Search

This paper presents a numerical investigation of lubricating slider bearings with conducting couple stress fluids using externally applied magnetics fields. The modified two-dimensional magnetohydrodynamic couple stress Reynolds-type equation is obtained. This governing equation is resolved numerically by using finite difference scheme, which involves the Gauss–Seidel method to compute the bearing characteristics. Numerical results using different considered values of the couple stress and Hartman number are presented. These results demonstrate that the transverse magnetic field and couple stress effects are significant.

A three-dimensional thermohydrodynamic model is developed to predict non-Newtonian lubricant behavior in slider bearings ,and channel flow. The generalized Reynolds equation is established using the concept of generalized Newtonian fluids (GNF) and the temperature field is determined with the energy equation. The chosen rheological models are the power-law, Bingham, and Hershel-Bulkley models. The last two models hold uniformly in yielded and unyielded regions using the approach proposed by Papanastasiou. The results present the evolution of the velocity, pressure, and thermal fields. The power loss, load capacity, and friction coefficient are analyzed. Comparisons are made with Newtonian lubricants and other recent non-Newtonian computational analyses. [DOI: 10.1115/1.2736732] Keywords: non-Newtonian lubricants, thermohydrodynamic lubrication, slider bearing flow, channel flow

The La0.8Na0.1MnO3 oxide was prepared by the solid-state reaction and annealed in air. The X-ray diffraction data reveal that the sample is crystallized in a rhombohedral structure with R3c space group. Magnetic study shows a second-order magnetic phase transition from ferromagnetic to paramagnetic state at the Curie temperature TC = 295 K. In addition, the magnetizations as a function of temperature and the magnetic field is used to evaluate the magnetic entropy change ΔSM. Then, we have deduced that the La0.8Na0.1MnO3 oxide has a large magnetocaloric effect at room temperature. Such effect is given by the maximum of the magnetic entropy change ΔSMmax=5.56, and by the Relative cooling power (RCP) factor which is equal to 235 under a magnetic field of 5 T. Moreover, the magnetic field dependence of the magnetic entropy change is used to determine the critical exponents β, γ, and δ which are found to be β=0.495, γ=1.083, and δ=3.18. These values are consistent with the prediction of the mean field theory (β=0.5, γ=1, and δ=3). Above all, the temperature dependence of electrical resistivity shows a metal-insulator transition at Tρ. The electrical resistivity decrease when we apply a magnetic field giving a magnetoresistance effect in the order of 60% at room temperature. [ABSTRACT FROM AUTHOR]

The La0.8Na0.1MnO3 oxide was prepared by the solid-state reaction and annealed in air. The X-ray diffraction data reveal that the sample is crystallized in a rhombohedral structure with R3c space group. Magnetic study shows a second-order magnetic phase transition from ferromagnetic to paramagnetic state at the Curie temperature TC = 295 K. In addition, the magnetizations as a function of temperature and the magnetic field is used to evaluate the magnetic entropy change ΔSM. Then, we have deduced that the La0.8Na0.1MnO3 oxide has a large magnetocaloric effect at room temperature. Such effect is given by the maximum of the magnetic entropy change ΔSMmax=5.56, and by the Relative cooling power (RCP) factor which is equal to 235 under a magnetic field of 5 T. Moreover, the magnetic field dependence of the magnetic entropy change is used to determine the critical exponents β, γ, and δ which are found to be β=0.495, γ=1.083, and δ=3.18. These values are consistent with the prediction of the mean field theory (β=0.5, γ=1, and δ=3). Above all, the temperature dependence of electrical resistivity shows a metal-insulator transition at Tρ. The electrical resistivity decrease when we apply a magnetic field giving a magnetoresistance effect in the order of 60% at room temperature. [ABSTRACT FROM AUTHOR]

