No. 76 FULL SOR HakfP Britainp's Best Selling Electronics Magazine ;AO sOtt,704ct; ovtI' C0010,1,11 isvoosts Discover the Secrets of How Semiconductors are Made IJM if)!Jf rj'i111 L.11111 .i?;_,;.f,-)1Y.:,-1` usive! Report Fuzzy Log eep Food Safe a Fridge Chec ersatile Video P mplifier for Yo Test Car Circuits "th a Simple Test New Series: Ho to Use Digital Pan N- N- The Maplin magazine binder holds twelve issues of Electronics - The Maplin Magazine. If you have not got the world of electronics covered, Double Tops now's your chance! This great quality binder comes in hard- wearing laminated art board, that not only looks good on your bookshelf or workshop desk, but is also a great way to build up an electronics reference library of your own! Now you can keep all of your copies of Electronics - The Maplin Magazine, together and on hand, for when you need that extra piece of information. To order your binder simply fill in the order coupon provided in this issue, or call the Credit Card Hotline on 0702 554161, or visit your local Maplin Store. Only £5.95. (Mail Order, please add £1.40 Handling charge.) Order code: XH99H. 4. *.a.' WIN170171 * oh MO iNlC ANMIP 7PVO 1O7M.41M11I- -, .1rerool J....TS/SR THE NEW MAPLIN MAGAZINE BINDER... A GREAT WAY TO GET THE WORLD OF ELECTRONICS COVERED! IN THIS !SS APRIL 1994 VOL 13. No. 76 11 THE INTELLIGENT PROJECTS FEATURES FOR YOU TO BUILD! ESSENTIAL READING! DATA NETWORK Tomorrow's technology or today's reality? FRIDGE CHECK THE SECRETS OF Can sensors and systems be configured 12 4 Guard against food poisoning with this SUPERCONDUCTIVITY to communicate intelligently? Stephen low-cost project. Once built and left in a EXPLAINED Waddington shows that they can, and fridge it will alert you to any excessive rise From floating magnets and particle reveals what local operating network in temperature - a condition that will, detectors to levitating trains and medical technology has to offer. unnoticed, promote bacteria growth in body scanners, superconductors are the food. magic ingredient. In this revealing article DESIGNING AND BUILDING Douglas Clarkson looks are what SUB -WOOFER SYSTEMS LOW NOISE HIGH superconductors are, how they were John Woodgate describes the processes FREQUENCY PREAMPLIFIER 28 discovered, the current state of the art, involved in designing and building high This Data File project based on the SL561 practical uses and what developments are performance sub -woofer systems - low noise preamplifier IC is ideal for video ahead. particularly with in -car and domestic applications. applications in mind. LATEST DEVELOPMENTS 20m CW TRANSMITTER IN FUZZY LOGIC 19 POWER ELECTRONICS IN & DIRECT CONVERSION Fuzzy logic, a comparatively new THEORY AND IN PRACTICE RECEIVER technology, is finding many applications Practical circuits, based on the silicon Get into amateur radio cheaply and easily in both domestic and industrial equipment. controlled rectifier, are presented in the with these two projects. Both can be This exclusive feature reveals some latest instalment of this informative series. exciting news about the latest combined in one case to form a single versatile unit. developments in this field. REGULARS NOT TO BE MISSED! SIMPLE CAR CIRCUIT TESTER HOW SEMICONDUCTORS Find car electrical faults fast with this JQ ARE MADE AISBSOUUET THIS HSUOBWS TCORIBE 17 CADVRTSED 53 low-cost. easy to build, project. aEnvde ra w 4o0n1d1e NreAdN hDo wga ate B sCta1r0te8d t rliafen,s wisetollr TwAETCCHHOLNOGY OCORDUEPRON 18 "kg-m 66 here's your chance to find out! DIARY n NEW 3 DATES BOOKS SSTIGRNAAYLS 67 WHAT ARE SCANNERS? in READERS' DID YOU Ian Poole explains what scanning radio REPORT IV LETTERS 42 MISS? 71 receivers are and what they can be used for. He also reviews a typical modern hand-held scanner. FIBRE DISTRIBUTED DATA INTERFACE EXPLAINED The future of computer communications or just another standard that nobody wants? Frank Booty answers these questions and more. HOW TO USE DIGITAL 37 PANEL METERS Ray Marston demonstrates how digital panel meters can be used in practical applications. Tried and tested circuits are included covering a multitude of possible uses. April 1994 Maplin Magazine 1 ABOUT THIS ISSUE... Hello and welcome to this month's issue of Electronics! powered the equipment through its end stops! An All of these safety critical systems rely on the correct 'A chain is only as strong as the weakest link...' This expensive and potentially dangerous design oversight. response being preprogrammed; questions arise as to saying can be applied to great many systems, processes What should of happened when the servo electronics lost how the system will respond to an event that has not been or methods encountered in every day life; it is especially one of its inputs was that the system failed sale- halting thought of. To be able cover for an almost infinite range true in the world of electronics and the related field of the motors in a predetermined way. of combinations of possible problems occurring in real-time computing. When designing systems to be fault or error Safety critical systems such as those used in power is very difficult and is helped to some extent by using tolerant, it is necessary to consider worst case scenarios, stations, chemical