High Temperature Thermal Aging Research Articles

Molecular dynamics study on the influence of thermal aging on the mechanical properties of epoxy resins for high voltage bushing.

Thermal aging significantly deteriorates the mechanical properties and service performance of epoxy resins used for the high-voltage bushing. Current studies on the thermal aging behavior of epoxy resins mainly focus on experimental observations. However, an in-depth understanding of the mechanism of thermal aging of epoxy resins requires the monitoring of structural evolution of epoxy resins during thermal aging at the molecular level. To thoroughly analyze the intrinsic factors affecting structural evolution and the effect of thermal aging on the mechanical properties of epoxy resin for high-voltage bushing, epoxy resin models with different crosslinking degrees were established and thermal aging treatments at various temperatures and time periods were carried out by molecular dynamics simulation. It was found that the tensile strength of the epoxy resin was enhanced with the increase of the crosslinking degree, which was related to the elevation of the proportion of C-N and O-H bonds in its structure. With the increase of thermal aging temperature, the tensile strength of the epoxy resin decreased, which was related to the formation of weak bonds. At the early stage of thermal aging and after a period of high-temperature thermal aging, the strength of epoxy resin significantly decreases. The thermal aging of the epoxy resin is accelerated under external loading. In addition, the crosslinking degree and curing agent also affect the thermal aging resistance of epoxy resins. The results of this study can provide guidance for predicting and improving the thermal aging resistance of epoxy resins. Materials Studio was used to construct molecular models and complete crosslinking reactions. DGEBA and 44DDS (or 33DDS) were mixed at a ratio of 2:1, followed by crosslinking reaction. During this process, the Nose method was used to control temperature, the Berendsen method was used to control pressure, and the polymer consistent force field (PCFF) was used to control the motion and force of atoms. Isobaric-isothermal ensemble (NPT ensemble) was used to heat up epoxy resin models to various thermal aging temperatures of 400K, 500K, 600K and 700K. The models were maintained at these temperatures for different thermal aging times of 100ps, 200ps, 300ps, 400ps, 500ps, 600ps, 700ps and 800ps. Afterwards, the models were cooled down to 300K and subjected to uniaxial tensile testing at this temperature with a strain rate of 1 × 109s-1. The structural configurations and stress-strain data during the tensile process were recorded. The flow stress of the material was derived by counting the average stress in the 20-50% strain interval.

Exploring the structure, characteristics, and thermal degradation of composites comprising thermoplastic polyolefin and graphene oxide under high-temperature dynamic shear conditions

Dynamic high-temperature shear aging tests were conducted on thermoplastic polyolefin (TPO)/graphene oxide (GO) composite materials using a Hakke torque rheometer. The study employed ATR-FTIR, DSC, SEM, oxygen performance testing, and TGA to analyze the structure, properties, thermal degradation behavior, kinetics, and mechanisms of TPO/GO composites. The results demonstrated that the high-temperature dynamic shear thermal oxidative aging of TPO/GO composite materials establishes a dual protective mechanism, incorporating physical (intercalation oxygen resistance) and chemical (inorganic oxygen resistance) effects. This can be attributed to the polar surface and honeycomb structure of GO, as well as the shear-induced more uniform physical distribution of GO within TPO. Consequently, TPO/GO composite materials exhibit significantly enhanced resistance to dynamic shear thermal oxidative aging compared to TPO alone.

Largely Improved Creep Resistance and Thermal-Aging Stability of Eco-Friendly Polypropylene High-Voltage Insulation by Long-Chain Branch-Induced Interfacial Constraints.

