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in the structure and that functional groups are being titrated in the intermediate pH range. These functional groups may be derived from the slow hydrolysis of second phase materials resulting from impurities in the ceramic. For example, alumina could slowly hydrolyze to AlOOH (Kennedy, 1959) which would probably be titratable in the mid-pH region. Silica should also hydrolyze, but we would expect silicic acid to titrate in a more acidic region. Even in the absence of impurities, segregation of yttria in grain boundaries could have a similar effect since Y (0H)3is stable relative to Yz03in water under our conditions (Shafer and Roy, 1959). In contrast to the above, ZrOz is stable relative to the hydrolyzed forms (Nakamura et al., 1977; Tani et al., 1983). While we are apparently dealing with a real phenomenon here, it does not seem to be a critical one at present with our best ceramics. We have, however, seen similar behavior with other ceramics when the pause associated with the "titration" has been over an hour in duration. It is suggested that this type of behavior, rather than an "acid error" may have been the cause of the observations of Tsuruta and Macdonald (1982). Conclusions The use of zirconia membrane sensors for the determination of the pH of high-temperature solutions is a viable application. Such sensors may also be useful for lower temperature measurements in highly alkaline media because of the absence of the "alkaline error" associated with glass electrodes. While the characteristics of selected ceramics are adequate for present applications, slow degradation of response time is observed with even the best. It appears likely that further improvement will be possible with additional attention to purity of materials and detail of fabrication.
599
1983,22, 599-603
It would also be desirable to achieve higher conductivities. This might be achieved through the substitution of alternative stabilizers for the present yttria, perhaps scandia. It appears less likely that alternative systems based on ceria or bismuth oxide will prove attractive because of the aggressiveness of high-temperature aqueous environments. Registry No. Zirconia, 1314-23-4; yttria, 1314-36-9.
Literature Cited Danielson, M. J. Corrosion 1979, 35, 201. Dell, R. M.; Hooper, A. I n "Solid Electrolytes", Hagenmuller, P.; Vangool, W., Ed.; Academic Press: New York, 1978; p 291. Etsell, T. H.; Flengas, S. N. Chem. Rev. 1970, 70, 339. Indig, M. E.; McIlree, A. R. Corrosion 1979, 35, 268. Kennedy, G. C. Am. J . Sci. 1959, 257, 568. Nakamura, K.; Hirano, S . ; Somiya, S. Am. Ceram. SOC. Bull. 1977, 56, 513. Nledrach, L. W. Science 198Oa, 207, 1200. Niedrach, L. W. J . Nectrochem. SOC. 1980b, 127, 2122. Niedrach, L. W.;Stoddard, W. H. "The Development of a High Temperature pH Electrode for Geothermal Fluids. Final Report-Task I " , prepared for the Pacific Northwest Laboratory operated by Battelle Memorial Institute under prime contract No. DE-Ac-06-76-RLO-1830 for the United States Department of Energy, Division of Geothermal Energy, Report No. PNL 3857 UC-66, Mar 1981. Nledrach, L. W.; Stoddard, W. H. "The Development of a High Temperature pH Electrode for Geothermal Fluids. Final Report-Task I11 and Year-End Summary" prepared for the Pacific Northwest Laboratory operated by Battelle Memorial Institute under prime contract No. DE-AC-06-76-RLO1630 for the United States Department of Energy, Division of Geothermal Energy, Report No. PNL 4651, Feb 1983. Shafer, M. W.; Roy, R. J . Am. Ceram. SOC. 1959, 42, 563. Tani, E.; Yoshimura, M.; Somiya, S. J . Am. Ceram. SOC. 1983, 66, 11. Tsuruta, T.; Macdonald, D. D. J . Nectrochem. SOC. 1982, 729, 1221. Vetter, K. J. "Electrochemical Kinetics", Translated by S. Bruckenstein and B. Howard, Academic Press: New York, 1967; p 17 ff.
Receiued for review May 10, 1983 Accepted May 28, 1983 Presented a t the 85th Annual Meeting of the American Ceramic Society, Chicago, IL, Apr 26, 1983.
High-Temperature Electrical Conductivity of Rigid Polyurethane Foam Ralph T. Johnson, Jr. Sandia National Laboratories, Albuquerque, New Mexico 87 185
The electrical conductivity of a rigid polyurethane foam, used for electronic encapsulation, has been measured during thermal decom osition to 340 O C . The conductivity below -150 O C is (ohm-cm)-l but it increases dramatically to IO- (ohm-cm)-' as the temperature is increased to 260-270 OC. At higher temperatures the conductance continues to increase, but some of this increase is due to changes in sample geometry resulting from sample softening. Experiments in air and nitrogen environments showed no significant dependence of the conductivity on the atmosphere. The insulating characteristics are compared to those of phenolic- and silicone-based materials (glass-fabric and glass microsphere reinforced) which are used for electronic case housings and component encapsulation. The insulating characteristics to -200 O C for the polyurethane foam are similar to those of the silicones and are better than that of the phenolics. At higher temperatures (2270 "C)the phenolics and particularly the silicones appear to be better insulators.
