Investigation of conductometric humidity sensors

Talanta ELSEVIER

Talanta 44 (1997) 1107-1112

Investigation of conductometric humidity sensors Jurgis Barkauskas Department o[ General and Inorganic Chemisto', Vih~ius University, L T-2734 Vilnius, Lithuania Received 24 January 1995: received in revised l%rm 31 March 1995; accepted 9 December 1996

Abstract Sensors for determining humidity in air have been described and investigated. Sensing film of the devices was prepared from polyvinylalcohol and graphitized carbon black disperse phase. The composition, thermal treatment and design of sensing films were investigated and optimized. An optimized humidity sensor has better metrological parameters as compared with its prototype (response time ~ 45 s, detection limit 0.17%, slope 6.25 + 0.05 ~)/R.H., standard deviation of measurement 0.15%, standard deviation of analytical signal in the graduation equation 8.29f~). Such construction of sensors have prospects in analytical practice. © 1997 Elsevier Science B.V.

Keywords: Air: Conductometric humidity sensors; Graphitized carbon: Polyvinylalcohol

!. Introduction Looking through the evolution in field of humidity sensors during the last years devices based on varying of resistance stand out. Majority of this kind are thin hydrophile polymer films sometimes including a disperse phase of first-class conductor [1-3]. The conductivity of thin film changes owing to the swelling process of hydrophile polymer layer [4]. First-class conductor dispersed in the polymer matrix improves metrological parameters of the sensor. There the resistance of thin film is significantly lower and the use of more simple and reliable measuring equipment is possible [5]. These sensors are used for measuring humidity in gaseous, liquid and solid phases [6,7]. Except named positive features these sensors are not lack of shortcomings limiting their appli-

cation. The most distinct are: relatively long response time, hysteresis and high level of uncertainty. Notwithstanding, only a few sources directly dealing with metrological improvement problem its possible to discover [8,9]. Getting clear that further development of polymer-film sensors with first-class conductor will advance only after elimination of enumerated shortcomings this work is an attempt to make a step in this direction.

2. Experimental Resistance of thin polymer film was measured using constant current ohmmeter (model V7-38, running voltage 1.0 V) and registered with recorder N 307. Thin polymer film (thickness of 30 Jam) was formed by means of spin-coating

0039-9140/97/$17.00 ¢~ 1997 Elsevier Science B.V. All rights reserved.

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J. Barkauskas / Talanta 44 (1997) 1107-1112

(device model OPn-8). The constant temperature in the system was maintained using thermostat MLW-UH. The air stream saturated up to fixed humidity was transported to the sensor by means of microcompressor (model AEN-3). Artificial aging of the sensors was carried out in device SKVS4.5.4, 54/ZI-1. This device consists from an insulated camera with controlled level of relative humidity (R.H.) and temperature inside. Sensors were kept at the R.H. 100% and temperature 50°C in this camera for 10 h. The chemicals used were analytical grade. Polyvinylalcohol, 10%, (PVA) (type PVS; av. tool. mass 60 000) solution was prepared after swelling and dissolving at 60°C. Graphitized carbon black (CB) for conducting disperse phase was synthesized according to Brauer [10] from gaseous carbon monoxide at 600 C using Fe as catalyst and purified by means of permanent hot extraction with HC1. The grain size of CB synthesized under above mentioned conditions is ~ 15 nm [10]. Dispersion of CB in PVA was observed using light microscope (Leitz-Wetzlar, type 307-107.002). There the conglomerates of inhomogeneity reaches 1.0-2.0 ~tm. Connection between the sensor's design and metrological parameters was examined. For that purpose sensors of different structure were prepared (Fig. 1A, B). Both kind consisted from two electrodes and thin sensing film. The sensors were designed on the glass plate. Two electrodes were made from A1 foil. In one of these modifications electrodes were coated with transition layer of high-concentrated CB dispersion in PVA, treated at 300°C (Fig. 1B). Etched glass (glass treated with 10% Na2SiO3 solution up to 300°C for 6 h; Fig. 1B) and heated PVA (at 300 C) sublayers were used to improve adhesion between polymer film and substrate. The morphology of etched glass was observed by means of light microscope. Surface roughness of etched samples reaches 0.5 ram, dimensions of chaotically scattered crystallites varies within 0.5-5.0 pm. Thin films heated at 300°C show gradual discoloration, embrittlement and reduction of water absorbency. There the polymer exists in cross-linked state incapable for swelling [11]. Thin films of PVA formed from aqueous solutions well when placed in contact

