Sintering behavior and microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics modified by B2O3–Li2O–Al2O3 and CeO2

A low temperature sintering method was used to avoid the relatively high sintering temperatures typically required to prepare 0.67CaTiO₃–0.33LaAlO₃ (CTLA) ceramics. Additionally, CeO₂ was introduced into the CTLA ceramics as an oxygen-storage material to improve their microwave dielectric properties. 0.67CaTiO₃–0.33LaAlO₃ ceramics co-doped with B₂O₃–Li₂O–Al₂O₃ and CeO₂ were prepared by a conventional two-step solid-state reaction process. The sintering behavior, crystal structure, surface morphology, and microwave dielectric proprieties of the prepared ceramic samples were studied, and the reaction mechanism of CeO₂ was elucidated. CTLA+0.05 wt% BLA+3 wt% CeO₂ ceramics sintered at 1360 °C for 4 h exhibited the optimal microwave dielectric properties: dielectric constant (ε_r)=45.02, quality factor (Q×f)=43102 GHz, and temperature coefficient of resonant frequency (τ_f)=2.1 ppm/°C. The successful preparation of high-performance microwave dielectric ceramics provides a direction for the future development and commercialization of CTLA ceramics.     

Effect of ZnO on Mg2TiO4–MgTiO3–CaTiO3 microwave dielectric ceramics prepared by reaction sintering route

ZnO-doped Mg₂TiO₄–MgTiO₃–CaTiO₃ microwave dielectric ceramics were successfully prepared by the reaction sintering route. The compact samples consisted of MgTiO₃, Mg₂TiO₄ and CaTiO₃, which was confirmed by X-ray diffraction and energy-dispersive spectra. ZnO efficiently lowered the sintering temperature and promoted the densification, as well as the improvements in the dielectric constant and the quality factor. At the level of ZnO?=?1 wt-%, the ceramics exhibited optimum microwave dielectric properties: a dielectric constant of 20.3, a high quality factor of 64,740 GHz (at 9.9 GHz) and a near-zero temperature coefficient of resonant frequency (–1.3 ppm/^oC) after sintering at 1320^oC for 4 h.     

Grain size effect on electrical properties of Mn-modified 0.67BiFeO3–0.33BaTiO3 lead-free piezoelectric ceramics

To investigate how grain size affects the dielectric, ferroelectric, and piezoelectric properties of Mn-modified 0.67BiFeO₃–0.33BaTiO₃ ceramics, we prepared samples with a wide variety of grain sizes from 4.1 μm to 0.59 μm via a conventional solid-state process that use the normal and the two-step sintering methods. Small-signal dielectric measurements show that all the samples exhibit a relaxor-like behavior and that grain size has little influence on the room-temperature dielectric permittivity. For grain sizes below 2 μm, the remanent polarization P_r and piezoelectric coefficient d₃₃ decrease with the grain size, whereas they remain almost constant near P_r = 27 μC/cm² and d₃₃ = 70 pC/N in samples with grain sizes exceeding 2 μm. The mechanism underlying the observed grain size effect is discussed in terms of the electric-field-induced formation of macroscopic ferroelectric domains.     

Microwave dielectric properties of ceramics based on CaTiO3–LnMO3 System (Ln–La,Nd; M–Al,Ga)

Structural features and microwave dielectric properties of LnMO₃–CaTiO₃ samples (where Ln stands for La or Nd, M stands for Al or Ga) are studied. Solid solutions with the rhombic perovskite structure are shown to be formed with increasing molar concentration of LnMO₃ up to ∼35% (for Ln–Nd, M–Al) or ∼40% (for Ln–La, M–Ga). Further increase of the neodymium aluminate or lanthanum gallate molar content in the solid solution up to 70% leads to formation of solid solutions with the tetragonal perovskite structure. A family of promising ceramics for application in the microwave technology with dielectric permittivity lying within the range from 43 to 48, the dielectric permittivity temperature coefficient being near to zero, and heightened quality factor (Q·f ⩾40,000 GHz) are obtained.     

