Glass and metal can bond together by purely mechanical means which gives weaker joints, or by chemical interaction where the oxide layer on the metal surface forms a robust bond with the glass. The acid-base reactions are main causes of interaction between glass-metal within the presence of metal oxides on the surface of metal. After complete dissolution of the surface oxides into the glass, further interaction depends on oxygen activity at the interface. The oxygen activity are often increased by diffusion of the molecular oxygen through defects. Also the reduction of the thermodynamically less stable components within the glass and releasing the oxygen ions can increase the oxygen activity at the interface. The redox reactions herein are the main causes of interaction between glass-metal within the absence of metal oxides on the surface of the metal.
To achieve a vacuum-tight glass metal seal, the glass metal seal must not contain any bubbles. Bubbles are typically created by gases escaping the metal at high temperature. Thus, degassing the metal before sealing is very important especially for nickel and iron and their alloys. This can typically be achieved by heating the metal in a vacuum or a hydrogen atmosphere or, in some cases, even air at temperatures above the sealing process. Oxidizing of the metal surface also reduces the gas evolution. Most of the evolved gas is produced because of the presence of carbon impurities within the metals. These carbon impurities are removed by heating the metal in hydrogen.
The glass-oxide bond is stronger than the glass-metal. The oxide forms a layer on the metal surface with the proportion of oxygen changing from zero within the metal to the stoichiometry of the oxide and therefore 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.
Metals Used in Glass Metal Seals
Metallic copper does not bond well to glass. Cuprous oxide is however wetted by molten glass and partially dissolves in it forming a strong bond. The oxide also bonds well to the underlying metal. But cupric oxide causes weak joints that may leak and its formation must be prevented.
For bonding copper metal to glass, the surface of the metal must be properly oxidized. The oxide layer is to also have the correct thickness - too little oxide wouldn't provide enough material for the glass to anchor to and an excessive amount of oxide would cause the oxide layer to fail. In both cases the joint would be weak and non-hermetic. To enhance the bonding of the metal to the glass, the oxide layer should be borated - this can be achieved by dipping the hot part into a concentrated solution of borax then heating it again. This treatment stabilizes the oxide layer by forming a skinny protective layer of sodium borate on its surface. Thus the oxide doesn't grow too thick during subsequent handling and joining. The layer should have uniform crimson to purple sheen. The boron oxide from the borated layer diffuses into glass and lowers its melting point. The oxidation occurs by the oxygen in the atmosphere diffusing through the molten borate layer and forming cuprous oxide, while formation of cupric oxide is inhibited. The copper-to-glass seal should look brilliant red, almost scarlet; pink, sherry and honey colors also are acceptable. Too thin an oxide layer appears light, up to the colour of metallic copper, while too thick oxide looks too dark.
Oxygen-free copper needs to be used if the metal comes in contact with hydrogen like in a hydrogen-filled tube or during handling within the flame. Normally the copper metal contains small inclusions of cuprous oxide. Hydrogen diffuses through the metal and reacts with the oxide reducing it to copper and yielding water. The water molecules however can't diffuse through the metal and are thus trapped within the location of the inclusion and cause embrittlement. As cuprous oxide bonds well to the glass, it's often used for combined glass-metal devices. The ductility of copper are often used for compensation of the thermal expansion mismatch in knife-edge seals. For wire feed throughs, dumet wire – a nickel-iron alloy plated with copper – is usually used. Its maximum diameter is however limited to about 0.5 mm because of its thermal expansion. Copper may be sealed to glass without the oxide layer, but the resulting joint is less strong.
Nickel can bond with glass either as a metal or via the primary nickel oxide layer. The nickel metal joint has a metallic color and inferior strength. The oxide-layer joint has a characteristic green-grey color. Nickel plating can be used in a similar way as copper plating to facilitate better bonding of the glass with the underlying metal.
Iron is rarely used for feedthroughs but frequently gets coated with a vitreous enamel where the interface is also a glass-metal bond. The glass-metal bond strength is also governed by the character of the oxide layer on its surface. A presence of cobalt 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 growing into the glass and improving the bond strength.
Iron cannot be directly sealed to lead glass as it reacts with the lead oxide and reduces it to metallic lead. For iron 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 impurities can be removed by heating in wet hydrogen. Plating with copper, nickel or chromium is also used.
Chromium 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.
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 or glass lasers. It can bond to glass via the intermediate oxide layer of nickel oxide and cobalt oxide. The proportion of iron oxide in this alloy 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 metal-glass seal. A metallic color indicates the lack of oxide, while a black color indicates an overly oxidized metal, in both cases leading to a weak glass-metal joint.
