Structural dependence of microwave dielectric properties of spinel structured Mg2(Ti1-xSnx)O4 solid solutions: Crystal structure refinement,Raman spectra study and complex chemical bond theory

Mg₂(Ti_1-xSn_x)O₄ (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg₂(Ti_1-xSn_x)O₄ average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg₂(Ti_1-xSn_x)O₄ compounds were obtained (${\epsilon }_r$= 12.18, $Q\times f$?=?170,130?GHz, ${\tau }_f$?=??53.1?ppm/°C, x?=?0.2).     

Densification,microstructure evolution,and microwave dielectric properties of Mg1-xCaxZrTa2O8 ceramics

In this study, the effects of the Mg²⁺ ions replaced by Ca²⁺ ions on the microwave dielectric properties of newly developed MgZrTa₂O₈ were investigated. Mg_1-xCa_xZrTa₂O₈ (x = 0–1.0) ceramics were prepared via a solid-state reaction method. Calcination of the mixed powders was performed at 1200 °C and sintering of the powder compacts was accomplished at temperatures from 1200 to 1550 °C. The substitution of Ca²⁺ significantly inhibited the densification of Mg_1-xCa_xZrTa₂O₈, led to the expansion of the unit cells, and triggered the formation of a second phase, CaTa₂O₆. The porosity-corrected relative permittivity increased almost linearly with the x value because of the replacement of the less polarizable Mg²⁺ ions by the more polarizable Ca²⁺ ions. The variation in the Q × f values followed a similar trend as that of the sintered density, and the change trend in the τ_f values was in accordance with that of relative permittivity. The best composition appeared to be Mg_0.9Ca_0.1ZrTa₂O₈, which showed excellent microwave dielectric properties of ε_r = 22.5, Q × f = 231,951 GHz, and τ_f = −32.9 ppm/°C. The Q × f value obtained is the highest among the wolframite dielectric ceramics reported in literature.     

Li+ enrichment to improve the microwave dielectric properties of Li2ZnTi3O8 ceramics and the relationship between structure and properties

Herein, Li⁺-enriched Li_(1+x)2ZnTi₃O₈ ceramics are prepared via the solid-phase methods. As x increases, the unit cell volume gradually increases, while the grain size initially increases and then decreases gradually. The Li_(1+0.06)2ZnTi₃O₈ ceramics exhibit the best dielectric properties: ε_r = 25.92, Q × f = 109534 GHz (@7.37 GHz, which is a 48 % increase compared with the stoichiometric counterpart.), and τ_f = ?8.21 ppm/°C. The complex chemical bond theory and Raman spectroscopy reveal that Ti-O bonds have a significant effect on the dielectric properties. An optimal Li⁺ enrichment leads to an overall reduction in the distortion of the Li/ZnO₄ tetrahedra, resulting in a reduction in τ_f. First-principles calculations demonstrate that a suitable excess of Li⁺ leads to an increase in the band-gap as well as an enhanced electron cloud density in the internal space of the Li1/ZnO₄ tetrahedra, thereby increasing the Q × f. In summary, Li⁺-enriched Li_(1+0.06)2ZnTi₃O₈ ceramics are promising for a wide array of applications in microwave communications.     

Crystal structure,chemical bond characteristics,and microwave dielectric properties of novel Sr2CaMoO6 ceramic at different sintering temperatures

This work prepared a novel Sr₂CaMoO₆ (SCM) ceramic through the conventional solid-state process. The crystal structure, chemical bond characteristics, and microwave dielectric properties of SCM ceramic were investigated for the first time. The X-ray diffraction patterns and Rietveld refinement indicate that SCM ceramic formed an orthorhombic phase with the space group Pmm2 (25) at temperatures above 1300 °C. The lattice vibrational modes of SCM ceramic were obtained through the Fourier transformed infrared spectrum (FTIR). The measured dielectric constant and dielectric loss of SCM ceramic is close to the theoretical values obtained by FTIR. According to the P–V–L theory, the chemical bond characteristics of SCM ceramic were calculated. The high ionicity and lattice energy of the Mo–O bond positively affect the properties of SCM ceramic. The outstanding microwave dielectric properties of ε_r = 19.37, Q·f = 32,970 GHz (at 5.96 GHz), and τ_f = ?32.56 ppm/°C were obtained in SCM ceramic sintered at 1450 °C. This work reveals the crystal structure of SCM ceramic, establishes the relationship between the properties and chemical bonds of the ceramic, and lays a foundation for studying the dielectric properties of related ceramic systems.     

