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Enhanced accuracy of palladium-nickel alloy based hydrogen sensor by in situ temperature compensation
Sensors & Actuators: B. Chemical 299 (2019) 126989
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced accuracy of palladium-nickel alloy based hydrogen sensor by in situ temperature compensation
T
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Kun Yu1, Xianqing Tian1, Xinfeng Wang , Fang Yang, Tianjiao Qi, Ji Zuo Institute of Chemical Materials, China Academy of Engineering Physics, P.O. box 919–327, Mianyang 621999, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen sensor PdNi alloy Temperature compensation Enhanced accuracy
The influence of working temperature on the accuracy of the PdNi alloy thin film hydrogen sensor was investigated. The PdNi hydrogen sensor was fabricated on a silicon chip together with a Pt temperature sensor by DC sputtering. The PdNi hydrogen sensor shows excellent response to H2 with concentration from 0.02% to 40% at room temperature. Results show that the response of PdNi hydrogen sensor is nonlinearly related to the hydrogen concentration and working temperature. And correction method only based on considering the baseline temperature correction cannot obtain high measurement accuracy in the whole range area. Therefore, in order to improve the full range detection accuracy of PdNi hydrogen sensor, a modified algorithm which takes baseline and sievert constant temperature compensation into account is presented in this work. After applying the algorithm, the PdNi hydrogen sensor exhibits enhanced H2 sensing accuracy and stability even in violent temperature fluctuate situations.
1. Introduction As an ideal replacement for fossil fuels, hydrogen is a key energy carrier of future for fuel cell powered vehicles, efficient energy storage devices, and etc [1,2]. In the meantime, H2 as a disease mediator [3,4] and/or detonation/degradation product of energetic materials [5], increasing research interests also have been raised in the asset health monitoring and safety applications, respectively. Hence, several sensing technologies have been employed to detect H2. Classification of H2 sensors based on a range of technologies including catalytic combustion, thermal conductivity, work-function based, mechanical effects and optical effects has been established and reviewed by Hübert [6] and other researchers [7–10]. Among them, one group of the technologies is based on palladium (Pd) [11,12] or Pd alloy [13–24]. Pd is popular for H2 sensing since it selectively absorbs H2. H2 molecules are dissociated into atomic H on the Pd surface and further absorbed by the Pd causing a volumetric expansion. The volumetric expansion increases linearly with increasing H concentration. H2 absorption of Pd results in two different solid phases: the so called α− and β− phase. This α−β phase change possesses several challenges in H2 sensing application, such as irreversible deformation of Pd film, stress severely the interface between the Pd film and the sensor, etc. In addition, the phase transition from the α phase to the β phase of PdHx
occurs at fairly low pH2, which caused irreversible behavior even in thin film (e.g. pure Pd film > 50 nm). Since longevity is one of the core demands for H2 sensors in many applications, lots of efforts have been formulated to avoid the phase change of Pd film [25]. Among them, Pd alloys with addition of Au [19], Ag [17], Ni [21,22,26], Cu [20], Co [14], Mg [13,16], Y [15,24], etc., have been proved to be an effective way to suppress the phase transition from α to β phase and widely utilized in commercial applications (such as H2 separation [27,28]). Also there is a considerable literature on the use of Pd alloys on hydrogen sensors [29–33]. Lately, we have designed a H2 sensor to detect H2 by monitoring the resistance of PdNi alloy thin film [34]. The PdNi hydrogen sensor exhibited excellent H2 sensing performances, but have severe limitations when used in practical applications. The primary limitation is the drift due to the temperature effect. Since we are interested in developing H2 sensors for practical applications, the resistance changes of PdNi hydrogen sensor, in particular, the hydrogen induced resistance change is a matter of vital importance. Because the resistance of the PdNi hydrogen sensor is depended both on the temperature and H2 concentration, especially the response of the PdNi thin film to H2 is also depended on the temperature. Although we can fix the PdNi hydrogen sensor in a constant working temperature condition which is accepted in some commercial products, it cannot utilize in those situations with
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Corresponding author. E-mail address: [email protected] (X. Wang). 1 Both of them contributed equally to this work. https://doi.org/10.1016/j.snb.2019.126989 Received 31 December 2018; Received in revised form 14 August 2019; Accepted 16 August 2019 Available online 21 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
Sensors & Actuators: B. Chemical 299 (2019) 126989
K. Yu, et al.
Scheme 1. Illustration of (A) the PdNi hydrogen sensor and (B) corresponding fabrication steps.
severe temperature fluctuation (e.g. dissolved gas analysis in the insulating mineral oil [35]). Therefore, we focus in this paper on finding temperature compensation method to improve the stability and accuracy of the PdNi hydrogen sensor even with serve temperature variation.
