Direct Current Arcjet Research Articles

The direct-current characteristics and surface repairing of a hydrogen-terminated free-standing polycrystalline diamond in aqueous solutions

As we know that more effective synthesis of diamond combined with physical and chemical properties of hydrogen termination in aqueous environment as well as device structure design can greatly facilitate the chemical and electrochemical applications of higher cost-performance diamond. For this purpose, the direct-current (DC) characteristics, surface reaction and reparation of a hydrogen-terminated DC arc jet plasma CVD polycrystalline diamond, which has a high cost-performance, were characterized by I-V experiments based on a FET-like structure device in multiple aqueous solutions. The variation trends of the I-V properties of device based on pH were similar in different aqueous solutions but could be affected by disparate ions (such as K+ ions) or organic molecules (such as citric acid radicals or a benzene ring). Especially, owing to the founded replacement of hydrogen termination with hydroxyl (OH), carboxyl radical (COOH) or carbon-oxygen bond (COC) in mixed solution, i.e., KHP + H2SO4 (and +NaOH) solutions, the resistance of the device was remarkably increased from 13.57 kΩ to 95.78 kΩ. However, the raised resistance of surface reacted diamond (SRD) can be reduced prominently by repairing hydrogen termination through negative potential sweeps (NPS) at a low negative potential (−1 to −3 V) if the SRD was introduced as an electrode in a strong inorganic acid. What's more, the NPS repaired device, which subsequently stored for four weeks, was more sensitive than the original hydrogen plasma-treated diamond in aqueous solution environments. This repaired result was coming out of NPS re-formed CH bonds with higher intensity. These findings will be the references of failure and reparation of diamond hydrogen termination in aqueous environment.

RF characteristic of MESFET on H-terminated DC arc jet CVD diamond film

Diamond has been considered to be a potential material for high-frequency and high-power electronic devices due to the excellent electrical properties. In this paper, we reported the radio frequency (RF) characteristic of metal-semiconductor field effect transistor (MESFET) on polycrystalline diamond films prepared by direct current (DC) arc jet chemical vapor deposition (CVD). First, 4in polycrystalline diamond films were deposited by DC arc jet CVD in gas recycling mode with the deposition rate of 14μm/h. Then the polished diamond films were treated by microwave hydrogen plasma and the 0.2μm-gate-length MESFET was fabricated by using Au mask photolithography and electron beam (EB) lithography. The surface conductivity of the H-terminated diamond film and DC and RF performances of the MESFET were characterized. The results demonstrate that, the carrier mobility of 24.6cm2/Vs and the carrier density of 1.096×1013cm−2 are obtained on the surface of H-terminated diamond film. The FET shows the maximum transition frequency (fT) of 5GHz and the maximum oscillation frequency (fmax) of 6GHz at VGS=−0.5V and VDS=−8V, which indicates that H-terminated DC arc jet CVD polycrystalline diamond is suitable for the development of high frequency devices.

Ohmic contact properties of p-type surface conductive layer on H-terminated diamond films prepared by DC arc jet CVD

With the advantages of high deposition rate and large deposition area, polycrystalline diamond films prepared by direct current (DC) arc jet chemical vapor deposition (CVD) are considered to be one of the most promising materials for high-frequency and high-power electronic devices. In this paper, high-quality self-standing polycrystalline diamond films with the diameter of 100 mm were prepared by DC arc jet CVD, and then, the p-type surface conductive layer with the sheet carrier density of 1011–1013 cm−2 on the H-terminated diamond film was obtained by micro-wave hydrogen plasma treatment for 40 min. Ti/Au and Au films were deposited on the H-terminated diamond surface as the ohmic contact electrode, respectively, afterwards, they were treated by rapid vacuum annealing at different temperatures. The properties of these two types of ohmic contacts were investigated by measuring the specific contact resistance using the transmission line method (TLM). Due to the formation of Ti-related carbide at high temperature, the specific contact resistance of Ti/Au contact gradually decreases to 9.95 × 10−5 Ω·cm2 as the temperature increases to 820°C. However, when the annealing temperature reaches 850°C, the ohmic contact for Ti/Au is degraded significantly due to the strong diffusion and reaction between Ti and Au. As for the as-deposited Au contact, it shows an ohmic contact. After annealing treatment at 550°C, low specific contact resistance was detected for Au contact, which is derived from the enhancement of interdiffusion between Au and diamond films.

