Thermal Aging Studies Research Articles

The effects of thermal aging on metallic interconnects of solid oxide fuel cells

For solid oxide fuel cells (SOFCs), metallic interconnects (ICs) are crucial components to add to the electrochemical catalyst electrodes. The high temperature operation condition in SOFCs is an extremely degrading factor for metallic ICs. Three metallic ICs have been selected in this thermal aging study. IC-1 and IC-2 contain similar Mn/Cr compositions. The thermal mechanical property of IC-2 is better than that of IC-1, attributed to the inclusion of rare earth elements. IC-3 has lower chromia composition, whereas IC-1 and IC-2 have higher Cr contents. Thermal aging is introduced under ambient atmosphere at 800°C from 0 to 100 h. X-ray diffraction study of the IC oxide scales shows growth of (MnCr)3O4 with the spinel structures. Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) reveal the thickness and spinel particles for the three ICs, the oxide layers at about 0·9–1·3 μm thick and the spinel sizes at 1·0–1·3 μm. The oxide weight gain studies have shown that IC-3 is the more readily oxidised IC, with an estimated oxidation rate of ∼1·98×10−7 g2 cm−4 s−1 at 800°C. Thus, IC-3 should be better coated with a protective layer for SOFC applications. For the thermal Cr diffusion study, we applied homemade glass sealant on the surface of IC-1. After thermal aging for 1000 h under ambient atmosphere at 800°C, we examined the sample with micro-Raman. The cross-sectional microstructures were studied with SEM/EDS. Both Raman and SEM/EDS results indicated that the Cr ions penetrated into the glass sealant by ∼65 μm. The oxide scales on ICs are influential on the electrical conductivity. The electrochemical impedance spectroscopy study on IC-2 samples after thermal aging is also shown.

Analysis of Thermal Aging and Structural Stability of Li[Lix(Ni0.3Co0.1Mn0.6)1-x]O2(x= 0.11) Cathode Active Material for Rechargeable Li-Ion Batteries

The high rate capability of Mn-rich Li[Lix(Ni0.3Co0.1Mn0.6)1-x]O2 (x = 0.11) cathode active materials is investigated by cycling the cell at a given rate for five cycles and keeping the cell idle under thermal control chamber for 10 h and the same process repeating up to 30 cycles. The before and after thermal aging of Mn-rich cathode materials deliver the initial discharge capacity of 153 and 157.32 mA h g-1 up to 30 cycles and also it is maintained the average specific discharge capacity of 140 mA h g-1 for before thermal aging and more than 90% capacity retention. After thermal aging of cathode materials have maintain the average specific discharge capacity of 155 mA h g-1 and more than 97% capacity retentions. During charging, they are not oxidized further; Ni2+ and at least part of Co3+ ions are oxidized to higher valence states. During the discharge reactions, the small amount of Mn3+ reduced to the Mn4+ and some part of Ni3+ ions are reduced to Ni4+. Also the Co3+ ions are fully reduced to the Co4+ state, which due to thermal aging studies does not have major affects in the Mn-rich layered structure under thermal control chamber. These thermal aging analyses are essential to achieve a deeper understanding of the structural defects and safety views for Li-ion batteries to use in electric vehicle technologies.

Analysis of Thermal Aging and Structural Stability of Li[Lix(Ni0.3Co0.1Mn0.6)1-x]O2 (x = 0.11) Cathode Active Material for Rechargeable Li-Ion Batteries

The high rate capability of Mn-rich Li[Lix(Ni0.3Co0.1Mn0.6)1-x]O2 (x = 0.11) cathode active materials is investigated by cycling the cell at a given rate for five cycles and keeping the cell idle under thermal control chamber for 10 h and the same process repeating up to 30 cycles. The before and after thermal aging of Mn-rich cathode materials deliver the initial discharge capacity of 153 and 157.32 mA h g-1 up to 30 cycles and also it is maintained the average specific discharge capacity of 140 mA h g-1 for before thermal aging and more than 90% capacity retention. After thermal aging of cathode materials have maintain the average specific discharge capacity of 155 mA h g-1 and more than 97% capacity retentions. During charging, they are not oxidized further; Ni2+ and at least part of Co3+ ions are oxidized to higher valence states. During the discharge reactions, the small amount of Mn3+ reduced to the Mn4+ and some part of Ni3+ ions are reduced to Ni4+. Also the Co3+ ions are fully reduced to the Co4+ state, which due to thermal aging studies does not have major affects in the Mn-rich layered structure under thermal control chamber. These thermal aging analyses are essential to achieve a deeper understanding of the structural defects and safety views for Li-ion batteries to use in electric vehicle technologies.

