Effect of 7YSZ on the long-term stability of YTaO4 doped ZrO2 system

The long-term stability of Ta_0.16Y_0.16Zr_0.68O₂ (TaYSZ) has been studied for possible application in thermal barrier coatings. X-ray diffraction was used for the characterization of the phase stability from 1100 to 1500 °C. Long-term stability of TaYSZ in presence of 7YSZ at 1250 °C has also been checked. At 1500 °C, TaYSZ remains as a single tetragonal phase. TaYSZ, however, when treated at 1250 °C for 600 h decomposes to m-ZrO₂ and contains a minor YTaO₄ phase and t-TaYSZ. In the presence of 7YSZ, decomposition of TaYSZ was suppressed. Decomposition of TaYSZ follows a different mechanism when treated in air and under vacuum. TaYSZ once pretreated at 1500 °C does not show any decomposition when treated at 1250 °C for 600 h, though weak reflections of Y₃TaO₇ are seen in the X-ray diffraction pattern. The onset of sintering and the coefficient of thermal expansion (CTE) of TaYSZ have been found to be ∼1200 °C and 11.24 × 10⁻⁶ K⁻¹, respectively.     

Structural features of Sc3TaO7

Conclusions The compound Sc₃TaO₇ synthesized at 1300°C has a monoclinic modification which is stable in vacuum in the 25–1700°C range. We detected a phase inversion of the monoclinic modification into the tetragonal at 1800°C, accompanied by a change in the chemical composition with respect to oxygen. We determined the temperature coefficients of linear expansion of the monoclinic modification of Sc₃TaO₇ in the 25–1700°C range, and detected a marked anisotropy of temperature expansion in the crystal lattice.The author wishes to thank O. N. Vasil'eva for assistance with the experiments.Translated from Ogneupory, No. 7/8, pp. 14–16, July–August, 1992.     

Synthesis of Thermally Stable Tetragonal Zirconia with Large Surface Area and Its Catalytic Activity in the Skeletal Isomerization of 1-Butene

Thermally stable zirconia with a large BET surface area was prepared by using zirconium atrane derivatives and commercial alkyltrimethylammonium bromide. The zirconia samples were characterized by wide-angle X-ray diffraction, thermal analysis and N₂ adsorption/desorption analysis. The results showed that the present zirconia existed as a thermally stable tetragonal crystallite and consisted of mesoporous material with microporous contribution. The BET surface area was found larger than 240 m²/g even after calcination at 600 °C. The present zirconia after sulfation was applied to the skeletal isomerization of 1-butene, and its catalytic activity was found superior to that of the conventional sulfated zirconia.     

Synthesis of CeAlO3/CeO2-Al2O3 for use as a solid oxide fuel cell functional anode material

A composite electrocatalyst was developed to be fitted for the purpose of satisfying the features required for use as a solid oxide fuel cell functional anode material. The main functionality searched for was the ability to make the direct oxidation of carbon containing fuels in an SOFC without being severely coked. The present paper deals with the synthesis and characterization of such material. Therefore, ceramic electrocatalysts composed of CeAlO₃, CeO₂ and Al₂O₃ were synthesized by the amorphous citrate method and calcined at temperatures ranging from 300 °C to 900 °C. The synthesis procedures were designed to produce nanometric sized powders for which the calcination conditions were selected in order to fulfill requirements such as ease to be sintered; formation of selected phases upon calcinations at different temperatures; particle size control; surface area and morphology well suited for the production of ceramic suspensions to be processed into an SOFC functional anode. The main results have shown that increasing the calcination temperature under an oxidizing atmosphere induces the CeAlO₃ phase with a tetragonal perovskite type structure to undergo a phase transformation to CeO₂ (and Al₂O₃) with cubic fluorite type structure. However, the structure is able to be reversed and reduced back to the CeAlO₃ phase if calcined under a hydrogen atmosphere. The increase in the calcination temperature increases the particle average size, reduces the surface area and increases the material density, considering the same phase and crystalline structure. It was shown that the synthesis and calcinations procedures hinder the crystallographic identification of the presence of alumina.     

