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At least 1,063 records · Page 59

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DOE OSTI2014

Elemental segregation and subsequent precipitation during solidification of continuous cast Nb–V–Ti high-strength low-alloy steels

In this study, elemental segregation during solidification and subsequent precipitation behaviour in a continuous cast Nb–V–Ti high-strength low-alloy steel was investigated by optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and thermodynamic modelling. It is known that for steels with low carbon contents the pearlite that forms on slow cooling does so where the interdendritic liquid was present prior to final solidification. The alloying elements of Nb, Ti, Mn and V segregate into the interdendritic liquid during solidification, while Al preferentially segregates into the solidifying solid phase. The composition analysis on the slab samples verified the predicted element segregation behaviour, with a smaller difference in the concentrations of Mn and V in the pearlite and dendritic ferrite regions being observed compared to the Nb levels. Small (30–100 nm) spherical or irregular shaped Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in the pearlite regions, while angular Al-rich (60–300 nm) precipitates were found in the dendritic ferrite regions, in the form of AlN and complex AlN–V(C, N) precipitates. Small isolated ferrite regions surrounded by pearlite were observed in the microstructure and has two origins: one type is dendritic ferrite that appears as an isolated island due to a sectioning effect when observing the two-dimensional microstructure; the other is a ferrite idiomorph that forms in the interdendritic region due to the low carbon content of the steel. Accordingly, in these isolated ferrite islands two different precipitation behaviours are found; predominantly Al-rich particles in the dendritic regions or predominantly Nb-rich precipitates in the interdendritic ferrite idiomorphs. No Al-rich precipitates were observed in the interdendritic regions (pearlite or isolated ferrite idiomorphs) despite the Thermo-Calc predictions indicating a higher volume fraction of AlN in these regions compared to the dendritic regions. This is believed to be due to back diffusion of nitrogen after solidification reducing the available nitrogen, which reacts with the high Nb levels present. - Highlights: • Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in pearlite. • Angular Al-rich precipitates (AlN and AlN–V(C, N)) were found in dendritic ferrite. • Precipitation behaviour in isolated ferrite islands within pearlite was revealed. • Segregation of substitutional elements during solidification is retained. • Back diffusion of interstitial elements (C, N) occurred.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Dynamic microstructural evolution of an Al–Zn–Mg–Cu alloy (7075) during continuous heating and the influence on the viscoelastic response

Highlights: • A TEM/HRTEM, FIB-SEM and APT analysis of an Al–Zn–Mg–(Cu) alloy is performed. • Microstructural evolution is analyzed during constant heating rate heat-treatment. • Composition, size, number density and vol. fraction of present phases are reported. • Vol. fraction and kinetic data from rate equations in the literature are validated. • Viscoelastic behavior of the alloy is associated to microstructural transformations. - Abstract: The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperature range of 298 to 648 K (25 to 375 °C) is characterized by focused ion beam-scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The chemical composition, dimensions, number density, and volume fraction of precipitates are measured comprehensively. Quantitative measurements of Guinier-Preston zones volume fraction and η′/η phase volume fraction are correlated to the viscoelastic response of the Al–Zn–Mg–Cu alloy with temperature in the studied range. This provides valuable evidence indicating that observed variations in the viscoelastic behavior of the Al–Zn–Mg–Cu alloy may be associated with phase transformations involving Guinier-Preston zones and phases η′ and η.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Simultaneously improving mechanical properties and corrosion resistance of pure Ti by continuous ECAP plus short-duration annealing

