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Strontium (Sr)-doped La0.67K0.33-xSrxMnO3 (LKSMO) ceramics were obtained by traditional sol-gel method. With increasing Sr content, full width at half maximum (FWHM) for temperature-dependent resistivity curves increased, resistivity decreased, and peak resistivity temperature (TP) shifted toward high temperature. Microstructure of LKSMO ceramics was characterized by X-ray diffraction, scanning electron microscopy, X-ray photoemission spectroscopy, energy dispersive spectrometry, and Fourier transform infrared spectroscopy, etc. Connection between microstructure and properties of ceramics was studied by combining double exchange mechanism, Jahn-Teller effect, and phenomenological percolation model. Electrical transport mechanism of ferromagnetic metal (FMM) and paramagnetic insulator (PMI) regions was employed to explain temperature coefficient of resistivity (TCR) and magnetoresistance (MR) behaviors of as-obtained samples. Various scattering factors, effective mass of itinerant electron, and energy difference between FMM and PMI phases affected FWHM of ρ-T curves in Sr-doped LKSMO ceramics. In sum, these findings provide physical mechanism and experimental guidance for synthesis of high-TCR and MR polycrystalline ceramics at room temperature.

In the presence of magnetic field, reducing the loss in temperature coefficient of resistance (TCR) and increasing metal−insulator transition point (TMI) to room temperature are the most important concerns for the application of perovskite manganites in high−precision thermistors. Based on emerging evidence, relevant factor to address these problems lies in the interaction between Jahn−Teller (JT) distortion and magnetic field, which results in spin−orbital coupling (SOC) effect and significantly influences TCR and TMI. In this work, we studied the magnetic field induced SOC effect in polycrystalline La1−xCaxMnO3 (x = 0.225–0.45) materials synthesized via sol−gel technique. Compounds undergo the JT distortions with increasing Ca dopant content, and the most pronounced distortion of 0.0144 at x = 0.40 is correlated with a basal−plane distortion mode. All samples exhibit TMI values between 262 K and 288 K, indicating the enhancement of doping−induced double−exchange interaction. In the magnetic field of 1 T, SOC effect sufficiently suppresses the deterioration of TCR caused by deficient magnetization in La0.6Ca0.4MnO3 to a value of 2.9%·K−1 at room temperature (287 K).

La0.71Ca0.29MnO3 is a strongly correlated electronic material, which has been extensively researched and has high application value. It is known that improvements of the temperature coefficient of resistance (TCR) and magnetoresistance (MR), which are important parameters for the application of such materials, promote their applications. Using other elements to replace Mn sites is one of the methods to improve the specific properties. Herein, we prepare a series of La0.71Ca0.29Mn1-xCoxO3 (x = 0, 0.01, 0.03, 0.05, 0.1) polycrystalline ceramics via sol-gel method, and explore the effect of Co doping on the properties of La0.71Ca0.29MnO3. The results show that a small amount of Co doping can significantly improve TCR and MR. Under our excellent preparation process, TCR of La0.71Ca0.29MnO3 was 22.5%·K−1 and MR was 52.9% (under 1T magnetic field). When Co doping is 0.01, TCR reaches 35.1%·K−1 value and MR reaches 64.8%. This can provide critical information for the future application of such materials in bolometers and magnetic sensors.

Reducing the loss of temperature coefficient of resistivity (TCR) in the magnetic field and increasing the low−field magnetoresistance (MR) are two crucial issues to be solved for large scale practical applications of perovskite materials. In this work, polycrystalline La0.67−ySmyCa0.33MnO3 (shortly LCMO:Smy, where y = 0.02, 0.04, 0.06, 0.08) ceramics were synthesized via optimized sol−gel technology. Colossal intrinsic MR up to 71 % was achieved in produced composites by applying only the magnetic field of 1 T. Lattice distortion was found to be a key factor governing the intrinsic MR response. In presence of magnetic field, the most unperturbed TCR up to 24 %·K−1 was attained in LCMO:Sm0.06 system, where the TCR−loss was only 1 %·K−1. This was explained by the opposite external field direction with respect to Jahn−Teller distortion, which enabled one to solve the problem of large attenuation of TCR under the magnetic field. These findings reveal great potential of the LCMO:Sm0.06 ceramic for applications in infrared bolometers and magnetic recording devices in complex electromagnetic environments.

