I've been meaning to get back to the dielectric materials topic with
some suggested alternate materials:
At 11:50 PM -0500 1/15/00, owner-amps-digest@contesting.com (RF
Amplifier Discussion Dige wrote:
>I have been advised NOT to use Teflon, Delryn, and a few other
>insulating materials for an RF choke. Instead only use ceramic. It was
>then stated the "D" factor was the reason.
>
>OK, but why as I could not find info on this subject in the ARRL
>Handbook. Any help to point me in the direction of knowledge on this
>subject would be appreciated. May as well learn something while
>building my amp. And I want to do it RIGHT.
The following reply is correct, thanks Dave for inserting facts into
that discussion.
>Teflon is a (fluorocarbon resin) or (flourine plastic) and has the same
>structure as polyethylene, in which the hydrogen is replaced by (flourine).
>OR a part may be replaced by (chlorine), in which case it is called a
>(fluorohalocarbon resin).
>
>Polymerized (tetrafluoro ethylene), aka PTFE, which carries the E.I. du Pont
>trade name of Teflon, formula (CF2*CF2)n is probably the most common
>solid form used for insulating materials, bearings, gaskets, insulating
>tapes, wire insulation, etc. This is a white waxy solid with specific
>gravity of up to 2.3, typical tensil strengths of up to 3500 psi, elongation
>of 250-350%, dielectric strength of 1000 v/mil and melting point of 594*F.
>
>Another commonly encountered material in electronic use and sometimes
>referred to generally as "Teflon", is Teflon FEP, which is a fluorinated
>(ethylene-propylene). In thin films (typically down to 0.0005"), it is most
>frequently found as insulation in capacitors. Typical tensil strength is
>3000 psi, elongation 250%, and dielectric strength 3200 V/mil.
>
>Neither PTFE or Teflon FEP emit phosgene upon decompositon at elevated
>temps.
>
>However, fluorothene plastic (which may be what Rich's info was referenced
>to) has the formula (CF2*CFCL)n and differ from (Teflon) in having one
>chlorine atom on every unit of the polymer chain, replacing the 4th
>fluorine atom. This is a (transparent) material with a specific gravity of
>~2.1, has tensil strengths of up to 9400 psi, but has a lower withstanding
>temp (as related to Teflon) of around 300*F. This material is sometimes
>called "Teflon" due to its similarity in chemical and polymer-structure
>make-up, and will emit phosgene upon decomposition at elevated temps.
>
>73, Dave, K1FK
I worked for E. I. DuPont in RF reseach in the late 1980s. We made a
LOT of dielectric measurements up to 3 GHz. We used Teflon* as a base
material (a substrate or boat) in a lot of heating experiments in
high power RF. It is one of the best materials to use. Even when
there is a direct arc, unless it burns deep into the material, it can
be reused without repeat arcing. Teflon* is also rather inert, so it
does not readily combine with other chemicals in experiments. Surface
arc-resistance of pure Teflon* PTFE (* DuPont trademark) is
excellent, and carbonized arc paths do not easily form on the
surface. As others have already mentioned, the dielectric losses are
among the best for polymeric materials (or even natural materials
besides gases like air and vacuum). It is often used in radomes and
in microwave windows. But it cold flows very easily, so if clamped or
mechanically held, it will eventually deform. So it is rarely used
where mechancal strength is needed. Coil forms should be no problem,
for smaller coils wound with magnet wire, however.
There is a sickness called the Teflon* flu. Machinists can get it
when working the material at higher temperatures near the smoking
point. This is to be avoided. It was not phosgene gas, but a fluorine
byproduct I think. I don't know how harmful it is, but sounds like
Rich's friend succumbed to an extreme case, or something else.
The following Teflon* safety information came from E. I. du Pont de
Nemours and Company website:
"Before using Teflon®, read the Material Safety Data Sheet and the
detailed information in the "Guide to the Safe Handling of
Fluoropolymer Resins, Latest Edition," published by the
Fluoropolymers Division of The Society of the Plastics
Industry-available from DuPont.
