Rapid low temperature synthesis of a titanate pyrochlore by molten salt mediated reaction

Ytterbium titanate pyrochlore, Yb₂Ti₂O₇, was prepared by molten salt mediated synthesis (MSS) from titanium oxide (TiO₂) and ytterbium oxide (Yb₂O₃) reagents. Potassium and sodium chloride mixtures were used as the molten salt medium and the effects of salt to reagent ratio, salt composition, synthesis temperature, reaction time, and TiO₂ particle size were explored. Synthesis temperatures and times required for formation of single phase Yb₂Ti₂O₇ were found to be lower than those required for solid state synthesis (SSS). Whereas MSS synthesis of single phase Yb₂Ti₂O₇ was achieved with micron-sized powders after a single reaction at 1200 °C for 1 h, SSS with micron-sized powders required an extended reaction time of 36 h at 1350 °C. Yb₂Ti₂O₇ micron-sized powder prepared by MSS showed similar particle size and morphology to that of the TiO₂ precursor demonstrating a template growth mechanism. However, the use of TiO₂ nano-sized powder changed the dominant synthesis mechanism from template growth to dissolution–precipitation and facilitated synthesis of near single phase Yb₂Ti₂O₇ at the remarkably low temperature of 700 °C in only 1 h. The potential application for lanthanide and actinide immobilisation from molten salt reprocessing wastes was demonstrated by preparation of Yb₂Ti₂O₇ by molten salt mediated synthesis from TiO₂ and ytterbium chloride (YbCl₃) reagents.     

Molten salt synthesis of La2Zr2O7 ultrafine powders

Ultrafine powders of pyrochlore-type La₂Zr₂O₇ were synthesized via a simple molten salt mediated process using zirconium oxide and lanthanum oxide as raw materials, and sodium chloride, potassium chloride and sodium fluoride to form a reaction medium. The effects of reaction temperature, salt/reactant ratio and salt type on the La₂Zr₂O₇ formation were investigated. Among the three attempted salt assemblies (KCl–LiCl, Na₂CO₃–K₂CO₃, and NaCl–KCl–NaF), NaCl–KCl–NaF showed the best accelerating effect on the La₂Zr₂O₇ formation. At a given temperature, the La₂Zr₂O₇ content in the final products increased with the increase in the salt amount. Phase pure submicron sized La₂Zr₂O₇ ultrafine powders were obtained after 3 h firing at 1100 °C with the salt/reactant weight ratio of 5:1 or at 1200 °C with salt/reactant weight ratio of 3:1. The synthesis temperature (1100 °C) was much lower than that required by the conventional solid-state mixing method or a wet chemical method. The “dissolution–precipitation” mechanism had dominated the synthesis process.     

Molten salt synthesis of zinc aluminate powder

ZnAl₂O₄ powder was synthesised by reacting equimolar ZnO and Al₂O₃ powders in alkaline chlorides (LiCl, NaCl or KCl). Formation of ZnAl₂O₄ started at about 700 °C in LiCl and 800 °C in NaCl and KCl. With increasing temperature, the amounts of ZnAl₂O₄ in the resultant powders increased with a concomitant decrease of ZnO and Al₂O₃. ZnAl₂O₄ powder was obtained by water-washing the samples heated for 3 h at 1000 °C (LiCl) or 1050 °C (NaCl and KCl). ZnAl₂O₄ formed in situ on Al₂O₃ grains from the surface inwards. The synthesised ZnAl₂O₄ grains retained the size and morphology of the original Al₂O₃ powders, indicating that a template formation mechanism dominated formation of ZnAl₂O₄ by molten salt synthesis.     

Catalyzed synthesis of rhombohedral boron nitride in sodium chloride molten salt

Although a variety of methodologies/techniques have been used to prepare h-BN powders with different sizes and purities, only a few methods are reported to synthesize r-BN. In this work, highly crystalline r-BN with a purity of 94 wt% was successfully synthesized in sodium chloride molten salt using Na₂B₄O₇ and Mg powders as starting materials at 1000 °C in nitrogen atmosphere. The sodium (Na) produced by the reaction of Mg and Na₂B₄O₇ has a positive effect on the formation r-BN in the molten salt. The effect of Na as a crystallization promoter to produce crystallized r-BN was demonstrated by heating a mixture of t-BN and Na at 800–1200 °C. The formation, dissolution and evaporation of Na in the melt was discussed. The influence of synthesis temperature on the phase composition and morphology of the final products in the melt was also investigated. The possible formation mechanism of r-BN is proposed.     

