Evaluation of the pore morphologies for piezoelectric energy harvesting application

Piezoelectric energy harvesting has attracted significant attention in recent years due to their high-power density and potential applications for self-powered sensor networks. In comparison to dense piezoelectric ceramics, porous piezoelectric ceramics exhibit superiority due to an enhancement of piezoelectric energy harvesting figure of merit. This paper provides a detailed examination of the effect of pore morphology on the piezoelectric energy harvesting performance of porous barium calcium zirconate titanate 0.5Ba(Zr0.2 Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 (BCZT) ceramics. Three different pore morphologies of spherical, elliptical, and aligned lamellar pores were created via the burnt-out polymer spheres method and freeze casting. The relative permittivity decreased with increasing porosity volume fraction for all porous BCZT ceramics. Both experimental and simulation results demonstrate that porous BCZT ceramics with aligned lamellar pores exhibit a higher remanent polarization. The longitudinal d33 piezoelectric charge coefficient decreased with increasing porosity volume fraction for the porous ceramics with three different pore morphologies; however, the rate of decrease in d33 with porosity is slower for aligned lamellar pores, leading to the highest piezoelectric energy harvesting figure of merit. Moreover, the peak power density of porous BCZT ceramics with aligned lamellar pores is shown to reach up to 38 μW cm-2 when used as an energy harvester, which is significantly higher than that of porous BCZT ceramics with spherical or elliptical pores. This work is beneficial for the design and manufacture of porous ferroelectric materials in devices for piezoelectric energy harvesting applications.</p

Enhanced energy harvesting performance in lead-free multi-layer piezoelectric composites with a highly aligned pore structure

The harvesting of mechanical energy from our living environment via piezoelectric energy harvesters to provide power for next generation wearable electronic devices and sensors has attracted significant interest in recent years. Among the range of available piezoelectric materials, porous piezoelectric ceramics exhibit potential for both sensing and energy harvesting applications due to their reduced relative permittivity and enhanced piezoelectric sensing and energy harvesting figures of merit. Despite these developments, the low output power density and the lack of optimized structural design continues to restrict their application. Here, to overcome these challenges, a lead-free multi-layer porous piezoelectric composite energy harvester with a highly aligned pore structure and three-dimensional intercalation electrodes is proposed, fabricated and characterized. The effect of material structure and multi-layer configuration of the porous piezoelectric ceramic on the dielectric properties, piezoelectric response and energy harvesting performance was investigated in detail. Since the relative permittivity is significantly reduced due to the introduction of aligned porosity within the multi-layer structure, the piezoelectric voltage coefficient, energy harvesting figure of merit and output power are greatly enhanced. The multi-layer porous piezoelectric composite energy harvester is shown to generate a maximum output current of 80 μA, with a peak power density of 209 μW cm−2, which is significantly higher than other porous piezoelectric materials reported to date. Moreover, the generated power can charge a 10 μF capacitor from 0 V to 4.0 V in 150 s. This work therefore provides a new strategy for the design and manufacture of porous piezoelectric materials for piezoelectric sensing and energy harvesting applications.</p

Enhanced energy harvesting performance in lead-free multi-layer piezoelectric composites with a highly aligned pore structure

The harvesting of mechanical energy from our living environment via piezoelectric energy harvesters to provide power for next generation wearable electronic devices and sensors has attracted significant interest in recent years. Among the range of available piezoelectric materials, porous piezoelectric ceramics exhibit potential for both sensing and energy harvesting applications due to their reduced relative permittivity and enhanced piezoelectric sensing and energy harvesting figures of merit. Despite these developments, the low output power density and the lack of optimized structural design continues to restrict their application. Here, to overcome these challenges, a lead-free multi-layer porous piezoelectric composite energy harvester with a highly aligned pore structure and three-dimensional intercalation electrodes is proposed, fabricated and characterized. The effect of material structure and multi-layer configuration of the porous piezoelectric ceramic on the dielectric properties, piezoelectric response and energy harvesting performance was investigated in detail. Since the relative permittivity is significantly reduced due to the introduction of aligned porosity within the multi-layer structure, the piezoelectric voltage coefficient, energy harvesting figure of merit and output power are greatly enhanced. The multi-layer porous piezoelectric composite energy harvester is shown to generate a maximum output current of 80 μA, with a peak power density of 209 μW cm−2, which is significantly higher than other porous piezoelectric materials reported to date. Moreover, the generated power can charge a 10 μF capacitor from 0 V to 4.0 V in 150 s. This work therefore provides a new strategy for the design and manufacture of porous piezoelectric materials for piezoelectric sensing and energy harvesting applications.</p

