Glass 8245 for X-Ray Sources
Linked by location and technology
A distance of just 40 km separates the home of SCHOTT AG in Jena, Thuringia from the Siemens Healthineers facility in Rudolstadt, Germany. Both tech companies can look back on more than 100 years of history and are now linked by close technological cooperation. Siemens Healthineers uses glass 8245 to manufacture vacuum components, X-ray tubes and X-ray beams, as well as detectors for diagnostic devices such as Computed Tomography (CT) scanners. The glass forms a gas-tight seal and insulator between the anode and cathode of the X-ray tube.
TaskIn an X-ray tube, a voltage of 25 to several hundred kilovolt (kV) in a high vacuum accelerates electrons from the cathode to the anode, where they crash into the anode metal and emit the characteristic X-rays. Glass is an ideal barrier also in form of a seal with metals since it prevents the entrance of air molecules that would inhibit the x-ray formation in the tube, the high ohmic resistance of the borosilicate glass SCHOTT 8245 (107.4 to 105.9 Ω.cm even at temperatures of 250 °C to 350 °C) electrically separates cathodes and anodes.
SolutionSiemens Healthineers acquires the glass as cylinders in a variety of diameters, for example, 75.5 mm. The further processing includes bonding rings made of the nickel-iron-cobalt alloy Kovar being connected at both ends and later assembled into the x-ray tube. The rings are first roughened and cleaned by sandblasting, then annealed. The glass is then heated, 3d formed according to the needs and fused with the sealing rings, which creates a vacuum-tight glass-metal connection.
Due to its sealing and electrical properties, the borosilicate glass SCHOTT 8245 is ideally suited for X-ray tubes.
In the first optional step the glass tube is heated and widened or blown into a mold to abtain the desired contour.
The glass is then fused with the sealing rings to create a vacuum-tight glass-metal connection.
Finally, the glass tube is blown into a mold to obtain the desired contour.
The final product is used for some of the most technically advanced radiographic devices in medicine.
Long-term stability and reliabilityThe stable vacuum-tight bond is particularly crucial to the long-term function of X-ray tubes. This is because high mechanical forces can occur in the glass tube at the glass-metal seal when the X-ray source is moved around the patient computed tomography. Moreover, unavoidable vibrations occur. “We need a glass of very high quality, without airlines or other impurities,” explains Daniel Kutschbach, a process engineer at Siemens Healthineers in Rudolstadt, who’s responsible for the components of X-ray tubes and glass processing. “We ensure this by closely monitoring each processing step.”
Used materialsThe permanent sealing of the borosilicate glass SCHOTT 8245 with a metal alloy is only possible because both materials have the same thermal coefficient of expansion as they are heated to 400 °C during operation. If the glass and metal expanded at different rates, mechanical stresses will occur and eventually lead to breakage.
Unlike many applications, the optical specifications of the glass are irrelevant since radiation can exit the tubes also through a radiolucent titanium window. During a typical three-year service time the glass even turns brown, but this does not impair its functionality.
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