The detection of hydrogen in molten aluminium

The presence of hydrogen in aluminium poses problems to the foundry and casting industries, because high residual hydrogen contents in molten aluminium cause significant porosity in the solid aluminium after casting. This usually renders the product useless as it may fail mechanically. Therefore, fast, accurate and reliable techniques are required for monitoring the dissolved hydrogen content in molten aluminium, but this particularly harsh environment places considerable restrictions on the equipment that can be used. Several methods are available for the determination of hydrogen in aluminium melts, but they either suffer problems of accuracy, reliability and longevity or are not applicable to industrial environments. It is considered that the most appropriate device for the hydrogen analysis in aluminium melts should be an electrochemical sensor, which employs a proton conducting solid electrolyte in conjunction with a measuring electrode and a suitable reference electrode. The electromotive force of such a cell allows direct calculation of the hydrogen concentration in the melt. However, all the electrochemical sensors reported in the literature thus far exhibit distinct drawbacks. This article discusses the various techniques for the determination of hydrogen in molten aluminium with particular emphasis on the benefits and shortcomings of the existing electrochemical sensors. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Anisotropic thermoelectric power of TTF-TCNQ

We have measured the thermoelectric power along the a-axis as well as the highly conducting b-axis in TTF-TCNQ crystals. Measurements were taken both parallel and perpendicular to the long axis on two sets of crystals. One set grew along the a-axis and the other along the b-axis. The thermopowers are of opposite sign except near 60°K where both cross zero at slightly different temperatures. The a-axis thermopower is consistent with non-metallic diffusive transport in the a direction.     

Chirped-pulse oscillators: theory and experiment

Theory of chirped-pulse oscillators operating in the positive dispersion regime is presented. It is found that the chirped pulses can be described analytically as solitary pulse solutions of the nonlinear cubic-quintic complex Ginzburg–Landau equation. Due to the closed form of the solution, basic characteristics of the regime under consideration are easily traceable. Numerical simulations validate the analytical technique and the chirped-pulse stability. Experiments with 10 MHz Ti:Sa oscillator providing up to 150 nJ chirped pulses, which are compressible down to 30 fs, are in agreement with the theory. PACS 42.65.Re; 42.65.Tg; 42.55.Rz     

Module-level design and characterization of thermoelectric power generator

Thermoelectric power generation provides us the unique capability to explore the deep space and holds promise for harvesting the waste heat and providing a battery-free power supply for IoTs. The past years have witnessed massive progress in thermoelectric materials, while the module-level development is still lagged behind. We would like to shine some light on the module-level design and characterization of thermoelectric power generators (TEGs). In the module-level design, we review material selection, thermal management, and the determination of structural parameters. We also look into the module-level characterization, with particular attention on the heat flux measurement. Finally, the challenge in the optimal design and reliable characterization of thermoelectric power generators is discussed, together with a calling to establish a standard test procedure.     

Steps on surfaces: experiment and theory

The properties of steps in thermal equilibrium are described in the context of prediction of the stability and evolution of nanostructures on surfaces. Experimental techniques for measuring the appropriate step parameters are described, and simple lattice models for interpreting the observations are reviewed. The concept of the step chemical potential and its application to the prediction of step motion (and therefore surface mass transport) is presented in depth. Examples of the application of this step-continuum approach to experimental observations of evolution of surface morphology are presented for morphological phase transitions, the decay of metastable structures, and the spontaneous evolution of metastable structure due to kinetic instabilities.     

K-shell photodetachment of He- : experiment and theory

High-resolution K-shell photodetachment measurements of He- giving rise to He+ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of He-. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.     

Oscillating neutrinos: theory vs experiment

Possible hints on neutrino masses are reviewed. They come from the deficits in the solar as well as atmospheric neutrinos and from need of a significant amount of hot component in the dark matter of the universe. The role of three generation mixing in simultaneously solving the solar and atmospheric neutrino problem is discussed. All the three hints can be reconciled if three neutrinos are almost degenerate. Models for neutrino masses and mixing implied by the above hints are briefly discussed.     

In situ observation and analysis of ultrasonic capillary effect in molten aluminium

An in situ synchrotron radiographic study of a molten Al–10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode.This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density.     

Preparation and thermoelectric power of SnBi4Se7

Polycrystalline samples of Bi₂Se₃ and a stoichiometric ternary compound in the quasi-binary system SnSe–Bi₂Se₃ have been prepared and characterized by X-ray powder diffraction analysis. At room temperature the carrier concentration values are n?=?1.1?×?10¹⁹?cm^?3 for Bi₂Se₃ and n?=?0.53?×?10¹⁹?cm^?3 for SnBi₄Se₇. The thermoelectric power has been measured over the temperature range 90–420?K. The thermoelectric power of Bi₂Se₃ is higher than that for SnBi₄Se₇, which shows that the Sn impurity has an acceptor character. Therefore, doping Bi₂Se₃ with tin atoms does not improve thermoelectric properties of this material, due to decrease the value of the power factor σS ². Transport properties of the studied polycrystalline samples are characterized by a mixed transport mechanism of free carriers. It is necessary to add more than one Sn atom to the Bi₂Se₃ compound in order to suppress the electron concentration by one electron. Such behaviour of the dopant is explained by the formation of various structural defects. Besides the dominant substitutional defect, Sn_Bi, tin atoms also form uncharged defects, corresponding to seven-layer lamellae of the composition Se–Bi–Se–Sn–Se–Bi–Se which corresponds to the structure of the SnBi₂Se₄.     

