41Synthesis of the precursor for aluminum-graphene composite

] ]

Rev. Adv. Mater. Sci. 39 (2014) 41-47

Corresponding author: V.G. Konakov, e-mail: glasscer@yandex.ru

SYNTHESIS OF THE PRECURSOR FORALUMINUM-GRAPHENE COMPOSITE

V.G. Konakov1,2 1,3, N.V. Borisova4, E.N. Solovyeva4, S.N. Golubev4,O.Yu. Kurapova1,2, N.N. Novik1,2 and I.Yu. Archakov1,3

1Research Laboratory for Mechanics of New Nanomaterials, St. Petersburg State Polytechnical University,Polytechnicheskaya 29, St.Petersburg 195251, Russia

2Insitute of Chemistry, St. Petersburg State University, Universitetskii pr. 26, Petrodvorets, St. Petersburg,198504, Russia

3Institute of Problems of Mechanical Engineering, Russian Academy of Sciences, Bolshoy pr. 61, V.O.,St. Petersburg 199178, Russia

4 ] ] ] ] ] ]

Received: October 28, 2014

Abstract. A technique is developed which provides synthesis of a precursor for aluminum-graphene composites with enhanced mechanical properties. The optimal conditions for produc-tion of such a precursor from aluminum and graphite powders are selected. The process ofgraphite-to-graphene transformation via micromechanical graphite splitting and the plastic prop-erties of the precursor are examined. The presence of graphene in the precursor is experimen-tally documented. The properties of the synthesized precursor (ductility (plasticity) and opportu-nity to compact a specimen by pressing) are consistent with the requirements for synthesis of thealuminum-graphene composites.

1. INTRODUCTION

In recent years, graphene - a new two-dimensionalcarbon material - has attracted a remarkable inter-est as a component that can significantly improvemechanical properties in metal-graphene compos-ites [1-9], ceramic-matrix composites containinggraphene nanoinclusions (nanoplatelets and few-layer graphene sheets) [10-18] and polymer-graphene composites [19-21]. This improvement isdue to several factors including unique mechanicalcharacteristics (say, superior tensile strength of 130 GPa and Young modulus of 1 TPa [22,23])exhibited by graphene, sheet-like geometry ofgraphene nanoinclusions and specific structural fea-tures of composites containing such nanoinclusions.

Several approaches aimed at graphene introduc-tion in metallic, ceramic and polymer matrices aswell as homogenization and densification of final

composites containing graphene nanoinclusonshave been considered in reviews [7,13,19-21]. It hasbeen demonstrated that mechanical characteristicsof graphene-reinforced composites are highly sen-sitive to methods of their fabrication/synthesis[7,13,19-21]. In general, this scientific area is in itsinfancy, and thereby there is huge potential in sig-nificant improvement of mechanical properties ex-hibited by graphene-reinforced composites throughboth optimization of existent methods and searchfor new routes for fabrication/synthesis of such novelcomposites.

Of a special interest is fabrication of aluminum-graphene composites, first of all, because of ex-traordinarily wide range of structural and functionalapplications of aluminum (Al). Recently, Wang withco-workers [1] have fabricated Al-graphenenanocomposites showing a dramatic enhancement

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in strength, as compared to pure aluminum and Al-matrix composites containing other reinforcing in-clusions. In doing so, they utilized the novel ap-proach based on flake powder metallurgy with itsessential stage being fabrication of Al flakes coatedby graphene nanosheets. In the experiment [1], theAl-graphene nanocomposite showed both a tensilestrength of 256 MPa and 13% elongation which areby 62% higher and around 2 times lower than thestrength (154 MPa) and elongation (27%) of pureAl, respectively. The dramatic increase in tensilestrength is reached due to insertion of 0.3 wt.%graphene nanosheets. Following Wang with co-work-ers [1], this strengthening with 0.3 wt.% grapheneis most effective, as compared to that of any otherreinforcing inclusions with the same weight fractionin Al. At the same time, the reported enhancementby 62% is much lower than that expected from theo-retical estimates of the potential effects caused bygraphene nanosheets on the mechanical propertiesof Al-graphene composites. As a corollary, there arereasonable expectations in achieving further remark-able progress in improvement of strength and othermechanical characteristics exhibited by Al-matrix

Fig. 1. PSD data on surface area for Al-PA-4 powder.

