Direct Current Arc Plasma Jet Research Articles

The uniform and robust 8-inch CVD diamond plate generated by dual-magnetic field controlled DC Jet CVD

Diamond is a highly attractive material for many applications in material science, engineering, chemistry, and biology because of its favorable properties. The preparation of large-area freestanding diamond plate can greatly reduce the production cost of diamond and promote the industrialization of diamond. In this study, the dual-magnetic field mode was introduced to optimize the magnetic field distribution in direct current arc plasma jet chemical vapor deposition (DC Jet CVD) system, and the arc plasma was effectively controlled under the coupling of electric field and magnetic field. The uniformity of the plasma distribution above the substrate surface is significantly improved, resulting in long-term stable and uniform growth of the 8-inch freestanding diamond plate. The influence of dual-magnetic field mode on magnetic field generation, arc feature, and reactive group distribution was studied. 8-inch freestanding diamond plates with an average thickness of 2.8 mm were prepared by dual-magnetic controlled DC Jet CVD. The results showed that the 8-inch diamond plate preferred a high (220) orientation, and the deviation of thickness of which was around 10 % of the averaged values. Raman spectroscopy of diamond revealed a 6.6 cm−1 of full width at half maximum (FWHM) and calculated 1.20 GPa compressive stress in the 8-inch diamond plate. By means of mechanical and thermal property measurements, the large area diamond plate shows a robust structure with the average values of three-point bending fracture strength of 987.2 MPa, and extremely high thermal conductivity of 1797.8 W/(m·k) as well. The deviation of three-point bending fracture strength and the thermal conductivity over the 8-inch diamond plate was around 7.9 % and 12.6 % of the respective averaged values.

Evaluation of the fracture strength of ultra-thick diamond plate by the three-point bending ISO standard method

In this study, standard diamond bars were prepared using an ultra-thick diamond plate, and the bars had sizes consistent with the International Organization for Standardization (ISO) 14704:2016(E) three-point bending method for measuring the fracture strength. Ultra-thick diamond plate with dimensions of 125 mm (diameter) × 6.5 mm (thickness) were deposited using direct current arc plasma jet chemical vapour deposition. After cutting, grinding, and polishing, 12 diamond bars meeting the requirements of the ISO standard were prepared (36 mm × 4 mm × 3 mm) and tested for fracture strength using the cited ISO standard and elastic modulus using deflection measurements. The fracture strength of the ultra-thick diamond plate was found to be 580 ± 66 MPa. In addition, for comparison, the fracture strengths of standard ZnS, sapphire, and alumina were also analysed. Crucially, this is the first report of the fracture strength of standard-sized diamond plate. The effects of the defects and grain size on the crack formation and failure of the diamond film were analysed. The diamond's fracture strength was affected by the sample thickness, which caused the gradual accumulation of defects as this dimension increased, but not by its length and width. The elastic moduli of sapphire and alumina were compared. Overall, the measurement of the fracture strengths of ultra-thick diamond plate via the ISO standard is important for industrial applications.

Au@SnO2-vertical graphene-based microneedle sensor for in-situ determination of abscisic acid in plants

For developing electrochemical plant sensors, in-situ detection of hormone levels in living plants is worth attempting. A microneedle array sensor based on Au@SnO2-vertical graphene (VG)/Ta microelectrodes was constructed for analyzing abscisic acid (ABA) in plants. Graphene was vertically grown on Ta wires with a diameter of 0.6 mm by direct current arc plasma jet chemical vapor deposition with SnO2 as the Au catalyst carrier. These VG nanosheets were embedded with core-shell Au@SnO2 nanoparticles, and the formation mechanism of the sensing layer was investigated. Three Au@SnO2-VG microelectrodes, one Ti wire, and one Pt wire were packed into a microneedle array sensor with a three-electrode system. ABA was then quantitatively detected by direct electrocatalytic oxidation, which involves the synergistic catalytic effects of the abundant catalytic active sites of the Au@SnO2 nanoparticles and the excellent conductivity of the VG nanosheets. The microneedle array sensor responds to ABA in the pH range 4–7, the response concentration range was 0.012 (or 0.024)–495.2 μM, and the detection limit varied between 0.002 and 0.005 μM. The small size, wide pH range, low detection limit, and wide linear concentration range allow the microneedle array sensor to be inserted into plants for in-situ detection of ABA.

Fracture strength and toughness of chemical-vapor-deposited polycrystalline diamond films

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25 °C, using a single-edge pre-cracked beam method.

Growth evolution of carbon film on the hydrocooling copper substrate by DC arc plasma jet

The growth process of carbon film on the surface of hydrocooling copper substrate in high-density direct current (DC) arc plasma was studied. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, nuclear magnetic resonance spectroscopy, and UV confocal Raman spectroscopy were used to analyze the structural constituents of the intermediate products generated at different growth periods. Results show that despite the remarkable heating effect of DC arc plasma to the copper substrate, one layer of cauliflower saturated carbon and double carbon bond as main component loose matter are first formed because of water-cooled copper substrate. With the increase in thickness of carbon film, the surface temperature gradually increases, the etching ability of the atomic hydrogen is enhanced and high concentration reactive units are maintained on the surface of carbon film. The cauliflower structure may effectively reduce the diamond nucleation barrier and promote the generation of nano-diamond. The diamond grains gradually grow up to the micron scale by the continuing influence of carbon-based active group and plasma. The growth pattern of carbon film on the hydrocooling copper substrate in high-density DC arc plasma may provide a new way to prepare large crack-free diamond films.

