Rajarshi Banerjee

High Capacity and Excellent Stability of Lithium Ion Battery Anode Using Interface-Controlled Binder-Free Multiwall Carbon Nanotubes Grown on Copper

We present a novel binder-free multiwall carbon nanotube (MWCNT) structure as an anode in Li ion batteries. The interface-controlled MWCNT structure, synthesized through a two-step process of catalyst deposition and chemical vapor deposition (CVD) and directly grown on a copper current collector, showed very high specific capacity, almost three times as that of graphite, excellent rate capability even at a charging/discharging rate of 3 C, and no capacity degradation up to 50 cycles. Significantly enhanced properties of this anode could be related to high Li ion intercalation on the carbon nanotube walls, strong bonding with the substrate, and excellent conductivity.

Direct laser deposition of alloys from elemental powder blends

Abstract The complexity in design of components used in advanced aerospace and automotive applications is continuously increasing. This has led to the development of near-net shape manufacturing techniques such as laser engineered net-shaping (LENSTM) which falls in the class of direct laser deposition processes from powder feedstock. Despite considerable advances in process optimization, there is a rather limited understanding of the role of metallurgical factors in laser deposition of alloys. This paper discusses the significant role played by the thermodynamic enthalpy of mixing in the deposition of alloys from elemental powder blends using LENSTM. This factor influences the homogeneity as well as the rate of solidification of the alloy and consequently the microstructure and properties of the deposit. The enthalpy of mixing could also serve as a very useful guideline in the design of novel alloys that are laser deposited from elemental powder blends.

Microstructural evolution in laser deposited compositionally graded α/β titanium-vanadium alloys

Abstract A graded binary Titanium-Vanadium alloy has been deposited using the laser engineered net-shaping (LENS™) process from a blend of elemental Ti and V powders. A compositional gradient in the alloy, from elemental Ti to Ti-25at%V, has been achieved within a length of ~25 mm. Subsequent to deposition, longitudinal sections of the deposit have been characterized in detail using scanning and transmission electron microscopy. Though the phases across the graded alloy correspond to those typically observed in α / β Ti alloys, the scale and morphology of the microstructural features varies substantially with composition. Several phase transformations, namely, β →Widmanstatten α , β → ω and martensitic β →hexagonal α ′, are encountered in the graded alloy sample during LENS™ deposition. The ability to achieve such substantial changes in composition across rather limited lengths make such graded alloys highly attractive candidates for investigating the influence of systematic compositional changes on phase transformations and concurrent microstructural evolution in these alloys.

Laser deposition of compositionally graded titanium–vanadium and titanium–molybdenum alloys

Compositionally graded binary titanium–vanadium and titanium–molybdenum alloys have been deposited using the laser engineered net-shaping (LENS™) process. A compositional gradient, from elemental Ti to Ti–25at.% V or Ti–25at.% Mo, has been achieved within a length of ∼25 mm. The feedstock used for depositing the graded alloy consists of elemental Ti and V (or Mo) powders. Though the microstructural features across the graded alloy correspond to those typically observed in α/β Ti alloys, the scale of the features is refined in a number of cases. Microhardness measurements across the graded samples exhibit an increase in hardness with increasing alloying content up to a composition of ∼12% in case of Ti–xV and up to a composition of ∼10% in case of the Ti–xMo alloys. Further increase in the alloying content resulted in a decrease in hardness for both the Ti–xV as well as the Ti–xMo alloys. A notable feature of these graded deposits is the large prior β grain size resulting from the directionally solidified nature of the microstructure. Thus, grains ∼10 mm in length grows in a direction perpendicular to the substrate. The ability to achieve such substantial changes in composition across rather limited length makes this process a highly attractive candidate for combinatorial materials science studies.

Direct laser deposition of in situ Ti–6Al–4V–TiB composites

Abstract Ti–6Al–4V–TiB composites have been in situ deposited from powder feedstocks consisting of a blend of pre-alloyed Ti–6Al–4V and elemental boron using the Laser Engineered Net-Shaping (LENS™) process. The microstructure of the as-deposited composites has been characterized in detail using scanning (SEM) and transmission electron microscopy (TEM). A homogeneous refined dispersion of TiB precipitates is formed within the Ti–6Al–4V α/β matrix. The scale of the microstructure is substantially refined as compared with composites produced using other recently developed powder processing techniques. In addition to the refined dispersion of the reinforcing phase, the influence of the TiB precipitation on the solid-state β→β+α transformation that takes place in the matrix has been examined using SEM and TEM. Finally, heat-treatments carried out post-LENS™ deposition, suggest that LENS™ fabricated composites are thermodynamically stable, exhibiting limited TiB coarsening.

