Flash method

A number of simplifying assumptions allow for a mathematical model for this method The radiation pulse is uniformly distributed across the front face of the specimen and is ab- 

Further assuming that at t = 0, no heat has propagated into the remainder of the specimen  

At the back wall x = l, the cosine function simply exchanges between 1 and -1, with successive values of n. This term can be rewritten as cos(n7r) = ( — ) . Substituting  

The function within the brackets goes from zero to unity with time, hence the maximum temperature of the back face is  

Given that the assumptions of the model have been met experimentally, by determining the time 11/2 corresponding to the temperature at half maximum, i.e. (Tmax — To)/2, the thermal diffusivity can be determined. 

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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... 

The two-layered laser flash method has been applied to some molten systems above 1000 K. In Fig. 30 an example of curve fitting is shown for molten calcium aluminosilicate at 1723 K. An analysis in which the radiative component is taken into account gives a good fit. The thermal conductivity and the radiative component parameter can be determined simultaneously by a curve-fitting procedure. 

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. 

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

Short triplet lifetimes, moreover, cause difficulties in applying conventional flash photolysis techniques, as will be shown later, and demand nanosecond flash methods. 

Lappalainen, J. Juovonen, R. Vaajasaari, K. Karp, M. A new flash method for measuring the toxicity of solid and colored samples. Chemosphere 1999, 38, 317-328. 

There are no ISO standards at present for polymers. However, a series of methods are being developed in TC 61 for conductivity and diffusivity of plastics. At the time of writing there are drafts for general principles, laser flash method, temperature wave analysis method and the Gustafsson method. The general principles draft is a bit misleading as it appears to deal only with transient methods, and the specific procedures so far drafted appear to have been selected at random from the many transient methods available. 

The laser flash method has been used by Foreman36 and by Agari et al60, the latter considering the effect of test conditions and sample size on accuracy. Papa et al61 developed a prototype apparatus that operated with step-down temperature perturbations and could be used with molten samples. 

ASTM E1461, 2001. Standard test method for thermal diffusivity of solids by the flash method. 

The desire for temporal resolution of photolysis led to the development of flash methods. In these experiments [70] the solution is exposed to a short (—10 ps width) burst of light at high intensity (several hundred joules dissipated in the flash lamp). Absorption by the photoactive solute creates a high initial concentration of the primary intermediate. Its decay with time often leads to the rise and fall of other transient species that appear later in the reaction scheme. Because these time dependencies tell much about the photolysis mechanism, flash methods are immensely valuable to photochemistry and have become very common. Usually, the intermediates are followed by UV or visible absorption spectroscopy. Berg and Schweiss were first to implement electrochemical monitoring [71], but Perone and his co-workers have been particularly active since the middle 1960s in the development and application of the technique [67,72-76]. 

The volatilization of arsenic during the thermal destruction of CCA-treated wood may be reduced by utilizing low-temperature pyrolysis. Low-temperature pyrolysis uses temperatures of approximately 300-400 °C with a limited air supply (Helsen and Van den Bulck, 2004, 286, 290 Helsen and Van den Bulck, 2003). Pyrolysis includes slow and flash methods (Helsen and Van den Bulck, 2004). Flash pyrolysis, which produces an oil byproduct, is not effective with CCA-treated wood because only 5-18% of the arsenic... 

The flash method entails a short pulse of high intensity energy, absorbed by the front surface of a small specimen shaped in the form of a disk. The radiant energy source can be a (xenon) flash lamp, laser, or electron beam. The energy absorbed on the front surface propagates (conduction, and at higher temperatures, radiation) toward the back surface, as depicted in Figure 9.8. 

W. J. Parker, R. J. Jenkins, C. P. Butler, and G. L. Abbott, Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity , Journal of Applied Physics, 32 (9) 1697-1684 (1961). 

High pressure grown single crystals of GaN [15] were used in the determination of thermal conductivity. The measurement was made using the laser-flash method. The result of these measurements was k = 1.7 W/(cm K), which is in agreement with the theoretical estimate of Slack [12]. 

InN single crystals of a size suitable for thermal conductivity measurements have not been obtained. The only measurement of the thermal conductivity has been made using InN ceramics [20], InN microcrystals obtained by microwave plasma were sintered under a pressure of 70 kbar at 700°C. The room temperature thermal conductivity was measured by the laser-flash method giving k = 0.45 W/(cm K) [20], This value is much below the estimate by Slack which gives k = 0.8 W/(cm K). This result indicates that the InN ceramic has a high impurity content and consists of small size grains. 

In the case of FeSi2, Fe and Si are melted at 1803 K (1530°C) and annealed in vacuum to form the /1-crystal of FeSi2. Then, we need to form the n- and p-type FeSi adding Co and Mn to the /> crystal of FeSi2, respectively. Another type of furnace (ULVAC Co. Ltd., Yokohama) is utilized to measure the Seebeck coefficient and the electrical resistivity in He atmosphere by increasing the temperature up to 900 K. The thermal conductivity is measured with the laser flash method (ULVAC Co. Ltd., Yokohama). 

Alkyl enol ethers polymerize under these conditions, and their reactivity, as that of other strong nucleophiles, has been determined with the LASER flash method (Table 7). Because this method of carbocation generation produces the nucleophilic counterions CL, eventual polymeriza-... 

The flow-flash method when applied, for example, in studies on cytochrome c oxidase, can be considered as a variant of the caged compound strategy. The reaction of reduced... 

By competitive methods Ishikawa and Noyes (226) (sensitized biacetyl phosphorescence) and Cundall and Davies (159) (butene-2 isomerization) both estimated the triplet lifetime to be of the order of 10 ys. In a reexamination of the butene-2 system, Lee (227) estimated a value of 100 ys, a finding confirmed by Cundall and Dunnicliff (105). An examination of the kinetics of the benzene photosensitized composition of cyclopentanone decomposition allowed a value of longer than 3 ys to be deduced. This type of experiment is far from satisfactory since photochemical processes can Intervene at low pressures, and impurities and the quenching effects of photoproducts can affect the results. These problems can only be overcome by some form of direct measurement. Parmenter and Ring (228) used a flash method in which 20 torrs of benzene and 0.01 torr of biacetyl were submitted to a 20 J,... 

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