Highlights: • Additional short-time annealing led to enhanced strength of UFG Ti due to texture strengthening. • UFG Ti showed improved corrosion resistance in artificial seawater compared with initial CG Ti. • Short-duration annealing enhanced corrosion resistance of UFG Ti due to stronger basal texture. - Abstract: This work investigated the effect of short-duration annealing on mechanical properties and corrosion behavior of bulk ultrafine-grained (UFG) pure Ti (grade 2) produced by high-efficiency equal-channel angular pressing (ECAP), serving the fast developing of marine industries. The initial coarse-grained (CG) Ti achieved higher strength after continuous ECAP at 420 °C for 4 passes, while additional short-duration annealing at 300 °C for 15 min of UFG Ti further led to extraordinary improvement of strength, which was associated with texture strengthening. The yield strength was increased from 362 MPa in as-received state to 561 MPa in post-ECAP annealed condition, and ultimate tensile strength from 450 MPa to 663 MPa. The ECAP-processed UFG Ti had better corrosion resistance in artificial seawater than the initial CG Ti. Short-duration annealing further improved the corrosion resistance of UFG Ti, which was related to stronger basal texture. The corrosion current density decreased from 68.37 nA cm{sup −2} for CG Ti in as-received condition to 41.77 nA cm{sup −2} for UFG Ti processed by ECAP plus short-duration annealing. The fraction of TiO{sub 2} within the surface oxide film for the post-ECAP annealed sample was increased. The bulk UFG Ti after post-ECAP short-duration annealing revealed an excellent combination of high strength, good ductility and superior corrosion resistance for marine applications.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Continuous microstructure evolution of ball rolled TWIP Ti15Mo alloy and its effect on phase precipitation behavior

Highlights: • Gradient microstructure was induced by ball rolling method in a Ti15Mo alloy. • Twin density increased and then decreased as increasing distance from surface. • Ultrafine (α + β) duplex structure was formed in the heavily deformed region. • α-phase in plate was precipitated in twined region in slightly deformed region. • Strengthening by formation of sub/nano-grains was notable than that by twins. - Abstract: A β-type Ti15Mo alloy with {332}<113> twinning-induced plasticity (TWIP) effect was utilized to investigate the continuous microstructure evolution of β-matrix by ball rolling method. Precipitation behavior of α-phase and strengthening mechanism were examined in regions with different strain. Amounts of intersected twins were present in the slightly deformed region of β-matrix, and their area fraction decreased with an increase of distance from rolled surface. The β-matrix close to surface was heavily deformed to result in the formation of abundant subgrains and nanograins, which was caused by cooperative occurrence of twinning refinement, intragranular dislocation activities and subgrain rotation. Mechanical twins provided the preferential nucleation sites for plate-like α-phase with strong variant selection. Equiaxed α-phase preferentially nucleated at the boundaries of subgrains and nanograins with weak variant selection to form the ultrafine (α + β) duplex structure. Due to the formation of subgrains and nanograins as well as the strain hardening, strengthening became more significant than that by twinning refinement solely. Similarly, the ultrafine (α + β) duplex structure exhibited a better strengthening effect than that by precipitation of plate-like α-phase. The diversities of deformation microstructure combined with different precipitation behaviors of α-phase are desirable for controlling the mechanical properties of β-type TWIP titanium alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Structural and electrical properties of high-quality 0.41 μm-thick InSb films grown on GaAs (1 0 0) substrate with In{sub x}Al{sub 1−x}Sb continuously graded buffer

High-quality InSb was grown on a GaAs (1 0 0) substrate with an InAlSb continuously graded buffer (CGB). The temperatures of In, Al K-cells and substrate were modified during the growth of InAlSb CGB. The cross-section TEM image reveals that the defects due to lattice-mismatch disappear near lateral structures in CGB. The measured electron mobility of 0.41 μm-thick InSb was 46,300 cm{sup 2}/Vs at 300 K. These data surpass the electron mobility of state-of-the-art InSb grown by other methods with similar thickness of InSb.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Photocatalytic CO{sub 2} reduction by CH{sub 4} over montmorillonite modified TiO{sub 2} nanocomposites in a continuous monolith photoreactor

