Research in the Novel Materials Laboratory




Materials Research


Solid-state materials synthesis has played an important part in improving our way of life. Improvements on existing materials have impacted the technological advances over the years; advances in many areas depend on the discovery and development of new materials. This is particularly true for power conversion technologies (such as thermoelectrics and photovoltaics) and alternative fuel technology. In addition, it is becoming increasingly urgent to produce materials designed to perform highly specific and cooperative functions. The Novel Materials Laboratory is designed for the synthesis and characterization of novel materials for these technologically significant applications. The emphasis is on an understanding of the structure-property relationships of material systems. The laboratory applies this understanding in the synthesis and crystal growth of new and novel materials.




Solid-State Power Conversion


Thermoelectrics

The potential of a material for thermoelectric (or heat-to-electrical conversion) applications is determined by the material's dimensionless figure of merit, ZT = (α2σ/κ)T, where α is the Seebeck coefficient, σ the electrical conductivity and κ the total thermal conductivity (κ = κL + κe, the lattice and electronic contributions, respectively). The power factor, α2σ, is typically optimized as a function of carrier concentration (typically around 1019 carriers/cm3), through doping, to give the largest ZT. Current TE devices are rather inefficient, with material parameters resulting in ZT ˜ 1, even though they are able to address many niche applications. To achieve their full potential, new materials and new material’s processing are needed in order to achieve higher performance or lighter weight TE devices.
Thermoelectric Power Generation




Skutterudites

The name skutterudite derives from a naturally occurring mineral, CoAs3, first found in Skutterud, Norway. This crystal structure is cubic - a schematic showing the basic structure is shown in the figure. Skutterudites have interesting properties useful for thermoelectric applications, including a large unit cell, heavy constituent atom masses, low electronegativity differences between the constituent atoms and large carrier mobilities. They form covalent structures with low coordination numbers for the constituent atoms and so can incorporate atoms in relatively large voids formed. Pioneering work on Yb and Eu skutterudite antimonides revealed high ZT compositions and 'filler' atoms for optimizing thermoelectric properties.
Type I Crystal


Amorphous Thermoelectrics

It has recently been shown that there are good prospects for increasing the thermoelectric figure of merit through boundary scattering of phonons in certain semiconductors. These are materials in which the effective mass of the charge carriers is relatively large. Thus, although a high power factor is essential for any thermoelectric material, this can sometimes be achieved through a combination of modest carrier mobility with a large effective mass, rather than a much higher mobility with a small effective mass. The material with a large effective mass may have a mean free path for the carriers that are not much higher than the mean free path of the phonons. This is one of the conditions for boundary scattering to enhance rather than diminish the figure of merit. Nolas and Goldsmid (see ‘The Figure of Merit in Amorphous Thermoelectrics’, Physica B, Vol. 194, p. 271, 2002) determined the conditions that must hold if the mean free path of the acoustic phonons is to be greater than that of the charge carriers. This should enable the selection of materials that are suitable candidates for production by a rapid quenching technique.


Multinary Chalcogenides

Multinary chalcogenides of varying compositions and structure types form a large class of minerals that are typically composed of earth-abundant and nontoxic elements. Although several different compositions have been studied over the past few years there are a large number of variations that have as yet not been investigated. Some of these materials have low thermal conductivity and are therefore of interest for thermoelectric applications while others have large band gaps that make them of interest for potential solar cell and photovoltaics applications [34-36]. The premise of this aspect of our research is in developing routes for electronic structure engineering that will help us achieve designed mulitnary chalcogenides with superior properties that can demonstrate enhance performance for energy-related applications.


Clathrates
Type II Clathrate Unit Cell
Clathrates are a class of novel materials that have frameworks built up from group IV atoms. These materials have crystal structures closely related to those of the clathrate hydrates. Many different compositions are possible within this class of materials. They are of fundamental interest from the perspective of both bonding and their physical properties. Tunability, i.e. simultaneously reducing the thermal conductivity while optimizing the electronic properties, is most prominent in materials with the clathrate crystal structure. In particular, type II clathrates have the ability to maintain partial or completely empty atomic cages. The “guest” species simultaneously act as electronic dopants and localized phonon scattering centers, allowing for a wider range of adjustable parameters in order to optimize the thermoelectric properties. This type of “tunability” is rare. Recently single crystals of different clathrate types have been grown, allowing for the intrinsic properties of these interesting materials to be investigated for the first time.



