Ultrasonic Milling of Thermoelectrical Nano-Powders
- Research has shown that ultrasonic milling can be successfully used for fabrication of thermoelectric nanoparticles and has the potential to manipulate the surfaces of the particles.
- Ultrasonically milled particles (e.g. Bi2Te3-based alloy) showed a significant size reduction and fabricated nano-particles with less than 10µm.
- Furthermore, sonication produces significant changes of the surface morphology of the particles and enable thereby to functionalize the surface of micro- and nano-particles.
Thermoelectric materials convert heat energy to electrical energy based on Seebeck and Peltier effect. Thereby it becomes possible to turn hardly usable or almost lost thermal energy effectively into productive applications. Since thermoelectric materials can be included in novel applications such as biothermal batteries, solid-state thermoelectric cooling, optoelectronic devices, space, and automotive power generation, research and industry is searching for facile and rapid techniques to produce environmental-friendly, economical, and high temperature-stable thermoelectric nanoparticles. Ultrasonic milling as well as bottom-up synthesis (sono-crystallization) are to promising routes to the fast mass production of thermoelectric nanomaterials.
Ultrasonic Milling Equipment
For the particle size reduction of bismuth telluride (Bi2Te3), magnesium silicide (Mg2Si) and silicon (Si) powder, the high-intensity ultrasonic system UIP1000hdT (1kW, 20kHz) was used in an open beaker setup. For all trials amplitude was set to 140µm. The sample vessel is cooled in a water bath, the temperature is controlled by thermo-couple. Due to sonication in an open vessel, cooling was used to prevent the evaporation of the milling solutions (e.g., ethanol, butanol, or water).
Ultrasonic milling for only 4h of Bi2Te3-alloy already yielded in a substantial amount of nanoparticles with sizes between 150 and 400 nm. Besides the size reduction to the nano range, sonication also resulted in a change of surface morphology. The SEM images in the figure below b, c, and d display that the sharp edges of the particles before ultrasonic milling have become smooth and round after ultrasonic milling.
To determine whether particle size reduction and surface modification are uniquely achieved by ultrasonic milling, similar experiments were conducted using a high-energy ball mill. The results are shown in Fig. 3. It is apparent that 200–800 nm particles were produced by ball milling for 48 h (12 times longer than ultrasonic milling). SEM shows that the sharp edges of the Bi2Te3-alloy particles remain essentially unchanged after milling. These results indicate that the smooth edges are unique characteristics of ultrasonic milling. Time-saving by ultrasonic milling (4 h vs 48 h ball milling) are remarkable, too.
Marquez-Garcia et al. (2015) conclude that ultrasonic milling can degrade Bi2Te3 and Mg2Si powder into smaller particles, the sizes of which range from 40 to 400 nm, suggesting a potential technique for industrial production of nanoparticles. Compared with high-energy ball milling, ultrasonic milling has two unique characteristics:
- 1. the occurrence of a particle-size gap separating the original particles from those produced by ultrasonic milling; and
- 2. substantial changes in surface morphology are apparent after ultrasonic milling, indicating the possibility of manipulating the surfaces of the particles.
Ultrasonic milling of harder particles requires sonication under pressure to generate intense cavitation. Sonication under elevated pressure (so-called manosonication) increases the shear forces and stress to the particles drastically.
A continuous inline sonication setup allows for a higher particle load (paste-like slurry), which improves the milling results since ultrasonic milling is based on inter-particle collision.
Sonication in a discrete recirculation setup allows to ensure a homogeneous treatment of all particles and therefore a very narrow particle size distribution.
A major advantage of ultrasonic milling is that the technology can be readily scaled up for production of large quantities—commercially available, powerful industrial ultrasonic milling can handle amounts up to 10m3/h.
Advantages of Ultrasonic Milling
- Rapid, time-saving
- Reproducible results
- No milling media (no beads or pearls)
- Low investment cost
High Performance Ultrasonicators
Ultrasonic milling requires high power ultrasonic equipment. In order to generate intense cavitational shear forces, high amplitudes and pressure are crucial. Hielscher Ultrasonics’ industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available. In combination with Hielscher’s pressurizable flow reactors, very intense cavitation is created so that intermolecular bondings can be overcome and efficient milling effects are achieved.
The robustness of Hielscher’s ultrasonic equipment allows for 24/7 operation at heavy duty and in demanding environments. Digital and remote control as well as automatic data recording onto a built-in SD card ensure precise processing, reproducible quality and allow for process standardization.
Advantages of Hielscher High Performance Ultrasonicators
- very high amplitudes
- high pressures
- continuous inline process
- robust equipment
- linear scale-up
- save and easy to operate
- easy to clean
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- Marquez-Garcia L., Li W., Bomphrey J.J., Jarvis D.J., Min G. (2015): Preparation of Nanoparticles of Thermoelectric Materials by Ultrasonic Milling. Journal of Electronic Materials 2015.
Facts Worth Knowing
Thermoelectric materials are characterized by showing the thermoelectric effect in a strong or convenient, usable form. The thermoelectric effect refers to phenomena by which either a temperature difference creates an electric potential or an electric potential creates a temperature difference. These phenomena are known as the Seebeck effect, wich describes the conversion of temperature to current, the Peltier effect, wich describes the conversion of current to temperature, and the Thomson effect, which describes the conductor heating/cooling. All materials have a nonzero thermoelectric effect, but in most materials it is too small to be useful. However, low-cost materials that show a sufficiently strong thermoelectric effect as well as other required properties to make them applicable, can be used in applications such as power generation and refrigeration. Currently, bismuth telluride (Bi2Te3) is widely used for its thermoelectric effect