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CHAPTER 2

EXPERIMENTAL STUDIES

2.1 INTRODUCTION

Over the last decade, significant improvements have been obtained

in the electrical and mechanical properties of nano dielectric materials. Hence

the selection of dielectric materials and fillers are very important in order to

make the composites to achieve the required electrical characteristics for

outdoor insulation applications. A variety of polymeric insulating materials

and fillers have been used to make the composites. The used dielectric

materials are PTFE (poly tetra fluoro ethylene), epoxy resins, polyethylene,

instant set polymers based urethane, various copolymers, ethylene -propylene

elastomers and ethylene propylene dine monomer (EPDM), room temperature

vulcanized (RTV) silicone rubber, high temperature vulcanized (HTV)

silicone rubber and silicone rubber. Similarly, the several nano fillers are

fumed and natural silica (SiO2), aluminium oxide (Al2O3), barium titanate

(BaTiO3), titanium dioxide (TiO2), and zinc oxide (ZnO) and alumina

trihydrate (ATH) or Aluminium hydroxide Al(OH)3). Out of this, the silicone

rubber dielectrics and fillers Al2O3, SiO2, Al(OH)3 are mostly used to make the

nano composites for outdoor insulation’s applications.

In this Section, the materials used and sample preparation,

experimental test setups, aging mechanism and material characterizations

using TG-DTG, SEM with EDAX and FTIR are discussed in detail.

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2.2 MATERIALS USED

High temperature cured Silicone Rubber (SIR) material (Dow

Corning, USA) is used in the present work as a base material. It is based on

polymers having a molecular backbone of alternate atoms of silicon and

oxygen. Some organic groups are attached to the main chains, which include

methyl, phenyl, or vinyl. The structure of silicone rubber provides properties

of good resistance to UV and ozone and other environmental conditions.

Apart from the base polymers, the shed materials of reinforcing fillers

Aluminum Oxide (Al2O3), Silica (SiO2), and Aluminium Hydroxide Al(OH)3

of size <80 nm and purity >99% supplied by Hefei Jiankun Chemical

Industry is used for making nano sized filler of SIR specimens. The

commercially available Al2O3 of size 5-10 µm is used to prepare the micro

sized filler of SIR specimens. Table 2.1 shows the characteristics of the micro

and nano fillers used in this research.

Table 2.1 Charecteristics of Micro and Nano Filler

Filler Averageparticle size

Specificsurface area

Density Meltingpoint

Aluminium Oxide(Al2O3)

70-80 nm 350-750m2/g 3500-000 kg/cm3 2300 K

Aluminium Oxide(Al2O3)

5-10 µm 630-700 m2/g 2000-2600 kg/cm3 1880 k

AluminiumhydroxideAl(OH)3

70-80 nm 350-750m2/g 3500-4000 kg/cm3 2300 K

Silica (SiO2) 70-80 nm 300-650m2/g 3000-4000 kg/cm3 2000 k

2.2.1 Sample Preparation

The viscosity of the silicone-filler mixtures increases with the

amounts of nano- and micro fillers increased. A high shear force mixer

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enables better dispersion within the mixture which, in turn, enhances the

electrical and mechanical properties of the composites. The base SIR and

fillers are weighed to make the formulation or mixture. This is denoted by the

weight percentage of each component. The ratio used for the silicone matrix

is 10 parts resin to 1 part curing agent. The desired amount of nano filler is

added with the matrix and mixed until lumps in the mixture are no longer

visible. The mixture is stirred again, poured into a mould and degassed. The

two stages of curing process namely primary and post curing processes are

carried out. In the primary curing, the mould with the material is allowed to

cure at 200 C for 2 hours. During the post curing process, it is kept in oven at

170 C for 16 hours. The two rectangular samples of size 34 cm (L) X 34 cm

(B) X 5 mm (H) are prepared from each composition and are cut into the

appropriate size according to IEC 60587 test procedures as shown in Figure

2.1. The samples are prepared in different filler concentrations of 5, 10, 20

and 30 % by weight of the micro and nano sized fillers. The same procedure

is also used to prepare the micro filled composites.

Figure 2.1 Silicone Rubber Sample

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2.3 EXPERIMENTAL TEST SETUP

This project consists of three sections of experiments, which are: 1.

Inclined-Plane Tracking and Erosion (IPT) test –AC and DC voltage

2.Leakage current measurement system 3. Aging mechanisms of silicone

rubber materials.

2.3.1 Experimental Test Setup – AC and DC Voltage

The IPT test is specifically designed to evaluate insulation materials

according to their resistance to electrical erosion and tracking. Material

degradation is induced allowing dry band arcing to take place on the surface

of the SIR specimen, thus causing the material to erode. In this research, the

test set up and method of tracking and erosion is totally based on the IEC-

60587 standard procedure. A set of six specimens each of size 34 cm (L) X 34

cm (B) X 5 mm (H) are tested to obtain the tracking resistance.

