Improvement of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript> thermoelectric performances by Na doping

Bi₂Sr_2-xNa_xCo₂O_y thermoelectric materials with x = 0, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15 have been prepared by the classical solid state reaction. Microstructure has shown an important grain growth when Na is added, leading to very high bulk densities confirmed through density measurements. These modifications have produced a drastic decrease of electrical resistivity without significant modification of Seebeck coefficient. As a consequence, Power Factor has been increased in all Na-doped samples, reaching the maximum value (0.21 mW/K².m at 650 °C) for 0.075 Na samples, which is fairly close to the reported for single crystals.     

Effect of Ca substitution by Fe on the thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> ceramics

(Ca_1-xFe_x)₃Co₄O₉ polycrystalline samples (x?=?0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1 and 0.2) have been prepared by solid state reaction and sintered by spark plasma sintering. Their thermoelectric properties have been studied between 323 and 1000 K. The substitution limit is low (< 3%). For higher iron contents, formation of Co₃O₄, Ca₂FeCoO₅ and a calcium ferrite compound occurs. Concerning electrical conductivity and power factor, an optimum value exists for x?=?0.03 substitution. This optimum power factor is circa 10% higher than the one of the unsubstituted sample. The optimum thermal diffusivity is measured for the sample with x?=?0.02 and is circa 9% lower than the one of the unsubstituted sample. Therefore, the dimensionless figure of merit ZT is finally increased by 14% for the (Ca_0.98Fe_0.02)₃Co₄O₉ and (Ca_0.97Fe_0.03)₃Co₄O₉ compositions.     

Structure and properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>-0.38PbTiO<Subscript>3</Subscript> ceramics with MPB composition

0.62Bi(Mg_1/2Ti_1/2)O₃-0.38PbTiO₃-xwt%Bi₂O₃ (BMT-0.38PT-xBi₂O₃) ceramics were prepared by conventional powder-processing method. It indicated that the morphotropic phase boundary (MPB) region located in 0.0?≤?x?≤?0.3. For x?=?0.3, it exhibited good piezoelectric properties, d₃₃ ~245pC/N and k_p ~40 %. With the increase of Bi₂O₃ content, the Curie temperature (T_c) was found to increase, and the dielectric loss was found to decrease above 200 °C compared with BMT-0.38PT sample. Finally, it can be found that depolarization temperature was around 350 °C by thermal depoling method.     

Polarization and piezoelectric properties of grain-oriented ferroelectric Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> ceramics prepared by magnetic-field-assisted electrophoretic deposition method

Magnetic-field-assisted electrophoretic deposition method has been employed for synthesizing a(b)-axis-oriented Bi₅FeTi₃O₁₅ ceramics, and the effects of grain orientation and microstructure on the polarization and piezoelectric properties have been investigated. Grain-oriented Bi₅FeTi₃O₁₅ ceramics with a high relative sintered density of 98% is shown to exhibit enhanced polarization and piezoelectric properties with a remanent polarization (P _r) of 19 μC/cm² and a piezoelectric strain constant (d ₃₃) of 23 pm/V, which are much superior to those of randomly-oriented ceramics (P _r of 7 μC/cm² and d ₃₃ of 5 pm/V).     

Studies of structural,dielectric and impedance properties of Bi<Subscript>9</Subscript>Fe<Subscript>5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>27</Subscript> ceramics

Bi₉Fe₅Ti₃O₂₇ is an eight-layered material belonging to the family of bismuth layered structured ferroelectromagnets. The polycrystalline sample of this compound was prepared by a standard solid-state reaction technique. The formation of the compound in an orthorhombic crystal structure was confirmed by an X-ray diffraction (XRD) technique (lattice parameters: a?=?5.5045[27] Å, b?=?5.6104[27] Å, c?=?76.3727[27] Å). Detailed studies of surface morphology of the compound using scanning electron microscopy (SEM) exhibit that the compound has domains of plate shaped grains. Studies of dielectric and electric properties in a wide temperature range (30–500 °C) at different frequencies (100 Hz–1 MHz) exhibit an anomaly at 291?±?2 °C, which is related to ferroelectric to paraelectric phase transition as suggested by hysteresis loop at room temperature. The values and nature of temperature variation of dc conductivity exhibit the NTCR behavior of the compound.     