with humid air. The state of polymer there is thermoplastic and swelling process is reversible. Humidity sensing CB/PVA film was prepared by means of spin-coating at 1000 turns min ~. In the process of calibration, the stream of air was saturated up to indicated value of relative humidity (R.H.) by means of blowing it through bubblers (stream velocity 25 cm min - 1). The bubblers were filled with solutions of salts and in common with measurement cell were placed into thermostat at 20 C. Salts for solutions used and R.H. reached (pointed out in blackets) were as follows [12]: K2SO 4 (99); ( N H 4 ) 2 S O 4 ( 8 1 ) ; N H 4 N O 3 (67); NaHSO4" H20 (52); CaC12.6H20 (33); LiCI.H20 (15); H2SO4 (0). Measurements were repeated five times.

3. Results and discussion

Parameters most often used for metrological description of humidity sensors are as follows [13]: response time (r), detection limit (AX, m), slope + confidence limits (b + Eb), standard deviation of measurement (ax), standard deviation of analytical signal in graduation equation (av). The slope was linear in all cases inside investigated R.H. interval (Table 1). Named parameters are sensitive to the composition and design of humidity sensor and could be used for the optimization. The contact quality between polymer film and substrate directly acts on the analytical signal. In the case of poor contact the sensor gradually becomes degraded at that time increasing the uncertainty of results (Table 1). Sublayers positively affect on the quality of sensor. This fact is evident comparing metrological parameters of the sensors after artificial aging: the one without sublayer degrades. There we can draw and conclusion that the sensors including sublayers are more stable in time. Heating of PVA reduces the hydrophobity and swelling capacity of the film due to forming of cross-linked structure [11]: -CH~-CHI O-H

[-H~O] H-O

{

-CH-CHz-

>

-CH2-CHI O

{

-CH-CH~-

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J. Barkauskas / Talonta 44 (1997) 1107 1112

A I. O0 c m

[ELectrodes ] Filled PVA/CB layer ISubstrate I

~ - ]Substrate I

~-- ~ec,rode,/

Side view

T o p view

B i

I. O0

,"m r'¢~--~

~-['ra.s~t'o. layer I

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/ [Etc~dgZ~',s I Etched glass [ Fig. I. Design of humidity sensors. (A) humidity sensing film consisting from CB dispersion in PVA matrix, (B) including composite two-layer humidity sensing film (optimized design of humidity sensor).

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J. Barkauskas / Talanta 44 (1997) 1107- I 112

Table I Metrological parameters of humidity sensors prepared by different ways Changed detail or construction and its parameters Sublayers Without sublayer Aged sensor Etched glass Etched glass + aged Heated PVA sublayer PVA+aged Heated sensing film (temperature, °C) 100 170 240 300

Resp. time (~);

Detection limit

s

(AXiom); %

Slope and confidence limits (b ,+ %); f~/R.H.

Stand. dev. of signal (av); f~

Stand. dev. of measurand (Crx); %

50 240 60 65 60 60

1.17 43.4 0.80 1.94 0.38 0.64

l 19.50 40.48 121.70 107.37 177.43 164.20

_+ 1.36 _+ 5.31 _+ 1.02 _+ 1.43 __+1.13 ± 1.40

O.98 36.3 O.67

58.4 220.1 43.8

65 30 40 40 40

1.39 0.58 4.64 8.04 14.8

34.91 10.54 4.67 0.31 0.23

_+ 0.42 _+ 0.05 ± 0.02 ,+ 0.01 ,+ 0.01

1.17 0.49 3.88 7.15 12.4

18.03 2.14 0.86 0.42 0.42

120 60 65 40 45

>20 2.21 1.54 -0.17

12 573 +_ 542 1842 ,+ 16 244.5 _+ 2.5 10.7 ,+ 2.0 16.25 _+ 0.05

> 18 1.73 1.30 > 80 0.15

23 265 687 107 86 8.29

1.62

61.3

0.32 0.54

48.5 60.0

CB amount in sensing film (%) 53 57 59 66

Two-layer sensing film (Fig. 1 B)

