Diffusion Coefficient Of Nitrogen Research Articles

Outgassing characteristics of a polycarbonate core material for vacuum insulation panels

Performance deterioration with time is one of the most important issues in a vacuum insulation panel (VIP), which is mostly due to the inner gas pressure rise. Outgassing from the interior of core materials is the major gas source when the core material is a polymer. Outgassing characteristics of a polycarbonate as the VIP’s core material are examined theoretically and experimentally. To measure the outgassing rate, specific outgassing tests are carried out using a pressure rise method. Diffusive outgassing mechanism is discussed based on the Fick’s law. As the result, the total amount of dissolved gas and the diffusion coefficients of various gases in the polycarbonate are obtained by using the measured outgassing rate. Temperature dependence of the diffusion coefficient of nitrogen is also examined. It is shown that the outgassing rate of polymer core materials can be significantly reduced to a negligible level by a baking pre-treatment in vacuum and/or by a metal coating on the polymer surface.

Modeling of nitrogen penetration in polycrystalline AISI 316L austenitic stainless steel during plasma nitriding

The nitrogen depth profile in polycrystalline AISI 316L austenitic stainless steel after plasma nitriding at temperatures around 400 °C is analyzed by the “trapping–detrapping” model. This model considers the diffusion of nitrogen under the influence of trap sites formed by local chromium atoms. Nitrogen depth profiles in polycrystalline AISI 316L steel simulated on the basis of this model are in good agreement with experimental nitrogen profiles. The enhanced nitrogen diffusivity as well as a plateau-type shape of nitrogen depth profile can be explained. The nitrogen diffusion coefficient at 400 °C is found to be D = 4.81 × 10 −12 cm 2/s and the diffusion pre-exponential factor D 0 (0.837 × 10 −3 cm 2/s) and detrapping activation energy E B (0.28 eV) were deduced from fitting experimental data. It is known that the nitrogen penetration depth (and nitrogen diffusivity) depends on the crystalline orientation and a tentative to take into account this anisotropy effect and describe nitrogen depth profiles in polycrystalline AISI 316L steel is proposed by using different diffusion coefficients characteristic for each crystallite orientation.

Gas transport in fluorothiophenyl modified PVC membranes

This work reports the modification of poly(vinyl chloride) by partially replacing the chlorine atoms of the chains with moieties derived from fluorinated nucleophilic compounds: 4-fluorothiophenol, 3,4-difluorothiophenol and pentafluorothiophenol. Membranes were cast from tetrahydrofuran solutions of the modified poly(vinyl chloride)s and the permeability and diffusion coefficients of hydrogen, nitrogen, oxygen, carbon dioxide and methane through the membranes were measured using permeation techniques. The influence of both the degree of modification and the number of fluorine atoms containing in functionalizing groups on the performance of the membranes was thoroughly investigated. A permselectivity study is also presented in order to evaluate the potential applicability of the membranes for gases separation.

Study of dimension dependent diffusion coefficient of titanium dioxide nanoparticles

In the present paper, we have studied size and dimension effect on diffusion coefficient of nitrogen and platinum doped TiO 2 nanoparticle using Arrhenius relation and Lindeman's criteria under their dynamic limit. The activation energy and melting point decreases with decrease in size. The TiO 2 diffused with metal and nonmetal gives rise to (meta-) stable structure and mid-gap state helps to increase surface to volume ratio and quantum confinement effects, and results in the increase of diffusion coefficient. The calculated diffusion coefficient of undiffused TiO 2 with size is in good agreement with available experimental data.

