High temperature thermal stabilization of alumina modified by lanthanum species

The effects of precursor pretreatment and addition methods of lanthanum species on stabilization of alumina (surface area loss, phase transformations and high temperature interaction with lanthanum species) have been investigated by BET specific surface area measurements (BET), X-ray powder diffraction (XRD), N₂ adsorption–desorption isotherms, thermal analysis and X-ray photoelectron spectroscopy (XPS) in the range of 600–1150°C. Although powder La₂O₃, which is mechanically mixed with γ-Al₂O₃ or pseudo boehmite, can effectively retard the α phase transformation by solid phase interaction with Al₂O₃, it does not show a positive effect on retarding the loss of surface area. Compared with the direct impregnation of γ-Al₂O₃, the gelation of pseudo boehmite by acidification accelerates phase transformations and weakens the stabilizing influence of lanthanum species. At 600°C and for atomic ratio of La/Al up to 0.1 or at 1150°C and La/Al≤0.02, the lanthanum species is highly dispersed in alumina. With the increase of calcination temperature or lanthanum content, lanthanum species is present as dispersed La₂O₃, LaAlO₃ and crystalline La₂O₃. At T≤1000°C the surface area loss of alumina is mainly attributed to the sintering of particles. The follow-up loss at T>1000°C results from both sintering and phase transformations. The highly dispersed lanthanum species retard both sintering and phase transformations, and their associated surface area loss. However, the formation of LaAlO₃ mainly retards the surface area loss resulting from the α phase transformation. Having considered the purely mechanical mixing effect of additive on the surface area loss of alumina, an influence criterion of lanthanum species on retarding the surface area loss whether resulting from sintering or from α phase transformation at high temperature ≥1000°C has been proposed in this paper.     

Synthesis,spray granulation and plasma spray coating of lanthanum phosphate nanorods for thermal insulation coatings

Nanorods of lanthanum phosphate obtained by a wet chemical precipitation route were granulated to obtain sizes in the range of 10–15 µm by spray drying from aqueous slurry of 35 wt% solid loading and 2 wt% of PVA binder. The powders thus obtained displayed enhanced flowability and were plasma sprayed on to stainless steel substrates resulting in the formation of adherent coatings of 150–180 µm thickness. These coatings were characterized using electron microscopy, X-ray diffraction analysis and Raman spectroscopy. X-ray analysis indicated phase instability of LaPO₄ during plasma spraying resulting in the formation of oxy and polyphosphates of lanthanum (La₂P₄O₁₃ and La₃PO₇). However, post deposition heat treatment of coated samples at 1100 °C for 2 h resulted in the reversible formation of stoichiometric lanthanum orthophosphate (LaPO₄). Raman spectral analysis was used to confirm the phase structure of the coatings deposited at various plasma input powers. The coatings obtained were found to effectively lower the thermal conductivity of the substrates from ~24 W/mK to less than 19 W/mK (~10%) even at 200 °C.     

Novel combustion synthesis of La3+-substituted MnFe2O4

La³⁺-substituted MnFe₂O₄ compounds have been prepared by using a novel combustion synthesis method. This process was found to yield homogeneous, finely crystalline powders without intermediate decomposition and/or calcination steps. Combustion-synthesized powders were sintered at 1000°C, and structural features of thus prepared materials were characterized by XRD analysis and FT-IR spectroscopy. The dc electrical conductivity of synthesized materials has been measured as a function of temperature up to 1000°C. The materials have shown semiconducting behavior at elevated temperatures. The ac electrical conductivity of synthesized samples was found to increase with increasing applied frequency. The dielectric constant and dielectric loss tangent have also been characterized. The article is published in the original.     

Improved resistance of lanthanum zirconate coatings to calcium-magnesium-alumina-silicate corrosion through composition tailoring

Calcium–magnesium–alumina–silicate (CMAS) corrosion resistance is an important issue on the design of next-generation thermal battier coatings. As one of the promising thermal battier coatings, the lanthanum zirconate coating has attracted continuous attention. In this work, three lanthanum zirconate coatings with different La/Zr composition, i.e., La_1.8Zr_2.2O_7.1, La₂Zr₂O₇, and La_2.5Zr_1.5O_6.75, are fabricated by laser-enhanced chemical vapour deposition, and their resistance to CMAS corrosion at 1250?°C is investigated. Among them, La_2.5Zr_1.5O_6.75 shows the best CMAS corrosion resistance because increased La content is beneficial to the formation of a dense and continuous apatite Ca₂La₈(SiO₄)₆O₂ layer, which effectively slows down the subsequent molten CMAS penetration. This study clarifies the significant role of rare earth on CMAS corrosion resistance and is expected to guide the future design of rare-earth-based thermal battier coatings through composition tailoring.     

