J. Cent. South Univ. (2021) 28: 1255-1265

DOI: https://doi.org/10.1007/s11771-021-4693-y

Improving effect of carbonized quantum dots (CQDs) in pure copper matrix composites

HUANG Xiao(黄啸)¹, BAO Rui(鲍瑞)^{1, 2}, YI Jian-hong(易健宏)^{1, 2}

1. Faculty of Materials Science and Engineering, Kunming University of Science and Technology,Kunming 650093, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: Carbon quantum dots (CQDs), which contain a core structure composed of sp² carbon, can be used as the reinforcing phase like graphene and carbon nanotubes in metal matrix. In this paper, the CQD/Cu composite material was prepared by powder metallurgy method. The composite powder was prepared by molecular blending method and ball milling method at first, and then densified into bulk material by spark plasma sintering (SPS). X-ray diffraction, Raman spectroscopy, infrared spectroscopy, and nuclear magnetic resonance were employed to characterize the CQD synthesized under different temperature conditions, and then CQDs with a higher degree of sp² were utilized as the reinforcement to prepare composite materials with different contents. Mechanical properties and electrical conductivity results show that the tensile strength of the 0.2 CQD/Cu composite material is ~31% higher than that of the pure copper sample, and the conductivity of 0.4 CQD/Cu is ~96% IACS, which is as high as pure copper. TEM and HRTEM results show that good interface bonding of CQD and copper grain is the key to maintaining high mechanical and electrical conductivity. This research provides an important foundation and direction for new carbon materials reinforced metal matrix composites.

Key words: carbon quantum dots; copper matrix; mechanical property; electrical property; interface bonding

Cite this article as: HUANG Xiao, BAO Rui, YI Jian-hong. Improving effect of carbonized quantum dots (CQDs) in pure copper matrix composites [J]. Journal of Central South University, 2021, 28(4): 1255-1265. DOI: https://doi.org/10.1007/s11771-021-4693-y.

1 Introduction

Carbon quantum dots (CQDs), very recently, a new type of quasi-zero-dimensional sphere or sphere-like carbon nanomaterial with diameter of 2-10 nm [1-4], have been creatively applied to the reinforcement of copper matrix [5]. Similar to carbon nano-reinforcers such as graphene and carbon nanotubes, they all contain a hexagonal network structure composed of sp² carbon [6-8], which gives these carbon nano-materials excellent mechanical and electrical properties (e.g., the ultimate tensile strength of graphene: ~130 GPa, elastic modulus: 1 TPa and room temperature electron mobility: ~2.5×10⁵ cm²/(V·s)) [9, 10].

CQD, moreover, as a reinforcement of metal-based material, has many advantages that graphene and carbon nanotubes cannot match. First of all, rich functional groups on the surface of the CQD and particle size less than 10 nm make it show colloidal characteristics in water and very good dispersibility [11]. This is unmatched by graphene and carbon nanotubes. In fact, both graphene and carbon nanotubes are more difficult to disperse due to the large specific surface area [12, 13], and it is usually necessary to add a large number of dispersing agents such as surfactants for surface modification to be dispersed in water solvents [14, 15]. This feature can make CQD uniformly dispersed in the salt solution of the metal matrix, which can avoid the problem of easy agglomeration of graphene and carbon nanotubes in the metal matrix. Secondly, in addition to having a graphene-like core structure, CQD has a large number of oxygen-containing functional groups on the surface, such as hydroxyl, carboxyl, amino and other groups [16, 17], which can effectively improve the wettability between carbon and copper matrix. Thus, the efficiency of load [18] and electron transfer [19] of the composites can be improved, and then the comprehensive properties of the composites will be enhanced due to the dependence on the interface. It can be seen that CQD is beneficial to solving the two problems of poor dispersion and weak interface bonding in composite materials [20-23].

Although CQD has very good application prospects in reinforcing copper-based composites, many issues have not been studied. For instance, the effect of the ratio of sp² to sp³ (that is, core-shell structure of CQD itself) and adding content of the CQD on the properties of composite materials is very limited. Therefore, in this study, molecular-level blending combined with SPS will be employed to prepare CQD/Cu composites, and mechanical and electrical properties of the composite will be explored. Furthermore, the microstructure of CQD and interface evolution will be analyzed and characterized by using TEM and HRTEM. At last, the rules and mechanisms of performance of CQD structure on composites will be discussed in detail.

