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Material performance trends

We have noted already that the areas of greatest research interest in materials science and engineering are not necessarily the areas of greatest current economic importance. Having said this, however, it is of interest to look at recent trends in three of the areas of current economic interest, namely high-temperature materials, light weight materials, especially high-strength composites, and "electronic" materials. Potential areas of future application will be noted.

Fig. 4.3 Man-made fibres performance index when values are non-dimensionalized by scaling between 0 and 1 (Source: Gordon and Munson 1982)

High-temperature materials

A requirement common to many material uses is a combination of toughness (i.e. ductility), strength at high temperatures, inflammability, corrosion and oxidation resistance, and minimum weight. Early uses for such materials were mainly for high-speed drilling and cutting tools (hence, "high-speed" steels). Jet engines and gas turbines currently exemplify this requirement. The essential point is that increased fuel economy and higher thrust-to-weight ratio are achieved by operating at higher temperature and pressures. A 150F increase in inlet temperatures yields a 20 per cent increase in thrust (Clark and Flemings 1986). (For comparison, the thrust-to-weight ratio for large jet engines has somewhat more than doubled in the past 30 years; Steinberg 1986.) Airframes and re-entry vehicles (RVs) also require a combination of high strength and low weight at high temperatures. Increased fuel economy for aircraft is obviously very important in terms of increasing long-term resource productivity.

Two radically different cases can immediately be distinguished, depending on whether exposure to air is also essential or not. Thus carbon fibre, one of the strongest and lightest of all materials, cannot be used in engines, for instance, because of its combustibility. On the other hand, non-metallic refractories such as oxides, carbides, or nitrides are quite strong and not affected by the presence of oxygen. On the other hand, they tend to be brittle, i.e. they lack ductility. Thus, two major lines of development can be discerned. The first is metallurgical. The problem is to find metallic alloys with better combinations of strength and ductility for applications in oxidizing environments, especially for turbine engines.

Here again, a panel of experts identified three relevant parameters (taking non-flammability for granted) and weighted them (Gordon and Munson 1982):

Parameter Panel weight
Rupture strength 0.333
Creep strength 0.333
Ductility 0.333
  1.000

Table 4.3 High temperature materials: Non-dimensionalized parameters and performance index

Material

Year

Rupture strength

Creep strength

Ductility

Index

Nimonic 1941 0 0 0.812 0.271
S-816 1943 0.348 0.476 0.246 0.357
Nimonic 80A 1944 0.022 0.110 0.391 0.174
Nimonic 90 1945 0.040 0.110 0.971 0.374
L-605 1947 0.203 0.079 0.551 0.278
M 252 1949 0.384 0.215 0.087 0.229
Rene 41 1950 0.312 0.319 0.478 0.370
Udimet 1955 0.384 0.424 0.275 0.361
GMR 235 1955 0.565 0.476 0 0.347
Alloy 713c 1955 0.710 0.581 0.246 0.512
Udimet 700 1957 0.529 0.267 0.420 0.405
Nimonic 105 1958 0.268 0.168 0.493 0.310
Nimonic 115 1959 0.529 0.288 0.290 0.369
IN-100 1960 0.855 0.843 0.043 0.580
B-1900 1962 0.855 0.895 0.058 0.603
Alloy 7131C 1964 0.674 0.581 0.275 0.510
MM 509 1964 0.493 0.476 0.333 0.434
MM 246 1965 0.964 1.000 0.072 0.679
HA-188 1966 0.167 0.079 1.000 0.415
MM 200 (DS) 1966 1.000 1.000 0.072 0.691
Unitemp 1970 0.746 0.895 0.174 0.605

Source: Gordon and Munson (1982).
Note: Index weight factors 1/3 each.

Data for a number of high-temperature alloys introduced since World War II are shown in table 4.3. In this case (since the application is relatively unchanged), the single composite index, illustrated in figure 4.4, seems to have some explanatory power. However, even here there were two different applications, namely turbine blades and vanes.

