Microwave dielectric properties of non-stoichiometric Sr1 + xNd2Al2O7 + x ceramic prepared by reaction-sintering process

Journal of Materials Science: Materials in Electronics - Sr1?+?xNd2Al2O7?+?x (0?≤?x?≤?0.04) ceramics were prepared using a more concise...     

Microwave dielectric properties of BaO–TiO2–TeO2 ternary system

Besides the applications as optical functional materials, tellurium oxides also have attracted interest as microwave dielectric materials. Most TeO₂-based binary and ternary system have large negative temperature coefficient of resonant frequency (τ_f), which is not compatible for the low-temperature cofired ceramic. To compensate τ_f close to zero, two single-phase predecessors of BaTe₄O₉ and TiTe₃O₈ are synthesized in air at 530–560 and 620–680 °C, respectively. The two predecessors show exceptional dielectric properties and their τ_f are opposite. The BaO–TiO₂–TeO₂ ternary system compounds are investigated by adjusting the ratio of BaTe₄O₉ and TiTe₃O₈ and sintered at 520–580 °C to develop the microwave properties and compensate the τ_f. After sintered at 560 °C, the ceramic sample with the composition of 0.47BaTe₄O₉–0.53TiTe₃O₈ exhibits a dielectric permittivity of 28, a Q × f-value of 12,200 GHz, and a τ_f of 4.0 ppm/°C measured at 10 GHz.     

Densification and dielectric properties of SrO–Al2O3–B2O3 ceramic bodies

The influence of SrO (0·0–5·0 wt%) on partial substitution of alpha alumina (corundum) in ceramic composition (95 Al₂O₃–5B₂O₃) have been studied by co-precipitated process and their phase composition, microstructure, microchemistry and microwave dielectric properties were studied. Phase composition was revealed by XRD, while microstructure and microchemistry were investigated by electron-probe microanalysis (EPMA). The dielectric properties by means of dielectric constant (ε _r ), quality factor (Q × f) and temperature coefficient of resonant frequency (τ _f ) were measured in the microwave frequency region using a network analyser by the resonance method. The addition of B₂O₃ and SrO significantly reduced the sintering temperature of alumina ceramic bodies to 1600 °C with optimum density (∼ 4g/cm³) as compared with pure alumina powders recycled from Al dross (3·55g/cm³ sintered at 1700 °C).     

Ferroelectric and dielectric properties of Bi4-xNdxTi3O12 thin films prepared by sol–gel method

Bi_4-xNd_xTi₃O₁₂ (BNT-x, x = 0, 0.25, 0.50, 0.75 and 1.0) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol–gel method. The microstructure, ferroelectric and dielectric properties of BNT-x thin films were investigated. The single-phase BNT-x thin films were obtained. With increasing Nd content, the preferred orientation changed from random to (117) and surface morphologies changed from the mixture of rod- and plate-like grains to rod-like grains. The Nd substitution improved the ferroelectric and dielectric properties of BTO films. BNT-x films showed better electrical properties at x = 0.50—1.0. BNT-0.75 film exhibited the best electrical properties with remanent polarization (2P _r) of 26.6 μC/cm², dielectric constant (ε _r) of 366 (at 1 MHz), dielectric loss (tanδ) of 0.034 (at 1 MHz), leakage current density (J) of ±3.0 × 10⁻⁶ A/cm² (at ± 5 V) and fatigue-free characteristics.     

Current–voltage nonlinear and dielectric properties of CaCu3Ti4O12 ceramics prepared by a simple thermal decomposition method

In this work, pure-CaCu₃Ti₄O₁₂ was successfully prepared by a simple thermal decomposition method. This can easily be achieved by direct firing of starting raw materials dissolved in an aqueous citric acid solution at 800 °C for 6 h. The results show that sintering conditions have a remarkable influence on the microstructure of the CaCu₃Ti₄O₁₂ ceramics. Interestingly, dense CaCu₃Ti₄O₁₂ ceramic sintered at 1,050 °C for 2 h exhibits a high dielectric constant of ~5.1 × 10³ with low loss tangent of ~0.048 at 30 °C and 1 kHz. The dielectric properties, electrical response of grain boundaries, and related nonlinear current–voltage behavior are found to be associated with the microstructure of CaCu₃Ti₄O₁₂ ceramics.     