Polypropylene (PP)-based composites have attracted numerous attention as a replacement of prevailing cross-linked polyethylene (XLPE) for high-voltage insulation due to their ease of processing, recyclability, and excellent electrical performance. However, the poor resistances against high-temperature creep and thermal aging are obstacles to practical applications of PP-based thermoplastic high-voltage insulation. To address these problems, in this Letter, we synthesized an impact polypropylene copolymer (IPC) containing multifold long-chain branched (LCB) structures in phases, especially the interfaces between the PP matrix and the rubber phase. The results indicated that the structural stability of LCBIPC was significantly enhanced under extreme conditions. In comparison to IPC (without LCB structures), 24.1% less creep strain and 75.2% less unrecoverable deformation are achieved in LCBIPC at 90 °C. In addition, the thermal aging experiments were performed at 135 °C for 48 and 88 days for IPC and LCBIPC, respectively. The results show that the resistance against thermal aging was also enhanced in LCBIPC, which showed a 133% longer thermal aging life compared to IPC. Further results revealed that the interfacial layer between the PP matrix and the rubber phase was constructed in LCBIPC. The two phases are tightly linked by chemical bonds in LCB structures, leading to enforced constraints of the rubber phase at the micro level and better resistance performance against creep and thermal aging at the macro level. Evidently, the reported eco-friendly LCBIPC thermoplastic insulation shows great potential for applications in high-voltage cable insulation.

Perovskite evolution on La modified Mn1.5Co1.5O4 spinel through thermal ageing with enhanced oxidation activity: Is sintering always an issue?

Non-precious metal oxides have great potential in catalytic deep oxidation of emitted hydrocarbons to help address various environmental pollution concerns. However, sintering issue is a stumbling block in practical application upon long-term high-temperature operation or thermal shock experience. Herein, La was applied to modify Mn1.5Co1.5O4 spinel to get a highly active oxidation catalyst with exceptional stability against high-temperature thermal ageing treatment. With thermal ageing at 750 °C for 100 h, the La modified Mn–Co composite reached 90 % conversion in toluene oxidation at 265 °C under the high WHSV of 120,000 mL g−1 h−1, while this value increased to 312 °C over blank Mn–Co spinel. The addition of La not only inhibited the growth of spinel nanocrystals, but also brought about perovskite formation through sintering, generating perovskite-spinel interfaces, thus enhancing both activity and stability. Meanwhile, the La modified Mn–Co spinel exhibited superior performance in presence of water or sulfur dioxide after thermal ageing treatment. This work offers a versatile strategy to design anti-sintering and active oxidation nanocatalysts based on non-precious metal oxides for industrial applications.

Research on dynamic characteristic of compressor RIP under thermal oxygen aging and variable preload conditions

The dynamic characteristics of rubber isolation pad (abbreviated as RIP) after service under the high temperature thermal oxygen aging and the variable preloads have preload dependence and thermal oxygen aging dependence, which is a crucial problem for matching the vibration isolation system of air conditioner compressor to reveal the dynamic characteristic mechanism of the RIP with different preloads and thermal oxygen aging conditions. The Peck model is first introduced to characterize the thermal oxygen aging factor, the fractional derivative Kelvin-Voigt thermal oxygen aging-perturbation model (FDKVTPM) and the Coulomb frictional thermal oxygen aging-perturbation model (CFTPM) are established to describe the frequency dependence and the amplitude dependence, respectively. The thermal oxygen aging-dynamic characteristic model of the RIP is built by considering the influence of variable preloads, the model parameters under different preloads are further identified, the validity of the model was examined by the experimental data. The concepts of the stiffness transition point (STP) and the stiffness transition frequency (STF) are innovatively proposed to better describe softening effect of the RIP under variable preload and variable amplitude working conditions. The results show that the static stiffness of RIP increases by 20.7%, the dynamic stiffness increases by 9.3%, and the loss factor decreases by 35% after thermal oxygen aging under different preload conditions, which can lay a theoretical foundation for in-depth study of the stiffness matching and optimization of air conditioner compressor with the RIP.

High temperature shear and thermal aging behavior of dissimilar transient liquid phase bonded Hastelloy X to Ni3Al intermetallic compound

