Dependence of dielectric properties on structural characteristics of (Zn1/3A2/3)0.5 (Ti1−xBx)0.5O2 (A = Nb5+, Ta5+, B = Ge4+, Sn4+) ceramics at microwave frequencies

The effects of structural characteristics on the dielectric properties of (Zn_1/3A_2/3)_0.5(Ti_1?xB_x)_0.5O₂ (A = Nb⁵⁺, Ta⁵⁺, B = Ge⁴⁺, Sn⁴⁺) (0.1 ≤ x ≤ 0.3) ceramics were investigated at microwave frequency. The sintered specimens showed solid solutions with a tetragonal rutile structure within the solid solution range of compositions. With an increase of BO₂, the temperature coefficient of resonant frequency (TCF) and dielectric constant (K) decreased with a decrease of oxygen octahedral distortion and dielectric polarizabilites, respectively. However, the quality factor (Qf) of the sintered specimens was increased with BO₂ due to the reduction of Ti⁴⁺ ions. The Qf value of the specimens with A = Ta was higher than that of the specimens with A = Nb.     

Intrinsic factors affecting the microwave dielectric properties of Mg2Ti1−x(Mg1/3Sb2/3)xO4 ceramics

The effect of crystal structure on the microwave dielectric properties of Mg₂Ti_1−x(Mg_1/3Sb_2/3)_xO₄ (0.025≤x≤0.15) ceramics was investigated. A single phase having a cubic inverse spinel structure formed over the entire range of compositions, in specimens sintered at 1450 °C for 4 h. The structural characteristics of these ceramics were quantitatively evaluated by applying the Rietveld refinement method to the X-ray diffraction data. The largest bond strength between the cation and the oxygen ion and hence the highest quality factor (Qf) of the specimens were obtained at x=0.05. Although the ionic polarizability (3.29 Å³) of (Mg_1/3Sb_2/3)⁴⁺ was larger than that (2.93 Å³) of Ti⁴⁺, the dielectric constant (K) of the specimens decreased owing to the decrease of rattling effect with increasing x. In addition, the temperature coefficient of resonant frequency (TCF) decreased with decreasing K values. Typically, a high Qf value of 229,000 GHz was obtained for the specimens with x=0.05.     

Layered composite thick films for dielectric applications

Electrophoretic deposition has been successfully used for the preparation of multilayer composite thick films of the temperature stable microwave dielectric ceramics BaLa₄Ti₄O₁₅ (BLT) (?_r = 44, Qf₀ = 44,000 GHz) and Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) (?_r = 82, Qf₀ = 10,000 GHz). The composites exhibit permittivity intermediate between the end members that may be tuned by modifying the ratio between and/or thickness of each layer, thereby tailoring the films for specific applications. In demonstrating an alternative way to tailor functional properties, these results have broad implications for a large number of thick film-based devices.     

Tunability and microwave dielectric properties of BaO–SrO–Nd2O3–TiO2 ceramics

Ferroelectrics such as barium–strontium titanates (BSTO) and Mg-doped BSTO are well known as promising candidates for the application in microwave tunable devices including phase shifters, filters, and others operating at microwave frequencies. In this study new bulk ceramics based on BaTiO₃ (BTO)–SrTiO₃ (STO) with addition of BaNd₂Ti₄O₁₂ (BNT) solid solution was investigated. The phase correlations, size, and nature of boundaries between phases were studied using scanning electron microscopy (SEM). The effect of compositional change on the unit cell parameters of the perovskite phase, microwave dielectric properties, and tunability under DC field had been studied. The materials with dielectric constant ∼320–700, Qf = 1950–3000 GHz at 3.5 GHz and tunability ∼8.0–31.7% at E = 1 V/μm were achieved.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

Origin of improvement of Q based on high symmetry accompanying Si–Al disordering in cordierite millimeter-wave ceramics

Cordierite (Mg₂Al₄Si₅O₁₈) is a candidate for millimetre-wave dielectrics because of the low dielectric constant ?_r of 6.19. This study has been focused on the effects of substituting Ni for Mg in octahedron. In the case of octahedral substitution, the quality factor Qf of the Ni-substituted analogue with single phase was improved from 39,900 to 99,110 GHz up to x = 0.1 in (Mg_1?xNi_x)₂Al₄Si₅O₁₈. Though Ni-substituted cordierite is single phase up to x = 0.1, compositions substituted with more than x = 0.15 have a secondary phase. The origin of improvement of Q comes from changing to the high symmetry which is from orthorhombic to hexagonal. Here, cordierite with orthorhombic crystal system Cccm (no. 66) transforms to indialite with hexagonal crystal system P6/mcc (no. 192). It is confirmed, based on the crystal structure obtained by Rietveld method, that the configuration of the (Si₄Al₂)O₁₈ ring changes to an equilateral hexagonal ring from a distorted ring according to the substitution Ni for Mg. The volumes and covalencies of SiO₄ and AlO₄ tetrahedra also change to similar values according to the substitution. The crystal structure shows a tendency to higher symmetry of a hexagonal ring accompanying Si–Al disordering. In this case, contribution of high symmetry to Q might be greater than that of ordering.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

