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Amperometric response of ambient temperature oxygen sensors based on insulating material-dispersed PbSnF4 layers
ELSEVIER
CHEMICAL
Sensors and Actuators B 24-25 (1995) 43&442
Amperometric response of ambient temperature oxygen sensors based on insulating material-dispersed PbSnF, layers Hironori Hanamoto, Department
of Industrial
Kazuhiro Tsuchiya, Jun Kuwano *
Chemisby, Faculty of Engineering, Science University of Tokyo, l-3 Kapnuaka,
Shinjuku-ky Tokyo 162, Japan
Abstract The response properties of amperometric oxygen sensors of the type AglA8&WO,lPbSnF,I insulating material (IM)-dispersed PbSnF,IPt, 02, are examined and the effects of the dispersed IMs are discussed. Powders of iron@) phtbaloqanine (FePc), oxides (ZrO,, A&OS, MgO), polymers (PVC, PTFE) and BN are used as IMs. The response properties depend on the material used as an IM and on the IM content. For example, when FePc is dispersed, the short-circuit current &) and 90% response time (rw) change similarly, having a maximum at 20 vol.% FePc and a minimum at 80 vol.%. For the oxides, there is an optimum oxide content at which tw and I,, take a minimum and a maximum, respectively. However, there is a common tendency that the dispersion of a proper amount of IM results in the improvement of t* A linear relation is found between 1, and PO2lR for the FePc-dispersed PbSnF.,, whereas I, increases linearly with Pm for the other IM-dispersed PbSnF,. The results suggest that the dispersed FePc acts as a catalyst for two-electron reduction of oxygen. Keyworck
Amperometric sensors; Oxygensensors; Insulating materials; Ambient temperature; Fluoride
1. Introduction Oxygen is the most important
element,
and its roles
in living organisms and technological processes have always attracted great interest. In various fields of science and technology, there is increasing demand for monitoring and controlling the concentration of oxygen gas under diversified conditions. However, commercially available oxygen sensors of galvanic-cell type and zirconia type have been unable to meet this demand sufficiently, because of the use of liquid electrolytes and high operating temperature, respectively. Against this background, several potentiometric and amperometric solid-state sensors based on inorganic fluorides have been developed [l-12]. The amperometric sensors [4-121 were superior to the potentiometric sensors in response time, operating pressure range and hysteresis. Recently, we have developed amperometric sensors of the type [P-11] Ag(AQaWO,(PbSnF&ensing
electrode, Oz
(1)
in which mixtures of iron(I1) phthalocyanine (FePc), platinum black and PbSnF, are used as a sensing electrode. The 90% response time (r& was shorter * Corresponding author. 0925-4005/95/$09.50 0 1995Elsevier SSDI 0925-4005(94)01391-T
Science S.A. All rights reserved
than 1 min at room temperature. The sensors were capable of measuring oxygen concentrations in a wide range of oxygen partial pressure (Pm), 2 kPa-7 MPa, where commercial oxygen sensors cannot be used. The sensing reaction was assumed to be as follows: 0,+2Ag+2F-
=20-
+24gF
A previous paper 1121has shown that some noble metals and carbon materials seme as a substitute for platinum black, and that their chemical nature rather than their physical properties, such as specific surface area, influences the response properties. However, the measuring principle and the roles of the dispersed FePc are not yet fully understood. In this study, we focus on dispersed FePc with a very low electrical conductivity and make sensors of the type Ag/A&I,WO.,IPbSnF,)IM-dispersed PbSnF,I sputtered Pt, 0,
(2)
The insulating materials (IMs) are FePc, ZrOz, A1203, MgO, BN, PTFE [poly(tetrafIuoroethylene)] and PVC [poly(vinyl chloride)]. For simplicity, in sensors of type (2) the sensing electrode of sensors of type (1) is separated into an IM-dispersed PbSnF, layer and a
H. Hanmwto
et al. f Sensors and Actuo~~rs B 24-25 (1995) 438-442
sputtered Pt electrode according to their functions. The amperometric response is examined as a function of the IM content. As a result, we find that the response properties are variously influenced by the materials dispersed in PbSnF, and by their content. The effects and roles of the IMs dispersed in PbSnF, are discussed for improvement of the response characteristics.
