Improved microwave dielectric characteristics in CaHf1−xTixO3 ceramics

In the present work, effects of Ti-substitution for Hf upon microwave dielectric characteristics for CaHfO₃ ceramics were investigated. CaHf_1−xTi_xO₃ solid solution in space group Pnma was determined in the present ceramics with x = 0–0.075, and only minor amount of secondary phases of CaHfO₃ (R$\overline{3}c$, 1.3 wt%) and Ca₂Hf₇O₁₆ (R$\overline{3}$, 0.8 wt%) was detected. The significantly improved microwave dielectric characteristics were achieved with Ti-substitution in CaHfO₃ ceramics. With increasing x, the temperature coefficient of resonant frequency τ_f was substantially tuned from negative to positive through zero, which was caused by the decrease of oxygen octahedral tilting, and the Qf value was apparently improved simultaneously and reached the maximum of 43670 GHz at x = 0.025, while the dielectric constant ε_r increased monotonously. The remarkable improvement of Qf was attributed to the enhancement of octahedral framework rigidity and the decreased nonuniform distribution of inner chemical bonding in crystal based on the change of chemical bond parameters calculated by the Phillips–Van Vechten–Levine bond theory, which was further confirmed by the higher frequency and narrower width of stretch mode of octahedron (i.e., A_1g(O) near 730 cm⁻¹) in Raman spectra. An interesting phenomenon was found in titanium-containing perovskite ceramics that they had consistent trend of τ_f changing with tolerance factor, which was helpful to design temperature-stable perovskite microwave dielectric ceramics. The best combination of microwave dielectric characteristics was achieved at x = 0.035:  ε_r = 27.2, Qf = 42 870, and τ_f = +3 ppm/°C. For practical applications of the present ceramics, further improvement of Qf is required, and it is expected to be achieved through microstructure optimization.     

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.     

Effect of (Zn1/3Nb2/3)4+ co-substitution on the microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

Ce₂[Zr_1-x(Zn_1/3Nb_2/3)_x]₃(MoO₄)₉ (CZ_1-x(ZN)_xM) (x = 0.02–0.08) compounds were successfully prepared to scientifically examine the effect of (Zn_1/3Nb_2/3)⁴⁺ doping on phase composition, microstructures, and properties. The XRD results showed that all compounds formed a pure phase with the space group of R-3c. SEM results indicated that all compounds were compact at 675 °C, and the lattice parameters and average grain size decreased with doping. Performance analysis illustrated that ε_r was closely related to the polarizability, and Q?f was affected by the lattice energy of the Mo–O bond. The τ_f was maintained at an excellent level. Far-infrared analysis indicated that the major dielectric contribution to CZ_1-x(ZN)_xM ceramics was related to the absorption of phonon oscillation. The optimum properties (ε_r = 10.72, Q?f = 59,381 GHz, τ_f = ?11.48 ppm/°C) were obtained when x = 0.04.     

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.     

Effects of Sn substitution on the crystal structures,microstructures, and microwave dielectric properties of Ce2Zr3(MoO4)9 ceramics

A series of Ce₂(Zr_1?xSn_x)₃(MoO₄)₉ (0.02 ≤ x ≤ 0.1) (CZ_1?xS_xM) ceramics were synthesized to investigate the effect of Sn⁴⁺ doping on the crystal structure, chemical bond parameters, and dielectric properties of Ce₂Zr₃(MoO₄)₉ ceramics. X-ray diffraction patterns illustrated the formation of the single-phase trigonal system solid solution in all samples. Rietveld refinement result showed that the lattice volume decreased linearly, which can be explained by the fact that the effective radius of Sn ion is smaller than that of Zr ion. As the Sn content increased, scanning electron microscope images showed that the CZ_1?xS_xM ceramics transformed from bar-like grains to disk-like grains and the grain size declined gradually. The structure–property correlation was estimated by using P–V-L theory; the descending ε_r was mainly consistent with the reduced polarizability and total bond ionicity. The Q × f was associated with the lattice energy of the Ce–O1 bond. The change of τ_f value was mainly attributed to the bond energy (E_Mo1–O1 and E_Mo1–O4) and the coefficients of thermal expansion (α_Mo1–O1 and α_Mo1–O4). Infrared analysis indicated that the dielectric properties of the CZ_1?xS_xM ceramics were primarily ascribed to the absorption of phonon oscillation. Notably, when x = 0.08, outstanding microwave dielectric properties could be achieved, namely, ε_r = 10.22, Q × f = 72,390 GHz, τ_f = ?7.54 ppm/°C.     

