Isothermal Time Research Articles

Enhancing carbon fibre recovery through optimised thermal recycling: Kinetic analysis and operational parameter investigation

Thermal recycling is an attractive method for recovering carbon fibre from carbon fibre-reinforced polymer (CFRP) composite waste. The main drawbacks of the thermal recycling approach are its high operational cost resulting from significant energy requirements and the risk of fibre surface damage caused by overheating. In this research, we will establish an optimal heating temperature and operating parameters (such as atmosphere, heating rates and isothermal dwelling time) through an analysis of the kinetic behaviour of CFRP thermal degradation to obtain clean and undamaged recycled fibres, while minimising energy requirements. The results show that a higher conversion fraction can be reached by applying a lower nitrogen flow rate, moderate heating rate and cap temperature of 425 °C. To ensure the complete removal of any residual matrix and pyrolytic carbon, and to obtain clean recycled carbon fibres, an additional recycling step is employed. In this case, oxidation is conducted with a 45-min isothermal period following pyrolysis. Using the proposed method, it was possible to retain 87.6 % of the fibre's tensile modulus and 80.3 % of its strength after the recycling process. The outcomes of this research will contribute to production of high-grade recycled carbon fibres with controlled mechanical performance and physical characteristics.

Kinetics of isothermal solidification during transient liquid phase bonding of Inconel 718 superalloy by differential scanning calorimetry

In the transient liquid phase bonding of Inconel 718 alloy, the time for isothermal solidification is measured by using the cyclic differential scanning calorimetry method. The enthalpy of solidification of the remnant liquid is used in the current investigation to determine the isothermal solidification rate and time. The differential scanning calorimetry results reveal a reduction in interlayer width during the heating and melting stage of transient liquid phase bonding due to the rapid interdiffusion of elements. The result also reveals that the isothermal solidification time for a eutectic-free joint depends upon the actual width of the interlayer material. The actual width of the interlayer material prior to isothermal solidification is found to be less than the initial width of the interlayer material due to rapid diffusional solidification. The measured isothermal solidification time and rate for transient liquid phase bonding of the Inconel 718 joint are in good agreement with the experimental and theoretical results.

Pyrolysis of Domestic Sewage Sludge: Effect of Process Parameters on Biochar Calorific Value

Abstract This research aims to look into a sustainable technique for the treatment, reuse and disposal of domestic sewage sludge (DSS). The purpose of the study was to examine the operating factors that influence the calorific value of the produced biochar from the pyrolysis of DSS. Based on the analysis of the full factorial design, the impacts of the pyrolysis conditions, specifically: temperature, heating rate, and isothermal time on the calorific value of biochar were evaluated. When the pyrolysis temperature was raised from 300 to 500 oC, the calorific value of biochar was decreased by 34%. A 14% decrease in the calorific content of the biochar was also noticed when the heating rate was increased from 5 to 35 oC/min. When the isothermal time was increased from 45 to 120 minutes, the calorific value of the biochar remained essentially unchanged. No interaction effects among process variables were found using the factorial design methodology. A first-order regression model was developed to predict the calorific value of biochar using the magnitude of the effects of the process factors and their interactions. The model predictions agreed very well with the obtained experimental results.

Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M23C6 and Mo‐rich M6C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models.

Optimizing temperature coefficient of Sm2Co17-type magnets through adjusting the isothermal aging process

The high-temperature magnetic performance and microstructure of Sm1–xGdx(CobalFe0.09Cu0.09Zr0.025)7.2 (x = 0.3, 0.5) magnets were investigated. With the isothermal aging time decreasing from 11 to 3 h, the temperature coefficient of intrinsic coercivity in the temperature range of 25–500 °C, β25–500 °C, was optimized from −0.167%/°C to −0.112%/°C for x = 0.3 magnets. The noticeable enhancement (∼33%) of temperature stability is correlated with the increased content of 1:5H cell boundary phase and its relatively high Curie temperature as well. However, for the x = 0.5 magnet, it is found that the presence of Sm5Co19 phases and wider nanotwin variants hinder the formation of 1:5H cell boundary phase. The insufficient 1:5H is not beneficial to the proper redistribution of Cu in cell boundary, making the x = 0.5 magnet difficult to achieve higher temperature stability. Consequently, the approach of adjusting the isothermal aging process can offer guidance for attaining superior magnetic performance in the temperature range from 25 to 500 °C for Gd-substituted Sm2Co17-type magnets.

