Synthesis,and measurement of structural and magnetic properties,of La<Subscript>1−x</Subscript>Cd<Subscript>x</Subscript>CoO<Subscript>3</Subscript> perovskite ceramic oxides

Perovskite-type Cd-doped LaCoO₃ materials were synthesized by a simple solution-based combustion process. The synthesized materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflectance (DR) in the UV-VIS, and magnetic property measurements. The parent LaCoO₃ compound showed spin-glass transition at low temperatures, and with progressive Cd doping, showed transition to paramagnetic ordering. The changes in magnetic properties of the materials are correlated to the changes in structural features resulting from the Rietveld structural refinement of the materials.     

Large signal electrical property of CuO-doped of a Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>–SrTiO<Subscript>3</Subscript>

This report describes the sintering and ferroelectric characteristic of 0.76 Bi_0.5Na_0.5TiO₃ (BNT)–0.26SrTiO₃ (ST) with CuO addition, synthesized by using a solid-state reaction. The XRD, microstructure, and density results reveal that Cu element reduces the sintering temperature from 1150 to 1000 °C. In dielectric study, CuO addition induces the increase of relaxation behavior for 26ST ceramic in the high temperature regime. In the low temperature regime (< 150 °C), the Cu doping to 26ST accelerates the degree in freezing of dipole, presumably due to the interaction between A site vacancy and nanodomain boundaries. The electromechanical properties of 26ST ceramics with CuO doping show a significant decay in frequency dependence of polarization and strain, which might be related to the formation of A-site cation vacancy.     

Structural,electrical, and multiferroic properties of Aurivillius Bi<Subscript>6</Subscript>Fe<Subscript>2</Subscript>(Ti<Subscript>3-x</Subscript>V<Subscript>x</Subscript>)O<Subscript>18+δ</Subscript> thin films prepared by chemical solution deposition

Aurivillius type five-layered V-doped Bi₆Fe₂(Ti_3-xV_x)O_18+δ (x = 0.00, 0.03, 0.06, and 0.09) thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. All the thin films were crystallized in Aurivillius structures, which have been confirmed by using X-ray diffraction and Raman spectroscopy studies. On comparing the thin films, the Bi₆Fe₂(Ti_2.94V_0.06)O_18+δ thin film showed the most enhanced electrical and multiferroic properties, with a leakage current density value about four orders of magnitude lower than that of the Bi₆Fe₂Ti₃O₁₈ thin film, for example. All of these thin films showed anti-ferromagnetic hysteresis loops at room temperature. The significant decrease in the concentration of oxygen vacancies and the formation of a stable structure caused by doping with the donor V⁵⁺-ion are related to the improved properties in the V-doped Bi₆Fe₂(Ti_3-xV_x)O_18+δ thin films.     

Preparation and characterization of negative temperature coefficient (Ni,Mn)<Subscript>3</Subscript>O<Subscript>4</Subscript>–La(Mn,Ni)O<Subscript>3</Subscript> composite

The composites made of spinel-structured (Ni,Mn)₃O₄ and perovskite-structured La(Mn,Ni)O₃ were investigated for potential application as negative temperature coefficient (NTC) thermistor. The composites were prepared using the standard ceramic route. The electrical resistivity of the composite at 25°C was found to decrease by one to two orders of magnitude depending the amount of the low-resistivity perovskite phase, while the thermal constant determining the temperature sensitivity of the NTC thermistor was still reasonably large in the range of 4,000 to 3,000 K, and the resistivity drift after annealing at 150°C for 1,000 h in air was relatively small (∼1.2%). The general effective media model was adopted to fit the electrical resistivity data of the composites, giving a value of 0.37 for the percolation volume fraction of the perovskite phase. This work demonstrates that it is possible to tune the electrical resistivity and thermal constant of the spinel-structured oxide through making composite with low-resistivity perovskite-structured oxide.     

