Copper and other metals used in telecommunications

Transcription

1 Copper and other metals used in telecommunications List the mechanical and physical properties that would be important in telecommunications conductors. 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 of material that is needed to carry current. It also means there are less insulating materials needed because the wires are thinner, sheathing costs are 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 during manufacture, extrusion of the insulation and the installation of the cable. Joining the conductors may be achieved through twisting, soldering or welding. Some materials are easier to join than others! In modules that you studied during the preliminary HSC course, you looked at the structure and atomic bonding of materials. Using this knowledge, explain why metals are normally conductors and why copper is an excellent conductor of electricity. Did you answer? Did you discuss the metallic bond that has the valence electrons in a cloud surrounding the ions and that conduction is due to the migration of these electrons? Did you mention how these free electrons easily transmit the flow of current? 8 Telecommunications

2 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 further development in wave theories has allowed a much clearer understanding of conductivity. While the individual valence electrons are involved in the movement of a current, the current moves in the form of a wave and these waves will move much more easily through a regular arrangement of obstacles. The regular arrangement of ions in the crystal lattice structure of an annealed metal, such as the face centred cubic arrangement of copper, provides little resistance to the passage of the current waves. Any amount of cold working or the introduction of alloying elements that sit in the spaces between the ions will increase the random nature of the obstacles and will increase the resistance of the material. Heating will cause the ions to vibrate and will increase the possibility of the migrating electrons hitting an ion and thus being slowed down. This explains the increase in resistivity noticed when the temperature of a conductor is raised. Copper Copper is the metal that has been traditionally used for communications wires and cables. It is ductile, has suitable tensile strength and is a very satisfactory conductor. As a conductor it is second only to silver and if the conductivity of silver is 100 units then pure copper would measure 97 units. Electrolytic tough pitch copper is used for wires and this grade of copper has a minimum copper content of 99.9 per cent with around 0.04 per cent of oxygen in the form of an oxide. This level of purity is essential as the introduction of some alloying elements or impurities can greatly reduce conductivity. For example only 0.04 per cent phosphorus will reduce the conductivity by 25 per cent. Other alloying elements, like cadmium, have little effect on the conductivity. The presence of cadmium, dissolved in the copper, increases both the strength and wear resistance of the transmission cable, so it is actually a favourable alloy in this application. The manufacturing process used to produce copper wires could easily induce stress and reduce the conductivity. To overcome this problem, the cables are cold drawn into wire then the roll of wire is fully annealed. Part 3: Telecommunications engineering materials 9

3 Copper is also an essential part of coaxial cables that are still used for some applications in telecommunications. Copper braid Solid copper Polymer skin Polymer layer Figure 3.6 The structure of coaxial cable In previous modules you looked at some of the alloys of copper. Some of these alloys have properties that make them suitable for use in telecommunications devices. Name some of these alloys, state the alloying element/s and suggest at least one use for each. Did you answer? Did you suggest sheet cartridge brass (copper with 30 per cent zinc) that could be used as contacts and cartridge brass cold formed screws and rivets. Even bronzes (copper with up to 11 per cent tin) could be used where extra strength is needed. Non-corroding nuts and bolts could be made in bronze. Aluminium Aluminium has three advantages over copper when used as conducting wires. It is lighter, less expensive and more abundant in nature than copper. With a density of only 2.7g/cm 3, compared to 9g/cm 3 for copper, aluminium is specially suitable for aerial power transmission cables. Only half the quantity of aluminum, by weight, is needed for conductors with the same resistance. However, it does not conduct as well as copper (only about 60 per cent of the conductivity of copper) so larger diameter cables are needed. The larger amount of insulation sheathing needed offsets some of the savings made on the conductor material. 10 Telecommunications

4 On the other hand, aluminium has some inferior properties to those of copper. These include marginally poorer ductility, tensile strength, jointing properties and corrosion resistance. This fact has retarded aluminium s general use in communication cables. Aluminium alloys are sometimes used for cables. A common alloy contains 0.5 per cent iron and 0.5 per cent cobalt. These alloying elements distort the normal aluminium structure and while this increases the strength of the cable, the conductivity is reduced. Gold The conductivity of gold is around that of copper and it is used for the linkage wires in some semiconductor devices. It is suitable for this application because while it is very expensive, only small quantities are used in these miniature circuits. The gold is ductile, doesn t oxidise and bonds easily to other metals such as aluminium and copper. Lead The outer layer on telecommunications cables is known as the sheath and is designed to create a stable environment for the cable core. Lead was once used extensively as it has good corrosion resistance, adequate strength and flexibility and is easy to join. It has been replaced with polymers because lead suffers from fatigue failures, is heavy and is relatively expensive. Lead alloys containing antimony and tin were used to reduce fatigue failures. Turn to the exercise sheet and complete exercise 3.2. Part 3: Telecommunications engineering materials 11

