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Toward solid-state switchable mirrors using a zirconium oxide proton conductor
Solid State Ionics 145 Ž2001. 17–24 www.elsevier.comrlocaterssi
Toward solid-state switchable mirrors using a zirconium oxide proton conductor Virginie M.M. Mercier ) , Paul van der Sluis Philips Research Laboratories, Prof. Holstlaan 4, (WA-11), 5656 AA EindhoÕen, The Netherlands Received 27 October 2000; received in revised form 1 February 2001; accepted 16 March 2001
Abstract The unique optical properties of magnesium rare-earth alloys are interesting for making new types of solid-state electrochromic devices, based on hydrogen transport. These devices can reversibly switch between a transparent and a reflective state, when a potential difference is applied. The all-solid state devices presently studied involve a zirconium oxide proton conductor. This paper discusses the deposition and preliminary characterisation of the solid-state electrolyte. It also gives some information about the performance of a symmetrical GdMg device tested in a hydrogen gas atmosphere. This stack is composed of a GdMgH xrPd bottom electrode, a ZrO 2 P ŽH 2 O. x P wH 2 x y proton conductor and a PdrGdMgH yrPd top-electrode. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal hydride; Electrochromic device; Solid-state electrolyte; ZrO 2 P ŽH 2 O. x P wH 2 x y
1. Introduction Lanthanide metals, capped with a palladium topcoat, show interesting optical properties. This was discovered five years ago at the Vrij Universiteit of Amsterdam w1,2x. Upon hydrogen insertion, they undergo a major optical change, reversibly switching between a non-transparent Žlow-hydrogen concentration. state and a transparent Žhigh-hydrogen concentration. state. The presence of the Pd top-layer is
)
Corresponding author. Tel.: q31-40-2742343; fax: q31-402744282. E-mail address: [email protected] ŽV.M.M. Mercier..
necessary both for protection and catalysis. The lanthanide metals are especially sensitive to oxidation. The lanthanide metal hydrides always lack colour neutrality in their transparent state: for example, YH 3 w3x or GdH 3 w4x are yellow-transparent. To improve the colour neutrality, van der Sluis et al. proposed to alloy the lanthanide metals with Mg w4x or to make lanthanide–magnesium multilayers w5x. The lanthanide–magnesium films have first been switched in hydrogen gas w4,5x. Unfortunately, for technological applications, this is not practical. Electrochemical switching has then been successfully carried out for various lanthanides or lanthanide–alloys in alkaline solutions w6–10x. This achievement opened opportunities for producing electrochromic devices. Such devices generally consist of three parts w11x: a main electrochromically active layer, capable
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V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
of reversible insertion of a guest ion ŽH, Li, . . . .; an electrolyte layer; and a ion storage layer, with an appropriate switching electrochromic behaviour. In the particular case of ‘switchable mirrors’, the main electrochromic layer will be a GdMg hydride. Different materials for the ion storage layer are available among which tungsten oxide is proven to be a good candidate w12,13x. The electrolyte can be a liquid, a gel or a solid. This last option is, of course, the most suitable for technological applications. That is the reason why the use of a zirconium oxide solid electrolyte is presently explored for the switchable mirror systems w12,13x. Usually, the electrolyte must fulfil some requirements w14x: it must be an electronic insulator Ž se 10y1 2 Srcm. and a good ionic conductor Žusually 10y7 - s i - 10y4 Srcm.. If the application requires that the device can be switched to the transparent state, then the proton conductor must also be transparent and colour neutral. The electrolyte should also not react with the two insertion electrodes in between which it is sandwiched. This chemical stability must be ensured both during deposition and during cycling. Finally, for certain applications Že.g. windows., the electrolyte must also be stable in temperature and under solar radiation. Generally, zirconium oxides, in the fluorite structure, are well known as good oxygen conductors at high temperatures w15x. However, ZrO 2 is mono˚ b s 5.17 clinic at ambient temperature Ž a s 5.12 A, ˚A, c s 5.29 A, ˚ b s 99.118., and non-conducting. The cubic fluorite structured ZrO 2 is generally stabilised using dopants like Y w15x. Hydrous oxides are usually good conductors w16x. They are described as a hydrated material consisting of irregular, amphoteric charged particles separated by aqueous solution. This material, initially prepared with a sol–gel-like technique, is pressed into a pellet and its conductivity is measured w16x. In a hydrated form ŽZrO 2 P 1.75H 2 O., ZrO 2 is said to have a conductivity of 2 ) 10y5 Vy1 cmy1 at 278C and an activation energy of 0.34 eV, with a surface liquidlike conductivity w16x. In this paper, zirconium oxide is deposited as a thin film by reactive sputtering in the presence of oxygen and hydrogen. This deposition technique would then allow us to obtain thin films with a proton conducting character.
