Microwave Dielectric Properties Research Articles

Enhanced Microwave Magnetic and Dielectric Properties of YBiIG Ferrite by Ca-Zr Co-substitution

Enhanced Microwave Magnetic and Dielectric Properties of YBiIG Ferrite by Ca-Zr Co-substitution

Structure/property dynamics of low-temperature sintering pure and Mg-modified Zn3V2O8 ceramics

The pure and Mg-modified Zn3V2O8 ceramics were successfully synthesized using the solid-state reaction. Zn3V2O8 is well crystallized as the orthorhombic phase with a space group of Cmca. The Zn site of Zn3V2O8 ceramic can be partially substituted by Mg to form a continuous solid solution. Appropriate substitution can tremendously enhance the dielectric chartcteristics of the samples. P-V-L bond theory was also employed to investigate the structural and chemical bond effects on the microwave dielectric properties of the specimens. Raman spectrum was sensitive to crystallinity, and can highly reflect the dielectric loss of the ceramics. The Zn3V2O8 ceramic sintered at 690°C reveals a high Q×f =59,000 GHz, along with an εr =13.8, and τf =–120 ppm/°C.Remarkably, the Zn2.99Mg0.01V2O8 specimen demonstrates outstanding properties, with εr =14.7, Q×f =86,000 GHz and τf =–82 ppm/°C. This signifies an approximately 46 % improvement in Q×f compared to pure Zn3V2O8, indicating the ceramic's great promise for low-temperature co-fired ceramic (LTCC) applications, especially in high-frequency regions where low dielectric loss is essential.

Crystal structure, chemical bonds and microwave dielectric properties of ultra-low loss Li2Mg2GaTi2O8F ceramics

Ceramics of Li2Mg2GaTi2O8F (LMGTOF) were prepared by the traditional solid-state method. XRD and TEM results confirmed that a single phase of Li2Mg2GaTi2O8F, with an Fd-3m space group and a cubic spinel structure, was formed by the ceramics. It was shown by the results that the lattice parameters of the ceramics at 1050 °C were measured as a = b = c = 8.36576 Å, with a volume of V = 585.485549 Å3. Furthermore, it was observed through the analysis of the Raman peak at 718 cm−1 that an inverse relationship was exhibited between the Raman shift and εr. Similarly, an opposing trend was also demonstrated between the Q × f value and FWHM. By bond valence analysis, it was shown that Mg2+, Ga3+, and Ti4+ ions were in a “rattling” state, and the large positive deviation between εcorr and εtheo was attributed to the strong “rattling” effect induced by the cations. At 1050 °C, the best microwave dielectric properties (εr = 14.58, Q × f = 76,689 GHz, and τf = −52 ppm/°C) were exhibited by LMGTOF ceramics.

Low-loss and low-temperature sintering of SrNb2V2O11 with a large positive τf value as an effective temperature compensator

Novel SrNb2V2O11 microwave dielectric was synthesized and proposed as a temperature compensator for Low-temperature cofired ceramics (LTCC)/Ultra-low-temperature cofired ceramics (ULTCC) applications. The X-ray diffractometer (XRD) patterns matched well with the SrNb2V2O11 phase, exhibiting a monoclinic structure and Cc space group over the entire experiment. The presence of the Nb-rich second phase at 850 °C was attributed to the melting of the samples. The microwave dielectric properties were highly correlated with the chemical bond parameters, which were intensively investigated using the P-V-L theory. The specimen sintered at 820 °C reveals dielectric properties, combining an εr of 74.5, Q × f of 13,000 GHz, and τf of 517 ppm/°C. The large positive τf can make the ceramic an effective temperature compensator for LTCC applications. Moreover, the medium-high εr value can also act as a practical material for dielectric resonators used in the base stations, where a stable environment and limited temperature changes are crucial.

