Slow Heating Rate Research Articles

Molecular Dynamics Study of Melting Behavior of Planar Stacked Ti–Al Core–Shell Nanoparticles

Selective laser sintering (SLS) is one of the most commonly used methods in additive manufacturing, due to its high prototyping speed and applicability to various materials. In the present work, molecular dynamics (MD) simulations were performed to study the thermodynamic behaviors of the planar stacked nanoparticles (NPs) model and explore the potential capability of the SLS process with nano-sized metal powders in the zero-gravity space environment. A multi-particle model of titanium–aluminum (Ti–Al) core–shell NP with a particle radius of 50 Å was constructed to investigate the characteristics of the melted pattern during sintering. Two patterns with different spatial densities were considered to study the influence of particle stacking on the melting process. Various core volume fractions and heating rates were examined to investigate their effects on the quality of the final sintered product. The stacked-NPs models with core volume fractions (CVFs) of 3%, 12%, and 30% were linearly heated up to 1100 K from room temperature (298 K) with heating rates of 0.04, 0.2, 0.5, and 1.0 K ps−1. The initial fusion temperature and final sintering temperature for each stacking pattern were obtained via the validation from the radial distribution function, mean squared displacement, and the radius of the gyration analysis. The 30% CVF yields the largest neck size before the melting point, while beyond the melting point, a larger core helps delay the formation of the fully-melted products. It is observed that using the close-packed stacked-NPs model under a slow heating rate (long melting duration) would help form a stable, completely sintered product with a relatively low final sintering temperature.

Characterization of charcoals produced from Acacia, Albizia and Leucaena for application in ironmaking

Fossil coal can be replaced with renewable charcoal to reduce CO2 emissions from ironmaking processes. Charcoals produced from three hardwood species, Acacia nilotica (W1), Albizia lebbeck (W2) and Leucaena leucocephala (W3), were evaluated as a reductant in ironmaking. Charcoals were produced with slow (5 °C/min) and fast heating rates (50 °C/min) at 600, 800 and 1100 °C. Charcoals were characterized using proximate, ultimate analysis, volume shrinkage, compressive loading, scanning electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy for their chemical, mechanical and physical properties. It was found that among the selected biomass species, fixed and total carbon content is higher in W1 (83–97; 84–91%) and W2 (83–96; 86–92%) than W3 (76–89; 78–84%) and increases with increase in carbonization temperature and heating rate. Charcoal yield in W1 (22–28%), W2 (22–26%) and W3 (23–28%) decreases with an increase in carbonization temperature and heating rate. Charcoals produced at various carbonization conditions have at most 10% ash content. Values from different tests indicate that W2 and W3 charcoal are potentially better reductant than W1.

In Situ Growth Mechanism for High-Quality Hybrid Perovskite Single-Crystal Thin Films with High Area to Thickness Ratio: Looking for the Sweet Spot.

The development of in situ growth methods for the fabrication of high‐quality perovskite single‐crystal thin films (SCTFs) directly on hole‐transport layers (HTLs) to boost the performance of optoelectronic devices is critically important. However, the fabrication of large‐area high‐quality SCTFs with thin thickness still remains a significant challenge due to the elusive growth mechanism of this process. In this work, the influence of three key factors on in situ growth of high‐quality large‐size MAPbBr3 SCTFs on HTLs is investigated. An optimal “sweet spot” is determined: low interface energy between the precursor solution and substrate, a slow heating rate, and a moderate precursor solution concentration. As a result, the as‐obtained perovskite SCTFs with a thickness of 540 nm achieve a record area to thickness ratio of 1.94 × 104 mm, a record X‐ray diffraction peak full width at half maximum of 0.017°, and an ultralong carrier lifetime of 1552 ns. These characteristics enable the as‐obtained perovskite SCTFs to exhibit a record carrier mobility of 141 cm2 V−1 s−1 and good long‐term structural stability over 360 days.

