Crystal structure and performance modification of a novel triclinic CaMgP2O7 microwave dielectric ceramic with low sintering temperature

In this study, crystal structure and microwave dielectric properties of phosphate CaMgP₂O₇ were comprehensively investigated. As a novel microwave dielectric ceramic, CaMgP₂O₇ consists of highly dense structure with optimal microwave dielectric properties (ε_r = 7.8 ± 0.124, Q×f = 13,165 ± 836 GHz, and τ_f = −85.04 ± 1.205 ppm/℃) at a low sintering temperature (950 ℃). The Rietveld refinement of XRD patterns revealed that CaMgP₂O₇ belongs to a triclinic system with P-1 symmetry type. Moreover, the substitution of Zn²⁺ for Mg²⁺ in CaMgP₂O₇ can further reduce the sintering temperature, effectively promote the densification process, and improve the Q×f value. The effects of porosity (or density) and chemical bond characteristics on the performance of CaMg_1-xZn_xP₂O₇ ceramics were carefully analyzed as well. Outstanding performance (ε_r = 8.05 ± 0.12, Q×f = 20,670 ± 923 GHz, and τ_f = −87.59 ± 3.24 ppm/℃) can be achieved for the CaMg_0.84Zn_0.16P₂O₇ ceramic sintered at 875 ℃ for 3 h.     

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.     

Effect of MgO/La2O3 additives on sintering behavior and microwave dielectric properties of (Sr,Ca)TiO3-(Sm,Nd)AlO3 ceramics

The effect of MgO/La₂O₃ additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃−0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT-3SNA) ceramics prepared via conventional solid-state route were systematically investigated. MgO/La₂O₃ as additives showed no obvious influence on the phase composition of the 7SCT-3SNA ceramics and all the samples exhibited pure perovskite structures. The presence of MgO/La₂O₃ additives effectively reduced the sintering temperature of 7SCT-3SNA ceramics due to the formation of a liquid phase at a relatively low temperature during sintering progress. The 0.5 wt% MgO doped 7SCT-3SNA sample with 0.5 wt% of La₂O₃, sintered at 1320 °C for 4 h, was measured to show superior microwave dielectric properties, with an ε_r of 45.57, a Q×f value of 46205 GHz (at 5.5 GHz), and τ_f value of −0.32 ppm/°C, which showed dense and uniform microstructure as well as well-developed grain growth.     

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.     

A novel temperature-stable Ba2-xCaxMgTi5O13 microwave dielectric ceramic

The Ba_2-xCa_xMgTi₅O₁₃ (0 ≤ x ≤ 0.3) microwave dielectric ceramics were for the first time prepared via a conventional solid-state reaction method. A small amount of Ca²⁺ can dissolve into the lattice by forming solid solutions with a monoclinic structure (C2/m) and further influence the sintering behavior, grain growth and microwave dielectric properties of Ba_2-xCa_xMgTi₅O₁₃ ceramics. Both increase of ε_r and decrease of Qxf with x should be associated with increased lattice distortion and uneven grain growth although the sample density and the ratio of the ionic polarizability to the molar volume show little variation. Moreover, the A-site bond valence and τ_f indicate a close relation in current study, such that the Ca²⁺substitution can induce an increase of τ_f values. The optimum microwave dielectric properties of ε_r ∼ 29.3, Qxf ∼ 30,870 GHz (6.5 GHz), and a near-zero τ_f ∼ +2.1 ppm/°C can be contained in the x = 0.15 ceramic sintered at 1160 °C.     

A low-permittivity microwave dielectric ceramic BaZnP2O7 and its performance modification

Complex pyrophosphates compounds have attracted much attention as promising candidates for substrate applications. In the work, a low-permittivity BaZnP₂O₇ ceramic was synthesized through solid-state reaction. The pure phase BaZnP₂O₇ was crystallized in the triclinic P−1 space group. Excellent microwave dielectric properties of the BaZnP₂O₇ ceramic with ε_r = 8.23, Qf = 56170 GHz, and τ_f = −28.7 ppm/°C were obtained at 870°C for 4 h. The substitution of Mg²⁺ for Zn²⁺ was found to have positive effects on grain morphology and dielectric properties. Optimized performance of ε_r = 8.21, Qf = 84760 GHz, and τ_f = −21.9 ppm/°C was yielded at 900°C for the BaZn₀.₉₈Mg_0.02P₂O₇ ceramic. Intrinsic dielectric properties of BaZn_1-xMg_xP₂O₇ ceramics were studied via Clausius–Mossotti equation and complex chemical bond theory.     