2. Experimental The IMs used are listed along with their tap densities (d,) and mean particle diameters (D,,,) in Table 1. The IM-dispersed PbSnF,, mixtures were prepared by mixing the powders of PbSnF, and one of the IMs. The volume percentages of IMs were calculated from their d,. The fast ion conductors a-PbSnF, and Ag&W04 were synthesized by a modification [6] of the precipitation reaction [13] and by the solid-state reaction [14], respectively. Figs. 1 and 2 show a photograph of one of the sensors and the schematic structure, respectively. A six-layered pellet (8 mm in diameter, 2 mm thick) was made by pressing and encapsulated in a PVC case fitted with two leads. A platinum sensing electrode was sputtered onto the whole surface of the IM-dispersed PbSnF, layer. The details of the assembly were described in previous papers [5,6,9,11]. Three of the sensors were fixed to a thermostatted test chamber with a capacity of 2.9 cm3. The short-circuit current (1,) response was simultaneously measured with ammeters (Toa PM-18U) under a dry 0,-N, gas flow of 100 ml min-I. The mixing ratio was changed stepwise with a pair of thermal mass-flow controllers (Kojima 3610). Prior to the measurements, the sensors were aged as short-circuited for about 48 h until I,, became steady.
439
3. Results and discussion We observed the different response properties that depended on the materials dispersed in PbSnF, and on their content. Figs. 3-6 show variations of tw and the magnitude, 1,,(20), of 1, at PO, = 20 kPa as a function of the IM content for the IM-dispersed PbSnF, layers. The sensor using a pure PbSnF, layer showed t,=6.5 min and 1,,(20) = 6.2 nA. For the FePc-dispersed PbSnF, (Fig. 3), the values of t9,,and 1,(20) increase similarly below 20 vol.%, reaching maxima of = 12 min and =40 nA, respectively at 20 vol.% FePc. The increase of I,= is probably due to the catalytic action of FePc for the reduction of oxygen. Afterwards, both decrease to minima of = 1.4 min and = 0.21 ~4, respectively, at 80 vol.% and slightly increase again in the range 80-100 vol.%. The sensors respond quickly in 3 min when the FePc content is more than ~60 vol.%. The variations of tso and 1,,(20) were similar to those of sensors of type (1) for more than =20 vol.% FePc [9,10], in spite of the differences in sensor structure. The electrical conductivity of FePc used here was very low, = lo-’ S cm-* at 25 “C. Thus, a large amount of FePc mixed with PbSnF, blocks the diffusion of reduced oxygen species towards PbSnF,,; this decreases I,, resulting in rapid response. The sensors using the ZrOz- and A.&O,-dispersed PbSnF, are similar in the variations of tw and 1,,(20) (Figs. 4 and 5). Both tw and 1,,(20) initially decrease with increasing oxide content, and only ZJE(20)increases after reaching a minimum at about 5-10 vol.%. There is an optimum oxide content (20 vol.% oxide) at which tm and 1&20) take a minimum and a maximum, respectively (i.e., t, = 3 min and Z,,(20) = 1.1 nA for ZrO,, t,=3.5 min and Z,,(20) ~0.6 nA for Al,O,). These variations are unlike those of the FePc-dispersed
Table 1 Response characteristics at room temperature for different IMdisperscd PbSnF, layers, and the tap densities (d,) and mean particle diameters (J&J of the IMs. An asterisk indicates that the results could not be determined because of drift
dt k cm-7
RI
AP
be’
WQ d
(Pm)
@A/102 kPa)
(do)
(W
FePc (80) FTFE (50)’ PVC (2O)i RN (2Q zm2 (20) ‘Go3 (2w WC’ (20)
0.20 0.35 0.47 0.37 0.99 1.07 0.65
0.1 10.0 0.5 0.75 0.08 3.2 0.2
Pure PbSnF,
0.50
0.2
WX) =
0.46 g 0.05 t
l
2.5 1.5 0.2
3 35 6
21.0
6.5
x vol.%. b AS: sensitivity.