Microstructure and mechanical properties of (Ti,W)C cermets prepared by ultrafast spark plasma sintering

To explore the impact of the sintering rate on the microstructure and mechanical properties of cermets, the preparation of (Ti,W)C cermets by ultrafast sintering via spark plasma sintering (SPS) is reported. Compared with a slow heating rate, the electric field produced by an ultrafast heating rate enhances the liquid phase mass transfer of the metal binder phase, thus achieving rapid densification of (Ti,W)C cermets and effectively inhibiting abnormal grain growth. However, an excessive heating rate will lead to an “overflow” phenomenon, which reduces the grain growth difficulty and the bonding strength between grains. The results show that when the heating rate is 500 °C/min, the liquid phase mass transfer is moderate, the densification degree is the highest and the mechanical properties are excellent. The flexural strength, Vickers hardness and fracture toughness are 1340.90 ± 23.55 MPa, 18.42 ± 0.46 GPa and 11.96 ± 0.23 MPa∙m1/2, respectively.

Microstructure evolution and mechanical properties of bimodal grain sized 301 stainless steel strip induced by reverse phase transformation

The sub-micron bimodal grain structure of 301 austenitic stainless steel stripe obtained by phase reversion process, and the relationship between grain size distribution and mechanical properties was described in this study. With the change of annealing heating rate under the industrial condition 10 °C/min, the bimodal grain size of submicron scale was obtained at 750 °C. By analyzing the micrograph of bimodal structure, it was found that the deformed martensite transformed to austenite through diffusion transformation, and the recovery austenite grain had obvious size distribution. It was found that the bimodal structure was easier to form under the condition of slow heating rate, and the ultra-fine grain materials with high performance were obtained by controlling the annealing parameter. The high-density grain boundary of ultrafine grain presents microvoid in the tensile fracture, and the streaks formed by the aggregation of holes can be observed, which is considered to be caused by deformation twinning. The large dimples in the coarse grain are observed, as the strain-induced martensite was formed in the large grain of the bimodal structure.

Interaction Between Ferrite Recrystallization and Austenite Formation in Dual-Phase Steel Manufacture

In this publication, the effect of heating rate on microstructural evolution of manganese segregated cold reduced dual phase steels is systematically studied for different inter-critical temperatures and holding times. At slow heating rate, completion of ferrite recrystallization before austenite formation led to the preferential formation of austenite on the ferrite grain boundaries leading to a necklace austenite (now martensite) morphology. The slower austenite formation kinetics has been attributed to longer diffusion paths dictated by larger ferrite grain sizes. In medium heating rate condition, microstructure before austenite formation had partially recrystallized ferrite and partially spheroidized cementite. Rapid austenite growth occurred along the rolling direction in carbon-rich cementite regions and dislocation-rich recovered ferrite regions. The presence of partially recrystallized ferrite grains restricted the austenite growth in the normal direction and therefore enabled the formation of thin martensite bands parallel to the rolling direction. At fast heating rate, the microstructure before austenite formation predominately contained un-recrystallized ferrite and un-spheroidized cementite and therefore enabled faster austenite formation kinetics. Thicker martensite bands are formed at fast heating rates due to the absence of recrystallized grains, thereby, enabling the growth of austenite in all directions with a higher preference to the rolling direction.

Tuning the Reactivity of Perfluoropolyether-Functionalized Aluminum Nanoparticles by the Reaction Interface Fuel-Oxidizer Ratio.

To deepen the oxidation depth and promote the exothermic reaction of aluminum nanoparticles (Al NPs), this work constructed perfluoropolyether-functionalized Al NPs by using a facile fabrication method. It was determined that perfluoropolyether (PFPE) was uniformly distributed on the surface of the Al NPs with no obvious agglomeration by micro-structure analysis. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), microcomputer automatic calorimeter (MAC), and combustion and ignition experiments were performed for varying percentages of PFPE blended with Al NPs to examine the reaction kinetics and combustion performance. It was revealed that the oxidation mechanism of PFPE-functionalized Al NPs at a slow heating rate was regulated by the reaction interface Fuel-Oxidizer ratio. Due to the enlarged Fuel-Oxidizer contact surface area, fluorine atoms could adequately decompose the inert alumina shell surrounding the Al NPs, optimizing the combustion process of Al NPs. The analytical X-ray diffraction (XRD) pattern results confirmed the existence of aluminum trifluoride in combustion products, providing insights into the oxidation mechanism of Al NPs. The obtained results indicated that PFPE participated in the oxidation of Al NPs and improved the overall reactivity of Al NPs.

