Zero-valent aluminum(ZVAl) possesses physical properties of good ductility and light weight, and has chemical properties of more negative redox potential. In the preparation process of new lightweight and high-strength composite materials, ZVAl has been taken preferentially as an ideal metal matrix. On the other hand, ZVAl, an excellent electronic donor, has been used for rapid hydrogen evolution and efficient removal of pollutants in the field of hydrogen production and environment remediation. Mechanical ball milling, is a novel material processing method which is simply operated and easy engineering. Furthermore, it can overcome the drawbacks of ununiform mixing and poor interface combination in the traditional preparation methods of aluminum-based metal materials. Moreover, it can destroy the dense oxide film on the surface of ZVAl, and break through the bottleneck that limits the electronic release of ZVAl. By ball milling ZVAl and grinding aids to control the grinding strength, the uniform dispersion and good interface bonding of the composites can be achieved,and a small amount of intermetallic compounds and chemically active substances are generated. As a result, the composites with excellent material properties can be obtained.Besides, the quantity of hydrogen production and the efficiency of pollutants removal could be increased through the function of the “cutter”, the mechanical chemical reaction such as displacement, carbonization or dechlorination, the change of microstructure during mechanical ball milling, and the pitting effect, the primary battery effect, the reduction of side reactions in the aqueous medium and other action mechanisms. Above all, this paper summarizes the principle of using mechanical ball milling to enhance the mechanical properties of ZVAl-based composite materials. In addition, the surface modification of ZVAl by mechanical ball milling and the mechanism of its application in hydrogen production and pollutant removal are systematically summarized. Afterward, the influence of ball milling parameters and water chemical conditions on the system are discussed. Finally, prospects of the research areas meriting for further investigation are pointed out. Supposing to promote the further development of mechanically modified ZVAl in the field of environment remediation through in-depth understanding of related fields of different disciplines.
Contents
1 Introduction
2 Enhanced mechanical properties of materials
2.1 Controlling grinding intensity
2.2 Intermetallic reaction
2.3 Formation of chemically active substances
2.4 Achievement of uniform dispersion of materials
Aluminum is the most abundant metallic element in the earth's crust, with good ductility, light weight (ρ=2.70 g/cm3) and other physical properties, amphoteric nature, extremely low redox potential (E0=-1.662 V) and higher thermodynamic driving force for electron transfer.It is an excellent electron donor (one atom produces three electrons) and other chemical characteristics, which has been studied by more and more scholars for the removal of pollutants in recent years[1,2][3,4].
It has been proved that ZVAl can directly reduce and degrade azo dyes into small molecular substances which are easy to be degraded by microorganisms[5]. Other auxiliary chemical substances or pretreatment such as pickling and the like can also be added to promote the direct reduction of the ZVAl to remove heavy metals and bromate[2,6,7][8,9]; In the bimetallic system constructed by Al and the second metal, it can be used to reduce and degrade chlorinated organic pollutants, and the potential difference between metals can be used to enhance the dechlorination reaction[10⇓~12]. In addition, the oxidation system based on ZVAl is also under development. In the ZVAl/O2/H+ system, Fenton-like reaction occurs to produce hydroxyl radical with strong oxidizing ability, which can effectively degrade phenolic organic pollutants and azo dyes in water, or add physical and chemical auxiliary means to promote the removal of heavy metals and chlorinated pollutants in water[13⇓~15][16,17][18,19]. In ZVAl/peroxide system, peroxide can be activated by ZVAl to produce hydroxyl radical or , which can oxidize and remove refractory organic pollutants in water[20,21].
The surface of ZVAl is covered with a dense alumina film, which will hinder the release of electrons on the surface of ZVAl, thus inhibiting the removal of pollutants by ZVAl. Mechanical ball milling, as a simple and easy engineering method, can destroy the oxide film on the surface of ZVAl by high energy grinding[22⇓~24]. The mechanical ball milling method was proposed by Benjamin in the study of oxide dispersion strengthened nickel-based superalloys, and the mechanical ball milling technology has attracted wide attention[25]. Mechanical ball milling technology has the advantages of low cost, simple equipment, short production cycle and suitable for operation at room temperature and pressure, which can be used to prepare functional materials, functional films and a variety of nanomaterials[26].
In the past two years, some scholars have studied the removal of pollutants by mechanical ball milling modified ZVAl, but the practice is less, and the mechanism is not clear[23,24,27]. In fact, the modification of ZVAl by mechanical milling has long been concerned in the field of materials and hydrogen production, and many scholars are still carrying out research. The modification mechanism has important reference significance for the development of new environmental functional materials[28⇓~30][31⇓⇓⇓~35][36⇓⇓~39]. Therefore, it is urgent to systematically summarize the mechanism of ZVAl modified by mechanical milling in the field of materials, hydrogen production and environment.
How to prepare ideal aluminum matrix composites by mechanical ball milling technology? How to modify the surface of ZVAl by mechanical ball milling? What is the effect mechanism of ZVAl surface in the process of mechanical milling and other milling parameters such as material type, milling time, milling speed and milling atmosphere? What is the mechanism of ZVAl modified by mechanical ball milling for hydrogen production and pollutant removal? What are the effects of pH, temperature and reaction by-products on water chemistry? At present, there is no systematic summary by scholars, so based on these problems, it is particularly important to review the surface modification and mechanism of mechanical milling technology for ZVAl.
Therefore, a systematic summary based on the above problems will be carried out for the first time in this paper: (1) ZVAl is milled with grinding AIDS by mechanical ball milling technology to obtain composites with excellent material properties.By controlling the grinding intensity, the uniform dispersion and good interface combination of the materials are realized, and a small amount of intermetallic compounds and chemically active substances are generated. (2) When the surface of ZVAl is modified by mechanical ball milling and applied to the field of hydrogen production and environment, the "cutter" effect in the process of mechanical ball milling, the mechanochemical reactions such as replacement, carbonization or dechlorination, the change of microstructure, and the internal effects such as pitting corrosion in aqueous medium, galvanic effect and the reduction of side reactions; (3) The effect of ball milling parameters (ball milling time, ball milling speed, the proportion of grinding AIDS, dopants and ball milling atmosphere) on the surface effect of ZVAl; (4) The influence of water chemical parameters (pH, temperature, reaction by-products) on hydrogen production and pollutant removal in aqueous media, and the future development direction of mechanical milling technology based on zero-valent aluminum was prospected, in order to promote more applications of mechanical milling ZVAl in the field of environment and play a greater application potential of mechanical milling modified ZVAl.
