M.L. Underwood

Effects of Scaling in SEE and TID Response of High Density NAND Flash Memories

Heavy ion single-event effect (SEE) measurements and total ionizing dose (TID) response for Micron Technology single-level cell 1, 2, 4, 8 Gb commercial NAND flash memory and multi-level cell 8, 16, 32 Gb are reported. The heavy ion measurements were extended down to LET 0.1 MeV-cm2/mg. Scaling effects in SEE and TID response are discussed. Floating gate bit error upset cross section does not scale with feature size at high LETs, except for single-level cell 8 Gb device which is built with 51 nm processes. The threshold LET does not change with scaling. Charge pump TID degradation and standby current improves with scaling. In general, the effect of radiation is either unchanged or is less severe for highly scaled NAND flash memories.

Recent advances in AMTEC recirculating test cell performance

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of thermal energy to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15% to 35% thermal to electric conversion efficiencies, and one experiment has demonstrated 19% efficiency for a short period of time. A recent experiment in a recirculating test cell (RCT) has demonstrated sustained conversion efficiencies as high as 13.2%. The cell was operated at lower current and 12% efficiency for over 1700 hours at the time of this writing. The cell required a maturation period of 355 hours at high temperature. During this period, the cell was operated once at 12% efficiency but was generally operated at lower powers. The maturation period ended with the formation of a reflective sodium film on the condenser surface which reduced the parasitic thermal losses in the cell. After maturation, the cell demonstrated the first experimental demonstration of the maximum efficiency occuring at a lower current than the maximum power. The cell also demonstrated an unexpected decrease in parasitic loss with increasing cell current. The decrease in parasitic loss resulted from the development of a more reflective sodium film at higher sodium fluxes.

Performance projections of alternative AMTEC systems and devices

The AMTEC is a device for the direct conversion of heat to electrical power with no moving parts. Most proposed AMTEC systems have focused on minimizing converter mass for space applications. This paper presents two AMTEC devices that focus on high conversion efficiencies (≳30% at 1100 K) and high volumetric power densities. These high current, low voltage modules could find use in small applications such as EM pumps or large systems requiring many modules to reach the desired power levels. A near term system called the tube bundle system produces a peak power of 426 W/1 and a peak efficiency (at lower power) of 34% at 1100 K hot zone temperature. A more advanced system called a flat plate system produces 2.4 W/1 and 30% efficiency at 1100 K. Further improvements are projected for these devices through the development of optimized porous electrodes or a potassium ion conducting sold electrolyte.

Lifetime Studies Of High Power Rhodium/tungsten And Molybdenum Electrodes For Application To Amtec (alkali Metal Thermal-to-electric Converter)

A detailed and fundamental model for the electrochemical behavior of AMTEC electrodes is developed which can aid in interpreting the processes which occur during prolonged operation of these electrodes. Because the sintering and grain growth of metal particles is also a well-understood phenomenon, the changes in electrode performance which accompany its morphological evolution may be anticipated and modeled. The grain growth rate observed for porous Mo AMTEC electrodes is significantly higher than that predicted from surface diffusion data obtained at higher temperatures and incorporated into the grain growth model. The grain growth observed under AMTEC conditions is also somewhat higher than that measured for Mo films on BASE (beta-alumina solid electrolyte) substrates in vacuum or at similar temperatures. Results of modeling indicate that thin Mo electrodes may show significant performance degradation for extended operation (greater than 10,000 h) at higher operating temperatures (greater than 1150 K), whereas W/Rh and W/Pt electrodes are expected to show adequate performance at 1200 K for lifetimes greater than 10,000 h. It is pointed out that current collection grids and leads must consist of refractory metals such as Mo and W which do not accelerate sintering or metal migration.

NASA's Soil Moisture Active Passive (SMAP) observatory

The Soil Moisture Active Passive (SMAP) mission, one of the first-tier missions recommended by the 2007 U.S. National Research Council Committee on Earth Science and Applications from Space, was confirmed in May 2012 by NASA to proceed into Implementation Phase (Phase C) with a planned launch in October 2014. SMAP will produce high-resolution and accurate global maps of soil moisture and its freeze/thaw state using data from a non-imaging synthetic aperture radar and a radiometer, both operating at L-band. Major challenges addressed by the observatory design include: (1) achieving global coverage every 2-3 days with a single observatory; (2) producing both high resolution and high accuracy soil moisture data, including through moderate vegetation; (3) using a mesh reflector antenna for L-band radiometry; (4) minimizing science data loss from terrestrial L-band radio frequency interference; (5) designing fault protection that also minimizes science data loss; (6) adapting planetary heritage avionics to meet SMAPs unique application and data volume needs; (7) ensuring observatory electromagnetic compatibility to avoid degrading science; (8) controlling a large spinning instrument with a small spacecraft; and (9) accommodating launch vehicle selection late in the observatorys development lifecycle.

