INTERNATIONAL MICROWAVE POWER INSTITUTEʼS

40TH ANNUAL SYMPOSIUM

PROCEEDINGSISSN: 1070-0129

AUGUST 9-11, 2006BOSTON, MASSACHUSETTS, USA

©International Microwave Power Institute, 2006. All rights reserved.

AN IMPI PUBLICATION

© International Microwave Power Institute, 2006 327

INTRODUCTION

Positive temperature coefficient (PTC) thermistors are widely utilized as current surge protectors, refrigerator motor starters, temperature sensors, etc. PTC elements mainly consist of BaTiO

3 and SrTiO

3 doped with Y

2O

3,

SiO2 and Mn(NO

3)

2. The PTC effect is highly influenced

by processing conditions such as sintering temperature, soaking time, cooling rate, and so on. The electrical characteristics of PTC resistors, for example voltage with-standing ability, large current impact property and PTC aging behavior are all affected by the microstructure of PTC resistors. Conventionally, PTC thermistors are sintered in resistant furnace at 1300-1375°C for 30-120 minutes. Microwave energy has been widely used to process ceramics and other materials [Agrawal, 1998][Agrawal, 2005]. Compared with conventional process, microwave process has some potential advantages such as high heating rates, shorter sintering time, lower sintering temperature, fine microstructures etc. [Agrawal et al, 2005][Agrawal, Cheng and Roy, 2004] Therefore, the feasibility of microwave processing of PTC thermistors was investigated and results are reported in this paper. The influence of processing conditions on microstructures and electrical properties

was studied in details.

Key words: microwave processing, thermistors, PTC effect, microstructure, BaTiO

3

Microwave sintering of positive temperature coefficient (PTC) thermistors was carried out in a 2.45 GHz microwave furnace without using any susceptor. Heating behavior, microstructure and electrical properties of the sintered samples were investigated. In the temperature range from 1275°C to 1350°C, optimum density and microstructures were achieved between 1300°C to 1325°C. Samples were also microwave sintered at 1300°C for various soaking times from 10 min to 90 min. It was found that soaking between 30 min to 60 min generated an uniform microstructure. Study also showed that the cooling rate has an influence on PTC effect but not on microstructure. Compared to conventional method, microwave processed samples exhibited better PTC effect.

EXPERIMENTAL

A 2kW industrial tube microwave furnace was used to sinter PTC samples. The green PTC specimens mainly consisting of BaTiO

3 and SrTiO

3, were circular pellets,

10.1 mm in diameter and 2.85 mm in thickness. For each run, 5 pellets stacked together were used. No microwave susceptor was used to pre-heat PTC samples. To test the heating behavior of the PTC materials, the output power was set at 2kW during the microwave sintering of PTC samples. All samples were sintered in ambient atmosphere at 1275°C, 1300°C, 1325°C, and 1350°C, respectively, with a 60-minute soaking time and cooling rate of 7.5°C/min. The cooling rate was controlled from sintering temperature to 800°C. Below 800°C, samples were allowed to cool naturally to room temperature. At the sintering temperature of 1300°C, the samples were soaked for 10min, 30min, 60min and 90min, respectively and cooled at 7.5°C/min. In order to check the influence of cooling rate on the properties, samples sintered at 1300°C/10min were cooled at 7.5°C/min, 3.75°C/min, 2.5°C/min, and 2°C/min, respectively. For comparison, some samples were also sintered by conventional method at 1300°C/60min with the cooling rate of 7.5°C/min. Scanning electron microscopy (SEM) was used to examine the microstructure of the sintered samples. X-ray diffraction (XRD) was used to identify phase composition of the samples sintered both by microwave and conventional methods. Samples were screen-printed with two layer electrodes: the bottom layer was ohmic silver-zinc electrodes and the top layer was silver electrodes. The R-T curves of PTC elements were measured by PTC resistance-temperature automatic measuring system.

MICROWAVE PROCESSING OF BATIO3-BASED

PTC THERMISTORS

Ming Fu1, Dinesh Agrawal2, and Yi Fang2

1Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan, China1Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania, USA

328 40th Annual Microwave Symposium Proceedings August 9-11, 2006 © International Microwave Power Institute, 2006 329

RESULTS AND DISCUSSION

PTC heating behavior

PTC samples were heated by microwave energy at a constant power from room temperature to the sintering temperature, without any pre-heating susceptor. The temperatures at and above 400°C only were recorded, because the limitation of IR pyrometer. The heating curve is shown in Figure 1. The heating curve shows that PTC materials absorb microwave energy very well. The heating rate rose with the increase of temperature. The heating rate was 30°C/min until 400°C, and 56°C/min until 1200°C. The thermal run-away didn’t occur during heating process.

