The sintering behavior and microwave dielectric properties of Mg4(Nb,Sb)2O9 ceramics

The sintering behavior, microstructure and microwave dielectric properties of Mg₄(Nb_2?x Sb _x )O₉ (0?≤?x?≤?2) solid solutions were investigated systematically by X-ray diffraction(XRD), scanning electron microscopy(SEM) and a network analyzer. The solid solutions of Mg₄(Nb_2?x Sb _x )O₉ was formed with x value being no more than 1.6. The dielectric constant (?) of the sintered ceramics decreased from 13.06 to 6.28 with Sb content x from 0 to 1.6. With a substitution of Sb⁵⁺ for Nb⁵⁺ (0.04?≤?x?≤?0.08), the sintering temperature of Mg₄Nb₂O₉ ceramics was decreased from 1400 to 1300 °C without degradation of the Qf values. The optimum microwave dielectric properties of ??～?12.26, Qf?～?168,450 GHz, and τ _f?～??56.4 ppm/°C were obtained in the composition of Mg₄(Nb_1.6Sb_0.4)O₉ sintered at 1300 °C.     

Characterization and dielectric properties of Ba5LnNiTa9O30 (Ln = La, Nd and Sm) ceramics

Three novel Ba₅LnNiTa₉O₃₀ (Ln = La, Nd and Sm) ceramics were prepared and characterized in the BaO-Ln₂O₃-NiO-Ta₂O₅ system. All three compounds adopted the filled tetragonal tungsten bronze (TB) structure at room temperature. The present ceramics exhibited relaxor behavior, and the Curie temperature (at 10kHz) were −130, −80 and −45°C for Ba₅LaNiTa₉O₃₀, Ba₅NdNiTa₉O₃₀, and Ba₅SmNiTa₉O₃₀ respectively. At room temperature, Ba₅LnNiTa₉O₃₀ ceramics have a high dielectric constants in the range 102∼118, a low dielectric loss in range 0.0019∼0.0036, and the temperature coefficients of the dielectric constant (τ_ɛ) in the range −320∼−460 ppm°C⁻¹ (at 1 MHz).     

稀土永磁电机的开发与应用(二)

(续上期) 5 稀土永磁电机的开发方向 近年来,随着永磁材料性能的不断提高和完善,特别是钕铁硼磁体的热稳定性和耐腐蚀性的改善以及电力电子元件的进一步发展和改进,加上永磁电机研究和开发经验的逐步成熟,除了大力推广和应用已有研究成果外,稀土永磁电机的应用和开发进入一个新阶段,目前正向大功率化(超高速、高转矩)、高功能化、微型化和环境适应性方向发展[7,11,13].     

Microwave dielectric properties of temperature stable Ca5A2Ti1−x Hf x O12 (A = Nb, Ta) ceramics

Ca₅A₂Ti_1−xHf_xO₁₂ (A = Nb, Ta) ceramics have been prepared as single-phase materials by conventional solid-state ceramic route. Their structure and microstructure were studied by X-ray diffraction and scanning electron microscopic methods and dielectric properties were characterised in the 4–6 GHz microwave frequency range. We observed an increase in cell volume and theoretical density with compositional variations. In Ca₅Nb₂Ti_1−xHf_xO₁₂ ceramics the dielectric constant varied from 48 to 22 and quality factor from 26000 to 16000 GHz whereas in Ca₅Ta₂Ti_1−xHf_xO₁₂ the variation in dielectric constant was from 38 to 17 and quality factor from 33000 to 18000 GHz with increase in x. In both the ceramic systems the temperature coefficient of resonant frequency shifted from positive to negative values with Hf ⁴⁺ substitution for Ti⁴⁺.     

