not uniformly distributed and therefore the paint suffers from the effect of the uneven layer thickness. TsAGI/OPTROD developed the proprietary two-luminophore PSP formulations, LPS B1 and LPS B12 (Bukov, et al. 1997; Lyonnet, et al. 1997). Three pressure sensitive paints with an internal temperature sensitive luminophore were also tested by Oglesby et al. (1996), where EuTTA, MgOEP and Ru(bpy) were used as temperature-sensitive reference luminophores. Hradil et al. (2002) used Ru(dpp) as a pressure probe molecule and manganese-activated magnesium fluorogermanate (MFG) as a thermographic phosphor. The preliminary results showed that two-luminophore PSPs indeed enabled point-by-point correction for the temperature effect of PSP.

Buck (1988, 1989, 1991) used a blue-green Radelin thermographic phosphor for aerothermodynamic testing that intrinsically exhibited two narrow-band emission peaks at 450 nm and 520 nm. It was found that a ratio of the blue to green emission intensity was a function of temperature, but independent of the UV illumination intensity. Another two-color phosphor system used a green-red mixture of rare-earth and Radelin phosphors for a broader range of temperatures. Buck (1988, 1989) gives a detailed description of the multiple-color phosphor thermography system developed at NASA Langley.

A challenging problem is application of PSP in cryogenic wind tunnels like the NASA Langley National Transonic Facility (NTF) and European Transonic Wind Tunnel Facility (ETW). Porous materials were usually used as binders for PSP at cryogenic temperatures. Porous material has a large exposure surface area where the probe luminophore can be directly applied and luminescence can be directly quenched by oxygen. The use of porous materials as binders allows PSP measurements at cryogenic temperatures and achieves fast time response as well (in the order of microseconds). Various porous materials were investigated as binders for PSP, including a thin layer chromatography (TLC) plate (Baron et al. 1993), hydrothermal coating (Bacsa and Gratzel 1996), sol-gel (MacCraith et al. 1995; Jordan et al. 1999a, 1999b), tape-casting (Scroggin 1999), anodized aluminum (Asai 1997a), anodized titanium and porous paper filter (Erausquin 1998; Erauquin et al. 1998).

Aluminum can be anodized to create a thin aluminum oxide layer on the surface through an electrochemical process. Anodized aluminum (AA) is highly porous with 10-100 nm micropores uniformly distributed on the surface. AA-PSP is made by adsorbing the luminophore into the pores on the AA surface (Asai 1997a). Figure 3.19 shows the Stern-Volmer plot for an AA-PSP, Ru(dpp) on anodized aluminum, compared with a conventional polymer-based PSP (Ru(dpp) in RTV) at cryogenic temperatures. This AA-PSP still exhibits good sensitivity to oxygen even at 100 K, whereas the conventional PSP loses its sensitivity to oxygen at 150 K. Upchurch et al. (1998) and Asai et al. (2000, 2002) developed a polymer-based cryogenic PSP that used highly porous Poly(TMSP) as a binder.

To compare the two cryogenic PSP formulations, Figure 3.20 shows the Stern – Volmer plots for the Poly(TMSP)-based PSP and AA-PSP that use Ru(dpp) as a luminescent dye; both PSPs exhibit the non-linear behavior in the Stern-Volmer plot.

A family of the luminescent molecules Ru(trpy) (see Fig. 3.21 for the chemical structure) has been studied for making cryogenic TSP formulations since it is found that these compounds are temperature sensitive at cryogenic temperatures (Campbell 1994; Erausquin 1998; Iijima et al. 2003). They have very intense emission at low temperatures, but they are nearly fully quenched at the room temperature. The absorption and emission spectra of the family of Ru(trpy) are
very similar to those of Ru(dpp) and Ru(bpy). The Ru(trpy) compounds, which can be excited by either a UV light or blue light, emit red luminescence. Various [Ru(trpy)2] molecules with different ligands were synthesized (Erausquin 1998). The dynamics of the metal-to-ligand bond, as well as the electron donating or accepting characteristics of the ligand, has a significant effect on the temperature sensitivity of a luminescent molecule. Thus, this would enable synthesis of a molecule specifically designed to respond with high sensitivity over a certain range of cryogenic temperatures.

