Comparative Assessment of Crystallographic Phase Stability of Anatase and Rutile TiO2 at Dynamic Shock Wave Loaded Conditions

In the present research article, authors have experimentally evaluated the shock wave resistant properties of technologically potential materials of the anatase and the rutile phase TiO₂ nanoparticles at the dynamic shock wave loaded conditions. The shock wave resistant behavior has been quantitatively drawn utilizing the crystallographic phase stability of the test samples for which the required crystallographic information has been extracted from the powder XRD patterns. Based on our observed experimental results as well as the respective interpretations, it is strongly authenticated that Rutile TiO₂ NPs are suitable candidates for aerospace and defense industrial applications of materials fabrications because of the outstanding shock resistant properties than that of Anatase TiO₂ NPs which undergo the crystallographic phase transition of rutile-TiO₂ at shocked conditions.

     

Effects of electrode arrangement and pressure on synthesis of diamond films by arc discharge plasma jet chemical vapor deposition

The setup and deposition conditions of electrode arrangement and pressure have been studied to synthesize diamond films at high growth rate on wide area efficiently by arc discharge plasma jet chemical vapor deposition. An apparatus has been used in which four plasma torches, one is used for cathode and the others for divided anodes, are arranged and the positions of these torches are changeable. Growth rate, deposition area and thickness of diamond films have increased with changing the electrode arrangements without improvement of thickness variation. Maximum growth rate of our apparatus has occurred at the pressure of 6.7 kPa and diamond films that have less variations of quality and surface roughness have been synthesized at lower pressure during deposition. Moreover, a high conversion rate, which is the ratio of carbon atoms that form diamond in supplied methane gas, of 16% has been obtained at the pressure of 6.7 kPa and methane concentration of 2%.     

Study of aluminium nitride/freestanding diamond surface acoustic waves filters

Highly oriented aluminium nitride (AlN) has been successfully deposited on silicon substrates and on the back side of unpolished, thick, freestanding, polycrystalline diamond films. Structural and electrical properties of the (0 0 2) oriented AlN films have been investigated. Using optimised AlN and chemical vapour deposited diamond films, high quality surface acoustic wave (SAW) filters were constructed by deposition of aluminium inter-digital transducers. The effects of the AlN thickness on the diamond-based SAW filter properties and the SAW propagation were investigated. A phase velocity of 17.1 km/s was observed, which is, to the best of our knowledge, the highest reported value for diamond. The dependence of the phase velocity (v) on the AlN thickness was compared to theoretical predictions. SAW filters with rather low insertion loss, high suppression and high electromechanical coefficient could be obtained. We also report on piezoelectric d₃₃ measurements of AlN films by atomic force microscopy.     

Dielectric property of thick freestanding diamond films by high power arcjet operating at gas recycling mode

Dielectric property of thick freestanding diamond films prepared by high power arcjet operating at gas recycling mode was measured by the high voltage electric bridge method at low frequencies (r.f.) and the wave guide resonance method at high frequencies (microwave). It was found that, with increasing frequencies, dielectric loss of freestanding diamond films increased at low frequencies, but decreased at high frequencies, with a maximum located at approximately 3 MHz. Measurements of dielectric loss of the freestanding diamond films at a microwave frequency of 5.2 GHz showed a strong dependence on the growth parameters such as substrate temperature and methane concentration. It was found that dielectric loss decreased with increasing substrate temperature, and increased with an increasing methane concentration in the feed gases. It is suggested that dielectric loss is closely related with the quality level of freestanding diamond film samples, as demonstrated by the results from Raman and SEM observations. Non-diamond carbon in the diamond films was found responsible for the increase in dielectric loss. Nitrogen was intentionally introduced into the Ar–H₂–CH₄ gas stream for diamond deposition to investigate the effect of impurities. It was shown that nitrogen addition to the feed gases seriously deteriorated the dielectric property of the resultant diamond films. This is again in agreement with our experimental observations by Raman and SEM, in that the addition of nitrogen also seriously deteriorated the quality of the diamond film. Mechanisms for the dielectric behaviour of the diamond films were discussed in detail.     

