Growth Of Diffusion Layer Research Articles

Corrosion behavior of Al2O3-modified Yb2SiO5 environmental barrier coating under water vapor conditions at 1500 °C

Al2O3-modified Yb2SiO5 was prepared as environmental barrier coating (EBC), and the water vapor corrosion behavior at 1500 °C was investigated. The residual phase, Yb2O3, was observed in the as-prepared coating besides Yb2SiO5 and Al2O3, which preferred to react with Al2O3 to form toughened Yb3Al5O12, meanwhile gave rise to the growth of diffusion layer (Yb2Si2O7) and prevented the generation of the porous region under EBC layer after exposure. The growth stress of the TGO layer and the thermal stress during cooling were the driving sources of delamination, which limited the service life of EBCs. Conclusions on phase transformation and interface evolution in this work are expected to guide the design of rare earth silicate EBCs.

Prospects of Application of Ultramicroelectrode Ensembles for Voltammetric Determination of Compounds with Close Standard Electrode Potentials and Different Diffusion Coefficients

The spherical diffusion that occurs when using ultramicroelectrodes (i.e., electrodes with a characteristic size of 1–10 µm) contributes to a higher mass transfer rate. This leads to equalization of the depletion rates of the near-electrode layer due to the electrochemical reaction and to the supply of the product from the solution depth. This is the reason why, for ultramicroelectrodes, a limiting size of the spherical layer exists in which the concentration gradient is localized (diffusion layer). Thus, a stationary mass transfer mode is achieved, which is expressed in the sigmoidal CV curve’s shape. In ultramicroelectrode arrays, when the diffusion hemispheres are separated, a steady-state diffusion is realized. However, with a decrease in the interelectrode distance, which leads to the diffusion spheres intersection, a mixed regime arises, which is not fully time-independent. The resulting voltammogram’s shape change can serve as an analytical signal in the study of substances with differing diffusion coefficients, since the diffusion layer growth rate and, consequently, the area of intersection of neighboring spheres, depends on it. This work shows the applicability of voltammetry using ensembles of ultramicroelectrodes operating in the transient mode for the analysis of mixtures of electrochemically active compounds with close electrode reaction parameters, such as exchange currents and electrode potential. Ferrocenemethanol esters are used as an example. The applicability of cyclic voltammetry on the UME array for analysis of mixtures was illustrated by means of finite element modelling. The reliability of the modelling results was experimentally proved for ferrocenemethanol esters with glycine and triglycine.

Thermal stability and interfacial structure evolution of Bi2Te3-based micro thermoelectric devices

Bi2Te3 based thermoelectric device is the only commercialized thermoelectric module. The thermal stability of the interface between the thermoelectric materials and barrier layer or electrode plays pivotal role for the stability and reliability of micro thermoelectric devices especially for power generation. In this work, the thermal stability, bonding strength and structure evolution of the interface between Bi2Te3 based materials (both n type and p type materials) and Ni barrier layer were systematically investigated. The interface between Ni and Bi2Te3 based materials demonstrates an ohmic contact with the contact resistivity of 3.64 μΩ cm2 and 5.31 μΩ cm2 for p-type and n-type element respectively. The bonding strength of p-type and n-type element with electrode is 13.43 MPa and 17.62 MPa, respectively. Upon annealing at 423 K for 7 days, due to the mass diffusion between Ni and Bi2Te3 based materials, a thin Ni-Te layer is formed. With further extending the annealing time at 423 K to 42 days, the thickness of the diffusion layer increases to 3 µm. The growth of diffusion layer enhances the bonding strength to 19.81 MPa. However, this is accompanied with a sharp increase in the contact resistivity to 14.48 μΩ cm2 for n-type element and to 6 μΩ cm2 for p-type element with Ni barrier. The increase in the contact resistance deteriorates the output power of thermoelectric module especially for the micro-device from 2.60 mW to 2.48 mW under the temperature gradient of 20 K. This work points out that for the application of Bi2Te3 based thermoelectric device in power generation above 423 K, appropriate barrier layer is indispensable and plays significant role for the performance of the device.

Swelling of Diffusive Fluid Threads in Microchannels.

The dynamics of viscous fluid threads concurrently flowing with miscible solvents is experimentally investigated in square microchannels. Diffusive fluid threads are found to significantly swell at low flow velocities due to large specific interfacial area and hydrodynamic lubrication. An approach based on bounded function analysis of confined thread diameter is developed to model diffusive behavior of viscosity-differing fluids at the small scale. This work shows the determination of a critical flow rate associated with each fluid pair and the use of dynamic similarity to calculate diffusion coefficients between oils and organic solvents. The thread divergence is estimated based on the growth of diffusion layers and related to diffusion-induced buckling instabilities of viscous threads in parallel flows.