DuPont ChromaClear (CC) is selected as a binder for cryogenic TSP due to its low oxygen diffusivity and good surface adhesion at cryogenic temperatures (Erausquin 1998). Figure 3.22 shows a comparison of several synthesized [Ru(trpy)2] compounds in the polymers CC and GP-197. In general, interaction between a polymer binder and a luminescent molecule can affect the mobility of the metal-to-ligand bonds, changing the temperature dependency of the paint. Figure 3.23 shows the temperature dependencies for [Ru(trpy)(phtrpy)](PF6)2 in two different polymer binders CC and GP-197. Other Ruthenium compounds have a good response at cryogenic temperatures as well, as shown in Table 3.3 that summarizes the temperature sensitivities and useful temperature ranges for cryogenic TSP formulations calibrated by Erausquin (1998).

Like PSP, TSP is prepared by dissolving a luminescent dye and a binder in a solvent. Many commercially available resins and epoxies can serve as polymer binders for TSP if they are not oxygen permeable and do not degrade the activity of the probe luminophore molecule. Table 3.2 lists some TSP formulations as well as the spectroscopic properties, temperature sensitivities and useful temperature measurement ranges. Data are collected from Campbell (1993) and Gallery (1993) and other sources. For a comparison between different TSP formulations, the maximum logarithmic slope max{ d[ln( I / Iref )] / dT } is used as an indicator of the temperature sensitivity for TSP over a certain temperature range, where Iref is the reference luminescence intensity. Here, the logarithmic

slope is used since it is independent of the reference intensity that is different in various sources. Two proprietary TSP formulations and two high-temperature thermographic phosphors are also included in Table 3.2 for comparison. Figure 3.13 shows typical temperature dependencies of the luminescent intensity for a number of TSP formulations. Some of them have been used to measure the temperature and heat transfer fields in various applications (Kolodner and Tyson 1982, 1983a, 1983b; Romano et al. 1989; Campbell et al. 1993, 1994; Liu et al. 1995a, 1995b, 1996; Hamner et al. 1994; Asai et al. 1996, 1997c).

Two typical TSP formulations are Ru(bpy) in an automobile clear coat (DuPont ChromaClear) binder and EuTTA in model airplane dope (see Appendix B); both are easy to prepare and use. Figure 3.14(a) shows the chemical structure of tris(2,2’-bipyridyl) ruthenium or Ru(bpy) and Figure 3.14(b) shows the absorption and emission spectra of Ru(bpy) that are similar to those of Ru(dpp) for PSP. Ru(bpy) can be excited by a UV lamp, nitrogen laser, argon laser, doubled YAG laser or blue LED array. Since the Stokes shift is large (the emission peak at about 620 nm), the excitation light can be easily separated from the luminescent emission using an optical filter. An automobile urethane clear coat, which is usually used as a top coat on most automobiles, is used as a polymer binder for Ru(bpy); particularly, DuPont ChromaClear 7500S is used, but other brands should work as well. The advantage of this binder is that it is oxygen impermeable, readily available, and easy to spray although some pressure sensitivity was observed at very high pressures and temperatures. Figure 3.13 shows the temperature dependency of the luminescent intensity for Ru(bpy) in DuPont ChromaClear along with other TSP formulations. Ru(bpy) can also mixed with a Shellac binder; the Ru(bpy)-Shellac TSP is similar to Ru(bpy) in DuPont ChromaClear in terms of the temperature sensitivity. It is also easy to apply. Figures 3.15 and 3.16 show, respectively, the Arrhenius plot and lifetime for the Ru(bpy)-Shellac TSP compared with the EuTTA-dope TSP.

(b) Wavelength (nm)

Fig. 3.17. (a) Chemical structure of EuTTA, (b) absorption and emission spectra of EuTTA

Another good TSP is based on Europium (III) Thenoyltrifluoroacetonate or EuTTA whose structure and the absorption and emission spectra are shown in Fig. 3.17. Obviously, a UV lamp or a nitrogen laser can be used for excitation. EuTTA has a high quantum yield and a large Stokes shift (the emission peak at about 620 nm). The binder used with this luminphore is a clear model airplane dope that, like DuPont ChromaClear and Shellac, is readily available, easy to spray, and oxygen impermeable. Figures 3.15 and 3.16 show, respectively, the Arrhenius plot and lifetime for the EuTTA-dope TSP along with those for the Ru(bpy)-Shellac TSP.

