Aerosol Deposition of Thermographic Phosphor Coatings

Abstract

Aerosol-deposited thermographic phosphors can be used for non-contact, two-dimensional temperature sensing in extreme environments. The fast response time and thermal/environmental stability of doped ceramic powders allow for temperature measurements up to the melting point of the phosphor on hot surfaces, such as rapidly rotating turbine components and combustors.

Claims

1. A method for measuring the temperature of a surface, comprising: aerosol depositing a thermographic phosphor on a surface, heating the surface to a temperature, and measuring the temperature of the deposited thermographic phosphor using a phosphor thermometry method.

2. The method of claim 1, wherein the phosphor thermometry method comprises a spectral ratio method or a lifetime decay method.

3. The method of claim 2, wherein the spectral ratio method comprises; exciting the emission of a thermographic phosphor with an excitation source, measuring the intensity of a first emission peak of the thermographic phosphor which increases with temperature at a first wavelength, measuring the intensity of a second emission peak of the thermographic phosphor which does not change substantially with temperature at a second wavelength, and ratioing the intensities of the first emission peak and the second emission peak to determine the temperature.

4. The method of claim 1, wherein the step of aerosol depositing comprises: providing a powder of a thermographic phosphor, and aerosol depositing the powder on a substrate.

5. The method of claim 1, wherein the thermographic phosphor comprises a ceramic phosphor.

6. The method of claim 5, wherein the ceramic phosphor comprises a ceramic host material doped with a rare-earth element.

7. The method of claim 6, wherein the ceramic phosphor comprises a rare-earth-doped yttrium-aluminum-garnet or rare-earth-doped ytrrium oxide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.

[0013] FIG. 1 is a schematic illustration of an aerosol deposition process.

[0014] FIG. 2 is a scanning electron microscope image of an aerosol-deposited YAG:Dy coating on a copper substrate.

[0015] FIGS. 3A and 3B are cross-sectional SEM images of YAG:Dy coating on stainless steel cylinder produced by aerosol deposition.

[0016] FIG. 4 is a graph of emission peak signal ratios as a function of temperature for various YAG:Dy phosphor coatings, compared to the results of Alden et al. See M. Alden et al., Prog. Energy Combust. Sci. 37, 422 (2010).

DETAILED DESCRIPTION OF THE INVENTION

[0017] A common thermographic phosphor is dysprosium-doped yttrium aluminum garnet (YAG:Dy). This thermographic phosphor has high temperature stability and phosphorescent range. However, application of the YAG:Dy phosphor is currently limited to techniques which result in poor adhesion, low emission signal, or crystallographic phase change. In particular, YAG:Dy coatings are commonly applied in two ways: paints/epoxies and plasma spray techniques. Paints/epoxies quickly burn away before the maximum operating temperature of otherwise stable ceramics (up to 2000° K) and result in much lower transmission efficiency and thermal conductivity. See W. Flores-Brito et al., “Measuring Temperatures in Fire Utilizing Thermographic Phosphors,” Sandia National Laboratories, SAND2018-5269 PE, (2018). Plasma-sprayed coatings rely on melting the particles during the deposition process, causing phase changes and rapid quenching, which results in undesirable stresses and damage to the substrate. See S. D. Parukuttyamma et al., J. Am. Ceram. Soc. 84(8), 1906 (2004).

