Slow Heating Process Research Articles

Experimental Study on Microstructural Changes of Nanoaluminum Particles during the Slow Heating Process in Vacuum and Aerobic Environments

Experimental Study on Microstructural Changes of Nanoaluminum Particles during the Slow Heating Process in Vacuum and Aerobic Environments

Dependence of martensitic stabilization on Ms temperature in Cu-Al-Ni shape memory alloys

The occurrence of martensitic stabilization shows a strong positive dependence on Ms temperature in Cu-Zn-Al alloys. Compared with Cu-Zn-Al alloys, Cu-Al-Ni alloys have a more excellent resistance to martensitic stabilization. However, whether the occurrence of martensitic stabilization is dependent on Ms temperature in Cu-Al-Ni alloys is still unknown. To clarify this question, we investigated the relationship between SME and Ms temperature in directly-quenched Cu-Al-Ni alloys. Results showed that the positive dependence of martensitic stabilization on Ms temperature indeed existed in directly-quenched Cu-Al-Ni alloys. The Cu-Al-Ni alloys with Ms≤488 K possess a good resistance to martensitic stabilization. For the directly-quenched Cu-Al-Ni alloys with remarkably higher than 488 K, they cannot obtain SME due to the occurrence of martensitic stabilization after recovery heating at <25 K/s. The original number of vacancies at martensite boundaries (Nvmb) rising with the increment in Ms temperature was responsible for the remarkable lowering in resistance to martensitic stabilization during the process of slow heating. To adjust the original Nvmb in alloys with remarkably high Ms temperature, we changed the Ms temperature through precipitating α phase. The precipitation of α phase can enrich the matrix with Al and Ni elements and thus decrease the Ms temperature. The reduction in Ms temperature to below ∼500 K significantly decreased the original Nvmb and improved the resistance to martensitic stabilization during the slow heating process.

A detailed exploration of microstructure evolution in a novel Ti–Mo martensitic steel through in-situ observation: The effect of heating rate

Rapid heating is a promising technique for achieving excellent mechanical properties. This study innovatively applies rapid heating to a novel Ti–Mo martensite steel through in-situ observation experiments. By comparing the microstructure evolutions at heating rates of 1 and 30 °C/s, the relationship between microstructure and property evolution was elucidated. The results indicate that the refined austenite grains resulting from rapid heating stem from enhanced grain nucleation, accumulated dislocations, and the increased formation of new carbides under the large superheat. Furthermore, carbide maturation predominates during the slow heating process, while rapid heating accelerates new carbide precipitation with limited carbide maturation. Moreover, during rapid heating, both coherent and non-coherent relationships were observed between the (Ti, Mo, Fe)C particles and the martensite matrix, demonstrating a remarkable pinning effect on grain boundary migration. Nevertheless, during slow heating, large-sized (Ti, Mo)C carbides were formed, presenting a non-coherent relationship with the martensite matrix. Additionally, the primary differences in strength caused by rapid heating compared to slow heating include grain refinement, dislocation, and precipitation strengthenings, with dislocation strengthening playing the dominant role.

Application of emission spectroscopy in plasma-assisted NH3/air combustion using nanosecond pulsed discharge

This work investigates the effects of atmospheric-pressure nanosecond (ns) pulsed plasma discharges on NH3/air flames using optical emission spectroscopy (OES). Firstly, spatially-resolved OES in the 200–800 nm range of NH3/air flames with and without discharges are presented to identify the plasma-induced excited species (such as N2* and H*). Then, time-resolved OES with ns timescale resolution are achieved to illustrate the discharge dynamics using a statistical imaging strategy, which is specifically designed to overcome the timing jitter of the discharge. From the time-resolved spectra of excited N2, the first and second emission events in NH3/air flames with a time interval of 80 ns are revealed, and each exhibits a profile with two exponential decays. Furthermore, spatial- and time-resolved rotational and vibrational temperatures are derived from the spectral fitting of N2(C→B) to quantify the thermal effects of ns plasmas, thus revealing the ultrafast and slow gas heating processes. In addition, the electron number density along the flame is obtained from the intense Hα line, which locally reaches up to 1 × 1017 cm−3. Besides, the spatial-resolved Hα line intensity suggests that the flame chemistry does not directly affect, but the discharge dominates the production of the atomic hydrogen. It provides insight into the kinetic effects of ns pulsed discharge on NH3 combustion enhancement.

