Sushil Kumar Mendiratta

Novel Anisotropic Magnetoelectric Effect on δ-FeO(OH)/P(VDF-TrFE) Multiferroic Composites

The past decade has witnessed increased research effort on multiphase magnetoelectric (ME) composites. In this scope, this paper presents the application of novel materials for the development of anisotropic magnetoelectric sensors based on δ-FeO(OH)/P(VDF-TrFE) composites. The composite is able to precisely determine the amplitude and direction of the magnetic field. A new ME effect is reported in this study, as it emerges from the magnetic rotation of the δ-FeO(OH) nanosheets inside the piezoelectric P(VDF-TrFE) polymer matrix. δ-FeO(OH)/P(VDF-TrFE) composites with 1, 5, 10, and 20 δ-FeO(OH) filler weight percentage in three δ-FeO(OH) alignment states (random, transversal, and longitudinal) have been developed. Results have shown that the modulus of the piezoelectric response (10-24 pC·N(-1)) is stable at least up to three months, the shape and magnetization maximum value (3 emu·g(-1)) is dependent on δ-FeO(OH) content, and the obtained ME voltage coefficient, with a maximum of ∼0.4 mV·cm(-1)·Oe(-1), is dependent on the incident magnetic field direction and intensity. In this way, the produced materials are suitable for innovative anisotropic sensor and actuator applications.

Magnetization behaviour of Gd lead borate and Fe lead borate glasses under high magnetic field (20 T)

Abstract Magnetization of Gd and Fe lead borate glasses of varying composition was measured in magnetic fields up to 20 T and temperatures up to 7 K. A convenient variable to represent the data is η = gμ B H / k B T and the data extend up to η 4. Low concentration data fit the Brillouin function, B J (η), and the value of effective magnetic moment per ion, P eff , thus deduced agrees well with that determined from the susceptibility measurement. For higher concentrations, where the magnetic interaction between ions had been deduced from susceptibility measurements, it is discussed how the data can be used, qualitatively at the moment, to infer the existence of magnetic clusters in the case of short ranged interaction of relatively closed Gd ions. In the case of Fe ions where it is believed that the interaction is stronger and is relatively long ranged, it is shown that M vs. H data at different temperatures do not collapse to single curve in M vs. η representation.

Dielectric behaviour of Nd ions in the lead borate glass

Abstract Accurate measurements of the relaxation in the time domain show that the low frequency relaxation processes in glasses are well described by the Williams-Watts (WW) relation. Measurement of the complex permittivity in the frequency domain have been oftein claimed, on the other hand, to satisfy the Cole-Davidson (CD) relation, e ∗ =e ∞ +[ (e 0 −e ∞ ) (1+jωρ) a ], 0 . In order to check with what accuracy the systems whose time domain behaviour is given by WW relation also satisfy the CD equation in the frequency measurements of the glasses in the frequency region 1 Hz-10 GHz are taken using different techniques. Absorption was observed in two different frequency regions and they are well described by the two relations. The temperature dependence of relaxation time parameters was also studied in order to characterize the physics of the relaxation process. The dependence of the parameters on the concentration of Nd sheds some light on the state of aggregation of the Nd ions in glass.

Poling of β-poly(vinylidene fluoride): dielectric and IR spectroscopy studies

Abstract β-Poly(vinylidene fluoride) (β-PVDF) exhibits ferroelectric properties due to the special arrangement of the chain units in the crystalline phase. The piezoelectric properties of the material can be optimised by poling the original stretched film. The main effect of the poling process is the alignment of the randomly organised dipolar moments against the applied field. In this work, poled and non-poled β- PVDF from the same batch are characterised by dielectric spectroscopy. The origin of the electrical and mechanical response of poled and non-poled β-PVDF were further explored by far IR spectroscopy and discussed on a molecular level. The main effect of the poling process on the dielectric response of the material is a small increase of the dielectric constant due to the preferential alignment of the main dipolar contribution and a slight decrease of the dielectric loss, due to the more organized amorphous structure. The conductivity is strongly increased by poling, especially the high-temperature conductivity, ascribed mainly to hopping conductivity due to free charges induced during poling. FTIR experiments indicate that the origin of these effects and also of the variations in the thermo-mechanical response of the material can be found in the reorientation of the crystalline dipoles along the poling field, together with a partial reduction of the amount of α phase and an increase of the amount of β phase. The α to β transformation, mainly due to the stretching process, seems to be optimized by the poling process.

Transient behaviour and rate effects in resistive detectors

Abstract The dependence of the detection efficiency (or charge gain) of a resistive detector on the counting rate has been considered by several authors. The influence of the resistivity of the electrodes, the counter geometry and the gas mixture composition have also been studied. Most of those measurements assume a stationary regime. In the present work we report on studies of the transient behaviour of detectors with a resistive cathode for various counting rates, charge gains and detector geometries. We show that in some cases the time-decay curves can be fitted by a single exponential plus a constant term, while in others (higher charge gains or higher counting rates) a sum of two or even three exponentials plus the constant term is needed to fit the experimental data. A study of the electric properties of the dielectrics used is also presented and a comparison is made between these results and the data obtained under irradiation conditions.

