ELECTROMAGNETIC RADIATION AND ABSORPTION

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CHAPTER 3 THEORETICAL BACKGROUND OF SELECTIVE SOLAR ABSORBERS

This chapter gives a general introduction to the fundamental theories used to build laser structured ferromagnetic nanorods embedded in alumina matrix. It starts with thin film optics and the physical background of spectrally selective absorbers. As any optical phenomenon is connected with the interaction of electromagnetic radiation with matter, it is worth to understand the basic electromagnetic wave theory that governs the physics of well-designed spectrally selective absorbing surfaces. This chapter will therefore describe how electromagnetic waves interact with matter in thin films; it also includes Effective Medium Theories Maxwell Garnett as well as the Bruggeman effective medium model for inhomogeneous materials. The last section is on surface structuring, the main interest of this section is to conduct a quick review of the available structuring methods and introduce the reader to the most applicable ones in the solar industry.

 THIN FILM OPTICS

An optical coating consists of a combination of thin film layers that create interference effects used to enhance transmission or reflection properties for an optical system. How well an optical coating performs is dependent upon the number of factors, including the number of layers, the thickness of each layer and the differences in refractive index at the layer interfaces. The transmission properties of light are predicted by wave theory. One outcome of the wave properties of light is that waves exhibit interference effects. Light waves that are in phase with each other undergo constructive interference, and their amplitudes are additive. Light waves exactly out of phase with each other (by 180°) undergo destructive interference, and their amplitudes cancel. It is through the principle of optical interference that thin film coatings control the reflection and transmission of light [3.1].
When designing a thin film, though the wavelength of light and angle of incidence are usually specified, the index of refraction and thickness of layers can be varied to optimize performance. As refraction and thickness are adjusted these will have an effect on the path length of the light rays within the coating which, in turn, will alter the phase values of the propagated light. As light travels through an optical component, reflections will occur at the two interfaces of refractive index change on either side of the coating [3.1]. In order to minimize reflection, ideally there should be a 180° phase shift between these two reflected portions when they recombine at the first interface. This phase difference correlates to a λ/2 shift of the sinusoid wave, which is best achieved by adjusting the optical thickness of the layer to λ/4. Fig. 3.1 shows an illustration of this concept.
Index of refraction influences both optical path length (and phase), but also the reflection properties at each interface. The reflection is defined through Fresnel’s Eqn. 2.1, which provides the amount of reflection that will occur from the refractive index change at an interface at normal incidence The intensity of reflected light is not only a function of the ratio of the refractive index of the two materials, but also the angle of incidence and polarization of the incident light. If the incident angle of the light is altered, the internal angles and optical path lengths within each layer will be affected, which also will influence the amount of phase change in the reflected beams [3.1]. It is convenient to describe incident radiation as the superposition of two plane-polarized beams, one with its electric field parallel to the plane of incidence (p-polarized) and the other with its electric field perpendicular to the plane of incidence (s-polarized). When a non-normal incidence is used, s-polarized and p-polarized light will reflect differently at each interface which will cause different optical performances at the two polarizations.
Determining the refractive index, n, and the absorptance (absorption coefficient), k, of a coating are two important parameters in thin film research. In real materials, the polarization does not respond instantaneously to an applied field [3.2].
Here, k indicates the amount of absorption loss when the electromagnetic wave propagates through the material. The term k is often called the extinction coefficient in physics. Both n and k are dependent on the wavelength. In most circumstances k > 0 (light is absorbed).

OPTICAL CHARACTERIZATION OF A SELECTIVE SOLAR ABSORBER

When incident light strikes a surface of a solid material, some part of the incident light is reflected, some part is absorbed, and some part is transmitted. Reflectance, transmittance and absorptance are the three parameters which describe characteristics of a selective solar absorber surface, they are wavelength dependent. For a given incident angle , reflectance, ( ) is the fraction of incident light at a specific wavelength that is reflected by the surface, absorptance, ( ) is the fraction of incident radiation that is absorbed by the surface and transmittance, ( )is the fraction of incident light that passes through the surface.
The actual performance of an absorber at high temperatures may not correspond to the calculated emittance [3.5-3.6]. This is because small errors in measured ρ can lead to large errors in small values of ε. In addition, for some materials the measured emittance data at two different temperatures may simply be different.
Solar reflectance measurements are usually performed in the wavelength range 0.3-2.5 µm at near normal (θ = 2) angle of incidence using standard spectrophotometers. The solar absorptance is characterized at near normal incidence for which the sun is at the zenith angle relative to the absorber [3.5-3.6]. When the sun is at other elevations than the zenith, i.e oblique incidence the near normal solar absorptance must be modified using the angle of incidence modifier.
To evaluate the performance of solar absorber material, the absorptance and emittance are usually combined in the figure of merit f called selectivity. High value of f indicates high performance,

