Dielectric and ferroelectric properties of Ba(ZrxTi1−x)O3 ceramics

It is known that Curie temperature of barium titanate system can be altered by the substitution of dopants into either A- or B-site. Dopants could pinch transition temperature, lower Curie temperature, and raise the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transition close to room temperature. This isovalent substitution could improve the ferroelectric properties of the BaTiO₃-based system. In this study, barium zirconate titanate Ba(Zr_xTi_1−x)O₃ (BZT; x = 0, 0.02, 0.05 and 0.08) ceramics were prepared by conventionally mixed-oxide method. The ferroelectric properties of BZT ceramics were investigated. Increasing Zr content in the BaTiO₃-based compositions caused a decrease in Curie temperature (T_c). At T_c, the highest relative permittivity of BZT with an addition of 0.08 mol% of Zr was 12,780. The BZT specimens with the additions of 0.05 mol% and 0.08 mol% of Zr presented the remanent polarization at 25 μC/cm² and 30 μC/cm², respectively.     

Microwave dielectric properties and co-firing with copper of (Bi1−xCux)(Nb1−xWx)O4 ceramics

Effect of substitution of CuO and WO₃ on the microwave dielectric properties of BiNbO₄ ceramics and the co-firing between ceramics and copper electrode were investigated. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ (x = 0.005, 0.01, 0.015, 0.02) composition can be densified between 900 and 990 °C. The microwave dielectric constants lie between 36 and 45 and the pores in ceramics were found to be the main influence. The Q values changes between 1400 and 2900 with different x values and sintering temperatures while Q_f values lie between 6000 and 16,000 GHz. The microwave dielectric losses, mainly affected by the grain size, pores, and the secondary phase, are discussed. The (Bi_1−xCu_x)(Nb_1−xW_x)O₄ ceramics and copper electrode was co-fired under N₂ atmosphere at 850 °C and the EDS analysis showed no reaction between the dielectrics and copper electrodes. This result presented the (Bi_1−xCu_x)(Nb_1−xW_x)O₄ dielectric materials to be good candidates for LTCC applications with copper electrode.     

Enhanced dielectric tunability properties of Ba(ZrxTi1−x)O3 thin films using seed layers on Pt/Ti/SiO2/Si substrates

The compositionally graded and homogeneous Ba(Zr_xTi_1−x)O₃ (BZT) thin films were fabricated on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si and Pt/Ti/SiO₂/Si substrates by a sol–gel deposition method, respectively. These films crystallized into a single perovskite phase. The BZT thin films deposited on LaNiO₃/Pt/Ti/SiO₂/Si substrates had a highly (1 0 0) preferred orientation and exhibited a preferred (1 1 0) orientation when the thin films were deposited on Pt/Ti/SiO₂/Si substrates. The LNO and Ba(Zr_0.30Ti_0.70) served as seed layer on Pt/Ti/SiO₂/Si substrates and analyze the relationship of seed layer, microstructure and dielectric behavior of the thin films. The compositionally graded thin films from BaTiO₃ to BaZr_0.35Ti_0.65O₃ were fabricated on LNO/Pt/Ti/SiO₂/Si substrates. The tunability behavior of compositionally graded films was analyzed in order to produce optimum effective dielectric properties. The dielectric constant of BaZr_xTi_1−xO₃ compositionally graded thin films showed weak temperature dependence. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.     

The homogeneity range and the microwave dielectric properties of the BaZn2Ti4O11 ceramics

Ceramics with a composition close to BaZn₂Ti₄O₁₁ were synthesized according to various substitutional mechanisms in order to verify an existence of a homogeneity range in the vicinity of this composition. Structural and microstructural investigations showed that the crystal structure of BaZn₂Ti₄O₁₁ was formed in the homogeneity range corresponding to the formula BaZn_2 − xTi₄O_11 − x (0 < x < 0.1). Densely sintered BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) ceramics exhibited a dielectric constant around 30, τ_f = −30 ppm/K and high Q × f values, which increased from 68,000 GHz at x = 0 to 83,000 GHz at x = 0.05. Structurally, the deficiency of Zn in BaZn_2 − xTi₄O_11 − x (0 < x < 0.1) resulted in a slight decrease in the unit-cell volume. The influence of secondary phases in the BaZn₂Ti₄O₁₁-based materials on the microwave dielectric properties was also investigated. A presence of small amounts of ZnO, BaTiO₃, hollandite-type solid solutions (Ba_xZn_xTi_8 − xO₁₆) and BaTi₄O₉ caused a decrease in Q × f values.     