Thermographic phosphors and thermochromic liquid crystals are also temperature sensitive coatings for measuring the surface temperature distributions. Similar to polymer-based TSPs, thermographic phosphors utilize the thermal quenching of the luminescent emission from ceramic materials that are doped or activated with rare-earth elements (Allison and Gillies 1997). However, they are usually in the form of insoluble powders or crystals in contrast to a polymer-based TSP where luminescent molecules are immobilized in a polymer matrix. The luminescent intensity (or lifetime) of thermographic phosphor and polymer-based TSP follows the same functional relation I ^ [1 + a0exp(-a1/T)]- . Figure 3.18 shows the measurement envelops of thermographic phosphors and polymer – based luminescent TSPs. A family of thermographic phosphors can cover a temperature range of 273-1600 K, which overlaps with the temperature range of the polymer-based TSP family from 90 to 423 K. Hence, a combination of thermographic phosphors and polymer-based luminescent TSPs can cover a very broad range from cryogenic to high temperatures. The measurement systems (intensity – and lifetime-based systems) for thermographic phosphors are essentially the same as those for polymer-based luminescent TSPs. The emission spectrum of certain phosphor has multiple distinct lines that have very different temperature sensitivities. Thus, an emission-intensity ratio between the temperature-sensitive and – insensitive lines can eliminate the effect of non­uniform illumination on a surface. Note that certain emission lines of certain phosphors are also temperature sensitive in cryogenic conditions. Thermochromic liquid crystals selectively reflect light depending on the surface temperature, and hence the dominant wavelength or hue of the reflected light varies monotonically with temperature over a relatively narrow temperature range of about 32-42oC (Smith et al. 2000). For comparison, Figure 3.18 plots the normalized hue of a typical thermochromic liquid crystal as a function of temperature; the temperature sensitivity of the thermochromic liquid crystal is high over a narrow temperature range.

A typical PSP is prepared by dissolving a luminescent dye and a polymer binder in a solvent; the order of mixing the components and the relative concentration of the components may change the characteristics of the paint. Chlorinated organic solvents such as dichloromethane and trichloroethane have been used for making PSP. The selection of a polymer binder for PSP is important, which should be based on a balanced consideration of its oxygen permeability, temperature effect, humidity effect, adhesion, mechanical stability, photodegradation, and other required properties. Silicone rubbers, GP-197, silica gel and sol-gel-derived coatings have been used as binders for PSPs and oxygen sensors (Wan 1993; Gallery 1993; Xu et al. 1994; MacCraith et al. 1995; Jordan et al. 1999a, 1999b). Other polymers and porous materials that are potentially useful for PSP can be found in the literature of polymers (Krevelen 1976; Mulder 1991; Fried 1995; Robinson and Perlmutter 1994). The permeability, solubility and diffusion coefficients of a polymer binder are related to the pressure sensitivity and time response of PSP. Furthermore, the behavior of PSP depends on interaction between a probe molecule and its surrounding polymer. The microenvironment of the probe molecule in the polymer binder can significantly affect the luminescence and quenching behavior (Hartmann et al. 1995; Meier et al. 1995; Xu et al. 1994; Lu and Winnik 2001; Lu et al. 2001). Also, it was observed that a basecoat might affect the behavior of PSP (Coyle et al. 1995). A useful review on quenching of luminescence by oxygen in polymer films was given by Lu and Winnik (2000), stressing on luminescent materials and polymers.

Table 3.1 lists some PSP formulations along with their spectroscopic properties and the Stern-Volmer coefficients. In Table 3.1, the Stern-Volmer coefficients A(T) and B(T) are the coefficients in the linear relation

Iref/1 = A(T) + B(T)p/pref at the room temperature of about 20oC, where the reference pressure pref is the ambient pressure of 1 atm. For certain PSPs that do not completely obey the linear Stern-Volmer relation, the coefficients A(T) and B( T ) are estimated by fitting data over a finite linear range. The results are collected from the theses and papers by Wan (1993), Burns (1995), Baron et al.