For turbine blades, nickel-based alloys were preferred because of higher strength and stress resistance, whereas for vanes, cobalt-based alloys were preferred (because of reduced environmental degradation). The only three cobalt-based alloys in the study were S-816, MM 509, and HA-188. They show almost no upward trend in the composite index. There was a clear and rapid upward trend in the index of performance for nickel-based alloys, on the other hand, up to the mid-1960s. Since then, improvements have been achieved mainly by the use of directional crystallization techniques in the investment casting process. Incremental improvements in high-temperature metallurgy have permitted gas turbine operating temperatures to increase at the rate of 10-12F (about 6-7C) per year since the 1960s (Clark and Flemings 1986). The development of gas turbines in the 120-150 MW range with turbine inlet temperatures of 2,600F is envisioned, thanks to developments in advanced casting systems.

Fig. 4.4 High-temperature materials performance index (Source: Gordon and Munson 1982)

Fig. 4.5 The steep climb m operating temperatures made possible by modern materials

The alternative line of research in high-temperature materials is focusing on advanced ceramics, such as silicon carbide, silicon nitride, and lithium aluminium silicate. Concern over possible shortages of cobalt, chromium, and other so-called "strategic" metals played a major role in accelerating the research effort in this field in the 1970s. Figure 4.5 shows the continuing trend in high-temperature materials capabilities. Based on their known properties, ceramic-matrix composites seem to offer a potential of raising turbine inlet temperatures from about 1,850F (1,000C) to as much as 2,700F or about 1,500C (Clark and Flemings 1986). This would increase theoretical maximum turbine fuel efficiency, if realized, by around 27 per cent.

As of 1996, the major applications of structural ceramics are still for cutting tools and mechanical seals. However, a decade ago ceramic automobile turbochargers were already being produced by Nissan in Japan and ceramic glow plugs and pre-combustion chambers for diesel engines are being made by Isuzu (Robinson 1986). Ford and Garret Corporation were about to test a 100 hp gas turbine engine with a metal housing and ceramic parts in contact with the hot gases (Robinson 1986). However, little further progress has been reported since then, at least in terms of practical applications.

Fig. 4.6 Typical strength variability curve for a ceramic (Source: NMABNRC 1975)

The problems of utilizing advanced ceramics such as silicon nitride for engines or other purposes where they compete with metals are not so much their well-known brittleness (i.e. lack of ductility) as their low fracture toughness and tendency to fail unpredictably. This, in turn, is because the distribution of microscopic defects - which concentrate and propagate stress cannot be predicted a priori, owing to scatter in the experimental data, as shown in figure 4.6. A theoretical possibility is to "proof test," namely to test all ceramic parts up to a certain level of performance and throw away those that fail. This greatly decreases the odds of random failure among the survivors, as shown in figure 4.7. However, under present conditions, yields are likely to be less than 20 per cent, which is far too low. Until yields of 70 per cent or better can be achieved in practice, the economics of advanced ceramics will remain unfavourable.

Part of the problem of unpredictability may have its origin in the traditional techniques of compaction and hot pressing (sintering). The quality of the product is dependent on the size of distribution and uniformity of the starting material. New processing techniques such as "sol-gel" processing may offer hope. A "sol" is a colloidal suspension of particles in sizes from 1 to 100 nanometers. As the "sol" loses liquid, it gradually becomes a "gel." Although the concept is old, this technique has been widely practiced for only about three years, and its popularity is growing rapidly (Robinson 1986). New approaches in the field of ceramic matrix composites are enhancing the fracture resistance of ceramics.

Fig. 4.7 Effect of a uniform tensile proof test on failure probability of a bar in bending (Source: NMABNRC 1975)

However, this growing interest in chemical-based techniques can be interpreted as evidence that the older physics-based techniques are reaching a dead end. At present, it appears safe to predict that advanced ceramics will rapidly grow in economic importance, but that they will not become serious competitors with metals (e.g. in auto, diesel, or jet engines) for at least another decade. This means that major technical improvements in engine performance - hence fuel economy - especially in large-scale applications cannot be expected for at least another 10 years, if not more. In the interim period, metal-matrix composites that contain ceramic particles or fibres will result in small incremental improvements in engine performance.

