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    AC plasma electrolytic oxidation of magnesium with zirconia nanoparticles

    R. Arrabal a, E. Matykina a, F. Viejo a, P. Skeldon a,*, G.E. Thompson a, M.C. Merino b

    a Corrosion and Protection Centre, School of Materials, The University of Manchester, Sackville Street, P.O. Box 88, Manchester M60 1QD, UKb Departamento de Ciencia de Materiales, Facultad de Ciencias Quimicas, Universidad Complutense, 28040 Madrid, Spain

    1. Introduction

    Plasma electrolytic oxidation (PEO) is used for the surface

    protection of magnesium alloys; several commercial processes are

    available[13]. Processes are also being developed that provide

    coatings with improved properties. Thecoatings areformed at high

    voltages, in various, usually aqueous, electrolytes, when micro-

    discharges are present on the alloy surface[4]. The mechanism of

    formation of the coating potentially involves anodic oxidation,

    thermal oxidation and plasma-chemical reactions[5]. The growth

    of the coating is accompanied by relatively profuse generation of

    oxygen gas. Coatings can be grown under DC and AC conditions,

    with a range of waveforms, leading to coatings that generally

    thicken roughly in proportion to thecharge passed [6]. Thicknesses

    of tens of microns arereadily produced,containing amorphous and

    crystalline constituents dependent on the particular alloy and

    electrolyte [7]. Magnesium oxide is generally present in the

    coatings, often accompanied by phases related to the electrolyte

    anions, for instance magnesium silicate in silicate electrolyte. In

    cross-section, two main layers of DC coatings may be discerned;

    the inner layer has a finer texture than the outer layer. At the base

    of the inner layer, a relatively thin, compact barrier region is

    located at the alloy/coating interface, with a thickness of up to

    lmm. Short-circuit transport of electrolyte components through

    the outer layer has been shown to occur[79]. This allows anion

    species of the electrolyte, or possibly derivatives formed in the

    microdischarge, ready access to the inner coating. Cation species

    can also be transported across the outer coating, although in

    greatly reduced amounts compared with the anion species[10]. In

    addition, nanoparticles in suspension in the electrolyte can

    reach the inner layer via the short-circuit paths, which has been

    demonstrated using monoclinic zirconia [11]. The nanoparticles

    are also incorporated into the outer part of the coating, which

    reveals a modified microstructure compared with coating

    formed in the absence of nanoparticles. Tetragonal zirconia is

    detected in significant amounts in the final coating, due to the

    high temperatures generated at microdischarge sites. In the

    present study, the influence of AC treatment on the incorporation

    of zirconia is investigated for magnesium treated in silicate

    electrolyte. The incorporation of nanoparticles is relevant to

    understand the mechanism of coating growth and for the potential

    to improve coating properties through modification of the coating

    microstructure [12]. The main interest of the work concerned

    the microstructure and morphology of the coatings, especially the

    distribution of zirconium and the types of zirconia-containing

    phases.

    Applied Surface Science 254 (2008) 69376942

    A R T I C L E I N F O

    Article history:

    Received 10 April 2008Received in revised form 29 April 2008

    Accepted 29 April 2008

    Available online 7 May 2008

    PACS:

    81.65.b

    81.65.Mq

    Keywords:

    Magnesium

    Coatings

    Plasma electrolytic oxidation

    Zirconium oxide

    Anodizing

    A B S T R A C T

    The incorporation of monoclinic zirconia nanoparticles and their subsequent transformation is examined

    for coatings formed on magnesium by plasma electrolytic oxidation under AC conditions in silicateelectrolyte. Thecoatings areshown to comprise two main layers, with nanoparticles enteringthe coating

    at thecoating surface andthrough short-circuit pathsto theregion of theinterface between theinner and

    outer coating layers. Under local heating of microdischarges, the zirconia reacts with magnesium species

    to form Mg2Zr5O12in the outer coating layer. Relatively little zirconium is present in the inner coating

    layer. In contrast, silicon species are present in both coating layers, with reduced amounts in the inner

    layer.

