Effects of synthesis route on the structural,magnetic and magnetocaloric properties of Pr0.8K0.2MnO3

Pr_0.8K_0.2MnO₃ ceramics are prepared by solid-state reaction and Pechini sol–gel method. The influence of the powder synthesis method on the structural, morphological, magnetic and magnetocaloric properties of the samples are investigated. The X-ray diffraction pattern shows reflections typical of the perovskite structure with orthorhombic symmetry. The experimental results reveal that both samples undergo a second-order phase transition. The maximum magnetic entropy changes, deduced from the M–µ₀H measurements, are 3.77 and 7.23 J/kg K under the magnetic field change of 5 T for Pr_0.8K_0.2MnO₃ synthesized by using solid-state reaction and sol–gel methods respectively. The field dependence of magnetic entropy change is analyzed showing the power law dependence, ΔS(T,µ₀H)=a(T)(μ₀H)^n(T,H) at the Curie temperature. Using the scaling laws of ∆S, the experimental ∆S collapse onto a universal curve for both ceramics. These results suggest that the physical properties of our samples are strongly depended on synthesis techniques. It is found that Pechini sol–gel method is more efficient and stable to obtain ceramic materials with good magnetic properties. Consequently, a substantial increase of the ferromagnetic to paramagnetic transition temperature and an enhancement of magnetocaloric properties are observed in the sol-gel made sample making it more suitable for magnetic refrigeration applications.     

Study of magnetic and magnetocaloric effect of REMnO3(RE=Dy,Eu)manganites

We have systematically investigated the magnetic and magnetocaloric effect (MCE) of REMnO₃ (RE = Dy, Eu) manganites sintered by solid-state reaction method. The Neel temperature (T_N), the maximum magnetic entropy change (-ΔS_m^max) and the relative cooling power (RCP) of the sample were obtained by measuring the magnetization and heat capacity. Under the magnetic field change of 50 kOe, large -ΔS_m^max of 8.93J/kg K and 9.31J/kg K are achieved around the T_N of 14 K and 50 K for DyMnO₃ and EuMnO₃, respectively. It is proved by the slope of the Arrott plot and the universal phenomenological curve that the sample undergoes second order phase transition. The critical behavior of antiferromagnetic to paramagnetic phase transition is studied by modifying Arrott plot, which shows that the spin order of the sample decreases at the phase transition temperature and tends to change into shortrange order. The large magnetic entropy change calculated by Landau theory and experimental data indicates that the sample has a large magnetocaloric effect and has potential application in low temperature magnetic refrigeration materials.     

Magneto-caloric properties of La0.8-xSmxSr0.2MnO3 (x=0.0, 0.05, 0.1, and 0.15)

In this work, the manganite's magnetocaloric properties are studied by changing their chemical composition for a group La_0.8-xSm_xSr_0.2MnO₃ (x = 0.0, 0.05, 0.10, and 0.15) ceramic synthesized by the solid-state reaction method. According to the X-ray powder diffraction (XRD) analysis, no sign of a secondary phase is detected. It is found that the prepared samples are formed in the rhombohedral crystal structure with an R-3c space group. There is a decreasing trend in the unit cell volume, which indicates the gradual replacement of Sm in La sites. The results obtained from the energy-dispersive X-ray (EDAX) spectroscopy test indicated that the ratio of elements in the compounds is close to their nominal values. Based on the M measurements (upon the change of temperature) of the as-prepared specimens, an FM transition is observed. The transition temperature decreases upon the increase of the x value. The measured magnetic entropies of all the La_0.8-xSm_xSr_0.2MnO₃ samples are an illustration of a magnetocaloric impact on these compounds. This observation introduces La_0.8-xSm_xSr_0.2MnO₃ as a potential candidate for applications related to magnetic refrigeration.     