Effect of structural and electrochemical properties of different Cr-doped contents of Li[Ni1/3Mn1/3Co1/3]O2

Layered Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ materials with x = 0, 0.01, 0.02, 0.03, 0.05 are prepared by a solid-state pyrolysis method. The oxide compounds were calcined with various Cr-doped contents, which result in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry, and SEM. XRD experiment revealed that the Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Cr_x]O₂ (x = 0, 0.01, 0.02, 0.03, 0.05) were crystallized to well layered -NaFeO₂ structure. The first specific discharge capacity and coulombic efficiency of the electrode of Cr-doped materials were higher than that of pristine material. When x = 0.02, the sample showed the highest first discharge capacity of 241.9 mAh g⁻¹ at a current density of 30 mA g⁻¹ in the voltage range 2.3–4.6 V, and the Cr-doped samples exhibited higher discharge capacity and better cycleability under medium and high current densities at room temperature.     

Microstructure and ferroelectric properties of sol–gel graded PZT (40/52/60) and (60/52/40) thin films

Graded Pb(Zr_x,Ti_1−x)O₃ films with Zr compositions varied across the thickness direction were deposited on Pt/Ti/SiO₂/Si substrate using a conventional spin-coating method. The up- and down-graded PZT films exhibited the perovskite polycrystalline structure. Microstructure investigations of the films showed a dense texture and successive layers of different compositions. The relative permittivities of the up- and down-graded PZT films measured at 1 kHz and room temperature were 1846 and 1019, respectively. Good dielectric and ferroelectric properties as well as the low-temperature processing suggested that the compositionally graded PZT films were promising for memory device applications.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

Sintering behaviour of La1−xSrxFeO3−δ mixed conductors

The sintering properties of La_1−xSr_xFeO_3−δ (x = 0.1, 0.25) mixed conductors have been investigated with particular emphasis on the effect of secondary phases due to cation non-stoichiometry (±5 mol% La excess and deficiency). Secondary phases, located at grain boundaries in cation non-stoichiometric materials, increased the sintering temperature compared to single-phase materials. Extensive swelling in final stage of sintering was observed in all materials, which resulted in micro-porous materials. The swelling was most pronounced in the phase pure and two-phase materials due to La-deficiency, while refractory secondary phases in La-excess materials inhibited both sintering, grain growth and swelling. In La-deficient materials, formation of molten secondary phases resulted in rapid swelling due to viscous flow. The present findings demonstrated the importance of controlling sintering temperature and time, as well as careful control of the cation stoichiometry of La_1−xSr_xFeO_3−δ in order to achieve fully dense and homogenous La_1−xSr_xFeO_3−δ ceramics.     

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Catalytic combustion of methane has been investigated over AMnO₃ (A = La, Nd, Sm) and Sm_1−xSr_xMnO₃ (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm³ g⁻¹ h⁻¹, CH₄ concentration = 0.4% v/v, O₂ concentration = 10% v/v.

Specific surface areas of perovskites were in the range 13–20 m² g⁻¹. XRD analysis showed that LaMnO₃, NdMnO₃, SmMnO₃ and Sm_1−xSr_xMnO₃ (x = 0.1) are single phase perovskite type oxides. Traces of Sm₂O₃ besides the perovskite phase were detected in the Sm_1−xSr_xMnO₃ catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO₃. The fraction of Mn(IV) in the Sm_1−xSr_xMnO₃ samples increased with x. TPR measurements on AMnO₃ showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO₃ > NdMnO₃ > SmMnO₃. In Sm_1−xSr_xMnO₃ the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO₂ below 1023 K. The activation energies of the AMnO₃ perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO₃ perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.     

Synthesis and properties of Bi0.5(Na1−x−yKxAgy)0.5TiO3 lead-free piezoelectric ceramics

Bi_0.5(Na_1−x−yK_xAg_y)_0.5TiO₃ piezoelectric ceramics were prepared by conventional ceramic processes. X-ray diffraction patterns show a pure perovskite structure, indicating that the K⁺ and Ag⁺ ions substitute for the Na⁺ ions in Bi_0.5Na_0.5TiO₃. The temperature dependence of the dielectric constant and dissipation factor shows all ceramics to experience two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric. The transition temperature from ferroelectric to anti-ferroelectric and the temperature at which the dielectric constant reaches its maximum value decrease with the increase of K⁺ amount. At room temperature, the ceramics containing 17.5–20 mol% K⁺ and 2 mol% Ag⁺ exhibit high piezoelectric constant (d₃₃ = 180 pC/N) and high electromechanical coupling factor (k_p = 35%).     