Nickel super alloys and intermetallic compounds (IMCs) are candidate as strategic materials for turbine industry, because of its excellent high temperature properties. In this study, mechanical properties and thermal aging behavior (post thermal exposure) of the transient liquid phase (TLP) bonds between Hastelloy X to Ni3Al IMC at temperature range of 800–900 °C were investigated. The microstructure and fracture surfaces of the joints were examined by optical and scanning electron microscopes as well as XRD analysis. The optimum joint bonding strength was achieved for the sample treated at 1100 °C–180 min equaling to 355 ± 4.5 MPa. XRD patterns of semi-cleavage fracture surfaces revealed nickel solid solution matrix containing BNi3 and Ni3Si compounds generated at ASZ/ISA interface. The ultimate tensile strength reaches 36.5 ± 1 and 20.5 ± 1 MPa at temperatures of 800 °C and 900 °C, respectively. Fracture occurred in the IMC substrates for both temperatures were due to shrinkage porosity during solidification of IMC and mismatch of crystal lattice constants with the matrix. The thermal aging process at 900 °C for 50–500 h had a minor effect on the joint microstructure but generated TCP phases in the Hastelloy X.

Study on the role of Fe species and acid sites in NH3-SCR over the Fe-based zeolites

Fe doped ZSM-5 zeolite catalysts, prepared with solution ion-exchange method, calcinated at various temperatures and aged with/without H2O, were tested for activity in the selective catalytic reduction of NOx with NH3 (NH3-SCR) and characterized by XRD, NMR, UV‐Vis, XPS, H2-TPR, NH3-TPD and in-situ DRIFTs for structure and species change to investigate the role of Fe sites and acidity in SCR reaction. The results showed that the SCR activity for the Fe-ZSM-5 was mainly dependent on the active Fe species rather than acidity. Fe-oxo related Lewis sites were important for the low temperature SCR reaction, while high Brønsted acidity was not necessary for a high SCR activity. It should be noticed that the inactive free Fe2+ ions could be converted to isolated Fe3+ species by high temperature during calcination and aging, which could be inhibited by H2O due to dealumination during hydrothermal aging. This resulted in the decreased SCR activity after aging in the presence of H2O and the improved NO conversion after high-temperature calcination and thermal aging due to that the isolated Fe3+ acted as the main active site at temperature <500 ℃. The in-situ DRIFTs study showed that NH3 adsorption on acid sites was not the necessary step for the low-temperature SCR reaction but essential to NH3 oxidation, and the Eley-Rideal (E-R) mechanism was the main pathway for standard NH3-SCR.

Overview on performance degradation behavior of 20Cr25NiNb steel for gas cooled reactor cladding during service

In this paper, the performance degradation behavior of fuel cladding material 20Cr25NiNb for the British Advanced Gas Reactor (AGR) is reviewed in detail, which is a strong guideline for the material selection of supercritical carbon dioxide cooled reactors. The degradation behavior during in-core service mainly includes high-temperature creep, thermal aging and mechanical property degradation caused by neutron irradiation (fission gas products, helium embrittlement and irradiation sensitization) and CO2 (oxidation and carburizing). Long-term service in AGR leads to coarsening of the second phase and precipitation of harmful phases such as σ, leading to performance degradation of the cladding. A point that should require special attention is that intergranular stress corrosion cracking (IGSCC) and intergranular attack (IGA) problems occur during wet storage of spent fuel.

Microstructural stability of a 9Cr oxide dispersion strengthened alloy under thermal aging at high temperatures

High temperature heat treatment is one of the critical steps in hot compression bonding (HCB), which was recently developed to scale up the fabrication of oxide dispersion strengthened (ODS) alloys. Understanding the microstructural evolution during heat treatments is required to refine the fabrication process to produce large-scale homogeneous ODS alloys. In this study, the effect of high temperature thermal aging treatments at representative temperatures (1000 °C and 1200 °C, respectively) on the microstructural stability of a 9Cr ODS alloy was investigated. The results show that the microstructure is relatively stable during thermal aging at 1000 °C up to 500 h. Within 50 h of aging at 1200 °C, δ-ferrite appears and its content increases with aging time. However, δ-ferrite completely disappears and hexagonal TiO2 particles with the size of 1–3 µm are uniformly distributed in the matrix after aging for 200 h. The nano-sized oxide particles coarsen slightly at 1000 °C, and the average size is ∼4.3 nm after aging for 500 h, whereas, significant coarsening takes place at 1200 °C after aging for more than 20 h, and the particle size increased from 3.76 nm at 20 h to 6.22 nm at 200 h. Additionally, the average grain size grows obviously after aging to 200 h. These results indicate that the HCB and hot working processes of 9Cr ODS alloy should be less than 20 h at 1200 °C or kept at lower temperatures to maintain the stability of nano-sized oxide particles and the overall microstructure.

