COLD SPRAY TECHNOLOGY FOR HIGH PERFORMANCE FREQUENCY …
Transcript of COLD SPRAY TECHNOLOGY FOR HIGH PERFORMANCE FREQUENCY …
Vol.104(3) September 2013 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 115
COLD SPRAY TECHNOLOGY FOR HIGH PERFORMANCE
FREQUENCY SELECTIVE CONDUCTIVE STRUCTURES
I. Hofsajer* and I. Botef**
* School of Electrical and Information Engineering,
** School of Mechanical, Industrial and Aeronautical Engineering,
University of the Witwatersrand, Johannesburg, Private Bag 3, Wits 2050, South Africa
E-mail: [email protected] , [email protected]
Abstract: Conductive structures and interconnects can be designed to exhibit a low pass filtering
characteristic. This is accomplished by making use of the skin and proximity effects which confine
high frequency currents to a reduced conductor cross sectional area. The efficacy of such filtering
structures can be enhanced through the use of composite multi layer conductors, each layer with
specific electric and magnetic properties. The widespread implementation of this type of filter has
been limited by the difficulty in the layer design, manufacture and availability of materials with the
correct properties. This paper discusses how each of these obstacles may be overcome and so doing
opens up a large area of new potential applications.
Keywords: cold spray technology, smart materials, dissipative filters, skin effect, proximity effect.
1. INTRODUCTION
Often electric circuits experience parasitic effects that are
detrimental to the circuit"s performance. Purely
conductive structures such as wires, cables and PCB
traces experience capacitive, inductive and resistive
parasitic effects because of the magnetic and electric
fields associated with current conduction. The capacitive
and inductive properties give rise to well known
transmission line effects. The series resistance of the
conductor gives rise to loss and is often seen as a very
undesirable property as it leads to a decrease in system
efficiency. Much effort is expended in minimising these
effects or in trying to mitigate effects once they are
present. However, sometimes it is possible to make good
use of these parasitic effects.
Therefore, in this paper a useful application of the series
resistance of conductive structures will be described. In
this respect, it is well known that under non-DC
conditions current will be distributed in a non-uniform
fashion throughout the conductor cross-sectional area.
This is described by the skin and proximity effects. As
the frequency of the current in a conductor rises and the
current distribution becomes non uniform the effective
resistance of the conductor for that particular frequency
rises as well. As the effective resistance rises it becomes
more difficult for the current to move through the
conductor. This effect can be usefully employed in order
to limit the frequency content of signals that may be
conducted via a conductive structure. This has been
recognised since the 1960s where it was used as a method
of suppressing noise[1,2,3,4]. In most cases round
conductors were coated with a material that would
dissipate high frequency energy.
More recently the effect has been applied to electronic
circuits that produce switching noise which may be
conducted onto the power lines. The conventional
solution for such a problem is the inclusion of a discrete
inductor-capacitor based electromagnetic interference
filter. It is also possible to achieve a similar effect using
conductive structures which allow the propagation of low
frequency currents but absorb higher frequency currents
due to an increase in the high-frequency resistance of the
conductor [5,6,7]. The preferred configuration of the
structure in these cases is a flat planar conductor, often
integrated as part of the terminal conductors either on
printed circuit boards or as separate off board conductors.
Often the multi layer conductive structures are included
as part of an integrated interference filter including
distributed inductors and capacitors [6]. These
applications report good performance, especially at the
higher frequency range.
Other applications consider the effects of transients on
high voltage conductors [8] and how the low pass
filtering effect of multilayer co-axial conductors reduces
bandwidth. Surges on high voltage transmission lines
may also be suppressed using this technique [9,10]. A
very novel application uses the concept via a transformer
in order to match the driving circuit to the gate of a
MOSFET via printed circuit board tracks [11]. Co-axial
anti interference cables are also still an area of much
activity, especially on aircraft where weight constraints
are important and the elimination of the weight of
discrete filters is advantageous [9].
Based on the above, section 2 discusses the basic
operation of the current filtering effect in materials.
Section 3 presents the analytical consideration for electric
and magnetic fields and current distribution inside a
conductor. Then, in section 4, the disadvantages of the
present application technologies are discussed and, in
section 5, a new enabling technology and its several
potential applications are introduced. Finally, section 6
draws conclusions and highlights practical implications.
