DAYS 4, 5
WEDNESDAY, THURSDAY, August 18-19, 2010
The XIX International Materials Research Congress 2010 (IMRC 2010) concluded on Thursday in Cancún, Mexico, after 5 days of intense activities and high quality science. Major events on Wednesday included the plenary talk by Prof. Donald Paul on glassy polymer films for gas separation applications and the conference banquet in the evening. The fifth and final plenary lecture of the conference was presented on Thursday by Dr. Miguel Contreras on current and future photovoltaic materials.
High Flux Membranes and the Behavior of Thin Glassy Polymer Films - Donald R. Paul
Practical gas separation membranes operate by a solution-diffusion mechanism wherein the gas molecules dissolve in the polymer and then diffuse through it rather than flowing through pores, as would be true in some types of membranes. Glassy polymers have been found to be most useful for gas separation. In his plenary talk on Wednesday, Prof. Donald R. Paul of the University of Texas, Austin, USA, presented recent results on the use of very thin glassy polymer films as membranes for gas separation applications.
Paul started by presenting some background on membrane technology including historical development. Some glassy polymers used for gas separation include polysulfone (PSF), Matrimid® (BTDA-DAPI) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). He discussed physical aging in these polymers. Glasses are not in a state of equilibrium by their very nature, thus they tend to change with time by the process of physical aging. In order to achieve high fluxes (thereby high productivity) these membranes need to be very thin, typically 100 nanometers in thickness or less. Paul's group has found that such thin films of glassy polymers undergo much more rapid physical aging than thick or bulk glassy polymers. As a result, thin films of glassy polymers, or membranes, do not have the same properties as the same polymer would have in bulk (or a thick film). However, for many years this difference in thick and thin were not recognized and most research on polymeric materials for membrane applications was done on very thick films because of the convenience of preparation and measurement. It is now clear that this is not adequate. Paul's work has clarified several of these differences. This is particularly important as membrane processes are being considered for applications such as purification of Natural Gas streams.
Another aspect covered by Paul in his talk is the effect of CO2 exposure on aging and consequently gas separation properties. Permeability and selectivity change more rapidly after CO2 exposure, and conditioning with CO2 seems to ‘restart’ aging behavior to some extent. This is important in real life applications such as Natural Gas processing. Currently, 100 trillion scf of Natural Gas is used worldwide annually. This requires pretreatment and amine absorption is currently the leading technology. Membranes have <5% market share. However, with appropriate optimization and modifications, the share of membrane technology could be significantly increased making this process more compact and efficient. Paul concluded with a brief discussion on aging behavior of multilayer films which was found to be similar to the behavior of the bulk material. Paul's talk demonstrated clearly that very thin glassy polymer films have an important role to play in gas separation applications.
Photovoltaic Materials: Past, Present, and Future - Miguel Contreras
Dr. Miguel Contreras became interested in renewable energy as a college student in Chile. He is now a senior scientist at the National Renewable Energy Laboratory located in Golden, CO and is recognized as a leader in the field of thin film CuInGaSe2 (CIGS) technology for photovoltaics. He was a recipient of an Outstanding Technical Achievement award from the Hispanic Engineer National Achievement Award Corporation (HENAAC) in 2008 for his contributions to the field of solar and renewable energy. Contreras began his plenary lecture by describing the energy landscape of the United States and world. Renewables only contribute about 7% to current US energy use, and don’t even figure into world calculations. He described his dream future where greater than 90% of energy would come from renewable sources. Currently, world energy demand is about 12 terawatts and is estimated to be about 30 terawatts by 2050. For comparison, the sun could meet a 30 terawatt energy demand more than 5000 times over. Clearly, this is a resource we should be taking advantage of.
