- Email: [email protected]
Application of hydrogen sensor using proton conductive ceramics as a solid electrolyte to aluminum casting industries
SOLID STATE ELXXIER
Solid State lonics 79 (1995) 333-337
IONICS
Application of hydrogen sensor using proton conductive ceramics as a solid electrolyte to aluminum casting industries Tamotsu Yajima a, Kunihiro Koide a, Haruki Takai a, Nirihiko Fukatsu b, Hiroyasu Iwahara ’ a TYK Corporation, Research and Development Center, 3-1, Ohbata-cho, Tajimi 507, Japan ’ Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan ’ Synthetic Crystat Research Laboratory, School of Engineering, Nagoya Uniuersi& Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Abstract A galvanic cell type of hydrogen sensor was developed using CaZrO,-based proton conductor as a solid electrolyte. It was clear that the hydrogen sensor could be used for the accurate determination of hydrogen concentration in molten aluminum alloys. The sensor probe exhibited stable EMF values and a fast response when the hydrogen concentration in the melt was changed. The application of this sensor to the real aluminum foundry was also investigated. This sensor could work very stably even when the flow rate of the liquid metal was very fast. In situ measurement was possible using this sensor anywhere in the casting shop, such as melting furnace, holding furnace, the degassing station, etc. Keywords:
Hydrogen sensor; Molten aluminum; Proton conductor; Solid electrolyte; Hydrogen concentration
1. Introduction Alkaline earth cerates [1,2] and zirconates [3-51 partially substituted by aliovalent cations for cerium or zirconium exhibit good protonic conduction under hydrogen-containing atmosphere at high tempera-
tures. SrCe,,,Yb,,,,O,
_ m, CaZr,,,,In,,,,O,
_ (I, etc.
belong to this class of conductors. Such high temperature-type oxide proton conductors find application as solid electrolytes in electrochemical devices such as hydrogen sensors [6,7], fuel cells [8,9], steam electrolyzers [ 101 for hydrogen production, etc. In the real casting process, hydrogen can be easily dissolved in molten alumium from water vapor in the surrounding atmosphere and can be absorbed during casting, fabrication and/or heat treatment. When the metal solidifies the dissolved hydrogen causes small pore defects owing to the large difference in hydro0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00083-6
gen solubility between liquid and solid aluminum. To produce high quality castings, it is necessary to reduce the hydrogen concentration in the molten metals during the casting process to an acceptable level 1111. Although many investigators have tried to determine the hydrogen content in liquid and solid aluminum alloys [12-U], the values were unreliable because hydrogen could easily be picked up from water vapor in the air and the concentration readily changed. To obtain highly accurate value of hydrogen concentration in molten aluminum, we developed the galvanic cell-type hydrogen sensor using CaZrO,-based proton conductor [16,17]. This sensor works well as hydrogen probe for continuous determination of in situ hydrogen solubility in molten aluminum. The real time and in situ measurement of hydrogen in molten aluminum alloys is required in order
T. Yajima et al./Solid State Ionics 79 (1995) 333-337
334
to control the hydrogen concentration in the practical aluminum casting process. It is impossible to carry out the real time and in situ measurement of hydrogen by other conventional methods. In this paper, the possibility of the real time measurement of hydrogen in molten aluminum alloys using this type of the sensor in practical foundries is investigated.
2. Principle minum
of hydrogen
sensor for molten
alup”(I)
A hydrogen concentration cell can be constructed using a proton conductor as a solid electrolyte. For the concentration cell. H( pu, (I)) [proton conductor I H( /.~u(II))
(1)
the equilibrium electromotive force measured across the electrode/electrolyte interface is given by ~~(11) - ~~(1)
~~(1) and ~~(11) are the chemical potentials of mono atomic hydrogen species at two electrodes, F, the Faraday constant and n, the number of electrons involved in the reversible reaction occurring at the interface between the electrolyte and electrode. IZ is taken to be a unity for atomic hydrogen. For the gaseous concentration cell, the chemical potentials may be replaced by the corresponding partial pressure of hydrogen:
(2)
I-
Hz (O.Olatm) ~~ Stainless tube
Alumina tube
ln PuiI)] In Pan],
y
(3) (4)
where Pu2(I) and P,$II) are hydrogen partial pressures at each electrode compartment and &, is the chemical potential of gaseous hydrogen at a standard state. The electromotive force of this cell, then, is given by the following equation:
Eln ‘H,(‘) 2F
P,?(II)
’
(5)
If one of the hydrogen parital pressures is known, this galvanic cell acts as a hydrogen gas sensor. On the other hand, the well-known Sieverts’ law [18] holds between hydrogen partial pressure in the atmosphere above the melt and hydrogen content in the liquid metal.