The effects of heat treatment on the extrusion-induced inhomogeneity of spray-formed aluminum alloy 7055

The local inhomogeneity along the thickness direction of the extruded plate of spray-formed aluminum alloy 7055 were investigated. The thermal extrusion process was simulated by using DEFORM-3D software to understand its formation mechanism. In order to reduce the extrusion-induced inhomogeneity and improve the mechanical properties, the evolution of the secondary phase during different heat treatments and its influence on the peak aged hardness and tensile strength of the alloy were emphatically analyzed. The results show that the strain rate and temperature during the thermal extrusion process tend to decrease from the surface to the center, which leads to the local inhomogeneity of the microstructure and properties in the thickness direction. Such inhomogeneity can be reduced by heat treatments combining long-time or slow-heating solution treatments (F2 and F3) with peak ageing at 120 °C, due to similar content of S (Al2MgCu) phase after solution treatment and η′ phase (peak-ageing) in different regions of the alloy. The alloy solution treated with a slow heating rate (F3) and then peak aged shows highest hardness and tensile strength, because the slow heating process during the solution treatment promotes the precipitation of Al3Zr particles, and there are less S (Al2MgCu) phases after solution treatment which is conducive to the η′ phase precipitation during the subsequent aging process.

Role of Kinetics and Thermodynamics in Controlling the Crystal Structure of Nickel Nanoparticles Formed on Reduced Graphene Oxide: Implications for Energy Storage and Conversion Applications

Ultrafast heating has emerged recently to speed up the synthesis processes of nanoparticles and control their morphology. However, it is not clear how the heating rate affects the formation of metal nanoparticles, particularly those formed on substrates. Here, we explored the formation of nickel (Ni) nanoparticles on graphene oxide (GO) substrates under slow (20 °C/min) and ultrafast (103 °C/s) heating rates. The experiments were performed in situ on heating microchip devices using an aberration-corrected transmission electron microscope. Interestingly, the GO structure was the most effective in controlling the stability of nanoparticles when ultrafast heating was employed, leading to a hexagonally close-packed Ni phase (hcp-Ni) because of less lattice mismatch with the graphitic substrate. On the contrary, fcc-Ni nanoparticles formed under a slow heating process where no strong correlation with the GO crystal structure was observed. Additionally, ultrafast heating resulted in smaller-size nanoparticles which could be ascribed to rapid reduction, nucleation rate, and higher diffusion barrier of hcp-Ni crystals on rGO. Nevertheless, the stability of the crystal structure of the nickel nanoparticles remains unaffected by their size. These results indicate the crucial role of the substrate on crystal structure during the nonequilibrium processing of materials and the competing effects of thermodynamics versus kinetics in creating novel phases of materials for energy storage and conversion applications.

Influence of pre-ignition technology on combustion and explosion characteristics of composite propellants

