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Langasite for high-temperature acoustic wave gas sensors
Sensors and Actuators B 93 (2003) 169–174
Langasite for high-temperature acoustic wave gas sensors Huankiat Seha,*, Harry L. Tullera, Holger Fritzeb a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Room 13-4010, 77 Massachusetts Avenue, Cambridge, MA 02139, USA b Department of Physics, Metallurgy and Materials Science, Technische Universita¨t Clausthal, D-38678 Clausthal-Zellerfeld, Germany
Abstract Langasite (La3Ga5SiO14), a piezoelectric material with no phase transitions to 1470 8C, holds promise as a high-sensitivity crystal microbalance gas sensor platform. In order to successfully implement langasite acoustic wave gas sensors, electrical losses at elevated temperatures need be minimized. The electrical conductivity of langasite is characterized as a function of oxygen partial pressure for temperatures of 700–1000 8C and correlated with the decrease in Q with increasing temperature. These results are used to define the operating limits for acoustic wave gas sensors based on langasite. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Langasite; BAW; Chemical gas sensors; High temperature
1. Introduction Bulk acoustic wave (BAW) devices, represented by the quartz crystal microbalance (QCM), characteristically provide exceptional mass sensitivity via measurable shifts in resonant frequency. Such devices can be converted into gas sensors by depositing films with selectivity to specific molecules onto the device surface. As gas molecules adsorb on the film, they are detected by monitoring the shift in the device resonant frequency. Because the QCM is relatively inexpensive and readily available, it is the system of choice for sensors that operate at or near ambient conditions. For many applications, however, higher-temperature operation is required. The absolute maximum operating temperature of quartz-based devices, for example, is limited by the destructive alpha-beta transition at 573 8C. To overcome this limitation, our group has undertaken the investigation of alternative materials, focusing initially on langasite (La3Ga5SiO14) which exhibits no phase transitions up to its melting point of 1470 8C and for which high-quality crystals are readily available. In initial investigations, a langasite bulk acoustic wave device was successfully operated as a microbalance to temperatures as high as 900 8C [1,2]. By adding a TiO2 film onto the langasite resonator, sensitivity to hydrogen gas was further demonstrated [3,4] and means for minimizing * Corresponding author. Tel.: þ1-617-258-5748; fax: þ1-617-253-2364. E-mail address: [email protected] (H. Seh).
temperature cross sensitivity was presented [5]. In these studies, a general drop in quality factor Q was observed with increasing temperature, resulting in reduced resolution in mass-induced frequency shifts. This drop in Q value is suspected, in large part, to be due to increasing electrical losses induced by corresponding increases in bulk conductivity. In a preliminary investigation of the electrical and transport properties of langasite, the results pointed to mixed ionic and electronic conduction with the overall electrical resistivity, as measured in air, decreasing exponentially with reciprocal temperature with an activation energy of the order of 1 eV [1]. Since the conductivity of most oxides is known to be dependent on the oxygen partial pressure and dopants as well as temperature, it is necessary to know these dependencies before being able to define the operating limits of such sensors. Consequently, in this work, we present the results of a detailed investigation of the bulk conductivity of langasite as function of temperature, and oxygen partial pressure and initial results related to the effects of dopants. These results enable the mapping of iso-conductivity lines, and the calculation of expected Q values taking into account the effect of the bulk resistance. These results thereby provide a guide to the operating limits for an acoustic wave gas sensor fabricated from langasite. These results also provide key information needed in the derivation of an appropriate defect model which allows for predictive capabilities in optimizing the sensor material. In this paper, we will provide a brief outline of the defect model necessary to
0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00189-8
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H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
explain the observed electrical behavior of langasite. A detailed analysis will be provided in a future paper.
