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In this work, perovskite manganites, La 0.7 (Ag 0.3- x Na x) 0.3 MnO 3 (LANMO, x = 0, 0.05, 0.1, 0.15, and 0.2), were synthesized using facile sol-gel method. Structural, electrical, and magnetoresistive (MR) properties of LANMO samples with different sodium contents were investigated. XRD and Rietveld refinement revealed that all samples crystallized in distorted rhombohedral system with R 3 ‾ c space group. The doping of sodium increased average particle sizes and A-site cation vacancies and also reduced formal charges of Mn. Electrical transport properties showed that peaks for temperature coefficient of resistance (TCR) and corresponding temperatures (T k) improved significantly due to increased Na content. TCR and MR values of 10.4% K−1 and 24.5% for x = 0.1 were obtained at room temperature (297.1 and 299.2 K, respectively). Results suggest that these LANMO ceramics are potentially attractive candidates for applications in room-temperature infrared detector. [ABSTRACT FROM AUTHOR]

In this study, La 0.7 Ca 0.3- x Sr x MnO 3 (LCSMO, 0.06 ≤ x ≤ 0.1) ceramics were synthesized via a sol-gel method. The perovskite microstructure, electrical transport properties, low-field magnetoresistance (MR) and peak temperature coefficient of resistivity (TCR) behaviors of the as-obtained materials were all studied and the results were discussed. Grain size of LCSMO ceramics expanded as more Sr was added but grain boundaries (GBs) decreased. Field emission scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and vibrating sample magnetometry (VSM) analyses suggested that addition of Sr greatly enhanced the electrical/magnetic properties of the prepared specimens. The metal-insulator transition temperature rose as doping concentration x enhanced, and increased Ca2+ substitution led to reduced peaks of MR and TCR. For single-phase LCSMO, large room-temperature TCR and low-field MR effects (0.8 T magnetic field) were obtained by controlling the level of Sr2+ substituted Ca2+. For x = 0.095, the optimized room-temperature TCR and MR reached 13.78% K−1 (299.2 K) and 26.41% (302.6 K), respectively. Overall, LCSMO ceramics have broad application prospects in photoelectronic or magnetic devices at room-temperature. [ABSTRACT FROM AUTHOR]

High density (Pr 1- x La x) 0.7 Sr 0.3 MnO 3 (PLSMO, x = 0, 0.10, 0.20, 0.25, and 0.30) polycrystalline ceramics were fabricated by the sol-gel method followed by sintering at 1723 K for 12 h. The doping effects of lanthanum on structural, surface morphology and electrical transport properties of obtained ceramics were systematically investigated. X-ray diffraction patterns (XRD) confirmed that all samples crystallized in typical single orthorhombic perovskite phase with Pnma space group. Field emission scanning electron microscopy (FESEM) showed slight increase in grain size, and energy dispersive spectroscopy (EDS) mapping displayed uniformly distributed elements on the sample surface. Resistivity dependence of temperature (R - T) curves of PLSMO specimens were investigated by standard four-probe method, and results indicated decline in resistivity with the increase in La content. In addition, as the lanthanum doped content increased, the metal-insulator transition temperature (T p) improved significantly. The average ionic radius increased as Pr3+ ions were replaced by La3+ ions with larger ion radius, which further affected the Mn–O band angle and Mn–O band length. These changes enhanced the double-exchange interaction, increasing T p value. The temperature coefficient of resistance (TCR) curves revealed that peak TCR temperature improved with La content rose. The peak TCR temperature of PLSMO at molar ratio x = 0.25 was significantly increased from 258.47 K to 299.37 K (room temperature 300 K) and value reached 5.03% K−1. Obviously, obtaining relatively high room temperature TCR value in Pr system materials was interesting as this was not observed previously. In sum, the proposed (Pr 0.75 La 0.25) 0.7 Sr 0.3 MnO 3 ceramics look potential candidates for use in advanced room temperature uncooled infrared bolometers. [ABSTRACT FROM AUTHOR]