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Introduction Rigid polyurethane foams have a number of important applications including encapsulants for electronics components. For situations where there may be an accidental exposure to a high-temperature environment, it is im-
portant to know how the electrical insulating characteristics of the foam degrade with temperature. This paper examines the change in the electrical conductivity of a rigid polyurethane foam, commonly used as an encapsulant, over the temperature range from 24 to 340 O C in air and ni-
0196-4321/83/1222-0599$01.50/0@ 1983 American Chemical Society
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trogen environments. The dependence of the sample current on voltage was measured as a function of temperature to determine the ohmic character of the conductance. Time-dependent changes in conductivity a t -210 "C were also examined. The results are compared with the electrical properties of other high-temperature insulating silicone and phenolic materials used for electronic case (or structural support) housings and a silicone material used for electronic encapsulation. These studies provide information regarding changes in electrical conduction associated with thermal decomposition. Experimental Section A polyester resin toluene diisocyanate based rigid polyurethane foam with a density of 290 kg/m3 was used in these studies. The material was prepared in the form of a slab of thickness 0.34 cm. Disk shaped samples ( - 3 cm diameter) were used for the electrical measurements. Electrodes for two-terminal conductance measurements were prepared using Ag paint. The electrodes were 1.0 cm in diameter and were placed on opposite faces of the sample. Electrical contact was made by using pressure-loaded platinum cylinders as shown in Figure 1. A three-terminal guard ring configuration was used for some experiments. The outer guard ring was on the periphery of the sample and was prepared with Ag paint. Experimental results showed no difference in conductance data obtained with the guard ring configuration indicating no detectable surface currents. This assured that the electrical properties of the bulk material were measured. For the experiments, the complete sample holder assembly (Figure 1) was placed in a tube furnace. The sample temperature was measured with a thermocouple in the sample support block. The temperature difference between the recording thermocouple and the sample was within 10 "C for the heating rate (2 OC/min) used in these experiments. For experiments in a nitrogen atmosphere, the system was maintained a t a gauge pressure of -70 kPa. The air atmosphere experiments were at atmospheric pressure with flowing dry air. The electrical measurement system utilized a 105-ohm resistor in series with the sample to limit short circuit currents. An automatic data acquisition system was used. Voltages of 100,300,600 and lo00 V (electric fields to -3 X lo3 V/cm) were sequentially applied to the sample for 2-6 s at each voltage setting as the temperature was increasing. Temperature and current readings were taken at each voltage setting. A constant voltage of 1000 V was used for the time-dependent measurements at constant temperature. The electrical conductivity was determined from l I 0 = k-AV where 1 is the sample thickness, A is the electrode area, I is the sample current, V is the applied voltage, and k is
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Figure 2. Sample current temperature dependence as a function of applied voltage for a rigid polyurethane foam. Dashed portion of curve denotes region where sample shape and electrode separation changed significantly, and reflects conductance changes observed under conditions of the experiment rather than material conductivity changes. 10-6-
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Figure 3. Conductivity temperature dependence for a rigid polyurethane foam in a nitrogen atmosphere, determined from the data in Figure 2. Dashed curve denotes conductance changes under conditions of the experiment rather than material conductivity changes, as noted in Figure 2 caption.
a geometric correction for fringing fields. For the geometry of Figure 1, lz was determined experimentally to have a value of -0.72. The accuracy of the conductivity data was found to be within a factor of 2 of the measured value. Results and Discussion The temperature dependence of the sample current for 100, 300, and 1000 V is shown in Figure 2. Analysis of the current-voltage characteristics at a given temperature showed a linear dependence of sample current on applied voltage, suggesting that conduction is ohmic in character. The temperature dependence of the conductivity is shown in Figure 3. The conductivity below 150 "C is (ohm-cm)-'. Above 150 "C, the conductivity increases dramatically with increasing temperature, approaching (ohm-cm)-' at -260 "C. In the 200-260
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Figure 4. Conductivity temperature dependence for rigid polyurethane foam in an air atmosphere. Dashed curve is for the first temperature cycle on as-prepared material to 210 "C. The solid curve is for the second temperature cycle on the same sample to 275
"C.
