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 ...
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.
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…
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 ...
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.
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 ...
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.
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…
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 ...
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.
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