Diffusion Coefficient Of Si Research Articles

A kinetic model of diffusional phase transformations in ternary line compounds and determination of diffusion coefficients: Application to the Nb–Mo–Si system

The diffusional phase transformations in inhomogeneous multicomponent systems play an important role in affecting the material properties. Ternary line compounds, which are ternary phases with components that have both large and small solid solubility, are often observed in applications. They are different from solid solutions with variable composition ratios and stoichiometric compounds with fixed composition ratios. However, the existing models cannot quantitatively characterize the diffusional phase transformations in ternary line compounds. Therefore, by introducing the state equation related to the chemical potential and mole fraction, a kinetic model of diffusional phase transformations was established for the first time to describe the mole fraction evolution and phase growth in ternary line compounds. Furthermore, a method to determine the tracer diffusion coefficients of components related to the mole fraction and crystal phase structure in ternary line compounds was proposed, and applied to the diffusion of Nb-Mo-Si ternary system composed of the MoSi2 and Nb. During the high-temperature diffusion process of Nb-MoSi2, the silicide-poor phase (Mo, Nb)5Si3 is formed, which is a typical ternary line compound. Based on the diffusion experiments and inverse calculations, the tracer diffusion coefficients of components in ternary line compound (Mo, Nb)5Si3 were experimentally obtained. Moreover, ab-initio calculations of defect energetics clarified the microscopic diffusion mechanisms in the silicide-poor phase, and explained the reason why that the diffusion coefficient of Si decreases significantly with the increase of Nb mole fraction in ternary line compound (Mo, Nb)5Si3. The research provides valuable guidance for modeling and analyzing the diffusional phase transformation of multi-component systems.

Diffusion Behavior of Carbon and Silicon in the Process of Preparing Silicon Steel Using Solid-State Decarburization

In this study, we investigated the relationship between the decarburization effect of the solid-state decarburization method for preparing silicon steel and the atomic diffusion behavior in the matrix, focusing on 1 mm thick Fe-0.18 wt% C-Si (1.5, 3.5 wt%) alloy strips as the research object. Solid-state decarburization experiments were carried out in an Ar-H2O-H2 atmosphere, and the self-diffusion behavior of C and Si in Fe-C-Si alloy system was simulated by molecular dynamics. The results show that the molecular dynamics simulation results are consistent with the decarburization experimental results. When the temperature is lower than 800 °C, the atoms maintain a compact bcc structure, so the migration rate of carbon is low. When the temperature rises, the atoms move violently, resulting in the destruction of the crystal structure. Because the atomic arrangement tends towards a disordered structure, the migration rate of C is high and the diffusion coefficient increases. The experimental results showed that the decarburization rate was accelerated. At the same temperature, the diffusion activation energy Q = 48.7 kJ·mol−1 of carbon in an Fe-3.5 wt% Si-C alloy matrix can be calculated. The diffusion activation energy of carbon Q = 47.3 kJ·mol−1 was calculated using a molecular dynamics simulation. When the content of Si is 3.5 wt% and 1.5 wt%, the diffusion series of Si can be expressed as D3.5Si, Si = 8.54 × 10−10 exp(−33,089.7/RT) m2/s and D1.5Si, Si = 2.06 × 10−9 exp(−46,641.5/RT) m2/s, respectively. When the Si content is 3.5 wt%, the diffusion coefficient of Si is larger. After diffusion to the near surface, it combines with the remaining O to form a continuous strip of SiO2. When the Si content is 1.5 wt%, the diffusion coefficient of Si is small. The remaining O diffuses in the matrix and will oxidize when encountering Si; it cannot aggregate in a highly-dispersed distribution. The lattice transition from face centered cubic lattice austenite to body centered cubic lattice ferrite occurred in the matrix of the thin strip. The diffusion coefficient of C in ferrite is much larger than that in austenite. Therefore, the decarburization rate suddenly increases before decarburization stagnation. With the increase in Si content, the diffusion activation energy of C decreases, which promotes the decarburization effect.

