Warsaw University of Technology Electrical Department. Laboratory of Materials Technology KWNiAE

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1 Warsaw University of Technology Electrical Department Laboratory of Materials Technology KWNiAE Practice 8 Determining The Resistivity of the Electroinsulating Materials

2 1. Behavior of the dielectric in the electric field Insulating materials, which are dielectrics materials, which do not contain any free charges. Lack of the free charges causes, that they oppose very high resistance to the flow of the electric current. From the whole group of the dielectrics, the widest practical application has solid dielectrics. The behavior of the dielectrics depends on the three following factors: Polarization Conductivity Ionization Polarization is a phenomenon depending on the structure of the dielectric. We can distinguish polar (dipolar) and non-polar structures. Polarization of the dielectric is resultant of the individual elements of the structure of the dielectric such that atoms, ions, molecules. For the non-polar dielectrics, their polarization consists in creating unstable, included dipoles from the individual atoms. After removing the electric field, there is a very fast decay of the polarization. Polar dielectrics are characterized by possession of the permanent dipoles, which exist independently of the induced dipoles. Under the influence of the electric field there is spatial arrangement of the permanent dipoles and it lasts longer time than after removing the filed. The polarization causes greater losses of the energy, because of maintaining of the arrangement of the dipoles. Relative electrical permittivity ε is a measure of the dielectric polarization and is defined as ratio of the capacity of the capacitor, which has specified dimensions with a given dielectric, to the capacity of the capacitor with identical dimensions, but which is filled by the vacuum. ε = C d C 0 C d capacity of the capacitor, which is filled by a given dielectric C 0 capacity of the capacitor, which is filled by the vacuum. If the polarization exists, there are losses of the energy, which are connected with overcoming resistances, when the position of the dipoles is changing. In the case of the nonpolar dielectrics, losses have bigger sense in the higher frequencies. For the non-polar dielectrics, losses can even reach considerable values just in the networking frequencies. Angle δ - is a measure of the power losses in the dielectric, when there is a sinusoidal voltage. So, it is often called as a loss angle. On the vector diagram of the flowing current in the dielectric, this angle is included between the vector of the current and its passive component. In the practical application, we use tgδ as a coefficient of the dielectric losses. It is dependent on the temperature, frequency, and applied voltage, when it s values is close to the ionization potential. Figure below presents equivalent scheme of the real dielectric and vector diagram

3 Equivalent scheme of the real dielectric and its vector diagram In reality, none of the dielectric is free of any charges, because there exist ions which come from pollution and defects of the structure and reaction of water, chemical compounds, UV radiation and γ radiation. In insulation, when such a charges exist and after placing the dielectric in the field, there is a systematic movement of charges and is called leakage current. For the solid dielectric leakage current consists of through current, which flow through the dielectrics and surface current, which flows on thin layer of the dielectric.. Through and surface resistivity In connection with the phenomenon of the leakage current and its division, on the surface and thorough current, we can define two parameters, which determine the state of the dielectric and it s usability to technical goals. Through resistivity ρ v is a ratio of the constant voltage, applied to the electrodes, to the fixed value of the current intensity flowing between electrodes, through the sample, except this part of current, which flows on the surface of the sample. This ratio is referred to the 1m surface of the electrode and 1m thickness of the sample. It is expressed in units. Ω m m lub [ Ω m] Surface resistivity ρs is a ratio of constant voltage, which is applied to the electrodes, to the fixed value of the current intensity through a layer, which is absorbed on the surface of the sample of damp, polluter and soot, referring to the 1m length of the electrodes and 1 m distance between electrodes. It is expressed in units: Ω m m lub [ Ω ] For the gaseous and liquid dielectric, we can determine only the through resistivity!

4 Through resistivity depends mainly on the type of the molecules, which go into the dielectric composition, amount of its defects of the structure and pollution. The highest values of the through resistivity demonstrate the non-polar dielectrics. With the increase of the temperature, through resistivity are decreases. Surface resistivity depends mainly on the state of the surface of the dielectric and pollution, especially the damp. The highest values of the surface resistivity demonstrate the non-polar dielectrics, which have hydrophobic properties. Some characteristics of the dielectrics are presented in the table below. Material Density g 3 cm Through resistivity Ω m [ ] Relative permittivity f = 50Hz Dielectric strength kv mm [ ] Crystalline mica,6-3, Micanites 1, -, Micolex,6-3, Samica 1, -, Porcelain,3 -, Stesit,6 -, Alundum ceramics 3,6-3, Calcium-sodium glass, Boron-silicate glass, -, ,5-8, Quartz glass, , - 4, 1-40 Polyethylene 0,,9-0, , -, Polypropylene 0,90-0, ,0 -, Polystyrene 1,0-1, ,4-3, PCV 1,5-1, ,0-5, Teflon,1 -, ,0 -, Natural caoutchouc 0,9-1, ,4-6, Synthetic caoutchouc Phenol formaldehyde resin 0,9-1, ,7-5, ,5-1, ,3-7, Epoxy resin 1,1-1, ,1-6, Polyurethane resin 1,15-1, ,5-5, Silicone resin 1,6 -, ,8-5,0 1-0 Cable paper 0,8-1, ,5-3, Oil-asphalt 1,0-1, , - 3,1 5-30