partial pressure of H2, when the dissolution equilibrium is reached, the partial pressure of H2 (pH2) in the mixed gas and the H atom content (xH) in the solid solution satisfy Sievert's law [26,36]:
pH0.52
xH = Ks (T )
(1)
where Ks(T) is Sievert constant, which is temperature dependent; xH is the number of hydrogen atoms assigned to each metal atom (Pd), where x is approximately proportional to the rate of resistance change and can be written as:
2. Experimental 2.1. Fabrication of the hydrogen sensor The schematic description of the PdNi hydrogen sensor (1000 μm × 1000 μm) is shown in Scheme 1, in which a Pt resistor (outer) and a PdNi resistor (inner) with line width of 40 μm was designed as the temperature sensor and hydrogen sensor, respectively. Four gold pads ohmic contacted with the two resistors were utilized to facilitate the electrical connection. And a compact film of silicon nitride was deposited to eliminate the influence of H2 to Pt resistor. The PdNi hydrogen sensor was accomplished with a standard MEMS process and details of the fabrication parameters could be found in our latest publication [34]. Since self-made PdNi mosaic target was utilized thought the whole work, the Ni content in the as-prepared PdNi thin film was controlled between 8.5 wt% and 10 wt%. In such a case, the resistance baseline of the PdNi resistance has a large distribution from 356 Ω to 514 Ω.
xH = a ΔRH , T RH 0, T
(2)
In formula (2), a is a proportional constant, and ΔRH,T is the resistance change of PdNi hydrogen sensor when the hydrogen partial pressure at pH2 and zero. Bring formula (2) into formula (1), thus formula (1) can be rewritten as:
ΔRH , T = aRH 0, T Ks (T ) pH0.52
(3)
Therefore, when the temperature is constant, aKs(T) is a constant. When the pressure of the mixed gas is constant, the pH2 is equal to hydrogen concentration (CH2). And the relationship between the CH2 and the resistance change of the PdNi film (ΔRH , T ) can be written as:
CH0.52 = aKs (T )ΔRH , T / RH 0, T
2.2. Sensor characterization
(4)
where RH 0, T is the resistance of the PdNi thin film at T C without H2. According to Eq. (4), the temperature has two effects on the hydrogen-sensitive PdNi thin film: on the one hand, it directly affects the resistance of PdNi thin film (RH0,T); on the other hand, it affects the distribution constant of hydrogen (Ks (T ) ) in the gas phase and the solid solution, and thus affects the hydrogen content in the solid solution followed with the resistance change of the PdNi thin film. Since the resistance of the PdNi thin film is very sensitive either to the environmental temperature and/or the hydrogen concentration, an effective processing method is necessary for obtaining the true resistance change (ΔRH2) caused by the hydrogen gas. Accordingly, the resistance change has two key components: H2 induced resistance change at T oC and temperature induced resistance change without H2. o
The performance of the PdNi hydrogen sensor was measurement on a homemade sensor evaluation platform which consists of a sensor chamber (10 ml) sealed in the thermostat water bath cauldron and a gas distribution instrument equipped with two high-accuracy mass flow meters. Various working temperatures and concentrations of the tested gases were all applied by this platform. And signals of the PdNi hydrogen sensor were transmitted through a flexible printed circuit cable and recorded by a homemade signal adjusting board. The gas pressure and flow rate in the sensor chamber were controlled at 1 atm and 50 ml min−1, respectively, throughout the whole experiment. Based on the principle of proportional dilution, different hydrogen concentration can be obtained by controlling the flow rate of hydrogen standard gas and diluent gas (N2).
(a) the effect of temperature on the resistance of the PdNi thin film. To simplify the calculation, temperature induced 3D size change of PdNi thin film is ignored. On the basis of the temperature coefficient of resistance (TCR) measurements (data see in Fig. 3), the relationship between the resistance of PdNi thin film and working
2.3. Methodology of TCR and NHS compensation On the surface of PdNi alloy, H2 molecules are dissociate into H atoms and penetrate into PdNi alloy to form solid solution. At lower 2
Sensors & Actuators: B. Chemical 299 (2019) 126989
K. Yu, et al.
Fig. 1. Relationship of the ΔRH , T -CH2-T curve.
Fig. 2. Responses of the PdNi hydrogen sensor to (a) 0.02%∼2%, (b) 0.6%∼3%, (c) 8%∼40% H2 in N2 and (d) relationship of PdNi resistance or ΔR and H2 concentrations at 20 °C.
hydrogen in the palladium alloy,
temperature could be written as:
RT = RT0 + αT +
βT 2
lnKs (T ) = ΔG / RT
(5)
where RT is the resistance of PdNi thin film at T °C, RT0 is the resistance of PdNi thin film at T0 °C, α and β are temperature coefficients. Synchronous results were also obtained from the Pt thin film, from which provide the key temperature information. (b) the effect of temperature on the distribution constant (Ks (T ) ). According to the thermodynamic relationship of the dissolution of
(6)
Ks (T ) is approximately proportional to the reciprocal of temperature. Once substituting Eq. (3) into Eq. (4) yields a sensor quantitative relationship curve with temperature terms. Combined with the hydrogen temperature response test data at different temperatures, the parameters could be obtained by multi-parameter least squares fitting. Fig. 1 shows the ΔRH , T -CH2-T relationship curve 3
