Tin atom formation in a graphite furnace atomizer

Absorbance measurements resolved in time and space are presented for tin vaporized under various conditions in a graphite furnace. Mass spectroscopie studies coupled with temporal and spatial absorbance measurements indicate that oxygen entrained in the inert sheath gas significantly attenuates the free atomic tin population through the rapid formation of SnO. Addition of 10% hydrogen to the sheath gas and enclosure of the atomizer, substantially reduce the effect of entrained oxygen. Similar release mechanisms are postulated with and without hydrogen, but it is suggested that the concentration of elemental tin on the surface at the time of vaporization is greater when hydrogen is included. The effects of added sulfate salts were also studied and it was concluded that the formation and prevolatilization of SnS is responsible for the extreme depressions reported in the literature.     

Quantitative analysis of trace bulk oxygen in silicon wafers using an inert gas fusion method.

This paper describes a method for removing oxide film from the surface of silicon wafers using an inert gas fusion impulse furnace and precise determination of bulk oxygen within the wafer. A silicon wafer was cut to about 0.35 g (6 x 13 x 2 mm) and dropped into a graphite crucible. The sample was then heated for 40 s at 1300 degrees C. The wafer's oxide film was reduced by carbon and removed as carbon monoxide. The treated silicon sample was taken out of the graphite crucible and maintained again with the holder of the oxygen analyzer. The graphite crucible was then heated to 2100 degrees C. The treated silicon sample was dropped into the heated graphite crucible and the trace bulk oxygen in the wafer was measured using the inert gas fusion infrared absorption method. The relative standard deviations of the oxygen in silicon wafer samples with the removed surface oxide film were determined to be 0.8% for 9.8 x 10(17) atoms/cm3, and 2.7% for 13.0 x 10(17) atoms/cm3.     

Physisorption of molecular oxygen on C60 thin films

The interaction of oxygen molecules with a fullerene surface has been studied using high resolution electron energy loss spectroscopy and temperature programmed desorption. Vibrational excitation of the adsorbed oxygen is observed at 190 meV, an energy value comparable with that for molecular oxygen in the gas phase. We take this to indicate physisorption of molecular oxygen on the C(60) surface. Thermal desorption results also show that the bonding of oxygen molecules to the C(60) overlayer is comparable to that on a graphite surface. A detailed study of the energy dependence of the vibrational excitation reveals an inelastic electron resonance scattering process. The angular dependence of the resonant vibrational excitation exhibits features distinctively different from those for molecular oxygen physisorbed on the related graphite surface, at a comparable coverage. One possible reason is that the corrugated surface potential, due to the curvature of the C(60) molecules, promotes the preferential ordering of the physisorbed oxygen molecules perpendicular to the surface plane of the C(60) overlayer.     

Oxygen‐barrier properties of cold‐drawn polyesters

The improvement of oxygen‐barrier properties of glassy polyesters by orientation was examined. Poly(ethylene terephthalate) (PET), poly(ethylene naphthalate), and a copolymer based on PET in which 55 mol % of the terephthalate was replaced with bibenzoate (PET‐BB55) were oriented by constrained uniaxial stretching. In a fairly narrow window of stretching conditions near the glass‐transition temperature, it was possible to achieve uniform extension of the polyesters without crystallization or stress whitening. The processes of orientation and densification correlated with the conformational transformation of glycol linkages from gauche to trans. Oxygen permeability, diffusivity, and solubility decreased with the amount of orientation. A linear relationship between the oxygen solubility and polymer specific volume suggested that the cold‐drawn polyester could be regarded as a one‐phase densified glass. This allowed an analysis of oxygen solubility in accordance with free‐volume concepts of gas permeability in glassy polymers. Orientation was seen as the process of decreasing the amount of excess‐hole free volume and bringing the nonequilibrium polymer glass closer to the equilibrium (zero‐solubility) condition. Cold drawing most effectively reduced the free volume of PET‐BB55. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 862–877, 2002     

Capacitive-discharge-heated anisotropic pyrolytic graphite furnace used for atomic absorption spectrometry Part. 1 Thoeretical Modelling of the Heating Characteristics of the Furnace Surface and of the Gas Phase

