Search NASA⌕ Search

SEARCH · Search NASA

Results for “dissolution mechanism”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Pinned Electrode/Electrolyte Interphase and Its Formation Origin for Sulfurized Polyacrylonitrile Cathode in Stable Lithium Batteries

Sulfurized polyacrylonitrile (SPAN) represents one of the most promising directions for high-energy-density lithium (Li)-sulfur batteries. However, the practical application of Li||SPAN is currently limited by the insufficient chemical/electrochemical stability of electrode/electrolyte interphase (EEI). Here, a pinned EEI layer is designed for stabilizing a SPAN cathode by regulating the EEI formation mechanism in an advanced LiFSI/ether/fluorinated-ether electrolyte. Computational simulations and experimental investigations reveal that, benefiting from the nonsolvating nature, the fluorinated-ether can not only act as a protective shield to prevent the Li polysulfides dissolution but also, more importantly, endow a diffusion-controlled EEI formation process. It promotes the formation of a uniform, protective, and conductive EEI layer pinning into SPAN surface region, enabling the high loading Li||SPAN batteries with superior cycling stability, wide temperature performance, and high-rate capability. Finally, this design strategy opens an avenue for exploring advanced electrolytes for Li||SPAN batteries and guides the interface design for broad types of battery systems.

25 ENERGY STORAGE↗
DOE OSTI2023

Microkinetic insights into the role of catalyst and water activity on the nucleation, growth, and dissolution during COF-5 synthesis

The chemical pathway for synthesizing covalent organic frameworks (COFs) involves a complex medley of reaction sequences over a rippling energy landscape that cannot be adequately described using existing theories. Even with the development of state-of-the-art experimental and computational tools, identifying primary mechanisms of nucleation and growth of COFs remains elusive. Other than empirically, little is known about how the catalyst composition and water activity affect the kinetics of the reaction pathway. In this work, for the first time, we employ time-resolved in situ Fourier transform infrared spectroscopy (FT-IR) coupled with a six-parameter microkinetic model consisting of ~10 million reactions and over 20 000 species. The integrated approach elucidates previously unrecognized roles of catalyst pK a on COF yield and water on growth rate and size distribution. COF crystalline yield increases with decreasing pK a of the catalysts, whereas the effect of water is to reduce the growth rate of COF and broaden the size distribution. The microkinetic model reproduces the experimental data and quantitatively predicts the role of synthesis conditions such as temperature, catalyst, and precursor concentration on the nucleation and growth rates. Furthermore, the model also validates the second-order reaction mechanism of COF-5 and predicts the activation barriers for classical and non-classical growth of COF-5 crystals. The microkinetic model developed here is generalizable to different COFs and other multicomponent systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Flexible, transparent, and sustainable cellulose-based films for organic solar cell substrates

Cellulose, often considered a highly promising substitute for petroleum-based plastics, offers several compelling advantages, including abundant availability, cost-effectiveness, environmental friendliness, and biodegradability. However, its inherent highly crystalline structure and extensive hydrogen-bonded network pose challenges for processing and recycling. Here, in this study, we introduce the concept of cellulose vitrimers (CVs), wherein dynamic bonds are incorporated to reconfigure the hydrogen-bonded network, resulting in a mechanically robust, highly transparent material. CVs exhibit exceptional malleability, thermal stability, and noteworthy resistance to water and solvents. Due to the dynamic bond disassociation, CVs can be effectively chemically recycled using a well-established “dissolution-and-reforming” process. Moreover, CVs have proven successful as flexible substrate materials for organic solar cells, outperforming traditional petroleum-based polyethylene naphthalate (PEN). Given these advantages, CVs have the potential to replace conventional petroleum-based materials as recyclable and environmentally friendly alternatives, particularly within the realm of electronic devices and displays.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Methanogens acquire and bioaccumulate nickel during reductive dissolution of nickelian pyrite