Locally periodic Timoshenko rod: experiment and theory

The flexural vibrations of a locally periodic rod, which consists of N unit cells, are discussed both from the experimental and theoretical points of view. Timoshenko's beam theory and the transfer matrix method are used to calculate the normal-mode frequencies and amplitudes. The theoretical values are then compared with the experimental ones, which are obtained using an electromagnetic acoustic transducer (EMAT). Good agreement between the numerical results and the experimental measurements is obtained. It is shown that as N grows, a band spectrum emerges.     

Phosphorus carbide thin films: experiment and theory

The recent finding that radio frequency plasma activation of CH₄/PH₃ gas mixtures [7] could lead to films with P:C ratios 3 (which also contain 10% hydrogen, distributed evenly throughout the bulk) has served to trigger further research into new amorphous phosphorus carbide materials. New theoretical and experimental results relating to these materials are presented here. The electronic structure and stability of different crystalline phosphorus carbide P_xC_y phases have been studied using first-principles density-functional theory methods. Calculations have been carried out for both P₄C₃ and PC and a range of the more likely periodic structures examined. The lowest energy pseudocubic P₄C₃ and GaSe PC phases have been further investigated as templates to discover the stability and the electronic and structural properties of these phosphorus carbide materials. Recent experimental studies have involved use of pulsed laser ablation (PLA) methods to produce hydrogen-free phosphorus carbide thin films. Mechanically hard, electrically conducting diamond-like carbon films containing 0-26 at.% P have been deposited on both Si and quartz substrates by 193 nm PLA of graphite/phosphorus targets (containing varying percentages of phosphorus), at a range of substrate temperatures (T_sub=25–400 °C), in vacuum, and analysed via laser Raman and X-ray photoelectron spectroscopy. PACS 68.55.Jk; 71.15.Nc; 81.15.Fg     

Multiple beam atom interferometer: theory and experiment

Received: 30 May 1997/ 27 August 1997     

Gate field emitter failures: experiment and theory

Intrinsic failures of gated field emitters have been studied. The gate-emitter voltage drops from typical values of 140 V to 10-70 V in less than 10 ns at the onset of a failure. Measurements with an electrostatic probe indicate that plumes of ions and electrons are ejected into vacuum. The measured ion current to the probe is typically 10% of the electron current. The voltage during the event and the ion-to-electron current ratio measured at the probe are characteristic of a cathodic vacuum arc plasma. For series resistors less than 1 kΩ, the arc is continuous, while the series resistors greater than 10 kΩ, the arc is intermittent. Initiation of the failure based on ion-space charge enhancement of the emitter electric field is modeled with the plasma simulation code PDS1. These structures provide a controlled geometry for studying arcs of micron size dimension     

Modulation of thermoelectric power of individual carbon nanotubes

Thermoelectric power (TEP) of individual single walled carbon nanotubes (SWNTs) has been measured at mesoscopic scales using a microfabricated heater and thermometers. Gate electric field dependent TEP modulation has been observed. The measured TEP of SWNTs is well correlated to the electrical conductance across the SWNT according to the Mott formula. Strong modulations of TEP were observed in the single-electron conduction limit. In addition, semiconducting SWNTs exhibit large values of TEP due to the Schottky barriers at SWNT-metal junctions.     

Activated barrier crossing: Comparison of experiment and theory

Photochemical isomerization in stilbene and diphenyl butadiene has been studied as a model for activated barrier crossing. Experiments have been carried out from isolated molecule conditions up to 3000 atm pressure in solution-phase samples. The qualitative features predicted by Kramers theory are observed. The system undergoes a transition from energy-controlled to diffusion-controlled behavior in the high-pressure gas phase. The influences of multidimensionality, intramolecular vibrational relaxation, and frequency dependent friction are discussed.Camille and Henry Dreyfus Teacher Scholar.     

Spectral imaging of time-resolved anisotropy: theory and experiment

Time-resolved fluorescence anisotropy on the nanosecond time scale is useful for the study of the rapid rotation of macromolecules. A system combining the capabilities of fluorescence spectral imaging with time-resolved fluorescence anisotropy and enabling the wide-field measurement of the spectroscopic parameters of fluorophores is discussed. The phasor approach is used to quantitatively analyze the time-resolved fluorescence anisotropy by transforming the polarized parallel and perpendicular components to the phasor space in the frequency domain, respectively, and a unique way to calculate the fluorescence rotational correlation time is put forward. Experimental results prove that the phasor approach is a proper model for the time-resolved fluorescence anisotropy.     

Zipper mechanism of nanotube fusion: theory and experiment

We propose a new microscopic mechanism to explain the unusually fast fusion process of carbon nanotubes. We identify the detailed pathway for two adjacent (5,5) nanotubes to gradually merge into a (10,10) tube, and characterize the transition states. The propagation of the fused region is energetically favorable and proceeds in a morphology reminiscent of a Y junction via a zipper mechanism, involving only Stone-Wales bond rearrangements with low activation barriers. The zipper mechanism of fusion is supported by a time series of high-resolution transmission electron microscopy observations.