Quartz Crystal Microbalance (QCM) used as humidity sensor

Quartz Crystal Microbalance (QCM) used as humidity sensor

Sensors and Actuators 84 Ž2000. 285–291 www.elsevier.nlrlocatersna Quartz Crystal Microbalance žQCM/ used as humidity sensor F. Pascal-Delannoy ) , B...

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Sensors and Actuators 84 Ž2000. 285–291 www.elsevier.nlrlocatersna

Quartz Crystal Microbalance žQCM/ used as humidity sensor F. Pascal-Delannoy ) , B. Sorli, A. Boyer Centre d’Electronique et de Micro-optoelectronique de Montpellier, UMR 5507 CNRS, UniÕersite´ Montpellier II, Place E. Bataillon, 34095 Montpellier cedex 05, France Received 27 October 1999; received in revised form 18 January 2000; accepted 25 January 2000

Abstract This paper describes an application of Quartz Crystal Microbalance ŽQCM. used as humidity sensor. Moisture apparition is detected by using a QCM associated with a Peltier module. When water condensation produced by the Peltier cooling appears on the QCM, a change of mass on the crystal sensitive surface results in the decrease of the resonant frequency. If we measure the delay time between the beginning of Peltier supply and the apparition of water condensation on the quartz, we determine the relative humidity and the condensation velocity. A study of thermal transfer is first presented. Relative humidity measurements are then realised in a climatic chamber. A theoretical approach is finally compared with the experimental results. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Humidity sensor; Quartz Crystal Microbalance

1. Introduction A quartz vibrating resonator along the transversal mode in thickness ŽAT cut. is highly sensitive to mechanical strains with a low temperature coefficient of resonant frequency w1x. Since a long time, those resonators have been used as gravimetric sensors called Quartz Crystal Microbalance ŽQCM. with a mass detection efficiency equal to a few nanograms per squared centimeter w2,3x. Applications in gaseous phase generally deal with vacuum deposit. Since the QCM can also work in the liquid phase Žliquid acoustic impedance measurement w4x and liquid phase viscosity w5x., it can be used as a humidity detector. Humidity measurements are very important concerns in environmental fields, such as medical or domestic applications for human comfort, industrial uses, agriculture, automobiles, textiles, etc. w6x. A sensor, using adsorption and absorption of water by quartz has already been worked out w7x; the apparent mass was modified and so was the

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Corresponding author. Tel.: q33-4-67-14-32-35; fax: q33-4-67-5471-34. E-mail address: [email protected] ŽF. Pascal-Delannoy..

resonant frequency. To increase the sensitivity of such device, a hygroscopic material has been laid on the quartz and the mass was sharply modified by humidity of ambient air. Other humidity sensors, using piezoelectric materials, such as surface acoustic wave ŽSAW., have been developed for humidity sensor applications w8,9x. The classical method of humidity measurement consists of doing an optical dew point detection w10x; an original system has also been developed in the laboratory making use of an optical emitter–detector and a Peltier module w11x. Our new sensor is made up of an AT cut QCM vibrating along the transversal mode w12x. This quartz does not require any absorbent material, and it is directly stuck on the Peltier element. The Peltier module allows the cooling of the quartz until the dew point is reached. When it occurs, water drops appear on the quartz, then the apparent mass of the quartz is modified and its resonant frequency decreases. This paper presents first the experimental equipment and a study of thermal transfer between the Peltier and the QCM. The experimental measurements, which have been done in a climatic chamber, are then exposed. Finally, the relative humidity H R is drawn as a function of time from

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The quartz is stuck directly on the Peltier element. The contact between both the quartz and the Peltier element is made up using circular ring of silver grease in order to improve thermal transfers, which are directly related to the sensor response time. The QCM is connected to a TTL clock oscillator. A thin thermocouple ŽOMEGA CHCO001, chromegarconstantan. is placed on the crystal surface in order to follow the quartz temperature. After each condensation, the quartz is heated by inverting the Peltier current in order to get a good reproducibility of the measurements. The system of measurement is given in Fig. 2.

Fig. 1. Schematic diagram of the QCM humidity sensor: active part.

the experimental measurements, the theory of humid air, and the condensation velocity.

2. Experimental set-up The sensor is described in Fig. 1. It is made up of an AT cut quartz vibrating at 5 MHz ŽBalzers QB 104. and a square hole Peltier element ŽMELCOR SH 0.8-28-05 L..

3. Measure principle The aim of the measurement is to compare the quartz sensor frequency to quartz reference frequency. Then, the difference between the two frequencies is analysed as a function of time for a fixed Peltier current and for different relative humidity levels. This is the microbalance principle. Water condensation is observed for a relative humidity between 10% and 95%. All the measurements have been made at 228C in a Secasi-controlled climatic chamber. The value of relative humidity is given by the reference sensor of the climatic chamber with an accuracy of 3% H R .

