Materials Evaluation of High Temperature Electrical Wires for Aerospace Applications

228 Pages·2014·12.9 MB·English

Materials Evaluation of High Temperature
Electrical Wires for Aerospace Applications

A thesis submitted to the University of Manchester for the degree of
Doctor of Philosophy
in the Faculty of Engineering and Physical Science
2014

ZIJING WANG
School of Materials
Table of Contents
Chapter 1 General Introduction ............................................................................. 1
1.1 Background ................................................................................................. 1
1.1.1 More electric aircraft - new opportunities and challenges .................. 1
1.1.2 Challenges of high temperature machine windings ............................ 2
1.2 State-of-the-art solutions to current problems ............................................... 3
1.3 Candidates for high temperature machine windings...................................... 3
1.3.1 SK-650, a glass-fibre insulated wire .................................................. 3
1.3.2 Cerafil-500, a ceramic-insulated wire ................................................ 4
1.3.3 Inorganic-organic composite insulation ............................................. 5
1.3.4 Summary of current technologies ...................................................... 6
1.4 Objectives of this study ................................................................................ 6
Chapter 2 Literature Review .................................................................................. 7
Part I Ni-Cu interdiffusion and its implications for Ni coated Cu conductors ..... 7
2.1 Electrical wire interconnect system (EWIS) ................................................. 7
2.2 Conductor materials in electrical interconnects ............................................ 7
2.2.1 Materials for wire conductors - why Cu? ........................................... 7
2.2.2 Cu, physical properties and applications ............................................ 8
2.2.3 Ni-coated Cu wires for high temperature applications ........................ 8
2.2.4 Diffusion, theory and fundamentals ................................................. 10
2.2.5 Factors influencing Ni-Cu interdiffusion ......................................... 14
2.2.6 Diffusion-controlled ageing behaviour of Ni-coated Cu conductors . 17
2.3 Summary ................................................................................................... 19
Part II Review of low temperature co-fired ceramics (LTCC) ............................ 20
2.4 Introduction to low temperature co-fired ceramics ..................................... 20
2.5 Sintering of ceramics, theory and practice .................................................. 21
2.6 Approaches to low temperature sintering .................................................... 22
2.6.1 Use of ultra-fine starting powders .................................................... 22
2.6.2 Sintering additives ........................................................................... 23
2.6.3 Advanced processing techniques ..................................................... 23
2.6.4 Ceramics with intrinsic low sintering temperatures .......................... 23
2.7 LTCC candidates for insulating applications .............................................. 25
2.7.1 Al O ............................................................................................... 25
2 3
2.7.2 BeO ................................................................................................. 26
2.7.3 AlN ................................................................................................. 26
2.7.4 Electrical porcelain .......................................................................... 27
2.7.5 Telluride ceramics ........................................................................... 28
2.7.6 Molybdate ceramics ........................................................................ 28
2.7.7 Charted summary of candidate LTCCs ............................................ 29
2.8 Summary ................................................................................................... 31
Part III Dielectric breakdown of ceramic materials ............................................ 32
2.9 Introduction to dielectric breakdown .......................................................... 32
2.10 Dielectric breakdown, theory and practice ................................................ 32
2.10.1 Electronic breakdown .................................................................... 32
2.10.2 Thermal breakdown ....................................................................... 35
2.10.3 Discharge breakdown .................................................................... 36
2.10.4 Long-term effects .......................................................................... 37
2.11 Measurement of dielectric strength ........................................................... 38
2.12 Factors influencing the dielectric strength ................................................ 40
2.12.1 Experimental conditions ................................................................ 40
2.12.2 Physical factors ............................................................................. 42
2.12.2 Microstructural factors .................................................................. 43
2.12.3 Effect of interfacial polarisation ..................................................... 44
2.13 Summary ................................................................................................. 48
Part IV Thermal properties of ceramics ............................................................... 49
2.14 Thermal properties - importance for high temperature machine windings . 49
2.15 Thermal conductivity, theory and application ........................................... 49
2.15.1 Fundamentals of thermal conductivity ........................................... 49
2.15.2 Mechanism of thermal conductivity in ceramics ............................ 51
2.16 Factors influencing the thermal conductivity ............................................ 52
