D.R. Chen

Dendrite growth in the recharging process of zinc- air batteries

To improve the cycling performance of rechargeable zinc–air batteries, the dendritic morphology of electrodeposited zinc should be effectively controlled. It is of crucial importance to understand the formation mechanism of the zinc dendritic structure. Here we show that an electrochemical phase-field model is established to simulate dendrite growth of electrodeposited zinc, and several measures including the pulsating current and the electrolyte flow are taken to suppress dendrite growth in the charging process. The results demonstrate that dendrite propagation is mainly controlled by diffusion dependent on overpotential and surface energy anisotropy, and dendritic morphology can also give rise to non-uniform distribution of the electric field and ion concentration in the electrolyte. The proposed model and solutions will be available for studying dendrite growth of metal–air batteries as well as metal electrodeposition.

Influence of passive potential on the electronic property of the passive film formed on Ti in 0.1 M HCl solution during ultrasonic cavitation

The influence of the applied passive potential on the electronic property of the passive film formed on Ti at different potentials in 0.1M HCl solution during ultrasonic cavitation, was investigated by electrochemical impedance spectra (EIS) and Mott-Schottky plot. The influence of the applied passive potential on the structure and composition of the passive film was studied by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The results showed that the applied passive potential can obviously affect the electronic property of the passive film formed on Ti during ultrasonic cavitation. The resistance of the passive film increased, and the donor density of the passive film decreased with increasing the potential. The flat band potential moved to positive direction and the band gap of the passive film moved to negative direction with increasing potential. AES and XPS results indicated that the thickness of the passive film increased evidently with applying passive potential. The passive film was mainly composed of the mixture of TiO and TiO2. While the TiO2 content increased with increasing the applied passive potential, and the crystallization of the passive film increased with the increased potential.

Effect of hydrodynamic pressures near solid surfaces in the incubation stage of cavitation erosion

Ultrasonic vibration cavitation erosion experiments were performed to study the erosion on a steel specimen at different pitch angles. Experimental results show that the number and size of the erosion pits decrease as the pitch angle increases. This result is thought to be related to the hydrodynamic pressures generated near the solid surface. Numerical analyses from the Reynolds equation prove that the hydrodynamic pressures are generated in the fluid film when a bubble is moving towards the surface of the specimen. Simulation of the bubble collapse process shows that the hydrodynamic pressures shorten the collapse time of a bubble and strengthen the microjet at the moment of bubble collapse. As the pitch angle of the specimen surface increases, the squeeze pressure due to normal velocity decreases, and the increased dynamic pressure due to tangential velocity cannot compensate for the loss of the squeeze pressure. Thus, the total hydrodynamic pressure decreases, and a decrement in the number and the size of the erosion pits occurs.

Ultrasonic cavitation erosion of Ti in 0.35% NaCl solution with bubbling oxygen and nitrogen.

The influences of oxygen and nitrogen on the ultrasonic cavitation erosion of Ti in 0.35%NaCl solution at room temperature, were investigated using a magnetostrictive-induced ultrasonic cavitation erosion (CE) facility and scanning electron microscopy (SEM). The roles of oxygen and nitrogen in the composition and the electronic property of the passive film on Ti, were studied by Mott-Schottky plot and X-ray photoelectron spectroscopy (XPS). The results showed that the mass loss of Ti in 0.35%NaCl solution increased with increasing cavitation time. Bubbling oxygen can evidently increase the resistance of ultrasonic cavitation erosion comparing with bubbling nitrogen. XPS results showed that the thickness of the passive film on Ti in 0.35%NaCl solution in the case of bubbling oxygen for 3 weeks, was about 7 nm, and the passive film was mainly composed of TiO2 with an anatase structure. While TiO2 with a rutile structure was found to be the major component of the passive film on Ti in 0.35%NaCl solution in the case of bubbling nitrogen for 3 weeks, and the film thickness was 5 nm. The results extracted from Mott-Schottky plot showed that the passive film on Ti in the case of bubbling oxygen had more donor density than the passive film on Ti in the case of bubbling nitrogen.

The role of passive potential in ultrasonic cavitation erosion of titanium in 1 M HCl solution.

The influence of the applied passive potential on the ultrasonic cavitation erosion of Ti specimen in 1 M HCl solution was investigated by mass loss experiment, scanning electron microscopy (SEM), electrochemical impedance spectra (EIS) and Mott-Schottky plot. The results showed that Ti was in the passive state within the potential region from -0.3 VSCE to 1.5 VSCE under ultrasonic cavitation erosion. The applied passive potential can obviously decrease the mass loss of Ti caused by ultrasonic cavitation erosion in 1 M HCl solution. The resistance of the passive film increased, the flat band potential moved to positive direction, and the donor density of the passive film decreased with increasing the passive potential. Finally, a physical model was provided to explain the experimental results based on energy band and semi-conductive theories.

