For mm-wave experiments executed at low temperatures, it is desirable to use a stainless-steel waveguide in the refrigerator for thermal isolation and to plate the waveguide with a higher conductive metallic layer to reduce microwave attenuation.
While the electroplating of metal on stainless-steel is a well-established procedure, there are a number of individuals and commercial establishments who have been unsuccessful in plating the interior of WR-10 waveguide sections of 10 cm or longer. This note points out that copper plating of WR-10 stainless waveguides is relatively easy with conformal electrodes.
Petencin1 electroplated short, approximately 8 cm long sections of stainless WR-10 waveguide with copper. He suspended a platinum anode wire under tension down the center of the waveguide and pumped plating solution through the guide. This procedure was previously used to strike 20 cm sections of waveguide with a nickel coating, followed by gold plating of the nickel. The results were intermittent, because the uneven potential distribution in the corners of the guide made the nickel strike problematic.
A successful procedure is reported here for copper plating of the guide. A copper ribbon, 0.5 mm by 1.76 mm in cross-section was used as an anode. The anode was inserted into the waveguide and insulated from it with beads of GE-7031 varnish, separated by 11 mm. The centered anode was separated from the cathodic guide by 0.4 mm on each side. This resulted in a more uniform potential distribution in the plating solution.
The guide was mounted vertically. A flange at the top of the guide was bolted to a Teflon block through which the plating solution was pumped with a peristaltic pump. A positive voltage was applied to a wire attached to the anode that extended through the top of the block. A cleaning solution of 30 percent by volume H2SO4 was passed down the guide with a 1000 A/m2 current applied to the anode for four minutes.
Next, a plating solution of 240 g/l H2SO4 and 112 g/l CuSO4 was pumped through the guide at the rate of 1 cc/sec with 100 A/m2 for five minutes. Then the anode was shifted vertically by 5 mm and the process was repeated to plate the areas under the insulated beads. Finally the guide was rinsed with distilled H2O. Plated waveguide sections of up to 40 cm were produced.
Longer sections should present no problem. Resistivity measurements indicated an average copper thickness of 4 μm. The copper plated guide had a mm-wave attenuation of 6.6 dB/m compared to 23 dB/m for the unplated guide.
It is believed that the use of conformal electrodes would also allow for efficient nickel striking of the interior of the guide followed by gold plating of the nickel. This was not attempted since copper was sufficient for the purpose.
Acknowledgment
The authors wish to thank J.A. Heilman and G. Petencin for helpful conversations. B.D. Shank was supported by an NSF REU grant. This work was supported in part by NSF grant EIA-0085922.
Reference
1. G. Petencin, private communication.
Benjamin Shank received his BS degree in physics from Case Western Reserve University, Cleveland, OH, in 2007. He is currently working in the physics department at Stanford University in pursuit of his PhD.
Jeremiah A. Heilman was awarded his BS degree in physics from the University of Notre Dame, South Bend, IN, in 1997, where he studied high energy states of nuclei with the particle accelerator group. He completed his MS degree developing a quantum computer using electrons on liquid helium with Arnold Dahm in 2005. He is currently pursuing his PhD thesis work with Mark Griswold on the topic of parallel transmission hardware in MRI at Case Western Reserve University, Cleveland, OH.
Arnold Dahm received his PhD from the University of Minnesota and held a postdoctoral position at the University of Pennsylvania. He taught physics at Case Western Reserve University (CWRU) from 1968 until 2001, with Fulbright awards for sabbaticals at the University of Sussex and the University of Mainz. He is currently institute professor of physics, emeritus at CWRU.