Tim R. Ohno

Deactivation of PdO–Al2O3 oxidation catalyst in lean-burn natural gas engine exhaust: aged catalyst characterization and studies of poisoning by H2O and SO2

A palladium oxide on alumina oxidation catalyst was employed to remove combustible pollutants from the exhaust of a spark-ignited, lean-burn natural gas engine. Rapid deactivation was seen for the oxidation of methane and ethane. Characterization results are consistent with sulfur as the primary source of catalyst activity loss. In microreactor studies, deactivation of the engine aged catalysts was only apparent if water was present in the feed stream. In dry feed gas, the activity of fresh and engine aged samples was the same. SO2 in dry gas was shown to cause both inhibition and deactivation for methane oxidation. This deactivation is partly reversible at 733 K and completely reversible at 793 K. Water inhibits the rate of methane oxidation and causes some permanent activity loss. Activity studies at 733 and 793 K indicate that activity loss is greater when both water and SO2 are present. Sulfur oxide groups on the surface increase both the amount of water sorbed and the water desorption temperature in TPD experiments. It is proposed that water and SO2 compete for adsorption sites on the alumina surface. Enhanced activity loss in the presence of both poisons is attributed to enhanced water inhibition and spillover of sorbed SO2 and SO3 species from alumina to the PdO surface.

Composition and oxidation resistance of Ti–B–C and Ti–B–C–N coatings deposited by magnetron sputtering

Abstract Nanocomposite Ti–B–C and Ti–B–C–N coatings were deposited from a TiB 2 –TiC target using RF magnetron sputtering. In this paper, the composition and oxidation kinetics of Ti–B–C and Ti–B–C–N coatings are presented. The film composition was characterized using XPS. Compared to the target composition, preferential sputtering of the carbon component was observed. Introducing nitrogen into the sputtering gas resulted in the formation of TiN, with nitrogen of approximately 30 at.%, and shifted the C 1s peak from the typical position for carbide to a higher binding energy position, which is typical of graphite. Both dynamic and isothermal oxidation kinetics were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The oxide compositional depth profile, structure and morphology were characterized by Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results show that: (1) catastrophic oxidation started at 920 K; (2) TiB 2 and TiC in Ti–B–C coatings oxidized in sequence; (3) isothermal oxidation of Ti–B–C coatings in the temperature range from 1173 to 1323 K obeyed a parabolic rate law with an activation energy of 1.64 eV/atom, indicating a diffusion-controlled mechanism; and (4) well-crystallized oxide scales formed after oxidation in air at 1173 K for 2 h are mainly rutile TiO 2 , with XRD-detectable hexagonal B 2 O 3 and hematite Fe 2 O 3 resulting from iron outward diffusion.

Wettability of NiAl, NiAlN, TiBC, and TiB–CN films by glass at high temperatures

Abstract The sticking/adhesion of glass to glass molding dies and forming tools is a critical problem which limits the quality of glass products and the performance and reliability of molding dies and forming tools. Depositing NiAl, NiAlN, TiBC, and TiBCN coatings and characterizing their wettability by glass at high temperature are part of an overall program that is being conducted to develop a non-sticking, oxidation resistant, and wear resistant coating system for glass molding dies and forming tools. The use of contact angle analysis for evaluation of wettability is described in this paper. The contact angles were measured by the sessile drop technique and analyzed by an image analyzer. The film microstructures were studied using cross-sectional TEM technique. Factors affecting the wettability are discussed. NiAl and NiAlN films seemed to offer more potential than TiBC and TiBCN films in terms of non-wettability by glass at high temperatures, they are promising ‘working’ layers for glass molding dies and forming tools.

Cu and CdCl2 influence on defects detected in CdTe solar cells with admittance spectroscopy

Admittance spectroscopy was used with a custom built temperature stage to study deep level defects in four polycrystalline thin-film CdTe solar cells that had postdeposition back contact treatments with and without Cu and CdCl2. One hole trap signature with activation energy Ea≈0.13eV was detected in all four cells and was attributed to a combination of VCd− and related complexes. A second hole trap with Ea≈0.30eV and detected only in Cu-treated cells was attributed to CuCd−. A third hole trap with Ea≈0.47eV was detected only in non-Cu-treated cells. The relationships and relative concentrations between these distinct trap levels are discussed.

