scholarly journals Microstructures of SiO2 Scales Formed on MoSi2

2006 ◽  
Vol 522-523 ◽  
pp. 595-602 ◽  
Author(s):  
Kazuya Kurokawa ◽  
Daichi Goto ◽  
Jyunichi Kuchino ◽  
Akira Yamauchi ◽  
Tamaki Shibayama ◽  
...  
Keyword(s):  
Growth Rate ◽  
Diffusion Coefficient ◽  
Oxide Scale ◽  
Oxidation Rate ◽  
Diffusion Mechanism ◽  
Lattice Diffusion ◽  
Oxide Scales ◽  
Amorphous Sio2 ◽  
O2 Diffusion ◽  
Diffusion Coefficient Of Oxygen

The microstructures of oxide scales formed on MoSi2 at medium-high temperatures in air were observed by TEM. Based on the observation, relationships between oxidation temperature and formation of MoO3 and crystallization of amorphous SiO2 scales were investigated. At 1273 K and 1373 K, the oxide scales had a structure consisting of amorphous SiO2 with small amounts of fine MoO3 particles. The oxide scales at 1573 K and 1773 K had a structure consisting of amorphous and crystalline SiO2. Growth rate of the oxide scale formed at 1773 K appreciably increased due to crystallization of amorphous SiO2. It was thought that the increase in the oxidation rate at 1773 K was caused by a change in the diffusion mechanism from O2 diffusion to lattice diffusion of O2- through SiO2. In addition, the diffusion coefficient of oxygen was estimated from the growth rate of SiO2 scale.

scholarly journals Analysis of oxide scale thickness and pores position of HCM12A steel in supercritical water

2021 ◽  
Vol 40 (1) ◽  
pp. 53-65
Author(s):  
Li Sun ◽  
Weiping Yan
Keyword(s):  
Diffusion Coefficient ◽  
Oxide Scale ◽  
Supercritical Water ◽  
Heating Surface ◽  
Oxidation Mechanism ◽  
Oxygen Activity ◽  
Oxide Scales ◽  
Spinel Layer ◽  
Magnetite Layer ◽  
Iron Diffusion

Abstract More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler. The oxidation mechanism of HCM12A steel in supercritical water is proposed and the growth of oxide film is simulated. The duplex scale contains an outer magnetite layer and an inner Cr-rich spinel layer. According to the data of Backhaus and Töpfer, the diffusion coefficient values of iron in magnetite layer are discussed and the function of R V, R I {R}_{\text{I}} for oxygen activity can be used for calculation of iron diffusion coefficients in Cr-rich spinel layer. Based on Wagner’s oxidation theory, the oxidation rate constants of HCM12A are calculated at 500 and 600°C in supercritical water, compared with experimental data of the relevant literatures. The oxygen activities at the interfaces of alloy/Cr-rich spinel oxide and magnetite/supercritical water are estimated. The simulation results of weight gain are matched with the test data. The iron diffusion mechanisms inside the magnetite layer and the Cr-rich spinel layer are analyzed. The iron diffusion coefficient at the interface of Cr-rich spinel/magnetite is discontinuous, while the oxygen activity is continuous in the whole double layer. The thickness of oxide scale on inner tube walls of the final superheater coils (T91) of a 600 MW supercritical boiler is calculated by using the calculation method provided by the paper. The modeling results, the measured data, and the calculation results by the method are compared. Accurate calculation of the thickness of the inner and outer oxide scales can provide a necessary basis for predicting the stress and exfoliation of oxide scales.

Calculation of Boundary Conditions between Internal and External Oxidation of Silicon or Chromium Containing Steels

2011 ◽  
Vol 696 ◽  
pp. 88-93 ◽  
Author(s):  
Shohei Nakakubo ◽  
Mikako Takeda ◽  
Takashi Onishi
Keyword(s):  
Boundary Condition ◽  
Diffusion Coefficient ◽  
Boundary Conditions ◽  
Oxygen Concentration ◽  
Internal Oxidation ◽  
Rate Equation ◽  
Oxidation Rate ◽  
Oxidation Rate Constant ◽  
External Oxidation ◽  
Diffusion Coefficient Of Oxygen

The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were calculated in order to clarify the formation mechanism of fayalite scale (Fe2SiO4) or chromite scale (FeCr2O4), which can form as a “sub-scale” in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, NO(s), which are constituents of the internal oxidation rate constant, (2DONO(s)/NB(O)n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2DONO(s)/NB(O)n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were also calculated by substituting the calculated values of DO and NO(s) into the rate equation.

scholarly journals Effect of Ti and Ta on Oxidation Kinetic of Chromia Forming Ni-Base Superalloys in Ar-O2-Based Atmosphere

2018 ◽  
Vol 37 (9-10) ◽  
pp. 801-806 ◽  
Author(s):  
Wojciech J. Nowak ◽  
Bartek Wierzba ◽  
Jan Sieniawski
Keyword(s):  
Oxide Scale ◽  
Oxidation Rate ◽  
Oxidation Kinetic ◽  
Isothermal Oxidation ◽  
Oxygen Flux ◽  
Oxide Scales ◽  
Lower Oxidation

