Preparation and microwave dielectric properties of BaLi1+xF3+x (x = 0–0.01) ceramics

BaLi_1+xF_3+x (x = 0–0.01) were successfully mechanosynthesized by a simple ball-milling process. The effects of excessive LiF and sintering method and/or annealing atmosphere on its sintering behavior, microstructure, and microwave dielectric properties have been investigated in this paper. The mechanosynthesized powder can be densified with relative densities of ∼95 % after sintering at 750–800 °C/2 h in N₂. The obtained ceramics exhibit excellent optimized microwave dielectric properties with ε_r of ∼11.46 ± 0.06, Q×f values of 83175 ± 1839 GHz and τ_f of ∼ − 70 ± 3 ppm/°C at the x = 0.006 composition. Its Q×f value could be improved to 94603 ± 2037 GHz) by post-annealing in N₂ after post annealing at 700 °C/2 h. The Q×f value could be further improved to (120,098 ± 2344 GHz) by hot-pressed sintering (HPS). Sintering in the ambient atmosphere or O₂ leads to lower Q×f values than those of the counterparts sintered in N₂ due to the introduction of F-vacancies by oxidation_, while little variation in ε_r andτ_f.     

Crystal structures,bond characteristics,and dielectric properties of novel middle-εr Ln3NbO7 (Ln = Nd,Sm) microwave dielectric ceramics with opposite temperature coefficients

This study synthesized two novel middle-ε_r Ln₃NbO₇ (Ln = Nd, Sm; named NNO and SNO) microwave dielectric ceramics through the classic solid-state process. The results of XRD and Rietveld refinement show that NNO and SNO ceramics formed pure phases with the space group Cmcm (63) and C222₁ (20), respectively. The properties of Ln-O and Nb–O bonds of NNO and SNO ceramics were calculated based on the P–V–L theory. The Nb–O bonds positively affect the crystal structure stability of the two ceramics. The optimum microwave dielectric properties were obtained (NNO: ε_r = 31.61, Q·f = 6,615 GHz (at 6.10 GHz), and τ_f = ?455.70 ppm/°C; SNO: ε_r = 34.55, Q·f = 11,625 GHz (at 5.77 GHz) and τ_f = 72.59 ppm/°C) when the samples sintered at 1550 °C. Notably, SNO ceramic shows a low dielectric loss and medium dielectric constant, and the opposite τ_f of NNO and SNO ceramics provide the possibility to fabricate microwave dielectric devices with good temperature stability.     

Ge-doped Li3+xMg2Nb1-xGexO6 ceramics with enhanced low loss and high temperature stability properties

New high-performance materials have attracted much attention due to ever-increasing demands for advanced communication technologies. In present work, Ge-doped Li_3+xMg₂Nb_1-xGe_xO₆ (0 ≤ x ≤ 0.08) ceramics are prepared via solid-state reaction route. Microstructural analysis and crystal structure refinement reveal that moderate substitution can promote grain growth and modify crystal structure, thus enhancing microwave dielectric properties of composites. In that sense, special attention is paid to the behavior of dielectric constant ε_r, quality factor Q×f, and frequency temperature coefficient τ_f of final products. In these systems, ε_r parameter depends on the density, miscellaneous phases, and polarizability; Q×f value is shown to be influenced by Nb-O bond energy, grain size, and bulk density; finally, τ_f characteristic refers to Nb-O bond valence and NbO₆ octahedral distortion. Among above ceramics, Li_3.02Mg₂Nb_0.98Ge_0.02O₆ composite sintered at 1250 °C exhibits outstanding microwave absorption performance with ε_r = 15.32, Q×f = 969 88 GHz, and τ_f = ?8.25 ppm/°C.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Microwave dielectric properties of silico-carnotite Ca3M2Si3O12 (M= Yb,Y) ceramics synthesized via high energy ball milling

The Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics with orthorhombic silico-carnotite structure were fabricated via high-energy ball milling and solid-state reaction route. Dense Ca₃Yb₂Si₃O₁₂ and Ca₃Y₂Si₃O₁₂ ceramics sintered at 1260 °C and 1240 °C revealed promising microwave dielectric properties with ε_r = 9.2 and 8.7, Q×f = 56,400 GHz and 29,094 GHz, τ_f = −77.5 ppm/°C and −76.8 ppm/°C, respectively. The connection between crystal structure and Q×f values of Ca₃M₂Si₃O₁₂ (M = Yb, Y) ceramics was discussed with respect to the packing fraction, and their intrinsic microwave dielectric properties were examined using the infrared reflectivity spectra analysis. The thermal stability of Ca₃Yb₂Si₃O₁₂ was improved successfully by forming 0.91Ca₃Yb₂Si₃O₁₂‐0.09CaTiO₃ composite ceramics with τ_f = +2.9 ppm/°C, ε_r = 12.93 and Q×f = 26,729 GHz.     

