Jose A. Carrasco

Alkoxide-intercalated CoFe-layered double hydroxides as precursors of colloidal nanosheet suspensions: structural, magnetic and electrochemical properties

Alkoxide-intercalated CoFe-layered double hydroxides (CoFe–LDHs) were synthesized via the non-aqueous methanolic route. According to powder X-ray diffraction and field emission scanning electron microscopy, they exhibit a nanosized plate-like morphology with a basal space of 9.21 A. The hydrolysis of the material in water leads to colloidal suspensions of nanosheets with lateral dimensions of about 20 nm and thicknesses of ca. 4 nm as demonstrated by atomic force microscopy and dynamic light scattering. Atomic resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy confirm the high crystalline quality of the crystals and the proper Co/Fe stoichiometry. The magnetic properties of the CoFe–LDH have been investigated by means of DC and AC magnetic susceptibility measurements and isothermal magnetisation, showing a low-temperature magnetic ordering below ca. 7 K with a size-dependent spin-glass like behaviour, and displaying hysteresis cycles at 2 K with a coercive field of 402 G. Moreover, the sample has been tested as an electrode material for supercapacitors in a three-electrode system by means of cyclic voltammetry and galvanostatic charge–discharge experiments, showing high capacitances and stability. Finally, we have explored the electrocatalytic behaviour towards water oxidation, demonstrating its efficient and persistent performance at basic pHs, highlighting their tremendous potential in energy storage devices.

Modeling the sequential switching shunt series regulator

This letter characterizes, in terms of the bandwidth and limit cycle frequency of its constituent subsystems, the sequential switching shunt series regulator -S/sup 4/R, a high-efficiency, low-mass and volume power cell devised to power the next generation of regulated power buses in telecommunication spacecrafts. Transconductance power source modeling is used to obtain linear and nonlinear models. These are used to establish a design control strategy which involves the dynamic response in large load requirements or at the end of the satellite life. Simulations and experimental results are also given to demonstrate the validity of the model.

High-Efficiency Regulation Method for a Zero-Current and Zero-Voltage Current-Fed Push–Pull Converter

A new high-efficiency regulation method for a zero-current and zero-voltage current-fed push-pull converter is presented. The method proposed is based on the use of a controlled transformer as a post-regulator, which adds or subtracts an additional voltage to the output filter of the converter. An auxiliary regulator, which only handles a percentage of the output power, controls this transformer, and therefore, results in much higher efficiency than a normal preregulator would have. The novelty of the presented converter is that regulation is achieved without affecting the zero-voltage and zero-current switching in any working conditions. Small- and large-signal models of the converter have been developed and a low-power prototype has been simulated and implemented. The good agreement, between the experimental simulations and theoretical results, validates the theoretical developments.

Small-signal modeling of a controlled transformer parallel regulator as a multiple output converter high efficient post-regulator

This paper presents a post-regulator based on the use of a controlled transformer, which adds or subtracts an additional voltage to the output filter of a converter in order to regulate its output voltage. So, their actuation is complementary to that of more known post-regulators, such as the magnetic amplifier (magamp) and synchronous switch post-regulator (SSPR), because the regulation is achieved by controlling the voltage across the filter inductor instead of its charge time. Besides, the post-regulator processes the power in parallel to the one flowing from input to output and only handles a percentage of it. The post-regulation by controlled transformer is suitable of being employed in any isolated PWM power converter and combines a good efficiency and the easiness of design of classical switched power supplies. The work describes the post-regulation strategy for obtaining two outputs independently regulated, and presents a model to obtain the control transfer function and the cross-impedance expressions.

Room Temperature Magnetism in Layered Double Hydroxides due to Magnetic Nanoparticles

Some recent reports claiming room temperature spontaneous magnetization in layered double hydroxides (LDHs) have been published; however, the reported materials cause serious concern as to whether this cooperative magnetic behavior comes from extrinsic sources, such as spinel iron oxide nanoparticles. The syntheses of crystalline Fe(3+)-based LDHs with and without impurities have been developed, highlighting the care that must be taken during the synthetic process in order to avoid misidentification of magnetic LDHs.

Application of a novel parallel regulation technique in a two output forward converter

Techniques based in parallel post-regulation eliminate cross-regulation between the different outputs of multi-output converters and achieve good efficiency by handling only a percentage of the power delivered to the output. This work describes a simple implementation of such a regulation mechanism in a two output forward converter. Moreover, the development of a complete steady state and low signal model ensures proper design and implementation. The model shows the inherent simplicity of the regulation procedure, similar to that of classical PWM regulators.

Alkoxide-intercalated NiFe-layered double hydroxides magnetic nanosheets as efficient water oxidation electrocatalysts

Alkoxide-intercalated NiFe-layered double hydroxides were synthesized via the nonaqueous methanolic route. These nanoplatelets exhibit high crystalline quality as demonstrated by atomic resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Moreover, the presence of the alkoxide moieties has been unambiguously demonstrated by means of thermogravimetric analysis coupled to a mass spectrometer. These NiFe-LDHs can be exfoliated in water or organic solvents and processed into homogeneous ultra-thin films (<3 nm thick) with the assistance of O2-plasma. The study of their behaviour as water oxidation electrocatalysts has shown an outstanding performance at basic pHs (small overpotential of ca. 249 mV and Tafel slopes in the range of 52–55 mV per decade).

Influence of the Parasitic Solar Array Capacitance in the Sequential Switching Shunt Series Regulator

This work deals with the effect produced by the solar array capacitance on the sequential switching shunt regulator, S4R. In the past, silicon solar cells were widely used with practically no effect upon the solar array regulator, so that the solar array capacitance was a minor problem in the design. Nowadays, the usage of triple junction GaAs solar cells with larger parasitic capacitance has prompted new questions about power losses, steady state and dynamic responses in the S4R. In this work, the effect of the solar array capacitance is quantified by analytical models. Simulations and experimental prototyping support the models

A new Sequential Switching Shunt Regulator - Digital Shunt Regulator (S3R-DSR) for Solar Array Regulators

This work describes the principles of the sequential switching shunt regulator-digital shunt regulator, S3R-DSR, a novel approach to implement the solar array regulator. This is especially devoted to overcome the problems derived from the high solar array parasitic capacitance that exhibits the present (high power) telecommunications satellites

High-frequency bi-directional three-phase rectifier with power factor correction

This article presents the study and experimental results of a high-frequency three-phase bi-directional rectifier and its control. With this structure, it is possible to find a low frequency modulation function that permits to obtain a sinusoidal average input current in phase with the main input voltage. The structure also permits to regulate the output voltage in a wide range of variation.