Silicon diffusion in AlN

In this study, we investigate the diffusion of Si donors in AlN. Amorphous Si1−xNx sputtered on the surface of bulk AlN with low dislocation density is used as a Si source. The diffusion experiments are conducted through isochronal and isothermal annealing in a protective N2 atmosphere at temperatures between 1500 and 1700 °C. The Si depth profiles measured by secondary ion mass spectrometry exhibit a convex box-like shape with a steep diffusion front. These concentration profiles are best described with a diffusion coefficient that depends on the square of local Si concentration. From the characteristic box-shaped Si profiles, we conclude that diffusion of Si in AlN is mediated by singly negatively charged dopant–vacancy pairs SiAlVAl−. The strong concentration dependence of Si diffusion is due to the electric field associated with the incorporation of Si donors (SiAl+1) on substitutional Al lattice sites and reflects that Si is fully electrically active at diffusion temperature. The experimentally obtained extrinsic Si diffusion coefficient is reduced to intrinsic doping conditions. The temperature dependence of Si diffusion for intrinsic conditions is described by an activation enthalpy of (10.34±0.32)eV and a pre-exponential factor of 235−203+1485cm2s−1. The migration enthalpy of the donor–vacancy pair SiAlVAl− is estimated to be around 3.5 eV. This estimation is based on the activation enthalpy of the transport capacity of SiAlVAl− and theoretical results concerning the formation energy of negatively charged vacancies on Al-sites in AlN.

Silicon and iron isotopes in components of enstatite chondrites: Implications for metal–silicate–sulfide fractionation in the solar nebula

AbstractSilicon and iron isotope compositions of different physically separated components of enstatite chondrites (EC) were determined in this study to understand the role of nebular and planetary scale events in fractionating Si and Fe isotopes of the terrestrial planet‐forming region. We found that the metal–sulfide nodules of EC are strongly enriched in light Si isotopes (δ30Si ≥ −5.61 ± 0.12‰, 2SD), whereas the δ30Si values of angular metal grains, magnetic, slightly magnetic, and non‐magnetic fractions become progressively heavier, correlating with their Mg# (Mg/(Mg+Fe)). White mineral phases, composed primarily of SiO2 polymorphs, display the heaviest δ30Si of up to +0.23 ± 0.10‰. The data indicate a key role of metal–silicate partitioning on the Si isotope composition of EC. The overall lighter δ30Si of bulk EC compared to other planetary materials can be explained by the enrichment of light Si isotopes in EC metals along with the loss of isotopically heavier forsterite‐rich silicates from the EC‐forming region. In contrast to the large Si isotope heterogeneity, the average Fe isotope composition (δ56Fe) of EC components was found to vary from −0.30 ± 0.08‰ to +0.20 ± 0.04‰. A positive correlation between δ56Fe and Ni/S in the components suggests that the metals are enriched in heavy Fe isotopes whereas sulfides are the principal hosts of light Fe isotopes in the non‐magnetic fractions of EC. Our combined Si and Fe isotope data in different EC components reflect an inverse correlation between δ30Si and δ56Fe, which illustrates that partitioning of Si and Fe among metal, silicate, and sulfidic phases has significantly fractionated Si and Fe isotopes under reduced conditions. Such isotope partitioning must have occurred before the diverse components were mixed to form the EC parent body. Evaluation of diffusion coefficients of Si and Fe in the metal and non‐metallic phases suggests that the Si isotope compositions of the silicate fractions of EC largely preserve information of their nebular processing. On the other hand, the Fe isotopes might have undergone partial or complete re‐equilibration during parent body metamorphism. The relatively uniform δ56Fe among different types of bulk chondrites and the Earth, despite Fe isotope differences among their components, demonstrates that the chondrite parent bodies were not formed by random mixing of chondritic components from different locations in the disk. Instead, the chondrite components mostly originated in the same nebular reservoir and Si and Fe isotopes were fractionated either due to gas–solid interactions and associated changes in physicochemical environment of the nebular reservoir and/or during parent body processing. The heavier Si isotope composition of the bulk silicate Earth may require accretion of chondritic and/or isotopically heavier EC silicates along with cumulation of refractory forsterite‐rich heavier silicates lost from the EC‐forming region to form the silicate reservoir of the Earth.

