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 category of materials, including rubber and plastics. Hermetic seals are essential to the correct and safe functionality of many electronic and healthcare products. Used technically, it is stated in conjunction with a specific test method and conditions of use. Colloquially, the exact requirements of such a seal varies with the application.
The word hermetic comes from the Greek god Hermes. A hermetic seal comes from alchemy in the tradition of Hermeticism. The legendary Hermes Trismegistus supposedly invented the process of making a glass tube airtight using a secret seal.[1]
Uses
A hermetically sealed battery
Some kinds of packaging must maintain a seal against the flow of gases, for example, packaging for some foods, pharmaceuticals, chemicals and consumer goods. The term can describe the result of some food preservation practices, such as vacuum packing and canning. Packaging materials include glass, aluminum cans, metal foils, and gas impermeable plastics.
In the funeral industry, some caskets and burial vaults are hermetically sealed by a rubber seal and being locked.
Types of epoxy hermetic seals
Typical epoxy resins have pendant hydroxyl (-OH) groups along their chain that can form bonds or strong polar attractions to oxide or hydroxyl surfaces. Most inorganic surfaces—i.e., metals, minerals, glasses, ceramics—have polarity so they have high surface energy. The important factor in determining good adhesive strength is whether the surface energy of the substrate is close to or higher than the surface energy of the cured adhesive.
Certain epoxy resins and their processes can create a hermetic bond to copper, brass, stainless steel, specialty alloys, plastic, or epoxy itself with similar coefficients of thermal expansion, and are used in the manufacture of hermetic electrical and fiber optic hermetic seals. Epoxy-based seals can increase signal density within a feedthrough design compared to other technologies with minimal spacing requirements between electrical conductors. Epoxy hermetic seal designs can be used in hermetic seal applications for low or high vacuum or pressures, effectively sealing gases or fluids including helium gas to very low helium gas leak rates similar to glass or ceramic. Hermetic epoxy seals also offer the design flexibility of sealing either copper alloy wires or pins instead of the much less electrically conductive Kovar pin materials required in glass or ceramic hermetic seals. With a typical operating temperature range of −70°C to +125°C or 150°C, epoxy hermetic seals are more limited in comparison to glass or ceramic seals, although some hermetic epoxy designs are capable of withstanding 200°C.[3]
Matched glass-to-metal seals
Types of glass-to-metal hermetic seals
When the glass and the metal being hermetically sealed have the same coefficient of thermal expansion, a "matched seal" derives its strength from bond between the glass and the metal's oxide. This type of glass-to-metal hermetic seal is generally used for low-intensity applications such as in light bulb bases.[4]
"Compression seals" occur when the glass and the metal have different coefficients of thermal expansion such that the metal compresses around the solidified glass as it cools. Compression seals can withstand very high pressure and are used in a variety of industrial applications.
Compared to epoxy hermetic seals, glass-to-metal seals can be operated up to much higher temperatures (250°C for compression seals, 450°C for matched seals). The material selection is however more limited due to thermal expansion constraints. The sealing process is performed at roughly 1000°C in an inert or reducing atmosphere to prevent discoloration of the parts.[5]
Ceramic-to-metal hermetic seals
Co-fired ceramic seals are an alternative to glass. Ceramic seals exceed the design barriers of glass to metal seals due to superior hermetic performance in high stress environments requiring a robust seal. Choosing between glass versus ceramic depends on the application, weight, thermal solution and material requirements.
Glassware sealing
Sealing solids
A taper-joint stopper with PTFE Sealing Ring. Optical transparency of the narrow sealing ring pressured by glass joint (right).
Glass taper joints can be sealed hermetically with PTFE sealing rings (high vacuum tight, air leakage rate 10−6 mBar×L/sec and below),[6]o-rings (optionally encapsulated o-rings), or PTFE sleeves,[7] sometimes used instead of grease that can dissolve into contamination. PTFE tape, PTFE resin string, and wax are other alternatives that are finding widespread use, but require a little care when winding onto the joint to ensure a good seal is produced.
Grease is used to lubricate glass stopcocks and joints. Some laboratories fill them into syringes for easy application. Two typical examples: Left – Krytox, a fluoroether-based grease; Right – a silicone-based high vacuum grease by Dow Corning.