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2. OPERATION
The basic operation of the filtering effect can best be
illustrated in the structure shown in Figure 1. For the sake
of simplicity a pair of flat planar conductors is shown
carrying a total differential current ITotal. At low
frequencies there is a uniform distribution of current
throughout the entire cross-sectional area of the
conductor (Figure 1a). The resistance of the conductor is
determined by the conductor dimension and the resistivity
of the material. As the frequency of the current rises the
current distribution tends towards the inner surfaces of
the conductor (for a common mode current this will tend
towards the outer surfaces). The rate at which the
migration of current takes place is determined by the
magnetic and electric properties of the conductor. For a
conductor with a single value of conductivity and
permeability this is described by the well-known skin and
proximity effects. A typical resistance versus frequency
plot of this effect is shown in the light trace of the graph
in Figure 2.
Normally the resistance of the conductor at a low
frequency must be small. This implies the use of a good
conductor such as copper. However at high frequencies
the resistive effects of such a material may not be good
enough to achieve sufficient filtering. In such a case it
may be possible to consider a composite structure
constructed of multiple layers of materials with different
electromagnetic properties.
Such a structure is shown in Figure 1b. On the inner
surfaces of the differential conductors a new material has
been introduced. At high frequencies as the current
migrates towards the inner surfaces, it experiences a more
pronounced increase in resistance. A typical situation is
shown by the dark trace in the graph of Figure 2. This has
been plotted for the case of a Brass conductor with a layer
of Nickel on the inner surfaces.
Figure 1: Configuration of differential mode conductive
structure.
Brass Layer Nickel layer
Conductivity 1.4x107 S/m 1.4x107 S/m
Rel. permeability 1 500
Thickness 100 m 50 m
Width 30mm 30mm
Length 2m 2m
Figure 2: Resistance increase of a planar conductor in
differential mode configuration. The parameters of the
conductors are shown beneath the graph.
3. ANALYTICAL CONSIDERATIONS
One of the biggest problems with the widespread
adoption of the dissipative low pass filter is the lack of a
sound theoretical description. There is much consensus as
to how the filtering structures operate. The idea of current
migrating from the inner, higher inductance core of the
conductor to the outer lower inductance parts as the
frequency increases is well understood [12,13,14]. If the
effective resistance of the current at the surface of the
conductor is large, then a large dissipation will take place,
hence the dissipative nature at elevated frequencies. This
concept leads to equivalent circuit diagrams consisting of
ladder networks of series/parallel inductors and resistors
to predict the move of current from within the internal
parts of the conductor to the exterior [12]. Quantifying
the equivalent circuit diagram has not been attempted.
In most of the reported literature such dissipative filters
have been analysed by means of experimental
measurement and more recently via eddy current finite
element simulations [12,15]. The design of such
structures has primarily been done via extensive
parametric sweeps using the FEM approach. This does
produce workable results, especially when coupled with
some design intuition as to how the structures function.
However the disadvantage of such a numerical approach
is the lack of insight into the internal physics of operation
as well as long parameter sweep times. While eddy
currents have received much attention in the past, often
focussed on minimising conduction loss in wound
magnetic components there has been very little work
done in treating composite conductors. Non destructive
eddy current testing of materials is a slightly different
ITotal ITotal
(a) (b)
y
z
Vol.104(3) September 2013 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 117
problem and in this area there has been much activity
especially in analytic models for multi layer testing
[16,17,18]. For this problem the conductive structure
itself carries no average current and the applied magnetic
field is normally at right angles to the surface of the
material under test. In this field the composite nature of
the conductor has been extended to the case of a
continuously variable parameter problem [19,20].
Multilayer conductor problems have also been addressed
in the context of low frequency shields. Here the goal is
effectively to exclude incoming electromagnetic fields
from an enclosure [21,22,23].
Early attempts at a solution for the multilayer conductor
problem included a wave based approach [24] but did not
produce a robust solution. More recently a closed form
analytical solution to the multi layer conductor problem
has become available [25,26]. The analytical method
described allows the field distributions within a
composite conductor with an arbitrary number of layers
to be determined. From these distributions it is possible to
determine derived parameters such as resistances and
internal inductances as a function of frequency. The
formulations are done for both cylindrical as well as
planar conductors. Planar conductors are considered
further here as they currently have greater immediate
application.