Investigation of chemical bonds in the ordered Ba3Zn(Nb2-xMox)O9+x/2 ceramics and its effects on the microwave performance

1:2 ordered Ba₃Zn(Nb_2-xMo_x)O_9+x/2(BZNM) ceramics with space group $\text{P}\overline{3}\text{ml}$ were prepared by solid-state method. The nature of chemical bonds in Ba₃Zn(Nb_2-xMo_x)O_9+x/2(BZNM) ceramics was investigated for the first time. Firstly, the bond energy was closely related to the lattice vibration. High bond energy would lead to weak non-harmonic interactions, which were the main factors of improving Qf. Secondly, polarization of the chemical bond was the principal contributor to the relative permittivity of microwave ceramics. Compared with the traditional method, the calculated result based on the P-V theory reduced the error from 81% to 4.4%. Through the discussion, it was confirmed that the analysis method based on chemical bond was highly feasible and scientific in the microwave ceramics. At last, the system of Ba₃Zn(Nb_1.992Mo_0.008)O_9.004 sintered at 1435?°C for 6?h and annealed at 1300?°C for 10?h had excellent microwave dielectric properties: ε_r?=?38.9, Qf?=?102,931?GHz, τ_f?=?19.2?ppm/°C, which, to our best knowledge, provided a alternative for the application of millimeter-wave communications.     

Crystal structure and microwave dielectric properties of NaPb2B2V3O12(B=Mg,Zn) ceramics

Two low temperature sintered NaPb₂B₂V₃O₁₂ (B?=?Mg, Zn) ceramics with garnet structure were synthesized through conventional solid state reaction route and their crystal structure and microwave dielectric properties were investigated for the first time. Rietveld refinements of XRD patterns show both the compounds belong to cubic symmetry with space group Ia-3d. Observed number of Raman bands and group theoretical predictions also confirm cubic symmetry with space group Ia-3d for both NPMVO and NPZVO. At the optimum sintering temperature of 725?°C NPMVO has a relative permittivity of 20.6?±?0.2, unloaded quality factor (Q_uxf) of 22,800?±?1500?GHz (f?=?7.7?GHz) and temperature coefficient of resonant frequency +25.1?±?1?ppm/°C while NPZVO has relative permittivity of 22.4?±?0.2, Q_uxf of 7900?±?1500?GHz (f?=?7.4?GHz) and near zero temperature coefficient of resonant frequency of -6?±?1?ppm/°C at 650?°C. The relative permittivity of the compounds is inversely related to the corresponding Raman shifts.     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.     

Investigation of crystal characteristics,Raman spectra,and microwave dielectric properties of Mg1-xZnxTa2O6 ceramics

Mg_(1-x)Zn_xTa₂O₆ (x = 0.00?0.08) dielectric ceramics were synthesized via the traditional solid-state reaction method. We used XRD and Rietveld refinement to demonstrate that a pure Mg_(1-x)Zn_xTa₂O₆ phase with trirutile structure was formed. Zn²⁺ substitution helped to decrease the Raman full width at half width of the A_1g mode at 703 cm^?1, which resulted in an increase in the order and rigidity of the TaO₆ octahedron, this in turn contributed to improving the Q×f values. Additionally, the introduction of Zn²⁺ significantly promoted grain growth and increased the dense, and the molecular polarizability, these factors lead to a higher permittivity. Moreover, enhanced Ta-O bond energy resulted in a more stable TaO₆ octahedron in the Mg_(1?x)Zn_xTa₂O₆ system, which contributed to enhanced τ_f values via substitution of Zn²⁺ doped on the A-site. Correspondingly, the microwave dielectric properties were significantly improved for 0.04-doped samples, obtaining: ε_r = 27, Q × f = 185,000 GHz (at 7.47 GHz), τ_f =32 ppm/°C.     

Characterization of crystal structure and microwave dielectric properties of AZrNb2O8 (A=Zn,Co, Mg,Mn) ceramics based on complex bond theory

Single phase AZrNb₂O₈ (A=Zn,Co,Mg,Mn) ceramics with monoclinic wolframite structure were prepared and characterized by crystalline structure refinement for the investigation on the correlations between crystal structure and microwave dielectric properties. A semiempirical method based on the complex bond theory was used to calculate the bond ionicity, lattice energy, and coefficient of thermal expansion of AZrNb₂O₈ ceramics. The crystal structure changed with the variation of A site ions and affected the Nb–O octahedral correspondingly. Due to the variation of crystal structure, the observed bond ionicity was helpful to understand the electric polarization. The variation of the Q·ƒ and τ_ƒ values could be attributed to the lattice energy and coefficient of thermal expansion respectively. The microwave properties of AZrNb₂O₈ ceramics were strongly dependent on the chemical bond ionicity, lattice energy and coefficient of thermal expansion.     

