Laser flash analysis

Abstract: Equations are derived giving the temperature distribution within a thin slab of material which has received a short pulse of energy on one surface, for the case in which energy loss at the surfaces (by radiation or convection) is not negligible. An analysis of these equations indicates that measurement of thermal diffusivity by the pulse method should be feasible even when losses are so large that the maximum temperature of the far face is only 10 or 20% of the no‐loss value; this includes nearly all materials, and temperatures to 2500°K or higher.

Abstract: The flash technique for measuring thermal diffusivity is analyzed for the case of a cylindrical‐shaped specimen of radius r0 and thickness a to determine the effects of radiation at high temperatures, finite duration of the heat pulse, and the feasibility of low temperature measurements. It is found that the flash diffusivity method is useful in two complementary limits: (1) pulse time τ short compared to the characteristic thermal response time tc, (2) τ/tc of the order 1 to 10. The former case corresponds to the original description of Parker, Jenkins, and Abbott, while the latter case is suitable at very low temperatures. Moreover, it is shown that there is an optimum specimen thickness a for a given material and pulse time τ, in the sense that a higher temperature can be reached before any corrections have to be made to the Parker et al. analysis.

Abstract: A frequency-domain thermoreflectance method for measuring the thermal properties of homogenous materials and submicron thin films is described. The method can simultaneously determine the thermal conductivity and heat capacity of a sample, provided the thermal diffusivity is greater, similar3x10(-6) m(2)/s, and can also simultaneously measure in-plane and cross-plane thermal conductivities, as well the thermal boundary conductance between material layers. Two implementations are discussed, one based on an ultrafast pulsed laser system and one based on continuous-wave lasers. The theory of the method and an analysis of its sensitivity to various thermal properties are given, along with results from measurements of several standard materials over a wide range of thermal diffusivities. We obtain specific heats and thermal conductivities in good agreement with literature values, and also obtain the in-plane and cross-plane thermal conductivities for crystalline quartz.

Abstract: Flash sintering occurs when an electric field is applied to a heated ceramic powder compact. At a critical combination of field and temperature, a power surge occurs (the “flash event”) and sintering takes place in a few seconds. This paper investigates the possibility that this surge occurs by runaway Joule heating. The resistivity of 3YSZ was measured under the relevant conditions. To a good approximation, resistivity was found to be history-independent and to have the same temperature dependence before and after the flash event. These data were used to model the thermal and electrical response of 3YSZ to an applied electric field. All electrical characteristics of the flash event observed experimentally were predicted with a high degree of accuracy. It is concluded that the thermal and electric characteristics of flash sintering are a classical consequence of the negative temperature coefficient of resistivity leading to runaway Joule heating at constant voltage.