Improving the magnetoresistance effect of perovskite ceramic materials under a low applied magnetic field to expand its application range is one of the main research directions of this type of material. In this study, La0.7Ca0.3MnO3 was doped with different levels of Sm by the sol-gel method to yield a series of La0.7-xSmxCa0.3MnO3 (LSCMO) polycrystalline ceramics. X-ray diffraction (XRD) results revealed that LSCMO ceramics possessed standard perovskite structures. Scanning electron microscopy (SEM) showed grains closely connected without obvious holes. In addition, the grain size gradually decreased with the increase in Sm doping content. The resistivity temperature curves displayed a clear metal-insulator transition behavior of LSCMO accompanied by a steep change from ferromagnetic to paramagnetic behavior (FM-PM). The metal-insulator transition temperature (Tp) values of the as-obtained LSCMO gradually shifted toward lower temperatures with increase in Sm content. Moreover, resistivity temperature coefficient (TCR) and magnetoresistance (MR) values also gradually increased with Sm doping content. The transport properties in polycrystalline ceramics could be adequately explained by the double exchange model, which would be useful for interpreting the CMR effects when used in magnetic devices.

The polycrystalline La1−xCaxMnO3 ceramics exhibit good electromagnetic performance, i.e., high temperature coefficient of resistivity (TCR), which can be tuned flexibly with respect to structures. Unfortunately, the magnetic field applied to these materials causes a massive decrease in TCR, which hinders their practical applications. In this study, polycrystalline ceramic La1−xCaxMnO3:Ag0.2 was fabricated by sol–gel and solid–phase doping methods, and subsequently vast TCR was obtained in the magnetic field for ceramic with x = 0.2. For this polycrystalline material, high value of TCR (58.65%·K−1, 62.00%·K−1) could be maintained with or without magnetic field with metal–insulator (M − I) transition temperature near room temperature range. Extremely high value of TCR in the presence of magnetic field is attributed to the spin–spin coupling effect, which is beneficial to the sensitivity of M − I transition, generating vast TCR in the magnetic field. Overall, these findings provide new prospects for future applications in infrared bolometers.

LCNMO (La0.7Ca0.26Na0.04MnO3) ceramics are prepared via the sol-gel method and sintered at different temperatures, ranging from 1000 to 1300 °C. The influences of sintering temperature on structure and electrical properties are systematically investigated. X-ray diffraction analysis reveals that uniform phase of perovskite structure is formed after sintering. Furthermore, scanning electron microscope (SEM) images demonstrate the presence of on the surface, a highly dense grain boundary and an average grain size of ~2.5 μm. The resistivity-temperature (ρ-T) measurement shows that the resistivity decreases with increased sinter temperature. Moreover, the metal-insulation transition temperature (Tp) and peak resistivity (ρmax) gradually decreased with increasing sintering temperature. The influence of various scattering mechanisms on resistivity at low temperatures is studied, revealing that the residual resistivity (ρ0) dictates the electrical transport performance of the LCNMO ceramics. Also, the different models employed to analyze the ρ-T curves in the temperature range of 100–300 K, confirming the occurrence of phase-separation mechanism in the entire temperature range. The resistance temperature coefficient (TCR) of 10.6%·K−1 and maximum magnetic resistance (MRmax) of 57.12% are achieved after sintering at 1200 °C. These results indicate that LCNMO are promising candidates for infrared detector and magnetic sensor.