On the kinetics of nitride and diffusion layer growth in niobium plasma nitriding

This work reports results of a study on niobium plasma nitriding kinetics, conducted at relatively low temperatures, regarding the N-Nb system (1523 K) eutectoid temperature, and short times (on the 1253–1453 K and 1–4 h ranges). Results confirm that the niobium plasma nitriding is diffusion-controlled. The obtained nitride (compound) layer is multiphase, mainly constituted of ε-NbN, γ-Nb4N3 and β-Nb2N nitrides. The estimated compound and diffusion layer growth activation energy was 97.11 and 120.11 kJ mol−1, respectively. Considering the hardness distribution all over the nitrided layer, the transition between the compound and diffusion layers is very sharp. As example, it can vary from 18.5 GPa (⁓1850 HV) at the surface compound layer to decreasing values from ⁓480 to 100 HV, along the diffusion layer, depending on the studied condition. All results obtained from the Sa and Sz roughness characterization indicated increasing measured values for longer treatment times and higher temperatures, for atom clusters islands-like surface morphology. Finally, apparently crack-free compound layers were yielded, highlighting the importance of the use of relatively low treatment temperatures, below the N-Nb system (1523 K) eutectoid temperature, and relatively short treatment times for the plasma nitriding treatment studied here.

Control of the Structure of Diffusion Layer in Carbon Steels Under Nitriding with Preliminary Deposition of Copper Oxide Catalytic Films

The effect of thin films of copper oxide deposited before nitriding on the phase composition and the kinetics of growth of diffusion layers in carbon steels is considered. The process of formation of an oxide film involves chemical reduction of pure copper on the surface of steel specimens from a salt solution and subsequent oxidation under air heating. The oxide film exerts a catalytic action in nitriding of low- and medium-carbon steels, which consists in accelerated growth of the diffusion layer, the nitride zone in the first turn. The kinetics of the nitriding process and the phase composition of the layer are controlled by the thickness of the copper oxide precursor, i.e., the deposited copper film.

DIFFUSION LAYER GROWTH MECHANISM IN ASPN METHOD USING AN IRON CAGE FOR ST52 STEEL

The mechanism of diffusion layer growth in plasma nitrided coatings applied on a St52 steel using an active screen is investigated. The nitriding was performed at 450, 500 and 550 ◦C temperature nitriding times of 5, 10 and 15 h, in a gas mixture containing 20 vol. % H2: 80 vol. % N2 and DC-pulsed plasma nitriding unit. The surface, cross section and the thickness of diffusion of specimens was studied in terms of optical and scanning electron microscopy. According to the measurements of diffusion layer thickness, values of Q and Do for nitrogen diffusion in substrate were calculated as 50585 (J/mol) and 4.11×10 (m/s), respectively. The variations of hardness depth during nitriding period were determined.

Bridging the gap between an isolated nanochannel and a communicating multipore heterogeneous membrane

To bridge the gap between single and isolated pore systems to multipore systems, such as membranes and electrodes, we studied an array of nanochannels with varying interchannel spacing that controlled the degree of channel communication. Instead of treating them as individual channels connected in parallel or an assembly like a homogeneous membrane, this study resolves the pore-pore interaction. We found that increased channel isolation leads to current intensification, whereas at high voltages electroconvective effects control the degree of communication via suppression of the diffusion layer growth.

Erosion Behavior of Al-12Si-2Mg-15Cu Alloy against 304 Stainless Steel Surface

The compatibility between phase change alloys and container steels at the high temperature is limiting their potential applications in energy storage systems. In this paper, we investigated the erosion behavior of Al-12Si-2Mg-15Cu alloy against 304 stainless steel surface. The effect of temperature and immersion time on the erosion behavior was studied. The erosion rate and erosion thickness increased with the temperature increasing. With the increase of time, the erosion layer thickness increased, but the erosion rate decreased. The mechanisms which accounted for the formation and growth of diffusion layer were studied. The results show that the corrosion is mainly due to the diffusion of Al. The presence of Si element resists the growth of Fe-Al compounds and make erosion layer brittle. Ni-compounds can decrease the diffusion rate of Al atoms. Furthermore, the erosion layer adjacent to the steel substrate is brittle and its micro-hardness is higher.

Microstructural characterization and kinetics of diffusion bonded AZ31/Al by hot press cladding

In the present investigation, magnesium alloy AZ31 sheet was covered by a commercial pure aluminium sheet by cladding using hot pressing. The influence of the applied pressure, temperature and holding time on the bond characteristics was studied. Microstructure examination of bonded specimens was experimentally investigated by optical microscope, SEM and EDX. The experimental investigation has revealed a good bond quality due to the effective mutual diffusion of Mg and Al. The phase analysis resulted in the formation of two equilibrium phases namely; Mg17Al12 and Mg2Al3. The initiation and growth mechanisms of the diffusion layers were determined by empirical equations and compared with the experimental results. It was observed that the growth kinetics obeyed a parabolic equation of the second order. Moreover, Arrhenius equation for the bonded couple AZ31/Al is derived to show the rate of diffusion layer growth.

