Publications

A structurally persistent bis-amido ligand framework capable of supporting nickel compounds in three different oxidation states has been identified. A highly unusual, isolable Ni(III) alkyl species has been prepared and characterized via a rare example of a two-electron oxidative addition of MeI to Ni(I).

The aging-related deterioration of the fracture properties of bone, coupled with higher life expectancy, is responsible for increasing incidence of bone fracture in the elderly; consequently, an understanding of how these fracture properties degrade with age is essential. In this study, ex vivo fracture experiments have been performed to quantitatively assess the effect of age on human cortical bone in the proximal–distal orientation, i.e., longitudinally along the osteons. Because cortical bone exhibits rising crack-growth resistance with crack extension, the toughness is evaluated in terms of resistance-curve (R-curve) behavior, measured for bone taken from wide range of age groups (34–99 years). Using this approach, both the crack-initiation and crack-growth toughness are determined and are found to deteriorate with age; the initiation toughness decreases some 40% over six decades from 40 to 100 years, while the growth toughness is effectively eliminated over the same age range. The reduction in crack-growth toughness is considered to be associated primarily with a degradation in the degree of extrinsic toughening, in particular, involving crack bridging in the wake of the crack. An examination of the micro-/nano-structural changes accompanying the process of aging, using optical microscopy, X-ray tomography, nanoindentation and Raman spectroscopy, is shown to support such observations.

The electrochemical reduction of CO 2 (CO 2 R) to value-added products is an attractive technology for tackling the rising atmospheric CO 2 levels and storing intermittent renewable energy into chemical bonds. Fundamental understanding of CO 2 R has progressed significantly in recent years and is critical in the development of CO 2 R to liquid-fuel electrolyzers, where gas-diffusion electrodes (GDEs) have been shown to be key enabling architectures. Various designs have been proposed and studied in the literature to enhance overall selectivity, rates, and maximize the conversion of CO 2 , the latter of which is only now being recognized as a critical issue. In this respect, there is a need to explore the governing phenomena inherent in these architectures to enable optimization. Mathematical modeling is ideally suited to tackle and explore these multiphysics interactions and provide virtual design analysis. In this talk, we discuss modeling methodologies and physics inherent in these devices and present our recent modeling of GDEs for CO 2 reduction. We specifically examine the impacts of multiphase flow and related phenomena on overall cell performance. We then explore the performance and limitations of various cell designs guided by simulation results and examine potential methods for improving water management and tuning catalyst selectivity including the use of different anion-exchange and bipolar membranes. Finally, we discuss the disparities in local environments between aqueous and GDE devices and propose strategies to reduce the gap in knowledge between the two systems. Acknowledgements This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

Deinococcus radiodurans, a nonpathogenic prokaryote able to withstand high doses of ionizing radiation, is being engineered for bioremediation and biostabilization of heavy metals and actinides. An actinide bioprecipitation system for D. radiodurans capable of polyphosphate accumulation, inducible degradation and secretion, and UO22+ precipitation, is being developed.The radioresistance of D. radiodurans to ionizing gamma radiation has been well studied, yet is not well understood. Several DNA repair mechanisms have been discovered, yet survivability studies are typically done under nutrient-rich conditions at a single growth stage. Although a minimal irradiation medium has been described, the effect of different variables on survivability in a nutrient-starved environment has not been systematically studied. Results indicate that there is a threshold below which the radiation dose is not sufficient to overcome the cells' passive defense mechanisms, such as radical scavenging and constitutive repair proteins. Additional radiation survivability is then from active resistance induced by cell damage, possibly related to aging. The effects of light-ion irradiation in aqueous suspension have also been studied using the 88" cyclotron at the Lawrence Berkeley National Laboratory and increasing lethality corresponding to increasing linear energy transfer (LET) values of the radiation is seen. Previous studies only examined heavy ion effects on cells supported on solid medium.To better understand the sorption chemistry of D. radiodurans, the interaction of strain R1 with UO22+ has been studied. The Gram-negative R1 uranyl sorption load is an order of magnitude less than that of non-engineered Gram-positive Bacillus sphaericus under similar conditions, and more than two orders of magnitude less than a polyphosphate accumulation engineered strain of Gram-negative Pseudomonas aeruginosa. Chemical studies of the cell-uranyl binding strength and pH sorption edges support spectroscopic data indicating that a carboxyl surface group, consistent with known characteristics of D. radiodurans' S-layer, interacts with and binds the uranyl. A strain engineered with the putative polyphosphate accumulation genes shows promise for use in applications of uranyl bioprecipitation and its efficacy and contrast to the non-engineered strain will be discussed.

We present observations of SN 2022joj, a peculiar Type Ia supernova (SN Ia) discovered by the Zwicky Transient Facility (ZTF). SN 2022joj exhibits an unusually red $g_\mathrm{ZTF}-r_\mathrm{ZTF}$ color at early times and a rapid blueward evolution afterward. Around maximum brightness, SN 2022joj shows a high luminosity ($M_{g_\mathrm{ZTF},\mathrm{max}}\simeq-19.7$ mag), a blue broadband color ($g_\mathrm{ZTF}-r_\mathrm{ZTF}\simeq-0.2$ mag), and shallow Si II absorption lines, consistent with those of overluminous, SN 1991T-like events. The maximum-light spectrum also shows prominent absorption around 4200 Å, which resembles the Ti II features in subluminous, SN 1991bg-like events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red ultraviolet minus optical colors at maximum luminosity ($u-v\simeq0.6$ mag and $uvw1 - v\simeq2.5$ mag), suggesting a suppression of flux at $\sim$2500--4000 Å. Strong C II lines are also detected at peak. We show that these unusual spectroscopic properties are broadly consistent with the helium-shell double detonation of a sub-Chandrasekhar mass ($M\simeq1 \mathrm{M_\odot}$) carbon/oxygen (C/O) white dwarf (WD) from a relatively massive helium shell ($M_s\simeq0.04$--$0.1 \mathrm{M_\odot}$), if observed along a line of sight roughly opposite to where the shell initially detonates. None of the existing models could quantitatively explain all the peculiarities observed in SN 2022joj. The low flux ratio of [Ni II] $λ$7378 to [Fe II] $λ$7155 emission in the late-time nebular spectra indicates a low yield of stable Ni isotopes, favoring a sub-Chandrasekhar mass progenitor. The significant blueshift measured in the [Fe II] $λ$7155 line is also consistent with an asymmetric chemical distribution in the ejecta, as is predicted in double-detonation models.

Bone fracture is a health concern for those with aged bone and brittle bone diseases. Mouse bone is widely used as a model of human bone, especially to investigate preclinical treatment strategies. However, little is known about the mechanisms of mouse bone fracture and its similarities and differences from fracture in human bone. In this work we present a methodology to investigate the fracture toughness during crack initiation and crack propagation for mouse bone. Mouse femora were dissected, polished on their periosteal surface, notched on the posterior surface at their mid-diaphysis, and tested in three-point bending under displacement control at a rate of 0.1mm/min using an in situ loading stage within an environmental scanning electron microscope. We obtained high-resolution real-time imaging of the crack initiation and propagation in mouse bone. From the images we can measure the crack extension at each step of the crack growth and calculate the toughness of the bone (in terms of stress intensity factor (K) and work to fracture (W f )) as a function of stable crack length (Δa), thus generating a resistance curve for the mouse bone. The technique presented here provides insight into the evolution of microdamage and the toughening mechanisms that resist crack propagation, which are essential for preclinical development of treatments to enhance bone quality and combat fracture risk.