plants, aircraft, train signalling systems knowledge based systems programmed with sets of however, it can be a great deal harder to think of all the usually adopt the principle of parallel redundancy, that 'rules'. These rules include a means of prioritising events possible 'what if' cases.... is the vital parts of a system are duplicated. Critical according to urgency. Testing such systems is even This can be particularly true of software based information is gathered by two sensors and the information harder, simulation systems are used prior to equipment systems which expect a particular input or a range of input carried by two separate cables following different physical going into final service. But what happens if the computer values. Typical simple examples include 'menu' options on routes - so if one sensor or its cable fails, the other can makes the wrong decision? ... Humans, although fallible, a computer program, e.g. 'Press A, B or C'. what happens still provide information. Parallel redundant processing generally cope better with unforseen circumstances. Faced if the user types 'X' or a lower-case letter instead of an systems have a supervisor system that monitors the with serious life threatening problems, humans deal with upper case letter? Good programmers ensure that such output from the parallel units (usually three). Under normal the most pressing matters first, temporarily ignoring the inputs are properly filtered, converted or error trapped. For circumstances the outputs from all of the processors unimportant ones (a rapidly approaching mountain is more example, inputs outside the range A to C are completely match. If however, a discrepancy occurs the supervisor important than landing gear that won't retract). Faced with ign red and the program waits for a valid entry, or takes the majority vote of the two processors that agree choices, humans act with some degree of reason and rule alternatively a suitable error message flashes on screen as being correct, shuts down the rogue processor, flags out the obviously incorrect options, the computer might just together with a 'beep' to alert the user to the invalid entry. the failure (sirens and lights) and maintains correct system metaphorically shrug its shoulders, or flash up 'call service 'User friendly enhancements to the program are desirable operation. The system can then be safely shut down and engineer'. I'm not advocating such systems are unsafe - and include the program being written to accept both repaired. there are official bodies to approve such systems - but uppercase and lower case letters as being equally valid. In some modem cars, the accelerator pedal operates it is worth thinking about the issues involved. It is also necessary to consider the deliberate wrong a potentiometer or a shaft encoder. The signal produced Perhaps in the future, when buying a car or boarding entry, by a user trying to outsmart the programmer, with is interpreted by the engine management system that a plane, we may adopt a similar maxim to that of computer combinations of 'system' keys such 'Ctrl' and 'Alf. Less controls the amounts of air admitted to, and fuel injected software purchasers - never trust software that is version easy to foresee scenarios include, the internal office mail into, the engine. The days of a direct cable linkage to a X.0, especially if it's version 1.0 - there's bound to be being placed on the keyboard and causing the keyboard carburettor are fast diminishing - at least if the cable snaps, some bugs! buffer to overrun. Poor programmers don't consider a retum spring is there to back -off the revs! If you have So until next month's issue I hope that you enjoy matters of this nature as being 'their problem'; programs a modem car, what would happen if the sensor or its cable reading this issue as much as the 'team' and I have then behave unexpectedly or lock up. went faulty? In theory it should fail-safe, but will it? enjoyed putting it together for you! A system I encountered some years ago employed In recent years, aircraft manufacturers have been the some very complex processor -based servo electronics, subject of criticism over fly -by -wire aircraft, in such an for rapidly and accurately positioning a piece of equipment. aircraft, the pilot's control column and various other levers To move the equipment swiftly, powerful motors were and pedals operate switches and positional sensors. The ABC needed; positional information was provided by high information is read by computer and passed onto the resolution shaft encoders. Most of the time it performed control surfaces (rudder, ailerons, etc.) and engines by UPI WNW CO 0.14,11.3 its task admirably. However, when it did fail, it really did data cables. In older aircraft the control surfaces were CONSUMER PRESS FAIL! What the system designer had not considered in his physically connected by steel cables, linkages, etc., and Front Cover Pictures: theoretical model, on which the physical system was based assisted by hydraulics. Fly -by -wire systems have been .