One major obstacle to human space exploration is the possible limitations imposed by the adverse effects of long-term exposure to the space environment. Even before human spaceflight began, the potentially brief exposure of astronauts to the very intense random solar energetic particle (SEP) events was of great concern. A new challenge appears in deep space exploration from exposure to the low-intensity heavy-ion flux of the galactic cosmic rays (GCR) since the missions are of long duration and the accumulated exposures can be high. Because cancer induction rates increase behind low to rather large thickness of aluminum shielding according to available biological data on mammalian exposures to GCR like ions, the shield requirements for a Mars mission are prohibitively expensive in terms of mission launch costs. Preliminary studies indicate that materials with high hydrogen content and low atomic number constituents are most efficient in protecting the astronauts. This occurs for two reasons: the hydrogen is efficient in breaking up the heavy GCR ions into smaller less damaging fragments and the light constituents produce few secondary radiations (especially few biologically damaging neutrons). An overview of the materials related issues and their impact on human space exploration will be given.

The Mars Pathfinder Wheel Abrasion Experiment (WAE) spun a specially prepared wheel with strips of aluminum, platinum, and nickel, in the Martian soil. These materials were chosen because of their differing hardnesses, their ability to stick to anodized aluminum, and their comparative chemical inertness under Earth launch and Mars landing conditions. Abrasion of those samples was detected by the change in their specular reflectances of sunlight as measured by a photovoltaic sensor mounted above the wheel. The degree of abrasion occurring on the samples is discussed, along with comparisons to the abrasion seen in Earth-based laboratory experiments using Martian soil analogs. Conclusions are reached about the hardness, grain size, and angularity of the Martian soil particles, and the precautions which must be undertaken to avoid abrasion on moving parts exposed to the Martian dust.

The thermal environment for Mars surface exploration provides unique challenges for materials for use in structure and thermal control. The Sojourner Mars Rover has a lightweight integrated structure/insulation that has been environmentally tested and qualified for the Pathfinder mission to Mars. The basic structure with insulation, called the Warm Electronics Box (WEB), accounts for only 10% of the total Rover mass. The WEB is a thermal isolating composite structure with co-cured thermal control surfaces and an ultra-lightweight hydrophobic solid silica aerogel which minimizes conduction and radiation. This design provides excellent thermal insulation at low gas pressures and meets the structural requirements for spacecraft launch loads and for a 60 g impact landing at Mars without damage to the insulation or structure. Since the Pathfinder mission, this basic design concept has been developed and improved for future Mars surface robotic missions.

The popular success of the Mars Pathfinder mission, especially in terms of individual access to “live”pictures over the internet, demonstrates substantial mass appeal of space exploration on a personal level. Teleoperation provides an inexpensive approach to remote presence, but is distance-limited due to finite signal propagation times. The proximity of the Lunar surface permits near real-time teleoperation, and through a Virtual Reality approach, presents a sense of “being there” without the difficulty of getting there (and back).

While limited single user teleoperated Lunar Rovers concepts have been proposed (Luna Corp., Inc.), an on-going and economically self supporting venture would require simultaneous teleoperation of many vehicles on a large scale. In addition, each vehicle would carry several simultaneous stereoscopic remote viewing television camera heads, whose position is controlled by the head movements of Earth bound viewers. The overall concept is similar to a small fleet of touring busses, although some smaller scout vehicles may be included, as well as a tow truck. An access fee structure for Earth bound teleoperators provides for return on investment.

Technical issues for an on-going operation in the Lunar environment include temperature extremes, high vacuum, solar radiation, micrometeorites, abrasive nature of Lunar dust and its capacity to electrostatically attach to surfaces are discussed, as are their impact on vehicle design and operation. Optical data links are considered to handle bandwidth requirements of 500 simultaneous users. The economics of scale, on multiple levels, demonstrates the feasibility of large scale teleoperation. Various base-line studies are presented.

The ultimate success of a permanent lunar base will depend upon the use of available resources. Two important resources available on the moon are oxygen and metals. We examine issues, including lunar resources, processing, power, and engine performance, surrounding the use of lunar resources for metal/oxygen engines, including the performance of metal alloys as rocket fuels. Results of this analysis have uncovered several promising candidates including alloys of Al-Ca, Al-Mg, Al-Si, Ca-Mg, and Ca-Si.

