That's a good question and I do not have an answer because manufacturers do not publish that information. That question came up a couple of years ago and things got kind of crazy. There was all sorts of speculation but no hard reliable information.
You CAN'T make heat pipes unless you can create a vacuum before filling the fluid. HOWEVER, you CAN buy some heat pipes here: http://www.enertron-inc.com/heatpipe.asp
I bought 3mm x 300mm Copper heat pipe from them for a Science Fair project my Freshmen year in high school.