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Vol. 8. Issue 1.
Pages 63-74 (January - March 2019)
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Vol. 8. Issue 1.
Pages 63-74 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2017.05.017
Open Access
Cryomilling: An environment friendly approach of preparation large quantity ultra refined pure aluminium nanoparticles
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Nirmal Kumar, Krishanu Biswas
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Corresponding author.
Department of Material Science & Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
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Tables (3)
Table 1. Composition of aluminium powder in atomic percent (estimated by XPS).
Table 2. Properties of aluminium used in the calculation.
Table 3. Milling parameters used in the calculation of rise in the temperature during milling.
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The preparation of aluminium nanoparticles in large quantity is a challenge for most of the synthesis processes available. The present investigation reports a top down approach, known as cryomilling to synthesize large quantity of aluminium nanoparticles (Al NPs). The cryomilling is known to be for ultra refinement of particles size as well as suppress of the rate of oxidation during synthesis. Aluminum is a reactive metal and highly prone to oxidization/nitridation in nanoscale. Therefore, a novel cryomill has been used to prepare large quantity Al NPs in which, the powder has been milled at extremely low temperature (<123K). This technique does not leave any hazardous by-product and is known to be environment friendly. The ultra refined Al NPs are promising candidate for application in various including explosive formulation, nanofluids, pigments, heat shield coating of aircrafts, etc. Thus, the bulk synthesis of Al NPs by cryomilling will satisfy the increasing demands of Al NPs on an industrial scale. The prepared nanoparticles have been characterized by host of advanced techniques to obtain shape, size, dispersion stability, and purity of the Al NPs. The results indicate that it is possible to prepare Al NPs having size ranging from 5 to 15nm. Additionally, the thermal stability of nanoparticles has been probed and Al NPs have been found to be thermal stable till 150°C. The results have been discussed using currently available theories.

Al nanoparticles
High purity
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The pure metal nanoparticles have attracted attention to the researchers in the last few decade due to their high dispersity in the polar solvents, making them potential candidates for a variety of applications, including flexible printed large-area electronic [1], nanofluids [2], surface enhanced Raman scattering photonic devices [3,4]. These applications demand large scale production of pure free standing metallic nanoparticles with narrow size distribution. Most of the synthesis routes, utilized to prepare free standing metallic nanoparticles, stabilize the nanostructure by using capping agents [3,5] to obtain narrow size distribution. However, the presence of a layer of the capping agent on the nanoparticles is detrimental for the desired properties in many applications. It is to be noted that the “free standing” nanoparticles are free from agglomeration without any surfactant or capping agents [6]. Ball milling at cryogenic temperatures (below 123K (−150°C)) has been reported to be an effective method to prepare pure free standing nanoparticles in large quantity without the usage of any capping agent [7]. Recently, the preparation of various free standing metallic nanoparticles (Fe, Cu, Zn, Ag) using this method has been reported in the literature [6–11]. However, free standing aluminium nanoparticles, due to their rapid propensity to oxidization and nitridation has not been reported earlier. It is to be noted that there is substantial discussion in literature over efficacy of the cryomilling process on increasing/decreasing of particles size. Earlier investigators have utilized attritor mills and wet milling (slurry of metallic powder with liquid nitrogen) to prepare metallic nanoparticles. Zheng et al. [12] have reported that it is possible to reduce the particle size of Mg alloy (AZ80) from 55μm to 10–30μm after 8-h cryomilling while average grain size is reported to be 40nm. In another study, particles size (55μm) of Ti alloy is reported to increase to 120μm in 2h cryomilling and later decrease to 44μm particles size with 20nm grain size during 8h cryomilling [13]. In another study reported average particles size of 99.9wt% aluminium cryomilled with process agent (0.2% stearic acid) of 75μm [14]. In the literature, the researchers predominantly utilized attritor mill for milling of powder with LN2 (slurry form). It is to be noted that aluminium nanoparticles can find various applications due to interesting physical, high enthalpy of combustion [15], enhance mechanical and electrical performance of epoxy based nanocomposite [16]. It is to be noted that aluminium powder is common ingredients for explosive formulation and it can ignite faster as the particle size gets smaller [17]. It has been reported that the addition of the nanoparticles in kerosene provides multiple heterogeneous nucleation sites to promote micro-explosions due to enhanced the evaporation rates of kerosene droplets at all tested temperature (673K (400°C)–1073K (800°C)) [18]. In addition, high pure free standing aluminium nanoparticles can find application in aluminium pigments as well as heat shielding coating of aircraft [19–21].

