Frequency mechanisms

Ultrasound can be used to supplement or replace the microwave radiation. Ultrasound effects a high frequency mechanical vibration that warms the interior of the exposed object. In this case, the sample can exposed, and heated, as a whole. However, because ultrasonic waves can be so readily focused, it is also possible to apply them in bundled form so that they act on certain selected regions of the blank, for instance by sweeping along a raster. 

This paper reviews the results of investigations into low-frequency mechanical and high-frequency (ultrasonic) vibration effects upon flowable polymeric systems, primarily, on molten commercial thermoplastics. We tried to systematize possible techniques to realize vibration in molding of polymers. Theoretical and experimental corroboration is provided for major effects obtained at cyclic (shear and bulk) strains of molten polymers and compositions based thereon. It is demonstrated that combined stress of polymeric media is attained under overlapping vibrations and this results in a decreased effective viscosity of the melts, a drop i the pressure required to extrude them through molding tools, increased critical velocities of unstable flow occurrence and a reduced load on the thrust elements of extruder screws. 

Plastic Deformation Plastic deformation results from the combination of thermal and mechanical effects. The thermoplastic excipient was subjected to a temperature above its glass transition temperature (Tg) and to a high-frequency mechanical pressure that can avoid the elastic recovery of the material. 

FIGURE 12J31 Shear modulus and d3mamic viscosity as a fiuiction of frequency. Mechanic behavior of a visco.elastic fluid and visco-elastic solid from models in Table 12.4. 

Where Tg is the glass transition temperature and C and are phenomenological parameters. In figure 8, we have arbitrarily chosen for Xb the value of Xj at 40.9 °C, i.e. 4.54 ns, and applied Equ. (12) together with the parameters C( and C2 given by Ferry from low frequency mechanical measurements. The corresponding WLF curve (dotted line) fits well the other measured values of Xi. Of course, a similar... 

The crucial aspect of NEMS fabricated structures is the realisation that one is dealing with mechanical vibrations of much higher frequencies. In the case of MEMS the resonance frequencies of the fabricated structures are in the range of tens to hundreds of kHz. For NEMS the frequencies are well into the RE range (1-100 MHz). Such a high frequency mechanical vibration needs investigation of physical properties (particularly mechanical properties) in a length scale that has not been done previously. 

As is the case with ceramic sintering, joining, and powder synthesis, microwave-assisted fabrication of ceramic coatings can offer unique benefits. To expose the material surfaces for reaction with a gas phase, fibers or powder particles may be suspended by the flow of gas in the fluidized bed. In addition to the flow of the fluidizing gas, low frequency mechanical vibrations (for example, 10-15 Hz) may be applied to the container to reduce the tendency for the powders or fibers to sinter together to form clumps. As the powder size decreases, the powder s sinterability increases. While vibration may not be necessary for particles as large as, say, 50 pm, it may be beneficial for powder particles that are a few microns in diameter. 

This paper considers Neoprene elastomers and their use as barriers to water penetration. Long time effects on permeability and composition will be discussed as will the effects on permeability of temperature and high-frequency mechanical (acoustic) agitation. 

The model described in this chapter can be applied to the calculation of the permittivity spectra in water in the broad frequency range 0-1000 cm-1 and to calculation of the ice far-IR spectra in the resonance region 50-1000 cm-1. As seen in Fig. 26 (curves I), in a nonresonance ice spectrum only the transverse-vibration mechanism (d) works. Indeed, we see from Fig. 24b that at v < 50 cm-1, namely in the submillimeter wavelength region and at lower frequencies, mechanisms a-c practically vanish. 

Signals from the transducers and microphones are amplified, filtered, and analyzed. Typically, charge amplifiers are used for the microphones, and voltage amplifiers, for the transducers. High-pass filters are applied to the signals to eliminate low-frequency mechanical noise, such as pipe vibration. The signals are analyzed by either a true RMS voltmeter or a wide-band spectrum analyzer. 

A generator for low-frequency mechanical oscillations 1 transforms the rotation of an electromotor to a translational motion with reversion and provides the possibility to regulate frequency and amplitude. The moving part of the generator was connected by means of a steel rod with the Teflon frame. The metal wire (0.05 mm diameter) covered by paraffin is mounted inside the frame. When the frame lay on the polished brims of the Teflon Langmuir trough the wire was wetted by the liquid and was in parallel to the liquid surface. In operation, the frame... 

Harrison R.J., Redfem S.A.T. The effect of transformation twins on the seismic-frequency mechanical properties of polycrystalline Cai.xStxTiOj perovskite. American Mineralogist 2003 88 574-82. 

In ultrasonic welding the molecules of the plastic to be welded are sufficiently disturbed by the application of ultrahigh-frequency mechanical energy to create frictional heat, thereby causing the plastics to melt and join quickly and firmly. 

Experimental apparatus Electromagnetic emission source, electromagnetic energy meter (tesla), high frequency mechanical wave recorder, coal and rock strain recorder, methane, ethane, carbon monoxide sensors, data logger. 

Low frequency mechanical resonances within elastic protein-based polymers occur near 3 kHz (within the frequency range of sound absorption) and 5 MHz (near radio frequencies). 

The Presence of Low Frequency Mechanical Resonances in Elastic Protein-based Polymers... 

S Low Frequency Mechanical Resonances Lower Free Energy of the Entropic Elastic Structure... 

Q.y = y — g are the transition frequencies. F is a phemenological dephasing rate which accounts for both homogeneous (e.g., an interaction with bath) and inhomogeneous (e.g., static distribution of molecular transition frequencies) mechanisms of line broadening (for a review see ref 76). 

Ultrasound can also be used to explore the viscoelastic properties of polymers in film form. Alig et al. [56] describe a shear rheometer operating in the range 1-40 MHz. on the shear reflection principle. A shear wave is sent through a quartz bar toward the interface between the bar and a polymer film. The film alters both the amphtude and phase of the reflected wave. The values of G and G" obtained when plotted against offset temperature from Tg (i.e., T — Tg), show broader transitions at higher temperatures than those obtained with low frequency mechanical DMA. One advan-... 

Ultrasonic nebulisers make use of piezo technology to create an aerosol from a solution of active substance. In such nebulisers the high frequency mechanical vibration of a piezoelectric element is transmitted to a solution of the medicine which creates standing capillary waves on the surface of that solution. Small droplets break free from the crests of these waves and constitute the aerosol. The mean droplet diameter is a function of the frequency of the acoustic signal, the surface tension, density and viscosity of the... 

Ultrasonic nebulizers utilize high-frequency mechanical waves to generate an aerosol. A distinct advantage of this approach is the ability to very efficiently generate small droplets in the aerosol. This results in... 

Kurowski V, Kaiser A, von Hof K, Killermann DP, Mayer B, Hartmann F. Apical and midventricular transient leftventricular dysfunction syndrome (tako-tsubo cardiomyopathy) frequency, mechanisms, and prognosis. Chest 2007 132(3) 809-16. 

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