Highlights: • TiO{sub 2}/MMT supported monolith photo-reactor was tested for CO{sub 2} reduction with CH{sub 4}. • MMT inhibited crystal growth and enhanced TiO{sub 2} photo-activity in monolith reactor. • CO{sub 2} was reduced by CH{sub 4} to CO, CH{sub 3}OH, C{sub 2}H{sub 6}, C{sub 3}H{sub 6} and C{sub 3}H{sub 8} in a monolith reactor. • CO yield over TiO{sub 2}/MMT was 237.5 μmol g-catal.{sup −1} h{sup −1}, a 2.52 fold than TiO{sub 2}. • Stability test revealed TiO{sub 2}/MMT partially lost photo-activity in reused cyclic runs. - Abstract: In this study, the performance of montmorillonite (MMT) modified TiO{sub 2} nanocomposites for photocatalytic CO{sub 2} reduction with CH{sub 4} in a continuous monolith photoreactor has been investigated. The MMT modified TiO{sub 2} nanocomposites were dip-coated over monolith channels and were characterized by XRD, SEM, TEM, XPS, N{sub 2}-adsorption–desorption and UV–vis spectroscopy. The MMT produced anatase phase of TiO{sub 2} and reduced TiO{sub 2} crystallite size from 19 nm to 13 nm. CO was the major reduction product with a yield rate of 237.5 μmol g-catal.{sup −1} h{sup −1} over 10 wt.% MMT-loaded TiO{sub 2} at 100 °C, and CO{sub 2}/CH{sub 4} feed ratio 1.0. The photoactivity of MMT-loaded TiO{sub 2} monolithic catalyst was 2.52 times higher than bare TiO{sub 2}. Likewise, low concentrations of C{sub 2}H{sub 6}, CH{sub 3}OH, C{sub 3}H{sub 6} and C{sub 3}H{sub 8} were detected in the products mixture. These results inferred MMT modified TiO{sub 2} and monolith photoreactor were beneficial for enhancing photocatalysis process with appreciable productivity. The stability test revealed photoactivity of MMT-loaded TiO{sub 2} nanocomposites partially diminished in recycle runs.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Mussel-inspired surface modification of untreated wasted husks with stable polydopamine/polyethylenimine for efficient continuous Cr(VI) removal

Highlights: • Functional rice husks with abundant amino groups was firstly modified with PDA/PEI for Cr(VI) removal. • Potentiality of monolithic samples for dynamic continuous adsorption. • Expectable higher adsorption capacity than most of reported similar bio-sorbents. • Excellent mechanical stability, reusability and convenient separation. - Abstract: A cost-effective and chemically modified biological adsorbent based on agricultural wasted husks for heavy metal removal was fabricated by coating with Polydopamine–Polyethylenimine via the mussel inspired chemistry. Batch and fixed bed column adsorption experiments were carried out to explore the sorption capacity and potency of industrial application. The batch results indicated that the maximum adsorption capacity reached 42.8 mg/g which is much larger than most adsorbents based on bioresource and agricultural wastes for Cr(VI) removal in aqueous solution simultaneously with excellent recycling performance. In column adsorption experiments, it exhibited high mechanical stability and applied potential. The equilibrium time was 9130 min under the flow rate of 1 ml/min, and reached the equilibrium adsorption capacity of 47.14 mg/g. The breakthrough curves ploted by the data of fixed bed experiments under different flow rates (1 ml/min & 2 ml/min) fitted the Thomas model with R{sup 2} of 0.9510 and 0.9437 respectively.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Realizing continuous cation order-to-disorder tuning in a class of high-energy spinel-type Li-ion cathodes

Conventional Li-ion cathode materials are dominated by well-ordered structures, in which Li and transition metals occupy distinct crystallographic sites. Here, we show in this paper that profoundly new degrees of freedom for the optimization of electrochemical properties may be accessed if controllable cation disorder is introduced. In a class of high-capacity spinel-type cathode materials, we identify cation to anion ratio in synthesis as a key parameter for tuning the structure continuously from a well-ordered spinel, through a partially ordered spinel, to rocksalt. We find that the varying degree of cation disorder modifies the voltage profile, rate capability, and charge-compensation mechanism in a rational and predictable way. Our results indicate that spinel-type order is most beneficial for achieving high-rate performance as long as the cooperative 8a to 16c phase transition is suppressed, while more rocksalt-like disorder facilitates O redox, which can increase capacity. Our findings reveal an important tuning handle for achieving high energy and power in the vast space of partially ordered cathode materials.