    2008 Elsevier B.V. All rights reserved.

    * Corresponding author. Tel.: +44 161 306 4872; fax: +44 161 306 4865.

    E-mail address: [email protected] (P. Skeldon).

    Contents lists available atScienceDirect

    Applied Surface Science

    j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c at e / a p s u s c

    0169-4332/$ see front matter 2008 Elsevier B.V. All rights reserved.

    doi:10.1016/j.apsusc.2008.04.100

    mailto:[email protected]://www.sciencedirect.com/science/journal/01694332http://dx.doi.org/10.1016/j.apsusc.2008.04.100http://dx.doi.org/10.1016/j.apsusc.2008.04.100http://www.sciencedirect.com/science/journal/01694332mailto:[email protected]
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    2. Experimental

    Specimens of 99.9% magnesium sheet were embedded in resin,

    with electrical connection provided through a shielded copper

    wire. The exposed surface, of area 1 cm2, was ground to 1200 grit

    SiC, using water as a lubricant, degreased in ethanol, rinsed in

    deionized water and dried in warm air. PEO treatment was carried

    out at 200 mA cm2 (rms) at 50 Hz with a sinusoidal waveform in

    0.025 M Na2SiO35H2O/0.05 M KOH for up to 2400 s. The electro-

    lyte was prepared from deionized water and high-purity chemi-

    cals. Additions of 10 g/l of monoclinic zirconia nanoparticles, of

    size 150300 nm, were made as required. The anodizing condition

    was selected following evaluation of a range of current densities

    and electrolytes compositions for incorporation of zirconia into the

    coating. The coatings were formed in a 1 l double-walled glass cell

    through which a cooledwater/glycol mixture waspumped in order

    to keep the electrolyte temperature close to 293 K. During the

    treatment, the electrolyte was stirred continuously, which kept the

    zirconia nanoparticles in suspension. A sheet of type 304 stainless

    steel, of size 7.5 cm 15 cm, was used as the cathode. Voltage

    responses were recorded electronically, with a sampling time of

    20 ms, employing an SCXI data acquisition system (National

    Instruments) with data analysis by Igor Pro (Wavemetrics). Aftercoating, specimens were rinsed with deionized water and dried in

    warm air. PEO-treated specimens were examined by field emission

    gun scanning electron microscopy, using a Philips XL30 instrument

    equipped with energy-dispersive X-ray (EDX) analysis facilities.

    Cross-sections were ground through successive grades of SiC

    paper, followed by finishing to 1 mm diamond. Phase composition

    was investigated by X-ray diffraction (XRD), using a Philips XPert-

    MPD (PW 3040) instrument with a step size 0.0058 and a scan

    range from 5 to 858 (in 2u).

    3. Results

    Voltagetime responses were practically the same during PEO

    without and with zirconia in the electrolyte, with commencementof sparking at 250 V coinciding with the major change in slope

    between the initial period of relatively rapid voltage rise and the

    start of the main period of slow voltage increase ( Fig. 1). Without

    zirconia in the electrolyte, the surface of the resultant coating

    comprises apparently overlapping, roughly circular features

    (Fig. 2(a)). The shapes of the features suggest outward flow of

    material along a channel through the underlying coating to the

    coating surface, followed by its spreading over the coating surface

    andsolidification.The largest of thefeatureshave dimensions of up

    to 50mm. The surface is significantly rough and pores and cracks

    are evident. In addition, micron-sized silicon-rich particles, with

    the appearance of a deposit, are also present; such particles are

    often found following PEO in silicate electrolyte [13].The coating roughness and pores are clear in cross-sections of

    the coating (Fig. 2(b)). The thickness of the coating at the thickest

    regions is about twice that at thethinnestregions,with inclusionof

    porosity in the thickness measurements. Two main layers are also

    revealed, with the inner layer being finely porous, whereas the

    outer layer has relatively large pores, cavities and channels. There

    are also regions of finer pores in the outer layer and of larger pores

    in the inner layer although these are of relatively minor extent.

