Kenneth H. Sandhage

High-strength alloys are intimately connected to human development, from the bronze age to the current applications in aerospace and energy. State-of-the-art alloys are engineered to harness strengthening mechanisms across scales, from crystal-level processes to complex hierarchical microstructures that are designed to hinder the mobility of dislocations and other carriers of plasticity. In this context, complex concentrated alloys (CCAs) are attractive since they can exhibit very high strength at the single-phase level, which can be further enhanced via incorporation of the second phase and microstructural optimization. Unfortunately, the optimization of CCAs is notoriously difficult due to the high dimensionality of the design space. We demonstrate that a combination of physics-based modeling, machine learning, experimental fabrication, and multi-resolution characterization results in the discovery of the hardest Al-containing BCC-based alloy, surpassing the current state of the art by 31%. Importantly, this is accomplished with only 24 experiments within a design space consisting of 67 536 possible candidates. The approach can be generalized to other alloys, and the resulting materials are of interest in applications ranging from aerospace to nuclear power.

Three‐dimensional macroporous titanium–aluminum–vanadium (TiAl6V4) implants produced by additive manufacturing (AM) can be grit blasted (GB) to yield microtextured exterior surfaces, with additional micro/nano‐scale surface features provided by subsequent acid etching (AE). However, the line‐of‐sight nature of GB causes the topography of exterior GB + AE‐modified surfaces to differ from internal GB‐inaccessible surfaces. Previous in vitro studies using dense TiAl6V4 substrates indicated that a nonline‐of‐sight, calciothermic‐reaction (CaR)‐based process provided homogeneous osteogenic nanotextures on GB + AE surfaces, suggesting it could be used to achieve a homogeneous nanotopography on external and internal surfaces of macroporous AM constructs. Macroporous TiAl6V4 (3D) constructs were produced by direct laser melting and modified by GB + AE, with the CaR process then applied to 50% of constructs (3DCaR). The CaR process yielded nanoporous/nanorough internal surfaces throughout the macroporous constructs. Skeletally mature, male Sprague–Dawley rats were implanted with these constructs using a cranial on‐lay model. Prior to implantation, a Cu++‐free click hydrogel was applied to half of the constructs (3D + H, 3DCaR + H) to act as a challenge to osseointegration. Osseointegration was compared between the four implant groups (3D, 3DCaR, 3D + H, 3DCaR + H) at 4w. 3D + H implants exhibited lower bone volume (BV) and percent bone ingrowth (%BI) than the 3D implants. In contrast, osseointegrated 3DCaR + H implants had similar BV and %BI as the 3DCaR implants. Implant pull‐off forces correlated with these results. In vitro analyses indicated that human bone marrow stromal cells (MSCs) exhibited enhanced production of osteoblast differentiation markers and factors associated with osteogenesis when grown on CaR‐modified 3D substrates relative to control (TCPS) substrates. This work confirms that the CaR process provides osteogenic nanotextures on internal surfaces of macroporous 3D implants, and suggests that CaR‐modified surfaces can promote osseointegration in cases where osteogenesis is impaired.