A comprehensive literature survey indicates that the aluminium nanoparticles have been synthesized by many other processes including electrical discharge [22], wire explosion process [23], wet chemical synthesis [24,25] arc plasma spray [26], aerosol synthesis [27], laser ablation [28], etc. In fact, a process called cryomelting; where spontaneous aluminium metal vapor condensed within a cryogenic medium has also been reported for preparation of aluminium nanoparticles. In this process, 60% of the particles is found to have size less than 70nm with 3nm alumina layer [29]. Similarly, another study using cryomelting has reported the preparation of aluminium particles having average grain size of 37nm [30]. It has also been reported that the commercial pure aluminium particles consisting 1wt% diamantane having average grain size of 22nm exhibit greater thermal stability than pure Al nanoparticles in the temperature range of 423 (150°C) to 773 (500°C) K [31]. Some research groups have reported cryomilling of different types of aluminium alloys followed by consolidation [32,33]. However, none of the studies reports the formation of pure free standing aluminium nanoparticles. In addition, the yield and purity of the nanoparticles are still a challenge, while the degree of agglomeration of the nanoparticles depends on the synthesis route. None of the above-mentioned techniques has reported successful synthesis of pure and free standing aluminium nanoparticles. On the other hand, the cryomilling involving mechanical grain refinement is expected to impart less contamination from the milling tools due to faster refinement at cryogenic temperature (<123K (−150°C)) as well as protection from oxidation and nitridation compared to the room temperature ball milling. The oxide layer formation over particles surface can either be inhibited or substantially reduced due to extremely low temperature and inert gas environment during milling, resulting in the formation of nanoparticles with virgin surfaces, which can allow them to easily disperse in the polar solvents. In addition, any applications involving usage of high temperature (>100°C) require the nanoparticles to be stable and, therefore, it is important to investigate the stability of the as processed nanoparticles.

In the present investigation, we report a detailed study on the synthesis as well as the dispersion stability of the aluminium nanoparticles using cryomilling. This systematic study will discuss the successful synthesis of aluminium nanoparticles and thermal stability of the free standing nanoparticles. The results will be discussed using the model by Mohamad dealing with early grain refinement and ultrafine refinement during cryomilling [34]. The sintering model by Alymov et al. [35] has been utilized to discuss the ball milling conditions required to be satisfied for the formation of free standing nanoparticles.

2Experimental details2.1Synthesis

The nanoparticles were prepared by cryomilling of aluminium powder 99.9% purity (Alpha Aesar, USA; −40+325 mesh). The custom built cryomill using single tungsten carbide (WC) ball has been utilized in which, cryo temperature (<123K) was maintained using liquid nitrogen (LN2) in surrounding the milling chamber [36]. The powder was milled in dry, i.e., the powder and LN2 did not come in contact during milling. The amplitude of the vibration in cryomilling was maintained 1.5mm throughout the milling. The details of cryomill and working principle have been reported elsewhere [36]. The milling has been carried out under inert gas atmosphere by purging Ar gas (1l/h) inside the milling vial. The real time temperature of the powder was monitored using a K-type thermocouple. A ball to powder ratio of 80:1 was utilized for the milling.

The milling carried out for 6h and 30min and the powder was intermittently collected each 30-min interval for structural as well as microstructural analysis. The milled powder was dispersed in high purity (better than 99.9% purity) methanol to check the free standing ability. Sufficient care was taken to avoid any foreign contamination during handling of the milled powder.


The X-ray diffraction patterns of synthesized powders were obtained using X-ray diffractometer (Bruker D8 Focus) with CuKα radiation (λ=0.154056nm) with step size 0.02°. The crystallite sizes and micro strain were estimated from peak broadening using Hall-Williamson approach [37]. The fine scale microstructure of the free-standing nanoparticles was obtained using transmission electron microscope (FEI, Tecnai G2 UT 20 operated at 200kV). A small amount of milled powder was dispersed in methanol and a few drops were placed on 400-mesh carbon coated copper TEM grid and dried in vacuum prior TEM analysis. In order to estimate foreign contamination due to handling of powder, nitridation, and oxidation of Al nanoparticles during ball milling, the milled powders were characterized using electron probe micro-analyzer (EPMA, JXA-8230, JEOL, Tokyo, Japan) as well as inductively coupled plasma mass spectrometry (ICP-MS, Thermo Scientific XSERIES 2 ICP-MS). The surface composition (few atomic layers) was estimated using X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Prob II, FEI Inc.) The cryomilled powder was annealed at different temperatures (323K (50°C), 373K (100°C), 423K (150°C), and 473K (200°C)) to study the thermal stability. The powder was kept in a vacuum sealed (10−6bar) quartz tube prior to the heat treatment at any specified temperature for 2h.