2.16.1 Material effect ............................................................................... 52
2.16.2 Temperature effect ......................................................................... 54
2.16.3 Grain size effect ............................................................................ 55
2.16.4 Porosity effect ............................................................................... 56
2.16.5 Secondary-phase effect .................................................................. 57
2.17 Thermal expansion, theory and practice ................................................... 58
2.18 Measurement of thermal expansion coefficient ......................................... 60
2.19 Factor influencing thermal expansion ....................................................... 61
2.19.1 Effect of temperature ..................................................................... 61
2.19.2 Effect of secondary phase .............................................................. 61
2.20 Summary ................................................................................................. 62
Part V Review of dip coating for the coating of ceramic films on metal substrates
................................................................................................................................ 63
2.21 Dip coating for applying ceramic insulation to conductor wire ................. 63
2.22 General process of dip coating ................................................................. 63
2.23 Implication of dip coating for wire coating ............................................... 64
2.24 General aspects of (ceramic) dip coating .................................................. 65
2.24.1 Effect of starting suspensions ........................................................ 65
2.24.2 Effect of withdrawal speed ............................................................ 67
2.24.3 Effect of multiple-deposition ......................................................... 68
2.25 Summary ................................................................................................. 69
Chapter 3 Ni-Cu interdiffusion and its implication for Ni-coated Cu conductors
................................................................................................................................ 70
3.1 Introduction ............................................................................................... 70
3.2 Experimental methods................................................................................ 71
3.2.1 Ageing experiments on Ni coated Cu wires ..................................... 71
3.2.2 Microstructural examination ............................................................ 72
3.2.3 Investigation of Ni-Cu interdiffusion ............................................... 72
3.3 Results and discussion ............................................................................... 74
3.3.1 Ageing experiments with Ni coated Cu wires .................................. 74
3.3.2 Ni-Cu interdiffusion experiments using foils ................................... 76
3.3.3 Effect of microstructural factors on Ni-Cu diffusion ........................ 82
3.3.4 Correction of (cid:1778) values based on the microstructure ........................ 84
c
3.3.5 Model for diffusion controlled ageing behaviour ............................. 87
3.4 Conclusions ............................................................................................... 93
Chapter 4 Development of an LTCC material for electrical insulation at high
temperatures .......................................................................................................... 95
4.1 Introduction ............................................................................................... 95
4.2 Experimental methods................................................................................ 97
4.2.1 Synthesis of zinc molybdate ceramics.............................................. 97
4.2.2 Material characterisations ................................................................ 98
4.2.3 Relative permittivity ........................................................................ 99
4.2.4 Breakdown measurement procedure ...............................................100
4.2.5 High temperature impedance analysis .............................................101
4.2.6 Coefficient of thermal expansion ....................................................102
4.2.7 Heat capacity ..................................................................................102
4.2.8 Thermal diffusivity .........................................................................104
4.2.9 Thermal conductivity......................................................................105
4.3 Results and discussion ..............................................................................106
4.3.1 Sintering behaviour ........................................................................106
4.3.2 Phase analysis ................................................................................108
4.3.3 Microstructure evaluation ............................................................... 112
4.3.4 Relative permittivity ....................................................................... 117
4.3.5 Dielectric strength ..........................................................................124
4.3.6 High temperature impedance analysis .............................................131
4.3.7 Effect of interfacial polarisation on dielectric strength ....................139
4.3.8 Proposed model for the breakdown of NSZM ceramics ..................140
4.3.9 Coefficient of thermal expansion (CTE) .........................................141
4.3.10 Thermal conductivity ....................................................................143
4.4 Conclusions ..............................................................................................148
Chapter 5 Application of ceramic insulation to high temperature wires by dip
coating ...................................................................................................................150
