Juanfang Liu

FIRST-PRINCIPLE INVESTIGATION ON THE BONDING MECHANISM OF THE SILICON PARTICLES ON THE COPPER FOIL IN COLD SPRAYING

For lithium–ion batteries, the composite silicon-based electrodes can prevent from losing electrical contact and hence retain the capacity over many cycles. To uncover the adhesion mechanism on the interface formed by the copper foil and the thin silicon coatings during the cold gas dynamic spraying (CGDS) at the microscopic level, the first-principle calculations are performed to investigate the interface properties between them. The ideal work of adhesion, fracture toughness and the interface electronic properties are analyzed. It is found that all the atoms on the interface have vertical displacements, and covalent and ionic bonds are formed between the interfacial Cu and Si atoms which increases the bonding strength. However, the ideal work of adhesion on the interface is lower than one of the Cu bulk and Si bulk, so that fracture would be easier to take place on the interface.

Effect of the Shape Factor on the Cold-Spraying Dynamic Characteristics of Sprayed Particles

Silicon powder was chosen to be deposited by cold spraying for the consideration of possible applications in lithium ion batteries. The influence of the silicon particle shapes other than spherical on the impact velocity and temperature for different working parameters of the gas streams have been numerically investigated by using computational fluid dynamics modeling. The results show that, for same equivalent diameter, the particle impact velocities increase to a maximum velocity when the shape factor increases to a certain value and then decreases to the impact velocity of spherical particles. In the cold-spraying process, the particle velocity profile for smaller shape factors is much closer to that of the gas stream due to the larger particle surface area. Furthermore, the particle impact velocity increment for smaller shape factors is much more remarkable with a higher main propulsion gas temperature and higher carrier gas pressure. The effect of raising the main propulsion gas pressure on the impact velocity of the particles with very smaller shape factors is negligible. The particle impact velocity and temperature can be altered by not only the change of the working parameters of the gas steams but also the change of the sizes and shapes of the sprayed particles.

Deposition characteristics and behaviour of high-pressure cold-sprayed silicon powder

ABSTRACT The high-pressure cold spraying technology was adopted to deposit silicon particles on the copper substrates for preparing the silicon-based electrode of lithium-ion batteries. The deposition of silicon powder was accomplished at two spray process parameter sets by impacting the particle stream or individual particle. The microstructure and the morphology of the silicon coatings were characterised by scanning electron microscopy. A single-silicon particle represents three different deposition behaviours. The mass gains of as-sprayed samples are negative over multiple spray passes, but the thin silicon coating with the particular covered structures is formed. Increasing the number of spray pass cannot cause the obvious increase in the coating thickness. The interfacial morphology of the silicon coatings indicates that, except for the plastic deformation of the copper substrate, both the mechanical interlock effect among the deposited silicon particles and even the chemical binding contributing to the effective bond of silicon particles to the substrate.

Behaviors of the wedge-shaped gas-lubricated film using the finite difference lattice Boltzmann method

In this article, the finite difference lattice Boltzmann method (FDLBM) is successfully applied to analyze the hydrodynamic properties of the wedge-shaped gas film lubrication for the high speed micro gas bearings by comparing with the macroscopic methods (solving the modified Reynolds equation coupled with the simplified energy (modified Reynolds equation) and the Navier–Stokes equations coupled with the energy equation). By comparison, it is found that the vertical flow across the gas film can weaken the gas backflow and thus improves the gas film pressure, as the Navier–Stokes equation and FDLBM are used to analyze the wedge-shaped film lubrication. The continuum assumption in the macroscopic methods leads to a larger gas film pressure, compared with the value predicted by the FDLBM. And, the high temperature and speed enlarge this difference between them. Furthermore, the FDLBM provides a good warm-up for the multiscale simulation on the complex flow in the micro gas bearings.

Steady Characteristics of High-Speed Micro-Gas Journal Bearings With Different Gaseous Lubricants and Extreme Temperature Difference

High-speed micro gas journal bearing is one of the essential components of the micro gas turbines. As for the operating conditions of the bearings, high speed, high temperature, ultra-high temperature difference along the axial direction and the species of gaseous lubricants are extremely essential to be taken into account and the effects of these factors are examined in this paper. The first-order modified Reynolds equation including the thermal creep, which results from the extremely large temperature gradient along the axial direction, is firstly derived, and coupled with the simplified energy equation to investigate the steady hydrodynamic characteristics of the micro gas bearings. Under the isothermal condition, it is found CO_2 cannot only improve the stability of the bearings, but also generate a relatively higher load capacity by some comparisons. Thus, CO_2 is chosen as the lubricant to further explore the influence of the thermal creep. As the eccentricity ratio and the rotation speed change, the thermal creep hardly has any effect on the gas film pressure. However, the shorter bearing length can augment the thermal creep. Compared with the cases without the thermal creep, the thermal creep could remarkably destroy the stability of the gas bearing, but it might slightly enhance the load capacity.

Hydrodynamic Behaviors of the Gas-Lubricated Film in Wedge-Shaped Microchannel

As for the micro gas bearing operating at a high temperature and speed, one wedge-shaped microchannel is established, and the hydrodynamic properties of the wedge-shaped gas film are comprehensively investigated. The Reynolds equation, modified Reynolds equation, energy equation, and Navier–Stokes equations are employed to describe and analyze the hydrodynamics of the gas film. Furthermore, the comparisons among the hydrodynamic properties predicted by various models were performed for the different wedge factors and the different wall temperatures. The results show that coupling the simplified energy equation with the Reynolds or modified Reynolds equations has an obvious effect on the change of the friction force acting on the horizontal plate and the load capacity of the gas film at the higher wedge factor and the lower wall temperature. The velocity slip weakens the squeeze of the gas film and strengths the gas backflow. A larger wedge factor or a higher wall temperature leads to a higher gas film temperature and thus enhances the rarefaction effect. As the wall temperature is elevated, the load capacity obtained by the Reynolds equation increases, while the results by the Navier–Stokes equations coupled with the full energy equation rapidly decrease. Additionally, the vertical flow across the gas film in the Navier–Stokes equations weakens the squeeze between the gas film and the tilt plate and the gas backflow.