A systematic investigation of the effect of Strontium (Sr) doping on the structure, magnetic behavior, and magnetocaloric properties of the manganite perovskites La 0.67 Ca 0.33 - x Sr x Mn 0.98 Ni 0.02 O 3 (LCSMNO) has been performed. The Sr content x takes three values, namely 0.15, 0.2, and 0.3. The sol–gel technique has been employed in the preparation of our compounds. The structural analysis has revealed that our samples exhibit a rhombohedral structure with R 3 ¯ c space group symmetry, close to ambient temperature. A second-order paramagnetic (PM) to ferromagnetic (FM) transition has been validated over the Curie temperature range TC between 319 K (x = 0.15) to 353 K (x = 0.3), based on Arrott's analysis with the behavior of the master curve. According to the calculations made, the maximal values of the magnetic entropy change (- Δ S M max ) resulting through Landau's theory and the ones obtained by the classic Maxwell relationship are very close. We find the value of (- Δ S M max ) is equal to 4.386, 4.469, and 4.250 J Kg−1 K−1, respectively, for x = 0.15, 0.2, and 0.3, when an external magnetic field μ0H = 5 T is applied. Based on the results of the study in this work, we find that controlled Sr doping adapts perfectly to the magnetic as well as the magnetocaloric properties of the LCMNO, which plays an important role in the implementation of many potential applications of the material in the field of near-ambient temperature magnetic refrigeration technology. [ABSTRACT FROM AUTHOR]

La0.8Ca0.2 - x[larger_open_square]xMnO3 (x = 0.00, 0.10, and 0.20) perovskite was prepared by the conventional solid-state reaction and annealed at 1473 K. X-ray diffraction and scanning electron microscopy shown the existence of a secondary phase attributed to the unreacted Mn3O4 oxide. The magneto transport properties have been investigated based on the temperature dependence of the resistivity ρ(T) measurements under several applied magnetic fields. We note that the La0.8Ca0.2MnO3 (x = 0.00) sample has a classical metal-insulator transition at Tρ. But we have observed that the lacunars samples (x = 0.10 and 0.20) include a metallic and insulator behavior simultaneously below Tρ and the resistivity is dominated by tunneling through the barriers associated with the insulating phase. In other words, the calcium deficiency favors the enhancement of the insulator behavior. The electrical resistivity is fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Furthermore, we found that the estimated results are in good agreement with experimental data. Above all, the resistivity dependence on the temperature and magnetic field data is used to deduce the magnetic entropy change. We have found that these magnetic entropy change values are similar to those calculated in our previous work from the magnetic measurements. Finally, we have found an excellent estimation of the magnetic entropy change based on the Landau theory. [ABSTRACT FROM AUTHOR]

CaCu2.8Ni0.2Ti4O12 ceramics were elaborated using the solid-state reaction technic. The pellets were annealed at 1000 °C for 24 h. The X-ray diffraction (XRD) analysis proves the main phase formation of our sample crystallize in the cubic structure with Im 3 ¯ space group. The diffuse reflectance analysis allow us to calculate the optical band gap energy which is equal to 3.172 eV. The dielectric properties of our compound were studied using complex impedance spectroscopy showing a lowering in dielectric loss (tan δ = 0.07) at 1 kHz and at room temperature. The impedance studies reveal the presence of temperature dependent dielectric relaxation. Thus, the electrical modulus studies show that the relaxation is associated with grain boundaries effects. The activation energy calculated from the electric modulus spectra, related to the electrical relaxation, is found to be 0.60 eV. This result suggests the hopping mechanism of oxygen vacancies produced at grain boundaries in relaxation processes. [ABSTRACT FROM AUTHOR]

The structural, magnetic and magnetocaloric properties of La 0.67 Ca 0.23 Sr 0.1 Mn 0.98 Ni 0.02 O 3 nano-crystalline manganite are investigated systematically. The compound is elaborated based on Pechini sol–gel method and then annealed at 1000 °C for 24 h. X-ray powder diffraction shows that, the sample crystallizes in a rhombohedral structure with R 3 ¯ C space group. A magnetization versus temperature study shows that the compound exhibits a second-order magnetic transition from a ferromagnetic to a paramagnetic state at a Curie temperature T C = 296 K. To evaluate the magnetic entropy change Δ S M , we measure the magnetization as a function of temperature and magnetic field. At ambient temperature, we find that La 0.67 Ca 0.23 Sr 0.1 Mn 0.98 Ni 0.02 O 3 sample has a very important magnetocaloric effect (MCE). In a magnetic field variation of 5 T, this MCE is explained by a maximum of magnetic entropy change Δ S M max and a big RCP values at 4.92 J. Kg - 1 K - 1 and 145 J/kg, respectively, around room temperature (296 K), which give the possibility of technologic application in magnetic refrigeration area. In addition, we find an admirable estimate of the magnetic entropy change, which is based on the Landau theory. Finally, we use the magnetic field dependence of Δ S M to determine the critical exponents of the system and we find that β = 0.39, γ = 1.35, and δ = 4.43, are very close to those predicted to the 3D-Heisenberg model. These results indicate that the present compound is one of candidate materials for magnetic refrigerators near room temperature. [ABSTRACT FROM AUTHOR]