The slow cook-off test is widely used for analyzing the safety of propellants under accidental thermal stimulation. To examine the influence of the pre-ignition technology of a composite propellant on thermal safety and combustion characteristics, a quantitative evaluation experiment of slow cook-off reaction intensity of a typical composite propellant charge was designed and executed, considering hydroxyl-terminated polybutadiene (HTPB) and hydroxyl‑terminated block copolyether (HTPE) propellants containing ammonium perchlorate (AP) and aluminum (Al). The experimental method for quantitative evaluation of the slow cook-off reaction intensity is highly essential to investigate the safety of ammunition. The influence of pre-ignition on reaction intensity was analyzed at different temperatures, the combustion behavior of the heated propellant was studied using a closed bomb experiment, and the influence of pre-ignition on combustion evolution was analyzed. Finally, the process was characterized by theoretical modeling. The results show that the slider speed was 169.3 m/s when the HTPB/AP/Al propellant was ignited at 234 °C. The slider speed was reduced to 43.4 m/s and 61.8 m/s by pre-ignition at 120 °C and 214 °C, respectively. When the spontaneous ignition temperature of HTPE/AP/Al propellant was 212 °C, the slider speed was 110.5 m/s. Pre-ignition at 120 °C and 192 °C reduced the slider speed to 39.4 m/s and 43.7 m/s, respectively. Pre-ignition can effectively reduce the reaction intensity. Under the same conditions, the reaction strength of HTPE/AP/Al propellant is marginally lower than that of HTPB/AP/Al propellant. In the slow heating process before ignition, with the increase in ignition temperature, when HTPB/AP/Al propellant was pre-ignition at 120 °C and 214 °C, the pressurization time was 9.8 ms and 6.4 ms, respectively, while before ignition, the pressurization time decreased to 5.5 ms. At 120 °C and 192 °C, the pressurization time of HTPE/AP/Al propellant was 50.9 ms and 36.6 ms, respectively, while before ignition, the pressurization time decreased to 26.9 ms. The reduction in ignition temperature decreased the pressure growth rate of propellant during combustion and reduced the pressurization time. A calculation model of the propellant combustion process was established to characterize the growth process of the combustion pressure at different ignition temperatures. This model can effectively predict the combustion evolution of composite propellants after the slow cook-off ignition and aid in ensuring propellant safety.

Algae Pyrolysis in Molten NaOH-Na2CO3 for Hydrogen Production.

Biomass pyrolysis within the alkaline molten salt is attractive due to its ability to achieve high hydrogen yield under relatively mild conditions. However, poor contact between biomass, especially the biomass pellet, and hydroxide during the slow heating process, as well as low reaction temperatures, become key factors limiting the hydrogen production. To address these challenges, fast pyrolysis of the algae pellet in molten NaOH-Na2CO3 was conducted at 550, 650, and 750 °C. Algae were chosen as feedstock for their high photosynthetic efficiency and growth rate, and the concept of coupling molten salt with concentrated solar energy was proposed to address the issue of high energy consumption at high temperatures. At 750 °C, the pollutant gases containing Cl and S were completely removed, and the HCN removal rate reached 44.92%. During the continuous pyrolysis process, after a slight increase, the hydrogen yield remained stable at 71.48 mmol/g-algae and constituted 86.10% of the gas products, and a minimum theoretical hydrogen production efficiency of algae can reach 84.86%. Most importantly, the evolution of physicochemical properties of molten NaOH-Na2CO3 was revealed for the first time. Combined with the conversion characteristics of feedstock and gas products, this study provides practical guidance for large-scale application of molten salt including feedstock, operation parameters, and post-treatment process.

Enhanced mechanical properties in metastable β titanium alloy via ω-assisted nucleation

In this study, we performed heat treatment with different solution cooling routes and aging heating rates were performed on the TB18 alloy (Ti-5.3Cr-4.9Mo-4.9V-4.3Al-0.9Nb-0.3Fe) to enhance its mechanical properties via ω-assisted nucleation. The results showed that the intragranular α plates that precipitated during aging appeared finer under slow heating than under fast heating because the ωiso particles formed during the slow heating process provide abundant ω/β interfaces, which can serve as heterogeneous nucleation sites for α precipitates and reduce the nucleation driving force. The corresponding phase transformation sequence is expected to be β + ωath → β + ωath + ωiso → β + ωiso + α → β + α during continuous heating. Further comparing the microstructures, we found that a faster solution cooling rate is more conducive to ω-assisted nucleation. During ω-assisted nucleation, the three-phase orientation relationship holds for: (112‾0)ω//(0001)α//(110)β, and [0001]ω//[112‾0]α//[11‾1]β. At the atomic scale, the ω/α interface appeared as a relatively straight atomic plane, whereas more defects, termed as disconnections, were observed at the α/β interface. Moreover, appropriate ω-assisted nucleation could enhance the ductility of the TB18 alloy without reducing its strength to obtain the optimal comprehensive mechanical properties, and the corresponding fracture mechanism was a mixed mode dominated by ductile fracture. Excessive ω-assisted nucleation could considerably strengthen the alloy but significantly deteriorated its ductility, and the mixed fracture mode was more inclined to brittle fracture.

