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Subject: Re: QUESTION: metal to glass seals and teflon
Date: Nov 14, 3:03 pm
Poster: d.preston

On Nov 14, 3:03 pm, d.preston wrote:

? Just a quick question, what is the metal alloy used in vacuum tubes for the pins, that bonds to the glass??. Is it similar to invar with a low expansion co-efficient???. Also are any special glasses needed???, I have been playing around with stainless wire and borosilicate and it does not work at all, mainly due to the contraction.

-the type of pin depends on the type of glass used. most small vacuum tubes use "soft glass". basically a soda lime for the shell (bulb portion) and a leaded glass for the stem (glass to metal seal portion). Lead glass will have a coe of 90 * 10-7. there are several alloys designed to seal to soft glass. the most common is Dumet alloy. Dumet is a copper clad iron wire that is typically borated.

-Larger glass power tubes are typically 'hard glass' reffering to a series of glasses with a lower coe in the range of 33-54 such a borosilicate and aluminosilicate type glasses.

-Pyrex brand borosilicate has a coe of 33 it will not seal directly to Kovar, in fact not to any metal in pin form. Kovar will seal to glass with a higher coe such a cornings 7052 (46 coe), 7056 (coe 51).

-Pyrex is generally not good for direct glass to metal seals although it can be sealed to SS if a specialized geometery is used (ring seal tube-tube with a special feathered edge on the ss.. feathered down to .0001 ") or to copper (housekeeper seal) also with a specialized ring to plate geometry. Pyrex can not be used to directly seal to pins.

-Tungsten witch is not appropriate for commercial (machine produced) seals can be oxidised and sealed to nonex or uranium glass (36 coe) and then sealed to a 33 pyrex.

-In general a glass to glass seal of up to 7pts missmatch is ok (seal geometry is most important).

-anyway I would reccomend that you read up on glass to metal seals before you waste your time and materials. good references would be:

-the art of sealing base metals through glass, w.g.housekeeper, j.am inst. elec. engrs.42,954,1923
-materials technology for electron tubes, w.a.kohl, reinhold publishing cp, ny,1959
-methode of sealing ss to glass, j.e.benbenck, rev.sci.instr.,31,460,1960
-glass to metal seals, j.h.partridge, soc.glass.tech., sheffield, england
-scientific glass blowing, e.l.wheeler, interscience publishers ny 1958
-handbook of electron tube and vacuum technique, f.rosenbury, amer.inst.phys.1964

>Finally how usefule is teflon for vacuum seals??? I realise it kind of flows under compression, but would it work if it was a fat, square cross section 'O' ring and located in grooves?.

-teflon may flow under mechanical pressure you inflict in your seal design, but it will not contunue to flow due to the 14.7#/sq " pressure exerted by your vacuum. teflon is suitable in small amounts depending on the situation down to almost 10-7. personally i never use ptfe only kel-f as it has better out gassing properties and not in systems below 10-4...

>I have a book on aluminium coatings on glass which requires a vacuum of high quality and they are using stuff like waxes for the bell jar seal, also i have read of vacuum epoxies used in laser tubes with helium, would it be suitable???

-the waxes are special (ie apezion). some specialy formulated and distilled waxes are suitable for use to better than 10-7 torr. there are also similar epoxies, although i have never like the use of either, they are 'fudge' techniques.


daniel preston