Fast growth of amorphous Si nanowires by direct current arc plasma jet chemical vapor deposition

Silicon (Si) nanowires were synthesized by direct current arc plasma jet chemical vapor deposition. It was found that growth temperature and growth time played important roles in the morphologies of the Si nanowires. The possible formation mechanism of Si nanowire was also proposed according to the corresponding results. In addition, the Ni particles are dispersed on the Si substrate with the help of alkaline etching, which enables the lateral growth of Si nanowires on a Si substrate. This offers a simple and practical way of synthesizing and positioning the Si nanowires on a substrate.

Fabrication of graphene/titanium carbide nanorod arrays for chemical sensor application

Vertically stacked graphene nanosheet/titanium carbide nanorod array/titanium (graphene/TiC nanorod array) wires were fabricated using a direct current arc plasma jet chemical vapor deposition (DC arc plasma jet CVD) method. The graphene/TiC nanorod arrays were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction spectroscopy. The TiO2 nanotube array was reduced to the TiC nanorod array, and using those TiC nanorods as nucleation sites, the vertical graphene layer was formed on the TiC nanorod surface. The multi-target response mechanisms of the graphene/TiC nanorod array were investigated for ascorbic acid (AA), dopamine (DA), uric acid (UA), and hydrochlorothiazide (HCTZ). The vertically stacked graphene sheets facilitated the electron transfer and reactant transport with a unique porous surface, high surface area, and high electron transport network of CVD graphene sheets. The TiC nanorod array facilitated the electron transfer and firmly held the graphene layer. Thus, the graphene/TiC nanorod arrays could simultaneously respond to trace biomarkers and antihypertensive drugs.

Structural evolution of Ti destroyable interlayer in large-size diamond film deposition by DC arc plasma jet

The addition of titanium (Ti) interlayer was verified to reduce the residual stress of diamond films by self-fracturing and facilitate the harvest of a crack-free free-standing diamond film prepared by direct current (DC) arc plasma jet. In this study, the evolution of the Ti interlayer between large-area diamond film and substrate was studied and modeled in detail. The evolution of the interlayer was found to be relevant to the distribution of the DC arc plasma, which can be divided into three areas (arc center, arc main, and arc edge). The formation rate of titanium carbide (TiC) in the arc main was faster than in the other two areas and resulted in the preferred generation of crack in the diamond film in the arc main during cooling. Sandwich structures were formed along with the growth of TiC until the complete transformation of the Ti interlayer. The interlayer released stress via self-fracture. Avoiding uneven fragile regions that formed locally in the interlayer and achieving cooperatively released stress are crucial for the preparation of large crack-free diamond films.

Synthesis and characterization of MgO nanocrystals for biosensing applications

MgO nanocrystals were prepared using a simple direct current arc plasma jet chemical vapor deposition method. Magnesium nitrate was used as source material and Mo film was used as a substrate and catalyst. The high-temperature plasma produced ensured rapid synthesis of the MgO nanocrystals. The as-prepared nanocrystals were characterized by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, Fourier transform infrared spectrometry, ultraviolet–visible spectrophotometry, and photoluminescence measurements. The as-synthesized samples were found to consist of cubic MgO nanobelts and nanosheets with large surface areas and low coordination oxide ions, and contained numerous contacts, rough edges, vacancies, and doping defects. The nanostructures exhibited excellent electrochemical sensing properties with high-sensing sensitivity toward ascorbic acid. Their high electrocatalytic activity was attributed to the effect of defects and the surface electron transfer ability of the one-dimensional MgO nanobelts.

Synthesis and optical properties of single-crystal MgO nanobelts

MgO nanobelts were obtained by the decomposition of magnesium nitrate in a direct current arc plasma jet chemical vapor deposition process, with a Mo substrate at 950°C for different durations. The products formed in the first 0.5min were nanobelts, some of which possessed spherical particles on tips of nanobelts, forming “tadpole-like” nanostructures. When the reaction time was extended to 1min, nanobelts become longer although spherical particles were still observed. When the reaction time was 3min, nanobelts became even longer, and no spherical particles were observed. Extending the reaction time to 5min significantly widened the nanobelts, with widths ranging from 10 to 100nm. The MgO nanobelt growth occurred along the 〈100〉 and the 〈111〉 axes. The growth mechanism of the MgO nanobelts was a combination of axial VLS and side VS growth. Nanobelts exhibited strong ultraviolet–blue photoluminescence emission under excitation at 350nm.

