10 Things to Consider When Buying copper composite material

Copper sheets have such a wide number of possible applications, thanks to its versatility and flexibility. From interior designers to industrial designers, construction contractors and architects, and even craftsmen and artisans, they all recognize how precious a material this is.

KLS are exported all over the world and different industries with quality first. Our belief is to provide our customers with more and better high value-added products. Let's create a better future together.

Of course, in order to bring to reality whatever it is you may envision to create using this material, the first thing you should do is to know where to buy copper sheets. Without it, then you obviously won&#;t be able to get started. Your task does not end there, however, because then you have to be able to pick out the right materials for your needs.

Certified Suppliers

If you want to use nothing but good quality materials, then find yourself a trusted and reputable supplier like Rotax Metals. It should also be good for you to build a steady and constant rapport with your supplier so that your transactions will be easier and smoother. Not only that, but these suppliers do sometimes offer promos, and can accommodate bulk orders, making it a whole lot more convenient and efficient for you.

Be wary of metal sheet sources that give you extremely low prices because if it&#;s too good to be true, then it probably is. It also isn&#;t advisable to skimp too much on these materials because they will have functional uses. Most definitely, do not source your materials from metal scrap yards. Don&#;t sacrifice quality for price. At least with trusted suppliers, not only do you get good quality items but also get topnotch service, including shipping.

Metal Sheet Type

Not all metal sheets are created equal. There are different kinds available on the market so it&#;s best if you do your research first as to which one is required for your project. This is especially crucial if you&#;re relatively new at handling this item. Since this hardware takes on a very thin form, unlike most other metal items, it can be easy to work with but also just as easy to mess up. Avoid unnecessary costs and redos by getting the right material for your job.

If you&#;re working on something for outdoors, for example, you&#;d want to get galvanized steel. It&#;s a lot more ideal for this environment because it can better resist the harsh outdoor elements.

How Much Do You Need

You don&#;t want to run out of supply, but at the same time you don&#;t want to buy too much and only end up wasting resources. Once you&#;ve identified the work you need to do, compute for the amount that you need to get. Some opt to get a little more than their actual computation for allowance, just in case something goes wrong and they&#;ll need to redo work.

Know Your Gauge Needs

There&#;s a wide variety of gauges for metal sheets, not only for copper but also other materials like brass, aluminum, and galvanized steel. It is important to know this because this pertains to the thickness of the metal sheet. In determining what gauge you need, just keep in mind that the lower the number, the thicker the sheets.

If you&#;ll be using the sheet on kitchen countertops, for example, you&#;d want to get something that&#;s thin but resilient because it will need to be able to put up with all the work that will be done on it, including chopping, pounding, rolling, and etcetera. A 30 gauge sheet should give you 10 mil, for example, which is just perfect for the job. The same goes for artwork or crafts, like jewellery; this is an ideal number because it can be easily cut through with a scissor.

For general home remodelling projects, you&#;ll need thicker gauge. The thickest gauge commonly available at retail is 24. To give you a clearer picture, this sheet is as thick as a credit card. It&#;s also the heaviest among the copper sheets.

With these tips, you should be able to make a smarter and more economic purchase decision when shopping around for metal sheets.

Sources:

Copper Sheets for Home Remodeling. TheSpruce.com.

A Few Tips for Buying Materials from Sheet Metal Suppliers. NetMarketSuccess.com.

A variety of processing techniques have been developed over the last 5 years in an effort to optimise the structure and properties of the newly emerging Cu/graphene composites. Irrespectively of the technique, the main challenges are always the attainment of a homogeneous dispersion of graphene in the matrix, the formation of a strong interfacial bonding and the retention of the structural stability of graphene. Powder metallurgy [12, 27,28,29, 32, 36, 38, 40, 46, 47, 49, 51,52,53, 55, 57, 60, 63,64,65,66, 68,69,70, 73, 101,102,103,104] and electrochemical deposition [19,20,21, 23, 24, 31, 33,34,35, 39, 43, 45, 48, 50, 54, 67, 71, 72, 74, 105,106,107,108,109,110,111,112,113,114] are by far the most extensively applied processing route for such composites. However, other processing techniques employed include CVD [22, 26, 30, 42, 44, 53, 56, 58], cold spraying [115], layer-by-layer assembly [19, 25], metal infiltration [61, 62], preform impregnation [41] and accumulative roll bonding [37].

