Investigation of the effect of ZrO2 and ZrO2/Al2O3 additions on the hot-pressing and properties of equimolecular mixtures of α- and β-Si3N4

In this work, hot-pressing of equimolecular mixtures of α- and β-Si₃N₄ was performed with addition of different amounts of sintering additives selected in the ZrO₂–Al₂O₃ system. Phase composition and microstructure of the hot-pressed samples was investigated. Densification behavior, mechanical and thermal properties were studied and explained based on the microstructure and phase composition. The optimum mixture from the ZrO₂–Al₂O₃ system for hot-pressing of silicon nitride to give high density materials was determined. Near fully dense silicon nitride materials were obtained only with the additions of zirconia and alumina. The liquid phase formed in the zirconia and alumina mixtures is important for effective hot-pressing. Based on these results, we conclude that pure zirconia is not an effective sintering additive. Selected mechanical and thermal properties of these materials are also presented. Hot-pressed Si₃N₄ ceramics, using mixtures from of ZrO₂/Al₂O₃ as additives, gave fracture toughness, K_IC, in the range of 3.7–6.2 MPa m^1/2 and Vicker hardness values in the range of 6–12 GPa. These properties compare well with currently available high performance silicon nitride ceramics. We also report on interesting thermal expansion behavior of these materials including negative thermal expansion coefficients for a few compositions.     

Study of AC electrical properties of V2O5–P2O5–B2O3–Dy2O3 glasses

The AC conductivity of glass samples of composition 60V₂O₅–5P₂O₅–(35−x)B₂O₃–xDy₂O₃, 0.4≤x≤1.2 has been analyzed. The samples were prepared by the usual melt-quench technique. The prepared compounds were analyzed by X-ray diffraction (XRD) and thermo gravimetric–differential thermal analysis (TG/DTA). The activation energies were evaluated using glass transition temperature (T_g) and peak temperature of crystallization (T_c) from TG/DTA. The dependence of activation energy on composition was discussed. The electrical conductance and capacitance were measured over a frequency range of 20 Hz to 1 MHz and a temperature range of 303–473 K; these reveal semiconducting features based predominantly on an ionic mechanism. The dielectric and complex-impedance response of the sample is discussed. The relaxation time was found to increase with increasing temperature. Jonscher's universal power law is applied to discuss the conductivity. The electrode polarization was found to be negligible and confirmed from electrical modulus.     

Effect of the Nb2O5 content on electrical properties of lead-free BaTiO3–Bi0.5Na0.5TiO3 ceramics

(1−x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (BT–BNT) ceramics were prepared by the solid-state reaction method. With an increase of BNT content, both the Curie temperature and the room temperature resistivity increased. At 1 mol% BNT addition, the sample was not semiconducting, due to Bi₂O₃ volatilization resulting from the decomposition of pre-calcined BNT during sintering. Appropriate extra Nb₂O₅ doping in the raw materials could offset Bi₂O₃ volatilization and neutralize the redundant acceptor Na⁺ ions. When the extra Nb₂O₅ content was 0.6 mg, the sample room-temperature resistivity was 6.3×10³ Ω cm, with the Curie point about 135 °C and a high PTC effect of ∼3 orders of magnitude.     

Effects of liquid phases on densification of TiO2-doped Al2O3–ZrO2 composite ceramics

This study investigated the densification behaviors and microstructural evolution of Al₂O₃–ZrO₂ (3Y) composite ceramics doped with four different amounts of TiO₂ (0, 1, 4, and 8 wt%; denoted as 0T, 1T, 4T, and 8T, respectively) to clarify the effect of TiO₂ dopants on densification. The shrinkage rate during densification increased with the increase in the amount of TiO₂. The development of grain boundary feature was also examined. The undoped ceramic showed clean grain boundaries. Thin liquid grain boundary phases were observed in 1T, whereas large liquid phases were found on the grain boundary and at the junction pockets in 4T and 8T. The results were discussed in terms of the relationship between densification and grain boundary feature.     