Low temperature fired CaF2-based microwave dielectric ceramics with enhanced microwave properties

Low-temperature-fired microwave ceramics are key to realizing the integration and miniaturization of microwave devices. In this study, a facile wet chemical method was applied to synthesize homogenous nano-sized CaF₂ powders for simultaneously achieving low-temperature sintering and superior microwave dielectric properties. Pure CaF₂ ceramics sintered at 950 °C for 6 h with good microwave dielectric properties (ε_r = 6.22, Q×f = 36,655 GHz, and τ_f = ?102 ppm/°C) was achieved. The microwave dielectric properties of the CaF₂ ceramics were further improved by introducing LiF as a sintering aid. The sintering temperature of CaF₂-based ceramics was effectively lowered from 950 °C to 750 °C with 10 wt% LiF doping, and excellent microwave dielectric properties (ε_r = 6.37, Q×f = 65,455 GHz, and τ_f = ?71 ppm/°C) were obtained.     

Modulation of microwave dielectric properties in melilite-structured SrREGa3O7 (RE = La,Pr) ceramics

Gallium-based SrREGa₃O₇ (RE = La, Pr) melilite ceramics were prepared and selected to modify their microwave dielectric properties. Sintered at 1425 °C for 6 h, SrLaGa₃O₇ (SLGO) and SrPrGa₃O₇ (SPGO) ceramics exhibited high relative densities of 98.33 and 95.23%, low ε_r values of 11.8 and 10.9, Q×f values of 32,500 GHz (at 12.1 GHz) and 26,400 GHz (at 12.7 GHz), negative τ_f values of ?32 and ?54 ppm/°C. As a compensator, CaTiO₃ can tune the τ_f values of SLGO and SPGO to near zero (+2 and ?4 ppm/°C). In SrREGa₃O₇ melilite ceramics, the ε_r and τ_f values are mainly dependent on ionic polarizability, crystal structure and the “rattling” effect. The micromorphology, XPS, Raman spectrum and A-site bond valence (V_RE) of SrREGa₃O₇ (RE = La, Pr) microwave dielectric ceramics have also been comprehensively reported.     

Structural characteristics and microwave dielectric properties of Zn1−xBixVxW1−xO4-based ceramics for LTCC applications

Herein, the improvement of the microwave dielectric properties and sintering characteristics of Zn_1?xBi_xV_xW_1?xO₄(x = 0–0.15)-based ceramics is reported. The results showed that an appropriate amount of doping could not only reduce the optimum sintering temperature from 1100° to 900°C, but also enhance the densification of the microstructures and increase the Q×f value from 5351 to 42525 GHz. Additionally, various structural parameters including the phase composition, crystal structure, vibrational and chemical bond characteristics that are correlated with the dielectric properties were systematically investigated. By considering the chemical bond characteristics, the first-principles calculations and the acquired Raman spectra, the interaction between W-O is stronger than Zn-O in the ZnWO₄ structure, while the interaction between V-O is stronger than Bi-O in BiVO₄. Interestingly, when the Zn_0.97Bi_0.03V_0.03W_0.97O₄-based ceramics were sintered at 900 °C, improved microwave dielectric properties were acquired (ε_r =18.32, Q×f=42525 GHz, τ_f=?67.51 ppm/°C), which provides a promising candidate in low-temperature co-fired ceramics technology.     

Low temperature sintering and microwave dielectric properties of Zn1.8SiO3.8 ceramics with BaCu(B2O5) additive for LTCC applications

The Zn_1.8SiO_3.8 (ZS) ceramics with BaCu(B₂O₅) (BCB) additive were synthesized by the conventional solid-state reaction route and the effect of BCB additive on the microwave dielectric properties of the ceramics was investigated. The results demonstrate that BCB could effectively decrease the sintering temperature from 1300?°C to 930?°C and does not induce obviously degradation of the microwave dielectric properties. The 6.wt% BCB added ZS ceramics exhibited a low sintering temperature (~ 930?°C) and excellent dielectric properties of ε_r =?6.79, Q×f =?33,648?GHz, and τ_f =??30?ppm/°C. To compensate the negative τ_f value of this system, TiO₂ powders were introduced. Particularly when 10.wt% TiO₂ was added, good microwave dielectric properties of ε_r=?8.175, Q×f=?21,252?GHz, and τ_f =?1.2?ppm/°C were obtained for the 6.wt% BCB added ZS ceramic sintered at 930?°C for 3?h. Moreover, BCB added ZS-TiO₂ ceramics have a chemical compatibility with silver, which indicate that the BCB added ZS ceramics are promising candidate for LTCC applications.     