Gd doped La0.8Sr0.2MnO3 (La0.8-xGdxSr0.2MnO3, LGSMO) ceramics were prepared by a sol-gel method. X-ray diffraction (XRD) patterns showed that all samples exhibited distorted perovskite structures, R3c. When the Gd3+ content x > 0.03, the crystal structure changed to orthorhombic, Pnma. Scanning electron microscopy (SEM) images showed that the ceramics characterize high density and grain boundary connectivity, and higher Gd3+ doping decreased the grain size from 26.72 μm to 7.42 μm. The temperature dependence of resistivity showed a transition from a low-temperature metal to a high-temperature insulator. The resistivity increased with Gd doping content, and the metal-insulator transition temperature, TP, increased first and then decreased, while the temperature coefficient of resistance (TCR) of the samples first decreased and then increased with Gd3+, and the magnetoresistance (MR) increased first and then decreased. The peak TCR at x = 0.06 was 5.18%·K−1, and MR at 0.04 was 34.57%. The electrical transport properties of the ceramics were explained based on the double exchange (DE) interaction mechanism. The obtained material may have application prospects in magnetic devices and infrared detectors.

La0.7Ca0.3MnO3 has been widely studied as a quintessential example showing colossal magnetoresistance, metal-insulator transition and related temperature coefficient of resistance (TCR). Ag doping was demonstrated as an effective way to enhance the TCR properties of bulk La0.7Ca0.3MnO3. In this article, La0.7Ca0.3MnO3:Ag0.2 thin films were prepared on LaAlO3 (LAO) substrates by pulsed laser deposition with different annealing temperatures. The structure, surface morphology, electrical properties, and magnetic properties of the obtained specimens were investigated. X-ray diffraction results showed that the annealed films exhibited stronger diffraction peaks compared to the unannealed ones, indicating enhanced crystallinity. Resistance-temperature curves revealed metal-insulator transition in thin films annealed at 900–1300 °C. However, the film annealing at 1400 °C exhibited an insulating characteristic due to the evaporation induced non-stoichiometry. A highest TCR of 31.6%·K−1 was achieved in the thin film annealed at 1200 °C. The excellent TCR properties of La0.7Ca0.3MnO3:Ag0.2 films in this study can promote its application in infrared detectors.

Manganites with large temperature coefficient of resistance (TCR) have been considered as potential candidate for infrared detecting. To date, A-site doping with alkaline-earth metal (+2 oxidation state) has been widely used to improve the TCR. However, doping with alkali ions (+1 oxidation state) was less explored. In this study, 2% K doped La0.7Ca0.28K0.02MnO3 (LCKMO) ceramics were prepared by sol-gel method and sintered within the temperature range of 1000–1300 °C for structural and electrical transport properties investigation. X-ray diffraction (XRD) analysis demonstrated that all samples exhibited an orthorhombic perovskite structure with space group Pnma. Elevated sintering temperature would lead to an increase in grain size, which could account for the concurrent decrease of the resistivity (ρ). By optimizing the sintering process, the magnetoresistance (MR) and TCR reached their maxima of 60.95% and 13.54% K−1, respectively, for sintering temperature of 1200 °C. By fitting the temperature dependence of resistivity (ρ-T), grain boundary scattering and small-polaron hopping (SPH) were found to dominate in the low-temperature ferromagnetic and high-temperature paramagnetic regions, respectively. In addition, the ρ-T in the whole temperature range of 100–300 K can also be explained by the percolation model.

La0.67Ca0.33MnO3:Ag0.2 thin films were obtained by pulsed laser deposition followed by post-annealing at 1200 °C for varied durations. Surface morphologies, structures, and electrical and magnetic properties of films prepared with different annealing durations were significantly different. X-ray diffraction results showed that annealed films exhibited stronger diffraction peaks and C-axis preferred growth. Regarding their electrical properties, metal-insulator transition temperature (TMI) and temperature coefficient of resistance (TCR) increased first and then decreased with the increase in annealing duration. In terms of their magnetic properties, thin films displayed ferromagnetic-paramagnetic transition. With the increase in annealing duration, Curie temperature increased first and then decreased. Specifically, the film annealed for 3 h showed excellent electromagnetic properties, with relatively high TCR of 20.3%·K-1 and near room temperature TMI of 285.7 °C. Owing to these excellent properties, as-prepared thin films have application potential in infrared detectors.