2 Mechanical properties of reinforcement
ZVAl has good material characteristics such as ductility and light weight, but in the process of adding reinforcements to prepare metal matrix composites, it is difficult to solve the problems of uneven mixing and poor interface bonding, and the ideal metal matrix composites can not be obtained[29]. Can be ball-milled together with a grinding aid to obtain a compound with excellent mechanical property and friction property, thereby being applied to the fields of aerospace, automobile and the like.
2.1 Control of grinding intensity
In the Process of ball milling ZVAl with grinding AIDS to prepare new materials, the most basic role of grinding AIDS is to control the grinding intensity as Process control agents (PCA) to prevent the highly ductile ZVAl from forming spherical particles due to excessive cold welding and self-sealing.
High energy ball milling was performed in a planetary ball mill using ZVAl powder and stearic acid, which was mixed with the ZVAl powder prior to milling, and stearic acid was used as the PCA[40]. The deformation during ball milling can introduce lattice defects such as grain boundaries and dislocations. The existence of defects leads to the broadening of X-ray diffraction (XRD) peaks. The higher amount of crystal defects in the lattice leads to wider peaks[41]. In addition, hexane and polyethylene glycol are also used as process control agents in ball milling technology[42][43]. The reactivity of aluminum powder depends on the specific surface area and microstructural changes. Aluminum powder can be milled with high energy to obtain highly reactive nanopowders. The addition of paraffin oil minimizes cold welding and improves grinding efficiency[44].
Three kinds of composite aluminum-metal oxide powders (Al·MoO3, Al·Bi2O3 and Al · CuO) and chemically inert compositions (Al · MgO and Al·Al2O3),Dupiano) were prepared by mechanical milling to generate hydrogen in the aluminum-water splitting reaction[42]. The results show that the reaction rate is affected by the specific composition. The reaction rate of the :Al·Bi2O3 composite is significantly faster than that of the other materials, while for the Al·MoO3 composite, the interruption of hydrogen generation can be observed at constant temperature due to the formation of MoO2.4(OH)0.6 (hydrated MoO3 phase). Nanocomposites of aluminothermic system Al-CuO were prepared by milling. The CuO was almost uniformly mixed into the aluminum and very few voids were observed, indicating that the density of the nanocomposites was close to its theoretical maximum density, and the shape and size of the samples prepared under different milling conditions did not change significantly[45]. Similarly, the synthesis of Al-MoO3 nanocomposites by using Al and MoO3 powders as starting materials can be used to prevent reactive milling, by using different milling media and different amounts of PCA to control the milling intensity, the crystallite size decreases as the milling intensity increases, but at the same time the increase in milling intensity causes side reactions between the starting materials[46].
2.2 Intermetallic reaction occur
ZVAl and other metals can greatly improve the mechanical properties of materials, and can be used in automotive and aerospace fields. Detecting the energy released by the combustion of metal particles is an active area of research. Zirconium (Zr) is one of the hottest combustion metal fuels with an adiabatic flame temperature of 4930 K. ZVAl is soft and ductile while Zr is hard and brittle. The use of high energy ball mill can take advantage of the difference in mechanical properties to better mix ZVAl and Zr, thus making full use of the intermetallic reaction between ZVAl and Zr and reducing the ignition threshold[47].
In addition, a series of ternary activated compounds containing Al-Sn-X were prepared by high-energy ball milling, and a small amount of intermetallic compounds were also produced. The AlSn phase was observed in the ternary Al-Bi-Sn composite, suggesting that the presence of Bi may have an effect on the behavior of the Al-Sn interparticle during mechanochemical processing (Fig. 1)[48]. The intermetallic compounds generated In the Al-Sn-In ternary composite include AlSn, Sn4In, or SnIn3[49].
2.3 Formation of chemically active substances
In the process of mechanical milling with ZVAl and carbon materials, chemically active species are formed on the surface of the particles. Streletskii et al. mechanically treat aluminum with 10 wt% -30 wt% graphite in a high energy vibratory mill to form a chemically active species,Dispersion of the components and accumulation of defects in the ZVAl and carbon materials occur, and at the same time, a carbon layer covers the surface of the aluminum nanoparticles, and then aluminum chemically reacts with carbon at the particle surface and forms aluminum carbide in the particle body[30].
2.4 Achieve uniform dispersion of material
Mechanical milling is an effective method to disperse the reinforcing material in the metal matrix. The mechanical milling technology is used to mix the ZVAl and the reinforcing material together, which mainly solves the following three problems: uniform dispersion of the reinforcing material on the surface of the ZVAl; Good interface bonding between ZVAl and reinforcement; Excellent mechanical and friction properties. Ball milling can also be combined with other techniques to obtain more desirable composites.
2.4.1 Separate ball milling method
In the process of preparing aluminum matrix composites with high hardness and high toughness, carbon materials are the main reinforcement materials, and graphene and carbon nanotubes are the most commonly used carbon materials in the preparation of ZVAl/carbon composites. Generally, there are three dispersion stages in the process of ball milling. In the initial stage of ball milling, the carbon material is first dispersed among the ZVAl powders, and only a small amount of carbon material is attached to the surface of the ZVAl powders. With the increase of milling time, almost all carbon materials were dispersed on the surface of ZVAl powders. If the ball milling time is further increased at this time, the carbon material will be enclosed and trapped in the cold welding particles.
2.4.1.1 Graphite
The metal graphite (MG) synthesized by mechanical grinding is repeatedly flattened, cold welded, broken and re-welded by the impact force of the grinding medium. The ductility of the metal makes welding and the formation of larger agglomerates possible. Further deformation leads to work hardening and mass fracture through fatigue failure mechanisms[28].