Microstructure and mechanical properties of 7075 aluminum alloy parts formed by semi-solid thixoextrusion

Hot-extruded 7075 aluminum alloy was used to form deep cavity cylinders by thixoextrusion. The effects of isothermal temperature and soaking time on the microstructure and mechanical properties of the formed parts were analyzed. The parameters of T6 heat treatment were optimized by orthogonal test, and the influence of T6 heat treatment on the microstructure and mechanical properties of the formed parts was studied. The results indicated that the deep cavity cylinder with a smooth surface could be successfully formed through thixoextrusion. The forming temperature and heating time had remarkable effects on the metallographic structure of the formed components. T6 heat treatment could greatly enhance the mechanical properties of the deep cavity cylinders. The optimum parameters of T6 heat treatment were achieved as: solution at 465 °C for 16 h and aging at 150 °C for 16 h. When semi-solid billets were heated at 600 °C for 10 min, the formed deep cavity cylinder had the best mechanical properties with the ultimate tensile strength of 573.57 MPa, the elongation of 13.44%, and the microhardness of HV 187.12.

Influence of the isothermal annealing time on structural morphological, and photocatalytic characters of BiT/ZnWO4 composite

The abbreviated composite Bi4Ti3O12/ZnWO4 (BIT/ZW) was produced using the molten salt technique at 800°C. The impact of isothermal annealing time, in the second step, on the structural, morphological, and photocatalytic properties was investigated. X-ray diffraction (XRD) confirmed the formation of composites with varying proportions (76.8% BIT, 20.2% ZW), (71% BIT, 22.8% ZW), and (56.4% BIT, 36.6% ZW) at annealing times of 4, 6, and 8 hours at 950°C, respectively. A minor amount of pyrochlore phases (<7%) was observed. Increasing the annealing time affected the crystallinity of the composites. The crystallite size and grain size of BIT and ZnWO4 decreased with longer annealing times, leading to a reduction in the band gap from 2.95 to 2.83 eV. The composite (76.8% BIT, 20.2% ZW) exhibited photocatalytic activity that was two times higher for degrading methylene blue (MB) in the dark compared to the other composites, but showed an improvement in degradation under solar irradiation from 70% to 72.18%. The results indicate that annealing isothermal time can effectively modify the properties of the composites, creating a proven heterojunction between BIT and ZnWO4.

Using an aromatic amide as nucleating agent to enhance the crystallization and dimensional stability of poly(3-hydroxybutyrate-co-3-hydroxyhexanate)

Biosynthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) has emerged as a promising biodegradable polymer with a great potential to compete with traditional petroleum-based plastics, however, the poor crystallization ability makes it challenge to transform into high-performance products via common melt-processing methods. Herein, we demonstrate that N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide (TMB) can serve as an efficient nucleating agent to significantly enhance the crystallization and resulting storage stability of PHBHHx. The results indicate that PHBHHx with small amounts of TMB (0.3–0.5 wt%) can crystallize completely even under a rapid cooling rate of 100 °C/min and the isothermal crystallization time is greatly reduced. As a result, the crystallinity of the injection-molded PHBHHx products is increased from 24.5 % to 39.5 %, without secondary crystallization after being stored at room temperature for 6 h. The products exhibit superior dimensional stability and the post-shrinkage can be decreased to as low as 0.1 %. Our work offers a feasible method to develop high-performance PHBHHx materials with remarkably enhanced crystallization ability.