微量的砷、硼和稀土Ce对黄铜冷凝管耐蚀行为的影响

采用扫描金相—微区成分分析,拉伸力学性能测试和x射线衍射生物相分析等手段对加入微量的砷、硼和稀土Ce黄铜冷凝管的腐蚀特性进行了观察和分析。结果表明:不仅单独加入微量的砷、硼和稀土Ce能抑制HSn70-1黄铜冷凝管的脱锌腐蚀,砷、硼和稀土Ce的联合加入能更有效地协同抑制HSn70-1黄铜冷凝管的脱锌腐蚀。     

Phase formation,microstructure and microwave dielectric properties of Li2SnO3-MO (M=Mg,Zn,) ceramics

Phase formation, microstructure and microwave dielectric properties of (1-x)Li₂SnO₃-xMO (M=Mg, Zn) ceramics have been investigated using x-ray powder diffractometer (XRD), scanning electron microscope (SEM) and a network analyzer at the frequency of about 8-12GHz in this paper. The results showed that Li₂SnO₃ formed limited range of solid solution ((β-Li₂SnO₃(ss)) with MgO doping (x?≤?0.1) or ZnO doping (x?≤?0.3). Multiphase of Li₄MgSn₂O₇, β-Li₂SnO₃ (ss) and α-Li₂SnO₃(ss) existed in the compositions of x?=?0.2?0.5 for MgO-added specimens. ZnO second phase appeared when x?>?0.3 for the ZnO–added specimens. Dense and homogeneous microstructure could be obtained for the ZnO-doped composition with x?=?0.3. The dielectric permittivity decreased with the increase of MgO doping content, but increased with the increase of ZnO dopant within the miscible compositional range (β-Li₂SnO₃(ss)). The presence of Li₄MgSn₂O₇ or ZnO second phase reduced the dielectric permittivity. The doping of ZnO improved the Q?×?f value of β-Li₂SnO₃(ss), whereas the doping of MgO slightly decreased the Q?×?f value. The improvement of Q?×?f value could be ascribed to the stabilization of the ordering-induced domain boundaries by the partial segregation of the larger doping cation. The τ _f value changed from positive into negative value with increasing MgO or ZnO addition and near zero τ _f value (4.67 or ?0.27 ppm/°C) could be obtained at x?=?0.3 composition for MgO or ZnO added specimens, respectively.     

Piezoelectric and dielectric aging in pb(zr,ti)o3 thin films and bulk ceramics

Abstract

Piezoelectric and dielectric aging was studied in Pb(Zr,Ti)O₃ (PZT) thin films and bulk ceramics. It was found that piezoelectric aging in thin films obeys the logarithmic time dependence with the relative aging rate much higher than that of the dielectric constant, while comparable aging rates of piezoelectric and dielectric constants were found in PZT ceramics. The origin of piezoelectric aging in PZT films was related to depolarization of the films rather than to suppression of the domain wall motion as was generally accepted for PZT ceramics.     

Dielectric relaxation phenomenon of rare earth Nd3+ modified bismuth layer ferroelectric ceramics fabricated by plasma active sintering method

Abstract

The crystalline structure, dielectric relaxation and ferroelectric properties of the solid solution, Nd _x Bi_4-x Ti₃O₁₂ (NBIT) compound were measured. The Curie temperature of the NBIT ceramics was determined to be 490°C from dielectric measurements. The dielectric constant of the NBIT ceramics shows a small anisotropic property. Polarization switching was observed using a Sawyer-Tower circuit at 50 Hz. Remnant polarizations and coercive fields could not be confirmed since the hysteresis loops were not saturated. The large dielectric relaxation is observed in the frequency range between 100 kHz and 1 MHz.     