2.4.1.2 Graphene
Graphene nanosheets (GNS) undergo a series of evolutions during mechanical milling. As shown in fig. 2, for the GNS, they are first dispersed between the ZVAl powders, and only a small amount of GNS adheres to the surface of the ZVAl powders; With the increase of milling time, almost all GNS was distributed on the surface of ZVAl powder. When the milling time is increased again, the aluminum flakes tend to cold weld together to form a larger powder, while the GNS is immersed in the ZVAl powder. At a longer milling time, the cold weld powder was pressed again into flaky particles, and the GNS was almost embedded in the ZVAl powder[50].
2.4.1.3 Carbon nanotube
Aluminum matrix composites reinforced with carbon nanotubes (CNTs) (vapor-grown carbon fibers (VGCFs) and multi-walled carbon nanotubes (MWCNTs)) can be prepared by ball milling (aluminum powder: average diameter 30.mu.m) and then heat-sealing the extruded powder encapsulated in an aluminum container[51]. CNTs are intended to improve the mechanical properties and thermal conductivity of aluminum composites formed by powder metallurgy. Mechanical milling is an effective method to disperse CNTs in the metal matrix. Generally, there are three stages of dispersion during ball milling: first, CNTs are dispersed in the spherical aluminum powder; Secondly, CNTs are dispersed on the surface of ZVAl induced by the micro-forging mechanism. Finally, the CNTs are trapped in the powder of the layered structure. The deformation of aluminum powder during ball milling includes three stages: flattening into flakes, cold welding into lamellar particles, and then splitting into smaller particles; While the CNTs are first dispersed on the surface of the aluminum sheet and then enclosed by the aluminum and trapped in the cold solder particles[50,52].
MWCNTs have high mechanical properties, but aggregation is caused by Van der Waals force. NiAl has a high temperature resistant structure and can be used in the aerospace field. Ni and Al powders were milled in a planetary ball mill, and then MWCNTs were dispersed in the milled powder.The results showed that MWCNTs were still aggregated, the ball milling could not overcome the strong van der Waals forces in the nanotubes, and the consolidated NiAl microstructure had pores, and MWCNTs could be observed in the pores of the composite[36].
Some scholars also used three different raw materials (ZVAl, ZVAl/CNT, powder (pre-milled and then milled with CNT)) to prepare nanocomposites at different milling time and rotation speed. After milling, the particle morphology did not change significantly, that is, the addition of CNT did not affect the particle morphology at low rotation speed, but the particle size of ZVAl increased slightly with the increase of rotation speed. FESEM shows that CNTs initially adhere to and disperse on the surface of the ZVAl powder, and increasing the rotation speed makes CNTs well dispersed on the surface of the ZVAl powder (Fig. 3A, B), while the ZVAlbm/CNT (Fig. 3C, d) is more well dispersed and contains a large number of CNTs and has the highest hardness[37].
The sheet powder metallurgy technique proposed by variable speed ball milling (SSBM), combined with the mechanisms of low and high speed ball milling (LSBM and HSBM) applied to CNTs, can enhance the toughness and ductility of aluminum composites. A longer initial LSBM is performed first to obtain CNT/ZVAl flakes with uniform dispersion, followed by a shorter HSBM to obtain cold solder particles with good CNT/ZVAl bonding without too much structural damage to the CNTs. In addition, due to the uniform dispersion of CNTs and the balance between interfacial bonding and structural integrity, the SSBM CNT/ZVAl composite doubles or triples the ductility of LSBM and HSBM CNT/ZVAl, while their strengths are nearly similar[52].
2.4.2 Chemical method + ball milling method
The uniform distribution of carbon materials on the surface of ZVAl can also be achieved by the combination of chemical method and ball milling method.
ZVAl was first coated on graphene using an organic aluminum chemical reduction, and then the aluminum-coated graphene and AlSi10Mg powder were ball-milled in vacuum using a ball mill. In the ZVAl-coated graphene, the graphene maintained its irregular curl and wrinkle, indicating that mechanical stirring did not affect the structure of graphene, and most of the ZVAl particles were uniformly coated on the graphene. ZVAl coated graphene is uniformly dispersed in AlSi10Mg powder after Al coated graphene and AlSi10Mg powder are ball milled in vacuum[53].
CNT-ZVAl composite powders were prepared by polymer thermal chemical vapor deposition (PP-CVD) at different temperatures using polyethylene glycol (PEG) as the carbon source, cobalt nitrate as the catalyst precursor, and fine aluminum powder as the starting material, and then were milled with aluminum powder by high-energy ball milling to obtain several CNT-ZVAl composite powders with different CNT contents (Figure 4).
Although the structure of CNTs is broken by ball milling to some extent, it increases the interfacial reaction between CNTs and ZVAl particles, and the mixing uniformity of CNT-ZVAl composite powders obtained by this method is significantly improved, and the ball milling time is also significantly reduced, resulting in less damage to the structure of CNTs[39]. In addition, the distribution of MWCNTs can be improved in a short time without any structural damage by pre-mixing (ultrasonic treatment, magnetic stirring and mechanical stirring) followed by high-energy ball milling, and the uniform distribution of MWCNTs is achieved[54].
Bismuth nanoparticles (Bi-NPs) modified graphene oxide composites (Bi-NPs @ GO) were prepared by a hydrothermal method, and then were ball-milled with ZVAl to obtain Bi-NP @ GO/Al and Bi- NPs/Al. For Bi-NPs @ GO (fig. 5), nano-Bi particles grow on GO with relatively uniform particle size; Bi-NPs are smooth and regular spherical, with large particle size and agglomeration. With the addition of ZVAl milling, the particles became irregular and the particle size increased (fig. 6)[55].
2.4.3 Ball milling method + other methods
In order to obtain the ideal composite materials, scholars have carried out pre-dispersion before ball milling and hot extrusion, compaction and sintering of the composite powder after ball milling. P P Érez-Bustamante et al. cold-consolidated and subsequently sintered a milled powder of graphene and aluminum powder and showed that GNP was also uniformly dispersed in the aluminum matrix[56]. High energy ball milling was used to study the effect of carbon nanotubes (CNTs) and graphene nanosheets (GNS) on the properties of Al-Mg alloys.The results show that CNTs and GNS are uniformly embedded and distributed in the composite powders without aggregation.The particle size of magnesium powders changes little before and after ball milling.The main purpose of ball milling is to disperse the reinforcing agent uniformly[57].
