Material Selection and Testing For the Magnet Wire

2 Comments

1. 

   Jonathan Demko

   August 1, 2008

   The RRR is the ratio of the resistance at 273K to the resistance at 4K. It can be simply measured using a 4 wire technique, by passing a small current through a sample and measuring the voltage drop at both of these temperatures. There is literature on the properties of different materials available. For copper, NIST Monograph 177, “Properties of Copper and copper alloys at Cryogenic Temperatures” contains data on thermal and electrical conductivity, and the effects of magnetic fields on electrical resistivity of copper. I suggest this would be a start.

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2. 

   Ray Radebaugh, National Institute of Standards and Technology

   August 1, 2008

   I can answer your question. For a solenoid you want to use a material with the lowest possible resistivity. Copper wire is normally used at room temperature and is also a good choice at cryogenic temperatures. The resistivity of metals decreases as the temperature is lowered until it reaches the residual value that is dependent on the amount and type of impurities as well as cold work. The residual value is usually measured at 4.2K. The ratio of the resistivity at room temperature to the residual value is known as the residual resistivity ratio (RRR). Higher purity materials have higher RRR. In your application you want the highest possible RRR. At 20K the resistivity of commercial coppers has just about reached the residual value; that is, it will not decrease any further as the temperature is lowered below 20K.
   Normal copper magnet wire is grade 100, known as electrolytic tough pitch copper. It has a RRR that can range from 50 to 150. Oxygen free copper comes in grade 101 (electronic OFC) and grade 102 (standard OFC). It is much harder to fnd wire of these materials. However, grade 102 has about the same RRR as grade 110. Grade 101 may have RRR values that range from about 100 to 300. The highest values would only be possible in the annealed condition.
   In addition to improving performance by using the highest RRR material, you could also improve performance by increasing the packing factor with the use of square wire. As far as insulations, most of the normal insulations work well at cryogenic temperatures. Certainly polyimide would be a great insulation at some increased cost. It has low outgassing, that may be a concern for some space applications with telescopes.
   For more information on materials at low temperatures you may want to look at “Materials at Low Temperatures,” by R.P. Reed and A.F. Clark, published by the American Society of Metals (1983).

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