Figure 2.2a shows the schematic diagram of the Inclined Plane Test

apparatus used for the AC voltage test. A 12 kVA, 230/20 kV step-up

transformer is used to generate the required test voltage. The SIR specimens

to be tested are clamped onto a stand orientates them at an angle of 45º to the

horizontal, with the surface facing downwards and the contaminant is allowed

to flow from high voltage to ground electrode over the surface of the plate

specimen. 0.1% NH4Cl is used as a contaminant. The non-ionic wetting agent

of 0.02 % weight (TritonX-100) is added to the contaminant to increase the

surface wetting of the insulating material under test. The conductivity of the

contaminant is measured with a EuTech conductivity meter. The flow rate of

the contaminant solution is maintained at 0.6 ml/min using a peristaltic pump

and the conductivity of the contaminant is kept at 2500 µS/cm. There are

eight layers of filter papers as a reservoir for the contaminant, which are

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clamped between the top electrode and the SIR specimen. The photograph of

peristaltic pump and its association set up are shown in Figure 2.3.

Two electrodes are fastened to the top and bottom of the sample.

The respective shapes and dimensions (all in mm) of the electrodes and filter

paper are shown in Figure 2.4. Electrodes in this experimental fixture such as

screws are made by stainless steel material. The top electrode is connected

with AC voltage of 4.5 kV and the bottom electrode is solidly grounded. A set

of six specimen failure time is calculated as the average of six failure times.

The deviation in failure times of this set of experiments is within 3 % of the

mean of the six failure times.

In order to do the tests using DC voltage (positive), the apparatus of

AC voltage test setup is modified by adding a rectifier circuit to the

experimental set up and these circuits are shown in Figure.2.2b. The

rectification is achieved by means of diodes. The rectifier used to convert H.V

AC into DC voltage of 4kV is connected to the top electrode. The 15KVA,

230/15kV step-up transformer is used to generate the required test voltage.

The experimental procedure is same as that of AC tests. According to the

standard, the constant tracking voltage has been applied to the SIR specimens

for the test duration of 6 hours in both AC and DC voltage tests. Figure 2.5

shows the photograph of the experimental test setup for AC and DC and

Figure 2.6 shows the photograph of the experimental test setup of the tracking

test.

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Figure 2.2 Schematic Diagram of Experimental Test Setup a) AC

Voltage b) DC Voltage

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Figure 2.3 Photographs of Peristaltic Pump and its Association Setup

Figure 2.4 General Shapes and Dimensions of a) High Voltage

Electrode b) Ground Electrode c) Filter Paper d) Electrodes

and Filter Paper Dimensions

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Figure 2.5 Photograph of Experimental Setup for Leakage Current

Measurement

Figure 2.6 Photograph of Experimental Test Setup for AC and DC

Tracking Test

2.3.2 Leakage Current Measurement System

An LC data acquisition system is developed to analyse the LC

flowing on the silicone rubber surface under laboratory conditions. It consists

of three parts:

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(1) Measuring Unit

(2) Signal Conditioning

(3) Analog to Digital Converter (ADC) with personal computer

The measuring unit is a component that is used to capture the LC

that flows along with the sample surface. It is done by measuring the voltage

drop across a 50 power resistor, which is connected in series with the

ground electrode. The shunt voltage across the 50 resistor is fed in to the

analog inputs of a high sampling rate data acquisition system (DAQ)(National

Instruments, 1.25 MSa/sec) by a coaxial cable. The analog signal is converted into

digital signal by ADC in DAQ with a sampling frequency of 5 kHz and stored in

personal computer (PC) through Lab VIEW software for further processing. In

order to access the reliability of the DAQ system, an 1GSa/sec Digital storage

oscilloscope (DSO) also provides visual display of the leakage current. The DAQ

was protected by back-to-back zener diode as a protection unit (PU).

Table 2.2 Technical Specifications of NI Card

Parameter SpecificationsNumber of channels 32Maximum sampling rate 1.25MS/sADC Resolution 16 bitTiming resolution 50 nsFIFO buffer size 4096 samplesBand width 1.5 MHzInput voltage range 10VInput coupling DCInput impedance 10 GInput bias current 100pAMaximum inrush current 500 mA

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Figure 2.7 Photograph of NI Card

A program based on virtual instrument LabVIEW package is

developed to communicate with the ADC to sort out the magnitude and LC

waveform pattern. The time domain representation of a signal is converted

into frequency domain for frequency spectral output by using a program of

Fast Fourier Transform (FFT) from LabVIEW software package. The

technical specifications for NI card are displayed in Table2.2 and the

photograph is shown in Figure 2.7

2.4 AGING MECHANISM OF THE SILICONE RUBBER

MATERIAL

In another set of experiments, the samples are aged by placing them

for 10 days in a hot water bath maintained at temperature of 60°C. To

investigate the thermal degradation of the material, the samples are aged by

placing the samples in an air-circulated oven maintained at 150°C for 30 days.

