Ruigang Wang

In situ environmental TEM studies of dynamic changes in cerium-based oxides nanoparticles during redox processes.

We apply in situ environmental transmission electron microscopy (ETEM) to study the dynamic changes taking place during redox reactions in ceria and ceria-zirconia nanoparticles in a hydrogen atmosphere. For pure ceria, we find that a reversible phase transformation takes place at 730 degrees C in which oxygen vacancies introduced during reduction order to give a cubic superstructure with a periodicity of roughly twice the basic fluorite lattice. We also observe the structural transformations taking place on the surface during reduction in hydrogen. The (110) ceria surface is initially constructed with a series of low-energy (111) nanofacets. Under strong reduction, the surface slowly transforms to a smooth (110) surface which was not observed to change upon re-oxidation. The surface transformation allows the reduced surface to accommodate a high concentration of oxygen vacancies without creating a strong perpendicular dipole moment. In the ceria-zirconia system, we are able to use ETEM to follow the redox activity of individual nanoparticles and correlate this property with structure and composition. We find considerable variation in the redox activity and interpret this in terms of structural differences between the nanoparticles.

Measuring the redox activity of individual catalytic nanoparticles in cerium-based oxides

We have followed the dynamic redox process taking place in individual ceria zirconia nanoparticles with changes in the oxygen chemical potential using an in situ environmental transmission electron microscopy (ETEM). We observe considerable variability in the redox activity and have been able to correlate these differences with nanoscale structural and compositional measurements. We find that the more active structure has predominantly disordered cations and shows no evidence for oxygen vacancy ordering during reduction.

Fabrication of Al2O3–Ti3SiC2 composites and mechanical properties evaluation

Al2O3 toughened with Ti3SiC2 has been investigated for improved toughness. Hot-pressed Al2O3–Ti3SiC2 composites were manufactured and characterized. Addition of Ti3SiC2 inhibited the growth of alumina grains though pinning mechanism. The experimental results showed that the Vickers hardness decreased with an increase in Ti3SiC2 content, and the fracture toughness and strength increased with Ti3SiC2 content. Maximum toughness (8.78±0.12 MPa m1/2) and maximum strength (500±10.2 MPa) were achieved through the addition of 50 wt.% Ti3SiC2 content, which is compared to alumina toughened by other materials.

Effect of LaPO4 content on the microstructure and machinability of Al2O3/LaPO4 composites

Abstract In the present work, microstructure and machinability of Al 2 O 3 /LaPO 4 composites are presented according to variations of lanthanum phosphate (LaPO 4 ) content (10 wt.%, 20 wt.%, 30 wt.%, 40 wt.%). X-ray diffraction analysis shows that only Al 2 O 3 and LaPO 4 phases exist in the Al 2 O 3 /LaPO 4 composites with the different LaPO 4 content at the sintering temperature (1600 °C for 2 h). The weak bonding of Al 2 O 3 and LaPO 4 phase can be responsible for the reduction of hardness and the improved machinability of the composites. Bulk density, hardness and microstructure of Al 2 O 3 /LaPO 4 composites are highly dependent on the LaPO 4 content. When the content of LaPO 4 increased up to 30 wt.%, Al 2 O 3 /LaPO 4 composites can be dilled using cemented carbide drills instead of conventional diamond tools.

