Molten salt synthesis of LaAlO3 powder at low temperatures

Rhombohedral LaAlO₃ powder was synthesised by reacting equimolar La₂O₃ and Al₂O₃ in a molten KF–KCl eutectic salt for 3 h between 630 and 800 °C. The lowest synthesis temperature (630 °C) is about 1000 °C lower than that of conventional mixed oxide synthesis, and close to or lower than those used by most wet chemical methods. The LaAlO₃ particle size increased from <3 to 3–7 μm with increasing temperature from 630 to 700 °C, but changed little on further increasing temperature to 800 °C. On the other hand, it decreased with increasing salt/oxide weight ratio from 1:1 to 6:1. The “dissolution–precipitation” mechanism played a dominant role in the molten salt synthesis of LaAlO₃.     

Preparation of spherical nanoparticles of LaAlO3 via the reverse microemulsion process

Spherical LaAlO₃ nanoparticles in a reverse microemulsion consisting of solution (water phase), Tween-80 and Span-80 (surfactant), n-butanol (cosurfactant, and cyclohexane (oil phase) were prepared. Precursor powders and calcined powders were characterized by differential thermal analysis (DTA), thermogravimetry analysis (TG), X-ray diffraction (XRD) and transmission electron microscopy (TEM). A pure perovskite LaAlO₃ formed when the precursor hydroxides calcined at 800 °C for 2 h. The particle size was about 50 nm and the shape of the monodisperse particles is spherical. The reverse microemulsion process can dramatically lower the crystallization temperature of LaAlO₃ about 700 °C than the classical solid-state reaction method.     

Synthesis of La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 using a polymeric method

Nanocrystalline La_0.9Sr_0.1Al_0.85Mg_0.1Co_0.05O_2.875 (LSAMC) powders were synthesized via a polymeric method using poly(vinyl alcohol) (PVA). The effect of PVA content on the synthesized powders was studied. When the ratio of positively charged valences (Mⁿ⁺) to hydroxyl groups (OH) is 1.5:1, crystalline LaAlO₃ could be obtained at such a low calcination temperature as 700 °C. While at 900 °C the ratio is of less importance, since pure LaAlO₃ perovskite could be formed for all powders after calcination at 900 °C. Thermal analysis (TG/DTA) was utilized to characterize the thermal decomposition behaviour of precursor powders. The chemical structure of the calcined powder was studied by Fourier transform infrared (FTIR) spectroscopy. The powder morphology and microstructure were examined by SEM. Dense pellets with well-developed submicron microstructures could be formed after sintering at 1450 °C for 5 h. Compared with the solid-state reaction method, the sintering temperature is substantially lower for powder prepared by the PVA method. This is due to the ultrafine and highly reactive powder produced.     

Mechanical behavior of ferroelastic LaAlO3

The present study investigates the mechanical deformation response of lanthanum aluminate (LaAlO₃) under various loading rates and temperatures. Ferroelastic domains were observed in samples of different porosity. Uniaxial compression tests were performed at room temperature and different loading rates ranging from 0.03 to 5.75 MPa/s. Temperature variation experiments were performed at 93 K, 193 K, 293 K, 393 K, and 553 K. LaAlO₃ shows non-elastic stress-strain behavior in which hysteresis loops are observed during loading–unloading cycles owing to ferroelasticity. The slope of the stress-strain curve became steeper with increasing loading rate and temperature. After unloading, remnant strain was stored in the material owing to ferroelastic domain switching.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Rheological properties of aqueous alumina–alumina-doped Y-PSZ suspensions

The effect of the substitution of alumina (Al₂O₃) by 0.3 wt% Al₂O₃-doped 3 mol% yttria-partially stabilized zirconia (Y-PSZ) on the rheological properties of concentrated aqueous slips was studied. Al₂O₃–Al₂O₃-doped Y-PSZ aqueous suspensions with different Al₂O₃-doped Y-PSZ contents: 0, 22 and 50 vol% were prepared using ammonium polyacrylate (NH₄PA) as dispersant. The particle size distributions of Al₂O₃ and Al₂O₃-doped Y-PSZ powders were similar; however, the particle shape and the surface coating of alumina conferred a markedly higher specific surface area to the Al₂O₃-doped Y-PSZ powder. The substitution of Al₂O₃ by Al₂O₃-doped Y-PSZ in the mixtures decreased the negative surface charge of the powders at pH 9, thereby increasing the amount of NH₄PA adsorbed and consequently the electrosteric repulsion between particles. However, the viscosity and yield stress values increased with increasing Al₂O₃-doped Y-PSZ content for all the solid loading studied. This could be explained by a larger interaction size of the Al₂O₃-doped Y-PSZ particles which resulted in a higher effective volume solid fraction and a lower amount of free-liquid available for flow.     