Influence of the processing rates and sintering temperatures on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The stoichiometric CaCu₃Ti₄O₁₂ pellets were prepared by the solid state synthesis. X-ray diffraction data revealed the tenorite CuO and cuprite Cu₂O secondary phases on the unpolished CaCu₃Ti₄O₁₂ samples regardless of the heating rates. Also, the dielectric constant marked the highest for the CaCu₃Ti₄O₁₂ sample sintered at the lowest heating rate (1°C/min), which was explained by the increased grain conductivity due to the cation reactions. On the other hand, Cu₂O phase was found only on the unpolished CaCu₃Ti₄O₁₂ sample sintered over 1100°C and those are considered as the remains reduced from the CuO phase. The higher sintering temperature showed the increased dielectric constant and the loss tangent of the CaCu₃Ti₄O₁₂ samples, and this result could be interpreted by the impedance measurement data. The relationship between the processing condition and the dielectric properties was discussed in terms of the cation non-stoichiometry and the defect chemistry in CaCu₃Ti₄O₁₂.     

Mechanical and electrical properties of Bi<Subscript>1.5-x</Subscript>La<Subscript>x</Subscript>Zn<Subscript>0.92</Subscript>Nb<Subscript>1.5</Subscript>O<Subscript>6.92</Subscript> pyrochlore ceramics

BiFeO₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (BF-PMN-PT) ternary ceramics with pure perovskite phase were prepared through a two-step solid reaction method. Based on structural analysis, the ternary phase diagram of BF-PMN-PT solid solution at room temperature has been established. The Curie temperature T_C, remnant polarization P_r and piezoelectric constant d₃₃ vary in the range of 138 to 225 °C, 15.12 to 23.65 μC/cm² and 129 to 276 pC/N, respectively. The coercive field Ec increases gradually from 5.77 to 29.56 kV/cm upon PT content increasing. The magnetic study suggests that the magnetism turns from diamagnetism for PMN-PT to paramagnetism for BF-PMN-PT by adding BiFeO₃ into PMN-PT and adding more content of BF does not change the paramagnetism further.     

Effect of Ca substitution on microwave dielectric properties of BaV<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics

Effects of Ca substitution for Ba on the phase composition, microstructure, sintering behavior and microwave dielectric properties of nominal ceramics Ba_1-xCa_xV₂O₆ (0.2?≤?x?≤?0.5) were investigated. The XRD, Raman and SEM results revealed that BaV₂O₆ and CaV₂O₆ composite ceramics were formed. Nominal ceramics Ba_1-xCa_xV₂O₆ could be well densified at about 550 °C via a solid-state reaction method. The microwave dielectric properties exhibited strong dependence on the composition and microstructure. Typically, the Ba_0.7Ca_0.3V₂O₆ ceramics sintered at 550 °C exhibited excellent microwave dielectric properties: ε_r?=?10.9, Qxf?=?17,100 GHz (at 9.9 GHz), and τ_f?=?4 ppm/°C. Meanwhile, Ba_0.7Ca_0.3V₂O₆ ceramics also showed good chemical compatibility with Al electrode. These results indicated that the Ba_0.7Ca_0.3V₂O₆ ceramics could be a promising candidate for the ULTCC technology.     

