Microwave dielectric properties of CaO–La2O3–Nb2O5–TiO2 ceramics

A series of ceramics with a general formula Ca_1+xLa_4?xNb_xTi_5?xO₁₇ (0 ≤ x ≤ 4) were fabricated using the solid-state ceramic route. The phase, microstructure, and microwave dielectric properties varied distinctly with composition or the value of x. X-ray diffraction results showed that the two end member phases, CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇, crystallized into single phases with orthorhombic and monoclinic crystal structure, respectively. For intermediate compounds with x = 1, 2, and 3, mixture phases CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇ coexisted and a trace amount of second phase was detected. The ceramics showed high ε _r in the range of 45–52, relatively high quality factors with Q × f in the range of 9,870–15,680 GHz and τ _f value in the range between ?38 and ?126.4 ppm/°C. τ _f of CaLa₄Ti₅O₁₇ can be tuned to a near-zero value by addition of suitable amount of TiO₂.     

Structure and microwave dielectric properties of 5BaO–2V2O5 binary ceramic system

5BaO–2V₂O₅ ceramic has been prepared by conventional solid state reaction method. The X-ray diffraction and Laser Raman studies have been carried out to ascertain the phase purity and crystal structure of the prepared composition. The microwave dielectric properties of the well sintered ceramic compacts were measured by Hakki & Colemann post resonator and cavity perturbation techniques using a vector network analyzer. The 5BaO–2V₂O₅ ceramic exhibits a low dielectric constant of 12.1, relatively good quality factor of 26,790 GHz, and a small temperature variation of resonant frequency of 7 ppm/°C in the microwave frequency region.     

Microwave dielectric properties of new SrLa4−xNdxTi5O17 ceramics

New microwave dielectric ceramics in the SrLa_4?xNd_xTi₅O₁₇ (0 ≤ x ≤ 4) composition series were prepared through a solid state mixed oxide route. All the compositions formed highly dense (～95%) single phase ceramics upon sintering at 1500–1580 °C. The molar volume and theoretical density decreased due to the substitution of small (1.27 Å) Nd ions for large (1.36 Å) La ions. This decrease was associated with a decrease in the dielectric constant (?_r), temperature coefficient of resonant frequency (τ_f) and quality factor (Q_uf_o). An analysis of properties achieved in the present study indicated that ceramics exhibiting nearly zero τ_f corresponding to ?_r ～ 54 could be fabricated in the SrLa_4?xNd_xTi₅O₁₇ composition series at x ～ 1.6; however, further work is required to improve Q_uf_o (～6000 GHz) for possible practical applications.     

Microwave dielectric properties of Ba2Ti3Nb4O18 ceramic doped with ZnO–B2O3–SiO2 glass and its compatibility with silver electrode

The effects of ZnO–B₂O₃–SiO₂ glass (ZBS) additions on the sintering behavior and microwave dielectric properties of Ba₂Ti₃Nb₄O₁₈ ceramic have been investigated. Due to the liquid phase effects resulting from the additives, the addition of ZBS glass can reduce the sintering temperature of Ba₂Ti₃Nb₄O₁₈ ceramic from 1,250 to 925 °C and induce no obvious degradation of the microwave dielectric properties. Typically, when 3wt% ZBS is added, a dense ceramic can be sintered at 925 °C with a ε_r of 33.2, a high of Q × f of 13,600 GHz and a low τ _f of −5.9 ppm/°C. Moreover, ZBS glass-added Ba₂Ti₃Nb₄O₁₈ ceramic has a chemical compatibility with silver electrode, which suggests that the ceramic could be applied to LTCC devices application.     

Microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics with Li2O–V2O5 additions

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 3:1:3 has been investigated. The compound is composed of two phases, the Li₂TiO₃ and “M-phase” solid solution phase. The microwave dielectric ceramic has low sintering temperature (∼1100 °C) and good microwave dielectric properties of a relatively high permittivity (∼51), high Q × f value up to 8700, and small temperature coefficient (∼37 ppm/°C). The low-amount doping of 0.83Li₂O–0.17V₂O₅ (LV) can effectively lower the sintering temperature from 1100 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 1 wt.% LV-doped ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 51.3, Q × f = 7235 GHz, τ _f  = 22 ppm/°C, which suggests that the ceramics can be applied in microwave LTCC devices.     

