Water-ion permselectivity of narrow-diameter carbon nanotubes

Carbon nanotube (CNT) pores, which mimic the structure of the aquaporin channels, support extremely high water transport rates that make them strong candidates for building artificial water channels and high-performance membranes. Here, we measure water and ion permeation through 0.8-nm-diameter CNT porins (CNTPs)—short CNT segments embedded in lipid membranes—under optimized experimental conditions. Measured activation energy of water transport through the CNTPs agrees with the barrier values typical for single-file water transport. Well-tempered metadynamics simulations of water transport in CNTPs also report similar activation energy values and provide molecular-scale details of the mechanism for water entry into these channels. CNTPs strongly reject chloride ions and show water-salt permselectivity values comparable to those of commercial desalination membranes.

Bimetallic Pt-Co Catalysts for the Liquid-Phase WGS

Bimetallic Pt-Co catalysts derived from cobalt aluminate spinel were investigated in the liquid-phase water–gas shift (WGS) reaction and CO hydrogenation. Liquid-phase WGS is a key reaction in the aqueous-phase reforming (APR) of polyols; thus, WGS activity is essential to formulate good APR catalysts. In this work, catalysts with different Pt/Co molar ratios were synthesized together with a reference Pt/alumina. All the synthesized catalysts were characterized by various techniques in order to gain knowledge on their structural and surface characteristics. WGS activity was tested with a feedstream of CO/H2O = 1/15 (space-time of 76.8 kgcat·s/molCO), isothermal operation at 260 °C and 50 bar, for 10 TOS. Bimetallic Pt-Co catalysts showed improved activity in liquid-phase WGS in comparison to bare Co or Pt catalysts, which was ascribed to the synergistic effect. Despite being subjected to an increased hydrogen concentration in the feedstream (H2/CO between 0 and 12/3), these catalysts maintained a preferential selectivity towards WGS activity. In addition, the effect of temperature (220–260 °C) and pressure (25–50 bar) was investigated over a catalyst with 0.3Pt/CoAl. CO conversion and CO2 yield were more sensitive to temperature, while a higher pressure favored methane production. The measured activation energy in the 220–260 °C temperature range was 51.5 kJ/mol.

The origin of the exceptionally low activation energy of oxygen vacancy in tantalum pentoxide based resistive memory

It is well known that collective migrations of oxygen vacancies in oxide is the key principle of resistance change in oxide-based resistive memory (OxRAM). The practical usefulness of OxRAM mainly arises from the fact that these oxygen vacancy migrations take place at relatively low operating voltages. The activation energy of oxygen vacancy migration, which can be inferred from the operational voltage of an OxRAM, is much smaller compared to the experimentally measured activation energy of oxygen, and the underlying mechanism of the discrepancy has not been highlighted yet. We ask this fundamental question in this paper for tantalum oxide which is one of the most commonly employed oxides in OxRAMs and try the theoretical answer based on the first-principles calculations. From the results, it is proven that the exceptionally large mobility of oxygen vacancy expected by the switching model can be well explained by the exceptionally low activation barrier of positively charged oxygen vacancy within the two-dimensional substructure.

Energetics of base flipping at a DNA mismatch site confined at the latch constriction of α-hemolysin

Unique, two-state modulating current signatures are observed when a cytosine–cytosine mismatch pair is confined at the 2.4 nm latch constriction of the α-hemolysin (αHL) nanopore. We have previously speculated that the modulation is due to base flipping at the mismatch site. Base flipping is a biologically significant mechanism in which a single base is rotated out of the DNA helical stack by 180°. It is the mechanism by which enzymes are able to access bases for repair operations without disturbing the global structure of the helix. Here, temperature dependent ion channel recordings of individual double-stranded DNA duplexes inside αHL are used to derive thermodynamic (ΔH, ΔS) and kinetic (EA) parameters for base flipping of a cytosine at an unstable cytosine–cytosine mismatch site. The measured activation energy for flipping a cytosine located at the latch of αHL out of the helix (18 ± 1 kcal mol−1) is comparable to that previously reported for base flipping at mismatch sites from NMR measurements and potential mean force calculations. We propose that the αHL nanopore is a useful tool for measuring conformational changes in dsDNA at the single molecule level.

Softening Behavior of Ti6Al4V Alloy during Hot Deformation

The effect of strain rate and strain on the hot compression behaviour of Ti6Al4V has been analysed to understand the microstructural evolution and restoration behaviour. Cylindrical samples with partially equiaxed grains were deformed in the α+β region at different thermo-mechanical conditions. EBSD has been used to study the microstructural behaviour and the restoration mechanisms. The microstructural evolution showed a complex restoration behaviour, where both fragmentation and nucleation of new grains have been observed. The volume fraction of the equiaxed grains increased with an increase in the strain, but oppositely decreased with the strain rate. At the same time the average grain size of the equiaxed grains decreased with an increase in both the strain and strain rate. The measured activation energy for deformation revealed a good agreement with reported values in the literature.

