A parametric study of electron energy distribution functions and rate and transport coefficients in nonequilibrium helium plasmas

Electron energy distribution functions in helium plasmas have been calculated by solving the Boltzmann equation at given values of reduced electric field, in the presence of superelastic and electron-electron collisions. Analytical expressions have been found connecting macroscopic coefficients to reduced electric field E/N, relative metastable concentration [He(2³S)]/N, and degree of ionization n_e/N.     

On the modulation of electron energy distribution function in radiofrequency SiH4, SiH4−H2 bulk plasmas

Electron energy distribution functions (EDF) in SiH₄, SiH₄–H₂ radiofrequency discharges have been calculated by solving the time-dependent Boltzmann equation in the presence of a sinusoidal field. Particular emphasis is given to the modulation of EDF as a function of the applied frequency (·10⁶/p ₀ ·10⁸ sec^–1 torr^–1) and of gas composition. The results show that at /p ₀ = ·10⁶ sec^–1 torr^–1 EDF follows in a quasistationary mode the variation of the field with the exception of a small range of electric field near to the zero crossing. Still, at the higher considered frequency (/p ₀ =·10⁸ sec^–1 torr^–1), we observe some modulation of EDF. The necessity of using a time-dependent approach is tested by comparing the present results with the corresponding ones obtained by using the effective field approximation (i.e., the approximation which solves instead of the time-dependent Boltzmann equation the corresponding stationary one at the effective values of the rf field). The two sets of results can differ by orders of magnitude in the tail of EDF, the differences decreasing with increasing molar fraction of H₂ and increasing field frequency. The role of excited states (second-kind collisions) is studied by inserting in the Boltzmann equation given concentrations of vibrational and electronic states. The results show that second-kind collisions strongly affect EDF especially in pure silane. Finally a satisfactory agreement has been found between theoretical and experimental results concerning the modulation of electrons of given energy in pure silane discharges.     

Decomposition of SiCl4 and deposition of Si in inductively coupled plasmas of H2+SiCl4

The mechanism of homogeneous reactions in plasmas of H₂+5%SiCl₄ was studied by mass spectrometry and was compared to the mechanism observed in plasmas of Ar+H₂+SiCl₄. Contrary to the behavior with Ar, the results indicate that without argon the SiCl₄ molecule undergoes only fragmentation and the deposition proceeds through SiCl₂. No polymerization was observed. The deposition rates of c-Si were lower and the amounts of chlorine incorporated in the films were higher in the plasma of SiCl₄+H₂ than in the argon-containing plasma.     

Deposition of Diamondlike Carbon Film and Mass Spectrometry Measurement in CH4/N2 RF Plasma

The deposition of diamondlike carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH₄/N₂ RF (13.56 MHz) plasma at 0.1 Torr. The film deposition rate greatly depended on both CH₄/N₂ composition ratio and RF power input. It was decreased monotonically as CH₄ content decreased in the plasma and then rapidly diminished to negligible amounts at a critical CH₄ content, which became large for higher RF power. The rate increased with increasing RF power, reaching a maximum value in 40% CH₄ plasma. The predominant ionic products in CH₄/N₂ plasma were NH⁺ ₄ and CH₄N⁺ ions, which were produced by reactions of hydrocarbon ions, such as CH⁺ ₃, CH⁺ ₂, CH⁺ ₅, and C₂H⁺ ₅ with NH₃ molecules in the plasma. It was speculated that the production of NH⁺ ₄ ion induced the decrease of C₂H⁺ ₅ ion density in the plasma, which caused a reduction in higher hydrocarbon ions densities and, accordingly, in film deposition rate. The N⁺ ₂ ion sputtering also plays a major role in a reduction of film deposition rate for relatively large RF powers. The incorporation of nitrogen atoms into the bonding network of the DLC film deposited was greatly suppressed at present gas pressure conditions.     

A comparative study of CH4 and CF4 rf discharges using a consistent plasma physics and chemistry simulator

A self-consistent, one-dimensional simulator for the physics and chemistry of radio frequency (rf) plasmas was developed and applied for CH₄ and CF₄. The simulator consists of a fluid model for the discharge physics, a commercial Boltzmann equation solver for calculations of electron energy distribution fuction (EEDF), a generalized plasma chemistry code, and an interface module among the three models. The CH₄ and CF₄ discharges are compared and contrasted: CH₄ plasmas are electropositive, with negative ion densities one order of magnitude less than those of electrons, whereas CF₄ plasmas are electronegative, with ten times more negative ions than electrons. The high-energy tail of tire EEDF in CH₄, lies below both the Druyvensteyn and Maxwell distributions, whereas tire EEDF high-energy tail in CF₄ lies between the two. For CH₄, the chemistry model was applied for four species, namely, CH₄ CH₃ CH₂, and H, whereas for CF₄, five species were examined namely CF₄, CF₃, CF₂, CF, and F The predicted densities and profiles compare favorably with experimental data. Finally, the chemistry results were fedback into the physics model until convergence was obtained.     

