![Vacuum capacitor with uranium glass](https://upload.wikimedia.org/wikipedia/commons/c/c3/Vacuum_capacitor_with_uranium_glass.jpg)
Glass-to-metal seals are a very important element of the construction of
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied.
The type kn ...
s, electric discharge tubes,
incandescent light bulb
An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire filament heated until it glows. The filament is enclosed in a glass bulb with a vacuum or inert gas to protect the filament from oxida ...
s, glass encapsulated
semiconductor diode
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other.
A diode ...
s,
reed switch
Reed or Reeds may refer to:
Science, technology, biology, and medicine
* Reed bird (disambiguation)
* Reed pen, writing implement in use since ancient times
* Reed (plant), one of several tall, grass-like wetland plants of the order Poales
* Re ...
es, pressure tight glass windows in metal cases, Vacuum Insulating Glass (Vacuum Glass) and metal or ceramic packages of
electronic component
An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
s.
Properly done, such a seal is
hermetic
Hermetic or related forms may refer to:
* of or related to the ancient Greek Olympian god Hermes
* of or related to Hermes Trismegistus, a legendary Hellenistic figure based on the Greek god Hermes and the Egyptian god Thoth
** , the ancient and m ...
(vacuum tight, good electrical insulation, special optical properties e.g. UV lamps). To achieve such a seal, two properties must hold:
# The molten glass must be capable of
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. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
the metal, in order to form a tight bond, and
# The
thermal expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions.
Temperature is a monotonic function of the average molecular kinetic ...
of the glass and metal must be closely matched so that the seal remains solid as the assembly cools.
Thinking for example about a metal wire in a glass bulb sealing, the metal glass contact can break if coefficients of thermal expansion (CTE) are not well aligned. For the case that the CTE of the metal is larger than the CTE of the glass, the sealing shows a high probability to break upon cooling. By lowering the temperature, the metal wire shrinks more than the glass does, leading to a strong tensile force on the glass, which finally lead to breakage. On the other hand, if the CTE of the glass is larger than the CTE of the metal wire, the seal will tighten upon cooling since compression force is applied on the glass.
![2017-11-16 14 50 51-Alloy](https://upload.wikimedia.org/wikipedia/commons/9/9d/2017-11-16_14_50_51-Alloy.png)
According to all requirements that need to be fulfilled and the strong necessity to align the CTE of both materials, there are only a few companies offering specialty glass for glass-metal sealing, such as
SCHOTT AG
Schott AG is a German multinational glass company specializing in the manufacture of glass and glass-ceramics. Headquartered in Mainz, Germany, it is owned by the Carl Zeiss Foundation. The company's founder and namesake, Otto Schott, is credi ...
and
Morgan Advanced Materials
Morgan Advanced Materials is a company which manufactures specialist products, using carbon, advanced ceramics and composites. The company is headquartered in Windsor, United Kingdom, and has 85 sites across 30 countries. A public limited compa ...
.
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 metal surface forms a strong bond with the glass (the glass itself is about 73% composed of a
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
(SiO
2)) . The
acid-base reactions are main causes of interaction between glass-metal in the presence of metal oxides on the surface of metal. After complete dissolution of the surface oxides into the glass, further progress of interaction depends on the oxygen
activity at the interface. The oxygen
activity can be increased by diffusion of
molecular oxygen
There are several known allotropes of oxygen. The most familiar is molecular oxygen (O2), present at significant levels in Earth's atmosphere and also known as dioxygen or triplet oxygen. Another is the highly reactive ozone (O3). Others are:
*A ...
through some defects like cracks.
Also, reduction of the thermodynamically less stable components in the glass (and releasing the oxygen ions) can increase the oxygen
activity at the interface. In other words, the
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
reactions are main causes of interaction between glass-metal in the absence of metal oxides on the surface of metal.
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 () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
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 is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
does not bond well to glass.
Copper(I) oxide
Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper, the other being or copper(II) oxide or cupric oxide (CuO). This red-coloured solid is a component of some antifoulin ...