The Effect of Soaking Time on Mechanical Properties of Roll-Bonded AA3003 and AA4045 Used for Heat Exchangers

Due to the rising need for energy saving, high-performing automotive heat exchangers, demand has significantly grown in recent years. As a result, effective fin-tube heat exchangers are becoming more popular. These tubes are typically made by rolling flat strips of AA3003 aluminum alloys that have either one or both sides coated with AA4xxx alloys. The AA3003 type of alloy is typically used as the core, which is then covered in either AA4045 or AA4343, which melts during the brazing process to adhere the fins to the tubes. To maintain the optimal size and distribution of manganese (Mn)-containing precipitates, preheating parameters are carefully controlled. Then, longer soaking times or higher soaking temperatures result in larger precipitates, which cause the final product to exhibit poor mechanical properties. Therefore, it is crucial to optimize the different manufacturing steps, such as homogenization, soaking time, and brazing in order to achieve a high quality product. Studies on the impact of homogenization temperature and time on the microstructure of AA3xxx aluminum alloys have been conducted. However, there has been little research on the impact of soaking (reheating) time on AA3003 cladded alloys. Hence, the effects of isothermal soaking time on the microstructure and mechanical properties of AA3003 cladded with AA4045 alloy were investigated in this work. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure and identify intermetallic phases. The final microstructure in terms of grain structure at various homogenization times was characterized by electron backscattered diffraction (EBSD). After the hot-rolling and cold-rolling of the as-received material, large particles of intermetallic (mainly in the form of Chinese script morphology consisting of Fe-Mn-Si) were broken into smaller particles with an increased Fe, Mn, and Si content, indicating the formation of an α-Al(Fe,Mn)Si phase. The α-Al(Mn,Fe)Si was found to be a dominant dispersoid precipitate in the modified AA3003 core. Coarsening of the Al(Mn,Fe)Si dispersoids at 505 °C was only observed at a 45 h homogenization time. The hardness trend with homogenization time was found to be similar to that after homogenization, cold working, and annealing, with exception of an increase in hardness in the latter possibly due to strain hardening (from cold-rolling).

Kinetic criterion for triple-shape memory effect in amorphous polymers undergoing heterogeneous glass transitions

The triple-shape memory effect (triple-SME) in amorphous polymers arises from the heterogeneous glass transitions of different components. However, due to the complex composition and structural relaxation of triple-shape memory polymers (triple-SMPs), existing models struggle to characterize the shape memory temperature range for each component and its dependence on thermal history. This presents a challenge in developing a kinetic criterion to judge the occurrence of the triple-SME. To address this issue, we propose a kinetic phase transition model to predict the shape memory temperature range for each transition phase in triple-SMPs under different thermal histories. By combining the Tool-Narayanaswamy-Moynihan (TNM) model with the Adam-Gibbs theory, the effect of cooling rate during programming on subsequent triple-SME is investigated. Subsequently, the effectiveness of the proposed model is evaluated by applying it to predict the shape memory performance and thermomechanical behavior of SMPs with dual- and triple- SMEs under different thermal histories. Using this modeling strategy enables the calculation of the critical isothermal shape recovery time for each transition phase under different thermal histories, effectively preventing shape recovery overlap. Consequently, the proposed model is expected to provide theoretical guidance for designing SMPs with triple-SME and promoting their application in engineering.

