Lsc24

% Theoretical Density

86.5±4.5

94.2±1.6

95.7+1.9

97.2±0.5

93.1±1.5

91.6±0.9

91.2±0.7

tively. The room temperature strength of the LCC samples increased with increasing calcium content, which is the result of higher densities and smaller pore sizes. The strength of the LSC samples showed no correlation to the strontium content as samples had similar densities. At elevated temperatures, the strength of the LCC samples decreased with increasing calcium content. Under these conditions, grain boundary-sliding or viscous flow of the grain boundary phases can extend existing flaws leading to decreased strength. [8] The strength of the LSC samples remained unchanged over the entire temperature range. The insensitivity of the porous materials to strength degradation (with temperature) is related to fracture initiated from pores. The pores act as initial flaw sites, and failure occurs via extension of-these flaws along the grain boundary. As the temperature increases, the grain boundary surface energy increases, resulting in an increase in the energy necessary to extend the pores along the grain boundary. The resulting increased resistance to pore-crack extension results in constant strength (LSC samples) or even increased strength (LCC-15). This behavior has been observed in UOj where fracture occurs by the extension of large pores along the grain boundaries. [8]

Figures 3 and 4 are plots of the room temperature strength as a function of P(02) for the treated LCC and LSC samples, respectively. The results indicate both acceptor content and type affect the mechanical perfomance of the material under the reducing conditions. LCC-25 and LCC-30 maintained a constant strength from ambient conditions to a P(C>2) of 10"10 atm. As the atmosphere became more reducing, both materials demonstrated a drastic decrease in strength with LCC-30, retaining only 10% of its original strength after exposure to a P(C>2) of 10"16 atm. LCC-20 showed similar behavior except the transition P(C>2) was lower (10"12 atm) and the material retained 55% of its original strength after exposure to the most reducing condition (10"18 atm). The data illustrates that samples with higher acceptor content show a decrease in strength at lower oxygen partial pressures and a greater loss of strength at more reducing conditions. This trend is consistent with the reported isothermal expansion behavior of LCC materials under high temperature reducing conditions. [2-4] Samples exhibited no expansion from ambient air to moderately reducing conditions with the onset of expansion occurring at a higher PiOj) for compositions with higher acceptor content. Similarly, the magnitude of expansion for a given loss in lattice oxygen was greater in samples with a higher acceptor content. Examining the model [2-4] used to interpret the isothermal expansion behavior can provide insight into the strength behavior. As the lattice loses oxygen and Cr4* ions are reduced to Cr3+, bonds are left unsatisfied and bond lengths increase from the repulsive forces of unshielded neighboring chromium ions. This expansion of the lattice has been documented via x-ray analysis. Armstrong et al. [4] measured an increase of 1.3% in the LCC-30 unit cell after reduction in a P(Oz) of 10"'8 atm at 1000°C. The forces required to rupture the bonds in this lattice, the cohesive strength, should be measurably less than that of a stoichiometric chromite lattice. Examination of fracture surfaces revealed that a transition in fracture mechanism, from intergranular to transgranular fracture, accompanied the decrease in strength. Fracture surfaces from the LCC samples exhibited small amounts of transgranular fracture just prior to the onset of strength degradation. The amount of transgranular fracture increased as the conditions became more reducing and corresponded to the decrease in strength., e.g. LCC-30 showed complete transgranular fracture at 10"16 atm. This supports the premise that the combination of oxygen vacancies and longer bond lengths resulted in a weaker lattice; reduced grains (crystallites) are substantially weaker than the unreduced grains and the grain boundaries.

The room temperature strength of the treated LSC samples remained a constant from ambient air to a P(02) of 10 atm. As the treatments became more reducing, however, an increase in strength was observed for all LSC samples, reaching a maximum at 10"14 atm. All samples demonstrated inter-granular fracture from ambient air to a P(02) of 10"12 atm. The increase in strength with decreasing P(C>2) can be attributed to an increase in strength of the grain boundary phase or a blunting of pore-grain boundary interfaces, thus limiting the extension of pores near critical size or inhibiting the formation of a critical flaw from several subcritical pores. [8] Similar to the trends observed in the LCC compositions, LSC samples with the highest acceptor content exhibited the greatest reduction in strength as the conditions became more reducing, e.g. LSC-24 retained only 40% of its unreduced strength (-30% of the maximum strength). In contrast, LSC-15 maintained a constant strength from 10"10 to 10"18 atm at 170% of the unreduced strength. The increase in strength from 10"8 to 10'14 atm was fairly consistent for all LSC compositions. The data shows that at more reducing conditions, samples with higher acceptor content experience a greater reduction in strength. This trend is consistent with the reported isothermal expansion under high temperature reducing conditions [4]. Relative to the LCC materials, LSC materials with similar acceptor contents display slightly smaller isothermal linear expansion at a given P(C>2) or oxygen deficiency. This corresponds to a smaller lattice expansion at a given oxygen deficiency, i.e. the measured change in the x-ray unit cell volume for LSC-30 after reduction in a P(02) of 10"18 atm at 1000°C is

