Thermal Cycling Test Research Articles

Improvement of solder interconnections applied on back contact solar cells with low-temperature copper paste busbars

This work addresses the solderability and the reliability of n-type IBC ZEBRA cells with screen printed copper paste busbars. Improvement of the solderability by Sn60Pb40 solder alloy coated PV ribbon using an industrial automated IR stringer is reported. For qualification of the module reliability, climate chamber thermal cycling (TC) and damp heat tests (DH) were performed on mini modules with variations in material combinations: low temperature silver and copper pastes, each of them demanding dedicated soldering process parameters and two different encapsulation materials: polyolefin elastomers (POE) and ethylene vinyl acetate (EVA). All tested modules passed 3 x IEC TC and 2 x IEC DH, while the modules with EVA encapsulant passed DH 3000 as well. The copper paste metallized modules with EVA encapsulant showed no degradation in pseudo fill factor (pFF) and open circuit voltage (VOC), proving the statement that no copper diffuses into the silicon bulk.

Bond Wire Fatigue of Au, Cu, and PCC in Power LED Packages.

Bond wire failure, primarily wire neck breakage, in power LED devices due to thermomechanical fatigue is one of the main reliability issues in power LED devices. Currently, the standard testing methods to evaluate the device's lifetime involve time-consuming thermal cycling or thermal shock tests. While numerical or simulation methods are used as convenient and quick alternatives, obtaining data from material lifetime models with accurate reliability and without experimental fatigue has proven challenging. To address this issue, a mechanical fatigue testing system was developed with the purpose of inducing mechanical stresses in the critical region of the bond wire connection above the ball bond. The aim was to accelerate fatigue cracks at this bottleneck, inducing a similar failure mode as observed during thermal tests. Experimental investigations were conducted on Au, Cu, and Pd-coated Cu bonding wires, each with a diameter of 25 µm, using both low- and high-frequency excitation. The lifetime of the wire bond obtained from these tests ranged from 100 to 1,000,000 cycles. This proposed testing method offers material lifetime data in a significantly shorter timeframe and requires minimal sample preparation. Additionally, finite element simulations were performed to quantify the stresses at the wire neck, facilitating comparisons to conventional testing methods, fatigue test results under various operating conditions, material models, and design evaluations of the fine wire bond reliability in LED and microelectronic packages.

Improved thermoelectric properties of Sb-doped Ti0.5Zr0.5NiSn alloy with refined structure induced by rapid synthesis processes

Thanks to the great advantage of preparation methods, the thermoelectric (TE) alloy materials can be fast synthesized with controllable structural morphology for optimizing charge and phonon transport properties, thereby improving the TE efficiency of the material. Herein, a series of Sb-doped Ti0.5Zr0.5NiSn1−xSbx based half-Heusler alloy was prepared via a combined fast procedure of arc melting (AC), melt spinning (MS) followed by spark plasma sintering (SPS). The detailed analyses of morphology, phase structure, and composition reveal high-quality samples through processes for enhanced TE properties. High ZT values over one are achieved for all samples with slight Sb doping content variation from x = 0.0025–0.02. The enhanced TE performance of the Sb-doped Ti0.5Zr0.5NiSn1−xSbx alloys here results from the synergistic between the approaches such as doping, alloying, nanostructuring, and the presence of nano-precipitates induced by MS. The highest ZT values of ∼1.2 at 847 K with an average ZT of 0.798 and estimated TE conversion efficiency reached 12.8% in the temperature range from 300 K to 847 K were achieved for the 1.25% Sb doped sample. In addition, the thermal cycling test through continuous measurement of five and a half cycles confirmed the high thermal stability of the bulk alloy. High ZT and thermal stability based on Hf-free Ti0.5Zr0.5NiSn1−xSbx alloy and fast synthesis are well matched for commercial requirements.

