ELECTRICAL-RESISTANCE METALS AND ALLOYS
This major family of metals, including alloys as well as pure metals, includes resistance alloys used in controls and instruments to measure or regulate electrical performance, heating alloys used to generate heat, and thermostat metals used to convert heat to mechanical energy. There are seven types of electrical-resistance alloys: (1) radio alloys, which contain 78 to 98% copper with the balance nickel; (2) manganins, 87% copper and 13 manganese or 83 to 85 copper, 10 to 13 manganese, and 4 nickel; (3) constantans, 55 to 57% copper and 43 to 45 nickel; (4) nickel-chromium-aluminum alloys, 72 to 75% nickel, 20 chromium, 3 aluminum, and either 5 manganese or 2 copper, iron, or manganese; (5) iron-chromium-aluminum alloys, 73 to 81% iron, 15 to 22 chromium, and 4 to 5.5 aluminum; (6) various other alloys, mostly nickel-base alloys, some of which contain substantial amounts of chromium or iron, chromium and iron, chromium and silicon, and, in some cases, manganese and aluminum; and (7) puremetals, notably aluminum, copper, iron, nickel, precious metals, and refractory metals.
Key characteristics of resistance alloys are uniform resistivity stable resistance, reproducible temperature coefficients of resistance, and low thermoelectric potential compared to copper. Less critical, but also important, are the coefficient of thermal expansion; strength and ductility; corrosion resistance; and joinability to dissimilar metals by welding, brazing, or soldering. Heating alloys require high heat resistance, including resistance to oxidation and creep in particular environments, such as furnaces, in which they are widely used; high electrical resistivity; and reproducible temperature coefficients of resistance. Also desirable are high emissivity low coefficients of thermal expansion, and low modulus to minimize thermal fatigue, strength, and resistance to thermal shock, and ductility for fabricabil-ity. Thermostat metals, two or more bonded materials of which one may be nonmetallic, are chosen based on different electrical resistivities and thermal expansivities so that applied heat can be converted to mechanical energy.
The electrical resistivity in ohm • circular mil/ft (n£l • m) for the alloys encompassed by the seven groups range from 9.6 (16) for silver to 872 (1,450) for an alloy made up of 72.5% iron, 22 chromium, and 5.5 aluminum. Iron has the highest resistivity, 583 (970), of the pure metals, followed by tantalum, 81 (135); platinum, 64 (106); nickel, 48 (80); and tungsten, 33 (55). The radio alloys are in the 30 to 180 (50 to 300) range, the manganins 228 to 289 (380 to 480), the constantans 295 to 300 (490 to 500), and most of the nickel-chromium-aluminum, iron-chromium-aluminum, and various other alloys are in the 610 to 872 (1,015 to 1,450) range. Temperature coefficients of resistance in parts per million per degree Fahrenheit (Celsius) range from ±10 to ±15 at 59 to 113°F (15 to 45°C) for the manganins to +6,000 at 68 to 95°F (20 to 35°C) for pure nickel. Thermoelectric potentials versus copper in the (xV/°F range from —43 to 77 to 221°F (25 to 105°C) for the 57 copper–43 nickel constantan to +12.2 at 32 to 167°F (0 to 75°C) for pure iron. The refractory metals have the lowest coefficients of thermal expansion, aluminum the highest, and most of the alloys are intermediate. Tensile strength and ductility also range widely depending on the alloy or metal. Maximum operating temperatures in air for the commonly used resistance-heating alloys range from 1700°F (927°C) for 43.5 Fe–35 Ni–20 Cr–1.5 Si alloy to 2505°F (1374°C) for 72.5 Fe–22 Cr–5.5 Al. For platinum, this temperature is 2750°F (1510°C). The refractory metals are suitable for still higher temperatures in vacuum and, in the case of molybdenum and tungsten, in select environments.
Electrical-resistance alloys are mainly wire products, and the alloys have been known by a multitude of trade names. The standard alloy for electrical-resistance wire for heaters and electrical appliances is nickel-chromium, but nickel-manganese and other alloys are used. For consumer products made in large quantities, cost and the relative scarcity of the alloying elements are important considerations. For high-temperature furnaces, tungsten, molybdenum, and alloys of the more expensive high-melting metals are employed. The much-used alloy with 80% nickel and 20 chromium resists scaling and oxidation to 2150°F (1177°C), but it is subject to an intergranular corrosion, known as green rot, which may occur in chromium above 1500°F (816°C) unless modified with other elements. The 80–20 alloy has a resistivity of 354 X 10~8 £1 • ft (108 X 10~8 £1 • m). The tensile strength of the annealed wire is 100,000 lb/in2 (689 MPa), with elongation of 35%, and the hardness is Rockwell B 80. The specific gravity is 8.412. In many appliances, high elongation is undesirable because it causes the wire to sag.