Molten salt attack on t′ yttria-stabilised zirconia by dissolution and precipitation

Degradation due to molten salt attack is one of the failure mechanisms of thermal barrier coatings. Thermochemical attack of the salt mixture Na₂SO₄–30 mol% NaVO₃ on ZrO₂–8 mol% YO_1.5 (8YSZ) at 950 °C was studied by two types of experiments. Sintered compacts were exposed to 25 mg cm^?2 salt dosage for up to 96 h. In the other set of experiments, 10–35 wt.% 8YSZ powder was mixed with the salts to study the dissolution of 8YSZ in the molten salt. The role of volatile losses was also examined. The results show that more than 25 wt.% 8YSZ dissolves in the sulphate-vanadate melt at 950 °C, followed by slow reactions to form YVO₄ and NaYV₂O₇ at 950 °C. The unreacted Y₂O₃ and monoclinic ZrO₂ precipitate out separately during rapid cooling (～300 °C/min). Slow cooling at ～3 °C/min leads to the formation of ZrOS apart from ZrO₂ and Y₂O₃.     

Microstructure evolution during air bonding of Al2O3 to Al2O3 joints using bismuth–borate–zinc glass

Alumina ceramics with 95 wt.% purity were sealed together using a bismuth based glass, 40Bi₂O₃–40B₂O₃–20ZnO (mol.%). The wettability of the glass on the Al₂O₃ substrate was investigated. The results showed a contact angle of ≤36.5° was achieved when the temperature was ≥630 °C. Subsequently, sealing cycles were performed at temperatures of 520–700 °C for 30 min. The dependence of microstructure evolution of the joints on temperature was investigated. Bi₂₄B₂O₃₉ was detected to be the product in the joints sealed at 530–580 °C, while ZnAl₂O₄ was identified to be the main product when sealing at temperature of ≥650 °C due to the reaction between the Al₂O₃ substrate and ZnO from the glass. The influence of dwelling time at 700 °C on microstructure evolution of the joints was also studied. The results showed that the size of ZnAl₂O₄ increased with increasing holding time.     

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.     

Fracture toughness,strength and creep of transparent ceramics at high temperature

Fracture toughness, four-point bending strength of transparent spinel, Y₂O₃ and YAG ceramics in function of temperature (from room temperature up to 1500° C) were measured. Creep resistance at 1500–1550° C was studied too. Grain size distribution was determined on polished and etched surfaces of samples. Fracture surfaces after tests were examined by scanning electron microscopy. The obtained results showed that: in the case of spinel ceramics fracture toughness and strength decreased from 20 to 800° C, increased from 800 to 1200° C and decreased at higher temperature; in the case of Y₂O₃ ceramics they increased from 400 to 800° C, and next kept constant up to 1500° C; in the case of YAG ceramics they kept constant from 20 to 1200° C and then decreased. The creep strain rate was measured for spinel and YAG but not for Y₂O₃ ceramics which appeared creep resistant. The hypotheses concerning toughening and creep mechanisms were proposed.     

Current limiting and negative differential resistance in indium oxide based ceramics

Indium oxide based ceramics with bismuth oxide addition were sintered in air in the temperature range 800–1300 °C. Current–voltage characteristics of In₂O₃–Bi₂O₃ ceramics sintered at different temperatures are weakly nonlinear. After an additional heat treatment in air at about 200 °C samples sintered at a temperature within the narrow range of about 1050–1100 °C exhibit a current limiting effect accompanied by low-frequency current oscillations. It is shown that the observed electrical properties are controlled by the grain-boundary barriers and the heat treatment in air at 200 °C leads to the decrease in the barrier height. Electrical measurements, scanning electron microscopy and X-ray photoelectron spectroscopy results suggest that the current limiting effect observed in In₂O₃–Bi₂O₃ may be explained in terms of a modified barrier model; the observed current limiting effect is the result of an increase of barrier height with increasing electric field, due to additional oxygen absorption. It is found that In₂O₃–Bi₂O₃–Co₃O₄–Cr₂O₃ ceramic exhibits current–voltage characteristics with negative differential resistance due to Joule microheating.     

