Highly ordered porous alumina with tailor-made pore structures fabricated by pulse

anodization

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Nanotechnology 21 (2010) 485304 (8pp) doi:10.1088/0957-4484/21/48/485304

Highly ordered porous alumina withtailor-made pore structures fabricated bypulse anodizationWoo Lee1,2,3 and Jae-Cheon Kim1

1 Korea Research Institute of Standards and Science (KRISS), Yuseong, 305-340 Daejeon,Korea2 Department of Nano Science, University of Science and Technology, Yuseong,305-333 Daejeon, Korea

E-mail: woolee@kriss.re.kr

Received 20 May 2010, in final form 22 October 2010Published 10 November 2010Online at stacks.iop.org/Nano/21/485304

AbstractA new anodization method for the preparation of nanoporous anodic aluminum oxide (AAO)with pattern-addressed pore structure was developed. The approach is based on pulseanodization of aluminum employing a series of potential waves that consist of two or moredifferent pulses with designated periods and amplitudes, and provides unique tailoringcapability of the internal pore structure of anodic alumina. Pores of the resulting AAOs exhibita high degree of directional coherency along the pore axes without branching, and thus aresuitable for fabricating novel nanowires or nanotubes, whose diameter modulation patterns arepredefined by the internal pore geometry of AAO. It is found from microscopic analysis onpulse anodized AAOs that the effective electric field strength at the pore base is a keycontrolling parameter, governing not only the size of pores, but also the detailed geometry ofthe barrier oxide layer.

S Online supplementary data available from stacks.iop.org/Nano/21/485304/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Nanoporous anodic aluminum oxide (AAO) formed by an-odization of aluminum has increasingly become a popular tem-plate system in the development of functional nanostructures.AAO membranes with tailor-made internal pore structurecould provide not only a new degree of freedom in template-based fabrications of advanced functional materials [1–4],but a model system for investigating separations of particlesand adsorption characteristics of molecules [5–9]. Previ-ously, fabrication of highly ordered AAO membranes withperiodically modulated diameter of nanopores was realizedby combining conventional mild anodization (MA) with lowcurrent density ( j = 1–5 mA cm−2) and hard anodization(HA) with high current density ( j = 30–250 mA cm−2),in which each modulation step required the exchange of the

3 Author to whom any correspondence should be addressed.

electrolyte solutions in order to satisfy both MA and HAprocessing conditions [10]. This work led to a development ofa process called ‘pulse anodization’, in which periodic pulsesconsisting a low potential (or current) pulse followed by a highpotential (or current) pulse were applied to achieve MA andHA conditions, respectively [11, 12]. Continuous engineeringof both the internal pore structure and the composition ofnanoporous AAO was demonstrated by utilizing the pulseanodization of aluminum for the sulfuric acid electrolyte,avoiding the tedious periodic replacement of the electrolytesolutions. However, it turned out that the process is noteffective in terms of processing time for other electrolytesystems (e.g., oxalic or phosphoric acid) due to the slowrecovery of current during MA-pulses especially at a lowtemperature (e.g., 1 ◦C). In an extension of the previousworks [11, 12], Losic et al reported a cyclic process [13],in which an anodizing current in the form of waves was

0957-4484/10/485304+08$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

Figure 1. Schematics showing (a) the experimental process for the fabrication of AAO with tailor-made pore structures by pulse anodizationof aluminum and (b) a generalized form of a potential pulse employed in pulse anodizations. U j and τi j define the repeating unit of potentialwaves, where U j = the potential at the time t j with U1 = U5 ( j = 1–4), τi j = t j+1 − t j , i = the pulse number (i = 1, 2, 3, . . .).

applied in order to combine MA and HA conditions. However,the resulting pores are distorted, losing their directionalcoherency, although the process demonstrates transformationof the current profiles into structural pore features.

Recently, we showed that AAOs which experiencedspontaneous current oscillations (amplitude ∼ 0.8 A cm−2)during HA conducted under a specific condition can havemodulated pore structures, in which the internal poregeometries follow exactly the details of the oscillating currentprofile [14]. The self-induced oscillatory kinetic behaviorsare not controllable per se, and thus the shape, amplitude,and period of the pore modulations are not controllable.Nevertheless, the experimental results give the importantinformation that one may achieve structural engineering ofnanoporous AAO by deliberately manipulating the anodizingcurrent during a potentiostatic anodization of aluminum.