Temperature Effects on the Crystallization and Coarsening of Nano-CeO2 Powders

The effect of temperature on nano-CeO2 particle coarsening is investigated. The nanoceria powders were synthesized using the microemulsion method and then exposed to temperatures in the range of 373–1273 K. It was found that the nanoparticles exhibited a strong tendency to form agglomerates and through the application of ultrasound these agglomerates could be broken into smaller sizes. In addition average nanoparticle sizes were determined by powder X-ray diffraction (XRD). The outcome of this work indicates that the initial nano-CeO2 powders are amorphous in nature. Annealing promotes CeO2 crystallization and a slight shift in the (111) XRD intensity peaks corresponding to CeO2. Moreover, at temperatures below 773 K, grain growth in nano-CeO2 particles is rather slow. Apparently, mass transport through diffusional processes is not likely to occur as indicated by an estimated activation energy of 20 kJ/mol. At temperatures above 873 K, the measured activation energy shifted to 105 kJ/mol suggesting a possible transition to Ostwald-Ripening type mass transport mechanisms.

Investigations of the νT=1 Exciton Condensate

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 (νT=1) have revealed the possibility of a superfluidic exciton condensate. We report on our experimental work involving the νT=1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We reproduce the previously reported zero bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρxx and ρxy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d/ℓB < 1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy. We establish a phase diagram of the excitonic condensate showing the enhancement of this state at slight imbalances.

Temperature-dependent Degradation Modes in CdS/CdTe Devices

A set of 24 identically made CdS/CdTe devices were subjected to accelerated lifetime testing (ALT) under open-circuit bias, 1 sun illumination, and temperatures of 60, 80, 100, and 120 °C. A total of 6 identical devices were tested for statistical purposes at each temperature. Current density-voltage (JV) measurements were made on stressed cells for up to 2000+ hours. Device performance parameters were calculated as a function of temperature and stress time using discrete element circuit models. Forward current behavior was represented by two parallel diodes to simulate recombination currents in the quasi-neutral (QNR) and space-charge (SCR) regions. Backcontact behavior was studied using a parallel combination blocking diode and shunt conductance. A systematic pattern of degradation was apparent with increased stress temperature. At 60 °C, degradation associated with the CdTe/backcontact dominates. At temperatures above 80 °C, greater losses in fill factor (FF) and open-circuit voltage (Voc) were observed. Recombination current modeling of JV data attributes this to increased space-charge recombination. Calculated diffusion lengths based upon an Arrhenius-derived activation energy of 0.63 eV in this temperature-range suggests Cu diffusion into the SCR is mechanistically responsible for the observed increased recombination, and decreased Voc and FF. At lower temperatures (60 to 80 ºC), degradation was considerably slower with a measured activation energy of 2.9 eV.

ONE DIMENSIONAL NATURE OF IONIC CONDUCTIVITY IN SELF-FLUX GROWN RbTiOPO4 SINGLE CRYSTALS

The ionic conductivity of self-flux grown RbTiOPO 4 single crystal along the crystallographic a, b and c (polar) axes in the frequency range 100 Hz–10 MHz and in the temperature range 300–1140 K has been studied. The measured activation energy indicates the existence of super ionic conduction behavior in RTP crystals and also reveals that the DC electrical conduction and dielectric polarization are governed by the same mechanism. Complex impedance measurement shows the existence of non-Debye type of relaxation in the crystals.

GaN Decomposition in Ammonia

GaN decomposition is studied as a function of pressure and temperature in mixed NH3 and H2 flows more characteristic of the MOVPE growth environment. As NH3 is substituted for the 6 SLM H2 flow, the GaN decomposition rate at 1000 °C is reduced from 1×1016 cm−2 s−1 (i.e. 9 monolayers/s) in pure H2 to a minimum of 1×1014 cm−2 s−1 at an NH3 density of 1×1019 cm−3. Further increases of the NH3 density above 1×1019 cm−3 result in an increase in the GaN decomposition rate. The measured activation energy, EA, for GaN decomposition in mixed H2 and NH3 flows is less than the EA measured in vacuum and in N2 environments. As the growth pressure is increased under the same H2 and NH3 flow conditions, the decomposition rate increases and the growth rate decreases with the addition of trimethylgallium to the flow. The decomposition in mixed NH3 and H2 and in pure H2 flows behave similarly, suggesting that surface H plays a similar role in the decomposition and growth of GaN in NH3.