Sensors & Actuators: B. Chemical 299 (2019) 126989
K. Yu, et al.
obtained by fitting according to Eq. (1). It can be seen that the influence of the response characteristics of the temperature on palladium nickel alloy is a complex curve, and the sensitivity of the PdNi thin film decreases as the temperature increases. Similar experimental results could be found in Fig. 3B. 3. Results and discussion 3.1. Sensor response at constant temperature Pt thin film has been widely used in applications of temperature measurement, however, it is very sensitive to H2 molecules which will interfere the accuracy of the temperature sensing. In our preliminary tests, we found the H2 molecules could diffuse into the Pt thin film and cause an apparent resistance change of the Pt resistor (see in Figure S1A). Such a shift of the H2 induced resistance is strongly relevant with the H2 concentration and exposure duration and results in false temperature outputs. Hence, it is vital important to isolate the Pt thin film from the H2 molecules. In this work, a sandwich structure (SiNx/Pt/ SiNx) utilizing the silicon nitride (SiNx) as the H2 molecule diffusion barrier was constructed to eliminate the interference of the H2 molecule on the Pt thin film. As we can see in Figure S1B, the sandwich structurebased Pt thin film exhibits stable temperature response with and without the presence of H2 gas. In such a case, accurate temperature of the PdNi hydrogen sensor can be insitu monitored without time and space delays which laid the groundwork for the following temperature compensation of the sensor. The H2 sensing properties of the PdNi sensor were recorded on our homemade sensor evaluation platform where high purity N2 gas as the background/dilution gas and 0.2%, 3%, 40% H2 (in N2) gases with certificates were used to generate different concentrations of H2 atmosphere (see in Fig. 2). In a typical measurement, several hours’ equilibrium in N2 gas was essential to acquire a stable sensor response baseline especially for those sensors have underwent H2 exposure at low working temperature (e.g. 20 °C). For example, as depicted in Fig. 2A, the ΔR of the PdNi sensor in the 0.02%∼0.2% H2 range is less than 0.27 Ω which could be easily blanketed by the working temperature fluctuation and/or the unrecovered sensor baseline. Nevertheless, obviously ΔR of the PdNi sensor could also be captured by changing over the H2 concentrations. The PdNi hydrogen sensor also exhibits repeatable responses no matter in H2 accumulation or pulse conditions demonstrating that the interaction between the H2 molecules and PdNi thin film is reversible and the ΔR and the H2 concentration (or H2 partial pressure) is corresponding one by one. The wide H2 response range (0.02%∼40%) and repeatable response make the PdNi sensor a promising candidate for H2 leakage detection and concentration monitoring. Nevertheless, the nonlinear relationship between the ΔR and the H2 concentration has raised the calibration difficulty especially when the working temperature is not constant. In order to find out how much the temperature fluctuation could affect the response of the PdNi hydrogen sensor, the relationships of the PdNi sensor and H2 concentrations at various working temperatures were examined and depicted in Fig. 3. Obviously, the Pd-Ni alloy film has a positive TCR and exhibits the similar H2 sensing tendency. But unfortunately, the nonconstant intercepts at the tested H2 concentration points of either two working temperatures implying that this issue should be examined more in depth. As can be seen from Fig. 3B, the TCR correction alone can only be effective in the low hydrogen concentration range. The measurement error increases significantly with the increase of hydrogen concentration. For example, the measurement error of 3% hydrogen concentration at 40 °C is 21.9% relative to that at 23 °C, and increases rapidly with the increase of temperature and concentration. From Fig. 3C, we can see that the deviation between the concentration point and the temperature point has been significantly reduced, even the measurement error of 40 °C has been reduced to less than 3% based on TCR and NHS algorithm. The above results show that
Fig. 3. Relationships of the PdNi resistance and H2 concentrations at various working temperatures (A) without, (B) with TCR and (C) with TCR and NHS corrections.
the accuracy of PdNi hydrogen sensor can be significantly improved by our proposed TCR and NHS correction algorithm.
3.2. Sensor response at dynamic temperature In initial tests, we found that the temperature coefficient of resistance (TCR) of the PdNi thin film is not negligible, hence it is 4
Sensors & Actuators: B. Chemical 299 (2019) 126989
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Fig. 4. The repeatability of PdNi hydrogen sensor to 3.00% H2 in N2 at various temperature environments.
of PdNi thin film under different H2 partial pressures [36]. Therefore, a modified temperature compensation method combined with TCR correction and NHS correction is provided and satisfied results in the whole range (0.02%∼40%) are obtained. Fig. 4 shows the dynamic sensing curves of the PdNi hydrogen sensor at different working temperature conditions. In the case of constant temperature situation (e.g. 20 °C, see in Fig. 4A from 0 h to 80 h), the PdNi hydrogen sensor presented excellent repeatability with relative standard deviation (RSD) of 3.8% at 3.00% H2/N2 with and/or without temperature compensation. In this case, complex modified compensation algorithm is not essential and can be easily wide used in commercial products. In the meantime, we also simulated the situation where working temperatures could not be well controlled. As shown in Fig. 4, we have checked out the response of PdNi hydrogen sensor at room temperature without deliberately control. Obviously, the temperature fluctuation has critical effect on the raw resistance data of the PdNi hydrogen sensor. Results of the hydrogen concentration only with simple TCR correction could cause relatively large deviation from the theoretical value. However, the results calculated using our modified temperature compensation method (see in Fig. 4B) exhibited moderate outputs with RSD of 5.8% at 3.00% H2/N2. This result demonstrating that our proposed method is effective and suitable for such cases. In the meantime, we also simulated the situation with violent temperature variable. Fig. 5 depicts the response profile of the PdNi hydrogen sensor equipped with temperature compensation algorithm to 2.40% H2/N2 from 55 °C to 25 °C. Obviously, the PdNi hydrogen sensor presented stable response in the constant temperature zone. Even during the temperature dramatically zone, the RSD of the PdNi sensor is
Fig. 5. Response of the PdNi hydrogen sensor to H2 in dynamic range of temperature. The temperature was applied by a thermostat water bath cauldron.
