copper and other metals used in...

8 Telecommunications

Copper and other metals used intelecommunications

List the mechanical and physical properties that would be important intelecommunications conductors.

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Did you list the following?

Low resistivity, suitable strength, ductility and ease of joining.

The lower the resistivity of a material, the smaller the amount ofmaterial that is needed to carry current. It also means there are lessinsulating materials needed because the wires are thinner, sheathing costsare lower and transport costs are lower.

To allow the material to be made into wire, it must exhibit ductility.

The material must be able to withstand the tensile stresses applied duringmanufacture, extrusion of the insulation and the installation of the cable.

Joining the conductors may be achieved through twisting, soldering orwelding. Some materials are easier to join than others!

In modules that you studied during the preliminary HSC course, youlooked at the structure and atomic bonding of materials. Using thisknowledge, explain why metals are normally conductors and why copperis an excellent conductor of electricity.

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Did you answer?

Did you discuss the metallic bond that has the valence electrons in a cloudsurrounding the ions and that conduction is due to the migration of theseelectrons? Did you mention how these ‘free’ electrons easily transmit the‘flow’ of current?

Part 3: Telecommunications engineering – materials 9

Metal ion (positively charged)

Electron (negatively charged)

Figure 3.5 Simple representation of the metallic bond

Of course this theory of current ‘flow’ is a bit too simplistic and furtherdevelopment in wave theories has allowed a much clearer understandingof conductivity. While the individual valence electrons are involved inthe movement of a current, the current moves in the form of a wave andthese waves will move much more easily through a regular arrangementof obstacles. The regular arrangement of ions in the crystal latticestructure of an annealed metal, such as the face centred cubicarrangement of copper, provides little resistance to the passage of thecurrent waves. Any amount of cold working or the introduction ofalloying elements that sit in the spaces between the ions will increase therandom nature of the obstacles and will increase the resistance of thematerial. Heating will cause the ions to vibrate and will increase thepossibility of the migrating electrons hitting an ion and thus being sloweddown. This explains the increase in resistivity noticed when thetemperature of a conductor is raised.

Copper

Copper is the metal that has been traditionally used for communicationswires and cables. It is ductile, has suitable tensile strength and is a verysatisfactory conductor. As a conductor it is second only to silver and ifthe conductivity of silver is 100 units then pure copper would measure 97units. Electrolytic tough pitch copper is used for wires and this grade ofcopper has a minimum copper content of 99.9 per cent with around 0.04per cent of oxygen in the form of an oxide. This level of purity isessential as the introduction of some alloying elements or impurities cangreatly reduce conductivity. For example only 0.04 per cent phosphoruswill reduce the conductivity by 25 per cent. Other alloying elements, likecadmium, have little effect on the conductivity. The presence ofcadmium, dissolved in the copper, increases both the strength and wearresistance of the transmission cable, so it is actually a favourable alloy inthis application.

The manufacturing process used to produce copper wires could easilyinduce stress and reduce the conductivity. To overcome this problem,the cables are cold drawn into wire then the roll of wire is fully annealed.


Copper is also an essential part of coaxial cables that are still used forsome applications in telecommunications.

Copper braid

Solid copper

Polymer skinPolymer layer

Figure 3.6 The structure of coaxial cable

In previous modules you looked at some of the alloys of copper. Some ofthese alloys have properties that make them suitable for use intelecommunications devices.

Name some of these alloys, state the alloying element/s and suggest atleast one use for each.

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Did you answer?

Did you suggest sheet cartridge brass (copper with 30 per cent zinc) that couldbe used as contacts and cartridge brass cold formed screws and rivets. Evenbronzes (copper with up to 11 per cent tin) could be used where extra strength isneeded. Non-corroding nuts and bolts could be made in bronze.

