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.     

Cold sintering co-firing of (Ca,Bi)(Mo,V)O4–PTFE composites in a single step

Because of large differences in the processing temperature windows between ceramics and polymers, the single-step co-sintering of microwave dielectric ceramic–polymer substrates remains challenging. In this work, a dense (Ca_0.65Bi_0.35)(Mo_0.65V_0.35)O₄ (CBMVO) ceramic was first prepared through cold sintering at 150°C, under a uniaxial pressure of 300 MPa for 60 min with Li₂MoO₄ (LMO) as a transient low-temperature solvent. Cold-sintered CBMVO–5 wt% LMO ceramic shows excellent microwave dielectric properties: ε_r ∼ 11.4, Q × f ∼ 7070 GHz, τ_f ∼ −7.4 ppm/°C. Moreover, the optimized cold sintering process enabled the preparation of a layered co-sintered (2–2 type) CBMVO–polytetrafluoroethylene composite, which maintained excellent microwave dielectric properties and showed a good heterogeneous interface bonding. The proposed cold sintering co-firing of ceramic–polymer composites in a single step shows great potential for application in the seamless integration between ceramics and polymer substrates.     

Cold sintering optimized SrF2 microwave dielectric ceramics for the development of dielectric resonator antenna at 5G millimeter-wave band

SrF₂ is a promising low-ε_r fluoride with outstanding microwave dielectric properties, while densification of SrF₂ ceramics is challenging via traditional thermal sintering (TTS). In this work, dense SrF₂ ceramics with 93.4%–97.2% relative density have been obtained via cold sintering (300 MPa–900 MPa, 150 °C, 1 h) and subsequent post-annealing at 950 °C/3 h. The pretreated cold sintering process is beneficial for microstructure optimization, where the maximum Qf value (62,037 GHz) is obtained at 750 MPa, nearly three times higher than the TTS sample (21,080 GHz). An ultra-low dielectric constant (ε_r) of 5.94 is simultaneously obtained, together with a temperature coefficient of resonant frequency τ_f = ?78.26 ppm/°C. Good chemical compatibility between SrF₂ ceramics and silver is verified, indicating great promise for their use in LTCC technology. Moreover, the low-ε_r and high Qf values of the cold sintering optimized SrF₂ ceramics suggest great potential in the 5G millimeter-wave antenna systems. A SrF₂-based dielectric resonator antenna is designed and fabricated, which resonates at the desired 24.5 GHz and exhibits an outstanding S₁₁ of ?43.95 dB and a broad bandwidth of 4.51 GHz.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

Crystal structure,Raman spectra,and modified dielectric properties of pure-phase ZnZrNb2O8 ceramics at low temperature

Low-temperature co-fired ceramics technology (LTCC) exhibits enormous superiorities in packaging, integration, and interconnection. However, the complex compositions of low-melting point sintering aids may react with ceramic matrix, which increases the difficulties of phase control and tape casting. In this work, the Li₂CO₃–B₂O₃–Bi₂O₃–SiO₂ (LBBS) sintering aid was adopted to sinter ZnZrNb₂O₈ ceramics with single phase at low temperatures. The LBBS glass could be used to fabricate pure-phase ZnZrNb₂O₈ ceramics at a low sintering temperature, promote the grain growth, and increase the densification of ZnZrNb₂O₈ ceramics. Furthermore, the unit cell volume, NbO₆ octahedral distortion, Raman shift, and FWHM changed along with LBBS addition, thereby affecting the microwave dielectric properties. Remarkably, ZnZrNb₂O₈ ceramics doped with 0.75 wt.% LBBS at 950°C were chemically compatible with the silver electrode and exhibited excellent microwave dielectric characteristics: ε_r = 27.1, Q × f = 54 500 GHz, and τ_f = −48.7 ppm/°C, providing candidates for LTCC applications.     

Cold sintering assisted CaF2 microwave dielectric ceramics for C-band antenna applications

A cold sintering process is adopted to pre-densify CaF₂ ceramics from 85.7% at 300 MPa to 91.7% at 750 MPa. Subsequent post-annealings at 1000–1150 °C lead to further improvements in densification, where great enhancements of grain size and crystallinity are also observed from the scanning and transmission electron micrographs. Significant advances in Qf values are achieved in the post-annealed CaF₂ ceramics. The optimum Qf value (80,522 GHz) is achieved after cold sintering at 750 MPa and post-annealing at 1000 °C, which is three times higher than the conventional sintered one at 1000 °C (26,448 GHz). Moreover, the obtained low-ε_r (5.9–6.5) of CaF₂ ceramics suggests broad application prospects in the high-band microwave communications. A microstrip patch antenna is fabricated using the CaF₂ ceramics as the substrate, which operates at 7.89 GHz in the C-band, with an S₁₁ of ?13.4 dB, simulated high gain and efficiency of 6.41 dBi and ?0.56 dB, respectively.     

