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Uranium glass used as lead-in seals in a vacuum capacitor

Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic components.

Contents

Mercury seal

The first technological use of a glass-to-metal seal was the encapsulation of the vacuum in the barometer by Torricelli. The liquid mercury wets the glass and thus provides for a vacuum tight seal. Liquid mercury was also used to seal the metal leads of early mercury arc lamps into the fused silica bulbs.

Platinum wire seal

The next step was to use thin platinum wire. Platinum is easily wetted by glass and has a similar coefficient of thermal expansion as typical soda-lime and lead glass. It is also easy to work with because of its non-oxidibility and high melting point. This type of seal was used in scientific equipment throughout the 19th century and also in the early incandescent lamps and radio tubes.

Dumet wire seal

In 1911 the Dumet-wire seal was invented which is still the common practice to seal copper leads through soda-lime or lead glass. If copper is properly oxidised before it is wetted by molten glass a vacuum tight seal of good mechanical strength can be obtained. Simple copper wire is not usable because its coefficient of thermal expansion is much higher than that of the glass. Thus, on cooling a strong tensile force acts on the glass-to-metal interface and it breaks. Glass and glass-to-metal interfaces are especially sensitive to tensile stress. The Dumet-wire is a copper wire with a core of an iron-nickel alloy with a low coefficient of thermal expansion. This way it is possible to make a wire with a coefficient of radial thermal expansion which is slightly lower than the linear coefficient of thermal expansion of the glass, so that the glass-to-metal interface is under a low compression stress. About 27% of the volume of the wire is copper. It is not possible to adjust the axial thermal expansion of the wire as well. Because of the much higher mechanical strength of the iron/nickel-core compared to the copper, the axial thermal expansion of the Dumet-wire is about the same as of the core. Thus, a shear stress builds up which is limited to a safe value by the low tensile strength of the copper. This is also the reason why Dumet is only useful for wire diameters lower than about 0.5 mm. In a typical Dumet seal through the base of a vacuum tube a short piece of Dumet-wire is butt welded to a nickel wire at one end and a copper wire at the other end. When the base is pressed of lead glass the Dumet-wire and a short part of the nickel and the copper wire are enclosed in the glass. Then the nickel wire and the glass around the Dumet-wire are heated by a gas flame and the glass seals to the Dumet-wire. The nickel and copper do not seal vacuum tight to the glass but are mechanically supported. The butt welding also avoids problems with gas-leakages at the interface between the core wire and the copper.

Copper tube seal

Another possibility to avoid a strong tensile stress when sealing copper through glass is the use of a thin walled copper tube instead of a solid wire. Here a shear stress builds up in the glass-to-metal interface which is limited by the low tensile strength of the copper combined with a low tensile stress. The copper tube is insensitive to high electrical current compared to a Dumet-seal because on heating the tensile stress converts into a compression stress which is again limited by the tensile strength of the copper. Also, it is possible to lead an additional solid copper wire through the copper tube. In a later variant, only a short section of the copper tube has a thin wall and the copper tube is hindered to shrink at cooling by a ceramic tube inside the copper tube.

If large parts of copper are to be fitted to glass like the water cooled copper anode of a high power radio transmitter tube or an x-ray tube historically the Houskeeper (not Housekeeper!) knife edge seal is used. Here the end of a copper tube is machined to a sharp knife edge, invented by O. Kruh in 1917. In the method described by W.G. Houskeeper the outside or the inside of the copper tube right to the knife edge is wetted with glass and connected to the glass tube.[1] In later descriptions the knife edge is just wetted several millimeters deep with glass, usually deeper on the inside, and then connected to the glass tube.

If copper is sealed to glass, it is an advantage to get a thin bright red Cu2O containing layer between copper and glass. This is done by borating. After W.J. Scott a copper plated tungsten wire is immersed for about 30 s in chromic acid and then washed thoroughly in running tap water. Then it is dipped into a saturated solution of borax and heated to bright red heat in the oxidizing part of a gas flame. Possibly followed by quenching in water and drying. Another method is to oxidize the copper slightly in a gas flame and then to dip it into borax solution and let it dry. The surface of the borated copper is black when hot and turns to dark wine red on cooling.