    Cracks are mostly evident in the outer layer. Magnesium, oxygen

    and silicon are detected in both layers by EDX point analyses; the

    atomic ratios of Si:Mg were 0.28 0.08 and 0.10 0.04 in the outer

    and inner layers, respectively (points A and B, Fig. 2(b)). The

    corresponding O:Mg ratios were 3.1 0.4 and 2.6 0.3.

    The surface of the coating formed in electrolyte containingzirconia nanoparticles revealed two distinct types of region in

    backscattered electron images (Fig. 3(a)). An extensive, irregular

    network of light regions corresponded to coating decorated by

    numerous zirconia nanoparticles. In contrast, relatively dark,

    roughly circular zones, of size 50100mm, are mainly free of

    nanoparticles, but disclose dendrites of size 12mm (Fig. 3(b)).

    Non-uniformity in the distribution of zirconia particles at the

    coating surface was also observed following DC treatment in an

    alkaline phosphate electrolyte[11]. Cracks and pores are found in

    both types of region of the coating. Further, EDX point analyses

    revealed the presence of silicon and zirconium, with Si:Mg and

    Fig. 1. Voltagetime responses during AC PEO treatment of magnesium at

    200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH, without and with

    addition of 10 g/l of monoclinic zirconia.

    Fig. 2.Scanning electron micrographs (secondary electrons) of magnesium following AC PEO treatment for 2400 s at 200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M

    KOH. (a) Plan. (b) Cross-section.

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    Zr:Mg ratios of 0.26 0.04 and 0.26 0.06, respectively in the

    region containing micron-sized dendrites (point C, Fig. 3(a)) and

    0.33 0.08 and 0.55 0.14, respectively, in the region covered by

    zirconia nanoparticles (point D,Fig. 3(a)).

    In cross-section, the coating formedwith zirconia in suspension

    in the electrolyte comprises two main layers (Fig. 4(a and b)). The

    inner layer has a relatively fine porosity. The outer layer is more

    compact, and with reduced porosity compared with theouterlayer

    of coatings formed in the absence of zirconia in the electrolyte.

    Cavities appear to be prevalent near the interface between the

    inner and outer layers. The X-ray elemental maps show the

    presence of magnesium, oxygen and silicon in most coating

    Fig. 3. Scanning electron micrographs (backscattered electrons) of the surface of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M

    Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) General view. (b) Region with relatively few nanoparticles.

    Fig. 4.(a) Scanning electron micrograph (backscattered electrons) of a cross-section of magnesium following AC PEO treatment for 2400 s at 200 mA cm 2 (rms) in 0.025 M

    Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (b) Detail of region between the inner and outer coating layers. EDX elemental maps are shown for

    magnesium, oxygen, silicon and zirconium of the cross-section of (a).

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    regions, with a reduced amount of silicon in the inner layer (Fig. 4).

    Zirconium was mainly a component of the outer layer. EDX point

    analyses disclosed Si:Mg and Zr:Mg ratios of 0.26 0.07 and

    0.16 0.05, respectively, in the outer layer (point E, Fig. 4(a)) and

    0.10 0.03 and 0.02 0.01, respectively, in the inner layer (point F,

    Fig. 4(a)), confirming the moderately reduced concentration of silicon

    andgreatlyreducedconcentration of zirconiumin theinnerlayer. TheSi:Mg ratiosin thetwo layersweresimilar to those determined forthe

    coating formed in zirconia-free electrolyte. Zirconia nanoparticles,

    similar in size to those present in the electrolyte, are prevalent near

    the interface between the inner and outer layers and within larger

    cavities and channels in the outer layer (Fig. 4(b)). Zirconium is also

    generally distributed throughout the outer coating at locations

    largely free of nanoparticles. The outer coating in these latter regions

    contains numerous dendrites roughly similar in size and shape to

    those observed at the coating surface (Fig. 5(a and c)). Observation at

    another region of the cross-section, corresponding to the network of

    nanoparticles observed at the film surface, revealed that the

    nanoparticles were incorporated into a layer of thickness 12mm

    (Fig. 5(b and d)). EDX point analysis indicated Si:Mg and Zr:Mg ratios

    of 1.33 0.31 and 1.09 0.46, respectively, suggesting trapping ofnanoparticles within silicon-rich material formed at the coating