5 3. Measurement of the through and surface resistivity In order to determine the resistivity of the dielectric, the sample should be placed in a suitable system of electrodes and it is applied the constant voltage the fixed value. It is measured the current or its components: surface and through ones. We can measure through or surface resistivity, and then we have to calculate it into value of the resistivity. Measured values are: Thickness of the sample d [mm] Diameter of the measuring electrodes D [mm] Distance between two electrodes, protective and measuring t [mm] Current intensity I [A] Voltage applied to the electrodes U [V] Instead of measuring values U, I it s possible to measure directly R r [Ω] Computed values are: Through resistivity ρ v [Ωm] Surface resistivity ρ s [Ω] ρ = ρ Π = Π ( D + t ) 4d v R V ( D + t ) t v R S For the measurements, we use teraohmmeter, which enable measurement of the resistance in the range from [Ω]. We executed measurement for the voltages of the value 500 or 1000V. To the clamps of the teraohmmeter we attach the system of electrodes with the sample in the middle, in accordance with the figure. By measuring of the through resistance of the protective electrode eliminates influence of the surface resistance, and by measuring of the surface resistance influence of the through resistance. Additionally, this electrode does matter as a shield for the measuring electrode from the strange fields. Figure presents the system of electrodes for measuring the resistance of the clamps. System for measuring of the through resistance E 1 measuring electrode (roller) E protective electrode (ring) E 3 voltage electrode (roller) System for measuring of the surface resistance E 1 measuring electrode (roller) E voltage electrode (ring) E 3 protective electrode (roller)

6 Dimensions of the electrodes have to fulfill the following requirements: Diameter D has to be value from the series 10, 5, 50, 100 mm in condition D>4d Width of the slit t is included 0,mm < t < d Width of the protective dielectric should be at least 10mm Width of the voltage electrode cannot be less than outer diameter of protective electrode. Generally, electrodes are made by using methods of adhesive a layer of metal or by gluing foil on the surface of the sample. For such electrodes it s supplied by the voltage thanks to brazen electrodes. Brasses electrodes can be used directly only for the samples in a form of the elastic sheet. In our practice we use brasses electrodes. Additionally, for the samples of laminates, we can use pegged electrodes, which have to be pushed in the holes, which are drilled in the sample. Because of the lack of time, in our practice we restrict ourselves to the measurements, in which we use stiff electrodes, made from the brass. They are closed in metal casing. This casing is grounded and equipped with automatic blockade of the voltage and automatic grounded machine. Both machines are mechanically coupled with cover of the casing. Opening the cover makes disconnecting of the measuring voltage and grounding the voltage clamp. Scheme of using the pegged electrodes 4. Samples In laboratory, we carry through comparative measurement of resistance of given material and that is why we examine one sample of each material. Samples have the circular or square shape. Their dimension are (diameter or the length of side) mm, and thickness below 10mm. Samples are placed in special box file, according to numbers and names, which are connected with numbers. Moreover, two unknown samples are examined to decide from which material the given sample is made.

7 5. Realization of measurements After taking out the sample from the file binder you should measure her thickness, with the help of micrometer. To place init in the system of electrodes, close the metal casing and switch-on teraohmmeter. The way, how to use teraohmmeter, will be given by lecturer in dependency on the type of the instrument, which is used for the measurement. After measuring, the voltage must be switched off. Cover of the case should be open and you have change the sample. After measurements series of through resistivity you need change the system of electrodes and start measuring the surface resistance. All measurements should be done for all samples from the box. After finishing measurements you have to measure all electrodes using slide caliper. The results of the measurements should be written in the table. No. 1 3 Name of material R V [Ω] R S [Ω] D [mm] d [mm] t [mm] ρ v [Ωm] ρ s [Ω] Measuring table after making direct measurement of the resistance of the samples No. Name of material U V [V] I V [A] U S [V] I S [A] D [mm] d [mm] t [mm] ρ v [Ω m] ρ s [Ω] 1 3 Measuring table after making indirect measurement of the resistance of the samples

Elizabethtown College Department of Physics and Engineering PHY104

Elizabethtown College Department of Physics and Engineering PHY104 Lab #7- Capacitors 1. Introduction The capacitor is one of the essential elements of analog circuitry. It is highly useful for its energy