Fig. 2. Experimental system of humidity measurements.

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starting from the ambient temperature on the Peltier surface. The temperature profile obtained on the quartz surface with the thermocouple is presented on Fig. 4, where Vth is the thermocouple voltage. The DTmax obtained on the quartz is equal to 32.58C. The experimental thermal response time t obtained is equal to 6.25 s Žsee Fig. 4. for the thermocouple method and 7 s for the IR camera measurement with an emissivity value of 0.81. The simulated response time calculated with the software Cosmos M gives a value of 5.5 s, which is close to the ideal transfer without thermal losses. Thus, we observe a good agreement between the experimental measurements Žthermocouple and IR camera. and the calculation. In these conditions, we are guaranteed to have a correct thermal transfer and a homogeneous temperature at the quartz surface. 4.2. Temperature effect

Fig. 3. Experimental procedure.

A series of measurements is following different steps, which are described during the manipulation procedure presented in Fig. 3.

A first set of measurements has been done in the climatic chamber for a Peltier current supply equal to 2.5 A and a relative humidity of 3% Ždried atmosphere.. This measure allows appreciating the effect of the temperature itself on the quartz. Fig. 5 represents the variation of the quartz resonant frequency vs. time due only to temperature variation. The frequency is converted in volt Vq Žthe conversion factor D frDVq s 375 Hz for 1 V., and the curve gives the evolution of Vq and Vth as a function of time. The Peltier is supplied during 30 s at 2.5 A. Vth follows the quartz temperature which decreases during 30 s to reach the maximum Vth s 1.92 mV, that is 32.58C against the room temperature Ž228C.. This temperature variation is not sufficient to induce condensation on the QCM for a relative humidity atmosphere of 3%. The theoretical calculation gives DT s 358C to reach the dew point for H R s 3%.

4. Experimental measurements 4.1. Study of thermal transfer The QCM thermal response has been realised to control the good thermal transfer between the Peltier and the quartz. Three methods have been used to analyse the variation of the QCM surface temperature as function of time during the Peltier cooling: Ø a thermocouple ŽOMEGA CHCO-001, chromegarconstantan, 59 mVr8C., Ø an infrared camera HGH, Ø a thermal simulation ŽSoftware COSMOS M.. With the thermocouple, we verified that a 2.5 A injected in the Peltier device provides a DTmax s y408C

Fig. 4. Evolution of the temperature profile obtained as a function of time using a thermocouple on the QCM for a Peltier supply equal to 2.5 A Ž DTma x sy32.58C starting from ambient temperature..

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Fig. 5. Variation of the converted frequency Vq and the thermocouple voltage Vth as a function of time for a Peltier supply equal to 2.5 A during 30 s and H R s 3%.

During these 30 s, Vq increases rapidly during 5 s then becomes stable reaching 0.7 V. This behaviour is typical for an AT cut QCM, the frequency variation corresponding to temperature effect only, this reference temperature curve will be subtracted afterwards from the curves taking account of the two phenomena: temperature and humidity. 4.3. RelatiÕe humidity measurements The second set of measurements has been done for a Peltier current supply of 2.5 A and a relative humidity

varying from 10% to 95%. Fig. 6 represents the variation of Vq vs. time for humidity levels starting from the different humidity levels ŽThe effect of temperature has been subtracted from each curve in order to keep only the effect of water condensation... It can be seen that Vq decreases rapidly. As a liquid that highly absorbs transversal vibrations, the quartz stops vibrating rapidly after the first drops appear. That is in agreement with the moisture effects observed by Glassford for AT cut QCM w13x. For humidity rates above 20%, condensation makes the quartz resonant frequency sharply decrease. The more the relative humidity is important, the more this frequency decreases rapidly. On these curves, a frequency threshold can be defined above which the condensation is considered to have occurred. In our case, we have chosen a threshold voltage equal to y2 V corresponding to 750 Hz. This reference voltage allows associating a relative humidity value Žfrom 20% to 95%. to a total delay time noticed t T that we define by: t T s t D q tC t D is the time necessary to reach the dew point, tC is the time of QCM humidity charge beyond the dew point. Starting from Fig. 6, the total delay time t T is deduced from the intersection between the threshold voltage at y2

Fig. 6. Variation of the converted frequency Vq as a function of time for different values of humidity from 10% to 95%.

