Porosity depth profiling of spin-coated silica thin films produced by different precursors sols

The structural evolution of nanoporous silica thin films was studied by Doppler broadening spectroscopy (DBS), 2-3 gamma ratio of positronium (3γ-PAS) and Fourier transform infrared spectroscopy (FT-IR). Four series of silica films with thickness in the 300-600 nm range were deposited by spin coating on Si substrate changing the content of sacrificial porogen in the sol precursors. The effect on the porosity of different amount of porogen and of the thermal treatments in the 400-900 °C temperature range have been highlighted. The evolution of the porosity is discussed considering the removal of porogen and of the silanol Si-OH groups by thermal treatments as pointed out by FT-IR. Pores with size from less than 1 nm up to sizes larger than 2.0 nm have been detected. In samples with maximum porogen load oPs escaping was observed indicating onset of connected porosity. At temperatures higher than 700 °C a decrease of the porosity due to a progressive pore collapsing was evidenced. A strong correlation was found between the shift of the Si-O-Si transversal optical (TO₃) mode in the FT-IR spectra and the pore size in the porous silica films as revealed by DBS and 3γ-PAS.     

Determination of positron beam parameters by various diagnostic techniques

Various diagnostic techniques have been applied at the neutron-induced positron source Munich NEPOMUC in order to determine the positron beam parameters such as intensity, beam shape and energy distribution. The positron beam intensity is determined by the detection of the annihilation radiation of positrons, which annihilate in a movable target. The use of a micro-channel plate (MCP) detector with a CCD-camera allows a direct measurement of the positron beam shape and the lateral resolved intensity distribution. At NEPOMUC a movable MCP-assembly inside the evacuated beam line enables a quick examination of the beam shape during operation. A retarding grid was mounted inside the homogeneous magnetic guiding field in order to determine the distribution of the longitudinal positron momentum, and hence estimate the energy distribution of the positrons.     

Behavior of cold-worked AISI-304 steel in stress-corrosion cracking process: Microstructural aspects

Austenitic stainless steel is one of the key structural materials for a wide-range of components for present nuclear power plants. Moreover, this type of steel is also foreseen as a key structural material in future reactor systems, the so-called Generation IV. However, for the successful application of these materials in new environmental conditions an integrated Research and Development program needs to be successfully completed. This work is focused to the evaluation of cold-worked AISI-304 stainless steel from 20 to 45% of cold-worked deformation by different spectroscopic techniques within the aim to study the microstructural characteristics. In particular, positron annihilation spectroscopy and small angle neutron scattering have been used for characterization of phase transformation and microstructural behavior. Furthermore, outcomes of corrosion properties of cold-worked AISI-304 stainless steel exposed for 100 and 500 h in super-critical water reactor conditions are correlated with the obtained results.     

Secondary electron spectra of gold under bombardment by very low-energy positrons

Measurements of the secondary electron energy spectra resulting from very low-energy positron bombardment of a polycrystalline Au surface are presented. The low-energy part of the secondary spectra contain significant contributions from two processes: (1) annihilation-induced Auger electrons that have lost energy before leaving the surface and (2) secondary electrons resulting from direct energy exchange with an incident positron. Our data indicate that the second process (direct energy exchange with the primary positron) is still important at and below 3 eV incident beam energy. Since energy conservation precludes secondary electron generation below an incident beam energy equal to the difference between the electron and positron work functions (∼3 eV), the fact that we still observe significant secondary electron emission at energies at or below this value provides strong evidence that the incident positrons are falling directly into the surface state and transferring all of the energy difference to an outgoing secondary electron. These measurements were also used to obtain the first experimentally determined upper limit on the intensity of the spectrum of Auger-induced secondary electrons.     

Fuzzy Impulsive Control of Permanent Magnet Synchronous Motors

The permanent magnet synchronous motors （PMSMs） may experience chaotic behaviours with systemic parameters falling into a certain area or under certain working conditions, which threaten the secure and stable operation of motor-driven. Hence, it is important to study the methods of controlling or suppressing chaos in PMSMs. In this work, the Takagi-Sugeno （T-S） fuzzy impulsive control model for PMSMs is established via the T-S modelling methodology and impulsive technology. Based on the new model, the control conditions of asymptotieal stability and exponential stability for PMSMs have been derived by the Lyapunov method. Finaily, an illustrated example is also given to show the effectiveness of the obtained results.     

Numerical Simulation of Solitary Kinetic Alfvén Waves

Using the two-fluid model in the case of α〉〉1 （α= β/2Q, β is the ratio of thermal pressure to magnetic pressure, and Q = me/mi ）, we numerically investigate the interactions between two solitary kinetic Alfvén waves （SKAWs） and between an SKAW and a density discontinuity. The results show that the two SKAWs would remain in their original shapes and propagate at their initiating speeds, which indicates that SKAWs behave just like standard solitons. The simulation also shows that SKAWs will reflect and refract when crossing a discontinuity and propagating into a higher density region. The transmission wave is an SKAW with increasing density, and the reverberation is a disturbance with lower amplitude.     

Influence of Dopants in ZnO Films on Defects

The influence of dopants in ZnO films on defects is investigated by slow positron annihilation technique. The results show S that parameters meet S_Al>S_un>S_Ag for Al-doped ZnO films, undoped and Ag-doped ZnO films. Zinc vacancies are found in all ZnO films with different dopants. According to S parameter and the same defect type, it can be induced that the zinc vacancy concentration is the highest in the Al-doped ZnO film, and it is the least in the Ag-doped ZnO film. When Al atoms are doped in the ZnO films grown on silicon substrates, Zn vacancies increase as compared to the undoped and Ag-doped ZnO films. The dopant concentration could determine the position of Fermi level in materials, while defect formation energy of zinc vacancy strongly depends on the position of Fermi level, so its concentration varies with dopant element and dopant concentration.     

Stability of periodic waves generated by subcritical oscillatory instability under the action of feedback control

Periodic solutions of a subcritical cubic complex Ginzburg-Landau equation are considered in the limit of large dispersion and nonlinear frequency shift. Results obtained formerly by Schöpf and Kramer are revisited and extended to the case of a defocusing nonlinearity. It is shown that a global feedback control can extend existence and stability regions of the stationary solutions in both focusing and defocusing cases.     

Back to the future: Polarised positron beams

Plans for the development of a polarised mono-energetic positron beam at the University of Bath are described. Pilot measurements with the existing beam, modified only by increasing the source-moderator separation, have demonstrated that the helicity of the present beam is sufficient to show sensitivity to ferromagnetism in bulk iron. However, an increase in flux and helicity is required if the beam is to be successfully used to probe magnetic structures of current technological interest, and strategies to achieve both are described. Potential applications in magnetic semiconductor structures and magnetic nanoparticles are discussed.     

Three-photon annihilation of the electron-positron pairs

Three-photon annihilation of the electron-positron pairs (=(e⁻,e⁺)-pairs) is considered in the electron rest frame. The energy of the incident positron can be arbitrary. The analytical expression for the cross-section of three-photon annihilation of the (e⁻,e⁺)-pair has been derived and investigated.