NEW MATERIALS IN ENGINEERING

NEW MATERIALS IN ENGINEERINGAuthor(s): WILLIS JACKSONSource: Journal of the Royal Society of Arts, Vol. 113, No. 5110 (SEPTEMBER 1965), pp. 758-781Published by: Royal Society for the Encouragement of Arts, Manufactures and CommerceStable URL: http://www.jstor.org/stable/41369528 .

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NEW MATERIALS IN ENGINEERING

The Trueman Wood Lecture by

PROFESSOR SIR WILLIS JACKSON , D.Sc.y D.Phil, F.R.S.y M.I.E.E., M.I.Mech.E F.Inst.P .,

Head of the Department of Electrical Engineering , Imperial College of Science and Technology , delivered to the Society on Wednesday 31 st March 1965, with Sir Hilary Blood , G.B.E. , K.C.M.G. , LL.D ., Chairman of Council of the Society , in the Chair

the chairman: The Trueman Wood Lecture is the most important of all the addresses delivered every year by distinguished people from this rostrum. Sir Henry Trueman Wood was Secretary of the Society for nearly forty years and later served as Chairman of the Council. In 19 17 a number of friends contributed sums to endow this lecture in his honour, and to become a memorial to him. Its scope is all matters relating to the application of science to civilization and particularly to industry. This is in common with the Society's traditional policy of seeking to apply knowledge for the benefit of mankind.

The lecture has provided the occasion for many outstanding addresses on a very wide variety of scientific, technological and industrial subjects. When Professor Auger spoke last year on this occasion I mentioned a number of previous Trueman Wood Lecturers ; I don't propose to repeat their names, but I do want to make one point - perhaps not so well known - and that is the interest which Trueman Wood himself had in industry in this country. He did much work in promoting and organizing various exhibitions: for example the Health Exhibition of 1884, the Inventions Exhibition of 1885, and the Colonial Exhibition in 1886. He also wrote a volume called Industrial England in the Middle of the Eighteenth Century , in which he described the state of each industry as it existed just about the time that this Society was founded. It is a valuable handbook of careful research and very useful information.

Throughout its long history the Society has been concerned with the use and development of new materials for industry. Industrial chemicals figured prominently amongst its early offers and awards. In 1762, on the instigation of John Wilkinson, the famous Shropshire iron master, the Society offered prizes for the production of high-grade pig or cast iron, and for bar and forged iron. A few years later it presented a gold metal to Abraham Darby for his famous iron bridge over the River Severn at Coalbrookdale. Later in the century awards were offered for crucibles, for assaying tin ore, for making white copper, and for all sorts of developments of that ldnd. Then much later came the period when the Society became interested in electricity, and many papers on that and its various uses were read. Later again, in a famous paper, the possibility of using petroleum in motor engines was also discussed, and in modern times atomic power has been the subject of notable addresses, several of them given under the Trueman Wood Trust.

It is very appropriate that the subject of to-day's lecture should be new materials in engineering. Sir Willis Jackson requires no introduction to an audience of this sort. He is as well known in the academic as in the industrial sphere, and as a research engineer has made many valuable contributions to human knowledge. He has been

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SEPTEMBER 1965 NEW MATERIALS IN ENGINEERING

generous with his advice, sitting on many committees and bodies set up to advise Government. His contribution to modern scientific and technological thought is of great significance.

The following lecture , which was illustrated with lantern slides , was then delivered.

THE LECTURE

INTRODUCTION

The past few decades have seen a quite remarkable range and pace of scientific discovery and technological development, and we who have been engaged in it have found ourselves in a dilemma. On the one hand, science and technology have been sub-dividing themselves into an increasing number of specializations, each progressing rapidly, so that to make an effective contribution our individual activities have had to become increasingly specialized. On the other hand, the various developments have become more and more interrelated: progress in technology has become more and more dependent on progress in science, and vice versa, making the work of the engineer increasingly conditioned by the work of the chemist, physicist and metallurgist. In order, therefore, that our specialist contribution should be most effective, it has become essential that we should be able to look intelligently well outside our own specialization into others which might prove to have an intimate bearing on it.

I like to think of engineering as the exploitation, by one technique or another, of the properties of materials. Certainly recent progress in many branches of

engineering has been bound up in the most intimate manner with the improvement of well-established materials and the discovery and development of new ones, and this will most certainly continue. There have been some remarkable examples during recent years, to some of which I shall be referring later, of how recognition of the potentialities of some special property of a material can lead to the evolution of a quite new branch of engineering development.

Speaking generally, the basic material of engineering construction - steel -

remains the same. But there have emerged during recent years a great diversity of new materials which, though relatively minute in their quantity, have been of major importance in particular applications. The discovery of the valuable mechanical

properties of steel was accidental in the scientific sense, for at the time there was no science on which it could be based. How different is the present situation, in which new materials can be evolved logically and systematically by the human brain from the system of ideas which constitutes the intellectual edifice known as natural science.

