Liquid Kr Research Articles

Precipitation of Kr In Ni at Room Temperature

AbstractThe fluence dependence of Kr precipitation in Ni at room temperature has been studied with the aid of Transmission Electron Microscopy. As in other metals, the Kr precipitates in small cavities. Electron diffraction demonstrates that the Kr precipitates are solid, fcc crystals aligned with each other and the Ni lattice. The trends are similar to those observed for Kr precipitation in Al at room temperature. The average Kr lattice parameter, determined from the electron diffraction, increases with increasing Kr fluence from 0.515 nm to an asymptotic value of 0.545 nm. The asymptotic limit is due to the melting of the larger Kr precipitates. The mismatch between the Kr and Ni lattices is as large as 55%. Diffuse electron scattering was observed from large, liquid Kr precipitaes. This occurs for Kr fluences above 5.1020 Kr+ m-2 in Ni and above 2.5.1020 Kr+ m-2 in Al. At room temperature, the largest solid Kr precipitate observed in dark field images was 8.3 nm in diameter compared to 4.7 nm in Al. The larger precipitates are liquid or gas. The solid Kr metals at the same lattice parameter in both Ni and Al suggesting that the melting is thermodynamic in nature and independent of the host material.

Drift Velocity of Hot Electrons in Liquid Ar, Kr, and Xe

The drift velocity of hot electrons in liquid Ar, Kr, and Xe near their triple points ia studied using a Cohen-Lekner equation in which an inelastic depolarized electron scattering process is newly considered. The saturation of the drift velocity at the high field strength is appropriately explained. The magnitude and onset energy of the effective inelastic cross section are compared with the experimental data of inelastic light scattering spectra.

Heating and diffusion of hot electrons in non-polar liquids

The drift velocities of excess electrons in liquid Ar, Kr and Xe under strong heating fields (up to field strengths of about 106V cm-1), their mean energy, longitudinal and transverse diffusion coefficients and respective characteristic energies are calculated in the framework of the Lekner-Cohen theory (1967). Computed values of the characteristic transverse energy are considerably greater than the mean electron energy and attain a few tens of eV. The transverse diffusion coefficient is larger than the longitudinal coefficient by a factor of a hundred. These effects are caused by the sharp increase of the structure factor of liquids for wavenumbers corresponding to energies of a few eV. The behaviour of the structure factor is also responsible for the non-monotonic behaviour of the electron drift velocity as a function of the electric field strength. The results of these calculations are compared with available experimental data.

Absorption spectra of cold dilute solid solutions

Infrared absorption spectra have been obtained for some compounds trapped in crystalline solids by freezing liquid Xe, Kr, Ar, or CH4 solutions. The optical quality of the solid solutions is good, and they have been cooled to ∼80 K in 1.35 cm sample thicknesses to study the absorption in fundamental vibrational bands of the solutes. In the cases discussed, the bands are narrow, with observed full widths at half-maximum absorbance 0.05–0.30 cm−1 greater than the instrumental resolution (0.18–0.29 cm−1). The spectra appear to be free of ‘‘multiple site’’ and solute aggregate absorptions. Spectra displaying isotropic splitting in bands of natural BCl3, SeF6, OsO4, TiCl4, and MoF6 are presented, and band frequencies are compared with some results obtained in evaporative matrices, in the gas phase, and in liquid solutions. For this comparison we have obtained some spectra of SeF6 and BCl3 gas.

Comments on Ascarelli’s papers on electron trapping in liquid Ar, Kr, and Xe

The criticisons of Ascarelli1,2 regarding Shibamura et al5’s measurements of electron drift velocity in liquid Ar, Kr and Xe are answered. (AIP)

A hydrogen-bonded system in a liquefied noble gas as an underdamped anharmonic stochastic oscillator

IR spectra of HCl complexes with acetonitrile, olefins have been studied and also for the comparison of methanol polymers in solutions of liquefied Xe, Kr, Ar and N 2 . The half-width of the band for HCl complexed with acetonitrile in liquid Kr is 35 cm −1 at 165 K; this is half of that value in Xe at the same temperature and an order of magnitude lower than in standard solvents. Temperature narrowing of this band down to 30 cm −1 has been found on cooling the solution to 120 K. The temperature shift of HCl bands in complexes of moderate strength not referring to self-association was observed. The results obtained are in agreement with the representation of a hydrogen-bonded system as an underdamped ahnarmonic stochastic oscillator.