A novel ultralow-loss Sr2CeO4 microwave dielectric ceramic and its property modification

A new ultralow-loss Sr₂CeO₄ microwave dielectric ceramic was prepared via a conventional solid-state method. The X-ray diffraction and Rietveld refinement results demonstrate that pure-phase Sr₂CeO₄ ceramics belong to the orthorhombic structure with a Pbam space group. Scanning electron microscopy analysis reveals dense and homogeneous microstructure. Optimum microwave dielectric properties of ε_r = 14.8, Q × f = 172,600 GHz (9.4 GHz) and τ_f = -62 ppm/°C were obtained as it was sintered at 1270 °C for 4 h. In addition, the substitution of a few amount of Ti⁴⁺ for Ce⁴⁺ was found to have significant influences on the grain morphology, sintering behavior, phase structure and microwave dielectric properties. Among them, the Sr₂Ce_0.65Ti_0.35O₄ ceramic sintered at 1350 °C for 4 h demonstrates near-zero τ_f of -1.8 ppm/°C, ε_r of 20.7 and Q×f of 115,550 GHz (8.1 GHz) because of its two-phase structure, showing large application potentials.     

A low-εr and high-Q microwave dielectric ceramic Li2SrSiO4 with abnormally low sintering temperature

A novel low-temperature co-fired ceramic Li₂SrSiO₄ with low-ε_r and high-Q was synthesized through solid-state reaction. XRD results showed that the Li₂SrSiO₄ ceramic formed a single hexagonal structure with a space group of P3₁21. The microwave dielectric properties of Li₂SrSiO₄ ceramic sintered at 880 °C were ε_r = 7.4 ± 0.1, Q × f = 100,700 ± 2000 GHz, and τ_f = −85.4 ± 2.4 ppm/℃. The PVL chemical bond theory indicated that the low sintering temperature of Li₂SrSiO₄ ceramic might be ascribed to the weak covalency (18 %) of SiO bond. A near-zero τ_f (‒1.9 ± 1.8 ppm/℃) combined with ε_r = 8.2 ± 0.1, and Q × f = 63,200 ± 1900 GHz was obtained by adding SrTiO₃ to form 0.94Li₂SrSiO₄ - 0.06SrTiO₃ composite ceramic. Given their chemical compatibility with Ag powders, the Li₂SrSiO₄ ceramics can be a candidate dielectric material for LTCC applications.     

Improved sintering behavior and microwave dielectric properties of SnO2-based ceramics and their LTCC materials with ultrawide temperature stability

Microwave dielectric ceramics with simple composition, a low permittivity (ε_r), high quality factor (Q × f) and temperature stability, specifically in the ultrawide temperature range, are vital for millimetre-wave communication. Hence, in this study, the improvements in sintering behavior and microwave dielectric properties of the SnO₂ ceramic with a porous microstructure were investigated. The relative density of the Sn_1-xTi_xO₂ ceramic (65.1%) was improved to 98.8%, and the optimal sintering temperature of Sn_1-xTi_xO₂ ceramics reduced from 1525 °C to 1325 °C when Sn⁴⁺ was substituted with Ti⁴⁺. Furthermore, the ε_r of Sn_1-xTi_xO₂ (0 ≤ x ≤ 1.0) ceramics increased gradually with the rise in x, which can be ascribed to the increase in ionic polarisability and rattling effects of (Sn_1-xTi_x)⁴⁺. The intrinsic dielectric loss was mainly controlled by r_c (Sn/Ti–O), and the negative τ_f of the SnO₂ ceramic was optimised to near zero (x = 0.1) by the Ti⁴⁺ substitution for Sn⁴⁺. This study also explored the ideal microwave dielectric properties (ε_r = 13.7, Q × f = 40,700 GHz at 9.9 GHz, and τ_f = ?7.2 ppm/°C) of the Sn_0.9Ti_0.1O₂ ceramic. Its optimal sintering temperature was decreased to 950 °C when the sintering aids (ZnO–B₂O₃ glass and LiF) were introduced. The Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic also exhibited excellent microwave dielectric properties (ε_r = 12.8, Q × f = 23,000 GHz at 10.5 GHz, and τ_f = ?17.1 ppm/°C). At the ultrawide temperature range (?150 °C to +125 °C), the τ_ε of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic was +13.3 ppm/°C, indicating excellent temperature stability. The good chemical compatibility of the Sn_0.9Ti_0.1O₂-5 wt% LiF ceramic and the Ag electrode demonstrates their potential application for millimetre-wave communication.     