(1993) , Kavandi et al. (1990), and McLachlan et al. (1993a, 1995), which documented the absorption spectra, emission spectra, and Stern-Volmer plots of oxygen-sensing luminophores and supporting polymer matrices. Table 3.1 also include a number of proprietary PSPs developed by TsAGI (Troyanovsky et al. 1993; Bukov et al. 1993), the former McDonnell Douglas (now Boeing at St. Louis) (Morris et al. 1993a, 1993b; Morris 1995), the University of Washington/NASA Ames (McLachlan and Bell 1995) and NASA Langley (Oglesby and Jordan 2000). Some PSP formulations of the former McDonnell Douglas have been patented (Schwab and Levy 1994). Generally speaking, a good PSP has the Stern-Volmer coefficient B(T) larger than 0.5, indicating acceptable pressure sensitivity for quantitative measurements (Oglesby et al. 1995a).

Three families of luminescent dyes, Platinum Porphyrins, Ruthenium Polypyridyls and Pyrene derivatives, have been commonly used for making PSP. Recipes of three PSP formulations are given in Appendix B. The Platinum Porphyrin compounds, which can be excited by either an UV light or a green light, emit red luminescence. They are very sensitive to oxygen, but they often have a long lifetime and low luminescent intensity at the atmospheric pressure. The Ruthenium compounds also emit red luminescence when excited by either an UV light or a blue light. They are very photo-stable, but are difficult to incorporate into polymer systems. The Pyrene derivatives, which are UV excited, emit blue
luminescence. The Pyrene derivatives have weak temperature sensitivity; however, they suffer from photodegradation and sublimation.

Figure 3.1 shows the chemical structure, and absorption and emission spectra for platinum meso-tetra(pentafluorophenyl)porphine (PtTFPP). Figures 3.2 and 3.3 show the Stern-Volmer plots and temperature dependencies for two PtTFPP PSP formulations: PtTFPP in the FIB polymer (poly-heptafluoro-n-butyl methacrylate-co-hexafluorisopropyl methacrylate) developed by the University of Washington (Gouterman and Carlson 1999) and PtTFPP in the FEM polymer (poly-tifluoro-ethylmethacrylate-co-isobutylmethacrylate) developed by NASA Langley (Oglesby and Jordan 2000). In these figures, the lower temperature sensitivity of PSP at vacuum represents the intrinsic temperature dependency of the luminophore, while the higher temperature sensitivity of PSP at the atmospheric pressure indicates an additional temperature effect on the oxygen diffusion in the polymer.

(b)

Fig. 3.2. (a) The Stern-Volmer plots, and (b) temperature dependency for PtTFPP in the FIB Polymer. From Oglesby and Jordan (2000)

(a)

(b)

Fig. 3.3. (a) The Stern-Volmer plots, and (b) temperature dependency for PtTFPP in the FEM polymer (NASA Langley). From Oglesby and Jordan (2000)

85 80 75 І5 70 .Q Ї 65 60 55 50
□ FIB A FEM + SOLGEL О UNICOAT • PAR
10 15 20 25 30 35 40 T (deg. C)
2.5

In order to examine the effect of a polymer binder on the properties of PSP, Mebarki and Le Sant (2001) calibrated five PSP formulations that used the same porphyrin molecule, PtTFPP, with different polymer binders. Two formulations, the PAR PSP from the Institute for Aerospace Research (IAR) of NRC in Canada (Mebarki 2000) and the FEM PSP from NASA Langley (Oglesby and Upchurch 1999), are not commercially available. Other paints, the FIB PSP originally developed by the University of Washington (Gouterman and Carlson 1999), sol – gel PSP (Jordan et al. 1999a, 1999b) and the Uni-Coat PSP (Mebarki 2000), were commercially produced by Innovative Scientific Solutions Inc. (ISSI). Except for the Uni-Coat PSP that did not require a primer layer, the commercial FIB and sol – gel PSP formulations were supplied with their respective primers. To simplify the application procedures and solve adhesion problems, the FIB active layer was applied on the top of the Tristar (DHMS C4.01TY3) white epoxy primer that was also used as a screen layer for both the FEM PSP and PAR PSP. It was found that the primer had no effect on the pressure or temperature sensitivity of the active layer. However, the polymer binder (or permeable matrix) in which the porphyrin molecule was immobilized affected both the pressure and temperature sensitivities of the paint. To evaluate the performance of PSP, the pressure sensitivity and temperature sensitivity are defined as SP = A(lref/l)/AP (in % per bar) and

