Amorphous arsenic

An attempt is made to unify current knowledge of the optical, vibrational, transport and defect-related properties of amorphous As in terms of its structure and the nature of the bonding. The importance of remnant mesomeric bonding, particularly in determining preparation-dependent variations in the optical gap, is stressed. Topological disorder is taken as a central theme and it is shown how the ring statistics dictate the radial distribution function. The relationship between atomic structure and the electron and phonon densities of states is discussed. A review is given of the transport properties of amorphous As: these point to a pinned Fermi level. Variations in s–p hybridization are considered in relation to the types of point defect likely to occur and it is shown how both paired and unpaired spin states are possible. Finally some experimental results on crystallization kinetics, specific heat and thermal conductivity are discussed.     

STUDIES OF ADHESION OF METAL PARTICLES TO SILICA PARTICLES BASED ON ZETA POTENTIAL MEASUREMENTS

The zeta-potentials of silica, copper, platinum and gold particles have been measured as a function of pH. The isoelectric points were found to be at pH 3.0, 5.8, 3.0 and 3.5, respectively. In the pH range 3.0 to 5.8 copper and silica particles are oppositely charged and accordingly the coating of silica with copper particles could be demonstrated. In the case of gold and platinum the sign of the charge is such that direct adhesion to silica particles cannot be expected and this was also demonstrated in the case of platinum.     

Evaluation of novel ZnO–Ag cathode for CO2 electroreduction in solid oxide electrolyser

CO₂ and steam/CO₂ electroreduction to CO and methane in solid oxide electrolytic cells (SOEC) has gained major attention in the past few years. This work evaluates, for the very first time, the performance of two different ZnO–Ag cathodes: one where ZnO nanopowder was mixed with Ag powder for preparing the cathode ink (ZnO_mix–Ag cathode) and the other one where Ag cathode was infiltrated with a zinc nitrate solution (ZnO_inf –Ag cathode). ZnO_mix–Ag cathode had a better distribution of ZnO particles throughout the cathode, resulting in almost double CO generation while electrolysing both dry CO₂ and H₂/CO₂ (4:1 v/v). A maximum overall CO₂ conversion of 48% (in H₂/CO₂) at 1.7 V and 700 °C clearly indicated that as low as 5 wt% zinc loading is capable of CO₂ electroreduction. It was further revealed that for ZnO_inf –Ag cathode, most of CO generation took place through RWGS reaction, but for ZnO_mix–Ag cathode, it was the synergistic effect of both RWGS reaction and CO₂ electrolysis. Although ZnO_inf –Ag cathode produced trace amount of methane at higher voltages, with ZnO_mix–Ag cathode, there was absolutely no methane. This seems to be due to strong electronic interaction between Zn and Ag that might have suppressed the catalytic activity of the cathode towards methanation.

     

Aerodynamic levitation and laser heating:

Aerodynamic levitation is an effective way to suspend samples which can be heated with CO₂ lasers. The advantages of this containerless technique are the simplicity and compactness of the device, making it possible to integrate it easily in different kinds of experiments. In addition, all types of materials can be used, including metals and oxides. The integration of aerodynamic levitation at synchrotron and neutron sources provides powerful tools to study the structure and dynamics of molten materials. We present here an overview of the existing techniques and of the developments made at the CEMHTI in Orléans, as well as a few examples of experimental results already obtained.     

Annihilation rates of positrons on aromatic molecules

We have measured positron annihilation rates for a selection of hydrocarbon molecules based on six carbon atoms, including substituted benzenes. These measurements illustrate the importance of molecular symmetry in determining the annihilation rate.     

The influence of large-scale seafloor slope and average bottom sound speed on low-grazing-angle monostatic acoustic scattering

Variations in large-scale seafloor slope and average seabed sound speed account for a significant portion of the variations in scattering intensity observed in low-grazing-angle monostatic reverberation. Numerical modeling using a finite-difference solution to the elastic wave equation is used to quantify the effect of these large-scale parameters in interpretations of reverberation data. For hard rough seafloor (e.g., basalt), the results of the modeling suggest that the monostatic backscattering strength increases with increasing large-scale seafloor slope up to a slope of about 15 degrees dipping toward the incident direction. Once the grazing angle of the incident wavefield exceeds the critical grazing angle for the flat reference seafloor the backscattering intensity increases only slowly with increasing grazing angle. Similarly, average subseafloor sound speed has a significant effect. Seafloor with low sound speeds characteristic of soft bottoms (e.g., sediment) generate significantly weaker backscatter signals than seafloor with sound speeds characteristic of hard bottoms (e.g., basalt). The difference is that the shear waves can always be passed into soft bottoms because even for a flat seafloor there is no shear wave critical grazing angle.     