Effect of Fe2O3 on properties and densification of Ce0.8Gd0.2O2−δ by PCAS

Dense Ce_0.8Gd_0.2O_2−δ was sintered by pulsed current activated sintering (PCAS) within 6 min from Ce_0.8Gd_0.2O_2−δ nanopowder prepared by co-precipitation method. Sintering was accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense Ce_0.8Gd_0.2O_2−δ with relative density of up to 96.3% was produced under simultaneous application of an 80-MPa pressure and the pulsed current. The effects of Fe₂O₃ additions on the sintering behavior, ionic conductivities, and mechanical properties of the Ce_0.8Gd_0.2O_2−δ were investigated.     

Dehydrogenation of propane in the presence of N2O over In2O3―Al2O3 mixed oxide catalysts

Dehydrogenation of propane coupled with N₂O over a series of binary In₂O₃―Al₂O₃ mixed oxides was investigated. In contrast to the poor performance for sole N₂O decomposition, a remarkable synergy was identified between N₂O decomposition and propane dehydrogenation. Among the catalysts tested, the In₂O₃―Al₂O₃ sample containing a 20 mol% In₂O₃ showed the highest activity for propane dehydrogenation in the presence of N₂O. Moreover, stability far superior to those of the conventional iron-based materials was observed, attributable to the moderate surface acidity of the In―Al―O composite. The essential role of N₂O is suggested to generate active oxygen species facilitating propane dehydrogenation.     

Influences of La and Er on structure and properties of Bi2O3–B2O3 glass

Bi₂O₃·2B₂O₃ glasses doped with La₂O₃ and Er₂O₃ were prepared by the melting-quenching method with AR-grade oxides. IR analysis was used to investigate the glass network structure. The characteristic temperatures including the glass transition temperature (T_g), crystallization temperature (T_p), and melting temperature (T_m) were estimated by DSC. The coefficient of thermal expansion (α), mass density (D), and Vickers hardness (H_v) were also measured. The results show that the basic network structure of Bi₂O₃·2B₂O₃ glasses doped with rare-earth oxides consists of chains composed of [BO₃], [BO₄], and [BiO₆] units. La₂O₃ and Er₂O₃ act as network modifiers. As the doping concentrations of the rare-earth oxides were increased, T_g increased and α decreased, indicating that a more rigid glass was obtained. Er₂O₃ reduces the melting temperature and prevents glass crystallization. La₂O₃ contributes to the improvement of the microhardness of Bi₂O₃·2B₂O₃ glass.     

Phase equilibrium of the ternary system of NH4Cl-CaCl2-H2O at 50°C

The equilibrium data on the ternary system of NH₄Cl—CaCl₂—H₂O at 50°C were investigated using the wet-residue method. The experimental results show that there are three pure phase crystal areas of NH₄Cl, 2NH₄Cl·CaCl₂·3H₂O and CaCl·2H₂O, two mixture phase crystal areas of NH₄Cl and 2NH₄Cl·CaCl·3H₂O, and 2NH₄Cl ·CaCl₂·3H₂O and CaCl·2H₂O in the system. A new hydration double salt (2NH₄Cl·CaCl·3H₂O) was found in the ternary equilibrium system for the first time.     

