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.     

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.     

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.     

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 excess lithium on the structure and microwave dielectric properties of LiMg0.5Ti0.5O2 ceramics

Herein, Li_1+xMg_0.5Ti_0.5O₂ (0.000 ≤ x ≤ 0.075) ceramics with excess lithium were prepared by solid-state reaction method to compensate for lithium volatilization, thus optimizing dielectric properties. The second phase Mg₂TiO₄ decreases with excess lithium content and eventually disappears at x = 0.05. Of these, Li_1.0375Mg_0.5Ti_0.5O₂ illustrates the best dielectric properties with ε_r = 16.58, Q × f = 120712 GHz (@9.18 GHz), and τ_f = −16.31 ppm/°C. From the point of view of non-intrinsic factors, excess lithium at optimum ratios slightly facilitates the sintering of the ceramics, improves densification, and increases the grain size, resulting in improved dielectric properties. For intrinsic factors, a slight increase in bond ionicity results in a slight improvement in ε_r. First-principles calculations demonstrate that suitable excess lithium increases electron cloud density in the internal space of Li/Mg/TiO₆ octahedron and increases band gap, thus optimizing the dielectric properties. The same results were obtained from the perspective of lattice vibrations. The modified Li_1.0375Mg_0.5Ti_0.5O₂ ceramics offer great potential for future microwave communication technology.     

Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics have been investigated. It is found that low-level doping of B₂O₃ (≤2 wt.%) can significantly improve the densification and dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics. Due to the liquid phase effect of B₂O₃ addition, LiNb_0.6Ti_0.5O₃ ceramics could be sintered to a theoretical density higher than 95% even at 880 °C. No secondary phase was observed for the B₂O₃-doped ceramics. There is no obvious degradation in dielectric properties for the ceramics with B₂O₃ additions. In the case of 1 wt.% B₂O₃ addition, the ceramics sintered at 880 °C show good microwave dielectric properties of ɛ_r = 70, Q × f = 5400 GHz, τ_f = −6.39 ppm/°C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) applications.     

Ferroelectric,dielectric, and impedance properties of Sm-modified Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

To investigate the effect of Sm doping on the electrical properties of Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 40, 50, 60) ceramics, three Sm-modified ceramics were prepared using the conventional solid-state reaction method. Related electrical measurements, including ferroelectric and dielectric investigations and impedance spectroscopy, were recorded for these ceramics. It was found that a tilted morphotropic phase boundary resulted from the addition of Sm, which induced the best piezoelectric properties and insulating behaviour in the Sm-BZT-60BCT sample. An abnormal P-E loop shrinkage appeared in the Sm-BZT-50BCT sample but not in the other two samples. This could be attributable to the different electronegativities between Ca²⁺ and Ba²⁺ and between Zr⁴⁺ and Ti⁴⁺, whose contents are different in varied samples and have an effect on defect-dipole alignment as well as spontaneous polarization. The activation energies for the bulk conductivity in the three composites were calculated to be 0.28 ± 0.01, 0.08 ± 0.01, and 0.36 ± 0.01 eV, confirming the existence of oxygen vacancies in our samples. The Sm dopant is responsible for the oxygen vacancies. This also leads to an increased Curie temperature in the three composites.     

Effects of ZrO2 substitution on crystal structure and microwave dielectric properties of Zn0.15Nb0.3(Ti1-xZrx)0.55O2 ceramics

In this study, the influence of substitution of Zr for Ti on crystal structure and microwave dielectric properties of Zn_0.15Nb_0.3(Ti_1-xZr_x)_0.55O₂ (0 ≤ x ≤ 0.7) ceramics were discussed through Rietveld refinement, normalized bond analysis and complex chemical bond theory. Rietveld refinement analysis indicated that a composite could be formed for x ≤ 0.22. Pure orthorhombic type (O-type) solid solutions are prepared in the region of x = 0.3–0.7. With the increasing amount of ZrO₂, cell volume of O-type phase increases, and it results in the enlarging and compressing behaviors of chemical bonds. Under the circumstances, the dielectric polarizability is deteriorated because of the reducing chemical bond covalency value (f_c), and this finding is also confirmed by Clausius-Mosotti equation. Lattice energy of Zr–O1 bonds and grain size distribution of sintered specimens are mainly responsible for the variations of quality factor (Q × f) value. Chemical bond energy (E) is closely related with the temperature coefficient of resonant frequency value (τ_f). Typical microwave dielectric properties of Zn_0.15Nb_0.3(Ti_1-xZr_x)_0.55O₂ ceramics (x = 0.22) were obtained when sintered at 1100 °C: ε_r = 46.31, Q × f = 30,297 GHz, τ_f = -8.24 ppm/°C.     