An aging global population is accelerating the need for better, longer-lasting orthopaedic and dental implants. Additive manufacturing can provide patient-specific, titanium-alloy-based implants with tailored, three-dimensional, bone-like architecture. Studies using two-dimensional substrates have demonstrated that osteoblastic differentiation of bone marrow stromal cells (MSCs) is enhanced on surfaces possessing hierarchical macro/micro/nano-scale roughness that mimics the topography of osteoclast resorption pits on the bone surface. Conventional machined implants with these surfaces exhibit successful osseointegration, but the complex architectures produced by 3D printing make consistent nanoscale surface texturing difficult to achieve, and current line-of-sight methods used to roughen titanium alloy surfaces cannot reach all internal surfaces. Here, we demonstrate a new, non-line-of-sight, gas/solid-reaction-based process capable of generating well-controlled nanotopographies on all open (gas-exposed) surfaces of titanium alloy implants. Dense 3D-printed titanium-aluminum-vanadium (TiAl6V4) substrates were used to evaluate the evolution of surface nanostructure for development of this process. Substrates were either polished to be smooth (for easier evaluation of surface nanostructure evolution) or grit-blasted and acid-etched to present a microrough biomimetic topography. An ultrathin (90 ± 16 nm) conformal, titania-based surface layer was first formed by thermal oxidation (600 °C, 6 h, air). A calciothermic reduction (CaR) reaction (700 °C, 1 h) was then used to convert the surface titania (TiO₂) into thin layers of calcia (CaO, 77 ± 16 nm) and titanium (Ti, 51 ± 20 nm). Selective dissolution of the CaO layer (3 M acetic acid, 40 min) then yielded a thin nanoporous/nanorough Ti-based surface layer. The changes in surface nanostructure/chemistry after each step were confirmed by scanning and transmission electron microscopies with energy-dispersive X-ray analysis, X-ray diffraction, selected area electron diffraction, atomic force microscopy, and mass change analyses. In vitro studies indicated that human MSCs on CaR-modified microrough surfaces exhibited increased protein expression associated with osteoblast differentiation and promoted osteogenesis compared to unmodified microrough surfaces (increases of 387% in osteopontin, 210% in osteocalcin, 282% in bone morphogenic protein 2, 150% in bone morphogenic protein 4, 265% in osteoprotegerin, and 191% in vascular endothelial growth factor). This work suggests that this CaR-based technique can provide biomimetic topography on all biologically facing surfaces of complex, porous, additively manufactured TiAl6V4 implants.

In an effort to lower the levelized cost of electricity generated by Concentrated Solar Power (CSP) plants, the heat-to-electricity conversion efficiency may be increased by operating such plants with higher turbine inlet temperatures using closed Brayton cycles with high-pressure, supercritical carbon dioxide (sCO2) as the working fluid. For example, if the turbine inlet temperature is raised to ≥750ᵒC, in combination with sCO2 closed Brayton cycles, the relative heat-to-electricity efficiency may be raised by more than 20% (compared to subcritical steam Rankine cycles operating with turbine inlet temperatures 20 MPa) decline rapidly at temperatures ≥550ᵒC. This project has been focused on demonstrating the desired high-temperature properties and attractive manufacturing characteristics of mechanically-, thermally-, and chemically-robustmore » ceramic/metal composites (cermets), in order to allow for the use of such composites in primary PCHEXs for heat transfer to sCO2 at ≥750ᵒC and >20 MPa in CSP plants. Prior work with high-temperature, co-continuous carbide/refractory metal cermets, such as ZrC/W composites, has already demonstrated: i) the attractive combination of high-temperature properties exhibited by such materials (e.g., high values of stiffness, failure strength, and thermal conductivity at 800 C), and ii) cost-effective processes for manufacturing PCHEX plates comprised of such cermets with tailorable channel patterns. However, ZrC/W cermets were not found to be inherently corrosion resistant in sCO2 at 750ᵒC and 20 MPa. While the application of a Cu coating to ZrC/W surfaces, along with a modest addition (50 ppm) of CO to CO2, rendered such composites resistant to corrosion for 1000 h in such sCO2-based fluids at 750ᵒC, such additional steps would increase the design complexity of such cermet-based components. An alternative approach (the focus of this project) is to develop mechanically-robust, cost- effective cermets that are inherently resistant to high-temperature oxidation in air and CO2. The overall goals of this project have been to demonstrate: i) the oxidation resistance of at least one cermet at 750ᵒC in air and in CO2 (i.e., with a projected annual corrosion 200 MPa), and iii) that the selected cermet can be manufactured via low-cost forming techniques. Key accomplishments include demonstrating: • the inherent oxidation resistances of Al2O3/Cr and SiC/Si cermets at 750ᵒC (with projected annual oxide scale thicknesses in air and CO2 1550ᵒC « less