Accelerated thermal ageing studies of polydimethylsiloxane (PDMS) rubber

Thermal ageing studies over relative high temperature range (from 250 °C to 85 °C) have been conducted on polydimethylsiloxane (PDMS) rubber with the aim of investigating ageing behavior. Tensile elongation, compression set and creep measurements are introduced to monitor the ageing process. Time temperature superposition is performed and the linear trends are found in the Arrhenius plots for all the approaches, revealing an identical process dominates the degradation process. Activation energies are found to be depended on the methods used, which are 88.5 kJ/mol for tensile tests, 67.7 kJ/mol for compression set measurements and 75.3 kJ/mol for creep measurements. The phenomenon that activation energy obtained from tensile elongation is always higher than other approaches is rationalized by the sensitive modification of thermal degradation on the rubbery network. Moreover, correlations between the results from different approaches have been examined. The linear relationships have been found between tensile elongation and creep or compression set measurements for the three methods are all influenced by the network structure.

Microstructure and Properties of High-Temperature Materials (1−x) Na0.5Bi2.5Nb2O9-xLiNbO3

Bismuth layer-structured ferroelectric ceramics of (1−x)Na0.5Bi2.5Nb2O9–xLiNbO3 (NBN–LN, 0≤x≤0.2) were synthesized by a traditional solid-state reaction. The effects of LN addition on the microstructure and electrical properties of ceramics were studied carefully. It was found that the ceramics with x≤0.10 have a bismuth oxide layered structure. With the increasing of LN addition, the mechanical quality factor Qm firstly increases, and then decreases with a maximum value of 4820 at x=0.02. The Curie temperature Tc increases gradually and the electrical properties of NBN-based ceramics are improved significantly by the addition of LN. The piezoelectric constant d33, dielectric loss tan δ, Curie temperature Tc, and remanent polarization Pr of 0.90Na0.5Bi2.5Nb2O9–0.10LiNbO3 ceramics were found to be 21 pC/N, 0.16%, 789°C, and 7.37 μC/cm2, respectively. Thermal annealing and aging studies indicate that the NBN–LN system possesses stable piezoelectric properties, demonstrating that the NBN–LN ceramics are the promising candidates for high-temperature applications.

Compression set in gas-blown condensation-cured polysiloxane elastomers

Accelerated thermal ageing studies on foamed condensation cured polysiloxane materials have been performed in support of life assessment and material replacement programmes. Two different types of filled hydrogen-blown and condensation cured polysiloxane foams were tested; commercial (RTV S5370), and an in-house formulated polysiloxane elastomer (Silfoam). Compression set properties were investigated using Thermomechanical (TMA) studies and compared against two separate longer term ageing trials carried out in air and in dry inert gas atmospheres using compression jigs. Isotherms measured from these studies were assessed using time-temperature (T/t) superposition. Acceleration factors were determined and fitted to Arrhenius kinetics. For both materials, the thermo-mechanical results were found to closely follow the longer term accelerated ageing trials. Comparison of the accelerated ageing data in dry nitrogen atmospheres against field trial results showed the accelerated ageing trends over predict, however the comparison is difficult as the field data suffer from significant component to component variability. Of the long term ageing trials reported here, those carried out in air deviate more significantly from field trials data compared to those carried out in dry nitrogen atmospheres. For field return samples, there is evidence for residual post-curing reactions influencing mechanical performance, which would accelerate compression set. Multiple quantum-NMR studies suggest that compression set is not associated with significant changes in net crosslink density, but that some degree of network rearrangement has occurred due to viscoelastic relaxation as well as bond breaking and forming processes, with possible post-curing reactions at early times.

Thermal ageing studies on low infrared emissivity composite coatings

Abstract The emissivity property of low infrared emissivity coatings was studied during the thermal ageing. The influence of roughness, surface structure and electrical conductivity on infrared emissivity was systematically investigated. The results of experimental investigation indicate that the infrared emissivity changes with roughness, surface structure and conductivity during thermal ageing. This effect was explained by thermal oxidation of Al particles and polysiloxane degradation in the composite coatings and confirmed by SEM and XRD. Due to the degradation of polysiloxane and oxidation of Al particles, roughness increases, surface structure forms cracks and gaps, and conductivity reduces obviously with different temperature, which leads to significantly increasing emissivity.

