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 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.     

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).     

Preparation of TiO<Subscript>2</Subscript> thin films deposited from highly dense targets with multi-oxide glass doping

Glass doped TiO₂ (GTO) thin films were deposited by radio frequency (RF) magnetron sputter at room temperature and annealed in a reductive atmosphere containing 90 % N₂ and 10 % H₂. Highly dense TiO₂ ceramic mixed with glass consisting of multi-metal oxides (as a sintering aid) was used as the sputtering target. This sintering aid allows low temperature densification of TiO₂ target through a liquid phase wetting mechanism, and also works as a doping resource. XRD and FESEM were carried out to characterize the microstructure of the GTO films and the results reveal that the doping of multi-metal ions enhances the crystallization and increases the grain size of TiO₂ films. TEM analysis also showed that these metal ions were dissolved into TiO₂ lattices. The electrical and optical properties of TiO₂ thin films at different glass concentrations were evaluated and compared to the films merely doped with MoO₃. The electrical resistivity of the GTO films reaches 9.1 × 10^–4 Ω·cm at 2 wt% glass doping, corresponding to a carrier density of 8.9 x 10²⁰ cm^-3 and a mobility of 7.1 cm²/Vs. Meanwhile, the electrical resistivity of the TiO₂ film doped with glass was found to be lower than that of MoO₃-doped film. This was mainly attributed to the increase in carrier concentration by double doping effect of glass. The optical band gap of the GTO films ranged from 3.34 to 3.42 eV, which is greater than that of the un-doped TiO₂ film. This blue shift of approximately 0.18 eV was due to the Burstein-Moss effect.     

Microwave dielectric properties of Na<Superscript>+</Superscript> and M<Superscript>3+</Superscript>-doped Ba(Mg<Subscript>0.5</Subscript>W<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics

In this study, phase evolution, microstructure, and microwave dielectric properties of (Ba_0.98Na_0.02)(Mg_0.48M³⁺_0.02W_0.5)O₃ (M³⁺?=?Al, Ga, Sc, In, Yb, Y, Dy, Gd, and Sm) ceramics sintered at 1700 °C for 1 h were investigated. All the compounds exhibited an ordered cubic perovskite structure. Regardless of the ionic radius of the doped M³⁺ ions, BaWO₄ was detected as the secondary phase in all the compounds. The field emission scanning electron microscopy (FE-SEM) images revealed a dense microstructure in all the compounds, except in the Al-doped compound, which exhibited an insufficient grain growth. The large and irregularly shaped grains indicated that the liquid phase sintering occurred. Splitting of the A_1g(O) mode was observed in the Raman spectra of large M³⁺ ion-doped compounds. Splitting of the F_2g modes did not occur and the bands were sharp, indicating that the cubic symmetry was retained. As the ionic radius of the doped M³⁺ ions increased, the dielectric constant (ε_r) increased slightly. The compounds doped with M³⁺?=?Sc, In, Yb, and Y exhibited a very high quality factor (Q?×?f₀) in the range of 250,000 ~ 280,000 GHz. In the case of the compounds doped with M³⁺?=?Al, Ga, Sc, In, Yb, Y, and Dy, the value of the temperature coefficient of resonant frequency (τ_f) was in the range of ?24 ~ ?19 ppm/°C, while the Gd and Sm-doped compounds exhibited positive values of 2.8 and 31.2 ppm/°C, respectively. The dielectric constant, quality factor, and temperature coefficient of resonant frequency of the In-doped compound, i.e., (Ba_0.98Na_0.02)(Mg_0.48In_0.02W_0.5)O₃, were 18.7, 286,557 GHz, and???24.4 ppm/°C, respectively.     

Evaluation of La<Subscript>2</Subscript>CoTi<Subscript>0.7</Subscript>Mg<Subscript>0.3</Subscript>O<Subscript>6</Subscript> as an electrode material for a symmetrical SOFC