leaving behind oxygen vacancies VO and excess electrons, e0 . This reduction reaction can be described by: OxO $ VO þ 2e0 þ 12 O2
2. Theory 2.1. Modified expression for Q At low-temperature operation, the resonator can be assumed to have bulk resistance large enough to be ignored in the resonant equivalent circuit. The quality value, Q, is then defined as: pffiffiffiffiffi Ls Q ¼ pffiffiffiffiffi (1) Rs Cs where Rs, Ls and Cs are the series components of the resonator equivalent circuit, and have the physical meanings as defined in [6,7]. However, at higher temperatures, the bulk resistance is sufficiently small so that it has to be considered in the modified equivalent circuit as shown in Fig. 1. A low bulk parallel resistance, Rp, should result in electrical losses and a corresponding drop in Q. It is therefore necessary to redefine Q to take into account the bulk resistance, Rp. The expression for the modified Q is given by [7]: rffiffiffiffiffi Rp Ls Qmodifed ¼ 2 (2) ðRs þ Rs Rp Þ Cs Eq. (2) shows a clear link between Rp (the bulk resistance) and Q. Since Fritze et al. [7] had determined that Ls and Cs have only slight dependences on temperature, Rs and Rp should have the most influence on Q at high temperatures. 2.2. Defect model Under equilibrium conditions, the oxygen stoichiometry of an oxide depends on the equilibrium oxygen partial pressure, pO2, under which it is equilibrated. Decreasing the pO2 will result in oxygen OxO leaving the lattice and
(3) 0
in which superscripts x, ( ), ( ) correspond to relative charges of 0, þ1 and 1, respectively. The corresponding mass action relation is given by: 1=2
KR ¼ ½VO n2 pO2
(4)
in which [VO ] and n correspond to oxygen vacancy and electron concentrations respectively. The neutrality equation: 2½VO ¼ ½A0 þ n
(5)
assumes that vacancies can be formed due to the introduction of acceptor impurities A0 as well as via the reduction reaction defined in Eq. (3). There are two cases for consideration: (a) At high pO2 where n is low, then (5) can be approximated by: 2½VO ½A0 6¼ f ðpO2 Þ
(6)
Substituting (6) into (4) yields: n / pO2 1=4
(7)
(b) At low pO2, where n becomes much larger than the background acceptor concentration, (6) can be approximated by: 2½VO n
(8)
Substituting (8) into (4) yields: n / pO2 1=6
(9)
Consequently, the ionic conductivity can be expressed as: sionic ¼ 2½VO qmi 6¼ f ðpO2 Þ
(10)
while the electronic conductivity would have a pO2 1=4 dependence in case (a): selectronic ¼ nqme / pO2 1=4
(11)
and a pO2 1=6 dependence in case (b): selectronic ¼ nqme / pO2 1=6
(12)
The total conductivity would therefore be: stotal ¼ sionic þ selectronic ¼ 2½VO qmi þ nqme
(13)
3. Experimental
Fig. 1. Modified resonator equivalent circuit, with the additional parallel bulk resistance, Rp, term. Rs, Cs and Ls are the series components of the resonator circuit. Cp is the parallel bulk capacitance.
Because single crystal langasite was found to exhibit slow reduction-oxidation kinetics, even at elevated temperatures, in this study we utilized polycrystalline specimens with interconnected porosity. Stoichiometric amounts of lanthanum, gallium and silicon oxide were mixed and ball-milled
H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
171
Fig. 2. Complex impedance plots of nominally undoped langasite at 700–1000 8C and oxygen partial pressure of 1 atm.
for approximately 24 h and then pressed into 1 in. pellets. Niobium oxide was added for donor doping. The pellets were fired at 1450 8C in air for 10 h. Densities of approximately 90% were obtained with a grain size on the order of
10 mm. X-ray diffraction showed the material to be langasite with no observable second phases. Pellets with effective cross sectional area of 6.55 mm2 and length of 5.6 mm were electroded with platinum using
Fig. 3. Bulk conductivity of nominally undoped langasite as function of oxygen partial pressure and temperature.