1.4 0.7
0.21 0.05 a2.5 no response 1.1 0.6 0.2 6.2
n=
IH’
l/2 1 =l
0.9999 0.9940 *
1 1 1
0.9997 0.9998 0.9995
1
0.9998
H. Hanamoto et al. I Sensors and Actuators B 24-25 (1995) 438-442
440
Volume % 2x0~ Fig. 1. Photograph
Fig. 4. Variations of fw (m) and 1,(20) (0) with the 2~0~ for the ZrOz-dispersed PbSnF,.
of one of the oxygen sensors.
content
ed PI Sensing Electrude IM-dispersed
PbSnF,
PbSnFa PbSrlF6 + A&wo, A&W04 A&WO,
Isc A
I-- \ Lead
+ As
At? Epuxy resin
Poly ( vinyl chlmde
) Case
Fig. 2. Schematic structure of the oxygen sensors.
1:15
Volume % Al*03 Fig. 5. Variations of k.,~(m) and L(20) (0) with the A1203 content for the A&O,-dispersed PbSnFa.
10
10 G 3
5 .E E 5‘ 0 0
50 Volume % FePc
d
I(
Fig. 3. Variations of 1% (a) and I&20) (0) with the FePc content for the FePc-dispersed PbSnF,.
0
PbSnF+ A similar optimum IM content was not found for the dispersion of the other IMs, FePc (molecular compounds) and the polymers, which will be described below. It is well known that the ionic conductivity in polycrystalline electrolytes is enhanced by an interfacial effect (i.e., interfacial transport) resulting from the dispersion of an ionic compound, such as A&O, or SiOz. [15]. The occurrence of an optimum oxide content is probably associated with such an interfacial effect of the dispersed oxides. No response was detected above ~30 vol.% for the ZrO,-dispersed PbSnF, and above =45 vol.% for the Al,O,-dispersed PbSnF,,
Fig. 6. Variations of tw (m) and I&O) for the PTFE-dispersed PbSnF4.
(0) with the PTFE content
because the electrical conductivities became very low. When the polymers were dispersed, the sensors exhibited drifts and hysteresis. For the PTFE-dispersed PbSnF, (Fig. 6), both &,, and 1,(20) decrease monotonically with increasing PTFE content. The sensors responded very quickly, but I,, gradually drifted in the direction opposite to the direction of normal response
H. Hanamoto et al. I Sensors and Actuators B 24-25 (1995) 438442
after reaching a 100% response value. For the PVCdispersed PbSnF,, we were unable to obtain a steady value of I,,, because considerable drift always occurred. These phenomena are associated with the slow equilibrium of dissolution of oxygen into the polymer particles. For the BN-dispersed PbSnF,, no response was detected, probably because of a side reaction associated with BN and PbSnF,. Fig. 7 shows the amperometric response curves of the oxygen sensor using the ZrO, (20 vol.%)-dispersed PbSnF, at room temperature as a typical example. The sensor exhibits slight hysteresis and drift. Figs. 8 and 9 show the plots of I, against PaIn or PO, for the optimal sensors which had the shortest tso: Fig. 8 is for the FePc-dispersed PbSnF4 and Fig. 9 for other IM-dispersed PbSnF,. They reveal excellent linear relations with a correlation coefficient, 14, of more than 0.9995, except for the sensor using the PTFE-dispersed PbSnF,. The linear relation between 1, and Po:/2 for the sensor using FePc (80 vol.%)-dispersed PbSnF, and for sensors of type (1) [9-111 indicates that the sensing electrode reaction changes from one-electron to twoelectron reduction of oxygen by the action of the dispersed FePc (see the number, II, of electrons as-
,‘, 101816040
19 Pq
40 /
M
81
101
kPa
Fig. 7. Response curve for the sensor using the ZrO, (20 vol.%)dispersed PbSnF,.
