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Multifunctional Advanced Carbon Aluminium Composite for Electricity Transport

Final Report Summary - MACE (Multifunctional Advanced Carbon Aluminium Composite for Electricity Transport)

The objectives of the MACE project have been:
- overcome the interfacial problems between carbon and molten aluminium which prevent production of the wire;
- facilitate its continuous production;
- research its use in a conductor of: high ampacity, high mechanical strength, low losses, low sag, long span;
- devise advanced conductor stranding techniques;
- facilitate expansion of the overhead electricity network with reduced environmental impact through fewer new lines (approximately 500 km less per year) and lower towers;
- promote exploitation of the composite in a wide range of applications (e.g. robotics, aerospace).

Metal matrix composites (MMCs) form a new class of materials of great current interest in which a metal bulk is reinforced or filled with other materials to improve mechanical, electrical or thermal properties. They can combine the best features of the component materials to give advanced properties that are unachievable by any alone, and can therefore be tailored for specific applications. For optimum properties, an interface of high integrity between the metal and reinforcement is essential. Processing is therefore crucial and subject to much research.

MMC aluminium wires (or endless fibre reinforced aluminium - EFRA) have major advantages in applications, many of them quite specialised, that demand high strength, low weight, and low thermal expansion. An application with high economic potential is needed to give the impetus to the research which can bring these materials to market. A prime candidate is advanced overhead conductors for electricity distribution.

Overhead conductors contain reinforcing strands in order to achieve long spans and withstand loadings from wind and ice. EFRA wires using ceramic fibre have low thermal expansion and high strength, and are being developed as an alternative to the commonly used steel reinforcement. Carbon fibre, however, offers far better properties even than ceramic. It is stronger, lighter, 90 % cheaper, electrically conductive and has lower thermal expansion, reducing sag. Carbon EFRA wire is not commercially available; however, because it is extremely difficult to achieve a good interface between the carbon and aluminium.

The project's detailed objectives can be described as follows:
- optimise plasma flux infiltration method for carbon EFRA wire manufacture;
- optimise plasma pre-treatment method for carbon EFRA wire manufacture;
- study the interface, minimise unwanted products, optimise the wire properties;
- develop a new fibre-rounding technique, with the targets of zero carbon exposed at surface, and a roundness of maximum ±10 %;
- devise and demonstrate the components of an integrated process, which can produce for the first time, continuous lengths;
- develop methods for stranding the highly stiff EFRA wire into conductor forms;
- produce a practical conductor sample, at least 30 m long. This should give an increase in current carrying capacity of 80 %;
- assess the feasibility and costs of manufacture. It was anticipated that the new conductor's advantages could justify a price premium of up to three times the cost of a conventional conductor;
- establish operational data (e.g. losses, corrosion resistance) for short sections;
- demonstrate the potential for a new generation conductor establish operational data for short sections of networks.

The research team worked in the following areas:

- Flux-assisted infiltration
Armines established a flux that satisfactorily achieved the required wetting of the carbon fibres by the molten aluminium and generated considerable understanding of the process. They established optimum operating conditions, including desizing temperature. This was evident in the production of wires that had the required combination of low porosity, high fibre fraction, high tensile strength and modulus. The problem of intermetallic deposits in the fibre / matrix interface was eliminated.

- Plasma pretreatment
C-Tech carried out the initial trials on carbon discs in order to allow convenient XPS analysis of the deposited material by UCL. Various combinations of precursors and plasma conditions (power, flow rate, carrier gas, etc.) were tested. Work centred on establishing reducing conditions, rather than the oxidising chemistry that is readily generated in a plasma. A range of conditions was found that produced coatings of acceptable physical properties. Confirmation of the precise chemical nature by XPS analysis was not practical due to the lengthy and involved procedure. Ultimately, the mechanical performance of composites incorporating the plasma-treated fibres is the best way to accurately assess the benefits of the process. C-Tech therefore modified the plasma for continuous treatment of carbon fibre tows from spool to spool. They then experimented with various glass constructions to expose the running fibres to activated gases emerging from the plasma unit in a controlled manner, and without unwanted ingress of oxygen into the treatment zone. Several long lengths of treated fibre tows were distributed to ARC and Armines for processing into composite wire, and subsequent mechanical testing. Further samples were sent to UCL, who formed composite wires in a less representative, but more convenient, 'squeeze casting' process. This did not give conclusive results.

