Inside this Issue

• Near-Future Ocean Modeling
• Art in Science
• Designing New Materials
• Summer Interns
• ARSC Cadet Program
• Expanding our Horizons
• Cray SX-6
• Modeling the Arctic Ice
• ARSC Training Workshops

Designing New Materials for the Future

Story by Leone Thierman

The discovery, design and development of new materials are of high priority interest for use in propulsion systems and vehicles of the U.S. Air Force space program. High-energy density compounds for use as rocket propellants, polyhedral oligomeric silsesquioxanes (POSS) compounds that are highly resistant to extreme environments, and porphyrin- and phthalocyanine-based non-linear optical (NLO) absorbers used as protection against optical threats are all being explored.

Computer modeling and simulation play key roles in the design of these novel materials. Computational chemistry is used along with theoretical chemistry to predict promising energetic systems, assess their stability and guide efficient synthesis of selected candidates.

The calculations required to determine the structures for such large and complex compounds are made possible using scalable quantum chemistry codes such as GAMESS (General Atomic and Molecular Electronic Structure System). Computational resources required for these calculations are provided by the Department of Defense High Performance Computing Modernization Program (DoD HPCMP) through a competitive, peer-reviewed process to select DoD Challenge projects. Supercomputers at the Arctic Region Supercomputing Center (ARSC) help provide the necessary computational resources needed to support these projects, including the resources to identify, design and model these new complex chemical components for the Air Force Research Laboratory (AFRL).

Advanced Rocket Propellants

One of the ways to make space vehicles lighter, safer, more reliable and better performing is to design advanced chemical propellants for use across the entire range of propulsion requirements. These new chemical species have significantly higher performance limits than conventional or near-term advanced propellants. Some candidate propellants include liquid hydrocarbons for heavy-lift booster rockets, cryogenic propellants for upper stages, liquid and solid oxidizers for boost and upper stages, and monopropellants for upper stages and satellite propulsion.

Dr. Jerry Boatz, a computational chemist at AFRL, is working with AFRL’s Propulsion Directorate High Energy Density Matter (HEDM) team to develop “next generation” rocket propellant systems aimed at doubling the payload capacity that can currently be put into orbit. The AFRL HEDM team uses a combination of experimental, theoretical and computational techniques to identify, synthesize, and characterize new chemical propellants.

After Boatz and the team identify potential new HEDM materials based on theoretical computations, the more promising candidates are made in large quantities for lab-scale testing that may ultimately lead to a recommendation for transition to the aerospace industry for further evaluation.

When a new chemical compound is in the development stage, its potential as a high-energy propellant ingredient is balanced against its reactivity, stability and compatibility with other materials. Propellant performance is measured by the Isp (specific impulse) or thrust delivered per mass flow rate of the propellant. The term Isp for a rocket is used in much the same way as the term “miles per gallon” is used for measuring the performance of a car. For a rocket, the higher the Isp, the better the performance. Developing stable propellants with high-energy densities and a high Isp is necessary to maximize the amount of payload carried by a particular vehicle, and to minimize the amount of weight devoted to dry mass, such as fuel tanks.

One of the future directions in HEDM research includes exploration of the use of energetic ionic liquids (ILs) that can be used in place of the more carcinogenic hydrazine, a proven fuel currently used in many satellite propulsion systems. One of the significant drawbacks of hydrazine is its extreme toxicity, which requires the use of self-contained suits for workers handling it. The new materials design team is also analyzing which chemical substituents lead to low melting points, increased heats of formation and the thermal stability of ILs.

Hybrid Polymers

The Air Force also has strong interest in a new class of compounds called Polyhedral Oligomeric Silsesquioxanes (POSS), which show great promise as additives that can significantly improve the thermal and physical properties of many plastics. POSS compounds are highly resistant to extreme environments and therefore hold great promise as lubricants and protective coatings for space vehicles. Since little is known experimentally about the mechanisms by which POSS compounds form, Boatz and the AFRL team, in collaboration with Professors Mark Gordon of Iowa State University and Sharon Hammes-Schiffer of Pennsylvania State University, have embarked on a long-term DoD Challenge project to develop rational design approaches for the synthesis of new POSS compounds. Their approach uses Gaussian and molecular dynamics codes requiring high-end computers to predict how POSS compounds form and to examine the factors that affect the reaction mechanisms and product distributions during POSS synthesis.

Compared to common fire retardant plastics, polymers containing POSS compounds show increased strength, delayed combustion and increased heat resistance. The use of the lightweight POSS additives is more beneficial and often eliminates the need to use common fillers such as silica. Depending on loading level, bulk density reductions of up to 10 percent have been observed with viscosity reductions of up to 24 percent relative to silica.

Polymers made with POSS components form a ceramic shell that can withstand radiation bombardment at least 10 times longer than other materials. Because of its chemical nature, POSS technology is easily incorporated into common plastics via co-polymerization, or blending, and therefore requires little or no alteration to existing manufacturing processes.

Because the reactions leading to the synthesis of POSS generally involve multiple proton transfer reactions, it is important to incorporate the quantum effects (such as tunneling) associated with this type of chemical reaction. The nuclear electronic orbital (NEO) method, which captures the quantum effects of nuclei (protons in particular), is being developed and is now implemented in GAMESS, so that the mechanisms of formation of POSS and the corresponding rates of reaction may be more fully understood.

The hybrid properties of POSS offer a variety of opportunity for use. As additives they can be used in heat- and abrasion-resistant paints and coatings, fire retardants, thermal modifiers and cross-linking agents. As next-generation plastics, they can be used for medical materials, space-resistant resins, packaging, electronic materials and optical plastics. And, as pre-ceramics they can be used for heat shields, nozzles and insulations, electronic materials and bonded metals.

Nonlinear Optical Materials

Nonlinear optical (NLO) devices used to mitigate optical threats, or for optical information processing, communications and data storage, are made of materials that allow light waves to be manipulated as light passes through them, much as transistors are the basis for signal processing in electronics.

Dr. Ruth Pachter of the AFRL Materials and Manufacturing Directorate, in collaboration with Boatz and other members of the New Materials Design team, is investigating NLO crystals, which are used to convert light from well-established lasers to light with a longer or shorter wavelength. This program primarily focuses on mid- and far-infrared regions, since a main concern for the Air Force is to enable infrared countermeasures for the protection of aircraft and laser radar systems used for the remote detection of chemical and biological agents.

Current work has shown that time-dependent density functional theory accurately predicts nonlinear absorbing materials in free-base porphyrins, phthalocynanines and their metal complexes with a mean absolute error of 0.11 eV for computed triplet-triplet excitation energies. Preliminary results of structure and energetics for extended systems are likewise encouraging.

Impact on the Future

The investment of time and effort in new materials research has paid off, as POSS technology is the only hybrid and nanostructured chemical feedstock technology developed to-date. The development and large-scale production of the first new polymer feedstocks in the past forty years has emerged from the laboratory and is now being offered by R&D chemical distributors.