S=k\lP,,
n
Proton conductor CaZm 9Ino 103. (I
Ceramic fiber
Glass and ceramic adhesive
Molten aluminum Fig. 1. Schematic
~~(11) = 1[ CL& +RT
E=
= -nFE.
= $[ poH>+RT
diagram of the sensor probe.
where S is the hydrogen concentration (ml-NTP/ 100 g-Al) in molten aluminum, k, the Sieverts’ coefficient and P,* the equilibrium hydrogen partial pressure in the atmosphere over the melt. If a small gas compartment isolated from the air can be created in the melt as shown in Fig. 1, hydrogen gas will come to the compartment from the melt and after equilibration, Sieverts’ law will be satisfied. It is possible to measure the equilibrium hydrogen partial pressure in the compartment by a galvanic cell-type hydrogen sensor using a proton conductor as a solid electrolyte. When this hydrogen partial pressure PHZ is taken as PH2(I) in Eq. (5) and this equation is substituted into Eq. (6), we obtain the following relation between hydrogen concentration in the molten aluminum and the sensor EMF: S = k/P,I(II)
exp(2FE/RT)
(7)
335
T. Yajima et al. /Solid State Ionics 79 (1995) 333-337
Using this equation, it is possible to determine the hydrogen content in molten aluminum from EMF of the hydrogen sensor if P,$II) is known and constant.
Table I Comparison of hydrogen concentrations determined by the sensor and by inert gas carrier extraction method. Ahoy: 99.99% Al Sample no.
1 2 3 4
3. Experimental A sensor probe was constructed using cap-shaped as the solid electrolyte as shown in CaZr,,In&-, Fig. 1. In the ceramic cap, alumina powder and carbon fiber are packed to reduce the dead volume of the gas phase. One percent of hydrogen gas balanced with argon was used as the standard gas for the concentration-type hydrogen sensor. 10 to 40 kg of aluminum alloys were melted in the graphite crucible by an electric furnace.
4. Results and discussion The EMFs of the sensor were measured under various conditions when the sensor was immersed into molten aluminum. At the same time, we measured hydrogen concentration in the melt by the hydrogen analyzer for molten aluminum produced by Sumitomo Light Metal which was based on the equilibrium partial pressure method reported by Ransley et al. [12]. Fig. 2 shows the relation between the sensor EMF and hydrogen concentration in
This sensor (ml (100 g-Al_’
)
Inert gas extraction (ml (100 g-Al)) 0.08 0.16 0.17 0.27
0.084 0.184 0.148 0.235
molten aluminum. The following derived from Eq. (7): RT E = G@
method
equation
can be
S* k2P,(II)
.
The solid line in Fig. 2 is the theoretical value calculated from Eq. (8) taking the hydrogen partial pressure in the standard gas, P,$II), is 0.01 atm. Each EMF value of the sensor is close to the theoretical one. From this result it is clear that the hydrogen concentration measurement in molten aluminum is possible by the new sensor developed in this study using proton conducting ceramic as a solid electrolyte. To calculate the hydrogen concentration from the sensor EMF, it is necessary to know the Sieverts’ coefficient k. In the temperature range from 670 to 850°C the Sieverts’ coefficient k for pure aluminum has been determined by Ransley and Neufeld [ll] and was given by the following equation: - 2760 log k = ~ + 1.356, T
(9)
where T is the absolute temperature. From Eqs. (7) and (9), the following equation can be derived:
PureAl
S = 10t(-2760/T)+ 1~3561/Pn~II) exp( 2FE/RT)
. (10)
---o----
:75()“c
-
:8OO”c
log@* / (ml/lOOg-Al)*) Fig. 2. Comparison of EMF of hydrogen sensor with theoretical EMF calculated from Sieverts’ law and Nemst equation. Solid lines show theoretical EMF value. Alloy is 99.8% aluminum.
