Shock-induced carbonization of phenanthrene at pressures of 7.9-32 GPa

The chemical behavior of phenanthrene during a reaction triggered by shock waves, and its influence on the physical property of phenanthrene were studied over the pressure range of 7.9-32.0 GPa. Chemical analyses showed that shocked phenanthrene included insoluble carbonaceous material containing amorphous carbon, polycyclic aromatic hydrocarbons (PAHs) with molecular weights ranging from 128 to 356, and unreacted phenanthrene. No diamond and fullerenes were detected in the shocked phenanthrene. The results indicate that reactions triggered by shock wave are dehydrogenation, which causes carbonization and radical addition reactions, and ring cross-linking. Carbonization is the most dominant and rapidly proceeded above 20.0 GPa. Thus, an abrupt increase of compressibility of phenanthrene above 20.1 GPa previously reported is caused by the drastic carbonization.     

Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

The polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high crystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to ∼1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5-40 μm single crystals. The p,T values of diamond formation here obtained are significantly lower than those previously known for direct graphite-diamond transformation.     

Shock-induced phase transitions of C70 fullerite

Shock-induced phase transitions of C₇₀ fullerite were studied at the pressure range up to 52 GPa with use of recovery assemblies of planar geometry. The starting material consists of two crystalline phases: phase with hexagonal close-packed (HCP) and phase with rhombohedral structure. We have found that C₇₀ fullerite undergoes a series of polymorphic phase transitions in conditions of step-like shock-wave compression. In the specimens, recovered from 9, 14 and 19 GPa, a dominant phase was fullerite C₇₀ with cubic structure. Also, some amount of C₇₀ with HCP structure was observed. The quantity of HCP phase was decreasing with increasing of intensity of shock loading. With further growth of shock pressure, destruction of C₇₀ molecules occurs. In the samples, recovered after shock loading, the main phase was graphite with a low degree of three-dimensional regularity.     

Conversion of carbon nanotubes to diamond by spark plasma sintering

The diamond phase has been converted directly from carbon nanotubes by spark plasma sintering (SPS), at 1500 °C under 80 MPa pressure, without any catalyst being involved. Well-crystallized diamond crystals, with particle sizes ranging from 300 nm to 10 μm were obtained. After sintering at 1200 °C, the tips of the carbon nanotubes were found to be open and the conversion from carbon nanotubes to diamond started. The mechanism for carbon nanotube to diamond conversion in SPS may be described as that from carbon nanotubes to an intermediate phase of carbon nano-onion, and then to diamond. It is believed that the plasmas generated by the low-voltage, vacuum spark, via a pulsed DC in the SPS process, played a critical role in the low pressure diamond formation. This SPS process provides an alternative approach to diamond synthesis.     

Highly oriented growth of n-type ZnO films on p-type single crystalline diamond films and fabrication of high-quality transparent ZnO/diamond heterojunction

We present the results on the fabrication and characterization of high-quality transparent heterojunction between n-type ZnO film and p-type diamond single crystalline film on the substrate of diamond bulk single crystal. The results indicated that the current density of the fabricated p-n junction reaches 110 A/m² when the forward bias voltage is 2.5 V, and the turn-on voltage value is about 0.75 V and agreement with the expected value. Moreover, a good rectification characteristic and transparent in the visible light range was obtained in the device.     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

Erosion resistance of polycrystalline diamond films to atomic oxygen

Polycrystalline diamond films deposited using hot filament chemical vapor deposition (CVD) technique have been investigated in atomic oxygen simulated as low earth orbit environment to examine their erosion resistance properties. After exposure to the atomic oxygen beam with a flux of 2.6×10¹⁶ atoms/cm² s, the diamond films only show a small mass loss. The reaction efficiency is estimated to be between 6.35×10⁻²⁶ and 8.28×10⁻²⁶ cm³/atom. Oxidation mechanism is investigated through the reaction temperature influence on the reaction rate. We suggest that atomic oxygen reacts with diamond surface and forms ether (C-O-C) and carbonyl (>CO) configurations besides eroding the surface.     