Current Development and Challenges of Mixed Matrix Membranes for CO2/CH4 Separation

In the early stage of membrane technology development in gas separation, utilization of polymeric membranes has gained attention due to their robustness and ease of fabrication. However, the performance of polymeric membranes is limited by the trade-off between permeability and selectivity. Meanwhile, inorganic membrane is capable to exhibit great enhancement in separation performance but unfortunately its fabrication process is hard and costly. Thus, development of mixed matrix membranes (MMMs) by incorporating inorganic fillers into the polymer matrix has become a potential alternative to overcome the limitations of polymeric and inorganic membranes in gas separation. Nevertheless, fabrication of defect-free MMMs with improved separation performance and without compromising the mechanical and thermal stability is extremely difficult and challenging. In the current review paper, various types of inorganic fillers for MMMs fabrication and recent reported efforts to tailor the underlying problems on MMMs fabrication were discussed. The future outlook to advance the performance of MMMs in gas separation especially for CO₂/CH₄ separation was highlighted.     

Enthalpy of solution and effect of temperature on activity coefficients in MSO4−H2SO4−H2O system

Results of calorimetric determination of integral enthalpies of solution of some hydrates (monohydrates and heptahydrates) of 3d transition metal sulphates such as FeSO₄, NiSO₄ and MnSO₄ in three-component systems at sulphuric acid concentrations up to 2M are reported. Measured values of integral enthalpies of solution are the basis for calculation of activity coefficient temperature dependences according to Pitzer's model.     

The mutual diffusion coefficients of Na2SO4−H2O and MgSO4−H2O at 25°C from Rayleigh interferometry

The volume-fixed mutual diffusion coefficients of Na₂SO₄–H₂O and MgSO₄–H₂O have been measured, from dilute solutions to near saturation, to an accuracy of 0.1–0.2% by free-diffusion Rayleigh interferometry. These results are compared to other available diffusion data for these salts. The diffusion coefficients of Na₂SO₄–H₂O and MgSO₄–H₂O decrease regularly with increasing concentration, while those of many other salts exhibit both a maximum and a minimum as a function of concentration. A few diffusion coefficients have also been measured for KCl–H₂O.Reference to a company or product names does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable.This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.University of Illinois at Urbana-Champaign; tenure served as a participating guest at Lawrence Livermore Laboratory.     

Modeling of H2 and H2/CH4 Moderate-Pressure Microwave Plasma Used for Diamond Deposition

One-dimensional transport models of moderate-pressure H ₂ and H ₂ /CH ₄ plasmas obtained in a diamond deposition microwave reactor are presented. These models describe the plasma as a thermochemically nonequilibrium flow with three different energy modes. The solution of the one-dimensional plasma transport equations enabled the estimation of plasma species concentrations and temperatures on the axis of the reactor. As far as pure H ₂ plasmas are concerned, results showed that the model predictions of gas and vibration temperatures are in good agreement with experimental measurements. The model also yields a relatively good qualitative prediction of the variations of H-atom mole fraction with the power density absorbed by the plasma. The results obtained for H ₂ /CH ₄ discharges showed that the model prediction on the variations of H-atom mole fraction with methane percentage in the discharge is in good qualitative agreement with experimental results. They also showed that methane is rapidly converted to acetylene before reaching the discharge zone. The concentrations of neutral hydrocarbon species in the reactor are mainly governed by thermal chemistry. The addition of methane strongly affects the ionization kinetics of the plasma. Three major ions are generally obtained in H ₂ /CH ₄ plasmas: C ₂ H ₂ ⁺ , C ₂ H ₃ ⁺ , and C ₂ H ₅ ⁺ . The relative predominance of these ions depends on the considered plasma region and on the discharge conditions. The ionic species concentrations are also mainly governed by chemistry, except very near the substrate surface. Finally the use of this transport model along with the surface chemistry model of Goodwin ⁽¹⁾ enabled us to estimate the diamond growth rate for several discharge conditions.     

