Sundeep Mukherjee

Measurement of thermophysical properties of molten silicon using an upgraded electrostatic levitator

Accurate measurements of thermophysical properties of molten silicon have been attempted using an upgraded electrostatic levitator. In order to reduce the temperature gradient in the levitated sample four laser beams of equal intensity with tetrahedral heating arrangement were used. A new image processing technique was adopted to get reliable sample volume (or density) measurements. In order to increase the accuracy of surface tension and viscosity measurements sample mass was continuously monitored throughout the experiment and the effect of mass loss was accounted for in the final analysis. Based on these new improvements, the results for the density, the specific heat over hemispherical total emissivity, the surface tension and the viscosity of molten silicon can he expressed by ρ(T) = 2.583 - 1.851 × 10 4 (T - T m ) - 1.984 × 10 7 (T T m ) 2 g/cm 3 (1370 K < T < 1830 K), C p /e T = 149.433 - 0.03735 × (T - T m ) + 8.0 × 10 5 (T - T m ) 2 - 1.1838 × 10 7 (T - T m ) 3 + 1.6342 × 10 9 (T - T m ) 4 J/mol K (1370 K < T < 1830 K). σ(T) = 721.13 - 0.0615 × (T - T m )mN/m (1459 K < T < 1845 K). and η(T) = 0.5572-5.39 × 10 4 (T - T m )mPas (1635 K < T < 1845 K) with T m = 1687 K. respectively. The hemispherical total emissivity of molten silicon at the melting temperature was determined to he 0.171.

Overheating threshold and its effect on time–temperature-transformation diagrams of zirconium based bulk metallic glasses

A pronounced effect of overheating is observed on the crystallization behavior for the three zirconium-based bulk metallic glasses: Zr41.2Ti13.8Cu12.5Ni10Be22.5, Zr57Cu15.4Ni12.6Al10Nb5, and Zr52.5Cu17.9Ni14.6Al10Ti5. A threshold overheating temperature is found for each of the three alloys, above which there is a drastic increase in the undercooling level and the crystallization times. Time–temperature-transformation (TTT) diagrams were measured for the three alloys by overheating above their respective threshold temperatures. The TTT curves for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and Zr57Cu15.4Ni12.6Al10Nb5 are very similar in shape and scale with their respective glass transition temperatures, suggesting that system-specific properties do not play a crucial role in defining crystallization kinetics in these alloys. The critical cooling rates to vitrify the alloys as determined from the TTT curves are about 2 K/s for Zr41.2Ti13.8Cu12.5Ni10Be22.5 and 10 K/s for Zr57Cu15.4Ni12.6Al10Nb5. The measurements were conducted in a high-vacuum electrostatic levitator.

Bulk Metallic Glass Micro Fuel Cell

Micro fuel cells (MFC) have been identifi ed as promising alternative power sources for portable electronics. Using noncorrosive electrolytes, they offer high theoretical power densities at low operating temperatures, with the potential for stable long-term operation. [ 1 ] Although these attributes make MFCs attractive for many portable device applications, [ 2 ] the primary design challenge is to identify the most effective lowcost materials and fabrication methods. [ 3 ] Here, we present a micro fuel cell in which the catalyst layer, gas diffusion layer, and fl ow fi elds are fabricated from bulk metallic glass (BMG) using thermoplastic forming (TPF). We show that TPF is a scalable and economical technique, for the fabrication of multi-scale BMG components of a MFC. BMGs have high electrical conductivity [ 4 ] and corrosion resistance, [ 5 ] and we demonstrate that end-plates with serpentine fl ow fi elds can be embossed into Zr 35 Ti 30 Cu 8.25 Be 26.75 (Zr-BMG) through a TPFbased process. The BMG fuel cell embodies the processing advantage of TPF into hierarchical structures involving length scales ranging from nanometers to centimeters, [ 6 ] and signifi es the fabrication of fuel cell components from a single material. We show that a hierarchical architecture fabricated through TPF-based embossing of Pt 57.5 Cu 14.7 Ni 5.3 P 22.5 (Pt-BMG) can function as a high-surface area catalyst as well as a porous gas diffusion layer, which allows us to demonstrate the concept of a metallic glass MFC. The ability to create structures over a wide range of length scales combined with remarkable electrochemical properties, suggests applications beyond MFCs, including sensors, lab-on-a-chip platforms, micro-reactors, and heterogeneous catalysis. [ 7 ]

Noncontact measurement of high-temperature surface tension and viscosity of bulk metallic glass-forming alloys using the drop oscillation technique

High-temperature surface tension and viscosities for five bulk metallic glass-forming alloys with widely different glass-forming abilities are measured. The measurements are carried out in a high-vacuum electrostatic levitator using the drop oscillation technique. The surface tension follows proportional mathematical addition of pure components’ surface tension except when some of the constituent elements have much lower surface tension. In such cases, there is surface segregation of the low surface tension elements. These alloys are found to have orders of magnitude higher viscosity at their melting points compared to the constituent metals. Among the bulk glass-forming alloys, the better glass former has a higher melting-temperature viscosity, which demonstrates that high-temperature viscosity has a pronounced influence on glass-forming ability. Correlations between surface tension and viscosity are also investigated.