La0.8Ca0.2MnO3 manganese oxides have been prepared by the conventional solid-state reaction. The samples crystallize in the orthorhombic structure with Pnma space group. A first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature that is found to be around 241 K. The magnetic and magnetocaloric properties have been simulated using a phenomenological model. The maximum of magnetic entropy change is of order 8.2 J/kg K for an applied magnetic field of 5 T, and the adiabatic temperature change is found to be of 4 K under a same magnetic field; this high value gives the sample the possibility of technologic application in the magnetic refrigeration area. The simulated results of magnetization and magnetic entropy change are compared with the experimental measurements. Finely, we found a good agreement between experimental and theoretical results. [ABSTRACT FROM AUTHOR]

La0.8Ca0.2MnO3 manganese oxides have been prepared by the conventional solid-state reaction. The samples crystallize in the orthorhombic structure with Pnma space group. A first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature that is found to be around 241 K. The magnetic and magnetocaloric properties have been simulated using a phenomenological model. The maximum of magnetic entropy change is of order 8.2 J/kg K for an applied magnetic field of 5 T, and the adiabatic temperature change is found to be of 4 K under a same magnetic field; this high value gives the sample the possibility of technologic application in the magnetic refrigeration area. The simulated results of magnetization and magnetic entropy change are compared with the experimental measurements. Finely, we found a good agreement between experimental and theoretical results. [ABSTRACT FROM AUTHOR]

A giant magnetocaloric effect induced by sodium-deficiency in a La0.8Na0.2−x□xMnO3 (0.00 ≤x≤ 0.15) polycrystalline sample is reported in the present research work. The ability to tune the temperature transition close to room temperature is revealed to be possible by changing the sodium-deficiency content as well. All samples were synthesized using solid–solid reaction. X-ray diffraction and magnetization measurements were performed to investigate their crystallographic structure and magnetocaloric properties. A rhombohedral structure with the R3̅c space group is deduced from pattern diffraction refinement of all samples. A second-order magnetic phase transition from the ferromagnetic to the paramagnetic state at the Curie temperature (Tc) is observed. Such a Tc decreased from 335 to 260 K when the sodium deficiency rate increased. Besides, the magnetic entropy change and the related Relative Cooling Power (RCP) values, sensitive to sodium-deficiency, were estimated. The magnetic entropy change ranges increased from 2.38 J kg−1 K−1 to 3.48 J kg−1 K−1 under a magnetic field of 2 T, whereas the RCP varied from 210 to 235 J kg−1 under an applied magnetic field of 5 T, when x increased from 0.00 to 0.15. The comparison of the values reported in the reference Gd material underlines that the proposed oxide material has substantial advantages for magnetic refrigeration. [ABSTRACT FROM AUTHOR]

The magnetic entropy change (Δ S) of polycrystalline samples LaCa□MnO ( x=0.00; x=0.10 and x=0.20) with a second-order phase transition has been investigated. The field dependence of the magnetic entropy change expressed as Δ S∝ H follows the phenomenological curves and at the temperature of the peak corresponds to a large field independent exponent of n=0.581; 0.642 and 0.671 for x=0.00; x=0.10 and x=0.20 samples, respectively. Then, we have constructed the phenomenological universal curve by normalizing the magnetic entropy change curves with respect to their maximum values, $\Delta S_{M_{\mathrm{max}}}$, and rescaling the temperature axis. These universal curves collapse onto a single curve for any applied magnetic field for all samples. Moreover, we note that the universal curve for x=0.10 and x=0.20 samples collapses with a small deviation compared with x=0.00 sample. This is consistent with our previous work where we demonstrated that the magnetic phase transition relative to the samples x=0.00 belongs to the 3D-Ising universal class and to the 3D-Heisinberg class for x=0.10 and x=0.20 samples. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory by extrapolations in field or temperature. [ABSTRACT FROM AUTHOR]