- Control of molten aluminium
Various combinations of permanent magnets or electromagnets with application of either AC or DC current to the aluminium test piece were tested. The aim was to maximise the pressure that could be applied, and hence the head of aluminium that could be supported as determined by calculation. Additional 'passive' systems, in which current was not separately applied to the molten aluminium, were also tested. One of these, using a multi-layer coil and a ferrite core, proved to generate the highest force per unit area, demonstrated the capability to support a head of over 50 mm. This is more than sufficient to allow the implementation of a horizontal process, with the fibre entering and exiting through the side walls of the aluminium bath. This would have great benefits in removing the need for pulleys, and their tendency to flatten the wire. However, the electromagnetic force also tends to wipe the aluminium from the fibre as it emerges from the melt, which is counter to the needs for a good composite wire. Although the flux method is adequate for ensuring wetting of the fibre by molten aluminium in normal circumstances, the effect is not sufficiently strong to counteract this 'wiping' action. During the first year of the project, both flux infiltration and rounding made promising progress, suggesting that pulleys could still be used without substantial detrimental effect. Further work on electromagnetic control was therefore halted.

- Wire rounding and process integration
ARC built an experimental version of an integrated rig that incorporated a desizing-furnace, flux infiltration bath, furnace for drying and a bath of molten aluminium, which is immediately followed by a set of rollers designed to impose a round cross-section on the wire. They experimented with several key parameters in optimising the performance of the rounding equipment, including speed and temperature control, profile and disposition of the rollers, material of the rollers, and applied roller pressure. They developed a temperature measurement system based on pyrometry, which is crucial to the operation of the system. ARC established that it is essential that the wire is rounded while it is still hot enough to be malleable, requiring very close proximity to the flux infiltration bath. Unfortunately, this is extremely difficult to achieve within the geometry of the continuous integrated process, and the system failed to produce the desired rounding effect in spite of trying many variations on the roller arrangement. ARC therefore carried out further, unscheduled work, using dies as an alternative rounding method. The dies were positioned at the exit of the aluminium bath, partly immersed in the melt. This work was done in close alliance with Armines. Some good results were obtained, producing small diameter wires of very good roundness, without porosity, which gave improved mechanical properties. Continuous lengths of up to 350 m were produced. However, it proved difficult to produce consistently round wires at larger diameters, particularly in the face of some persistent problems were encountered.

- Property assessment
The aim of the plasma treatment was to desize the fibres and to produce a coherent coating that is elemental, rather than an oxide, and which is mutually compatible with both carbon and aluminium. XPS analysis of disc samples by UCL, and on fibre samples by KEMA confirmed a reasonable amount of the required element deposited, but gave little evidence of its form, a variety of compounds being identified. SEM analysis and pyrolysis of coated fibres indicated coherent coatings of about one micrometre thickness. Important new techniques for quality assessment for these specialised materials were established, including highly valuable online monitoring for voids. The mechanical measurements show that at present the EFRA wire has achieved the required properties, which would have implications for both the winding of the conductors and for performance in service. Bending stress, is the most important parameter, and these measurements dictate the necessary modifications to the winding equipment.

- Conductor design, winding and testing
KEMA developed a model for conductor performance. A key output is the prediction of sag, which is a prime parameter affecting application and installation. They added an allowance for pre-stress as a conductor production feature, and investigated the sag of the conductor as a function of temperature for a range of variables including fibre volume fraction of the wire, pre-tension, and various mechanical parameters.

The model showed that as the temperature increases, it reaches a point at which the modulus of the core will become equal to that of the carbon fibre. For fibre volume fractions (Vf) greater than 40 %, the effect on sag is relatively modest at lower temperature, but as temperature increases the higher volume fraction starts to reduce sag significantly. For example, at 200 degrees Celsius a Vf of 75 % gives a sag that is over 20 % less than for a Vf of 40 %. Initial tension has a much stronger effect. As well as flattening the response of sag to temperature, it introduces a marked resistance to sag. The new EFRA wire has mechanical properties that lend it well to application of this technique.

Both the theoretical and test results from stranding and winding indicate the potential of the EFRA wire as an advanced conductor core material. The non-optimised wire caused difficulties due to its non-round nature and fragility, but the project has indicated that if the EFRA wire can be improved then a competitive conductor would be possible. For the future, the project identified various enhancements that could improve the mechanical properties of the composite wire, but the crucial element is to find a means of ensuring consistent roundness without generating breakages.

- Techno-economic assessment
The technico-economic assessment of advanced high capacity overhead conductor lines is an important output of MACE, and was led by KEMA. European asset managers and high voltages experts of the grids were interviewed about their demands for advanced conductors. The main issues discussed were cost factors, the expected development of their grid and a review of the capacity increase. Some countries have specific topics because of the laws and rules in their regions. The starting point was to estimate the expected capacity within a period of 30 - 40 years.

An advanced conductor such as MACE has a good opportunity for the replacement market if the price is less than twice that of the steel core conductor. There is a need for pilot projects because a first application is always considered as experimental. It is highly desirable that there is a European competitor in the field for developing advanced conductors.

The MACE project website can be found at: http://www.mace-conductor.com .It contains basic information about the background to the project, and links to the websites of each of the participants.

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