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。     

Magnetic and electrical properties of La0.7Ca0.3Mn0.95Co0.05O3 epitaxial layers by pulsed laser deposition

Epitaxial La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (LCMCO) thin films were prepared on (1 0 0) LaAlO₃ single-crystal substrates by pulsed laser deposition (PLD). We have been studied using X-ray diffraction (XRD), electrical transport magneto-transport and dc magnetization. XRD pattern reflects that all films have c-axis epitaxial growth on LaAlO₃ substrates. The decrease in out-of-plane cell parameter specifies a progressive relaxation of in the plane compressive strain as the thickness film is increases. From the dc magnetization measurements, it is observed that ferromagnetic to paramagnetic transition temperature increases with increase in the film thickness. Magneto-resistance and temperature coefficient of resistance increases with thickness of film and have maximum value near its metal to insulator transition temperature.     

Characterization of microwave and terahertz dielectric properties of single crystal La2Ti2O7 along one single direction

New generation wireless communication systems require characterisations of dielectric permittivity and loss tangent at microwave and terahertz bands. La₂Ti₂O₇ is a candidate material for microwave application. However, all the reported microwave dielectric data are average value from different directions of a single crystal, which could not reflect its anisotropic nature due to the layered crystal structure. Its dielectric properties at the microwave and terahertz bands in a single crystallographic direction have rarely been reported. In this work, a single crystal ferroelectric La₂Ti₂O₇ was prepared by floating zone method and its dielectric properties were characterized from 1 kHz to 1 THz along one single direction. The decrease in dielectric permittivity with increasing frequency is related to dielectric relaxation from radio frequency to microwave then to terahertz band. The capability of characterizing anisotropic dielectric properties of a single crystal in this work opens the feasibility for its microwave and terahertz applications.     

Effect of BaCu(B2O5) on the sintering and microwave dielectric properties of Ca0.4Li0.3Sm0.05Nd0.25TiO3 ceramics

The sintering behaviors and microwave dielectric properties of the Ca_0.4Li_0.3Sm_0.05Nd_0.25TiO₃ (abbreviated CLSNT) ceramics with different amounts of BaCu(B₂O₅) addition were investigated in this paper. Adding BaCu(B₂O₅) to CLSNT lowered its sintering temperature from 1300 °C to 925 °C. No secondary phase was observed in the CLSNT ceramics and complete solid solution of the complex perovskite phase was confirmed. The CLSNT ceramics with small amounts of BaCu(B₂O₅) addition could be well sintered at 925 °C without much degradation in the microwave dielectric properties. Especially, the 1.75 wt.% BaCu(B₂O₅)-doped CLSNT ceramic sample sintered at 925 °C for 3 h had optimum microwave dielectric properties of ε_r = 93.5 ± 3.2, Q × f = 6486 ± 434 GHz, and τ_f = 5 ± 1.5 ppm/°C (at 3–4 GHz), enabling it a promising candidate material for LTCC applications. Obviously, BaCu(B₂O₅) could be a suitable sintering aid to facilitate the densification and microwave dielectric properties of the CLSNT ceramics.     

Modified self-propagating high-temperature synthesis of nanosized La0.7Ca0.3MnO3

A modified solid-state combustion route was developed for the preparation of nanocrystalline manganite La_0.7Ca_0.3MnO₃ in a single-step process, using metal nitrates and glucose/KNO₃ redox mixture. The obtained sample was found to crystallize within O′ type of orthorhombic perovskite structure (space group Pnma), without the presence of other structural phases or impurities. Nanoparticles are found to have particle size in the range 12–35 nm, and to be highly crystalline without the presence of amorphous surface layer. Magnetic measurements show that nanoparticles display bulk-like magnetic properties, with ferromagnetic phase transition at 125 K and the absence of superparamagnetic or spin-glass behavior.     

Piezoelectric and dielectric properties of Ce substituted La2Ti2O7 ceramics

The ferroelectric and dielectric properties of cerium (Ce) substituted La₂Ti₂O₇ (LTO) have been investigated. Single phase, dense La_2?xCe_xTi₂O₇ (x = 0.15, 0.25, 0.35) ceramics were prepared by spark plasma sintering. The solubility limit of Ce in La_2?xCe_xTi₂O₇ was found to be between 0.35 and 0.5. The a-, b- and c-axes of the unit cell decrease with increasing Ce substitution. The Curie point (Tc) of La_2?xCe_xTi₂O₇ (x = 0, 0.15, 0.25, 0.35) decreases and dielectric constant and loss increase with increasing Ce substitution. Cerium can increase the d₃₃ of La₂Ti₂O₇. The highest d₃₃ was 3.9 ± 0.1 pC/N for La_1.85Ce_0.15Ti₂O₇ textured ceramic.     

Effect of layered structure on dielectric properties and energy storage density in xBa0.7Sr0.3TiO3-SrTiO3 multilayer ceramics

xBa_0.7Sr_0.3TiO₃-SrTiO₃ (BST-ST) multilayer ceramics with different BST layers (x=1, 3, 5) were designed and fabricated by lamination of Ba_0.7Sr_0.3TiO₃ (BST) and SrTiO₃ (ST) tapes. Dielectric and energy storage properties of the multilayer ceramics were investigated. BST-ST multilayer ceramics exhibited enhanced temperature- and frequency-stability of dielectric properties, accompanying high permittivity (~2000) and low dielectric loss (<0.005) at room temperature. P-E loops revealed that BST-ST multilayer ceramics displayed low remnant polarization and favorable maximum polarization. The optimal energy storage performance was obtained in the composition of x=5 with dielectric breakdown strength of 220 kV/cm and energy storage density of 2.3 J/cm³. These results indicate that BST-ST multilayer ceramics can be a favorable candidate for dielectric capacitor applications.     