Influence of V2O5 additions to NdAlO3 ceramics on sintering temperature and microwave dielectric properties

The effects of sintering aids addition on the microstructures and microwave dielectric properties of NdAlO₃ ceramics were investigated. V₂O₅ was selected as liquid phase sintering aid to lower the sintering temperature of NdAlO₃ ceramics. With V₂O₅ addition, the densification temperature of NdAlO₃ can be effectively reduced from 1650 °C to 1390∼1410 °C. The crystalline phase exhibited no phase differences at low addition level while Nd₄Al₂O₉ and NdAl₁₁O₁₈ present as second phases as the addition level was over 1 wt.%. The quality factor Q×f was strongly dependent upon the amount of additions. Q×f values of 64,000 GHz and 48,000 GHz could be obtained at 1390∼1410 °C with 0.25 wt.% and 0.5 wt.% V₂O₅ addition, respectively. During all addition ranges, the relative dielectric constants were not significantly different and ranged from 21.6 to 22.5.The temperature coefficients varies from –31∼–43 ppm/°C.     

Low-temperature preparation and microwave dielectric properties of cold sintered Li2Mg3TiO6 nanocrystalline ceramics

This study aims to fabricate Li₂Mg₃TiO₆ ceramics with ultrafine grains using a novel cold sintering process combined with a post-annealing treatment at a temperature <?950?°C. In this study, phase composition, sintering behavior, microstructure evolution, and microwave dielectric properties of the resultant nanocrystalline ceramics were investigated for the first time. The as-compacted green pellets at 180?°C yielded a high relative density of ~ 90% and the ceramics that were post-sintered over a broad temperature range (800–950?°C) possessed highly dense microstructure with a relative density of ~ 96%. The average grain size varied from 100 to 1200?nm for the samples sintered at 800–950?°C. Furthermore, the quality (Q × f) values of the obtained specimens exhibited a strong positive dependency on the grain size, which increased from 17,790 to 47,960?GHz for grain sizes ranging between 100 and 1200?nm, while the dielectric permittivity (ε_r) and temperature coefficient of the resonant frequency (τ_f) values did not undergo any significant changes over this range of grain size.     

Low‐Temperature Sintering and Microwave Dielectric Properties of ZnNb2O6–TiO2 Ceramics with BaCu(B2O5) Additions

The influence of BaCu(B₂O₅) (BCB) on densification, phases, microstructure and microwave dielectric properties of ZnNb₂O₆–xTiO₂ (x = 1.70–1.90) composite ceramics have been investigated. Undoped ZnNb₂O₆–1.8TiO₂ ceramics sintered at 1200°C exhibit temperature coefficient of resonant frequency (τ_f) ~9.25 ppm/°C. When BaCu(B₂O₅) was added, the sintering temperature of the ZnNb₂O₆–1.8TiO₂ composite ceramics was effectively reduced to 950°C. The results indicated that the permittivity and Q × f were dependent on the sintering temperature and the amounts of BaCu(B₂O₅). Addition of 3.0 wt% BaCu(B₂O₅) in ZnNb₂O₆–1.8TiO₂ ceramics sintered at 950°C showed excellent dielectric properties of ε_r = 40.9, Q × f = 12,200 GHz (f = 5.015 GHz) and τ_f = +0.3 ppm/°C.     

Microstructure and mechanical properties of cold sintered porous alumina ceramics

New innovative approach to fabricate porous alumina ceramics by cold sintering process (CSP) is presented using NaCl as pore forming agent. The effects of CSP and post-annealing temperature on the microstructure and mechanical strength were investigated. Al₂O₃–NaCl composite with bulk density of 2.92 g/cm³ was compacted firstly using CSP and then a porous structure was formed using post-annealing at 1200°C–1500°C for 30 min. Brazilian test method and Vickers hardness test were used to determine the indirect tensile strength and hardness of the porous alumina, respectively. Meanwhile, the phases and the microstructure were respectively examined using X-ray diffractometer and scanning electron microscope (SEM) complemented by the 3D image analysis with X-ray tomography (XRT). SEM structural and XRT image analysis of cold sintered composite showed a dense structure with NaCl precipitated between Al₂O₃ particles. The NaCl volatization from the composite was observed during the annealing and then complete porous Al₂O₃ structure was formed. The porosity decreased from 48 vol% to 28 vol% with the annealing temperature increased from 1200 °C to 1500 °C, while hardness and mechanical strength increased from 14.3 to 115.4 HV and 18.29–132.82 MPa respectively. The BET analysis also showed a complex pore structure of micropores, mesopores and macropores with broad pore size distribution.     