La₂CoTi_0.7Mg_0.3O₆ (LCTM) material has been prepared at 1473 K for 24 h in air. X-ray powder diffraction study has revealed that it contains two orthorhombic perovskite phases (in a ratio ~1:4) with close unit cell parameters. Annealing of LCTM in reducing (Ar/H₂, 8%) atmosphere at 1173 K for 12 h has resulted in the preparation of a single-phase material containing the GdFeO₃-type perovskite phase with the unit cell parameters of a?=?5.5631(3) Å, b?=?5.5462(3) Å, c?=?7.8522(5) Å. LCTM material exhibits a reversible transformation of a mixture of two perovskite phases with close cation content in air and a single perovskite phase in a reducing atmosphere. Both as-prepared and reduced LCTM samples have been studied by thermogravimetric analysis and dilatometry in air and Ar/H₂ (8%). No chemical interaction between the as-prepared LCTM and standard electrolyte materials for SOFC like GDC and YSZ has been observed up to 1273 K. High-temperature electrical conductivity of the as-prepared LCTM at variable oxygen partial pressure (10^?4-0.21 atm) showed weak dependence over pO₂ with E_a?=?0.48?±?0.01 eV. AC impedance study of the symmetrical cells LCTM/GDC/LCTM has revealed ASR value at 1173 K of ~8.1?±?0.1 Ω?cm² in air and 0.24?±?0.05 Ω?cm² in a reducing atmosphere. These results allow to consider LCTM as a promising electrode material for a symmetrical SOFC.     

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.     

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 I: Defect chemistry

Oxygen nonstoichiometry and the defect chemistry of the SrSn_1-xFe_xO_3-x/2+δ (SSF) system were examined by means of thermogravimetry as a function of oxygen partial pressure in the temperature range of 700–1000 °C and compared against the corresponding mixed ionic-electronic conducting titanate, SrTi_1-xFe_xO_3-x/2+δ (STF) system. The alternate B site host cation, Sn, was selected to replicate and extend the STF studies, given its distinct band structure and higher electron mobility associated with its 5s derived conduction band as compared to the 3d nature of the conduction band in the titanate. Though shifted slightly by the larger size of Sn, the defect equilibria – including the oxygen vacancy concentration – were found to be largely dominated by Fe oxidation state, and thus differed only in a limited way from those in STF. Key thermodynamic parameters for SrSn_0.65Fe_0.35O_2.825+δ (SSF35) were derived including the reduction enthalpy (4.137 ± 0.175 eV), the high temperature electronic band gap (1.755 ± 0.015 eV) and the anion Frenkel enthalpy (0.350 ± 0.350 eV). The implications these observations have for cathode behavior in solid oxide fuel cells are briefly discussed.     

Ionic conduction and crystal structure of aluminum doped NASICON-type LiGe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic crystallized at different times and temperatures

In this communication, NASICON-type glass-ceramic (lithium germanium phosphate, LiGe₂(PO₄)₃) was prepared as lithium super ionic conductor using aluminum as dopant for ionic conduction improvement. The solid solution was Li_1?+?xAl_xGe_2-x(PO₄)₃ (x?=?0.5) that Ge⁴⁺ ions were partially substituted by Al³⁺ ions in crystal structure. Initial glasses were converted to glass-ceramics at different times and temperatures for maximum ionic conduction achievement. The crystals were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS) methods. The maximum lithium ion conductivity for glass-ceramic, 5.32?×?10^?3 S/cm at 26 °C was obtained for specimen crystallized at 850 °C for 8 h with minimum activation energy of 0.286 eV. Increasing the crystallization temperature results in secondary phase formation in grain boundary and increasing in crystallization time results in microcracks formation in specimen. Both phenomena decreased the ionic conductivity.     

Multiferroic,magnetoelectric and magneto-impedance properties of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/(Pb,Sr) TiO<Subscript>3</Subscript> bilayer films

We prepared NiFe₂O₄/(Pb, Sr)TiO₃ (NFO/PST) bilayer films by the chemical solution method and investigated their multiferroic, magnetoelectric and magneto-impedance properties. Multiferroic properties have been observed at room temperature. The bilayer films exhibit saturation polarization P _s ? 26.6 μC/cm² and saturation magnetization M _s ? 134 emu/cm³. With increasing Sr content, M _s. and P _s values of the NFO/PST bilayer films decrease. The variation may be ascribed to the influence of interfacial strain and decrease in tetragonality. High magnetoelectric coupling effect has been observed in the NFO/PST bilayer films with maximum value of α _E  = 6.35 Vcm^?1 Oe^?1 measured at H _DC  ~ 1 kOe and f ~ 10 kHz for Sr ~ 10% (NFO/PST10). The magneto-impedance measurements establish a strong dependence on magnetic field, further confirming magnetoelectric response in NFO/PST bilayer films. An alternative approach for impedance analysis of NFO/PST10 bilayer film provides direct evidence of strain mediated magnetoelectric coupling at room temperature. The results indicate that NFO/PST bilayer films can be considered as a potential multiferroic magnetoelectric material.     

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.     

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.     

