Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

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.     

Low temperature sintering and microwave dielectric properties of Zn1.8SiO3.8 ceramics with BaCu(B2O5) additive for LTCC applications

The Zn_1.8SiO_3.8 (ZS) ceramics with BaCu(B₂O₅) (BCB) additive were synthesized by the conventional solid-state reaction route and the effect of BCB additive on the microwave dielectric properties of the ceramics was investigated. The results demonstrate that BCB could effectively decrease the sintering temperature from 1300?°C to 930?°C and does not induce obviously degradation of the microwave dielectric properties. The 6.wt% BCB added ZS ceramics exhibited a low sintering temperature (~ 930?°C) and excellent dielectric properties of ε_r =?6.79, Q×f =?33,648?GHz, and τ_f =??30?ppm/°C. To compensate the negative τ_f value of this system, TiO₂ powders were introduced. Particularly when 10.wt% TiO₂ was added, good microwave dielectric properties of ε_r=?8.175, Q×f=?21,252?GHz, and τ_f =?1.2?ppm/°C were obtained for the 6.wt% BCB added ZS ceramic sintered at 930?°C for 3?h. Moreover, BCB added ZS-TiO₂ ceramics have a chemical compatibility with silver, which indicate that the BCB added ZS ceramics are promising candidate for LTCC applications.     

A synergistic effect of Li-B-Si glass and La-B on low-temperature sintering of BaO-0.15ZnO-4TiO2 dielectric ceramic

In the present work, the synergistic effect of Li₂O-B₂O₃-SiO₂ glass (LBS) and La₂O₃-B₂O₃ (LaB) addition on the wetting behavior, activation energy, phase composition, microstructure and microwave dielectric properties of the BaO-0.15ZnO-4TiO₂ ceramic (Ba-Zn-Ti) has been investigated to understand the low temperature mechanism of the sintering kinetic. The results show that co-doping with LBS and LaB decreases the activation energy and therefore accelerates the sintering process of the Ba-Zn-Ti due to the liquid phase formed at a low sintering temperature. Moreover, the co-doping of LBS and LaB restrains the appearance of BaTi(BO₃)₂ caused by the reaction of Ba-Zn-Ti and LBS, which has a deleterious effect on the densification and microwave dielectric properties of Ba-Zn-Ti. With LBS and LaB co-doped, the sintering temperature of Ba-Zn-Ti significantly dropped from 1150 °C to 850–900 °C and remained excellent microwave dielectric properties.     

Impact of additives and processing on microstructure and dielectric properties of willemite ceramics for LTCC terahertz applications

The paper presents the fabrication procedure, microstructure and dielectric properties of the low temperature cofired ceramics (LTCC) based on Zn₂SiO₄ doped with AlF₃, CaB₄O₇, Li₂TiO₃ and MgTiO₃. The heating microscope studies and differential thermal analysis were used for characterization of the behavior of the green tapes and ceramic samples during heating up to high temperatures. The microstructure and composition were analyzed by scanning electron microscopy, X-ray energy dispersive spectroscopy and XRD method. The dielectric properties were investigated in three frequency regions: 100 Hz–2 MHz, 90–140 GHz and 0.15–3 THz. The developed materials are promising candidates for the LTCC submillimeter wave applications due to a low sintering temperature of 900–980 °C, good compatibility with silver pastes and good dielectric properties – a low dielectric permittivity of 6–6.8, a relatively low dissipation factor of 0.005–0.008 at 1 THz, and a weak temperature dependence of dielectric permittivity.     

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.     

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.     

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.     

Impact of sintering temperature on phase composition,microstructure, and porosification behavior of LTCC substrates

Phase development and changes in crystalline composition of LTCC material during the sintering process were investigated using in-situ X-ray diffraction (XRD) measurements. CeramTape GC was chosen as the chemically simplest model system composed of alumina particles and glass for the investigations. The chemical characterization and microstructural analyses of the tapes sintered with some representative firing profiles were performed by techniques such as (scanning) transmission electron microscope, energy-dispersive X-ray spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and XRD. Moreover, the porosification behavior of LTCC substrates fired at different peak temperatures was studied. These investigations are important for the subsequent wet chemical etching, representing an approach which allows to reduce locally the permittivity of LTCC tapes. Treatment with a KOH solution shows non-selective etching behavior for all substrates. In addition, highly porous silica structures corresponding to Ca and Al depletion from the anorthite phase were observed in all samples after etching treatment.     