441
PozildkPa Fig. 9. Plots of I, against PO2for the sensors using IM-dispersed PbSnF,. IM: 1, ZrO, (20 vol.%); 2, Al,4 (20 vol.%); 3, PI-FE. (50 vol.%).
sociated with the sensing-electrode reaction in Table 0 Table 1 summarizes the response characteristics of the optimal sensors with the shortest tw: they are sensitivity (AS), tw, Z,,(20), n and jrj. In general, the dispersion of a proper amount of IMs resulted in a decrease in tw, though the sensitivity decreased. The sensor using the ZrOz (20 vol.%)dispersed PbSnF, exhibited a sensitivity value approximately 2-50 times higher than the others. However, the sensor using FePc (80 vol.%)-dispersed PbSnF, was superior to the sensors using the oxide-dispersed PbSnF, in terms of the response time and the long-term stability of the I,= signal. The sensitivity of the FePc (80 vol.%)dispersed PbSnF., was rather small, but still large enough to ensure precise measurements. The response properties for the AlzO,- and MgO-dispersed PbSnF, were worse than those for the ZrO,-dispersed PbSnF, in terms of AS, tw, hysteresis and drift. We took SEM-EDX images of the IM-dispersed PbSnF, layers to examine the microstructure, but did not find any correlation between the microstructure and the response properties.
4. Conclusions
0.4
0.6 (P@ / loz ,ay
0.8
1
Fig. 8. Plots of I, against Pm " for the sensor using the FePc (80 vol.%)-dispersed PbSnF,.
We conclude that the effects and roles of the dispersed IMs are as follows:* (1) The dispersed FePc acts as a catalyst for twoelectron reduction of O2 to 20- in the sensing-electrode reaction. In the range 0 to ~30 vol.%, this leads to a linear relation between 1, and Pmln and an increase in Z,, and tso. However, a large amount of almost nonconductive FePc dispersed in PbSnF, blocks the diffusion of reduced oxygen species towards PbSnF,; this decreases Z,,, resulting in rapid response. (2) For the oxide-dispersed PbSnF,, there is an optimal oxide content at which tm and r&20) have a
H. Hanam
442
minimum explained dispersion (3) The dispersed content.
et al. / Sensors and Actuators B 24-25 (1995) 438242
and a maximum, respectively. This can be as an interfacial effect resulting from the of oxides. response properties of the sensors using IMPbSnF, depend on the type of IM and its
Acknowledgements J.K. thanks Riken Keiki Company, Ltd. for providing galvanic cell-type oxygen sensors and the sensor holders.
References VI E. Siebert and J. Fouietier, Characteristics of an oxygen gauge
at temperatures lower than 200 “C, Solid State Ionics, 9/10 (1983) 1291-1294. 121 E. Siebert, J. Fouletier, M. Kanier and M. Kleitz, Oxygen sensing with solid electrolyte cells from room temperature up to 250 “C, 1. Elecbwchem. SIX., X34 (1987) 1573-1578. [A N. Yamazoe, J. Hisamoto, N. Miura and S. Kuwata, Potentiometric solid-state oxygen sensor lanthanum fluoride operative at rmrn temperature, Sensors andActuators, 12 (1987) 415-423. 141 T. Yoshida, T. Isomura, J. Kuwano and M. Kato, Ambient temperature all-solid-state oxygen sensor using vacuum deposited Las films and high ionic-conductivity glasses, DenkiKaguku, 57 (19S9) 1087-1093. 151 J. Kuwano, M. Asano, K. Shigehara and M. Kato, Ambient temperature solid-state oxygen sensor using fast ion conductors, PbSnF, and Ag&W04, Solid State Ion&, 40141 (1990) 472-475. 161 M. Asano, J. Kuwano and M. Kato, Ambient temperature solid-state oxygen sensor using fast ion conductors, L Ceram. Sot. Jpn., 97 (1989) 12X-1261. (71 J. Kuwano, Ambient temperature solid-state oxygen sensors using fluoride ion conductors, in I’. Vincenzini (ed.), Ceramics Today - Tomorrow’s Ceramics, Part D, Elsevier, Amsterdam, 1991, pp. 2487-2493. I81 J. Kuwano, H. Arakawa and M. Kato, Cobalt(H)-doped pyrolyzcd polyacrylonitrile electrode materials for ambient temperature solid-state oxygen sensors, Rep. Prog. Polym. Phys. Jpn., 34 (1991) 289-292.
