Impedance/Dielectric Spectroscopy of Electroceramics—Part 2: <Emphasis Type="Italic">Grain Shape Effects and Local Properties of Polycrystalline Ceramics</Emphasis>

The reduction of grain size from the microcrystalline regime into the nanocrystalline regime is known to produce significant changes in the transport properties of polycrystalline ceramics. Part 1 of this series [1] described the development of a pixel-based finite-difference nested-cube model (NCM), which was used to evaluate existing composite models for the electrical/dielectric properties of polycrystalline ceramics over the entire range of grain core vs. grain boundary volume fractions, from the nanocrystalline regime to the microcrystalline regime. Part 2 addresses grain shape and periodicity effects in such composite modeling, and the extraction of local materials properties (conductivity, dielectric constant) from experimental impedance/dielectric spectroscopy data.     

Temperature Sensitive Properties of the La(Ti<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>)O<Subscript>3</Subscript> System

The electric mechanisms of perovskite-type LaMnO₃ was investigated with B-site substitution in this paper. Samples of La(Ti_xMn_{1 − x})O₃ (0.1 ≰ x ≰ 0.7) were sintered at different temperature. The voltage-temperature (V-T) curves of the samples were tested from room temperature (25^∘C) to 300^∘C, then the electric properties were measured and analyzed. The experimental results showed that the resistivity-temperature (ρ-T) curves of the samples matched NTC characteristic. The resistivity increased slightly with the increase of Ti amount as x was less than 0.5, however, it rose greatly after x exceeded 0.5; The sintering temperatures have a little influence on the resistivity, except for the sample with x = 0.7.     

Microstructural and High-Temperature Electrical Characterization of La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3 − δ</Subscript>

Strontium-doped lanthanum ferrites (LSF) were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), 4-point D.C. electrical conductivity and bulk property measurements. The results were compared to those of previous studies as well as selected processing conditions. The investigation focused on effects of sintering temperature, time, atmosphere (air, O₂ and N₂) and composition of La_1–xSr_xFeO_3– (x = 0.2–0.9), on the sintering behavior, microstructural development and electrical conductivity. An oxalate precipitation method was used to prepare lanthanum ferrite powders. Simultaneous thermogravimetric and differential thermal analysis (TGA/DTA) studies found calcination temperatures of 800 and 850^C were necessary to form single-phase crystalline powders, as determined by XRD. Specimens were sintered from 1300 to 1400^C with dwell times from to 2 hrs. Results from SEM/EDS analysis showed the presence of a second phase in the samples fired in air or oxygen. The second phase was not detected by x-ray diffraction due to the small amount of material present. Samples fired in nitrogen had the lowest conductivity while those fired in oxygen had the highest. A composition of x = 0.5 resulted in the highest conductivity, 352 S/cm, at an operating temperature of 550C in air. High strontium additions (x = 0.9) lowered the linear shrinkage of LSF.     

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.     

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.     

Partial Electronic Conductivity and Chemical Diffusivity of Li<Subscript>3<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>2/3−<Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> Solid Solution

Partial electronic conductivity and chemical diffusivity of Li have been measured on the system of Li_3xLa_2/3–xTiO₃ (LLT) with x = 0.13, a prospective Li⁺ electrolyte, against oxygen activity in the range of 10^–22 < a_O2 < 0.21 at 557, 610 and 663C, respectively by an ion-blocking polarization technique. It is found that the electronic conductivity of LLT, which in air is essentially an ionic conductor, varies as a_O2^–1/4 to render it mixed-conductive in reducing atmospheres, say, in a_O2 < 10^–12. The chemical diffusivity of component Li also increases from a value of the order of 10^–8 cm²/s in air atmosphere up to a maximum on the order of 10^–3 cm²/s as the electronic conductivity increases with decreasing oxygen activity. This is attributed to the variation of the electronic transference number and the thermodynamic factor with oxygen activity. The latter has been evaluated to be on the order of 10–10³.     

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.     

<Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> mapping of cerebrospinal fluid: 3 T versus 7 T

Object

Cerebrospinal fluid (CSF) T ₂ mapping can potentially be used to investigate CSF composition. A previously proposed CSF T ₂–mapping method reported a T ₂ difference between peripheral and ventricular CSF, and suggested that this reflected different CSF compositions. We studied the performance of this method at 7 T and evaluated the influence of partial volume and B ₁ and B ₀ inhomogeneity.

Materials and methods

T ₂-preparation-based CSF T ₂-mapping was performed in seven healthy volunteers at 7 and 3 T, and was compared with a single echo spin-echo sequence with various echo times. The influence of partial volume was assessed by our analyzing the longest echo times only. B ₁ and B ₀ maps were acquired. B ₁ and B ₀ dependency of the sequences was tested with a phantom.

Results

T _2,CSF was shorter at 7 T compared with 3 T. At 3 T, but not at 7 T, peripheral T _2,CSF was significantly shorter than ventricular T _2,CSF. Partial volume contributed to this T ₂ difference, but could not fully explain it. B ₁ and B ₀ inhomogeneity had only a very limited effect. T _2,CSF did not depend on the voxel size, probably because of the used method to select of the regions of interest.