ABSTRACT Nickel (Ni) is a key component of the active site metallocofactors of numerous enzymes required for methanogenesis, including [NiFe]-hydrogenase, carbon monoxide dehydrogenase, and methyl CoM reductase, leading to a high demand for Ni among methanogens. However, methanogens often inhabit euxinic environments that favor the sequestration of nickel as metal-sulfide minerals, such as nickelian pyrite [(Ni,Fe)S 2 ], that have low solubilities and that are not considered bioavailable. Recently, however, several different model methanogens ( Methanosarcina barkeri , Methanococcus voltae , Methanococcus maripaludis ) were shown to reductively dissolve pyrite (FeS 2 ) and to utilize dissolution products to meet iron and sulfur biosynthetic demands. Here, using M. barkeri Fusaro, and laboratory-synthesized (Ni,Fe)S 2 that was physically isolated from cells using dialysis membranes, we show that trace nickel (<20 nM) abiotically solubilized from the mineral can support methanogenesis and limited growth, roughly fivefold less than the minimum concentration known to support methanogenesis. Furthermore, when provided direct contact with (Ni,Fe)S 2 , M. barkeri promoted the reductive dissolution of (Ni,Fe)S 2 and assimilated solubilized nickel, iron, and sulfur as its sole source of these elements. Cells that reductively dissolved (Ni,Fe)S 2 bioaccumulated approximately fourfold more nickel than those grown with soluble nickel and sulfide but had similar metabolic coupling efficiencies. While the mechanism for Ni uptake in archaeal methanogens is not known, homologs of the bacterial Nik uptake system were shown to be ubiquitous across methanogen genomes. Collectively, these observations indicate that (Ni,Fe)S 2 is bioavailable in anoxic environments and that methanogens can convert this mineral into nickel-, iron-, and sulfur-containing metalloenzymes to support methanogenesis and growth. IMPORTANCE Nickel is an essential metal, and its availability has changed dramatically over Earth history due to shifts in the predominant type of volcanism in the late Archean that limited its availability and an increase in euxinic conditions in the early Proterozoic that favored its precipitation as nickel sulfide minerals. Observations presented herein indicate that the methanogen, Methanosarcina barkeri , can acquire nickel at low concentration (<20 nM) from soluble and mineral sources. Furthermore, M. barkeri was shown to actively reduce nickelian pyrite; use dissolution products to meet their iron, sulfur, and nickel demands; and bioaccumulate nickel. These data help to explain how M. barkeri (and possibly other methanogens and anaerobes) can acquire nickel in contemporary and past anoxic or euxinic environments.

(NiFe)S2↗
DOE OSTI2025

Insight Into Sulfur‐Containing Additive to Boost Anti‐Oxidation Ability of the Ether‐Based Electrolyte for Sodium‐Ion Full Batteries

Ether-based electrolytes are considered as promising candidates for sodium-ion batteries (SIBs) due to their high ionic conductivity and good compatibility with hard carbon (HC) anode. However, they suffer from poor anti-oxidative ability with unstable cathode electrolyte interface, inducing successive electrolyte decomposition and rapid capacity fading. Herein, the sulfur-containing additive (1,3-propanediol cyclic sulfate, PCS) is introduced to boost the stability of the electrode-electrolyte interface (EEI) in ether-based electrolyte. PCS largely participates in the inner Na + sheath, causing more diglyme (G2) molecules and fewer PF 6 - anions to occupy the inner Na + solvation sheath, avoiding the decomposition of G2 molecules at high operation voltage. Moreover, PCS is beneficial for simultaneously constructing thin and robust sulfur-containing EEI on both Prussian blue (PB) cathode and HC anode, maintaining the structural stability of PB and alleviating the dissolution of transition metals. These enable PB||HC pouch cells to deliver a capacity retention of 60.3% after 400 cycles, significantly higher than the 32.1% in PCS-free electrolytes. In conclusion, this work discloses the underlying mechanism of PCS to evoke the anti-oxidation ability of ether-based electrolyte and provides a promising method for realizing advanced sodium-ion full cells.

anti-oxidation ability↗
DOE OSTI2021

Liquid state properties of SEI components in dimethoxyethane

The solid-electrolyte interphase (SEI) layer is a critical constituent of battery technology, which incorporates the use of lithium metals. Since the formation of the SEI is difficult to avoid, the engineering and harnessing of the SEI are absolutely critical to advancing energy storage. One problem is that much fundamental information about SEI properties is lacking due to the difficulty in probing a chemically complex interfacial system. One such property that is currently unknown is the dissolution of the SEI. This process can have significant effects on the stability of the SEI, which is critical to battery performance but is difficult to probe experimentally. Here, we report the use of ab initio computational chemistry simulations to probe the solution state properties of SEI components LiF, Li 2 O, LiOH, and Li 2 CO 3 in order to study their dissolution and other solution-based characteristics. Ab initio molecular dynamics was used to study the solvation structures of the SEI with a combination of radial distribution functions, discrete solvation structure maps, and vibrational density of states, which allows for the determination of free energies. From the change in free energy of dissolution, we determined that LiOH is the most likely component to dissolve in the electrolyte followed by LiF, Li 2 CO 3 , and Li 2 O although none were favored thermodynamically. Furthermore, this indicates that dissolution is not probable, but Li 2 O would make the most stable SEI with regard to dissolution in the electrolyte.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Understanding the Degradation of La 1−x Sr x FeO 3−δ (0 ≤ x ≤ 1) Perovskite Oxides during the Oxygen Evolution Reaction in Alkaline Solution