Effect of reaction sintering process on the microwave dielectric properties of Ba2Ti9O20 materials

The pre-reaction step, which is the holding time at 1000 °C in reaction-sintering process, was observed to influence the characteristics of the Ba₂Ti₉O₂₀ materials profoundly. For the materials possessing high sintered density and of pure Hollandite-like structure, the uniformity of granular structure is the prime factor, altering the microwave dielectric properties of the samples. Utilizing the BaTi₄O₉ + BaTi₅O₁₁ mixtures to replace for the 2BaTiO₃ + 7TiO₂ ones pronouncedly improved the uniformity of the microstructure and thus markedly increased the Q × f-value for the Ba₂Ti₉O₂₀ materials. The possible explanation for such a phenomenon is that the B-series mixture forms the Ba₂Ti₉O₂₀ Hollandite-like phase in a simpler way and results in more uniform microstructure then the A-series mixture. The microwave properties of B-series materials are superior to those of A-series ones, achieving K = 38.2 and Q × f-value = 36,000 GHz for the samples, which were pre-reacted at 1000 °C for 6 h and were sintered at 1410 °C for 4 h by reaction-sintering processes.     

Crystal structure and microwave dielectric properties of garnet-type Ca2YZr2-xTixAl3O12 ceramics for dual-band bandpass filters

Microwave dielectric ceramics with a high-quality factor are vital materials for substrates, dielectric resonators, and filters in millimeter-wave communication systems. Here, a novel microwave dielectric ceramic based on a garnet-type Ca₂YZr₂Al₃O₁₂ compound for bandpass filter was prepared using the solid-state reaction method. Sintering the Ca₂YZr₂Al₃O₁₂ ceramics at 1600 ℃ for 5 h resulted in excellent microwave dielectric properties of ε_r = 10.81 ± 0.16, Qf = 87,628 ± 4000 GHz and τ_f = ?34.3 ± 0.5 ppm/℃. Increasing the Ti⁴⁺ content of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics significantly improved the sintering process. Superior microwave dielectric properties (ε_r = 11.93 ± 0.15, Qf = 121,930 ± 2600 GHz and τ_f = ?30.8 ± 0.4 ppm/℃) were obtained for Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) because of its dense microstructure, large grain size and high lattice energy. The Ca₂YZr_2-xTi_xAl₃O₁₂ (x = 0.3) ceramics were used to fabricate a dual-band bandpass filter with a hairpin structure that exhibited a large return loss (|S₁₁| > 12 dB) and a small insertion loss (|S₂₁| < 0.73 dB). The high performance of the Ca₂YZr_2-xTi_xAl₃O₁₂ ceramics and the corresponding bandpass filter makes this material a potential candidate for millimeter-wave devices.     

Microwave dielectric properties of La3Ti2TaO11 ceramics with perovskite-like layered structure

The first characterization of the microwave dielectric properties of the La₃Ti₂TaO₁₁ ceramics is presented. An ordinarily sintered ceramic at 1560 °C exhibits good microwave dielectric properties with ?_r = 46, Q × f = 7500 GHz and τ_f = ?47 ppm/°C. An alternative approach to tailor the temperature coefficient of resonate frequency of La₃Ti₂TaO₁₁ ceramics is also presented. Textured La₃Ti₂TaO₁₁ ceramics were fabricated using spark plasma sintering (SPS). By controlling the sintering temperature, orientation degree increased together with the steadily increase in ?_r and Q × f. A noteworthy change in τ_f from ?43.1 ppm/°C to ?13.6 ppm/°C with increasing orientation degree was observed. These results suggest that grain-orientation control was an effective way to tailor the microwave dielectric properties of La₃Ti₂TaO₁₁ ceramics.     

Temperature-dependent dielectric and Raman spectra and microwave dielectric properties of gehlenite-type Ca2Al2SiO7 ceramics

Gehlenite-type Ca₂Al₂SiO₇ ceramics were prepared by the conventional solid-state reaction. Two anomalies were found in the plot of dielectric constant vs temperature, which were associated with space charge polarization. Pure phase crystal structure and no phase transition were observed in the temperature-dependent X-ray diffraction (XRD) patterns and Raman spectra from room temperature (RT) to 900°C. There was relevant relation between Q × f and τ_ƒ with the stretching vibrations of Ca-O bond and O-Ca-O bending in CaO₈ polyhedron. Excellent microwave dielectric properties (ε_r = 8.86, Q × f = 22 457 GHz, and τ_f = −51.06 ppm/°C) were obtained for Ca₂Al₂SiO₇ sintered at 1440°C in air, which had the potential application to use in microwave and millimeter-wave devices such as capacitors and substrates.