Electrical properties of BaTiO3-based NTC thermistors doped by BaBiO3 and La2O3

In our research it has been firstly found that BaTiO₃ materials doped with BaBiO₃ only show negative temperature coefficient effect over a wide temperature range. Major phases present in the sintered bodies are BaTiO₃ compounds with a perovskite structure and BaBiO₃ compounds with a monoclinic structure. Also, at a given BaTiO₃ and BaBiO₃ content, the influence of La₂O₃ content on the microstructure and electrical properties has been investigated. The mean grain size of samples decreases with an increase in La₂O₃ content. However the mean grain size remains unchanged with a further increase in La₂O₃ content when the La₂O₃ content in material is more than 0.20. As the amount of La₂O₃ in BaTiO₃-based ceramics thermistors increases, the resistivity decreases to a minimum value and then slowly increases again.     

Interface Oxygen Vacancy-Enhanced Co3O4/WO3 Nanorod Heterojunction for Sub-ppm Level Detection of NOx

Herein, a cobalt oxide/tungsten oxide (Co₃O₄/WO₃) p–n heterojunction for NO_x detection is developed and optimized. Field-emission scanning electron microscopy shows that the WO₃ nanorods are embellished with Co₃O₄ nanostructure. X-ray photoelectron spectroscopy reveals the presence of oxygen vacancy on the Co₃O₄ coupled with WO₃ heterojunction. Compared to bare WO₃ and pure Co₃O₄, the Co₃O₄/WO₃ heterojunction sensor shows significant sensitivity to NO_x at 200 °C. The sensor exhibits higher linearity from 0.4 to 10 ppm of NO_x, with a sensing response of 4.4–93%. The NO_x sensitivity of the Co₃O₄/WO₃ heterojunction sensor is ninefold higher than that of the pure WO₃ sensor. Even in a high humidity (84%) environment, the Co₃O₄/WO₃ heterojunction sensor demonstrates high NO_x sensitivity. The sensor maintains remarkable stability measured for up to 4 weeks. The possible NO_x sensing mechanism of the Co₃O₄/WO₃ heterojunction is additionally discussed.     

Electronic structures of Mg3Pn2 (Pn=N, P, As, Sb and Bi) and Ca3N2 calculated by a first-principle pseudopotential method

Electronic structure calculations for Mg₃N₂, Mg₃P₂, Mg₃As₂ (low and high temperature modifications), Mg₃Sb₂, Mg₃Bi₂, and Ca₃N₂ have been performed. Mg₃Sb₂ is predicted to be an indirect semiconductor with the gap value of about 0.41 eV. Mg₃As₂ with a high temperature modification is also predicted to be a semiconductor with the gap value of about 1.1 eV, but the valence band maximum and the conduction band minimum of Mg₃Bi₂ contacts at Γ which would make it a semimetal. Mg₃N₂, Mg₃P₂, and Mg₃As₂ (low temperature phase) are semiconductors with the direct band gaps of 1.64 eV, 1.73 eV, and 1.57 eV, respectively. Ca₃N₂ is a semiconductor with a gap of about 1.2 eV.     

Effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of materials of different functional groups

This paper describes the synthesis of Ba-doped PbZn_1/3Nb_2/3O₃-PbMg_1/3Nb_2/3O₃-PbNi_1/3Nb_2/3O₃-PbTiO₃ multicomponent solid solutions rich in PbNi_1/3Nb_2/3O₃, which include ferroelectric relaxors, ferroelectrics with a diffuse phase transition, and classic ferroelectrics, and presents the phase diagrams along three cuts through a section near PbNi_1/3Nb_2/3O₃. We examine the effect of sintering temperature on the density, piezodielectric response, and mechanical and elastic properties of ceramics of different functional groups; optimize the conditions for the fabrication of such ceramics, and compare the performance parameters of the materials obtained with those of their commercially available analogs.     