Sorption, diffusion, and perm-selectivity of toluene vapor/nitrogen mixtures through polydimethylsiloxane membranes with two cross-linker densities

The sorption, diffusion, and permeation properties of toluene and nitrogen from binary gaseous mixtures were studied using polydimethylsiloxane (PDMS) membranes with various cross-linker levels. The prepared PDMS membranes were characterized using X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and positron annihilation lifetime spectroscopy (PALS). The PDMS made with 10:1 pre-polymer/cross-linker ratio contained a negligible amount of unreacted cross-linker. The cross-linking reaction for curing PDMS with 10:2.5 pre-polymer/cross-linker ratio, however, only consumed 60% of the cross-linker and the actual cross-linker density was 15% (pre-polymer/cross-linker ratio). The resulting 10:1 membrane has a cross-linked network of dominant high-molecular-weight polymer chains linked by shorter cross-linker components. Although the PDMS made with 10:2.5 pre-polymer/cross-linker ratio has a higher cross-linking density and possibly a smaller fractional free volume, the excessive embedded short-chain cross-linker served as a plasticizer and counter-balanced the depressed free volume. The toluene sorption isotherms were similar for these two membranes and could be predicted using the Flory–Huggins model. The toluene diffusion coefficients were correlated with its concentration using the Long's equation for both PDMS samples. The toluene vapor permeability in the PDMS membrane made with a pre-polymer/cross-linker ratio of 10:1 was slightly higher than that of the film with a 10:2.5 ratio. However, the nitrogen permeability from gaseous mixtures was significantly lower in the sample made with more cross-linker (pre-polymer/cross-linker ratio of 10:2.5). The nitrogen permeabilities depended on the toluene activity, decreasing to several tens of Barrer when the toluene vapor activity increased from 0.2 to 1.0. The decreased nitrogen diffusion coefficient might be ascribed to a reduction in effective fractional free volume due to the free volume occupied by the adsorbed toluene molecules, especially at high toluene activity levels. The suppressed nitrogen permeability contributed to toluene/nitrogen selectivity being higher than the ideal selectivity value in the toluene-enriched systems. When the toluene vapor activity increased from 0.2 to 1.0, the selectivities were increased from 16 to 1480 (95-fold increase) and from 110 to 1170 (10-fold), respectively, using PDMS membranes made with 10:1 and 10:2.5 ratios. The selectivity dependence on vapor activity (or vapor concentration) suggests that the ideal selectivity obtained from single permanent (or saturated vapor) measurements may not truly reflect the separation efficiency in binary vapor/gas systems.

Evaluation of the Diffusion Coefficient of N in<i> γ' </i>Iron Nitride: Influence of the Nitriding Potential

This work deals with a study of the nitriding potential effect on development of the compound layer during the gas nitriding of Armco Fe samples. The gas nitriding experiments were performed in an atmosphere of partially dissociated gas ammonia (NH3) at 520 °C under a nitriding potential varying from 0.25 to 3.5 atm-0.5 during 2 h. Through this experimental work including XRD analysis, optical and SEM observations of the cross-sections of the treated samples, it is shown that the microstructural nature of the compound layer depends upon the nitriding potential value. By use of the inverse problem based on a diffusion model previously published, it was possible to estimate the diffusion coefficient of N in ' iron nitride as a function of the applied nitriding potential. XRD analysis has shown that the compound layer was composed of iron nitride. A linear semi-logarithmic relationship relating the nitriding potential to the diffusion coefficient of nitrogen in iron nitride was also derived.

Determination of the concentration dependent diffusion coefficient of nitrogen in expanded austenite

AbstractThe concentration dependent diffusion coefficient of nitrogen in expanded austenite was determined from the rate of retracting nitrogen from thin initially N-saturated coupons. Nitrogen saturated homogeneous foils of expanded austenite were obtained by nitriding AISI 304 and AISI 316 in pure ammonia at 693 K and 718 K. Denitriding experiments were performed by equilibrating the foils with a successively lower nitrogen activity, as imposed by a gas mixture of ammonia and hydrogen. The concentration depen-dent diffusion coefficient of nitrogen in expanded austenite was approximated in the composition range where nitrogen can be extracted by hydrogen gas at the diffusion temperature. Numerical simulation of the denitriding experiments shows that the thus determined concentration dependent diffusion coefficients are an accurate approximation of the actual diffusivity of nitrogen in expanded austenite.