Here, we report a generic anodization method forthe fabrication of highly ordered nanoporous AAOs withtailor-made pore structures, in which diameters of oxidenanopores are modulated with designated patterns. Themethod is based on pulse anodization of aluminum underpotentiostatic conditions. Pores of the resulting AAOs exhibita high degree of directional coherency along the pore axeswithout any bifurcations, and thus are suitable for fabricatingnovel nanowires or nanotubes, whose diameter modulationpatterns are predefined by the internal pore geometry ofAAO. We show further that AAO membranes with three-dimensional (3D) porous architectures can readily be utilizedas templates by demonstrating the fabrication of novel pattern-addressed gold nanowires that can be used not only asparticle scaffolds for multiplexed bioassays, but also asmodel systems for investigating topography-induced opticalproperties [1, 2, 4, 15].

2. Experimental section

As-received aluminum discs (2 cm in diameter, Goodfellow,99.999%) were used in anodization experiments without the

annealing step. The aluminum discs were electrochemicallypolished in a vigorously stirred 1:4 mixture solution of 65%HClO4 and 99.5% ethanol (5 ◦C) in order to exclude theeffect (e.g., localized field concentration) that could arise fromthe surface roughness during the anodization. The finishedaluminum disc was placed in an electrochemical cell with anO-ring, so that one side of metal could be anodized. Thearea exposed to the electrolyte solution was 1.96 cm2. Allanodization experiments in the present work were performedby using an electrochemical cell equipped with a cooling stagethat is in thermal contact with the aluminum substrate toremove the reaction heat. The experimental process comprisestwo consecutive anodizations using 0.3 M oxalic acid asan electrolyte. The first-step hard anodization (HA) wasconducted at 140 V for 20 min according to the methodreported previously [10], which enables a uniform growthof AAO during the subsequent pulse anodization (step (i) infigure 1(a)). Subsequently, the second-step pulse anodizationwas carried out at temperatures ranging from 0 to 15 ◦Cto address patterns into the oxide nanopores by applying aseries of potential waves with different shapes (step (ii) infigure 1(a)). The current densities ( j ) reported in this paperwere calculated by dividing the measured values of current (I )by the anodized sample area, not by the effective surface areaconsidering the detailed hemispherical geometry of the barrierlayer. For microscopic characterizations of AAOs, aluminumsubstrate was removed by using a mixture solution of CuCl2and HCl.

Free-standing porous AAO membranes with controlledinternal pore structures obtained by stepwise voltage reductiontechnique were used for electrodeposition of metals [16]. Athin layer (20 nm) of silver was sputter deposited on thebottom side of the AAO membrane in a sputter coater (208HR,Cressington, UK) equipped with a high resolution thicknessmonitor (MTM-20, Cressington, UK) in order to make thesurface electrically conductive. This silver layer servesas a working electrode in the subsequent electrodepositionof the desired metal. Firstly, nickel was deposited into

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

the pores from a solution comprised of 8.41 × 10−2 MNiCl2·6H2O, 1.59 M Ni(H2NSO3)2·4H2O, 0.33 M H3BO3,sodium acetate buffer (pH 3.4). Secondly, gold wasdeposited by using a commercially available plating solution(Auruna 5000). The current densities for nickel and golddeposition were 3.0 mA cm−2 and 2.5 mA cm−2, respectively.The total membrane area in contact with the electrolytewas 1.0 cm2. After gold electrodeposition, the resultingsample was immersed into a concentrated HNO3 solutionto remove the silver working electrode layer and the nickelsegments. Pattern-addressed gold nanowires could be isolatedby completely dissolving AAO template using 0.01 M KOHsolution at room temperature.

