Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Direct synthesis of La9.33Si6O26 ultrafine powder via sol–gel self-combustion method

Single phase La_9.33Si₆O₂₆ ultrafine powder, as a kind of highly activated precursor to prepare medium-to-low temperature electrolyte for solid oxide fuel cells (SOFCs), has been successfully synthesized via a non-aqueous sol–gel and self-combustion approach from the starting materials: lanthanum nitrate (La(NO₃)₃·6H₂O), citric acid, ethylene glycol (EG), tetraethyl orthosilicate (TEOS) and ammonium nitrate. The details of gel's self-combustion were investigated by DTA–TG and the structural characterization of as-synthesized powder from self-combustion was performed by XRD and SEM. The results show that La_9.33Si₆O₂₆ single phase of apatite-type crystal structure can be directly synthesized by sol–gel self-combustion method without further calcinations on the condition that the molar ratio (R) of NO₃⁻ to citric acid and ethylene glycol being 6:1. Such powders composed of well-dispersed particles with an average size of 200 nm and a specific surface area of 5.54 m²/g. It can be sintered to 90% of its theoretical density at 1500 °C for 10 h, about 200 °C lower than the sintering temperature for the powder derived from traditional solid reactions. The sintered material has a thermal expansion coefficient of 9.2 × 10⁻⁶ K⁻¹ between room temperature and 800 °C.     

Sm0.5Sr0.5CoO3 cathode material from glycine-nitrate process: Formation, characterization, and application in LaGaO3-based solid oxide fuel cells

Cathode material Sm_0.5Sr_0.5CoO₃ (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm_0.5Sr_0.5CoO₃ were investigated. The single cell performance was also tested using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm₂O₄ phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10⁻⁶ K⁻¹ from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm² at 800 and 850 °C, respectively.     

Low temperature synthesis of nanocrystalline lanthanum monoaluminate powders by chemical coprecipitation

Nanocrystalline lanthanum monoaluminate (LaAlO₃) powders were prepared by chemical coprecipitation using 25 vol.% of NH₄OH, 0.05 M La(NO₃)₃·6H₂O and 0.05 M Al(NO₃)₃·9H₂O aqueous solutions as the starting materials. Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analyses (TGA/DTA), X-ray diffraction (XRD), Raman spectrometry, specific surface area (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED) were utilized to characterize the LaAlO₃ powders prepared by chemical coprecipitation. The crystallization temperature of the LaAlO₃ precursor gels precipitated at pH 9 is estimated as 810 °C by TG/DTA. The XRD pattern of the LaAlO₃ precursor gels precipitated at pH 8–12 and calcined at 700 °C for 6 h shows a broad arciform continuum exist between 24° and 32° and sharp peaks of LaAlO₃ except the precursor gels precipitated at pH 9. For the LaAlO₃ precursor gels precipitated at pH 9 and calcined at 700 °C for 6 h, the formation of the perovskite LaAlO₃ phase occurs and the presence of crystalline impurities is not found. The crystallite size of LaAlO₃ slightly increases from 37.8 to 41.5 nm with calcination temperature increasing from 700 to 900 °C for 6 h. The LaAlO₃ powders prepared by chemical coprecipitation have a considerably large specific surface of 30 m²/g. The relative density greater than 97% is obtained when these nanocrystalline LaAlO₃ powders are sintered at 1550 °C for 2 h.     

Sintering of alumina-niobium carbide composite

Several studies have been focused on particulate-dispersed Al₂O₃ composites in order to improve both room and high temperature mechanical properties and wear resistance. In the present work Al₂O₃-NbC composites have been pressureless sintered and their microstructures analysed as a function of NbC and Y₂O₃ concentration, the latter added as sintering aid. The compositions used in this study were Al₂O₃-xNbC and (Al₂O₃ 3%Y₂O₃)-xNbC, (x = 10, 20 and 40 wt%) and the sintering was performed at 1650 °C/30 min and 1750 °C/15 min. A density greater than 96% of the theoretical density was reached even for those materials sintered at 1650 °C. The observed microstructure was more homogeneous for the samples with Y₂O₃ addition and the Y₃Al₅O₁₂ phase was detected. The Al₂O₃ grain growth restraining due to the NbC concentration was more pronouncedly in samples sintered at 1750 °C.     