Perovskite oxides are an emerging class of highly active catalysts for the oxygen evolution reaction (OER); however, their electrochemical stability remains poorly understood. Here, we report a systematic evaluation of the OER activity and stability of La 1−x Sr x FeO 3-δ perovskites in 1 M KOH. Their initial OER activity first increases with increasing Sr content (fromx= 0 to 0.8), and then decreases when the Sr content is increased to 1. Their stability evaluated by monitoring the element leaching from the electrodes show that La does not leach at a detectable rate, but Sr and Fe leach substantially. The leaching of Sr occurs at similar rates under open circuit potential (OCP) and OER potential, suggesting a nonelectrochemical dissolution process. The leaching of Fe is, however, strongly dependent on the electrode potential. More Fe leaching is observed under the OER potential than OCP. Additionally, the electrode with higher initial OER activity leaches more Fe. These results indicate that OER facilitates the dissolution of Fe from the electrode. The leaching of Fe, in turn, is considered responsible for the activity loss of La 1-x Sr x FeO 3−δ during OER. This study brings new insight into the degradation mechanism of La 1-x Sr x FeO 3−δ and their related perovskite oxides during electro-oxidation processes.

Electrochemistry↗
DOE OSTI2024

Impact of Electrolyte Additives on the Lifetime of High Voltage NMC Lithium-Ion Pouch Cells

This work involves improving the lifetime of lithium-ion cells during high voltage cycling using electrolyte additives. Three generations of electrolyte additives were investigated and screened in NMC442/graphite pouch cells using a 24 h voltage-hold protocol at 40 degrees C to accelerate oxidative reactions occurring at 4.4 V. Once promising additives and combinations were identified, they were then tested in cobalt-free NMC640/graphite cells for long-term cycling to upper cutoff voltages of 4.3, 4.4, and 4.5 V at temperatures of 20, 40, and 55 degrees C. Degradation mechanisms were probed using dV/dQ analysis, micro-X-ray fluorescence spectroscopy, and electrochemical impedance spectroscopy. The primary failure mode of cells held at high voltages is due to increase in cell impedance, which is correlated to the dissolution of transition metals, specifically manganese, originating from the positive electrode. We believe this dissolution is presumably due to the formation of a high impedance rock salt surface layer on the NMC positive electrode particles. Such deleterious outcomes can be limited by selecting an appropriate electrolyte additive package. It is hoped that this paper can provide a starting point for developing NMC Li-ion cells that can operate to voltages as high as 4.4 V and still display long lifetimes.

Azam, Saad↗
DOE OSTI2021

Isotopic fractionation as in-situ sensor of subsurface reactive flow and precursor for rock failure

Greater utilization of subsurface reservoirs perturbs in-situ chemical-mechanical conditions with wide ranging consequences from decreased performance to project failure. Understanding the chemical precursors to rock deformation is critical to reducing the risks of these activities. To address this need, we investigated the coupled flow-dissolution- precipitation-adsorption reactions involving calcite and environmentally-relevant solid phases. Experimentally, we quantified (1) stable isotope fractionation processes for strontium during calcite nucleation and growth, and during reactive fluid flow; (2) consolidation behavior of calcite assemblages in the common brines. Numerically, we quantified water weakening of calcite using molecular dynamics simulations; and quantified the impact of calcite dissolution rate on macroscopic fracturing using finite element models. With microfluidic experiments and modeling, we show the effect of local flow fields on the dissolution kinetics of calcite. Taken together across a wide range of scales and methods, our studies allow us to separate the effects of reaction, flow, and transport, on calcite fracturing and the evolution of strontium isotopic signatures in the reactive fluids.