Shock-induced phase transition of oriented pyrolytic graphite to diamond at pressures up to 15 GPa

In studies of shock-induced phase transition of ordered pyrolytic graphite to a diamond-like phase, the lowest transition onset pressure was observed at 19.6 GPa. The phase transition in that case was considered to be martensitic. In the present study ordered pyrolytic graphite with voids between particles was loaded at pressures up to 15 GPa using a planar shock wave propagating along the basal plane of the graphitic crystal structure. As a result, both diamond-like carbon and diamond were observed in the postshock sample. The phase transition of graphite to diamond was assumed to occur by the release of distortional energy stored in the graphite particles, that is, diffusional-controlled reconstructive mechanism, on the basis of the data by high resolution electron microscopy together with electron energy loss spectroscopy.     

Growth of microcrystalline and nanocrystalline diamond films by microwave plasmas in a gas mixture of 1% methane/5% hydrogen/94% argon

Well-faceted microcrystalline diamond (MCD) films were deposited along with nanocrystalline diamond (NCD) films on the same substrate by a microwave plasma in the gas mixture of 1% CH₄+5% H₂+94% Ar. This was achieved by forcing a microwave plasma ball generated at 170 torr gas pressure to touch a silicon substrate that was pre-seeded by nanocrystalline diamond powder resulting in a high concentration of atomic hydrogen on the surface of growing diamond. Previously reported compositional mapping of the argon–methane–hydrogen system for MCD and NCD growth was not valid in this process parameter space. The non-uniform concentrations of atomic hydrogen and carbon containing radicals such as C₂ as well as varied local substrate temperature resulted in the simultaneous deposition of well-faceted MCD films in some areas with nanograined NCD films in others. Dilution of methane/hydrogen microwave plasmas by as much as 94% of argon alone could not suppress the growth of MCD.     

Measurement of the adhesion of diamond films on tungsten and correlations with processing parameters

Adhesion between diamond films and tungsten substrates is reported as a function of the deposition processing parameters. Diamond films were grown by a hot filament method as a function of seven different processing parameters: substrate scratching prior to diamond deposition, substrate temperature, methane content of the input gas mixture, filament temperature, filament-substrate distance, system pressure, and total gas flow rate. Adhesion was measured by using a Sebastian Five A tensile pull tester. Testing was complicated by the non-uniformity of the film thickness, diamond quality, film cohesion, and surface preparation across the full substrate surface area. Various types of film failure mode were observed, which did not correlate with the film processing parameters. The measured adhesion values showed larger variations from point to point across the sample surface and from identically prepared samples than variations as a function of the film processing parameters. Weak correlations of adhesion with the processing parameters were found using statistical analysis of the results from multiple pulls on a large number of samples. The statistical results suggest that substrate preparation, gas flow rate, and gas pressure are the most important processing parameters affecting the film adhesion, while the temperature of the hot filament has little or no effect on the adhesion of the film. However, improvements in film processing and adhesion testing need to be made before true quantitative adhesion testing of high-quality diamond films can be accomplished.     

NEXAFS characterization of nanocrystalline diamond thin films synthesized with high methane concentrations

Diamond thin films were deposited on silicon in gas mixtures of methane and hydrogen with different methane concentrations ranging from 1% to 100% using microwave plasma assisted chemical vapor deposition. Both Raman spectroscopy and synchrotron near edge extended X-ray absorption fine structure spectroscopy (NEXAFS) were used to characterize the electronic structure and chemical bonding of the synthesized films. The NEXAFS spectra of the nanocrystalline diamond (NCD) films exhibit clear spectral characteristics of diamond. Close observation reveals that the films (10% CH₄ or above) exhibit a slightly broadened exciton transition with a 0.25 eV blue shift. With the increase in methane concentration, the growth rate, the surface smoothness, and the sp² carbon concentration of the films increase while the grain size decreases. Well-faceted microcrystalline diamond films were synthesized with a methane concentration of 5% or lower, while NCD films were formed with a methane concentration of 10% or higher. Diamond thin films with low surface roughness and fine nanocrystalline structure have been synthesized with high methane concentrations (50% or above). It has been observed that the diamond growth rate increases with methane concentration. The growth rate at 100% methane concentration is approximately 10 times higher than at 1%.     

