J. Jaarinen

Photothermal determination of vertical crack lengths in silicon nitride

In this paper we describe the use of thermal microscopes in measuring the lengths of vertical cracks in silicon nitride. Results of theoretical calculations for temperature profiles in samples near the end of the cracks are demonstrated. These calculations showed that the effect of the crack decreases to a non-measurable level at distances of about 10 μm from the end of the crack. Experimental measurements showed that thermally obtained crack lengths were at least 34% longer than those that were optically measured. The obtained crack length was independent of the heat modulation frequency used.

The effects of experimental parameters in the thermal diffusivity measurements of oriented polymer films using mirage effect: Computer simulation

Computer simulations have been carried out to study the effects of the experimental parameters when the mirage method has been applied to thermal diffusivity measurements of oriented polymer films. The parameters under study are the thermal diffusivity of the fluid surrounding the sample, the modulation frequency and the radius of the heating beam, the height and the radius of the probe beam, and the sample thickness and thermal diffusivity. Proposals for the optimum parameter values to maximize the measurement sensitivity for the sample diffusivity are made and the difficulties arising from the low diffusivity of the samples are described. It is also concluded that because the thermal properties of the fluid surrounding the sample have a strong contribution to the mirage signals, the signals do not include any simple feature corresponding to the sample diffusivity. Therefore it should be determined from the entire measurement data using regression methods.

Determination of thermal diffusivity of polymers by means of multivariate calibration of mirage data

In this paper we describe how multivariate calibration techniques including experimental design and partial least‐squares regression, PLS, can be used to determine thermal diffusivity from mirage data. In order to illustrate the utility of the theory a case study is presented where photothermal mirage data were simulated for a series of polymeric film samples. A PLS model was calculated that predicts thermal diffusivity with high precision.

The Determination of Thermal Diffusivity of Oriented Polymer Films by Means of Optical Beam Deflection

The understanding of the structure-properties relationships in polymers is of increasing importance in the development of new engineering polymers. This places requirements to the characterization of physical properties of differently treated polymers. In this work we present a method for determination of the thermal diffusivity of oriented polymer films using the mirage (optical beam deflection,OBD) technique. We present here the application of chemometrics, in the form of multivariate calibration, to obtain the thermal diffusivity from the mirage data.

A New Infrared Measurement Method for Determination of Anisotropic Thermal Conductivities of Plastic Foils

The use of different polymers is gaining increasingly more room also in the areas that have been before dominated by metals. This progress sets more demands on the physical characterization of new plastic materials and products. Plastics have already been widely used because of their excellent insulating properties but unfortunately there has been a lack of fast, simple, and accurate methods to determine their thermal conductivity, especially when the samples are not in cast form but thin foils. In many cases it is important to determine the thermal conductivity of anisotropic samples. Drawing of polymer foils changes the orientation of polymer chains and thus affects the thermal conductivity causing anisotropy. The ratio of anisotropy depends on the drawing ratio and the structure of the molecules. The conductivity perpendicular to the foil surface can be measured with flash methods [1] but measurements in other directions are more difficult [2,3,4].

A Thermal Wave Technique to Determine Thermal Diffusivities of Polymer Foils

The increasing use of polymers sets more demands on nondestructive characterization and testing of both raw materials and ready products. Thus far, plastics have been considered to be poor electric and thermal conductors but manipulation of electric conductivity has already become an important factor for many applications. Also the more profound knowledge of thermal conductivity mechanisms and the measurement of thermal characteristics is gaining increasing importance in the future.

Computer Simulation of the Thermal Diffusivity Measurements of Oriented Polymer Samples Using the Mirage Effect

The effect of the measurement parameters on the mirage signal obtained when determining the thermal diffusivity of oriented polymer samples has been studied in order to clarify the difficulties arising from the low diffusivity of the samples and to find out the best measurement conditions. Because the mirage data obtained from these samples do not include any single feature to determine the thermal diffusivity, it is concluded that statistical methods applied to the entire measurement data should be used for this purpose.

Photothermal Length Measurement of Vickers Cracks in SI3N4

Ceramic materials are gaining ever increasing popularity in different high—technology applications, especially in those where high temperatures are used. However, the mechanical strength of ceramics has thus far set limitations on their utilizing. The critical size of cracks in ceramic material depends on the force applied, but in typical applications it is less than 100 µm. This small size in addition to the fact that critical cracks are often closed makes the use of conventional NDT methods in crack detection very difficult or even impossible. More nuisance comes from the porosity of the material which takes methods needing immersing in liquids out of consideration.

Detection of Vertical Cracks in Ceramic Materials by the Mirage Effect

Results of numerical calculations of the temperature distribution near the end of a closed, semi-infinite vertical crack are presented with a comparison of optical and thermal length measurements of vertical cracks made by Vickers indentations in Si3N4.