Electrochemical characteristics of electrospun La0.6Sr0.4Co0.2Fe0.8O3−δ-Gd0.1Ce0.9O1.95 cathode

The poor activity of cathode materials for electrochemical reduction of oxygen in intermediate and low temperature regime (<700 °C) is a key obstacle to reduced-temperature operation of solid oxide fuel cells (SOFCs). In our previous work, the direct methane fuel cell exhibits approximately 1 W cm⁻² at 650 °C in hydrogen atmosphere without any functional layers when the electrospun LSCF–GDC cathode was applied into the La₂Sn₂O₇–Ni–GDC anode-supported cell, which is approximately two times higher performance than 0.45 W cm⁻² of the cell with the conventional LSCF–GDC cathode. For detailed analysis of the fibrous cathode, the symmetrical cells with the electrospun and conventional LSCF–GDC cathode are fabricated, and then their electrochemical characteristics are measured by using electrochemical impedance spectroscopy (EIS). Each resistance contribution is determined by equivalent circuit consisting of a series resistance (R_s) and three arcs to describe the polarization resistance of the cathode. Total polarization resistance of the electrospun LSCF–GDC cathode is approximately two times lower than that of the conventional LSCF–GDC cathode at 650 °C, which is attributed to fibrous microstructures and large amount of pores in 100–200 nm. The results correspond to the difference in the cell performances obtained from our previous work.     

Effect of yttrium and praseodymium on properties of Ce0.75Zr0.25O2 solid solution for CH4–CO2 reforming

Ce_0.75Zr_0.25O₂ solid solutions doped with Y³⁺ or Pr⁴⁺/Pr³⁺ were prepared by the co-precipitation method, and their physicochemical properties were characterized by means of N₂ adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, FT-Raman, and H₂ temperature-programmed reduction and thermogravimetric analysis. Their performance in CH₄–CO₂ reforming was also tested in an atmospheric fixed-bed reactor. Ce_0.75Zr_0.25O₂ and Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂ solid solutions are of CaF₂ structure, and the thermal stability of Ce_0.75Zr_0.25O₂ is enhanced by doping Y³⁺ or Pr⁴⁺/Pr³⁺. Comparing with Ce_0.75Zr_0.25O₂, the migration of bulk lattice oxygen species become easier and the content of surface oxygen species is higher in the doped Ce_0.75Zr_0.25O₂, which is due to either oxygen vacancies or/and structural distortion resulted from the doping. The activity of the solid solutions in CH₄–CO₂ reforming is closely related to the surface oxygen species. Y³⁺ or Pr⁴⁺/Pr³⁺ doped Ce_0.75Zr_0.25O₂, especially the former, show higher activity than Ce_0.75Zr_0.25O₂, and Y³⁺ doped Ce_0.75Zr_0.25O₂ possesses better stability. All of the catalysts have good coke resistance. The catalyst deactivation is mainly due to the catalyst sintering.     

Determination of the eutectic solubility lines of the ternary system NaHCO3–Na2CO3–H2O

A crystallizer was built and a procedure developed to accurately measure the eutectic solubility lines where ice and salt coexist in equilibrium with the solution, for potential application of Eutectic Freeze Crystallization. The eutectic solubility lines of the ternary system NaHCO₃–Na₂CO₃–H₂O were determined experimentally and calculated with the extended UNIQUAC model. The extended UNIQUAC model describes the experimental data quite well. Anhydrous NaHCO₃ and Na₂CO₃·10H₂O were the only two types of crystals present in equilibrium with ice crystals in the ternary system. At the quadruple point NaHCO₃ and Na₂CO₃·10H₂O are in equilibrium with a solution of about 4.34 wt% of Na₂CO₃ and 4.77 wt% of NaHCO₃ at −3.32 °C. The anhydrous NaHCO₃ crystals were needle shaped with lengths between 5 and 10 μm, that were agglomerated into particles of about 100–300 μm, while the Na₂CO₃·10H₂O crystals were hexagonally shaped with sizes between 100 and 500 μm.     

The effect of [(η-C3H5)(η-C5H5)Mo(CO)2] on drying of solvent-borne alkyd paints

Allyl molybdenum complex [(η³-C₃H₅)(η⁵-C₅H₅)Mo(CO)₂] (Mo-A) was examined as a new drier for solvent-borne alkyd binder. The drying activity of pure Mo-A and the of combinations of Mo-A with cobalt(II) 2-ethylhexanoate (Co-Nuodex) was examined using standard methods for measurements of the drying time and hardness development of the alkyd film. The detailed study of the drying process was studied by time-resolved infrared spectroscopy on ethyl linoleate as well as on thin coating of alkyd binder.     

