Dependence of particle volume fraction on sound velocity and attenuation of EPDM composites

Particulate Metal Composites as Backing for Ultrasonic Transducers for Continuous Nondestructive Measurements at Moderate and High Temperatures

There is an increasing need for ultrasonic transducers able to operate continuously at high temperatures in many industrial sectors. Operating temperatures may be 600 °C and higher. There is not any commercial solution to deal with such applications, and only a few ultrasonic prototype probes exist. It is, therefore, important to find new materials suitable for the manufacture of high-temperature ultrasonic probes. One important element of ultrasonic probes is the backing as it enables the control of both sensitivity and bandwidth. The goal of this article is to explore the possibility of using particulate metal composite as a high-temperature backing elements. Several tin-tungsten composites with different tungsten volume fractions were produced by uniaxial pressing. The backings were characterized by ultrasonic spectroscopy. The backings have an impedance ranging from 25 to 31 MRayl with attenuation at least equal to 20 dB/cm and up to 191 dB/cm. In order to operate at 600 °C, a sample of Al/W composite was produced. Its impedance ranged from 8 to 10 MRayl with a mean attenuation of around 156 dB/cm at 4.5 MHz. The metallic composites, therefore, have ultrasonic properties suitable for use as backing. Moreover, they are relatively easy to manufacture. These are, therefore, very interesting materials for high-temperature ultrasonic probe applications.

Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m-1 K-1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake-PU and Ag nanosphere-PU TIMs provide significantly smaller κ (7.9 and 15.0 W m-1 K-1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF-PU TIM dramatically increases (κlat = 22.2 W m-1 K-1) compared with Ag nanoflake-PU and Ag nanosphere-PU (κlat = 0.2 and 1.2 W m-1 K-1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF-PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.