Ce1−xCuxO2−x/Al2O3/FeCrAl catalysts for catalytic combustion of methane

A series of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts (x = 0–1) were prepared. The structure of the catalysts was characterized using XRD, SEM and H₂-TPR. The catalytic activity of the catalysts for the combustion of methane was evaluated. The results indicated that in the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts the surface phase structure were the Ce_1−xCu_xO_2−x solid solution, -Al₂O₃ and γ-Al₂O₃. The surface particle shape and size were different with the variety of the molar ratio of Ce to Cu in the Ce_1−xCu_xO_2−x solid solution. The Cu component of the Ce_1−xCu_xO_2−x/Al₂O₃/FeCrAl catalysts played an important role to the catalytic activity for the methane combustion. There were the stronger interaction among the Ce_1−xCu_xO_2−x solid solution and the Al₂O₃ washcoats and the FeCrAl support.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Microwave dielectric properties of rutile (Zn1/3Nb2/3)0.40(Ti1−xSnx)0.60O2 (0.15 ≤ x ≤ 0.30) ceramics

Microwave dielectric properties of (Zn_1/3Nb_2/3)_0.40(Ti_1−xSn_x)_0.60O₂ ceramics were investigated as a function of SnO₂ content (0.15 ≤ x ≤ 0.30). A single phase with tetragonal rutile structure was obtained through the entire composition. The unit-cell volume of the specimens was increased with SnO₂ content, due to the larger ionic radius of Sn⁴⁺ (0.69 Å) than that of Ti⁴⁺ (0.605 Å) for octahedral site. Dielectric constant (K) of the sintered specimens was affected by the dielectric polarizability. Quality factor (Qf) was dependent on the degree of reduction of Ti⁴⁺ ion. With an increase of SnO₂ content, the temperature coefficient of resonant frequency (TCF) of the specimens decreased due to the decrease of the octahedral distortion of rutile structure.     

Hydrothermal synthesis of Ba(Ti, Sn)O3 fine powders and dielectric properties of the corresponding ceramics

Fine powders of submicron-sized crystallites of BaTiO₃ were prepared at 85–130°C by the hydrothermal method, starting from TiO₂.ξH₂O gel and Ba(OH)₂ solution. The products obtained below 110°C incorporated considerable amounts of H₂O and OH⁻ in the lattice. As-prepared BaTiO₃ is cubic and converts to the tetragonal phase after heat treatment at 1200°C, accompanied by the loss of residual OH⁻ ions. Hydrothermal reaction of SnO₂.ξH₂O gel with Ba(OH)₂ at 150–260°C gives rise to the hydrated phase, BaSn(OH)₆.3H₂O, due to the amphoteric nature of SnO₂.ξH₂O which stabilises Sn(OH)₆²⁻ anions in basic media. On heating in air or releasing the pressure in situ at 260°C, BaSn(OH)₆.3H₂O converts to BaSnO₃ through an intermediate, BaSnO(OH)₄. Solid solutions of Ba(Ti,Sn)O₃ are directly formed from (TiO₂ + SnO₂)..ξH₂O gel up to 35 mol% SnO₂. At higher Sn contents, the hydrothermal products are mixtures of BaSn(OH)₆.3H₂O and BaTiO₃, which on annealing at 1000°C result in monophasic Ba(Ti,Sn)O₃. The sintering characteristics and the dielectric properties of the ceramics prepared out of these fine powders are presented. The dielectric properties of fine-grained Ba(Ti,Sn)O₃ ceramics are explained on the basis of the prevailing diffuse phase transition behaviour.     

