FERROELECTRIC CERAMICPOLYMER COMPOSITE FOR 铁电ceramicpolymer复合材料.doc

Sensors & Transducers Magazine, Vol.39, Issue 1, 2004, pp.112-120Sensors & TransducersISSN 1726- 5479 2004 by IFSAFerroelectric Ceramic/Polymer Compositefor Soil-Humidity DetectionWalter Katsumi SAKAMOTO, Jos Antonio MALMONGE,Srgio Henrique FERNANDESDepartamento de Fsica e Qumica, Faculdade de Engenharia,Universidade Estadual Paulista UNESP, Av. Brasil, 56, 15385-000 Ilha Solteira (SP), Brasil.Fax: +55 18 3742 4868, tel: +55 18 3743 1081, e-mail: sakamotofqm.feis.unesp.br http:/www.fqm.feis.unesp.brReceived: 2 December 2004 /Accepted: 14 January 2004 /Published: 18 January 2004Abstract: Ferroelectric ceramic + polymer composites using castor oil-based polyurethane (PU) as matrix and lead zirconate titanate (PZT) as dispersed powder were fabricated in the thin films form by spin-coating method. The composites were poled with appropriated electric field and its piezoelectric property was studied by conventional method. Using the ability of the composite to response with an electric signal to a variation of pressure, a detection system was made to substitute the manometer to measure the soil humidity.Keywords: composite, soil humidity, pressure sensor, tensiometer_1. IntroductionIn soil study measures of soil water content are needed and in the field knowledge of the water available for plant growth requires a measure of some index of water content 1. Soil water can be characterised by an energy state. Since the soil water movement is very slow the kinetic energy can be neglected and the energy state, which is a function of position and internal condition of the water, is defined by potential energy. The energy state of soil water can be described by the Gibbs free energy thermodynamics function 2 that is called “total potential of water” in soil-plant-atmosphere system.The first thermodynamics law associated with entropy can be written 3,(1)where dU is the internal energy variation, T is the temperature, dS the entropy variation, Pe the external pressure, dV is the change in volume, is the chemical potential and is any other kind of energy.The differential equation of Gibbs free energy for soil-plant-atmosphere system, (dG = dy, y is the total potential of water) is:,(2)where g is the gravity, n is the number of moles of water, w define the relations with soil/water/air interactions, q is the water content, z the height related to an arbitrary referential.Actually y, the potential of water in particular state of the system is a relative value in relation to the pattern value stated equal zero. Summarising, the total potential of water can be written:(3)It can be seen from eq. 3 that the total potential of water has thermal, pressure, gravimetric, osmotic and matric components, i.e.,(4)The matric component is the component of interest because it is related with the water content (q). That potential results from capillary forces and adsorption because of the interaction between water and solid particles, i. e., the soil matrix. These forces attract and fix the water in the soil reducing its potential energy related with the free water energy. The matric potential is generally determined experimentally.For each sample of homogeneous soil, Ym has a characteristic value to each water contents (q). The relation between Ym and q is called characteristic curve of soil-humidity or simply retention curve. In this paper we propose to characterise a ferroelectric ceramic/polymer composite as piezoelectric sensor for use in the tensiometer for soil water content measurements.2. Experiments2.1 The TensiometerFigure 1 shows the essential parts of a tensiometer for field use. A porous cup is sealed to the PVC tube. A removable airtight cap is used to seal the tube at the top. A mercury manometer is attached near the upper end of the tube, which is filled with water.Fig. 1. TensiometerTensiometer measures the energy status of soil water. When the water contents of soil, surrounding the tensiometer decreases its energy decreases relative to the water inside the tensiometer and water moves out from tensiometer to the soil and the pressure is reduced in the tensiometer tube. If the soil receives water, the flux path becomes inverse and again the pressure inside the tube changes.2.2 Ceramic/Polymer Piezoelectric CompositeComposite films were made by mixing lead zirconate titanate (PZT) powder with vegetable based polyurethane (PU). The mixture was placed on a glass substrate and using a spin-coating system, flexible composite films were obtained in the thickness range of 80 200 mm. The composition used was 33 vol % of ceramic and 67 vol % of PU. After polymerisation, the composite sample was cut in the appropriated size and aluminium electrodes were evaporated onto both sides for electrical contact. Using suitable temperature and electric field, the composite was poled during 1.0 h.2.3 The SensorFigure 2 shows the composite film placed in a special cell to work as a sensor. A frequency generator is connected to a steel-carbon film, which excite the composite film. That way any change in the pressure applied to the sensor will give an electric signal because of the piezoelectric activity of the composite.Fig. 2. Special cell using the composite film as sensor3. Results and DiscussionFigure 3 shows the electrical response of the sensor as a function of applied pressure. The applied frequency was 2.4 kHz square waves with 5.0 V peak-to-peak amplitude. The pressure range was 0 0.8 atm, which is the usual work range of tensiometer 4.Fig. 3: Sensor response to the pressure variationThe composite film was cut in an appropriated size and after aluminium electrode deposition, the sample was poled during 1.0 h. Figure 4 shows the behaviour of d33 as a function of temperature of polarisation (TP). Two different PZT/PU sample compositions were used: 27/73 vol. % and 33/67 vol.