Dynamic Aging Research Articles

Nanotubular polypyrrole: Reversibility of protonation/deprotonation cycles and long-term stability

An alteration of electrical conductivity of the conducting polymers from a conducting to a non-conducting form and vice versa is a feature of the high importance. Here, the changes in conductivity of the polypyrrole nanotubes (PPy-NT) associated with a deprotonation/reprotonation in alkaline (various concentrations of ammonium or sodium hydroxide) or acidic (1 M hydrochloric or hydrobromic acid) solutions are presented. All experiments were followed by energy dispersive X-ray spectroscopy (EDX) elemental analyses and van der Pauw conductivity measurements. Extensive measurements of conductivity of samples during 2-years' timespan have also been made in order to determine the kinetics of natural ageing of the deprotonated/reprotonated PPy-NT. EDX measurements have allowed for a deeper insight into the process of the deprotonation/reprotonation of PPy-NT, which is generally characterized by the exchange of counter-ions and by the increase in the oxygen content. Correlations among conductivities of original, deprotonated and reprotonated samples revealed the universal behaviour, which can be used for the precise prediction of the ability of PPy-NT to be reprotonated by acids. Kinetics of the degradation of PPy-NT by natural ageing is faster for deprotonated sample and, on the contrary, the reprotonation by acids had a stabilizing effect.

The Kinetics of Aging and Reducing Processes of Cr(VI) in Two Soils.

To investigate the aging process and reduction of Cr(VI) in two soils. The adsorption behavior of the soils demonstrated that the paddy soil had higher adsorption capacity for Cr(VI), but the capacity was lower for Cr(III), which contrasted the results for fluro-aquic soil. The mobilizable Cr was assessed using EDTA extraction. The results suggested that the reduction of Cr(VI) to Cr(III) and the aging process occurred simultaneously. A simplified kinetic model was established and the rate constants of the reduction and aging processes were obtained. The aging process and reduction of Cr(VI) were faster in the paddy soil, due to a higher adsorption capacity and stronger reducing ability, as indicated by the organic matter and amorphous Fe oxides. The Cr(III) aging was faster in the fluro-aquic soil due to the low solubility of Cr(III) at a high pH. The modeling study provides a fundamental understanding of the dynamics of Cr mobility in a complicated soil system.

Non-isothermal thermal cycle process model for predicting post-weld hardness in friction stir welding of dissimilar age-hardenable aluminum alloys

A challenge in welding age hardenable aluminum alloys is the strength loss in the heat affected zone (HAZ) due to transformation of precipitates upon non-isothermal heating cycles. The present work focuses on understanding the evolution of the HAZ microstructure of age hardened Al-Mg-Zn and Al-Mg-Si alloys joined together by dissimilar friction stir welding (FSW). An analytical model was applied to describe the thermal history of the dissimilar alloy FSW joints processed at 125, 250 or 500 mm min−1 and measured by embedded thermocouples. The Shercliff-Ashby age hardening model was fit to the isothermal ageing kinetics obtained from Vickers microhardness measurements of each individual alloy. The FSW process cycles were modelled in terms of an equivalent isothermal treatment and coupled to the Shercliff-Ashby model to predict the hardness values, and compared to Vickers microhardness profiles and microstructures observed after FSW. The model predicts softening in the HAZ, which correlated to the precipitate evolution observed by transmission electron microscopy.

Divergent Aging of Isogenic Yeast Cells Revealed through Single-Cell Phenotypic Dynamics

Although genetic mutations that alter organisms' average lifespans have been identified in aging research, our understanding of the dynamic changes during aging remains limited. Here, we integrate single-cell imaging, microfluidics, and computational modeling to investigate phenotypic divergence and cellular heterogeneity during replicative aging of single S.cerevisiae cells. Specifically, we find that isogenic cells diverge early in life toward one of two aging paths, which are characterized by distinct age-associated phenotypes. We captured the dynamics of single cells along the paths with a stochastic discrete-state model, which accurately predicts both the measured heterogeneity and the lifespan of cells on each path within a cell population. Our analysis suggests that genetic and environmental factors influence both a cell's choice of paths and the kinetics of paths themselves. Given that these factors are highly conserved throughout eukaryotes, divergent aging might represent a general scheme in cellular aging of other organisms.