The La□CaMnO compounds were prepared by the solid-state reaction with 0.00≤ x≤ 0.20. Morphologic, X-ray diffraction (XRD), magnetic, and electrical measurements were performed to examine the effect of the lanthanum deficiency on the physical properties of these materials. The scanning electron microscopy shown the existence of a secondary phase attributed to the MnO oxide. The XRD data have been analyzed by the Rietveld refinement technique. We have found that all samples crystallize in an orthorhombic symmetry with Pnma space group and the unit cell volume decreases with lanthanum deficiency. A magnetization versus temperature study has shown that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with increasing temperature. The Curie temperature T increases with the lanthanum deficiency from 236 K to 247 K for the samples x=0.00 and x=0.20, respectively. Moreover, the temperature dependence of electrical resistivity shows a metal-insulator transition at T for all samples which increase with vacancy rate. The electrical resistivity is fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Thus, show that the conduction mechanist was explained by the electron-electron and electron-phonon scattering process at low temperature and by small polaron hopping at high temperature. Furthermore, we found that the estimated results are in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

We have investigated the effect of iron substitution on the magnetocaloric properties of manganites with LaCaSrMnFeO ( x=0, 0.075, 0.15, 0.175, and 0.2) nominal composition. The magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (−Δ S), using the M( T, μ H) data and employing the thermodynamic Maxwell equation. The large magnetic entropy in AMnFeO is attributed to the field-induced suppression of short-range charge-orbital ordering and antiferromagnetic fluctuations present above T. The increase of the Fe concentration x is accompanied by a decrease of (−Δ S), from 5.205 to 0.95 J/kg K for x=0 and 0.20, respectively, with μ H=5 T. For all samples, we find quite large values of (−Δ S), which are very close to that provided for Gd, the prototypical magnetocaloric material. [ABSTRACT FROM AUTHOR]

In this paper, a new method to broaden the range of the magnetic refrigeration temperature by changing the annealing temperature was proposed. Series of LaCa□MnO(0.00≤ x≤0.20) compounds were prepared by solid-state reaction and annealed firstly at a temperature of 1473 K (S1) and then at 1073 K (S2). Morphologic and structural studies have revealed that the decrease of the annealing temperature modifies the grain size and the structure of these compounds. The magnetic measurements have shown that the annealing at low temperature (1073 K) increases the magnetization and enhances the one-electron bandwidth, which induces an increase of the Curie temperature for 183 K (S1) to 241 K (S2) for the x=0.00 sample. The magnetocaloric investigation has exposed that the decrease of the annealing temperature induces a change from a second-order magnetic phase transition to first-order one for S1 and S2 compounds, respectively. Also, we have found that the Relative Cooling Power (RCP) factor remains almost constant as a function of calcium-deficiency concentration ( x) and the annealing temperature. Finally, we have deduced that we can use composite magnetocaloric compounds, exploiting a mixing of the same compounds annealed at two different temperatures (1473 K; S1) and (1073 K; S2), for refrigeration over the temperature range 175-264 K. [ABSTRACT FROM AUTHOR]

The authors developed a numerical study to predict the influence of non-Newtonian character of incompressible lubricants on the operation of hydrodynamic thrust bearings. The chosen rheological models are the Power law (pseudo-plastic behaviour), the Bingham, and the Herschel-Bulkley (viscoplastic behaviour). The mathematical model is based on the solution of a generalized Reynolds equation established using the concept of Generalized Newtonian Fluids. The temperature effect is taken into account through the resolution of energy equation in the film and Laplace's equation in the pad. The evolution of pressure and temperature fields, according to the characteristics of non-Newtonian lubricants, is analysed. Load capacity, friction torque, and power loss are also calculated as interesting parameters from an industrial point of view. [ABSTRACT FROM AUTHOR]

A numerical procedure is developed for the analysis of thermohydrodynamic behaviour of the hydrodynamic (HD) flow in the groove of a journal bearing. The Navier–Stokes and energy equations are written in terms of the primitive variables u, v, p, and T and solved simultaneously using the incremental load method and the finite element formulation. The numerical model is applied to the analysis of the velocities, the pressure, and the temperature patterns that characterize the lubricant flow in the HD groove. The effects of the runner velocity and the feeding pressure are investigated. [ABSTRACT FROM AUTHOR]

The corresponding author would like to indicate his correction to his affiliation. [ABSTRACT FROM AUTHOR]