French physicist Alexandre-Edmond Becquerel discovered the photovoltaic effect in 1839, but it was not until 1954 that the first highly efficient solar cell was realized. Today, there are a variety of technologies being developed. Contreras went on to describe first, second, and third generation solar cells. First generation solar cells represent a silicon-based technology including crystalline and polycrystalline single junction cells. Polycrystalline cells make up about 94% of the market share today but, as Contreras argued, it takes a lot to make silicon out of sand, and a by-product of common processing techniques is carbon dioxide. Second generation, also called thin-film, solar cells use direct band gap materials such as CdTe, CIGS, and amorphous Si which can absorb light with only several microns of material. These types of cells are becoming major players in the PV market. The third generation is a mixture of many alternative approaches including multi-junction cells and organic-based active layers. The goal moving forward with any member of this generation is to make highly efficient cells at very low cost.
As a future direction, Contreras suggested that photovoltaics could take inspiration from nature. While he cautioned against trying to hook electrodes up to the electrical system in a salamander (!), he emphasized learning how natural systems function so we can design our own. Finally, he stressed the importance of long term vision and policies to bolster growth of renewable energies and our duty as scientists to continue reminding our leaders that change is necessary.
Symposium 1: Nanostructured Materials and Nanotechnology
Patterning Flat Silicon Surfaces by Chemical Self-Assembly: Deposition of Nanomaterials
Future devices for a wide range of applications, including biosensors, optics, catalysis, and electronics, are likely to use hybrid organic/inorganic junctions, said Damien Aureau of the University of Texas at Dallas, USA. One method commonly used to tether nanoparticles to a surface is to functionalize an oxidized silicon surface with silanes, molecules that form a self-assembling monolayer, or SAM. The formation of a SAM on oxide-free silicon can lead to higher chemical stability and better organization of the SAM on the surface. A higher level of gold nanoparticle coverage was achieved using the oxide-free method compared to traditional methods, shown both by AFM and SEM imaging. The organic SAM layer also completely passivates the silicon surface and protects it from oxidation during treatment with nanoparticle solutions, making the final surface highly homogenous in chemical composition. This high quality surface was used in experiments to study energy transfer through the organic layer. Data was shown that proved a monolayer of quantum dots was deposited on the surface of the silicon, a feat achievable because of the high quality organic interface. The quantum dot monolayer makes it possible to study the energy transfer from quantum dot to silicon without confounding the signal of transfer between quantum dots in different layers. Aureau suggested that this may find viable applications in photovoltaic technology, and that the gold patterning could be used in developing a one-electron transistor using a gold nanoparticle as the electron island.
Low Friction Coefficient Coatings in Ni-Cr by Magnetron Sputtering
J. Morales Hernandez of Centro de Investigacion y Desarrollo Tecnologico en Electroquimica, Mexico, wants to avoid sticky situations. Specifically, Hernandez wants to improve the anti-stick technology of medical devices. Specialty surgical devices that expose tissue to high heat or pressure can cause a significant amount of damage simply through tissue sticking to the contact surface. A successful anti-stick coating would be compatible with series 300 stainless steel, stable at temperatures up to 115°C, electrically conductive, safe for human contact, and minimally affected by current sterilization techniques. Results were shown describing the generation of a solid solution of Ni-20wt%Cr though mechanical alloying processes and extensive characterization of sputtered coatings using this material. Coatings were sputtered onto stainless steel substrates and subjected to a variety mechanical property testing. Film hardness was found to be directly related to film thickness. Strong adherence of the coating to the substrate was demonstrated by minimal crack formation under a series of loads. When friction tests were performed, the best coefficient of friction obtained was 0.08, which is about 3 times lower than a similarly processed coating using a commercially available alloy.
Symposium 3: Structural and Chemical Characterization of Metal Alloys and Compounds
Calculation of Spectral Reflectance of Butterfly Wings
Materials scientists at Facultad de Ingenieria Quimica BUAP in Pueblo, Mexico are getting in touch with nature. Carolina Osnaya Gamboa explained how the team studied spectral reflectance patterns from Papilio blumei and Albulina metallica butterfly wings in a poster presentation on Wednesday evening. The beautiful iridescent colors that are the signature of butterfly wings are due to the intricate architecture of scales on the wing surface which are composed of periodic multilayers of chitin and air. Gamboa measured the reflectance spectra at normal incidence and found that the maximum reflectance corresponds to wing color which varied between the two species based on the number of layers in each wing. For example, the dorsal wing surface of P. Blumei has a band of iridescent green scales which are composed of ten layers of chitin alternating with layers of air. In contrast, A. metallica wing scales consist of six layers of chitin also separated by air, which appear as iridescent blue. The experimental reflectance spectra were consistent with multilayer theory computations based on models of the heterostructure. The results show that wing color varies with incidence of light, layer thickness and number of layers. Gamboa stated that these principles can be applied to the design of photonic crystal structures in the visible spectrum for application in various devices.
