Diffusion Constant D0 Research Articles

Study on the performance of Tungsten–Titanium alloy film as a diffusion barrier for iron in a flexible CIGS solar cell

A Tungsten–Titanium (W–Ti) diffusion barrier was prepared on an iron (Fe) substrate by means of magnetron sputtering with subsequent annealing at different temperatures. Rutherford Backscattering Spectrometry (RBS) was used to analyze the Fe distribution in the W–Ti layer. The diffusion constant D0=3.4×10−16m2/s and the activation energy Q=43kJ/mol of the Fe diffusing in the W–Ti layer were subsequently calculated based on Fick’s Second Law. The results demonstrated that a 291nm-thick W–Ti barrier layer can be used to block Fe from diffusing into a CIGS solar cell for 20years at a work temperature of 308K (35°C).

Potential step study of electrooxidation of methanol in 1-butyl-3-methylimidazolium tetrafluoroborate

A potential step method was used to characterize the electrooxidation of methanol on a chemically modified electrode in an ionic liquid solvent. Two major findings were reported from this study. Firstly, the oxidation was dominant 2.2s after the potential step. Before that, the double layer charging and adsorption were dominant. Therefore, there should be a waiting time of a few seconds if a methanol sensor is developed with a potential step method. Secondly, the oxidation of methanol on the electrode was diffusion controlled. The concentration of methanol affected the diffusion. The diffusion constant D0 was 8.37×10−17m2/s when the concentration was lower than 0.5M and was 2.66×10−13m2/s when the concentration was higher than 1.0M. This suggests that the methanol concentration should be kept higher than a threshold in an ionic liquid based fuel cell.

Arrhenius relationship and two-step scheme in AF hyperdynamics simulation of diffusion of Mg/Zn interface

The accelerating factor (AF) method is a simple and appropriate way to investigate the atomic long-time deep diffusion at solid-solid interface. In the framework of AF hyperdynamics (HD) simulation, the relationship between the diffusion coefficient along the direction of z-axis which is normal to the Mg/Zn interface and temperature was investigated, and the AF's impact on the diffusion constant (D0) and activation energy (Q*) was studied. Then, two steps were taken to simulate the atomic diffusion process and the formation of new phases: one for acceleration and the other for equilibration. The results show that: the Arrhenius equation works well for the description of Dz with different accelerating factors; the AF has no effect on the diffusion constant D0 in the case of no phase transition; and the relationship between Q* and Q conforms to Q*=Q/A. Then, the new Arrhenius equation for AFHD is successfully constructed as Dz=D0exp[-Q/(ART)]. Meanwhile, the authentic equilibrium conformations at any dynamic moment can only be reproduced by the equilibration simulation of the HD-simulated configurations.

Long-Range Diffusion of Hydrogen in Solid-Solution PdCe Alloys as Deduced from Absorption Experiments

The diffusion of H and the thermal decomposition of hydrides have been investigated at high temperatures in two PdCe alloys of composition 5% and 9% Ce. It has been found that the H diffusion coefficient obeys an Arrhenius-type of law with the following values of the activation energy W and diffusion constant D0, ( )     = ± × = ± − s m D W eV 2 7 0 2 2 10 0.20 0.02 (Pd95Ce5 alloy) ( )     = ± × = ± − s m D W eV 2 7 0 2 1 10 0.24 0.01 (Pd91Ce9 alloy) The high-temperature absorption data match the low-temperature ones deduced from internal friction measurements, indicating that Ce atoms do not act as strong trapping centres for H. Thermal decomposition of hydrides in the Pd95Ce5H0.008 alloy occurs in a single stage showing a homogeneous solid solution state of the H-Me system.

Investigation of the diffusion processes in a self-processing acrylamide-based photopolymer system.

