Mengyu Huang

Spreading behaviors of silicone droplet impact on flat solid surface: Experiments and VOF simulations

Silicone, as the carrier of phosphor powder in the phosphor coating process of white light emitting diode (LED) packaging, its spreading behaviors will influence the morphology of phosphor layer and thus affect the optical and thermal performances of light emitting diodes (LEDs). In this paper, the spreading behaviors of a silicone droplet impact on a flat silicon surface was experimentally and computationally studied. Droplets with the same volume (R=1.1±1%mm) were deposited on a flat silicon substrate at a range of weber number from 5 to 20, morphology change of silicone droplets was captured using a high-speed digital camera. A computational fluid dynamics (CFD) model, based on the volume of fluid (VOF) approach, was used to simulate spreading behaviors of a silicone droplet using the same boundary condition getting from experiments. Time evolution of dynamic radius R(t) and dynamic contact angel θ(t) were analyzed. The CFD simulation results were compared with the experiment results, and the simulation results showed good agreement with the experimental data which indicated that a VOF-based computational model was able to capture key features of the interaction of a silicone droplet with flat solid surfaces.

Fabrication and Thermal Characterization of the Modularized Thermal Storage Unit

Phase change materials (PCMs), due to their extraordinary latent heat storage characteristic, have been widely investigated in the thermal management of electronic devices. However, the current PCM-based heat sinks generally suffer a long heat dissipation time after heat absorption. To address this issue, the modularized thermal storage unit (MTSU) was proposed in this paper. The online charging and offline discharging working modes of the MTSU allow it to cool the intermittent electronic devices with a high duty cycle. In this paper, the MTSU samples were fabricated with paraffin wax as latent heat storage core unit and polymer encapsulation shell. The specific latent heat storage and morphology of the MTSU samples were studied. Afterward, the thermal storage performance of each individual MTSU sample was specially characterized with the sample subjected to the constant heat flux. The MTSU samples showed desirable performance on encapsulating the PCMs and stabilizing the heat source temperature. The safe operation time of the heat source was prolonged by 181% with 16 g of PCM in the vacuum environment. Furthermore, the performance of the MTSU samples on cooling of the electronic devices with a relatively high duty cycle was investigated. The results show that owing to the modularization and replaceability of the samples, the ON/OFF working time ratio of the heat source was greatly enlarged. When cooled by the MTSU sample with 11.42 g of paraffin, the heat source could work with an ON/OFF time ratio up to 6.7 at 4 W and 14.9 at 3 W.

Simulation of droplet spreading process on heterogeneous striped surface by lattice Boltzmann method

Wettability as a surface property has a great effect on the droplet spreading on substrate. The droplet motion control on surface can be achieved by combining hydrophobicity and hydrophilicity regions on substrate surface. And the possibility of using the heterogeneous surface formed by appropriate hydrophobic regions to guide the droplet spreading or splitting along certain directions has recently attracted much attention. In this paper, based on the lattice Boltzmann method, a flow model is established to simulate the droplet spreading process on heterogeneous surface. Using the proposed model, the dynamic morphology evolution of droplet on heterogeneous surface was obtained, and parameters affecting the equilibrium morphology were investigated, such as the width of the single stripe and the number of hydrophobic stripe.

A remote phosphor coating by lens wetting for phosphor-converted white light-emitting diodes

In our previous study, we demonstrated a remote phosphor coating method by lens wetting for pcLEDs. For the proposed method, the phosphor gel droplet is dropped onto the concave inner surface of a hemispherical lens, then the phosphor gel wets the lens surface driven by gravity and surface tension and finally reaches to a stable geometry. In this study, fluid flow simulations based on Volume of Fluid (VOF) model were applied to investigate the geometry evolution of phosphor gel droplets after been dropped into the lens, and the final geometries with phosphor gel volume range from 4 μl to 20 μl were studied. Optical simulations based on Monte Carlo ray tracing method was applied to investigate the optical performance of LED samples with the final geometries obtained from VOF simulations. The VOF simulation results show that when the phosphor volume is less than 10 μl, the proposed lens wetting method tends to realize uniform thickness phosphor geometry, and when the phosphor volume is more than 10 μl, the proposed method tends to form the non-uniform thickness phosphor geometry. Optical simulation results show that the vertical LED chip modules with remote phosphor geometry formed by the proposed method have better ACU performance than those with spherical cap phosphor geometry fabricated by the dispensing phosphor coating method. When the average CCT is about 5000 K, a smallest angular CCT deviation of 275 K was achieved when the volume is 17μl, while the CCT deviation of the conventional spherical cap phosphor geometry is 2499 K.

Locally reinforced polymer-based composites for efficient heat dissipation of multiple heat sources

A locally reinforced heterogeneous composite with conductive particles concentrated at the preferential path of heat flux was fabricated and proposed to cool the hot spots of electronics. The local reinforcement was achieved by using magnetically responsive particles as reinforcing elements and a specific magnetic field to organize the elements into the predefined structure. The results of thermal tests show that compared to the homogeneous composites, the proposed composites with local reinforcement can significantly enhance the heat dissipation of local heat sources. When heat flux is 15912.5 W m-2, the locally reinforced composites with a fillers volume fraction of 5% can reduce the hot spot temperature 3.9°C.

Simulation of stamp-printed coating process in light-emitting diodes packaging by Lattice Boltzmann method

To get white light emission, its usual to use a blue LED chip covered with yellow emitting phosphor via a dispensing process. Stamp printing is such a process of free-surface flow, which decides the morphology of the phosphor gel, and thus has a great effect on the optical performance of LEDs. Therefore, it is important to study the phosphor gel flow in the stamp-printed coating process. In this paper, a flow model was established to simulate the coating process based on the two-dimensional Lattice Boltzmann method (LBM). With the model, dynamic morphology of phosphor gel was obtained. Two factors, the ratio of LED chip width to the width of stamp bump and phosphor gel volume, were investigated through simulations. In addition, complex phosphor layer morphology was achieved by adapting the two factors.

Experimental study of chip offset on the packaging consistency of high power light-emitting diodes

In this study, we experimentally investigated the effect of chip offset on the packaging consistency of high power LED. A series of LED modules with different chip offset distances and phosphor mass concentration were packaged, and two comparative experiments were conducted. Compared with the conventional modules, the maximum CCT deviation of off-centered modules increased from 128K to 667K. The results show that chip offset contributes significantly to the consistency of LED packaging.