The Fabrication and Performance of CVD Diamond-Based X-Ray Detectors

In this study, X-ray detectors with coplanar metal-semiconductor-metal structure, were fabricated employing high quality chemical vapour deposited (CVD) diamond film grown by a direct current arc jet plasma system. In which the electrical contacts with dimension of 25 µm in width with a 25 µm inter-electrode spacing, were patterned on the growth side of the diamond film using lift-off technology. The performance of the fabricated detectors was evaluated by steady-state X-ray illumination. The photoconductivity of the diamond detectors was found to linearly increase with increase in the X-ray photon flux. This demonstrates that high quality CVD diamond can be used for X-ray detectors.

Nitrogen in diamond films grown by direct current arcjet plasma process using gas recycling

The nitrogen in polycrystalline diamond films prepared using the direct current arcjet plasma process has been studied using scanning electron microscopy, X-ray diffraction and electron spin resonance (ESR). The ESR spectrum of single substitutional nitrogen was observed, whose intensity showed that the spin concentration decreases with an increase in the substrate temperature. The preferentially orientated growth is influenced by nitrogen. The result indicates that nitrogen plays a special role in the formation and structure of the polycrystalline diamond films studied in this report.

Emission spectroscopic measurement of ammonia or mixture of nitrogen and hydrogen plasma in a direct-current arc jet generator with an expansion nozzle

Spectroscopic measurement was carried out to understand the plasma features in a 10-kW-class water-cooled direct-current arcjet generator with a supersonic expansion nozzle. Ammonia or a mixture of nitrogen and hydrogen was used as the working gas. In the mixture of N/sub 2/+nH/sub 2/, the H/sub 2/ mole fraction n was varied from 0.5-3.0, in which a H/sub 2/ mole fraction of 3.0 corresponded to that of simulated ammonia. The discharge voltage and the vacuum tank pressure for N/sub 2/+3H/sub 2/ were higher than those for NH/sub 3/ at a constant discharge current and a constant input power, respectively. These characteristics agreed with those of H-atom electronic excitation temperatures in the constrictor of the nozzle throat. The NH/sub 3/ and N/sub 2/+3H/sub 2/ plasmas in the constrictor were expected to be nearly in a temperature-equilibrium condition. On the other hand, the plasmas in the expansion nozzle were in thermodynamical nonequilibrium state because the electron number densities rapidly decreased downstream. As a result, the H-atom excitation temperature and the N/sub 2/ rotational excitation temperature decreased from 7000-11000 K in the constrictor to about 4000 K and to 1000-1500 K, respectively, on the nozzle exit with mass flow rates of 0.1-0.2 g/s at input powers of 7-12 kW, although the NH rotational excitation temperature did not show a significant axial decrease.

Enhanced deposition rate of diamond in atmospheric pressure plasma CVD: Effects of a secondary discharge

Abstract Diamond deposition in an atmospheric pressure direct current arcjet was enhanced through induced nonequilibrium chemistry. A secondary discharge was created in the thin boundary layer above the growth substrate by means of a positive potential applied to the growth surface (thus forming a strong electric field across the boundary layer). This applied electric field provides a targeted energy addition to the free electrons in the plasma, heating them above the bulk gas temperature (Te > Tg). These energetic electrons are then able to promote electron impact dissociation of molecular species, specifically molecular hydrogen, to yield an enhanced flux of radicals important in the diamond growth process to the substrate surface. A factor of 7 increase in growth rate was observed with 4.9 A/cm2 and 170 V across the secondary discharge as compared with an unbiased (floating) substrate case. The enhanced growth rate observed with an active secondary discharge may be attributable to increased fluxes of atomic hydrogen and hydrocarbon radicals to the growth surface. Two-dimensional imaging of the plasma jet found a substantial increase in the Hα emission from the boundary layer with a positive potential applied. This result may indicate that the secondary discharge is in fact increasing the electron temperature and atomic hydrogen concentration in the very near surface region as compared with the unbiased substrate case. SEM and Raman spectroscopy analysis indicated that uniform, high quality films were deposited in all cases reported here.

