James Kulleck

Crystal Structure of Li2B12H12: a Possible Intermediate Species in the Decomposition of LiBH4

The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH4. Our results reveal that the structure of Li2B12H12 differs from other known alkali-metal (K, Rb, and Cs) derivatives.

Assessment of Zr–V–Fe getter alloy for gas-gap heat switches

Abstract A commercial Zr–V–Fe alloy (i.e. SAES Getters trade name alloy St-172) has been assessed as reversible hydrogen storage material for use in actuators of gas-gap heat switches. For applications involving hydride compressors in closed-cycle Joule–Thomson sorption cryocoolers, the actuator need to produce a conducting (i.e. ON) state pressure above 670 Pa and an insulating (i.e. OFF) state pressure below 0.13 Pa in the gas-gap switch with switching times ∼200 s between the two states. Pressure–composition–temperature isotherms have been measured for the SAES St-172 material to define power efficient baseline performance at appropriate hydrogen concentration for these heat switch actuators. Two prototype actuators containing the SAES St-172 material were built and operated for several thousand cycles to evaluate performance of the metal hydride system under conditions simulating heat switch operation.

Invited Article: Physical and chemical analyses of impregnated cathodes operated in a plasma environment

Destructive analyses of impregnated-cathode assemblies from an ion thruster life test were performed to characterize erosion and degradation after 30,472 h of operation. Post-test inspection of each cathode included examination of the emitter (insert), orifice plate, cathode tube, heater, anode assembly, insulator, and propellant isolator. The discharge-cathode assembly experienced significant erosion due to ion sputtering from the discharge plasma. The keeper electrode plate was removed and the heater and orifice plate were heavily eroded at the conclusion of the test. Had the test continued, these processes would likely have led to cathode failure. The discharge cathode insert experienced significant tungsten transport and temperature dependent barium oxide depletion within the matrix. Using barium depletion semiempirical relations developed by Palluel and Shroff, it is estimated that 25,000 h of operation remained in the discharge insert at the conclusion of the test. In contrast, the neutralizer insert exhibited significantly less tungsten transport and barium oxide depletion consistent with its lower current operation. The neutralizer was estimated to have 140,000 h of insert life remaining at the conclusion of the test. Neither insert had evidence of tungstate or oxide layer formation, previously known to have impeded cathode ignition and operation in similar long duration hollow-cathode tests. The neutralizer cathode was in excellent condition at the conclusion of the test with the exception of keeper tube erosion from direct plume-ion impingement, a previously underappreciated life-limiting mechanism. The most critical finding from the test was a power dependent deposition process within the neutralizer-cathode orifice. The process manifested at low-power operation and led to the production of energetic ions in the neutralizer plume, a potential life-limiting process for the neutralizer. Subsequent return of the engine and neutralizer operation to full-power removed the deposits and energetic ion production ceased.

The thermal stability of sodium beta'-Alumina solid electrolyte ceramic in AMTEC cells

A critical component of alkali metal thermal-to electric converter (AMTEC) devices for long duration space missions is the beta″-alumina solid electrolyte ceramic (BASE), for which there exists no substitute. The temperature and environmental conditions under which BASE remains stable control operational parameters of AMTEC devices. We have used mass loss experiments in vacuum to 1573K to characterize the kinetics of BASE decomposition, and conductivity and exchange current measurements in sodium vapor filled exposure cells to 1223K to investigate changes in the BASE which affect its ionic conductivity. There is no clear evidence of direct thermal decomposition of BASE below 1273K, although limited soda loss may occur. Reactive metals such as Mn or Cr can react with BASE at temperatures at least as low as 1223K.

Mars Lander Engine plume impingement environment of the Mars Science Laboratory

The Mars Science Laboratory (MSL) Mission will land a 900-kg rover on the surface of Mars in 2010. Four Mars Lander Engines (MLEs) will be fired during the final propulsive descent to maintain a 0.75 m/s vertical rate of descent, in support of a tethered landing approach referred to as the “Sky-Crane”. At 20 m above the surface the rover will be lowered on a bridle as it continues to descend. At touch-down, a minimum of 6.5 m of vertical separation are provided between the engines nozzle exit plane and the ground-surface below [1]. This maneuver was chosen in part to minimize the ground/soil interaction that occurs when rocket engine plumes are fired into a soil media. In spite of the 6.5 m altitude above the surface, surface impingement pressures are expected to reach in excess of 2000 Pa, a metric previously established by the Viking program to mitigate soil bearing capacity failure. Plume-ground interaction has been a concern of Lunar and Mars propulsive landings for some time, but was not an issue for the Mars Pathfinder and Mars Explorer Rover era due to their use of airbag landing systems [2][3].This was also a concern of the Phoenix lander program, which fired twelve pulsed hydrazine monopropellant thrusters for its final descent and touch-down [4]. Phoenix was concerned with plume impingement soil interaction due to its high surface impingement pressure and potential for diffused gas eruptions. Phoenix was also concerned with landing site alteration due to its lack of mobility as well as instrument and solar array contamination issues. As MSL will operate in a regime that will result in ground-soil erosion a plume-ground interaction program has been undertaken to quantify the amount of soil erosion, namely the trajectory and number flux of particulates and the contamination and erosion this can impart to sensitive instruments and thermal surface coatings.

