Energy transfer efficiencies

Finally, an estimate can be made of the amount of energy expended by the gunpowder lifting charge in aspects such as warming up the mortar tube and overcoming the friction of the shell. 

Assuming that a 75 mm shell is fired by a 13 g lifting charge to give a muzzle velocity (F) of 105 m s for a shell of mass (w) 0.21 kg, the kinetic energy (KE) of the shell is given by equation (4.20). 

Therefore, the kinetic energy of the shell at the muzzle will be  

The muzzle energy can be compared with the overall explosion energy from the gunpowder lifting charge, which is typically about 2000 kJ kg , and hence the available energy from the 0.013 kg lifting 

An efficiency of 4.5% might seem surprisingly low but it should be remembered that the shell is never a good fit in the bore of the mortar tube, there are no gas-tight seals around the shell, and that the shell is not perfectly spherical (or cylindrical). 

The gunpowder lifting charge beneath a shell provides a source of chemical energy, a proportion of which, depending on the efficiency of the mortar, is converted into kinetic energy in the shell. 

However, in practice, the energy of the hot combustion products of the gunpowder is never fully utilised in providing forward motion to the shell. Losses occur unavoidably in several ways - as radiation as residual energy of motion of the partially expanded gases as leakage of gas around the shell and as wave motion (noise) in the surrounding atmosphere. 

Modern guns are more than 30% efficient, relying on smokeless propellants (where the gas volume at STP is around lOOOcm g and heat of explosion is at least 3000 Jg ). On the other hand, gunpowder will only produce about 400 cm g gas at STP, while the corresponding heat of explosion is approximately 1550 Jg  

Hence the efficiency, E, of black powder, in comparison to smokeless powder, as a gun propellant can be estimated on the basis of the above 

The above estimate at least partly explains why early 12-bore gun cartridges, containing 82 grains (5.3 g) of black powder, could alternatively be loaded with 26 grains (1.7 g) of double-base powder to give similar ballistic performance. 

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Figure 6-25. Fluorescence spectra at 4.2 K of T,. thin lilms with morphology characterized by a) grains b) layers c) islands d) sub-monolayer. The lower density ol aggregate states in the sub-monolayer architecture decreases the energy transfer efficiency to low energy stales and the fluorescence acquires dominant excitonie character (sec Section 6.6.2.2J.

Figure 6.8. Dependence of the energy transfer efficiency (E = etin the figure) on distance. The slope of 5.9 is in excellent agreement with the r e dependence for Forster-type transfer. From Stryer and Haugh-land.(45) Reprinted by permission of Proc. Nat. Acad. Sci. U.S.

Polivka, T., M. Pellnor, E. Melo, T. Pascher, V. Sundstrom, A. Osuka, and K. R. Naqvi. 2007. Polarity-tuned energy transfer efficiency in artificial light-harvesting antennae containing carbonyl carotenoids peridi-nin and fucoxanthin. J. Phys. Chem. C 110 467 -76. 

The energy transfer efficiency exhibits a very steep dependence on the distance separating two fhiorophores, R ... 

Growth of the degree of fluorescence polarization (the Weber s effect) and a decrease of energy transfer efficiency while shifting the excitation wavelength to the red edge. 

This is the most common method for measuring the energy transfer efficiency. 

Posokhov, Y. O., Merzlyakov, M., Hristova, K. and Ladokhin, A. S. (2008). A simple proximity correction for Forster resonance energy transfer efficiency determination in membranes using lifetime measurements. Anal. Biochem. 380, 134—6. 

Fig. 19.10 (a) Normalized spectra of FRET in free space forR6G (donor) andLDS722 (acceptor). The bottom curve (A/D 1.0/0 mM) is acquired in the absence of the donor. The donor concentration for the remaining curves is fixed at 0.1 mM. The CW pump wavelength 532 nm. A/D acceptor to donor ratio, (b) Energy transfer efficiency, e, calculated from (a). Solid line is the theoretical curve for the efficiency with a parameter c0 1.7 mM, corresponding to R0 6.2 nm. Reprinted from Ref. 18 with permission. 2008 Optical Society of America... 

Host) (Guest) Poor energy transfer Efficient energy transfer... 

The first study using HBMs in PHOLEDs was conducted by O Brien et al. when they studied the energy transfer efficiency in a PtOEP-doped PHOLED [349]. They observed that... 

The definition used depends on the phenomenon under study. For instance, the intensity-averaged lifetime must be used for the calculation of an average colli-sional quenching constant, whereas in resonance energy transfer experiments, the amplitude-averaged decay time or lifetime must be used for the calculation of energy transfer efficiency (see Section 9.2.1). 

Three steady-state methods can be used to determine the energy transfer efficiency. In the following description of these methods, the fluorescence intensity is indicated with two wavelengths in parentheses the first one is the excitation wavelength, and the second is the observation wavelength. Because the characteristics of the donor and/or acceptor are measured in the presence and in the absence of transfer, the concentrations of donor and acceptor and their microenvironments must be the same under both these conditions. 

The energy transfer efficiency for this system can be calculated from relative quantum yield of the donor... 

It should be mentioned that data for energy-transfer-based probes simulated in this chapter are generic and can apply to any D-A system regardless of the decay time of the donor. The optimal modulation frequency will be determined by the decay time, but the magnitude of changes in phase and modulation will depend only on absolute energy transfer efficiency. 

The energy transfer efficiency is therefore increased with a larger acceptor extinction coefficient, better spectral overlap between the donor emission and the acceptor absorbance, and higher quantum efficiency of the donor. The orientation term k2 can vary from 0 to 4, and for randomly oriented molecules is 2/3. Random orientation is, in fact, generally assumed when calculating the Ro-... 

Using laser flash photolysis with a frequency-quadrupled neodynium laser, Stevens and al 161b) measured the lifetime of the triplet state of fluoro- and pentafluoro-benzene in the gas phase along with the energy transfer efficiencies to cis-2-butane and oxygen. The triplet transient absorption decay was found to be predominantly first order with a... 

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