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Vacuum 83 (2009) 699–702

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Vacuum

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New hydrogen sensor based on sputtered Mg–Ni alloy thin film

Kazuki Yoshimura a,*, Shanhu Bao a, Naoki Uchiyama b, Hiroyuki Matsumoto b, Tomomi Kanai b,Seigo Nakabayashi b, Hiroshi Kanayama c

a National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japanb Atsumitec CO., Ltd. 7111 Yuto-cho, Nishi-ku, Hamamatsu-shi, Shizuoka 431-0192, Japanc Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, Fukuoka 819-0395, Japan

Keywords:Hydrogen sensorSwitchable mirrorSputteringMagnesium–nickel alloyPalladium

* Corresponding author. Fax: þ81 52 736 7315.E-mail address: k.yoshimura@aist.go.jp (K. Yoshim

0042-207X/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.vacuum.2008.04.047

a b s t r a c t

Pd-capped magnesium–nickel alloy thin films were prepared by magnetron sputtering and their hy-drogen sensing properties were investigated. By monitoring the resistance change or the transmittancechange of the film, we can obtain the information on hydrogen concentration in air. The sensing range ofthis sensor is quite wide and it can measure the hydrogen concentration range from 10 ppm to 10%without heating. Using a certain protective coating, the durability of the film can be much improved. Alsothere is a unique application of Pd/Mg–Ni thin film as ‘hydrogen check sheet’, which can visualize thehydrogen flow.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Nowadays, more attention is paid to hydrogen as a source of cleanenergy. According to this trend, a hydrogen sensor is becoming moreimportant because hydrogen should be treated with stringent cau-tion. There are several types of hydrogen sensors which have beeninvestigated for practical use [1,2]. The most popular hydrogensensor uses an oxide-semiconductor such as SnO2. Although oxide-semiconductor gas sensors have high sensitivity and high reliability,the operation temperature of these sensors is high (approximately600 K), and they consume relatively large power for device opera-tion. Also generally, these sensors are very expensive.

In contrast, our group has been involved in the research on‘Switchable Mirror’ thin film which can be switched from themirror state to the transparent state by exposure to hydrogencontaining atmosphere [3]. This phenomenon is based on the hy-drogenation of metal thin film at room temperature [4–7]. In thecourse of this research, our group found that Pd-capped Mg2Ni thinfilm can be used as a hydrogen sensor which can be operated atroom temperature [8]. In this paper, we have investigated the detailof the hydrogen sensing properties of Pd/Mg–Ni thin films pre-pared by magnetron sputtering to obtain the technological per-spective for practical applications of this sensor.

2. Experiments

Mg–Ni alloy thin films were prepared by DC magnetron sput-tering. Metal targets of magnesium and nickel were set to two of

ura).

All rights reserved.

guns. After evacuation, co-sputtering of Mg and Ni targets wascarried out for the deposition of Mg–Ni alloy thin films on glasssubstrates. Then a thin Pd layer, which is necessary for the en-hancement of hydrogen uptake kinetics and protection of the un-derlying Mg–Ni layer was subsequently coated onto the alloy film insitu. After the composition analysis by using Rutherford backscat-tering spectroscopy (RBS), we adjusted the power ratio of the Mgtarget to the Ni target to obtain MgxNi (2 � �� 6) thin films. Thethickness of Mg–Ni layer was about 40 nm and that of Pd layer wasabout 4 nm.

Fig. 1 shows the shape of the sample for measurements of hy-drogen sensing property. There are two common methods for ob-serving the hydrogenation of Pd/Mg–Ni thin films; one is resistancemeasurement and the other is optical measurement. In this paper,two methods were tested and compared. For the resistance mea-surements, aluminum electrode was coated on both the edge of theglass substrate before Mg–Ni layer deposition. The active area ofsample was 5 mm� 15 mm. The measurements of resistance weredone by two probe method, which we adopted consideringa practical application. Resistance between two aluminum elec-trodes was measured by Keithley 2000 digital multimeter directly.For the optical measurements, the transmittance of Pd/Mg–Ni thinfilm was monitored by using a semiconductor laser (l¼ 670 nm)and a Si photodiode. For hydrogen sensing property measurements,another glass plate was put on the sample with silicone spacer. Themixture gas of hydrogen and dry air was introduced into theclearance of the sample and the glass plate, and then resistance andtransmittance changes were measured. After some interval, hy-drogen flow was stopped and air entered the cleavage of two glasslayers, which causes the dehydrogenation. All measurements havebeen done at room temperature (25 �C).

Glass Substrate Aluminum Electrode

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Fig. 1. A schematic illustration of Pd/Mg–Ni thin film prepared on a glass substrate.

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Fig. 2. Hydrogen sensing property of Pd/Mg2Ni thin film by resistance measurements.

K. Yoshimura et al. / Vacuum 83 (2009) 699–702700

3. Results and discussion

Fig. 2 shows the sensing properties of Pd/Mg2Ni thin film bymonitoring the resistance change. At t¼ 0s, hydrogen containinggas is introduced to the clearance of two glasses, and it is stopped att¼ 200s. The resistance of the sample changes by hydrogenationand dehydrogenation of Mg2Ni layer in the film. These graphs showthat this sensor responds the hydrogen containing gas with wideconcentration range from 10 ppm to 100% H2 in air. It should benoted that all measurements were at room temperature and noheating is necessary even for the low concentration hydrogen of10 ppm to detect with good S/N ratio. The response time has largedependence on hydrogen concentration range. From 10% to 100%H2, the dehydrogenation process is very slow and it takes about 2 hto recover, although hydrogenation process is fast (about 5 s). From1000 ppm to 10% H2, the dehydrogenation process takes about 50 sand the dehydrogenation process takes about 100 s. From 10 ppmto 1000 ppm H2, both the dehydrogenation and hydrogenationprocesses are fast. This sensor shows best performance in thisrange. Below 10 ppm H2, the hydrogenation process is not saturatedand resistance is increasing gradually. The reason of the fast re-sponse for low concentration range hydrogen may be a solid statesolution of hydrogen into Mg2Ni layer instead of a formation ofhydride of Mg2NiH4, because the Mg2Ni can absorb hydrogen inlattice solution up to H/Mg2Ni¼ 0.3 [9].