5.1 Introduction ..............................................................................................150
5.2 Experimental methods...............................................................................152
5.2.1 Preparation of ceramic suspensions for dip coating .........................152
5.2.2 Rheological behaviour ....................................................................153
5.2.3 Ceramic coating on Ni disc by dip coating ......................................153
5.2.4 Characterisation of the coating morphology ....................................154
5.2.5 Breakdown characteristics of dip-coated films on Ni discs .............155
5.2.6 Ceramic coating on Ni-coated Cu wires ..........................................156
5.2.7 Breakdown tests of dip-coated wires ..............................................157
5.3 Results and discussion ..............................................................................158
5.3.1 Rheological behaviour of starting suspension .................................158
5.3.2 Effect of the solid loading on the coating morphology ....................161
5.3.3 Effect of multiple-deposition ..........................................................166
5.3.4 Breakdown characteristics of dip-coated films ................................167
5.3.5 Morphology of ceramic-coated wires..............................................169
5.3.6 Breakdown voltage of dip-coated wires ..........................................170
5.4 Conclusions ..............................................................................................171
Chapter 6 General Conclusions ...........................................................................173
Chapter 7 Future Work ........................................................................................177
References .............................................................................................................183
Curriculum Vita ...................................................................................................203
List of Figures
Figure 1.1 Electrical windings around the stator of an electric motor [6]. ........... 2
Figure 1.2 Scanning electron micrograph of a cross-sectional view of the SK-650
wire............................................................................................................ 4
Figure 1.3 Scanning electron micrograph of a cross-sectional view of Cerafil-500
wire............................................................................................................ 5

Figure 2.1(a) Schematic diagram of the interstitial mechanism under which the
atoms in the interstitial sites of the lattice move to an adjacent site due to
thermal activation; (b) schematic diagram showing the vacancy mechanism
under which the diffusion occurs in the form of the diffusant moving towards
the adjacent vacancy of the lattice. ............................................................ 11
Figure 2.2 Typical concentration-distance profile across a diffused interface. The
dashed line is the Matano interface. .......................................................... 12
Figure 2.3 Ni-Cu interdiffusivity as a function of Ni alloy concentration. From the
work of Hayashi et al [37]. ....................................................................... 13
Figure 2.4 Concentration-distance profiles, calculated by the ‘thick film solution’
after times, t and t . The thickness of the slab, illustrated by dashed lines and
1 2
located at the origin, is h. ......................................................................... 14
Figure 2.5 Arrhenius plots of the Ni-Cu interdiffusivity data. After Divinski et al
[48], Schwarz et al [39] and Zhao et al [40]. ............................................. 15
Figure 2.6 Arrhenius plot of volume and grain boundary diffusivity as a function
of grain size. From the work of Kaja [49]. ................................................ 16
Figure 2.7 Geometric aspect of the three-annular-diffusion-zone model proposed
by Loos and Haar [32].............................................................................. 17
Figure 2.8 Schematic diagram of the geometric concept of Powell’s annular
model for wire resistivity. ......................................................................... 18
Figure 2.9 Sintering temperature versus packing density for selected ceramic
materials. From various authors [63, 75, 78-83]. ...................................... 24
Figure 2.10 Microstructure of electrical porcelain, composed of quartz grains,
mullite precipitates (in needle shape) and residual glass flux [94]. ............ 27
Figure 2.11 Band structures of (a) an insulator in which a large gap is present
between the valence band and the conduction band, (b) a semiconductor in
which an intermediate band gap is present and (c) a metallic conductor in
which the valence band and the conductors overlap with each other thus a
band gap is no longer present. .................................................................. 33
Figure 2.12 Schematic diagram showing three stages of an intrinsic breakdown (in
the case of the chemical doping): (a) few conduction electrons tend to diffuse
to a higher level of the conduction band at a strong electric field; (b) The
electrons lose their energy to the valence electrons when it reaches the upper
limit of the conduction band; (c) The valence electrons, which gain energy,
are activated to the conduction band, resulting in an increase in the number of
the conduction electrons. .......................................................................... 34
Figure 2.13 Digital graph showing electrical treeing in a polymer insulator
subjected to long-term exposure under high voltage. From the work of
Schurch et al [106]. .................................................................................. 37
Figure 2.14 Schematic diagram of two different specimen geometries for electrical
breakdown tests: (a) Disc shaped specimen with electrode on surface of each
side. (b) Recessed specimen in which the central part is slightly removed. 39
Figure 2.15 Dielectric strength of the alumina ceramics versus the test temperature.