Effect of Sn and In on the natural ageing kinetics of Al–Mg–Si alloys

The deleterious effect of natural ageing (NA) on subsequent artificial ageing (AA) of Al–Mg–Si alloys can be markedly reduced by adding small amounts of impurities such as Sn or In owing to their strong interactions with vacancies. The retarding effect of Sn on clustering after quenching and during NA was verified in this study. The largest effect was found in a solute-lean Al–Mg–Si alloy containing 70 ppm Sn and solutionised at 570 °C. Employing the same strategy, the delayed clustering kinetics was also observed in alloys in which Sn was replaced by In. Based on the data obtained from positron annihilation lifetime, hardness and electrical resistivity experiments, we introduce some concepts describing the mechanisms of NA cluster formation in the presence of Sn or In based on vacancy-solutes interactions.

Yttria Ceria Co-Stabilized Zirconia Reinforced with Alumina and Strontium Hexaaluminate

Zirconia materials are frequently used in dental applications due to their excellent strength and their tooth-like aspect. Standard yttria stabilized zirconia (Y-TZP) ceramics suffer, however, from moderate toughness and vulnerability to low-temperature degradation. In this study, 1Y6Ce-TZP materials reinforced with different amounts of alumina and/or strontium hexaaluminate were manufactured by slip casting and pressureless sintering at different temperatures to assess their mechanical properties, microstructure, phase composition, and low-temperature degradation stability. Results show that these materials exhibit a high fracture resistance of 10–12 MPa√m, a bending strength between 700–950 MPa, and a Vickers hardness of 1100–1200 HV10. Strontium hexaaluminate (SA6) precipitates were formed in situ by reaction of alumina and strontium zirconate. Although crack deflection at SA6 platelets was clearly visible, a net toughening was not observed. Accelerated ageing tests at 134 °C/3 bar water vapor pressure showed best results for mixed alumina/SA6 reinforcements and a sintering temperature of 1500 °C. Mehl-Avrami-Johnson plots used to describe the ageing kinetics showed clear indications of different ageing mechanisms due to the introduction of the SA6 phase.

Impact of ZnO nanoparticle morphology on relaxation and transport properties of PLA nanocomposites

In this work we study the effect of zinc oxide (ZnO) nanoparticle morphology and concentration on the resulting relaxation and transport properties of polylactide (PLA) nanocomposites. Films containing spherical and rod-shaped ZnO nanoparticles were incorporated into an amorphous polylactide (PLA) matrix through a solvent-precipitation and compression moulding method. Morphological analyses carried out by scanning electron microscopy (SEM) together with ultraviolet–visible (UV-Vis) spectroscopy and thermogravimetric analysis (TGA). Results indicate a much better distribution of rod-shaped ZnO within PLA matrix. Relaxation experiments reveal faster physical aging kinetics of PLA in presence ZnO nanoparticles notwithstanding their shape, suggesting the presence of non-interacting surfaces between the amorphous PLA matrix and the ZnO nanoparticles. Interestingly, both helium and oxygen permeability remained stable or increase upon nanoparticle addition, and anisole sorption kinetics showed faster mass transport in the nanocomposites, suggesting that the low interfacial adhesion between PLA and ZnO brings supplementary voids to the material increasing mass transport. Overall, the experimental findings here reported provide a deeper understanding on the influence of metal oxide nanoparticle morphology on the resulting relaxation and gas transport properties of amorphous polymeric nanocomposites.

Effect of Cerium and Erbium on the Aging Kinetics and the Properties of an IMV7-1 Alloy of the Mg–Y–Gd–Zr System

The effect of additional alloying of an IMV-7 alloy of the Mg–Y–Gd–Zr system with other rare-earth metals, in particular, cerium and erbium, is studied. The hardness and the electrical resistivity of the alloys, which characterize the kinetics of supersaturated solid solution decomposition upon aging at 200°C, are presented. The hardness and the mechanical properties of the alloys at high temperatures (250, 300°C) are measured. The microstructure of the alloys after heat treatment is examined. Cerium in the content range under study is found to exert no effect on the properties of alloys, such as IMV7-1, whereas erbium is undesirable and can decrease their hardening upon aging.