Dielectric breakdown of cryogenic liquids in terms of pressure, polarity, pulse width and impurity

In liquid Ar (liq.Ar) and liquid Kr (liq.Kr), the breakdown voltage is insensitive to pressure and the electron avalanche seems to play an important role. In liquid N 2 (liq.N 2), the breakdown voltage increases with pressure, but much less in nsec range. The polarity effect of breakdown in liq.N 2, reverses with pulse width, pressure and electrode distance. Bubble process plays an important role in long time range and under low pressure, but positive streamer plays predominant role in short time range and under high pressure. In liquid He (liq.He), material and surface condition of cathode metal influence strongly on breakdown but that of anode influences little. Breakdown characteristics in superfluid and supercritical states are also studied. Breakdown characteristics of liquid Ne (liq.Ne) is similar to those of liq.He. Impurities also influence breakdown of cryogenic liquids.

Comment on the paper ‘‘The role of shallow traps on the mobility of electrons in liquid Ar, Kr, and Xe,’’ by G. Ascarelli, J. Chem. Phys. 71, 5030 (1979)

The electron drift velocity in liquid Ar, Kr and Xe is furthe discussed. It is proposed that the electron transport occurs via shallow traps. Some experimental evidence is cited for the presence of hot electrons in liquefied rare gases. (AIP)

Mobility of positive ions in liquid krypton

New experimental data of positive ion mobility in Kr, along the liquid branch of the coexistence curve, are reported. The results are compared with the theory of Davis et al. and the agreement is very satisfactory if we use experimental radial distribution functions. The validity of such a theory in the case of self-diffusion in liquid Kr is also discussed.

Response to the critique to the paper ‘‘The role of shallow traps on the mobility of electrons in liquid Ar, Kr, and Xe’’

In response to the critique of the previous papers by Freeman and some of the comments by Schnidt et al., it is shown that the electric field dependence of the drift velocity is inconsistent with the hot electron model. The information obtained from the addition of polyatomic impurities to the liquified rare gases is inconsistent with a picture of an average electron energy that is a large fraction of 1 eV. In the following paper it will be shown that the field dependence of the trapping rate constant of electrons with SF6 and O2 can be quantitatively explained with no reference to hot electrons. (AIP)

On the role of shallow traps in electron transport in liquid Ar, Kr, and Xe

The drift velocity of electrons in gaseous or liquid argon, krypton, or xenon is roughly independent of applied electric field strength in the vicinity of 1×10−18 V cm2/molecule. The drift velocity ’’saturation region’’ is not completely flat and varies somewhat with density. The value of the saturation drift velocity is independent of density in the gas phase nearly up to critical, but in the subcritical liquid phase it increases roughly in proportion to the density. The behavior is not consistent with the participation of thermal electrons and shallow traps. It is consistent with electron heating by the field, and the energy dependence of the elastic scattering cross section of the fluid. (AIP)

