Method of producing porous copper plugs combines hydrogen annealing and oxygen annealing. Plugs have high thermal conductivity and small pore size

Holdeman, L. B.

English

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August 1975	 1375-10163 
NASATECH BRIEF 
Marshall Space Flight Center
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Fabrication of Porous Plugs for Control of Liquid Helium 
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The problem: 
Porous-plug phase separators prevent the unneces-
sary loss of liquid helium stored in Dewars. The plugs, 
which are inserted in the vent lines, must have high 
thermal conductivity and small pore size, apprccxi-
mately 1 to 2 jim in diameter. Currently used plugs 
have high thermal conductivity, but the pore size 
ranges between 10 and 15 pm. On the other hand, 
plugs with pore diameters in the 1-Mm to 2-jim range 
are available with ceramic materials which have very 
poor thermal conductivity. 
The solution: 
Copper plugs can be made with pore sizes ranging 
between 1 and 2 pm using a newly-proposed 
fabrication method. 
How It's done: 
The new method takes advantage of certain 
properties of copper and combines two well-known 
procedures: oxygen annealing and hydrogen anneal-
ing. Copper has a very low free energy of surface 
oxidation: If copper is heated at an oxygen pressure of 
10 torr or less, a surface oxide is not formed, but 
instead oxygen diffuses into the bulk to form a solid 
solution. However, impurities in the copper with a 
higher free energy of oxidation will be oxidized. This 
process of oxygen annealing is commonly used to 
produce copper of high electrical purity. 
Under certain circumstances, these oxidized impu-
rities can be made to precipitate at the grain 
boundaries. Oxides of copper can also be made to 
precipitate at the grain boundaries. If such copper is 
subsequently heated in hydrogen, the hydrogen 
diffuses into the copper, reacts with the precipitated 
oxides, and produces separations at the grain 
boundaries. These separations form the necessary 
pores.
The proposed fabrication procedure is as follows: 
a. Copper with the desired grain size is prepared by 
the selected addition of impurities and appropri-
ate heat treatment. 
b. The copper is machined to the desired size and 
shape, and then it is electrochemically polished to 
remove surface damage caused by the machining. 
c. This part is then oxygen annealed. Precipitation at 
the grain boundaries is controlled partially 
through the choice of impurities and the regulation 
of oxygen pressure during anneal, but it is con-
trolled primarily through control of the anneal 
temperature. 
d. If the plug is a disk, an additional refinement at 
this stage is possible. The plug as a whole will be 
larger after firing in hydrogen than before firing 
because of the grain boundary separation. A ring 
of OFHC (oxygen-free, high-conductivity) copper 
can be machined and placed around the disk in the 
furnace before heating in hydrogen. The inner 
diameter of the OFHC ring will be slightly less 
than the outer diameter of the disk after the 
hydrogen anneal. Then upon firing, the plug will 
expand into the OFHC ring and thus will be per-
manently mounted in a retainer ring with an 
expansion coefficient perfectly matching that of 
the plug at all temperatures. The plug can then be 
installed by standard techniques (soldering, 
indium 0-ring seal, or the like) without changing 
the effective cross section of the plug. 
e. The final step is the hydrogen anneal. 
In summary, by the consecutive annealing of 
copper in two different gases, porous plugs can be 
fabricated with the desired pore size. The ratio of 
channel area to metal area is controlled through the 
control of grain size, and total channel area is 
controlled by controlling the oxide precipitation at the 
grain boundaries.
(continued overleaf) 
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Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	
information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19750000163 2020-03-19T20:26:02+00:00Z
. 
Note: 
Requests for further information may be directed 
to:
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Technology Utilization Officer	 Patent Counsel 
Marshall Space Flight Center	 Marshall Space Flight Center 
Code ATO1	 Code CCOI 
Marshall Space Flight Center, Alabama 35812 	 Marshall Space Flight Center, Alabama 35812 
Reference: B75-10163
Source: L. B. Holdeman of 
National Research Council

(MFS-23218) 
Categories: 04 (Materials) 
B75-10163	 03 (Physical Sciences)


Fabrication of porous plugs for control of liquid helium

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