Tribological properties of Al6061–Al2O3 nanocomposite prepared by milling and hot pressing

Tribological properties of bulk Al6061–Al₂O₃ nanocomposite prepared by mechanical milling and hot pressing were investigated. Al6061 chips were milled for 30 h to achieve a homogenous nanostructured powder. A 3 vol.% Al₂O₃ nanoparticles (∼30 nm) were added to the Al6061 after 15 and 30 h from the beginning of milling. The milling times with Al₂O₃ in these two samples were then 15 h and 30 min, respectively. Additionally, 3 vol.% Al₂O₃ (1 μm and 60 μm) was added to the Al6061 after 15 h of milling; where, the micron size Al₂O₃ in these two samples, was milled 15 h with the matrix. Hot pressing of milled samples was executed at 400 °C under 128 MPa pressure in a uniaxial die. The hot pressed samples were characterized by micro-hardness test, bulk density measurements, pin on disc wear test, and finally scanning electron microscopy observations. Fifteen hour-milled nanocomposite with nanoscale Al₂O₃, showed improvement in wear resistance and bulk density compared with that of 30 min-milled nanocomposites due to better dispersion of Al₂O₃ nanoparticles, improved surface quality of nanocomposite particles before pressing and more grain refinement of Al matrix. Moreover, increasing the reinforcement size increased the wear rate because of reduction in relative density, hardness and inter-particle spacing.     

Photoluminescence properties of β-Ga2O3:Tb nanofibers prepared by electrospinning

One-dimensional Tb³⁺-doped β-Ga₂O₃ nanofibers were prepared by a simple and cost-effective electrospinning process. Field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman technique, and photoluminescence (PL) were used to characterize the electrospun nanofibers. FE-SEM results indicated that the diameters all of the nanofibers ranged from 100 to 300 nm, and the lengths of nanofibers reached up to several millimeters. The XRD and Raman results showed that the Ga₂O₃ phase belongs to the monoclinic phase. Under ultraviolet excitation, the β-Ga₂O₃:Tb³⁺ samples showed green emission with the strongest peak at 550 nm, corresponding to ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions. The luminescence intensity had been further studied as a function of the doping concentration of Tb³⁺ in the β-Ga₂O₃ samples.     

Structural and dielectric properties of Li2O–ZnO–BaO–B2O3–CuO glasses

Glass samples of the system (15Li₂O–30ZnO–10BaO–(45 − x)B₂O₃–xCuO where x = 0, 5, 10 and 15 mol%) were prepared by using the melt quenching technique. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, a.c. conductivity and dielectric properties (constant εφ, loss tan δ, a.c. conductivity, σ_ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of copper ion concentration. The analysis of the results indicate that the density increases while molar volume decreases with increasing of copper content indicates structural changes of the glass matrix. The glass transition temperature, T _g, and crystallization temperature, T _c, increase with the variation of concentration of CuO referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter (T _c − T _g) decreases with increasing CuO content, indicates an increasing concentration of copper ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BO₃, BO₄, and ZnO₄. The structural changes observed by varying the CuO content in these glasses and evidenced by FTIR investigation suggest that the CuO plays a network modifier role in these glasses while ZnO plays the role of network formers. The dielectric constant decreased with increase in temperature and CuO content. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 5 mol%. In the high temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction.     

