Serene C. Farmer

High temperature creep deformation of directionally solidified Al2O3/Er3Al5O12

Abstract The microstructure of directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 (19.5 mol% Er 2 O 3 ) is analyzed and high temperature creep deformation studied using fibers in tension between 1400° C and 1550° C. The directionally solidified Al 2 O 3 /Er 3 Al 5 O 12 system is an in situ composite and has a fine eutectic- microstructure with sub-micron phase spacing. The microstructure is elongated in the direction of growth. Transmission electron microscopy observations revealed well-bonded interfaces and scatter within the crystallographic alignment of the constituent phases. The creep resistance of the system was very high, comparable to c-axis sapphire, and failure initiated at the lamella interfaces. The influence of the different elastic and plastic behaviors of the eutectic components on creep is examined. A critical discussion on the origin of the high stress dependence of the creep rate, the existence of steady state creep, and the relevant microscopic deformation mechanisms is presented.

Tensile strength and microstructure of Al2O3–ZrO2 hypo-eutectic fibers

Abstract Al 2 O 3 –ZrO 2 (Y 2 O 3 ) eutectic materials possess good fracture strengths and creep resistance. Increased Al 2 O 3 content is one means to further improve creep resistance. The objective of this study is to examine fracture strength of Al 2 O 3 -rich (hypoeutectic) compositions at varying Y 2 O 3 contents. Fibers 160–220 μm in diameter with 68 m/o Al 2 O 3 and 1.1–7.6 m/o Y 2 O 3 (30.5 to 16 m/o ZrO 2 ) were directionally solidified at 0.11 mm/s using the laser-heated float-zone process. Defect populations increased in size and severity with higher Y 2 O 3 contents. However, fibers maintained 1 GPa fracture strength in the presence of numerous pores and shrinkage cavities, which extend with crack-like morphology along the fiber axis.

High Temperature Mechanical Properties of Al2O3/ZrO2(Y2O3)Fibers

In-situ composite fibers produced by directional solidification of two phase oxide eutectics are one means of producing fibers with good strength and higher creep resistance than single crystal fibers. In this work, directionally solidified alumina-yttria stabilized zirconia eutectic fibers have been grown by the laser heated float zone (LHFZ) method at NASA Lewis. The average tensile strength of the alumina-zirconia (60.8 m/o Al 2 O 3 ; 39.2 m/o ZrO 2 (9.5 m/o Y 2 O 3 )) eutectic fibers was 1.2 GPa at room temperature. The high temperature tensile strength and creep resistance of the eutectic fiber were determined and compared to single crystal Al 2 O 3 .

Development of Thin Film Ceramic Thermocouples For High Temperature Environments

The maximum use temperature of noble metal thin film thermocouples of 1100 C (2000 F) may not be adequate for use on components in the increasingly harsh conditions of advanced aircraft and next generation launch technology. Ceramic-based thermocouples are known for their high stability and robustness at temperatures exceeding 1500 C, but are typically found in the form of rods or probes. NASA Glenn Research Center is investigating the feasibility of ceramics as thin film thermocouples for extremely high temperature applications to take advantage of the stability and robustness of ceramics and the non-intrusiveness of thin films. This paper will discuss the current state of development in this effort.

Directionally Solidified Mullite Fibers

Directionally solidified fibers with nominal mullite compositions of 3Al{sub 2}O{sub 3} {center_dot} 2SiO{sub 2} were grown by the laser heated float zone (LHFZ) method at NASA Lewis. High resolution digital images from an optical microscope evidence the formation of a liquid-liquid miscibility gap during crystal growth. Experimental evidence shows that the formation of mullite in aluminosilicate melts is in fact preceded by liquid immiscibility. The average fiber tensile strength is 1.15 GPa at room temperature. The mullite fibers retained 80% of their room temperature tensile strength at 1,450 C. SEM analysis revealed that the fibers were strongly faceted and that the facets act as critical flaws. Examined in TEM, these mullite single crystals are free of dislocations, low angle boundaries and voids. Single crystal mullite showed a high degree of oxygen vacancy ordering. Regardless of the starting composition, the degree of order observed in polycrystalline fibers was lower than that observed in the mullite single crystals.