Commercial aluminum powders, Part I: Particle size characterization and slow heating rate thermal analysis

We characterized nine commercial aluminum (Al) powders using several methods to measure particle characteristics and thermal analysis, with the goal to understand how these parameters influence energy release. Although it is well-known that lot-to-lot variations in commercial nanoparticles are common, the Al powders were more heterogeneous than anticipated – both with regards to particle size distributions and impurities. Manufacturer specifications – often quoted in the literature without confirmation – were not always accurate for the specific sample lots we investigated. In several cases, different conclusions could be drawn from individual particle size techniques; a combination of multiple techniques provides a clearer picture of the powder properties. Thorough characterization of Al powders is required prior to interpretation of experimental results from a variety of applications. In particular, previous studies have shown contradictory results on the influence of Al on detonation performance, perhaps partially due to insufficient characterization of the Al powders themselves.

Bio-FLASHCHAIN® theory for rapid devolatilization of biomass. 5. Interpreting AAEM catalysis in primary cellulose devolatilization

This paper extends cel-FC, the reaction mechanism for the devolatilization of mineral-free cellulose formulated in Part 4 of this series, to cover catalysis by alkali and alkaline earth metals (AAEMs) in parallel with the spontaneous reaction channels. Quantitative comparisons with data cover temperatures to 600 °C, heating rates from 100 to 5000 °C/s, contact times after heatup to 30 s, pressures of vacuum and 0.1 MPa, and K and Ca loadings to 0.0415 mol cation/mol glucose. Collectively the catalytic processes are responsible for an enormous breadth of devolatilization behavior over the range of typical AAEM loadings in whole biomass. AAEMs catalyze depolymerization via scission and unzipping as well as monomer decomposition and charring but with much different sensitivities. AAEM catalysis lowers the activation energies for all four reaction processes. For scission and unzipping, the shift moves the onset of devolatilization toward cooler temperatures. Monomer decomposition has the strongest shift to explain both suppressed oil production, due to the accelerated conversion of labile bridges into refractory char links, and enhanced gas production, as noncondensables are the explicit products of this process. Oil molecular weight distributions (MWDs) become heavier for progressively heavier AAEM loadings due primarily to suppression of isolated modified levoglucosan units. Consequently, the lighter halves of the oil MWDs for vacuum and atmospheric pressure are very similar for the heaviest AAEM loadings, contrary to the preponderance of monomers and dimers in oils from mineral-free cellulose at atmospheric pressure. Depolymerization accelerates compared to monomer decomposition in extrapolations to faster heating rates, which enhances oil yields. Consequently, the impacts of AAEM catalysis are much stronger at slower heating rates than at faster rates.

Development of cube recrystallization textures in high-purity Al

Abstract High-purity (99.99 wt.%) Al containing 50 wt. ppm Cu was cold rolled to 98% reduction in thickness and annealed by varying the heating rate of annealing and annealing cycles. Textures developed at various stages of annealing were investigated with the orientation distribution function analysis. The observed results were compared with those obtained on standard high purity (99.99 wt.%) Al without Cu addition. In slow heating rate annealing, it was found that in high purity Al the addition of 50 wt.ppm Cu significantly enhanced the development of {100}<001> recrystallization textures. Intensities of {100}<001> developed in high purity Al with 50 wt. ppm Cu were about twice as high as those observed in high purity Al without Cu addition. {123}<634> R orientations were completely eliminated by the addition of 50 wt. ppm Cu. In rapid heating annealing, on the other hand, annealing at temperatures above 325 °C resulted in the formation of very coarse recrystallized grains with weak {100}<001> recrystallization textures irrespective of Cu content. Such grain coarsening could be suppressed by pre-annealing at temperatures between 225 and 300 °C. In the specimens subjected to this pre-annealing, strong {100}<001> recrystallization textures could be obtained, even if they were annealed with a rapid heating rate in final annealing.