Preparation and characterisation of mixed matrix Al2O3- and TiO2-based ceramic membranes prepared using polymeric synthesis route

Al₂O₃- and TiO₂-based ceramic membranes prepared using polymeric synthesis route were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and gas permeability tests. The influence of the final calcination temperature and the systematic investigation of the properties of the membranes are provided. The calcination temperature affected morphological, structural and chemical properties, as well as the gas permeability of the ceramic membranes. XRD analysis revealed rhombohedral and tetragonal structures of Al₂O₃ and TiO₂-based ceramic, respectively, prepared at calcination temperatures of 1100 and 1200 °C. The TiO₂-based ceramic matrix calcined at temperatures of 1100 and 1200 °C exhibited a well-defined crystalline microstructure with the grains increasing in size as a function of temperature. FTIR analysis revealed that phosphorus additives in orthoclase clay tend to form phosphonate groups during the calcination process. The decomposition of organic source was not fulfilled as tested at calcination temperatures of 1000, 1100 and 1200 °C.     

Fabrication of Lu2O3:Eu transparent ceramics using powder consisting of mono-dispersed spheres

Mono-dispersed spherical Lu₂O₃:Eu (5 mol%) powders for transparent ceramics fabrication were synthesized by urea-based homogeneous precipitation technique. The effects of the doped-Eu³⁺ on the synthesis of Lu₂O₃:Eu particles were investigated in detail. The results show that the doping of Eu³⁺ ions into Lu system can significantly decrease the particle size of the resultant precursor spheres. Owing to the sequential precipitation in Lu/Eu system, there are compositional gradients within each of the resultant precursor spheres. Well dispersed, homogeneous and spherical/near spherical Lu₂O₃:Eu powders were obtained after calcination at 600–1000 °C for 4 h. The powder calcined at 600 °C shows better sintering behavior and can be densified into transparent ceramic after vacuum sintering at 1700 °C for 5 h. The luminescence properties of the obtained Lu₂O₃:Eu powder and transparent ceramic were also studied.     

Microstructure and mechanical properties of ceramics obtained from chemically co-precipitated Al2O3-GdAlO3 nano-powders with eutectic composition

An efficient and flexible chemical co-precipitation method has been used to synthesize nanoscale Al₂O₃-GdAlO₃ powders with eutectic composition. The as-synthesized powders exhibit a highly dispersive and homogeneous distribution with an average particle size of 50 nm. The phase transition in the resulting powders strongly depends upon the calcination temperature. GdAlO₃ undergoes complete crystallization after calcination at 1050 °C, however, the diffraction peaks of α-Al₂O₃ are found at a relatively high calcination temperature of at least 1300 °C. The fully-densified Al₂O₃-GdAlO₃ ceramic with eutectic composition obtained by hot pressing the nanoscale powders at 1500 °C exhibits a room temperature flexural strength of 556 MPa, a Vickers hardness of 17.3 GPa and a fracture toughness of 7.5 MPa m^1/2. The high temperature flexural strength of the as-sintered Al₂O₃-GdAlO₃ ceramic is measured to be 515 MPa after bending tests at 1000 °C.     

