LAFEO3 AND LACOO3 BASED PEROVSKITES: PREPARATION AND PROPERTIES OF DENSE OXYGEN PERMEABLE MEMBRAN... more LAFEO3 AND LACOO3 BASED PEROVSKITES: PREPARATION AND PROPERTIES OF DENSE OXYGEN PERMEABLE MEMBRANES. Kjell Wiik, Anita Fossdal, Lise Sagdahl, Hilde L. Lein, Mohan Menon, Sonia Faaland, Ivar Waernhus, Nina Orlovskaya, Mari-Ann ...
An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical ... more An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca 3 Co 4Àx O 9+d and CaMnO 3Àd exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca 3 Co 1+y Mn 1Ày O 6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca 3 Co 1+y Mn 1Ày O 6 varies and is close to 1 (y ¼ 0) at the Ca 3 Co 1+y Mn 1Ày O 6-CaMnO 3Àd boundary. The existence of a wide homogeneity range of 0 # y # 1 for Ca 3 Co 1+y Mn 1Ày O 6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.
Abstract Gd2Zr2O7 ceramic is considered as a potential candidate for thermal barrier coatings. He... more Abstract Gd2Zr2O7 ceramic is considered as a potential candidate for thermal barrier coatings. Here we present hot corrosion behavior of single layer Gd2Zr2O7 (GZO), Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ), YSZ and double-layer YSZ/Gd2Zr2O7 + 50 wt% YbSZ (YSZ + GZO/YbSZ) coatings prepared by air plasma spraying on Inconel 625 substrates. Prior to hot corrosion the fracture toughness of the coatings was assessed by Vickers indentation showing a variation between 0.6 and 2.3 MPa.m1/2 where the GZO/YbSZ coating exhibited the highest fracture toughness. The hot corrosion test was conducted by exposing the coatings to an equimolar mixture of Na2SO4 and V2O5 at 750 °C. After the hot corrosion test, the type of phases, chemical composition, microstructure and crack formation were investigated. Formation of GdVO4 and YVO4 was observed as the corrosion products in the GZO containing coatings and the YSZ coating, respectively. All coatings showed the formation of monoclinic zirconia after the corrosion test. The Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ) coating showed the best hot corrosion resistance due to reduced reactivity and enhanced fracture toughness and represents a new composition with properties promising for TBC applications.
Journal of The European Ceramic Society, Apr 1, 2019
Schemes, diagrams both SEM and TEM micrographs were created using Orig-inPro 9.1G, ImageJ, Diamon... more Schemes, diagrams both SEM and TEM micrographs were created using Orig-inPro 9.1G, ImageJ, Diamond and Digital Micrograph. Figures were arranged, merged and saved using PowerPoint 2010 and Photoshop CS5. Table S 1 shows the ionic radii of the substituted elements and inserted dopants. According to the similarity of the ionic radii of the elements used, doping should be possible. The XRD patterns of the Na x CoO 2 (NCO) and Bi 2 Ca 2 Co 2 O 9 (BCCO) phases, shown in Figure S 1 refer to Figure 2 in the main text. A step size of 0.003942, a time per step of 1.1 seconds, a voltage of 40 kV and a current of 40 mA were used in the XRD experiments. Pure NCO and nearly pure BCCO phases were subjected to SEM and elemental distribution analyses of polished cross-sections, as shown in Figure S 2a-d. Vibration-polished cross-section specimens were prepared by a multistep (30 µm, 15 µm, 6 µm, 3 µm and 1 µm diamond lapping films) polishing program using a Techprep from Allied-High Tech Products, Inc., followed by vibration polishing using a Buehler Vibromet-2 and a 50 nm colloidal alumina suspension. TEM specimens were prepared similar to SEM specimens and put on a TEM grid. The specimens were pinched out using a precision ion polishing system (Ar-ion) Model 691 from Gatan. The BCCO phase decomposes at approximately 1023 K, and Ca-containing phases are formed; see Figure S 1 [1]. Figure S 3 gives detailed elemental distribution information referring to Figure 4a-c in the main document. The interdiffusion of Ca into the NCO phase is clearer, and the very thin layers of NCO and BCCO are clearly visible in the Na, Ca and Bi signals shown in Figures S 3d-f and 4a-d. The insets of Figure 4d-f in the main document are enlarged for a better readability in Figure S 5a-f. Additional TEM micrographs of other sites of the CCO-30-35-10 nanocomposite ceramic are shown in Figures S 6, 7 and 8. These other sites clarify the composition and thickness of different layers within the material. These sequences continue throughout the ceramic. Comparing Figure S 6 and Figure S 7 shows that the amount of interdiffusion of Ca into the NCO phase is not constant, suggesting that the degree of interdiffusion might also depends on thickness and surroundings (e.g., being embedded between BCCO phases). Figure S 9 illustrates the measured heat capacities C P of CCO and nanocomposite ceramics as a function of temperature in the range from 313 K to 1173 K. The C P values reached approximately 0.83 J • g-1 • K-1 at 1073 K for a CCO-30-35-10 nanocomposite ceramic. The nanocomposite ceramics showed only small differences in C P from those of undoped CCO. The system was calibrated and the sapphire method was used. The sensitivity S and heat capacity C P were calculated as described by Jankovsky et al. [2]. Samples of bar geometry were
Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can se... more Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can selectively separate oxygen from air at elevated temperatures. These membranes have several potential applications that require a continuous supply of oxygen. For example, they may be an alternative for cryogenic production of oxygen or alternative electrode materials in solid oxide fuel cells. Of particular significance is the partial oxidation of methane to syngas (CO + H2). By combining air separation and partial oxidation of natural gas into a single step, the need for expensive oxygen production by cryogenic means may be eliminated. Combined with existing processes for gas-to-liquid production such as Fisher-Tropsch and methanol synthesis, the MIEC membrane technology represents a very attractive route for conversion of natural gas to liquid fuels.The research in this field was initially concerned with the search for materials with the optimum oxygen flux. Today, the long term stability of the membranes is probably the main issue. The membranes have to be stable under operating conditions, which include mechanical stability and chemically compatibility with other materials like sealing and support materials. However, the current understanding of the long term chemical and mechanical reliability is poor and this is one of the major challenges for solid state ionic research. The aim of this work has been to investigate the mechanical properties and the chemical stability of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0, 0.5, 1) materials when they are exposed to thermal and chemical gradients.The chemically induced stresses due to reduction of the valence state of the transition metals are of particular importance with respect to the mechanical stability. In paper I, the oxygen non-stoichiometry, investigated by thermogravimetrical analysis, and thermal end chemical expansion, studied by dilatometry and high temperature X-ray diffraction, of La0.5Sr0.5Fe1-xCoxO3-δ materials are reported. The oxygen deficiency was observed to increase with decreasing partial pressure of oxygen and increasing temperature corresponding to expectations and previous reports. At ambient temperature the thermal expansion coefficient of the materials were in the range 15- 18·10-6 K-1. Above a certain temperature thermal reduction of the material take place, and the thermal expansion coefficient due to chemical expansion raise to 16-36·10-6 K-1. The chemical expansion ec, defined as the linear expansion due to a change in partial pressure of oxygen at constant temperature, reached a maximum in the range 0.036-0.039 for the materials studied at 800oC. The change in ionic radii of the transition metals is the main contribution to the chemical expansion. The crystal structure of the perovskite materials were shown to be slightly rhombohedral at ambient temperatures and a transition to cubic phase were observed above 300oC.This non-linear thermal expansion behavior is a major challenge for the applications of the mixed conductor materials. La0.5Sr0.5Fe1-xCoxO3-δ membranes in an oxygen partial pressure gradient will have different oxygen deficiency on either side of the membrane. The increasing oxygen deficiency is accompanied by a volume expansion as shown in paper I, and this will lead to chemically induced stresses. These stresses and the failure that might follow can be prevented by creep of the materials. Creep is also important due to dimensional stability. In paper II, the steady-state creep performance under compression of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 1) as a function of temperature, atmosphere, load and two different grain sizes is reported. The stress exponent found for the materials was close to unity and an unusual low inverse grain size exponent close to one was found for one of the materials. The activation energy of the two materials was not equal and the influence of secondary phases on the creep was discussed. The obtained creep behavior and microstructural investigation after measurements point to a diffusion related mechanism for the creep. Higher creep rates are found under reducing conditions and this suggest that creep relaxation of mechanical or chemical induced stresses may enhance the mechanical stability of oxygen permeable membranes.In Paper III, the mechanical properties of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 0.75, 1) were investigated by several methods. Fracture strength was measured by four-point bending, fracture toughness was measured by SENB and SEVNB methods and finally Young’s modulus were investigated by four-point bending and resonant ultrasound spectroscopy. Four-point bending showed a non-linear ferroelastic behavior at ambient temperature due to rhombohedral crystal structure. Above the ferroelastic to paraelastic transition temperature the materials showed elastic behavior, however, at temperatures from about 800oC a non-elastic respond was observed due to creep. The measured fracture strength and fracture…
Doped Gd 2 Zr 2 O 7 materials have interesting properties as thermal barrier coatings (TBC) to re... more Doped Gd 2 Zr 2 O 7 materials have interesting properties as thermal barrier coatings (TBC) to replace the YSZ topcoats traditionally used. Here we investigate the thermomechanical properties and phase relations of Gd 2 Zr 2 O 7 (GZO) alloyed with 5 mol% Yb 2 O 3 stabilized ZrO 2 (YbSZ) in the composition range (Gd 2 Zr 2 O 7) 1−x (YbSZ) x , 0 ≤ x ≤ 0.98. With increasing YbSZ content, phase transformations from ordered to disordered pyrochlore to fluorite and tetragonal structures were observed. The thermal expansion coefficient (TEC) and Vickers hardness were correlated showing a maximum hardness (~ 11.5 GPa) and minimum TEC at x = 0.82. At 1000 °C, the TEC for the end members, x = 0 and 0.98, were 11.4 and 11.3 × 10-6 K −1 , respectively. The fracture toughness, K IC , showed an average value around 1.5 MPa m 0.5 for x ≤ 0.93 and increased significantly at x = 0.98 reaching 5.4 MPa m 0.5 due to the presence of a ferroelastic phase. For TBC applications, compounds with x = 0.98 show promise due to high TEC and high K IC .
