TUESDAY, August 17, 2010
The XIX International Materials Research Congress 2010 (IMRC 2010) completed its third day in Cancún, Mexico, on Tuesday. The day's full slate of activities included technical sessions, the second poster session, a science luncheon with a talk on Mayan mathematics and the second day of the exhibit.
Poster award winners from the 2010 MRS Spring Meeting in San Francisco Hunter McDaniel (Univ. Illinois) and Stephen Morin (Univ. Wisconsin) presented their posters as part of an agreement between MRS and the Mexican-MRS. The other award winner Ho Won Jang (Univ. Wisconsin and KIST, Korea) could not attend. Poster award winners from this Cancun meeting will similarly present their posters at an MRS meeting.
Controlling Electronic, Chemical and Magnetic Properties via Super-orbitals: Towards Nanoscale Materials with Controllable Functionalities - Shiv Khanna
Prof. Shiv Khanna is Commonwealth Professor of Physics at Virginia Commonwealth University, Richmond, USA, and a Fellow of the American Physical Society. His work involves theoretical studies of electronic structure, magnetic properties, and catalytic properties of metal clusters, cluster assemblies, and nanoscale materials. In the third plenary lecture of the meeting, he took us along on a scientific journey from the first descriptions of metal clusters of a few atoms and their intriguing properties to cluster assemblies and the prospect of tunable materials made from these assembly building blocks. Much the same way as a single atom has defined energy levels, small clusters of atoms (a few dozen or less) do, too. These clusters were found to be stable at certain sizes termed “magic numbers” that have non-traditional properties. For example, aluminum which ordinarily quickly oxidizes at the surface does not oxidize at all when in a cluster size of 13 with a negative charge (Al13-). In fact, it has similar chemical properties to an inert gas. Take away one electron to get Al13, and it behaves much like a halogen. The key difference is that orbital filling of electrons is different from that of conventional atoms.
One major attribute of these “superatoms” that determine their reactivity is geometry. Again, using aluminum clusters as an example, Khanna outlined theory and experiments showing that certain sizes of Al superatoms can react with water to produce hydrogen due to positioning of reactive sites on the cluster. The fact that individual clusters have tunable properties based on size and geometry leads to the idea of using these clusters as building blocks to generate highly tunable materials. Several examples of band gap tunability were discussed including the variation of counterions used to assemble clusters and whether assemblies were extended in one, two, or three dimensions. Further investigations into superatom assemblies and the design rules for tunability could lead to an “era of designer reactants” and precise control over material properties, concluded Khanna.
Symposium 1: Nanostructured Materials and Nanotechnology
Quantum Dynamical Simulation of Plasmon Excitation in Metal Nanoparticles
Cristian Sanchez (Universidad Nacional de Cordoba, Argentina) presented results from quantum mechanical time dependent simulations of plasmons on nanoparticles. The goal of this work was to be able to model particles that contain thousands of atoms (sizes up to 5 nm). This “medium range” of particle size is difficult to access using traditional modeling techniques which can model either very small or very large particles depending on the choice. One scenario highlighted in the talk is that of a single atomic layer of silver on a gold nanoparticle. Simulations show that even just a single layer of silver can shift the resonance peak very close to that of pure silver, with very little difference other than a broadening of the peak attributed to increased scattering at the interface. Other results presented showed that particle shape, alloying, and surface adsorbates can all have strong non-classical effects on plasmon resonances.
From Carbon Nanotubes to Carbon Atomic Chains
Various forms of carbon nanoscale structures are currently under intense study. However, there are some allotropes of carbon that are not amenable to study because of the difficulty in synthesizing them. An example is carbyne which is a allotrope of carbon made of a linear chain of carbon atoms. It is formed by a linear arrangement of carbon atoms with sp-hybridization. Due to its unique geometry, it is anticipated to have many interesting properties as carbon nanotubes and graphene. In a poster presentation, G. Casillas García and M. Jose Yacaman of the University of Texas at San Antonio, USA, described a reliable and reproducible method to obtain these carbon atomic chains using few-layer-graphene (FLG) sheets and a High Resolution Transmission Electron Microscope (HRTEM, Jeol JEM-2011F at 200 KV).
