The last article focussed primarily on mechanical properties. We focussed on the most important parameters for assessing the mechanical properties. Today we are looking at the electrical and thermal properties.
As boron nitride crystals have a pronounced platelet shape, the way they are orientated in the sintered body plays a major role. This is also referred to as the anisotropy of hexagonal boron nitride, i.e. a direction-dependent property profile. Depending on the orientation, which can be perpendicular or parallel to the pressing direction of the sintered body, the respective parameter differs. This plays a decisive role in both the mechanical and thermal design of a component.
Thermal shock resistance: Many materials are sensitive to thermal shock. This means that sudden changes in temperature can cause the material to fail. The reasons for this sensitivity are internal stresses caused by an increase or decrease in temperature and also the comparatively high brittleness of ceramics in general. Cracks or spalling are therefore the result. In addition to the geometric boundary conditions, influencing factors include the coefficient of thermal expansion, the modulus of elasticity, the thermal conductivity and the strength. For the HeBoSint® SL-N 300, it has already been possible to prove, on the basis of exemplary tests and through use at the customer's premises, that it is resistant to thermal shock when heated rapidly (for example by sudden flooding with hot inert gas or contact with molten metal) and retains its mechanical properties at temperature differences of more than 1600 K.
Thermal conductivity: Thermal conductivity describes the heat flow through a material. The thermal conductivity of boron nitride is comparatively high for an electrical insulator. In the case of HeBoSint® SL-N 300, the thermal conductivity parallel to the pressing direction, i.e. through the platelet plane, is 27 W/mK and perpendicular to the pressing direction, in the platelet plane, is 45 W/mK, both measured at 20 °C
Coefficient of thermal expansion: This material-specific value describes how the spatial dimensions of a component change with the change in temperature. The HeBoSint® SL-N 300 achieves a maximum value of 4.6 [10-6/K] parallel to the pressing direction and a maximum value of 2.5 [10-6/K] perpendicular to the pressing direction, measured in the temperature range of 20°C-1500 °C
Maximum operating temperature: The maximum operating temperature depends on the process gas. HeBoSint® SL-N 300 is stable up to approx. 900°C in air or in an oxidising environment. At this temperature, the material begins to oxidise, which can initially be observed as an increase in weight. As the temperature rises, a decrease in weight can be seen, which is due to the successive decomposition. In an inert environment or under vacuum, HeBoSint® SL-N 300 is stable up to 1800°C.
Specific electrical resistance: This measured variable indicates how much voltage is required to allow current of a certain amperage to flow through a material. The great strength of boron nitride as an insulator is particularly evident in applications at higher temperatures. With many other ceramics, the specific electrical resistance collapses much faster at higher temperatures. In the case of our HeBoSint® SL-N 300, the value is >10-15 Ohm*cm.
Dielectric strength: Dielectric strength describes the resistance to high voltage. In other words, it is the field strength at which a sudden current flow occurs because strongly accelerated electrons ionise and "break through" the material. The HeBoSint® SL-N 300 also impresses in terms of dielectric strength. Parallel to the pressing direction, the value is approx. 100 kV/mm and perpendicular to the pressing direction approx. 80 kV/mm.
To summarise, we have now highlighted the mechanical, thermal and electrical properties and it can be seen how the individual properties influence and interact with each other. In the next article, we will look at chemical resistance and gas tightness. Once all the pieces of the puzzle have been put together, it will become clear what characterises this material as a whole.
If you would like to find out more or would like advice, our technical sales team will be happy to help. We will be happy to advise you and work with you to find out whether our HeBoSint® SL-N 300 could be the right solution for you.
Link to technical advice: https://www.henze-bnp.de/service/technischer-vertrieb.php