In this study, La0.8-xYxSr0.2MnO3 (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y3+ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (Tp) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y3+. At x = 0.06, peak TCR and peak MR reached 4.85% K−1 and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.

La0.72Ca0.28MnO3:Ag0.2/LaAlO3 (100) thin films, prepared by pulsed laser deposition, were post-annealed at different temperatures in air. X-ray diffraction technique were used to analysize the orientation and lattice parameter, atomic force microscope (AFM) and scanning electronic microscope (SEM) were employed to characterize the surface morphology of the thin films. Resistance vs. temperature (R-T) behaviors of the thin films were tested with standard four-probe technique. The surface morphology results show that the grain size increases and crystalline quality of the films are improved with increased annealing temperature up to 1200 °C, while the amount of grains also increases; R-T results illustrate that both the temperature coefficient of resistance (TCR) and metal-insulator transition temperature (TMI) of the films, increase with annealing temperature. In particular, TCR of thin film (post-annealing at 1200 °C) reaches 28.8%·K−1, which makes such films be promisingly applicated in thermal detectors near the room temperature.

La0·7Ca0.3−xSrxMnO3:Ag0.2 (LCSMO:Ag) ceramics with varying strontium (Sr) content (x = 0.0165, 0.03, 0.05, 0.07, and 0.1) were synthesized using a combination of sol–gel and solid-state doping methods and studied as colossal magnetoresistance materials. Significant improvements in the peak temperature coefficient of resistivity (TCR) and resistivity (ρ) were observed across all ceramic samples when the mol% of Sr was adjusted. The low ρ and large TCR values resulted from a combination of two factors. First, the presence of Sr led to the increase in the grain size and decrease in the number of boundaries, which improved the electrical transport and reduced the boundary scattering effects. This resulted in additional improvement in the grain arrangement order. Second, the cell volume (Vcell) and octahedron volume (Voct) both decreased first and then increased. The MnO6 octahedron underwent gradual deformation into a flattened shape, which facilitated carrier migration and increased electrical sensitivity of the material. The ceramic with x = 0.03 maintained high and stable TCR values in different working environments, both without a magnetic field (TCRo, 54.76% K−1) and in the presence of a vertical field (TCR⊥, 33.33% K−1). Furthermore, the material exhibited a low resistivity and a higher insulator–metal transition temperature (Tp). Performance differences among the samples were comprehensively and systematically explained by conducting cross magnetic field tests with field directions parallel and perpendicular to the flat surface of the bulk material, respectively. Overall, the findings of this study indicate that these ceramics act as promising candidates for future applications in information storage devices and magnetic sensors.

In this account, polycrystalline La0.7(Ca0.27Sr0.03)MnO3:Ag0.2 (LCSMO:Ag) ceramics were synthesized by the sol-gel method followed by solid-state doping. The Ag amounts doped into grain boundary and cell lattice could be adjusted by changing the sintering temperature from 1000 °C to 1500 °C. The temperature coefficient of resistivity (TCR) and magnetoresistance (MR) of the obtained LCSMO:Ag ceramics were tested under cross magnetic field with directions parallel and perpendicular to the flat of bulk. The difference between TCR and MR values reached their maxima at sintering temperature of 1450 °C, meaning that degree of lattice distortion reached maximum value. The combined data from X-ray diffraction (XRD) and scanning electron microscopy (SEM) demonstrated that Ag was doped into the grain boundary and lattice cell, and Ag played an important role during the process. The influence of Ag-doping on TCR and MR suggested that degree of lattice distortion can be adjusted by doping, leading to change in isotropic ceramics into anisotropic ceramics without damage. Application of parallel magnetic fields shifted the application temperature to room temperature, and response sensitivity of the ceramics to magnetic field further increased. Overall, these findings look promising for future applications in photoelectric and magnetic devices.

