The integration of electro-ionically active inorganic species in polymer matrices allows for the design of either electrode or electrolyte materials depending on the conducting or insulating properties of the polymer used. Conducting polymers can be used as the basis for a variety of hybrid electrode systems, whereas other polymers such as polybenzimidazoles have been used as electrolyte membranes by themselves or in combination with inorganic solid acids. We will discuss the general approach of hybrid design with this in mind and specifically we will describe our recent results on the use of polyoxometalate-containing hybrids in energy storage and conversion devices. In this respect we have worked in our laboratory on electrochemical supercapacitors and fuel cells but emphasis should be made on the broader potential fields of application of this type of materials.
We seek to use electrically conducting polymers, such as those commonly utilized in polymeric LEDs, as hosts for silicon nanoparticles. The proper design of multilayered devices based on these materials will yield efficient light-emitters in which charge carriers localize and recombine within the nanoparticles. Furthermore, these may combine the flexibility and processability of polymeric LEDs with the reliability of inorganic materials. We have synthesized luminescent silicon nanoparticles and have characterized their photoluminescence (PL) using continuous-wave and time-resolved spectroscopy. These particles have been, incorporated into a variety of transparent solid hosts. The photoluminescence obtained from particle-containing poly(methyl methacrylate) (PMMA) matrices is very similar to that of the particles in solution, both in spectral content and PL decay characteristics. However, when incorporated into a variety of conducting polymers, such as poly(N-vinylcarbazole) (PVK), the nanoparticles do not retain their photoluminescence properties. A variety of chemical species have been reported as effective PL quenchers for porous silicon. We believe that these polymers quench the luminescence through similar mechanisms. Protective passivation of the nanoparticle surface is suggested as a strategy for overcoming this quenching.
TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media as functions of temperature and strain rate. Environmental media used in this study were vacuum, air, water vapor, a mixture gas of 5 vol%H2+Ar, O2 gas, N2 gas and Ar gas. All the TiAl-based intermetallic alloys showed low tensile strength or tensile elongation in air, water vapor and a mixture gas of 5 vol%H2+Ar compared to those in vacuum. The reduction of tensile strength or tensile elongation (i.e. environmental embrittlement) was observed not only in low temperature range mostly reaching 600 K but also in high temperature range mostly from 600 K to 1000 K (sometimes temperatures higher than 1000 K). The low- and high-temperature environmental embrittlement depended on the alloy composition (or microstructure). The low-temperature environmental embrittlement diminished at higher strain rates. The high-temperature environmental embrittlement diminished not only at higher strain rates but also at lower strain rates. The possible species causing the high-temperature environmental embrittlement is hydrogen atoms decomposed from water vapor (H2O) or hydrogen gas (H2), similar to those causing the low-temperature environmental embrittlement. Also, it is suggested that the oxidized scale is effective in reducing the high-temperature environmental embrittlement.
Ni Schottky-diodes (SDs) 300 μm in diameter were fabricated by thermal evaporation using contact lithography on cubic GaN and AlxGa1-xN epilayers. Phase-pure cubic GaN and c-Al0.3Ga0.7N/GaN structures were grown by plasma assisted molecular beam epitaxy (MBE) on 200 µm thick free-standing 3C-SiC (100) substrates. The quality of the cubic group III-nitride epilayers was checked by high resolution X-ray diffractometry, atomic force microscopy and photoluminescence at room temperature and at 2 K. Large deviations from the thermionic emission transport were observed in the current voltage (I–V) behavior of these SDs. Detailed analysis of the I–V characteristics at 300 K and at low temperature showed that a thin surface barrier is formed at the Ni semiconductor interface. Thermal annealing in air at 200°C alters the composition of this thin surface barrier and reduces the leakage current by three orders of magnitude. The doping density dependence of breakdown voltages derived from the reverse breakdown voltage characteristics of c-GaN SDs is in good agreement with theoretically calculated values and follows the expected trend. From these experimental data a blocking voltage of higher than 600V is extrapolated for c-GaN films with a doping level of ND = 5×1015 cm−3.
The utility of superelastic Ni-Ti alloys in the medical industry has been rather dramatically demonstrated in recent years. A great number of devices are now in production, and still others are staged to enter production during the next few years. This surge in interest from the medical community stems from an increased acceptance of Ni-Ti because of its biocompatibility, advances in micromachining techniques and trends towards less-invasive surgical techniques. In addition, there are a variety of new developmental concepts that will have a major influence on this and other markets during the next 5 years of commercialization. This review will highlight many of the properties of Ni-Ti by illustration in a variety of recent medical applications, and then discuss some of the newer developmental concepts. Medical applications that will be presented here include guidewires, laparoscopic surgical instruments, implants, stents, retrieval baskets, and bone anchors. Some of the new concepts and capabilities that are reviewed include microvalves made from thin films, high temperature alloys, fatigue resistant composites, and robotic actuators with tactile feedback.
