A synchrotron XPS study of the vanadia–titania system as a model for monolayer oxide catalysts

The deposition of vanadium metal onto stoichiometric TiO₂(110) has been studied with photoelectron spectroscopy from low to high coverages of vanadium. A synchrotron source was employed in XPS experiments for the study of submonolayer coverages of vanadium in order to determine the oxidation state of the vanadia species formed at submonolayer coverages. The exposure of the titania surface to vanadium metal results in charge-transfer from vanadium to titania at the interface. At low doses of the metal vigorous interaction between the metal and titania surface yields reduction of the Ti⁴⁺ species to Ti³⁺ at the interface, as evidenced by both changes in the lineshape of the Ti 2p XPS spectra and Ti LIII-edge spectra. Concurrent with this surface reduction vanadium metal is oxidized. At higher vanadium doses the vanadium 2p binding energy indicates the formation of metallic vanadium. When metallic vanadium deposition is followed by exposure of the surface to oxygen, only one vanadium species remains on the surface, the binding energy of which corresponds to that of the oxide present initially at low doses of vanadium metal. By comparison of the V 2p binding energies to those of bulk oxides, it appears that the oxidation state of the vanadium in the oxide species is +3, suggesting the formation of V₂O₃ on the surface. Vanadium LIII-edge data also suggest that V₂O₅ is not formed by the oxidation of predosed vanadium metal.     

Effect of grain size on the strength of deformed 36NKhTYu alloy aged by a discontinuous mechanism of γ′-phase precipitation

A specially developed method of heat treatment has made it possible to study the effect of grain size on the yield stress and the deformation stress of 36NKhTYu alloy which has undergone practically complete discontinuous decomposition. The well-known Hall-Petch equation is shown to be completely applicable. The parameters ₀ and K of this equation are determined and compared with those for material aged by the mechanism of continuous precipitation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 55–58, September, 1977.     

Numerical study of friction-induced instability and acoustic radiation – Effect of ramp loading on the squeal propensity for a simplified brake model

This paper presents a numerical study of the influence of loading conditions on the vibrational and acoustic responses of a disc brake system subjected to squeal. A simplified model composed of a circular disc and a pad is proposed. Nonlinear effects of contact and friction over the frictional interface are modelled with a cubic law and a classical Coulomb?s law with a constant friction coefficient. The stability analysis of this system shows the presence of two instabilities with one and two unstable modes that lead to friction-induced nonlinear vibrations and squeal noise. Nonlinear time analysis by temporal integration is conducted for two cases of loadings and initial conditions: a static load near the associated sliding equilibrium and a slow and a fast ramp loading. The analysis of the time responses shows that a sufficiently fast ramp loading can destabilize a stable configuration and generate nonlinear vibrations. Moreover, the fast ramp loading applied for the two unstable cases generates higher amplitudes of velocity than for the static load cases. The frequency analysis shows that the fast ramp loading generates a more complex spectrum than for the static load with the appearance of new resonance peaks. The acoustic responses for these cases are estimated by applying the multi-frequency acoustic calculation method based on the Fourier series decomposition of the velocity and the Boundary Element Method. Squeal noise emissions for the fast ramp loading present lower or higher levels than for the static load due to the different amplitudes of velocities. Moreover, the directivity is more complex for the fast ramp loading due to the appearance of new harmonic components in the velocity spectrum. Finally, the sound pressure convergence study shows that only the first harmonic components are sufficient to well describe the acoustic response.     

Effect of water–methanol content on the structure of Nafion in the sandwich model and solvent dynamics in nano-channels; a molecular dynamics study

Continuing an ongoing study, molecular dynamics (MD) simulations were performed to investigate the effects of methanol concentration on Nafion morphology, such as the size of solvent cluster, solvent location, and polymer structure via the sandwich model. Our survey shows that high methanol concentrations resulted in increment of solvent cluster size in Nafion membrane. The sulfonic acid clusters also befall much in order as subsequent layers of such ionic clusters are formed. The number of neighbouring hydronium ions around a sulfur atom is independent of methanol concentration, but the first shell of hydronium and water around sulfonic acid clusters is broader. Although methanol would prefer to interact with water molecules rather than sulfonic acid groups, gathering of methanol molecules via hydrophobic self-aggregation is preferred. Methanol is located closer to the hydrophobic part of the polymer than water, while water is located closer to the hydrophilic part of the polymer. It was found that methanol distributes specifically more than water in nano-channels. Investigation of solvent dynamics in nano-channels shows that diffusion coefficients (D) of water, methanol, and hydronium decrease with increasing methanol concentration and they may be ordered as follows: D _Water?>?D _Methanol?>?D _Hydronium (D _Water?≈?1.6–2.0D _Methanol and D _Methanol?≈?2.1–3.0D _Hydronium).     

A terahertz time-domain study on the estimation of opto-mechanical properties of pharmaceutical tablets using the Hashin–Shtrikman bounds for refractive index: a case study of microcrystalline cellulose and starch acetate compacts

This work highlights the use of Hashin–Shtrikman (H–S) bounds in the prediction and verification of the effective refractive index, the height and the Young’s modulus of given training sets of pharmaceutical compacts using the measured time delay of a THz pulse traversing the compacts. Set A consisted of 13 microcrystalline cellulose (MCC) compacts whereas set B was made up of 5 starch acetate (SA) compacts. MCC is a typical ingredient of many pharmaceutical tablets. In the case of the MCC compacts, tight and closely matched bounds were obtained between the experimental, the calculated upper, lower bound values for the effective refractive index, and the height values. This promising outcome has shown the high possibility of utilizing H–S bounds in the verification and prediction of the decision level of useful parameters, which can serve as a quality check for pharmaceutical tablets. For the SA compacts, although less tight bounds were observed, the experimental values for the effective refractive index and the Young’s modulus were closely matched with the upper and the lower bounds, respectively. We therefore speculate based on the above observations that the MCC tablets contain an almost evenly distributed spherically shaped air voids whereas in the SA compacts, this assumption might not necessary be true.     

