Approaches for the Discovery and Design of New Permanent Magnets

Apr 22, 2014 9:30am ‐ Apr 22, 2014 10:00am

Identification: WW1.06

Concern for supply restrictions of rare-earth metals has spurred intense interest in the discovery of new compounds that do not contain critical elements yet still exhibit high saturation magnetization and intrinsic coercively. There have also been efforts in optimizing magnetic properties in older alloys. Both are daunting tasks given the high energy product of existing rare-earth based alloys. Criticality of Dy, in particular, is driving the need to developing new alloys for the higher operating temperature regime of traction motors and some generators. Discovery of new compounds, however, requires a more sophisticated approach than simple “trial and error”. Ames Laboratory, in collaboration with a number of universities and laboratories have been embarking on a comprehensive research program to combine a series of integrated computational and experimental efforts to both discover and design new compounds with promising magnetic properties. Experimental materials discovery will include both bulk and thin film combinatorial synthesis. Additionally, we are developing high throughput thermal analysis and in situ XRD capabilities to characterize the phase space of these multi-elemental libraries as a function of temperature. The computational efforts of this research include both density functional theory and adaptive genetic algorithms to identify new compounds. Specific examples of materials discovery and new insights into improvements of existing alloys will be presented.

Origami-Inspired Compliant Mechanisms: Test Beds and Applications for Shape Programmable Materials

Apr 22, 2014 9:30am ‐ Apr 22, 2014 10:00am

Identification: XX2.01

Compliant mechanisms achieve their motion from the deflection of flexible components rather than from traditional motion elements such as hinges and bearings. Origami models can be thought of as compliant mechanisms during folding because they achieve their motion from bending at folds and flexing panels. Advantages of compliant mechanisms include characteristics associated with efficiency such as reduced part count and ease of manufacture, as well as compactness (also shared with origami), low weight, low wear, reduced maintenance, improved recyclability, and high precision motion. Uniting origami and compliant mechanisms principles could enable innovative and cost-effective devices that are capable of accomplishing sophisticated mechanical tasks.

Origami-inspired compliant mechanisms have the potential advantages of planar fabrication (they can be fabricated from planar sheets of material and allow the use of planar fabrication methods); a flat initial state (which allows compactness for volume critical applications); and monolithic composition (which provides the advantages associated with compliant mechanisms noted above). Many recent origami-inspired compliant mechanisms have been manually or passively actuated, but some applications would be improved, and many others made feasible, if they were actuated by integrated actuators.

Origami-inspired compliant mechanisms have characteristics that make them an ideal test bed for shape programmable materials, including the following:

  1. Because of their nature, it may be possible to make entire compliant mechanisms, or at least many of their elements, of programmable materials.
  2. They provide a cost effective way to evaluate, validate, and refine programmable materials.
  3. Their basic designs (e.g. crease patterns) are transferrable, enabling sharing between labs across the world
  4. Their applications offer specifications and performance goals to guide material development.
  5. Successful integration with applications can lead to mechanisms that can make a positive societal, scientific, or economic impact.

Specific compliant mechanisms proposed as potential test beds include the following:

  1. Bistable waterbomb base (a straightforward origami base that has interesting bistable behavior).
  2. Lamina emergent mechanisms (compliant mechanisms that are fabricated in a plane but have motion that emerges out of the plane of fabrication).
  3. Two-degree-of-freedom positioner (a monolithic compliant mechanism originally developed for space applications).
  4. Deployable solar panel array (compact when stored and a large surface area when deployed).
  5. Minimally invasive surgery devices (compact during transport, then deployed when at the surgery site).

