Structural evolution,grain growth kinetics and microwave dielectric properties of Li2Ti1-x(Mg1/3Nb2/3)xO3

Li₂Ti_1-x(Mg_1/3Nb_2/3)_xO₃ ceramics were prepared by conventional solid state process. Their structural evolution, grain growth kinetics and microwave dielectric properties have been studied in this paper. The results show that continuous solid solution could be formed within the experiment compositional range. The structure changed from long range ordered monoclinic into short range ordered cubic phase as the increase in x. Small levels of substitution for Ti⁴⁺by (Mg_1/3Nb_2/3)⁴⁺ slightly decreased the dielectric permittivity, while considerably improved the Q × f value. The temperature coefficient of resonant frequency changed from positive into negative value. The grain growth kinetics during sintering process and Q × f value of the sintered body were affected by different calcining temperature of mixed powders. Excellent combined microwave dielectric properties with ε_r ～21.0, Q × f ～ 200 000 GHz and τ_f value of ?1 ppm/ °C could be obtained after optimizing calcining temperature for the x = 0.24 composition after sintering at 1250 °C/2 h.     

Structure and microwave dielectric properties of Li(Al1-xLix)SiO4-x ceramics

Li(Al_1-xLi_x)SiO_4-x (x = 0.005, 0.01, 0.015, and 0.02) ceramics were synthesized via a traditional solid phase reaction method with different sintering temperatures. To determine the positions occupied by Li⁺ in the lattice, the defect formation energies and total energies of various sites of LiAlSiO₄ (LAS) occupied by Li⁺ were examined, and the energy of LAS systems were calculated using density functional theory of first-principle with the CASTEP module. The results demonstrated that the Al-sites occupied by Li⁺ had the lowest formation energies and total energy, so Li ⁺ should substitute Al³⁺. The impacts of replacing Al³⁺ with Li⁺ on the bulk density, sintering properties, phase composition, microstructure, and microwave dielectric properties of Li(Al_1-xLi_x)SiO_4-x (0 = x ≤ 0.02) ceramics were thoroughly studied. With Li⁺-doping, the sintering temperature decreased from 1300 °C (x = 0) to 1175 °C (x = 0.02), while the Q × f and τ_f values of LAS ceramics significantly increased. The Li(Al_0.99Li_0.01)SiO_3.99 ceramic was fully sintered at 1250 °C for 10 h to obtain excellent microwave dielectric properties: ε_r = 3.49, Q × f = 51,358 GHz, and τ_f = ?51.48 × 10^?6 °C^?1.     

Bond characteristics and microwave dielectric properties on Zn3-xCux(BO3)2 ceramics with ultralow dielectric loss

The crystal structure and microwave dielectric properties of Zn_3-xCu_x(BO₃)₂ (x = 0–0.12) ceramics prepared via a traditional solid-state reaction method were investigated by means of X-ray diffraction (XRD) utilizing the Rietveld refinement, complex chemical bond theory, and Raman spectroscopy. XRD showed that all samples were single phase. The samples maintained a low permittivity, even at higher Cu²⁺ contents, which is conducive to the shortening of signal delay time, and intimately related to the average bond ionicity and Raman shift. Moreover, proper Cu²⁺ substitution greatly reduced the dielectric loss associated with the lattice energy. Cu²⁺ entering the lattice optimized the temperature coefficient of resonance frequency (τ_f) values and improved the temperature stability of samples by affecting the bond energy. Optimal microwave dielectric properties were: ε_r = 6.64, Q × f = 160,887 GHz, τ_f = ?42.76 ppm/°C for Zn_2.96Cu_0.04(BO₃)₂ ceramics sintered at 850 °C for 3 h, which exhibited good chemical compatibility with silver and are therefore good candidate materials for Low temperature co-fired ceramic applications.     

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.     