Results from the investigation of the diffusion processes in a dry acrylamide-based photopolymer system are presented. The investigation is carried out in the context of experimental research on optimization of the high-spatial-frequency response of the photopolymer. Tracing the transmission holographic grating dynamics at short times of exposure is utilized to measure diffusion coefficients. The results reveal that two different diffusion processes contribute with opposite sign to the refractive-index modulation responsible for the diffraction grating buildup. Monomer diffusion from dark to bright fringe areas increases the refractive-index modulation. It is characterized with diffusion constant D0 = 1.6 x 10(-7) cm2/s. A second diffusion process takes place during the recording. It decreases the refractive-index modulation and we ascribe it to diffusion of short-chain polymer molecules or radicals from bright to dark fringe areas. The estimated diffusion coefficient for this process is D0 = 6.35 x 10(-10) cm2/s. The presence of the second process could be responsible for the poor high-spatial-frequency response of the investigated photopolymer system. Comparison with the diffusion in photopolymer systems known for their good response at high spatial frequencies shows that both investigated diffusion processes occur in a much faster time scale.

Permeability and conductivity of platelet-reinforced membranes and composites.

We present large-scale simulations of the diffusion constant D of a random composite consisting of aligned platelets with aspect ratio a/b>>1 in a matrix (with diffusion constant D0) and find that D/D(0)=1/(1+c(1)x+c(2)x(2)), where x=av(f)/b and v(f) is the platelet volume fraction. We demonstrate that for large aspect ratio platelets the pair term (x(2)) dominates suggesting large property enhancements for these materials. However, a small amount of face-to-face ordering of the platelets markedly degrades the efficiency of platelet reinforcement.

Diffusion of Cs atoms in Ne buffer gas measured by optical magnetic resonance tomography

Optical magnetic resonance tomography uses optical pumping and the paramagnetic Faraday effect to image spin density distributions in optically thin media. In this paper we present an apparatus that allows to measure the distribution of spin-polarized Cs atoms, which we applied to study the diffusion of Cs in Ne buffer gas by time-resolved 2D-mapping of the evolution of an initial inhomogeneous spin distribution. The diffusion constant D0 for Cs in a Ne buffer gas of 1013 mbar is determined as 0.20(1) cm2\s.

Optical and Structural Properties of Proton-Exchanged Waveguides in Y-Cut ZnO:LiNbO 3

Fabrication and characterisation of proton exchanged planar optical waveguides in Y-cut ZnO:LiNbO3 crystal using adipic acid as proton source are reported in this paper. These waveguides have a linear-step index profile with a surface index increase of 0.138 at the wavelength of 0.633 µ m. The activation energy Q and diffusion constant D0 for the proton exchange process are characterised optically to be 95.32 KJ/mol and 9.61×109 µ m2/h. A critical thickness of about 2 µ m for proton exchanged y-cut ZnO:LiNbO3 waveguides is optically determined in our experiments. Surface damage is observed for those waveguides with layer thickness larger than this critical value. The strains in y-cut waveguides are measured by using double-crystal X-ray symmetric and asymmetric diffraction method. Based on the measured data, an explanation is given for the surface damage occurred on y-cut samples.

Proton-Exchanged Waveguides for Surface Acoustic Wave on Z-cut LiNbO3 Using Octanoic Acid

The properties of the surface acoustic wave (SAW) on proton-exchanged Z-cut Y-propagation LiNbO3 waveguides using octanoic acid are investigated. The distribution of hydrogen in the PE samples measured by secondary-ion mass spectrometry (SIMS) shows a step like profile with the diffusion constant D0 and the activation energy Q of about 1.18×106 µ m2/ h and 63.21 kJ/mol, respectively. The results show that the acoustic velocity (V p) of SAW on PE samples decreases monotonically with increased kd, where k is the wave number (2π/λ) and d is the waveguide depth. Furthermore, the variation of the electromechanical coupling coefficient (K2) is similar to that of V p. The SAW insertion loss (IL) becomes saturated at about kd=0.13, with a maximum increase of about 5-6 dB. The temperature coefficient of delay (TCD) calculated from the frequency change of the output of the SAW delay line for the temperature ranging from -10 to 50° C shows a clear increase in the PE layer.