Powder metallurgy (PM)

Powder metallurgy is a very versatile process for manufacturing of composites with graphene due to its simplicity, flexibility and near-shape capability [1]. The process basically involves mixing graphene with raw metallic powders to prepare the composite powders followed by their consolidation into a bulk shape. This last step comprises the compaction of the composite process and/or densification processes such as sintering, pressing and/or rolling [1, 2]. The raw metallic powders used tend to be pure Cu powders or Cu alloys powders, consisting of atomised Cu powders mixed with powders of the alloying elements [116].

Mixing

The composite powders can be prepared by simple mixing techniques including mechanical stirring, magnetic stirring, sonication and vortex mixing [28, 29, 32, 47, 49, 51, 52, 55, 57, 60, 63, 65, 66, 68,69,70, 73, 103, 104]. However, high-energy processes such as ball milling (BM) or mechanical alloying (MA) have been also employed [12, 27, 28, 36, 38, 40, 53, 60, 61, 64, 101, 102]. Mechanical alloying is the solid-state processing of powder materials which is often used to produce alloys and composites that are difficult to obtain from conventional melting and casting techniques [1]. The process of MA starts with mixing graphene with the metallic powders in the desired proportion and then loading the powder mix into a mill (shaker mill, planetary mill or attritor) along with the grinding medium (generally steel balls) [116]. The mix is milled for the desired length of time, usually in a protective atmosphere to prevent Cu oxidation. During mixing, the impacted powders undergo repeating fracture, deformation and welding processes, which leads to the intimate mixing of the constituent powder particles on an atomic scale [1]. The total milling energy can be tailored by varying the charge ratio (the ratio of the weight of balls to the powder), ball mill design, milling atmosphere, time, speed and temperature. In certain cases, a process control agent (PCA), such as stearic acid or petroleum ether, is added to the powder mixtures to prevent excessive sticking and agglomeration of Cu powders during milling [12, 28, 36, 60]. The PCA adsorbs on the surface of the powder particles and minimises cold welding between impacted particles, thereby preventing agglomeration [1]. Moreover, mixing techniques such as mechanical stirring, magnetic stirring and sonication and, occasionally, BM are performed in certain organic solvents (e.g. ethanol, acetone, etc.), which hinders the agglomeration of graphene into clusters. The solvents must be then evaporated to obtain dry composite powders before compaction and/or consolidation. For this purpose, vacuum-drying, air-drying and rotary evaporation are commonly used, although other less common techniques such freeze&#;drying or vacuum infiltration have been also employed.

Mechanical alloying can produce composites with finer microstructures and a better distribution of graphene in the Cu matrix [1]. However, the processing steps must be handled with care in order to retain the structural integrity of graphene. Yue et al. [60] reported that with increase in BM time the size of the composite powders decreases and the dispersion of graphene improves, but the damage of the graphene intrinsic structure inevitably increases. Figure 1 shows SEM micrographs of Cu-0.5 wt% GO powders after BM for different times varying from 1 h to 7 h. It can be seen that the shape of the Cu/GO powders undergoes a change from flake-like to more granular morphology with increase in BM time due to the shearing effect of the balls (Fig. 1) [60].

Reproduced with permission from [60]

SEM images of Cu-0.5 wt% GO powder after ball milling for a, b 1 h, c, d 3 h, e, f 5 h and g, h 7 h.

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Figure 2 displays Raman spectra of the composite powders after BM for different times [60]. These spectra show the typical D and G of the GO nanosheets located at around and  cm&#;1, respectively. It can be seen that the ratio of ID/IG increases from 0.84 to 1.42 with increase in BM time, indicating that the degree of damage of the GO increases with increase in BM time. Additionally, Cui et al. [27] found, for CMCs reinforced with GNPs, that the higher the milling speed, the higher the degree of exfoliation of GNPs. Nevertheless, the ID/IG values increased with increase in the milling speed, indicating that the degree of structural damage of graphene also increases with increase in the speed of BM. Thus, the BM conditions need to be balanced to obtain a uniform dispersion of fine graphene particles in Cu matrix while reducing structural damage to the graphene [60].

Reproduced with permission from [60]

Raman spectra of Cu-0.5 wt% GO powder after ball milling for different times.

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Realising that the most critical issues in processing graphene-reinforced MMCs are the dispersion of graphene and the interfacial bond strength between the graphene and the matrix, many researchers have adopted modified steps in their approach [2]. Gao et al. [47] coated Cu powders with hexadecyl trimethyl ammonium bromide (CTAB), a cationic surface agent, to obtain a positive surface charge. The results showed that GO, with a negative charge, is adsorbed on the surface of CTAB coated Cu powder, realising the homogeneous dispersion of graphene in the CMCs [47]. A schematic of the fabrication process of Cu/graphene composites following this approach is given in Fig. 3.