Strong light materials

De-materialization depends to some extent on the substitution of lighter materials for conventional ones, especially in structural applications. Strength-to-weight and (Young's) modulus-to-weight are obviously important characteristics in this context. For most practical purposes "strength" is a combination of two characteristics, namely resistance to stretching and resistance to bending (stiffness). The first is commonly measured in terms of the amount of pulling or tensile stress required to cause the sample to break (usually measured in psi, or pounds per square inch, of cross-section). The second is measured in terms of the tensile stress required in principle to stretch the sample to twice its original length, also measured in psi. This number is called "Young's modulus." For purposes of comparison, typical values of breaking strength and stiffness for standard engineering materials are as follows:

 

Tensile strength (x103) psi

Stiffness (modulus) (x103) psi

Wood (spruce, along grain) 15 2,000
Bone 20 4,000
Glass (window or bottle) 5-25 10,000
Aluminium 10 10,500
Carbon steel (mild) 60 30,000

In principle, it seems obvious that these numbers must bear some relation to the attractive forces between atoms of the material. But if only inter-atomic forces were involved, materials should be 10 to 50 times stronger than they actually are. Very careful experiments in the 1940s and 1950s showed that flawless microscopic crystals or whiskers or very thin fibres of glass approached theoretical breaking strength much more closely than macro materials (Gordon 1973). Figure 4.8 shows that the strength-to-density ratios of today's engineering materials have increased by more than 50-fold, as compared with materials available at the beginning of the industrial revolution. This trend can be expected to continue for some time to come.

In the 1950s, theory (supported by newly available empirical data from Xray microscopy and other new research tools) began to catch up' and the essential mechanisms of defect propagation in brittle materials and "crack-stopping" behaviour in ductile metals and natural composites (such as wood and bone) were finally understood (Gordon 1973). "Composites" are composed of two or more components, namely very strong small fibres (oriented or not) embedded in a much weaker matrix. A factor of 5 or so difference in strength between the two components is actually essential. This insight opened the door to synthetic composites, of which the first commercially important one was fiber glass-reinforced plastic (FRP). FRP is still by far the most important composite commercially, but by the beginning of the 1970s a large family of new high-performance composites had been developed, largely by the aerospace industry.

Fig. 4.8 Progress in materials strength-density ratio, showing a 50-fold increase (Source: NAS/NRC 1989)

The key to a practical composite material is the stronger and stiffer component, which can be a glass fibre, a mineral crystal such as sapphire (Al2O3), or boron (B), or graphite (C) fibre, a metallic crystal ("whisker"), or even a complex structure consisting of a silicon carbide-coated boron fibre or a core of thin (e.g. tungsten) wire on which a coating of boron (B) or a boron carbon compound (B4C) has been vapour-deposited.

For almost all commercial applications, the matrix or binder is an epoxy or phenolic resin that can be easily moulded. However, if the composite material must also be heat resistant and non-inflammable, only mineral materials or metals can be used. In such cases, manufacturing techniques may be similar to those used in ceramic manufacturing (casting, powder compaction, followed by isostatic compression and sintering). As noted above, recent trends in advanced structural ceramic applications research suggest that physical techniques may be supplanted by chemical methods, such as the "solgel" method (Robinson 1986).

Another approach to the creation of metallic composites is to arrange a two phase system of metallic crystals with the requisite difference in strength and stiffness. Such a system can be created by powder-forming techniques (metalmatrix composites) or by dissolving one metal in another and allowing it to crystallize as a separate phase within the melt under controlled conditions. The result is called a eutectic or "intermetallic" alloy. A number of combinations have been identified that have the requisite characteristics (e.g. Lynch 1975). In recent years a great deal of attention has been given to composites with intermetallic compounds as matrix materials reinforced by strong fibres. The most promising example at present is nickel aluminide (Ni3Al), with small amounts of boron added to increase cohesion and small amounts of hafnium to increase yield strength (Claasen and Girifalco 1986). Other promising matrix materials include NiAl and TiAl.

The list of possible metal-matrix composites and eutectics may get much longer in time, but it is difficult to say whether significant improvements in absolute performance are likely. In any case, the primary objective of R&D over the next few years is to improve predictability, consistency, and formability, in order to decrease the cost per unit performance. Well over a decade after their initial introduction into the aerospace industry (for specialized uses in military aircraft and spacecraft), ultra-strong graphite-based composites finally appeared in a few selected commercial products such as tennis rackets, skis, and golf clubs in the late 1970s. They are gradually increasing market share as prices come down and designers learn how to utilize the new materials to best advantage. However, there are many other potential "civil" applications where strength, light weight, and corrosion resistance will make a difference. Bicycle frames, motorcycles, and small light aircraft would probably be the next obvious applications, followed by substantial use in commercial aircraft.

