Zero-valent aluminum (ZVAl), an excellent electron donor due to its chemical properties of very low redox potential, which is a potential zero-valent metal in the field of environmental engineering. However, because of its strong reducibility, ZVAl can be easily to form a dense oxide layer, passivated again when expose to oxygen or water medium even if the surface oxide film has destroyed, and react with impurities in medium to reduce utilization of electrons, which will constrain the reaction with pollutants. The studies have shown that ZVAl modified by carbon materials could not only improve the reaction efficiency of ZVAl by initiating galvanic and intergranular corrosion, strengthening mass transfer; but also endow the composite with excellent mechanical strength, overcome its own disadvantages, shield against oxygen and corrosive media, maintaining the durability of composite; moreover, the adjustable hydrophobic properties, surface charge, functional groups of carbon materials improve the specific adsorption to pollutants, the high catalytic activity enables the substrate to achieve directional transformation, which improves the electron utilization efficiency of the ZVAl system. Above all, this paper systematically summarizes the effects of carbon materials such as activated carbon, graphite, carbon nanotubes and graphene on ZVAl under different modification methods; and discusses the influence by parameters that include the ratio and type of carbon materials, the type of process control agents temperature and time of heat treatment, the geometry of ZVAl during the modified process, furthermore based on accurately control processing parameters and deeply explore underlying mechanism to realize the selective preparation of functionalized composites to broaden their application value. Through the in-depth understanding of related fields of different disciplines to promote the further application of aluminum-carbon composites in the field of environmental pollution control.
Contents
1 Introduction
2 Carbon materials
2.1 Activated carbon
2.2 Graphite
2.3 Carbon nanotubes
2.4 Graphene
3 Major modification methods
3.1 Ball milling
3.2 Chemical vapor deposition
3.3 Ultrasonic atomization process
3.4 High pressure torsion
3.5 Friction stir processing
3.6 Melt method
3.7 Spark plasma sintering
4 Enhanced reaction activity
4.1 Galvanic cells
4.2 Local corrosion
4.3 Accelerating electron transfer
5 Maintaining long-term effectiveness
5.1 Improving mechanical properties
5.2 Improving stability
6 Increased the efficiency of electronic utilization
Aluminum is the most abundant metal element in the earth's crust, which is an excellent electron donor due to its extremely low redox potential (E0=-1.662 V) and strong reduction activity. Because of its unique properties, zero-valent aluminum (ZVAl) has attracted the attention of scholars and become a potential zero-valent metal in the field of environmental engineering pollutant degradation[1,2][3⇓⇓~6].
It has been confirmed that nano-zero-valent aluminum (nZVAl) can directly reduce and degrade azo dyes and hexabromododecane (HBCD)[7][8]. Heavy metal ions, oxyanions, bromate and chlorinated organic compounds can also be removed by reduction, adsorption and coprecipitation in the pretreated and activated ZV-Al reduction system and Al/Fe bimetallic system[2,9,10][11]. The advanced oxidation system with ZVAl as the core has also attracted much attention in recent years. ZVAl can form strong oxidative free radicals or through Fenton-like reaction or activation of peroxides in oxygen-containing systems, which can oxidize and degrade phenolic organic compounds, azo dyes, heavy metal ions, chlorinated pollutants and other refractory organic compounds such as HBCD in water[12,13][14,15][16,17][18,19][8,20⇓ ~22]. In addition, the hydroxyl hydrate AlOOH formed by the aging of ZVAl in aqueous medium can also remove perchlorate by adsorption[23].
Although ZVAl has made substantial progress in the field of pollutant removal, there are still some problems: (1) The reactivity of oxidized ZVAl is low. The dense oxide film grown in situ on the aluminum surface is still the bottleneck hindering the reaction. (2) The activated ZVAl is easy to be passivated again. The newly formed ZVAl surface after membrane rupture will react with oxygen in the environmental medium again to prevent the degradation reaction from continuing. (3) The utilization rate of electrons is low. The essence of pollutant degradation by ZVAl is the process of electron transfer to the target (pollutant or oxidant) as an electron donor. In the actual reaction, the side reactions of ZVAl with H2O, H+ and OH- (formulas 1 – 3) are the main reasons for the low electron selectivity of ZVAl[24].
Activated carbon (AC) has developed pore structure, large specific surface area and abundant surface chemical groups, so it has strong adsorption capacity and is commonly used in water treatment. Jeon et al. Mixed ZVAl, AC and ammonium thiosulfate enrichment solution containing dissolved gold in a constant temperature water bath oscillator at 25 ℃, which greatly promoted the recovery of gold. More than 99% of the gold ions were reduced, and the recovered gold was mainly deposited on the AC layer on the surface of ZVAl[55]. With the assistance of AC and NaCl, Jiang et al. Mechanically milled micro-aluminum powder (mZVAl) at 300 R/min for 1 H to obtain a new carbon-modified material AC@mZVAlbm/NaCl, which realized the efficient reductive removal of completely unreactive HBCD[36].
2.2 Graphite
Graphite is an allotrope of carbon, which can be divided into natural graphite flake, expanded graphite (EG) and artificial graphite powder (Gr) according to the differences in structure and preparation methods. It is soft, lubricity, good thermal and electrical conductivity, chemical stability and thermal shock resistance, and has great application potential in the fields of materials and environment.
Graphite is often used as a modifier for ZVAl due to its excellent conductivity, thus shortening the hydrolysis induction time to obtain the maximum hydrogen generation rate. Streletskii et al. Mechanically treated mZVAl in argon atmosphere by adding 10 wt% ~ 30 wt% graphite. After the mechanochemical activation of graphite, the reactivity of aluminum with water increased significantly with the formation of aluminum carbide phase in high-speed grinding[42]. Xiao et al. Milled mZVAl with graphite and nano-metal bismuth to test the hydrogen production performance of the composite material. After scanning, the formed material showed a flaky morphology, the carbon element was uniformly distributed on the surface of the aluminum matrix, and the hydrolysis rate was significantly increased[56]. Huang et al. Prepared aluminum/graphite composites with core-shell structure by ball milling method with NaCl as grinding aid. By increasing the amount of graphite, the induction time of hydrogen evolution was shortened, the temperature required for hydrolysis reaction was reduced, and the reaction efficiency was improved[39].
