A simple atomization approach enables monolayer dispersion of nano graphenes in cementitious composites with excellent strength gains

Carbon nano additives(CNAs)are critical to achieving the unique properties of functionalized composites,however,controlling the dispersion of CNAs in material matrix is always a challenging task.In this study,a simple atomization approach was successfully developed to promote the dispersion efficiency of graphene nanoplatelets(GNPs)in cement composites.This atomization approach can be integrated with the direct,indirect and combined ultrasonic stirrings in a homemade automatic stirring-atomization device.Mechanical and microstructure tests were performed on hardened cement pastes blended with GNPs in different stirring and mixing approaches.Results show that the direct ultrasonic stirrings enabled more homogeneous dispersions of GNP particles with a smaller size for a longer duration.The atomized droplets with the mean size of~100μm largely mitigated GNPs’agglomerations.Monolayer GNPs were observed in the cement matrix with the strength gain by up to 54%,and the total porosity decrease by 21%in 0.3 wt%GNPs dosage.The greatly enhanced dispersion efficiency of GNPs in cement also raised the cement hydration.This work provides an effective and manpower saving technique toward dispersing CNAs in engineering materials with great industrialization prospects.

Numerical investigation of airborne transmission of respiratory infections on the subway platform

Underground subway platforms are among the world’s busiest public transportation systems,but the airborne transmission mechanism of respiratory infections on these platforms has been rarely studied.Here,computational fluid dynamics(CFD)modeling is used to investigate the airflow patterns and infection risks in an island platform under two common ventilation modes:Mode 1-both sides have air inlets and outlets;Mode 2-air inlets are present at the two sides and outlets are present in the middle.Under the investigated scenario,airflow structure is characterized by the ventilation jet and human thermal plumes.Their interaction with the infector’s breathing jet imposes the front passenger under the highest exposure risk by short-range airborne route,with intake fractions up to 2.57%(oral breathing)or 0.63%(nasal breathing)under Mode 1;oral breathing of the infector may impose higher risks for the front passenger compared with nasal breathing.Pathogen are efficiently diluted as they travel further,in particular to adjacent crowds.The maximum and median value of intake fractions of passengers in adjacent crowds are respectively 0.093%and 0.016%(oral breathing),and 0.073%and 0.014%(nasal breathing)under Mode 1.Compared with Mode 1,the 2nd mode minimizes the interaction of ventilation jet and breathing jet,where the maximum intake fraction is only 0.34%,and the median value in the same crowd and other crowds are reduced by 23–63%.Combining published quanta generation rate data of COVID-19 and influenza infectors,the predicted maximum and median infection risks for passengers in the same crowds are respectively 1.46%–40.23%and 0.038%–1.67%during the 3–10 min waiting period,which are more sensitive to ventilation rate and exposure time compared with return air.This study can provide practical guidance for the prevention of respiratory infections in subway platforms.

Numerical investigation of impinging jet ventilation in ICUs:Is thermal stratification a problem?

Intensive care units(ICUs)are the high incidence sites of hospital-acquired infections,where impinging jet ventilation(IJV)shows great potential.Thermal stratification of IJV and its effect on contaminants distribution were systematically investigated in this study.By changing the setting of heat source or the air change rates,the main driving force of supply airflow can be transformed between thermal buoyancy and inertial force,which can be quantitatively described by the dimensionless buoyant jet length scale(l_(m)^(-)).For the investigated air change rates,namely 2 ACH to 12 ACH,l_(m)^(-)varies between 0.20 and 2.80.The thermal buoyancy plays a dominant role in the movement of the horizontally exhaled airflow by the infector under low air change rate,where the temperature gradient is up to 2.45℃/m.The flow center remains close to the breathing zone of the susceptible ahead,resulting into the highest exposure risk(6.6‰for 10-μm particles).With higher heat flux of four PC monitors(from 0 W to 125.85 W for each monitor),the temperature gradient in ICU rises from 0.22℃/m to 1.02℃/m;however,the average normalized concentration of gaseous contaminants in the occupied zone is reduced from 0.81 to 0.37,because their thermal plumes are also able to carry containments around them to the ceiling-level readily.As the air change rate was increased to 8 ACH(l_(m)^(-)=1.56),high momentum weakened the thermal stratification by reducing the temperature gradient to 0.37℃/m and exhaled flow readily rose above the breathing zone;the intake fraction of susceptible patient located in front of the infector for 10-μm particles reduces to 0.8‰.This study proved the potential application of IJV in ICUs and provides theoretical guidance for its appropriate design.