[91 J. Kuwano, A. Wakagi and M. Kato, Fast response of amperometric oxygen sensors based on an iron phthalocyaninebased sensing electrode at ambient temperature,J: Electrochem. sot., 139 (1992) L113-L115. 1101 J. Kuwano, A. Wakagi and M. Kato, Amperometric oxygen sensors based on fast ion conductors for rapid detection at ambient temperature, Sensors and Achratots 5, 13-14 (1993) 608-609. WI A. Wakagi and J. Kuwano, Amperometric PbSnF,-based oxygen sensors: rapid response at room temperature in the operating pressure range 10 kPa-7.2 MPa, J. Mater. Chem., 4 (1994) 973-97s. P21 A. Wakagi, J. Kuwano, M. Kato and H. Hanamoto, Fast amperometric response of ambient temperature oxygen sensors based on PbSnF,; iron(H) phthakxyanine-based sensing electrodes containing carbon microbeads, Solid State Ionics, 70/71 (1994) 601-605. 1131 G. Perez, S. Vilminot, W. Granier, L. Cot, C. Lucat, J.M. Reau and J. Portier, About the allotropic transformations of PbSnF,, Mater. Rex Bull., 15 (1980) 587-593. P41 T. Takahashi, S. Ikeda and 0. Yamamoto, Solid-state ionics: a new high ionic conductivity solid electrolyte Ag&WO, and use of this compound in a solid-electrolyte cell, J. Electmchem six,
WI
120 (1973) 647-651.
J.B. Wagner, Jr., Composite solid ion conductors, in T. Takahashi (ed.), High Conductiviry Solid Ionic Conducton, World Scientific, Singapore, 1989, pp. 146-165.
Biographies Hironori Hanamoto and Kazuhiro Tsuchiya received their B. Eng. degrees in 1992 and in 1993, respectively, from Science University of Tokyo (SUT) and are presently graduates in the M.S. course. Their area of interest is the development of solid-state oxygen sensors. Jun Kuwuno is a research associate in SUT. He obtained his MS. degree from the Department of Chemistry in 1972. His current research activities are directed toward the development and applications of fast ion conductors, inorganic ion exchangers and conducting polymers, in particular, toward solid-state oxygen sensors.
CHEMICAL
Sensors and Actuators B 24-25 (1995) 43&442
Amperometric response of ambient temperature oxygen sensors based on insulating material-dispersed PbSnF, layers Hironori Hanamoto, Department
of Industrial
Kazuhiro Tsuchiya, Jun Kuwano *
Chemisby, Faculty of Engineering, Science University of Tokyo, l-3 Kapnuaka,
Shinjuku-ky Tokyo 162, Japan
Abstract The response properties of amperometric oxygen sensors of the type AglA8&WO,lPbSnF,I insulating material (IM)-dispersed PbSnF,IPt, 02, are examined and the effects of the dispersed IMs are discussed. Powders of iron@) phtbaloqanine (FePc), oxides (ZrO,, A&OS, MgO), polymers (PVC, PTFE) and BN are used as IMs. The response properties depend on the material used as an IM and on the IM content. For example, when FePc is dispersed, the short-circuit current &) and 90% response time (rw) change similarly, having a maximum at 20 vol.% FePc and a minimum at 80 vol.%. For the oxides, there is an optimum oxide content at which tw and I,, take a minimum and a maximum, respectively. However, there is a common tendency that the dispersion of a proper amount of IM results in the improvement of t* A linear relation is found between 1, and PO2lR for the FePc-dispersed PbSnF.,, whereas I, increases linearly with Pm for the other IM-dispersed PbSnF,. The results suggest that the dispersed FePc acts as a catalyst for two-electron reduction of oxygen. Keyworck
Amperometric sensors; Oxygensensors; Insulating materials; Ambient temperature; Fluoride
1. Introduction Oxygen is the most important
element,
and its roles