Decomposition Mechanism of Prior Austenite in Low‐Temperature Zone and Its Relationship to Fracture Behavior of High‐Strength Medium‐Mn Steel

Herein, the isothermal and continuous cooling transformation mechanisms of prior austenite and its relationship to fracture behavior of high‐strength medium‐Mn steel are studied. Furthermore, the precipitation behavior of cementite is also analyzed. The results indicate that the short rod‐like cementite forms in the martensite matrix during the air‐cooling process, the solubility difference of supersaturated elements provides a driving force for the element diffusion. Accordingly, it is proved that no isothermal transformation of prior austenite occurs during the 360 °C × 7 days isothermal treatment. The lath‐like microstructure is the martensite, which is produced during the subsequent continuous cooling process. The prior austenite is “divided” by the preferential martensite (transformed via heterogeneous nucleation), and the interfaces between preferential martensite and untransformed austenite provide nucleation sites for the subsequent martensite. The introduction of initial martensite before the isothermal treatment leads to the reversible brittleness of medium‐Mn steel. The fracture strength decreases with the increase of the initial martensitic fraction and isothermal time. The grain boundary binding force of prior austenite grains (PAGs) increases in the experimental steels subjected to annealing, and the plastic deformation of the martensite matrix is prior to grain boundary cracking, which inhibits the intergranular brittle fracture.

Nanocrystallization of Cu46Zr33.5Hf13.5Al7 Metallic Glass

The recently discovered Cu46Zr33.5Hf13.5Al7 (at.%) bulk metallic glass (BMG) presents the highest glass-forming ability (GFA) among all known copper-based alloys, with a record-breaking critical casting thickness (or diameter) of 28.5 mm. At present, much remains to be explored about this new BMG that holds exceptional promise for engineering applications. Here, we report our study on the crystallization behavior of this new BMG, using isochronal and isothermal differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). With the calorimetric data, we determine the apparent activation energy of crystallization, the Avrami exponent, and the lower branch of the isothermal time–temperature–transformation (TTT) diagram. With XRD and TEM, we identify primary and secondary crystal phases utilizing samples crystallized to different degrees within the calorimeter. We also estimate the number density, nucleation rate, and growth rate of the primary crystals through TEM image analysis. Our results reveal that the crystallization in this BMG has a high activation energy of ≈360 kJ/mole and that the primary crystallization of this BMG produces a high number density (≈1021 m−3 at 475 °C) of slowly growing (growth rate < 0.5 nm/s at 475 °C) Cu10(Zr,Hf)7 nanocrystals dispersed in the glassy matrix, while the second crystallization event further produces a new phase, Cu(Zr,Hf)2. The results help us to understand the GFA and thermal stability of this new BMG and provide important guidance for its future engineering applications, including its usage as a precursor to glass–crystal composite or bulk nanocrystalline structures.

Fatigue Properties of Thixoformed Al-6wt%Si-2.5wt%Cu

One of the main functions of semisolid metal alloy forming processes, notably rheocasting and thixoforming, is the manufacture of parts, casings and frames for mechanical assemblies and systems. These parts not only must have the required minimum mechanical properties in terms of yield strength and elongation, but also must be able to withstand cyclic tensile and compressive forces. However, there is little fatigue strength data for the materials used for these parts. The present work seeks to fill this gap by determining the fatigue limit at 107 cycles of Al-6.0wt%Si-2.5wt%Cu alloy, or simply Al6Si2.5Cu, thixoformed in a pneumatic press at 585 °C (the temperature corresponding to 40 % solid fraction) with isothermal treatment times of 30 and 60 seconds. The parts were also subjected to T6 heat treatment, for which they were solution heat treated at a temperature of 520 °C for 4 hours followed by aging at 180 °C for 10 hours. For all the conditions tested, the microstructures were characterized to determine the grain size, appearance and shape of the silicon particles, in addition to the residual porosity. For the best conditions observed, 30 s holding time and T6 heat treatment, the grain size varied between 100 µm and 130 µm; the shape factor was around 0.60, indicating an excellent degree of roundness; and there was low residual porosity of around 0.3 %, resulting in a yield strength of up to 240 MPa with 4.5 % elongation. The average fatigue strength was estimated by the staircase method and was between 95 MPa and 98 MPa for 107 cycles.