Deposition of crackless freestanding diamond films on Mo substrates with Zr interlayer

The Mo substrate with Zr interlayer, namely composite substrate, was employed to solve the problem of crack formation in the freestanding diamond film deposition. Freestanding diamond films deposited on the composite substrates by the direct current arc plasma jet chemical vapor deposition (CVD) method were investigated with scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Raman spectroscopy. In addition, the stress distribution during the large area freestanding diamond film deposition on the composite substrate was analyzed based on the finite element model ANSYS. The results reveal that Zr interlayer can be easily destroyed during the post-deposition cooling process, which is helpful for stress release and crack avoiding in diamond films.

Thermal Stress in Free-standing Diamond Films with Cr Interlayer Destroyed

Thermal stress in large area free-standing diamond films was remarkable during the post-deposition cooling of direct current (DC) arc plasma jet chemical vapor deposition (CVD) process. In this research, the stress release caused by delamination of Cr interlayer was of great importance to ensure the integrity of free-standing diamond film. The effects of Cr interlayer on Mo substrate, namely composite substrate, on thermal stress were investigated. Thermo-mechanical coupling analysis of the thermal stress was applied by finite element analysis (FEA) using ANSYS code. It was found that the interlayer could be destroyed first by the large thermal stress, and then the stress could be released and the probability of diamond film crack initiation would be reduced. The stress concentration at the bent edge of diamond film was also discussed. In addition, diamond films deposited on Mo substrates with and without Cr interlayer were prepared by DC arc plasma jet CVD system and experimental measurements were used to characterize these films. It was found that composite substrate could be an effective method of growing free-standing crack-free diamond films by DC arc plasma jet CVD system when there is no special requirement to the film strength.

Role of arc plasma instability on nanosynthesis

Recent studies have shown that use of direct current arc plasma jet is a promising technique for bulk generation of nanostructures. This paper presents a dynamical study on such systems during the synthesis and attempts to correlate the role of inherent arc fluctuations on the properties of nanostructures produced. Different fluctuations detected during the process of nanosynthesis have been characterized in terms of behavior in real time, phase space, frequency space, fractal dimension, Lyapunov exponent of evolution and diagnosed to be chaotic. For the first time, it has been shown that size of nanoparticles are strongly related to the Lyapunov exponent of inherent voltage fluctuations realized during the synthesis. Interesting features of such dependences under different process conditions have been brought out. Since chaotic systems are controllable, the study brings out the possibility of development of new size control strategies for nanosynthesis.

Plasma Characteristics of Supersonic Nitrogen/Hydrogen-Mixture and Ammonia Plasma Jets and Nitrided Material Properties

Spectroscopic and electrostatic probe measurements were made to examine plasma characteristics under nitriding for a 10-kW-class direct-current arc plasma jet generator with a supersonic expansion nozzle in a low pressure environment. Heat fluxes into the plate from the plasma were also evaluated. Ammonia and mixtures of nitrogen and hydrogen were used as the working gas. The H-atom electronic excitation temperature and the N2 molecule-rotational excitation temperature intensively decreased downstream in the nozzle although the NH molecule-rotational excitation temperature did not show an axial decrease. As approaching the titanium plate, the thermodynamical nonequilibrium plasma came to be a temperature-equilibrium one. Both the electron number density near the plate and the heat flux increased with H2 mole fraction for mixtures gases. In cases with mixtures of N2 and H2, a radical of NH with a radially wide distribution is considered to contribute to the better nitriding as a chemically active and non heating process.

Nanocrystalline diamond films produced by direct current arc plasma jet process

Nanocrystalline diamond films have been prepared by using a direct current arc plasma jet system. Hydrogen flow rate of the chemical vapor deposition process was decreased to induce the transition from micro- to nanocrystalline diamond films. The deposition rate was found to decrease from 20 μm/h for the microcrystalline to 8 μm/h for the nanocrystalline diamond films. It is believed that this proves that the growth of diamond films is somehow facilitated by atomic hydrogen abstractions. On the other hand, the growth of nano diamond films was found to be a quenched process, with a result of formation of colony-featured microstructures. For the nanocrystalline diamond films, the problem of surface roughness remains, not because of faceting of individual diamond grains, as in the case of microcrystalline diamond films, but because of suppressed diffusion of depositing atoms. It is shown that there exists a close relationship between surface roughness of the deposited nanocrystalline diamond films and the substrates, and an improvement in flatness of substrate surfaces could lead to a decrease in roughness of the deposited nanocrystalline diamond films.

Periodic vapor-phase fluctuation influence on growth of diamond films

A computer control system was designed to work with diamond film deposition apparatus to control methane flow automatically. In experiments, the methane flow can fluctuate with a fixed period and fixed amplitude cosine curve. Diamond films were deposited by direct current arc plasma jet and their morphologies were examined by scanning electron microscopy. This proved that the reaction vapor-phase periodic fluctuation had an obvious influence upon the growth and nucleation of diamond. The phenomena of alternate growth and nucleation were observed; especially interesting are the two-dimensional fractal carbon clusters that have been observed for the first time.