A thin layer of grease made for this application can be applied to the ground glass surfaces to be connected, and the inner joint is inserted into the outer joint such that the ground glass surfaces of each are next to each other to make the connection. In addition to making a leak-tight connection, the grease lets two joints be later separated more easily. A potential drawback of such grease is that if used on laboratory glassware for a long time in high-temperature applications (such as for continuous distillation), the grease may eventually contaminate the chemicals.[8] Also, reagents may react with the grease,[9][10] especially under vacuum. For these reasons, it is advisable to apply a light ring of grease at the fat end of the taper and not its tip, to keep it from going inside the glassware. If the grease smears over the entire taper surface on mating, too much was used. Using greases specifically designed for this purpose is also a good idea, as these are often better at sealing under vacuum, thicker and so less likely to flow out of the taper, become fluidic at higher temperatures than Vaseline (a common substitute) and are more chemically inert than other substitutes.
Cleaning
Ground glass joints are translucent when clean and free of debris. Solvents, reaction mixtures and old grease appear as transparent spots. Grease can be removed by wiping with an appropriate solvent; ethers, methylene chloride, ethyl acetate, or hexanes work well for silicone- and hydrocarbon-based greases. Fluoroether-based greases are quite impervious to organic solvents. Most chemists simply wipe them off as much as possible. Some fluorinated solvents can remove fluoroether greases, but are costlier than laboratory solvents.
Testing
Standard test methods are available for measuring the moisture vapor transmission rate, oxygen transmission rate, etc. of packaging materials. Completed packages, however, involve heat seals, joints, and closures that often reduce the effective barrier of the package. For example, the glass of a glass bottle may have an effective total barrier but the screw cap closure and the closure liner might not.
See also
Look up hermetic seal in Wiktionary, the free dictionary.
↑ Rob Toreki (2006-12-30). "Glassware Joints". Interactive Learning Paradigms Inc.
↑ Haiduc, I., "Silicone Grease: A Serendipitous Reagent for the Synthesis of Exotic Molecular and Supramolecular Compounds", Organometallics 2004, volume 23, pp. 3–8. doi:10.1021/om034176w
↑ Lucian C. Pop and M. Saito (2015). "Serendipitous Reactions Involving a Silicone Grease". Coordination Chemistry Reviews. 314: 64–70. doi:10.1016/j.ccr.2015.07.005.
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 analytical laboratories. Many laboratories have training programs to demonstrate how glassware is used and to alert first–time users to the safety hazards involved with using glassware.
In organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane. They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking utensils, thermal insulation, and electrical insulation. Some common forms include silicone oil, grease, rubber, resin, and caulk.
An O-ring, also known as a packing or a toric joint, is a mechanical gasket in the shape of a torus; it is a loop of elastomer with a round cross-section, designed to be seated in a groove and compressed during assembly between two or more parts, forming a seal at the interface.
A stopper, bung, or cork is a cylindrical or conical closure used to seal a container, such as a bottle, tube, or barrel.
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 1×10−6 pascals. UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
Silicone grease, sometimes called dielectric grease, is a waterproof grease made by combining a silicone oil with a thickener. Most commonly, the silicone oil is polydimethylsiloxane (PDMS) and the thickener is amorphous fumed silica. Using this formulation, silicone grease is a translucent white viscous paste, with exact properties dependent on the type and proportion of the components. More specialized silicone greases are made from fluorinated silicones or, for low-temperature applications, PDMS containing some phenyl substituents in place of methyl groups. Other thickeners may be used, including stearates and powdered polytetrafluorethylene (PTFE). Greases formulated from silicone oils with silica thickener are sometimes referred to as silicone paste to distinguish them from silicone grease made with silicone oil and a soap thickener.
Vacuum engineering is the field of engineering that deals with the practical use of vacuum in industrial and scientific applications. Vacuum may improve the productivity and performance of processes otherwise carried out at normal air pressure, or may make possible processes that could not be done in the presence of air. Vacuum engineering techniques are widely applied in materials processing such as drying or filtering, chemical processing, application of metal coatings to objects, manufacture of electron devices and incandescent lamps, and in scientific research. Key developments in modern science owe their roots to exploiting vacuum engineering, be it discovering fundamental physics using particle accelerators, the advanced analytical equipment used to study physical properties of materials or the vacuum chambers within which cryogenic systems are placed to execute operations in solid state Qubits for quantum computation. Vacuum engineering also has its deep bearings in manufacturing technology.