Shock compression of an emulsion matrix at pressures up to 37 GPa

This paper presents an experimental study of the shock compression of an emulsion matrix based on an aqueous solution of ammonium and sodium nitrates at pressures up to 37 GPa, which is significantly higher than the calculated detonation pressure. The data obtained were used to determine the parameters of the Hayes equation of state and calculate the shock heating temperature of the matrix. At a pressure of more than 17 GPa, the input pressure profiles shows a rise associated with the chemical transformation of the emulsion.     

Diffusion of cobalt in diamond films synthesized by combustion flame method

It is well known that Cobalt (Co) plays an important role during diamond deposition on cemented carbide substrates (WC). The presence of cobalt on the substrate lead to decrease adhesion and increase the formation of non-diamond compounds phases. However, the diffusion phenomenon of cobalt in diamond coatings is not well understood.We have carried out a detailed study to investigate the diffusion of cobalt during the nucleation and growth of diamond on WC-Co substrate by combustion-flame method, and the influence of it on the structure and quality of the diamond coatings. At high substrate temperature T_s>800 °C, ball-sharp of cobalt with a ball size about 0.7 μm was observed on the top surface of diamond coatings (thickness >50 μm). The constraints in the coating are very high, the Raman peak appearance at 1341.1 cm⁻¹. At relatively low substrate temperature, T_s is about 550 °C, ball sharp of cobalt was not observed by MEB but a lot of cobalt particles dissolution carbon films were detected by EDX.Based on the above results, the influence of cobalt on the structure, the quality and the constraints in the diamond films are discussed, a model suggesting the nucleation and growth mechanisms of diamond, to explain the cobalt diffusion in diamond films, is presented.     

Thermionic emission of amorphous diamond and field emission of carbon nanotubes

Thermal-field emission characteristics from nano-tips of amorphous diamond and carbon nanotubes at various temperatures are reported in this study. Amorphous diamond emitted more than 13 times more electrons at a temperature of 300 °C than at room temperature. In contrast, CNTs exhibited no increase of emitted current upon heating to 300 °C. The thermally agitated emission of amorphous diamond is attributed to the presence of defect bands. The formation of these defect bands raises the Fermi level into the upper part of the band gap, and thus reduces the energy barrier that the electrons must tunnel through. From defect bands within the band gap, the conduction band electrons were significantly increased due to electron tunnels from defect bands. The enhanced thermal-field emission originating from defect bands was observed in this study. This thermally agitated behavior of field emission for amorphous diamond was highly reproducible as observed in this research.     

Preparing SiC/diamond coatings via chemical vapor deposition of SiC on diamond-coated graphite and their frictional properties

SiC/diamond coatings with excellent frictional properties were successfully prepared using graphite as substrate. Diamond particles with size of 25–38 μm were firstly bonded on graphite substrate through PVA glue, followed by chemical vapor deposition (CVD) of SiC with varied MTS flow on the diamond-coated graphite substrate to enhance the adhesion of diamond particles. The influence of the MTS flow on the SiC coatings was investigated. The results showed that polycrystalline SiC coating with good crystallinity has been obtained. With MTS flow increasing, the SiC grains feature increased surface roughness and greater sizes of the SiC crystallite resulting from the co-deposition of SiC and carbon with increased carbon containing species. Reciprocating sliding wear tests were conducted to investigate the coefficient of friction. With increasing applied load, while the low-flow specimens showed a remarkable increase in the friction coefficient resulting from degradation of the SiC coatings, the high-flow specimens maintained a relatively low friction coefficient during wear tests indicating strong holding force to diamond particles of the SiC coatings. The reason for low friction coefficient of the high-flow specimens was that GCr15 steel ball was wearing by the SiC/diamond coatings with good affinity to the substrate resulting in a flat–flat contact on the contact area.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

Volume phase transition in poly(N‐isopropylacrylamide) gels in seawater at high pressures

