Seizure before and after cardiac arrest

Accepted Manuscript Electrodeposited Cu/buckypaper composites with high electrical conductivity and ampacity Xiaofeng Chen, Jingmei Tao, Jianhong Yi, Yichun Liu, Rui Bao, Caiju Li PII:

S0925-8388(17)33786-6

DOI:

10.1016/j.jallcom.2017.11.040

Reference:

JALCOM 43733

To appear in:

Journal of Alloys and Compounds

Received Date: 29 July 2017 Revised Date:

3 November 2017

Accepted Date: 4 November 2017

Please cite this article as: X. Chen, J. Tao, J. Yi, Y. Liu, R. Bao, C. Li, Electrodeposited Cu/buckypaper composites with high electrical conductivity and ampacity, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Electrodeposited Cu/buckypaper composites with high electrical conductivity and ampacity Xiaofeng Chen, Jingmei Tao*, Jianhong Yi, Yichun Liu，Rui Bao, Caiju Li

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Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China Abstract:

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Cu/buckypaper composites were prepared through electrodeposition of Cu onto buckypaper

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from a Cu sulfate electrolyte. The oxygen-containing functional groups were introduced to the surface of carbon nanotubes (CNTs) through acidification and oxidation. Incorporation of functional groups was shown to facilitate the spontaneous reduction of Cu ions on the surface of CNTs during immersion treatment. Under small deposition current density of 1 mA·cm-2, Cu

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ions could penetrate into the reticulate structure of buckypaper and nucleate homogeneously on the CNTs, which led to the formation of a thicker transition layer. Functionalization also increased the nucleation rate of Cu on the CNTs. When the current density was 1 mA·cm-2, the

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electrical conductivity of Cu/oxidized buckypaper composite reached the maximum value of

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4.09×105 S·cm-1 which was 76% of pure Cu. The ampacities of all the composites were obviously improved compared with electrodeposited pure Cu. The Cu/oxidized buckypaper composite had the maximum ampacity of 15437 A·cm-2, which was 65% and 14% higher than that of pure Cu and Cu/original buckypaper composite, respectively. Good interface bonding and reticulate structure of CNTs formed in the transition layers are responsible for the relatively high electrical conductivities and high ampacities of Cu/buckypaper composites.

Key words: Buckypaper; Composite; Electrodeposition; Electrical conductivity; Ampacity

ACCEPTED MANUSCRIPT 1. Introduction Since discovered in 1991[1], carbon nanotubes (CNTs) have been found to possess excellent electrical, thermal and mechanical properties[2-4]. In the past two decades, due to the unique

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physical structure and excellent properties, CNTs have attracted increasing interests in the scientific community. Buckypapers, the thin macroscopic freestanding films, are fabricated from an aggregation of CNTs which entangle with each other at the tube-tube junctions by weak van der

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Waals interactions. Buckypapers have been widely used in many areas, such as actuators,

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hydrogen gas storage, electrodes for supercapacitors, batteries and strain sensors[5-9]. However, due to the large electrical resistances between CNTs, the conductivity performance of CNTs is not well represented by buckypapers. The conductivities of buckypaper have been reported to be in the order of ~103 S·cm-1[10, 11]. Progress has been made in enhancing the

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electrical properties of buckypaper by some approaches, such as purification of CNTs, improvement of CNT alignment and introduction of guest between CNTs [7, 12-14]. Anso´n-Casaos et al. [12] prepared buckypapers from purified CNTs with carboxyls by vacuum

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filtration. They found that the electrical conductivity of carboxylated buckypapers was 1.08×103

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S·m, which was 2-4 times higher than that of as-grown buckypapers. The research of Xing et al.[15] indicated that composite buckypapers with high conductivity could be prepared by co-deposition of multi-walled CNTs and 75 wt.% Cu nanowires. The study of Su and co-workers[16] indicated that the silver-coated buckypaper exhibited a distinct step transition in electrical conductivity with silver content of 6 vol.%. The electrical conductivity of silver-coated buckypapers increased by 15-fold compared with that of pure buckypapers, which are still two to three orders of magnitude lower than that of traditional metal materials, such as Cu and Al.

