Magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: Magnetite-based magnetic liquids and suspensions

The magnetocaloric effect (MCE) and heat capacity of magnetite-based magnetic liquids and suspensions of magnetite in cyclohexane and water were studied calorimetrically at various temperatures and magnetic inductions. It was found that the magnetocaloric effect in the systems under study increases nonlinearly with the magnetic induction. In contrast to monocrystalline magnetite, the inverse temperature dependence was observed for the MCE in the nanosystems studied over the entire temperature range covered; i.e., the effect decreases with increasing temperature. It was found that the dependence of the specific heat on the magnetic induction passes through a maximum for all the systems at all temperatures tested; its height increases with the temperature. The extremal character of the dependence can be explained by the formation of chain structures of magnetite nanoparticles in the presence of a magnetic field.     

The magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: High-dispersity magnetite

The magnetocaloric effects of aqueous and ethanolic high-dispersity magnetite suspensions and the magnetite magnetic liquid were determined calorimetrically over the temperature range 15–80°C. The temperature dependence of the magnetocaloric effect of suspensions was evidence of the thermal oxidation of magnetite to maghemite. The temperature dependences of the magnetocaloric effect of the magnetic liquid passed extrema related to the second-order magnetic phase transition.     

Magnetocaloric effect and heat capacity of high-spin manganese complexes in a disperse state

Magnetothermal properties of high-spin chloro(2,3,7,8,12,13,17,18-octaethylporphyrinato)manganese(III), chloro(5,10,15,20-tetraphenylporphyrinato) manganese(III), bromo(5,10,15,20-tetraphenylporphyrinato)manganese(III), and (acetato)(5,10,15,20-tetraphenylporphyrinato)manganese(III) complexes as 6% water suspensions were determined by the microcalorimetric method at 298 K in a magnetic field of 0–1.0 T. It was established that when the magnetic field was applied, the temperature of the systems increases, leading to positive values of the magnetocaloric effect: the higher the magnetic field induction, the higher the values. It is shown that the dependences of the heat capacity of the complexes’ solid particles on the magnetic field induction are of an extreme nature with a heat capacity in the area above 0.6 T less than that in the zero field. The regularities of the dynamics of the numerical values of the change in enthalpy and magnetic entropy of the manganese complexes when a growing magnetic field was applied and the regularities of the influence of the acidoligand in pentacoordinated complexes on their magnetothermal properties were considered.     

A microcalorimeter for studying the magnetocaloric effect and the heat capacity in magnetic fields

The design of an automated microcalorimeter for determining the magnetocaloric effect and the heat capacity of suspensions and magnetic colloids in magnetic fields of from 0 to 1 T over a wide temperature range and the corresponding experimental procedure were described.     

Binary fluids in planar nanopores: Adsorptive selectivity, heat capacity and self-diffusivity

Monte Carlo and molecular dynamics simulations are performed to study fluid adsorption of a two component fluid in slit pores of nanoscopic dimensions. The slit pores are immersed in a binary fluid bath, which is comprised of spherical molecules having a size ratio of 1.43, at constant temperature and composition. Pore width is varied to determine how the heat capacity and self-diffusion coefficient are linked to the composition and structure of the adsorbed fluid. In pores where the fluid structure is most pronounced, we observe: perfect (or near perfect) exclusion of one component by the other component, a heat capacity that rapidly oscillates and is of greater magnitude than in the fluid bath, and self-diffusion coefficients on the order of 10^–8 cm²/s. The behavior of the heat capacity and diffusion coefficients appears to arise from a near solid-like layering of OMCTS that occurs at certain favorable pore widths.     

Low temperature heat capacity and magnetic properties of UF3

The low temperature heat capacity of UF(3) has been measured using an adiabatic low temperature calorimeter in the temperature range from 10 to 350 K. These data are complemented at the lowest temperature region with data obtained with a Quantum Design PPMS-14 device in the temperature range from 0.5 to 20 K. Good agreement between both techniques has been found, and from these experimental results the absolute entropy of UF(3) at 298.15 K has been determined as 126.8 ± 2.5 J K(-1) mol(-1). On the basis of the specific heat data and the magnetization measurements performed on a SQUID device, a transition at 1.59 K attributed to Curie temperature of a ferromagnetic transition has been found in this study. This observation makes UF(3) a unique compound with an unusually low ferromagnetic ordering temperature.     

Destabilizing effect of time-dependent oblique magnetic field on magnetic fluids streaming in porous media

The present work studies Kelvin-Helmholtz waves propagating between two magnetic fluids. The system is composed of two semi-infinite magnetic fluids streaming throughout porous media. The system is influenced by an oblique magnetic field. The solution of the linearized equations of motion under the boundary conditions leads to deriving the Mathieu equation governing the interfacial displacement and having complex coefficients. The stability criteria are discussed theoretically and numerically, from which stability diagrams are obtained. Regions of stability and instability are identified for the magnetic fields versus the wavenumber. It is found that the increase of the fluid density ratio, the fluid velocity ratio, the upper viscosity, and the lower porous permeability play a stabilizing role in the stability behavior in the presence of an oscillating vertical magnetic field or in the presence of an oscillating tangential magnetic field. The increase of the fluid viscosity plays a stabilizing role and can be used to retard the destabilizing influence for the vertical magnetic field. Dual roles are observed for the fluid velocity in the stability criteria. It is found that the field frequency plays against the constant part for the magnetic field.     