[0018] The present invention is directed to aerosol deposition to produce coatings of thermographic phosphors. Aerosol deposition is an attractive technique due to the zero thermal energy input and lack of binders to produce coatings. Aerosol deposition is a relatively new technique that relies solely on the velocity of particles to create highly dense coatings at room temperature through plastic deformation and successive tamping. See J. Akedo, J. Am. Ceram. Soc. 89(6), 1834 (2006); J. Akedo and M. Lebedev, Materia 41(7), 459 (2002); J. Akedo, J. Therm. Spray Tech. 17, 181(2008); and D. Hanft et al., J. Ceram. Sci. Technol. 6(3), 147 (2015), which are incorporated herein by reference. Resulting coatings are typically characterized by low porosity which can result in less light scattering and higher thermal conductivity compared to other fabrication methods. Coatings are also nano-crystalline due to the nature of the impact consolidation mechanism which relies on particle fracture to small enough sizes to induce plastic deformation. Aerosol deposition was initially proposed as an alternative method to fabricate ceramic-coated materials without the need for high sintering temperatures. This would broaden the range of applications, allowing ceramics to be bonded with materials that have lower melting points. Some of the applications and components on which aerosol deposition has been successfully tested include: aluminum, piezoelectric materials, titanium dioxide, biocomponents, magnetic materials, fuel cells, sensing materials, and batteries. See D. Hanft et al., J. Ceram. Sci. Technol. 6(3), 147 (2015); J. Akedo et al., “Aerosol Deposition (AD) and Its Applications for Piezoelectric Devices,” in Advanced Piezoelectric Materials, pp. 575-614 (2017); P. Sarobol et al., J. Therm. Spray Technol. 25, 82 (2015); and J. Akedo, J. Am. Ceram. Soc. 89, 1834 (2006).

[0019] Aerosol deposition uses the kinetic energy (supersonic velocities) of the particles to fracture ceramic particles to small enough sizes to plastically deform and build coatings layer-by-layer. A schematic illustration of an aerosol deposition process is shown in FIG. 1. The aerosol deposition process uses nano-to-micron-sized feedstock particles, which are accelerated to supersonic velocities to create metallic, ceramic, and carbide coatings. The carrier gas usually comprises N.sub.2 or He, and transports dry particles of the desired material to a nozzle, where they are accelerated into a vacuum chamber. Aerosol deposition is conducted in a vacuum to reduce the effect of bow shock, a phenomenon common in similar kinetic coating techniques such as cold spray. The bow shock (or pressure wave) can re-direct the path of the nano-sized particles which is used as feedstock and prevent impact with the substrate. In the absence or reduction of bow shock, the supersonic particles impinge on the surface of the substrate resulting in fragmentation and subsequent plastic deformation (ceramics included) of the particles. This results in a strong mechanical bond between coating and substrate while producing microstructures with grain sizes on the order of lOs of nanometers. See J. Akedo, J. Am. Ceram. Soc. 89, 1834 (2006); J. Akedo and M. Lebedev, Mater. Jpn. 41, 459 (2002); P. Sarobol et al., J. Therm. Spray Technol. 25, 82 (2015); and E. Calvié et al., J. Eur. Ceram. Soc. 32, 2067 (2012), which are incorporated herein by reference. Each subsequent collision tamps the surface to produce a highly dense, void-free, adherent coating, capable of large thickness (up to 10s of microns). In addition, the aerosol deposition method can be automated to print a variety of patterns for other applications, e.g., dot patterns for stress and stain diagnostics such as digital image correlation (DIC).

[0020] The aerosol-deposited thermographic phosphor can typically be an inorganic ceramic host material doped with an activator, such as a rare-earth element. The doping concentration can be low enough (e.g., few %) so that the dopant atoms are isolated from one another in the host matrix. The host material can be substantially transparent to the excitation radiation such that the activator element absorbs and emits the radiation. A number of temperature-dependent ceramic phosphors can be used. See G. Särner et al., Meas. Sci. Technol. 19, 125304 (2008), which is incorporated herein by reference. As described above, a common thermographic phosphor that can be deposited by aerosol deposition is YAG:Dy. Other doped ceramic phosphors, such as YAG:Tm, YAG:Tb, and red-emitting europium-doped yttrium oxide (Y.sub.2O.sub.3:Eu) and manganese-doped magnesium fluorogermanate (Mg.sub.3F.sub.2GeO.sub.4:Mn), can be used to increase the working range of temperature sensing. See A. H. Khalid and K. Kontis, J. Lumin. 131 (7), 1312 (2011); and W. Flores-Brito et al., “Study of Sensitivity vs. Excitation Time of LED Excited Thermographic Phosphor,” AIAA SciTech Forum (2019), which are incorporated herein the reference.

