Polyacrylonitrile (PAN) solution containing the iron oxide precursor iron (III) acetylacetonate (AAI) was electrospun and thermally treated to produce electrically conducting, magnetic carbon nanofiber mats with hierarchical pore structures. The morphology and material properties of the resulting multifunctional nanofiber mats including the surface area and the electric and magnetic properties were examined using various characterization techniques. Scanning electron microscopy images show that uniform fibers were produced with a fiber diameter of ∼600 nm, and this uniform fiber morphology is maintained after graphitization with a fiber diameter of ∼330 nm. X-ray diffraction (XRD) and Raman studies reveal that both graphite and Fe(3)O(4) crystals are formed after thermal treatment, and graphitization can be enhanced by the presence of iron. A combination of XRD and transmission electron microscopy experiments reveals the formation of pores with graphitic nanoparticles in the walls as well as the formation of magnetite nanoparticles distributed throughout the fibers. Physisorption experiments show that the multifunctional fiber mats exhibit a high surface area (200-400 m(2) g(-1)) and their pore size is dependent on the amount of iron added and graphitization conditions. Finally, we have demonstrated that the fibers are electrically conducting as well as magnetically active.
Laser-induced forward transfer (LIFT) is a nozzle-less printing technique where a controlled amount of material is transferred from a thin film to a receiver substrate with each laser pulse. Conventionally, each laser pulse is directed to a different spot on the donor ink film as the donor substrate is moved together with the receiver surface after each pulse. In this letter, we demonstrate that it is possible to do the LIFT printing of industrial grade silver paste using multiple pulses on the same spot on the donor film due to the healing of the silver paste film. We modify the rheology of the silver paste by adding a lower viscosity solvent and show that the change in material rheology allows for printing in different regimes.
The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses. In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface. Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.
As technology becomes smaller and smaller, the need for micro-energy sources becomes increasingly imperative. One promising technology to address this need is piezoelectrochemical harvesting, a recently identified mechanism to directly convert mechanical energy to electrochemical potential [1-4] . In piezoelectrochemical (PEC) materials, the chemical potential of ions is affected by an applied stress, and under such circumstances these materials can be used in a thermodynamic cycle to harvest energy, at a relatively slow rate commensurate with the kinetic transport in electrochemical systems. Previous work [1] has demonstrated that commercial lithium cobalt oxide (LCO) batteries exhibit the PEC effect, as both the lithium cobalt oxide cathode and lithium-intercalated graphite anode are PEC materials. The coupling factor between the change in equilibrium potential and applied mechanical stress has been found to be linear. In this presentation, we will discuss our research to use piezoelectrochemical energy harvesting to increase the voltage generated from commercial lithium ion batteries. We measured the differential expansion and differential voltage of a lithium ion battery, and used this data to estimate the coupling factor as a function of state-of-charge (SOC). We analyzed the coupling factor for commercial LCO batteries, and found the SOC where the coupling factor was maximized. At this SOC, batteries were placed under a mechanical load to harvest energy. The voltage generated was quantified by measuring the voltage drop across a resistor. To understand how the PEC effect operates in multiple batteries, we wired cells in series and parallel, and performed similar mechanical load experiments. As expected, the PEC voltage can be increased by compressing batteries in series. Increasing the PEC voltage generated would allow the effect to be used in practical applications such as micro-energy devices. References: [1] J. Cannarella and C. B. Arnold, “Toward Low-Frequency Mechanical Energy Harvesting Using Energy-Dense Piezoelectrochemical Materials," Advanced Materials , 27 , 7440 (2015). [2] S. Kim, S. J. Choi, K. Zhao, H. Yang, G. Gobbi, S. Zhang, and J. Li, “Electrochemically driven mechanical energy harvesting," Nature Communications , 7 , 10146 (2016). [3] N. Muralidharan, M. Li, R. E. Carter, N. Galioto, and C. L. Pint, “Ultralow Frequency Electrochemical−Mechanical Strain Energy Harvester Using 2D Black Phosphorus Nanosheets,” ACS Energy Lett ., 2 , 1797 (2017). [4] E. Jacques, G. Lindbergh, D. Zenkert, S. Leijonmarck, and M. H. Kjell, “Piezo-Electrochemical Energy Harvesting with Lithium-Intercalating Carbon Fibers,” ACS Appl. Mater. Interfaces, 7 , 13898 (2015).
Direct laser writing with ultrafast lasers surpasses limitations of conventional fabrication techniques that have low resolution, require multiple post-processing steps or are restricted to fabrication in 2D. The first part of the talk discusses fabrication of 3D metal-dielectric nanocomposite structures of tunable dimensions ranging from hundreds of nanometers to micrometers. By directly reducing metal ions with femtosecond pulses, direct laser written high-quality single-crystals, polymer-matrix embedded diffraction gratings are fabricated. The next part discusses methods of direct laser writing graphene structures. Graphene patterns written with ultrafast lasers show higher conductivity, indicating that limited thermal processes can help achieve better quality direct-laser-written graphene patterns.
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