Aluminum matrix composites (AMC) are widely used in aerospace. Silicon carbide (SiC) is the most common reinforcement used in AMC. SiC can be used to enhance mechanical properties and ensure ductility at the same time, but it is usually compacted and sintered after ball milling to obtain stronger mechanical properties. The main problem to be solved is how to disperse SiC uniformly in Al matrix. Generally, it is uniformly dispersed in the organic solvent by ultrasonic or magnetic stirring, and then ball milled[58⇓~60].
3 Improving the efficiency of chemical reaction
ZVAl has excellent reduction performance, but the dense oxide film on the surface will hinder the release of electrons, so the surface of ZVAl can be modified by mechanical ball milling, which can be used in the field of hydrogen production and environment to improve the hydrogen production and the removal rate of pollutants. The typical cases of mechanical ball milling surface modification based on ZVAl in aqueous medium are summarized in Table 1.
3.1 "Cutter" action
When ZVAl is ball milled with chloride, the chloride particles have sharp edges and are harder than the ZVAl powder, acting as cutters during the milling process. The presence of the chloride additive creates microstructural defects and reduces the particle size thereby increasing the surface area to improve the efficiency of the milling process[61]. During ball milling, the chloride is driven into the ZVAl matrix and destroys the surface oxide layer, which activates the powder and prevents cold welding of the aluminum powder during ball milling due to repeated cracking and cold welding (resulting in defects),It also provides a fresh surface, a smaller particle size and a deformed microstructure for the reaction of aluminum with water, thus accelerating the reaction rate of aluminum with water[62].
Comparing the hydrogen production from the reaction of ZVAl with water after ball milling with chloride, KCl is significantly better than NaCl as a grinding aid, and the addition of 50 wt% KCl to ZVAl is even better than the addition of 75 wt% NaCl, and the milling time required to activate ZVAl is also shorter, which saves energy and reduces the final cost.The reason may be that KCl has a higher hardness compared to NaCl (KCl: 2.5, NaCl: 2, Mohs scale), which means that KCl is more brittle and breaks into smaller segments by impact, which can produce finer particles after grinding and thus higher hydrogen generation rates[35].
In addition, Liu et Al. Found that LiCl, NiCl2 and MgCl2 can also be used as grinding AIDS to react with water to produce hydrogen after high-energy ball milling with ZVAl. Among them, NiCl2 shows the best performance and can be used as a cutter in the grinding process, which helps to reduce the particle size of ZVAl and produce defects in Al particles[63].
3.2 Mechanochemical reaction takes place
3.2.1 Displacement reaction
ZVAl reacts with metal compounds during ball milling or hydrolysis to form a microbattery. The NiCl2 is helpful to prevent the cold welding and agglomeration of ZVAl particles during ZVAl-NiCl2 milling. Secondly, metal Ni (formula (1), (2)) will be produced during the formation of Al3Ni2, and the Al/Ni micro-galvanic effect will aggravate the corrosion of Al. Also, the dissolution of NiCl2 in water produces Cl-, which can damage dense hydroxides through the pit corrosion process[33].
2Al+3NiCl2→3Ni+2AlCl3
3Al+2Ni→Al3Ni2
In that hydrolysis of the Al/NaBH4, the Ni2B,Ni2B particle generated dure the hydrolysis of the NiCl2 are deposited on the surface of the ZVAl powder and act as a cathode of a microbattery to stimulate the hydrolysis of ZVAl (formula (3));Ni2B catalyzes the NaBH4 hydrolysis,Hydrolysis of the NaBH4 is initiated (formula (4))[64].
In addition, the Bi2O2CO3 can prevent cold welding of the aluminum powder at the time of ball milling of the ZVAl with the Bi2O2CO3, the Bi2O2CO3 disappears after ball milling, and Bi and Al2O3 are formed (Formulas (5), (6))[65].
3.2.2 Carbonization reaction
ZVAl was ball milled with organic compounds to form Al/C galvanic cells. When ZVAl and oxide are milled, the mixed powder can be mixed with high-purity ethanol solution and then milled with high energy[66]. Ethanol, as a process control agent, is partially carbonized by mechanochemistry and remains in the ZVAl particles, while a large number of microscopic galvanic cells can be spontaneously generated between the ZVAl (anode) and the carbon (cathode), thereby enhancing the ability of electrons to migrate from the ZVA1 to the catalyst or contaminant[22].
3.2.3 Dechlorination reaction
Mechanochemical treatment (MCT) is a promising method to reduce environmental hazards, especially in the field of persistent organic pollutants (POPs) disposal. Rapid dechlorination of chlorinated pollutants can be achieved by mechanochemical reaction through mechanical ball milling of ZVAl, oxidant and pollutants.
In the mechanochemical process, when HCB was milled together with ZVAl and powders, the mechanochemical and photochemical dechlorination mechanisms of HCB were different, and the intermediate products were also different. The former was based on chlorobenzene oxygen radical (CB-O ·), while the latter was based on chlorobenzene radical (CB ·)[67].
When ZVAl and quartz sand (SiO2) are used as additives for co-milling with DP, the mechanochemical destruction of DP can be promoted[68]. The destruction of hydrocarbon skeleton after sequential dechlorination should be the main degradation pathway of DP with low mass ratio, while the mechanically induced thermit reaction can destroy DP with high mass ratio more effectively. The final products of C and Cl in DP are mainly graphite, amorphous carbon, AlCl3·6H2O and some compounds polymerized with Si — Cl bonds, while the gaseous products are mainly composed of CH4, C2H6, C2H4 and C3H8[69].
Municipal Solid Waste Incineration (MSWI) fly ash (FA) is classified as hazardous waste and contains pollutants such as heavy metals and dioxins (PCDD). CaO-ZVAl can promote the degradation of PCDD to improve the degradation of dioxins and furans, and dechlorination is the main degradation mechanism[70].
3.3 Change the microstructure
3.3.1 Dry Ball Milling
When ZVAl is ball-milled with oxidizer-type grinding AIDS, it can be activated by grinding, resulting in further phase transformation or crystal structure change.
