All MDC quartz viewports are constructed from UV grade fused silica. Sleeve and flange materials are manufactured from high purity 304 stainless steel and vacuum tube grade braze alloys are designed and rated for high and ultrahigh vacuum applications. Fused silica is a polycrystalline, isotropic material with no crystal orientation. The physical, thermal, dielectric and optical properties are uniform in all directions of measurement. Synthetic fused silica is manufactured by flame hydrolysis of silicon tetrachloride. These viewports are available up to 7.78 inch view diameters and are preferred over glass viewports because of their transparency to ultraviolet radiation. Other advantages include a higher abrasion resistance and a low coefficient of thermal expansion, making them very resistant to thermal shock. The main limitation with fused silica viewports is that they cannot be heated to temperatures over 200°C. The lead-silver alloy used to seal and bond the window to a stainless steel sleeve melts at 305°C.
Sapphire is a synthetic, hexagonal single crystal anisotropic material which displays
substantially different physical, thermal, dielectric and optical characteristics when
measured along different axes. MDC sapphire viewports are constructed using 90°
crystal orientation, where the crystal orientation is parallel to a window’s surface.
Viewports with crystal orientation parallel to the optical axis, or 0° orientation,
can be custom ordered from Insulator Seal. MDC offers a wide selection of sapphire viewport
assemblies suitable for high and ultrahigh vacuum applications. The use of high purity
sapphire and nickel-iron low expansion alloys enables these viewports to be repeatedly baked to temperatures as
high as 450°C. All sapphire viewports up to 1 inch in diameter use ultraviolet grade sapphire
material. Those exceeding 1 inch diameters are supplied with regular grade sapphire.
Please reference the transmission curves on this page for transmission characteristics
and comparisons. Also note that limitations in current sapphire crystal growth technology make it difficult to
guarantee constant transmission performance at wavelengths below 0.25µm.
In general, poor surface finish can contribute as much as 10% to a window’s total
transmission losses. The sapphire viewports offered in this catalog are supplied
with flat faces polished to a 50-20 scratch-dig while quartz viewports are 80-50 scratch-dig.
Although these finishes are suitable for most optical applications, the optics industry
recommends a 20-10 scratch-dig finish for wavelengths below 0.25µm.
MDC Pyrex® viewports provide a convenient and economical solution for high vacuum
systems requiring periodic glass substrate replacement. Some applications like vacuum
coating will deposit coating materials on system viewports which will eventually
render them useless. The Pyrex® substrate is sealed against an elastomer gasket and
clamped in place with an aluminum retaining flange. It is important to note that
materials are susceptible to point stressing if clamped directly against a metal surface.
For this reason MDC Pyrex® viewport retaining flanges are fitted with a cushion gasket that
prevents metal to glass contact. These viewports are offered in a weldable 4 inch
view diameter half-nipple geometry. They are constructed with Corning type 7740
glass and are suitable for service to 1x10-8 Torr.
Viewport Shutter Assemblies
VPS Series viewport shutters provide quick and efficient shielding for
all standard viewports. These shutters are ideally suited for service
in high and ultrahigh vacuum coating applications. These swing type
pivoting shutters feature a double-sided sealing design and are
designed to mount between a port flange and the viewport. The
shape of each shutter takes advantage of the maximum aperture of
the inside of a standard port tube. The actual amount of shutter
swing is dependent on the mating chamber’s tube port diameter. The
table below gives the maximum swing angle for each shutter when
mounted on a standard Del-Seal™ flanged half nipple. To provide
maximum shielding of viewports each shutter body has a step
machined to match the shape and contour of the shutter’s flapper
plate. Actuation of the shutter is provided through an MDC bellows
sealed rotary drive. Improved design features include all stainless
steel body allowing for bakeout up to 230°C. A positive click-stop
action has also been added to the rotary driver that allows for partial
opening of the shutter’s flapper plate between the fully opened and
fully closed positions.