Graphite is cheap, easy to get, and has more application prospects. As shown in Figure 1, some researchers tried to use graphite instead of carbon nanotubes to produce H2O2 by in-situ activation O2. After graphite and aluminum powder were mixed and activated by a ball mill, polyethylene glycol was added to the ball mill tank step by step for further grinding, and then heat treated in a tube furnace at 600 ~ 720 ℃ for 0.5 ~ 2 H.Aluminum/graphite (Al-Gr) composite with porous structure was successfully prepared, and the results showed that the material could effectively promote the production of H2O2, which could be used to degrade a variety of organic pollutants in different water matrices, and had important environmental significance[33].
In addition, EG made from natural graphite flake is a new type of functional carbon material, which not only has the properties of natural graphite, but also has good forming characteristics such as resilience and plasticity, which makes it have great application potential in electrical materials. Zhao et Al. Prepared EG by chemical intercalation method, co-milled with aluminum and acetone, and heated the obtained mixture to 550 ℃ and kept it for 30 min under argon protection. Acetone was decomposed into amorphous carbon coated on the surface of aluminum particles (forming C @ Al nanoparticles), and C @ Al nanoparticles were dispersed in EG to obtain C @ Al-EG composite[57]. The experimental results show that C @ Al-EG as a cathode material for lithium-ion batteries exhibits high capacity, excellent cycling stability and rate capability, and exhibits the best conductivity when the aluminum content is 30%.
2.3 Carbon nanotube
Carbon nanotube is a kind of one-dimensional quantum material with special structure, which is light in weight and consists of coaxial circular tubes of carbon atoms arranged in a hexagonal pattern[58]. At present, the main preparation methods are arc discharge, laser ablation and chemical vapor deposition. Due to the difference of preparation methods, the formed carbon nanotubes can be divided into single-walled and multi-walled. Single-walled carbon nanotubes have higher chemical inertness and purer surface, while multi-walled carbon nanotubes have more active surface and a large number of surface groups and amorphous carbon[59]. Carbon nanotubes have great application prospects in the fields of energy hydrogen production, pharmaceutical medicine, material science and environmental engineering because of their special specific strength, excellent electrical and chemical properties[60⇓⇓~63].
Some researchers added 5 wt% carbon nanotubes to aluminum powder, and prepared a disc-shaped porous composite material by spark plasma sintering under an applied stress of 20 MPa, which achieved rapid hydrogen production in alkaline solution.And the hydrogen generated by the hydrolysis of the carbon nanotube/aluminum composite material has high purity and does not generate any unwanted CO; and the result shows that the hydrolysis rate of the aluminum modified by the carbon nanotube is improved by 6 times, so that the carbon nanotube/aluminum composite material is a promising hydrogen source for a fuel cell[41].
The curvature of the carbon nanowall transfers the π electron density of the graphene layer from the inner surface to the outer surface, which provides a good interface site for anchoring aluminum on the outer wall of the carbon nanotube, showing good catalytic activity[25]. Tan et Al. Mixed multi-walled carbon nanotubes (MWCNT), aluminum powder and polyethylene glycol at a given mass ratio under argon atmosphere, and then rotated and milled for 4 H at a rotation speed of 400 R/min. The milled mixed powder was transferred to a tube furnace at 500 ~ 900 ℃ for heat treatment for 1 H to prepare MWCNT-Al composite, which was used to generate H2O2 in situ in the system containing O2 for the removal of glyphosate[34]. As shown in Fig. 2, Liu et Al. First synthesized MWCNT-Al by ball milling-sintering process, then mixed it with CuSO4 solution to prepare suspension, reduced it with KBH4 and freeze-dried it in vacuum, and prepared composite Al0-CNTs-Cu2O by reduction-oxidation method.The in situ catalytic formation of · OH and from O2 can degrade sulfamonomethoxine antibiotics in wastewater by Fenton-like method in a wide pH range without the addition of H2O2, which is a promising alternative technology for the degradation of toxic organic pollutants in water and wastewater[35].
Carbon nanotubes are often used in the preparation of aluminum-based reinforced materials because of their unique mechanical properties, which make up for the low stiffness and strength of light metal aluminum while maintaining the light weight and ductility of aluminum. The preparation methods of composites mainly include mechanical ball milling, friction stir processing and so on[51,64][50]. The main problems affecting the mechanical properties of the composites are as follows: to realize the uniform dispersion of carbon nanotubes in the aluminum matrix; And that wettability of the aluminum-carbon interface is improve; Keep the structure of carbon nanotubes intact. In recent years, researchers have continuously improved the preparation methods and achieved some results[47,49,65⇓ ~67].
2.4 Graphene
Graphene (GE) is a two-dimensional carbon nanomaterial with hexagonal honeycomb lattice composed of carbon atoms with sp2 hybrid orbitals. The commonly used preparation methods include mechanical exfoliation, liquid phase exfoliation and chemical vapor deposition. Different preparation methods make graphene different in structure and function, which are mainly divided into single-layer graphene (SLG), multi-layer graphene (MLG), graphene oxide (GO), reduced graphene oxide (rGO) and graphene platelets[68,69]. Its ultra-high electronic conductivity (200 000 cm2·V-1·s-1), good thermal conductivity (about 5300 W·m-1·K-1), high strength (Young's modulus 0.1 TPa, breaking strength 130 GPa), high specific surface area (theoretical specific surface area of graphene monolayer 2630 m2·g-1) and impermeability make graphene a research hotspot in photocatalysis, electricity, environmental remediation, fuel cell and anti-corrosion[70][71][26,27].
