Mechanical Properties Of Lanthanum And Yttrium Chromites

S.W. Paulik and T.R. Armstrong Pacific Northwest National Laboratory* P. O. Box 999 Richland, WA 99352

Introduction

In an operating high-temperature (1000°C) solid oxide fuel cell (SOFC), the interconnect separates the fuel (P(02)=10"16 atm) and the oxidant (P(02)=10°-2 atm), while being electrically conductive and connecting the cells in series. Such severe atmospheric and thermal demands greatly reduce the number of viable candidate materials. Only two materials, acceptor substituted lanthanum chromite and yttrium chromite, meet these severe requirements. In acceptor substituted chromites (Sr2+ or Ca2+ for La3+), charge compensation is primarily electronic in oxidizing conditions (through the formation of Cr4*). Under reducing conditions, ionic charge compensation becomes significant as the lattice becomes oxygen deficient. The formation of oxygen vacancies is accompanied by the reduction of Cr4* ions to Cr3* and a resultant lattice expansion [1-4], The lattice expansion observed in large chemical potential gradients is not desirable and has been found to result in greatly reduced mechanical strength [5,6].

Few investigations of the mechanical properties of lanthanum chromite have evaluated the strength in both air and after reduction. Of these investigations, the results are not consistent and comparison of individual results are difficult due to inconsistent sample size, test method and test conditions. Reliable measurements of Young's modulus, Poisson's ratio, and mechanical strength are needed in air and reducing environments so that accurate models can be developed to predict the possible stresses the interconnect may experience in an operating SOFC. Therefore, it is the purpose of this program to measure the mechanical properties of acceptor substituted lanthanum and yttrium chromite to determine the mechanical properties as a function of material chemistry, as well as temperature and atmosphere.

Experimental

La|.xCaxCrOj and Lai.xSrxCr03 (x varying between 0.15 and 0.30) powders were synthesized using the glycine nitrate process. Following a 1 hr calcination at 1000°C in air, powders were isos-tatically pressed into 34 by 34 by 64 mm billets and sintered between 1600 and 1690 °C for 2 to 6 hr. Samples were machined into 3 by 4 by 45 mm military standard 1942B [7] bars. Four-point bend strength was measured (Instron model 1125) in air at 25,600,800, and 1000°C using a cross-head speed of 0.508 mm/min on a 20 mm top and 40 mm bottom span. Prior to testing at room temperature, selected samples were exposed to high temperature reducing conditions (1000°C and an oxygen partial pressure varying between 10"8 to 10"' atm) for 2 hr using a buffered C02/Ar-4%H2 gas system and cooled to room temperature, maintaining the P(02) to approximately 700°C.

Results and Discussion

Table 1 displays the percent theoretical density for the sintered samples. In addition to having low densities, the LCC-15 and LSC samples had pores larger than the grains which will result in lower strengths from a smaller cross-sectional area and large initial flaw size. Both internal and intergran-ular pores were found in all compositions typical for a liquid phase sintered material. Figures 1 and 2 show the strength as a function of temperature for the lanthanum calcium chromite (denoted LCC, e.g. Lao 7Cao_3Cr03 is written LCC-30) and lanthanum strontium chromite samples, respec-

* Operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC06-76RLQ 1830.

Table 1: Percent theoretical density of the indicated LCC and LSC compositions.

Composition

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