Golden Kumar

Nanomoulding with amorphous metals

Nanoimprinting promises low-cost fabrication of micro- and nano-devices by embossing features from a hard mould onto thermoplastic materials, typically polymers with low glass transition temperature. The success and proliferation of such methods critically rely on the manufacturing of robust and durable master moulds. Silicon-based moulds are brittle and have limited longevity. Metal moulds are stronger than semiconductors, but patterning of metals on the nanometre scale is limited by their finite grain size. Amorphous metals (metallic glasses) exhibit superior mechanical properties and are intrinsically free from grain size limitations. Here we demonstrate direct nanopatterning of metallic glasses by hot embossing, generating feature sizes as small as 13 nm. After subsequently crystallizing the as-formed metallic glass mould, we show that another amorphous sample of the same alloy can be formed on the crystallized mould. In addition, metallic glass replicas can also be used as moulds for polymers or other metallic glasses with lower softening temperatures. Using this ‘spawning’ process, we can massively replicate patterned surfaces through direct moulding without using conventional lithography. We anticipate that our findings will catalyse the development of micro- and nanoscale metallic glass applications that capitalize on the outstanding mechanical properties, microstructural homogeneity and isotropy, and ease of thermoplastic forming exhibited by these materials.

Bulk Metallic Glass Nanowire Architecture for Electrochemical Applications

Electrochemical devices have the potential to pose powerful solutions in addressing rising energy demands and counteracting environmental problems. However, currently, these devices suffer from meager performance due to poor efficiency and durability of the catalysts. These suboptimal characteristics have hampered widespread commercialization. Here we report on Pt(57.5)Cu(14.7)Ni(5.3)P(22.5) bulk metallic glass (Pt-BMG) nanowires, whose novel architecture and outstanding durability circumvent the performance problems of electrochemical devices. We fabricate Pt-BMG nanowires using a facile and scalable nanoimprinting approach to create dealloyed high surface area nanowire catalysts with high conductivity and activity for methanol and ethanol oxidation. After 1000 cycles, these nanowires maintain 96% of their performance-2.4 times as much as conventional Pt/C catalysts. Their properties make them ideal candidates for widespread commercial use such as for energy conversion/storage and sensors.

Tunable Tensile Ductility in Metallic Glasses

Widespread adoption of metallic glasses (MGs) in applications motivated by high strength and elasticity combined with plastic-like processing has been stymied by their lack of tensile ductility. One emerging strategy to couple the attractive properties of MGs with resistance to failure by shear localization is to employ sub-micron sample or feature length scales, although conflicting results shroud an atomistic understanding of the responsible mechanisms in uncertainty. Here, we report in situ deformation experiments of directly moulded Pt57.5Cu14.7Ni5.3P22.5 MG nanowires, which show tunable tensile ductility. Initially brittle as-moulded nanowires can be coerced to a distinct glassy state upon irradiation with Ga+ ions, leading to tensile ductility and quasi-homogeneous plastic flow. This behaviour is reversible and the glass returns to a brittle state upon subsequent annealing. Our results suggest a novel mechanism for homogenous plastic flow in nano-scaled MGs and strategies for circumventing the poor damage tolerance that has long plagued MGs.

Critical fictive temperature for plasticity in metallic glasses

A long-sought goal in metallic glasses is to impart ductility without conceding their strength and elastic limit. The rational design of tough metallic glasses, however, remains challenging because of the inability of existing theories to capture the correlation between plasticity, composition and processing for a wide range of glass-forming alloys. Here we propose a phenomenological criterion based on a critical fictive temperature, Tfc, which can rationalize the effect of composition, cooling rate and annealing on room-temperature plasticity of metallic glasses. Such criterion helps in understanding the widespread mechanical behaviour of metallic glasses and reveals alloy-specific preparation conditions to circumvent brittleness.

Thermoplastic blow molding of metals

While plastics have revolutionized industrial design due to their versatile processability, their relatively low strength has hampered their use in structural components. On the other hand, while metals are the basis for strong structural components, the geometries into which they can be processed are rather limited. The “ideal” material would offer a desirable combination of superior structural properties and the ability to be precision (net) shaped into complex geometries. Here we show that bulk metallic glasses (BMGs), which have superior mechanical properties, can be blow molded like plastics. The key to the enhanced processability of BMG formers is their amenability to thermoplastic forming. This allows complex BMG structures, some of which cannot be produced using any other metal process, to be net shaped precisely.

Write and erase mechanisms for bulk metallic glass

Microfeatures are imprinted on Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass (BMG) using thermoplastic forming. Subsequent erasing is carried out by annealing in the supercooled liquid region. The driving force for the erasing process is the capillary force controlled by the curvature and surface tension of the liquid-vacuum surface. Sluggish crystallization kinetics in this alloy permit experimental observation at temperatures where the viscosity is sufficiently low to completely erase small surface features on a time scale smaller than the crystallization time. The kinetics of the writing and erasing processes suggest that BMGs may offer a viable alternative rewritable high-density data storage technology.

Three-Dimensional Shell Fabrication Using Blow Molding of Bulk Metallic Glass

A blow molding method based on thermoplastic forming of bulk metallic glasses (BMGs) is used to fabricate 3-D microshells. The 3-D microshells are attached to the Si wafer through mechanical locking, which is achieved in the same processing step. Versatile sizes and shapes of the 3-D shells can be precisely controlled. High strength ( >; 1 GPa), elasticity (~ 2%), and controlled surface roughness (<; 2 nm), which are achievable for BMGs, suggest their potential use in devices, including resonators, microlenses, microfluidic, and packaging.

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 ]

Functionalization of metallic glasses through hierarchical patterning.

Surface engineering over multiple length scales is critical for electronics, photonics, and enabling multifunctionality in synthetic materials. Here, we demonstrate a sequential embossing technique for building multi-tier patterns in metals by controlling the size-dependent thermoplastic forming of metallic glasses. Sub-100 nm to millimeter sized features are sculpted sequentially to allow an exquisite control of surface properties. The process can be integrated with net-shaping to transfer functional patterns on three-dimensional metal parts.

Atomically smooth surfaces through thermoplastic forming of metallic glass

We demonstrate that atomically smooth surfaces can be generated by thermoplastic forming of metallic glasses. This is enabled by the flow associated with the contact-line motion which removes rough surface layer from the advancing metallic glass-air interface. The thermoplastically formed surface is two orders of magnitude smoother than a polished surface of the same alloy. This process is capable of generating atomically smooth surfaces and replicating nanoscale features in a single processing step, providing a versatile toolbox for nanofabrication.