In the hydrogen generation experiments of ZVAl powders modified by different oxides, different modifiers have different effects on the reaction kinetics of ZVAl with water. Because of the different induction time at the beginning of the reaction,The hydrogen generation time of ZVAl powder modified by γ-Al2O3 is significantly shorter than that of ZVAl powder modified by α-Al2O3 and TiO2. The phase transformation of the surface layer of the ZVAl phase particles and the resulting weakening during processing are strongly dependent on the nucleation energy barrier on the modified oxide particles. Due to the low nucleation energy barrier of γ-Al2O3, the phase transformation of the surface layer of ZVAl particles is relatively complete, so the ZVAl powder modified by γ-Al2O3 has short induction time and complete reaction time with water. This means that the surface modification of ZVAl may originate from the phase transformation of its surface layer[66].
Andr André et al. Used a milling procedure in which air was introduced in a controlled manner to increase the specific surface area of the powder and change the microstructure of the aluminum particles, resulting in a powder with a specific surface area greater than the 10 m2/g and particles in the form of platelets.The particles are made of polycrystalline ZVAl and surrounded by a thin layer of amorphous alumina with a thickness of 4.5 ± 0.5 nm, which can be used as propellants, rate modifiers and explosive additives[44].
SiO2 nanoparticles are easy to synthesize and widely used in various industries. Crystalline silica nanoparticles can be synthesized at low temperature by mechanical activation of amorphous silica and aluminum powder. With the increase of ball milling time, amorphous silica was transformed into crystalline silica, and the intensity of crystalline silica peak increased, and the strongest peak was formed after mechanical activation for 60 H[71].
3.3.2 Wet Ball Milling
By simply surface coating the aluminum nanoparticles with various organic dispersants using wet ball milling, the stable nanocolloids can be applied to various fields. Terpineol is stable during ball milling and can be used as a universal solvent for dispersing surface-coated nanocolloids. Three dispersants are highly soluble in terpineol. Polyethylene glycol diacrylate and ethoxylated trimethylolpropane triacrylate, which are commonly used as dispersants, both have C = O groups. The superpolymer KD-2 has —NH2 groups. The coating thickness of the dispersant on the aluminum nano-colloid is 10 to 13 nm. The mechanism is that in the process of ball milling, ZVAl nanoparticles collide with zirconia balls, and the Al3+ is activated to form chemical bonds with the functional groups of the dispersant, forming dispersant-coated nanoparticles, that is, nano-colloids[72].
3.4 Pitting corrosion
When ZVAl is ball-milled with chlorides or some oxides, these grinding AIDS will distribute on the surface of ZVAl and form a "salt gate" during the hydrolysis process to produce a more active powder. In the system of hydrogen generation from the reaction of ZVAl-NaCl mixture with water, NaCl surrounds the activated particles and prevents their exposed surfaces from reoxidation. During ball milling, NaCl is driven into the ZVAl matrix and destroys the surface oxide layer. NaCl particles are uniformly distributed on the surface of ZVAl and form "salt gates". In an aqueous environment, these "salt gates" are exposed to water. As these salts dissolve into active spots, more "salt gates" can produce more active powders. Smaller aluminum particles lead to increased hydrolysis reaction kinetics, accelerating the aluminum-water reaction[62,73]. In a system in which a ZVAl-TiO2 mixture reacts with water to generate hydrogen gas, the TiO2(P90) nanocrystals having a high surface area cause pitting corrosion on the surface of ZVAl by allowing the particles to contact and mix with the surface of aluminum for a short time (fig.And the nanocrystals are kept in close contact with the ZVAl surface, the anodic polarization of the TiO2∶Al2O3 of the contact point makes the mobile defect substances in the Al2O3 layer move more easily and the reactivity is increased, thereby maintaining the continuous reaction of molten aluminum[74].
3.5 Galvanic effect
3.5.1 ZVAl/other metals
ZVAl and other metals can form many tiny galvanic cells in the process of hydrolysis after ball milling, thus improving the effect of hydrogen production. In the hydrolysis system of ZVAl/Bi/X (other grinding AIDS) composites, the smaller Bi particles attached to the aluminum particles can form more current reactions with aluminum, thus improving the hydrolysis reaction rate of the composites[55]. A series of ZVAl-Bi-Sn ternary composites were prepared using high energy mechanochemistry and their reactivity to pure water was determined under standard ambient conditions. Bi and Sn invaded the ZVAl particles, resulting in sustained electrical interaction between the anode ZVAl and the cathode Bi/Sn during the hydrolysis reaction[48]. In addition, a series of ternary activated compounds containing ZVAl-Sn-In were prepared by high-energy ball milling. SEM-EDS analysis shows that Sn and In can be distributed on and inside the ZVAl particles (Fig. 9). The presence of Sn and In on the surface destroys the protective oxide layer of ZVAl, while Sn and In located on the surface and inside of ZVAl promote the electrochemical activity between ZVA1 and Sn/In[49].
3.5.2 ZVAl/C
When ZVAl was milled with carbon materials, many small galvanic cells were formed in aqueous solution. Some scholars use the MWCNT-Al/O2 system to generate H2O2 in situ, as shown in Figure 10. Through high-energy ball milling, the defects on the surface of MWCNT provide more active sites for the reduction of O2 in the MWCNT-ZVAl/O2 system. The direct contact between ZVAl and MWCNT forms a large number of corrosion cells, which can accelerate the transfer of electrons from ZVAl to O2[43].
3.6 Reduce the occurrence of side reaction
3.6.1 Elimination of side reaction
The ball milling of ZVAl and hydroxide (oxide) can promote the removal of the by-product layer and realize the continuous aluminum-water reaction. Liu et al. Found that the addition of CaH2 and NiCl2 was an important factor affecting the hydrolysis process, which produced a hydrocalumite Ca2Al(OH)6Cl-(H2O)2 and promoted the removal of the by-product layer[63]. Fan et Al. studied the hydrogen production of Al-Sn-Zn-hydride mixture ground at room temperature in water. The addition of MgH2 and CaH2 improved the reactivity of the mixture and quickly simulated their hydrolysis rate in water. The system with CaH2 could produce hydrogen rapidly within 5 minutes, and the reaction rate was faster[75]. It has also been found that continuous aluminum-water reaction can be achieved when aluminum powder is milled with other additives, such as Al(OH)3 and Al2O3[66,74].
