Steven Chu

Space radiation resistant hybrid and polymer materials for solar cells

The emergence of new radiation resistant organic-polymer and hybrid materials for potential extra-terrestrial and terrestrial application to photovoltaic solar cell technology is reported. New materials that exhibit resistance to induced effects caused by natural or even man-made ionizing radiation that could be encountered in aerospace, near-Earth orbits and in inter-planetary space missions is of vital importance to DOD, commercial and NASA space programs. Experimental data reflecting the relative stability of new polymer and hybrid polymer materials exposed to simulated total dose conditions that could be experienced in extraterrestrial regions such as within the Van Allen belts or on Lunar or Martian surfaces are the focus of this paper.

Irradiation of hydrophobic coating materials by gamma-rays and protons: Space applications

The responses of hydrophobic silicone-based coatings following irradiation by Co-60-gamma-rays are reported. The dimethylsilicone (DMS) resin coatings consisted of neat samples and samples incorporating semiconductor metal oxide (SMO) irradiated at photon energies of 1.17 and 1.33 MeV. Pre-and post-irradiation measurements indicated that at a total dose of ~ 185 krad(Si) there was no significant change to the coating static hydrophobic contact angles, surface molecular structure and biocide neutralization efficiency. The data was compared with previously irradiated and reported DMS/SMO coatings at a proton fluence of ~1.5 x 1012 p/cm2. Potential space applications for the radiation resistant coating self- cleaning properties are presented as well as a discussion of other environmental testing required to qualify the technology for transition to photonic space applications.

Tailoring of superhydrophilic to superhydrophobic coating morphologies for space exploration contamination control

Dust and ice contamination is a serious problem for equipment and vehicles for air and space mission applications. Dust contamination gathers on photonic sensors inhibiting motion and data gathering. Photonic devices that require transparency to light for maximum efficiency, such as solar photovoltaic power systems, video cameras and optical or infrared detectors, can be seriously affected by dust accumulation. The lunar thermal and radiation environment also pose unique challenges because of its large temperature variations and its interaction with the local plasma environment and solar UV and X-rays induced photoemission of electrons. Superhydrophilic materials are composed of polar molecules and have been used to defog glass, enable oil spots to be swept away easily with water, as door mirrors for cars and coatings for buildings. Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules often cluster together. Hydrophobic surfaces contain materials that are difficult to wet with liquids, with superhyrophobic surfaces having contact angles in excess of 150° (the equilibrium angle of contact of a liquid on a rigid surface where liquid, solid and gas phases meet). This paper presents an overview of the fundamental forces (van der Waals) which allows certain contamination to adhere to critical photonic surfaces and the various passive coatings phenomenology (hydrophilic to hydrophobic) that is used to minimize this contamination.

Modeling & simulation of nanostructures for superhydrophobic coatings

Materials with surfaces that are difficult to wet with water are called hydrophobic. Hydrophobic molecules tend to be nonpolar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules in water often cluster together. Superhydrophobic surfaces are generally made by additionally controlling the surface chemistry and surface roughness of various hydrophobic materials. Superhydrophobic surfaces can be caused by protrusions (or papillae) present on a hydrophobic surface Generally, two types of wetting are usually considered for rough surfaces, the drop suspended on the papillae (Cassie-Baxter) or resting on both the papillae and surface between protrusions (Wenzel case). Similar microstructures can be artificially produced using a number of processing techniques, including physical vapor deposition and physical wet chemistry. These techniques can control the papillae size, particle size distribution as well as the papillae shape. These parameters can also be altered, in conjunction with the molecular properties of the surface or coating matrix material, to produce other physical properties (transparency over a certain wavelength band, strength, stiffness, coefficient of expansion) to meet specific application requirements. Recently there has been great interest in the development of superhydrophobic surfaces for a wide variety of applications, such as minimizing corrosion. Some methods for the production of superhydrophobic surfaces have been based on the formation of densely packed high-aspect-ratio structures such as polymer nanofibers, aligned carbon nanotubes, Si pillars fabricated by photolithography and plasma etching, and Si nano-rod arrays fabricated by Si vapor deposition techniques. The surface morphology is, in general, very sensitive their mechanical properties and directly related to the type of fabrication process and materials used to create the superhydrophobic surface. We have investigated superhydrophobic surfaces comprised of randomly distributed roughness versus those produced via ordered deposition of particles or microposts. We have modeled the contact angle, the equilibrium angle of contact of a liquid on a rigid surface where liquid, solid and gas phases meet, in order to determine the advantages of ordered versus random superhydrophobic surface roughness and the correlation to superior surface mechanical properties as a function of coating fabrication approach.

Tailoring of Hydrophilic to Hydrophobic Coating Properties for Space Exploration Contamination Control

On the lunar and Martian surface, dust contamination is a serious problem for equipment and vehicles since the lunar and Martian soils have fine texture compared to terrestrial dust particle size distributions. Dust contamination is a serious problem for equipment and vehicles for space mission applications and gathers on photonic sensors inhibiting motion and data gathering. Photonic devices that require transparency to light for maximum efficiency, such as solar photovoltaic power systems, video cameras and optical or infrared detectors, can be seriously affected by dust accumulation. The thermal and radiation environment also pose unique challenges because of its large temperature variations and its interaction with the local plasma environment and solar UV and X-rays induced photoemission of electrons. Superhydrophilic materials are composed of polar molecules and have been used to defog glass, enable oil spots to be swept away easily with water, as door mirrors for cars and coatings for buildings. Hydrophobic molecules tend to be non-polar and thus prefer other neutral molecules and nonpolar solvents. Hydrophobic molecules often cluster together. Hydrophobic surfaces contain materials that are difficult to wet with liquids, with superhyrophobic surfaces having contact angles in excess of 150° (the equilibrium angle of contact of a liquid on a rigid surface where liquid, solid and gas phases meet). This paper presents an overview of the fundamental forces (van der Waals) which allows certain contamination to adhere to critical photonic surfaces and the various passive coatings phenomenology (hydrophilic to hydrophobic) that is used to minimize this contamination. Gamma and proton radiation testing of these coatings demonstrate their molecular, contact angle and neutralization property resistance to space radiation.