Aluminium

Aluminium has three advantages over copper when used as conductingwires. It is lighter, less expensive and more abundant in nature thancopper. With a density of only 2.7g/cm3, compared to 9g/cm3 for copper,aluminium is specially suitable for aerial power transmission cables.Only half the quantity of aluminum, by weight, is needed for conductorswith the same resistance. However, it does not conduct as well as copper(only about 60 per cent of the conductivity of copper) so larger diametercables are needed. The larger amount of insulation sheathing neededoffsets some of the savings made on the conductor material.


On the other hand, aluminium has some inferior properties to those ofcopper. These include marginally poorer ductility, tensile strength,jointing properties and corrosion resistance. This fact has retardedaluminium’s general use in communication cables.

Aluminium alloys are sometimes used for cables. A common alloycontains 0.5 per cent iron and 0.5 per cent cobalt. These alloyingelements distort the normal aluminium structure and while this increasesthe strength of the cable, the conductivity is reduced.

Gold

The conductivity of gold is around that of copper and it is used for thelinkage ‘wires’ in some semiconductor devices. It is suitable for thisapplication because while it is very expensive, only small quantities areused in these miniature circuits. The gold is ductile, doesn’t oxidise andbonds easily to other metals such as aluminium and copper.

Lead

The outer layer on telecommunications cables is known as the sheath andis designed to create a stable environment for the cable core. Lead wasonce used extensively as it has good corrosion resistance, adequatestrength and flexibility and is easy to join. It has been replaced withpolymers because lead suffers from fatigue failures, is heavy and isrelatively expensive. Lead alloys containing antimony and tin were usedto reduce fatigue failures.

Turn to the exercise sheet and complete exercise 3.2.


Ceramics as insulation materials

From the information available in previous modules, define a ceramic andexplain why ceramics are often used as electrical insulators.

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Did you answer?

Did you mention that ceramics contain both metal and non-metal phases? Didyou also discuss that they often contain both ionic and covalent bonds and thatboth these types of primary bonds do not have free valence electrons to allowfor the ’flow’ of electrons?

In insulating materials, there is a large gap between the full valence bandand the next electron energy level. For an electron to be free to transmita current, it must move up to this next energy level. Under normalconditions, the gap is so large that electrons are unable to cross.

At high temperatures there is a greater chance that an occasional electronwill possess the energy needed to cross the gap and allow someconduction.

In ionically bonded materials, ions may migrate, rather than electrons.This will provide a small degree of conductivity. At elevatedtemperatures, ions can become more mobile and conductivity mayincrease.

Very high voltages may cause the break-down of some insulators. Thisoccurs because the electric field is sufficient to raise the energy of someelectrons and ‘free’ them across the gap allowing electron flow.

Surface breakdown is more common and the presence of moisture oraccumulation of dirt may allow conduction. The glazing of ceramicinsulators helps eliminate moisture because water runs off easily. It alsois less susceptible to dirt build up because it is smooth. The use of acorrugated design greatly increases the length that the current musttravel.


Semiconductors

Some materials are known as semiconductors because the gap betweenthe filled valence band and the empty conduction band is relatively small.Conduction can occur through two mechanisms. Heating for intrinsicsemiconductors, and doping in extrinsic semiconductors.

Intrinsic semiconductors

Silicon and germanium are semiconductors due solely to the distributionof electron energies within the pure material. When one valence electronis freed to cross the energy gap it will mean that one atom within thecrystal lattice only has three bonds as shown in figure 3.7. This gap isknown as an electron hole. The freed bonding electrons are constantlymoving and can even switch from one atom to another. This movementof the electron in one direction means that the hole ‘moves’ in theopposite direction. This could be considered as a positively chargedcarrier. Both these movements allow the material to conduct.

Heat may be used to provide the initial energy to free the electron. So, incontrast to metals, increasing the temperature of an intrinsicsemiconductor will increase conductivity.

An electron is freed froma covalent bond creatingan electron hole at A.

Electron transfer to A fromadjacent site,B. Holeeffectively moves to B.

Electron transfer from Cto B. Hole moves to C.