Microscopic Reaction Mechanisms of Formic Acid Generated During Pyrolysis of Cellulosic Insulating Paper

Cellulosic insulating paper is the essential component of the insulation in power transformers. Under the thermal stress inside the transformer, the cellulosic insulating paper degrades and generates formic acid. Meanwhile, the presence of formic acid further accelerates the aging process of cellulosic insulating paper. This study takes the cellulose molecule composed of D-glucose as the research object. The ReaxFF reactive force field is applied to simulate the high-temperature thermal aging process of cellulosic insulating paper. During pyrolysis process, the number of formic acid molecules presents short-term fluctuations and continuous increases. Though simulation, the main reaction pathways of cellulose pyrolysis to generate formic acid are obtained. In addition, an accelerated thermal aging experiment of oil-paper insulation is designed. The formic acid in the experimental samples is detected by the silanization derivatization method. Through the combination of simulation and experimental results, the feasibility of formic acid as an aging indicator for cellulosic insulating paper is further demonstrated.

Tensile Behavior of Incoloy-800HT Exposed to High-Temperature Molten FLiNaK Salt and Thermal Aging

Tensile Behavior of Incoloy-800HT Exposed to High-Temperature Molten FLiNaK Salt and Thermal Aging

Oxide-water interaction and wetting property of ceria surfaces tuned by high-temperature thermal aging

The high transmittance, environmentally robust, and unique interaction with water make lanthanide series rare-earth oxides promising for various applications such as next-generation self-cleaning coatings. However, the underlying mechanism of the anomalous hydrophobicity in lanthanide oxides remains controversial due to the lack of molecular-level studies in a well-controlled environment, which greatly limits the possibility to engineer their wettability for broader technical applications. Here, we systematically studied the mechanism that governs the wetting behaviors of ceria films by in-situ X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and water contact angle measurements under the environmentally controlled conditions. It was found that the hydrophobicity of ceria was mainly contributed by the adsorbed hydrocarbon, and the wettability could be enhanced on cleaner ceria surfaces with more degree of hydroxylation. Based on the significant dependence of wetting property on surface contamination, we further developed an in-situ annealing method to significantly improve the water-splitting performance of ceria powders by removing hydrocarbon contamination. These results suggest that the removal of surface contamination by high-grade oxidation is the key factor to facilitate the water dissociation of ceria films, providing new insights into the control of their wetting property and design of high-performance devices based on ceria films.

Clarifying the effect of moisture absorption and high-temperature thermal aging on structure and properties of polyimide film at molecular dynamic level

A series of pyromellitic dianhydride (PMDA)/4,4′-oxydianiline (ODA) polyimide (PI) films with different moisture content were experimentally prepared and corresponding PI/H2O composite models were constructed and analyzed based on molecular dynamic (MD) simulation. The elongation at break increased from 47.2% to 59.7% when the moisture content of the PI film reached 2%, which was attributed to the presence of water molecules that increased the mobility of molecular chains. The fractional free volume and XRD patterns of the composite models illustrated that the average packing distance of PI chains increased with moisture absorption and weakened intermolecular charge-transfer complex formation, therefore endowing the wet PI films better transmittance. The thermal decomposition mechanism of PMDA/ODA, 3,3′,4,4′-biphenyldianhydride (BPDA)/ODA and BPDA/p-phenylene diamine (PDA) based PI films were also investigated by thermogravimetric-infrared radiation (TG-IR) and MD simulation. The IR spectra of gas products indicated that the thermal decomposition of PI mainly generated CO2, CO, NH3 and HCN, and the PI with different chemical structures had the same thermal decomposition products. The MD results showed that the C–O bond in the ODA structural unit was broken first during the thermal decomposition, which could well explain that the BPDA/PDA exhibited better thermal stability compared with PMDA/ODA and BPDA/ODA. This work paved the way for the investigation of the macromolecular structural evolution of PI materials during moisture absorption and thermal aging process.

