DEVELOPMENT OF NANOSTRUCTURED DIELECTRIC HYDROPHOBIC THIN FILMS FOR HIGH VOLTAGE INSULATOR

Abstract

High voltage insulators are the backbone of any power system network. They can be defined as the devices which are used on transmission lines, transformers and distribution substations to support, separate or contain current carrying conductors at high voltage. All insulators have dual functions, mechanical and electrical, which commonly present conflicting demands to the designer. A major problem of these insulators is the accumulation of air borne contamination on their surface. The major cause of pollutant accumulation on the insulator surface is the high surface energy due to their strong electrostatic bond among the various atoms in the material. The chief sources of contamination are coastal areas, salt industries, cement industries, volcanic activity areas, industrial burning and chemical industries. During a light rain or fog or mist, these contaminations get moistened and thus, form a conducting layer through which leakage current flows. Dry bands are formed as a consequence of the warming-up of the insulation surface layer. Partial arc appear throughout the dry bands which ultimately lead to surface flashover of the insulator. Thus, complete breakdown of the power system . ii The main objective of present work was to synthesize nanostructured dielectric hydrophobic thin films especially, Hafnium oxide (HfO2), Hafnium oxynitride (HfOxNy), and Hafnium-titanium oxide (HfTiO) on glass and Quartz substrates by DC/RF magnetron sputtering technique and to investigate the effect of sputtering process parameters on structural, optical, hydrophobic and electrical properties of these materials in order to mitigate the problem of contamination. A chapterwise summary of the thesis is given below. Chapter 1 gives an overview about the high voltage insulators used in electrical power system. It discusses the role of insulators and also the problem of contamination being by these insulators. The surface flashover phenomenon was described and its consequences were also highlighted. The economic loss incurred due to contamination was also highlighted in this chapter. Various mitigation techniques and their pros-cons has also been discussed in this chapter. Nanotechnology based coatings was put forward as a effective remedial measure and dielectric hydrophobic coating was suggested as a solution to this problem. Hafnium based oxide, oxynitride and composite was selected as a material to resolve this problem. Magnetron Sputtering was selected as a synthesis technique method Chapter 2 presents the details of synthesis and characterization techniques, employed for the present research work. Section 2.1- A brief description as to the thin film growth modes, influenced by the interaction energies of substrate and film atoms is included in this section. Section 2.2 – The process iii description and mechanistic details of DC/RF magnetron sputtering technique used for the deposition of thin films in the present work is discussed in this section. Section 2.3- The methodology for the characterization of deposited films by different techniques such as X-Ray Diffraction for the phase identification and grain size, surface morphology of the films by using techniques such as Atomic Force Microscopy (AFM) and elemental analysis using Electron Probe Microscopy (EPMA) and electron diffractin scattering (EDS) are discussed. The hydrophobic properties of deposited films were measured by water contact angle goniometer. The four probe measurement unit and impedance analyser was used to measure electrical properties of the sputter deposited thin films in the present work. UV-vis-NIR spectrophotometer used for optical property measurement was also highlighted in this chapter. Chapter 3 describes the synthesis and characterization of hafnium oxide at different sputtering Parameters. Section 3.1 give a brief introduction about the hafnium oxide and discusses the work of different research group contributed in synthesizing hafnium oxide by sputtering. Section 3.2 discusses the effect of sputtering gas on structural, morphological, hydrophobic, optical and electrical properties of deposited nanostructured hafnium oxide thin film over glass insulators. All the deposited films were found to be hydrophobic as well as dielectric. The argon gas was found to be optimum sputtering gas at a O2/Ar=0.5. Section 3.3 discussed about the effect of sputtering pressure on different properties of HfO2 films. The optimum pressure was obtained as 15m Torr where the film was monoclinic crystalline, hydrophobic, dielectric and possesses high resistivity of order 104 ohm-cm. Section 3.4 discusses the different properties of HfO2 as sputtering power is varied from 30 to 60 W. iv Higher hydrophobicity was achieved at 50 and 60 W power with dielectric and insulating characteristics. However, due to economical reason 50 W was selected as a optimum power for HfO2 coatings. Section 3.5 deals with the effect of substrate temperature on the structural, hydrophobic, optical and electrical properties of HfO2 films. The temperature was varied from room to 500 °C and higher hydrophobicity and crystallinity was obtained at 500 °C but the electrical resistivity was drastically reduced. Hence, 50 W, 15 mTorr, room temperature was selected as a optimum sputtering parameter for hydrophobic dielectric HfO2 coating over glass insulators. Section 3.6 deals with the electrical breakdown study on uncoated glass and optimized coated glass. An enhancement was obtained in the breakdown strength from 20 kV/mm to 23 kV/mm. Chapter 4 presents in detail the synthesis and characterization details of hafnium oxynitride films. Section 4.1 deals with the properties of hafnium oxynitride and also highlights the work done by the various research group. Section 4.2 describes the effect of sputtering gas and oxygen partial pressure on the structural, morphological, hydrophobic and electrical property of HfOxNy. The optimum parameter were obtained for Ar gas at 10% oxygen partial pressure. Section 4.2 deals with the substrate temperature effect on different properties of HfOxNy. The bi-phase characterization were obtained for all the deposited films with higher hydrophobicity at 400 °C. However, the dielectric constant and resistivity were less in comparison to the room temperature coated v film. Thus, from technical as well as economical point of view, room temperature was considered as optimum temperature. In section, 4.3 variation of sputtering pressure was studied. The sputtering pressure was varied from 10 to 30 mTorr in steps of 5 mTorr. Higher hydrophobicity and resistivity was obtained for 20 mTorr sputtering pressure. Section 4.4 describes the effect of power on various properties of HfOxNy coated at room temperature. The power was varied from 20 to 60 W. The optical, electrical and hydrophobic was same and higher for power 50 and 60 W. Hence, 50 W power was considered as the optimum economical power. Section 4.5 deals with electrical breakdown study of uncoated and optimized coated glass insulator. The breakdown strength was found to be 25 kV/mm which is slightly higher in comparison to the HfO2. Chapter 5 deals with the synthesis and characterization of HfTiO nanocomposite coating over glass substrate. Section 5.1 gives introduction about nanocomposite coating. It also highlights the work of different research group over HfTiO coating. Only two parameter were studied for nanocomposite coating namely temperature and power. Section 5.2 deals with the effect of temperature on hydrophobic, electrical and optical properties of HfTiO coating. Higher hydrophobicity, large dielectric constant and less resistivity were obtained at 500 °C temperature. Section 5.3 deals the variation vi of power of Ti target from 110 to 150 W in steps of 20 W keeping Hf target power constant equals to 50 W. The coating were hydrophobic for all power. The higher hydrophobicity and high dielectric properties were obtained at a Ti power of 150 W. The contact angle was 107.6° and resistivity was also of order 104 at this power. Section 5.4 describes the breakdown study of coated and HfTiO coated glass insulators. The breakdown strength was 37 kV/cm for optimized HfTiO coated glass.