In principle, composites can replace aluminium for most of the structural parts of any aircraft, including the exterior "skin," and a significant part of the engine. Even small savings in weight in aircraft (or spacecraft) have a significant pay-off in terms of fuel economy or, equivalently, increased payload.

Undoubtedly, these materials will ultimately have a significant impact on the economics of air transportation. Commercialization has been slow, up to now, because of the long product "life cycle" in the aircraft industry, the specialized knowledge involved, and the fact that most of it was initially proprietary to the aerospace industry. All of these factors result in rather high costs. However, most of the basic patents have already expired and the key "process'' patents are currently expiring. This will open up the field to more intense competition. It can be expected that the ratio of "composites" to metals in newly designed subsonic aircraft will rise rapidly through the 1990s. For example, all the control surfaces on the Boeing 757 and 767 aircraft are made of graphite-epoxy composites, yielding a saving of 856 lb in weight and a 2 per cent saving in fuel (Clark and Flemings 1986).

Beyond aircraft applications, there will eventually also be important applications in automobiles. Until 1980 or so, only fiber glass (FRP) had found a significant automotive use (in the Chevrolet Corvette body). But an increasing number of bumpers and body panels and some complete metal automobile bodies are being replaced by unreinforced thermoplastic polymers, so the first major opportunity for lightweight composites may be to replace steel in the chassis and frame. The overall proportion of plastics in the weight of an average auto has increased quite sharply in recent years - it is now between 10 and 15 per cent - and this ratio can be expected to continue to grow in the future. Use of plastics in automobiles will accelerate if ways are developed to recycle the plastics more effectively than at present. (Currently, the plastics from junked cars are mainly dumped in landfills or incinerated, whereas the metals are largely recycled.)

Meanwhile, as noted above, ceramics may ultimately replace much of the metal in the conventional auto engine, and high-strength low alloy steel will continue to replace mild steel in chassis and frame. The benefits of weight reduction in automobiles (and trucks) are not as great as in the case of aircraft but are nevertheless significant. Most of the increased fuel economy observed in automobiles since 1970 is attributable to lighter weight and better tyres - not to more efficient engines. However, it is clear that a great deal remains to be learned about large-scale manufacturing with composite materials before they can replace metals in mass-produced products.

Up until now, the auto industry has not invested much effort in this field. In view of the long lead-times in the industry, polymer-matrix composites (except FRP) cannot be expected to begin to replace steel in major automobile structural parts such as the chassis and frame until probably after 2010. However, ultra light metal-matrix composites such as aluminium-silicon carbide are beginning to replace old materials such as cast-iron in brake rotors, brake calipers, and engine blocks. The use of aluminium and magnesium will significantly increase in the next generation of motor vehicles, which will be half to two thirds the weight of the current generation of cars. The weight reductions, together with engine performance improvements and continuing aerodynamic improvements (thanks to CAD) and continuing tyre performance improvements, will cut fuel consumption per vehicle-kilometre by at least a factor of two.

Another application would be for second-generation supersonic aircraft, now being developed by several countries. Such an aircraft would probably utilize up to 50 per cent polymer-matrix composites, plus 10 per cent meta-matrix composites, 15 per cent aluminium lithium alloy, and 25 per cent other metals such as steel, aluminium, and titanium (Steinberg 1986).

Electronic materials

The category of electronic materials includes "ordinary" conductors, semiconductors, superconductors, photoconductors, photoelectrics, photovoltaics, photomagnetics, ferromagnetics, diamagnetics, paramagnetics, magnetostrictives, piezo-electrics, laser materials, and a host of others. Even a brief summary of the physical phenomena involved would be far too long for a chapter such as this.

Since the development of the transistor in 1947 - as a substitute for the electron tube or "vacuum tube" - research in the field of semiconductors has grown spectacularly. The rapid growth of basic knowledge about the materials has been driven by burgeoning demand for electronic devices, from telephone switchboards to radio, television, radar, sonar, and computers. The last application has proved the most important, especially after the successive development of integrated circuits (c. 1960) followed by the "microprocessor" (c. 1970), and then large-scale integration (LSI), very large-scale integration (VSLI), and now ultra large-scale integration (ULSI). Table 4.4 summarizes these dramatic changes.