Co-substitution at the Mn-site in YMnO3: Structural stability and physical properties

The YMnO₃ oxide, crystallizing in the P6₃cm space group is one of the most studied multiferroic material. While these properties can be modified by substitutions at the Mn-site, the peculiar fivefold coordination appears to limit the M solubility range in YMn_1?xM_xO₃. This motivated the preparation by solid state reaction of several series of co-substituted YMn_1?x(M′,M″)_xO₃, which were studied using both powder X-ray diffraction and transmission electron microscopy. The cation pairs have been chosen for their ability to adopt a fivefold coordination similar to that of Mn³⁺ in YMnO₃, to keep a constant average trivalent oxidation state and to maintain an average cationic radius similar to that of Mn³⁺. For YMn_1?x(Cu_1/2Ti_1/2)_xO₃, a complete solid solution is observed without cation ordering. For all other studied cation pairs, the solubility range is always reaching a minimum value of 25% for the Zn containing pairs (Zn_2/3V_1/3 and Zn_3/4Mo_1/4), a maximum of 50% being reached for the Cu containing pairs (Cu_2/3V_1/3, Cu_2/3Nb_1/3 and Cu_3/4W_1/4). Interestingly, the non-centrosymmetric P6₃cm space group is retained within the solubility range without generating cation ordering phenomena. These co-substitutions at the Mn³⁺ are found to induce a decrease of the antiferromagnetic ordering temperature, the T_N variations being similar to the temperature decrease of the dielectric anomaly. Thus, these oxides are interesting candidates for the study of the multiferroic behavior of YMnO₃ derivatives.     

Luminescence properties of BaLaGa3O7

The luminescence properties of undoped and Fb²⁺-, Tb³⁺-, and Ce³⁺-doped BaLaGa₃O₇ have been studied. At LHeT undoped BaLaGa₃O₇ shows an emission with a maximum at 490 nm. The emission of Ba_0.99Pb_0.01LaGa₃O₇ consists of a broad band with a maximum at 400 nm, that of BaLa_0.89Tb_0.02Ga₃O₇ of the well-known ⁵D₃ and ⁵D₃ Tb³⁺ emission lines. The last compound shows a strong afterglow. BaLa_0.98Ce_0.02Ga₃O₇ did not luminescence; a possible explanation is presented.     

Interfacial Electron Transfer Barrier at Compact TiO2/CH3NH3PbI3 Heterojunction

Low‐temperature solution‐processed CH₃NH₃PbI₃ interfaced with TiO₂ has recently been demonstrated as a highly successful type‐II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH₃NH₃PbI₃ to TiO₂ is expected. However, by probing the photoexcited charge carrier dynamics in CH₃NH₃PbI₃/quartz, CH₃NH₃PbI₃/TiO₂ (compact), and CH₃NH₃PbI₃/PCBM in a comparative study, an electron transfer potential barrier between CH₃NH₃PbI₃ and the compact TiO₂ (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH₃NH₃PbI₃ photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO₂. The electron accumulation within CH₃NH₃PbI₃ needed to overcome this interfacial potential barrier results in the undesirable large current–voltage hysteresis observed for CH₃NH₃PbI₃/TiO₂ planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit.     

Exfoliated HNb3O8 nanosheets of enhanced acidity prepared by efficient contact of K2CO3 with Nb2O5

