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To solve the heat dissipation problem of electronic devices with high heat flux hotspots,a diamond microchannel heat sink consisting of 37 parallel triangular microchannels with channel lengths of 45 mm and hydraulic diameters of 280 μm was designed.The flow boiling heat transfer characteristics of ammonia in the microchannels were investigated under high heat fluxes of 473.9-1000.4 W/cm2.Saturated flow boiling experiments with saturation temperatures of 25℃,30℃,and 35℃ and mass fluxes of 98-1200 kg/m2s were conducted,as well as subcooled flow boiling with inlet subcooling of 5℃ as a comparison.The temperature and pressure drop measurements were analyzed.The main conclusions below can be drawn.(1) At a given heat flux,the heat source temperature first decreased and then increased with the mass flux,and there existed an optimum mass flux to optimize the cooling performance of the heat sink.(2) The heat transfer performance under the saturated inlet condition was obviously better than that under the subcooled inlet condition.(3) A larger saturation temperature leaded to weakening of both the heat transfer capacity and the stability of the microcharnel heat sink.Notably,with the high heat diffusion ability of the diamond substrate and the great heat transfer capacity of ammonia flow boiling in microchannels,the heat sink can achieve a heat removal capacity of up to 1000.4 W/cm2.