Wire resistance heating elements for vacuum duty
High vacuum wire wound thin film substrate heating elements are effective for local heating of substrates, or for wide area heating of Vacuum furnaces. Refractory metal wire is wound around ceramic formers or embedded in an electrically insulating housing. The ceramics provide support for the material when it is in its high temperature state. The refractory metals that are commonly used are Kanthal or Molybdenum. These materials are capable of working at temperatures of 1000°C in air, or1200°C in an oxygen free atmosphere or vacuum.
When used in a reactive environment, a hard oxide layer builds up on the surface of the wire. This protects the material from further oxidation. Because of this most sample heaters of this type are first 'run in'. If the wire is not supported well the oxide layer can flake / crack off, which exposes bare metal to the reactive gases. if this is allowed to happen , eventually the material will wear away or will crack, then would need replacing.
The main drawback with this type of heater element is that is ages very quickly. The material will tend to become brittle and hence very fragile. To overcome these drawbacks, elaborate ceramic housings and formers have been designed to support the wire whilst hot.
Very small resistance elements can be formed by using these materials, so they are very popular in sample heating for surface science experiments.
The main benefits of using wire elements is their ability to be used at temperatures high than un- coated graphite in a reactive atmosphere, and that they are very easy to design into wide area furnace heating. Control of these elements is also very straight forward. They need only burst firing thyristors and a thermocouple for feedback to the temperature controller.
Pictured below is a Kanthal coiled element supported by Macor ceramic former. This element was designed and used to heat Quartz Ø150mm substrates in an oxygen atmosphere during a reactive Sputtering process. It has been running at up to 800°C for extended periods. The element was installed in a Sputtering system 4 years ago, and still shows no major wear.
If you have an application or need some technical data on this type of element please contact us. Our engineers are glad to help.