Although exfoliated HNb₃O₈ nanosheet (e-HNb₃O₈) is known to be a powerful catalyst in acid-catalyzed reactions, the preparation technique has limitations in its high-yield production. We herein report the enhanced exfoliation efficiency and recovery of e-HNb₃O₈ by contacting Nb₂O₅ solid with a diluted K₂CO₃ solution before ball milling (mechanochemical activation). The so-obtained e-HNb₃O₈ showed a larger specific surface area, a higher density of total acid sites and an improved catalytic performance in the dehydration of 2-heptanol and formic acid, compared to the corresponding sample prepared by the conventional solid-state mixing. The ball-milled K₂CO₃–Nb₂O₅ mixtures, layered KNb₃O₈, ion-exchanged HNb₃O₈ and exfoliated HNb₃O₈ samples were characterized by PXRD, TEM-EDS, TGA-MS and SEM-EDS. The characterization results revealed more intercalated K⁺ ions in KNb₃O₈ particles as well as more homogeneous K₂CO₃–Nb₂O₅ mixture when the employed preparation technique was used. This is achieved by more infiltration of K₂CO₃ into Nb₂O₅ lattice before ball milling, thereby resulting in the above positive findings. Therefore, our approach has great potential to be extended to other niobate-containing nanosheet-type materials.     

Reaction chemistry and subsolidus phase equilibria in lead-based relaxor systems: Part I Formation and stability of the perovskite and pyrochlore compounds in the system PbO-MgO-Nb2O5

The reaction chemistry involved in the synthesis of perovskite Pb(Mg_1/3Nb_2/3)O₃ [Pb₃MgNb₂O₉] was studied by the solid state reaction technique using precursor oxides as reactants. At the initial stage of the reaction process, a large fraction of PbO present in the mixtures combined with Nb₂O₅ and a small amount of MgO to form an oxygen-deficient pyrochlore phase with a composition Pb_1.714(Mg_0.286Nb_1.714)O_6.286 [Pb₆MgNb₆O₂₂]. The pyrochlore phase thus formed further reacted with the remaining PbO and MgO to yield the perovskite Pb(Mg_1/3Nb_2/3)O₃. The pyrochlore Pb_1.714(Mg_0.286Nb_1.714)O_6.286 accomodates a small amount of PbO into its lattice and forms a narrow homogeneity range which extends from the composition Pb_1.714(Mg_0.286Nb_1.714)O_6.286 [Pb₆MgNb₆O₂₂] to a composition Pb₂(Mg_0.286Nb_1.714)O_6.571 [Pb₇MgNb₆O₂₃] with a corresponding increase in the lattice constant value from a = 10.586 to 10.601 Å. The pyrochlore phase melts incongruently at a temperature near 1230°C to yield Mg₄Nb₂O₉ and a liquid. Below this temperature, the perovskite Pb(Mg_1/3Nb_2/3)O₃ coexists with the pyrochlore solid solutions. However, the compound Pb(Mg_1/3Nb_2/3)O₃ is not compatible with Nb₂O₅ and these two phases react with one another to form the pyrochlore Pb_1.714(Mg_0.286Nb_1.714)O_6.286 and MgO.     

Properties of hetero-structured SiCX films deposited by hot-wire CVD using SiH3CH3 as carbon source

The hetero-structured SiC_X films have been deposited by hot-wire CVD using SiH₃CH₃ as the carbon source gas. Although the carbon source gas ratio and filament temperature in the deposition using SiH₃CH₃ were smaller than those using C₂H₆, the carbon content in the sample deposited using SiH₃CH₃ was similar to that deposited using C₂H₆. The optical energy gap in the sample deposited using SiH₃CH₃ was larger than that deposited using C₂H₆. The sample deposited using SiH₃CH₃ under optimized condition showed a wide optical energy gap of 1.99 eV and a large dark conductivity of 15.1 S/cm. The p-type sample deposited using SiH₃CH₃ under the optimized condition has been used as a window layer material in p-i-n a-Si:H based solar cells.     