2 Experimental

2.1 Fabrication of CQD/Cu composite

Citric acid (C₆H₈O₇ AR, ≥99.5%), copper acetate monohydrate (C₄H₆CuO₄·H₂O (CuAc) AR, ≥99.0%), glucose anhydrose (C₆H₁₂O₆ AR, ≥99.0%) and sodium hydroxide (NaOH AR, ≥99.0%) were purchased from Shanghai Macklin Biochemical Co., Ltd. Ethylene diamine anhydrous (C₂H₈N₂ AR, ≥98%) was obtained from Tianjin Damao Chemical Reagent Factory. Dialysis bag (MD44) was run through a cellulose ester membrane with molecular weight cut-off of 1000 purchased from VAKE. Firstly, CQD was prepared by using hydrothermal method as follows (see Figure 1): 12 g anhydrous citric acid and 2 mL ethylenediamine were added in 200 mL distilled water and stirred uniformly in a reaction kettle for 10 min, then placed the kettle in a drying oven at 160, 170 and 180 °C for 6 h. After the reaction was completed, the resulting product was dialyzed in a dialysis bag for 10 h (deionized water was changed every 2 h) to remove the raw materials which were not completely reacted. After dialysis, freeze-drying was performed to obtain the CQD powder material as expected. Secondly, CQDs were dispersed in CuAc aqueous solution by ultrasonic dispersion method at 80 °C in a water bath for 10 min, then NaOH solution and glucose were added slowly in turn. When the color of the mixture turned brick red, we stopped mixing and quickly put it into ice water for cooling. After filtration and vacuum drying, CQD/Cu₂O composite powder was obtained.Thirdly, the obtained CQD/Cu₂O composite powder was mixed by ball milling method with the ball-milled copper powder and reduced in argon hydrogen gas as the protective reducing gas at 280 °C for 500 min to obtain CQD/Cu composite powder. Finally, the CQD/Cu powder was placed in a cylindrical graphite die with an inner diameter of 20 mm, and then sintered by spark plasma sintering (SPS) at 700 °C to prepare bulk composite. The heating rate was 50 °C/min and an axial pressure of 50 MPa was applied throughout the sintering cycle. According to the mass fraction of CQD in the composites, the samples were labeled as pure-Cu, 0.1 CQD/Cu, 0.2 CQD/Cu, 0.3 CQD/Cu and 0.4 CQD/Cu, respectively.

Figure 1 Schematic illustration of preparation process of CQD/Cu composite

2.2 Characterizations

Phase composition of the composite powder was determined by X-ray diffraction (XRD, RINT, Rigaku, Japan) with Cu K_α radiation. Surface defects were recorded by Raman spectroscopy (a LabRam HR Evolution). Functional groups on the CQD prepared under different temperature conditions were characterized by Fourier transform infrared spectroscopy (FTIR, Nicolet iS 10). The ¹³C solild-state nuclear magnetic resonance (NMR) experiments were conducted at resonance frequencies of 150 MHz, on a Bruker DSX400 spectrometer with a magic angle spinning (MAS) double resonance probe at ambient temperature. Microstructures of the samples were carefully observed by transmission electron microscope (TEM, FEI Tecnai G2-TF30, S-Twin). Tensile test of the pure copper and the composites was measured by a mechanical tester (AG-X-100 kN, SHIMADZU) at the crosshead speed of 0.2 mm/min, and the SPS prepared sample was processed into the dog-bone sample with a gauge length of 10 mm and gauge width of 3 mm. Relative density of the composites was evaluated according to the principle of Archimedes. Electrical conductivity of the composites was measured by direct current low resistance test instrument (Sigma 2008B). Fracture surface of the samples after tensile fracture was observed by field-emission scanning electron microscopy (FE-SEM, Nova Nano-450, FEI). Atomic scale model is finished by using Materials Studio software.

3 Results and discussion

3.1 Optimization of CQD material

CQD nanoparticles were synthesized by oxidation polymerization in a hydro-thermal reactor at 160, 170 and 180 °C. They all have good colloidal stability at room temperature, and there is still no precipitation after being placed for 10 d (as shown in Figure 2(a)). The three brown colloids all show obvious “Tyndall effect” under the irradiation of the laser pointer. Besides, all samples show obvious green fluorescence under ultraviolet irradiation, which cannot be found under natural light. These macroscopic phenomena all show that we have successfully synthesized CQD. Further, the microstructure of these samples was characterized systematically and carefully.

