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Vacuum 80 (2006) 684–687

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Optical switching property of Pd-capped Mg–Ni alloy thin filmsprepared by magnetron sputtering

Kazuki Yoshimura�, Shanhu Bao, Yasusei Yamada, Masahisa Okada

National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, 463-8560, Japan

Abstract

Pd-capped Mg–Ni alloy thin films were prepared by magnetron sputtering, on glass substrates, and the dependence of the optical

switching property of these films on alloy composition and temperature has been investigated using diluted hydrogen gas and dry air for

changing. The transition from the transparent state to the mirror state by dehydrization shows strong dependence on both factors, while

the hydrization transition has weak dependence. At higher temperature, the dehydrization is much faster for Mg-rich Mg–Ni thin film

whose transition is very slow at room temperature. By using heated air to change from the transparent state to the mirror state, a fast

switching can be done for Mg-rich Mg–Ni alloy thin film which has a wide optical modulation range.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Switchable mirror; Magnesium–nickel alloy; Palladium; Optical switching; Temperature dependence

Some metal thin films with a Pd-cap layer showremarkable optical switching from the mirror state to thetransparent state by alternate exposure to diluted hydrogenand oxygen gases [1]. They are called ‘Switchable Mirror’and have attracted much attention for their potentialapplications such as smart windows, displays, opticalswitches, etc. These optical changes caused by hydrizationand dehydrization of the metal film with the aid of catalyticaction of Pd.

The thin films of rare earths [1–3], mixture of rare earthand magnesium [4–6], and magnesium–transition-metalalloys [7] show optical switching between the mirror stateand the transparent state. Among these materials, Mg–Nialloy thin film is expected to be one of the most promisingmaterials in the view point of resource and cost, especiallyfor large window applications.

Recently our group found that Mg-rich Mg–Ni alloythin films shows good optical switching [8]. For example,the transmittance of Pd-capped Mg6Ni in the hydride stateis over 50% in the visible range after exposure to dilutedhydrogen gas, while that of Pd-capped stoichiometric

ee front matter r 2005 Elsevier Ltd. All rights reserved.

cuum.2005.11.013

ing author. Fax: +81 52 736 7315.

ess: k.yoshimura@aist.go.jp (K. Yoshimura).

Mg2Ni is only 20%. However, the switching speed fromthe transparent state to the mirror state is slow (�severalminutes), while the reversed change is fast (�severalseconds).We prepared the films with various conditions and

characterized the optical switching property of preparedsamples to improve the responsibility. After these investi-gations, we found that a fast switching can be done using acharacteristic dependence of the switching property ontemperature. In this paper, we report the temperature andthe composition dependence of optical switching for Pd-capped Mg–Ni thin films using diluted hydrogen gas.Pd/Ni–Mg alloy thin films were prepared by DC

magnetron sputtering. The details of the sputteringconditions were described elsewhere [8]. The depositionchamber has three magnetron sputtering guns with metaltargets of magnesium, nickel and palladium. Personalcomputer controlled DC power supplies were used fordischarge. After evacuation, co-sputtering of Ni and Mgtargets has been done for deposition of Mg–Ni alloy thinfilms with the thickness of 40 nm on glass substrates(35mm� 35mm� 1mm). Then a thin Pd layer (4 nm) wassubsequently coated onto the alloy film in situ. Byadjusting the discharge power ratio of Mg to Ni, we

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prepared three kinds of Mg–Ni alloys; Mg2Ni, Mg4Ni andMg6Ni. The composition of each sample was checked byRutherford backscattering (RBS) measurements.