Perovskites in the system La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ exhibit high stability at severe reduci... more Perovskites in the system La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ exhibit high stability at severe reducing conditions and high temperatures. Here we report the processing and oxygen transport properties of two compositions: La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ (x = 0.2, 0.4). Single-phase powders were synthesized by spray pyrolysis (SP) and solid state reaction (SSR). The resulting average particle sizes (BET) ranged from 140 nm (SP) to 1090 nm (SSR). Samples were prepared by CIP and pressureless sintered between 1200 and 1250 • C for 8-12 h. Sintered densities ranged from 97 (SSR) to 99% (SP) and grain size was less than 1.5 m. The average TEC varied in the range (13.8-14.6) × 10 −6 K −1 at 300-600 • C and (21.3-28.9) × 10 −6 K −1 at 600-800 • C. Surface exchange coefficients (k chem) and bulk diffusion coefficients (D chem) in terms of temperature and p O 2 were assessed by the method of electrical conductivity relaxation. A simple defect model was applied to explain electrical conductivity and bulk diffusion behaviour.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Dense nanocomposites of high electrical conductivity, thermoelectric power factor. • Highly align... more Dense nanocomposites of high electrical conductivity, thermoelectric power factor. • Highly aligned, semi-coherently grown 2D nanostructures built a 3D bulk material. • Co-doped In 2 O 3 of high electrical conductivity and thermoelectric power factor. • The Ioffe plot is suitable to evaluate applicability of materials power generation. • Designed power materials maximize power output of thermoelectric generators.
Journal of the American Ceramic Society, Dec 22, 2004
Formation of secondary phases and diffusion of cations in diffusion couples of yttria‐stabilized ... more Formation of secondary phases and diffusion of cations in diffusion couples of yttria‐stabilized zirconia and lanthanum manganite substituted with 0 to 60 mol% strontium have been studied by scanning electron microscopy and energy dispersive X‐ray spectroscopy. Only the primary phases were observed after 120 h at 1200°C, while formation of secondary phases was identified already after 1 h heat treatment at 1350°C. The phase composition of the reaction layer altered from La2Zr2O7 to SrZrO3 at increasing Sr content in LaxSr1‐xMnO3. The thickness of the reaction layer was increasing with heat treatment time. In diffusion couples of La0.4Sr0.6MnO3 formation of manganese oxide was observed in the perovskite layer after 1 h heat treatment at 1350°C, while isolated grains of SrZrO3 relatively deep inside the zirconia were observed after longer heat treatment time. Diffusion of Mn into zirconia was observed preferenced along grain boundaries in the early stage of the interface reaction.
Oxygen flux through La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5 and 1) membranes has been determi... more Oxygen flux through La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5 and 1) membranes has been determined as a function of oxygen partial pressure, temperature and time. The flux was diffusion controlled for low pO 2 gradients while larger pO 2 gradients caused a surface exchange ...
Cation tracer diffusion coefficients in La 1−x Sr x FeO 3−δ , (x = 0 and 0.1) at temperatures ran... more Cation tracer diffusion coefficients in La 1−x Sr x FeO 3−δ , (x = 0 and 0.1) at temperatures ranging from 1173 to 1373 K have been investigated by secondary ion mass spectrometry (SIMS) for both A-site and B-site diffusion applying Y 3+ and Cr 3+ as tracer ions. B-site impurity (represented by Cr) diffusion was found to be faster than A-site impurity (represented by Y) diffusion for LaFeO 3 in the whole temperature interval investigated and for La 0.9 Sr 0.1 FeO 3 at temperatures less than 1273 K. The obtained activation energies for bulk diffusion in La 0.9 Sr 0.1 FeO 3−δ were 168 ± 24 kJ/ mol for the Y tracer and 150 ± 30 kJ/mol for the Cr tracer, which are significantly lower than activation energies reported from diffusion couple experiments. The large variation in reported activation energies can be explained in terms of the difference in cation vacancy populations between the two experimental techniques. The diffusion coefficients were also clearly affected by a first order phase transition at approx. 1263 K in pure LaFeO 3. The obtained grain boundary diffusion coefficients were about five order of magnitude higher than the bulk diffusion coefficients, with an activation energy of 128 ± 28 kJ/mol.