Heteroepitaxy of Diamond on Cubic Boron Nitride

The epitaxial growth process of diamond from the gas phase on a cubic boron nitride (c-BN) {111} surface has been investigated. At the initial growth stage, carbon adsorption progressed on a boron-terminated surface of c-BN ({111}_B). The coordination of the carbon atoms was found to be the same as that observed in diamond, as confirmed by electron energy loss spectroscopy (EELS). The epitaxial growth of diamond particles has been observed after formation of the carbon layer. On the other hand, on the nitrogen-terminated surface ({111}_N), neither stable adsorption of carbon nor nucleation of diamond has been observed. The stability of adsorbed carbon atoms in the chemical vapor deposition (CVD) ambient, in which large amounts of atomic hydrogen are supplied to the substrate heated at high temperature, is quite important for the nucleation of diamond. Using cross-sectional transmission electron microscopy (TEM), numerous crystal defects were observed, both in c-BN and diamond. Formation of the epitaxial diamond particles has been observed especially at defect sites on c-BN. The misfit dislocation has been observed near the interface with the diamond particle. Even though there exist misfit dislocations that relieve the stress caused by the lattice mismatch between diamond and c-BN, the epitaxial film involved retains a tensile strain of about 0.29% for a film thickness of about 200 nm.     

High rate of diamond deposition through graphite etching in a hot filament CVD reactor

Although a hot filament diamond CVD reactor has many advantages over other processes, the relatively low growth rate of the films has been a crucial drawback. We developed a new hot filament process that uses no hydrocarbon gas for diamond deposition. The graphite plate was placed below the silicon substrate and only hydrogen was supplied during the process. We could achieve the growth rate of 9 μm/h, which is approximately 9 times higher than that of conventional hot filament CVD using a gas mixture of methane and hydrogen. In spite of the high growth rate, the quality of diamond films was not degraded. Besides, the diamond films consisted of small crystallites with a smooth surface while the conventional diamond films of the same thickness tend to have a columnar structure with a rough surface.     

Comparative study of diamond films grown on silicon substrate using microwave plasma chemical vapor deposition and hot-filament chemical vapor deposition technique

Diamond films on the p-type Si(111) and p-type(100) substrates were prepared by microwave plasma chemical vapor deposition (MWCVD) and hot-filament chemical vapor deposition (HFCVD) by using a mixture of methane CH₄ and hydrogen H₂ as gas feed. The structure and composition of the films have been investigated by X-ray Diffraction, Raman Spectroscopy and Scanning Electron Microscopy methods. A high quality diamond crystalline structure of the obtained films by using HFCVD method was confirmed by clear XRD-pattern. SEM images show that the prepared films are poly crystalline diamond films consisting of diamond single crystallites (111)-orientation perpendicular to the substrate. Diamond films grown on silicon substrates by using HFCVD show good quality diamond and fewer non-diamond components.     

In situ plasma diagnostics of the chemistry behind sulfur doping of CVD diamond films

Microwave plasma enhanced chemical vapour deposition (CVD) has been used to grow sulfur doped diamond films using a 1% CH₄/H₂ gas mixture with various levels of H₂S addition (100–5000 ppm), upon undoped Si substrates. X-Ray photoelectron spectroscopy has shown that S is incorporated into the diamond at number densities (≤0.2%) that are directly proportional to the H₂S concentration in the gas phase. Four-point probe measurements showed the resistivity of these S-doped films to be a factor of three lower than undoped diamond grown under similar conditions. Sulfur containing diamond film was also obtained using a 0.5% CS₂/H₂ gas mixture, although the high resistivity of the sample indicated that the sulfur had been incorporated into the diamond lattice in a different manner compared with the H₂S grown samples. Molecular beam mass spectrometry has been used to measure simultaneously the concentrations of the dominant gas phase species present during growth, for a wide range of H₂S doping levels (1000–10 000 ppm in the gas phase). CS and CS₂ have been detected in significant concentrations in the plasma region as a result of gas phase reactions. Additional measurements from a 1% CS₂/H₂ plasma gave similar species mole fractions except that no CS was detected. These results suggest that CS may be the first step toward CS bond formation in the film and thereby a pathway allowing S incorporation into diamond. Optical emission spectroscopy has shown the presence of S₂ in both gas mixtures, consistent with the observed deposition of sulfur on the cool chamber walls.     

