The nucleation and morphology of diamond crystals and films synthesized by the use of a combustion flame have been investigated. By operating an oxy-acetylene torch in a fuel-rich mode, diamond crystals and films have been deposited on mechanically abraded molybdenum, on in situ created molybdenum carbide, and on thin diamond-like carbon (DLC) layers synthesized on molybdenum. Scanning electron microscopy, Auger and Raman spectroscopy have been used to characterize the films and crystals. Diamond is found to be uniformly deposited in the region of the substrate that intersects the inner, acetylene-rich region of the flame. The nucleation density, the growth rate, and the morphology of the diamond crystals and films are found to be strongly influenced by the surface condition of the substrate. On mechanically abraded molybdenum, abraded with 600 mesh silicon carbide, and on molybdenum carbide, well-formed cubo-octahedrons of diamond, up to 45 μm in diameter, are formed for deposition times of 90 min. Film formation is seldom observed under these conditions. To enhance nucleation, thin layers of DLC were formed on molybdenum substrates by reducing the O2/C2H2 ratio in the gas mix to ∼ 0.75 for short periods of time under 30 s. This was followed by increasing the O2/C2H2 ratio to conditions that produce diamond (an O2/C2H2 ratio of ∼ 0.9). Under these conditions the nucleation density of diamond was increased by an order of magnitude and the growth rates by about 60%, as compared to diamond deposited on abraded molybdenum and molybdenum carbide. In addition, the morphology of the diamond crystals and films was substantially affected with indications of dendritic growth. The DLC layer is effective in promoting diamond nucleation due to the high surface defect density and the high hydrogen concentration of these films. The combination of surface defects, in the form of dangling bonds, and the evolution of hydrogen from the DLC layer during the diamond deposition process, which is characterized by higher temperatures, result in a high concentration of active surface sites for diamond nucleation. The nucleation density, the distribution on the substrate, and the morphology of diamond crystals and films are not driven by the transport of reactive specie in the flame to the substrate, but rather by nucleation processes, temperature distribution across the surface, and attendant surface phenomena.

The nucleation density and the morphology of diamond crystals and films, synthesized by the combustion flame technique, are shown to be strongly influenced by the nucleation processes at work. Nucleation of diamond on a mechanically abraded molybdenum surface results in well-formed cubo octahedrons with a relatively low nucleation density. Nucleation on an initially formed, diamond-like carbon layer markedly increases the nucleation density and alters the morphology of the diamond crystals and film. The enhancement of nucleation by diamond-like carbon layers is postulated to be a result of the high surface defect density and the high hydrogen concentration of these materials.

A novel approach for the rapid determination of changes in the impurity profile of oligonucleotide therapeutics has been developed. The straightforward data treatment and the speed and simplicity of the approach make the method easy to implement and use. Possible quality control applications include drug substance and drug product stability studies, and the assessment of batch-to-batch variability.

The electrical conductivity, from room temperature to 1000 °C, of combustion flame synthesized diamond films and free-standing diamond slabs are demonstrated to be up to two orders of magnitude lower than that of type IIa natural diamond crystals. The low conductivity, indicative of high purity, has been achieved at diamond growths rates of 5–10 μm/h, considerably higher than that achievable with other diamond synthesis techniques. These high growth rates have been achieved over areas of 5 cm×5 cm and both thin (10 μm) films on silicon substrates and thick (∼80 μm), free-standing diamond slabs exhibit similar electrical behavior. The high purity of this diamond is attributed to the presence of oxidizing species in the flame ambient which are more effective than hydrogen in removing any nondiamond forms of carbon and other impurities from the growing diamond film.

Among the various low pressure techniques being developed for the synthesis of diamond films and bulk diamond slabs the combustion flame synthesis process has some distinct advantages. In this approach the combustion reaction between acetylene and oxygen is utilized to generate the requisite energy to activate excess acetylene in the gas mix leading to the deposition of diamond films on a temperature controlled substrate brought into contact with the flame. Other diamond synthesis approaches, such as microwave enhanced and the filament assisted chemical vapor deposition processes, and the various arc jet techniques utilize mixtures of hydrogen and methane as the process gases. Oxygen and oxidizing specie ( such as OH radicals) in the flame ambient may be much more effective than atomic hydrogen in promoting the growth of diamond over the growth of graphite and other non- diamond forms of carbon. In addition this technique enables the growth of diamond at high rates and is relatively easily scaled for large area synthesis. In this paper a discussion of this technique is presented drawing upon recent research by the authors as well as published work to present a general discussion of the issues involved in the development of this technique of low pressure diamond synthesis.