ACCEPTED MANUSCRIPT Copper is widely used in the microelectronic industry due to its superior electrical and thermal conductivities. Due to the shrinking size of devices, pathways for carrying current to operate these components have been intensively reduced, which requires the conductors to possess

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higher current-carrying capacity. However, the current-carrying capacity of conventional conductors (such as Cu and Ag) can no longer meet the needs of present devices. It is urgently necessary to find new conductors with higher current-carrying capacity. Ampacity represents the

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maximum current-carrying capacity of the object that depends both on the structure and material.

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However, high ampacity and high conductivity are mutually exclusive properties. This is because the former requires a strongly bonded system, whereas the latter requires the free electrons from a weakly bonded system[17]. Therefore, achieving high electrical conductivity and ampacity in the same material has been impossible. However, a high electrical conductivity and ampacity could be

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expected to achieve simultaneously in the composite[17, 18].

The present work aims to prepare Cu/buckypaper composites through electrodeposition. Buckypapers were subjected to different pretreatments to improve the interface bonding between

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Cu matrix and CNTs. Influence of deposition parameters on the properties of Cu/buckypaper

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composites were discussed. The prepared composites were underwent microstructure investigations, electrical conductivity and ampacity measurements.

Experimental

Buckypapers used in the present research were commercially available (Chengdu Institute of

Organic Chemistry, Chinese Academy of Sciences)，which were synthesized by floating catalyst chemical vapor deposition. The thickness and porosity of the buckypapers were specified by the manufacturer, which are 6 ± 1 µm and 64%, respectively. The typical inner diameter, outer

ACCEPTED MANUSCRIPT diameter and length of the multi-walled CNTs that constitute the buckypaper are 5-15 nm, 30-50 nm and 10-20 µm, respectively. The typical SEM image of the as-received buckypaper is shown

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in Fig.1.

Fig.1 SEM image of the as-received buckypaper

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Before electrodeposition, buckypapers were subjected to acidification and oxidation treatment, respectively. For acidification, buckypapers were immersed in 3M HNO3 aqueous solution at 338 K for 12 h. For oxidation, thermogravimetric analysis (TGA) was carried out using a STA 409 PC analyzer to determine the oxidation temperature, and the buckypaper was heated

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from room temperature to 950 °C at a rate of 10 °C/min in a simulated air. The temperature for oxidation treatment was then determined according to the TGA result. After pretreatment, the

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buckypapers were immersed in the plating bath for 72 h to improve the penetration depth of Cu

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ions in the reticular pores of buckypapers.

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Fig.2 Schematic process of spreading out buckypaper flatly and preparing Cu/buckypaper composites

To make the buckypaper keep flat during the electrodeposition process, a strategy for spreading out buckypaper was proposed by fabricating a polyethylene die. Fig.2 schematically

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depicts the process, by which the buckypaper can be spread out flatly. The buckypaper was first spread out onto the two-layer holder with different length and width. Since the buckypaper is

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free-standing onto the hollow of the holder and can not attach onto any substrates, the buckypaper can be easily drawn on all sides in a plane and keep flat. After drawing the buckypaper flatly, the

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cap was covered onto the lower die to fix the buckypaper. The composition of the plating bath used in the present study was 0.64M

CuSO4·5H2O+0.94M H2SO4+0.16M C6H12O6. A commercially available electrolytic cell with size of 70 mm×70 mm×100 mm was employed for the electrodeposition. Two copper plates containing a small amount of phosphorus were used as the anodes and buckypaper was used as the cathode. A DC power (PS-305DM, Long Wei) was used to provide electrical energy, and mechanical stirring with a speed of 350 rpm was applied during deposition process. The prepared Cu/buckypaper

ACCEPTED MANUSCRIPT composites were cleaned with deionized water and dried in vacuum drying chamber (DZF-6260) for 10 h at 343 K. Raman spectroscopy data were recorded using a LabRam HR Evolution, scanning from 100

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cm-1 to 2000 cm-1, and the wavelength of the laser probe was 532 nm. The buckypaper was exposed to the laser for 5 s during the spectrum acquisition. The microstructure of the composites was characterized by a Nano Nova 45 FESEM microscope. The electrical conductivity of the

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composites was measured using a four-point probe system (ST-2258C, Suzhou, China) with the

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specimen size of 10 mm×10 mm at 303 K. The thickness of the composites was measured under optical microscope by mosaicking the sample. In order to calculate the ampacity of the composites, a DC power (PAS10-105, KIKUSUI) and a digital multimeter (U3606B, KEYSIGHT) were used to measure the changes of current and voltage on the specimen. The size of the specimen used for

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ampacity test was 40 mm × 1 mm × the deposited composite thickness. The composites were put into a tube and connected to the DC power via wires. The voltage change between the two ends of

fused.