Synthesis and characterization of freestanding diamond/carbon nanoflake hybrid films

Freestanding diamond/carbon nanoflake hybrid films have been synthesized by generating variable gas chemistries near a microstructured substrate in a conventional diamond deposition reactor. A multi-cathode direct current plasma enhanced CVD reactor, designed to deposit thick diamond wafers in its conventional configuration, has been used. The deposition conditions are identical to that for diamond films (10%CH₄ in H₂ gas, 100 Torr, 700–800 °C) except for a novel substrate. A sacrificial layer of silica micro-spheres (diameter 10–30 μm) was close-packed through gentle agitation of the spheres on a 100 mm diameter molybdenum disc. The growth of diamond and graphitic phases was observed on the front and back surfaces of the micro-spheres respectively. TEM observation confirmed that they were chemically bonded hybrid films without any distinguishable interlayer between the two phases.     

Reaction Behavior of Polytetrafluoroethylene/Al Granular Composites Subjected to Planar Shock Wave

The shock‐compression responses of PTFE (polytetrafluoroethylene)/Al granular composites subjected to planar shock waves of various pressures are investigated. A 57‐mm diameter single‐stage gas‐gun and 50‐mm diameter plane‐wave lenses are employed to perform planar shock wave experiments. High frequency manganin piezoresistance stress gauges are used to monitor the stress (regarded as pressure in consideration of the high pressure state) at four Lagrangian positions of the PTFE/Al granular composites specimens. Planar shock wave experiments show characteristics of densification at measured input pressure of 0.5 GPa to 1.27 GPa using single‐stage gas‐gun and shock‐induced reaction (SIR) indicated by growth of shock pressure and specific volume expansion at measured input pressure of 7.29 GPa to 12.25 GPa using plane‐wave lenses. The pressure and relative volume states behind the shock wave front are calculated from the experimental recorded pressure profiles using Lagrangian analysis method, which are used to determine the reaction ratios under different shock pressures by comparing with partial reacted Hugoniot calculations. It was shown that the reaction ratios obtained in this research have good agreement with the thermochemical modeling calculations. The corresponding results indicate that the shock‐induced reactions of PTFE/Al granular composites occur in the shock wave rising period and the reaction ratios are intimately related to the shock wave pressure.     

Effects of nitrogen addition on morphology and mechanical property of DC arc plasma jet CVD diamond films

Nitrogen-doped diamond films have been synthesized by 100 KW DC arc plasma jet chemical vapor deposition using a CH₄/Ar/H₂ gas mixture. The effect of nitrogen addition into the feed gases on the growth and surface morphology and mechanical property of diamond film was investigated. The reactant gas composition was determined by the gas flow rates. At a constant flow rate of hydrogen (5000 sccm) and methane (100 sccm), the nitrogen to carbon ratio (N/C) were varied from 0.06 to 0.68. The films were grown under a constant pressure (4 KPa) and a constant substrate temperature (1073 K). The deposited films were characterized by scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The fracture strength of diamond films was tested by three point bending method. The results have shown that nitrogen addition to CH₄/H₂/Ar mixtures had led to a significant change of film morphology, growth rate, crystalline orientation, nucleation density and fracture strength for free-standing diamond films prepared by DC arc plasma jet.     

Diamond films with preferred <110> texture by hot filament CVD at low pressure

The effects of gas pressure on the textured growth of diamond films were investigated in a hot filament chemical vapor deposition (HFCVD) system. Diamond thin film with the growth rate of 1.3 μm/h and with high <110> texture was obtained at 5 Torr when lowering the gas pressure from 40 Torr to 1 Torr. The formation of high density nanocrystalline diamond nuclei elongated along the <110> direction in the nucleation stage and its consequent growth at lower pressure were considered to be responsible for the formation of <110> textured diamond thin film.     