From the XRD spectra of the CQD, it can be seen that only one broad peak appears at about 2θ=20°, which is caused by the highly disordered carbon atoms [2] (see Figure 2(b)). Moreover, as the temperature increases, the broad peaks in the XRD spectrum gradually become sharper, accompanied by a certain blue shift, that is, the lattice constant becomes larger [24]. This may be due to the increased carbonization caused by the increased temperature [25, 26]. Figure 2(c) shows the Raman spectra of the synthesized CQD at different temperatures. Typical G and D bands can be observed at ~1586 and ~1350 cm^-1, which are caused by sp² crystal carbon and sp³ disordered carbon, respectively [27].

Besides, I_D/I_G value can reflect the degree of structural defects of carbon materials, the higher I_D/I_G value means the higher defect density [28]. In our study, the I_D/I_G values of the CQD at different temperatures are 0.85, 0.87 and 0.87, respectively, which are very similar and close to the values of CNT (0.74-0.95) and graphene oxide (0.78-0.81) in literatures [29, 30], indicating a certain degree crystallinity of the carbon atoms.

Figure 2 (a) CQD dispersion under the irradiation of the laser pointer and ultraviolet lamp; (b) XRD patterns, (c) Raman spectra and (d) FTIR spectra of CQD prepared under different temperature conditions; (e-g) ¹³C HMW of CQD prepared by 160, 170 and 180 °C respectively; TEM image (h, h₁) and particle size statistic of the CQD (h₂); HRTEM image (i) of CQD, corresponding inverse FFT image of CQD (i₁, i₂)

The Fourier transform infrared absorption spectrum of the CQD shows several obvious absorption peaks (see Figure 2(d)). The stretching vibration peaks of hydroxyl and amine groups are observed at ~3397 cm^-1. The characteristic peak at ~2945 cm^-1 is the stretching vibration of —CH, whereas the C=O bending vibration and NH stretching vibration appear at ~1708 and ~1652 cm^-1, respectively. The stretching vibration peak of C=O and the stretching vibration peak of N=H appear at ~1708 and ~1652 cm^-1, respectively [5]. The characteristic peaks at ~1546 and ~779 cm^-1 are assigned to the bending vibration of N—H, and the wide absorption band at ~1403 cm^-1 corresponded to the asymmetric stretching vibration of carboxylic acid anion. However, the explanation and quantitative analysis of the spectrum are impossible due to the contribution from different functional groups and the strength of the absorption band is very weak, hence the ¹³C nuclear magnetic resonance (¹³C NMR) has been carried out to quantitative analysis to the CQD (as shown in Figures 2(e)-(g)). The basic structure of CQD can be determined according to literature report [31]. In the signal range of 30-45 ppm, it contains sp³ hybridized carbon atoms; In the range of 401-185 ppm, it contains sp² hybridized carbon atoms, and in the range of 170-185 ppm, it contains carboxyl and amide groups. In the synthesis process of the CQD, the sp² carbon content increases first and then decreases with the reaction temperature, which is ascribed to the low polymerization reaction at low temperature, and the sp² carbon content is increased obviously with the increase of temperature, which is due to the intensification of the polymerization and carbonization reaction process. As for the decrease of the sp² content at 180 °C may be caused by the faster reaction rate of carbonization than the polymerization [32]. The reactants are carbonized before the polycondensation reaction occurs. TEM images of CQD prepared by hydrothermal conditions is presented in Figures 2(h)-(i). The as-obtained CQD shows near-spherical structure of discrete nanoparticles and well monodispersed (Figure 2(h₁)). Through statistical analysis of particle size, it is found that the average particle size of CQD is ~2.4 nm, and the particle size distribution is uniform (Figure 2(h₂)). The smallest particle is only ~1 nm, the diameter of CQD mainly presents in the narrow distribution range of 1.0-5.0 nm. The microstructure of the CQD is further studied by HRTEM and corresponding fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) (see Figures 2(i₁ )-(i₂)). The FFT recorded from the marked box shows diffraction spots of CQD and the corresponding IFFT image depicts the clear lattice fringes with the measured inter-planar spacing of 0.24 nm, which corresponds to the plane of (100)CQD.

In short, we can see that CQDs have a typical microstructure of core part and polymer chain shell part. The core is mainly composed of sp² hybrid carbon atoms, and the surface is composed of a variety of functional groups, mainly hydrophilic groups, such as hydroxyl, amino, carboxyl and so on. The graphitization degree is the highest when the reaction temperature is 170 °C, so the CQD prepared under this condition is selected as the reinforcement of the pure copper-based composite material for further research.