Analysis of Free Energy Increment for Insulation Aging in Crosslinked Polyethylene DC Cable

Electrical aging limits the reliability of crosslinked polyethylene DC cable. The study on the reaction kinetics of electrical aging is an important basis of improving the reliability of DC cable. Free energy increment (FEI) under electric field reflects the reaction rate of electrical aging, which is very useful to analyze the aging characteristics of materials. However, there is no direct way to get the FEI, which causes that the FEI stays at the stage of theoretical analysis and cannot be obtained by experiment. In this paper, an easy method including experiment and calculation to get the FEI is proposed, and a simulation method to obtain the FEI is also introduced. The experiment and simulation both show that the FEI of crosslinked polyethylene (XLPE) is proportional to the degree of polymerization, is approximately linear with the electric field, and grows with the temperature. According to the FEI obtained, we find that the molecular branches of the XLPE whose plane of carbons is perpendicular to electric field are more likely to age. Improving molecular chain orientation and reducing the length of branches are two of the methods to improve the aging characteristics of the XLPE.

Study on Aging Characteristics of Cast A357 Alloy

Cast A357 alloy is a typical heat-treated strengthened aluminium alloy. Secondary dendrite arm spacing (SDAS) has a very significant effect on the mechanical properties of the alloy. It is important to study the effect of SDAS on the aging kinetics of the alloy. The effect of SDAS on aging kinetics of A357 alloy was studied through hardness tester and optical microscopy. It was found that under the same solid solution treatment conditions, with the increasing of secondary dendrite arm spacing, the hardness of A357 alloy decreases after solid solution treatment; the aging peaks of alloy samples with different wall thicknesses delay with the increase of SDAS; the mathematical model of aging mechanical properties was obtained by regression analysis of the experimental results of aging treatment.

Influence of Initiation Site and Lamellar Divergency on the Overall Kinetics of Cellular Growth and Coarsening in Aged U-Nb Alloys

Overall kinetics of lamellar overaging reactions in U-5.5Nb and U-7.5Nb were analyzed by Avrami-Arrhenius analyses of volume fractions measured from an extensive temperature-time (T-t) matrix of specimens. The cellular initiation site (grain boundaries, inclusions) and regimes of lamellar divergency-cum-slowing growth rate were explicitly accounted for. Avrami exponents n from T-t regimes of constant-growth rate were consistent with theory (1<n<3); those from divergent T-t regimes were smaller, n~0.7, which is not surprising given their different growth rate behavior. The apparent activation energies Q were similar for grain-boundary and inclusion-nucleated discontinuous precipitation, indicating that their nucleation site does not alter their overall kinetics. Avrami Analysis of Isothermal Aging Kinetics

The kinetics of ageing in dry-stored seeds: a comparison of viability loss and RNA degradation in unique legacy seed collections.

Determining seed longevity by identifying chemical changes that precede, and may be linked to, seed mortality, is an important but difficult task. The standard assessment, germination proportion, reveals seed longevity by showing that germination proportion declines, but cannot be used to predict when germination will be significantly compromised. Assessment of molecular integrity, such as RNA integrity, may be more informative about changes in seed health that precede viability loss, and has been shown to be useful in soybean. A collection of seeds stored at 5 °C and 35-50 % relative humidity for 1-30 years was used to test how germination proportion and RNA integrity are affected by storage time. Similarly, a collection of seeds stored at temperatures from -12 to +32 °C for 59 years was used to manipulate ageing rate. RNA integrity was calculated using total RNA extracted from one to five seeds per sample, analysed on an Agilent Bioanalyzer. Decreased RNA integrity was usually observed before viability loss. Correlation of RNA integrity with storage time or storage temperature was negative and significant for most species tested. Exceptions were watermelon, for which germination proportion and storage time were poorly correlated, and tomato, which showed electropherogram anomalies that affected RNA integrity number calculation. Temperature dependencies of ageing reactions were not significantly different across species or mode of detection. The overall correlation between germination proportion and RNA integrity, across all experiments, was positive and significant. Changes in RNA integrity when ageing is asymptomatic can be used to predict onset of viability decline. RNA integrity appears to be a metric of seed ageing that is broadly applicable across species. Time and molecular mobility of the substrate affect both the progress of seed ageing and loss of RNA integrity.