On the theory of birefringence in magnetic fluids

The influence of homogeneous correlations between the positions and orientations of ferroparticles on the effect of light birefringence in magnetic fluids has been studied. It has been demonstrated that, for typical magnetic fluids, the optical effects associated with the homogeneous correlations can be stronger than the effects caused by elongated primary aggregates formed at the stage of ferrofluid synthesis, as well as heterogeneous chains resulting from magnetic attraction between the largest particles of a magnetic fluid.     

Low temperature heat capacity and magnetic studies on DyCu5

Low temperature heat capacity studies on DyCu₅ revealed a λ-anomaly at 6.55 K. Evaluation of the entropy indicated that the ground state is not (2J + 1) fold degenerate. High field magnetization data yield a moment of 9.28 μ_B at 4.2 K and 120 kOe.     

Magnetocaloric effect in La1.25Sr0.75MnCoO6

The magnetocaloric properties of La_1.25Sr_0.75MnCoO₆ (LSCM) manganites that were synthesized at 750 and 1,300 °C have been investigated. It is found that magnetic entropy change distribution of the LSCM is much more uniform than that of gadolinium. This feature is desirable for an Ericson-cycle magnetic refrigerator. It is suggested that LSCM can be used in an active magnetic regenerative nitrogen liquefier to cool nitrogen gas from room temperature to 77 K.     

On the nonlinear rheology of magnetic fluids

The influence of chain aggregates on the rheological properties of magnetic fluids is studied theoretically. The dependence of effective viscosity on the shear rate of stationary flow is investigated. The character and the relaxation time of the viscosity of fluids after the jumpwise changes in the shear rate are determined.     

Dipole interparticle interaction in magnetic fluids

The temperature dependence of the saturation magnetization of a magnetite-based magnetic fluid has been directly measured with a vibrating-coil magnetometer equipped with a superconducting solenoid. The magnetization varies in accordance with the 1 ? αT ² law. Coefficient α = 1.4 × 10^?6 is almost twice as high as that of monolithic magnetite. The results of measuring the susceptibility of magnetic fluids stabilized with oleic and linoleic acids have been analyzed using novel corrections to the temperature dependence of particle magnetization. The susceptibility of ultimately concentrated samples is in good agreement with the Ivanov-Huke-Lücke and Morozov theories. The susceptibility of samples with a medium concentration is adequately described by the Ivanov theory alone. The susceptibility of low-concentrated samples increases to the level predicted by the Morozov theory in the case of particle aggregation. The widening of the particle size distribution leads to a reduction in the level of the interparticle interactions.     

Morphology and thermodynamics of selenium-containing nanosystems: The effect of polymer stabilizers

Nanosystems based on zero-valent selenium and biocompatible polymer stabilizers (polyvinylpyrrolidone with molecular weight (MW) М _w = (10–55) × 10³, poly-N,N,N,N-trimethylacryloyloxyethylammonium methylsulfate with М _w = (30–250) × 10³ and polyethylene glycol with М _w = (1–40) × 10³) are studied by means of static and dynamic light scattering, and the resulting data are compared. Dense spherical multimolecular nanosystems are found to be formed. Morphological and thermodynamic characteristics of selenium-containing nanosystems, depending on the nature and MW of the polymer stabilizer, are determined. It is shown that the properties of nanosystems can be adjusted by varying the molecular weight of the polymer stabilizer.     

Different methods for the fractionation of magnetic fluids

 Magnetic fluids are used in many fields of application, such as material separation and biomedicine. Magnetic fluids consist of magnetic nanoparticles, which commonly display a broad distribution of magnetic and nonmagnetic parameters. Therefore, upon application only a small number of particles contribute to the desired magnetic effect. In order to optimize magnetic fluids for applications preference is given to methods that separate magnetic nanoparticles according to their magnetic properties. Hence, a magnetic method was developed for the fractionation of magnetic fluids. Familiar size-exclusion chromatography of two different magnetic fluids was carried out for comparison. The fractions obtained and the original samples were also magnetically characterized by magnetic resonance and magnetorelaxometry, two biomedical applications. The size-exclusion fractions are similar to those of magnetic fractionation, despite the different separation mechanisms. In this respect, magnetic fractionation has several advantages in practical use over size-exclusion chromatography: the magnetic method is faster and has a higher capacity. The fractions obtained by both methods show distinctly different magnetic properties compared to the original samples and are therefore especially suited for applications such as magnetorelaxometry. Received: 12 July 1999/Accepted in revised form: 9 November 1999     

