JOURNAL OF MATERIALS SCIENCE LETTERS 19 (2 0 0 0 ) 603 – 605 Polyurethane-Bi-system a conducting composite W. K. SAKAMOTO, D. H. F. KANDA, C. L. CARVALHO Universidade Estadual Paulista-UNESP-C ˆ ampus de Ilha Solteira-Departamento de F´ısica e Qu´ ımica-DFQ, Av. Brasil, 56-C. P. 31-15385-000 Ilha Solteira, S˜ ao Paulo-Brazil E-mail: sakamoto@fqm.feis.unesp.br Technological applications have motivated the study of blends and composites as alternatives to conven- tional materials. Combining appropriate properties of two different materials, composites can provide very interesting characteristics making their study not aca- demic only but with real possibilities for applications in various devices. For example, electrically conduct- ing organic polymers are used for gas and vapor de- tection [1, 2]. Some metal-oxide-semiconductor thick films also are used as gas sensors [3, 4]. There are many possibilities to use polymers or blends or composites to improve the quality of devices. Nowadays rechargeable batteries using polyaniline as electrodes are commer- cialized [5]. Other possible industrial applications are: smart electronic window [6]; cable coating [7]; electro- magnetic and corrosion protection [8]. Superconducting materials are also investigated due to their electric and magnetic characteristics for tech- nological applications. In this case the Bi-system, particularly Bi:Pb:Sr:Ca:Cu:O/2223, is a well-known promising material to use like a superconductor ce- ramic [9–11]. In this work a composite using ceramic powder based on bismuth oxide and conventional insu- lating polymer based on vegetable oil was prepared and preliminary characterization of the sample shows that combining the properties of two materials the resulting composite shows flexibility and mechanical resistance plus electric conductivity, at room temperature, nine orders of magnitude higher than pure polyurethane. Thick green powder was obtained from solution made of bismuth, lead, strontium, calcium and copper nitrate in dilute acid, using stoichiometric composition [(1.6; 0.4):2 : 2 : 3], after increasing the temperature of the solution very slowly and maintaining it for a long time around 373 K to evaporate the liquid components. To eliminate the organic compounds the mixture was treated at 773 K. The Bi-2223 phase that has transition superconducting temperature of 110 K is present in this powder that was submitted to various steps of calcina- tion, around 873 K for 12 h. After structural and electric characterization, the pellets that showed the best results were pulverized into powder form. The average size of the ceramic grain is less than 1 μm [12]. The powder is mixed with a polyol based on cas- tor oil. To this homogeneous solution was added diiso- cyanate (MDI). The composite film was prepared onto glass plate and after polymerization it was immersed in pure water to remove the sample [13]. After cutting it to the appropriate size, an aluminum electrode was evap- orated onto both sides of the sample to make electric contact. The sample composition is 10 wt % of ceramic and 90 wt % of polymer. The electrical conductivity of the composite sam- ple was measured in a LF Impedance Analyser Model 4192A from Hewlett Packard. The measurement was carried out at room temperature and 0.01 to 10 MHz frequency range. The electrical resistivity measure- ment was carried out using a d.c. four-probe method. The sample was cut in rectangular from (1.0 × 2.0 × 10.0 mm) and four electrodes using conductor silver were painted with specific distance between then. The current source model 228A and nanometer model 181 both of Keithley Instruments have been used together with a computer system for automatic data acquisition. A silicon diode of LakeShore Cryotronics was used to measure the low temperature. A Rigaku X-ray diffractometer model RU-200B with CuK α filters was used to identify superconductor crys- talline phases of the ceramic and of the composite sam- ples. The pellets that showed a superconducting transi- tion were pulverized in an agate mortar and submitted to this technique. The composite sample was utilized in film format with area about 1.0 cm 2 . The DSM model 940 A from Zeiss was used to observe the structure of the composite film. Fig. 1 shows the real conductivity of the composite with 10 wt % of ceramic content as a function of fre- quency from 0.01 to 10 MHz range. The conductivity Figure 1 Electrical conductivity of composite as a function of frequency at room temperature. 0261–8028 C  2000 Kluwer Academic Publishers 603