The δ value dependence of photoluminescence in green-emitting Ba3Si6O12−δN2+2/3δ:Eu2+ phosphors for white light-emitting diodes

Green emitting Eu²⁺-doped Ba₃Si₆O_12?δ N_2+2/3δ phosphors with δ value varying in the range of ?0.6 to 1.8 were synthesized by firing the raw materials of α-Si₃N₄, SiO₂, BaCO₃ and Eu₂O₃ at 1,350 °C for 12 h under a N₂/H₂ atmosphere. The solid-solution and luminescence properties of as-prepared Ba₃Si₆O_12?δ N_2+2/3δ :Eu²⁺ phosphors were investigated by XRD, O/N automatic analyzer and UV–Vis spectrophotometer. The composition-dependent solid solution of Ba₃Si₆O_12?δ N_2+2/3δ with small range of δ is established. The emission wavelength with a range of 522–545 nm by varying δ value as a function of dopant Eu²⁺ is observed in this phosphor. The emission band shows a shift to higher energy and a diverse quenching concentration with an increase of N/O ratio in Ba₃Si₆O_12?δ N_2+2/3δ :Eu²⁺ phosphors. The measured results suggest that Ba₃Si₆O_12?δ N_2+2/3δ :Eu²⁺ possesses broad applications as an effective green phosphor in producing white light-emitting diodes.     

Crystal-chemical anomalies in nonsuperconducting PrBa2Cu3O7

Studies of the crystal chemistry of nonsuperconducting PrBa₂Cu₃O₇ indicate that this compound is strictly isostructural with its superconducting RBa₂Cu₃O₇ (R = Y, rare earth) analogs. Crystallographically, Pr is present in the trivalent state according to the structural trends exhibited by the RBa₂Cu₃O₇ series as a function of R³⁺ ionic radius. The sole structural anomaly attributable to the presence of Pr³⁺ in the YBa₂Cu₃O₇ structure is a next-next-nearest neighbor effect and consists of an unusually short axial Cu-O distance, i.e., a short bond length between the in-plane copper and the chain oxygen. The correlation of this anomaly with the nonmetallic/nonsuperconducting properties of PrBa₂Cu₃O₇ supports a variety of literature reports, both theoretical and experimental, suggesting that the apical oxygen in the YBa₂Cu₃O₇ structure plays a critical role in mediating the appearance of superconductivity. The mechanism by which the f-electrons in Pr³⁺ (f ²) interact with the Cu-O manifold to produce the nonmetallic behavior of PrBa₂Cu₃O₇ remains unknown; however, superconductivity is “turned back on” for Nd³⁺ (f ³), immediately next to Pr and just slightly smaller. Careful comparative studies of superconducting NdBa₂Cu₃O₇ and nonmetallic PrBa₂Cu₃O₇ are needed to elucidate the critical difference in the behavior of the f-electrons and may shed light on the fundamental mechanism of high-temperature superconductivity in copper oxides.     

Catalytic effect of Ti<Subscript>5</Subscript>Si<Subscript>3</Subscript> on thermal decomposition of Li<Subscript>3</Subscript>AlH<Subscript>6</Subscript>

Fine Ti₅Si₃ powder has been mechanochemically synthesized from a mixture of elemental Ti and Si powders. When Ti₅Si₃ is added as a catalyst into Li₃AlH₆, it shows a good catalytic ability by reducing the decomposition temperature and improving the decomposition kinetics as well. Although its catalytic effect is not as good as well-known TiCl₃, the use of Ti₅Si₃ has a benefit of releasing more hydrogen than TiCl₃ during dehydrogenation. This can be explained by that Ti₅Si₃, unlike TiCl₃, does not incur any chemical reactions with Li₃AlH₆ and thus remains inert during milling for dispersion.     

Solubility and coprecipitation of barium and strontium nitrates in HNO<Subscript>3</Subscript> solutions and multicomponent systems

The solubility of Ba(NO₃)₂ and Sr(NO₃)₂ in HNO₃ solutions at 25–95°C is characterized by the power dependence on the total concentration of the nitrate ion with the exponent for Ba(NO₃)₂ equal to −2 in ∼9 M HNO₃ and −6 in more concentrated acid solutions. The latter exponent is also characteristic of the more soluble Sr(NO₃)₂ throughout the examined range of HNO₃ concentrations. In strongly acidic solutions, Ba(NO₃)₂ coprecipitates with Sr(NO₃)₂. The solubility curve for Ba(NO₃)₂ in NH₄(Na)NO₃ solutions suggests formation of a double salt, whereas in UO₂(NO₃)₂ solutions the dependence is the same as in HNO₃ solutions.     

