Impurities diamond synthesis

The most frequently employed starting explosives in indnstrial-scale production of diamond by detonation synthesis are, as a rnle, trinitrotoluene (TNT) and hexogen (also called RDX = Research Department Explosive), with detonation performed in a water medium to reach a higher productivity. The most complicated stage in the industrial process is chemical isolation of NC diamond from the detonation carbon prodnced in an explosion, which is actually a mixture of micro- and nanoparticles of graphite, varions forms of sp -hybridized carbon and impurities originally contained in the explosive itself, and construction materials of the synthesis vessel. For details of the technology used in detonation diamond synthesis, the reader can be referred to Ref. 19 and Section 9.23.2 for the structure of the detonation diamond particles, their properties, and applications. 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

Depending on the method of their preparation, the individual nanodiamond particles do not exist as isolated crystallites, but they form tightly bound agglomerates. Apart from unordered sp - and sp -hybridized carbon, they may also include other impurities. The latter may originate either from synthesis or purification, for example, finely dispersed material from the reactor walls may contaminate the sample (Section 5.3). This is especially true for material produced by the detonation or shock wave method, whereas hydrogen-terminated diamond nanoparticles do not show this effect. 

Carbon in the structural form of diamond is the only element used industrially as a hard material. Each year about ten tons of natural diamond and about twenty tons of synthetic diamond (produced via high temperature high pressure synthesis) are marketed as hard materials. While pure diamond is transparent, a yellow tint results from the replacement of some carbon atoms by nitrogen, and a blue, yellow, or even green tint through substitution of carbon by boron atoms. Polycrystalline diamond with impurities, used as an abrasive, is often black. 

The possible success of a synthesis route using nongraphitic carbon would seem to depend upon the suppression of graphite nucleation and purity factors, since growth of high quality diamond requires low levels of impurities. This is more difficult to achieve in a carbon which has not been graphitized. 

There are several challenges associated with the synthesis of BDD suitable for electrochemistry. Since diamond is a semiconductor with exceptional properties, precise control of dopant impurities and extended defects is required to dope the diamond lattice with sufficient boron to make the material conduct. However, as the boron levels increase, it can be harder to maintain crystallinity and control the amount of nondiamond carbon (NDC) both within crystal defects and at grain boundaries. While NDC can increase material conductivity, it is be detrimental to a diamond electrochemist, as the widely recognized electrochemical properties of BDD (wide solvent window, low background currents, reduced susceptibility to electrode fouling, corrosion resistance) are impaired and the electrochemical response becomes more akin to glassy carbon. If the presence of NDC is unaccounted for, electrical resistivity measurements will mislead the user into believing that there is more boron than actually present in the matrix. 

Exceptional process control is required to produce large (>1 carat) sc HPHT diamond relatively free from impurities [21]. Small heavily boron-doped HPHT diamond crystals <10 mm have been fabricated using ultrahigh pressures, 8-20GPa, and temperatures of >2500K, and diamond superconductivity at 4K has been reported. However, such synthesis conditions require highly specialized HPHT capabilities and to date these materials remain a research curiosity [22]. 

Less glamorous than the synthesis of diamonds but far more important is the industrial manufacture of three impure graphite forms of carbon carbon black (soot), coke, and activated charcoal. 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

Besides the diamond phase, the condensed products of explosion recovered from the armored chamber after the explosion of a charge contain the nondiamond modifications of carbon and metal impurities. Depending on the method of synthesis, the diamond phase in the condensed carbon products of explosion is 30 to 75% of the weight of these products. Optimization of the detonation synthesis by the ratio of trinitrotoluene and hexogen in the mixture, by the ratio of the weight of exploded charge and the volume of the chamber and also the use of special coolants enables a stable 75% yield of the diamond phase in the condensed products of explosion. 

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