Phase transition behavior and electrical properties of lead-free (1-x)KNLNS-xBNKZ piezoelectric ceramics

Lead-free (1-x)(K_0,48Na_0,48Li_0,04)(Nb_0,95Sb_0,05)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (KNLNS-xBNKZ) piezoelectric ceramics (x?=?0, 0.02, 0.04, 0.06, 0.08, and 0.1) were successfully prepared by the two-step sintering method. The effects of different BNKZ contents on the crystal structure, ferroelectric phase transition, and electrical properties of the as-prepared ceramics were systematically researched. The ceramics with a perovskite structure possessed an orthorhombic phase (space group?=?Amm2) at x?=?0, a mixed orthorhombic-tetragonal phase (Amm2?+?P4/mbm) at x?=?0.02, a mixed rhombohedral-tetragonal (R-T) phase (R3m?+?P4/mbm) at x?=?0.04–0.06, and a rhombohedral phase (R3m) at x>0.06. At x?=?0.04 (rhombohedral and tetragonal (R-T) phases coexisted), the ceramics manifested the best electrical properties with d₃₃?=?224 pC/N, k_p = 0.44, k_t = 0.47, T_C = 218^oC, and remanent polarization (P_r)?=?11.5µC/cm². The improvement of the electrical properties of KNLNS-BNKZ ceramics was achieved by controlling their phase compositions and microstructures.

     

Composition dependent electrocaloric behavior of (SrxBa1-x)Nb2O6 ceramics

This article presents electrocaloric effect in (Sr_xBa_1-x)Nb₂O₆ ceramics (where x = 0.25, 0.50 and 0.75) using an indirect approach based on Maxwell's relations. Here, we have calculated various parameters of electrocaloric effect like temperature change (ΔT), entropy change (ΔS) and heat carrying capacity (ΔQ) of material due to the change in polarization under two different electric fields of 30 kV/cm and 20 kV/cm. (Sr_xBa_1-x)Nb₂O₆ ceramics is well known pyroelectric material, where by increasing Sr/Ba ratio the ferroelectric behavior turns towards relaxor behavior. While in terms of electrocaloric effect performance (temperature change ΔT) is increase as the Sr/Ba ratio increases. Additionally, maximum ΔT (0.30 K) was found for (Sr_xBa_1-x)Nb₂O₆ ceramic having x=0.75 molar concentration under the electric fields of 30 kV/cm.     

DFT studies of electronic structure and dielectric properties in layered perovskite $$\hbox {LaSrAlO}_{4}$$

The structural, electronic, dielectric and optical properties of tetragonal \(\hbox {LaSrAlO}_{4}\) are studied in detail using density functional theory calculations. The energy band structures and density of states are predicted by generalized gradient approximation (GGA) and local density approximation (LDA) respectively. The fundamental band gaps of \(\hbox {LaSrAlO}_{4}\) are all indirect by GGA (2.860 eV) and LDA (2.863 eV) calculations. The complex dielectric function was calculated. There are two peaks in the real part \(\varepsilon _{1}(\omega )\) and three peaks in the imaginary part \(\varepsilon _{2}(\omega )\). The optical spectra are assigned to the interband transition from O valence to La and Sr conduction bands in the low energy region. In addition, the electron energy-loss spectrum, optical conductivity, reflectivity spectrum, and refractive index, are given to support the potential applications for microwave dielectric ceramics.     

Effects of structural characteristics on microwave dielectric properties of (1−x)CaWO4 –xLaNbO4 ceramics

Dependences of microwave dielectric properties on the structural characteristics of (1−x)CaWO₄–xLaNbO₄ ceramics were investigated as a function of LaNbO₄ content (0.0 ≤ x ≤ 0.5). A single phase with tetragonal scheelite structure was obtained up to x = 0.35, and then the mixture phases with scheelite and fergusonite structure were detected. With the increase of LaNbO₄, the deviation of the observed dielectric polarizabilities calculated by the Clausius-Mosotti equation from the theoretical values calculated by the additivity rule of dielectric polarizability, was decreased due to the decrease of oxygen bond valence in ABO₄ scheelite structure. Dielectric constant (K) and temperature coefficient of resonant frequency (TCF) were increased with LaNbO₄ content due to the decrease of oxygen bond valence. Q ⋅ fvalue was dependent on the atomic packing fraction of unit cell as well as the grain size. Typically, K = 13.3, Qf = 50,000 GHz and TCF = −8.7 ppm/^oC were obtained for the specimens with 0.3 mol of LaNbO₄ sintered at 1150^oC for 3 h.     