5 Ceramics as insulation materials From the information available in previous modules, define a ceramic and explain why ceramics are often used as electrical insulators. Did you answer? Did you mention that ceramics contain both metal and non-metal phases? Did you also discuss that they often contain both ionic and covalent bonds and that both these types of primary bonds do not have free valence electrons to allow for the flow of electrons? In insulating materials, there is a large gap between the full valence band and the next electron energy level. For an electron to be free to transmit a current, it must move up to this next energy level. Under normal conditions, the gap is so large that electrons are unable to cross. At high temperatures there is a greater chance that an occasional electron will possess the energy needed to cross the gap and allow some conduction. In ionically bonded materials, ions may migrate, rather than electrons. This will provide a small degree of conductivity. At elevated temperatures, ions can become more mobile and conductivity may increase. Very high voltages may cause the break-down of some insulators. This occurs because the electric field is sufficient to raise the energy of some electrons and free them across the gap allowing electron flow. Surface breakdown is more common and the presence of moisture or accumulation of dirt may allow conduction. The glazing of ceramic insulators helps eliminate moisture because water runs off easily. It also is less susceptible to dirt build up because it is smooth. The use of a corrugated design greatly increases the length that the current must travel. 12 Telecommunications

6 Semiconductors Some materials are known as semiconductors because the gap between the filled valence band and the empty conduction band is relatively small. Conduction can occur through two mechanisms. Heating for intrinsic semiconductors, and doping in extrinsic semiconductors. Intrinsic semiconductors Silicon and germanium are semiconductors due solely to the distribution of electron energies within the pure material. When one valence electron is freed to cross the energy gap it will mean that one atom within the crystal lattice only has three bonds as shown in figure 3.7. This gap is known as an electron hole. The freed bonding electrons are constantly moving and can even switch from one atom to another. This movement of the electron in one direction means that the hole moves in the opposite direction. This could be considered as a positively charged carrier. Both these movements allow the material to conduct. Heat may be used to provide the initial energy to free the electron. So, in contrast to metals, increasing the temperature of an intrinsic semiconductor will increase conductivity. B B C A A An electron is freed from a covalent bond creating an electron hole at A. Electron transfer to A from adjacent site,b. Hole effectively moves to B. Electron transfer from C to B. Hole moves to 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, there will be electron holes left in the lattice structure. Conduction due to these holes can occur, and the majority carriers in this type of semiconductor, are these positive electron holes. Aluminium in silicon is an example of this type that is commonly known as a p-type semiconductor (p- for Part 3: Telecommunications engineering materials 13

7 positive). These dope atoms are introduced in the ratio of around one atom to a million base material atoms. Alternatively, if an element like phosphorus, that has five electrons in its outer shell, is added to the silicon structure there will be an extra electron for each phosphorus atom added. Only four of the electrons are bonded to both the phosphorus and the silicon so the fifth valence electron can easily move in the conduction energy band and allow conduction to take place. The electrons are the majority carriers in this type of semiconductor, so it is known as an n-type semiconductor. Turn to the exercise sheet and complete exercise 3.3. The p-n junction When a piece of n-type semiconductor is joined to a piece of p-type semiconductor a type of one way valve results. The normal method is to introduce p-type and n-type impurities into opposite ends of a crystal of 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 are negatively charged and the n-type atoms have lost an electron and become a positive ion. This depleted zone has a positive charge on one side and a negative charge on the other. When a voltage is applied across the component containing the p-n junction, it will either conduct or insulate. If the p-type end is made positive compared to the n-type end then the current will flow easily. If the voltage is reversed, the positive holes and electrons are attracted away from the depleted layer and it becomes very hard for charged particles to move across the junction. Depletion layer Forward bias Reverse bias Figure 3.8 A p-n junction exposed to a voltage 14 Telecommunications

8 This simple type of semiconductor device is known as a diode. When three layers of semiconductor material are combined, npn or pnp, a transistor 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 are made from wafer thin layers of pure silicon into which the many individual microelectronic circuits are formed. This chip is then packaged so that it can be fitted into a printed circuit board and used in different electronic applications. Polymers as insulation materials Drawing on knowledge and understanding that you gained in previous modules, briefly explain why polymers are insulators. You should refer to the type of primary bonds found in polymers. Did you answer? Did you talk about the covalent bonds normally found in polymers and the fact that all the valence electrons are involved in the bond and are therefore not free to transmit electrical flow? Nucleus Electron Figure 3.9 Cl + Cl = Cl 2 Simple representation of the covalent bond Many of the insulating materials in personal telecommunication devices are made from polymers. They are subject to low voltages and low temperatures and are therefore quite suitable for these applications. Part 3: Telecommunications engineering materials 15