Diffusion of Hydroxylated Si Vacancies in Olivine, and Its Relevance to Si Diffusion

AbstractThe diffusion coefficient associated with the 3,612 cm−1 infrared absorbance band in H‐bearing olivine, attributed to a defect involving a vacant Si site balanced by four H+ (i.e., ), was determined in H in‐diffusion experiments at 1200°C–1400°C and 1.5–3.0 GPa in piston‐cylinder and multi‐anvil apparatuses. The retrieved values are in good agreement with experimentally determined diffusion coefficients ascribed to the same process from previous experiments, despite those being conducted at atmospheric pressure and representing out‐diffusion rather than in‐diffusion. Overall, the diffusivity of the Si vacancy is several orders of magnitude lower than that of the M‐site vacancy (∼4 orders of magnitude at 1000°C), with a higher activation energy (∼450–500 vs. 200–250 kJ mol−1), when both are charge‐compensated solely by H+. From these data, the diffusion coefficient of Si in H‐bearing olivine can be estimated using the relationship between vacancy concentration, vacancy diffusivity, and diffusivity of an ion moving by a vacancy mechanism. However, these diffusivities are inconsistent with published results for Si diffusivity, with a considerable discrepancy of around two orders of magnitude. This suggests that simple relationships between diffusion coefficients of vacancies and species moving by vacancy diffusion mechanisms may be inappropriate, highlighting again the complexity of H incorporation and diffusion mechanisms in olivine.

Features of the Oxidation of Multilayer (TiAlCrSiY)N/(TiAlCr)N Nanolaminated PVD Coating during Temperature Annealing

A nano-multilayer Ti0.2Al0.55Cr0.2Si0.03Y0.02N/Ti0.25Al0.65Cr0.1N PVD coating was deposited on Kennametal carbide K 313 inserts. These coatings are widely used to protect cutting tools under severe exploitation conditions. Under equilibrium conditions, it was found that the Al2O3 oxide possessed better adhesive properties than the TiO2. The addition of chromium further enhanced the oxidation resistance of the coatings. Silicon significantly increased the oxidation resistance of this type of coating. The properties of the diffusion process in this coating have not been sufficiently investigated, despite the considerable number of articles published on this topic. For the purpose of this study, a multilayer ion-plasma (TiAlCrSiY)N/(TiAlCr)N coating was oxidized under equilibrium conditions; its chemical inhomogeneity was studied by time-of-flight mass spectroscopy using a TOF SIMS5-100 instrument. The data was collected from an area of 100 × 100 µ. A D-300 profilometer (KLA-Tencor Corp., Milpitas, California 95035, USA) was used to determine the rate of ion etching. It was found that oxidation commenced at the surface nanolayer of a TiAlCrN nitride, forming loose films of Cr2O3, TiO2, and Al2O3 oxides. This passivating film had a thickness of around 140 nm. For the first time, the interlayer diffusion coefficients of Si and Y were determined in multilayer coatings based on Ti0.2Al0.55Cr0.2Si0.03Y0.02N/Ti0.25Al0.65Cr0.1N, under open air annealing at 700 °C. The physical nature of the differences in the diffusion of these elements is discussed. The diffusion rate in the near-surface volumes was lower than in the deep layers of the multilayer coating, most likely due to the formation of passivating oxide films on the surface.

Water Enhancement of Si Self‐Diffusion in Wadsleyite

AbstractWe investigated the H2O‐content dependence of Si self‐diffusion coefficient in Fe‐free wadsleyite using multi‐anvil experiments at pressures of 19–20 GPa, temperatures of 1573–1873 K, and H2O‐content ranging from ∼10 to 5,300 wt. ppm by the isotopic thin‐film diffusion‐couple method. The 29Si‐doped diffusion profiles were measured by nanoscale secondary ion mass spectrometry in the depth profiling mode. The H2O contents in the samples were analyzed by Fourier transformation infrared spectroscopy. The experimental results show a H2O enhancement of Si diffusion coefficient with a H2O content exponent of 0.8 ± 0.1. The activation enthalpy was found to be 270 ± 40 kJ/mol. The diffusion coefficients in the [100], [010], and [001] directions are indistinguishable. The temperature and H2O‐content dependences of Si diffusion indicate that H2O incorporation dramatically reduces the rheological strength of wadsleyite, whereas temperature has a relatively small effect. The viscosity in the mantle transition zone could be significantly reduced by H2O incorporation in wadsleyite. The viscosity contrast between mantle plumes and surroundings may control the evolution of plume shapes at 410–660 km depths.