Enhancement of cavitation erosion resistance of 316 L stainless steel by adding molybdenum

The influence of Mo on ultrasonic cavitation erosion of 316 L stainless steel in 3.5% NaCl solution were investigated using an ultrasonic cavitation erosion (CE) facility. The morphologies of specimen after cavitation erosion were observed by scanning electron microscopy (SEM). The results showed that the addition of Mo can sharply decrease the mean depth of erosion (MDE) of 316 L SS, implying the increased resistance of cavitation erosion. In order to better understanding the influence of Mo on the cavitation erosion of 316 L SS, the semi-conductive property of passive films on 316 L SS containing different concentrations of Mo were studied by Mott-Schottky plot. Based on Mott-Schottky results and semiconductor physics, a physical model was proposed to explain the effect mechanism of Mo on cavitation erosion of 316 L SS.

Effect of micro-particles on cavitation erosion of Ti6Al4V alloy in sulfuric acid solution

The influences of micro-particles on ultrasonic cavitation erosion of Ti6Al4V alloy in 0.1M H2SO4 solution were investigated using mass loss weight, scanning electron microscopy (SEM) and white light interferometer. Mass loss results revealed that the cavitation erosion damage obviously decreased with increasing particle size and mass concentration. Open circuit potential recorded during cavitation erosion shifted to positive direction with the decreased mass loss. Meanwhile, the mass loss sharply decreased with applying a positive potential during the entire ultrasonic cavitation erosion, and the relationship between the open circuit potential and the cavitation erosion resistance was discussed.

Magnetic field induced motion behavior of gas bubbles in liquid.

The oxygen evolution reaction generally exists in electrochemical reactions. It is a ubiquitous problem about how to control the motion of oxygen bubbles released by the reaction. Here we show that oxygen bubbles during oxygen evolution reaction exhibit a variety of movement patterns in the magnetic field, including directional migration and rotational motion of oxygen bubbles when the magnet in parallel with the electrode, and exclusion movement of oxygen bubbles when the magnet perpendicular to the electrode. The results demonstrate that the direction of oxygen bubbles movement is dependent upon the magnet pole near the electrode, and the kinetics of oxygen bubbles is mainly proportional to intensity of the electromagnetic field. The magnetic-field induced rotational motion of oxygen bubbles in a square electrolyzer can increase liquid hydrodynamics, thus solve the problems of oxygen bubbles coalescence, and uneven distribution of electrolyte composition and temperature. These types of oxygen bubbles movement will not only improve energy saving and metal deposition for energy storage and metal refinery, but also propel object motion in application to medical and martial fields.

Influence of Molybdenum on Tribo-Corrosion Behavior of 316L Stainless Steel in Artificial Saliva

The influence of molybdenum on the tribo-corrosion behavior of 316L stainless steel in artificial saliva was investigated using potentiodynamic polarization curve, electrochemical impedance spectroscopy and sliding wear testing. The results showed that the passive capability of 316L stainless steel in artificial saliva was enhanced with increasing Mo. The anti-corrosion property of the passive film on 316L stainless steel was improved with increasing Mo via increasing transfer resistance of the passive film. Sliding wear testing results showed that the friction coefficient of 316L stainless steel in artificial saliva decreased with the increased Mo.

Effect of Transverse Surface Topography on Cavitation Erosion

Experimental and numerical investigations on the effect of transverse surface topography on cavitation erosion were performed. Specimens made in 45 steel with different transverse surface topographies, which were finished by milling under different processing parameters respectively, were tested in the water-tunnel experimental devices. Scanning electronic microscope and photon microscope were adopted to analyze the cavitation erosion degree on the specimens’ surface after 120 minutes’ tests. Results showed that the degree of the cavitation erosion was affected by the distribution and shape of the surface topography, and the interval between the topography units, which represented the topographies’ distribution, played a more important role than the shape of the units. Numerical simulation results of the flow over flat surfaces with transverse topographies, which were triangle or trapezoid and proportional spacing, were obtained to explain the effect of the transverse surface topographies on cavitation erosion. The variation of the average pressure distribution in the vicinity of the surface and pressure fluctuation caused by the surface topographies both affect the collapse behavior of the cavities which usually exist in the flow. Experimental and numerical results indicate that the degree of cavitation erosion on the specimens in the water-tunnel can be reduced by proper topography design.