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RADIATIVE PROPERTIES OF REAL MATERIALS. Electromagnetic Theory: ▪ ideal, optically smooth surface ▪ valid for spectral range larger than visible Real Materials: surface condition ▪ contaminants or oxides ▪ surface roughness ▪ thin coating on substrate.

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Presentation Transcript
slide1

RADIATIVE PROPERTIES OF

REAL MATERIALS

  • Electromagnetic Theory:
  • ▪ ideal, optically smooth surface
  • ▪ valid for spectral range larger than
  • visible
  • Real Materials: surface condition
  • ▪ contaminants or oxides
  • ▪ surface roughness
  • ▪ thin coating on substrate
slide2

Material Dependence

  • ▪ opaque nonmetals
  • ▪ opaque metals
  • ▪ selective and directional opaque
  • surfaces
  • ▪ selectively transparent materials
  • Parameter Dependence
  • ▪ spectral and directional variations
  • ▪ variation with temperature
  • ▪ effect of surface roughness
  • ▪ effect of surface impurities
slide3

Radiative Properties of

Opaque Nonmetals

Effect of Coating Thickness

Emissivity of zinc oxide coatings on oxidized stainless steel substrate. Surface temperature, 880 ± 8 K

slide4

Effect of Substrates

Effect of substrate reflection characteristics on the hemispherical characteristics of a TiO2 paint film for normal incidence: Film thickness, 14.4 mm; volume concentration of pigment 0.017,

slide5

Spectral Dependence

Spectral reflectivity of paint coatings. Specimens at room temperature

slide7

Variation with Temperature

Effect of surface temperature on emissivity of dielectrics

slide9

Effect of Surface Roughness

Bidirectional total reflectivity of typewriter paper in plane of incidence. Source temperature, 1178 K

slide10

Bidirectional total reflectivity for visible lightin plane of incidence for mountainous regions of lunar surface

slide11

constant

q varies from 0° to 90° as the position of the incident energy varies from the center to the edge of the lunar disk

uniform brightness: constant intensity reflected to earth

Reflected energy at full moon

slide12

Semiconductor

Normal spectral emissivity of a highly doped silicon semiconductor at room temperature

slide13

follow theoretical trend

Radiative Properties of Metals

Directional Dependence

EM theory doesn’t hold at shorter wavelengths.

Effect of wavelength on directional spectral emissivity of pure titanium. Surfaceground to 0.4 mm rms

slide14

infrared region

Spectral Dependence

Peak e near the visible region

gray body assumption

Variation with wavelength of normal spectral emissivity for polished metals

slide15

Peak appears near the visible region

H-R relation holds

1.27 mm

Effect of wavelength and surface temperature on hemispherical spectral emissivity of tungsten

X point: iron,1.0 mm; nickel,1.5 mm; copper,1.7 mm; platinum,0.7 mm

slide16

Hagen – Rubens

Variation with Temperature

Effect of temperature on hemispherical total emissivity of several metals and one dielectric

slide17

approach to optically

smooth surface

follow theoretical trend

Effect of Surface Roughness

Precise definition of surface characteristics

geometric shape,

method of preparation,

distribution of size around rms

s0/l > 1: multiple reflection

Roughness effects for small optical roughness, s0/l < 1; effect of surface finish on directional spectral emissivity of pure titanium. Wavelength, 2 mm

slide18

behave more like a polished surface

smoother relative to the incident radiation for large l

Effects of roughness on bidirectional reflectivity in specular direction for ground nickel specimens. Mechanical roughness for polished specimen, 0.015 mm

slide19

s0/l = 2.6

For large q, peak qr shifted to larger angles

Off-specular reflection at larger rough-ness and incident angle

Bidirectional reflectivity in plane of incidence for various incidence angles; material, aluminum (2024-T4), aluminum coated; rms roughness s0 = 1.3 mm; wavelength of incident radiation, l =0.5 mm

slide20

0.69

0.45

Effect of Surface Impurities

Effect of oxide layer on directional spectral emissivity of titanium. Emission angle, q = 25°; surface lapped to 0.05 mm rms; temperature, 294 K.