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Table 1 Values of t T , tC , and t D corresponding to the relative humidity levels from 20% to 95% t T s tC q t D is the time determined by the voltage threshold Vq at y2 V. tC is the time of QCM charge beyond the dew point. t D is the time corresponding to the dew point. H R Ž%.

t T Žs.

tC Žs.

t D Žs. calculated value

20 30 40 50 60 70 80 90 95

16.5 9.03 6 4.45 4 3.05 2.75 2.37 1.95

5 3.33 2.25 2 1.66 1.42 1.25 1.11 1.05

11.5 5.7 3.75 2.45 2.34 1.63 1.5 1.26 0.9

Fig. 8. Variation of the humidity level as a function of time for the voltage threshold y2 V. Theoretical curve and experimental points: H R vs. t D .

V and the frequency variation for each humidity level. The experimental points are reported in Table 1. 4.4. Condensation Õelocity On the other hand, it is possible to draw the quartz charge velocity n Q ŽHzrs. as a function of humidity level Žsee Fig. 7.. Each experimental point reported represents the value of the slope for each curve at Vq s y2 V. The fit gives a linear variation for which n Q ŽHzrs. is related to H R Ž%. by the equality:

n Q s 7.5 = H R

Ž 1.

tC is then calculated for each humidity value as: tC s

f threshold 7.5 = H R

Ž 2.

with f threshold as the frequency corresponding to the threshold y2 V and equal to 750 Hz. So, it is easy to calculate

t D . tC and t D are reported in Table 1. This table will be useful for the theoretical interpretation. Starting from the variation of n Q vs. H R , it is possible ˚ .. The value to calculate the condensation velocity n C ŽArs of the condensation velocity is deduced from the microbalance formula wLUx:

nC s

De Dt

1 s

=

0.474

1

r water

=

Df Dt

Ž 3.

with r water s 1 as the water density, 0.474 is a constant characteristic of the QCM dimension, and D e is the thickness variation. Finally:

nC s

1 0.474

= nQ .

Ž 4.

Or also, using Eq. Ž1.:

n C s 15.82 = H R .

Ž 5.

For a relative humidity of 95%, the condensation velocity ˚ is equal to 1500 Ars.

5. Theoretical approach Relative humidity calculation is based on the Clapeyron formula giving saturation vapour pressure P Vsat of water as a function of ambient temperature TA . In human environment application domain, 08C - TA - 358C, Clapeyron formula is reduced to: P Vsat Ž TA . s C = exp Fig. 7. Evolution of the quartz charge velocity n Q ŽHzrs. as a function of humidity level. Experimental points obtained for the voltage threshold y2 V and linear fit: n Q s 7.5= H R Ž%..

TA

ž / B

Ž 6.

with C s 643.03 Pa and B s 15.798 K, C and B are fixed constant.

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The definition of relative humidity is given by: HR s

P Vp Ž TA .

Ž 7.

P Vsat Ž TA .

with P Vp ŽTA . as the vapour partial pressure of water in the mixing at the temperature TA and P Vsat ŽTA . as the saturation vapour pressure of water at temperature TA . At dew point temperature, T D , the pressures verify: P Vsat Ž T D . s P Vp Ž TA . s H R = P Vsat Ž TA . .

Ž 8.

it is based on frequency measurement. It is possible to improve such a sensor by reducing the quartz thickness in order to work with higher value of resonant frequency.

Acknowledgements This work has been carried out as a part of a research program sponsored by the Direction Generale de l’Armem´ ´ ent ŽFrance. whose support is gratefully acknowledged.

The Eq. Ž6. injected in Eq. Ž8. gives: TD s TA q B = ln H R References

and also:

DT s TA y TD s yB = ln H R .

Ž 9.

On the other hand, the temperature variation is linked to the thermal response time measured on the quartz ŽFig. 4; t s 6.25 s. by a mathematical approximation: yt

DT s DTmax 1 y exp

ž /

Ž 10 .

t

with DTmax as the maximum temperature variation obtained on the QCM for the chosen I Peltier . In our case, DTmax s 32.58C for 2.5 A. In the end, Eqs. Ž9. and Ž10. give: ln H R s

ž

DTmax B

yt

/

= exp

ž / t

y1

Ž 11 .

or H R s 100 = exp

ž

DTmax B

yt

/ ž ž / / = exp

t

y1

.

Ž 12 .

Eq. Ž12. links directly relative humidity level to time for a fixed Peltier current. The corresponding curve is drawn in full line in Fig. 8. At the dew point, experimental points are represented by the t D vs. H R issue from Table 1. In the end, if the delay time t D is known, it is possible to determine the relative humidity using the drawing. The variation of t D Ž H R . is close to the theoretical curve indicating that the experiment and the calculated model are in good agreement. Considering these final curves, we obtain a total response time for the sensor varying from 11.5 to 1 s if the humidity is ranging between 20% and 95%. This result, which concerns the accuracy and the velocity of sensor response, is very interesting.