necessary to reduce the influence of the temperature fluctuation during practical application [34]. Our previous research demonstrated that although simple TCR based correction could gain satisfied results at low H2 concentration range, significant deviations with enlarged tendency at high H2 concentration range are unelectable. The reason caused for this deviation is that we have ignored the nonlinear H2 solubility (NHS) 5
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Wang, Preparation and characterization of Pd/Ni thin films for hydrogen sensing, Sens. Actuators B Chem. 30 (1996) 11–16. [27] R. Sharma, A. Kumar, R.K. Upadhyay, Characteristic of a multi-pass membrane separator for hydrogen separation through self-supported PdAg membranes, Int. J. Hydrogen Energy 43 (2018) 5019–5032. [28] Y.H. Ma, I.P. Mardilovich, E.E. Engwall, Thin composite palladium and Palladium/ Alloy membranes for hydrogen separation, Ann. N. Y. Acad. Sci. 984 (2003) 346–360. [29] J.L. Rodriguez, R.C. Hughes, W.T. Corbett, P.J. Mcwhorter, Robust, Wide Range Hydrogen Sensor, Electron devices Meeting, 1992 IEDM’ 92 Technical Digest, International, 1992, pp. 521–524. [30] A.R. Usgaocar, C.H.D. Groot, C. Boulart, A. Castillo, V. Chavagnac, Low power hydrogen sensors using electrodeposited PdNi–Si schottky diodes, Sens. Actuators B Chem. 170 (2012) 176–181. [31] R.C. Hughes, W.K. Schubert, T.E. Zipperian, J.L. Rodriguez, T.A. Plut, Thin‐film palladium and silver alloys and layers for metal‐insulator‐semiconductor sensors, J. Appl. Phys. 62 (1987) 1074–1083. [32] M.W. Jenkins, R.C. Hughes, S.V. Patel, Long Term Drift Studies of Sandia H2 Sensor in Reducing Atmospheres, Office of Scientific & Technical Information Technical Reports, (2000). [33] R.C. Thomas, R.C. Hughes, Sensors for detecting molecular hydrogen based on Pd metal alloys, J. Electrochem. Soc. 144 (1997) 3245–3248. [34] H. Jiang, M. Huang, Y. Yu, X. Tian, X. Zhao, W. Zhang, et al., Integrated Temperature and Hydrogen Sensors with MEMS Technology, Sensors 18 (2018) 94–103. [35] B. Jang, M.H. Kim, J. Baek, W. Kim, W. Lee, Highly sensitive hydrogen sensors: Pdcoated Si nanowire arrays for detection of dissolved hydrogen in oil, Sens. Actuators B Chem. 273 (2018) 809–814. [36] F.A. Lewis, The palladium-hydrogen system: structures near phase transition and critical points, Int. J. Hydrogen Energy 20 (1995) 587–592.
Fig. 6. Long time stability evaluation of the PdNi hydrogen sensor. The same reference material (3.00% H2 in N2) within the validity period was utilized at 20 °C.
less than 10%. The increased deviation is mostly originated from the evolution of the hydrogen molecules from PdNi thin film lagged behind the change of temperature. As shown in Fig. 6, despite of the wide fluctuation in ambient temperatures, the PdNi hydrogen sensor showed excellent stability to 3.00% H2/N2 with drift RSD of 0.1% over the 140 days evaluation process. All of the above results demonstrating that the PdNi hydrogen sensor equipped with temperature compensation algorithm is very promising for practical application of hydrogen detection. 4. Conclusions In summary, a PdNi hydrogen sensor together with a Pt temperature sensor was fabricated by DC sputtering. SiNx was utilized as H2 barrier to suppress the penetration of H2 into Pt thin film and obtain a H2-free response of the temperature. The PdNi hydrogen sensor shows excellent response to H2 with concentration from 0.02% to 40% at room temperature. Results indicating that the response of the PdNi hydrogen sensor is a function of hydrogen concentration and working temperature. In order to obtain the true H2 response of the PdNi hydrogen sensor, an insitu temperature compensation method based on the experimental data approximations are presented. After applying the algorithm, the PdNi hydrogen sensor exhibits enhanced H2 sensing accuracy and stability even in violent temperature fluctuate situations. Acknowledgment This study was financially supported by the Foundation of China Academy of Engineering Physics (CAEP). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126989. References [1] S. Apak, E. Atay, G. Tuncer, Renewable hydrogen energy and energy efficiency in Turkey in the 21st century, Int. J. Hydrogen Energy 42 (2017) 2446–2452. [2] I. Dincer, C. Acar, Smart energy solutions with hydrogen options, Int. J. Hydrogen Energy 43 (2018) 8579–8599. [3] K.-H. Kim, S.-J. Kim, H.-J. Cho, N.-H. Kim, J.-S. Jang, S.-J. Choi, et al., WO3 nanofibers functionalized by protein-templated RuO2 nanoparticles as highly sensitive exhaled breath gas sensing layers, Sens. Actuators B Chem. 241 (2017) 1276–1282. [4] K. Nguyen, C.M. Hung, T.M. Ngoc, D.T. Thanh Le, D.H. Nguyen, D. Nguyen Van, et al., Low-temperature prototype hydrogen sensors using Pd-decorated SnO2 nanowires for exhaled breath applications, Sens. Actuators B Chem. 253 (2017) 156–163.