25 ENERGY STORAGE↗

DOE OSTI2016

Acid-functionalized mesoporous carbons for the continuous production of 5-hydroxymethylfurfural

The stability of solid acid catalysts is essential in the development of sustainable biomass conversion processes. In this report we study the activity and stability of acid-functionalized periodic mesoporous carbons (CMK-3 and CMK-5) for the continuous production of 5-hydroxymethylfurfural from fructose. The acid-functionalized mesoporous carbon catalysts show comparable selectivities and turnover frequency (TOF) to that of propylsulfonic acid functionalized SBA-15. However, the first-order deactivation rate coefficient of the carbon-based materials are up to 60 times more stable than that for the acid-functionalized SBA-15 catalyst and up to 3 times more stable than commercial Nafion SAC-13. Of the catalysts studied, phenylsulfonic acid CMK-5 showed both the highest TOF for HMF production and lowest deactivation rate coefficient.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2016

Method for analyzing passive silicon carbide thermometry with a continuous dilatometer to determine irradiation temperature

Silicon carbide is used as a passive post-irradiation temperature monitor because the irradiation defects will anneal out above the irradiation temperature. The irradiation temperature is determined by measuring a property change after isochronal annealing, i.e., lattice spacing, dimensions, electrical resistivity, thermal diffusivity, or bulk density. However, such methods are time-consuming since the steps involved must be performed in a serial manner. This work presents the use of thermal expansion from continuous dilatometry to calculate the SiC irradiation temperature, which is an automated process requiring minimal setup time. Analysis software was written that performs the calculations to obtain the irradiation temperature and removes possible user-introduced error while standardizing the analysis. In addition, this method has been compared to an electrical resistivity and isochronal annealing investigation, and the results revealed agreement of the calculated temperatures. These results show that dilatometry is a reliable and less time-intensive process for determining irradiation temperature from passive SiC thermometry.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2021

Continuous evolution of electromagnetic field in heavy-ion collisions

In relativistic heavy-ion collisions the electromagnetic field exists before the hot nuclear matter emergence. Requiring the field continuity, we compute it in the central rapidity region by taking into account the electromagnetic response of the Quark Gluon Plasma. We show that the electromagnetic field is nearly time-independent from about 1 fm/c after the collision until the freezeout. In particular, its value is about 10% of the initial magnitude of the magnetic field. In conclusion, our result provides a theoretical basis for the common assumption that the electromagnetic field produced in relativistic heavy-ion collisions varies slowly with time.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2019

Investigation of continuous carbothermal reduction of magnesia by magnesium vapor condensation onto a moving bed of solid particles

Magnesium metal production by carbothermic reduction (CTR) can reduce process emissions and energy requirements relative to silicothermic or electrolytic production, if high magnesium metal yields are achieved. A novel process was investigated where product magnesium gas condensed onto a moving bed of solid particles at high temperatures (≥300 °C) and in vacuum to minimize reversion and promote crystal growth. Using a half-system in which CTR product gases were artificially generated, magnesium continuously condensed onto steel, oxide, or carbide particles at yields >85% for P Mg = 0.5 kPa. Steel particles exhibited the greatest bed retention and ease of separation, so this medium was used for scale-up to a full-system in which MgO CTR in a fixed bed gasifier at 1400 °C–1550 °C produced Mg (g) and CO. The initial reduction resulted in high Mg yield (>80%), but yield decreased as the reaction proceeded. The accumulation of Mg (s) and reversion product in the condenser promoted further reversion. The process yield could likely be improved by using high surface area particles to promote heat and mass transfer, allowing for shorter solid and gas residence times in the condenser. A finite volume model on the reactor tube and pellet bed described the kinetics, heat transfer, and mass transfer phenomena in the reduction reactor. The model predicted that the center of the bed was >50 °C cooler than the furnace, and the product gas pressures were near equilibrium. A wide, directly heated reactor could overcome these heat and mass transfer limitations.