0.2., almost 1/6 of the LCC-30 expansion. [6] This is believed to be due the larger unit cell of LSC materials, which can accommodate oxygen deficiency with less bond lengthening. The cohesive energy in oxygen deficient LSC should remain higher at higher oxygen deficiencies compared to LCC. Fracture surfaces from the LSC samples indicated an onset P(C>2) of 10"14 atm for transgran-ular fracture. An increasing amount of transgranular fracture accompanied the decrease in strength from 10"14 to 10"18 atm for the LSC-20 and LSC-24 samples, with approximately 60% transgranular fracture for the LSC-24 sample after the 10"i8 atm treatment. These results are evidence that the loss of lattice oxygen has weakened the lattice but also that the Si2+ ion stabilizes the chromite lattice more than Ca .

Conclusions

The strength of the LCC samples decreased with increasing temperature and increased acceptor content. The reduction in strength at elevated temperatures is believed to be caused by a grain boundary-sliding mechanism. The LSC samples showed a constant strength with temperature as failure occurred by extension of the pores along the grain boundaries. The room temperature strength of the reduced LCC and LSC samples was dependent on the P(C>2). Samples with the highest acceptor content showed a precipitous drop in strength after exposure to the most reducing conditions. The loss of lattice oxygen and the reduction of Cr4* to Cr3+ resulted in a reduced bond strength in the lattice and a larger bond length. This resulted in a decrease in the cohesive strength of the lattice, and the corresponding decrease in fracture strength as the individual crystals lost mechanical integrity. This was supported by changes in the microstructure (transition from inter-granular to transgranular fracture) as the treatments became more reducing.

References

1. S. Srilomsak, D. P. Schilling, and H. U. Anderson, in Proceedings of the 1st Int. Symposium on Solid Oxide Fuel Cells, Hollywood, FL1989, ed. S. C. Singhal, (The Electrochemical Society, Pennington, NJ, 1989) p. 129.

2. T. R. Armstrong, J. W. Stevenson, P. E. Raney, and L. R. Pederson, in 1994 Fuel Cell Seminar Abstracts, San Diego, CA, (Courtesy Associates 1994) p. 105.

3. T. R. Armstrong, J. W. Stevenson, L. R. Pederson, and P. E. Raney, in Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells, Yokohama Japan 1995 ed. S. Sin-ghal, (The Electrochemical Society, Pennington, NJ 1995) p. 944

4. T. R. Armstrong, J. W. Stevenson, R. E. Raney and L. R. Pederson, "Dimensional Stability of Doped Lanthanum Chromite in Reducing Environments," submitted J. Electrochem. Soc. 1996.

5. C. Milliken, S. Elangovan and A. Khandkar, in Proceedings of the Third International Symposium on Solid Oxide Fuel Cells, 1993, Edited by S.C. Singhal and H. Iwahara, pp. 335-343.

6. S.W Paulik and T.R. Armstrong, "Mechanical Properties of Acceptor Substituted Lanthanum Chromites," submitted J. Mat. Sci. 1996.

7. G.D. Quinn, F.I. Baratta, and J. A. Conway, "Commentary on U.S. Army Standard Test Method for Flexural Strength of High Performance Ceramics at Ambient Temperature," Army Materials and Mechanics Research Center Technical Report 85-21, Aug. 1985.

8. R. W. Davidge, Mechanical Behavior of Ceramics, pp. 93-103, Cambridge University Press, Cambridge England, 1979.

Figure 1. Strength of the LCC compositions as a function of temperature.

Temperature (°C)

Figure 1. Strength of the LCC compositions as a function of temperature.

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