The swelling behavior of poly(N‐isopropylacrylamide) (PNIPA) gels in seawater at high pressures up to ～40 MPa is examined in terms of human activity at deep sea. The neutral gel in seawater undergoes a continuous volume transition at 26°C at atmospheric pressure. Addition of the ionic group (sodium acrylate) does not have much effect on the swelling properties of the gels in seawater except that the transition temperature is somewhat increased. At high constant pressures up to ～40 MPa, the gels undergo a continuous volume transition at 26–28°C as the temperature varies. Normally, the gel takes a swollen state at deep sea. The ratio of the volume change associated with the transition is more than 10, which is 3 times larger than that obtained by changing the pressure at a constant temperature. The results suggest a possibility that the phase transition of PNIPA gels is utilized for producing mechanochemical energy at deep sea. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1069–1072, 2005     

Characteristics of HPHT diamond grown at sub-lithosphere conditions (10-20 GPa)

We have conducted high pressure-high temperature (HPHT) diamond synthesis experiments at the conditions of growth of superdeep diamonds (10-20 GPa), equivalent to the transition zone, using MgCO₃ carbonate (oxidising) and FeNi (reducing) solvent catalysts. High rates of graphite-diamond transformation were observed in these short duration experiments (20 min). Transformation rates were higher using the metallic catalyst than in the carbonate system. High degrees of carbon supersaturation at conditions significantly above the graphite-diamond stability line, led to a high nucleation density. This resulted in the growth of aggregated masses of diamond outlined by polygonised diamond networks, resembling carbonado. Where individual crystals are visible, grown diamonds are octahedral in the lower pressure experiments (≤ 10 GPa in MgCO₃ and ≤ 15 GPa in FeNi) and, cubo-octahedral at higher pressure. All grown diamonds show a high degree of twinning. The diamonds lack planar deformation features such as laminations or slip planes, which are commonly associated with natural superdeep diamonds.     

A study of phase transformation between diamond and graphite in P-T diagram of carbon

The phase transformation between diamond and graphite has been studied by calculating the transiting probability of the carbon atoms over a potential barrier. For the first time, the boundaries of the proposed metastable regions of diamond and graphite have been estimated theoretically, and the currently used characteristic paths of pressure-temperature for synthesizing diamond using a high pressure and high temperature (HPHT) method can be understood.     

Pore size distribution model derived from a modified DR equation and simulated pore filling for nitrogen adsorption at 77 K

The Dubinin-Radushkevich (DR) equation has been used extensively to characterize microporous materials. However, it assumes a continuous pore filling mechanism and does not reduce to Henry’s law at low adsorptive pressures. In this study, we modify the DR equation by introducing adsorption density and the correlation between pore filling pressure and critical pore size for N₂ adsorption at 77 K. This modified DR equation is used as the local isotherm in the generalized adsorption isotherm (GAI) to interpret N₂ adsorption isotherm at 77 K. To solve the GAI, a modified normal distribution is developed and used as a distribution function of pore size on the domain [0, ∞). Pore size distribution (PSD) analyses of several adsorbent samples are carried out using this model. The results are found comparable with other popular PSD methods (e.g. MP, JC, HK and DFT).     

The phase transitions of n-alkanes in mesoscopic pores of graphite

The phase transitions of two n-alkanes of different parity, C₁₆H₃₄ and C₁₇H₃₆, confined in the mesoscopic pores of exfoliated graphite have been studied by differential scanning calorimetry. For samples from one statistical monolayer up to about four layers of paraffin, microcalorimetry shows a characteristic signature from the “surface phase” alone. At higher fillings the melting and phase transition temperatures and enthalpies of the additional “included” phase were depressed compared to those of the bulk paraffin. The depressions in the phase transition temperatures and the neperian logarithm of the enthalpies are to a good approximation inversely proportional to the number of included layers. Depending on the coverage, competing effects between the graphite surface and the degree of filling of the pores are observed for both the surface phase and the included phase. The results for the included phase are consistent with the Gibbs-Thompson equation down to pore sizes of the order of a few nanometres.