Electron densities and temperatures in a diamond-depositing direct-current arcjet plasma

Langmuir probe measurements of electron density and temperature are made in the plume of a dc arcjet reactor. A dc arc is struck in an argon or argon/hydrogen mixture at 6 atm pressure and expands through a converging/diverging nozzle into the reactor with a pressure of 25 Torr. Methane and methane/nitric oxide are added in the diverging nozzle, and diamond film grows on a substrate in the gas plume. Electron temperatures of 1–2 eV are significantly hotter than the neutral gas temperature in the plume. Electron densities range from 1010 to 1013 cm−3, well above the Saha equilibrium for the gas temperature and pressure and well below the equilibrium for the electron temperature.

A simplified analytical model of diamond growth in direct current arcjet reactors

A simplified model of a direct current arcjet-assisted diamond chemical vapor deposition reactor is presented. The model is based upon detailed theoretical analysis of the transport and chemical processes occurring during diamond deposition, and is formulated to yield closed-form solutions for diamond growth rate, defect density, and heat flux to the substrate. In a direct current arcjet reactor there is a natural division of the physical system into four characteristic domains: plasma torch, free stream, boundary layer, and surface, leading to the development of simplified thermodynamic, transport, and chemical kinetic models for each of the four regions. The models for these four regions are linked to form a single unified model. For a relatively wide range of reactor operating conditions, this simplified model yields results that are in good quantitative agreement with stagnation flow models containing detailed multicomponent transport and chemical kinetics. However, in contrast to the detailed reactor models, the model presented here executes in near real-time on a computer of modest size, and can therefore be readily incorporated into process control models or global dynamic loop simulations.

Increased deposition rate of chemically vapor deposited diamond in a direct-current arcjet with a secondary discharge

A secondary discharge was used to enhance chemical nonequilibrium in the boundary layer of a stagnation point flow reactor during the atmospheric pressure deposition of chemically vapor deposited (CVD) diamond with a direct-current (dc) arcjet. The secondary discharge was induced by means of a positive potential on the deposition surface to drive a current through the boundary layer and produce energetic electrons. These electrons can promote superequilibrium concentrations of radicals at the growth surface. It was found that with 3.5 A/cm2 at 115 V in the secondary discharge, the growth rate of diamond in this reactor increased by a factor of 6 as compared with the floating or grounded substrate case.

Plasma diagnostics of a direct current arcjet diamond reactor. I. Electrostatic probe analysis

Plasma parameters were determined for a direct current arcjet diamond chemical vapor deposition reactor. Electron temperatures and densities were measured using a floating double probe technique, yielding values ranging from 2.3 eV and 7×1011 cm−3 near the plasma gun nozzle to 0.4 eV and 1×109 cm−3 in the downstream and peripheral regions of the plasma. These results show that the plasma is weakly ionized, indicating that reactions involving electrons play a minor role in the macroscopic gas–phase chemistry though high energy products from these reactions may significantly affect film quality.

Diamond growth using remote methane injection in a direct current arcjet chemical vapor deposition reactor

A significant enhancement in the quality of diamond films grown using a remote CH4 carbon source was observed in a direct current arcjet chemical vapor deposition reactor. The linewidths of the diamond peak at 1332 cm−1 in the Raman spectra from these films were reduced from 9.0 to 6.5 cm−1 with no reduction in mass deposition rate as compared to films grown using premixed reactor gas feeds. For diamond growth conditions, the percentage of the reactor feed carbon reaching the substrate was measured to be ∼35%. The enhanced quality of the films is attributed to reduced residence times of the carbon source in the plasma, which would limit the gas phase chemistry from proceeding beyond CH3 formation before the gas reaches the substrate.

Supersonic DC-arcjet synthesis of diamond

A supersonic, direct-current arcjet, designed for expansion into low pressures (⪡ 100 Pa) and operating entirely on hydrogen, is employed to produce diamond films on molybdenum substrates. This technique features strongly reduced boundary layer effects due to the near rarefied flow conditions, and short residence times for post-plasma injected reactants, thereby providing better control of the gas-phase chemistry. The results from a parametric study of diamond growth for methane as a carbon source gas are presented. Despite the low pressures, relatively high diamond growth rates (up to 10 μm h −1) are obtained. A “saturation” is observed in the variation of the diamond film thickness with methane to hydrogen flow ratio. Non-diamond phases become more pronounced at higher methane flow ratios, where the film thickness appears to increase linearly with methane concentration. The results are interpreted within the framework of a simplified five-step heterogeneous mechanism that accounts for the competition between the growth of diamond and non-diamond carbon.