© Copyright 1994 Pictor (perhaps because he had not been taught to consider blamed, in part, as possibly contributing to plane crashes. International, and Tony Stone it at university), was the chance of an input variable Pilots have reported CRT screens, on which graphical Images. Virtually, every aspect disappearing! In this instance, the flexible wire that carried representations of traditional instruments are displayed, of our lives has been affected by semiconductor devices one the positional information became open circuit. When this going blank at critical periods. Bugs in computer software way or another, the technvivyy happened, the servo system got a little confused, applied at this level are a little more serious than bugs in a word is new, but the raw material is full power to the motors, ignored the limit switches and processing program. as old as the earth. EDITORIAL Copyright 1994 Maplin Electronics plc. happily issue you with one. Payment can be made by credit card. Editor Robert Ball AMIPRE Copyright: Al matenal IS subject to worldwide copyright protection, and II you have a tone dal (DTMF) telephone or a pocket tone dialler you reproduction or imitation in whole or part is expressly forbidden. Permission to can arss our computer system and place orders directly onto the Maplin Technical Authors Robin Hall FBIS, G4DVJ, reproduce pnnted circuit board layouts commercially or. marketing of kits must computer 24 hours a day by simply dialing (0702) 556751 You will need Mike Holmes be sought from the publisher. a Maple customer number and a personal identification number (PIN) to Print Co-ordinators John Craddock, Mike Brett Advertleaments: Whist every reasonable precaution is undertaken to protect access the system. Ifyoudo not have a customer number or a PIN number News Editor Stephen Waddington BEng (I -ions) the interests d readers by ensuring, as tar as possible, that advertisements Tel: (0702) 552911 and we call happy issue you vat one. Technical Illustrators Ross Nisbet, astpapffe oafn tnhge inm athgea zciunrere cnat nisnsout eg iovfe E alencyt ruonndicesrt aakrein bgosn ina rfiedsep, etchte o pf usbtalistehmere anntsd Fcuartr deentta Milsa pofli na tC oaf ttahleo gm..e.thods of ordering from Maplin can be found in the Lesley Foster, Paul Evans, Nicola Hull or dans made by advertisers. whether on printed page or on loose insert. Project Development Chris Barlow, Readers who have reasonable grounds to believe that they have been misled Prices are advised to contact they local Trading Standards Office. 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CashTer supports 300-, &dronesand suggestions for ddezts, ledges, seem et Due to the sheer United Magazine Distribution Ltd., 1204 and 2400 -baud MODEMS using CCITT tales The format S 8 data bits, 1 vciume of letters received, we are unfortunately unable to reply to every Idler, Caste House, 37-45 Paul Street, London EC2A 4PB. stop bit, no panty, ful duplex with XoriXorf hardshalung. NI evrstng customers however. every letter is read -your tem and conch is gasify appreciated. Leen Tel: (071) 490 2020. Fax: (071) 490 1239. w55it2h9 a4 1M. aIfp ylionu c duos tnoomt ehra nveu ma bceurs ctoamn earc ncuemssb ethr eT seyl:s (t0e7m0 2b)y 5 s5im29p1ly1 daantdin wge ( 0w7il0l2) oMfy p caartriecsuplaorn indteenrcees tr aidn Wd aegnrdiferadn fooxr pmaaliyc abteio np umbulissht ebde adte tahrely Emdaitrsk eddis carse tsiounch.. 2 Maplin Magazine April 1994 TECHI1OLOGY = Cold Christmas Turkey computer. While - yes - they have the same at all? For that matter, what's the point of a components inside, and - yes - they have a real world? In the pre -Christmas run-up last year. Sega monitor (or at least a TV screen) in all but a and Nintendo spent some £20 million or so in couple of notable cases they don't do any Turning around the UK alone on advertising. Now while that standard computer tasks at all. What do you do when you finish that cool, sounds a lot (and indeed, it would pay my Like computers themselves, on the other thirst -quenching (aluminium) can of cola? wife's Visa card bill for a month or two) you have to take it in context of the products - that hand, their capabilities all mirror the trends What do you do when you've emptied a is games consoles and games themselves - inherent to the microprocessing hardware (glass) milk -bottle? How do you get rid of that which the companies were likely to take. They they are built around. In the beginning were pile of old (paper) newspapers? When the were, in fact, each aiming for the lion's share the table tennis -type games. They very soon (CFC-filled) fridge finally gives up its ghost, of something around half a billion pounds - evolved to 8 -bit microprocessor -based games, what do you do with it? The answer to all worth of business - and in this respect which in turn have given way to the 16 -bit these questions - or should be; as any £20 million is peanuts. Around four percent microprocessor -based consoles. In the short- responsible citizen of the 90s will know - is of total UK revenue. One game alone (Super term these will move over for 32 -bit systems, that you recycle. By recycling, earth's valuable Mario 3) has made more than £500 million in and ultimately systems based on the but dwindling resources can be persuaded to revenue world-wide, and on its launch last PowerPC and (maybe) Pentium last a little longer, smaller amounts of waste October over 500,000 copies of another game microprocessors will be