2. Experimental Deposition of layers and stack are carried out in a custom-made RF magnetron sputtering apparatus Žtargets are about 5 cm in diameter and their distance to the substrate is around 3 cm.. Deposition of GdMg is done from a Gd metal target with inserts of Mg, so that the ratio between Gd and Mg in the deposited films is near 1:1. The deposition of the GdMg hydride is then carried out in an ArrH 2 mixture. Pd is also deposited in the same ArrH 2 atmosphere from a Pd metal target. The electrolyte is deposited from a Zr target in an ArrO 2rH 2 mixture. Individual mass flow controllers allow an estimation of the pressures of each gas involved in the deposition. The purity of the sputtered gases is better than 99.9995% for O 2 and H 2 and 99.9999% for Ar. The base pressure before deposition is about 8 ) 10y7 mbar. The substrate holder is water-cooled so that deposition can be performed as close as possible to room temperature. Substrates are placed face-down in order to avoid contamination by dust. Float glass and silicon substrates Ž10 ) 40 mm2 . are used for zirconium oxide characterisation. For stacks, float glass substrates of 30 ) 30 mm2 are used. It is possible to simultaneously deposit four devices Ža–d., using a mask system. Chemical compositions are determined by Rutherford Backscattering Spectrometry ŽRBS.. Elastic Recoil Detection ŽERD. is used to estimate the total amount of hydrogen in the sample. Layer thicknesses are usually approximated from RBS measurements, taking into account a theoretical density for the material. In the case of zirconium oxide films, optical measurements in the 300–2500 nm wavelength range are performed in a spectrophotometer ŽPerkin Elmer l19.. XRD ŽX-Ray Diffraction. measurements are performed in a PW 1800 powder diffractometer with CuK a radiation Ž108 - 2 u - 808; DŽ2 u . s 0.028.. To measure the optical properties of the complete symmetric-GdMg stack, a monochromatic He–Ne laser Ž633 nm. has been used. Reflection measurements are carried out from the substrate side. Electrical measurements Žcurrent in function of applied potential. are made with a Keithley 2400 sourcemeter controlled by a PC.
V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
3. Results 3.1. Zirconium oxide film 3.1.1. Deposition and chemical characterisation Two different zirconium oxides have been prepared. One film has been deposited in an ArrO 2 atmosphere Žnamed ‘Sample A’. and the other in an ArrO 2rH 2 atmosphere Žnamed ‘Sample B’.. The flows of the different gases during deposition were, respectively: 100r0.8 and 100r0.8r40 sccm Žstandard cubic centimetre per minute.; the total deposition pressures were 46.7 and 60.5 mbar, respectively. The power was kept constant at 50 W. These samples have been deposited during the same amount of time Ž2500 s. but their deposition rates were different Žsample A: 0.11 nmrs and sample B: 0.07 nmrs., resulting in layer thicknesses of 280 and 170 nm for samples A and B, respectively. RBS on the two different samples gave OrZr ratio of around 2.0 " 0.1 for sample A and 2.1 " 0.1 for sample B. The RBS profile also showed that Hf was present as impurity in the films with a concentration of around 0.02 at.%. The main difference between the two samples came essentially from their hydrogen content, determined by ERD. Both samples showed a profile in the hydrogen atomic concentration and two zones could be distinguished: the first zone Žlater called ‘lower’. corresponded to about half of the layer closest to the substrate. The second zone Žlater called ‘upper’. corresponded to the other half of the film closer to the surface. An average hydrogen concentration was measured in both zones for Samples A and B. From RBS and ERD, the following corresponding compositions were found: for Sample A—ZrO 2 H 0.14 Ž‘lower’. and ZrO 2 H 0.24 Ž‘upper’., for Sample B—ZrO 2.1 H 0.92 Ž‘lower’. and ZrO 2.1 H 0.62 Ž‘upper’.. The hydrogen concentration was relatively high in the film deposited without H 2 in the sputtering environment. It appeared later on that the presence of hydrogen in the film is probably related to the important contamination of the Zr metal target by hydrogen. The measured chemical compositions suggest that part of the hydrogen cannot be chemically bonded as water or as hydroxyl groups in the zirconium oxide. Indeed, the oxygen concentration should be higher in
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that case. As Zr 4q is known to be a very stable cation, the presence of Zr 3q in the film is unlikely. Part of the hydrogen therefore has to come from H 2 gas molecules, weakly interacting with the oxide inside pores. This hydrogen does not participate in the ion conduction process. An appropriate formula for this zirconium oxide would then be ZrO 2 P ŽH 2 O. x P wH 2 x y . Finally, the difference between the ‘lower’ and the ‘upper’ zones of the sample suggests a possible interaction of the material with the atmosphere. It should be kept in mind that these layers are porous. The porosity is estimated to be 30% w13x. In the case of Sample A, the increase in the H atomic percentage near the surface could be related to adsorption of water. This water would then be weakly bound to the zirconium oxide inside the pores. As for Sample B, a hydrogen gas release process can explain the decrease of the H atomic concentration in the ‘upper’ zone of the material. 3.1.2. UV r VIS r NIR characterisation The two samples A and B have been characterised in order to determine the transparency Žcurves b and c in Fig. 1.. A glass reference substrate has also been measured for comparison and is displayed as curve a in Fig. 1. The two films are colour-neutral and have a transmittance of 80–90%. Being of different thicknesses, they show different interference patterns.
Fig. 1. UVrVISrNIR spectra for the glass substrate Žcurve a., a sputter-deposited ZrO 2 in a plasma ArrO 2 —sample A— Žcurve b., and a sputter-deposited ZrO 2 in a plasma ArrO 2 rH 2 —sample B— Žcurve c..
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The two zirconium oxide films, deposited without and with H 2 , both show a good transparency and colour neutrality. That makes this material suitable for application in switchable mirrors devices, where the switching is between a reflecting and a transparent state. 3.2. ‘Symmetric-GdMg stack’
Fig. 2. XRD spectra for samples A and B. The curve from sample B has been shifted upward Žq200 counts. to provide a convenient reading of the two spectra, which would overlap otherwise. The XRD spectra corresponding to monoclinic baddelehite ŽJCPDS Ref. 37.1484. and cubic ŽJCPDS Ref. 27.0997. ZrO 2 have also been given.