One of the key technological driving forces, whose impact seems to have been consistently underestimated, is the close relationship between operating speed, power consumption, cost, and scale. The original motivation for the invention of the transistor was to cut down on the electric power consumption of the telephone switching systems. Miniaturization and large scale required the solution of many difficult technological problems such as controlling even smaller line widths (fig. 4.9). However, as these technical problems were solved, it proved to be a powerful cost-cutting strategy, because manufacturers' sharply declining semiconductor circuitry costs, in turn, generated steady increases in demand, including wholly new applications (fig. 4.10).

Table 4.4 The development of semiconductors

     

Integrated circuit

   

Period of diffusion

Vacuum tube to 1945

Transistor

SSI

MSI

LSI

VLSI

   

1955

1965

1975

1985

lntegration (elements per unit or per chip) 1 unit 1 unit 2-100 100-1,000 1,000-100,000- 100,000-1,000,000
        1,000 100,000 1,000,000
Functions per unit or per chip 1-2 1 100   10,000 100,000
Reliability per function 0.05 1 30   1,000 10,000
Price per function(per chip or per unit) Y300 Y10 Y1   Y0.05 <Y0.05

Source: NIRA (1985).

Fig. 4.9 Changes in the scale of integration and minimum line width (Data source: Electronic Industries Association of Japan)

The growth of demand for more "computer power" seems to be continuing unabated, as costs continue to fall. In fact, major new categories of computer and communications applications, such as voice processing, vision processing, and "artificial intelligence," are just beginning to emerge (table 4.5). The silicon chip continues to dominate all challengers; its eventual replacement by other more exotic materials continues to be delayed into the indefinite future. However other (faster) semiconductor materials, such as gallium arsenide, may eventually have their day.

Fig. 4.10 Changes in computing power and computer usage (Source: Moravec 1991)

It is literally impossible to forecast with any confidence the "winners" and "losers" in this intense competition. A few conclusions can be drawn, however:

- Switching speeds and micro-miniaturization can still be increased by orders of magnitude, in principle, exploiting optical technologies now becoming ever more important (table 4.6).

- Manufacturing techniques are becoming more and more critical. The need for microscopic tolerances and ultra-low levels of impurity contamination require increasingly sophisticated (and expensive) and totally automated robotics facilities.

- Design complexity is becoming the limiting factor. Sophisticated CAD is already essential for "chip" design and every successive generation of more powerful memory or microprocessor chips5 will certainly require correspondingly more powerful CAD software, probably including artificial intelligence (AI) to perform some of the functions performed by human designers at present. This, in turn, will emphasize the role of the very few research institutions capable of assembling a "critical mass" of front-rank AI researchers, applied mathematicians and logicians, and electronics and software specialists.

For the above reasons, the semiconductor, telecommunications, computer, and software industries are now inextricably linked and marching together. "New starts," small firms, and small countries are now essentially out of the game, as far as leading-edge microelectronics technology is concerned. (This is not true for software, of course.)

One of the major apparent opportunities for research in the field of electronic materials has been superconductors.6 The advent of a practical commercial helium liquefied in the early 1950s resulted in an explosion of exploratory research in this field. Only a few superconductors were known up to that time, but by 1970 several hundred superconducting compounds and alloys had been identified. Moreover, by that time superconducting magnets were being sold commercially (by Westinghouse). Such magnets are now standard laboratory research tools, and will be used for any new large particle accelerators or, probably, for future magnetic levitation ("mag-lev") rail systems.

Table 4.5 Breakthroughs expected in electronics

 

Field

Technological need

Current technology

New technology

Performance comparison

Communications Large-volume transmission Microwaves Milliwaves 10 times
Information-processing High-speed processing Silicon LSI GaAs 5-6 times
    Josephson-junction device At least 10 times
High-density memory Horizontal magnetic recording Perpendicular magnetic recording At least 10 times
Larger-scale integration devices Planar integration Three dimensional circuit devices -
Instrumentation and control Improved sensitivity - Josephson junction device (10-6-10-7G)
  Improved resolution - Ultrasonics (microscope) (1 An or less)

Source: Hitachi Research Institute (n.d.).