Bidentate ligands are potentially useful to modify metal alkoxides precursors for sol-gel processing. However, anionic bidentate ligands are needed to get a strong coordination, while diamines as neutral bidentate ligand form coordination polymers [Ti 2 (OR) 8 (diamine) 2 ]∞ when reacted with Ti(OR) 4 . The Lewis acidity of metal alkoxides may cause organic side reactions. The first is the formation of ester and water in the reaction with carboxylic acids, and therefore carboxylate-substituted metal oxide clusters of the general composition M n O x (OH/OR) y (OOCR) z are obtained instead of substituted monomeric precursors. The second side reaction discussed is the cleavage of β-diketones by metal alkoxides. Despite this side reaction, 3-acetyl-6-trimethoxy-silyl-hexane-2-one is a useful compound to prepare single-source precursos for silica/metal oxide composites.
The ordered intermetallic NiAl with aluminum levels near 36% undergoes a B2 to martensite transformation. Shape memory alloys based on NiAl+Fe have the potential for transition temperatures of greater than 150°C. While binary alloys appear inherently brittle, alloying with iron and boron results in two phase alloys with L12 and B2 phases and with about 7% room temperature tensile ductility. These alloys show a two-way shape memory effect over a range of transition temperatures with austenite peak temperature, Ap, between 100 to 200°C based on composition. Unfortunately, the B2 phase and its low temperature body centered tetragonal martensitic form are not stable and both can transform to Ni5Al3 with a loss in ductility. These alloys with a constant tensile load show a two way shape recovery of up to 0.6% during temperature cycling between 100 and 200°C. A thorough survey of the shape memory properties of one such alloy with a composition of Ni-25.5 Al-16 Fe-0. 12 B (at.%) as a function of prior cold work, tensile loading and other training steps is presented. Nanoindentation was used to independently measure the mechanical properties of the two phases.
High angular resolution electron channeling x-ray spectroscopy (HARECXS) was examined as a practical tool to locate lattice-ions in spinel crystals. The orientation dependent intensity distribution of emitted x-rays obtained by HARECXS is so sensitive to lattice-ion configuration in the illuminated areas that the occupation probabilities on specific positions in the crystal lattice can be determined accurately through comparison with the theoretical rocking curves. HARECXS measurements have revealed partially disordered cation arrangement in MgO·nAl2O3 with n=1.0 and 2.4. Most Al3+ lattice-ions occupy the octahedral (VI) sites with 6-fold coordination, while Mg2+ lattice-ions reside on both the tetrahedral (IV) and the octahedral (VI) sites. The structural vacancies are enriched in the IV-sites. Further evacuation of cations from the IV-sites to the VI-sites is recognized in a disordering process induced by irradiation with 1 MeV Ne+ ions up to 8.9 dpa at 870 K.
Various GaP nanostructures such as nanowires, nanobelts, nanocables, and nanocapsules were synthesized by sublimation of ball-milled powders. They have a single-crystalline zinc blende structure with [111] growth direction. The morphology and structure were controlled by reactant gas, growth time, flow rate, and growth temperature. The size, morphology and properties of the nanostructures were examined by scanning electron microscopy, transmission electron microscopy, electron energy-loss spectroscopy (EELS), electron diffraction, energy dispersive x-ray spectroscopy, powder x-ray diffraction, and Raman spectroscopy using a 514.5 nm argon ion laser. The photoluminescence was carried out using the 458 nm line of an argon ion laser as the excitation source. The GaP nanowires are straight, cylindrical, and smooth in surface, with mean diameter of 40 nm and length up to 300 mm. The nitrogen-doped nanobelts and nanowires were synthesized by ammonia ambient gas. EELS data reveals that the nitrogen doping occurs mainly in the surface region. The PL spectrum shows the typical isoelectronic bound exciton peaks in the range of 2.11~2.25 eV, suggesting a concentration of (1018 cm−3 nitrogen atoms. We also synthesized two types of GaP nanocables; GaP nanowire sheathed with the amorphous silicon oxide layers and with the graphite layers. The core-shell diameter is under 30 nm and the outerlayer can be removed by acid treatment to produce the 10 nm diameter GaP nanowires. The GaP encapsulated with BCN nanotubes were synthesized under the ammonia flow using the ball-milled carbon-containing boron oxide powders. The number of BCN layers is typically 10~20.
Corrosion behavior is a key issue in the assessment of disposal performance for activated waste such as spent fuel assemblies (i.e., hulls and end-pieces) because corrosion is expected to initiate radionuclide (e.g., C-14) leaching from such waste. Because the anticipated corrosion rate is extremely low, understanding and modeling Zircaloy (Zry) corrosion behavior under geological disposal conditions is important in predicting very long-term corrosion. Corrosion models applicable in the higher temperature ranges of nuclear reactors have been proposed based on considerable testing in the 523-633 K temperature range. In this study, corrosion tests were carried out to confirm the applicability of such existing models to the low temperature range of geological disposal, and to examine the influence of material, environmental, and other factors on corrosion rates under geological disposal conditions. A characterization analysis of the generated oxide film was also performed. To confirm applicability, the corrosion rate of Zry-4 in pure water with a temperature change from 303 K to 433 K was obtained using a hydrogen measuring technique, giving a corrosion rate for 180 days of 8 × 10−3 μm/y at 303 K. To investigate the influence of various factors, corrosion tests were carried out. The corrosion rates for Zry-2 and Zry-4 were almost same, and increased with a temperature increase from 303 K to 353 K. The influence of pH (12.5) compared with pure water was about 1.4 at 180 days at 303 K.