developed. Well, I say need to be tipped onto our ever-expanding (Mortal Kombat) were sold around the ultimately, but that's just a figure of speech, of dumps, fewer noxious and harmful by- world in a single day. course. There will be generations of games products are released into our fragile It's a sign of the times that fortunes of systems based on every future generation ecosystem, and quite a few people are monies like these are being spent and earned of microprocessor family that's developed as given a job. from what amounts to little more than toys, time goes by. In the short-term again, there'll But answer this ft )..v. What do you do with your old telly, or radio, or Hi-Fi. or personal and it's a sign of the times that parents and be more and more CD -ROM -based systems, guardians of these youngsters (or the of which a few have got on to the market stereo, or digital clock, or battery -driven toy, or - while we've just been on the topic - last youngsters themselves, for that matter) recently. year's games console? Chances are ,he have the available funds to keep their kids' While there's been little of it so far, I predict answer is simply to dump it. But doesr,'t that addictions supplied. that there'll be a combination of games And what are these games? Little more available to date only on these games strike you as a waste? Pun intended. It's a waste because those devices (or at than reaction testing arcade -style cartoon - consoles and ordinary computer systems. based teasers, which create goggle-eyed In the past this has been possible but least many of their constituent parts) can be physically underfit motor -morons of our economically unviable because of the cost of recycled. Just like glass, aluminium, steel, youth? Or do they teach children how to apply upgrading personal computers to effectively a paper and the rest, electronic parts are themselves methodically to problems and multimedia format to be able to take valuable. In many existing cases some of increase concentration levels? Obviously, it's advantage of the games in question. But with those parts can even be reused, for the not in my hands to answer those questions, lower computer prices, and many systems benefit of society in general, and for the and it's not in my hands to criticise a society available now with the technical potential to benefit of the manufacturers involved. which allows its youngsters to spend or play games, it's just a question of time before All it would take is an initiative to encourage persuade their parents to spend these it happens. There always have been computer manufacturers to design products which are fortunes in the first place. games, of course (that is, games played on easier to recycle, and a structured method On the other hand, it is my brief to look at true computers - not the games console -type of actually recycling the products in the first the technologies involved and comment on of machine), but what Sega and Nintendo place. The Industry Council for Electronic possible future scenarios. Games consoles - (among others) have to offer is really in a Equipment Recycling, comprising and that includes any of the available formats, different league. Still,I somehow can't see manufacturers, users, recyclers, suppliers and not just Sega and Nintendo - are based of me stopping off half -way through writing local authorities is attempting to persuade the Government to take steps to set up such an course on microprocessors. The very same Technology Watch to play Streetfighter 14. microprocessors you get in computers. Some Where v,r1I it stop? Indeed, will it stop at all? initiative. This makes considerable sense in parents or guardians have even mistaken With mass oroduced virtual reality just around the light of impending European regulations the act of buying a games console as being the corner, and games consoles now (well, which are set to cover disposal of electronic the equivalent of buying a home computer nearly, anyway) able to generate it, what's appliances. I'll keep readers posted about the (believing that little Johnny is upstairs doing the future for our youth? What's the future for situation as things happen. his homework when really he's battling it out the rest of us? If we can just slip on our VR with Bubonic the Hedgehog, or Super Harry -0 headset and be in a different (interactive, 25) except that most of these games consoles sensory, exciting, stimulating, perceptory and The opinions expressed by the author are not aren't in any way, shape or form, like a so on) world, why come back to the real world necessarily those of the publisher or the editor. /1111 WITH M;cRo MR ftl, crin,Q. wk244 kids" Hove- to Auwes I 23M yes it has viisevvect fodzsm ave- addicted to to stop' 4 arkci i4ecAgeflagcl r mos Eqt-.... lck.a_ us ,-tko-, I iro_'s 1-oo rov-H,ke, weekvAx sk-cx,o (22. oor t abat, Cesi" i sir, e 4obrt-- life awl wko$42, ..i<U2-yci52.., Foy 1-. - Acrrsimrkt coysuVriving lar o'roNa'vw5t, 'Ct'-- April 1994 Maplin Magazine 3 by Douglas Clarkson So much of today's civilisation relates to Everyone, however, will be familiar Above: Computer -generated image of the the technology of the use of electricity with the basic concept of superconduc- crystal structure of the high -temperature