Composite structure of YSZ embedded in NiCoCrAlTaY bond coat induces thin and multilayered Al2O3 film to extend the thermal cycle life of thermal barrier coatings

This study employs cold spraying technology to introduce supersonic-YSZ ceramic particle impacts onto the bond coat, enabling control the coating structure and interface morphology of the bond coat of thermal barrier coatings. The TBCs with YSZ ceramic particle-dispersed NiCoCrAlTaY bond coat were deposited and the dispersed distribution of YSZ ceramics changes the nucleation and growth behavior of oxides of the bond coat during high-temperature oxidation, which inducing the formation of thin and multilayered structure TGO that effectively suppresses the cracking phenomenon caused by concentrated oxide growth at coating interface during thermal cycling test at 1100 °C. The thermal cycling life of the specimens with NiCoCrAlTaY-mixed YSZ composite structure bond coat reached 68 days, which was 2.5 times longer than that of the specimens with APS NiCoCrAlTaY bond coat. Significantly enhancing oxidation resistance and service life of the coating.

Thermal shock behavior and high-temperature oxidation performance of PDC-based environmental barrier coatings on AISI 441 stainless steel

The present work aimed to investigate the influence of different testing conditions on high-temperature oxidation performance of selected polymer-derived ceramic (PDC) environmental barrier coatings on AISI 441 stainless steel substrates. For this purpose, two types of double-layer PDC-based glass-ceramic coatings containing passive and glass fillers were fabricated using a combination of dip coating and spray coating processes. To evaluate the performance of the coatings in harsh environments simulating real applications, thermal cyclic tests in dry air, long-term static oxidation tests in humidified air, and thermal shock tests were performed. YAG, monoclinic ZrO2, cubic ZrO2, and a newly formed BaAl2Si2O8 phase were identified from XRD patterns of the tested PDC coatings. The SiO2 reflections, detected in the coatings after pyrolysis, were absent after both oxidation and thermal shock resistance tests. The SEM examination indicated that the performance of the studied PDC coatings was mainly affected by the microstructural modifications induced by differential sintering of the glass filler particles, leading to an expansion of existing pores, which represented preferential failure sites in the coating. In contrast, the existence of internal porosity relieved the residual stresses and reduced the driving force for crack extension, resulting in an increased thermal shock resistance. A combined scratch test based on the acoustic emission signal record along with the SEM observation of the resulting scratch tracks were also carried out. The most significant damage event in both tested PDC coatings was partial decohesion and surface chipping, which, however, did not translate into a critical adhesive failure. These findings confirm that the developed PDC coatings can be used for the protection of stainless steel in oxidizing environments up to 900 °C.

2D MXene based nanocomposites for solar driven renewable energy storage utilizing binary eutectic phase change material

In this study, we incorporated eutectic phase change materials (EUPCM) into MXene nanopowder to strengthen the thermal properties of EUPCM. Here, Ti3C2 MXene was synthesized from Ti3AlC2 MAX phases via mild chemical etching using microwave and doped at concentrations ranging from 0.2 % to 1 % into binary eutectic of PEG 6000 and PEG10000. The FESEM validated the existence of adequate pores in the MXene structure and DLS revealed that approximately 66 % of the measured nanoparticles had a size of 218 nm. The FTIR and XRD investigations affirmed that the interaction between EUPCM and MXene was predominantly physical in nature. Differential scanning calorimetry (DSC) analysis revealed fusion enthalpies ranging from 163.5 J/g to 159.3 J/g and freezing enthalpies varying from 155.1 J/g to 146.2 J/g for the nanocomposites, keeping melting and freezing temperature. The incorporation of MXene resulted in a consistent decrease in time required for heating and cooling of EUPCM. Furthermore, the nanocomposites exhibited durability as demonstrated by thermal cycling tests, with no significant variation in the chemical structure and only slight fluctuations in the enthalpies after 500 melt/freeze cycles. Consequently, MXene-based nanocomposites display relatively high heat transfer rates compared to EUPCM for the same amount of solar radiation and present promising prospects for integration into solar energy storage systems.