Effect of microwave sintering on structural, morphological and dielectric properties of Ba(FeNb)0.5O3 ceramics

Polycrystalline Ba(FeNb)_0.5O₃/BFN ceramics were sintered conventionally and in a microwave (MW) furnace, respectively. Conventional and microwave sintering temperatures were same with different soaking times. Microwave sintering of BFN ceramics showed enhanced grain growth with improved dielectric properties. Highest dielectric constant (~29,913 at 1 kHz) at room temperature (RT) was observed in BFN ceramics sintered in MW furnace for 30 min. At RT, a non-Debye type of dielectric relaxation was observed in both conventionally and MW sintered BFN ceramics. The observed giant dielectric constant of conventionally and MW sintered BFN ceramics was attributed to intrinsic (space charge polarization) and extrinsic (Maxwell-Wagner type polarization) effects, respectively.     

Piezoelectric properties of SrBi2M2− xVxO9 (M = Nb and Ta) ceramics

Vanadium-substituted strontium bismuth tantalate, Sr_0.8Bi_2.2Ta_{2− x}V_xO₉ (SBTVx), and strontium bismuth niobate, SrBi₂Nb_{2− x}V_xO₉ (SBNVx), ceramics were synthesized by a low-temperature processing, and their dielectric, ferroelectric and piezoelectric properties were characterized. With the partial substitution of tantalum or niobium by vanadium cations, the single phase of the ABi₂M₂O₉-type structure was preserved and the sintering temperature was significantly decreased. For the SBNV ceramics, the T _c of 437^∘C for x = 0.0, the vanadium content hardly changed. On the other hand, the T _c of the SBTV ceramics increased from 408^∘C for the non-substituted SBTV to 414^∘C for x = 0.05 and then with the increasing vanadium content, the T _c decreased to 379^∘C for x = 0.20. The remanent polarizations, P _r, of SBTV and SBNV at room temperature were 4.9 and 5.4 μC/cm², respectively. All the obtained independent electromechanical coupling factors of the SBTV0.05 ceramics were as follows: k _p = 0.119, k ₃₁ = 0.073, k ₃₃ = 0.165, k ₁₅ = 0.051 and k _t = 0.134, and the SBNV0.05 ceramics were as follows: k _p = 0.074, k ₃₁ = 0.045, k ₃₃ = 0.175, k ₁₅ = 0.106 and k _t = 0.140. These coupling factors were higher than those of the non-substituted materials. From these results, the vanadium-substituted SBT and SBN-based materials can be expected to be lead-free piezoelectric resonator materials that can be prepared at low sintering temperatures.     

Piezoelectric and dielectric properties of (Na0.54K0.46)0.96Li0.04(Nb0.80Ta0.20)O3 ceramics substituted with Co

Lead-free [(Na_0.54K_0.46)_0.96Li_0.04](Nb_0.80Ta_0.20)O₃(abbreviated as NKL-NT) ceramics were synthesized by conventional solid state sintering method and the effect of Co substitution on the microstructure and piezoelectric properties were investigated. The crystal structure and microstructure were investigated by X-ray diffraction (XRD) and SEM images. The crystal structure exhibited the tetragonal structure at the composition ceramics less than 1 mol% Co. However, with increasing Co substitution from 1.5 to 2.0 mol%, the ceramics showed morphotropic phase boundary (MPB) and the crystal structure of the ceramic possessed orthorhombic structure at 3 mol% Co. The grain size was increased with increasing of Co contents. The phase transition temperature tetragonal-cubic(T_C) and orthorhombic-tetragonal(T_O-T) were shifted to downward and upward with increasing Co contents, respectively. The high piezoelectric properties of d₃₃?=?259[pC/N], k_p?=?0.43 and Qm?=?109 were obtained from the 1.5 mol% Co substituted ceramics sintered at 1060 °C for 5 h.     

