Anita M. Weiner

Measurement of stable species present during filament‐assisted diamond growth

We have measured mole fractions of two of the major stable species at the surface of a silicon substrate during filament‐assisted diamond growth as a function of the filament‐to‐substrate distance. Input gases were methane and hydrogen. A quartz probe withdrew gases at the growing surface, and the gases were sampled with an on‐line mass spectrometer. Close to the filament the methane is largely consumed, with most of the remaining gas phase carbon in the form of acetylene. Mass spectral results are compared to compositions calculated with a detailed chemical kinetics model. Our initial analysis suggests that diamond growth comes mainly from reaction of acetylene, ethylene, methane, or methyl radical.

Surface Growth of Soot Particles in Premixed Ethylene/Air Flames

Abstract The surface growth rate of soot increases steeply with increasing equivalence ratio. We have found that the increased growth rate is accounted for primarily by the increased surface area available for growth rather than by increased concentrations of surface growth species. We have shown that the surface growth species are not depleted by the process of surface growth. Our data suggest that the surface growth seen in our flames can be accounted for primarily by reaction of acetylene with the soot particles.

In situ characterization of diamond nucleation and growth

Filament‐assisted chemical vapor deposition (CVD) diamond film growth on Si(100) was studied using x‐ray photoelectron spectroscopy (XPS) to examine the sample at selected intervals during the nucleation and growth processes. The sample was transferred under vacuum from the growth chamber to the attached XPS analysis chamber without exposure to air. Before growth XPS showed that the Si sample is covered by a layer of SiO2 and carbonaceous residue; however, after 15 min of growth both of these substances are removed and replaced by a distinct SiC layer [Si(2p)=100.3 eV and C(1s)=282.7 eV].

Methyl radical and H‐atom concentrations during diamond growth

The gas‐phase composition at the surface of a growing diamond film was measured as a function of the initial methane (CH4) fraction and, for a 2% methane fraction, as a function of added oxygen (O2). The results were modeled with a one‐dimensional reactor flow code that includes diffusion and detailed chemical kinetics. We found that most of the ethylene (C2H4) and ethane (C2H6) that was detected was actually not present in the growth chamber but was instead formed in the probe by recombination of methyl radicals (CH3) that were present in the gas phase. Thus, C2H4 and C2H6 acted as surrogates for CH3 in our system, and measurement of those two stable species allowed us to estimate the mole fraction of the CH3 radical. We then took advantage of the fact that CH3, CH4, H2, and H were in partial equilibrium in the diamond growth chamber in order to estimate the concentration of H. A comparison between the mole fractions of CH3 and H, as determined from our experiments, and the mole fractions calculated from...

Formation of small aromatic molecules in a sooting ethylene flame

Abstract The chemical route to the formation of soot and PAH is poorly understood, in part because rate constants for reactions of aromatic species at flame temperatures are largely unknown. In this work we used a quartz sampling probe to measure the concentration profiles of the single-ring aromatics benzene, phenylacetylene, and styrene in heavily sooting premixed ethylene flames. A detailed chemical kinetics model was then constructed for the purpose of explaining the flame chemistry. The model, which uses estimated rate constants for many of the reactions involving aromatic species, gives good predictions for benzene and better than order-of-magnitude predictions for styrene and phenylacetylene. A sensitivity analysis has isolated a particular chemical reaction which controls their rate of formation, and it shows that even large errors in the other aromatic rate constants have relatively little effect on the predictions. The value for this rate constant is consistent with measurements made in a low pressure flame and in shock tubes.

Determination of the Rate Constant for Soot Surface Growth

Abstract The total mass of soot in a premixed flat flame increases steeply with time through a process known as surface growth. In this paper we show that the surface growth rate is first order in the acetylene concentration, and we measure the apparent first order rate constant. We also show that the surface growth rate is not strongly temperature dependent, It is suggested that the C/H ratio of the soot may be important in determining its reactivity.

Effects of oxygen on diamond growth

In situ mass spectral measurements of gas composition at the substrate surface were made during filament‐assisted diamond growth. The input gases were various mixtures of CH4, O2, and H2 chosen in order to discern the effects of oxygen addition on diamond formation and growth. The gas phase chemistry was modeled as a one‐dimensional flow reactor, and the measured and calculated species mole fractions were in good agreement. The model was then used to estimate mole fractions of several atomic and radical species which could not be measured. We find that addition of O2 has only a small effect on the radical mole fractions. However, O2 can reduce the effective initial hydrocarbon mole fraction, which is important because higher quality diamond is grown at a lower initial hydrocarbon mole fraction. Most important, perhaps is that O2 addition leads to the formation of sufficient gas phase OH to remove nondiamond (pyrolytic) carbon from the film. Thus, O2 addition allows diamond films to be grown under composit...

Effects of the ratio of hardness to Young's modulus on the friction and wear behavior of bilayer coatings

We present a study of the effects of the ratio of hardness to Young’s modulus on the friction and wear behavior of layered composite coatings. Layered coating structures with the same surface coating but different interlayers were prepared by physical vapor deposition. We found that the ratio of hardness to Young’s modulus plays an important role in determining the friction coefficient and wear resistance of layered composite coatings. A low friction coefficient and high wear resistance can be achieved in structures with high ratio of hardness to Young’s modulus and moderately high hardness.

Tribology of metal-containing diamond-like carbon coatings

Abstract Ball-on-disk tests were run for several commercial and in-house metal-containing diamond-like carbon (Me:DLC) coatings on steel coupons. The balls, which were made from either WCCo or 52100 steel, developed normal stresses between roughly 100 MPa and 1 GPa during the course of the experiments. The wear resistance of the Me:DLC coatings from the various sources (running against WC balls) differed by up to nearly an order of magnitude, but each case the wear coefficient was constant until the coating wore away. Abrasiveness of the Me:DLC coatings (running against steel balls) differed by up to almost three orders of magnitude. The rates at which the steel wore away fell drastically for some Me:DLC coatings during the course of the tests. Some possible explanations for this fall are considered. Coating morphology did not correlate with any other measured properties.

Soot Particle Growth in Premixed Toluene/Ethylene Flames

Abstract Surface growth of soot was examined in several premixed flat flames. The fuel was a mixture of toluene and ethylene, with more than half of the carbon supplied by the toluene. We found: (I) Acetylene in the burned gases supplied most of the mass for surface growth, and the growth mechanism in toluene/ethylene flames was the same as in the ethylene flames that we examined previously; i.e., the rate constants that we found in the earlier work predicted the surface growth rates in these flames. (2) In comparing ethylene and toluene/ethylene flames, the fluorescence intensity from polycyclic aromatic hydrocarbons (PAH) was very different for flames with the same C/O ratios but similar for flames with the same soot volume fraction. (3) For a given C/O ratio the amount of soot produced in the particle inception stage in the toluene/ethylene flames was much higher than in the ethylene flames. However, because the acetylene concentrations were higher in the ethylene flames, the rate of surface growth per...