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Effects of heavy density (p = 9.2 x 103 kg/m3) Yb2O3 fine dopant (16 nm in diameter) on the acoustic properties of a high-temperature-vulcanization (HTV) silicone rubber have been investigated, to develop a new acoustic lens material with a low acoustic attenuation (alpha) for the medical array probe application. The HTV silicone rubber has advantages in that it shows a lower alpha than that of a room-temperature-vulcanization (RTV) silicone rubber and it can be mixed by applying shear stress, using roll-milling equipment. Roll-milling time dependence of the HTV silicone rubber indicates that the alpha is closely affected by the dispersion of nanopowders in the rubber matrix. The 8 vol% Yb2O3-doped HTV silicone rubber mixed for 30 min showed the lowest alpha of 0.73 dB/mmMHz with an acoustic impedance [AI = sound speed (c) times density (p)] of 1.43 times 106 kg/m2s at 37degC. Moreover, simulation results reveal that a 5 MHz linear probe using the HTV silicone rubber doped with Yb2O3 powder showed relative sensitivity around 2.6 to 3.0 dB higher than a probe using RTV silicone rubber doped with Yb2O3 powder or SiO2-doped conventional silicone rubber for the ultrasonic medical application.