Orest Ostash

Influence of Treatment Temperature on Microstructure and Properties of YSZ–NiO Anode Materials

The cyclic treatment technique (redox cycling) comprising stages of material exposition in reducing and oxidizing high-temperature environments and intermediate degassing between these stages has been developed to improve the structural integrity of YSZ–NiO ceramic anode substrates for solid oxide fuel cells. A series of specimens were singly reduced in a hydrogenous environment (the Ar–5xa0vol% Н2 mixture or hydrogen of 99.99xa0vol% H2 purity) under the pressure of 0.15xa0MPa or subjected to redox cycling at 600 or 800xa0°C. The influence of redox cycling at the treatment temperatures of 600 and 800xa0°C on the structure, strength and electrical conductivity of the material has been analysed. Using the treatment temperature 600xa0°C, a structure providing improved physical and mechanical properties of the material was formed. However, at the treatment temperature 800xa0°C, an anode structure with an array of microcracks was formed that significantly reduced the strength and electrical conductivity of the material.

Thermal Stability and Mechanical Characteristics of Densified Ti3AlC2-Based Material

The mechanical properties and temperature stability in air and hydrogen of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) have been investigated. At room temperature the samplesexhibited microhardness HV = 4.6 GPa (at 5 N), hardness HV50 = 630 MPa (at 50 N ) and HRA=70 (at 600 N), Young modulus was 140 ± 29 GPa, fracture toughness K1C=10.2 MPa·m0.5compression strength 700 MPa and bending strength 500 MPa. After 1000 hours of exposition at 600 °C the oxide film (containing mainly Al2O3 and TiO2) formed on the surface and material demonstrated a higher oxidation resistance than chromium ferrite steels. Due to the surface oxidation the defects self-healing took place and the bending strength of the porous Ti3AlC2 (22% porosity) after exposition for 3 h at 600 oC in air slightly (for 3%) increased as compared to that at 20 oC. Besides, the porous Ti3AlC2 material resisted to high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 %.

Study of the Thermal Stability and Mechanical Characteristics of MAX Phases of Ti-Al-C(N) System and their Solid Solutions

The DTA and TG study in air of Ti2Al (C1-xNx) and Ti3AlC2 synthesized under Ar 0.1 MPa pressure and densified in thermobaric conditions at 2 GPa, 1400 °C, for 1 h showed that the increase of the amount of TiC layers in Ti-Al-C MAX phases structures leads to the increase of their stability against oxidation: 321 MAX phase Ti3AlC2 are more stable than Ti2AlC and Ti2Al (C1-xNx) solid solutions both before and after thermobaric treatment. The oxide film formed on the surface of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) made the material highly resistant in air at high temperatures: after 1000 hours of exposition at 600 °C it demonstrated a higher resistance to oxidation than chromium ferrite steels (Crofer GPU and JDA types). Due to the surface oxidation self-healing of defects took place. Besides, the Ti3AlC2 material demonstrated resistance against high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 % only. At room temperature the Ti3AlC2 bulk exhibited microhardness Hμ = 4.6 GPa (at 5 N), hardness HV50 = 630 (at 50 N ) and HRA = 70 (at 600 N), Young modulus was 140 ± 29 GPa, bending strength =500 MPa, compression strength 700 MPa, and fracture toughness K1C=10.2 MPa·m0.5.

Effect of Corrosion Environment on the Fatigue Behavior of WC — Co Hard Alloy Teeth of Drill Bits

The fatigue crack growth in WC-Co hard alloys with cobalt content from 6 to 12 wt pct has been investigated in air and fluids of pH from 4.5 to 10.5. Significant difference in crack growth rates at low the stress intensity factor range was detected. It was established that the negative influence of corrosion environment on fatigue crack growth resistance of WC-Co hard alloy increases significantly for compositions with higher cobalt content. This effect is observed in fluids of various pH-values. Lower operating life of hard alloy inserts of drill bits in service environment corresponds to hydrogen embrittlement of cobalt phase.