Spin-glass behaviour of x Fe2O3 (0.4 − x) NiO PbO·2B2O3 glasses

Abstract We have measured the zero-field a.c. susceptibility of five glasses, containing varying amounts of Fe and Ni ions, in the temperature region 1 K to 150 K. Most of the glasses show spin-glass behaviour with spin-freezing temperature in the region of 1 K to 4 K. From the overall data we conclude that: 1) the interaction between the iron ions in these glasses is antiferromagnetic; 2) the introduction of Ni ions reduces the antiferromagnetic interaction between Fe ions.

Dielectric relaxation in glasses containing different relaxing species

Abstract Recent EXAFS, Mossbauer, ESR and other spectroscopic studies here indicated that the continuous random network model of glass as proposed by Zachariasen is not correct. These and other studies point towards a structure for oxide glasses in which modifier ions maintain the coordination of crystalline polymorphs and at higher concentrations molecular clusters appear in which two modifier ions share a bridging oxygen ion. It has been shown previously how ESR can be used to infer the existence of these ‘connected’ ions. In the present paper, data are given on macroscopic properties, dielectric constant and depolarisation current, to show how the relaxation parameters of the stretched exponential form of the depolarisation current are affected by the presence of two types of relaxing ions.

Effect of microwave fields on the germination period and shoot growth rate of some seeds

Electromagnetic fields in the microwave frequency have been used to sort good seeds from the bad ones and the technique makes use of the absorptive component of the dielectric constant of the water molecules material in this frequency region. On the other hand, microwaves are also used for accelerating the culture growth of a bacterial medium. One theory holds that the beneficial effects of microwave derive principally from the local temperature enhancement. The exact bio-chemical mechanism by which microwaves affect the functioning of a living organisms are not very well understood and the mechanism may vary depending upon the amplitude of the field and its frequency and, in the general case, on the duty cycle. We have started our research in this field by focussing on small organisms that can be easily submitted to the well characterised microwave fields. Another motivating factor is if the effect is beneficial it can be easily implemented. In this communication we describe a set-up and methodology that we have used to study the effect of microwave fields in the GHz region, more precisely 2.45 GHz fields; this band of frequencies is used in household microwave appliances.

Growth of the Bi-2223 phase after a short nucleation stage at high temperature

Abstract A new two-step annealing procedure is investigated in order to accelerate the kinetics of transformation and reaction: a short stage at high temperature (870°C), to induce nucleation of the high-transition temperature phase “2223”, is followed by a second stage at lower temperature (830°C) to promote its growth. Magnetic susceptibility measurements on fibres submitted to this heat treatment cycle show a transition onset to superconductivity state at temperatures higher than that observed for samples annealed without nucleation stage. The increase of Tc values are due to the early development of the “2223” phase. Electrical resistance measurements corroborate that this new two-step procedure is an alternative process to develop the “2223” phase without the detrimental effect of liquid-phase formation.

Effect of microwave fields on the growth rate and mitosis of a single cell in spirogyra algae

There is a growing concern about the effect of electric and magnetic fields in the microwave range frequencies on human health. The common microwave oven emits radiation in the frequency region of 2.45 GHz. On the other hand RF radiation (~27 MHz to 1 GHz, microwave diathermy) is beneficially used in physiotherapy for muscular relaxation. There are weapons based on microwave beams and microwaves are also used to accelerate the culture of some bacteria. In most of the cases the exact mechanism of effect of microwave fields on cells and organs (network of cells) is not known. Though this absence of clear understanding has not prevented the ever increasing discovery of many empirical effects and multiple uses, it seems there is a clear need of carefully collected data to delimit the range of two relevant parameters, amplitude and frequency, that may affect the normal functioning of a cell, an organ or an organism. The effect on an organ may be due to effect on the cells and due to effect on the inter-cell junctions and chemical transporters. We have started a systematic investigation of the effect of microwave fields on simple organisms whose cells are easy to see and analyse in-vivo by optical spectral microscopy and that can be easily subjected to microwave fields of known and variable strength of a given frequency. We have chosen common filamentary algae as the first target of our research for the following reasons: (1) the culture protocol of algae is easy to follow and implement; (2) the structure of this species is simply a one dimensional line-up of identical cells with each cell having common cell-walls with its two nearest neighbours; and (3) the two important parameters that characterise the functioning of cell, i.e., the growth rate and cell-division (mitosis) rate can be measured through quantitative imaging and image analysis. In this communication we describe the method of culturing the cells, the equipment details and some preliminary results.