Logarithm Of Aging Time Research Articles

Interplay between chemical and physical aging in diethanolamine cured diglycidyl ether of bisphenol-A (DGEBA) epoxy

Thermomechanical properties during isothermal aging of diethanolamine (DEA) cured diglycidyl ether of bisphenol-A (DGEBA) were tracked using uniaxial compression and differential scanning calorimetry. The epoxy was cured at 70 °C for 24 h, following standard procedure, and then aged at one of four temperatures, T: 55 °C, 65 °C, 76 °C or 105 °C. The initial glass transition of cured DGEBA/DEA was 70 °C and increased proportionally with the logarithm of time indicating that additional crosslinks had formed. Similarly, the yield stress, σy, increased, by up to 80%, proportionally with the logarithm of aging time until approximately 600 h. After that time period, the increase in σy slows significantly. Aging also narrowed the yield peak and increased the post-yield flow stress. The changes in σy due to physical aging were separated from those due to chemical aging by testing samples combining both physical and chemical aging and comparing the results to those from (only) chemically aged samples. The proposed method for separating out the physical contributions can be used to predict the mechanical changes due to physical aging in slowly chemically evolving materials. The aging data are also utilized along with data on σy as a function of strain rate for unaged material to generate a tentative expression for σy as a function of strain rate, aging time and temperature. The derivation of this expression requires only the common assumption that the rate of change of σy/T with respect to log(strain rate) is independent of temperature and of aging time.

The kinetics of the glass transition and physical aging in germanium selenide glasses

The kinetics associated with the glass transition is investigated using differential scanning calorimetry (DSC) for germanium selenide glasses with Ge content ranging from 0 to 30at.% and average coordination numbers ranging from 2.0 to 2.6. As Ge content increases, the glass transition region broadens and the step change in heat capacity at Tg decreases. As a result of physical aging, enthalpy overshoots are observed in DSC heating scans and the corresponding change in enthalpy can be calculated as a function of aging time. The change in enthalpy linearly increases with the logarithm of aging time and then levels off at an equilibrium value that increases with decreasing aging temperature. The time required to reach equilibrium increases with decreasing aging temperature and, at a given distance from Tg, the time increases with decreasing germanium content. The results indicate that all samples show expected physical aging behavior, and no evidence is found for a Boolchand intermediate phase characterized by high stability and absence of physical aging.

Enthalpy Relaxation and Embrittlement of Poly(l-lactide) during Physical Aging

The enthalpy relaxation behavior of poly(l-lactide) (PLLA) below glass transition temperature (Tg) and its effects on the mechanical properties were studied using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), and tensile test. Both the enthalpy loss (δH) and peak endothermic temperature (Tp) in the glass transition region show a linear increase with the logarithm of aging time (ta). On the basis of DSC results, the kinetics of the enthalpy relaxation process were analyzed, and the kinetic parameters of PLLA were obtained: relaxation rate of enthalpy at 40 °C βH = 1.77 J g-1 per decade, and apparent activation energy Δh* = 1107 kJ mol-1. Besides, analysis on the kinetic of enthalpy relaxation for poly(dl-lactide) (PDLLA) shows that the Δh* value of PDLLA is similar to that of PLLA. Moreover, it was found that the strength and modulus of PLLA increase gradually; on the contrary, the toughness reduces dramatically during the physical aging. The yield strength and tensi...

Physical aging of glassy normal and waxy rice starches: thermal and mechanical characterization

Physical aging of glassy amorphous rice (normal and waxy) starches (14.5% moisture) was characterized in terms of thermal and mechanical properties, using a differential scanning calorimeter (DSC), dynamic thermal analyzer (DMTA), and Instron texturometer. The starch samples were prepared by compressing under heat (100 °C, 2000 psi), and subsequently aged at 25 °C for up to 29 days. Relaxation enthalpy, as measured by DSC, increased gradually with aging time, reaching a structural equilibrium. The peak temperature of the relaxation endotherm in DSC thermogram linearly increased with the logarithm of aging time. Relaxation kinetics revealed that relaxation rate was much slower for normal rice starch than for waxy rice starch. DMTA showed that storage modulus increased by aging, but the tan δ peak height reduced, indicating that the chain mobility of glassy starch was reduced by aging. Dual glass transitions were observed in both DSC and DMTA thermograms for normal rice starch samples, whereas a single transition was observed for waxy rice starch samples. It suggests that the amorphous regions in normal starch samples are heterogeneous. Breaking strength, measured using an Instron texturometer, increased gradually with aging time, in accordance with the change in relaxation enthalpy. The relaxation kinetics of glassy starch aging could be explained by thermal transition enthalpy and by mechanical properties such as storage modulus and breaking strength.