Structural,magnetic, and dielectric properties in Aurivillius phase SrxBi6-xFe1-x/2Co1-x/2Ti3+xO18

Sr_xBi_6-xFe_1-x/2Co_1-x/2Ti_3+xO₁₈ (x = 0, 0.25, 0.5, 0.75, 1)(SBFCT-x) ceramics were prepared by the sol-gel auto-combustion method, and their microstructures, ferroelectric, magnetic and dielectric properties were investigated. All samples show layer-perovskited Aurivillius phase, which confirms that Sr doping does not affect the structure of SBFCT ceramics. The coexistence of ferroelectricity and ferromagnetism were observed at room temperature for all the samples. The largest remnant polarization (2P_r ˜ 17.4 μC/cm²) is observed in the SBFCT-1 ceramic, while the SBFCT-0.5 ceramic shows the highest remnant magnetization (2M_r ˜ 0.74 emu/g). To explore the effect of valance states of magnetic ions on the properties, we analysed the content variation of Fe²⁺, Co²⁺, and oxygen vacancies by the X-ray photoelectron spectroscopy results. Furthermore, dielectric anomalies have been found around 400 K, which can be ascribed to the hopping process of oxygen vacancies. The effects of Sr and Ti substitution on ferroelectric and magnetic properties have been investigated and discussed.     

Structure dependence of microwave dielectric properties in Zn2-xSiO4-x-xCuO ceramics

In this work, the Zn_2-xSiO_4-x-xCuO (x = 0, 0.04, 0.08, 0.12, 0.16 and 0.20) ceramics were synthesized through solid state reaction. The dependence of microwave dielectric properties on the structure was investigated through X-ray diffraction (XRD) with Rietveld refinements, Scanning electron microscope (SEM) and Raman spectra. The melting of CuO can reduce the densification temperature of Zn_2-xSiO_4-x ceramics. In comparison with x = 0, the x = 0.08 ceramics were densified at 1150℃ and the excellent microwave dielectric properties with low dielectric constant (ε_r = 6.01), high quality factor (Qf = 105 500 GHz) and τ_f = ?28 ppm/°C, were obtained. The ε_r, Qf and τ_f value are dominated by covalency of Si-O bond and secondary phase, crystallinity and lattice energy, respectively. This provides a theoretical basis to further adjust the microwave dielectric property (especially τ_f value) from the structural point of view.     

Ultrahigh Q values and atmosphere-controlled sintering of Li2(1+x)Mg3ZrO6 microwave dielectric ceramics

Ultrahigh-Q Li_2(1+x)Mg₃ZrO₆ microwave dielectric ceramics were successfully prepared by means of atmosphere-controlled sintering through simultaneously adopting double crucibles and sacrificial powder. This technique played an effective role in suppressing the lithium volatilization and further promoting the formation of the liquid phase, as evidenced by the X-ray diffraction, microstructural observation and the density measurement. Both dense and even microstructure, and the suppression of detrimental secondary phases contributed to low-loss microwave dielectric ceramics with Q×f values of 150,000–300,000 GHz. Particularly, desirable microwave dielectric properties of ε_r=12.8, Q×f=307,319 GHz (@9.88 GHz), and τ_f=−35 ppm/°C were achieved in the x=0.06 sample as sintered at 1275 °C for 6 h.     

Structure and microwave dielectric properties of 1:1 ordered Nd[(Mg1-xZnx)1/2Ti1/2]O3 complex perovskite ceramics

Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ perovskite ceramics (x = 0, 0.2, 0.4, 0.6, 0.8) are prepared by the solid-state reaction method. The effects of Zn²⁺ substitution on the structure, microstructure, especially the B-site 1:1 cation ordering and microwave dielectric properties have been investigated. Sintered Nd[(Mg_1-xZn_x)_1/2Ti_1/2]O₃ ceramics all adopt dense microstructure, along with increased dimensional uniformity as Zn²⁺ substitution. All the ceramics are confirmed to have B-site 1:1 ordered monoclinic perovskite structure with P2₁/n space group. Atomic mass difference of B-site elements might be an important factor affecting the B-site 1:1 cation ordering. HRSTEM observation suggest that the doped Zn²⁺ cations have roughly entered the Mg²⁺ sites to promote 1:1 cation ordering. The degree of the 1:1 cation ordering can be negatively reflected by the full width at half maximum (FWHM) of F_2g(B) mode at 372 cm^?1 in Raman spectra. With Zn²⁺ doping, the degree of the 1:1 cation ordering first increases then decreases, and reaches its maximum at x = 0.6. Meanwhile the best combination of microwave dielectric properties is obtained, as ε_r = 31.4, Q × f = 74,000 GHz, τ_f = ?44 ppm/°C. It is found that the long-range ordering not only decreases the dielectric loss but also affects the dielectric constant, providing a theoretical foundation to understand further the correlation between ionic configuration and microwave dielectric properties.     