Oxygen deficiency, thermal and chemical expansion of La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5,... more Oxygen deficiency, thermal and chemical expansion of La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5, 1) have been measured by thermogravimetry, dilatometry and high temperature X-ray diffraction. The rhombohedral perovskite materials transformed to a cubic structure at 350 ± 50°C. The thermal expansion of the materials up to the onset of thermal reduction was 14-18 × 10 − 6 K − 1. Above 500°C in air (400°C in N 2), chemical expansion contributed to the thermal expansion and the linear thermal expansion coefficients were significantly higher, 16-35 × 10 − 6 K − 1. The chemical expansion, ε c , showed a maximum of 0.0045 for x = 0.5 and 0.0041 for x = 1 at 800-900°C. The normalized chemical expansion, ε c /Δδ, was 0.036 for x = 0.5 and 0.035 for x = 1 at 800°C. The chemical expansion can be correlated with an increasing ionic radius of the transition metals with decreasing valence state.
Journal of the American Ceramic Society, Jan 21, 2005
Crystal structure and thermal properties of La 1؊x Sr x FeO 3؊␦ (x ؍ 0, 0.1, 0.3, 0.4, 0.5, and... more Crystal structure and thermal properties of La 1؊x Sr x FeO 3؊␦ (x ؍ 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high-temperature X-ray diffraction and thermal analysis in air and nitrogen (p(O 2) Ϸ 10 ؊3 atm) atmosphere. The first-order phase transition from orthorhombic-to-rhombohedral La 1؊x-Sr x FeO 3؊␦ (x ؍ 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase-transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO 3 and the observed volume contraction of the phase transition. Rhombohedral La 1؊x Sr x-FeO 3؊␦ (x ؍ 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second-order phase-transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3-SrFeO 3؊␦ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr-rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.
The present challenge with all-oxide thermoelectric modules is their poor durability at high temp... more The present challenge with all-oxide thermoelectric modules is their poor durability at high temperatures caused by the instability of the metal-oxide interfaces at the hot side. This work explains a new module concept based on a hybrid p−n junction, fabricated in one step by spark plasma co-sintering of Ca 3 Co 4−x O 9+δ (CCO, p-type) and CaMnO 3−δ /CaMn 2 O 4 (CMO, n-type). Different module (unicouple) designs were studied to obtain a thorough understanding of the role of the in situ formed hybrid p−n junction of Ca 3 CoMnO 6 (CCMO, p-type) and Co-oxide rich phases (ptype) at the p−n junction (>700°C) in the module performance. A time-enhanced performance of the modules attributed to this p−n junction formation was observed due to the unique electrical properties of the hybrid p−n junction being sufficiently conductive at high temperatures (>700°C) and nonconductive at moderate and low temperatures. The alteration of module design resulted in a variation of the power density from 12.4 (3.1) to 28.9 mW/cm 2 (7.2 mW) at ΔT ∼ 650°C after 2 days of isothermal hold (900°C hot side). This new concept provides a facile method for the fabrication of easily processable, cheap, and high-performance hightemperature modules.
Journal of the American Ceramic Society, Aug 21, 2006
Steady‐state compressive creep rate of La0.5Sr0.5Fe0.5Co0.5O3−δ (LSFC) and La0.5Sr0.5CoO3−δ (LSC)... more Steady‐state compressive creep rate of La0.5Sr0.5Fe0.5Co0.5O3−δ (LSFC) and La0.5Sr0.5CoO3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N2 compared with air. Relaxation by creep of chemical‐induced stresses in oxygen‐permeable membranes is addressed, especially at low partial pressure of oxygen.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
LAFEO3 AND LACOO3 BASED PEROVSKITES: PREPARATION AND PROPERTIES OF DENSE OXYGEN PERMEABLE MEMBRAN... more LAFEO3 AND LACOO3 BASED PEROVSKITES: PREPARATION AND PROPERTIES OF DENSE OXYGEN PERMEABLE MEMBRANES. Kjell Wiik, Anita Fossdal, Lise Sagdahl, Hilde L. Lein, Mohan Menon, Sonia Faaland, Ivar Waernhus, Nina Orlovskaya, Mari-Ann ...