Effect of SrZrO3 on phase structure and electrical properties of 0.974(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3 lead-free ceramics

(0.974−x)(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–xSrZrO₃ lead-free piezoelectric ceramics have been prepared by the conventional solid state sintering method. Systematic investigation on the microstructure, crystalline structures as well as electrical properties of the ceramics was carried out. With the addition of SrZrO₃, the rhombohedral–orthorhombic phase transition temperature of the ceramics increases. Both the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions of the ceramics were modified to be around room temperature when x~0.05, and as a result remarkably strong piezoelectricity has been obtained in 0.924(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–0.05SrZrO₃ ternary system, whose piezoelectric parameters were d₃₃=324 pC/N and k_p=41%.     

Synthesis and properties of core–shell structured BaCe0.9Y0.1O2.95:BaZr0.9Y0.1O2.95

The perovskite proton conductor BaZr_0.9Y_0.1O_2.95 (BZY10) shows better chemical stability but lower conductivity than BaCe_0.9Y_0.1O_2.95 (BCY10). In this paper we attempted to synthesize BCY10:BZY10 core–shell materials in which BCY10 particles prepared by solid reaction were wrapped by a sol–gel deposited thin layer of BZY10 with ZnO as sintering aid to improve the sinterability of the materials. The effects of the BCY10/BZY10 ratios on the phase purity, microstructure, chemical stability and electrical conductivity of the samples were characterized by XRD, TEM, SEM, TGA and electrochemical impedance spectroscopy. A dense core–shell structure was formed after being sintered at 1300 °C for 10 h. The core–shell samples displayed improved stability against CO₂ and water vapor at high temperature. With BCY10/BZY10 ratio varying from 9:1 to 7:3, the core–shell samples became more stable, and the total conductivities decreased.     

Structure and electrical properties of Nd2O3-doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 ceramics

Nd₂O₃ doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (abbreviated to BNKT) binary lead-free piezoelectric ceramics were synthesized by the conventional mixed-oxide method. The results show that the BNKT ceramics with 0–0.15 wt.% Nd₂O₃ doping possesses a single perovskite phase with rhombohedral structure. The grain size of BNKT decreased with the addition of Nd₂O₃ dopant. The temperature dependence of the dielectric constant ?_r revealed that there were two-phase transitions from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric. A diffuse character was proved by linear fitting of the modified Curie–Weiss law. At room temperature, the specimens containing 0.0125 wt.% Nd₂O₃ with homogeneous microstructure presented excellent electrical properties: the piezoelectric constant d₃₃ = 134 pC/N, the electromechanical coupling factor K_p = 0.27, and the dielectric constant ?_r = 925 (1 kHz).     

Physical properties of pseudo quaternary Na2B4O7 – SiO2 – MoO3 – Dy2O3 glasses

Pseudo quaternary Na₂B₄O₇–SiO₂–MoO₃–Dy₂O₃ glasses with various concentrations of Dy₂O₃ were synthesized and characterized by ultrasound, DTA and X-ray diffraction (XRD). It was found that, the density, the molar volume and the elastic moduli increased while the ultrasonic velocities and the packing density decreased with increasing of Dy₂O₃ concentration. The lower values of the radii and the bond length of B₂O₃, compared to those of Dy₂O₃, decreased the interatomic forces between the reactant glass forming cations and oxygens inside the glassy network. Accordingly, this will increase the molar volume and in the same time will decrease the average cross-link density, the ultrasonic velocities, the glass transition temperature and the thermal stability. Also, this may result in a high density glass sample which will increase the elastic moduli. Based on Lasocka's model, a good correlation between the temperature of both glass transition ($T_{\mathit{gl}}$) and crystallization peak ($T_{\mathit{pc}})$was observed.     