Effect of Ce3+ doping on phase and microwave dielectric properties of 0.94MgTiO3−0.06(Ca0.8Sr0.2)TiO3 ceramics

In this study, 0.94Mg_(1-3x/2)Ce_xTiO₃−0.06(Ca_0.8Sr_0.2)TiO₃ (MCe_xT−CST, 0≤x≤0.01) composite ceramics were prepared at a low temperature of 1175°C by using the 50-nm-sized powders. The effects of Ce³⁺ doping on crystalline phase, microstructure, and microwave dielectric properties of MCe_xT−CST were studied. A main ilmenite (Mg,Ce)TiO₃ phase and a minor perovskite (Ca_0.8Sr_0.2)TiO₃ phase coexist well with the appearance of impurity MgTi₂O₅ phase in MCe_xT−CST. The dielectric properties of MCe_xT−CST are affected by the molecular polarizability, the impurity phase, and the Ce³⁺ doping. The replacement of Mg²⁺ by high valence Ce³⁺ could effectively inhibit the formation of oxygen vacancy, resulting in the enhancement of Q×f. When x = 0.005, MCe_xT−CST exhibits microwave dielectric properties with a moderate ε_r of 21.5, a high Q×f of 67 000 GHz, and a near-zero τ_f of −0.74 ppm/°C. The results reveal that the Ce³⁺ substitution is a prospective approach to optimize the microwave dielectric properties of MgTiO₃-based ceramics.     

Effect of Er3+ substitution on the quality of Mg–Fe spinel pigments

Inorganic pigments containing erbium cations and based on the spinel structure of MgFe₂O₄ were prepared. This type of spinel compound provides pigments of red to brown colour; an increase in the content of trivalent erbium cations results in pigments of a light brown hue. The pigments displayed good resistance to sunlight but this was reduced as the content of erbium increased. Pigments prepared using mechanoactivation possessed good resistance to sunlight over a range of erbium cation content.     

Effects of Ni2+ substitution on the crystal structure,bond valence,and microwave dielectric properties of BaAl2–2xNi2xSi2O8–x ceramics

BaAl_2?2xNi_2xSi₂O_8?x (x = 0, 0.005, 0.01, 0.02, 0.03) ceramics were prepared using traditional solid phase reaction method. The microwave dielectric properties, including permittivity (ε_r), quality factor (Q × f), and temperature coefficient of resonant frequency (τ_f), were discussed based on the bond valence theory. The first-principle calculation was adopted to determine the site (Ba, Al, and Si) where doping element (Ni²⁺) would be inclined to occupy. The substitution of Ni²⁺ for Al³⁺ contributed to the breaking of Al-O and Si-O bonds and then facilitated the BaAl₂Si₂O₈ (BAS) hexacelsian-celsian transformation. Moreover, this substitution could change the bond strength between cation and oxygen anion due to the variation of the bond valence, which reasonably explained the variation of ε_r, Q × f, and τ_f values. Well-sintered and completely transformed celsian ceramics can be obtained after doping with Ni²⁺. When x = 0.01, compact BaAl_1.98Ni_0.02Si₂O_7.99 ceramic exhibited highly promising microwave dielectric properties: ε_r = 6.89, Q × f = 53, 287 GHz and τ_f = -25.31 × 10^?6 /°C.     

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3–CaTiO3 system

(1−x)La(Mg_0.5Ti_0.5)O₃ (LMT)–xCaTiO₃ (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT–0.1CT composition was refined using P2₁/n space group, which allows taking into account B-site ordering. The compounds having x⩾0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT–CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.     

Mechanistic study of the effect of Ca–Sn co-doping on the microwave dielectric properties and magnetic properties of YIG

To investigate the crystal structure, electrical properties, and magnetic properties of Ca–Sn co-doped Y_3-xCa_xFe_5-xSn_xO₁₂ (x = 0.00–0.25 in steps of 0.05), solid-state reaction experiments, first principles calculations, and complex crystal bonding theoretical calculations were performed. The relative permittivity (ε_r) is strongly correlated with the average bond ionicity when Ca²⁺ is added. Furthermore, appropriate Sn⁴⁺ substitution significantly lowers the dielectric loss (tanδ_ε) associated with the lattice energy. The right amount of Ca–Sn co-doping can change the saturation magnetization (4πM_S) and improve the microscopic morphology of YIG, lowering the ferromagnetic resonance linewidth (ΔH) of YIG. The optimized microwave dielectric and magnetic properties are as follows: ε_r = 14.7, tanδ_ε = 4.15 × 10^?4, 4πM_S = 1680 G, and ΔH = 53 Oe for Y_2.8Ca_0.2Fe_4.8Sn_0.2O₁₂ sintered for 6 h at 1425 °C. Based on this material, a simple 3D model of a strip-line circulator with an insertion loss of less than 0.3 dB at each port and isolation greater than 20 dB in the 10–12 GHz range was developed, indicating the potential of the material for microwave high-frequency components such as circulators.     