Review of heat transport properties of solar heat transfer fluids

The present article reviews the test techniques for some of the important heat transport properties of oils such as viscosity, density, specific heat capacity and thermal conductivity mainly used for characterization of heat transfer fluids. It can be seen that while density of oils can be tested at higher temperatures, the other heat transport properties of oils like viscosity, specific heat capacity and thermal conductivity have a limitation of being tested at low temperatures below 100–150 °C. While quite a few number of researchers have reported evaluation of heat transfer properties like specific heat capacity and thermal conductivity of oils by different methods, there remains a huge scope of debate and discussions on the repeatability and reproducibility of such tests, especially in case of oils used in high-temperature applications. A lot of insight has been gathered with respect to testing of thermal conductivity of oils, and several common test methods have been compared with each other. Lastly, two mathematical models, reported in the literature in open domain, have been reviewed and compared with each other. If the oils are to be used at elevated temperatures, like heat transfer fluids used in concentrated solar power generation where temperatures go as high as 400 °C and beyond, there is an urgent need to standardize a laboratory test method for performance evaluation of heat transport properties, which can help in formulating new generation oils based on novel chemistries and technologies like nanofluids, synthetic oils of novel chemistries, molten salts and molten metals.     

Modeling the heat flow and heat capacity of modulated differential scanning calorimetry

Modulated differential scanning calorimetry (MDSC) uses an abbreviated Fourier transformation ?r the data analysis and separation of the reversing component of the heat flow and temperature signals. In this paper a simple spread-sheet analysis will be presented that can be used to better understand and explore the effects observed in MDSC and their link to actual changes in the instrument and sample. The analysis assumes that instrument lags and other kinetic effects are either avoided or corrected for.     

Low-temperature susceptibility of concentrated magnetic fluids

The initial susceptibility of concentrated magnetic fluids (ferrocolloids) has been experimentally investigated at low temperatures. The results obtained indicate that the interparticle dipole-dipole interactions can increase the susceptibility by several times as compared to the Langevin value. It is shown that good agreement between recent theoretical models and experimental observations can be achieved by introducing a correction for coefficients in the series expansion of susceptibility in powers of density and aggregation parameter. A modified equation for equilibrium susceptibility is offered to sum over corrections made by Kalikmanov (Statistical Physics of Fluids, Springer-Verlag, Berlin, 2001) and by B. Huke and M. Lucke (Phys. Rev. E 67, 051403, 2003). The equation gives good quantitative agreement with the experimental data in the wide range of temperature and magnetic particles concentration. It has been found that in some cases the magnetic fluid solidification occurs at temperature several tens of kelvins higher than the crystallization temperature of the carrier liquid. The solidification temperature of magnetic fluids is independent of particle concentration (i.e., magneto-dipole interparticle interactions) and dependent on the surfactant type and carrier liquid. This finding allows us to suggest that molecular interactions and generation of some large-scale structure from colloidal particles in magnetic fluids are responsible for magnetic fluid solidification. If the magnetic fluid contains the particles with the Brownian relaxation mechanism of the magnetic moment, the solidification manifests itself as the peak on the "susceptibility-temperature" curve. This fact proves the dynamic nature of the observed peak: it arises from blocking the Brownian mechanism of the magnetization relaxation.     

Determination of maximum particle size in magnetic fluids

Higher-order moments of particle size distribution functions are determined for magnetic fluids from analysis of initial segments of magnetization curves. It is shown that the higher-order moments calculated using approximation of real particle size distributions by the Γ distribution are strongly overestimated. Agreement between the measured and calculated moments can be radically improved by truncating maximum particle size X_max. A relation between X_max and the parameters of the Γ distribution is proposed taking into account the degree of polydispersity of a magnetic fluid. Namely, the ratio between the maximum and most probable particle diameters is equal to the ratio between the mean-square magnetic moment of a particle and its squared average magnetic moment.     

Time-dependent heat capacity in the glass transition region

Time-dependent, apparent heat capacities of glucose, poly(vinyl chloride), polystyrene, selenium, poly(methyl methacrylate), and poly(2,6-dimethyl-1,4-phenylene ether) in the glass transition region were determined by differential thermal analysis. The thermal history was set by linear cooling at rates between 0.007 and 160°C/min. Linear heating for analysis was carried out at rates between 0.3 and 600°C/min. Average activation energies of 52, 81, 90, 54, 77, and 108 kcal/mole, respectively, were evaluated by using the hole theory of glasses previously developed. Within experimental limitations all data could be described quantitatively by the theoretical expressions using only one parameter, the number of frozen-in holes, to described the thermal history. Experimental and theoretical limitations are discussed.     

Estimation of magnetic particle clustering in magnetic fluids from static magnetization experiments

The volume fraction dependence of the static magnetization of two magnetic fluids with different degrees of steric stabilization was measured at low field values (0-10 kA/m) and it was found to be nonlinear for both magnetic fluids. The nonlinearity is more pronounced in the case of the less stabilized magnetic fluid. The experimental data were processed by nonlinear regression using an analytical model for the formation of chain-like magnetic particle aggregates in magnetic fluids. The calculated dependence on the degree of steric stabilization, magnetic field, and sample concentration of the mean number of particles per chain was in the range (1-1.04).