Eco-friendly tape casting of borosilicate glass/Al2O3 sheets for LTCC applications

This work reports the innovative development of a borosilicate glass/Al₂O₃ tape for LTCC applications using an eco-friendly aqueous tape casting slurry. Polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) were the respective dispersants, while carboxymethyl cellulose (CMC) and styrene acrylic emulsion (SA) were the respective binders. The results showed that PVP was more suitable than PAA as the dispersant for the aqueous casting slurry, and that 1.5 wt% PVP would achieve well dispersion of CABS glass/Al₂O₃ powder in the aqueous slurry. Moreover, a small amount of 2.0 wt% CMC binder could yield smooth CABS glass/Al₂O₃ tapes crack free. A high-quality CABS glass/Al₂O₃ tape with a smooth surface was made from an aqueous slurry containing 1.5 wt% PVP dispersant, 2.0 wt% CMC binder, and 2.0 wt% PEG-400 plasticizer. The density, tensile strength, and surface roughness of the green tape were 2.05 g/cm³, 0.87 MPa, and 148 nm, respectively. The resulting CABS glass/Al₂O₃ composites sintered at 875 °C exhibited a bulk density of 3.14 g/cm³, a dielectric constant of 8.09, a dielectric loss of 1.0 × 10^?3, a flexural strength of 213 MPa, a thermal expansion coefficient of 5.30 ppm/^°C, and a thermal conductivity of 3.2 W m^?1 K^?1, thus demonstrating its broad prospects in LTCC applications.     

Synthesis and characterization of LTCC compositions with middle permittivity based on CaO-B2O3-SiO2 glass/CaTiO3 system

CaTiO₃ ceramics with the addition of CaO-B₂O₃-SiO₂ (CBS) glass (45–55 wt%) composites were sintered at 830 °C, 850 °C, 875 °C and 900 °C. To illustrate influence mechanism of the different glass contents and sintering temperatures on the properties of the composites, we focused on the multiple performances of the composites by employing different qualitative and quantitative instruments. Composites with 50 wt% glass sintered at 875 °C presented fairly ideal performance: the bulk density was 3.20 g/cm³, the dielectric constant was 25.7 and the dielectric loss was 0.0009 at 7 GHz. Micro-Structure analysis of the composites showed a dense and pore-less microstructure except for few pores with size around 1 μm. In addition, the composite could meet the shrinkage requirement of Ag electrodes and could not possibly react with Ag electrodes any more. This makes them suitable for various dielectric applications at low sintering temperature.     

Fabrication and performance of dielectric tape based on CaO-B2O3-SiO2 glass/Al2O3 for LTCC applications

A non-aqueous tape-casting process for fabricating CaO-B₂O₃-SiO₂ glass/Al₂O₃ dielectric tape for LTCC applications was investigated. An isopropanol/ethanol/xylene ternary solvent-based slurry was developed by using castor oil, poly(vinyl butyral), and dibutyl phthalate as dispersant, binder, and plasticizer, respectively. The effects of dispersant concentration, binder content, plasticizer/binder ratio, and solid loading, on the properties of the casting slurry and resultant tape were systematically investigated. The results showed that the optimal values for the dispersant and binder contents, plasticizer/binder ratio, and solid loading were 2.0 wt%, 7.5 wt%, 0.6, and 62 wt%, respectively. The resultant flexible and uniform, 120-μm-thick CaO-B₂O₃-SiO₂ glass/Al₂O₃ tape had a density of 1.90 g/cm^?3, tensile strength of 1.66 MPa, and average surface roughness of 310 nm. Laminated tapes sintered at 875 °C for 15 min exhibited excellent properties: relative density of 97.3%, ε_r of 7.98, tan δ of 1.3 × 10^?3 (10 MHz), flexural strength of 205 MPa, and thermal expansion coefficient of 5.47 ppm/°C. The material demonstrated good chemical compatibility with Ag electrodes, indicating a significant potential in LTCC applications.     