AbstractIn the present study, two commercially available Ni-base superalloys, namely Rene 80 which contains Ti and IN 792 containing Ti and Ta, were investigated during isothermal oxidation at 1,050°C in Ar–20% O2 gas up to 50 h. It was found that IN 792 showed a slightly lower oxidation rate as compared to Rene 80. Also differences in oxide scale microstructures and thicknesses were found. Both alloys formed qualitatively similar oxide scales consisting of outer TiO2 on top of a Cr2O3 scale and internally oxidized Al2O3. The IN 792 formed a thinner oxide scale in general as well as a thinner outer TiO2 scale. Also the depth of the internally oxidized Al2O3 precipitates is smaller in case of IN 792. These findings were correlated with the formation of TiTaO4, which in turn hampered outer Ti and inward oxygen flux.

Effect of Perovskite Coating on Oxide Scale Growth on Fe-22Cr

2006 ◽  
Vol 258-260 ◽  
pp. 372-377
Author(s):  
Åase Persson ◽  
Peter V. Hendriksen ◽  
Lars Mikkelsen ◽  
Marcel A.J. Somers
Keyword(s):  
Growth Rate ◽  
Oxide Scale ◽  
Oxidation Rate ◽  
Oxidation Mechanism ◽  
Scale Growth ◽  
Spinel Layer ◽  
Oxide Layers

A coating consisting of La0.85Sr0.15MnO3 (LSM) was deposited onto two Fe 22 wt % Cr alloys Crofer 22APU and Sandvik 1C44Mo20. The evolution of the oxide layers developing underneath the coatings during oxidation was investigated. The effect of the LSM coating on oxidation rate and microstructure of the oxide scale was investigated, and possible effects on the overall oxidation mechanism are discussed. It was found that the growth rate for coated Crofer 22APU was decreased by a factor of 3 in comparison with the uncoated samples, and the thickness of the chromia layer in the scales decreased in favor for a (Mn,Cr)-spinel layer.

Precipitation Behavior of MN(M=V,Nb) Phase in the 7Cr-1.5Mo Ferritic Heat Resistant Steels

2007 ◽  
Vol 26-28 ◽  
pp. 1093-1096
Author(s):  
Deuck Seung Bae ◽  
Woo Sang Jung
Keyword(s):  
Diffusion Coefficient ◽  
Ostwald Ripening ◽  
Diffusion Mechanism ◽  
Effective Diffusion ◽  
Lattice Diffusion ◽  
Precipitation Behavior ◽  
Heat Resistant ◽  
Heat Resistant Steels ◽  
Fossil Power Plant ◽  
The Relationship

The relationship between precipitation behavior of MX(M=V, Nb) nitrides and mechanical property during aging in 7Cr-1.5Mo ferritic heat resistant steels were investigated in this study. Artificial aging was carried out at 700°C for different time from 100 to 10000 hours to simulate the environment of fossil power plant. The Ostwald ripening equation is related to the containing elements can explain the accelerated coarsening of MX in the model alloys. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that V, Nb and Ti might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism.

The stirring of the medium by bull spermatozoa

1963 ◽  
Vol 157 (969) ◽  
pp. 461-472 ◽  
Keyword(s):  
Diffusion Coefficient ◽  
Short Range ◽  
Short Range Order ◽  
Effective Diffusion ◽  
Sperm Head ◽  
Special Shape ◽  
Bull Semen ◽  
Sperm Density ◽  
Bull Sperm ◽  
Diffusion Coefficient Of Oxygen

1. Bull semen diluted 1/5 or 1/10 respired at the same rate whether the manometers were stationary or shaken. 2. Respirometric experiments using a manometer flask of special shape showed that bull sperm suspensions achieve this result by increasing the effective diffusion coefficient of oxygen in the suspending medium by 900%. 3. The hypothesis is put forward ( a ) that these results are caused by the existence of short-range order, as opposed to disorder, in bull sperm suspensions, even at comparatively low sperm densities (dilution 1/8 to 1/20); ( b ) that this order produces group sperm velocities greater than those of isolated spermatozoa; and ( c ) that as a result, larger volumes of the suspending medium are convected with the ordered sperm groups, causing an augmented ‘diffusion’ of oxygen. 4. This hypothesis was examined by taking photomicrographs of sperm suspensions at different dilutions and temperatures and determining from them the distributions of (i) the distance between pairs of nearest spermatozoa; (ii) the angle of inclination of a sperm head relative to that of the spermatozoon nearest to it; and (iii) the relative position of the nearest spermatozoon. 5. Comparison of the observed distributions and the corresponding random ones showed that the spermatozoa attracted each other, so that transient sperm groups were formed, in which the spermatozoa tended to swim in the same direction. A reduction in temperature or sperm density decreased the sperm order.