%. It can be seen from figure 3 that at 383 K the polarisation of the samples is more efficient. The reduction of the piezoelectric activity for higher poling temperature could be due to the beginning of polymer degradation.Fig. 4 Piezoelectric coefficient as a function of poling temperature.10 MV/m applied electric field for 1.0 hThe longitudinal piezoelectric coefficient increases with increasing poling electric field. The highest applied electric field was 20 MV/m and the limitation was the sample breakdown. The PZT content is responsible for the higher piezoelectric coefficient since the polymer matrix does not display any piezoelectric activity. In figure 5 is showed the variation of d33 for different electric field. The temperature of polarisation was kept constant (= 383 K). That coefficient was measured with a d33 Piezo Tester model 8000 from APC (American Piezo Ceramics).Fig. 5. Piezoelectric coefficient for different poling field. Poling temperature of 383 K for 1,0 hAlthough the transversal piezoelectric coefficient (d31) was also measured, using the null method 5 and was found the value of d31 = 3.0 pC/N for a sample poled with 20 MV/m at 383 K for 1.0 h., that coefficient is not important for the use of the composite as pressure sensor.For testing the composite as sensor in substitution of the conventional Hg manometer, a special cell (fig. 2) was constructed and connected to the tensiometer. Figure 6 shows the apparatus for using the PZT/PU composite as soil humidity detector.Fig. 6. Apparatus for soil humidity detection using the PZT/PU composite as sensorThe tests were carried out using a soil sample analysed by the Agricultural Department of Universidade Estadual Paulista UNESP with conventional tensiometer. Table 1 shows the matric potential (Ym) and the volumetric humidity at three different depths, commonly used for soil analyses. To fit the experimental data, a computer program was used together the Van Genuchten 7 model. Table 2 shows the parameters used to fit the retention curve of the soil sample.Table 1: Variation of the matric potential and the volumetric humidity.ys(mm Hg)Volumetric humidity (cm3.cm-3)15 cm (deep)Volumetric humidity (cm3.cm-3) 30 cm (deep)Volumetric humidity (cm3.cm-3)45 cm (deep)00.2810.3300.3700.7350.2810.3200.3502.9410.2800.2900.2807.3530.2000.2400.23022.0590.1700.2100.20036.7650.1500.1800.18173.5290.1400.1500.170367.6470.1020.1400.1501102.9410.1000.1300.130Table 2. Parameters of Genuchten ModelParameters15 cm30 cm45 cma0.02860.0180.0643m0.01830.46020.2128n27.4431.1822.085qr0.1000.1300.130qs0.2810.3300.370where qr and qs are the residual and saturation volumetric humidity, respectively. a, m and n are soil parameters. All these parameters were provided by the Agricultural Department of UNESP.Figure 7 shows the behaviour of retention curve fitted with the parameters from Van Genuchten model, using experimental data obtained with a conventional tensiometer, i. e., with Hg manometer.Fig. 7. Retention curve using conventional manometer and Van Genuchten modelUsing the apparatus with PZT/PU composite sensor (fig. 6) and the same soil sample, similar results were obtained as showed in figure 8. It can be seen the good agreement between the two measurements indicating that the PZT/PU composite can be used as sensor for soil humidity detection.Fig. 8. Retention curve using PZT/PU sensor and Van Genuchten modelConclusionsFlexible and free-standing composite films were obtained by mixing PZT powder and castor oil-based polyurethane. In spite of the low ceramic content (33 vol. %), the composite shows enough piezoelectric activity and could be used as sensor. Using the ability of the sensor to detect pressure variation, a special cell was constructed to use the composite film as a diaphragm and connecting the cell to the tensiometer, the soil humidity was detected. The retention curve, which gives the soil humidity condition, obtained with the new sensor agrees very well with those obtained with Hg manometer. The sensibility of the PU/PZT sensor is higher in the low-pressure range (0 100 mmHg). These results give a real possibility to think in an automated irrigation system.AcknowledgementsThanks are due to Fundao de Amparo Pesquisa do Estado de So Paulo FAPESP for financial support. The authors also are grateful to Dr. Fernando B. T. Hernandes for the soil data and discussion.