A stopcock is a form of valve used to control the flow of a liquid or gas. The term is not precise and is applied to many different types of valve. The only consistent attribute is that the valve is designed to completely stop the flow when closed fully.
A glass-ceramic-to-metal seal is a type of mechanical seal which binds glass-ceramic and metal surfaces. They are related to glass-to-metal seals, and like them are hermetic (airtight).
In physics and engineering, permeation is the penetration of a permeate through a solid. It is directly related to the concentration gradient of the permeate, a material's intrinsic permeability, and the materials' mass diffusivity. Permeation is modeled by equations such as Fick's laws of diffusion, and can be measured using tools such as a minipermeameter.
Electronic packaging is the design and production of enclosures for electronic devices ranging from individual semiconductor devices up to complete systems such as a mainframe computer. Packaging of an electronic system must consider protection from mechanical damage, cooling, radio frequency noise emission and electrostatic discharge. Product safety standards may dictate particular features of a consumer product, for example, external case temperature or grounding of exposed metal parts. Prototypes and industrial equipment made in small quantities may use standardized commercially available enclosures such as card cages or prefabricated boxes. Mass-market consumer devices may have highly specialized packaging to increase consumer appeal. Electronic packaging is a major discipline within the field of mechanical engineering.
A Schlenk flask, or Schlenk tube, is a reaction vessel typically used in air-sensitive chemistry, invented by Wilhelm Schlenk. It has a side arm fitted with a PTFE or ground glass stopcock, which allows the vessel to be evacuated or filled with gases. These flasks are often connected to Schlenk lines, which allow both operations to be done easily.
Ground glass joints are used in laboratories to quickly and easily fit leak-tight apparatus together from interchangeable commonly available parts. For example, a round bottom flask, Liebig condenser, and oil bubbler with ground glass joints may be rapidly fitted together to reflux a reaction mixture. This is a large improvement compared with older methods of custom-made glassware, which was time-consuming and expensive, or the use of less chemical resistant and heat resistant corks or rubber bungs and glass tubes as joints, which took time to prepare as well.
Glass-to-metal seals are a type of mechanical seal which joins glass and metal surfaces. They are very important elements in the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass-encapsulated semiconductor diodes, reed switches, glass windows in metal cases, and metal or ceramic packages of electronic components.
Dry lubricants or solid lubricants are materials that, despite being in the solid phase, are able to reduce friction between two surfaces sliding against each other without the need for a liquid oil medium.
Cannula transfer or cannulation is a set of air-free techniques used with a Schlenk line, in transferring liquid or solution samples between reaction vessels via cannulae, avoiding atmospheric contamination. While the syringes are not the same as cannulae, the techniques remain relevant.
A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting it to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.
Glass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, e.g., accelerometers or gyroscopes. This technique utilizes low melting-point glass and therefore provides various advantages including that viscosity of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching or deposition. A low viscosity promotes hermetically sealed encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon. This results in low stress in the bonded wafer pair. The glass has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures occurs. The usual temperature of achieving flowing and wetting is between 450 and 550 °C.
Materials for use in vacuum are materials that show very low rates of outgassing in vacuum and, where applicable, are tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the vacuum chamber. The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed on the material surface. Materials may sublimate in vacuum. Or the gases can be released from porous materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.
Cryogenic seals provide a mechanical containment mechanism for materials held at cryogenic temperatures, such as cryogenic fluids. Various techniques, including soldering and welding are available for creating seals; however, specialized materials and processes are necessary to hermetically entrap cryogenic constituents under vacuum-tight conditions. Most commonly used are liquid helium and liquid nitrogen, which boil at very low temperatures, below −153 °C, as well as hydrocarbons with low freezing points and refrigerating mixtures. Pure indium wire or solder preform washers are accepted as the most reliable low temperature sealing materials. When correctly formed, indium will afford leak rates of less than 4.0x10 -9 mbar- liter/sec. Alternative cryogenic seal materials include silicone grease conical seals, and Pb/Sn (lead-tin) wire seals.
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