3. Results and discussion

The current in anodization of aluminum under a potentiostaticcondition is related to the passage of ions through the barrieroxide layer at the pore bottom. Previous studies indicate thatthe anodization current is dependent on the thickness and thechemical composition of the barrier oxide [10, 11]. For a givenanodization potential (U ), the current density ( j ) is inverselyproportional to the logarithm of the barrier layer thickness (tb)through the following equation;

j = jo exp(β E) = jo exp(β�U/tb),

where jo and β are the material-dependent constants and�U/tb is the effective electric field strength (E) across thebarrier layer of thickness tb [17, 18]. On the other hand,for a given electrolyte system, the barrier layer thickness(tb) increases with anodization potential (U ) at a rate ofζ MA

tb∼ 1.2 nm V−1 for MA and ζ HA

tb∼ 0.6–1.0 nm V−1 for

HA [10, 19–23]. Therefore, when the anodization potentialis switched from a higher value satisfying the HA condition(i.e., UHA) to a lower MA one (i.e., UMA), the current dropsabruptly to a very small value and then gradually increaseswith time to a steady value corresponding to UMA (i.e., currentrecovery) [11]. The recovery behavior of the current can berelated to the gradual decrease of the barrier layer thickness(i.e., the gradual increase of the effective electric field strength,E) due to the field-induced viscous flow of oxide material fromthe center of the pore base toward the cell boundary [24, 25]and/or to the slow dissolution of the oxide layer into theelectrolyte [26].

In pulse anodization of aluminum, current recovery withina reasonable period of time is a prerequisite for the continuousengineering of the internal pore structure of AAO. For a givenelectrolyte system, the time required for a complete currentrecovery is dependent on the temperature and the potentialdifference between UMA and UHA. For three major poreforming acid electrolytes (i.e., H2SO4, H2C2O4, and H3PO4),the current recovery time is governed by the chemical natureof the barrier oxide (i.e., the degree of incorporation of acidanion) and increases in the order H2SO4 < H2C2O4 <

H3PO4 [27]. Accordingly, unlike the case of H2SO4-basedpulse anodization [11], it is difficult to achieve a continuouspulse anodization of aluminum in H2C2O4 or H3PO4 solution

due to the very slow recovery of current, especially at a lowtemperature and a large potential difference.

In the present study, the problem associated with the slowcurrent recovery could be overcome by increasing the potentialgradually prior to pulsing a high potential. Figures 1(a) and (b)show schematically the fabrication procedure of AAOs withtailor-made pore structures and a generalized form of thepotential pulses employed, respectively. The experimentalprocess comprises two consecutive anodizations using 0.3 Moxalic acid as an electrolyte. The first-step hard anodization(HA) was conducted at 140 V for 20 min according to themethod reported previously [10], which enables a uniformgrowth of AAO during the subsequent pulse anodization(step (i) in figure 1(a)). Subsequently, the second-steppulse anodization was carried out to address patterns into theoxide nanopores by applying a series of potential waves withdifferent shapes (step (ii) in figure 1(a)). A repeating unit ofpotential waves, which determines the modulation pattern ofpores, can be constructed by combining pulses with designatedperiods and amplitudes. Each pulse consists of four segmentsof potential ramps defined by the potentials (U j ) and the timewidths (τi j ), where U j = the potential at the time t j ( j = 1–5),t5 − t1 = the period of a pulse, τi j = t j+1 − t j , and i = thepulse number (i = 1, 2, 3, . . .) (figure 1(b)). An arbitrary formof potential wave (e.g., square, triangle, or sawtooth) can begenerated by appropriately varying U j and τi j . The length ofoxide nanopores with a smaller diameter can be controlled byvarying the time width τi1 (i.e., the first segment in figure 1(b)),during which the recovery of current takes place. On theother hand, the length and the internal pore geometry of oxidenanopores with a larger diameter can be tuned by appropriatelyvarying the pulse duration and the amplitude, which are definedby (τi2, τi3, τi4) and (U2, U3, U4), respectively.