Effect of zirconium deficiency on structure characteristics,morphology and microwave dielectric properties of Li2Mg3Zr1-xO6 ceramics

To suppress the impurity phases and porous microstructure caused by lithium volatilization, the Li₂Mg₃ZrO₆ and Zr-deficiency Li₂Mg₃Zr_1-xO₆ (x = 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were successfully synthesized via the solid-state method assisted with atmosphere-controlled sintering. The influences of non-stoichiometric on structure characteristics, morphology and microwave dielectric properties of Li₂Mg₃Zr_1-xO₆ ceramics were investigated. The XRD and SEM results proved that the Zr-deficiency restrained the formation of impurity phases and remarkably improved the densification of Li₂Mg₃Zr_1-xO₆ samples. The variation trend of dielectric constant (ε_r) was explained by the relative density and theoretical dielectric polarizability. The quality factor (Q × f) was strongly associated with the impurity phases and relative density. Additionally, the temperature coefficient of resonant frequency (τ_f) showed the same trend as the total bond energy E_total, indicating the bond energy might play vital roles in thermal stability of Li₂Mg₃Zr_1-xO₆ samples. Typically, the Li₂Mg₃Zr_1-xO₆ sample obtained at x = 0.06 possessed remarkable dielectric performances: ε_r = 13.13, Q × f = 116,400 GHz (10.14 GHz) and τ_f = ?26.30 ppm/°C.     

Structural characteristics and dielectric properties of Ti4+-substituted Li2Mg3SnO6 ceramics

To analyze the impacts of intrinsic factors on the dielectric properties, LiF was introduced to eliminate impurity phase Mg₂SnO₄, and a set of Li₂Mg₃Sn_1-xTi_xO₆-4 wt.% LiF (x = 0.0–1.0) ceramics were prepared by the solid-state reaction method. The XRD and SEM results indicated that all the compositions displayed a pure cubic phase (space group: Fm-3m, 225) and compact microstructure with increasing Ti⁴⁺ concentration, accompanied by a decrease in the average crystallite size from 2.54 μm to 1.92 μm. Based on the chemical bond theory and refinement data, several structural factors were determined to evaluate the relationship between the structural characteristics and dielectric properties. The increase in the dielectric constant (ε_r) was ascribed to the improved average ionic polarizability α_th/V_m. The trend of the change in the quality factor (Q × f) was closely related to the packing fraction and lattice energy. Additionally, the temperature coefficient of the resonant frequency (τ_f) changed within a narrow range, which could be attributed to the minor deviation in the octahedron bond energy. These results demonstrated that structural characteristics are key factors influencing the dielectric properties.     

Composition dependent structure,dielectric and energy storage properties of Pb(Tm1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3 antiferroelectric ceramics

In order to stabilize the perovskite structure and improve the storage energy density (U) of Pb(Tm_1/2Nb_1/2)O₃ (PTmN) based materials, Pb(Mg_1/3Nb_2/3)O₃ (PMN) was introduced into PTmN to form binary (1-x)PTmN-xPMN solid solution ceramics. The XRD patterns show that all the compositions belong to orthorhombic phase with space group Pbnm. The Curie temperature (T_C) gradually decreases while the dielectric constant (ε') increases for (1-x)PTmN-xPMN with increasing PMN content. The ε' of each composition above T_C obeys the Curie-Weiss law. The appearance double hysteresis loop confirms the antiferroelectric nature of (1-x)PTmN-xPMN (x = 0.02–0.18) ceramics. With the increase of PMN concentration, the maximum polarization slowly increases from 8.58 μC/cm² to 29.5 μC/cm² while the threshold electric field (E_A-F) gradually declines from 290 kV/cm to 120 kV/cm. The maximum of U (3.12 J/cm³) is obtained in 0.92PTmN-0.08PMN ceramic with moderate E_A-F = 220 kV/cm, which makes (1-x)PTmN-xPMN ceramics safe in practical application.     