The subject of materials science and technology is now becoming recognized as an essential component in undergraduate courses in engineering; and as an

extremely important field for inter-departmental postgraduate study and research, affording outstanding opportunities for fruitful collaboration between the

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JOURNAL OF THE ROYAL SOCIETY OF ARTS SEPTEMBER 1965

Figure i

universities and the research and development establishments of Government and of industry. It promises, perhaps, more than any other subject, to break through the barriers between the traditional academic disciplines in science and technology, and to bridge the still serious gap between the educational and industrial domains. These trends are of particular importance to the electrical engineer in view of his very wide interest in the properties of materials, covering not only those of electrical conduction, semi-conduction and insulation, but equally magnetic, nuclear and structural properties. I cannot hope in a single lecture to deal at all comprehensively with the debt which the members of my profession of electrical engineering owe to those chemists, physicists, metallurgists and others who have contributed so abundantly towards a better understanding of the factors governing these various properties, and to the provision of improved and new materials among which one or other of them proved to be of outstanding practical significance. We feel, or at least hope, that our own contributions may have given something to the progress of their subjects in return.

I shall now try to illustrate the nature and extent of our debt.

CONDUCTING MATERIALS AND SUPERCONDUCTIVITY

The theoretical clarification of the mechanism of electrical conduction in metals which occurred a few decades ago has not affected the place of copper as the

760




dominant material for construction of the conducting circuits of electrical equipment generally. This situation is unlikely to change for a long time to come, and yet we may now contemplate with confidence a growing impact on it of a branch of physics which would seem to involve far too many difficulties to be of significance to electrical engineering practice - that of low temperature physics and of superconductivity.

It is now some fifty years since the discovery that below a characteristic critical temperature close to absolute zero the electrical conductivity of certain metallic elements increased discontinuously, apparently to infinity. The value of this critical temperature for a selection of these elements is given in Table I.

TABLE i

SUPERCONDUCTING ELEMENTS AND THEIR CRITICAL TEMPERATURES

Degrees Element absolute

Niobium ... ... ... 8.0 Lead ... ... ... 7-22 Tantalum ... ... ... 4.4 Tin 3.73 Zinc 0.91 Titanium ... ... ... 0.4

In 19 1 6 it was observed that the superconducting state could be destroyed by the

application of a critical magnetic field, provided either by a current of sufficient value passing through the material itself, or imposed from an external source. The variation of this critical field with temperature for niobium, lead, tantalum and zinc is shown in Figure 1.

In 1933 it was further observed that when in the superconducting state magnetic flux is completely excluded from the body of the material.

The application which has been found for these superconducting phenomena in the preparation of storage and other elements for use in digital computers will be dealt with later, but it is appropriate to mention here that the ability to destroy the resistance of electrical conductors- and with it the prospect of removing the

'copper losses', and the reduction of efficiency and the thermal problems of operation which these losses involve - has tremendous attractions to the electrical engineer. Exploratory analytical and experimental studies are now being carried out in respect of transformers, direct current electrical generators, and cables for the lossless transmission of electrical power. No doubt it will be some years before these take

practical form, but the production of electromagnets, incorporating energizing coils of superconducting material, to provide magnetic flux densities as high as

70 kilogauss is already within the realm of achievement. This requires, of course, the availability of materials which will remain superconducting at the field strengths of this high order, and the alloy niobium-tin, with a critical temperature of about

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JOURNAL OF THE ROYAL SOCIETY OF ARTS SEPTEMBER 1965

i8°K, has obliged by affording a critical magnetic field of the order of 150 kilogauss. It need not be doubted that this is only the beginning of what solid state physics will provide in due course for the engineering exploitation of superconductivity.

SEMI-CONDUCTING MATERIALS

Semi-conductor Rectifiers and the Transistor As their name implies, semi-conductors exhibit an electrical conductivity

intermediate between that of good conductors - metals such as copper - and insulators, such as mica and the plastics. As early as 1835 it was observed that the contact between the semi-conductor silver sulphide and a metal exhibited assymetric electrical conduction, and this characteristic appears to have been used first for the rectification of alternating to direct current in 1874. A selenium to metal contact was used for this purpose in 1883, and the copper to copper oxide rectifying junction studied in 1904. In this year silicon carbide (carborundum) crystal with a metal contact wire - the famous cat's whisker - was first employed for the rectification and reception of wireless signals. Selenium and copper oxide rectifiers did not become commercially available until 1925, and their slow development was no doubt related to the fact that the behaviour of semi-conductors had only just begun to receive the serious attention of theoretical physicists. Even by the late 1930s the theoretical models for semi-conductor to metal rectifying junctions which had been devised made possible no very useful deductions as to how improved selenium and copper oxide rectifiers might be made.

However, what was to prove an extremely important observation, that of rectifier action at a metallic point contact with germanium, had been made in 1925, and this led, through intervening studies with material of increasing purity, to the germanium crystal diode which played such an important part in the war-time development and operation of centimetre-wave radar. By 1946 the techniques of purifying and growing single crystals of germanium had been so perfected in the Bell Telephone Laboratories of America that the major physical parameters in the theory of semi-conductors could be measured systematically, known impurities could be added under controlled conditions, and a fairly complete correspondence was established between the observed conduction phenomena and the refined theory which the experimental results showed to be necessary. This understanding of the conduction process made possible the synthesis of a new and much improved type of rectifier, the junction of oppositely conducting forms, n and p> of the material, as shown in Figure 2a, and led around 1950 to the junction transistor, illustrated in Figure 2b.

