Optimizing the radiation properties of thin films

Master's Thesis of Mechanical Engineering - Energy Conversion

Abstract

Covering with thin layers plays a very important role in semiconductor industries and microelectromechanical and nanoelectromechanical equipment. By adding a thin layer to the surface due to interference of electromagnetic waves, the radiation properties of the surface will be completely different. In this project, using electromagnetic methods, the radiation properties of a thin multilayer structure are calculated, and by using genetic algorithm and simulated heat treatment, the properties of such a structure are optimized by changing the material and thickness of the layers according to practical issues.

One ??of the issues investigated in this project is radiation cooling. It has been determined that if the humidity is not high, the earth's atmosphere acts as a thermal well in the range of 8 to 13 micrometers, and as a result, if a selective coating is used, cooling can be done without consuming energy in such a way as to limit the energy exchange to this range. The use of coatings that provide cooling under direct sunlight has remained a challenge until now. In this project, a number of coatings have been introduced, with the help of which it is possible to partially cool down by 2 to 3 degrees Celsius under direct sunlight. Also, a large number of optimal coatings for cooling at night have been introduced. In addition, the idea of ??using potassium bromide coated on both sides as a very suitable coating for cooling at night has been proposed for the first time. The temperature drop will increase by about 123% using such a cover.

Also, optimal structures for use as a thermal mirror have been introduced. Besides, BaTiO3 has been investigated as a very suitable thermal mirror for the first time.

Key words: heat transfer, thin films, radiation heat transfer in nano dimensions, radiation properties, radiation cooling, thermal mirrors, optimization

Considering the wide applications of thin films, the use of this technology has become common in many optical, electronic devices and equipment related to solar energy. On the other hand, knowing the radiation properties of multilayer structures [1] including thin layers is of key importance in many practical applications such as rapid thermal processes [2] (RTP) [1, 2] and solar cells. Finding the optimal thickness of the layers to achieve the desired radiation properties has important applications in radiation cooling equipment [3], thermal mirrors [4], solar collectors and solar cells, but despite this, it has rarely been investigated. style="direction: rtl;">around it (or only on one side) are thin layers. One of the important features of these structures is that their radiation properties can be adjusted. The radiation properties of such structures depend on several factors that are listed below [3]:

Number of layers

Type of layers

How to arrange layers

Thickness of layers

Incidence angle

temperature of the layers

polarization of the incident beam

according to the spectral changes of the radiation properties of these layers, it is possible to change the radiation properties in different wavelength ranges by using various combinations of different layers. As a result, if the material and thickness of the layers are chosen correctly, it is possible to access various selective coatings with the help of thin multilayer structures. 1-1 Radiation cooling A part of the energy emitted from the sun is absorbed in the earth's atmosphere, which will lead to the emission of energy from the atmosphere. As a result, the radiant energy flux entering the earth's surface consists of 2 parts: solar radiation and sky radiation (Figure 2-1).In this figure, the radiation flux is given in terms of GW/m3 (energy flux per surface unit in the wavelength range of 1 micrometer equivalent to 1000 W/m2) and MW/m3 (energy flux per unit surface in the wavelength range of 1 micrometer equivalent to 1 W/m2). About 95% of the sun's radiation enters in the range of 0.3-2.4 ?m, while the sky radiation is mainly in the range of 4-85 ?m and completely in the infrared range. If the humidity is not too high, the sky radiation is very low in the range of 8-13 ?m. At other wavelengths, the sky radiation almost conforms to a Planck distribution at a temperature of about 300 K. The range of 8-13 ?m is called the atmospheric window [5]. In this period, the atmosphere acts as a heat sink and the radiation emitted from the objects located on the surface of the earth is not balanced with the incoming radiation of the atmosphere. This fact is the basis of radiation cooling. In this way, cooling will be possible without energy consumption [4]. This method is used in preserving food and medicine, providing cool water, cooling buildings [5, 6, 7] and condensing air humidity [8, 9, 10]. But due to the displacement heat exchange with air, it is not possible to cool more than 10-20 ?C [12]. By using a displacement coating [6], by reducing the displacement heat transfer coefficient in an ideal state, a temperature of about 30-40 ?C lower than the environment can be reached. But the different spectral radiation properties of the coating compared to air reduce the cooling power. The transmission coefficient of an ideal coating should be equal to 1 in the range of 8-13 ?m and zero in other wavelengths. During the day, a considerable radiation flux in the range of 0.3-2.4 ?m enters the earth's surface, which makes cooling very difficult. Therefore, an ideal coating for cooling during the day, in addition to the previous conditions, must have a very high reflection coefficient in the range of solar radiation.

The goal of most of the activities of the last 3 decades has been to achieve a suitable coating for cooling under direct sunlight, but this issue still remains as a challenge.

1-2 Thermal mirrors

The meaning of the mirror Thermal insulation is a coating that allows visible light to pass through while preventing the transfer of radiant heat in the infrared range. As a result, by using such a cover, in addition to providing the light required for lighting the building, it will prevent the loss of energy in the form of radiation. In addition, such coatings will be used to increase energy absorption in solar cells and solar collectors. The transmission coefficient in the visible light range (0.4-0.7 ?m) and the reflection coefficient in the infrared range (wavelengths higher than 0.7 ?m) for an ideal thermal mirror is equal to one [4, 13]. The layers, the thickness of the layers and the number of layers are optimized. Optimization will be done according to practical issues and in one or more wavelength intervals.

In this project, optimal structures for use in radiation cooling and thermal mirrors will be introduced. Also, thin layer structures with maximum absorption, reflection and transmission coefficients in the range of solar radiation will be introduced. Such structures can be used in solar collectors, solar cells and solar water heaters.

1-4 Research Objectives

The objectives of this study are:

< > Calculation of radiation properties of a thin multilayer structure, introduction of optimal thin layer coatings for diverse applications, considering a wide range of materials Presentation of a comprehensive theoretical review about radiation cooling and the use of thin layer coatings as a displacement coating [7] Introduction of optimal coatings for radiation cooling during the day and night Introduction of optimal structures for use as thermal mirrors 1-5 Research methods

In this project, optimization will be done using two methods of genetic algorithm[8] and simulated heat treatment[9]. The radiation properties of thin multilayer structures are calculated using electromagnetic methods