Preparation and characteristic of the thermistor materials in the thick-film integrated temperature�/humidity sensor

Jing Huang a,*, Yongde Hao a, Hong Lin b, Daoli Zhang a, Jiaojiao Song a,Dongxiang Zhou a

a Department of Electronic Science and Technology, Engineering Research Centre for Functional Ceramics, MOE, Huazhong University of Science and

Technology, No. 1037 Luoyu Road, Hongshan District, Wuhan, Hubei 430074, People’s Republic of Chinab Shenyang Institute of Instrumentation Technology, No. 242 Beihai Street, Dadong District, Shenyang, Liaoning 110043, People’s Republic of China

Received 14 June 2002; received in revised form 23 September 2002; accepted 21 October 2002

Abstract

The designing, preparation, and characteristic of the thermistor materials in a thick-film integrated temperature�/humidity sensor

were investigated in detail. This article presented the relationship between resistance�/temperature characteristics and the

composition of the negative temperature coefficient thermistor materials, which matched with humidity sensitive material in the

integrated sensor. The effects of the fabrication processing on the R �/T characteristic and the material granularity, granularity

uniformity were presented. Finally this article provided some experimental data of the thermistor materials and the characteristic

curves of the thick-film integrated temperature�/humidity sensor prepared from the material.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Integrated temperature�/humidity sensor; NTC thermistor materials; Slurry

1. Introduction

The thick-film integrated temperature�/humidity sen-

sor, in which the thick film thermistor and the macro-

molecule humidity sensitive resistor are fabricated in a

simple ceramic body, not only can measure the tem-

perature and humidity at the same time but also reduce

the volume along with enhancing the measuring preci-

sion by compensating the humidity sensor through the

negative temperature coefficient resistor (NTCR) di-

rectly. The thick-film temperature�/humidity sensor has

many traits such as little volume, high-speed response,

high sensitivity, simple structure, and convenience to

use, suitable to volume product and with good fore-

ground [1,2]. This article is based on the key points such

as the characteristic, designing and the fabrication

technology of NTC materials, and the fabrication

technology of the thick film.

2. Experiment

2.1. Design of NTCR material

NTCR material was mainly mixed with some transi-

tion metal oxides such as Mn, Co, Ni according to

proper proportion and sintered at high temperature,

during which the spinel phase with the thermistor

characteristics was formed. In order to make thick

film slurry, the powders had to be grinded till the grain

size ranged around 0.1�/10 mm. We could obtain

complete spinel phase only if we controlled and adjustedrestrictly the composition ratio, grinding time and

sintering parameters and ensured thermistor composi-

tion to have favorable chemistry and electric stability.

The properties of slurry had direct influences on the

characteristics of thick-film thermistors. It was mainly

the mixture of thermistor powder, glass binder, organic

medium and electroconductive phase powder, which

were sufficiently mixed to definite proportion. Thechemical composition and line-coefficient of the glass

binder, the soakage performance of the thermistor and

electroconductive powder had an important influence

* Corresponding author. Tel.: �/86-27-875-42894; fax: �/86-27-875-

42994.

E-mail address: syhj_sy@163.com (J. Huang).

Materials Science and Engineering B99 (2003) 523�/526

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0921-5107(02)00547-0

on the film quality and adhesion intension of thethermistor film and the stability of thermistors. Electro-

conductive powder mainly contained noble metal mate-

rials. It could adjust the specific resistance of thermistors

at definite range and improve the stability of thermistors

along with reducing electronics noise of them. The

organic medium must have moderate fluidity, aiming

to not only ensure good prompting performance during

printing slurry on silk screen, but also guaranteecomplete volatilization and burning out during drying

and sintering process. The viscosity of slurry, thermistor

constant, specific resistance and stability could be

adjusted by the composition of the slurry.

2.2. Fabrication of NTCR material

2.2.1. Fabrication of powders

The studied thermistor powders could be prepared by

chemistry precipitation process, solid reaction and

nitrate decomposition process, respectively. We investi-

gated the powders by the above three methods through

X-ray diffraction. Comparing the resulting spinel phases

with each other, and then changing the powders, wefurthermore fabricated them into thick film.

2.2.2. Relationship between powder composition, material

constant and specific resistance

For Mn�/Co�/Ni three components system, one way

was to verify the relationships between material con-

stant, specific resistance and the composition of materi-als by changing their ratios. The other was to add

additive RuO2 in order to improve the electric perfor-

mance of powder.

2.2.3. Influences of powder size on thick film thermistor

performance

The size of powders was related to grinding time.

During grinding procedure, we took out powder at

interval and made the same composition into slurry.

Then sintering into thick film thermistor, thus we could

find out the influences of the powder size on thermistor

materials through measuring the square resistance Rs .

3. Results and analysis

In the solid reaction process, the specified ratio Mn,

Co, Ni, Rn series compounds are mixed, formed, fired

and pulverized, to get temperature-sensitive powders.The powders show good temperature-sensitive proper-

ties for the high temperature solid reaction. Thus the

used powders for experiment were prepared from solid

reaction process.

3.1. The relationships between material composition and

material constant B and resistivity r

Tables 1 and 2 were the experimental results for the

relationships between material compositions and mate-

rial constant B and resistivity r . Experiments showed

that additives could obviously diminish specific resis-

tance and B , what is more, reducing of B is lesscompared with specific resistance.

