Enhancement of structural and microwave properties of Zn2+ ion-substituted Li2MgSiO4 ceramics for LTCC applications

In this study, Zn²⁺-substituted Li₂MgSiO₄ ceramics (Li₂(Mg_1-xZn_x)SiO₄, x = 0.00, 0.05, 0.10, 0.15, and 0.20) were synthesized using a traditional solid-state method. A fixed amount of LiF sintering aid (1.5 wt%) was added to the ceramics for decreasing the sintering temperature and adjusting their microwave dielectric properties. X-ray diffraction (XRD) results revealed no secondary phases, and scanning electron microscopy (SEM) data suggest that the Zn²⁺ ion substitution increased the size and uniformity of the grains, thereby affecting the densification of the prepared ceramics. The maximum bulk density (2.94 g/cm³) was found in a Zn²⁺ ion-substituted ceramic with x = 0.10 at a relative density of 94.2% (compared with the XRD theoretical density). Excellent microwave dielectric properties (ε_r = 6.28, Q × f = 50400 GHz, and τ_f = ?145 ppm/°C) can also be obtained at this zirconium content. We believe that the developed ceramics are promising for use as antenna substrates or transmit/receive modules in low-temperature co-firing ceramic applications.     

Li4Mg3Ti2O9: A novel low-loss microwave dielectric ceramic for LTCC applications

Low-loss novel Li₄Mg₃Ti₂O₉ dielectric ceramics with rock-salt structure were prepared by a conventional solid-state route. The crystalline structure, chemical bond properties, infrared spectroscopy and microwave dielectric properties of the abovementioned system were initially investigated. It could be concluded from this work that the extrinsic factors such as sintering temperatures and grain sizes significantly affected the dielectric properties of Li₄Mg₃Ti₂O₉ at lower sintering temperatures, while the intrinsic factors like bond ionicity and lattice energy played a dominant role when the ceramics were densified at 1450 °C. In order to explore the origin of intrinsic characteristics, complex dielectric constants (ε^’ and ε^’’) were calculated by the infrared spectra, which indicated that the absorptions of phonon oscillation predominantly effected the polarization of the ceramics. The Li₄Mg₃Ti₂O₉ ceramics sintered at 1450 °C exhibited excellent properties of ε_r=15.97, Q·f=135,800 GHz and τ_f=−7.06 ppm/°C. In addition, certain amounts of lithium fluoride (LiF) were added to lower the sintering temperatures of matrix. The Li₄Mg₃Ti₂O₉−3 wt% LiF ceramics sintered at 900 °C possessed suitable dielectric properties of ε_r=15.17, Q·f =42,800 GHz and τ_f=−11.30 ppm/°C, which made such materials promising for low temperature co-fired ceramic applications (LTCC).     

Phase composition,microstructure and microwave dielectric properties of rock salt structured Li2ZrO3-MgO ceramics

A novel series of rock salt structured (1-x)Li₂ZrO₃-xMgO ceramics were prepared via the conventional solid state method. The tetragonal-cubic phase transition can be observed in the case of 0.5?≤?x?≤?0.6, which has been testified by the results of XRD and SEM-EDS. Relatively dense and homogeneous microstructure can be obtained for all the compositions sintered at 1500?°C. With the x value increasing from 0.5 to 0.8, the relative permittivity linearly decreases from 16.50 to 12.65, and the τ_f value decreases from ～?10?ppm/°C to ～?35?ppm/°C. The addition of MgO stabilizes the crystal structure and increases the bond energies in Li₂ZrO₃-MgO system, so there is an upward tendency in Q·f values from ～77,000?GHz to ～166,000?GHz. Typically, the Li₂Mg₄ZrO₇ ceramics sintered at 1500?°C possesses excellent properties with ε_r?=?12.65, Q·f?=?165,924?GHz and τ_f=-34.66?ppm/°C, which makes these materials good candidates for microwave devices.     

Significantly enhanced sinterability and temperature stability of Li3Mg4NbO8-based microwave dielectric ceramics with LiF and Ba3(VO4)2 addition

Li₃Mg₄NbO₈-basic composite ceramics were elaborated via the solid-state reaction process, in which LiF and Ba₃(VO₄)₂ were utilized as a sintering aid and reinforcement phase, respectively. The sinterability, phase assemblage, microstructures, and microwave dielectric performances of Li₃Mg₄NbO₈–LiF–Ba₃(VO₄)₂ composite ceramics were thoroughly researched. The co-addition of LiF–Ba₃(VO₄)₂ can simultaneously lower the sintering temperature and improve the thermal stability of Li₃Mg₄NbO₈-basic ceramics. Solid state activated sintering is responsible for the low-temperature densification of the present ceramics. The coexistence of rock-salt structural Li₃Mg₄NbO₈/Li₄Mg₄NbO₈F and hexagonal structural Ba₃(VO₄)₂ phases was demonstrated by the combinational XRD and SEM-EDS analysis results. The 0.65(Li₃Mg₄NbO₈–LiF)-0.35Ba₃(VO₄)₂ ceramics fired at 825 °C/5 h exhibited promising microwave dielectric performances: τ_f = 0.5 ppm/°C along with ε_r = 13.8 and Qxf = 68500 GHz. The good compatibility of the developed ceramics with Ag demonstrates it potential for use in LTCC technology.     