Eq. (10) holds for pure aluminum and corrections need to be made for other alloys. Hydrogen measurement results obtained by this sensor in 99.99% Al are shown in Table 1 with the analyzed values by inert gas extraction method which is one of the most popular methods of hydrogen measurement in molten aluminum alloys. In this case, hydrogen concentrations by this sensor were calculated from the sensor EMFs and Eq. (10). The
336
T. Yajima et al. /Solid
Table 2 Typical hydrogen Alloys
a
measurement
results before and after degassing
Degassing
procedure
(JIS No.) AC4C AC4C AC4CH AC2B AC2B 99.999% Al
Degassing flux Inert gas bubbling by lance pipe Rotary gas bubbling Rotary gas bubbling Vacuum degassing Rotary gas bubbling
a Alloy names are Japanese
industrial
State Ionics 79 (1995) 333-337
processes
Treatment time (min)
Before degassing (ml (100 g-AI_‘)
After degassing (ml (100 g-AI_‘)
10 40
0.298 0.286
0.275 0.114
10 4 30 _
0.220 0.181 0.398 0.264
0.076 0.065 0.048 0.083
standard.
hydrogen concentration values determined by the sensor are close to the value by the inert gas extraction method although the value by the sensor was slightly lower than that by the inert gas extraction method. In the latter method, the values are often overestimated because of uptake of hydrogen from humid air during sampling and/or measuring process. As we reported earlier [17], the in situ measurement of hydrogen concentration is possible during the degassing process while using this sensor. There are several methods for hydrogen degassing of molten aluminum alloy in a practical foundry. Here, we tried to measure the decreasing hydrogen concentration during several kinds of degassing treatments as shown in Fig. 3 and Table 2. This hydrogen analyzer worked
stably and reproducibly. In these cases about 40 kg of AC4C alloy (i.e. Japanese industrial standard alloy) was used. It is possible to measure the variation of hydrogen concentration in the melt continuously during many kinds of degassing treatments as shown in Fig. 3. In the case of the degassing flux treatment, hydrogen concentration in the melt decreased from 0.28 ml/100 g-Al to 0.18 ml/100 g-Al after the treatment. Lower hydrogen level could be achieved by the inert gas bubbling treatment using argon gas. The rotary-type degassing machine has the best performance in these treatments and lowered the hydrogen concentration from 0.28 to 0.06 ml/100 g-Al within 8 min. The stable and reproducible performance of this sensor is attributed to the essential qualities of this proton-conducting ceramics which are the chemical and thermal stabilities and almost pure proton conductivity in this condition. The results described above suggest that the real time and in situ measurement of hydrogen concentration in molten aluminum alloys is possible using the galvanic cell-type hydrogen sensor using proton conducting CaZr,,,,In,~,O, _ LI. In the future studies, this proton-conducting ceramic-type hydrogen sensor will be applied to other molten metals.
Ar gas bubbling by lance pipe
5. Conclusion
Ar ga; bubbling by rotary type degassinp machine
’
’
’
I ’ 2.500
‘
3
Time / set Fig. 3. Comparison of hydrogen decreasing behavior during degassing by different treatments. Alloy is AC4C which is Japanese industrial standard. Temperature is 740°C.
It is clear that a galvanic cell-type hydrogen sensor using proton conducting ceramics as a solid electrolyte can be useful to measure hydrogen concentration in molten aluminum alloys. The sensor developed in this study makes it possible to measure
T. Yajima et al. /Solid
State Ionics 79 (1995) 333-337
hydrogen content in molten Al continuously in practical aluminum foundry processes such as the degassing treatments, etc. Such in situ hydrogen measurement is impossible by other conventional methods. The results described above suggest that this sensor makes it possible to control the degassing process by feeding the measured data back to a process controller.
References HI H. Iwahara, T. Esaka, H. Uchida and N. Maeda, Solid State Ionics 3/4
(1981) 359.
