Ar-H2 Gas Mixture Research Articles

Formation of iron boride layers on steel by d.c.-plasma boriding and deposition processes

Boriding and deposition of iron borides on Armco iron and the steels Ck45 and 42CrMo4 have been investigated at temperatures below 833 K using BCl3–H2–Ar gas mixtures activated by pulsed d.c. glow discharges. For the layer deposition FeCl3 and Fe(CO)5 were used as precursors. A boriding depth of several micrometres was achieved, in which the microhardness amounts to 1500–2000 HV0.02. FeB and Fe2B layers could only be obtained with Fe(CO)5 as iron precursor. The incorporation of carbon in the layers can be controlled by the process pressure. Microhardness values above 2000 HV0.02 are achieved at low carbon content in the layers. In order to deposit well-adherent Fe2B layers on borided substrates intermediate FeB layers or inclusions have to be avoided.

Pore nucleation in solidifying high-purity copper

High-purity copper (6 or 7 N) was melted and solidified unidirectionally in the atmosphere of a H2-Ar gas mixture for the purpose of studying the mechanism of pore nucleation in solidifying metal. Hydrogen content in the melt was controlled by changing the partial pressure in the atmosphere. Pores were formed when the hydrogen partial pressure in the atmosphere was 0.3 atm or more. Oxides of aluminum and silicon were observed at the bottom of the pores and the pores were nucleated heterogeneously. Water vapor with a very low partial pressure existed in the furnace atmosphere, and the melt must have contained a small amount of oxygen in equilibrium with this water vapor. The solid/liquid (S/L) interface was planar and convection was eliminated. The redistribution of the solute during solidification can, therefore, be estimated. The concentration of oxygen in the liquid at the S/L interface is estimated to be much larger than its initial concentration, due to the very small equilibrium distribution coefficient of oxygen in copper, and aluminum and silicon were oxidized even though their concentrations were very low. The probability of homogeneous nucleation by α particles was very small in these experiments.

D.c. arc plasma deposition of smooth nanocrystalline diamond films

We report on chemical vapour deposition growth of nanocrystalline (grain size 30–50 nm) diamond films of 100 nm to 2 μm thickness in methane-rich mixtures. A d.c. arc discharge in CH4H2Ar gas mixtures with a methane percentage CH4(CH4 + H2) varied from 10% to 100% was used for diamond deposition on Si substrates seeded with ultrafine (5 nm) diamond particles. The films obtained were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and X-ray excited Auger electron spectroscopy. Remarkably well-crystallized diamond films were produced even in hydrogen-free gas mixtures. Raman spectra confirmed the nanocrystalline structure of all the films examined. The film hardness measured with a nanoindenter was in the range 70–85 GPa typical for diamond, the highest values corresponding to 100% methane content. The films were very smooth with surface roughness Ra < 20 nm. The essential improvement in surface smoothness was obtained by means of laser-induced disintegration of coalesced seeding particles.

Deoxidatlon rate of copper droplet levitated in Ar-H2 gas stream

Levitated copper droplets, 5 mm in diameter with initial oxygen contents of 0.036 to 1.9 wt pct, were deoxidized at about 1970 K in an Ar-H2 gas stream. The Ar-H2 gas mixture having hydrogen partial pressure less than 4 kPa was introduced into a silica reaction tube of 11-mm ID at gas flow rates up to 2 x 10-4 Nm3s-1. The effects of initial oxygen content of the droplets, hydrogen partial pressure, and gas flow rate on the deoxidation process were examined. A mixed control model for the deoxidation rate involving both gas and liquid film mass-transfer resistances was combined with a thermodynamic relationship for the dissolved species in molten copper. The value of 2 × 10-4 m 73x00D7; s-1 was assigned to the liquid film mass-transfer coefficient of dissolved oxygen throughout all experimental conditions. Under the experimental conditions of low initial oxygen content and high hydrogen partial pressure, the liquid film mass-transfer resistance was significant. When a droplet of high initial oxygen content was deoxidized, transition phenomena from gas to liquid film mass-transfer control were noticed in the later stage of reaction. It was deduced from the present model that the accumulation of dissolved hydrogen was indispensable to these phenomena.

Mechanical properties of a-Si:H films studied by Brillouin scattering and nanoindenter

A series of a-Si:H films has been prepared by rf sputtering in a H2-Ar gas mixture. To obtain films with different hydrogen content the hydrogen portion of the gas mixture was changed from 0% to 20%. The shear modulus μ was then measured by the frequency of the surface phonon (Rayleigh wave). The stiffness S and the ultramicrohardness H were measured by using a nanoindenter. From the shear modulus μ and the stiffness S, the Young’s modulus E and Poisson’s ratio ν were calculated. The intrinsic mechanical stress was measured by the bending-beam method. With increasing hydrogen content of the films, the decrease of Young’s modulus, microhardness, and internal stress are observed.

Microstructural characterization of plasma-sprayed oxide ceramics.

