Glass-to-metal seal - AbsoluteAstronomy.com

Glass-to-metal seals are a very important element of the construction of vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s, electric discharge tubes, incandescent light bulb

Incandescent light bulb

The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...

s, glass encapsulated semiconductor diodes, reed switch

Reed switch

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope...

es, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic component

Electronic component

An electronic component is a basic electronic element and may be available in a discrete form having two or more electrical terminals . These are intended to be connected together, usually by soldering to a printed circuit board, in order to create an electronic circuit with a particular function...

Glass-to-metal bonds

Glass and metal can bond together by purely mechanical means, which usually gives weaker joints, or by chemical interaction, where the oxide layer on the glass surface forms a strong bond with the glass.

For achieving a vacuum-tight seal, the seal must not contain bubbles. The bubbles are most commonly created by gases escaping the metal at high temperature; degassing the metal before its sealing is therefore important, especially for nickel and iron and their alloys. This is achieved by heating the metal in vacuum or sometimes in hydrogen atmosphere or in some cases even in air at temperatures above those used during the sealing process. Oxidizing of the metal surface also reduces gas evolution. Most of the evolved gas is produced due to the presence of carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

impurities in the metals; these can be removed by heating in hydrogen.
The glass-oxide bond is stronger than glass-metal. The oxide forms a layer on the metal surface, with the proportion of oxygen changing from zero in the metal to the stoichiometry of the oxide and the glass itself. A too thick oxide layer tends to be porous on the surface and mechanically weak, flaking, compromising the bond strength and creating possible leakage paths along the metal-oxide interface. Proper thickness of the oxide layer is therefore critical.

Copper

Metallic copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

does not bond well to glass. Its oxide

Copper(I) oxide

Copper oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper. This red-coloured solid is a component of some antifouling paints. The compound can appear either yellow or red, depending on the size of the particles, but both forms...

, , however, is wetted by molten glass and partially dissolves in it, forming a strong bond. The oxide also bonds well to the underlying metal. Copper(II) oxide

Copper(II) oxide

Copper oxide or cupric oxide is the higher oxide of copper. As a mineral, it is known as tenorite.-Chemistry:It is a black solid with an ionic structure which melts above 1200 °C with some loss of oxygen...

causes weak joints that may leak and its formation has to be prevented.

For bonding copper to glass, the surface has to be properly oxidized. The oxide layer has to have the right thickness; too thin oxide would not provide enough material for the glass to anchor to, too thick oxide would fail in the oxide layer, and in both cases the joint would be weak and possibly non-hermetic. To improve the bonding to glass, the oxide layer should be borated; this is achieved by e.g. dipping the hot part into a concentrated solution of borax

Borax

Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

and then heating it again for certain time. This treatment stabilizes the oxide layer by forming a thin protective layer of sodium borate on its surface, so the oxide does not grow too thick during subsequent handling and joining. The layer should have uniform deep red to purple sheen. The boron oxide from the borated layer diffuses into glass and lowers its melting point. The oxidation occurs by oxygen diffusing through the molten borate layer and forming copper(I) oxide, while formation of copper(II) oxide is inhibited.

The copper-to-glass seal should look brilliant red, almost scarlet; pink, sherry and honey colors are also acceptable. Too thin oxide layer looks light, up to the color of metallic copper. Too thick oxide looks too dark.

Oxygen-free copper

Oxygen-free copper

Oxygen-free copper or Oxygen-free high thermal conductivity copper generally refers to a group of wrought high conductivity copper alloys that have been electrolytically refined to reduce the level of oxygen to .001% or below....

has to be used if the metal comes in contact with hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

(e.g. in a hydrogen-filled tube

Gas-filled tube

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass-lined metal...

or during handling in the flame). Normally, copper contains small inclusions of copper(I) oxide

Copper(I) oxide

. Hydrogen diffuses through the metal and reacts with the oxide, reducing it to copper and yielding water. The water molecules however can not diffuse through the metal, are trapped in the location of the inclusion, and cause embrittlement

Hydrogen embrittlement

Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...

.

As copper bonds well to the glass, it is often used for combined glass-metal devices. The ductility of copper can be used for compensation of the thermal expansion mismatch in e.g. the knife-edge seals. For wire feed throughs, dumet wire - nickel-iron alloy plated with copper - is frequently used. Its maximum diameter is however limited to about 0.5 mm due to its thermal expansion.

Copper can be sealed to glass without the oxide layer, but the resulting joint is less strong.

Platinum

Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

has similar thermal expansion as glass and is well-wetted with molten glass. It however does not form oxides, its bond strength is lower. The seal has metallic color and limited strength.