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the specimen was recorded with the current increase rate of 0.1 A per time until the specimen was

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3. Results and discussion

3.1 Pretreatments of buckypapers Fig.3 is the weight-loss curve of the buckypaper in TGA. As shown, the curve appears two

different stages as the temperature rose from the ambient temperature to 950 °C. Below 500 °C, a gentle descent of residual weight from 100% to 86% can be observed, which could be attributed to the oxidation of amorphous carbon and impurities at this stage. In addition, the evaporation of water at this stage also caused the loss of weight. Above 500 °C, the weight-loss curve has a sharp

ACCEPTED MANUSCRIPT descent, indicating that CNTs began to oxidize[19, 20]. Based on the weight-loss curve, the buckypapers were heated in air at 500 °C for oxidation treatment, and the oxidation time was 2 h.

100

80

500 °C, 86%

70 60 50 40 30 20 10 0 200

300

400

500

600

700

Temperature(°C)

800

900

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100

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Residual weight (%)

90

GGGG

(a)

1100 1000

DDDD

(b)

IIII

ID/IG=0.38

Acidized buckypaper

ID/IG=0.24

Oxidized buckypaper

ID/IG=0.19

Original buckypaper

Intensity(a.u.)

900

DDDD

Intensity(a.u.)

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Fig.3 TGA of original buckypaper

Original buckypaper Oxidized buckypaper Acidized buckypaper

800 700 600 500 400

D′

300 200

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100

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000

-1 Raman Shift(cm )

0

1580 1590 1600 1610 1620 1630 1640 1650 1660 1670

Raman Shift(cm-1)

Fig.4 (a) Raman spectra of buckypapers with different pretreatment methods; (b) D＇bands of

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different buckypapers on the Raman spectra from (a).

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Fig.4a shows the Raman spectra of original, oxidized and acidized buckypapers. The D bands and G bands of all the buckypapers appear apparently at 1345 cm-1 and 1580 cm-1, respectively. The D＇bands at about 1610 cm-1can also be seen in all buckypapers. It is believed that D, G and D＇ bands are the characteristic peaks of CNTs. The ID/IG value is a reflection of integrity degree of the CNTs surface structure[21]. It can be seen that the ID/IG ratio increased from 0.19 for original buckypaper to 0.24 for oxidized buckypaper and 0.38 for acidized buckypaper, which indicated that the oxygen-containing functional groups partially recovered the nature of CNTs[22]. D＇bands

ACCEPTED MANUSCRIPT of the buckypapers are further detailed in Fig.2d, which appear at about 1610 cm-1. Usually, the D＇ band is the characteristic peak of amorphous carbon[19], which is weaker than the D band and is difficult to detect and distinguish. The amorphous carbon has high reaction activity and poor

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oxidative resistance, which could be removed from the surface of CNTs through oxidation treatment[23]. As shown, the D＇peak of oxidized buckypaper is smoother than that of the original

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and acidized buckypapers, suggesting that the amorphous carbon had been partially removed.

Fig.5 Morphologies of different buckypapers after immersed in plating bath for 72 h: (a) original, (b) acidized, (c)oxidized.

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Fig.5 shows the SEM images of the original, acidized and oxidized buckypapers after immersion in the plating baths for 72 h, respectively. Cu particles could be seen on the surface of acidized and oxidized buckypapers (Fig.5b and c), which clearly demonstrates the spontaneous

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reduction of Cu ions on the surface of CNTs during immersion treatment. The spontaneous

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reduction of metal ions on CNTs has been shown previously[24-26]. The functional groups introduced by acidification and oxidization caused an increase in hydrophilicity and negative charge at the CNT surface due to oxygen-containing functional groups, which led to an improvement in adsorption and reduction of Cu ions at the surface of CNTs. Once the buckypaper was immersed in the plating bath, the Cu ions diffused through the double layer and adsorbed at the inner and outer Helmholtz planes aided by functional groups through electrostatic attraction and ion-exchange[27]. Finally, the spontaneous reductions of Cu ions took place at the surface of

ACCEPTED MANUSCRIPT CNTs, as can be seen in Fig.5b and c. These Cu particles generated due to the spontaneous reduction could be served as the nucleation sites during the subsequent electrodeposition process. In addition, Fig. 5 also shows that compared with original and acidized buckypapers, the oxidized

ions during electrodeposition process.