A model of the temperature distribution in the tube wall and in the gas phase of an anisotropic pyrolytic graphite furnace heated by capacitive discharge technique is proposed. The heat loss from the graphite tube by conduction via the contact electrodes to the water-cooled electrode supports and by radiation to its surroundings are the main processes condidered. In calculating the gas temperature, heat transfer by conduction from the tube wall to the gas phase is the only mode taken into account. The instantaneous temperature of a graphite furnace is expressed in the form of a differential equation. A finite-difference form of the differential equation is used in a computer program to calculate the temperature at each time step. The computer simulation offers the capability of studying the factors affecting the characteristics of temperature/time profiles and the distribution of surface and gas phase temperatures of the furnace. The results of simulation studies of the effect of the capacitance and the initial voltage of the capacitor bank on the heating characteristics of the furnaces show a reasonable agreement with those obtained experimentally.     

The hydrogen bond and free volume property of poly(ether‐urethane) irradiated by neutron

Poly(ether‐urethane) (PEU) was irradiated by neutron in different atmospheres. The hydrogen bonding interaction was analyzed by Fourier transform infrared (FTIR), and the microstructure of PEU had been investigated by positron annihilation lifetime spectroscopy (PALS). The gas products were detected by gas chromatography after irradiation. The results demonstrated that the irradiation led to more hydrogen bonded carbonyl in PEU, smaller relative free volume fraction, and narrower free volume distribution. It suggested that increasing hydrogen bonds would result in the collapse of free volume. The irradiation induced micro‐phase merging together and the presence of oxygen would accelerate this tendency, which was revealed by PALS. All the results indicated that the chain relaxation led to more hydrogen bonds, and the hydrogen bonding interaction suppressed the free volume. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 381–388, 2010     

Vacuum fusion determination of oxygen in gadolinium,terbium metal and iron-terbium alloy

The extraction conditions for the accurate determination of oxygen in gadolinium, terbium and iron-terbium alloy using vacuum fusion analysis were studied. The influence of the gettering effect, the analyzing temperature and the weight ratio of the bath metal to the sample were investigated. Oxygen values of gadolinium and terbium were measured by the graphite crucible, the graphite capsule, the tin bath, the iron-tin bath and the platinum-tin bath techniques in the temperature range of 1500–2100 °C using vacuum fusion analysis. These oxygen values were compared with those obtained by inert gas fusion analysis. In inert gas fusion analysis, the samples were analyzed with iron and tin in a tin capsule, and the samples with platinum in a tin capsule were analyzed in a graphite capsule enclosing with carbon powder. Oxygen values of both metal samples in the graphite capsule at 2000 °C, with an iron-tin bath at 1850 °C and a platinum-tin bath at 2000 °C in vacuum fusion analysis, were respectively in good agreement within their errors; the oxygen values of gadolinium were also in good agreement with that from inert gas fusion analysis in the iron-tin bath, but those of terbium were not in agreement. This agreement for gadolinium guarantees the reliability of the conditions for the accurate determination, and the difference of oxygen values for terbium suggests a need for further consideration on the conditions of the inert gas fusion analysis.     

Combined molecular mechanical and quantum mechanical potential study of a nucleophilic addition reaction in solution

