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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Thickness measurements of nanoscale brine films on silica surfaces under geologic CO 2 sequestration conditions using synchrotron X-ray fluorescence

In reservoirs used for geologic CO 2 sequestration, brine films remaining on mineral surfaces can influence flow, diffusion, and reactions. We have examined how the capillary (disjoining) potential influences the thickness of a KCsI 2 brine film on both smooth and rough SiO 2 surfaces [root mean square roughness (Rrms), 1.6 and 330 nm, respectively], under confinement with supercritical (sc) CO2. The thicknesses of brine films coating interior surfaces of SiO 2 windows in a high-pressure cell were determined through synchrotron X-ray fluorescence of two tracer ions (I - and Cs +) at 7.8 MPa and 40°C (representative of conditions at about 0.78 km below the land surface), with scCO 2 as the immiscible confining fluid. The measured area-averaged film thicknesses on the 330 nm Rrms silica surface ranged from 265 to 249 nm for capillary potentials measured within a narrow range from 0.18 to 3.7 kPa. Over this same range of potentials, film thicknesses measured on the smooth (1.6 nm Rrms) silica surface were about 2 nm, although equilibrium does not seem to have been achieved. The measured average brine film thicknesses were critically controlled by surface roughness, with very weak variation in response to the fairly narrow range of tested capillary potentials.

58 GEOSCIENCES↗
DOE OSTI2012

Microstructure and electrical properties of LaNiO{sub 3} thin films by RF sputtering for the growth of (Pb,La)(Zr,Ti)O{sub 3} films on silicon and nickel substrates

Conductive LaNiO{sub 3} (LNO) thin films were deposited on Si substrates by RF sputtering at room temperature to 600 Degree-Sign C. X-ray diffraction of the LNO films showed that preferred orientation varied from (100) below 500 Degree-Sign C to (110) at 600 Degree-Sign C at deposition temperature of 200-600 Degree-Sign C. The room temperature resistivity was 2250-3400 {mu}{Omega}-cm at the range of 200 Degree-Sign C {approx}500 Degree-Sign C. The LNO films, deposited at room temperature, were postannealed in air from 500 to 800 Degree-Sign C, and the resistivity value was 280 {mu}{Omega}-cm at a postannealing temperature of 600 Degree-Sign C. Subsequently, sol-gel derived (Pb,La)(Zr,Ti)O{sub 3} thin films were deposited on LNO-buffered nickel substrates and characterized, the results were compared to those of the same films on silicon substrates. The comparison demonstrated that the performance characteristics of the LNO-buffered nickel foils are similar to those of LNO-buffered silicon substrates. Thus, LNO films prepared by the sputtering method can act as an effective barrier layer on nickel substrates for embedded microelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Vibrational spectra of CO adsorbed on oxide thin films: A tool to probe the surface defects and phase changes of oxide thin films

Thin films of iron oxide were grown on Pt(111) single crystals using cycles of physical vapor deposition of iron followed by oxidative annealing in an ultrahigh vacuum apparatus. Two procedures were utilized for film growth of ∼15–30 ML thick films, where both procedures involved sequential deposition+oxidation cycles. In procedure 1, the iron oxide film was fully grown via sequential deposition+oxidation cycles, and then the fully grown film was exposed to a CO flux equivalent to 8 × 10{sup −7} millibars, and a vibrational spectrum of adsorbed CO was obtained using infrared reflection-absorption spectroscopy. The vibrational spectra of adsorbed CO from multiple preparations using procedure 1 show changes in the film termination structure and/or chemical nature of the surface defects—some of which are correlated with another phase that forms (“phase B”), even before enough of phase B has formed to be easily detected using low energy electron diffraction (LEED). During procedure 2, CO vibrational spectra were obtained between deposition+oxidation cycles, and these spectra show that the film termination structure and/or chemical nature of the surface defects changed as a function of sequential deposition+oxidation cycles. The authors conclude that measurement of vibrational spectra of adsorbed CO on oxide thin films provides a sensitive tool to probe chemical changes of defects on the surface and can thus complement LEED techniques by probing changes not visible by LEED. Increased use of vibrational spectra of adsorbed CO on thin films would enable better comparisons between films grown with different procedures and by different groups.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Anomalous grain growth in the surface region of a nanocrystalline CeO2 film under low-temperature heavy ion irradiation

