Van Son Nguyen

Effect of ultrasonication and dispersion stability on the cluster size of alumina nanoscale particles in aqueous solutions

This study deals with the deagglomeration of nanoparticles in low concentration suspensions in water, protic polar solvent for polymers such as poly(vinyl alcohol) (PVA). The influence of the main parameters of ultrasonication such as time, power and irradiation modes (continuous, pulsed) on the cluster size of aluminium oxide nanoparticles 1 mg/ml in aqueous solutions was investigated. Power-law dependence of size reduction on ultrasonic time was observed. The study indicated an optimum power input, i.e. at higher vibration amplitude the break up of nanoparticle clusters was no better and there was a risk of reagglomeration occurring during a long ultrasonication. Under optimal conditions, continuous and pulsed irradiations showed almost the same efficiency of deagglomeration over a given time. This result provides alternative operating conditions for attaining the smallest size of the alumina aggregates in suspension. The influence of stabilization on the cluster size was also studied. Alumina nanoparticles were stabilized by electrostatic forces against reagglomeration without the need for dispersants, and the enhancement of dispersion stability using electrostatic, steric effects had no significant effect on the aggregate size. On the contrary, the adsorption of high molecular weight polyelectrolytes onto the particle surface could lead to reagglomeration due to material bridges between particle surfaces and even flocculation.

Optimized structure for next-to-next balancing of series-connected lithium-ion cells

Voltage balancing between series-connected cells of rechargeable lithium-ion batteries is important for battery life, autonomy and security. Active balancing is the designated choice for applications that are sensitive to energy losses. A well-known next-to-next balancing technique is presented and its design and operating limitations are analyzed. This paper then presents a new technique for next-to-next balancing that improves on the conventional technique by offering better performances while being simpler to implement. Experimental results show an important reduction of the size of the magnetic coupler while maintaining efficiency at a level above 90%.

Dispersion of nanoparticles: from organic solvents to polymer solutions.

This work is devoted to a systematic study of nanoparticle dispersion by ultrasonication in different solutions: from organic solvents to polymer solutions. The cluster size of nanoparticles at different concentrations in both organic solvents and polymer solutions were directly characterized by Dynamic Light Scattering to study the effect of solid concentration, surfactant and polymer on the dispersion. It reveals that in stabilized suspensions, the smallest attainable size or aggregate size of nanoparticles is independent of solvent type and solid content over the tested range. Furthermore, nanoparticles in simple solvent and in polymer solutions had the similar evolution of cluster size and almost the same final size, which could be very helpful to optimize the dispersion of nanofillers in polymer solutions and nanocomposites. It is also shown that, with appropriate sonication amplitudes, the dispersion procedure developed for very dilute suspensions could be transferred to higher concentration suspensions or even to polymer suspensions.

Preparation and characterization of P(VDF-TrFE)/Al 2 O 3 nanocomposite

Hybrid nanocomposites based on crystalline nanoparticles dispersed in polymer matrix have been widely studied in the past few years because of the ability of these materials to combine the properties of organic polymer and inorganic nanoparticles. The aim of this work is to tune the mechanical properties of a piezoelectric polymer by adding nanoparticles to the matrix. In this paper, alumina nanoparticles were dispersed in the copolymer P(VDF-TrFE), which exhibits high piezoelectric coefficient after polarization under high electric field without needing stretching during the polarization process. Transmission electron microscopy and scanning electron microscopy demonstrate the high rate of welldispersed nanoparticles with 10% of alumina nanoparticles added to the matrix. Piezoelectric measurements indicate that P(VDF-TrFE) may be filled by up to 10 wt% of alumina while retaining its high piezoelectric properties and increasing its elastic constant by more than 20%, measured by Brillouin spectroscopy. This work opens a wide range of applications using nanoparticles with nonlinear optical, pyroelectric, magnetic, or ferroelectric properties.