Improved microwave dielectric properties of LaZn1/2Ti1/2O3 through Y3+ substitution

LaZn1/2Ti1/2O3 is a double perovskite with promising microwave dielectric properties for applications towards dielectric resonators and antennas. Its dielectric properties can be enhanced through the substitution of suitable ions in either A- or B-sites. In this paper, we report a significant improvement in the quality factor (Q) by substituting Y3+ in the La site with a marginal reduction in dielectric constant (εr). A single phase is confirmed in XRD indicating the formation of La1-xYxZn1/2Ti1/2O3 (0 ≤ x ≤ 0.25) solid solutions. The substitution results in an expansion of lattice along the b direction while there is a contraction in both a and c directions, leading to a reduction in unit cell volume. The dielectric constant, εr, is observed to reduce from 33.4 to 29.6 due to the polarizability of A-site atoms. The temperature coefficient of resonant frequency (τf) increases with an increase in x which does not appear to correlate with reducing tolerance factor (t). This non-conventional behaviour of τf in this perovskite based solid solutions brings bond valence of O site (VO) into picture. VO appears to show competing effect on τf. The non-monotonic behaviour is observed in Q×f with a maximum value of 41,400 GHz at x = 0.15 and a sharp decline for x > 0.15. This observation is correlated with variations in the A-site bond valence (VA) and a long-range ordering (LRO) of B-site ions. LRO is associated with Ag mode at 710 cm−1 in the Raman spectra. The low frequency (< 250 cm−1) Raman modes shift to a higher frequency with the increase in Y3+. Y3+ substitution also induces lattice distortion which is discussed in terms of variations in FWHM and relative intensity of the Raman modes.

Microwave dielectric properties of a temperature-stable and ultra-low sintering Li3AlMo3O12/Li0.5Bi0.5MoO4 composite ceramics for ULTCC application

Temperature-stable ULTCC composite (1-x)Li3AlMo3O12-xLi0.5Bi0.5MoO4 ceramics were prepared by solid-state reaction method. X-ray and EDS results showed that Li3AlMo3O12 phase and Li0.5Bi0.5MoO4 phase exist independently, and no other phase is generated. Due to the low sintering temperature of both two phases, the composite ceramics can obtain high density at 560 ℃. Microwave dielectric properties mainly depend on the ceramic Li0.5Bi0.5MoO4 content x, the addition of Li0.5Bi0.5MoO4 effectively increases the τf of Li3AlMo3O12 from -65.05 ppm/℃ to 4.96 ppm/℃, but caused the dielectric loss increase. When x=0.45 and sintered at 560 ℃, microwave dielectric properties of 0.55Li3AlMo3O12-0.45Li0.5Bi0.5MoO4 ceramics with close zero τf: εr = 15.32, Q×f = 13,158GHz, τf = 4.96 ppm/℃.

A novel microwave dielectric ceramic CaB2O4 with low dielectric constant

A novel microwave dielectric ceramic CaB2O4 with low dielectric constant was synthesized by the solid-state reaction method. The impact of diverse Ca/B ratios on the phase composition, microstructure and dielectric properties of CaB2+xO4+3x/2 (x = 0 ∼ 0.6) ceramics was investigated. The bond ionicity and the lattice energy of CaB2+xO4+3x/2 ceramics were calculated by the P-V-L theory. The appropriate excess of boron facilitated the densification of CaB2O4 ceramics, resulting in the stable crystal structure and enhanced dielectric properties. CaB2.3O4.45 ceramic sintered at 950 °C had excellent properties: εr = 6.03, Q×f = 37,962 GHz, and τf = -24.7 ppm/°C. The influence of TiO2 on the microwave dielectric properties of CaB2O4 was investigated, 87wt% CaB2O4 - 13wt% TiO2 ceramics sintered at 950 °C showed a zero τf value.