Ab initio calculation of 4-ClC6H4CH2Cl and peculiarities of electron density distribution in this molecule

Ab initio calculation of the 4-ClC₆H₄CH₂Cl molecule was performed by the restricted Hartree-Fock method in the split valence 6–31 G* basis set with complete optimization of its geometry. Populations of p-orbitals of atoms of this molecule were analyzed.³⁵Cl NQR frequencies and asymmetry parameters of the electric field gradient on³⁵Cl nuclei were estimated on the basis of the populations of valent p-orbitals of CI atoms and their components. Good conformity with the experimental values was obtained when only less diffuse components of p-orbitals were used in calculations of populations.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 823–826, April, 1996.     

Synthesis,characterization and thermal analysis of [Fe(N2H4)2(CH3COO)2]

[Fe(N₂H₄)₂(CH₃COO)₂] was synthesised and characterized for the first time by chemical analysis, magnetic measurements, electronic and IR spectral studies. Its thermal reactivity was ascertained by thermogravimetric (TG) and derivative thermogravimetric (DTG) techniques and it has been concluded that unlike some other metal carboxylate hydrazinates, it does not show any autocatalytic behaviour. The decomposition was also subjected to kinetic analysis using the equations of Coats-Redfern and Horowitz-Metzger by the method of weighted least-squares.     

Reforming of CH4 with CO2 over Co/Mγ-Al oxide catalyst

A series of Co/Mg–Al oxide samples,CoMgAl-x(x=(Mg+Co)/Al molar ratio of 1–5),were prepared by the self-combustion method followed by H2reduction.The catalytic performance and stability of the samples were studied in dry reforming of CH4.XRD and H2-TPR characterization results showed that the reduced CoMgAl-x samples mainly consisted of solid solution and spinel phases with cobalt particles.The spinel phases contained Co3O4 and Con Mg1-n Al2O4(0≦n≦1)varying with the(Mg+Co)/Al ratio.The effect of (Mg+Co)/Al molar ratio on the catalytic behavior was investigated in detail and CoMgAl-3 exhibited the highest catalytic activity and stability among the catalysts studied.     

Comparative study of LiMn2O4 thin film cathode grown at high, medium and low temperatures by pulsed laser deposition

LiMn₂O₄ thin films with different crystallizations were respectively grown at high, medium and low temperatures by pulsed laser deposition (PLD). Structures, morphologies and electrochemical properties of these three types of thin films were comparatively studied. Films grown at high temperature (?873 K) possessed flat and smooth surfaces and were highly crystallized with different textures and crystal sizes depending on the deposition pressure of oxygen. However, films deposited at low temperature (473 K) had rough surfaces with amorphous characteristics. At medium temperature (673 K), the film was found to consist mainly of nano-crystals less than 100 nm with relatively loose and rough surfaces, but very dense as observed from the cross-section. The film deposited at 873 K and 100 mTorr of oxygen showed an initial discharge capacity of 54.3 μAh/cm² μm and decayed at 0.28% per cycle, while the amorphous film had an initial discharge capacity of 20.2 μAh/cm² μm and a loss rate of 0.29% per cycle. Compared with the highly crystallized and the amorphous films, nano-crystalline film exhibited higher potential, more capacity and much better cycling stability. As high as 61 μAh/cm² μm of discharge capacity can be achieved with an average decaying rate of only 0.032% per cycle up to 500 cycles. The excellent performance of nano-crystalline film was correlated to its microstructures in the present study.     

Decomposition of P2S2N4(Si(CH3)3)4A12C14 as Studied by FT-Raman and Quantum Chemistry Means

The dimeric title compound decomposes upon heating to give the monomer and desulphurized monomer as shown by FT-Raman and quantum chemical means.     

Mutual diffusion coefficients,electrical conductances,osmotic coefficients,and ionic transport coefficientsl ij for aqueous CuSO4 at 25°C

Experimental thermodynamic and transport data are presented for aqueous CuSO₄ solution at 25°C from low concentrations to near saturation. Included are diffusion coefficients (Rayleigh interferometry), electrical conductances (DC method), and osmotic coefficients (isopiestic). These data and corresponding literature data are critically compared, as are literature density data. The resulting best values, together with previously compared transference number results, provide an extensive set of critically reviewed data for aqueous CuSO₄. Onsager coefficientsl _ij have been calculated for this salt and are compared with data for other valence types.     