Molecular dynamics studies on spark plasma sintering of Ge–Si based thermoelectric material

The development of new fabrication technologies for Ge–Si thermoelectric materials requires a corresponding theoretical description of physical processes lying behind the synthesis. In the present paper, we investigated the interdiffusion of Si and Ge atoms at the Ge/Si interface, which takes place during spark plasma sintering of Ge and Si powders for fabrication of thermoelectric bulk. The calculation was performed using numerical simulation based on the classical molecular dynamics method. The diffusion coefficients of Si in Ge and vice versa were found at sintering temperatures of 900 K–1300 K and an external pressure of 7 MPa. The calculation results were used to analyze the experimental data derived from the measurements of Ge and Si profiles at the interface of thin Ge/Si plates subjected to spark plasma sintering at the temperature of 1160 K (887 °C). The comparison of measured and calculated diffusion profiles has shown good agreement with one another.

Pressure dependence of Si diffusion in γ-Fe

Abstract The pressure dependence of Si diffusion in γ-Fe was investigated at pressures of 5–15 GPa and temperatures of 1473–1673 K using the Kawai-type multi-anvil apparatus to estimate the rate of mass transportation for the chemical homogenization of the Earth's inner core and those of small terrestrial planets and large satellites. The obtained diffusion coefficients D were fitted to the equation D = D0 exp[−(E* + PV*)/(RT)], where D0 is a constant, E* is the activation energy, P is the pressure, V* is the activation volume, R is the gas constant, and T is the absolute temperature. The least-squares analysis yielded D0 = 10-1.17±0.54 m2/s, E* = 336 ± 16 kJ/mol, and V* = 4.3 ± 0.2 cm3/mol. Moreover, the pressure and temperature dependences of diffusion coefficients of Si in γ-Fe can also be expressed well using homologous temperature scaling, which is expressed as D = D0exp{–g[Tm(P)]/T}, where g is a constant, Tm(P) is the melting temperature at pressure P, and D0 and g are 10-1.0±0.3 m2/s and 22.0 ± 0.7, respectively. The present study indicates that even for 1 billion years, the maximum diffusion length of Si under conditions in planetary and satellite cores is less than ∼1.2 km. Additionally, the estimated strain of plastic deformation in the Earth's inner core, caused by the Harper–Dorn creep, reaches more than 103 at a stress level of 103–104 Pa, although the inner core might be slightly deformed by other mechanisms. The chemical heterogeneity of the inner core can be reduced only via plastic deformation by the Harper–Dorn creep.

Diffusion models for the early-stage SON68 glass dissolution in a hyper-alkaline solution

Understanding the dissolution of nuclear waste glass in hyper-alkaline conditions is critical to predicting its safe disposal in cement-dominated disposal conditions. In this work, solution data from the early-stage SON68 glass dissolution in a KOH solution (pH 13.5 and 30 °C) were interpreted with models based on silicon (Si) diffusion controlled stoichiometric dissolution of the glass. The solution data show a linear increase of boron (B) concentration with the square root of time and a decrease of the glass dissolution rate with an increase in the ratio of glass surface area to solution volume (SA/V). Three models from the literature, based on either aqueous pore diffusion or solid diffusion with/without resistance of the surface boundary layer, can fit the evolution of solution concentrations for the major elements of the glass equally well. However, the fitted Si diffusion coefficients differ by several orders of magnitude. Since all these fitted diffusion coefficients are lower than diffusion coefficients generally reported for aqueous pore diffusion, they are more indicative of solid diffusion processes. These results show that even under such hyper-alkaline conditions where dissolution and precipitation reactions are usually considered to prevail over solid diffusion, solid diffusion can still be an important dissolution mechanism. Those two dissolution mechanisms do not necessarily contradict each other. It is postulated that incorporation of potassium (K) from the solution onto the glass surface provides a diffusion barrier against the release of glass components into the solution. Therefore, the early-stage glass dissolution rate under hyper-alkaline conditions should be lower than the predicted glass network dissolution rates extrapolated based simply on pH effects.

