Punnathat Bordeenithikasem

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.

High quality factor metallic glass cantilevers with tunable mechanical properties

The resonant properties of bulk metallic glass (BMG) microcantilevers were studied. Pt57.5Cu14.7Ni5.3P22.5 BMG was used to fabricate the cantilevers, on the wafer scale, using a thermoplastic forming technique. The resonant behaviors of the cantilevers were then measured in air and various vacuum levels and annealing conditions. The quality factors increase predictably after annealing at temperatures below the glass transition temperature (Tg) for different times. The increase of quality factor of annealed cantilevers is attributable to the decrease in internal friction of the BMG due to thermally activated structural relaxation. Annealing above Tg resets the quality factor to values comparable to that of the as-formed cantilever. Therefore, the quality factor and mechanical properties of a cantilever could be tuned by the selection of suitable annealing temperatures and times. The measured quality factors are in excess of 2000 in air and 8100 in vacuum.

Determination of critical cooling rates in metallic glass forming alloy libraries through laser spike annealing

The glass forming ability (GFA) of metallic glasses (MGs) is quantified by the critical cooling rate (RC). Despite its key role in MG research, experimental challenges have limited measured RC to a minute fraction of known glass formers. We present a combinatorial approach to directly measure RC for large compositional ranges. This is realized through the use of compositionally-graded alloy libraries, which were photo-thermally heated by scanning laser spike annealing of an absorbing layer, then melted and cooled at various rates. Coupled with X-ray diffraction mapping, GFA is determined from direct RC measurements. We exemplify this technique for the Au-Cu-Si system, where we identify Au56Cu27Si17 as the alloy with the highest GFA. In general, this method enables measurements of RC over large compositional areas, which is powerful for materials discovery and, when correlating with chemistry and other properties, for a deeper understanding of MG formation.

Microscale three-dimensional hemispherical shell resonators fabricated from metallic glass

A novel use of bulk metallic glasses in microresonator applications is reported and a method for the fabrication of ultra-smooth and highly symmetric three-dimensional wineglass structures with integrated electrode structures is presented. A shell with N=2 modes at 9.393 kHz and 9.401 kHz was characterized for both mounted and unmounted (to the electrode structure) conditions. Prior to mounting, the resonator was found to have quality factors of 7800 and 6500 for the two degenerate modes. After mounting, capacitive readout of the shell structure in both the time and frequency domain showed the resonances shifted to 9.461 kHz and 9.483 kHz respectively, for the N=2 modes increasing the frequency split by 15 Hz. The quality factor was also found to be reduced with values of 5400 and 5300 respectively. This investigation demonstrates that a device such as the one reported here promises robust platforms for complete MEMS inertial sensors with low cost and simplified fabrication.

Electrocatalysts: Guided Evolution of Bulk Metallic Glass Nanostructures: A Platform for Designing 3D Electrocatalytic Surfaces (Adv. Mater. 10/2016).

On page 1940, A. D. Taylor and co-workers demonstrate nanoporous bicontinuous structures using controlled structural evolution of metallic glass. By using techniques such as dealloying, galvanic replacement, and under-potential deposition, bulk-metallic-glass alloys can be pushed beyond their compositional limitations and tuned for a wide variety of interfacial and electrochemical reactions. Examples are illustrated for hydrogen and methanol oxidation, as well as oxygen reduction reactions.

Multiscale patterning of a metallic glass using sacrificial imprint lithography

Bulk metallic glasses (BMGs) have been developed as a means to achieve durable multiscale, nanotextured surfaces with desirable properties dictated by topography for a multitude of applications. One barrier to this achievement is the lack of a bridging technique between macroscale thermoplastic forming and nanoimprint lithography, which arises from the difficulty and cost of generating controlled nanostructures on complex geometries using conventional top-down approaches. This difficulty is compounded by the necessary destruction of any resulting reentrant structures during rigid demolding. We have developed a generalized method to overcome this limitation by sacrificial template imprinting using zinc oxide (ZnO) nanostructures. It is established that such structures can be grown inexpensively and quickly with tunable morphologies on a wide variety of substrates out of solution, which we exploit to generate the nanoscale portion of the multiscale pattern through this bottom-up approach. In this way, we achieve metallic structures that simultaneously demonstrate features from the macroscale down to the nanoscale, requiring only the top-down fabrication of macro/microstructured molds. Upon detachment of the formed part from the multiscale molds, the ZnO remains embedded in the surface and can be removed by etching in mild conditions to both regenerate the mold and render the surface of the BMGs nanoporous. The ability to pattern metallic surfaces in a single step on length scales from centimeters down to nanometers is a critical step toward fabricating devices with complex shapes that rely on multiscale topography for their intended functions, such as biomedical and electrochemical applications. Biomedical and optical devices stand to benefit from a multiscale patterning technique developed by researchers in the USA. Bulk metallic glasses (BMGs) are extremely strong and corrosion-resistant alloys that can be thermoplastically molded with features spanning centimeter to nanometer dimensions. The generation of multiscale molds with conventional lithography, however, requires several costly steps. To simplify BMG patterning from the bottom-up, Chinedum Osuji from Yale University and co-workers from Yale and Rutgers grew tunably packed nanowires and nanosheets of zinc oxide (ZnO) directly onto mold surfaces. When a BMG is formed then detached from this modified mold, the nanostructures embed themselves into the metallic surface. A subsequent mild etching procedure removes the ZnO and gives the BMG a nanoporous surface—an economical route to biomimetic textures for applications ranging from biomanipulation to fuel cells.