Switchable adhesives using stimuli-responsive systems have many applications, including transfer printing, climbing robots, and gripping in pick and place processes. Among these adhesives, electroadhesive surface can spontaneously adjust their adhesion in response to an external electric field. However, electroadhesives usually need high voltage (e.g. kV) and the adhesion disappears upon turning off the signal. These limitations make them complicated and costly. In this research, we demonstrated a gold-coated silica microsphere (GCSM) with highly switchable and memorable adhesion triggered by a relatively small voltage (<30 V). In the experiment, a silica microsphere with a diameter of 15 μm was glued to a tipless atomic force microscope (AFM) cantilever. The nanoscale thick gold coating was sprayed on the surface of the microsphere by a sputter coater. AFM was used to explore the tunable adhesion with an external voltage at different relative humidity (RH). The results revealed that when applying a positive electrical bias at high RH, the adhesive force increased dramatically while it decreased to almost zero after applying a negative potential. Even if the bias was turned off, the adhesive force state could still be kept and erased on demand by simply applying a negative voltage. The adhesive force can be …

The method of optical emission spectroscopy has been used with Fe-Ni and other complex alloys to investigate in-situ compositional control for additive manufacturing. Although additive manufacturing of metallic alloys is an emerging technology, compositional control will be a challenge that needs to be addressed for a multitude of industries going forward for next-gen applications. This current scope of work includes analysis of ionized species generated from laser and metal powder interaction that is inherent to the laser engineered net shaping (LENS) process of additive manufacturing. By quantifying the amount of a given element's presence in the electromagnetic (EM) spectrum, this amount can be compared to the actual amount present in the sample via post-processing and elemental dispersive x-ray (EDX) data analysis. For this work a commercially available linear silicon CCD camera captured metallic ion peaks found within the ultraviolet (UV) region to avoid background contamination from blackbody radiation. Although the additive manufacturing environment can prove difficult to measure in-situ due to time dependent phenomena, extreme temperatures, and defect generation, OEM was able to capture multiple data points over a time series that showed a positive correlation between an element's peak intensity and the amount of that element found in the …

Piezoelectricity in two-dimensional (2D) transition metal dichalcogenides (TMDs) has attracted significant attention due to their unique crystal structure and the lack of inversion centers when the bulk TMDs thin down to monolayer. Although the piezoelectricity effect in atomic-thickness TMDs has been demonstrated, they are not scalable. Herein, we demonstrate a piezoelectric effect from large-scale, sputtered MoS2 and WS2 using a robust defect-engineering based on the thermal-solvent annealing and solvent immersion process. This yields a higher piezoelectric output over 20 times after annealing or solvent immersion. Indeed, the piezoelectric responses are strengthened with the increases of defect density. Moreover, the MoS2 or WS2 piezoelectric device array shows an exceptional piezoelectric sensitivity with a high-level uniformity and excellent environmental stability under ambient conditions. A detailed study of the sulfur vacancy-dependent property and its resultant asymmetric structure-induced piezoelectricity is reported. The proposed approach is scalable and can produce advanced materials for flexible piezoelectric devices to be used in emerging bioinspired robotics and biomedical applications.

Chiral nematic liquid crystals or cholesteric liquid crystals (CLC) can be obtained by adding a chiral dopant into a nematic liquid crystal. Liquid crystal molecules spontaneously rotate along a long axis to form helical structures in CLC system. Both pitch size and orientation of the helical structure is determined by the boundary conditions and can be further tuned by external stimuli. Particularly, the uniform lying helical structure of CLC has attracted intensive attention due to its beam steering and diffraction abilities. Up to now, studies have worked on controlling the in-plane orientation of lying helix through surface rubbing and external stimuli. However, it remains challenging to achieve steady and uniform lying helical structure due to its higher energy, comparing with other helical configurations. Here, by varying the surface anchoring, uniform lying helical structure with long-range order is achieved as thermodynamically stable state without external support. Poly (6-(4-methoxy-azobenzene-4'-oxy) hexyl methacrylate) (PMMAZO), a liquid crystalline polymer, is deposited onto the silicon substrate to fine-tune the surface anchoring. By changing the grafting density of PMMAZO, both pitch size and orientation of lying helical structure are precisely controlled. As the grafting density increases, the enhanced titled deformation of helical structure suppresses the pitch size …

High entropy alloys (HEAs), also referred to as complex concentrated alloys (CCAs), are a relatively new class of alloys that have gained significant attention since 2010 due to their unique balance of properties that include high strength, ductility and excellent corrosion resistance. HEAs are usually based on five or more elements alloyed in near equimolar concentrations, and exhibit simple microstructures by the formation of solid solution phases instead of complex compounds. HEAs have great potential in the design of new materials; for instance, for lightweight structural applications and elevated temperature applications. The relation between grain size and yield strength has been a topic of significant interest not only to researchers but also for industrial applications. Though some research papers have been published in this area, consensus among them is lacking, as the studies yielded different results. Al atom being a large atom causes significant lattice distortion. This work attempts to study the Hall-Petch relationship for Al0.3CoFeNi and Al0.3CoCrFeNi and to compare the data of friction stress σ0 and Hall-Petch coefficient K with published data. The base alloys for both these alloys are CoFeNi and CoCrFeNi respectively. It was observed by atom probe tomography (APT) that clustering of Al-Ni atoms in …