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H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
Fig. 4. The pO2 independent conductivity as a function of temperature for both undoped and 5% Nb-doped langasite.
platinum ink. The ac complex impedance measurements were conducted using a frequency response analyzer (Solartron 1260). A typical complex impedance plot for an oxygen partial pressure of 1 atm is shown in Fig. 2. The bulk conductivity values were extracted by fitting the spectra to the appropriate RC circuits. Samples were heated in a tube furnace to temperatures of 700–1000 8C with the oxygen partial pressure controlled with Ar/O2 (for high pO2 range) and CO/CO2 (for low pO2 range) gas mixtures.
4. Results and discussions The bulk conductivity of nominally undoped langasite was extracted from the complex impedance plots and plotted in Fig. 3 as function of oxygen partial pressure and temperature. The oxygen partial pressure independent component of the total conductivity of the nominally undoped langasite was assumed to be ionic, as predicted by the defect model, Eq. (10). Fig. 4 shows the ionic conductivity plotted
Fig. 5. Electronic conductivity of undoped langasite, with pO2 1=4 dependence.
H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
173
Fig. 6. Electronic conductivity of undoped and 5% Nb-doped langasite extrapolated to pO2 ¼ 1 atm as function of temperature.
versus reciprocal temperature for nominally undoped langasite. Given a fixed vacancy density, the activation energy of 0.81 eV must represent the migration energy of oxygen vacancies in the nominally undoped langasite. The electronic conductivity of the nominally undoped langasite can be obtained by subtracting the ionic conductivity from the bulk conductivity (see Eq. (13)). Fig. 5 shows the electronic conductivity of nominally undoped langasite, following, within experimental error, a pO2 1=4 dependence, consistent with case (a) of the defect model. Fig. 6 shows the electronic conductivity, extrapolated to pO2 ¼ 1 atm, plotted versus reciprocal temperature. The 3 eV activation energy for undoped langasite corresponds to half of the reduction enthalpy describing reaction (3). The addition of Nb, as expected, contributed to a significant increase in n (see Fig. 6). Surprisingly, it did not have the expected depressing effect on the oxygen independent conductivity (Fig. 4). We suspect that the pO2 independent conductivity in the Nb-doped material is actually electronic rather than ionic in nature. This will be confirmed once a more systematic examination of the effect of donor concentration on the electrical conductivity is completed. From langasite resonator studies in [7] and, in part, from this study, Rs and Rp were found to have significant temperature dependences. A series of iso-Q values lines are plotted as function of Rs and Rp in Fig. 7. In addition, the measured Rs and Rp values, at various temperatures, are also indicated in Fig. 7 for reference. One observes two regimes. One in which Q is dependent only on Rs, i.e. where the iso-Q lines are vertical and the second in which Q becomes
dependent on both Rs and Rp. For the given data, one expects significant influence of Rp on Q only above a temperature of approximately 800 8C where Rp drops to approximately 5 103 O which corresponds to log s of 4.5 S/cm. With the obtained conductivity data, a map of iso-conductivity lines could be plotted as a function of temperature and oxygen partial pressure as in Fig. 8. This now provides a guide to the operating limits of an acoustic wave resonant gas sensor fabricated from langasite, based on the maximum tolerable conductivity for a desired value of Q. For example, to insure that one limits log conductivity to less than 4.5, one would have to operate in the right hand upper quadrant defined by the curve designated by the solid black squares, i.e. below about 830 8C and above log pO2 of about 15 to
Fig. 7. Iso-Q values plot. The markers (&) show the Rp and Rs values obtained for a langasite resonator studied in [7] at various temperatures.
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H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
Fig. 8. Iso-conductivity plot for undoped langasite.
