Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

Efficient Terpene Production by Marine Thraustochytrids: Shedding Light on the Thermodynamic Driving Force

Terpenoids, such as squalene, are valuable compounds for cosmetic and drug industries, the supply of which is often limited by natural sources. Alternative production strategies have been investigated for decades but remain challenging due to low yields.

Thermodynamic and Kinetic Modeling of Co-utilization of Glucose and Xylose for 2,3-BDO Production by Zymomonas mobilis

Prior engineering of the ethanologen Zymomonas mobilis has enabled it to metabolize xylose and to produce 2,3-butanediol (2,3-BDO) as a dominant fermentation product. When co-fermenting with xylose, glucose is preferentially utilized, even though xylose metabolism generates ATP more efficiently during 2,3-BDO production on a BDO-mol basis. To gain a deeper understanding of Z. mobilis metabolism, we first estimated the kinetic parameters of the glucose facilitator protein of Z. mobilis by fitting a kinetic uptake model, which shows that the maximum transport capacity of glucose is seven times higher than that of xylose, and glucose is six times more affinitive to the transporter than xylose. With these estimated kinetic parameters, we further compared the thermodynamic driving force and enzyme protein cost of glucose and xylose metabolism. It is found that, although 20% more ATP can be yielded stoichiometrically during xylose utilization, glucose metabolism is thermodynamically more favorable with 6% greater cumulative Gibbs free energy change, more economical with 37% less enzyme cost required at the initial stage and sustains the advantage of the thermodynamic driving force and protein cost through the fermentation process until glucose is exhausted. Glucose-6-phosphate dehydrogenase (g6pdh), glyceraldehyde-3-phosphate dehydrogenase (gapdh) and phosphoglycerate mutase (pgm) are identified as thermodynamic bottlenecks in glucose utilization pathway, as well as two more enzymes of xylose isomerase and ribulose-5-phosphate epimerase in xylose metabolism. Acetolactate synthase is found as potential engineering target for optimized protein cost supporting unit metabolic flux. Pathway analysis was then extended to the core stoichiometric matrix of Z. mobilis metabolism. Growth was simulated by dynamic flux balance analysis and the model was validated showing good agreement with experimental data. Dynamic FBA simulations suggest that a high agitation is preferable to increase 2,3-BDO productivity while a moderate agitation will benefit the 2,3-BDO titer. Taken together, this work provides thermodynamic and kinetic insights of Z. mobilis metabolism on dual substrates, and guidance of bioengineering efforts to increase hydrocarbon fuel production.

Designing microbial communities to maximize the thermodynamic driving force for the production of chemicals

Microbial communities have become a major research focus due to their importance for biogeochemical cycles, biomedicine and biotechnological applications. While some biotechnological applications, such as anaerobic digestion, make use of naturally arising microbial communities, the rational design of microbial consortia for bio-based production processes has recently gained much interest. One class of synthetic microbial consortia is based on specifically designed strains of one species. A common design principle for these consortia is based on division of labor, where the entire production pathway is divided between the different strains to reduce the metabolic burden caused by product synthesis. We first show that classical division of labor does not automatically reduce the metabolic burden when metabolic flux per biomass is analyzed. We then present ASTHERISC (Algorithmic Search of THERmodynamic advantages in Single-species Communities), a new computational approach for designing multi-strain communities of a single-species with the aim to divide a production pathway between different strains such that the thermodynamic driving force for product synthesis is maximized. ASTHERISC exploits the fact that compartmentalization of segments of a product pathway in different strains can circumvent thermodynamic bottlenecks arising when operation of one reaction requires a metabolite with high and operation of another reaction the same metabolite with low concentration. We implemented the ASTHERISC algorithm in a dedicated program package and applied it on E. coli core and genome-scale models with different settings, for example, regarding number of strains or demanded product yield. These calculations showed that, for each scenario, many target metabolites (products) exist where a multi-strain community can provide a thermodynamic advantage compared to a single strain solution. In some cases, a production with sufficiently high yield is thermodynamically only feasible with a community. In summary, the developed ASTHERISC approach provides a promising new principle for designing microbial communities for the bio-based production of chemicals.

