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.     

Correlation between crystal structure and modified microwave dielectric characteristics of Cu2+ substituted Li3Mg2NbO6 ceramics

Structural characteristics exert significant influences on microwave dielectric properties, and ion substitution is widely adopted to modify material performances by adjusting the crystal structure. In this work, low loss Li₃Mg_2-xCu_xNbO₆ (x?=?0–0.04) ceramics were prepared by Cu²⁺ substitution. The impacts of Cu²⁺ substitution for Mg²⁺ sites on the microwave dielectric characteristics and crystal structure were discussed in detail. Rietveld refinement results implied that a single Li₃Mg₂NbO₆ phase was formed. Additionally, a dense and homogeneous microstructure with grain sizes of 7–9?μm could be achieved, and moderate Cu²⁺ substitution could significantly promote the grain growth. The correlation between microwave dielectric characteristics and crystal structure was discussed by calculating some structural parameters. The ε_r was determined by the polarizability. The Q?×?f was influenced by the packing fraction. The τ_f value was dependent on the NbO₆ octahedron distortion, and the τ_f value could be adjusted to near zero for x?=?0.02. Typically, the Li₃Mg_2-xCu_xNbO₆ (x?=?0.02) composition exhibited remarkable microwave dielectric performances: ε_r?=?15.75, Q?×?f?=?92,134?GHz (9.86?GHz) and τ_f?=??2?ppm/°C, making it a promising candidate for temperature-stable millimeter-wave applications.     

Structure,bond characteristics and Raman spectra of CaMg1-xMnxSi2O6 microwave dielectric ceramics

The CaMg_1-xMn_xSi₂O₆(x = 0–0.08)ceramics were reported here for the first time. The relationships among structural characteristics, vibrational modes and dielectric properties for the ceramics were researched based on complex chemical bond theory and Raman vibrational spectroscopy. The formation of a single phase with clinopyroxene structure when x = 0 to 0.08 was detected by X-ray diffraction. The monotonous increase of ${\epsilon }_r$ is ascribed to the average bond covalency, polarizability and Raman shift. The $Q\times f$ value is influenced by total lattice energy and full width at half maximum of Raman spectra which are both connected with the intrinsic loss. The variation of ${\tau }_f$ is related to thermal expansion coefficient and M1-site bond valence. Furthermore, the CaMg_0.98Mn_0·02Si₂O₆ ceramic sintered at 1300 °C possessed optimal microwave dielectric properties of ${\epsilon }_r$ = 8.01, $Q\times f$ = 83469 GHz and ${\tau }_f$ = −45.27 ppm/°C.     

Crystal structure,bond energy,Raman spectra,and microwave dielectric properties of Ti-doped Li3Mg2NbO6 ceramics

Crystal structure exerts dominant influence on the microwave dielectric performance enabling satisfying the demands for 5G communication system. In this study, the Ti-doped Li₃Mg₂Nb_1-xTi_xO_6-x/2 (x = 0.0-0.1) ceramics were prepared by the solid-state reaction procedure. Crystal structure refinement and microstructure analysis indicate pure phase with orthorhombic structure and homogeneous microstructure with grain size (~14 μm). The relative permittivity was affected by the relative density, cell volume, and polarizability. The Q × f value was dominated by the Nb-O bond energy and grain size. The τ_f value was correlated with the NbO₆ octahedral distortion and Nb-O bond valence. Particularly, the composition (x = 0.04) exhibited remarkable microwave dielectric performance: ε_r = 15.88, Q × f = 131 000 GHz and τ_f = −26.8 ppm/°C, providing a promising candidate for millimeter-wave applications.     

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.     

Raman spectra,bond characteristics,and microwave dielectric properties of Gd2Mo3O12 ceramics

Gd₂Mo₃O₁₂ ceramics were prepared using the traditional solid-phase reaction method. All samples were found to possess an orthorhombic crystal structure with a space group of Pba2, as revealed by refined XRD results. The ceramic sintered at 1000 °C exhibited a high relative density of 96.95 % and superior microwave dielectric properties, including ε_r of 9.42 ± 0.05,  × f of 49258 ± 1200 GHz, and τ_f of −71.8 ± 0.7 ppm/°C. The results suggest a correlation between an increase in ε_r and higher relative density, and a more compact and uniform microstructure can lead to higher  × f value. Chemical bonding theories and Raman spectroscopy analysis reveal that Mo-O bonds, rather than Gd-O bonds, dominate the microwave dielectric properties. Furthermore, the ε_r of Gd₂Mo₃O₁₂ ceramic was closely related to bond ionicity, while  × f and τ_f were mainly determined by lattice energy and bond energy, respectively.     