Enhanced energy storage properties and large electrostrictive effect of Bi0.35Na0.35Ba0.09Sr0.21Ti(1-x)(Al0.5Nb0.5)xO3 relaxor ceramics

Ferroelectric ceramics have good piezoelectric and ferroelectric properties and can be used for energy storage equipment and actuators. Nevertheless, current research on dielectric capacitors has only focused on the energy storage density, but ignored efficiency. Moreover, conventional piezoelectric materials have a large strain hysteresis. In this work, (Al_0.5Nb_0.5)⁴⁺ (AN) complex ions doped 0.7Bi_0.5Na_0.5TiO₃-0.3Ba_0.3Sr_0.7TiO₃ (BNBST) ceramics were prepared. Doping AN destroyed the long-range ordered ferroelectric domains and generated polar nano regions, resulting in a gradual thinning and inclination of polarization hysteresis loops and an increase in relaxor degree. For BNBST-3AN ceramics, a W_rec of 1.52 J/cm³ and a η of 92.1% were achieved at 150 kV/cm. Meanwhile, BNBST-3AN ceramics had good energy storage temperature stability and cycling performance. The AN doping reduced the strain hysteresis in BNBST ceramics. BNBST-2AN ceramics exhibited a longitudinal electrostrictive coefficient Q₃₃ ～ 0.0292 m⁴/C² and a field-induced strain of 0.25% with low strain hysteresis (6.67%). Furthermore, BNBST-4AN ceramics had superior dielectric temperature stability from 24 to 270 °C. All results show that BNBST-100xAN ceramics have great promise for energy storage devices and actuators.     

Energy storage properties and transparency in (Ba0.6Sr0.4)1-1.5xBixTi1-x(Mg1/3Nb2/3)xO3 ceramics

Fully densified (transparent) ceramic with small grain size is highly desired to improve the field breakdown strength (BDS) and its scattering. Sintering behavior, microstructural evolution, electric, dielectric, and energy storage properties of (Ba_0.6Sr_0.4)_1-1.5xBi_xTi_1-x(Mg_1/3Nb_2/3)_xO₃ (x = .04–.10) ceramics have been studied in this paper. Phase pure cubic perovskite is observed for the x = .04 composition. Nb-rich tungsten bronze type and Ti-rich barium titanate secondary phases are present in the x > .05 compositions. A multiphasic transparent ferroelectric ceramic with ∼74.2% (780 nm) transmittance and a high refractive index of ∼2.3 within the visible region could be successfully obtained for the x = .10 composition by traditional ceramic process. The x = .09 composition demonstrates good energy storage performance (recoverable energy density W_rec = 3.74J/cm³, efficiency η = 77% and BDS = 390kV/cm) with extremely low scattering in BDS, suggesting potential application in large sized energy storage capacitor.     

Correlations between the crystal structure and microwave dielectric properties in Ce2[Zr1-xMx]3(MoO4)9 (M = Mn1/3Nb2/3, Mn1/3Ta2/3) solid-solution ceramics

Ce₂[Zr_1-xM_x]₃(MoO₄)₉ (M = Mn_1/3Nb_2/3, Mn_1/3Ta_2/3; x = 0.02, 0.04, 0.06, 0.08 and 0.10) (abbreviated as CZ_1-xN_x and CZ_1-xT_x) ceramics were prepared through the solid-state reaction method. Effects of (Mn_1/3Nb_2/3)⁴⁺ and (Mn_1/3Ta_2/3)⁴⁺ ions on the sintering characteristics, crystal structures, microwave dielectric properties and infrared vibrational modes were studied in detail. X-ray diffraction (XRD) results reveal the formation of solid solutions for all components. Based on the chemical bond theory and Rietveld refinement, intrinsic structure parameters including the polarizability (P), the packing fraction (P.F.) and the octahedral distortion (Δ_octa.), and bond parameters including the lattice energy (U), bond energy (E) and thermal expansion coefficient (α) were calculated. Interestingly, the Ce–O bond plays a major role in the bond ionicity (f_i), while Mo–O bond dominates the contributions in the lattice energy (U), bond energy (E) and thermal expansion coefficient (α). In addition, these parameters are used to explain the variations of the microwave dielectric properties of ceramics either changing the doping contents or replacing different ions at x = 0.06. Furthermore, far infrared (FIR) spectra uncover that the phonon modes provide the major polarization contribution of 68.59% in the CZ_0.9T_0.1 ceramic, implying that the main contribution to ε_r stems from the ionic polarization instead of the electronic polarization. Typically, the optimum microwave dielectric properties are achieved for the CZ_0.9N_0.1 and CZ_0.9T_0.1 ceramics with ε_r = 10.76, Q × f = 85,893 GHz (at 9.52 GHz), τ_f = −14.83 ppm °C⁻¹ and ε_r = 10.72, Q × f = 87,355 GHz (at 9.81 GHz) and τ_f = −8.68 ppm °C⁻¹, respectively. Notably, the CZ_0.9T_0.1 ceramic has a markedly increased Q × f while maintaining a good τ_f = −8.68 ppm °C⁻¹ and a low sintering temperature of 700 °C.     