ELECTROMAGNETIC RADIATION AND ABSORPTION

A plane electromagnetic wave propagating along the x-axis through an absorbing medium at time t is defined by its electric field component, E(x,t) described by the following progressive wave equation 3.12 [3.5-3.7
From Maxwell’s equations on electromagnetic theory, the speed of light in a vacuum is related to the permittivity of free space, (the degree to which a medium can resist the flow of charge, defined by the ratio of the electric displacement to the intensity of the electric field that produces it), and the permeability of free space (the ratio of the magnetic flux density in a solid to the external magnetic field strength inducing)  where the last exponential term is the measure of the damping factor or extinction coefficient
The power or intensity of an incident wave through a solid is the conductivity of the solid multiplied by the square of the electric field vector [3.5-3.7]. Using the damping factor term, the fraction of the incident power that has propagated from position (0) to a distance through the material .

CHAPTER ONE INTRODUCTION
1.1 BACKGROUND TO THE RESEARCH STUDY
1.2 STATEMENT OF THE PROBLEM
1.3 SOLAR RADIATION
1.4 SOLAR-THERMAL COLLECTORS
1.5 SPECTRALLY SELECTIVE SOLAR ABSORBERS.
1.6 AIMS AND OUTLINE
1.7 REFERENCES
CHAPTER TWO LITERATURE SURVEY OF SELECTIVE SOLAR ABSORBERS
2.1. EVAPORATION
2.2 SPUTTERING
2.3 ELECTROCHEMICAL METHODS
2.4 PAINTING AND OTHER METHODS
2.5 SOLUTION-BASED METHOD
2.6 COMMERCIALLY AVAILABLE SOLAR SELECTIVE COATINGS
2.7. REFERENCES
CHAPTER THREE THEORETICAL BACKGROUND OF SELECTIVE SOLAR ABSORBERS
3.1 THIN FILM OPTICS
3.2 OPTICAL CHARACTERIZATION OF A SELECTIVE SOLAR ABSORBER
3.3 ELECTROMAGNETIC RADIATION AND ABSORPTION
3.4 MULTILAYER REFLECTION
2.5 EFFECTIVE MEDIUM THEORY
2.6 DIFFERENT TECHNIQUES OF SURFACE STRUCTURING
3.7 REFERENCES
CHAPTER FOUR EXPERIMENTAL METHODS
4.1 SAMPLE PREPARATION
4.2 PREPARATION SETUP
4.3 ANODIZATION
4.4 FACTORS AFFECTING THE GROWTH OF NANORODS
4.5 BARRIER AND POROUS ALUMINA
4.6 ELECTRO-DEPOSITION OF MAGNETIC NANOWIRES
4.7 OPTICAL CHARACTERISATION
4.8 NON-OPTICAL CHARACTERISATION
4.9 DIRECT LASER STRUCTURING
4.10 REFERENCES
CHAPTER FIVE RESULTS AND DISCUSSIONS
5.1 STRUCTURAL CHARACTERISATION OF TUBULAR COBALT NANOCOMPOSITES
5.2 XRD OF TUBULAR COBALT NANOCOMPOSITES
5.3 EDS ANALYSIS TUBULAR COBALT NANOCOMPOSITES
5.3RBS ANALYSIS OF TUBULAR COBALT NANOCOMOSITES
5.5. TOTAL REFLECTANCE AND THE INFRARED EMISSIVITY ( OF TUBULAR COBALT NANOCOMOSITES
5.6 THE EFFECT OF ANNEALING ON THE PROPERTIES OF LASER SURFACE STRUCTURED Co- Al2O3 SOLAR ABSORBER
5.7 REFERENCES
CHAPTER SIX CONCLUSION AND RECOMMENDATIONS
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