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3.2 Surface morphologies of the composites

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buckypaper has a comparatively looser structure which is likely to facilitate the diffusion of Cu

Fig.6 Surface SEM images of Cu/original buckypaper composites electrodeposited with the same coulomb quantity of 4.8C·cm-2under current densities of (a) 1, (b) 10 and (c) 20 mA·cm-2,

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respectively.

Fig.6 shows the surface SEM images of Cu/original buckypaper composites with electrodeposited coulomb quantity of 4.8 C·cm-2 under different current density of 1, 10 and 20

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mA·cm-2, respectively. As shown in Fig.6a, the nanoscale Cu nuclei distribute homogenously on

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the surface of buckypaper and have relatively higher density compared with that in Fig.6b and c. It is worth nothing that some Cu nuclei in Fig.6a formed not only on the surface but also inside the pores of buckypaper, which suggests that Cu ions could penetrate into the reticulate structure of buckypaper and nucleate on the CNTs during electrodeposition under the small current density of 1 mA·cm-2. In comparison, when the current density increased to 10 mA·cm-2, the Cu nuclei on the surface of buckypaper present uneven distribution and have lower density, while the size of Cu nuclei gets bigger (Fig. 6b). As the current density increased to 20 mA·cm-2, the amount of Cu

ACCEPTED MANUSCRIPT nuclei on the surface of buckypaper increases again, and the size of nucleus gets smaller compared with that deposited under current density of 10 mA·cm-2 (Fig. 6c). However, as can be seen, the majority of Cu nuclei deposited under current density of 20 mA·cm-2 only distribute on the outer

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surface of the buckypaper compared with that deposited under current density of 1 mA·cm-2.

Fig.7 Schematic illustrations of the electrodeposition behavior of the Cu/original buckypaper composites with different current densities.

Fig.7 shows the schematic illustrations of the electrodeposition behavior of Cu on the original

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buckypaper under different current densities. When the current density is relatively low, such as 1 mA·cm-2 in the present study, Cu can electrodeposit on the surface of CNTs with relatively slow nucleation and growth rate, which results in a good distribution of Cu nuclei on the surface of

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buckypaper. In addition, small current density could facilitate the penetration of Cu ions into the

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pores of the reticulate structure composed of CNTs. At the initial stage of electrodeposition, Cu can deposit not only on the CNTs but also on the protruding tips of the Cu grains. Due to the small current density, the corresponding cathode overpotential of CNT (ηCNT) may be small and the potential of a protruding tip of the Cu grain (ECu) on CNTs may be more negative than the electrodeposition potential of Cu on the protruding tip of the Cu grain (Edepo)[28, 29]. It is considered that the overpotential needed for Cu deposition on the protruding tip of the Cu grain (ηtip-depo) is smaller than that on the CNTs, which is ηCNT. This resulted in that newly arrived Cu

ACCEPTED MANUSCRIPT ions are deposited onto the existing Cu grain surface. When the applied current density increases to 10 mA·cm-2, Cu electrodeposits with relatively faster nucleation rate. The corresponding overpotential on the CNTs (ηCNTs) may be large and the potential of a protruding tip of the Cu

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grain (ECu) may be more negative than Edepo. That means ηCNTs may be higher than ηtip-depo. Then Cu deposits not only on the existing Cu grain surface but also on the CNT surface, which leads to an uneven distribution of Cu nuclei on the surface of CNTs. When the applied current density

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increases to 20 mA·cm-2, the corresponding cathode overpotential on the CNTs (ηCNTs) gets larger,

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which would accelerate the electrodeposition process because the negative cathode overpotential generally increases with the increasing current density. However, the nucleation rate of Cu is so fast that newly arrived Cu ions almost all deposited on the outer surface of buckypaper and can not penetrate deeply into the pores of the buckypaper. It is generally recognized that the nucleation

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rate is affected by the deposition rate. With a larger current density, the nucleation rate increases with increasing of deposition rate, therefore, the size of Cu nuclei become smaller than that deposited at lower current density. In order to obtain homogeneous Cu nucleus distribution and

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deeper nucleation sites, the current density of 1 mA·cm-2 was chosen to prepare the Cu/acidized

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buckypaper and Cu/oxidized buckypaper composites.