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Kun Yu received M.S. degree from Sichuan University in 1992. Afterwards, she joined the Institute of Chemical Material (CAEP) to work on trace level gas detection and analyze. Currently, she is working as the associate research fellow.
Fang Yang received her B.S. degree from Xiamen University (China) in 2012 and M.S. degree from Nankai University (China) in 2015. Afterwards, she joined in ICM, CAEP to work on the preparation of functional materials for gas sensor applications.
Xianqing Tian received M.S. degree from Beijing Institute of Technology (China) in 2009 and B.S. degree from Sichuan University (China) in 2012, respectively. Since July 2012, he is a research assistant in Institute of Chemical Materials (CAEP). His research interests include trace level gas detection and gas sensors made of metal oxides and MEMS.
Tianjiao Qi received her B.S. degree from Chengdu University of Technology (China) in 2007 and M.S. degree from CAEP in 2010. Now, she is a Research Associate in ICM, CAEP. She is engaged in the synthesis and characterization of semiconducting functional materials and gas sensors.
Xinfeng Wang received M.S. degree from Northwestern Polytechnical University in 1994 and B.S. degree from Institute of Chemical Materials in 2003. His interests are sensor signal processing, modeling and trace level gas detection. Currently, he is an associate research fellow working on Sensor array algorithm.
Ji Zuo is a senior worker in Institute of Chemical Materials (CAEP). Her research interests include design and construct different experimental setup for gas analysis.
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Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Enhanced accuracy of palladium-nickel alloy based hydrogen sensor by in situ temperature compensation
T
⁎
Kun Yu1, Xianqing Tian1, Xinfeng Wang , Fang Yang, Tianjiao Qi, Ji Zuo Institute of Chemical Materials, China Academy of Engineering Physics, P.O. box 919–327, Mianyang 621999, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen sensor PdNi alloy Temperature compensation Enhanced accuracy
The influence of working temperature on the accuracy of the PdNi alloy thin film hydrogen sensor was investigated. The PdNi hydrogen sensor was fabricated on a silicon chip together with a Pt temperature sensor by DC sputtering. The PdNi hydrogen sensor shows excellent response to H2 with concentration from 0.02% to 40% at room temperature. Results show that the response of PdNi hydrogen sensor is nonlinearly related to the hydrogen concentration and working temperature. And correction method only based on considering the baseline temperature correction cannot obtain high measurement accuracy in the whole range area. Therefore, in order to improve the full range detection accuracy of PdNi hydrogen sensor, a modified algorithm which takes baseline and sievert constant temperature compensation into account is presented in this work. After applying the algorithm, the PdNi hydrogen sensor exhibits enhanced H2 sensing accuracy and stability even in violent temperature fluctuate situations.
1. Introduction As an ideal replacement for fossil fuels, hydrogen is a key energy carrier of future for fuel cell powered vehicles, efficient energy storage devices, and etc [1,2]. In the meantime, H2 as a disease mediator [3,4] and/or detonation/degradation product of energetic materials [5], increasing research interests also have been raised in the asset health monitoring and safety applications, respectively. Hence, several sensing technologies have been employed to detect H2. Classification of H2 sensors based on a range of technologies including catalytic combustion, thermal conductivity, work-function based, mechanical effects and optical effects has been established and reviewed by Hübert [6] and other researchers [7–10]. Among them, one group of the technologies is based on palladium (Pd) [11,12] or Pd alloy [13–24]. Pd is popular for H2 sensing since it selectively absorbs H2. H2 molecules are dissociated into atomic H on the Pd surface and further absorbed by the Pd causing a volumetric expansion. The volumetric expansion increases linearly with increasing H concentration. H2 absorption of Pd results in two different solid phases: the so called α− and β− phase. This α−β phase change possesses several challenges in H2 sensing application, such as irreversible deformation of Pd film, stress severely the interface between the Pd film and the sensor, etc. In addition, the phase transition from the α phase to the β phase of PdHx
occurs at fairly low pH2, which caused irreversible behavior even in thin film (e.g. pure Pd film > 50 nm). Since longevity is one of the core demands for H2 sensors in many applications, lots of efforts have been formulated to avoid the phase change of Pd film [25]. Among them, Pd alloys with addition of Au [19], Ag [17], Ni [21,22,26], Cu [20], Co [14], Mg [13,16], Y [15,24], etc., have been proved to be an effective way to suppress the phase transition from α to β phase and widely utilized in commercial applications (such as H2 separation [27,28]). Also there is a considerable literature on the use of Pd alloys on hydrogen sensors [29–33]. Lately, we have designed a H2 sensor to detect H2 by monitoring the resistance of PdNi alloy thin film [34]. The PdNi hydrogen sensor exhibited excellent H2 sensing performances, but have severe limitations when used in practical applications. The primary limitation is the drift due to the temperature effect. Since we are interested in developing H2 sensors for practical applications, the resistance changes of PdNi hydrogen sensor, in particular, the hydrogen induced resistance change is a matter of vital importance. Because the resistance of the PdNi hydrogen sensor is depended both on the temperature and H2 concentration, especially the response of the PdNi thin film to H2 is also depended on the temperature. Although we can fix the PdNi hydrogen sensor in a constant working temperature condition which is accepted in some commercial products, it cannot utilize in those situations with
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Corresponding author. E-mail address: [email protected] (X. Wang). 1 Both of them contributed equally to this work. https://doi.org/10.1016/j.snb.2019.126989 Received 31 December 2018; Received in revised form 14 August 2019; Accepted 16 August 2019 Available online 21 August 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Illustration of (A) the PdNi hydrogen sensor and (B) corresponding fabrication steps.