Nitrogen and Argon in Borosilicate Glass: Solubility, Diffusivity, Residual Gas Concentration and Behaviour of Bubbles in Glass Melts, Containing Both Gases

Diffusivities of nitrogen and argon in a borosilicate glass were determined with two different methods: (1) from gas exchange experiments between molten glass and bubbles containing nitrogen and argon, and (2) from solution rates of nitrogen and argon in glass during saturation experiments. Between 1200°C and 1580°C the diffusion coefficients of nitrogen and argon yielded the following equations:      − = − RT s m DN 134900 exp 10 22 . 1 ] / [ 6 . 2 2 and      − = − RT s m DAr 125300 exp 10 08 . 1 ] / [ 6 . 2 , with R=8.314 J/(mol.K). The solubilities and residual gas concentration in the glass which are necessary for the calculation of the diffusivities were determined with the high temperature vacuum extraction method.

Nitrogen Diffusion in the Nb-1.0wt%Zr Measured by Mechanical Spectroscopy

The scientific and technological development in the area of new materials contributed to several applications of niobium and its alloys in nuclear power plants as well as in aerospace, aeronautics, automobile and naval industries. This paper presents the interstitial diffusion coefficients of nitrogen in solid solution in the Nb-1.0wt%Zr alloy using internal friction measurements obtained by mechanical spectroscopy, which uses a torsion pendulum operating at an oscillation frequency between 1.0 Hz and 10.0 Hz. The temperature range varies from 300K to 700K, at a heating rate of 1 K/min and vacuum better than 2 x 10-6 Torr. The results showed an increase of the interstitial diffusion coefficient of nitrogen that was correlated with configurational considerations for the octahedral interstitials.

Nitrogen diffusion and nitrogen depth profiles in expanded austenite: experimental assessment, numerical simulation and role of stress

The present paper addresses the experimental assessment of the concentration dependent nitrogen diffusion coefficient in stress free expanded austenite foils from thermogravimetry, the numerical simulation of nitrogen concentration depth profiles on growth of expanded austenite into stainless steel during gaseous nitriding, a qualitative discussion of the role of stress on local equilibrium conditions of growing expanded austenite and a discussion of the erroneous concentration dependent diffusivity of nitrogen in expanded austenite as obtained from applying the Boltzmann–Matano method to composition–depth profiles.

Phase transitions in surface layers of polycrystalline solids

The fundamental equations of the thermodynamics of films were applied to describe the properties of polycrystalline solids, namely, first-order phase transitions of grain boundaries with the formation of two-dimensional liquids. The generalized equation obtained was used to calculate the temperature of this phase transition (the premelting temperature) for metals; it was found to lie between 0.55 and 0.86 of the temperature of fusion. The experimental nitrogen diffusion coefficient in steel over this temperature range was the same as in liquid steel.

Kinetics of Nitriding in Titanium Bearing Austenitic Stainless Steel

The kinetics of nitriding in titanium bearing austenitic stainless steels in cylindrical coordination has been investigated. Nitriding at nitrogen partial pressure of 0.5 atm and temperature from 980°C up to 1160oC resulted in the formation of titanium and chromium nitride and above the Cr2N solvus temperature (1100°C), no chromium nitride was formed. The nitrided case may consist of up to three zones: nitrogen in solid solution, precipitation of TiN and finally mixed TiN+Cr2N precipitation. These are in consistent with the affinity of titanium and chromium towards nitrogen which has been thermodynamically justified. To assess the kinetics of nitriding, a mathematical model for nitrogen diffusion in cylindrical system has been developed via solving Fick’s equation for cylindrical coordinate by numerical method. For this purpose the use has been made of the austenite stabilizing effect of nitrogen to correlate the movement of the boundary of the nitrogen stabilized austenite and plastic deformation induced martensite. Since the crystal structure of the material used in this study is of fcc type and it is nitrided in a fully annealed and unstrained state, therefore the effect of excess nitrogen on the kinetics of nitride fronts growth has been assumed as negligible. The results are in good agreement with the previous investigations. Unlike previous works, the present model takes into account the change of nitrogen diffusion coefficient due to nitrogen concentration profile and