An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical ... more An all-oxide thermoelectric generator for high-temperature operation depends on a low electrical resistance of the direct p-n junction. Ca 3 Co 4Àx O 9+d and CaMnO 3Àd exhibit p-type and n-type electronic conductivity, respectively, and the interface between these compounds is the material system investigated here. The effect of heat treatment (at 900 C for 10 h in air) on the phase and element distribution within this p-n junction was characterized using advanced transmission electron microscopy combined with X-ray diffraction. The heat treatment resulted in counter diffusion of Ca, Mn and Co cations across the junction, and subsequent formation of a Ca 3 Co 1+y Mn 1Ày O 6 interlayer, in addition to precipitation of Co-oxide, and accompanying diffusion and redistribution of Ca across the junction. The Co/Mn ratio in Ca 3 Co 1+y Mn 1Ày O 6 varies and is close to 1 (y ¼ 0) at the Ca 3 Co 1+y Mn 1Ày O 6-CaMnO 3Àd boundary. The existence of a wide homogeneity range of 0 # y # 1 for Ca 3 Co 1+y Mn 1Ày O 6 is corroborated with density functional theory (DFT) calculations showing a small negative mixing energy in the whole range.
Abstract Gd2Zr2O7 ceramic is considered as a potential candidate for thermal barrier coatings. He... more Abstract Gd2Zr2O7 ceramic is considered as a potential candidate for thermal barrier coatings. Here we present hot corrosion behavior of single layer Gd2Zr2O7 (GZO), Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ), YSZ and double-layer YSZ/Gd2Zr2O7 + 50 wt% YbSZ (YSZ + GZO/YbSZ) coatings prepared by air plasma spraying on Inconel 625 substrates. Prior to hot corrosion the fracture toughness of the coatings was assessed by Vickers indentation showing a variation between 0.6 and 2.3 MPa.m1/2 where the GZO/YbSZ coating exhibited the highest fracture toughness. The hot corrosion test was conducted by exposing the coatings to an equimolar mixture of Na2SO4 and V2O5 at 750 °C. After the hot corrosion test, the type of phases, chemical composition, microstructure and crack formation were investigated. Formation of GdVO4 and YVO4 was observed as the corrosion products in the GZO containing coatings and the YSZ coating, respectively. All coatings showed the formation of monoclinic zirconia after the corrosion test. The Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ) coating showed the best hot corrosion resistance due to reduced reactivity and enhanced fracture toughness and represents a new composition with properties promising for TBC applications.
Journal of The European Ceramic Society, Apr 1, 2019
Schemes, diagrams both SEM and TEM micrographs were created using Orig-inPro 9.1G, ImageJ, Diamon... more Schemes, diagrams both SEM and TEM micrographs were created using Orig-inPro 9.1G, ImageJ, Diamond and Digital Micrograph. Figures were arranged, merged and saved using PowerPoint 2010 and Photoshop CS5. Table S 1 shows the ionic radii of the substituted elements and inserted dopants. According to the similarity of the ionic radii of the elements used, doping should be possible. The XRD patterns of the Na x CoO 2 (NCO) and Bi 2 Ca 2 Co 2 O 9 (BCCO) phases, shown in Figure S 1 refer to Figure 2 in the main text. A step size of 0.003942, a time per step of 1.1 seconds, a voltage of 40 kV and a current of 40 mA were used in the XRD experiments. Pure NCO and nearly pure BCCO phases were subjected to SEM and elemental distribution analyses of polished cross-sections, as shown in Figure S 2a-d. Vibration-polished cross-section specimens were prepared by a multistep (30 µm, 15 µm, 6 µm, 3 µm and 1 µm diamond lapping films) polishing program using a Techprep from Allied-High Tech Products, Inc., followed by vibration polishing using a Buehler Vibromet-2 and a 50 nm colloidal alumina suspension. TEM specimens were prepared similar to SEM specimens and put on a TEM grid. The specimens were pinched out using a precision ion polishing system (Ar-ion) Model 691 from Gatan. The BCCO phase decomposes at approximately 1023 K, and Ca-containing phases are formed; see Figure S 1 [1]. Figure S 3 gives detailed elemental distribution information referring to Figure 4a-c in the main document. The interdiffusion of Ca into the NCO phase is clearer, and the very thin layers of NCO and BCCO are clearly visible in the Na, Ca and Bi signals shown in Figures S 3d-f and 4a-d. The insets of Figure 4d-f in the main document are enlarged for a better readability in Figure S 5a-f. Additional TEM micrographs of other sites of the CCO-30-35-10 nanocomposite ceramic are shown in Figures S 6, 7 and 8. These other sites clarify the composition and thickness of different layers within the material. These sequences continue throughout the ceramic. Comparing Figure S 6 and Figure S 7 shows that the amount of interdiffusion of Ca into the NCO phase is not constant, suggesting that the degree of interdiffusion might also depends on thickness and surroundings (e.g., being embedded between BCCO phases). Figure S 9 illustrates the measured heat capacities C P of CCO and nanocomposite ceramics as a function of temperature in the range from 313 K to 1173 K. The C P values reached approximately 0.83 J • g-1 • K-1 at 1073 K for a CCO-30-35-10 nanocomposite ceramic. The nanocomposite ceramics showed only small differences in C P from those of undoped CCO. The system was calibrated and the sapphire method was used. The sensitivity S and heat capacity C P were calculated as described by Jankovsky et al. [2]. Samples of bar geometry were
Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can se... more Ceramic membranes made from mixed oxygen-ionic and electronic conducting perovskite oxides can selectively separate oxygen from air at elevated temperatures. These membranes have several potential applications that require a continuous supply of oxygen. For example, they may be an alternative for cryogenic production of oxygen or alternative electrode materials in solid oxide fuel cells. Of particular significance is the partial oxidation of methane to syngas (CO + H2). By combining air separation and partial oxidation of natural gas into a single step, the need for expensive oxygen production by cryogenic means may be eliminated. Combined with existing processes for gas-to-liquid production such as Fisher-Tropsch and methanol synthesis, the MIEC membrane technology represents a very attractive route for conversion of natural gas to liquid fuels.The research in this field was initially concerned with the search for materials with the optimum oxygen flux. Today, the long term stability of the membranes is probably the main issue. The membranes have to be stable under operating conditions, which include mechanical stability and chemically compatibility with other materials like sealing and support materials. However, the current understanding of the long term chemical and mechanical reliability is poor and this is one of the major challenges for solid state ionic research. The aim of this work has been to investigate the mechanical properties and the chemical stability of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0, 0.5, 1) materials when they are exposed to thermal and chemical gradients.The chemically induced stresses due to reduction of the valence state of the transition metals are of particular importance with respect to the mechanical stability. In paper I, the oxygen non-stoichiometry, investigated by thermogravimetrical analysis, and thermal end chemical expansion, studied by dilatometry and high temperature X-ray diffraction, of La0.5Sr0.5Fe1-xCoxO3-δ materials are reported. The oxygen deficiency was observed to increase with decreasing partial pressure of oxygen and increasing temperature corresponding to expectations and previous reports. At ambient temperature the thermal expansion coefficient of the materials were in the range 15- 18·10-6 K-1. Above a certain temperature thermal reduction of the material take place, and the thermal expansion coefficient due to chemical expansion raise to 16-36·10-6 K-1. The chemical expansion ec, defined as the linear expansion due to a change in partial pressure of oxygen at constant temperature, reached a maximum in the range 0.036-0.039 for the materials studied at 800oC. The change in ionic radii of the transition metals is the main contribution to the chemical expansion. The crystal structure of the perovskite materials were shown to be slightly rhombohedral at ambient temperatures and a transition to cubic phase were observed above 300oC.This non-linear thermal expansion behavior is a major challenge for the applications of the mixed conductor materials. La0.5Sr0.5Fe1-xCoxO3-δ membranes in an oxygen partial pressure gradient will have different oxygen deficiency on either side of the membrane. The increasing oxygen deficiency is accompanied by a volume expansion as shown in paper I, and this will lead to chemically induced stresses. These stresses and the failure that might follow can be prevented by creep of the materials. Creep is also important due to dimensional stability. In paper II, the steady-state creep performance under compression of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 1) as a function of temperature, atmosphere, load and two different grain sizes is reported. The stress exponent found for the materials was close to unity and an unusual low inverse grain size exponent close to one was found for one of the materials. The activation energy of the two materials was not equal and the influence of secondary phases on the creep was discussed. The obtained creep behavior and microstructural investigation after measurements point to a diffusion related mechanism for the creep. Higher creep rates are found under reducing conditions and this suggest that creep relaxation of mechanical or chemical induced stresses may enhance the mechanical stability of oxygen permeable membranes.In Paper III, the mechanical properties of La0.5Sr0.5Fe1-xCoxO3-δ (x = 0.5, 0.75, 1) were investigated by several methods. Fracture strength was measured by four-point bending, fracture toughness was measured by SENB and SEVNB methods and finally Young’s modulus were investigated by four-point bending and resonant ultrasound spectroscopy. Four-point bending showed a non-linear ferroelastic behavior at ambient temperature due to rhombohedral crystal structure. Above the ferroelastic to paraelastic transition temperature the materials showed elastic behavior, however, at temperatures from about 800oC a non-elastic respond was observed due to creep. The measured fracture strength and fracture…
Doped Gd 2 Zr 2 O 7 materials have interesting properties as thermal barrier coatings (TBC) to re... more Doped Gd 2 Zr 2 O 7 materials have interesting properties as thermal barrier coatings (TBC) to replace the YSZ topcoats traditionally used. Here we investigate the thermomechanical properties and phase relations of Gd 2 Zr 2 O 7 (GZO) alloyed with 5 mol% Yb 2 O 3 stabilized ZrO 2 (YbSZ) in the composition range (Gd 2 Zr 2 O 7) 1−x (YbSZ) x , 0 ≤ x ≤ 0.98. With increasing YbSZ content, phase transformations from ordered to disordered pyrochlore to fluorite and tetragonal structures were observed. The thermal expansion coefficient (TEC) and Vickers hardness were correlated showing a maximum hardness (~ 11.5 GPa) and minimum TEC at x = 0.82. At 1000 °C, the TEC for the end members, x = 0 and 0.98, were 11.4 and 11.3 × 10-6 K −1 , respectively. The fracture toughness, K IC , showed an average value around 1.5 MPa m 0.5 for x ≤ 0.93 and increased significantly at x = 0.98 reaching 5.4 MPa m 0.5 due to the presence of a ferroelastic phase. For TBC applications, compounds with x = 0.98 show promise due to high TEC and high K IC .