6. Conclusion This sensor is based on the microbalance principle, and its feasibility has been proved. A quartz, directly fixed on a Peltier element is able to measure a relative humidity between 20% and 95%. Quartz humidity sensor has one of the best response times of different hygrometers listed by Yamazoe and Shimizu w6x. It is also very accurate because

w1x J. Zelenka, Piezoelectric resonators and their applications, in: Studies in Electrical and Electronic Engineering Vol. 24 Elsevier, Amsterdam, 1986, p. 211. w2x C. Lu, A.W. Czanderna, Applications of Piezoelectric Quartz Crystal Microbalances, Elsevier, Amsterdam, 1984. w3x R.P. Chiarello, J. Krim, C.T. Thompson, Quartz crystal microbalance and synchrotron X-ray reflectivity study of water and liquid xenon adsorbed on gold and quartz, Surf. Sci. 306 Ž1994. 359–366. w4x L. Tessier, M. Lethiecq, D. Certon, F. Patat, Impedance acoustique d’un liquide a` la surface d’un quartz coupe AT ŽEnglish title: Acoustic impedance of a liquid at the surface of an AT cut quartz., Journal Physique IV, Colloque C5, suppl. au Journal de Physique III vol. 4 Ž1994. 1205–1209. w5x L.V. Rajakovic, B.A. Cavic-Vlasak, V. Ghaemmaghami, K.M.R. Kallury, A.L. Kipling, M. Thompson, Mediation of acoustic energy transmission from acoustic wave sensors to the liquid phase by interfacial viscosity, Anal. Chem. 63 Ž1991. 615–621. w6x N. Yamazoe, Y. Shimizu, Humidity sensors: principles and applications, Sens. Actuators 10 Ž1986. 379–398. w7x N. Ichinose, T. Kobayashi, Guide Pratique des Capteurs, wPractical Guide of Sensorsx 1990, pp. 135–137, Edition Masson. w8x T. Nomura, K. Oobushi, T. Yasuda, S. Furukawa, Humidity sensor using surface acoustic wave delay line with hygroscopic dielectric film, Jpn. J. Appl. Phys. 32 Ž1993. 4205–4208. w9x M. Hoummady, C. Bonjour, J. Collin, F. Lardet-Vieudrin, G. Martin, Surface acoustic wave ŽSAW. dew point sensor: application to dew point hygrometry, Sens. Actuators, B 26–27 Ž1995. 315–317. w10x R.S. Jachowicz, Dew point hygrometer with heat injection principle of construction and operation, Sens. Actuators, B 7 Ž1992. 455–459. w11x F. Pascal-Delannoy, A. Sackda, A. Giani, A. Foucaran, A. Boyer, Fast humidity sensor using optoelectronic detection on pulsed Peltier device, Sens. Actuators, A 65 Ž1998. 165–170. w12x J.R. Vig, Introduction to quartz frequency standards, SLCET-TR-921 Žrev. 1., October 1992. w13x C. Lu, A.W. Czanderna, Applications of piezoelectric quartz crystal microbalances, Response of the Quartz Crystal Microbalance to Liquid Deposits, Elsevier, Amsterdam, 1984, pp. 325–346.

Biographies Andre´ Boyer was born in Perpignan ŽFrance.. He is Doctor ‘‘es Sciences Physiques’’ from Montpellier University Ž1975.. Since then, he works in the Center of Electronic and Micro-optoelectronic of Montpellier, Montpellier University. He is a specialist in thermocouple temperature measurement, preparation, and properties of thin solid films and ultrasonic method in Solid State Physics. Actually, he is involved in the fundamental studies of sensor phenomena and thermal transport processes in small structures.

F. Pascal-Delannoy et al.r Sensors and Actuators 84 (2000) 285–291 Frederique Pascal-Delannoy was born in Avignon, France. She is a ´ ´ Doctor in Electronics from Montpellier University since 1988. Since then, she has been with the laboratory of Electronic and Micro-Optoelectronic Center of Montpellier, Montpellier University where she is a specialist of MOCVD growth of antimonides-based materials for optoelectronic applications. Until now, she is involved in the preparation and study of low size sensors based on thermal transport phenomena like humidity sensors using Peltier device.

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Brice Sorli was born in Montpellier, France. He received the «Doctorat» degree in Electronics from Montpellier University in 1998. Since then, he is preparing a PhD thesis on «Integration de capteurs d’humidite´ a base de micromodule Peltier.» in the Center of Electronic and Micro-Optoelectronic of Montpellier where he works on electronic measurements, instrumentation, simulation systems, and porous silicon.