Levitation technique

The ultrasonic levitation technique was introduced in the 1930s and does not rely on any specific properties of the sample except size and mass, and has been used in bioanalytical and analytical chemistry applications6-10. The acoustic levitator consists of an ultrasonic transducer and a solid reflector supporting standing waves (see Fig. 17.5). 

Delhaye and Dhamelincourt (1975) were the first to combine a Raman spectrometer with a microscope. Kiefer (1988) described the Raman spectroscopy of single particles of aerosols by the optical levitation technique, an approach which is even possible with a compact spectrometer (Hoffmann et al., 1992). Raman spectra recorded with NIR FT Raman microscopes have proven the value of this technique (Messerschmidt and Chase, 1989 Bergin and Shurwell, 1989 Simon and Sawatzki, 1991). Examples of micro Raman spectra obtained from different spots on certain biological samples have been published (Schrader, 1990 Puppels et al., 1991). 

Figure 8.5. Main levitation techniques. (Reproduced with permission of Elsevier, Ref. [80].)...

In contrast to most levitation techniques such as levitation in electrostatic or magnetic fields, acoustic levitation requires no specific properties of the sample, so almost every substance, whether solid or liquid, can be acoustically levitated. The maximum possible diameter for a levitated sample is a function of the ultrasonic wavelength and turns out to be about one-half the wavelength under ambient atmospheric conditions. Usually, levi-tators are operated by ultrasound frequencies between 15 and 100 kHz resulting in wavelengths from 2.2 to 0.34 cm. 

Glasses of Y3A15012 composition can be made by levitation techniques [106] and other glass compositions can be made at compositions close to that of the metastable eutectic (Fig. 4) using a Xe-arc image furnace. The precursors are made via sol-gel route [115]. 

Homogeneous and heterogeneous condensation is an ubiquitous phenomenon in the atmosphere and proceeds via nanometer sized critical nuclei. The application of levitation techniques and laser spectroscopy initially developed for cluster reseeurch allows to observe these processes in the laboratory under well controlled and realistic conditions. The application of appropriate models edlows to deduce the size and surface energy of the critical nuclei as a function of temperature. 

This means no restriction of any kind, because knowledge of the actual energy input is unnecessary for viscosity and surface tension measurements using levitation techniques. 

Bubble pickup-. A bubble is formed at the tip of a capillary tube and introduced into a stirred suspension of particles. Hydrophobic particles will adhere to the bubble. Under standardized conditions, the amount of material collected on the bubble(s) can be compared with other suspensions. Microflotation-. Any small-scale flotation cell device can be used to contact a suspension with rising air bubbles onto which hydrophobic particles will become attached. The particle-laden froth that is produced either flows to a collection tube (as in HaUimond or similar type flotation tubes) or can be drawn off into a separate collection vessel (as in small-scale Denver, levitation device or similar type flotation cells). This is also referred to as a levitation technique. 

Containerless levitation techniques [114,152] have become increasingly popular and useful for studying the structures and thermodynamic properties of... 

The relatively recent development of contactless processing, such as aerodynanuc [32], aero-acoustic [42], and electrostatic [38] levitation techniques has enabled the structural determination of amorphous materials that were previously experimentally challenging or even unachievable. Levitation methods have been used for both in situ structural measurements [9, 23] and ex situ for atypical sample preparation this includes sample purification [43, 47] and single crystal growth [6], but is perhaps most often applied to the fabrication of new glasses which are unobtainable by other methods. 

In this thesis I have shown how levitation techniques can be combined with nentron diffraction, XAS, and computer simulations, in order to determine the strnctures of a diverse range of amorphous materials. The structural information reported relates to several distinct areas of condensed matter the novel glass systems of BaTi20s (BTO) and rare earth doped BTO the controversial issue of isocompositional first order liquid-liquid phase transitions and polyamorphism in yttria aluminate liquids and the extension of Invar into the liqnid state to provide further constraints on crystalline models. 

Kawamura et al. ]50] successfully measured the normal spectral emissivity over a wavelength range between 550 and 1600 nm using an electromagnetic levitation technique, which assures measurements for molten silicon in undercooled conditions from 1553 to 1797 K, as shown in Fig. 4.13. 

Przyborowski etal. [79] applied the levitation technique for the first time to measure the surface tension of molten silicon and reported a high level of surface tension at the melting temperature and a steep temperature dependence as a = 783.5-0.65(T-1410) mN/m. The rather large absolute value of temperature... 

D. A. Winborne, P. C. Nordine, D. E. Rosner, and N. F. Marley, 1976, Aerodynamic levitation technique for containerless high temperature studies on liquid and solid samples , Metall. Trans. B 7, 711-713. 

M. Watanabe, Y. Katayama, and K. Kuribayashi, 2005, Precise measurement of density and stmcture of undercooled molten silicon by using synchrotron radiation combined with electromagnetic levitation technique ,... 

P.F. Paradis, T. Ishikawa, R. Fujii, S. Yoda, Physical properties of liquid and undercooled tungsten by levitation techniques. Appl. Phys. Lett. 8 4), 041901 (2005)... 

To check the efficiency (practically 100 %), the above authors treated samples in which was introduced by means of fusion by the levitation technique. 

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