3.1.3. XRD characterisation Deposited on silicon substrates, samples A and B show some crystallinity and two very different XRD spectra can be seen from Fig. 2. The curve of sample A is shifted up Žq200 counts. so that the differences between the two spectra are more striking. In sample A, the monoclinic baddelehite phase appears dominant. However, a certain amount of face centred cubic ZrO 2 or the quite similar Žin XRD. tetragonal ZrO 2 can be found. This last cubic phase appears highly dominant in the sample deposited in the presence of hydrogen in the plasma. The intensity ratios of the diffraction lines imply the absence of any strong preferred orientation. A rough estimation of the amount of both phases in sample A and B has been made. In Sample A, deposited without hydrogen, the estimated ratio monoclinicrcubic is ; 0.75r0.25 while in sample B, it is ; 0.25r0.75.
3.2.1. Description of the stack and deposition This multilayer assembly has been named ‘symmetric-GdMg stack’ because it involved not only one GdMg alloy layer as the electrochromically active layer, but also a very similar layer as charge storage electrode on the opposite side of the electrolyte. These two GdMg layers have been deposited in presence of a small amount of H 2 . Indeed, a major improvement in the durability of the films has been noticed when the film was deposited in a hydrided state w9x; this improvement could be related with the amount of stress created in the film during insertionrextraction. The addition of a small amount of H 2 to the argon plasma was not enough to provide a fully ‘hydrided’ state, but was supposed to suppress the irreversible stress introduced by an irreversible GdMg´ GdMgH 2 transition during the first hydrogen insertion. So, the ‘symmetric-GdMg stack’ studied here consisted of three main parts Žcf. cross-section in Fig. 3a., shifted with respect to each other along the diagonal of a square representing one device Žcf.
Fig. 3. Schematic representation of the ‘symmetric-GdMg’ stack: GdMgH x rPdrZrO 2 P ŽH 2 O. x P wH 2 x y rPdrGdMgH y rPd: Ža. cross-section; Žb. bottom view.
V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
Fig. 3b.. This shift was required in order to easily access and contact the bottom layer, which would be otherwise embedded below the other layers. Moreover, stacking all the layers exactly on top of each other would also result in an increased probability of short-circuiting along the edges. The first layer, close to the glass substrate was the GdMg alloy, deposited with a bit of H 2 inside the sputtering chamber. The gas flow ratio between Ar and H 2 was 20r10 sccm and the total pressure inside the sputtering chamber was 16.5 mbar. The power used for deposition of this layer was 30 W. The GdMg hydride film has been deposited during 200 s and its thickness was approximately 64 nm. At this hydrogen concentration the atomic ratio between Gd and Mg in the deposited film was 66r34, respectively, as extrapolated from RBS measurements. Directly on top of this metal hydride layer, a Pd layer was deposited, also in presence of H 2 in the plasma. The power and the gas flows were the same as for the GdMg hydride layer and the total pressure during deposition was 16.6 mbar. The time of deposition was 11 s and the measured thickness was ; 6 nm. Then the second layer Žzirconium oxide electrolyte. was deposited using the power and gas flow conditions close to those used for sample B: 50 W, ArrO 2rH 2 s 100r0.4r40 sccm. The time for deposition was 1100 s, providing a thickness of around 110 nm. The total pressure during deposition was 64.7 mbar. Finally, the top electrode was composed of three layers, all deposited using the same conditions: a RF power of 30 W, a gas mixture of ArrH 2 Ž20r10 sccm. and a total pressure approximately equal to 16.3 mbar. The first Pd layer, just on top of the ZrO 2 P ŽH 2 O. x P wH 2 x y electrolyte layer, had a protective and catalyst function and was deposited during 10 s. Its thickness was estimated to be ; 6 nm. Then, the second GdMg hydride layer was sputtered in 50 s providing a film with an estimated thickness of 16 nm. The final layer was palladium, covered the two previous layers and had a protective and catalyst function for the GdMg hydride in contact with the atmosphere. The electronic conductivity of this top electrode was high enough to be used as an electrical contact. To complete the description of the stack, it should be mentioned that the active area was represented by
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the overlap of all layers Žcf. Fig. 3a.. The surface of such an overlap is usually 25 mm2 . In Fig. 3b, the device is viewed from the glass substrate side, so that the first layer to be seen is the bottom GdMg metal hydride layer Žnoted 1., then the transparent zirconium oxide layer Žnoted 2. and finally the top block PdrGdMgH yrPd electrode Žnoted 3.. 3.2.2. Electrical and optical properties The top part of the ‘symmetric-GdMg stack’ is not capped, which means that when exposed to an H 2 gas atmosphere, the top-Pd layer allows the dissociation of H 2 molecules. The top GdMg layer gets hydrided, turning into a transparent state and staying so, as long as H 2 is present in the atmosphere. The metal hydride top-layer, as soon as hydrided, serves as hydrogen storage layer. When a potential is applied between the bottom and the top electrode, it is possible to reversibly transport the H ions through the electrolyte, into the bottom metalhydride layer. The device is a two-electrode system; potentials presented below are cell potentials and their values are progressively raised—respectively decreased—in order to reach the optical transitions. The chosen values should be low enough to avoid parasitic reactions, leading to a degradation of the sample. No particular precautions, i.e. potential ramping, have been taken in that case to avoid high current peaks. The current densities, associated to the anodic Ua and cathodic Uc potential steps, have been measured. A few cycles ŽUa ´ Uc ´ Ua . have been performed on this sample, and the first three are presented in Fig. 4. The potential has been kept constant in each direction during 450 s. It was sufficient to allow the optical transition between a transparent and a mirror state of the GdMg layer at these potentials. The values for the anodic and the cathodic potentials were q1 and y0.5 V, respectively Žcf. Fig. 4.. The measured current response as a function of the time is also displayed in Fig. 4. Fig. 5 gives the transmission and the reflection for the corresponding cycles. A potential step in the anodic direction corresponds to the extraction of H from the bottom GdMg layer, initially in a fully hydrided GdMgH 5 state, leading to the formation of the reflective GdMgH 2 hydride. On the other hand, the cathodic potential step causes the material to get back to a fully hy-
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V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
Fig. 4. Cell voltage and current density of the ‘symmetric-GdMg’ device shown in Fig. 3.
