Sintering and magnetic properties of Co-Nb ferrites

A series of samples of the system Co Nb_x Fe_2-x O₄ (x=0, 0.2, 0.4, 0.6 and 0.8) are prepared by the usual ceramic technique. X-ray analysis shows that they are cubic spinel (single phase). The lattice parameter, theoretical densityD _x, bulk densityD and the porosityP are measured for the samples. The magnetic susceptibility is measured and conducted with the additions of niobia. Results were evaluated on the bases of the exchange electrons at the octahedral sites.

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Zusammenfassung Mittels herkömmlicher keramischer Verfahren wurde eine Reihe von Proben des Systemes Co Nb_xFe_2-xO₄ (mitx=0, 0.2, 0.4, 0.6 und 0.8) gefertigt. Röntgenographische Untersuchungen zeigen ein kubisches Spinell (single-phase). Gitterkonstanten, theoretische Dichte D_x, Raumdichte D und das relative Porenvolumen der einzelnen Proben wurden bestimmt. Die magnetische Suszeptibilität wurde bestimmt. Die Ergebnisse wurden auf der Grundlage von Elektronenübergängen an den oktaedrischen Stellen gewertet.

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The author is indebted to Prof. Dr. A. Tawfik and Dr. G. Gaballa for their helpful discussions and cooperation during this work.     

Effect of highly filled ferrites on non-isothermal crystallization behavior of polyamide 6 bonded ferrites

The aim of this study is to characterize the non-isothermal crystallization of polyamide 6 bonded highly filled ferrites which were prepared by the melt extrusion. Especially, the effect of ferrite concentration and its surface property on the non-isothermal crystallization were investigated by means of differential scanning calorimetry. The highly filled ferrite particles acting as obstacles could severely hinder the motion of surrounding chain segments, which were irrespective of surface nature. The ferrite could be modified by silane and obtain a visually enhanced interaction with polymer matrix which evoked the heterogeneous nucleation. Increasing this enhanced interfacial area between polymer-particle can promote the heterogeneous nucleation. However, a strong interaction can slow the motion of surrounding chain segments of particles, thereby producing a competitive effect on the crystallization rate and crystallinity. The plot of crystallization activation energy against concentration also can evaluate dispersion performance of hydrophilic fillers within hydrophobic polymer.     

Thermal conductivity of the ferrites Ni0.65Zn0.35CuxFe2−xO4

A series of samples of the type Ni_0.65Zn_0.35Cu_xFe_2?xO₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by a ceramic technique. The existence of a single phase was confirmed by X-ray studies. The thermal conductivity and specific heat were measured at 53°C. The thermal conductivity had a minimum value atx=0.3, due to the maximum porosity at this composition. The phonon frequency was estimated to have an optimum value at x=0.3, due to the increase in phonon scattering.     

Effect of Sm3+ substitution on dc electrical resistivity and magnetic properties of Ni–Co ferrites

Sm³⁺ substituted Ni–Co ferrite materials have been developed with the chemical compositions Ni_0.5Co_0.5Fe_2-xSm_xO₄ for x = 0.0, 0.02, 0.04, and 0.06 by solid-state method. A spinel with a single-phase cubic structure was found through XRD analysis. Crystallite size and lattice parameter fluctuation with Samarium content, decreasing with Sm³⁺ doping. When Sm³⁺ is doped, SEM shows a uniform distribution of grains with increasing agglomeration, and the grain size decreases with samarium concentration. Two peaks of absorption mode FTIR spectra show their spinel nature. The samples' negative temperature coefficient measured by the two probe method indicates their semiconducting nature. The DC electrical resistivity of the Ni–Co ferrite system was found to increase from 5.99 × 10³ ($\text{Ω}$-cm) to 9.89 × 10⁹ ($\text{Ω}$-cm) as an increase in Sm concentration. VSM measurement gives the saturation magnetization for the concentration up to x = 0.04 decreased and then increased for x = 0.06. All these properties show our samples' higher applicability in various electronic devices.     

Investigation of structural and magnetic properties of nanocrystalline manganese substituted lithium ferrites

Nanocrystalline manganese substituted lithium ferrites Li_0.5Fe_2.5−xMn_xO₄ (2.5≤x≥0) were prepared by sol-gel auto-combustion method. X-ray diffraction patterns revealed that as the concentration of manganese increased, the cubic phase changed to tetragonal. Magnetic properties were measured by hysteresis loop tracer technique. All the compositions indicated ferrimagnetic nature. The surface morphology of all the samples was studied by using scanning and transmission electron microscopy. The substitution of manganese ions in the lattice affected the structural as well as magnetic properties of spinels.     