42 ENGINEERING↗

DOE OSTI2023

A Three-Dimensional, Analytical Wind Turbine Wake Model: Flow Acceleration, Empirical Correlations, and Continuity

A new, three-dimensional, analytical, steady-state wake model is presented that includes local flow acceleration near the rotor, improving the wake description compared to existing models. Wake structures such as the momentum deficit and regions of accelerated flow are concisely described with compound and normal Gaussian functions. Large-eddy simulations (LES) are used as training data to develop the model using two in-line turbines under various inflow conditions parameterized by hub-height wind speed and turbulence intensity. Mass conservation is considered by fixing two components of the wake velocity model and optimizing the third to best satisfy continuity; after which, the model performs comparably if not better than existing work with regards to both relative error and mass consistency. The final model demonstrates a high degree of flexibility, making use of empirical correlations to scale across different inflow conditions. The inclusion of these effects is capable of revealing unused opportunities for enhanced power generation by aligning wake trajectories with these regions of accelerated flow.

modeling↗

DOE OSTI2025

Abiotic influences on continuous conifer forest structure across a subalpine watershed

Understanding the abiotic drivers of high-elevation forest physiognomy is essential for forecasting how mountain ecosystems will respond to emerging environmental pressures. Most prior studies of these relationships have relied on small samples of the full landscape, resulting in limited power to detect dominant covariates and their interactions. Here we report the first evaluation of abiotic influences on a complement of accurate, wall-to-wall estimates of conifer forest structure and composition at the watershed scale. In a subalpine conifer domain in the Colorado Rocky Mountains (USA), we developed a novel method for deriving stand structure metrics from waveform LiDAR data, which showed high fidelity with field inventory. We quantified the relationships between structural and compositional metrics and climate, topographic, edaphic, and geologic factors. Our results showed that peak snow water equivalent (SWE), snow disappearance rate, and elevation explained most of the variation in forest structure. The highest stand density, basal area, maximum canopy height, and quadratic mean diameter occurred in sites with SWE around one standard deviation below mean, but with long snow residence times. Stand density decreased linearly with elevation, while other metrics peaked between 3000 m and 3200 m. Substrate properties had weaker influence. Continuous mapping of through-canopy forest structure enabled our novel findings of the dominant role of snowpack in explaining structural and compositional variation, and of elevation thresholds. Our reproducible approach facilitates assessment of forest-topoclimate relationships in other conifer-dominated landscapes and improves understanding of the baseline patterns controlling forest structure, which is needed for predicting long-term ecological change.

forest ecology↗

DOE OSTI2018

Nowcasting, predictive control, and feedback control for temperature regulation in a novel hybrid solar-electric reactor for continuous solar-thermal chemical processing

A novel 15 kW reactor for the hybridization of concentrated solar and conventional electric heat was fabricated for renewable and continuous chemical processing at temperatures up to 1700 °C. Solar-electric controllers based on feedback or predictive linear models were used to regulate the device at 925 °C for the production of syngas via the gasification of carbon. The system was challenged with cloud transients programmed on a 45 kW high-flux solar simulator that approximated weather observed in the San Luis Valley (Colorado, USA). In experiment it was found that model predictive control with a 1 min ahead nowcast of incipient clouding best regulated solar-electric reactor temperatures, potentially averting thermal fatigue. Upon clouding, model predictive control with a nowcast yielded temperature disturbances of ±10 °C, whereas feedback control alone featured ±25 °C excursions. Overall, the performance of model predictive control with a nowcast was 75% better than feedback as indicated by the integral squared error of temperature excursion residuals. The importance of nowcast accuracy was explored, and although the model-based controller was robust to forecast amplitude inaccuracy, temporal forecast inaccuracy wholly negated the benefits of predictive control.