Figures 3(a)-(c) show the TEM and HRTEM images of the CQD/Cu composite. The CQD are distributed uniformly in the copper matrix and no agglomeration phenomenon is found. Besides, good interface bonding between CQD and copper matrix is observed in Figure 3(a). Figures 3(b) and (c) are step-by-step enlarged views of Figure 3(a). The selected regions at/near the interfacial are carefully analyzed by FFT and IFFT. In region I, the FFT shows the characteristic (100) diffraction spots of CQD (see Figure 3(b)). According to the noise-filtered IFFT image, the lattice inter-planar spacing is measured to be 0.24 nm, best matching the d-spacing of (100) CQD plane. In interface area II (Figure 3(d)), the FFT/IFFT patterns confirm the presence of (100) plane of CQD and (111) plane of Cu. A strong interface bonding derived from a low degree of mismatch between (100) CQD and (111) Cu (d-spacing of 0.24 and 0.21 nm, respectively). This may also be related to the fact that the CQD surface contains a large number of functional groups and reacts with the copper surface. Based on the selected area electron diffraction pattern, an orientation relationship of (100)CQD//(111)Cu is observed. Based on the measured inter-planar spacing from the IFFT image, the misfit (ε) of (100)CQD//(111)Cu interface can be determined to be 12.9%, indicating the formation of a semi-coherent interface according to the Bramfitt lattice matching theory (5%<ε<15%, semi-coherent interface) [33]. Misfit dislocations are clearly visible at the (100)CQD//(111)Cu interface in Figure 3(e). These dislocations, originating from different lattice parameters of CQD and Cu matrix, can play an important role in strengthening the composites. The increased interface energy of the semi-coherent interface and the strong interaction between dislocations during metal deformation are beneficial to the improvement of interface strength between Cu and CQD [28]. Moreover, improving the interface bonding is of great importance in composites, because it will improve the load-transfer and energy-exchange efficiency between reinforcement and matrix. Correspondingly, Figure 3(f) is verified as the matrix of Cu by FFT diffraction pattern and corresponding IFFT lattice spacing measurement. The results show that the plane spacing of these lattice fringes is ~0.209 nm, which is the (111) plane of Cu. The improvement of interface structure may enhance the electrical and mechanical properties of the CQD/Cu composites [34].

Figure 3 TEM and HRTEM images of CQD-Cu composite (a-c) and corresponding FFT and inverse FFT image of area I (d), area II (e) and area III (f) in (b)

3.2 Mechanical and electrical property

In order to study the strengthening effect of CQD reinforcement, a tensile test was carried out on CQD/Cu composite material. Composites with four different mass fraction and pure copper sample were prepared with the same preparation process and conditions for better comparative analysis. The stress-strain curves of the prepared samples are shown in Figure 4. Obviously, the yield strength of the composite is much higher than that of pure copper. The 0.4 CQD/Cu composite obtains tensile strength of ~270 MPa, which is ~31% higher than that of pure Cu sample. However, the elongation of the 0.4 CQD/Cu composite is only 1/3 that of the carbon materials. And as the carbon content increases, the strength increases while the elongation decreases.

Figure 4 Tensile stress-strain curves

Figures 5 show the fracture morphology of the different samples. All of them display numerous dimples implying a ductile fracture. The enlarged image shows that the dimples are uniform in size and there is no obvious agglomeration/re-stacking. However, the diameter and the depth of the dimples are decreased with the increase of CQD content.

Relative density and electrical conductivity of the Cu and CQD/Cu composites are given in Figure 6. The relative density of all samples exceeds 97.8%, and the 0.2 CQD/Cu sample even reaches 98.3%, which can be regarded as close to full density for solid phase sintered material. More importantly, the electrical conductivity of all composite is not as low as expected [35], and the conductivity of the composite increases with the additional content of the CQD. The conductivity of the 0.4 CQD/Cu sample is 94.8% IACS (International Annealed Copper Standard), which is very close to that (96.6%) of pure copper prepared under the same condition.

Like graphene and carbon nanotubes to enhance the metal matrix, the contribution of CQD to the enhancement of the metal matrix mainly comes from three aspects, which are load transfer △σLT [36], grain refinement △σGR [37] and dislocation strengthening △σD [38], respectively. Among them, △σLT is considered to be the most important strengthening contribution due to the excellent mechanical and electronic property of the reinforcements. However, the load transfer efficiency is also very dependent on the interface combination of the reinforcement with the matrix in addition to their own strength [15]. Different from graphene and carbon nanotubes, our prepared CQDs contain a very large number of chemical functional groups. In the process of molecular-level mixing, plenty of Cu—O bonds are formed at the interface of the CQD/Cu composites, which is conducive to the interface bonding and load transfer between the matrix and the carbon.

