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Amorphous Metals in Electric-Power Distribution Applications

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Amorphous Metals in Electric-Power Distribution Applications Amorphous Metals in Electric-Power Distribution Applications Nicholas DeCristofaro Originally Published by: Materials Research Society MRS Bulletin, Volume 23, Number 5 (1998 ) , P. 50 - 56 www.mrs.org/publications/bulletin re-published electronically 2002 , Metglas® Solutions www.metglas.com Nicholas DeCristofaro INTRODUCTION These factors necessitated changes to the On April 13, 1982, the Duke Power transformer manufacturing process and Company energized an experimental mandated both laboratory and field testing pad-mount distribution transformer in before energy savings could be made Hickory, North Carolina.' The transformer, available at a commercially practical cost manufactured by General Electric, provided and with an acceptable reliability. This electric power to a local residence. That same article traces the developments leading to month, the Georgia Power Company installed the commercialization of amorphous-metal a similar transformer, made by Westinghouse distribution transformers. Electric, 2 atop a utility pole in Athens, Georgia. It supplied electricity for the The Discovery of exterior lights at the Westinghouse Newton Amorphous Metals Bridge Road plant. These devices shown in Figure 1 were unique among the nearly 40 Metal alloys typically possess crystalline million distribution transformers in service in atomic structures in which individual atoms the United States because their magnetic are arranged in ordered, repeating patterns. cores were made from an Fe-B-Si Amorphous-metal alloys differ from their amorphous-metal alloy. This new material, crystalline counterparts in that they consist produced by Allied Signal (formerly Allied of atoms arranged in near random Chemical), was capable of magnetizing more configurations devoid of long-range order. efficiently than any electrical steel. By Although such noncrystalline structures are replacing grain-oriented silicon steel in the common in nature, they have normally been transformer cores, the amorphous metal re- associated only with nonmetallic systems. duced the core losses of the transformers by For example, noncrystalline solids can be 75%. formed from silicates by continuous cooling Although distribution transformers are from the liquid state. The disordered liquid relatively efficient devices, often operating at structure is preserved when the cooling rate efficiencies as high as 99% at full load, they is sufficiently great to prevent atoms or lose a significant amount of energy in their molecules from aligning into ordered, use. Because of the number of units in crystalline configurations. In silicates, service, coupled with the fact that the core which consist of three-dimensional atomic material is continuously magnetized and clusters, the liquid state is viscous. demagnetized at line frequency, transformers Individual molecules have limited mobility account for the largest portion of the energy Figure 1. (a) Experimental General and crystallization proceeds slowly. Only losses on electric power distribution systems. electric pad-mount amorphous -metal modest cooling rates are required to 9 transformer. (b) Experimental It is estimated that over 50 X 10 kWh are Westinghouse Electric pole-mounted suppress crystallization entirely. dissipated annually in the United States in the amorphous metal distribution By contrast, liquid metal alloys are form of distribution transformer core losses.3 transformer characterized by low viscosity and high At today's average electricity generating cost diffusivity, partly because they consist of of $0.035/kWh, that energy is worth over loosely bonded atoms rather than bulky $1,500 million. clusters or molecules. The individual atoms Currently over 1,250,000 amorphous metal in a liquid metal alloy can move about distribution transformers have been installed freely. On cooling, atomic rearrangement worldwide, helping electric power utilities and crystallization occur rapidly, suggesting improve the efficiency of their transmission that extraordinary cooling rates would be and distribution systems. However the necessary to bypass crystallization. application of Fe base amorphous metal to The first observation of metallic alloys with transformers would not be a straightforward noncrystalline atomic structures was made substitution for grain-oriented silicon steel. in 1950 at the National Bureau of 4 The amorphous metal used in transformers is Links of Science and Technology describes the Standards. A. Brenner reported that thinner, harder, and more fragile than the origin, science, technology, and unique amorphous Ni-P alloy films could be silicon steel it replaces. The amorphous metal accomplishments of advances in materials