Symposium 5: New Trends in Polymer Chemistry and Characterization
Constitutive Model for Glassy-Active Phase Transformation on Shape Memory Polymers
Shape memory polymers (SMPs) are a special class of “smart” polymers that after deformation recover their original shape in response to an externally applied stimulus. Interest in these unique polymers is growing as an inexpensive replacement for shape memory alloys in a variety of applications. Ignacio Varela (Tecnológico de Monterrey, Mexico) explained in a talk on Wednesday that the design of SMP-based devices requires characterization and modeling of the thermomechanical behavior to facilitate predication of the material’s behavior. Therefore, the storage and recovery of these unique polymers must be characterized.
The goal of Varela’s research is to develop a constitutive model of flow stress of acrylate SMPs using a temperature-induced glassy-active phase transformation approach. This approach assumes that the polymer is composed of a “glassy” (hard, crosslinked) phase and an “active” (soft, uncrosslinked) phase. It is considered that the SMP phase transformation is a sigmoidal function of temperature and that the volume fraction of the glassy and active phases both equal 0.5 at a characteristic temperature Tc. Strain-temperature data for three different SMPs were obtained by dynamic mechanical analysis and were used to determine the volume fractions of the glassy phases. A fitting parameter, B, was determined for each SMP by the least squares method, which was found to be related to the temperature sensitivity of the polymer, a function of composition. The final model describes stress on SMPs based on a glassy-active phase transformation as function of strain, temperature, and glass transition temperature, and the predicted results were consistent with experimental data. Varela stated that this model can be applied to other smart materials such as Ni-Ti alloys, magnetorheological fluids and electroactive polymers.
Fabrication of Polyvinylidene Fluoride Films for Application in Microsystems Development
Mathieu Hautefeuille of the Universidad Nacional Autónoma de México gave a talk on Wednesday about his work with a ferroelectric polymer, polyvinylidene fluoride (PVDF), for use in microsystems. PVDF is inexpensive, strong, flexible, easy to process, biocompatible, and stable under harsh conditions, in addition to having piezoelectric properties, which gives it an edge over comparable ceramic materials. Fabrication of PVDF films in their favorable crystalline form (b-phase) remains a challenge that has limited its use in areas where this polymer would otherwise be well suited. Hautefeuille and his colleagues have therefore used a direct approach to make these films whereby PVDF powder is dissolved in a polar solvent and deposited from solution onto various substrates using a heat-controlled spin-coating process. This technique reproducibly orients the polymer chains in the desired b-phase without additional processing. The films were characterized using FTIR-ATR and AFM as well as polarization and dynamic out-of-plane measurements with promising results. According to Hautefeuille, this direct method of fabrication could pave the way for PVDF film use in non-volatile memory, curvature sensors, artificial muscles, acoustic-wave propagation substrates and other specialty microsystem applications.
Symposium 8: Composite and Hybrid Materials
Evaluation of Chitosan/Poly(DL-Lactide-Co-Glycolide) and Chitosan/Poly(DL-Lactide) Composites for Bone Engineering
S.A. Martel-Estrada from Centro de Investigacíon en Materiales Avanzados, Mexico, described her work on polymer scaffolds for bone engineering in a poster presentation. She compared two composite materials, chitosan/poly(dl-lactide) (Ch/PDLLA) and chitosan/poly(dl-lactide-co-glycolide) (Ch/PDLLGA), as potential support structures for biomineralization and osteoblast growth. The scaffolds were prepared through thermally induced phase separation (TIPS) and sterilized using ethanol and UV light. They were then seeded with osteoblasts (bone cells) from a Winstar rat. The scaffolds were evaluated for cell activity, alkaline phosphatase (ALP) activity, and mineralization. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) were used to determine cell proliferation throughout the porous structure, cell adherence to the polymer was measured through a separate assay. Fourier Transform Infrared (FTIR) spectroscopy was used to measure the levels of CO3 and PO4 produced by the cells. The results showed ALP activity increased over time indicating cell differentiation. The highest activity levels were seen in the Ch/PDLLGA composite. At seven days out, FTIR showed the cells were producing significant levels of CO3 and PO4. SEM images revealed the spread of osteoblasts throughout the matrix over a period of 14 days. It was concluded that the inclusion of the hydrophilic chitosan polymer improved the growth and attachment of bone cells.