Liquid alumina: detailed atomic coordination determined from neutron diffraction data using empirical potential structure refinement

The neutron scattering structure factor S(N)(Q) for a 40 mg drop of molten alumina (Al2O3) held at 2500 K, using a laser-heated aerodynamic levitation furnace, is measured for the first time. A 1700 atom model of liquid alumina is generated from these data using the technique of empirical potential structural refinement. About 62% of the aluminum sites are 4-fold coordinated, matching the mostly triply coordinated oxygen sites, but some 24% of the aluminum sites are 5-fold coordinated. The octahedral aluminum sites found in crystalline alpha-Al2O3 occur only at the 2% level in liquid alumina.     

Energy and line width in the induced emission of Mössbauer radiation

Summary Some promising schemes proposed for extending the range of lasers to the X-ray and gamma-ray regions are those involving a transition from a long-lived nuclear excited state. While metastable states allow the creation of conditions for population inversion, they also give rise to problems due to the narrowness of the expected line width. We discuss the line width of the radiation due to gamma transitions between excited states and derive a corrected relation for the cross-section in this case. Criteria for choosing an appropriate isotope are deduced from the corrected crosssection, showing that the problems derived from the narrowness of the line width can be avoided in a transition from an isomeric state to a lower short-lived excited state in which case the line width of the radiation is totally determined by the breadth of the lower excited state.

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Riassunto Alcuni schemi promettenti proposti per estendere il campo dei laser alle regioni dei raggi X e dei raggi gamma sono quelli che coinvolgono una transizione da uno stato eccitato nucleare a vita lunga. Mentre gli stati metastabili permettono la creazione delle condizioni per l’inversione della popolazione, essi danno luogo anche ai problemi dovuti alla ristrettezza dell’ampiezza della linea attesa. Si discute l’ampiezza di linea della radiazione dovuta alle transizioni gamma tra gli stati eccitati e si deduce una relazione corretta per la sezione d’urto in questo caso. Si deducono i criteri per scegliere un isotopo appropriato dalla sezione d’urto corretta, mostrando che i problemi che derivano dalla ristrettezza dell’ampiezza di linea si possono evitare in una transizione da uno stato isomerico a uno stato eccitato piú basso a vita breve, nel qual caso l’ampiezza di linea della radiazione si determina totalmente dalla larghezza dello stato eccitato piú basso.

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Резюме Были предложены обнадеживающие схемы для расширения области лазеров в рентгеновскую и гамма-области. Эти схемы вкяючают переход из долгоживущего ядерного возбужденного состояния. Так как метастабильные состояния допускают создание условий для инверсной заселенности, то они также приводят к проблемам, обусловленным узостью ожидаемой ширины линии. Мы обсуждаем ширину линии излучения, обусловленную гамма-переходами между возбужденными состояниями и выводим соотношение для поперечного сечения в этом случае. Выводятся критерии для выбора соответствующего изотопа из полученного поперечного сечения. Показывается, что проблемы, связанные с узостью ширины линии, можно обойти в случае перехода из изомерного состояния в низшее короткоживущее возбужденное состояние, причем в этом случае ширина линии излучения полностью определяется шириной низшего возбужденного состояния.

     

The Structural Characterization of Tin- and Titanium-Dopedα-Fe2O3Prepared by Hydrothermal Synthesis

The structural characteristics of tin- and titanium-dopedα-Fe₂O₃prepared by hydrothermal methods have been investigated by Rietveld structure refinement of the X-ray powder diffraction data. The analysis reveals that the dopant ions adopt two distinct sites: in addition to partially substituting at the octahedral Fe sites, they also occupy the interstitial octahedral sites which are vacant in theα-Fe₂O₃structure. The structural model deduced involves clusters of three substituted cations and is rational in that it represents microstructural regions of the rutile structure within a matrix ofα-Fe₂O₃.