Timothy

Strobel

Extreme Materials

Timothy Strobel's research is centered around the synthesis and characterization of novel materials for energy and advanced applications. New materials are synthesized using unique pressure-temperature conditions and through innovative processing pathways. Of particular interest are extended structures rich in carbon, silicon, and germanium, as well as hydrogen-rich molecular systems.

Timothy Strobel subjects materials to high pressures to understand chemical processes and interactions and to create new, advanced energy-related materials.

For instance, silicon is the second most abundant element in the Earth’s crust and a mainstay of the electronics industry. But normal silicon is not optimal for solar energy. In its conventional crystalline form, silicon is relatively inefficient at absorbing the wavelengths most prevalent in sunlight. Strobel made a discovery that may turn things around. Using the high-pressure techniques pioneered at Carnegie, he created a novel form of silicon with its atoms arranged in a cage-like structure. Unlike normal silicon, this new form has near-ideal properties for absorbing sunlight and could potentially usher in an entirely new generation of solar cells.

Understanding the chemical reactions that can create tiny molecular cages that hold other “guest” molecules—structures called clathrates—is key to creating a new generation of electronic devices and possible energy materials. Strobel and his team were the first to use high-pressure synthesis to create a stable clathrate of sodium (Na) and silicon (Si)—the least understood system of the so-called group 14 clathrates. Strobel also created a new clathrate of hydrogen sulfide (H2S) and molecular hydrogen (H2). Both findings open the door for major advances in materials science.

In addition to these experiments, Strobel and his team calculated how the materials would behave. Calculations and experiments revealed that sodium clathrates are thermodynamically stable at high pressure. The consistency suggests that scientists can use theoretical calculations to predict new synthesis routes for other compounds.

In other work, a team including Strobel reported the synthesis of an ionic semiconductor —Mg2C—that is fully recoverable to ambient conditions. Semiconductors are substances with electrical conductivity between insulators and metals used in electronic circuits.

For any given pressure, temperature and composition, we can now predict and synthesize ground-state materials—those that lie on the convex hull– with remarkable success. However, numerous polytypes are predicted to exist above the convex hull, sometimes within only a few kJ/mol. In many cases, there are hundreds or thousands of “energetically plausible” and dynamically stable structures. Given the vast number of potential metastable structures compared with the singular ground state, it is likely that the material with the “best” property possible for a particular application is not the ground state. To what extent are metastable structures accessible? What criteria define accessibility and are there fundamental limits on synthesizability? Are there rational design principles and experimental methods to access these materials?

Of all physical variables, pressure possesses one of the greatest ranges—over sixty orders of magnitude. Pressure is thus a phenomenal tool to characterize fundamental interactions/processes and to gain access to new materials. As pressure is increased, the free energy of the system also increases through contributions to the pV term. At pressures approaching one million times atmospheric pressure, this contribution becomes comparable to the energy of a covalent bond and remarkable atomic arrangements become stabilized.

While the true energy-configurational landscape exists in high-dimensional (3N) space, it can be visualized in three dimensions by reducing the overall dimensionality. The global energy minimum represents the thermodynamic ground state, but many higher-energy local minima may be stabilized by the system's inability to overcome kinetic barriers. Take carbon for example. At low pressure, the lowest-energy structure is graphite, while diamond is the lowest-energy structure at high pressure. Once formed under high-pressure, high-temperature conditions, diamonds may be metastably recovered to ambient conditions, with robust kinetic persistence. Many new materials become thermodynamically stabilized under high-pressure conditions and can be recovered to ambient conditions.

New oxides, nitrides, and carbides –virtually every class of compounds– are waiting to be discovered.

The LABstar glovebox allows for experiments to be conducted and manipulated under an inert Ar atmosphere. The glove box is equipped with two antechambers, one small and one large, to allow for more efficient cycling. The box is equipped with a single column inert gas purification system that maintains O2 and H20 levels below 1 ppm. The interior of the box is equipped with a diamond anvil cell loading stage (camera, monitor, light sources), a balance accurate to 0.001 g, pellet press, and a vacuum sealing system. Available chemicals are maintained in an inventory list stored next to the glovebox.

A variety of furnaces and ovens are available for synthesis. The Mellen microtherm furnace is an industrial box furnace setup for solid-state synthesis. The furnace can maintain temperatures up to 1800 °C and heats a volume of 6”x6”x6”. The oven is controlled by an Omega CN7800 controller and can run a variety of temperature profiles including slow heating and cooling. The Mellen split tube furnace operates at temperatures up to 1250ºC in air. The standard orientation of the furnace is horizontal, however, a vertical stand or a bench stand is available. The furnace has fast heat-up rates (30ºC per minute standard) and an integrated dual-display controller, making it easy to use. Tubes are easily attached to the turbo pump for dynamic pumping experiments. The Thermolyne small muffle furnace is the ideal choice for quick runs up to 1100ºC. The digital setpoint controller allows for a variety of custom heating profiles. The isotemp vacuum oven is an ideal setup for general vacuum drying and heating applications. The oven has a depth of 18” and a capacity of 1.5 cu. Ft. The oven can maintain temperatures up to 280 °C and can be monitored with the built-in LED controller. The controller can be set up for programs as long as seven days and can cycle on and off up to eight times per day.

The vacuum sealing line is used to isolate solid-state experiments under vacuum in glass tubing. The line is setup to receive various sizes of glass tubing, evacuate them, and seal the tubing with a hydrogen/oxygen torch. The system can maintain vacuum to at least 10-3 Torr during sealing.

The Fritsch pulverisette is suited for loss-free grinding to 100 nm of hard, medium-hard, and brittle materials. Grinding can be performed dry, in suspension, or in inert gas. The system maintains main disk rotational speeds of 100-1100 rpm and features a SelfLOCK sample chamber for absolutely safe operation. The system can run simultaneously on two samples with a capacity of 30 ml each.