3.6.2 Surface coating reduces side reaction
Ball milling of ZVAl with hydrophobic organic or carbon materials can effectively cover the surface of aluminum particles and reduce the occurrence of side reactions. During the ball milling process, the carbon material can hinder the cold welding of the aluminum particles. Graphite, for example, has a lubricating effect that prevents crystallite size from decreasing and forms a fine protective layer of fresh surfaces[61]. However, when the aluminum particles are hydrolyzed, the interface between graphene oxide and Al acts as a channel for water, and at the same time, the internal carbon material is gradually detached from the aluminum, thereby exposing fresh aluminum present inside[55]. The exposed aluminum can increase the contact area of the aluminum-water reaction and improve the water reaction performance[55].
Fluorocarbon has good hydrophobicity, when ZVAl is milled with fluorocarbon, the reaction between fluorine and aluminum generates high heat, fluorine can react with alumina shell and form AlF3, and the reaction between aluminum and fluorine is prior to that between aluminum and oxygen, so the formation of AlF3 is superior to that of Al2O3. At the same time, the sublimation temperature of AlF3(1277℃) is much lower than that of Al2O3(3000℃), and the sublimation of AlF3 will destroy the particles and reduce the agglomeration of aluminum[76]. The organic fluoride can effectively cover the surfaces of the aluminum particles in the ball milling process, and the hydrophobic fluorine atoms can prevent the internal aluminum from rapidly reacting with water vapor in the air, so that the aging resistance is good; and after the metal Bi is added for ball milling,Many micro-galvanic cells can be formed and the hydrolysis reaction can be accelerated by using Bi and ZVAl. By combining the catalyst Bi with fluorocarbon with good hydrophobicity, an active aluminum powder which can react quickly with water and has good aging resistance in air can be obtained[77].
In addition, wet ball milling with ZVAl/B/ethanol can make the surface of boron particles become rough and have some cracks, and make the boron particles tightly and uniformly wrapped by nano-aluminum, remove the inherent oxide layer on the surface of boron, and improve the combustion rate[78].
4 Effect of ball milling parameters
In the process of high-energy mechanical milling, the milling parameters such as milling time, milling speed, the proportion of grinding AIDS, dopants and milling atmosphere play an important role in the milling process.
4.1 Ball Milling Time
The milling time is an important parameter in the milling process. When the milling time is short, the ZVAl is not activated and the fresh surface can not be exposed. When the ball milling time is long, the ZVAl with ductility is easy to agglomerate. Therefore, the milling time has an important effect on the properties of the composites.
In the ZVAl-salt milling system, the NaCl-containing powder mixture appears to flatten, flatten, and fracture the primary mechanism of milling particles by plastic deformation with increasing milling time, and has a flaky geometry after milling, but very long milling times negatively affect hydrogen and reduce hydrogen production[73]. With the increase of ball milling time, the hydrogen generation rate is improved in the initial stage. However, as the ball milling time continued to increase, it was close to the theoretical limit, and further grinding did not significantly increase the amount of hydrogen produced[35]. When the alloy is added to the ball mill, NaCl in the alloy is dissolved in water, and the solution enthalpy and a large amount of Na+ and Cl- are generated; and increasing the temperature and the free ion concentration is beneficial to the micro-battery effect in a micro region, and the generated Cl- can improve the negative potential of the aluminum alloy[79].
In the ZVAl-organic ball milling system, the particle size increases first and then decreases with increasing ball milling time. The morphology change of the mixed powder during ball milling can be explained by assuming that there is competition between two mechanisms: fracture and cold welding. In the early stage of ball milling, the ZVAl particles are ductile and tend to deform and cold weld, and the impact of the ball causes the ZVAl particles to flatten and they cold weld to form larger disk-shaped particles; Then, the plastic deformation makes the particles hard and brittle, and the fracture mechanism is activated. Thus, the large plate-like particles break down to form smaller plate-like particles; As the milling time continues to increase, cold welding predominates, where the flaky particles become equiaxed and the particle size of the powder decreases, allowing new surfaces to be introduced within the particles; After high grinding, more impact can compress the particles, giving them a more uniform matrix with no pores or interlaminar spaces[40].
In the ZVAl-CNT variable-speed ball milling system, as shown in fig. 11, CNT clusters were continuously torn apart and CNTs were gradually dispersed on the surface as the LSBM time increased during the LSBM. Almost all CNTs were well dispersed on the surface after LSBM/8 H, and the dispersion was still present after LSBM/9 H. When the powder was observed quickly after HSBM/2 H, a large number of CNT clusters could be seen laying on the surface. The flakes were then cold welded into cake-like particles, and CNTs were no longer observed on the surface of the powder[52].
4.2 Ball milling speed
4.2.1 Uniform speed
Increasing the rotation speed will increase the collision speed and the number of collisions per unit time, both of which are beneficial to the reaction between the co-milling reagent and the organic pollutants, but there is also an optimal rotation speed, and the removal efficiency of the pollutants remains almost constant at higher milling speeds. This means that in a coupled system, high rotational speeds are not necessary, which reduces energy consumption[80]. In the ZVAl-NaCl milling system, the increased collision velocity and number of collisions per unit time can promote the breakdown of NaCl on ZVA1, which is helpful to enhance the reactivity of ZVA1[24].
4.2.2 Variable-speed
In recent years, in order to obtain Al/C composites with uniform dispersion and good interfacial bonding, scholars began to study the variable speed ball milling process. The LSBM is a mild ball-powder collision process, and the spherical aluminum powder is slowly flattened into a sheet, so that the carbon nanotubes are uniformly dispersed on the surface of the aluminum sheet, and little damage is caused to the carbon nanotubes; HSBM is a stronger ZVAl flake collision and cold welding, resulting in the aggregation of CNTs within the welded ZVAl particles with severely damaged but better inter-flake adhesion. SSBM enables intelligent dispersion of CNTs, embedding them within the particles[52].