Developments in Amtec Devices, Components and Performance

Improvement of the performance of an AMTEC device requires improvement and development of components as well as of device geometry and construction. The research and development effort at JPL includes studies which address both overall device construction and studies of components. This paper discusses recent studies on components and devices which have been carried out at JPL. Components investigated include the electrode materials titanium nitride (TiN) and rhodium‐tungsten (RhW) and the electrolyte materials sodium β“‐alumina and potassium β” ‐alumina. We have studied the mechanical characteristics of sodium and potassium β“‐alumina ceramic and conditions for fabrication of potassium β”‐alumina. Device studies include fabrication and operation of a wick fed cell using a graded, sintered wick, a higher voltage vapor‐vapor multicell which includes three “ subcells” which are internally series connected, and an AMTEC which uses potassium as the working fluid.

Efficiency of an AMTEC Recirculating Test Cell, Experiments and Projections

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of heat to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15 to 35 percent thermal to electric conversion efficiencies, and one experiment has demonstrated 19 percent efficiency for a short period of time. Recent experiments in a recirculating test cell (RTC) have demonstrated sustained conversion efficiencies as high as 10.2 percent early in cell life and 9.7 percent after maturity. Extensive thermal and electrochemical analysis of the cell during several experiments demonstrated that the efficiency could be improved in two ways. First, the electrode performance could be improved. The electrode for these tests operated at about one third the power density of state of the art electrodes. The low power density was caused by a combination of high series resistance and high mass flow resistance. Reducing these resistances could improve the efficiency to greater than 10 percent. Second, the cell thermal performance could be improved. Efficiencies greater than 14 percent could be realized through reducing the radiative thermal loss. Further improvements to the efficiency range predicted by systems studies can be accomplished through the development and use of an advanced condenser with improved reflectivity, close to that of a smooth sodium film, and the series connecting of individual cells to further reduce thermal losses.

High temperature conductivity of potassium-β''-alumina

Abstract Potassium β″-alumina single crystals have been reported by several groups to have higher ionic conductivity than sodium β″-alumina crystals at room temperature, and similar conductivities are obtained at temperatures up to 600–700 K. Potassium β″-alumina ceramics have been reported to have significantly poorer consuctivities than those of sodium β″-alumina ceramics, but conductivity measurements at temperatures above 625 K have not been reported. In this study, K + -β″-alumina ceramics were prepared from Na + -β″-alumina ceramic using a modified version of the exchange reaction with KCl vapor reported by Crosbie and Tennenhouse, and the conductivity has been measured in K vapor at temperatures up to 1223 K, using the method of Cole, Weber and Hunt. The results indicate reasonable agreement with earlier data on K + -β″-alumina ceramic measured in a liquid K cell, but show that the K + -β″-alumina ceramics conductivity approaches that of Na + -β″-alumina ceramic at higher temperatures, being within a factor of four at 700 K and 60% of the conductivity of Na + -β″-alumina at T > 1000 K. Both four-probe dc conductivity and four probe ac impedance measurements were used to characterize the conductivity. A rather abrupt change in the grain boundary resistance suggesting a possible phase change in the intergranular material, potassium aluminate, is seen in the ac impedance behavior.

AMTEC flight experiment progress and plans

An experiment is being developed to validate the performance of AMTEC technology in the space microgravity environment. A group of AMTEC cells have been fabricated and assembled into an experiment module and instrumented for operation. The experiment is manifested as a Hitchhiker payload on STS-88 now planned for flight in July 1998. The AMTEC cells will be operated in space for up to ten days. The microgravity developed distribution of the sodium working fluid will be frozen in place before the cells are returned to Earth. Upon return the cells will be destructively evaluated to determine the location of the sodium and to assure that the sodium has been properly controlled by the sodium control elements. This paper describes the experiment purpose, status and plans for the flight operations and data analysis. An overview of how this experiment fits into the overall AMTEC development is also provided.

AMTEC cell testing, optimization of rhodium/tungsten electrodes, and tests of other components

Electrodes, current collectors, ceramic to metal braze seals, and metallic components exposed to the high ‘‘hot side’’ temperatures and sodium liquid and vapor environment have been tested and evaluated in laboratory cells running for hundreds of hours at 1100–1200 K. Rhodium/tungsten electrodes have been selected as the optimum electrodes based on performance parameters and durability. Current collectors have been evaluated under simulated and actual operating conditions. The microscopic effects of metal migration between electrode and current collector alloys as well as their thermal and electrical properties determined the suitability of current collector and lead materials. Braze seals suitable for long term application to AMTEC devices are being developed.