Synthesis and phase stability of precursor derived HfO2/Si-C-N-O nanocomposites

Hafnium alkoxide modified polysilazane was synthesized by the drop-wise addition of hafnium tetra(n-butoxide) to polysilazane. The solid state thermolysis (SST) temperature and the ceramic yield for both the polysilazane and modified polysilazane were determined by performing thermogravimetry. Fourier transform infrared spectroscopy was performed to understand the polymer to ceramic conversion as well as the bonding characteristics of the ceramics. The modified polymer after crosslinking was subjected to SST at 800 °C at a constant heating rate of 5 °C/min for a dwell time of 2 h in atmospheric ambience. From the X-ray diffractograms, the as-thermolysed ceramics were observed to remain X-ray amorphous and on heat-treatment resulted in the crystallization of tetragonal hafnia. However, on heat-treatment at 1500 °C, reverse phase transformation from tetragonal to monoclinic hafnia was observed. Raman spectroscopy and transmission electron microscopy were employed to further understand the phase evolution. The thermal stability and the influence of amorphous matrix on the coarsening of HfO₂ were also evaluated.     

High piezoelectric properties of BaTiO3-xLiF ceramics sintered at low temperatures

BaTiO₃-xLiF ceramics were prepared by a conventional sintering method using BaTiO₃ powder about 100 nm in diameter. The effects of LiF content (x) and sintering temperature on density, crystalline structure and electrical properties were investigated. A phase transition from tetragonal to orthorhombic symmetry appeared as sintering temperatures were raised from 1100 °C to 1200 °C or as LiF was added from 0 mol% to 3 mol%. BaTiO₃-6 mol% LiF ceramic sintered at 1000 °C exhibited a high relative density of 95.5%, which was comparable to that for pure BaTiO₃ sintered at 1250 °C. BaTiO₃-4 mol% LiF ceramic sintered at 1100 °C exhibited excellent properties with a piezoelectric constant d₃₃ = 270 pC/N and a planar electromechanical coupling coefficient k_p = 45%, because it is close to the phase transition point in addition to high density.     

Structure,dielectric and piezoelectric properties of Ba0.90Ca0.10Ti1−xSnxO3 lead-free ceramics

Lead-free piezoelectric ceramics Ba_0.90Ca_0.10Ti_1−xSn_xO₃ have been prepared by a conventional ceramic fabrication technique and the effects of Sn⁴⁺ on the structure, dielectric and piezoelectric properties of the ceramics have been investigated. All the ceramics exhibit a pure perovskite structure. After the substitution of Sn⁴⁺, the crystal structure of ceramics is transformed gradually from a tetragonal to an orthorhombic phase, and becomes a pseudo-cubic phase at x≥0.14. The substitution also decreases the Curie temperature greatly from 138 °C at x=0 to 33 °C at x=0.12, and shifts the orthorhombic–tetragonal phase transition to higher temperatures. Coexistence of the orthorhombic and tetragonal phases is formed in the ceramic at x=0.10, leading to significant improvements in the piezoelectric properties: d₃₃=521 pC/N and k_p=45.5%. Our results also reveal that the ceramics sintered at higher temperatures contain larger grains, and thus exhibit more noticeable tetragonal–orthorhombic phase transition and enhanced ferroelectric and piezoelectric properties.     

Microstructure of Fe-ZrSiO4 solid solutions prepared from gels

Microstructural changes associated with chemical and structural evolution from gels to Fe_x-ZrSiO₄ solid solutions are reported. Mineralizer-free Fe_x-ZrSiO₄ gels in the compositional range 0 ≤ x ≤ 0.15 were prepared by sol-gel liquid-phase route from mixtures of alkoxides of silicon and zirconium, and iron (III) acetylacetonate, and annealed at different temperatures and/or times.The first step on the whole process to the final Fe_x-ZrSiO₄ solid solutions was the formation of aggregated of tetragonal Fe-doped ZrO₂ nanocrystals with diameters smaller than 50 nm. At this stage the tetragonal Fe-ZrO₂ were embedded in amorphous silica resulting nanocomposite materials. The formation of the final Fe_x-ZrSiO₄ solid solution nanocrystals at around 1100 °C occurred almost simultaneously to the Fe-ZrO₂ solid solution phase transformation from the tetragonal to the monoclinic form. The microstructural examination of specimens after annealing at 1200 °C revealed the development of Fe-doped zircon nanoparticles smaller than 100 nm.     

Phase stability and hot corrosion behavior of ZrO2–Ta2O5 compound in Na2SO4–V2O5 mixtures at elevated temperatures

For thermal barrier coating (TBC) applications, yttria stabilized zirconia (YSZ) is susceptible to hot corrosion. This paper examines the hot corrosion performance of ZrO₂/Ta₂O₅ compounds. Different compositions of ZrO₂–Ta₂O₅ samples in the presence of molten mixture of Na₂SO₄+V₂O₅ at 1100 °C were tested. The compositions were selected to form tetragonal and orthorhombic phases of zirconium-tantalum oxides. Results show that orthorhombic zirconium-tantalum oxide is more stable, both thermally and chemically in Na₂SO₄+V₂O₅ media at 1100 °C, and shows a better hot corrosion resistance than the tetragonal phase.     