30 depending on temperature. The lower the temperature, the lower the accessible oxygen partial pressure. 5. Conclusion We have characterized the electrical behavior of langasite polycrystalline samples. A defect model was used to describe the electrical behavior of nominally undoped langasite and the dominant sources of electrical conduction contributing to electrical losses. Iso-conductivity lines, depending on the tolerable electrical losses, were plotted as function of temperature and oxygen partial pressure, and the expected Q values were plotted as a function of the bulk resistance, Rp. These results suggest that Q values in langasite remain insensitive to the magnitude of the electrical conductivity to temperatures of at least 800 8C. Reducing the conductivity of langasite would improve Q and enable operation to even higher temperatures. We are now investigating means to achieve this by modifications in the material’s composition. Acknowledgements This work was supported by the National Science Foundation under Grant nos. DMR-9701699 and INT-9910012,
the Germany Exchange Service (DAAD) and the German Government (BMBF).
References [1] H. Fritze, H.L. Tuller, G. Borchardt, T. Fukuda, High temperature properties of langasite, in: R. Gotthardt, K. Uchino, Y. Ito, M. WunFogle (Eds.), Proceedings of the Symposium on Smart Materials, MRS Symposium Proceedings, vol. 604, Materials Research Society, Warrendale, PA, 2000, pp. 65–70. [2] H. Fritze, H.L. Tuller, Langasite for high temperature bulk acoustic wave applications, Appl. Phys. Lett. 78 (2001) 976–977. [3] H. Fritze, H.L. Tuller, H. Seh, G. Borchardt, High temperature nanobalance sensor based on langasite, Sens. Actuators B 76 (2001) 103–107. [4] H. Fritze, H. Seh, H.L. Tuller, G. Borchardt, Operation limits of langasite high temperature nanobalances, J. Eur. Ceram. Soc. 21 (2001) 1473–1477. [5] H Fritze, Gas Sensor Concept Based on High Temperature Piezoelectric Materials, in: Proceedings of the Conference Sensoren und Mess-Systeme, Ludwigsburg, 11–12 March 2002. [6] R. Lucklum, P. Hauptmann, Determination of shear modulus with quartz crystal resonators, Faraday Dis. 107 (1997) 123–140. [7] H. Fritze, O. Schneider, G. Borchardt, High temperature bulk acoustic wave properties of gallium orthophosphate and langasite, in: Proceedings of the International Meeting on Chemical Sensors, Boston, 9–10 July 2002.
Langasite for high-temperature acoustic wave gas sensors Huankiat Seha,*, Harry L. Tullera, Holger Fritzeb a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Room 13-4010, 77 Massachusetts Avenue, Cambridge, MA 02139, USA b Department of Physics, Metallurgy and Materials Science, Technische Universita¨t Clausthal, D-38678 Clausthal-Zellerfeld, Germany
Abstract Langasite (La3Ga5SiO14), a piezoelectric material with no phase transitions to 1470 8C, holds promise as a high-sensitivity crystal microbalance gas sensor platform. In order to successfully implement langasite acoustic wave gas sensors, electrical losses at elevated temperatures need be minimized. The electrical conductivity of langasite is characterized as a function of oxygen partial pressure for temperatures of 700–1000 8C and correlated with the decrease in Q with increasing temperature. These results are used to define the operating limits for acoustic wave gas sensors based on langasite. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Langasite; BAW; Chemical gas sensors; High temperature
1. Introduction Bulk acoustic wave (BAW) devices, represented by the quartz crystal microbalance (QCM), characteristically provide exceptional mass sensitivity via measurable shifts in resonant frequency. Such devices can be converted into gas sensors by depositing films with selectivity to specific molecules onto the device surface. As gas molecules adsorb on the film, they are detected by monitoring the shift in the device resonant frequency. Because the QCM is relatively inexpensive and readily available, it is the system of choice for sensors that operate at or near ambient conditions. For many applications, however, higher-temperature operation is required. The absolute maximum operating temperature of quartz-based devices, for example, is limited by the destructive alpha-beta transition at 573 8C. To overcome this limitation, our group has undertaken the investigation of alternative materials, focusing initially on langasite (La3Ga5SiO14) which exhibits no phase transitions up to its melting point of 1470 8C and for which high-quality crystals are readily available. In initial investigations, a langasite bulk acoustic wave device was successfully operated as a microbalance to temperatures as high as 900 8C [1,2]. By adding a TiO2 film onto the langasite resonator, sensitivity to hydrogen gas was further demonstrated [3,4] and means for minimizing * Corresponding author. Tel.: þ1-617-258-5748; fax: þ1-617-253-2364. E-mail address: [email protected] (H. Seh).