3Results3.1X-ray diffraction (XRD)

The Al powder (particle size ∼10μm) was cryomilled for up to 390min and the samples were collected at 30-min interval for X-ray diffraction analysis. The XRD patterns of the cryomilled powder are shown in Fig. 1a. All the peaks in the patterns can consistently be indexed using reflections due to FCC aluminium (lattice parameter, a=0.40494nm: JCPDF No. 00-004-0787). No peak due to oxide or nitride could be detected to the best of the resolution of the XRD. The XRD pattern of the as-received powder is also shown at the bottom of Fig. 1a for reference. It is evident that the diffraction peaks of the cryomilled powder show peak broadening (Fig. 1a inset). In the process of cryomilling of powder, peak broadening can be due to nano-sized particle as well as micro-strain induced and therefore, one needs to carefully deconvolute the broadening due to particle size as well as micro-strain. The crystallite size and RMS (root mean square) micro-strain of the cryomilled powder have been estimated from the peak broadening using Hall–Williamson approach after subtracting broadening due to the XRD machine.

Fig. 1.

(a) X-ray diffraction pattern of Al cryomilled powder with different time of milling; inset shows the broadening of (200) peak; (b) crystallite size and strain variation of Al nanoparticles with time of milling.


Fig. 1b shows the variation of the crystallite size and the strain as a function of the time of milling.

The particle size decreases monotonically as a function of milling time. This is expected because of the fact the cryomilling leads to nanocrystal formation fast. The crystallite size of the cryomilled powder is observed to vary from 10 to 15nm after 390min of ball milling. The micro-strain initially increases reaching maximum value (0.21) at 210min after cryomilling, as shown in Fig. 1b, and then decreases to 0.17at 390min. This behaviour is due to change in the deformation mechanism of plastic deformation as the particle size decreases during cryomilling. It is to be noted that this strain is due to plastic deformation occurring within aluminium grains via generation and movement of dislocations. The increase of micro-strain in the particle is mainly due to dislocation-dominated deformation occurring in bigger particles (>100nm), leading to the formation of small angle boundaries. As shown in Fig. 1b, after 210min of milling, crystallite size reaches in nanometric range (<100nm). Nano-sized particles attribute to the energy release accompanying the fracture of the crystals after reaching a critical value of strain [38,39]. In addition, we need to consider stacking fault in FCC aluminium. The stacking faults in nano grain size will be eliminated due to the instability of partial Shockley dislocations when grain size becomes comparable with the equilibrium distance between two partial dislocations [40].

3.2Dispersion stability (free standing behaviour)

In order to check free standing or dispersion stability (without surfactant), the cryomilled powder was dispersed in ultra pure methanol, following ultrasonication for 10min. Fig. 2 shows the results of the investigation on the powder milled for 390min. Fig. 2a is the optical image of the test tube containing Al NPs dispersed in methanol. For clarity, the optical image of a test tube containing only methanol is also shown in both Fig. 2a and b. The good dispersion of Al NPs is visible in the image. In fact, the inverted image (inverse contrast) is also shown for betterment of the contrast. The nanoparticles are observed to be stable in methanol after 10 days (as shown in Fig. 2b). In fact, the nanoparticles have been found to remain well dispersed in methanol even after 2 months. Thus, it is evident that the Al nanoparticles are highly stable in methanol and there is no tendency to fast agglomeration. Similar experiments carried out using other polar solvents, such as ethanol, ethylene glycol, etc.; indicate the similar behaviour of the nanoparticles.

Fig. 2.

Cryomilled Al powder disperses in ultra high pure methanol (time of milling mentioned over test tube): (a) just after preparation Al NPs and (b) after 10 days of dispersion.