A series of La1−xCaxMnO3:Ag0.2 (0.25 ≤ x ≤ 0.33) polycrystalline ceramic materials were prepared by sol–gel method using methanol as solvent. X-ray diffraction analysis showed that all samples consisted of pure perovskite structurewith no second phase. The grain size and morphology of the samples were studied by scanning electron microscopy. The results showed the samples with good crystallinity, large grain size, and high density. X-ray photoelectron spectroscopy measurement results shows that Ag exists in manganese oxides in two forms (Ag and Ag+). The resistivity temperature curves showed that all samples exhibited a sharp metal–insulator transition. At x = 0.28, the temperature coefficient of resistance was 78.8%·K−1. The magnetization-temperature curves also showed a sharp ferromagnetism–paramagnetism transition, and the Curie temperature increased gradually. These ceramics act as promising candidates for application in infrared and bolometric detectors.

La1−xSrxMnO3 (0.1 ≤ x ≤ 0.25) high density ceramics were prepared by sol-gel method using methanol as solvent. X-ray diffraction analysis showed that all samples exhibited single perovskite structure and no second phase was detected. Scanning electron microscopy images exhibited good particle connectivity on the surface of sample, and grain size increased with the increase in Sr doping. Resistivity-temperature curves of samples were measured by standard four-probe method, and curves exhibited significant differences in studied range of Sr doping. Magnetic measurement results indicated that the variation of susceptibility of different samples was quite different, and the Curie temperature of samples increased with the increase in Sr content. For x = 0.2, temperature coefficient of resistance value of the sample was larger, and corresponding peak TCR temperature was 307.1 K, which is very close to room temperature. Thus, La0.8Sr0.2MnO3 ceramics exhibited high TCR value close to room temperature. Combined with its excellent magnetic properties, La0.8Sr0.2MnO3 ceramics may potentially act as effective candidates for uncooled radiation calorimeter and uncooled magnetic sensor. Applications of La0.8Sr0.2MnO3 ceramics in uncooled infrared radiation calorimeter at room temperature will be highly beneficial.

In this study, facile sol-gel method was used to prepare La0.625Ca0.375-xSrxMnO3:Ag0.2 (LCSMO:Ag0.2, 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:Ag0.2 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 MR were both optimized by Sr doping, providing further understanding of CMR mechanism and application of manganite materials in electromagnetic equipment under room temperature.

Structural, electrical, and magnetic properties of La0.67Ca0.33MnO3 with Ag doped (LCMO:Agx, 0 ≤ x ≤ 0.5) polycrystalline ceramics were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), four-probe resistance-temperature measurements (RT), X-ray photoemission spectroscopy (XPS), and vibrating sample magnetometry (VSM). Significant improvements in temperature coefficient of resistance (TCR) and magnetoresistance (MR) were observed in LCMO:Agx (x = 0.2) samples on addition of Ag. TCR and MR reached maximum values of 26.0% K−1 (269.5 K) and 47.4% (271.8 K) respectively, for x = 0.2. Interestingly, XPS along with XRD and SEM results indicated that silver existed in LCMO:Agx composites in two forms, at A sites (La3+/Ca2+) as Ag+ ion and segregated at grain boundaries (GBs) as metallic silver. All evidences supported the fact that Ag doping was beneficial in improving the electrical and magnetic transport properties of LCMO:Agx ceramics. Hence, they can be used in infrared detectors or magnetic sensor devices. Double-exchange (DE) mechanism and Jahn-Teller (JT) distortion were used to explain the optimal physical performance of LCMO ceramics with Ag doping.