The effects of temperature,holding time and salt amount on formation of nano CaZrO3 via molten salt method

Nano-particles of CaZrO₃ were successfully synthesized at 800 °C using the molten-salt method, and the effects of processing parameters, such as temperature, holding time and amount of salt on the crystallization of CaZrO₃ were investigated. Na₂CO₃, CaCl₂ and nano-ZrO₂ were used as starting materials. On heating, Na₂CO₃ reacted with CaCl₂ to form NaCl and in situ CaCO₃. Na₂CO₃–NaCl molten eutectic salt provided a liquid medium for reaction of CaCO₃ and ZrO₂ to form CaZrO₃. The results demonstrated that CaZrO₃ started to form at about 700 °C and that, after the temperature was increased to 1000 °C, the amounts of CaZrO₃ in the resultant powders increased with a concomitant decrease in CaCO₃ and ZrO₂ contents. After washing with hot-distilled water, the samples heated for 3 h at 800 °C were single-phase CaZrO₃ with 70–90 nm particle size. Furthermore, the synthesized CaZrO₃ particles retained the size and morphology of the ZrO₂ powders, which indicated that a template formation mechanism dominated the formation of CaZrO₃ by molten-salt synthesis.     

The effects of raw materials particle size and salt type on formation of nano-CaZrO3 from molten salts

Nano-CaZrO₃ was successfully synthesized at 800 °C using the molten-salt method, and the effects of salt type and raw materials particle size on the formation of CaZrO₃ were investigated. Na₂CO₃, CaCl₂, nano-ZrO₂ and micro-ZrO₂ were used as starting materials. On heating, Na₂CO₃ reacted with CaCl₂ to form NaCl and in situ CaCO₃. Na₂CO₃–NaCl molten eutectic salt provided a liquid medium for reaction of CaCO₃ and ZrO₂ to form CaZrO₃. The results demonstrated that in both nano- and micro-ZrO₂ inclusive samples, CaZrO₃ started to form at about 700 °C and that, after the temperature was increased to 1000 °C, the amounts of CaZrO₃ in the resultant powders increased with a concomitant decrease in CaCO₃ and ZrO₂ contents. After washing with hot-distilled water, the samples containing nano- and micro-ZrO₂ heated for 3 h at 800 °C and 1000 °C, were single-phase CaZrO₃ with 70–90 nm and 400–450 nm particle size, respectively. Also, the synthesis process was completed in lower temperatures using eutectic salts. Furthermore, the synthesized CaZrO₃ particles retained the size and morphology of the ZrO₂ powders, which indicated that a template formation mechanism dominated the formation of CaZrO₃ by molten-salt synthesis.     

Degradation of plasma-sprayed yttria-stabilized zirconia coatings via ingress of vanadium oxide

V₂O₅ reaction and melt infiltration in plasma-sprayed 7 wt% Y₂O₃–ZrO₂ (YSZ) coatings were investigated at temperatures ranging from 750 °C to 1200 °C using SEM and TEM combined with EDS. The interlamellar pores and intralamellar cracks, common in plasma-sprayed materials, provide pathway for the molten species. The microstructure of the contaminated coatings is therefore the result of the interplay between the dissolution/reaction rates of the V₂O₅ with YSZ coating and the infiltration rates of the molten species. Near the coating surface, the reaction front proceeds in a planar fashion, via dissolution of the lamella and precipitation of fine-grained reaction products composed of ZrV₂O₇ (for reactions at 750 °C and below), m-ZrO₂ and YVO₄. The thickness of this planar reaction zone or PRZ was found to increase as reaction time and temperature increased. The melted V₂O₅ was observed to infiltrate along the characteristic microstructure of plasma-sprayed coatings, i.e. the interconnected pores and cracks, and react with the YSZ. The thickness of this melt infiltrated reaction zone or MIRZ ranged from 5 μm for reactions at 750 °C for 30 min to 130 μm for reactions at 1000 °C for 90 min. At 1200 °C, only a PRZ was observed (i.e. the thickness of the MIRZ was nominally zero), suggesting that the dissolution reaction within the pores/cracks and subsequent formation of reaction products may limit infiltration. Fifty-hour heat-treatments at 1000 °C and 1200 °C prior to reaction with the V₂O₅ at 800 °C for 90 min were used to change the microstructural features of the coating, such as crack connectivity and pore size. The heat-treatment at 1000 °C was found most deleterious to the coating due to large cracks created via a desintering process that afforded deep penetration of the molten V₂O₅.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Tribological properties of NiCr–ZrO2(Y2O3)–SrSO4 composites at elevated temperatures