, however, is wetted by molten glass and partially dissolves in it, forming a strong bond. The oxide also bonds well to the underlying metal. But
copper(II) oxide
Copper(II) oxide or cupric oxide is an inorganic compound with the formula CuO. A black solid, it is one of the two stable oxides of copper, the other being Cu2O or copper(I) oxide (cuprous oxide). As a mineral, it is known as tenorite. It i ...
causes weak joints that may leak and its formation must be prevented.
For bonding copper to glass, the surface needs to be properly oxidized. The oxide layer is to have the right thickness; too little oxide would not provide enough material for the glass to anchor to, too much oxide would cause the oxide layer to fail, 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 is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form, ...
and then heating it again for certain time. This treatment stabilizes the oxide layer by forming a thin protective layer of
sodium borate Sodium borate is a generic name for any salt of sodium with an anion consisting of boron and oxygen, and possibly hydrogen, or any hydrate thereof. It can be seen as a hydrated sodium salt of the appropriate boroxy acid, although the latter may n ...
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 an oxide layer appears light, up to the color of metallic copper, while too thick oxide looks too dark.
Oxygen-free copper
Oxygen-free copper (OFC) or oxygen-free high thermal conductivity (OFHC) copper is a group of wrought high-conductivity copper alloys that have been electrolytically refined to reduce the level of oxygen to 0.001% or below.
Specification
Ox ...
has to be used if the metal comes in contact with
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
(e.g. in a
hydrogen-filled tube or during handling in the flame). Normally, copper contains small inclusions of
copper(I) oxide
Copper(I) oxide or cuprous oxide is the inorganic compound with the formula Cu2O. It is one of the principal oxides of copper, the other being or copper(II) oxide or cupric oxide (CuO). This red-coloured solid is a component of some antifoulin ...
. 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
Embrittlement is a significant decrease of ductility of a material, which makes the material Brittleness, brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such ...
.
As copper(I) oxide 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
Fernico describe a family of metal alloys made primarily of iron, nickel and cobalt. The family includes Kovar, FerNiCo I, FerNiCo II, and Dumet. The name is made up of the chemical symbols of its constituent three elements. "Dumet" is a portma ...
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
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
has similar thermal expansion as glass and is well-wetted with molten glass. It however does not form oxides, so its bond strength is lower. The seal has metallic color and limited strength.
Gold
Like platinum,
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
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(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO3. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light ...
and oxides of lanthanum, aluminum and zirconium, exist.
Silver
Silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
forms a thin layer of
silver oxide
Silver oxide is the chemical compound with the formula Ag2O. It is a fine black or dark brown powder that is used to prepare other silver compounds.
Preparation
Silver oxide can be prepared by combining aqueous solutions of silver nitrate and a ...
on its surface. This layer dissolves in molten glass and forms
silver silicate, facilitating a strong bond.
Nickel
Nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
can bond with glass either as a metal, or via the
nickel(II) oxide
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to r ...
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
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
is only rarely used for feedthroughs, but frequently gets coated with
vitreous enamel
Vitreous enamel, also called porcelain enamel, is a material made by fusing powdered glass to a substrate by firing, usually between . The powder melts, flows, and then hardens to a smooth, durable vitreous coating. The word comes from the Lati ...
, 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 is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
in the glass leads to a chemical reaction between the metallic iron and
cobalt oxide Cobalt oxide is a family of chemical compounds consisting of cobalt and oxygen atoms.
Compounds in the cobalt oxide family include:
* Cobalt(II) oxide (cobaltous oxide), CoO
*Cobalt(III) oxide
Cobalt(III) oxide is the inorganic compound with the ...
, yielding iron oxide dissolved in glass and cobalt alloying with the iron and forming
dendrites
Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...
, growing into the glass and improving the bond strength.
Iron can not be directly sealed to
lead glass
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically als ...
, 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
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
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.
Kovar
Kovar
Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K betwe ...