Examination of selectivity of templates for the textured potassium sodium niobate ceramics

To achieve the surface-orientation-controlled microstructure of K_0.5Na_0.5NbO₃, not only the template must exhibit the high anisotropy like a plate-shape but the reactive stability as well as the crystallographic coherency between the template and the K_0.5Na_0.5NbO₃ must be carefully considered. In this work, we focused on what kinds of ceramics are suitable for the fabrication of the surface-orientation-controlled K_0.5Na_0.5NbO₃. First of all, BaTiO₃, SrTiO₃, Ba(Zr_0.1Ti_0.9)O₃, KNbO₃, NaNbO₃, KTaO₃ and NaTaO₃ were chosen as the candidates in the regard of the crystallographic coherency. To verify the chemical stability between candidate ceramics and K_0.5Na_0.5NbO₃, the prepared candidate ceramic bulks were embedded by the K_0.5Na_0.5NbO₃ powder, they were annealed at 1100 °C, and then the interface regions between candidate ceramics and K_0.5Na_0.5NbO₃ were investigated using Secondary Electron Microscopy (SEM) equipped with Energy Dispersive Spectrometer (EDS). From these experimental results, BaTiO₃, SrTiO₃, Ba(Zr_0.1Ti_0.9)O₃, KNbO₃ and NaTaO₃ were found to be not appropriate for the templates for the tailoring of K_0.5Na_0.5NbO₃, while the NaNbO₃ and KTaO₃ showed a good crystallographic coherency and a chemical stability with K_0.5Na_0.5NbO₃.     

H3ONb3O8 and HNb3O8 : New protonic oxides with a layer structure involving ion exchange properties

The oxide H₃ONb₃O₈, with a layer structure, was synthesized by ion exchange reaction of the niobate KNb₃O₈ with an acid solution. H₃ONb₃O₈ is a good ion exchanger : the ion exchange reaction is reversible, and NH₄Nb₃O₈ can be obtained by action of an ammonium salt solution. The study of the thermal stability of H₃ONb₃O₈ allowed us to isolate the layer oxide HNb₃O₈ whose hydration is reversible, the R-Nb₂O₅ form, and a new form of niobium pentoxide closely related to the R-form, called R′. The structural characteristics of these oxides are studied and their structural relationships are discussed.     

Effects of additives on the nitridation process of foamed materials containing silicon and silicon carbide powders

Without using any additive, the nitridation process of silicon powder was slow and the main product was α-Si₃N₄ due to the cycling production of SiO species. The addition of Al₂O₃ and Y₂O₃ could facilitate the nitridation process resulting in a higher β-Si₃N₄ content presumably due to the liquid phase formed between Al₂O₃, Y₂O₃ and surface silica on silicon powder. When a small amount of 1.76% Fe₂O₃ was added, the accelerated nitridation process was attributed to the FeSi₂ liquid phase produced by reaction Fe element with surface silica at a lower temperature of 1212 °C, but the Al₂O₃ and Y₂O₃ additives could still be active for sustaining the nitridation process at higher temperature. At a higher Fe₂O₃ concentration of 3.46%, the nitridation process was mainly controlled by the formed FeSi₂ liquid phase. This study has demonstrated the active role of using Al₂O₃ and Y₂O₃ combination and Fe₂O₃ on the nitridation process, which could be helpful for further investigation on reaction bonding of SiC and Si₃N₄ ceramics.     