Formation of a KNbO<Subscript>3</Subscript> single crystal using solvothermally synthesized K<Subscript>2-m</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6-m/2</Subscript> pyrochlore phase

A K_2-mNb₂O_6-m/2 single crystal with a pyrochlore phase formed when the Nb₂O₅?+?x mol% KOH specimens with 0.6?≤?x?≤?1.2 were solvothermally heated at 230 °C for 24 h. They have an octahedral shape with a size of 100 μm, and the composition of this single crystal is close to K_1.3Nb₂O_5.65. The single-crystal KNbO₃ formed when the single-crystal K_2-mNb₂O_6-m/2 was annealed at a temperature between 600 °C and 800 °C with K₂CO₃ powders. When annealing was conducted at 600 °C (or with a small amount of K₂CO₃), the KNbO₃ single crystal has a rhombohedral structure that is stable at low temperatures (< ? 10 °C). The formation of the rhombohedral KNbO₃ structure can be explained by the presence of the K⁺ vacancies in the specimen. The KNbO₃ single crystal with an orthorhombic structure formed when the K_2-mNb₂O_6-m/2 single crystal was annealed at 800 °C with 20 wt% of K₂CO₃.     

Influence of fillers on the re-crystallization and dielectric properties of 60ZnO-30B<Subscript>2</Subscript>O<Subscript>3</Subscript>-10SiO<Subscript>2</Subscript> glass

This paper reports re-crystallization behaviour of 60ZnO-30B₂O₃-10SiO₂ pure glass. Influence of different fillers on the re-crystallization is also highlighted. Irrespective of the nature of the filler, glass transition temperature (T_g) remains the same as that of the pure glass at 570 °C. The onset crystallization temperature is enhanced by 15 to 25 °C owing to filler addition. The peak crystallization temperature corresponding to Zn₃B₂O₆, ZnB₂O₄ and willemite occurs at 735, 810 and 851 °C for the pure glass. In the presence of alumina, gahnite emerges first at 700 °C followed by ZnB₂O₄ phase. With mullite as filler, initially ZnB₂O₄ and willemite formed at 700 °C and gahnite phase emerged only at 750 °C and peaks at 787 °C. In the presence of fused silica only ZnB₂O₄ and willemite phases were observed. The dielectric properties of the pure glass and that of the glass + fillers are also reported. Re-crystallization of various low K phases ensures good dielectric porpertis with reduced glass content after sintenring.     

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.     

Structural evolution and dopant occupancy preference of yttrium-doped potassium sodium niobate thin films

Sodium potassium niobate (KNN) is the most promising candidate for lead-free piezoelectric material, owing to its high Curie temperature and piezoelectric coefficients among the non-lead piezoelectric. Numerous studies have been carried out to enhance piezoelectric properties of KNN through composition design. This research studied the effects of yttrium concentrations and lattice site occupancy preference in KNN films. For this research, the yttrium-doped KNN thin films (mol% = 0, 0.1, 0.3, 0.5, 0.7 and 0.9) were fabricated using the sol-gel spin coating technique and had revealed the orthorhombic perovskite structures. Based on the replacement of Y³⁺ ions for K⁺/ Na⁺ ions, it was found that the films doped with 0.1 to 0.5 mol% of yttrium had less lattice strain, while films with more than 0.5 mol% of Y³⁺ ions had increased strain due to the tendency of Y³⁺ to occupy the B-site in the perovskite lattice. Furthermore, by analysing the vibrational attributes of octahedron bonding, the dopant occupancy at A-site and B-site lattices could be identified. O-Nb-O bonding was asymmetric and became distorted due to the B-site occupancy of yttrium dopants at high dopant concentrations of >0.5 mol%. Extra conduction electrons had resulted in better resistivity of 2.153× 10⁶ Ω at 0.5 mol%, while higher resistivity was recorded for films prepared with higher concentration of more than 0.5 mol%. The introduction of Y³⁺ improved the grain distribution of KNN structure. Further investigations indicated that yttrium enhances the surface smoothness of the films. However, at high concentrations (0.9 mol%), the yttrium increases the roughness of the surface. Within the studied range of Y³⁺ _, the film with 0.5 mol% Y³⁺ represented a relatively desirable improvement in dielectric loss, tan δ and quality factor, Q_m.     