Enhanced resistivity and strain stability of BiFeO3–BaTiO3 ceramics by hot-press sintering in oxygen atmosphere

Even though BiFeO₃–BaTiO₃ (BF–BT) with high Curie temperature and excellent piezoelectric properties is very suitable for high-temperature applications, its rapid reduction in resistivity with temperature limits its further application. So far, there is no effective method to improve the resistivity of BF–BT at a high-temperature state. In this work, hot-press sintering combined with an oxygen atmosphere was used to prepare (1 − x)BF–xBT (x = 0.2–0.33) ceramics for the first time, which reduced the sintering temperature from 1000 to 920°C. The controllable grain size can be achieved by adjusting the sintering temperature and the applied pressure. The X-ray photoelectron spectroscopy results confirmed that using hot-press sintering effectively avoided the generation of heterovalent Fe ions, and the resistivity of BF–BT ceramics at the high-temperature stage was improved by two orders of magnitude. It was found that hot-press sintering can cause the oriented growth of the sample along the (1 1 0) direction, and further refined X-ray diffraction was used to accurately analyze the changes in the lattice structure. The hot-press sintered samples obtained larger polarization strength, especially the electro-induced strain showed excellent temperature stability in the wide temperature range of 30–170°C. Hot-pressing sintering combined with an oxygen atmosphere is more suitable for preparing high insulation and electrical breakdown resistance ceramics.     

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.     

Mechanism study of the Mn-substituted magnesium borate: Decreased sintering temperature and improved dielectric property

The influence of manganese substitution on the sintering and dielectric properties of Mg₃B₂O₆ (MBO) was studied, and the detailed mechanism of variation was discussed using density functional theory calculations. The characterization method involved a network analyser, differential–thermal analyses, thermomechanical analyses, X-ray diffraction, and scanning electron microscopy. Manganese substitution formed a two-phase system (MBO and Mg₂B₂O₅), improved the dielectric property, densified the microstructure, lowered the activation energy, decreased the crystallite size, modified the band structure property, and strengthened the covalency of MgO₆ octahedron. The 7% mole substitution of manganese to magnesium improved the dielectric properties of MBO to 7.06 for ε_r and 61 100 GHz at 15 GHz for Q × f, −54.8 ppm/°C for τ_f. The intrinsic densification temperature decreased from 1350 to 1175°C with 97.7% relative density.     

Low-temperature-sintered MgO-based microwave dielectric ceramics with ultralow loss and high thermal conductivity

With the development of 5G/6G communication, the requirements of portable devices for miniaturization and multifunction make low-temperature co-fired ceramic (LTCC) more and more important. In the area of high-frequency high-density passive integration, microwave dielectric ceramics with a low dielectric loss and high thermal conductivity are urgently needed to ensure the effective signals transmission and system reliability. However, most microwave dielectric ceramics with a low dielectric loss were not applicable for the LTCC technology due to the high sintering temperature. In this work, a series of MgO-based ceramics [(100 − x) wt.% MgO–x wt.% (0.2SrF₂–0.8LiF) (x = 5,7,10)] were prepared by solid-state reaction method. The addition of sintering aid 0.2SrF₂–0.8LiF (S₂L₈) decreased the sintering temperature below 880°C without degrading the microwave dielectric properties of ceramics. Microwave dielectric properties of ceramics, including quality factor Q × f, relative permittivity ε_r, and temperature coefficient of resonant frequency τ_f, were investigated to find the optimum composition and sintering temperature. In general, MgO–7 wt.% S₂L₈ ceramic sintered at 860°C exhibits outstanding properties of Q × f = 180 233 GHz, ε_r = 9.11, τ_f = −40.33 ppm/°C, and a high thermal conductivity of 24.02 W/(m K). This series of ceramics are suitable to be co-fired with Ag electrodes. With all those great properties, this series of MgO-based ceramics are expected to be the candidates for LTCC applications in 5G/6G technology.     

Low‐Temperature‐Fired ReVO4 (Re = La,Ce) Microwave Dielectric Ceramics

We report a series of ReVO₄ (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid‐state reaction method. X‐ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO₄ ceramics with a monazite structure and tetragonal CeVO₄ ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO₄ and CeVO₄ ceramics sintered at 850°C and 950°C for 4 h possessed out‐bound microwave dielectric properties: ε_r = 14.2, Q × f = 48197 GHz, τ_f = ?37.9 ppm/°C, and ε_r = 12.3, Q × f = 41 460 GHz, τ_f = ?34.4 ppm/°C, respectively. The overall results suggest that the ReVO₄ ceramics could be promising materials for low‐temperature‐cofired ceramic technology.     