drided, transparent state. The current, measured in both directions, seems to display two main contributions. Indeed, a different current regime appears some 60–150 s after the potential step. The first abrupt decrease of the current response seems associated with a steep decrease of the transmission. Furthermore, the slower variation of the current values corresponds to a slower decrease in the transmission and an important increase in the reflection. Leakage currents can be estimated on that sample by measuring the value of current just before a subsequent potential step. The average values are
q225 and y304 mArcm2 , respectively, in the anodic and the cathodic direction for the three cycles presented in Fig. 4. The cathodic leakage current is slightly higher than the anodic one. This imbalance has already been seen in other devices w13x. The presence of leakage currents is usually related to electronic short circuits due to either the presence of sharp edges on the sample sides or pinholes in the active area. The total charge measured for the anodic and the cathodic steps are 3.6 and 3.9 mCrcm2 , respectively. The slight imbalance between these two va-
Fig. 5. Transmission and reflection of the ‘symmetric-GdMg’ device shown in Fig. 3.
V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
lues is due to the different values of leakage currents in the anodic and the cathodic direction. The transmission Žcf. Fig. 5. reached by the sample is varying between 0.5% at the end of the anodic potential step and 6% at the end of the cathodic one. This is relatively low due to the presence of the three Pd layers Žin total 18 nm thick. and the large number of interfaces in this device. The reflection signal needs more explanation: each potential step, whatever its direction, gives an increase of the reflection. However, the shapes of the curves are quite different: in the case of anodic potential step, the reflection increases progressively, then when the current is reversed, decreases first then increases very rapidly again. If the device, presented here, really worked the way a ‘switchable mirror’ would be expected to, between a transparent and a reflective state, a maximum in transmission should be correlated with a minimum in reflection. Such a high reflection during the cathodic step Žcf. Fig. 5., can only be explained by the presence of the three Pd metal layers, which have quite a high reflectivity Žabout 50%.. This is even larger than the one from the GdMg layer itself Ž; 43%., which clearly appears when the transmission is minimal Ž0.5%.. To get closer to a real switchable mirror, the reflectivity in the transparent state has to go down, and the reflectivity in the reflecting state has to go up. To that end, the Pd layer thicknesses could be reduced or the GdMg thickness enlarged or the Mg content of the GdMg alloy increased.
4. Conclusions A ‘symmetric-GdMg’ all-solid state device has been deposited on a glass substrate, and was constituted by the following layers: a GdMgH xrPd bottom electrode, a ZrO 2 P ŽH 2 O. x P wH 2 x y electrolyte and a PdrGdMgH yrPd storage top electrode. When exposed to an H 2 atmosphere, this device showed a reversible optical switching. These preliminary results obtained on this ‘symmetric-GdMg’ stack, and those previously presented for another solid-state device w13x, showed the promising character of ZrO 2 P ŽH 2 O. x P wH 2 x y as a proton conductor for switchable mirror devices.
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The composition, transparency and structure of the zirconium oxide, sputter-deposited in an Arr O 2rH 2 gas mixture, have been investigated in this paper. However, more work is needed for this material so that a wider range of data Žchemical composition, structure, and conductivity measurements. can be obtained also for films with different hydrogen concentrations.
Acknowledgements The authors thank the European TMR Research Network: ‘Metal hydride films with switchable physical properties’ and the Dutch BTS program: ‘Switchable mirrors’ for their financial support.
References w1x J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Decker, D.G. de Groot, N.J. Koeman, Nature 380 Ž1996. 231. w2x J.N. Huiberts, J.H. Rector, R.J. Wijngaarden, S. Jetten, D. De Groot, B. Dam, N.J. Koeman, R. Griessen, B. Hjorvarsson, ¨ S. Olafsson, Y.S. Cho, J. Alloys Comp. 239 Ž1996. 158. w3x E.S. Kooij, A.T.M. van Gogh, R. Griessen, J. Electrochem. Soc. 146 Ž1999. 2990. w4x P. van der Sluis, M. Ouwerkerk, P.A. Duine, Appl. Phys. Lett. 70 Ž1997. 3356. w5x P. van der Sluis, Appl. Phys. Lett. 73 Ž1998. 1826. w6x P.H.L. Notten, M. Kremers, R. Griessen, J. Electrochem. Soc. 143 Ž1996. 3348. w7x M. Ouwerkerk, Solid State Ionics 113–115 Ž1998. 431. w8x A.-M. Janner, P. van der Sluis, in: E.D. Wachsman, J.R. Akridge, M. Liu, N. Yamazoe ŽEds.., Proceedings of the International Symposium: Solid State Ionic Devices, The Electrochemical Society, Pennington, 1999, p. 289. w9x A.-M. Janner, P. van der Sluis, V.M.M. Mercier, International Meeting on Electrochromism IME-4, August 2000, Uppsala, Electrochim. Acta 46 Ž2001. 2173. w10x K. von Rottkay, M. Rubin, F. Michalak, R. Armitage, T. Richardson, J. Slack, P.A. Duine, Electrochim. Acta 44 Ž1999. 3093. w11x C.G. Granqvist ŽEd.., Handbook of Inorganic Electrochromic Materials. Elsevier, Amsterdam, 1995. w12x R. Armitage, M. Rubin, T. Richardson, N. O’Brien, Y. Chen, Appl. Phys. Lett. 75 Ž1999. 1863.