Symposium 11: New Catalytic Materials
Photocatalysis for Residual Water of Coffee Processing Plants
The harvesting of coffee beans produces destructive effects on the local environments of many coffee producing nations. With over 122 million 60 kg bags of coffee expected to be produced between 2010-2011, the organic waste by-product generated by harvesting is an ever growing threat. G. Corro of Benemerita Universidad Autonoma de Puebla, Mexico, gave a presentation on a novel treatment that uses a copper titanium photocatalyst to degrade coffee waste water in a cost efficient and ecologically sustainable manner. The extraction of the coffee bean from its fruit requires large amounts of water to de-pulp the fruit and wash the beans. The residual water from this process contains organic matter and mucilage, a mix of proteins and sugar. These sugars are broken down into ethanol which further reacts with oxygen to be converted into acetic acid. The acetic acid lowers the pH of the surrounding ecosystem and harms arable land by killing off helpful bacteria that aid in digestion of biomass. Unfortunately current solutions to treat organic waste are too expensive for most farmers.
Corro proposed a solution wherein acetic acid is quickly converted to CO2 and H2O using a photocatalytic reaction. In her experiment, she compared the use of a titanium oxide (TiO2) catalyst with a 10% copper/titanium oxide catalyst (Cu/TiO2). Both used solar radiation as their energy source; however TiO2 can only be excited by ultraviolet (UV) radiation. During the reaction process, bubbling air mixed 1 gram of the catalyst with 1 liter of the waste water. The mixture was exposed to 6 hours of sunlight at 1000 W/m2 each day. In the absence of electricity, solar panels were used to power the pump that circulated the system. Samples were analyzed for their concentration of polluting molecules and byproducts using UV-visible (UV-Vis) and Fourier Transform Infrared (FTIR) spectroscopy. The results revealed that over a period of 50 days, the activity level of the Cu/TiO2 catalyst was greater than that of TiO2 alone and was able to maintain high amounts of CO2 production over a much longer period of time.
Photocatalytic Hydrogen Production: Nanostructured Copper-Titania
Rosendo Lopez of Universidad Autónoma Metropolitana, Mexico, presented a poster on a renewable low cost method of generating hydrogen (H2). Current avenues of H2 production usually rely on steam reforming or electrochemical methods. But lately, researchers in this field have taken a keen interest in hydrogen production using semiconductor materials, by photoexcitation of the semiconductor, which causes the material to form electron pairs that can split water molecules into hydrogen and oxygen. The current work by Lopez called for the use of mesoporous titanium dioxide (TiO2) semiconductors doped with Cu at 0.1 to 5%. Semiconductor samples were made using a sol-gel reaction followed by an annealing thermal treatment. The final product was a dried powder that was finely ground. The photocatalytic reaction was initiated when the reaction vessel containing the methanol-water solution was irradiated by UV light at 254 nm. Thermal conductivity detector (TCD) gas chromatography revealed that H2 production was highest with the 5% doped Cu/TiO2 semiconductor. Lopez stated that the level of H2 produced was heavily reliant on the degree of Cu substitution of the Ti. He concluded that Cu/TiO2 as a photocatalyst has the potential to take on a significant role in the hydrogen economy.