A comparative study of thermal aging of transformer insulation paper impregnated in natural ester and in mineral oil

AbstractAging of two kinds of transformer insulation paper in natural ester is compared to that in conventional transformer oil (mineral oil). Sealed aging vessels containing copper, insulation paper (Kraft paper and thermally upgraded paper), and dielectric fluid (mineral oil and natural ester) were aged at 90, 110, and 130°C for 399, 371, and 329 days. Under the same aging condition, both the degree of polymerization (DP) and the leveling‐off degree of polymerization (LODP) for natural ester aged paper were higher than the ones for mineral oil aged paper, and the micro surface of natural ester aged paper was destroyed less seriously than that of the mineral oil aged paper. The aging time to reach LODP for natural ester aged paper was only 1/4 to 2/3 of that for mineral oil aged paper. When water content in natural ester for natural ester aged Kraft paper samples reached the maximum, acids concentration in natural ester increased abruptly. Water content in natural ester decreased in the whole aging process for natural ester aged thermally upgraded paper samples. Six physical and chemical mechanisms for the slower aging rate of natural ester aged paper are proposed. Evidence for transesterification between natural ester and paper was presented with infrared spectra instrument. Although the acids in natural ester were far more than in mineral oil, the natural ester can significantly slow the thermal aging of the paper. Three mechanisms of this abnormal phenomenon were presented. Copyright © 2009 John Wiley & Sons, Ltd.

Embrittlement of irradiated F82H in the absence of irradiation hardening

Neutron irradiation of 7–12% Cr ferritic/martensitic steels below 425–450 °C produces microstructural defects and precipitation that cause an increase in yield stress. This irradiation hardening causes embrittlement, which is observed in a Charpy impact or fracture toughness test as an increase in the ductile-brittle transition temperature. Based on observations that show little change in strength in steels irradiated above 425–450 °C, the general conclusion has been that no embrittlement occurs above these temperatures. In a recent study of F82H steel, significant embrittlement was observed after irradiation at 500 °C, but no hardening occurred. This embrittlement is apparently due to irradiation-accelerated Laves-phase precipitation. Observations of the embrittlement of F82H in the absence of irradiation hardening have been examined and analyzed with thermal-aging studies and computational thermodynamics calculations to illuminate and understand the embrittlement during irradiation.

Silane‐crosslinked ethylene–octene copolymer blends: Thermal aging and crystallization study

AbstractThermally stable materials can be achieved by crosslinking. This article presents the thermal aging and thermal energy storage properties of ethylene–octene copolymer (EOR) and low‐density polyethylene (LDPE) blends as affected by silane crosslinking. Fourier transform infrared spectroscopy revealed a similar degree of silane grafting among the various blend compositions. However, the highest crosslink content was observed in EOR, whereas the lowest was found for LDPE. From melting temperature and heat of fusion data, a linear relationship between the amount of the crystalline component and the crosslink content was found. The decrease in crystallinity due to crosslinking was very limited, which implied a high thermal energy storage capacity of the silane‐crosslinked products and their good mechanical properties at room temperature. Furthermore, a strong ability to retain the properties after thermal aging indicated good thermal stability of the materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Effect of Hygrothermal Aging on Interfacial Reliability of Silicon/Underfill/FR-4 Assembly

The reliability issues have been converted to the underfill adjacent interfaces since the introduction of the underfill to flip chip package in 1990's. Both thermal cycling and hygrothermal conditioning severely attack the interfaces to de- laminate. The moisture migrating into the underfill decreases the adhesion strength, swells to deform the assembly, and weakens the mechanical and thermal properties of the material. In this study, interfacial reliability of a silicon/underfill/FR-4 assembly exposed at 85degC/85%RH was studied using moire interferometry and micro-digital image speckle correlation (mu-DiSC) techniques. A thermal aging study was simultaneously performed to understand the long-term reliability of the assembly. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, whereas increased the strains induced by hygrothermal swelling. It indicated the time effect is not negligible when the assembly is subjected to the moisture conditioning, otherwise, the deformation induced by the swelling could be overestimated. The mu-DiSC technique was applied to measure the critical interfacial fracture toughness of the silicon/underfill interface. The results showed that the moisture could significantly reduce the interfacial strength due to the break of hydrogen bonding. By combining the moire and mu-DiSC results, it was concluded that the hygrothermal loading could increase the possibility of interfacial delamination in a flip chip package. Finally, the morphologies of the fractured surface were studied with the aid of scanning electron microscope. Remarkable changes of the failure mode were observed.