Low temperature sintering of barium titanate based ceramics with high dielectric constant for LTCC applications

The sintering temperature of BaTiO₃ powder was reduced to 900 °C due to the ZnO-B₂O₃-Li₂O-Nb₂O₅-Co₂O₃ addition. Excellent densification was achieved after sintering at 900 °C for 2 h. The low sintering temperature of newly developed capacitor materials allows a co-firing with pure silver electrodes. The dielectric constant and the temperature stability of the dielectric constant are strongly correlated with the composition of the ZnO-B₂O₃-Li₂O additives. A high dielectric constant up to 3000 and a dielectric loss less than 0.024 were measured on multilayer capacitors sintered at 900 °C with silver inner electrodes.     

Cold sintered LiMgPO4 based composites for low temperature co-fired ceramic (LTCC) applications

Cold sintered, Li₂MoO₄-based ceramics have recently been touted as candidates for electronic packaging and low temperature co-fired ceramic (LTCC) technology but MoO₃ is an expensive and endangered raw material, not suited for large scale commercialization. Here, we present cold sintered temperature-stable composites based on LiMgPO₄ (LMP) in which the Mo (and Li) concentration has been reduced, thereby significantly decreasing raw material costs. Optimum compositions, 0.5LMP-0.1CaTiO₃-0.4K₂MoO₄ (LMP-CTO-KMO), achieved 97% density at <300°C and 600 MPa for 60 minutes. Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray mapping confirmed the coexistence of end-members, LMP, CTO, and KMO, with no interdiffusion and parasitic phases. Composites exhibited temperature coefficient of resonant frequency ~ –6 ppm/°C, relative permittivity ~9.1, and Q × f values ~8500 GHz, properties suitable for LTCC technology and competitive with commercial incumbents.     

Low-temperature sintering and microwave dielectric properties of CaWO4 ceramics for LTCC applications

Microwave dielectric properties of CaWO₄ ceramics were investigated as a function of H₃BO₃ and/or Bi₂O₃ content and sintering temperature. For a single addition of H₃BO₃ (1 ≤ x (wt.%) ≤ 5), the density of specimen increased up to 3 wt.% H₃BO₃, and then decreased. The dielectric constant (K) and the quality factor (Q × f) of the specimens sintered at 850 °C showed lower value than those of specimens sintered above 900 °C due to the poor sinterability. With the increase of H₃BO₃ content of 0.5 wt.% Bi₂O₃–yH₃BO₃ (5 ≤ y (wt.%) ≤ 20), the sintering temperature of CaWO₄ ceramics could be effectively reduced from 1100 to 850 °C without degradation of dielectric properties. For the specimens sintered at 850 °C for 30 min, K was not changed remarkably with Bi₂O₃–H₃BO₃ content; however, Q × f value increased up to 9 wt.% H₃BO₃ of 0.5 wt.% Bi₂O₃–yH₃BO₃, and then decreased. The temperature coefficient of resonant frequency (TCF) shifted to the positive value with increasing Bi₂O₃–H₃BO₃ content. Typically, K of 8.7, Q × f of 70,220 GHz and TCF of −15 ppm/°C were obtained for the specimens with 0.5 wt.% Bi₂O₃–9 wt.% H₃BO₃ sintered at 850 °C for 30 min.     

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.     

Effect of complex Si4++Mg2+ additive on sintering and properties of undoped YAG ceramics

The paper is devoted to studying of Si⁴⁺+Mg²⁺ complex additive for obtaining transparent YAG ceramics for laser applications. Ceramics were fabricated by reactive vacuum sintering of commercial Y₂O₃, Al₂O₃ powders taken in a stoichiometric mixture with TEOS and MgO as sintering aids. Microstructure and optical properties of YAG:Si⁴⁺,Mg²⁺ ceramics were investigated as a function of the Si⁴⁺/Mg²⁺ ratio. It was found that the influence of complex additive does not correspond to the direct superposition of known Si⁴⁺- and Mg²⁺-induced sintering mechanisms and involves interaction between Si⁴⁺ and Mg²⁺ ions during sintering. It was shown that C_Si/C_Mg> 1 provides more effective pore elimination and uniform microstructure when C_Si/C_Mg< 1 gives more intense inhibition of grain grown which may be important for scaling the size of ceramics.     

Effect of TiO2 addition on sintering,microstructure and properties of anorthite-based ceramics derived from reduced copper slag

Reduced copper slag is a promising substitute constituent in anorthite-based ceramics. To reduce the sintering temperature of this type of slag-derived ceramics, the role of TiO₂ addition in the sintering, microstructure, physical and mechanical properties, and environmental and appearance performance was studied by experimental and theoretical methods. The results show that the Ti element was dissolved in the glassy phases and anorthite crystals instead of producing new phases. This behavior allows the liquid appearance at earlier temperatures and accelerates the ion diffusion, thus significantly enhancing the sintering and contributing to a dense microstructure with fine closed pores. Besides, this improvement is achieved at a relatively slight sacrifice of anorthite grains. The addition of 4 wt% TiO₂ results in a decrease in the vitrification temperature by 100 ^oC and an enhancement of flexural strength by 50%. Moreover, the slag-derived ceramics show extremely low leaching toxicity and a relatively higher whiteness, ensuring the cleanliness and quality of anorthite-based ceramics. However, the ceramics show lower bulk density and obvious surface defects with the TiO₂ content up to 6 wt%.     