Low-dielectric-constant LiAlO2 ceramics combined with LBSCA glass for LTCC applications

This study used Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) glass to reduce the sintering temperature of LiAlO₂ ceramics and to realise the low dielectric constants (ɛ_r<5) of low-temperature co-fired ceramic (LTCC) materials. LBSCA glass remarkably enhanced the densification of LiAlO₂ ceramics. X-ray diffraction patterns indicated that only the γ-LiAlO₂ phase occurred within the doping range of 1 wt% to 3.5 wt%. Scanning electron microscopy images showed dense and uniform grains in samples with 3.0 wt% LBSCA glass. These samples also exhibited low dielectric constants and low dielectric loss when sintered at 900 °C and 950 °C (i.e., ɛ_r=4.48, Qf=35,540 GHz and τ_f=−53 ppm/°C at 900 °C; ɛ_r=4.50, Qf=38,979 GHz and τ_f=−55 ppm/°C at 950 °C, respectively). The material prepared was chemically compatible with silver and showed potential in applications of high-frequency LTCC microwave substrates.     

Microwave dielectric properties,low-temperature sintering mechanism,and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li₂Zn₃Ti₄O₁₂-xSr₃(VO₄)₂ (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO₃ and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τ_f value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti³⁺, and V⁴⁺, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li₂Zn₃Ti₄O₁₂-0.3Sr₃(VO₄)₂ ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of ε_r = 17.8, Q × f = 41,891 GHz, and τ_f = ?4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.     

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.     

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).     

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.     

Structural evolution,grain growth kinetics and microwave dielectric properties of Li2Ti1-x(Mg1/3Nb2/3)xO3

Li₂Ti_1-x(Mg_1/3Nb_2/3)_xO₃ ceramics were prepared by conventional solid state process. Their structural evolution, grain growth kinetics and microwave dielectric properties have been studied in this paper. The results show that continuous solid solution could be formed within the experiment compositional range. The structure changed from long range ordered monoclinic into short range ordered cubic phase as the increase in x. Small levels of substitution for Ti⁴⁺by (Mg_1/3Nb_2/3)⁴⁺ slightly decreased the dielectric permittivity, while considerably improved the Q × f value. The temperature coefficient of resonant frequency changed from positive into negative value. The grain growth kinetics during sintering process and Q × f value of the sintered body were affected by different calcining temperature of mixed powders. Excellent combined microwave dielectric properties with ε_r ～21.0, Q × f ～ 200 000 GHz and τ_f value of ?1 ppm/ °C could be obtained after optimizing calcining temperature for the x = 0.24 composition after sintering at 1250 °C/2 h.     

Tape casting and characterization of Li2.08TiO3-LiF glass free LTCC for microwave applications

Li_2.08TiO₃-LiF Glass-free Low temperature co-fired ceramic (LTCC) green tapes were prepared by tape casing technique. The rheology of the slurry was characterized using rheometer. The slurry exhibited pseudoplastic behavior. The sintering kinetics of the green tape was investigated using heating microscope. The sintering activation energy was determined to be ∼173 kJ/mol. The green tape could be densified at 900 °C/2 h. Microwave dielectric properties of the sintered tape were characterized in a split-post dielectric resonator using a network analyzer. The ceramic sheet with thickness of 0.11 mm demonstrated good microwave dielectric properties: ε_r = 22.4 and Q × f = 35,490 GHz. The cross sectional microstructure of the cofired multilayer stack was observed by scanning electron microscopy (SEM). The green tape demonstrated good chemical and shrinkage compatibilities with Ag electrode during sintering process. The thermal expansion coefficient and thermal conductivity of the ceramic is 22.4 ppm/∘C and 4.75 W m⁻¹ K ⁻¹, respectively.     

Low-temperature sintering and microwave dielectric properties of B2O3-added ZnO-deficient Zn2GeO4 ceramics for advanced substrate application

ZnO-deficient Zn_2-xGeO_4-x ceramics with 0.05?≤?x?≤?0.15 were synthesized because a ZnO secondary phase is formed in the stoichiometric Zn₂GeO₄ ceramics synthesized using micrometer-sized ZnO and GeO₂ powders. The Zn_1.9GeO_3.9 ceramic sintered at 1000?°C showed a homogeneous Zn₂GeO₄ phase with good microwave dielectric properties: ε_r of 6.8, Q?×?f of 49,000?GHz, and τ_f of ?16.7?ppm/°C. However, its sintering temperature was still too high for it to be used as an advanced substrate for low-temperature co-fired ceramic devices. Therefore, various amounts of B₂O₃ were added to the Zn_1.9GeO_3.9 ceramics to reduce their sintering temperature. Owing to the formation of a B₂O₃-GeO₂ liquid phase, these ceramics were well sintered at low temperatures between 925?°C and 950?°C. In particular, 15?mol% B₂O₃-added Zn_1.9GeO_3.9 ceramic sintered at 950?°C showed promising microwave dielectric properties for advanced substrates without the reaction with an Ag electrode: ε_r?=?6.9, Q?×?f?=?79,000?GHz, and τ_f?=??15?ppm/°C.     