Polycrystalline La0.67Ca0.33MnO3 (LCMO) ceramics were prepared by the sol-gel method. LCMO/mol%(Sm2O3)x (LCMO/(SO)x, x = 0, 0.01, 0.03, 0.05, 0.10) ceramics composites were synthesized using solid state sintering. Effects of Sm2O3-adding on the structures, electrical transport and magnetoresistance properties of the as-prepared LCMO/(SO)x samples were studied. The refinement of the X-ray diffraction (XRD) patterns showed that the lattice parameters (a, b, c, V) of LCMO are slightly decreased, indicating Sm3+ ion substitution of A-site rose. Field emission scanning electron microscope (FE-SEM) study confirmed the composite structure of LCMO/(SO)x ceramics samples. At x ≤ 0.03, the temperature dependence of resistivity shows a double insulator-metal (I-M) transition. While at x ≥ 0.05, it exhibits a single I-M transition with broad transition width at low temperature. An interesting and important result for the present LCMO/(SO)x ceramics composites is the observation of the enhanced transversal low-field magnetoresistance (LFMR) at low applied magnetic field of 300 and 500 mT over a broad temperature range of below ∼ 260 K.

In this study, La0.7Ca0.3-xKxMnO3 (LCKMO) polycrystalline ceramics were synthesized by sol–gel method. Their structures, surface morphologies, and electromagnetic transmission characteristics were studied and discussed. X-ray diffraction results showed that all prepared specimens highly crystalized as single phase with orthorhombic perovskite structure and Pnma space group. Characterization by scanning electron microscopy revealed that grain size increased with the increase in the K+ content. Electrical resistivity as function of temperature (ρ-T) was investigated through standard four-probe method. With the increase in the K+ content, metal–insulator transition temperature (Tp) and the Curie temperature (TC) shifted to higher temperatures, while the resistivity (ρ) reduced. Careful analysis of data indicated that peak temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by K+ substitution. At x = 0.02, peak TCR and MR values reached 6.03%·K−1 and 30.26%, respectively. Characteristics of resultant ceramics were explained based on double exchange interaction. Overall, LCKMO materials act as promising candidates for broad range of applications in magnetic devices and infrared detectors at room temperature.

In this study, high-density La1-xSrxMnO3:Ag0.2 (x = 0.1, 0.125, 0.15, 0.175, and 0.2) ceramics were prepared by the conventional sol-gel method. The peak values of temperature coefficient of resistivity (TCR) in all La1-xSrxMnO3:Ag0.2 samples were systematically controlled by changing the Sr content. A significant improvement in peak TCR have been observed through adjusting the Sr content. At doping molar ratio of x = 0.15 in La1-xSrxMnO3:Ag0.2 ceramics, the peak TCR value reached 14.7% K−1, and peak TCR temperature (TK, 288.2 K) was estimated to room-temperature (290 K). Visibly, it is encouraging to get such a high room-temperature TCR value. These findings suggested that La0.85Sr0.15MnO3:Ag0.2 ceramics could be used to prepare room-temperature uncooled infrared bolometers.

To obtain comprehensive materials with both high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at low magnetic fields, the polycrystalline La0.72−xNdxCa0.28MnO3(x = 0.02, 0.04, 0.06, and 0.08) were synthesized using the sol–gel method, which is a significant practical application in magnetic sensors and bolometers. We observe an enhancement of TCR and MR by substituting La3+ with Nd3+. The excellent TCR (19.21 %·K−1) and MR (73.10 %) are observed in La0.68Nd0.04Ca0.28MnO3 at the 1 T magnetic field. Meanwhile, the maximum TCR reaches 22.68 %·K−1 without a magnetic field. This substitution increased the total magnetic moment and a slight increase in average grain size. The Mn3+ and Nd3+ spin within the disordered region realign along the field direction with a magnetic field. Therefore, Nd-doped La0.72Ca0.28MnO3 ceramic materials can effectively improve the material properties.