Effect of magnetic CoFe2O4 component on sintering densification process of Bi3.15Nd0.85Ti3O12 ceramics

Sintering densification processes of the composite ceramics (Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT)/CoFe₂O₄ (CFO)) have been investigated using dilatometric experiments combining with the TG-DTA, density measurements and microstructure studies. Microstructures analyses and quantitative calculations show that the composite ceramics achieve densification at low temperatures (<1150 °C). The formation of coherent-lattice interfaces between (200)/(020)_BNdT and (310)_CFO are considered to play an important role on such densification. The intrinsic preferred orientation of BNdT grains is suppressed by CFO phase because of this coherent relationship. Although the sintering activation energies of 0.8BNdT-0.2CFO are about 2.7 times larger than those of pure BNdT due to the pinning effect, the composite ceramic could still be densified, indicating the formation energy of coherent-lattices provided the extra sintering force. The even coercive electric fields of the resulting pure BNdT and 0.8BNdT-0.2CFO ceramic are approximately 89 and 97 kV/cm, respectively, at 250 kV/cm. The polarization of 0.8BNdT-0.2CFO reaches saturation around 430 kV/cm.     

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.     

Influence of TiO2 additive on sintering temperature and microwave dielectric properties of Mg0.90Ni0.1SiO3 ceramics

(1-x)Mg_0.90Ni_0.1SiO₃-xTiO₂ (x = 0, 0.01, 0.03, 0.05) ceramics were successfully formed by the conventional solid-state methods and characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and their microstructure and microwave dielectric properties systematically investigated. It was observed that when TiO₂ content increased from 0 to 5 wt%, the Q_uf_o of the sample decreased from 118,702 GHz to 101,307 GHz and increases the τ_f value from −10 ppm/°C to +3.14 ppm/°C accompanied by a notable lowering in the sintering temperature (125 °C). A good combination of microwave dielectric properties (ε_r ∼ 8.29, Q_uf_o ∼ 101,307 GHz and τ_f ∼ −2.98 ppm/°C) were achieved for Mg_0.90Ni_0.1SiO₃ containing 3 wt% of TiO₂ sintered at 1300 °C for 9 h which make this material of possible interest for millimeter wave applications.     

烧结气氛对合成MgAl2O4-Ti(C,N)复合陶瓷的影响

以金属铝粉、钛白粉和轻烧MgO细粉为原料,通过设计100%焦炭粒(简称C气氛),10%钛白粉 90%焦炭粒(简称TC气氛),以及10%硅微粉 90%焦炭粒(简称SC气氛)3种埋粉条件下的高温烧结还原性气氛,采用X射线衍射仪(XRD)、扫描电镜(SEM)和微区电子探针分析(EPMA)等手段,研究了铝热还原氮化法(1600℃3h)制备MgAl2O4-Ti(C,N)复合陶瓷在不同烧结气氛下的相组成和显微结构的变化。结果表明在不同气氛下,烧后试样的主要物相均为MgAl2O4和Ti(C,N),Ti(C,N)可能会固溶氧,气氛对Ti(C,N)的影响较大。和单纯埋炭气氛下相比,在TC气氛下有助于Ti(C,N)的生成,但晶粒细小;在SC气氛下不利于Ti(C,N)的生成,且有玻璃相存在。     

High-Q microwave dielectric properties of Li(Zn0.95Co0.05)1.5SiO4 ceramics for LTCC applications

This study investigated the effects of Li₂O-MgO-ZnO₂-B₂O₃-SiO₂ (LMZBS) glass on the microstructure, sintering behaviour and microwave dielectric properties of Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics. Li(Zn_0.95Co_0.05)_1.5SiO₄ powders were synthesised by a traditional solid-state route and added with different amounts of LMZBS glass (0–4 wt%) to decrease the sintering temperature of the ceramics to approximately 900 °C. The XRD patterns showed that no chemical reactions occurred between the Li(Zn_0.95Co_0.05)_1.5SiO₄ ceramics and the LMZBS glass within the doping range. The SEM images indicated that the sample added with 1.5 wt% glass and sintered at 900 °C exhibited a compact and uniform microstructure. In particular, the microwave dielectric properties of the products were related to LMZBS glass content and sintering temperature. The sample with 1.5 wt% LMZBS glass exhibited excellent microwave dielectric properties, namely, ${\epsilon }_r$=6.12, $Q\times f$=83,600 GHz and ${\tau }_f=-39.1\mathrm{ppm}/\mathrm{℃}.$