304 Stainless steel forms bonds with glass via an intermediate layer of chromium oxide and ferric oxide. Further reactions of chromium forming chromium silicide dendrites are also possible. The thermal expansion coefficient of steel is however fairly different from the glass like that of copper. This can be alleviated by using knife-edge seals.
Molybdenum bonds to the glass via the intermediate layer of molybdenum dioxide. Due to its low thermal expansion coefficient which is matched to glass, molybdenum is often used for glass-metal bonds especially in conjunction with aluminium-silicate glass. It's high electrical conductivity makes it much superior to nickel-cobalt-iron alloys. It is favoured by the lighting industry as feedthroughs for light bulbs and other devices. Molybdenum oxidises much faster than tungsten and quickly develops a thick oxide layer that does not adhere well. It's oxidation should therefore be limited to just yellowish or at most a blue-green color. The oxide is volatile and evaporates as a white smoke above 700 °C. Excess oxide can be removed by heating it in inert gas like argon at 1000 °C. Molybdenum strips are used instead of wires where higher currents and higher cross-sections of the conductors are needed.
Tungsten bonds to the glass via the intermediate layer of tungsten trioxide. A properly formed glass-metal bond has a characteristic coppery/orange/brown-yellow color in lithium-free glasses. In lithium-containing glasses, the bond is blue in colour due to the formation of lithium tungstate. Due to its low thermal coefficient of expansion which is matched to glass, tungsten is frequently used for glass-metal bonds. Tungsten forms satisfying bonds with glasses with similar thermal coefficient of expansion 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.
Mercury is a metal which is liquid at normal temperature. It was used as the earliest glass-to-metal seal and is still in use for liquid seals for applications like rotary shafts.
Platinum has similar thermal expansion as glass and is well-wetted with molten glass. It however does not form oxides and its bond strength is lower. The glass-metal seal has a metallic color and limited strength.
Like platinum, gold does not form any oxides that could assist in glass-metal bonding. Glass-gold bonds are therefore metallic in color and weak. Gold tends to be used for glass-metal seals only very rarely. Special compositions of soda-lime glasses that match the thermal expansion of gold containing tungsten trioxide and oxides of lanthanum, aluminum and zirconium do exist.
Silver forms a thin layer of silver oxide on its surface. This layer dissolves in molten glass and forms silver silicate facilitating a strong bond.
Zirconium 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 much like zirconium can be sealed to some types of glasses with just a little treatment.
Indium and some of its alloys can be used as a solder capable of wetting glass, ceramics, and metals and joining them together. Indium has a low melting point and is very soft. The softness of indium 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 is a soft metal with a melting point at 30 °C. It readily wets glasses and most other metals and can be used for seals that can be assembled or 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 like galinstan.
Types of Glass Metal Seals
The first technological use of a glass-to-metal seal was the encapsulation of 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.
A less toxic and more expensive alternative to mercury is gallium. Mercury and gallium seals can be used for vacuum-sealing rotary shafts.
Platinum Wire Seal
The next step in glass-metal seal technology was to use thin platinum wire for this. 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-oxidability and high melting point. This type of seal was most 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. 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. A 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 called the lead-in-wire. 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
To avoid a strong tensile stress when sealing copper through glass a thin walled copper tube is used instead of a solid wire. In this seal, a shear stress builds up in the glass-to-metal interface which is limited by the low tensile strength of the copper combined with 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 use 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, the knife edge seal is used. In the knife edge seal, the end of a copper tube is machined to a sharp knife edge. In this method, 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 improvements, the knife edge is wetted just 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 copper oxide containing layer between copper and glass. This is done by borating. After a copper plated tungsten wire is immersed for about 30s in chromic acid it is 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, followed by quenching in water and drying. Another method of borating 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 copper oxide 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 like 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 copper oxide which react with hydrogen that diffuses into the copper to water 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 copper which is both free of copper oxide and deoxidising additives which might evaporate at high temperature in vacuum.
Copper Disc Seals
In the copper disc seal, the end of a glass tube 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 0.3mm. Best mechanical strength is obtained if both sides of the disc are fused to the same type of glass tube 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 key to success here 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.
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 thermal expansion coefficient 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 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 iron-nickel 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. 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.
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 carbon monoxide when it is sealed to glass under oxidizing conditions. Both are a major source of problems for the 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.
Molybdenum Foil Seals
Another widely used method to seal through glass with low thermal expansion coefficient 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. 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.
Another type 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. Because glass is extremely strong in compression, compression seals can withstand very high pressures.
Silver Chloride Seals
Silver chloride, which melts at 457 °C bonds to glass, metals and other materials and is 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 of Glass-to-Metal Seals
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 Housekeeper 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.
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