A A

B B C

Figure 3.7 Electrical conduction by the movement of ‘holes’

Extrinsic semiconductors

Silicon and germanium have four outer shell electrons per atom but if an‘impurity’ element, that only has three outer electrons is introduced, therewill be electron holes left in the lattice structure. Conduction due to theseholes can occur, and the majority carriers in this type of semiconductor,are these positive electron holes. Aluminium in silicon is an example ofthis type that is commonly known as a p-type semiconductor (p- for


positive). These ‘dope’ atoms are introduced in the ratio of around oneatom to a million base material atoms.

Alternatively, if an element like phosphorus, that has five electrons in itsouter shell, is added to the silicon structure there will be an extra electronfor each phosphorus atom added. Only four of the electrons are bondedto both the phosphorus and the silicon so the fifth valence electron caneasily move in the conduction energy band and allow conduction to takeplace. The electrons are the majority carriers in this type ofsemiconductor, so it is known as an n-type semiconductor.


The p-n junction

When a piece of n-type semiconductor is joined to a piece of p-typesemiconductor a type of ‘one way’ valve results. The normal method isto introduce p-type and n-type impurities into opposite ends of a crystalof silicon or germanium. At the junction of the two types of materials,the positive holes in the p-type are filled with electrons from the n-type.In this region the p-type atoms have gained an electron and arenegatively charged and the n-type atoms have lost an electron andbecome a positive ion. This ‘depleted’ zone has a positive charge on oneside and a negative charge on the other.

When a voltage is applied across the component containing the p-njunction, it will either conduct or insulate. If the p-type end is madepositive compared to the n-type end then the current will flow easily. Ifthe voltage is reversed, the positive holes and electrons are attractedaway from the depleted layer and it becomes very hard for chargedparticles to move across the junction.

Forward bias Reverse bias

Depletion layer

Figure 3.8 A p-n junction exposed to a voltage


This simple type of semiconductor device is known as a diode. Whenthree layers of semiconductor material are combined, npn or pnp, atransistor is formed. Now you will have an idea of how they work.

These semiconductor devices form the basis of the integrated circuits that‘drive’ the modern telecommunications industry. These devices aremade from wafer thin layers of pure silicon into which the manyindividual microelectronic circuits are formed. This ‘chip’ is thenpackaged so that it can be fitted into a printed circuit board and used indifferent electronic applications.

Polymers as insulation materials

Drawing on knowledge and understanding that you gained in previousmodules, briefly explain why polymers are insulators. You should referto the type of primary bonds found in polymers.

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Did you answer?

Did you talk about the covalent bonds normally found in polymers and the factthat all the valence electrons are involved in the bond and are therefore not freeto ‘transmit’ electrical ‘flow’?

Cl + Cl = Cl2

Nucleus

Electron

Figure 3.9 Simple representation of the covalent bond

Many of the insulating materials in personal telecommunication devicesare made from polymers. They are subject to low voltages and lowtemperatures and are therefore quite suitable for these applications.


If you can find an old broken telephone, pull it apart! If you don’t havean old phone, look at the one in your home and answer the followingactivity.

Suggest those parts of the telephone that are made from polymer.Indicate with an ‘I’ those parts that must be insulators.

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Did you answer?

The body, buttons/dial, receiver are all moulded in polymer. They couldpossibly be high impact polystyrene which is a copolymer of polystyrene andthe rubbery polymer, polybutadiene. It doesn’t break when you drop it on thefloor! Other tough polymers that would be used for telephone bodies are ABS(acrylonitrile butadiene styrene) and polycarbonate.

The printed circuit boards (epoxy resin), wire insulation (polyethylene),integrated circuit bodies (polyurethane) and transistor bodies are all polymer sothat they insulate.

In telecommunication cables, an insulating layer covers the surface of theconductor material. Traditionally, paper was used to insulatetelecommunication cables and while it has high insulation resistance, if itgets wet, immediate and complete failure usually results. Paper containsa high proportion of the polymer, cellulose. Various polymers arecurrently used in place of the traditional paper.