ＳＵＪ２の浸炭窒化処理後の窒素濃度分布予測

We have been investigating the nitrogen penetration mechanism carbonitriding into high carbon chromium bearing steel JIS-SUJ2 on carbonitriding processes. Assuming that the penetration mechanisms of nitrogen and carbon are the same, we developed a calculation method in order to estimate nitrogen concentration distribution profiles, which are based on our through experimental data. In our method, the dependencies on carbon activity, partial pressures of H2 and undecomposed NH3, and diffusion coefficient of nitrogen are taken into consideration. It was confirmed that the nitrogen concentration distribution profiles predicted by our method agreed well with the experiment results under various conditions. Hence, we deduce that our method is effective in a practical range on carbonitriding processes.

The first order phase-transition of polycrystal solid surfaces with nanothickness

The fundamental equations of thermodynamics of a film have been used for describing a fundamental property of solid crystalline materials i.e. the first-order phase transition on the grain boundaries by the formation of two-dimensional liquid. The generalized equation that is obtained is used for calculating the premelting temperature of any metal, which has a value in the range of 0.55–0.86 of the melting point. The experimental diffusion coefficient of nitrogen in steel at premelting temperature is the same as in the liquid phase. The described phenomenon of phase transition on the grain boundaries decreases in case of radical modification of the existing process engineering of handling metals. It also provides a precise physical explanation to the super plasticity of fine-structure metal alloys.

Nitrogen in silicon: Transport and mechanical properties

A novel dislocation locking technique is applied to nitrogen-doped float-zone silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen diffuses to the dislocations. The stress required to bring about dislocation motion is then measured, generally at 550°C. The segregation of nitrogen to dislocations is found to be stable to at least 1200°C, and the dislocation unlocking stress measured at 550°C is of similar magnitude to that for oxygen in Czochralski silicon. At all annealing temperatures studied, the measured unlocking stress as a function of annealing time initially rises linearly before taking a constant value. The rate of the initial rise is found to be strongly dependent on temperature, with an activation energy of 1.5eV. In a separate set of experiments, the magnitude of the dislocation unlocking stress measured is also found to depend upon the temperature at which the unlocking process takes place in the 500–700°C temperature range. The dislocation locking technique is also used to give some measure of nitrogen out-diffusion, by measuring the unlocking stress as a function of material etched away after annealing. For specimens annealed at 750°C for 15h, the dislocation unlocking stress is found to rise with increasing depth away from the specimen’s surface, before it takes an approximately constant value. By fitting an error function to these data, the diffusion coefficient of nitrogen in silicon is deduced to be approximately 4×10−11cm2s−1 at 750°C. This value is consistent with a SIMS investigation by Itoh and Abe.

Anelastic Relaxation due to Interstitial Solutes Diffusion in Nb and Nb-1 wt% Zr

Metals and alloys containing solute atoms dissolved interstitially often show anelastic behavior due to a process know as stress-induced ordering. The application of mechanical spectroscopy measurements to diffusion studies in body-centered cubic metals has been extensively used in the last decades. However the kind of preferential occupation of interstitial solutes in bodycentered cubic metals is still controversial. The anelastic properties of the Nb and Nb-1 wt% Zr polycrystalline alloys were determined by internal friction and oscillation frequency measurements using a torsion pendulum inverted performed between 300K and 650K, operating in a frequency oscillation in the hertz bandwidth. The interstitial diffusion coefficients of oxygen and nitrogen in Nb and Nb-1 wt% Zr samples were determined at two distinct conditions: (a) for low concentration of oxygen and (b) for high concentration of oxygen.