Fig.8 Surface SEM images of (a) Cu/acidized buckypaper and (b) Cu/oxidized buckypaper composites with the electrodeposited coulomb quantity of 4.8 C·cm-2 under current densities of 1 mA·cm-2.

ACCEPTED MANUSCRIPT Fig.8 shows the surface morphologies of the Cu/acidized buckypaper and Cu/oxidized buckypaper composites with the electrodeposited coulomb quantity of 4.8 C·cm-2 under current densities of 1 mA·cm-2. Compared with Cu/original buckypaper composite, the amounts of Cu

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nuclei on the surfaces of acidized and oxidized buckypapers increased significantly, which were in accordance with the results of immersion test. Fig.8 is another indication of that the amount of Cu nuclei on the surface of buckypaper is strongly influenced by the degree of functionalization. The

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number of adsorption sites was increased with the increasing amount of functional groups due to

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acidification and oxidation treatments, therefore the sites available for Cu reduction were increased. The rate of nucleation is derived from the probability of formation of critical nucleus by:


=  · exp (

∆

 

)

(1)

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Where K is a constant that takes into account the number of adsorption sites and the rate of attachment of atoms,  is critical free energy,  is the Boltzman constant and T is the absolute temperature[30]. An increase in nucleation rate can therefore be expected from increasing the

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adsorption sites by functionalization.

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3.3 Cross-sectional morphologies of the composites

ACCEPTED MANUSCRIPT Fig.9 Cross-sectional SEM images of Cu/original buckypaper composites electrodeposited with coulomb quantity of 50 C·cm-2 under current densities of (a) 1, (b) 10 and (c) 20 mA·cm-2; (d), (e) and (f) are enlarged images from (a), (b) and (c).

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Fig.9 shows the cross-sectional SEM images of the Cu/original buckypaper composites electrodeposited with coulomb quantity of 50 C·cm-2 under current densities of 1, 10 and 20 mA·cm-2, respectively. It is evident that the transition layers have formed between buckypapers

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and Cu matrix for all the composites. However, the thickness of the transition layer decreased with

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the increasing of current density. As discussed in the previous section, when current density was as small as 1 mA·cm-2, Cu ions could penetrate into the reticulate structure composed of CNTs during electrodeposition process. The small current density was selected due to expected favoring of nucleation uniformity and low nuclei growth rate. Therefore, the composite prepared under

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current density of 1 mA·cm-2 could obtain thicker transition layer in contrast with the composites prepared under higher current density. However, major defects such as voids and gaps in the transition layers could be observed for the Cu/original buckypaper composites prepared under any

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current density, which indicates that the interface bonding between original CNTs and Cu matrix is

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weak due to the strongly hydrophobic nature of CNTs[31].

Fig.10 Cross-sectional SEM images of (a) Cu/acidized and (b) Cu/oxidized buckypaper composites electrodeposited undercurrent density of 1 mA·cm-2 with coulomb quantity of 50 C·cm-2.

ACCEPTED MANUSCRIPT Fig.10 shows the cross-sectional SEM images of Cu/acidized and Cu/oxidized buckypaper composites electrodeposited under current density of 1 mA·cm-2 with coulomb quantity of 50 C·cm-2. As shown, thicker transition layers have formed between pretreated buckypapers and Cu

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matrix, and no obvious voids or gaps could be observed in the transition layers in both of the composites. The functional groups introduced through oxidation and acidification treatments make the CNTs become hydrophilic, and in consequence the Cu ions can easily penetrate into the

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reticulate structure of the buckypapers. In addition, as predicted by theoretical studies that the

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incorporation of any oxygen-containing functional group serves to bind adsorbed Cu atoms more strongly on the CNTs surface[32]. Therefore, better transition layers with less defects formed between the CNTs and Cu matrix after oxidation and acidification treatment. 3.4 XRD of the as-prepared composites

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I(111)/I(220)=2.51

-2

I(111)/I(220)=1.48

-2

I(111)/I(220)=2.06

-2

I(111)/I(220)=2.72

Cu/acidized buckypaper 1mA.cm

Cu/original buckypaper 20mA.cm

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Cu/original buckypaper 10mA.cm

-2

I(111)/I(220)=2.66

Cu/original buckypaper 1mA.cm

10

20

30

Cu(222)

Cu(200)

-2

Cu/oxidized buckypaper 1mA.cm

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Intensity(a.u.)