Evolution of structural,magnetic and magnetocaloric effect in TmFe1-xMnxO3 (x≤0.3)

The influence of Mn doping on structure, magnetic behaviors and magnetocaloric effect in TmFeO₃ polycrystalline ceramics has been explored. X-ray powder diffraction proves that TmFe_1-xMn_xO₃ (x ≤ 0.3) ceramics maintain an orthorhombic structure, and the space group is Pbnm. Compared with the original TmFeO₃ sample, structural parameters change slightly and magnetic properties are effectively tuned with the gradual substitution of Mn at Fe site. The spin reorientation temperature region shifts from 90.3 to 73.2 K for TmFeO₃ to 180.0–156.0 K for TmFe_0.7Mn_0.3O₃. Besides, for TmFe_1-xMn_xO₃ (x ≤ 0.3), the maximum magnetic entropy changes dependent on the Mn composition are 6.29 J/kg K, 6.56 J/kg K, 6.79 J/kg K and 7.22 J/kg K for 0–70 kOe, respectively. The refrigeration capacities are 159.3 J/kg, 168.9 J/kg, 176.7 J/kg and 184.4 J/kg, respectively. For a better assessing the magnetocaloric performance of TmFe_1-xMn_xO₃ (x ≤ 0.3), we have calculated the temperature average entropy change, refrigerant capacity and normalized refrigerant capacity, and their values become larger with increasing Mn doping. Our experimental results can provide valuable references for the application and development of RFeO₃ (R = rare earth) as multifunctional materials.     

A comparative study of the structural,optical, magnetic and magnetocaloric properties of HoCrO3 and HoCr0.85Mn0.15O3 orthochromites

In the present study, the physical properties including structural, optical and magnetic of HoCr_1-xMn_xO₃ (x = 0, 0.15) have been thoroughly investigated and compared. A detailed structural analysis of the compounds provided a way to correlate the optical and magnetic properties with the change in Cr-site ionic radius, tilt angles and distortion in CrO₆ octahedra due to Mn substitution. Further, the shift in the Raman phonon modes was associated with the increased structural distortion owing to Mn substitution in HoCrO₃ compound. The variation in the Urbach energy values, measured from the diffuse reflectance spectroscopy, was found to be closely related to the octahedral distortion. The optical band gap reduced from 3.22 eV to 2.01 eV after Mn substitution. Other optical parameters such as skin depth, refractive index, extinction coefficient and optical conductivity were also perceived to vary noticeably in the substituted compound. The overall structural distortion with Mn substitution resulted in the decrease of antiferromagnetic transition temperature of the pristine compound. The strong magnetic entropy change was observed at low temperatures in pure HoCrO₃ compound and its value was enhanced on Mn substitution. Therefore, the present work provides a convenient method to tune the multifunctional properties of HoCrO₃ compound, which has practical applications such as photocatalyst and low-temperature magnetic refrigerant.     

Effects of temperature and electric field on upconversion luminescence in Er3+‐Yb3+ codoped Ba0.8Sr0.2TiO3 ferroelectric ceramics

Optical and electrical properties of 1%Er³⁺ and different Yb³⁺ content (1ExY) codoped Ba_0.8Sr_0.2TiO₃ (BST) ferroelectric ceramics fabricated by the solid‐phase reaction were investigated. Under 980 nm pump condition, two green emission bands at 525 and 549 nm wavelength corresponding to, ²H_11/2→⁴I_15/2 and ²S_3/2→⁴I_15/2 transitions, and two red emission bands at 655 and 668 nm wavelength attributed to ⁴F_9/2→⁴I_15/2 transition are observed. The temperature‐sensing behaviors, calculated by the intensity ratio I₅₂₅/I₅₄₉ suggested that, the maximum sensitivity of the green emission of the 1E8Y‐BST ceramics is 1.07×10^?2 K‐1 at 293 K. Furthermore, the maximum sensitivity of 1E6Y‐BST and 1E11Y‐BST ceramics were obtained around the Curie temperature. The fluorescence lifetime of 1E8Y‐BST ceramics for ²H_11/2 level and ²S_3/2 level shortened with the increase in the temperatures. Moreover, the upconversion (UC) luminescence intensity of 1E8Y‐BST decreased with the increase in the external electric field and had a mutation at the coercive electric field (E_c) of about 1.24 kV/cm, which revealed that the electric field had influence on the UC luminescence.     