Polycrystalline La 0.67 (Ca 0.27 Sr 0.06 )MnO 3 :mol%Ag x (LCSMO:Ag x , x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by conventional sol-gel method. Effects of Ag-doping ( x ) on structures, surface morphologies, electrical and magnetic transport properties of the as-prepared LCSMO:Ag x samples were evaluated. X-ray diffraction (XRD) showed that all samples crystallized in orthorhombic structures with Pnma space group. As x content increased, main diffraction peak ( 121 ) shifted downwards lower angles, giving rise to A site ions (Ca 2+ or Sr 2+ ) substitution by Ag + . This, in turn, caused slight expansion of the lattice and improved Mn 4+ ion concentration. Meanwhile, grain size gradually increased and grain boundaries (GBs) reduced, the resistivity decreased and metal-insulator transition temperature ( T MI ) almost kept constant at room temperature. On the other hand, the magnetoresistance ( MR ) and temperature coefficient of resistance ( TCR ) increased during first stages and then decreased as x further rose. An optimum doping molar ratio of x = 0.2 was obtained, with room temperature MR and TCR reaching up to 44.5% and 22.5% K −1 , respectively. Curve fitting results indicated that in low temperature ferromagnetic metallic (FM) region, conductive behavior of LCSMO:Ag x was mainly related to electron-electron, electron-phonon, and electron-magnon scattering. In high temperature paramagnetic insulating (PI) region, data followed the small-polaron-hopping model. Conductive behavior in the entire temperature region was fitted with phenomenological equation under percolation approach, and found to depend on segregation of FM and PI phases. These findings suggest promising applications of LCSMO:Ag x materials in photoelectric (uncooled bolometer or infrared detectors) or magnetic devices (uncooled magnetic sensors) at room temperature.

A series of polycrystalline composites La0.70Ca0.3-xSrxMnO3:mol-%Ag0.2 (LCSMO:Ag, x = 0.00, 0.0165, 0.03, 0.05, 0.07, and 0.10) were prepared by sol-gel method using methanol as solvent. The structural, electrical, and magnetic properties were investigated. All of the LCSMO:Ag composites showed orthorhombic perovskite structure with a space group of Pnma. Electrical and magnetic transport properties curve show that both the insulator-metal transition temperature (Tp) and the Curie temperature (TC) increase from 263 K to 250 K correspondingly for LCSMO:Ag (x = 0.00) to 331 K and 302 K, respectively, for LCSMO:Ag (x = 0.10). The temperature coefficient of resistance (TCR) value reduce from 68.8%·K−1 to 11.2%·K−1 with increase of Sr content. At Sr doping x = 0.07, the value of Tp higher than room temperature (312 K), and the maximum values of TCR reached up to 19.9%·K−1, which is significantly higher than that previously reported.

A series of polycrystalline ceramic samples of La 0.7 Ca 0.25 X 0.05 MnO 3 (X = Sr, Pb, and Ba) were prepared using sol-gel method with methanol as the solvent. Resistivity-temperature curves show sharp metal-insulator transitions; furthermore, magnetization-temperature curves reveal a transition from ferromagnetism to paramagnetism. After doping with Sr, Pb, and Ba, both average ionic radius of A site and tolerance factor increase. Double-exchange also increases, which supports the metallicity of polycrystalline ceramic. However, larger lattice distortion is also introduced via substitution of larger ions at A site, and this tends to localize carriers. There is competing relationship between tolerance factor enhancement and lattice distortion; in combination, they determine electronic and magnetic properties of polycrystalline ceramics. As a result, after doping with Sr, Pb, and Ba, resistance temperature coefficient decreases, and metal-insulator transition temperature ( T IM ) increases. Meanwhile, ferromagnetic-paramagnetic transition temperature shows the same tendency as T IM .

