AMORPHOUS SILICON

Amorphous Silicon is a very promising material for photovoltaic solar cells. We are investigating fundamental aspects of this material, such as computer generation of amorphous Si samples, energy depth of hole traps, and light-induced degradation. (Images: Lucas Wagner and Eric Johlin).

Figure: strain causes formation in a-Si (left). Under illumination, defect states form that contribute to the material's degradation (right).

THREE-DIMENSIONAL PHOTOVOLTAICS

The architecture of currently used photovoltaic solar cells is inherently 2-dimensional. Thin films of materials constitute the panels, that are then assemble in 2-dimensional modules. This allows to save material and avoid inter-cell shading. When a solar tracker (i.e. a device that can favorably orient solar cells at different times of the day) is not used, the power production peaks at noon and decreases dramatically at different times of the day.

On the other hand, natural light-capturing structures, like trees, plants, and flowers, are 3-dimensional, and use the different orientation of the absorbing structures (like the leaves for example) to capture the light at different times of the day. For a number of technological reasons, our solar cells are not (yet!) 3-dimensional (3D)...but what would their ideal shape be, if they were 3D? What would their performance be?

What are the best insolation conditions for 3DPV? In testing our experimental prototypes, in many cases more than 30% of the energy generated by a single structure is due to internal and external reflections, i.e. between panels in the structure and with the surrounding environment. Furthermore a significant contribution comes from diffuse illumination (for example in a cloudy day). As it is shown in the figure below, the major benefits of 3DPV seem to be in such conditions of poor illumination. or where the interaction between panels and 3DPV system is significant. There is a tremendous, yet unexplored potential for this technology. Stay tune for more!

M. Bernardi, N. Ferralis, J.H. Wan, R. Villalon, J.C. Grossman, Energy Environ. Science 5, 6880 (2012)

B. Myers, M. Bernardi, and J.C. Grossman, Appl. Phys. Lett. 96, 071902 (2010).

U.S. Patent 2012/0007434

(Image: N. Ferralis, M. Bernardi)

CEMENT

Cement manufacturing is responsible for a large share of CO2 emission deriving from human activities. To manufacture cement in a more environment friendly way, fundamental studies of the structure and transformation of cement must be undertaken. We are part of the "Liquid Stone" project at MIT and are performing Density Functional Theory calculations to investigate certain aspect of cement and its structure. (Image: Engin Durgun).

HEAT DISSIPATION AT THE NANOSCALE

Carbon nanotubes show anomalous heat dissipation. Sharp resonance effects allow for near complete and highly efficient energy transfer. Our recent work provides the mechanistic basis for a theoretical description of lattice vibration mediated heat flow in carbon nanotubes and nanoscale materials in general.

(Image: Alex Greaney ).

SURFACES

High duty nuclear power reactors face the risk of Axial Offset Anomaly (AOA), a problem usually associated with the formation of corrosion related products (CRUD) on nuclear fuel rods. AOA refers to an unequal axial power distribution along the fuel rods, with a neutron flux depression in the upper region of the rod. Sub-cooler nucleate boiling, circulating corrosion products and boron deposition in the CRUD are believed to be the major causes for the occurrence of AOA. With reference to this industry-specific problem, we are studying the mechanism of boron incorporation in CRUD.



(Image: Priyank Vljaya Kumar)

A simple model to explain boron deposition is a two-step adsorption-precipitation process. We are studying the surface affinity of different CRUD substrates (Nickel oxide, Zirconia, Magnetite, Nickel ferrite) towards boric acid, and trying to understand the adsorption and surface reactions through ab-initio calculations. In our current research, we use density functional theory in particular to obtain the energetics. We also employ the nudged elastic band (NEB) calculations to obtain activation barriers and rates of several surface reactions. We hope to construct a simple model to explain the overall mechanism of boron incorporation in CRUD, and aim to extend the same in future.