Influence of the sputtering gas on the preferred orientation of nanocrystalline titanium nitride thin films

Abstract Nanocrystalline titanium nitride thin films have been deposited by high pressure reactive magnetron sputtering from an elemental titanium target using a mixture of an inert gas and nitrogen. The mean crystallite or grain size in these films is in the range 8–12 nm as measured from X-ray line broadening. Interestingly, the type of inert gas used in the sputtering gas mixture significantly influences the microstructure and preferred orientation in these films. Thus, using a 70% He+30% N2 gas mixture results in a strongly (002) oriented film whereas using a 70% Ar+30% N2 gas mixture results in a strongly (111) oriented film with a similar grain size. In addition, films have also been deposited using pure nitrogen as the sputtering gas. These films exhibited a strong (002) orientation and had a significantly larger grain size as compared with those deposited using a mixture of an inert gas and nitrogen. Details of the microstructure in these films have been investigated by transmission electron microscopy. The ability to tailor the size and preferred orientation of grains in the TiN thin films (by proper choice of sputtering gas) is expected to have a significant impact on the properties of these films in a variety of technological applications.

Lattice expansion in nanocrystalline niobium thin films

High-purity nanocrystalline niobium (Nb) thin films have been deposited using high-pressure magnetron sputter deposition. Increasing the pressure of the sputtering gas during deposition has systematically led to reduced crystallite sizes in these films. Based on x-ray and electron diffraction results, it is observed that the nanocrystalline Nb films exhibit a significantly large lattice expansion with reduction in crystallite size. There is however, no change in the bcc crystal structure on reduction in crystallite size to below 5 nm. The lattice expansion in nanocrystalline Nb has been simulated by employing a recently proposed model based on linear elasticity and by appropriately modifying it to incorporate a crystallite-size-dependent width of the grain boundary.

Mechanism of the Size Dependence of the Superconducting Transition of Nanostructured Nb

In nanocrystalline Nb films, the superconducting Tc decreases with a reduction in the average particle size below 20nm. We correlate the decrease in Tc with a reduction in the superconducting energy gap measured by point contact spectroscopy. Consistent with the Anderson criterion, no superconducting transition was observed for sizes below 8 nm. We show that the size-dependence of the superconducting properties in this intermediate coupling Type II superconductor is governed by changes in the electronic density of states rather than by phonon softening.

Ultrathin alumina-coated carbon nanotubes as an anode for high capacity Li-ion batteries

Alumina-coated carbon nanotubes (CNTs) were synthesized on a copper substrate and have been used as an anode in Li-ion batteries. CNTs were grown directly on the copper current collector by chemical vapor deposition and an ultrathin layer of alumina was deposited on the CNTs by atomic layer deposition, thus forming the binder-free electrode for the Li-ion battery. While CNTs, which form the core of the structure, provide excellent conductivity, structural integrity and Li-ion intercalation ability, the aluminium oxide coating provides additional stability to the electrode, with further enhancement of capacity. The anode showed very high specific capacity, good capacity retention ability and excellent rate capability. This novel anode may be considered as an advanced anode for future Li-ion batteries.

Laser Deposition of In Situ Ti – TiB Composites

Due to their enhanced mechanical properties and potentially wide applicability, there is considerable interest in the development of metal-matrix composites consisting of titanium borides in a titanium alloy matrix. Despite the development of a variety of different processing routes for these composites, there are relatively few ones capable of processing a fully dense, near-net shape component with a relatively fine dispersion of boride precipitates. This paper will discuss the in situ laser deposition of Ti-TiB composites using the laser engineered net-shaping (LENS™) process from a blend of elemental titanium (or titanium alloy) and boron powders. The microstructure of the LENS™ deposited Ti-TiB composite has been compared with that of a conventionally cast in situ composite of the same composition. The conventionally cast composite exhibits a significantly coarser scale microstructure. Thus, the ability to produce a fine dispersion of TiB precipitates in dense Ti-TiB composites of near-net shape using LENS™ processing can be attributed to the rapid solidification effects during such processing.