Electrical isolating, thermal conductive material

Hi,
Can anybody help me find a material that is an isolator for electricity and that conducts heat?
It has to be "sandwiched" between a PCB (electronical printed circuit
board) and an aluminium holder.
It also needs to be machinable, cheap (if possible ;-) ) and obtainable.
Thanks in advance!
Greets, Johan
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Commercially available material: Gap Pad by Bergquist. We use VO Ultra soft. It is a two ply silicone, one of low durometer to fill in and conform around the PCB, the other a harder version with embedded fiberglass mat for puncture resistance. They have a variety of materials available.
www.bergquistcompany.com/objects/Thermal_Materials/Gap_Pad/ Gap_Pad_Tables/Gap_Pad_Comparison_Table.pdf
Mark
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If you require an insulating ceramic, I suggest either BeO (beryllium oxide) or AlN (aluminum nitride), both commercially available and machinable by grinding/polishing (for BeO, let the vendor do the machining, since the dust is toxic). Either material will give you much improved thermal conductivity over alumina or quartz.
Grant

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Although I can't suggest a material that meets your specs, I can point you to a web site that may be helpful: www.matweb.com. You can enter material property limits (i.e. Thermal conductivity > X and Electrical resistivity < Y) and their material property database will be searched for matches.
--

Dave
dvt at psu dot edu
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