Preconditioning of the YSZ-NiO Fuel Cell Anode in Hydrogenous Atmospheres Containing Water Vapor

The YSZ–NiO ceramics for solid oxide fuel cells (SOFCs) anode have been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (Ar–5xa0vol%xa0H2 mixture) at 600xa0°C under the pressure of 0.15xa0MPa or subjected to ‘reduction in the mixture–oxidation in air’ (redox) cycling at 600xa0°C. The YSZ–Ni cermets formed in both treatment conditions were then aged in ‘water vapor in Ar–5xa0vol%xa0H2 mixture’ atmosphere at 600xa0°C under the pressure of 0.15xa0MPa. Additionally, the behaviour of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar–5xa0vol%xa0H2 mixture (water vapor pressure below 0.03xa0MPa) does not affect the reduction of the nickel phase in the YSZ–NiO ceramics, but causes some changes in the YSZ–Ni cermet structure. In particular, nanopore growth in tiny Ni particles takes place. At higher concentration of water vapor in the mixture (water vapor pressure above 0.03–0.05xa0MPa), converse changes in the kinetics of reduction occur. The best physical and mechanical properties were revealed for the material treated by redox cycling after holding at 600xa0°C in water depleted gas mixture. The dual effect of water vapor on nickel-zirconia anode behaviour is discussed basing on scanning electron microscopy analysis data, material electrical conductivity, and strength.

Effect of the Additive of Y 2 O 3 on the Structure Formation and Properties of Composite Materials Based on AlN–SiC

The results of studies on the strength at bending and volumetric electrical resistance of composite materials based on AlN–SiC with additions from 2 to 6 wt % Y2O3. It is shown that at increasing the content of Y2O3 in the mixture from 2 to 6 wt % the compaction of the composites intensifies their electrical resistance from (1.4–5.4) × 106 to (1.8–5.94) × 107 Ohm·cm (at 20°C), which at the increasing temperature decreases exponentially and at 800°C for all composites is (5–6) × 104 Ohm·cm. It was determined that materials with the smaller content of Y2O3 have somewhat higher value of the ultimate strength during bending, namely, 110 MPa.

Presence of Oxygen in Ti-Al-C MAX Phases-Based Materials and their Stability in Oxidizing Environment at Elevated Temperatures

Presence of Oxygen in Ti–Al–C MAX Phases-Based Materials and their Stability in Oxidizing Environment at Elevated Temperatures T. Prikhnaa,∗, O. Ostash, V. Sverdun, M. Karpets, T. Zimych, A. Ivasyshin, T. Cabioc’h, P. Chartier, S. Dub, L. Javorska, V. Podgurska, P. Figel, J. Cyboroń, V. Moshchil, V. Kovylaev, S. Ponomaryov, V. Romaka , T. Serbenyuk and A. Starostina Institute for Superhard Materials of the National Academy of Sciences of Ukraine, 2 Avtozavodskaya Str., Kiev, 04074, Ukraine Karpenko Physical-Mechanical Institute of the National Academy of Sciences of Ukraine, 5, Naukova Str. Lviv, 79060, Ukraine Université de Poitiers, CNRS/Laboratoire PHYMAT, UMR 6630 CNRS Université de Poitiers SP2MI, BP 30179, F-86962 Chasseneuil Futuroscope Cedex, France The Institute of Advanced Manufacturing Technology, Wroclawska 37A, 30-011 Krakow, Poland EDL “Proton 21”, 48a, Chernovola Str., Kiev’s region 08132, Vishnevoe, Ukraine Lviv Polytechnic National University, 12 Bandera Str., Lviv, 79013, Ukraine

Effect of a Hydrogen Sulfide-Containing Atmosphere on the Physical and Mechanical Properties of Solid Oxide Fuel Cell Materials