The effect of SrSO₄ content on the tribological properties of NiCr–30wt%ZrO₂(Y₂O₃) (NC30Z) cermet was evaluated over a wide temperature range from room temperature to 1000 °C. The results indicated that the inclusion of SrSO₄ effectively improved the friction coefficients and wear rates of NC30Z cermet above 400 °C. NC30Z–5SrSO₄ composite against alumina ball exhibited satisfactory tribological performance, which was attributed to synergistic lubrication of pseudocubic-SrZrO₃ and NiCr₂O₄ between 400 °C and 800 °C and cubic-SrZrO₃, NiCr₂O₄, NiO and Cr₂O₃ at 1000 °C.     

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.     

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.     

Enhanced nitridation of silicon compacts by Yb2O3 addition

The catalytic effect of ytterbium oxide (Yb₂O₃) on the nitriding reaction of Si compacts was investigated. Si powder mixtures containing Yb₂O₃ were prepared and nitrided in the form of compacts with a multi-step heating schedule over the range of 1200 °C–1450 °C. The nitriding profiles of the powder mixture with increasing temperature indicated that Yb₂O₃ clearly promoted the nitridation of Si compacts at 1200 °C compared with the pure Si compact containing no additives. The critical role of Yb₂O₃ on the nitridation of Si, was elucidated that Yb₂O₃ promotes the loss of initial SiO₂ of the raw Si powder via the measurement of the weight changes at low temperature (1100 °C) and thermogravimetric analysis under N₂ atmosphere. It was also found that the β-ratio of fully nitrided Si was closely related to the intermediate degree of nitridation at 1200 °C and 1300 °C.     

Molten salt synthesis of PZT powder for direct write inks

The preparation of lead zirconate titanate (PZT) powder by molten salt synthesis (MSS) is described and a mechanism proposed. The effect of process parameters, such as reaction time and temperature and heating rate, on particle size and shape was investigated. A reaction mechanism for the synthesis of PZT by molten salt method is proposed, where dissolved Pb initially reacts with insoluble TiO₂ to form intermediate PbTiO₃. Subsequently Zr diffuses into and reacts with PbTiO₃ to form PZT. Spherical particles, ～340 nm in size, were obtained, using a NaCl/KCl salt, by heating the starting materials at 850 °C for 60 min, with a ramp rate of 3.3 °C min^?1.     

Low-temperature molten salt synthesis of YAlO3 powders assisted by an electrochemical process

YAlO₃ (YAP) powders were successfully synthesized by a unique molten salt method, where YAP precursor was prepared by an electrochemical method at room temperature, followed by calcining it at a temperature of not exceeding 400 °C for 8 h using LiNO₃ as the molten salt medium. XRD analysis and TEM observation show that well-crystallized YAP powders can be obtained at 400 °C for a holding time of 8 h with 1:16 ratio of YAP precursor to LiNO₃ by weight. Greatly reduced temperature of forming YAP should be attributed to the incorporation of LiNO₃ salt in preparing process.     

Microstructure and mechanical properties of Ni/10 mol% Sc2O3–1 mol% CeO2–ZrO2 cermet anode for solid oxide fuel cells

The mechanical properties of anode materials play an important role in the reliability and durability of solid oxide fuel cells operating at high temperatures in a reducing environment. In this paper, we produced the results of the mechanical properties investigation of Ni/10 mol% Sc₂O₃–1 mol% CeO₂–ZrO₂ cermet anodes. Young's modulus as well as strength and fracture toughness of non-reduced and reduced anodes has been measured, both at room and at high temperatures. High temperature experiments were performed in the reducing environment of forming gas. It is shown that while at 700 °C and 800 °C, the anode specimens exhibited purely elastic deformation and brittle fracture, a brittle-to-ductile transition occurred for heating above 800 °C and the anode deformed plastically at 900 °C. Fractography of the anode specimens were performed to identify the fracture modes of anodes tested at different temperatures.