AISI 420 martensitic stainless steel low-temperature plasma assisted carburizing kinetics

The present paper reports the experimental results on martensitic stainless steel low-temperature plasma assisted carburizing kinetics. The treatments were carried out using a gas mixture of 99.5% (80% H2+20% Ar)+0.5% CH4 at temperatures of 623, 673, 723 and 773K and for times of 4, 8, 12 and 16h. The peak voltage and the pulse period of the pulsed DC power supply were kept constant at 700V and 240μs, respectively. Temperature was controlled by adjusting the duty cycle. The treated samples were characterized by confocal laser scanning microscopy, X-ray diffractometry and microhardness measurements. Results indicate that low-temperature plasma carburizing is a diffusion controlled process. The calculated activation energy for outer and diffusion layer growth was 29 and 85kJmol−1, respectively. An apparently precipitation-free layer can be produced only when the processing temperature is sufficiently low (≤723K) and time sufficiently short (≤12h).

Modelling of processes of thermochemical treatment of metals: traditions of Russian scientific school

The paper is devoted to 100th anniversary of the outstanding Russian scientist professor Yuriy Mikhailovich Lakhtin – the founder of the world-famous scientific school of surface strengthening of metals and alloys. Lakhtin's scientific school is recognised for its contribution into research of processes of thermochemical treatment of metals and especially of nitriding. Today at the Department of Metal Science and Heat Treatment of MADI his followers continue the traditions of Lakhtin's scientific school. The development of technologies of surface engineering is based on complex modelling of physical processes realised by thermochemical treatment of metals. Thermodynamic models describe the interaction between metals and components of saturating atmosphere and predict phase composition of diffusion layer. Diffusion models of kinetics of saturation of metals allow us to calculate the rate of growth of diffusion layer and regulation of its depth and structure. Structural models determine quantitative dependence between parameters of structure (grain size, dispersed particles, etc.) and mechanical properties. These models allow us to estimate the level of strengthening by control of structural specifics of strengthened layer. On the basis of this complex of models, new efficient technologies of surface strengthening are developed.

Growth of diffusion layers at liquid Al–solid Cu interface under uniform and gradient high magnetic field conditions

The effect of high magnetic fields on interfacial reactive diffusion in liquid/solid (Al/Cu) couples was experimentally investigated at a temperature of 973 K. Regardless of any magnetic field, compound layers consisting of the δ, ξ 2, η 2 and θ phases were produced at the interface. The magnetic flux density, B , exerted a non-monotonic influence on the growth of the diffusion layers. Moreover, the mean thickness of the diffusion layers (parallel to B ) was found to be always greater than that of the diffusion layers (perpendicular to B ) under the applied magnetic fields. These phenomena should be attributed to the effects of two types of the Lorentz force under a uniform high magnetic field on diffusion behavior. In addition, the growth of intermetallic phases could be retarded by a magnetic field gradient due to the magnetic force in the axial direction.

Irradiation-enhanced interdiffusion in the diffusion zone of U-Mo dispersion fuel in Al

Uranium-molybdenum (U-Mo) alloy fuel particles dispersed in an aluminum (Al) matrix, designated as U-Mo/Al dispersion fuel, is in the development stage in the worldwide RERTR (Reduced Enrichment for Research and Test Reactors) program. The main issue in developing U-Mo/Al dispersion fuel is the diffusion reaction occurring at the interface between the fuel particles and matrix. To accurately analyze fuel performance, a model to predict the diffusion kinetics is necessary. For this purpose, the authors developed a diffusion layer growth rate correlation for out-of-pile annealing tests and a similar correlation for in-reactor tests. The correlation for in-reactor tests is considerably different from that of out-of-pile tests because it contains factors that amplify diffusion kinetics by fission damage in the diffusion reaction zone. This irradiation enhancement was formulated by a combination of the fission rate in the fuel and fission fragment damage distribution in the diffusion reaction zone. Using a computer code, fission damage factors were obtained as a function of diffusion reaction layer thickness and composition. The model correlation was established and fitted to the in-reactor data. As a result of this data fitting, the interaction layer growth rate is found to be proportional to the square root of the fission fragment damage rate and to have a temperature dependence characterized by the effective activation energy of 46 to 76 kJ/mole, which is smaller by a factor of 4 to 7 than that of out-of-pile tests.

