Enhanced room-temperature TCR of La0.67Ca0.33-xSrxMnO3 (0.06 ≤ x ≤ 0.11) polycrystalline ceramics by Sr content adjustment

Herein, the influence of Sr doping on chemical composition, microstructure, and electrical transport properties of La_0.67Ca_0.33-xSr_xMnO₃ (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.     

La1-xSrxMnO3:Ag0.2 (0.1 ≤ x ≤ 0.2) ceramics with large room-temperature TCR for uncooled infrared bolometers

In this study, high-density La_1-xSr_xMnO₃:Ag_0.2 (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 La_1-xSr_xMnO₃:Ag_0.2 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 La_1-xSr_xMnO₃:Ag_0.2 ceramics, the peak TCR value reached 14.7% K⁻¹, and peak TCR temperature (T_K, 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 La_0.85Sr_0.15MnO₃:Ag_0.2 ceramics could be used to prepare room-temperature uncooled infrared bolometers.     

Utilization of Ag ions to improve room-temperature TCR of La0.85-xSr0.15AgxMnO3 polycrystalline ceramics

In this work, silver-doped La_0.85-xSr_0.15Ag_xMnO₃ (LSAMO, 0.05 ≤ x ≤ 0.20) polycrystalline ceramics were prepared by the sol-gel method. The experimental characterization of ceramics revealed that Ag ions were bounded with the lattice matrix. The resistivity and the corresponding peak temperature coefficient (TCR) of LSAMO ceramics were systematically tuned by changing the Ag dopant content. Adjusting the proportion of Ag enabled one to vary the metal-insulator transition temperature (T_MI) for LSAMO specimens in a wide range, and their peak resistivity temperature (T_p) was increased from 299.6 to 335.7 K. At the same time, the peak TCR value achieved its maximum of 15.2% K^?1 at the doping molar ratio x of 0.15 and the peak resistivity temperature (T_k) of 294.1 K. In addition, the electrical transport was described in the context of the small polaron hopping (SPH) model and the phenomenological percolation model (PP) over the metal-insulator transition region. Under the combined action of the Jahn-Teller (JT) effect, the double exchange (DE) mechanism and the PP model, LSAMO ceramics possessing high TCR at room temperature were obtained by varying the amount of Ag doping. The observed properties suggest LSAMO material can be used in advanced uncooled infrared bolometers.     

Improvement of electrical and magnetic properties in La0.67Ca0.33Mn0.97Co0.03O3 ceramic by Ag doping

For the perovskite manganite La_1-xCa_xMnO₃, achieving high temperature coefficient of resistance (TCR) and magnetoresistance (MR) is the key to realize its potential applications. In this study, high-quality La_0.67Ca_0.33Mn_0.97Co_0.03O₃:Ag_x polycrystalline ceramics were prepared by the sol-gel method. The results show that Ag doping has important impact on metal-insulator and ferromagnetic-paramagnetic transitions. A Ag doping amount increases, the grain size of the samples increases at x = 0.05 and then decreases. The doping of Ag can improve the crystalline quality of the samples and enhance the connectivity between grains, thereby improving the metallicity of the system. Additionally, with Ag doping amount increases, the resistivity of the samples gradually decreased, while the Curie temperature and the metal-insulator transition temperature gradually increased. Especially after Ag doping, both the TCR peak (TCR_peak) and the MR peak (MR_peak) values are significantly improved. The TCR_peak reaches 65.2%·K^?1 at x = 0.1, while the MR_peak is as high as 82.6% at x = 0.05 under 1 T magnetic field. Doping perovskite manganite ceramics with Co and Ag can greatly optimize their TCR and MR, favoring the potential applications of these materials.     

Enhanced room temperature coefficient of resistivity (RT-TCR) and broad metal-insulator transition temperature (TMI) of La0.67Ca0.33-xAgxMnO3 polycrystalline ceramics

Polycrystalline La_0.67Ca_0.33-xAg_xMnO₃ (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.     

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La_0.8-xY_xSr_0.2MnO₃ (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 (T_p) 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.     