Microwave dielectric properties of mixed phase ceramics, Ba(Zn1/3Ta2/3)O3–xCaTiO3 and xMgTiO3–yCaTiO3–z(Nd2O3,wTiO2)

The effect of different chemical compositions on the microwave dielectric properties of Ba(Zn_1/3Ta_2/3)O₃–xCaTiO₃ and xMgTiO₃–yCaTiO₃–z(Nd₂O₃,wTiO₂) was studied. High fQ dielectrics were designed by optimizing composition and firing conditions. Adding up to 1 mol % CaTiO₃ to Ba(Zn_1/3Ta_2/3)O₃ increased ε_r from 25 to 30 at a firing temperature of 1450 °C, and produced very high fQ values of more than 100 000 GHz at a firing temperature of 1550 °C. EPMA and XRD suggested that ceramics based on Ba(Zn_1/3Ta_2/3)O₃ with CaTiO₃ had mixed phases of Ba(Zn_1/3Ta_2/3)O₃ and Ca–Ti–Zn–O. Addition of CaTiO₃ increased the Ba(Zn_1/3Ta_2/3)O₃ peak observed in XRD and decreased the Ba₃Ta₂O₈ peak. Prolonged sintering of Ba(Zn_1/3Ta_2/3)O₃ with CaTiO₃ increased the fQ value but kept ε_r constant. 0.5MgTiO₃–0.5CaTiO₃–z(Nd₂O₃,wTiO₂) showed a high dielectric constant ε_r>40 and fQ>20 000 GHz. When w=1, τ_f decreased linearly with z around 0 ppm/ °C in 0.5MgTiO₃–0.5CaTiO₃–z(Nd₂O₃,TiO₂) (0.25≤z≤0.5). X-ray and EDX analysis revealed a mixed phase matrix of MgTiO₃ and (Ca_1?αNd_2α/3)Ti_βO₃. It was concluded that ε_r of the high fQ materials Ba(Zn_1/3Ta_2/3)O₃–xCaTiO₃ and xMgTiO₃–yCaTiO₃–z(Nd₂O₃,wTiO₂) would be increased by varying their chemical compositions, x, y, z, and w, and that their fQ value would be improved by appropriate choices of heating temperature and time.     

Hydrothermal Crystallization in the Systems Nd2O3(Nd(NO3)3 ⋅ 6H2O)-CaCO3(BaCO3)-R-H2O (R = Na2CO3, K2CO3, NaCl, NH4Cl, NaHCO3, KHCO3, Na2CO3 + NaCl, Na2CO3 + NH4Cl, Na2CO3 + CO(NH2)2)

Hydrothermal crystallization in the systems Nd₂O₃(Nd(NO₃)₃ ? 6H₂O)-CaCO₃(BaCO₃)-R-H₂O (R = Na₂CO₃, K₂CO₃, NaCl, NH₄Cl, NaHCO₃, KHCO₃, Na₂CO₃ + NaCl, Na₂CO₃ + NH₄Cl, Na₂CO₃ + CO(NH₂)₂) was studied in the range 400–480°C. All of the precipitates were found to contain crystalline NdOHCO₃, NaNd(CO₃)₂, and Nd₂(OH)₄CO₃. The lattice parameters of NdOHCO₃ were refined, and a detailed scheme for its thermal decomposition was proposed.     

Microwave dielectric properties of (1 − y)Nd(1−2x/3)Bax(Mg0.5Sn0.5)O3–yCa0.8Sr0.2TiO3 ceramic

The (1 ? y)Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little Nd₂Sn₂O₇ as the second phase. An apparent density of 6.89 g/cm³, a dielectric constant (ε _r ) of 19.1, a quality factor (Q × f) of 212,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?68 ppm/°C were obtained when the Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,550 °C for 4 h. The temperature coefficient of resonant frequency (τ _f ) increased from ?68 to +55 ppm/°C as y increased from 0 to 0.7 when the (1 ? y)Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were sintered at 1,600 °C for 4 h. 0.4Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–0.6 Ca_0.8Sr_0.2TiO₃ ceramic that was sintered at 1,600 °C for 4 h had a τ _f of ?7 ppm/°C.