severe temperature fluctuation (e.g. dissolved gas analysis in the insulating mineral oil [35]). Therefore, we focus in this paper on finding temperature compensation method to improve the stability and accuracy of the PdNi hydrogen sensor even with serve temperature variation.
partial pressure of H2, when the dissolution equilibrium is reached, the partial pressure of H2 (pH2) in the mixed gas and the H atom content (xH) in the solid solution satisfy Sievert's law [26,36]:
pH0.52
xH = Ks (T )
(1)
where Ks(T) is Sievert constant, which is temperature dependent; xH is the number of hydrogen atoms assigned to each metal atom (Pd), where x is approximately proportional to the rate of resistance change and can be written as:
2. Experimental 2.1. Fabrication of the hydrogen sensor The schematic description of the PdNi hydrogen sensor (1000 μm × 1000 μm) is shown in Scheme 1, in which a Pt resistor (outer) and a PdNi resistor (inner) with line width of 40 μm was designed as the temperature sensor and hydrogen sensor, respectively. Four gold pads ohmic contacted with the two resistors were utilized to facilitate the electrical connection. And a compact film of silicon nitride was deposited to eliminate the influence of H2 to Pt resistor. The PdNi hydrogen sensor was accomplished with a standard MEMS process and details of the fabrication parameters could be found in our latest publication [34]. Since self-made PdNi mosaic target was utilized thought the whole work, the Ni content in the as-prepared PdNi thin film was controlled between 8.5 wt% and 10 wt%. In such a case, the resistance baseline of the PdNi resistance has a large distribution from 356 Ω to 514 Ω.
xH = a ΔRH , T RH 0, T
(2)
In formula (2), a is a proportional constant, and ΔRH,T is the resistance change of PdNi hydrogen sensor when the hydrogen partial pressure at pH2 and zero. Bring formula (2) into formula (1), thus formula (1) can be rewritten as:
ΔRH , T = aRH 0, T Ks (T ) pH0.52
(3)
Therefore, when the temperature is constant, aKs(T) is a constant. When the pressure of the mixed gas is constant, the pH2 is equal to hydrogen concentration (CH2). And the relationship between the CH2 and the resistance change of the PdNi film (ΔRH , T ) can be written as:
CH0.52 = aKs (T )ΔRH , T / RH 0, T
2.2. Sensor characterization
(4)
where RH 0, T is the resistance of the PdNi thin film at T C without H2. According to Eq. (4), the temperature has two effects on the hydrogen-sensitive PdNi thin film: on the one hand, it directly affects the resistance of PdNi thin film (RH0,T); on the other hand, it affects the distribution constant of hydrogen (Ks (T ) ) in the gas phase and the solid solution, and thus affects the hydrogen content in the solid solution followed with the resistance change of the PdNi thin film. Since the resistance of the PdNi thin film is very sensitive either to the environmental temperature and/or the hydrogen concentration, an effective processing method is necessary for obtaining the true resistance change (ΔRH2) caused by the hydrogen gas. Accordingly, the resistance change has two key components: H2 induced resistance change at T oC and temperature induced resistance change without H2. o
The performance of the PdNi hydrogen sensor was measurement on a homemade sensor evaluation platform which consists of a sensor chamber (10 ml) sealed in the thermostat water bath cauldron and a gas distribution instrument equipped with two high-accuracy mass flow meters. Various working temperatures and concentrations of the tested gases were all applied by this platform. And signals of the PdNi hydrogen sensor were transmitted through a flexible printed circuit cable and recorded by a homemade signal adjusting board. The gas pressure and flow rate in the sensor chamber were controlled at 1 atm and 50 ml min−1, respectively, throughout the whole experiment. Based on the principle of proportional dilution, different hydrogen concentration can be obtained by controlling the flow rate of hydrogen standard gas and diluent gas (N2).
(a) the effect of temperature on the resistance of the PdNi thin film. To simplify the calculation, temperature induced 3D size change of PdNi thin film is ignored. On the basis of the temperature coefficient of resistance (TCR) measurements (data see in Fig. 3), the relationship between the resistance of PdNi thin film and working
2.3. Methodology of TCR and NHS compensation On the surface of PdNi alloy, H2 molecules are dissociate into H atoms and penetrate into PdNi alloy to form solid solution. At lower 2
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Fig. 1. Relationship of the ΔRH , T -CH2-T curve.