Densification,flexural strength and dielectric properties of CaO-MgO-ZnO-SiO2/Al2O3 glass ceramics for LTCC applications

Novel glass ceramics for LTCC applications with high flexural strength can be achieved by CaO-MgO-ZnO-SiO₂(CMSZ) glass cofiring with Al₂O₃. The sintering shrinkage behavior, crystalline phases, mechanical and dielectric properties, and thermal expansion of the CMZS/Al₂O₃ glass ceramic were determined. The X-ray diffraction results revealed that multiphases (CaMgSi₂O₆, Al₂Ca(SiO₄)₂ and ZnAl₂O₄) formed in the sintering process of the CMZS/Al₂O₃ glass ceramic. The flexural strength of CMZS/Al₂O₃ glass ceramics first increases and then decreases with increasing Al₂O₃ content. The CMZS/Al₂O₃ glass ceramic with 50 wt % Al₂O₃ sintered at 890 °C for 2 h achieved the best performance, with a maximum flexural strength of 256 MPa, dielectric constant (ε_r) of 7.89, dielectric loss (tan δ) of 3.41 × 10⁻³ (12 GHz), temperature coefficient of resonance frequency (τ_f) of −29 ppm/°C, and the CTE value of 7.93 × 10⁻⁶/°C.     

Synthesis and characterization of low CTE value La2O3-B2O3-CaO-P2O5 glass/cordierite composites for LTCC application

Low-softening-point La₂O₃-B₂O₃-CaO-P₂O₅ (LBCP) glass-ceramic/cordierite composite systems have been prepared in this work. Influence of the ratio of La₂O₃ to B₂O₃ and the content of cordierite on the sintering behavior, microstructure, sintering quality, thermal properties and dielectric properties of composites are studied. The results show that high La₂O₃/B₂O₃ ratio improves the crystalline quality of LBCP glass-ceramic, but also narrows its process window. The increase of cordierite content reduces the coefficient of thermal expansion (CTE) value of composites obviously. However, excess cordierite is detrimental to the densification of the composite microstructure, and too low cordierite content causes serious foaming. Sample containing 30?wt% LBCP1 glass-ceramic and 70?wt% cordierite sintered at 850?°C shows excellent properties: relative density of 95.26%, CTE value of 4.12?ppm/°C, dielectric constant of 4.78 (1?MHz)/4.52 (12.8?GHz), dielectric loss of 2.3?×?10^?3 (1?MHz)/2.5?×?10^?3 (12.8?GHz) and the ability to co-fire with silver, which suggests that LBCP glass/cordierite composite system has potential to meet the requirements of LTCC substrate material.     

Manufacturing a high performance film of CaO-B2O3-SiO2 glass-ceramic powder with surface modification for LTCC application

The present work was oriented to develop a high-performance film for LTCC application through using a kind of silane coupling agent (SCA) to modify the CBS (CaO-B₂O₃-SiO₂) glass-ceramic powders in tape casting process. The coupling mechanism of the silane coupling agent interacting with the powders was illustrated in detail, and the optimum amount of additions was determined by the infrared transmittance and contact angle experiment. The rheological properties of suspensions, the compactness and tensile breaking strength of the green tape were texted by using a number of quantitative and qualitative instruments. Furthermore, when the green tapes sintered at 820 °C, the sintering characteristics and microwave properties of sintered body with different SCA additions was also analyzed. Based on the experimental results, it revealed that the powders modified by SCA with 1.5 wt% additions could induce the most excellent performance of film, with the density at 1.57 g/cm³ and the tensile strength at 2.57 MPa, possessing the highest density and best microwave properties of sintered body: the density of sintered body was 2.48 g/cm³, the dielectric constant and dielectric loss was ε_r = 5.93, tanδ=8 × 10^?4 at 12 Hz.     

Ultralow-permittivity glass /Al2O3 composite for LTCC applications

In the field of low temperature co-fired ceramic (LTCC), it remains a challenge to design the performance of LTCC with low permittivity less than 5. Here, a novel glass mixture of K-Al-B-Si-O (KABS) and Zn-B-Si-O (ZBS) is introduced as a sintering aid of alumina to obtain ultralow-permittivity glass/Al₂O₃ composite. Meanwhile, the factors of glass mixture component on microstructure, phase structure and dielectric properties of the composites are considered systematically. The crystal structure measured by X-ray diffraction (XRD) shows that pure crystalline phase of ZnAl₂O₄ spinel can be attained by tailoring the component of the glass mixture. In case of mass ratio of KABS: ZBS equal to 6:4, it favors to efficiently increase the sintering densification of composite, and significantly benefit the low dielectric loss, good mechanical and thermal performances. In detail, the optimal glass/ceramic composites sintered at 850 °C for 2 h exhibit the bulk density of 2.89 g/cm3, ε_r of 4.92 at 14 GHz and Q × f of 6873 GHZ, flexural strength of 202 MPa, thermal expansion coefficient of 5.5 ppm/°C. The above study provides an effective approach for preparing the novel composites as a promising candidate for LTCC applications.     