El H. Iwahara, H. Uchida, K. Kondo and K. Ogaki, J. Electrochem. Sot. 135 (1988) 529. 131 T. Yajima, H. Kazeoka, T. Yogo and H. Iwahara, Solid State Ionics 47 (1991) 271. [41 T. Yajima, H. Suzuki, T. Yogo and H. Iwahara, Solid State Ionics 51 (1992) 101.
337
[5] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki and H. Suzuki, Solid State Ionics 61 (1993) 65. [6] H. Iwahara and H. Uchida, Proc. Intern. Meeting Chemical Sensors, Fukuoka, Japan (Kodansha-Elsevier, Tokyo, 1983) p. 227. [7] T. Yajima, H. Iwahara, K. Koide and K. Yamamoto, Sensors Actuators BS (1991) 145. [8] H. Iwahara, H. Uchida and S. Tanaka, J. Appl. Electrochem. 16 (1986) 663. [9] H. Iwahara, H. Uchida and K. Morimoto, 1. Electrochem. Sot. 137 (1990) 462. [lo] H. Iwahara, T. Esaka, U. Uchida, T. Yamauchi and K. Ogaki, Solid State Ionics 18/19 (1986) 1003. [ll] C.E. Ransley and H. Neufeld, J. Inst. Metals 74 (1948) 599. [12] C.E. Ransley, D.E.J. Tablot and H.C. Barlow, Aluminium 34 (1958) 643. [13] D.J. Neil and A.C. Burr, Trans. Am. Foundrymen’s Sot. 69 (1961) 272. [14] G. Ohira and V. Kondic, Foundry Trade J. 96 (1954) 331. [15] R. Gee and D.J. Fray, Metal. trans. B 9B (1978) 427. [16] T. Yajima, H. Iwahara, N. Fukatsu, T. Ohashi and K. Koide, J. Japan Inst. Light Met. 42 (1992) 263. [17] T. Yajima, K. Koide, N. Fukatsu, T. Ohashi and H. Iwahara, Sensors Actuators B13/14 (1993) 697. [18] A. Sieverts, Z. Metallk. 29 (1929) 37.
Solid State lonics 79 (1995) 333-337
IONICS
Application of hydrogen sensor using proton conductive ceramics as a solid electrolyte to aluminum casting industries Tamotsu Yajima a, Kunihiro Koide a, Haruki Takai a, Nirihiko Fukatsu b, Hiroyasu Iwahara ’ a TYK Corporation, Research and Development Center, 3-1, Ohbata-cho, Tajimi 507, Japan ’ Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan ’ Synthetic Crystat Research Laboratory, School of Engineering, Nagoya Uniuersi& Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Abstract A galvanic cell type of hydrogen sensor was developed using CaZrO,-based proton conductor as a solid electrolyte. It was clear that the hydrogen sensor could be used for the accurate determination of hydrogen concentration in molten aluminum alloys. The sensor probe exhibited stable EMF values and a fast response when the hydrogen concentration in the melt was changed. The application of this sensor to the real aluminum foundry was also investigated. This sensor could work very stably even when the flow rate of the liquid metal was very fast. In situ measurement was possible using this sensor anywhere in the casting shop, such as melting furnace, holding furnace, the degassing station, etc. Keywords:
Hydrogen sensor; Molten aluminum; Proton conductor; Solid electrolyte; Hydrogen concentration
1. Introduction Alkaline earth cerates [1,2] and zirconates [3-51 partially substituted by aliovalent cations for cerium or zirconium exhibit good protonic conduction under hydrogen-containing atmosphere at high tempera-
tures. SrCe,,,Yb,,,,O,
_ m, CaZr,,,,In,,,,O,
_ (I, etc.