The superior electrical properties of graphene make it an attractive material in the field of hydrogen production from aluminum water. Zhang et Al. used an ultrasonic atomizer to generate a suspension mist of graphene oxide and nZVAl, which was carried by a nitrogen flow through a heated horizontal furnace to prepare aluminum nanoparticles (Al @ rGO) wrapped in graphene oxide bags, which could react rapidly with pure water under infrared irradiation, and the hydrogen production efficiency was much higher than that of pure nZVAl, without additional heat treatment or addition of acid and alkali[40]. In addition, AlOOH, the by-product of Al @ rGO and water, shows high adsorption for toxic ions and is a useful recyclable material, which realizes green hydrogen production. Xiao et Al. Explored the effect of different carbon materials on the hydrolysis performance of aluminum, and the results showed that the hydrogen production efficiency of aluminum-7. 5% nano-bismuth-2. 5% graphene nanosheets composite milled at 800 R/min for 4 H was up to 23.3 mL·s-1·g-1 at 30 ℃, and the synergistic effect of graphene nanosheets (GNSs) and metal bismuth greatly promoted the hydrolysis effect of Al[56]. Fei et al. First dispersed graphene oxide in ethylene glycol by ultrasonic method, then added Bi(NO3)3 solution, and heated at 150 ℃ for 8 H in a reaction kettle to prepare bismuth nanoparticle-modified graphene oxide composite (Bi-NPs @ GO).Bi-NPs @ GO/Al composite was formed by co-milling with mZVAl at 800 R/min for 4 H. The results show that the composite has hydrogen generation performance even at 0 ℃ and shows a high hydrogen generation rate at room temperature[37].
The thermal stability and impermeability of graphene make it a good candidate for barrier coatings in anti-corrosion coatings to achieve anti-corrosion protection of metals[26]. Liu et al. Dispersed the treated graphene oxide into absolute ethanol by ultrasonic method to prepare a suspension, and released the suspension onto the polished aluminum alloy by spin coating method.The superhydrophobic graphene film coated aluminum matrix composite was obtained by drying in a vacuum furnace for more than 12 H, and the material showed excellent mechanical wear resistance and corrosion resistance by electrochemical impedance spectroscopy[45]. Alwahib et al. treated the aluminum surface with a pulsed laser (850 mJ) before anodization, converted the graphene oxide into a reduced graphene oxide composite rGO-Ag with silver nitrate, and added it to the electrolyte solution for anodization of aluminum at 15 V. The results showed that the corrosion potential of the composite was increased to 395.4 mV, and the protection of the aluminum surface was significantly improved compared with the untreated sample[72]. Kumar et al. Used the electrodeposition method to obtain an aluminum-graphene oxide composite coating by dispersing graphene oxide in an aluminum electrolytic cell. After adding graphene oxide, the aluminum coating became more compact, and the corrosion resistance of the rGO-Ag composite coating was twice as high as that of pure aluminum[44].
Other researchers first used alkali solution to etch aluminum alloy to obtain aluminum groups with negative hydroxyl charges on the surface, and then alternately immersed in polyallyldimethylammonium chloride (PDDA) aqueous solution with positive charges and GO aqueous solution with negative charges to obtain (PDDA-GO)10 modified substrates by self-assembly method. The pH of the tin dichloride dihydrate solution was then adjusted to 10 to obtain tin hydroxide, and the SnO2 film was synthesized on the modified substrate by hydrothermal method (Fig. 3). The composite showed good corrosion resistance in a mixed acidic solution of 0.5 mol·L-1H2SO4 and 2 ppm HF[73]. In addition, some researchers have prepared polymer-graphene composite coatings by chemical vapor deposition (CVD), including two layers of single-layer graphene (SLG) sandwiched by three layers of polyvinyl butyral, which can provide complete corrosion protection for commercial aluminum alloys even after exposure to simulated seawater for 120 days[74].
Similar to carbon nanotubes, graphene has excellent mechanical properties, which makes it commonly used to prepare aluminum matrix composites with good mechanical strength and thermal properties. The commonly used synthesis technology is powder metallurgy[75⇓⇓~78].
3 Main Methods of Carbon Materials Modifying ZVAl
The difference of modification methods will change the existing state, structure and interfacial bonding strength of the two phases of aluminum and carbon, which greatly affects the function of the composite, so the appropriate modification methods will be selected according to the different uses in various fields. Among them, mechanical ball milling and vapor deposition are the most widely used modification methods. In addition, there are ultrasonic atomization, high pressure torsion, friction stir processing, melting and spark plasma sintering in powder metallurgy.
3.1 Mechanical ball milling
Mechanical ball milling is one of the most commonly used methods for modifying ZVAl with carbon materials because of its simple and controllable operation[33⇓⇓⇓~37,39,46,56,57,79]. That is to say, the carbon material is dispersed on the surface or inside of ZVAl by the pressure of the grinding medium. By adjusting the process parameters, the preparation of aluminum-carbon composite materials with different uses in various fields is realized[80⇓⇓⇓⇓~85].
In the process of mechanical milling, the effect of carbon materials on ZVAl will go through three stages: (1) simple mixing in a short time, the carbon powder will first disperse in the ZVAl powder, and a small amount of carbon powder will adhere to its surface; (2) With the increase of milling time, almost all the carbon is distributed on the surface of ZVAl; (3) If the time is prolonged, the carbon will invade into the ZVAl powder, and the cold welding will form a larger powder, and the composite will lose its reactivity[80]. In the field of environment and energy, the modification effect is generally controlled in the first two steps by adjusting the process parameters, increasing the amount of carbon, using the protective effect of the carbon layer, and retaining the fresh aluminum surface to improve the activity of the composite[33,39,56]. In the preparation of electronic devices and alloy workpieces, the mechanochemical process will continue to the third step, and the mechanical strength and excellent conductive effect of composite materials will be the priority target properties[46,80,81].
3.2 Chemical vapor deposition (CVD)
CVD is a method of producing thin films by chemical reaction on the surface of a substrate using one or more gas-phase compounds or simple substances containing thin film elements. CVD method can effectively maintain the overall structure of carbon materials and obtain good interfacial bonding with aluminum matrix. Because of its low cost, simple operation and large-scale production, it is the main method for commercial preparation of aluminum-carbon composites, which is often used to construct anti-corrosion films and conductive materials[63][80,86,87].