It is also possible to make a bright seal between copper and glass where it is possible to see the blank copper surface through the glass, but this gives less adherence than the seal with the red Cu2O containing layer. If glass is melted on copper in a reducing hydrogen atmosphere the seal is extremely weak.

Copper disc seal

In the copper disc seal, as proposed by W.G. Houskeeper, the end of a glass tube is closed by a round copper disk. An additional ring of glass on the opposite side of the disc increases the possible thickness of the disk to more than 0.3 mm. Best mechanical strength is obtained if both sides of the disk are fused to the same type of glass tube and both tubes are under vacuum. The disk seal is of special practical interest because it is a simple method to make a seal to low expansion borosilicate glass without the need of special tools or materials. The keys to success are proper borating, heating of the joint to a temperature as close to the melting point of the copper as possible and to slow down the cooling, at least by packing the assembly into glass wool while it is still red hot.

Matched seal

In a matched seal the thermal expansion of metal and glass is matched. Copper-plated tungsten wire can be used to seal through borosilicate glass with a low coefficient of thermal expansion which is matched by tungsten. The tungsten is electrolytically copper plated and heated in hydrogen atmosphere to fill cracks in the tungsten and to get a proper surface to easily seal to glass. The borosilicate glass of usual laboratory glassware has a lower coefficient of thermal expansion than tungsten, thus it is necessary to use an intermediate sealing glass to get a stress-free seal.

There are combinations of glass and iron-nickel-cobalt alloys (Kovar) where even the non-linearity of the thermal expansion is matched. These alloys can be directly sealed to glass, but then the oxidation is critical. Also, their low electrical conductivity is a disadvantage. Thus, they are often gold plated. It is also possible to use silver plating, but then an additional gold layer is necessary as an oxygen diffusion barrier to prevent the formation of iron oxide.

While there are Fe-Ni alloys which match the thermal expansion of tungsten at room temperature, they are not useful to seal to glass because of a too strong increase of their thermal expansion at higher temperatures.

Reed switches use a matched seal between an iron-nickel alloy (NiFe 52) and a matched glass. The glass of reed switches is usually green due to its iron content because the sealing of reed switches is done by heating with infrared radiation and this glass shows a high absorption in the near infrared.

Historically, some television cathode ray tubes where made by using ferritic steel for the funnel and glass matched in expansion to ferritic steel. The steel plate used had a diffusion layer enriched with chromium at the surface made by heating the steel together with chromium oxide in a HCl containing atmosphere. In contrast to copper, pure iron does not bond strongly to silicate glass. Also, technical iron contains some carbon which forms bubbles of CO when it is sealed to glass under oxidizing conditions. Both is a major source of problems for the technical enamel coating of steel and makes direct seals between iron and glass unsuitable for high vacuum applications. The oxide layer formed on chromium containing steel can seal vacuum tight to glass and the chromium strongly reacts with carbon. Silver plated iron was used in early microwave tubes.

It is possible to make matched seals between copper or austenitic steel and glass, but silicate glass with that high thermal expansion is especially fragile and has a low chemical durability.

Molybdenum foil seal

Another widely used method to seal through glass with low coefficient of thermal expansion is the use of stripes of thin molybdenum foil. This can be done with matched coefficients of thermal expansion or unmatched after Houskeeper. Then the edges of the strip also have to be knife sharp. The disadvantage here is that the tip of the edge which is a local point of high tensile stress reaches through the wall of the glass container. This can lead to low gas leakages. In the tube to tube knife edge seal the edge is either outside, inside, or buried into the glass wall.