Effect of dose on radiation-induced conductivity in polymers

Numerical simulation of radiation-induced conductivity in polymers upon long-term irradiation on the basis of the generalized Rose-Fowler-Vaisberg model, which allows for both dipolar carrier transport and generation of radiation traps during irradiation, was performed. The unusual properties of radiation-induced conductivity, such as the appearance of a maximum on current transients, the absence of a steady state, and a substantial difference between these curves for the first and subsequent irradiation, are rationalized in terms of the formation of free radicals, the major feature of radiolysis in the chemical aspect. This interpretation does not require the involvement of degradation or crosslinking processes, unlike other interpretations that appear in the literature. With the use of low-density polyethylene as an example, it was shown that radiation-induced conductivity both upon pulse and continuous irradiation can satisfactorily be described with the unified set of parameters of the generalized Rose-Fowler-Vaisberg model.     

Structural and magnetic properties of Zn-substituted cobalt ferrites prepared by co-precipitation method

Zn substituted cobalt ferrite spinels with the general formula Zn(x)Co(1-x)Fe(2)O(4) (with x varying from 0 to 0.5) were synthesized by a co-precipitation method and calcined at 500 °C and 800 °C. It was found that Zn substitution has a big effect in decreasing the Curie temperature (T(c)), from around 440 °C for the undoped sample to ~180 °C with x = 0.5. However, these values were also strongly affected by the pre-calcination temperature of the samples, thus T(C) shifts from ~275 °C for the x = 0.3 sample to ~296 °C after calcination at 500 °C and 800 °C respectively. These effects are due to facilitation of demagnetisation by substitution of the non-magnetic Zn ions and by production of very small nanoparticles. The latter are removed by higher temperature calcinations and so T(C) increases.     

Effect of pressure on the thermal conductivity of polymers

The thermal conductivity of polyolefins and halogen-substituted polymers was studied in a broad temperature interval spanning both solid and melt states, in the range of pressures from 0.1 up to 100 MPa with the aid of a high-pressure-calorimeter in the continuous heating regime. Treatment of data on the pressure dependence of the thermal conductivity of melts in terms of Barker's equation yielded the values of quasilattice Grueneisen parameter _B which exhibited the same dependence on molecular structure of a polymer as the parameter 3C/p from the Simha-Somcynsky equation of state (number of external degress of freedom per chain repeat unit). Analysis of the dependence of the thermal conductivity of polyethylene on the degree of crystallinity revealed the inadequacy of the current two-phase model which does not account for the microheterogeneity of the amorphous phase. It was concluded that interchain heat transfer makes the dominant contribution to the thermal conductivity of polymers both in amorphous and in crystalline states.

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Zusammenfassung Mit Hilfe eines Hochdruck-- Kalorimeters mit kontinuierlicher Aufheizung wurde im Druckintervall 0,1 bis 100 MPa und in einem breiten Temperaturbereich, in den sowohl feste als auch flüssige Zustände gehören, die Wärmeleitfähigkeit von Polyolefinen und halogenierten Polymeren untersucht. Drückt man die Druckabhängigkeit der Wärmeleitfähigkeit der Schmelzen mit Hilfe der Barkerschen Gleichung aus, erhält man die Werte für den Quasigitter Grueneisen-Parameter_b, der die gleiche Abhängigkeit von der Molekular-struktur eines Polymers zeigt, wie der Parameter 3C/p aus der Gleichung von Simha-Somcynsky (Zahl der externen Freiheitsgrade geteilt durch Kettenstruktureinheit). Eine Untersuchung der Abhängigkeit der Wärmeleitfähigkeit von Polyethylen von Kristallinitäts-grad zeigt die Mängel dieses Zwei-Phasen-Modelles, was die Mikroheterogenität der amorph-en Phase nicht erklärt. Man zog die Schlußfolgerung, daß ein Wärmetransport zwischen den Ketten sowohl im amorphen als auch im kristallinen Zustand den entscheidenden Beitrag zur Wärmeleitfähigkeit von Polymeren liefert.

     

Effect of strain on the thermal conductivity of solids

We present a systematic study of the effect of strain (equivalent to uniform pressure) on the thermal conductivity of an insulating solid. Following a theoretical analysis that uncovers the dependence of the thermal conductivity on temperature and strain, we present classical molecular dynamics calculations of the thermal conductivity. We find that the molecular dynamics results closely match the theoretical result.     

Cation distribution and magnetic properties in chromium-substituted nickel ferrites prepared using aerosol route

Cation distribution have been investigated using X-ray diffraction, magnetic and Mössbauer spectral studies in chromium-substituted nickel ferrites prepared by aerosol route. Cation distribution indicates that the chromium atom occupy octahedral site upto x=0.8, and then also enters into tetrahedral site. The saturation magnetization decreases linearly with the increase of chromium concentration due to the diamagnetic nature of the Cr³⁺. However, interesting behaviour is observed in the coercivity. Initially it increases slowly with the chromium concentration but when x>0.8 a very large increase has been observed. This was attributed to the specific cation distribution of Cr³⁺ which results an unquenched orbital angular momentum and a large anisotropy. Room temperature Mössbauer spectra of as obtained samples exhibited a broad doublet resolved into two doublets corresponding to the surface and internal region atoms. The samples annealed at 1200 °C show broad sextets, which were fitted with different sextets, indicating different local environment of both tetrahedral and octahedral coordinated iron cation.     