9 If you can find an old broken telephone, pull it apart! If you don t have an old phone, look at the one in your home and answer the following activity. Suggest those parts of the telephone that are made from polymer. Indicate with an I those parts that must be insulators. Did you answer? The body, buttons/dial, receiver are all moulded in polymer. They could possibly be high impact polystyrene which is a copolymer of polystyrene and the rubbery polymer, polybutadiene. It doesn t break when you drop it on the floor! 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 so that they insulate. In telecommunication cables, an insulating layer covers the surface of the conductor material. Traditionally, paper was used to insulate telecommunication cables and while it has high insulation resistance, if it gets wet, immediate and complete failure usually results. Paper contains a high proportion of the polymer, cellulose. Various polymers are currently used in place of the traditional paper. Polyethylene Polyethylene has superior insulation resistance to paper, is suitable for high frequency cables, can be accurately made to size in a variety of colours, has good jointing properties and maintains good electrical properties under humid conditions. Its main disadvantages are cost and low softening temperature. When used as an outer sheathing on groups of cables, polyethylene allows water vapour to penetrate and is difficult to join. For these reasons it is only used for interior cables or as the outer layer on sheaths with a wound aluminium foil inner and polyethylene outer. 16 Telecommunications

10 Polyvinyl chloride Polyvinyl chloride (PVC) has poorer electrical properties than either paper or polyethylene but is tougher, withstands higher temperatures and survives better in a fire. Under extreme temperatures and combustion, hydrogen chloride fumes are liberated and may cause corrosion problems. It is a suitable alternative to polyethyelene. Polypropylene Polypropylene has similar electrical properties to polyethylene but is tougher and has a higher softening temperature. It is not as flexible and is more expensive than either PVC or polyethylene. Nylon Nylon is often used as an insect resistant outer layer or sheath on cables that are used underground. The hard, smooth surface of the nylon makes it difficult for an insect or termite to grip the cable. Turn to the exercise sheet and complete exercise 3.4. Part 3: Telecommunications engineering materials 17

11 Fibre-optics Light has been used throughout history to convey messages over long distances. Identify historical long-distance communication methods that have used light. Did you answer? Did you suggest bonfires and mirrors (using the sun)? What about smoke signals? History Up until the 1840s, both bonfires and mirrors were used to relay messages from one hilltop to the next. The electric telegraph quickly replaced these simple light methods as the wires carried the message regardless of the weather or the terrain. Light travels very fast, around kilometres per second, and it has long been known that the shorter the wavelength, the more information a wave could carry. Light waves are only millimetres to nanometres long and can carry a huge amount of information. Early experiments saw lasers being fired between towers but fog or rain blocked the message and it quickly became obvious that the light beam should be guided through a cable or pipe. Optical fibres were chosen for this purpose. Typical optical fibres are very fine fibres of glass hairs made of pure silica. The method of manufacturing optical fibres had been patented back in the 1930s just in case someone ever finds a use for it. Initially it was difficult to keep the transmitted light inside the glass fibre but eventually the glass core was enclosed in a glass sleeve or cladding. The cladding has a different refractive index to the core and causes the light energy to be reflected back off the core-cladding interface. This total internal reflection means that all the light is reflected and continues to zig-zag along the core of the fibre. The optical fibres guide the light beam so wherever the fibre goes, the light follows. These fibres can be made to make the light bend around corners. Materials used for optical fibres must: be able to be formed into long thin structures be flexible enough to go around bends 18 Telecommunications

12 allow light to travel through them and so need to be transparent. Only silica glass and some polymers have these properties. Buffer coating Core 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 red end of the visible light spectrum. This length of wave is less susceptible to attenuation in the glass. The light is generated by a little semiconductor laser (Light Amplification by the Stimulated Emission of Radiation) made from gallium, aluminium and arsenic. This device produces a stream of electromagnetic radiation, light, at a constant frequency. The pulses generated in the laser by the transmitter are sent down the glass fibre and converted back to electrical impulses by the receiver. Movement of light in the fibre As light beams move down the core of the glass fibre they bounce from side to side. As long as they only hit the junction between the core and the cladding at a low angle the total energy of the light rays is reflected back into the core and none escapes into the cladding. The rays bounce to the other side and again, as long as the angle is low, bounce back and continue to be transmitted to the end of the fibre. Part 3: Telecommunications engineering materials 19