The regions of the germanium marked n contain a small proportion, of the order of a few parts in io8, of impurity atoms such as phosphorus, which endow the germanium crystal lattice with one excess electron per added impurity atom. These excess electrons may be regarded as free to move about within the lattice, and the electrical conduction in the n regions takes place by means of these electrons. The p regions contain similarly small proportions of atoms such as gallium which produce an electron deficiency of one electron per impurity atom - so-called positive holes -

762




Figure 2

and the current flow in these regions is known as positive hole conduction. Positive holes and electrons move in opposite directions in similarly directed electric fields.

The transistor operates on the basis that a variation of current in a circuit connected between the emitter and base electrodes can produce a sympathetic, and amplified, current variation in a circuit between the base and collector electrodes. It can therefore perform all the functions associated with the thermionic valve, and its great advantage of much smaller size and of the absence of an in- dependently heated cathode as the source of its carriers of electrical charge led to its rapid replacement of the thermionic valve in all forms of electronic and telecommunication equipment. Within little more than a decade the transistor has revolutionized the form of this equipment.

Extreme thinness of the n region shown in Figure 2b is a crucial factor in transistor operation. In modern transistors it is of the order of only io"4 cm. The commercial production of these transistors and of their associated circuit components is a remarkable technological achievement, as may be judged from the fact that the amplifier circuit illustrated in Figure 3, containing 14 transistors and 8 other components, can be 'printed* by appropriate diffusion techniques in a piece of silicon little larger than 0.1 X 0.1 X 2 X io"3cm.

The sphere of application of the germanium and silicon rectifier and transistor

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JOURNAL OF THE ROYAL SOCIETY OF ARTS SEPTEMBER 1 965

Figure 3

is not restricted, however, to this so-called light-current side of electrical engineering. Thus the mercury arc rectifiers previously used on electric locomotives to convert the high voltage alternating current supply to direct current for the traction motors have now largely been replaced by silicon rectifiers with current ratings up to 250 amperes. Further, the direct current excitation of large steam turbine driven alternators, now being installed at ratings up to 500 megawatts, which has traditionally been provided by a direct current generator carried on the alternator shaft, is now being supplied via an exciting alternator and silicon rectifier combination of ratings up to 5000 kilowatts, with considerable improvement in reliability and saving in maintenance charges. Moreover, a quite recently developed three-electrode silicon device, the thyristor, somewhat analogous to the switching form of transistor, is being installed in current ratings up to 250 amperes for controlling the speed of the driving motors in paper and steel rolling mills, printing machines, etc. It also affords a means of converting direct to alternating current - a process known as inversion - the difficulties of which at high voltages and high powers using previously existing means have so far restricted the development and denied the economic advantages of long distance d.c. transmission.

In these connections the science of semi-conductors has been developed by physicists and metallurgists during the past fifteen to twenty years from being a mere commentary on an empirical practice in rectification to one providing devices which are predeterminate, more efficient and reliable, and vastly more extensive in scope than would ever have been achieved by common-sense observation and

764




Figure 4

improvement. As already touched upon in respect of Figure 3, the electrical engineer has brought to bear on its potentialities new techniques of repetitive manufacture under conditions of laboratory cleanliness and precision, and has devised new types of electric circuit engineering. The circuit components -

resistors, capacitors, diodes, transistors - and their means of assembly, have been taken through successive stages of miniaturization, sub-miniaturization and now micro-miniaturization, to the point where in the telecommunication earth satellite the density of these components has reached the order of io8 per cubic foot. Indeed, even greater component densities could be realized were it not for the difficulty of dissipating the heat generated in them by the current flow.

The electronic digital computer has often been referred to as an electronic brain. This is a highly flattering description of its present capabilities - Remarkable though these may be - and of its dimensions, taken in relation even to the capabilities and the size of brains of much more elementary form than the human one. But with the rapidity of development of new electronic components and component assemblies of ever decreasing size, and of the progress of our biological and medical colleagues towards an understanding of the operation of the nervous system, it would perhaps be unwise to conceive undue limitations for its ultimate capabilities.

Hall Effect Devices The impact of semi-conductors on electrical engineering is only at an early stage.

The extremely wide range of properties likely to be exhibited by binary and ternary semi-conducting alloys is only beginning to be explored comprehensively. One example of the possibilities is afforded by indium arsenide formed from elements of the III and V groups of the periodic table. The Hall Effect discovered in 1879 concerns the production of a voltage mutually at right angles to a current and a

magnetic field passed through a piece of conducting material, as illustrated in

Figure 4. Until recently this effect had seemed to have no technological significance because of the smallness of the Hall Coefficient and its sensitivity to change of

temperature. Both these limitations have been sufficiently removed in synthetic crystals of highly purified indium arsenide to open up the prospect of new methods

7^5




Figure 5

of voltage, current and power measurement up to the highest radio frequencies, a multiplier for use in analogue computers, and a modulator for telecommunication purposes.

Thermoelectric Generators

Another important and promising field is that of thermoelectric power generation. The starting point of this possibility was the discovery by Seebeck in 1822 that a potential difference was produced by heating the junction between two dissimilar metals. The effectiveness with which electrical power can be generated in this way is governed by what is called the 'figure of meriť Z, which is related as -

SV Z =

К to the Seebeck coefficient, or thermoelectric power, S which denotes the voltage developed at the junction per degree Centigrade temperature difference between the hot and cold junctions, to the electrical conductivity of the junction materials, o-, and to К their thermal conductivity. With metallic junctions, Z and the associated

766




efficiency, of about i per cent, for the conversion from heat to electrical power, is much too low to be of technological significance. However, the improvement found possible with semi-conducting alloy materials, Figure 5, has transformed the subject of thermoelectric power generation into one of considerable electrical engineering interest. The materials referred to in Figure 5 are n and p type lead telluride doped with silver and samarium respectively - which means the incorporation of appropriate proportions of these elements as impurities - germanium telluride doped with bismuth, and/> and n forms of manganese telluride and cerium sulphide respectively. Many other compounds are being studied. A particularly promising one is iron disilicide, FeSi2, which, although showing a conversion efficiency of only 4 per cent as yet, has the big advantages that it is easy to manufacture and can be operated at high temperature in free air.