Low operating temperature CO sensor prepared using SnO<Subscript>2</Subscript> nanoparticles

A low operating temperature CO (carbon monoxide) sensor was fabricated from a nanometer-scale SnO₂ (tin oxide) powder. The SnO₂ nanoparticles in a size range 10–20 nm were synthesized as a function of surfactant (tri-n-octylamine, TOA) addition (0–1.5 mol%) via a simple thermal decomposition method. The resulting SnO₂ nanoparticles were first screen-printed onto an electrode patterned substrate to be a thick film. Subsequently, the composite film was heat-treated to be a device for sensing CO gas. The thermal decomposed powders were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD), and surface area measurements (BET). The CO-sensing performance of all the sensors was investigated. The experimental results showed that the TOA addition significantly decreased the particle size of the resulting SnO₂ nanoparticle. However, the structure of the powder coating was crucial to their sensing performance. After heat-treatment, the smaller particle tended to cause the formation of agglomeration, resulting in the decline of surface area and reducing the reaction site during sensing. However, the paths for the sensed gas entering between the agglomerated structure may influence the sensing performance. As a CO sensing material, the SnO₂ nanoparticle (~12 nm in diameter) prepared with 1.25 mol% TOA addition exhibited most stable electrical performance. The SnO₂ coating with TOA addition >0.75 mol% exhibited sensor response at a relatively low temperature of <50°C.     

Optimization of the thermoelectric properties of Bi<Subscript>2</Subscript>O<Subscript>2</Subscript>Se ceramics by altering the temperature of spark plasma sintering

The n-type polycrystalline Bi₂O₂Se ceramics were fabricated by solid state reaction and SPSed at different temperatures (873–973 K). The grains of the sample grow up gradually and the grain size enlarges from about 200 nm to micrometer level with the increase of sintering temperature. The highest electrical conductivity of 6.23 S/cm is obtained for the sample sintered at 898 K which benefits from grain orientation along (00l) plane and the highest measured density. The electrical transport properties tend to decline with further increase of the sintering temperature due to the decrease of density and the orientation degree. The maximum power factor of 78.39 μWK^?2m^?1 is obtained at 773 K. The thermal conductivity is intrinsically suppressed owing to the layered crystal structure of Bi₂O₂Se and fine grains within the nanometer size. The ZT value reaches 0.09 at 773 K for the sample SPSed at 898 K and the optimal temperature during the SPS process has been determined.     

The effect of growth atmosphere and Ir contamination on electric properties of La<Subscript>3</Subscript>Ta<Subscript>0.5</Subscript>Ga<Subscript>5.5</Subscript>O<Subscript>14</Subscript> single crystal grown by the floating zone and Czochralski method

The influence of growth atmosphere and Ir contamination on electrical resistivity of langatate (La₃Ta_0.5Ga_5.5O₁₄; LTG) was studied. LTG single crystals were grown via the Czochralski method under different oxygen partial pressures with Ir contamination from the Ir crucible. In addition, LTG crystals were grown by the floating zone method and they were not contaminated by Ir. The electrical resistivity and ionic transport number of these crystals were measured in the temperature range 300–1000°C. The conduction mechanism of LTG changed at about 720°C. At T < 720°C, electronic conduction was dominant, and the resistivity was affected by growth atmosphere as well as Ir contamination. In contrast, at T > 720°C, ionic conduction was dominant, and the resistivity was affected only by Ir contamination. In both temperature regions, Ir contamination decreased the resistivity by an order of magnitude.     

Normal-to-relaxor ferroelectric phase transition and electrical properties in Nb-modified 0.72BiFeO<Subscript>3</Subscript>-0.28BaTiO<Subscript>3</Subscript> ceramics

Preparation and electrical properties of sintered bodies consisting of monophase cubic spinel oxides, Cr_XMn_1.5Co_(1.0-X)Ni_0.5O₄ (0 ≦ X ≦ 0.42), were investigated. Specimens with compositions within X = 0.42 were prepared as starting materials. The element of Cr was used to exchange Co³⁺ in octahedral sites (B sites) with Cr³⁺ so that the hopping mechanism can be discussed. The sintered bodies with mono cubic spinel structure were confirmed to be prepared by heat-treatment for 48 h in air at 1000 °C to convert them into a cubic spinel structure after sintering at 1400 °C. The semiconductive characteristics of the sintered bodies were determined as p-type because the Seebeck coefficients were all positive. The electrical conduction of the sintered bodies was concluded to be controlled by the small polaron hopping mechanism. In the region 0.1 ≦ X ≦ 0.42, the lattice constant increases and electrical conduction (σ) decreases linearly with increasing Cr concentration. The decrease in σ and the increase in the lattice constant corresponded to the increase in Cr concentration by which the jumping distance of electrons between Mn³⁺ and Mn⁴⁺ is lengthened.