Laser pulse method, thermal measurement

Figure 6.6 Thermal measurement by laser pulse method, (a) Laser pulse on the sample, (b) Temperature response of back face of sample...

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. 

Several techniques are available for thermal conductivity measurements, in the steady state technique a steady state thermal gradient is established with a known heat source and efficient heat sink. Since heat losses accompany this non-equilibrium measurement the thermal gradient is kept small and thus carefully calibrated thermometers and heat source must be used. A differential thermocouple technique and ac methods have been used. Wire connections to the sample can represent a perturbation to the measurement. Techniques with pulsed heat sources (including laser pulses) have been used in these cases the dynamic response interpretation is more complicated. 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

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... 

Selected scaled ks results have been presented in Table VII. A more extensive listing follows in Appendix B. Many thermal hydrogen abstraction and olefinic addition reactions have been tabulated that are too fast to be characterized using the available direct measurement methods. Based on comparisons with the scaled MNR data, the HF chemiluminescence (52), electron spin resonance (53,66), and laser pulse decay (54,67), absolute teclmiques apparently involve large systematic errors. 

Besides pulse calorimetry, the laser flash (LF) technique is an established tool for directly measuring the thermal diffusivity by penetrating a sample with a short laser pulse on one side and monitoring the change in temperature on the opposite site. More details about pulse methods for diffusivity measurements can be found in, e.g., [72]. 

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). 

The laser flash method is used for thermal diffusivity measurements. In this technique, a laser is used to pulse one side of a test specimen uniformly. The temperature rise of the other side is measured using an infrared detector. This transient is then used to calculate the thermal diffusivity. While the technique is simple in concept, nonidealities such as heat loss from the front and back surfaces complicate the resulting data analysis, so that fairly complex models need to be used to extract the thermal dilfu-sivity. These calculations can be easily performed by the computers used to run the instruments. 

Thermal Diffusivity. A measure of the rate of change of temperature at a point in a material when heat is applied or removed at another point, e.g. the rate of rise of temperature at the centre of a furnace wall when the furnace is heated. The thermal diffusivity is the ratio of the thermal conductivity of the material to the product of the bulk density and the specific heat. Two methods of measurement used for ceramics are the Angstrom Method, in which a sinusoidal variation in temperature is applied to one face of a specimen, and the changes in amplitude and phase of the temperature wave are measured on the opposite (parallel) face and the Flash Method, in which a short duration heat pulse (from a flash tube or laser) is applied to one face, and the temperature rise on the opposite face is recorded electronically as a function of time. (ASTM E1461-92). Both methods are rapid and suitable for relatively small samples, which may be heated in a small furnace to measure the variation of thermal diffusivity with temperature. Theimal Endurance. The ability of a piece of glass-ware to resist thermal shock. An attempt to assess this in terms of the physical properties of the glass is offered by the winkelmann and SCHOTT EQUATION (q.V.). 

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

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