Study on the growth mechanism of the internal oxide layer in 9% Ni cryogenic steel

The oxidation behavior of the Ni-rich layer in the internal oxide layer (IOL) in 9% Ni cryogenic steel is investigated at 1,150°C for 0–240 min in the air atmosphere. The morphology and phase composition of the Ni—rich layer are analyzed with energy dispersive spectroscopy, scanning electron microscopy, metallographic microscopy, and X—ray diffraction. The results show that the Ni—rich layer mainly consists of gray Fe3O4/FeO and white Ni–Fe particles, with a small amount of black Fe2SiO4. The morphologies of Ni–Fe particles undergo the following changes with isothermal oxidation time: dot—like → strip—like → net-like; at the same time, layered Ni–Fe particles were formed at about 1/3 of the thickness of the Ni—rich layer. Compared with the dot-like Ni–Fe particle, the net-like and layered Ni–Fe particles provide a fast path for the diffusion of O in the Ni—rich layer. However, the experimental steel still has a much lower oxidation rate because of the hindrance of Ni–Fe particles on the out-diffusion of Fe. During the oxidation process, the Kirkendall effect induces pores/cavities in the IOL, which weakens the stability of the IOL. In the end, the spalling phenomenon of the layered Ni–Fe particle occurs at 1,150°C for 180 min.

Why Volume and Dynamics Decouple in Nanocomposite Matrices: Space that Cannot Be Accessed is Not Free.

Polymer nanocomposites have important material applications and are an ongoing focus of many molecular level investigations, however, puzzling experimental results exist. For example, specific volumes for some polymer nanocomposite matrices are 2% to 4% higher than for the neat polymer; in a pure polymer melt this would correspond to a pressure change of 40 to 100MPa, and a decrease in isothermal segmental relaxation times of 3 to 5 orders of magnitude. However, the nanocomposite segmental dynamics do not show any speed up. We can explain this apparent uncoupling of dynamics from specific volume, and the key is to consider the system expansivity, i.e., the temperature dependence of the volumetric data, together with the concept of limiting volume at close liquid packing. Using pressure, volume, temperature data as a path to both, we are able to predict the effect of nanoadditives on the accessible, i.e., free, space in the material, which is critical for facilitating molecular rearrangements in dense systems. Our analysis explains why an increase in specific volume in a material may not always lead to faster segmental dynamics.

Controllable cell structures of poly(ether-block-amide) foams via isothermal melt crystallization-foaming in supercritical CO2

A simple isothermal melt crystallization-foaming method was proposed to regulate the crystallinity (Xc) of PEBA to prepare poly(ether-block-amide) (PEBA) foams with uniform or bimodal cell structures by supercritical CO2. PEBA with different Xc values was obtained by controlling the isothermal crystallization temperature (Tiso) and time (tiso). The rheological and DSC results showed that the viscoelasticity and Xc of the samples increased with decreasing Tiso and increasing tiso. PEBA foams with Xc above 1.1% exhibited a distinct bimodal cell structure. The increased crystals acted as heterogeneous nucleation sites to increase the cell density from 6.8 × 106 cells/cm3 to 3.58 × 1010 cells/cm3. The size of large cells decreased from 206.4 µm to 48.0 µm, and the number ratio of small to large cells increased from 2.34 to 65 times. Finally, a mechanism to explain the cell structure variation in PEBA foams was proposed, providing a new strategy for regulating the cell structure of polymeric foams.

Effect of chemical composition and isothermal treatment time on the microstructure and mechanical properties of high-strength carbide-free bainitic steel

Recent works about the development of carbide-free bainitic (CFB) steels use chemical compositions that require too long isothermal treatments for the bainitic transformation, making it difficult their manufacture under industrial continuous annealing processes. The present work reports an approach for designing CFB steels based on a computer simulation of phase transformations as a function of chemistry. The effect of chemistry and isothermal bainitic treatment (IBT) time on the microstructure and mechanical properties of CFB steels are also investigated. Results show that CFB steels of third generation, containing about 96% of bainite, can be obtained from short IBT times, opening the possibility to obtain these steels under the said conditions.

