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C. Thermal simulation resultsThe simulation results of the porous silicon based microsensing device showed very good correlation with themeasurements. The result of the steady state simulations ofthe built thermal model (see fig.7), in case of empty poresand in case of pores filled with water showed significantdifference between the two investigated cases. In case of drypores the highest temperature elevation was 25.42 °C whilein case of wet pores the elevation dropped to 24.13°C. Theresults of the transient simulations were also similar to themeasured curves. The two transient responses started todiverge at early times, app. at 100 µs, which should be thetime constant of the porous sensing layer and the heaterelement together.24-26 September 2008, Rome, ItalyIII. CONCLUSIONSIn this paper we have introduced a relative humiditysensing device, based on the thermal conductivity changes ofa porous silicon sensing layer. The thermal conductivitychanges of a 60 – 80 µm thin adsorbent layer were measuredwith the application of the thermal transient methodology.The sensor is proven to react to the RH changes of theenvironment. Both the measured thermal transient curvesand the generated structure functions are a suitable output ofthe measurements. There is a significant shift in one of thepeaks of the time constant spectrums of the measured devicein the driest and wettest state. Such shift also indicatesstructural changes and based on its mathematical meaning,with more precise measurements probably it will be used toestimate the amount of the adsorbed water.Thermal simulations have also proven the applicability ofthis measurement method. Both the steady state and thetransient simulation results of the model structure haveshown significant difference between a completely dry stateand the case when all the pores are filled with water.ACKNOWLEDGMENTFig. 7. Thermal model of the microstructure ( left) and the temperaturedistribution of the platinum filament ( right). The warmest point is 24.13°CCth [Ws/K]1e61000010010.011e-41e-61e-8Dry PoresWet PoresT3Ster Master: cumulative structure function(s)1e-100 5000 10000 15000 20000 25000Rth [K/W]Fig. 8. Structure functions generated from the simulated transient response.The graph indicates the decrease in the thermal resistance due to the moistureelevationThe transient curves were once again turned into structurefunctions, see fig.8. These derived functions show that in theregion of the heater the curves run together and start todiverge as the heat enters the porous layer. The heater regionshows a very high thermal resistance, which is true as theheat cannot spread well on the suspension of the heaterelement, only through the porous structure. In case of thestructure functions of the measured curves this region issmaller, probably due to some physical effects which werenot simulated, e.g. natural convection.This work was supported by the PATENT IST-2002-507255 Project and the NANOPACK FW7 No.216176/2007 IP Project of the EU and partly supported bythe NKFP3-00021/2005 project of the Hungarian NationalResearch and Development Program.REFERENCES[1] Z. M. Rittersma: Recent achievements in miniaturized humiditysensors – a review of transduction techniques; Sensors andActuators A 96 (2002) p. 196-210[2] Tímárné Horváth Veronika, Juhász László, Vass-Várnai András,Perlaky Gergely “Usage of porous Al 2O 3 layers for RH sensing”,Proceedings of DTIP'07 Stresa, 25-27-April, (2007)[3] M. Rencz, V. Székely: “Determining partial thermal resistances in aheat flow path with the help of transient measurements”, Proc. ofthe 7th THERMINIC Workshop, Paris, France, Sept 24-27,2001,pp. 250-256[4] J. J. Mares, J. Kristofik, E. Hulicius: Influence of humidity ontransport in porous silicon, Thin Solid Films 255 (1995) pp. 272-275[5] Stephen Brunauer, P. H. Emmett, Edward Teller, “Adsorption ofGases in Multimolecular Layers”, J. Am. Chem. Soc., (1938), 60,309. doi:10.1021/ja01269a023[6] W. N. Dos Santos, “Experimental investigation of the effect ofmoisture on thermal conductivity and specific heat of porousceramic materials”, Journal of Materials Science 35 (2000), 3977 -3982[7] Barsony I, Furjes P, Adam M, Ducso C, Vizvary Z, Zettner J, Stam,“Thermal response of microfilament heaters in gas sensing”,SENSOR ACTUAT B CHEM 103: 442-447 (2004)[8] T. Lu, C. Chen, “Uncertainty evaluation of humidity sensorscalibrated by saturated salt solutions”, Measurement (2006),doi:10.1016/j.measurement.2006.09.012[9] www.micred.com/t3ster.html[10] V. Székely et al. “New way of thermal transient testing”Proceedings of the XVth SEMI-THERM Symposium, March 9-11,1999, San Diego, CA, USA, pp. 182-188[11] M. Rencz, V. Székely, “Structure function evaluation of stackeddies”, Proceedings of the XXth SEMI-THERM Symposium, March9-11, 2004, San Jose, CA,USA, pp 50-54.©EDA Publishing/THERMINIC 2008 203ISBN: 978-2-35500-008-9