, an iron-nickel-cobalt alloy, has low thermal expansion similar to
high-borosilicate glass and is frequently used for glass-metal seals especially for the application in
x-ray tubes
An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast ...
or glass lasers. It can bond to glass via the intermediate oxide layer of
nickel(II) oxide
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to r ...
and
cobalt(II) oxide
Cobalt(II) oxide is an inorganic compound that has been described as an olive-green or gray solid. It is used extensively in the ceramics industry as an additive to create blue colored glazes and enamels as well as in the chemical industry for pr ...
; 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
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
bonds to the glass via the intermediate layer of
molybdenum(IV) oxide
Molybdenum dioxide is the chemical compound with the chemical formula, formula MoO. It is a violet-colored solid and is a metallic conductor. The mineralogical form of this compound is called tugarinovite, and is only very rarely found.
Structur ...
. Due to its low thermal expansion coefficient, matched to glass, molybdenum, like tungsten, is often used for glass-metal bonds especially in conjunction with aluminium-silicate glass. 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
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
bonds to the glass via the intermediate layer of
tungsten(VI) oxide
Tungsten(VI) oxide, also known as tungsten trioxide is a chemical compound of oxygen and the transition metal tungsten, with formula WO3. The compound is also called tungstic anhydride, reflecting its relation to tungstic acid . It is a light ye ...
. 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
Lithium tungstate is the inorganic compound with the formula Li2WO4. It is a white solid that is soluble in water. The compound is one of the several orthotungstates, compounds that feature the tetrahedral WO42− anion.
Structure
The salt consis ...
. 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 such as
high-borosilicate glass. 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 is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...
forms bonds with glass via an intermediate layer of
chromium(III) oxide
Chromium(III) oxide (or chromia) is an inorganic compound with the formula . It is one of the principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.
Structure and properties
has the corundum ...
and
iron(III) oxide
Iron(III) 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(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturally ...
. Further reactions of chromium, forming chromium silicide dendrites, are possible. The thermal expansion coefficient of steel is however fairly different from the glass; like with copper, this can be alleviated by using knife-edge (Houskeeper) seals.
Zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
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
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
, like zirconium, can be sealed to some glasses with just little treatment.
Indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts p ...
and some of its alloys can be used as a
solder
Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
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
and cryogenic applications.
Gallium
Gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
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 (R) is a brand name for a alloy composed of gallium, indium, and tin which melts at and is thus liquid at room temperature. However, it is not a eutectic alloy but a near eutectic alloy. In scientific literature, galinstan is also used ...
).
Mercury
Mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
in the
barometer
A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis ...
by
Torricelli. The liquid
mercury
Mercury commonly refers to:
* Mercury (planet), the nearest planet to the Sun
* Mercury (element), a metallic chemical element with the symbol Hg
* Mercury (mythology), a Roman god
Mercury or The Mercury may also refer to:
Companies
* Merc ...
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 lamp
A mercury-vapor lamp is a gas-discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz
Fused quartz, fused silica or quartz glass is a glass consi ...
s into the
fused silica
Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...
bulbs.
A less toxic and more expensive alternative to mercury is
gallium
Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...
.
Mercury and gallium seals can be used for vacuum-sealing rotary shafts.
Platinum wire seal
The next step was to use thin
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
wire
Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm
A wire is a flexible strand of metal.
Wire is c ...
. Platinum is easily
wetted
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with t ...
by glass and has a similar
coefficient of thermal expansion
Thermal expansion is the tendency of matter to change its shape
A shape or figure is a graphics, graphical representation of an object or its external boundary, outline, or external Surface (mathematics), surface, as opposed to other pro ...
as typical
soda-lime and
lead glass
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically als ...
. 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
Fernico describe a family of metal alloys made primarily of iron, nickel and cobalt. The family includes Kovar, FerNiCo I, FerNiCo II, and Dumet. The name is made up of the chemical symbols of its constituent three elements. "Dumet" is a portma ...
-wire seal was invented which is still the common practice to seal
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
leads through
soda-lime or
lead glass
Lead glass, commonly called crystal, is a variety of glass in which lead replaces the calcium content of a typical potash glass. Lead glass contains typically 18–40% (by weight) lead(II) oxide (PbO), while modern lead crystal, historically als ...