Effect of yttria concentration on low strain rate flow stress of cubic zirconia single crystals

The effects of Y₂O₃ solute concentration, strain rate, and temperature on solid-solution strengthening in single crystal yttria-stabilized cubic zirconia was investigated. Previous work was extended by studying the flow behaviour at strain rates from 1.5 × 10⁻⁵ s⁻¹ to 8 × 10⁻⁸ s⁻¹ at 1200, 1300 and 1400°C in the harder 001 orientation. Solute hardening in this system is sensitive to strain rate down to 8 × 10⁻⁸ s⁻¹, but at 1400°C there was no difference in flow stress between a 9.4 mol% alloy and 21 mol% alloy at a strain rate of 8 × 10⁻⁸ s⁻¹. The results were explained by the solute drag model.     

Study on phase formation and sintering kinetics of BaTi0.6Zr0.4O3 powder synthesized through modified chemical route

BaTi_0.6Zr_0.4O₃ powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO₂. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO₃ (BT) and BaZrO₃ (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi_0.6Zr_0.4O₃ (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol⁻¹) formation was found to be less than that of BZ (167.5 kJ mol⁻¹) formation. BTZ formation requires 503.6 kJ mol⁻¹ of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol⁻¹.     

Preparation of nano-sized SrAl2O4 using an amorphous hetero-nucleus complex as a precursor

Nano-crystalline SrAl₂O₄ with spinel structure was successfully prepared at 700 °C using amorphous SrAl₂(diethylenetriaminepentaacetic acid (DTPA)_1.6)(H₂O)₄ as precursor. The precursor was synthesized by a simple inorganic reaction and decomposed into SrAl₂O₄ at temperatures above 500 °C, which was proved by DTA–TGA and X-ray photoelectron spectroscopy (XPS) analysis. X-ray diffraction (XRD) results illustrated that a crystalline SrAl₂O₄ phase can form at 700 °C, which is about 600 °C lower than that used in the traditional method. The crystalline SrAl₂O₄ prepared at 900 °C for 2 h had a crystal size of about 28 nm and a grain size of about 80 nm, and its BET surface area can reach 28.056 m²/g. Calcination temperature and time had a weak effect on crystal size.     

Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy

In this paper we present a transmission electron microscopic study on the effect of annealing on the microstructure of a cold-rolled Ni_50.2Ti_49.8 ribbon. Transmission electron microscopy of the as-received sample shows the presence of alternating amorphous and crystalline bands. The crystalline bands have widths of the order of a few microns and contain amorphous nanopockets and B2 nanograins, the latter at around 20 nm diameter and preferentially oriented with their normal along the 111 direction and perpendicular to the strip surface. As-received samples were annealed for 30 min at different temperatures up to 800 °C. Crystallization starts in the amorphous bands at around 350 °C and finally ends up with the coarsening of the grains in the entire sample. Annealing of the samples at 450 °C entirely transforms the amorphous bands into crystalline bands. At 800 °C the grain size increases to 30–50 μm with a formation of a tweed kind of morphology inside the grains when observed at room temperature. Diffraction patterns from such grains reveal the presence of diffuse intensity around 1/3110^* indicating the formation of the R-phase. NiTi₂ precipitates form at 450 °C while annealing at 600 °C and higher yields Ni₃Ti₂ precipitates. For samples annealed at 500 °C for a longer time, Ni₄Ti₃ precipitates have been observed along with the austenite to martensite transformation in the grains.     

Nickel electrodeposition from novel citrate bath

A new type of electroplating bath suitable for nickel electrodeposition was developed. Trisodium citrate was used as a complexing agent and a buffer in the bath. The buffering capacity between trisodium citrate and boric acid were compared. The effects were investigated under different conditions of bath composition, current density, pH and temperature on the potentiodynamic cathodic polarization curves, cathodic current efficiency and throwing index, as well as the electrical conductivity of these baths. The optimum conditions for producing sound and satisfactory nickel deposits were： NiSOn6H2O350 g/L, NiCl2·6H2O 45 g/L and Na3C6H5 O7 30 g/L at pH=4 and 55 ℃. The surface morphology of the as-plated Ni deposit was examined by SEM. The results reveal that the nickel deposition obtained from the optimum conditions are composed of compact, non-porous fine grains covering the entire surface. X-ray analysis shows that nickel deposits obtained from the citrate bath have a fine crystal structure compared with deposits from the Watts bath.     