Layered perovskite structured La5−xSrxTi4+xSc1−xO17 microwave ceramics for dielectrically loaded antennas

The crystal structure, microstructure and dielectric properties of layered perovskite structured La_5−xSr_xTi_4+xSc_1−xO₁₇ (x=0–0.8) ceramics prepared by a solid state reaction route were investigated. X-ray diffraction and Raman spectroscopy analysis revealed the formation of single phase orthorhombic (Pnnm) symmetry. The microwave dielectric properties (ε_r, Q×f_o and τ_f) of samples have been investigated as a function of composition. ε_r and τ_f increased while Q×f_o decreased with an increase in x. The sample x=0.6 exhibited ε_r=54.8, Q×f_o= 12,260 GHz and τ_f=6.7 ppm/°C which may be a suitable candidate material for dielectrically loaded antenna applications. The complex impedance spectroscopy analysis of the sample x=0.6 revealed that the sample is highly insulative with two electro-active regions (bulk and grain boundary).     

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.     

A new type of microwave dielectric ceramic based on K2O–SrO–P2O5 composition with high quality factor and low sintering temperature

A new type of microwave dielectric ceramics with low dielectric loss was fabricated through a traditional solid-phase method. X-ray diffraction and density tests showed that KSrPO₄ ceramics with a single orthorhombic phase could be synthesized and densified at 950 °C, and the crystal structure of KSrPO₄ was further confirmed by Rietveld refinement analysis. The densification temperature of KSrPO₄ was lower than 961 °C, indicating the ceramics could be used in LTCC devices. Additionally, based on the complex chemical bond theory, some internal parameters of KSrPO₄ ceramics were calculated and the effects of these parameters on the properties of KSrPO₄ were systematically analyzed for the first time. Furthermore, the composite dielectric constant and loss of KSrPO₄ ceramics were analyzed by infrared reflectance spectroscopy, and the theoretical loss and the actual loss were compared. Finally, a vector network analyzer was employed to measure the microwave dielectric properties of all samples. The results showed that KSrPO₄ sintered at 950 °C obtain the best microwave dielectric properties, including ε_r = 7.85, Q·f = 34,527 GHz (at 10.43 GHz) and τ_f = ?14.82 ppm/°C.     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 ppm/^°C.     

NaTaO3 microwave dielectric ceramic a with high relative permittivity and as an excellent compensator for the temperature coefficient of resonant frequency

NaTaO₃ ferroelectric ceramics sintered at 1550 °C exhibit high relative permittivity and outstanding microwave dielectric properties of ε_r = 113.76, Q × f = 8824 GHz, and τ_? = +645 ppm/°C in the low-frequency band (3.5195 GHz). The high relative permittivity is advantageous for the miniaturization of modern wireless communication devices. A near zero temperature coefficient of the resonance frequency (τ_? = +1.05 ppm/°C) and good microwave dielectric properties (Q × f = 54,680 GHz, ε_r = 19.42) were obtained for 0.92MgTiO₃-0.08NaTaO₃ ceramics. Moreover, the sintering temperature of MgTiO₃ could be decreased from 1350 °C to 1200 °C. Hence, NaTaO₃ is a high-potential microwave dielectric material for adjusting systems with a negative temperature coefficient of resonance frequency.     

Correlation between crystal structure,bond characteristics,Raman vibrations,and improved microwave dielectric properties of high-performance Zn0.5Zr0.5NbO4 ceramics: First principle calculation and experiment

The structure–performance mechanism provides new insights into performance modification and materials discovery. Herein, the electronic structure, Raman vibration, chemical bond factors and enhanced microwave dielectric properties of Zn_0.5Zr_0.5NbO₄ ceramics through oxygen-assisted reaction sintering were investigated by Raman spectroscopy, first-principle calculations, and complex P–V-L theory. Pure-phase Zn_0.5Zr_0.5NbO₄ ceramics were synthesized under oxygen-assisted reaction sintering, confirmed by XRD refinement and Raman analysis. Systematic vibration analysis was first introduced to provide complete mode assignments and Raman shift. Optimized microstructure with full density was obtained through morphology and EDS analysis. First-principle calculations indicated that the d orbit exerts the main contribution to Fermi energy with energy gap of 3.51 eV and Nb–O bonds may possess strong vibration, exhibiting a remarkable effect on dielectric loss. P–V-L results showed that Nb–O bonds have a significant influence on the dielectric constant and Q×f value while the Zn–O bonds dominate the τ_f value. In addition, high-performance Zn_0.5Zr_0.5NbO₄ ceramics were fabricated through oxygen-assisted reaction sintering at 1250 °C, with ε_r = 28.4, Q×f = 79,800 GHz, and τ_f = −47.7 ppm/^°C, exhibiting tremendous superiorities for commercial production.     