slide22

Effect of surface condition and oxidation on normal total emissivity of stainless steel type 18-8

slide25

At low source temperature, incident radiation in long wavelength region

→ Barely influenced by the thin oxide layer on the anodized surface

→Acts like a bare metal

Approximate directional total absorptivity of anodized aluminum at room temperature relative to value for normal incidence

slide26

Hemispherical spectral reflectivity for normal incident beam on aluminum coated with lead sulfide. Coating mass per unit surface area, 0.68 mg/cm2

slide27

Selective and Directional Opaque Surfaces

Modification of Surface Spectral Characteristics

lc: cutoff wavelength

lc

Characteristics of some spectrally selective surfaces

slide28

Ex 5-1

arriving from the sun at Ts = 5780 K

An ideal selective surface with a cutoff wavelength lc = 1 mm

Energy balance

: absorbed energy

: emitted energy

slide29

dw

ib,s

R = 6.95 × 108 m

earth

sun

L = 1.50 × 1011 m

TS = 5780 K

slide30

arriving from the sun at Ts = 5780 K

An ideal selective surface with a cutoff wavelength lc = 1 mm

slide33

Cutoff wavelengthlc, mm

0.6

0.8

1.0

1.2

1.5

Equilibrium temperatureTeq, K

1811

1523

1334

1210

1041

393

By trial and error

slide34

approximation

0.95

0.05

1.5

  • extracted energy for use in a power-
  • generating cycle
  • for the black surface at the same
  • temperature

Ex 5-3

SiO-Al selective surface

Wavelength l

Solar energy absorber

at TA = 393 K

slide35

0.95

0.05

1.5

approximation

Energy balance

SiO-Al selective surface

Wavelength l

For black surface

No energy available

slide36

reflect well at short wavelengths

radiate well at long wavelengths

Reflectivity of white paint coating on aluminum

slide37

lc

lc

Selective Transmission

ordinary glasses: typically two strong cutoff wavelengths

Normal overall spectral transmittance of glass plate (includes surface reflection) at 298 K

slide38

1

r

r3 (1-r)t3(1-t)

r (1-r)t(1-t)

(1-r)(1-t)

r2(1-r)t2(1-t)

A transmitting Layer with Thickness L >l

r (1-r)2t2

r3 (1-r)2t4

1-r

r2(1-r)t2

r3 (1-r)t4

r4 (1-r)t4

r (1-r)t2

(1-r)t

r (1-r)t

r2 (1-r)t3

r3 (1-r)t3

(1-r)2t

r2 (1-r)2t3

Reflectance

Transmittance

slide39

1

r

r (1-r)2t2

r3 (1-r)2t4

1-r

r2(1-r)t2

r3 (1-r)t4

r4 (1-r)t4

r (1-r)t2

r3 (1-r)t3(1-t)

r (1-r)t(1-t)

(1-r)(1-t)

r2(1-r)t2(1-t)

(1-r)t

r (1-r)t

r2 (1-r)t3

r3 (1-r)t3

(1-r)2t

r2 (1-r)2t3

Absorptance

R + T + A =1

spectral transmittance

total transmittance

slide40

Transmittance depends on thickness.

95% of solar energy can be trapped.

Effect of plate thickness on normal overall spectral transmittance of soda-lime glass (includes surface reflection) at 298 K

slide42

Selectively transparent coating may also be useful in the collection of solar energy.

Transmittance and reflectance of 0.35-mm-thick film of Sn-doped In2O3 film on Corning 7059 glass. Also shown is the effect on Tl of an antireflection coating of MgF2

slide43

Modification of Surface Directional Characteristics

Directional emissivity of grooved surface with highly reflecting specular side walls and highly absorbing base; d/D = 0.649. Results in plane perpendicular to groove direction; data at 8 mm

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