The procedure of combined semiempirical quantum mechanical (AM1) and molecular mechanical potential⁷ was used to study the nucleophilic addition of hydroxide to formaldehyde in solution. The gas phase AM1 potential surface is approximately 26 kcal/mol more exothermic than the corresponding ab initio 6-31 + G* calculation results. The free energy profile for the reaction in solution was determined by means of molecular dynamic simulations. The resulting free energy of activation is approximately 5 kcal/mol. The difference of the free energy of solvation between the reactant and the product states is about 38 kcal/mol. As the reaction goes on, the number of hydrogen bonds formed by the hydroxide oxygen with the surrounding water molecules decreases, whereas the number of hydrogen bonds formed by the carbonyl oxygen increases. There is no significant change in the total number of hydrogen bonds between the solute and the solvent molecules, and the average number of these hydrogen bonds is between five and six during the entire reaction process. These results are consistent with previous studies using a model based on ad initio 6-31 + G* calculations in the gas phase. The reaction path in solution is different from the gas phase minimum energy reaction path. When the two reactants are at a large distance, the attack route of the hydroxide anion in solution is close to perpendicular to the formaldehyde plane, whereas in the gas phase the route is collinear with the carbonyl group. These results suggests that although AM1 does not yield accurate energies in the gas phase, valuable insights into the solvent effects can be obtained through computer simulations with this combined potential. This combined procedure could be applied to chemical reactions within macromolecules, in which a quantitative estimation of the effects of the environment would not be easily attainable by another technique. © 1994 by John Wiley & Sons, Inc.     

Graphitic structures by design

It is shown that self-supporting graphitic structures of specific shape can be grown in a variety of forms, from nanoscale to macroscale, on metal templates, in a fuel-rich mixture of ethylene and oxygen at temperatures between 750 and 900 K. The evidence presented suggests graphite can be grown in any shape created from catalytic metals (e.g., Ni) under the proper conditions of temperature and gas composition. Structures produced include macroscale bodies, centimeters in dimension, composed of micrometer-scale graphite elements such as graphite "foam" and regular graphite "lattices". Nanoscale hollow graphite spheres were also produced. The production rate in the apparatus employed was roughly shown to be 1 layer/s and was steady with time over several hours. The process of producing self-supporting bodies generally produces hollow graphite structures, as the underlying metal template must be removed by acid following the completion of graphite growth. The process is believed to be possible only in an environment, such as combustion, in which a high concentration of particular radical species is present in the vicinity of the template surface. The following process is postulated: (i) a single layer of graphite is formed from gas-phase radicals by the catalytic action of the metal template, (ii) additional graphite growth is "autocatalytic" and occurs via the decomposition of radicals on the surface and the incorporation of "free" carbon atoms, or other radical fragments, into "edge sites" on the graphite surface.     

Wettability and surface free energy of electrolytically oxidized graphite fibers

Wettability of electrolytically oxidized graphite fibers has been investigated by contact angle measurements employing the Wilhelmy method. The atomic ratio of oxygen to carbon, O/C, in the surface layer of the graphite fiber increased with increasing electric specific charge. Contact angle hysteresis was not observed for the untreated graphite fiber (O/C=0.01). The contact angles decreased with increasing O/C, especially for the receding angle, and approached constant for O/C>0.2. The nondispersive and dispersive surface free energies of the oxidized graphite fibers were calculated from the experimentally determined contact angles. The nondispersive surface free energy increased by the oxidation, whereas the dispersive one decreased. From the results of surface analysis, it was found that the changes in the nondispersive and dispersive surface free energies were caused by the increase in O/C ratio and the decrease in surface crystallinity, respectively.     

Impact of crystallinity of poly(lactide) on helium and oxygen barrier properties

The helium and oxygen gas barrier properties of poly(lactide) were investigated as a function of stereochemistry and crystallinity degree. Poly(l-lactide) and poly(d,l-lactide) films were obtained by extrusion and thermally cold crystallized in either α′- or α-crystalline form with increasing crystallinity degree. Annealing of the films at low temperatures yielded to α′-crystals as well as the creation of a rigid amorphous fraction in the amorphous phase. Unexpectedly, the quantity of the rigid amorphous fraction was highest in poly(l-lactide) crystallized under α′-form. Unexpectedly, the gas permeability increased with increasing quantity of α′-crystals in poly(l-lactide) and remained constant with increasing quantity of α′-crystals in poly(d,l-lactide). A gain in gas barrier properties was obtained upon crystallization at higher temperatures yielding α-crystals. The analysis of the oxygen transport parameters, in particular the diffusion and the solubility coefficient showed that the diffusion was accelerated upon crystallization, while the solubility coefficient decreased in an expected manner which led to conclude that it remained constant in the amorphous phase. The acceleration of the diffusion seems to be correlated to the occurrence of the rigid amorphous fraction, which holds larger free volume. To conclude, for optimization of poly(lactide) gas barrier properties by focussing on the decrease of the diffusion coefficient it can be suggested to work with poly(d,l-lactide) and to aim a crystallization in α-form avoiding the formation of a rigid amorphous fraction.     