The graphene film synthesized in situ has a higher anti-corrosion effect on metals, while the film subjected to mechanical migration has a poor anti-corrosion effect due to surface damage and the formation of pores, and the high chemical activity of aluminum also makes the composite interface easy to contain oxides and impurities, which affects electron transfer[88]. Therefore, at present, some researchers use polymer thermal chemical vapor deposition (PP-CVD) to prepare carbon coatings with few defects on the surface of aluminum matrix at different temperatures, as shown in Figure 4, which improves the interfacial contact and improves the mechanical properties of the composite[74,89,90].
3.3 Ultrasonic atomization
Ultrasonic atomization is the use of ultrasonic waves to turn materials into fine droplets. Graphene nano-encapsulated nZVAl composite was obtained by ultrasonic atomization and pyrolysis in a horizontal furnace with the assistance of nitrogen, which avoided rapid passivation of nZVAl during reaction with water[40].
3.4 High pressure torsion method
High pressure torsion (HPT) is one of the severe plastic deformation processing methods, which is a molding process that applies a torque on the cross section of the material through active friction while applying pressure in the longitudinal direction of the material, so that the material produces axial compression and shear deformation. Some researchers used this method to compound graphite and aluminum powder for hydrogen production. At room temperature, 0.5 G of the powder mixture was consolidated into a disk with a diameter of 10 mm and a thickness of 0.9 mm by adjusting the pressure to 6 GPa. The results showed that the Al-30 wt% Bi-10 wt% Gr composite showed the highest hydrogen production rate of 270 mL·min-1·g-1[38].
3.5 Friction stir processing
Friction stir processing (FSP) is a solid-state joining and microstructure modification process. In which a rotating tool pin is inserted into and moved along the surface of the metal to be machined, the friction and plastic deformation it exerts heats and softens the workpiece, promoting mixing of the material in localized areas. FSP does not require multiple processing steps, does not rely on a precursor powder, has the effect of refining the crystal size in the aluminum matrix, and increases with the addition of carbon materials[50,93]. Some researchers have found that increasing the indentation depth and rotation speed will break the carbon nanotubes during the stirring process and destroy the carbon structure, but reduce its parameters. Because the CNTs entangled by van der Waals attraction can not be uniformly distributed in the aluminum matrix, it is necessary to further improve the dispersion of carbon materials through multiple pores to improve the mechanical properties of aluminum-carbon materials[50].
3.6 Fusion method
Nanoscale dispersion and melt processes involving squeeze casting can improve carbon material dispersion and mechanical properties of the composite, but aluminum-based systems using these processes can only disperse a limited amount of carbon material due to the low solubility of the two phases even in high temperature environments[94,95].
3.7 Spark plasma sintering
Spark plasma sintering (SPS) is a technique for densifying difficult-to-sinter materials such as CNTs by applying a specific sintering power supply and compaction pressure to the sintered powder using an energized electrode, which is an effective method for achieving good interfacial bonding between the matrix and the reinforcement[96][97]. Aluminum treated by SPS has a porous structure with large specific surface area, and its porosity can be controlled by changing the applied uniaxial stress. It is often used in the preparation of aluminum-carbon reinforcement, and some scholars have used it in the field of energy hydrogen production[98]. However, similar to the congruent method, the carbon materials that can be dispersed in this method are limited, and too much carbon materials will lead to agglomeration and insufficient interfacial bonding.
4 Enhanced reactivity
The oxide film is the primary obstacle to the ZVAl reaction, and the main mechanism is that the low conductivity limits the electron transfer. Invading active sites shields good contact with contaminants; Hard quality, high stability and high melting point (2050 ℃) aggravate the difficulty of destruction and transformation. Compactness disrupts the reaction channel. Carbon materials with high porosity and strong electrical conductivity provide a transfer medium for electrons by coating and loading ZVAl.
4.1 Galvanic effect
4.1.1 Galvanic corrosion
The huge potential difference between the carbon material and ZVAl makes the two phases spontaneously produce a large number of microscopic galvanic cells when they are in good contact. Aluminum acts as the anode and the carbon material acts as the cathode, and the electrons of ZVAl are transferred to the carbon material through the galvanic reaction. However, due to the high temperature and strong mechanical force in some preparation processes, carbon materials are easy to react with aluminum at the structural disorder, amorphous carbon layer and nano-defects with dangling carbon bonds to form Al3C4 (4),Al3C4), which is easy to form a Al0 poor area around the 4),Al3C4, resulting in a galvanic effect between the Al0 poor area (anode) and the Al3C4 (cathode), and aggravating the galvanic corrosion[99][100]. Studies have shown that the conductive graphene anticorrosive coating prepared by CVD method will aggravate the galvanic corrosion in long-term contact with the metal substrate, compared with the natural passive surface generated in situ, which will terminate the required electron transfer and hinder the continuous corrosion[101]. The galvanic current of aluminum connected with graphite is 1.5 times that of aluminum in bulk state, which improves the chemical reactivity[41]. In the experiment of hydrogen production from the reaction of ZVAl with water, the carbon material accelerates the hydrolysis reaction efficiency through galvanic corrosion effect through the electrochemical inert phase formed in the aluminum matrix[37,39⇓ -41,56,102,103].
4 Al + 3 C → Al 4C3
4.1.2 Accelerated mass transfer rate
Carbon materials affect the mass transfer rate mainly in two aspects: (1) Surface defects, which can promote the diffusion of corrosive medium[26]. Wu et al. tested the corrosion of graphene coatings with different degrees of defects on metal substrates, and found that high-density defects accelerated the mass transfer efficiency of the corrosion medium and the anodic corrosion of the metal by scanning vibrating electrode technology[104]. (2) Pore size. According to Hagen-Poiseuille law, large pore size increases the diffusion rate of reactants, corrosive media and electrons at the same time, and accelerates the corrosion of metal aluminum[33,40,41,105].
4.2 Localized corrosion
4.2.1 Intergranular corrosion
The low compatibility and potential difference between carbon phase and ZVAl lead to intergranular delamination in alloy preparation and composite application, and then cause intergranular corrosion. At the same time, the interface product Al3C4 will accelerate the fatigue crack growth rate of alloy materials in humid environment, develop into crevice corrosion in composites, and then form corrosion grooves deep into composites[106]. Moreover, the Al3C4 can lead to the generation of an anode grain boundary product Al0 poor area, and a corrosion cell of the Al0 poor area (anode) and the Al3C4 (cathode) is formed,The difference of corrosion potential between the two phases will further form intergranular corrosion, which will lead to the deterioration of the properties of the alloy and reduce the service life of the material, but also enhance the corrosion current of the composite and accelerate the electron transfer[33,100,107,108].