直流アークジェット発生装置の内部流れ場の数値解析

A simple computational code for simulating flowfields in direct-current arcjet generators was developed to understand inner discharge mechanisms and to clarify the correlation between operational characteristics and arc features in the discharge chamber. In this analysis, a quasi-one dimensional core-flow model including dissociation and ionization processes in thermodynamic equilibrium was used. The results showed that the arc radii for Ar gradually increased downstream in the constrictor and that the arcs attached to the constrictor wall. An increase in the mass flow rate for Ar located the arc attachment point more downstream. However, the arcs for N2 passed through the constrictor and did not attach even to the expansion nozzle because of dissociation process. The calculated arc features for Ar and N2 agreed with the experimental ones although a suitable interface condition between the arc and cold gas regions was required in the present analytical model.

Diamond synthesis in supersonic direct-current arcjet plasma at subtorr pressures

A new approach to diamond thin film synthesis is presented. A supersonic, direct-current (d.c.) arcjet designed for expansion into low pressures (approximately 1 Torr), operating on mixtures of methane (0.5–25%) in hydrogen and argon, was employed to produce diamond films on silicon and molybdenum substrates. At subtorr pressures, free molecular flow conditions are approached, resulting in strongly reduced boundary layer effects. Large area coatings (up to 20 cm 2) due to the wide expansion of the plasma plume were obtained. The high molecular velocity, on the order of 10 km s −1 at the arcjet exit, and the high activation energies by the thermal plasma provide a high reactive species flux, both of the above giving rise to relatively high diamond growth rates (1–20 μm h −1, despite the lower pressures. Results of parametric studies of diamond growth over ranges of substrate temperatures and reactant mixture ratios show good agreement with commonly observed trends in other vapor deposition systems.

Experimental study on a low-power direct current arcjet

An experimental investigation on a low-power dc arcjet with helium propellant was conducted. Objectives were to evaluate the thermal efficiency, to assess the performance of various cathode materials, and to investigate the effects of transpirational cooling from the electrodes. The anode was made of porous tungsten so that the propellant could be supplied through the anode. The cathode materials used in this experiment were porous tungsten, thoriated tungsten, lanthanum hexaboride, and porous tungsten impregnated with barium oxide (BaO/W). Two discharge modes of low and high voltages were found, which corresponded to low and high thermal efficiency, respectively. The BaO/W cathode was the best of all the tested materials, demonstrating a thermal efficiency as high as 64.9%. The transpirational cooling resulted in a great decrease of the thermal efficiency.

Development of a regeneratively cooled 30-kw arcjet engine

The development of a fully regenerative-cooled 30-kw arcjet engine including a summary of the successful continuous 500-hr endurance test, is presented. This direct current arcjet engine, which uses hydrogen as the propellant, operated satisfactorily at an average measured thrust of 0.745 Ib and demonstrated a total impulse of 1.34 million Ib-sec during the described endurance test. Numerous analytical as well as experimental heat-transfer studies were required in order to substantiate the basic design philosophy, full regenerative-cooling using gaseous hydrogen. This electrothermal engine demonstrated a performance of 55% in over-all thruster efficiency at a specific impulse of 1000 sec, resulting in a 20% improvement in efficiency over a radiation-cooled 30-kw arcjet engine developed during 1962. The results of this development program indicated that the 30-kw arcjet engine described in this paper is one of the most advanced electrothermal propulsion devices at this power level yet reported.

Current Status of the Nuclear-Powered Arc Jet Rocket Engine

The present paper is in the nature of a status report on the performance and possible application to space missions of the thermal arc jet rocket engine. The following areas are discussed: 1. Arc Jet Operation and Performance 2. Preliminary Arc Jet System Considerations 3. Orbit Transfer of a 24-Hour Communications Satellite The paper includes discussions of electrode configuration, liquid versus radiation-cooled engines, and arc engine starting, and contains speculations on a battery-powered flight test, cryogenic versus noncryogenic propellants, and alternating versus direct-current arc jet operation.