Paper structures become 3D semiconductors after slow-heating process

Paper structures become 3D semiconductors after slow-heating process

Rapid and Energetic Solid-State Metathesis Reactions for Iron, Cobalt, and Nickel Boride Formation and Their Investigation as Bifunctional Water Splitting Electrocatalysts

Metal borides have long-standing uses due to their desirable chemical and physical properties such as high melting points, hardness, electrical conductivity, and chemical stability. Typical metal boride preparations utilize high-energy and/or slow thermal heating processes. This report details a facile, solvent-free single-step synthesis of several crystalline metal monoborides containing earth-abundant transition metals. Rapid and exothermic self-propagating solid-state metathesis (SSM) reactions between metal halides and MgB2 form crystalline FeB, CoB, and NiB in seconds without sustained external heating and with high isolated product yields (∼80%). The metal borides are formed using a well-studied MgB2 precursor and compared to reactions using separate Mg and B reactants, which also produce self-propagating reactions and form crystalline metal borides. These SSM reactions are sufficiently exothermic to theoretically raise reaction temperatures to the boiling point of the MgCl2 byproduct (1412 °C). The chemically robust monoborides were examined for their ability to perform electrocatalytic water oxidation and reduction. Crystalline CoB and NiB embedded on carbon wax electrodes exhibit moderate and stable bifunctional electrocatalytic water splitting activity, while FeB only shows appreciable hydrogen evolution activity. Analysis of catalyst particles after extended electrocatalytic experiments shows that the bulk crystalline metal borides remain intact during electrochemical water-splitting reactions though surface oxygen species may impact electrocatalytic activity.

Numerical investigations of turbulent premixed flame ignition by a series of Nanosecond Repetitively Pulsed discharges

Nanosecond Repetitively Pulsed (NRP) discharges are an efficient way to promote turbulent flame ignition in lean regimes. The energy released by NRP discharges leads first to an ultra-fast species dissociation and heating phenomena, followed by a slow heating process. A phenomenological plasma model is presented to capture the influence of NRP discharges on the combustion process at low CPU cost. The model is here implemented in a LES flow solver to simulate the ignition sequence of a bluff-body turbulent premixed flame by a series of NRP discharges. Two numerical computations are performed. First, only the thermal effects of the discharge (ultra-fast heating and slow heating due to vibrational energy relaxation) are taken into account. Then both the thermal and chemical effects (mainly O2 dissociation into O) are considered. The results show that in the first simulation the ignition never occur, whereas in the second simulation flame ignition occurs after only 5 pulses. The ignition success or failure results from a competition between the residence time of the reacting gases in the discharge channel and the combustion chemistry time scale. A low-order model based on a perfectly stirred reactor (PSR) is then derived. It confirms that the atomic O produced during the discharge enhances the methane oxidation reactions, reducing the combustion chemistry time scale and leading to a successful ignition. PSR results are used to build-up a plasma-assisted ignition diagram which indicates the number of pulses required to form a turbulent flame kernel.

Contactless Thermal Diagnostics of Acoustically Levitated Biomass under Uniform High Flux Radiation

This study reports a contactless optical system developed for investigating the fast-thermal processes of biomass under high-flux radiation, particularly for understanding the synergy of renewable biomass and concentrated solar energy. A biomass tablet was successfully suspended using a home-built acoustic levitator in a well-controlled oxygen-lean atmosphere and was irradiated under uniform radiation with a high flux of approximately 1 MW/m², i.e., ∼1000 suns. The biomass temperature profile was spatio and temporally recorded using an infrared thermographic camera. Several key thermal parameters of the biomass were determined, including the time-resolved heating rate and the ignition temperature. Three different thermal processes were identified from the temperature profiles. These are an initial fast-heating process, a following slow-heating process, and a final biomass ignition depending on the flux of radiation. At high flux, these three processes are merged, and only a steep linear increase in temperature was observed. The contactless apparatus provides the high-fidelity data of the heating rates of biomass and can benefit the understanding of the fast-thermal process associated with biomass under high flux concentrated solar radiation.