Polymeric Adsorbents for CO2 Capture with Tunable Ultra-Microporosity and Mesoporosity

Apr 22, 2014 9:45am ‐ Apr 22, 2014 10:00am

Identification: I1.05

Porous polymers are getting increasingly more studied as candidate adsorbents, membranes and parts in membrane composites for CO2 capture from flue gas or natural gas mixtures. As adsorbents they are relevant as both physisorbents and chemisorbents for CO2. As physisorbents targeting CO2 capture from flue gas it is crucial that the polymers have a significant amount of (ultra)micropores, which enable a high working capacity for CO2 removal. The ultramicropores leads to a high mass transport restriction and micro-/mesoporous organic polymers (MMOPs) could offer a way to lessen those.

Three armed monomers of 1,3,5-tris(4-aminophenyl)benzene and 1,3,5-benzenetricarboxaldehyde were used to synthesize MMOPs by Schiff base condensation reactions. The fraction of micro- and mesopores in the MMOPs depended strongly on the amine/aldehyde ratio used. A mechanism based on oligomeric self-templating is proposed that rationalize the dependency on the amine/aldehyde ratio on the mesopores formation. The MMOPs had specific surface areas and pore volumes up to 694 m2/g and 0.67 cm3/g. They exhibited a high CO2 uptake (21~38 cm3/g at 0.15 bar and 49~76 cm3/g at 1 bar; 273 K), and the CO2-over-N2 selectivity was 31~90.

The mesopores appeared to be of an “ink-bottle” type as revealed by cavitation on N2 desorption. The ultramicropores appeared to form by templating by either DMSO or by an excess of the aldehyde. The MMOPs could potentially be relevant for applications in carbon capture and storage (CCS), where the mesopores would facilitate a rapid mass transport. We will conclude with comparing the MMOPs with zeolites, metal organic frameworks and alike.


Inkjet Printing Graphene-Based Transparent Conductive Films

Apr 22, 2014 9:45am ‐ Apr 22, 2014 10:00am

Identification: LL2.02

Graphene is a strong contender material for the replacement of indium tin oxide (ITO) as the transparent conductor of choice for electronic applications due to its exceptional electrical and optical properties [1]. For practical manufacturing applications, large scale production of graphene materials is necessary. To produce large quantities of graphene materials, reduction of exfoliated graphene oxide sheets is favoured because it is a solution phase method with potential low cost. The resulting graphene oxide suspensions can be processed as graphene inks and deposited to form graphene films via large scale and low cost solution process such as inkjet-printing. In this work, we present a study of conductive reduced graphene oxide films produced by inkjet-printing. Highly stable graphene ink (up to 6 months) was prepared by dispersing graphene oxide in water with a stabilizing surfactant at pH ≈ 10 by adding ammonia. This was subsequently reduced in solution-phase using hydrazine monohydrate. Printed film electrical and optical properties are shown to be strongly dependent on the mean flake size used in the ink. By using large area size of graphene oxide sheets and adjusting the number of printing layers films with electrical sheet resistance of 6 kΩ/sq and optical transparency of 65% could be achieved. These properties corresponded to a ratio between the DC (σDC) and optical (σOp) conductivities (σDC/σOp) of 0.13, which was comparable with solution processed pristine-graphene films that have been reported previously [2]. This indicates that the flake size of the ink is at least of equal importance as the quality of the graphene in determining printed transparent film properties.

[1] S.-K. Bae, H.-K. Kim, Y.B. Lee, et al: “Roll-to-Roll Production of 30-inch Graphene Films for Transparent Electrodes”, Nature Nanotechnology, 5 (8), 574-578 (2010).

[2] S. De, P.J. King, M. Lotya, et al: “Flexible, Transparent, Conducting Films of Randomly Stacked Graphene from Surfactant-Stabilized, Oxide-Free Graphene Dispersions”, Small, 6 (3), 458-464 (2010).