Dependence of Dielectric Breakdown of Liquids on Imolecular Sturcture

High field carrier transport, electron multiplication and breakdown in liquids of various molecular structure are discussed. Breakdown characteristics depend on the molecular structure of liquids. Breakdown strength and its polarity effect are determined by the combination of the energy input from electric field and energy loss which depend on electron mobility and internal vibrational mode respectively, therefore on molecular structure. In rare gas liquids with high electron mobility like liquid Ar, Kr and Xe the electron avalanche is the mechanism of the breakdown. However, in liquid He, electron injection from cathode is the determining process for breakdown and the polarity effect of breakdown in liquid He is completely different from other liquids. In hydrocarbon liquids of ultra-high purity, which have high electron mobility in the case of spherical molecules, and also even in the case of linear molecules at high fields, the electron avalanche (or positive streamer) is also important in the short time range (nsec). However, thermal effects are significant in the long time range. Change of breakdown mechanism from bubble process to positive streamer process with decreasing pulse width and increasing pressure is indicated clearly in liquid N2 as an example. Effect of impurities on breakdown is also dependent on the molecular structure of liquids and solutes. Laser induced breakdown is strongly dependent on the molecular structuire and also can be explained in terms of electron avalanche by the high-frequency breakdown theory.

Are there hot carriers in liquid argon, krypton, and xenon?

It is shown that the saturation of the drift mobility observed in liquid Ar, Kr, Xe, and other insulating liquids is not a hot electron effect but can instead be understood as an effect due to shallow traps. Estimates of the binding energy of such traps in Ar indicate a value approx. 90 MeV. The effect of the addition of impurities can be understood as enhancing a Schottky ionization process.

The role of shallow traps on the mobility of electrons in liquid Ar, Kr, and Xe

It is shown that the saturation of the drift mobility observed in liquid Ar, Kr, Xe and other insulating liquids is not a hot electron effect but can instead be understood as an effect due to shallow traps. Estimates of the binding energy (B.E.) of such traps in Ar indicate a value ∼90 meV. The effect of the addition of impurities can be understood as enhancing a Schottky ionization process.

The long wavelength limit of the liquid structure factor-a theory for the compressibility of liquid rare gases and metals

A theory for the isothermal compressibility of monatomic liquids at temperatures close to their triple points is presented. Assuming that the atoms interact through pairwise potentials, an exact relation for the direct correlation function of the fluid in terms of that of a reference fluid is derived. This is used to construct approximations for the long wavelength behaviour of the liquid structure factor a(q). By dividing the pairwise potential into a reference part, representing the short range repulsive forces, and a perturbation part, representing the long range 'attraction', the compressibility can be expressed in a mean field fashion in terms of an effective hard sphere contribution modified by the perturbation potential. The theory has been used to calculate the long wavelength limit a(q=0) in liquid Ne, Ar, Kr and CH4 and in a wide range of liquid metals.

Analysis of translational band observed in gaseous and liquid Kr–Ar mixtures

The translational absorption spectrum of Kr–Ar gaseous and liquid mixtures, arising from the collision-induced dipole moment, has been measured in the 13–300 cm−1 frequency region. From the translational band obtained in the gaseous phase we have derived the intensity and the range ρo of the induced dipole moment. The high frequency part of the spectrum observed in the gaseous phase is similar in shape to that observed in the liquid phase. The low frequency part of the spectrum obtained in the latter case is dominated by the intercollisional interference effect and is discussed in terms of a simple model for the motion of the impurities in the liquid. We have also derived the value of the mean square force 〈F2〉 acting on the impurity dissolved in the liquid matrix.

Effect of molecular solutes on the electron drift velocity in liquid Ar, Kr, and Xe

Measurements of the electron drift velocity in liquid argon, krypton, and xenon were performed in an electric field up to 100 kV ${\mathrm{cm}}^{\ensuremath{-}1}$. At higher field strengths saturation velocities were observed in agreement with other authors. The addition of a small concentration of molecular solutes leads to an increase of the electron drift velocity above the saturation value of the pure liquid. The drift velocity either reaches a higher constant value or passes through a maximum at field strengths greater than ${10}^{4}$ V ${\mathrm{cm}}^{\ensuremath{-}1}$. This effect was investigated as a function of solute concentration for ${\mathrm{N}}_{2}$, ${\mathrm{H}}_{2}$, methane, ethane, propane, and butane. Inelastic energy losses in collisions of electrons and solute molecules are assumed and by means of the Cohen-Lekner theory the energy dependence of the loss processes is derived.