Conclusion

CSF T ₂ mapping is feasible at 7 T. The shorter peripheral T _2,CSF is likely a combined effect of partial volume and CSF composition.

     

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.     

Effect of the air–fuel mixing on the NO<Subscript> <Emphasis Type="Italic">х</Emphasis> </Subscript> yield in a low-emission gas-turbine plant combustor

The article deals with construction of a simplified model of inhibition of nitric oxides formed in the combustors of the gas-turbine plants (GTPs) operating on natural gas. A combustor in which premixed, lean air–fuel mixtures are burnt is studied theoretically and experimentally. The research was carried out using a full-scale combustor that had parameters characteristic of modern GTPs. The article presents the results computed by the FlowVision software and the results of the experiments carried out on the test bench of the All-Russia Thermal Engineering Institute. The calculations and the tests were conducted under the following conditions: a flow rate of approximately 4.6 kg/s, a pressure to 450 kPa, an air temperature at the combustor inlet of approximately 400°C, the outlet temperature t₃ ≤ 1200°C, and natural gas as the fuel. The comparison of the simulated parameters with the experimental results underlies the constructed correlation dependence of the experimental NO_x emission on the calculated parameter of nonuniform fuel concentration at the premixing zone outlet. The postulate about a weak dependence of the emission of NO_x formed upon combustion of a perfectly mixed air–fuel mixture—when the methane concentration in air is constant at any point of the air–fuel mixture, i.e., constant in the mixture bulk—on the pressure in the combustor has been experimentally proven. The correctness and the practicability of the stationary mathematical model of the mixing process used to assess the NO_x emission by the calculated amount of the air–fuel mixture generated in the premixing zone has been validated. This eliminates some difficulties that arise in the course of calculation of combustion and formation of NO_x.     

Selective 3D ultrashort TE imaging: comparison of “dual-echo” acquisition and magnetization preparation for improving short-<Emphasis Type="Italic">T</Emphasis><Subscript>2</Subscript> contrast

Objective The objective of this study was to compare two different schemes for long-T ₂ component suppression in ultrashort echo-time (UTE) imaging. The aim was to increase conspicuity of short-T ₂ components accessible by the UTE technique. Materials and methods A “dual-echo” and a magnetization-preparation approach for long-T ₂ and fat suppression were implemented on clinical scanners. Both techniques were compared in 3D UTE exams on healthy volunteers regarding short-T ₂ Signal-to-noise ratio (SNR), long-T ₂ suppression quality, and scan efficiency. A quantitative SNR evaluation was performed using ankle scans of six volunteers. T ₂ suppression profiles were simulated for both approaches to facilitate interpretation of the observations. Results At 1.5 T, both techniques perform equally well in suppressing long-T ₂ components and fat. Magnetization preparation requires more shimming effort due to the use of narrow-band pulses, while the “dual-echo” technique requires a post-processing step to form a subtraction image. For scans with a short repetition time (TR), the “dual-echo” approach is much faster than the magnetization preparation, which depends on slow T ₁ recovery between preparation steps. The SNR comparison shows slightly higher short-T ₂ SNR for the “dual-echo” approach. At 3.0 T, magnetization preparation becomes more challenging due to stronger off-resonance effects. Conclusion Both techniques are well suited for long-T ₂ suppression and offer comparable short-T ₂ SNR. However, the “dual-echo” approach has strong advantages in terms of scan efficiency and off-resonance behavior.     

Exploration of Na<Superscript>+</Superscript>,K<Superscript>+</Superscript>-ATPase ion permeation pathways <Emphasis Type="Italic">via</Emphasis> molecular dynamic simulation and electrostatic analysis

Biologically-inspired nanodevices can serve as “natural” alternatives to conventional semiconductor devices in many applications from information storage to mechanical rotors. In this work we consider an ATP-powered transmembrane protein, the Na⁺,K⁺-ATPase, which has appealing functionality but still lacks an “atomistic” picture capable of elucidating its operation. The vast collection of experimental literature on the Na⁺,K⁺-ATPase gives a unique advantage to this protein in developing and validating computational tools. We have performed extensive molecular dynamic simulations of the Na⁺,K⁺-ATPase in an accurate biological environment, followed by time-averaged electrostatic analysis, to investigate the ion-binding loci and access/egress pathways that cations may take through the protein as they are transported across the membrane.     

Surface roughness induced device variability: 3D <Emphasis Type="Italic">ab initio</Emphasis> Monte Carlo simulation study

It is expected that published results from drift diffusion simulation of oxide thickness fluctuations in nano-scale devices underestimates the true intrinsic device parameter variation by neglecting local variations in surface roughness scattering. We present initial results from 3D ‘bulk’ Monte Carlo simulation including an ab initio treatment of surface roughness scattering capable of capturing such transport variation. The scattering is included directly through the real space propagation of carriers in the fluctuating potential associated with a randomly generated interface. We apply this approach to simulate inversion layer mobility in order to validate the model before its possible application in device variability simulations. Qualitative agreement is found with universal mobility data and avenues for possible calibration of surface and simulation parameters are highlighted.     

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.     

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.     

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.