Researchers have also successfully prepared aluminum matrix composites with fullerene soot by variable speed ball milling technology. The milling process was divided into two stages: the first stage was operated at 200 R/min for 60 min, and the second stage was operated at 600 R/min for 2 H, which was contrary to the conventional long time low speed ball milling and short time high speed ball milling, but the obtained composites also had excellent strength properties and ductility[38].
In addition, the uniform dispersion, good interfacial bonding and minimum structural strain of MWCNTs after dispersion can also be achieved by using the variable speed ball milling technology of two ball mills to disperse MWCNTs in aluminum alloy. When milled by a low speed ball mill for a certain time, the MWCNTs milled for a shorter time (6 H) were dispersed in the aluminum alloy particles to a certain extent, but the dispersed MWCNTs were agglomerated in the interstices of the alloy particles.It may be that the energy is not enough to overcome the strong van der Waals force and disentangle the MWCNTs, and the MWCNTs milled for a longer time (8 H) are relatively due to the greater impact energy applied to the powder.After milling for 1 H by a high speed ball mill, MWCNTs were uniformly dispersed in the alloy.With good interfacial bonding between the powder mixtures, SAED showed that many spots were formed on the rings, which may be due to the formation of amorphous carbon caused by crystal defects and dislocations in the MWCNT structure during the dispersion process[81].
4.3 Proportion of grinding aid
The ratio of grinding AIDS is another important parameter for obtaining the desired material, and there will generally be an optimum ratio. Effect of increasing the molar ratio of salt to aluminum in the ZVAl-salt milled powder on the rate and efficiency of hydrogen production in the hydrolysis reaction: the more salt, the higher the rate and efficiency[62]. This suggests that higher amounts of salt in the mixture significantly increase the reaction rate, and that when the salt phase is dissolved in hot water, the number of pores in the microstructure of the aluminum particles is greater, and therefore, a higher specific surface area is obtained at a higher percentage of salt, which improves the kinetics of hydrogen generation[35]. However, some scholars believe that there is an optimal addition amount, and the excessive salt added will cover the surface of ZVAl particles, thus hindering the hydrolysis process[63].
In the ZVAl-hydride ball milling system, the addition of more hydrides leads to higher hydrogen yield. The increase in hydrogen yield may be due to the decrease in particle size during ball milling, as hydrides with brittle characteristics can act as cutters and prevent the aggregation of ZVAl particles. The shape of the ZVAl powder changes from a large solid volume to smaller particles as the amount of hydride added increases. This is because with the increase of the amount of hydride added, the fracture portion on the surface of ZVAl becomes more and more obvious, and as a result, more defects are generated within the ZVAl particles[82].
In the ZVAl-CNT ball milling system, as shown in Fig. 12, with the increase of CNT content (1 wt% ~ 7 wt%), the particle size obtained after ball milling gradually decreases. CNT has the performance similar to graphite, but is more stable than graphite and is also considered as a good process control agent, which is equivalent to adding more PCA in CNT-ZVAl + ZVAl[39]. The higher the ratio of ZVAl and multi-walled carbon nanotubes, the higher the catalytic performance of the composite and the higher the amount of H2O2, but there is an optimal ratio. This is because excess ZVAl can cover the surface of MWCNT and block the pores of MWCNT, reducing the effective surface area of MWCNT to reduce the O2 and hindering the transfer of O2[43].
In the reaction system of ZVAl(mZVAlbm) activated with stearic acid (SA) and water, when the dosage of SA was changed from 0.50 wt% to 2.50 wt%, the H2 increased firstly and then decreased (Fig. 13). The natural oxide film on the original mZVAl surface is destroyed by mechanical force, exposing fresh surface and active sites, thus promoting the release of electrons and the generation of H2. However, as the SA dosage increases to 2.50 wt%, the excess SA will cover the resulting fresh surface and reduce its active sites. In addition, SA as a hydrophobic substance can also improve the hydrophobicity of the surface of the ZVAlbm, thereby inhibiting the surface corrosion of the ZVAlbm[23].
4.4 Dopant
In addition to ZVAl and common grinding AIDS, the addition of some special substances during ball milling can significantly improve the reactivity or mechanical properties of the composite powder.
In the reaction system of aluminum-metal-salt with water, metal Li is often added, which can significantly increase the amount and rate of hydrogen production, and with the increase of Li amount, the amount and rate of hydrogen production also increase, which can be attributed to the existence of AlLi phase during grinding. In the hydrogen generation reaction with water, the new phase formed will act as an active reaction center, resulting in a complete and rapid hydrogen generation reaction (equation (7)). However, in the early stages of the hydrolysis process, the hydrogen production rate of the mixture with low Li dopant is higher than that of the mixture with high Li dopant, and this reason has not been explained[83].
In the milling system of ZVAl-MWCNT, the addition of appropriate surfactant can increase the surface affinity between ZVAl and MWCNT, and promote the mechanical bonding between them. Improving the dispersion of MWCNT in the ZVAl matrix; The ZVAl particles are reduced, the dense oxide layer on the surface of the ZVAl powder is removed, and the activity of the ZVAl is improved; Resulting in strong plastic deformation of the MWCNT[43].
4.5 Ball milling atmosphere
The milling atmosphere also has a certain effect on the milling process. In the study of mechanically milled activated ZVAl (MA-ZVAl) system, Zhang et al. Found that there was no significant difference in the removal rate of Cr (Ⅵ) between MA-ZVAl prepared under N2 or air (Fig. 14)[24]. Even if the milling atmosphere is a high-purity O2, the removal efficiency is only slightly reduced, and ZVAl can be slightly oxidized by the high-purity O2. These results indicate that the atmosphere condition is a factor affecting the activation of ZVAl in the ball milling system, but it is not the most critical factor. Regardless of the atmosphere, mechanical milling can activate ZVAl in the presence of NaCl, greatly increasing the importance of MA-ZVA1 for practical applications. Similarly, the same conclusion was obtained in the system of oxidative degradation of phenol by ZVAl activated by mechanical ball milling[27].