Open and use containers only in well-ventilated
areas using local exhaust ventilation (LEV). Vapors and fumes
liberated during hot processing, or from smoking tobacco or
cigarettes contaminated with Teflon® or Tefzel® fluoropolymer resins,
may cause flu-like symptoms (chills, fever, sore throat) that may not
occur until several hours after exposure and that typically pass
within about 36 to 48 hours. Vapors and fumes liberated during hot
processing should be exhausted completely from the work area;
contamination of tobacco with polymers should be avoided. Mixtures
with some finely divided metals, such as magnesium or aluminum, can
be flammable or explosive under some conditions. "
A lot of flexible conveyor belts for RF heating systems in industry,
use glass-reinforced Teflon* FEP polymers with silicone rubber. They
don't get very hot in the intense RF fields used to heat polymers.
To circumvent the weak mechanical properties, microwave industry uses
crosslinked polystyrene, such as Rexolite, Polymer Q200.5, and maybe
a few other tradenames. This stuff is the hard brittle plastic that
is translucent, although it can be polished to be like glass. We use
it to carry 3 Megawatts of RF, as the spider in our 14 inch coaxial
lines in my present work. We buy it in 4 foot sheets, and cut and
turn the discs. We have a special polishing wheel that looks like a
phonograph. Rexolite does leave a carbon track when arced, and it can
really melt into a goo if ignited. But the dielectric properties are
only slightly inferior to Teflon*. K is 2.55 instead of 2.05-2.1.
Loss tangent is still 0.000x range. Its good mechanically. By the
way, 3 Megawatts is about 12 kV RMS, assuming perfect match in the
coax line. It is worse than this, because of the high VSWR during off
tune (Q of our load is 60,000).
Another material used a lot is ultra high molecular weight
polyethylene or polypropylene -also called UHMWPE. This stuff is soft
like Teflon*, a lot cheaper, and has good low loss, if you are
careful in what you spec and buy. However, it is very easy to get the
wrong material, so a dielectric test is really needed before
designing with a piece. This stuff is used for waveguide windows
sometimes.
Amateur applications of lower RF voltage and frequency can get by
with materials such as PVC, G10, maybe even Delrin (very questionable
to me though). In high voltage RF service, in the VHF region, none of
these can be used, due to excessive losses. Believe me, I have tried
them all. I have a standard test i perform, in a 90 MHz dielectric
heater, putting about 15 kV peak RF across a small block. In a few
minutes, most of those materials will either burst in flames, or will
begin to smoke and stink, as they are internally cooked.
If you want to use fiberglass materials, G7 has lower loss than G10
or G11, due to the use of silicone instead of the more lossy epoxy
resins in the other. It is harder to machine and cannot be turned for
coil forms, we tried it 2 years ago and it just delaminated and fell
apart. For the RF choke in the 100 kW 2.8 MHz amplifier, where the
coil is exposed to the E field alternating at the plate of the tube,
I chose polysulfone rod. It is easier to machine than Rexolite
(crosslinked Polystyrene), and has fairly good dielectric properties.
It is sold under one tradename UDEL* or Thermalux*. We are now using
this same material for HV insulator posts for a 90 kV klystron power
supply (not AC dielectric problem, only DC standoff).
Another interesting new material is polyetherimide. This is
trademarked as ULTEM* by GE and Westlake Polymers and Hydex* by
Polypenco/DSM Polymers, I think. It can be had in glass reinforced
sheets, and has good RF properties. It is expensive but very strong
for mechanic support, such has holding an 80 pound tetrode so that it
does not crash into the finger stock at the bottom of the filament
connector in the socket.
There are numerous other materials that are good, mostly more
expensive, like polyimides Torlon* and Vespel* with incredible
strength due to glass loading, etc. These are really beyond the range
of most of us. They should be considered in cases where the ionizing
radiation (gamma, Xrays) is high, and temperatures are high.
The BEST reference book on dielectrics is still one of A. Von
Hipple's books "Dielectric Properties and Materials", MIT. Long out
of print, but I heard that they got reprinted not too long ago. Of
course, these texts don't have all the modern miracle polymers that I
refer to above.....
If it works for you, then use it. But be aware that it may smoke now,
or later, when moisture is absorbed in your material. Pick something
that is not very hydroscopic if you live in a more humid climate than
I do in New Mexico.
73
John
K5PRO
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