and the limitations currently encoun- tivity where the electrical resistance of a superconducting material yttrium -barium - tered in its generation, transmission, material vanishes to zero. Supercon- copper -oxide. Yttrium atoms are shown as silver, barium as green, copper as blue and storage and utilisation. Sources of ductivity was discovered by Kamerlingh oxygen as red. (Photograph reproduced power remote from centres of civilisa- Onnes in 1911. Initially working with a courtesy of, and ©Copyright, IBM solid mercury wire, it was found that tion such as hydroelectric power tend Research.) not to be utilised due to problems of resistance vanished at a temperature of around 4 Kelvin (K). power losses in transmission lines. The superconductivity. The temperature at Subsequently, it was found that as potential of superconductivity to alter which a substance becomes supercon- many as twenty-seven of the chemical radically this perspective is widely ducting is called the Transition tempera- elements demonstrated low temperature appreciated and numerous groups are ture (T-). Table 1 indicates the values of making a determined effort to move T, for a range of chemical elements. from a recent era of startling laboratory Element Tc (Kelvin) The early demonstrations of super- findings to one which can demonstrate Tungsten 0.015 conducting materials, however, did not initially produce any practical applica- a range of matching applications. Zinc 0.88 tions. as the initial superconducting ma- Aluminium 1.20 terials could only sustain relatively small Indium 3.41 currents flowing through them. More and THERE is great diversity within the more laboratories around the world Tin 3.72 groups seeking to exploit the tech- became equipped to undertake low tem- nology. The more 'favourable' Mercury 4.15 perature research although this did not applications, for example include the Lead 7.19 lead to any remarkable discoveries. By development of lower cost Nuclear 1933, only eleven superconducting ele- Niobium 9.20 Magnetic Resonance scanners and the ments and around fifty superconducting development of low loss power trans- Table 1. Transition temperatures (Ti) alloys and compounds had been dis- mission cables. of some superconducting elements. covered. 4 Maplin Magazine April 1994 Small Beginnings shown the way forward for practical super- ments were repeated and checked, and it conducting magnets using liquid helium was then decided to publish a paper on Two researchers in the USA, John HuIm temperatures. what they had discovered. They pub- (initially from Cambridge University) and The development of superconducting lished their findings in a slightly obscure Bernard Matthias, shared a keen interest magnets was especially vital for research journal. This would cause them little in searching for new superconducting in fundamental particle physics where embarrassment if they were mistaken in compounds. Matthias was a renowned very high values of magnetic fields were their conclusions. but would still secure expert in the field of ferroelectrics while required to be maintained over excep- their precedence in the discovery. Such is Hulm had also independently done work tionally long distances. Within CERN. for the nature of the scientific establishment. in this field. The two workers began a example, experiments require rings some period of research for new superconduct- 27km long and with high values of mag- ing materials. Two years were spent 'bak- netic fields present. The wider commercial ing' a range of compounds such as advantage, however, of using supercon- niobium and carbon in a vacuum oven ductors did not develop. The significant and then investigating the conductivity of cost of liquid helium and the difficulty of the compounds at low temperatures. maintaining its ultra cold temperature of Technology, however, was on hand to 4.2K were to remain great limitations. help, when HuIm came across a refer- While there was still significant interest ence to an electric arc furnace which in superconductivity around the world, could very rapidly achieve the tempera- subsequent research in the 60s. 70s and tures required to fuse together the com- early 80s did not see any significant pounds he wished to investigate. breakthrough for materials with much At this stage Hulm was joined by higher values of T,. George Hardy, an able graduate chemist in the process of investigating compounds The Swiss Connection for superconductivity. One initial surprise was the discovery that niobium monoxide Work had been progressing at IBM's was a superconductor. Until then, it had impressive Thomas J. Watson Laboratory been considered that oxides did not at Yorktown. some 30 miles north of New exhibit this property. The compounds York, on the Josephson junction - an ultra formed by silicon and vanadium were fast switch incorporating superconduct- Photo 2. The small magnet floats in air extensively investigated. The 'winning' ing material. A visiting physicist, K. Alex above the superconductor surface because formula of three atoms of vanadium for Muller from IBM's research facility at of a physical phenomenon known as the every atom of silicon with a transition Zurich became interested in this field of Meissner effect. whereby