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w13x P. van der Sluis, V.M.M. Mercier, International Meeting on Electrochromism IME-4, August 2000, Uppsala, Electrochim. Acta 46 Ž2001. 2167. w14x Ph. Colomban, A. Novak, in: Ph. Colomban ŽEd.., Proton Conductors, Solids, Membranes and Gels—Materials and Devices, Cambridge Univ. Press, Cambridge, 1992. w15x R.M. Dell, A. Hooper, in: P. Hagenmuller, W. van Gool
ŽEds.., Chap. 18: Oxygen Ion Conductors, Solid Electrolytes: General Principles, Characterisation, Materials, Applications, Material Science Series, Academic Press, New York, 1978, p. 291. w16x W.A. England, M.G. Cross, A. Hamnett, P.J. Wiseman, J.B. Goodenough, Solid State Ionics 1 Ž1980. 231.
Toward solid-state switchable mirrors using a zirconium oxide proton conductor Virginie M.M. Mercier ) , Paul van der Sluis Philips Research Laboratories, Prof. Holstlaan 4, (WA-11), 5656 AA EindhoÕen, The Netherlands Received 27 October 2000; received in revised form 1 February 2001; accepted 16 March 2001
Abstract The unique optical properties of magnesium rare-earth alloys are interesting for making new types of solid-state electrochromic devices, based on hydrogen transport. These devices can reversibly switch between a transparent and a reflective state, when a potential difference is applied. The all-solid state devices presently studied involve a zirconium oxide proton conductor. This paper discusses the deposition and preliminary characterisation of the solid-state electrolyte. It also gives some information about the performance of a symmetrical GdMg device tested in a hydrogen gas atmosphere. This stack is composed of a GdMgH xrPd bottom electrode, a ZrO 2 P ŽH 2 O. x P wH 2 x y proton conductor and a PdrGdMgH yrPd top-electrode. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Metal hydride; Electrochromic device; Solid-state electrolyte; ZrO 2 P ŽH 2 O. x P wH 2 x y
1. Introduction Lanthanide metals, capped with a palladium topcoat, show interesting optical properties. This was discovered five years ago at the Vrij Universiteit of Amsterdam w1,2x. Upon hydrogen insertion, they undergo a major optical change, reversibly switching between a non-transparent Žlow-hydrogen concentration. state and a transparent Žhigh-hydrogen concentration. state. The presence of the Pd top-layer is
)
Corresponding author. Tel.: q31-40-2742343; fax: q31-402744282. E-mail address: [email protected] ŽV.M.M. Mercier..
necessary both for protection and catalysis. The lanthanide metals are especially sensitive to oxidation. The lanthanide metal hydrides always lack colour neutrality in their transparent state: for example, YH 3 w3x or GdH 3 w4x are yellow-transparent. To improve the colour neutrality, van der Sluis et al. proposed to alloy the lanthanide metals with Mg w4x or to make lanthanide–magnesium multilayers w5x. The lanthanide–magnesium films have first been switched in hydrogen gas w4,5x. Unfortunately, for technological applications, this is not practical. Electrochemical switching has then been successfully carried out for various lanthanides or lanthanide–alloys in alkaline solutions w6–10x. This achievement opened opportunities for producing electrochromic devices. Such devices generally consist of three parts w11x: a main electrochromically active layer, capable
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 9 0 6 - 7
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V.M.M. Mercier, P. Õan der Sluis r Solid State Ionics 145 (2001) 17–24
of reversible insertion of a guest ion ŽH, Li, . . . .; an electrolyte layer; and a ion storage layer, with an appropriate switching electrochromic behaviour. In the particular case of ‘switchable mirrors’, the main electrochromic layer will be a GdMg hydride. Different materials for the ion storage layer are available among which tungsten oxide is proven to be a good candidate w12,13x. The electrolyte can be a liquid, a gel or a solid. This last option is, of course, the most suitable for technological applications. That is the reason why the use of a zirconium oxide solid electrolyte is presently explored for the switchable mirror systems w12,13x. Usually, the electrolyte must fulfil some requirements w14x: it must be an electronic insulator Ž se 10y1 2 Srcm. and a good ionic conductor Žusually 10y7 - s i - 10y4 Srcm.. If the application requires that the device can be switched to the transparent state, then the proton conductor must also be transparent and colour neutral. The electrolyte should also not react with the two insertion electrodes in between which it is sandwiched. This chemical stability must be ensured both during deposition and during cycling. Finally, for certain applications Že.g. windows., the electrolyte must also be stable in temperature and under solar radiation. Generally, zirconium oxides, in the fluorite structure, are well known as good oxygen conductors at high temperatures w15x. However, ZrO 2 is mono˚ b s 5.17 clinic at ambient temperature Ž a s 5.12 A, ˚A, c s 5.29 A, ˚ b s 99.118., and non-conducting. The cubic fluorite structured ZrO 2 is generally stabilised using dopants like Y w15x. Hydrous oxides are usually good conductors w16x. They are described as a hydrated material consisting of irregular, amphoteric charged particles separated by aqueous solution. This material, initially prepared with a sol–gel-like technique, is pressed into a pellet and its conductivity is measured w16x. In a hydrated form ŽZrO 2 P 1.75H 2 O., ZrO 2 is said to have a conductivity of 2 ) 10y5 Vy1 cmy1 at 278C and an activation energy of 0.34 eV, with a surface liquidlike conductivity w16x. In this paper, zirconium oxide is deposited as a thin film by reactive sputtering in the presence of oxygen and hydrogen. This deposition technique would then allow us to obtain thin films with a proton conducting character.
