Author Topic: Choosing Mica sheet for high temp  (Read 2329 times)
Ash
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Choosing Mica sheet for high temp « on: January 04, 2017, 03:48:57 PM » Author: Ash
I want to get a Mica sheet from Ebay (those that look like thin Cardboard made of Mica) to make an assembly for a terminal to which heater wire is connected, similar to how the heater is stretched on Mica sheet in a toaster

Are all Mica sheets the same thermally ? As in - Is their max temperature that of the Mica mineral itself, or is there some cement or epoxy in them that would limit the max temp to a lower value ? If so, then what to look for ?

If i use Aluminum parts (like an ordinary Aluminum rivet with pulling stem) in the assembly - Does Aluminum behave "nasty" at high temps ? As in - is such use of an Aluminum part, at temp of maybe few 100s degC but below 660degC, susceptible to fail after some slow degradation process in the Aluminum ?
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Medved
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Re: Choosing Mica sheet for high temp « Reply #1 on: January 06, 2017, 02:42:09 AM » Author: Medved
Are all Mica sheets the same thermally ? As in - Is their max temperature that of the Mica mineral itself, or is there some cement or epoxy in them that would limit the max temp to a lower value ? If so, then what to look for ?

You are probably talking about mica based paper (normally the mica mineral is almost transparent). So obviously the thermal robustness will be somewhat worse than with the pure mica. But with a reasonable designed product, it should still be usable.
The product datasheet shall list all the figures, but I do understand, it will be rather hard to get it from most ebay sellers...



If i use Aluminum parts (like an ordinary Aluminum rivet with pulling stem) in the assembly - Does Aluminum behave "nasty" at high temps ? As in - is such use of an Aluminum part, at temp of maybe few 100s degC but below 660degC, susceptible to fail after some slow degradation process in the Aluminum ?

Aluminium should not be used that much above about 100degC (or so; it highly depend on the alloy composition and structure) for long term. The thermal stress causes the surface oxide layer to crack, so the oxidation then continues through bulk of the material, so gradually chew through the whole material
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Ash
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Re: Choosing Mica sheet for high temp « Reply #2 on: January 06, 2017, 03:56:03 AM » Author: Ash
What about M3 screws/nuts made of unknown Steel and plated with unknown something else ? (they come in a few shades, so the plating is probably some alloy that varies)
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Medved
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Re: Choosing Mica sheet for high temp « Reply #3 on: January 08, 2017, 10:57:18 AM » Author: Medved
I think steel is the only "generic hardware" metal suitable for such high temperatures.
If you don't care how the nuts/bolts will be looking, any form will be OK.
If the material and/or surface treatment are not designed for high temperature (high temp stainless steel alloy,...), it will just change color and maybe permanently stuck together (so you may not be able to loosen it afterwards), but otherwise any steel will hold the thing well.
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Re: Choosing Mica sheet for high temp « Reply #4 on: January 08, 2017, 11:04:23 AM » Author: Ash
I am concerned about possibility of the Steel massively rusting (which i seen happening where inappropriate Steel screws are used in high temp places). Is there way to prevent that and prevent the screw and nut from getting stuck too hard (which i think is caused by the bulging of the rust) ?
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Re: Choosing Mica sheet for high temp « Reply #5 on: March 03, 2017, 04:22:07 PM » Author: Lodge
You could use brass rivets, they seem to last about three years in my toaster and it cooks about a loaf a day.. The aluminium will oxidize over time and fail, just like aluminum wiring connected to copper only you have aluminium connected to ni-chrome or kanthal, different wire but the same problem and with the addition of heat it will make it fail faster.   

Or you can cut a strip off the mica and lay it over the coils in the middle and just rivet mica to mica, pinching the wires in place, and just nip little slits in the end of sheet to hold apart so they don't slide, doing this shouldn't affect aluminum rivets unless you get it hot enough to melt them. But leave enough slack on the wires to allow for a bit of expansion and contraction with the heating and cooling..

If you use cement you can get refractory type cement which will survive very high temperatures, commonly used in kilns, and it can be found in most hardware stores that sell fireplaces or you could use a high temp glue like Sodium silicate (waterglass) it's cheap like $15 / gallon, simple to apply and once it's dry it won't melt until 1900 F and it's commonly sold as gasket adhesive for fireplaces or as concrete sealant or egg preserver (who knew you can keep an egg for like 18 months in a root cellar without any issues, if you dip the egg in water glass let it dry and keep it cool your good to go in the Zombie Apocalypse, if only you could keep bacon that long)       

What are you building / repairing ?
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Ash
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Re: Choosing Mica sheet for high temp « Reply #6 on: March 03, 2017, 05:44:36 PM » Author: Ash
In the end i used big staples bent out of heavy gauge Galvanised Steel wire and pressed into place with a vise

I tried to make a heater panel out of a heating element of a toaster
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James
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Re: Choosing Mica sheet for high temp « Reply #7 on: March 18, 2017, 09:56:23 AM » Author: James
There are indeed differences in grades of Mica.  There are basically two different sorts ; natural and man made.  The best is pure natural mica, which is transparent.  This used to be used as the heat baffle plates in the necks of old lamps but it is expensive, because it is split into sheets by hand in Madagascar, where practically all of it comes from.  Then there is man-made mica, which is made by shredding rough blocks of the mineral into small pieces and then rebonding together to form man-made sheets.  The thermal performance and strength of this material is limited by the bonding materials and production process.  This mica is not transparent and often has a texture pressed into the surface, it is much cheaper and is more common in modern lamps.