Nanosized perovskite-type oxides La1−xSrxMO3−δ (M = Co, Mn; x = 0, 0.4) for the catalytic removal of ethylacetate

Nanometer perovskite-type oxides La_1−xSr_xMO_3−δ (M = Co, Mn; x = 0, 0.4) have been prepared using the citric acid complexing-hydrothermal-coupled method and characterized by means of techniques, such as X-ray diffraction (XRD), BET, high-resolution scanning electron microscopy (HRSEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and temperature-programmed reduction (TPR). The catalytic performance of these nanoperovskites in the combustion of ethylacetate (EA) has also been evaluated. The XRD results indicate that all the samples possessed single-phase rhombohedral crystal structures. The surface areas of these nanomaterials ranged from 20 to 33 m² g⁻¹, the achievement of such high surface areas are due to the uniform morphology with the typical particle size of 40–80 nm (as can be clearly seen in their HRSEM images) that were derived with the citric acid complexing-hydrothermally coupled strategy. The XPS results demonstrate the presence of Mn⁴⁺ and Mn³⁺ in La_1−xSr_xMnO_3−δ and Co³⁺ and Co²⁺ in La_1−xSr_xCoO_3−δ, Sr substitution induced the rises in Mn⁴⁺ and Co³⁺ concentrations; adsorbed oxygen species (O⁻, O₂⁻, or O₂²⁻) were detected on the catalyst surfaces. The O₂-TPD profiles indicate that Sr doping increased desorption of the adsorbed oxygen and lattice oxygen species at low temperatures. The H₂-TPR results reveal that the nanoperovskite catalysts could be reduced at much lower temperatures (<240 °C) after Sr doping. It is observed that under the conditions of EA concentration = 1000 ppm, EA/oxygen molar ratio = 1/400, and space velocity = 20,000 h⁻¹, the catalytic activity (as reflected by the temperature (T_100%) for EA complete conversion) increased in the order of LaCoO_2.91 (T_100% = 230 °C) ≈ LaMnO_3.12 (T_100% = 235 °C) < La_0.6Sr_0.4MnO_3.02 (T_100% = 190 °C) < La_0.6Sr_0.4CoO_2.78 (T_100% = 175 °C); furthermore, there were no formation of partially oxidized by-products over these catalysts. Based on the above results, we conclude that the excellent catalytic performance is associated with the high surface areas, good redox properties (derived from higher Mn⁴⁺/Mn³⁺ and Co³⁺/Co²⁺ ratios), and rich lattice defects of the nanostructured La_1−xSr_xMO_3−δ materials.     

Development of new catalysts for N2O-decomposition from adipic acid plant

Direct decomposition of N₂O was investigated using simulated and real industrial gas stream coming from an adipic acid plant. Two different kinds of catalysts were studied: (i) LaB_1−xB′_xO₃ and CaB_1−xCu_xO₃ (B = Mn, Fe and B′ = Cu, Ni) perovskites (PVKs) and (ii) supported PVKs (10 or 20 wt.%) on γ-Al₂O₃ and CeO₂–ZrO₂. The structural modifications induced by the composition of PVK samples affect the catalytic performances: mixed oxide formation in CaMn_0.7Cu_0.3O₃ samples allows to reach the highest values of N₂O conversion while the effect of PVK phases is more controversial. The importance of copper on catalytic activities is confirmed by the investigation on CaMn_1−xCu_xO₃ samples. The best results were obtained with a CaMn_0.6Cu_0.4O₃ catalyst calcined at 700 °C for 5 h, in which the presence of copper maximises the Ca₃CuMnO₆ phase formation. The increase in Cu-content produces a large segregation of CuO despite PVK formation. The best catalyst was tested using industrial gas stream, showing good stability also in the presence of H₂O and O₂ (8% v/v ) after 1400 h on-stream. To increase surface area, Cu-containing PVKs were deposed on γ-Al₂O₃ and CeO₂–ZrO₂, and this latter has been recognised as the best support. Indeed, the activity of the PVKs supported on ceria–zirconia is comparable to and even better than that of the bulk catalysts. A possible explanation regards the support contribution in terms of activity and/or promotion of O₂ mobility which enhances the overall activity of the catalyst.     

Catalytic reduction of sulfur dioxide using hydrogen or carbon monoxide over Ce1−xZrxO2 catalysts for the recovery of elemental sulfur

This study focuses on the direct sulfur recovery process (DSRP), in which SO₂ can be directly converted into elemental sulfur using a variety of reducing agents over Ce_1−xZr_xO₂ catalysts. Ce_1−xZr_xO₂ catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO₂ reduction were as follow: the space velocity (GHSV) was 30,000 ml/g_-cat h and the ratio of [CO (or H₂, H₂ + CO)]/[SO₂] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce_0.5Zr_0.5O₂ catalyst and CO, respectively. In this case, the SO₂ conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO₂ removal and elemental sulfur recovery was achieved in the reduction of SO₂ with CO as a reducing agent than that with H₂. In the reduction of SO₂ by H₂ over the Ce_0.5Zr_0.5O₂ catalyst, SO₂ conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO₂ using synthetic gas with various [CO]/[H₂] molar ratios over the Ce_0.5Zr_0.5O₂ catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO₂ reduction using the synthetic gas with various [CO]/[H₂] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce_1−xZr_xO₂ catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H₂.     