Low temperature sintering and characterization of La2O3-B2O3-CaO glass-ceramic/LaBO3 composites for LTCC application

An interesting attempt to develop low temperature sintering glass-ceramic/ceramic composite based on La₂O₃-B₂O₃-CaO (LBC) glass-ceramic and LaBO₃ ceramic, which was reported to be the main crystalline phase precipitated from La₂O₃-B₂O₃-based glass-ceramics, has been taken. The sintering behavior, phase evolution, microstructures and dielectric properties of LBC/LaBO₃ composites have been studied. The densification of LBC/LaBO₃ composite is achieved by partially reactive sintering. The LaBO₃ filler is directly involved in the sintering process of glass/ceramic composite as additional liquid phase provider at high sintering temperature, and it will suppress the formation of other crystalline phases so that the produced LBC/LaBO₃ composites exhibit unusual simple phase structures, which is beneficial to regulate the performance of composites. LBC/LaBO₃ composite with 50 wt% LaBO₃ sintered at 950 ºC for 2 h has a dielectric constant ε_r = 10.12, a dielectric loss tanδ = 1.82 × 10^―3, a Q × f = 9312 GHz (at 16.95 GHz), and shows good chemical compatibility while co-firing with Ag electrode. This indicates that LBC glass/LaBO₃ ceramic composites have a potential to meet the requirements of microwave LTCC applications.     

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.     

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.     

Fabrication of high-performance Y2O3 stabilized hafnium dioxide refractories

Y₂O₃ stabilized HfO₂ ceramics with 5 mol% and 8 mol% Y₂O₃, respectively, were successfully fabricated by pressureless sintering at 1600 °C for 1 h. The phase distribution and transformation, microstructure, and relative density were investigated for Y₂O₃ stabilized HfO₂ ceramics. Cubic phase was observed in the Y₂O₃ stabilized hafnium dioxide while pure HfO₂ showed monoclinical phase only. The SEM images of fractured surface indicated two kinds of structures existed in modified HfO₂, the solid solution region and uniform polygon grains, and some holes caused by Kirkendall effect. Refractoriness test showed that high temperature volumetric stability of the material can be effectively enhanced by adding Y₂O₃ into HfO₂.     

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.     

Structural and dielectric properties of B2O3/Li2O-added Ba0.6Sr0.4TiO3 multilayer ceramics for tunable devices

B₂O₃ and Li₂O (B/L)-added Ba_0.6Sr_0.4TiO₃ (BST) ceramics sintered at 940 °C exhibited a dense microstructure with large grains. The amount of B/L additive was 4.5 wt% with a B/L ratio of 1.5:1. The B/L-related liquid phase assisted the densification of the BST ceramics. This BST ceramic displayed a large dielectric constant (ε_r) of 2834 with a low dielectric loss (tan δ) of 0.21% at 1.0 MHz. It also displayed a large tunability (28.2% at 10 kV/cm) and a high figure of merit (FOM) of 134. BST thick-films were synthesized using the tape casting method. The thick-film densified at 940 °C exhibited a large tunability of 18.7% at 10.0 kV/cm and an FOM of 208; these are higher than the values reported in the literature. Multilayer ceramics (MLCs) consisting of five layers of 40-μm-thick BST thick-films and Ag electrodes were also fabricated at 940 °C. No diffusion occurred between the Ag electrode and BST thick-film. A large tunability of 67.6% at 52 kV/cm with a high FOM of 294 was obtained from this MLC. This verified that the B/L-added BST ceramic is effective for application in tunable multilayer devices.     

Microstructure evolution and cooperative reinforcement mechanisms of Al2O3/Al2O3 joints brazed by low-melting borosilicate glass

The Al₂O₃/SiO₂–B₂O₃–Al₂O₃–Na₂O glass/Al₂O₃ joints reinforced cooperatively by glass matrix and in-situ Al₄B₂O₉ whiskers were obtained via a low-melting borosilicate glass braze. The composition of glass seam transformed from SiO₂–B₂O₃–Na₂O to SiO₂–B₂O₃–Al₂O₃–Na₂O due to continuous diffusion and dissolution of Al₂O₃. An appropriate amount of [AlO₄] units introduced into the glass braze played a vital role in strengthening the glass network structure resulting to considerably improved mechanical strength of the glass seam. Meanwhile, plenty of in-situ Al₄B₂O₉ whiskers growing from the Al₂O₃/glass braze interface to the center of glass seam in various directions generated. Three-dimensional crisscross structures were fabricated at the Al₂O₃/glass braze interface domains, where were enhanced by crack-bridging and pull-out effect of the whiskers. Generally, ascribed to the cooperative reinforcement of the glass matrix in the seam and in-situ Al₄B₂O₉ whiskers at Al₂O₃/glass braze interface domains through reactions of Al₂O₃ and borosilicate glass braze, strength of the as-brazed joints was promoted prominently. The shear strength of the joints reached a maximum of 61 MPa brazed at 1050 °C for 60 min.     