Influence of sintering temperature on dielectric properties and crystal structure of corundum-structured Mg4Ta2O9 ceramics at microwave frequencies

Microwave dielectric properties of corundum-structured Mg₄Ta₂O₉ ceramics were investigated as a function of sintering temperatures by an aqueous sol–gel process. Crystal structure and microstructure were examined by X-ray diffraction (XRD) technique and field emission scanning electron microscopy (FE-SEM). Sintering characteristics and microwave dielectric properties of Mg₄Ta₂O₉ ceramics were studied as a function of sintering temperature from 1250 °C to 1450 °C. With increasing sintering temperature, the density, ε_r and Qf values increased, saturating at 1300 °C with excellent microwave properties of ε_r=11.9, Qf=195,000 GHz and τ_f=?47 ppm/°C. Evaluation of dielectric properties of Mg₄Ta₂O₉ ceramics were also analyzed by means of first principle calculation method and ionic polarizability theory.     

Microwave dielectric properties of (Zn1/3B2/3)0.5(Ti0.8M0.2)0.5O2 (B = Nb5+, Ta5+, M = Sn4+, Ge4+) ceramics

Dielectric properties of (Zn_1/3Nb_(2?x)/3Ta_x/3)_0.5(Ti_0.8Sn_0.1Ge_0.1)_0.5O₂ (x = 0, 1, 2) and/or (Zn_1/3Nb_1/3Ta_l/3)_0.5(Ti_0.8Sn_0.2(l?y)Ge_0.2y)_0.5O₂ (y = 0, 0.5, 1) were investigated at the microwave frequencies. For the compositions with single phase of rutile structure, the dielectric constant (K) of specimens was not only dependent on the dielectric polarizabilities, but also on the bond length ratio of apical bond (d_apical) to equatorial bond (d_equatorial) of oxygen octahedron in the unit cell. Temperature coefficients of the resonant frequencies (TCF) of the specimens with B = Nb⁵⁺ and/or M = Sn⁴⁺ was larger than those with B = Ta⁵⁺ and/or M = Ge⁴⁺. These results could be attributed to the changes of the degree of oxygen octahedral distortion. Quality factors (Qf) of the specimens with B = Ta⁵⁺ and/or M = Sn⁴⁺ were larger than those with B = Nb⁵⁺ and/or M = Ge⁴⁺.     

Effect of Ni substitution on the microwave dielectric properties of cordierite

Cordierite (Mg₂Al₄Si₅O₁₈) is one of the silicates with low dielectric constant (ɛ_r), which are expected for good candidate of millimeter wave dielectrics. Microwave dielectric properties of Ni substituted cordierite solid solutions; (Mg_1−xNi_x)₂Al₄Si₅O₁₈ have been investigated. (Mg_1−xNi_x)₂Al₄Si₅O₁₈ with no secondary phase was obtained in the compositions x range from 0 to 0.1. It was found that a very small amount of Ni substitution was effectively increased of the quality factor (Qf) value and the highest Qf value of 99,110 GHz was obtained in the composition x = 0.1. No remarkable composition dependence of the temperature coefficient of resonance frequency (τ_f) was observed in the range from x = 0.05 to 0.2, On the other hand, τ_f abruptly shifted toward negative value with increasing x from 0.3 to 0.5. The correlation between the crystal structure of cordierite and the microwave dielectric properties, particularly Qf is discussed.     

Crystal structure and microwave dielectric properties of Zn0.9Ti0.8−xSnxNb2.2O8 ceramics

The crystal structure and microwave dielectric properties of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.00, 0.05, 0.10, 0.15) ceramics sintered at temperatures ranging from 1100 °C to 1140 °C for 6 h were investigated. A single phase with ixiolite structure was obtained. With the increase of Sn content, the dielectric constant decreased attributed to the decrease of dielectric polarizability. The Qf value decreased with the decrease of packing fraction and grain size. The temperature coefficient of resonant frequency (τ_f) increased due to the increase of the bond valence of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ ceramics. The excellent microwave dielectric properties of ? = 35.05, Qf = 49,100 GHz, τ_f = ?27.6 × 10^?6/°C were obtained for Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.05) specimens sintered at 1120 °C for 6 h.     