Figure 5 Fracture morphology after tensile test of samples:

Figure 6 Relative density and electrical conductivity of Cu and CQD/Cu composites

It can be seen from Figures 7(a) and (b) that the surfaces of graphene/Cu and carbon nanotube/Cu are not wetted, and there is a large distance between them. Therefore, these reinforcements are pulled away easily when they are stressed, and a large number of dislocations will appear at the interface [39]. Meanwhile, electrons are easily scattered or blocked when they encounter these interfaces. But the interface between CQD and copper is very different (see Figure 7(c)). The distance between them is shortened due to the existence of a large number of Cu—O—C chemical bonds. This composite material can get good load transfer when subjected to stress. At the same time, the existence of chemical bonds can also provide electron transmission channel [40, 41].

In addition, since the CQD content of the composite carbide is increased to generate a more continuous proton path, this reduces the transfer distance and potential barrier, and leads to an increase in the electrical conductivity of the composite material [42].

4 Conclusions

Our study demonstrated that the CQD/Cu composites with good mechanical and electrical conductivity were fabricated by the molecular level mixing method combined with the SPS method. The composites with 0.4 CQD exhibited a ~31% tensile strength enhancement and as high electrical conductivity as pure copper due to the strong bonding between CQD and Cu matrix, which is ascribed to the formation of large number of Cu—O—C chemical bonding from the CQD surface functional group. Next, the effect of CQD surface functional groups on the electrical properties of composite materials should be to quantitatively studied.

Contributors

HUANG Xiao provided the concept and edited the draft of manuscript. BAO Rui conducted the literature review and wrote the first draft of the manuscript. YI Jian-hong edited the draft of manuscript.

Conflict of interest

HUANG Xiao, BAO Rui, and YI Jian-hong declare that they have no conflict of interest.

Figure 7 Atomic interface structures:(Cu, O, N and C atoms are represented by pink, red, blue and black balls, respectively)

References

[1] BAKER S, BAKER G. Luminescent carbon nanodots: Emergent nanolights [J]. Angewandte Chemie International Edition, 2010, 49(38): 6726-6744. DOI: 10.1002/anie. 200906623.

[2] ZHU Shou-jun, MENG Qing-nan, WANG Lei, ZHANG Jun-hu, SONG Yu-bin, JIN Han, ZHANG Kai, SUN Hong-chen, WANG Hai-yu. Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging [J]. Angewandte Chemie International Edition, 2013, 52(14): 3953-3957. DOI: 10.1002/anie.201300519.

[3] ROY P, CHEN P, PERIASAMY AP, CHEN Y, CHANG H. Photoluminescent carbon nanodots: Synthesis, physicochemical properties and analytical applications [J]. Materials Today, 2015, 18(8): 447-458.

[4] ZHOU Jin, ZHOU Hui, TANG Jin-bao, DENG Shu-e, YAN Fang, LI Wen-jing, QU Mei-hua. Carbon dots doped with heteroatoms for fluorescent bioimaging: A review [J]. Microchimica Acta, 2017, 184(2): 343-368. DOI: 10.1007/ s00604-016-2043-9.

[5] ZHAO Wen-min, BAO Rui, YI Jian-hong, FANG Dong, LI Cai-ju, TAO Jing-mei, LI Feng-xian, LIU Yi-chun, YOU Xin. Improving mechanical and thermal property of pure copper matrix simultaneously by carbonized polymer dots (CPD) cluster reinforcement [J]. Materials Science and Engineering A, 2021, 805: 140573. DOI: 10.1016/j.msea. 2020.140573.

[6] SHAMSIPUR M, BARATI A, TAHERPOUR A A, JAMSHIDI M. Resolving the multiple emission centers in carbon dots: From fluorophore molecular states to aromatic domain states and carbon-core states [J]. The Journal of Physical Chemistry Letters, 2018, 9(15): 4189-4198. DOI: 10.1021/acs.jpclett.8b02043.

[7] FANG Qing-qing, DONG Yong-qiang, CHEN Ying-mei, LU Chun-hua, CHI Yu-wu, YANG Huang-hao, YU Ting. Luminescence origin of carbon based dots obtained from citric acid and amino group-containing molecules [J]. Carbon, 2017, 118: 319-326. DOI: 10.1016/j.carbon.2017.03.061.

[8] FU Ming, EHRAT F, WANG Yu, MILOWSKA K Z, RECKMEIER C, ROGACH A L, STOLARCZYK J K, URBAN A S, FELDMANN J. Carbon dots: A unique fluorescent cocktail of polycyclic aromatic hydrocarbons [J]. Nano Letters, 2015, 15(9): 6030-6035. DOI: 10.1021/ acs.nanolett.5b02215.