Ultra-low fired fluoride composite microwave dielectric ceramics and their application for BaCuSi2O6-based LTCC

A total of 14 fluoride composite ceramics were prepared through solid-state method and their microwave dielectric properties were investigated. Among the fluoride composite ceramics, 0.36LiF–0.39MgF₂–0.25SrF₂ (LMS) had the lowest sintering temperature (600°C) and presented a dielectric constant (ε_r) of 6.24 ± 0.05, a quality factor (Q × f) of 33 274 ± 900 GHz, and a temperature coefficient resonant frequency (τ_f) of −86.74 ± 8 ppm/°C. As the LMS ceramic had a low melting point (646°C), it can be used as sintering aid for LTCC applications. The sintering temperature of BaCuSi₂O₆ decreased from 1050°C to 875°C with 2 wt% LMS doped and excellent microwave dielectric properties of ε_r = 8.16 ± 0.04, Q × f = 24 351 ± 300 GHz, and τ_f = −9.74 ± 1 ppm/°C were obtained. Moreover, BaCuSi₂O₆-2 wt% LMS can be co-fired with Ag powders, which makes it a potential new candidate for LTCC applications.     

Low‐Temperature Sintering and Microwave Dielectric Properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–(Ca0.8Sr0.2)TiO3 Composite Ceramics

0.9(Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄–0.1(Ca_0.8Sr_0.2)TiO₃ (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited ε_r = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τ_f = 15 ppm/°C. The effects of LiF–Fe₂O₃–V₂O₅ (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF₂/0.28MgF₂/0.6LiF and MgV₂O₆ were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca₅Mg₄(VO₄)₆ as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: ε_r = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τ_f = ?10 ppm/°C.     

Highly dense lead titanate ceramics from refined processing

Lead titanate (PbTiO₃, PT) ceramic is a useful pyroelectric and piezoelectric material for high-temperature applications. However, it is very difficult to prepare a pure-phase and dense PT as a result of a high c/a ratio. In this study, conformable PT ceramics with 97% of the theoretical density were successfully synthesized by means of carefully controlled processing parameters that include sintering temperature, soaking time and heating/cooling rates. It can be found that these ceramics exhibit highly densified and uniform microstructure with the optimum conditions of 1225 °C sintering temperature, 2 h soaking time and 1 °C min⁻¹ heating/cooling rates. Moreover, relatively inexpensive laboratory grade lead oxide (PbO) and titanium oxide (TiO₂) can be used as starting materials.     

Novel Temperature Stable Li2MnO3 Dielectric Ceramics with High Q for LTCC Applications

Novel microwave dielectric ceramics in the Li₂MnO₃ system with high Q prepared through a conventional solid‐state route had been investigated. All the specimens exhibited single phase ceramics sintered in the temperature range 1140°C–1230°C. The microwave dielectric properties of Li₂MnO₃ ceramics were strongly correlated with sintering temperature and density. The best microwave dielectric properties of ε_r = 13.6, Q × f = 97 000 (GHz), and τ_f = ?5.2 ppm/°C could be obtained as sintered at 1200°C for 4 h. BaCu(B₂O₅) (BCB) could effectively lower the sintering temperature from 1200°C to 930°C and slightly induced degradation of the microwave dielectric properties. The Li₂MnO₃ ceramics doped with 2 wt% BaCu(B₂O₅) had excellent dielectric properties of ε_r = 11.9, Q × f = 80 600 (GHz), and τ_f = 0 ppm/°C. With low sintering temperature and good dielectric properties, the BCB added Li₂MnO₃ ceramics are suitable candidates for LTCC applications in wireless communication system.     

A low-sintering temperature microwave dielectric ceramic for 5G LTCC applications with ultralow loss

In next-generation mobile and wireless communication systems, low sintering temperature and excellent dielectric properties are synergistic objectives in the application of dielectric resonators/filters. In this work, Li₂Ti_0·98Mg_0·02O_2·96F_0.04–1 wt% Nb₂O₅ (LTMN) ceramics were fabricated, and their sintering temperature was successfully lowered from 1120 °C to 750 °C by adjusting the mass ratio of B₂O₃–CuO (BC) additive. The optimum dielectric properties (ԑ_r ~ 24.44, Q × f ~ 60,574 GHz and τ_f ~ 22.8 ppm/°C) were obtained in BC-modified LTMN ceramics sintered at 790 °C. Even if their sintering temperature was lowered to 750 °C, the lowest temperature among the Li₂TiO₃-based dielectric ceramics currently used for LTCC technology, excellent dielectric properties (ԑ_r ~ 23.77, Q × f ~ 51,636 GHz) were still maintained. Additionally, no extra impurity phase was detected in BC-modified LTMN ceramics co-fired with Ag at 790 °C, indicating that BC-modified LTMN ceramics have a bright prospect in high-performance LTCC devices for 5G applications.