Characteristics of Surface Acoustic Waves on Proton-Exchanged and Annealed Proton-Exchanged Z-Cut LiNbO3 Using Pyrophosphoric Acid

The characteristics of surface acoustic waves (SAWs) on proton-exchanged (PE) and annealed proton-exchanged (APE) Z-cut Y-propagated LiNbO3 waveguides using pure pyrophosphoric acid are investigated and compared. The distribution of hydrogen in the PE samples measured by secondary-ion mass spectrometry (SIMS) shows a step-like profile with the diffusion constant D0 and the activation energy Q of about 9×107 µ m2/ h and 82.18 kJ/mol, respectively. The velocity (V p) of SAW on PE samples decreases with the increase of kd, where k is 2π/λ and d is the waveguide depth, and almost exhibits an exponential decay. However, post-annealing will result in a restoration of the decreased velocity and an improvement of the SAW insertion loss (IL). On the other hand, the electromechanical coupling coefficient (K2) almost decreases with the increase of kd and shows greater sample-to-sample discrepancy.

High-index proton-exchanged MgO:LiNbO3 optical waveguides using adipic acid

We report the fabrication and characterization of proton-exchanged optical waveguides in z-cut MgO:LiNbO3 using adipic acid as a new proton source. These waveguides exhibit propagation losses of less than 1 dB/cm, a linear-step index profile, and a surface index increase of 0.149 measured at 0.633 μm wavelength. This is the largest surface index change reported for proton-exchanged MgO:LiNbO3 waveguides. The diffusion parameters were characterized optically, and the diffusion constant D0 and the activation energy Q were found to be 5.40×1010 μm2/h and 103.27 kJ/mol, respectively. The diffusion rate is slower than that of the popular phosphoric acid.

Translational diffusion of chain polymers. II. Effect of internal friction

The translational diffusion constant D0(λ) of chain polymers has been evaluated for models that contain internal friction measured by a strength parameter λ. D0(λ) is an upper bound to the true diffusion constant D(λ). The method of evaluation is based on the facts that the power series expansion of D0(λ) in λ is a series of Stieltjes, and the coefficients in this power series permit a rapid evaluation from equilibrium simulations. Convergent Padé approximants are therefore available. Results are reported for theta point chains with N beads, including Gaussian and lattice models, with N≤1792. In the limit λ→∞, D0(λ) approaches the translational diffusion constant of a chain that diffuses as a rigid body, and this diffusion constant D0(∞) is known to be a lower bound to D(λ). The two main findings are these: First, D0(∞) is approached very closely by D0(λ) for fairly modest values of λ, comparable in size to ordinary bead friction constants. Second, the ratio RG/RH of radius of gyration to hydrodynamic radius, which is the same as D0(λ)/D0(0), seems to have a limit at N→∞ which depends on the backbone model, and probably depends on the bead friction constant. That is, as a result of fluctuations in hydrodynamic interaction the translational diffusion constant fails to satisfy universality. The deviations seem to be rather small for this transport property, on the order of 5%.

On the Kinetics of a Paramagnetic Oxygen Centre in CaF2

AbstractIn calcium fluoride crystals, doped with oxygen, centres of the VK type are generated at T = 77 K by X‐irradiation. The VK type centres recombine at T ≈ 150 K and paramagnetic oxygen fluorine‐vacancy centres (O–FV)− originate in the same concentration due to hole transfer whereas X‐irradiation at T = 300 K generates the (O–FV)− centres directly. The generation kinetics and optical and thermal bleaching of the centres are investigated. The activation energy of thermal bleaching EA ≈ 0.6 V indicates a fluorine‐vacancy mechanism. The oxygen diffusion in undoped CaF2 is studied by means of the (O–FV)− centres. The activation energy of the diffusion amounts to E = (1.8 ± 0.2) eV, the diffusion constant D0 depends on the crystal lattice defects.

Spin Relaxation in Optical Pumping

The spin relaxation of optically aligned rubidium vapor has been studied as a function of helium buffer gas pressure. Relaxation times as long as 0.68 sec were observed in helium at ½-atm pressure. The diffusion constant D0 for rubidium in helium at 50°C was evaluated as 0.54 cm2 sec—1. The disorientation cross section σ for rubidium-helium collisions was found to be 6.2×10—25 cm2. A mechanism for spin relaxation in optically aligned alkali vapors is proposed and discussed. It is found that the relaxation arises from the coupling of the alkali electron spin to orbital and rotational motion accompanying the collision. The variation of σ for rubidium in the presence of other inert gases is also discussed.