temperature cross sensitivity was presented [5]. In these studies, a general drop in quality factor Q was observed with increasing temperature, resulting in reduced resolution in mass-induced frequency shifts. This drop in Q value is suspected, in large part, to be due to increasing electrical losses induced by corresponding increases in bulk conductivity. In a preliminary investigation of the electrical and transport properties of langasite, the results pointed to mixed ionic and electronic conduction with the overall electrical resistivity, as measured in air, decreasing exponentially with reciprocal temperature with an activation energy of the order of 1 eV [1]. Since the conductivity of most oxides is known to be dependent on the oxygen partial pressure and dopants as well as temperature, it is necessary to know these dependencies before being able to define the operating limits of such sensors. Consequently, in this work, we present the results of a detailed investigation of the bulk conductivity of langasite as function of temperature, and oxygen partial pressure and initial results related to the effects of dopants. These results enable the mapping of iso-conductivity lines, and the calculation of expected Q values taking into account the effect of the bulk resistance. These results thereby provide a guide to the operating limits for an acoustic wave gas sensor fabricated from langasite. These results also provide key information needed in the derivation of an appropriate defect model which allows for predictive capabilities in optimizing the sensor material. In this paper, we will provide a brief outline of the defect model necessary to
0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00189-8
170
H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
explain the observed electrical behavior of langasite. A detailed analysis will be provided in a future paper.
leaving behind oxygen vacancies VO and excess electrons, e0 . This reduction reaction can be described by: OxO $ VO þ 2e0 þ 12 O2
2. Theory 2.1. Modified expression for Q At low-temperature operation, the resonator can be assumed to have bulk resistance large enough to be ignored in the resonant equivalent circuit. The quality value, Q, is then defined as: pffiffiffiffiffi Ls Q ¼ pffiffiffiffiffi (1) Rs Cs where Rs, Ls and Cs are the series components of the resonator equivalent circuit, and have the physical meanings as defined in [6,7]. However, at higher temperatures, the bulk resistance is sufficiently small so that it has to be considered in the modified equivalent circuit as shown in Fig. 1. A low bulk parallel resistance, Rp, should result in electrical losses and a corresponding drop in Q. It is therefore necessary to redefine Q to take into account the bulk resistance, Rp. The expression for the modified Q is given by [7]: rffiffiffiffiffi Rp Ls Qmodifed ¼ 2 (2) ðRs þ Rs Rp Þ Cs Eq. (2) shows a clear link between Rp (the bulk resistance) and Q. Since Fritze et al. [7] had determined that Ls and Cs have only slight dependences on temperature, Rs and Rp should have the most influence on Q at high temperatures. 2.2. Defect model Under equilibrium conditions, the oxygen stoichiometry of an oxide depends on the equilibrium oxygen partial pressure, pO2, under which it is equilibrated. Decreasing the pO2 will result in oxygen OxO leaving the lattice and
(3) 0
in which superscripts x, ( ), ( ) correspond to relative charges of 0, þ1 and 1, respectively. The corresponding mass action relation is given by: 1=2
KR ¼ ½VO n2 pO2
(4)
in which [VO ] and n correspond to oxygen vacancy and electron concentrations respectively. The neutrality equation: 2½VO ¼ ½A0 þ n
(5)
assumes that vacancies can be formed due to the introduction of acceptor impurities A0 as well as via the reduction reaction defined in Eq. (3). There are two cases for consideration: (a) At high pO2 where n is low, then (5) can be approximated by: 2½VO ½A0 6¼ f ðpO2 Þ
(6)
Substituting (6) into (4) yields: n / pO2 1=4
(7)
(b) At low pO2, where n becomes much larger than the background acceptor concentration, (6) can be approximated by: 2½VO n
(8)
Substituting (8) into (4) yields: n / pO2 1=6
(9)
Consequently, the ionic conductivity can be expressed as: sionic ¼ 2½VO qmi 6¼ f ðpO2 Þ
(10)
while the electronic conductivity would have a pO2 1=4 dependence in case (a): selectronic ¼ nqme / pO2 1=4
(11)
and a pO2 1=6 dependence in case (b): selectronic ¼ nqme / pO2 1=6
(12)
The total conductivity would therefore be: stotal ¼ sionic þ selectronic ¼ 2½VO qmi þ nqme
(13)
3. Experimental
Fig. 1. Modified resonator equivalent circuit, with the additional parallel bulk resistance, Rp, term. Rs, Cs and Ls are the series components of the resonator circuit. Cp is the parallel bulk capacitance.
