Stabilization of diamond

The electrochemical and chemical stability of diamond makes it an ideal electrode material for electrochemical fluorination reactions. The installation of fluorine a to heteroatom-substituted positions can be anodically performed by hydrogen fluoride/triethylamine mixtures. The Fuchigami group studied several electrode materials for the fluorination of oxindole 20. In this transformation to 21 only a... 

In addition to thin-film electrodes, compact diamond single crystals grown at high temperature and high pressure have become the object of electrochemical study in recent years. These so-called HTHP crystals can be obtained by crystallization from a carbon solution in a metal melt (e.g., based on the Ni-Fe-Mn system) at /arranges that correspond to the conditions of thermodynamic stability of diamond. These crystals can be also doped with boron in the course of growth. 

The major advantage of an arylation or alkylation lies in the very strong covalent bonding of the functional group to the diamond surface by way of a C-C-bond, which also reflects in the high stability of diamond films functionalized in this manner. 

Pseudomorphic stabilization of diamond is possible on tiie following substrates ... 

Fig. 7. For carbon the BC8 phase is predicted to be more stable than all previously considered simple metallic phases (Ref. 127). Here we show that calculated enthalpy H = E + PV vs. P gives a transition from diamond to BC8 at 12 Mbar. This may set the limit of stability of diamond, (from Ref. 30). 

Chemical Stability of Diamond Tools. Diamond reacts with carbideforming metals such as iron. In contact with these, diamond dissolves rapidly and is considered unsuitable to machine steel and cast iron. Likewise, it is not recommended for superalloys. 

The idea underlying the dynamic synthesis of nanometer-sized diamonds essentially consists of providing the pressure and tanperature needed to drive the graphite-diamond-phase transition by means of a shock (explosive) wave. Since both the pressure and temperature in a shock wave are characteristic of the region of thermodynamic stability of diamond, the time of synthesis is necessarily very short (as a rule, a few fractions of a microsecond), and, hence, the diamond crystallites produced usually remain nanometer sized. 

An analysis of the above pnbUcations indicates a wide range of experimental conditions (laser parameters, target, and type of Uqnid) tested to determine the combination favorable for formation of diamond structures by PLA. It is believed that this process evolves formation of the so-called laser plume or a cloud of reaction products consisting of the evaporated substrate material and, partially, the surrounding liquid. These evaporated substances form bubbles inside the liquid. As the amount of the evaporated material increases, the bubbles expand and, as the pressure and temperature reach a certain critical combination, they collapse. At the collapse of the bubbles, the temperature and pressure may reach in the range of thermodynamical stability of diamond. 

Like all dynamic methods, the detonation-based synthesis of ND is basically a version of classical synthesis adapted to the high-pressure and temperature conditions providing the required thermodynamic stability of diamond. It is the short time during which these conditions have to be maintained in the shock wave in dynamic synthesis (not over a few microseconds) that is the main factor determining the size of the diamond nanocrystals fabricated by such methods. 

The shell is formed in the process of reverse diamond-to-graphite transition in the concluding stages of detonation synthesis, after the shock wave has passed and the pressure has dropped below the limit of thermodynamic stability of diamond. The thickness of the shell is largely determined by the conditions of the DND synthesis and in the course of DND isolation from detonation carbon, the thickness of the shell decreases. In the strongest regimes of i p -oxidation, the shell can be ranoved completely, except for separate single-layer 5p2 caj-, Q j islands, which result, as shown by calculations, from natural reconstruction of the free surface of diamond nanoparticles. 

Bar-Yam, Y, Monstakas, T.D., 1989, Defect-induced stabilization of diamond films. Nature 342, 786-787. 

Fabrication of abrasive tools containing mesh diamonds, particularly with metal bonds, involves processing at temperatures of up to 900°C or even higher. The thermal stability of the crystals is therefore very important for such applications. The most common way to characterize thermal stabUity is to perform a toughness measurement after a high-temperature exposure— for example, 1100°C for 10 minutes in an inert atmosphere—yielding a thermal toughness index (TTl). The thermal stability of diamonds is mainly determined by the... 

Diamond, due to its rigid crystalline network, is expected to be more stable than existing dimensionally stable anodes (DSAs). There are several reports on the stability and electrochemical treatment applications of diamond electrodes. Swain and coworkers have studied the morphological structural stability of diamond electrodes in both acidic and basic media [25,26]. The diamond films in their study were found to be dimensionally stable in both media, even at a current density of 0.5 A cm 2 for 20 h. However, when low quality diamond films, with significant sp impurities, were used, this harsh treatment resulted in the formation of pits at the grain boundaries. Ramesham and Rose [27] demonstrated the high corrosion resistance of diamond films in comparison to molybdenum, as well as noble metals such as Pt and Au. 

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