Effect of (Ba1/3Ta2/3)4+ substitution on microstructure, bonding properties and microwave dielectric properties of zirconium-cerium molybdate ceramics and the fabrication of S-band patch antennas

In this study, Ce2[Zr1x(Ba1/3Ta2/3)x]3(MoO4)9 (CZ1xTx, x = 0.01 ~ 0.1) ceramics were synthesized via the solid-state method at temperatures ranging from 650°C to 850°C. XRD and Rietveld refinement analyses revealed that CZ1xTx ceramics exhibit a single phase with the space group R3-c, and the cell volume increases with higher doping content. SEM results indicate that CZ0.98T0.02 ceramics exhibit a highly regular and uniform grain shape. The CZ0.98T0.02 ceramics displayed optimal performance at 800°C, exhibiting εr = 10.40, Q×f = 76,893GHz, and τf = 14.61 ppm/°C. Additionally, the P-V-L theory was employed to establish correlations between ceramic chemical bonds and performance attributes. The results showed that the Ce-O bond predominantly influences εr while the Mo-O bond correlates with Q×f and τf. Furthermore, far-IR reflection spectra unveiled ion displacement polarization as the primary mechanism in CZ1xTx ceramics. Finally, based on CZ0.0.98T0.02 ceramics, a patch antenna applicable to the S-band was designed and fabricated, evidencing the vast potential of this ceramic in industrial applications.

Temperature-dependent dielectric behavior of A6B2O17 (A = Zr, Hf; B = Nb, Ta) phases

We report on temperature-dependent dielectric behavior of disordered ternary A6B2O17 (A = Zr, Hf; B = Nb, Ta)-form oxides in the GHz frequency range. The microwave dielectric properties including relative permittivity, dielectric loss, and temperature-dependent relative permittivity were characterized using cylindrical dielectric resonators using a resonant post measurement technique. Dielectric measurements through the resonant post method approach generally agree with dielectric measurements of A6B2O17 bulk ceramics measured through standard resonant post techniques. Coefficients describing the temperature-dependent relative permittivity for ternary A6B2O17 phases are strongly positive, suggesting contributions to polarizability arising from long-range mechanisms potentially associated with structural disorder. These observations support the working hypothesis that material functionality can be engineered by the chemical diversity and structural disorder possible in high configurational entropy A6B2O17 phases.

Relationship between chemical bond characteristics and microwave dielectric properties of low temperature sintering CoO-V2O5 ceramics

The two most fundamental compositions within the CoO-V2O5 family, namely CoV2O6 and Co2V2O7 ceramics, were synthesized using the traditional solid-state method. The microwave dielectric properties of both ceramics were investigated for the first time. The P-V-L bond theory was used to analyze the correlation between their microwave dielectric properties and chemical bond characteristics. The CoV2O6 ceramic exhibits a γ phase (εr = 11.6, Q×f = 39,177 GHz, and τf = −15 ppm/°C) when sintered at 660 °C. However, it experiences a phase transformation from the γ phase to the α phase at 690 °C, which causes the dielectric properties to degrade. The Co2V2O7 ceramic demonstrates excellent dielectric characteristics (εr = 9.4, Q×f = 88,645 GHz, and τf = −37 ppm/°C) at 720 °C. Importantly, its sintering temperature can be lowered to 660 °C while still maintaining a good combination of microwave dielectric properties (εr = 7.7, Q×f = 75,887 GHz, and τf = −58 ppm/°C). It is found that Co2V2O7 exhibits a low dielectric loss due to relatively high lattice energy resulting from the contribution of Co–O bonds. CoV2O6, on the other hand, shows a lower sintering temperature because it contains more V2O5, which acts as an effective sintering aid. These two ceramics both exhibit good chemical compatibility with aluminum and are promising candidates for applications in low-temperature and ultra-low-temperature co-fired ceramics (LTCCs and ULTCCs) that operate in the microwave frequency range or higher.