Figure 2 presents the effect of the pulse parameters onthe structure of the resulting AAO; ((a), (c)) τ11 = 30 s,τ12 = 2 s, τ13 = τ14 = 0 s, U1 = 40 V, U2 = 140 V,U3 = U4 = 160 V, ((b), (d)) τ11 = 54 s, τ12 = 1 s,τ13 = 0.5 s, τ14 = 0 s, U1 = 80 V, U2 = 140 V, U3 =U4 = 160 V. Current–time transients during pulse anodizationof aluminum commonly show that potential pulsing results inthe sharp increase of current density and correspondingly theintegrated charge (figures 2(a) and (b)). As a consequenceof the periodic current surges, the resulting AAOs exhibitmodulated pore structures without disordering or branchingof pores (figures 2(c) and (d)), in which the length of eachmodulation is proportional to the corresponding integratedcharge. A typical recovery behavior of the anodizing currentcan be observed during the potential sweep from U1 to U2. Itappears from the insets of figures 2(a) and (b) that the actualcurrent recovery starts at an anodizing potential around 112 V,below which the current recovery is negligible. Therefore, thecontribution of the potential below 112 V to the formation ofanodic oxide can be ignored under the present electrochemicalconditions. Our control experiments revealed that the rateof potential sweep from U1 to U2 does not affect the onsetpotential of the current recovery, while it affects the currentefficiency for the formation of AAO (i.e., the amount of anodicoxide formed per unit charge density); the current efficiency

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

Figure 2. ((a), (b)) Representative current–time (left axes) and charge–time (right axes) transients during pulse anodizations of aluminumusing 0.3 M H2C2O4 (5 ◦C); (a) τ11 = 30 s, τ12 = 2 s, τ13 = τ14 = 0 s, U1 = 40 V, U2 = 140 V, U3 = U4 = 160 V. (b) τ11 = 54 s, τ12 = 1 s,τ13 = 0.5 s, τ14 = 0 s, U1 = 80 V, U2 = 140 V, U3 = U4 = 160 V. The insets show potential–current plots of the respective anodizations.((c), (d)) Cross-section SEM micrographs of the resulting respective AAOs with modulated pore structures. The repeating units of thepotential wave are shown as insets of the respective SEM images. Scale bars = 1 μm.

was observed to decrease with the rate of potential sweep (seesupporting information, figure S1 available at stacks.iop.org/Nano/21/485304/mmedia).

In general, the pore diameter (Dp) and the cell size(i.e., the interpore distance Dint) of porous AAO are linearlyproportional to the anodization potential for both MA andHA [10, 28]. But the potential dependence of the formeris known to be not as sensitive to the interplay of currentdensity ( j ), temperature, concentration, and the nature of theelectrolyte used [29–32]. In fact, our recent study showedclearly that pore diameter (Dp) increases with the currentdensity ( j ) even at a fixed potential, that is, with the effectiveelectric field strength (E) across the barrier layer [14]. In orderto see the effect of the potential pulsing on the structure of theresulting AAO, we have performed a comparative investigationon the internal morphologies of oxide nanopores that wereprepared from two separate anodization experiments, whose

reactions were terminated at U2 and U4 during anodization ofaluminum.

Figures 3(a) and (b) present the barrier layer structuresof AAOs prepared by stopping the anodization reactions at(a) 140 V ( j = 0.0667 A cm−2) and (b) 160 V ( j =1.2760 A cm−2). A schematic cross-section of AAO onaluminum and the parameters defining the geometry of thebarrier layer are shown in figure 3(c) and the table infigure 3(d), respectively. It was found from the structuralanalysis of the samples that the potential dependences of boththe interpore distance (ζDint = 2.14 nm V−1 for (a) and1.93 nm V−1 for (b)) and the barrier layer thickness (ζtb =0.91 nm V−1 for (a) and 0.77 nm V−1 for (b)) are smallerthan those (ζ MA