Microwave sintering of IR-transparent Y2O3–MgO composite ceramics

A method for preparation of dense Y₂O₃–MgO composite ceramics by the microwave sintering was developed. The initial powders were obtained by glycine-nitrate self-propagating high-temperature synthesis (SHS) with different oxidant-to-fuel ratio. Density and IR-transmission of microwave sintered Y₂O₃–MgO ceramics increase with respect to dispersity of the SHS-powders and reach its maximum values for the powder prepared in a 20% fuel excess. The sintering behavior of Y₂O₃–MgO compacts was investigated by optical dilatometry and measuring an electric conductivity upon heating. Significant microwave radiation power surges at temperatures of 900–1000 °C, caused by the decomposition of magnesium carbonate, have been found. As a result of matching the conditions for the synthesis of powders and sintering modes, a transmission of composite ceramics of 78% at a wavelength of 6 μm was achieved at a maximum processing temperature of 1500 °C.     

Structure sensitivity of the catalytic oxidative coupling of methane on lanthanum oxide

The oxidative coupling of methane (OCM) has been found to be structure sensitive on La₂O₃ catalysts exhibiting different crystallite morphologies. Thin plates obtained by thermal decomposition of lanthanum nitrate at 650 °C are more selective on OCM reaction performed at 750 °C than the particles obtained by decomposition of the nitrate at 800 °C. It is assumed that the oxycarbonate observed is formed from the methane deep oxidation on the catalyst surface. This compound appears to act as an intermediate in the production of CO₂ and is thus important hi the resulting selectivity.     

Synthesis andcharacterisation of nanocrystalline MgAl2O4 ceramic powders by use of molten salts

Abstract

Nanocrystalline MgAl₂O₄ powders were prepared by a thermal decomposition method, i.e. by use of molten salts. This method involves co-melting stoichiometric amounts of magnesium nitrate hydrate Mg(NO₃)₂.6H ₂O and aluminium nitrate hydrate Al(N O₃) ₃.9H₂O at 500°C. The spinel content of the co-melted and calcined powders at different firing temperatures up to 1000°C was determined by chemical analysis and the powders were characterised with respect to spinel formation, crystallite and particle sizes by X RD, T EM , and IR spectroscopy. The results obtained revealed that the co-melted materials were amorphous. After heat treatment of the amorphous materials at up to 1000°C, pure spinel powder was obtained, reaching over 98% spinel content. During calcination at different firing temperatures up to 1000°C the amorphous material progressively crystallised, forming nanocrystalline spinel with a maximum crystallite size of about 10 nm and particle size of around 300 nm. Bands in IR spectra were observed corresponding to the ex istence of AlO₆ groups prior to magnesium spinel formation, which was the only crystalline phase at 1000°C.     

Preparation and corrosion resistance of nonstoichiometric lanthanum zirconate coatings

Lanthanum zirconate is a promising thermal barrier coating material owing to its excellent thermophysical properties and La plays the key role in its corrosion resistance. Here, an amorphous precursor is used as raw feedstock material so as to synthesize lanthanum zirconate coatings with tailorable composition by atmospheric plasma spray (APS). Three lanthanum zirconate coatings of La_1.7Zr_2.3O_7.15, La_2.0Zr_2.0O_7.0 and La_2.3Zr_1.7O_6.85 are fabricated. Furthermore, the corrosion resistance of the as-sprayed coatings against CaO-MgO-Al₂O₃-SiO₂ at 1250℃ is investigated. The increased La content promotes the formation of a sealing layer of the crystalline Ca₂La₈(SiO₄)₆O₂ apatite, which slows down the penetration of molten CaO-MgO-Al₂O₃-SiO₂. Therefore, the infiltration rate of the La_2.3Zr_1.7O_6.85 coating decreased up to 42.6 % compared with the other two coatings. This work develops a feasible preparation strategy to control the La composition for the improved corrosion resistance, which is expected to guide the future coating design and synthesis for the materials with big composition changes during the APS process.     