A novel Li3Mg4(Nb1-xVx)O8 ceramic with high Q×f value and good temperature stability with low sintering temperature

A series of low-loss Li3Mg4(Nb1-xVx)O8 (0.02 ≤ x ≤ 0.08) ceramics has been synthesized in this work, with the optimal sintering temperature lowered to 950°C due to the incorporation of V5+ ions. Moreover, the effects of substituting V5+ for Nb5+ on the sintering, composition, structure and properties of the ceramics have been systematically analyzed. It has been discovered that the permittivity (εr) and quality factor (Q×f) of Li3Mg4(Nb1-xVx)O8 ceramics exhibit a correlation with the relative density, which is influenced by the variation in grain size distribution resulting from the substitution of V5+ ions. Additionally, the total lattice energy of Li3Mg4(Nb1-xVx)O8 ceramics is also affected by the bond characteristics within the Nb-O octahedra, thus influencing the Q×f value. The resonant frequency temperature coefficient (τf) fluctuates within a small range. The Li3Mg4(Nb0.98V0.02)O8 ceramics exhibit excellent microwave dielectric properties at 950°C: εr∼14.03, Q×f∼93,425 GHz, τf∼-6.12 ppm/°C, with a relative density reaching 98.3 %. Compared to unsubstituted Li3Mg4NbO8 ceramics, the sintering temperature is reduced by 200°C without forming impurity phases. While adjusting the τf value to nearly zero, it maintains a high Q×f value, indicating promising applications in Low-Temperature Co-fired Ceramics (LTCC).

Microwave dielectric properties of normal, fibroelastotic, and malignant human lung tissue

Technological development of microwave treatment and detection techniques for lung cancer requires accurate and comprehensive knowledge of the microwave dielectric properties of human lung tissue. We characterize the dielectric properties of room temperature human lung tissue from 0.5 to 10 GHz for three lung tissue groups: normal, fibroelastotic, and malignant. We fit a two-pole Debye model to the measured frequency-dependent complex permittivity and calculate the median Debye parameters for the three groups. We find that malignant lung tissue is approximately 10% higher in relative permittivity and conductivity compared to normal lung tissue; this trend matches previously reported normal versus malignant data for other biological tissues. There is little contrast between benign lung tissue with fibroelastosis and malignant lung tissue. We extrapolate our data from room temperature to 37 °C using a temperature-dependence model for animal lung tissue and use the Maxwell-Garnett dielectric mixing model to predict the dielectric properties of inflation-dynamic human lung tissue; both approximations correspond with previously reported dielectric data of bovine and porcine lung tissue.

Influence of structural characteristics of Ca1-xNdxW1-xVxO4 ceramics on microwave dielectric properties

Dense Ca1-xNdxW1-xVxO4 (x = 0−0.4) microwave dielectric ceramics were obtained when sintered at 1150 °C for 4 h. A tetragonal scheelite could be formed in the range of x ≤ 0.3, while a composite region with tetragonal scheelite and tetragonal zircon was found at x ≥ 0.4. The relative permittivity (εr) increased from 10.15 to 13.66, mainly attributed to an increase in the ionic polarizability. The quality factor (Q×f) decreased from 61,060 to 32,204 GHz, correlated to the variations in lattice energy and the Raman FWHM. The temperature coefficient of resonant frequency (τf) gradually increased from −54.03 to −23.50 ppm/℃, mainly affected by the rattling effect of W6+/V5+ and the temperature coefficient of ion polarizability (ταm). Ca0.7Nd0.3W0.7V0.3O4 with excellent comprehensive properties (εr = 13.21, Q×f = 35,570 GHz, and τf = −23.50 ppm/°C) were obtained at 1150 °C.

