Preparation,structure and microwave dielectric properties of 3MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–3TiO<Subscript>2</Subscript> ceramics

3MgO–Al₂O₃–3TiO₂ (MAT) ceramics were prepared by a conventional solid-state reaction method. The crystal structure, sintering behavior and microwave dielectric properties of ceramics were investigated using X-ray diffraction, scanning electron microscopy and network analyzer. MAT ceramics contained the coexistence of three phases, including MgAl₂O₄, MgTiO₃ and MgTi₂O₅. The ceramics sintered at 1350 °C for 4 h presented excellent comprehensive performances with relative permittivity (ε _r ) of 15.4, quality factor (Q × f) of 91,000 GHz and temperature coefficient of resonant frequency (τ _f ) about ?55.1 ppm/°C.     

Enhanced microwave dielectric properties of Ba<Subscript>0.40</Subscript>Sr<Subscript>0.60</Subscript>TiO<Subscript>3</Subscript>–Zr<Subscript>0.80</Subscript>Sn<Subscript>0.20</Subscript>TiO<Subscript>4</Subscript> composite ceramics

The (1−x) Ba_0.40Sr_0.60TiO₃ (BST)−xZr_0.80Sn_0.20TiO₄ (ZST) composite ceramics with x = 10, 20, 30, and 40 wt% were fabricated by conventional solid-state reaction method. With increasing of ZST content, the dielectric constant of composite ceramics was decreased and dielectric loss increases. The effect of ZnO addition to 70 wt% BST–30 wt% ZST composition on the microstructure and dielectric properties was investigated. The improvements in dielectric constant, dielectric loss, and microwave dielectric properties of composite ceramics can be achieved by ZnO addition. The sample with 98 wt% (70 wt% BST–30 wt% ZST)–2 wt%ZnO composition exhibits promising dielectric properties, with dielectric constant, loss tangent and tunability at 4 kV/mm, of 125, 0.0016 and 12%, at 10 kHz and room temperature. At ~2 GHz, it possesses a dielectric constant of 101 and a Q factor of 187, which makes it a good candidate for tunable microwave device applications.     

Influence of Bi substitution on microwave dielectric properties of BaO–La<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sm<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> ceramics

The influences of Bi substitution on microwave dielectric properties of Ba₄(La_0.5Sm_0.5)_9.33Ti₁₈O₅₄ solid solutions were investigated. Dielectric ceramics with general formula Ba₄(La_(0.5−z)Sm_0.5Bi _z )_9.33Ti₁₈O₅₄, z = 0.0–0.2 were prepared by conventional solid state route. The structural analysis of all the samples was carried out by X-ray diffraction and scanning electron microscopy. The dielectric properties were investigated as a function of Bi contents using open-ended coaxial probe method in the frequency range 0.3–3.0 GHz at room temperature. Dielectric constant varies from 83 to 88 and loss tangent from 2.1 × 10⁻³ to 5.5 × 10⁻³ at 3 GHz with temperature coefficient of resonant frequency changing from 106.7 to −8.4 ppm/oC as Bi contents increases from z = 0.00–0.20. It has been found that dielectric constant and temperature coefficient of resonant frequency improve whereas loss tangent is adversely affected with increase in Bi substitution.     

Microwave dielectric properties of Sr<Subscript>0.7</Subscript>Ce<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript>–Sr(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> ceramics

xSr_0.7Ce_0.2TiO₃–(1???x)Sr(Mg_1/3Nb_2/3)O₃ ceramics, referred to xSCT–(1???x)SMN, were successfully produced by conventional solid-state sintered technology. The compounds, belonging to perovskites with a secondary phase of CeO₂, can be detected even with x down to 0.1 of SCT composition. The overall trend for grain growth illustrates the increase with increasing SCT doping level. The Raman peak at 825 cm^?1 splits into two peaks and causes red shift phenomenon. XPS spectra indicate that Ti and Nb ions exist respectively in tetravalence and pentavalence, and Ce ions exist in trivalence and tetravalence. Dielectrics constant (ε _r ) of SCT–SMN ceramics gradually increases with increasing theoretical dielectric polarizabilities. A wider width of the 825 cm^?1 for FWHM of A_1g mode Raman peaks suggests to a lower Q?×?f value. The increasing tolerance factor in agreement with temperature coefficient of resonant frequency (τ _f ), denotes that the rise of perovskite symmetry. The 0.1SCT–0.9SMN ceramic sintered at 1450?°C for 4 h illustrates excellent microwave dielectric properties with ε _r ?~?35.4, Q?×?f?~?11282 GHz and τ _f ?~?1.7 ppm/°C. Activation energies of 0.1SCT–0.9SMN ceramic at 100, 300 and 500 V, are ~0.436, 0.427 and 0.331 eV, respectively, indicative of a decreased trend with external electric field.     