Compact, isolated and simple to implement gate driver using high frequency transformer

This paper presents a simple, ultra-compact and isolated gate driver system used to drive power switches. Using two legs of a CMOS inverter, a high frequency transformer and two zener diodes connected with the gate of power switch, this driver provides an optimal gate driver waveform with a high gate voltage to switch on the transistor, and a negative bias gate voltage during OFF state. In the paper, the proposed gate driver will be theoretically analyzed; simulation and experimental results for the system implemented to drive a high side MOSFET will be also showed and discussed.

Preparation and characterization of P(VDF-TrFE)/Al 2 0 3 nanocomposite

Hybrid nanocomposites based on crystalline nanoparticles dispersed in polymer matrix has been widely studied in the past few years because of the ability of these materials to combine both properties of organic polymer and inorganic nanoparticles. The aim of this work is to tune mechanical properties of piezoelectric polymer by adding nanoparticles to the matrix. In this paper alumina nanoparticles were dispersed in the copolymer P(VDF-TrFE) which exhibits high piezoelectric coefficient after polarization under high electric field without needing stretching during the polarization process‥ Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) demonstrate the high rate of well-dispersed nanoparticles. with ten percent of alumina nanoparticles added in the matrix. Piezoelectric measurements indicate that P(VDF-TrFE) may be filled up to 10 wt.% of alumina while keeping high piezoelectric properties and increasing of more than 10% the acoustic wave velocity of the material, measured by Brillouin spectroscopy. This work opens a wide range of applications using nanoparticles with nonlinear optical, pyroelectric, magnetic or ferroelectric properties.

Flexible over-moded resonators based on P(VDF-TrFE) thin films with very high temperature coefficient

This work presents for the first time a flexible over-moded resonator (OMR) based on P(VDF-TrFE) thin films. The devices were manufactured on commercially available elastic substrate with inkjet-printed electrodes. The sensing copolymer films used in the devices were polarized by the corona method after electrode deposition. The main performance parameters of the component were then determined. The manufactured OMRs on P(VDF-TrFE) exhibited a linear variation of frequency versus temperature and a very large value of temperature coefficient of frequency (TCF > 1600 ppm/°C). These properties suggest a great potential for using such components as low-cost and high-precision temperature sensors. The electromechanical coupling coefficient and the quality factor of the resonator were also characterized versus temperature.

P(VDF-TrFE)/Al 2 O 3 piezoelectric nanocomposite thin films

Alumina nanoparticles were incorporated into copolymer of vinylidene difluoride and trifluoroethylene (P(VDF-TrFE)) to form nanocomposite. Thin films were prepared on silicon wafers and some acoustic, optical and mechanical properties were studied by Brillouin spectroscopy. The morphology of the polymer and the dispersion quality of the nanoparticles were observed by Transmission Electron Microscopy. The refractive index was shown to be relatively constant with the concentration of nanoparticles. The acoustic wave velocity and the elastic constant of the material increase linearly with weight percentage of nanoparticles. Such improvement in material properties might be attributed to the intermolecular hydrogen bonding between hydroxyl groups of the surface of alumina nanoparticles and the copolymer matrix.

Brillouin spectroscopy of polymer nanocomposites

Abstract Brillouin spectroscopy (BS), or Brillouin light scattering spectroscopy, is a very useful tool for studying mechanical properties of polymer nanocomposites. After a general presentation of the BS technique, with its theoretical basis, some technical considerations and the main scattering geometries used, some applications to polymers and polymer nanocomposites will be presented in this chapter. This chapter gives indications about what the important parameters for the sample are, and which should be considered by a reader aiming to perform a BS experiment: such as opacity, dimension, homogeneity, etc. The acoustic wave velocities, the elastic constants, the refractive index, various phase transitions, and in particular the glass transition temperature of polymers and nanocomposites are investigated from Brillouin scattering measurements. The variation of the storage modulus with the nanoparticle concentration is also explored. The fruitful coupling of Brillouin spectroscopy with other techniques is done with two examples. Brillouin and Raman spectroscopies monitor the in situ polymerization and the cross-linking processes during the network formation of epoxy, whereas the Brillouin scattering and friction experiment of nanocomposite correlate elastic modulus and tribological properties. Some references about the studies of biological systems are also provided.