scholarly journals Methanesulphonic acid movement in solid ice cores

2004 ◽  
Vol 39 ◽  
pp. 540-544 ◽  
Author(s):  
Barbara T. Smith ◽  
Tas D. Van Ommen ◽  
Mark A. J. Curran
Keyword(s):  
Diffusion Coefficient ◽  
Biological Activity ◽  
Sea Ice ◽  
Diffusion Mechanism ◽  
Ice Core ◽  
Ice Cores ◽  
East Antarctica ◽  
The Core ◽  
Central Value

AbstractMethanesulphonic acid (MSA) is an important trace-ion constituent in ice cores, with connections to biological activity and sea-ice distribution. Post-depositional movement of MSA has been documented in firn, and this study investigates movement in solid ice by measuring variations in MSA distribution across several horizontal sections from an ice core after 14.5 years storage. The core used is from below the bubble close-off depth at Dome Summit South, Law Dome, East Antarctica. MSA concentration was studied at 3 and 0.5 cm resolution across the core widths. Its distribution was uniform through the core centres, but the outer 3 cm showed gradients in concentrations down to less than half of the central value at the core edge. This effect is consistent with diffusion to the surrounding air during its 14.5 year storage. The diffusion coefficient is calculated to be 2 ×10–13 m2 s–1, and the implications for the diffusion mechanism are discussed.

Molecular Dynamics Simulation Study of the Effect of Ni, Fe, Cu on Cryolite Molten Salt System 78% Na3AlF6-9.5%AlF3-5.0%CaF2-7.5%-Al2O3

2021 ◽  
Vol 1026 ◽  
pp. 39-48
Author(s):  
Han Bing He ◽  
Yu Si Wang ◽  
Ze Xiang Luo ◽  
Jing Zeng
Keyword(s):  
Molecular Dynamics ◽  
Diffusion Coefficient ◽  
Molten Salt ◽  
Diffusion Mechanism ◽  
Research Work ◽  
Dynamics Simulation ◽  
Salt System ◽  
Electrolyte System ◽  
Aluminum Composite ◽  
Molten Salt System

The effect of different additives Ni, Fe, Cu on the structure and properties of electrolyte system 78% Na3AlF6- -9.5%AlF3-5.0%CaF2-7.5%Al2O3 at 1200K and 1.01Mpa was studied by molecular dynamics method. The radial distribution function, coordination number, diffusion coefficient, conductivity, and viscosity of the system were discussed in detail. The results demonstrated that the order of the self-diffusion coefficient of ions in the electrolyte system is: Na+ > F- > O2- > Ca2+ >Al3+. The addition of Ni and Fe connected the free aluminum composite ion groups in the system through fluorine bridges, which enhanced the interaction between Al3+ and Al3+. The addition of Cu weakened the interaction between Al3 + and Al3+ and the F-. The interaction between Al3+ and Na+, [AlF7]4- ionic groups might appeared in the melt system. After adding NiO, Fe2O3, and Cu, the electrical conductivity of the system increased, and the viscosity decreased. The research work revealed the influence of Ni, Fe, Cu on the ion existence form, mobility, inter-ion interaction and diffusion mechanism of cryolite molten salt system, which has important guiding significance for aluminum electrolysis production.

Anisotropic diffusion of oxygen in slices of rat muscle

1984 ◽  
Vol 246 (1) ◽  
pp. R107-R113 ◽  
Author(s):  
L. D. Homer ◽  
J. B. Shelton ◽  
C. H. Dorsey ◽  
T. J. Williams
Keyword(s):  
Diffusion Coefficient ◽  
Fluorescence Quenching ◽  
Muscle Fiber ◽  
Extracellular Space ◽  
Anisotropic Diffusion ◽  
Muscle Fibers ◽  
Rat Muscle ◽  
Hamstring Muscles ◽  
Standard Deviations ◽  
Diffusion Coefficient Of Oxygen

The diffusion coefficient of oxygen (D) and the fluorescence quenching coefficient (K') of pyrenebutyric acid (PBA) were measured in sections of rat hamstring muscles. Values of D and K' at temperatures (Tc) of 20, 30, and 40 degrees C were determined and referred to the values in water. In sections cut parallel to the muscle fibers, D = DH2O (0.380 +/- 0.038), whereas in sections cut across the grain of the fibers, D = DH2O (0.985 +/- 0.039). Oxygen diffuses along the length of a muscle fiber over twice as rapidly as it diffuses in directions perpendicular to the long axis of the fiber. This suggests that fibers, myofibrils, or myofilaments offer substantial barriers to the diffusion of oxygen, whereas extracellular space and spaces around fibers or myofibrils or myofilaments offer no more resistance than water to the diffusion of oxygen. Corresponding estimates for K' were K' = K'H2O[0.14 (1 + 0.25 Tc)] and K' = K'H2O[0.21 (1 + 0.25 Tc)] for slices cut parallel to the long axis of muscle fibers and across the long axis, respectively. Standard deviations of K' were 9%.

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