Table 4.6 Breakthroughs expected in optics

Field

Technological need

Current technology

New technology

Performance comparison

Communications Large-volume transmission Milliwaves (1011 Hz) Laser light (1014 Hz) 1,000 times
Long-distance transmission (relaying distance) Electromagnetic waves (1 km) Laser light (10-100 km) 10-100 times
Transmission cost reduction (cable weight) Coaxial cable (130 kg/m) Optical fibre cable (70 g/m) About 1/2000
Information-processing High-speed processing Josephson junction device (6-7 pieoseconds) Laser light (10 picoseconds) 0.6 times
Intra-CPU transmission (data volume/second) Sequential processing Parallel processing Several dozen times
Spatial image information- processing Unidimensional development needed Parallel processing of two-dimensional image possible Advantageous for image information processing
High-density recording Perpendicular magnetic recording (TOM bits/cm2) Magneto-optic recording (20M bits/cm2 minimum) At least 2 times
Instrumentation and control High reliability:     Light is advantageous
Electromagnetic interference Present Absent  
Crosstalk Present Absent  
Short circuits Present Absent  

Source: NIRA (1985).

On the other hand, the cost of liquid helium has not fallen significantly since 1960 and is not likely to. Many once active projects such as the development of a superconducting computer (IBM) have been dropped. As of 1975 the highest known critical temperature (Al0.8Ge0.2Nb3) was only 20.7K, which was still below the boiling point of liquid hydrogen (22.7K).

The first sign of a major breakthrough was the discovery in 1986 of a new class of barium-copper-lanthanum oxide superconductors, which achieved superconductivity at 35K. In January 1987 partial superconductivity in a similar compound was reported at 52K, under very high pressure. Only a few weeks later another metallic oxide compound was reported to be super conductive at 98K. More discoveries are to be expected. Dozens of laboratories around the world are now said to be searching for new compounds capable of superconductivity at even higher temperatures, and many physicists are now optimistic about the possibility of achieving superconductivity at room temperature (Sullivan 1987).

However the 77K barrier, which has now been exceeded, was the truly significant one. Below that temperature only liquid helium is a feasible coolant (except in space), whereas above that point liquid nitrogen (77K) can be used. Liquid nitrogen is available in industrial quantities as a by-product of the production of liquid oxygen used by the steel industry and for rocket propulsion. It costs only 10 per cent as much as liquid helium and is far less volatile. Thus, it is now realistic to think in terms of large-scale applications of superconductivity, e.g. power generation and transmission and magnetic levitation of high-speed trains. Neither of these applications is imminent. However, on a time scale of 50 years, both are rather good bets.

Photovoltaic (PV) materials are another category of potential importance as solar cells. Major candidates include silicon (crystalline or amorphous) and thin films. The latter may be made from gallium arsenide, copper indium diselenide, cadmium telluride, or other combinations not yet discovered. Silicon is by far the most widely used, at present, with achievable solar conversion efficiency of nearly 20 per cent for the crystalline form. Amorphous silicon has achievable conversion efficiency of at least 15 per cent, but it can be manufactured at much lower cost. Laboratory cells have already achieved over 30 per cent conversion efficiency, using concentrator cells, and 40 per cent or more is now regarded as likely by the end of the 1990s.

Some experts think that an 80 per cent conversion rate for sunlight to electricity is ultimately conceivable. This development is of the greatest possible importance. Each unit of electricity generated by photovoltaics instead of coal-burning eliminates emissions of sulphur and nitrogen oxides, volatile organics, coal ash carrying toxic trace metals, and carbon dioxide into the atmosphere.

Apart from progress in the fundamental science, there has been very rapid progress on the technological side. A number of new techniques for coating thin films of semi-conductive materials onto a glass (or other) substrate have been developed, e.g. by Mobil-Tyco, Westinghouse, and Honeywell. Spectacular progress has also been made in reducing film thickness by a factor of 100, as compared with early cells (Zweibel 1987).