On the growth of diffusional layer driven by solid solution formation

Abstract The kinetics of diffusion layer growth is considered for the case when the supply of the diffusing component from an external source to the specimen surface is driven by solid solution formation. The growth law is shown to be X S ∼ t 2/3 rather than the classical parabolic behavior ( X S ∼ t 1/2 ).

In situ processing of TiB 2/TiC ceramic composites by thermal explosion under pressure: experimental study and modeling

A model describing non-isothermal reactive diffusion in B 4C–Ti powder blends leading to thermal explosion (TE/SHS) in the conditions of enhanced heat transfer into the surrounding ambience was developed. The model explicitly incorporates the reaction kinetics and is based on previously reported experimental kinetics of isothermal growth of diffusion layers at a flat B 4C/Ti interface. In spite of a number of simplifications (spherical geometry, sequential reactions of TiB and TiB 2 formation, etc.), a good agreement between the model calculations and experimental results on TE/SHS in B 4C–3Ti dense samples placed between the preheated rams of a press has been obtained. The B 4C powder particle size was found to significantly affect the ignition of TE, and the combustion process could be controlled by addition of small amounts of very fine powder fraction.

FACTORS DETERMINING GROWTH OF DIFFUSION LAYERS ON LOW CARBON STEEL AND IRON DURING VACUUM CHROMISING

Layers produced on low carbon steel and Armco iron during vacuum chromising have been investigated. In particular, the influence of two fundamental factors, the surface area of the sublimating metallic chromium and the pressure within the furnace chamber, is discussed. Layer thickness and morphology, and the chromium and iron concentrations in the diffusion zone of chromised layers, have been studied. Layers consisting of solid solutions of chromium in iron resulted in all instances. Chromising conducted at low pressure, with a static rather than a dynamic vacuum, was found to be most effective for maximising the rate of growth of diffusion layers. Scanning electron micrographs of the surfaces of chromised low carbon iron alloys show the presence of microsteps characteristic of screw dislocations. Hence, it is concluded that the growth of diffusion layers during vacuum chromising depends on the generation and growth of dislocations according to Frank‘s spiral lnechanism model.

The measurement of transient dendrite tip interface supersaturation in NH4Cl− H2O using optical techniques

Using laser optical techniques, the time-dependent concentration field ahead of a (transient) solidifying dendritic interface was measured in the NH 4 Cl− H 2 O system. Dendrite tip interface supersaturation was calculated from the measured concentrations. It increases with time until convective effects disrupt diffusion layer growth. The rate of increase of the supersaturation is a strong function of cooling rate but has only a slight dependence on temperature gradient.

金属間化合物ＳｍＣｏ５およびｍ２Ｃｏ１７におけるＳｍの拡散係数

For the purpose of obtaining basic information on the reduction-diffusion process, diffusion experiments were carried out for the Sm-Co binary system at 1000, 1050, 1100, 1150, and 1200°C. Sm was made diffuse into Co by contacting a vapor from a molten Sm-Ca alloy with a polished surface of a Co block. The use of the Sm-containing vapor was effective to suppress the formation of a Co-containing melt on the surface of the Co block. The formation of the Co-containing melt hindered the uniform growth of diffusion layers and blurred the time of initiation of the diffusion. Though several intermetallic compounds are present in the Sm-Co binary system. only SmCo5 and Sm2Co17 phases were found to grow systematically. Diffusion coefficients of Sm for the reactive diffusion in these phases at the respective temperatures were calculated from the relationships between the diffusion layer thickness and the diffusion time by utilizing Sm concentrations at the boundaries of the phases which were read from Sm-Co binary phase diagrams. The diffusion coefficient of Sm in SmCo5 has been found at least three times larger than that in Sm2Co17 at each temperature of this study, and the reason for their difference was qualitatively discussed by considering the difference in their crystal structures. Also a reasonable explanation could be given for contradictory results reported by other investigators about the preferential growth of the Sm2Co17 phase.

Cracking and layer growth in Nb<inf>3</inf>Sn Bronze route material

A sensitive AC technique has been used to investigate T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and layer growth in Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn diffusion layer material. This method and the more frequently used resistive T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> measurement have been used in conjunction to examine several pertinent topics in diffusion layer growth. Firstly, it has been shown that the 37% volume expansion that occurs when Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn is formed can be accommodated without cracks forming. Secondly, by deliberately deforming the layer the effects of cracks have been investigated; the inductive and resistive methods being sensitive to cracks along and across the layer respectively. Thirdly, by deconvoluting the increase in amplitude of the inductive signal with annealing time (t) the layer thickness (δ) of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn in a standard multifilamentary wire can be plotted; for IMI All Bronze (3000 × 5μm filaments) at 750°C <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\delta\sim t^{0.25}</tex> .