J. Gibkes

Extremum method: Inverse solution of the two-layer thermal wave problem

An inverse solution of the two-layer thermal wave problem has been derived, which allows us to determine the relevant thermal transport parameters, the thermal diffusion time and the thermal reflection coefficient, respectively, the ratio of the effusivities of the two layers, deduced from the relative minimum or maximum of the calibrated phase lags measured between the periodically modulated excitation of the thermal wave and the detected thermal response. Applying a functional transformation by multiplying the calibrated phase lags with the variable (1∕f1∕2)q, where f is the modulation frequency of excitation and q a positive or negative real number close to zero, the inversion method is extended to other values of the calibrated phase lags measured in the neighborhood of the phase minimum or maximum. The application potential of these two solution methods is studied by analyzing the phase lags measured as a function of frequency for two-layer systems of technological importance, e.g., different plasma-...

Temperature-dependent quantitative 3ω scanning thermal microscopy: Local thermal conductivity changes in NiTi microstructures induced by martensite-austenite phase transition

We develop the theoretical description of 3omega signals from the resistive Wollaston thermal probe (ThP) of a scanning thermal microscope (SThM) in terms of an equivalent low-pass filter. The normalized amplitude and phase frequency spectra are completely characterized by a single parameter, the crossover frequency f(c)(k) depending on the sample thermal conductivity k. The application concerns polycrystalline NiTi shape memory alloy microstructured by focused Ga ion beam milling and implantation. The calibration of the ThP combined with a novel two-step normalization procedure allowed quantitative exploitation of 3omega signal variations as small as -1.75% in amplitude and 0.60 degrees in phase upon heating the sample from room temperature to 100 degrees C. This corresponds to k increase of 23.9% that is consistent with the expected thermal conductivity variation due to martensite-austenite structural phase transition. To our knowledge this is for the first time that SThM 3omega phase information is used quantitatively as well. The static, calibrated 3omega measurements are complementary to 3omega SThM images of the patterned sample surface. The local SThM measurement of temperature-dependent thermal conductivity opens the possibility to imaging structural phase transitions at submicron scale.

Radiometric Analysis of Laser Modulated IR Properties of Semiconductors

A photothermal technique for the characterization of semiconductor materials is presented, in combination with the theoretical description of the signal generation process of the effects of the charge carrier density on the IR optical properties. It relies on the excitation of charge carrier density waves by modulated laser irradiation in the visible spectrum, leading to periodical variations of the IR optical properties. The detection is based on sensing the ir transmission of the semiconductor sample. The modulated laser irradiation in the visible simultaneously leads to small temperature variations and additional signal contributions due to the modulation of the internal IR radiation, which can be minimized and eliminated by appropriate focussing conditions. A principal understanding of the signal generation mechanism has been achieved by time-dependent measurements with a gradually increasing intensity of the external IR radiation source, while frequency-dependent measurements of the modulated IR transmission signal provide quantitative information on the semiconductor properties.

Photothermal characterization of thin-layer chromatography plates

The potential of photothermal beam deflection spectrometry (PTDS), photoacoustic spectroscopy (PAS) and photothermal radiometry (PTR) for the characterization of thin-layer chromatography (TLC) plates with respect to the surface and in-depth distribution of different compounds inside the sorbent was investigated. Photothermal measurements of TLC chromatograms of the Camag III test dye mixture demonstrate that accurate and comparable values for thermal diffusivity of spots in chromatograms and of the sorbent can be obtained by PTDS and PAS. PAS has been demonstrated to be the most reliable and least time-consuming among the three investigated techniques. Differences observed between the PAS signal and the phase-frequency scans obtained from the spots of the same compound on different TLC plates could be due to the irreproducibility of chromatogram development or the inhomogeneousness in different TLC plates.