In times of nickel stringency, or for cost reduction, various nickel-chromium-iron alloys are used. An alloy of 60% nickel, 16 chromium, and 24 iron has a resistivity of 675 £1 with oxidation resistance to 1950°F (1066°C). Tophet C is this alloy. The alloy with 30% nickel, 20 chromium, and 50 iron is resistant to 1560°F (849°C). The resistivity of the low-nickel, chromium-iron alloys is high, and the heat resistance is ample for some types of appliances, but the strength is lower, with a tendency for the hot wire to sag. Cromel AA, of Hoskins Mfg. Corp., is an 80–20 nickel-chromium alloy for continuous service to 2150°F (1127°C). It is modified with small amounts of cobalt, manganese, columbium, silicon, and iron, the columbium stabilizing the chromium to prevent green rot. It is also resistant to carbon pickup which tends to make the chromium-iron alloys brittle. The resistance of the wire is 381 X 10~8 £1 • ft (116 X 10-8ft-m).
The chromium-aluminum-iron alloys have high resistivity and high oxidation resistance, but have a tendency to become brittle. Hoskins alloy 870 contains 22.5% chromium, 5.5 aluminum, 0.5 silicon, 0.10 carbon, and the balance iron. The resistivity is 466 X 10~8 O • ft (142 X 10~8 £1 • m). It is used as wire or ribbon in furnaces to 2350°F (1288°C). The Kanthal alloys marketed as wire and ribbon by Kanthal Corp. have 20 to 25% chromium with some cobalt and aluminum, and the balance iron. Kanthal A, with 5% aluminum, will withstand temperatures to 2370°F (1299°C), has a resistivity of 456 X 10~8 £1 • ft (139 X 10~8 £1 • m), and is resistant to sulfuric acid. The tensile strength is 118,000 lb/in2 (813 MPa) with elongation of 12 to 16%. Kanthal A-1, for furnaces, has a resistance of 872 £1 and an operating temperature to 2505°F (1373°C). The Nikrothal alloys of this company are nickel-base modifications of Kanthal. They have higher tensile strength, up to 200,000 lb/in2 (1,378 MPa), to permit rapid winding of tape without breakage. Nikrothal 6 has 60% nickel, 15 chromium, and 25 iron. Heating tape, of Rogers Corp., designed for heating rocket batteries, is also used for panel heating and is operable at continuous temperatures up to 250°F (121°C). It has three flat wires of copper-nickel alloy encapsulated in Mylar tape 0.008 in (0.020 cm) thick and 0.375 in (0.953 cm) wide in lengths to 250 ft (76 m). The rating is 2 W/ft (6.6 W/m), and the dielectric strength is 2,400 V.
A series of alloys of Westinghouse Electric Corp., called Hirox alloys, contain 6 to 10% aluminum, 3 to 9 chromium, up to 4 manganese, with the balance iron except for small additions of boron and zirconium to reduce the size of the aluminum-iron grains and refine the structure. The alloy with 9% aluminum and 9 chromium has a resistivity of 850 £1 and a tensile strength of 118,000 lb/in2 (813 MPa). At 1300°F (704°C) the tensile strength is 15,000 lb/in2 (103 MPa) with elongation of 94%. Wire will give continuous service in air at 2350°F (1288°C) without failure.
Resistance alloys are generally specified for specific uses rather than by composition. Controlled resistivity over a temperature range instead of high heat resistance is desired for instrument use, while a definite coefficient of expansion is required for spark-plug wire and other uses where the wire is embedded. In some cases, good heat resistance with selected low resistivity is desired. Oxalloy 28, of GTE Corp., is copper wire clad with 28% by weight of chromium-iron alloy. It withstands continuous service at 1300°F (704°C). The resistivity at 1100°F (593°C) is 28 X 10~8 £1 • ft (8.6 X 10~8 £1 • m). Neyoro G, of J. M. Ney Co., used for fine resistance wire in potentiometers and electronic applications where high cost is not a factor, has a high gold content with platinum, silver, and copper. The drawn wire has a tensile strength of 185,000 lb/in2 (1,275 MPa) and high corrosion resistance.
