Vacuum Optics ❯ Glass-to-metal seals & glass working
Glass-to-metal seals & glass working
Introduction to glass working
Within the glass shop at Torr Scientific, a large variety of vacuum viewports and glass fabrications undergo glass-to-metal sealing. This is the process for how vacuum optics are connected to metal flanges to create hermetic seals.
Glass-to-metal seals may seem simple, but there is a lot of science and engineering know-how required to manufacture them.
Torr Scientific can offer vacuum brazed, induction heated and bonded seals for our vacuum optics.
For more information, please contact our sales team.
More information/Custom orders
Glass tubulations
Torr Scientific manufactures glass tubulations for various purposes with a range of styles and types of glass.
The product “tubulation” refers to the open-ended glass tube, sealed to a Conflat® flange (CF). Available in kodial glass (borosilicate) and Pyrex.
Tubulated viewports, these consist of a Conflat® flange attached to a glass tube with a flat window at the top. The standard product is available in kodial glass (borosilicate).
Domed tubulation, these tubulations are connected to a Conflat® flange, meeting at a dome shape at the top. The standard product is available in kodial glass (borosilicate).
What is thermal expansion?
Thermal expansion is the degree to which materials change in volume when exposed to a change in temperature. Materials all have different thermal expansion coefficients and this will dictate what can be successfully sealed together.
Matched seals work on the idea of using materials with similar thermal expansion coefficients, as this means there is reduced strain for the optic and the metal.
For the Pyrex glass tubulations in our catalogue it is necessary to utilise a “graded seal”. A graded seal uses a sequence of intermediate glasses, each with a slightly different expansion coefficient to fill the gap between Pyrex and borosilicate.
Facilities at Torr Scientific
The glass shop at Torr houses a variety of expertise and equipment surrounding glass working. Together, the team have a collective 75 years’ experience with the field. All kodial glass viewports are produced in-house in the glass shop.
Equipment we have at Torr includes:
Induction heaters
These are used for joining the weld-rings to the optic, they allow for heating equally around the optic for a secure and uniform connection.
Glass-working lathes
The purpose of lathes is to hold glass products while glass blowing occurs. They allow the materials to be rotated. During the process the metal and glass tubes are pressed together to form the seal.
Ovens
The main purpose of the ovens is to remove strain from optics, in a process called “annealing”. Manufactured parts are placed in the oven at a high temperature and allowed to cool to room temperature overnight.
Test equipment
The glass shop is home to a strain viewer. Completed optic assemblies are positioned between a pair of crossed polarisers; residual strain in the assemblies can be clearly seen as visually distinct colours.
What are the different methods of joining optics and metal optic carriers together?
Optics are joined to an intermediate metal optic carrier by a variety of means, depending on the materials involved. Brazing, induction heating and bonding are the methods Torr uses for sealing glass optics into metal carriers.
Bonding
The alternative method to brazing is bonding. This technique sets the optic into the flange at room temperature using a vacuum-compatible, low outgassing bonding material.
One advantage of this technique is the original flatness of the optic is maintained after sealing; however, the drawback is that the maximum temperature for bakeouts is decreased to 120°C.
Induction heating
For borosilicate viewports we use a kovar optic carrier and a borosilicate optic; these have matched coefficients of expansion. The optic carrier is heated up to a point where the edge of the borosilicate optic melts forming a hermetic seal between the two.
Brazing
Brazing uses an intermediate braze material to join the optic to the optic carrier. We braze our fused silica, sapphire and CVD diamond viewports.
Different grades of braze material are used depending on the materials of the optic carrier and the optic involved. For example, we use different braze materials to manufacture our low and high temperature sapphire viewports.
Brazing imparts stress between the optic and the metal carrier. This manifests itself in the optic changing shape, resulting in the optic’s flatness pre-brazing being reduced post-brazing.
However, the effects of reducing the flatness does not have a significant impact on the transmitted wavefront error.
Seals for vacuum optics
For more information regarding vacuum viewports and the types of seals available please visit the Vacuum Viewports webpage or enquire with our sales team: sales@torrscientific.co.uk.
Custom orders
In the past we have manufactured customised glass products, see the links below.
For any non-standard product requests, please contact our sales team: sales@torrscientific.co.uk.
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