Chemical oxygen demand oxidation via sustained-release persulfate balls: a rate-compatibility study of flow velocity, releasing, and oxidation

Identification of chemical oxygen demand(COD)in municipal solid waste(MSW)landfill leachates is a challenging problem.This paper investigated the feasibility of using sodium persulfate(PS),a strong oxidant,as a permeable reactive barrier(PRB)filling material.Firstly,sustained-release persulfate balls were manufactured to adjust the release rate of persulfate,the oxidation agent.In addition,Fe(II)-loaded activated carbon(Fe-AC)was used to help with an even distribution of Fe(II)in the porous medium(PRB in this case).Then,the oxidation efficiency and kinetic rate of COD removal by the sustained-release balls were subjected to batch tests.A mass ratio of 1:1.4:0.24:0.7 for PS:cement:sand:water was the most efficient for COD removal(95%).The breakthrough curve for a 5 mm sustained-release ball revealed that the retardation factor was 1.27 and that the hydrodynamic dispersion coefficient was 15.6 cm^(2)/d.The corresponding half-life of COD oxidation was 0.43 d,which was comparable with the half-life of PS release from sustained-release balls(0.56 d).The sustained-release persulfate balls were shown to be an economical material with a simple recipe and production method when catalyzed by Fe-AC.Compared with cutting-edge methods,sustained-release balls used in PRBs offer significant advantages in terms of both effectiveness and economy for the preparation of sustained-release and catalytic materials.These results verified the feasibility of using sustained-release persulfate balls as a PRB material for COD removal.

A real-time sensing system based on triboelectric nanogenerator for dynamic response of bridges

The strain of bridges under traffic loads is time-varying and of small amplitude(~10^(-6)),which is a type of cumulative response and needs long-term continuous monitoring.To precisely capture the time-varying responses,a dynamic strain triboelectric nanogenerator(TENG)sensor with superior response capability,sensitivity,self-powered,and long-term stability is proposed in this paper.An analytical correlation between the structural strain response signal and the detected electrical signal is established for long-term continuous quantitative strain measurements based on the principles of contact electrification and electrostatic induction.A series of experiments are conducted to investigate the output performance of the proposed lateral-sliding mode TENG sensors.The results reveal that,with the loading condition with frequencies lower than 10 Hz,the time-varying strain responses of the steel bridge within the range of 3 to 150 microstrains can also be detected with high precision of 0.1 microstrains.And it achieves long-term stability after 10000 loading cycles compared with commercial sensors.The proposed novel sensing theory and method based on TENG technology can be applied as a new alternative approach for monitoring realtime structural strain information quantitatively with general applicability and feasibility for bridges.

Effect of intracellular water release on hydro-mechanical behaviors of high kitchen waste content municipal solid waste

The release of intracellular water during degradation process contributes to the great leachate production and settlement of landfilled high kitchen waste content MSW(HKWC-MSW). An oven-drying and absorbent-paper combined method was proposed to measure the intracellular and interparticle water contents of HKWC-MSW. Two degradation experiments were carried out to study the release process of intracellular water and its effect on the hydro-mechanical behaviors of HKWC-MSW.It was found that the two degradation experiments showed similar degradation behaviors with BOD/COD decreasing with time in the early stage. In the first degradation experiment, most intracellular water was released during the first two months, and the degradation of degradable matter in kitchen waste(KW) was much slower than the release process of intracellular water. The particle size became smaller and the overall grain specific gravity increased during the fast release process of intracellular water.In the second degradation experiment, after two-year degradation, the total leachate production was about 45.2% of the initial wet weight of HKWC-MSW specimen. Water retention capacity θ_f increased from 0.23 to 0.58 during 1–69 d, which might be caused by the decrease of particle size and compression of waste skeleton. As almost all the intracellular water was released after 80-day degradation, during the latter stage of leachate drainage under gravity, θ_f decreased and was close to the total volumetric water content. The total compression strain was about 0.39. The secondary compression strain during 1–80 d(i.e., about 0.07)was considered to be mainly resulted by the release of intracellular water and the subsequent drainage of leachate, and it accounted for about 22.6% of the total secondary compression strain.