Synthesis and properties of magnetic solid superacid: SO4/ZrO2–B2O3–Fe3O4

A magnetic SO₄²⁻/ZrO₂–B₂O₃–Fe₃O₄ solid superacid catalyst is prepared via a simple chemical co-precipitation approach. The obtained materials were characterized in detailed by X-ray powder diffraction, thermogravimetric analysis–different scanning calorimetry, Fourier transform infrared spectroscopy (FTIR), electron microscopy (SEM and TEM), and Mossbauer spectra. Powder X-ray diffraction patterns show that in this composite oxide the transformation temperature of ZrO₂ from tetragonal to monoclinic phase is higher compared to the pristine SO₄²⁻/ZrO₂ material. The introduction of Fe₃O₄ endows the superacid with a super-paramagnetic property while in a ferromagnetic state after calcination. The superacid exhibits high catalytic activity in forming ethyl acetate by esterification.     

CoAl2O4/Al2O3纳米复合陶瓷的结构与表征

利用溶胶-凝胶方法制备了的CoAl2O4/Al2O3纳米复合陶瓷,并用X-ray分析(XRD)、红外光谱(IR)和扫描电镜分析(SEM)对其结构进行了分析.结果表明,随Al2O3含量增多,CoAl2O4尖晶石相从CoAl2O4/Al2O3凝胶中的析晶温度升高.SEM形貌也说明,随Al2O3含量增多,CoAl2O4/Al2O3陶瓷中形成一定的层状结构.     

Low-temperature sintering and microwave dielectric properties of 5Li2O–0.58Nb2O5–3.23TiO2 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of the 5Li₂O–0.58Nb₂O₅–3.23TiO₂ (LNT) ceramics have been investigated. It is found that the LNT ceramics could be sintered well at ∼880 °C with low-level doping of B₂O₃ (≤2 wt.%). Only Li₂TiO₃ solid solution (Li₂TiO₃ss) crystal structure could be detected for all the ceramics with various amounts of B₂O₃ addition from the X-ray diffraction (XRD) results. And interestingly, two phases with different color in SEM images are observed in B₂O₃-doped LNT ceramics. EDS results suggest that the two different phases are two Li₂TiO₃ss phases with different amount of Nb. In addition, there is no much degradation in the microwave dielectric properties with the B₂O₃ adding. In the case of 0.5 wt.% B₂O₃-doped samples sintered at 880 °C, good microwave dielectric properties of ?_r = 22, Q × f = 32,000 GHz, τ_f = 9.5 ppm °C⁻¹ are obtained.     

Optical properties and structure of R2O–Ga2O3–SiO2 and RO–Ga2O3–SiO2 glasses

Refractive index and molar refraction of Li₂O–, Na₂O–, CaO–, and BaO–Ga₂O₃–SiO₂ glasses have been used to test the validity of a structural model of silicate glasses containing Ga₂O₃ glasses. Ga₂O₃ enters these types of glass in a similar manner as Al₂O₃. It is assumed that, for (SiO₂/Ga₂O₃) >1 and (Ga₂O₃/R₂O) ≤1, Ga₂O₃ associates primarily with modifier oxides to form GaO₄ units. The rest of modifier oxide forms silicate units with non-bridging oxygen ions. Silicate structural units have the same factors as found for binary alkali- and alkaline earth silicate glasses. Differences between experimental and model values suggest another structure for (Ga₂O₃/SiO₂) ≥1.     

电熔MgAl2O4对Al2O3-MgAl2O4复合耐火材料高温蠕变性能的影响

以烧结板状刚玉和电熔镁铝尖晶石为原料、磷酸为结合剂,在1680℃下制备了刚玉-镁铝尖晶石复合耐火材料样品。在1500℃、0.2 MPa的条件下保温50 h测试试样高温蠕变性能,采用XRD、SEM和EDS分析蠕变前后试样的物相组成及显微结构,分析镁铝尖晶石添加量对刚玉-镁铝尖晶石复合耐火材料高温蠕变性能的影响。结果表明:刚玉-镁铝尖晶石复相材料较纯刚玉材料有着更好的抗蠕变性。镁铝尖晶石骨料在蠕变过程中会与氧化铝基质之间发生固溶反应而在尖晶石颗粒周围形成二次尖晶石层,有效连接了基质与骨料,提高了试样的抗蠕变性。在二次尖晶石层形成的过程中由于Mg;有着更高的迁移速率和在反应界面两侧较高的厚度比,会诱发柯肯达尔效应,导致界面处空位大量积累和孔隙的产生。     