Sintering behaviors and microwave dielectric properties of Ti-modified Ba<Subscript>3</Subscript>Ti<Subscript>5</Subscript>Nb<Subscript>6</Subscript>O<Subscript>28</Subscript> ceramics with 35BaO–35ZnO–30B<Subscript>2</Subscript>O<Subscript>3</Subscript> addition

Effect of 35BaO–35ZnO–30B₂O₃ (BZB) addition on the sintering behaviors, phase evolution and microwave dielectric properties of Ti-modified Ba₃Ti₅Nb₆O₂₈ (Ba₃Ti_5.1Nb_5.9O_27.95, BTNO) ceramics had been investigated. BZB addition effectively reduced the sintering temperature of BTNO from 1250 °C to about 900 °C. With increasing BZB addition, the crystal phase of the present ceramics changed from single phase Ba₃Ti₅Nb₆O₂₈ to mixing phases of Ba₃Ti₅Nb₆O₂₈ and Ba₃Ti₄Nb₄O₂₁. And the major phase gradually became from Ba₃Ti₅Nb₆O₂₈ to Ba₃Ti₄Nb₄O₂₁. Dielectric constant and temperature coefficient increased with the rising of BZB addition. But the Qf value gradually declined from 28000 to about 6000 GHz. The BTNO ceramics with 15% BZB addition exhibited excellent microwave dielectric properties as following: ε?=?46.3, Qf?=?5887 GHz and τ_f?=?35.5 ppm/°C, which was potential of candidate materials for LTCC application. And the variation of microwave dielectric properties had been also discussed with the phase and microstructure evolution.     

Effect of ZnO/WO<Subscript>3</Subscript> additives on sintering behavior and microwave dielectric properties of (Sr,Ca)TiO<Subscript>3</Subscript>–(Sm,Nd)AlO<Subscript>3</Subscript> ceramics

The effect of ZnO/WO₃ additives on phase composition, microstructures, sintering behavior, and microwave dielectric properties of 0.7(Sr_0.01Ca_0.99)TiO₃–0.3(Sm_0.75Nd_0.25)AlO₃ (7SCT–3SNA) ceramics prepared via conventional solid-state route were systematically investigated. All the samples exhibited pure perovskite structures, and Ti⁴⁺ ions could be substituted by W⁶⁺ ions. While further increasing WO₃ additives, the W⁶⁺ ions migrated into the lattice. The τ _f values of samples first became more positive, and then tended to move toward negative direction with increasing WO₃ addition. Moderate ZnO/WO₃ additives not only effectively reduced the sintering temperature from 1500 to 1330 °C but also improved the dielectric properties of 7SCT–3SNA ceramics. The 0.50 wt% ZnO doped 7SCT–3SNA sample with 1.00 wt% of WO₃, sintered at 1330 °C for 4 h, was measured to show optimum microwave dielectric properties, with an ε _r of 45.12, a Q?×?f value of 51200 GHz (at 5.4 GHz), and τ _f value of +?2.68 ppm/°C.     