Study on the critical behavior and magnetocaloric effect of RCrO3 (R = Gd,Dy)

In this study, we aim to examine the critical behavior and magnetocaloric effect of RCrO₃ (R = Gd, Dy) polycrystal synthesized by solid-state sintering. As per the heat capacity fitting curves, GdCrO₃ and DyCrO₃ fit the 3D-XY and 3D-Ising models, respectively. The analysis of heat capacity curves, field cooling (FC), and zero-field cooling (ZFC) curves reveals two magnetic phase transitions in the samples, with T_N^Cr = 174 K, T_N^Gd = 3.7 K and T_N^Cr = 152 K, T_N^Dy = 3.5 K for GdCrO₃ and DyCrO₃, respectively. Under a 50 kOe magnetic field, the maximum magnetic entropy change (−ΔS_m^max) and relative cooling efficiency (RCP) of the sample are 39.9 J/kg K, 373.4 J/kg and 15.9 J/kg K, 276 J/kg for the two samples, respectively. We believe that the significant magnetocaloric effect of GdCrO₃ is associated with its critical behavior. This provides an additional reference for the origin of the large magnetocaloric effect of materials.     

Effect of sintering temperature on the ferroelectric property and electrocaloric effect of Pb0.8Ba0.2ZrO3 ceramics

This work presents the effects of sintering temperature ranging from 1200 °C to 1300 °C at intervals of 20 °C on the crystal structure, ferroelectric properties, and electrocaloric effect (ECE) of Pb_0.8Ba_0.2ZrO₃. Samples sintered at 1240 °C, 1260 °C, and 1280 °C have large remanent polarization and small coercive field. Meanwhile, samples sintered at 1260 °C, 1280 °C, and 1300 °C possess large breakdown field strength. Samples sintered at 1260 °C for 4 h exhibit the optimal ferroelectric properties. Antiferroelectricity-ferroelectricity (AFE-FE) phase transition occurs at room temperature T₁ (279 K). Directly examining ECE at this temperature is meaningful, and the temperature change is 0.068 K at approximately 60 °C and 30 kV/cm. Results laid the foundation for studying the performance of ferroelectric and ECE within this phase-transition temperature range and provide a reference for new solid-state refrigeration technology.     

Impact of the A-site rare-earth ions (Ln3+ – Sm3+, Eu3+, Gd3+) on structure and electrical properties of the high entropy LnCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3 perovskites

High entropy perovskites LnCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O₃ ceramics were produced by solid-state reactions from oxides. The B-site chemical composition was fixed (Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2) and A-site composition was varied by the rare-earth ions (Ln = Sm³⁺, Eu³⁺ and Gd³⁺). The entropy of B-sublattice mixing was 1.609R J/(mol*K). The dependences of the lattice parameters, microstructure features, and electrical properties were discussed as function of the A-site rare-earth ions. The correlation of the lattice parameters with the nature of the A-site rare earth ions was demonstrated. Impact of the rare-earth ions in A-site on microstructural parameters was observed. Charge conduction mechanisms were discussed in details for a wide range of temperatures.     

Influence of lattice defects on the high pressure properties of Ni0.66Mn2.34O4 NTC ceramics

The properties and hence the applications of negative temperature coefficient (NTC) oxides, Ni_xMn_3-xO₄, strongly depend on the Ni:Mn ratio, valence states, defects and cation distribution in the spinel structure. Here, the accuracy of determination of nickel and manganese cations distribution in the spinel structure of Ni_0.66Mn_2.34O₄ oxide, has been improved by taking into account the presence of tetrahedral vacancies. Additionally, the structural stability of cubic Ni_0.66Mn_2.34O₄ was investigated as a function of pressure up to 9.5 GPa using synchrotron radiation angle-dispersive X-ray powder diffraction and a diamond anvil cell. The bulk modulus and its first derivative were determined by fitting the Birch-Murnaghan equation of state (EoS) model to the experimental pressure-volume data. Rietveld refinement of the X-ray powder diffraction data reveals that the cubic spinel structure is stable upon compression to 9.5 GPa. The XRD data yielded a bulk modulus of K₀ = 126(7) GPa, with a pressure derivative K′ = 12(2). The obtained data were discussed in terms of defects in the cationic sublattice and compared with the elastic parameters of NiMn₂O₄.     