Resonance-driven microwave heating for improved methane conversion to hydrogen

Efficient hydrogen production through methane pyrolysis is hindered by high temperature required and frequent catalyst deactivation due to carbon deposition. This study proposes a novel use of variable-frequency microwave heating to enhance the efficiency and stability of methane pyrolysis by leveraging the resonance effects between microwave frequencies and dielectric properties of activated charcoal (AC). A solid-state variable-frequency microwave reactor was employed to modulate microwave frequencies to match the dielectric properties of AC, with a peak tanδ of 0.19 at 4600 MHz. Experimental results showed that at a resonance frequency of 4650 MHz and a microwave power of 100 W, the reaction temperature of AC reached 960 °C within 2 min, achieving a 100% methane conversion rate. However, carbon deposition reduced the conversion rate to 35.1% after 150 min. By cyclically adjusting the microwave frequencies between 4650 MHz and 4640 MHz, the catalyst was successfully reactivated, restoring the conversion rate to 100%. This frequency modulation method enabled continuous methane pyrolysis for 1300 min, highlighting the potential of variable-frequency microwave heating as a novel and efficient approach for a sustainable production of green hydrogen.

Structures and properties of LiBaF3 microwave dielectric ceramics for ULTCC applications

Perovskite structural LiBaF3 ceramics with excellent microwave dielectric properties were synthesized using traditional solid-state method. By regulating the sintering temperatures from 600 °C to 675 °C, LiBaF3 ceramics exhibit tunable dielectric constant (εr) from 12.42 to 13.72, adjustable Q × f value from 55000 GHz to 73000 GHz, and regulable temperature coefficient of resonance frequency (τf) from −36.78 ppm/°C to −34.68 ppm/°C. The properties are strongly associated with the density, and can be further optimized by 0.5 wt%TiO2 addition (εr = 14.43, Q × f = 56000 GHz, τf = -1.26 ppm/°C). The LiBaF3 ceramics also exhibit excellent compatibility with Ag and Al electrodes. Furthermore, relationships between multiscale structures and microwave dielectric properties of the ceramics were revealed. Oxidation behaviors between LiBaF3 and O2 would result in the lattice shrinkage, the introduction of defect level, the decrease of bandgap and density, and then deteriorating the microwave dielectric properties. All the findings make the LiBaF3 ceramic a promising candidate for ULTCC application, and may facilitate the development of other novel microwave dielectric materials.

Influence of crystal system, bond valence and ionic polarizability on the microwave dielectric properties of BaIn2O4 ceramic

BaIn2O4 ceramics with the monoclinic crystal system of the P21/c space group were synthesized using a solid-state reaction method. The sample sintered at 1250 °C exhibits a relative density (94.3 %) and the following microwave dielectric properties: εr = 16.3, Q × f = 49,500 GHz, τf = −73.1 ppm/°C. The bond valence results shows that the deviation of permittivity (24.9 %) between the experimental (εr = 16.3) and calculated values (εrC-M = 13.05) obtained using the Clausius−Mosotti equation may be ascribed to the rattling effect of Ba2+ and In3+. Compared with orthorhombic CaIn2O4 and SrIn2O4 ceramics, BaIn2O4 ceramics have lower Q × f values possibly because of the intrinsic loss caused by nonharmonic lattice vibration. The Edc value (0.43 eV) of BaIn2O4 demonstrates that the defects are caused by the single ionized oxygen vacancies. The intrinsic εr (14.7) and Q × f (105,600 GHz) values of the BaIn2O4 ceramic was inferred through infrared reflectivity spectroscopy. All results demonstrate that the dielectric properties are affected not only by density, ionic polarizability, and rattling effect of cations, but also by the crystal system, and particularly the intrinsic factors.

High-performance SiO2-Sr3(PO4)2 low-εr dielectric ceramics for MW/THz applications

We developed high-performance SiO2-based low-εr microwave dielectric composites via adding Sr3(PO4)2 and investigated their phase assemblages, sintering behaviors, microwave dielectric properties, thermal expansion coefficients (CTE), and mechanical strengths in this paper. Adding Sr3(PO4)2 into SiO2 could successfully suppress the occurrence of microcracks in SiO2, tune the values of both temperature coefficient of resonant frequency (τf) and CTE, and enhance the flexural strength of the composite simultaneously. The composite with 22.5% volume fraction (Vf) of Sr3(PO4)2 exhibits excellent combined properties with a low-εr (~5.35), high Q×f value ( ~73,717GHz), near-zero τf value(~ -1.1 ppm/°C), suitable high CTE value (~17ppm/°C), and acceptable high flexural strength (~85Mpa) after sintering at the temperature of 1150°C/3h.