2. Experimental Deposition of layers and stack are carried out in a custom-made RF magnetron sputtering apparatus Žtargets are about 5 cm in diameter and their distance to the substrate is around 3 cm.. Deposition of GdMg is done from a Gd metal target with inserts of Mg, so that the ratio between Gd and Mg in the deposited films is near 1:1. The deposition of the GdMg hydride is then carried out in an ArrH 2 mixture. Pd is also deposited in the same ArrH 2 atmosphere from a Pd metal target. The electrolyte is deposited from a Zr target in an ArrO 2rH 2 mixture. Individual mass flow controllers allow an estimation of the pressures of each gas involved in the deposition. The purity of the sputtered gases is better than 99.9995% for O 2 and H 2 and 99.9999% for Ar. The base pressure before deposition is about 8 ) 10y7 mbar. The substrate holder is water-cooled so that deposition can be performed as close as possible to room temperature. Substrates are placed face-down in order to avoid contamination by dust. Float glass and silicon substrates Ž10 ) 40 mm2 . are used for zirconium oxide characterisation. For stacks, float glass substrates of 30 ) 30 mm2 are used. It is possible to simultaneously deposit four devices Ža–d., using a mask system. Chemical compositions are determined by Rutherford Backscattering Spectrometry ŽRBS.. Elastic Recoil Detection ŽERD. is used to estimate the total amount of hydrogen in the sample. Layer thicknesses are usually approximated from RBS measurements, taking into account a theoretical density for the material. In the case of zirconium oxide films, optical measurements in the 300–2500 nm wavelength range are performed in a spectrophotometer ŽPerkin Elmer l19.. XRD ŽX-Ray Diffraction. measurements are performed in a PW 1800 powder diffractometer with CuK a radiation Ž108 - 2 u - 808; DŽ2 u . s 0.028.. To measure the optical properties of the complete symmetric-GdMg stack, a monochromatic He–Ne laser Ž633 nm. has been used. Reflection measurements are carried out from the substrate side. Electrical measurements Žcurrent in function of applied potential. are made with a Keithley 2400 sourcemeter controlled by a PC.
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3. Results 3.1. Zirconium oxide film 3.1.1. Deposition and chemical characterisation Two different zirconium oxides have been prepared. One film has been deposited in an ArrO 2 atmosphere Žnamed ‘Sample A’. and the other in an ArrO 2rH 2 atmosphere Žnamed ‘Sample B’.. The flows of the different gases during deposition were, respectively: 100r0.8 and 100r0.8r40 sccm Žstandard cubic centimetre per minute.; the total deposition pressures were 46.7 and 60.5 mbar, respectively. The power was kept constant at 50 W. These samples have been deposited during the same amount of time Ž2500 s. but their deposition rates were different Žsample A: 0.11 nmrs and sample B: 0.07 nmrs., resulting in layer thicknesses of 280 and 170 nm for samples A and B, respectively. RBS on the two different samples gave OrZr ratio of around 2.0 " 0.1 for sample A and 2.1 " 0.1 for sample B. The RBS profile also showed that Hf was present as impurity in the films with a concentration of around 0.02 at.%. The main difference between the two samples came essentially from their hydrogen content, determined by ERD. Both samples showed a profile in the hydrogen atomic concentration and two zones could be distinguished: the first zone Žlater called ‘lower’. corresponded to about half of the layer closest to the substrate. The second zone Žlater called ‘upper’. corresponded to the other half of the film closer to the surface. An average hydrogen concentration was measured in both zones for Samples A and B. From RBS and ERD, the following corresponding compositions were found: for Sample A—ZrO 2 H 0.14 Ž‘lower’. and ZrO 2 H 0.24 Ž‘upper’., for Sample B—ZrO 2.1 H 0.92 Ž‘lower’. and ZrO 2.1 H 0.62 Ž‘upper’.. The hydrogen concentration was relatively high in the film deposited without H 2 in the sputtering environment. It appeared later on that the presence of hydrogen in the film is probably related to the important contamination of the Zr metal target by hydrogen. The measured chemical compositions suggest that part of the hydrogen cannot be chemically bonded as water or as hydroxyl groups in the zirconium oxide. Indeed, the oxygen concentration should be higher in
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that case. As Zr 4q is known to be a very stable cation, the presence of Zr 3q in the film is unlikely. Part of the hydrogen therefore has to come from H 2 gas molecules, weakly interacting with the oxide inside pores. This hydrogen does not participate in the ion conduction process. An appropriate formula for this zirconium oxide would then be ZrO 2 P ŽH 2 O. x P wH 2 x y . Finally, the difference between the ‘lower’ and the ‘upper’ zones of the sample suggests a possible interaction of the material with the atmosphere. It should be kept in mind that these layers are porous. The porosity is estimated to be 30% w13x. In the case of Sample A, the increase in the H atomic percentage near the surface could be related to adsorption of water. This water would then be weakly bound to the zirconium oxide inside the pores. As for Sample B, a hydrogen gas release process can explain the decrease of the H atomic concentration in the ‘upper’ zone of the material. 3.1.2. UV r VIS r NIR characterisation The two samples A and B have been characterised in order to determine the transparency Žcurves b and c in Fig. 1.. A glass reference substrate has also been measured for comparison and is displayed as curve a in Fig. 1. The two films are colour-neutral and have a transmittance of 80–90%. Being of different thicknesses, they show different interference patterns.