4.2.2 Surface Defect Corrosion
The surface defects of carbon materials are the key factors to accelerate the corrosion when they are used to modify ZVAl. As shown in Fig. 5, although the graphene film can prevent metal corrosion, the metal is significantly corroded at its cracks and defects, and the corrosive medium is easy to penetrate the defects, oxidize or corrode the underlying surface, and due to the uneven distribution of defects on its surface, the oxide on the metal base is further corroded after cracking[86,109,110]. Therefore, some researchers believe that compared with the anti-corrosion performance of graphene film, the damaged area caused by local defects will form corrosion micropores, which will extend the corrosion to the depth direction and aggravate the destructive effect of corrosion[101,111]. Therefore, at present, dense or hydrophobic polymers are mostly used in the preparation of corrosion protection materials for aluminum[112]. Or adding a densifying agent to modify graphene and adding a polymer barrier layer to make up for defects and reduce the high reactivity caused by local corrosion[45,72].
4.3 Accelerated electron transfer
4.3.1 Increase conductivity
The reactive nature of aluminum chemistry makes it difficult to improve conductivity by purification and refining. The modification of ZVAl by carbon materials with high conductivity is a feasible method to enhance the conductivity of composites, but the improvement of conductivity depends on the good interfacial contact between the two, and the poor interfacial bonding, such as the existence of voids, cracks or impurities, is the main factor leading to the decrease of conductivity[113]. The weak adsorption of metal aluminum on the carbon material not only does not affect the special electronic structure of the carbon material itself, but also the graphene film prepared by the CVD method can produce good lattice matching when in contact with pure aluminum, optimize the orientation relationship and improve the conductivity of the composite material[53,54,87,114][53]. In addition, Araujo et al. Confirmed that the formation of the aluminum-carbon interface bonding product Al3C4 will enhance the doping and coupling of metal to carbon nanotubes, increase the charge density of the composite, enhance the transfer of electrons to carbon materials, become an effective electron donor of carbon nanotubes, and promote the conductivity of the composite[115].
4.3.2 Reduced electron scattering
Alloying of aluminum is a necessary means to improve its mechanical strength in practical applications. Alloying of ordinary metals or semiconductors will produce more electron scattering centers. Perturbations (including defects) in metal crystals will aggravate the electron scattering effect by affecting the microstructure of metals[116,117]. However, the doping of carbon nanotubes or graphene with high conductivity can greatly reduce the electron scattering in the composite material, because the carbon lattice contains strong sp2 conjugated bonds, at room temperature, electrons can move extremely fast in the carbon layer without scattering[118]. In addition, if a clear and compact covalent compound Al3C4 is formed at the aluminum-carbon interface, it is also helpful to reduce electron scattering and enhance the conductivity of the composite[54,115].
5 Maintain the longevity of the material
Aluminum has low stability when used alone because of its soft nature, easy to be passivated by oxygen, and its amphoteric nature, which is not resistant to acid and alkali. Carbon materials can not only strengthen the load transfer through their excellent mechanical strength and the load transfer of aluminum-carbon interface layer, but also improve the mechanical properties of composite materials and prolong the service life of industrial devices. It can also retard the erosion of oxygen and corrosive medium, and maintain the effectiveness of aluminum-carbon composite materials under extreme conditions.
5.1 Optimize the mechanical strength of materials
5.1.1 Improved load transfer
The strengthening mechanism of aluminum-carbon reinforced materials mainly comes from the mechanical strength of carbon materials and the load transfer of mechanical stress, so maintaining the integrity of carbon structure is the prerequisite for maintaining mechanical properties. Preventing the agglomeration of carbon nanomaterials and uniformly dispersing them into the aluminum matrix can effectively refine the grains of the composite, enhance the compactness of the material, and improve the penetration depth of load transfer[47,50,67,79,92].
5.1.2 Carbide bridge ”Al4C3.
The properties of aluminum-based reinforced materials largely come from the orientation relationship between the carbon-based material and the interface transition layer, and the formation of a small amount of Al4C3 at the Al interface is beneficial to the bonding efficiency of aluminum and carbon.The load transfer and dislocation strengthening are enhanced, the shear strength of the interface is improved, the sliding deformation of the dislocation is prolonged, the thermal mismatch of the interface between the carbon material and the aluminum matrix is reduced, the compactness of the composite material is improved, and the mechanical properties of the composite material are optimized[46][91]. Some researchers believe that the formation of chemical bonds at the carbon-metal interface is of great significance to the establishment of aluminum-carbon interfacial bonding and the improvement of the mechanical properties of composites[119]. Due to the relatively low formation free energy of Al4C3, it is easy to form at high temperature, high pressure, strong mechanical force or carbon defects. Therefore, some advanced processing technologies such as mechanical milling, powder metallurgy, friction stir and so on will promote the formation of local Al3C4, improve the weak interface wettability of aluminum and carbon, and improve the load transfer effect by controlling the speed or time, adjusting the temperature or pressure and other parameters[51,64][79][50].
5.2 Increased stability
5.2.1 Retarding agglomeration
During the milling process, carbon materials can control the morphology of aluminum particles by inhibiting their cold welding, so that the composite can obtain a more stable contact surface, reduce the particle size of the composite, and form a larger specific surface area, which is conducive to the further formation of active sites[38,40,41,120]. It is found that the defects formed by graphite during ball milling make the aluminum powder more easily embedded in the carbon particles, thus slowing down the agglomeration effect of ZVAl, promoting the dispersion of ZVAl, expanding the contact between the two phases, and compounding into a more stable aluminum-carbon material[33]. In addition, the carbon-coated structure can also release the agglomeration effect caused by aluminum nanoparticles. The ultrasonic atomization method makes the graphene oxide collapse during evaporation, forming a large number of π-π stacking structures. The crumpled graphene nanopackages effectively avoid the aggregation of aluminum nanoparticles by preventing the van der Waals attraction between aluminum particles[57][40].