The approach is based on considering the standard eddy
current diffusion equation which is applied separately to
each homogeneous layer of material. Considering the
planar conductor of Figure 1, it is clear that if the width
of the conductor is wide relative to its height then the
current distribution will only vary with the height
dimension (y). Hence a one dimensional solution is
sufficient [27]. The electric field will only have a
component in the x-direction.
With the axes defined as in Figure 1b for the multilayer
conductor structure, the eddy current diffusion equation
for the electric field for sinusoidal excitation with a
frequency of is given by[28]:
Which has a general solution of:
))(sinh())(cosh()( tyDMtyDLyEx
Where:
2)1( jD
t is the thickness of the layer
is the conductivity of the layer
is the permeability of the layer
M and L are boundary conditions
This solution will give the normal skin effect distribution
of the current profile in a conventional conductor. All that
is required to solve the simple solution is to determine the
coefficients L and M from the boundary conditions. For a
single conductor carrying a known current the boundary
conditions of the magnetic fields at the edges of the
conductor are easily determined. This is not the case for a
composite conductor where there is no prior knowledge
about the split in current between the layers. The
approach taken by [25] is to set up equations each with
their own boundary conditions for each layer of the
composite conductor and then simultaneously solve for
all the boundary conditions. Continuity of the electric and
magnetic fields is enforced across the boundaries between
adjacent layers.
Using this approach it is possible to determine the electric
field, magnetic field and current distributions inside a
conductor. The current distribution profile for a Nickel-
Brass composite conductor previously discussed is shown
in Figure 3 for three different frequencies. At low
frequencies the distribution is almost uniform. As the
frequency increases the current distribution between the
two different materials distorts and most of the current
migrates quickly into the thin Nickel layer.
4. APPLICATIONS
Given the general operation of these low pass filtering
structures, and the analytical design tools, it is possible to
synthesize a multilayer structure that can have almost any
arbitrary resistance versus frequency profile. The only
real limitation imposed by the physics of the structure is
that the gradient of the resistance profile is always
positive. It is not possible to cause a decrease in
resistance with increasing frequency. This is true for real
materials with positive conductivities and permeabilities.
Figure 3: Current distribution for the Nickel- Brass
composite conductor. The inner Nickel surface is at the
origin of the x axis.
2
2
x
x
d Ej E
dy 100kHz
10kHz
1kHz
Brass Nickel
Figure 3: Current distribution for the Nickel- Brass
composite conductor. The inner Nickel surface is at the
origin of the x axis.
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The conductive structures may be constructed with a
variety of different technologies. The simplest is merely
layering sheets of different conductors on top of one
another. While easy to construct, this is not very robust.
Electroplating one metal on top of another has shown to
give good results [29,30]. Electroplating however does
have limitations on the maximum thicknesses that are
achievable. Also, not all materials lend themselves
equally to electroplating. Electroplating anything other
than a pure material is difficult.
It is quite possible that during the design procedure
materials are required that are not readily available with
arbitrary conductivities and permeabilities. This greatly
limits the possible applications of the technology. It is
however becoming possible to make use of cold spray
metal deposition technology artificially to synthesize
materials with the correct properties.
Cold spray metal deposition technology is finding
application in the area of power electronics where it is
often necessary to deposit conductors that need to carry a
large current. Conventional printed circuit board
techniques are quite limited and the maximum
thicknesses achievable are limited by the etching
processes needed in order to ensure well defined
conductor edges. In [31] cold spray technology is used to
be able directly to deposit copper conductors up to 0.5
mm thick onto alumina substrates.
5. COLD SPRAY TECHNOLOGY
The phenomenon of cold gas dynamic spraying, which is
more commonly referred to as the cold spray (CS)
process was discovered at the Institute of Theoretical and
Applied Mechanics of the Siberian Branch of the Russian
Academy of Sciences in the early 1980s as a result of
work done on models subjected to a supersonic flow
consisting of gas along with solid particles in a wind
tunnel [32].
CS is a process of applying coatings using a supersonic
jet of compressed gas. The jet of gas is loaded with small
particles of material between 5 m and 50 m in size.
These particles are accelerated to between 300 m/s and
1200 m/s, before impacting on the substrate being coated.
