Entropy and Fragility in Supercooling Liquids

We review the Kauzmann paradox and what it implies about the configuration space energy hypersurface for “structural glassformers.” With this background, we then show how the relaxation expression of Adam and Gibbs qualitatively accounts for most of the phenomenology of liquid and polymeric glassformers including the strong/fragile liquid pattern, and the behavior of non-ergodic systems. Extended temperature range relaxation studies are consistent with a relaxation time pre-exponent on the quasi-lattice vibration time scale. When this boundary condition is imposed on Vogel-Fulcher-Tammann fittings, correspondence of T₀ with T_K is found for liquids with T_g ranging over 1000 K. When it is imposed on the WLF equation C₁ is obliged to become ~16, and the corresponding force-fitted C₂ provides a measure of the polymer fragility which is generally not available from thermodynamic studies. Systems which exhibit discontinuous changes in configurational entropy on temperature increase, which include unfolding proteins, are briefly reviewed.     

Morphology, Structural, Magnetic, and Magnetocaloric Properties of Pr0.7Ca0.3MnO3 Nanopowder Prepared by Mechanical Ball Milling Method

A sample of Pr_0.7Ca_0.3MnO₃ nanopowder was prepared by the ball milling method. The crystal structure examined by X-ray powder diffraction indicates that the sample is single phase and crystallizes in the orthorhombic perovskite system with Pnma space group at room temperature. The average crystallite size of 29 nm was obtained by X-ray diffraction. Magnetic measurements showed that the sample exhibits a ferromagnetic-to-paramagnetic transition at a Curie temperature close to 120 K. The magnetic entropy change |ΔS _M | has been deduced by the Maxwell relation method. The maximum value of the magnetic entropy change $\vert \Delta {S}_{M}^{\max} \vert$ obtained from the M(H) plot data is found to be 0.86 J/kg?K for an applied magnetic field of 2 T. At this value of magnetic field the relative cooling power (RCP) is 44.05 J/kg. At low temperature, large change in magnetic entropy has been observed in the sample. Our result on magnetocaloric properties suggests that Pr_0.7Ca_0.3MnO₃ nanopowder is attractive as a possible refrigerant for low-temperature magnetic refrigeration.     

Magnetic,magnetocaloric properties,and critical behavior in a layered perovskite La<Subscript>1.4</Subscript>(Sr<Subscript>0.95</Subscript>Ca<Subscript>0.05</Subscript>)<Subscript>1.6</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript>

We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La_1.4(Sr_0.95Ca_0.05)_1.6Mn₂O₇. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature T _C = 248 K, a Neel transition at T _N = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??S _M) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T _C = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M _S(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??S _M) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??S _M) vs. M ²] and the classical extrapolation from the Arrott curves (µ₀H/M vs. M ²), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well.     

Anisotropic Magnetocaloric Effect in Single-crystalline Pr0.52Sr0.48MnO3

We report the magnetocaloric effect (MCE) in a Pr_0.52Sr_0.48MnO₃ single crystal estimated from the isothermal magnetization curve using the Maxwell relation. Isothermal magnetization curves are measured over the range 20 K to 320 K where the field was applied parallel (??) and perpendicular (??) to the [110] direction of the perovskite structure with Pbnm space group. A peak in the temperature (T) dependence of magnetic entropy change (??S _M) with a fairly large negative value (???3.3 J/kg?K) is observed at 275 K close to the Curie temperature (T _C) for a change in field of ??H=40 kOe. The ?? and ?? components of ??S _M deviate from each other below ??260 K and an inverse MCE is observed below ??150 K. We note that the Landau theory of phase transitions satisfactorily explains the ??S _M vs T plot around the second-order transition at T _C.     

Magnetic State and Magnetocaloric Effect of SmMnO3

We present a study of magnetization and magnetocaloric effect for the SmMnO₃ compound. This compound was synthesized by combustion reaction and its magnetic and structural properties studied by X-ray powder diffraction (XRD) and magnetization (M) measurements as a function of temperature and under magnetic fields. The XRD pattern at room temperature confirmed the presence of a single phase with orthorhombic structure. From magnetization versus temperature, we observe two magnetic orderings, the first one at 6 K due to Sm³⁺, and the other one at T _N =57(2) K is the anti-ferromagnetic long-range ordering. The magnetic entropy change, ΔS _M , was obtained from magnetization isotherms close to T _N where it reaches a maximum value of about 8.0 J/kg K for an applied field of 7 T.     

