Ca0.7Ti0.7La0.3Al0.3O3微波介质陶瓷微观组织结构与介电性能

采用固相反应法制备斜方晶系钙钛矿结构Ca07Ti07La0.3Al0.3O3微波介质陶瓷,研究了Al3+、Ca2、Ba2+和La3+离子掺杂对CTLA陶瓷微观组织结构和介电性能的影响.研究结果表明不同掺杂离子对于CTLA陶瓷的微观结构和介电性能有很大的影响,不同离子掺杂CTLA陶瓷的晶粒尺寸、气孔率、晶界析出相有很大的不同.Al3+、Ca2+、Ba2+和La3+离子掺杂可以有效降低CTLA陶瓷的谐振频率温度系数,但Ca2+、Ba2+离子掺杂同时也降低了CTLA陶瓷的致密度和Q×f值,Al3+、La3+离子掺杂不仅有效提高了CTLA陶瓷的致密度和Q×f值,并且有效降低了谐振频率温度系数.适量掺杂La3离子可以有效促进CTLA陶瓷的致密化,提高了CTLA陶瓷的微波介电性能.掺杂0.15mol％ La3+的CTLA陶瓷在4.7 GHz下测试介电性能为:εr=48.39,Q×f=32560 GHz,τf=23.68 ppm/C.     

溶胶-凝胶法制备La_(0.8)Sr_(0.2)MnO_3涂层的研究

采用溶胶-凝胶法在氧化铝基体制备La0.8Sr0.2MnO3涂层。通过热重分析、差示扫描量热分析、X射线衍射分析、扫描电镜等手段对涂层制备工艺、相组成以及表面形貌进行了分析。结果表明,用溶胶-凝胶法可制备La0.8Sr0.2MnO3单相涂层;涂层表面致密性好,但有裂纹,且不平整;涂层均匀且与基体间界面不明锐,涂层厚度约为0.5μm;另外,随Sr含量增加,La1-xSrxMnO3涂层方块电阻减小,当x=0.3时达到最小值。     

溶胶-凝胶法制备La0.65Sr0.35MnO3及其光催化性能

采用溶胶-凝胶法合成了以Sr取代部分稀土金属La的La0.65Sr0.35MnO3样品,经XRD衍射分析确定其具有钙钛矿的晶体结构,并对其进行了光催化活性实验.结果表明:在700℃时,La0.65Sr0.35MnO3样品平均粒径为19 nm;在太阳光照射下,LA0.65Sr0.35MnO3降解亚甲基蓝溶液120 min时,其降解率达到90%.     

Investigation of structural,optical, electrical,and magnetic properties of Fe-doped La0.7Sr0.3MnO3 manganites

In this work, the physical properties of nanocrystalline samples of La_0.7Sr_0.3Mn_1−xFe_xO₃ (0.0 ≤ x ≤ 0.20) perovskite manganites synthesized by the reverse micelle (RM) technique were explored in detail. The phase purity, crystal structure, and crystallite size of the samples were determined using X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. All the samples had rhombohedral crystal structure and crystallite size increased with increase in Fe content in La_0.7Sr_0.3MnO₃. The scanning electron micrographs (SEMs) exhibited smooth surface morphology and nonuniform shape of the particles. The optical properties studied using UV-visible absorption spectroscopy revealed a decrease in the absorbance and optical band gap with an increase in Fe content in La_0.7Sr_0.3MnO₃ compound. The temperature-dependent resistivity measurements revealed semiconducting nature of x = 0 and 0.1 samples up to the studied temperature range, while a metal-to-insulator transition was observed at higher Fe doping. Magnetic studies revealed weak ferromagnetism in all the samples and a reduction in the maximum magnetization with an increase in Fe content. A close correlation between electrical transport and magnetic properties was observed with the doping of Fe ion in La_0.7Sr_0.3MnO₃ at Mn site. These results advocate strong interactions associated with the double exchange mechanism among Fe³⁺ and Mn³⁺ ions.     