3.3Transmission electron microscopic (TEM) observation

To obtain finer scale micro structural information (size, shape, distribution, defect structure, etc.), the milled powder was investigated using TEM. The cryomilled powder dispersed in methanol was taken out from the test tubes with the help of micropipette and dropped on carbon coated 400-mesh Cu grid. The grid was subsequently dried in vacuum over night prior to TEM observation. Fig. 3a shows a typical bright field micrograph showing Al NPs, free from agglomeration with nearly spherical geometry. Fig. 3b shows the corresponding histogram, indicating narrow distribution of nanoparticles (7±3nm) after cryomilling. The histogram was obtained using a large number of high resolution bright field TEM micrographs. Fig. 3d shows a high resolution image of one such nano-particle with inset showing FFT (Fast Fourier Transform) of the selected region (marked on the figure).

Fig. 3.

TEM images: (a) bright field TEM image of aluminium nanoparticles; (b) histogram showing distribution of nanoparticles; (c) correspondence diffraction pattern; (d) high resolution image of bigger particles; inset shows FFT of selected region. (e) FFT filtered (masked) image showing edge dislocations.


It shows large number of dislocation accumulation in nanoparticles during cryomilling. The SAED pattern (Fig. 3c) indicates the spotty diffraction rings due to aluminium only, indicating the presence of pure aluminium nanoparticles.

3.4Purity of nanoparticles

It has already been reported that the cryomilling can be used for the preparation of high purity metal nanoparticles for a variety of applications [36]. A comprehensive compositional measurement of the milled powder has been carried out to confirm the purity of the nanoparticles. The composition of cryomilled powder analyzed using electron probe micro analyzer (EPMA) as well as inductively coupled plasma mass spectroscopic (ICP-MS) technique. Fig. 4 shows the results obtained using EPMA investigation on Al powder cryomilled for 390min. The presence of carbon peak, arising from the carbon tape used for mounting the powder sample is observed. Thus, no other impurity has been detected within the detection limit of EPMA (100ppm). Further, the milled powder was investigated using ICP-MS, which is sensitive to presence of the trace elements (detection limit of 1ppb). The detailed ICP-MS investigation shows the presence of W (3ppm) in the milled powder. The impurities from milling tool (W from ball and vial) cannot be avoided. This is negligible amount compared to room temperature ball milling [41].

Fig. 4.

EPMA spectra (WDS) of powder cryomilled for 390min (carbon from LDE crystal, Al from TAP crystal).

3.5X-ray photoelectron spectroscopy

The surface oxygen has been estimated using XPS and compared with as received Al powder. It is to be noted that the process has not used any hazardous chemical during the preparation of nanoparticles and thus, it can be considered a green synthesis process [42] for the preparation of high purity aluminium nanoparticles. In this process, the nanoparticles have not been capped by any surfactant. Therefore, XPS has been utilized, XPS is highly sensitive to surface analysis, ideal for analysis of few outermost atomic layers [43]. Here, all the peaks has been referenced (charge correction) with adventitious carbon 284.8eV. Therefore, Fig. 5a and b shows that oxide layer present over surface of the as received Al powder particles. The total O concentration is contributed by adsorbed contamination (533.5eV) [44], aluminium in hydroxide/oxyhydroxide state (532.4eV) [45,46] as well as aluminium oxide layer (531.33eV) [47]. In fact, a careful analysis reveals that the oxygen contribution from the oxide is 9 atom% out of total oxygen concentration. Further comparing with cryomilled powder, Fig. 5c and d shows that over all Al metal concentration is slightly increased and oxygen concentration (quantification shown in Table 1) remains almost unchanged during cryomilling. Thereafter, O 1s peak of cryomilled powder was further deconvoluted and shown in Fig. 5e. It indicates that the oxygen contribution from the oxide layer is about 21.5 atom% out of total O concentration. Therefore, the oxide layer contributed O concentration has increased 12.5 atom% after cryomilling. The increased oxide content is mainly due to large number of nanoparticles (large surfaces area), containing thin layer of oxide. The cryomilled powder does not have any other impurities. C is inadvertently present as shown (Fig. 5f) in survey spectra of cryomilled Al powder. The sample has been exposed in environment during sampling in XPS for a short time, which might be reason for C and thin layer formation of oxide over nanoparticles. In addition, surfaces of the nanoparticles are highly prone to formed oxide (reduce to energy with binding oxygen and organic contamination). Thus, it can be concluded that the oxide thin layer always present over the aluminium nanoparticles surfaces. The cryomilling has not introduced extra contamination and this technique has capability to protect powders from further oxidation.

Fig. 5.

As received aluminium powder XPS spectra: (a) Al 2p, (b) O 1s, (c) Al 2p, (d) O1s, (e) deconvolution of cryomilled Al O 1s peak and (f) survey spectra of cryomilled Al nanopowder.

