Boron Nitride
Classification
Boron Nitride is a ceramic material that is made of Boron and nitrogen. It can be produced in many forms (liquid, solid or powder) and the structure is changed based on the application.
History
Boron Nitride was first made possible by W.H. Balmain in 1842, where he synthesized it using boric acid and calcium cyanide. The material had little to almost no significance up until the 1940s where good quality of it became possible to produce using better processing techniques. Now a days Boron Nitride is being developed by ESK Ceramics GmbH & Co. KG.
Structure
Hexagonal Boron Nitride (h-BN) is one of Boron nitride (BN) structures, it has a very similar structure to graphite. It has a very strong covalent bonds, and since it is layered as seen in figure 1, the layers are kept together by weak Van Der Waals bonds these bonds makes the layers move easily which gives this structure properties that can be used for lubrication.Cubic Boron Nitride (C-BN) has a diamond cubic structure which gives it a compressive strength and hardness that is similar to diamond

Properties
Boron nitride in its different structures gives different properties. Hexagonal boron nitride is the most widely used in the world, it is not easily wetted by glasses or metals which makes ideal for the use in metal casting due to it high melting point and lubrication as seen in table 1 which gives a maximum and minimum range of values based on each of BN structures in which each structure gives different property value. Cubic boron nitride with its diamond shape has mechanical properties that classify it as a strong and tough material. BN is considered as an effective electrical insulator, due to its high thermal conductivity and its ability to resist thermal shock.

Table 1. Properties of Boron Nitride

Property Minimum value Maximum value Units
Density 1.9 3.1 Mg/m3
Tensile Strength 27 83.3 Mpa
Young’s Modulus 19.5 100 Gpa
Melting Point 3150 3400 K
Thermal Conductivity 19 52 W/m.K
Resistivity 1e+018 1e+021 10-8ohm.m

Processing
Hexagonal Boron Nitride is synthesized using other materials such as boric oxide, boric acid and boron trichloride for boron, and an ammonia gas or any other organic compound that carries nitrogen such as urea or biuret for nitrogen. The two substances are then mixed together in specific ratios and then heated up to about 500 OC to create a precursor. The resulted precursor is an oligomer with high concentrations of monovalent groups that has very similar ratios between the ingredients used. For example, if a precursor was formed from boric acid and urea, a very similar ratio between B-N and B-O bonds is present. Ignoring the high concentration of other elements in the precursor, the precursor still contains the layered character of boron nitride. Next, the microcrystalline oxide-based boron nitride is further nitrided in ammonia which will result in a material that withstands a temperature range from 900 oC to 1100 oC, to get rid of all other elements present in the resulting compound and further increase the concentration of boron nitride, the compound is exposed to high temperature evaporation with temperatures above 1500 oC which will give a high thermal stability.
Performance and applications
Boron nitride applications and performance differs depending on the structure, some applications requires a structures the can yield a required performance. One structure is capable of resisting temperatures over 2000 oC, which makes it ideal as a lubricant for metals. BN can be transformed from its powder form into a solid by hot pressing it and making it into rods, bars and plates as seen in figure 2 that can be useful since BN has a high young modulus and tensile strength. Due to its high temperature resistance it can be also utilized in sealing and brazing of furnaces, as a heat sink or a general high temperature insulator. Also, Boron nitride nanotubes (BNNT) are one of major areas of research which gave great rise to the importace of Boron nitride. Some of the research done on the BNNT shows that it has a stable wide band gap, high mechanical strength, high thermal conductivity, and an utra-violet emmition. BNTT can be thought of as rolled hollow cylinder as in carbon nanotubes where Boron and Nitrogen atoms replaces Carbon atoms as shown in figure 3.

Related pages
Titanium Nitride is a ceramic that is related to Boron Nitride since both are ceramics and made of one part Nitrogen. An application that both materials have is being used as a coating. A common property is that both ceramics have a high and similar melting points.

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