A-site mixed-valence co-doping to optimize room-temperature TCR of polycrystalline La0.8K0.04Ca0.16-xSrxMnO3 ceramics

Herein, polycrystalline La_0.8K_0.04Ca_0.16-xSr_xMnO₃ ceramics (A-site = La, K, Ca and Sr, 0.00 ≤ x ≤ 0.16) are successfully synthesized via the sol-gel process and the influence of Sr content on microstructural and electrical transport properties is investigated in detail. The results reveal that the replacement of Ca-ions by Sr-ions resulted in a decreased resistivity and enhanced temperature coefficient of resistance (TCR) peak temperature (T_k). The influence of A-site mixed-valence co-doping, where A-site is occupied by monovalent alkali-metal, divalent alkaline-earth element and trivalent rare-earth element, on electrical transport properties can be explained by using different conduction mechanisms in different temperature regions. At optimal doping content of 0.12, peak TCR of the resistivity was found to be 12.04% K⁻¹ at 290.38 K, which is the best reported TCR for A-site mixed-valence co-doped perovskite manganese oxides. These results confirm that high room-temperature TCR values can be achieved by optimizing A-site mixed-valence co-doping and demonstrate the potential of polycrystalline La_0.8K_0.04Ca_0.04Sr_0.12MnO₃ ceramic in uncooling infrared bolometers.     

Optimization of room-temperature TCR of polycrystalline La0.9-xSrxK0.1MnO3 ceramics by Sr adjustment

High-density La_0.9-xSr_xK_0.1MnO₃ ceramics (LSKMO, A-site = La, Sr and K, 0 ≤ x ≤ 0.25) are successfully fabricated by using facile sol-gel method. Electrical properties are performed by using combination of phenomenological percolation (PP) model, double exchange (DE) mechanism, and Jahn-Teller (JT) effect. Moreover, X-ray diffraction and scanning electron microscopy are employed to analyze the structure and morphology of LSKMO ceramics. Valence states and ionic stoichiometry are assessed by using X-ray photoemission spectrometry. Results reveal that Sr²⁺ ions, substituting La³⁺ ions, significantly influenced DE mechanism and JT effect. In addition, Sr-doping plays essential role in improving electrical properties of LSKMO ceramics. At optimal doping content of x = 0.09, peak temperature coefficient of resistance (TCR) of the resistivity is found to be 11.56% K^?1 at 297.15 K, which is optimal TCR for A-site K-occupied perovskite manganese oxides. These results confirm that polycrystalline LSKMO ceramics render high room-temperature TCR values due to Sr-doping.     

Electrical transport properties of Sm-doped La0.7Ca0.3MnO3 polycrystalline ceramics

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, La_0.7Ca_0.3MnO₃ was doped with different levels of Sm by the sol-gel method to yield a series of La_0.7-xSm_xCa_0.3MnO₃ (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 (T_p) 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.     

Effect of Fe substitution on temperature coefficient of resistance and magnetoresistance of La0.67Ca0.33MnO3 polycrystalline ceramics

La_0.67Ca_0.33MnO₃ perovskite manganate exhibits high temperature coefficient of resistance (TCR) and large magnetoresistance (MR) effect, these enable novel multifunctionalities. Mn site substitution can change magnetic order of the manganates, thereby tailoring both electrical and magnetic transport. Herein, La_0.67Ca_0.33Mn_1-xFe_xO₃(0 ≤ x ≤ 0.06) polycrystalline ceramics were prepared by sol-gel route. The effect of Fe substitution on TCR and MR of La_0.67Ca_0.33MnO₃ was studied. Fe replacing Mn ions, would weaken double exchange, and significantly reduced Curie temperature and ferromagnetism. Additionally, Fe doping promoted the development of grain. TCR increased first with Fe content x and then decreased, and reached 45.2%·K^-1 at x = 0.01, which is the highest value reported. Notably, with Fe doping, MR gradually increased and reached 81.1% (1 T) at x = 0.06. Fe doping can significantly enhance TCR and MR, which generates promising potential in (uncooled) thermistor/infrared detecting and magnetic sensors.     