Laser assisted high entropy alloy coating on aluminum: Microstructural evolution

High entropy alloy (Al-Fe-Co-Cr-Ni) coatings were synthesized using laser surface engineering on aluminum substrate. Electron diffraction analysis confirmed the formation of solid solution of body centered cubic high entropy alloy phase along with phases with long range periodic structures within the coating. Evolution of such type of microstructure was a result of kinetics associated with laser process, which generates higher temperatures and rapid cooling resulting in retention of high entropy alloy phase followed by reheating and/or annealing in subsequent passes of the laser track giving rise to partial decomposition. The partial decomposition resulted in formation of precipitates having layered morphology with a mixture of high entropy alloy rich phases, compounds, and long range ordered phases.

Amorphous Coatings and Surfaces on Structural Materials

Metallic glasses show a unique combination of high strength, excellent corrosion, and wear resistances because of their amorphous structure having a short-range order. In spite of excellent properties, the application of metallic glasses is restricted because of their inherent limitations in the bulk form, including limited tensile ductility. Using metallic glasses as the coatings for structural applications is an attractive way of taking advantage of their superior properties. Additionally, metallic glass-based composites having crystalline phases embedded in a amorphous matrix have also shown improved properties. Thus, metallic glasses can be synthesized as the coatings or subjected to surface modification to provide functionally superior surfaces. This article is a review of metallic glass-based coatings and surface modification of metallic glasses to achieve functionally superior surfaces for structural applications. Essential theoretical concepts were discussed which influence the processing. Common ways of processing along with the influence of various processing parameters were explored. Some non-conventional techniques which emerged as a result of continued efforts were also taken into account. Corrosion and wear properties of these materials along with the underlying mechanisms were discussed in detail. Focus was given to the recent product level applications explored in the open literature. Current challenges in the field were reviewed and guidelines for the future developments were provided.

Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces

Electrochemical devices such as fuel cells, electrolyzers, lithium-air batteries, and pseudocapacitors are expected to play a major role in energy conversion/storage in the near future. Here, it is demonstrated how desirable bulk metallic glass compositions can be obtained using a combinatorial approach and it is shown that these alloys can serve as a platform technology for a wide variety of electrochemical applications through several surface modification techniques.

Effect of melt convection at various gravity levels and orientations on the forces acting on a large spherical particle in the vicinity of a solidification interface

Numerical modeling was undertaken to analyze the influence of both radial and axial thermal gradients on convection patterns and velocities during solidification of pure Al and an Al-4 wt% Cu alloy. The objective of the numerical task was to predict the influence of convective velocity on an insoluble particle near a solid/liquid (s/l) interface. These predictions were then be used to define the minimum gravity level (g) required to investigate the fundamental physics of interactions between a particle and a s/l interface. This is an ongoing NASA funded flight experiment entitled particle engulfment and pushing by solidifying interfaces (PEP). Steady-state calculations were performed for different gravity levels and orientations with respect to the gravity vector. The furnace configuration used in this analysis is the quench module insert (QMI-1) proposed for the Material Science Research Facility (MSRF) on board the International Space Station (ISS). The general model of binary alloy solidification was based on the finite element code FIDAP. At a low g level of 10 -4 g 0 (g 0 = 9.8 m/s 2 ) maximum melt convection was obtained for an orientation of 90°. Calculations showed that even for this worst case orientation the dominant forces acting on the particle are the fundamental drag and interfacial forces.

Finite size effects in the crystallization of a bulk metallic glass

We explore finite size effects in the crystallization of a bulk metallic glass with nm-scale dimensions. Nanorods of Pt57.5Cu14.7Ni5.3P22.5 are produced by thermoplastic extrusion of supercooled liquid through a nanoporous template. The nanorods exhibit remarkable differences in their crystallization behavior above the glass transition. Crystallization for 100 and 200 nm diameter nanorods occurred at 6 and 24 °C lower, respectively, than the nominal crystallization temperature for bulk material while the glass transition temperatures were unchanged from the bulk value. Size dependent crystallization kinetics is discussed within a framework of classical nucleation theory, as well as possible shear and surface-induced effects.

Size-dependent viscosity in the super-cooled liquid state of a bulk metallic glass

We experimentally investigate the role of sample size on the viscosity of a bulk metallic glass by examining pressure driven flows in nm-scale confinement. A Pt57.5Cu14.7Ni5.3P22.5 metallic glass in the super-cooled liquid state is extruded into isolated cylindrical pores of varying nano-scale dimensions, down to 40 nm. The apparent viscosity of the liquid as a function of sample size is determined from the filling depth by appropriate corrections to the Hagen-Poiseuille equation. We observe a striking, sudden increase of the apparent viscosity for dimensions below approximately 100 nm. Results are discussed in the framework of confinement of collective shear events.