An Evaluation of the Discharge Chamber of the 30,000 Hr Life Test of the Deep Space 1 Flight Spare Ion Engine

A 30-cm-diameter ring cusp ion thruster built as a flight spare unit for the DS1 mission was operated for over 30,352 hours to determine the ultimate service life capability of this thruster design. Post-test examinations indicate that molybdenum films up to 10 um in thickness were deposited inside the discharge chamber and that these films adhered well to the sputter containment mesh. The magnetic field internal to the engine was unchanged with respect to magnetic field measured prior to the start of the life test. Propellant isolators were destructively analyzed and test data validated that the isolators were performing properly at the end of the test.

Hydride Compressor Sorption Cooler and Surface Contamination Issues

A continuous‐duty hydrogen sorption cryocooler is being developed for the Planck spacecraft, a mission to map the cosmic microwave background beginning in 2007. This cryocooler uses six individual compressor elements (CEs) filled with the hydriding alloy LaNi4.78Sn0.22 to provide high‐pressure (50 bar) hydrogen to a Joule‐Thomson (J‐T) expander and to absorb low‐pressure (∼0.3 bar) gas from liquid hydrogen reservoirs cooled to ∼18K. Quadrupole Mass Spectrometry (QMS) showed methane in these hydride beds after cycling during initial operation of laboratory tests of the Planck engineering breadboard (EBB) cooler. These contaminants have caused problems involving plugged J‐T expanders. The contaminants probably come from reactions with residual hydrocarbon species on surfaces inside the hydride bed. The hydride bed in each CE is contained in an annular volume called a “gas‐gap heat switch,” which serves as a reversible, intermittent thermal path to the spacecraft radiator. The gas‐gap is either “off” (i.e., ...

Thin Film Platinum Alloys for Use as Catalyst Materials in Fuel Cells

Introduction: In state-of-the-art polymer electrolyte fuel cells (PEFCs) using an acid polymer electrolyte, platinum (Pt) and platinum group metal (PGM) alloy catalysts are used as the cathode materials for the reduction of oxygen. Some challenges limiting the widespread application of PEFCs, that utilize PGM catalysts are: 1) slow kinetics for oxygen reduction; 2) insufficient long-term durability manifest by metallurgical effects (e.g., Ostwald particle ripening, and surface area loss due to corrosion); and 3) the high cost of platinum. Motivated by these challenges, we report the results of a study designed to discover new Pt-based, transition metal alloy catalysts that are stable in acid and electrochemically active for the oxygen reduction reaction (ORR). Results: Using a high-throughput, co-sputtering, synthesis technique [1], an array of thin film specimens in the ternary series (Pt3Co)1-xZrx, 0 ≤ x ≤ 30 (At.%), were simultaneously prepared. The individual films were deposited onto an 18-segment current collector structure comprised of nanostructured Au thin films, with average Au grain size of 40-50 nm. The Au films were strongly oriented, with a (111) crystallographic orientation, as shown in Fig.-1. The XRD patterns for representative films from the array, with 0 < x < ~20 (At.%) [some labeled w/nominal compositions] are shown in this figure, indicating that the Pt-Co-Zr thin films are also ordered, with a (111) crystallographic orientation. The decreased intensity for Pt56Co24Zr20 is consistent with the reduced thickness of the films in this part of the array.

Thermal stability of beta″-alumina solid electrolyte under AMTEC operating conditions

A critical component of alkali metal thermal-to electric converter (AMTEC) devices for long duration space missions is the sodium beta″-alumina solid electrolyte ceramic (BASE), for which there exists no substitute. The major phase in this ceramic, sodium beta″-alumina shows no evidence of thermal decomposition in AMTEC environments including clean liquid sodium and low pressure sodium gas, at temperatures below 1173K, or in vacuum below 1273K. This paper presents additional results of ionic conductivity and exchange current studies in sodium exposure test cells (SETCs) to characterize the changes occurring in BASE below 1273K in low pressure sodium vapor. Also presented are additional annealing studies to characterize the kinetics of processes occurring in the BASE ceramic in the AMTEC operating regime.