Frequency dependence of dielectric properties of CaO–SrO–Li<Subscript>2</Subscript>O–Ln<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiO<Subscript>2</Subscript> ceramics

CaO–SrO–Li₂O–Ln₂O₃–TiO₂ ceramics were prepared by solid state reaction method, where Ln₂O₃ consists of equal proportions of Nd₂O₃ and Sm₂O₃. Dielectric properties and crystal structure were investigated with respect to the content of TiO₂. Single phase with an orthorhombic perovskites structure was formed within the composition range of investigation. The frequency dependence of dielectric properties of the present ceramics was extensively investigated. Dielectric constant was less sensitive to frequency. However, dielectric loss and temperature coefficients were both very sensitive to frequency and gradually decreased with increasing frequency, such as the variation was more than ten times between 1 MHz and several GHz. The relationship between the temperature coefficient and dielectric loss was also discussed at different frequencies. And the mechanism of the frequency dependence was discussed in term of the role of Li ions.     

Phase formation and dielectric properties of ceramics in the BiFeO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript>–Bi(Mg<Subscript>0.5</Subscript>Ti<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> system

We have studied the effect of Bi(Mg_0.5Ti_0.5)O₃ additions on the phase formation, structural parameters, microstructure, and dielectric properties of solid solutions in the region of a morphotropic phase boundary in the BiFeO₃–BaTiO₃ system. Single-phase samples with the perovskite structure have been obtained and the addition of Bi(Mg_0.5Ti_0.5)O₃ has been shown to raise the Curie temperature of the ceramics and improve their dielectric properties.     

Microstructure,dielectric and photoluminescence properties of Tm-doped Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3 </Subscript>thin films fabricated by sol–gel method

Ba_0.8Sr_0.2TiO₃ thin films doped by Tm from 0 to 7 mol% were fabricated by sol–gel method on silicon and Pt/Ti/SiO₂/Si substrates. X-ray diffraction, scanning electron microscopy, and Raman spectroscopy have been used to study variations of crystal structure, surface morphologies, and phase stability of Tm-doped BST films, respectively. The residual stress in BST films on silicon substrates can be reduced by Tm doping, as demonstrated by the blueshift of phonon peaks in Raman spectra. The dielectric measurements were conducted on metal-insulator-metal capacitors at the frequency from 1 kHz to 1 MHz. The grain size and dielectric constant decreased with increasing Tm concentration. While the variation of dielectric loss, tunability and the figure of merit were nonlinear with increasing Tm concentration. In addition, the photoluminescence property of 0.2 mol% Tm-doped BST was also studied. The effect of Tm doping on the microstructure, dielectric and photoluminescence properties were analyzed.     

Microwave dielectric properties of temperature-stable (Mg<Subscript>0.95</Subscript>Co<Subscript>0.05</Subscript>)<Subscript>2</Subscript>TiO<Subscript>4</Subscript>–Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> composite ceramics for LTCC applications

In this paper, the effects of Li₂O–B₂O₃–Bi₂O₃–SiO₂ (LBBS) glass on the phase formation, sintering characteristic, the microstructure and microwave dielectric properties of temperature-stable (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ ceramics were investigated. (Mg_0.95Co_0.05)₂TiO₄–Li₂TiO₃ powders were obtained by using the traditional solid-state process. A small amount of LBBS doping can effectively reduce sintering temperature and promote the densification of the ceramics. X-ray diffraction analysis revealed not only the primary phase (Mg·Co)₂TiO₄ associated with Li₂TiO₃ minor phase but also a third phase (Mg·Co)TiO₃. The dielectric constant and Qf values vary with the doping amount of LBBS and sintering temperatures. With the compensation of the positive temperature coefficient (τ _f ) of Li₂TiO₃ and the negative τ _f of (Mg_0.95Co_0.05)₂TiO₄, the τ _f of the specimens fluctuates around zero. The (Mg_0.95Co_0.05)₂TiO₄ ceramic with 2.5 wt% LBBS addition and sintering at 900?°C for 4 h exhibited excellent microwave dielectric properties: ? _r ?=?19.076, Qf?=?126100 GHz, and τ _f ?=?0.98 ppm/°C.     