belong to this class of conductors. Such high temperature-type oxide proton conductors find application as solid electrolytes in electrochemical devices such as hydrogen sensors [6,7], fuel cells [8,9], steam electrolyzers [ 101 for hydrogen production, etc. In the real casting process, hydrogen can be easily dissolved in molten alumium from water vapor in the surrounding atmosphere and can be absorbed during casting, fabrication and/or heat treatment. When the metal solidifies the dissolved hydrogen causes small pore defects owing to the large difference in hydro0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00083-6
gen solubility between liquid and solid aluminum. To produce high quality castings, it is necessary to reduce the hydrogen concentration in the molten metals during the casting process to an acceptable level 1111. Although many investigators have tried to determine the hydrogen content in liquid and solid aluminum alloys [12-U], the values were unreliable because hydrogen could easily be picked up from water vapor in the air and the concentration readily changed. To obtain highly accurate value of hydrogen concentration in molten aluminum, we developed the galvanic cell-type hydrogen sensor using CaZrO,-based proton conductor [16,17]. This sensor works well as hydrogen probe for continuous determination of in situ hydrogen solubility in molten aluminum. The real time and in situ measurement of hydrogen in molten aluminum alloys is required in order
T. Yajima et al./Solid State Ionics 79 (1995) 333-337
334
to control the hydrogen concentration in the practical aluminum casting process. It is impossible to carry out the real time and in situ measurement of hydrogen by other conventional methods. In this paper, the possibility of the real time measurement of hydrogen in molten aluminum alloys using this type of the sensor in practical foundries is investigated.
2. Principle minum
of hydrogen
sensor for molten
alup”(I)
A hydrogen concentration cell can be constructed using a proton conductor as a solid electrolyte. For the concentration cell. H( pu, (I)) [proton conductor I H( /.~u(II))
(1)
the equilibrium electromotive force measured across the electrode/electrolyte interface is given by ~~(11) - ~~(1)
~~(1) and ~~(11) are the chemical potentials of mono atomic hydrogen species at two electrodes, F, the Faraday constant and n, the number of electrons involved in the reversible reaction occurring at the interface between the electrolyte and electrode. IZ is taken to be a unity for atomic hydrogen. For the gaseous concentration cell, the chemical potentials may be replaced by the corresponding partial pressure of hydrogen:
(2)
I-
Hz (O.Olatm) ~~ Stainless tube
Alumina tube
ln PuiI)] In Pan],
y
(3) (4)
where Pu2(I) and P,$II) are hydrogen partial pressures at each electrode compartment and &, is the chemical potential of gaseous hydrogen at a standard state. The electromotive force of this cell, then, is given by the following equation:
Eln ‘H,(‘) 2F
P,?(II)
’
(5)
If one of the hydrogen parital pressures is known, this galvanic cell acts as a hydrogen gas sensor. On the other hand, the well-known Sieverts’ law [18] holds between hydrogen partial pressure in the atmosphere above the melt and hydrogen content in the liquid metal.
S=k\lP,,
n
Proton conductor CaZm 9Ino 103. (I
Ceramic fiber
Glass and ceramic adhesive
Molten aluminum Fig. 1. Schematic
~~(11) = 1[ CL& +RT
E=
= -nFE.
= $[ poH>+RT
diagram of the sensor probe.
where S is the hydrogen concentration (ml-NTP/ 100 g-Al) in molten aluminum, k, the Sieverts’ coefficient and P,* the equilibrium hydrogen partial pressure in the atmosphere over the melt. If a small gas compartment isolated from the air can be created in the melt as shown in Fig. 1, hydrogen gas will come to the compartment from the melt and after equilibration, Sieverts’ law will be satisfied. It is possible to measure the equilibrium hydrogen partial pressure in the compartment by a galvanic cell-type hydrogen sensor using a proton conductor as a solid electrolyte. When this hydrogen partial pressure PHZ is taken as PH2(I) in Eq. (5) and this equation is substituted into Eq. (6), we obtain the following relation between hydrogen concentration in the molten aluminum and the sensor EMF: S = k/P,I(II)
exp(2FE/RT)
(7)
335
T. Yajima et al. /Solid State Ionics 79 (1995) 333-337
Using this equation, it is possible to determine the hydrogen content in molten aluminum from EMF of the hydrogen sensor if P,$II) is known and constant.
Table I Comparison of hydrogen concentrations determined by the sensor and by inert gas carrier extraction method. Ahoy: 99.99% Al Sample no.
