Mechanochemical synthesis of nanocrystalline titanium monoxide

Nanocrystalline cubic titanium monoxide, TiO_x (0.92 < x < 1.19), with mean crystallite size of ≈ 6 nm, was synthesized by mechanochemical treatment of Ti and TiO₂ (rutile) powder mixtures with molar ratios of 1:1, 1.10:1 and 1.25:1. The mechanochemical solid state reaction in a high-energy planetary ball mill was completed for 2 h in either air or argon atmosphere. During heating in vacuum at 900 and 1000 °C for 24 h, nanocrystalline TiO_x transforms to a well-crystallized, cubic or monoclinic TiO_x. The materials prepared were characterized by XRPD, TGA/DSC and SEM/EDS analysis.     

Discrete time sliding mode approach to synchronization of modulated two-phase harmonic oscillator

This paper explores two-phase harmonic oscillator synchronization with a referent oscillator, whose amplitude and phase are modulated by two arbitrary smooth functions, respectively. Synchronization between modulated referent and controlled oscillator is based on amplitude and phase control, and pursued by employing a discrete time variable structure control system. Synchronization is achieved by organizing a quasi-sliding mode along intersection of two appropriate nonlinear sliding surfaces. Discrete-time variable structure controllers are proposed, which provides the desired motion. Mathematical analysis as well as experimental results are presented.     

Sliding Manifold Design for Higher-order Sliding Mode Control of Linear Systems

International Journal of Control, Automation and Systems - The paper considers sliding manifold design for higher-order sliding mode (HOSM) in linear systems. In this case, the sliding manifold...     

Kinematic redundancy and sensor redundancy for enhancement of robot tracking performance

Imprecision of robot's internal sensors and its influence on trajectory tracking tasks along with high dynamic requirements are the main issues in this paper. The solution proposes redundancy introduction (in the shape of small mechanism at the top of the robot) for resolving both problems. The inverse kinematics of the redundant robot has been solved at the tactical level of control via the Distributed Positioning (DP) concept. The redundancy applied reduces the tracking error caused by dynamic requirements and partly by imprecision of internal sensors. However, additional solutions with external position sensor could be used in order to reduce sensor imprecision influence. All solutions are very conservative considering changes in basic (non-redundant) robot. Simulation results provide clear insight in the validity of proposed solutions.     

Complex landslide in the Rječina valley (Croatia): origin and sliding mechanism

This paper discusses the development of the Grohovo landslide on the north-eastern slope of the Rječina valley, the largest active landslide along the Croatian part of the Adriatic Sea coast. This complex retrogressive landslide was reactivated in December 1996. Thirteen separate slide bodies have been identified. The slide surface is considered to be on the upper flysch bedrock. Monitoring indicated that the magnitude of displacements was very different in time and space. The maximum movements were recorded on the upper part of the slope. The limestone mega-block and separated rocky blocks on top of the slope have also moved, which is not a typical phenomenon of the flysch slopes in the area of Rijeka.      

Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi₂O₃ and TiO₂. Only Bi₄Ti₃O₁₂ phase was formed after 3 h of milling time. The excess of 3 wt% Bi₂O₃ added in the initial mixture before milling does not improve significantly the formation of Bi₄Ti₃O₁₂ phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time. The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi₄Ti₃O₁₂ did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi₄Ti₃O₁₂ crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis.     

A new design of servomechanisms with digital sliding mode

A new control algorithm based on Golo's approach (Systems & Control Letters 41(2000)), for high-precision servomechanism synthesis, is offered in this paper. In the proposed algorithm, an additional integral action, activated at the predefined system state, is introduced. Servomechanisms with the proposed control and by introducing a compensation of disturbance caused by reference derivative, are capable to track cubic references. Theoretically investigated features of the proposed control are verified by simulations and experiments on a servomechanism with DC motor.     

Electrical properties of barium titanate stannate functionally graded materials

Barium titanate stannate (BTS) functionally graded materials (FGMs) with different tin/titanium concentration gradient were prepared by the powder-stacking method and uniaxially pressing process, followed by sintering. Impedance spectroscopy (IS) was used to determine the electrical characteristics of FGMs and ingredient BTS ceramics, as well as to distinguish the grain-interior and grain boundary resistivity of the ceramics. Activation energies of FGMs and ingredients were calculated. It has been established that for BTS ceramics the activation energy deduced from grain-interior conductivity (0.73–0.75 eV) is defined by chemical composition, while activation energy for grain boundary conductivity (1.07–1.25 eV) is influenced by microstructural development (density and average grain size). Furthermore, for FGMs, activation energy for grain-interior conductivity kept the intrinsic properties (0.74–0.78 eV) and did not depend on tin/titanium concentration gradient, while activation energy (1.03–1.29 eV) for grain boundary was determined by the microstructural gradient. No point dissipation was observed by IS, accordingly, no insulator interfaces (cracks and/or delamination) between graded layers were detected.     

The influence of mechanochemical treatment of the Bi2O3–ZrO2 system on the structural and dielectric properties of the sintered ceramics

A powder mixture of -Bi₂O₃ and ZrO₂, both monoclinic, in the molar ratio 2 : 3, was mechanochemically treated in a planetary ball mill in an air atmosphere for up to 20 h, using steel vial and hardened-steel balls as the milling medium. Mechanochemical reaction leads to the gradual formation of an amorphous phase. After 5 h of milling the starting -Bi₂O₃ and ZrO₂ were transformed fully into a non-crystalline phase. After milling for various times the powders were compacted by pressing and isothermal sintering. The pressed and sintered densities depended on the milling time. Depending on the duration of the mechanochemical treatment and sintering temperature, the phases: -Bi₁₂(Zr _x Fe_1–x )O₂₀; Bi(Zr _x Fe_1–x )O₃ and Bi₂(Zr _x Fe_1–x )₄O₉ were obtained by reactive sintering, whereby the Fe originates from vial and ball debris. The dielectric permittivity of the sintered samples significantly depends on the milling time. Samples milled for 10 and 15 h and subsequently sintered at 800 °C for 24 h exhibit a hysteresis dependence of the dielectric shift (in altering electric fields higher than 10 kV/cm at room temperature), confirming that the synthesized materials possess ferroelectric properties.