In-situ synthesis of silicide coatings on molybdenum substrates by electrodeposition in chloride-fluoride molten salts

Silicide coatings including MoSi2 and Si-MoSi2 were prepared on molybdenum substrates by electrodeposition in NaCl-KCl-NaF-K2SiF6 molten salt. The experimental was conducted under different cathodic current densities at the temperature of 1073 K. The effect of the current density on the microstructure, phase composition, cross-section morphologies and elemental distribution of the as-prepared coatings were investigated by means of SEM, XRD and EDX. The results revealed that the type of the silicide coatings was strongly dependent on the current density and the relevant deposition mechanism was discussed. Besides, the electrochemical behavior of silicon ion in the chloride-fluoride molten salts was also studied using cyclic voltammetry and chronopotentiometry to reveal the electroreduction mechanism. The reduction of Si(IV) to Si was proved to be a quasi-reversible or irreversible diffusion-controlled single-step reaction and the diffusion coefficient of Si(IV) calculated from chronopotentiograms was (1.28 ± 0.25) × 10−5 cm2·s−1.

Electrochemical and Phase Analysis of Si(IV) on Fe Electrode in Molten NaCl-NaF-KCl-SiO2 System

AbstractThe electrochemical behaviour of Si(IV) ions on Fe electrode in the NaCl-NaF-KCl-SiO2 molten salt system (0.177 mol·L−1 SiO2) at 1,103 K was studied by using cyclic voltammetry, square wave voltammetry, chronoamperometry, and chronopotentiometry．Experiments conducted using the first two methods indicated that the reduction of Si(IV) occurred in two steps: Si(IV) → Si(II) → Si(0). Furthermore, the electrochemical reaction is a quasi-reversible process, which is controlled by both the ion diffusion and electron transport rates. The electrochemical crystallization of Si was found to be a transient, the three-dimensional nucleation process. The cyclic voltammetry curves indicate that the diffusion coefficient of Si(IV) is 1.16×10–5 cm2·s−1. The phases formed on the surface of the deposit were analysed by scanning electron microscopy and X-ray diffractometer. The results show that Fe and Si have formed intermetallic compounds Fe3Si, FeSi, and Fe5Si3.

Silicon Electrodeposition in a Water-Soluble KF–KCl Molten Salt: Effects of Temperature and Current Density

The effects of temperature and current density on the electrodeposition of Si films in molten KF–KCl–K2SiF6 were investigated at 923–1073 K. The peak current density of Si deposition in cyclic voltammetry increased as the temperature rose. The diffusion coefficient of Si (IV) ions measured by chronoamperometry increased through a rise in temperature. The activation energy for the diffusion of Si (IV) ions, 28.0 kJ mol−1, agreed with that for the viscosity of KF and KCl. The crystallinity of the Si films prepared by galvanostatic electrolysis on Ag substrates was measured based on electron backscatter diffraction. The largest crystallite size in the deposited Si increased with the deposition temperature, from a submicron size at 923 K, to several tens of microns at 1073 K. Moreover, even at the same temperature of 1073 K, larger crystallite sizes were observed for the Si deposit at 100 mA cm−2 as compared with that at 300 mA cm−2, which was explained based on the crystallization rate of Si.

Doping Si, Mg and Ca into GaN based on plasma stimulated room-temperature diffusion

It is demonstrated by use of the secondary ion mass spectroscopy (SIMS) some impurities, including Si, Mg and Ca, can be doped into GaN films with depths of ten nanometer and quite high surface densities in plasma without any external bias at room-temperature (RT). The physical mechanism of plasma doping without any external bias (PDWOEB) is RT diffusion stimulated by the plasma, which is very different from that of the plasma doping method with an external bias, which was called plasma immersion ion implantation in literature, whose physical mechanism is low energy ion implantation. Stimulated by a plasma with power of 750 W in 2 min, the RT diffusion coefficients of Si, Mg and Ca in GaN obtained by fitting the complementary error function distribution with their SIMS concentration distributions are 1.3E−15, 1.9E−16 and 1.2E−16 cm2/s, respectively. The RT diffusion coefficients of Si and Mg have been enhanced 18 and 22 orders of magnitude, respectively, compared with the RT extrapolation values of the high temperature diffusion coefficients reported in literature.