Variation in structures and photoluminescence spectra of BaxEu1−xAl2O4 powders

Barium europium(II) aluminate (Ba_xEu_1?xAl₂O₄) powders were prepared by a solid-state reaction among barium carbonate (BaCO₃), europium oxide (Eu₂O₃), and alumina (Al₂O₃) powders at 1400 °C for 3 h under a mixed gas flow of H₂ and N₂. The powders were characterized by powder X-ray diffraction (XRD), infrared and Raman spectroscopy, and photoluminescence (PL). With increasing Ba²⁺ content in Ba_xEu_1?xAl₂O₄, the structure of Ba_xEu_1?xAl₂O₄ changed from a monoclinic (P2₁) to hexagonal (P6₃) phase. The hexagonal (P6₃22) phase was also observed between the two phases. The XRD pattern of a single Ba_0.6Eu_0.4Al₂O₄ phase, which has not been reported in the literature, was refined by the Rietveld method and its structure was confirmed by selected-area electron diffraction. With increasing x value, the emission peak in the PL spectra of Ba_xEu_1?xAl₂O₄ became weaker (x = 0–0.4) and then more intense (x = 0.6–0.98), and its position showed a blue shift from 520 to 498 nm.     

Stabilizing effect of the carbon shell on phase transformation of the nanocrystalline alumina particles

Intensive phase transformations of alumina are known to occur at temperatures above 1000 °C. In the present work, high temperature behaviour of pure Al₂O₃ and the carbon coated Al₂O₃@C sample with core-shell structure was comparatively studied using low-temperature nitrogen adsorption, transmission electron microscopy, powder X-ray diffraction (XRD) analysis and solid-state nuclear magnetic resonance (NMR). The solid-state NMR ²⁷Al method has allowed us to identify and estimate the concentration of all phases appeared during the transformation of pseudoboehmite γ-Al₂O₃ into corundum α-Al₂O₃. The data obtained correlate well with the results of XRD analysis and low-temperature nitrogen adsorption. It is shown that the deposition of carbon coating with formation of core-shell Al₂O₃@C system stabilizes the size of oxide core and prevents the formation of corundum phase until the temperatures of 1350–1400 °C, which are close to the temperature of carbothermal reduction of alumina. The stabilization of the size of the oxide core nanoparticles was considered as a main factor preventing the formation of corundum phase at high temperatures. At the same time, the carbon coating does not affect the phase transformation of γ-Al₂O₃ to δ-Al₂O₃.     

Magnetic and electrical properties of La0.7Ca0.3Mn0.95Co0.05O3 epitaxial layers by pulsed laser deposition

Epitaxial La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (LCMCO) thin films were prepared on (1 0 0) LaAlO₃ single-crystal substrates by pulsed laser deposition (PLD). We have been studied using X-ray diffraction (XRD), electrical transport magneto-transport and dc magnetization. XRD pattern reflects that all films have c-axis epitaxial growth on LaAlO₃ substrates. The decrease in out-of-plane cell parameter specifies a progressive relaxation of in the plane compressive strain as the thickness film is increases. From the dc magnetization measurements, it is observed that ferromagnetic to paramagnetic transition temperature increases with increase in the film thickness. Magneto-resistance and temperature coefficient of resistance increases with thickness of film and have maximum value near its metal to insulator transition temperature.     

Extraction of europium and electrodeposition of Al–Li–Eu alloy from Eu2O3 assisted by AlCl3 in LiCl–KCl melt

The work presents an electrochemical study on preparation of Al–Li–Eu alloys on a tungsten electrode in molten LiCl–KCl–AlCl₃–Eu₂O₃ system at 753 K and 953 K. Gibbs energy shows that AlCl₃ can chloridize Eu₂O₃, with a discharge in the form of Eu(III) ions on the cathode. The electrochemical behavior of Al(III), Li(I) and Eu(III) and alloy formation processes were investigated by cyclic voltammetry, square wave voltammetry, and chronopotentiometry. Cyclic voltammetry indicated that the underpotential deposition of europium on pre-deposited Al forms two Al–Eu intermetallic compounds at electrode potentials around ?2.00 V and ?2.34 V, respectively. And the underpotential deposition of lithium on Al surface at about ?2.24 V leads to a formation of Al–Li alloy. X-ray diffraction (XRD) indicated that Al–Li–Eu alloys with different phases were obtained via galvanostatic electrolysis. The microstructure and micro-zone chemical analysis of Al–Li–Eu alloy were characterized by scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS), respectively. The analysis of EDS showed that element Eu mainly distributes on needle-like precipitate, and not homogeneously in the Al–Li–Eu alloy. Composition of the alloys was analyzed by inductive coupled plasma analysis, and current efficiency was also determined with respect to the alloy composition.     