A study of thermal aging of steels for nuclear power plant reactor vessels aimed at prolonging the service life of the equipment to 60 years

The effect of thermal aging in the range of 250–400°C on the critical brittle point of pearlitic steels for nuclear power plant reactor vessels is studied. The structural mechanisms of the embrittlement of the steels are investigated by determining the activation energy. Methods are suggested for predicting the shift of the critical brittle point due to thermal aging of vessel steels for determining the design service life of nuclear power plants of a new generation.

Adhesive Characterization & Durability of Self-Adhered Flashings

Abstract Self-adhered flashing products can very effectively maintain a durable moisture-tight seal at the window-wall interface, which is highly vulnerable to moisture intrusion. However, it is essential that these products be installed under conditions where adequate adhesion to the substrate is achieved. Also, the self-adhered flashing products must have thermal durability and dimensional stability to maintain their performance at the high temperatures that can exist behind siding. This study gives a preliminary adhesion characterization assessment of six self-adhered flashing products, three with modified-asphalt based adhesives and three with butyl based adhesives, onto several common building substrates and installed at a range of temperatures, including moist and dusty conditions. What is found is that the butyl based adhesive systems have a broader window of installation surface conditions and temperatures where “adequate adhesion” is achieved without the use of a primer than the modifiedasphalt based adhesive products. Also, a thermal aging study shows that the butyl based adhesive products are more thermally stable than the modified-asphalt based products at typical temperatures behind siding. Also, film topsheets are more prone to deformation and curling after thermal aging than nonwovens composite or foil laminate based topsheets.

High pretilt four-domain twisted nematic liquid crystal display by microrubbing: Process, characterization, and optical simulation

A microrubbing (μ-rubbing) technique was utilized to create four-domain alignment in a liquid crystal display sample. A small metallic sphere under sufficient load was used to directly rub a homeotropic polyimide alignment layer to create a large surface pretilt angle. We demonstrate a 47-μm linewidth and a variable pretilt angle with respect to the substrate plane for different loads. The homeotropic polyimide surface was μ rubbed in such a way that neighboring alignment regions are rubbed in opposite directions. liquid crystal cells were constructed with two μ-rubbed substrates where the rubbing directions are orthogonal to each other. With this cell configuration we obtained four domains consisting of two left-handed and two right-handed twisted nematic subpixels. We report on the electro-optic performance properties, viewing angle characteristics, the relationship between pretilt angle versus load, and thermal aging study of the four-domain liquid crystal cell. The optical simulations of the liquid crystal director orientation, viewing angle characteristics, and the contrast ratio of the four domains are also presented. The experimental results are in agreement with the simulation results.

Study of the diffusion of pyrene in silicone polymer coatings by steady state fluorescence technique: effects of pyrene concentration

The concentration dependence of the diffusion coefficient of pyrene in single component and two-component room temperature curing silicone polymer coatings is investigated by the steady state fluorescence technique by measuring the pyrene excimer fluorescence intensity. At pyrene concentrations lower than 10 mM, the intensity of excimer fluorescence is proportional to the concentration and at higher concentrations it deviates from this trend due to concentration quenching. Thermal aging studies show that this concentration quenching can be removed by thermal annealing and the excimer emission intensity approaches the value expected from the trend at lower concentrations. The diffusion coefficient of pyrene at low concentrations in silicone polymer coatings is obtained using the approximate solution of one-dimensional diffusion equation. A modified approach is employed to estimate the diffusion coefficient at higher pyrene concentrations. In this method, the excimer intensity and time scale are shifted, respectively to I max the maximum value of excimer intensity attained during annealing and t max, the time taken to reach this. The estimated diffusion coefficients at different pyrene concentrations show a negligible dependence on pyrene concentration in both types of polymers. These results are attributed to the high structural mobility of silicone polymer chains due to their molecular structure.