    surface (point G, Fig. 5(b)). The underlying coating microstructure

    disclosed discrete light particles, of size suggestive of incorporated

    nanoparticles, mainly within a relatively light network of matrix

    material (Fig. 5(d)). The latter surrounded micron-sized islands of

    relatively dark material. EDX point analyses indicated Zr:Mg ratios of

    0.16 0.06 and 0.18 0.05 and Si:Mg ratios of 0.16 0.03 and

    0.17 0.03 in thedarker and lighter materials, respectively (points H

    and I, Fig.5(d)), although theresults are probably affected by thelarge

    size of the analysis volume relative to the dimension of the zones and

    hence, represent average compositions for the microstructure.

    The kinetics of coating growth were approximately linear,

    with average rates of 15.8 and 18.0 nm s1 without and with

    zirconia, respectively (Fig. 6). The thickness of both the inner and

    outer coating layers increased with increase of treatment time.

    The inner layer thickness appeared to increase relative to the

    total film thickness, from 20% of the film at 10 min to 40% at

    later times. XRDrevealed strongpeaks forMgO andweaker peaks

    for Mg2SiO4 in the coating formed in electrolyte without

    nanoparticles (Fig. 7). These peaks were also found for the

    coating produced with nanoparticles, with relatively reducedpeaks for Mg2SiO4 and the additional presence of peaks for

    monoclinic zirconia and Mg2Zr5O12.

    Coatings formed for 10 min in electrolyte with nanoparticles

    followed by 10 min in electrolyte free of nanoparticles, and vice

    versa, also resulted in two-layered coatings (Fig. 8). For the

    Fig. 5.Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for 2400 s at 200 mA cm2 (rms) in 0.025 M

    Na2SiO35H2O/0.05 M KOH with addition of 10 g/l of monoclinic zirconia. (a) Surface region with relatively few nanoparticles. (b) Surface region with nanoparticle layer. (c)

    Detail of (a). (d) Detail of (b).

    Fig. 6. Dependenceof coatingthicknesson time forAC PEOtreatment of magnesium

    at 200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without and with

    addition of 10 g/l of monoclinic zirconia.

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    former sequence, occasional regions of material with incorpo-

    rated nanoparticles were present in the final coating, usually at

    the coating surface, within cavities and near the inner layer/

    outer layer interface (Fig. 8(a)). EDX analyses indicated the

    presence of zirconium species in the outer layer material, with a

    Zr:Mg ratio of 0.09 0.05 (point J, Fig. 8(a)); no significant

    amount of zirconium was detected in the inner layer (point K,

    Fig. 8(a)). The respective Si:Mg ratio were 0.21 0.04 and

    0.19 0.05, indicating less silicon in the inner layers. For the

    reverse sequence, increased amounts of zirconia nanoparticles

    were present at the coating surface, within cavities and near the

    inner layer/outer layer interface (Fig. 8(b)). Surface regions with

    much reduced numbers of nanoparticles were also identified,

    similar to the findings for the previous single PEO treatment withnanoparticles. Point analyses through the coating thickness

    disclosed reducing Zr:Mg ratios of 0.07 0.05, 0.06 0.05 and

    0.03 0.01 with increasing depth in the coating (points LN,

    Fig. 8(b)). A similarly reducing trend was found for Si:Mg ratios,

    namely 0.23 0.03, 0.20 0.05 and 0.14 0.05.

    4. Discussion

    The two-layered coatings observed previously following DC PEO

    treatment of magnesium in silicate electrolyte in the presence and

    absence of a zirconia nanoparticle suspension[11]are also formed

    under the present AC conditions. Similarly, the transport of zirconia

    nanoparticles along short-circuit paths, through the outer layer to

    the inner layer, is also revealed. Irrespective of the presence of

    zirconia, the coatings contained magnesium species from the

    substrate and silicon species from the electrolyte, with detection

    of crystalline MgO and Mg2SiO4. Further, theinnerlayer contained a

    reducedamount of silicon, with typical Si:Mg ratios from EDX point

    analyses of 0.10 0.04 compared with 0.26 0.04 in the outer layer.