[32]. The gas temperature is always lower than the
melting point of the particle's material, so, upon impact
with the substrate, these high-velocity 'cold' particles
plastically deform and bond with the underlying material
resulting in the formation of the coating of particles in the
solid state.
The cold spray process has been used to produce dense,
hard, thick, well bonded, wear and corrosion resistant
coatings, with minimum oxidation and phase
transformations of coatings, of many metals and alloys
such as aluminium, copper, nickel, tantalum, titanium,
silver, and zinc, as well as stainless steel, nickel-base
alloys (Inconels, Hastalloys), and bond-coats, such as
MCrAlYs. Cold spray can produce composites, such as
metal-metal like copper-tungsten (Cu-W) or copper-
chromium, metal-carbides like aluminium-silicon carbide
(Al-SiC), and metal-oxides like aluminium-alumina. Cold
spray has been used to produce all manner of protective
coatings and performance enhancing layers, very thick
coatings, freeform and near net shape substrates [33].
Electrical conductivity is a good indicator of coating
quality [34]. Therefore, Figure 4 highlights some of the
advantages of the cold spray process and the resulting
coating quality. The electrical properties such as the
conductivity, permeability and permittivity (for non-
conductive materials) of the coatings can be utilised in
the same way as conventional materials. This leads to
applications such as good conductors, resistive
conductors, magnetic components, insulators and
resistance heating. These coatings could be applied
locally or over very large areas, or be built in multilayer
materials systems. Consequently, based on the cold
spraying advantages, the next sub-sections will highlight
few potential cold spray electrical and electronic possible
applications.
Figure 4: Cold spray advantages.
5.1 Coatings for Power Electronics
By combining insulating substrates as well as conductive
coatings of various conductivities, possibilities exist to
apply the cold spray process in the power electronics
industry [35]. A typical power electronic assembly
conducts currents of several hundred amperes that lead to
thermal stresses in soldered connections which in turn
lead to a reduction in device lifetime. However, soldering
could be replaced by metallic coatings deposited by cold
spraying onto an insulator such as Al2O3. This process
would form both the layout conductive patterns as in
conventional Direct Copper Bonding as well as the
device interconnections. The cold-sprayed Cu layers
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(deposited with a standard de-Laval-type nozzle WC-Co,
HSU/CGT, 3 MPa stagnation gas pressure, and 600 °C
gas temperature) were dense, with good adhesion to the
ceramic substrate, and a high electrical conductivity. The
electrical conductivity has been reported to achieve up to
98% IACS in the as-sprayed condition on substrates
heated to 280 °C. 90% of the IACS value has been
obtained by spraying on cold substrates. In this case an
aluminium bond-coat and additional heat-treatment were
applied [35].
The results are also in line with other published research.
Figure 5 shows electrical conductivity of coatings using
the high-pressure (HP) and low-pressure (LP) cold spray
systems in as-sprayed and heat-treated state. IACS %
values are given as an average of values measured by
using four-point measurements for the following sample
conditions: (c1) on steel as-sprayed, (c2) on steel heat-
treated at 400 °C, (c3) on ceramic heat-treated at 280 °C,
and (c4) on ceramic heat-treated at 280 °C.
Figure 5: Electrical conductivity for CS coatings.
The high-pressure cold spray system produces coatings
with higher electrical conductivities than the low-pressure
cold spray system available at the University of the
Witwatersrand, Johannesburg. However, the low-pressure
cold spray system can deposit, for example, Cu+Al2O3
coatings that can reach relatively high electrical
conductivity levels in as-sprayed and heat treated states,
and so meeting the requirements for electronic
applications. The main function of Al2O3 particle addition
is to activate (cleans and roughens) the sprayed surfaces
which become more receptive to fresh impact of sprayed
particles and better adhere to the surface. In addition, due
to the hammering effect, the collision of the ceramic
particles increases deformation of the metallic particles
which in turn affects the coating properties and
deposition efficiency. In addition, heat treatment
significantly enhanced the conductivity of the coatings
due to densification by void reduction and
recrystallization which occur [34].
5.2 Coating of Copper on Aluminium for Heat Sinks
An important commercial application is the deposition of
a thin layer of copper onto the bonding surface of finned
aluminium heat sinks used to cool computer chips. The
cold-sprayed copper layer makes it easy to solder the
aluminium heat sink to other components in the heat sink
assembly - it is difficult to solder aluminium due to its
very stable surface oxide. In contrast, the relatively high
porosity and oxide levels inherent in most thermal
sprayed copper would degrade the thermal conductivity
and solder wetability of such copper coatings to the point
that they would not be suitable for the application[36].
