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.     

Microwave dielectric properties of CaWO4–Li2TiO3 ceramics added with LBSCA glass for LTCC applications

In this work, 0.86CaWO₄–0.14Li₂TiO₃ ceramics were prepared via a traditional solid-state process. The effects of Li₂O–B₂O₃–SiO₂–CaO–Al₂O₃ (LBSCA) addition on the phase formation, sintering character, microstructure and microwave dielectric properties of the ceramics were investigated. A small amount of LBSCA addition could effectively lower the sintering temperature of the ceramics. X-ray diffraction analysis revealed that CaWO₄ and Li₂TiO₃ phases coexisted without producing any other crystal phases in the sintered ceramics. The dielectric constant and Qf values were related to the amount of LBSCA addition and sintering temperatures. All specimens could obtain near-zero temperature coefficient (τ_f) values through the compensation of the positive τ_f of Li₂TiO₃ and the negative τ_f of CaWO₄. The 0.86CaWO₄–0.14Li₂TiO₃ ceramic with 0.5 wt% LBSCA addition and sintered at 900 °C for 3 h exhibited excellent microwave dielectric properties of ε_r=12.43, Qf=76,000 GHz and τ_f=−2.9 ppm/°C.     

Influence of Li2O–B2O3–SiO2 glass on the sintering behavior and microwave dielectric properties of BaO–0.15ZnO–4TiO2 ceramics

This paper reports the investigation of the performance of Li₂O–B₂O₃–SiO₂ (LBS) glass as a sintering aid to lower the sintering temperature of BaO–0.15ZnO–4TiO₂ (BZT) ceramics, as well as the detailed study on the sintering behavior, phase evolution, microstructure and microwave dielectric properties of the resulting BZT ceramics. The addition of LBS glass significantly lowers the sintering temperature of the BZT ceramics from 1150 °C to 875–925 °C. Small amount of LBS glass promotes the densification of BZT ceramic and improves the dielectric properties. However, excessive LBS addition leads to the precipitation of glass phase and growth of abnormal grain, deteriorating the dielectric properties of the BZT ceramic. The BZT ceramic with 5 wt% LBS addition sintered at 900 °C shows excellent microwave dielectric properties: ε_r=27.88, Q×f=14,795 GHz.     

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).     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

LTCC system with new high-ɛr and high-Q material co-fired with conventional low-ɛr base material for wireless communications

The authors have developed a new LTCC material with characteristics of high dielectric constant (ɛ_r), high quality factor (Q) and low temperature coefficient of capacitance (TCC). This material can be co-fired with a conventional base LTCC material and buried resistors with low temperature coefficient of resistance (TCR). The base material which consists of Al₂O₃ filler and glass, has low ɛ_r of 8.7 at 3 GHz. The newly developed LTCC material, which consists of Ba–(Re)–Ti–O filler, Al₂O₃ filler, and glass, has the following characteristics of ɛ_r of 15.1, Q of 900 at 3 GHz, and TCC of −10 ppm/K. The buried resistors consist of RuO₂ and glass. Two different LTCC materials, a resistor material and a silver electrode paste can be co-fired as multi-layer substrates and are regarded as a new LTCC system.Constrained sintering could be applied to this LTCC system and the dimensions of substrates could be controlled with quite high accuracy. This LTCC system is expected to contribute to further miniaturization of RF circuits and the reduction of electrical loss.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

High-Q microwave dielectrics in low-temperature sintered (Zn1−xNix)3Nb2O8 ceramics

The effects of Ni substitution for Zn on microwave dielectric properties of (Zn_1−xNi_x)₃Nb₂O₈ (x = 0.02–0.08) ceramics were investigated in this study. The XRD patterns of the sintered samples reveal single-phase formation with a monoclinic structure. The tremendous improvement of Q × f value can be achieved by a small level of Ni substitution (x = 0.05). The τ_f value was found to decrease with a decreasing A-site bond valence. In addition, B₂O₃ and CuO were used as a sintering aid to lower the sintering temperature from 1180 to 900 °C. Excellent microwave dielectric properties (ɛ_r ∼ 20.7, Q × f ∼ 98,000 GHz and τ_f ∼ −85.2 ppm/°C) and a chemical compatibility with Ag electrodes can be obtained for 4 wt% B₂O₃–CuO doped (Zn_0.95Ni_0.05)₃Nb₂O₈ ceramics sintered at 930 °C for 2 h. This constitutes a very promising material for LTCC applications.     

Characterization and dielectric behavior of willemite and TiO2-doped willemite ceramics at millimeter-wave frequency

Willemite ceramics (Zn₂SiO₄) have been successfully prepared in the temperature range from 1280 to 1340 °C. It is found that willemite ceramics possess excellent millimeter-wave dielectric properties: a dielectric constant ɛ_r value of 6.6, a quality factor Q × f value of 219,000 GHz and a temperature coefficient of resonant frequency τ_f value of −61 ppm/°C. By adding TiO₂ with large positive τ_f value (450 ppm/°C), near zero τ_f value can be achieved in a wide sintering temperature range. With 11 wt% of TiO₂, an ɛ_r value of 9.3, a Q × f value of 113,000 GHz, and a τ_f value of 1.0 ppm/°C are obtained at 1250 °C. The relationships between microstructure and properties are also studied. Our results show that willemite with appropriate TiO₂ is an ideal temperature stable, low ɛ_r and high Q × f dielectric for millimeter-wave application.     