Reproduced with permission from [47]

Schematic of the fabrication process of Cu/graphene composites using hexadecyl trimethyl ammonium bromide (CTAB) modified Cu powders.

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Consolidation

Most researchers have used sintering to consolidate the composite powders. In a few works, green compacts, generally prepared using a press or a testing machine, were sintered in a conventional [38, 52, 57, 63, 65, 73, 104] or microwave furnace [57]. The major advantage of the microwave sintering over conventional sintering is that it provides rapid heating, resulting in much finer grain sizes. A larger number of researchers have used hot pressing (HP) consolidation of powders or compacts [12, 36, 47, 52, 60, 63, 64, 102, 103]. This is a high-pressure consolidation technique working at a temperature high enough to induce sintering. It is conducted by placing either the composite powders or the composite compacts into a suitable die, typically graphite, and applying uniaxial pressure, while the entire system is held at an elevated temperature. So, by hot pressing, consolidation is achieved by the simultaneous application of heat and pressure. Spark plasma sintering (SPS), a comparatively new sintering technique, has also been explored [27, 29, 32, 40, 49, 51, 53, 55, 66, 68,69,70, 101]. In this process, a pulsed direct current is passed through a graphite die where the powder mixtures or compacts are pressed uniaxially [1]. When a spark discharge appears at the contact point between the particles of a material, a local high-temperature condition is created, resulting in rapid heating and hence increasing the sintering rate [1], so that grain growth, graphene agglomeration and thermal decomposition of graphene can be minimised during consolidation [23]. Efficient densification can be achieved by applying a combination of spark impact pressure, joule heating and electrical field diffusion [32, 117].

Kim et al. [28] rolled composite powders to achieve a better density and distribution of graphene in a Cu matrix; the powders were balled milled, followed by encapsulation in a pure Cu tube and degasification, and then subjected to equal speed rolling (ESR) or conventional rolling and to high-ratio differential speed rolling (HRDSR). All the ESR- and HRDSR-processed Cu and Cu composites showed high densities between 98.8 and 99.4%, indicating that almost full densification was obtained after rolling.

Electrochemical deposition

Traditional processes of PM cannot always effectively prevent agglomeration of graphene in the metal matrix because graphene is prone to segregate from the metal particles due its poor affinity to metal in the absence of any binding sites [23]. Thus, novel dispersion methods, such as electrochemical deposition, are needed. These techniques can be divided into electrodeposition and electroless deposition processes; both of which have been used for Cu/graphene fabrication. Electrodeposition, also known as electroplating, required the use of an electrochemical cell and a power source in which an applied current flows between the anode and cathode [1, 2]. The composite film or coating is deposited onto the cathode surface. In contrast, the second technique, known as electroless plating, does not require electricity for the occurrence of reactions in the bath [1, 2]. This is basically a chemical process, in which thermochemical decomposition of metallic salts takes place in the bath to release metallic ions to form a composite with graphene [1].

Electrodeposition

The electrodeposition technique is an easy, cost-effective and scalable method to fabricate Cu/graphene composite coatings [72, 112,113,114] and foils or films [19,20,21, 24, 31, 33, 48]. In addition, electrodeposition being a low temperature process preserves the properties of graphene during the preparation of the composites, unlike in the conventional sintering processes, which may damage graphene because they may involve temperatures higher than its decomposition temperature (>&#;600 °C) [31]. Electrodeposition takes place from a dispersion of graphene in an electrolytic bath consisting of copper sulphate as a source of Cu2+ ions, the graphene content in the Cu/graphene composites depending on the amount of dispersed graphene in the bath. To disperse graphene sheets uniformly into the electrolyte is one of the main challenges to synthesise graphene enhanced nanocomposites by electrodeposition [72]. Stirring [24, 31, 33, 48, 114] can be used to keep graphene in suspension during electrodeposition. Additions of anionic or polymeric surfactants have also been used to improve the wettability of the substrate to be coated and to prevent agglomeration [31, 113, 114]. These additions may, however, introduce heterogeneous impurities, that weaken the interfacial bonding of graphene sheets and matrix, adversely affecting the mechanical and physical properties of the composite coatings. As an alternative, Mai et al. [72] proposed a surfactant-free colloidal solution comprising copper (II)-ethylene diamine tetra acetic acid ([CuIIEDTA]2&#;) complexes and GO sheets to prepare Cu/RGO composites. The anionic complexes stably coexist with negatively charged GO sheets due to the electrostatic repulsion between them, facilitating the electrochemical reduction and the uniform dispersion of RGO sheets into the Cu matrix.