ST = – A( I / lref )/ AT (in % per degree), respectively. The pressure sensitivity was calculated in a pressure range of 0.15-2 bars and the temperature sensitivity was determined in a temperature range of 10-35°C. Figure 3.4 shows the pressure sensitivity SP as a function of temperature and the temperature sensitivity ST as a function of pressure. The pressure sensitivity SP varied from 55% to nearly
80% per bar, depending on the polymer binder used and temperature as well. The FIB PSP formulation had nearly constant pressure sensitivity over a temperature range of 10-40°C. The Uni-Coat and sol-gel PSP formulations had a similar linear dependency of the pressure sensitivity SP on temperature; the temperature sensitivity ST ranges from 0.6% to 1.6% per degree. The temperature sensitivity was somewhat affected by pressure for all the PSP formulations except the FIB PSP. The FIB PSP also has the lowest temperature sensitivity among them.

Figure 3.5 shows the chemical structure, and absorption and emission spectra of Bathophen Ruthenium Chloride (Ru(ph2-phen) or Ru(dpp)). Ruthenium-based oxygen sensors have been studied extensively by analytical chemists (Bacon and Demas 1987; Carraway et al. 1991a, 1991b; Sacksteder et al. 1993; Xu et al. 1994; Klimant and Wolfbeis 1995). The Ruthenium-based PSP formulations have been developed and used for wind tunnel testing by the former McDonnell Douglas (now Boeing at St. Louis) (Schwab and Levy 1994). Figure 3.6 shows the Stern – Volmer plots for Ru(dpp) in GE RTV 118 added with silica gel particles at different temperatures; Figure 3.7 shows the luminescent lifetime as a function of pressure for Ru(dpp) in GE RTV 118 at 22oC.

(b)

Figure 3.8 shows the chemical structure and absorption and emission spectra of Pyrene. The Pyrene-based PSP formulations were developed by TsAGI/OPTROD in Russia (Fonov et al. 1998). One of them was the binary paint (B1 PSP) in which a pressure-insensitive reference component was added to correct the excitation light variations on a surface in performing a ratio between the wind-on and wind-off images. Figure 3.9 shows the Stern-Volmer plots at different temperatures for Pyrene complex in GE RTV 118. Obviously, this Pyrene-based

PSP exhibits weak temperature dependency over a temperature range of 17-40oC. Note that Perylene and its derivatives like Green Gold (perylene dibutylate) were also used as a luminescent dye for PSP. Besides TsAGI, ONERA in France and DLR in Germany developed Pyrene-based PSP formulations as well (Engler et al. 2001a). The PyGd PSP formulation developed by ONERA contained Pyrene as a pressure-sensitive dye and a gadolinium oxysulfide as a reference component. Figure 3.10 shows the emission spectrum of the PyGd PSP excited at 325 nm. The two components in the paint absorbed an ultraviolet excitation light and emitted at sufficiently different wavelengths such that the emissions from the two components can be separated using optical filters. Figure 3.11 shows the Stern – Volmer plots at the ambient temperature for three Pyrene-based PSP formulations: PyGd, B1 and PdGd. Because the temperature sensitivity of the reference component was similar to that of the Pyrene dye in the PyGd PSP, the temperature effect can be compensated by taking a ratio between the luminescent intensities from the pressure-sensitive component (Pyrene) and reference component. As a result, the PyGd PSP displayed very low temperature sensitivity of 0.05%/K. A number of ‘Gottingen Dyes’ (GD) were developed by DLR and the University of Gottingen, and three stable Pyrene-based paints, GD145, GD146 and GD147, were tested in wind tunnels (Engler and Klein 1997b). Figure 3.12 shows the pressure sensitivities of the Gottingen PSP formulations. A shortcoming of Pyrene-based paints is that sublimation may occur when temperature is greater than 40oC.

