Electrical Insulation combined
with high Thermal Conductivity
Hexagonal Boron Nitride and Graphite have a similar crystalline structure. However, Boron Nitride is an excellent electrical insulator, because its outer electrons are firmly bound by the nitrogen atoms.
Boron Nitride, in addition, is a good heat conductor. However there are some differences in thermal conductivity between Boron Nitride sintered bodies and Boron Nitride powder. These differences are due to the different production processes.1.Boron Nitride Sintered Bodies (HeBoSint®)
2.Boron Nitride Powder (HeBoFill®)
When Boron Nitride is sintered with the aid of a – transient – liquid phase, this phase will constrain the heat transfer within the crystalline structure. Nevertheless, thermal conductivity of sintered bodies made of Boron Nitride is one of the highest of all ceramic materials.
The thermal conductivity of pure Boron Nitride differs from that of mixed ceramics. BN-ZrO2 (HeBoSint®) has a lower, BN-AlN (HeBoSint® A) a higher thermal conductivity than pure Boron Nitride. Moreover, thermal conductivity of Boron Nitride also depends on temperature. At extremely low temperature it is equivalent to that of high grade steel, at high temperatures it matches that of Beryllium Oxide (BeO), and above 700°C it even exceeds the latter. The electrical conductivity of Boron Nitride also depends on temperature (this is illustrated below for selected HeBoSint® types).
Typical physical values of sintered bodies made of boron nitride
| Type: | Hot pressed Boron Nitride | Isostatically hot pressed Boron Nitride | Hot pressed BN/ZrO2- ceramic composite |
| Thermal Conductivity (Wm –¹K–¹)a) |
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| 20°C | 65/45 | 50 | 35 – 25/20 – 18 b) |
| 400°C | 50/30 | 40 | 31 – 21/17 – 15 |
| 700°C | 30/20 | 30 | 28 – 18/15 – 13 |
| 1000°C | 15/10 | 20 | 25 – 15/13 – 18 |
| Specific resistance, W cm (20 °C) |
> 1012 | > 1012 | > 1012 |
| Dielectric strength, kV/mm kV/mm (20°C) | > 6 | > 6 | > 6 |
| a) Direction of measurement vertical/parallel to direction of pressing. b) Depends on BN/ZrO2-ratio Values applicable to HeBoSint® products are stated in the appropriate data sheets |
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(extracted from Ullmann’s Encyclopedia of Industrial Chemistry, Sixth Edition, Electronic Release 1999)
A comparison of properties of HeBoSint® O820 with those of glass-bonded mica can be found under Download: Technical Information Boron Nitride vs. Glass Bonded Mica.
2. Boron Nitride Powder (HeBoFill®)
1. Boron Nitride Sintered Bodies (HeBoSint®)
The thermal conductivity of Boron Nitride powder is significantly higher than that of other ceramic fillers such as fused Silica, Aluminum Oxide or Aluminum Nitride. This is essentially due to the inherent anisotropy of the hexagonal microstructure of Boron Nitride.
Thermal conductivity measured in the z-plane is a mere 2 Wm–¹K–¹. However in the basal plane (x-y) a value exceeding 400 Wm–¹K–¹ applies! Boron Nitride materials optimised for thermal conductivity will show an overall value of 300 Wm–¹K–¹. Thus the thermal conductivity of Boron Nitride is only slightly higher than that of Aluminum Nitride. However, in polymers containing these materials this small difference usually is strongly enhanced. Consequently, the performance of compounds incorporating Boron Nitride often exceeds expectations.
This is because Boron Nitride is a solid lubricant that improves the rheological properties of the polymer system. This increases the mobility of the Boron Nitride platelets which, in turn, promotes their natural tendency to attach themselves to each other by their basal planes. Consequently thermal bridges are formed across these planes throughout the system.
Typical Physical Values of Ceramic Materials as found in Technical Literature
| Property | Hexagonal Boron Nitride | AlN | Al2O3 | fused SiO2 |
| Thermal Conductivity Wm –1 K –1 at 25°C |
>300 | 260 | 30 | 1.4 |
| Dielectric Constant | 3.9 | 8.8 | 9.7 | 3.8 |
| Volume Resistivity ohm-cm | 1015 | 1014 | 1014 | 1014 |
| Loss Tangent at 1 Mhz |
< 0.0002 | 0.0004 | 0.0001 | 0.0002 |
| Knoop Hardness kg/mm² | 11 | 1200 | 1500 | 500 |
| Coefficient
of Expansion, Linear x 10-4/mm³ |
< 1 | 4.4 | 6.7 | 0.5 |
| Specific
Heat J/kgK at 25°C |
794 | 734 | 798 | 689 |
| Theoretical Density g/cm³ | 2.25 | 3.26 | 3.98 | 2.20 |
| Values applicable to HeBoFill® products are stated in the appropriate data sheets | ||||