From the work of Yoshimura et al [110], Miyazawa et al [114] and Britt et al
[115]. ....................................................................................................... 40
Figure 2.16 Dependence of breakdown voltage on atmospheric pressure. From the
work of Leader [111]. ............................................................................... 41
Figure 2.17 Different polarisations including electronic polarisation, ionic
polarisation, dipolar polarisation and space-charge polarisation [52]. ....... 45
Figure 2.18 Variation of ε’ and ε’’ with frequency, indicating the contribution of
r r
each polarisation mechanism to the relative permittivity of a dielectric
material. ................................................................................................... 46
Figure 2.19 Schematic diagram showing a polarised interface between the
domains of different conductivities (σ) and relative permittivities (ε). ..... 47
r
Figure 2.20 Thermal conductivities of different ceramic materials at room
temperature, From the work of Slack et al [86]. ........................................ 53
Figure 2.21 Thermal conductivity of AlN as a function of temperature. From the
work of Slack et al [86]. ........................................................................... 54
Figure 2.22 Grain size dependent thermal conductivity in nano-crystalline yitria
stabilized zirconia (YSZ). From the work of Soyez et al [146]. ................ 55
Figure 2.23 Thermal conductivity values of alumina with different porosities.
From the work of Francl and Kingery [147]. ............................................ 56
Figure 2.24 Variation of the thermal conductivity with the volume fraction of
secondary in AlN [92]. ............................................................................. 57
Figure 2.25 (a) Microstructure of Y O added AlN ceramic, slowly cooled to room
2 3
temperature and (b) microstructure of Y O added AlN ceramic, cooled with
2 3
a very high cooling rate. The micrographs are after [149] ......................... 58
Figure 2.26 Temperature dependent thermal expansion coefficient of Cu. From the
work of Nasekovski [157]. ....................................................................... 61
Figure 2.27 Schematic diagram of a reel-to-reel drive system for continuous dip
coating of ceramic materials on metallic tape. From the work of Kandle et al
[169]. ....................................................................................................... 64
Figure 2.28 SEM micrograph of an alumina insulated platinum wire, processed by
dip coating. From the work of Kreidler and Bhallamudi [166]. ................. 65
Figure 2.29 Variation of the (alumina) film thickness with viscosity of starting
suspensions. From the work of Lee et al [167].......................................... 66
Figure 2.30 The withdrawal speed effect on the thickness of the dip-coated films.
After the work of Zwinkel et al [179]. ...................................................... 68
Figure 2.31 Variation of the dip-coated (alumina) film thickness with the number
of depositions. After the work of Lee et al [167]. ...................................... 69

Figure 3.1 Electrical resistivity data of AWG20-Class3 and AWG18-Class27 Ni
coated Cu wires being thermally aged at 400°C as a function of ageing time.
................................................................................................................ 74
Figure 3.2 Elementary mappings (by electron probe micro-analyser) of (a)
as-received Ni coated Cu wire (b) the Ni coated Cu wire thermally aged at
400°C for 600 hours and (c) the Ni coated Cu wire thermally aged at 400 °C
for 1200 hours. ......................................................................................... 75
Figure 3.3 Resistivity data of Ni-Cu binary alloys at 400 °C as a function of Ni
concentration. From the work of Ho et al [185]. ....................................... 76
Figure 3.4 Back scattered electron micrograph of a typical Ni-Cu diffusion couple
(thermally aged at 500 °C for 96 hour). The scale bar is 10 μm. Following
annealing process, several EDX line scans were performed across the
diffused interface. .................................................................................... 77
Figure 3.5 Typical Ni concentration profile obtained across the Ni-Cu diffused
interface by EDS. This diffusion couple was annealed at 500 ºC for 96 hours
in Ar. ........................................................................................................ 77
Figure 3.6 Composition-dependent Ni-Cu interdiffusion coefficients in the
temperature range 400 to 600 °C. ............................................................. 79
Figure 3.7 Arrhenius fitting of Ni-Cu interdiffusivity data determined from the foil
experiment at 50At% Ni concentration. .................................................... 82
Figure 3.8 Ni-Cu interdiffusivity data together with average grain sizes of Cu foils
used in the previous and present work [39, 40, 48]. .................................. 84
Figure 3.9 Plot of interdiffusivity values, from the literature [39, 40] and present
work, against the geometric factor of the Cu foils used. ............................ 85
Figure 3.10 Optical micrographs of the etched cross-sections of AWG20-Class3
(left) and AWG18-Class27 (right) Ni coated Cu wires. The scale bars are