Effects of pH and sintering temperature on the synthesis and electrical properties of the bilayered LaSr2Mn2O7 manganite prepared by the sol–gel process

The bilayered LaSr₂Mn₂O₇ manganite was synthesized by the sol–gel process at different pH values (3.5, 7, and 9.5) and different sintering temperatures (1553, 1623, and 1723 K) to investigate the effects of growth conditions upon the structural and electrical properties of the samples under investigation. X-Ray diffraction and FT-IR spectroscopy techniques confirm the phase formation for all samples. However, samples sintered at 1553, 1623, and 1723 K, and formed at pH = 7 are single phased having a tetragonal structure. Study of these single-phased samples indicates that with increase in the sintering temperature from 1553 to 1723 K, the average grain size increases from ~200 to ~1000 nm. To investigate the influence of grain size on the conduction mechanism, resistivity of the samples was measured as a function of temperature. The data obtained was analyzed by the adiabatic small polaron hopping model. These analyses show the influence of grain size on the parameters obtained by fitting the data by the above models.     

La2O3添加对MgAl2O4-CaAl4O7-CaAl12O19复合材料烧结行为的影响

为适应材料轻量化的发展需要,在1 400~1 600℃温度下开发了MgAl₂O₄-CaAl₄O₇-CaAl₁₂O₁₉（MA-CA₂-CA₆）复合材料,并考察了La₂O₃添加对该复合材料烧结行为、显微结构和力学性能的影响。结果表明,La₂O₃添加剂优先固溶到MA-CA₂-CA₆复合材料组成晶相CA₆中,促使CA₆相发生晶格畸变,有效抑制了CA₆晶粒沿基面的异常长大,其形貌由片状向等轴状趋势转变,促使MA-CA₂-CA₆复合材料制备过程中由于CA₆晶粒异常长大而导致的多孔网状显微结构得以有效消除,因此也极大地改善了Mg²⁺的扩散条件,在一定程度上间接促进了MA晶粒的发育,有效促进了MA-CA₂-CA₆复合材料的烧结。经1 200℃预烧、1 600℃保温2 h烧成后,当La₂O₃的添加量为4wt%时,MA-CA₂-CA₆复合材料试样的显气孔率由19.2%下降至6.1%,体积密度由2.78 g/cm³上升至3.18 g/cm³,制得了MA、CA₂、CA₆晶相呈现交织分布、显微结构致密、有利于其力学性能改善的La₂O₃/MA-CA₂-CA₆复合材料,经1 200℃预烧、-1 600℃保温2 h烧成后的4wt% La₂O₃添加试样,其冷态抗压强度由317 MPa增加到了501 MPa。     

Morphological and magnetic properties of the hydrothermally prepared α-Fe2O3 nanorods

α-Fe₂O₃ nanorods were synthesized via hydrothermal method. X-ray powder diffraction revealed the formation of rhombohedral α-Fe₂O₃ single crystal phase with fiber texture. Scanning and transmission electron micrographs analyses showed that the rhombohedral α-Fe₂O₃ has nanorods in shape with diameters of 40–85 nm and lengths of 150–45,000 nm. Isothermal magnetization vs. applied magnetic field curves measured at room and liquid nitrogen temperatures displayed a variation on magnetic ordering: from weak ferromagnetism at room temperature to not hysteretic behavior at liquid nitrogen temperature that is well described by a Langevin function. Moreover, the zero field cooling-field cooling curves under applied magnetic field of 100 Oe confirms the decreasing of Morin temperature transition due to nanometric size of the samples.     