Low dielectric constant porous BN/SiCO made by pyrolysis of filled gels

BN/SiCO composite ceramics having a dielectric constant, ɛ_r, as low as 1.9 have been made by pyrolysis of filled gels at temperature ≥1000 °C. Such a low value of ɛ_r is supposed to be due to a combination of factors: the low ɛ_r of BN and of SiCO itself and the high amount of residual porosity present in the samples. The porous microstructure – and the related dielectric properties – obtained at 1000 °C show a very good stability up to 1400 °C. This result has been ascribed to the high viscosity of the SiCO glass and to the platelet shape particles of BN. Both factors hinder the sintering and prevent the closure of the pores.     

Novel microwave dielectric LTCCs based uponV2O5 doped M2+Cu2Nb2O8 compounds (M2+ = Zn,Co, Ni,Mg and Ca)

The copper-niobates, M²⁺Cu₂Nb₂O₈ (M²⁺ = Zn, Co, Ni, Mg or Ca) have good microwave dielectric properties when sintered between 985–1010 °C and 1110 °C for CaCu₂Nb₂O₈. Therefore, they would be potential dielectric LTCC materials if they could be made to sinter below 960 °C (melting point of silver). To this end, additions of 3 wt.% V₂O₅ were made to ZnCu₂Nb₂O₈, CoCu₂Nb₂O₈, NiCu₂Nb₂O₈, MgCu₂Nb₂O₈ and CaCu₂Nb₂O₈, and their sintering and dielectric behaviour was investigated for samples fired between 800 and 950 °C. Doping lowered sintering temperatures to below the 960 °C limit in all cases. Doping had the general effect of reducing ɛ_r, density, Qf and τ_f, although doped CaCu₂Nb₂O₈ had a Qf value of 9300 GHz, nearly four times that of the best undoped sample. Doped ZnCu₂Nb₂O₈ fired to 935 °C had Qf = 10,200 GHz, and for doped CoCu₂Nb₂O₈ fired to 885 °C Qf = 7500 GHz. When doped and undoped samples all fired to 935 °C were compared, all doped samples had greater ɛ_r and density, and all except ZnCu₂Nb₂O₈ had a smaller τ_f. All doped samples had a more linear relationship between frequency and temperature in the range 250–300 K.     

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.     

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 low-fired Li2MnO3 ceramics co-doped with LiF–TiO2

Li₂MnO₃ ceramics co-doped with 2 wt% LiF and x wt% TiO₂ (x=0, 3, 5, 7, 10) were prepared by solid-state reaction for low-temperature co-fired ceramics (LTCC) applications. The sintering temperatures of Li₂MnO₃ ceramics were successfully lowered to 925°C due to the formation of a LiF liquid phase. Their temperature stability was improved by doping with TiO₂. A typical Li₂MnO₃-2 wt% LiF-5 wt% TiO₂ sample with well-densified microstructures displayed optimum dielectric properties (ε_r=13.8, Q×f= 23,270 GHz, τ_f=1.2 ppm/°C). Such sample was compatible with Ag electrodes, which suggests suitability of the developed material for LTCC applications in wireless communication systems.     

Low temperature sintering and microwave dielectric properties of Ba3Ti5Nb6O28 with ZnO–B2O3 glass additions for LTCC applications

The effects of ZnB₂O₄ glass additions on the sintering temperature and microwave dielectric properties of Ba₃Ti₅Nb₆O₂₈ have been investigated using dilatometer, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and a network analyzer. The pure Ba₃Ti₅Nb₆O₂₈ system showed a high sintering temperature (1250 °C) and had the good microwave dielectric properties: Q × f of 10,600 GHz, ɛ_r of 37.0, τ_f of −12 ppm/°C. It was found that the addition of ZnB₂O₄ glass to Ba₃Ti₅Nb₆O₂₈ lowered the sintering temperature from 1250 to 925 °C. The reduced sintering temperature was attributed to the formation of ZnB₂O₄ liquid phase and B₂O₃-rich liquid phases. Also the addition of ZnB₂O₄ glass enhanced the microwave dielectric properties: Q × f of 19,100 GHz, ɛ_r of 36.6, τ_f of 5 ppm/°C. From XPS and XRD studies, these phenomena were explained in terms of the reduction of oxygen vacancies and the formation of secondary phases having the good microwave dielectric properties.     

Sintering behavior and microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics modified by B2O3–Li2O–Al2O3 and CeO2

A low temperature sintering method was used to avoid the relatively high sintering temperatures typically required to prepare 0.67CaTiO₃–0.33LaAlO₃ (CTLA) ceramics. Additionally, CeO₂ was introduced into the CTLA ceramics as an oxygen-storage material to improve their microwave dielectric properties. 0.67CaTiO₃–0.33LaAlO₃ ceramics co-doped with B₂O₃–Li₂O–Al₂O₃ and CeO₂ were prepared by a conventional two-step solid-state reaction process. The sintering behavior, crystal structure, surface morphology, and microwave dielectric proprieties of the prepared ceramic samples were studied, and the reaction mechanism of CeO₂ was elucidated. CTLA+0.05 wt% BLA+3 wt% CeO₂ ceramics sintered at 1360 °C for 4 h exhibited the optimal microwave dielectric properties: dielectric constant (ε_r)=45.02, quality factor (Q×f)=43102 GHz, and temperature coefficient of resonant frequency (τ_f)=2.1 ppm/°C. The successful preparation of high-performance microwave dielectric ceramics provides a direction for the future development and commercialization of CTLA ceramics.     

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 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 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.     

Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.