Helium-hydrogen Gas Mixtures Research Articles

Anomalous heat transport in binary hard-sphere gases.

Equilibrium and nonequilibrium molecular dynamics (MD) are used to investigate the thermal conductivity of binary hard-sphere fluids. It is found that the thermal conductivity of a mixture can not only lie outside the series and parallel bounds set by their pure component values, but can lie beyond even the pure component fluid values. The MD simulations verify that revised Enskog theory can accurately predict nonequilibrium thermal conductivities at low densities and this theory is applied to explore the model parameter space. Only certain mass and size ratios are found to exhibit conductivity enhancements above the parallel bounds and dehancement below the series bounds. The anomalous dehancement is experimentally accessible in helium-hydrogen gas mixtures and a review of the literature confirms the existence of mixture thermal conductivity below the series bound and even below the pure fluid values, in accordance with the predictions of revised Enskog theory. The results reported here may reignite the debate in the nanofluid literature on the possible existence of anomalous thermal conductivities outside the series and parallel bounds as this Rapid Communication demonstrates they are a fundamental feature of even simple fluids.

Stark broadening of the hydrogen Paschen ? transition at electron densities of the order of $ \mathsf {10^{15}}$ cm $ \mathsf {^{-3}}$

Stark broadening measurements and calculations of the Paschen γ spectral line of hydrogen ( λ = 1.094[: MU :] m) are reported. Investigations have been performed at plasma electron densities between 1.4×10 15 cm -3 and 3.7×10 15 cm -3 . As the light source a wall-stabilized arc operated in a helium-hydrogen gas mixture at atmospheric pressure has been applied. The radiation of the plasma emitted from nearly homogeneous plasma layers in end-on direction, was measured with the use of a grating spectrometer equipped with a charge coupled device (CCD) detector. The radiance calibration was carried out against light outputs originating from a tungsten strip radiation standard. The measured FWHM are compared with results of our calculations based on computer simulation techniques (full computer simulation method -- FCSM). Our broadening data are also compared with results of other theoretical approaches (MM-method, quasi-static approximation) and with experimental data obtained at electron densities about one order of magnitude larger than ours.

Experimental and theoretical Stark broadening studies of the hydrogen Paschen beta line.

Experimental and theoretical Stark broadening studies of the Paschen beta line of hydrogen (lambda=1.28 microm) are reported. Line shape measurements were performed at electron densities of the plasma between 3.5 x 10(15) cm(-3) and 7.5 x 10(15) cm(-3), applying as the light source a wall-stabilized arc operated at atmospheric pressure in a helium-hydrogen gas mixture. The radiation of the plasma, emitted from nearly homogeneous plasma layers in the end-on direction, was registered with the use of a grating spectrometer equipped with a charge coupled device detector. The measured light outputs were calibrated against signals obtained from a tungsten strip radiation standard. The experimentally determined line profiles are compared with results of new Stark broadening calculations based on simulation techniques. The measured broadening, shift, and asymmetry parameters are also compared with results of previous Stark broadening calculations and other experimental data obtained at electron densities higher as well as lower than ours.

Kinetics of reduction of nickel oxide with helium–hydrogen gas mixtures in the range 300–400°C

The effect of hydrogen pressure on the reduction kinetics of nickel oxide was investigated by thermogravimetry at temperatures of 300, 350 and 400°C. Pore-diffusion resistance to the reduction was minimized by the use of thin, dense, flat specimens (12 mm × 12 mm × 54 µm) of NiO(s). The partial pressure of H2 in the helium-hydrogen gas mixtures was varied from 0.109 to 1.0 atm (11 044 to 101 325 Pa) in each of the three sets of isothermal experiments. The reduction of nickel oxide appears to be first-order with respect to the H2 pressure.

A note on the spin-polarization of hydrogen atoms in a helium–hydrogen gas mixture

In a helium–hydrogen mixture at T = 300 K, the polarization degree of hydrogen spins, both electron and nuclear, was determined to be as high as (5 ± 1)%.The role of the main processes giving rise to the formation of spin-polarized H atoms has been discussed. It is shown that an effective transfer of angular momentum from the preliminarily optically oriented He*(23S1) atoms to the H atoms is not due to the direct chemi-dissociation of the H2 molecules by the He* atoms alone, or due to Penning ionization of these molecules followed by the ion-molecular rearrangement reaction H2+ + H2 → H3+ + H alone, but to their summary action. The relative importance of the two mechanisms depends on the rotational temperature and total gas pressure.

