A comparison study has been conducted on the formation of catalyst nanoparticles on a high surface tension metal and low surface tension oxide for carbon nanotube (CNT) growth via catalytic chemical vapor deposition (CCVD). Silicon dioxide (SiO2) and tantalum have been deposited as supporting layers before deposition of a thin layer of iron catalyst. Iron nanoparticles were formed after thermal annealing. It was found that densities, size distributions, and morphologies of iron nanoparticles were distinctly different on the two supporting layers. In particular, iron nanoparticles revealed a Volmer-Weber growth mode on SiO2 and a Stranski-Krastanov mode on tantalum. CCVD growth of CNTs was conducted on iron∕tantalum and iron∕SiO2. CNT growth on SiO2 exhibited a tip growth mode with a slow growth rate of less than 100nm∕min. In contrast, the growth on tantalum followed a base growth mode with a fast growth rate exceeding 1μm∕min. For comparison, plasma enhanced CVD was also employed for CNT growth on SiO2 a...

Comparison study of catalyst nanoparticle formation and carbon nanotube growth: Support effect

https://hal.archives-ouvertes.fr/hal-03059610/document

Reducing hot tearing by grain boundary segregation engineering in additive manufacturing: example of an AlxCoCrFeNi high-entropy alloy

Diffusivities of an Al–Fe–Ni melt and their effects on the microstructure during solidification

The diffusion of impurities in solid and liquid silicon is critically reviewed and assessed in this paper. The activation energies and pre-exponential factors in theArrhenius equation have been evaluated using the least-squares analysis and semi-empirical correlations. Impurity diffusion coefficients for Ag, Al, As, Au, B, Bi, C, Co, Cr, Cu, Fe, Ga, In, Li, Mn, N, Ni, 0, P, S, Sb, Te, Ti, and Zn in both solid and liquid silicon have been obtained. The current assessed impurity diffusivities can be coupled with the assessed thermochemical properties for the simulation of diffusion phenomena in the production of solar grade cell silicon feedstock. The assessed diffusivities have been applied to simulate the impurity diffusion profiles and the denuded zone in the intrinsic gettering annealing.

Critical Assessment of the Impurity Diffusivities in Solid and Liquid Silicon

Earth’s magnetic field is generated by convective motions in its liquid metal core. In this fluid, the heat diffuses significantly more than momentum, and thus the Prandtl number is well below unity. The thermally driven convective flow dynamics of liquid metals are very different from moderate- fluids, such as water and those used in current dynamo simulations. In order to characterise rapidly rotating thermal convection in low- number fluids, we have performed laboratory experiments in a cylinder of aspect ratio using liquid gallium ( ) as the working fluid. The Ekman number varies from to and the Rayleigh number varies from to . Using spectral analysis stemming from point-wise temperature measurements within the fluid and measurements of the Nusselt number , we characterise the different styles of low- rotating convective flow. The convection threshold is first overcome in the form of container-scale inertial oscillatory modes. At stronger forcing, sidewall-attached modes are identified for the first time in liquid metal laboratory experiments. These wall modes coexist with the bulk oscillatory modes. At well below the values where steady rotating columnar convection occurs, the bulk flow becomes turbulent. Our results imply that rotating convective flows in liquid metals do not develop in the form of quasisteady columns, as in moderate- fluids, but in the form of oscillatory convective motions. Thus, thermally driven flows in low- geophysical and astrophysical fluids can differ substantively from those occurring in models. Furthermore, our experimental results show that relatively low-frequency wall modes are an essential dynamical component of rapidly rotating convection in liquid metals.

Rotating thermal convection in liquid gallium: multi-modal flow, absent steady columns

By combining the modified Stokes-Einstein formula with the authors’ model for the melting-point viscosity, the authors present a model for accurate predictions of self-diffusivity of liquid metallic elements. The model is expressed in terms of well-known physical quantities and has been applied to various liquid metallic elements for which experimental data are available. The results of calculations show that agreement with experimental data is excellent; the uncertainties in the calculations of the self-diffusivities in various liquid metallic elements are equal to the uncertainties associated with experimental measurements. Also, the authors propose an expression for the temperature dependence of self-diffusivity in liquid metallic elements in terms of melting-point temperature. Using the model, self-diffusivity data are predicted for liquid iron, cobalt, nickel, titanium, aluminum, magnesium, silicon, and so forth.

A model for accurate predictions of self-diffusivities in liquid metals, semimetals, and semiconductors

Recently, the authors presented two models for accurate predictions of the viscosity for pure liquid metals. In this article, the authors checked the models against the viscosities of various liquid high-melting-point metals. These two models give very good agreement with experiment. Using these two models, the viscosities of liquid transition and Group IIA metals were predicted for dysprosium, erbium, molybdenum, neodymium, platinum, scandium, vanadium, yttrium, barium, and strontium. Finally, recommended data were collected for the experimental viscosities of pure liquid metals.

Accurate predictions for the viscosities of several liquid transition metals, plus barium and strontium

The current article presented appropriate models using a new parameter recently introduced by the authors to accurately predict the atomic transport coefficients, i.e. viscosity and self-diffusivity, of liquid metallic elements at their melting points. The models for both the meltingpoint viscosity and self-diffusivity are expressed in terms of well-known physical quantities; atomic mass, atomic volume, melting point, melting-point surface tension, and the new parameter T ξ . Moreover, the authors derived expressions for the temperature dependence of the atomic transport coefficients of liquid metallic elements in terms of melting point temperature. These two models give very good agreement with experimental data for various metallic liquids. Using the models, self-diffusivities were predicted for liquid aluminum, calcium, and magnesium.

Nagendra Tripathi

Papers

A model for accurate predictions of self-diffusivities in liquid metals, semimetals, and semiconductors

Accurate predictions for the viscosities of several liquid transition metals, plus barium and strontium

Models and Equations for Atomic Transport Coefficients of Liquid Metals: Viscosity and Self-Diffusivity