Structural,morphological, and magnetic properties of sol-gel derived La0.7Ca0.3MnO3 manganite nanoparticles

La_0.7Ca_0.3MnO₃ (LCMO) manganite nanoparticles are synthesized via a sol-gel route at different annealed temperatures. Their structural, morphological, and magnetic properties are investigated. The X-ray diffraction patterns coupled with electron diffraction confirm that all the LCMO samples are single phase and crystallize in the orthorhombic perovskite structure (Pnma space group). The morphology of the samples observed by TEM, reveals a spherical shape with an average grain size lower than 50 nm. The resolved lattice fringes in high-resolution TEM images also reveal the single crystalline nature of the LCMO nanoparticles. Magnetization measurements versus temperature under low magnetic field (0.01 T) show a paramagnetic - ferromagnetic transition for all the samples. The Curie temperature (T_c) is found to be decreased with increasing the annealed temperature. A bifurcation is observed in the zero field-cooled and field-cooled magnetizations, indicating a competition between ferromagnetic and antiferromagnetic interactions in the nanoparticles at low temperatures. Field-cooled hysteresis measurements suggest a cluster glasslike behavior of the nanoparticles. Room temperature and low temperature M - H loops demonstrate that all the samples exhibit ferromagnetic behavior at 5 K, whereas a paramagnetic behavior at room temperature. Resistivity behavior of the LCMO samples shows that they exhibit a metal - insulator transition. Magnetoresistance of ~ 50% at the field up to 8 T was observed at 2 K in the LSCO samples annealed at 600 °C.     

Ferroelectric and ferromagnetic properties of Mn-doped 0.7BiFeO3–0.3BaTiO3 solid solution

Binary solid solutions 0.7BiFeO₃–0.3BaTiO₃–x wt.% MnO₂ (x = 0, 0.2, 0.3, and 0.5) were prepared by a traditional ceramic process. All ceramic samples show single perovskite phase. The effect of manganese doping on structure, dielectric, ferroelectric and ferromagnetic properties, and resistivity was investigated. Results show that Mn-dopant can improve the sintering ability of the materials when MnO₂ content is below 0.3 wt.%. When MnO₂ content exceeds 0.3 wt.%, the sintering ability is weakened and the phase structure of 0.7BiFeO₃–0.3BaTiO₃ solid solution changes from rhombohedral into tetragonal phase. With increasing concentration of MnO₂, the resistivity increases at first and then decreases. Whereas the coercive electric field decreases at first and then increases, the remanent magnetization M_r increases and the coercive magnetic field decreases.     

Structural characterization and magnetic properties of single-crystalline (La0.6Pr0.4)0.67Ca0.33MnO3 nanowires

Herein, structural and magnetic properties of single-crystalline (La_0.6Pr_0.4)_0.67Ca_0.33MnO₃ nanowires synthesized via a hydrothermal process are reported. The nanowires are crystallized in an orthorhombic structure (Pnma space group). Their lattice parameters follow the relationship a ≈ c ≈ b/√2 These nanowires exhibited a clean and smooth surface with diameters of 60-120 nm and an average length of approximately 2.0 μm. High-resolution transmission electron microscopy images confirmed the single-crystalline nature of the nanowires growing along the [100] direction. The nanowires demonstrated magnetic hysteresis loops at low temperatures and a weak exchange bias (EB) effect. Paramagnetic (PM)–ferromagnetic (FM) phase transition occurred at a Curie temperature (T_C) of 224 K, and strong irreversibility between zero-field-cooled (ZFC) and field-cooled (FC) magnetization (M_ZFC and M_FC, respectively) curves was observed at 273 K. The M_ZFC curve exhibited a significantly broad peak with a maximum at a freezing temperature (T_f) of 134 K. Relative difference between M_FC and M_ZFC in the nanowires [(M_FC − M_ZFC)/M_FC] rapidly increased below T_f and reached approximately 50% below 35 K. The effective magnetic moment deduced from the Curie constant is larger than the theoretical value, indicating short-range FM interactions in the nanowires. A positive PM T_C (θ_p) implies dominant FM interactions in the nanowires, and θ_p > T_C observed herein indicates the existence of short-range ordered states above T_C.     

La0.8Sr0.2MnO3 MgAl2O4丝网蜂窝催化剂用于低浓度甲烷催化燃烧

首先,采用沉积-沉淀法制备了与镁铝尖晶石(Mg Al2O4,简称MA)载体复合的镧锶锰钙钛矿(La_(0.8)Sr_(0.2)MnO_3,简称LSM)催化剂LSMn,考察了MA与LSM的物质的量比(n)、前驱体老化和焙烧温度系列制备条件的影响,在程序升温微反应器中测试了LSMn粉体对低浓度甲烷催化燃烧反应的活性,并用FTIR、H2-TPR、BET、XRD和SEM手段对催化剂进行表征。结果表明:在10℃老化和800℃下焙烧的LSM4性能最佳,T50为489.4℃,活化能Ea=112.5 kJ/mol,BET比表面积仍有81.6 m2/g,表现出较好的抗烧结性能。然后,用浆料涂覆法依次将第二载体(MA)和LSM4粉体涂覆在Fe-Cr-Al合金丝网上制得了丝网蜂窝催化剂。考察了第二载体及LSM/4MA粉体涂覆量的影响,在空速40000 h–1条件下测试了蜂窝催化剂的活性,结果表明,MA的最佳涂覆厚度约20mm,LSM/4MA的最佳涂覆量约为2.65 mg/cm2,甲烷转化率30%以上。     

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.     