Adam-Gibbs Formulation of Enthalpy Relaxation Near the Glass Transition

The entropically based nonlinear Adam-Gibbs equation is discussed in the context of phenomenologies for nonlinear enthalpy relaxation within the glass transition temperature range. In many materials for which adequate data are available, the nonlinear Adam-Gibbs parameters are physically reasonable and agree with those obtained from linear relaxation data and thermodynamic extrapolations. Observed correlations between the traditional Tool-Narayanaswamy-Moynihan parameters are rationalized in terms of the Adam-Gibbs primary activation energy (Δμ) determining how close the kinetic glass transition temperature can get to the thermodynamic Kauzmann temperature. It is shown that increased nonlinearity in the glass transition temperature range is associated with greater fragility in the liquid/rubber state above T_g.     

Pinning and dissipative effects in Bi-Pb-Sr-Ca-CuO superconductor

Pinning and dissipative effects have been investigated in Bi-Pb-Sr-Ca-CuO sintered compounds by means of DC magnetization and magnetic relaxation measurements. Hysteresis cycles have been performed up to 5 T from 18 to 50 K, and the time decay of the zero field cooled magnetization (M _zfc) has been recorded both as a function of field and temperature. The observed temperature dependence of the reduced pinning force and the crossover from a logarithmic to a nonlogarithmic time decay ofM _zfc (forT>50 K), are discussed in relation to the collective pinning and the related extensions to flux creep.     

Structural, Magnetic, and Magnetocaloric Properties of La0.5M0.1Sr0.4MnO3 (M=Bi, Eu, Gd, and Dy) Perovskite Manganites

Structural, magnetic, and magnetocaloric properties of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) powder samples, synthesized using the solid-state reaction at high temperature, have been experimentally investigated. X-ray diffraction analysis using the Rietveld refinement show that La_0.5Bi_0.1Sr_0.4MnO₃ sample is single phase and crystallizes in the rhombohedral system with $R\overline{3}c$ space group whereas a mixture of orthorhombic (Pbnm) and rhombohedral ( $R\overline{3}c$ ) phases is observed for M=Eu, Gd, and Dy compounds. The Curie temperature, T _C , shifts to lower temperature with decreasing the average A-site ionic radius 〈r _A 〉, which is consistent with large cationic disorder. Arrott plots show that all our samples exhibit a second order magnetic phase transition. From the measured magnetization data of La_0.5M_0.1Sr_0.4MnO₃ (M=Bi, Eu, Gd, and Dy) samples as a function of magnetic applied field, the associated magnetic entropy change |ΔS _M | has been determined. In the vicinity of T _C , |ΔS _M | reached, in a magnetic applied field of 1 T, maximum values of 0.98 J/kg?K, 1.01 J/kg?K, 0.81 J/kg?K, and 0.77 J/kg?K for M=Bi, Eu, Gd, and Dy, respectively.     

Above room temperature magnetocaloric effect in perovskite Pr0.6Sr0.4MnO3

The dependence of magnetization M on the applied magnetic field H and temperature T was measured carefully, near the Curie temperature T_C for the perovskite manganite sample Pr_0.6Sr_0.4MnO₃. The experimental results indicate that this specimen exhibit ferromagnetic (FM) to paramagnetic (PM) transition at T_C ~ 320 K. In the 200 K–45 K temperature range the spontaneous magnetization was decreasing, probably due to spin canted state between manganese and praseodymium spin systems. At 46 K the magnetization presents a second little transition, which can be ascribed to very weak traces of secondary Mn₃O₄ phase, and remains constant between 10 K and 46 K. The maximum value of the magnetic entropy change obtained from the M(H) plot data is |ΔS_M^max| = 2.3 Jkg^? 1 K^? 1 for applied magnetic field of 2.5 T. At this value of magnetic field the relative cooling power (RCP) is 34.5 Jkg^? 1. The temperature corresponding to ΔS_M maximum value is almost equal to T_C. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. It is suggested by the results that this compound can be used as the working material in an active magnetic regenerative refrigerator above room temperature.     

Magnetocaloric Effect in LaFe10.7Co0.8Si1.5 Compound Near Room Temperature

Magnetocaloric properties of LaFe_10.7Co_0.8Si_1.5 with the cubic NaZn₁₃-type structure were investigated around their Curie temperature T _C . By the help of the phenomenological model, the magnetization curves for LaFe_10.7Co_0.8Si_1.5 at several magnetic fields were simulated. The magnetic entropy change and specific heat are obtained. The values of maximum magnetic entropy change, full-width at half-maximum, and relative cooling power, at several magnetic field variation, were calculated. It is shown that for LaFe_10.7Co_0.8Si_1.5 the magnetic entropy change follows an asymmetrical broadening of ΔS _M peak with increasing field. The maximum magnetic entropy change of this compound is 7.10 JK^?1?kg^?1 and relative cooling power is 201.37 J/K under a magnetic field of 2 T. The magnetocaloric effect of this material is large and tunable, suggesting a possible technical application of the material at moderate magnetic fields near room temperature.     