Ionic conductivity and Raman investigations on the phase transformations of Na4P2O7

The ionic conductivity and thermo-Raman spectra of anhydrous sodium pyrophosphate Na₄P₂O₇ were measured dynamically in the temperature range from 25 to 600 °C with a heating rate of 2 °C min⁻¹ to understand the structural evolution and phase transformation involved. The DSC thermogram was also measured in the same thermal process for the phase transformation investigation. The spectral variations observed in the thermo-Raman investigation indicated the transformation of Na₄P₂O₇ from low temperature phase () to high temperature phase () proceeded through pre-transitional region from 75 to 410 °C before the major orientational disorder at 420 °C and minor structural modifications at 511, 540 and 560 °C. The activation energies and enthalpies of the proposed phase transformations were determined. The possible mechanism for temperature dependent conductivity in Na₄P₂O₇ was discussed with the available data.     

Preparation of epitaxial and polycrystalline bismuth titanate thin films on single crystal (100) MgO by chemical solution deposition

Epitaxial and polycrystalline Bi₄Ti₃O₁₂ thin films were prepared on single crystal (100) MgO substrates by a chemical solution deposition process using metal naphthenates as starting materials. Pyrolyzed films (at 500°C) were annealed for 30 min in air at 650, 700, 750 and 800°C, respectively. The effects of annealing temperature on the crystallinity, epitaxy and surface morphology of the films were investigated by X-ray diffraction θ-2θ scans, pole-figure analysis, and atomic force microscopy (AFM). Epitaxially grown films annealed at 700 and 750°C, respectively, showed growth of three-dimensional needle-shaped grains. During annealing at 800°C, grain growth of Bi₄Ti₃O₁₂ may be suppressed by the formation of a titanium-rich phase such as Bi₂Ti₂O₇ owing to Bi volatilization, resulting in lower root mean square roughness than that of film annealed at 750°C.     

Structural and luminescent properties of nanostructured KGdF4:Eu synthesised by coprecipitation method

The single-phase low-temperature cubic form of KGdF₄, with the average crystallites size of 19 nm precipitates from a solution. The cubic phase is stable up to approximately 460 °C. At higher temperatures the cubic KGdF₄ transforms, first to the orthorhombic and then to the trigonal phase, however, the transformation is not complete and the cubic form is still present. The highest concentrations of orthorhombic and trigonal phases were observed at temperatures of 540 and 720 °C, respectively. The single-phase high-temperature cubic form of KGdF₄ was obtained when the as-received sample was heated at 790 °C. In contrast to a coprecipitation (CP) method, the single-phase orthorhombic modification of KGdF₄ was obtained by a solid state (SS) reaction conducted at 650 °C. In excitation spectra recorded for CP samples the characteristic Eu³⁺–O²⁻ CT bands expected at 260 nm are not present, which indicates that oxygen impurities are practically absent. However, both IR spectra and the ⁵D_1,2,3/⁵D₀ emission branching ratio points to the presence of OH⁻ groups incorporated into a fluoride lattice and a heat treatment at temperature of at least 650 °C is indispensable to get rid of them. Emission spectra for samples with different processing conditions were recorded and are discussed.     

Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.8Ti0.2O3−δ solid solutions

Series of perovskite-type compounds La_1−aCa_aCr_0.8Ti_0.2O_3−δ (a=0–1.0) were synthesized by the ceramic technique in air (final heating 1350 °C). The crystal structure of the compounds after cooling in air to room temperature was characterized as orthorhombic in space group Pbnm. Analysis of the lattice constants shows a noticeable decrease with increasing Ca content. All compounds prepared were stable in air and in a stream of Ar/1 Pa O₂ at 20–1400 °C, as also in Ar/5% H₂ (pH₂O/pH₂=0.01) at 850–1000 °C. Oxygen stoichiometry and electrical conductivity of the solid solutions with a=0.0–1.0 are investigated. Increasing Ca contents decrease the stability of the oxides in respect to the thermal dissociation of oxygen. All compounds are p-type semiconductors in the temperature range 20–1000 °C at oxygen partial pressures of 10⁻¹⁵ to 0.21×10⁵ Pa. A maximum conductivity of about 30 S/cm in air at 1000 °C is observed for the composition with a=0.6 corresponding to a ratio of Cr³⁺/Cr⁴⁺=1 at an oxygen stoichiometry near 3.0, and oxidation states of La, Ca, Ti, and O ions of 3+, 2+, 4+, and 2−, respectively.     