Both direct and pulse reverse current have been used by Pavithra et al. [31, 48] for electrodeposition of Cu/graphene nanocomposite films (Fig. 4). Pulse reverse (PR) is advantageous over direct current (DC) electrodeposition because it allows the optimisation of several key processing parameters including applied current, pulse duration and duty cycle that enables a smooth, highly dense, uniform deposit, while minimising hydrogen embrittlement. This in turn improves the properties of the deposited material. The forward pulse restricts the mass transfer and hence controls the grain size, whereas the reverse pulse minimises the dendritic morphology and helps in the removal of extended graphene and loosely adsorbed Cu or graphene, in addition to removal of entrapped hydrogen during each pulse. Furthermore, PR electrodeposition facilitates a uniform distribution of graphene sheets into the Cu matrix, where they spread around the grain boundaries to achieve an improved interface with the Cu throughout the composite.

Reproduced with permission from [31]

Experimental setup of electrodeposition (a) and schematic representation of the current waveforms and the co-deposition of Cu and graphene by direct (b) and pulse reverse (c) current.

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In the case of DC electrodeposition, the deposition is rapid at the most active nucleation sites and, due to the continuous application of current, the continuous incorporation of graphene along with the Cu deposition results in a rough surface with graphene clusters in the matrix.

Electroless deposition

An electroless plating process consisting in situ chemical or thermal reduction has been used to manufacture graphene-metal nanoparticles (MNPs) hybrids [29, 32, 35, 50, 55, 64, 66, 105,106,107, 110, 111] or sandwich-like 2D Cu/RGO nanocomposites composed of continuous Cu layers on both sides of the central RGO [108]. Copper-nanoparticle/graphene composite powders fabricated by this technique were further consolidated by SPS to obtain bulk Cu/graphene composites [35] or used as such for different applications [105,106,107, 110, 111]. Graphene decorated with other metallic nanoparticles such as Ag or Ni was also fabricated and afterwards successfully introduced as fillers into Cu matrices by processing techniques such as PM routes or molecular level mixing (MLM) in order decrease the contact angle of Cu on graphene and thus to improve the wettability between graphene and the Cu matrix [29, 32, 50, 55, 64, 66]. The fabrication of graphene-MNPs hybrids (Fig. 5) usually consists of the in situ nucleation of MNPs on the graphene sheets by reducing a mixture of GO and metallic ions. Metal ions prefer to nucleate at the sites of functional groups. For this reason, when GNPs are used as precursor materials, they are sensitised and activated before being decorated with the metallic particles [50, 55, 66].

Reproduced with permission from [29]

Schematic of the preparation of GNPs decorated with Ni nanoparticles.

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Another simple, but usually multi-step electroless plating technique, molecular level mixing (MLM), has been used to fabricate Cu/graphene composite powders which are subsequently consolidated by SPS [23, 34, 39, 43, 50, 54, 67, 74, 109]. A schematic diagram of a fabrication process of Cu/RGO nanocomposites by MLM is given in Fig. 6 [23]. Firstly, GO and Cu ions are homogeneously mixed in deionised water. Chemical bonds are then formed between the functional groups of GO and the Cu ions. Finally, Cu/GO nanocomposites are thermally reduced in H2, the as-reduced Cu/RGO composite powders being subsequently consolidated by SPS. However, additional steps involving the generation of copper oxides (CuO and Cu2O) as intermediate products are usually required. Graphene-MNPs hybrids or GNPs can be also used as raw material [34, 43, 50, 54, 67]. However, since formation of Cu&#;O&#;C chemical bonds (whose origin is in the reaction between the carboxyl or hydroxyl groups and Cu) plays a major role in the adsorption of graphene on the Cu surface, GNPs are usually sensitised and activated by a hydrochloric acid solution of SnCl2 and PdCl2, respectively, beforehand.

Reproduced with permission from [23]

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Schematic of fabrication process of Cu/RGO nanocomposites by a molecular level mixing method. a Pristine graphite. b Graphene oxide obtained by the Hummers method. c Dispersion of Cu salt in GO solution. d Oxidation of Cu ions to CuO on graphene oxide. e Reduction of CuO and GO. f Sintering of the Cu/RGO powders.