Deposition and characterization of NiAl and Ni-Al-N thin films from a NiAl compound target

Abstract NiAl and Ni–Al–N thin films have been deposited from a dense and homogeneous NiAl compound target onto various substrates, including stainless steel, glass, and Si 100 wafer, by using RF magnetron sputtering. The films have been characterized using X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy. Both the NiAl and Ni–Al–N thin films exhibited the near equiatomic NiAl phase. The Ni–Al–N thin films showed an increasing nitrogen content with increasing the amount of N2 in the sputtering atmosphere during deposition. XPS spectra confirmed the possible formation of aluminum nitride in the Ni–Al–N films. The texture, composition, and microstructure of the NiAl films change with the discharge power used. The NiAl thin films deposited using 500 W RF power exhibited the microstructure of a 0.5–0.7-μm amorphous layer adjacent to the substrate and a dense and columnar zone T crystalline microstructure which had a preferred orientation [110]. The Ni–Al–N films showed a homogeneous microstructure of very fine (nano scale) NiAl (110) grains distributed into an amorphous matrix. The results confirm the feasibility of producing high-quality NiAl and Ni–Al–N thin films from a NiAl compound PVD target.

Simulated admittance spectroscopy measurements of high concentration deep level defects in CdTe thin-film solar cells

Computer simulations of thin-film p-CdTe∕n-CdS solar cells using the SCAPS software program are examined to offer explanations for a number of experimentally observed admittance spectroscopy results. We show that for a high concentration of deep level defects, the magnitude of the admittance spectroscopy signal which is an indicator of defect concentration is also affected by the cell thickness. For CdTe cells thinner than 3μm, the signal for defects within 0.25eV of the band edge can be weak and may not be detected at all. We also show that Fermi level pinning resulting from high concentrations of deep level defects can distort the measured activation energy and apparent capture cross section. Finally, we show that decreasing capacitance values with increasing temperature can be caused by the interaction between the CdTe cell back contact Schottky barrier, a defect concentration gradient adjacent to the back contact, and a small shallow acceptor concentration relative to the defect concentration.

Methane partial oxidation by silica‐supported iron phosphate catalysts. Influence of iron phosphate content on selectivity and catalyst structure

Selective oxidation of methane to methanol and formaldehyde at atmospheric pressure was studied over a series of silica‐supported FePO4 catalysts, with iron phosphate content ranging from 2 to 16 wt%. Performance was evaluated over the range T=773–963 K, GHSV=25,000–65,000 h−1, and CH4 : O2=1. The main products were formaldehyde, carbon monoxide and carbon dioxide. Small, but quantifiable amounts of methanol were also observed. Catalytic activity exhibited a clear dependence on the iron phosphate content. The highest selectivity and space time yield (STY) to formaldehyde and methanol were observed for 2 wt% FePO4 on silica (STY of 622 and 25 g/kgcat h, respectively). The selectivity–conversion pattern suggests that methane is oxidized directly to methanol and formaldehyde, and sequentially to carbon oxides. Characterization was performed by X‐ray powder diffraction, X‐ray photoelectron spectroscopy, and Mössbauer spectroscopy. Crystalline FePO4 is observed at all loading levels, however, a significant fraction of the iron (58% at 2 wt% FePO4) is present in an X‐ray amorphous phase. Mössbauer spectra suggest that this phase contains iron in five‐fold coordination, and with a higher electron density relative to bulk FePO4. The amount of this five‐coordinate phase present is roughly 1 wt% Fe, independent of total iron loading. XPS confirms the lower effective oxidation state of iron, and indicates that at low loading the surface is enriched in phosphorus relative to bulk FePO4. It is proposed that iron in five‐fold coordinate sites, isolated by phosphate groups, more selectively activates methane than crystalline FePO4. As loading increases, so does the amount of crystalline FePO4, which is proposed to more rapidly catalyze sequential oxidation of the selective products.