Relaxor and strain behavior in BaTi1-x(Li2/3Nb2/3)xO3 ceramics

A new lead-free perovskite system of BaTi_1?x(Li_2/3Nb_2/3)_xO₃ (x = 0.02, 0.05, 0.07 and 0.10) has been synthesized by the mixed-oxide route. Temperature and frequency dependences of permittivity show a crossover from ferroelectric to relaxor behavior. For the samples with x ≥ 0.05, the frequency-dependent T_m satisfying the Vögel–Fulcher formula indicates a relaxor behavior. The increase with orders of magnitude in E_a with respect to x is possibly ascribed to more Ba-vacancies. Remnant polarization and strain decrease with increasing x due to phase transformation from ferroelectric tetragonal to paraelectric pseudocubic at room temperature.     

Characterization of structure and chemical bond in high-Q microwave dielectric ceramics LiM2GaTi2O8 (M = Mg,Zn)

LiM₂GaTi₂O₈ (M = Mg, Zn) ceramics with the Fd-3m space group were synthesized using the solid-state method. In comparison with Mg²⁺ that fully occupied the tetrahedral (A) site in LiMg₂GaTi₂O₈, LiZn₂GaTi₂O₈ was jointly occupied by Zn²⁺ and Li⁺ at the A site. Excellent microwave dielectric properties of Q×f = 133,400 ± 500 GHz, 101,800 ± 500 GHz, ε_r = 17.1 ± 0.2, 15.8 ± 0.2, and τ_f = ?60.1 ± 3.0 ppm/°C, ? 42.2 ± 3.0 ppm/°C for LiZn₂GaTi₂O₈ and LiMg₂GaTi₂O₈ were obtained, respectively. The large deviations (30.3% for LiMg₂GaTi₂O₈ and 19.6% for LiZn₂GaTi₂O₈) between the corrected ε_corr and theoretical ε_th were observed, which might be attributed to the underestimated Shannon’s ionic polarizability of Ti⁴⁺ in Ti-containing spinels. Their intrinsic microwave dielectric properties were discussed based on bond valence, lattice energy (U), and B-site bond energy (E). Besides, their large negative τ_f values were compensated to near-zero by CaTiO₃.     

Improvement of microwave dielectric properties for Ba(Ni1/3Nb2/3)O3 ceramics by Zr-substitution

Effects of Zr-substitution on the structure, microstructure and microwave dielectric properties of Ba(Ni_1/3Nb_2/3)O₃ ceramics have been investigated. A small amount of Zr-substitution facilitates the densification of Ba(Ni_1/3Nb_2/3)O₃ ceramics. Within x≤0.05, the densification temperature decreases with increasing x in Ba[(Ni_1/3Nb_2/3)_1−xZr_x]O₃, while it turns to increase for x>0.05. With increasing x, the grains become more homogeneous and closely contacted, and significantly increase in size for x=0.15–0.20. The B-site cations 1:2 ordering is destroyed by Zr-substitution, and only stabilizes for x≤0.04. B-site cations 1:1 ordering starts to form in x=0.04, and the 1:1 ordering degree first increases and then decreases with increasing x. Qf value decreases slightly in x=0.01 and then increases monotonously with x increasing from 0.02 to 0.20. The destroyed 1:2 ordering structure is responsible for the decreased Qf value in x=0.01, while the improved grain configuration dominates the increase of Qf value for x=0.02–0.20. The dielectric constant ε_r increases monotonously with increasing x, due to the higher polarizability of Zr ion than the average value of Ni/Nb ions. The temperature coefficient of resonant frequency τ_f shifts from negative to positive through zero with increasing x, which is ascribed to the highly positive τ_f value of the end member BaZrO₃. The significant improvement of microwave dielectric properties has been achieved for x=0.10, higher ε_r, higher Qf as well as near zero τ_f value have been obtained: ε_r=31.8, Qf=36,100 GHz, τ_f=7.8 ppm/°C.