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Figure 7 displays the evolution of the nanocomposite powders during the MLM and SPS process as proposed by Hwang et al. [23]. Figure 7a shows an atomic force microscopy (AFM) image of GO fabricated by the Hummers method. Figure 7b shows that after mixing the GO and Cu salts, the GO layer was not agglomerated and was homogeneously mixed with the Cu ions. After oxidation, GO particles were fully covered with ellipsoidal CuO particles of about 500 nm in size (Fig. 7c). The Cu/RGO nanocomposite powders obtained by H2 thermal treatment of Cu/CuO powders are shown in Fig. 7d. CuO particles that were formed on GO were reduced to form islands with average size of 30 nm, while CuO particles that were formed without GO were reduced to form large Cu particles and connected to each other during the thermal treatment. The fine size of Cu particles on the RGO originated from the difficulty of Cu diffusion on the surface of RGOs. After consolidation by SPS, the RGO layers were dispersed homogeneously in the Cu matrix without further agglomeration (Fig. 7e). The Raman spectra in Fig. 7f illustrate the evolution of defects in GO and RGO. The ID/IG ratio increased from 0.78 for GO to 0.81 for Cu2+/GO, indicating an increase in defects in the GO structure after mixing with Cu ions. Graphene oxide with Cu ions could be more defective because the interaction of the Cu ions with the GO surface could damage the sp2 bonding network of the graphene further. The continuous, conformal coating of CuO on the GO flakes immediately after the oxidation process blocked the characteristic Raman signals of GO (i.e. D and G bands) from the CuO/GO samples. The ID/IG ratio of the Cu/RGO nanocomposite powders was markedly lower (i.e. 0.40) because the reduction process removed functional groups and partially recovered the graphene structure.

Reproduced with permission from [23]

a AFM image of GO prepared using the Hummers method. SEM images of b Cu2+&#;+&#;GO powders, c CuO/GO powders, d Cu/RGO powders and e Cu/RGO bulk nanocomposite. f Raman spectra of the GO and different nanocomposite powders.

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It is important to note that the reaction products, as well as their morphology, in a MLM process strongly depend on the reaction conditions, with the pH and the reaction temperature being the most critical factors. Yang et al. [71] carried out experiments at different pH values of 5.9, 6.6, 12.8 and 13.6, respectively. XRD patterns of the as-collected samples are given in Fig. 8a. It can be seen that the phase constitution of the composite powders is pH-sensitive. When the pH value is lower than 6.6, the diffraction peaks are assigned to the crystal planes of Cu2(OH)3Ac. However, once the pH value is increased to 12.8, the major diffraction peaks match well with Cu(OH)2 and CuO. Figure 8b, c shows the morphology change of the composite powders at carious pH values as indicated by SEM analysis. As shown in Fig. 8b, Cu2(OH)3Ac is in the form of sheets of about 5 μm in size. When pH value is increased to 12.8 and 13.6, the Cu2(OH)3Ac sheets transform into Cu(OH)2 and CuO nanofibers (Fig. 8c). The nanofibers are uniformly dispersed on GO sheets with diameters of a few tens of nanometres. Most importantly, the edges of GO sheets can be easily observed and are distributed almost parallel to each other, which can be considered the ideal framework of micro-layered composites with nacre-inspired architecture [74]. Nacre is a natural inorganic/organic composite material that gains its toughness from a microstructure that consists of sheets of calcium carbonate separated by layers of elastic biopolymers.

Reproduced with permission from [71]

a XRD patterns of composite powders fabricated by MLM at different pH values. SEM images of the composite powders fabricated at b pH 6.6 and c pH 13.6.

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A modified MLM process, comprising the self-assembly, reduction and consolidation of CuO/GO/CuO or 2D Cu/CuO sandwich-like nanosheets, has been employed to fabricate multilayer Cu/graphene composites with a nacre-inspired architecture [45, 71, 74]. This process leads simultaneously to a uniform dispersion and high alignment of graphene in the metal matrices. An example of such processing technique is shown schematically in Fig. 9 [45]. First, GO is synthesised from natural graphite flakes by a modified Hummers method. In order to assist the dispersion of GO in aqueous media and direct the deposition of CuO on the surface of GO, surfactant sodium dodecyl sulphate (SDS) was chosen to adsorb electrostatically and self-assemble onto the surface of the GO. Cu cations were bound to the surfactant assembled onto the GO, forming CuO/GO/CuO sandwich-like nanosheets in alkaline solution with the decomposition of the added urea in at elevated temperature. CuO was deposited on both sides of the GO (Fig. 9a) and prevented them from restacking. Subsequently, the bottom-up assembly of CuO/GO/CuO sandwich-like nanosheets was carried out by vacuum filtering the parent solution (Fig. 9b). Afterwards, by reducing the assembled CuO/GO/CuO films (Fig. 9c), Cu/RGO/Cu films were achieved. Finally, these films were stacked and consolidated by HP to produce bulk nano-laminated composites (Fig. 9d).