A dramatic reduction in man-made CO2 emissions could be achieved if the cost of electricity generated from concentrated solar power (CSP) plants could become competitive with fossil-fuel-derived electricity. The solar heat-to-electricity conversion efficiency of CSP plants may be significantly increased (and the associated electricity cost decreased) by operating CSP turbines with inlet temperatures >750 C instead of 750 C to allow for rapidly dispatchable and/or continuous electricity production. Unfortunately, earth-abundant MgCl2-KCl-based liquids currently being considered as low-cost media for large-scale, high-temperature TES are susceptible to oxidation in air, with associated undesired changes in liquid composition and enhanced corrosion of metal alloys in pipes and tanks containing such liquids. In this paper, alternative high-temperature, earth-abundant molten chlorides that are stable in air are identified via thermodynamic calculations. The oxidation resistance, and corrosion-resistant containment, of such molten chlorides at 750 C are then demonstrated. Such chemically-robust, low-cost TES media and effective containment provide critical advances towards the higher-temperature operation of, and lower-cost electricity generation from, CSP plants.

The efficiency of generating electricity from heat using concentrated solar power plants (which use mirrors or lenses to concentrate sunlight in order to drive heat engines, usually involving turbines) may be appreciably increased by operating with higher turbine inlet temperatures, but this would require improved heat exchanger materials. By operating turbines with inlet temperatures above 1,023 kelvin using closed-cycle high-pressure supercritical carbon dioxide (sCO₂) recompression cycles, instead of using conventional (such as subcritical steam Rankine) cycles with inlet temperatures below 823 kelvin^1,2,3, the relative heat-to-electricity conversion efficiency may be increased by more than 20 per cent. The resulting reduction in the cost of dispatchable electricity from concentrated solar power plants (coupled with thermal energy storage^4,5,6) would be an important step towards direct competition with fossil-fuel-based plants and a large reduction in greenhouse gas emissions⁷. However, the inlet temperatures of closed-cycle high-pressure sCO₂ turbine systems are limited⁸ by the thermomechanical performance of the compact, metal-alloy-based, printed-circuit-type heat exchangers used to transfer heat to sCO₂. Here we present a robust composite of ceramic (zirconium carbide, ZrC) and the refractory metal tungsten (W) for use in printed-circuit-type heat exchangers at temperatures above 1,023 kelvin⁹. This composite has attractive high-temperature thermal, mechanical and chemical properties and can be processed in a cost-effective manner. We fabricated ZrC/W-based heat exchanger plates with tunable channel patterns by the shape-and-size-preserving chemical conversion of porous tungsten carbide plates. The dense ZrC/W-based composites exhibited failure strengths of over 350 megapascals at 1,073 kelvin, and thermal conductivity values two to three times greater than those of iron- or nickel-based alloys at this temperature. Corrosion resistance to sCO₂ at 1,023 kelvin and 20 megapascals was achieved¹⁰ by bonding a copper layer to the composite surface and adding 50 parts per million carbon monoxide to sCO₂. Techno-economic analyses indicate that ZrC/W-based heat exchangers can strongly outperform nickel-superalloy-based printed-circuit heat exchangers at lower cost.

Cellulose nanocrystals (CNCs) can be attractive templates for the generation of functional inorganic/organic nanoparticles, given their fine sizes, aspect ratios, and sustainable worldwide availability in abundant quantities. Here we present for the first time a scalable, surfactant-free, tailorable wet chemical process for converting commercially-available CNCs into individual aspected gold nanoshell-bearing particles with tunable surface plasmon resonance bands. Using a rational cellulose functionalization approach, stable suspensions of positively-charged CNCs have been generated. Continuous, conductive, nanocrystalline gold coatings were then applied to the individual, electrostatically-stabilized CNCs via decoration with 1-3 nm diameter gold particles followed by electroless gold deposition. Optical analyses indicated that these core-shell nanoparticles exhibited two surface plasmon absorbance bands, with one located in the visible range (near 550 nm) and the other at near infrared wavelengths. The near IR band possessed a peak maximum wavelength that could be tuned over a wide range (1000-1300 nm) by adjusting the gold coating thickness. The bandwidth and wavelength of the peak maximum of the near IR band were also sensitive to the particle size distribution and could be further refined by fractionation using viscosity gradient centrifugation.