Boron nitrides, applications electronic

Non-oxide ceramics such as silicon carbide (SiC), silicon nitride (SijN ), and boron nitride (BN) offer a wide variety of unique physical properties such as high hardness and high structural stability under environmental extremes, as well as varied electronic and optical properties. These advantageous properties provide the driving force for intense research efforts directed toward developing new practical applications for these materials. These efforts occur despite the considerable expense often associated with their initial preparation and subsequent transformation into finished products. 

Uses. In spite of unique properties, there are few commercial applications for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses in the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particulady useful as cathodes in electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

The synthesis of diamond and cubic boron nitride has strongly motivated improvements in the development of high-pressure equipment and increased the interest in these materials, which have exceptional properties. Single crystals are required for optical and electronic applications. Consequently, specific crystal-growth processes have been set up under very high-pressure conditions. The principle is similar to that described, at lower pressures, for the preparation of single crystals of a-Si02. 

Boron nitride, when crystallized in its cubic form, has interesting electronic applications [53]. In addition to its chemical inertness, it has high thermal conductivity. This is important in microelectronic applications where the dissipation of heat is one of the main problems. An increase in temperature can induce the diffusion and reaction between the different layers of the electronic device and reduce its performance. The boron nitride coating in this case is protective in at least three ways mechanical protection due to its hardness, chemical protection, and thermal protection. 

Fiber reinforced ceramic matrix composites (CMCs) are under active consideration for large, complex high temperature structural components in aerospace and automotive applications. The Blackglas resin system (a low cost polymer-derived ceramic [PDC] technology) was combined with the Nextel 312 ceramic fiber (with a boron nitride interface layer) to produce a sihcon oxycarbide CMC system that was extensively characterized for mechanical, thermal, and electronic properties and oxidation, creep mpture, and fatigue. A gas turbine tailcone was fabricated and showed excellent performance in a 1500-hour engine test. 

The future applications of 2D nanocrystals will be many and varied. It is possible to speculate over potential uses such as in electronic components and high performance nanocomposites. It is not, of course, necessary to use different individual types of 2D nanocrystals and hybrid systems have already been investigated. It is possible to create multilayer heterostructures and devices with designed electronic properties by stacking various 2D atomic nanocrystals crystals, such as graphene and boron nitride, on top of each other. These are essentially new forms of matter and the scope for developing hybrid systems is clearly vast and a whole host of unexpected applications of these novel materials will no doubt be forthcoming. 

CVD of boron nitride films on silicon or germanium or on printed circuit boards is now a common practice in the electronic industry [154 to 162]. The high thermal conductivity combined with the excellent electrical insulation properties are most valuable for these applications [163] see additional references in Section 4.1.1.10.8, p. 129. The use of a-BN layers is of particular importance in the manufacture of electrophotographic photoreceptors (such as solar cells) and of X-ray lithographic masks (see Section 4.1.1.10.8, p. 129). In the last mentioned application, structural aspects of the deposited films are of importance. In films still containing hydrogen, (N)H moieties are depleted by annealing at about 600°C, while (B)H moieties are depleted above 1000°C [164]. Also, elastic stiffness and thermal expansion of boron nitride films have to be viewed in connection with the temperature-dependent stress of CVD-deposited boron nitride films [165]. Reviews of properties and electronic applications of boron nitride layers have appeared in Polish [166] and Japanese [167]. 

As stated in Ch. 12, boron nitride exists in two crystalline forms hexagonal and cubic with much different properties. Nis] ]( was first synthesized as a powder in 1842 but for many years remained a laboratory curiosity since the powder was thought too difiGcult to mold into useful shapes. In the 1950 s, the Carborundum Co. found a way to hot-press the material and the Raytheon Co. developed a chemical vapor dqiosition process.1 1 Boron nitride is now used extensively as a solid lubricant, as a chemically resistant container, and as a dielectric in electronic applications. It should be stressed that the reported property values often vary considerably and the values given here are a general average. 

Carbon and boron nitride (BN) nanotubes are two typical classes of one-dimensional tubular nanostructures, which are of great importance in the fundamental research and in developing nanoscale mechanical and electronic applications (lijima, 1991 Chopra et ah, 1995). They are composed of concentric and seamless cylinders formed by rolling up the... 

---