Physical properties of dielectrics for high voltage applications

Physical properties of dielectrics for high voltage applications

During operation, an electrical insulation system can be exposed to several kinds of stress factors, which can be of thermal, electrical, ambient and mechanical nature, often referred to as the “T.E.A.M.” stresses, each leading to corresponding ageing mechanisms. Accordingly, the functionality of insulation systems can generally be characterized by two main categories of material’s properties:
• mechanical properties;
• chemical properties.

Mechanical properties

Depending on the application, mechanical properties like the mechanical strength of dielectric material can be of importance as well. In case of epoxy, winding insulations of electric machines would be a prominent example of an application where high mechanical stresses act on the insulation system during operation and especially during transient states.

Based on the application for which the dielectric material is used, it is always of importance to find an adequate balance between rigidity and flexibility of the insulation system, according to possible stresses during operational states.

Chemical properties

Further important properties for dielectrics are their resistance to environmental (ambient) aspects, like UV radiation or air pollution for outdoor insulators, as well as hydrophobicity. In some applications the chemical resistance to degradation due to reactants caused by partial discharge (PD), for instance in gas insulated lines (GIL) filled with SF6 gas, embodies one more point of interest for insulating materials (Küchler, 2009).

From micro to nano – or is it sub-micro really? 

When is “nano” really nano? 

Nanodielectrics are commonly defined as electrical insulation materials, with incorporated fillers of particle sizes in the range of some tens of nanometers, forming structures of less than 100 nm in one dimension. Composites showing dimensions above 100 nm up to 500 nm are specified as mesocomposites and those with internal structures exceeding 500 nm are considered microcomposites.

Introduction to nanocomposites 

It is commonly agreed today that the key point for the improved or altered properties of nanocomposites (NC) lies within the particle-host interfaces (Fréchette, 2009; Lewis, 2005; Nelson et Hu, 2005; Raetzke et Kindersberger, 2006; Roy et al., 2005), which become more and more prominent with decreasing filler sizes . Those interfacial layers, or interaction zones are estimated to be of approximately 10 nm thickness around the particle surfaces, and possibly more if certain prerequisites are met (Lewis, 2004). Models like the “Multi-Core Model” of Tanaka et al. (Tanaka et al., 2005), the “Polymer Chain Alignment Model” of Andritsch et al. (Andritsch et al., 2011) or the “Overlap Model” of Preda et al. (Preda et al., 2014) try to predict the properties of NC or explain their structural composition, respectively.

Often, such nanocomposites expose very distinct properties which cannot easily be explained and might derive from the characteristics of their meso- and microscopic pendants, respectively. The origins of those comportments are thought to be due to several factors (Nelson, 2010; Roy et al., 2005):
• the large surface area of nanoparticles;
• the surface of particles affecting the polymer morphology;
• a reduction in the internal field caused by the decrease in size of particles;
• change in space charge distribution;
• scattering mechanisms .

In addition many other aspects like filler size, aspect ratio, dispersion and distribution of introduced particles in the polymer matrix, may strongly affect the resulting composites’ electric, dielectric, thermal as well as mechanical parameters.

The polymeric matrix in NC can be of diverse origin: rubbers, Polyethylene and epoxy resins, to name just a few important polymeric materials in electrical insulation engineering. This thesis focuses on epoxy based composites and their characteristics, although many of the following properties and models are generally applicable to polymeric NCs.

Theories and models for nanocomposites

Several models for describing the special dielectric characteristics of NC exist, all of them stressing the importance of the particle-matrix interface. But as there are still many unknowns to be understood, probably none of these following models can claim to fully explain all processes in NC.

Electric double layer 

The electric double layer introduced by Lewis (Lewis, 2004) describes a coulombic interaction between a filler particle and the surrounding polymer matrix. It consists of two layers formed at the particle-host interface, the inner Stern layer where ions are bound to the particle surface and the outer layer, also called diffuse Gouy Chapman layer, which represents a diffuse region where the charge decreases exponentially with the distance from the particle.

Any ions beyond the so called slipping plane are not affected by the charges surrounding the particle. The electric double layer forms a long distance dipole, with a slow time response, which affects electrical conduction and dielectric properties in the low frequency domain. Regarding the triboelectric series, epoxy resins tend to get negatively charged (Andritsch, 2010; Nelson, 2010).

