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A typical Hall thruster is powered by a constant-voltage power supply. In this study, to ensure the normal operation of Hall thrusters at lower power conditions, we applied pulse modulation technology to supply power to a 600-W Hall thruster. The results show that, when the input power of the thruster was within the rated power range, the thrust and efficiency of the thruster were slightly reduced under pulsating mode compared with those of direct current (DC) operation. However, the thrust-to-power ratio of the thruster under pulsating mode was superior to that under DC operation at low power (<350 W). The thrust-to-power ratio under pulsating mode was improved to 77.5 mN/kW at 210 W compared with that under DC operation (70 mN/kW). Meanwhile, we found that the stable operating boundary of the thruster under pulsating mode significantly widened at high frequencies. A 600-W Hall thruster under pulsating mode can operate at a lowest power of 66.8 W, which is an order of magnitude lower than the rated power. The thrust and anode efficiency under pulsating mode at 68.8 W were 4.3 mN and 7.8%, respectively. [ABSTRACT FROM AUTHOR]

The current trend in the asphalt industry is to increase the utilization of recycled asphalt pavement (RAP). In general, there are two types of RAP binder in RAP aggregates, the mobilized RAP binder and the immobilized RAP binder. The former can be blended into the virgin binder to coat aggregates during asphalt blending, while the latter can be treated as black rocks because they have already become an outer crust of RAP aggregates. Studies have focused on the mobilized RAP binder quantification and the significance of mobilized RAP binder to mixture performances. However, it remains unknown whether the immobilized RAP binder affects mixture performances. This study developed a laboratory procedure for immobilized RAP aggregates preparation and investigated the contribution of immobilized RAP binder to asphalt-aggregate interaction as well as the performance of 100% recycled asphalt mixtures. The work of adhesive energy and the K-B-G* model were adopted to characterize the asphalt-aggregate interaction for asphalt mortar. Resilient modulus and dissipated creep strain energy were used to evaluate the stiffness and cracking resistance of 100% recycled asphalt mixtures. Results showed that the immobilized RAP binder can strengthen the asphalt-aggregate interaction and improve the cracking resistance of 100% recycled asphalt mixtures by increasing total asphalt content. However, if the total asphalt content remains the same, the immobilized RAP binder increases stiffness but decreases the cracking resistance of 100% recycled asphalt mixtures. [ABSTRACT FROM AUTHOR]

The modified boiling test based on image processing has gained significant attention in recent years, aimed at replacing subjective visual observation with an objective measurement. However, there are still some unsolved problems influencing the accuracy of the test such as the shadow between aggregate particles and the selection of threshold values for binary image conversion. Based on these challenges, an innovative modified boiling test procedure based on digital imaging has been successfully developed for the purpose of rating asphalt mix ingredients via moisture susceptibility. The new method focuses on a single piece of aggregate with a newly cut smooth surface, and then the aggregate coated by asphalt is subjected to the boiling test. Changes to the thickness of the asphalt coating on the fresh surface before and after boiling can be captured indirectly by digital images and reflected by the changing average grayscale values. The experimental results of 20 asphalt-aggregate combinations clearly showed the moisture damage resistance of different samples and the effect of liquid antistripping agent. In addition, the test results correlated well with energy ratios based on the surface free energy measurements of asphalt and aggregate. A decreased stripping rate over time were also observed for all samples and no significant stripping was found after 90 min of boiling. The selection of a suitable boiling time is critical to the accuracy of the test, which can be determined by trial-and-error methods. The proposed modified boiling test avoids the disadvantages of previously developed methods and provides a quick way to evaluate the asphalt mixture ingredients by moisture susceptibility, making it possible to test a large amount of samples within a short period of time. The new method can be used to generate an initial selection of asphalt-aggregate combinations in place of the surface free energy method. [ABSTRACT FROM AUTHOR]