.
If copper is properly oxidised before it is
wetted
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with t ...
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
Borax is a salt (ionic compound), a hydrated borate of sodium, with chemical formula often written . It is a colorless crystalline solid, that dissolves in water to make a basic solution. It is commonly available in powder or granular form, ...
solution, as borating the copper helps prevents over-oxidation when reintroduced to a flame. Simple copper wire is not usable because its CTE 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 (25% of copper by weight) with a core of
nickel-iron alloy 42 (42% of nickel by weigth). The core having low CTE makes it possible to produce a wire with a radial CTE lower than a linear CTE 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 CTE the wire is about the same as of the core. Therefore, 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
![Glass to metal seals 1922](https://upload.wikimedia.org/wikipedia/commons/4/43/Glass_to_metal_seals_1922.png)
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 electric 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
Glass tubes are mainly cylindrical hollow-wares. Their special shape combined with the huge variety of glass types (like borosilicate, flint, aluminosilicate, soda lime, lead or quartz glass), allows the use of glass tubing in many applications. Fo ...
. 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
Glass tubes are mainly cylindrical hollow-wares. Their special shape combined with the huge variety of glass types (like borosilicate, flint, aluminosilicate, soda lime, lead or quartz glass), allows the use of glass tubing in many applications. Fo ...
.
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 meant 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 copper (OFC) or oxygen-free high thermal conductivity (OFHC) copper is a group of wrought high-conductivity copper alloys that have been electrolytically refined to reduce the level of oxygen to 0.001% or below.
Specification
Ox ...
(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
Glass tubes are mainly cylindrical hollow-wares. Their special shape combined with the huge variety of glass types (like borosilicate, flint, aluminosilicate, soda lime, lead or quartz glass), allows the use of glass tubing in many applications. Fo ...
is closed by a round copper disc. An additional ring of glass on the opposite side of the disc increases the possible thickness of the disc to more than 0.3 mm. Best mechanical strength is obtained if both sides of the disc are fused to the same type of
glass tube
Glass tubes are mainly cylindrical hollow-wares. Their special shape combined with the huge variety of glass types (like borosilicate, flint, aluminosilicate, soda lime, lead or quartz glass), allows the use of glass tubing in many applications. Fo ...
and both tubes are under vacuum. The disc 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
![Matched glass-to-metal seals](https://upload.wikimedia.org/wikipedia/commons/a/a9/Matched_glass-to-metal_seals.JPG)
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 refers to a variety of equipment used in scientific work, and traditionally made of glass. Glass can be blown, bent, cut, molded, and formed into many sizes and shapes, and is therefore common in chemistry, biology, and anal ...
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 (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K betwe ...
) 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 light yellow lamps for street lighting, are made of niobium alloyed with 1% of zirconium.
Historically, some television cathode ray tubes were made by using ferric steel for the funnel and glass matched in expansion to ferric 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. 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
Container glass is a type of glass for the production of glass containers, such as bottles, jars, drinkware, and bowls. Container glass stands in contrast to ''flat glass'' (used for windows, glass doors, transparent walls, windshields) and ''gl ...
. 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. Compression seals can withstand extremely high pressures and physical stress such as mechanical and thermal shock.
Silver chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heating, ...
, which melts at 457 C bonds to glass, metals and other materials and has been used for vacuum seals. Even if it can be a convenient way to seal metal into glass it will not be a true glass to metal seal but rather a combination of a glass to silver chloride and a silver chloride to metal bond; an inorganic alternative to wax or glue bonds.
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 is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
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
Solder (; NA: ) is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable ...
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 ---------.
See also
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Hermetic seal
A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...
Notes
References
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External links
Glass-To-Metal Hermetic Sealing
{{DEFAULTSORT:Glass-To-Metal Seal
Seals (mechanical)
Industrial processes
Glass applications
Glass engineering and science
Glass compositions