in living organisms and technological processes have always attracted great interest. In various fields of science and technology, there is increasing demand for monitoring and controlling the concentration of oxygen gas under diversified conditions. However, commercially available oxygen sensors of galvanic-cell type and zirconia type have been unable to meet this demand sufficiently, because of the use of liquid electrolytes and high operating temperature, respectively. Against this background, several potentiometric and amperometric solid-state sensors based on inorganic fluorides have been developed [l-12]. The amperometric sensors [4-121 were superior to the potentiometric sensors in response time, operating pressure range and hysteresis. Recently, we have developed amperometric sensors of the type [P-11] Ag(AQaWO,(PbSnF&ensing
electrode, Oz
(1)
in which mixtures of iron(I1) phthalocyanine (FePc), platinum black and PbSnF, are used as a sensing electrode. The 90% response time (r& was shorter * Corresponding author. 0925-4005/95/$09.50 0 1995Elsevier SSDI 0925-4005(94)01391-T
Science S.A. All rights reserved
than 1 min at room temperature. The sensors were capable of measuring oxygen concentrations in a wide range of oxygen partial pressure (Pm), 2 kPa-7 MPa, where commercial oxygen sensors cannot be used. The sensing reaction was assumed to be as follows: 0,+2Ag+2F-
=20-
+24gF
A previous paper 1121has shown that some noble metals and carbon materials seme as a substitute for platinum black, and that their chemical nature rather than their physical properties, such as specific surface area, influences the response properties. However, the measuring principle and the roles of the dispersed FePc are not yet fully understood. In this study, we focus on dispersed FePc with a very low electrical conductivity and make sensors of the type Ag/A&I,WO.,IPbSnF,)IM-dispersed PbSnF,I sputtered Pt, 0,
(2)
The insulating materials (IMs) are FePc, ZrOz, A1203, MgO, BN, PTFE [poly(tetrafIuoroethylene)] and PVC [poly(vinyl chloride)]. For simplicity, in sensors of type (2) the sensing electrode of sensors of type (1) is separated into an IM-dispersed PbSnF, layer and a
H. Hanmwto
et al. f Sensors and Actuo~~rs B 24-25 (1995) 438-442
sputtered Pt electrode according to their functions. The amperometric response is examined as a function of the IM content. As a result, we find that the response properties are variously influenced by the materials dispersed in PbSnF, and by their content. The effects and roles of the IMs dispersed in PbSnF, are discussed for improvement of the response characteristics.
2. Experimental The IMs used are listed along with their tap densities (d,) and mean particle diameters (D,,,) in Table 1. The IM-dispersed PbSnF,, mixtures were prepared by mixing the powders of PbSnF, and one of the IMs. The volume percentages of IMs were calculated from their d,. The fast ion conductors a-PbSnF, and Ag&W04 were synthesized by a modification [6] of the precipitation reaction [13] and by the solid-state reaction [14], respectively. Figs. 1 and 2 show a photograph of one of the sensors and the schematic structure, respectively. A six-layered pellet (8 mm in diameter, 2 mm thick) was made by pressing and encapsulated in a PVC case fitted with two leads. A platinum sensing electrode was sputtered onto the whole surface of the IM-dispersed PbSnF, layer. The details of the assembly were described in previous papers [5,6,9,11]. Three of the sensors were fixed to a thermostatted test chamber with a capacity of 2.9 cm3. The short-circuit current (1,) response was simultaneously measured with ammeters (Toa PM-18U) under a dry 0,-N, gas flow of 100 ml min-I. The mixing ratio was changed stepwise with a pair of thermal mass-flow controllers (Kojima 3610). Prior to the measurements, the sensors were aged as short-circuited for about 48 h until I,, became steady.
439
