简介概要

Effect of the duty cycle of pulsed current on nanocomposite layers formed by pulsed electrodeposition

来源期刊：Rare Metals2010年第2期

论文作者：M.Aliofkhazraei Sh.Ahangarani A.Sabour Rouhaghdam

文章页码：209 - 213

摘要：Nickel-tungsten/carbon nanotube nanocomposite layers with high content and uniform dispersion of carbon nanotubes were fabricated using pulsed electrodeposition technique.Nanocomposite layers were analyzed by scanning electron microscopy, atomic force microscopy, microhardness, and Tafel polarization tests.The effect of the duty cycle of pulsed current or concentration of carbon nanotubes in the metallic matrix on electrochemical and mechanical properties of obtained layers has been investigated.It has been shown that both the electrochemical and mechanical properties of nanocomposite layers that formed by pulsed current were improved significantly with respect to un-composed Ni-W layer.The results were not only concerned by the concentration of carbon nanotubes in the layer but also influenced by the distribution of nanoparticulates in the metallic matrix.

Rare Metals 2010,29(02),209-213

Effect of the duty cycle of pulsed current on nanocomposite layers formed by pulsed electrodeposition

M.Aliofkhazraei Sh.Ahangarani A.Sabour Rouhaghdam

Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University

Department of Advanced Materials & Renewable Energies, Iranian Research Organization for Science and Technology(IROST)

作者简介：M. Aliofkhazraei E-mail: khazrayie@modares.ac.ir;

收稿日期：26 July 2009

基金：financial support from Iranian Nanotechnology Initiative;

Effect of the duty cycle of pulsed current on nanocomposite layers formed by pulsed electrodeposition

Abstract：

Nickel-tungsten/carbon nanotube nanocomposite layers with high content and uniform dispersion of carbon nanotubes were fabricated using pulsed electrodeposition technique.Nanocomposite layers were analyzed by scanning electron microscopy, atomic force microscopy, microhardness, and Tafel polarization tests.The effect of the duty cycle of pulsed current or concentration of carbon nanotubes in the metallic matrix on electrochemical and mechanical properties of obtained layers has been investigated.It has been shown that both the electrochemical and mechanical properties of nanocomposite layers that formed by pulsed current were improved significantly with respect to un-composed Ni-W layer.The results were not only concerned by the concentration of carbon nanotubes in the layer but also influenced by the distribution of nanoparticulates in the metallic matrix.

Keyword：

nanocomposite; corrosion; nanostructure; carbon nanotubes;

Received： 26 July 2009

1. Introduction

Numerous investigations have been done on the formation of nanocomposite layers during recent years,such as papers about Ni-P-TiO₂[1]as lubricious layers,Ni-SiC[2]and Ni-Co-SiC[3]as wear and corrosion-resistant coatings,Ni-Ti O₂[4]as photocatalytic layers,and Ni-Si O₂[5]as corrosion-resistant layers.The most important features of a well-performed layer are constant concentration along the nanocomposite layer and uniform distribution of nanoparticulates in the matrix.Some modifications in electrodeposition such as using pulsed current and ultrasonic bath were usually employed for better dispersion of nanoparticulates in obtained nanocomposite layers[2,6-7].

Some papers reported usage of the ultrasonic bath during nanocomposite electrodeposition process.Results about the effect of the ultrasonic condition outside of the cell during electrodeposition[8]demonstrated that the ultrasonic condition increases uniform distribution of Al₂O₃ nanoparticulates but decreases their concentration in the metallic matrix.Also,some reports were published about the effect of pulsed current on electrochemical coating process[9-10].It has been revealed that usage of pulsed current will lead to fabrication of harder nanocomposite layers[11].In this investigation,a pulse generator has been utilized for fabricating nanocomposite layers in order to achieve more concentration of carbon nanotubes and to increase the uniform distribution of nanoparticulates in deposited layers.

There is no wide study on specific nickel-tungsten/carbon nanotube(Ni-W/CNT)nanocomposite layer formation by electrodeposition.The aim of this study is to perform Ni-W/CNT nanocomposite layers by pulsed current and study the concentration of nanoparticulates and process effective parameters on the electrochemical and mechanica properties of coated samples.Distribution of nanoparticulates in nanocomposite layers has also been investigated in this paper.