(a)

(b)

Fig. 3.8. (a) Chemical structure of Pyrene, and (b) absorption and emission spectra of Pyrene. From Mebarki (2001)

Fig. 3.10. Emission spectra of a binary Pyrene-based PSP (PyGd) at 1 bar and vacuum. From Engler et al. (2001a)

3.1. Calibration

In order to quantitatively measure air pressure with PSP, the relationship between the luminescence signal (intensity, lifetime or phase) and air pressure should be experimentally determined by calibration. Apparatus for calibration of PSP over a temperature range of 90-423 K are described in Appendix A. A generic calibration set-up consists of a pressure chamber, excitation light source and photodetector. A PSP sample is placed in the pressure chamber where pressure can be adjusted from vacuum to high pressures. The surface temperature of the PSP sample is controlled using a heating/cooling device and measured using a temperature sensor. The PSP sample is excited by an illumination source (e. g. UV lamp, LED array or laser) through a window of the pressure chamber. The luminescent emission from the paint sample, after filtered by a band-pass optical filter, is measured using a photodetector (e. g. photodiode, PMT or CCD camera), and the photodetector output is acquired with a PC over a range of pressures and temperatures. Therefore, the correspondence between the luminescence signal and pressure, which is usually described by the Stern-Volmer equation, is established over a range of temperatures. Typical calibration results for a number of PSP formulations based on Platinum Porphrins, Ruthenium complexes and Perlene/Pyrene are given in the following sections. The calibration set-up for PSP can be used for TSP calibration when the surface temperature of a paint sample varies while pressure in the chamber is kept constant. Calibration data for TSP are typically presented as an Arrhenius plot over a certain temperature range; typical calibration results for TSP formulations are given in Sections 3.3 and 3.4.

The most common calibrations for PSP and TSP are based on measurements of the luminescent intensity as a function of pressure and/or temperature. As discussed before, however, the luminescent lifetime (or phase) is also a function of pressure and/or temperature. In a lifetime calibration apparatus, a pulsed (or modulated) excitation light source is used, and after an exciting pulse light ceases the exponential decay of the luminescent intensity is measured using a fast – responding photodetector and recorded with a PC or an oscilloscope. The luminescent lifetime can be determined by fitting data with a single exponential function or multiple-exponential function for certain paints over a range of pressures and temperatures. Early instrument for measuring the luminescent

lifetime was described by Brody (1957) and Bennett (1960), and the current state of luminescent lifetime measurement systems in photochemistry and medical applications was comprehensively reviewed by Lakowicz (1991, 1999). The Stern-Volmer relations between the lifetime and oxygen partial pressure (or concentration) for oxygen sensitive luminescent materials were determined by Gewehr and Delpy (1993), Gord et al. (1995), Sacksteder et al. (1993), Xu et al.

(1994) , and Papkovsky (1995). Lifetime calibration results for TSPs and thermographic phosphors were also reported by Sholes and Small (1980), Grattan et al. (1987), Bugos (1989), Noel et al. (1985), and Lakowicz (1999). A more detailed discussion on the lifetime and phase methods is given in Chapter 6.

For TSP where the paint layer is not oxygen-permeable such that no oxygen quenching occurs, from Eq. (2.8), the quantum yield of luminescence is simply given by

Ф = 4- = – TkT – . (2.64)

I a kr + krn

The temperature dependency of the non-radiative processes knr can be decomposed into a temperature-independent term and a temperature-dependent term modeled by the Arrhenius relation (Bennett and McCartin 1966; Song and Fayer 1991; Schanze et al. 1997)

e

knr = knr0 + knr1exp(—-ziz) , (2.65)

where k^0 = knr(T = 0) and km1 are the rate constants for the temperature – independent and temperature-dependent processes, respectively, Enr is the activation energy for the non-radiative process, R is the universal gas constant, and T is the absolute temperature in Kelvin. From Eqs. (2.64) and (2.65), we have

where I(0) = I(T = 0) is the luminescent intensity at the absolute zero temperature. For ЩТ) – I(Tf M/I(0) << 1 and I(T)I(Tf ) /[I(0)]2 << 1

over a certain temperature range, a relation between the luminescent intensity and temperature can be approximately written in the Arrhenius form

Theoretically speaking, the Arrhenius plot of ln[I(T)/ I(Tref )] versus 1/T gives

a straight line of the slope EJR. Experimental results indeed indicate that the simple Arrhenius relation Eq. (2.67) is able to fit data over a certain temperature range. However, for some TSPs, experimental data may not fully obey the simple Arrhenius relation over a wider range of temperature. Thus, as an alternative, an empirical functional relation between the luminescent intensity and temperature is

-IjTP – = f(T/Tf) , (2.68)

11ref->

where f(T /Tref) could be a polynomial, exponential or other function to fit

experimental data over a working temperature range. Either Eq. (2.67) or Eq. (2.68) can serve as an operational form of the calibration relation for TSP in practical applications.