superconducting temperature of 17K was eventually dis- endeavour. Upon returning to Zurich in material excludes magnetic fields from the covered. the summer of 1983. he interested a material's interior. When chilled in liquid Matthias had by now taken up an young crystallographer Johann Bednorz nitrogen, the superconductor acts as a appointment at Bell Labs in New Jersey in helping him investigate the supercon- perfect mirror to the magnet. causing it and when eventually told by Hulm of his ducting properties of likely compounds. to levitate as it sees its reflection in the superconductor. (Photograph reproduced success with the electric arc furnace, Over two years were spent researching courtesy of. and ©Copyright. IBM established this facility in his own labora- compounds containing mainly nickel, but Research.) tory. Through links which Matthias had without success. A reference. however. with the other research establishments the to a new ceramic containing barium. lan- electric arc method of fusing compounds thanum copper and oxygen alerted the The Race Begins became widely adopted and the pace of researchers to a compound which had a After the Swiss paper was published. superconducting research increased sig- high probability of being superconduct- nificantly. ing. The samples were duly prepared major research teams around the world began to crank up their activity. The Using all the technical resources avail- before the Christmas holidays and tested Swiss researchers were soon after able at Bell Labs, however, a solution was in the new year of 1986. The resistance found. What was unknown was the criti- of the sample fell to zero at 35K. This was awarded the Nobel Prize for Physics. One cal field which the wire could withstand. the breakthrough that had been so long group at the University of Houston under Tests subsequently proved that it could awaited. the leadership of Professor Paul Chu had been seeking a breakthrough in super- withstand a field of 8.8 Tesla - the value Bednorz and Muller were delighted but of field produced by the largest copper at the same time very cautious. Photo 1 conductivity studies since heading his solenoid at Bell Labs. Thus Matthias had shows the two researchers. Measure - team in Houston in 1979. Chu was almost on the point of giving up when the work of Bednorz and Muller was finally pub- lished. Working in collaboration with a former student at Alabama University. Maw- Kuen Wu, the two teams began to inves- tigate variations on the Swiss recipe. Efforts at both labs began to concentrate on compounds of yttrium. barium, cop- per and oxygen. It was at Wu's lab in Alabama, however. that the initial break- through was made on 29 January 1987. where evidence of high temperature superconductivity was observed. Over the next few days improved samples gave routine T values around 90K - well within the cooling range of liquid nitrogen. The era of high temperature supercon- ductivity had arrived. The discovery was written up as a sci- entific paper in Physical Reciew Letters Photo 1. K. Alex Muller (left) and J. George Bednorz of IBM's Zurich Research Laboratory and dated 6 February for publication on provided the spark that ignited the excitement of the world scientific community by their 2 March. It transpired. however, that discovery, in 1986, of high -temperature superconductivity in a class of oxide materials. They received the 1987 Nobel Prize in Physics for their contribution. (Photograph reproduced numerous groups had been playing courtesy of, and ©Copyright, IBM Research.) around with the same compounds in early April 1994 Maplin Magazine 1987. At Bellcore, for example, the 'cor- rect' mix had been synthesized on 3 January, but had not been tested for 140 superconductivity until 25 February. HgBaCaCuO Schilling(93) The bake of chemicals was found to consist of a green phase and a black phase. 130 It was soon discovered that the black phase Sheng and Herman was the superconducting material. The T12 Ba2Ca2Cu3010 (Feb 88) atomic structure of the famous compound is shown in the title photograph on page 4. 120 Bi2Sr2Ca2Cu3010 Maeda (Jan 88) Following the initial discovery of the Houston compound, researchers at IBM laboratories subsequently discovered that 110 the new superconducting compounds could carry currents up to 100 times higher 112 Bat CaCu 208 than previously believed. 100 The events between 29 January and 18 March - the date of a major meeting YBa2 Cu3 07 Chu and Wu (Mar 87) on the topic of superconductivity in New 90 York. will long be remembered as one of the most dynamic periods in basic Bi2Sr2CaCu208 research this century. 