Reliability of the hybrid bonding level using submicrometric bonding pads

Hybrid bonding is a major enabler for devices requiring 3D stacking with high interconnection density. An in-depth reliability study is performed for the specific Cu/SiO2 integration to identify the potential failure mechanisms. A high robustness is evidenced down to bonding pitch 0.81 μm for the standard reliability tests such as Thermal Cycling (TC) tests, High Temperature Storage (HTS) test, Stress induced Voiding (SiV), Time Dependent Dielectric Breakdown (TDDB) or electromigration. The hybrid bonding interface is stable under thermal stress with no atomic or ionic Cu diffusion observed through the hybrid bonding interface. The role of the thin self-formed cuprous oxide at the Cu/SiO2 interface in the case of misaligned bonding pads is believed to play a major role in the high robustness to reliability tests.

Enhancing the performance of composite phase change materials using novel triply periodic minimal surface structures

Organic phase change materials (PCMs) are widely used in thermal management applications owing to their high latent heats of fusion and low cost. Unfortunately, their poor thermal conductivities severely hamper the thermal transport process. Hence, PCMs are often hybridized with high thermal conductivity materials that promote phase change for time-sensitive applications and potentially increase the power density of thermal energy storage (TES) devices. Optimized thermally conductive networks with high-latent-heat PCM matrices are critical in the thermal management of high heat flux applications. With advancements in additive manufacturing, it has become easier to create complex topologies, such as triply periodic minimal surfaces (TPMS), that can further augment the performance of composite PCMs. TPMS surfaces have shown great promise in improving thermal transport performance but investigations into their thermal properties remain limited. In this study, the thermal performances of PCM infused TPMS structures of wall thicknesses varying from 0.4 mm to 0.8 mm were systematically investigated. The TPMS structures were designed using level-set equations, and each system of structures was fabricated by selective laser melting, a metal additive manufacturing technique. Numerical investigations were performed to understand the phase change dynamics of PCM in TPMS cells and experiments were performed to determine their thermal properties and phase change performance for thermal energy storage applications. Our simulations revealed that for structures with uniform wall thickness of 0.4 mm, the novel Neovius structure resulted in the highest effective thermal conductivity enhancement. For a given thickness, it is thus hypothesized that more complex TPMS topologies have higher surface areas and volumes which enhance their thermal conductivities. However, the complex shape of the Neovius structure could not be further uniformly thickened without self-intersecting geometries. Ultimately, a balance of both the surface area and thickness of the TPMS cell determines the thermal conductivity enhancement. Overall, the 0.6 mm thick IWP cells had the highest thermal conductivity of 13.84 W/m K, which was consistently verified in experiments. This is likely due to the fact that it has a larger surface area than the P structure but is also thicker than the 0.4 mm Neovius structure. To further demonstrate the potential application of our PCM-based TPMS design, low temperature thermal cycling tests were performed. Our results showed the IWP cell structure promoted a more uniform temperature distribution as compared to the P cell. Phase change in the 0.6 mm thick IWP cell structure was completed 1.4 times faster than the 0.8 mm thick P cell and 1.8 times faster than the 0.6 mm thick P cell. The purpose of this study is to provide useful guidelines for the selection of TPMS structures for various applications such as battery packs and spacecraft and to open doors for the co-optimization of materials and device geometry for enhanced energy and power density.

A matching design co-enhancing thermal barrier and thermal cyclic performances of sintering-resistant coatings