Origins of the Instability of Non-precious HER Catalysts at Open Circuit Potential

<p>Non-precious hydrogen evolution reaction (HER) catalysts commonly suffer from severe dissolution under open circuit potential (OCP). In this work, using calculated Pourbaix diagrams, we quantitatively analyze the stability of a set of well-known active HER catalysts (MoS₂, MoP, CoP, Pt in acid, and Ni₃Mo in base) under working conditions. We determine that the large thermodynamic driving force towards decomposition created by the electrode/electrolyte interface potential is responsible for the substantial dissolution of non-precious HER catalysts at OCP. Our analysis further shows the stability of HER catalysts in acidic solution is ordered as Pt ∼ MoS₂<i>> </i>MoP <i>> </i>CoP, which is confirmed by the measured dissolution rates using an inductively coupled plasma mass spectrometer. Based on gained insights, we suggest strategies to circumvent the catalyst dissolution in aqueous solution.</p>

Origins of the Instability of Non-precious HER Catalysts at Open Circuit Potential

<p>Non-precious hydrogen evolution reaction (HER) catalysts commonly suffer from severe dissolution under open circuit potential (OCP). In this work, using calculated Pourbaix diagrams, we quantitatively analyze the stability of a set of well-known active HER catalysts (MoS₂, MoP, CoP, Pt in acid, and Ni₃Mo in base) under working conditions. We determine that the large thermodynamic driving force towards decomposition created by the electrode/electrolyte interface potential is responsible for the substantial dissolution of non-precious HER catalysts at OCP. Our analysis further shows the stability of HER catalysts in acidic solution is ordered as Pt ∼ MoS₂<i>> </i>MoP <i>> </i>CoP, which is confirmed by the measured dissolution rates using an inductively coupled plasma mass spectrometer. Based on gained insights, we suggest strategies to circumvent the catalyst dissolution in aqueous solution.</p>

Stability of a Nonequilibrium Biochemical Cycle Revealed by Single-Molecule Spectroscopy

Biological machinery relies on nonequilibrium dynamics to maintain stable directional fluxes through complex reaction cycles. In stabilizing the reaction cycle, the role of microscopic irreversibility of elementary transitions, and the accompanying entropy production, is of central interest. Here, we use multidimensional single-molecule spectroscopy to demonstrate that the reaction cycle of bacteriorhodopsin is coupled through both reversible and irreversible transitions, with directionality of trans-membrane H+ transport being ensured by the entropy production of irreversible transitions. We observe that thermal destabilization of the process is the result of diminishing thermodynamic driving force for irreversible transitions, leading to an exponentially increasing variance of flux through the transitions. We show that the thermal stability of the reaction cycle can be predicted from the Gibbs-Helmholtz relation.

The Effect of the Compounding Procedure on the Morphology and Mechanical Properties of PLA/PBAT-Based Nanocomposites

Poly(lactic acid) (PLA)/poly(butylene adipate-co-terephthalate) (PBAT)-based nanocomposites filled with 1 vol.% silicon dioxide nanoparticles (nano-SiO2) were prepared using a co-rotating twin-screw extruder and injection molding. The nanocomposites with various blending sequences were investigated using PLA-based and PBAT-based nanocomposite masterbatches. Morphology of the PLA/PBAT/SiO2 nanocomposites was examined using a scanning electron microscope (SEM) and a focused ion beam (FIB) SEM. It is found that the nano-SiO2 locates in the original polymer phase, in which it is firstly incorporated in the masterbatch process, as well as at the interface between the two polymers. However, as the residence time in the extrusion process increases, the nanoparticles migrate from the original phase to the interface, governed by the thermodynamic driving force. The best optimization of mechanical properties is achieved by using the PBAT-based masterbatches with a one-step process or short residence time. The processing history, therefore, has a tremendous impact on the final properties of the resulting materials.