Influence of Ag on TCR and MR of La0.7(Ca0.27Sr0.03)MnO3:Ag0.2 ceramics subjected to cross magnetic fields

In this account, polycrystalline La_0.7(Ca_0.27Sr_0.03)MnO₃:Ag_0.2 (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.     

Effect of Gd doping on electrical transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

Gd doped La_0.8Sr_0.2MnO₃ (La_0.8-xGd_xSr_0.2MnO₃, LGSMO) ceramics were prepared by a sol-gel method. X-ray diffraction (XRD) patterns showed that all samples exhibited distorted perovskite structures, R_3c. 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, T_P, 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.     

Structural,electrical, and magnetic transport properties of La0.72Ca0.28Mn1−xCrxO3 (0 ≤ x ≤ 0.06) ceramics

To obtain comprehensive materials with both high temperature coefficient of resistivity (TCR) and magnetoresistance (MR) at low magnetic fields, polycrystalline La_0.72Ca_0.28Mn_1?xCr_xO₃ (x = 0, 0.02, 0.04, 0.06) ceramics were prepared herein by sol–gel method. Electronic configuration of Cr³⁺ ions is similar to that of Mn⁴⁺ ions, therefore, successive substitution of Mn with Cr increases electrical resistivity and decreases metal–insulator transition temperature of ceramics, even yielding hump-like feature for Cr-rich (x = 0.06) samples. The best TCR (28.50%·K^?1) and MR (72.37%) values were obtained simultaneously at Cr dopant content of 0.02 (La_0.72Ca_0.28Mn_0.98Cr_0.02O₃). Strong response of the material to temperature and magnetic field was caused by minimal symmetry of orthorhombic structure and the most robust Jahn–Teller distortion. With increasing Cr content, Mn³⁺/Mn⁴⁺ or Mn³⁺/Cr³⁺ double exchange was diluted, and Cr³⁺/Cr³⁺ or Cr³⁺/Mn⁴⁺ superexchange was promoted. However, the internal competition effect was not conducive to the improvement of material properties.     

Nd1-xSrxMnO3 (x = 0.3): A perovskite manganite ceramic that exhibits PTC and NTC double properties

Nd_1-xSr_xMnO₃ (NSMO, x = 0.280, 0.300, 0.330, 0.350, and 0.375) polycrystalline ceramics were fabricated using the sol-gel method. The crystal structure, surface morphology, valence state, elements distribution, and electrical properties were examined to understand the effect of Sr doping on NdMnO₃ ceramics. The resistivity of the NSMO ceramics was measured using a standard four-probe method. The results obtained revealed that Sr doping significantly decreased the resistivity of the ceramics, which can be explained by the double exchange (DE) interaction and small-polaron hopping (SPH) model. The Nd_0.70Sr_0.30MnO₃ ceramic had a positive temperature coefficient (PTC) of resistivity (16.69% K^?1) at 197.5 K, and is expected to be used for preparing electronic switches with high sensitivity. Additionally, the NSMO ceramics maintained a stable negative temperature coefficient (NTC) of resistivity (?1% K^?1) for x = 0.300 in the temperature range of 210.6–342.5 K. In conclusion, it is worth exploring materials with a high PTC and NTC over an extended temperature range, possessing the double potential function for high sensitivity or wide-temperature detection for electronic switches or infrared bolometers.     

Optimized electrical transport properties of La0.7Ca0.18Sr0.12MnO3 film by adjusting sintering temperature

