The application of surface second harmonic sensor to probe laser induced modification of titanium dioxide

The effects of high power pulsed laser light on a TiO2 photocatalyst have been investigated using a surface second harmonic generation (SSHG) sensor. When TiO2 is irradiated with a laser at 355mm a visible change in colour from white to dark blue crystals was observed. X-ray diffraction studies indicate that the crystal structure of the TiO2 developed a more rutile form following laser exposure.

Facile synthesis of nanocrystalline LiFePO4/graphene composite as cathode material for high power lithium ion batteries

We describe a simple strategy, which is based on the idea of space confinement, for the synthesis of carbon coating on LiFePO₄ nanoparticles/graphene nanosheets composites in a water-in-oil emulsion system. The prepared composite displayed high performance as a cathode material for lithium-ion battery, such as high reversible lithium storage capacity (158 mA h g^-1 after 100 cycles), high coulombic efficiency (over 97%), excellent cycling stability and high rate capability (as high as 83 mA h g ^-1 at 60 C). Very significantly, the preparation method employed can be easily adapted and be extended as a general approach to sophisticated compositions and structures for the preparation of highly dispersed nanosized structure on graphene. 

Ultrafast charge dynamics initiated by high-intensity, ultrashort laser-matter interaction

The interaction of high‐intensity laser pulses with matter releases instantaneously ultra‐large currents of highly energetic electrons, leading to the generation of highly‐transient, large‐amplitude electric and magnetic fields. We report results of recent experiment in which such charge dynamics have been studied by using proton probing techniques able to provide maps of the electrostatic fields with high spatial and temporal resolution. The dynamics of ponderomotive channelling in underdense plasmas have been studied in this way, as also the processes of Debye sheath formation and MeV ion front expansion at the rear of laser‐irradiated thin metallic foils. An application employing laser‐driven impulsive fields for energy‐selective ion beam focusing is also presented.

Determination of the thermal parameters of high-power batteries by local heat

A new approach to the determination of the thermal parameters of high-power batteries is introduced here. Application of local heat flux measurement with a gradient heat flux sensor (GHFS) allows determination of the cell thermal parameters in di_erent surface points of the cell. The suggested methodology is not cell destructive as it does not require deep discharge of the cell or application of any charge/discharge cycles during measurements of the thermal parameters of the cell. The complete procedure is demonstrated on a high-power Li-ion pouch cell, and it is verified on a sample with well-known thermal parameters. A comparison of the experimental results with conventional thermal characterization methods shows an acceptably low error. The dependence of the cell thermal parameters on state of charge (SoC) and measurement points on the surface was studied by the proposed measurement approach.

Control of bistability in a directly modulated semiconductor laser using delayed optoelectronic feedback

We show numerically that direct delayed optoelectronic feedback can suppress hysteresis and bistability in a directly modulated semiconductor laser. The simulation of a laser with feedback is performed for a considerable range of feedback strengths and delays and the corresponding values for the areas of the hysteresis loops are calculated. It is shown that the hysteresis loop completely vanishes for certain combinations of these parameters. The regimes for the disappearance of bistability are classified globally. Different dynamical states of the laser are characterized using bifurcation diagrams and time series plots.

Chaotic Dynamics of Semiconductor Laser with Current Modulation and optoelectronic feedback

Chaotic dynamics of directly modulated semiconductor lasers have been studied extensively over the last two decades because of their application in secure optical communication. However, chaos is generally suppressed in such systems when the nonlinear gain reduction factor is above 0.01 which is very much smaller than the reported values in semiconductors like InGaAsP. In this paper we show that by giving an optoelectronic feedback with appropriate delay one can increase the range of the values of the gain reduction factor for which chaos can be observed. Numerical studies show that negative feedback is more efficient in producing chaotic dynamics.

Subjective preference for a high power hearing aid utilizing a directional microphone

This paper discusses the use of a directional microphone by hearing aid users.