Cu(111)

40

50

2θ/degree

60

70

80

Fig.11 XRD patterns of the as-prepared Cu/buckypaper composites electrodeposited with coulomb quantity of 50 C·cm-2 Fig.11 shows the XRD patterns of the as-prepared composites. All of the diffraction peaks could be assigned to face-centered-cubic (fcc) Cu, and no CNTs peaks are observed owing to their

ACCEPTED MANUSCRIPT weak intensity. The relative intensities of the diffraction peaks associated with (111), (200) and (222) crystal planes are almost the same for all composites. Crystallite orientations is identified by the XRD (111)/(200) peak intensity ratio. In the present study, no obvious textures are detected on

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the surface of Cu layers, indicating that the surface morphologies of the Cu layers have less effect

100

4.4

90 80 70 60

40 30 20 10

0

10

20

30

40

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Coulomb quanity(C⋅cm-2)

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3.6 3.2

Cu/original buckypaper composites Cu/oxidized buckypaper composites Cu/acidized buckypaper composites

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50

(b)

4.0

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(a)

Conductivity / (105S⋅cm-1)

5.6 5.2 4.8 4.4 4.0 3.6 3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 0.0

Conductivity/(% IACS)

Conductivity/(105S⋅cm-1)

3.5 Electrical conductivities of the composites

0 80

65 60 55 50 45

2.4

40

2.0

35 30

1.6

2

4

6

8

75 70

2.8

0

80

Conductivity / (% IACS)

on the electrical conductivities of the composites.

10

12

14

16

18

20

Curent density / (mA⋅cm-2)

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Fig.12 (a) Relationship between the electrical conductivity and coulomb quantity of the Cu/original buckypaper composites electrodeposited under current density of 1 mA·cm-2; (b) the variation of electrical conductivity with current density for Cu/buckypaper composites with the

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coulomb quantity of 50 C·cm-2.

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To study the variation of electrical conductivity with coulomb quantity, a series of experiments were carried out based on Cu/original buckypaper composites. Fig.12a shows the relationship between electrical conductivity and coulomb quantity of the Cu/original buckypaper composites with the deposition current density of 1 mA·cm-2. The electrical conductivity of the original buckypaper was 0.55×103 S·cm-1. As shown, and with increase of coulomb quantity, the electrodeposition process could be divided into three stages with the increase of deposited coulomb quantity. In the initial stage (≤ 20 C·cm-2), Cu deposited with slow nucleation growth

ACCEPTED MANUSCRIPT rate on the surface of buckypaper, and the filling ratio of Cu in the pores of the buckypaper increased with the increasing of deposited coulomb quantity. Therefore, the electrical conductivities of the composites improved rapidly at the initial stage. When the deposited

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coulomb quantity is in the range of 20‐50 C·cm-2, the increasing rate of electrical conductivity slows down. This could be attributed to that the newly arrived Cu ions were deposited on the existing Cu surface, and make the composite surface keep flatly and fill adequately the voids and

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gaps of the deposited Cu layer. At the last stage, when the deposited coulomb quantity is over 50

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C·cm-2, the newly arrived Cu ions were deposited on the outer surface. It is worth noting that the electrical conductivity does not change visibly with the increase of coulomb quantity at this stage, which demonstrates that the increase of electrical conductivity does not stem from the Cu deposited on the outer surface. Actually, the combination status between buckypaper and Cu

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matrix is the key factor that would affect the electrical conductivity of the composites. The electrical conductivity of Cu/original buckypaper composite reached the stable value of about 3.67×105 S·cm-1 with coulomb quantity of 50 C·cm-2.