The spheres of application of this form of electrical generation may well be limited. Nevertheless, it should be mentioned as an example of otherwise unattainable engineering achievement at the time, that a U.S. Navy navigational satellite launched in 1961, and still in operation, is powered for telecommunication purposes by a thermoelectric generator weighing only 4.6 lb., and that its plutonium pellet is capable of ensuring the production of 2.7 watts of steady electrical power for much longer than the five years for which the satellite is designed to operate. By comparison the weight of mercury batteries (the only alternative) to produce the equivalent power over five years would have been about 1 J tons, which would have meant that there would have been no such satellite. A power of 2.7 watts is, of course, microscopic compared with what is needed for normal, ground-based, purposes, and the thermoelectric means of power generation may never be of

importance for these purposes taken relative to the available alternatives. The

significant thing, however, is the way in which the preparation of new semi-conducting materials has brought thermoelectric power generation to the

fringe of technological relevance. If there is still a long way to go, the exploratory stages of the route are clearly enough defined. A similar story might be told of other

possibilities - those of thermionic generation, magnetohydrodynamic generation, and the fuel cell - with all of which the production of new materials in appropriate context is likely to provide the key to success.

Photo Devices - The Semi-conductor Laser

A further field of application of semi-conductors of immense potentiality was

opened up by the announcement in late 1962 that coherent infra-red radiation had been observed from p-n junctions of gallium arsenide - a semi-conductor alternative to the helium-neon gas and the ruby lasers, themselves discovered only a few years previously. The name laser is an abbreviation for Light Amplification by Stimulated Emission of Radiation.

The form of the device is illustrated in Figure 6. It comprises a tiny pellet of

gallium arsenide in which a p-n junction has been formed by diffusing a heavy concentration of zinc to afford a p region in initially n type material. When current of density 1000-5000 amperes per square cm. is passed through the junction, light is produced in a region of some 104 cm. thickness at the junction of wavelength

767




Figure 6

characterized by the change of potential energy which the charge carriers experience in passing through this region. The emission occurs in the plane of the junction and optical gain and coherence is achieved by making the two faces A and В accurately parallel to each other and perpendicular to the plane of the junction. These two faces form the reflecting surfaces of an interferometer, and their separation determines the frequencies of the possible resonant modes of the device and of the coherent light which emerges from it. The other faces С and D are deliberately made irregular and not perpendicular to the junction plane in order to discourage other resonant modes. The separation between the corresponding faces is typically 0.2 to 0.5 mm. Beam widths of less than one degree in the plane perpendicular to that of the junction, and of a few degrees in the latter plane, have been obtained, at peak powers up to 30 watts.

The high current densities required have so far made it essential to pulse the current, normally in pulses of microsecond duration and of repetition rate a few hundred per second, and to cool the laser to the temperature of liquid nitrogen, when the wavelength of the emission is 0.84 ¡i . However, it is rumoured that semi- conductor lasers capable of working at room temperature have been achieved in the United States, and, in any case, this achievement and that of producing light sources over a wide range of wavelength, cannot be more than a matter of time. The device offers a considerable number of technological possibilities among which are those of modulating the laser beam at very high frequency for communication purposes, and use of the beam in surgical operating techniques, notably in eye surgery.

768




ELECTRICAL INSULATING, OR DIELECTRIC, MATERIALS

In the spheres to which I have referred so far the electrical engineer has been mainly indebted to the physicist and the metallurgist, but in the next, that of electrical insulating or dielectric materials, his prime benefactor has been the chemist. This is not to say that in searching for, discovering and developing new synthetic materials the chemist has had the needs of the electrical engineer particularly in mind - at least not in the first instance - but that several of the synthetic materials he has produced during recent years have proved to have quite excellent dielectric properties and associated physical and mechanical ones, and to be of considerable value to the electrical engineer. The consequent close collaboration which has developed between the two has permitted a correlation of data about the relevant properties of dielectric materials which affords an invaluable guide to further improvements, and it is not too much to claim that considerable progress has been made towards the 'tailor-making' of insulating materials to previously specified characteristics.

High Polymers In the high polymer field the debt of electrical engineering to chemistry includes

polythene - the polymer of ethylene - without which, in its application to high frequency cables, our war-time development of radar would have been seriously handicapped and retarded, and which made possible the achievement of the first transatlantic telephone cable laid in 1956; polystyrene for mouldings; Poly- vinylchloride for cable sheathing; polytetrafluorethylene for high-frequency high-temperature purposes ; and the silicones which have opened up the possibility of operating electrical equipment at considerably increased temperatures. In addition, a wide range of synthetic paints, varnishes, imprégnants and moulding compounds have found extensive application in electrical manufacture.