Optical switching property was measured by thecharacterization setup as shown in Fig. 1. There are twokinds of methods of switching for switchable mirrormaterials. One is using liquid or solid electrolyte likeelectrochromic materials. Another way is using hydrogengas, which is called ‘gasochromic’ method [9]. Gasochro-mic has an advantage from the view point of productioncost because the layer structure is much simpler than thatof an electrochromic device. In this work we investigate thegasochromic property of Mg–Ni alloy thin films. Adeposited sample is attached to another glass plate withthe coated side inside. They are separated by siliconerubber spacer with the thickness of 2mm. A glass pipe forgas introduction is connected to the glass plate. The flow ofintroduction gas is controlled by a mass flow controller.The whole glass setup is heated by using an air blower forthe measurements at higher temperatures of 40 and 60 1C.The change of optical transmittance is measured by thecombined use of a laser diode (l ¼ 670 nm) and a Siphotodiode.

H2+Ar

Laser

Mass Flow Controller

GlassGlass

Mg-NiPd

O2+N2

Spacer

Hot Air

Si Photodiode

Heater(modified setup)

(a)

(b)

Fig. 1. Characterization setup of optical switching property: (a) the

schematic diagram, (b) the photograph of the setup.

Firstly, we investigate the structure of Mg–Ni alloy thinfilms. XRD analysis shows that all prepared samples haveamorphous like structure. To obtain the information onthin film structure, a heat treatment has been done. Fig. 2shows the XRD patterns of Mg2Ni, Mg4Ni and Mg6Niafter annealing at 150 1C for 60min. XRD pattern ofMg2Ni has a sharp peak at 20.11 and a small peak at 40.91,which correspond to Mg2Ni (0 0 3) and (0 0 6) peaks,respectively. It means that this film consists of an almostsingle phase of Mg2Ni. Very small peak at 34.41 which isattributed to Mg (0 0 2) peak appears in XRD pattern ofMg4Ni. It shows that small amount of Mg grain is includedin the film. This Mg peak is clearer in the XRD pattern ofMg6Ni. These results imply that Mg-rich Mg–Ni thin filmconsists of Mg2Ni and Mg grains and the content ratio ofMg increases with increase of Mg composition.All samples have shiny metallic surface in the as-

deposited state. Diluted hydrogen gas causes the drasticchange from the metallic state to the transparent state byhydrization of Mg–Ni alloy and their transmittanceincreases rapidly. Subsequent introduction of dry aircauses the slow change from the transparent state to themetallic state by dehydrization and their transmittancedecreases gradually. Fig. 3 shows the optical switchingproperty of Pd-capped Mg2Ni, Mg4Ni and Mg6Ni at threedifferent temperatures for gasochromic switching using 4%H2 in Ar. The maximum transmittance shows thedependence on composition. The Mg-rich samples havelarger transmittance in the fully hydrated state comparedwith that of Mg2Ni, as reported before [8]. As mentionedabove, Mg–Ni thin film consists of Mg2Ni and Mg grains.Richardson et al. [7] reported that in the hydride state ofthe Mg–Ni thin films, there are two kinds of hydride,Mg2NiH4 and MgH2. Therefore, the following reactionsare supposed to occur at hydrization and dehydrization.

Mg2Niþ 2H2!Mg2NiH4 ðhydrizationÞ (1)

50454035302520152θ

Mg2Ni

Mg4Ni

Mg6Ni

Mg

2N

i (0

03

)

Mg

2N

i (0

06

)

Mg

(0

02

)

Fig. 2. XRD patterns of annealed Pd-capped Mg2Ni, Mg4Ni and Mg6Ni.

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20

10

0

Tran

smit

tan

ce (

%)

Tran

smit

tan

ce (

%)

Tran

smit

tan

ce (

%)

100806040200Time (s)

Time (s)

Time (s)

H2+Ar

H2+Ar

H2+Ar

Dry Air

Dry Air

Dry Air

Mg2Ni

Mg4Ni

Mg6Ni

50

40

30

20

10

0

12080400

60

40

20

0

6004002000

20˚c40˚c

60˚c

20˚c40˚c

60˚c

20˚c40˚c

60˚c

Fig. 3. Optical switching properties of Pd-capped Mg2Ni, Mg4Ni and

Mg6Ni at three different temperatures for gasochromic switching using

4% H2 in Ar.