A Phenomenological Explanation to the Dielectric Aging Mechanism of a Lead Magnesium Niobate-Based Ceramic

We measured the dielectric permittivity (ε) of a Pb(Mg1/3Nb2/3)O3-based ceramic as a function of aging time (t) and external dc bias (field strength E up to 20 kV/cm). By comparing the ε–t and ε–E relationships, we obtained an E–t relationship that assumedly reflects the evolution of the internal field in the ceramics during aging. The internal field was found to gradually increase during aging and keep a nonlinear relationship with logarithm of aging time. A phenomenological model was proposed to semi-quantitatively explain why and how the internal bias causes the decrease in the dielectric permittivity and consequently the aging effect.

Physical Aging in Amorphous Poly(ethylene terephthalate). 2. Isothermal Volume Recovery After Two Temperature Jumps.

The dependence of physical aging kinetics on glass-forming histories and thermal treatments was examined for amorphous poly (ethylene terephthalate) (PET) glasses. The samples were subjected to two-step thermal treatment : first, temperature drop from equilibrium at 80°C to 60°C and then isothermal annealing ; second, temperature jump to aging temperature Ta. Isothermal volume recovery was measured after the second step to Ta (70°C> Ta>60°C) below the glass transition temperature (T). The volume recovery curves were represented by a plot of normalized fractional free volume (=(V-V∞)/V∞, where V∞ is the equilibrium value of specific volume V) vs. logarithm of aging time. All curves, showing the well known “memory effect” characteristic, could be described nearly by a six-parameter phenomenological model postulating a multiplicity of independent relaxation modes with distinct relaxation times. The model parameters determined to fit the data were found to be greatly dependent on the glass-forming history, but weakly dependent on the thermal treatment conditions. Furthermore, the individual recovery curves could be superimposed by horizontal shifts along the logarithm axis of aging time.

Effect of the quenching rate on the magnetic permeability of annealed non-magnetostrictive CoFeMnMoSiB glass

The dependence of the initial magnetic permeability and its stability during aging on the quanching ability of a quenchant (brine, water, oil and air) applied after annealing (708 K/10 min) of non-magnetostrictive Co 71.5Fe 2.5Mn 2Mo 1Si 9B 14 glass characterized by a medium value of T c was studied. It was found that the heat treatment enhances magnetic permeability μ 0.4 of the studied glass by a factor of 4–13 depending on the quenching medium. The highest value of μ 0.4 (>25 000) was obtained for the glass quenched with an intermediate quenching rate achieved in oil. An approximately linear decrease in μ 0.4 with the logarithm of aging time t of the studied glass was observed in as-quenched state as well as after heat treatment. The highest stability of μ was found for the glass annealed and slowly and slowly cooled in open air. The obtained results are explained on the basis of changes of local magnetic anisotropy during annealing and aging.

Long-Term Aging Effect on Coercive Force in Amorphous Magnetic Cores and Estimation of Its Deterioration at Operating Temperature

The long-term aging effect on the coercive force was studied for Co-based and Fe-based amorphous cores. Both amorphous cores show the same tendency in the aging process of their coercivities; they increase linearly with the logarithm of aging time over one year. The nonsaturated aging kinetics of the coercive force were analyzed by a method based on the pair ordering model reported by Taniguchi. The method does not need the saturation value of the coercive force and enables us to estimate the deterioration of the cores at operating temperatures which are lower than the aging temperatures. The increase in the coercivity for our cores was estimated to be less than 15 mOe after 106 h at 100°C.

Piezoelectric properties and temperature stability of poly(vinylidene fluoride-trifluorethylene) copolymers

Piezoelectric measurements were made on several copolymers of vinylidene fluoride and trifluorethylene having a vinylidene fluoride concentration in the range of 65-70%. The hydrostatic g constant showed only a slight dependence on pressure, and pressures up to 35 MPa caused no apparent loss of the piezoelectric activity. A significant increase in the value of the hydrostatic g constant was observed both at temperatures above and considerably below the room temperature. The anomalous result obtained upon cooling below the room temperature can be attributed to the glass transition temperature of the trifluoroethylene comonomer. Temperature aging studies were performed at high temperatures using films of these copolymers. A significant loss of piezoelectric activity occurred after long-time exposure. Isothermal studies at various aging temperatures revealed that this decay continued over a long time span. The aging behavior characteristically followed a linear dependence on the logarithm of aging time.