We have demonstrated single dot spectroscopy of InAs/GaAs self-assembled quantum dots embedded in a bias controlled Schottky diode. The photoluminescence spectra exhibit discrete lines depending on bias, which we attribute to the recombination of positively charged, neutral, and negatively charged confined excitons. With excitation directly into the dot, large circular polarization memory is exhibited by the two charged exciton (trion) lines. This indicates long spin lifetimes for both the electron and the heavy hole in the quantum dots.
Detailed microstructural examination by TEM of fine-grained polycrystalline lamellar Ti-44Al-8Nb-1B after fatigue testing (R=0.1) at room temperature has been carried out. The operative slip systems were identified as 1/2 < 110 > ordinary dislocations and 1/6 < 112 > twinning. The results showed no strong relation between operative slip systems and macroscopic Schmid factor and it is believed the local stress conditions control the operation of the slip systems. Translamellar cracking was observed to be associated with fine transverse twins in the gamma lamellae.
In Ni-Ti-Nb ternary system, there are some geometrically close-packed (GCP) phases with long-period stacking sequences of a close-packed plane (CPP). Among them, our focus is on the Ni3(Ti0.90Nb0.10) crystals with Pb3Ba-type rhombohedral structure with nine-fold stacking sequence. Compression tests were conducted using the single crystals and the temperature and orientation dependences of plastic deformation behavior were investigated in comparison with those of D024-Ni3Ti crystals with the four-fold stacking sequence. The K-W locking of screw dislocation was found to occur not only in the compounds such as Ni3Al and Ni3Ti with a relatively small unit cell, but also even in complex compounds with longer-period stacking structures by slip on the common CPP in the GCP structures.
Protocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield.
Poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO-PPO-PEO) was reacted with iron ethoxide in a molar ratio of 1:2. This modified surfactant was used as a template for the synthesis of mesostructured, nanocomposite iron oxide-silica films and powders. Grazing incidence (GI)-SAXS measurements of the films showed that depending on the concentration of the surfactant wormhole-like, 2d-centered rectangular, orthorhombic and lamellar nanocomposite structures were formed. Interestingly, the mesoscopic ordering of the orthorhombic and 2d-centered rectangular pore structure is retained even after calcination at 1000 °C for one hour. The influence of heat treatment on the mesostructure was investigated by GI-SAXS, TEM and N2- sorption measurements.
Off-axis electron holography and micromagnetic calculations that involve solutions to the Landau-Lifshitz-Gilbert equations are used to study magnetization reversal processes in lithographically patterned submicron-sized Co and Co/Au/Ni magnetic elements.
Laser ablation has been used to grow silicon nanowires with an average diameter of 6.7 nm ± 2.7 nm surrounded by an amorphous SiOx sheath of 1–2 nm. This paper reports the imaging, chemical and structural analysis of these wires. Due to the growth temperature and the presence of calcium impurities and trace oxygen, two distinct types of wires are found. They appear to grow by two different processes. One requires a metal catalyst, the other is catalyzed by oxygen.
Heat treatment of bare single-domain LiNbO3 plates near the Curie temperature induces a local polarization reversal, thereby yielding a ferroelectric inversion layer. For a sufficiently long heat treatment the inversion layer thickness becomes equal to one half of the plate thickness. Similar domain inversion is also induced by heat treatment of protonexchanged LiTaO3. These domain inversion phenomena and a model for explaining the mechanism are reviewed. The inversion layer has piezoelectric constants opposite in sign to those of the uninveiled domain. Therefore, a LiNbO3 plate with such an inversion layer can be used as a piezoelectric actuator similar to the bimorph, because bending motion or torsional motion can be piezoelectrically produced, depending on the plate orientation. The measured characteristics of these actuators are presented and demonstrated to exhibit excellent linearity and no hysteresis.
By compositionally grading AlGaN layers over different thicknesses, high mobility electron gases are produced by polarization-induced doping. Temperature dependent Hall and capacitance-voltage measurements were performed on these AlGaN layers, and two degrees of freedom are found for choosing the carrier concentration of these slabs. Carrier mobilities determined from Hall measurements are observed to be much higher than impurity doped structures of similar carrier densities. Alloy and phonon scattering are determined to be the major contributors limiting the mobility of the electron in the graded layers. This form of polarization-induced doping offers an attractive alternative to the traditional doping techniques, and may be used for highly conductive AlGaN layers with high Al composition, both for lateral and vertical transport.
This paper reviews the history and present state of magnetostrictive devices capable of controlled motion over large linear distances. Magnetostriction imparts the most force per unit weight of any technology however, the successful development of practical devices requires a multi-disciplinary effort involving materials science, magnetics, innovative mechanical design, electrical power engineering, and system control engineering. Advances to date have included demonstrations linear motors capable of 115 Newtons of force at 2.54 centimeters/second.