5.2.2 Erosion resistance
Carbon materials can cover the surface of aluminum particles to form a physical barrier to prevent the surface of aluminum from being eroded by oxygen and corrosive media[39,40]. Gr and EG, which have lubricating effect, can also form a fine protective layer on the fresh surface of ZVAl, which is well protected from oxidation at high temperature or reaction with electrolyte, resulting in more aluminum nanoparticles in an active state[57,121].
The wrapping effect of graphene prevents metal aluminum from forming a dense passive film on the surface, as shown in Fig. 6. The sp2 hybrid ultrathin hexagonal structure of graphene has an inert physical barrier effect to block corrosive substances, which not only has a strong barrier to water and other corrosive media, but also leads to slow down the diffusion of O2[40][86]. The limit of monolayer graphene is a chemically stable and conductive film, which is impermeable to gases, and the anti-corrosion film prepared by CVD method shows good corrosion resistance to aluminum substrates[122][45,74,101]. Through electrochemical impedance spectroscopy and other experiments, the researchers found that the modified graphene film, which makes up for the surface defects, significantly reduces the corrosion of metals, and the increase of the number of graphene coatings will prolong the propagation path of the corrosive medium, and the greater the impedance, the stronger the impermeability.
6 Improve the efficiency of electron utilization
The strong reduction activity of ZVAl makes it easy to react with H2O, H+ and OH- in the environmental medium after membrane breaking activation, which reduces the available electrons. ZVAl modified by carbon materials with different hydrophilic-hydrophobic properties, charges and surface functional groups enhances the specific adsorption of the target. In addition, the high catalytic activity of carbon materials also realizes the directional conversion of target substances and improves the utilization efficiency of electrons.
6.1 Construction of electron channel
6.1.1 Al/C microcell
The formation of a corrosion cell, enhances the ability of electrons to migrate from ZVAl to the catalyst or pollutant. As shown in Fig. 7, Jeon et Al. Found that the formation of Al/C galvanic cell induced synergy and promoted the cementation of gold when using ZVAl @ AC composite to recover gold, and the attached activated carbon acted as an electron channel from ZVAl to gold-thiosulfate complex, which improved the recovery of gold[55].
As shown in Fig. 8, the researchers prepared the MWCNT-Al composite by ball milling to generate H2O2 in situ, and the direct contact of the two formed a large number of micro-electrolytic phases, which provided more active sites for the reduction of O2 and accelerated the transfer of electrons from ZVAl to O2[34]. After that, Liu et al. Added Cu2O to prepare Al0-CNTs-Cu2O composites on this basis, and the generated Cu2+ was quickly reduced to Cu2O by aluminum-carbon nanotubes in the composites.The decomposition reaction of the O2 is promoted, and the free radical is efficiently formed, so that the Al0-CNTs-Cu2O composite material can be repeatedly used, the catalytic activity of the composite material is maintained, and the leaching of copper ions into the solution is reduced[35].
6.1.2 Influence reaction channel
Carbon materials can achieve effective contact with reactants by changing the specific surface area, surface morphology and pore size of the composite to affect electron transfer and reaction path. As shown in Figure 9, the researchers used graphene oxide to coat ZVAl, and the carbon material on the surface will provide a diffusion path (about 4 nm pores) for small molecular reactants and products to enter and leave the internal nanocavity during the reaction process, completing the interaction of electron transfer and improving the hydrogen generation rate[40]. Other researchers transferred the carbon material into the aluminum matrix through ball milling to form a multi-layer structure inside the aluminum particles. The interface between graphene oxide and ZVAl acts as a channel for water. During the hydrolysis reaction of aluminum, the carbon material desorbs and exposes fresh aluminum, which increases the contact with water and improves the reaction performance with water[123].
The existence of AC layer on the surface of ZVAl can realize the enrichment of HBCD, strengthen the mass transfer and promote the release and migration of electrons, and realize the efficient debromination degradation of HBCD by adsorption first and then reduction[36]. The Al-Gr composite with porous structure on the surface promotes the mass transfer of molecular oxygen from solution to the composite[33]. In addition, the hydrophobic and porous structure of carbon nanotubes modified ZVAl can also accelerate the mass transfer rate of oxygen and improve the generation efficiency of H2O2[34,35].
6.2 Hydrophilic and hydrophobic modification
ZVAl coated or loaded with carbon materials with different hydrophilic and hydrophobic properties can change the hydrophilic and hydrophobic properties of the surface of the composite material to improve its wettability in the solution system and the adsorption of pollutants, and realize the directional transfer of electrons. Graphene oxide is widely used as a carrier of nanoparticles because of its good dispersion in water due to the interaction of carboxyl and hydroxyl groups[124]. The researchers used hydrophilic graphene oxide to modify ZVAl, which increased the wettability of the composite, promoted the charge transfer between aluminum and water, and improved the efficiency of hydrogen production[37]. In addition, another way of hydrophilic modification of ZVAl by carbon materials is the generation of active substance Al4C3. Al4C3 can react with water, and the hydrolysate Al(OH)3 shows hydrophilicity due to the presence of oxygen-containing functional group — OH, which promotes the adsorption of composite materials. The related equation is shown in Equation 5[33,40].
Al4C3 + 12 H2O → 3 CH 4 + 4 Al(OH)3
As shown in fig. 10, the aromaticity of the ZVAl composite supported by the hydrophobic porous carbon nanotubes is beneficial to the ionic bond or hydrogen bond interaction with organic molecules, which promotes the degradation of sulfadiazine antibiotics[35]. In addition, in the field of corrosion protection, researchers have prepared superhydrophobic graphene coating to coat aluminum alloy, which prevents hydrogen bonding with water and reduces the transfer of corrosive media, thus improving the protection of aluminum substrate[45,88].