Ionic and electronic transport in calcium-substituted LaAlO3 perovskites prepared via mechanochemical route

The present work explores mechanosynthesis of lanthanum aluminate-based perovskite ceramics and corresponding effects on ionic-electronic transport properties. La_1-xCa_xAlO_3-δ (x = 0.05–0.20) nanopowders were prepared via one-step high-energy mechanochemical processing. Sintering at 1450 °C yielded dense ceramics with submicron grains. As-prepared powders and sintered ceramics were characterized by XRPD, XPS and SEM. Electrochemical studies showed that partial oxygen-ionic conductivity in prepared La_1-xCa_xAlO_3-δ increases with calcium content up to 10 at.% in the lanthanum sublattice and then levels off at ∼6 × 10⁻³ S/cm at 900 °C. La_1-xCa_xAlO_3-δ ceramics are mixed conductors under oxidizing conditions and ionic conductors with negligible contribution of electronic transport in reducing atmospheres. Oxygen-ionic contribution to the total conductivity is 20–68% at 900 °C in air and increases with Ca content, with temperature and with reducing p(O₂). Impedance spectroscopy results showed however that electrical properties of mechanosynthesized La_1-xCa_xAlO_3-δ ceramics below ∼800 °C are determined by prevailing grain boundary contribution to the total resistivity.     

Chemical interactions between 3 mol% yttria-zirconia and Sr-doped lanthanum manganite

The chemical interactions between porous (La_0.8Sr_0.2)MnO₃ (LSM) film and 3 mol% yttria tetragonal zirconia (TZ3Y) substrate have been investigated over the temperature range of 1300–1500 °C in air. Two distinct reaction layers of fluorite-type cubic zirconia solid solution c-(Zr,Mn,La,Y)O₂ and lanthanum zirconate pyrochlore (La,Sr)₂(Zr,Y)₂O₇ were observed at the interface of LSM/TZ3Y. It has been found that the diffusion/dissolution of Mn ions in TZ3Y leads to the formation of the fluorite-type cubic zirconia solid solution, while the interaction of lanthanum with TZ3Y results in the formation of the lanthanum zirconate pyrochlore phase. Phase studies in the (ZrY)O₂–La₂O₃–Mn₃O₄ system show that the fluorite-type cubic zirconia solid solution phase c-(Zr,Mn,La,Y)O₂, rather than the tetragonal 3 mol% Y₂O₃–ZrO₂ phase, is in equilibrium with LSM perovskite at high temperatures. A ternary phase diagram of the system at the (ZrY)O₂-rich end at 1400 °C in air was proposed based on the experimental results. It is suggested that the fundamental reason for the beneficial effect of A-site non-stoichiometry or Mn excess of LSM in the inhibiting of the lanthanum zirconate formation is due to the fact that Mn₃O₄ does not equilibrate with lanthanum zirconate at high temperatures.     

Effect of La2O3 addition and sintering mode on the mechanical properties and microstructural evolution on an 8YSZ ceramic alloy

In this research, the influence of La₂O₃ addition on the microstructure, phase stability and mechanical properties of 8?mol% yttria stabilized zirconia (8YSZ) was studied. 8YSZ with La₂O₃ (9, 12 and 15?wt%) ceramics were fabricated by microwave and conventional sintering at 1400?°C/ 20?min and 1400?°C/ 5?h, respectively. Irrespective of the sintering technique, the relative sintered density was found to decrease with increasing amount of La₂O₃. The grain growth of 8YSZ was enhanced significantly by the addition of La₂O₃. The XRD results demonstrated that addition of La₂O₃ up to 15?wt% did not disrupt the cubic 8YSZ phase regardless of sintering technique; additionally evolution of pyrochlore phase, La₂Zr₂O₇ was observed in all sintered specimens. Vickers hardness of 8YSZ ceramic compacts were also found to decrease with increasing amount of La₂O₃.     

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.     