Synthesis and characterization of MgAl2O4–ZrO2 composites

Different types of dense 5–97% ZrO₂–MgAl₂O₄ composites have been prepared using a MgAl₂O₄ spinel obtained by calcining a stoichiometric mixture of aluminium tri-hydroxide and caustic MgO at 1300 °C for 1 h, and a commercial yttria partially stabilized zirconia (YPSZ) powder as starting raw materials by sintering at various temperatures ranging from 1500 to 1650 °C for 2 h. The characteristics of the MgAl₂O₄ spinel, the YPSZ powder and the various sintered products were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), BET surface area, particle size analysis, Archimedes principle, and Vickers indentation method. Characterization results revealed that the YPSZ addition increases the sintering ability, fracture toughness and hardness of MgAl₂O₄ spinel, whereas, the MgAl₂O₄ spinel hampered the sintering ability of YPSZ when sintered at elevated temperatures. A 20-wt.% YPSZ was found to be sufficient to increase the hardness and fracture toughness of MgAl₂O₄ spinel from 406 to 1314 Hv and 2.5 to 3.45 MPa m^1/2, respectively, when sintered at 1600 °C for 2 h.     

The influence of In2O3 on electrical characteristics of iron mixed Li2O–PbO–B2O3–SiO2–Bi2O3 multi-component glass system

Glasses of the composition 19Li₂O–20PbO–20B₂O₃–30SiO₂–(10−x) Bi₂O₃–1Fe₂O₃: xIn₂O₃ with six values of x (0 to 5.0) were synthesized. Dielectric properties viz., dielectric constant, ε′(ω), loss, tan δ, ac conductivity, σ_ac, electric modulus, M(ω) over wide ranges of frequency and temperature and also dielectric break down strength have been studied as a function of In₂O₃ concentration. The temperature dispersion of real part of dielectric constant, ε′(ω) has been analyzed using space charge polarization model. The dielectric loss (and also the electric moduli) variation with frequency and temperature exhibited relaxation effects and these effects were attributed to the divalent iron ion complexes. The ac conductivity exhibited maximal effect, whereas the activation energy for the conductivity demonstrated minimal magnitude at about 1.0 mol% of In₂O₃. The conductivity mechanism is understood due to the polaronic transfer between Fe²⁺ and Fe³⁺ ions. The low temperature ac conductivity mechanism is explained following the quantum mechanical tunneling model. Spectroscopic studies viz., optical absorption and ESR spectra have revealed that the redox ratio (Fe²⁺/Fe³⁺) is maximal when the concentration of In₂O₃ is ~1.0 mol%. The higher values of dielectric parameters observed at 1.0 mol% of In₂O₃ are attributed to the presence of iron ions largely in divalent state and act as modifiers. The analysis of these results together with spectroscopic studies has indicated that when In₂O₃ is present in the glass matrix in higher concentrations (more than 1.0 mol%) iron ions predominantly exist in trivalent state, occupy substitutional positions and make the glass more rigid. Such enhanced rigidity of the network is causing the decrease of dielectric parameters with the concentration of In₂O₃. Finally it is concluded that In₂O₃ mostly participate in the glass network in octahedral positions and make act as reducing agent (for iron ions) in the studied glass matrix when its concentration is ≤1.0 mol%.     

Preparation of micrometer-sized α-Al2O3 platelets by thermal decomposition of AACH

Micrometer-sized α-Al₂O₃ platelets with hexagonal shape were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) using AlF₃ as an additive. The precursor and the calcined product were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The α-Al₂O₃ platelets with the size of 2-3 μm were obtained by calcining AACH at 1200 °C with 5 wt.% AlF₃. The morphology modification is attributed to the various growth rates along different crystal orientations due to the adsorption and uneven distribution of AlF₃. A step growth mode is responsible for the formation of the platelets.