A Wiener model for memory high power amplifiers using B-spline function approximation

In this paper we introduce a new Wiener system modeling approach for memory high power amplifiers in communication systems using observational input/output data. By assuming that the nonlinearity in the Wiener model is mainly dependent on the input signal amplitude, the complex valued nonlinear static function is represented by two real valued B-spline curves, one for the amplitude distortion and another for the phase shift, respectively. The Gauss-Newton algorithm is applied for the parameter estimation, which incorporates the De Boor algorithm, including both the B-spline curve and the first order derivatives recursion. An illustrative example is utilized to demonstrate the efficacy of the proposed approach.

Adaptive B-spline neural network based nonlinear equalization for high-order QAM systems with nonlinear transmit high power amplifier

High bandwidth-efficiency quadrature amplitude modulation (QAM) signaling widely adopted in high-rate communication systems suffers from a drawback of high peak-toaverage power ratio, which may cause the nonlinear saturation of the high power amplifier (HPA) at transmitter. Thus, practical high-throughput QAM communication systems exhibit nonlinear and dispersive channel characteristics that must be modeled as a Hammerstein channel. Standard linear equalization becomes inadequate for such Hammerstein communication systems. In this paper, we advocate an adaptive B-Spline neural network based nonlinear equalizer. Specifically, during the training phase, an efficient alternating least squares (LS) scheme is employed to estimate the parameters of the Hammerstein channel, including both the channel impulse response (CIR) coefficients and the parameters of the B-spline neural network that models the HPA’s nonlinearity. In addition, another B-spline neural network is used to model the inversion of the nonlinear HPA, and the parameters of this inverting B-spline model can easily be estimated using the standard LS algorithm based on the pseudo training data obtained as a natural byproduct of the Hammerstein channel identification. Nonlinear equalisation of the Hammerstein channel is then accomplished by the linear equalization based on the estimated CIR as well as the inverse B-spline neural network model. Furthermore, during the data communication phase, the decision-directed LS channel estimation is adopted to track the time-varying CIR. Extensive simulation results demonstrate the effectiveness of our proposed B-Spline neural network based nonlinear equalization scheme.

Single-phase high power-factor boost ZCS pre-regulator operating in critical conduction mode

This paper presents a new single-phase interleaved high power factor boost pre-regulator operating in critical conduction mode, where the switches and boost diode performing zero-current commutations during its turn-off, eliminating the disadvantages related to the reverse recovery losses and electromagnetic interference problems of the boost diode, when operating in the continuous conduction mode. The interleaving technique is applied in the power cell, providing a significant input current ripple reduction in comparison to discontinuous mode of operation, due to its input current continuous conduction operation. This paper presents a complete modeling for the converter operating in critical conduction mode, resulting in an improved design procedure for interleaved techniques with high input power factor, a complete design procedure, and main simulation results from a design example with two interleaved cells rated at 1kW, 400V output voltage and 220V rms input voltage.

A novel single-phase ZCS-PWM high-power-factor boost rectifier

This paper presents a novel single-phase high-power-factor (HPF) pulsewidth-modulated (PWM) boost rectifier featuring soft commutation of the active switches at zero current (ZC), It incorporates the most desirable properties of conventional PWM and soft-switching resonant techniques.The input current shaping is achieved with average current mode control and continuous inductor current mode.This new PWM converter provides ZC turn on and turn off of the active switches, and it is suitable for high-power applications employing insulated gate bipolar transistors (IGBT's),The principle of operation, the theoretical analysis, a design example, and experimental results from a laboratory prototype rated at 1600 W with 400-Vdc output voltage are presented. The measured efficiency and the power factor were 96.2% and 0.99%, respectively, with an input current total harmonic distortion (THD) equal to 3.94%, for an input voltage with THD equal to 3.8%, at rated load.