Dielectric properties of vacuum-deposited bismuth oxide films

The dielectric properties of bismuth oxide films prepared by vacuum deposition have been studied in the frequency range 0.1–10 kHz and the temperature range 90–298.5 K. The capacitances C_p and C_s and the loss factor show dependences on the frequency, temperature and aging of the samples. The loss factor exhibits a flat maximum in its temperature variation curve at about 170 K. Interfacial polarization, which is caused by the excess bismuth and by various defects and impurities, is thought to be the main relaxation mechanism operating in the low frequency region.     

Equilibrium vacancy parameters and limit temperature of nonequilibrium melting in osmium

For osmium, based on experimental data on enthalpy and the averaged heat capacity obtained by the method of mixing within the temperature range of 1150?C2960 K, for the first time the parameters of the equilibrium vacancies for this metal have been determined: the vacancy formation energy E = 1.8 eV, the vacancy concentration at melting c = 3.3%, and the vacancy formation entropy S = 25.6 J/(mol K). The limit temperature of the onset of nonequilibrium melting of osmium T _m = 4256 K and its relative value T _lim/T _m = 1.30 has been calculated.     

Study on the homogeneity of hydrogenation for LaFe11.5Si1.5 intermetallic compound

The homogeneity of hydrogen absorption for LaFe_11.5Si_1.5 intermetallic compound was investigated. The hydrides remained NaZn₁₃-type structure when the hydrogenation temperature varied from 423 K to 923 K under 1 atm hydrogen atmosphere. The Differential Scanning calorimetric (DSC) measurements revealed slightly inhomogeneous distribution of hydrogen atoms in the hydrides hydrogenated at a low temperature. The activation before hydrogenating at a higher temperature could improve the homogeneity of hydrogen absorption. Uniform magnetic transition temperature, corresponding to the homogeneous hydrogen absorption, was observed by hydrogenating the compound at 823 K, or at 523 K after activation. The maximum magnetic entropy change ΔS_M (ΔH = 2 T) for the compound hydrogenated at 823 K is about 15.5 J/kg K with the latent heat of 5.12 × 10³ J/kg during the phase transition.     

NMR Studies of 3He Droplets in Dilute 3He-4He Solid Solutions

We have performed measurements of the temperature dependence of the nuclear spin-lattice relaxation times (T ₁) for a wide range of ³He concentrations for dilute mixtures of ³He in solid ⁴He. The temperatures for phase separation are determined for ³He concentrations 500<x ₃<2000 ppm for a molar volume V _M =20.7 cm³. We report the temperature dependence of the nuclear spin-lattice relaxation times for ³He in the droplets formed after phase separation at low temperatures. The temperature dependence suggests that the interface ³He atoms responsible for the relaxation are degenerate, not solid-like.     

Preparation structural and thermal properties of MAlSiO4 (M = Li,Na, K,Rb, Cs,Tl, Ag) of ABW-type

The preparation of new, hydrated and anhydrous members of the ABW class of aluminosilicates using hydrothermal methods or ion exchange in molten salts are described. Of these, NaAlSiO₄·0.8 H₂O, AgAlSiO₄·0.8 H₂O, and AgAlSiO₄·0.68 H₂O contain zeolitic water. Dehydration occurs, within rather narrow temperature intervals, at 450, 330 (partially), and 380 K, respectively. Upon complete dehydration, new, probably ABW type, modifications of AgAlSiO₄ and NaAlSiO₄ are obtained. Thermal expansion behavior and temperature-induced phase transitions are described for MAlSiO₄·x H₂O (M = Li, Na, Ag) and MAlSiO₄ (M = Li, Na, Rb, Tl, and Ag). The different-sized unit cells, and, accordingly, the axial ratios, of the ABW-type compounds, are discussed in relation to the turning of tetrahedra, which are the essential building units of the crystal structure. Minimum and maximum unit cells, with respect to volume, are described. The differences in thermal expansion behavior found for different MAlSiO₄ are discussed in terms of the same concept. At high temperatures, MAlSiO₄, M = Li, Na, K, and Ag, transforms into other polymorphs, being, respectively, γ-eucryptite, nephelin, kalsilite, and a cubic F-centered phase related to the cristobalite type. On cooling, cubic AgAlSiO₄ transforms reversibly into a triclinic modification of the carnegeite type.     