Microwave dielectric properties of ceramics with compositions along the La2/3TiO3(STAB)–LaAlO3 tie line

The stabilization of La2/3TiO3 in the perovskite structure by 4 mol% LaAlO3 resulted in maximum values for the relative permittivity (k72) and temperature coefficient of resonant frequency (f=123 ppm K-1). The Q-value quality factor of these ceramics were over 5300 measured at 4.5 GHz. Higher contents of LaAlO3 cause a decrease in both the permittivity and f values, accompanied by an increase in the Q-value. Typical values measured at 6.0 GHz for ceramics with the composition 70 mol% La2/3TiO3 – 30 mol% LaAlO3 are k>44, f=+7 ppm K-1 and Q>5500. The results depend on the processing parameters. The sintering temperature increases with increasing LaAlO3 content. An oxidizing sintering atmosphere improves the Q-value, whereas the permittivity and f are not significantly affected.     

Microwave dielectric properties of Mg2SiO4–Ca1−ySm2y/3TiO3 composite ceramics

In this work, (1?x)Mg₂SiO₄–xCa_1?ySm_2y/3TiO₃ ceramics were fabricated using the conventional solid-state reaction method. When y?=?0.2, the effects of Ca_1?ySm_2y/3TiO₃ contents on microstructure, phase evolution, and microwave dielectric properties of the samples were studied. The X-ray diffraction showed that Mg₂SiO₄ and Ca_0.8Sm_0.4/3TiO₃ co-existed in composite ceramics. The dielectric constant (ε_r) and temperature coefficient of resonant frequency (τ_f) increased with the increasing content of Ca_0.8Sm_0.4/3TiO₃, while the quality factor (Q?×?f) decreased. At x?=?0.22, the (1?x) Mg₂SiO₄–xCa_0.8Sm_0.4/3TiO₃ ceramics sintered at 1325?°C for 3?h possessed optimal microwave dielectric properties: ε_r?~?11.89, Q?×?f value?~?32,703?GHz and τ_f?~?+?0.49?ppm/°C. As a result, the ε_r of (1?x) Mg₂SiO₄?+?xCa_1?ySm_2y/3TiO₃ ceramics could be adjusted in the range of 10.83 to 13.88 while the τ_f is near-zero.

     

Effect of borosilicate glasses on the microwave dielectric properties of ZnO–Nb2O5–Ta2O5 system

(1 ? y)[0.5ZnNb₂O₆–0.5Zn₃Nb₂O₈]–yZnTa₂O₆ with y = 0.91 (ZNT) ceramic have been prepared by conventional solid state ceramic route. The effect of glass additives on the microstructure, densification, and microwave dielectric properties of the ZNT ceramic for low temperature co-fired ceramic applications was investigated. Different weight percentages of quenched glass such as ZnO–B₂O₃–SiO₂, BaO–B₂O₃–SiO₂, LiO–B₂O₃–SiO₂ and MgO–B₂O₃–SiO₂ were added to ZNT powder. The crystal structure of the ceramic–glass composites was studied by X-ray diffraction and microstructure by scanning electron microscopy. The microwave dielectric properties such as relative permittivity (ε_r), quality factor (Q_uxf) and co-efficient of temperature variation of resonant frequency (τ_f) of the ceramics have been measured in the frequency range 4–6 GHz. The 5 wt% ZnO–B₂O₃–SiO₂ added ZNT ceramic sintered at 900 °C showed: ε_r = 28.1, Q_uxf = 32820 GHz (at 4.92 GHz), and τ_f = ?7.7 ppm/^oC respectively.     