Perovskites in the system La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ exhibit high stability at severe reduci... more Perovskites in the system La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ exhibit high stability at severe reducing conditions and high temperatures. Here we report the processing and oxygen transport properties of two compositions: La 0.2 Sr 0.8 Fe 1−x Ti x O 3−δ (x = 0.2, 0.4). Single-phase powders were synthesized by spray pyrolysis (SP) and solid state reaction (SSR). The resulting average particle sizes (BET) ranged from 140 nm (SP) to 1090 nm (SSR). Samples were prepared by CIP and pressureless sintered between 1200 and 1250 • C for 8-12 h. Sintered densities ranged from 97 (SSR) to 99% (SP) and grain size was less than 1.5 m. The average TEC varied in the range (13.8-14.6) × 10 −6 K −1 at 300-600 • C and (21.3-28.9) × 10 −6 K −1 at 600-800 • C. Surface exchange coefficients (k chem) and bulk diffusion coefficients (D chem) in terms of temperature and p O 2 were assessed by the method of electrical conductivity relaxation. A simple defect model was applied to explain electrical conductivity and bulk diffusion behaviour.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Dense nanocomposites of high electrical conductivity, thermoelectric power factor. • Highly align... more Dense nanocomposites of high electrical conductivity, thermoelectric power factor. • Highly aligned, semi-coherently grown 2D nanostructures built a 3D bulk material. • Co-doped In 2 O 3 of high electrical conductivity and thermoelectric power factor. • The Ioffe plot is suitable to evaluate applicability of materials power generation. • Designed power materials maximize power output of thermoelectric generators.
Journal of the American Ceramic Society, Dec 22, 2004
Formation of secondary phases and diffusion of cations in diffusion couples of yttria‐stabilized ... more Formation of secondary phases and diffusion of cations in diffusion couples of yttria‐stabilized zirconia and lanthanum manganite substituted with 0 to 60 mol% strontium have been studied by scanning electron microscopy and energy dispersive X‐ray spectroscopy. Only the primary phases were observed after 120 h at 1200°C, while formation of secondary phases was identified already after 1 h heat treatment at 1350°C. The phase composition of the reaction layer altered from La2Zr2O7 to SrZrO3 at increasing Sr content in LaxSr1‐xMnO3. The thickness of the reaction layer was increasing with heat treatment time. In diffusion couples of La0.4Sr0.6MnO3 formation of manganese oxide was observed in the perovskite layer after 1 h heat treatment at 1350°C, while isolated grains of SrZrO3 relatively deep inside the zirconia were observed after longer heat treatment time. Diffusion of Mn into zirconia was observed preferenced along grain boundaries in the early stage of the interface reaction.
Oxygen flux through La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5 and 1) membranes has been determi... more Oxygen flux through La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5 and 1) membranes has been determined as a function of oxygen partial pressure, temperature and time. The flux was diffusion controlled for low pO 2 gradients while larger pO 2 gradients caused a surface exchange ...
Cation tracer diffusion coefficients in La 1−x Sr x FeO 3−δ , (x = 0 and 0.1) at temperatures ran... more Cation tracer diffusion coefficients in La 1−x Sr x FeO 3−δ , (x = 0 and 0.1) at temperatures ranging from 1173 to 1373 K have been investigated by secondary ion mass spectrometry (SIMS) for both A-site and B-site diffusion applying Y 3+ and Cr 3+ as tracer ions. B-site impurity (represented by Cr) diffusion was found to be faster than A-site impurity (represented by Y) diffusion for LaFeO 3 in the whole temperature interval investigated and for La 0.9 Sr 0.1 FeO 3 at temperatures less than 1273 K. The obtained activation energies for bulk diffusion in La 0.9 Sr 0.1 FeO 3−δ were 168 ± 24 kJ/ mol for the Y tracer and 150 ± 30 kJ/mol for the Cr tracer, which are significantly lower than activation energies reported from diffusion couple experiments. The large variation in reported activation energies can be explained in terms of the difference in cation vacancy populations between the two experimental techniques. The diffusion coefficients were also clearly affected by a first order phase transition at approx. 1263 K in pure LaFeO 3. The obtained grain boundary diffusion coefficients were about five order of magnitude higher than the bulk diffusion coefficients, with an activation energy of 128 ± 28 kJ/mol.