Studies of Diffusion Processes in AlAs/GaAs Resonant-Tunneling Heterostructures

The problem of reliability prediction and assurance is characteristic of wireless devices based on nanoscale multilayer heterostructures because of the sensitivity of heterostructures’ parameters to the degradation processes due to the thinness of layers. In the current work, the degradation of the nanoscale AlAs/GaAs resonant-tunneling heterostructures due to the diffusion of the constituent elements was investigated. Analysis and comparison of data on Al and Si diffusion coefficients in GaAs shows that they strongly vary depending on the conditions of heterostructure fabrication. This happens while the defect density of the grown heterostructures depends on a large number of technological factors such as the substrate temperature during molecular beam epitaxy, chamber pressure, annealing temperature and time, defect density in the initial substrate, and many others. The values of the diffusion coefficients obtained by the authors of this article by IR spectral ellipsometry are consistent with the data of foreign researchers. This allows their use to predict the reliability of resonant tunneling diodes and nonlinear radio transmitters based on them.

Assessment of the resistance to diffusion destruction of AlAs/GaAs nanoscale resonant-tunneling heterostructures by IR spectral ellipsometry

A technique for assessing the quality of AlAs/GaAs nanoscale resonant-tunneling heterostructures from the viewpoint of their resistance to diffusion destruction is developed. The diffusive spreading of AlAs/GaAs heterostructure layers is revealed by infrared (IR) spectral ellipsometry and the Al and Si diffusion coefficients in GaAs are determined.

Bottom-up Filling of Damascene Trenches with Cobalt By Electroplating Process

1. Introduction: In advanced semiconductor chips, filling of the circuit such as contact vias, trenches and conductive interconnects typically consists of Cu due to its high electrical conductivity. However, Cu can diffuse into Si and SiO2and form silicides to alter the properties of the circuit. The Cu diffusion can be much severer in small critical dimensions (CD < 10nm) as it can cause an electrical connection between two interconnects, resulting in an electric shot to damage the circuit. Cobalt with low diffusion coefficient in Si and SiO2 was considered as a filling material to replace Cu. In this study, Co filling was obtained by electroplating at a current density of 6.25mA/cm2 in a CoSO4electrolyte. For the first time, Co bottom-up and conformal growth in the damascene trenches with a CD range of 48-130nm can be achieved by electroplating in the Co electrolyte with different additives. The current efficiency, morphology, resistivity, uniformity, reflectivity and the deposition rate of the Co film on the blanket coupons (with Co 150Å) were also determined in this study. 2. Experimental: The plating electrolyte was made of 120g/L CoSO4.7H2O, 30g/L H3BO3 and 50mg/L Cl- at pH<4. The effect of different additives such as CUPUR CSFX, CUPUR DTX and CUPUR DTK was examined as a function of various concentration. The electrochemical method reported by Broekmann et al [1] was used to characterize the additives. The electrochemical experiment was performed in a three-electrode cell with a Co rod as anode, a standard Ag/AgCl reference electrode and a Pt RDE as cathode. Galvanostatic plating with the current density range of 6-12 mA/cm2was used for the Co deposition. 3. Results and Discussion: 3.1 Polarisation/depolarisation behaviours of the additives Figure 1 shows the electrochemical transient curve of the Co electrolyte and additives. The cathodic overpotential of the Co electrolyte was increased by dosing CUPUR DTX, indicating a polarisation occurred during the plating process. The cathodic overpotential of the Co electrolyte was gradually decreased by dosing CUPUR CSFX, indicating a depolarisation occurred during the plating process. We also found the effect of [Co2+] and [H3BO3] on the polarisation/depolarisation was not obvious, but the depolarisation of CUPUR CSFX was faster by increasing the concentration of [Cl-]. Figure 1 Electrochemical transient measurement for Co electrolyte, CUPUR DTX and CUPUR CSFX. 3.2 Partial fill results Different Co filling structure can be obtained by adding different additives. Bottom-up Co filling in the 130nm trenches (with Cu seeds) shown in Fig 2(a) was obtained by plating in the Co electrolyte with CUPUR DTX and CUPUR CSFX. Superconformal Co filling shown in Fig 2(b) was obtained by plating in the Co electrolyte with CUPUR DTK and CUPUR CSFX. Figure 2 SEM image of Co partial fill in the 130nm damascene trenches (a) bottom-up filling; (b) superconformal with V-shape filling. Further study will be focused on the Co filling performance in the trenches with Co seed. 4. Reference: P. Broekmann, et al, Electrochimica Acta 56 (2011) 4724. Figure 1

