Crystallographic characteristics and microwave dielectric properties of Ni-modified MgTa2O6 ceramics

Ni²⁺ modified MgTa₂O₆ ceramics with a trirutile phase and space group P4₂/mnm were obtained. The correlations between crystallographic characteristics and microwave dielectric performance of MgTa₂O₆ ceramics were systematically studied based on the chemistry bond theory (PVL theory) for the first time. The results indicate that the introduction of Ni²⁺ causes a change in polarizability and the Mg–O bond ionicity, which contributes to the variation of dielectric constant. Moreover, the lattice energy, and packing fraction, full width at half maximum of the Raman peak of Ta–O bond, as the quantitative characterization of crystallographic parameters, regulate the dielectric loss of MgTa₂O₆ ceramics in GHz frequency band. In addition, the study of sintering behavior shows that the densification and micromorphology are the crucial factors affecting the microwave dielectric performance. Typically, Ni²⁺ doping on the A-site of MgTa₂O₆ can effectively promote the Q × f values to 173,000 GHz (at 7.43 GHz), which ensures its applicability in 5G communication technology.     

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.     

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 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).     

The synthesis and microwave dielectric properties of Mg3B2O6 and Mg2B2O5 ceramics

Single-phase Mg₃B₂O₆ and Mg₂B₂O₅ ceramics were synthesized and then structurally and dielectrically characterized. The highest Qxf value of 230,900 GHz was obtained for a Mg₃B₂O₆ ceramic with a density of 97% and 1000-μm grains. Considerably lower Qxf values (10,000–32,000 GHz) were determined for the Mg₂B₂O₅ ceramic. Mg₃B₂O₆ and Mg₂B₂O₅ exhibited permittivities (?) of 7.2 and 6.2–7.0, respectively. Both ceramics showed negative temperature coefficients of resonant frequency (τ_f) of ?18 to ?45 ppm/°C.     

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.     

The microwave dielectric properties and crystal structure of low temperature sintering LiNiPO4 ceramics

The LiNiPO₄ ceramic for the LTCC technology was prepared via the traditional solid-state reaction route and its dielectric properties were investigated for the first time. The best dielectric properties of LiNiPO₄ ceramics with a ε_r of 7.18, Q×f value of 27,754?GHz and τ_f of ?67.7?ppm/°C were obtained in samples sintered at 825?°C for 2?h. Rietveld refinement was firstly employed to study the crystal structure and dielectric properties of LiNiPO₄ ceramics. Unfortunately, the relatively large negative τ_f was unfavorable to practical applications. Therefore, we introduced TiO₂, which possessed a considerable positive τ_f, to obtain a desired τ_f value. The prepared LiNiPO₄ ceramics with 15?wt% TiO₂ sintered at 900?°C for 2?h exhibited excellent dielectric properties of ε_r～11.49, Q×f～10,792?GHz, τ_f～?2.8?ppm/°C. The Ag co-fired experiments confirmed the excellent chemical compatibility with LiNiPO₄-TiO₂ ceramics which might be potential dielectric LTCCs for high frequency applications.     

Synthesis and microwave dielectric properties of new high quality Mg2NdNbO6 ceramics

New high‐quality microwave dielectric ceramics Mg₂NdNbO₆ were prepared by conventional solid‐state sintering method. The phases, micro‐structures and microwave dielectric properties of Mg₂NdNbO₆ ceramics were investigated at sintering temperature in the range of 1275°C‐1400°C. The X‐ray diffraction patterns showed that the peaks of the compounds were attributed to two phases, including the main crystalline phase of NdNbO₄ that was indexed as the monoclinic phase and MgO as the second phase. Well‐developed microstructures of Mg₂NdNbO₆ ceramics can be achieved, and the grain size reached the maximum value (1.63 μm) at 1375°C. As the sintering temperature increased, the dielectric constant, temperature coefficient of resonant frequency and apparent density remained almost unchanged, however, the significant change in the quality factor was observed. At 1375°C, Mg₂NdNbO₆ ceramics possessed excellent microwave dielectric properties: ε_r = 16.22, Q × f = 116 000 GHz and τ_f = ?30.96 ppm/°C.     

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.     

Relations on synthesis,crystal structure and microwave dielectric properties of SrZnP2O7 ceramics

Different experimental conditions were applied to investigate the optimum sintering properties of SrZnP₂O₇ ceramics, and its crystal structure and microstructure were investigated by X-ray powder diffraction and scanning electron microscope, respectively. The microwave dielectric properties were measured using a network analyzer. In calcination process, SrZnP₂O₇ powders were synthesized at different temperatures. The sintering characterizations and electric properties of the SrZnP₂O₇ ceramics prepared from the different calcined SrZnP₂O₇ powders were studied systematically. Better microwave dielectric properties can be obtained when using well calcined powders and optimum sintering conditions.     

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.     

Structure and properties of low temperature sintered ZnTa2O6 microwave dielectric ceramics

ZnTa₂O₆ microwave dielectric ceramics have been prepared using ZnTa₂O₆ nano-powders synthesized by sol–gel processing in this study. The crystal structure and microstructure of the ZnTa₂O₆ powders and ceramics were characterized by XRD and SEM techniques. ZnTa₂O₆ ceramics can be densified at a lower sintering temperature of 1200 °C. Microwave dielectric properties show that both of Q × f and ?_r values are lower than those of ceramics prepared by solid state route, and the τ_f values do not show different from that of solid state route. ZnTa₂O₆ ceramics sintered at 1200 °C exhibit good microwave dielectric properties: Q × f = 50,600 GHz, ?_r = 35.12 and τ_f = 9.69 ppm/°C.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Correlation between crystal structure and dielectric characteristics of Ti4+ substituted CaSnSiO5 ceramics