Crystal structure and microwave dielectric properties of BaZn(1-x)CoxP2O7 ceramics

BaZn(1-x)CoxP2O7 (0 x 0.125) ceramics were synthesized using the solid-state reaction method, with sintering temperatures ranging from 875 °C to 975 °C. The study systematically investigated the phase composition, crystal structure, and microstructure of the ceramics, and correlated these characteristics with their dielectric properties. Additionally, density functional theory (DFT) was employed to examine the energy band structure, electron density, and density of states of the materials. The dielectric constant values were primarily influenced by polarizability, while the Q × f values of BaZn(1-x)CoxP2O7 ceramics were predominantly affected by the packing fraction and full width at half maximum (FWHM). The temperature coefficient of resonant frequency was influenced by a decrease in bond valence and an increase in PO4 tetrahedral distortion. Notably, the BaZn0.975Co0.025P2O7 ceramic demonstrated promising microwave dielectric properties, with = 8.15, Q × f = 42,431 GHz, and = -27.5 ppm/ °C, when sintered at 950 °C for 4 h.

Microwave dielectric properties of Li2Mg2Ga2Ti3O12 ceramics with spinel-type

Novel spinel-type Li2Mg2Ga2Ti3O12 ceramics were prepared by the solid-state method. Rietveld refinement revealed that the Li2Mg2Ga2Ti3O12 ceramics possess a cubic spinel structure with the space group. Raman spectrum analysis indicated that the Q×f and FWHM of ceramics exhibit an inverse relationship. According to the P-V-L theory, the Ti-O bond has the highest percentage of contribution to both of bond ionicity and lattice energy, indicating a significant impact on εr and Q×f, respectively. The Li2Mg2Ga2Ti3O12 ceramics showed the best microwave dielectric properties (εr = 15.72, Q×f = 69,633 GHz, and τf = −20 ppm/°C), sintering at an optimum temperature of 1200 °C. Additionally, a dielectric resonance antenna, on the basis of Li2Mg2Ga2Ti3O12 ceramics was designed and simulated.

Structure and microwave dielectric characteristics of (Ba,Sr,Ca)HfO3 ceramics

In the present work, the structure and microwave dielectric properties of (Ba,Sr,Ca)HfO3 ceramics were systematically investigated to understand the general mechanism of tuning the temperature coefficient of resonant frequency, τf in perovskite ceramics. Ba1–xSrxHfO3 and Sr1–yCayHfO3 could form continuous solid solutions while the solid solubility of Ca in Ba1–yCayHfO3 was about 20% (in mole). τf changed nonlinearly with increasing tolerance factor as the result of competition between the increase in the restoring force on the ions and the increase in polarizability. Under the guidance of three microscopic mechanisms affecting τf, a preliminary attempt was made to explore the suitable parameters to predict the variation trend of τf. Normalized ionic radii and the τf values of the end members were selected as independent variables, and τf was calculated by using multiple linear regression method. For Ba1–xSrxHfO3 and Sr1–yCayHfO3 with orthorhombic structures, the root mean square error between the calculated and measured τf was only 6.8×10–6 °C–1. The good agreement between the calculated τf values and the measured ones in Ba1–x–ySrxCayHfO3 ceramics confirmed its validity where three elements jointly occupy A-site, but it only works when there is no structural phase transition. Progresses in this research field would not only deepen our understanding of mechanism of regulating properties in multi-ion solid solutions, but also boost the developments of closely related fields such as machine learning-assisted design and high-entropy ceramics. Finally, a good combination of microwave dielectric properties was achieved in Sr0.15Ca0.85Hf0.96Ti0.040O3: εr = 27.8, Qf = 36,470 GHz, τf = +5×10–6 °C–1.