Diamond crystals

A thin layer of dark green beryl had been grown by a hydrothermal technique over the surface of a pale beryl to imitate emerald. It has been suggested that such stones should be called synthetic emerald-beryl doublets (16). The abiHty to grow thin, but not thick, single-crystal diamond on the surface of natural diamond (17) leads to the possibiHty of growing such a thin film colored blue with boron this has been done experimentally (18). 

Fig. 6. Synthesized high quaUty single-crystal diamonds.

Siace these masses of polycrystaUine diamond possess extensive diamond-to-diamond bonding, they have, ia contrast to siagle-crystal diamond, excellent crack resistance, siace any crack that begias ia oae crystal oa an easy cracking plane (parallel to an octahedral face) is halted by neighboring crystals that are unfavorably oriented for their propagation. 

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]... 

Heat sinks, in the form of thin slices prepared from single-crystal natural diamond, are already used commercially but are limited in size to approximately 3x3x1 mm. These single-crystal diamonds are gradually being replaced by CVD diamond, which is now available in shapes up to 15 cm in diameter. P6]-[28] gQg - gf cVD diamond may remain a... 

Infrared optics is a fast growing area in which CVD plays a maj or role, particularly in the manufacture of optical IR windows. 1 The earths atmosphere absorbs much of the infrared radiation but possesses three important bandpasses (wavelengths where the transmission is high) at 1-3 im, 3-5 im and 8-17 pm. As shown in Table 16.2, only three materials can transmit in all these three bandpasses single crystal diamond, germanium, and zinc selenide. 

Single-crystal diamond is the ideal material with remarkable optical properties, high heat resistance, extreme hardness, and excellent chemical resistance. But, because of its high cost and size limitation, it is only used in exceptional cases, such as the window... 

For example, in the lengthy Natural History [2], by the famous Roman natural historian Gains Plinius Secundus, there are descriptions of polyhedral forms of beryl, rock-crystal, diamond, etc. 

Figure 2.1. Various forms exhibited by crystals, (a) Polyhedral crystals (b) hopper crystal (c) dendritic crystal (snow crystal, photographed by the late T. Kobayashi) (d) step pattern observed on a hematite crystal (0001) face (e) internal texture of a single crystal (diamond-cut stone, X-ray topograph taken by T.Yasuda) (f) synthetic single crystal boule. Si grown by the Czochralski method (g) synthetic corundum grown by the Verneuil method.

Natural single-crystal diamond and carbonado can now be replaced in many industrial uses by sintered diamond tool blanks. Such tool blanks are available in disks and cores. The disks, or sectors of disks, consist of a thin (0.5—1.5 mm) layer of sintered diamond up to about 50 mm diameter on a cemented tungsten carbide-base block about 3—6 mm thick. Using diamond abrasive, such blanks can be formed into cutting tools of various shapes. Typical tool blanks are shown in Figure 9. The wire dies have diamond cores up to 10 mm in diameter and 10 mm in length, which are encased in a cemented tungsten carbide sleeve up to 25 mm in diameter. 

Tabor D., 1965, The influence of temperature on the mechanical properties of hard crystals, Diamond Research, Inf. Diam. Review. Supplement, 2-5. 

When a single crystal diamond (synthetic or natural) is the substrate, epitaxial growth occurs the growing diamond replicates the substrate crystal lattice and turns to single crystal film. The film thickness usually comes to a few microns however, films of 1 mm in thickness were reported. The diamond-coated area would achieve 10 cm in diameter by order for industrial applications, much larger areas (e.g. 40 by 60 cm) are covered. Samples destined for the electrochemical measurements used to have dimensions ca. 1 by 1 cm. 

Fig. 8. Cyclic voltammogram of background current on a single-crystal diamond film in 0.1 M H2SO4 [40],...

The above-described situation is but an exception rather than the rule. Generally, the diamond electrode capacitance is frequency-dependent. In Fig. 12 we show a typical complex-plane plot of impedance for a single-crystal diamond electrode [69], At lower frequencies, the plot turns curved (Fig. 12a), due to a finite faradaic resistance Rp in the electrode s equivalent circuit (Fig. 10). And at an anodic or cathodic polarization, where Rf falls down, the curvature is still enhanced. At higher frequencies (1 to 100 kHz), the plot is a non-vertical line not crossing the origin (Fig. 12b). Complex-plane plots of this shape were often obtained with diamond electrodes [70-73],... 

Fig. 12. (a) Complex-plane plot of impedance spectrum for a single-crystal diamond thin-film electrode, taken in 0.5 M H2SO4 at open-circuit potential (b) its high-frequency portion. Frequency/kHz shown on the figure [69]. 

Typical values of o and a, as well as the faradaic resistance Rf, series resistance Rs (and the film resistivity p calculated thereby, assuming that the electrolyte resistance can be neglected) are presented in Table 3 for polycrystalline and single-crystal diamond electrodes and a DLC electrode [69-77]. By comparing the impedance... 

As shown in Section 3.2, polycrystalline diamond film is a heterogeneous system comprising diamond crystallites and intercrystallite boundaries, presumably consisting of amorphous carbon. This brings up the question To what extent do intercrystallite boundaries affect the electrochemical behavior of polycrystalline diamond electrodes To answer this question, the electrochemical properties of polycrystalline and single crystal diamond and amorphous carbon should be compared. In such a comparison, a model material of the intercrystallite boundaries should be chosen. 

Kondo, T., Honda, K., Tryk, D.A. and Fujishima, A. (2005) Covalent modification of single-crystal diamond electrode surfaces. J. Electrochem. Soc. 152, E18-E23. 

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