Statistics and other results accompanying the heating of sensing film at various temperatures are presented in Table 1. These data show strict correlation between heating temperature and standard deviation of analytical signal. This could be explained taking into account interaction between CB grains and PVA matrix. At lower temperatures the chemical interaction is weak and CB grains are not fixed in the polymer matrix very steady. Working with humidity-sensing films prepared under these conditions we observe standard deviation of analytical signal reaching significant values. One of the possible explanation of this phenomena could be that during multiple process of swelling and contraction of the polymer matrix CB grains are affected by the irreversible process of spatial orientation. This could occur where bonds between PVA molecules and CB grains are weak. The conjecture is affirmed by the fact that standard deviation of humidity-sensing film prepared at higher temperatures reduces to insignificant values. Taking into account that under

higher temperatures - O H groups from PVA chain reacts with surface of CB grains (see reaction above), these grains being fixed more steadily in the polymer matrix. Under these conditions we could expect that the irreversible process of grains spatial orientation has no place. The swelling capacity of humidity-sensing films, treated at higher temperatures, is reduced as well as their sensitivity. Detection limit being the function of slope reaches minimum values after heating at IO0°C. The amount of conducting CB phase in polymer matrix affects the metrological parameters of humidity sensors too (Table 1). In the case of less amounts CB grains are insulated by PVA matrix and resistance of the film is high. The standard deviation of analytical signal there is significant apparently due to lower reliability of equipment as well as to high values of resistance and less amounts of microcontacts between CB grains. In the case of greater amount when carbon grains prevails over polymer in the bulk of sensing film

J. Barkauskas /Talanta 44 (1997) 1107 1112

sensitivity should be improved. To solve this problem is possible using composite sensing film with separated CB and PVA layers. • Quality of adhesion between sensing film and substrate affects on the sensor's metrological parameters (Table 1). For better adhesion the sublayer of etched glass is used. Taking into account these notes above design of humidity sensor has been changed (Fig. 1B). New one included composite two-layer humidity sensing film. Additionally a sublayer of etched glass and inert transition layer on the electrodes were formed. Metrological parameters of the sensor are given in Table 1. Response transient and calibration curve of this sensor are presented comparing with analogous data of other sensor in Fig. 2. It's evident, that precision and sensitivity are increased using two layer humidity sensors. Thus, the conclusion about correction direction in sensor's design optimization could be done.

1 R max

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

R(90%)

/ A

/

,

o~

i

2o

~i

40 tresl3

i 60

i 80 time

i

lOO~

1111

i lOO (S)

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Acknowledgements 0

20

40

60

80

100

R.H. (%)

Fig. 2. Response transients (A) and calibration curves (b) of two-layer humidity sensor (1) and heated (at 100°C) sensing film (2).

The author wishes to express his gratitude for Lithuanian Innovation Center (LIC) for the financial support.

References the sensitivity to humidity falls abruptly. Mass, 57 59, percent amount of CB in PVA matrix is an optimum for humidity sensing films. Design of this humidity sensor has some joints we could distinguish as 'weak points' in the circuit of resistance measurement. Those are (see Fig. IA): • Construction allows direct interaction between electrodes, e.g., by reaching a dew point. Electrode material should be insulated using an inert coating. Chemical inactive CB coating is used for improvement the sensor's quality. • CB grains in PVA bulk are scattered and insulated by the polymer. During the swelling process positions of CB grains changes in three-dimensional coordinates. Limiting this process in two-dimensional coordinates the

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J. Barkauskas / Talanta 44 (1997) 1107-1112

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