Mingmei Sun

Open microchannels are capable of significantly lowering pressure drop and improving the stability of two-phase flows. That said, how gap height (H_g ) and channel aspect ratio (AR) affect flow boiling behaviors in such open microchannels is yet to be investigated. For this research, four copper-made open microchannel heat sinks were constructed, with H_g values of 0.4 mm and 0.7 mm, and AR values of 1:1 and 2:1. Flow boiling experiments were carried out under mass fluxes of 305, 507, and 708 kg/m²s. The inlet temperature of deionized water was kept constant at 80 °C. The visualization results revealed the presence of bubbly flow, slug flow, stratified flow, and completely stratified flow within the open microchannels. These shifts in flow patterns were accompanied by corresponding changes in the heat transfer coefficient (HTC). At high mass fluxes (G > 305 kg/m²s), the subcooled liquid in open microchannels delayed the generation of a stratified flow, which resulted in a decrease in the HTC. Low H_g (H_g = 0.4 mm) reduced the influence of subcooled liquid on stratified flow and significantly improved HTC at a large heat flux. High AR (AR = 2:1) considerably enhanced HTC and reduced the pressure drop.

The use of a micro heat sink is an effective means of solving the problem of high-power chip heat dissipation. Diamond/Cu composites exhibit high thermal conductivity and a linear thermal expansion coefficient that is compatible with semiconductor materials, rendering them ideal micro heat sink materials. The aim of this study was to fabricate diamond/Cu and Cu separately as heat sinks and subject them to flow boiling heat transfer experiments. The results indicate that the diamond/Cu heat sink displayed a decrease in wall superheat of 10.2–14.5 °C and an improvement in heat transfer coefficient of 38–51% compared with the Cu heat sink under identical heat fluxes. The heat sink also exhibits enhanced thermal uniformity. Secondary diamond particles are incorporated into the gaps of the main diamonds, thereby constructing a three-dimensional heat conduction network within the composite material. The diamond network enhances the internal heat flux of the material while also creating more nucleation sites on the surface. These increase the boiling intensity of the diamond/Cu heat sink, leading to better heat transfer performance. By combining the transient thermal model with computational fluid dynamics, a heat transfer model based on the diamond/Cu heat sink is proposed. The efficient heat dissipation capability of diamond/Cu heat sinks can lower the working temperature of microelectronic devices, thereby improving device performance and reliability during operation.

The interface microzone characteristics determine the thermophysical properties of diamond/Cu composites, while the mechanisms of interface formation and heat transport still need to be revealed. Here, diamond/Cu-B composites with different boron content were prepared by vacuum pressure infiltration. Diamond/Cu-B composites up to 694 W/(mK) were obtained. The interfacial carbides formation process and the enhancement mechanisms of interfacial heat conduction in diamond/Cu-B composites were studied by HRTEM and first-principles calculations. It is demonstrated that boron can diffuse toward the interface region with an energy barrier of 0.87 eV, and these elements are energetically favorable to form the B₄C phase. The calculation of the phonon spectrum proves that the B₄C phonon spectrum is distributed in the range of the copper and diamond phonon spectrum. The overlapping of phonon spectra and the dentate structure together enhance the interface phononic transport efficiency, thereby improving the interface thermal conductance.