The demand for spacecraft power has dramatically increased recently. Higher efficiency, multi-junction devices are being developed to satisfy the demand. The multi-junction cells presently being developed and flown do not employ optimized bandgap combinations for ultimate efficiency due to the traditional constraint of maintaining lattice match to available substrates. We are developing a new approach to optimize the bandgap combination and improve the device performance that is based on relaxing the condition of maintaining lattice match to the substrate. We have designed cells based on this approach, fabricated single junction components cells and tested their performance. We will report on our progress toward achieving beginning-of-life AMO multi-junction device conversion efficiencies above 30%.

Realization of high quality GaAs photovoltaic materials and devices by Metal-organic Molecular Beam Epitaxy (MOMBE) with growth rates in excess of 3 microns/ hours is demonstrated. Despite high growth rates, the optimization of III/V flux-ratio and growth temperatures leads to a two dimensional layer by layer growth mode characterized by a (2×4) RHEED diagrams and strong intensity oscillations. The not intentionally doped layers exhibit low background impurity concentrations and good luminescence properties. Both n(Si) and p(Be) doping studies in the range of concentrations necessary for photovoltaic device generation are reported. Preliminary GaAs (p/n) tunnel diodes and solar cells fabricated at growth rates in excess of 31µm/h exhibit performances comparable to state of the art and stress the potential of the high growth rate MOMBE as a reduced toxicity alternative for the production of Space 111-V solar cells.

One of the critical components in a thermophotovoltaic (TPV) system is the infrared-sensitive photovoltaic (PV) semiconductor device that converts the emitted radiation to electricity. Currently, several semiconductor material systems are under development by various workers in the field. The most common are InGaAs/InP, GaSb, and InGaSbAs/GaSb. These devices normally have electronic energy bandgap values in the range of 0.50-0.74 eV. In addition, the design and structure of these devices fall into two distinct formats: conventional planar and monolithic interconnected module (MIM). The conventional planar devices normally have one semiconductor junction and are high-current/low-voltage devices. In a MIM, small area PV cells are connected in series monolithically, on a semi-insulating substrate. This results in the formation of a single high-voltage/low-current module. Electrical and optical performance results for MIMs with electronic energy bandgaps of 0.60 and 0.74 eV will be presented.

The Photovoltaic Engineering Testbed (PET) is a facility to fly on the International Space Station to test advanced solar cell types in the space environment. The purpose is to reduce the cost of validating new technologies and bringing them to spaceflight readiness by measuring them in the in-space environment. The facility is scheduled to be launched in 2002.

NASA's Genesis Discovery Mission is designed to collect solar matter and return it to earth for analysis. The mission consists of launching a spacecraft that carries high purity collector materials, inserting the spacecraft into a halo orbit about the L1 sun-earth libration point, exposing the collectors to the solar wind for two years, and then returning the collectors to earth. The collectors will then be made available for analysis by various methods to determine the elemental and isotopic abundance of the solar wind. In preparation for this mission, potential collector materials are being characterized to determine baseline impurity levels and to assess detection limits for various analysis techniques. As part of the effort, potential solar wind collector materials have been analyzed using resonance ionization mass spectrometry (RIMS). RIMS is a particularly sensitivity variation of secondary neutral mass spectrometry that employs resonantly enhanced multiphoton ionization (REMPI) to selectively postionize an element of interest, and thus discriminates between low levels of that element and the bulk material. The high sensitivity and selectivity of RIMS allow detection of very low concentrations while consuming only small amounts of sample. Thus, RIMS is well suited for detection of many heavy elements in the solar wind, since metals heavier than Fe are expected to range in concentrations from 1 ppm to 0.2 ppt. In addition, RIMS will be able to determine concentration profiles as a function of depth for these implanted solar wind elements effectively separating them from terrestrial contaminants. RIMS analyses to determine Ti concentrations in Si and Ge samples have been measured. Results indicate that the detection limit for RIMS analysis of Ti is below 100 ppt for 106 averages. Background analyses of the mass spectra indicate that detection limits for heavier elements will be similar. Furthermore, detection limits near 1 ppt are possible with higher repetition rate lasers where it will be possible to increase signal averaging to 108 laser shots.