Properties of surface compounds in methanol conversion on γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Data of in situ IR spectroscopy

In situ IR spectroscopic studies show that a formate, an aldehyde-like complex, and bridging and linear methoxy groups exist on the alumina surface involved in methanol conversion. In the absence of methanol in the gas phase, the interaction between two bridging methoxy groups yields dimethyl ether in the gas phase. When methanol is present in the gas phase, it interacts with methoxy groups on the surface. This reaction makes the main contribution to the formation of dimethyl ether. The linear methoxy group undergoes conversion via several routes. The main route is desorption with methanol formation in the gas phase, and no more than 10% of the linear methoxy groups are converted into formate and aldehyde, which are CO₂ sources in the gas phase. In the absence of methanol in the gas phase, the conversion rate of the methoxy groups is independent of the presence of water and oxygen. A scheme of the surface reactions is suggested to explain the conversion of the methoxy groups.     

Determination of micro amounts of oxygen in silicon by inert-gas fusion

A chromatographic inert-gas fusion method using an Ni-Sn fusion bath and helium as carrier gas has been developed for determining micro amounts of oxygen in silicon. With the Ni-Sn bath, the oxygen determination can be done at lower temperatures (1650-1700 degrees ) in a heated graphite crucible than in an empty crucible (with no molten metal bath) in which the sample is directly in contact with the carbon. Four samples can be analysed in succession in a single crucible with a relatively short time for oxygen extraction (5 min). Careful control of experimental conditions, and the use of a water-cooled quartz tube and a small unshielded graphite crucible have resulted in a lower blank (0.1 mug of oxygen), and better reproducibility, enabling oxygen in silicon to be determined down to 1 ppm. A calibration curve for determining oxygen in single crystals of silicon by measuring the infrared absorption at 9 mum has been constructed and gives results agreeing with those obtained by alpha-particle activation analysis.     

Sputtered alumina as a novel stationary phase for micro machined gas chromatography columns

Silica and graphite sputtering have previously been reported as novel solid stationary phase deposition techniques for micro gas chromatography columns. As a conventional solid stationary phase in gas chromatography, compatible with sputtering yet so far unreported, alumina was evaluated in this study. Alumina sputtered semi-packed micro columns were fabricated (including an activation step) and proved able to separate a mixture of volatile alkanes (C₁–C₄ with isomers) in less than 1 min. Kinetic and a thermodynamic evaluation led to calculation of 4,500 theoretical plates for ethane in 1.1 m (HETP_min?=?250 μm) and a Gibbs free energy for propane of 30.2 kJ mol^?1, making this stationary phase’s properties very close to those observed with silica-sputtered micro columns.     

新型气体扩散电极体系高效产H2O2的研究

以自制新型石墨/聚四氟乙烯(PTFE)气体扩散电极在无隔膜体系中进行双氧水发生工艺的优化研究, 主要探讨了不同石墨和PTFE质量比、阴极电位、pH值和氧气流速对H2O2产率的影响. 结果表明, 以石墨和PTFE质量比为2:1的气体扩散电极为阴极, 在pH=3, Na2SO4浓度为0.1 mol•L−1, 氧气流速为0.4 L•min−1, 阴极电位为−0.55 V (vs SCE)时, 2 h后H2O2可以达到60 mg•L−1. 该新型体系有较高的H2O2产率和电流效率(可达60%以上), 且pH值适用范围较广, 可望应用于水中污染物的处理.     

