| Sustainable energy has attracted much attention both in scientific research communities and industries alike, with an international competition to clean fossil energy, hydrogen, renewable energy, as well as water reuse and recycling. Electricity is the most convenient form of energy today. With increasing energy demand, depleting fossil fuel reserves, and growing concern about the climate and environment, there is an urgent need to increase electricity generation efficiency and to develop renewable energy sources. Fuel cell is an energy conversion device, which converts directly the chemical energy of fuels such as hydrogen, methanol, ethanol, natural gas, and hydrocarbons to electricity. Therefore, fuel cells inherently have a significantly higher efficiency than conventional energy conversion technologies, and solid oxide fuel cells(SOFCs) are considered to be the most efficient.The primary function of the SOFC interconnector is to connect the anode of one cell to the cathode of the next cell in electrical series. The interconnector also separates the fuel from the oxidant in adjoining cells of a stack. Thus, the interconnect material must be stable in both the reducing and oxidizing environment, impermeable to gases, and sufficiently conductive to support electron flow at the operating conditions.Currently, La Cr O3 is the most common interconnect material for SOFCs. La Cr O3 is a perovskite oxide(ABO3) with highly refractory properties. The substitution of lanthanum and chromium in La Cr O3 with another cation significantly influences the electrical properties and the thermal expansion properties. In the paper, the SOFC interconnect materials La0.75Sr0.25Cr1-xO3-δ(x=0, 0.01, 0.02, 0.03, 0.04) were prepared using an auto-ignition process. The influences of Cr deficiency on their sintering, thermal expansion and electrical properties were investigated. All the samples were pure perovskite phase after sintering at 1400℃ for 4 h. The cell volume of La0.75Sr0.25Cr1-xO3-δ decreased with increasing Cr deficient content. The relative density of the sintered bulk samples increased from 93.2%(x=0) to a maximum value of 94.7%(x=0.02) and then decreased to 87.7%(x=0.04). The thermal expansion coefficients of the sintered bulk samples were in the range of 10.60~10.98×10-6 K-1(30~1000℃), which are compatible with that of YSZ. Among the investigated samples, the sample with 0.02 Cr deficiency had a maximum conductivity of 40.4 Scm-1 and the lowest Seebeck coefficient of 154.806 μVK-1 at 850℃ in pure He. The experimental results indicate that La0.75Sr0.25Cr0.98O3-δ has the best properties and is much suitable for SOFC interconnect material application.Further, dense La0.75Sr0.25Cr0.98O3-δ ceramics were prepared using the spark-plasma sintering(SPS) method. The SPS process on their sintering, thermal expansion and electrical properties were investigated. The sintering of the powders was conducted using a spark plasma apparatus over the temperature range 1300~1500℃ for 5 min with a constant loading pressure of 45 MPa. The starting powder was densified to 98.5% of the theoretical density with much shorter sintering period(5min) at 1500℃. The maximum electrical conductivity was 15.3 S/cm at 850℃ after sintered at 1500℃ for 5min, which is lower than that of the bulk La0.75Sr0.25Cr0.98O3-δ prepared by pressureless sintering. The fracture toughness of the bulk La0.75Sr0.25Cr0.98O3-δ prepared by SPS were 1.71±0.12 MPa·m1/2(1300℃), 1.66±0.12 MPa·m1/2(1400℃), 1.43±0.11 MPa·m1/2(1500℃), respectively, which are higher than that of the bulk prepared by pressureless sintering. The thermal expansion coefficients of the sintered bulk samples decreased from 10.50×10-6 K-1(1300℃) to 9.8×10-6 K-1(1400℃) and then decreased to 9.4×10-6 K-1(1500℃). Among the investigated samples, the experimental results indicate that La0.75Sr0.25Cr0.98O3-δ by pressureless sintering has the best properties and is much suitable for SOFC interconnect material application. |
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