Oxidative coupling of methane in a mixed-conducting perovskite membrane reactor

Ionic-electronic mixed-conducting perovskite-type oxide La0.6Sr0.4Co0.8Fe0.2O3 was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the pO2-range of 10¿3¿1 bar at 1073¿1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m¿2 s¿1 at 1173 K with a corresponding activation energy of 130¿140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1¿3% at 1073¿1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30¿35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized

Oxygen permeation through oxygen ion oxide-noble metal dual phase composites

Oxygen permeation behaviour of three composites, yttria-stabilized zirconia-palladium, erbia-stabilized bismuth oxidenoble metal (silver, gold) was studied. Oxygen permeation measurements were performed under controlled oxygen pressure gradients at elevated temperatures. Air was supplied at one side of a dense sintered disk specimen, while helium was fed at the opposite side to sweep away the permeated oxygen. This research has demonstrated that in addition to the presence of percolative metal phase in the oxide matrix, a large ionic conductivity of the oxide phase and a high catalytic activity of the metal phase towards surface oxygen exchange are required for the dual phase composite to possess high oxygen permeability. The bismuth oxide-silver composite fulfils these requirements, hence showing the best oxygen permeability

Sinter forging of zirconia toughened alumina

Sinter forging experiments have been carried out on powder compacts of zirconia toughened alumina (ZTA) Ceramics Alumina-15 wt% zirconia was prepared by a gel precipitation method and calcined at temperatures of 900 or 1100°C. Full densification of ZTA ceramics was obtained within 15 min at 1400°C and 40 MPa. A homogeneous microstructure can be observed with an alumina grain size of 0.7 mgrm and a zirconia grain size of 0.2 mgrm. Almost no textural evolution occurred in the microstructure. During sinter forging the densification behaviour of the compacts was improved by an effective shear strain, for which values of more than 100% could be obtained. As a result of the shear deformation the densification of ZTA in the agr alumina phase stage shifted to lower temperature. During pressureless sintering the gamma to agr alumina transformation temperature was dependent of the preceding calcination temperature, while during sinter forging this phase transformation was independent of calcination temperature and took place at a lower temperature

Improvement of mechanical properties of zirconia-toughened alumina by sinter forging

ZTA powder with a composition of 85 wt% alumina/15 wt% zirconia was prepared by a gel precipitation method. Sinter forging was performed with this powder to enhance the mechanical properties of ZTA materials. The influence of processing flaws on mechanical properties of sinter forged materials and pressureless sintered materials was investigated. Sinter forging at 40 MPa effectively decreases process flaw sizes resulting in a homogeneous microstructure and improves the grain boundary structure because of large shear applied in this process. Sinter forging resulted in an increase in strength and toughness by a factor of 1.5–2 when compared with pressureless sintered compacts. The fracture energy is enhanced by a factor of two. The predominate mechanism for improvement of mechanical properties of these sinter-forged ZTA materials is grain boundary strengthening

Electrochemical characterisation of 3Y-TPZ-Fe2O3 composites

The influence of the addition of ferric oxide to 3Y-TZP on the conductivity and microstructure of sintered Y-stabilised tetragonal zirconia ceramics (3Y-TZP) was investigated. A comparison was made between two different dense 3Y-TZP¿¿-Fe2O3 composites. Compacts were made by pressureless sintering at 1150 °C or by sinterforging at 1000 °C and 100 MPa. The sinterforging process resulted in smaller zirconia and hematite grains and a higher monoclinic zirconia content as compared to the compact that was sintered pressureless. The high monoclinic content led to loss of ionic conductivity. The addition of ferric oxide caused electronic conductivity. The sinterforging resulted in a high concentration of metastable defects in the zirconia¿hematite composite, leading to a relatively high electronic conductivity. Heating above 380 °C caused irreversible loss of these defects and a large decrease in electronic conductivity

Effects of a second phase on the tribological properties of Al2O3 and ZrO2 ceramics

The tribological properties of four different materials are investigated, tetragonal zirconia (Y-ZTP), Al2O3 dispersed in Y-TZP (ADZ), ZrO2 dispersed in Al2O3 (ZTA) and Al2O3 (with 300 ppm MgO). These materials are used as a cylinder sliding against a plate of Y-TZP (TZ-3Y)). Compared to Y-TZP, the wear resistance of ADZ composites is increased by a factor of 4¿10. At a contact pressure of 230 MPa, a wear transition for Y-TZP is observed from plastic deformation to microchipping and microfracture due to the high interfacial temperature (450°C¿550°C) generated by frictional heating. Because of the higher elastic modulus, hardness and fracture toughness at high temperature, ADZ composites show better wear resistance and a higher transition contact pressure (over 400 MPa) under the present conditions. For Al2O3, the transition from mild to severe wear occurs when the contact pressure is changed from 250 to 400 MPa. For ZTA ceramics, the wear behaviour does not change because of the presence of a compressive layer due to the zirconia phase transformation during sliding.\ud \ud In water the wear resistance for ADZ and ZY5 is almost two orders of magnitude higher than the results under dry conditions. Reduction of the interfacial temperature by using water and the formation of a hydroxide layer at the contact surface by the tribochemical reaction of water with the ceramic, as observed by XPS, gives a positive effect on wear resistance

Plasticity of nanocrystalline zirconia ceramics and composites

The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real superplastic deformation (strain rate > 10−4 s−1) is observed in these materials at relative low temperature (1100–1200 °C). Grain-boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation.\ud \ud Uniaxial pressure-assisted sintering techniques (=sinter-forging) provide the opportunity of large shear strains during densification. Sinter-forging experiments on zirconia-toughened alumina (15 wt% ZrO2/85 wt% Al2O3) resulted in a dense composite within 15 min at 1400 °C and 40 MPa, with effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. These improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at the grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. K1C values of 10 MPa√m are obtained for Y-TZP, while no classical stress-induced phase transformation toughening is observed. Sinter-forged ZTA samples showed a better wear resistance than free sintered ones.\u

Production of defect-poor nanostructured ceramics of yttria-zirconia

For the production of nanostructured ceramics of yttria-zirconia four powders differing in agglomerate strength, agglomerate size and crystallite size are compared. An ultra-fine-grained ceramic with a final density of 98% and a grain size of 0.18 μm could be produced from a hydrothermally crystallized ethanol-washed powder. The remaining porosity is caused by some residual defects which are present due to the irregular shape of the agglomerates and which cause improper die filling. A commercially available powder was also investigated. This powder consists of homogeneous porous, spherical, weak agglomerates. The resulting ceramic has a high density (≥ 99%) but cannot be obtained with ultra-fine grain size (minimum grain size is 0.3 μm). The air-crystallized ethanol-washed powder resulted, after sintering, in larger porosities. In this case the powder consists of weak and some strong agglomerates and a few defect clusters are found in the sintered ceramic which limit the maximum attainable density to 92%. The air-crystallized water-washed powder consists of agglomerates which are too strong to be fractured during compaction. The sintered ceramics contain a large amount of porosity (20%) which is attributed to the presence of inter-agglomerate pores.\u