Sintering-induced healing of intrinsic micropores is the main cause of thermal insulation degradation in plasma-sprayed thermal barrier coatings (TBCs). Although large pores can effectively retain the thermal-insulation performance, they can lower the mechanical performance during thermal cyclic tests. Therefore, balancing the tradeoff between the thermal and mechanical performances is a challenging task. Herein, sintering-resistant coatings were achieved through a matching design that co-enhances the thermal insulation and thermal cycling span. TBCs with different contents of sintering-resistant pores were prepared. The thermal-insulation performance was found to be positively correlated with the contents of sintering-resistant pores. However, in thermal cyclic tests, the lifespan increased as the pore content increased from 0 vol% to 20 vol% but drastically decreased as the pore content increased further above 20 vol%. Next, the competing effects of the newly formed pores on the crack driving force and fracture toughness, which might hasten TBC failure, are discussed. Based on the critical pore content, the matching design enhances the thermal insulation and lifespan of the resultant TBCs by 50% and 40%, respectively, from those of conventional TBCs. Therefore, the sintering-resistant structure is expected to considerably contribute to next-generation applications of TBCs.

Development and evaluation of cenospheres/polyethylene glycol composite microcapsule for temperature-regulation of asphalt pavements

ABSTRACT In this study, a phase change material microcapsule was produced by using cenospheres as the microcapsule shell, polyethylene glycol 2000 as the phase change material, and silica sol as the sealing material. The produced CenoPCM microcapsule was analyzed using scanning electronic microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The thermal properties and stability of the CenoPCM were evaluated using differential scanning calorimetry (DSC), thermogravimetric analysis (TG), and a thermal cycling test. The results showed that the phase change temperature and enthalpy of the CenoPCM were 55.7°C and 101.4 J/g, respectively, with a mass loss of 2.35% at 200°C. After 200 thermal cycles, the mass change remained stable at 2.05%. An outdoor test was conducted to assess the impact of CenoPCM on the thermal performance of asphalt mixtures. In comparison with conventional asphalt mixture specimens, the maximum temperature differences at depths of 2, 5, and 8 cm below the surface of the phase change asphalt mixture specimens were 4.9°C, 6.4°C, and 5.9°C, respectively. Finally, the influence of CenoPCM on the performance of asphalt mixtures was evaluated, revealing a promising future for its use in asphalt pavements.

Enhancing properties of Li2TiO3/Li4SiO4 tritium breeding ceramics by chitosan addition

Li2TiO3/Li4SiO4 ceramics have been regarded as promising tritium breeders due to their numerous merits. In this work, chitosan was employed as an effective binder to provide adhesion of Li2TiO3 particles to Li4SiO4 particles, resulting in Li2TiO3/Li4SiO4 ceramics with improved performance. Chitosan could be removed during the sintering process and had little influence on the phase composition. The resulting Li2TiO3/Li4SiO4 ceramics had higher density (92.5%T.D.), greater crushing loads (115.2 ± 20.8 N) and superior thermal properties (3.61 W/m∙K) when compared to the samples prepared without chitosan. SEM analyses displayed a dual-scale structure and the inter- and trans-granular fracture mode. Thermal cycling tests revealed that the phase composition was stable but the crushing load fell slightly. The crushing load remained at 92.4 ± 17.1 N after 12 cycles. Overall, chitosan plays an important role in the amelioration of interface combination between Li4SiO4 and Li2TiO3, and thus significantly improve density, crushing load and thermal conductivity of Li2TiO3/Li4SiO4.

Thermal storage performance of NaNO3/cordierite-mullite ceramic (CMC) composites deriving from metallurgical slag