6.3 Catalytic activation
In the process of modifying aluminum, carbon materials produce more active sites of micro-electrolytic phase and active rich and poor electron centers to activate materials and accelerate the reaction rate, and the adsorption and doping of metal aluminum on carbon materials will make the Fermi level of carbon materials far away from the conical point, resulting in doping electrons and holes, which is conducive to the mineralization of organic pollutants[87,115,125,126]. The modification of activated carbon on ZVAl promotes the quantum tunneling effect, so that electrons can be transferred through the hydroxyl oxide layer of aluminum[55]. The researchers used the excellent electrocatalytic properties of multi-walled carbon nanotubes and multi-walled carbon nanotube-based materials to modify ZVAl to prepare composite materials, and used the two-electron pathway to catalyze O2 reduction for in-situ production of H2O2 in Fenton system[33⇓~35]. The excellent Photoelectrochemical activity of graphene can enhance the light absorption, increase the temperature of the reaction system and the current of the composite. The researchers used graphene to modify aluminum to increase the reaction temperature and enhance the efficiency of hydrogen production by absorbing infrared radiation[40].
In addition, graphene is prone to form wrinkles in preparation to reduce the surface energy due to surface defects, and the wrinkled reduced graphene oxide-coated nanoparticles or "nanopackets" can be used as nanoreactors to accelerate specific chemical reactions[127]. For example, Zhang et al. Used this strategy to design reduced graphene oxide-coated aluminum particles to promote the aluminum-water reaction[40].
7 Factors affecting the modification of ZVAl by carbon materials
7.1 Kinds of carbon materials
The properties of Al/C composites are closely related to the type of carbon materials modified by ZVAl. The developed pore structure of activated carbon enhances the mass transfer rate of the composite material and realizes the efficient adsorption-reduction process of the target substance[36,55]; The soft and porous graphite also has good catalytic performance, which can not only improve the hydrogen production rate of molten aluminum, but also enhance the H2O2 of the composite material on the two-electron reduction of O2, and the expanded graphite formed by processing can be used as an electrode material because of its good plasticity and resilience, thus improving the cycle stability of the electrode[39,42,56][33][57]. Multi-walled carbon nanotubes (MWNTs), which are relatively inexpensive, are widely used because of the presence of a large amount of amorphous carbon on the surface, which can form a good Al-C interface[25,59]. Its high electrical conductivity accelerates the hydrogen production rate of aluminum, its high mechanical strength enhances the mechanical properties of the composite material, and its high catalytic performance promotes the in-situ production of H2O2[41][47,49,65⇓ ~67][25,34]. Graphene has the advantages of high conductivity and high specific surface area of carbon materials, and has a wide variety of properties[68⇓⇓~71]. Hydrophilic GO, with more oxygen-containing functional groups on the surface, provides more active sites for ZVAl, improves the stability of the material, enhances its wettability with water, and accelerates the rate of hydrogen production from aluminum water[37,56]. Hydrophobic rGO has few surface defects, strong impermeability and corrosion resistance, and is mostly used to prepare anticorrosive coatings[44,72]. Multilayer graphene is relatively inexpensive and has high mechanical strength, which can optimize the mechanical properties of composites[75⇓⇓~78].
7.2 Proportion of carbon materials
The proportion of carbon materials is affected by the preparation technology, which is an important parameter to obtain ideal composites. In the field of energy hydrogen production, increasing the mass ratio of carbon materials to aluminum can reduce the induction time and temperature required for the hydrolysis reaction and improve the reaction efficiency[37,39]. The addition of carbon materials can increase the specific surface area of the composite and improve the hydrolysis rate. However, some researchers have found that the addition of excessive carbon materials will cover the surface of ZVAl particles and hinder the hydrolysis reaction[56].
In the preparation process of the reinforced material, the carbon material has the function of relieving the cold welding of ZVAl. With the increase of carbon material content (1 wt% ~ 7 wt%), the particle size of the composites obtained by CNT ball milling ZVAl becomes smaller. CNT has similar properties to graphite, but its chemical properties are more stable, so it is considered to be a good process control agent. Increasing the content of CNT prevents the agglomeration of the composites, slows down the shear and damage of CNT during high-energy ball milling, and improves the mechanical properties of the composites[90].
In the aerobic system, the higher the proportion of carbon materials in the ZVAl and multi-walled carbon nanotube composites, the higher the catalytic performance and the higher H2O2, but there is an optimal C/Al ratio, because excessive aluminum powder will cover the surface of MWCNT and block the surface pores of carbon materials, resulting in the reduction of the effective surface area of MWCNT, which hinders the transfer of O2[34].
7.3 Process control agent
The incompatibility of the surface properties of carbon materials and ZVAl leads to the poor interface bonding and uneven dispersion of the two, which reduces the performance of the composite. The process control agent can not only alleviate the above problems, but also control the cold welding effect of ZVAl to improve the performance of the composite[67,128]. Common process control agents fall into two categories, polyols and salts.
Polyols are mainly used as adhesives and dispersants for Al/C composites. Commonly used polyols are ethanol, polyethylene glycol, polyvinyl alcohol, etc[98]. The researchers used hydrophilic polyvinyl alcohol membrane to modify the surface of aluminum powder, which made the surface properties of aluminum more compatible with functionalized carbon nanotubes, enhanced the strong hydrogen bonding interaction between the two phases, and made the adsorption uniformity of carbon materials much better than that of untreated aluminum powder.This prevents them from migrating with water during the subsequent rinsing and drying process, and the uniform dispersion of aluminum on the surface of carbon nanotubes can be well maintained even after the polyvinyl alcohol film is completely removed by pyrolysis[67,79]. The polyethylene glycol is connected to the aluminum powder through the reaction between one end of the polyethylene glycol and the aluminum, and the other end of the polyethylene glycol is connected to the graphite particles with a plurality of active sites due to the pi-pi conjugate adsorption effect, so that the wetting effect between the aluminum powder and the graphite particles is increased; and moreover, the polyethylene glycol can be decomposed into gas in the sintering process, and the gas is separated out to form a vacuum atmosphere,The penetration of aluminum into graphite is promoted, which further promotes the welding interface between aluminum and graphite. Moreover, the addition of a large amount of polyethylene glycol will produce excessive amorphous carbon, which promotes the bridging of ZVAl and Gr, not only improves the galvanic corrosion effect, but also promotes the mass transfer of oxygen from the solution to the surface of the composite[90][33].