2. Experimental

2.1. Nanocomposite layer formation

The composition of the Ni-W bath and the conditions of electrodeposition were selected similar to our previous work[12].An electrolyte was prepared with pure 0.14 mol?L^-1nickel sulfate(NiSO₄?6H₂O),0.44 mol?L^-1 tri-sodium citrate(Na₃C₆H₅O₇)as the complexing agent,0.1-0.3 mol?L^-1 sodium tungstate(Na₂WO₄?2H₂O)with 0.01 g/L saccharin(C₇H₅NO₃S),0.01 g/L SDS(C₁₂H₂₅Na O₄S)(MERCK analytical grade material),and 50 g/L multi-walled carbon nanotube(MWCNT or CNT in this paper)(with an average diameter of less than 90 nm and length of up to 20?m and purchased from PlasmaChem Company(Germany)).Fig.1illustrates the nanostructure of used carbon nanotubes.The p H of all solutions,measured with a p H meter(model METTLER MP230),was adjusted to 8.0 by addition of sulfuric acid or sodium hydroxide at room temperature.Pure copper sheets(50 mm×10 mm×1 mm)were used as cathodic electrodes.The preparation process of all specimens was first mechanically polishing with different grades of emery papers up to#4000,then degreasing in sodium hydroxide solution and inserting in 10%HCl solution for surface activating,and finally rinsing with pure acetone.Operating conditions for electrodeposition were 10 A/dm² of average current density,200 r/m of stirring rate,and 1000 Hz of frequency,while duty cycle of pulsed current was adjusted at its relative desired levels.Monopolar cathodic pulsed current was used throughout this research.The electrodeposition cell was filled with 200 cm³ of prepared electrolyte.The distance between the cathodic plate and anodic cylinder was approximately 1 cm.

Fig.1.SEM nanostructure of used CNTs for performing nanocomposite layers.

2.2. Nanostructure and elemental analysis

Nanostructure and elemental analysis of Ni-W/CNT nanocomposite layers were examined by scanning electron microscopy(SEM)(Philips XL30)and its energy-dispersive X-ray detector,respectively.For a better study of fabricated nanocomposite layers in nanometric scale,atomic force microscopy(AFM)has been used.The AFM part was a Nano Scope II from Digital Instruments USA and non-scraping Si₃N₄ tips were used throughout.Nanostructures of layers have been analyzed also by transmission electron microscope(TEM)(CM200-FEG-Phillips).

2.3. Microhardness measurements

Vickers microhardness of nanocomposite layers was measured by a microhardness tester rig(BUHLLER Micromet 1).0.49 N of load was applied for approximately 15s and averages of five microhardness measurements were reported for minimizing its error.

2.4. Electrochemical tests

Studies of corrosion properties of obtained layers have been performed by an EG&G 273A potentiostat/galvanosta system.The corrosion potential(E_corr)and corrosion current density(i_corr)were calculated by performing Tafel polarization tests and analyzed by Softcorr III software using PAR calculations.Tafel polarization tests were carried out at 25±0.2°C in a Ben-Marry bath for fixing the temperature.Corrosive electrolyte was 3.5 wt.%NaCl solution prepared with distilled water and MERCK analytical grade material.A standard EG&G flat cell with three electrode holders was used.The reference electrode consisted of a saturated calomel electrode,and a platinum rod connected to a Pt shee was used as the counter electrode.The exposed area of nanocomposite layers to corrosive electrolyte was 0.785 cm²(a circle with 1 cm diameter).The working electrode was exposed to corrosive electrolyte about 1 h prior to the Tafe polarization test in order to reach equilibrium conditions The scanning rate was fixed on 0.1 mV?s^-1.

3. Results and discussion

3.1. Nanostructure and elemental analysis

Fig.2 illustrates the nanostructures of nanocomposite layers formed by different(low,medium,and high)duty cycles of pulsed current.The comparison of nanostructures of obtained nanocomposite layers shows that increasing the duty cycle significantly alters the distribution and conten percentage of carbon nanotubes in nanocomposite layers.I has been revealed that the carbon nanotube content will increase from 4.3 wt.%to 13.1 wt.%by increasing the duty cycle from 20%to 80%,respectively,and agglomeration of nanoparticulates will decrease in higher duty cycles.The first mentioned result was predictable since in higher duty cycles the electrochemical reaction for deposition of the metallic matrix has longer times for its occurrence;hence deposition of nanoparticulates in the layer has longer times to occur(in each cycle of pulsed current).By considering ideal distributed nanoparticulates in electrolyte,it can be concluded that increasing the duty cycle will lead to longer“on time”(of applied pulsed current in each cycle)and lower applied potential(for obtaining constant average current density),which means lower power for embedment of nanoparticulates into the nanocomposite layer,so agglomeration is less than that in lower duty cycles that act in the opposite manner.

Fig.2.Nanostructures of Ni-W/CNT nanocomposite layers formed by different duty cycles of pulsed current:(a)20%(AFM);(b)50%(AFM);(c)50%(TEM);(d)80%(AFM).