The Titanate Ceramics and Thin Film Capacitors

By comparison with this extensive development of new organic insulating materials, that of inorganic ones has been rather modest, though of considerable technical importance for particular purposes. Thus new glass and ceramic compositions exhibiting low dielectric loss at centimetre wavelengths and siiitable for the production of effective glass-metal seals have been developed, and means have been found for producing crystalline quartz and mica synthetically. However, perhaps the most interesting of the new ceramics are those based on barium titanate BaTi03 - interesting not only because of their remarkably wide range of

properties, but also because the technical exploitation of these properties was, in the main, overtaken by the possibilities afforded by alternative materials.

As is well known, capacitance is expressed by the relation - A

С = е.- d

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Figure 7

where A denotes the area of the electrode surfaces; d their separation; and e the permittivity (dielectric constant) of the insulating material between them; so that the higher the value of e, the smaller the electrode area for a given separation, and the smaller the overall dimensions required for a desired capacitance.

The materials traditionally used as the dielectric of capacitors were mica and wax or oil-impregnated paper. The permittivity of these materials is of the order of only 4-6, so that considerable interest was aroused in the mid 1930s by the commercial development in Germany of capacitors incorporating a ceramic form of titanium dioxide, Ti02, of permittivity around 100, and even more interest when in 1943 a report from the United States announced that a new material - barium titanate - had been produced in ceramic form and found to have a permittivity of over 1000 at room temperature, as shown in Figure 7. This was potentially of great importance, in view of the war-time drive towards the miniaturization of electronic components for incorporation in telecommunication and radar equipment to which I have already referred.

Successful attempts were made by the incorporation of proportions of such components as strontium titanate, magnesium and zirconium oxides, to increase the room temperature permittivity to still higher values. The result has been a range of miniaturized, high value, capacitors which have found many applications. However, their dielectric losses and their capacitance variation with temperature are too high for other purposes, and another technology has had to be invoked and developed to overcome these limitations, namely the production of thin dielectric films by evaporation in a vacuum. In this case the capacitance is increased by the smallness of the factor d in the above formula. Thin film capacitors having very low

770




losses, using such materials as silicon monoxide and zinc sulphide in thicknesses down to 10"5 cm., are now used extensively in low- voltage transistor circuits.

Both structurally and in its properties, barium titanate bears a close similarity to ferromagnetic materials, and in consequence an analogous terminology has been applied to it. It exhibits a domain structure, dielectric hysteresis and electrostrictive properties at temperatures below a crystallographic transition point at I20°C, which is known as the Curie Point, and below which it is said to be ferroelectric. In consequence, it is potentially suitable for use as a dielectric analogue of the magnetic amplifier, as a dielectric store in digital computers, and as an electro-acoustic transducer - as replacement for quartz in high power ultrasonic generators, and for Rochelle Salt in gramophone pick-up units. In the event, however, its amplifier and storage potentialities were overtaken by those of the Ferrites to which I must now give attention.

FERROMAGNETIC MATERIALS When an electrically conducting material is subjected to an alternating magnetic

field, eddy currents are set up within it with a consequential power loss expressed by the relation -

W = K.B2max.f2.d2. & .V where Bmax is the maximum magnetic flux density; f the frequency; d the thickness of the material normal to the flux direction; <r its electrical conductivity; and v its volume.

For given values of Bmax, f and v, this power loss may be reduced by laminating the material, as is done with the silicon iron used to provide the magnetic circuit in rotating electrical machines and power transformers, and by reducing the thickness of the mutually insulated laminations. At the higher frequencies involved in telephony, however, this procedure is inadequate and the nickel-iron alloys there employed are sub-divided into small grains which are compressed together to form the 'dust cores' of the transformers and inductors required. The Ferrites

Additionally, the eddy-current loss may be reduced by decreasing the conductivity of the material. However, notwithstanding extensive metallurgical research, it has not proved possible to produce magnetic iron alloys with conductivities lower than about five times that of pure iron, which is about io5 mho cm. units. By comparison the conductivity of magnetic oxide of iron Fe304 - which served as the historic Lodestone for navigational purposes, and which is now referred to as ferrous ferrite - is of the order of io2 mho. cm.; that is, it is a poorer conductor by a factor of 1000. Even so, it is not a poor enough conductor to make it serviceable at high radio frequencies, and it was an event of great technological importance when it was discovered during the war in the research laboratories of the Philips Company, Eindhoven, that the replacement of one of the iron atoms in Fe304 by atoms of such elements as magnesium, nickel, copper, zinc, manganese, produced materials having conductivities of the order of io"5 to io"7 mho. cm., and a remarkable range of magnetic properties. With conductivities of this order the

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Figure 8

need for lamination or sub-division in other ways does not arise up to the highest radio frequencies, and the materials can be produced in ceramic form to whatever shape is desired. In this form they are used extensively in radio and television receivers and telecommunication equipment generally, and have found a number of more specialized applications at microwave frequencies up to io10 cycles per second. Unfortunately, as shown in Table II, their saturation magnetization is too low to compete with iron for use in electrical power equipment.