Understanding the impact of diffusion of CO in the astrochemical models

Abstract The mobility of lighter species on the surface of interstellar dust grains plays a crucial role in forming simple through complex molecules. Carbon monoxide is one of the most abundant molecules, its surface diffusion on the grain surface is essential to forming many molecules. Recent laboratory experiments found a diverse range of diffusion barriers for CO on the grain surface, their use can significantly impact the abundance of several molecules. The impact of different diffusion barriers of CO, in the astrochemical models, is studied to understand its effect on the abundance of solid CO and the species for which it is a reactant partner. A gas-grain network is used for three different physical conditions; cold core and warm-up models with slow and fast heating rates. Two different ratios (0.3 and 0.5) between diffusion and desorption barrier are utilised for all the species. For each physical condition and ratio, six different models are run by varying diffusion barriers of CO. Solid CO abundance for the models with the lowest diffusion barrier yields less than 0.1% of water ice for cold clouds and a maximum of 0.4% for slow and fast warm-up models. Also, solid $\textrm{CO}_2$ in dense clouds is significantly overproduced ( ${\sim}140\%$ of water). The abundance of H2CO and $\textrm{CH}_3\textrm{OH}$ showed an opposite trend, and HCOOH, $\textrm{CH}_3\textrm{CHO}$ , $\textrm{NH}_2\textrm{CO}$ , and $\textrm{CH}_3\textrm{COCH}_3$ are produced in lower quantities for models with low diffusion barriers for CO. Considerable variation in abundance is observed between models with the high and low diffusion barrier. Models with higher diffusion barriers provide a relatively better agreement with the observed abundances when compared with the models having lower diffusion barriers.

Comparative Study of the Measured and Two Ideal Glass Transition Temperatures Corresponding to the Limit of Infinitely Slow Heating Rate and the Infinite Barriers with Respect to Flow in Metallic Glasses and Glass-Forming Liquids

Comparative Study of the Measured and Two Ideal Glass Transition Temperatures Corresponding to the Limit of Infinitely Slow Heating Rate and the Infinite Barriers with Respect to Flow in Metallic Glasses and Glass-Forming Liquids

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Highly Dispersive Metal Atoms Anchored on Carbon Matrix Obtained by Direct Rapid Pyrolysis of Metal Complexes

Study on the residual performance of RC beams exposed to processed temperatures and fire

PurposeThe Reinforced Concrete(RC) elements are known to perform well during exposure to elevated temperatures. Hence, RC elements are widely used to resist the extreme heat developing from accidental fires and other industrial processes. In both of the scenarios, the RC element is exposed to elevated temperatures. However, the primary differences between the fire and processed temperatures are the rate of temperature increase, mode of exposure and exposure durations. In order to determine the effect of two heating modalities, RC beams were exposed to processed temperatures with slow heating rates and fire with fast heating rates.Design/methodology/approachIn the present study, RC beam specimens were exposed to 200 °C, to 800 °C temperature at 200 °C intervals for 2 h' duration by adopting two heating modes; Fire and processed temperatures. An electrical furnace with low-temperature increment and a fire furnace with standard time-temperature increment is adapted to expose the RC elements to elevated temperatures.FindingsIt is observed from test results that, the reduction in load-carrying capacity, first crack load, and thermal crack widths of RC beams exposed to 200 °C, and 600 °C temperature at fire is significantly high from the RC beams exposed to the processed temperature having the same maximum temperature. As the exposure temperature increases to 800 °C, the performance of RC beams at all heating modes becomes approximately equal.Originality/valueIn this work, residual performance, and failure modes of RC beams exposed to elevated temperatures were achieved through two different heating modes are presented.