Polycrystalline Ag-doped [La0.625(Ca0.315Sr0.06)MnO3]1-x:Agx (LCSMO) ceramics with (x = 0, 0.03, 0.05, 0.10, 0.15, and 0.20) were prepared by sol-gel method, and their structures and properties were characterized. X-ray diffraction results indicated that all bulk samples had single phase with orthorhombic phase (space group of Pbnm) without impurities. With the increase of Ag doping content, the resistivity of the samples decreased, while the remanent magnetization and coercive field increased. The metal to insulator transition temperature (Tp), temperature coefficient of resistance (TCR) and Curie temperature (Tc) for x = 0.20 were determined as 300 K, 9.38% (292.6 K) and 291.86 K respectively. The highest MR value of 28.36% (295.03 K) was obtained at x = 0.15. XPS data revealed that substitution between A-site ions and Ag+ could increase the ratio of Mn4+ ion. Double exchange effect (DE) enhanced by changing Mn–O bond distance, Mn–O–Mn bond angle, and increasing Mn4+ ion concentration. These features promoted the transfer of itinerant electron between Mn3+ and Mn4+ ions. However, the magnetization obtained at x = 0.20 was less than that at x = 0, as diamagnetic Ag released magnetism of the samples. The results suggested that the LCSMO polycrystalline ceramics could be used as a candidate to prepare room temperature infrared detectors, magnetic sensors or magneto-electric devices, and so on.

A series of La 0.72 Ca 0.28− x Sr x MnO 3 :Ag 0.2 ( x = 0.00, 0.01, 0.03, 0.05, 0.07, and 0.10) composites was prepared by sol–gel method using methanol as solvent. Sample composition was analyzed by X-Ray diffraction, which revealed that all samples are pseudo-perovskite. Grain size and morphology were characterized by scanning electron microscopy. The results showed that all samples are of high density and large grain size in the range of 50–100 µm. All samples exhibited sharp metal–insulator (M-I) transitions, along with a drastic transition from paramagnetism to ferromagnetism. With increased Sr doping, radius of A-site ion increased, both Curie temperature T C and M-I transition temperature shift to higher temperature increases, and transition width turns widener which makes a decreased temperature coefficient of resistance (TCR). Larger ionic radius of Sr 2+ , which resulted in enhancement of double exchange, enlarges Mn–O–Mn bond angle, hence improves electrical and magnetic properties. It was found that TCR could reach 24.3% K −1 by Ag addition at near room temperature, which makes this composite promising potential applications in infrared detection and night vision.

Perovskite manganite La1−yCayMnO3 has giant and tunable magnetoresistance (MR) and anisotropic magnetoresistance (AMR), still the following two challenges are of great concern: (i) large magnetic field is required for achieving excellent electromagnetic performance; and (ii) metal–insulator (M–I) transition temperature (TMI) significantly different from room temperature; both these factors limit its practical applications. In this study, polycrystalline La0.7Ca0.3MnO3:Agx composites were fabricated by sol–gel and solid–phase addition methods, and large MR and AMR (84.1% and 32%, respectively) were obtained at near room temperature in 1 T magnetic field when x = 0.2. Evidently, it was found that the grain boundary scattering effects were weakened due to Ag−addition, which resulted in an enhancement of MR. The tremendously high AMR value was attributed to the anisotropic spin–orbit coupling (SOC) effect. The electron–scattering and small–polaron hopping models were used to successfully fit the resistivity data, which support the SOC effect under different magnetic fields that enhance the AMR. These results support further development of the perovskite manganese oxide with potential applications in manganite devices.

Polycrystalline YBa 2 Cu 3 O 6+ δ bulks were synthesized by sol-gel method. Sintering processing played a vital role in the evolution of phase structure and microstructure, and thus significantly influenced their superconducting properties. The influence of calcination temperature, sintering temperature, on the bulks structure, morphology and superconducting behaviors were investigated. The results showed that the oxygen content drastically increased with calcination temperature and sintering temperature. The SEM images revealed that the grains grew up monotonously with increase of calcination temperature. With increased calcination and sintering temperature, the resistivity was reduced gradually and the superconducting properties increased. Moreover, it was found that the optimal superconducting properties (with the highest superconducting transition temperature T c onset and the narrowest transition width Δ T ) were obtained at calcination temperature of 900 °C and sintering temperature of 950 °C.