Influence of substitution on dielectric diffuseness in Ba<Subscript>(1-x)</Subscript>Bi<Subscript>(2+2x/3)</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics prepared by chemical precursor decomposition method

Barium bismuth niobate, Ba_(1-x)Bi_(2+2x/3)Nb₂O₉ (BBN with x = 0.0, 0.1, 0.2, 0.3, 0.4) ceramic powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). The single phase layered perovskite was prepared throughout the composition range studied. No intermediate phase was found during heat treatment at and above 600°C. The crystallite size and the particle size, obtained from XRD and TEM respectively, were in the range of 15–30 nm. The addition of Bi₂O₃ substantially improved the sinterability associated with high density (96%) which was otherwise difficult in the case of pure BaBi₂Nb₂O₉ (BBN x = 0.0). The sintering was done at 900°C for 4 h. The relative permittivity of BBN ceramics at both room temperature and in the vicinity of the temperature of maximum permittivity (T_m) has increased significantly with increase in bismuth content and loss is also decreased to a certain level of bismuth doping. T_m increased with increase in Bi₂O₃. The diffuseness (γ) in the phase transition was found to increase from 1.54 to 1.98 with the increase in Ba²⁺ substitution level from x = 0.0 to x = 0.3.     

Dielectric properties of pure and Mn-doped CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics over a wide temperature range

The effect of manganese doping on the dielectric properties of CaCu₃Ti_4-xMn_xO₁₂ (x?=?0, 0.02, 0.04) were investigated over a broad temperature range (93–723 K) in the frequency range from 100 Hz to 10 MHz. Two dielectric relaxations and two dielectric anomalies were observed. The low-temperature relaxation appearing in the temperature range below 200 K is the characteristic relaxation for CaCu₃Ti₄O₁₂. This relaxation was attributed to the polaron relaxation due to electron hopping between Ti³⁺ and Ti⁴⁺ states. Due to the negative factors of notable decreases in the Ti³⁺/Ti⁴⁺ and Cu³⁺/Cu²⁺ ratios and the concentration of oxygen vacancies as revealed by X-ray photoemission spectroscopy, Mn-doping was found to gradually destroy rather than move this relaxation to a higher temperature. The high-temperature relaxation occurring around room temperature was found to be a Maxwell-Wagner relaxation caused by grain boundaries. Our results confirm that the colossal dielectric behavior in the tested samples results from both polaron and Maxwell-Wagner relaxations, but is predominated by the latter relaxation. The low-temperature anomaly behaves as a phase-transition-like behavior. It was argued to be created by oxygen vacancies transition from static disorder to dynamic disorder. The high-temperature anomaly is an artificial effect caused by negative capacitance.     

Crystallization,microstructures and properties of low temperature co-fired CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass-ceramic

CaO-Al₂O₃-SiO₂ glass-ceramic were prepared by melt quenching technique. The crystallization behavior and properties were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The influence of sintering temperature on phase formation, microstructure, bending strength, dielectric and thermal properties were determined. The activation energy of crystallization and the Avrami parameter were also discussed. The X-ray diffraction results show that SiO₂ phase could be found in all samples and CaSiO₃ and anorthite phases could only be observed in the samples sintered at above 875°C. The densification of glass-ceramic starts at 730°C after the liquid glass is formed and stops at 803°C. Complete densification was achieved at 875°C and the highest mechanical strength was obtained at 850°C, but density significantly decreased at higher temperatures. The coefficient of thermal expansion and the dielectric constant increase with the increasing sintering temperature. The value of the Avrami parameter (n) is ~1.6 and the apparent activation energy (E) is 298 kJ/mol.     

Influence of the calcination procedure on the thermoelectric properties of calcium cobaltite Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript>

Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca₃Co₄O₉ powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.     