TABLE II

Relative values of Material magnetization

Iron ... ... ... 1.00 MnFe204 0.24 Fe.Fe204(magnetite) ... 0.28 Co.Fe204 0.23 Ni.Fe204 ... ... ... 0.16 Cu.Fea04 ... ... ... 0.08 Mg.Fe?04 ... ... ... 0.06

The Ferrite Store

However, one of the most important applications has been as a storage element in electronic digital computers, arising from the fact that manganese, or mixed

core manganese-magnesium, ferrite affords a hysteresis loop in alternating magnetic fields which is a close

X ̂ - ■ - "7^ approach to rectangular shape, as shown in Figure 8. /

" The principle governing the application is as follows. Basically a digital store requires two clearly defined stable states for the binary digits i and o.

Magnetising These are afforded by the remanent points X and current y of the hysteresis loop which are attained by

applying and then removing polarizing magnetic fields in the positive and negative direction to W and Z respectively. If X is designated as the

Y state of polarization in which a binary 1 is stored, 2 Y is then the state in which о is stored.

When it is desired to read the store, that is to determine in which of these states the material has been placed, a negative pulse - the so-called read-out pulse - is applied to it. If the material is in state Y when the read-out pulse is applied, the polarization changes from Y to Z and back to Y upon termination of the pulse. The change of polarization, and the voltage induced in an associated secondary winding, is therefore small. If, however, the material is in state X, the loop is

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Figure 9

traversed from X to Z and then to Y, with a consequent much greater change of polarization and induced secondary voltage.

Figure 9 shows the nature of the assembly and the dimensions of a typical ferrite store comprising 960 cores for a digital computer. Small though this is, still smaller ferromagnetic storage systems are being produced, as another facet of the miniaturization process to which I have already referred. I have mentioned the application of the vacuum deposition of thin films of dielectric materials to the production of capacitors. The same technique has been applied to the preparation and study of thin films of ferromagnetic materials for use as storage elements, and affords another example of the inter-play between science and technology in that this applied objective has stimulated and made possible a new approach to the fundamental study of ferromagnetic phenomena.

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Figure 10

The Superconducting Store - The Cryotron

Although not strictly relevant, it is appropriate to conclude this section with a brief reference to the use of superconductive devices for computer storage purposes. This was touched upon in the second main section of this lecture, where it was mentioned that the temperature below which metals become superconducting depends upon the ambient magnetic field strength. If the field strength exceeds some critical value depending on the particular metal and the temperature, super- conduction ceases. Thus one superconductor may be used to set up a magnetic field which will 'switch' another superconductor into the normal resistive state. Such a switch was first produced in 1935, and is illustrated in Figure 10a.

A current of sufficient value passed through the superconducting niobium or lead 'control' coil will switch the tantalum or tin 'gate' into the resistive state without destroying its own superconductivity, and in doing so will effectively discontinue the flow of current in the gate. The device thus acts in a similar manner to a relay, the contacts of which have been opened by the current in the control coil. A

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combination of two such devices interconnected so that the current in the control coil of one flows through the gate of the other, and vice versa, provides a bistable device which may be used to store information in binary form. Further switching devices must be incorporated to 'write in' and 'read ouť the information, but it must suffice here merely to refer to Figure 10b, which illustrates still further the application of thin film technique as a means of reducing the dimensions and increasing the speed of operation of the device. With this form of Cryotron switching speeds faster than 107 per second have been achieved. It is probably too early to say whether, in view of the problems of low temperature operation involved, the superconducting store will supercede the ferrite one, but there can be no doubt that it heralds the emergence of a new branch of electrical technology.

NUCLEAR MATERIALS

It is only about 25 years since the laboratory experiment which established, as was expected, that the fission of the nucleus of uranium 235 by the entry of a neutron was productive of a considerable amount of free energy, and also, as was perhaps not expected, that the fission process resulted in the release, on average, of г' new neutrons which could be used for a continuation of the fission process. Within five years there followed the first atomic bomb, and within seventeen years the first major nuclear reactor powered electrical generating station, the 60 megawatt station at Calder Hall in Cumberland. In the quite near future some 3000 megawatts of electrical power will be derived in this country from uranium fuel, and by 1970 this source of power is likely to become economically competitive with that produced from fossil fuels.

In this short period uranium, thorium and plutonium have attained immense technological importance. The exploitation of their power-producing potentialities is possible, however, only through the association with them of other materials serving a variety of purposes, notably those of moderator, for slowing down the fission neutrons to thermal energies; of coolant, for absorbing the released free energy and transferring it through water and steam to the electrical generator; and for canning the fuel when this is solid or, in mobile fuel systems, for holding the fissile material in liquid solution or suspension.

The specifications requisite for these purposes were technologically abnormal and they could be met only by materials which, in general, had been of little, if any, previous engineering significance. It must suffice for me merely to mention some of the materials which have found application, or which have been produced in sufficient quantities for their properties to be studied with this objective, in this fantastically rapid translation of scientific discovery into engineering achievement. They are, highly purified graphite, heavy water, various magnesium alloys, zirconium, beryllium, niobium, and liquid sodium, potassium, and bismuth - and no doubt many others of which only the experts in this field will be aware. Never has there been, or perhaps been needed, so comprehensive a collaboration between chemists, physicists, mathematicians, biologists, civil, mechanical, electrical and chemical engineers, or so perfect an example of how the material gifts of nature can

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JOURNAL OF THE ROYAL SOCIETY OF ARTS SEPTEMBER 1 965 be utilized through the creative efforts of scientists and technologists for the benefit or the ill of mankind.