Fig. 1. UVrVISrNIR spectra for the glass substrate Žcurve a., a sputter-deposited ZrO 2 in a plasma ArrO 2 —sample A— Žcurve b., and a sputter-deposited ZrO 2 in a plasma ArrO 2 rH 2 —sample B— Žcurve c..
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The two zirconium oxide films, deposited without and with H 2 , both show a good transparency and colour neutrality. That makes this material suitable for application in switchable mirrors devices, where the switching is between a reflecting and a transparent state. 3.2. ‘Symmetric-GdMg stack’
Fig. 2. XRD spectra for samples A and B. The curve from sample B has been shifted upward Žq200 counts. to provide a convenient reading of the two spectra, which would overlap otherwise. The XRD spectra corresponding to monoclinic baddelehite ŽJCPDS Ref. 37.1484. and cubic ŽJCPDS Ref. 27.0997. ZrO 2 have also been given.
3.1.3. XRD characterisation Deposited on silicon substrates, samples A and B show some crystallinity and two very different XRD spectra can be seen from Fig. 2. The curve of sample A is shifted up Žq200 counts. so that the differences between the two spectra are more striking. In sample A, the monoclinic baddelehite phase appears dominant. However, a certain amount of face centred cubic ZrO 2 or the quite similar Žin XRD. tetragonal ZrO 2 can be found. This last cubic phase appears highly dominant in the sample deposited in the presence of hydrogen in the plasma. The intensity ratios of the diffraction lines imply the absence of any strong preferred orientation. A rough estimation of the amount of both phases in sample A and B has been made. In Sample A, deposited without hydrogen, the estimated ratio monoclinicrcubic is ; 0.75r0.25 while in sample B, it is ; 0.25r0.75.
3.2.1. Description of the stack and deposition This multilayer assembly has been named ‘symmetric-GdMg stack’ because it involved not only one GdMg alloy layer as the electrochromically active layer, but also a very similar layer as charge storage electrode on the opposite side of the electrolyte. These two GdMg layers have been deposited in presence of a small amount of H 2 . Indeed, a major improvement in the durability of the films has been noticed when the film was deposited in a hydrided state w9x; this improvement could be related with the amount of stress created in the film during insertionrextraction. The addition of a small amount of H 2 to the argon plasma was not enough to provide a fully ‘hydrided’ state, but was supposed to suppress the irreversible stress introduced by an irreversible GdMg´ GdMgH 2 transition during the first hydrogen insertion. So, the ‘symmetric-GdMg stack’ studied here consisted of three main parts Žcf. cross-section in Fig. 3a., shifted with respect to each other along the diagonal of a square representing one device Žcf.
Fig. 3. Schematic representation of the ‘symmetric-GdMg’ stack: GdMgH x rPdrZrO 2 P ŽH 2 O. x P wH 2 x y rPdrGdMgH y rPd: Ža. cross-section; Žb. bottom view.
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Fig. 3b.. This shift was required in order to easily access and contact the bottom layer, which would be otherwise embedded below the other layers. Moreover, stacking all the layers exactly on top of each other would also result in an increased probability of short-circuiting along the edges. The first layer, close to the glass substrate was the GdMg alloy, deposited with a bit of H 2 inside the sputtering chamber. The gas flow ratio between Ar and H 2 was 20r10 sccm and the total pressure inside the sputtering chamber was 16.5 mbar. The power used for deposition of this layer was 30 W. The GdMg hydride film has been deposited during 200 s and its thickness was approximately 64 nm. At this hydrogen concentration the atomic ratio between Gd and Mg in the deposited film was 66r34, respectively, as extrapolated from RBS measurements. Directly on top of this metal hydride layer, a Pd layer was deposited, also in presence of H 2 in the plasma. The power and the gas flows were the same as for the GdMg hydride layer and the total pressure during deposition was 16.6 mbar. The time of deposition was 11 s and the measured thickness was ; 6 nm. Then the second layer Žzirconium oxide electrolyte. was deposited using the power and gas flow conditions close to those used for sample B: 50 W, ArrO 2rH 2 s 100r0.4r40 sccm. The time for deposition was 1100 s, providing a thickness of around 110 nm. The total pressure during deposition was 64.7 mbar. Finally, the top electrode was composed of three layers, all deposited using the same conditions: a RF power of 30 W, a gas mixture of ArrH 2 Ž20r10 sccm. and a total pressure approximately equal to 16.3 mbar. The first Pd layer, just on top of the ZrO 2 P ŽH 2 O. x P wH 2 x y electrolyte layer, had a protective and catalyst function and was deposited during 10 s. Its thickness was estimated to be ; 6 nm. Then, the second GdMg hydride layer was sputtered in 50 s providing a film with an estimated thickness of 16 nm. The final layer was palladium, covered the two previous layers and had a protective and catalyst function for the GdMg hydride in contact with the atmosphere. The electronic conductivity of this top electrode was high enough to be used as an electrical contact. To complete the description of the stack, it should be mentioned that the active area was represented by