5.3 Coatings of Ferromagnetic Materials
Coatings of ferromagnetic materials such as Fe, Ni, and
Co are very useful where high permeabilities are
required. Soft magnetic materials based on alloys of these
materials are becoming available [37]. This is important
as it is normally the high permeability layers which
dictate the current distribution at high frequencies.
Additionally, it is possible to construct all manner of low
profile planar magnetic components. It is not envisaged
that spray deposition of magnetic materials will replace
bulk conventional materials.
5.4 Coatings of Oxygen-Sensitive Materials
It is difficult to thermally-spray oxygen-sensitive
materials such as aluminium, copper, magnesium,
titanium, and their alloys because increasing the
temperature exponentially accelerates the oxidation.
However, with the cold spray process it is possible to
create coatings such as titanium on aluminium. With
conventional thermal spray techniques a brittle phase
would be formed at the substrate/coating interface [38].
Also, microstructural defects such as oxides and porosity
in most traditional thermal spray coatings could
significantly degrade the mechanical, electrical, and
thermal properties of sprayed materials [39]. For
example, although copper possesses excellent electrical
and thermal conductivity, the conductivity of the plasma-
sprayed copper is only 15 % of the conductivity of
oxygen-free-high-conductivity (OFHC) copper. In
contrast, the relatively defect-free cold-sprayed copper
has a conductivity that is 85 % of the conductivity of
OFHC copper [36]. Thermal spray techniques were used
in the past for the deposition of planar conductors and
exhibited the problems of poor conductivity [40].
5.5 Nanostructured Coatings
Unlike other powder consolidation processes such as
powder-metallurgy and thermal spray which lead to
unacceptable grain growth, cold spraying produces
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nanostructured coatings without any appreciable grain
growth [41]. As a result, desirable properties are retained
in the bulk materials using cold-spray.
Therefore, cold spraying could offer a practical way to
consolidate nanostructured powder materials into
structures of useful size without destroying the fine grain
size that imparts the unique properties if these materials
[42]. The importance of powder properties is also
highlighted in the sense that high purity as-received
powder leads to lower resistivity and therefore, higher
electrical conductivity [34].
5.6 Higher Conductivity Coatings
Coatings with lower levels of porosity have higher
conductivity [34]. A comparison of two copper coatings
deposited using the same feedstock powder, but two
different processes, showed 5 % porosity in the plasma-
sprayed coating and less than 1 % porosity in the cold-
sprayed material. The much higher porosity in plasma-
spraying resulted from splashing of molten droplets upon
impact and failure of the molten metal completely to fill
surface irregularities during deposition. However, the
greatly reduced level of porosity in the cold-sprayed
material resulted from the fact that cold spay is a solid-
slate process, with no splashing, and a process viewed as
a combination of particulate and microscopic vapor
deposition processes [36].
5.7 Hybrid Electric Machines
Rotors for hybrid electric machines could be cold spray
coated with copper on a sinusoidal contour. Advantages
of cold sprayed coatings include the excellent electrical
conductivity of the copper layer, excellent bonding
between coating and substrate, low heating of the parts
during the coating process, application of the coating
exactly in the necessary geometry with masks, and the
effective automation for cost reduction [43]. Combining
this application with the frequency selective approach
described earlier, makes it possible to tune the rotor
resistance of an induction machine to be different at
different values of slip, creating much improved speed
torque characteristics.
5.8 Tin-Bronze Coatings
Tin-bronze coatings are widely used in industries such as
aviation, navigation and automotive as they exhibit good
abrasive resistance and corrosion resistances and have
high strength and good elasticity. As discussed in section
5.4 conventional thermal spray technology cannot be
used for bronze coatings because of oxidization at
elevated temperatures [44]. To apply this type of coating
(Cu!6 wt.% Sn and Cu!8 wt.% Sn) it is necessary to use
cold spray processes.
Bronze has a relatively high strength as opposed to pure
Cu. The makes it difficult for the bronze to completely
deform and fill in all the gaps in between deposited
particles. As such there is a higher porosity in bronze
coatings as compared to pure Cu. The as-sprayed CuSn6
and CuSn8 coatings exhibit the same porosity of 4.7 %.