Phase transition and dielectric properties of PbLa(Zr,Sn,Ti)O3 antiferroelectric ceramics under hydrostatic pressure

The influences of temperature and hydrostatic pressure on the dielectric constant (? _r) and loss (tanδ) of PbLa(Zr,Sn,Ti)O₃ antiferroelectric ceramics have been studied. Temperature-induced transition from tetragonal antiferroelectric phase to cubic paraelectric phase and low-temperature frequency dispersion are observed. The dielectric behavior as a function of temperature was determined by XRD measurement. It has been found from experimental results that the ferroelectric–antiferroelectric phase transition temperature decreased and antiferroelectric–paraelectric phase transition temperature increased with increasing pressure.     

Structure and dielectric properties of Cu and (K,Na) doped SrBi2Nb2O9 ferroelectric ceramics

Ferroelectric ceramics, SrBi₂Nb₂O₉ (SBN), Sr_0.8Cu_0.2Bi₂Nb₂O₉ (SCBN) and Sr_0.8K_0.1Na_0.1Bi₂Nb₂O₉ (SKNBN) were prepared by a solid state reaction process. X-ray diffraction analysis shows that the alkali and Cu almost diffuse into the SBN lattice to form a solid solution during sintering and some slight secondary phases was detected. The effect of alkali and Cu on dielectric properties of the SBN ceramics was discussed. The dielectric loss factor of (K,Na) doped SBN ceramics degraded considerably to 0.01 and their frequency and temperature stabilities were enhanced. The dielectric constant was enhanced by approximately 60% and the Curie temperature (Tc) was also improved for Cu doped SrBi₂Nb₂O₉ ceramics.     

Thermal and dielectric properties of ZnO-B2O3-MO3glasses (M = W, Mo)

Glasses in the ZnO-B₂O₃-MO₃(M = W, Mo) ternary were examined as potential replacements to PbO-B₂O₃-SiO₂-ZnO glass frits with the low firing temperature (500–600^∘C) for the dielectric layer of a plasma display panels (PDPs). Glasses were melted in air at 950–1150^∘C in a narrow region of the ternary using standard reagent grade materials. The glasses were evaluated for glass transition temperature (T _g ), softening temperature (T _d ), the coefficient of thermal expansion (CTE), dielectric constant (ε _r ), and optical property. The glass transition temperature of the glasses varied between 470 and 560^∘C. The coefficient of thermal expansion and the dielectric constant of the glasses were in the range of 5–8 × 10^{− 6}/^∘C and 8–10, respectively. The addition of MO₃to ZnO-B₂O₃binary could induce the expansion of glass forming region, the reduction of T _g and the increase in the CTE and the dielectric constant of the glasses. Also, the effect of the addition of MO₃to ZnO-B₂O₃binary on the transmittance in the visible-light region (350–700 nm) was investigated.     

Effects of BaWO4 and MnWO4 on the microwave dielectric properties of (Zr0.8 Sn0.2)TiO4 ceramics

Abstract

The effects of BaWO₄ and MnWO₄ on the microwave dielectric properties for (Zr_0.8Sn_0.2)TiO₄ ceramics have been investigated as a function of additives concentrations (0–5 mol%). As the amount of BaWO₄ and MnWO₄ increase up to 1 mol%, the unloaded Q increases due to reduction of the oxygen vacancy concentration. Further addition of BaWO₄ or MnWO₄ causes the unloaded Q to decrease due to the presence of secondary phases, either BaWO₄ itself or a needle shaped undetermined MnWO₄ phase. The temperature coefficient of resonant frequency can be controlled by the volume mixing rule for (Zr_0.8Sn_0.2)TiO₄ and secondary phases.