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[October 2007]
Collaboration Solves Challenge With Development of Metallised Ceramic Parts for Neutron Source
ISIS, a world-class spallation neutron source based at the CCLRC Rutherford Appleton Laboratory (Oxon, England), has commissioned a series of highly specialist metallised ceramic components from Morgan Advanced Ceramics (Rugby, Warks, England) as part of a major expansion project. The components are a fundamental element of new monitoring equipment to be installed inside the extracted proton beam forming part of the second target station currently being added to the facility.
ISIS has contributed significantly to many of the major breakthroughs in materials science, physics and chemistry since it was commissioned in 1985. The £145 million Second Target Station Project (TS-2) will provide a second neutron source at the facility to double capacity and enable the ISIS science programme to expand into the key research areas of soft condensed matter, bio-molecular sciences, advanced materials and nano-scale science.
One of many challenges for the project team was in the construction of the instrumentation to monitor the intensity of the extracted proton beam (EPB). Ceramic vacuum tubes used in the first target station were sealed with Indium wire, but experience had shown that these could become unreliable if disturbed. The ISIS team wanted to find a robust, zero-maintenance solution for TS-2.
Eamonn Quinn, a Senior Project Manager at Rutherford explains: "This is a radiation environment, so reducing the need for maintenance staff to enter the area was a key driver. We needed to find a way of producing a 100% reliable vacuum seal within the very tight tolerances of the design, and came up with the idea of trying a metallised ceramic".
Morgan Advanced Ceramics, a major manufacturer of technical ceramic materials and particularly in metallisation of ceramics, was the obvious choice of partner. Following initial discussions the two companies collaborated closely to develop a solution to what was essentially a unique engineering challenge.
"There were severe design constraints on the project,” said Eamonn Quinn, “because we insisted the new vacuum tubes should retrofit to the first target station, as well as be suitable for TS-2. The previous design based on alumina ceramic and mild steel worked well, apart from the Indium seal, and we wanted to stick with it as closely as possible."
There were two key challenges for Morgan Advanced Ceramics. This first was to come up with a design and a manufacturing process that would produce a robust, high integrity vacuum seal (leak rate 10-8mbar l/s) across a large component (200mm diameter). The second was to solve the problem of the differences in thermal co-efficient between the alumina ceramics of the tube and its mild steel flanges. Further, a very tight specification was set for the physical dimensions and cleanliness of the components.
Martin Davidson, the Morgan Advanced Ceramics Senior Engineer who led the ISIS project, explains: "In a development like this you don't have the luxury of being able to produce a prototype to refine the process – there is neither time nor budget. We had to rely heavily on our experience to get it right first time. We worked very closely with the ISIS team to come up with a design that we knew we could manufacture, and that met all the project requirements".
The ISIS assembly is 158mm long with two nickel-plated mild steel flanges 240mm diameter insulated from each other by a pre-formed diamond ground alumina ceramic insulator. To ensure hermetic integrity of the assembly the ceramic is brazed in a hydrogen/nitrogen furnace at 850°C to two flanges made of nickel iron cobalt steel, chosen because it provides the best thermal expansion match to the ceramic. This process is achieved by applying a moly-manganese coating which is sintered at 1400°C, then electroplating a layer of nickel. The ceramic/metal brazed sub-assembly is then welded to the mild steel flanges with a stainless steel interface and machined to the final dimensions. The geometric tolerances are challenging, and critical.
ENDS