Structure,far-infrared reflectance spectra,and microwave dielectric properties of Ba2MGa11O20 (M = Bi,La) ceramics

Ba₂MGa₁₁O₂₀ (M = Bi, La; called BBG and BLG, respectively) ceramics with monoclinic space group I2/m were prepared through a solid-state reaction method. BBG ceramic sintered at 1150 °C for 6 h has the best microwave dielectric properties with low ε_r = 10.68, Q × f = 41,756 GHz, and negative τ_f = ?61.3 ppm/°C. BLG ceramic sintered at 1440 °C for 6 h exhibits ε_r = 13.94, Q × f = 45,592 GHz, and near-zero τ_f = ?16.3 ppm/°C. The large deviation between ε_r and ε_th was ascribed to the “rattling” effect of the cations and the existence of lone pair ions of Bi³⁺. The difference in Q × f of the two ceramics was discussed in terms of packing fraction, and the τ_f of BLG was closer to zero than that of BBG due to the smaller τ_ε value. Their intrinsic dielectric properties were analyzed through far-infrared reflectivity spectroscopy.     

Two novel low permittivity microwave dielectric ceramics Li2TiMO5 (M = Ge,Si) with abnormally positive τf

Two tetragonal natisite structured Li₂TiMO₅ (M = Ge, Si) ceramics fabricated using the conventional solid-state reaction method were investigated in terms of the thermal stability, sintering behavior and dielectric properties at radio (RF) and microwave frequency region. At the optimum sintering temperature of 1140 °C, Li₂TiGeO₅ (LTG) has ε_r ˜ 9.43, Q × f ˜ 65,300 GHz (at 14.7 GHz), and τ_f ˜ +24.1 ppm/°C, while Li₂TiSiO₅ (LTS) sintered at 1180 °C exhibits ε_r ˜ 9.89, Q × f ˜ 38,100 GHz (at 14.2 GHz), and τ_f ˜ +50.1 ppm/°C. The positive τ_f values of the present LTG and LTS are abnormal and extremely important for low-ε_r microwave dielectric ceramics, which could behave as a promising τ_f compensator. Moreover, the dielectric spectra of both ceramics revealed a phase transition at low-temperature, exhibiting a dielectric peak, which could account for the negative τ_ε and positive τ_f in operating temperature ranges.     

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.     

Microwave dielectric properties of (0.75ZnAl2O4–0.25TiO2)–MgTiO3 ceramics prepared using digital light processing technology

ZTM ceramics comprising of 0.75ZnAl₂O₄–0.25TiO₂ and MgTiO₃ at a ratio of 90:10 wt.% are widely used in the field of communication as filters and resonators owing to their excellent microwave dielectric properties. However, the development of such dielectrics with complex structures, as required by microwave devices, is difficult using traditional fabrication methods. In this study, ZTM microwave dielectric ceramics were prepared using the digital light processing (DLP) technology. The influence of the sintering temperature on the phase composition, microstructure, and microwave dielectric properties of ZTM ceramics was investigated. Results showed that with an increase in the sintering temperature, the dielectric constant (ε_r) and quality factor (Q × f) of ZTM ceramics initially increased owing to the increase in the density and diffusion of ions. However, when the sintering temperature was excessively high, the abnormal growth of crystal grains and micropores led to a decrease in ε_r and Q × f. The ZTM ceramics sintered at 1450°C exhibited the optimum microwave dielectric properties (ε_r = 12.99, Q × f = 69 245 GHz, τ_f = −9.50 ppm/°C) owing to the uniform microstructure and a high relative density of 95.02%. These results indicate that DLP is a promising method for preparing high-performance microwave dielectric ceramics with complex structures.     

Correlations between the structural characteristics and enhanced microwave dielectric properties of V–modified Li3Mg2NbO6 ceramics

Novel low-temperature fired Li₃Mg₂Nb_1-xV_xO₆ (x?=?0.02??0.08) microwave dielectric ceramics were synthetized by the partial substitution of V⁵⁺ ions on the Nb⁵⁺ sites. The effects of V⁵⁺ substitution on structure and microwave dielectric properties were investigated in detail. XRD patterns and Rietveld refinement demonstrated that all of the samples exhibited a single orthorhombic structure. The structural characteristics such as the polarizability, packing fraction and NbO₆ octahedron distortion were determined to establish the correlations between the structure and the microwave dielectric characteristics. The ?_r values presented a tendency similar to that of the polarizability. The high Q×f values were mainly attributed to the effects of the grain sizes and density rather than the packing fraction. The variation in the τ_f values was attributed to NbO₆ octahedron distortion. Notably, the Li₃Mg₂Nb_1-xV_xO₆ (x?=?0.02) ceramics sintered at 900?°C had outstanding microwave dielectric properties: ε_r=?16, Q×f=?131,000?GHz (9.63?GHz), and τ_f=???26?ppm/°C, making these ceramics promising ultralow loss candidates for low temperature co-fired ceramics (LTCC) applications.