Microstructural and microchemical characterization has been carried out on plasma-sprayed zircon (ZrSiO4) and alumina-chromia (Al2O3–5wt%Cr2O3) by transmission electron microscopy and energy dispersive X–ray analysis. The as–sprayed zircon coatings consist of zirconia and silica glass as a result of phase separation during rapid cooling in the plasma–spraying process. Zirconia shows a tetragonal structure for small particles and a monoclinic for large particles. The zircon coatings annealed at 1 300°C for 96 h in air show a mixture of zircon and monoclinic zirconia phases. Alumina–chromia coatings plasma–sprayed with N2 gas consist mainly of a polycrystalline alumina–chromia solid solution with a typical grain size of 0.4 μm. Fine precipitates are observed frequently along the grain boundaries in the coatings sprayed with Ar–H2 gas mixture or N2–H2 gas mixture. The fine precipitates are a metallic Cr–rich Cr–Al alloy formed by reduction of Cr2O3 and Al2O3 during the plasma-spraying process. These precipitates disappear during annealing at temperatures higher than 1 300°C for 96 h in air. The precipitates affect the wear resistance of the coatings.

The rate of formation of SiO by the reaction of CO or H2 with silica and silicate slags

The rate of formation of SiO(g) by the reaction of CO and H2 with silica or silicate slags containing SiO2, CaO, and A12O3 has been measured. The rate with hydrogen for both pure silica and silica in slags is controlled by gas phase mass transport. The rate changed with flow conditions, sample size, and gas properties as predicted by the appropriate mass transfer equations. In particular, the rate is faster in H2-He than in H2-Ar gas mixtures with the same hydrogen pressure, which proves conclusively the rate is controlled by gas phase mass transport. The rate of formation of SiO by the reaction of silica with CO did not change with flow conditions or particle size and the rate was the same in CO-He and CO-Ar with the same pressure of CO. In addition, the calculated rate with CO for mass transfer is considerably faster than was observed. These observations strongly indicate the rate is controlled by chemical kinetics on the surface. Similar results were obtained for the rate of reaction of CO with silica in slags and the rate was found to be proportional to the activity of SiO2 and CO pressure. The present results are discussed in relationship to silicon transfer in slag-metal reactions.

Mössbauer spectroscopy of amorphous silicon–tin–hydrogen alloys

Amorphous–Si1−x Snx :H alloys have been prepared by dc and rf sputtering of composite Si–Sn targets in Ar–H2 gas mixtures. 119Sn conversion electron Mössbauer spectroscopy and x-ray diffraction reveal the Sn to be incorporated into the films in at least four distinct sites, the relative populations of which depend strongly on preparation conditions. The Mössbauer resonance from the substitutional Sn in the a-Si:H matrix is well characterized by a quadrupole splitting of 0.46±0.05 mm/s independent of x and preparation conditions. Experimental procedures have been found such that samples can be produced which contain only the substitutional Sn site.

Control of Optical Gap in a-SixC1-x: H Alloy Films Produced by Reactive Sputtering Method

Hydrogenated amorphous silicon-carbon alloy (a-SixC1-x: H) films are prepared by simultaneous if reactive sputtering of silicon and graphite in a H2–Ar gas mixture. The optical gap E0 of a-SixC1-x: H depends on alloy composition x and preparation conditions. The optical gap E0 of a-SixC1-x: H prepared under a same condition is ∼1.7 eV at x=1.0 (a-Si: H), increases with the decrease in x, reaches a maximum value of 2.2 eV at x=0.6 and then decreases with decrease in x, reaching ∼1.4 eV at x=0 (a-C: H). For films prepared at lower substrate temperature, the optical gap becomes larger. This behavior of the optical gap is discussed referring to the results of X-ray photoemission spectroscopy, infrared absorption measurements, etc., on a-SixC1-x: H.

Compositional and structural properties of amorphous SixC1−x : H alloys prepared by reactive sputtering

Amorphous SixC1−x : H alloys are prepared by simultaneous rf reactive sputtering of silicon and graphite in a H2-Ar gas mixture. Silicon, carbon, and hydrogen contents are measured for the entire range of x by electron spectroscopy for chemical analysis (ESCA), Rutherford-backscattering method, and thermal evolution of hydrogen. Evolution temperature dependence of the number of evolved hydrogen atoms is measured. The hydrogen-evolution behavior and the optical gap are x dependent. These phenomena are discussed in the light of chemical-bonding states.

Determination of Radial Temperature Distribution in Argon-Hydrogen Plasma Jet

In the case when the plasma temperature is below the calibrating temperature required in the Fowler-Milne method, a new method for determining the radial temperature distribution in an Ar-H2 gas mixture plasma jet is described. It is shown that the radial temperature distribution can be determined by combining the radial intensity distribution for the λ4200ArI line and the Stark profile for the Hβ line. This method requires only measurement of the relative intensity for a spectral line.