Synthesis,lattice energy and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics

BaCu_2-xCo_xSi₂O₇ solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu_2-xCo_xSi₂O₇ ceramics were investigated. Single BaCu₂Si₂O₇ phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi₂O₆ phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu²⁺ was replaced by Co²⁺ and the maximum solubility of BaCu_2-xCo_xSi₂O₇ was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu_2-xCo_xSi₂O₇ ceramics. The corrected dielectric constant (ε_r-corr) of BaCu_2-xCo_xSi₂O₇ ceramics decreased monotonously with the susceptibility (Σχ^μ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (U_cal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τ_f) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: ε_r = 8.45, Q×f =58958 GHz and τ_f = -34.4 ppm/°C.     

Microwave dielectric properties of Pb2MoO5 ceramic with ultra-low sintering temperature

Ultra-low firing microwave dielectric ceramic Pb₂MoO₅ with monoclinic structure was prepared via a conventional solid state reaction method. The sintering temperature ranged from 530 °C to 650 °C. The relative densities of the ceramic samples were about 97% when the sintering temperature was greater than 570 °C. The best microwave dielectric properties were obtained in the ceramic sintered at 610 °C for 2 h with a permittivity ∼19.1, a Q × f value about 21,960 GHz (at 7.461 GHz) and a temperature coefficient value of −60 ppm/°C. From the X-ray diffraction, backscattered electron image results of the co-fired samples with 30 wt% silver and aluminum additive, the Pb₂MoO₅ ceramics were found not to react with Ag and Al at 610 °C for 4 h. The microwave dielectric properties and ultra-low sintering temperature of Pb₂MoO₅ ceramic make it a promising candidate for low temperature co-fired ceramic applications.     

Co-effects of Nb2O5 and stoichiometric deviations on the microwave dielectric properties of Y3Al5O12

Four control experiments (S1: Y₃Al₅O₁₂, S2: Y₃Al₅O₁₂+1 wt%Nb₂O₅, S3: Y₃Al_4.955Nb_0.045O₁₂, S4: Y_2.955Al₅Nb_0.045O₁₂) were designed to investigate the co-effects of Nb₂O₅ and non-stoichiometry on the microwave dielectric properties of Y₃Al₅O₁₂. The beneficial results of Nb₂O₅ doping are demonstrated based on X-ray patterns, electron diffraction information, Rietveld refinement, and micromorphological characterization. Differences in dielectric properties were explored using lattice defects, ion polarizability, bond valence, and Raman spectroscopy. In contrast, the Y-deficient sample (S4) possesses the best dielectric properties (ε_r ～ 10.37, Q × f ～ 161,724 GHz and τ_f ～ -33.8 ppm/°C).     

Structural evolution and microwave dielectric properties in Sr2(Ti1-xSnx)O4 ceramics

The structure and microwave dielectric properties of Sr₂(Ti_1-xSn_x)O₄ ceramics were determined in the entire composition range of x?=?0–1.0. X-ray diffraction patterns and Raman spectra indicated a composition-induced onset of octahedral tilting at x?=?0.75, and the crystal structure transformed from tetragonal (I4/mmm) to orthorhombic (Pccn). An obvious change of grain morphology was observed in the phase transformation region as well. The variations of the microwave dielectric properties with composition were systematically investigated and the effect of octahedral tilting on the evolution of τ_f value was emphasized. Moreover, the relationship between τ_ε and tolerance factor of the present ceramics was revealed and compared with the empirical rule in perovskite structure. The role of tolerance factor in designing the materials with required performance was highlighted.     

