[Amps] Use Caution when using Silicon Dielectric Greases

[Roger (K8RI)]

I didn't find any reference ti Silicon Carbide in Tom's write-up.

There is no carbon involved and unless it's carbon contacts, decomposing 
Silicone dioxide does not form silicon carbide.  The formation of 
Silicon Carbide "normally" requires a high temperature and always the 
presence of carbon.  Typical formation is decomposing HSiCl3 on red hot 
carbon (roughly around 1100C IIRC) The normal result with Silicone 
greases being decomposed by an electric arc is Silicon Dioxide (SiO2) 
which is as abrasive as Silicon Carbide. It just doesn't last quite as 
long.   Automotive greases (hydrocarbon based) will carbonize in arcs, 
or at high temperatures.

I agree with almost everything Tom said except the way he uses the term, 
Viscosity. Having worked in the Silicone and Silicon industry since 
1961, I have seen Silicones with viscositys ranging from 5 to well over 
6000 Cs.  5 is used for dry fly treatment for all you fly fishermen. 
6000 is like a very thick taffy
5 is essentially like alcohol except it forms a water repellent coating 
causing the "dry fly" to float.  I don't know what something with a 
viscosity of 6000 would be used for, although there are several types of 
Silicone rubber with some used at fairly high temperatures. At any rate, 
I'd not call a grease, low viscosity. It's maybe a little less resilient 
than what is used in a standard grease gun.

His uses do not go back far enough, as I said earlier, DC4 Compound 
(dielectric grease) was developed to use in the spark plug seals on B17s 
in WWII by Dow Corning. Its use prevented arcing at very high altitudes. 
Other compounds including Silastic RTVs were developed from that.  DC 
did not patent the compound, but rather kept the constituents and 
process proprietary.  This kept them ahead of the game for over 50 years 
until a competitor reverse engineered the compound. Patent life was far 
shorter back then.  Once the product was reverse engineered, silicones 
became a commodity instead of a specialty product which made for a 
drastic reduction in price.

Oh! Depending on what he means by high power, but Flooded UHF connectors 
will work well beyond the legal limit.  OTOH I've been told it is not 
normally a good idea to flood all the connectors in a coax run.

I do have one caveat when using Silicone dielectric grease. Even 
fingerprints with a filim of Silicone grease on them will prevent 
adhesives and epoxies from sticking to the.  Electrical tape will not 
even stick to rubber like cable jackets where those fingerprints exist.

73, Roger (K8RI)
.
On 7/6/2018 3:42 PM, Jim Thomson wrote:
> Date: Thu, 5 Jul 2018 22:15:51 -0400
> From: "Roger (K8RI)" <k8ri at rogerhalstead.com>
> To: "amps at contesting.com" <amps at contesting.com>
> Subject: [Amps] Use Caution when using Silicon Dielectric Greases
> Message-ID: <c163d4f4-1cf6-7391-4d0f-0a592ccc4c61 at rogerhalstead.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> <Silicone dielectric greases work just fine as long as there is no
> <arcing. If there is an arc in the grease, the grease will break down
> into its constituent components, one of which is Silicon Dioxide.? This
> is not a problem as long as the wiper(s) maintain a constant contact.
> I've used it for years. Even used both DC4 and DC5 in ignition systems
> with only one exception. After all, these compounds were designed for
> and used on the ignition systems of B17s.
>
> However, one distributor cap had a flaw and the grease leaked through to
> the inside. The rotor wiper was fine, but each connection to the spark
> plug wires has a tiny gap. Each gap had a "growth" of Silicon Dioxide
> which eventually grew to the point where the end of the rotor hit it,
> breaking off mostly small pieces which quickly ground the insides into a
> mess. The same thing would happen with an RF arc.
>
> I haven't used a roller inductor in many years, but I used Silicone
> grease with no problems when I did.
>
> For those who aren't aware, Silicon Dioxide is Quartz. Fine Quartz is a
> very tough abrasive!? IIRC it was used for the Space Shuttle windows in
> its crystal form which were made by decomposing HSiCl3 at high
> temperature. I do not know the process was implemented.
>

-- 
Roger (K8RI)


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