Figure 2. Schematic diagram of a specific heat flux imposed through one individual graphene nanoribbon in a shape of square. The red region has a higher value of temperature than the blue regions and that the property being transported in the graphene nanoribbon therefore flows from the red region to the blue regions. The arrows indicate the direction of heat transfer between the hot and cold regions. Linear momentum of the atoms is transferred to maintain a constant heat flux between the red region and the blue regions and a constant temperature gradient across the graphene nanoribbon.
Graphite generally comprises multiple layers of carbon atoms arranged in planar hexagonal lattices. In its highly oriented pyrolytic form, the hexagonal lattice sheets have an angular spread of less than one degree. This structure results in properties that are highly desirable for use in vacuum electron devices. For example, highly oriented pyrolytic graphite is well suited for use as a vacuum barrier. When used to manufacture components that form part of the vacuum seal, highly oriented pyrolytic graphite maintains vacuum integrity. Highly oriented pyrolytic graphite also possesses the lowest sputtering rate of all materials. Thus, electrodes made from highly oriented pyrolytic graphite will emit far fewer contaminating trace elements during operation than will copper or molybdenum electrodes. Further contributing to this property is that fact that highly oriented pyrolytic graphite has an extremely high melting point. It is refectory and changes state at a temperature of 3650 °C as compared to copper’s melting point of just 1080 °C. Highly oriented pyrolytic graphite is thus grown on a graphite substrate in reactor vessels at temperatures of up to 3000 °C, and contaminates are simply precipitated out, resulting in an extremely pure finished product.
Highly oriented pyrolytic graphite also exhibits a very low ion erosion rate compared to copper or molybdenum [37]. For microwave devices that exhibit failure modes due to ion erosion, highly oriented pyrolytic graphite dramatically improves operational lifetime [38]. Highly oriented pyrolytic graphite also exhibits a very low vapor pressure, which reduces electron ionization of the residual gas in vacuum electron devices [39]. It is likely that gas ionization, ionization of sputtered elements, and secondary electron yield are responsible for presenting charge that is out of favorable phase to the output electrodes of vacuum devices, resulting in degraded operation [40]. In particular, this out-of-favorable-phase charge collected at the output electrodes manifests as spurious radio frequency output noise. Electrodes made from highly oriented pyrolytic graphite will thus result in vacuum devices that exhibit lower radio frequency noise at comparable operating conditions when compared with standard devices employing copper or molybdenum electrodes. Highly oriented pyrolytic graphite also possesses a very high thermal conductivity close to that of diamond and at least four time greater than that of copper. This enables highly oriented pyrolytic graphite components to dissipate far higher thermal loads before exhibiting thermal damage. This enables vacuum electron devices to be designed for and to operate at much higher power densities. Nichrome wire exhibits a thermal expansion coefficient different enough from copper and steel to ensure that the assembly is maintained under pressure even when the assembly is brought to braze temperature. The pressure applied by the fixture forces the braze alloy through the gaps between the pyrolytic graphite and the copper and effectively eliminates voids. When excess alloy is allowed to contact the fixture, it invariably bonds to the fixture. Highly oriented pyrolytic graphite is a highly pure and ordered form of synthetic graphite.