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Harrell, Zach

Publications and source records attributed to Harrell, Zach.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Strain Enhanced Functionality in a Bottom-Up Approach Enabled 3D Super-Nanocomposites

The ability to control nanoparticle size, concentration, and distribution in epitaxial nanocomposite films has been a formidable challenge in the synthesis of nanostructured composite materials. In this paper, a novel 3D super-nanocomposite (3D-sNC) architecture is successfully demonstrated by integrating superlattice and vertically aligned nanocomposite structures. In the 3D-sNC architecture, the feature size and distribution of the nanocylinders such as the height/lateral dimension and the vertical/lateral spacing of nanocylinders can be precisely controlled. The microstructure parameters such as nanocylinder height and spacing modulated interfacial area control the lattice strain, which further tunes the magnetotransport property. These results demonstrate that 3D-sNC is a simple and yet effective architecture to achieve controlled functionalities via the precise control of nanocylinder size, spacing, concentration, and distribution. Lastly, such a 3D-sNC structure can be used to design advanced nanostructures with desired physical properties for a variety of material systems.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Role of temperature and oxygen content on structural and electrical properties of LaBaCo 2 O 5+δ thin films

Here, the role of temperature and the oxygen content in the structural transformation and electrical conductivity of epitaxial double perovskite LaBaCo 2 O 5+δ (0≤ δ ≤ 1) thin films was systematically investigated. Reciprocal space mapping and ω-2θ x-ray diffraction performed at different temperatures in vacuum indicate that oxygen vacancies in the films become ordered at high temperature in a reducing environment. The changes of the oxygen content and the degree of oxygen vacancy ordering in the films result in a strong in-plane anisotropic lattice deformation and a large thermal expansion coefficient along the c-axis direction. The electrical conductivity measurements reveal that these behaviors are related to the degree of oxygen vacancy formation and lattice deformation in the films.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Roles of reaction kinetics of CO 2 on a PrBaCo 2 O 5.5+δ surfaces

A symmetric PBCO/YSZ/PBCO cell (where PBCO refers to PrBaCo 2 O 5.5+δ , and YSZ to yttria stabilized zirconia) was designed and fabricated for the investigation of the catalytic nature and reaction kinetics of CO 2 on PBCO surfaces. The electrochemical impedance spectroscopy was employed to probe the CO 2 reaction behavior on the PBCO electrode. The symmetric equivalent circuit cell characterization reveals that the surface resistance of CO 2 on PBCO is only 30.2 Ω cm 2 and the maximum surface exchange coefficient k chem is 2.0 × 10 -4 cm s -1 at 1147 K, suggesting that PBCO can be an excellent candidate for CO 2 reduction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Hidden Interface Driven Exchange Coupling in Oxide Heterostructures

A variety of emergent phenomena have been enabled by interface engineering in complex oxides. The existence of an intrinsic interfacial layer has often been found at oxide heterointerfaces. However, the role of such an interlayerin controlling functionalities is not fully explored. Here, we report the control of the exchange bias (EB) in single-phase manganite thin films with nominallyuniform chemical composition across the interfaces. The sign of EB depends on the magnitude of the cooling field. A pinned layer, confirmed by polarized neutron reflectometry, provides the source of unidirectional anisotropy. The origin of the exchange bias coupling is discussed in terms of magnetic interactions between the interfacial ferromagnetically reduced layer and the bulk ferromagnetic region. The sign of EB is related to the frustration of antiferromagnetic coupling between the ferromagnetic region and the pinned layer. Our results shed new light on using oxide interfaces to design functional spintronic devices.

epitaxial thin films↗
DOE OSTI2017

Oxygen Vacancy-Tuned Physical Properties in Perovskite Thin Films with Multiple B-site Valance States

Controlling oxygen content in perovskite oxides with ABO 3 structure is one of most critical steps for tuning their functionality. Notably, there have been tremendous efforts to understand the effect of changes in oxygen content on the properties of perovskite thin films that are not composed of cations with multiple valance states. Here, we study the effect of oxygen vacancies on structural and electrical properties in epitaxial thin films of SrFeO 3-δ (SFO), where SFO is a compound with multiple valance states at the B site. Various annealing treatments are used to produce different oxygen contents in the films, which has resulted in significant structural changes in the fully strained SFO films. The out-of-plane lattice parameter and tetragonality increase with decreasing oxygen concentration, indicating the crystal structure is closely related to the oxygen content. Importantly, variation of the oxygen content in the films significantly affects the dielectric properties, leakage conduction mechanisms, and the resistive hysteresis of the materials. These results establish the relationship between oxygen content and structural and functional properties for a range of multivalent transition metal oxides.

36 MATERIALS SCIENCE↗