Had a little search on Inspec and compendex (research paper databases)
This paper:
THE ALUMINUM FLAT HEAT PIPE USING CYCLOPENTANE AS WORKING FLUID
Takahiro Shimura, Hitoshi Sho and Yoshio Nakamura
The Furukawa Electric Co., Ltd.
Ecology & Energy Laboratory
2002 Inter Society Conference
on Thermal Phenomenir
For two heat pipes cyclopentane and hcfc123 obtained dT was:
dt = dT[initial] + a(1- e ^ (-B*t))
where a (kelvin) and b (10^-3 / hours) are constants.
For cyclopentane in their experiment they list
a= 1.73
dt(initial) =1
B = 1.39
for a period up to 1500hrs (realively low amount of time)
How this corresponds to computer heatsinks is unclear but the extrapolation shows that while they degrade over time they only degrade to a lower performance level not to infinity.
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