Effects of TiO2 additive on ultra-low-loss MgO–LiF microwave dielectric ceramics

MgO ceramics have good microwave dielectric properties, but the high sintering temperatures limit its application. The effects of TiO₂ additive on the phase composition and microwave dielectric properties of MgO ceramics with 4mol%LiF were investigated by solid state reaction method. TiO₂ and MgO form Mg₂TiO₄ in a magnesium-rich environment with 4mol%LiF at about 900 °C, which as a solid solution or second phase had a huge impact on MgO ceramic with 4mol % LiF. When the content of TiO₂ less than 2mol %, Mg₂TiO₄ as a solid solution in MgO ceramics, which made the grain of MgO larger. When the content of TiO₂ more than 2mol %, Mg₂TiO₄ as a second phase in MgO ceramics, which made the microwave dielectric properties of MgO ceramics bad. Typically, the MgO-4mol%LiF-0.5mol%TiO₂ ceramic sintered at 1075 °C for 6 h acquired the best dielectric properties: ε_r = 9.7, Qf = 617,000 GHz and τ_f = −59.49 ppm/°C.     

Effect of La3+ substitution on the structural and thermoelectric properties of Ca3-xLaxCo4O9+δ

The thermoelectric properties of Ca₃Co₄O₉ were optimized by the substitution of La³⁺ for Ca²⁺ in Ca₃Co₄O₉. The La³⁺ substitution significantly enhanced the thermoelectric power factor and reduced the lattice thermal conductivity. The lattice thermal conductivities at 800 °C for x = 0 and 0.3 samples were 1.80 and 1.34 Wm⁻¹ K⁻¹, respectively. The reduced thermal conductivity was mainly attributed to mass and strain field fluctuations in the crystal lattice. Ca_2.7La_0.3Co₄O_9+δ showed the largest dimensionless figure-of-merit (0.282 at 800 °C) by combining high power factor and the lowest lattice thermal conductivity. This work demonstrates that the La³⁺ substitution is a highly effective approach for improving high-temperature thermoelectric properties.     

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of SnxTa0.025Ti0.975-xO2 ceramics

The Sn_xTa_0.025Ti_0.975-xO₂ (x%Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn⁴⁺ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO₂ phase were observed as x% increased. The mean grain size was slightly reduced (～17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x%, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x%Sn(TTO) ceramics remained unchanged owing to the same Ta⁵⁺ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x% increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ～0.02 and high ε′～10⁴ with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn⁴⁺–isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO₂ without the creation of complex defect clusters inside the grains.     

Effect of ZnO–WO3 additives on sintering behavior and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics

The effect of WO₃ addition on the phase formation, the microstructures and the microwave dielectric properties of 1 wt% ZnO doped 0.95MgTiO₃–0.05CaTiO₃ ceramics system were investigated. Formation of second phase MgTi₂O₅ could be effectively restrained through the addition of WO₃, but should be in right amount. WO₃ as additives could not only effectively lower the sintering temperature of the ceramics to 1310 °C, but also promote the densification. A dielectric constant ε_r of 20.02, a Q×f value of 62,000 (at 7 GHz), and a τ_f value of −5.1 ppm/°C were obtained for 1 wt% ZnO doped 0.95MgTiO₃–0.05CaTiO₃ ceramics with 0.5 wt% WO₃ addition sintered at 1310 °C.     

Preparation and microwave dielectric properties of BaLi1+xF3+x (x = 0–0.01) ceramics

BaLi_1+xF_3+x (x = 0–0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95 % after sintering at 750–800 °C/2 h in N₂. The obtained ceramics exhibit excellent optimized microwave dielectric properties with ε_r of ∼11.46 ± 0.06, Q×f values of 83175 ± 1839 GHz and τ_f of ∼ − 70 ± 3 ppm/°C at the x = 0.006 composition. Its Q×f value could be improved to 94603 ± 2037 GHz) by post-annealing in N₂ after post annealing at 700 °C/2 h. The Q×f value could be further improved to (120,098 ± 2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O₂ leads to lower Q×f values than those of the counterparts sintered in N₂ due to the introduction of F-vacancies by oxidation_, while little variation in ε_r andτ_f.     