Photovoltaics

The semiconductor Si is the most important semiconductor material for current electronic devices. The relatively small and indirect band gap however makes it unsuitable for many possible applications, in particular for opto-electronic applications. One approach used for “band gap engineering” has been to alter the geometry thereby tuning the properties in an attempt to realize the desired results. Another interesting way is by making Si in porous form. The clathrates have the appropriate crystal structure for just this application. The elemental Si-clathrate Si136 has a 2 eV band gap with semiconducting properties. Recent theoretical predictions indicate that clathrate alloys with the type II clathrate structure have a tunable direct band gap of between 1.2 and 2.0 eV that is dependant on the composition. This represents a most exciting technological development for opto-electronic semiconductor device applications. The controlled synthesis of clathrate alloys as a function of “guest” species and concentration, and the subsequent determination of their electrical optical properties are essential for the development of these clathrate alloys as viable opto-electronic materials.




Solid State Refrigeration

Solid State Refrigeration_web figure Magnetocaloric Effect

Solid state refrigeration based on the magnetocaloric effect (MCE) receives increased attention as an alternative to the compression-evaporation techniques. MCE is a magneto-thermodynamic phenomenon in which an externally applied changing magnetic field can strongly affect the spin degrees of freedom in a solid that results in a reversible change in temperature in a given specimen. Magnetic refrigeration is also an environmentally friendly cooling technology, in this way similar to thermoelectric refrigeration, using no ozone depleting chemicals, hazardous chemicals, or green house gasses, and possesses higher efficiency as compared to the conventional gas-compression techniques. A magnetocaloric material with a large magnetic entropy change over a wide temperature range, i.e., a large refrigerant capacity (RC), is of therefore interest for magnetic refrigeration applications. Clathrates-I and VIII Eu8Ga16Ge30 are two such materials. The type-I clathrate–EuO composite shows the largest RC in this group of magnetocaloric materials, while possessing nearly zero thermal and field hysteresis losses. These magnetocaloric properties make it one of the best candidate materials for active magnetic refrigeration around 70 K.



Nanoscale Materials Research

Some of our recent progress in higher efficiency thermoelectric materials can be attributed to nanoscale enhancement. Physically, nanostructured TE enhancement aims to split the interdependence of the electrical and thermal transport, allowing for greater optimization of the thermoelectric properties. One consequence of nanostructure is the increase of interfaces. Interface scattering of phonons and charge carriers plays an important role in understanding the fundamental physics behind this enhancement. The presence of interfacial energy barriers filters the carrier energy traversing the interface, restricting those energies that limit the mean carrier energy. This increases the Seebeck coefficient, as its value depends on the mean carrier energies relative to those at the Fermi level. Understanding the physical mechanism responsible for Seebeck enhancement in these materials is one of our goals. We are currently investigating the transport properties of doped and undoped nanocomposites, i.e. nanocrystals densified within a macro-scale nanocomposite, in order to promote further insight and stimulate fundamental research into the transport of polycrystalline bulk semiconductor grain boundaries. To this end, we have developed a novel approach to prepare chalcogenide dimensional nanocomposites by densifying nanocrystals (10 - 100 nm) synthesized employing an aqueous solution-phase reaction with high yield and low cost. The carrier concentration of these nanocomposites is adjusted by directly doping the nanocrystals, necessary for thermoelectric optimization. An enhancement in the TE properties has been realized in these nanocomposites, as compared to that of similar bulk compositions.

TEM images of nanocrystalline materials synthesized in the Novel Materials Laboratory





Hydrogen Storage


In spite of great progress in most of the last century in solid state materials and the developments of highly refined porous solids, our ever growing technological needs require novel approaches in order to develop the required novel materials. The ability to design tunable porous solids with zeolite-like topologies will allow for the exploration of unique “designer” materials for hydrogen storage applications. One such system has the clathrate hydrate structure. Given the novel “open-structured” framework of clathrates and the corresponding ability to vary the absorption into these polyhedra, together with the potential adsorption on the framework sites, there exists a genuine opportunity to study the hydrogen storage rate and capacity in three-dimensional inorganic framework materials that is rare in most physical systems. Most importantly, this material system is ideal in allowing for conclusive evidence as to the potential of inorganic complexes as hydrogen storage materials. As such it makes this part the investigation both very timely and very important.



Fundamental Research


Building on the current scientific knowledge base is an important aspect of the new materials research. The knowledge obtained through the investigation of the materials research described is an important aspect of our research. An example is the work currently underway on clathrate material systems. Clathrates provide an opportunity to investigate novel phonon and charge scattering phenomena and strucutre-property relationships. Not only is this important from the standpoint of investigating these materials for potential applications, but it also presents an opportunity to investigate fundamental properties of group-IV elements in novel crystal structures. While structural, mechanical, optical, electrical and thermal properties will be studied, the primary goal is to understand the fundamental physics of these properties as a function of structure and stoichiometry.
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