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Fig.12b shows the relationship between the electrical conductivity and the current density of

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the composites with the deposited coulomb quantity of 50 C·cm-2. The electrical conductivity of all the composites decreased with the increasing of current density, and the electrical conductivity of Cu/oxidized buckypaper composite has the maximum value of 4.09×105 S·cm-1 when the current density is 1 mA·cm-2. According to Matthiessen’s rule [33] the total resistivity of a structure can be separated into the phonon contribution and the defect part:  =  + 

(2)

The transition layer between the buckypaper and the Cu matrix can act as the conduction path of

ACCEPTED MANUSCRIPT phonons and electrons in the composite. A good interface bonding is favorable for enhancing the electrical conductivity through reducing the electron scattering and interaction between electron and phonon, and reduces the interface contact resistance[34]. On the other hand, the defect part

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including impurities, defects of CNTs, voids and gaps, would have adverse effects on the electrical conductivity of the composite. Impurities like amorphous carbon and carbon nanoparticles on the surface of CNTs are most likely to be removed by oxidation due to their poor oxidative resistance.

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The defects of CNTs, including non hexatomic rings and two ends of CNTs, will be oxidized

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preferentially during the oxidation treatment. This also explains why the Cu/oxidized buckypaper composites possess the best electrical conductivity when deposited under the same current density. In addition, the existence of voids and gaps will also greatly deteriorate the electrical conductivity of the composites [35-38]. Composites with voids and gaps would lose momentum for the

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scattering of electrons collided in the place with the voids and gaps, and the loss of momentum is a primary cause for the decrease of electrical conductivity[39]. Therefore, the conductivities of Cu/oxidized buckypaper and Cu/acidized buckypaper composites are higher than that of

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Cu/original buckypaper composites due to their better transition layers.

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3.6 Ampacities of the composites

7

0.6

Resistivity (µΩ µΩ ⋅ cm)

(a)

0.5

U (V)

0.4

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0.2

0.1

0.0

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4

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I (A)

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7

-2 9384A⋅ cm

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(b)

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20000

Current density(A.cm-2)

25000

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9

(c)

(d)

1.5

5

6

2

Ampacity / (10 A⋅ cm )

7 6

4

4 3

13559A⋅ cm-2

2

2 1.2 1.1 1.0

0

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10000

15000

20000

25000

30000

-2

Current density (A ⋅ cm )

3

1.3

1 Electrodeposited Cu -2 2 Cu/original buckypaper, 20 mA⋅cm -2 3 Cu/original buckypaper, 10 mA⋅cm -2 4 Cu/original buckypaper, 1 mA⋅cm -2 5 Cu/acdized buckypaper, 1 mA⋅cm -2 6 Cu/oxidized buckypaper, 1 mA⋅cm

0.9 1.0

1.5

2.0

2.5

1

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5

1

1.4

4

Resistivity (µΩ ⋅ cm)

8

3.0

3.5

4.0

4.5

5.0

5.5

Conductivity / (105S⋅ cm-1)

Fig.13 (a) Voltage (U)-current (I) curve of pure Cu for ampacity test; (b) and (c) resistivity-current

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density curves for pure Cu and Cu/original buckypaper composites, respectively; (d) ampacity vs.

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conductivity of the pure Cu and the Cu/buckypaper composites.

Fig.13 shows the ampacity test results of the composites with deposited coulomb quantity of 50 C·cm-2 and electrodeposited pure Cu under the same deposition condition was used for comparison. The voltage (U)‐current (I) curve of pure Cu are plotted in Fig.13a. As shown, at the

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initial stage, the voltage increases monotonically with the increasing of current, which indicate that the resistance of Cu at this stage is a constant according to the Ohm’s law. When the current exceeds a critical point, the voltage then increases exponentially with the increasing of current.

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Fig.13b shows the variation of pure Cu with current density. As shown, the resistivity-current

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density curve can be divided into two stages, which is in accordance with the variation trend of the U‐I curve (Fig.13a). The resistivity first increased linearly and then exponentially with the increasing of current density, which meant the conduction mechanism of the specimen had changed during test process. The ampacity is define as the maximum current density where the resistivity remains constant[17]. However, considering the Joule heat generated during the measurement, the ampacity is regarded as the maximum current density in the first stage. According to Fig.13b, the ampacity of pure Cu was estimated to be 9384 A·cm-2. Using the same

ACCEPTED MANUSCRIPT method, the ampacity of the composites was measured and plotted in Fig.13d. It can be seen that for Cu/original buckypaper composites, with the decreasing of current density, the ampacity increased as well as the electrical conductivity, which could be attribute to the increased thickness

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of transition layers. When the applied current density was 1 mA·cm-2, the ampacity of the Cu/original buckypaper composite was 13559 A·cm-2, which was approximately 45% higher than that of pure Cu. In addition, the Cu/oxidized buckypaper composite had the maximum ampacity of

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15437 A·cm-2, which was approximately 65% and 14% higher than that of pure Cu and

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Cu/original buckypaper composite, respectively. At the same time, the Cu/oxidized buckypaper composite had maintained a relatively high electrical conductivity which reached 76% of pure Cu. The electrical conductivities and ampacities of the original buckypaper, pure Cu and the composites deposited under different current densities are listed in Table 1.