Flexible PVDF-TrFE Nanocomposites with Ag-decorated BCZT Heterostructures for Piezoelectric Nanogenerator Applications

Flexible piezoelectric nanogenerators are playing an important role in delivering power to next-generation wearable electronic devices due to their high-power density and potential to create self-powered sensors for the Internet of Things. Among the range of available piezoelectric materials, poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE)-based piezoelectric composites exhibit significant potential for flexible piezoelectric nanogenerator applications. However, the high electric fields that are required for poling cannot be readily applied to polymer composites containing piezoelectric fillers due to the high permittivity contrast between the filler and matrix, which reduces the dielectric strength. In this paper, novel Ag-decorated BCZT heterostructures were synthesized via a photoreduction method, which were introduced at a low level (3 wt %) into the matrix of PVDF-TrFE to fabricate piezoelectric composite films. The effect of Ag nanoparticle loading content on the dielectric, ferroelectric, and piezoelectric properties was investigated in detail, where a maximum piezoelectric energy-harvesting figure of merit of 5.68 × 10-12 m2/N was obtained in a 0.04Ag-BCZT NWs/PVDF-TrFE composite film, where 0.04 represents the concentration of the AgNO3 solution. Modeling showed that an optimum performance was achieved by tailoring the fraction and distribution of the conductive silver nanoparticles to achieve a careful balance between generating electric field concentrations to increase the level of polarization, while not degrading the dielectric strength. This work therefore provides a strategy for the design and manufacture of highly polarized piezoelectric composite films for piezoelectric nanogenerator applications.</p

How does auditors’ work stress affect audit quality? Empirical evidence from the Chinese stock market

AbstractWith reference to the Job Demands–Control Model, we empirically examine the effect of auditors’ work stress on audit quality using a sample of Chinese A-share listed companies and their signature auditors from 2009 to 2013. The results show that (1) there is generally no pervasive deterioration in audit quality resulting from auditors’ work stress; (2) there is a significant negative association between work stress and audit quality in the initial audits of new clients; and (3) the perception of work stress depends on auditors’ individual characteristics. Auditors from international audit firms and those in the role of partner respond more strongly to work stress than industry experts. Auditors tend to react more intensively when dealing with state-owned companies. We suggest that audit firms attach more importance to auditors’ work stress and rationalize their allocation of audit resources to ensure high audit quality

Recent durability studies on concrete structure

Trabajo presentado en el 14th International Congress on the Chemistry of Cement (ICCC), celebrado en Beijing (China) del 13 al 16 de octubre de 2015The durability of concrete has attracted significant attention over the past several decades and is still a research hotspot until now. This paper reviews and discusses recent research activities on the durability of concrete, including: 1) major durability problems such as alkali aggregate reaction, sulfate attack, steel corrosion and freeze-thaw; 2) durability of concrete in marine environment; and 3) coupling effects of mechanical load and environmental factors on durability of concrete. Moreover, the consideration of durability in concrete structure design (DuraCrete and performance-based specifications) is also briefly reviewed.The support from the Hong Kong Research Grant Council under grant of 615412 and from the China Ministry of Science and Technology under grant of 2015CB655104 is gratefully acknowledged. Prof. Yao also thanks the finical support from National Natural Science Foundation of China (No. 51320105016)

Residual Saturation Effects on CO₂ Migration and Caprock Sealing: A Study of Permeability and Capillary Pressure Models