In this study, facile sol-gel method was used to prepare La 0.625 Ca 0.375- x Sr x MnO 3 :Ag 0.2 (LCSMO:Ag 0.2 , x = 0.09–0.15) polycrystalline ceramics. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) indicated an increase in grain size of LCSMO:Ag 0.2 ceramics with orthorhombic structure as a function of x. Four-probe resistivity temperature system was employed to characterize electrical and magnetic properties of all prepared specimens. Temperature coefficient of resistivity (TCR) and magnetoresistance (MR) yielded room temperature maxima of 4.9% K−1 at 299.2 K and 17.5% at 299.1 K. The enhancement in TCR and MR was mainly attributed to Jahn-Teller (JT) distortion and double exchange (DE) effect. X-ray photoemission spectroscopy (XPS) and inductively coupled plasma-optical emission spectrometer (ICP-OES) all confirmed the existence of silver in metallic forms and the increase of Sr content. On the other hand, satellite peaks in low-temperature range of resistivity-temperature (ρ - T) curves gradually weakened as Sr-doped amount increased. The fitting of ρ - T curves in low-temperature region revealed that gradual vanishing of satellite peak may be due to presence of Sr, able of inhibiting various types of interactions: electron-electron, electron-phonon, and Kondo-like spin-dependent scattering. Overall, room temperature TCR and M R were both optimized by Sr doping, providing further understanding of CMR mechanism and application of manganite materials in electromagnetic equipment under room temperature. [ABSTRACT FROM AUTHOR]

Herein, the influence of Sr doping on chemical composition, microstructure, and electrical transport properties of La 0.67 Ca 0.33- x Sr x MnO 3 (LCSMO, 0.06 ≤ x ≤ 0.11) polycrystalline ceramics is systematically investigated. The study was performed by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), elemental mapping, energy dispersive spectrometry (EDS), X-ray photoemission spectroscopy (XPS), and four-probe method (ρ - T). XRD results verified the synthesis of high-purity orthogonal perovskite structure with Pnma space group. EDS and XPS results confirmed presence and uniform distribution of La, Ca, Sr, Mn and O. Furthermore, ρ - T curves indicated that the resistivity decreased with increasing Sr content, whereas metal-insulator transition temperature (T MI) gradually increased. Moreover, cell volume (V) and electron bandwidth (W) increased with the increase in Sr content, which effectively reduced the resistivity of LCSMO ceramics. One should note that double exchange (DE) enhancement between Mn3+ and Mn4+ is responsible for gradually increasing T MI. Finally, maximum TCR value of 14.3% K−1 was achieved at x = 0.08, which rendered TCR peak temperature of 299.0 K. High room-temperature TCR of LCSMO ceramics is promising for next-generation infrared bolometers, operating at room-temperature. [ABSTRACT FROM AUTHOR]

Polycrystalline La 0.67 Ca 0.33- x Ag x MnO 3 (LCAMO, x = 0, 0.06, 0.15, 0.18, and 0.24) ceramics were fabricated by conventional sol-gel route at relatively low sintering temperature of 1100 °C for 12 h. Effects of silver content (x) on crystal structure, grain size, resistivity and magnetic properties of as-prepared LCAMO specimens were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), elemental mapping and energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), and standard four-probe method (ρ - T). The data from XPS, XRD and EDS revealed that silver existed as Ag+ ions in the lattice matrix position of LCAMO ceramics. Broad metal-insulator transition temperature (T MI) values ranging from 267.0 K (x = 0) to 302.6 K (x = 0.24) were obtained with LCAMO specimens prepared with variable Ag+ added contents. Peak temperature coefficient of resistivity (TCR) enhanced from 4.1% K−1 at 263.2 K (for x = 0) to 10.9% K−1 at 278.5 K (for x = 0.18), and reached 7.5% K−1 at room-temperature (295.9 K) for x = 0.24. Meanwhile, magnetoresistance (MR) of materials reached 17.7% at room-temperature (299.2 K) for x = 0.24. Overall, these findings demonstrated that Ag doping was beneficial for improving electrical and magnetic properties of LCAMO materials. In summary, LCAMO ceramics achieved RT-TCR and MR at optimal Ag stoichiometric ratio, promising for applications in infrared bolometers or magnetic sensors. [ABSTRACT FROM AUTHOR]