Reactivity of Zircaloy-4 with Ti3SiC2 and Ti2AlC in the 1100–1300 °C temperature range

Herein, we report on the interactions between zirconium, in the form of Zircaloy-4 (Zr-4) and the MAX phases, Ti3SiC2 and Ti2AlC in the 1100–1300°C range. In both cases, the A-group element, Al or Si, diffused out of the MAX phases and formed intermetallics with the Zr-4. Under comparable conditions, Si penetration into the Zr-4 was roughly an order of magnitude less than that of the Al from the Ti2AlC. The effective diffusion coefficient of Si can be described by Deff_Sim2s=7.4×10-9exp-170kJ/molRT; that of Al by Deff_Alm2s=9.3×10-12exp-38kJ/molRT. At 38kJ/mol, the activation energy for the Ti2AlC/Zr-4 diffusion bonding process was significantly lower than that of the Ti3SiC2/Zr-4 diffusion couples, at 170kJ/mol.

Investigation on fluoroacidity of molten fluorides solutions in relation with mass transport

Equilibria between species in solution and its corresponding gaseous species were investigated in various molten fluorides mixtures. From the measurements of boron and silicon kinetic constant of gas release, a relative fluoroacidity scale of solvents was proposed. Due to very high kinetic rates of SiF4(g) and BF3(g) release for the most acidic melts, another approach to evaluate fluoroacidity based on mass transport was considered. Relationships between bridging fluorine atoms, viscosity and fluoroacidity was demonstrated. Diffusion coefficients of Si(IV) and B(III) ions were measured and correlated to the viscosity through the Schmidt number (Sc) defined as the ratio between solvent viscosity and solute diffusivity. These results confirmed the fluoroacidity scale previously obtained, validating the original approach developed on mass transport.

Diffusivity of dissolved silica in rock pore water at 25°C as characterized by through-diffusion experiments

[1] The diffusivity of dissolved Si in rock pore water at 25°C was evaluated by through-diffusion experiments for three types of rock (Fontainebleau sandstone, Berea sandstone, and Tenjyosan rhyolite). Comparisons of the effective diffusion coefficients of Si, K+, and Cl− ( , , and ) at neutral pH (unbuffered) showed that the values obtained for the three rocks were 2.3–3.0 times smaller than those of and . The small value of compared with and is primarily attributed to the difference in the diffusion coefficients of K+, Cl−, and Si in free water (a factor of 1.7) but might be also partly due to the precipitation of Si in the pores. The chemical form of the dissolved Si is altered depending on the solution pH. To examine how the diffusivity of Si is affected by its chemical form, the values were also determined at pH 6 and pH 11 (buffered solutions containing 0.5 M NaCl) using a Fontainebleau sandstone sample. The dominant chemical forms of Si predicted from thermodynamic calculations are an electrically neutral monomer (Si(OH)4) at pH 6 and an anionic monomer (SiO(OH)3−) with a certain amount of anionic multimers (e.g., dimer, trimer, tetramer) at pH 11 (total Si concentration ≤20.4 mM). The values of obtained at pH 6 and pH 11 were quite similar (1.94 × 10−7 to 1.97 × 10−7 cm2 s−1), and was hardly affected by the change in the charge state and the presence of multimeric species.