Gold

Like platinum, gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

does not form oxides that could assist in bonding. Glass-gold bonds are therefore metallic in color and weak. Gold tends to be used for glass-metal seals only rarely. Special compositions of soda-lime glasses that match the thermal expansion of gold, containing tungsten trioxide

Tungsten trioxide

Tungsten oxide, also known as tungsten trioxide or tungstic anhydride, WO3, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals. Tungsten ores are treated with alkalis to produce WO3...

and oxides of lanthanum, aluminium and zirconium, exist.

Silver

Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

forms a thin layer of silver oxide on its surface. This layer dissolves in molten glass and forms silver silicate, facilitating a strong bond.

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

can bond with glass either as a metal, or via the nickel(II) oxide

Nickel(II) oxide

Nickel oxide is the chemical compound with the formula NiO. It is notable as being the only well characterized oxide of nickel . The mineralogical form of NiO, bunsenite, is very rare. It is classified as a basic metal oxide...

layer. The metal joint has metallic color and inferior strength. The oxide-layer joint has characteristic green-grey color. Nickel plating can be used in similar way as copper plating, to facilitate better bonding with the underlying metal.

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

is only rarely used for feedthroughs, but frequently gets coated with vitreous enamel

Vitreous enamel

Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

, where the interface is also a glass-metal bond. The bond strength is also governed by the character of the oxide layer on its surface. A presence of cobalt

Cobalt

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

in the glass leads to a chemical reaction between the metallic iron and cobalt oxide, yielding iron oxide dissolved in glass and cobalt alloying with the iron and forming dendrites

Dendrite (metal)

A dendrite in metallurgy is a characteristic tree-like structure of crystals growing as molten metal freezes, the shape produced by faster growth along energetically favourable crystallographic directions. This dendritic growth has large consequences in regards to material properties.Dendrites form...

, growing into the glass and improving the bond strength.
Iron can not be directly sealed to lead glass

Lead glass

Lead glass is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40 weight% lead oxide , while modern lead crystal, historically also known as flint glass due to the original silica source, contains a minimum of 24% PbO...

, as it reacts with the lead oxide and reduces it to metallic lead. For sealing to lead glasses, it has to be copper-plated or an intermediate lead-free glass has to be used. Iron is prone to creating gas bubbles in glass due to the residual carbon impurities; these can be removed by heating in wet hydrogen. Plating with copper, nickel or chromium is also advised.

Chromium

Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

is a highly reactive metal present in many iron alloys. Chromium may react with glass, reducing the silicon and forming crystals of chromium silicide growing into the glass and anchoring together the metal and glass, improving the bond strength.

Iron-nickel-cobalt

Kovar

Kovar is a nickel-cobalt ferrous alloy designed to be compatible with the thermal expansion characteristics of borosilicate glass in order to allow direct mechanical connections over a range of temperatures...

, an iron-nickel-cobalt alloy, has low thermal expansion similar to glass and is frequently used for glass-metal seals. It can bond to glass via the intermediate oxide layer of nickel(II) oxide

Nickel(II) oxide

and cobalt(II) oxide

Cobalt(II) oxide

Cobalt oxide or cobalt monoxide is an inorganic compound that appears as olive-green to red crystals, or as a greyish or black powder...

; the proportion of iron oxide is low due to its reduction with cobalt. The bond strength is highly dependent on the oxide layer thickness and character. The presence of cobalt makes the oxide layer easier to melt and dissolve in the molten glass. A grey, grey-blue or grey-brown color indicates a good seal. A metallic color indicates lack of oxide, while black color indicates overly oxidized metal, in both cases leading to a weak joint.

Molybdenum

Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...

bonds to the glass via the intermediate layer of molybdenum(IV) oxide. Due to its low thermal expansion coefficient, matched to glass, molybdenum, like tungsten, is often used for glass-metal bonds. Its high electrical conductivity makes it superior over nickel-cobalt-iron alloys. It is favored by the lighting industry as feedthroughs for lightbulbs and other devices. Molybdenum oxidizes much faster than tungsten and quickly develops a thick oxide layer that does not adhere well, its oxidation should be therefore limited to just yellowish or at most blue-green color. The oxide is volatile and evaporates as a white smoke above 700 °C; excess oxide can be removed by heating in inert gas (argon) at 1000 °C. Molybdenum strips are used instead of wires where higher currents (and higher cross-sections of the conductors) are needed.