In CO2 geological storage, multiphase flow plays a vital role in the movement and distribution of CO2. However, due to the limitations of fluid buoyancy and capillary forces, CO2 encounters challenges in penetrating the caprock, and the potential for leakage remains a concern due to variations in injection conditions. The migration and distribution of CO2 in the process of CO2 geological storage in saline formations are determined by relative permeability and capillary pressure, which are key factors. Consequently, this study focuses on two essential models: relative permeability and capillary pressure models. A two-dimensional isothermal reservoir–caprock model was constructed, utilizing data from the Shenhua CCS demonstration project. The analysis indicates that the core parameters in the model are residual gas saturation and residual water saturation. Specifically, residual gas saturation governs the diffusion distance of CO2 within the reservoir–caprock system, while its combined effect with residual water saturation affects the permeation rate of CO2. Through the application of the Analytic Hierarchy Process (AHP) to analyze the impact of different models on caprock integrity, it was determined that when selecting caprock models and optimizing parameters, precedence should be given to models with lower residual saturation and caprocks that offer sufficient capillary pressure for optimal sealing effects. These research findings can serve as references for practical CO2 storage projects, providing guidance on activities such as adjusting water injection strategies and controlling gas injection pressures to optimize geological storage efficiency

Physical activity level of urban pregnant women in Tianjin, China: a cross-sectional study.

To determine the physical activity level and factors influencing physical activity among pregnant urban Chinese women.This prospective cross-sectional study enrolled 1056 pregnant women (18-44 years of age) in Tianjin, China. Their socio-demographic characteristics were recorded, and the Pregnancy Physical Activity Questionnaire was used to assess their physical activity during pregnancy. The data were analyzed by multinomial logistic regression with adjustment for potential confounders.Median total energy expenditure of pregnant women in each of the three trimesters ranged from 18.50 to 21.90 metabolic equivalents of task (METs) h/day. They expended 1.76-1.85 MET h/day on moderate and vigorous activities and 0.11 MET h/day on exercise. Only 117 of the women (11.1%) met the international guideline for physical activity in pregnancy (≥150 min moderate intensity exercise per week). The most frequent reason given for not being more physically active was the fear of miscarriage. Higher education level (OR: 4.11, 95% CI: 1.59-10.62), habitual exercise before pregnancy (OR: 2.14, 95% CI: 1.39-3.28), and husbands who exercised regularly (OR: 2.21, 95% CI: 1.33-3.67) significantly increased the odds of meeting the guideline (p<0.001). A low pre gravid body mass index (OR: 0.42, 95% CI: 0.20-0.87) significantly decreased the odds (p<0.001).Few urban Chinese pregnant women met the recommended physical activity guideline. They also expended little energy exercising. Future interventions should be based on the clinic environment and targeting family members as well as the subjects. All pregnant women should be targeted, not just those in high-risk groups

A Mathematical Model for the Electrical Resistivity of Cement Paste at Early Ages Considering the Partially Saturated State

For cementitious materials, electrical resistivity is often used in the study of the cement hydration process at early age, as one of the few indicators that can be continuously and non-destructively monitored. Variation characteristics of resistivity are widely reported to interact with the early-age performance of cement paste, such as hydration kinetics parameters and setting time. However, there is no reasonable mathematical model to predict the resistivity at early ages, especially within the first 24 h, due to significant changes in the porosity and degree of saturation. In this work, a mathematical model was developed by considering the partially saturated state and density change of C-S-H (calcium silicate hydrate). To verify the model, two experimental methods were chosen, including the non-contact electrical resistivity test and isothermal calorimetry test. The hydration heat and resistivity of cement paste with a water&ndash;cement ratio of 0.35 and 0.45 were continuously monitored for 3 days. In the resistivity test, embedded temperature sensors were used to monitor the internal temperature and temperature correction was treated carefully in order to obtain accurate data. The test results prove that the mathematical model can accurately predict electrical resistivity and describe the saturation state of early-age cement pastes under sealed curing