Fundamental understanding of high strain rate deformation behavior of materials is critical in designing new alloys for wide-ranging applications including military, automobile, spacecraft, and industrial applications. High entropy alloys, consisting of multiple elements in (near) equimolar proportions, represent a new paradigm in structural alloy design providing ample opportunity for achieving excellent performance in high strain rate applications by proper selection of constituent elements and/or thermomechanical processing. This dissertation is focused on fundamental understanding of high strain-rate deformation behavior of several high entropy alloy systems with widely varying microstructures. Ballistic impact testing of face centered cubic Al0.1CoCrFeNi high entropy alloy showed failure by ductile hole growth. The deformed microstructure showed extensive micro-banding and micro-twinning at low velocities while adiabatic shear bands and dynamic recrystallization were seen at higher velocities. The Al0.7CoCrFeNi and AlCoCrFeNi2.1 eutectic high entropy alloys, with BCC and FCC phases in lamellar morphology, showed failure by discing. A network of cracks coupled with small and inhomogeneous plastic deformation led to the brittle mode of failure in these eutectic alloys. Phase-specific mechanical behavior using small-scale techniques revealed higher strength and strain rate sensitivity for the B2 phase compared to the L12 phase. The interphase boundary demonstrated good stability without any …

In this dissertation, small and large NaCl particle-derived surfaces (Ra > 40 microns) were generated on 2D Zn materials, and the surfaces were carefully studied concerning topography, corrosion behavior, and bone cell compatibility. Increases in surface roughness accelerated the corrosion rate, and cell viability was maintained. This method was then extended to 3D porous scaffolds prepared by a hybrid AM/casting technique. The scaffolds displayed a near-net shape, an interconnected pore structure, increasing porosity paralleled to an increased corrosion rate, an ability to support cell growth, and powerful antibacterial properties. Lastly, nano/micro (Rz 0.02–1 microns) topographies were generated on 2D Zn materials, and the materials were comprehensively studied with special attention devoted to corrosion behavior, biocompatibility, osteogenic differentiation, immune cell response, hemocompatibility, and antibacterial performance. For the first time, the textured nonhemolytic surfaces on Zn were shown to direct cell fate, and the micro-textures promoted bone cell differentiation and directed immune cells away from an inflammatory phenotype.

The design and fabrication of porous ceramic materials with anisotropic properties has, in recent years, gained popularity due to their potential application in various areas that include medical, energy, defense, space, and aerospace. Freeze-casting is an effective, low-cost, and safe method as a wet shaping technique to create these structures. To control the morphology of these materials, many critical factors were found to play an important role. In this dissertation, the processing parameters of the magnetic field-assisted freeze-casting method were optimized with a focus on comparing the structure obtained using vertical and horizontal magnetic fields and understanding the mechanisms that occur under different freezing modes. More specifically, this processing method was used to produce Al2O3 and B4C porous ceramics materials with unidirectionally-aligned pore channels. The effect of the vertical and horizontal magnetic field strength and direction, concentration of magnetic material (Fe3O4), cooling rate, and freezing time were examined. The resulting ceramics with highly aligned pore channels were infiltrated with molten metal to create metal matrix composites. The mechanical properties of these structures were measured and were subsequently correlated to their morphology and composition.

This dissertation presents an assortment of research aimed at understanding the composition-dependence of deformation behavior and the response to thermomechanical processing, to enable efficient design and processing of low stacking fault energy (SFE) high entropy alloy (HEAs). The deformation behavior and SFE of four low SFE HEAs were predicted and experimentally verified using electron microscopy and in-situ neutron diffraction. A new approach of employing a minimization function to refine and improve the accuracy of a semi-empirically derived expression relating composition with SFE is demonstrated. Ultimately, by employing the minimization function, the average difference between experimental and predicted SFE was found to be 2.64 mJ m-2. Benchmarking with currently available approaches suggests that integrating minimization functions can substantially improve prediction accuracy and promote efficient HEA design with expansion of databases. Additionally, in-situ neutron diffraction was used to present the first in-situ measurement of the interspacing between stacking faults (SFs) which were correlated with work hardening behavior. Electron transparent specimens (< ~100 nm thick) were used in order to resolve nanoscale planar faults instead of the thicker sub-sized specimens (on the order of millimeters in thickness) which exhibit the classical stages III work hardening behavior characteristic of low SFE metals and alloys. …