Study of bulk and grain-boundary conductivity of Ln<Subscript>2+x</Subscript>Hf<Subscript>2−x</Subscript>O<Subscript>7−δ</Subscript> (Ln = Sm-Gd; x = 0, 0.096) pyrochlores

The electrical conductivity of new solid electrolytes Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ have been compared with those for different pyrochlores including titanates and zirconates Ln_2+xМ_2−xO_7−δ (Ln = Sm-Lu; M = Ti, Zr; x = 0−0.81). Impedance spectroscopy data demonstrate that Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ synthesized from mechanically activated oxides have high ionic conductivity, comparable to that of their zirconate analogues. The bulk and grain-boundary components of conductivity in Sm_2.096Hf_1.904O_6.952 (Т_synth = 1600oС), Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ (Т_synth = 1670oС) have been determined. The highest bulk conductivity is offered by the disordered pyrochlores prepared at 1600oC and 1670oC: ~1.5 × 10⁻⁴ S/cm for Sm_2.096Hf_1.904O_6.952, 5 × 10⁻³ S/cm for Eu_2.096Hf_1.904O_6.952 and 3 × 10⁻³ S/cm for Gd₂Hf₂O₇ at 780oС, respectively. The conductivity of the fluorite-like phases at the phase boundaries of the Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x ~ 0.286) solid solutions, as well as that of the high-temperature fluorite-like phases Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.286), is lower than the conductivity of the disordered pyrochlores Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.096).     

The effect of sintering temperature and time on the growth of single crystals of 0.75 (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)TiO<Subscript>3</Subscript>–0.25 SrTiO<Subscript>3</Subscript> by solid state crystal growth

Materials in the (Na_0.5Bi_0.5)TiO₃–SrTiO₃ system are of interest for use as lead-free piezoelectric actuators due to high electric-field induced strains. Piezoelectric properties may be further improved by growing single crystals but as yet work on single crystal growth in this system is limited. In the present work, single crystals of composition 0.75 (Na_0.5Bi_0.5)TiO_3?0.25 SrTiO₃ were grown by solid state crystal growth (SSCG) on [001] SrTiO₃ seed crystals and the dependence of crystal growth distance and matrix grain growth on sintering temperature investigated. Electron backscattered diffraction and X-ray diffraction analysis show that the single crystals grow epitaxially on the seed crystals. Energy dispersive spectroscopy indicates that the grown crystals are slightly Na-deficient, while X-ray photoelectron spectroscopy indicates the presence of oxygen vacancies. Single crystal growth distance, mean matrix grain size and grain size distribution as a function of sintering temperature and time are presented. Increasing the sintering temperature increases both single crystal and matrix grain growth rates. The optimum single crystal growth temperature is found to be 1250°C. The effect of sintering temperature on the single crystal and matrix grain growth behavior is explained using the mixed control mechanism of microstructural evolution.     

Electromechanical properties of ternary BiFeO<Subscript>3</Subscript>−0.35BaTiO<Subscript>3</Subscript>–BiGaO<Subscript>3</Subscript> piezoelectric ceramics

In the present work, composition dependent crystal structure, ferroelectric, piezoelectric, and temperature dependent dielectric properties of the BiGaO₃-modified (1–x)(0.65Bi_1.05FeO₃–0.35BaTiO₃) (BFBT35–xBG, where x?=?0.00–0.03) lead-free ceramics were systematically investigated by solid-state reaction method, followed by water quenching process. The substitution of BG successfully diffuses into the lattice of the BFBT ceramics, without changing the pseudo-cubic structure of the samples. The scanning electron microscopy (SEM) results revealed that the average grain size was increased with BG-content in BFBT system. The BFBT–xBG ceramics showed a maximum in permittivity (?_max) at temperatures (T_max) above 500 °C in the compositional range of 0.00?≤?x?≤?0.03. The electro-strain is measured to be 0.125% (d^*₃₃ ~ 250 pm/V) under unipolar fields (5 kV/mm) for BFBT–0.01BG ceramics. The same composition (x?=?0.01), large static piezoelectric constant (d₃₃ ~ 165 pC/N) and electromechanical coupling factor (k_p ~ 25%) were obtained. The above investigated characterizations suggests that BFBT–BG material is favorable for piezoelectric and high temperature applications.     