Fig.3 also shows that the W content in the metallic matrix did not change significantly by increasing the duty cycle of pulsed current.The changing trend of W content is the same as carbon nanotube content.The W content increased from 10.8 wt.%to 12.1 wt.%.It can be assumed that the interaction of nanoparticulates and pulsed current has an influence on W content in the metallic matrix.It can easily be concluded that the effect of carbon nanotubes is much more than that of duty cycle,and decreasing the carbon nanotube content will also lead to a decrease in W content of the metallic matrix.

3.2. Electrochemical tests

Plots of Tafel polarization tests for Ni-W and nanocomposite layers formed by different duty cycles(or different concentrations of carbon nanotubes in the metallic matrix)are shown in Fig.4.The changing trends of corrosion potential(E_corr)and corrosion current density(i_corr),which were analyzed through Tafel plots,are illustrated in Fig.5.Addition of nanoparticulates will shift corrosion potentials of electroplated layers towards noble direction(positive values).Increasing the concentration of carbon nanotubes will have an effect in an opposite manner,but the changing amount is ignorable.The changing trend of corrosion current density shows that increasing the amount of carbon nanotubes has an optimum level for decreasing them.It was found that 9.1wt.%of carbon nanotubes in the metallic matrix will show the minimum corrosion current density of nanocomposite layers.Surfaces have approximately no porosity,so it can be concluded that differences in electrochemical properties are just related to the content of carbon nanotubes and their distribution and amount of agglomeration in the metallic matrix.When carbon nanotubes distribute uniformly,they will protect nanocomposites and substrates from corrosive agents and decrease the corrosion current density.In an opposite manner,the agglomerated nanoparticulates will decrease the electrochemical properties of the obtained layer.

Fig.3.Influence of the duty cycle of pulsed current on CNT nanoparticulate content in obtained nanocomposite layers and W content in the metallic matrix of nanocomposite layers.

Fig.4.Tafel plots of(□)Ni-W and Ni-W/CNT nanocomposite layers formed by different duty cycles of pulsed current((?)20%of duty cycle(or 4.3 wt.%of carbon nanotubes content),(×)50%of duty cycle(or 9.1 wt.%of carbon nanotubes con-tent),and(○)80%of duty cycle(or 13.1 wt.%of carbon nano-tubes content)).

Fig.5.Changing trends of Ecorr and icorr with respect to ap-plied duty cycle(or concentration of CNT nanoparticulates in the metallic matrix).

3.3. Microhardness

The microhardness of Ni-W and nanocomposite layers with respect to different concentrations of carbon nanotubes as well as different applied duty cycles is reported in Table 1,which increases from 522 Hv for Ni-W alloy to 779 Hv for nanocomposite layer with 13.1 wt.%of carbon nanotubes.

Table 1.Microhardness of nanocomposite layers 下载原图

Table 1.Microhardness of nanocomposite layers

Also,the W content in nanocomposite layers will not change by changing the duty cycle of pulsed current,so increasing the microhardness of the obtained nanocomposite layers should be concerned by the presence of carbon nanotubes.As mentioned before,there is less carbon nanotubes in nanocomposite layers,which are formed by lower duty cycles,but the microhardness of nanocomposite layers wil not change significantly by changing the applied duty cycles(Table 1).Thus,increasing the duty cycle will lead to the mutual effect of higher contents of carbon nanotubes in the metallic matrix with simultaneous less norma distribution,which in total will lead to approximately constant microhardness of the obtained layer.Fig.6illustrates the distribution of carbon nanotubes in a 500 nm×500 nm area of analyzed SEM nanostructures.The changing trend of distribution in this figure confirms our conclusions.

Fig.6.Distributions of CNT nanoparticulates in the metallic matrix of nanocomposite layers for different applied duty cy-cles of pulsed current:(a)20%;(b)50%;(c)80%.

4. Conclusion

Duty cycle of pulsed current significantly alters the distribution of carbon nanotubes in nanocomposite layers bu does not change W content in the metallic matrix.The influence of carbon nanotubes is much more than that of the duty cycle of pulsed current,and decreasing the carbon nanotubes will also lead to a decrease in W content of the metallic matrix.

Addition of nanoparticulates will shift corrosion potentials(E_corr)of electroplated layers towards noble direction(positive values).Increasing the concentration of carbon nanotubes will have an effect in the same way,but the changing amount is ignorable.

Microhardness increased from 522 Hv for Ni-W alloys to779 Hv for nanocomposite layers with 13.1 wt.%of carbon nanotubes.The microhardness of nanocomposite layers wil not change significantly by changing the duty cycle.

Increasing the duty cycle apparently will lead to the mutual effect of higher contents of carbon nanotubes in the metallic matrix and less normal distribution,simultaneously which in total will lead to approximately constant microhardness of nanocomposite layers obtained by different applied duty cycles.