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Hielscher Ultrasonics: Dispersing and de-agglomeration of nanoparticles for solar panels

During the last years the solar cell energy market grew explosively. Due to the advances in photovoltaic technology, the favourable cost factors for the installation of solar panels and the State incentives the use of this green energy generating technology became very popular.
The solar cell technology is an alternative, innovative method of energy generation. Thereby the sunlight energy is directly converted into electricity by the photovoltaic effect.
Solar cells mostly manufactured from silicon (Si). Differences in quality, efficiency and cost are caused by the use of monocrystalline, polycrystalline and amorphous solar silicon. The monocrystalline solar cells are the most expensive Si solar cells. They achieve an efficieny of approx. 16-18% and consist in a wafer of a highly pure silicon monocrystal. They are recognizable due to their dark blue or black color. The polycrystalline solar cell consists in a wafer of casted silicon. During the cooling of the liquid silicon, differently aligned and separated crystalls are generated. With an efficiency of approx. 14%, the polycrystalline cells are slightly lower priced than the monocrystalline solar cells. Their visible distinctive mark is the crystal structure and the blue color. The amorphous solar cells are produced by evaporating a thin Si layer on a backing material (e.g. on a glass plate). Compared to the other mentioned manufacturing techniques, production costs of the amorphous solar cells are the lowest, but also the effectiveness is with 6-8% is the lowest.
The thin-film solar cells are based on cadmium telluride (CdTe) or on copper indium gallium selenide (CIGS), the crystalline solar cells convert the energy using – especially monocrystalline - silicon wafers. For Si thin-film cells, a very thin layer of Si is deposited on a low-cost substrate. In comparison with conventional bulk silicon solar cells, for the thin-film solar cells only a low ratio of high-grade silicon has to be used. Thus, thin-film solar cells become a promising technique to make photovoltaics more cost-effective for various applications.
For all different types of solar cells, the use of nanomaterials has a significant importance regarding quality and efficiency the photovoltaic panels and the achieved energy output. Nanomaterials have three unique advantages which can improve the solar-to-electric energy conversion: First, nanoparticles offer very large surface areas per unit volume or per unit mass, so there are large interfaces between the particles. The second advantage are the optical and physical characteristics, that differ significantly to the properties of bulk material. The third important reason for nanomaterials is the cost factor. Nanomaterials can be used economically as thin coatings, using special coating, printing and spray-on techniques.
The recent advantage of creating low-coat, highly efficient, lightweight solar panels, is empowered by inclusion of nano-sized materials, such as Carbon Nanotubes, nanowires and nanoantennas. Especially nanowires and nanoantennas are able to enhance the efficiency of solar cells up to 80% and more (regarding the capture of the mid infrared sun rays).
To achieve functional nanomaterials with the desired characteristics, they have to be de-agglomerated and highly fine dispersed. The majority of nanomaterials are available as powders. To process them, commonly the powder must be dispersed into liquid. But most nanomaterials tend to agglomerate when mixed into water or organic solvent. To overcome the high bonding forces ultrasonic cavitation is tested as an appropriate method to create surfactant-stabilized suspensions of dispersed nano-sized particles.
Hielscher offers ultrasonic devices to untangle/detangle, deagglomerate and disperse nanoparticles efficiently. Dispersion and deagglomeration by ultrasonication are a result of ultrasonic cavitation. When exposing liquids to ultrasound the sound waves that propagate into the liquid result in alternating high-pressure and low-pressure cycles. This applies mechanical stress on the attracting forces between the individual particles. Ultrasonic cavitation in liquids causes high speed liquid jets of up to 1000km/hr (approx. 600mph). Such jets press liquid at high pressure between the particles and separate them from each other. Smaller particles are accelerated with the liquid jets and collide at high speeds (interparticular collision). This makes ultrasound an effective means for the dispersing but also for the milling of micron-size and sub micron-size particles.
For processing nanomaterials, an ultrasonic reactor is recommended. In the reactor chamber the material can be exposed to ultrasonic cavitation at a controlled intensity. The ultrasonic inline processing eliminates by-passing because all particles pass the reactor chamber following a defined path. As all particles are exposed to identical sonication parameters for the same time during each cycle, ultrasonication typically shifts the particle distribution curve rather than widening it.
Further applications for ultrasonication are the degassing (dissolved gas) and defoaming (entrapped bubbles) of liquids. As ultrasonic dispersing technology can be used on lab, bench-top and production level, allowing for throughput rates over 10 tons/hour it is being applied in the R&D stage and in the commercial production. Process results can be scaled up easily (linear). Hielscher ultrasonic devices are very energy efficient. The devices convert approx. 80 to 90% of the electrical input power into mechanical activity in the liquid. This leads to substantially lower processing costs.

Hielscher Ultrasonics
Warthestr. 21
14513 Teltow, Deutschland
Tel.: +49 3328 437 420
Fax: +49 3328 437 444
Email: info@hielscher.com
Web: www.hielscher.com

Literature:
• Aydil, Eray S.: Nanomaterials for Solar Cells. In: Nanotechnology Law & Business, Fall 2007; 275-291.
• Gedanken, Aharon: Using sonochemistry for the fabrication of nanomaterials. In: Ultrasonics Sonochemistry 11/ 2004; 47-55.
• Koshio, Akira, Yudasaka, Masako, Zhang, M., Iijima, Sumio: A Simple Way to Chemically React Single-Wall Carbon Nanotubes with Organic Materials Using Ultrasonication. In: Nano Letters, Vol. 1, No. 7, 2001; 361-363.
• Zhu, Jun-Jie; Xu, Shu; Wang, Hui; Zhu, Jian-Min; Chem, Hong-Yuan: Sonochemical Synthesis of CdSe Hollow Spherical Assemblies Via an In-Situ Template Route. In: Advanced Materials 15/ 2003; 156-159.

Source: Hielscher Ultrasonics on pressbot.org | Date: 2010/06/08 - 13:14 | 956 Hits