The properties of mica can be further changed by various thermal treatments in different atmospheres.  Clear mica has one drawback in that it is transparent to infrared radiation.  Its surface infrared reflectivity can be improved by heating to about 1050°C in wet hydrogen for about 15 minutes - that makes the sheets expand in thickness and become partly opaque.  They lose some mechanical strength in the process.

For connections, the standard practice is to drill two or three small holes in the mica, and tie a short length of nickel wire to the sheet.  Then by spot-welding, crimping or simple twisting, connect one end of that wire to the resistance wire.  Stainless steel wire is also pretty good for this application.
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Ash
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Re: Choosing Mica sheet for high temp « Reply #8 on: March 18, 2017, 10:38:49 AM » Author: Ash
I have opened once a big 80s voltage regulator, there was a transparent sheet of isolator under a 3055 power transistor. between it and a heatsink. The thing was sorta delaminating into thinner layers around the edges. I guess that is what natural Mica is ?

Then if this sheet is by itself made of multiple layers, how the sheet thickness is controlled ? (between which layers will the block separate)

I assume the IR transparency is a problem for lamps, where the IR heats the insides of the screw cap. But then why is Mica used in the 1st place ? Why not a Metallic disk ?



The heater longs ok so far, but there is one thing that concerns me a bit :

The heater wire (its actually like a tape, with flat rectangle cross section) was a bit loose on the section that goes to the terminal, so i bent a small "w" in it (like the schematic symbol of a resistor) to stretch it

Now, that spot is getting hotter due to more wire length close together, and it is bright Orange, towards Yellow, while the rest is the normal dimmer Orange. So far it does not appear to reach to fusing temperature, but perhaps it won't last too long like that.. if it fuses i would try to reconnect it there with a crimp tube (like what is used to connect stranded wire to a terminal), hopefully the tube would provide some heatsinking so the resulting connection is colder than the rest of the wire without making a hot spot
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Re: Choosing Mica sheet for high temp « Reply #9 on: March 18, 2017, 01:38:24 PM » Author: Lodge
Ash, the mica found on most older metal can 2 pin TO-3 style transistors is natural mica, these can also be found on TO-220 style 3 pin transistors as well, but now they like to use synthetic materials and in some cases ceramics (Consider this a warning, if your playing with anything operating in radio frequencies or high voltages like 1500 V and the transistor contains ceramic insulators or they are made of ceramic there is good chance it's Beryllium Oxide because it prevents detuning and loss of signal, and it's a health concern on exposure and they meet ROHS standards so don't think ROHS  makes it all safe to tear apart, if you don't know 100% it's an aluminum oxide ceramic treat it as though it's beryllium oxide and handle them with care, wash your hands and avoid breakage) 

Mica thickness you can just peel them apart layer by layer to get the desired thickness, if your plus or minus a few layers it won't make much difference, and they can be peeled by hand or with a little assistance of a razor blade to pry a layer off.

Why they would use Mica in a lamp over metal, if some thing was to happen like a hard impact causing the disk to move a metal disk would cause a short and destroy the light and potentially cause a fire, the Mica disk being non-conductive won't do anything but move and mica also withstands extreme temperature gradients better then metal before suffering any damage so there is less expansion and contraction movement to stress the leads, and it's also corona resistant so it will withstand any surge voltages, even though the lamp filament may not it reduce the chance of an arc flash over in the lamp. Also mica is lighter so it will make less noise when someone shakes the lamp then a lose fitting metal disk so it cause less consumer confusion because unlike most consumers the people on this website know shaking a light and listening for noise is not the way to check if the light is working but it truly is common place outside the lighting circles..   
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tolivac
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Re: Choosing Mica sheet for high temp « Reply #10 on: March 19, 2017, 12:43:50 AM » Author: tolivac
BO semiconducter and tube envelope material-besides being an excellent electrical insulator BO is great at transferring heat to a heat sink.Some types of transmitting tubes use a block of BO to transfer heat from the tube to an outside heat sink.The BO electrically insulates the heat sink.In color the material is either white or purple(RCA-Burle Electronics)I deal with this stuff in tube envelopes and some vacuum capacitor envelopes.
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