Oxygen deficiency, thermal and chemical expansion of La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5,... more Oxygen deficiency, thermal and chemical expansion of La 0.5 Sr 0.5 Fe 1−x Co x O 3−δ (x = 0, 0.5, 1) have been measured by thermogravimetry, dilatometry and high temperature X-ray diffraction. The rhombohedral perovskite materials transformed to a cubic structure at 350 ± 50°C. The thermal expansion of the materials up to the onset of thermal reduction was 14-18 × 10 − 6 K − 1. Above 500°C in air (400°C in N 2), chemical expansion contributed to the thermal expansion and the linear thermal expansion coefficients were significantly higher, 16-35 × 10 − 6 K − 1. The chemical expansion, ε c , showed a maximum of 0.0045 for x = 0.5 and 0.0041 for x = 1 at 800-900°C. The normalized chemical expansion, ε c /Δδ, was 0.036 for x = 0.5 and 0.035 for x = 1 at 800°C. The chemical expansion can be correlated with an increasing ionic radius of the transition metals with decreasing valence state.
Journal of the American Ceramic Society, Jan 21, 2005
Crystal structure and thermal properties of La 1؊x Sr x FeO 3؊␦ (x ؍ 0, 0.1, 0.3, 0.4, 0.5, and... more Crystal structure and thermal properties of La 1؊x Sr x FeO 3؊␦ (x ؍ 0, 0.1, 0.3, 0.4, 0.5, and 0.75) have been studied by high-temperature X-ray diffraction and thermal analysis in air and nitrogen (p(O 2) Ϸ 10 ؊3 atm) atmosphere. The first-order phase transition from orthorhombic-to-rhombohedral La 1؊x-Sr x FeO 3؊␦ (x ؍ 0, 0.1) was strongly shifted to lower temperatures with increasing Sr content. The phase-transition temperature was observed significantly lower in polycrystalline ceramics compared with fine powders. The temperature depression of the phase transition in the ceramics was qualitatively explained by stresses induced both by the anisotropic thermal expansion of LaFeO 3 and the observed volume contraction of the phase transition. Rhombohedral La 1؊x Sr x-FeO 3؊␦ (x ؍ 0.3, 0.4, 0.5) were observed to transform to the cubic perovskite structure during heating. The second-order phase-transition temperature decreased with increasing Sr content and decreasing partial pressure of oxygen. On the basis of the present findings, a pseudobinary phase diagram of the LaFeO 3-SrFeO 3؊␦ system is presented. Finally, a severely nonlinear thermal expansion was observed for the Sr-rich materials at high temperature. The high thermal expansion in this region is due to a chemical expansion resulting from a reduction of the valence state of Fe.
The present challenge with all-oxide thermoelectric modules is their poor durability at high temp... more The present challenge with all-oxide thermoelectric modules is their poor durability at high temperatures caused by the instability of the metal-oxide interfaces at the hot side. This work explains a new module concept based on a hybrid p−n junction, fabricated in one step by spark plasma co-sintering of Ca 3 Co 4−x O 9+δ (CCO, p-type) and CaMnO 3−δ /CaMn 2 O 4 (CMO, n-type). Different module (unicouple) designs were studied to obtain a thorough understanding of the role of the in situ formed hybrid p−n junction of Ca 3 CoMnO 6 (CCMO, p-type) and Co-oxide rich phases (ptype) at the p−n junction (>700°C) in the module performance. A time-enhanced performance of the modules attributed to this p−n junction formation was observed due to the unique electrical properties of the hybrid p−n junction being sufficiently conductive at high temperatures (>700°C) and nonconductive at moderate and low temperatures. The alteration of module design resulted in a variation of the power density from 12.4 (3.1) to 28.9 mW/cm 2 (7.2 mW) at ΔT ∼ 650°C after 2 days of isothermal hold (900°C hot side). This new concept provides a facile method for the fabrication of easily processable, cheap, and high-performance hightemperature modules.
Journal of the American Ceramic Society, Aug 21, 2006
Steady‐state compressive creep rate of La0.5Sr0.5Fe0.5Co0.5O3−δ (LSFC) and La0.5Sr0.5CoO3−δ (LSC)... more Steady‐state compressive creep rate of La0.5Sr0.5Fe0.5Co0.5O3−δ (LSFC) and La0.5Sr0.5CoO3−δ (LSC) is reported in the temperature region 900°–1050°C and stress range 5–28 MPa. The stress exponents for the two materials were 1.71±0.18 and 1.24±0.15, respectively. The activation energy for creep was considerably higher for LSC (619±56 kJ/mol) than for LSFC (392±28 kJ/mol). The grain size exponent for LSC was 1.28±0.14. Considerably higher creep rates were observed for both materials in N2 compared with air. Relaxation by creep of chemical‐induced stresses in oxygen‐permeable membranes is addressed, especially at low partial pressure of oxygen.
Users may download and print one copy of any publication from the public portal for the purpose... more Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
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