42 ENGINEERING↗
DOE OSTI2026

Binder-jetted AISI M2 tool steel during hot isostatic pressing: Densification and carbide transformation

Binder jetting (BJ) enables fabrication of complex components from high-alloy steels such as AISI M2; however, residual porosity after sintering limits mechanical performance. This study investigates the coupled effects of binder chemistry, sintering conditions, cooling rate, and subsequent hot isostatic pressing (HIP) on densification, microstructure, and mechanical response in binder-jetted M2 tool steel. HIP increased relative density from ∼93 to 95% to >99% and improved compressive strength by ∼40-70%. Densification was governed by initial pore morphology, where closed porosity was effectively eliminated, while interconnected porosity limited full consolidation. Microstructural analysis (XRD, EBSD, SEM) shows that HIP promotes dissolution of metastable carbides and redistribution of alloying elements (W, Mo, V), transforming heterogeneous carbide networks into finer and more uniformly distributed M 2 C, MC, and M 6 C phases through diffusion-assisted homogenization. Among the investigated conditions, the FluidFuse binder combined with sintering at 1270 °C for 60 min and furnace cooling produced the most balanced response, achieving ∼99.7% density, ∼855 HV hardness, ∼4130 MPa compressive strength, and ∼24% strain. In contrast, higher sintering temperatures promoted carbide coarsening, reducing ductility despite high density. HIP reduces microstructural heterogeneity and drives the system toward a near-equilibrium state with reduced sensitivity to prior processing history. A comparative assessment with conventional and additive manufacturing routes (LPBF, DED, EBM, FFF) shows that the BJ-HIP approach achieves competitive densification and mechanical performance. These findings provide a mechanistic basis for controlling densification and microstructure in high-alloy steels processed via BJAM.

AISI M2 tool steel↗
DOE OSTI2023

Microstructure Evolution of a Multimodal Gamma-Prime Ni-Based Superalloy Characterized by In Situ Diffraction

The relationship between the evolution of microstructure, deformation micromechanisms and mechanical properties is difficult to establish in multimodal size distribution $_γ$$'$ superalloys, as the microstructure evolves with both temperature and time, and multiple strengthening mechanisms across each size distribution contribute to mechanical performance. In situ X-ray scattering can offer unparalleled insight regarding microstructure evolution at the temperatures and stresses of importance to gas-turbine applications; however, in situ X-ray diffraction has not been applied to the study of multimodal $_γ$$'$ distribution superalloys. Herein, lattice parameter evolution of secondary and tertiary $_γ$$'$ precipitates in a representative superalloy, Nimonic 115, is determined between 750 °C and 950 °C and correlated to room-temperature SEM and microhardness values. A large positive lattice parameter misfit of secondary $_γ$$'$ induces precipitate splitting, and the tertiary $_γ$$'$ goes into dissolution at ~ 800 °C, but with little apparent change in hardness values. The volume fraction of $_γ$$'$ decreases above 900 °C and precipitate-matrix coherency is lost, and there is a corresponding decrease in microhardness values. Importantly, the diffraction analysis demonstrates the capability to determine critical microstructural parameters of both precipitate size distributions in situ, representing an additional tool for determining microstructure-mechanical property relationships of multimodal superalloys.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Microstructural evolution and strengthening mechanisms in a heat-treated additively manufactured Al–Cu–Mn–Zr alloy

In this report the microstructural and strength evolution of an additively manufactured Al-8.6Cu-0.5Mn-0.9Zr alloy upon aging at 300, 350, and 400 °C is investigated. The strengthening phases of the alloy evolve significantly upon aging, with breakdown and spheroidization of the interconnected θ-Al 2 Cu network, dissolution of metastable θ'-Al 2 Cu precipitates, and precipitation of nanometric L1 2 -Al 3 Zr from a matrix supersaturated in Zr. In the peak-aged states, the alloy displays a favorable combination of strength and ductility, with a room-temperature yield strength of 314–341 MPa and ductility of 11–13%. The measured yield strengths for microstructures with different aging treatments are compared to predictions of yield strengths from grain boundary, solid solution, and particle strengthening contributions. The observed strain hardening behavior is related to fundamental precipitate and dislocation interactions. Comparison between predicted and measured strength values indicates a continued need for strengthening models specifically developed for the heterogeneous microstructures of additively manufactured alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Achieving Stable Molybdenum Oxide Cathodes for Aqueous Zinc-Ion Batteries in Water-in-Salt Electrolyte

