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.

[email protected], [email protected]

Vol.104(3) September 2013SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS116

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


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.


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


(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


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


of different compositions of sprayed conductors is

ongoing.

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