Reliability-Based Evaluation of the Suitability of Polymers for Additive Manufacturing Intended for Extreme Operating Conditions.

A reliability engineering program must be implemented from the conceptual phase of the physical asset to define the performance requirements of the components and equipment. Thus, in this work, the aim is to find the most optimal solution to manufacture polymer-based parts for the nuclear power industry using additive manufacturing routes. This case study application has been selected because polymers processed by additive manufacturing (AM) can be well suited for nuclear applications. The methodology includes—firstly—an analysis of the suitability of materials based on high-temperature resistance, thermal aging and irradiation tolerance, considering operation conditions. Secondly, an analysis of materials’ processability considering their associated AM routes is performed based on thermal analysis and evaluation of physical properties of materials. A final assessment integrating the in-service suitability and AM processability is performed using a reliability approach, solving different emerging objective conflicts through defined constraints and selection criteria. According to the integrated in-service performance evaluation: Polypropylene-ethylene polyallomer (PPP), Epoxy (EP), Phenolics (Ph), Polyurethane (PU) and Acrylonitrile butadiene rubber (NBR) are the best options for mild operation conditions and EP, Ph and PU, considering high temperature along with radiation exposure. Considering AM techniques: EP and Ph can be manufactured using VAT photopolymerization-stereolithography (VP-SLA) with a good expected processability being these materials valid for high temperature environments. Consequently, this research work analyzes the viability, processability and in-service behavior of parts.

Nature of Active Sites on Cu–CeO2 Catalysts Activated by High-Temperature Thermal Aging

Cu clusters supported on CeO2 nanorods (NRs) were found to exhibit significantly enhanced activity and thermal/hydrothermal durability for low-temperature CO oxidation when the catalyst was calcine...

Influence of thermal ageing on creep rupture mechanism and creep life of P92 ferritic steel

In order to analyze the effect of thermal ageing on creep rupture mechanism and establish a method for predicting the creep rupture life of P92 heat-resistant steel under thermal ageing, creep tests were performed on P92 steel specimens, aged at 650 °C for different times under different applied stresses. Optical microscopy and scanning electron microscopy were utilized to observe the microstructure after thermal ageing and creep rupture. Voids and cracks were distributed in the grains of P92 steel before thermal ageing, whereas the voids were clearly distributed around M23C6 precipitates and Laves phase along sub-grain boundaries after thermal ageing. The creep damage tolerance factor of the unaged and short-time aged samples ranged from 2.5 to 5, and the creep rupture was caused by dislocation movement. After high-temperature thermal ageing for 3000 h, the damage tolerance factor increased to > 5 and creep rupture was caused by precipitate coarsening. A theoretical method was established to predict the creep rupture life of heat-resistant P92 steel under thermal ageing, providing consistent results with the American Society of Mechanical Engineers (ASME) long-term test and trend.

Controlled synthesis of CeO2 nanorods and their promotional effect on catalytic activity and aging resistibility for diesel soot oxidation

Nanostructure of catalysts plays a critical role in catalytic activity and durability. Herein, a ceria nanorod catalyst, CeO2(NR), was designed and synthesized, and the relationships between nanostructure and catalytic activity as well as aging resistibility were investigated. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction results indicate that the nanorod structure of CeO2(NR) can be well maintained after aging at 900 °C for 8 h and can prevent the excessive expansion of crystals during high-temperature aging. X-ray photoelectron spectroscopy and hydrogen temperature-programmed reduction results suggest that the CeO2(NR) nanorod has distinctly more oxygen vacancies than the CeO2 nanoparticle; moreover the nanorod structure of CeO2(NR) and CeO2(NR)-A can enhance the lattice oxygen mobility and contribute to the spillover of lattice oxygen to form surface active oxygen during high temperature reaction. Finally, the catalytic activity tests indicate that the synthesized CeO2(NR) nanorod catalyst displays significantly better soot purification efficiency and intrinsic activity for soot oxidation; more than that, this superior catalytic activity of CeO2(NR) is well maintained after high-temperature thermal aging. Thus, this work suggests that the nanorod structure of ceria-based catalysts plays a key role in enhancing aging resistibility and catalytic soot oxidation activity.