Effect of tool pin geometry and weld pass number on microstructural, natural aging and mechanical behaviour of SiC-incorporated dissimilar friction-stir-welded aluminium alloys

The aim of this investigation was to study the effect of tool pin geometry and pass number on mechanical and microstructural behaviour of SiC-incorporated dissimilar friction stir welding of 6-mm-thick plates of AA6061-T6 and AA2024-T351. Two tool pin geometries, square and triangle frustum, were used. It was found that the weld made by square frustum pin and two passes with rotation in the same direction produces more homogenized and finer distribution of SiC particles. Also, the grain size of stir zone in the weld made by two passes is finer than that made by one pass. The weld made by triangle frustum pin shows higher thermo-mechanically affected zone hardness, natural aging kinetics and tensile strength than the others.

Effect of reinforcement content and aging treatment on microstructure and mechanical behavior of B4Cp/6061Al composites

The effects of B4C particle (B4Cp) content and aging treatment on the microstructure and mechanical behavior of B4Cp/6061Al composites prepared by powder metallurgy were investigated. The results demonstrated that the relative density of the B4Cp/6061Al composites decreased slightly by increasing reinforcement content from 0 to 15 wt%. No obvious pores or reaction products were observed at the B4Cp/matrix interfaces by transmission electron microscopy, suggesting that a good, clean interface was achieved. Mg2Si precipitates and dislocations were detected both near the interfaces and in the matrix. Moreover, the aging kinetics could be significantly accelerated by the presence of B4C particles; this behavior was associated with the enhanced dislocation density. The variation in hardness and electrical conductivity as a function of aging time in composites with various B4Cp contents was examined. For the peak-aged composites with increasing B4Cp content from 0 to 15 wt%, the ultimate tensile strength increased from 342.2 to 451.0 MPa, whereas the failure strain decreased from 18.4% to 7.9%. The grain refinement, dislocation strengthening, and work hardening, which contributed to the improvement in yield strength, were theoretically calculated, and the results agreed well with the experimental results.

Microstructure, mechanical properties and aging behaviour of nanocrystalline copper–beryllium alloy

A complex study of aging kinetics for both coarse-grained and nanostructured by severe plastic deformation Cu – 2 wt% Be alloy is reported. It is shown that aging of a coarse-grained alloy leads to continuous formation of nanosized CuBe body centred cubic (bcc, CsCl – type) semi-coherent particles with the {220} Cu // {200} CuBe crystallographic orientation relationship. These particles created significant internal stress fields and became obstacles for dislocation glide that resulted in a change in the hardness from 95 Vickers hardness (HV) for the solubilized alloy to 400 HV for the aged one. The severe plastic deformation led to the formation of a single-phase nanograined microstructure with an average grain size of 20 nm and 390 HV. It was found that this grain size was slightly driven by grain boundary segregation. Further aging of the nanocrystalline alloy led to the discontinuous formation of precipitates on the former Cu grain boundaries and skipping of metastable phases. Significant age hardening with a maximum hardness of 466 HV for the aged nanostructured alloy was observed. Mechanical tests result revealed a strong influence of microstructure and further aging on strength capability of the alloy for both coarse-grained and nanostructured alloy. A good thermal stability in the nanostructured alloy was also noticed. Theoretical calculations of the hardness value for the CuBe phase are provided. It was shown that Be as a light alloying elements could be used for direct change of microstructure of severely deformed copper alloys.

Microstructure and High Temperature Tensile Properties of Mg–10Gd–5Y–0.5Zr Alloy after Thermo-Mechanical Processing

The microstructure, high-temperature tensile properties and fracture behavior of the Mg-10Gd-5Y-0.5Zr alloy after thermo-mechanical processing (pre-tension between solution and aging treatment) were investigated. The pre-deformed alloy shows the accelerated aging kinetics compared to the un-deformed alloy. Microstructure of pre-deformed samples showed not only the homogeneous nucleation of the precipitate but also heterogeneous nucleation of precipitates on the dislocation and twin boundaries. Tensile results show that the pre-deformation enhanced the strength of the alloy, while it deteriorates the ductility of the alloy. The ultimate tensile strength (UTS) of the T6 treated un-deformed and pre-deformed alloy at room temperature are 331 MPa and 366 MPa, respectively. Tensile strength of the T6 treated alloy in both un-deformed and deformed conditions was enhanced by raising the test temperature and then reduced by further raising the test temperature. The higher strength of the pre-deformed alloy could be related to the higher density of the precipitates, which grow on the twin boundaries and can hinder the dislocation movement and strengthen the alloy. The results shows that thermo-mechanical processing can significantly improve the room- and high-temperature mechanical properties and enhance the formation of precipitates in Mg-10Gd-5Y-0.5Zr alloy, which can lead to wider application of the alloy in industries such as aerospace or powertrains that need better room- and high-temperature mechanical properties.

