Benjamin S. Chao

Effect of hydrogen dilution on the structure of amorphous silicon alloys

We investigate why high levels of hydrogen dilution of the process gas lead to enhanced light soaking stability of amorphous silicon (a-Si) alloy solar cells by studying the microstructural properties of the material using high-resolution transmission electron microscopy (TEM) and Raman spectroscopy. The TEM results show that a-Si alloy (with or without hydrogen dilution) is a heterogeneous mixture of amorphous network and linear-like objects that show evidence of order along their length. The volume fraction of these ordered regions increases with increasing hydrogen dilution.

In-plane epitaxial alignment of YBa2Cu3O7-x films grown on silver crystals and buffer layers

Superconducting YBa2Cu3O7−x (YBCO) films were grown by laser ablation on Ag(001), Ag(110), and Ag(111) single‐crystal surfaces. X‐ray diffraction measurements showed that, in all cases, the films were aligned with specific in‐plane epitaxial orientation with respect to the crystallographic axes of the substrate. The observed orientations were consistent with predictions of near‐coincident site lattice models of the YBCO‐Ag interface. This technique for achieving three‐dimensional film alignment was extended to include a Ag epitaxial buffer layer on mica in place of a bulk Ag crystal. In‐plane epitaxial alignment on metal substrates and buffer layers has important consequences in practical applications for inhibiting weak‐link behavior caused by high‐angle grain boundaries.

Characterization of as‐prepared and annealed hydrogenated carbon films

Hydrogenated amorphous carbon films were obtained from the decomposition of methane using a rf‐couple glow‐discharged system at power densities ranging from 0.4 to 4.8 W/cm2 . The structure of as‐prepared and annealed films were characterized by Raman spectroscopy, infrared absorption, photoluminescence, energy‐loss spectroscopy, and x‐ray diffraction. The results indicate that the incorporated hydrogen concentration is an important parameter in determining the structure and properties of the films. Carbon films deposited at a lowest rf power density contain a large amount of hydrogen with most of the C–H bonds in CH3 configurations, whereas films produced at higher rf powers reveal dominant CH2 bonding structures. According to Raman scattering measurements the sp2 domains in as‐prepared samples are disordered due perhaps to bond‐angle distortions. Upon annealing, hydrogen leaves the film at a temperature that depends on the initial hydrogen concentration. Once most of the hydrogen has been driven out, cr...

Intermediate order in tetrahedrally coordinated silicon: evidence for chainlike objects

Abstract In this report, we describe the nature of intermediate order in silicon as determined by recent measurements on thin films using transmission electron microscopy (TEM) and Raman scattering. The TEM images show in addition to the expected continuous random network (CRN), the presence of highly ordered quasi-one-dimensional “chain-like objects” (CLOs) that are 1– 2 nm wide and tens of nm long that meander and show some evidence of cross-linking with each other. The presence of these objects correlate to a Raman feature centered at 490 cm −1 whose width is 35– 40 cm −1 , and is used to quantify the heterogeneity in terms of the CLO and CRN (=475 cm −1 scattering) concentrations. The 490 and 35 cm −1 values are consistent with bond angle deviations approaching 0°, and thus reinforces an association with the CLOs. We find that in reference quality a-Si:H (made using pure SiH4), the CLO concentration is about 5 vol % , while in state-of-the-art material using high H2 levels of dilution during processing, it increases to about 15%. Increased stability of such material to light-soaking is thus not mediated by a direct volumetric replacement of poor with high-quality components. Rather, an important characteristic of intermediate order in silicon is the low-dimensional aspect of its order, which allows it to influence more total volume than which it is itself composed. Consistent with these and other recent findings, we propose a tensegrity model of amorphous silicon.

Recent Advances in Solid Hydrogen Storage Systems

Hydrogen energy offers great promise as an energy alternative. Hydrogen technologies can reduce and eliminate the release of carbon dioxide from fossil-fuel combustion, the main cause of global warming. One of the main challenges is hydrogen storage. Storing hydrogen in the solid-state hydride form holds a volumetric advantage over compressed and liquid hydrogen states. Solid hydrogen storage systems also have features of low-pressure operation, compactness, safety, tailorable delivery pressure, excellent absorption /desorption kinetics, modular design for easy scalability, and long cycle life. In this paper, solid hydrogen storage systems (such as portable power canisters, lightweight fiber wrapped vessels, and aluminum tubular vessels, developed by Texaco Ovonic Hydrogen Systems LLC) will be discussed. A system of four canisters each storing approximately 80 grams of reversible hydrogen is shown to run a 1 kW PEM fuel cell for more than 247 minutes at full power. Canisters show no plastic deformation after more than 500 charge/discharge cycles. The measured strain on canister surfaces indicates that DOT stress limits are not exceeded. The canisters are in the early commercialization stage for uninterrupted power supply (UPS) and auxiliary power unit (APU) applications. A lightweight fiber-wrapped vessel engineered with metal hydride and internal heat exchanger is being developed for onboard applications. At the system level, the vessel has a volumetric energy density of 50 grams of hydrogen per liter and a gravimetric density of 1.6 wt.%. The vessel is capable of storing 3 kg of hydrogen with a fast refueling capability. Ninety percent of the storable hydrogen can be refueled in 10 minutes at 1500 psig. The vessel can easily release the hydrogen at a rate of 350 slpm at 70°C. Aluminum tubular vessels are being designed and tested for bulk storage and infrastructure applications including stationary power, hydrogen shipment and hydrogen service stations. The tubular vessel dimensions may be designed for specific applications. For example, a tubular vessel 6 inches in diameter and 62 inches in length can store up to 1 kg of hydrogen.