Aging change in mechanical properties of (Pb, La)TiO3ceramics

Abstract The fracture toughness of the hot-pressed tetragonal unpoled Pb0.85La0.10 TiO3 ceramics with tetragonality c/a= 1.028, a grain size of 3 um, a volume fraction of porosity less than 0.1% and a Curie point of 340°C was measured using a micro-indentation technique with various loads of indent. An internal stress is induced at the Curie point during the cooling process after sintering or after heat-treating above the Curie point. It was found that the compressional internal stress was 140MN/m2 at just after the thermally depolarization was linearly decreased with the logarithm of aging time at below the Curie point. The aging change seems to be a stress releasing process with the domain rearrangements.

Facet oxidation of InGaAsP/InP and InGaAs/InP lasers

The facet oxidation of InGaAsP/InP and InGaAs/InP lasers is investigated after aging under high constant optical output power or high current stress. Facet oxidation in InGaAsP/InP lasers is negligibly small under several thousand hours of practical operation. The thickness of oxide film increases in proportion to optical output power and logarithm of aging time. The growth rate of facet oxide film weakly depends on the junction temperature and the activation energy is estimated to be 0.07 eV within a experimental range between 25 and 150°C. The facet of InGaAs/InP lasers are oxidized more easily than that of InGaAsP/InP lasers but are about two orders of magnitude more stable against oxidation than that of AlGaAs/GaAs lasers.

The physical aging of isotactic polypropylene

AbstractThe microstructural changes in isotactic polypropylene (PP), subsequent to quench from the melt to around Tg were studied using dynamic mechanical loss, volume dilatometry, small and wide angle X‐ray scattering, infrared and NMR spectroscopy. The β‐transition loss tangent and the specific volume decreased linearly with logarithm of aging time. Dynamic mechanical loss and NMR spectroscopy results established that amorphous chain mobility reduced during aging. X‐ray and IR techniques showed that the crystallinity, the crystalline density, and the average chain conformation do not change during aging. Tensile tests indicated that diffusion of air or moisture into the polymer is not a competitive mechanism for the aging phenomenon. A simple free‐volume model is quantitatively consistent with these observations. The fraction of the material which cannot age increased as the quench temperature decreased. Further, at lower temperatures a portion of the nonageable fraction is shown to reside in the amorphous fraction.

Some considerations on isothermal aging curves of aluminum alloys

Isothermal aging curves of various aluminum alloys have been often plotted against logarithm of aging time. However, the discussion on the shape and slope of these curves is usually done without taking serious considerations on the time scale. In this paper, some significant differences between the logarithmic and the linear plotting were discussed. The results obtained were as follows:(1) Even in the situation where the aging occurs monotonously with an increasing rate, which itself decreases as the aging proceeds, the aging curve can take a complicated form when the logarithmic time scale is adopted; it can have an incubation period, abrupt acceleration, a two stage change, etc, depending on the increasing rate.(2) It has been found that if most of the aging curves, reported previously, are replotted in the linear time scale, the shapes of these curves become very simple, and the complicated features seen in the logarithmic expression disappear.(3) It is proposed, therefore, that the various previous considerations on the shape of the aging curve in the logarithmic expression should be reexamined.

Mechanism of Aging in Polycrystalline BaTiO3

Microscopic observation of the domain structure in polycrystalline BaTiO 3 is carried out. After the specimen is cooled down to room temperature through the Curie point the domain structure changes gradually from an initial irregular one to a final one consisting of fine stripes of 90°-domains. Such a change is named “90°-splitting.” The aging of dielectric constant may be due to an increase in clamping effect by the 90°-splitting. This “90°-splitting model” may explain also the agings in dielectric loss and hysteresis loop. The dielectric constant decreases linearly with logarithm of aging time. It was shown theoretically that the displacement of a 90°-wall towards the mechanically most stable position is proportional to the logarithm of time in the case of whisker crystal. The “logarithmic time dependence” of dielectric constant in polycrystalline specimen may be explained as due to the 90°-splitting which progresses at a rate proportional to logarithm of time.