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.     

Bond characteristics,sintering behavior and microwave dielectric properties of Ce2[Zr1−x(Ca1/3Sb2/3)x]3(MoO4)9 ceramics

Ce₂[Zr_1?x(Ca_1/3Sb_2/3)_x]₃(MoO₄)₉ (CZ_1?x(CS)_xM) (x = 0.02–0.10) ceramics were prepared by the conventional solid-state reaction method. The correlations between the chemical bond parameters and microwave dielectric properties were calculated and analyzed by using the Phillips–Van Vechten–Levine (P–V–L) theory. Phase composition and microstructures were evaluated by scanning electron microscopy and X-ray diffraction patterns. Lattice parameters were obtained by Rietveld refinements based on XRD data. Excellent properties for Ce₂[Zr_0.96(Ca_1/3Sb_2/3)_0.04]₃(MoO₄)₉ ceramic sintered at 775 °C: ε_r = 10.68, Q×f = 85,336 GHz and τ_f = ?7.58 ppm/°C were achieved.     

Relationship between bond characteristics and microwave dielectric properties of REVO4 (RE = Yb,Ho) ceramics

Two novel low-ε_r REVO₄ (RE = Yb, Ho) microwave dielectric ceramics with the symmetry of the zircon structure, space group I4₁/amd, were prepared using the solid-state method. Dense REVO₄ (RE = Yb, Ho) ceramics sintered at 1200 °C and 1160 °C performed ε_r ～ 12.3 ± 0.1 and 13.3 ± 0.1, Q × f ～ 28,200 ± 300 GHz and 24,100 ± 300 GHz, τ_f ～ ?18.8 ± 0.5 ppm/°C and ?17.4 ± 0.5 ppm/°C, along with thermal expansion coefficient (α_L) of 9.0 ppm/°C and 8.1 ppm/°C, respectively. Bond valence results indicated that the slightly rattling RE³⁺ cations at the A-site and compressed V⁵⁺ at the B-site occurred in both ceramics. The positive deviations (Δε_r) of porosity corrected ε_r(Corr) from those calculated by the Clausius-Mosotti equation ε_r(C-M), 8.1% for YbVO₄ and 17.7% for HoVO₄, were observed, implying that the rattling effect of RE³⁺ in dodecahedral A-site were greater than those of compressed V⁵⁺ in tetrahedral B-site. Rattling effect also led REVO₄ (RE = Yb, Ho) to develop higher ε_r, and smaller τ_ε and τ_αm, then closer to zero τ_f values than other zircon-structured REVO₄ (RE = Ce, Nd, Sm, Eu) ceramics with large negative τ_f. The differences in sintering temperature and microwave dielectric performance of both ceramics were discussed using the packing fraction, full width at half maximum (FWHM) of Raman modes and Phillips-Van Vechten-Levine (P–V-L) theory.     

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.     

Structural and chemical bond characteristics of microwave dielectric ceramics Sm3-xBixGa5O12

Herein, the Bi substitution for Sm in garnet Sm_3-xBi_xGa₅O₁₂ (x = 0–0.4) microwave dielectric ceramics is reported. A single garnet-structure phase could be achieved when Bi³⁺ content is in the range of 0 ≤ x ≤ 0.3. The addition of Bi³⁺ effectively reduces the sintering temperature of Sm₃Ga₅O₁₂ ceramics from 1440 °C to 1140 °C. Furthermore, the relationship between the structure and properties of Sm_3-xBi_xGa₅O₁₂ ceramics was investigated by Raman, SEM, TEM, and complex chemical bonding theory. The dielectric constant of Sm_3-xBi_xGa₅O₁₂ (0 ≤ x ≤ 0.3) ceramics increases slightly from 12.68 to 13.35, which is closely related to an increase in the polarizability and the bond susceptibility χ^μ. The Q × f increases from 107,617 GHz to 137,069 GHz, which is related to the lattice energy and the Raman FWHM values. The τ_f value of Sm_3-xBi_xGa₅O₁₂ gradually shifted in the positive direction with the increase of Bi content and the best performance (ε_r = 13.35, Q × f = 137,069 GHz and τ_f = ?15.37 ppm/°C) was obtained at x = 0.3.     

