Micro-module Peltier
elaborated by flash
technique and used as a
humidity sensor
The detection and measurement of humidity are very important in environmental fields such as medical or domestic applications for human comfort[1]. Humidity sensors are based on various principles: quartz oscillator[2-4], macromolecular sensors[1,2,5],
electrolyte[1,2], semi-conductors[2,6,7], porous materials: ceramics[1,2] and silicon[8].
With increasing interest in thin film thermoelectric devices, we have studied a thin film humidity sensor based on the Seebeck effect[9]. This phenomenon is due to the creation of an electromotive force in a conducting material subjected to a
temperature gradient. In previous work[10] the development and characterization of micro-module Peltier (MMP) elaborated by a flash technique have been studied. The MMP is a P-N junction realised by a deposition of (Bi2Te3-Bi2Se3) alloys for N type materials
and (Bi2Te3-Sb2Te3) alloys for P type
materials on polyimide substrate. In order to investigate the performance of our flash evaporated layers, we tested three different structures of MMP, and with these first results we optimised in this work a ``butterfly'' MMP structure (B-MMP) that we used as a humidity sensor.
2. Optimisation of the MMP structure
With the results obtained on the three previous MMP we have realised a new MMP structure. This structure which we have called ``butterfly'' (B-MMP) is represented in Figure 1 and more details are given in Table I. The B-MMP structure is designed to resolve some of the disadvantages of the previous MMP:
. stabilisation of the temperature reference; . reduced contacts resistance;
. reduction of the Joule effect; . augmentation of the temperature
difference between the cold side and the hot side.
The thermal gradient is improved by the geometrical modification of the MMP structure which gives to the Peltier effect[11]
The authors
B. Sorli, A. Foucaran, A. Giani, F. Pascal-Delannoy
andA. Boyerare all with the Centre d'EleÂctronique et de Micro-optoeÂleÂctronique de Montpellier, Universite Montpellier II, France.
Keywords
Humidity, Sensors, Thin film
Abstract
In this work an original humidity sensor is described. It is based on the study of Seebeck voltage evolution during the water evaporation of a micro-module Peltier (MMP). The measurement principle is to detect (after cooling) the small temperature decrease created when total water evaporation occurs over the MMP. All the active thin layers of the sensor are made from (Bi2Te3)0.9(Bi2Se3)0.1
(N) films and (Bi2Te3)0.25(Sb2Te3)0.75(P) films flash
evaporated. Experimental measures were performed in a climatic chamber for several values of relative humidity (50 to 90 per cent). The phenomenon (evaporation) appears after a delay time. This delay time is the
response time of the sensor. Therefore it is possible to draw the evaporation delay time as a function of relative humidity.
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Research articles
Received: 11 February 2000 Accepted: 10 April 2000
This work is supported by the Direction GeÂneÂrale des ArmeÂes.
332 Sensor Review
Volume 20 . Number 4 . 2000 . pp. 332±335
an advantage over the Joule effect. Indeed, current density of the B-MMP structure is decreased by the use of thin P and N layers; therefore the Joule effect is also reduced. Moreover, the Peltier effect increases on the thermal-electrical shunt because of the increase in the current density. With this B-MMP structure a temperature difference between the hot and cold side of 9.5ëC is achieved for an injected cooling current (Ip) of 50mA as shown in Figure 2.
3. Measurement principle
A humidity sensor is constructed using this B-MMP structure. The measurement principle is described below:
. water condensation is produced by Peltier effect applied on the P-N junction; . after stopping the Peltier cooling, the
evaporation is detected by the structure when used as a simple thermocouple (Seebeck effect).
In fact, the flash elaborated B-MMP also produces a very high thermoelectrical sensitivity of 440V/ëC allowing the
temperature variations specifically related to evaporation to be followed.
This sensor was characterized in a climatic chamber SECASI controlled by SIRPAC software and using Labview software.
4. Experimental measurements
Experimental measurements are achieved in a climatic chamber for several values of relative humidity from 50 to 95 per cent and for a B-MMP current Ip = 40mA. The temperature was maintained constant and equal to 20ëC; no temperature compensation was necessary. The experimental measurement apparatus is shown in Figure 3. To verify that our results Figure 1MMP with ``butterfly'' structure: B-MMP
Table IThermoelectric and geometrical characteristics of each MMP structure
No. of MMP structure
MMP1 MMP3 MMP4 B-MMP
e(m) thickness of
active layers 20 20 20 20
L (m) length of
active N or P layer 100 500 1,000 300
(.m)
experimental resistivity of P type
layer 12 12 12 12
(.m)
experimental resistivity of N
type layer 15 15 15 15
REXP() experimental
resistance of MMP 3.36 6.3 15.32 2.1
RC() contact
resistance () 2.75 3.57 9.92 0.52
(V.K-1)
experimental Seebeck coefficient
of N type layer 200 200 200 200
(V.K-1)
experimental Seebeck coefficient
of P type layer 240 240 240 240
333 Micro-module Peltier elaborated by flash technique
B. Sorli, A. Foucaran, A. Giani, F. Pascal-Delannoy and A. Boyer
Sensor Review
are repeatable we use the Peltier reversibility effect to pre-heat the water detection zone for 4 seconds by applying a heating current Ih = 40mA, then we reverse the current to apply Ip = 40mA for 4 seconds. The analysis of the Seebeck voltage supplied by the
thermocouple begins only at the end of the Ip injection.
Figure 4 shows the current injection process and the Seebeck voltage obtained as a function of time for a humidity of 70 per cent. We think that the steep decrease in the Seebeck voltage observed on this curve correlates with when water evaporation occurs. In Figure 5 we show the measure of this delay time,, versus different humidity levels. The higher the humidity level, the longer is time. The experimental conditions are described as follows:
. heating current Ih = 40mA, heating time Th = 4s;
. cooling current Ip = 40mA, cooling time Tc = 4s;
. ambient temperature 20ëC.
We have presented here the first results of this structure used as a humidity sensor.
The delay time of the Seebeck voltage decreases rapidly with humidity (Figure 6).
Then it is possible to draw the evaporation delay time as a function of relative humidity.
Figure 4Applied B-MMP current process and Seebeck voltage typical curve versus time observed at B-MMP connections
Figure 2Temperature difference between hot and cold sides produced by Peltier effect versus the input current
Figure 3Sensor testing system
Figure 6Delay timeversus humidities
Figure 5Seebeck voltage versus time for different humidity levels
334 Micro-module Peltier elaborated by flash technique
B. Sorli, A. Foucaran, A. Giani, F. Pascal-Delannoy and A. Boyer
Sensor Review
5. Conclusion
In this work we have shown how, with the optimisation of contact and experimental resistance for the MMP structure (B-MMP), we obtained a maximum value for the temperature drop between hot and cold sides of about 9.5K. So we used this B-MMP structure as a humidity sensor for high humidity levels. These first results, coupled with those obtained in our laboratory by Pascal-Delannoyet al.[12], are very promising and offer a very wide domain of possible applications in the sensors field.
References
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335 Micro-module Peltier elaborated by flash technique
B. Sorli, A. Foucaran, A. Giani, F. Pascal-Delannoy and A. Boyer
Sensor Review