Effects of strontium and calcium simultaneous substitution on electrical and structural properties of Pb1?x?yCaxSryTiO3 thin films

Ferroelectric Pb_1−x−y Ca _x Sr _y TiO₃ thin films (denoted by PCST90, PCST70, and PCST30) were deposited on the Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. Their properties were investigated from the viewpoint of crystal structure, microstructure, dielectric, and ferroelectric properties. X-ray diffraction patterns revealed the formation of PCST90, PCST70, and PCST30 thin films without any secondary phases. Infrared and Raman spectroscopy results show that a gradual phase transition from tetragonal to pseudocubic or cubic perovskite structure may occur in PCST thin films with the simultaneous increase of Ca²⁺ and Sr²⁺ contents. Both substitution of isovalent Ca²⁺ and Sr²⁺ at Pb²⁺-site enhanced the dielectric constant and reduced the remnant polarization. In addition, ferroelectric test analyses show that the PCST thin films undergo a ferroelectric-to-paraelectric phase transformation with an amount of Pb²⁺, Ca²⁺, and Sr²⁺ at 30%, 35%, and 35% mol, respectively. Hence, the absence of ferroelectric property may be attributed to a decreasing of the octahedron distortion in the perovskite structure accompanied by a weakening of long-range ferroelectric order.     

Structural complexity of disordered surfaces: Analyzing the porous silicon SFM patterns

This paper introduces a relative structural complexity measure for the characterization of disordered surfaces. Numerical solutions of 2d+1 KPZ equation and scanning force microscopy (SFM) patterns of porous silicon samples are analyzed using this methodology. The results and phenomenological interpretation indicate that the proposed measure is efficient for quantitatively characterize the structural complexity of disordered surfaces (and interfaces) observed and/or simulated in nano, micro and ordinary scales.     

Tunable visible photoluminescence of powdered silica glass

Intense photoluminescence in the visible region was observed at room temperature in standard soda-lime-silica glass powder, mechanically milled in a high-energy attrition mill. The emission band maximum shows an interesting dependence on the exciting wavelength, suggesting the possibility to tune the PL emission. These findings indicate that the photoluminescence may be directly related to unsatisfied chemical bonds correlated with the high surface area. The Raman scattering and ultraviolet–visible optical reflectance measurements corroborate this assertion. Transmission electron microscopy measurements indicate that samples milled more than 10 h present the formation of nanocrystallites with about 10–20 nm.     

Chaos control of a nonlinear oscillator with shape memory alloy using an optimal linear control: Part I: Ideal energy source

In this paper, an optimal linear control is applied to control a chaotic oscillator with shape memory alloy (SMA). Asymptotic stability of the closed-loop nonlinear system is guaranteed by means of a Lyapunov function, which can clearly be seen to be the solution of the Hamilton–Jacobi–Bellman equation, thus guaranteeing both stability and optimality. This work is presented in two parts. Part I considers the so-called ideal problem. In the ideal problem, the excitation source is assumed to be an ideal harmonic excitation.     

Theoretical and infrared spectroscopy study of the conformational preferences for some 3-monosubstituted-2-methylpropenes

The infrared spectra of 3-X-2-methylpropenes (X = Cl, Br, I, NMe(2), NEt(2), OH, OMe, OEt, SH, SMe and SEt) have been recorded at room temperature in CCl(4) solution. The C=C stretching mode was analyzed and compared to theoretically calculated data to give insight about the conformational isomerism of these compounds. A combination band systematically appears in all spectra (except for amines); the remaining ν(C=C) band and the corresponding intensities were used to obtain the conformer populations. For second-period atoms bonded to C-3, two or more conformers are observed in CCl(4) solution, but when substitution by heavier atoms takes place, only one conformer is observed under the tested conditions, i.e., the gauche form with respect to the C=C-C-X dihedral angle. Therefore, steric hindrance between X and =CH(2) in the s-cis form is strongly dependent on the heteroatom size, while the effect of the alkyl chain bonded to the heteroatom seems to be of secondary importance.     

A non-ideal portal frame energy harvester controlled using a pendulum

A model of energy harvester based on a simple portal frame structure is presented. The system is considered to be non-ideal system (NIS) due to interaction with the energy source, a DC motor with limited power supply and the system structure. The nonlinearities present in the piezoelectric material are considered in the piezoelectric coupling mathematical model. The system is a bi-stable Duffing oscillator presenting a chaotic behavior. Analyzing the average power variation, and bifurcation diagrams, the value of the control variable that optimizes power or average value that stabilizes the chaotic system in the periodic orbit is determined. The control sensitivity is determined to parametric errors in the damping and stiffness parameters of the portal frame. The proposed passive control technique uses a simple pendulum to tuned to the vibration of the structure to improve the energy harvesting. The results show that with the implementation of the control strategy it is possible to eliminate the need for active or semi active control, usually more complex. The control also provides a way to regulate the energy captured to a desired operating frequency.