Polyethylene

Polyethylene has superior insulation resistance to paper, is suitable forhigh frequency cables, can be accurately made to size in a variety ofcolours, has good jointing properties and maintains good electricalproperties under humid conditions. Its main disadvantages are cost andlow softening temperature.

When used as an outer sheathing on groups of cables, polyethyleneallows water vapour to penetrate and is difficult to join. For thesereasons it is only used for interior cables or as the outer layer on sheathswith a wound aluminium foil inner and polyethylene outer.


Polyvinyl chloride

Polyvinyl chloride (PVC) has poorer electrical properties than eitherpaper or polyethylene but is tougher, withstands higher temperatures andsurvives better in a fire. Under extreme temperatures and combustion,hydrogen chloride fumes are liberated and may cause corrosionproblems. It is a suitable alternative to polyethyelene.

Polypropylene

Polypropylene has similar electrical properties to polyethylene but istougher and has a higher softening temperature. It is not as flexible andis more expensive than either PVC or polyethylene.

Nylon

Nylon is often used as an insect resistant outer layer or sheath on cablesthat are used underground. The hard, smooth surface of the nylon makesit difficult for an insect or termite to grip the cable.



Fibre-optics

Light has been used throughout history to convey messages over longdistances.

Identify historical long-distance communication methods that have usedlight.

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Did you answer?

Did you suggest bonfires and mirrors (using the sun)? What about smokesignals?

History

Up until the 1840s, both bonfires and mirrors were used to relaymessages from one hilltop to the next. The electric telegraph quicklyreplaced these simple ‘light’ methods as the wires carried the messageregardless of the weather or the terrain.

Light travels very fast, around 300 000 kilometres per second, and it haslong been known that the shorter the wavelength, the more information awave could carry. Light waves are only millimetres to nanometres longand can carry a huge amount of information. Early experiments sawlasers being fired between towers but fog or rain blocked the messageand it quickly became obvious that the light beam should be guidedthrough a cable or pipe. Optical fibres were chosen for this purpose.

Typical optical fibres are very fine fibres of glass – ‘hairs’ made of puresilica. The method of manufacturing optical fibres had been patentedback in the 1930s ‘just in case someone ever finds a use for it.’ Initiallyit was difficult to keep the transmitted light inside the glass fibre buteventually the glass core was enclosed in a glass sleeve or cladding. Thecladding has a different refractive index to the core and causes the lightenergy to be reflected back off the core-cladding interface. This totalinternal reflection means that all the light is reflected and continues tozig-zag along the core of the fibre.

The optical fibres guide the light beam so wherever the fibre goes, thelight follows. These fibres can be made to make the light bend aroundcorners. Materials used for optical fibres must:

• be able to be formed into long thin structures

• be flexible enough to go around bends


• allow light to travel through them and so need to be transparent.

Only silica glass and some polymers have these properties.

Core

Buffer coating

Cladding

Figure 3.10 The structure of fibre-optic cable

The light source

The ‘light’ used in fibre-optic systems is either at or just beyond the redend of the visible light spectrum. This length of wave is less susceptibleto attenuation in the glass. The light is generated by a littlesemiconductor laser (Light Amplification by the Stimulated Emission ofRadiation) made from gallium, aluminium and arsenic. This deviceproduces a stream of electromagnetic radiation, light, at a constantfrequency. The pulses generated in the laser by the transmitter are sentdown the glass fibre and converted back to electrical impulses by thereceiver.

Movement of light in the fibre

As light beams move down the core of the glass fibre they bounce fromside to side. As long as they only hit the junction between the core andthe cladding at a low angle the total energy of the light rays is reflectedback into the core and none escapes into the cladding. The rays bounceto the other side and again, as long as the angle is low, bounce back andcontinue to be transmitted to the end of the fibre.

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