NASA, DOD, and Bell Laboratories supported much of the early R&D work in this field to obtain long-lived solar cells for application in satellites. The first solar cells, used mainly by NASA, cost US$1,000 per watt. An array of solar cells in 1975 cost about US$75 per watt of peak capacity (Wp) compared with US$5 per watt for a large nuclear power plant in 1975US$. The "energy crisis" of 19731974 precipitated an accelerated programme of R&D in this area, focused mainly on bringing down the cost of manufacturing. The US R&D programme was cut back sharply in the 1980s (from US$150 million in 1980 to US$43 million in 1987), but not before major progress had been made, as shown in figure 4.11 (Maycock 1982). The modest goals of US efforts to tap solar energy in recent years have been to achieve a competitive final price level of 6 cents per kWh by the year 2010, with module efficiencies of 15-20 per cent. Up until 1990 or so, the market for solar PV power had been restricted to remote locations and special purpose applications (although the market was clearly growing). But the energy utility industry had not shown much interest. However it now appears likely that some big firms (notably Enron Corp.) have decided to invest in mass production of solar cells with the deliberate intention of bringing the price down more rapidly, perhaps even by 2000. More recent progress in this field is discussed in chapter 7 in this book.

Another important category that is worth discussing briefly is ferromagnetic materials (White 1985). In a way, this is surprising, because the phenomenon of ferromagnetism has been known for such a long time. However, as in the case of "strong materials," the relationship between magnetic fields on the micro (inter-atomic) scale and the macro scale was not adequately understood until the 1940s when Neel, Kittel, and others developed the basic physical concepts that have dominated subsequent R&D in magnetism. The progress in basic physics of ferromagnetism was rapidly translated into increased practical interest, especially because of the growing importance of magnetic materials.

Photovoltaic module and system price goals (Source: Maycock 1982)

Ferrites- a new class of magnetic oxide materials, mainly Fe2O3 were first used for data recording in the 1930s.7 Ferrites also rapidly found applications in transformers, radar, communication equipment, and (by the late 1950s) computer memories. Discrete ferrite "core" memories have long been superseded by high-speed semiconductors; but ferrite-based magnetic tapes and disks remain the major form of read-in/read-out medium- to long-term data storage system. (It is not yet clear to what extent optical storage devices will ultimately replace magnetic devices, if ever.)

New non-iron-based ferromagnetic alloys for permanent magnets also began to be discovered in the 1930s, beginning with the Al-Ni-Co family. This was mainly research by trial and error. In the 1950s Phillips Laboratories produced permanently magnetized ferrites based on iron oxides combined with strontium or barium, aligned in powder form, then compacted and sintered. The rare-earth-cobalt (SmCo5) based permanent magnets (REPMs) were discovered in 1967 and first commercialized by 1970. A second generation series based on Sm2Co17 was introduced around 1981, and an important boron-based compound Fe14Nd2B appeared in 1983.

For permanent magnets there are two important parameters, namely energy product (the amount of stored magnetic energy8) and coercivity (the resistance to reversal or demagnetization by an external field, Hc). Progress since 1900 in these two areas is shown in figures 4.12 and 4.13, respectively. It is interesting to note that the theoretical maximum value of stored magnetic energy for iron would be 107 mega-gauss oersteds (MGOe) (if all the microdomains could be completely aligned), and in the case of other alloys it may well be much higher. Thus, there is still room for significant progress in this area.

Applications of permanent magnets are widespread in many types of devices, but perhaps the most important single application is for special purpose electric motors. Recent improvements in magnet performance can be expected to be reflected in improved electric motor performance. In fact, a whole new class of compact motor designs now appears practical (White 1985). This, in turn, will result in at least some significant new applications. For instance, compact high-power electric motors could replace hydraulic motors in robots, resulting in a substantial increase in speed of operation. However, the most attractive application for new types of powerful compact electric motors would be to propel electric vehicles. This is a major topic in itself, however, and we cannot discuss it at length here.

Fig. 4.12 Change in energy of various permanent magnet materials (Source: NAS/NRC 1989)

Fig. 4.13 Change in coercivity of various permanent magnet materials (Source: NAS/NRC 1989)

One final example of electronic materials worth mentioning is a class of organic liquids whose viscosity is strongly dependent on the imposed electric field. When a transverse field (voltage) is imposed, such a liquid becomes extremely viscous - almost glassy; yet when the field is removed it flows freely. This class of materials could conceivably become the basis for electrically controlled clutches, brakes, or robotic grippers, thus eliminating much of the mechanical complexity that now plagues such devices. However, much research remains to be done, primarily in the optimization of the molecular synthesis and the scale-up of manufacturing technology.


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