Because single crystal langasite was found to exhibit slow reduction-oxidation kinetics, even at elevated temperatures, in this study we utilized polycrystalline specimens with interconnected porosity. Stoichiometric amounts of lanthanum, gallium and silicon oxide were mixed and ball-milled
H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
171
Fig. 2. Complex impedance plots of nominally undoped langasite at 700–1000 8C and oxygen partial pressure of 1 atm.
for approximately 24 h and then pressed into 1 in. pellets. Niobium oxide was added for donor doping. The pellets were fired at 1450 8C in air for 10 h. Densities of approximately 90% were obtained with a grain size on the order of
10 mm. X-ray diffraction showed the material to be langasite with no observable second phases. Pellets with effective cross sectional area of 6.55 mm2 and length of 5.6 mm were electroded with platinum using
Fig. 3. Bulk conductivity of nominally undoped langasite as function of oxygen partial pressure and temperature.
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H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
Fig. 4. The pO2 independent conductivity as a function of temperature for both undoped and 5% Nb-doped langasite.
platinum ink. The ac complex impedance measurements were conducted using a frequency response analyzer (Solartron 1260). A typical complex impedance plot for an oxygen partial pressure of 1 atm is shown in Fig. 2. The bulk conductivity values were extracted by fitting the spectra to the appropriate RC circuits. Samples were heated in a tube furnace to temperatures of 700–1000 8C with the oxygen partial pressure controlled with Ar/O2 (for high pO2 range) and CO/CO2 (for low pO2 range) gas mixtures.
4. Results and discussions The bulk conductivity of nominally undoped langasite was extracted from the complex impedance plots and plotted in Fig. 3 as function of oxygen partial pressure and temperature. The oxygen partial pressure independent component of the total conductivity of the nominally undoped langasite was assumed to be ionic, as predicted by the defect model, Eq. (10). Fig. 4 shows the ionic conductivity plotted
Fig. 5. Electronic conductivity of undoped langasite, with pO2 1=4 dependence.
H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
173
Fig. 6. Electronic conductivity of undoped and 5% Nb-doped langasite extrapolated to pO2 ¼ 1 atm as function of temperature.