Fig. 2. PSD data on volume distribution for Al-ASD powder.

nanocomposites due to their strengthening bygraphene. There is high interest in development ofnew methods for fabrication of Al-graphene compos-ites, that is, methods different from the approachexploited in the experiment [1].

The main aim of this paper is to develop the firststage of a new approach for fabrication of Al-graphenecomposites with enhanced mechanical properties.The first stage in question represents synthesis ofa ductile precursor for fabrication of novel Al-graphenecomposites.

2. EXPERIMENTAL

Commercial thermally expanded graphite (TEG) wasused as a source for graphene production. Two kindsof aluminum powders - fine-grained ASD Aluminum(Al-ASD) and PA-4 Aluminum (Al-PA-4) - were ex-ploited and examined as a base for the aluminummatrix of the composite. In order to diminish theaverage grain size of Al-PA-4 powder, it was meshedusing 40 m sieve. The Al powder fraction selectedin this procedure was used in further experiments.Both aluminum powders were characterized for par-

43Synthesis of the precursor for aluminum-graphene composite

Fig. 3. PSD data on size distribution for Al-ASD powder.

Fig. 4. PSD data on surface area for Al-ASD powder.

Fig. 5. XRD patterns for Al-ASD+3 wt.% TEG and Al-PA-4+3 wt.% TEG mixtures treated for 1 hour (milling+ mechanical activation).

ticle size distribution (laser PSD analyzer HoribaLA-950), and TEG+aluminum mixtures containing3 and 5 wt.% TEG were prepared.

According to Ref. [12], micromechanical split-ting is an effective method for graphite-to-grapheneconversion. This approach was used in the present

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work. In doing so, TEG+aluminum mixtures weremilled and mechanically activated in the planetarymill (Pulverisette 6, Fritch) during 1-3 hours at 450-500 rpm.

The treated mixtures were characterized by XRD(Shimadzu XRD-6000) to control the graphite-to-graphene transformation. PDF-2 database [24] wasused to identify phase composition. Electron mi-croscopy (Leica DM 4500 P LED, reflected beam)was used to characterize both shape and agglom-erate sizes of aluminum, graphite, and graphenegrains in the mixtures.

3. RESULTS AND DISCUSSION

Fig. 1 demonstrates the particles surface distribu-tion in the sieved Al-PA-4 powder according to PSD

Sample Peak intensity, Halfwidth of the Integral peak Integral peaka.u. peak, rad. intensity, a.u. halfwidth, rad

1 Al-PA-4+3 wt.% TEG 4120 0.197 52041 0.253Treatment for 1 hat 500 rpm

2 Sample 1, additional 4451 0.179 51844 0.250treatment for 1 h at 500rpm, total treatment

3 Sample 2, additional 3372 0.195 41515 0.257treatment for 1 h at 500rpm, total treatment

4 Al-PA-4+3 wt.% TEG 2936 0.200 38252 0.265Continuous treatmentfor 3 h at 450 rpm

5 Al-ASD+3 wt.% TEG 739 0.225 11371 0.308Treatment for 1 h at450 rpm

6 Al-ASD+3 wt.% TEG 3 300 0.199 2825 0.188cycles of 1 h treatmentat 450 rpm, total

]7 Al-ASD+3 wt.% TEG 351 0,1927 3189 01817

Continuous treatmentfor 3 h at 450 rpm

8 Al-ASD+5 wt.% TEG 1435 0.224 21290 0.301Treatment for 1 h at450 rpm

9 Al-ASD+5 wt.% TEG 587 0.214 5157 0.203Continuous treatmentfor 3 h at 450 rpm

Table 1. Characterization of 2 ] ] ]mill.

Fig. 6. Microphoto of Al-PA-4 powder.

data. As it is seen from Fig. 1, the powder consistsof particles with the diameters lying in the 10-100

45Synthesis of the precursor for aluminum-graphene composite

mm range, and the relative surface area is evalu-ated as 1904 m2/g. In contrast to Al-PA-4 case, dis-tributions for Al-ASD powder are not so uniform.Particle volume distribution (Fig. 2) indicates theexistence of two fractions in Al-ASD powder. Onefraction consists of particles with the average diam-eter ~ 3-30 m, while the typical diameter for theother fraction particles is 200-3000 m. However,as it follows from the quantitative distribution (seeFig. 3 showing Al-ASD powder, particle size distri-bution), the average particle size is ~ 5-7 m. Fig. 4presents the surface area distribution, the relativesurface area evaluated from these data is 4390m2/g, this value is twice higher than that for Al-PA-4. With analysis of the data on Al-ASD distributions,one can assume that Al-ASD powder consists ofrather crumply agglomerates.