Effects of the solution temperature and the pH on the electrochemical properties of the surface oxide films formed on Alloy 600

The surface oxide films on Alloy 600 have been investigated as a function of the solution temperature and the pH by using a cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and a depth profiling by Auger electron spectroscopy (AES). H₃BO₃, Na₂SO₄ and NaOH aqueous solutions with temperatures in the range of 30–300 °C were used as the test solutions. As the solution temperature of the 0.5 M H₃BO₃ increased, the thickness of the passive film increased but the resistance of the passive film was diminished, which is coincident with a solution temperature dependency of the passive current in the potentiodynamic curve. The inner oxide film on Alloy 600 was distinguishable from the Cr-rich outer oxide film above 100 °C. From the Mott–Schottky relation, the oxide formed in 0.5 M H₃BO₃ at 300 °C showed a p-type semiconductor property, accompanied by a Cr-rich oxide film throughout the whole oxide film unlike the n-type oxide films up to 250 °C. The oxide resistance of the passive film decreased in the order of 0.5 M H₃BO₃, 0.1 M NaOH and 0.5 M Na₂SO₄, which is consistent with the pH dependency of the passive current. Ni-rich oxide films of a p-type were formed in the 0.5 M Na₂SO₄ or 0.1 M NaOH.     

BH复合催渗程控系统的构建及数值模拟

通过对可控气氛多用炉设备进行改造,将闲置不用的氨气管道改为BH催渗用滴注管道并设计了催渗剂滴注控制系统,实现了BH渗碳工艺的自动化控制。研究了20Cr2Ni4A钢930 ℃超级渗碳和BH渗碳两种工艺的渗碳动力学,并模拟了渗层深度与碳浓度分布曲线。结果表明: BH渗碳时碳扩散系数要大于超级渗碳,BH渗碳的扩散系数为2.084×10^-9 mm²·s^-1,超级渗碳为1.667×10^-9 mm²·s^-1。本文模拟的20Cr2Ni4A钢在930 ℃进行BH渗碳及超级渗碳时的碳浓度分布和渗层厚度变化与实际验证情况相符。实际渗碳后要达到相同渗层厚度,采用超级渗碳所需的时间要高于BH渗碳,BH渗碳的渗碳速率提高了22.6%。     

Structural phase transformations of rare-earth modified transition alumina to corundum

The effect of rare-earth dopant on transformations of the γ→θ→ phases in fine alumina powders under vacuum was investigated by in situ neutron diffraction from 500 to 1300 °C. La-doped (1 mol%) Al₂O₃ powders (surface area 170 m²/g) were prepared by an impregnation technique. Below 800 °C both samples contain a dominant γ-phase. Above 1000 °C, transformation of the γ-phase to the intermediate θ-phase, and then completely to the -phase (corundum) was observed. Addition of 1 mol% of La in Al₂O₃ effectively shifts the -phase formation temperature from 1125 °C for pure alumina to 1250 °C, probably due to the larger size of La compared to Al ions, which hinders ionic diffusion in the processes of sintering and transformation. Consequently, doping La in alumina improves the surface-area and thermal stability at high temperatures, which is important for catalytic applications.     

Influence of ageing treatment on work hardening behaviour of a Ni-base superalloy

Thus it may be summarised that work hardening behaviour of the alloy superni 263 can not be analysed using the simple power law equation. The work hardening behaviour is satisfactorily analysed using the modified power law equation (Ludwigson equation). There is systematic variation in the different work hardening parameters K₁, n₁, K₂, n₂ and _C with the period of ageing at 800 °C. The drastic lowering of the parameters n₁ and e_up, from ageing even for a short duration, suggests that this material should be formed in fully solution treated condition and any precipitation of γ′ must be avoided for good formability.     

Electrical properties of silver vanadate electrochemical cells

The silver iodide based ternary system xAgI-yAg₂O-zV₂O₅ (x = 10, 20, 30, …, 90; y/z = 2) was prepared by rapid quenching of the melt at liquid nitrogen temperature. X-ray diffraction confirmed the glassy or polycrystalline nature of the powdered phases. The 70AgI-20Ag₂O-10V₂O₅ phase has the highest room temperature (300 K) electrical conductivity of 0.011 S m⁻¹ at 1 kHz. Scanning electron microscopy showed that the surface of the 70AgI-20Ag₂O-10V₂O₅ as-quenched phase contains separate agglomerates. IR spectroscopy revealed bands at approximately 960, 920, 890, 850, 820 and 700 wavenumbers, indicating the possible existence of [VO₄]³⁻ clusters. Solid electrochemical cells fabricated from the phase with the highest electrical conductivity showed that the transference number is almost unity and that the phase is an ionic conductor. The internal resistance of this battery is approximately 460 ω. When discharged at a load current of 30 μA, the current density is 0.04 mA cm⁻², the discharge capacity is 3.78 C, the power density is 0.012 W kg⁻¹ and the energy density is 1.512 J g⁻¹ for a circular cell of mass 1.55 g and a surface area of 1.3 cm².