Reproduced with permission from [45]

Schematic representation of the fabrication of Cu/RGO nano-laminated composites by assembling sandwich-like units. a Deposition of CuO on both sides of graphene oxide (GO) to form CuO/GO/CuO sandwich-like nanosheet. b Assembling sandwich-like nanosheet via vacuum filtration. c Reduction of CuO/GO in H2/Ar mixed atmosphere. d Stacking the Cu/RGO films followed by hot pressing to obtain bulk composites.

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Chemical vapour deposition (CVD)

Most of the techniques commonly used to fabricate bulk Cu/graphene composites, including PM and MLM routes, consist of dispersing and combining 2D graphene on the surface of metal powders. This technique often fails to produce good dispersions of graphene into the matrix or good interfacial bonding and may even, as in the case of the BM technique, lead to the damage of the graphene structure. Hence, novel methods based on covering the Cu powders with graphene, mainly by CVD, followed by compaction and/or consolidation are being developed to fabricate bulk composites. These methods solve the above-mentioned disadvantages of other processing techniques and, in addition, can lead to a more ideal structure of graphene within the metal matrix.

Babul et al. [42] fabricated Cu/3D-graphene composites through the following steps: (1) fluidisation under gases containing hydrocarbons in a working chamber, (2) high-temperature decomposition of hydrocarbons that act as the carbon source and (3) nucleation and growth of carbon structures on the surface of the Cu powders. Afterwards, the composite powders obtained were consolidated by HP. However, the synthesis of graphene onto the Cu powders generally takes place by CVD. For example, graphene was synthesised on the surface of micron-sized copper powder by CVD using ethylene as a carbon source in the temperature range from 700 to 940 °C [22, 26]. The composite powders synthesised were then mixed with a certain amount of plain Cu particles, and, in order to obtain compact materials, the mixture was subjected to hot rolling in two stages to a total thickness reduction of 70%. The bulk composites exhibited a grain size around 7 μm elongated in the rolling direction with fine carbon layers located around the boundaries. SEM images of the Cu powder treated at 890 and 940 °C in the presence of ethylene are presented in Fig. 10. As seen from the images, the Cu particles are covered by a smooth layer of carbon.

Reproduced with permission from [22]

Copper powder particles treated in the presence of ethylene at a 890 °C and b 940 °C.

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Graphene has also been grown on the surface of the Cu powders by in situ CVD [44, 56, 58]. This way, Cu/3D-graphene composites were fabricated through an approach involving BM of Cu powders with poly(methyl methacrylate) (PMMA) as a solid carbon source, in situ growth of graphene on the Cu powders by heating under Ar and H2 atmosphere and consolidation of the composite powders [56]. During the BM process, PMMA powders are transformed into extremely small particles and dispersed on the Cu powders. In addition, nacre-inspired Cu matrix nano-laminated composites were fabricated by a similar process comprising in situ growth of graphene on flaky metal powders after BM followed by self-assembly assembling and consolidation of the Cu flakes cladded with in situ grown graphene [44, 58] (Fig. 11).

Reproduced with permission from [58]

Schematic illustration of fabrication of Cu/graphene composite with nacre-inspired structure. Spherical Cu powder a was first transformed into Cu flake b by a ball-milling process. c The as-obtained Cu flakes were soaked in an anisole solution of PMMA and then dried in vacuum, forming a uniform PMMA film on the surface. d The PMMA-coating was used as carbon source for in situ growing graphene at elevated temperature. e The Cu/graphene composite powders were self-assembled into green compact by gravity because of its large aspect ratio. f A nacre-inspired composite was finally obtained by a hot-pressing and a hot-rolling process.

