High density graphites and Carbon Carbon Composites (CCC) are ideal materials for in vacuum heating elements. Chemically the same, high density graphite and carbon carbon composite materials are very inert, get stronger with temperature, has low expansion coefficient and will not seize after heating. High density graphite is brittle but inexpensive and machined conventionally from large blocks, therefore very large sized graphite elements can be produced in a variety of shapes and sizes. Our high strength ultra-fine grained graphite enables small very intricate elements to be manufactured also. Graphite has a low expansion coefficient and is not degraded by constant heating and cooling, and also gets stronger as its temperature increases.
Its low resistivity means it requires high current power supplies and therefore large feedthroughs and cables which can be expensive. It can operate over 2000 °C in an inert atmosphere or in vacuum and <500°C if oxygen is present. Graphite elements have the ability to take very high power density, and therefore very fast ramp up times can be achieved. Its relatively high specific heat capacity means that cool down times in vacuum can be quite long. Apart from reacting with oxygen from 500C, graphite is otherwise very inert and can therefore operate in very corrosive or aggressive atmospheres without degradation. Particle contamination and open porosity can be a problem with graphite, but this can be overcome with coatings, or impregnations detailed below. Graphite elements are suitable for UHV applications, but must undergo initial out-gassing process due to its open porosity.
Carbon Carbon Composite is much stronger than graphite and is not brittle due to its fibrous grain structure. Carbon Carbon Composite elements can therefore be made in very thin sections, typically 1mm thick, which overcome a number of the problems associated with high density graphite elements. Thin carbon carbon composite elements have a much higher resistance than high density graphite elements, allowing lower current, higher voltage power supplies to be used, and smaller power feedthroughs and cables, thereby reducing costs. The lower mass of the thin carbon carbon composite elements means that they heat up much quicker and also cool down much faster in vacuum. Carbon Carbon composite is produced in sheets (typically 1m²) of various thicknesses from 1.0mm to 30mm. Modern CNC machining techniques mean that carbon carbon composite elements can be produced very cheaply. We stock standard designs of carbon carbon composite elements from Ø1″ to Ø6″ which are manufactured in quantity and therefore very cost effective. Carbon carbon composite also has extremely low thermal conductivity which is beneficial in reducing heat loss through the power contact points, thereby increasing element uniformity. This low thermal conductivity also means that carbon carbon composite is ideal for use as a heat shield, both in vacuum and also in inert atmosphere.
For larger sized elements it will be necessary for the graphite element or carbon carbon composite element to be supported, as these materials do not have the rigidity to support themselves without sagging. Ceramisis can supply a range of ceramic materials suitable for supporting graphite elements and carbon carbon composite elements. Ceramisis can also manufacture ceramic bases with a recess machined to the same pattern as the element. A high thermal conductivity ceramic lid can then be fixed in position, completely encapsulating the graphite element. This not only supports the element but also protects it from deposition product, and eliminates high temperature arcing in low vacuum.
To electrically insulate graphite elements it is also possible to apply a SiC coating. This can be applied by CVD or by painting and firing. The CVD method has better and more uniform adhesion, and can withstand up to 1400C. It is however very expensive and the surface tension of the coating does apply a high stress to the graphite element meaning the element can distort and thin section elements are not strong enough to be coated. The paint on SiC coating is inexpensive and easy to apply but has a maximum operating temperature of 1100C. Other coatings can be applied to graphite and carbon carbon composite to seal the porosity and reduce particle count. These coatings are pyrolytic graphite, vitreous carbon, and pyrolytic boron nitride. These coatings can improve oxidation resistance, reduce particle count and improve chemical resistance of the graphite element. Pyrolytic graphite coating is the only coating that can be applied to carbon carbon composite. The table below details the properties and applications for each coating.
Silicon Carbide, Pyrolytic Graphite, Vitreous Carbon and PBN Coatings, Plus Vitreous Carbon Impregnation On Graphite
To stop the problems of out gassing, particle contamination and oxidation that occurs with high density graphite and carbon carbon composite elements, there are various coatings that can be applied as follows:
Pyrolytic Graphite Coating ( PG ):
This can be applied by a CVD method to high density graphite and carbon carbon composite elements (see picture right). It is still electrically conductive, but it totally seals the surface porosity and therefore traps any particles. Pyrolytic graphite coating is chemically the same as high density graphite and ccc and so will still react chemically in the same way and with oxygen at 500°C.
PG coating is preferred by some for UHV applications, because it seals the open porosity of the graphite. However should the coating have a pin hole then the underlying graphite will take forever to outgas and thus act as a virtual leak. We therefore recommend uncoated graphite for UHV, as without the coating the graphite can initially outgas freely. PG coating would only be recommended for UHV if particle contamination was an issue.
Vitreous Carbon Coating / Impregnation:
Vitreous carbon surface treatment is a cheap alternative to pyrolytic graphite coating, and is produced by vitrifying a resin applied to the surface of the high density graphite component. It seals in the particles but does not totally seal the porosity, although it can be drastically reduced. Vitreous carbon coating is better at sealing the porosity than the impregnation and gives a nice black glassy appearance to the component. It is chemically the same as high density graphite and so will still react chemically in the same way and with oxygen at 500°C.
Silicon Carbide Coating ( SiC ):
Is a dark grey coating applied by a CVD method to specific grades of high density graphite. This silicon carbide ( SiC ) coating is an electrical insulator and therefore cannot be applied to the electrical contact points on the element (see picture right). We can supply a SiC paint that can be applied to connection points after connection has been made. This paint is then thermally cured. The silicon carbide coating can operate in oxygen environments up to 1500°C and can resist some chemically corrosive environments better than graphite.
Pyrolytic Boron Nitride Coating ( PBN ):
This white Pyrolytic Boron Nitride coating can be applied to very specific grades of high density graphite (see picture right) to seal the porosity and improve the oxidation and chemical resistance of the element. It will oxidise at 900°C if oxygen is present, but can withstand 2000°C in an inert atmosphere or vacuum (with N2 present).