Transport numbers and oxygen permeability of SrCe(Y)O3-based ceramics under oxidising conditions

Partial conductivities in the SrCe(Y)O_3−δ system have been studied in oxidising conditions in the temperature range 923–1273 K. Compositions with variable Y content (5 and 10 at.%), Sr deficiency (3 at.%), and with the addition of Fe₂O₃ as sintering aid (2 mol%) were analysed. A modified Faradaic efficiency method and oxygen permeation measurements were employed to appraise the oxide-ionic transport. Oxide-ion transference numbers in air lie in the range 0.19–0.80 and decrease with increasing temperature in the range 973–1223 K. Modelling of total conductivity as a function of oxygen partial pressure (p(O₂)) confirmed that protonic transport is minor under the studied conditions. SrCe_0.95Y_0.05O_3−δ exhibits greater oxide-ion conductivity than SrCe_0.9Y_0.1O_3−δ, indicative of dopant–vacancy association at high dopant contents. Conversely, oxygen permeability is slightly higher for SrCe_0.9Y_0.1O_3−δ as a result of faster surface-exchange kinetics. The oxygen flux through Fe-free membranes is dominated by the bulk in low p(O₂) gradients, when the permeate-side p(O₂) is higher than 0.03 atm, but surface exchange plays an increasing role with increasing p(O₂) gradient. Addition of Fe₂O₃ to SrCe(Y)O_3−δ lowers the sintering temperature by 100 K but results in the formation of intergranular second phases which block oxide-ionic and electronic transport, and thus oxygen permeation. The average thermal expansion coefficients (TECs) are (10.8–11.6) × 10⁻⁶ K⁻¹ in the temperature range 373–1373 K for all studied compositions.     

Characterisation of ceria–zirconia solid solutions after hydrothermal ageing

Ce_1−xZr_xO₂ (x=0–0.84) solid solutions prepared by co-precipitation were characterised after calcination at 700 or 900°C, and after hydrothermal ageing at 1000 or 1200°C. The solid solutions were formed at 700°C, and crystallise as cubic or tetragonal phases depending on their compositions. Despite the rather high surface areas obtained after calcination at 700°C, the sintering is important at 900°C, and tremendous after hydrothermal ageing at 1000°C. For all compositions between 16 and 83 mol% ceria, complete de-mixing of the solid solutions into two phases was observed after ageing at 1200°C: one Zr-rich, tetragonal phase, and one Ce-rich, cubic phase. XPS and ISS measurements show that the phase separation takes place with surface enrichment in Zr, the Zr-rich phase being formed at the periphery of the particles, whereas the core is composed of the Ce-rich phase.     

Microstructure development and properties of novel Ba-doped phase Sialon ceramics

Herein, we report the microstructure and properties of the newly developed near monophasic S-Sialon ceramic, based on the composition of Ba₂Si_12−xAl_xO_2+xN_16−x (x = 20.2). Appropriate amount of the precursor powders (BaCO₃, -Si₃N₄, AlN, Al₂O₃) with a targeted composition of BaAlSi₅O₂N₇ was ball milled and hot pressed to full density in the temperature range of 1600–1750 °C for 2 h in nitrogen atmosphere. Extensive transmission electron microscopy (TEM) study has been conducted to understand the microstructure development and characterise the various morphological features in hot pressed S-Sialon. The sintering mechanism is based on the liquid phase sintering route, which involves the formation of a Ba–Al silicate liquid (<5%) with dissolved nitrogen at intergranular pockets. The experimental observation suggests that the S-phase crystallises in elongated platelet morphology with preferred growth parallel to the orthorhombic ‘c’ axis and primary facet planes parallel to (1 0 0) and (0 1 0). The Ba-S-phase ceramic has an acoustically measured Young modulus of 210–230 GPa, a hardness of 13 GPa and a fracture toughness of 4 MPa m^1/2, little lower than typical of a ceramic with morphologically anisotropic grains contributing to bridging and pullout mechanisms.