Symposium 15: Photovoltaics, Solar Energy Materials, and Technologies
Interfacial Modification of Oxides for Enhanced Performance of OPV Devices
In the current family of photovoltaic materials, Organic Photovoltaics (OPV) represent an important component, and a low cost, scalable approach for producing renewable energy. Current module efficiencies are nearing 4% supporting scalability and there is now a clear pathway for developing acceptors and donors to achieve 10% or greater efficiency. One of the key questions is that of the stability of OPV devices and the potential for their lifetime to be sufficient for commercial use. Critical to this is both the intrinsic stability of the donor/acceptor phase separated mixture and the stability of the interfaces especially those between the inorganic and organic phases. The interfacial structure between the transparent conductor and the absorber appears to be critical to both performance and stability. In his invited talk, Dave Ginley of the National Renewable Energy Laboratory, USA, (and president of the Materials Research Society) discussed how the use of metal oxide intermediate contact layers, such as NiO, TiO2 and WO3 can significantly improve device characteristics.
Ginley discussed this in the context of both the standard architecture for OPV devices and the newer inverted polarity devices that can use higher work function contacts. In the inverted architecture, ZnO and Ag can be used as contacts, thereby significantly reducing cost and improving the ease of processing. The present data indicates that there is no inherent instability in the bulk heterojunction and that solving the interfacial issues may lead to devices of sufficient stability for commercial viability. Ginley described the use of a combinatorial system for evaluating stability. In summary, the talk indicated that metal oxides provide control for contact properties resulting in increased performance and lifetimes in OPV devices. Scalable processes were demonstrated for NiO as alternatives for pulsed laser deposition, including sputtering and solution deposition. Enhanced performance and lifetime for NiO was demonstrated with deep HOMO donor material.
Symposium 23: Nanostructure Applications in Cross-over Scientific and Technological Fields
Nanomaterials Toxicity Concerns
Emilio Rojas of the Biomedical Institute at Universidad Nacional Autonoma de Mexico shared interesting perspectives on the toxicity of nanomaterials. Recently, there has been a dramatic increase in inquiries into the safety aspects of nanomaterials. This is a complex question because so many factors must be taken into account including attributes of the nanomaterial, size, surface properties, charge, as well as environmental factors such as duration of exposure and route of administration. Rojas provided an overview of research into developing reliable biomarkers as indicators of nanomaterial toxicity currently being conducted at the Biomedical Institute. He also discussed some details of work in his own laboratory into cellular responses to multi-walled carbon nanotubes and 2 nm gold nanoparticles. By studying DNA damage and generation of reactive oxygen species, both bad situations for cells, differential effects of nanomaterials can be determined. Rojas impressed upon the audience that little is known about realistic exposure limits to date, and there is discussion as to what units are appropriate metrics to monitor. To this end, he stressed, we should aim to study in depth the toxicity phenomena associated with nanomaterials in order to better understand how to responsibly develop new materials in our increasingly nano-world.
Manufactured Nanomaterials: Understanding Possible Environmental Impacts
Ron Turco of Purdue University, USA, wonders if we should be looking at the environmental impact of a tennis racket in soil. His point is that many nanomaterials have now become mainstream by incorporation into manufactured products. Turco, with expertise in soil microbiology, is interested in what happens to the soil food web when different types of nanomaterials are introduced. In soil, there is a rich community of bacteria and fungi that help support plant life and other important environmental processes such as nitrification, and assessing the effects of nanomaterials on this system is a key part of understanding overall environmental impact. His group has studied materials composed of carbon, fullerenes and single wall carbon nanotubes, as well as metals including nano-silver, indium, and gallium. Methods of assessment include soil sorption tests, microbial community diversity, the respiratory response of fungi, and plant growth assays. Results suggest that carbon-based materials have little effect on soil communities, and fullerenes may even be degraded by certain types of fungi when first photo-oxidized to form hydroxyfullerenes. Overall the impacts of nano metals are “more negative, immediate, and warrant further study,” though indium and gallium showed milder effects than silver. There was some discussion in the audience about whether it was possible to assess the vast number of nanomaterials that are being developed, and Turco responded that it is indeed difficult to keep up. He stressed the importance of developing a system to classify different types of materials and maximize the impact of environmental studies such as these.
Scanning the Conference
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