The effect of chemical distribution on the magnetocaloric effect: A case study in second-order phase transition manganites

Ferromagnetic manganites of general formula ${\text{ABMnO}}_3$ (where A is a trivalent rare-earth ion and B is a divalent dopant) are candidates for magnetic cooling applications, since they can present either second- or first-order magnetic phase transitions, and the perovskite structure allows for substantial chemical substitution and tuning of the magnetocaloric properties. The consequent chemical distribution from substitution affects the magnetic and magnetocaloric properties of the compound. The change of relative cooling power with chemical disorder is discussed by the use of the molecular mean-field model. We present experimental results of the ferromagnetic, second-order phase transition ${\text{La}}_{0.70-x}(\text{Er,}\text{Eu}{)}_x{\text{Sr}}_{0.30}{\text{MnO}}_3$ system.     

Correlations between apparent activation energy and thermostability and glass forming ability for Fe based metallic glasses

Apparent activation energies (E_g) of glass transition, glass transition temperature (T_g) and crystallization temperature (T_x) of amorphous alloys of composition Fe_91?xB_xZr₅Nb₄ (FBZN, 5 ? x ? 30 at.%) and Fe_61?xCo_xZr₅B₃₀Nb₄ (FCZBN, 0 ? x ? 15 at.%) were obtained by using differential scanning calorimeter (DSC) measurement and Kissinger equation, and correlations between E_g and T_g, T_x and glass-forming ability (GFA) were studied in the present work. It was found that the T_g and T_x are not independent each other for each glass composition in the two alloy systems, but related by a formula, T_x = αT_g+β, where α and β are constants, and were measured by nonisothermally scanning in the DSC together with the Lasocka’s equation. The E_g was found to be directly proportional to α and β, respectively, and had a correlation with T_x and T_g, $T_{\text{x}}=\frac{E_{\text{g}}-1.09}{3.527}{T}_{\text{g}}-\frac{E_{\text{g}}-4.86}{0.0041}$, indicating that E_g determines linear relationship between T_x and T_g. Supercooled liquid region ΔT_x is used as characterization of GFA of the Fe based metallic glasses and related to E_g and T_g by a formula: $\mathrm{\Delta }{T}_{\text{x}}=E_{\text{g}}(\frac{T_{\text{g}}}{3.527}-234.9)+1185.37-1.309T_{\text{g}}$, indicating that E_g and T_g can characterize GFA of the Fe based metallic glass well.     

High-quality Cu2O crystals with various morphologies grown by thermal oxidation

We present an improved preparation method for, and experimental results on, high-quality crystals of cuprous oxide (${\mathrm{Cu}}_2\mathrm{O}$) grown by thermal oxidation. X-ray diffraction and low-temperature optical measurements at 2 K confirm excellent characteristics that are on par with natural-growth cuprite crystals. Synthetic ${\mathrm{Cu}}_2\mathrm{O}$ crystals were prepared in different geometries including platelets, cylindrical wires, and spheroids. Interestingly, we observed that the oxidation of Cu wires at high temperatures leads to the formation of hollow tubules of ${\mathrm{Cu}}_2\mathrm{O}$. Such unconventional structures of ${\mathrm{Cu}}_2\mathrm{O}$ could be utilized to confine excitonic matter, or serve as high-density exciton–polariton cavities.     

Free volume in poly(propylene glycol) and its relationships to spin probe reorientation

In this work we carried out positron annihilation lifetime spectroscopy (PALS) on pure poly(propylene glycol) (PPG 4000) and an electron spin resonance (ESR) study on PPG 4000 doped with the spin probe TEMPO. The spectral and dynamic features of both the microscopic probes such as the ortho-positronium (o-Ps) lifetime, τ₃, the spectral parameter, T_50 G, and the correlation time of spin probe, τ_c, as a function of the temperature were compared. New empirical relationships between the characteristic PALS temperatures ($T_{\text{b}1}^{\text{L}}$ and $T_{\text{b}2}^{\text{L}}$) and the characteristic ESR temperatures ($T_{\text{X}1}^{\text{L}}$ and $T_{\text{X}2}^{\text{L}}$) have been revealed. In addition, these characteristic PALS and ESR temperatures can be connected with the Schönhals and Stickel ($T_{\text{B}}^{\text{SCH}}$, $T_{\text{B}}^{\text{ST}}$) temperatures of the primary α relaxation process of PPG 4000 as obtained by dielectric spectroscopy (DS) studies in the literature. Next, the mean free volume hole sizes of the matrix were related to the spin probe size and to the Arrhenius or the non-Arrhenius motional regimes of TEMPO. Finally, the temperature dependences of the reorientational correlation time have been analysed using the Vogel–Fulcher–Tamman–Hesse (VFTH) equation and its generalized form, i.e., the Bendler–Shlesinger–Fontanella (BSF) one, the latter derived from the defect diffusion model of the dynamics and transport properties. The fitting parameters are then compared with those of the primary α relaxation process.