Low-fired fluoride microwave dielectric ceramics with low dielectric loss

Low-fired fluoride microwave dielectric ceramics (LiF, CaF₂, SrF₂ and BaF₂) were prepared through a simple one-step sintering process. Fluoride ceramics, especially LiF, which had the lowest sintering temperature of 800?°C, could be well sintered below 1050?°C. Rietveld refinement results showed that LiF, CaF₂, SrF₂ and BaF₂ ceramics crystallized into a cubic structure with space group Fm-3m. The relative permittivity (ε_r), quality factor (Q?×?f) and temperature coefficient of the resonant frequency (τ_f) of the fluoride ceramics were closely related to relative density, the ionic polarizability of the primitive unit cell, the packing fraction and the bond valence. In this series of low-permittivity fluoride ceramics, LiF, CaF₂ and BaF₂ could be co-fired with Ag powders, and LiF ceramic exhibited the highest Q×f value of 73880?GHz, which is comparable to those of traditional oxide microwave dielectric ceramics.     

Microwave dielectric properties of LiNb3O8 ceramics with TiO2 additions

The microwave dielectric properties of LiNb₃O₈ ceramics were investigated as a function of the sintering temperature and the amount of TiO₂ additive. LiNb₃O₈ ceramics, which were calcined at 750 °C and sintered at 1075 °C for 2 h, showed a dielectric constant (ɛ_r) of 34, a quality factor (Q × f₀) of 58,000 GHz and a temperature coefficient of resonance frequency (τ_f) of −96 ppm/°C, respectively. The density of the samples influenced the properties of these properties. As the TiO₂ content increased in the LiNb₃O₈–TiO₂ system, ɛ_r and τ_f of the material were increased due to the mixing effect of TiO₂ phase, which has higher dielectric constant and larger positive τ_f. The 0.65LiNb₃O₈–0.35TiO₂ ceramics showed a dielectric constant ɛ_r of 46.2, a quality factor (Q × f₀) of 5800 GHz and a temperature coefficient of resonance frequency τ_f of near to 0 ppm/°C.     

Microwave dielectric properties of Co2P2O7 ceramics

Co₂P₂O₇ ceramics were prepared through the traditional solid-state sintering technique. The phase composition, grain size distribution, and densification were researched via X-ray diffraction and scanning electron microscope. The influence of pores on permittivity was described by various models. The dielectric loss was found highly dependent on porosity. Moreover, the low ε_r (<10) values of Co₂P₂O₇ ceramics were explained by the covalent feature of P–O bonds. Raman spectroscopy was used for exploring the relationship between polar phonon modes and dielectric properties in terms of intrinsic factors. The optimum dielectric properties (ε_r = 6.76, Qf = 36,400 GHz and τ_f = ?23.9 ppm/°C) were obtained at 1160 °C for 4 h.     

Microwave dielectric properties of glass-ceramic composites for low temperature co-firable ceramics

Ba–B–Si glass was added to Ba–Nd–Sm–Bi–Ti–O (BRT₁₁₄) microwave dielectric material for LTCC applications. Conventional one-step processing method for preparing glass-BRT₁₁₄ composite materials yields low dielectric constant, since the glass was easy to react with BRT₁₁₄ and forms a low dielectric constant phase, Ba₃B₆Si₂O₁₆. A large proportion of pores appeared. The nature of glass, whether it is sol-gel derived or fused, shows marked influence on the microstructure and microwave dielectric properties of the composites. A two-step process containing precoating the BRT₁₁₄ powders with a thin layer of glass, followed by conventional samples preparation process, tremendously improved the densification behaviour of the material. The formation of pores and interactions between glass and BRT₁₁₄ was greatly suppressed such that materials with high dielectric constant (ε_r=40) were achieved by sintering 9 wt.% glass-containing composite at 950 °C for 2.5 h.     

Microwave dielectric properties of MgTiO3–SrTiO3 layered ceramics

MgTiO₃–SrTiO₃ layered ceramics with different stacking were fabricated and the microwave dielectric properties were evaluated with TE₀₁₁ mode. With increasing SrTiO₃ thickness fraction, the resonant frequency (f₀) decreased, while the effective dielectric constant (ɛ_r,eff) and temperature coefficient of resonant frequency (τ_f) increased for the bi-layer ceramics. The stacking arrangement also had significant effect on the microwave dielectric properties. For the same SrTiO₃ thickness fraction of 0.333, the tri-layer MgTiO₃/SrTiO₃/MgTiO₃ ceramics had lower f₀, higher ɛ_r,eff and τ_f. The result was not consistent with the previous report on the layered ceramics with TE₀₁₁ mode [Cho, J. Y., Yoon, K. H. and Kim, E. S., Effect of stress on microwave dielectric properties of layered Mg_0.93Ca_0.07TiO₃–(Ca_0.3Li_0.14Sm_0.42)TiO₃ ceramics. Mater. Chem. Phys. 2003, 79, 286; Cho, J. Y., Yoon, K. H. and Kim, E. S., Correlation between arrangement of dielectric layers and microwave dielectric properties of Mg_0.93Ca_0.07TiO₃–(Ca_0.3Li_0.14Sm_0.42)TiO₃ ceramics. J. Am. Ceram. Soc. 2003, 86, 1330], where the effective dielectric constant was only determined by the thickness fraction and was independent of the stacking arrangement. Finite element analysis gave an explanation for the different microwave dielectric behaviors of the bi- and tri-layer ceramics in the present experiment.     