WO3/BiOCl0.7I0.3光催化剂的制备及其光催化降解机理

通过简单的煅烧法和原位沉淀法制备了WO₃/BiOCl_0.7I_0.3复合光催化材料,利用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱(XPS)、紫外可见漫反射光谱(UV-Vis DRS)等表征手段对合成材料的微观形貌、化学组成等进行表征。并通过可见光催化降解20mg/L盐酸四环素来评价材料的光催化性能,结果显示,WO₃/BiOCl_0.7I_0.3复合材料较单一的BiOCl_0.7I_0.3和WO₃具有更优的光催化性能,其中W与Bi摩尔比为1∶15时,复合材料具有最高的光降解率,可见光下60min达到最高值93.84%,且四次循环试验后仍具有较好的光催化活性。对自由基捕获试验结果及电子自旋共振光谱进行分析,明确了h⁺和·O₂^-为光催化主要活性物质,提出WO₃/BiOCl_0.7I<...     

Synthesis and magnetic properties of complex oxides La0.67Sr0.33MnO3 nanowire arrays

As a new approach of melt-injection-decomposition method, it has been successfully adopted for the synthesis of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays. X-ray diffraction studies confirmed the formation of perovskite manganite phase of the sample. Transmission electron microscope and scanning electron microscope characterizations showed a large quantity of one-dimensional nanowires is obtained and the nanowires are continuous, concrete, oriented and rather uniform with an average diameter of 170 nm and length of several tens of micrometers. Magnetic measurements exhibited good ferromagnetic properties at the temperature of 10 K and 300 K respectively. The transition temperature of the complex oxides La_0.67Sr_0.33MnO₃ nanowire arrays is about 350 K, which will endow their great potential applications in magnetoresistance, spintronics or sensor fields at room temperature.     

Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La_0.7Na_0.3MnO₃ sample synthesized by microwave (MW) irradiation (Na_0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na_0.3LMO_SS). Single phase Na_0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na_0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at T_C ～324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La_0.7Na_0.3MnO₃.     

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.     

Co3+掺杂对La0.7Sr0.3Mn1-xCoxO3的结构及光催化性能的影响

本文采用溶胶-凝胶法制备了钙钛矿型复合氧化物La0.7Sr0.3Mn1-xCoxO3。利用XRD,SEM对纳米颗粒的结构、形貌进行了表征。结果表明,合成的样品为钙钛矿型,呈立方颗粒状,粒径均匀,边长约为1μm。根据甲基橙的降解情况对其光催化活性进行了研究。结果表明,经过B位Co3+离子掺杂的La0.7Sr0.3Mn1-xCoxO3样品,光催化活性明显提高。     

固相微波法制备Pd/La0.5Pb0.5MnO3催化剂及其催化性能

以MnCO3P、PbCO3和La2O3为原料,利用固相微波法制备La0.5Pb0.5MnO3钙钛矿型复合氧化物载体,并用XRD、SEM以及WJL激光粒度仪进行表征.通过沉淀法得到负栽型钯催化剂,以氧化羰基化合成碳酸二苯酯为评价对象,考察不同载体制备工艺对催化性能的影响.实验结果表明,载体最佳制备条件是微波烧结功率450...     

Influence of the processing way for La3+-doping on crystal structure,microstructure, and microwave dielectric properties of Ca0.7Ti0.7La0.3Al0.3O3 ceramics

Perovskite ceramics with a formula of Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) were produced through a conventional solid-state reaction procedure following three different La³⁺-doping methods using powders of La₂O₃, or La₂O₃/Al₂O₃ powder mixture, or LaAlO₃. La³⁺ doping favored grain growth and densification, affected the grain size distribution, and improved the dielectric properties of the produced sintered CTLA ceramics. The doping methods had a strong influence on these properties. More specifically, doping with La₂O₃ and La₂O₃/Al₂O₃ resulted in formation of solid solution, while a secondary phase formed in the CTLA ceramics doped with LaAlO₃, which caused a coarsening of the microstructure and lowered the La³⁺ doping effects on the dielectric properties. The experimental results suggest that La³⁺ doping improves the dielectric properties of the sintered CTLA perovskite ceramics, which are further enhanced by doping with Al³⁺ ions in small amounts. However, further increase of Al³⁺ ions content jeopardizes them.     