Grain growth and phase stability of nanocrystalline ceria are investigated under ion irradiation at different temperatures. Irradiations at temperatures of 300 and 400 K result in uniform grain growth throughout the film. Anomalous grain growth is observed in thin films of nanocrystalline ceria under 3 MeV Au+ irradiation at 160 K. At this low temperature, significant grain growth is observed within 100 nm from the surface, no obvious growth is detected in the rest of the films. While the grain growth is attributed to a defect-stimulated mechanism at room temperature and above, a defect diffusion-limited mechanism is significant at low temperature with the primary defect responsible being the oxygen vacancy. The nanocrystalline grains remain in the cubic phase regardless of defect kinetics.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2013

Structural and Physical Property Studies of Amorphous Zn-In-Sn-O Thin Films

The structures in amorphous (a-) Zn, Sn co-doped In 2 O 3 (ZITO) thin films grown by pulsed laser deposition on glass under varying oxygen pressure or with varying Sn:Zn ratios were determined using X-ray absorption spectroscopy and anomalous X-ray scattering. Typical structures around cations in a-ZITO films are described and compared with crystalline (c-) ZITO films. The results show that the Zn cations are fourfold coordinated with Zn–O bond lengths of 1.98 ± 0.02 Å, which is close to that in bulk ZnO. As a consequence, the second coordination shells around Zn contract. At longer distances away from Zn, the structure is commensurate with the averaged structure. The unit volume around In also contracts slightly compared to bulk In 2 O 3 , whereas the Sn–O bond length is similar to the one in bulk SnO 2 . These unique structural characteristics may account for the films' superior thermal stability over amorphous Sn-doped In 2 O 3 , and suggest that Zn and Sn act as network-forming cations. Like in c-ZITO, coordination numbers (N) around Sn, In, and Zn follow the order N Sn > N In > N Zn . Unlike in c-ZITO, where electrical properties change significantly with a slight variation in the Sn:Zn ratio, this variation does not markedly alter the electrical properties, or the local structures, of a-ZITO films. Dramatic changes in the electrical properties occur for films grown under various oxygen pressures, which point to oxygen “defects” as the source of charge carriers.

Proffit, Diana E↗
DOE OSTI2012

Cation Off-Stoichiometry Leads to High p-Type Conductivity and Enhanced Transparency in Co2ZnO4 and Co2NiO4 Thin Films

We explore the effects of cation off-stoichiometry on structural, electrical, optical, and electronic properties of Co{sub 2}ZnO{sub 4} normal spinel and Co{sub 2}NiO{sub 4} inverse spinel using theoretic and experimental (combinatorial and conventional) techniques, both at thermodynamic equilibrium and in the metastable regime. Theory predicts that nonequilibrium substitution of divalent Zn on nominally trivalent octahedral sites increases net hole density in Co{sub 2}ZnO{sub 4}. Experiment confirms high conductivity and high work function in Co{sub 2}NiO{sub 4} and Zn-rich Co{sub 2}ZnO{sub 4} thin films grown by nonequilibrium physical vapor deposition techniques. High p-type conductivities of Co{sub 2}ZnO{sub 4} (up to 5 S/cm) and Co{sub 2}NiO{sub 4} (up to 204 S/cm) are found over a broad compositional range, they are only weakly sensitive to oxygen partial pressure and quite tolerant to a wide range of processing temperatures. In addition, off-stoichiometry caused by nonequilibrium growth decreases the optical absorption of Co{sub 2}ZnO{sub 4} and Co{sub 2}NiO{sub 4} thin films, although the 500-nm thin films still have rather limited transparency. All these properties as well as high work functions make Co{sub 2}ZnO{sub 4} and Co{sub 2}NiO{sub 4} thin films attractive for technological applications, such as hole transport layers in organic photovoltaic devices or p-type buffer layers in inorganic solar cells.