Although inorganic salts are considered outstanding thermal storage materials, drawbacks such as low thermal conductivity, corrosion issues, and leakage severely hinder their practical industrial application. Porous ceramics are suitable to serve as the supporting skeleton for inorganic salt due to good corrosion resistance, high thermal conductivity and high porosity. In this study, secondary aluminum dross (SAD) and ferronickel slag (FNS) were used as raw materials to prepare cordierite-mullite porous ceramic (CMC). The NaNO3/CMC composites were synthesized using spontaneous melting-infiltrating method, and their thermal storage performance were studied in detail. The results indicated that CMC had a favorable pore structure with high porosity and micro-sized pores, as well as high compressive strength exceeding 30 MPa. Excellent chemical compatibility between CMC and NaNO3 was demonstrated by XRD analysis. The latent heat of the NaNO3/CMC5, NaNO3/CMC15 and NaNO3/CMC30 composites, as determined by DSC, were 76.4 J/g, 76.7 J/g and 73.6 J/g, respectively, accounting for 43.5%,43.6% and 41.9% of pure NaNO3. The supercooling of NaNO3 was significantly decreased after infiltration into CMC supporting skeleton. A 100-cycle thermal cycles experiment was conducted to verify the long-term thermal reliability of the NaNO3/CMC composites, including latent heat and heat transfer property. The melting latent heat of the NaNO3/CMC5, NaNO3/CMC15 and NaNO3/CMC30 decreased by nearly 5.6%, 3.7% and 2.9%, respectively, after 100 thermal cycling tests, indicating the NaNO3/CMC composites had good thermal reliability. The thermal conductivity of all the NaNO3/CMC composites exceeded 1.8 W m−1 K−1 at 350 °C. Additionally, the compressive strength of all the composites showed only minimal decrease after thermal cycling test, remaining higher than 100 MPa. The NaNO3/CMC composites offer promising prospects in the medium temperature thermal storage utilization field, such as industrial waste heat recovery, owing to low production cost, high thermal conductivity and high mechanical strength.

Effect of Elastomeric Coating on the Properties and Performance of Myristic Acid (MA) Phase Change Material (PCM) Used for Photovoltaic Cooling

Nitrile butadiene rubber (NBR) latex exhibits excellent tensile properties, chemical resistance, and thermal stability in applications such as gloves and safety shoes due to vulcanization. In this research work, attempts have been made to manipulate the vulcanization to produce thin and compact elastomeric NBR coating on myristic acid (MA) phase change material (PCM) to produce shape-stabilized PCM. The proposal for the use of latex-based elastomeric coating for PCM has been rarely considered in the literature due to a lack of understanding of the crosslink of elastomers. Thus, in this research, the effects of sulfur formulation on the coating performance of NBR on the PCM in terms of latent heat and thermal stability were determined. Leakage analysis indicates that the MA pellet coated with 0.5 phr of sulfur-cured NBR layer (MA/NBR-0.5) successfully eliminates the leakage issue. A tensile analysis revealed that a durable PCM coating layer must possess a combination of the following criteria: high tensile strength, ductility, and flexibility. Fourier transform infrared analysis (FTIR) and electron microscopy images showed the formation of thin, compact, and continuous NBR coating when 0.5 phr of sulfur was used. The further increment of sulfur loading between 1.0 and 1.5 phr causes the formation of defects on the coating layers, while non-vulcanized NBR layers seem to be very weak to withstand the phase-change process. The recorded latent heat values of melting and freezing of MA/NBR-0.5 are 142.30 ± 1.38 and 139.47 ± 1.23 J/g, respectively. The latent heat of the shape-stabilized MA/NBR-0.5 PCM is reduced by 32.24% from the pure MA latent heat density. This reduction is significantly lower than the reported latent heat reduction in shape-stabilized PCMs in other works. The thermal cycle test highlights the durability of the coated PCMs by withstanding up to 1000 thermal cycles (2.7 years) with less than 2% changes in latent heat value. Cooling performance test on photovoltaic (PV) module shows that the fabricated shape-stabilized PCM could reduce the temperature of the PV module up to 17 °C and increase the voltage generation by 7.92%. Actual performance analysis of shape-stabilized PCMs on the cooling of the PV module has been rarely reported and could be considered a strength of this work.