Intensity model 

Proposed by Lewis in 2005 (Lewis, 2005), the intensity model is built on the idea that a material property α embodied by its intensity Iα can only change gradually with the distance over an interfacial area between two different phases , meaning there are no abrupt boarders for those properties. The intensity Iα does not necessarily have to stay between the values for phases A and B, but can reach intensities exceeding or even deceeding both of those values, depending on the nature of the phases.

Multi-core model 

The multi-core model developed by Tanaka et. al (Tanaka et al., 2005) is a theoretical approach to explain the possible interactions of filler particle and matrix. It is based on a three layer geometry , which are overlapped by a fourth, the electric double layer :

Bonded layer:
This is a transition layer physically bonded to the particle surface, approximately of 1nm thickness. It is formed by ionic, covalent, hydrogen and Van-der-Waals bindings (with strength decreasing from first to last).

Bound layer:
Interfacial region between a layer of polymer chains strongly bound to the first layer, as well as the surface of the inorganic particle. A perpendicular alignment of surrounding polymer chains to the filler surface results in a rather structured morphology, and therefore affects the mobility of polymer chains in the proximity of filler particles. The thickness of this layer is assumed to measure between 2 and 9nm.

Loose layer:
Measuring several tens of nm, this layer is supposed to be loosely coupled with the second layer, consisting of polymer chains with affected morphology due to the presence of the inorganic filler, leading to changes in chain formations and mobility, as well as free volume or crystallinity.