Direct Observation of Doping Sites in Temperature-Controlled, p-Doped P3HT Thin Films

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: C4.02

Over recent years, solution-processed molecular dopants have received much attention due to their potential for the realization of stable and controllable doped transport layers for both p- and n-type materials. Such doped layers are indispensable for a variety of applications including organic light-emitting diodes (OLED), organic photovoltaics (OPV) and transparent conducting electrodes. Yet despite the increasing availability of p- and n-type materials from organic synthesis, the solution doping process and overall distribution of dopants within thin films are poorly understood. To this end, we investigate the relationship between solution-state doping and the corresponding phase separation between dopant and host molecules in thin film.

Here we focus specifically on the p-type doping of a prototypical, high performance semiconducting polymer poly(3-hexylthiophene) (P3HT) using F4TCNQ (7,7,8,8-Tetracyano-2,3,5,6-tetrafluoroquinodimethane). Using conducting-AFM (c-AFM), we are able to directly map dopant sites in blended films and observe the dopant-polymer phase separation as a function of doping concentration and solution temperature. Our data, firstly, confirm the existence of the “weak” and “strong” doping regimes and the resultant phase separation previously reported. In the “weak doping” regime, F4TCNQ remains as a neutral species in solution and becomes homogeneously dispersed among amorphous domains in thin films. In the “strong doping” regime, F4TCNQ molecules bind to the polymer backbone in solution, which leads to the formation of new crystalline domains in the solid state. Furthermore, we find that it is possible to tune the doping strength by controlling the solution of the spin casting solutions. A 2% doped film spin casted at 30oC, for instance, exhibits a doping efficiency of nearly one order of magnitude higher than a film spin casted at 80oC. The observed change in doping efficiency also resulted in a drastic change in phase separation in thin film. To our knowledge, this is the first report to successfully image doped sites in F4TCNQ:P3HT thin films and substantiates c-AFM as a powerful tool for characterizing doped polymer systems.


Photoelectrochemical Water Splitting Using Adapted Thin Film Silicon Tandem Junction Solar Cells

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: D1.07

For the application as photocathodes in integrated photoelectrochemical water splitting devices the thin film silicon technology stands out as an attractive choice, because it combines low-cost production, earth-abundance and versatility. Since the electrochemical potential to electrolyze water generally lies above 1.23 V, great importance is given to the latter characteristic, as thin film silicon solar cells can be adjusted to provide an extended range of achievable voltages, without impairing device efficiency. Nevertheless, as integrated water splitting devices additionally require chemical-resistant electrodes, stability issues of the silicon solar cells in contact with aqueous solutions need to be addressed.

We report on the optimization and usage of thin film silicon tandem junction solar cells. Tandem junction solar cells consist of two sub-cells connected in series. In this work, we investigate two types of tandem solar cells: (i) two amorphous (a-Si:H/a-Si:H) sub-cells with an open circuit voltage VOC of 1.87 V and a solar conversion efficiency of 10.0% (ii) and amorphous connected to microcrystalline (a-Si:H/µc-Si:H) sub-cells with a VOC of 1.42 V and an efficiency of 10.8%.

a-Si:H and µc-Si:H layers were deposited by plasma enhanced chemical vapor deposition, using a mixture of SiH4, H2, CH4, B(CH3)3 and PH3 gases. The optical band gap E04 was evaluated using photothermal deflection spectroscopy measurements and the crystallinity ICRS of µc-Si:H was determined by means of Raman spectroscopy. Solar cells were investigated by current-voltage measurements under AM 1.5 illumination. The photoelectrochemical performance of the electrodes was evaluated in an aqueous 0.1M H2SO4 solution under Xe halogen lamp irradiation (100 mW/cm2). By carrying out cyclic voltammetry measurements, we demonstrate the performance of the developed silicon based photocathodes, with respect to photocurrent densities and onset potentials for water reduction. a-Si:H/µc-Si:H photocathodes with a Pt back contact, for instance, exhibit a photocurrent onset potential of 1.3 V vs. the reversible hydrogen electrode (RHE) and a high photocurrent of 9.0 mA/cm2 at 0 V vs. RHE. However, the poor stability of the photocathodes, evaluated using chronoamperometric measurements, suggests that the application of protective layers on the silicon surface will be essential. During operation at 0 V vs. RHE, photocathodes without back contacts, i.e. direct contact of the silicon surface to the acidic electrolyte, generate stable photocurrents only for two hours. In this regard, various back contact interface designs are investigated, including silicon-silicon (µc-SiC:H, µc-SiOx:H), silicon-metal (Pt, Ag, Al, Ni, Mo) and silicon-TCO-metal interfaces. The corrosion behavior of both single layers and complete photovoltaic devices are studied in a broad pH range and in different electrolyte concentrations. Thereby, the electrochemical stability of the respective interfaces is evaluated.