Crystal structure,far-infrared spectra,and microwave dielectric properties of bazirite-type BaZr(Si1-xGex)3O9 ceramics

Novel BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) microwave dielectric ceramics were prepared by solid-state reaction sintering at 1200–1450 °C for 5 h Ge⁴⁺ ions occupied the Si⁴⁺ positions, and BaZr(Si_1-xGe_x)₃O₉ solid solutions were obtained. The BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics exhibited hexagonal structures with P-6c2 space groups and octahedral layers and [Si/Ge₃O₉]^6- rings. Owing to these structural characteristics, the ceramics exhibited low permittivity. With an increase in x, the relative permittivity (ε_r) values of the BaZr(Si_1-xGe_x)₃O₉ (0 ≤ x ≤ 1.0) ceramics increased from 7.68 (x = 0) to 9.45 (x = 1.0), while their quality factor (Q × f) values first increased and then decreased. The Q × f value (10,300 GHz at 13.43 GHz) of the BaZrSi₃O₉ (x = 0) ceramic improved with the substitution of Si⁴⁺ by Ge⁴⁺. A high Q × f value (36,100 GHz at 13.81 GHz) was obtained for the BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic, and the Q × f values of the BaZr(Si_1-xGe_x)₃O₉ ceramics could be controlled by varying the Si/Ge-site bond valence. The temperature coefficient of resonance frequency (τ_f) values of the BaZr(Si_1-xGe_x)₃O₉ ceramics were mainly affected by the O2-site bond valence, and the optimum τ_f value (?22.8 ppm/°C) was achieved for the BaZrSi₃O₉ ceramic. The BaZr(Si_1-xGe_x)₃O₉ (x = 0.2) ceramic showed the optimum microwave dielectric properties (ε_r = 8.36, Q × f = 36,100 GHz at 13.81 GHz, and τ_f = ?34.5 ppm/°C).     

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.     

Temperature-dependent dielectric and Raman spectra and microwave dielectric properties of gehlenite-type Ca2Al2SiO7 ceramics

Gehlenite-type Ca₂Al₂SiO₇ ceramics were prepared by the conventional solid-state reaction. Two anomalies were found in the plot of dielectric constant vs temperature, which were associated with space charge polarization. Pure phase crystal structure and no phase transition were observed in the temperature-dependent X-ray diffraction (XRD) patterns and Raman spectra from room temperature (RT) to 900°C. There was relevant relation between Q × f and τ_ƒ with the stretching vibrations of Ca-O bond and O-Ca-O bending in CaO₈ polyhedron. Excellent microwave dielectric properties (ε_r = 8.86, Q × f = 22 457 GHz, and τ_f = −51.06 ppm/°C) were obtained for Ca₂Al₂SiO₇ sintered at 1440°C in air, which had the potential application to use in microwave and millimeter-wave devices such as capacitors and substrates.     

Optimized crystal structure and microwave dielectric properties of BaZnSi3O8-based ceramics

In this study, we synthesized BaZnSi₃O₈-based compounds with monoclinic structures (P21/a) using a solid-state method. The crystal structure, phase composition, and microwave dielectric properties of BaZnSi₃O₈-based ceramics were systematically investigated systematically. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images proved that the maximum solubility of BaZn_1-xMg_xSi₃O₈ ranged between 0.3 and 0.4. Rietveld refinement and Phillips–Van Vechten–Levine complex chemical bond theory were used to illustrate the relationship between the microwave dielectric performance and lattice parameters. To further improve the properties, we substituted Ba²⁺ with Sr²⁺ in BaZn_0.8Mg_0.2Si₃O₈. Ba_1-ySr_yZn_0.8Mg_0.2Si₃O₈ remained in a single-phase as y increased from 0 to 1.0. We achieved thermal stability of the resonance frequency of the BaZnSi₃O₈-based ceramics by adjusting TiO₂ to form composite ceramics. After sintering at 1020°C for 5 h, excellent microwave dielectric properties with ε_r = 7.44, Q×f = 57,400 GHz, and τ_f = − 0.2 ppm/°C were realized in the SrZn_0.8Mg_0.2Si₃O₈+8 wt %TiO₂ system.     

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.     

Effects of W6+ substitution on crystal structure and microwave dielectric properties of Li3Mg2NbO6 ceramics

Li₃Mg₂(Nb_1-xW_x)O_6+x/2 (0 ≤ x ≤ 0.08) ceramics were synthesized by the solid-state reaction route. The effects of W⁶⁺ substitution on the phase composition, microstructure and microwave dielectric properties of Li₃Mg₂NbO₆ ceramics were investigated systematically. The XRD results showed that all the samples formed a pure solid solution in the whole doping range. The SEM iamges and relative density revealed the dense structure of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The relationship between the crystal structure and dielectric properties of Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics was researched through polarizability, average bond valence, and bond energy. The substitution of W⁶⁺ for Nb⁵⁺ in Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics significantly promoted the Q × f values. In addition, the increase of W⁶⁺ content improved the thermal stability of the Li₃Mg₂(Nb_1-xW_x)O_6+x/2 ceramics. The Li₃Mg₂(Nb_0.94W_0.06)O_6.03 ceramics sintered at 1175 °C for 6h possessed excellent properties: ε_r ~ 15.82, Q × f ~ 124,187 GHz, τ_f ~ −18.28 ppm/°C.     

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.     

Sintering characteristics and microwave dielectric properties of ultralow-loss SrY2O4 ceramics

A SrY₂O₄ microwave dielectric ceramic suitable for 5G systems is synthesised via a solid-state reaction in a sintering temperature range of 1425–1525 °C. X-ray diffraction patterns and Rietveld refinement analysis show that the ceramic has an antispinel orthorhombic crystal structure belonging to the Pnma space group. Scanning electron microscopy images show that the ceramic particles are closely connected, the grain boundaries are clear, and the particles are uniform at the optimal sintering temperature of 1475 °C. The optimal microwave dielectric performances are ε_r = 14.78, Q × f = 84090 GHz, τ_? = ?14.98 ppm/°C. The relatively low dielectric constant, high Q × f value, low τ_? value, and easily available raw materials indicate that it is a good choice for 5G equipment.