Electrical Characteristics Cufe 2 o 4 Thick Film Ceramics with Different Glass Concentrations Fired at 1000 ° C for Negative Thermal Coefficient ( Ntc ) Thermistor

Fabrication of CuFe2O4 thick film ceramics utilizing Fe2O3 derived from yarosite using screen printing technique for NTC thermistor has been carried out. Effect of glass frit addition (0, 2.5, 5 weight %) has been studied. X-ray diffraction analyses (XRD) was done to know crystal structure and phases formation. SEM analyses were carried out to know microstructure of the films. Electrical properties characterization was done through measurement of electrical resistance at various temperatures (room temperature to 100°C). The XRD data showed that the films crystalize in tetragonal spinel. The SEM images showed that glass frit addition made the grain size smaller. Electrical data showed that the larger the glass frit concentration, the larger the resistance, thermistor constant and sensitivity. From the electrical characteristics data, it was known that the electrical characteristics of the CuFe2O4 thick film ceramics followed the NTC characteristic. The value of B and RRT of the produced CuFe2O4 ceramics namely B = 2215-2807K and RT = 6,9-16,7 M Ohm, fitted market requirement.


Introduction
NTC thermistor is widely applied in many applications such as temperature sensor, electric current limiter, flowrate meter and pressure sensor (Anonymous, 2008).It is generaly known that the NTC thermistor is made of ceramic having structure of spinel of AB 2 O 4 where A is the ion occupies tetrahedral position and B is the ion occupies octrahedral position (Fritsch, 2004;Hamada, 2001;Jung, 1993;Matsuo, 1982;Moulson & Herbert, 1990;Na et al., 2001;Park, 2003;Park & Bang, 2003;Park & Han, 2005;Schmidt et al., 2004;Wiendartun & Syarif, 2007;Wiendartun et al., 2008;Wiendartun & Syarif, 2008).Many efforts can be performed in order to improve the characteristic of the NTC thermistor having spinel structure.One of the spinel ceramics may be applied as NTC thermistor is CuFe 2 O 4 ceramic.The thermistor may be produced in the form of disk or thick film.
Here, the object of study is the thick film thermistor.
Technology generally used for fabrication of thick film thermistor is screen printing which is technically simple.Some parameters in the screen printing namely viscosity of paste, screen size, paste composition and firing parameter such as time and temperature significantly influence the characteristic of the thick film produced.In this work, a study on fabrication of thick film thermistor based on CuFe 2 O 4 with different glass frit concentration was performed.The effect of different glass frit concentration on the characteristics, especially the electrical characteristics, of the CuFe 2 O 4 thick film ceramics for NTC thermistor is the focus of the study.

Experimental Procedures
Powder of Fe 2 O 3 derived from yarosite mineral (chemical composition is shown in Table 1), CuO powder and glass frit made of SiO 2 , B 2 O 3 and PbO were crushed and sieved with a sieve of 38 m (Hole size of 38m).The method for processing the Fe 2 O 3 powder is described elsewhere (Wiendartun & Syarif, 2007).The sieved Fe 2 O 3 and CuO powder and (0, 2.5, 5 weight %) glass frit were mixed.The mixture of Fe 2 O 3 , CuO and glass frit was mixed with organic vehicle containing alpha terpineol and ethyl cellulose with composition of 90 weight % and 10 weight %, respectively, to form a paste.The paste was screen printed on alumina substrates using screen printing technique.The films were fired at 1000°C for 1 hour in air.The crystal structure of the fired thick films was analyzed with x-ray diffraction (XRD) using K radiation.The films were investigated by SEM.A couple of parallel electrodes which is 1 mm apart are made on the sensor side of the fired thick film by using Ag paste.After the paste was dried at room temperature, the Ag coated-thick films were heated at 600°C for 10 minutes.The resistance was measured at various temperatures from 25 to 100°C in steps of 5 o C using a digital multimeter and a laboratory made chamber equipped with a digital temperature controller.Thermistor constant (B) was derived from Ln resistivity vs. 1/T curve where B is the gradient of the curve based on (1) (Park & Han, 2005): where,  is the electrical resistivity,  o is a constant or the resistivity at T is infinite, B is the thermistor constant and T is the temperature in Kelvin.Room temperature resistance (R RT ) was determined as the electrical resistance at room temperature (25 o C) and sensitivity () was calculated using (2) (Moulson & Herbert, 1990).
where,  is the sensitivity, B is the thermistor constant and T is the temperature in Kelvin.
Microstructure and structural analyses were carried out by using a Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD), respectively.All steps of the procedure can be seen in Figure 1.Glass frit: (0, 2.5, 5%)

Results and Discussions
Figure 2 shows the appearance a typical thick film.Figure 3, Figure 4 and Figure 5 show the XRD profiles of CuFe 2 O 4 thick film ceramics fired at 1000°C for 1 hour with different glass frit concentrations of 0; 2,5 and 5 %, respectively.As shown in the figure Figure 3, Figure 4 and Figure 5 the profiles are similar.The XRD profiles show that the structure of the thick film ceramics is tetragonal spinel after being compared to the XRD standard profile of CuFe 2 O 4 from JCPDS No. 34-0425.No peaks from second phases observed.Peaks from alumina substrate (with A sign) are observed.The electrical data of the CuFe 2 O 4 thick film ceramics is shown in Figure 8 and Table 2.The electrical data of Figure 8 shows that the Ln resistivity increases linearly as the 1/T increases, indicating that the electrical characteristics of the ceramics follows the NTC tendency expressed by equation ( 1).As shown in Table 2, The increase of the glass frit concentration from 0% to 5% increases the room temperature resistance (R RT ), thermistor constant (B) and sensitivity ().The thermistor constant (B) for market requirement is ≥ 2000K.So, the value of thermistor constant (B) of the ceramics in this work fitted market requirement.

Figure 1 .Fe
Figure 1.Flow diagram of the experiment procedure

Figure 2 .
Figure 2. A typical thick film thermistor

Figure 8 .
Figure 8.The relation between ln Electrical Resistivity and 1/T of CuFe 2 O 4 based-thick film fired at 1000°C for 1 hour with 0, 2.5 and 5% glass frit addition

Table 1 .
Chemical composition of Fe 2 O 3 powder derived from yarosite

Table 2 .
Electrical characteristics of the CuFe 2 O CuFe 2 O 4 thick film ceramics utilizing Fe 2 O 3 derived from yarosite mineral have been well fired at 1000°C for 1 hour with 0, 2.5 and 5% glass frit addition.All of the thick films crystallize in tetragonal spinel.Glass frit addition makes grain size of the films smaller.The larger the glass frit concentration, the larger the resistance, thermistor constant and sensitivity.The electrical characteristics of the CuFe 2 O 4 thick film ceramics follow the NTC characteristic.The value of thermistor constant (B) = 2215-2807 o K and room temperature resistance (R RT ) = 6,9-16,7 MOhm.Compared to the value of B for market requirement namely ≥ 2000K, the B value of the produced CuFe 2 O 4 ceramics fits market requirement.