Microwave dielectric properties of Ca1-xBaxMgSi2O6 ceramics

Ca_1-xBa_xMgSi₂O₆(x = 0–0.4) ceramics were prepared through a traditional solid-state reaction sintering route with various sintering temperatures. The effects of substituting Ba²⁺ for Ca²⁺, the relative density, phase composition, crystal morphology, and microwave dielectric properties of Ca_1-xBa_xMgSi₂O₆ (x = 0–0.4) ceramics were thoroughly studied. X-ray diffraction patterns indicate a single phase was formed in the samples when x ≤ 0.2, and the second phase BaMg₂Si₂O₇ appeared at x = 0.4. As the amount of Ba²⁺ substitution increases, the Q×f value first increases and then decreases due to the combined effects of FWHM of peak v₁₁ and atomic packing density, and the ${\epsilon }_r$ value was increased continuously which was closely corrected with the relative density and molecular polarization. The ${\tau }_f$ value improved slightly with the substituting Ba²⁺ for Ca²⁺. Typically, the Ca_0.88Ba_0.12MgSi₂O₆ ceramic can be well sintered at 1275 °C for 4 h with a maximum relative density of 99.3%, and possesses optimal microwave dielectric properties: ${\epsilon }_r=7.49$, $Q\times f=64310$ GHz, ${\tau }_f=-44.02$ ppm/°C.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Microwave dielectric properties of La3Ti2TaO11 ceramics with perovskite-like layered structure

The first characterization of the microwave dielectric properties of the La₃Ti₂TaO₁₁ ceramics is presented. An ordinarily sintered ceramic at 1560 °C exhibits good microwave dielectric properties with ?_r = 46, Q × f = 7500 GHz and τ_f = ?47 ppm/°C. An alternative approach to tailor the temperature coefficient of resonate frequency of La₃Ti₂TaO₁₁ ceramics is also presented. Textured La₃Ti₂TaO₁₁ ceramics were fabricated using spark plasma sintering (SPS). By controlling the sintering temperature, orientation degree increased together with the steadily increase in ?_r and Q × f. A noteworthy change in τ_f from ?43.1 ppm/°C to ?13.6 ppm/°C with increasing orientation degree was observed. These results suggest that grain-orientation control was an effective way to tailor the microwave dielectric properties of La₃Ti₂TaO₁₁ ceramics.     

Synthesis,characterization, and dielectric properties of low loss LaBO3 ceramics

The preparation, sintering behaviour, and dielectric properties of low loss LaBO₃ ceramics have been investigated. Single-phase LaBO₃ powder was synthesized by the conventional solid-state ceramics route and dense ceramics (relative density >96%) with uniform microstructure (grain size ～30 μm) were obtained by sintering at 1300 °C in air. The electrical conductivity of LaBO₃ follows the Arrhenius law and the related activation energies for electrical conduction of bulk and grain boundary are 0.62 eV and 0.90 eV, respectively. The LaBO₃ ceramics sintered at 1300 °C exhibit excellent microwave dielectric properties with a relative permittivity, ?_r ～ 11.8, a quality factor, Q × f₀ value ～76,869 GHz (at ～15 GHz), and a negative temperature coefficient of resonant frequency τ_f ～ ?52 ppm/°C.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Microwave dielectric properties of low loss microwave dielectric ceramics: A0.5Ti0.5NbO4 (A = Zn,Co)

The microwave dielectric properties of low-loss A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics prepared by the solid-state route had been investigated. The influence of various sintering conditions on microwave dielectric properties and the structure for A_0.5Ti_0.5NbO₄ (A = Zn, Co) ceramics were discussed systematically. The Zn_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as ZTN) showed the excellent dielectric properties, with ɛ_r = 37.4, Q × f = 194,000 (GHz), and τ_f = −58 ppm/°C and Co_0.5Ti_0.5NbO₄ ceramic (hereafter referred to as CTN) had ɛ_r = 64, Q × f = 65,300 (GHz), and τ_f = 223.2 ppm/°C as sintered individually at 1100 and 1120 °C for 6 h. The dielectric constant was dependent on the ionic polarizability. The Q × f and τ_f are related to the packing fraction and oxygen bond valence of the compounds. Considering the extremely low dielectric loss, A_0.5Ti_0.5NbO₄ (A = Zn and Co) ceramics could be good candidates for microwave or millimeter wave device application.     