Thin film LiCoO2 anodes and SnO2 cathodes on the order of 1-10 µtm thick have been synthesized using a spray decomposition technique, employing a lithium-nickel nitrate/methanol solution and tin ethylhexanoate/methanol solution, respectively. Preliminary electrochemical test results on the films indicate that the LiCoO2 anodes in general display a relatively high open circuit voltage (OCV) of ∼4.10-4.25V, good specific capacity on the order of 120 mAh/g, and acceptable cycle-ability, with a 16-25% decay in operating voltage after 50 cycles. The SnO2 films display excellent performance characteristics, upon an expected initial drop in capacity, with an OCV of 1.00-1.10V, specific capacity of ∼600 mAh/g, and virtually no decay in operating voltage after 50 cycles. The solid-state electrolyte, lithium phosphate, was prepared in thin film form by a similar spray processing technique. Additionally, it has been demonstrated that spray decomposition can be used to sequentially deposit the anode, electrolyte, and cathode film layers to form a layered thin film battery (TFB), although electrochemical testing of this thin film device has not yet been conducted.

Highly transparent films with tailorable sheet resistivity were prepared by ion-beam sputtering of indium tin oxide (ITO) with MgF2 or SiO2 in the presence of high-purity air. Sheet resistivities of 103−101 ohms/square (ω/–) and visible transmittances as high as 92% (not corrected for substrate absorption) were obtained in films ∼30 nm thick. Resistivity increased by as much as two orders of magnitude in the first year after preparation; however, thicker films (e.g. 80 nm) were much more stable but somewhat less transparent. Preliminary data from exposure of film samples to atomic oxygen in a plasma asher indicate minimal degradation in optical properties. Heat-treating pure ITO in air produced transparent, slightly conductive films but with poorer stability of sheet resistivity in air than co-deposited ITO with either SiO2, or MgF2. Electrical transport measurements yielded new information on the electronic properties of ITO and related materials. These films show promise as low-absorption static bleedoff coatings for space photovoltaic arrays as well as CRT faceplates and other commercial applications.

As a result of their electron structure, rare earth ions in crystals at high temperature emit radiation in several narrow bands rather than in a continuous blackbody manner. This study presents a spectral emittance model for films and cylinders of rare earth doped yttrium aluminum garnets. Good agreement between experimental and theoretical film spectral emittances was found for erbium and holmium aluminum garnets. Spectral emittances of films are sensitive to temperature differences across the film. For operating conditions of interest, the film emitter experiences a linear temperature variation whereas the cylinder emitter has a more advantageous uniform temperature. Emitter efficiency is also a sensitive function of temperature. For holminum aluminum garnet film the efficiency is 0.35 at 1446K but only 0.27 at 1270 K.

We have been investigating the synthesis of novel materials and device structures based on CuInSe2 or CIS system We are interested in developing CIS for use in thin film photoltaic solar cells used in space power systems. Non-stoichiometric native defects allow CIS to be selectively doped n or p type by controlling the Cu to In ratio. The Cu to In ratio of an electrodeposited CIS thin film is shown to be directly proportional to the deposition voltage. This behavior has allowed us to deposit different semiconductor-type films, pn junctions, and multilayer structures from the same aqueous solution. The affect of annealing in an Argon atmosphere and the addition of a wetting agent to our deposition solutions on the structural and electrical behavior of our films was measured. A dramatic improvement in the crystallinity, which was proportional to the annealing temperature was observed. The addition of a wetting agent to the solution was shown to improve the surface morphology and provide uniformity in the grain size. The electrical characterization of Schottky barriers on the CIS showed a decrease in several orders of magnitude of the carrier density with annealing. The ability to incorporate our CIS-based films in pn junction devices using CdS window layers was demonstrated by the rectifying current versus voltage behavior of these junctions.

There is considerable interest in the deposition of compound semiconductors by methods which involve relatively low capital expense and are technically undemanding on the experimentalist. One process to meet these criteria is Chemical Bath Deposition (CBD). Such processing methods are particularly appropriate for the production of devices for which large areas and low cost are essential, such as the BP Solar Ltd “Apollo” cells based on CdS:CdTe heterojunctions.

The electrical and optical properties of these devices have been investigated with respect to impurity profiles of 16-O, 34-S, 35-Cl and 12-C in the CdTe and CdS:CdTe interface. Device characteristics (e.g. Voc and Rs) have been correlated with SIMS data. The distribution of the carbon contaminant appears to influence the chloride-promoted recrystallisation of CdTe. XPS analysis of CdS layers appears to indicate that the annealing process induces the formation of a chloride-rich surface layer, which may promote the n- to p- type thermal conversion of CdTe.

As the performance demands on metal alloys of interest to the aerospace community continue to increase, the complexity of these alloys has tended to grow as well, with corresponding increases in the cost and time needed to develop future generations of materials. While the traditional empirical approach to designing these materials has proven effective, as the complexity of the materials grows, such methods will become less efficient, limiting prospects for further improvement. The incorporation of theoretically- and computationally-based input into the design process has the potential to both improve the quality of, and reduce the time to design, new materials.