Salts with sharp edges can be used as both a dispersant and a cutter to destroy the oxide film. When the salt phase dissolves in water, more fresh aluminum surface is exposed.The grinding aid can control the cold welding of aluminum powder by reducing the particle size of ZVAl and the generated defects in the grinding process, so that the aluminum and the carbon material can better interact with each other and the electron transfer can be improved. NaCl is a common grinding aid[34,39].
In addition, sodium dodecyl sulfate surfactant can improve the aggregation property of carbon materials by slowing down the van der Waals force of carbon materials themselves, and promote the dispersion effect of carbon materials in aluminum matrix[57,129,130].
7.4 Temperature and time of heat treatment
Heat treatment can change the surface morphology and the wettability of the two phases of the composite. At high temperature, the stability of carbon materials will change, and the surface will produce more reactive amorphous carbon and surface defects, where ZVAl is easy to react and form interfacial products, which improves the interfacial connectivity of the two phases[49,131,132]. However, some researchers have found that multi-walled carbon nanotubes have strong reactivity with oxygen at high temperatures above 800 K, and carbon is gasified into carbon oxides.Because the walls of multi-walled carbon nanotubes are connected by van der Waals force, the structure of multi-walled carbon nanotubes is inevitably damaged during exposure to oxygen or active elements at high temperature, which deteriorates the reinforcement effect of carbon materials[48].
The extension of heat treatment time will promote the formation of Al4C3, form good chemical bonds, and promote the diffusion of atoms. When the researchers prepared the Al0/Gr composite, they found that the longer sintering time made the aggregation and growth of Al0, which increased the diffusion of Al0 from grain to grain boundary (according to Scheler's formula, the particle size of Al0 in the composite was calculated to increase from 22.9 nm to 99.2 nm), thus reducing the Al0 depleted area in the grain boundary and weakening the intergranular corrosion[33].
7.5 Geometry of ZVAl.
ZVAl can be divided into nZVAl and mZVAl according to the particle size, and can be divided into spherical and flaky according to the shape. It is found that nZVAl has better wettability with carbon materials and can produce better interface phase. Flake aluminum powder has better geometric compatibility with carbon materials, and its larger specific surface area can provide more adsorption sites for carbon materials. In the ball milling process, the aluminum particles are extruded by the grinding balls to form a large number of flaky particles to increase the contact area of the aluminum-carbon phase and realize the effective combination of the two phases[56,67].
8 Conclusion and prospect
The high reduction activity of ZVAl will form a compact oxide film on its surface to inhibit the reaction activity, and even after acid and alkali activation, it will be passivated again to hinder the utilization of ZVAl, and the activated ZVAl will continue to react with H+ and OH- in the system, which will reduce the electron utilization efficiency of the non-hydrolytic reaction system. Through the modification of carbon materials, the above disadvantages can be overcome from three aspects: improving the conductivity and enhancing the reaction activity, maintaining the long-term effectiveness of the anti-oxidation and anti-corrosion medium, and constructing the electron channel to improve the utilization rate. At present, carbon materials modified ZVAl composites are mainly used in the production of reinforcement, energy hydrogen production and electronic devices, and there are few studies in the field of environmental engineering degradation of pollutants. As the modification of zero-valent metals by carbon materials has become a research hotspot in recent years, the modification of ZVAl by carbon materials has broad prospects for development, and the following problems need to be further studied[33,133⇓⇓ ~136].
(1) The application of carbon material modified ZVAl in the environmental field is further expanded.
In recent years, carbon materials have been used to modify ZVAl, mostly to form a Al/C/O2 oxidation system to degrade pollutants under oxygenated conditions, and the auxiliary oxidant often added is H2O2, which produces oxidizing hydroxyl to construct a Fenton-like system. However, the excellent catalytic utility of carbon materials, which can generate different kinds of free radicals alone or with different oxidants, is applicable to a wider range of pollutants[137]. Therefore, how to construct an efficient Al/C oxidation system is worth further exploring.
In addition, there is a lack of Al/C reduction system to remove pollutants at this stage, which is mostly concentrated in the field of energy hydrogen production. However, both the free electrons and holes produced by the doping of carbon materials by ZVAl and the specific adsorption of carbon materials with different hydrophilic and hydrophobic properties on the target create the possibility of directional electron migration and reserve a direct reaction channel for the degradation of pollutants. Therefore, the efficient utilization of electrons by modifying ZVAl with different carbon materials is worthy of further study.
(2) Control the process parameters in the preparation of Al/C composites
The properties of Al/C composites are affected by the preparation method and process parameters. At present, the preparation of reinforcement is mostly studied. Researchers study the preparation technology of Al/C composites, control the preparation process, and add various process control agents to achieve reinforced materials with good dispersion, high interfacial bonding and excellent mechanical properties. In contrast, the preparation methods of Al/C composites in the field of environmental pollution are relatively single, but the research ideas of the two are the same, maintaining a good carbon structure can give full play to its own performance advantages, and a good direct contact between the two phases can maintain the stability of the material and enhance electron transfer. Therefore, the excellent performance of carbon materials and ZVAl needs more advanced technical advantages and more precise process control to be further developed. How to realize the coating effect of carbon materials on ZVAl and reduce side reactions? How to select the type of carbon materials, the geometric shape of ZVAl and additives to increase the compatibility; How to control the temperature and time, adjust the material structure, and how to realize the directional preparation of functional composites by pretreatment or post-processing need further study.
(3) In-depth study of the mechanism of carbon materials on ZVAl
At present, the action mechanism of carbon materials on ZVAl in the fields of energy hydrogen production, metal protection, metallurgical materials and so on has its own disciplinary limitations, such as only considering the enhancement of reaction activity, only protecting aluminum-based materials from any corrosive medium damage, and only improving the mechanical strength of composite materials. However, the comprehensive mechanism of carbon materials in the modification of ZVAl degradation system is rarely involved, such as the effect of modification on the formation of ZVAl oxide film and hydroxide film; Mechanism of action on the selection and degradation of pollutants by composite materials; The release of ZVAl electrons and the effectiveness of the reaction pathway have not yet been explained, and are worthy of further study.
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