The crystal structure and dielectric properties of Ti⁴⁺-substituted CaSnSiO₅ ceramic were investigated. Ti⁴⁺ entirely substituted Sn⁴⁺ of CaSnSiO₅, and the solid solutions were formed at Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 1.0) ceramics. The evolutions of crystal structure were analysed through Rietveld refinement and transmission electron microscopy, and the phase transition from A2/a to P2₁/a space groups at Ca(Sn_1-xTi_x)SiO₅ (0.9 ≤ x ≤ 1.0) ceramics was clarified. The change in dielectric properties was related to the structural evolution of Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 1.0). The τ_f values of Ca(Sn_1-xTi_x)SiO₅ (0 ≤ x ≤ 0.4) ceramics initially decreased to +49.8 ppm/°C and then increased to +98.3 ppm/°C because of Sn/TiO₆ octahedral distortion. The temperature coefficient of capacitance and ε_r anomaly peak were controlled by Ti⁴⁺ substitution for Sn⁴⁺ at Ca(Sn_1-xTi_x)SiO₅ (0.4 ≤ x ≤ 1.0) ceramics. The results provided a way to control the τ_f value of microwave dielectric ceramics.     

Electronic states and cation distributions of MgAl2O4 and Mg0.4Al2.4O4 microwave dielectric ceramics

The electronic state and microwave dielectric properties of MgAl₂O₄ prepared using solid-state (MA-S) and molten salt (MA-M) methods and those of Mg_0.4Al_2.4O₄ (M04A24) were investigated. The λ values, which correspond to the fraction of Al³⁺ cations in tetrahedral sites, for MA-S, MA-M, and M04A24 were 0.23, 0.41, and 0.60, respectively. In molecular orbital calculations, a larger overlap was observed between Al-3s or Al-3p in tetrahedral sites and O-2p orbitals for M04A24, and the bond order for AlO at tetrahedral sites of M04A24 (0.241) was higher than those for MA-S (0.178) and MA-M (0.205). The dielectric constant, ε_r, for M04A24 (7.6) was lower than those for MA-S and MA-M (both 7.9), and the highest quality factor, Q·f, was obtained for M04A24 (235, 800 GHz). It was found that the covalency of the AlO bonds in the MO₄ tetrahedra is closely related to the Q?f values of the present ceramics.     

Preparation and microwave dielectric properties of low-loss MgZrNb2O8 ceramics

A novel low-loss microwave dielectric material MgZrNb₂O₈ was reported for the first time. Single-phase MgZrNb₂O₈ was prepared by a conventional mixed-oxide route and sintered in the temperature range of 1280–1360 °C. The microstructure and microwave dielectric properties were investigated systematically. The X-ray diffraction results showed that all samples exhibit a single wolframite structure. When the sintering temperature was lower than 1340 °C, the Q×f value mainly depended on the relative density. However, when the sintering temperature was above 1340 °C, the Q×f value mainly relied on the grain morphology in addition to the density. The MgZrNb₂O₈ ceramic sintered at 1340 °C for 4 h exhibited excellent microwave dielectric of ε_r=26, Q×f=120,816 GHz (where f=6.85 GHz), and τ_f=?50.2 ppm/°C. These results demonstrate that MgZrNb₂O₈ could be a promising candidate material for the application of highly selective microwave ceramic resonators and filters.     

Tailoring the microwave dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramics

Ba(Mg_1/3Ta_2/3)O₃ (BMT) is a very important microwave dielectric resonator material with a high dielectric constant of about 25, high quality factor and small temperature variation of resonant frequency. The preparation temperature of BMT ceramics is relatively high, about 1650 °C. The microwave quality factor of BMT depends on the ordering of Mg–Ta–Ta ions on the perovskite B-site. In the present paper we report how one can tailor the properties of BMT ceramics by glass addition, slight non-stoichiometry and by dopant addition. (a) It is found that addition of small amount of glasses such as B₂O₃, ZnO–B₂O₃, ZnO–2B₂O₃, ZnO–B₂O₃–SiO₂ reduces the sintering temperature, increases density, order parameter and quality factor. (b) Slight Mg or Ba deficiency improves densification, order parameter and quality factor whereas excess Mg or Ba deteriorated the properties (c) Small amounts of dopants such as Sb₂O₅, MnCO₃, ZrO₂, WO₃, SnO₂ and ZnO improve the microwave dielectric properties. It is found that the quality factor is maximum when the ionic radii of the dopant ions are close to the weighted average ionic radii of B site ions (i.e. Mg_1/3Ta_2/3), which is 0.653 Å.     

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.     

Synthesis and properties of TiO2 added NiNb2O6 microwave dielectric ceramics using a simple process

TiO₂ added NiNb₂O₆ ceramics produced using a reaction-sintering process were investigated. Pure columbite NiNb₂O₆ could be obtained without TiO₂ addition. With 30 and 40 mol% TiO₂ addition, a phase with the same structure of Ni_0.5Ti_0.5NbO₄ formed. Grain growth is easier in pellets with 30 and 40 mol% TiO₂ addition than in the NiNb₂O₆ pellets. Microwave dielectric properties: ?_r = 20.7, Q × f = 19,800 GHz (at 9 GHz) and τ_f = ?31.9 ppm/°C were obtained for NiNb₂O₆ pellets sintered at 1300 °C/2 h. ?_r around 45, Q × f = 5400–7700 GHz (at 6 GHz) and τ_f ～ 73 ppm/°C were obtained in pellets with 30 mol% TiO₂ addition. ?_r around 50, Q × f = 3800–5700 GHz (at 6 GHz) and τ_f ～ 99 ppm/°C were obtained in pellets with 40 mol% TiO₂ addition.