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Table 1

Average thicknesses of transition layers and samples, electrical conductivities and ampacities of buckypaper, pure Cu and the composites deposited under different current densities

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Average

thickness of

(mA·cm-2)

transition layer

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Sample

Current density

(µm)

Average thickness of sample (µm)

Electrical conductivity

Ampacity

(S·cm-1)

(A·cm-2)

−

6

0.55×103

−

1

−

~18.3

5.30×105

0.94×104

Cu/original buckypaper

1

~8.0

~22.7

3.67×105

1.36×104

Cu/original buckypaper

10

~2.5

~24.3

1.76×105

1.26×104

Cu/original buckypaper

20

~1.6

~25.0

1.63×105

1.23×104

Cu/acidized buckypaper

1

~8.5

~22.6

3.87×105

1.51×104

Cu/oxidized buckypaper

1

~11.5

~20.4

4.09×105

1.54×104

Buckypaper Pure Cu

−

The ampacity of the Cu/buckypaper composites is related to the electromigration of Cu in the composites. A low diffusion coefficient would restrain the electromigration[17]. The diffusion coefficient

∗

could be calculated by the equation:

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Where

"

=

&'( " #$% (  )

(3)

is the diffusion coefficient at infinity[40], ) is the activation energy, K is the

Boltzmann constant and T is the mean temperature. For bulk Cu, diffusion occurs at surfaces and

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grain boundaries that possess much lower activation energy[41, 42]. However, theoretical studies have proposed that the activation energy for the carbon-doped Cu diffusion increased by suppressing surface and grain boundary pathways[43]. In the transition layers of Cu/buckypaper

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composites, the reticulate CNTs covered the grain boundaries of Cu, which would suppress Cu

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diffusion and therefore resulted in high ampacities. Enhancing the thickness of transition layers and the interfacial bonding strength would raise the activation energy for Cu diffusion in the composites, therefore the Cu/oxidized buckypaper and Cu/acidized buckypaper composites obtained higher ampacities compared to Cu/original buckypaper composites.

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Conclusions

The Cu/buckypaper composites were prepared by Cu sulfate electrodeposition process. The oxygen-containing functional groups were introduced on the surface of CNTs through

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pretreatments of acidification and oxidation. Spontaneous reduction of Cu ions on the surface of

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CNTs can take place during immersion treatment due to the introduced functional groups. Small deposition current density of 1 mA·cm-2 was beneficial to the homogeneous nucleation of Cu, and can facilitate the formation of thicker transition layers. The increased nucleation rate of Cu could be attributed to the functionalization through acidification and oxidization pretreatments. The electrical conductivities of the composites increased with decreasing of deposition current density. When the current density was 1 mA·cm-2, the electrical conductivity of Cu/oxidized buckypaper composite reached the maximum value of 4.09×105 S·cm. The relatively

ACCEPTED MANUSCRIPT high electrical conductivity could be attributed to the good interface bonding. The ampacities of all the composites improved obviously compared with pure Cu. The ampacity of Cu/oxidized buckypaper composite was 15437 A·cm-2, which was 65% and 14% higher than that of pure Cu

Acknowledgement

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and Cu/original buckypaper composite, respectively.

Grant no. 51401098, 51561014 and 51301079.

References

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This work was supported by the National Natural Science Foundation of China (NSFC) under

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ACCEPTED MANUSCRIPT Highlights 1. Cu/buckypaper composites were prepared by Cu sulfate electrodeposition process. 2. Oxygen-containing functional groups were introduced by acidification and oxidation. 3. Functional groups contributed to good interface bonding and thicker transition layers.

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4. Small deposition current density helped to obtain high electrical conductivity.

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5. High ampacity obtained due to the reticulate structure formed in the transition layer.