The overall objective of this project is the development of NLO-active materials with optical clarity and mechanical strength. These materials are intended for laser eye protection. By combining χ2 and χ3 optical properties, the intensity of incident laser radiation may be efficiently reduced. Using an in-plane poling technique, aligned films of liquid crystal poly(benzyl-Lglutamate), PBLG, were made which showed higher second harmonic generation (SHG) values compared to quartz. Silver sols in the 10-90 nm diameter size range were complexed with tricyanovinyl aniline, TCVA, resulting in composite PBLG/Ag Sol films with higher than at least an order of magnitude of χ3 values materials such as polydiacetylenes and nitroanilines. These polymeric NLO materials offer definite advantages in terms of easy processability into films for the manufacture of the optical elements necessary for laser eye protection.

In recent years, a great deal of interest has been directed toward the use of organic materials in the development of high-efficiency optoelectronic and photonic devices. There is a myriad of possibilities among organic materials, which allows flexibility in the design of unique structures with a variety of functional groups. The use of nonlinear optical (NLO) organic materials as thin-film waveguides allows full exploitation of their desirable qualities by permitting long interaction lengths and large susceptibilities allowing modest power input. There are several methods in use to prepare thin films such as Langmuir-Blodgett (LB) and self-assembly techniques,2-4 vapor deposition.5-7 growth from sheared solution or melt,8,9 and melt growth between glass plates.10 Organic-based materials have many features that make them desirable for use in optical devices, such as high second- and third-order nonlinearity, flexibility of molecular design, and damage resistance to optical radiation. However, processing difficulties for crystals and thin films has hindered their use in devices.

We discuss the potential role of microgravity processing of a few organic and polymeric materials. It is of interest to note how materials with second- and third-order NLO behavior may be improved in a diffusion-limited environment and ways in which convection may be detrimental to these materials. We focus our discussion on third-order materials for all-optical switching, and second-order materials for frequency conversion and electro-optics. The goal of minimizing optical loss obviously depends on processing methods. For solution-based processes, such as solution crystal growth and solution photopolymerization, it is well known that thermal- and solutal-density gradients can initiate buoyancy-driven convection. Resultant fluid flows can affect transport of material to and from growth interfaces and become manifest in the morphology and homogeneity of the growing film or crystal. Likewise, buoyancy-driven convection can hinder production of defect-free, high-quality crystals or films during crystal and film growth by vapor deposition.

Gravity can affect the physical vapor growth of mercuric iodide in two distinct ways. First, gravity will induce convection during growth, which strongly mixes residual impurities and any elementary gases resulting from imperfect stoichiometry, either of which can then form precipitates in the growing crystal. Second, gravity loads the resulting crystal, which is particularly soft while still hot, especially in the absence of precipitates. We have investigated the effects of these processes on the resulting crystalline regularity and the effects of various types of irregularity, in turn, on performance.

High resolution synchrotron x-radiation diffraction imaging of three generations of crystals, grown both in microgravity and in full gravity, provide graphic evidence of the influence of gravity on mercuric iodide crystal growth. These images tie together the results of other characterization studies, identifying the crystallographic sources of the observed property enhancement in microgravity. The first process, convection, is found to be particularly important, both in its influence on observed crystalline regularity and in the resulting electronic performance of detectors made from these crystals.

As a result of these investigations, the crystalline regularity and performance of terrestrial crystals has been substantially improved, although the resulting crystals have not yet achieved parity with the performance of crystals grown in microgravity. We propose new experiments in microgravity for property optimization.

The magnetic levitation of diamagnetic materials in a high field superconducting magnet is presented. The magnetic force is determined from numerical calculations of the magnetic field components and the stability of levitation is studied using energy calculations. Amongst the numerous materials that can be levitated, experiments are shown for ethanol droplets.

An AC modulation calorimetry (ACMC) experiment was performed on two bulk metallic glass forming alloys, Ti34Zr11CU47Ni8 (VIT 101) and Zr57Nb5Ni12.6Al10Cu15.4(VIT 106) on the recent MSL-1 shuttle flight using the TEMPUS hardware. VIT 106 exhibited a maximum undercooling of 140 K in free radiative cooling, while VIT 101 exhibited a smaller undercooling of 50 K. Specific heat measurements were done in both the stable and undercooled liquid regions. These results will be combined with further ground-based measurements in an electrostatic levitator for modeling the nucleation kinetics of these alloys and for calculation of the free energies and entropies of the undercooled liquids in these systems.