K. Yoshimura et al. / Vacuum 80 (2006) 684–687686

Mg2NiH4 þO2!Mg2Niþ 2H2O

ðdehydrizationÞ (2)

MgþH2!MgH2 ðhydrizationÞ (3)

2MgH2 þO2! 2Mgþ 2H2O

ðdehydrizationÞ (4)

Mg2NiH4 is a semiconductor with a band gap of 2.0–3.5 eV[7] and its bulk crystal is red. MgH2 is an insulator with aband gap of 5 eV [10], and its bulk crystal is supposed to betransparent. Therefore we speculate that the difference ofthe transmittance in the fully hydrated state comes from

the difference of content ratio of Mg2NiH4 and MgH2 inthe film.As shown in Fig. 3, hydrization speed from the mirror

state to the transparent state is almost the same for Mg2Ni,Mg4Ni and Mg6Ni. It takes several seconds for the change.Also, hydrization time shows weak dependence ontemperature ranged from 20 to 60 1C. On the contrary,dehydrization speed from the transparent state to themirror state shows strong dependence both on compositionand temperature. At room temperature, the dehydrizationtime is getting longer with increase of Mg content inMg–Ni thin films. The dehydrization takes about 20 s forMg2Ni, while it takes about 90 s for Mg4Ni and 550 s forMg6Ni. For each sample, elevating the temperature causesthe improvement of the response time of dehydrization.For example, the dehydrization of Mg2Ni takes only 1 s at60 1C. This effect is more prominent for Mg-rich samples.Although the dehydrization of Pd/Mg6Ni thin film takes550 s at room temperature, it is drastically reduced to 10 sat 60 1C.The weak dependence of hydrization process on

composition and temperature may indicate that thereaction rates of Eqs. (1) and (3) are close and have weakdependence on temperature. Although the dehydrizationprocess is very complicated in this system and the details ofthe mechanism are not clear, we think that these results canbe explained if we assume that the reaction rate of Eq. (2) isfaster than that of Eq. (4) and both reactions have strongdependence on temperature.These results show that the switching property can be

improved at higher temperature. This tendency is suitablefor practical use as an energy efficient window becausenormally we will change the glass from the transparentstate to the mirror state at hot condition to shade sunshine.We should point out that it is not necessary to heat up thewhole glass for fast switching. The temperature of the thinfilm determines the response time. So the practical way is tointroduce heated air for dehydrization. By using the thinspacer and making the gap between the two glasses small,only a small amount of hot air is enough to heat up thecoated film.We verified this method using a modified setup, as shown

in Fig. 1. A heater is added to the pipe to heat up the gasintroduced. The thickness of silicone spacer is 0.5mm.Unheated H2+Ar is used for hydrization. After thatheated dry air (60 1C) is introduced for dehydrization. Theresulting optical switching of Pd/Mg6Ni thin film coatedglass is shown in Fig. 4. In this case, the dehydrization isalso as fast as the hydrization, which is within 10 s. Byusing this method the sample is highly transparent in thehydride state, and its switching speed is also acceptably fastfor practical use.In conclusion, we prepared Pd-capped Mg–Ni thin film

by using magnetron sputtering and investigated thedependence of the optical switching property on composi-tion and temperature. The hydrization change shows smalldependence on composition and temperature. On the

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60

50

40

30

20

10

0

Tran

smit

tan

ce (

%)

806040200Time (s)

Mg6Ni

4%H2+ Ar

Heated Dry Air

Fig. 4. Optical switching property of Pd-capped Mg6Ni for gasochromic

switching using 4% H2 in Ar and heated dry air.

K. Yoshimura et al. / Vacuum 80 (2006) 684–687 687

contrary, the dehydrization change shows strong depen-dence on both composition and temperature. At highertemperatures, the dehydrization speed is much improved.We speculate that such dependence comes from thedependence on temperature of the dehydrization reaction.

By using heated air to change from the transparent state tothe mirror state, quick switching can be realized for Mg-rich Mg–Ni alloy thin film which has a wide optical-modulation range.

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