The AC impedance and variable temperature dielectric spectroscopic analysis of MnO<Subscript>2</Subscript> doped and un-doped ZnO–V<Subscript>2</Subscript>O<Subscript>5</Subscript> ceramics

The electrical and dielectric properties of MnO₂ doped and un-doped ZnO–V₂O₅ ceramics were studied by ac impedance and variable temperature dielectric spectroscopy. The results show that V and Mn ions simultaneously segregated at the grain boundaries to form an intergranular phase, increasing the resistivity of the intervening layer and the Schottky barrier at the grain boundaries, and then improving the varistor performance. An obvious loss peak appeared in all the samples, which means an effective depletion layer has formed. As for the samples sintered at 1,000 °C for 2 h, the activation energy of the characteristic relaxation process is about 0.339 eV for 99.5 mol% ZnO + 0.5 mol% V₂O₅ and 0.352 eV for 99.0 mol% ZnO + 0.5 mol% V₂O₅ + 0.5 mol% MnO₂, respectively, which means this relaxation process is associated with oxygen vacancy \textV_\textO ^· {\text{V}_{\text{O}}}^{ \cdot } .     

Micro-structural,dielectric, ferroelectric and piezoelectric properties of mechanically processed (Pb<Subscript>1−x</Subscript>La<Subscript>x</Subscript>)(Zr<Subscript>0.60</Subscript>Ti<Subscript>0.40</Subscript>)O<Subscript>3</Subscript> ceramics

In this work, (Pb_1?xLa_x)(Zr_0.60Ti_0.40)O₃ (PLZT x/60/40, x?=?at.%) ceramics were prepared by using high energy mechanical ball milling followed by cold isostatic pressing (CIP), investigated for their micro-structural, dielectirc, ferroelectric and piezoelectric properties. Mechanical activation results in the highly reactive nature of the nano size milled PLZT powders, which enables the partial perovskite phase formation, confirmed by room temperature XRD patterns. CIP leads to a higher density with a closely packed dense microstructure of sintered PLZT ceramics shown in SEM images. The grain size of PLZT x/60/40 ceramics was found to be decreasing with increasing La³⁺ content. The highest relative density of ~?97% was found to be for PLZT 8/60/40 ceramics with grain size of ~?1.35 µm. The PLZT 8/60/40 system also shows the highest dielectric constant of ~?1976, remnant polarization of 29.1 µC/cm², piezoelectric coefficients (d₃₃?\(~ \cong ~\)?570 pC/N, g₃₃?\(~ \cong ~\)?28.03?×?10^?3 Vm/N) and electromechanical coupling factors (k_p?=?k₃₃?=?64.1% and k₃₁?=?54%). The elastic compliances for the PLZT x/60/40 ceramics were also obtained.     

Compositional dependence of the structural and dielectric properties of Li<Subscript>2</Subscript>O–GeO<Subscript>2</Subscript>–ZnO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

(10Li₂O–20GeO₂–30ZnO–(40-x)Bi₂O₃–xFe₂O₃ where x = 0.0, 3, 6, and 9 mol%) glasses were prepared. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, DC and AC conductivities, and dielectric properties (constant ε′, loss tan δ, AC conductivity, σ _ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of iron ion concentration. The analysis of the results indicate that, the density and molar volume decrease with an increasing of iron content indicates structural changes of the glass matrix. The glass transition temperature T _g and onset of crystallization temperature T _x increase with the variation of concentration of Fe₂O₃ referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter ΔT decrease with increase Fe₂O₃ content, indicates an increasing concentration of iron ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BiO₃, BiO₆, ZnO₄, GeO₄, and GeO₆. The structural changes observed by varying the Fe₂O₃ content in these glasses and evidenced by FTIR investigation suggest that the iron ions play a network modifier role in these glasses while Bi₂O₃, GeO₂, and ZnO play the role of network formers. The temperature dependence of DC and AC conductivities at different frequencies was analyzed using Mott’s small polaron hopping model and, the high temperature activation energies have been estimated and discussed. The dielectric constant and dielectric loss increased with increase in temperature and Fe₂O₃ content.     

