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.