Rotational relaxation of symmetric tops CH3D and CD3H in some inert liquids

Infra-red spectra of CH3D and CD3H dissolved in liquid Ar, Kr, Xe, N2, O2 and CH4 or CF4 have been recorded. From the band shapes, rotational correlation functions, band moments and intermolecular mean square torques have been calculated. To describe the tumbling motion, involved in the parallel band profiles, we have computed the theoretical correlation functions in the m or j diffusion limits. The validity of these Gordon models is discussed. In an attempt to obtain some information on the rotational diffusion tensor, experimental correlation times have been estimated from A1 and E bands of CD3H.

Charge Transport in Solid and Liquid Ar, Kr, and Xe

This paper reports an investigation of the drift velocity of excess electrons in solid and liquid Ar, Kr, and Xe. After purification of the commercially available gas, thin crystal specimens (100-600 \ensuremath{\mu}m) were grown from the liquid between parallel electrodes in a chamber attached to a miniature cryostat. Pulses of 40-keV electrons were used to generate the charge carriers in both liquids and solids. This technique overcomes the limitations inherent in previously applied methods and has made it possible to investigate the drift velocity over a range of applied fields from 10 V ${\mathrm{cm}}^{\ensuremath{-}1}$ to 100 kV ${\mathrm{cm}}^{\ensuremath{-}1}$. Near the triple point, the low-field mobility ${\ensuremath{\mu}}_{0}$ in solid Ar, Kr, and Xe was found to be 1000, 3700, and about 4500 ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$, respectively. In the liquids the corresponding mobilities were 475, 1800, and 2200 ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$. The temerature dependence of ${\ensuremath{\mu}}_{0}$ has been measured on Ar crystals, and the results indicate that ${\ensuremath{\mu}}_{0}$ is determined by acoustic scattering. The electron lifetime appears to be controlled predominantly by oxygen impurities. Pronounced hot-electron effects are observed in drift-velocity---versus---field curves for both liquids and solids, and their fit to the Shockley hot-electron theory has been investigated. In the high-field region all the curves show an almost complete saturation with field. The theory of Cohen and Lekner applied to liquid Ar fits the results over most of the field range, suggesting that the deviations from the Shockley theory at higher electron temperatures are associated with an increase in the value of the structure factor. In solid Ar or Kr, positive holes do not appear to be mobile, but in Xe crystals a hole mobility of about 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{2}$${\mathrm{sec}}^{\ensuremath{-}1}$${\mathrm{V}}^{\ensuremath{-}1}$ was found. The implications of these results are briefly discussed.

Free-Carrier Drift-Velocity Studies in Rare-Gas Liquids and Solids

Free-carrier drift-velocity studies were made in liquid and solid Ne, Ar, Kr, and Xe samples, using a crystal counter technique. Electron-ion or electron-hole pairs were generated in the samples by means of a ${\mathrm{Po}}^{210}$ $\ensuremath{\alpha}$-particle source which was electrochemically deposited on one electrode of the parallel electrode arrangement. Transit times of carriers drifting across the electrode spacing were determined from pulses displayed on an oscilloscope and recorded photographically. The number of ion pairs escaping from each $\ensuremath{\alpha}$-particle track was found to be electric-field-dependent, and the values obtained were less than half the corresponding number observed using $\ensuremath{\alpha}$-particle excitation in the gaseous phase of the same materials. In fields greater than about 10 kV/cm, saturated electron drift velocities were observed. Values of the saturated electron drift velocities in solid Ne, Ar, Kr, and Xe are 1.8, 1.36, 0.95, and 0.6\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/sec, respectively, while in liquid Ar and Kr, the respective values were 0.6 and 0.35\ifmmode\times\else\texttimes\fi{}${10}^{6}$ cm/sec. Charge transport by holes as well as electrons were observed only in solid Xe. An argument is given which suggests that the results observed can be qualitatively explained in terms of a hot-electron model, with inelastic scattering by molecular impurities proposed as the velocity-limiting interaction.