The FLG sheets were synthesized from worm-like exfoliated graphite and then drop-casted on a lacey-carbon copper grid. Once in the TEM, two holes were opened near each other in a FLG sheet by focusing the electron beam into a small spot. Due to the radiation, the carbon atoms rearranged themselves between the two holes to form carbon fibers (i.e., a multiwall carbon nanotube (MWCNT)). The beam was concentrated on the carbon fibers in order excite the atoms and induce a tension until an MWCNT formed. As the radiation continued, the MWCNT broke down until there was only a single wall carbon nanotube (SWCNT). Then, when the SWCNT broke, an atomic carbon chain was formed that lasted for several seconds under the radiation and finally disintegrated. This demonstrated the stability of the carbon atom chains under 200 kV electron irradiation in the TEM.
Symposium 3: Structural and Chemical Characterization of Metal Compounds and Alloys
Influence of Process Parameters on Microstructure of Wide Gap Vacuum Brazing Stainless Steel 304 with Ni–Cr-Fe–B-Si and Inconel 738 Filler Metal
Brazing is a metal joining process that unites solid metal pieces with a filler metal in the liquid state just above its melting temperature. Careful selection of the filler alloy, clean parent metal surfaces and appropriate joint design are essential for creating sound brazed joints, with vacuum brazing providing superior strength and integrity compared to other brazing methods. Isidro Guzmán Flores of Corporacion Mexicana de Investigacion en Materiales (COMIMSA) described vacuum brazing of stainless steel 304 using an 80/20 blend of commercial Ni–Cr-Fe–B-Si filler alloy paste Nicobraz 160 and Inconel 738 powder filler metal. Isothermal solidification and microstructure development were examined at two brazing temperatures (1120°C, 1160°C) and two holding times (2 hours, 4 hours). Flores concluded from his results that using a Ni based filler metal alloy containing B and Si lead to the formation of a eutectic centerline in brazed joint microstructures. Increasing holding time reduced the eutectic centerline by allowing isothermal solidification and appropriate diffusion of B towards the metal base. This effect was enhanced further by decreasing the gap size. Finally, diffusion-affected zone precipitate formation was significantly affected by brazing temperature, as the joint/base metal interface was almost precipitate-free with increasing brazing temperature.
Symposium 4: Advanced Structural Materials
Quantitative 3-D Imaging of Single/Double Sheet Graphene
Graphene is being intensively investigated by numerous researchers. Since graphene is a 2-D layer of carbon atoms, it has been sunjected to TEM investigations, particularly high resolution TEM, to image the structure including defects. Researchers have used spherical aberration correction in TEM and STEM modes. However, no quantitative analysis of the electron dose has thus far been carried out even though the dose is critical. Prof. Hector A. Calderon (ESFM-IPN, Mexico) gave a presentation on Monday on using exit wave reconstruction and a 80 kV accelerating voltage to successfully quantitatively image the structure of graphene. This is a collaborative effort between ESFM-IPN and FEI Company, Eindhoven, The Netherlands. 80 kV was used to minimize damage and maximize contrast of light elements. In a FEI Titan G2 TEM, focal series reconstruction (FSR) was used wherein 20 images at exposure time interval of 0.1 s were obtained, and the images formed a complex exit wave image. This still included residual aberration and is not directly interpretable. This residual aberration was then corrected using software yielding a corrected phase image that was directly interpretable and could be analyzed quantitatively.
This method was used to resolve and image a double layer of graphene, as confirmed using simulations. The difference between a single carbon atom position and a double carbon atom position, and an upper and lower layer single carbon atom could be distinguished, indicating that an atomic 3D structure was resolved. In fact the contrast difference between the upper and lower layer carbon atoms matched a focus change of 0.3 nm which is equivalent to the distance between two carbon sheets. The results suggest that graphene sheets can be quantitatively imaged in 3-D using FSR and exit wave reconstruction in the TEM. This method will be very valuable to investigate the structure of graphene and defects.