Accelerated thermal ageing studies on nitrile rubber O-rings

Poly(acrylonitrile-butadiene) rubber (NBR) O-rings have been aged in air whilst under 12.5% compression at temperatures up to 110 °C. The compression set properties together with oxygen uptake of the O-rings under the applied load conditions were measured and results analysed using time-temperature superposition and Arrhenius kinetics. The compression set results suggest a single degradation process with an activation energy of 81.03 kJmol −1. This compares well with the activation energy of 74.3 kJmol −1 obtained from oxygen consumption. Results from solvent swell and oxygen uptake tests show that the main influence on ageing of the rubber is oxidative cross-linking, leading to the material becoming hard and brittle. The approach presented here predicts that the rubber will reach full compression set by 40 (+54/−7) years based on 95% confidence bands.

Studies on Electrical and Thermal Properties of Vinyloxyaminosilane Grafted Ethylene–Propylene–Diene Terpolymer/Linear Low Density Polyethylene Blends for Cable Applications

A new terpolymer of vinyloxyaminosilane (VOS) grafted onto ethylene–propylene–diene terpolymer (EPDM) has been synthesised in a Haake Rheocord-90, torque rheometer. Vinyloxyaminosilane grafted ethylene–propylene–diene terpolymer/linear low density polyethylene (EPDM-g-VOS/LLDPE) blends with different compositions were prepared using a two roll mixing mill. The electrical properties such as surface and volume resistivities, dielectric strength and arc resistance are decreased and dielectric constant and dielectric loss are increased with increasing percentage composition of LLDPE in EPDM-g-VOS/LLDPE blends due to enhanced crosslink density. The incorporation of VOS moiety onto EPDM improves the inception decomposition and final decomposition temperatures due to stable three dimensional network and high bond energy of –Si–O–Si– linkage. The values of Tg are increased with increasing concentration of LLDPE in the EPDM-g-VOS/LLDPE blends due to enhanced crystallinity and crosslink density. Thermal aging studies on mechanical properties show that the tensile strength, elongation at break and Young modulus are decreased due to disruption of crosslinking at LLDPE sites.

Studies on the thermal ageing of organic coatings

The ageing of organic protective coatings depends on environmental impacts and is associated with a variety of chemical and physical processes. The results of former thermal ageing studies indicate that evaporation and polymer degradation are the decisive processes for changes in coating properties. For the verification of these assumptions the impact of the thermal exposure temperature T L and the length of the exposure time t L on properties of clear lacquer coatings was determined by further studies. The gradation of T L was aimed at creating conditions where on the one hand only evaporation processes occur, while on the other hand evaporation and polymer degradation reactions occur in parallel. The objective was to find the time t L and the temperature T L at which degradation starts to prevail.

Studies on the thermal stability of natural rubber/polystyrene interpenetrating polymer networks: thermogravimetric analysis

The thermal degradation of natural rubber/polystyrene (NR/PS) interpenetrating polymer networks (IPNs) was studied and the effects of blend ratio, crosslink level and initiating system were analysed. The thermogravimetric analysis (TGA) shows that the IPNs are more stable than the pure components. The full-IPNs have better stability than semi-IPNs which is due to higher entanglement density of full-IPNs. The initial decomposition temperature and temperature for 50% decomposition ( T 50) increase with increase in concentration and crosslinker level of the plastic phase. The kinetics of degradation were analysed using nine mechanistic equations. The kinetic parameters such as activation energy, Arrhenius parameter and entropy of activation were determined. Thermal ageing studies of the samples were also conducted. The IPNs aged for 72 h at 100°C showed enhanced mechanical strength due to crosslinking on post curing.

A mass spectrometry and electron paramagnetic resonance study of photochemical and thermal aging of triterpenoid varnishes.

Photochemical and thermal aging of triterpenoid dammar and mastic resins used as varnishes on paintings were studied using graphite-assisted laser desorption/ionization mass spectrometry. This extends an earlier study on similar materials (Zumbühl et al., Anal. Chem. 1998, 70, 707-715) that focused on photoaging. Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as well as simultaneous loss of hydrogen. Oligomers up to tetramers are formed, while cleavage reactions lead to increased signal intensities in the mass ranges between the oligomers and below the monomers. No major differences were found between the mass spectra of samples aged in light or darkness, except that deterioration was faster in light. Electron paramagnetic resonance spectroscopy revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radical reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings lead to a unified explanation for yellowing of natural resin varnishes, one of the major degenerative changes in the appearance of paintings. It is also shown that the commercially available, nominally fresh resins are already in an advanced stage of oxidation and degradation. Energy-rich substances are formed upon irradiation with sunlight and are believed to restart the autoxidative chain reactions, regardless of storage conditions. As a result, varnishes are oxidized quite quickly (months) even when kept in darkness.