versus reciprocal temperature for nominally undoped langasite. Given a fixed vacancy density, the activation energy of 0.81 eV must represent the migration energy of oxygen vacancies in the nominally undoped langasite. The electronic conductivity of the nominally undoped langasite can be obtained by subtracting the ionic conductivity from the bulk conductivity (see Eq. (13)). Fig. 5 shows the electronic conductivity of nominally undoped langasite, following, within experimental error, a pO2 1=4 dependence, consistent with case (a) of the defect model. Fig. 6 shows the electronic conductivity, extrapolated to pO2 ¼ 1 atm, plotted versus reciprocal temperature. The 3 eV activation energy for undoped langasite corresponds to half of the reduction enthalpy describing reaction (3). The addition of Nb, as expected, contributed to a significant increase in n (see Fig. 6). Surprisingly, it did not have the expected depressing effect on the oxygen independent conductivity (Fig. 4). We suspect that the pO2 independent conductivity in the Nb-doped material is actually electronic rather than ionic in nature. This will be confirmed once a more systematic examination of the effect of donor concentration on the electrical conductivity is completed. From langasite resonator studies in [7] and, in part, from this study, Rs and Rp were found to have significant temperature dependences. A series of iso-Q values lines are plotted as function of Rs and Rp in Fig. 7. In addition, the measured Rs and Rp values, at various temperatures, are also indicated in Fig. 7 for reference. One observes two regimes. One in which Q is dependent only on Rs, i.e. where the iso-Q lines are vertical and the second in which Q becomes
dependent on both Rs and Rp. For the given data, one expects significant influence of Rp on Q only above a temperature of approximately 800 8C where Rp drops to approximately 5 103 O which corresponds to log s of 4.5 S/cm. With the obtained conductivity data, a map of iso-conductivity lines could be plotted as a function of temperature and oxygen partial pressure as in Fig. 8. This now provides a guide to the operating limits of an acoustic wave resonant gas sensor fabricated from langasite, based on the maximum tolerable conductivity for a desired value of Q. For example, to insure that one limits log conductivity to less than 4.5, one would have to operate in the right hand upper quadrant defined by the curve designated by the solid black squares, i.e. below about 830 8C and above log pO2 of about 15 to
Fig. 7. Iso-Q values plot. The markers (&) show the Rp and Rs values obtained for a langasite resonator studied in [7] at various temperatures.
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H. Seh et al. / Sensors and Actuators B 93 (2003) 169–174
Fig. 8. Iso-conductivity plot for undoped langasite.
30 depending on temperature. The lower the temperature, the lower the accessible oxygen partial pressure. 5. Conclusion We have characterized the electrical behavior of langasite polycrystalline samples. A defect model was used to describe the electrical behavior of nominally undoped langasite and the dominant sources of electrical conduction contributing to electrical losses. Iso-conductivity lines, depending on the tolerable electrical losses, were plotted as function of temperature and oxygen partial pressure, and the expected Q values were plotted as a function of the bulk resistance, Rp. These results suggest that Q values in langasite remain insensitive to the magnitude of the electrical conductivity to temperatures of at least 800 8C. Reducing the conductivity of langasite would improve Q and enable operation to even higher temperatures. We are now investigating means to achieve this by modifications in the material’s composition. Acknowledgements This work was supported by the National Science Foundation under Grant nos. DMR-9701699 and INT-9910012,
the Germany Exchange Service (DAAD) and the German Government (BMBF).
References [1] H. Fritze, H.L. Tuller, G. Borchardt, T. Fukuda, High temperature properties of langasite, in: R. Gotthardt, K. Uchino, Y. Ito, M. WunFogle (Eds.), Proceedings of the Symposium on Smart Materials, MRS Symposium Proceedings, vol. 604, Materials Research Society, Warrendale, PA, 2000, pp. 65–70. [2] H. Fritze, H.L. Tuller, Langasite for high temperature bulk acoustic wave applications, Appl. Phys. Lett. 78 (2001) 976–977. [3] H. Fritze, H.L. Tuller, H. Seh, G. Borchardt, High temperature nanobalance sensor based on langasite, Sens. Actuators B 76 (2001) 103–107. [4] H. Fritze, H. Seh, H.L. Tuller, G. Borchardt, Operation limits of langasite high temperature nanobalances, J. Eur. Ceram. Soc. 21 (2001) 1473–1477. [5] H Fritze, Gas Sensor Concept Based on High Temperature Piezoelectric Materials, in: Proceedings of the Conference Sensoren und Mess-Systeme, Ludwigsburg, 11–12 March 2002. [6] R. Lucklum, P. Hauptmann, Determination of shear modulus with quartz crystal resonators, Faraday Dis. 107 (1997) 123–140. [7] H. Fritze, O. Schneider, G. Borchardt, High temperature bulk acoustic wave properties of gallium orthophosphate and langasite, in: Proceedings of the International Meeting on Chemical Sensors, Boston, 9–10 July 2002.