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Table des matières

INTRODUCTION 
0.1 Research objectives and approach
0.2 Specific research objectives & structure of PhD thesis
0.3 Methodology
CHAPTER 1 DIELECTRICS VS. NANODIELECTRICS 
1.1 Physical properties of dielectrics for high voltage applications
1.1.1 Thermal properties
1.1.2 Electrical properties
1.1.3 Mechanical properties
1.1.4 Chemical properties
1.2 From micro to nano – or is it sub-micro really?
1.2.1 When is “nano” really nano?
1.2.2 Introduction to nanocomposites
1.3 Theories and models for nanocomposites
1.3.1 Electric double layer
1.3.2 Intensity model
1.3.3 Multi-core model
1.3.4 Water shell model
1.4 Overview of the impact of nanoparticles on some electrical properties of NC
1.4.1 Impact on AC breakdown strength
1.4.2 Impact on the resistance to corona discharges
1.5 Materials Review
1.5.1 Epoxy resins
1.5.2 Boron nitride
1.5.3 Polyhedral oligomeric silsesquioxanes (POSS)
CHAPTER 2 PHYSICAL PROPERTIES OF POLYMER DIELECTRICS 
2.1 Electrostatics of dielectrics
2.2 Polarization mechanisms in dielectrics
2.2.1 Electronic or optical polarization
2.2.2 Atomic or ionic polarization
2.2.3 Dipolar or orientational polarization
2.3 Dielectric relaxation in polymers
2.3.1 Complex relative permittivity and the dielectric losses
2.3.2 Complex permittivity vs. polarization mechanisms in NC
2.3.3 Dielectric relaxation theory and models
2.3.4 Dielectric relaxation processes in polymers
2.4 Conduction mechanisms in polymers
2.4.1 Electronic conduction
2.4.2 Ionic conduction
2.4.3 Thermal conduction
CHAPTER 3 EPOXY/BN MICRO- AND SUBMICRO-COMPOSITES:
DIELECTRIC AND THERMAL PROPERTIES OF ENHANCED
MATERIALS FOR HIGH VOLTAGE INSULATION SYSTEMS 
3.1 Introduction
3.2 Materials and sample preparation
3.3 Experimental methods
3.3.1 Microstructure analysis
3.3.2 Differential scanning calorimetry
3.3.3 Dielectric spectroscopy
3.3.4 AC breakdown strength
3.3.5 Resistance to electrical discharge
3.3.6 Thermal conductivity
3.4 Experimental results and discussion
3.4.1 Microstructure analysis
3.4.2 Differential scanning calorimetry
3.4.3 Dielectric spectroscopy
3.4.4 AC breakdown strength
3.4.5 Resistance to electrical discharge
3.4.6 Thermal conductivity
3.5 Conclusions
3.6 Acknowledgment
CHAPTER 4 NANOSTRUCTURED EPOXY/POSS COMPOSITES:
ENHANCED MATERIALS FOR HIGH VOLTAGE INSULATION
APPLICATIONS 
4.1 Introduction
4.2 Materials and sample preparation
4.3 Experimental methods
4.3.1 Microstructure analysis
4.3.2 Differential scanning calorimetry
4.3.3 AC breakdown strength
4.3.4 Resistance to corona discharges
4.3.5 Thermal conductivity
4.3.6 Dielectric spectroscopy
4.4 Experimental results and discussion
4.4.1 Microstructure analysis
4.4.2 Differential scanning calorimetry
4.4.3 AC breakdown strength
4.4.4 Resistance to corona discharge
4.4.5 Thermal conductivity
4.4.6 Dielectric spectroscopy
4.4.7 Relaxation behavior
4.5 Concluding remarks
4.6 Acknowledgment
CHAPTER 5 ENHANCED ELECTRICAL AND THERMAL PERFORMANCES
OF NANOSTRUCTURED EPOXY/POSS COMPOSITES 
5.1 Introduction
5.2 Materials and sample preparation
5.3 Experimental methods
5.3.1 Microstructure analysis
5.3.2 Differential scanning calorimetry
5.3.3 AC breakdown strength
5.3.4 Resistance to corona discharges
5.3.5 Thermal conductivity
5.3.6 Dielectric spectroscopy
5.4 Results and discussion
5.4.1 Microstructure analysis
5.4.2 Differential scanning calorimetry
5.4.3 AC breakdown strength
5.4.4 Resistance to corona discharges
5.4.5 Thermal conductivity
5.4.6 Dielectric spectroscopy
5.4.7 Relaxation behavior
5.5 Concluding remarks
5.6 Acknowledgment
CHAPTER 6 FUNCTIONAL EPOXY COMPOSITES FOR HIGH VOLTAGE
INSULATION INVOLVING C-BN AND REACTIVE POSS AS
COMPATIBILIZER 
6.1 Introduction
6.2 Materials and sample preparation
6.2.1 Materials
6.2.2 Compounding
6.3 Experimental methods
6.3.1 Microstructure analysis
6.3.2 Dielectric spectroscopy
6.3.3 Differential scanning calorimetry
6.3.4 AC breakdown strength
6.3.5 Thermal conductivity
6.4 Results and discussion
6.4.1 Microstructure analysis
6.4.2 Dielectric spectroscopy
6.4.3 Differential scanning calorimetry
6.4.4 AC breakdown strength
6.4.5 Thermal conductivity
6.5 Concluding remarks
6.6 Acknowledgment
CHAPTER 7 MATHEMATICAL VALIDATION AND ESTIMATION OF
MATERIAL PROPERTIES
7.1 Theoretical models for the complex permittivity of composite materials
7.1.1 Two phase models
7.1.2 Three phase models
7.2 Theoretical models for the thermal conductivity of composite materials
7.3 Comparison of theoretical models with the experimental data
7.3.1 Estimation of the thermal conductivities of h-BN composites
7.3.2 Estimation of the thermal conductivities of c-BN composites
7.4 Conclusion on the applicability of mixing laws to estimate material properties
CHAPTER 8 3D FEM MODELING OF THE THERMAL CONDUCTIVITY OF
COMPOSITE MATERIALS 
8.1 Basics of heat transfer in solids
8.2 Modeling the thermal conductivity of epoxy/c-BN composites
8.3 Unraveling the thermal conductivity of the multiphase and functional POSS
composites
8.4 Evaluation of POSS’ contribution to the thermal conductivity in the multi-phase
composites
8.4.1 Simulations based on the assumption that POSS is represented only
by its silica core
8.4.2 Proposal of the Interfacial Restructuration Model (IFRM) for
conductive heat transfer in functional POSS composites
8.4.3 Evaluation of the interfacial restructuration zone (IFRZ) of POSS by
means of 3D FEM simulations
8.4.4 Validation of the proposed model of the interfacial restructuration
zones
8.5 Concluding remarks on the interfacial restructuration zones in functional POSS
composites
CONCLUSION

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