A set of thermally aged samples is removed intermittently in a time period of

24 hours and quenched in liquid nitrogen bath, and then thermal aging is

continued.

Figure 2.8a shows photograph of the water aging of SIR specimens

processed by using the laboratory type small oven. This oven is provided with

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a continuous variable thermostat which controls the temperature 10°C to

150°C within the accuracy of 1°C. The 500 watts heating element is wound

along with sides and bottom of the oven. It operates on a voltage of

220/230,50Hz AC supply. The thermal aging of SIR specimens is done by

using the hot air oven. Figure 2.8b shows the photograph of hot air oven. The

specifications of this oven are as shown below:

Sturdy double walled units with the outer chamber made of

M.S. sheet duly powder coated with the inner made up of

Stainless Steel sheet.

The gap between inner and outer chamber is filled with high

grade glass wool to avoid thermal losses.

Beaded 2000 Watts heating elements are placed in ribs, at the

bottom and sides.

Inner chamber is provided with ribs for adjusting perforating

shelves to convenient height.

Temperature is controlled by a digital temperature controller.

Temperatures are up to 250oC.

Air ventilators are also provided on the sides of the unit.

To work on 220/230 volts. AC supply.

Air circulating fan 1.2 KW

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Figure 2.8 Photograph of Aging Experimental Setup a) Water Aging

b) Thermal Aging

2.5 MATERIAL CHARACTARIZATION STUDIES

In order to understand the thermal, physical and chemical behavior

in the silicone rubber composites the following analytical measurements are

carried out.

2.5.1 Thermo Gravimetric and Derivative Thermo Gravimetric

Analysis

Thermal stability of the material is important as it influences the

tracking and erosion performance which directly depends on the thermal

decomposition of the material during the inclined plane test. The Thermo-

Gravimetric Analysis (TGA) and Derivative Thermo Gravimetric (DTG)

Analysis are used to check the thermal stability of the material. The TGA

gives the change in weight loss of silicone rubber specimens with respect to

temperature.

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Figure 2.9 Photograph of Perkin Elmer TG-DTG Analysis (SAIF IIT

Bombay)

The DTG gives the 1st order derivative weight (wt % /min) values

with respect to temperature. The DTG can be constructed with the TGA data

and it indicates the rate of weight loss or degradation of the material with

temperature. A Perkin Elmer Pyris Diamond TG/DTA instrument made in

USA is used for the research as shown in Figure 2.9. The sample weight of

about 15 mg is used for the TGA analysis. The measurement is made in a

nitrogen gas atmosphere at warm room temperature to 800 ºC with the rate of

20 ºC/minute. The temperature is increased from 100ºC to 800ºC.

2.5.2 Scanning Electron Microscopy Analysis

To ascertain the degree of filler dispersion of the nano and micro

fillers in the silicone rubber materials, a JEOL make and JSM 6390 model

electron microscope system as shown in Figure 2.10 is used to analyse the

morphology of nano and micro filled silicone composites. The function of this

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electron microscope is similar to its optical counterpart except that a focused

beam of electron is employed instead of light to image the specimen and to

gain information on its structure and composites.

Figure 2.10 Photograph of JEOL JSM 6390 Scanning Electron

Microscopy with EDAX (Karunya University Coimbatore)

2.5.3 Energy Dispersive X-ray Analysis

The EDAX is done using the JEOL make and JSM 6390 model

equipped with an EDAX system. The accelerating voltage is 20 KV in all

cases. The samples are placed on carbon conductive tape and coated with the

platinum film of 20 nm thickness using a high vacuum sputter. EDAX

analysis collects the X-ray generated by the electron beam of the SEM and

provides information on the composition of the material.

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2.5.4 Universal Attenuated Total Reflectance Fourier Transform

Infrared Analysis

The FTIR system used in this work is a Spectrum Two FTIR

UATR (Universal Attenuated Total Reflectance) spectrometer- perkin Elmer

USA which is shown in Figure 2.11. The silicone rubber specimens are

analyzed at 4 cm-1 resolution, and 16 scans are averaged in the Transmittance

mode. The spectrum analysis is done with the software for the instrument.

FTIR spectroscopy is used to determine the concentration of functional

groups in nano and micro fillers. In this method, the instrument is used to

measure the spectrum in transmittance mode from 500 – 4000 cm-1.

Figure 2.11 Photograph of Spectrum Two FTIR Instrument (Perkin

Elmer Hyderabad Lab)