science produced by electrodeposition. Although transformer must be compatible with the that have significant practical value amorphous Ni-P is widely used as a existing power system distribution system and must survive 30 years continuous service. hard-surfacing material, Brenner's startling and through the use of extraordinary observation of the amorphous structure has processing techniques. The rapid gone relatively unnoticed. solidification method employed by Duwez The discovery of amorphous metals is yielded cooling at rates of approximately generally credited to P. Duwez, who in 1960 105 K/s at the instant of solidification. produced amorphous samples by rapid Within the splat-quenched sample, the quenching an AU75Si25 alloy from the liquid solidification interface traveled at speeds state. 5 Duwez used a pressurized gas gun to approaching 100 mm/s. By comparison, propel small droplets of the molten alloy onto continuously cast slabs of steel experience a polished copper plate. On impact, each cooling rates of only about 1-10 K/s and droplet deformed into a thin film. Intimate exhibit solidification interface velocities of contact with the highly conductive copper about 1-10 mm/s. plate allowed the molten film to cool rapidly These extremes highlight a fundamental and solid1ify into flake or "splat" form. Ironi- limitation of rapid solidification processing. cally, this discovery came as a surprise. To achieve cooling rates of 105 K/s, at least Duwez adopted the rapid solidification, "splat one dimension of the quenched material quenching" process to study solid solubility must be small so that heat can be efficiently and phase separation in crystalline extracted. Typically, the small dimension of metal-alloy systems. the rapidly solidified material is 25-50µ tm. H. Cohen and D. Turnbull suggested that To facilitate heat extraction, the material the formation of the amorphous AU75Si25 must also be in contact with a highly structure was related to the presence of a conductive medium. deep eutectic in the Au-Si alloy system near In Duwez's case, the quenched sample 25 at.'% Si coupled with the rapid was in the shape of a flattened droplet, and solidification rate achieved in Duwez's the highly conductive medium was a copper experiment.6 The incorporation of 25% Si plate. Subsequent studies of amorphous into molten Au reduces the melting point of metal attempted to produce larger samples, Au from 1336 K to about 970 K .7 Thus the either as multiple droplets or as elongated molten AU75Si,, alloy could cool to a splats. The conductive medium also took on relatively low temperature without a variety of shapes such as an inclined solidifying. At the reduced temperature, plane, a piston and anvil, twin pistons, and atomic diffusion would proceed slowly, counter-rotating rollers. Ultimately, these permitting the alloy to solidify without efforts led to the production of continuous, crystallization. The association of amorphous metal filaments via a process amorphous-metal formation with both rapid known as chill block melt spinning. solidification rates and eutectic compositions Modeled after the work of E.M. Lang," who has formed the basis for virtually all in 1871 produced solder wire by casting a subsequent studies of this material form. stream of molten alloy onto the outer In the early 1970s, H.S. Chen and D.E. Polk surface of a rotating drum, chill-block melt at AlliedSignal conducted the most spinning has taken on many forms. In each exhaustive Study of amorphous-metal case, a stream of molten metal is directed at formation.8 This work defined alloy com- a moving substrate. The stream forms a positions that-on rapid solidification formed "puddle" on impact with the surface of the stable, amorphous structures. The alloys were moving substrate, and the puddle serves as a Figure 2(a) Chill block melt spinning by described by the general formula M70-90 Y10 - local reservoir from which a filament or free-jet casting. (b) Free Jet Cast amorphous metal ribbon. 30 Z0.1-15 ,"'here M is one or more transition ribbon is continuously formed and chilled metals, Y is a nonmetallic element (such as as shown iii Figure 2. B, P, or Q, and Z is a metalloid (such as Si, Two common and historically important On impact with the surface of the moving Al, or Ge). Virtually all amorphous-metal variations of chill-block melt spinning are substrate, large cylindrical jets produced products manufactured follow this basic free-jet melt spinning and planar flow nonuniform puddles and hence recipe. casting. In the free-jet process, molten metal nonuniform ribbon shapes. Driven by the is ejected under pressure from a cylindrical high surface tension and low viscosity of Rapid Solidification nozzle to form a free jet. Using this process, molten metal, rectangular jets tended to Process Development the first commercial amorphous
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