[0021] As an example of the invention, micron-sized particles of YAG:Dy (e.g., 3% dysprosium) were coated onto a variety of substrates with thicknesses ranging from 5 to 30 microns depending on the number of passes that were used during aerosol deposition. No pre-processing of the powder was needed prior to aerosol deposition. Coatings were applied using a custom aerosol deposition chamber. See J. Mahaffey et al., “Aerosol Deposition (AD) for Advanced Manufacturing of Functional Materials,” SAND2018-0179 D. Helium was used as a carrier gas at a flow rate of ˜30 SCFH (standard cubic feet of gas per hours). A custom diverging nozzle was used to deposit the coatings. YAG:Dy powder was injected into the gas stream at using a rotating brush aerosol generator. YAG:Dy was deposited onto copper, stainless-steel, alumina, and borosilicate glass substrates. 1 cm×1 cm square patterns were made by rastering. The vacuum chamber was kept at less than 10 torr during the spray run. The spray deposition parameters are tabulated in Table 1.

TABLE-US-00001 TABLE 1 Spray parameters for Aerosol Deposition of YAG:Dy coatings. Substrate Materials Copper, Glass, Alumina, Stainless Steel Inlet Pressure 15 psi Chamber Pressure 8.1 torr Number of passes 1 Powder Feed 20 mm/hr Traverse Speed 900 mm/min Step Size 0.2 mm Nozzle Type DeLaval Brush Speed 1200 RPM Carrier Gas Helium

[0022] In FIG. 2 is shown a scanning electron microscope (SEM) image of a YAG:Dy deposit on a copper substrate. The surface roughness of the deposits were analyzed with a 3D laser scanning confocal laser microscope to ensure that the method deposits films with consistent thickness and uniformity. Coatings on all substrates had similar roughness measurements (arithmetic mean height, R.sub.a˜1.6 μm, and the maximum height of profile, R.sub.z˜20.5 μm).

[0023] An important feature of aerosol-deposited coatings is that the impact mechanism results in a nano-crystalline microstructure (due to fracture down to a size that can be plastically deformed). See D. Hanft et al., J. Ceram. Sci. Technol. 6(3), 147 (2015). Without the proper velocity of the particles, this fracturing mechanism cannot occur. To ensure that the coatings were in the plastic deformation range, X-ray diffraction (XRD) was used to determine crystallite size. The as-deposited coatings were found to have an average crystallite size of 25 nm, a good indication that the coating was produced using the impact consolidation mechanism proposed in literature. See J. Akedo, J. Therm. Spray Technol. 17(2), 181 (2008). In-situ XRD coupled with heat treatment of the as-deposited coating was performed in order to determine grain growth and phase transformation for typical working temperatures of the coatings. Heat treatment of the resulting coating indicated a general shift upward in crystallite size at temperatures starting at 1000° C., suggesting grain coarsening occurs at elevated temperatures.

[0024] Uniform sub-micron particles can be obtained by ball milling of the powder, albeit at the expense of additional processing. Both ball-milling and heat treatment can be used to produce uniform coatings with nano-crystalline microstructure and high density. It has been suggested that ball-milling not only reduces particle size through fracturing of the particles, but also induces defects into the crystalline lattice which can be aligned to produce grain boundaries during subsequent heat treatment, thus reducing crystallite size in each particle. See H. Park et al., J. Therm. Spray Technol. 5(22), 882 (2013); Y. Kawakami et al., J. Cryst. Growth 275, 1295 (2005); J. Exner et al., Adv. Powder Technol. 26(4), 1143 (2015); and J. Exner et al., J. Eur. Ceram. Soc. 39(2-3), 592 (2019). The addition of a post ball-mill heat treatment results in more consistent powder flowability. For example, a post ball-milled powder was heated for 8 hours at 200° C. before being heat treated at 800° C. for 4 hours. The heat treated powder was used to deposit a YAG:Dy coating onto a ¾ in stainless steel tube. Additional passes on the cylindrical tube were conducted to get a similar thickness in coating comparable to the flat substrates described previously. Scanning electron microscopy (SEM) images of a coating produced on the cylinder with a post ball-mill heat-treated powder are shown in FIGS. 3A and 3B. The coating on a four-pass section of the cylinder was roughly 10-20 microns in thickness. The original surface is nearly preserved. This indicates very little damage is occurring to the underlying substrate. The lack of damage is a direct result of the starting sub-micron particle size. The momentum transfer to the substrate is simply not high enough (as long as erosion is not occurring) to induce significant damage.