14 SOLAR ENERGY↗

DOE OSTI2022

t 1 -noise elimination by continuous chemical shift anisotropy refocusing

We report that due to their high gyromagnetic ratio, there is considerable interest in measuring distances and correlations involving protons, but such measurements are compounded by the simultaneous recoupling of chemical shift anisotropy (CSA). This secondary recoupling adds additional modulations to the signal intensities that ultimately lead to t 1 -noise and signal decay. Recently, Venkatesh et al. demonstrated that the addition of CSA refocusing periods during 1 H-X dipolar recoupling led to sequences with far higher stability and performance. Herein, we describe a related effort and develop a symmetry-based recoupling sequence that continually refocuses the 1 H CSA. This sequence shows superior performance to the regular and t1-noise eliminated D-HMQC sequences in the case of spin-1/2 nuclei and comparable performance to the later for half-integer quadrupoles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Model-based and Model-free Designs for an Extended Continuous-time LQR with Exogenous Inputs

We present an extended linear quadratic regulator (LQR) design for continuous-time linear time-invariant (LTI) systems in the presence of exogenous inputs. We first propose a model-based solution with cost minimization guarantees for states and inputs using dynamic programming (DP). The control law consists of a combination of the optimal state feedback and an additional optimal term dependent on the exogenous inputs. The control gains for the two components are obtained by solving a set of matrix differential equations. We provide these solutions for both finite horizons and steady-state cases. In the second part of the paper, we formulate a reinforcement learning (RL) based algorithm which does not need any model information except the input matrix, and can compute an approximate steady-state LQR gain using measurements of the states, the control inputs, and the exogenous inputs. Both model-based and data-driven optimal control algorithms are tested with a numerical example under different exogenous inputs showcasing the effectiveness of the designs.

Mukherjee, Sayak↗

DOE OSTI2011

Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions

Highlights: > High-solids (dry) anaerobic digestion is attracting a lot of attention these days. > One reactor was fed with food waste (FW) and paper waste. > Maximum biogas production rate of 5.0 m{sup 3}/m{sup 3}/d was achieved at HRT 40 d and 40% TS. > The other reactor was fed with FW and livestock waste (LW). > Until a 40% LW content increase, the reactor exhibited a stable performance. - Abstract: With increasing concerns over the limited capacity of landfills, conservation of resources, and reduction of CO{sub 2} emissions, high-solids (dry) anaerobic digestion of organic solid waste (OSW) is attracting a great deal of attention these days. In the present work, two dry anaerobic co-digestion systems fed with different mixtures of OSW were continuously operated under mesophilic conditions. Dewatered sludge cake was used as a main seeding source. In reactor (I), which was fed with food waste (FW) and paper waste (PW), hydraulic retention time (HRT) and solid content were controlled to find the maximum treatability. At a fixed solid content of 30% total solids (TS), stable performance was maintained up to an HRT decrease to 40 d. However, the stable performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again. In further operation, instead of decreasing HRT, solid content was increased to 40% TS, which was found to be a better option to increase the treatability. The biogas production rate (BPR), CH{sub 4} production yield (MPY) and VS reduction achieved in this condition were 5.0 m{sup 3}/m{sup 3}/d, 0.25 m{sup 3} CH{sub 4}/g COD{sub added}, and 80%, respectively. Reactor (II) was fed with FW and livestock waste (LW), and LW content was increased during the operation. Until a 40% LW content increase, reactor (II) exhibited a stable performance. A BPR of 1.7 m{sup 3}/m{sup 3}/d, MPY of 0.26 m{sup 3} CH{sub 4}/g COD{sub added}, and VS reduction of 72% was achieved at 40% LW content. However, when the LW content was increased to 60%, there was a significant performance drop, which was attributed to free ammonia inhibition. The performances in these two reactors were comparable to the ones achieved in the conventional wet digestion and thermophilic dry digestion processes.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