A few years ago some scientists appeared confident of the effective control and application to electrical power generation of the reverse process - that of the fusion of the light nuclei of deuterium. In the event this seems likely to be deferred for several decades, but there can be no doubt that mankind can look forward with confidence to the availability of unlimited power for unlimited time. The greater uncertainty is whether it will choose to utilize this power for constructive rather than destructive purposes.

STRUCTURAL MATERIALS

I do not intend to attempt to relate this concluding section to electrical engineering specifically, beyond making the point that good mechanical engineering is essential to good electrical engineering, and that mechanical failure usually arises from the failure of a structural material.

According to the theory of atomic structure, solid materials should be far stronger than they prove to be, by at least an order of magnitude and often by several. It is convenient to write the tensile strength TS in the form -

TS = a.E where E is Young's Modulus. The atomic force-displacement relations indicate that a should have a value of about o.i, and therefore TS about E/io for almost all solids. Professor Cottrell has recently tabulated some of the highest observed values of tensile strength and of the associated values of a from which the figures in Table III have been selected as examples.

TABLE III

RELATIVE VALUES OF OBSERVED TENSILE STRENGTHS

Relative Material TS a

Graphite whisker (diameter of order io"4 cm.) ... ... ... ... i .00 0.024

А12Оз ... ... ... ... ... 0.63 0.028 Iron whisker 0.54 0.044 Drawn high С steel wire 0.17 0.02 Glass fibre ... ... ... ... 0.15 °«°35 Mild steel 0.02 0.003 Cast iron 0.0 1

The low stresses at which bulk solids fail are due either to their being brittle and containing cracks or other stress concentrations, or because they are ductile and contain glissile dislocations. Under stress the movements or slip of these dislocations

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Figure ii

in the atomic structure reach a stage where they allow one plane of atoms to slide over another and the material 'gives'. The improvement of the strength of iron resulting from the presence of a small percentage of carbon, though it was not so understood at the time of its discovery, illustrates one of the means by which the movement of the dislocations can be impeded. More recently this has been achieved by reinforcing the metal or alloy concerned - or, in other words, stabilizing the imperfections in the crystal lattice - by the precipitation during appropriate heat treatment of dispersions of small immobile particles of other materials of diameter 100-1000 ângstroms.

Precipitation-hardened Nimonic alloy and duralumin, and various dispersion- hardened alloys of aluminium, molybdenum and nickel are examples of this process. The result is not so much an increase of strength as a retention of strength to temperatures much closer to the softening point of the basic material. An important disadvantage of this method is, however, that the fine dispersions tend to be thermally unstable and to dissolve or coarsen at high temperatures and at grain boundaries, with a resulting weakening of the material.

Attention is now being concentrated on an alternative form of dispersion strengthening, based on utilizing the exceptional properties of thin fibres and whiskers indicated in Table III. Figure 1 1 illustrates the advantage gained at high temperatures by the incorporation in silver of whiskers of A1203 as compared with small particle dispersions of the same material. So far this so-called filament

strengthening has proved most successful and found its main application in non- metallic systems, of which fibre-glass - Table III - is an early example. But the

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Figure 12

prospects in respect of metals may be judged from Figure 12, taken from a recent lecture by Sir Robert Cockburn on the likely effects of new materials on further progress in the field of aeronautics.

It seems appropriate for me to conclude this brief section by a quotation from the contribution made by Sir Denning Pearson, of Rolls Royce Limited, to the series of speculative articles about the likely scientific and technological situation in 1984, which appeared in New Scientist during last year. He wrote, following his reference to the potentialities of the filament strengthening of materials,

In ways such as these we shall be evolving towards the engineering equivalent of biological materials, with polyphase structures having specialized constituents to perform specialized functions. Since the technology of any age is founded upon the materials of that age, this new era of composite materials will have a profound effect upon the technology of the future. To the designer, limited by the uniform nature of present-day materials, these concepts will give a new degree of freedom and there will be opened up a field of micro-engineering design. Just as, in making an engine, we do not start with one large piece of uniform material and machine the whole engine out of it, but design each component in the most suitable material and then assemble them; so with composite materials the engineer will be able to design in detail the insides of components, giving non-uniform properties such as greater strength in a preferred direction or differences in properties in various parts of the same component.

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CONCLUSION

If in this lecture I have skated too superficially over too broad a field to satisfy anyone interested particularly in some part of it, I hope I may have succeeded in revealing the profound effect which the search for and production of new materials is having on the progress of my own subject, and of engineering and technology as a whole, and the extent of the debt which my professional colleagues owe to those scientists and others who have made it possible. In thanking them I would wish to say that there is much more we still want them to do.

DISCUSSION

MR. cecil H. Robinson: The other day I was reading a report of a lecture by Sir Ronald German on 'The Future of Telecommunications'. He was talking about reduction in size of components, and seemed to suggest that instead of needing repairs, we were getting to such a state of efficiency in these very small units that we could in fact produce them so that they could be locked away in the complete certainty that they had a life of ten years. After that, if you want to replace the system, you scrap the lot. Is this going to be a trend likely to extend to other fields besides telecommunications, and is it going to affect our ordinary domestic life?

the lecturer: It certainly is the case in digital computers, for example, where a new ferrite core store would be slotted in to replace one that had become faulty- you would not, as it were, trouble to repair it. Similarly, an assembly of capacitors and transistors would be replaced in its entirety. I should not be surprised if the practice extended into the domestic field in due course. I guess the electrical industry would encourage it !