Superplastic deformation of directionally solidified nanofibrillar Al2O3–Y3Al5O12–ZrO2 eutectics

Nanofibrillar Al₂O₃–Y₃Al₅O₁₂–ZrO₂ eutectic rods were manufactured by directional solidification from the melt at high growth rates in an inert atmosphere using the laser-heated floating zone method. Under conditions of cooperative growth, the ternary eutectic presented a homogeneous microstructure, formed by bundles of single-crystal c-oriented Al₂O₃ and Y₃Al₅O₁₂ (YAG) whiskers of ≈100 nm in width with smaller Y₂O₃-doped ZrO₂ (YSZ) whiskers between them. Owing to the anisotropic fibrillar microstructure, Al₂O₃–YAG–YSZ ternary eutectics present high strength and toughness at ambient temperature while they exhibit superplastic behavior at 1600 K and above. Careful examination of the deformed samples by transmission electron microscopy did not show any evidence of dislocation activity and superplastic deformation was attributed to mass-transport by diffusion within the nanometric domains. This combination of high strength and toughness at ambient temperature together with the ability to support large deformations without failure above 1600 K is unique and shows a large potential to develop new structural materials for very high temperature structural applications.     

Synthesis and characterization of rhombohedral- and tetragonal-lanthanum oxyfluoride powders

Rhombohedral- and tetragonal-lanthanum oxyfluoride (LaOF) powders were prepared by thermal decomposition of lanthanum(III) carbonate fluoride (LaFCO₃) powders that had been obtained from boiling an aqueous solution containing lanthanum(III) salt, fluoride ion (F^?), and urea. The conversion process of LaFCO₃ to LaOF was monitored by thermogravimetry, powder X-ray diffraction, and ¹⁹F magic-angle spinning nuclear magnetic resonance spectroscopy. The crystal structure (rhombohedral, tetragonal) of LaOF depended on the mole ratio of F^? to La³⁺ ions in the preparation of LaFCO₃, and tetragonal-LaOF was completely transformed into rhombohedral-LaOF at 1000 °C. The difference in crystal structures was reflected in the intensity of the ⁵D₀→⁷F₀ transition peak in the luminescence spectra of Eu³⁺-doped LaOF powders.     

The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl₂O₄) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO₃), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO₃ intermediate phase was not observed and La₄Al₂MgO₁₀ was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

Phase diagram of the Al2O3–ZrO2–La2O3 system

The phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Three new ternary and two new binary eutectics were found. The minimum melting temperature is 1665 °C and it corresponds to the ternary eutectic LaAlO₃ + T-ZrO₂ +  La₂O₃·11Al₂O₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. The latter fact is the theoretical basis for creating new composite ceramics with favorable properties in the Al₂O₃–ZrO₂–La₂O₃ system.     

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu₂Ti₂O₇) bodies were fabricated by spark plasma sintering using Lu₂O₃ and TiO₂ powders calcined from 700 °C to 1200 °C. No solid-state reaction was identified after calcination at 700 °C, whereas single-phase Lu₂Ti₂O₇ powder was prepared at 1100 and 1200 °C. The calcination at 700 °C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu₂Ti₂O₇ bodies with low transparency. Low-density and opaque Lu₂Ti₂O₇ bodies formed owing to the coarsening of the powder calcined at 1200 °C. The Lu₂Ti₂O₇ body sintered using the powder calcined at the moderate temperature of 1100 °C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550 nm and 2000 nm, respectively.     

A new method for the fabrication of MgO-Y2O3 composite nanopowder at low temperature based on bioorganic material

MgO-Y₂O₃ composite nanopowders were synthesized by agarose at low calcination temperature. The influences of agarose (A) to transition metals (TM) mole ratio and calcination temperature on the properties of the composite nanopowder were investigated. As-synthesized samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), thermal gravimetric-differential thermal analysis (TG/DSC) and Fourier transform infrared (FTIR) analysis. The optimized sample synthesized with A to TM mole ratio of 1:1, had the average particle size of 18 nm with 59 m²/g specific surface area. Furthermore, using agarose led to reducing calcination temperature from 600 to 400 °C and the particle size reduced from 18 nm to 8.6 nm. The FESEM results showed that MgO and Y₂O₃ phases had a uniform distribution phase in MgO-Y₂O₃ composite.