The rapid solidification of a Fe- 12wt%Cr- 16wt%Ni alloy was investigated under containerless processing conditions using both ground-based electromagnetic levitation equipment and aboard the shuttle Columbia using the TEMPUS facility. A high-speed digital video technique was used to image growth of the metastable ferritic phase and the stable austenitic phase into the undercooled melt. Above a critical undercooling, the metastable phase nucleates first. After a delay, a second thermal rise is observed during transformation to the stable phase. Double recalescence events were observed at temperatures consistent with the To, temperature of the bcc phase thus defining a value of the critical undercooling for metastable nucleation which is significantly lower than previously predicted. For a given liquid temperature the velocity of the stable fcc phase is greater than that of the metastable bcc phase. The velocity for growth of the stable phase into the semi-solid which forms during primary metastable recalescence was also measured and found to be independent of the initial undercooling. A model based on competitive growth of the two phases successfully predicts the limit where double recalescence events may be detected.

Exactly one year ago, the Isothermal Dendritic Growth Experiment (IDGE) completed its third and final orbital space flight aboard the United States Microgravity Payload (USMP) on STS-87. The IDGE conducted 180 experiments on dendritic growth in 5-9's succinonitrile (SCN), a BCC material used on USMP-2 and USMP-3, and over 100 experiments on 4-9's pivalic acid (PVA), an FCC material used on USMP-4. IDGE film and telemetry data provide benchmark tip velocity and radii versus supercooling for critically testing transport theory and the interfacial physics of diffusion-limited dendritic growth. Post-flight application of optical tomography is providing the first tip shape data allowing quantitative tests of three-dimensional phase field calculations. Several new discoveries were made during each flight concerning the behavior of dendrites at low driving forces, and the influences of time-dependent pattern features and noise. A summary of these scientific highlights will be provided.

The IDGE instrument was upgraded on each successive flight, improving its optics and electronics, especially the capability for teleoperational control. Near real-time, full gray-scale video was accommodated on USMP-4, allowing investigation of non-steady-state features and time-dependent growth dynamics. A short example of video from space will be shown. USMP-4 science was teleoperated by a student cadre for 16 days from a remote site established by NASA at RPI. This operational experience provides valuable insights, which will be drawn upon for future microgravity experiments to be conducted on the International Space Station.

The extensive use of zeolites and their impact on the world's economy has resulted in many efforts to characterize their structure and improve the knowledge base for nucleation and growth of these crystals. Zeolite crystal growth (ZCG) experiments have been conducted in space by many researchers. The results have been varied with little overall constancy with respect to the types of zeolites grown, the procedures used, and methodologies employed for comparison of terrestrial and microgravity processed crystals. Results from this laboratory have indicated that solutions must be mixed in space, and that the nucleation mechanism as well as the growth mechanism is affected by the “suppression” of fluid motion. Earlier flight experiments (STS-43, STS-50 and STS- 57) indicated that by controlling the nucleation event, the size of the crystals could be increased with improved structural quality compared to the ground-based controls. This was hypothesized to be the result of hindrance of the solution/dissolution mechanism caused by reduced fluid motion. USML-2 (STS-73) zeolite experiments were designed to further enhance the understanding of nucleation and growth of zeolite crystals, while attempting to provide a means of controlling the defect concentration in microgravity. Zeolites A, X, and Silicalte were grown during the 16-day United States Microgravity Laboratory number 2 (USML-2) mission. All zeolites were successfully grown in the 16 day mission. The zeolite A and X crystals were in general larger in dimension (10-50%), but not substantially different in crystal perfection as measured by their lattice parameters. However, They were significantly smoother as indicated by AFM. Zeolite β as well as Silicalite were only slightly larger in size but, appear to have different distributions of Aluminum atoms as determined by selective catalytic reactions.

This paper covers findings obtained from the microgravity directional solidification of immiscible aluminum-indium alloys during the Life and Microgravity Spacelab Mission in 1996. Three alloys, one of monotectic composition and two alloys containing an excess of indium above the monotectic (i.e., hypermonotectic compositions) were solidified using the Advanced Gradient Heating Facility (AGHF). Samples were processed in specialized ampoule assemblies containing pistons and a high temperature spring in a partially successful attempt to prevent void formation due to thermal contraction of the melt and solidification shrinkage. A comparison of compositional variations between microgravity processed and ground processed samples revealed compositional variations along the length of ground processed samples which were representative of results anticipated due to convective mixing in the melt. Flight samples showed an initial compositional variation indicative of minimal mixing in the melt. However, a discontinuity in the microstructure was observed which coincided with the presence of a void in the flight sample.

Recent experimental measurements of various microgravity experiments have been taken on the Mir and Space Shuttle under different conditions of microgravity using the Microgravity-vibration Isolation Mount (MIM). The results to date show a clear difference when the experimental measurements are taken from g-levels offered by the Mir and the Space Shuttle (non isolated) to g-levels offered by MIM (isolated) which have been reduced by two orders of magnitude. Concern for the International Space Station (ISS) experimental facilities arises when the quality of microgravity on the Mir and Space Shuttle (non isolated), which is believed to be not good enough, has been measured to be better than the ISS Requirement established by NASA for isolated racks, which will be significantly better than those racks not isolated.