sir charles GOODEVE, F.R.s. (British Iron & Steel Research Association): One serious question, and if the Chairman will permit, one frivolous. The serious one is that rumours keep occurring that super-conductivity at room temperature is likely to be discovered. Is there any evidence of that? The second is, why are semi-conductors known as feminine?

the lecturer : I know of no evidence to support the rumour you mention. On the other hand, I have been told that the Americans have produced a semi-conductor laser capable of operating at room temperature. As to the sex of semi-conductors, I do not know.

sir charles GOODEVE: I would have thought that your students would have told you that it is because their resistance decreases as they warm up ! At the Bell telephone laboratories semi-conductors are known as 'misconductors ' .

the chairman : We shall make a special note of this for the Journal. MR. I. P. HAIGH, M.A., M.i.c.E. (Sir Alexander Gibb & Partners): In connection

with digital computers, may I ask what is the electronic life as opposed to the economic life of a computer?

the lecturer : The electronic life of a computer is vastly longer than the economic life, interpreting economics in perhaps rather unconventional terms. Mr. Duckworth may know more.

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JOURNAL OF THE ROYAL SOCIETY OF ARTS SEPTEMBER 1 965 MR. j. c. Duckworth, M.A., M.I.E.E., F.iNST.p., F.iNST.F. (Managing Director,

National Research Development Corporation): I think the life could be almost indefinite if one kept replacing the parts, but this is not a significant point because at the moment it is economics entirely which decide the useful life. We are, for instance, just donating one of the first computers to the Science Museum (not to be outdone by Leo who also donated theirs to the Science Museum). Both these machines could in fact still be made to work, but it is just not worth while to do so, as they would be hopelessly uneconomic by modern standards.

MR. G. CONRADI, A.M.i.E.E. (British Central Electrical Co. Ltd.) : We have heard with great interest of the increasing acceleration of miniaturization, but there does seem to be one aspect of electrical engineering where we have done nothing towards miniaturization. I am referring to heat-resistance generating plant. I should like to ask Sir Willis Jackson if there is any evidence that we may be reaching a point where it could conceivably be possible to generate electricity directly from nuclear energy without going through the process that we now have of generating by turbines?

the lecturer: Certainly it has not reached the stage of miniaturization, but I suppose for major generating plant the weight per megawatt has gone down by a factor of three since 1948. So this must be counted as a major change towards miniaturization.

So far as direct generation of electricity is concerned, I should not have thought there was an early chance direct from nuclear power. On the other hand, there is a possibility (though it remains to be established whether it would be economic) of using the magneto hydro-dynamic generator. Whether this would replace the more conventional types may well be doubtful, but in association with conventional types it could, so I have heard, increase the efficiency of major units by several per cent, which would be economically very significant. The thermo-electric generator is a direct generator, the thermionic generator is another, and the fuel cell still another. All these are, as it were, at the exploratory stage, and while they may not compete in the bulk power field they may nevertheless have important applications in a lower power situation. Then, of course, there is ultimately the fusion-type nuclear reactor, but whether this will be a direct generator, I do not know.

JAMES TAYLOR, М.В.Е., PH.D., D.sc., F.R.I.C., F.iNST.p., A.i.MiN.E. (a Member of Council of the Society) : Sir Willis Jackson pointed out that a good deal of advance- ment in theory had to precede these practical advances, and that has helped the demand of the engineers for new materials. One of the things which struck me in connection with all these materials is that the demands now are for extremely pure ones, where impurities or additions of the order of one in ten million parts or even less are commonly involved and may often have very serious effects ; and I think one of the most interesting things that has happened during the last ten or fifteen years is the enormous improvement in analytical methods which has occurred by the side. So without the demand of engineers for these new materials of construction throughout the period for metal conduction and semi-conduction, and without this enormous improvement in analysis, I don't think we would have got so far.

the lecturer: The whole process is really one of leap-frogging isn't it? A step forward on the analytical side leads to a step forward in the practice ; this feeds back new techniques of experimentation, and the process goes on. The preparation of highly purified materials, and the ability then to control their electrical properties by the deliberate incorporation of known amounts of selected materials, has of course been crucial to the whole of recent semi-conductor development.

the chairman : It remains for me on your behalf to thank Sir Willis for his lecture, 780




It would be completely impertinent of me to comment on it. I am no scientist. The second law of thermodynamics, echoes of which have come resounding down the Corridors of Power, means no more to me than does the third or the first law thereof, or, indeed, the science of thermodynamics itself. But the audience has shown how very much it has appreciated what Sir Willis has had to say.

We are a comparatively small audience to-day because this lecture unfortunately coincides with a conference at Cambridge, organized by the Royal Society and the engineering institutions, to consider, with headmasters, engineering as a career. Nevertheless, those unable to be here will be able to read Sir Willis's lecture in the Journal.

If I have understood correctly what he said, I suppose the day may come when the Trueman Wood Lecture is given by a computer with the equivalent of a human brain, with a chairman who is a computer with the potentialities of a human brain, and a similarly composed audience. And if all those computers are going to get smaller the whole event could take place in an area the size of this table. That, I think, will be a great pity, because this is such a delightful room !

Thank you very much, Sir Willis, for delivering this Trueman Wood Lecture to-day.

The vote of thanks to the Lecturer was carried with acclamation , and the meeting then ended.

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NEW MATERIALS IN ENGINEERING