1 2 3 4
3. Experimental A sensor probe was constructed using cap-shaped as the solid electrolyte as shown in CaZr,,In&-, Fig. 1. In the ceramic cap, alumina powder and carbon fiber are packed to reduce the dead volume of the gas phase. One percent of hydrogen gas balanced with argon was used as the standard gas for the concentration-type hydrogen sensor. 10 to 40 kg of aluminum alloys were melted in the graphite crucible by an electric furnace.
4. Results and discussion The EMFs of the sensor were measured under various conditions when the sensor was immersed into molten aluminum. At the same time, we measured hydrogen concentration in the melt by the hydrogen analyzer for molten aluminum produced by Sumitomo Light Metal which was based on the equilibrium partial pressure method reported by Ransley et al. [12]. Fig. 2 shows the relation between the sensor EMF and hydrogen concentration in
This sensor (ml (100 g-Al_’
)
Inert gas extraction (ml (100 g-Al)) 0.08 0.16 0.17 0.27
0.084 0.184 0.148 0.235
molten aluminum. The following derived from Eq. (7): RT E = G@
method
equation
can be
S* k2P,(II)
.
The solid line in Fig. 2 is the theoretical value calculated from Eq. (8) taking the hydrogen partial pressure in the standard gas, P,$II), is 0.01 atm. Each EMF value of the sensor is close to the theoretical one. From this result it is clear that the hydrogen concentration measurement in molten aluminum is possible by the new sensor developed in this study using proton conducting ceramic as a solid electrolyte. To calculate the hydrogen concentration from the sensor EMF, it is necessary to know the Sieverts’ coefficient k. In the temperature range from 670 to 850°C the Sieverts’ coefficient k for pure aluminum has been determined by Ransley and Neufeld [ll] and was given by the following equation: - 2760 log k = ~ + 1.356, T
(9)
where T is the absolute temperature. From Eqs. (7) and (9), the following equation can be derived:
PureAl
S = 10t(-2760/T)+ 1~3561/Pn~II) exp( 2FE/RT)
. (10)
---o----
:75()“c
-
:8OO”c
log@* / (ml/lOOg-Al)*) Fig. 2. Comparison of EMF of hydrogen sensor with theoretical EMF calculated from Sieverts’ law and Nemst equation. Solid lines show theoretical EMF value. Alloy is 99.8% aluminum.
Eq. (10) holds for pure aluminum and corrections need to be made for other alloys. Hydrogen measurement results obtained by this sensor in 99.99% Al are shown in Table 1 with the analyzed values by inert gas extraction method which is one of the most popular methods of hydrogen measurement in molten aluminum alloys. In this case, hydrogen concentrations by this sensor were calculated from the sensor EMFs and Eq. (10). The
336
T. Yajima et al. /Solid
Table 2 Typical hydrogen Alloys
a
measurement
results before and after degassing
Degassing
procedure
(JIS No.) AC4C AC4C AC4CH AC2B AC2B 99.999% Al
Degassing flux Inert gas bubbling by lance pipe Rotary gas bubbling Rotary gas bubbling Vacuum degassing Rotary gas bubbling
a Alloy names are Japanese
industrial
State Ionics 79 (1995) 333-337
processes
Treatment time (min)
Before degassing (ml (100 g-AI_‘)
After degassing (ml (100 g-AI_‘)
10 40
0.298 0.286
0.275 0.114
10 4 30 _
0.220 0.181 0.398 0.264
0.076 0.065 0.048 0.083
standard.