Mobility in SnO 2 :F Thin Polycrystalline Films: Grain Boundary Effect and Scattering in the Grain Bulk

Basic electronic properties relevant to the carrier mobility were studied in tin oxide thin films doped with fluorine, prepared by atmospheric pressure chemical vapor deposition. Electrical resistivity, Hall and Seebeck effects, plasma and collision frequencies were measured (the last two by using multiangle spectral ellipsometry) and analyzed for films with carrier concentrations from 1.8×10 20 to 5.6×10 20 cm −3 . Scanning over the sample area of resistivity (four-point probe method) and Seebeck coefficient (thermoprobe) monitored uniformity of electronic properties in plane. Ellipsometry was used to check uniformity over the film thickness. Films with a thickness above 400 nm demonstrated high spatial uniformity and were used for further studies. Effective mass was determined from combined Hall and plasma frequency measurements and was found to be independent of carrier concentration, which indicates a parabolic band spectrum. Its value was very close to the literature data. In films with carrier concentration ≥3×10 20 cm −3 the Hall mobility was very close to the optical mobility calculated based on collision frequency and effective mass values. This indicates a very small contribution of grain boundaries to the total resistivity of films. Thus the measured mobility is close to the electron mobility in the grain bulk. The scattering parameter value derived from thermopower measurements along with the temperature independent mobility indicated that electron scattering by impurity ions screened by free carriers is the dominating scattering mechanism. Theoretical estimates of mobility are very close to the highest measured mobility values (≥ 30 cm 2 /Vs) if the spatial dispersion of the dielectric constant is taken into account. Comparison of differently processed films showed that compensation of donor dopant with uncontrolled acceptor centers significantly impacted mobility.

Development of a Si/C diffusion barrier layer based on the Mo-Si-C-N system

Abstract A critical component of a high-temperature, oxidation-resistant coating system being developed for molybdenum is a diffusion barrier layer which would minimize interdiffusion between the MoSi 2 + X SiC coating and the molybdenum substrate. Previous results have shown that both silicon and carbon diffuse into the substrate, necessitating the development of a barrier layer for both elements. This paper will examine the possibility of using reactive rf sputtering for synthesizing a ‘nitrided’ layer of MoSi 2 or MoSi 2 + X SiC. The deposition process consisted of rf magnetron sputtering of either a pure MoSi 2 or a MoSi 2 + 1.96 SiC composite target in a nitrogen ambient. A MoSi x N y layer, synthesized by reactive sputtering of a pure MoSi 2 target, has been shown to be an effective silicon diffusion barrier layer up to 1100 °C, by researchers at Los Alamos National Laboratory. In the present study, the feasibility of producing a diffusion barrier layer for both silicon and carbon has been examined in the quaternary system of Mo-Si-C-N. It has been shown that reactive, rf magnetron sputtering of a composite target (MoSi 2 + 1.96 SiC) in a nitrogen ambient results in the formation of an amorphous, diffusion barrier layer for both silicon and carbon diffusion from the overlying coating to the substrate. The barrier layer remains amorphous after a heat treatment at 1000 °C for 30 min. A ball-cratering technique was used, in conjunction with Auger line scans, to assess the efficacy of the barrier layer, following a diffusion anneal treatment. Details of Auger electron spectroscopy, scanning/transmission electron microscopy and X-ray diffraction analyses are presented.

Optically activated sub-millimeter dielectric relaxation in amorphous thin film silicon at room temperature

Knowing the frequency-dependent photo-induced complex conductivity of thin films is useful in the design of photovoltaics and other semi-conductor devices. For example, annealing in the far-infrared could in principle be tailored to the specific dielectric properties of a particular sample. The frequency dependence of the conductivity (whether dark or photo-induced) also gives insight into the effective dimensionality of thin films (via the phonon density of states) as well as the presence (or absence) of free carriers, dopants, defects, etc. Ultimately, our goal is to make low-noise, phase-sensitive room temperature measurements of the frequency-dependent conductivity of thin films from microwave frequencies into the far-infrared; covering, the frequency range from ionic and dipole relaxation to atomic and electronic processes. To this end, we have developed a high-Q (quality factor) open cavity resonator capable of resolving the complex conductivity of sub-micron films in the range of 100–350 GHz (0.1–0...