TFAWS 2015 Thermal Coatings Seminar Series Training Part 1 : Properties of Thermal Coatings NASA GSFC Contamination and Coatings Branch – Code 546 Hosted by: Jack Triolo - SGT, Inc. August 6, 2015 1 Agenda • The Relationship of Coating Properties That We Can “Easily” Measure vs. the Properties We Need…ɑS & ɛH • How Solar Absorptance is Determined • Description of Solar Reflectance measurement techniques • Typical data • How Thermal Hemispherical Emittance is Determined • Conversion of normal emittance to hemispherical emittance • Emittance vs. temperature • Description of measurement techniques • Typical data • Factors that Influence Thermal Radiative Properties • BRDF – Specular and Diffuse • GSFC Instruments Overview • Types of Coatings Used at GSFC August 6, 2015 2 Thermal Radiative Properties of Coatings • Reflectance • Transmittance • Absorptance • Emittance August 6, 2015 3 Thermal Radiative Properties of Coatings (Information Obtained From Thermal Radiative Properties Coatings, Thermaphysical Properties of Matter, Volume 9) • Radiant energy is reflected, transmitted and/or absorbed by a surface or material r + t + a = 1, for materials, where t = 0, r + a = 1, or a = 1- r Where: Reflectance = r, Transmittance = t, and Absorptance = a • Emittance (e) is the rate at which a body radiates energy (heat) at a given temperature in relation to the rate a black body radiator radiates energy (heat) at the same temperature • Kirchhoff’s Law • Ideal radiator, when in thermal equilibrium, the body emits radiant energy at the same rate at which it absorbs = e • In the Aerospace Industry, a and e are never directly measured – THEY ARE CALCULATED! August 6, 2015 4 Solar Absorptance Property Measurement • At GSFC, the instrumentation used to calculate the solar absorptance measures over the spectral range of 250 to 2800 nanometers (.25 to 2.8 microns). An integrating sphere is used to measure the coating’s reflectance for the solar absorptance calculation • Solar Absorptance is the total solar energy absorbed by the surface divided by the total solar energy integrated as a function of the wavelength 2800 R( ) S( ) d 250 1 s 2800 S( ) d 250 • Where R = reflectance, S = solar energy, as = solar absorptance, and l = wavelength • The reflectance measurement is performed near-normal (angle of incidence = 15º). This measurement is typically sufficient for most surfaces up to approximately 45º • Whereas, when measuring cylindrical surfaces, spherical surfaces or angle of incidence greater than 45º, variations in the angle of incidence will influence the solar absorptance value and must be measured • Typically the Johnson curve is used to represent the total solar energy over the solar spectrum August 6, 2015 5 Reflectance and the Johnson Curve Johnson curve (blue) and the Polyrip clear/VDA (red) Solar Absorptance value = .405 August 6, 2015 6 Directional Total Reflectivity Incident Enegry rs rd rd rd rd August 6, 2015 7 Two Types of Integrating Spheres Reference Beam Incidence Angle Sample Beam Detector Sample Reference Beam sample Sample Beam Transmittance Transmittance Sample Reference Sample Loc ation Loc ation Detector Reference Edwards Type Integrating Sphere Four Port C omparsion Integrating Sphere August 6, 2015 8 LPSR-300 Reflectometer Optical Schematic 115mm 2 Position Beam Integrating Deflector Sphere Si and PbS Detectors Sample Under 0.4μm to 2.8 μm High Test 160 Hz Tuning Pass Dichroic Filter Fork Chopper Tungsten 1/6.5 Prism Monochromator Filament for 0.25 μm to 2.80 μm Range Source Variable Variable Collection Entrance Slit Exit Slit Optics Deuterium 3 Position Source Filter Wheel August 6, 2015 9 Reflectance Curves of Various Thermal Coatings 1 0.9 0.8 0.7 0.6 0.5 0.4 Reflectance 0.3 0.2 0.1 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 Wavelength (nm) Silver Composite Coating (CCAg)/Al a = .07 e(n) = .67 09 Jul 2003 Z93P White Paint a = .17 e(n) = .93 18 Sep 2002 2-mil Kapton/VDA a = .42 e(n) = .83 09 Jul 2003 Germanium/Black Kapton a = .49 e(n) = .85 11 Mar 2000 Aeroglaze Z306 Black Paint a = .93 e(n) = .91 13 Mar 2001 August 6, 2015 10 Emittance Property Calculation • Normal Emittance • At GSFC, the instrumentation used to calculate the normal reflectance measures over the spectral range of 5 to 100 microns at room temperature • The normal emittance is calculated by measuring the reflectance of a material’s surface in the infrared region of the spectrum and subtracting the measured reflectance from one (for opaque coatings only) • Hemispherical Emittance • For thermal modeling and analysis, the emittance must be in terms of a hemispherical (total body) emittance value. Converting normal emittance to hemispherical emittance can be accomplished by using a conversion table and chart by E. Schmidt, E. Eckert, and M. Jakob • Hemispherical