Identifying, Visualizing and Modifying Reaction Pathways of Oxygen Reduction on Lanthanum Manganite (LSM) Model Electrodes

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: L1.07

Sr-doped lanthanum manganite (LSM) is a widely used cathode material in commercially produced solid oxide fuel cells (SOFC). Despite being a poor ion conductor, LSM electrodes may reduce oxygen via different pathways: a path which includes surface diffusion of oxygen species (surface path) and a path based on oxygen bulk diffusion (bulk path). The relevance of each path can be expected to depend on geometry and microstructure, temperature, overpotential and partial pressure. However, separation of effective reaction rates on LSM cathodes into contributions of each path is experimentally nontrivial. Hence, a detailed knowledge of the rate limiting steps and their dependence on experimental parameters is still missing.

In this contribution, several different methods are employed to identify, visualize and modify the oxygen reduction paths of (La0.8Sr0.2)MnO3 and (La0.8Sr0.2)0.95MnO3 thin films and thin film microelectrodes deposited by pulsed laser deposition (PLD):i) LSM films on strontium titanium oxide (STO) and yttria-stabilized zirconia (YSZ) with differentmicrostructures, from epitaxial to fine columnar textured, were investigated by 18O tracer diffusion. Numerical analysis allowed to separate surface resistance and bulk diffusion properties of both, grains and grain boundaries.ii) Material parameters calculated from 18O depth profiles were compared to results gained from impedance spectroscopy measurements on microelectrodes.iii) 18O incorporation upon cathodic bias visualizes different reaction pathways and their changing contributions for microelectrodes under different measurement conditions.iv) Current-voltage studies on microelectrodes with variation of geometry, bias and oxygen partial pressure allows separating reaction pathways, identifying the rate limiting step and gives information on the current voltage characterization of different kinetic steps.The combination of these complementary tools under various operation conditions thus lead to a substantially improved understanding of the oxygen reduction kinetics on LSM thin films.


Molecular Functionalization of Exfoliated Graphene and Transferred CVD Graphene

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: LL2.03

Monolayer graphene, single-atom sheets of carbon atoms arranged in a honeycomb pattern is an attractive candidate due to its high transparency (97.7%) and flexibility. However the intrinsic sheet resistance of graphene (~ 6.5 kilo-ohms per square) is two orders of magnitude too high for transparent electrode applications. Further, the sensitivity of the electrical properties of monolayer graphene to ambient adsorbates or organic processing residue (from transfer or patterning) represents a serious obstacle to exploitation of this novel nanomaterial. Molecular functionalization offers a route to overcome these difficulties, through passivation and/or controlled adsorbate doping via charge transfer. We report a systematic study of molecular functionalization of graphene comprising optical microscopy, atomic force microscopy, scanning electron microscopy, Raman spectroscopy, together with initial electrical characterization results of as-fabricated and post-functionalized graphene field-effect devices. Candidate molecules screened included both p-dopants and n-dopants and deposition techniques include evaporation, spin-coating and drop-casting.