Effects of MnO<Subscript>2</Subscript> addition on microstructure and electrical properties of (Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>) <Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> ceramics

In this letter, MnO₂-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT-6) lead-free piezoelectric ceramics were synthesized by solid state reaction, and the microstructure and electrical properties of the ceramics were investigated. X-ray diffraction (XRD) reveals that all specimens take on single perovskite type structure, and the diffraction peaks shift to a large angle as the MnO₂ addition increases. Scanning electron microscopy shows that the grain sizes increases, and then decreases with increasing the MnO₂ content. The experiment results indicate that the electrical properties of ceramics are significantly influenced by the MnO₂ content, and the ceramics with homogeneous microstructure and excellent electrical properties are obtained with addition of 0.3 wt% MnO₂ and sintered at 1160°C. The piezoelectric constant (d₃₃), the electromechanical coupling factor (k _p ), the dissipation factor (tan δ) and the dielectric constant (ɛ _r ) reach 160 pC/N, 0.29, 0.026 and 879, respectively. These excellent properties indicate that the MnO₂-doped BNBT-6 ceramics can be used for actuators.     

Microwave Dielectric Properties of CuO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Doped ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

Microwave dielectric properties of low temperature sintering ZnNb₂O₆ ceramics doped with CuO-V₂O₅-Bi₂O₃ additions were investigated systematically. The co-doping of CuO, V₂O₅ and Bi₂O₃ can significantly lower the sintering temperature of ZnNb₂O₆ ceramics from 1150 to 870C. The secondary phase containing Cu, V, Bi and Zn was observed at grain boundary junctions, and the amount of secondary phase increased with increasing CuO-V₂O₅-Bi₂O₃ content. The dielectric properties at microwave frequencies (7–9 GHz) in this system exhibited a significant dependence on the relative density, content of additives and microstructure of the ceramics. The dielectric constant ( _r) of ZnNb₂O₆ ceramics increased from 21.95 to 24.18 with increasing CuO-V₂O₅-Bi₂O₃ additions from 1.5 to 4.0 wt%. The quality factors (Q× f) of this system decreased with increasing CuO-V₂O₅-Bi₂O₃ content and ranged from 36118 to 67100 GHz for sintered ceramics, furthermore, all Q× f values of samples with CuO-V₂O₅-Bi₂O₃ additions are lower than that of un-doped ZnNb₂O₆ ceramics sintered at 1150C for 2 h. The temperature coefficient of resonant frequency ( _f) changed from –33.16 to –25.96 ppm/C with increasing CuO-V₂O₅-Bi₂O₃ from 1.5 to 4.0 wt%     

The Similar Defect Chemistry of Highly-Doped SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>

The equilibrium electrical conductivities of undoped SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) have been shown to behave quite differently. SBT has the behavior expected for a 1% acceptor-doped oxide, while SBN behaves like a 1% donor-doped oxide. This difference has been related to the substantial cation place exchange that occurs between the Bi^{+ 3} and Sr^{+ 2} ions in the alternating layers of the structure. It was proposed that this place exchange is not entirely self-compensating, as would be expected for a simple, isotropic oxide, but that there is some local compensation within each layer by lattice and/or electronic defects. It is now shown that the equilibrium conductivity of 3% donor-doped SBT is similar to that of undoped SBN, while the equilibrium conductivity of 3% acceptor-doped SBN resembles that of undoped SBT. Thus the defect chemistrys of the two compounds are quite similar, but the equilibrium conductivities are displaced along a doping axis.     

Properties and crystallization kinetics of low temperature co-fired Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> electroceramics

The glass-ceramic in the Li₂O-Al₂O₃-SiO₂ system has been prepared by melt quenching route. The crystallization kinetics was studied by differential scanning calorimetry. The effects of sintering temperature on the phase transformation, sintering behavior, bulk density, microstructure, thermal expansion, bending strength and dielectric properties were also investigated by X-ray diffractometry and scanning electron microscopy. (Li, Mg, Zn)_1.7Al₂O₄Si₆O₁₂ is the first crystalline phase forming in the glass-ceramic and transforms to LiAlSi₃O₈ phase at 800 °C. The other two crystalline phases of ZrO₂ and CaMgSi₂O₆ precipitate at 700 and 750 °C, respectively. The densification of this LAS glass-ceramic starts at around 730 °C and stops at about 805 °C. The coefficient of thermal expansion increases with the increasing sintering temperature. The sample sintered at 800 °C for 30 min exhibited excellent properties. The nonisothermal activation energy of crystallization is 149 kJ/mol and the values of Avrami constant (n) are in the range of 3.2 to 3.9. The LAS glass-ceramic sintered at 800 °C for 30 min showed excellent properties. This makes that this material suitable for a number of LTCC applications.     