Effects of LiF addition on the densification and microwave dielectric properties of LiInO2 ceramics

Low-temperature co-fired ceramics (LTCC) LiInO₂ + xwt% LiF (x = 0, 1, 2, 3, 4, 5) was synthesized by a traditional solid-state reaction method. XRD, TEM, and SEM show that all specimens form a pure-phase tetragonal structure with a space group of I4₁/amd. The addition of LiF can effectively optimize the microstructure and improve the relative density of LiInO₂ ceramics. As x value increased, the ε_r increased from 9.6 to 13.6, Q×f increased from 39,600 GHz to 52,500 GHz, and all the specimens exhibit a positive τ_f value at the range of 9.6–19.1 ppm/°C. The low-ε_r and positive τ_f in LiInO₂ ceramics was investigated by bond valence and P–V-L chemical bond theory, indicating that it was closely related to the rattling effect of In and Li. The concentration of oxygen vacancies was studied by dielectric spectroscopy, indicating that doping LiF can compensate for the volatilization of Li during high temperature sintering. Notably, excellent microwave dielectric properties (ε_r ~13.6, Q×f = 52,500 GHz, and τ_f ~ 18.1 ppm/°C) were achieved in the LiInO₂ + 3 wt% LiF sintered at 890 °C.     

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.     

Structure and microwave dielectric properties of novel walstromite-type MCa2Si3O9 (M = Ba,Sr) ceramics

Novel walstromite-type MCa₂Si₃O₉ (M = Ba, Sr) ceramics, with triclinic space group P-1, were prepared through a solid-state reaction method. The P–V-L theory proves that the lattice energy and bond energy of the Si–O bond play a leading role in the quality factor and the dielectric constant is mainly determined by the ionic polarization. Excellent microwave dielectric properties of BaCa₂Si₃O₉ and SrCa₂Si₃O₉ ceramics could be obtained: ε_r = 8.99 ± 0.23, Q × f = 44,542 ± 500 GHz, and τ_f = −25.9 ± 3.0 ppm/°C and ε_r = 7.39 ± 0.23, Q × f = 48,772 ± 500 GHz, and τ_f = −27.5 ± 3.0 ppm/°C, when sintered at 1240/1280 °C for 4 h. Then SrCa₂Si₃O₉ ceramic is applied to a new microstrip bandpass filter, because of its high microwave dielectric properties and low thermal expansion coefficient. With reduced dimension, the filtering performance of the circuit is also highly improved, including reduced capacitor parasitic effect and the optimized stopband insertion loss. Accordingly, the SrCa₂Si₃O₉ ceramic is a promising candidate for sub-6 GHz a filter of microstrip bandpass applications.     

Crystal structure,infrared-reflectivity spectra,and microwave dielectric properties of novel Ce2(MoO4)2(Mo2O7) ceramics

Novel Ce₂(MoO₄)₂(Mo₂O₇) (CMO) ceramics were prepared by a conventional solid-state method, and the microwave dielectric properties were investigated. X-ray diffraction results illustrated that pure Ce₂(MoO₄)₂(Mo₂O₇) structure formed upon sintering at 600 °C-725 °C. [CeO₇], [CeO₈], [MoO₄], and [MoO₆] polyhedra were connected to form a three-dimensional structure of CMO ceramics. Analysis based on chemical bond theory indicated that the Mo–O bond critically affected the ceramics’ performance. Furthermore, infrared-reflectivity spectra analysis revealed that the primary polarisation contribution was from ionic polarisation. Notably, the optimum microwave dielectric properties of ε_r = 10.69, Q·f = 49,440 GHz (@ 9.29 GHz), and τ_f = −30.4 ppm/°C were obtained in CMO ceramics sintered at 700 °C.     

