Insulators, diamonds

The electrical properties of undoped diamond crystals make it suitable to be used as an insulator. It can support electric field strengths as high as 10 MV/cm before conduction and has a resistivity value as high as 10 Q. Its higher band-gap of 5.46 eV makes it difficult to promote an electron to the conduction band. This makes diamond a first-class insulator. Diamond insulators are being used in electronics as an intrinsic interface with doped diamond. 

FIGURE 5.2 Heat capacity of an atom (He), some molecules (Nj, COj, CH4), diamond insulator, and copper metal as a function of temperature. 

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. 

Electrical and Electronic. Diamond is an electrical insulator (-- lO H/cm) unless doped with boron when it becomes ap-ty e semiconductor with a resistivity in the range of 10 to 100 Q/cm. n-Ty e doping has often been claimed but is less certainly estabUshed. The dielectric constant of diamond is 5.58. 

The covalent compounds of graphite differ markedly from the crystal compounds. They are white or lightly colored electrical insulators, have Hi-defined formulas and occur in but one form, unlike the series typical of the crystal compounds. In the covalent compounds, the carbon network is deformed and the carbon atoms rearrange tetrahedraHy as in diamond. Often they are formed with explosive violence. 

C fi3 diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film. 

Hollomon s ethos, combined with his ferocious energy and determination, and his sustained determination to recruit only the best researchers to join his group, over the next 15 years led to a sequence of remarkable innovations related to materials, including man-made diamond, high-quality thermal insulation, a vacuum circuit-breaker, products based on etched particle tracks in irradiated solids, polycarbonate plastic and, particularly, the Lucalox alumina envelope for a metal-vapour lamp. (Of course many managers besides Hollomon were involved.) A brilliant, detailed account of these innovations and the arrangements that made them possible was later written by Guy Suits and his successor as director, Arthur Bueche (Suits and Bueche 1967). Some of these specific episodes will feature later in this book, but it helps to reinforce the points made here about Hollomon s coneeption of broad research on materials if I point out that the invention of translucent alumina tubes for lamps was... 

Materials in which there is a substantial difference in energy between occupied and vacant MOs are poor electron conductors. Diamond, where the gap between the filled valence band and the empty conduction band is 500 kj/mol, is an insulator. Silicon and germanium, where the gaps are 100 kj/mol and 60 kj/mol respectively, are semiconductors. 

In diamond, each carbon atom is sp3 hybridized and linked tetrahedrally to its four neighbors, with all electrons in C C cr-bonds (Fig. 14.30). Diamond is a rigid, transparent, electrically insulating solid. It is the hardest substance known and the best conductor ol heat, being about five times better than copper. These last two properties make it an ideal abrasive, because it can scratch all other substances, yet the heat generated by friction is quickly conducted away. 

This chapter is a review of the two major allotropes graphite and diamond, which are both produced extensively by CVD. The properties of these two materials can vary widely. For instance, diamond is by far the hardest-known material, while graphite can be one of the softest. Diamond is transparent to the visible spectrum, while graphite is opaque diamond is an electrical insulator, while graphite is a conductor. 

CVD is a maj or process in the production of thin films of all three categories of electronic materials semiconductors, conductors, and insulators. In this chapter, the role of CVD in the fabrication of semiconductors is reviewed. The CVD production of insulators, conductors, and diffusion barriers is reviewed in the following chapter. The major semiconductor materials in production or development are silicon, germanium, ni-V and II-VI compounds, silicon carbide, and diamond. 

CVD plays an increasingly important part in the design and processing of advanced electronic conductors and insulators as well as related structures, such as diffusion barriers and high thermal-conductivity substrates (heat-sinks). In these areas, materials such as titanium nitride, silicon nitride, silicon oxide, diamond, and aluminum nitride are of particular importance. These compounds are all produced by CVD. 1 1 PI... 

Heat dissipation can be effectively dealt with by using substrate materials such as aluminum nitride, beryllia and, more recently, diamond which combine electrical insulation with high thermal conductivity. The relevant properties of these three materials are shown in Table 14.1. 

Diamond is an electrical insulator with the highest thermal conductivity at room temperature of any material and compares favorably with beryllia and aluminum nitride. P3]-P5] jg undoubtedly the optimum heat-sink material and should allow clock speeds greater than 100 GHz compared to the current speed of less than 40 GHz. 

Metallic lead is dark in color and is an electrical conductor. Diamond, the most valuable form of carbon, is transparent and is an electrical insulator. These properties are very different yet both lead and carbon are in Group 14 of the periodic table and have the same valence configuration, s p Why, then, are diamonds transparent insulators, whereas lead is a dark-colored conductor ... 

Carbon in the form of diamond is an electrical insulator because of its huge band gap. hi fact, its band gap of 580 kJ/mol substantially exceeds the C—C bond energy of 345 kJ/mol. In other words, it requires more energy to promote an electron from band to band in diamond than to break a covalent bond. Lead, in contrast, is a metallic conductor because it has... 

This FLAPW-SIC scheme has been applied to the CP calculations of Cu, Si and diamond. The semiconductor Si and the insulator diamond have energy gaps and the most upper valence electrons are regarded as being a slightly bound state. The noble metal Cu has tightly bound d-electrons. 

The size of the bandgap can vary from a fraction of an eV (in the IR region of the spectrum) to ca. 4 eV or more (wide-bandgap semiconductors). The upper limit is somewhat arbitrary a substance commonly thought of as an insulator such as diamond has a large bandgap of 5.5 eV, but it can nevertheless be doped with elements such as B, N, or P to become an electrically-conducting semiconductor. 

Notes on some peculiar applications of diamond. Diamond has a very interesting and important range of material properties. It is the hardest and stiffest material known, it has a very high thermal conductivity and it is a very good electrical insulator. It is transparent to ultraviolet, visible and infrared light, and it is chemically inert to nearly all acids and bases. Large crystals may therefore find applications not only in jewellery tiny diamonds are used in saw blades, in drill bits, etc. Electronic properties and colour of diamond depend on the impurities and their distribution within the crystal. 

The next point to realize is that the best emitter is a metal. Many forms of carbon initially studied are semiconductors or even insulators, including nanodiamond [8-11] and diamond-like carbon (DLC) [12-13,4]. Combine this with local field enhancement means that there is never uniform emission from a flat carbon surface, it emits from local regions of field enhancement, such as grain boundaries [8-11] or conductive tracks burnt across the film in a forming process akin to electrical breakdown [13]. Any conductive track is near-metallic and is able to form an internal tip, which provides the field enhancement within the solid state [4]. Figure 13.2 shows the equipoten-tials around an internal tip due to grain boundaries or tracks inside a less conductive region. 

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