Effects of structural characteristics on microwave dielectric properties of (1 − x)Ca0.85Nd0.1TiO3–xLnAlO3 (Ln = Sm,Er and Dy) ceramics

Dependence of microwave dielectric properties on the structural characteristics of (1 − x)Ca_0.85Nd_0.1TiO₃–xLnAlO₃ (Ln = Sm, Dy and Er) ceramics were investigated as a function of LnAlO₃ content (0.05 ≤ x ≤ 0.25). For the specimens with SmAlO₃, a single phase with orthorhombic perovskite was obtained through the entire composition, however, Dy₂Ti₂O₇ and Er₂Ti₂O₇ were detected as a secondary phase along with the orthorhombic perovskite phase for the specimens with DyAlO₃ (x = 0.25) and ErAlO₃ (0.10 ≤ x ≤ 0.25), respectively. With an increase of LnAlO₃ content, the dielectric constant decreased due to the smaller ionic polarizability of LnAlO₃ than Ca_0.85Nd_0.1TiO₃. The temperature coefficient of the resonant frequency (TCF) decreased with LnAlO₃ content resulted from an increase of oxygen octahedral distortion.     

The correlation between the structure and the dielectric properties of KxBa1−xGa2−xGe2+xO8 ceramics

A study of K_xBa_1−xGa_2−xGe_2+xO₈ ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P2₁/a ⇔ C2/m phase transition. The temperature of this phase transition decreases with an increase in x, in a similar way to the sintering temperature, which decreases from 1100 °C (x = 0) to 970 °C (x = 1). The temperature of the P2₁/a ⇔ C2/m phase transition is below the sintering temperatures of K_xBa_1−xGa_2−xGe_2+xO₈ (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x (x = 0.4 and 0) remain in the P2₁/a modification over a wide temperature range above the sintering temperature. Compared to the C2/m modification of K_xBa_1−xGa_2−xGe_2+xO₈ (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P2₁/a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q × f values of ∼100,000 GHz (at ∼12 GHz). The temperature coefficient of the resonant frequency is ∼−25 ppm/K, regardless of the composition.     

Enhanced piezoelectric properties of (Na0.53K0.47)(Nb1−xTax)O3 ceramics by Ta substitution

The effects of Ta substitution for B-site Nb in (Na_0.53K_0.47)(Nb_1?xTa_x)O₃ (NKNT) ceramics were investigated in the range of x = 0–0.6. It was found that polymorphic phase transitions (PPT) were significantly influenced by Ta substitution. Transitions among orthorhombic, tetragonal, and cubic phases in sequence with temperature, T_O-T and T_C, respectively, decreased linearly with x. At x = 0.45, T_O-T was reduced to room temperature from 182 °C at x = 0, and subsequently piezoelectric coefficient (d₃₃) at room temperature was enhanced up to 284 pC/N from 120 pC/N at x = 0 due to the coexistence of ferroelectric orthorhombic and tetragonal NKNT phases. With x further increasing beyond x = 0.45, d₃₃ decreased due to there being no orthorhombic but only a tetragonal NKNT phase at room temperature with T_O-T below room temperature.     

Microwave properties and structure of La–Ti–Si–B–O glass-ceramics for applications in GHz electronics

A dielectric bulk glass-ceramic of the La₂O₃–TiO₂–SiO₂–B₂O₃ system is developed which is able to fulfill the requirements for dielectric loading-based mobile communication technologies. It is shown that the given dielectric requirements can be fulfilled by glass-ceramic materials without being dependent on ceramic processing techniques. The material exhibited permittivity values of 20 < ɛ_r < 30, quality factor 2000 GHz < Qf < 10,000 GHz and a temperature coefficient of resonance frequency −100 < τ_f < +180 ppm/K. A zero τ_f material with a Qf value of 9500 GHz and ɛ_r = 21.4 could be achieved at a ceramming temperature T_cer = 870 °C. The material is aimed to provide an alternative to existing, commercially used sintered ceramic materials. Further focus is laid on the investigation of the dominant dielectric loss mechanisms in the GHz frequency range and how they are correlated with the microstructure.     

Effect of Zn2+ substitution on the microwave dielectric properties of LiMgPO4 and the development of a new temperature stable glass free LTCC

The LiMg_(1?x)Zn_xPO₄ ceramics have been prepared by the solid state ceramic route. The LiMg_(1?x)Zn_xPO₄ ceramic retains the orthorhombic structure up to x = 0.2. The compositions with 0.3 ≤ x ≤ 0.8 exist as a mixture of orthorhombic and monoclinic phases. When Mg²⁺ is fully replaced with Zn²⁺ (x = 1.0) complete transition to monoclinic phase occurs. The ceramic with x = 0.1 (LiMg_0.9Zn_0.1PO₄) sintered at 925 °C exhibits low relative permittivity (?_r) of 6.7, high quality factor (Q_u × f) of 99,700 GHz with a temperature coefficient of resonant frequency (τ_f) of ?62 ppm/°C. The slightly large τ_f is adjusted nearly to zero with the addition of TiO₂. LiMg_0.9Zn_0.1PO₄–TiO₂ composite with 0.12 volume fraction TiO₂ sintered at 950 °C shows good microwave dielectric properties: ?_r = 10.1, Q_u × f = 52,900 GHz and τ_f = ?5 ppm/°C. The ceramic is found to be chemically compatible with silver.     

