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1.
J Appl Crystallogr ; 52(Pt 5): 1189-1201, 2019 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-31636522

RESUMO

The neutron powder diffractometer POWGEN at the Spallation Neutron Source has recently (2017-2018) undergone an upgrade which resulted in an increased detector complement along with a full overhaul of the structural design of the instrument. The current instrument has a solid angular coverage of 1.2 steradians and maintains the original third-generation concept, providing a single-histogram data set over a wide d-spacing range and high resolution to access large unit cells, detailed structural refinements and in situ/operando measurements.

2.
Rev Sci Instrum ; 89(9): 092904, 2018 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-30278698

RESUMO

High fluxes available at modern neutron and synchrotron sources have opened up a wide variety of in situ and operando studies of real processes using scattering techniques. This has allowed the user community to follow chemistry in the beam, which often requires high temperatures, gas flow, etc. In this paper, we describe an integrated gas handling system for the general-purpose powder diffraction beamline Powgen at the Spallation Neutron Source. The Automated Gas Environment System (AGES) allows control of both gas flow and temperature (room temperature to 850 °C), while measuring the partial pressure of oxygen and following the effluent gas by mass spectrometry, concurrent with neutron powder diffraction, in order to follow the structural evolution of materials under these conditions. The versatility of AGES is illustrated by two examples of experiments conducted with the system. In solid oxide fuel cell electrode materials, oxygen transport pathways in double perovskites PrBaCo2O5+δ and NdBaCo2O5+δ were elucidated by neutron diffraction measurements under atmosphere with oxygen partial pressures (pO2) of 10-1 to 10-4 (achieved using mixtures of nitrogen and oxygen) and temperatures from 575 to 850 °C. In another example, the potential oxygen storage material La1-xSrxFeO3 was measured under alternating flows of 15% CH4 in N2 and air (20% O2 in N2) at temperatures from 135 to 835 °C. From the oxygen stoichiometry, the optimal composition for oxygen storage was determined.

3.
Angew Chem Int Ed Engl ; 55(42): 13195-13199, 2016 10 10.
Artigo em Inglês | MEDLINE | ID: mdl-27652577

RESUMO

Crystals of the first sodalite-type zeolite containing an all-iron framework, a ferrolite, Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O, were synthesized using the hydroflux method in nearly quantitative yield. Ba8 (Fe12 O24 )Nay (OH)6 ⋅x H2 O crystallizes in the cubic space group Pm3‾m with a=10.0476(1) Å. Slightly distorted FeO4 tetrahedra are linked to form Fe4 O4 and Fe6 O6 rings, which in turn yield channels and internal cavities that are characteristic of the sodalite structure. Barium, sodium, and hydroxide ions and water molecules are found in the channels and provide charge balance. Magnetic measurements indicate that the ferrolite exhibits magnetic order up to at least 700 K, with the field-cooled and zero-field-cooled curves diverging. Analysis of the 57 Fe Mössbauer spectra revealed two spectral components that have equal spectral areas, indicating the presence of two subsets of iron centers in the structure. Dehydrated versions of the ferrolite were also prepared by heating the sample.

4.
ACS Nano ; 6(10): 8887-95, 2012 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-22947081

RESUMO

Core-shell nanowires (NWs) composed of silicon (Si) and germanium (Ge) are key structures for realizing high mobility transistor channels, since the site-selective doping and band-offset in core-shell NWs separate the carrier transport region from the impurity doped region, resulting in the suppression of impurity scattering. Four different types of Si/Ge (i-Si/n-Ge, p-Si/i-Ge) and Ge/Si (n-Ge/i-Si, i-Ge/p-Si) core-shell NWs structures were rationally grown. The surface morphology significantly depended on the types of the core-shell NWs. Raman and X-ray diffraction (XRD) measurements clearly characterized the compressive and tensile stress in the core and shell regions. The observation of boron (B) and phosphorus (P) local vibrational peaks and the Fano effect clearly demonstrated that the B and P atoms are selectively doped into the shell and core regions and electrically activated in the substitutional sites, showing the success of site-selective doping.


Assuntos
Cristalização/métodos , Germânio/química , Nanotubos/química , Nanotubos/ultraestrutura , Silício/química , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Tamanho da Partícula , Estresse Mecânico , Propriedades de Superfície
5.
Rev Sci Instrum ; 82(7): 075107, 2011 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-21806227

RESUMO

An in situ acoustic emission (AE) and x-ray diffraction cell for use in the study of battery electrode materials has been designed and tested. This cell uses commercially available coin cell hardware retrofitted with a metalized polyethylene terephthalate (PET) disk, which acts as both an x-ray window and a current collector. In this manner, the use of beryllium and its associated cost and hazards is avoided. An AE sensor may be affixed to the cell face opposite the PET window in order to monitor degradation effects, such as particle fracture, during cell cycling. Silicon particles, which were previously studied by the AE technique, were tested in this cell as a model material. The performance of these cells compared well with unmodified coin cells, while providing information about structural changes in the active material as the cell is repeatedly charged and discharged.