Isopiestic determination of the osmotic and activity coefficients of the {yKCl + (1 − y)K2HPO4}(aq) system at T = 298.15 K

The osmotic coefficients of aqueous mixtures of KCl and K₂HPO₄ have been measured at T = (298.15 ± 0.01) K by the isopiestic vapor pressure method over the range of ionic strengths from (2.3700 to 11.250) mol · kg⁻¹ using CaCl₂(aq) as the reference solution. Our new experimental results were modeled with an extended form of Pitzer’s ion-interaction model equations, both with the usual mixing terms and with Scatchard’s neutral–electrolyte mixing terms, and with the Clegg–Pitzer–Brimblecombe equations based on the mole-fraction-composition scale. There is a dearth of previously published isopiestic data for mixtures containing salts of and, consequently, no previous measurements are available for comparison with the present results. The present study yields mixing parameters for these three models that are needed for modeling the thermodynamic activities of solute components of natural waters and other complex aqueous electrolyte mixtures.     

The mutual diffusion coefficients of NaCl−H2O and CaCl2−H2O at 25°C from Rayleigh interferometry

The volume-fixed mutual diffusion coefficients of NaCl–H₂O and CaCl₂–H₂O have been measured to an accuracy of 0.1–0.2%, from dilute solutions to high concentrations, by free diffusion Rayleigh interferometry. These diffusion coefficients are compared to other diffusion data for these salts, obtained from Guoy interferometry and the conductometric method. Discrepancies between data in the literature are resolved using the present results. Some diffusion coefficients have also been measured for KCl–H₂O and NH₄Cl–H₂O.Work performed in part under the auspices of the U.S. Department of Energy by the Lawrence Livermore Laboratory under contract number W-7405-ENG-48.Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Department of Energy to the exclusion of others that may be suitable.This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.University of Illinois at Urbana-Champaign; tenure served as a participating guest at Lawrence Livermore Laboratory.     

Viscosity of aqueous NH4Cl and tracer diffusion coefficients of ions,water and non-electrolytes in aqueous NH4Cl,KI, and MgCl2 at 25°C

Viscosities for aqueous NH₄Cl and tracer diffusion coefficients for²²Na⁺,³⁶Cl^–, HTO, and CH₃OH, acetone and dimethylformamide (all¹⁴C-labelled) in NH₄Cl supporting electrolyte are reported for 25°, together with tracer diffusion coefficients for²²Na⁺,³⁶Cl^–, and¹⁴CH₃OH in 1M KI, and for¹⁴CH₃OH in 1M MgCl₂. The diffusion coefficient of HTO in NH₄Cl solutions is slightly larger, for most of the concentration range investigated (0.05 to 4.5 M), than the value for pure water and is almost unaffected by the supporting electrolyte up to about 4M. Similar behavior is shown by CH₃OH, acetone and dimethylformamide in NH₄Cl solutions. Onsager limiting law behavior is approached by Cl^– at NH₄Cl concentrations in the 0.05–0.1M region but at much lower concentrations by Na⁺.     

Ab initio study of the potential energy surface and product branching ratios for the reaction of O(D) with CH3CH2F

The potential energy surface of O(¹D) + CH₃CH₂F reaction has been studied using QCISD(T)/6-311++G(d,p)//MP2/6-311G(d,p) method. The calculations reveal an insertion–elimination reaction mechanism of the title reaction. The insertion process has two possibilities: one is the O(¹D) atom inserting into C–F bond of CH₃CH₂F produces one energy-rich intermediate CH₃CH₂OF and another is the O(¹D) atom inserting into one of the C–H bonds of CH₃CH₂F produces two energy-rich intermediates, IM1 and IM2. The three intermediates subsequently decompose to various products. The calculations of the branching ratios of various products formed though the three intermediates have been carried out using RRKM theory at the collision energies of 0, 5, 10, 15, 20, 25 and 30 kcal/mol. CH₃CH₂O is the main decomposition product of CH₃CH₂OF. HF and CH₃ are the main decomposition products for IM1; CH₂OH is the main decomposition product for IM2. Since IM1 is more stable and more likely to form than CH₃CH₂OF and IM2, HF and CH₃ are probably the main products of the O(¹D) + CH₃CH₂F reaction. Our computational results can give insight to reaction mechanism and provide probable explanations for future experiments.     

Thermodynamics of the system H2O−NaCl−Na2SO4−MgSO4 to the saturation limit of NaCl at 25°C

Isopiestic results are reported for the quaternary system H₂O–NaCl–Na₂SO₄–MgSO₄. The excess free energies for mixing the double salt Na₂ Mg(SO₄)₂ with NaCl are fairly large and negative, as also are the free energies for mixing the three salts to form the quaternary aqueous system.