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The use of piezoelectric materials based Electromechanical Impedance (EMI) technique for monitoring the hydration of cementitious materials has caught much attention recently. However, very few literatures have explored the feasibility of using this method on monitoring the stiffness development and compressive strength gain at the very early age properties (4th–8th h) of cementitious materials. This research serves as a comprehensive study to verify the reliability of using lead zirconate titanate (PZT) based EMI method in monitoring the compressive strength gain and elastic modulus of mortar at both very early age (4th–8th h) and early age (1st, 3rd, and 7th day). Extensive experiments and data analysis have been done on ten different mixes with various water-to-cement ratio and Type I & III cement. The EMI signatures are measured for each sample at the period of interest, and post-processed with three statistic model including the root mean square deviation (RMSD), correlation coefficient deviation (CCD), and mean absolute percentage deviation (MAPD) as indices. To examine the correlation and linearity between the compressive strength/elastic modulus obtained via conventional cubic testing using ASTM C109 and the EMI indices, a linear least square regression analysis is performed. As the authors postulated, all the mixes display a good linear correlation of R2. Among all three statistical indices, RMSD index is proved as the most accurate statistical index on strength gain monitoring of cementitious materials. The results indicated the feasibility of using piezoelectric-based EMI method for monitoring the cementitious material’s strength gain at very early age, regardless the concrete mix design.

This paper investigated the spatial distribution of steel fibers and air bubbles in UHPC cylinder specimen by using X-ray CT system. Four levels of superplasticizer dosage were adopted to obtain cement mortar mixtures with different rheological properties and then steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cylindrical specimens with diameter of 100 mm and height of 200 mm were prepared under the same condition for every mixture and the content of steel fibers and air bubbles at different casting depth was determined by X-ray CT measurement. On the other hand, the cylinder specimen was cut into four pieces for evaluating the splitting tensile strength variation with the casting depth. The results reveal that steel fibers and air bubbles are uniformly distributed in UHPC specimen when the fresh mixture is prepared by adding a lower dosage of superplasticizer. This homogeneity is also proved by the splitting tensile strength for samples at different depths. When the yield stress of fresh mortar was reduced by the higher dosage of superplasticizer, the steel fiber sedimentation and air bubble rise occurred and the splitting tensile strength was significantly decreased at the top portion of cylinder specimen. The influence of steel fiber content on splitting tensile strength can be regressed into a good quadratic equation. Therefore, the balance between homogeneity and flowability should be fully considered for manufacturing UHPC elements with a high depth.

This paper aims to study the distribution of steel fibers in UHPC specimens prepared by different rheological cement mortar. Four levels of superplasticizer dosage were used for every water to binder ratio (0.18, 0.20, 0.22, 0.24) to adjust rheological properties of cement mortar mixtures (without steel fibers). Steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cube specimens (70.7 × 70.7 × 70.7 mm3) were cast and cured under the same condition for every mixture and the distribution of steel fibers along casting depth was determined by digital image processing. The experimental results reveal that the flowability of fresh cement mortar increases and the yield stress decreases with the higher water to binder ratio and superplasticizer dosage. The plastic viscosity of fresh mixture is decreased by the higher water to binder ratio and little influenced by the variation of superplasticizer dosage. The fiber distribution along depth mainly depends on rheological parameters of fresh mixture, especially the yield stress. Moderate rheological parameters are necessary for obtaining an uniform distribution of steel fibers in UHPC. On the basis of this study, the optimal yield stress of fresh matrix ranges among 900–1000 Pa, 700–900 Pa and 400–800 Pa respectively for UHPC mixtures with 1%, 2% and 3% fiber volume fraction.