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the overlap of all layers Žcf. Fig. 3a.. The surface of such an overlap is usually 25 mm2 . In Fig. 3b, the device is viewed from the glass substrate side, so that the first layer to be seen is the bottom GdMg metal hydride layer Žnoted 1., then the transparent zirconium oxide layer Žnoted 2. and finally the top block PdrGdMgH yrPd electrode Žnoted 3.. 3.2.2. Electrical and optical properties The top part of the ‘symmetric-GdMg stack’ is not capped, which means that when exposed to an H 2 gas atmosphere, the top-Pd layer allows the dissociation of H 2 molecules. The top GdMg layer gets hydrided, turning into a transparent state and staying so, as long as H 2 is present in the atmosphere. The metal hydride top-layer, as soon as hydrided, serves as hydrogen storage layer. When a potential is applied between the bottom and the top electrode, it is possible to reversibly transport the H ions through the electrolyte, into the bottom metalhydride layer. The device is a two-electrode system; potentials presented below are cell potentials and their values are progressively raised—respectively decreased—in order to reach the optical transitions. The chosen values should be low enough to avoid parasitic reactions, leading to a degradation of the sample. No particular precautions, i.e. potential ramping, have been taken in that case to avoid high current peaks. The current densities, associated to the anodic Ua and cathodic Uc potential steps, have been measured. A few cycles ŽUa ´ Uc ´ Ua . have been performed on this sample, and the first three are presented in Fig. 4. The potential has been kept constant in each direction during 450 s. It was sufficient to allow the optical transition between a transparent and a mirror state of the GdMg layer at these potentials. The values for the anodic and the cathodic potentials were q1 and y0.5 V, respectively Žcf. Fig. 4.. The measured current response as a function of the time is also displayed in Fig. 4. Fig. 5 gives the transmission and the reflection for the corresponding cycles. A potential step in the anodic direction corresponds to the extraction of H from the bottom GdMg layer, initially in a fully hydrided GdMgH 5 state, leading to the formation of the reflective GdMgH 2 hydride. On the other hand, the cathodic potential step causes the material to get back to a fully hy-
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Fig. 4. Cell voltage and current density of the ‘symmetric-GdMg’ device shown in Fig. 3.
drided, transparent state. The current, measured in both directions, seems to display two main contributions. Indeed, a different current regime appears some 60–150 s after the potential step. The first abrupt decrease of the current response seems associated with a steep decrease of the transmission. Furthermore, the slower variation of the current values corresponds to a slower decrease in the transmission and an important increase in the reflection. Leakage currents can be estimated on that sample by measuring the value of current just before a subsequent potential step. The average values are
q225 and y304 mArcm2 , respectively, in the anodic and the cathodic direction for the three cycles presented in Fig. 4. The cathodic leakage current is slightly higher than the anodic one. This imbalance has already been seen in other devices w13x. The presence of leakage currents is usually related to electronic short circuits due to either the presence of sharp edges on the sample sides or pinholes in the active area. The total charge measured for the anodic and the cathodic steps are 3.6 and 3.9 mCrcm2 , respectively. The slight imbalance between these two va-
Fig. 5. Transmission and reflection of the ‘symmetric-GdMg’ device shown in Fig. 3.
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lues is due to the different values of leakage currents in the anodic and the cathodic direction. The transmission Žcf. Fig. 5. reached by the sample is varying between 0.5% at the end of the anodic potential step and 6% at the end of the cathodic one. This is relatively low due to the presence of the three Pd layers Žin total 18 nm thick. and the large number of interfaces in this device. The reflection signal needs more explanation: each potential step, whatever its direction, gives an increase of the reflection. However, the shapes of the curves are quite different: in the case of anodic potential step, the reflection increases progressively, then when the current is reversed, decreases first then increases very rapidly again. If the device, presented here, really worked the way a ‘switchable mirror’ would be expected to, between a transparent and a reflective state, a maximum in transmission should be correlated with a minimum in reflection. Such a high reflection during the cathodic step Žcf. Fig. 5., can only be explained by the presence of the three Pd metal layers, which have quite a high reflectivity Žabout 50%.. This is even larger than the one from the GdMg layer itself Ž; 43%., which clearly appears when the transmission is minimal Ž0.5%.. To get closer to a real switchable mirror, the reflectivity in the transparent state has to go down, and the reflectivity in the reflecting state has to go up. To that end, the Pd layer thicknesses could be reduced or the GdMg thickness enlarged or the Mg content of the GdMg alloy increased.
4. Conclusions A ‘symmetric-GdMg’ all-solid state device has been deposited on a glass substrate, and was constituted by the following layers: a GdMgH xrPd bottom electrode, a ZrO 2 P ŽH 2 O. x P wH 2 x y electrolyte and a PdrGdMgH yrPd storage top electrode. When exposed to an H 2 atmosphere, this device showed a reversible optical switching. These preliminary results obtained on this ‘symmetric-GdMg’ stack, and those previously presented for another solid-state device w13x, showed the promising character of ZrO 2 P ŽH 2 O. x P wH 2 x y as a proton conductor for switchable mirror devices.
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The composition, transparency and structure of the zirconium oxide, sputter-deposited in an Arr O 2rH 2 gas mixture, have been investigated in this paper. However, more work is needed for this material so that a wider range of data Žchemical composition, structure, and conductivity measurements. can be obtained also for films with different hydrogen concentrations.
Acknowledgements The authors thank the European TMR Research Network: ‘Metal hydride films with switchable physical properties’ and the Dutch BTS program: ‘Switchable mirrors’ for their financial support.
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