The effect of hydrogen sulfide content in a hydrogenous atmosphere on the structure, physical, and mechanical properties of solid oxide fuel cell (SOFC) anode materials has been studied. A series of specimens of porous nickel and YSZ–Ni cermet have been investigated. In order to obtain the corresponding YSZ–Ni cermet structure, specimens of the YSZ–NiO ceramics were singly reduced in a hydrogenous atmosphere (either Ar–5 vol% H2 mixture or hydrogen of 99.99 vol% H2 purity) for 4 h at 600 °C under the pressure of 0.15 MPa. A part of the specimens of each series was then aged in “hydrogen sulfide in Ar–5 vol% H2 mixture” atmosphere for 4 h at 600 °C. According to a test mode, the atmosphere contained 7 or 18 vol% Н2S. Material microstructure and fracture surface morphology of the specimens as well as the physical and mechanical behaviors were investigated. It was revealed that the atmosphere containing up to 7 vol% Н2S does not affect the strength and electrical conductivity of the YSZ–Ni cermet. Increased content of Н2S (18 vol%) causes some changes in the YSZ–Ni cermet structure. A large number of completely reduced tiny nickel particles are formed. These nickel particles react with hydrogen sulfide. Sulfur is segregated on the boundaries between the zirconia and nickel phases and pores. Finally, multiple breaking of the zirconia–nickel bonds occurs that results in reduced strength of the cermet (by 39% as compared to as-received ceramics).

Effect of Water Vapor Amount in Hydrogenous Atmospheres on Reducing Ability of the YSZ–NiO Fuel Cell Anode Material

Anode ceramics of YSZ–NiO system for solid oxide fuel cells (SOFCs) has been investigated. A series of specimens were singly reduced in a hydrogenous atmosphere (the Ar–5 vol%Н2 mixture) at 600°С under the pressure of 0.15 MPa or subjected to “reduction in the mixture–oxidation in air” (redox) cycling at 600°С. After both the treatment conditions, corresponding structures of YSZ–Ni cermets were formed and then subjected to dwell in “water vapor–Ar–5 vol% Н2 mixture” atmosphere at 600°С under the pressure of 0.15 MPa. Additionally, behavior of the as-received material in this atmosphere was studied. It was revealed that small amount of water vapor in Ar–5 vol% Н2 mixture (water vapor pressure below 0.03 MPa) does not affect the reduction of a nickel phase in YSZ–NiO ceramics but causes some changes in the YSZ–Ni cermet structure, in particular, growth of nanopores on tiny Ni particles. A higher concentration of water vapor in the mixture (water vapor pressure above 0.03–0.05 MPa) causes a converse change in the reduction kinetics. For as-received material, such amount of water vapor in the mixture is an obstacle for its reduction and causes reoxidation of a nickel phase at 600°С. For the material treated by redox cycling, better physical and mechanical properties were revealed after dwelling at 600°С in a water-depleted gas mixture. Contrary to this, after dwelling at 600°С in a water-enriched gas mixture, the material showed lower resistance against reoxidation. Based on the scanning electron microscopy and the data on the conductivity and strength, the dual effect of water vapor on durability of a nickel–zirconia anode is discussed.

Oxidation Resistance of Materials Based on Ti3AlC2 Nanolaminate at 600 °C in Air.

The oxidation behavior of Ti3AlC2-based materials had been investigated at 600xa0°C in static air for 1000xa0h. It was shown that the intense increase of weight gain per unit surface area for sintered material with porosity of 22xa0% attributed to oxidation of the outer surface of the specimen and surfaces of pores in the bulk material. The oxidation kinetics of the hot-pressed Ti3AlC2-based material with 1xa0% porosity remarkably increased for the first 15xa0h and then slowly decreased. The weight gain per unit surface area for this material was 1.0xa0mg/cm2 after exposition for 1000xa0h. The intense initial oxidation of Ti3AlC2-based materials can be eliminated by pre-oxidation treatment at 1200xa0°C in air for 2xa0h. As a result, the weight gain per unit surface area for the pre-oxidized material did not exceed 0.11xa0mg/cm2 after 1000xa0h of exposition at 600xa0°C in air. It was demonstrated that the oxidation resistance of Ti3AlC2-based materials can be significantly improved by niobium addition.