80 At this time superconductivity was Liquid N2 headline news with photographs of 'float- ing' magnets such as shown in Photo 2 70 Layered cuprates being given great prominence. kU Techniques were developed at IBM for the plasma spraying of superconducting 60 films onto component surfaces. This was a key element of the production of SQUIDs (Superconducting QUantum LaSrCuO Chu Interface Devices) an example of which is 50 under pressure shown in Photo 3. T12 Ba2CuO 40 Rbx Tly C60 (Sept 91) Bednorz and Muller(Sept 86) LaBaCuO 30 BaKBi03(April 88) NbAIGe Nb3Ge V3 Si Nb3Sn 20 Liquid H2 NbN K3C60 (Feb 91) Nb0 Ba(PbBi)03 (BED-TTF) Organic 10 Pb Nb PbM06 S8 Hg Liquid He 1910 1930 1950 1970 1990 2100 1_1 Conventional 1=IFullerenes Organics Lcuayperareteds Figure 1. This figure highlights the rapid rise in critical temperature since the initial breakthrough by Muller and Bednorz in 1986. (Figure reproduced courtesy of, and ©Copyright 1991, Physics World.) Subsequent however, is a highly toxic element. In the drive to produce practical wires and films Developments for real applications thallium and bismuth compounds are more promising. A group of researchers at the University Figure 1 shows how superconducting of Arkansas in October 1987 discovered research has rapidly raised values of criti- superconducting material using thallium as a substitute for yttrium, and they cal temperatures of such compounds. In the spring of 1993, researchers achieved superconductivity at about 82K found that mercury incorporated into - still above the temperature of liquid superconducting compounds increased nitrogen. On the day of their press con- critical temperatures still further to ference announcing their result on 22 Photo 3. The upper part of the photograph 133 5K. This new wave of superconduct- shows two 3 x 4mm counterwound January 1988, the Japanese, however, ing research using mercury is perhaps a superconducting pickup coils. In the announced superconducting materials reminder that science sometimes goes centre, shown enlarged in the lower part using bismuth, barium, calcium, copper of the photograph, is a small 250 x 25011m and oxygen in two phases at 75K and round in circles - in 1911 mercury was the superconducting square. It is the multilayer 105K. The 'bismuth breakthrough' had, first element to demonstrate supercon- spiral input coil that focuses the magnetic already been discovered 'in secret' by the ducting properties. field being measured onto the German company Hoechst in November Exploiting Technology superconducting quantum interference and subsequently patented. The group at device (SQUID) that is hidden directly Arkansas subsequently improved the per- While groups are searching for new HTC underneath. (Photograph reproduced courtesy of, and ©Copyright, IBM formance of their thallium superconduc- (High Critical Temperature) compounds, Research.) tor - raising its T to 125K. Thallium, a considerable amount of effort is being Maplin Magazine April 1994 6 train was in operation in the early 1980s in Lower Saxony near the Dutch border. 108 The track was extended to 31-5km in 77.3K 1987 and the prototype train achieved a YBaCuO thin film world record of 435km/hr (laser deposition) The Japanese, however, have been 10 YBaCuO thin film imaginative in developing superconduc- (sputtering) tivity in new areas. The superconducting ship, the Janato- I which weighs 280 , tonnes and attains a speed of 20 knots ,BiSrCaCuO thin seeks to harness an entirely new method 106 film (sputtering) of propulsion involving superconductivity. device% Various groups are investigating the direct storage of energy in the high values of magnetic fields established by superconducting magnets. The energy in 105 a cubic metre of space in which is estab- lished a magnetic field of strength 1 Tesla is around 800,000 Joules and is propor- tional to the square of the field strength! Figure 3 illustrates potential energy stor- 104 age as a function of field strength. So far experimental storage units with a diameter of about 4 metres have been March 91 constructed with a capacity of 30MJ. It can be appreciated, however, that if the 10 BiPbSrCaCuO size is increased by a factor of 5 and the wire field by a factor of 4, then the storage T capability would be increased by a factor of 400 to around 12.000MJ. This could deliver 100MW over a period of two min- 102 Early utes at 100% efficiency. There is clearly I ceramics some way to go before this mode of I July 88 energy storage is viable. The level of sensing of extremely low magnetic fields attained with liquid helium 10 0 10-2 10-1 10 102 cooled SQUIDS has now been achieved 1 with SQUID devices cooled by liquid nitro- Magnetic field (testa) gen. Signals of amplitude around l OpTesla (10-12) - typical of the amplitude Figure 2. Variation of critical current density as a function of magnetic field for wire and of signals generated by the human heart thin film superconductors. The circular areas indicate the bounds within which specific can now be routinely monitored. If an application parameters can be met. (Figure reproduced courtesy of, and ©Copyright 1991, alternating current of amplitude IA was Physics World/ Sumitomo.) flowing along a long wire, then such a SQUID device could detect the magnetic directed towards harnessing the technol- developing so called MAGLEV (Magnetic field signal at a distance of 10km. SQUIDs ogy which is already able to be demon- Levitation) systems where superconduct- are already finding application in strated. ing magnets are used to levitate an entire advanced electronics instrumentation, The key parameters of a supercon- train which is then free to 'float on air'. non-destructive testing, geological sur- ducting wire or film are the critical current Japan's MAGLEV train, the MLLI-002 is veying and biological and potentially density (the highest current density one of her superconductivity technology