Aging behavior of the extruded SiCp-reinforced AZ91 Mg alloy composite

Abstract

Predict the rheological properties of aged asphalt binders using a universal kinetic model

In this study, universal models were proposed to predict the aging behaviors of five different asphalt binders, where the rheological properties were emphasized on. The carbonyl content and crossover modulus of binders during oxidative aging process were predicted using the proposed universal kinetic models. The carbonyl content test using the Fourier Transform infrared spectroscopy (FTIR) technique and the frequency sweep test using a Dynamic Shear Rheometer (DSR) were performed to obtain the optimum model parameters. From the testing results, the crossover modulus of binders under oxidative aging effect showed similar growth trend to the carbonyl content that follows the two-reaction kinetics of aging. An approximately linear relation was established between the log crossover frequency and the inverse of log crossover modulus. A sufficient good agreement was achieved between the measured and predicted results of carbonyl content and crossover modulus of asphalt binders at given aging temperature and duration. However, it lost a little prediction accuracy when using the universal model parameters, compared to the specific model parameters. Moreover, based on the calculated crossover modulus and crossover frequency, the complex shear modulus and phase angle were successfully predicted based on the Christensen-Anderson model. The proposed universal model provides an effective approach of predicting the rheological properties of aged asphalt binders.

Ageing of aqueous TEMPO-oxidized nanofibrillated cellulose dispersions: a rheological study

The study concerns the development of rheological properties of aqueous TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-oxidized nanofibrillated cellulose (NFC) suspensions prepared from a commercially available TEMPO-NFC powder. Ageing of hydrogels with concentrations from 1 to 3 wt% of TEMPO-NFC was investigated by monitoring shear flow and linear viscoelastic properties after different days from sample preparation. For quantitative representation of the results time dependencies of zero shear viscosity and storage modulus are given. The increase of rheological parameters is described by a sigmoidal mathematical model. By superposition of individual data sets master curves displaying reduced values of the most significant parameters are constructed, showing the effect of ageing kinetics. The reduced storage modulus and zero-shear viscosity values increase for 2 and 3 decades, respectively, before reaching final values. Obtained results are useful for several applications of nanocellulose hydrogels, especially for those, where pumping, mixing, coating or injecting is necessary. For example, in biomedical field, a less viscous suspension can be injected to the spot of interest, where the final gel properties are developed.

Towards more Durable Electrochemical Capacitors by Elucidating the Ageing Mechanisms under Different Testing Procedures.

Electrical double‐layer capacitors (EDLCs) commonly denoted supercapacitors are rechargeable energy storage devices with excellent power and energy delivery metrics intermediate to conventional capacitors and batteries. High‐voltage aqueous electrolyte based EDLCs are particularly attractive due to their high‐power capability, facile production, and environmental advantages. EDLCs should last for thousands of cycles and evaluation of future cell chemistries require long‐term and costly galvanostatic cycling. Voltage holding tests have been proposed to shorten evaluation time by accelerating cell degradation processes. Whether voltage holding can replace cycling completely remains undemonstrated. In this work, a systematic investigation of the influence of testing procedure on cell performance is presented. The state‐of‐the‐art post‐mortem and operando experimental techniques are implemented to elucidate ageing mechanisms and kinetics inside EDLC cells under different testing procedures. Carbon corrosion occurring on the positively polarized electrode leads to the lower active surface area and higher oxygen content. On the contrary, an increase of surface area and micropore volume are observed on the negatively polarized electrode. Repeated galvanostatic cycles at U<1.6 V appears to facilitate the depletion of oxygen species on the positively polarized electrode in comparison with voltage holding, which indicates a more complex degradation mechanism during cycling. Caution is advised when comparing results from different test procedures.