"C range, the sample softened, became porous, and was yellowish to brownish in color. At 260 "C there is a kink in the conductivity curve and at this point in the heating cycle there was significant sample distortion (puffed up appearance). As the temperature approached 340 "C, the sample became black from carbonizing and the electrodes pushed in to the sample, resulting in shorting between the electrodes. The large changes in sample dimensions and decrease in electrode separation make interpreting the data above -270 "C in terms of material bulk conductivity difficult. Therefore all the data above -270 OC (dashed portion of curves) should be interpreted as only conductance (not conductivity) changes observed under the conditions of these experiments. To determine if the electrodes retarded decomposition and affected the materials bulk properties, experiments were performed on electroded and unelectroded samples. The samples were annealed (temperature cycled) in air to 250 "C following the normal heating and cooling cycle experienced during the conductivity experiments. After this temperature cycle, Ag electrodes were put on the sample cycled without electrodes and the conductivity of both samples was measured. The results were identical with those in Figure 3. This showed that the electrodes did not significantly affect the electrical properties during decomposition. Experiments were also conducted to determine if the environment affected the conductivity results. Previous studies of silicone and phenolic materials showed that above 450-500 "C the presence of oxygen accelerated decomposition and significantly affected the electrical properties (Johnson and Biefeld, 1982,1983). The same effects were examined in this study on the polyurethane foam to 340 OC. Conductivity data to -260 "C taken on as-prepared material in nitrogen and air environments (Figures 3 and 4, respectively) showed the same conductivity temperature dependence. Data from similar experiments to 340 "C on annealed samples are shown in Figure 5. In these experiments the conductivity temperature dependence was measured after annealing samples with and without electrodes to 250 "C for 3 h. There was no significant difference in results obtained in air and
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i 1 1
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Figure 5. Conductivity temperature dependence for rigid polyurethane foam after annealing at 250 "C for 3 h in air and nitrogen atmospheres. There was no significant dependence of the conductivity characteristicson the annealing atmosphere (air or nitrogen).
nitrogen environments. There was also no difference in sample appearance after cycling to 340 "C in both nitrogen and air environments. All of these results suggest that the environment does not significantly affect the conductivity of these materials to 340 "C. The results in Figure 5 from the 250 "C, 3 h annealing experiments show a shift in the conductivity characteristics to lower temperatures and the elimination of the kink in the conductivity curve at -260 "C. The kink in the conductivity characteristics for the as-prepared material probably resulted from decomposition, in a manner similar to that observed on phenolic and silicone materials (Johnson and Biefeld, 1982, 1983). The effects of successive temperature cycles and the time dependence of the conductivity were also examined on as-prepared material. The data in Figure 4 were obtained from temperature cycling the sample twice. The first cycle was to 210 OC. (The time at 210 "C was -10 min.) After cooling to room temperature, the sample was cycled on the second run to 275 "C. The results show that the temperature-dependent data were the same within experimental error. The time dependence of the conductivity at 210 "C was examined in air over a period of 6 h (after an initial temperature stabilization period of -30 min). During this time the sample current increased by a factor of about 4. However, the material softened resulting in a factor of 2 decrease in the electrode separation. This suggests that the conductivity in air increased by no more than a factor of about 2 over a period of 6 h at 210 OC. The fact that the electrical properties do not change significantly with time suggests that the material remains a relatively good insulator below 200 OC. Between 200 and 300 "C, however, there are dramatic changes in both the conductivity (increasing in value) and materials physical properties (degradation). Comparison with Other Materials The conductivity temperature dependence for the rigid polyurethane foam is compared with that for Sylgard 184/GMB in Figure 6. The Sylgard material is a silicone-based encapsulant for electronic components. It is syntactic foam composed of Sylgard 184 (manufactured by Dow Corning) and hollow glass micropheres (manufactured by 3M Company). The conductivity data in
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
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Figure 6. Comparison of conductivity temperature dependence for rigid polyurethane foam and the silicone Sylgard 184/GMB.
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forcing. Although these materials decompose at high temperature, they generally retain their structural shape since they are stabilized by the glass reinforcing. The conductivity temperature dependence for the silicones and phenolics was obtained in the same manner as for the polyurethane foam except that a heating rate of 10 "C/min was used. A change in heating rate does introduce a change in results, but the magnitude of the changes in conductance are not of significant for the comparisons made in this study. The data in Figures 7 and 8 show that the insulating characteristics of polyurethane foam below -270 "C are similar to those for the silicones and are better than for the phenolics. Above 270 "C, it is difficult to compare the electrical characteristics of polyurethane foam with the phenolics and silicones because of uncertainties in the polyurethane data due to sample softening and electrode contact and shorting problems. However, taking into account the combined physical and electrical characteristics, the phenolics appear to be better insulators to at least 500 "C and the silicones to at least 600 "C. Conclusions Reliable material bulk conductivity data were obtained on a rigid polyurethane foam below -270 " C . The conductivity below this temperature is