Among the most fundamental issues in new materials design, the prediction of the crystallographic structure of solids has long been a goal of researchers in the field of computer simulation. To become an effective tool for assisting in the process of alloy design, a computational scheme must make use of an energy method fast enough to allow consideration of a large number of candidate structures, and an efficient energy minimization scheme.

In this work, we discuss our application of Monte Carlo methods, using the BFS semi-empirical energy method developed in our laboratory, to the problem of prediction of the crystallographic structure of multi-component metal alloys. Using a simple Metropolis Monte Carlo method in conjunction with an accurate and transferable method for computing energetics, we have investigated the crystallographic structure of a variety of fcc- and bcc-based intermetallic compounds, and have considered the influence of alloying additions in compounds having up to five constituents. We discuss the successes and limitations of the methodology, and the prospects for obtaining accurate prediction of physical properties from our results. We also emphasize the role of this type of methodology as part of a synergistic experimental-theoretical effort for alloy design.

Material variations on an atomic scale enable the quantum mechanical functionality of devices such as resonant tunneling diodes (RTDs), quantum well infrared photodetectors (QWIPs), quantum well lasers, and heterostructure field effect transistors (HFETs). The design and optimization of such heterostructure devices requires a detailed understanding of quantum mechanical electron transport. The Nanoelectronic Modeling Tool (NEMO) is a general-purpose quantum device design and analysis tool that addresses this problem. NEMO was combined with a parallelized genetic algorithm package (PGAPACK) to optimize structural and material parameters. The electron transport simulations presented here are based on a full band simulation, including effects of non-parabolic bands in the longitudinal and transverse directions relative to the electron transport and Hartree charge self-consistency. The first result of the genetic algorithm driven quantum transport calculation with convergence of a random structure population to experimental data is presented.

Silicon ions were implanted into fused silica substrates at doses of 1×1021, 2×1021, 5×1021, and 1×1022 ions/cm3. The implanted substrates were annealed at 1100°C for one hour in a reducing atmosphere (95% Ar+5% H2). Optical absorption spectra recorded after the annealing treatment showed absorption onsets at 3.86, 3.73, 2.86 and 2.52 eV for substrates implanted with 1×1021, 2×1021, 5×1021, and 1×1022 ions/cm3, respectively. Static photoluminescence (PL) measurements indicated red emission between 1.72 and 1.61 nm with a slightly increasing red shift with ion dose. Time resolved PL at room temperature revealed slow (∼50 μs) and fast (×20 μs) lifetimes which increased with decreasing temperature. TEM studies showed that the particle size increased with increasing ion dose. Typical particle sizes ranged between 2 and 5 nm indicating quantum confinement of the exciton, which can account for the blue shift in the absorption edge with decreasing ion dose. However, the maxima in the PL spectra for all ion doses are relatively independent of the ion dose and are strongly shifted from the absorption spectra. This suggests that radiative recombination occurs from a common luminescent center, possibly a surface or interfacial state in thexs SiOx, layer surrounding the nanocrystal.

Highly reflective and surface conductive silvered polyimide films have been prepared by the incorporation of the (1,1,1-trifluoro-2,4-pentanedionato)silver(I) complex into a dimethylacetamide solution of the poly(amic acid) formed from 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) and 4,4'-oxydianiline (ODA). Thermal curing of the silver(I)- containing poly(amic acid) films leads to cycloimidization with concomitant silver(I) reduction yielding a reflective and conductive silvered film surface.

The analysis of nucleation and growth processes relies mostly on circular arguments since basic thermophysical properties necessary, such as the Gibbs free energy (enthalpy of crystallization, specific heat), the density, emissivity, thermal conductivity (diffusivity), diffusion coefficients, surface tension, viscosity, interfacial crystal / liquid tension, etc. are generally unknown with sufficient precision and therefore often deduced from insufficient linear interpolations from the elements. The paucity of thermophysical property data for commercial materials as well as research materials is mostly a result of the experimental difficulties arising from the unwanted convection and reactions of melts with containers at high temperatures. An overview will be given on the results of thermophysical property measurements during several different space flights using containerless processing methods. Furthermore, a perspective on a future measurement program of thermophysical properties supported by the European Space Agency is described. In this regard, the International Space Station is considered as the ideal laboratory for high precision measurements of thermophysical properties of fluids which help to improve manufacturing processes for a number of key industries.