Unraveling the Correlation between the Synthesis Time and Electrochemical Performance of Transition Metal Layered Oxides by In Situ Neutron Powder Diffraction

Ni-rich layered transition metal oxides are promising cathodes for Li-ion batteries due to their low cost and high theoretical capacity. However, their practical applications are hindered by the capacity fading caused by intrinsic lattice structure variations, such as changes in the atom arrangement and valence. In situ neutron powder diffraction is a powerful technique for studying the structure of battery materials and is expected to provide more information than other techniques due to its nondestructive, high-resolution, and light-element probing capability. Herein, we employed neutron powder diffraction to probe the structural evolution during the synthesis of LiNi0.5Co0.2Mn0.3O2 in real time, including the transition metal–oxygen/lithium–oxygen (TM–O/Li–O) bond, phase formation, and lattice parameters as a function of temperature and isothermal dwelling time. The results revealed that the lattice structure of cathode materials is a function of temperature and isothermal dwelling time. Variations of the TM–O/Li–O bond and lattice parameters with the dwelling time at an annealing temperature of 850 °C indicated that the instability of the structure of the layered transition metal oxides may be a possible mechanism for the changes in the discharging capacity and cycle stability of the battery performance. Our findings provide insights into the correlation between the annealing time of NCM cathodes and the electrochemical performance and can be very helpful for synthesizing high-performance transition metal layered oxide materials.

Fine demolition wastes as Supplementary cementitious materials for CO2 reduced cement production

Construction and demolition waste accounts for a significant amount of the total solid waste produced worldwide, and its recycling is challenging. Although some demolition waste is processed into recycled sand and rubble, the finer fractions resulting from screening and washing of recycled aggregates are not used. This research investigates the potential of use of real demolition wastes, namely concrete screening fines (CS), mixed concrete-ceramic screening fines (MS), and mud from recycled aggregates washing (WM), as supplementary cementitious materials (SCMs) in eco-efficient blended cement. The study employed various experimental methods, such as isothermal calorimetry, thermogravimetric analysis (TGA), and setting time tests, to evaluate the hydraulic activity of waste materials and the Chapelle test and TGA to assess their pozzolanic activity. The mechanical properties and microstructure of mortars containing 20% of waste powders were evaluated using compressive strength tests and scanning electron microscopy (SEM). The results showed that thermal treatment of waste materials at 500 °C improved the mechanical properties of mortars, increasing Strength Activity Index (SAI) by 10% for CS and MS and by 6% for WM after 90 days of curing. All three waste types achieved similar mechanical properties, with compressive strengths of at least 37.93 MPa, 46.25 MPa, and 51.33 MPa after 7, 28, and 90 days of curing, respectively. The contribution of waste powders to mortar strength was due to filler effect and partially dehydrated C-S-H products. However, pozzolanic ceramic inclusions in waste powders did not affect mortar strength at a 20% substitution rate. Therefore, the research findings indicate that waste materials derived from demolition can potentially be used as environmentally friendly materials in construction. Their use as SCMs with a substitution rate of 20% can reduce the CO2 emissions of cement production by at least 10.7%.

Influence of Heat Treatment Parameters of Austempered Ductile Iron on the Microstructure, Corrosion and Tribological Properties.

The influence of heat treatment parameters such as the annealing time and austempering temperature on the microstructure, tribological properties and corrosion resistance of ductile iron have been investigated. It has been revealed that the scratch depth of cast iron samples increases with the extension of the isothermal annealing time (from 30 to 120 min) and the austempering temperature (from 280 °C to 430 °C), while the hardness value decreases. A low value of the scratch depth and a high hardness at low values of the austempering temperature and short isothermal annealing time is related to the presence of martensite. Moreover, the presence of a martensite phase has a beneficial influence on the corrosion resistance of austempered ductile iron.