Tungsten

Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

bonds to the glass via the intermediate layer of tungsten(VI) oxide. A properly formed bond has characteristic coppery/orange/brown-yellow color in lithium-free glasses; in lithium-containing glasses the bond is blue due to formation of lithium tungstate. Due to its low thermal expansion coefficient, matched to glass, tungsten is frequently used for glass-metal bonds. Tungsten forms satisfying bonds with glasses with similar thermal expansion coefficient. The surface of both the metal and glass should be smooth, without scratches. Tungsten has the lowest expansion coefficient of metals and the highest melting point.

Stainless steel

304 Stainless steel

Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

forms bonds with glass via an intermediate layer of chromium(III) oxide

Chromium(III) oxide

Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...

and iron(III) oxide

Iron(III) oxide

Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main...

. Further reactions of chromium, forming chromium silicide dendrites, are possible. The thermal expansion coefficient of steel is however fairly different to the glass; like with copper, this can be alleviated by using knife-edge (Houskeeper) seals.

Zirconium

Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...

wire can be sealed to glass with just little treatment - rubbing with abrasive paper and short heating in flame. Zirconium is used in applications demanding chemical resistance or lack of magnetism.

Titanium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

, like zirconium, can be sealed to some glasses with just little treatment.

Indium

Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...

and some of its alloys can be used as a solder

Solder

Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

capable of wetting glass, ceramics, and metals and joining them together. Indium has low melting point and is very soft; the softness allows it to deform plastically and absorb the stresses from thermal expansion mismatches. Due to its very low vapor pressure, indium finds use in glass-metal seals used in vacuum technology.

Gallium

Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...

is a soft metal with melting point at 30 °C. It readily wets glasses and most metals and can be used for seals that can be assembled/disassembled by just slight heating. It can be used as a liquid seal up to high temperatures or even at lower temperatures when alloyed with other metals (e.g. as galinstan

Galinstan

Galinstan is a family of eutectic alloys mainly consisting of gallium, indium, and tin, which are liquids at room temperature, typically freezing at . Due to the low toxicity and low reactivity of its component metals, it finds use as a replacement for many applications that previously employed...

Mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

is a metal liquid at normal temperature. It was used as the earliest glass-to-metal seal and is still in use for liquid seals for e.g. rotary shafts.

Mercury seal

The first technological use of a glass-to-metal seal was the encapsulation of the vacuum

Vacuum

In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

in the barometer

Barometer

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather...

by Torricelli

Evangelista Torricelli

Evangelista Torricelli was an Italian physicist and mathematician, best known for his invention of the barometer.-Biography:Evangelista Torricelli was born in Faenza, part of the Papal States...

. The liquid mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

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.

A less toxic and more expensive alternative to mercury is gallium

Gallium

.

Mercury and gallium seals can be used for vacuum-sealing rotary shafts.

Platinum wire seal

The next step was to use thin platinum

Platinum

wire

Wire

A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...

. Platinum is easily wetted

Wetting

Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. The degree of wetting is determined by a force balance between adhesive and cohesive forces.Wetting is important in the bonding or adherence of...

by glass and has a similar coefficient of thermal expansion as typical soda-lime

Soda-lime glass

Soda-lime glass, also called soda-lime-silica glass, is the most prevalent type of glass, used for windowpanes, and glass containers for beverages, food, and some commodity items...

and lead glass

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

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. After copper is oxidized, it is often dipped in a borax solution, as borating the copper helps prevents over-oxidation when reintroduced to a flame. 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. Dumet-wire is a copper clad wire (about 25% of the weight of the wire is copper) with a core of nickel-iron alloy 42, an alloy with a composition of about 42% nickel. The core has a low coefficient of thermal expansion, allowing for 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. It is not possible to adjust the axial thermal expansion of the wire as well. Because of the much higher mechanical strength of the nickel-iron 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 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. 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 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 containing layer. If glass is melted on
copper in a reducing hydrogen atmosphere the seal is extremely weak. If copper is to be heated in hydrogen-containing atmosphere e.g. a gas flame it
needs to be oxygen-free to prevent hydrogen embrittlement. Copper which is ment to be used as an electrical conductor is not necessarily oxygen-free
and contains particles of which react with hydrogen that diffuses into the copper to which cannot diffuse out-off the
copper and thus causes embrittlement. The copper usually used in vacuum applications is of the very pure OFHC (oxygen-free-high-conductivity)
quality which is both free of and deoxidising additives which might evaporate at high temperature in vacuum.

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

Laboratory glassware

Laboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...

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

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.

The electrical connections of high-pressure sodium vapour lamps, the yellow lamps for street lighting, are made of niobium alloyed with 1% of zirconium.

Historically, some television cathode ray tubes were 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 are a major source of problems for the technical enamel coating of steel and make 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 strips 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

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

Vitreous enamel

Vitreous enamel, also porcelain enamel in U.S. English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C...

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 ---------.

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