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Layer-by-layer assembly

This is a time-consuming, although very versatile method has been employed to produce different kinds and scales of multilayer Cu/graphene composite films. For example, it was used to fabricate nanolayered composites consisting of alternating layers of Cu and monolayer graphene with 70&#;200 nm repeat layer spacing following the steps schematically shown in Fig. 12 [25]. First, single-atomic-layer graphene was grown on a 25-μm-thick foil by a previously reported chemical vapour deposition (CVD) method [118]. The graphene was then transferred onto a deposited Cu layer to fabricate the metal&#;graphene multilayer structures. A supporting polymer (PMMA) was spin-coated onto the graphene on the Cu foil to prevent damage to the graphene during transfer. The Cu foil was etched by an aqueous solution (ammonium persulphate), thereby detaching the graphene from the Cu foil. The PMMA with attached graphene was floated in the aqueous solution and cleaned several times with distilled water. The graphene films were transferred by scooping the PMMA/graphene films with a Cu-deposited Si/SiO2 substrate. Finally, the substrate was heated to 80 °C for 5 min and then cleaned with acetone to remove the PMMA. This process was repeatedly performed to fabricate the alternating layers of graphene and Cu.

Reproduced with permission from [25]

Schematic of a metal&#;graphene multilayer system synthesis. Graphene is first grown using CVD and transferred onto the evaporated metal thin film on an oxidised Si substrate via a PMMA support layer. The PMMA layer is then removed, and the next metal film layer is evaporated. The mechanical properties of the resulting Cu&#;graphene nanolayered composites produced by repeating the metal deposition and graphene transfer process were studied by compressing nanopillars etched by FIB. The scale bar for the floating graphene is 10 nm and that for the TEM is 20 nm.

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A multilayer film composite has also been prepared comprising several layers of Cu/graphene deposited on a Cu substrate [19]. GO was syphoned from a suspension in isopropyl alcohol and deposited on the Cu substrate using a 3-mm-diameter glass tube. The GO particulate deposition was repeated several times to achieve a uniform dispersion on the surface after the evaporation of the solvent. In the next step, a Cu film was deposited on the top of the GO particulates by laser physical vapour deposition (LPVD). The thickness of the resulting film with GO dispersion was found to be between 0.8 and 0.85 μm. The substrate with the Cu film and GO dispersion was subjected to flowing hydrogen atmosphere at 400 °C for 4 h to reduce GO to graphene. This procedure was repeated to deposit six layers of Cu film containing the dispersion of graphene on the Cu substrate with a final thickness of&#;~&#;5 μm.

Metal infiltration

Melt infiltration is a liquid metallurgy process involving the infiltration of molten metal into a reinforcing preform, which serves to prepare materials that are not accessible by other preparation methods owing to insolubility (e.g. WCu alloys) [1, 119]. WCu/graphene composites have been fabricated by liquid phase sintering (LPS) above the Cu melting point [61, 62], which could be considered a variant of pressureless melt infiltration where the W preform is prepared by BM of graphene with a mixture of almost pure W and Cu followed by pressing [119]. In brief, the as-prepared graphene was dispersed in a C2H5OH solution. At the same time, W and Cu powders were mixed in ethanol solvent by mechanical stirring. The graphene dispersion solution was then added slowly into the WCu powder solution and the mixture was agitated for several minutes. Afterwards, the graphene and WCu mixture powders solution was ball milled under a high-purity Ar atmosphere and the resultant mixture was dried in a vacuum oven. The composite powders were then compacted into cylindrical bars with a universal testing machine. These green compacts were sintered at a temperature of  °C, exceeding the Cu melting point. In these conditions, W solid grains coexist with Cu liquid and sintering takes place by particle rearrangement [120], as shown in Fig. 13.

Reproduced with permission from [120]

A schematic of the microstructure changes during liquid phase sintering starting with mixed powders and pores between the particles. During heating the particles sinter, but when a melt forms and spreads the solid grains rearrange. Subsequent densification is accompanied by coarsening. For many products, there is pore annihilation as diffusion in the liquid accelerates grain shape changes that facilitate pore removal.

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Metal infiltration process has been proved to be very effective at getting dense WCu alloys with a homogeneous distribution of W and Cu [121, 122]. However, some of the drawbacks of the process include reinforcement damage, a coarse grain size, contact between reinforcement particulate and undesirable interfacial reactions [1, 2, 15]. It was also found that during sintering, the crystal structure of graphene was heavily damaged. Example Raman spectra of W70Cu30-1 wt% graphene composite powders and sintered composite are shown in Fig. 14 [61]. In the latter, the intensity IG/ID ratio decreases dramatically compared to graphene, suggesting that defects or disorder of the graphene structure increased during BM. Unfortunately, after infiltration sintering, the IG/ID ratio further decreases.