However after annealing at 600 °C the porosity of the
coating is reduced to 2.4 % for CuSn8 and to1.4 % for
CuSn6.
However, in order to improve coating porosity and so
improve electrical properties, tin-bronze/TiN and tin-
bronze/quasicrystal (AlCuFeB) may be used. These
coatings may be applied by cold spray processes [45].
Thermal spray cannot be used for these new materials due
to their phase transformation and oxidization at high
temperatures. However, CuSn8/TiN and CuSn8/QC cold
spray composite coatings show a microhardness and
density that is significantly increased compared to the
pure tin-bronze coating [45], and so has improved
electrical properties.
6. CONCLUSIONS
There are current and many potential applications for low
pass frequency selective conductive structures. These
applications have been known for a long time, but due to
several difficulties in their implementation, have not
received widespread deployment. It is shown in this paper
that the two biggest difficulties in the design and
manufacture of these structures can be overcome by
combining recent advances in multiple fields.
The availability of new analytical electromagnetic
procedures allows for the rapid design of the dissipative
nature of the conductors. These tools also allow for the
verification and optimization of existing structures.
From a manufacturing perspective, cold spraying enables
the production of dense and low oxygen content metallic
coatings without the presence of porosity, inclusions and
impurities that compromises electrical and thermal
conductivity and corrosion resistance of coatings. The
reported high electrical conductivity values for cold
sprayed copper coatings of over 90% of the value of pure
bulk copper has been regarded as a great advantage in
obtaining optimum electrical and thermal conductivity of
coatings. As electrical conductivity is a good indicator of
coating quality, the emerging cold spray process could be
used in a range of applications such as power electronics,
connecting plates, heat sinks in electronics, and cooling
devices.
The possible applications of this multilayer conductor
technology can easily be analysed theoretically by
calculating the current distributions and the frequency
dependency of the conductor resistance. While it has been
mentioned that it is possible to synthesize materials with
a wide variety of electrical properties, much work in this
direction still needs to be completed. The experimental
measurement of the high frequency electrical properties
Cu. That makes it difÞ cult for the bronze to completely
Vol.104(3) September 2013 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 121
of different compositions of sprayed conductors is
ongoing.
REFERENCES
[1] F. Mayer, "Electromagnetic compatibility: anti-
interference wires,cables and filters," IEEE Trans.
on EMC, vol. EMC-8, no. 3, pp. 153-160, 1966.
[2] H.M. Hoffart, "Electromagnetic interference
reduction filters," IEEE Trans. on EMC, vol. EMC-
10, no. 2, pp. 225-232, 1968.
[3] H.M. Schlicke and H. Weidmann, "Compatible EMI
filters," IEEE Spectrum, pp. 59-68, October 1967.
[4] P. Schiffres, "A Dissipative Coaxial RFI Filter,"
IEEE Trans. on EMC, vol. 6, no. 1, pp. 55-61, 1964.
[5] T. Sato, S. Ikeda, K. Yamasawa, and T. Mizoguchi,
"Transmission line low-pass filter for switching
power supplies," in Proc. of IEEE Power Electronics
Specialists Conference, 1998, pp. 1972-1978.
[6] L. Zhao, R. Chen, and J.D. van Wyk, "An Integrated
Common Mode and Differential Mode Transmission
Line RF-EMI Filter," in Proc of Power Electronics
Specialists Conference, 2004, pp. 4522 - 4526.
[7] L. Zhao and J.D. van Wyk, "Electromagnetic
modeling of an integrated RF EMI filter," in Proc of
Industry Applications Society Annual Meeting, 2003,
pp. 1601-1607.
[8] I.R. Jandrell and J.P. Reynders, "Consideration of
skin effect in high-voltage coaxial systems under
transient and steady-state conditions, and its impact
on steep traveling waves ," Generation,
Transmission and Distribution, IEE Proceedings C ,
vol. 138, no. 5, pp. 445 - 451, 1991.
[9] F. Mayer, "Electrical power and signal distribution
in modern aircrafts, combines weight advantages
and EMC compatiblity," in IEEE Int. Symp.
Electromagn. Comp., 1998, pp. 281-283.