Annealing Influence on the Microstructure and Magnetic Properties of Ni–Mn–In Alloys Ribbons

We report on the structure, microstructure and inverse magnetocaloric effect associated with the first-order martensitic phase transition, in Heusler Ni_50.0Mn_35.5In_14.5 alloy ribbons. We have studied the short-time vacuum annealing influence at 1048?K, 1073?K, 1098?K, and 1123?K in these properties. At room temperature, an increase in the degree of structural order for ribbons annealed up to 1098 K was observed, corresponding to cubic L2₁ austenite phase. Meanwhile, for the sample annealed at 1123?K a monoclinic 10M martensitic phase was detected. A comparison of magnetic entropy change as a function of the applied field, after using zero-field-cooling thermomagnetic and isothermal magnetization measurements, has been made for the sample annealed at 1073?K.     

Magnetic properties of nano-crystalline Li0.35Cd0.3Fe2.35O4 ferrite prepared by modified citrate precursor method

Single phase nano-crystalline lithium cadmium ferrite Li_0.35Cd_0.3Fe_2.35O₄ is synthesized by a modified citrate gel precursor technique in different pH media. The modified citrate precursor technique reduces the formation of the impurity phase α-Fe₂O₃ in the inverse spinel phase of lithium ferrite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the average crystallite size. As-prepared sample shows the paramagnetic behaviour of M–H curve measured by vibrating sample magnetometer (VSM). The coercivity (H_c) and magnetization (M) both increase with decrease in temperature from 300 K to 80 K. Temperature dependent magnetic properties below the Curie point are defined by the Bloch's law and Neel relaxation relations. The effect of annealing on magnetic properties at different temperature is studied.     

Low Temperature Giant Magnetocaloric Effect and Critical Behavior in Amorphous Co100−xEr x (x = 55, 65) Alloys

We report on the magnetic properties, magnetocaloric effect (MCE) and critical exponents in amorphous Co_100?xEr _x (x = 55 and 65), prepared by liquid quenching technique. The transition temperature from ferromagnetic to paramagnetic state has been evaluated according to M(T) measurements, and it is found to be 26 and 15 K for Co₄₅Er₅₅ and Co₃₅Er₆₅, respectively. The magnetization dependence M(H, T) on the temperature T and magnetic field H was measured carefully in the critical region. Magnetic entropy change (– ΔS _M ) allowing estimation of the MCE was determined based on thermodynamic Maxwell’s relation. The magnetocaloric study exposes a quite large value of the magnetic entropy change, which decreases when increasing Er concentration. For an applied magnetic field of 5 T,the values of (– ΔS _Max) peak are about 10.8 and 9.8 J kg ^?1 K ^?1 for Co₄₅Er₅₅ and Co₃₅Er₆₅, respectively. From the field dependence of the magnetic entropy ΔS _M (ΔS _M α H ⁿ ) and the relative cooling power (RCP) (RCP α H ^1+1/δ), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott method.     

Swelling behaviors, tensile properties and thermodynamic interactions in APS/HEMA copolymeric hydrogels

A series of hydrogels was synthesized from hydrophobic allyl phenyl sulfone (APS) and hydrophilic 2-hydroxyethyl methacrylate (HEMA) by bulk free radical copolymerization. The effects of APS content and temperature were studied on network parameters such as effective crosslink density (v _e), molar mass between crosslinks (M _c) and polymer-water interaction parameter (x) of hydrogels. The increase in APS content was shown to enhance hydrophobic bonding within hydrogel, leading to the decrease in equilibrium water content (EWC) and the increase in volume fraction of polymer in hydrogel (ϕ ₂), tensile strength and Young’s modulus. At the same time, the increases in v _e and x and the decrease in M _c were also observed. When the temperature is increased from 273 to 343 K, the hydrogel A/H3 undergoes decreasing in EWC and increasing in ϕ ₂ and x values. The thermody namic analysis indicated that the swelling process is an exothermic process.     

Inhomogeneous magnetic structure in clean magnetic superconductors

Clean magnetic superconductors are considered in which the phase transition into the ferromagnetic state takes place at a temperature ? in the absence of superconductivity, with ??T _c1, whereT _c1 is the superconducting critical temperature. The exchange and electromagnetic interactions of electrons and localized magnetic moments are taken into account, as well as magnetic anisotropy. We show that below the temperatureT _M≈? in the superconducting state the inhomogeneous magnetic structure of transverse one-dimensional domain type (DS phase) should occur at real values of the exchange interaction and anisotropy. In the DS phase gapless superconductivity is realized at temperatures sufficiently far fromT _M. Here the equilibrium direction of the magnetic structure wave vectorQ can be changed by applying a supercurrent across the sample. The behavior of this DS phase in an external magnetic field is also considered.