Influence of high pressure on the structure,hardness and brittle-to-ductile transition of NbSi2 ceramics

We apply first-principles calculations to study the influence of pressure on the structure, elastic modulus, hardness, brittle-or-ductile behavior and melting point of NbSi₂. Four NbSi₂ phases: C40, C11_b, C54 and C49 are considered based on the structural feature. The results show that the calculated formation enthalpy of the four NbSi₂ phases is negative within the pressure range of 0–60 GPa, indicating that they are thermodynamically stable in whole pressure. In particular, the calculated formation enthalpy of the C54 NbSi₂ is smaller than the other NbSi₂ phases, indicating that the C54 NbSi₂ is more thermodynamically stable than the other NbSi₂ phases. The calculated elastic modulus and Vickers hardness of NbSi₂ increase with increasing pressure. Note that the pressure results in brittle-to-ductile transition of the C40 NbSi₂, C11_b NbSi₂ and C54 NbSi₂ between 30 GPa and 60 GPa. Naturally, the increasing of mechanical properties is that the pressure enhances the electronic hybridization between Nb and Si, which is demonstrated by the chemical bonding such as Nb–Si bond and Si–Si bond.     

Enhanced energy storage properties and large electrostrictive effect of Bi0.35Na0.35Ba0.09Sr0.21Ti(1-x)(Al0.5Nb0.5)xO3 relaxor ceramics

Ferroelectric ceramics have good piezoelectric and ferroelectric properties and can be used for energy storage equipment and actuators. Nevertheless, current research on dielectric capacitors has only focused on the energy storage density, but ignored efficiency. Moreover, conventional piezoelectric materials have a large strain hysteresis. In this work, (Al_0.5Nb_0.5)⁴⁺ (AN) complex ions doped 0.7Bi_0.5Na_0.5TiO₃-0.3Ba_0.3Sr_0.7TiO₃ (BNBST) ceramics were prepared. Doping AN destroyed the long-range ordered ferroelectric domains and generated polar nano regions, resulting in a gradual thinning and inclination of polarization hysteresis loops and an increase in relaxor degree. For BNBST-3AN ceramics, a W_rec of 1.52 J/cm³ and a η of 92.1% were achieved at 150 kV/cm. Meanwhile, BNBST-3AN ceramics had good energy storage temperature stability and cycling performance. The AN doping reduced the strain hysteresis in BNBST ceramics. BNBST-2AN ceramics exhibited a longitudinal electrostrictive coefficient Q₃₃ ～ 0.0292 m⁴/C² and a field-induced strain of 0.25% with low strain hysteresis (6.67%). Furthermore, BNBST-4AN ceramics had superior dielectric temperature stability from 24 to 270 °C. All results show that BNBST-100xAN ceramics have great promise for energy storage devices and actuators.     

Effect of Al3+ on the photoluminescence and radioluminescence properties of Mn2+-Doped high silica glass

Mn²⁺/Al³⁺-, Eu²⁺/Mn²⁺-, and Eu²⁺/Mn²⁺/Al³⁺-doped high silica glasses were prepared to investigate their photoluminescence and radioluminescence properties. Particularly, the effect of Al³⁺ on the energy transfer from Eu²⁺ to Mn²⁺ and the regulation law of Mn²⁺ luminescence properties in Mn²⁺/Al³⁺- and Eu²⁺/Mn²⁺/Al³⁺-doped glasses were investigated. A strategy to improve the radioluminescence intensity of glasses was developed, and the radioluminescence intensity formula was improved. The dispersion caused by Al³⁺ improves the luminous intensity of Mn²⁺ and lowers the efficiency of the Eu²⁺→Mn²⁺ energy transfer in high silica glass. Al³⁺ can significantly broaden the excitation spectrum of Mn²⁺ by affecting the coordination number of Mn²⁺ and changing the field intensity around Mn²⁺ ions. Notably, the developed glass can potentially be applied in laser and LED lights. In addition, nonbridging oxygens (NBOs) are considered to be the main factor leading to the low efficiency of glass radioluminescence. Al³⁺ is able to reduce the relative amount of NBOs by forming [AlO_4/2]^?1 tetrahedra, thereby improving the radioluminescence intensity effectively.     

Competition effects of grain boundary and aging on the hysteresis loop behavior of (Ba0.8Sr0.2)(Ti,Mn)O3 ceramics

Hysteresis loop is very important for the application of ferroelectric ceramics. Here the competition effects of grain boundary and aging on the hysteresis loop behavior of Mn doped (Ba_0.8Sr_0.2)TiO₃ ceramics were studied. The results show that a double hysteresis loop appears after aging. Nevertheless, P_max of small grained ceramics decreased clearly but P_r decreased little with aging time increases; large grained ceramics have the opposite result. Such difference is caused by the competition effect on their domain switching behavior, from aging-induced time-dependent internal defect field and grain boundary generated time-independent back field. This study may provide help on the control of hysteresis loop by grain size and aging.     