In the ball-milling of the aluminum powder-paraffin oil system, it was found that without the addition of air, a powder with a low specific surface area was produced, which was a micron-sized particle, whereas once air was introduced during the ball-milling process, the specific surface area increased, and the particles after ball-milling were made of polycrystalline ZVAl,It is surrounded by an amorphous layer, and the thickness of the surrounding alumina layer is greater than that typically reported for nanopowders synthesized using other techniques. The continuous regeneration of the alumina layer maintains the hardness of the powder, which in turn is more fragile[44].
5 Influence of hydrochemical parameters
Hydrochemical parameters are important parameters for corrosion of ZVAl. When ZVAl is used for hydrogen production or pollutant removal after ball milling with grinding AIDS, the effects of hydrochemical parameters such as pH, temperature and reaction by-products on ZVAl are different.
5.1 pH
The pH value has an important effect on the corrosion of ZVAl metal, thereby affecting its power supply capability. For the ball-milled ZVAl-grinding aid -H2O system, a large amount of corrosion was observed at neutral pH (tap water pH = 6.5 to 7.1), and the pH change was hardly measurable when a small amount of powder and an excess of water were used, however, the pH change was measurable after reducing the ratio of water to powder. In the system, immediately after the powder comes into contact with water, the oxide/solution reaction occurs at the interface, resulting in the entry of OH- species into the solution and an immediate rise in pH[84]. Excessive corrosion of ZVAl can significantly lead to an increase in pH. The protective oxide layer can prevent excessive consumption of the ZVAl substrate. The pH controls the competition between aluminum corrosion and alumina formation, allowing the system to self-regulate[10].
5.2 Temperature
The aluminum corrosion reaction progresses very slowly in cold water, with the fastest rate in boiling water (T = 100 ℃). The aluminum-water reaction is an exothermic reaction, and the water temperature at which the corrosion occurs increases due to the relatively rapid corrosion of the milled aluminum salt powder, especially when the ratio of water to powder is small and the reaction rate is high. Therefore, it is speculated that at the micro scale, the reaction heat generated at the ZVAl-water interface (especially at the micropores) will increase the local temperature, accelerate the local corrosion rate, and promote the reaction and hydrogen generation[84]. Increasing the temperature results in a rapid increase in the chemical reaction rate, and it has been reported that the lower activation energy accelerates the corrosion of aluminum[34][32].
Higher water temperature is beneficial to increase the hydrogen production rate. The alloy was stable at lower temperature, and the hydrolysis kinetics increased significantly with the increase of temperature[79]. However, the hydrogen generation process by the reaction of ZVAl-Li alloy with water is an exothermic reaction type, and a large amount of heat is released in the hydrogen generation process, thereby heating the surrounding water, even if the temperature of the water is initially 0 ° C, as long as the reaction can be initiated, the water temperature will rise. Therefore, the initial water temperature has no effect on the hydrogen generation of ZVAl-Li alloy, which indicates that ZVAl-Li alloy has stronger adaptability to different water temperatures[31].
5.3 Reaction by-product
In the process of hydrogen production by aluminum-water reaction, the addition of dopants and the aluminum-water reaction itself will produce some by-products, some of which are conducive to accelerating the hydrolysis. In the hydrolysis of ZVAl/NaBH4, Al(OH)3, BNi2 and NaBO2·2H2O will be produced, which have a great impact on the hydrolysis of Al/NaBH4.The alkalinity of NaBO2·2H2O can accelerate the hydrolysis of Al, and Al(OH)3 and BNi2 are good promoters for the hydrolysis of NaBH4[64].
6 Conclusion and prospect
ZVAl has a lower redox potential and a higher thermodynamic driving force for electron transfer, but the surface of ZVAl is covered with a dense alumina film, which hinders the release of electrons. The surface modification of ZVAl can be achieved by mechanical ball milling technology through the force generated by the high-speed rotation of the ball milling medium. At present, this technology is mainly used in the field of material synthesis and hydrogen production, but the research on pollutant removal is still less. Therefore, the research on the modification of ZVAl by mechanical milling and its application in pollutant removal is still in its infancy, and the following problems need to be further studied.
(1) Expanding the wide application of ZVAl modified by mechanical ball milling in the field of environment. ZVAl modified by mechanical ball milling is mostly used in the field of materials and hydrogen production at present, but only in the field of environment in recent two years. However, whether ZVAl is used for hydrogen production or pollutant removal, both of them are essentially the same, which is the redox reaction of ZVAl losing electrons. Mechanical ball milling has technical advantages, and ZVAl has excellent reducibility, therefore, the application of mechanical ball milling ZVAl in the field of environment has a good prospect, and more applications of mechanical ball milling ZVAl in the field of environment deserve our further study.
(2) precisely regulate and controlling that mechanical ball mil process. In order to obtain composites with uniform dispersion, good interfacial bonding, high hardness, high strength and other excellent material properties, some scholars have begun to study the process of variable speed ball milling in recent years, but the mechanical ball milling technology achieves the purpose of modification by high-energy ball milling of materials in a ball milling tank. Under the action of high-speed rotation and high energy, the interaction between materials may occur rapidly in a short time, and limited by the current development level of high-energy ball mill products, we can not observe and judge this process, which also has a certain impact on the study. Therefore, the research and development of more ball mill products, through the precise control of the ball milling process, to achieve directional modification of materials is the next step to be studied.
(3) The mechanism of the oxide film on the surface of ZVAl before and after mechanical milling was studied. When the ZVAl modified by the mechanical ball milling is applied to hydrogen generation and pollutant removal, the starting point is to destroy the oxide film on the surface of the ZVAl, and if the purpose is achieved,So when choosing the type of grinding AIDS, we should first consider the relative size of ZVAl (or the Al2O3 on the surface of ZVAl) and the hardness of grinding AIDS, and preliminarily judge whether the grinding AIDS can destroy the oxide film according to its hardness, which is rarely discussed in the current literature. In addition, scholars have found that the oxide film on the surface of ZVAl is not completely removed after mechanical milling of ZVAl, while most scholars have studied the effect of fresh surface exposed on the surface of composite powder after milling in aqueous medium.Whether and how the oxide layer which is not completely removed on the surface of ZVAl after mechanical milling will change in the water medium has not been explained clearly[23,24].
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