The power adsorbed per unit volume sample, P (W/m3), is given by [Sutton, 1989]

P = σ|E|2 = 2πfε0ε

r tan δ |E|2 (1)

where E (V/m) is the magnitude of internal field; ε

0 is the permittivity of free space; ε

r is the relative

dielectric constant; f is the frequency (GHz); σ is the total effective conductivity (S/m) caused by conduction and displacement currents. Equation (1) shows that the power absorbed varies linearly with frequency, relative dielectric constant, and tan δ, and varies with the square of electric field. PTC materials mainly consist of BaTiO

3 and SrTiO

3.

The ε and tan δ of PTC green pellets are 0.84×10-9 and 0.012, respectively at 10 kHz. The ε and tan δ of PTC ceramics are 0.36×10-6, 261.6, respectively at 10 kHz. During heating, the ε and tan δ of PTC materials increase rapidly with temperature, and this accelerates the absorption of microwaves [Agrawal et al, 2004]. Therefore, heating rate rises accordingly with temperature.

The influence of sintering temperature on microstructures

The SEM micrographs of samples sintered at different temperatures are shown in Figure 2. Sintering temperature varied from 1275°C to 1350°C, while the soaking time was kept at 60 min and the cooling rate at 7.5°C/min. Clearly, the sample sintered at 1275°C has more pores than other samples. The microstructures of samples sintered at 1275°C and 1350°C are not uniform. Besides, the sample sintered at 1350°C exhibits an

Figure 1. Heating curve of PTC samples during microwave heating

Figure 2. SEM micrographs of samples sintered at different

temperatures (a) 1275°C; (b) 1300°C; (c) 1325°C; (d) 1350°C.

Figure 3. SEM micrographs of samples sinered at different

soaking times (a) 10 min; (b) 30 min; (c) 60 min; (d) 90 min.

Figure 4. SEM micrographs of samples microwave processed at (a) 7.5ºC/min; (b) 3.75ºC/min;

(c) 2.5ºC/min; (d) 2ºC/min.

328 40th Annual Microwave Symposium Proceedings August 9-11, 2006 © International Microwave Power Institute, 2006 329

abnormal grain growth. Samples sintered at 1300°C and 1325°C are very dense and uniform. The density of samples (a), (b), (c), and (d) are 5.07, 5.32, 5.36, and 5.18g/cm3, respectively. Therefore, 1275°C is too low to prepare high density PTC ceramics, while 1350°C is too high, resulting in a non-uniform microstructure. The suitable temperature for microwave sintering PTC is 1300°C through 1325°C.

Effects of soaking time on microstructures

Samples were processed at various soaking times from 10 min to 90 min, while the sintering temperature was kept constant at 1300°C with the cooling rate of 7.5°C/min. The SEM micrographs of samples sintered at different soaking times are shown in Figure 3. The results show that the sample sintered for 10 min still has some porosity and that sintered for 90 min has a secondary grain growth. Samples sintered at 30 min and 60 min are uniform and dense. The densities of samples (a), (b) (c) and (d) are 5.10, 5.27, 5.32, and 5.43g/cm3, respectively. Therefore, the optimal soaking time for microwave processing PTC is 30min to 60min.

Effects of cooling rate on microstructures and PTC effect

The SEM micrographs of samples processed at different cooling rates are shown in Figure 4. Samples sintered for 10 min at 1300°C were cooled at 7.5°C/min, 3.75°C/min, 2.5°C/min, and 2°C/min, respectively. SEM shows that cooling rate did not significantly affect microstructure, but the porosity reduced with the increase of cooling time. The sintered densities of samples (a), (b), (c) and (d) are 5.10, 5.25, 5.30, and 5.34 g/cm3, respectively. PTC effect is mainly characterized by resistance temperature coefficient (α

T) calculated with equation (2):