(Cu1/3Nb2/3)4+ ionic substituting effects,low-temperature sintering behaviors,and improved temperature stability of Ba3Nb4Ti4O21 microwave dielectric ceramics

In this study, (Cu_1/3Nb_2/3)⁴⁺ complex cation and BaO–ZnO–B₂O₃ glass frit were adopted to solve the high sintering temperature and poor temperature stability of Ba₃Nb₄Ti₄O₂₁ ceramics. It is shown that pure Ba₃Nb₄Ti₄O₂₁ phase was formed when Ti site was partially replaced by (Cu_1/3Nb_2/3)⁴⁺ cation. The increasing number of dopants decreases the dielectric polarizability, correspondingly, the dielectric constant and temperature coefficient of the resonance frequency values are reduced consistently. The variation of the Q × f value is determined by internal ionic packing fraction and external sintering densification. The (Cu_1/3Nb_2/3)⁴⁺ cation effectively decreases the suitable sintering temperature from 1200 to 1050 °C while greatly improving the temperature stability. BaO–ZnO–B₂O₃ glass was used to further improve the low-temperature sintering characteristics of Ba₃Nb₄Ti₄O₂₁ ceramics. It is proven that the addition of glass frits effectively decreases the temperature to 925 °C with combinational excellent microwave dielectric properties: ε_r ～55.6, Q × f ～5700 GHz, τ_f ～3 ppm/^°C, making the Ba₃Nb₄Ti₄O₂₁ ceramics promising in the applications of low-temperature cofired ceramic technology.     

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.     

Effects of (Cr0.5Ta0.5)4+ on structure and microwave dielectric properties of Ca0.61Nd0.26TiO3 ceramics

The Ca_0.61Nd_0.26Ti_1-x(Cr_0.5Ta_0.5)_xO₃ (CNT-CTx) ceramics with orthorhombic perovskite structure were prepared using the conventional solid-state method. The X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectra (XPS) were employed to investigate the correlations between crystal structure and microwave dielectric properties of CNT-CTx ceramics. The XRD results showed that all CNT-CTx samples were crystallized into the orthorhombic perovskite structure. The SEM micrographs indicated that the average grain size of samples depended on the sintering temperature. As (Cr_0.5Ta_0.5)⁴⁺ concentration increased, there was a significant decrease in the average grain size of samples. The short range order (SRO) structure and structural distortion of oxygen octahedra proved to exist in CNT-CTx crystals according to the analysis of Raman spectra results. The microwave dielectric properties highly depended on the full width at half maximum (FWHM) of Raman spectra, oxygen octahedra distortion, reduction of Ti⁴⁺ to Ti³⁺ and bond valence. At last, the CNT-CT0.05 ceramic sintered at 1420?°C for 4?h exhibited the good and stable comprehensive microwave dielectric properties: relative permittivity of 96.5, quality factor of 14,360?GHz, and temperature coefficient of resonant frequency of +153.3?ppm/°C.     

Structure and microwave dielectric properties of Li2Mg3Ti1-x(Al1/2Nb1/2)xO6 ceramics

Aiming to establish relationships between intrinsic structure factors and dielectric characteristics, a series of Li₂Mg₃Ti_1-x(Al_1/2Nb_1/2)_xO₆ (x = 0.0, 0.04, 0.08, 0.12, 0.16, 0.20) ceramics were synthesized to investigate the influences of (Al_1/2Nb_1/2)⁴⁺ substitution on the dielectric properties of Li₂Mg₃TiO₆ ceramics. The XRD and SEM results revealed that the pure rock salt phase (space group: Fm-3m) with a dense microstructure could be obtained with increasing the (Al_1/2Nb_1/2)⁴⁺ concentration, which is accompanied by an increase in the grain size from 11.69 to 22.81 μm. Meanwhile, some intrinsic factors, such as the average ionic polarizability, bond energy, packing fraction and lattice energy were calculated according to the complex chemical bond theory and refinement results. The unusual change in the dielectric constant (ε_r) was explained by the combined effects of the average ionic polarizability and relative density. The variation in the quality factor (Q × f) was ascribed to the packing fraction and lattice energy. The temperature coefficient of the resonant frequency (|τ_f|) reduced gradually with the increase in the octahedral bond energy, which enhanced the system thermal stability. Particularly, the Li₂Mg₃Ti_0.92(Al_1/2Nb_1/2)_0.08O₆ sample exhibited outstanding dielectric characteristics:ε_r = 15.256, Q × f = 174,300 GHz and τ_f = −19.97 ppm/°C.     

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).