Effect of Dy<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on microstructure,electrical properties,and aging behavior of ZnO–V<Subscript>2</Subscript>O<Subscript>5</Subscript>–MnO<Subscript>2</Subscript>–CoO varistor ceramics

The microstructure, electrical properties, and aging behavior of ZnO–V₂O₅–MnO₂–CoO–Dy₂O₃ varistor ceramics were investigated for different contents of Dy₂O₃. The microstructure consisted of ZnO grain as a main phase and secondary phases such as Zn₃(VO₄)₂, ZnV₂O₄, and DyVO₄. The average grain size increased from 7.6 to 10.1 μm and the sintered density slightly increased from 5.53 to 5.57 g/cm³ with the increase of Dy₂O₃ content. The varistor ceramics added with 0.05 mol% Dy₂O₃ exhibited the most nonlinear properties, with nonlinear coefficient of 30, and the highest stability against DC-accelerated aging stress. The Dy₂O₃ acted as an acceptor due to the decrease of donor density in the range of 2.73 × 10¹⁸/cm³ to 1.28 × 10¹⁸/cm³.     

Mechanical and acid-etching properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZnO–B<Subscript>2</Subscript>O<Subscript>3</Subscript> glass-containing ceramic fillers

The properties of the composite, having a complicated microstructure, are decided by many factors such as those of glass matrix, crystal phases, fillers, and holes. We investigated how the addition of ceramic fillers to the glass matrix affects the mechanical and etching properties of the glass composite by forming new crystal phases. Different amounts of two fillers, ZnO and Al₂O₃, were added to a glass frit consisting of Bi₂O₃–ZnO–B₂O₃. It was sintered at 550 °C for 30 min. Based on the results of this study, the porosity and degree of crystallization of the composites could be controlled by adjusting the content of the ZnO and Al₂O₃ fillers. Therefore, porosity and degree of crystallization formed by the reaction between a glass matrix and fillers influence the mechanical and etching properties of the composite.     

Growth and structure of Mg(Fe<Subscript>0.8</Subscript>Ga<Subscript>0.2</Subscript>)<Subscript>2</Subscript>O<Subscript>4 − δ</Subscript> films

Films 150–200 nm in thickness, with the nominal composition Mg(Fe_0.8Ga_0.2)₂O_{4 − δ} have been grown on (100) single-crystal silicon substrates by ion-beam sputtering in vacuum. The effect of growth and annealing conditions on the crystal structure and morphology of the films has been studied, and the thermal conditions for the growth of spinel-structure films have been optimized.     

Spectroscopic characterisation of local structure in Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses doped with gadolinium

Gadolinium doped bismuth borate glasses containing up to 30 mol% Y₂O₃ were prepared by fast melt quenching method. The effect of yttrium on the local order in 3B₂O₃ · Bi₂O₃ and B₂O₃ · Bi₂O₃ glass matrices, particularly on the bismuth sites, was investigated by infrared (IR) spectroscopy and electron paramagnetic resonance (EPR) of Gd³⁺ ions. The IR results show that the local structure is more ordered in the glass system with higher bismuth content and the progressive addition of yttrium increases the local disorder in both bismuth–borate glass matrices. The EPR results indicate that Gd³⁺ ions occupy both bismuth and yttrium sites and reflect the same structural disorder like that suggested by IR results.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Crystal structure and properties of BaTiO<Subscript>3</Subscript>–(Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript> ceramic system

(1 − x)BaTiO₃–x(Bi_0.5Na_0.5)TiO₃ (x ranged from 0.01 to 0.96) ceramics were fabricated by the conventional ceramic technique. The crystal structure, as well as dielectric and piezoelectric properties of the ceramics were studied. All the ceramics formed single-phase solid solutions with perovskite structure after sintering in air at 1150–1250 °C for 2–4 h. The crystal structure and microstructure varied gradually with the increase of (Bi_0.5Na_0.5)TiO₃ (BNT) content. The Curie temperature, T _c, shifted monotonously to high temperature as BNT increased. The ceramics with 20–90 mol% BNT had relatively low and stable dielectric loss characteristics. The piezoelectric constant, d ₃₃, enhanced with the increase of BNT content through a maximum value in a composition of 93 mol% BNT and then tended to decrease. The maximum value, 148 pC/N, of piezoelectric constant d ₃₃ together with the electromechanical coupling factors, k _t, 19.8% and k _p, 15.8%, were obtained when BNT was 93 mol%.