Single Goss grain growth by isothermal annealing in rolled Fe–Al–Ga–NbC sheets

The (Fe82Al13.5Ga4.5)99.9(NbC)0.1 sheets with strong Goss texture and large magnetostriction were prepared by rolling and isothermal annealing processes. Single Goss grain growth was observed in sheets undergoing isothermal annealing at 1050 °C for 1 h using Ar as protecting gas. With annealing time increasing to 8 h at 1050 °C, a largest magnetostriction of 154 × 10−6 is obtained in sheet and a large λ// (the maximum magnetostriction with magnetic field parallel to rolling direction) of 160 × 10−6 along the rolling direction is also observed. The fact that a λ// of 160 × 10−6 along RD can be achieved with only 4.5 at% Ga in the Fe-18 at% (Al, Ga) sheet is of practical importance. Compared with previous studies in Fe–Al and Fe–Ga sheets, this paper provides a more efficient approach to prepare strong Goss textured sheets. In this way, a slow continuous heating process, sulfur-induced surface energy control and high-temperature annealing above 1200 °C are no more necessary. It shows a good prospect for industrial production of high-performance magnetostrictive alloy sheets.

Chromospheric Response during the Precursor and the Main Phase of a B6.4 Flare on 2005 August 20

Abstract Solar flare precursors depict a constrained rate of energy release, in contrast to the imminent rapid energy release, which calls for a different regime of plasma processes to be at play. Due to the subtle emission during the precursor phase, its diagnostics remain delusive, revealing either nonthermal electrons (NTEs) or thermal conduction to be the driver. In this regard, we investigate the chromospheric response during various phases of a B6.4 flare on 2005 August 20. Spatiotemporal investigation of flare ribbon enhancement during the precursor phase, carried out using spectra images recorded in several wavelength positions on the Hα line profile, revealed its delayed response (180 s) compared to the X-ray emission, as well as a sequential increment in the width of the line profile, which are indicative of a slow heating process. However, the energy contained in the Hα emission during the precursor phase can reach as high as 80% of that estimated during the main phase. Additionally, the plasma hydrodynamics during the precursor phase, resulting from the application of a single-loop one-dimensional model, revealed the presence of a power-law extension in the model-generated X-ray spectra, with a flux lower than the RHESSI background. Therefore, our multiwavelength diagnostics and hydrodynamical modeling of the precursor emission indicates the role of a two-stage process. First, reconnection-triggered NTEs, although too small in flux to overcome the observational constraints, thermalize in the upper chromosphere. This leads to the generation of a slow conduction front, which causes plasma heating during the precursor phase.

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells.

As a convenient preparation technique, a two-step method, which is normally done by spin-coating CH3NH3I onto PbI2 film followed by a thermal annealing, is generally used to prepare solution-processed CH3NH3PbI3 films for planar perovskite solar cells. Here, we prepare the compact CH3NH3PbI3 thin films by the two-step method at a low temperature (<80 °C) and investigate the effects of PbI2 crystallization on the structure-property correlation in the CH3NH3PbI3 films. It is found that the importance of the crystallization in PbI2 matrix lies in governing the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI2 film has a much stronger ability to react with CH3NH3I solution and produces larger CH3NH3PbI3 grains with a higher crystallinity. The CH3NH3PbI3/TiO2 planar solar cell derived from a better crystallized PbI2 film exhibits significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI2 film matrix subjected to the annealing after a slow heating process prior to contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The results in this paper provide a guide for preparing high-quality CH3NH3PbI3 thin films for efficient perovskite solar cells and CH3NH3PbI3 interfacial films over the layers susceptible to temperature.