CeO2 nanorods-supported transition metal catalysts for CO oxidation

A catalytically active oxide support in combination with metal catalysts is required in order to achieve better low temperature activity and selectivity. Here, we report that CeO2 nanorods with a superior surface oxygen release/storage capability were used as an active support of transition metal (TM) catalysts (Mn, Fe, Co, Ni, Cu) for CO oxidation reaction. The as-prepared CeO2 nanorods supported 10 wt% TM catalysts were highly active for CO oxidation at low temperature, except for the Fe sample. It is found that the 10%Cu-CeO2 catalyst performed best, and it provided a lower light-off temperature with T50 (50% conversion) at 75 °C and T100 (100% conversion) of CO to CO2 at 194 °C. The atomic level surface structure of CeO2 nanorods was investigated in order to understand the improved low temperature catalytic activity. The richness of surface roughness and various defects (voids, lattice distortion, bending, steps, twinning) on CeO2 nanorods could facilitate oxygen release and storage. According to XRD and Raman analysis, copper species migrate into the bulk CeO2 nanorods to a greater degree. Since CO adsorbed over the surface of the catalyst/support is detrimental to its catalytic activity, the surface defects on the CeO2 nanorods and CeO2-TM interactions were critical to the enhanced activity.

Combustion Synthesis of Nanoparticulate LiMgxMn1-xPO4 (x=0, 0.1, 0.2) Carbon Composites

A combustion synthesis technique was used to prepare nanoparticulate LiMgxMn1-xPO4 (x=0, 0.1,0.2)/carbon composites. Powders consisted of carbon-coated particles about 30 nm in diameter, which were partly agglomerated into larger secondary particles. The utilization of the active materials in lithium cells depended most strongly upon the post-treatment and the Mg content, and was not influenced by the amount of carbon. Best results were achieved with a hydrothermally treated LiMg0.2Mn0.8PO4/C composite, which exhibited close to 50percent utilization of the theoretical capacity at a C/2 discharge rate.

Ultrafast Microwave Nano-manufacturing of Fullerene-Like Metal Chalcogenides.

Metal Chalcogenides (MCs) have emerged as an extremely important class of nanomaterials with applications ranging from lubrication to energy storage devices. Here we report our discovery of a universal, ultrafast (60 seconds), energy-efficient, and facile technique of synthesizing MC nanoparticles and nanostructures, using microwave-assisted heating. A suitable combination of chemicals was selected for reactions on Polypyrrole nanofibers (PPy-NF) in presence of microwave irradiation. The PPy-NF serves as the conducting medium to absorb microwave energy to heat the chemicals that provide the metal and the chalcogenide constituents separately. The MCs are formed as nanoparticles that eventually undergo a size-dependent, multi-stage aggregation process to yield different kinds of MC nanostructures. Most importantly, this is a single-step metal chalcogenide formation process that is much faster and much more energy-efficient than all the other existing methods and can be universally employed to produce different kinds of MCs (e.g., MoS2, and WS2).

Graded machinable Si3N4/h-BN and Al2O3/LaPO4 ceramic composites

This paper provides a new material design method for machinable ceramics within an overall methodology. Examples are provided to illustrate the use of the methodology in specific ceramic materials, and machinability of the materials is characterized with cemented carbide drills instead of diamond tools. This method enables the use of composition compound and structure design combination for the purposes of improving machinability of ceramics while allowing for retaining mechanical properties of the materials. At the same time, this method also provides the basic design principle for the other comparable, but intrinsically different, ceramic systems based on chemical compatibility and thermodynamic properties. Graded machinable Si3N4/h-BN and Al2O3/LaPO4 composites were designed, fabricated and characterized.

Seed-mediated synthesis of shape-controlled CeO2 nanocrystals

We report the effect of varying the starting materials and experimental conditions on the shape control of CeO2 nanocrystals using hydrothermal methods, including using different cerium precursors (Ce3+: Ce(NO3)3 and Ce4+: Ce(NH4)2(NO3)6), stirring treatment, and oxidation of cerium(III) hydroxide. It was demonstrated that the formation of Ce(OH)3 nuclei is a key step to growth of different morphological CeO2 nanocrystals.

Improved low-temperature reducibility in ceria zirconia nanoparticles by redox treatment

It is found that optimized redox treatment can improve the low-temperature reducibility in ceria zirconia. We have studied the atomic-level structure and chemistry of individual nanoparticles for the raw and redox-treated ceria zirconia nanopowders to understand the origin of this improved performance.