Control of Microwave Dielectric Properties in the System BaO ⋅ Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>—BaO ⋅ Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ⋅ 4TiO<Subscript>2</Subscript>

BaO ⋅ Nd₂O₃ ⋅ 4TiO₂—based ceramics were prepared by the mixed oxide route. Specimens were sintered at temperatures in the range 1200–1450^∘C. Microstructures were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microwave dielectric properties were determined at 3 GHz by the Hakki and Coleman method. Product densities were at least 95% theoretical. The addition of up to 1 wt% Al₂O₃ to the starting mixtures reduced the sintering temperatures by at least 100^∘C. Incorporation of small levels of Al into the structure (initially Ti sites) led to an increase in Q × f values, from 6200 to 7000 GHz, a decrease in relative permittivity (ε_r) from 88 to 78, and moved the temperature coefficient of resonant frequency (τ_f) towards zero. The addition of 0.5 wt% Al₂O₃ with 8 wt% Bi₂O₃ improved densification, increased both ε_r (to 88) and Q× f (to 8000 GHz) and moved τ_f closer to zero. Ceramics in the system (1 − x)BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + xBaO ⋅ Al₂O₃ ⋅ 4TiO₂ exhibited very limited solid solubility. The end member BaO ⋅ Al₂O₃ ⋅ 4TiO₂ was tetragonal in structure with the following dielectric properties: ε_r = 35; Q× f = 5000 GHz; τ_f = −15ppm/^∘C. Microstructures of the mixed Nd-Al compositions contained two distinct phases, Nd-rich needle-like grains and large Al-rich, lath-shaped grains. Products with near zero τ_f were achieved at compositions of approximately 0.14BaO ⋅ Nd₂O₃ ⋅ 4TiO₂ + 0.86BaO ⋅ Al₂O₃ ⋅ 4TiO₂, where Q× f = 8200 GHz and ε_r = 71.     

Dielectric and piezoelectric properties of Bi<Subscript>1/2</Subscript>Na<Subscript>1/2</Subscript>TiO<Subscript>3</Subscript>–SrTiO<Subscript>3</Subscript> lead–free ceramics

This study investigated the microstructure, crystal structure, and electrical properties of (1???x)Bi_1/2Na_1/2TiO₃–xSrTiO₃ (BNST100x; x?=?0.20, 0.22, 0.24, 0.26, 0.28, and 0.30) lead?free piezoceramics. The average grain size of BNST100x ceramics decreased with increasing SrTiO₃ content. A phase transition from nonergodic relaxor (NER) to ergodic relaxor (ER) was observed at x?=?0.26, and the highest unipolar strain under 4 kV/mm electric field, of 0.25% (d₃₃^*?≈?620 pm/V), was obtained at x?=?0.28. We found that the BNST26 and BNST28 compositions yielded the competitive advantage of larger strain values under lower operating fields compared with other BNT–based lead–free piezoelectric ceramics. Therefore, we regard these ceramics as promising candidates for actuator applications.     

Low temperature sintering of lead–free (Bi<Subscript>1/2</Subscript>Na<Subscript>1/2</Subscript>)TiO<Subscript>3</Subscript>-SrTiO<Subscript>3</Subscript>-BiFeO<Subscript>3</Subscript> piezoelectric ceramics by adding excess CuO

This study examined the microstructures, crystal structures, and electrical properties of 0.01 mol CuO–added (1–x)(Bi_1/2Na_1/2)TiO₃–xSrTiO₃–2BiFeO₃ (BNST100x–2BF, x?=?0.20 ~ 0.28) ceramics synthesized at two different sintering temperatures. The sintering temperature of the BNST100x–2BF ceramics could be decreased from 1175 °C to 1000 °C by adding a 0.01 mol CuO excess. Low–temperature sintering led to a decrease in average grain size. The dielectrics, polarization hysteresis (P–E), switching current, and electric–field induced strain (S–E) curves changed with increasing SrTiO₃ content and decreasing sintering temperature. Interestingly, the highest reduction ratio of d₃₃^* was calculated to be somewhere in between the high–temperature sintered and low–temperature sintered BNST26–2BF ceramics. These results were attributed to the difference in the stabilized relaxor state and closely related to the electric field–induced reversible phase transition from the relaxor and ferroelectrics.     

