D. Robbes

In this work, two types of perovskite oxide materials with high temperature coefficients have been chosen as thermometers for use in bolometric applications: superconducting YBa2Cu3O7-δ (YBCO) and colossal magnetoresistive La2/3Sr1/3MnO3 (LSMO) thin films. Two different temperature ranges are concerned: around 90 K for YBCO, since they were operated at the superconducting-metal transition and around the room temperature for LSMO. The

ABSTRACT A pair of amorphous magnetic wires has been used in a balanced arrangement, as usually done to get gradiometers, but this was intended to better sense the effect of a longitudinal stress applied to the wire as the magnetoimpedance effect is electronically compensated. It was shown that the sensitivity to the stress applied on one wire whereas no large field gradient is applied at the same time on both wires was quite large. This effect is also sensed through the wire impedance variation. We then show how the device can easily be transformed in a spirometer arrangement, which was thought to be useful in practical daily diagnostic of asthma for example. The reference wire is just glued on the external side of a metallic tube and the stress sensitive one is connected along the tube axis, between the center of a small membrane and the pin of SMA connector. The two wires are used as reactive elements of two Colpits oscillators, the outputs of which are doubly demodulated using two rapid diodes each, before passing two differential low noise amplifiers. The balanced output is then obtained using a third difference amplifier, while the output of the reference channel leads to the magnetic field information. Because the membrane is hung upon 50 m diameter elastic metallic wires, it has not to stretch out all the cross section of the tube. It follows that the obstacle to some gas flow inside the tube can be rather low. Further more, the mass of the membrane may be chosen very low, as obtained for example using mylar materials, which, conjugated with light and tight elastic wires, led to a rather rapid response to time varying air flows. Examples of the fast responses are clearly shown by monitoring forced cough through the tube. Details varying at a time scale less than 100 ms are shown. These results which where shown to some lung specialists, will be further commentated, together with the needed improvements to get practical devices in the field of spirometry.

The bolometric nature of the optical sensitivity of YBCO (YBa2Cu3O(7 - delta) thin-film microstrips in the superconducting state is demonstrated. Below the critical temperature T(sub c) the critical current temperature dependence of the device provides a suitable temperature sensor. A theoretical thermal model that gives reliable forecasts for the sensor time constant and sensitivity over the 10-90 K temperature range

The low frequency noise in patterned La0.7Sr0.3MnO3 thin film bridges with different geometries was measured at 300 K in ambient magnetic field. Films were deposited by pulsed laser ablation on (100) SrTiO3 substrates. The measured /f noise was well described by the semiempirical Hooge relation, which indicates that the low frequency noise could be originated by resistance fluctuations. The measured noise magnitude was compared with bibliographic data for epitaxial La1-xSrxMnO3 (LSMO) and La1-xCaxMnO3 thin films. The normalized Hooge parameter alpha/n for our films was in the range of 2-4×10-28 m3, which is the order of magnitude of the best results reported in literature for oxygen annealed films. Finally the noise equivalent temperature of a thermometer fabricated with such LSMO films was estimated to be 3.4×10-6 K/Hz at 300 K, 10 Hz, and I=100 muA. These results demonstrate that the intrinsic low frequency noise of LSMO is not a limitation and that LSMO thin films can advantageously be used as performant thermometers for the fabrication of room temperature microbolometers.

ABSTRACT Our results measuring I-V characteristics of YBCO/MgO microbridges show that large current sensitivities (10 mA/K) as a function of temperature can be obtained, roughly independently of the applied voltage V up to a few hundred mV. This offers the possibility of working with high values of the dynamic resistance so that the phonon noise associated to the thermal boundary resistance, located at the YBCO/MgO interface, becomes dominant. A noise equivalent temperature of 10-8 K/√Hz (T=85 K, f>1 kHz) has already been obtained in a 0.2×12×10 μm3 microbridge. We describe a process involving the periodical sampling of the I-V characteristics that eliminates 1/f amplifier noise and returns the current value (temperature dependent) at fixed voltage bias (±1 mV). This output, locked to a reference voltage through a commercial temperature regulator, leads to temperature fluctuations less than 100 μKpp in a 10 Hz bandwidth. These results are promising to enhance performances of high-Tc microbolometers as infrared detectors.