Effects of Different Carbonization Conditions on the Color Change of Biochar

The aim of this research was to determine the effects of different carbonization temperature, gas flow rate and heating rates on biochar's color change. Three different carbonization temperatures (400°C, 500°C, and 600°C), two different gas flow rates (0.2 l min-1 and 0.5 l min-1) and two different heating temperature rates (30°C min-1 and 60°C min-1) were used in the experiments. The color changes of biochar were examined utilizing the international approved L*, a*, b* system. Atriplex nitens Schkuhr was used as a biomass source in the experiments. High carbonization temperature and high gas flow rate caused a decrease in the “L” value of biochar. It is an indication that the color is getting darker, when the L value approaches zero. In the study, only the effect of gas flow rate on the "a" value was found to be statistically significant (P≤0.05). The increase in gas flow rate caused the biochar to become darker by increasing the deep red tone. Heating rate and gas flow speed significantly influenced the "b" values of biochar. The slow heating rate and high gas flow rates made the biochar color darker. At end of the research, it can be said that the biochar produced at high carbonization temperature, low heating rate and high gas flow rates will have darker tones.

Investigating the Structural Dynamics and Crack Propagation Behavior under Uniform and Non-Uniform Temperature Conditions.

The robustness and stability of the system depend on structural integrity. This stability is, however, compromised by aging, wear and tear, overloads, and environmental factors. A study of vibration and fatigue cracking for structural health monitoring is one of the core research areas in recent times. In this paper, the structural dynamics and fatigue crack propagation behavior when subjected to thermal and mechanical loads were studied. It investigates the modal parameters of uncracked and various cracked specimens under uniform and non-uniform temperature conditions. The analytical model was validated by experimental and numerical approaches. The analysis was evaluated by considering different heating rates to attain the required temperatures. The heating rates were controlled by a proportional-integral-derivative (PID) temperature controller. It showed that a slow heating rate required an ample amount of time but more accurate results than quick heating. This suggested that the heating rate can cause variation in the structural response, especially at elevated temperatures. A small variation in modal parameters was also observed when the applied uniform temperatures were changed to non-uniform temperatures. This study substantiates the fatigue crack propagation behavior of pre-seeded cracks. The results show that propagated cracking depends on applied temperatures and associated mass. The appearance of double crack fronts and multiple cracks were observed. The appearance of multiple cracks seems to be due to the selection of the pre-seeded crack shape. Hence, the real cracks and pre-seeded cracks are distinct and need careful consideration in fatigue crack propagation analysis.

Structural transformations for a subbituminous coal, impact of temperature on gold-tube pyrolysis chars evaluated using HRTEM

Structural transformations accompanying coalification can be explored by gold-tube pyrolysis. There the pyrolysis products are retarded by the high gas pressure and the slow-heating with temperature soak can somewhat simulate in situ conditions in a laboratory setting. Coalification process under geological conditions can be explained. While there has been considerable research expenditure on bituminous and anthracite coals, the subbituminous coals have been less-well explored. Here, the structural transformations accompanying subbituminous coal during heating were followed using HRTEM image analysis. Specifically the distributions of fringe: length, orientation, stacking, and curvature were quantified. Pyrolysis was performed under N2 at a slow heating rate of 20 °C/h with a 50 MPa initial (cold) confining pressure. Temperatures between 325 and 590 °C were explored. Each sample was temperature soaked for 1 h and allowed to cool slowly to room temperature. The structural transformations accompanying pyrolysis could be divided into three stages by the fringe orientation within a 45° bin capturing any thermoplastic transformations. In stage one: 325∼432 °C there is some growth in the lattice fringe lengths and the distribution broadens. In stage two: 456∼504 °C there is an enhanced partial ordering (orientation and stacking) indicating some thermoplastic transformations occur for this subbituminous coal. In stage three: 524∼590 °C the partial ordering extent is reduced. Presumably at higher temperatures the appropriate balance between mass loss, tar viscosity, and other properties are not in balance for significantly improved thermoplastic behavior—although there is a recovery at highest temperatures explored here. These results can provide some structural transformations for a subbituminous coal and could influence the strategy for attempting to produce coke from subbituminous coal.