Nitriding of AISI 4140 steel by a low energy broad ion source

A comprehensive study of the thermochemical nitriding process of steel AISI 4140 by low energy ion implantation (Kaufmann cell) is reported. Different times of implantation were employed and the studied samples were characterized by x-ray diffraction, in situ photoemission electron spectroscopy, scanning electron microscopy, and hardness (nanoindentation) measurements. The linear relationship between nitrogen content and hardness was verified. The structure of the nitrided layer was characterized yielding that the compound layer is formed by coarse precipitates, around small grains, constituted principally by ε-Fe2–3N and γ-Fe4N phases and the diffusion zone is formed by fine precipitates, around big grains of the original martensitic phase, constituted principally by γ-Fe4N phase. Finally, a diffusion model for multiphase systems was applied to determine effective diffusion coefficients of nitrogen in the different phases.

Transport Behavior of Oxygen and Nitrogen through Organasilicon-Containing Polystyrenes by Molecular Simulation

Molecular dynamics (MD) simulations have been used to study the transport properties of oxygen and nitrogen in the para-substituted polystyrenes which possess one to four Si atoms in each substituent. The Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field was used to construct the polymers. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) with up to 3 ns simulation times. After molecular dynamic simulation, the trajectories of the small molecules in the polymer matrix were obtained. Then diffusion coefficients have been calculated from the Einstein relationship revealing a considerable agreement between the simulated and the experimental data. And solubility coefficients have been calculated by the Grand Canonical Monte Carlo (GCMC) method. The solubility of oxygen increased with increasing Si content in the polymer membrane. The para-substituted polystyrenes with a branched substituent at the alpha-position showed higher permeability than those of the nonbranched ones. The higher the glass transition temperature (T(g)) of the membrane, the larger the diffusion coefficients of oxygen and nitrogen obtained.

Laser-enhanced diffusion of nitrogen and aluminum dopants in silicon carbide

The diffusivities of different types of dopant atoms in silicon carbide wafers are generally very low. Nd:YAG and excimer lasers have been used to dope silicon carbide with nitrogen and aluminum, respectively. Mathematical models have been presented for the temperature distributions in the wafers to understand the diffusion mechanisms in the laser doping process. Since the silicon carbide substrate reaches its peritectic temperature (3100 K) at irradiances of 80.6 and 61 MW/cm 2 for Nd:YAG and excimer lasers, respectively, lower irradiances were used to achieve solid-state diffusion. The Nd:YAG laser doping process doped nitrogen to a depth of 800 nm; the KrF excimer laser doping process produced aluminum dopant depths of 200 and 450 nm for different numbers of laser pulses. Two distinct diffusion regions, near-surface and far-surface regions, were identified in the dopant concentration profiles, indicating different diffusion mechanisms in these two regions. The effective diffusion coefficients of nitrogen and aluminum were determined for both regions and found to be 2.4 × 10 −5 and 9.2 × 10 −6 cm 2/s in the near- and far-surface regions for nitrogen, respectively, and 1.2 × 10 −5 and 1.3 × 10 −6 cm 2/s in the near- and far-surface regions for aluminum, respectively. The calculated diffusivities are at least six orders of magnitude higher than the typical values for nitrogen and aluminum, which indicate that the laser doping process enhances significantly the diffusion of dopants in silicon carbide.

Measurement of gas solubility and diffusivity in polylactide

The solubilities of carbon dioxide and nitrogen in a polylactide melt were determined at temperatures from 180 to 200 °C and pressures up to 28 MPa using a magnetic suspension balance (MSB). Both Sanchez–Lacombe (SL) and Simha–Somcynsky (SS) equations-of-state (EOS) were applied to predict the amount of volume swelling in the polylactide (PLA)/gas mixture due to gas dissolution. With the proper prediction of volume swelling from both SS-EOS and SL-EOS, the “corrected solubility” was determined based on the experimentally measured “apparent solubility” from the MSB. After measurement of the gas sorption curve in the PLA melt, the theoretical sorption model based on Fick's second law was adopted to extract the diffusion coefficients of carbon dioxide and nitrogen in polylactide at both 180 and 200 °C.