Symposium 5: New Trends in Polymer Chemistry and Characterization
Synthesis, Characterization and Properties of Poly(3-hydroxybutyrate-g-acrylamide): Effect of Solvents
Polyhydroxybutyrate (PHB) is a nontoxic, biocompatible, biodegradable polymer synthesized by microorganisms that has been studied for biomedical applications such as drug delivery devices and scaffolding for tissue engineering. In a poster presentation on Tuesday evening, Maykal González-Torres (Universidad Popular Autonoma de Puebla) described radiation-induced graft co-polymerization of acrylamide (AAm) onto PHB. The graft copolymer was synthesized in various solvents via the simultaneous irradiation method using a Co60-source and verified by the appearance of an N-H stretching signal in FTIR spectra. The degree of grafting was confirmed by thermogravimetric analysis. Differential scanning calorimetry revealed a decrease in crystallinity with AAm grafting and an increase in glass transition temperature. AAm is one of five monomers studied by González-Torres to render PHB more hydrophilic. Swelling study results showed that water uptake increased significantly with increasing graft degree which further validated the success of the grafting reaction. González-Torres stated that synthesis of the copolymer is the first phase of his research. Future work will include examining the use of PHB-AAm copolymer microparticles for controlled drug delivery in the treatment of diabetes.
Some Unique Applications of Carbon Nanotube Composites and Hybrids
Carbon nanotubes (CNTs) have been extensively studied because of their unique combination of physical, electrical, thermal and mechanical properties. On Tuesday afternoon, Somenath Mitra, Professor of Chemistry and Environmental Science at New Jersey Institute of Technology, gave an invited talk on some novel uses of CNTs. He discussed several key properties of CNTs, e.g. functionalizability, excellent sorption, support for other materials, and self-assembly on substrates, and presented highlights from his work with CNTs over the past several years. Mitra’s primary research focus is finding new ways to assemble and modify CNTs with the ultimate goal of creating novel materials for a variety of applications. His other research interests include analytical techniques and sensors to detect low level trace elements in soil, water and air. Mitra discussed the application of self-assembled CNTs in long capillary tubes for gas chromatography whereby high-resolution separation of a number of organic molecules was achieved. He also described the use of CNT-immobilized membranes for desalination of sea water and pervaporation applications. The use of multiwalled CNT-C60 fullerene complexes in bulk heterojunction organic photovoltaic cells was also reported which exploits the electron acceptor properties of C60 and the efficient charge transport properties of multiwalled CNTs. Mitra also described the use of CNT hybrids such as CNT-Pt and CNT-Pd hybrids for applications in catalysis, and also zirconium-multiwalled CNT hybrids as a novel sorbent for defluoridation of water. Mitra’s talk shed light on the versatility of CNTs for a variety of engineering applications.
Symposium 9: Advances in Semiconducting Materials
Silicon Heterojunction Solar Cells
The reduction of greenhouse gases, specifically carbon dioxide (CO2) emissions is a pressing global concern. Minimizing the consumption of fossil fuels through the use of alternate energy sources such as photovoltaics is one of the most feasible options. With the rate of photovoltaics (PV) production growing at 40% a year to meet this demand, the trend is towards thinner silicon materials with good passivation properties to improve efficiency and decrease cost. Stuart Bowden, from Arizona State University, USA, gave an inspired talk on how the Silicon Heterojunction (SHJ) Solar Cell will be part of this revolution.
Though the utilization of photovoltaics made from crystalline silicon (c-Si) solar cells is widespread because of its abundance and high efficiencies possible, there is still a need for a material that will further increase the current maximum efficiency while improving affordability. SHJ solar cells achieve this by using amorphous silicon (a-Si) deposition in place of the high-temperature diffused junction in current c-Si cells. These heterojunction cells are composed of an intrinsic a-Si layer and c-Si substrate sandwiched between p- and n-type amorphous silicon, a design that allows for better passivation. Enhanced passivation yields an increase in the open-circuit voltage (Voc), and conversion efficiency. SHJ solar cells manufactured by Sanyo have shown efficiencies as high as 24%. Additionally, the cell exhibits good thermal stability over a period of 6 months and employs low temperature production methods. Bowden stated that these factors, among others such as wafer thickness and feedstock cost, have been identified by a national panel as targets for improvement in future photovoltaics.