Effect of Heating Rate on Carbothermic Reduction and Melting Behavior of Iron Ore-Coal Composite Pellets

In the ironmaking processes via the carbothermic reduction with the tall-bed reactor, the composite pellets are rapidly reduced to highly metallized direct reduced iron (DRI) with high productivity under high temperature. Without coke making and ore sintering process, carbothermic reduction are not only help broaden the selection of raw materials but an environmentally friendly approach for the ironmaking process. In this study, experiments were conducted for iron ore-coal composite pellets under four heating rates. During the reduction, the reaction was quenched at different temperatures by quickly moving the specimens to room temperature. After the experiments, we performed phase analysis by X-ray diffraction, chemical composition analysis by wet method, and microstructure observation and element distribution analysis by scanning electron microscopy with energy dispersive spectrum. The pellet morphology during the experiments was also recorded. The pellets under fast heating rates (80°C/min and 40°C/min) collapsed over 1300°C. The pellets under the slow heating rate (20°C/min) maintained their spherical morphology until 1400°C. We found that the formation and melting of fayalite (2FeO–SiO2 or Fe2SiO4) played a key role in the collapsing behavior of pellets, which can be classified into three different stages, (a) between 1000°C and 1200°C, a large amount of fayalite compound (melting point at 1178°C) was formed. (b) above 1178°C, the fayalite compound started to melt and became a liquid phase. (c) between 1178°C and 1400°C, the pellets lost their strength, and collapsed due to the excess amount of liquid phase present.

Formation Mechanism of Tempering-Induced Martensite in Ti-10Mo-7Al Alloy

The formation mechanism of α′′-martensite (α′′Mt) induced by tempering at 450-550℃ for a short time was investigated using Ti-10Mo-7Al alloy. The solution treated and quenched (STQ) sample was composed of β phase and a small amount of α′′Mq, and a large amount of α′′Mt was generated by rapid tempering at 550℃-3 s using a salt bath. However, α′′Mt was completely transformed into a single β phase by aging at 200℃ for 3 min. Reversibility was observed between the α′′Mt transformation and the β reverse transformation. In-situ high-temperature X-ray diffraction measurements revealed that α′′Mq → β reverse transformation occurred at 200℃ and that a thermally activated α′′iso was generated at 450℃ due to the slow heating rate. In-situ optical microscopic observation of STQ sample with rapid lamp heating revealed that α′′Mt was formed during heating process. However, α′′Mt did not generate under following conditions; that is, a slow heating rate, thin sample plate, and a small temperature difference until tempering by preheating. On the other hand, rapid tempering using thick plate from liquid nitrogen (−196℃) to 250℃ was performed to ensure a sufficient temperature difference, but α′′Mt was not generated at all.

Determination of ignition temperature and kinetics and thermodynamics analysis of high-volatile coal based on differential derivative thermogravimetry

In order to accurately determine the ignition temperature (Ti) of coal, Coal spontaneous combustion (CSC) process was tested by a simultaneous thermal analyzer. Next, the abrupt change point of the differential derivative thermogravimetric curve (DDTG) of coal after the high adsorption temperature was taken as the Ti of CSC. Besides, variations of kinetics and thermodynamics during the CSC before and after the Ti were calculated. It was found that the mass loss, heat releases and gaseous products of coal changed slowly before the Ti and surged sharply after the Ti. The coal at ignition temperature was in thermodynamic equilibrium with low activity. However, when the temperature was greater than Ti, the aromatic hydrocarbons in coal begin to decompose, resulting in the increased of active sites and the release of volatiles, and the coal enters an irreversible combustion stage. At this time, upward trends were obtained for the apparent activation energy, pre-exponential factor, enthalpy change, and entropy change, while a gradual decrease was observed for the Gibbs free energy change. Moreover, reaction intensity between oxygen and coal would be increased due to increased of active sites in the coal and enhanced of volatiles release under a slower heating rate.