neurological monitoring. achieved at superconducting tempera- showpieces. In Germany a 20-6km test Commercial success has been tures) and the maximum value of mag- netic field which can be tolerated. Figure 2 shows how year by year curves of BiPbSrCuCiO wire at liquid nitrogen tem- peratures are inching forwards to meet 1000 - requirements of power cables. The disad- vantage of existing high voltage power transmission lines is that they are unsightly and dissipate energy in the con- 7 50 - ducting wires. The advantages of super- Energy conducting power transmission lines are per that they can be sited underground and unit volume would reduce transmission losses. One (MJ) potential problem, however, in centres of 500 - population is that the likely size of the magnetic fields surrounding such wires is likely to be much higher than for con- ventional cables. Considerable research 250 - requires to be undertaken to both evalu- ate possible adverse effects of higher field values and determine if magnetic fields can be reduced by active shield tech- niques. 0 5 10 15 20 25 30 35 40 45 50 It has been anticipated that trans- portation systems will be considerably (5,10) (15,90) (25,250) (40,640) (50,1000) influenced by superconducting technol- ogy. As for land based systems, con- Figure 3. Variation of stored energy per unit volume (m3) as a function of the value of the siderable effort has been expended in magnetic field in Tesla. April 1994 Maplin Magazine 7 achieved using HTC materials in microwave applications. The group at r5.40 t Birmingham in collaboration with indus- 0 trial partners has developed short dipole antennae and microwave band pass fil- it ters which outperform 'copper' equiva- lents. As higher and higher magnetic fields are generated using superconducting magnet technology; the energies of par- ticles that can be held within a circular accelerator of a given diameter can be increased. One option with projects such as the Superconducting Super Collider is to wait until higher field magnets are avail- able so that smaller sized (and cheaper) accelerators can be constructed. One of the early applications of super- conductivity was in Magnetic Resonance Imaging where images are obtained from mapping nuclear magnetic resonance of atoms within the body. In order to pro- duce images of highest quality it is essen- tial that the magnetic field is highly uniform within the volume being scanned. High homogeneity fields at the level of variation of 5 parts per million within a 50cm sphere can be readily achieved. Field strengths achieved are typically Photo 4. Active shield superconducting magnet. (Photograph reproduced courtesy of, and between 0.5 Tesla and 2 Tesla. While there ©Copyright, Oxford Magnets.) are advantages to scan patients at higher field strengths, such high fields tend to net. Newer technology so called 'active ventional magnetic and 3 metres from an influence cardiac function and are not shield' magnets use ancillary magnetic active shield type. used in routine clinical practice. circuits to cancel the field of the main Photo 5 shows a typical active shield A conventional superconducting mag- magnet. Typically at a core field of 2 liquid helium superconducting magnet net will employ an iron core to help limit Tesla, the 5 gauss line in the radial direc- assembly. Helium is still used as the cool- the extent of the stray field from the mag- tion will appear at 11 metres from a con - ing liquid of choice since only liquid helium superconducting wire elements can withstand very large mechanical stresses due to the 'pull' on the current carrying wire in the strong magnetic field. Photo 6 shows some typical MRI scans made possible with such superconducting magnet technology. With the MRI scan, a 'slice' through an area to be imaged can be scheduled as required. Also, MRI sys- tems do not involve any dose of ionising radiation to the patient. This is making them increasingly popular as a diagnos- tic tool. Also, MRI is undergoing a rapid 4b phase of technological development while 40 CT systems are relatively static. 9 Summary Around the world something like $1 billion is being spent in the field of super- conductivity research. Attention is being directed both to the discovery of new HCT materials and the technological exploita- tion of materials already discovered. Such technology is likely to have the biggest impact on the management of energy - its generation, utilisation and storage. The rapid growth of superconductivity research has highlighted the need for 'interdisciplinary co-operation' in science generally. The physicist, the chemist, the materials scientist, the crystallographer all have a role to play in achieving progress. Further Reading Physics World, Institute of Physics, July 1993, Mercury sends superconducting critical temperature soaring. Superconductors: The Breakthrough, 1 Robert M. Hozen, Unwin, 1988. Photo 5. Typical MRI head scan images. Specific 'slices' through the head can be selected The Path of No Resistance, Bruce and image details produced base on a range of tissue characteristics. The most basic one is proton density. (Photograph reproduced courtesy of, and ©Copyright, Oxford Magnets.) Schechter, Simon and Schuster, 1989. 8 Maplin Magazine April 1994