Effect of ZnO nano-particulate modification on properties of PZT–BLT ceramics

This research was conducted to study the effect of ZnO nano-particulate modification on properties of Pb(Zr_0.52Ti_0.48)O₃ (PZT)–(Bi_3.25La_0.75)Ti₃O₁₂ (BLT) ceramics prepared by a mixed-oxide solid-state sintering method. ZnO nano-particulate was added into PZT–BLT ceramics to obtain PZT–BLT/xZnO (x = 0, 0.1, 0.5 and 1.0 wt%). The PZT–BLT/xZnO ceramics were investigated in terms of phase, microstructure, physical, electrical, and mechanical properties. Tetragonality of PZT–BLT crystal structure tended to increase with increasing ZnO content. ZnO addition obviously increased the density of PZT–BLT ceramics while the grain size slightly decreased. Intergranular fracture mode was observed for pure PZT–BLT ceramic while the samples contained ZnO nano-particles showed a mixed-mode inter-/trans-granular fracture. Addition of ZnO also affected hardness and fracture toughness values. Addition of ZnO nano-particulate into PZT–BLT ceramics was found to improve room temperature dielectric constant but did not have a significant effect on ferroelectric properties. These observed results were expected to be caused by the behaviors similar to a donor-doped system.     

Effect of WO3 doping on dielectric and ferroelectric properties of 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics

WO₃(0–6 mol%)-doped 0.94Bi_0.5Na_0.5TiO₃–0.06BaTiO₃ lead-free ceramics were synthesized by conventional solid-state reaction. The effect of WO₃ addition on the structure and electrical properties were investigated. The result revealed that a small amount of WO₃ (≤1 mol%) can diffuse into the lattice and does not significantly affect the phase structure, however, more addition will result in distortion and enlargement of the unit cells. The maximum permittivity temperature (T_m) is suppressed dramatically as the dopant increasing, while the depolarization temperature (T_d) fall to the minimum with 1 mol% WO₃ additive. The remanent polarization (P_r) was enhanced and coercive field (E_c) was reduced by doping with WO₃. The strain shows the largest value for 1 mol% doped sample, which is due to a field-induced antiferroelectric–ferroelectric phase transition.     

Effect of tungsten doping on the dielectric response of PZN–PT–BT ceramics with the morphotropic phase boundary composition

In this paper, effect of tungsten doping on the dielectric property of the Pb(Zn_1/3Nb_2/3)O₃–BaTiO₃–PbTiO₃ system around morphotropic phase boundary(MPB) composition is presented. Samples were prepared according to the formula Pb_0.85−xBa_0.15[(Zn_1/3Nb_2/3)_0.7Ti_0.3]_1−xW_xO₃ assuming the compensation for W is achieved via the appearance of the lead vacancies. X-ray diffraction results show that a nearly complete perovskite phase was maintained as W was progressively added up to 5 wt.%. Introduction of W stabilizes the tetragonal phase against the rhombohedral one, resulting in the displacement of MPB composition region towards relaxor end. Moreover, lattice distortion of the tetragonal phase is enlarged, that of the rhombohedral phase is lowered. W- doping leads to an increase in the dielectric permittivity maximum and a decrease in the phase transition temperature (T_m). The frequency dispersion of T_m is succesively weakened as W ions are gradually incorporated, reflecting the strengthened couplings among ferroactive oxygen octahedra. A maximum on the degree of diffuse phase transition is observed at 1 mol% WO₃, which can be interpreted in terms of competing effects of chemical inhomogeneity and ferroelectric couplings. W-doping also induces an increase in the tendency towards Curie–Weiss behavior above T_m, which is associated with the growth of ferroelectric domains.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

Microstructure effect on dielectric Properties of MgO-doped BaTiO3–BiYO3 ceramics

MgO-doped 0.97BaTiO₃–0.03BiYO₃ (0.97BT–0.03BY) polycrystalline ceramics were prepared by the solid-state sintering method. Then the structural, dielectric and resistant properties were investigated as functions of MgO addition. Microstructure was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS). The results show that Bi³⁺, Y³⁺ and Mg²⁺ ions exhibit nonuniform distribution behavior in BT–BY ceramics, demonstrating the existence of a “core–shell” structure, which plays important roles in the capacitance-temperature characteristics, where 0.97BT–0.03BY with the addition 2.2–2.8 at% MgO meets the Electronic Industries Association (EIA) X8R (−55 to 150 °C, ΔC/C_25 °C=±15% or less) specification. Moreover, the fine-grained samples with core–shell structure show much higher bulk resistance than the coarse-grained samples over the studied temperature range, which is attributed to the higher proportion of grain boundaries and the lower concentration of the effective acceptor.