Dint= 2.5 nm V−1, ζ MA

tb = 1.2 nm V−1) ofmild anodized AAOs [19, 20, 33], but close to those (ζ HA

Dint=

1.8–2.0 nm V−1, ζ HAtb = 0.6–1.0 nm V−1) of hard anodized

ones [10, 21–23]. Since the inverse of the potential dependence

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

Figure 3. Cross-section SEM micrographs of AAOs as-prepared from two separate anodization experiments using 0.3 M H2C2O4 (15 ◦C),whose reactions were terminated at (a) U = 140 V ( j = 0.0667 A cm−2) and (b) U = 140 V, j = 1.2760 A cm−2. Scale bars = 300 nm. Theinner and outer concentric circles in both SEM images are introduced to depict the e/o and o/m interface profiles, respectively. (c) A schematiccross-section of AAO on aluminum. (d) The parameters defining the geometry of the barrier layer shown in (c): Dp = the pore diameter,Dint = 2a = the interpore distance = the cell size, tb = the thickness of the barrier layer (=b + c), R = the radius of curvature (=a/ sin θ),θ = the angle from the pore axis to the ridge-top (= cos−1[1 − 2b2/(a2 + b2)]), ζDint = the potential dependence of Dint, ζtb = the potentialdependence of tb, E = the effective electric field strength across the barrier layer. (e) A schematic illustrating the evolution of the barrier layermorphology upon increase of the cell size (2a), Dp, and tb of AAO without changing the pore axes.

of the barrier layer thickness (i.e., ζ−1tb ) is equivalent to the

effective electric field strength (E) at the pore base, the barrierlayer structures shown in figures 3(a) and (b) correspondto those of AAOs formed at a lower E and a higher E ,respectively. Upon close examination of the SEM micrographs,we found that the detailed shapes of the electrolyte/oxide (e/o)and oxide/metal (o/m) interfaces are dependent on the electricfield strength (E). AAO formed at a lower E turned out tohave an elliptical e/o interface as it deviates from the innerconcentric circle (see the white circles in figure 3(a)), whichwas suggested to be an important shape for controlling theuniformity of the interface speed in the scalloped region of thebarrier layer [34], while the e/o interface of AAO formed ata higher E could be more properly depicted by a circular one

(figure 3(b)). In addition, the curvature radius (R = a/ sin θ )of the o/m interface was observed to be larger for AAO formedat a high E compared to that at a low E (i.e., θ at low E >

θ at high E ).The observed evolution of the radius (R) of curvature of

the o/m interfaces can be explained by a simple geometricconsideration (figure 3(e)). During pulse anodization,switching of the anodization potential from a low value (i.e.,a low E) to a higher one (i.e., a high E) will accompanyincrease of the cell size (2a), the pore diameter (Dp), and thebarrier layer thickness (tb) of AAO (dashed lines in figure 3(e)).Since the increase of these structural parameters takes placewithout changing the pore axes, the o/m interfaces of theindividual oxide cells will overlap with those of their six

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

Figure 4. Cross-section SEM micrographs of nanoporous AAOs with modulated pore structures prepared by pulse anodization of aluminum;(a) τ11 = 36 s, (b) τ11 = 144 s, (c) τ11 = τ21 = 36 s, τ31 = τ41 = 144 s, and (d) τ11 = τ21 = 144 s, τ31 = 36 s, τ41 = 144 s, τ51 = τ61 = 36 s.Other parameters were fixed at U1 = 80 V, U2 = 140 V, U3 = U4 = 160 V, τi2 = τi4 = 0 s, τi3 = 0.2 s, where i denotes the pulse number(i = 1, 2, 3, . . .). The repeating units of pulses are shown as insets in the respective images. Anodization was conducted using 0.3 M H2C2O4

(1 ◦C). Subsequently, pores were widened by immersing the resulting samples into 0.3 M H2C2O4 (15 ◦C) for 16 h.

nearest neighbors, lowering the ridge height (i.e., γ at low E1 >

γat high E1 in figure 3(d)). As a result, the curvature radius of

the o/m interface of AAO at a high E is larger (i.e., R′ > R;θ ′ < θ in figure 3(e)).