In interplanetary travel and high altitude flight, humans will be exposed to high energy charged particles from solar flares and galactic cosmic rays. These particles lose energy in a material by Coulomb interactions and nuclear collisions. In nuclear collisions, large amounts of energy are transferred and secondary particles are formed from both the projectile and the struck nucleus. A significant portion of these particles are neutrons which can only lose energy by collisions or reactions with a nucleus. Hydrogen-containing materials, such as polymers, are most effective in reducing the neutron energy. When reduced to very low energies, neutrons have a high probability of reacting with a nucleus. Such reactions are dangerous in the human body, and can cause electronic equipment failure. Low energy neutrons react particularly well with a stable isotope of boron, 10B. To test structural materials which contain both hydrogen to reduce the energy of neutrons and boron to absorb neutrons of reduced energy, samples of two polyimides were made which contained varying amounts of either amorphous boron powder or boron carbide whiskers. The polymers used were a thermoset, PETI-5 from Imitec, and a thermoplastic, K3B from Fiberite. Both materials were made in pure form and with up to 20% by weight of the boron additives. The addition of boron in either form did not change the thermal properties of these materials significantly. However, the compressive yield strength and the tensile strength were both affected by the addition of the boron materials. A neutron absorption test using a PuBe thermal neutron source showed that a 0.5 cm thick sample of K3B containing 15% amorphous boron powder absorbed over 90% of the incident neutrons.

Aluminum matrix composites reinforced with high thermal conductivity vapor grown carbon fiber (VGCF) were developed for improved thermal efficiencies in electronic devices. The carbon fiber was heat treated to increase its thermal conductivity. Various aluminum matrix composites were fabricated by the densification of fiber preforms using a pressure casting technique. Uniformity of the density was examined using optical microscopy. A scanning electron microscope equipped with a microprobe was utilized to examine the mechanical integrity of the composite. Mechanical properties, including tension, compression and flexural properties, were measured. While the results of the mechanical property measurements indicate moderate values, the composite exhibited remarkable thermal conductivity that reached 642 W/m.K, three times that of aluminum, at a fiber volume fraction of 36.5%, following closely the rule of mixture.

At Marshall Space Flight Center (MSFC), a new aging technique has been developed that can significantly enhance the cryogenic fracture toughness (CFT) of Alloy 2195, as well as reducing the statistical spread of fracture toughness values. This technique improves cryogenic properties by promoting T1 nucleation and growth in the matrix, so that T1 nucleation and growth can be suppressed in the subgrain boundaries. It also minimizes mechanical property scatter due to unavoidable variations in thermomechanical processing parameters and alloy chemistry.

A new material for structures in space, which have a high energy absorbability has been developed using a powder particle assembling technique. Powder particles of polystyrene coated with nickel-phosphorus alloy layers using electroless plating were sintered at high temperature. A metallic closed cellular material containing polystyrene was then constructed.

The mechanical and ultrasonic properties of this material were measured at both room and high temperatures. The compressive tests of this material show a low Young's modulus and high energy absorption. Ultrasonic measurement shows that the attenuation coefficient of this cellular material is very large and would change due to increasing temperature. These results indicate that this metallic closed cellular material can be used for the space applications.

Several multilayer test coatings for hard X-ray telescopes were fabricated using DC magnetron sputtering. The process parameters were selected from a series of trials of single layer depositions. The samples were characterized using X-ray specular reflectivity scans, AFM, and cross-sectional TEM. Additional measurements (stylus profilometry, RBS, and Auger analysis) were used in the optimization of the deposition rate and of the thin film properties (density, composition, surface/interface microroughness). The X-ray reflectivity scans showed that the combinations of reflector and spacer materials tested so far (W/Si and W/C) are suited for graded d-spacing multilayer coatings that present a constant reflectivity bandpass up to 70 keV.

Samples of Ti and Zr metals have been carbonized or nitrided at high temperature by means of reactive plasma processing. Thickness and hardness of modified layers increased with the processing temperature and time in an experimental range below 1500°C for less 5h. The maximum hardness of TiC-layer modified at 1300°C for 3h was about 5000Hv(kg/mm2) and the maximum thickness of ZrC-layer formed at 1400°C for 5h reached about 100μm. A TiC-layer was analyzed with EPMA, XRD, EXAFS and PAS(Positron Annihilation Spectroscopy). It was confirmed that the sub-surface layer of Ti metal was the mixture of TiC ceramics and Ti metal, and that the thickness of TiC with NaCI type structure was very large.