[9] LEE C, WEI X, KYSAR J W, HONE J. Measurement of the elastic properties and intrinsic strength of monolayer graphene [J]. Science, 2008, 321(5887): 385-388. DOI: 10.1126/ science.1157996.

[10] SONG Gui-ming, GUO Ying-kui, ZHOU Yu, LI Qiang. Preparation and mechanical properties of carbon fiber reinforced-TiC matrix composites [J]. Journal of Materials Science Letters, 2001, 20(23): 2157-2160. DOI: 10.1023/A: 1013788701303.

[11] SHARMA S, UMAR A, SOOD S, MEHTA S K, KANSAL S K. Photoluminescent C-dots: An overview on the recent development in the synthesis, physiochemical properties and potential applications [J]. Journal of Alloys and Compounds, 2018, 748: 818-853. DOI: 10.1016/j.jallcom.2018.03.001.

[12] YUE Hong-yan, YAO Long-hui, GAO Xin, ZHANG Shao-lin, GUO Er-jun, ZHANG Hong, LIN X, WANG Bao. Effect of ball-milling and graphene contents on the mechanical properties and fracture mechanisms of graphene nanosheets reinforced copper matrix composites [J]. Journal of Alloys and Compounds, 2017, 691: 755-762. DOI: 10.1016/j.jallcom. 2016.08.303.

[13] CHEN Xiang-yang, BAO RUI, YI Jian-hong, FANG Dong, TAO Jing-mei, LIU Yi-chun. Enhancing interfacial bonding and tensile strength in CNT-Cu composites by a synergetic method of spraying pyrolysis and flake powder metallurgy [J]. Materials (Basel, Switzerland), 2019, 12(4): E670. DOI: 10.3390/ma12040670.

[14] LIU Liang, BAO Rui, YI Jian-hong, FANG Dong. Fabrication of CNT/Cu composites with enhanced strength and ductility by SP combined with optimized SPS method [J]. Journal of Alloys and Compounds, 2018, 747: 91-99. DOI: 10.1016/ j.jallcom.2018.03.029.

[15] XIONG Ni, BAO Rui, YI Jian-hong, TAO Jing-mei, LIU Yi-chun, FANG Dong. Interface evolution and its influence on mechanical properties of CNTs/Cu-Ti composite [J]. Materials Science and Engineering A, 2019, 755: 75-84. DOI: 10.1016/j.msea.2019.03.128.

[16] BOURLINOS A B, STASSINOPOULOS A, ANGLOS D, ZBORIL R, KARAKASSIDES M, GIANNELIS E P. Surface functionalized carbogenic quantum dots [J]. Small, 2008, 4(4): 455-458. DOI: 10.1002/smll.200700578.

[17] ZHU Shou-jun, SONG Yu-bin, ZHAO Xiao-huan, SHAO Jie-ren, ZHANG Jun-hu, YANG Bai. The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): Current state and future perspective [J]. Nano Research, 2015, 8(2): 355-381. DOI: 10.1007/s12274-014-0644-3.

[18] SUBRAMANIAM C, YAMADA T, KOBASHI K, SEKIGUCHI A, FUTABA D N, YUMURA M, HATA K. One hundred fold increase in current carrying capacity in a carbon nanotube-copper composite [J]. Nat Commun, 2013, 4: 2202. DOI: 10.1038/ncomms3202.

[19] LU Lei, SHEN Yong-feng, CHEN Xian-hua, QIAN Li-hua, LU K. Ultrahigh strength and high electrical conductivity in copper [J]. Science, 2004, 304(5669): 422-426. DOI: 10.1126/science.1092905.

[20] WANG Bei-bei, FU Qian-gang, LIU Yue, YIN Tao, FU Ye-wei. The synergy effect in tribological performance of paper-based composites by MWCNT and GNPs [J]. Tribology International, 2018, 123: 200-208. DOI: 10.1016/ j.triboint.2018.03.014.

[21] YANG Ming, WENG Lin, ZHU Han-xing, ZHANG Fan, FAN Tong-xiang, ZHANG Di. Leaf-like carbon nanotube-graphene nanoribbon hybrid reinforcements for enhanced load transfer in copper matrix composites [J]. Scripta Materialia, 2017, 138: 17-21. DOI: 10.1016/j.scriptamat.2017.05.024.