Compression seal

Another possibility of seal construction is the compression seal. This type of glass-to-metal seal can be used to feed through the wall of a metal container. Here the wire is usually matched to the glass which is inside of the bore of a strong metal part with higher coefficient of thermal expansion.

Design aspects

Also the mechanical design of a glass-to-metal seal has an important influence on the reliability of the seal. In practical glass-to-metal seals cracks usually start at the edge of the interface between glass and metal either inside or outside the glass container. If the metal and the surrounding glass are symmetric the crack propagates in an angle away from the axis. So, if the glass envelope of the metal wire extends far enough from the wall of the container the crack will not go through the wall of the container but it will reach the surface on the same side where it started and the seal will not leak despite the crack.

Another important aspect is the wetting of the metal by the glass. If the thermal expansion of the metal is higher than the thermal expansion of the glass like with the Houskeeper seal, a high contact angle (bad wetting) means that there is a high tensile stress in the surface of the glass near the metal. Such seals usually break inside the glass and leave a thin cover of glass on the metal. If the contact angle is low (good wetting) the surface of the glass is everywhere under compression stress like an enamel coating. Ordinary soda-lime glass does not flow on copper at temperatures below the melting point of the copper and, thus, does not give a low contact angle. The solution is to cover the copper with a solder glass which has a low melting point and does flow on copper and then to press the soft soda-lime glass onto the copper. The solder glass must have a coefficient of thermal expansion which is equal or a little lower than that of the soda-lime glass. Classically high lead containing glasses are used, but it is also possible to substitute these by multi-component glasses e.g. based on the system Li2O-Na2O-K2O-CaO-SiO2-B2O3-ZnO-TiO2-BaO-Al2O3.

References

Eldred, B.E., "Compound metal", US 1083070, published 1911, issued 1913

Eldred, B.E., "Incandescent lamp", US 1140134, published 1914, issued 1915

Eldred, B.E., "Process for the production of compound metal articles", US 1140135, published 1914, issued 1915

Eldred, B.E., "Low-expansion wire", US 1140136, published 1913, issued 1915

Kraus, C.A., "Conducting-seal for vacuum-containers", US 1093997, published 1914, issued 1914

Fink, C.G., "Evacuated container", US 1498908, published 1915, issued 1924

Van Keuren, W.L., "Leading-in conductor", US 1268647, published 1918, issued 1918

Kruh, O., "Luftdichter Metallkappenanschluß für die Stromzuführung in Glashohlkörper", DE 424133, published 1917, issued 1926

Houskeeper, W.G., "Combined metal and glass structure and method of forming same", US 1293441, published 1918, issued 1919

Houskeeper, W.G., "Combined metal and glass structure and method of making same", US 1294466, published 1918, issued 1919

Houskeeper, W.G. (1923), "The art of sealing base metals through glass", J. Am. Inst. Elec. Engrs. 42: 954–960  

Hall, R.D., "Method of borating dumet wire", US 1647620, published 1926, issued 1927

Reimann, A.L. (June 1946), "Coppered-tungsten seals through hard glass", J. Sci. Instrum. 23: 121–124, doi:10.1088/0950-7671/23/6/305  

Scott, W.J. (September 1946), "Glass-to-metal seal design", J. Sci. Instrum. 23: 193–202, doi:10.1088/0950-7671/23/9/301  

Egyesuelt Izzolampa, HU, "Stromzuführungsdraht für vakuumtechnische Glasgeräte", DE 1817839U, published 1959, issued 1960

Mönch, G.C. (Berlin 1961), Neues und Bewährtes aus der Hochvakuumtechnik  

Roth, A. (Oxford 1966), Vacuum sealing techniques  

Kohl, W.H. (New York 1967), Handbook of Materials and Techniques for Vacuum Devices  

Chabin, et al., "Method and device for integrating a glass part and metal part", US 6324870, published 1997, issued 2001

Brix, et al., "Lead-free glass tubing, especially for encapsulating diodes and diodes encapsulated with same", US 7102242, published 2005, issued 2006

See also

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