14 SOLAR ENERGY↗
DOE OSTI2016

Environmentally induced chemical and morphological heterogeneity of zinc oxide thin films

Zinc oxide (ZnO) thin films have been reported to suffer from degradation in electrical properties, when exposed to elevated heat and humidity, often leading to failures of electronic devices containing ZnO films. This degradation appears to be linked to water and oxygen penetration into the ZnO film. However, a direct observation in the ZnO film morphological evolution detailing structural and chemical changes has been lacking. Here, we systematically investigated the chemical and morphological heterogeneities of ZnO thin films caused by elevated heat and humidity, simulating an environmental aging. X-ray fluorescence microscopy, X-ray absorption spectroscopy, grazing incidence small angle and wide angle X-ray scattering, scanning electron microscopy (SEM), ultra-high-resolution SEM, and optical microscopy were carried out to examine ZnO and Al-doped ZnO thin films on two different substrates—silicon wafers and flexible polyethylene terephthalate (PET) films. In the un-doped ZnO thin film, the simulated environmental aging is resulting in pin-holes. In the Al-doped ZnO thin films, significant morphological changes occurred after the treatment, with an appearance of platelet-shaped structures that are 100–200 nm wide by 1μm long. Synchrotron x-ray characterization further confirmed the heterogeneity in the aged Al-doped ZnO, showing the formation of anisotropic structures and disordering. X-ray diffraction and X-ray absorption spectroscopy indicated the formation of a zinc hydroxide in the aged Al-doped films. In conclusion, utilizing advanced characterization methods, our studies provided information with an unprecedented level of details and revealed the chemical and morphologically heterogeneous nature of the degradation in ZnO thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural and chemical ordering of Heusler C o x M n y G e z epitaxial films on Ge (111): Quantitative study using traditional and anomalous x-ray diffraction techniques

Epitaxial films of C o x M n y G e z grown on Ge (111) substrates by molecular-beam-epitaxy techniques have been investigated as a continuous function of composition using combinatorial synchrotron x-ray diffraction (XRD) and x-ray fluorescence (XRF) spectroscopy techniques. A high-resolution ternary epitaxial phase diagram is obtained, revealing a small number of structural phases stabilized over large compositional regions. Ordering of the constituent elements in the compositional region near the full Heusler alloy C o 2 MnGe has been examined in detail using both traditional XRD and a new multiple-edge anomalous diffraction (MEAD) technique. Multiple-edge anomalous diffraction involves analyzing the energy dependence of multiple reflections across each constituent absorption edge in order to detect and quantify the elemental distribution of occupation in specific lattice sites. Results of this paper show that structural and chemical ordering are very sensitive to the Co : Mn atomic ratio, such that the ordering is the highest at an atomic ratio of 2 but significantly reduced even a few percent off this ratio. The in-plane lattice is nearly coherent with that of the Ge substrate, while the approximately 2% lattice mismatch is accommodated by the out-of-plane tetragonal strain. The quantitative MEAD analysis further reveals no detectable amount (<0.5%) of Co-Mn site swapping, but instead high levels (26%) of Mn-Ge site swapping. Increasing Ge concentration above the Heusler stoichiometry ( C o 0.5 M n 0.25 G e 0.25 ) is shown to correlate with increased lattice vacancies, antisites, and stacking faults, but reduced lattice relaxation. The highest degree of chemical ordering is observed off the Heusler stoichiometry with a Ge enrichment of 5 at.%.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Magnetic Nonuniformity and Thermal Hysteresis of Magnetism in a Manganite Thin Film [Depth profiling of magnetization and coupling of strain with magnetization in (La 0.4 Pr 0.6 ) 0.67 Ca 0.33 MnO 3 films]