Bond characteristics and microwave dielectric properties on Zn3-xCux(BO3)2 ceramics with ultralow dielectric loss

The crystal structure and microwave dielectric properties of Zn_3-xCu_x(BO₃)₂ (x = 0–0.12) ceramics prepared via a traditional solid-state reaction method were investigated by means of X-ray diffraction (XRD) utilizing the Rietveld refinement, complex chemical bond theory, and Raman spectroscopy. XRD showed that all samples were single phase. The samples maintained a low permittivity, even at higher Cu²⁺ contents, which is conducive to the shortening of signal delay time, and intimately related to the average bond ionicity and Raman shift. Moreover, proper Cu²⁺ substitution greatly reduced the dielectric loss associated with the lattice energy. Cu²⁺ entering the lattice optimized the temperature coefficient of resonance frequency (τ_f) values and improved the temperature stability of samples by affecting the bond energy. Optimal microwave dielectric properties were: ε_r = 6.64, Q × f = 160,887 GHz, τ_f = ?42.76 ppm/°C for Zn_2.96Cu_0.04(BO₃)₂ ceramics sintered at 850 °C for 3 h, which exhibited good chemical compatibility with silver and are therefore good candidate materials for Low temperature co-fired ceramic applications.     

Microwave dielectric characteristics of SrLaGaO4 and SrNdGaO4 ceramics

SrLnGaO₄ (Ln = La and Nd) ceramics with K₂NiF₄ structure were prepared by solid-state reaction approach, and the microwave dielectric properties and microstructures were characterized. The SrLaGaO₄ and SrNdGaO₄ ceramics with minor secondary phase, Sr₃Ga₂O₆, were obtained by sintering at 1250–1350 °C for 3 h, and good microwave dielectric characteristics were achieved: the ceramics had (1) ɛ = 20.3, Q × f = 16,219 GHz, and τ_f = −33.5 ppm/°C for SrLaGaO₄; and (2) ɛ = 21.4, Q × f = 16,650 GHz, and τ_f = 7.1 ppm/°C for SrNdGaO₄.     

Microwave dielectric properties of tetragonal scheelite-structured (NaY)1/2MoO4 ceramics

(NaY)_1/2MoO₄ was fabricated via the solid-state reaction method of Na₂CO₃, Y₂O₃, and MoO₃. Scanning electron microscopy results demonstrated that all the (NaY)_1/2MoO₄ ceramics could be densified well in the sintering temperature range of 900–960°C. Results of X-ray diffraction analysis demonstrated that the (NaY)_1/2MoO₄ ceramics crystallized into tetragonal scheelite structure. Sintering (NaY)_1/2MoO₄ at 940°C for 2 h optimized the microwave dielectric properties of the ceramics. The microwave permittivity, Q × f, and TCF of the (NaY)_1/2MoO₄ were 10.9, 29 000 GHz and −40.7 ppm/°C, respectively.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

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.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

Preparation and dielectric properties of polymer-derived SiCTi ceramics

SiCTi ceramics were prepared by a polymer-derived-ceramic route, with allylhydridopolycarbosilane (AHPCS) and bis(cyclopentadienyl) titanium dichloride (Cp₂TiCl₂) as starting materials. The cross-linking and ceramization of the AHPCS/Cp₂TiCl₂ hybrid precursors were characterized by means of FT IR, NMR, TGA and EDS. The results indicate that the cross-linking of hybrid precursors was significantly catalyzed by using Cp₂TiCl₂ as a catalyst, which might be responsible for a high ceramic yield of 80.8% at 1200 °C. The polymer-to-ceramic conversion was completed at 900 °C to give an amorphous ceramic. The chemical composition of the final ceramics could be tailored by the weight ratio of Cp₂TiCl₂ to AHPCS in feed. The microstructure and dielectric properties of final SiCTi ceramics were investigated by means of XRD, Raman spectroscopy and vector network analyzer. The results indicate that the 1600 °C SiCTi ceramics are composed of amorphous SiCTi, SiC crystal, TiC crystal and graphite. The dielectric loss of SiCTi is up to 0.34, which is 6 times higher than that of SiC (0.058), indicating that the SiCTi ceramics are promising wave-absorbing materials.     

Preparation and microwave dielectric properties of cristobalite ceramics

Dense SiO₂ ceramics with cristobalite phase were prepared by the solid state sintering route, and the microwave dielectric properties were evaluated. The dielectric constant (?_r) and temperature coefficient of resonant frequency (τ_f) of the pure cristobalite ceramics showed little dependence on the sintering temperature. While, the Qf value increased significantly with increasing the sintering temperature, and it was due to the increasing grain size. The optimized microwave dielectric properties with very low ?_r of 3.81, high Qf value of 80,400 GHz and low τ_f of ?16.1 ppm/°C were obtained for the cristobalite ceramics sintered at 1650 °C for 3 h. It was indicated that cristobalite ceramic was a promising candidate as a low-dielectric-constant microwave material for applications in microwave substrates.