α

T = 2.303 × lg(R

p/R

b)/( T

p - T

b) (2)

where Tp = T

c+25, T

b = T

c+15, R

p is the resistance at T

p,

and Rb is the resistance at T

b. Curie temperature (T

c) is the

temperature when R equals 2 times the room temperature resistance (R

25). The R

25 of samples (a), (b), (c) and (d)

are 5.6, 6.7, 10.4, and 12.7Ω, respectively. And αT of

samples (a), (b), (c), and (d) are 8.5, 10.9, 11.6, and 12.7, respectively. The results show that with the increase of cooling time, PTC effect is enhanced. PTC effect comes from the grain boundary barrier, and a slower cooling rate favors formation of a higher barrier. Therefore, a slower cooling rate is supposed to be chosen when PTC is sintered by microwave energy, as long as the R

25 of samples

matches

with the requirements of the specified applications.

Comparison of microwave and conventionally processed samples

Microstructure

The SEM micrographs of samples sintered by conventional and microwave methods are shown in Figure 5. Both samples were sintered at 1300°C/60 min, and cooled at 7.5°C/min. SEM shows that the conventional sample has more porosity than the microwave sample. The relative density (theoretical density is 5.7g/cm3

for PTC ceramics) of microwave sample was 93.5%, compared to 90.2% of the conventional sample.

PTC Effect

The PTC effect of the samples is reflected by the resistance-temperature (R-T) relationship. The main parameters of R-T curve is R

25, T

c, α

T, and resistance

jump orders (β). The R-T curves of both the microwave and conventionally processed samples are shown in Figure 6. The R-T characteristics show that R

25, α

T, β,

Tc, and R

max of the microwave sample are 17.6 Ω, 15.8,

6.7×105, 88.0°C, and 11.5 MΩ, respectively, compared to 12.1 Ω, 14.6, 1.1×105, 88.1°C, and 1.3 MΩ, respectively, for the conventional sample. β is the resistance rise ratio defined by

Figure 5. SEM micrographs of samples sintered by (a) conventional and (b) microwave process.

Figure 6. R-T curves of microwave and conventional samples.

330 40th Annual Microwave Symposium Proceedings August 9-11, 2006

β = Rmax

/Rmin

(3)

Rmax

is the maximum of sample resistance and Rmin

is the minimum of sample resistance. The experimental results show that under the same processing condition, the microwave samples have better PTC effect than that of conventional ones. This is mainly attributed to the better grain boundary structure of the microwave sample as shown in Figure 6(b). Phase identification

XRD patterns show that the samples sintered by both microwave and conventional methods have the same major phase composition: Ba

0.70Sr

0.30TiO

3, indicating that

microwave processing improved microstructure and PTC property but did not change the composition of the sintered samples.

CONCLUSIONS

PTC materials can be easily heated by microwave energy at 2.45 GHz. Compared to conventional process, microwave processing achieved a better PTC effect. The optimal processing parameters for microwave sintering PTC were found to be as the temperature in the range of 1300°C-1325°C and soaking time 30-60 minutes, with the cooling rate as low as possible.

ACKNOWLEDGEMENTS

We acknowledge Shenzhen Ampron Sensitive Components Co. Ltd. for providing PTC green pellets, and Wuhan Supernano Optoelec Technology Co. Ltd. for providing silver electrodes pastes for this study.

REFERENCES

Agrawal, D. K. (1998). “Microwave processing of ceramics: A review,” Current Opinion in Solid State & Mat Sci, 3 (5), pp.480-486.

Agrawal, D. (2005). “Microwave sintering developments spur emergence of new materials and technologies," Industrial Heating, 72 (6), pp.37-39.

Agrawal, D., J. Cheng, and R. Roy (2004c). “Microwave sintering, brazing and melting of metallic materials,” Proc. Intl. Symp. On Heating by Electromagnetic Sources, pp.717.

Agrawal, D., J. Cheng, M. Jain and G. Skandan, R. Dowding, K. Cho, B. Klotz, and D. Kapoor (2004). “Microwave sintering of tungstren and its alloys,” Eighth Intl Conf. Sci. Hard Mats., pp. 143-144.

Agrawal, D., J. Cheng, Y. Fang, and R. Roy (2005). “Microwave processing of ceramics, composites and metallic materials,” Am. Cer. Soc. Publ., pp. 205-228.

Sutton, H. W. (1989). “Microwave Processing of Ceramic Materials,” Ceramic Bulletin, 68(2), pp.376-386.