Lead-free K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>–Bi<Subscript>0.5</Subscript>Li<Subscript>0.5</Subscript>ZrO<Subscript>3</Subscript>–BiAlO<Subscript>3</Subscript> ternary ceramics: Structure and piezoelectric properties

Lead-free perovskite (0.995–x)(K_0.5Na_0.5)NbO₃–x(Bi_0.5Li_0.5)ZrO₃–0.005BiAlO₃ ternary piezoelectric ceramics were projected and prepared by a conventional solid-state method. A research was conducted on the effects of (Bi_0.5Li_0.5)ZrO₃ content on the structure and piezoelectric properties of the ceramics. By combining the X-ray diffraction patterns with the temperature dependence of dielectric properties, a rhombohedral–orthorhombic–tetragonal phase coexistence was identified for the ceramics with 0.02 ≤ x ≤ 0.025, and a rhombohedral–tetragonal phase boundary was determined in the composition x = 0.03. Upon further increasing the (Bi_0.5Li_0.5)ZrO₃ content, the rhombohedral–tetragonal phase boundary transformed to a single rhombohedral structure with x ≥ 0.035. An obviously improved piezoelectric activity was obtained for the ceramics with compositions in and around the rhombohedral–tetragonal phase boundary, among which the composition x = 0.025 exhibited the maximum values of piezoelectric constant d ₃₃, and planar and thickness electromechanical coupling coefficients (k _p and k _t), of 252 pC/N, 0.366, and 0.466, respectively. In addition, the ceramic with x = 0.025 was found to possess a relatively high Curie temperature of 368 °C, suggesting it may have a prospect for applications at elevated ambient temperatures.     

Electrical and mechanical properties of MgO added 0.5Ba(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript>–0.5(Ba<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>)TiO<Subscript>3</Subscript> (BZT–0.5BCT) composite ceramics

MgO (0–2.0 vol%) added Ba(Zr_0.2Ti_0.8)O₃–0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT) ceramics have been prepared by the conventional solid-state reaction method. The effects of MgO powder on the phase formation, densification, dielectric, piezoelectric and mechanical properties (flexural strength, hardness) of the BZT-0.5BCT ceramics have been studied systematically. The synthesized powder could be densified to 97 % of true density at a temperature of 1350 °C. The MgO addition also provided materials with better mechanical properties. The most interesting aspect of MgO added samples is their relative permittivity vs. temperature response. MgO additions effectively suppress the relative permittivity around phase transition temperature. The aging rate of d₃₃ observed for BZT-0.5BCT is 14 %/decade. MgO addition reduces the ageing rate and for 1 vol% MgO added, BZT-0.5BCT shows aging rate of 3 %/decade. BZT-0.5BCT/MgO ceramics possesses good mechanical properties viz., flexural strength 93 MPa, which is almost 25 % higher than that of monolithic BZT-0.5BCT (73 MPa).     

A first-principles study of the structural,elastic, electronic,vibrational, and optical properties of BaSe<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The structural, elastic, electronic, vibrational, and optical properties of BaSe_1?xTe_x alloys are investigated by means of the full-potential linearized augmented plane wave method. The exchange–correlation effects are treated with the local density approximation, as well as the GGA-PBE, GGA-PBEsol, and GGA?+?mBJ schemes of the generalized gradient approximation. Ternary BaSe_1?xTe_x compounds have not yet been synthesized. Improved predictions of the structural parameters are obtained using the GGA-PBEsol approach. Calculations of the electronic and optical properties with the GGA?+?mBJ approach yield accurate results. Ternary BaSe_1?xTe_x alloys are wide-band-gap semiconductors with a direct gap Γ–Γ. The upper valence band is mainly due to Se p and Te p states, while the bottom of the conduction band results essentially from Ba d states. The dielectric function, refractive index, reflectivity, absorption coefficient, and energy-loss function are calculated in the range 0–35 eV. The increase in x gives rise to a redshift of the optical spectra. BaSe_1?xTe_x alloys exhibit reflective properties of metals in some energy ranges. The static dielectric constant ?₁(0) and the static refractive index n₀ are calculated. The investigation of the elastic and vibrational properties shows that ternary BaSe_1?xTe_x should be mechanically and dynamically stable, elastically anisotropic, brittle, and relatively soft.     

Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>3-x/2+δ</Subscript> Part III: Thermal and chemical expansion

The thermal and chemical expansion of a potential solid oxide fuel cell (SOFC) cathode material SrSn_0.65Fe_0.35O_{3–0.35/2+δ} (SSF35) were investigated to assess its thermo-chemo-mechanical stability at SOFC operating temperatures and to establish the correlation between defect concentrations (oxygen vacancies and electrons) and chemical expansion with the aid of the defect chemical model reported in part I of this study. Thermochemical expansion was measured as a function of temperature and oxygen partial pressure. The chemical expansion of SSF35 showed a strong correlation with changes in oxygen nonstoichiometry associated with changes in Fe valence state. Coefficients of both chemical (CCE) and thermal (CTE) expansion were calculated and found to be smaller than that of the closely related mixed conducting perovskite oxide SrTi_0.65Fe_0.35O_{3–0.35/2+δ} (STF35). The thermal expansion coefficient of SSF was found to be close to that of YSZ (most popular solid oxide electrolyte), which makes SSF35 more attractive in terms of overall thermo-chemical stability. The chemical expansion of SSF35 showed decreasing CCE with increasing temperature and decreasing CTE with increasing oxygen deficiency, both opposite to the trends observed for STF35. Distortion in symmetry from the cubic structure seems to be responsible for the smaller coefficients and increasing asymmetry with expansion seems accountable for opposite trends of CCE and CTE compared to the STF counterpart.     

Preparation and electrochemical properties of La<Subscript>1.0</Subscript>Sr<Subscript>1.0</Subscript>FeO<Subscript>4+δ</Subscript> as cathode material for intermediate temperature solid oxide fuel cells

Cathodic material La_1.0Sr_1.0FeO_4+δ for an intermediate temperature solid oxide fuel cell (IT-SOFC) was prepared via the glycine-nitrate process and characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM). XRD results showed that no reaction occurred between the La_1.0Sr_1.0FeO_4+δ electrode and Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1000 °C. SEM results showed that the electrode formed good contact with the SDC electrolyte after sintering at 1000 °C for 2 h. The electrochemical properties of La_1.0Sr_1.0FeO_4+δ were measured using electrochemical impedance spectroscopy (EIS) and steady state polarization measurement. At 700 °C, the polarization resistance was about 3.90 Ωcm², and the lowest polarization overpotential was 57 mV at a current density of 55 mA cm⁻².     

Evolution of ferroelectric and piezoelectric response by heat treatment in pseudocubic BiFeO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript> ceramics

Heat treatment of ceramics is an important process to tailor the fine electromechanical properties. To explore the criteria for optimized heat treatment in a perovskite structure of (1–x)Bi_1.05FeO₃–xBaTiO₃ (BF–BT100x) system, the structural phase relation, ferroelectric and piezoelectric response of BF–BT36 and BF–BT40 ceramics prepared by furnace cooling (FC) and quenching process were investigated. The X-ray diffraction examination showed single pseudocubic perovskite structure for all the ceramics. The homogenous microstructure was obtained for all ceramics with relatively large grain size in the furnace cooled samples. Well saturated ferroelectric hysteresis loops and enhanced piezoelectric constant (d₃₃?=?97 pC/N) were achieved by quenching process. Dielectric curve of BF–BT36 showed large dielectric constant at its Curie temperature, however, BF–BT40 showed diffused relaxor-like dielectric anomalies. Quenched BF–BT36 samples showed typical butterfly like field induced strain curves, however negative strain decreased in BF–BT40 ceramics. From these investigated study, it is observed that BF–BT ceramics are very sensitive to the heat treatment process (furnace cooling and quenching) on the dielectric, electromechanical properties.     

Study of the pyroelectric behavior of BaTi<Subscript>1−<Emphasis Type="BoldItalic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="BoldItalic">x</Emphasis></Subscript>O<Subscript>3</Subscript> piezo-ceramics

In this work, the thermal square wave method at single-frequency (TSWM) was applied to functionally graded BaTi_1−x Sn _x O₃ samples with a tin gradient of 0.075 ≤ x ≤ 0.15. The samples were fabricated by sintering of bi-, tri- and tetramorph green bodies. The polarization at the back side with a higher tin content was in opposite direction to the polarization of the surface with a lower tin content. This was attributed to the appearance of a space charge layer. The determined depth profiles of the pyroelectric coefficient illustrate the appearance of a nearly constant polarization in the region of lower tin concentration of tri- and tetramorph samples. The pyroelectric coefficient profiles were in good agreement with the averaged over sample thickness pyroelectric coefficient obtained separately by the quasistatic method.     

Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn<Subscript>1-x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>3-x/2+δ</Subscript> part II: Electrical conductivity and cathode performance

The bulk electrical conductivity of the mixed ionic-electronic conducting perovskite-structured SrSn_1-xFe_xO_3-x/2+δ (SSF) was measured to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact defect charge carrier density and ultimately electrode performance. These results complement a defect chemical model for SSF investigated and reported in Part I of this study. The electrical properties of SSF were found not to differ significantly from the corresponding composition in SrTi_1-xFe_xO_3-x/2+δ (STF). It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted in energy due to the larger size of Sn, the defect equilibria and therefore the electrical conductivity of SSF were found to be largely dominated by Fe and thus differed only in a limited way from that in STF. Key kinetic parameters obtained include the migration enthalpy of oxygen vacancies (0.772 ± 0.204 eV), the activation energy of area-specific-resistance for oxygen exchange (1.65 ± 0.03 eV) and the magnitudes of electron (0.0002 ± 0.00005 cm²/V?s) and hole (0.0037 ± 0.0015 cm²/V?s) mobilities.     

Effect of mechanical loading on the tuning of acoustic resonances in Ba<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> thin films

The effect of mechanical loading on the tuning performance of a tunable Thin Film Bulk Acoustic Wave Resonator (TFBAR) based on a Ba_0.3Sr_0.7TiO₃ (BST) thin film has been investigated experimentally and theoretically. A membrane-type TFBAR was fabricated by means of micromachining. The mechanical load on the device was increased stepwise by evaporating SiO₂ on the backside of the membrane. The device was electrically characterized after each evaporation step and the results were compared to those obtained from modeling. The device with the smallest mechanical load exhibited a tuning of − 2.4% and − 0.6% for the resonance and antiresonance frequencies at a dc electric field of 615 kV/cm, respectively. With increasing mechanical load a decrease in the tuning performance was observed. This decrease was rather weak if the thickness of the mechanical load was smaller or comparable to the thickness of the active BST film. If the thickness of the mechanical load was larger than the thickness of the active BST layer, a significant reduction in the tuning performance was observed. The weaker tuning of the antiresonance frequency was due to a reduced tuning of the sound velocity of the BST layer with increasing dc bias. The resonance frequency showed a reduced tuning due to a decrease in the effective electromechanical coupling factor of the device with increasing mechanical load. With the help of the modeling we could de-embed the intrinsic tuning performance of a single, non-loaded BST thin film. We show that the tuning performance of the device with the smallest mechanical load we fabricated is close to the intrinsic tuning characteristics of the BST layer.     

Microwave dielectric properties of (1-<Emphasis Type="Italic">x</Emphasis>) BiVO<Subscript>4</Subscript>–<Emphasis Type="Italic">x</Emphasis>Ln<Subscript>2/3</Subscript>MoO<Subscript>4</Subscript> (Ln=Er,Sm, Nd,la) ceramics with low sintering temperatures

A series of microwave dielectric ceramics of (1-x) BiVO₄ -xLn_2/3MoO₄ (Ln = Er, Sm, Nd and La; x = 0.06, 0.08, 0.10) sintered below 900 °C were prepared via solid-state reaction. As the x values increase, the monoclinic scheelite continuously changes to a tetragonal structure at x = 0.10. The incorporation of Ln_2/3MoO₄ into the BiVO₄ matrix increases the product (Q × f) of quality factor (Q) and resonance frequency (f), and temperature coefficient (τ _f ), but lowers the dielectric constant (ε _r). Microwave dielectric ceramics with low sintering temperatures (<900 °C) are obtained: ε _r of ~71.1, 81.6, 75.6 and ~75.3; Q × f values of ~8292, 5508, 8695 and 9043 GHz; τ _f of ~ ?51, 134, 149 and 158 ppm/°C, for 0.94BiVO₄–0.06Er_2/3MoO₄, 0.92BiVO₄–0.08Sm_2/3MoO₄, 0.9BiVO₄–0.1Nd_2/3MoO₄ and 0.9BiVO₄–0.1La_2/3MoO₄ ceramics, respectively. Moreover, (1-x) BiVO₄ -xLn_2/3MoO₄ (Ln = Er, Sm, Nd and La; x = 0.06, 0.08 and 0.10) ceramics are chemically compatible with both Ag and Cu powders at their sintering temperatures. The series of microwave dielectric ceramics might be potential candidates for low temperature co-fired ceramics (LTCC) technology applications.