Microwave Absorbing by Conducting Hybrid Nanocomposites Based on Magnetite Nanoparticles

Microwave absorbing properties of hybrid nanocomposites of Fe3O4 nanoparticles with carbon nanotubes and polyaniline (PANI) doped with dodecylbenzenesulfonic acid have been studied. It has been found that the the addition of nanostructured components increases the absorbtion of the nanocomposites. Moreover, CNTs affect the absorbtion in high frequency range, and Fe3O4 nanoparticles – in low frequency range. Introduction of conductive PANI into the matrix enhances the influence of nanomaterials on the absorbance of the films. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3491

Molecular Dynamics Study of the Primary Ferrofluid Aggregate Formation

Investigations of the phase transitions and self-organization in the magnetic aggregates are of the fundamental and applied interest. The long-range ordering structures described in the Tom\'anek's systematization (M. Yoon, and D. Tom\'anek, 2010 [1]) are not yet obtained in the direct molecular dynamics simulations. The resulted structures usually are the linear chains or circles, or, else, amorphous (liquid) formations. In the present work, it was shown, that the thermodynamically equilibrium primary ferrofluid aggregate has either the long-range ordered or liquid phase. Due to the unknown steric layer force and other model idealizations, the clear experimental verification of the real equilibrium phase is still required. The predicted long-range ordered (crystallized) phase produces the faceting shape of the primary ferrofluid aggregate, which can be recognized experimentally. The medical (antiviral) application of the crystallized aggregates has been suggested. Dynamic formation of all observed ferrofluid nanostructures conforms to the Tom\'anek's systematization.Comment: Revised in the JMM

Ultra-high field transport in GaN-based heterostructures

This paper describes measurements of the velocity of electrons at electric fields up to 100 kV/cm in GaN/AlGaN heterostructures. In order to avoid the Joule heating effect, a pulse technique with a time sweep of 10-30 ns was used. The experimental results indicate that overheating of the 2DEG does not exceed 1000 K in this electric field range and drift velocity as high as ~10⁷ cm/s was obtained. Additionally, the low frequency 1/f noise spectra measured for a different bias voltage are analyzed with respect to field-induced contribution of hopping conductivity in AlGaN barrier region

Нелінійність дифузійних резисторів при струмі високої щільності

У роботі представлено результати експериментального дослідження залежності опору дифузійних резисторів (ДР) від струму, виготовлених за технологією "кремній з діелектричною ізоляцією", з різними геометричними характеристиками, зокрема, довжиною та товщиною, при густині струму до 105 A/см2. Проведено аналіз отриманих результатів і визначено три області на кривих залежності опору резистора від струму R(I). Перша область – омічна ділянка, на якій значення опору дифузійного резистора лінійно залежить від струму. Друга область кривої R(I) характеризується наявністю сильної нелінійності та стрибками і різким збільшенням опору ДР, що пов'язано із виникненням області високого електричного поля в ДР. Третя область залежності R(I) характеризується зменшенням величини опору від струму. Чим товщим є ДР, тим менше пікове значення опору. Показано, що зміна довжини і зменшення товщини дифузійних резисторів призводять до зміни протяжності лінійної ділянки залежності R(I). Чим менша довжина, тим менший опір зразка і тим довша ділянка сталого диференційного опору. Із залежності диференціального опору від струму визначено області лінійності опору дифузійного резистора. Область лінійності опору була визначена як область, де зміна диференціального опору не перевищувала 10 % від його значення при малому електричному полі: найбільша область лінійності резистора має місце в зразку довжиною 2.4 мкм та товщиною 8.4 мкм. Виявлені особливості поведінки опору дифузійного резистора пояснюються зміною його фізичних характеристик в результаті значного самонагрівання.The paper presents the results of an experimental study of the current dependence of the resistance of diffusion resistors (DR) produced by the "silicon with dielectric insulation" technology, with different geometric characteristics, in particular, length and thickness, with a current density of 105 A/cm2. The analysis of the obtained results is carried out and three areas on the dependence of resistance on current R(I) are determined. The first region is an ohmic plot, on which the resistance value of the diffusion resistor is linearly dependent on the current value. The second region of the curve R(I) is characterized by the presence of strong nonlinearity, jumps and a sharp increase in the resistance of the DR, which happen due to the emergence of a high electric field in the DR. The third region on R(I) is characterized by a decrease in the value of the current resistance: the thicker the DR, the lower the peak value of the resistance. It is shown that the change in the length and the decrease in the thickness of the diffusion resistors lead to a change in the length of the linear region on R(I): the smaller the length, the lower the resistance of the sample and the longer the area of stable differential resistance. The region of the linearity of the diffusion resistor is determined on the basis of the differential resistance on the current dependence. The linearity of the resistance was defined as the region where the change in the differential resistance did not exceed 10 % of its value at some small electric field: the largest region of the linearity of the resistor is present in a specimen with a length of 2.4 μm and a thickness of 8.4 μm. The revealed characteristics of the behavior of the resistance of the diffusion resistor are due to the change in its physical characteristics as a result of significant self-heating

The behavior of magnetic nanoparticles in liquid in magnetic field

The behavior of magnetic nanoparticles in a liquid free of anti-coagulants in magnetic field has been studied in two container types (round and flat). Application of a specifically configured gradient magnetic field to the magnetic nanoparticles in a round capillary makes it possible to form a periodic structure consisting of needle-like clusters of the magnetic particles. In a flat container, a fairly homogeneous distribution of clusters is observed. In some conditions, clusters of larger size (mega-clusters) appear within a two-dimensional ensemble of the needle-like clusters. The mega-clusters are of dumbell-like shape with asymmetric (presumably spiral) constriction

1/f noise and mechanisms of the conductivity in carbon nanotube bundles

Experimental results are reported of the investigation of conductivity mechanisms in metallic single-wall carbon nanotube (SWCNT) bundles in a wide temperature range from 4.2 K to 300 K. The temperature dependence of the resistance and noise parameters – the logarithmic slope of the current dependence of noise as well as the normalized current noise – are compared. Remarkable changes in noise characteristics are registered at temperatures typical of the transition from hopping conductivity to Luttinger liquid conductivity and the transition from Luttinger liquid conductivity to diffusion conductivity. In the first transition region, the slope of the normalized noise level of the current changes significantly as a function of temperature. In the region of diffusion conductivity, a stronger variation of the normalized noise level is revealed. These changes in noise properties are correlated with changes in the transport characteristics of SWCNT bundles that allow us to adequately explain the mechanisms of conductivity in the system.