[22] HWANG J, YOON T, JIN S H, LEE J, KIM T S, HONG S H, JEON S. Enhanced mechanical properties of graphene/copper nanocomposites using a molecular-level mixing process [J]. Advanced Materials, 2013, 25(46): 6724-6729. DOI: 10.1002/adma.201302495.

[23] AKBARPOUR M R, ALIPOUR S, FARVIZI M, KIM H S. Mechanical, tribological and electrical properties of Cu-CNT composites fabricated by flake powder metallurgy method [J]. Archives of Civil and Mechanical Engineering, 2019, 19(3): 694-706. DOI: 10.1016/j.acme.2019.02.005.

[24] JIAN Wang, RONG Sheng-li, HONG Zhi-zhang, NI Wang, ZHENG Zhang, HUANG Cheng -zhi. Highly fluorescent carbon dots as selective and visual probes for sensing copper ions in living cells via an electron transfer process [J]. Biosensors and Bioelectronics, 2017, 97: 157-163. DOI: 10.1016/j.bios.2017.05.035.

[25] WANG Huan, LIU Chao-qun, LIU Zhen, REN Jin-song, QU Xiao-gang. Specific oxygenated groups enriched graphene quantum dots as highly efficient enzyme mimics [J]. Small, 2018, 14(13): 1703710. DOI: 10.1002/smll.201703710.

[26] XU Zhi-wei, CHEN Lei, LIU Liang-sen, WU Xiao-qing, CHEN Li. Structural changes in multi-walled carbon nanotubes caused by γ-ray irradiation [J]. Carbon, 2011, 49(1): 350-351. DOI: 10.1016/j.carbon.2010.09.023.

[27] RASHAD M, PAN Fu-sheng, TANG Ai-tao, ASIF M, AAMIR M. Synergetic effect of graphene nanoplatelets (GNPs) and multi-walled carbon nanotube (MW-CNTs) on mechanical properties of pure magnesium [J]. Journal of Alloys and Compounds, 2014, 603: 111-118. DOI: 10.1016/j.jallcom.2014.03.038.

[28] WEI Xia, TAO Jing-mei, LIU Yi-chun, BAO Rui, LI Feng-xian, FANG Dong, LI Cai-ju, YI Jian-hong. High strength and electrical conductivity of copper matrix composites reinforced by carbon nanotube-graphene oxide hybrids with hierarchical structure and nanoscale twins [J]. Diamond and Related Materials, 2019, 99: 107537. DOI: 10.1016/j.diamond. 2019.107537.

[29] HWANG J, YOON T, JIN S H, LEE J, KIM T S, HONG S H, JEON S. Enhanced mechanical properties of graphene/copper nanocomposites using a molecular-level mixing process [J]. Advanced Materials, 2013, 25(46): 6724-6729. DOI: 10.1002/adma.201302495.

[30] CHEN Xiang-yang, BAO Rui, YI Jian-hong, FANG Dong, TAO Jing-mei, LI Feng-xian. Enhancing mechanical properties of pure copper-based materials with Cr_xO_y nanoparticles and CNT hybrid reinforcement [J]. Journal of Materials Science, 2021, 56(4): 3062-3077. DOI: 10.1007/s10853-020-05440-6.

[31] DUAN P, ZHI B, COBURN L, HAYNES C L, SCHMIDT-ROHR K. A molecular fluorophore in citric acid/ethylenediamine carbon dots identified and quantified by multinuclear solid-state nuclear magnetic resonance [J]. Magnetic Resonance in Chemistry, 2020, 58(11): 1130-1138. DOI: 10.1002/mrc.4985.

[32] LI Li-qing, YAO Xiao-long, LI Hai-long, LIU Zheng, MA Wei-wu, LIANG Xin. Thermal stability of oxygen-containing functional groups on activated carbon surfaces in a thermal oxidative environment [J]. Journal of Chemical Engineering of Japan, 2014, 47(1): 21-27. DOI: 10.1252/jcej.13we193.

[33] BRAMFITT B L. The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron [J]. Metallurgical Transactions, 1970, 1(7): 1987-1995. DOI: 10.1007/BF02642799.

[34] LIU Liang, BAO Rui, YI Jian-hong. Mono-dispersed and homogeneous CNT/Cu composite powder preparation through forming Cu₂O intermediates [J]. Powder Technology, 2018, 328: 430-435. DOI: 10.1016/j.powtec.2018.01.055.

