Thermal conductivity Diamond

Fig. li-.l-ti-B Diamond. Thermal conductivity vs. temperature for three type la diamonds [1.39]... 

In diamond, thermal conductivity occurs by aflow of phonons (see Ch. 11, Sec. 5.2). These phonons are scattered by imperfections such as isotopic imperfections and the scattering varies as the fourth power of the phonon frequency. Thus, the exclusion of C should result in a significant increase in thermal conductivity. This increase was confirmed experimentally as C diamond is reported to have a 50% higher thermal conductivity than natural diamond.i l i It is also reported to be harder than natural diamond by a few percent as determined by the relation between hardness and the elastic coefficients (see Ch. 11, Sec. 10.2). 

Thermal Conductivity. The value of 2000 W/(m-K) at room temperature for Type Ila natural stones is about five times that of Cu, and recent data on 99.9% isotopicaHy pure Type Ila synthesized crystals ate in the range of 3300—3500 W/(m-K) (35). This property combined with the high electrical resistance makes diamond an attractive material for heat sinks for electronic devices. 

Electronic. Diamonds have been used as thermistors and radiation detectors, but inhomogeneities within the crystals have seriously limited these appHcations where diamond is an active device. This situation is rapidly changing with the availabiHty of mote perfect stones of controUed chemistry from modem synthesis methods. The defect stmcture also affects thermal conductivity, but cost and size are more serious limitations on the use of diamond as a heat sink material for electronic devices. 

By beginning with methane, the diamonds formed have only in them. These tiny diamonds may then be used as the carbon source to form large (5 mm) single crystals by growth from molten catalyst metal in a temperature gradient. The resulting nearly pure crystals have outstanding thermal conductivities suitable for special appHcations as windows and heat sinks (24). 

The compact structure of diamond accounts for its outstanding properties. It is the hardest of all materials with the highest thermal conductivity. It is the most perfectly transparent material and has one of the highest electrical resistivities and, when suitably doped, is an outstanding semiconductor material. The properties of CVD and single-crystal diamonds are summarized in Table 7 2.[1][18]-[20]... 

Cubic boron nitride (c-BN) is a different material altogether from h-BN, with a structure similar to that of diamond, which is characterized by extremely high hardness (second to diamond) and high thermal conductivity.As such, it is a material of great interest and a potential competitor to diamond, particularly for cutting and grinding applications. Its characteristics and properties are shown in Table 10.3... 

The Keyes figure of merit, based on thermal conductivity as an additional factor, (see following section) predicts the suitability of a semiconductor for dense logic circuit applications. Again, diamond is far superior to other materials. 

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. 

Recently Butler et al. [4] reported the deposition of nanocrystalline diamond films with the conventional deposition conditions for micrometer-size polycrystalline diamond films. The substrate pretreatment by the deposition of a thin H-terminated a-C film, followed by the seeding of nanodiamond powder, increased the nucleation densities to more than 10 /cm on a Si substrate. The resultant films were grown to thicknesses ranging from 100 nm to 5 fim, and the thermal conductivity ranged from 2.5 to 12 W/cm K. 

Diamond. In this structure (see Chapter 7) all the atoms are equivalent each atom being surrounded by a perfect tetrahedron of four other carbons, forming with each one of them a localized two-electron bond. Diamond has a high density and refraction index and thermal conductivity and the highest melting point ( 4000°C) of any element. 

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. 

Diamond coatings as bulk diamonds may have important applications due to the unique properties of this substance (high hardness, low thermal expansion, high-thermal conductivity). 

A pure form of sp3 hybridized carbon is known as diamond and this may also be synthesized at the nanoscale via detonation processing. Depending on their sizes, these are classified as nanocrystalline diamond (10 nm 100 nm), ultrananocrystalline diamond (< 10 nm) and diamondoids (hydrogenated molecules, 1 nm-2 nm). Nanodiamond exhibits low electron mobility, high thermal conductivity and its transparency allows spectro-electrochemistry [20,21]. However, ultrananocrystalline diamond exhibits poor electron mobility, poor thermal conductivity and redox activity [21,22]. 

G. A. Slack. Thermal Conductivity of Prue + Impru e Silicon, Silicon Carbide + Diamond , Journal of Applied Physics 35 (1964), 3460-3466. 

---