High performance Zr-based metal hydride alloys for nickel metal hydride batteries

Based upon Ovonics multi-element, atomic engineering approach, two families of alloys are being used in commercial Nickel Metal Hydride (NiMH) rechargeable batteries, i.e., the mischmetal (Mm) based AB{sub 5} and Zr based AB{sub 2} alloys. While Mm based alloys are faster to activate, they are limited by a discharge capacity of only 320--340 mAh/g. The Zr based alloy, although slightly slower to activate, provides a much higher discharge capacity. In this paper, the authors first discuss the use of Ovonics multi-element approach to generate a spectrum of disordered local environments. They then present experimental data to illustrate that through these atomically engineered local environments, they are able to control the hydrogen site occupancy, discharge capacity, kinetics, and surface states. The Zr based alloy with a specific discharge capacity of 465 mAh/g and excellent rate capability has been demonstrated.

The effect of the viscosity in the precursor solution on the structure of sol-gel derived silica films containing cobalt

Silica coatings containing cobalt have been prepared from sol-gel precursor solutions having a viscosity in the range of 3 to 40 cP. The different values of viscosity were achieved by aging the solutions with various amount of solvent (ethanol). The structure of these samples was analyzed using x-ray diffraction, Raman scattering, infrared absorption and Auger depth profiles. These measurements show that, upon heat treatments in air at 500°C, the cobalt oxidizes to form small oxide particles. However, when the coatings are prepared from precursor solutions with viscosity lower than about 9 cP, the cobalt migrates to the outer surface of the coating were it is oxidized to form the particles. On the other hand, if the coatings are prepared from solutions with a higher viscosity, the oxide particles are formed in the bulk of the coating. The infrared and Raman spectra indicate that the structure of this later set samples is more ordered with fewer amount of voids.

Superconductivity in Fluorinated Copper Oxide Ceramics

It is difficult to do justice to the scientific and technological impact of the recent discoveries in high Tc superconducting ceramic oxides. It is now clear that the oxides that were the basis of the lanthanum copper oxide ceramics which moved the superconducting temperature from 23 to 30 to 40K (1) have been around for some time.(2,3)

Thermal stability of encapsulated tungsten/carbon multilayer thin films

Tungsten/carbon (W/C) multilayer thin films prepared by sputtering on unheated Si (100) substrates were encapsulated with two types of layers having low x‐ray absorption. The encapsulating layers are SiNx and carbon prepared by plasma enhanced chemical vapor deposition and sputtering techniques, respectively. Isochronal annealings for 1 h in the temperature range from 300 to 600 °C and isothermal annealings at 300 and 400 °C were carried out under ambient conditions (in air) on coated and uncoated multilayers. Previous studies have shown that unprotected W/C multilayers annealed in air exhibit oxidation at relatively low temperatures (∼300 °C). In the present study, we have used Raman scattering, Auger depth profiling, and scanning electron microscopy to investigate the effects of thermal treatments on the encapsulated W/C multilayers. The results indicate that oxidation of both W and C layers takes place during annealing at temperatures as high as 600 °C on the SiNx coated multilayer.

Formation of a Co3O4 Top Layer in SiO2 Cobalt Containing Coatings Sol-gel Obtained

SiO 2 coatings containing cobalt were prepared on glass substrates using the sol-gel method. It has been found that in coatings with a Si to Co ratio of 1.3, a layer of Co 3 O 4 is formed at the free surface of the coating upon thermal annealings in air. The properties of the coatings were studied using optical, x-ray and Auger depth profile measurements. The thermal annealings in the temperature range of 300 to 500 o C, in steps of 50 o C, were performed for 10 min. Some samples were subjected to an isothermal annealing at 400 °C for different times from 10 to 210 min. From the x-ray diffraction patterns the cubic spinel structure of Co 3 O 4 was detected after the thermal treatments. The optical reflection and transmission spectra for each annealing temperature and annealing time, are described with an air-Co 3 O 4 -SiO 2 :Co 2+ -substrate system. From this, the cobalt oxide thickness was obtained as a function of the annealing temperature and annealing time. From the Arrhenius plots, in the temperature range studied, it was found that the activation energy for the growth of the cobalt oxide layer is 0.41 eV. The layer thickness follows a parabolic behavior with time, which suggests a diffusion-controlled process. The Auger depth profiles obtained from a sample annealed at 500 °C confirm the optical model used.