3.2. Powder size versus thermistor performance

The relationship curve of square resistance between

grinding time (t) for thermistor powders is shown in Fig.1. From Fig. 1 we can see that the number of thermistor

powders was increasing as granularity reduction, along

with better distribution of glass phase. Furthermore, the

Table 1

The relationships between material constant B and Mn�/Co�/Ni three component system

Number Composition of Mn:Co:Ni Specific resistance r25 (V cm) Material constant B (K)

1 45.0:29.0:26.0 490.0 3280

2 48.0:39.0:13.0 450.0 3207

3 50.0:30.0:20.0 314.3 3155

4 55.0:30.0:15.0 353.7 3410

5 33.3:33.3:33.3 2399.0 2860

6 74.0:20.0:6.00 52 400.0 3770

Table 2

The relationship between material constant, specific resistance r25 and Mn�/Co�/Ni system, with additive RuO2

Number Composition of Mn:Co:Ni Specific resistance r25 (V cm) Material constant B (K)

1 50:20:15:15 40 2502

2 50:20:10:20 60 2482

J. Huang et al. / Materials Science and Engineering B99 (2003) 523�/526524

ratio of electroconductive chain was up while square

resistance was down, when the grinding time was up to

t3, the grain sizes reduced to utmost, the grain sizes and

square resistance no long changed with prolonging

grinding time, namely saturation condition.

The sintered and grinded thermistor powders were

blended with Ag powder, RuO2 powder according to

following composition: thermistor powders 60�/90%,

glass powder 10�/30%, Ag powder 5�/10%, and RuO2

powder 5�/10%.The blended powder was mixed with

organic binder (ratio: 70�/80%, 20�/30%) by milling with

commix for 8�/12 h. Thermistor results were shown in

Table 3 and Fig. 2.

The following equation could help to verify the R �/T

characteristics of NTCR [4]

RT �R0 exp

�B

T

�(1)

Both sides were taken logarithms and we obtained

B�T1T2

T1 � T2

lnR2

R1

(2)

aT ��B

T2(3)

The value of B calculated according to Eq. (2) was �/

2295.09.

Fig. 1. The relationship between square resistance and grinding time

of thermistors.

Table 3

The results of resistance�/temperature characteristics of thermistor

Temperature (8C) 0 23.9 42.3 55.1 69.8

Resistance (V) 3208.4 1629.1 1072.1 805.5 597.4

Fig. 2. Curve between resistance and temperature of thermistor.

Fig. 3. The humidity-sensitive proety curves of sensors.

Table 4

The temperature properties of copolymer humidity sensors

Temperature (8C) Humidity/resistance (V)

11%RH 33%RH 55%RH 75%RH 95%RH

20 1762 321 31.7 10.2 2.21

1660 302 28.2 8.86 2.21

0 2116 357 32.2 (60%RH) 10.63 2.45

25 1590 157 29.7 (53%RH) 8.99 1.81

50 569 90.7 21.4 5.71 1.32

70 116 28.2 10.8 (50%RH) 3.51 �/

J. Huang et al. / Materials Science and Engineering B99 (2003) 523�/526 525

3.3. Influence of materials on characteristic of sensors

The thermistor prepared from the studied materials in

the present paper could have another particular functionthat NTCR was able to directly compensate temperature

for humidity sensors. In the humidity-sensitive element,

there commonly exists temperature shift which has the

bad influences on measuring precision [3]. The resistance

of humidity sensor was related to temperature by

exponent, which was due to saturation�/vapor-pressure

curve and material itself. On the other hand, the

temperature characteristic of NTC thick-film thermistorwas negative exponent curve. By adjusting parameters,

we could carry out the desired temperature compensa-

tion for temperature shift of copolymer humidity-

sensitive components and enhance measuring precision

of humidity with the volume reduction of elements. The

temperature properties of copolymer humidity sensors

are showed in Table 4 and Fig. 3. When the descent

trend of NTC thermistor component was close to that ofpolymer humidity-sensitive element, we could put them

on two bridge arms of the bridge circuit, respectively, to

achieve good temperature compensation.

4. Conclusion

Thermistor material design and preparation method

had been investigated for thick-film integratedtemperature�/humidity sensors. The results clearly in-

dicated that sensitive performance of thermistor materi-

als was obviously improved. Thus we increased its log-

term stability, and then achieved effective temperature

compensation for thick-film temperature�/humidity sen-

sors.

References

[1] Hitachi Manufacture Co. Ltd, Temperature or moisture-sensitive

element has pair of temp. sensors and humidity sensor with heat-

cleaning function integrally formed on mutual Insulating substrate

JP58105046.

[2] Shinyeikaisha Co. Ltd, Humidity sensor with temperature com-

pensation thermistor C8-TM3 RHU-215.

[3] T. Masumoto, IEEE Vol CHMT-6, 1 March 1983.

[4] D.X. Zhou, B.R. Li, X.L. Zhang, Semiconducting Ceramics and

Their Applications, Huazhong University of Science and Technol-

ogy Press, Wuhan, 1991.

J. Huang et al. / Materials Science and Engineering B99 (2003) 523�/526526