On chemical vapour deposition of Si1-(x+y)GexBy thin films with very high or very low boron contents. 2. Very high boron contents and amorphous state

For part I see ibid, vol.9, p.1666 (1994). Total growth rate and composition data of amorphous Si1-(x+y)GexBy thin films grown from silane, germane, diborane and helium-hydrogen gas mixtures at 500 degrees C and 0.2 Torr are the subject of a closer examination. Data are interpreted against the background of the 'three partial rates' model taking into account the results published in part 1. Besides the partial pressures of the source gases, total pressure and temperature, the total atom number per cm3 of the thin film also controls layer growth rate. Additional silicon deposition is principally caused by the presence of germane or diborane in the gas phase. At a deposition temperature of 500 degrees C, however, germane does not cause additional silicon deposition, but diborane does. In the binary Si/Ge and Si/B systems the rate of additional silicon deposition depends on both the partial pressure of the respective source gas and the composition of the thin film. In the ternary Si/Ge/B system, however, the partial growth rate of additionally deposited silicon remains constant as long as, for instance, the diborane partial pressure is unchanged, though the boron content of the film is varied by a variation of the germane partial pressure.

Application of SAES and HWT gas purifiers forthe removal of impurities from helium-hydrogen gas mixtures

An experimental programme is described which is performed in the framework of a tritium technology task for the Next European Torus (NET). The aim is to investigate the applicability of commercial gas purifiers for the removal of gaseous impurities such as N 2, CO, and CO 2 from a helium-hydrogen or a helium-tritium gas stream. In addition, it is intended to recover chemically bound hydrogen isotopes by decomposition of hydrocarbons, ammonia and water. Experimental results obtained in the PEGASUS facility are reported for the sorption of CH 4 by three non-uranium getters as a function of temperature and gas flow rate, the enhancement of CH 4 sorption efficiency by continuous removal of hydrogen, and cosorption behaviour of CO, N 2 and CH 4 in the presence of hydrogen. Gettering of CO and N 2 was found to be much more effective than that of CH 4. At temperatures above 200°C, an increasing tendency for the reaction 2H 2+CO→CH 4+1/2O 2 was found to occur during the passage of the gas through the getter. This reaction can be suppressed by using two getters in series: the first is operated at about 250°C to remove O 2, H 2O, CO and CO 2, and the second at 400°C or more for the removal of residual amounts of these species as well as for the removal of N 2 and CH 4.

Measurements of the Stark Broadening of Two Neutral Helium Lines in a Plasma

A detailed study has been made of the broadening of the lines $\ensuremath{\lambda}3889$ \AA{} (width only) and $\ensuremath{\lambda}5016$ \AA{} (shift and width) of neutral helium in the plasma produced in an electromagnetically driven T tube operating in helium-hydrogen gas mixtures. The electron density in the plasma was found from the broadening of ${\mathrm{H}}_{\ensuremath{\beta}}$, while the temperature was determined from the ratio of the intensity of ${\mathrm{H}}_{\ensuremath{\beta}}$ to that of the underlying continuum. Temperatures were in the range 20 000-30 000\ifmmode^\circ\else\textdegree\fi{}K; electron densities varied from ${10}^{16}$ to 6\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. The broadening of the line $\ensuremath{\lambda}3889$ \AA{} was found to increase linearly with electron density; and it compared well with calculations based on the generalized impact theory, except for (constant) correction factors ranging from about 0.9 to 1.0 (depending on which of the various calculations is referred to). For the line $\ensuremath{\lambda}5016$ \AA{}, neither of the previous calculations, nor a correction discussed here, predicts either the width or the shift variations satisfactorily; all the calculations give significantly wider lines above electron densities of ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ than are found experimentally. Also, as the electron density increases, the calculated shifts all become increasingly larger than those found experimentally. However, at the low densities most deviations are small (\ensuremath{\lesssim}20%).