Initial data on exfoliated graphene indicate that films formed by evaporation of small molecules, e.g. tetracyanoquinodimethane (TCNQ, a p-dopant) grow by nucleation and coalescence of ultra-thin islands (< 2 nm layer thicknesses). Efficient coupling between TCNQ and graphene is evidenced by molecular signatures observed in Raman data acquired from ultra-thin films deposited on both exfoliated graphene and graphene grown using chemical vapor deposition (CVD) on copper foil. Initial data from micron-scale field-effect devices fabricated from CVD graphene show net p-doping after evaporation of TCNQ, albeit with reduced mobility (ie increased sheet resistance).

This work was supported by the European Commission under the FP7 project GO-NEXTs (309201), and by the Irish Government HEA PRTLI programmes (INSPIRE & TYFFANI)


Formation Conditions for Epitaxial Graphene on Diamond (111) Surfaces

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: OO5.02

The phase transformation from a non-terminated diamond (111) surface to graphene, has in the present study been simulated using ab initio Molecular Dynamic calculations at different temperatures and under various reaction conditions. For vacuum conditions, the graphitization process was found to start at about 800 K, with a final graphene-like adlayer obtained at 2500 K. The C-C bonds across the interface were found to be broken gradually when increasing the temperature. The resulting graphene-like adlayer at 2500 K was observed to chemisorb to the underlying diamond surface with 33% of the initial C-C bonds, and with a C-C covalent energy value of 3.4 eV. The corresponding density of states spectra show a p-doped character, as compared with graphene. When introducing H radicals during the annealing process, a graphene-like adlayer started to be formed at a much lower temperature; 500K. The completeness of the diamond-to-graphene process was found to depend on the concentration of H radicals in the lattice. When the number of H radicals reached 34 within a super cell, a final free-standing graphene monolayer was formed at 1000 K. When introducing a larger concentration of H radicals into the lattice in the initial part of the annealing process, the formation of a free-standing graphene layer was found at an even lower H concentration and lower temperature (17 H within the supercell, and at 1000 K).

Graphene-Hydroxyapatite Biocompatible Nnanomateriales

Apr 22, 2014 10:00am ‐ Apr 22, 2014 10:15am

Identification: W1.07

Tissue engineering is a branch of science that deals with the design and synthesis of biomaterials with properties such that promote tissue regeneration by cell proliferation and differentiation. Biocompatible polymers have been widely investigated for the development of biomaterials due to their low cytotoxicity, together with its ability to form polymeric scaffolds with morphology suitable for mechanical support of the tissue. Also, various techniques have been developed which allow functionalization of polymeric scaffolds in order to improve such mechanical and biological properties, thus yielding, high quality hybrid materials with application in the area of tissue engineering.

In this study, graphene oxide sheets (GrO) were functionalized with hydroxyapatite nanoparticles (nHAp) through a simple and effective hydrothermal treatment and a new physicochemical process. Microstructure and crystallinity of the new hybrid nanomaterial GrO/nHap were investigated by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to characterize the morphology of the functionalized material.

To analyze biological properties, the composite material was functionalized using three different GrO:nHap ratios. In order to evaluate the cytotoxicity and cell proliferation the obtained materials were subjected to a MTT assay using the NIH-3T3 cell line. Polymeric scaffolds based in chitosan-polyvinyl alcohol (PVA-Ch) co-polymer were fabricated, integrating the hybrid material GrO/nHap at different concentrations (1-5 wt %), using a physical technique. The morphological characteristics of the resulting biomaterials were observed by SEM. The technique parameters were modified to increase the surface area and pore size of the biopolymer scaffolds. Confocal electron microscopy and SEM were performed to observe cell adhesion on the composite. Cell viability of the polymeric scaffolds reinforced with the hybrid materials was measured by MTT assay, using the same cell line described before.

The mechanical and physical properties will be characterized using thermal analysis (TGA and DSC) and mechanical tester. The resulting novel materials combine the biocompatibility of the nHap with the strength and physical properties of the graphene improving the properties of the polymer matrices, thereby obtaining a biomaterial with potential applications in tissue engineering.