A layered MoO 3 material with large interlayer spacing represents a promising cathode for aqueous rechargeable Zn-ion batteries (ARZIBs), but the implementation of this material is limited due to the intrinsically low conductivity and poor structural stability. A 30 m ZnCl 2 water-in-salt electrolyte (WISE) was introduced to a MoO 3 nanobelt cathode for the first time, significantly increasing the stability of MoO 3 cathodes compared to those in 3 M ZnSO 4 and 3 M ZnCl2 electrolyte. The Zn/MoO 3 cell in WISE unambiguously demonstrated significantly improved rate performance delivering 349, 253, and 222 mAh/g at 100, 500, and 1000 mA/g, denoting a 2×, and 12× capacity increase of those achieved in 3 M electrolytes at 500 and 1000 mA/g, respectively. A capacity retention rate of 73% was achieved after (dis)charging at 100 mA/g for 100 cycles, and no obvious capacity fading was observed at higher current densities of 500 mA/g and 2A/g. A compilation of structural and morphological characterization was systematically performed to provide insight into the mechanisms of the improved performance in ZnCl 2 WISE for the MoO3 cathode materials. Specifically, our data collectively suggested that the drastic fading in 3M electrolytes can be attributed to the parasitic surface deposits on Zn originated from Mo dissolution and H 2 formation due to Zn corrosion and hydrogen evolution reaction (HER), which were significantly suppressed in the ZnCl 2 WISE. The direct visualization of these side reactions was achieved for the first time in the Zn-MoO 3 system, using an in situ optoelectrochemical measurement.

25 ENERGY STORAGE↗
DOE OSTI2023

Coupled Lattice Boltzmann Modeling Framework for Pore-Scale Fluid Flow and Reactive Transport

In this paper, we propose a modeling framework for pore-scale fluid flow and reactive transport based on a coupled lattice Boltzmann model (LBM). We develop a modeling interface to integrate the LBM modeling code parallel lattice Boltzmann solver and the PHREEQC reaction solver using multiple flow and reaction cell mapping schemes. The major advantage of the proposed workflow is the high modeling flexibility obtained by coupling the geochemical model with the LBM fluid flow model. Consequently, the model is capable of executing one or more complex reactions within desired cells while preserving the high data communication efficiency between the two codes. Meanwhile, the developed mapping mechanism enables the flow, diffusion, and reactions in complex pore-scale geometries. We validate the coupled code in a series of benchmark numerical experiments, including 2D single-phase Poiseuille flow and diffusion, 2D reactive transport with calcite dissolution, as well as surface complexation reactions. The simulation results show good agreement with analytical solutions, experimental data, and multiple other simulation codes. In addition, we design an AI-based optimization workflow and implement it on the surface complexation model to enable increased capacity of the coupled modeling framework. Compared to the manual tuning results proposed in the literature, our workflow demonstrates fast and reliable model optimization results without incorporating pre-existing domain knowledge.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Degradation mechanism of blended cement pastes in sulfate-bearing environments under applied electric fields: Sulfate attack vs. decalcification

Applied electric fields, the reason behind stray currents, accelerate the ingress of sulfate into cementitious materials. To identify mitigation approaches, this study investigates the effects of slag and fly ash on sodium sulfate attacks of cement pastes under a constant electric current. The mineralogical alterations induced by the attacks were analyzed using X-ray diffractometry, thermogravimetric analysis, scanning electron microscopy, and thermodynamic modeling. The dissolution of aluminates in the slag and fly ash induced a monosulfate-rich area (>~20 mm from the cathode surface) to form next to the ettringite-rich area on the sample surface (<~20 mm). This effect reduced the availability of SO 4 2– in the pore solution, thereby hindering the penetration of sulfate. Meanwhile, the consumption of portlandite by the pozzolanic reaction lowered the decalcification resistance of the materials. This produced a wide area that endured the decomposition of portlandite and carbonate-AFm. Altogether, blending 30% fly ash did not improve the resistance of the material to either sulfate attacks or decalcification; blending 50% slag can effectively mitigate sulfate ingress, though decalcification may become a governing mechanism in the degradation process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Elucidating microstructural evolution and hardness variation across friction self-piercing riveted Al-7055 using synchrotron X-ray scattering and advanced microscopy techniques