emittance can also be determined by calorimetric emittance measurement • With the addition of an ellipsoidal attachment, GSFC also has the capability of calculating hemispherical emittance as a function of temperature by radiometric reflectance measurement August 6, 2015 11 Directional Emissivity Curve For a Conductor Angle of Emission 0 10 10 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 0.5 0.4 0.3 0.2 0.1 0 0.1 0.2 0.3 0.4 0.5 Directional Emissivity Directional emissivity curve for a conductor with an index of refraction of n= 5.7+i9.7 August 6, 2015 12 Hemispherical Emissivity Coordinate System d 1 2 = ( , , )cos d H 0 August 6, 2015 13 Ratio of Hemispherical to Normal Emissivity for Conductors Ratio of Hemispherical to Normal Emittance for Conductors 1.4 1.35 1.3 1.25 1.2 n / 1.15 h 1.1 1.05 1 0.95 0.9 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Normal Emissivity (n) August 6, 2015 14 Directional Emissivity Curve for a Dielectric Angle of Emission 0 10 10 20 20 30 30 40 40 50 50 60 60 70 70 80 80 90 90 1.0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1.0 Directional Emissivity Directional emissivity curve for a dielectric with an index of refraction of n=1.5 August 6, 2015 15 Ratio of Hemispherical to Normal Emissivity for Insulators Ratio of Hemispherical to Normal Emittance for an Insulator 1.1 1.09 1.08 1.07 1.06 1.05 1.04 1.03 1.02 1.01 n / 1 h 0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.9 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 Normal Emissivity (n) August 6, 2015 16 DB 100 Optical Diagram sample to be measured unheated cavity ~7-10° Black Anodize interior cavity rotation heated cavity ~ 43C Selective Filter Base Plate folding mirror folding mirror Vacuum CsI Thermocouple Illumination: Hemispherical Detector: Directional ~7-10 deg focusing mirror Detector type: CsI vacuum thermocouple Detector Range: 5?-40μm? Accuracy: ±0.02 sample must be gray Measurement: Hemispherical-Directional Reflectance August 6, 2015 17 SOC 100 Optics Illumination: Hemispherical Detector: 10˚ -80˚ Detector type: FTIR: Si, KBr, Pe Detector Range: 2-100μm Accuracy: ± ? Measurement: Hemispherical-Directional Spectral August 6, 2015 18 Temp200A Optical Diagram Electronics Mirror Pyroelectric 15 Detector Ellipsodial Illumination: 15˚ Collector Detector: Hemispherical Chopper Detector type: Pyroelectric Detector Range: 3-35μm Infrared Sample Accuracy: ±0.01 for gray samples Source ±0.03 for non-gray samples Measurement: Directional-Hemispherical Reflectance Temp 2000A Optical System August 6, 2015 19 Emittance by the Calorimetric Technique Heat Flow Manganin Wires Heat Qlead loss Radiated conduc ted ra d iat ed Epox y Vacuum Chamber Silicon Diode -7 P = 1*10 torr Q sd Qgas Gas M o lec u le A1100 Aluminum LN2 Shroud Aluminum Door LHe Shroud Manganin Wires Infrasil Windows LHe Shroud Manganin Wires Quartz Window Aluminum Plate Sample Sample Aluminum Sample Door -8 P = 2*10 torr Aluminum Door P = 760 torr LeH Shroud Top View Liquid Helium Liquid Helium Dewar Dewar LeH Shroud Front View Dewar Supports Base Plate HV Valve & Turbo Pump August 6, 2015 20 Dielectrics over Metals Emittance of SiOx Coated Aluminum Emittance of Al2O3 coated Aluminum as a Function of Oxide Thickness* as a Function of Oxide Thickness* 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 Emissivity Emissivity e 0.2 0.2 e e(n) 0.1 0.1 e(n) 0 0 0 4 8 12 16 20 24 28 32 36 40 0 4 8 12 16 20 24 28 32 36 40 Oxide Thickness** Oxide Thickness** * Charts reproduced from Heaney, Triolo, and Hass, “Evaporated Thin Films For Spacecraft Temperature Control Applications”, July 1977. ** Oxide Thickness is represented as /4 at 550 nm. August 6, 2015 21 Spectral Reflectance August 6, 2015 22 Blackbody Spectral Radiance August 6, 2015 23 Emittance of a Hypothetical Coating and Two Black Body Temperature Curves 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Hemisphercal Emittance 0.2 0.1 0 0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35 37.5 40 42.5 45 47.5 50 W avelength (microns) Emittan ce of Coatin g Black Bod y 290° K Black Bod y 90° K August 6, 2015 24 Calorimetric Results for A276 Aeroglaze A276 (3.0 mils) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 Hemispherical Emittance 0.3 0.2 0.1 0.0 0 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 Temperature (°K) Figure 6.1 August 6, 2015 25 Infrared Reflectance of A276 1 0.9 0.8 0.7 0.6 0.5 0.4 Reflectance 0.3 0.2 0.1 0 0 20 40 60 80 100 120 Wavelength (microns) August 6, 2015 26 Factors that Influence Thermal Radiative Properties • Solar Absorptance and/or Emittance Values Influencing Factors: • Surface Finishes • Highly Polished (mirror-like/optical surface) • Polished • Buffed • Matt • Machined • Substrate Texture • Rough versus Smooth • Woven • Bead Blasted (sand, glass, etc…)