We believe that the detailed geometry of the barrier layerplays an important role, governing the local distributions of theelectric field and the current within the barrier oxide, and thusthe evolution of mechanical stresses at the barrier layer as wellas the rates of the e/o and o/m interface motions [14]. In fact

recent theoretical modeling by Houser et al [34, 35] has shownthat the rate of o/m interface motion at the scalloped regionand the ridge sensitively varies with the elliptic e/o interfaceprofile, and predicted the pseudoconvective motion of mobileanions from the pore center towards the cell boundary, whichaccounts for both recession of the e/o interface at the porebase and also accumulation of oxide at the pore wall, unlikeprevious models based on field-assisted chemical dissolutionof oxide [28, 36, 37]. A complete understanding of the effect

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

Figure 5. SEM micrographs of pattern-addressed gold nanowires prepared by electrodeposition of gold into AAO templates with the porestructures (a) shown in figures 4(a) and (b) shown in figure 4(d). Panels (c) and (d) show magnified SEM images of the respective samples.

of the interface profiles on the anodization kinetics needs moresystematic study.

The present pulse anodization method enables continuousengineering of the pore structure of AAOs. To demonstrate itsfacile tailoring capability of the internal pore structures, AAOswith a variety of pore modulation patterns were fabricatedby employing a series of potential waves that consist oftwo or more different pulses with the desired periods andamplitudes. Cross-sectional SEM images of representativeexamples of AAO are presented in figures 4(a)–(d). Fourdifferent repeating units of potential waves (shown as insetsof the respective images) were applied during anodization ofaluminum: (a) τ11 = 36 s, (b) τ11 = 144 s, (c) τ11 = τ21 =36 s, τ31 = τ41 = 144 s, and (d) τ11 = τ21 = 144 s, τ31 = 36 s,τ41 = 144 s, τ51 = τ61 = 36 s; the other parameters werefixed at U1 = 80 V, U2 = 140 V, U3 = U4 = 160 V,τi2 = τi4 = 0 s, τi3 = 0.2 s (i = the pulse number). Asmanifested by the present examples, the modulation patternsof the oxide nanopores follow exactly the profile details ofthe repeating potential units as a result of the periodic surgesof current density, i.e., the electric field strength E (seesupporting information, figure S2 available at stacks.iop.org/Nano/21/485304/mmedia), indicating successful addressingof modulation patterns into the oxide nanopores. It isexpected that fabrication of AAOs with even more complexporous architectures can readily be achieved by appropriatelydesigning the repeating unit of the applied potential.

It is worth mentioning that pores of AAOs fabricatedby the present pulse anodization method are well orderedand exhibit a high degree of directional coherency along

their axes without disordering (see supporting information,figure S3 available at stacks.iop.org/Nano/21/485304/mmedia).Therefore, AAO membranes with tailor-made pore structuresare suitable as templates for preparing novel nanowires ornanotubes, of which the patterns of diameter modulation arepredefined by the internal pore structure of AAOs. Theresulting pattern-addressed nanostructures may have potentialof utilization not only as particle scaffolds (i.e., nano-barcodes) for multiplexed bioassays, but also as model systemsfor investigating topography-induced optical, electronic, andmagnetic properties [1, 2, 4, 15, 38]. In fact, we wereable to fabricate novel shape-addressed gold nanowires byperforming electrodeposition of gold into AAO templates bytaking advantage of the tailoring capability of the pore structureand also the highly ordered nature of oxide nanopores (seefigure 5). Apart from template preparation, we expect that ouranodization method will offer a promising fabrication strategyfor AAOs with 3D porous architectures that could be used forphotonic applications [39, 40].

4. Conclusions

In summary, a new tailoring method of the pore structureof anodic aluminum oxide (AAO) has been developed.The approach is based on pulse anodization of aluminumemploying a series of potential waves that consist of two ormore different pulses with deliberately chosen periods andamplitudes. As a consequence of periodic surges of theanodizing current in compliance with the profile details of the

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Nanotechnology 21 (2010) 485304 W Lee and J-C Kim

applied potential, the pores of the resulting AAO are modulatedalong their pore axes. Microscopic investigation on pulseanodized AAOs reveals that the effective electric field strength(E) at the pore base has profound implications on the structureof AAO, governing the detailed geometry of the barrier oxidelayer as well as the size of pores.

Acknowledgment

This work is supported by Korea Research Council ofFundamental Science and Technology (KRCF) through theKRISS project of ‘Development of Advanced IndustrialMetrology’.

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