Occurrence of lithium plating on the anode is a severe side reaction in the lithium-ion batteries, which brings cell capacity degradation and reduces the cell safety. This paper focuses on 37Ah commercial lithium-ion batteries and clarifies the evolution of lithium plating during long-term low temperature (−10 °C) cycling. The tested cells are analyzed at different degradation stages, named “fixed-point analysis”, to evaluate the evolution process of lithium plating. It is found that the capacity fade and the internal resistance increase exhibit a decelerated trend with a turning point around 450 cycles. The loss of cyclable lithium caused by lithium plating is deemed to be the main reason behind the battery degradation. Post-mortem analysis including scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) is conduced to reveal the mechanism. The thickness and morphology change of plated lithium, and the plated lithium covered by solid electrolyte interface film, are disclosed by the surface and cross-section SEM images from the fixed-point analysis. XPS analysis further reveals the composition of solid electrolyte interface film, and the plated lithium existing. In a nutshell, lithium plating is inhomogeneous and has high spatial dependence.

It is critical to develop an accurate non-destructive testing (NDT) methods to evaluate the real-time concrete strength gain due to the fast pace of construction. Among the NDTs for concrete strength monitoring, piezoelectric materials based electromechanical impedance (EMI) method has attracted much attention recently. Some statistical approaches are proposed for impedance signal processing, such as root mean square deviation (RMSD) and mean absolute percentage deviation (MAPD). However, these methods require the baseline value for calculation, which is not convenient, particularly for the very early strength gain monitoring. In this study, three features extracted directly from phase angle spectrum, including resonant frequency of the first local phase angle peak (PARF), local phase angle peak value (PA), and the corresponding width of half-prominence (HPW) are employed as indicators to monitor the strength gain process. Among these three indicators, the first peak of phase angle resonant frequency (PARF) is found the most reliable. The PARF index can directly be obtained from the impedance spectrum without baseline calculation. Five types of different cement mortar with various water-to-cement ratios from 0.38 to 0.46 were prepared. The EMI test and conventional cubic testing (ASTM 109) were conducted concurrently at a very early age (4th to 8th hour) and an early age (1st, 3rd, and 7th day). To examine the correlation between the compressive strength obtained and the EMI-PARF indices, a linear least-squares regression analysis was performed. Furthermore, the finite element analysis (FEA) was conducted to discuss the relationship between the features obtained from the phase angle spectrum and the stiffness of the host structure. The PARF index exhibits a high correlation with the mechanical properties of cementitious materials. Both experiments and FEA results ascertain that piezoelectric-based EMI coupled with the PARF index is a reliable method for in-place strength gain evaluation of cementitious materials.

A novel phenol–formaldehyde carbon with ordered/disordered bimodal mesoporous structure is synthesized by the facile evaporation induced self-assembly strategy under a basic aqueous condition with SiO2 particles as template. The prepared bimodal mesoporous carbons (BMCs) are composed of ordered mesoporous and disordered mesoporous with diameter of about 3.5 nm and 7.0 nm, respectively. They can be employed as supercapacitor electrodes in H2SO4 aqueous electrolyte after the simple acid-treatment. BMC exhibits an exceptional specific capacitance of 344 F g−1 at the current density of 0.1 A g−1, although it has a relatively low surface area of 722 m2 g−1. And the BMC electrode displays an excellent cycling stability over 10,000 cycles.

Highly ordered, body-centered cubic mesoporous phenol–formaldehyde carbon C-FDU-16 is prepared by a facile evaporation induced self-assembly strategy under a basic aqueous condition. Afterwards, in order to increase the active site of surface electrochemical reactions and promote the wettability in aqueous electrolyte, a simple chemical surface modification is carried out on the C-FDU-16 by nitric acid treatment. The C-FDU-16 and the acid-modified C-FDU-16 (N-FDU-16) are used as the active materials for supercapacitors. Although C-FDU-16 has unique 3D mesoporous network combined with high specific surface area, it do not show well supercapacitor properties for its bad wettability with the aqueous electrolyte. However, the N-FDU-16 with acid treatment exhibits the largest specific capacitance of 219 F g−1 at a current density of 0.1 A g−1 in 1 M H2SO4 electrolyte which is 7 times larger than that of the C-FDU-16 and it also shows good cycling stability as well. Therefore, based on the above investigations, ordered mesoporous phenol–formaldehyde carbon such as C-FDU-16 can be a potential candidate for supercapacitors after the simply acid treatment.