Enhancement of thermoelectric performance in rare earth-doped Sr<Subscript>3</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> by symmetry restoration of TiO<Subscript>6</Subscript> octahedra

Aiming at the realization of the enhancement of thermoelectric performance through structural modification, the present work has clarified the significant effects of rare earth (RE=Gd, Sm, Nd, and La) doping at Sr-sites in Sr₃Ti₂O₇, both on the structural restoration of distorted TiO₆ octahedra and on the Seebeck coefficient, especially at high temperatures. The preferential substitution of RE ³⁺ at the nine-coordinate Sr-sites can facilitate the degeneration of the conduction band (Ti 3d-t _2g ) orbital, owing to its special capability in restoring TiO₆ octahedra to a higher state of symmetry and thus enhance the density of states (DOS) effective mass of the carriers, which gives rise to a rather large increase in the Seebeck coefficient. The present findings have affirmed the effectiveness of structural restoration in enhancing the Seebeck coefficient by Sr-site-doping, which will help establish a useful solution for Ti-based thermoelectric oxides with inherently distorted TiO₆ octahedra to achieve high thermoelectric performance.     

Raman and infrared spectroscopy of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> - BaTiO<Subscript>3</Subscript> ceramics

Lead-free Na_0.5Bi_0.5TiO₃ -BaTiO₃ ceramics have been prepared in the whole range of concentrations and studied at room-temperature by means of X-ray, Raman scattering and infrared techniques. X-ray measurements revealed rhombohedral, rhombohedral-tetragonal boundaries and tetragonal modifacations depending on the contents of BaTiO₃. The distinct changes of the Raman and infrared spectra with increasing of BaTiO₃ content, which were correlated with X-ray results, were observed. The broad phonon spectra indicated the disorder in the A site of Na_0.5Bi_0.5TiO₃ -BaTiO₃ system.     

Phase transition,microstructure and properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>1-x</Subscript>Ba<Subscript>x</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

In this article, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction. The influence of Ba contents on phase structures, compositional distribution and electrical properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were systematically investigated to further understand the nature of phase transition. It was found that the phase structure of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ transforms from rhombohedral to tetragonal symmetry at x = 0.06 ~ 0.07 and Ba²⁺ segregation forms the coexistence of Ba-rich tetragonal and Ba-deficient rhombohedral phases close to MPB. The electrical properties of prepared samples regularly changed with Ba content, which is closely related to the distribution of rhombohedral and tetragonal phases. The prepared sample near MPB exhibited the largest dielectric constant and the excellent piezoelectric properties (the maximal measuring field reached 78 kV/cm and the piezoelectric constant d ₃₃ = 151pC/N).     

Synthesis of (Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>)(Ti<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics and effect of Sr content on room temperature dielectric properties

The compositions in the system (Ba_1−x Sr _x )(Ti_0.5Zr_0.5)O₃ with different Sr (x) content, were synthesized through solid oxide reaction route. The phase formation behaviors in the system were investigated by XRD. The room temperature dielectric properties of the compositions were investigated in the frequency range 10 Hz to 13 MHz. The solid solution system Ba_1−x Sr _x Ti_0.5Zr_0.5O₃ remains as cubic perovskite up to x < 0.6 and transforms into the tetragonal structure above x > 0.6. Composition with x = 0.6 contains a mixture of cubic and tetragonal phases with broadened diffraction pattern. It is observed that the increasing of Sr substitution results in the decreasing of bulk density, average grain size and dielectric constants etc. in the composition system. The AC dielectric conductivity of the ceramics also decreases with the increase in Sr-substitution due to decrease in loss as well as grain size with that substitution.