Crystal structure,chemical bond characteristics,and microwave dielectric properties of novel Sr2CaMoO6 ceramic at different sintering temperatures

This work prepared a novel Sr₂CaMoO₆ (SCM) ceramic through the conventional solid-state process. The crystal structure, chemical bond characteristics, and microwave dielectric properties of SCM ceramic were investigated for the first time. The X-ray diffraction patterns and Rietveld refinement indicate that SCM ceramic formed an orthorhombic phase with the space group Pmm2 (25) at temperatures above 1300 °C. The lattice vibrational modes of SCM ceramic were obtained through the Fourier transformed infrared spectrum (FTIR). The measured dielectric constant and dielectric loss of SCM ceramic is close to the theoretical values obtained by FTIR. According to the P–V–L theory, the chemical bond characteristics of SCM ceramic were calculated. The high ionicity and lattice energy of the Mo–O bond positively affect the properties of SCM ceramic. The outstanding microwave dielectric properties of ε_r = 19.37, Q·f = 32,970 GHz (at 5.96 GHz), and τ_f = ?32.56 ppm/°C were obtained in SCM ceramic sintered at 1450 °C. This work reveals the crystal structure of SCM ceramic, establishes the relationship between the properties and chemical bonds of the ceramic, and lays a foundation for studying the dielectric properties of related ceramic systems.     

Ultra-low temperature sintering and microwave dielectric properties of a novel temperature stable Na2Mo2O7-Na0.5Bi0.5MoO4 ceramic

The novel ultra-low temperature sintering (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ ceramics have been obtained via solid-state reaction method for passive integration use. The Na₂Mo₂O₇ and Na_0.5Bi_0.5MoO₄ crystal phases are found to be compatible with each other from the results of XRD and SEM-EDS. With the x value changing from 0.36 to 1.00, the ε_r increases from 16.0 to 32.0 and the τ_f value varies from ?58 to 47 ppm/°C. At x = 0.75, the 0.25Na₂Mo₂O₇-0.75Na_0.5Bi_0.5MoO₄ ceramic sintered at an ultra-low sintering temperature of 580 °C can be densified (>96%) and possesses good microwave dielectric properties of an ε_r of 24.0, a Q × f value of 13,000 GHz (at 6.2 GHz), and a τ_f value of 3 ppm/°C. The theoretical ε_r and τ_f of the (1-x)Na₂Mo₂O₇-xNa_0.5Bi_0.5MoO₄ composites were calculated using the mixing law and in accordance with the measured values.     

Effects of structural transition on microwave dielectric properties of Sr3(Ti1-xSnx)2O7 ceramics

Sr₃(Ti_1-xSn_x)₂O₇ (x = 0–1.0) ceramics were prepared via a standard solid-state reaction method. X-ray diffraction patterns and Rietveld refinement results indicated a composition induced onset of octahedral tilting when x > 0.2, and the crystal structure transformed in sequence: tetragonal (I4/mmm) → coexistence of tetragonal and orthorhombic (I4/mmm + Amam) → orthorhombic (Amam). The τ_f value could be successfully tuned towards zero and the effects of octahedral tilting on the evolution of τ_f value were emphasized. Meanwhile, the role of tolerance factor in tailoring the resultant τ_ε of the present ceramics was revealed and compared with the empirical rule for complex perovskites. Qf value decreased monotonously with increasing x, which could be elucidated by the variations of extrinsic parameters and intrinsic dielectric loss extrapolated from the infrared reflectivity spectra. The optimum microwave dielectric properties were achieved at x = 0.8 (ε_r = 18.6, Qf = 45,250 GHz, τ_f =–14 ppm/^oC).     

A novel high-Q oxyfluoride Li4Mg2NbO6F microwave dielectric ceramic with low sintering temperature

Novel low-fired Li₄Mg₂NbO₆F ceramics were synthesised using a conventional solid-state reaction method. X-ray diffraction and Rietveld refinement confirmed that the Li₄Mg₂NbO₆F compound had a face-centred-cubic rock salt structure [Fm-3 m(225)] above 625 °C. Li₄Mg₂NbO₆F ceramics sintered between 875 °C and 950 °C displayed the optimised density (> 97.5 %). The theoretical ε_theo was calculated based on the refined crystal parameters, closing to the measured ε_r. The ceramic sintered at 900 °C exhibited excellent microwave dielectric properties with ε_r of 15.53 ± 0.03, Q × f value of 93,300 ± 1100 GHz (at 7.7 GHz) and τ_f value of ?39.8 ± 0.8 ppm/°C. The compatibility with Ag powders makes the oxyfluoride a potential candidate for LTCC applications.     