Influence of pressure on the state of poly(N-isopropylacrylamide) and poly(N,N-diethylacrylamide) derived polymers in aqueous solution as probed by FTIR-spectroscopy

We investigated the hydration and swelling properties of poly(N-isopropylacrylamide) and poly(N-isopropylacrylamide)-poly(N,N-diethylacrylamide) derived microgels by Fourier transform infrared- (FTIR-) spectroscopy in a wide region of the temperature-pressure plane. These systems are known to show a swollen-to-collapsed-transition upon temperature elevation. Our data reveal that pressure favours the swollen, hydrated state over the collapsed state in all systems investigated. A detailed analysis of the fractions of the respective IR sensitive amide-I′-subbands allowed the calculation of ΔG^o and ΔV^o for the pressure-induced swelling process as well as evaluation of various intra- and intermolecular hydrogen bonding connectivities in the different systems. In fact, considerable differences exist between different polymer or microgel types with regards to their hydrogen bonding pattern as a function of temperature and pressure, and the microgels may even exhibit a biphasic swelling behavior. Notably, the thermodynamic parameters derived reveal to be in the same order of magnitude as measured for the pressure and cold denaturation of proteins.     

微量K掺杂对样品La0.75Sr0.25-xKxMnO3的磁性、电输运及室温磁电阻的影响

采用溶胶-凝胶方法制备了系列样品La0.75Sr0.25-xKxMnO3(x=0.00,0.05,0.07,0.10).对系列样品进行了相结构、磁性和电输运性质的研究.XRD数据表明:系列样品具有完整的钙钛矿锰氧化物结构.通过对磁性和电阻率的研究发现:与双交换作用相比,A位离子的无序度2对样品的磁性和电输运的影响起主要作用.随掺杂量的增加,磁电阻的峰值逐渐增大;峰值温度逐渐下降,更接近室温.在210～280 K的温区内,磁电阻基本保持稳定.这些结果都有利于磁电阻的实际应用.     

Effects of Nb,Mn doping on the Structure,Piezoelectric, and Dielectric Properties of 0.8Pb(Sn0.46Ti0.54)O3–0.2Pb(Mg1/3Nb2/3)O3 Piezoelectric Ceramics

Nb and Mn were doped, respectively, to 0.8Pb(Sn_0.46Ti_0.54)O₃–0.2Pb(Mg_1/3Nb_2/3)O₃ (PST–PMN) to improve electrical properties for meeting the requirement in various fields. The additions of Nb and Mn influence in a pronounced way the structure, and improve the densities of the ceramics. Nb‐doped PST–PMN increased the piezoelectric coefficient d₃₃, planar electromechanical coupling k_p, and relative dielectric constant ε, indicating “soft” piezoelectric behavior. Mn doping played a “hard” part, which significantly enhanced the mechanical quality factor Q_m without deteriorating other piezoelectric properties. The most excellent properties of Nb‐doped PST–PMN were obtained with doping amount of 0.75 mol%, specifically d₃₃, k_p, being on the order of 455 pC/N, 57.5% and 3560, respectively. The addition of 0.75 mol% Mn for PST–PMN presented the optimum electrical properties, with Q_m of 554, d₃₃ of 430 pC/N, k_p of 57.0%, ε of 2770. It was proposed that the addition of Nb, Mn generated different defect dipoles involved in the domain walls motion and intrinsic piezoelectric responses, leading to different effects on electrical properties.     

Experimental study on the effect of H2S and SO2 on high temperature corrosion of 12Cr1MoV

Aiming at the high temperature corrosion in a coal-fired boiler, the effect of H₂S and SO₂ on the corrosion of 12Cr1MoV under the water wall condition has been investigated by experiments. The results indicate that H₂S can promote the corrosion significantly, and the coarse porous oxide film formed cannot stop the progress of corrosion. While SO₂ presents little effect on the corrosion. The main composition of the surface of 12Cr1MoV corrosion products is Fe₂O₃. With H₂S in the atmosphere, the corrosion gradually develops into deeper layers by forming FeS, FeO and Fe₂O₃ alternately. The corrosion rate is doubled for every 50 °C increase in temperature at 400–500 °C.