Structural and electrical properties of 0.57PSN-0.43PT ceramics prepared by mechanochemical synthesis and sintered at low temperature

In this investigation we show that the dielectric, ferroelectric and piezoelectric properties of stoichiometric 0.57Pb(Sc_1/2Nb_1/2)O₃-0.43PbTiO₃ (0.57PSN-0.43PT) ceramics prepared by mechanochemical synthesis are comparable or even better than the properties of 0.57PSN-0.43PT ceramics with Nb doping, which was proposed to enhance the electrical properties. Here, the stoichiometric ceramic was sintered to 97% of theoretical density at a temperature of 1000 °C, which is 200-300 °C lower than previously reported. The dielectric constant ?, remnant polarization P_r, piezoelectric coefficient d₃₃, coupling coefficients k_p and k_t and mechanical quality factor Q_m of the ceramics prepared by mechanochemical synthesis were 2200, 43 μC/cm², 570 pC/N, 0.71, 0.56 and 38, respectively.The effects of the poling field on the structural and electrical properties of the 0.57PSN-0.43PT ceramics were investigated. The results show that the ratio of the monoclinic to the tetragonal phases is influenced by the application of the poling electric field. The non-poled ceramics contain 71% of the monoclinic phase and 29% of the tetragonal phase. The highest d₃₃, k_p and k_t were measured for ceramics poled at an electric field of 3 kV/mm. For these poled ceramics a phase determination of 86% monoclinic phase and 14% tetragonal phase was obtained from Rietveld refinements.     

Influence of zirconium salt precursors on the crystal structures of zirconia

Procedures have been developed for preparing zirconia to yield either tetragonal or monoclinic phases following low (400–600 °C) temperature calcination. The data in this note emphasize the need for paying attention to the precursor zirconium salt in preparing the monoclinic or tetragonal phases. A salt precursor obtained from a supplier at different times may produce different results. Generally, it is more difficult to obtain a precursor that will produce the monoclinic phase than the one that will produce the tetragonal form.     

Phase structure and nano-domain in high performance of BaTiO3 piezoelectric ceramics

BaTiO₃ ceramics were prepared by conventional sintering technique with a special emphasis on the effects of sintering temperature (1100-1230 °C) on the crystalline structure and piezoelectric properties. XRD patterns indicated that the crystallographic structure changed from tetragonal phase to orthorhombic one with raising sintering temperature from 1160 °C to 1180 °C. Domains were shaped in a stripe and a herringbone in orthorhombic samples for BaTiO₃ ceramics. The domain width and domain density increased with raising sintering temperature. The BaTiO₃ ceramic sintered at 1190 °C showed the excellent electrical properties, d₃₃ = 355 pC/N, k_p = 40%, P_r = 10.2 μC/cm², respectively, which are originated to the contributions of both the crystallographic structure transition and nano-domain.     

Effect of manganese oxide on the sintered properties and low temperature degradation of Y-TZP ceramics

The sinterability of yttria-tetragonal zirconia polycrystals (Y-TZP) containing small amounts of MnO₂ as sintering aid was investigated over the temperature range of 1250–1500 °C. Sintered samples were evaluated to determine bulk density, Young's modulus, Vickers hardness and fracture toughness. In addition, the tetragonal phase stability of selected samples was evaluated by subjecting the samples to hydrothermal ageing in superheated steam at 180 °C/10 bar for up to 24 h. The results showed that the addition of MnO₂, particularly ≥0.3 wt% was effective in aiding densification, improving the matrix stiffness and hardness when compared to the undoped Y-TZP sintered at temperatures below 1350 °C. On the other hand, the fracture toughness of Y-TZP was unaffected by MnO₂ addition except for the 1 wt% MnO₂-doped Y-TZP samples sintered above 1400 °C. The hydrothermal ageing resistance of Y-TZP was significantly improved with the additions of MnO₂ in the Y-TZP matrix.     