Improvement of high-temperature thermal aging reliability of Ag–Au joints by modifying Ni/Au surface finish

Sintering Ag paste on Ni/Au surface finish (Ag–Au joint) is a hot topic because Ni/Au surface finish is one of the most commonly used metallizations in the electronics industry. However, it is still difficult to achieve a reliable Ag–Au joint after high-temperature thermal aging due to severe Ag–Au interdiffusion. In this work, we investigated thermal aging reliability of Ag–Au joint via high-temperature storage at 250 °C up to 1000 h. The Ag–Au joints with 0.1 and 0.8 μm Au layer coated substrates were compared to study the Au layer thickness effect on thermal aging reliability of joints, and a substrate preheating treatment was proposed to improve thermal aging reliability of Ag–Au joint. The result clearly shows that preheating treatment can improvement of bonding performance including thermal aging resistance and initial shear strength for both the thickness of Au layer. The shear strength of 1000 h thermally aged Ag–Au joint with the 0.1 and 0.8 Au substrate has been further improved to 11.7 and 16.5 MPa via the substrate preheating treatment, respectively. The substrate preheating treatment can lead to further growth of Au grains by eliminating fine grains, which results in the improvement of thermal aging reliability of joints. In addition, the thicker Au layer showed superior thermal aging reliability. This is attributed to the thicker Au layer less susceptive to massive Ag–Au interdiffusion and can prohibit severe oxidation of Ni because it hosts larger Au grains and fewer grain boundaries. The study would provide a convenient way to improve the thermal aging reliability of Ag–Au joints.

Validated prediction of weld residual stresses in austenitic steel pipe girth welds before and after thermal ageing, part 2: Modelling and validation

An extensive finite element simulation campaign was undertaken to examine the complete manufacturing history and high temperature thermal ageing of thick-walled girth-welded austenitic steel pipes fabricated from Esshete 1250 austenitic steel. The simulations examined the impacts of prior quenching of pipe material, fabrication of closely adjacent welds, and axial restraint during welding. The simulations considered both simple isotropic and kinematic hardening behaviour, and a large matrix of Lemaitre-Chaboche mixed isotropic-kinematic hardening material constitutive models, with a focus on examining the most accurate evolutionary hardening behaviour for weld metal. High temperature (650 °C) service exposure was modelled using an RCC-MR type creep model, and the sensitivity of the predicted relaxation to variability in the model parameters was assessed. The predicted residual stresses were validated using measurements made with the deep hole and incremental deep hole drilling techniques and the contour method.

Significant improvement of thermal oxidative mechanical properties in phthalonitrile GFRP composites by introducing microsilica as complementary reinforcement

High performance composites are not only pursued with the high modulus, high strength and high service temperature, but also most of their mechanical performances should be maintained after the high-temperature thermal aging. To improve thermal oxidative mechanical properties of phthalonitrile (PPN) composites, microsilica (MS) as complementary reinforcement was introduced into PPN to form particle-filled GFRP (MS/PPN/GF) composite. DSC and DRA studies showed that the introduction of MS could lower the curing temperature and increase melt complex viscosity of MS/PPN prepolymers. Thermal oxidative stabilities of the MS/PPN composites with 16 wt% MS content were improved by 25.4 °C in T5%, 41.3 °C in T10% and 14.7% in char residue at 800 °C compared those of PPN matrix, respectively. Mechanical performances of MS/PPN/GF composites were significantly improved with the increasing MS content. More importantly, most of their mechanical performances could be maintained after being aged at 400 °C for various durations. After 400 °C/12 h thermal aging, all MS/PPN/GF composites showed a retention ratio of flexural modulus and strength higher than 90% and 70%, respectively. These improvements can be mainly attributed the complementary reinforcement of MS and better thermo-oxidative stabilities of the MS/PPN composites. Fracture surface images of MS/PPN/GF-8 composite reveal an obvious interfacial structure transformation as the thermal oxidative process goes. This study demonstrate that the introduction of particles like MS into polymer-based GFRP composite as complementary reinforcement can be an easy and effective way to improve the thermal oxidative mechanical properties.