A study on the determination of mechanical properties of a power law material by its indentation force–depth curve

In this study a novel optimization approach is proposed to extract mechanical properties of a power law material whose stress–strain relationship may be expressed as a power law from its given experimental indentation P–h curve. A set of equations have been established to relate the P–h curve to mechanical properties E, σ _y and n of the material. For the loading part of a P–h curve this approach is based on the assumption that the indentation response of an elastic–plastic material is a linear combination of the corresponding elastic and elastic–perfect plastic materials. For the unloading part of the P–h curve it is based on the assumption that the unloading response of the elastic–plastic material is a linear combination of the full contact straight line and the purely elastic curve. Using the proposed optimization approach it was found that the mechanical properties of an elastic–plastic material usually cannot be decided uniquely by using only a single indentation P–h curve of the material. This is because in general a few matched sets of mechanical properties were found to produce a given P–h curve. It is however possible to identify the best matched set of mechanical properties by knowing some background information of the material. If the best matched material is identified, the predictions of mechanical properties are quite accurate.     

Effect of γ-irradiation and semiconducting oxide (tio2) on the thermal decomposition of mgc2o4: A comparative study

The simultaneous rising temperature (DTA-TG) technique and the gas evolution method are adopted for studying the thermal decomposition of unirradiated and irradiated MgC₂O₄ and MgC₂O₄ + TiO₂ mixtures. The data are applied to theories of different solid state reaction models and the best fit is obtained for the Avrami-Erofeev mechanism (n=2) suggesting that both the nucleation and growth processes occur at the reactant product interface in a two dimensional chain branching manner. Low irradiation doses decrease the rate of reaction remarkably whereas the reverse phenomenon takes place at higher doses. The n-type semiconducting oxide, TiO₂ (5-40 mol%) enhances the rate of decomposition which increases with increasing concentration of the catalyst. The influence of n -irradiation is explained in the light of defects, dislocations and electron-hole (e^?, h⁺) pairs generated in the lattice, whereas the influence of TiO₂ is understood on the basis of electron transfer process involved in the reaction.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A_0A (M688N_PsaA) and A_0B (M668N_PsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N_PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N_PsaA mutant. In particular, the P ₇₀₀ ^·+ decay kinetics in the M688N_PsaA mutant is significantly slower than in the WT or the M668N_PsaB mutant. The analysis of the out-of-phase electron–electron dipolar electron spin echo envelope modulation shows that in the M668N_PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair measured in the X-ray crystal structure. In the M688N_PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7–0.3, which agree well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair and the 24.6 Å distance for the P ₇₀₀ ^·+ A _1B ^·? radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron–sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors.     

Exact quantum dynamics study of the H(2S)+SiH+(X1Σ+) reaction on a new potential energy surface of SiH2+(X2A1)

Based on a new global potential energy surface of SiH₂⁺(X²A₁), the exact quantum dynamical calculation for the H(²S)+SiH⁺(X¹Σ⁺)→H₂+Si⁺ reaction has been carried out by using the Chebyshev wave packet method. The initial state specified (ν_i=0, j_i=0) probabilities, integral cross sections (ICS) and thermal rate constants of the title reaction are calculated. All partial wave contributions up to J=90 are calculated in exact quantum calculation including the full Coriolis coupling (CC) effect. The dynamical behaviors of probabilities, ICSs and rate constants are found to be in accord with an exothermic reaction without potential barrier. By comparing the probabilities of CC with the corresponding centrifugal sudden (CS) approximation ones, it can be concluded that neglecting CC effect will decrease the collision time, increase the amplitude of oscillation and lead to overestimation or underestimation of the reaction probability. For ICSs and rate constants, it is found that the deviation of CC and CS ICSs is small in the most of collision energy range except for the range of 0 eV-0.05 eV, while the deviation of both rate constants is considerable in the temperature range of 16 K-1000 K.     

Reply to “A comment on “A test of general relativity using the LARES and LAGEOS satellites and a GRACE Earth gravity model,by I. Ciufolini et al.””

In 2016, we published “A test of general relativity using the LARES and LAGEOS satellites and a GRACE Earth’s gravity model. Measurement of Earth’s dragging of inertial frames [1]”, a measurement of frame-dragging, a fundamental prediction of Einstein’s theory of General Relativity, using the laser-ranged satellites LARES, LAGEOS and LAGEOS 2. The formal error, or precision, of our test was about 0.2% of frame-dragging, whereas the systematic error was estimated to be about 5%. In the 2017 paper “A comment on “A test of general relativity using the LARES and LAGEOS satellites and a GRACE Earth’s gravity model by I. Ciufolini et al.”” by L. Iorio [2] (called I2017 in the following), it was incorrectly claimed that, when comparing different Earth’s gravity field models, the systematic error in our test due to the Earth’s even zonal harmonics of degree 6, 8, 10 could be as large as 15%, 6% and 36%, respectively. Furthermore, I2017 contains other, also incorrect, claims about the number of necessary significant decimal digits of the coefficients used in our test (claimed to be nine), in order to eliminate the largest uncertainties in the even zonals of degree 2 and 4, and about the non-repeatability of our test. Here we analyze and rebut those claims in I2017.