大孔SiO_2载体制备La_(0.8)Sr_(0.2)CoO_3及其对NO+CO的催化性能

以具有高比表面积的大孔SiO_2为载体,采用浸渍法,通过控制浸渍液浓度的不同原位制备不同负载率的稀土钙钛矿型复合氧化物La_(0.8)Sr_(0.2)CoO_3/SiO_2样品。利用X射线衍射、比表面积、扫描电镜和X射线光电子能谱等分析方法对样品进行表征,结果表明,载体表面均匀负载La_(0.8)Sr_(0.2)CoO_3,且随着负载率增加,样品比表面积下降。考察不同负载率的La_(0.8)Sr_(0.2)CoO_3/SiO_2样品对NO+CO的催化活性,结果表明,与比表面积和结晶度相比,负载率在该催化实验体系中是影响最大的因素,综合催化效果最好的样品负载La_(0.8)Sr_(0.2)CoO_3质量分数为50.47%。     

Double perovskite Sr2FeReO6 oxides: Structural,dielectric, magnetic,electrical, and optical properties

Double perovskite Sr₂FeReO₆ (SFRO) powders were synthesized by so-gel process and annealed in argon atmosphere. Their structural, dielectric, magnetic, electrical, and optical properties were comprehensively investigated. It was found that the SFRO powders possessed a tetragonal crystal structure with I4/m space group and exhibited spherical shapes with some agglomeration due to the magnetic interactions between particles of the powders. Quantitative energy dispersive X-ray spectrometer data revealed the atomic ratio of Sr, Fe, Re, and O elements close to the nominal values of 2:1:1:6. X-ray photoemission spectroscopy spectra reveal two species of Re⁵⁺ and Re^6-7+ coexist in the SFRO powders. Sr, Fe, and O elements are present as Sr²⁺, Fe³⁺, and lattice oxygen, respectively. Dielectric property measurements revealed a Maxwell–Wagner type dielectric dispersion in the SFRO ceramics. Ferromagnetic behavior was verified by the observed magnetic hysteresis loops in the SFRO powders at 2 K and 300 K. The remanent magnetization and coercive field at 2 K were 8.23 emu/g and 3152 Oe, respectively, and the saturated magnetization was estimated to be 21.8 emu/g (or 2.0 μ_B/f.u.), smaller than the theoretical value of 3.0 μ_B/f.u. owing to the presence of the anti-site defects. Magnetic Curie temperature (T_C) was estimated to be 432.3 K. Intergranular tunneling magnetoresistance and hysteresis phenomena were observed in the SFRO powders at low temperatures, and the MR (2 K, 6 T) was measured to be −15% and −10% for MR (100 K, 6 T). Electrical transport and optical absorption measurements demonstrate the semiconducting nature of the SFRO with optical band gap of 1.39 eV. The electrical transport process follows the small polaron variable range hopping theory. The unique combination of high T_C ferromagnetism with the semiconductivity enables the SFRO to be a promising candidate for spintronic devices.     

Effect of manganese stoichiometry at B-site on magneto-transport and magnetic properties of La0.67Sr0.33MnO3manganites

To study the effect of manganese non-stoichiometry at B-site, a series of manganites with compositional formula La_0.67Sr_0.33Mn_1±xO₃ (where x = 0, 0.05 and 0.1) was synthesized by oxalate precursor method. X-ray diffraction data confirm the rhombohedral structure of La_0.67Sr_0.33Mn_1±xO₃ along with minor phases of Mn₃O₄. The average grain size is found to be 266 nm for x = 0 whereas its magnitude decreases with excess or deficiency in manganese concentration. An increase in the manganese non-stoichiometry leads to the coexistence of ferromagnetic and antiferromagnetic interactions. The effect of Mn_1±x on the magnetotransport properties could be understood on the basis of collective behaviour of magnetic spins, double exchange mechanism and ratio of Mn⁴⁺/Mn³⁺ ions. A crossover from negative to positive magnetoresistance behavior above metal-insulator transition temperature was observed for LSP-0.95 sample, whereas a positive magnetoresistance over the entire temperature region was noticed for LSP-1.10 sample.