Fig. 3 shows the sensing properties of Pd/Mg2Ni thin film bymonitoring the optical transmittance. The transmittance change issimilar to the resistance change, while the signal is noisy for the10 ppm hydrogen. It means that Pd/Mg2Ni thin film can be used asan optical sensor for hydrogen, although the sensing range is nar-rower than that of the resistance measurement.

We also characterized the sensing property of the sampleswith different composition. The Pd/Mg4Ni and Pd/Mg6Ni thinfilms show almost the same sensing properties as Pd/Mg2Ni bothfor electrical and optical measurements. There is a tendency thatthe Mg-rich samples show larger resistance and transmittancechange than that of Pd/Mg2Ni, while dehydrogenation processtakes longer time. These tendencies are consistent with the be-havior of gasochromic switching of Pd/Mg–Ni thin films [10].

These results indicate that Pd/Mg–Ni thin film has possibility torealize low cost room temperature hydrogen sensor because it hassuperior hydrogen sensing property and the structure of this sensoris simple. However, this sensor has problem in its durability. Theresistance or transmittance modulation level by hydrogen exposuredecreases gradually when the sample is kept in air. For example, thebroken line in Fig. 4 shows the change of transmission modulationlevel of Pd/Mg6Ni thin film by exposure to 100% hydrogen. In thisgraph, the initial modulation level is defined as 100%. After 120days, it shows almost no response to hydrogen by degradation.Such degradation is mainly caused by the oxidation of Mg–Ni layer[11].

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Fig. 3. Hydrogen sensing property of Pd/Mg2Ni thin film by optical transmittancemeasurements.

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Fig. 4. The change of the response of transmittance of Pd/Mg6Ni thin films with andwithout polymer coating for exposure to 100% hydrogen.

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Fig. 5. Optical response of Pd/Mg6Ni thin film for H2 and propane (C3H8) gases.

K. Yoshimura et al. / Vacuum 83 (2009) 699–702 701

Through the development of switchable mirror window glass,our group found that the surface coatings of some polymers areeffective to reduce the oxidation of the film and improve its dura-bility [12]. We have tested the various materials as a protectivecoating for switchable mirror type hydrogen sensor, and found thata certain fluorinated polymer is quite effective to improve the du-rability. The solid line in Fig. 4 shows the durability for the fluori-nated polymer coated Pd/Mg6Ni thin film. In this case, themodulation level is still more than 95% of the initial level after 120days.

Another important factor as a hydrogen sensor is a selectivity tohydrogen. Pd/Mg–Ni thin films have high selectivity to hydrogen.

Fig. 5 shows the comparison of the optical response of Pd/Mg6Nithin film to hydrogen gas and propane gas (C3H8). It shows almostno response to propane gas. We tested the response for some othergases such as CH4, CO and N2 gases and confirmed that the sensordoes not respond to these gases.

In addition to a function of conventional hydrogen sensor de-vice, Pd/Mg–Ni thin films can be used as ‘hydrogen check sheet’.The appearance of the film immediately changes by exposure tohydrogen containing atmosphere and we can check the existence ofhydrogen by our own eyes. Also hydrogen flow or hydrogen diffu-sion can be visualized by using Pd/Mg–Ni thin films. For suchpurposes, transparent plastic sheet is suitable as a substrate.

Fig. 6 shows an example of the application of the hydrogencheck sheet to validate the simulation result. Fig. 6(a) shows theassumed model used for hydrogen diffusion simulation. Hydrogenis introduced to the box from an inlet at bottom left. There area door vent and a roof vent. The simulation of hydrogen diffusionwas done based on the compressible Navier–Stokes equation. Thedetail of the calculation is described elsewhere [13]. Fig. 6(b) showsthe simulated hydrogen distribution at 10 s after hydrogen in-troduction. The black area means that the hydrogen concentrationis high around there. We assembled a plastic box with the sameshape as the assumed model. Pd/Mg6Ni thin film is deposited ontransparent plastic sheets and the coated sheets are put inside ofthe box. Hydrogen gas is introduced from the inlet and the changeof appearance is observed. Fig. 6(c) shows the plastic box at 10 s

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Fig. 6. Validation of hydrogen diffusion simulation; (a) assumed model, (b) simulationresult and (c) experimental result using Pd/Mg6Ni thin film.

K. Yoshimura et al. / Vacuum 83 (2009) 699–702702

after hydrogen inlet. Black area means that the hydrogen concen-tration is high around there, which corresponds to the black area inFig. 6(b). The experimental result corresponds to the simulationresult very well. By this way, we can validate the simulation resultdirectly using Pd/Mg6Ni coated sheet.

4. Conclusions

The detail of the hydrogen sensing properties of Pd/Mg–Ni thinfilms prepared by magnetron sputtering has been investigated. Bymonitoring the resistance change or the transmittance change ofthe film, we can obtain the information on hydrogen concentrationin air. The hydrogen sensor using Pd/Mg–Ni thin film has severaladvantages: wide sensing range (10 ppm to 10%), quick response,high selectivity, simple structure, low material cost etc. Usinga fluorinated polymer coating, the durability of the film can bemuch improved. Also there is a unique application of Pd/Mg–Nithin film as ‘hydrogen check sheet’, which can visualize the hy-drogen flow.

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