Perovskite manganites La_x(Ca_1-ySr_y)_1-xMnO₃ have conspicuous electrical transport properties used for the application of uncooled bolometers. Thus, considerable attention has paid to the researches on electrical transport properties of La_x(Ca_1-ySr_y)_1-xMnO₃. Temperature coefficient of resistivity (TCR) and the corresponding peak temperature (T_k) are crucial parameters for uncooled bolometers. Optimal La_0.7Ca_0.18Sr_0.12MnO₃ (LCSMO) films with highly oriented growth were prepared on (00l) LaAlO₃ substrate at different sintering temperature (T_s) using a facile sol-gel spin coating method. Based on the analysis of microstructure, ionic valence, surface morphology and electrical transport properties, LCSMO films exhibit high crystallinity and large room-temperature TCR at higher T_s. Theoretical Curie temperature and the coupling of the electron lattice weaken gradually with the increase in T_s, which is the dominant factor for the optimization of electrical transport properties. Additionally, the results suggest that the active energy and theoretical Curie temperature play a crucial role in adjusting TCR and T_k. With the increase in T_s, the former increases while the latter decreased, resulting in a high TCR value (12.10%/K) at room temperature (297.38 K). The peak TCR value of LCSMO sintered at 1673 K is about 25% higher than that of the film sintered at 1473 K. The resulting LCSMO films demonstrate great potential for application in uncooled bolometers. Further theoretical basis on the effects of T_s on electrical transport properties was provided.     

Co-optimization of Na and K doping for improved room-temperature TCR of La0.7(Na0.3-xKx)MnO3 polycrystalline ceramics

Polycrystalline La_0.7(Na_0.3-xK_x)MnO₃ (LNKMO, x = 0.10, 0.15, 0.20, 0.25, and 0.26) ceramics were successfully compounded by adopting conventional sol-gel technology. The physical properties of as-prepared specimens were closely related to their morphology and internal structure, which were characterized and analyzed via X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. Results confirmed that La⁺ ions located at A-sites in crystal lattice were partially substituted by doped Na⁺ and K⁺ ions, which resulted in rotation and distortion of MnO₆ octahedron. Lattice distortion was primary factor behind double exchange (DE) mechanism and Jahn-Teller (JT) effects. In addition, Na and K dopants altered relative amount of Mn³⁺ and Mn⁴⁺ ions, causing intensity variation in DE effect. These changes contributed to a decline in resistivity and an increase in peak resistance temperature (T_k) with increasing K doping level. Meanwhile, optimal temperature coefficient of resistance (TCR) value of LNKMO ceramics reached 8.48% K^?1 at 292.14 K when x = 0.25. This work reveals the mechanism of Na and K co-doping to optimize electrical transport properties of LNKMO manganese oxides and provides excellent material for the fabrication of uncooled infrared bolometers.     

A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn³⁺/Mn⁴⁺ pairs, distortion of MnO₆ octahedron, and electrical transport properties of La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ materials, room-temperature coefficient of resistivity (TCR) of La_0.7Ca_0.3-xSr_xMnO₃ (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO₃ (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K^-1 at 296.1 K in La_0.7Ca_0.18Sr_0.12MnO₃ films.     

Evolution of microstructure and electrical properties of Aurivillius phase (CaBi4Ti4O15)1-x(Bi4Ti3O12)x ceramics

(CaBi₄Ti₄O₁₅)_1-x(Bi₄Ti₃O₁₂)_x (CBT-xBIT) Aurivillius phase ceramics were synthesized by the conventional solid reaction method. The evolution of the structure and the electrical properties of CBT-xBIT ceramics were systematically investigated. Due to the enhanced spontaneous polarization induced by internal stresses on the Bi₂O₂ layers in the CBT-xBIT structure, the optimal piezoelectric coefficient (d₃₃ ~ 13?pC/N) was obtained in the ceramics with x?=?0.3 while exhibiting a relatively good thermal stability in the temperature range of 20–700?°C. The dc resistivity (ρ_dc) of the CBT-xBIT ceramics exhibited a higher value (≥?10⁹ Ω?cm) at room temperature, and the tan δ value of CBT-xBIT (x= 0, 0.1 and 0.3) within the temperature range of 20–500?°C maintained stability as a result of the domain structure and point defect concentration in the ceramics. In addition, a distinctive double dielectric peak anomaly was observed in the ε_r-T curves of the CBT-xBIT (x= 0.3, 0.5 and 0.7) ceramics, and it plays a remarkable role in the thermal stability of the piezoelectricity of CBT-xBIT ceramics. As a result, such research can benefit high temperature practical piezoelectric devices.