Microwave synthesis and properties of NaPO3–SnO–Nb2O5 glasses

Tin niobiophosphate glasses were produced using a domestic microwave oven under a nitrogen flow. The fast microwave melting method and the protective atmosphere prevent the oxidation of SnO. After 10 min of heating, the NaPO₃, SnO, and Nb₂O₅ mixtures are homogeneous and permit to obtain transparent glasses. Three series of glasses with different Sn/Nb ratio were studied to determine the influence of each oxide. The glass transition temperature increases linearly with the amount of Nb₂O₅ and SnO. These variations are more important for compositions with high metallic cation proportions and with a low Sn/Nb ratio. The same evolutions were observed for the density, Vickers hardness, and elastic modulus while the thermal expansion coefficient decreases monotonously. The simultaneous insertion of SnO and Nb₂O₅ in phosphate glass matrix leads to a progressive strengthening of the glass network. The chemical durability of the glasses also increases as a function of the amount of metal oxides. We prepared a bulk glass sample with a dissolution rate of about 3.3 × 10⁻⁸ g cm⁻² min⁻¹ in renewed water conditions at 95 °C. This durability is equivalent to those of the window glass whereas the glass transition temperature remains lower than 485 °C.     

Low-firing of BiSbO4 microwave dielectric ceramic with V2O5–CuO addition

Effects of 1.0 wt.% V₂O₅–CuO mixture addition on the sintering behavior, phase composition and microwave dielectric properties of BiSbO₄ ceramics have been investigated. BiSbO₄ ceramics can be well densified below temperature about 930 °C with 1.0 wt.% V₂O₅–CuO mixtures addition with different ratios of CuO to V₂O₅. The formation of BiVO₄ phase and substitution of Cu²⁺ can explain the decrease of sintering temperature. Dense BiSbO₄ ceramics sintered at 930 °C for 2 h exhibited good microwave dielectric properties with permittivity between 19 and 20.5, Qf values between 19,000 and 40,000 GHz and temperature coefficient of resonant frequency shifting between ?71.5 ppm °C^?1 and ?77.8 ppm °C^?1. BiSbO₄ ceramics could be a candidate for microwave application and low temperature co-fired ceramics technology.     

Effects of SrO–B2O3–SiO2 glass additive on the microstructure and dielectric properties of CaCu3Ti4O12

The effect of SrO–B₂O₃–SiO₂ glass additive (SBS) on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics was investigated. This SBS–added CCTO ceramics were prepared by the solid state reaction. The undesirable impurity phases Ca₃SiO₅ started appearing in the XRD patterns, suggesting a possible chemical reaction between CaTiO₃ and SiO₂ (the devitrification production of SBS glass). The SBS glass additive promoted the grain growth and densification of CCTO ceramics. Cole–Cole plots of conductance suggested that the resistivity grain boundary decreased with increasing amount of SBS glass (when x = 0–2 wt%), then increased (when x = 2–3 wt%). The addition of SBS glass was desirable to increase the dielectric constants (up to 10⁴) and lowered the dielectric losses of CCTO over the frequency range of 450–40 kHz at the relatively lower sintering temperature for relatively shorter sintering time (1,050 °C, 12 h).     

Microwave dielectric ceramics in the BaO–TiO2–ZnO system doped with Nb2O5

The microwave dielectric properties of BaO–TiO₂–ZnO (BTZ) system ceramics were studied as a function of the amount of Nb₂O₅ dopant. With the addition of 0–0.025 mol% Nb₂O₅, the substitution of Ti⁴⁺ ions with Nb⁵⁺ ions decreased the sintering temperature and increased the dielectric constant ε_r and quality factor Q of the system due to the similar atomic radius of Nb⁵⁺ and Ti⁴⁺ ions. When the amount of Nb⁵⁺ increased further (>0.025 mol%), Q was decreased by increasing the titanium vacancies. When the system doped with 0.025 mol% Nb₂O₅ was sintered at 1160 °C for 6 h, the ε_r, Qf₀, and the temperature coefficient of resonant frequency (τ_f) were 36.7, 41,000 GHz, and −5.0 ppm/°C, respectively, at 5 GHz.