Improvements for ZCS-PWM commutation cell in high-power-factor preregulator application

A Summary of different topological arrangements concerning a ZCS-PWM cell is presented, based on the analysis of its application in boost rectifying preregulators, controlled by the technique of instantaneous average values of input current, with the purpose of obtaining high-input-power-factor rectifier and high efficiency in single-phase applications in telecommunication systems. The main characteristics of each switching cell are described, providing conditions to establish a qualitative comparison among the structures. In addition, experimental results are presented for a prototype of the latest version of the ZCS-PWM boost rectifier, implemented for processing normal values of 1200 W output power and 400 V output average voltage, at 220 V Input RMS voltage and 50 kHz switching frequency.

Novel high-power-factor ZCS-PWM preregulators

This paper introduces novel zero-current-switching (ZCS) pulsewidth-modulated (PWM) preregulators based on a new soft-commutation cell, suitable for insulated gate bipolar transistor applications. The active switches in these proposed rectifiers turn on in zero current and turn off in zero current-zero voltage. In addition, the diodes turn on in zero voltage and their reverse-recovery effects over the active switches are negligible. Moreover, based on the proposed cell, an entire family of de-to-de ZCS-PWM converters can be generated, providing conditions to obtain naturally isolated converters, for example, derived buck-boost, Sepic. and Zeta converters. The novel ac-to-dc ZCS-PWM boost and Zeta preregulators are presented in order to verify the operation of this soft-commutation cell, In order to minimize the harmonic contents of the input current, increasing the ac power factor, the average-current-mode control is used, obtaining preregulators with ac power factor near unity and high efficiency at wide load range. The principle of operation, theoretical analysis, design example, and experimental results from test units for the novel preregulators are presented. The new boost preregulator was designed to nominal values of 1.6 kW output power, 220 V(rms) input voltage, 400 V(dc) output voltage, and operating at 20 kHz. The measured efficiency and power factor of the new ZCS-PWM boost preregulator were 96.7% and 0,99, respectively, with an input current total harmonic distortion (THD) equal to 3.42% for an input voltage with THD equal to 1.61%, at rated load, the new ZCS-PWM Zeta preregulator was designed to voltage step-down operation, and the experimental results were obtained from a laboratory prototype rated at 500 W, 220 V(rm), input voltage, 110 V(dc) output voltage, and operating at 50 kHz. The measured efficiency of the new ZCS-PWM Zeta preregulator is approximately 96.9% and the input power factor is 0.98, with an input current THD equal to 19.07% while the input voltage THD is equal to 1.96%, at rated load.

Root surface treatment using diode laser in delayed tooth replantation: radiographic and histomorphometric analyses in rats

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Pressure field along the axis of a high-power klystron amplifier

The pressure field in a high-power klystron amplifier is investigated to scale the ionic vacuum pump used to maintain the ultra high-vacuum in the device in order to increase its life-time. The investigation is conducted using an 1.3 GHz, 100 A - 240 keV high-power klystron with five reentrant coaxial cavities, assembled in a cylindrical drift tube 1.2 m long. The diffusion equation is solved to the regime molecular flow to obtain the pressure profile along the axis of the klystron drift tube. The model, solved by both analytical and numerical procedures, is able to determine the pressure values in steady-state case. This work considers the specific conductance and all important gas sources, as in the degassing of the drift tube and cavities walls, cathode, and collector. For the drift tube degassing rate equals to q(deg) = 2x10(-12) (-)mbar.L.s(-1) cm(-2) (degassing rate per unit area), to cavities q(cavity) = 3x10(-13) mbar.L.s(-1)cm(-2), to the cathode q(cathode) = 6x10(-9)_mbar.L.s(-1) and to the collector q(collector) = 6x10(-9) mbar.L.s(-1), it was found that a 10 L.s(-1) ionic vacuum pump connected in the output waveguide wall is suitable. In this case, the pressure obtained in the cathode is p(cathode) = 6.3x10(-9) mbar, in the collector p(collector) = 2.7x10(-9) mbar, and in the output waveguide p = 2.1x10(-9) mbar. Although only the steady-state case is analyzed, some aspects that may be relevant in a transient situation, for instance, when the beam hits the drift tube walls, producing a gas burst, is also commented.