The contact heat conductance at diamond-OFHC copper interface with GaIn eutectic as a heat transfer medium Page: 3 of 7

The contact heat conductance at diamond-OFHC copper interface with
Gain eutectic as a heat transfer medium
L. Assoufid and A.M. Khounsary
Experimental Facilities Division, Advanced Photon Source, Argonne National Laboratory, Argonne,
Illinois, 60439, USA
(Presented on 19 October 1995)
The results of an experimental study of the contact heat conductance across a single diamond
crystal interface with OFHC copper (Cu) are reported. Gallium-indium (GaIn) eutectic was used as
an interstitial material. Contact conductance data are important in the design and the prediction of
the performance of x-ray diamond monochromators under high-heat-load conditions.
Two sets of experiments were carried out. In one, the copper surface in contact with diamond was
polished and then electroless plated with 1 gm of nickel, while in the other, the copper contact
surface was left as machined.
The measured average interface heat conductances are 44.7 8 W/cm2 -K for nonplated copper
and 23.0 3 W/cm2-K for nickel-plated copper.
For reference, the thermal contact conductances at a copper-copper interface (without diamond)
were also measured, and the results are reported.
A typical diamond monochromator, 0.2 mm thick, will absorb about 44 W under a standard
undulator beam at the Advanced Photon Source. The measured conductance for nickel-plated copper
suggests that the temperature drop across the interface of diamond and nickel-plated copper, with a 20
mm2 contact area, will be about 10 C. Therefore temperature rises are rather modest, and the
accuracy of the measured contact conductances presented here are sufficient for design purposes.

I. INTRODUCTION
Contact cooling of optical substrates is a common
technique for removal of the absorbed x-ray heat from those
substrates, and is accomplished by placing cooling pads in
contact with the substrate. A thin layer of liquid gallium-
indium eutectic is often used at the interface to improve the
thermal contact and also to provide a strain-free mount for
crystal monochromators and mirrors.1-3 This technique has
been proven to be particularly useful in cooling of diamond
monochromators.3 Considering the small size of single
crystal diamond that can be procured at this time (typically
less than 1 cm2 in area), only a small part of which is
available for contact cooling, efficient cooling is necessary to
avoid high temperature gradients. For design purposes and
in order to predict temperature and the performance of a
diamond monochromator, it is necessary to know the contact
heat conductance at the interface diamond-GaIn-heat sink. A
simple experimental setup was devised and used to measure
this quantity.
It is well known that liquid gallium, as well as its
eutectics, reacts with most metals. For example, copper,
which is very often used as a heat sink, is easily chemically
attacked by liquid gallium and its eutectics even at low
temperature. To prevent this, copper surfaces are usually
plated with a thin layer of nickel. The chemical reaction
between nickel and gallium does not occur up to a
temperature of about 600*C,4 which is much higher than the
operating temperature of a typical monochromator (less
100*C).

Nickel plating introduces an additional resistance to the
heat flow, and, although the thermal conductivity of nickel is
relatively poor compared to that of Cu (0.91 versus 3.98
W/cm-K at 300 K), the thickness of the plating is too small
to make any significant contribution to the temperature drop
at the interface. Therefore, the resistance can mainly come
from the Cu-Ni and Ni-GaIn-diamond interfaces. In order to
evaluate the influence of nickel plating, two sets of
experiments were carried out. In one, the OFHC Cu surface
in contact with diamond was polished and then electroless
plated with 1 pm of nickel, while, in the other the OFHC Cu
surfaces were left as machined.
II. CONTACT CONDUCTANCE
It is well known that, at a solid/solid interface, contact
occurs at a very few discrete spots, which may result in a
large resistance to the heat flow and consequently, a
substantial temperature drop at the interface. Contact heat
conductance, hc, is defined by 5

hc= (P/Aa )/AT ,

(1)

where P is the total power crossing the interface, Aa is the
interface apparent contact area, and AT is the temperature drop
across the interface. The experimental setup was to measure
this temperature drop.

e1995 American Institute of Physics 1

Rev. Sci. Instrum. 66 (9), September 1995

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Assoufid, L. & Khounsary, A.M. The contact heat conductance at diamond-OFHC copper interface with GaIn eutectic as a heat transfer medium, article, January 1, 1996; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc670814/m1/3/ocr/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.

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