[10] F. Mayer, "Absorptive low-pass cables: state of the
art and an outlook to the future," IEEE Trans on
EMC, vol. EMC-28, no. 1, 1986.
[11] M. Hartmann, A. Musing, and J.W. Kolar,
"Switching transient shaping of RF power
MOSFETs for a 2.5 MHz, three-phase PFC ," in 7th
International Conference on Power Electronics,
2007 , pp. 1160 - 1166.
[12] J.D. van Wyk, W.A. Cronje, J.D. van Wyk, C.K.
Campbell, and P.J. Wolmarans, "Power Electronic
Interconnects: Skin- and Proximity Effect-Based
Frequency Selective Multipath Propagation,"
Transactions on Power Electronics, vol. 20, no. 3,
pp. 600-610, May 2005.
[13] H.M. Schlicke, "Theory of Simulated-Skin-Effect
Filters a Thin Film Approach to EMI," IEEE Trans.
on EMC, vol. 6, no. 1, pp. 47-54, 1964.
[14] L. Hwang and I. Turlik, "A review of the skin effect
as applied to thin film interconnections," IEEE
Trans. on Comps., Hybrids, Manufact. Technol., vol.
15, no. 1, pp. 43-54, 1991.
[15] Y. Liang, J.D. van Wyk, and K.D.T. Ngo,
"Parametric Characterisation of Differential-Mode
Transmission Line EMI Filters," in Proc of Appl.
Power Electron. Conf., 2007, pp. 1217-1223.
[16] Li Yong, T. Theodoulidis, and Tian Gui Yun,
"Magnetic Field-Based Eddy-Current Modeling for
Multilayered Specimens," IEEE Trans. on
Magnetics, vol. 43, no. 11, pp. 4010-4015, 2007.
[17] J.W. Luquire and W.E. Deeds, "Alternating Current
Distribution between Planar Conductors," J. Appl.
Phys, vol. 41, no. 10, pp. 3983-3991, 1970.
[18] J.F. Hoburg, "A Computaitional Methodology and
Results for Quasistatic Multilayered Magnetic
Shielding," IEEE Trans. on EMC, vol. 38, no. 1, pp.
92-103, 1996.
[19] T.P. Theodoulidis, T.D. Tsiboukis, and E.E. Kriezis,
"Analytical solutions in eddy current testing of
layered metals with continuous conductivity
profiles," IEEE Trans. Magnetics, vol. 31, no. 3, pp.
2254-2260, 1995.
[20] E. Uzol, J.C. Moulder, and J.H. Rose, "Impedance of
coils over layered metals with continuously variable
conductivity and permeability: Theory and
experiment," J. Appl. Phys., vol. 74, no. 3, pp. 2076-
2089, 1993.
[21] T.K. Liotopoulos, C.S. Antonopoulos, and E.E.
Kriezis, "Low-frequency generalised solution for the
shielding effectiveness of a multicoated system of
coaxial cylindrical shells ," Science, Measurement
and Technology, IEE Proceedings A , vol. 140, no.
4, pp. 257-262, 1993.
[22] L. Sandrolini, A. Massarini, and R. Ugo, "Transform
Method for Calculating Low-Frequency Shielding
Effectiveness of Planar Linear Multilayered
Shields," IEEE Trans. on Magnetics , vol. 36, no. 6,
pp. 3910-3919, 2000.
[23] Y. Trenkler and L.E. McBride, "Shielding
improvement by multi-layer design ," in IEEE
International Symposium on Electromagnetic
Compatibility, 1990, pp. 1-4.
[24] K. de Jager, L. Dalessandro, I.W. Hofsajer, and
W.G. Odendaal, "Wave Analysis of Multilayer
Absorptive Low-Pass Interconnects," in Porc of
Power Electronics Specialists Conference, 2007, pp.
2121-2127.
[25] E.A. Brink, Aspects of Electromagnetic Field
Distributions in Multipath Conductive Structures,
PhD Thesis, Ed.: University of Witwatersrand, 2011.
[26] E.A. Brink and I.W. Hofsajer, "General Approach
for Determining the Frequency Dependant Current
and Field Distributions Inside Multi-layer
Conductors," In Press.
[27] P.L. Dowel, "Effects of Eddy Currents in
Transformer Windings," Proc. of the IEE, vol. 113,
no. 8, pp. 1387-1394, 1966.