Diamond nucleation and growth under very low-pressure conditions

The synthesis of thin diamond films using various chemical vapor deposition methods has received significant attention in recent years due to the unique characteristic of diamond, which make it an attractive candidate for a wide range of applications. In order to grow diamond epitaxially, the proper control of diamond nucleation on mirror-polished Si is essential. Adding the negative bias voltage to the substrate is the most popular method. This paper has proposed a new method to greatly enhance the nuclear density. Under very low pressure (1 torr), the high-density nucleation of diamond is achieved on mirror-polished silicon in a hot-filament chemical vapor deposition (HFCVD). Scanning electron microscopy has demonstrated that the nuclear density can be as high as 10¹⁰–10¹¹ cm⁻². Raman spectra of the sample have shown a dominant diamond characteristic peak at 1332 cm⁻¹. The pressure effect has been discussed in detail and it has been shown that the very low pressure is a very effective means to nucleate and grow diamond films on mirror-polished silicon. Extraordinary pure hydrogen (purity=99.9999%) was used as the source. Compared with the highly pure hydrogen (purity=99.99%), we found that the density of nucleation was greatly increased. The residual oxygen in the hydrogen displayed a very obvious negative effect on the nucleation of diamond, although it can accelerate the growth of diamond. Based on these results, it was suggested that the enhanced nucleation at very low pressure should be attributed to an increased mean free path, which induced a high density of atomic hydrogen and hydrocarbon radicals near the silicon surface. Atomic hydrogen can effectively etch the oxide layer on the surface of silicon and so greatly enhance the nucleation density.     

Hydrogen incorporation in diamond films

Hydrogen in a variety of forms (molecular hydrogen, atomic hydrogen, hydrocarbon radicals) is involved in diamond evolution and its incorporation may give insight into the growth processes of diamond films and their related properties.The present work studies the incorporation of hydrogen in hot filament chemical vapor deposited (HF CVD) diamond films with different grain sizes and compares it to that occurring in nano-diamond films deposited from energetic species. Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and high resolution electron energy loss spectroscopy (HREELS) were applied to investigate the hydrogen trapping in the films. The hydrogen retention of the diamond films increases with decreasing grain size, indicating that most likely hydrogen is bonded and trapped in grain boundaries. Raman and HREELS analysis show that at least part of this hydrogen is bonded to carbon forming typical C–H vibrations. Partial or full isotopic exchange of H by D in the CH₄/H₂ feeding gas mixture used for growth substantiated the assignment of both the Raman and HREELS C–H related modes. This isotopic exchange also indicated that most of the hydrogen incorporated in the diamond films originates from the molecular hydrogen component of the feeding gas.     

Microcrystalline diamond growth in presence of argon in millimeter-wave plasma-assisted CVD reactor

The additions of argon and oxygen to H₂–CH₄ feed gas and high-electron-density plasma generated by the millimeter-wave power were used to deposit microcrystalline diamond films having high quality and high growth rate simultaneously. Microcrystalline diamond films were grown on silicon substrates with 60–90 mm diameter in the millimeter-wave plasma-assisted CVD reactor based on 10 kW gyrotron operating at a frequency of 30 GHz. The growth process and morphology of diamond films at wide variation of parameters (gas pressure, substrate temperature, microwave power, argon and oxygen concentrations in gas mixtures Ar–H₂–CH₄ and Ar–H₂–CH₄–O₂) are investigated. For understanding of growth conditions the investigations of the plasma parameters (electron density and gas temperature) in novel CVD reactor are presented.     

Synthesis of CVD diamond at atmospheric pressure using the hot-filament CVD method

At low pressure, chemical vapor deposition (CVD) diamond growth by conventional techniques such as micro-wave plasma and hot-filament have been achieved by metastable precursor species. Moreover, bulk diamond at extremely high pressures and temperatures was consistently originated by the nature of diamond-graphite phase transition. CVD diamond growth has four problems with these conventional techniques. Excluding contaminated air from low pressure reactive systems has been problematic. It is very difficult to control the concentration of atomic hydrogen at high pressures. The growth rate is unsatisfactory and the running cost of gases are high.However, the hot-filament CVD technique at atmospheric pressure overcomes these problems. We have found that in order to control the concentration of atomic hydrogen, the residence time of the input gas and the methane-hydrogen concentration ratio needed to be varied at each pressure. The relationship between the quality of deposited diamond and the pressure have been also investigated by Raman spectroscopy and X-ray diffraction patterns (XRD).The growth rate at atmospheric pressure (106 000 Pa) was found to be greater than that at the conventional pressure (5000 Pa). At atmospheric pressure, the growth rate abruptly increases with the residence time. XRD analysis revealed that the quality of diamonds grown at atmospheric pressure was higher than that of diamonds produced at low pressures. Furthermore, high quality diamond growth was achieved with a long residence time of the input gas at atmospheric pressure.