6.
J Nanosci Nanotechnol ; 10(12): 8298-306, 2010 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-21121331

RESUMO

Average crystallite sizes of microbially synthesized pure, metal-, and lanthanide-substituted magnetite (bio-magnetite) were determined for a variety of incubation times and temperatures, substitutional elements and amounts, bacterial species, and precursor types. The intriguing difference between nanoparticle bio-magnetite and chemically synthesized magnetite (chem-magnetite) was that powder X-ray diffraction (XRD) data showed that the bio-magnetite exhibited slightly smaller lattice parameters, however, Raman Spectroscopy exhibited no difference in Fe-O bonding. These results indicate that bio-magnetite likely exhibits a more compact crystal structure with less uncoordinated iron on the surface suppressing negative pressure effects. The bio-magnetite with decreased lattice parameters could have potential technological advantages over current commercial chemically synthesized magnetites.


Assuntos
Óxido Ferroso-Férrico/química , Nanopartículas de Magnetita/química , Óxido Ferroso-Férrico/metabolismo , Nanopartículas de Magnetita/ultraestrutura , Nanopartículas/química , Nanopartículas/ultraestrutura , Tamanho da Partícula , Shewanella/metabolismo , Análise Espectral Raman , Thermoanaerobacter/metabolismo , Difração de Raios X
7.
ACS Nano ; 4(7): 3807-16, 2010 Jul 27.
Artigo em Inglês | MEDLINE | ID: mdl-20565120

RESUMO

Impurity doping is the most important technique to functionalize semiconductor nanowires. The crucial point is how the states of impurity atoms can be detected. The chemical bonding states and electrical activity of boron (B) and phosphorus (P) atoms in germanium nanowires (GeNWs) are clarified by micro-Raman scattering measurements. The observation of B and P local vibrational peaks and the Fano effect clearly demonstrate that the B and P atoms are doped into the crystalline Ge region of GeNWs and electrically activated in the substitutional sites, resulting in the formation of p-type and n-type GeNWs. This method can be a useful technique for the characterization of semiconductor nanowire devices. The B-doped GeNWs showed an increasingly tapered structure with increasing B concentration. To avoid tapering and gain a uniform diameter along the growth direction of the GeNWs, a three step process was found to be useful, namely growth of GeNWs followed by the deposition of an amorphous Ge layer with high B concentration and then annealing.

8.
J Am Chem Soc ; 130(45): 14958-9, 2008 Nov 12.
Artigo em Inglês | MEDLINE | ID: mdl-18921981

RESUMO

We report an approach for growing aligned ZnO nanowire arrays with a high degree control over size, orientation, dimensionality, uniformity, and possibly shape. Our method combines e-beam lithography and a low temperature hydrothermal method to achieve patterned and aligned growth of ZnO NWs at <100degreesC on general inorganic substrates, such as Si and GaN, without using catalyst. This approach opens up the possibility of applying ZnO nanowires as sensor arrays, piezoelectric antenna arrays, two-dimensional photonic crystals, IC interconnects, and nanogenerators.


Assuntos
Gálio/química , Nanofios/química , Silício/química , Óxido de Zinco/química , Catálise , Temperatura Baixa , Cristalização , Polimetil Metacrilato/química , Difração de Raios X
9.
Nanotechnology ; 19(43): 435302, 2008 Oct 29.
Artigo em Inglês | MEDLINE | ID: mdl-21832689

RESUMO

A simple, scalable, and cost-effective technique for controlling the growth density of ZnO nanorod arrays based on a layer-by-layer polyelectrolyte polymer film is demonstrated. The ZnO nanorods were synthesized using a low temperature (T = 90 °C), solution-based method. The density-control technique utilizes a polymer thin film pre-coated on the substrate to control the mass transport of the reactant to the substrate. The density-controlled arrays were investigated as potential field emission candidates. The field emission results revealed that an emitter density of 7 nanorods µm(-2) and a tapered nanorod morphology generated a high field enhancement factor of 5884. This novel technique shows promise for applications in flat panel display technology.

10.
Nanotechnology ; 19(44): 445708, 2008 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-21832750

RESUMO

For the first time, the growth of ZnO nanobelts was monitored in situ using x-ray diffraction. The growth was carried out by heating metallic zinc powder in air at temperatures ranging from 368 to 568 °C. The morphology depends on both the growth temperature and the rate of heating to that temperature. A morphology diagram for the synthesized products was generated after systematic study of the experimental parameters. Higher temperatures and faster heating rates favor one-dimensional growth. Faster growth was observed for samples with higher growth temperatures, lower heating rates, and one-dimensional growth. These results give insight into the mechanism for the growth of ZnO nanobelts by metal oxidation.

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