Fig. 2. Responses of the PdNi hydrogen sensor to (a) 0.02%∼2%, (b) 0.6%∼3%, (c) 8%∼40% H2 in N2 and (d) relationship of PdNi resistance or ΔR and H2 concentrations at 20 °C.
hydrogen in the palladium alloy,
temperature could be written as:
RT = RT0 + αT +
βT 2
lnKs (T ) = ΔG / RT
(5)
where RT is the resistance of PdNi thin film at T °C, RT0 is the resistance of PdNi thin film at T0 °C, α and β are temperature coefficients. Synchronous results were also obtained from the Pt thin film, from which provide the key temperature information. (b) the effect of temperature on the distribution constant (Ks (T ) ). According to the thermodynamic relationship of the dissolution of
(6)
Ks (T ) is approximately proportional to the reciprocal of temperature. Once substituting Eq. (3) into Eq. (4) yields a sensor quantitative relationship curve with temperature terms. Combined with the hydrogen temperature response test data at different temperatures, the parameters could be obtained by multi-parameter least squares fitting. Fig. 1 shows the ΔRH , T -CH2-T relationship curve 3
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obtained by fitting according to Eq. (1). It can be seen that the influence of the response characteristics of the temperature on palladium nickel alloy is a complex curve, and the sensitivity of the PdNi thin film decreases as the temperature increases. Similar experimental results could be found in Fig. 3B. 3. Results and discussion 3.1. Sensor response at constant temperature Pt thin film has been widely used in applications of temperature measurement, however, it is very sensitive to H2 molecules which will interfere the accuracy of the temperature sensing. In our preliminary tests, we found the H2 molecules could diffuse into the Pt thin film and cause an apparent resistance change of the Pt resistor (see in Figure S1A). Such a shift of the H2 induced resistance is strongly relevant with the H2 concentration and exposure duration and results in false temperature outputs. Hence, it is vital important to isolate the Pt thin film from the H2 molecules. In this work, a sandwich structure (SiNx/Pt/ SiNx) utilizing the silicon nitride (SiNx) as the H2 molecule diffusion barrier was constructed to eliminate the interference of the H2 molecule on the Pt thin film. As we can see in Figure S1B, the sandwich structurebased Pt thin film exhibits stable temperature response with and without the presence of H2 gas. In such a case, accurate temperature of the PdNi hydrogen sensor can be insitu monitored without time and space delays which laid the groundwork for the following temperature compensation of the sensor. The H2 sensing properties of the PdNi sensor were recorded on our homemade sensor evaluation platform where high purity N2 gas as the background/dilution gas and 0.2%, 3%, 40% H2 (in N2) gases with certificates were used to generate different concentrations of H2 atmosphere (see in Fig. 2). In a typical measurement, several hours’ equilibrium in N2 gas was essential to acquire a stable sensor response baseline especially for those sensors have underwent H2 exposure at low working temperature (e.g. 20 °C). For example, as depicted in Fig. 2A, the ΔR of the PdNi sensor in the 0.02%∼0.2% H2 range is less than 0.27 Ω which could be easily blanketed by the working temperature fluctuation and/or the unrecovered sensor baseline. Nevertheless, obviously ΔR of the PdNi sensor could also be captured by changing over the H2 concentrations. The PdNi hydrogen sensor also exhibits repeatable responses no matter in H2 accumulation or pulse conditions demonstrating that the interaction between the H2 molecules and PdNi thin film is reversible and the ΔR and the H2 concentration (or H2 partial pressure) is corresponding one by one. The wide H2 response range (0.02%∼40%) and repeatable response make the PdNi sensor a promising candidate for H2 leakage detection and concentration monitoring. Nevertheless, the nonlinear relationship between the ΔR and the H2 concentration has raised the calibration difficulty especially when the working temperature is not constant. In order to find out how much the temperature fluctuation could affect the response of the PdNi hydrogen sensor, the relationships of the PdNi sensor and H2 concentrations at various working temperatures were examined and depicted in Fig. 3. Obviously, the Pd-Ni alloy film has a positive TCR and exhibits the similar H2 sensing tendency. But unfortunately, the nonconstant intercepts at the tested H2 concentration points of either two working temperatures implying that this issue should be examined more in depth. As can be seen from Fig. 3B, the TCR correction alone can only be effective in the low hydrogen concentration range. The measurement error increases significantly with the increase of hydrogen concentration. For example, the measurement error of 3% hydrogen concentration at 40 °C is 21.9% relative to that at 23 °C, and increases rapidly with the increase of temperature and concentration. From Fig. 3C, we can see that the deviation between the concentration point and the temperature point has been significantly reduced, even the measurement error of 40 °C has been reduced to less than 3% based on TCR and NHS algorithm. The above results show that
Fig. 3. Relationships of the PdNi resistance and H2 concentrations at various working temperatures (A) without, (B) with TCR and (C) with TCR and NHS corrections.
the accuracy of PdNi hydrogen sensor can be significantly improved by our proposed TCR and NHS correction algorithm.
3.2. Sensor response at dynamic temperature In initial tests, we found that the temperature coefficient of resistance (TCR) of the PdNi thin film is not negligible, hence it is 4
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Fig. 4. The repeatability of PdNi hydrogen sensor to 3.00% H2 in N2 at various temperature environments.