Development of nanomodified eco-friendly thermal energy storing cementitious composite using PCM microencapsulated in biosourced encapsulation shell

This study presents the development of a thermal energy storing cementitious composite that incorporates phase change material (PCM) microencapsulated in biosourced polymer shell (m-PCM). The biodegradability of the encapsulated PCM was assessed using Nuclear Magnetic Resonance (NMR), which confirmed the high biodegradability of the m-PCM shell under specific environmental conditions. To examine the thermophysical properties of the m-PCM, Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA) were performed. The DSC results revealed exothermic enthalpies of 84.7 J/g and 140.2 J/g, with associated peak temperatures of − 3.2 °C and − 22.9 °C, respectively. Subsequently, the m-PCM was incorporated into cement-based mortar at proportions of 5%, 10%, and 15% by weight of binder. Uniaxial compression tests were conducted to evaluate the effect of the m-PCM on the mechanical properties of the mortar. The results indicated a significant decrease in mechanical strength upon incorporating the m-PCM. However, this reduction in strength was mitigated by the addition of silica fume (SF) and multiwalled carbon nanotubes (MWCNTs) in combination. Furthermore, a thermal cycling test was performed to examine the behavior of the nanomodified m-PCM incorporated mortar under varying ambient conditions. The results showed that the addition of MWCNTs improved both the mechanical performance and thermal performance of the composite.

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

Thermal cycle test performances of environmental barrier coatings of HfO2-SiO2/Yb2Si2O7 in the steam environment at 1475 °C

Composited MAO coating with SiO2 particles on TA2 alloy by laser treatment

In order to decrease defects and improve protective performance, micro-arc oxidation (MAO) coating was modified by adding SiO2 particles to form a composite coating on TA2 alloy via laser treatment. The microstructural morphology, phase structure, cohesive strength and corrosion resistance of the composite coatings were investigated and analyzed by using SEM, XRD, thermal cycle, and electrochemical testing, respectively. The results showed that SiO2 particles were firstly adsorbed onto the MAO surface with the aid of ultrasonic treatment and then sealed micro-pores of the MAO coating after laser treatment. The composite coating mainly consisted of R-TiO2, A-TiO2 and SiO2 phases and exhibited a dense multilayer structure with a rich Si element in the outer layer. After subjecting the coatings to 300 thermal cycles, clacking or marked spalling zones could not be observed in the composite coating; nevertheless, an obvious destruction appeared in the single MAO coating without SiO2 particles. The corrosion potential (Ecorr) and pitting potentials (Epit) of the composite coating were −0.303 V and 1.765 V compared to that of −0.682 V and 1.401 V for the single MAO coating. Additionally, the corrosion rate decreased to 1.40 × 10−4 A·cm−2 with the highest resistance (76,788.67 Ω·cm2) from2.19 × 10−4 A·cm−2, indicating a stronger corrosion resistance in the composite coating. The results of simulation from Comsol Multiphysics demonstrated that a laser power as 80 W was appropriate for melting SiO2 particles into the MAO coating to form a dense and cohesive composite coating.

Electrically conductive adhesive-free interconnection of shingle solar cells

This work deals with the usage of electrically conductive adhesives (ECA) for the interconnection of shingle solar cells. In a detailed study on small-format shingle modules characterized by current-voltage, electroluminescence and magnetic field imaging measurements, the impact of an ECA reduction (usage of ECA dashes instead of a continuous ECA line) as well as the complete omission of ECA is investigated with regards to the electrical interconnection quality. Inter alia, it is demonstrated that lamination can improve interconnection quality in ECA-containing shingle joints and establish interconnection in ECA-free shingle joints by pressing adjacent shingles together and thereby forming a busbar-to-busbar contact of decent electrical quality. In a follow-up experiment, the approach of simply omitting the usage of ECA for shingle interconnection is tested with regards to thermal cycling, damp heat and dynamic mechanical load tests finding only minor compromises, if at all, in terms of long-term stability.