Microstructure evolution of 7085 aluminum alloy with different initial deformation energy storage in fast and slow heating process

The effect of initial deformation stored energy, target temperatures and heating rates on the microstructure and texture as well as the hardness and conductivity of 7085 aluminum alloy were investigated through hardness test, conductivity test, x-ray diffraction (XRD) analysis and electron backscatter diffraction scans (EBSD) measurement. The results revealed that with increasing the target temperature, the dislocation of the alloy was consumed, and the consumption of hot-pressed 7085 alloy was much faster than that of extruded alloy. Moreover, for the extruded alloy with low stored energy, the slow heating at rate of 3.6 °C h−1 could effectively suppress the recrystallization, while for hot-pressed 7085 alloy with high stored energy, the rapid heating at rate of 180 °C h−1 could not suppress the recrystallization. The different heat treatment also had great effect on the hardness and conductivity of 7085 alloy.

Effect of Solution Treatment on the Properties and Microstructural Evolution of Al-Zn-Mg-Cu-Er-Zr Alloy

Effect of solution treatment on the properties and microstructural evolution of Al-Zn-Mg-Cu-Er-Zr alloy was investigated by hardness test, conductivity test, exfoliation corrosion test, scanning electron microscope (SEM), transmission electron microscopy (TEM). The results show that the hardness of single-stage solution treated samples first increase and then decrease, the conductivity and volume fraction of the residual phase particles decrease with increasing solution temperature. Compared to conventional multi-stage solution, slow heating process solution treatment can dissolve plenty of residual phases and further improve the hardness and decrease conductivity. Besides, the effect of solution treatment on EXCO is discussed, the result of exfoliation corrosion of SHT8 sample test is judged as EA. Moreover, the effect of the presence of nanometer-sized Al3(Er,Zr) particles on the microstructural evolution during solution treatment has also been discussed in detailed.

Sequential control of step-bunching during graphene growth on SiC (0001)

We have investigated the relation between the step-bunching and graphene growth phenomena on an SiC substrate. We found that only a minimum amount of step-bunching occurred during the graphene growth process with a high heating rate. On the other hand, a large amount of step-bunching occurred using a slow heating process. These results indicated that we can control the degree of step-bunching during graphene growth by controlling the heating rate. We also found that graphene coverage suppressed step bunching, which is an effective methodology not only in the graphene technology but also in the SiC-based power electronics.

Extended Validation of Dynamic Irreversible Thermoporation: A Novel Thermal Process for Microbial Inactivation

AbstractA novel thermal treatment for microorganism inactivation, characterized by a very rapid temperature increase (up to 30°C/s) and a low final temperature (up to 65°C) maintained for a relatively short holding time, has been recently presented and tested by the authors, showing microbial load reduction greater than 5 log units against several common bacteria and yeasts. With the aim of extending the possible use of the new thermal treatment to a wider microorganisms class, in this work the dynamic irreversible thermoporation (DIT) treatment was further tested on a well‐known thermoresistant strain, the Enterococcus hirae: The results of these new experimental tests confirmed the reliability of the process, which allowed to reach the 5 log microbial reduction once the adequate holding time was employed.The comparison with simple immersion in a thermostatic bath, where the very slow heating process with 0.3°C/s has been performed, confirmed the crucial role of the thermal shock for the success of the treatment. The inactivation kinetics of E. hirae in isothermal conditions immediately after the application of thermal shock has also been studied. Finally, the morphological analysis performed by using a scanning electron microscope clearly revealed the rupture of the cell membrane, leading to identification of the process called dynamic irreversible thermoporation (DIT).Practical ApplicationsThe main features of the dynamic irreversible thermoporation (DIT) process produce two major advantages with respect to the traditional heat treatments commonly employed for beverages: one perfectly agrees with production economics principles, while the other concerns the quality of the product. First, the DIT process can be implemented directly on bottled products because its low maximum temperature of 65°C is compatible with the plastic materials usually employed for sealed beverages: Its online application in the filling process could hence be conveniently considered, thus avoiding both the use of an expensive aseptic environment and the risk of post‐processing contamination. Second, by using such a low final temperature, the thermal degradation of the food components is avoided, with consequent preservation of the sensory and nutritional quality of the fresh‐like food products.