3. Results and discussion We observed the different response properties that depended on the materials dispersed in PbSnF, and on their content. Figs. 3-6 show variations of tw and the magnitude, 1,,(20), of 1, at PO, = 20 kPa as a function of the IM content for the IM-dispersed PbSnF, layers. The sensor using a pure PbSnF, layer showed t,=6.5 min and 1,,(20) = 6.2 nA. For the FePc-dispersed PbSnF, (Fig. 3), the values of t9,,and 1,(20) increase similarly below 20 vol.%, reaching maxima of = 12 min and =40 nA, respectively at 20 vol.% FePc. The increase of I,= is probably due to the catalytic action of FePc for the reduction of oxygen. Afterwards, both decrease to minima of = 1.4 min and = 0.21 ~4, respectively, at 80 vol.% and slightly increase again in the range 80-100 vol.%. The sensors respond quickly in 3 min when the FePc content is more than ~60 vol.%. The variations of tso and 1,,(20) were similar to those of sensors of type (1) for more than =20 vol.% FePc [9,10], in spite of the differences in sensor structure. The electrical conductivity of FePc used here was very low, = lo-’ S cm-* at 25 “C. Thus, a large amount of FePc mixed with PbSnF, blocks the diffusion of reduced oxygen species towards PbSnF,,; this decreases I,, resulting in rapid response. The sensors using the ZrOz- and A.&O,-dispersed PbSnF, are similar in the variations of tw and 1,,(20) (Figs. 4 and 5). Both tw and 1,,(20) initially decrease with increasing oxide content, and only ZJE(20)increases after reaching a minimum at about 5-10 vol.%. There is an optimum oxide content (20 vol.% oxide) at which tm and 1&20) take a minimum and a maximum, respectively (i.e., t, = 3 min and Z,,(20) = 1.1 nA for ZrO,, t,=3.5 min and Z,,(20) ~0.6 nA for Al,O,). These variations are unlike those of the FePc-dispersed
Table 1 Response characteristics at room temperature for different IMdisperscd PbSnF, layers, and the tap densities (d,) and mean particle diameters (J&J of the IMs. An asterisk indicates that the results could not be determined because of drift
dt k cm-7
RI
AP
be’
WQ d
(Pm)
@A/102 kPa)
(do)
(W
FePc (80) FTFE (50)’ PVC (2O)i RN (2Q zm2 (20) ‘Go3 (2w WC’ (20)
0.20 0.35 0.47 0.37 0.99 1.07 0.65
0.1 10.0 0.5 0.75 0.08 3.2 0.2
Pure PbSnF,
0.50
0.2
WX) =
0.46 g 0.05 t
l
2.5 1.5 0.2
3 35 6
21.0
6.5
x vol.%. b AS: sensitivity.
1.4 0.7
0.21 0.05 a2.5 no response 1.1 0.6 0.2 6.2
n=
IH’
l/2 1 =l
0.9999 0.9940 *
1 1 1
0.9997 0.9998 0.9995
1
0.9998
H. Hanamoto et al. I Sensors and Actuators B 24-25 (1995) 438-442
440
Volume % 2x0~ Fig. 1. Photograph
Fig. 4. Variations of fw (m) and 1,(20) (0) with the 2~0~ for the ZrOz-dispersed PbSnF,.
of one of the oxygen sensors.
content
ed PI Sensing Electrude IM-dispersed
PbSnF,
PbSnFa PbSrlF6 + A&wo, A&W04 A&WO,
Isc A
I-- \ Lead
+ As
At? Epuxy resin
Poly ( vinyl chlmde
) Case
Fig. 2. Schematic structure of the oxygen sensors.
1:15
Volume % Al*03 Fig. 5. Variations of k.,~(m) and L(20) (0) with the A1203 content for the A&O,-dispersed PbSnFa.
10
10 G 3
5 .E E 5‘ 0 0
50 Volume % FePc
d
I(
Fig. 3. Variations of 1% (a) and I&20) (0) with the FePc content for the FePc-dispersed PbSnF,.
0
PbSnF+ A similar optimum IM content was not found for the dispersion of the other IMs, FePc (molecular compounds) and the polymers, which will be described below. It is well known that the ionic conductivity in polycrystalline electrolytes is enhanced by an interfacial effect (i.e., interfacial transport) resulting from the dispersion of an ionic compound, such as A&O, or SiOz. [15]. The occurrence of an optimum oxide content is probably associated with such an interfacial effect of the dispersed oxides. No response was detected above ~30 vol.% for the ZrO,-dispersed PbSnF, and above =45 vol.% for the Al,O,-dispersed PbSnF,,
Fig. 6. Variations of tw (m) and I&O) for the PTFE-dispersed PbSnF4.