Mechanismus der protonkatalysierten gasphasendehydratisierung von furfurylalkohol

The gas phase proton catalysed water elimination of tetrahydrofurfuryl alcohol (1) gives exclusively the ring-enlarged oxonium-ion 12. There is no experimental indication for the generation of 11 via a [1.2]-hydrogen migration. From the analysis of appropriately ¹³C-labelled precursors in combination with collisional activation mass spectrometry the mechanism for the ring-enlargement has been derived, clearly indicating that the oxygen/carbon bond is cleaved. A comparison of the experimental results with computational investigation (ab initio molecular orbital calculations at the 4-31G level) suggests that free tetrahydrofurfuryl cation 7 is not generated in the gas phase solvolysis. The elimination of H₂O from the protonated molecule of 1 is a process characterised by anchimeric assistance of the ether oxygen 15.     

Quantitative Structure-Property Relationship: XIII. Properties of Aliphatic Alcohols

A simple quantitative structure-property relationship was used to calculate a set of properties of aliphatic alcohols including the boiling point; enthalpy of vaporization; heat capacity of liquids at constant pressure; entropy, enthalpy, and free energy of formation; free energy of transfer from the gas phase to water; distribution factor in the octanol-water system; dielectric constant; surface tension; viscosity; thermal conductivity; diamagnetic susceptibility; ionization potential; self-association index; distribution factor between the gas and hexadecane; distribution factor between water and dodecyl sulfate mixelles; solubility of HCl in alcohols; and solvatochromic parameter. The rate constants, activation entropies, and activation energies of the Menshutkin reaction in alcohols were also calculated. The calculated values are in good agreement with the experimental data. The suggested relationship well reproduces the proton affinites of a series of alcohols calculated by rigorous methods.     

The fundamentals of the chemical crystallization of diamond from the gas phase

The basic steps of researches for the synthesis of diamond from the gas phase is reviewed. The experimental results obtained on the thermal activation of a medium by using diamond powders as nucleation centers are considered. It has been established that the diamond rate from hydrocarbons exceeds that of graphite. Fractionation of the stable carbon isotopes is observed. The presence of hydrogen influences differently the growth of diamond and that of graphite.

The method of periodical pulse supersaturations is described, enabling one to obtain epitaxial films and isometrical diamond crystals. The method of activation of the gas medium by using an electric discharge allowed to obtain polycrystalline diamond films on the surface of metals and semiconductors. A possibility of the homogeneous formation of diamond in the gas phase has been established. The possibility of realization of this process in the cosmic space is discussed.     

Structures,g-tensors,and hyperfine coupling constants of L-α-alanine radicals in radiation dosimetry: An ab initio molecular dynamics simulation study

Alanine is used as a transfer standard dosimeter for gamma ray and electron beam calibration. An important factor affecting its dosimetric response is humidity which can lead to errors in absorbed dose calculations. Ab initio molecular dynamics calculations were performed to determine the environmental effects on the electron paramagnetic resonance (EPR) parameters of L-α-alanine radicals in acidic and alkaline solutions. A new result, not dissimilar to the closed-shell amino acid molecule alanine, is that the non-zwitterionic form of the alanine radical is the stable form in the gas phase while the zwitterionic neutral alanine radical is not a stable structure in the gas phase. Geometric and EPR parameters of radicals in both gas and solution phases are found to be dependent on hydrogen bonding of water molecules with the polar groups and on dynamic solvation. Calculations on the optimized free radicals in the gas phase revealed that for the neutral radical, hydrogen bonding to water molecules drives a decrease in the magnitudes of g-tensor components g _xx and g _yy without affecting neither g _zz component nor the hyperfine coupling constants (HFCCs). The transfer from the gas to solution phase of the alanine radical anion is accompanied with an increase in the spin density on the carboxylic group's oxygen atoms. However, for the neutral radical, this transfer from gas to solution phase is accompanied with the decrease in the spin density on oxygen atoms. Calculated isotropic HFCCs and g-tensor of all radicals are in good agreement with experiment in both acidic and alkaline solutions.     

二元共聚物热解碳包覆的石墨负极材料

以苯萘二元共聚物包覆天然石墨后进行热处理，用所制备的包覆石墨作为锂离子电池的负极材料，与包覆前石墨材料相比较，可逆容量提高了10％，不可逆容量降低了7％，循环性能也得到了大的改善. X射线分析表明，改性后该石墨晶体中三方石墨的含量增高.