Research on the aging process of styrene-butadiene-styrene block copolymers (SBS) block copolymers modifier has practical significance in guiding the recycling of SBS-modified asphalt pavement. The objective of this study is to investigate the aging process of SBS modifier under various oxygen and thermal conditions to address the variations of physical properties and chemical reaction during the service life of recycled asphalt pavement (RAP). In the proposed testing process, SBS modifier was thermal oxidative and vacuum thermal aged at different temperatures and time prior to blending with base asphalt to prepare SBS-modified asphalt. SBS modifier was first dissolved by carbon tetrachloride (CCl4) and then tested through infrared (IR), gel permeation chromatography (GPC), and thermogravimetric (TGA) tests to investigate the variations on microstructure and chemical compositions of aged SBS modifier. The effect of aged SBS modifier on performance of SBS-modified asphalt was evaluated by comparing the mechanical and chemical properties of base and SBS-modified asphalts. Based on the results of the experimental program, appearance variation, modifying effects, and chemical reaction of SBS modifier under oxygen condition were obviously different from that of vacuum condition. The chemical reaction of SBS modifier was not significant at vacuum and high temperature condition, instead, a small amount of degradation was observed. Degradation of mechanical performance of SBS-modified asphalt was mainly caused by aging of base asphalt. Therefore, the SBS modifier in the RAP was suggested to be considered in the mixture design. [ABSTRACT FROM AUTHOR]

This work investigated the effects of the aging process on the performance of asphalt binder under multiple environmental conditions based on multitechnique experimental results. The aging process of matrix asphalt binder with the increase of temperature for 96 h, 192 h, and 288 h was simulated by an aging oven. Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were used to quantify the microcosmic characteristics of asphalt after aging. The results showed that the aging process of asphalt is an oxidation process, and the oxygen-containing functional groups (C═O and S═O) increased with aging. At the macroscopic level, the changes of adhesion properties and bond strength of asphalt were studied by testing the changes in three major indexes. The penetration and ductility of asphalt decreased, and the softening point increased with aging. The changes of rheological performance were tested by dynamic shear rheometer (DSR) and Brookfield viscometer. Mutual transformation of aromatic phenol, colloid, and asphaltene led to an increase in asphalt viscosity and decrease in workability. Antifatigue performance gradually decreased under the action of load with the extension of time. The elastic properties were finally damaged by fatigue. [ABSTRACT FROM AUTHOR]

The application of photocatalysts on pavement engineering to degrade automobile exhaust has become a hot topic for researchers. However, the effect of photocatalysts on pavement materials has not been fully investigated. The primary purpose of this study is to explore the effect of a typical photocatalyst titanium dioxide (TiO2) on the chemical structure and properties of asphalt through laboratory experiments and molecular dynamics (MD) simulation. Performance tests indicated that the physical properties of asphalt modified by TiO2 changed more significantly compared with that of calcium carbonate (CaCO3) and silica (SiO2), which might be due to the smaller particle size of TiO2. Atomic force microscopy (AFM) and Fourier infrared spectrum (FI-IR) analyses demonstrated that the addition of TiO2 resulted in a smoother asphalt surface and a disappearance of the bee structure. Using TiO2 would not accelerate the aging process of asphalt. Moreover, the MD simulation results indicated that a more significant interaction existed between TiO2 and asphalt than that of CaCO3 and SiO2 , which resulted in a tighter asphalt molecule and smaller free volume distribution in the TiO2-asphalt system. [ABSTRACT FROM AUTHOR]

With the continuing increase in the use of recycled asphalt pavement (RAP) and recycled asphalt shingle (RAS) for highway construction, there is a concern over how much of the aged asphalt binder inside RAP/RAS can be blended into virgin asphalt binder. In this study, the RAP/RAS binder mobilization rates of 13 different mixtures produced by three different types of asphalt plants in Tennessee were determined using fluorescence microscopy and gel permeation chromatography (GPC). A multiple regression model was developed to explore the factors critical to enhancing the mobilization rate of the RAP/RAS binder. Results showed that the RAP/RAS binder mobilization rate was significantly lower than 100% for most mixtures, indicating that only part of the RAP/RAS binder was mobilized during the production process. The mixing and delivery temperatures had significant influence on the mobilization rate of RAP/RAS binder. In addition, the aging degree of RAP/RAS binder also significantly affected the mobilization rate. [ABSTRACT FROM AUTHOR]