Effect of Zn2TiO4 and ZnB2O4 additions on the microstructure and dielectric properties of AgNb1 ? xTaxO3 solid solutions

We report the preparation and characterization of AgNb_0.6Ta_0.4O₃ (ANT) based materials. The addition of Zn₂TiO₄ and ZnB₂O₄ influences the sintering temperature, phase composition, and microstructure of ANT ceramics. ANT doped with 1 wt % Zn₂TiO₄ or ZnB₂O₄ has high dielectric permittivity (400–470), low dielectric losses (tanδ ∼ 10⁻³), and a nonlinearity coefficient n _R ≅ 3–9% (at a field strength E = 3 × 10⁶ V/m).     

Ultralow-temperature hydrothermal synthesis of Zn–Mn spinel nanocrystals: Its defect spinel of λ-MnO2 prepared by a soft chemical method

An ultralow-temperature hydrothermal strategy was for the first time reported to synthesize spinel-type zinc manganese oxide (ZnMn₂O₄) nanocrystals at 70 °C. The synthetic parameters including hydrothermal temperature, zinc or manganese source, hydrothermal time, and NaOH concentration were systematically investigated. The structure and morphology characterization revealed that the resulting Zn–Mn spinels are tetragonal-phase, and have well-defined crystalline structure with the size of 10–20 nm. Furthermore, a defect spinel of λ-MnO₂ nanocrystal (10–20 nm) was directly synthesized from ZnMn₂O₄ nanocrystal upon acid leaching. Finally, a redox mechanism for the formation of λ-MnO₂ was discussed.     

Thermal properties and crystallization of PbO–MoO3–P2O5 glasses

Thermal properties and crystallization of glasses from PbO–MoO₃–P₂O₅ ternary system were studied in three compositional series (100 − x)[0.5PbO–0.5P₂O₅]–xMoO₃ (A), 50PbO–yMoO₃–(50 − y)P₂O₅ (B), and (50 − z)PbO–zMoO₃–50P₂O₅ (C). Glass transition temperature, crystallization temperature, coefficient of thermal expansion, and dilatation softening temperature of the studied glasses were determined by differential thermal analysis and dilatometry. Crystallization products of annealed glass samples were investigated by X-ray diffraction and Raman spectroscopy. X-ray diffraction analysis of crystallized glasses revealed the formation of PbP₂O₆, Pb₃P₄O₁₃, and PbMoO₄ in the samples of the B series. In the series A and C in the samples with a high MoO₃ content, crystalline compounds of Pb(MoO₂)₂(PO₄)₂ and (MoO₂)(PO₃)₂, respectively, were identified. Raman spectra of crystalline samples confirmed the results of X-ray diffraction measurements and provided also information on thermal stability of glasses and formation of glass-crystalline phases in the studied glass series.     

Preparation and magnetic properties of SrFe12O19/Ni0.5Zn0.5Fe2O4 nanocomposite ferrite microfibers via sol–gel process

SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrite microfibers with diameters of 1–2 μm have been prepared by the sol–gel process. The SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrites are formed after the precursor calcined at 850 °C for 2 h, fabricating from nanosized particles with a uniform phase distribution. The ferrite grain size increases with the calcination temperature. The magnetic properties for the nanocomposite ferrite microfibers are mainly influenced by the chemical composition and grain size. The nanocomposite ferrite microfibers obtained at 900 °C show the enhanced specific saturation magnetization (M_sh) of 64.8 Am² kg⁻¹, coercivity (H_c) of 146.5 kA m⁻¹ and remanence (M_r) of 33.6 Am² kg⁻¹ owing to the exchange–coupling interaction. This exchange–coupling interaction in the SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrite microfibers has been discussed.