(0) with the PTFE content
because the electrical conductivities became very low. When the polymers were dispersed, the sensors exhibited drifts and hysteresis. For the PTFE-dispersed PbSnF, (Fig. 6), both &,, and 1,(20) decrease monotonically with increasing PTFE content. The sensors responded very quickly, but I,, gradually drifted in the direction opposite to the direction of normal response
H. Hanamoto et al. I Sensors and Actuators B 24-25 (1995) 438442
after reaching a 100% response value. For the PVCdispersed PbSnF,, we were unable to obtain a steady value of I,,, because considerable drift always occurred. These phenomena are associated with the slow equilibrium of dissolution of oxygen into the polymer particles. For the BN-dispersed PbSnF,, no response was detected, probably because of a side reaction associated with BN and PbSnF,. Fig. 7 shows the amperometric response curves of the oxygen sensor using the ZrO, (20 vol.%)-dispersed PbSnF, at room temperature as a typical example. The sensor exhibits slight hysteresis and drift. Figs. 8 and 9 show the plots of I, against PaIn or PO, for the optimal sensors which had the shortest tso: Fig. 8 is for the FePc-dispersed PbSnF4 and Fig. 9 for other IM-dispersed PbSnF,. They reveal excellent linear relations with a correlation coefficient, 14, of more than 0.9995, except for the sensor using the PTFE-dispersed PbSnF,. The linear relation between 1, and Po:/2 for the sensor using FePc (80 vol.%)-dispersed PbSnF, and for sensors of type (1) [9-111 indicates that the sensing electrode reaction changes from one-electron to twoelectron reduction of oxygen by the action of the dispersed FePc (see the number, II, of electrons as-
,‘, 101816040
19 Pq
40 /
M
81
101
kPa
Fig. 7. Response curve for the sensor using the ZrO, (20 vol.%)dispersed PbSnF,.
441
PozildkPa Fig. 9. Plots of I, against PO2for the sensors using IM-dispersed PbSnF,. IM: 1, ZrO, (20 vol.%); 2, Al,4 (20 vol.%); 3, PI-FE. (50 vol.%).
sociated with the sensing-electrode reaction in Table 0 Table 1 summarizes the response characteristics of the optimal sensors with the shortest tw: they are sensitivity (AS), tw, Z,,(20), n and jrj. In general, the dispersion of a proper amount of IMs resulted in a decrease in tw, though the sensitivity decreased. The sensor using the ZrOz (20 vol.%)dispersed PbSnF, exhibited a sensitivity value approximately 2-50 times higher than the others. However, the sensor using FePc (80 vol.%)-dispersed PbSnF, was superior to the sensors using the oxide-dispersed PbSnF, in terms of the response time and the long-term stability of the I,= signal. The sensitivity of the FePc (80 vol.%)dispersed PbSnF., was rather small, but still large enough to ensure precise measurements. The response properties for the AlzO,- and MgO-dispersed PbSnF, were worse than those for the ZrO,-dispersed PbSnF, in terms of AS, tw, hysteresis and drift. We took SEM-EDX images of the IM-dispersed PbSnF, layers to examine the microstructure, but did not find any correlation between the microstructure and the response properties.
4. Conclusions
0.4
0.6 (P@ / loz ,ay
0.8
1
Fig. 8. Plots of I, against Pm " for the sensor using the FePc (80 vol.%)-dispersed PbSnF,.
We conclude that the effects and roles of the dispersed IMs are as follows:* (1) The dispersed FePc acts as a catalyst for twoelectron reduction of O2 to 20- in the sensing-electrode reaction. In the range 0 to ~30 vol.%, this leads to a linear relation between 1, and Pmln and an increase in Z,, and tso. However, a large amount of almost nonconductive FePc dispersed in PbSnF, blocks the diffusion of reduced oxygen species towards PbSnF,; this decreases Z,,, resulting in rapid response. (2) For the oxide-dispersed PbSnF,, there is an optimal oxide content at which tm and r&20) have a
H. Hanam
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minimum explained dispersion (3) The dispersed content.
et al. / Sensors and Actuators B 24-25 (1995) 438242
and a maximum, respectively. This can be as an interfacial effect resulting from the of oxides. response properties of the sensors using IMPbSnF, depend on the type of IM and its
Acknowledgements J.K. thanks Riken Keiki Company, Ltd. for providing galvanic cell-type oxygen sensors and the sensor holders.
References VI E. Siebert and J. Fouietier, Characteristics of an oxygen gauge
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Biographies Hironori Hanamoto and Kazuhiro Tsuchiya received their B. Eng. degrees in 1992 and in 1993, respectively, from Science University of Tokyo (SUT) and are presently graduates in the M.S. course. Their area of interest is the development of solid-state oxygen sensors. Jun Kuwuno is a research associate in SUT. He obtained his MS. degree from the Department of Chemistry in 1972. His current research activities are directed toward the development and applications of fast ion conductors, inorganic ion exchangers and conducting polymers, in particular, toward solid-state oxygen sensors.