Hot corrosion mechanism and thermal cycling performance of air-plasma-sprayed LaYbZr2O7 thermal barrier coatings in the vanadate-containing molten salts

LaYbZr2O7 has been regarded as a promising thermal barrier coatings (TBC) material due to its excellent mechanical and thermal properties. In this study, the LaYbZr2O7 coating was prepared by atmospheric plasma spraying (APS), and its hot corrosion mechanism and thermal cycling performance in vanadate-containing molten salts (V2O5 and Na2SO4 + V2O5) were investigated. Results indicated the hot corrosion reaction of LaYbZr2O7 coatings in molten V2O5 was temperature dependent, attributed to the thermal decomposition of ZrV2O7 above 800 °C. A continuous dense corrosion layer could form on the top surface of LaYbZr2O7 coatings after hot corrosion in molten V2O5 salt, effectively preventing the further penetration of molten salt. However, the dense corrosion layer could not form when exposed to Na2SO4 + V2O5 molten salts at 1000 °C, since the formation of NaVO3 could improve the mobility of molten salts and slow the precipitation reaction rate of corrosion products. Furthermore, thermal cycling tests indicated that the Na2SO4 + V2O5 mixture could be more detrimental to LaYbZr2O7 coatings than single V2O5 salt, and the dense corrosion layer could act as a sacrifice layer to protect the inner coating from corrosion degradation. The hot corrosion mechanism of LaYbZr2O7 was discussed in detail based on the Lewis acid-base and dissolution-precipitation rule.

Microstructural characteristics evolution of asphalt mastics under cyclic environmental temperature variations

Environmental temperature-effect is the primary factor responsible for asphalt pavement failure. Asphalt pavement exposed to the atmosphere is constantly subjected to the thermal cycles induced by environmental temperature fluctuations, and the resulting thermal cracks significantly reduce the durability of asphalt pavement. Nevertheless, the influence of cyclic temperature variations on the performance degradation of asphalt binder materials is often overlooked. To address this, accelerated thermal cycling test was conducted on two asphalt mastics to simulate repeated environmental temperature variations. Dynamic shear rheological test (DSR) and atomic force microscopy test (AFM) were performed to characterize the evolution of the macroscopic rheological properties and microstructural characteristics under five cyclic temperature ranges. Pearson correlation analysis was utilized to explore the correlation between macro and micro properties. Following thermal cycling, it was observed that the two asphalt mastics experienced a decline in their viscous properties and became more elastic, resulting in a decrease in their stress relaxation capacity and asphalt-aggregate interaction on a macroscopic level. The microstructure transitioned from a multiphase to a homogeneous structure, while the expansion of the valley structure indicated the accumulation of microscopic defects. The larger the temperature variations and the higher the cyclic temperatures are, the more severe the deterioration in the viscoelasticity and microstructures of asphalt mastics is; the degradation of the micromechanical characteristics directly affects the macroscopic rheological properties. The bee structure ratio, the adhesion, and relaxation properties are sensitive to cyclic temperature variations and recommended to characterize the thermal cycling resistance. In comparison to 90# asphalt mastic, the SBS modified asphalt mastic exhibits great resistance to thermal deterioration. This study deepens the understanding of the causes of asphalt pavement performance deterioration in regions with frequent environmental temperature fluctuations.

Waste acceptance criteria testings of a phosphate-based geopolymer waste form to immobilize radioactive borate waste

Immobilization of radioactive borate waste (BW) is an urgent issue in South Korea because of the lack of appropriate waste forms. This study aimed to investigate whether phosphate-based geopolymers could satisfy the waste acceptance criteria established by Korean government. Geopolymers with 40 wt% BW did not exceed the criterion of compressive strength (3.445 MPa) after the water immersion and thermal cycling tests, whereas geopolymers containing 30 wt% BW still showed higher than 3.445 MPa after waste acceptance criteria tests. In particular, the compressive strength increased and the total porosity significantly decreased after the thermal cycling and gamma irradiation tests. The leachability index of boron calculated according to the ANSI/ANS 16.1 test was found to be higher than the criterion value (>6.0), and the controlling leaching mechanism was diffusion. The characteristic analyses indicated that water-soluble borate phases were leached from the geopolymer during the water immersion and leaching tests. However, the structurally incorporated 4-coordinated boron was also present as a sparingly soluble phase. Consequently, phosphate-based geopolymers can be used as stable binder materials to immobilize radioactive BW.