We measured the chemical and magnetic depth profiles of a single crystalline film grown on a NdGaO 3 substrate using x-ray reflectometry, electron microscopy, electron energy-loss spectroscopy and polarized neutron reflectometry. Our data indicate that the film exhibits coexistence of different magnetic phases as a function of depth. The magnetic depth profile is correlated with a variation of chemical composition with depth. The thermal hysteresis of ferromagnetic order in the film suggests a first order ferromagnetic transition at low temperatures

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

General method for simultaneous optimization of light trapping and carrier collection in an ultra-thin film organic photovoltaic cell

We describe a general method for maximizing the short-circuit current in thin planar organic photovoltaic (OPV) heterojunction cells by simultaneous optimization of light absorption and carrier collection. Based on the experimentally obtained complex refractive indices of the OPV materials and the thickness-dependence of the internal quantum efficiency of the OPV active layer, we analyze the potential benefits of light trapping strategies for maximizing the overall power conversion efficiency of the cell. This approach provides a general strategy for optimizing the power conversion efficiency of a wide range of OPV structures. In particular, as an experimental trial system, the approach is applied here to a ultra-thin film solar cell with a SubPc/C{sub 60} photovoltaic structure. Using a patterned indium tin oxide (ITO) top contact, the numerically optimized designs achieve short-circuit currents of 0.790 and 0.980 mA/cm{sup 2} for 30 nm and 45 nm SubPc/C{sub 60} heterojunction layer thicknesses, respectively. These values correspond to a power conversion efficiency enhancement of 78% for the 30 nm thick cell, but only of 32% for a 45 nm thick cell, for which the overall photocurrent is actually higher. Applied to other material systems, the general optimization method can elucidate if light trapping strategies can improve a given cell architecture.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Doping CdSexTe1-x/CdTe Graded Absorber Films with Arsenic for Thin-Film Photovoltaics

CdTe thin-film photovoltaics have demonstrated some of the lowest costs of electricity generation owing to its low material cost and ease of manufacturing. However, the full potential of polycrystalline CdTe photovoltaics can only be realized if the open-circuit voltage can be increased beyond 1 V Open-circuit voltage ~850-900 mV has been consistently observed for state-of-the-art polycrystalline CdTe solar cells. Open-circuit voltage of over 1V has been demonstrated for single crystal CdTe devices by doping with Group V elements. Therefore, this study is aimed at understanding behavior of polycrystalline CdTe devices with arsenic doping, its activation and process and performance optimization in order to overcome current voltage limitations in CdTe solar cells.

14 SOLAR ENERGY↗
DOE OSTI2020

Pulsed laminar falling films in vertical tubes: Maintaining a continuous liquid film with reduced film thickness

Falling-liquid-film heat transfer is important to a wide variety of industrial applications, including two-phase thermosyphons, falling-film evaporators in the food industry, heat exchangers in fuel-ethanol processing, and the chemical service industry. Particularly for laminar falling films, the heat transfer increases as the film thickness decreases, hence it is desirable to keep the films as thin as possible. This article presents a periodically-pulsed flow concept to reduce the average mass flow inside a vertical tube. The technique provides thinner films than can be achieved with continuous flow. A drainage model is adopted to predict the film thickness as a function of flow rate and location along the tube. The concept is confirmed experimentally by measuring the film thickness in real time in a circular tube using a laser-based absorption technique. A new heat-transfer-based film continuity measurement is also integrated into the optical film measurement setup. Water at ambient temperature and pressure is used for this work. Film thickness reductions of up to 60%, and liquid-film volume-flow reductions of up to 80% compared to the minimum continuous-flow film have been obtained. The time-varying film thickness profile can also be controlled to a limited extent, which may be of interest for process industries.

42 ENGINEERING↗