thermally conductive silicone rubber mixture? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 01:40:15 am
content: I have a part A/B silicone that I wanted to make a small but some what thick thermally conductive pad with (say 3-4 mm thick with a bevel), for gentle human body slightly deformable thermal contacts that work at temperature ranges near the human body range.The closest I have seen to what I want came from a plasma TV, the tube element of the TV was thermally bonded to a panel in the back with a type of thermal transfer tape that felt a little bit like gritty silpad. I assume they mixed something thermally conductive with whatever they make silpads out of. When I rolled it up, the 'slugs' were significantly massive and felt cold to the touch.What would be a good starting point for a additive to the silicone epoxy to get started on this? My first thought was some kind of copper powder (spherical geometry). I also have some fine silver powder I could try (low micron sized). Has anyone tried something remotely similar before?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 09, 2022, 02:03:51 am
content: As with magnetically loaded resin, you're probably going to have a hard time doing a good job without specific materials (grain shape and size), and special molding processes that tolerate the high viscosity or gelled material (i.e., it's more work than just stirring and pouring).I wouldn't think silver or copper is very important here, unless perhaps in some long format (chop wire? fiber?) so it's able to carry heat some distance through the matrix. Round-ish grains, you're just looking at passing heat from grain to grain through a film of silicone matrix and it just matters that the filler has thermal conductivity much higher than the matrix, not that it's particularly high at all. So, alumina powder is fine here, or zinc oxide, or whatever like any other thermal paste.On the upside, skin's conductivity isn't fantastic, nor is the permissible heat flux, so the conductivity of something applied to it doesn't matter very much...Tim

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 02:11:46 am
content: hmm so if I cut a shit load of fine wires to the correct size and stacked them upright in the mold that might be pretty good.. i wonder if laser cut wire has a nice round non sharp nosethis is small, so if its possible to blunt wire some how, that would work pretty well. Some method like how you use flame on cut glass to get rid of the edge. Or maybe just make it over sized then freeze it and sand it. Then I can pack the mold with cuttings from fine wire, encapsulate it and sand it. I don't like the idea of sharp wire cuttings having human body contact... it seems like it would cause irritation.

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 02:25:44 am
content: I wonder if you can make the mold, solidify the wires in ice, sand it at reasonably low temperature (not sanding rubber), let it dry out, bake, then use silicone. That should give nicely deburred wires. I wonder how well something sands in a block of ice. Otherwise you need to cryosand silicone, if that even works.

author: bill_c
author_rank: Regular Contributor
timestamp: February 09, 2022, 04:33:01 am
content: Just as an example, sure you can find it many other places:https://www.amazon.com/pure-copper-powder/s?k=pure+copper+powderhttps://www.mcmaster.com/powdered-metals/But like T3sl4co1l said, it may not matter.

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 04:43:53 am
content: well I was wondering if it will just fall apart or have shitty mechanical properties. Not sure what proportion to add, what mesh to use, what procedures to use (i.e. preclean).

author: SeanB
author_rank: Super Contributor
timestamp: February 09, 2022, 08:10:59 am
content: You get really thick silpads, so try those, or just take your sensor and pot it into the silicone resin, around half way in, with a bit of open cell foam behind to act as thermal shield, to get a better temperature measurement.

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 09:23:06 pm
content: the silpad would require machining to conform, I want it a slightly weird concave shapeI wonder if rubberiszed abrasive would machine silpad by hand at room temp.

author: wraper
author_rank: Unknown
timestamp: February 09, 2022, 09:31:47 pm
content: Quote from: coppercone2 on February 09, 2022, 09:23:06 pmThermal pads basically come in 3 varieties. 1. Those which can have only a little bit of deformation and are quite stiff. 2. are very soft but still rubbery but regain original shape after pressure is released. Mostly used on GPU RAM and similar applications. Thermal conductivity is not that good. 3. Pads which get squeezed to the shape and don't regain original shape. Have good thermal conductivity and usually used on VRM and similar high thermal dissipation devices.

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2022, 09:45:11 pm
content: I have seen the deformable ones and they are not suited for human body contact, it eventually turns into playdough and contaminates things. That is like a use once and leave alone thing... it seemed chemically susceptible to body fluids. "kneed" one for a while and you might see this, it left residue on fingers. Feels quite 'cold' though.

author: bill_c
author_rank: Regular Contributor
timestamp: February 10, 2022, 02:48:40 am
content: Quote from: coppercone2 on February 09, 2022, 04:43:53 amI would just try the silicone as is before overthinking the solution. It usually conducts heat well enough.Properties will depend on how much you add, more fillers will be less flexible. Fillers are often added to silicone rubber, it is common to add iron oxide to give silicone that red color. Procedures may involve multi axis rotation to keep the copper from falling to the bottom of mold during cure.And you can buy 2 part thermally conductive silicone, here is an example (random site):https://siliconesolutions.com/electrically-and-thermally-conductive.html