Didier Leclercq

Simultaneous fabrication of superhydrophobic and superhydrophilic polyimide surfaces with low hysteresis.

Polyimide is of great interest in the field of MEMS and microtechnology. It is often used for its chemical, thermal, mechanical, and optical properties. In this paper, an original study is performed on controlled variation of polyimide film wettability. A two-step microtexturing method is developed to transform hydrophilic polyimide surfaces into a superhydrophobic surface with low magnitude of hysteresis (Δθ ≈ 0° and contact angle θ ≈ 158°). This method is based on the conception of a new kind of fakir surface with triangular cross-section micropillars, the use of a two-scale roughening, and a C(4)F(8) coating. We demonstrate that the absence of hysteresis is related to a combination of two scales of structuring and the pillar shape. The technology that has been developed results in the simultaneous fabrication of adjacent superhydrophobic and superhydrophilic small areas, which allows an effect of self-positioning of water droplets when deposited on such a checkerboard-like surface.

Thin foil planar radiometers: application for designing contactless sensors

This paper is devoted to describing a new sensor allowing one to measure the net radiant flux exchanged by the wall surface it is mounted on. The device is constructed by mounting a thermopile-type radiometer on a larger thin metallic foil support. When the emissivity of the paint covering the support is the same as that of the wall surface on which the sensor is applied, a direct reading (positive or negative emf) of the radiant flux (absorbed or emitted) by the wall surface is given, whatever the convective losses. The calibration is carried out in a simple and useful apparatus designed to produce a prescribed total radiant exchange between two metallic plates at different temperatures and is estimated to be accurate to within two per cent. Simplicity and ruggedness make the radiometer appropriate for direct measurement of heat exchanged between surfaces heated up to 500 K. Notable applications include use as a traditional total hemispheric radiometer and a contactless temperature difference sensor.

Direct measurement of the latent heat of evaporation by flowmetric method

This paper describes an original and very simple device dedicated to measure the latent heat of evaporation of various liquids at ambient temperature. This apparatus is composed of two very thin heat flow sensors glued to a heat sink. One of these sensors is covered with oiled textile, and the other one with an identical dry textile intended to receive the liquid to be characterized. This differential system allows compensating the disturbances due to environment. A thermodynamic model is established to determine the heat flow induced by evaporation. By integrating this flow, one calculates the quantity of latent heat absorbed by evaporation. Numerous experiments with different liquids such as alcohol, acetone, water, and trichloroethylene have been carried out. The results are compared with theoretical values, and the maximum error is less than 6%.

Design of Textile Heat Flowmeter Combining Evaporation Phenomena

This article describes the development of a new thermal flowmeter based on the principle of thermopiles measuring heat flow through a wall. The principal advantage of this sensor is the use of a textile auxiliary wall, which gives it flexibility and enables it to be applied to rounded, deformable surfaces. Another property, related to this type of wall, is that it is permeable to moisture. In fact, this sensor takes into account the phenomena of evaporation which, until now, was not carried out. This allows a better measurement of the energy during heat exchange.

A new sensor to measure fluid velocities: the Peltier anemometer

This paper introduces a new approach to the measurement of low fluid velocities. A new type of anemometer is presented, resorting to a sensor directed to measuring low velocities ( in air) with a precision to the nearest . The device works owing to the thermoelectric effects, especially the Peltier and Seebeck ones. It is non-invasive since its increase in average temperature remains lower than 15 K. Therefore low-velocity measurements can be carried out without any significant perturbation. A coherent design rationale is formulated and the various stages in the technical development of the system are delineated. A compensating technique is described in order to provide reliable performance over temperature variations. In most cases this sensor can be instrumental in studying mechanisms of natural convection.

Power dissipated measurement of an ultrasonic generator in a viscous medium by flowmetric method.