Analysis of multifunctional oxycarbide and oxynitride thin films by modulated IR radiometry

Multifunctional coatings consisting of transition metal oxycarbides and oxynitrides deposited by physical vapour deposition techniques on tool steel are analysed in this work by means of modulated IR radiometry (MIRR), a non-contact non-destructive thermal wave measurement technique, with respect to the thermal transport properties relevant for time-dependent surface heating processes of coating–substrate systems. In order to interpret the measured data quantitatively, an inverse solution of the two-layer thermal wave problem is applied, which relies on the thermal wave phase lag data measured as a function of modulation frequency of the periodically modulated laser beam heating intensity. Based on these measurements and their quantitative interpretation, correlations between the thermal transport properties of the coatings and their deposition conditions have been found, which can be used to monitor deposition processes. For a second objective of this work, namely to determine the film thickness by means of MIRR, different sets of thin films of approximately constant thermal transport properties, but differing film thickness, have been measured. To discuss the limitations and error limits of these non-contact non-destructive measurements of the coating thickness, the results obtained by MIRR are compared with the coating thickness determined by destructive measurements.

Noncontacting laser photocarrier radiometric depth profilometry of harmonically modulated band bending in the space-charge layer at doped SiO2‐Si interfaces

Laser infrared photocarrier radiometry (PCR) was used with a harmonically modulated low-power laser pump and a superposed dc superband-gap optical bias (a secondary laser beam) to control and monitor the space-charge-layer (SCL) width in oxidized p‐Si–SiO2 and n‐Si–SiO2 interfaces (wafers) exhibiting charged interface-state related band bending. Applying the theory of PCR-SCL dynamics [A. Mandelis, J. Appl. Phys. 97, 083508 (2005)] to the experiments yielded various transport parameters of the samples as well as depth profiles of the SCL exhibiting complete ( p-type Si) or partial (n-type Si) band flattening, to a degree controlled by widely different minority-carrier capture cross section at each interface. The uncompensated charge density at the interface was also calculated from the theory.

Thermal wave characterization of plasma-facing materials by IR radiometry

An experiment based on the IR detection of thermal waves has been developed, which allows the measurement of periodic temperature oscillations of 3–7 mK at average sample temperatures of 450–1000 K. Measurements have been performed on various plasma-facing materials. The thermal diffusivity, effusivity and thermal depth profiles of graphite have been determined at different temperatures, thus demonstrating the potentiality of this method for remote in situ measurements of thermal properties and nondestructive evaluation of plasma surface modifications in nonaccessible hostile environments.

Interface resistance in copper coated carbon determined by frequency dependent photothermal radiometry

The heat transfer in copper-carbon flat model systems was studied by frequency dependent photothermal radiometry. A novel approach which relies on the frequency dependence of the photothermal signal phase and amplitude at intermediate frequencies was introduced to determine the thermal interface resistance between the Cu-film and the substrate. The frequency dependent amplitude and phase of the photothermal signals were analyzed in the frame of a model of a one- dimensional heat flow perpendicular to the film plane. The interface resistance of the investigated CuC-sample with a Ti-bonding layer was found to increase by a factor two on heat treatment.

Thermophysical characterization of fine-grain graphites based on thermal waves

Abstract Laser-induced thermal waves are a useful tool to characterize the thermophysical properties of materials. One method of photothermal characterization is frequency-dependent photoacoustics. Here a high-temperature photoacoustic cell is described which has been developed to measure the thermal diffusivity and the effusivity of porous materials like fine-grain artificial graphites. The device allows measurements of the thermal properties continuously between room temperature and 1000 K. Measurements on different graphites used as limiter materials in Tokamak devices are presented.

Effects of He irradiation in plasma-facing materials

Abstract The effects of He irradiation on the thermal properties of polycrystalline nickel and graphite have been measured by means of thermal waves. Both the amplitude and the phase of the thermal waves have been interpreted in the frame of a two-layer model, where the changes of the thermal properties of the first layer serve to characterize the effects of irradiation and erosion.