History of Electrochemical and Energy Storage Technology Development at NASA Glenn Research Center

AbstractThe National Aeronautics and Space Administration Glenn Research Center (GRC) has a rich heritage of developing electrochemical technologies and energy storage systems for aerospace. Primary and rechargeable batteries, fuel cells, flywheels, and regenerative fuel cells are among the GRC’s portfolio of energy storage devices and primary power systems. These technologies have been developed for missions and applications such as low Earth orbit and geosynchronous Earth orbit satellites, space shuttle, astronaut spacesuit, International Space Station, landers and rovers, and lunar and planetary habitats. The desire for lower mass, lower volume, higher efficiency, and more reliable power systems has most often been the driving force behind the development of these technologies. Often, as with fuel cells for the early Gemini and Apollo missions, development of the technology has been mission enabling. Although many of these technologies were initially developed for applications in space, the existence o...

FRACTURE CHARACTERISTICS OF SINGLE CRYSTAL AND EUTECTIC FIBERS

Abstract Single-crystal fibers are attractive for functional ceramic applications as active devices and are equally important for structural ceramic components as load bearing applications. The fracture characteristics of single-crystal fibers from a variety of crystal systems including the Al2O3/Y3Al5O12 eutectic were examined. The Young moduli of 〈0001〉 Al2O3, 〈111〉 Y3Al5O12 and 〈111〉 Y2O3 fibers were 453, 290, and 164 GPa, respectively, and agreed well with the literature. Single crystals of 〈111〉 Y2O3 were the weakest fibers and their strength did not exceed 700 MPa. The moderate tensile strength of single-crystal 〈111〉 Y3Al5O12 was controlled by the facet forming tendency of the cubic garnet structure and in some cases by the precipitation of cubic perovskite phase YAlO3. High-strength single-crystal 〈0001〉 Al2O3 fibers did not retain their strength at elevated temperatures. The data suggest that single-crystal 〈0001〉 Al2O3 failure is dependent on slow crack growth at elevated temperatures. The high-temperature tensile strength of Al2O3/Y3Al5O12 eutectic fibers is superior to sapphire (1.3 GPa at 1100°C) and demonstrably less prone to slow crack propagation. The Al2O3/Y3Al5O12 eutectic interphase boundary is of a coherent nature with strong bonding.

Development Status for a Combined Solid Oxide Co-Electrolyzer and Carbon Formation Reactor System for Oxygen Regeneration

A critical component in spacecraft life support loop closure is the removal of carbon dioxide (CO2, produced by the crew) from the cabin atmosphere and chemical reduction of this CO2 to recover the oxygen. In 2015, we initiated development of an oxygen recovery system for life support applications consisting of a solid oxide co-electrolyzer (SOCE) and a carbon formation reactor (CFR). The SOCE electrolyzes a combined stream of carbon dioxide (CO2) and water (H2O) gas mixtures to produce synthesis gas (e.g., CO and H2 gas) and pure dry oxygen as separate products. This SOCE is being developed from a NASA GRC solid oxide fuel cell and stack design originally developed for aeronautics longduration power applications. The CFR, being developed by pHMatter LLC, takes the CO and H2 output from the SOCE, and converts it primarily to solid carbon (C(s)) and H2O and CO2. Although the solid carbon accumulates in the CFR, the innovative design allows easy removal of the carbon product, requiring minimal crew member (CM) time and low resupply mass (1.0 kg/year/CM) for replacement of the solid carbon catalyst, a significant improvement over previous Bosch reactor approaches. In this work, we will provide a status of our Phase I efforts in the development and testing of both the SOCE and CFR prototype units, along with an initial assessment of the combined SOCE-CFR system, including a mass and power projections, along with an estimate of the oxygen recovery rate.