Fig. 5 shows XRD patterns for Al-ASD+3wt.%TEG and Al-PA-4+3 wt.% TEG powders treatedin a planetary mill for 1 hour (milling+mechanicalactivation). Most peaks in these patterns are attrib-uted to aluminum, while the reflex corresponding to

Fig. 7. Microphoto of Al-PA-4+3 wt.% TEG mixturetreated for 1 h at 450 rpm, region I.

Fig. 9. Microphoto of Al-PA-4+5 wt.% TEG mixturetreated for 1 h at 450 rpm.

Fig. 8. Microphoto of Al-PA-4+3 wt.% TEG mixturetreated for 1 h at 450 rpm, region II.

Fig. 10. Microphoto of Al-ASD+3 wt.% TEG mix-ture treated for 1 h at 450 rpm.

2 = 26.5 indicates the presence of graphite. Thenarrow peak shape gives an opportunity to considerpure graphite phase, while peak broadening corre-sponds to scaly structure formation. The higher isthe number of scales with the dimensions close tomonoatomic layer, the broader is the peak shape.Hence, one can assume the peak broadening asan indirect indicator of graphite-to-graphene trans-formation.

Table 1 presents the results of XRD analysisperformed for the aluminum-graphite mixtures withdifferent compositions and treatment conditions.After analysis of the data presented in the table,one can conclude that graphite-to-graphene trans-formation is enhanced in powders with higherdispersity (lower average particle size). This factmanifests itself for all treatment conditions. Theoptimal treatment procedure is continuous milling/mechanical activation for 3 hours at 450 rpm.

The results of electron microscopy analysis per-formed for Al-PA-4+3 wt.% TEG treated mixtures iswell consistent with the results of PSD analysis,

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see Fig. 6. The size of the powder grains (whitegrains in Fig. 6) is less than 50 mm. In addition,they look like some agglomerated structure formedfrom compacted particles of lower size.

Figs. 7 and 8 demonstrate the images of tworegions of Al-PA-4+3 wt.% TEG sample treated for1 h at 450 rpm. As it follows from these figures,aluminum (white) and graphite (black) grains arepresent in the sample. At the same time, black ordark grey broken lines prove the presence ofgraphene nanoplatelets in the sample. Similar im-ages were shown to be typical for graphene synthe-sized in Si

3N-graphite system [4]. So, our sugges-

tion on graphite-to-graphene transformation, derivedfrom XRD data, is proved by results of electron mi-croscopy analysis. Note that similar observationsfor the powder having the composition with 5 wt.%TEG (Fig. 9) did not show graphene formation. Im-ages of Al-ASD+ 3 wt.% TEG are presented in Fig.10. As it is seen in the figure, the powder consistsof crumpled agglomerated aluminum particles. Thisdata is in fine accordance with PSD results andproves the assumption about agglomerate structure.Similar to Al-PA-4+3 wt.% TEG sample, black graph-ite grains are distributed in between the particles ofaluminum. However no graphene formation is ob-served. The data obtained gives an opportunity toconclude that it is the high dispersity of the alumi-num powder that, most likely, induces graphite-to-graphene transformation during micromechanicalsplitting.

Note that pure aluminum powders, both ultrafine-and fine-grained, cannot be compacted into a tab-let. On the contrary, the Al-graphene samples werecompacted by pressing at 2125 kg/cm2 for 10 min,tablets with 30 mm diameter and 8-10 mm in heightwere manufactured for further study.

4. CONCLUDING REMARKS

To summarize, the results of the present work arebriefly as follows: The graphene content in the precursors producedfrom mechanoactivated powders containing alumi-num and thermally expanded graphite increases withincrease in the aluminum powder dispersity, i.e. withdecrease in the average particle size in the powder. The graphene content is maximal in the initial pow-der mixture specified by the graphite content of 3wt.%. In contrast to pure aluminum powders (having par-ticle size < 40 m), synthesized precursor contain-ing graphene possess ductility (plasticity) requiredfor fabrication of aluminum-graphene composites.

ACKNOWLEDGEMENTS

This work was supported by the Russian ScienceFoundation (Research Project 14-29-00199).

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