of PdNi thin film under different H2 partial pressures [36]. Therefore, a modified temperature compensation method combined with TCR correction and NHS correction is provided and satisfied results in the whole range (0.02%∼40%) are obtained. Fig. 4 shows the dynamic sensing curves of the PdNi hydrogen sensor at different working temperature conditions. In the case of constant temperature situation (e.g. 20 °C, see in Fig. 4A from 0 h to 80 h), the PdNi hydrogen sensor presented excellent repeatability with relative standard deviation (RSD) of 3.8% at 3.00% H2/N2 with and/or without temperature compensation. In this case, complex modified compensation algorithm is not essential and can be easily wide used in commercial products. In the meantime, we also simulated the situation where working temperatures could not be well controlled. As shown in Fig. 4, we have checked out the response of PdNi hydrogen sensor at room temperature without deliberately control. Obviously, the temperature fluctuation has critical effect on the raw resistance data of the PdNi hydrogen sensor. Results of the hydrogen concentration only with simple TCR correction could cause relatively large deviation from the theoretical value. However, the results calculated using our modified temperature compensation method (see in Fig. 4B) exhibited moderate outputs with RSD of 5.8% at 3.00% H2/N2. This result demonstrating that our proposed method is effective and suitable for such cases. In the meantime, we also simulated the situation with violent temperature variable. Fig. 5 depicts the response profile of the PdNi hydrogen sensor equipped with temperature compensation algorithm to 2.40% H2/N2 from 55 °C to 25 °C. Obviously, the PdNi hydrogen sensor presented stable response in the constant temperature zone. Even during the temperature dramatically zone, the RSD of the PdNi sensor is
Fig. 5. Response of the PdNi hydrogen sensor to H2 in dynamic range of temperature. The temperature was applied by a thermostat water bath cauldron.
necessary to reduce the influence of the temperature fluctuation during practical application [34]. Our previous research demonstrated that although simple TCR based correction could gain satisfied results at low H2 concentration range, significant deviations with enlarged tendency at high H2 concentration range are unelectable. The reason caused for this deviation is that we have ignored the nonlinear H2 solubility (NHS) 5
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Fig. 6. Long time stability evaluation of the PdNi hydrogen sensor. The same reference material (3.00% H2 in N2) within the validity period was utilized at 20 °C.
less than 10%. The increased deviation is mostly originated from the evolution of the hydrogen molecules from PdNi thin film lagged behind the change of temperature. As shown in Fig. 6, despite of the wide fluctuation in ambient temperatures, the PdNi hydrogen sensor showed excellent stability to 3.00% H2/N2 with drift RSD of 0.1% over the 140 days evaluation process. All of the above results demonstrating that the PdNi hydrogen sensor equipped with temperature compensation algorithm is very promising for practical application of hydrogen detection. 4. Conclusions In summary, a PdNi hydrogen sensor together with a Pt temperature sensor was fabricated by DC sputtering. SiNx was utilized as H2 barrier to suppress the penetration of H2 into Pt thin film and obtain a H2-free response of the temperature. The PdNi hydrogen sensor shows excellent response to H2 with concentration from 0.02% to 40% at room temperature. Results indicating that the response of the PdNi hydrogen sensor is a function of hydrogen concentration and working temperature. In order to obtain the true H2 response of the PdNi hydrogen sensor, an insitu temperature compensation method based on the experimental data approximations are presented. After applying the algorithm, the PdNi hydrogen sensor exhibits enhanced H2 sensing accuracy and stability even in violent temperature fluctuate situations. Acknowledgment This study was financially supported by the Foundation of China Academy of Engineering Physics (CAEP). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.126989. References [1] S. Apak, E. Atay, G. Tuncer, Renewable hydrogen energy and energy efficiency in Turkey in the 21st century, Int. J. Hydrogen Energy 42 (2017) 2446–2452. [2] I. Dincer, C. Acar, Smart energy solutions with hydrogen options, Int. J. Hydrogen Energy 43 (2018) 8579–8599. [3] K.-H. Kim, S.-J. Kim, H.-J. Cho, N.-H. Kim, J.-S. Jang, S.-J. Choi, et al., WO3 nanofibers functionalized by protein-templated RuO2 nanoparticles as highly sensitive exhaled breath gas sensing layers, Sens. Actuators B Chem. 241 (2017) 1276–1282. [4] K. Nguyen, C.M. Hung, T.M. Ngoc, D.T. Thanh Le, D.H. Nguyen, D. Nguyen Van, et al., Low-temperature prototype hydrogen sensors using Pd-decorated SnO2 nanowires for exhaled breath applications, Sens. Actuators B Chem. 253 (2017) 156–163.
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Kun Yu received M.S. degree from Sichuan University in 1992. Afterwards, she joined the Institute of Chemical Material (CAEP) to work on trace level gas detection and analyze. Currently, she is working as the associate research fellow.
Fang Yang received her B.S. degree from Xiamen University (China) in 2012 and M.S. degree from Nankai University (China) in 2015. Afterwards, she joined in ICM, CAEP to work on the preparation of functional materials for gas sensor applications.
Xianqing Tian received M.S. degree from Beijing Institute of Technology (China) in 2009 and B.S. degree from Sichuan University (China) in 2012, respectively. Since July 2012, he is a research assistant in Institute of Chemical Materials (CAEP). His research interests include trace level gas detection and gas sensors made of metal oxides and MEMS.
Tianjiao Qi received her B.S. degree from Chengdu University of Technology (China) in 2007 and M.S. degree from CAEP in 2010. Now, she is a Research Associate in ICM, CAEP. She is engaged in the synthesis and characterization of semiconducting functional materials and gas sensors.
Xinfeng Wang received M.S. degree from Northwestern Polytechnical University in 1994 and B.S. degree from Institute of Chemical Materials in 2003. His interests are sensor signal processing, modeling and trace level gas detection. Currently, he is an associate research fellow working on Sensor array algorithm.
Ji Zuo is a senior worker in Institute of Chemical Materials (CAEP). Her research interests include design and construct different experimental setup for gas analysis.
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