A new flowmetric method of the power dissipated by an ultrasound generator in an aqueous medium has been developed in previous works and described in a preceding paper [V. Mancier, D. Leclercq, Ultrasonics Sonochemistry 14 (2007) 99-106]. The works presented here are an enlargement of this method to a high viscosity liquid (glycerol) for which the classical calorimetric measurements are rather difficult. As expected, it is shown that the dissipated power increases with the medium viscosity. It was also found that this flowmetric method gives good results for various quantities of liquid and positioning of the sonotrode in the tank. Moreover, the important variation of viscosity due to the heating of the liquid during experiments does not disturb flow measurements.

Thermoelectric infrared microsensor using suspended membranes made by silicon micromachining

The main activity of our laboratory is based on the design and the fabrication of thermal microsensors whose infrared radiometers are a significant part. These microsensors work by converting infrared radiation into spatial periodical temperature gradients patterned on substrate. An array of plated bimetallic microthermocouples is designed to produce the output voltage from the maximum generated temperature differences. Until now, planar sensors featuring a metering area from 3 to 100 mm2 have been realized on glass or kapton (submitted for SPIE 2001) substrates. To take advantages of silicon technology, it is necessary to adapt the structure of the sensors to such a material. Indeed, the sensitivity of this kind of sensor is approximately proportional to the thermal resistance between hot and cold junctions. Thus, taking into account the low thermal resistivity of silicon, membranes must be recessed what makes it possible to strongly increase the thermal resistance of the zone located under the junctions of the thermocouples. However, to preserve the advantage of immunity against convection, these membranes must present quite particular properties of symmetry and periodicity compared to the thermocouples. In this case, the microthermopile is constituted of alternatively doped N and P polysilicon layers. Using a simple model, operation principle is analyzed and the microradiometer sensitivity can be computed. The microsensors are manufactured within the framework of a national project (CNRS Interlab) and the successive steps of the technological process are described. Lastly, the first experimental results are presented.

Probing technique for localized thermal conductivity measurement

A low-cost and non-destructive measurement technique based on the combination of a temperature sensor and a heater integrated in a very sharp tip is proposed for the determination of thermal conductivity of planar materials. The thermal sensor is fabricated by means of microtechnology technique. Associated to the system, an analytical thermal model is developed to express the measured Seebeck voltage as a function of the material thermal resistance. A numerical analysis based on COMSOL MultiphysicsⓇ is then developed to extract the thermal conductivity from the thermal resistance. To validate the approach proposed, experiments on planar dielectric materials and metals are conducted. Precision around 0.1 W m−1 K−1 for thermal conductivities lower than 2 W m−1 K−1 is obtained.

A new method to improve the efficiency of the heat flow path of a micro thermoelectric generator

This work proposes a new method to improve the efficiency of the heat flow path of a micro thermoelectric generator (μTEG). A silicon heat concentrator is placed on a planar μTEG after alignment to guide the heat flow from the heat source to the hot junctions. The results show that the heat concentrator can efficiently isolate the heat source from the cold junctions and guide the heat flux to pass through the planar thermocouples in order to generate a temperature gradient. When a 4W/cm2 power is injected to a fabricated device, it builds an output voltage of 29V/cm2 which can supply an output power of 41μW/cm2 to a matched load resistance.

Distribution-patterned radiometers: a new paradigm for irradiance measurement

This paper introduces a new approach to measuring infrared radiant flux density. Thermal sensors featuring a significant metering area from 3 mm2 to 25 cm2 can be achieved by way of distributing the sensing surface upon an appropriate plated-planar thermopile. Despite a low figure of merit regarding bimetallic structures, low noise, rugged, thin and even flexible devices are made in the laboratory. Such sensors have neither to be covered with a protective widow nor to be placed in an insulating gas, thanks to their inherent immunity against convection afforded by the differential behavior of their structure. Hence wide spectrum infrared measurements, and experiments undergoing a wide range of pressure, are allowed with distribution-patterned radiometers. Current techniques of manufacture are reviewed together with the philosophical arguments concerning the distributed layout of monolithic thermopiles. Since such devices can be directly deposited upon various dielectric materials, many an application in military and space research can be expected. As regards industrialization, those multipurpose sensors meet the necessary requirements of self-calibrating ability, good reproducibility, fast response (#20 ms), ruggedness, and low cost. It is expected that the versatility of the device will result in a wide number of industrial applications.