Studies on the promotion of nickel—alumina coprecipitated catalysts: II. Lanthanum oxide

Two series of lanthanum promoted nickel—alumina catalysts have been prepared by coprecipitation of the metal nitrates, using potassium carbonate. The molar ratio between nickel and the sum of aluminium and lanthanum was kept constant at 2.5 or 9.0 within each series. The calcination and reduction of these samples were studied by thermogravimetry and their structures before and after calcination and reduction were examined by X-ray diffraction. The methanation activities of the final catalysts were determined by differential scanning calorimetry. The results showed clearly that the methanation of carbon monoxide over nickel—alumina catalysts is enhanced by the presence of La₂O₃. With low percentages of lanthanum, the promoter is built into the precursor structure during the coprecipitation process. This is a meta-stable situation; phase separation occurs during hydrothermal treatment. In both series there was an optimum amount of lanthanum at which the activity per gram of nickel reached a maximum. The optimum specific activity of a lanthanum promoted nickel—alumina catalyst was twice as large as that of the unpromoted material. Above these optimum values, the activity per gram of nickel decreased because of two effects: an increase in the nickel particle sizes and an increase in the amounts of potassium remaining from the precipitation step. Alumina is needed to stabilize the nickel crystallites against sintering. The promoting action of La₂O₃ is slightly higher after reduction at 400°C than after reduction at 600°C. Lanthanum increased the amount of carbon monoxide which was adsorbed slowly; the amount of carbon monoxide which was rapidly adsorbed, however, was not altered. The increase in activity was accompanied by an increase in the apparent activation energy.     

Facile synthesis of LaAlO3 and Eu(II)-doped LaAlO3 powders by a solid-state reaction

Lanthanum aluminate (LaAlO₃) powder was prepared by a solid-state reaction between lanthanum(III) carbonate fluoride (LaFCO₃) and alumina (Al₂O₃) powders at elevated temperatures, and characterized by powder X-ray diffraction (XRD), ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The formation temperature (1000 °C) of LaAlO₃ was much lower than that in other solid-state reactions. The Eu(II)-doped LaAlO₃ powder was prepared by calcination of a mixture of Eu(III)-doped LaFCO₃ and Al₂O₃ in a nitrogen atmosphere and characterized by powder XRD, and photoluminescence and X-ray photoelectron spectroscopy. The intensity of blue emission due to Eu(II) ions in the LaAlO₃ powder increased with increasing calcination temperature up to 1200 °C but decreased with further increases in temperature. The origin of the Eu(II) ions was explained by the thermal decomposition of EuF₃.     

Synthesis and structural characterization of ultrafine terbium oxide powders

Glycine-nitrate self-propagating high-temperature synthesis was used to synthesize terbium oxide nanopowders whose phase transformations were investigated by methods of high-temperature differential scanning calorimetry, dilatometry, and X-ray diffraction analysis. The as-prepared powders consisting of Tb₇O₁₂ and Tb₁₁O₂₀ phases were converted to Tb₂O₃ after calcination at 600–800 °C in reducing atmosphere. The sintering behavior of Tb₂O₃ was studied under microwave heating up to 1780 °C. The microstructure of the powders and ceramics was investigated by scanning electron microscopy. Near full-density material was obtained at about 1620 °C. Further temperature increases causes a deterioration of the ceramics microstructure due to monoclinic Tb₂O₃ phase formation.     

Effect of applied pressure on microstructure and properties of hot-pressed Ca-doped LaCrO3 ceramics

High-density chromium deficient calcium-doped lanthanum chromite-based ceramics (La_0.8Ca_0.2Cr_0.98O₃) were prepared by hot pressing (HP) at different sintering pressures, and the highest density can reach 98.8%. The effects of sintering pressure on the microstructure, mechanical properties, and electrical conductivity of La_0.8Ca_0.2Cr_0.98O₃ materials were studied. The experimental results show that HP can increase the density of lanthanum chromite-based ceramic materials and significantly inhibit the growth of grain size. As the sintering pressure increases, the strength and hardness gradually increase, but the fracture toughness decreases. When the sintering pressure is greater than 58 MPa, the presence of the second phase CaCr₂O₄ can be detected in the XRD results of the sintered ceramics. The SEM results showed that CaCr₂O₄ had two completely different morphologies in the sintered ceramics, and it was initially speculated that the possible causes were due to two different generation pathways. The electrical conductivity decreases with increasing sintering pressure, whereas the maximum electrical conductivity obtained is 18.61 S/cm in vacuum at 800°C for pressureless sintering ceramic.     

