Laser flash heating

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Fig. 20. left) Optical emission spectra from a suspension of A1 nanoparticles in nitrocellulose oxidizer (NC) after laser flash heating at the indicated fluences. Higher resolution spectra of the features indicated by i)-(iv) are shown in the insets. These features are attributed to AlO emission at surfaces or gas pockets. Reproduced from ref. [201]. 

In the laser flash method, the heat is put in by laser flash instead of electric current in the stepwise heating method mentioned above. Thus this method may be classified as a stepwise heating method. A two-layered laser flash method was developed by Tada et al. " The experimental method and the data analysis, including a case involving radiative heat flow, are described in detail in the review article by Waseda and Ohta. A thin metal plate is placed at the surface of a melt. A laser pulse is irradiated onto a metal plate of thickness / having high thermal conductivity. The sample liquid under the metal plate and the inert gas above the plate are designated as the third and first layers, respectively. The temperature of the second layer becomes uniform in a short time" and the response thereafter is expressed by... 

In the laser flash method, a melt of interest is placed between two parallel plates. The upper plate is heated stepwise and the thermal diffusiv-ity is measured from the rise in temperature. The specific design for molten materials and especially slags employed by Ohta et al. is based on the differential three-layer technique utihzing a special cell that can be accommodated in the system. A schematic diagram of the principle of the measurement section is shown in Fig. 31. A laser pulse irradiates the upper (platinum) crucible and the temperature response of the surface of the lower platinum crucible is observed, a liquid specimen being sandwiched between the two. 

The chlorine atom adds in the gas phase to propadiene (la) with a rate constant that is close to the gas-kinetic limit. According to the data from laser flash photolysis experiments, this step furnishes exclusively the 2-chloroallyl radical (2a) [16, 36], A computational analysis of this reaction indicates that the chlorine atom encounters no detectable energy barrier as it adds either to Ca or to Cp in diene la to furnish chlorinated radical 2a or 3a. A comparison between experimental and computed heats of formation points to a significant thermochemical preference for 2-chloroal-lyl radical formation in this reaction (Scheme 11.2). Due to the exothermicity of both addition steps, intermediates 2a and 3a are formed with considerable excess energy, thus allowing isomerizations of the primary adducts to follow. 

LANGMUIR TROUGH AND BALANCE LAPLACE TRANSFORM LARMOR PRECESSION LASER-FLASH KINETIC ANALYSIS LATENT ACTIVITY LATENT HEAT Latent heat of fusion LATENT HEAT Latent heat of vaporization LATENT HEAT Lateral binding proteins,... 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

In this chapter, five methods of determining the thermal conductivity of solids are described. The final technique, laser flash, is a method of measuring the thermal diffusivity, from which the thermal conductivity may be obtained if the specific heat and density are known. In the following sections, the operating principles of each technique are described. Novel techniques for measurements of this form appear every year— reference [l] is suggested for a start on contemporary literature. 

A laser flash technique can be used for determining the thermal conductivity of fibers (Whittaker et al., 1990). The laser flash technique provides us the dilfusivity, a, which, in turn, is a function of the thermal conductivity, specific heat, and density of the material. Thus,... 

In the laser flash photolysis/PIMS technique the radicals are generated uniformly along the entire length of the heated flowtube in two ways, either by the direct photolysis of a suitable precursor... 

In this study, thermal diffusivity and specific heat of Be/Cu sintered compacts were measured by laser flash method. In those measurement, specimens were loaded with laser which had constant energy under vacuum. The degree of vacuum was less than 1X lO Pa in order to avoid the oxidation of specimens. Themial diffusivity and specific heat of these compacts were... 

Thermal conductivity was evaluated by the product of the density, specific heat, and thermal diffusivity. Thermal diffusivity was measured by the Laser Flash method. 

The thermal diffusivities of each composite were measured at 673K, 1073K, 1273K, 1373K and 1473K by laser-flash method. The thermal conductivity coefficient, the product of thermal diffusivity, specific heat and density, are plotted as a function of the concentration of PSZ and test temperature in Fig. 2 and Fig.3. In Fig. 2, the thermal conductivity coefficient at room temperature is plotted simultaneously. From... 

The Seebeck coefficient were calculated from measurement of electromotive force with temperature difference of lOK. The electrical resistivity and Hall measurement were performed by van der Pauw method. The thermal conductivity were calculated from the thermal diffusivity, the specific heat and the density. The thermal diffusivity and the specific heat were measured by laser flash method and differential scanning calorimeter (DSC), respectively. 

Figure 22 shows the time dependence of NO2 disappearance in a flash-heated nanoaluminum sample [201]. NO2 disappearance is measured using CARS spectroscopy to monitor the ONO2 totally symmetric stretching transition near 1300 cm . At the lowest fluence there is practically no NO2 consumption, but at higher fluences, NO2 consumption is seen to occur in two phases. The faster phase is characterized by an -300 ps time constant, and the amplitude of the faster phase increases with laser intensity. The slower phase occurs over -2 ns. The faster phase is associated with consumption of the shell of NO2 near each nanoaluminum particle. The slower phase is associated with NO2 consumption between the hot spots. 

Several techniques are used to measure A,h- One method that has gained popularity recently is the laser flash technique. In principle the technique attempts to measure the time evolution of the temperature on one side of the sample as the other side is very rapidly heated by a laser pulse. As it passes through the solid, the signal will be altered in two ways There will be a time lag between the time at which the solid was pulsed and the maximum in the response. This time lag is directly proportional to the thermal diffusivity, of the material. The second effect will be a reduction... 

The heat capacity measurements of Wallace [1960WAL] from 298.15 to 1273 K merge smoothly with the low temperature data of [1953GRI/SKO]. However, Nakamura et al. [1980NAK/TAK] measured the heat capacity of thorium containing 0.05 mass% impurities using a laser-flash technique. This a valuable study, as not only... 

The heat capacities of metallic uranium and thorium were determined from at 84 to 991 K by laser-flash calorimetry. 

Thermal Diffusivity - The thermal diffusivity [D = k/(pCp)] of S10C-N312 BN 2-D composites was determined by the laser flash method in which a laser is used as a heat source and the thermal pulse transmission speed is measured in the desired orientation. Thermal diffusivity measurements were made both in-plane and through-plane ofthe 2-D composite. The specimen size was 9x9x2 mm square. The thermal diffusivity was calculated from solution of the diffusion equation for heat flow with the known boundary conditions. Details of this procedure are found in ASTM Standard Test Method E37.05 (Thermal Diffusivity by the Flash Method). 

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