Computer tomography is based on the fundamental mathematical principles of the transform developed by the Austrian scientist Johann Radon in 1917 and subsequently named after him.
Its technical realization in the form of the computer tomography scanners used today for medical diagnostics goes back to developments since around 1972. The constant further development of the performance of these scanners over several generations since this time led in the 1990s to the application of high-purity, dense-sintered Al2O3 ceramic in electrical insulators. Background is the rising thermal stress on the radiation unit with increasing precision of the optical image resolution, as the radiation source only delivers 1 % X-ray radiation and 99 % heat.
The use of a component made of Al2O3 ceramic in the X-ray emitter requires a joining method for the substance-to-substance bond with a metal component. Such a ceramic-metal composite (CMC) has to exhibit reliable mechanical and thermal resistance in use, be high-vacuum-tight, have high geometric precision and should not be contaminated with any impermissible material and surface impurities. The requirements for the cleanness and reliability of the production equipment for the bonded components are therefore correspondingly high.
A CMC for an X-ray emitter generally consists of three components:
- Dense-sintered Al2O3 ceramic with a minimum purity of 95 %.
- Vacuum brazing alloy, usually AgCu28 (eutectic: 779 °C) or AgPd5Cu26,6 (melting range: 807 – 810 °C).
- Metal, usually 1.3981 (NiCo2918). The thermal expansion of this alloy is well fitted to the thermal expansion behaviour of the Al2O3 ceramic in the range of the brazing curve of the CMC.
With the use of these components, the ceramic is not brazed directly to the metal part. The positions to be brazed are therefore first coated with a metallization, for example according to MoMn process known for more than 70 years, and after firing, they are strengthened with a nickel film measuring several µm in thickness. As a result, reliable wetting of the metallization is ensured by the brazing material.
After assembly of the three components, brazing is performed in reducing conditions or in a vacuum of sufficient quality at temperatures above 800 °C. The strength of a material composite produced in this way regularly reaches more than 100 MPa when test specimens are exposed to bending loads.
An X-ray emitter component produced in this way has the following properties:
- Leakage rate for helium in the leakage test in normal conditions ≤1*10-9 mbar*l/s.
- Reliable compliance with the geometric tolerances that frequently lie in the order of 0.01 mm.
- Contamination- and defect-free ceramic and metallic surfaces in accordance with the specifications of the equipment manufacturer.
Al2O3-based CMC components for the X-ray emitter of the computer tomography scanners and other equipment components, like, for example, electrical feedthroughs, have been the established and proven standard in this field for far more than 10 years.
Plant components made of dense-sintered Al2O3 ceramics are used in angiography systems like, for example, X-ray image intensifiers (XRII), in positions where they must meet high requirements for electrical insulation, geometric precision, high-vacuum tightness of ceramic-metal composites (CMC), freedom from contamination by extraneous material components and freedom from functional surface defects.
Insofar, the principal requirements and the data of the materials are comparable with the CMCs used in computer tomography scanners.
Typical material and component data are:
- Al2O3 ceramic:
- Specific electrical resistance at room temperature >1013 Ω*cm
- Dielectric strength >30 kV/mm
- Component diameter measuring 200 mm and much larger with tolerances up to 0.01 mm range
- Ceramic-metal composite: composite produced by means of brazing with 1.3981 material with a leakage rate for helium ≤10-9 mbar*l/s
For the use of a CMC in an XRII, an additional requirement is the coating of the vacuum-side ceramic surfaces so that they are electrically conductive in order to prevent electrostatic charging during the operation of the equipment and accordingly undesirable brightening of the image. The electrically conductive surfaces can be obtained, for example, with the use of a metal-based suspension by means of thick film technology. In this way, areas measuring 500 cm² and larger can be coated defect-free.
The use of high-vacuum-tight brazed components for imaging processes in angiography with high-grade Al2O3 ceramic as an electrical insulator goes back to developments in the late 1980s. Today, components of this type are the established standard for such equipment with correspondingly wide distribution in the industry.