Spectroscopic study of the competition between dehydration and carbonation effects in La2O3-based materials

La₂O₃ takes part in a variety of technologically interesting solids, but it has a known tendency to react with water and CO₂, which may result in nonstoichiometry of the final product and the appearance of impurities. We apply Raman spectroscopy to study hydration and carbonation effects of La₂O₃-based materials, using La(OH)₃ as starting material. The thermal evolution of La(OH)₃ reflects the competition between dehydration and carbonation kinetics, and thus is highly dependent upon environment and experimental conditions. In vacuum, La(OH)₃ evolves first to LaOOH and then to a mixture of C- and A-La₂O₃ until, upon further heating, only A-La₂O₃ phase remains. In air, La(OH)₃ yields first LaOOH, but under slow heating LaOOH converts to I, Ia and II-type oxycarbonates, in increasing order of thermal stability. II-La₂O₂CO₃, in turn, decomposes to La₂O₃ at 700 °C. Structural relations governing phase conversion within hexagonal or orthogonal-like compounds are discussed.     

The Effect of La2O3 Addition on Sol–Gel Derived Mullitization

Monophasic gel with stoichiometric 3Al₂O₃·2SiO₂ composition and gels with 0.99, 1.96, and 2.91 mol% La₂O₃ added were sol–gel derived. The crystallization path, structure evolution, microstructure, and morphology of calcined premullite powders and sintered ceramic bodies have been investigated as a function of La₂O₃ content and sintering temperature. In addition to mullite, spinel phase at about 980°C, and α‐alumina at above 1000°C were determined; however, neither La₂O₃ nor La‐related compounds had crystallized. The La₂O₃ predominately incorporated into the glassy phase, enhanced with La₂O₃ level, which affected both mullite structure and composition, as confirmed by electron microscopy, Rietveld structure refinement, determination of unit cell parameters, electron microscopy, and achieved density of the sintered bodies. Increased thermal treatment changes the alumina/silica ratio in mullite (towards 3:2 below 1200°C, and toward 2:1 above), and decreases the mullite/amorphous ratio. Sintered dense ceramic bodies revealed a positive densification effect and increased sinterability as a result of the lanthanum‐induced increase in glassy phase.     

Influence of La3+ additions on the phase behaviour and antibacterial properties of ZrO2–SiO2 binary oxides

The present study reports the influence of lanthanum (La³⁺) content on the phase stability and antibacterial activity of ZrO₂–SiO₂ binary oxides. Four different concentrations of La³⁺ additions in ZrO₂–SiO₂ binary oxides were synthesized using a sol–gel technique. Heat treatment of the synthesized powders resulted in the formation of t-ZrO₂ phase at 1000 °C. Heat treatment beyond 1000 °C resulted in the phase degradation of t-ZrO₂ to yield m-ZrO₂ and ZrSiO₄. Results from antibacterial tests confirmed the potential activity of La³⁺ doped ZrO₂–SiO₂ binary oxides in countering the microbial invasion.     

Phase relation studies in the CeO2–La2O3–Er2O3 system at 1500°C

Materials based on CeO₂–La₂O₃–Er₂O₃ system are promising candidates for a wide of applications, but the phase relationship has not been studied systematically previously. To address this challenge, the isothermal section of the phase diagram for 1500 °C was investigated. The phase relations in the CeO₂–La₂O₃–Er₂O₃ ternary system at 1500 °C were studied by X-ray diffraction and scanning electron microscopy in the overall concentration range. To study phase relationships at 1500 °C the as-repared samples were thermally treated in two stages: at 1100 °C (for 300 in air) and then at 1500 °C (for 70 h in air) in the furnaces with heating elements based on Fecral (H23U5T) and Superkanthal (MoSi2), respectively. The solid solutions based on various polymorphous forms of constituent phases and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions were revealed in the system. No new phases were found. The isothermal section of the phase diagram for the CeO₂–La₂O₃–Er₂O₃ system has been constructed. It was established that in the ternary CeO₂–La₂O₃–Er₂O₃ system there exist fields of solid solutions based on hexagonal (A) modification of La₂O₃, cubic modification of CeO₂ with fluorite-type structure (F), cubic modification Er₂O₃ and with perovskite-type structure of LaErO₃ (R) with orthorhombic distortions. The maximal solubility of ceria in LaErO₃ was found to be around ∼ 2 mol% CeO₂ along the section CeO₂–(50 mol % La₂O₃ –50 mol% Er₂O₃).