    For coating containing zirconium species, the local high temperatures

    of coating growth result in formation of dendrites in the outer coating

    layer, andgeneration of Mg2Zr5O12, whichis reported tobe stablein the

    temperature range 21232373 K[14]. This phase is presumed to be a

    constituent of the outer coating, since, relatively little zirconium is

    found within the inner layer, for which EDX point analyses indicated a

    Zr:Mg ratio of 0.02, compared with 0.090.16 in the outer layer.

    Mg2Zr5O12 was not detected by XRD in coatings formed in silicate

    electrolyte under DC conditions with a current density of 30 mA cm2

    and a reduced concentration of zirconia in the electrolyte, 2 g/l [11].

    However, generation of tetragonal zirconia occurred under DC

    treatments with silicate electrolyte and both tetragonal zirconia and

    Mg2Zr5O12 in phosphate electrolyte. The absence of tetragonal zirconia

    in the AC coatings, contrary to findings for a DC coating, is due to the

    different thermal conditions in the coatings, related to the energies

    dissipated in the individual discharge events. The latter will depend on

    the local current densityof the discharge sites and thesizeand lifetime

    of the discharge. The presence of tetragonal zirconia under the DC

    conditions compared with the presence of Mg2Zr5O12 under AC

    conditions suggestshighertemperature in thedischarges for thelatter.

    Monoclinic zirconia is also detected in the present coatings, and those

    formed previously under DC conditions, due to incorporation of

    zirconia that either does not transform under the local conditions of

    temperatureand pressure, or revertsto themonoclinicform on cooling.

    The occurrence of dendrites indicates local melting in the outer layer.

    The melting point of the outer layer material will depend upon the

    precisecomposition of thecoating.The meltingpointsof relatively pure

    MgO and Mg2SiO4 are 3073 and 2163 K, respectively. The extensivecracking of the coatings is presumed to arise from stresses in the

    coating associated with volume changes due to coating growth and

    phase transformation, differential thermal expansion within the

    coating and substrate, and the pressure of oxygen and hydrogen

    generated during anodic and cathodic half-cycles, respectively[15].

    Zirconia nanoparticles were incorporated into the coating

    surface. In some regions of the surface, the particles are entrained

    within a thin layer of silicon-rich material,which possibly contains

    silica formed either by precipitation, due to reduction in pH of the

    surface electrolyte, or thermolysis. The particles appear to be

    Fig. 7. XRD data for magnesium following AC PEO treatment for 2400 s at

    200 mA cm2 (rms) in 0.025 M Na2SiO35H2O/0.05 M KOH without (bottom) and

    with (top) addition of 10 g/l of monoclinic zirconia.

    Fig. 8. Scanning electron micrographs (backscattered electrons) of cross-sections of magnesium following AC PEO treatment for at 200 mA cm2 (rms) in 0.025 M

    Na2SiO35H2O/0.05 M KOH. (a) Treatment for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia then for 600 s in zirconia-free electrolyte. (b) Treatment for

    600 s in zirconia-free electrolyte then for 600 s in electrolyte with addition of 10 g/l of monoclinic zirconia.

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    subsequently incorporated into the underlying melted coating

    material. At other locations, the surface appeared relatively-free of

    particles and silicon-rich material, which may be due to instability

    of silica at the locally high pH generated during cathodic

    polarization. Alternatively, the silicon-rich material and zirconia

    nanoparticles may have been incorporatedinto thecoating at these

    sites, without sufficient time being subsequently available to re-

    establish their presence. In addition to incorporation at the coating

    surface, nanoparticles of zirconia were incorporated following

    transport to the inner layer/outer layer interface region. Further,

    cavities within the coating were often lined with nanoparticles,

    suggesting that cavities may be sections of channels that provide

    access to the inner coating regions. However, cavities may also

    arise from bridging of open pores that were located at the coating

    surface at an earlier stage of coating growth.

