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Materials with superior strength, density and resiliency properties are important for the harsh environments in which Department of Defense platforms, weapons and their components operate. Recent scientific advances have opened up new possibilities for material design in the ultrahigh pressure regime (up to three million times higher than atmospheric pressure). Materials formed under ultrahigh pressure, known as extended solids, exhibit dramatic changes in physical, mechanical and functional properties and may offer significant improvements to armor, electronics, propulsion and munitions systems in any aerospace, ground or naval platform.
Despite the dramatic performance improvements—both demonstrated and predicted—for extended solids, the ultrahigh pressures currently required for synthesis and stabilization of such materials prevent scalability for any practical use. DARPA created the Extended Solids (XSolids) program to address the key challenges in synthesis and scale-up necessary for manufacture, through both computational and experimental approaches, with the intention of opening a vast new material design space for the DoD.
Interdisciplinary research teams are working to develop multi-step, barochemical processes that can reduce the peak pressure needed to achieve scalable synthesis of target materials. Performers are working in parallel on computational exploration of high-pressure material structures and properties, and the small-scale synthesis of a variety of materials to experimentally verify their properties.
Potential DoD applications of extended solids are pervasive. If the program is successful, it will create breakthrough improvements in properties such as strength, stiffness, energy content, thermal conductivity, electromagnetic and optical properties, with associated performance improvements in a wide range of defense applications.
Understanding of condensed matter phenomena at extreme conditions offers unique opportunities for designing
and synthesizing novel materials. These materials, both stable and metastable ones, possess with optimized
properties over a wide range of length scales — far beyond those achieved by other methods such as varying
temperatures and chemical composition. Among novel phenomena uniquely occurring at high pressures is the
pressure-induced electron delocalization from a simple molecular solid to a non-molecular extended solid such as a
metal and a three-dimensional (3D) network polymer (e.g., diamond).
DARPA’s material initiatives
DARPA has different approaches to create new materials through it’s Materials with Controlled Microstructural Architecture (MCMA) program. This initiative aims to control architectures of the material microstructres to enhance structural efficiency and imbue characteristics that cannot be achieved in single substances, like steel strength and weight of plastic. MCMA also aims to incorporate important properties like high heat diffusion for thermal management and the ability to tailor thermal expansion to enable fusion of conventionally incompatible materials.
DSO’s Materials for Transduction (MATRIX) program intends to realize the beneficial properties of novel materials at the system or device level, by developing materials for transduction – conversion of energy from one form to another. MATRIX hopes to make a difference by boosting the development of new capabilities and enabling weight and size reductions of military systems.
DSO is aiming at a different class of materials which can only be made at ultrahigh pressures with their Extended Solids (Xsolids) program. The materials subjected to high pressure display drastic improvements in their physical, mechanical and functional characteristics. These ‘polymorphs’ may improve performances in semiconductor electronics, propulsion, structural application, from aerospace to ground vehicles.
A2P, MATRIX and Xsolids address various challenges of scaling innovation to higher dimensions. Additionally DSO’s Local Control of Material Synthesis (LoCo) seeks to bring advances in thin-film material and surface coatings, used in military applications ranging from advanced electronics to optics. LoCo program researchers are coming up with new strategies and tools toward ordered material deposition at near room temperatures.
“All of these programs reflect a fundamental shift,” said Stefanie Tompkins DSO Director, “from bulk-process to architected materials – a shift we believe has the potential to introduce a new ‘Designer Age’ of materials development.”
DSO’s Extended Solids (XSolids) program
The Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) is requesting information on scalable techniques for the synthesis of extended solid materials characterized by extensive covalent bond networks. Extended solid materials include polymorphs and/or phases of metals, intermetallics, oxides, nitrides, and carbides.
Xsolids program takes aim at a different class of materials—those that currently can be made and exist only at ultrahigh pressures up to millions of times atmospheric pressure. Materials formed under ultrahigh pressure, known as extended solids, exhibit dramatic changes in physical, mechanical and functional properties and may offer significant improvements to armor, electronics, propulsion and munitions systems in any aerospace, ground or naval platform.
These new “polymorphs” may provide significant performance enhancements in areas as diverse as semiconductor electronics and propulsion, and in structural applications ranging from aerospace to ground vehicles. “The discovery and fabrication of new materials has long been based on the application of heat,” said Goldwasser. “The development of high-pressure chemistry—or barochemistry—could open up a new era in materials discovery and development featuring an entirely new palette of materials for exploitation.”
Early work already hints at unique materials and properties that may emerge when everyday gasses such as carbon dioxide as well as silicon- and carbon-based solids are compressed under extreme conditions, Goldwasser noted. But because their synthesis and stabilization is so demanding, production of these materials for practical use has proven difficult. So in addition to materials discovery, XSolids is researching processing techniques to make their fabrication practical.
The DARPA Extended Solids (XSolids) program has identified a number of materials with exceptional properties that are stable at ambient temperatures after the synthesis pressure has been released. For example, tough B 4 C ( J. Mater. Chem. C, 2015, 3, 11705; Chem. Mater., 2015, 27, 2855; J. Phys. Chem. Lett., 2014, 5, 4169) and a direct-bandgap silicon polymorph Si 24 ( Nature Materials, 2015, 14, 169) were recently reported. These materials currently require high pressures (>1 GPa) for fabrication. Such high-pressure conditions can only be achieved in diamond anvil presses that, even at large scale, produce only extremely small quantities (<1 mg) of material over the course of several hours.
The XSolids program has pioneered the use of metastable synthetic intermediates, but even these approaches ultimately require extreme temperatures and pressures that are intrinsically not scalable to continuous or large-scale batch production. Broadly, scalable production is only possible if the extended covalent bond networks characteristic of extended solids can be obtained using processes that are accessible at near or below atmospheric pressure and at temperatures below 1000° C. Scalable production technologies for the synthesis of extended solids are needed to make practical applications possible.
If the program is successful, it will create breakthrough improvements in properties such as strength, stiffness, energy content, thermal conductivity, electromagnetic and optical properties, with associated performance improvements in a wide range of defense applications.