Copper-manganese alloys have high resistivity, an alloy with 96 to 98% manganese having a resistance of more than 16,400 (ifMn3 (1,000 (jXl/cm3). But when the manganese content is high, they are brittle and difficult to make into wire. Addition of nickel makes them ductile, but lowers resistivity. A typical alloy contains 35% manganese, 35 nickel, and 30 copper. A resistance alloy developed by the National Bureau of Standards, called Therlo, contains 85% copper, 9.5 manganese, and 5.5 aluminum. Fecraloy has 15% chromium, 5 aluminum, and the balance iron. It is for temperatures to 1400°F (760°C). Sparkaloy is a spark-plug wire and is a manganese-nickel alloy. The spark-plug wire of Hoskins Mfg. Co., called Hoskins alloy 667, contains 4% manganese, 1 silicon, and the balance nickel. The resistance is 82 X 10~8 Q, ■ ft (25 X 10~8 Q, ■ m), specific gravity 8.4, and coefficient of expansion 15.1 X 10~6/°F (27 X 10~6/K). Manganin contains 80 to 85% copper, 2 to 5 nickel, and 12 to 15 manganese. It has a tensile strength of 70,000 lb/in2 (482 MPa) and a resistance of 157 X 10~8 £1 • ft (48 X 10~8 £1 • m). It is used for coils and shunt wires in electrical instruments and in sheet form for instrument springs. Tophet A is a standard 80–20 nickel-chromium alloy. The tensile strength is 120,000 lb/in2 (827 MPa) and resistance 354 X 10~8 £1 • ft (108 X 10~8 £1 • m). Electrical-resistance alloys, developed by Inco Alloys International, contain 60 to 80% nickel plus chromium and iron. They are used for heater elements, resistors, rheostats, resistance thermometers, and in potentiometers.
Calorite, of General Electric Co., has 65% nickel, 8 manganese, 12 chromium, and 15 iron. Excello metal contains 85% nickel, 14 chromium, and 0.5 each manganese and iron. It is used in electric heaters for temperatures up to 2000°F (1093°C). Alumel, of Hoskins Mfg. Co., intended for temperatures up to 2282°F (1250°C), has 94% nickel, 2.5 manganese, 0.5 iron, and small amounts of other elements. Calido, of Driver-Harris Co., contains 59% nickel, 16 chromium, and 25 iron. Nichrome V, of the same company, is the 80–20 alloy. Nichrome S contains 25% nickel, 17 chromium, and 2.5 silicon. It is marketed in sheet form for temperatures up to 1800°F (982°C). Comet metal, of the same company, used for rheostats, contains 30% nickel, 5 chromium, and the balance iron. It has high strength, up to 160,000 lb/in2 (1,103 MPa), and a resistivity of 570 ft. The Driver-Harris resistance alloy known as Karma contains 20% chromium, 3 iron, 3 aluminum, 0.30 silicon, 0.15 manganese, 0.06 carbon, and the balance nickel. Its melting point is 2552°F (1400°C), its resistivity 800 ft, and the annealed wire has a tensile strength of 130,000 lb/in2 (896 MPa) with elongation 25%. Hytemco, of the same company, is an iron-nickel alloy used for low-temperature wire. The resistance is 6.6 X 10~8 ft • ft (2.0 X 10~8ft • m). Magno is a 95% nickel, 5 manganese alloy of the same company; Climax metal has 74% iron, 25 nickel, and 1 manganese.
ELECTRORHEOLOGICAL (ER) FLUIDS. Suspensions of fine particles, usually polymers, in nonconducting oils or other liquids. When an electric current is passed through them, they turn from liquid to gellike solids or vice versa in 0.001 to 0.0001 s. With the amount of applied voltage governing the degree of solidity, the fluid itself can perform various control functions, such as damping shock and vibration. Particles include aluminosilicate zeolites and polyacene quinones. The most common fluids are silicone oils, gasoline, and kerosene. Advanced Fluid Systems, Ltd., of England, supplies a 20 to 40% solution of lithium-polymethacrylate particles in chlorinated paraffins, silicone, or mineral oil depending on property requirements. ER Fluid Development Ltd., of England, has a series of ERFs that used crosslinked lithium polymethacrylate particles in low-molecular-weight fluorosilicone fluids. Asahi Chemical Industries, of Japan, has developed specially plated and coated nylon spheres in dimethyl silicone.

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