Comprehensive sulfation model verified for T-T sorbent clusters during flue gas desulfurization at moderate temperatures

An empirical sulfation model for T-T sorbent clusters was developed based on amassed experimental results under moderate temperatures (300-800 °C). In the model, the reaction rate is a function of clusters mass, SO₂ concentration, CO₂ concentration, calcium conversion and temperature. The smaller pore volume partly results in a lower reaction rate at lower temperatures. The exponent on SO₂ concentration is 0.88 in the rapid reaction stage and then decreases gradually as reaction progresses. The exponent on the fraction of the unreacted calcium is 1/3 in the first stage and then increases significantly in the second stage. The CO₂ concentration has a negative influence on SO₂ removal, especially for the temperature range of 400-650 °C, which should be avoided to achieve a high effective calcium conversion. The sulfation model has been verified for the T-T sorbent clusters and has also been applied to CaO particles. Over extensive reaction conditions, the predictions agree well with experimental data.     

Synthesis of nanocrystalline SnO2 powder from SnCl4 by mechanochemical processing

The rapid synthesis of nanocrystalline SnO₂ powder using a mechanochemical reaction of SnCl₄ (instead of the widely used tin (II) compounds) with (NH₄)₂CO₃ and the subsequent annealing of the product in air and under an H₂O/NH₃ atmosphere has been investigated using X-ray powder diffraction, TG and TEM. The reaction was complete within 5 min. Additional milling of the product at a higher milling intensity for 120 min led to the crystallisation of tetragonal SnO₂. The NH₄Cl salt matrix was removed by annealing at 300 °C. The average crystallite size of tetragonal SnO₂ was in the range of 2-48 nm and it can be controlled by variation heating temperatures and annealing atmospheres in the range of 300-700 °C.     

Effects of chloride ions on hydrothermal synthesis of tetragonal BaTiO3 by microwave heating and conventional heating

Effects of the chloride ions on the synthesis of tetragonal BaTiO₃ in the microwave-hydrothermal (MH) and conventional hydrothermal (CH) processes were investigated. Such effects were found to be dependent of the synthesis method and temperature. In the MH process performed at 240 °C, tetragonal BaTiO₃ powders of c/a ratios larger than 1.009 were readily prepared whether the chloride ions were present or not. At the lower temperature of 220 °C, however, the c/a ratio in the MH BaTiO₃ was only 1.0061 in the absence of the chloride ions but increased to 1.0084 aided by the chloride ions. In the CH process, the chloride ions were observed to play an invariably important role in the preparation of tetragonal BaTiO₃. In contrast, the chloride ions appeared to have less influence on the particle size of the hydrothermal BaTiO₃ in a relatively short period of time.     

Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

Preparation and characterization of nanometer-sized (Pb1−x,Bax)TiO3 powders using acetylacetone as a chelating agent in a non-aqueous sol-gel process

Nanometer-sized lead barium titanate (Pb_1−xBa_xTiO₃, PB_xT) powders were prepared by a non-aqueous sol-gel process using lead acetate, barium acetate, and titanium isopropoxide as precursors and ethylene glycol as the solvent. In this procedure, Ti-isopropoxide was chelated with acetylacetone. The samples were characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermogravimetric analysis/differential thermal analysis (TGA/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) specific surface area analysis. The results indicate that perovskite PB_xT phases were obtained by heat treatment at 450 °C for 5 h, and a pure perovskite was examined at 600 °C. The average particle sizes of perovskite PB_xT powders calcined at 600 °C were approximately about 40-80 nm, and BET analysis showed that the surface areas of the powders obtained at 600 °C were approximately 6-16 m²/g. In addition, the phase transition from the tetragonal ferroelectric phase to the cubic paraelectric phase occurred in a range of approximately 0.6 < x < 0.8.     

Soot-free synthesis of C60

A new synthetic medium for the production of C₆₀ has been found that does not produce soot. C₆₀ was produced in the liquid phase of an aerosol of precursor soot at 700 °C. The precursor soot aerosol, a high temperature stable form of hydrocarbon, was produced by pyrolysis of acetylene at atmospheric pressure in a flow tube reactor. At 700 °C, the effluent particles were found to contain PAHs, small hydrocarbons and fullerenes but no observable black material. However, when the reactor temperature was changed to 800 °C, soot was also produced in the effluent particles along with PAHs and other small hydrocarbons, and the fullerene product disappeared. These results show a clear competition between the production of fullerenes and other forms of carbon. The filter-collected effluent was shown to be completely soluble in conventional solvents suggesting the possibility of an efficient cyclic synthetic process. Fullerenes were only found in the particle phase implying the first observed liquid phase synthesis of C₆₀.