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Dr. Hui Wu

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Hui Wu
NIST Center for Neutron Research
100 Bureau Dr.
Gaithersburg, MD

Ph: (301) 975-2387
Fx: (301) 921-9847

BT1 Instrument
BT4 Instrument
My principal research interests lie in the synthesis, structure, solid state chemistry, and properties of complex oxides and hydrides. As part of the Crystallography and Diffraction Applications Team at the NIST center for neutron research, we routinely use Neutron and X-ray diffractometers and spectrometers in our research work. Shown below is a brief description of my main research areas.
"Crystallographic investigation of novel materials using Neutron and X-ray diffraction and vibrational spectroscopy"
Previous experience with neutron scattering not required.


I. Neutron and X-ray Crystallography and Spectroscopy

Neutron and X-ray crystallography and spectroscopy have been fundamental in the development of condensed matter science. In our work, we widely use these methods to study the atomic structures and vibrational properties of new materials, such as novel hydrides, oxides, and metal organic frameworks (MOFs).

II. Novel Hydrogen-Storage Materials

Successful development of hydrogen as a primary fuel will simultaneously reduce the dependence on fossil fuel and emissions of greenhouse gases and pollutants. This is especially critical in this century with increasing concern on energy security and global warming. One of the major challenges to widespread use of hydrogen is the lack of suitable hydrogen storage materials with the on-board operating storage capabilities for fuel-cell vehicular applications. Current hydrogen storage materials being intensively investigated include metal hydrides (complex hydrides), chemical hydrides and high surface area adsorbents, which can chemically or physically store hydrogen in the form of atoms, ions or molecules.

Our research is focused on synthesis, characterization, and development of advanced materials for hydrogen storage, including:

Metal Hydrides and Complex Hydrides

Metal hydrides have the potential for reversible on-board hydrogen storage and release at low temperatures and pressures. Onboard hydrogen storage systems must meet vehicle manufacturers goals for safe and cost-effective storage while enabling a vehicle to be in operation for at least 300 miles between refueling. Light-metal hydrides have received considerable attention in recent years as possible candidates for hydrogen storage because of their relatively high hydrogen-storage densities (>5 wt%). However, so far most of these hydrides have not been considered practically competitive because of rather low absorption kinetics, relatively high thermal stability, and/or problems with the reversibility of hydrogen absorption/desorption cycling. Our group is working to address these challenges using various strategies.

Metal Organic Frameworks (MOFs)

MOFs, consisting of metal ions or clusters connected by organic linkers, are a relatively new class of crystalline, nanoporous materials. These materials have exhibited many interesting gas adsorption properties and great potential for gas storage and separation applications, mainly due to their large surface areas and pore volumes. The work in our group focuses on understanding the fundamental physics and chemistry of these novel materials and their exciting gas adsorption properties.

Useful links:

DOE Hydrogen Storage Program

DOE Energy Efficiency and Renewable Energy (EERE) office

II. Complex Perovskites and Their Applications

"ABO3" Perovskites constitute the most important and widely studied class of dielectric and ferroelectric materials due to their ability to sustain outstanding microwave dielectric properties and tolerate extensive chemical substitution. In practice, by tailoring the chemistry of the A and B sublattice the electronic properties can be tuned to produce a ferroelectric, relaxor, anti-ferroelectric, or low-loss dielectric response and serve as critical components in a number of smart devices.

The physical properties of perovskite materials are quite diverse, covering a wide range of scientific areas, from electrical (ferroelectricity, superconductivity, dielectricity, ionic and electronic conductivity) to magnetic, optical, and catalytic properties. Due to their comparatively simple crystal structure and large possibility of ionic substitutions, perovskite compounds have been used as models to investigate the relationship between crystal structure and the physical properties.

Particularly, the number of possible perovskite compounds can be significantly expanded when the A and/or B- sub-lattice contain a mixture of two (or more) different cations. An ordered arrangement can be stabilized if the species occupying the same site differ sufficiently in charge and/or size, but in most cases the symmetry and the unit cell will change. The study of the phenomena associated with atomic order and disorder on the local scale in complex perovskite systems is an interesting and important topic at the forefront of the current materials research.

Dielectric Ceramics for Microwave Wireless Communication

Low-loss dielectric ceramics have important applications as resonators, filters, and other key components in microwave communication systems. The size of the resonator at any particular frequency depends on the inverse of the square root of the dielectric constant (er). Therefore, materials with the highest dielectric constant seem to be optimal for the requirement of miniaturization. However, to be useful, the resonant frequency of the filters must be temperature independent. The practical requirement for the temperature coefficient of resonant frequency (tf) is tf = ± 5ppm/°C. Furthermore, the dielectric loss in the microwave frequency range (about 500 MHz to 20 GHz) must be very low, with a quality factor (Q) (Q = 1/tand) greater than 1000, preferably an order of magnitude higher.

However, in practice, these requirements: high dielectric constant, low dielectric loss, low temperature coefficient of resonate frequency are almost always mutually exclusive in dielectric materials and only a few materials satisfy the stringent requirements for application in microwave communication devices. In addition, the fundamental physics that gives rise to the desired properties, especially dielectric loss and temperature stability, are not well understood.

Our target is to develop new temperature stable dielectric ceramics with low loss and high dielectric constants, to explore current processing challenges in known complex perovskites, and to investigate the correlations between material processing, cation ordering, microstructure and microwave dielectric response.

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