Direct tape casting of Al2O3/AlN slurry for AlON transparent ceramic wafers via one-step reaction sintering

In this work, transparent aluminate oxynitride (AlON) ceramic wafers were successfully fabricated by the direct non-aqueous tape casting of Al₂O₃/AlN slurry and the one-step reaction sintering for the first time. The reaction sintered AlON ceramic wafer exhibits high transmittance of 73.2 % at the wavelength of 1600 nm. This fabricating route realizes smooth and flexible tape without cracks or pinholes in Al₂O₃/AlN system and efficiently shortens the preparation cycle of transparent AlON wafers, which is a feasible way to prepare high-quality transparent AlON ceramics with large lateral sizes and thin thicknesses by reaction sintering, might also promote the application of transparent AlON ceramic wafers.     

Fabrication and dielectric properties of barium titanate-based glass ceramics for tunable microwave LTCC application

Low-fired ferroelectric glass ceramics were fabricated from glass powders with a basic composition of 0.65BaTiO₃·0.27SiO₂·0.08Al₂O₃. The combined addition of SnO₂ (or ZrO₂) and SrCO₃ was conducted to modify the dielectric properties of the glass ceramics. The Sr-component could be incorporated preferentially in the perovskite structure after heating at 1000 °C. The bulk and thick film samples obtained by sintering glass powder with a starting composition of 0.65(Ba_0.7Sr_0.3)(Ti_0.85Sn_0.15)O₃·0.27SiO₂·0.08Al₂O₃ at 1000 °C for 24 h showed a broadened ɛ_r–T relation with T_c ≈ 10 °C and ɛ_r(max) ≈ 280 and microwave tunability of 32% at 3 GHz, respectively.     

Improved microstructure and high quality factor of Li2Ti0.9(Zn1/3Ta2/3)0.1O3 microwave ceramics with LiF additive for LTCC applications

In this study, LiF was utilized to decrease sintering temperature, improve microstructure, enhance Q×f, and regulate τ_f of Li₂Ti_0.9(Zn_1/3Ta_2/3)_0.1O₃ (abbreviated as LTZT) ceramics. A complete solid solution together with a phase transition from monoclinic to cubic rock salt structure occurred. The cell volume of LTZT ceramics decreased as the LiF content increased. Relatively dense and uniform microstructures were observed for the ceramics as the LiF content was not less than 2 wt%. The dielectric constant of LTZT ceramics initially increased and then decreased with the increasing LiF content. The FWHM of the Raman band at about 808 cm^?1 was closely related to the Q×f value. Notably, the samples with 3 wt% LiF exhibited the highest relative density of 97.4 % and satisfactory microwave dielectric properties of ε_r = 23.14 ± 0.16, Q×f = 110,090 ± 1100 GHz, and τ_f = +3.25 ± 1.45 ppm/°C when sintered at 950 °C. Good chemical compatibility with silver indicated the ceramic is a promising candidate in LTCC applications.     

A new BaB2O4 microwave dielectric ceramic for LTCC application

To satisfy the requirements of miniaturization and integration of microwave devices, microwave dielectric ceramics with low sintering temperatures and good microwave dielectric properties are particularly important for LTCC materials. In this study, low-cost BaB₂O₄ ceramics were prepared with different Ba/B ratios using a solid-phase method. Combined with the Raman spectra, the effects of the Raman shift and FWHM of the vibration peaks on the microwave dielectric properties were determined. As a novel microwave dielectric ceramic, BaB₂O₄ consists of a highly dense structure with optimal microwave dielectric properties (ε_r = 4.06, Q×f = 23845 GHz, and τ_f = −7.2 ppm/℃) at a low sintering temperature (840 ℃). In addition, BaB₂O₄ ceramic is chemically compatible with Ag, making it a promising candidate substrate for microwave communications.