Friction self-piercing riveting (FSPR) is a unique hybrid joining technique that combines the advantages of mechanical interlocking, frictional heat, and solid-state joining (if metallurgically compatible) to produce crack free joints in high strength and/or low-ductility alloys at room temperature. Here, in the current study, Al-7055 sheets were joined using FSPR for lightweight automotive applications and significant microhardness variations were observed across the joint cross-section. A detailed microstructural characterization at multiple length scales was carried out using advanced electron microscopy and X-ray scattering techniques to provide a fundamental understanding of the process-structure-property relationships. The relative contributions of microstructural characteristics at various length scales (i.e., grain size, dislocation density, solute concentration, precipitate nature) to strengthening were estimated using existent formulations (i.e., Hall-Petch, Taylor, precipitate bypass/shear equations) and correlated to the observed microhardness values across different regions. Small-angle X-ray scattering and scanning transmission electron microscopy revealed significant changes in the size and volume fraction of precipitate species, i.e., GP-I Zones, η′, and Mg/Zn solute co-clusters, depending on the process region. It was observed that the dissolution of the small η′/GP-I zones (T ∼ 150–200 °C) in the heat-affected zone were the key reason for the hardness drop. Further, it was shown that solid-solution, dislocation, grain size and solute co-cluster strengthening played a key role in the thermo-mechanically affected zone and grain-refined zone (GRZ). Finally, these observations were leveraged along with the Zener-Holloman relationship and grain size in the GRZ to estimate the peak joining temperature of the GRZ (∼ 350 °C) near the steel rivet.

aluminum 7xxx alloy↗
DOE OSTI2022

Microstructural pattern formation during liquid metal dealloying: Phase-field simulations and theoretical analyses

In recent years, liquid metal dealloying has emerged as a promising material processing method to generate micro- and nanoscale bicontinuous or porous structures. Most previous studies focused on the experimental characterization of the dealloying process and on the properties of the dealloyed materials, leaving the theoretical study incomplete to fully understand the fundamental mechanisms of the liquid metal dealloying process. In this paper, we use theoretical models and phase-field simulations to clarify the kinetics and pattern formation during liquid metal dealloying. Our investigation starts from a theoretical analysis of the 1D dissolution of a binary precursor alloy, which reveals that the 1D dissolution process involves two regimes. In the first regime, due to the low solubility of one of the elements in the melt, it accumulates at the solid-liquid interface, which reduces the dissolution kinetics. In the second regime, the interface kinetics reaches a stationary regime where both elements of the precursor alloy dissolve into the melt. Previous works revealed that in the early dealloying stage, the dealloying front is destabilized by an interfacial spinodal decomposition, which triggers the formation of interconnected ligaments. Here we extend this line of work by proposing a linear stability analysis able to predict the initial length scale of the ligaments formed in the initial stage of the dealloying. Combining this analysis with the 1D dissolution model proposed here enables us to better understand the initial conditions (composition of the precursor alloy and the melt) leading to a planar dissolution without interface destabilization. Finally, we report a strong influence of solid-state diffusion on dealloying. Although the solid-state diffusivity is four to five orders of magnitude smaller than in the liquid phase, it is found to affect both dissolution kinetics and ligament morphologies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

A re-examination of the mechanism of whiting events: A new role for diatoms in Fayetteville Green Lake (New York, USA)

Whiting events—the episodic precipitation of fine-grained suspended calcium carbonates in the water column—have been documented across a variety of marine and lacustrine environments. Whitings likely are a major source of carbonate muds, a constituent of limestones, and important archives for geochemical proxies of Earth history. While several biological and physical mechanisms have been proposed to explain the onset of these precipitation events, no consensus has been reached thus far. Fayetteville Green Lake (New York, USA) is a meromictic lake that experiences annual whitings. Materials suspended in the water column collected through the whiting season were characterized using scanning electron microscopy and scanning transmission X-ray microscopy. Whitings in Fayetteville Green Lake are initiated in the spring within the top few meters of the water column, by precipitation of fine amorphous calcium carbonate (ACC) phases nucleating on microbial cells, as well as on abundant extracellular polymeric substances (EPS) frequently associated with centric diatoms. Whiting particles found in the summer consist of 5–7 μm calcite grains forming aggregates with diatoms and EPS. Simple calculations demonstrate that calcite particles continuously grow over several days, then sink quickly through the water column. In the late summer, partial calcium carbonate dissolution is observed deeper in the water column. Settling whiting particles, however, reach the bottom of the lake, where they form a major constituent of the sediment, along with diatom frustules. The role of diatoms and associated EPS acting as nucleation surfaces for calcium carbonates is described for the first time here as a potential mechanism participating in whitings at Fayetteville Green Lake. This mechanism may have been largely overlooked in other whiting events in modern and ancient environments.

54 ENVIRONMENTAL SCIENCES↗