Temperature-dependent quantitative 3omega 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.

Detection of EPR in metalloorganic complexes using the photoacoustic effect.

Electron paramagnetic resonance spectra from polycrystalline samples of Fe III tetraphenylporphyrin with gaseous molecular oxygen at atmospheric pressure have been studied by means of the photoacoustic effect. The different dependency of the signal amplitudes on the modulation frequency and the phase shift between the absorption lines of the solid and gaseous paramagnetic species were analyzed in the present investigation.

Extended Helmholtz resonator in low-temperature photoacoustic spectroscopy.

The acoustic transfer function of resonant PA cells is investigated experimentally and theoretically in the 10-300-K temperature range. The underlying cell design consists of two cavities that are interconnected by a cylindrical tube. The acoustic properties are treated in the scope of a generalized Helmholtz resonator model, which includes elements of acoustic transmission lines. For the low-temperature measurements the cell construction was modified to fit into a commercial optical cryostat. Photoacoustic signals are recorded from carbon and graphon samples as a function of the chopping frequency and of the temperature. With decreasing sample temperature the photoacoustic signal amplitude increases proportional to T(-2.75), as the phase angle and the resonance frequencies are simultaneously reduced.