Dissolution temperatures, diamond

A new, low-pressure, plasma-assisted proeess for synthesising diamonds has been found by Roy et al [83,84]. An intimate mixture of various forms of carbon with one of many metals (e.g., Au, Ag, Fe, Cu, Ni) is exposed to a microwave plasma derived from pure hydrogen at temperatures ranging from 600-1000 °C. Roy et al postulate a mechanism in which a solid solution of atomic hydrogen and the metal. Me, facilitates dissolution of carbon to form molten droplets of Me -Cj,-H. Diamonds nucleate at the surface of the droplets as the temperature is reduced. 

Germanium — (Ge, atomic number 32) is a lustrous, hard, silver-white metalloid (m.p. 938 °C), chemically similar to tin. Ge is a low-band-gap - semiconductor that, in its pure state, is crystalline (with the same crystal structure as diamond), brittle, and retains its luster in air at room temperature. Anodic dissolution of the material occurs at potentials more positive than ca. -0.2 V vs SCE. Peaks in the voltammograms of germanium in acidic electrolyte are ascribed to a back-and-forth change between hydrogenated and hydroxy-lated surfaces [i]. Studies are often conducted at p-doped and n-doped Ge electrodes [ii] or at Ge alloys (e.g., GeSe) where photoelectrochemical properties have been of considerable interest [iii]. 

The position marked B at 4 GPa and temperatures between 1775 and 2075 K, gives the condition under which well faceted diamonds of about 200 pm diameter have been crystallized using specially treated phenolic resins as the source of carbon and molten cobalt as solvent [30]. The notable point of this particular crystallization is that it occurred well into the graphite stable region of Fig. 7 and is thus an example of the metastable growth of diamond . An explanation for this may be found in consideration of the relative solubilities of phenolic resins and diamond in molten cobalt allowing dissolution of one metastable form and precipitation of another, namely diamond. This is an example where rules governing the transitions between metastable states, such as the Ostwald and Ostwald-Volmer rules, can be applied [31]. 

The essence of the purification method used in the production of diamond nanopowders is to dissolve impurities of metals and their compounds and oxidize nondiamond forms of carbon by chromic anhydride in the presence of sulfuric acid. The use of such a strong oxidant in the presence of a strong acid makes it possible to combine in one stage the purification of diamonds both from nondiamond carbon forms and from metal impurities. The suspension of the nanodiamond-containing mixture is filtered through a set of sieves to remove mechanical impurities, the metal part is removed by magnetic treatment, and the solid phase is concentrated by nutsch filters. Dissolution of impurities of metals and their compounds and oxidation of nondiamond forms of carbon is carried out in a reactor (further on, this operation is called oxidation ). One run of oxidation to purify 3.3-3.7-kg solid phase of the mixture requires 24-27-kg sulfuric acid and 6.9-7.5-kg chromic anhydride. In the oxidation, when a solution of chromic anhydride is added, the temperature in the reactor reaches 125-130°C the mixture is kept in the reactor with sulfuric acid and chromic anhydride for 3-4 h. Following the oxidation, the reaction mixture is washed with water to remove chromium and sulfuric-acid salts. The yield is a nanodiamond suspension that contains 2.3-2.5-kg solid phase. 

---