Semiconductor Composite Creation From Reactions of Mixed Organo-Silicon Gels and Aniline
Héctor Olmos from the Universidad de Guanajuato, Mexico, gave a poster presentation on the feasibility of forming a hybrid organic-inorganic semiconductor using a sol-gel process. To synthesize this product, a variety of silicon alcoxides were used to undergo a hydrolysis-polycondensation reaction to form the silicon layer. In conjunction with this reaction, polyaniline was prepared by the oxidation of aniline with ammonium persulfate to form a glass substrate firmly linked to the conducting polyaniline. The two polymerization reactions competitively formed the composite. The hybrid material was analyzed for optical and thermoelectrical properties using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and resistance measurements. The composite material displayed characteristics favorable for light sensitizers. However, of the five silicon alcoxides used, only (3-trimethoxysilyl) propylethylenediamine (TMSPEDA) showed good electrical performance and thermal stability.
Symposium 16: Renewable Energy and Sustainable Development
Trends in Energy Storage
Symposium 19: Solid State Chemistry
Polar Inorganic Materials: Design Strategies and Functional Properties
Shiv Halasyamani from the University of Houston, USA, gave an in-depth review of the factors that play a role in determining which elements influence polarity in a titanium iodate crystal structure. Polar oxide materials are of great interest because of the unique properties polarity embodies a material with, such as pyroelectricity and ferroelectricity. These are of particular importance in various sensing devices, capacitors, and power generation applications. However, polarity only exists in solid state materials if it falls into one of 10 crystal classes. Halasyamani noted that the foundation for building solid state materials with polarity is using oxides that contain octahedrally coordinated d0 transition metal ions such as Ti4+ and lone-pair cations TI+. In recent work he synthesized a new series of alkali titanium iodates using five of the group 1 alkali metals and thallium (TI). However he found that cations with this configuration do not always result in polarity. The larger cations created non-polar centrosymmetric crystals whereas the smaller ions, Na+ and Li+, were polar but noncentrosymmetric. Additionally, he found that while Na2Ti(IO3)6 and Li2Ti(IO3)6 are polar, they were not ferroelectric, since polarity was not reversible in the presence of an electric field. This is most likely due to unfavorable energy requirements.
The Fascinating, Powerful, and Ludic Mayan Mathematics
Dr. Luis Fernando Magaña (Universidad Nacional Autonoma de Mexico) gave a fascinating lecture on Tuesday on Mayan Mathematics at a special science luncheon. The Mayans are well known for their accurate calendar system and their extremely sophisticated grasp of astronomical events. Modern calculations show the length of the year is 365.2422 days. Magaña playfully noted that our current Gregorian calendar uses a value of 365.2425 days, a value less accurate than the Mayan calculation of 365.2420. None of this understanding of the world around them would be possible without a well-developed system of mathematics, which they clearly had.
The Dresden Codex, one of the few Mayan documents left to us today, has been key in determining the system of everyday mathematics that Mayans used, incorporating dots, bars, and seashells. It is a system of columns and images that requires knowing only how to count and recognize patterns, without memorization of addition or multiplication tables. A dot represents one unit, a bar five, and a seashell zero. While the Mayans used base 20 notation, with each “slot” in a column representing an increasing power of 20, this lecture was conducted in base 10, a simple conversion to make in Mayan math. In one hour Magaña taught the audience how to add, subtract, multiply, divide, and even calculate square roots using Mayan notation.
Looking around the room during the lecture, the joy of discovery and understanding was apparent on many faces, and one could hear some friendly competition to see who could figure out the next solution first. Many questions after the lecture were asked involving more complex mathematics. Did the Mayans have a system of negative numbers? What about logarithms? What were the details of their geometry? Sadly, few Mayan documents survive today to provide direct archeological evidence for these, but Magaña did say this: when a mathematical system is as elegant as this one, it is easy to imagine that more complex operations are possible and were known to the Mayan people.
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