Ba缺位和烧结工艺对Ba1-xZn1/3Nb2/3O3陶瓷离子有序度和微波介电性能的影响

利用常规固相法制备了Ba1-xZn1/3Nb2/303(x=O～0.02)陶瓷,研究了Ba缺位对Ba1-xZn1/3Nb2/3O3陶瓷的相成分、B位离子长程有序度(LRO)和微波介电性能的影响规律.X射线衍射(XRD)结果显示,适量的Ba缺位可以提高材料的阳离子有序度,x=0.01时陶瓷具有最大的阳离子有序度;Ba缺位...     

Influence of Cr3+ substitution for Mg2+ on the crystal structure and microwave dielectric properties of CaMg1-xCr2x/3Si2O6 ceramics

The CaMg_1-xCr_2x/3Si₂O₆ (0?≤?x?≤?0.1) microwave dielectric ceramics were synthesized via conventional solid state reaction. In this study, the effects of Cr³⁺ substituting for Mg²⁺ on morphology, crystal structure and microwave dielectric properties of CaMg_1-xCr_2x/3Si₂O₆ ceramics were explored. XRD diffraction patterns exhibited that the CaMg_1-xCr_2x/3Si₂O₆ ceramics possessed the pure phase of CaMgSi₂O₆ when x?≤?0.06 and a small amount of secondary phase Ca₃Cr₂(SiO₄)₃ for 0.08?≤?x?≤?0.1. SEM micrographs revealed that the substitution of Mg²⁺ with Cr³⁺ could decrease the grain size. The apparent density was affected by the concentration of Mg vacancies. The correlation between crystal structure and microwave dielectric properties was investigated through the Rietveld refinement and Raman analysis. The microwave dielectric properties were mainly dependent on relative density, ionic polarizabilities, internal strain ?, disordered structure and MgO₆ octahedron distortions. Finally, CaMg_1-xCr_2x/3Si₂O₆ (x?=?0.02) ceramics sintered at 1270?°C for 3?h exhibited excellent microwave dielectric properties of ε_r?=?8.06, Q?×?f?=?89054?GHz, τ_f?=??44.92182?ppm/ºC.     

Effect of non-stoichiometry on the densification,phase purity,microstructure, crystal structure,and dielectric loss of Ba(Co1/3Nb2/3)O3 ceramics

The structural, vibrational, densification, and microwave properties of Ba(Co_1/3Nb_2/3)O₃ ceramics with small compositional variations along several tie lines in the ternary BaOCoONb₂O₅ diagram were studied. The results showed that very small deviation from stoichiometric Ba(Co_1/3Nb_2/3)O₃ composition has profound effect on Q × f, degree of ordering, densification, and phase assemblage. The 0.94 Ba(Co_1/3Nb_2/3)O₃–0.06 Ba₅Nb₄O₁₅ ceramic has the highest Q × f value (71 THz) – a value two times larger than that of stoichiometric Ba(Co_1/3Nb_2/3)O₃ (36 THz). Transformation from the (partial) disordered distribution of Co and Nb cations to 1:2 ordered arrangement in the octahedral sites was found to increase the Q factor of the high density and single phase ceramics. It was also observed that formation of very small amount of Ba₉CoNb₁₄O₄₅ second phase degraded Q × f value severely for the dense and highly ordered Nb-rich and Ba-deficient ceramics.     

Crystal structure,defect relaxation,and microwave dielectric properties of Ba[(Mg1/3Nb2/3)1−xHfx]O3 solid solutions

Ba[(Mg_1/3Nb_2/3)_1?xHf_x]O₃ (BMNH, x = 0.05, 0.1, 0.15, 0.2) solid solutions were prepared via the solid‐state reaction method. The effect of BaHfO₃ on the crystal structure, microwave dielectric performance, and defect relaxation behavior of Ba(Mg_1/3Nb_2/3)O₃ (BMN) were studied. BaHfO₃ additions degraded the sintering activity of BMN powder, requiring a high sintering temperature (T_s) ~ 1650°C; but it could be effectively improved by a prolonged sintering process at a lower T_s of 1600°C. The well‐sintered BMNH ceramics (1600°C for 30 h) possessed a high densification >96%, and exhibited cubic perovskite structures without 1:2 cation ordering. Once doped with Hf, the low‐temperature relaxation in dielectric spectroscopy and thermally stimulated depolarization current (TSDC) for pure BMN disappeared, further indicating such relaxation is related to cation‐ordered structure. Oxygen vacancies, namely showing in‐grain and across‐grain‐boundary relaxation of ‐related defects, were the main defect types in BMNH. The concentrations of in‐grain decreased as x increased, which is beneficial to BMNH to maintain high Q × f values of 69 400‐73 000 GHz. Accompanied by a high ε_r of 33.27‐33.59 and a low τ_f of +13.6 to +20.7 ppm/°C, these materials have a good potential for applications in microwave components and devices.