Microwave dielectric characteristics of SrLaGaO4 and SrNdGaO4 ceramics

SrLnGaO₄ (Ln = La and Nd) ceramics with K₂NiF₄ structure were prepared by solid-state reaction approach, and the microwave dielectric properties and microstructures were characterized. The SrLaGaO₄ and SrNdGaO₄ ceramics with minor secondary phase, Sr₃Ga₂O₆, were obtained by sintering at 1250–1350 °C for 3 h, and good microwave dielectric characteristics were achieved: the ceramics had (1) ɛ = 20.3, Q × f = 16,219 GHz, and τ_f = −33.5 ppm/°C for SrLaGaO₄; and (2) ɛ = 21.4, Q × f = 16,650 GHz, and τ_f = 7.1 ppm/°C for SrNdGaO₄.     

Electrical properties of [Li0.04(K0.5Na0.5)0.96−xAgx](Nb1−ySby)O3 ceramics

Dependence of electrical properties on the structural characteristics of Li_0.04(K_0.5Na_0.5)_0.96(Nb_1?ySb_y)O₃ (LKNNS (x = 0, 0.00 ≤ y ≤ 0.10)) and [Li_0.04(K_0.5Na_0.5)_0.96?xAg_x](Nb_0.925Sb_0.075)O₃ (LKNANS (0.01 ≤ x ≤ 0.05, y = 0.075)) were investigated. The oxygen octahedral distortion was dependent on Ag⁺ and/or Sb⁵⁺ content which affected to the phase transition temperature of LKNNS and LKNANS ceramics. The orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures (T_O–T, T_C) of the specimens were decreased with increasing of average octahedral distortion. With increasing of Sb⁵⁺ content, the electromechanical coupling factor (k_p), piezoelectric constant (d₃₃) and dielectric constant (?_r) of the sintered specimens were increased up to y = 0.075, and then decreased. These results could be attributed to the shift of T_O–T to near room temperature for Li_0.04(K_0.5Na_0.5)_0.96(Nb_0.0925Sb_0.075)O₃.     

Phase transitions and microwave dielectric properties of Bi3NbO7 ceramics with Bi4B2O9 addition

The effects of Bi₄B₂O₉ on the phase transitions, sinterability and microwave dielectric properties of Bi₃NbO₇ ceramics were investigated. Densities around 96% theoretical could be achieved at 900 °C for samples with up to 20 wt% Bi₄B₂O₉ addition. Phase transitions of cubic→tetragonal→cubic with the increase of sintering temperature were observed for the samples with Bi₄B₂O₉ addition. Moreover, the Bi₄B₂O₉ addition effectively accelerated the phase transition from cubic Bi₃NbO₇ to tetragonal Bi₃NbO₇. Bi₄B₂O₉ addition and the sintering temperature significantly affected the microwave dielectric properties mainly due to the phase transitions. When 20 wt% Bi₄B₂O₉ was added, a dense ceramic could be sintered at 900 °C with relative permittivity ε_r=79, microwave quality factor Qf₀=1010 GHz, and temperature coefficient of resonance frequency τ_f=+8 ppm/°C, which makes it a promising candidate for LTCC applications.     

Microwave dielectric properties of BiNbO4 ceramics with CuO–V2O5 addition

BiNbO₄ ceramics were developed by using CuO–V₂O₅ as a liquid phase sintering agent. The resultant dielectric properties were analyzed in terms of the densification and the amount of CuO–V₂O₅ sintering agent. The addition of 0.8 wt.% CuO–V₂O₅ as its sintering agent was observed to perform most satisfactory. At 850 °C, uniform and enhanced microstructure was observed for the BiNbO₄ specimen with 0.8 wt.% CuO–V₂O₅ addition. Furthermore, the effect of CuO–V₂O₅ addition on the microwave dielectric properties of BiNbO₄ was also investigated. As the sintering temperature increased to 900 °C, the dielectric constant increased but nearly constant and the quality factor (QF) showed a maximum at 850 °C and then decreased for all compositions of the 900 °C sintered specimens. With an increase in CuO–V₂O₅ content, the temperature coefficient of frequency (TCF) increased in accordance with the dielectric mixing rule and microstructural behavior.