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.     

Microwave dielectric properties,microstructure, and bond energy of Zn3-xCoxB2O6 low temperature fired ceramics

Low temperature co-fired ceramics (LTCCs) technology plays an important role in modern wireless communication. Zn_3-xCo_xB₂O₆ (x = 0–0.25) low temperature fired ceramics were synthesized via traditional solid-state reaction method. Influences of Co²⁺ substitution on crystal phase composition, grain size, grain morphology, microwave dielectric properties, bond energy, and bond valence were investigated in detail. X-ray diffraction analysis indicated that the major phase of the ceramics was monoclinic Zn₃(BO₃)₂. Solid solution was formed with Co²⁺ substituted for Zn²⁺ because no individual phase that contained Co was observed. An increase in the amount of Co²⁺ substitution changed average grain sizes, and regrowth of grains were observed with Co²⁺ substitution. Appropriate amount of Co²⁺ substitution improved densification. With changes in Co²⁺ substitution, bond energy of major phase and average bond valence of B–O were positively correlated to temperature coefficient of resonant frequency. The Zn_2.927Co_0.075B₂O₆ ceramic sintered at 875 °C for 4 h exhibited excellent microwave properties with ε_r = 6.79, Q × f = 140,402 GHz, and τ_f = −87.42 ppm/°C. This ceramic is regarded as candidate for LTCC applications.     

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.     

Co2O3 substitution effects on the structure and microwave dielectric properties of low-firing (Zn0.9Mg0.1)TiO3 ceramics

Low-firing (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ (x = 0.02–0.10) (ZMC_xT) microwave dielectric ceramics with high temperature stability were synthesized via conventional solid-state reaction. The influences of Co₂O₃ substitution on the phase composition, microstructure and microwave dielectric properties of ZMC_xT ceramics were discussed. Rietveld refinement results show the coexistence of ZnTiO₃ and ZnB₂O₄ phases at x = 0.02–0.10. (Zn_0.9Mg_0.1)_1?xCo_xTiO₃ ceramic with x = 0.06 (ZMC_0.06T) obtains the best combination microwave dielectric properties of: ε_r = 21.58, Q × f = 53,948 GHz, τ_f = ? 54.38 ppm/°C. For expanding its application in LTCC field, 3 wt% ZnO-B₂O₃-SiO₂ (ZBS) and 9 wt% TiO₂ was added into ZMC_0.06T ceramic, great microwave dielectric properties were achieved at 900 °C for 4 h: ε_r = 26.03, Q × f = 34,830 GHz, τ_f = ? 4 ppm/°C, making the composite ceramic a promising candidate for LTCC industry.     

Microwave dielectric properties of LBBS glass added (Zn0.95Co0.05)2SiO4 for LTCC technology

The effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the sintering characteristics and microwave dielectric properties of (Zn_0.95Co_0.05)₂SiO₄ were investigated in this study. (Zn_0.95Co_0.05)₂SiO₄ powders were fabricated by traditional solid-state preparation, and LBBS glass was synthesised by quenching method. The LBBS glass can effectively reduce the sintering temperature of (Zn_0.95Co_0.05)₂SiO₄ from 1300 °C to 900 °C and thus promote the densification and uniformity of the specimens. XRD patterns indicated that no other secondary phases existed in our doping range (0–2 wt%). To obtain the highest sintering density and a uniform microstructure when the samples were sintered at 900 °C, the optimal doping content was set to be 1.5 wt%. The sample also demonstrated the following excellent microwave dielectric properties: ɛ_r=6.16, Qf=33,000 GHz and τ_f=−59 ppm/°C.     

Room temperature densified H3BO3 microwave dielectric ceramics with ultra-low permittivity and high quality factor for dielectric substrate applications

The densification and microwave dielectric properties of H₃BO₃ ceramics prepared by dry pressing at room temperature were studied. The results show that pressure is the key factor of densification of H₃BO₃ ceramics. No second phase appears in all the as-fabricated H₃BO₃ ceramic samples. A dense H₃BO₃ ceramic (relative density~97.6%) was obtained by uniaxial compression of 384 MPa for 300s and the optimal microwave dielectric properties are ε_r = 2.83, Q × f = 59,400 GHz (f = 16 GHz), τ_f = −91 ppm/°C, which make it as a prospective candidate for microwave and millimeter-wave devices such as substrates for 5G communication technology.