Nuclear magnetic relaxation study of the establishment of order in the ammonium halides

The temperature dependence and thermal hysteresis of proton and deuteron spin—lattice relaxation times at high fields and a study of the recovery of the proton and deuteron magnetizations in ammonium halides are reported. The anomalies occurring near the order—disorder phase transition are related to structural, dielectric and Raman data. The hysteresis effect and the establishment of long range order are shown to be related.     

Glass crystallization synthesis of ultrafine hexagonal M-type ferrites: Particle morphology and magnetic characteristics

This review concerns the synthesis and functional properties of ultrafine particles of M-type hexagonal ferrites prepared by the most advanced process of oxide glass crystallization. Hexaferrite phase formation during the heat treatment of multicomponent oxide glasses of various chemical compositions containing boron and/or silicon oxides as glass formers is considered. This route is useful to prepare assemblies of single-crystal strontium barium hexaferrite particles in the range of average particle sizes from tens of nanometers to several micrometers. The resulting glass ceramics and magnetic particle assemblies recovered from them are characterized by high coercive forces, approaching the theoretical limit for such compounds, and high magnetizations, close to the magnetization value for coarse-grained materials.     

Enhanced conductivity and magnetic ordering in isostructural heavy atom radicals

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P42(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E2---E2' contacts. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E2 position, with sigma(300 K) reaching a maximum (for E1 = E2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E(act) follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E2 position, order ferromagnetically, with Curie temperatures of T(c) = 12.8 and 17.0 K, respectively. The coercive fields H(c) at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Hückel Theory band structure calculations.     

Structural phase transitions and magnetic order in SrTcO3

The structure of the perovskite SrTcO(3) has been investigated using both synchrotron X-ray and neutron powder diffraction. At room temperature SrTcO(3) is orthorhombic as a consequence of cooperative tilting of the corner sharing TcO(6) octahedra. The tilts are sequentially removed as the sample is heated with the oxide displaying the sequence of structres Pnma→Imma→I4/mcm→Pm ?3m. Neutron powder diffraction data collected in the temperature range 4-1023 K indicate that SrTcO(3) has G-type antiferromagnetic structure, in which each Tc moment is antiparallel to its six nearest neighbours, below ～1000 K. The magnetic structure is collinear antiferromagnetic with the technetium moments parallel to c-axis and can be described by the propagation vector k = [0,0,0] and the basis vector (0,0,A(z)). The same magnetic structure is observed in each of the four crystal structures.     

Electric conductivity of polymer films filled with magnetic nanoparticles

The conductivity of polymer composites with magnetic nanoparticles (MNP) containing magnetite and other MNP (Ni, Cu–Ni) in the layers and planar cells with Al electrodes is studied. For soluble polymers (polyvinylpyrrolidone and polyvinyl alcohol) containing 1–10 wt % of magnetite MNP, a substantial effect of MNP on surface conductivity is detected over a wide range (from 10^–10 to 10^–3 Ω^–1). It is shown that the addition of magnetite MNP not only results in a considerable change in cell conductivity, but also leads to its partially irreversible variation (by an order of magnitude or more) via minor modifications of the experimental conditions (temperature, electric field). For high-resistance samples with low probabilities of conducting chain formation, temperature current peaks are observed upon moderate heating (up to 350 K). These peaks are similar to the maxima observed upon polymer electret thermodischarges when the charges are captured by the deep centers associated with separate MNP or MNP aggregates. The type and position of the maxima are determined by the characteristics of the polymer matrix. For polyvinylpyrrolidone composites, the maxima are observed some time after heating (the echo effect). With composites based on solventborne polymers (polyalkanesterimides, soluble polyimide) and Ni, Cu–Ni MNP, no change in film conductivity measured electrophotographically is observed, due to the formation of a dielectric coating formed by polymer macromolecules adsorbed on the MNP surface. An explanation based on the possible formation of magnetic aggregates of magnetite MNP and conducting chains is proposed. Magnetic aggregation IPM is proposed as one way of controlling cell conductivity.     

Effect of copper nanoparticle aggregation on the thermal conductivity of nanofluids

The thermal conductivity of water and glycerol is investigated via the transient hot wire method by adding small amounts of copper nanoparticles to solutions. At a 0.2% copper nanoparticle concentration, the thermal conductivity coefficient rises to 25% for the Cu + glycerol system, and to 35% for Cu + water system. A mechanism and mathematical model for describing the nanoparticle aggregation effect on the thermal properties of nanofluids are proposed, based on an analysis of the accumulated experimental data. It is shown that the enhancement of nanofluid thermal conductivity at low nanoparticle concentrations is directly proportional to their volume fraction and thermal conductivity coefficient, and (in accordance with the literature data) is inversely proportional to the radius and the aggregation ratio. The proposed model describes the existing experimental data quite well. The results from this work can be applied to the rapid cooling of electronic components, in the power engineering for ensuring the rapid and effective transfer of thermal energy in a nuclear reactor, and in the oil industry for thermal stimulation.