hydrogen concentration values determined by the sensor are close to the value by the inert gas extraction method although the value by the sensor was slightly lower than that by the inert gas extraction method. In the latter method, the values are often overestimated because of uptake of hydrogen from humid air during sampling and/or measuring process. As we reported earlier [17], the in situ measurement of hydrogen concentration is possible during the degassing process while using this sensor. There are several methods for hydrogen degassing of molten aluminum alloy in a practical foundry. Here, we tried to measure the decreasing hydrogen concentration during several kinds of degassing treatments as shown in Fig. 3 and Table 2. This hydrogen analyzer worked
stably and reproducibly. In these cases about 40 kg of AC4C alloy (i.e. Japanese industrial standard alloy) was used. It is possible to measure the variation of hydrogen concentration in the melt continuously during many kinds of degassing treatments as shown in Fig. 3. In the case of the degassing flux treatment, hydrogen concentration in the melt decreased from 0.28 ml/100 g-Al to 0.18 ml/100 g-Al after the treatment. Lower hydrogen level could be achieved by the inert gas bubbling treatment using argon gas. The rotary-type degassing machine has the best performance in these treatments and lowered the hydrogen concentration from 0.28 to 0.06 ml/100 g-Al within 8 min. The stable and reproducible performance of this sensor is attributed to the essential qualities of this proton-conducting ceramics which are the chemical and thermal stabilities and almost pure proton conductivity in this condition. The results described above suggest that the real time and in situ measurement of hydrogen concentration in molten aluminum alloys is possible using the galvanic cell-type hydrogen sensor using proton conducting CaZr,,,,In,~,O, _ LI. In the future studies, this proton-conducting ceramic-type hydrogen sensor will be applied to other molten metals.
Ar gas bubbling by lance pipe
5. Conclusion
Ar ga; bubbling by rotary type degassinp machine
’
’
’
I ’ 2.500
‘
3
Time / set Fig. 3. Comparison of hydrogen decreasing behavior during degassing by different treatments. Alloy is AC4C which is Japanese industrial standard. Temperature is 740°C.
It is clear that a galvanic cell-type hydrogen sensor using proton conducting ceramics as a solid electrolyte can be useful to measure hydrogen concentration in molten aluminum alloys. The sensor developed in this study makes it possible to measure
T. Yajima et al. /Solid
State Ionics 79 (1995) 333-337
hydrogen content in molten Al continuously in practical aluminum foundry processes such as the degassing treatments, etc. Such in situ hydrogen measurement is impossible by other conventional methods. The results described above suggest that this sensor makes it possible to control the degassing process by feeding the measured data back to a process controller.
References HI H. Iwahara, T. Esaka, H. Uchida and N. Maeda, Solid State Ionics 3/4
(1981) 359.
El H. Iwahara, H. Uchida, K. Kondo and K. Ogaki, J. Electrochem. Sot. 135 (1988) 529. 131 T. Yajima, H. Kazeoka, T. Yogo and H. Iwahara, Solid State Ionics 47 (1991) 271. [41 T. Yajima, H. Suzuki, T. Yogo and H. Iwahara, Solid State Ionics 51 (1992) 101.
337
[5] H. Iwahara, T. Yajima, T. Hibino, K. Ozaki and H. Suzuki, Solid State Ionics 61 (1993) 65. [6] H. Iwahara and H. Uchida, Proc. Intern. Meeting Chemical Sensors, Fukuoka, Japan (Kodansha-Elsevier, Tokyo, 1983) p. 227. [7] T. Yajima, H. Iwahara, K. Koide and K. Yamamoto, Sensors Actuators BS (1991) 145. [8] H. Iwahara, H. Uchida and S. Tanaka, J. Appl. Electrochem. 16 (1986) 663. [9] H. Iwahara, H. Uchida and K. Morimoto, 1. Electrochem. Sot. 137 (1990) 462. [lo] H. Iwahara, T. Esaka, U. Uchida, T. Yamauchi and K. Ogaki, Solid State Ionics 18/19 (1986) 1003. [ll] C.E. Ransley and H. Neufeld, J. Inst. Metals 74 (1948) 599. [12] C.E. Ransley, D.E.J. Tablot and H.C. Barlow, Aluminium 34 (1958) 643. [13] D.J. Neil and A.C. Burr, Trans. Am. Foundrymen’s Sot. 69 (1961) 272. [14] G. Ohira and V. Kondic, Foundry Trade J. 96 (1954) 331. [15] R. Gee and D.J. Fray, Metal. trans. B 9B (1978) 427. [16] T. Yajima, H. Iwahara, N. Fukatsu, T. Ohashi and K. Koide, J. Japan Inst. Light Met. 42 (1992) 263. [17] T. Yajima, K. Koide, N. Fukatsu, T. Ohashi and H. Iwahara, Sensors Actuators B13/14 (1993) 697. [18] A. Sieverts, Z. Metallk. 29 (1929) 37.