Investigation of the non-linear loss properties of magnetic fluids subject to large alternating fields

The non-linear relaxational properties of a water-based magnetic fluid are investigated by means of measuring the frequency dependent complex susceptibility, χ(ω) in the presence of an external potential. The results obtained are discussed in terms of the magnetic analogue of the Coffey and Paranjape model as modified by Déjardin. The loss processes in the magnetic fluid in the non-linear region are investigated in the context of the loss tangent, tan(δ), and the power dissipation per unit volume. These measurements are of importance because of the continuing interest in the clinical applications of magnetic fluids, where large alternating magnetic fields can be applied to magnetic fluids to induce loss processes and heating effects, often driving the magnetic fluid into the non-linear region of magnetisation. We evaluate the increment of the susceptibility, Δχ, due to the non-linear response, through measurement of χ(ω), and extend this formulism to the non-linear increment of the loss tangent, Δ tan(δ) and the increment of the heating rate, ΔU_heat.     

Characteristic times of relaxation peaks of magnetic fluids

The use of the Fokker–Planck equation in theoretical studies of relaxation processes in single-domain particle systems is a well-established technique. However this method has a particular disadvantage in that it can give analytical results solely in some limiting cases. An alternative method, which avoids this difficulty, is that of the use of the integral relaxation time, τ_int, which is presented in this work.     

ESI‐MS/MS of expanded porphyrins: a look into their structure and aromaticity

Electrospray mass spectrometry/mass spectrometry was used to investigate the gas‐phase properties of protonated expanded porphyrins, in order to correlate those with their structure and conformation. We have selected five expanded meso‐pentafluorophenyl porphyrins, respectively, a pair of oxidized/reduced fused pentaphyrins (22 and 24 π electrons), a pair of oxidized/reduced regular hexaphyrins (26 and 28 π electrons) and a regular doubly N‐fused hexaphyrin (28 π electrons). The gas‐phase behavior of the protonated species of oxidized and reduced expanded porphyrins is different. The oxidized species (aromatic Hückel systems) fragment more extensively, mainly by the loss of two HF molecules. The reduced species (Möbius aromatic or Möbius‐like aromatic systems) fragment less than their oxidized counterparts because of their increased flexibility. The protonated regular doubly fused hexaphyrin (non‐aromatic Hückel system) shows the least fragmentation even at higher collision energies. In general, cyclization through losses of HF molecules decreases from the aromatic Hückel systems to Möbius aromatic or Möbius‐like aromatic systems to non‐aromatic Hückel systems and is related to an increase in conformational distortion. Copyright © 2016 John Wiley & Sons, Ltd.     

Microwave propagation parameters in magnetic fluids

Complex dielectric permittivity and complex magnetic permeability measurements of two magnetic fluids (as microwave propagation media), in the approximate range 0.2-5GHz were performed. The two samples consisted of magnetite nanoparticles, dispersed in kerosene and in water, respectively. Based on the dielectric and magnetic measurements, the frequency (f ) dependence of the attenuation parameter, , the phase constant, , the propagation constant, , the intrinsic impedance, Z_m, the refractive index, n , the reflection coefficient, R , the wavelength, and the skin depth, , of the investigated samples were determined.     

Microwave specific loss power of magnetic fluids subjected to a static magnetic field

This paper reports on the frequency dependence of the magnetic and electric power dissipation in a magnetic fluid sample, in the microwave frequency range (0.5 to 8GHz), at various values of the static magnetic field (0 to 167.8kA/m). The computation of the power dissipation relies on the experimental values measured for the complex dielectric permittivity, ɛ = ɛ′ - iɛ″, and the complex magnetic permeability, μ = μ′ - iμ″, over the same frequency range. The results show that the magnetic power dissipation is much larger than the electric one for the investigated sample. At a specific frequency, f (Hz) , the power dissipation, p, depends on the external magnetic field, and exhibits a maximum. The result obtained suggests the possibility of controlling the energy absorption in the microwave range by means of the application of an external magnetic field.