Reproduced with permission from [61]

Raman spectra of graphene, W70Cu30-1 wt% graphene powder and W70Cu30-1 wt% bulk composite.

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Other processing methods

Preform impregnation

This novel technique was employed by Xiong et al. [41] to fabricate a nature-inspired CMC, where RGO was chosen as &#;brick&#; because of its inherent 2D geometry and good mechanical properties and Cu was used as mortar. The entire process consisted of three steps: replication of the ordered porous structure of fir wood with Cu, absorption of RGO into the porous Cu preform and HP compaction. Fir wood has a highly ordered layer porous structure, in which pores are in rectangular shape with an average size of&#;~&#;20&#;×&#;30 μm and a wall thickness of 1.5 μm. The authors applied a chemical route comprising copper oxide replication and subsequent reduction to replicate the porous structure of fir wood with Cu.

Cold spraying

Cold spraying (CS) is a relatively new technique in which the composite powders are accelerated to very high velocities (~&#;500&#; ms&#;1) at low temperature and impacted on a substrate [1, 2]. During the process, powders are accelerated by injection into a stream of a gas in a converging diverging de-Laval type nozzle. The gas is heated, without using combustion, only to increase the gas and particle velocity [123]. The particles are in solid state when they impact the surface, where they undergo severe plastic deformation. The high kinetic energy upon impact ensures good adhesion of the particles on the substrate. Since the temperature of the process is below the melting point, oxidation and phase transformations can be avoided. Cold spraying in conjunction with ball milling has been shown to be successful in the fabrication of Cu coatings, where non-agglomerated and uniformly distributed GNPs were embedded [115].

Accumulative roll bonding (ARB)

Accumulative roll bonding (ARB) is a severe plastic deformation technique consisting of multiple cycles of cutting, stacking and roll bonding [124]. Hence, large strains can be accumulated in the material and significant structural refinement can be achieved [125]. As a result, good mechanical properties at low and high temperatures have been observed for different metals and alloys over their coarse-grained counterparts [125]. In addition, several researchers have also used ARB for the successful fabrication of particle reinforced MMCs [126,127,128,129]. So, by manually distributing SiC or Al2O3 particles between the two metallic strips prior to each roll-bonding steps, Al or Cu matrix composites with excellent distributions of reinforcing particles, good interfacial bonding and no porosity were obtained after several ARB cycles. More recently, Liu et al. [37] adopted a similar approach to fabricate GNPs reinforced CMCs. In this way, pure Cu was ARB up to eight cycles at RT, a GNPs dispersion being sprayed on the surface of the Cu strips before each rolling step.

Densification

Obtaining sufficiently high densification is a common key difficulty of processing particulate MMCs. The absolute density of the composite should reduce with increase in graphene content due to the relative densities of graphene (2.2 g/cm3) and copper (8.9 g/cm3). However, graphene also has an effect on their degree of densification or relative densities (ratio of the measured experimental density to the maximum theoretical density). Hence, although Cu/graphene composites have high relative densities (usually higher than 96%), they are usually lower than that of the unreinforced matrix and decreases with increase in the graphene content [12, 28, 32, 38, 43, 55, 57, 103]. This is usually attributed to the presence of graphene agglomerates because they form obstacles in composite consolidation, increasing the distance between Cu powder particles and thus reducing their sintering ability or restricting the matrix material to flow. Both factors result in the formation of pores or voids in the composites.

In contrast, graphene has been reported to improve the densification behaviour of WCu alloys. Figure 15 shows the variation of theoretical, measured and relative density of W70Cu30/graphene composites containing different weight fractions of graphene [61]. As expected, the theoretical density of the W70Cu30/graphene composites decreases with an increasing amount of graphene since the density of graphene is far less than that of W (19.35 g/cm3) and Cu (8.96 g/cm3). It is clear that the gap between the W-W skeleton was not completely filled with molten Cu during the infiltration-sintering process because the highest relative density is 98.4% at 1 wt% graphene loading. Nevertheless, the relative density of W70Cu30 was improved with the additive amount of graphene, was explained by the authors due to the following two main reasons; firstly, owing to the good wettability of graphene, W particle rearrangement could be promoted and accelerated to some extent and secondly, graphene has very good wettability on Cu at high temperature, which will promote greatly the ability of liquid filling the W skeleton.

Reproduced with permission from [61]

Theoretical, measured and relative density of W70Cu30/graphene composites containing different amounts of graphene.

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