    The sequentially treated specimens reveal fewer zirconia

    particles at the coating surface when thesecond stage of treatment

    uses the zirconia-free electrolyte. In contrast, a thin layer of

    zirconia particles is established during the second stage of

    treatment with the zirconia suspension. It appears that new

    coating material maybe formedat the coating surface on topof the

    zirconia-rich layer, which is suggested by the lighter, zirconium-

    containing material at location L ofFig. 8(b) that is separated fromthe slightly darker material M, by a line of zirconia nanoparticles

    connecting to the adjacent zirconia-rich layer. Zirconium-contain-

    ing material also appears to form at the inner/outer coating

    interface, which is suggested by the presence of lighter material

    near the interface. However, some zirconia nanoparticles remain

    near the interface following the second stage treatment in

    zirconia-free electrolyte (Fig. 8(a)), suggesting that lower tem-

    peratures are achieved in this region.

    The coating kinetics ofFig. 6indicate that both the outer and

    inner coating layers continue to grow during the PEO process,

    with incorporation of silicon, but little zirconium, into the inner

    layer. Early studies of PEO using sequential treatments in DC

    conditions in nanoparticle-free electrolytes with different anion

    species revealed transport of electrolyte anions, or derivativesproduced by the microdischarge, to the inner coating regions,

    paralleling the transport of zirconia nanoparticles to the inner

    coating/outer coating interface of the present films [7]. Further,

    the inner coating material appeared to be readily substituted by

    new material following change of the electrolyte, whereas the

    outer coating underwent relatively slow change in composition.

    Componentsof coating material formed in thefirst electrolyte,i.e.,

    containing anion or derivative species of the firstelectrolyte, were

    also detected in the second electrolyte, consistent with an

    accelerated dissolution of prior formed coating[16]. Studies are

    in progress to determine whether similar loss of inner coating

    material occurs during AC PEO, when the lifetimes of discharges

    may be restricted compared with DC conditions. The inability to

    incorporate significant amounts of zirconium into the innercoating layer under the present conditions of PEO contrasts with

    the ready incorporation of silicon species into the inner coating

    material. Further, work is required on the nature and growth

    mechanism of the inner layer in order to understand the differing

    behaviours. However, the difference in distributions of silicon and

    zirconium may relate to the transport of the zirconium in particle

    form, rather than atomic, ionic or molecular form, to the inner

    layer/outer layer interface region. Such relatively large particles

    appear not to be melted readily. Further, the electric field will

    oppose the migration of zirconium ionsinto the innerlayer during

    anodic polarization.

    A thin region of compact material, with relatively uniform

    thickness, is present at the base of the inner coating layer. The

    thickness is consistent with development of a barrier film, with a

    formation ratio of the order 1 nm V1, at the final anodic voltage of

    430 V achieved at the particular microdischarge site. The

    material is formed at the termination of coating growth, during

    cooling of the overlying main coating material. The growth of the

    barrier film indicates that electrolyte species can penetrate the

    inner coating layer, enabling film formation at the metal/coating

    interface.

    5. Conclusions

    1. Zirconia nanoparticles can be incorporated into PEO coatings

    formed on magnesium under AC conditions in silicate electro-

    lyte with monoclinic zirconia in suspension.

    2. The coatings comprise two main layers, with the inner layers

    revealing a relatively fine porosity. Both layers thicken as the

    coating grows, with the inner layer representing about 40% of

    the coating for a total coating thickness of40mm.

    3. Magnesium, oxygen and silicon are present within the coatings,

    which contain crystalline MgO and Mg2SiO4. The concentration

    of silicon is less in the inner layer compared with theouter layer.4. Zirconia nanoparticles are incorporated into the coating at the

    coating surface and following transport along short-circuit

    paths to the inner coating region. Zirconium species are

    subsequently incorporated into the coating material, resulting

    in a modified microstructure of the outer coating layer.

    Relatively little zirconium is incorporated into the inner coating

    layer.

    5. Due to locally high temperatures at sites of microdischarges

    Mg2ZrO5is formed within the coatings following incorporation

    of monoclinic zirconia from the electrolyte.

    Acknowledgements

    The authors are grateful to the Engineering and Physical

    Sciences Research Council (U.K.) and the Spanish Ministry of

    Education (grant no. EX2006-1371) for support of this work.

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