Besides the collision-controlled quenching, an adsorbed oxygen molecule on a porous surface can also quench the luminescence; if this is the dominant mechanism, the oxygen quenching is controlled by adsorption and surface diffusion of the adsorbed oxygen on the porous surface. The oxygen concentration on a porous surface, [OJad, can be described by the fractional coverage of oxygen on the porous surface

Q [ O 2 ] ads

[O2] adsM

where [O2]adSM is the maximum oxygen concentration on the porous surface. The Stern-Volmer equation is then written as

I – _ 1 + kqT0[O2]adM Q, (2.38)

and accordingly the convenient form of the Stern-Volmer relation for aerodynamic applications is

Q

EaL _ A(T) + B(T)——, (2.39)

I Qref

The rate constant kq for the oxygen quenching, which is surface-diffusion – controlled, can be described by (Freeman and Doll 1983)

kq = 2nRAB *bd = К exP(-Esdiff /RT), (2.41)

where Rab is the relative distance between an adsorbed oxygen and an adsorbed luminophore, and D is the diffusivity and the parameter AB is a ratio of the modified first-order and second-order Bessel functions of the second kind. Basically, kq is temperature-dependent due to the Arrhenius relation

D = D0 eXP(-Esdiff /RT) .

To describe в, either the Langmuir isotherm or the Freundlich isotherm can be used (Carraway et al. 1991b). The Langmuir isotherm relates в to the partial pressure of oxygen pOq in the working gas by

b pO

в= °2 . (2.42)

1 + bPo2

The factor b in Eq. (2.42) is a function of temperature (Butt 1980)

The coefficient A[nngmut has the same temperature dependency as that for a conventional polymer PSP and that in the collision-controlled model, i. e.,

and the linearized form for Ar. is

Langmuir

Hence, Eq. (2.48) indicates that A[angmur is related to the temperature dependency of the non-radiative processes of the luminophore. On the other hand, B[amgmidr has the following temperature dependency

where E, — Esdiff + Eads. Rewriting Eq. (2.49) in an exponential form yields

and furthermore, linearization of Eq. (2.50) at T = T gives

Where El — E, – R Tref / 2 — Ediiff + E ads – RTref/ 2 . С^аГ^ the temperature

dependency of the coefficient B[angmiit, Eq. (2.51), is associated with both surface diffusion and adsorption; but it has the similar form to Eq. (2.23) for a conventional polymer layer. The reference Stern-Volmer coefficients ALangmuir, ref

and BLcngmuir, ref (their lengthy expressions are not given here) satisfy the

constraint Ahangmuir, ref + B Langmuir, ref = 1 .

The Freundlich isotherm can serve as another model for surface adsorption

0 — bp(Po2)r (2.52)

where the coefficient and exponent are

і RT

bf —~b= exp(-Eads /RT) and у ——————– . (2.53)

■^TY EadsM

The exponent /is an empirical parameter that is temperature-dependent. For a known yref at a known reference temperature Trep EdisU is given by

(2.54)
Substituting Eqs. (2.52), (2.53) and (2.54) into Eq. (2.39), we obtain the non-linear Stern-Volmer equation
(2.55)
where
(2.56)
and the linearized form for „иш1йЛ is
T – Tre T ref
(2.58)
AFreundlich = AFreundlich, ref
The coefficient BFremdlich has the temperature dependency kqbF(pO2ref)Y
(2.59)
Substituting Eqs. (2.41) and (2.53) into (2.59) yields
(2.60)
where Ef _ Esiiff + Eads. When an approximation Yref ~ Y is used for a small temperature change, the expression for BFreun<mch becomes
(2.61)
B Freundlich BFreundlich, ref ■

which is similar to B^ . After rewriting all the terms in Eq. (2.61) in an

exponential form, linearization at T = Tref yields

where

Similar to the Langmuir-type model, the coefficient BFrmndlich has the temperature dependency associated with surface diffusion and adsorption. However, the photophysical model Eq. (2.55) describes the non-linear behavior of the Stern – Volmer plot for a porous PSP.