Bond characteristics and microwave dielectric ceramic of rare-earth tantalite NdTaO4 ceramic

A type of rare-earth tantalite ceramic NdTaO₄ was synthesized by the solid-state reaction method and investigated for the relationship between the structure and microwave dielectric properties for the first time. X-ray diffraction and Rietveld refinement confirmed that an M-fergusonite phase was formed. The dense micromorphology and uniform grain distribution obtained at 1500 °C are advantageous for the microwave dielectric properties. Based on the different bonding properties in the crystal, we investigated the dependence between the crystal structure and the dielectric properties of NdTaO₄, where the ionicity of Nd–O bond in the crystal structure mainly determines the dielectric constant compared with Ta–O bond, and the lattice energy of Ta–O contributes significantly to the quality factor, the TaO₄ and NdO₈ polyhedron contributes collectively to the thermal stability. The optimum microwave dielectric properties were obtained for 1500 °C - sintered samples: ε_r = 18, Q × f = 13,000 GHz (at 8.9 GHz), τ_f = ?21 ppm/^°C.     

Effects of MnO2 and WO3 co-doping and sintering temperature on microstructure,microwave dielectric properties of Ba2Ti9O20 microwave ceramics

Ba₂Ti₉O₂₀ (short for B2T9) ceramics doped with 0.9 mol% MnO₂ and y mol% WO₃ were prepared by solid-state reaction. The influence of sintering temperature, content of WO₃ dopant and the molar ratio x of TiO₂: BaCO₃ on crystal structure, microstructures as well as microwave dielectric properties of B2T9 ceramics was systematically investigated. The major phase of all samples is B2T9, and the minor phase is BaWO₄, respectively. The content of impurity TiO₂ alternates with the variation of compositions and sintering temperature, which also leads to different microwave dielectric properties. With the continuous increase of the sintering temperature, the B2T9 phase grains gradually grow larger and transform from rod grains to plate-like grains. The enlargement and flattening of grains also result in the decrease of compactness and deterioration of microwave dielectric properties. It is found that B2T9 ceramics possess better performance when the sintering temperature is 1340°C, which is related to lower TiO₂ content, BaWO₄, B2T9 grain size, aspect ratio of B2T9 phase and high compactness. When x = 4 and y = 0.2, the relative dielectric constant, quality factor and the temperature coefficient of resonant frequency are 38, 23758 and 7 ppm/°C, respectively.     

A novel low-firing microwave dielectric ceramic Li2ZnGe3O8 with cubic spinel structure

A Li₂ZnGe₃O₈ ceramic was investigated as a promising microwave dielectric material for low-temperature co-fired ceramics applications. Li₂ZnGe₃O₈ ceramic was prepared via the conventional solid-state method. X-ray diffraction data shows that Li₂ZnGe₃O₈ ceramic crystallized into a cubic spinel structure with a space group of P4₁32. Dense ceramic with a relative densities of 96.3% were obtained when sintered at 945 °C for 4 h and exhibited the optimum microwave properties with a relative permittivity (ε_r) of 10.3, a quality factor (Q × f) of 47,400 GHz (at 13.3 GHz), and a temperature coefficient of resonance frequency (τ_f) of −63.9 ppm/°C. The large negative τ_f of Li₂ZnGe₃O₈ ceramic could be compensated by rutile TiO₂, and 0.9Li₂ZnGe₃O₈–0.1TiO₂0·1TiO₂ ceramic sintered at 950 °C for 4 h exhibited improved microwave dielectric properties with a near-zero τ_f of −1.6 ppm/°C along with ε_r of 11.3 and a Q × f of 35,800 GHz (11.6 GHz). Moreover, Li₂ZnGe₃O₈ was found to be chemically compatible with silver electrode when sintered at 945 °C.