Vol.104(3) September 2013SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS122
[28] R.L. Stoll, The Analysis of Eddy Currents.: Oxford
University Press, 1974.
[29] C.K. Campbell, J.D. van Wyk, and P. Wolmarans,
"Improved transmission-line attenuators for
integrated power filters in the RF band," IEEE
Trans. on Components and Packaging Technologies,
vol. 27, no. 3, pp. 311-316, 2004.
[30] P.J. Wolmarans, J.D. van Wyk, J.D. van Wyk jr, and
C.K. Campbell, "Technology for Integrated RF-EMI
Transmission Line Filters for Integrated Power
Electronics Modules," in Industry Applications
Society Annual Meeting, 2002, pp. 1774-1780.
[31] E. Rastjagaev and J. Wilde, "Development and
Testing of Cold gas Sprayed Circuit Boards for
Power Electronics Applications," in Proceedings,
Conference on integrated power systems, 2012.
[32] A. Papyrin, V. Kosarev, S. Klinkov, A. Alkhimov,
and V. Fomin, Cold Spray Technology, 1st ed.:
Elsevier, 2007, ch. 1.
[33] MIL-STD-3021, Materials Deposition, Cold Spray,
Department of Defense Manufacturing Process
Standard., July 2011.
[34] H. Koivuluoto, A. Coleman, K. Murray, M. Kearns,
and P.l Vuoristo, "High Pressure Cold Sprayed
(HPCS) and Low Pressure Cold Sprayed (LPCS)
Coatings Prepared from OFHC Cu Feedstock:
Overview from Powder Characteristics to Coating
Properties," Journal of Thermal Spray Technology,
vol. 21, no. 5, pp. 1065-1075, 2012.
[35] K.R. Donner, F. Gaertner, and T. Klassen,
"Metallization of Thin Al2O3 Layers in Power
Electronics Using Cold Gas Spraying," Journal of
Thermal Spray Technology, vol. 20, no. 1-2, pp.
299-306, 2011.
[36] M.F. Smith, "Overview of Cold Spray," Sandia
National Laboratories, 1999.
[37] W. Cherigui1 et al., "Microstructure and magnetic
properties of FeSiBNbCu-Al cold spray coatings,"
Eur. Phys. J. Appl. Phys, vol. 43, pp. 79!86, 2008.
[38]
R. Morgan, P. Fox, J. Pattison, C. Sutcliffe, and W.
O'Neill, "Analysis of cold gas dynamically sprayed
aluminium deposits," Materials Letters, vol. 58, pp.
1317-1320, 2004.
[39] R.G. Maev and V. Leshchynsky, Introduction to
Low Pressure Gas Dynamic Spray, Physics and
Technology.: Wiley, 2008.
[40] P.A. Janse van Rensburg, J.D. van Wyk, M.F.K.
Holm, and J.A. Ferreira, "On the technology of
planar integrated capacitive inductive structures for
hybrid power electronics," in Proc of Ind. Appl.
Conf., 1997, pp. 1104-1119.
[41] H.J. Kim, C.H. Lee, and S.Y. Hwang, "Superhard
nano WC-12%Co coating by cold spray deposition,"
Materials Science and Engineering A, pp. 243-248,
2005.
[42] J. Karthikeyan, "The advantages and disadvantages
of cold spray coating process," in The cold spray
materials deposition process: fundamentals and
applications.: Woodhead Publishing Limited, 2007,
ch. 4.
[43] S. Hartmann, "New industrial applications for cold
spraying," in Cold Spray North American
Conference, 2010.
[44] X. Guo et al., "Microstructure, microhardness and
dry friction behavior of cold-sprayed tin bronze
coatings," Applied Surface Science, vol. 254, pp.
1482!1488, 2007.
[45] X. Guo et al., "Investigation of the microstructure
and tribological behaviour of cold-sprayed tin-
bronze-based composite coatings," Applied Surface
Science, vol. 255, pp. 3822!3828, 2009.
[46] C. Hebedean, C. Munteanu, A. Racasan, and O.
Antonescu, "Technologies to increase HF losses in
planar structures and their limitations ," in Proc. of
13th Intl. Conf on Optimization of Electrical and
Electronic Equipment , 2012, pp. 48 - 53.