[0025] Thermal conductivity of the resulting as-deposited coatings was performed using frequency domain thermoreflectance (FDTR). The measured volumetric heat capacity ρc.sub.p of the aerosol-deposited film was 2.69±0.18 MJ/m.sup.3K, within one standard deviation compared to the literature value. See P. H. Klein and W. Croft, J. Appl. Phys., 38(4), 1603 (1967). This was expected as the density of the aerosol-deposited coating was estimated to be >95%. The thermal conductivity was found to be 1.43±.28 W/mK, which is about 1/7.sup.th of bulk material. See P. H. Klein and W. Croft, J. Appl. Phys., 38(4), 1603 (1967); N. Padture and P. Klemens, J. Am. Ceram. Soc. 80(4), 1018 (1997); and J. Lu et al., J. Alloys Compd. 341(1-2), 220 (2002). The reduction of the thermal conductivity from bulk is due to the small crystallite size resulting in a large number of grain boundaries and slight porosity, both of which are expected to lower the thermal conductivity in solids. See G. Soyez et al., Appl. Phys. Lett. 77(8), 1155 (2000); and K. W. Schlichting et al., J. Mater. Sci. 36(12), 3003 (2001). Higher thermal conductivity can be desirable in order to provide faster and more accurate temperature measurements of the substrate. Added heat treatment of the coatings can produce a higher conductivity due to grain growth and reduction of pores. Further, thin coatings can be used to reduce the insulating heat transfer properties may occur due to the low thermal conductivity.

[0026] Phosphorescent properties were measured using two color pyrometry. A Q-smart 850 Nd:YAG laser beam, frequency tripled to 355 nm, was used to excite the deposited phosphor. See A. C. Eckbreth, “Laser Diagnostics for Temperature and Species in Unsteady Combustion,” in Unsteady Combustion, pp. 393-410 (1996). A PI-Max4 intensified camera from Princeton Instruments was used for data collection. An image doubler (or stereoscope) from LaVision with 460 nm (10-nm FWHM, ASAHI) and 500 nm (10-nm FWHM, ASAHI) filters was mounted to the intensified camera to obtain and compare emission data with the spectral ratio method. With this image doubler, two images, one at each wavelength, can be obtained simultaneously without the need for two synchronized cameras. These images can then be processed using a MATLAB code to obtain the intensity ratio of 460 nm/500 nm emissions at each temperature. This code first averages the background images, and the background noise average is subtracted from each of the signal images. The code then adds and averages the signal for each temperature. The signals from both wavelengths are in the same image. Therefore, the area for each signal is determined by choosing a small set of pixels (e.g., 672 pixels in total for each set) that align best on top of each other. This results in single-pixel intensity ratio maps. Calibration ratios were also obtained from a small stainless-steel coupon coated with the phosphor-ethanol mix, which was simply painted/brushed onto the surface.

[0027] The samples were imaged for a range of sensing temperatures while excited with the 355 nm Nd:YAG beam. FIG. 4 displays these average intensity ratios per temperature for the three samples imaged (aerosol-deposited YAG:Dy on copper and stainless-steel samples, and the painted-on phosphor-ethanol mix sample), compared to results from Alden et al. See M. Alden et al., Prog. Energy Combust. Sci. 37, 422 (2010).

[0028] The present invention has been described as aerosol deposition of thermographic phosphor coatings. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.