[35] YANG Ping, YOU Xin, YI Jian-hong, FANG Dong, BAO Rui, SHEN Tao, LIU Yi-chun, TAO Jing-mei, LI Cai-ju. Simultaneous achievement of high strength, excellent ductility, and good electrical conductivity in carbon nanotube/copper composites [J]. Journal of Alloys and Compounds, 2018, 752: 431-439. DOI: 10.1016/j.jallcom.2018.03.341.

[36] GEORGE R, KASHYAP K T, RAHUL R, YAMDAGNI S. Strengthening in carbon nanotube/aluminium (CNT/Al) composites [J]. Scripta Materialia, 2005, 53(10): 1159-1163. DOI: 10.1016/j.scriptamat.2005.07.022.

[37] NAM D H, CHA S I, LIM B K, PARK H M, HAN D S, HONG S H. Synergistic strengthening by load transfer mechanism and grain refinement of CNT/Al-Cu composites [J]. Carbon, 2012, 50(7): 2417-2423. DOI: 10.1016/j.carbon.2012.01.058.

[38] ZHU Jian-die, LIU Xia, ZHOU Xiao-huan, YANG Qing-sheng. Strengthening effect of graphene-edge dislocation interaction in graphene reinforced copper matrix composites [J]. Computational Materials Science, 2021, 188: 110179. DOI: 10.1016/j.commatsci.2020.110179.

[39] AKBARPOUR M R, MOUSA MIRABAD H, ALIPOUR S, KIM H S. Enhanced tensile properties and electrical conductivity of Cu-CNT nanocomposites processed via the combination of flake powder metallurgy and high pressure torsion methods [J]. Materials Science and Engineering A, 2020, 773: 138888. DOI: 10.1016/j.msea.2019.138888.

[40] YANG Qian, LIN Chen-xiao, LIU Fang-hua, LI Ling, ZHANG Qiu-gen, ZHU Ai-mei, LIU Qing-lin. Poly (2, 6-dimethyl-1, 4-phenylene oxide)/ionic liquid functionalized graphene oxide anion exchange membranes for fuel cells [J]. Journal of Membrane Science, 2018, 552: 367-376. DOI: 10.1016/j.memsci.2018.02.036.

[41] ZHANG Hao-qin, MA Chuan-ming, WANG Jing-tao, WANG Xu-yang, BAI Hui-juan, LIU Jin-dun. Enhancement of proton conductivity of polymer electrolyte membrane enabled by sulfonated nanotubes [J]. International Journal of Hydrogen Energy, 2014, 39(2): 974-986. DOI: 10.1016/j.ijhydene. 2013.10.145.

[42] ZHU Hong, LI Rui, CHEN Nan-jun, WANG Fang-hui, WANG Zhong-ming, HAN Ke-fei. Electrorheological effect induced quaternized poly(2, 6-dimethyl phenylene oxide)-layered double hydroxide composite membranes for anion exchange membrane fuel cells [J]. RSC Advances, 2016, 6(88): 85486-85494. DOI: 10.1039/c6ra14177c.

(Edited by HE Yun-bin)

中文导读

碳量子点(CQD)在纯铜基复合材料中的增强作用

摘要：碳量子点(CQD)的核心结构为sp²碳，可以像石墨烯和碳纳米管一样作为金属基体中的增强相。本文采用粉末冶金法制备了CQD/Cu复合材料，先用分子共混法和球磨法制备复合粉末，再用放电等离子烧结(SPS)致密化成块状材料。利用X射线衍射、拉曼光谱、红外光谱和核磁共振对不同温度条件下合成的CQD进行表征，然后利用sp²含量较高的CQD作为增强剂制备不同含量的复合材料。力学性能和电导率结果表明：0.2 CQD/Cu复合材料的抗拉强度比纯铜试样高~31%，0.4 CQD/Cu复合材料的电导率为~96% IACS，与纯铜相当。TEM和HRTEM结果表明，CQD与铜晶粒良好的界面结合是保持高力学性能和电导率的关键。本研究为新型碳材料增强金属基复合材料的研制提供了重要的基础和方向。

关键词：碳量子点；铜基；力学性能；电学性能；界面结合

Foundation item: Project(52064032) supported by the National Natural Science Foundation of China; Projects(2019ZE001, 202002AB080001) supported by the Yunnan Science and Technology Projects, China; Project (YNWR-QNBJ-2018-005) supported by the Yunnan Ten Thousand Talents Plan Young & Elite Talents, China

Received date: 2021-01-18; Accepted date: 2021-03-15

Corresponding author: BAO Rui, PhD, Professor; Tel: +86-13888480327; E-mail: baorui@kust.edu.cn; ORCID: https://orcid.org/0000-0002-5826-2316