Table 1.

Composition of aluminium powder in atomic percent (estimated by XPS).

Element  As received (Al) (atom%)  Cryomilled (Al) (atom%) 
Al  33.73±0.5  35.54±0.5 
3.6Thermal stability

As the metal nanoparticles are highly prone to agglomerates and grain growth due to the large surface area to volume ratio, it is worthwhile to study the thermal stability of the cryomilled nanoparticles. In addition, many potential applications require the usage of the aluminium nanoparticles at elevated temperature (>373K (100°C)) and the thermal stability of the nanoparticles plays a critical role in deciding the usage. Thus, annealing or heat treatment at different temperatures (323K (50°C), 373K (100°C), 423K (150°C), and 473K (200°C)) for 2h in inert atmosphere has been used to systematically study the thermal stability of the cryomilled aluminium nanoparticles. The annealing temperatures have been selected in such a way that it varies from 0.075 to 0.3 Tm, where Tm is the melting temperature of pure aluminium. It is expected that nanocrystalline grains of the cryomilled powder will exhibit growth during the heat treatment. Subsequently, the microstructural investigation of the annealed powder has been carried out using TEM. Fig. 6a–d shows the bright field TEM micrographs, whereas, Fig. 6a′–d′ illustrates the histograms showing the particle size or grain size distribution. Fig. 5b inset shows the SAED pattern from a nanoparticle, which is evidence of FCC aluminium. One can clearly observe the coarsening of particles with increasing heat treatment temperature. Initially, the nanoparticles with narrow size distribution (Fig. 3b) undergo coarsening with wide size distribution, indicating anomalous growth. The detailed particle size analysis indicates that the nanoparticles do not grow much until 373K (100°C). However, annealing at 423K (150°C) and 473K (200°C) leads to the formation of particles having sizes more than 100nm. The annealing at 473K (200°C) shows a bimodal distribution of particles with some particles even growing up to 200nm. The anomalous coarsening behaviour of nano-crystalline aluminium particles is responsible for the bimodal distribution of the particles.

Fig. 6.

Transmission electron microscope image after heat treatment with correspond size distribution: (a) at 323K (50°C) for 2h; (b) at 373K (100°C) for 2h with inset showing SAD pattern from nanoparticle; (c) at 323K (150°C) for 2h; (d) at 473K (200°C) for 2h, lower inset in each figure showing corresponding particles size distribution; (e) coarsening of nanoparticles with temperature.


Some nanoparticles coarsen rapidly due to favourable surface energy. The temperature of 200°C has been found to be enough for the nanoparticles to grow fast while particles are in contact. Some other particles do grow relatively slowly and thus, it leads to bimodal distribution of particles. Fig. 6e shows the coarsening behaviour of the nanoparticles as a function of annealing temperature. Another important observation of the annealing treatment is the change of shape of the nanoparticles. The as prepared nanoparticles having near spherical shape undergo shape change to cuboidal shape at a higher temperature (373K (100°C), 423K (150°C) and 473K (200°C)). The higher magnification micrographs as shown in the inset of Fig. 6a–d reveal the shape of the particles. Initially particles remain in spherical shape and then achieve cuboidal shape at higher temperature. This is because of the fact that post necking, the particles rotate to minimize grain boundary energy (inset Fig. 6d). It is to be noted that these images have been obtained when the nanoparticles are oriented along 〈001〉 direction. This aspect is under investigation and will be communicated separately.


The present investigation, for the first time, shows the successful synthesis of high pure Al NPs using ball milling at cryogenic temperature. The Al NPs have been found to be of high purity, devoid of oxides or nitrides. The only impurity in minute quantity observed in the cryomilled nanoparticles is tungsten, originating from the milling media. The nanoparticles have also been found to be stable (retainment of size in the nanometric domain) during annealing treatment up to 373K (100°C).

These novel experimental findings need explanation. In the following, we shall discuss the results in the light of available literature. Free standing or isolated nanoparticles have been studied extensively in the past decades in order to extract their intrinsic properties, which are useful for device applications and thus, these are of foremost importance to the scientific and technological communities. The bulk preparation of Al NPs is scarcely reported in the literature [6,7,9,10,36]. It is expected that ball milling at cryogenic temperature can be used for the synthesis of different metallic nanoparticles in large quantity. Therefore, it is imperative to understand the synthesis of the free standing nanoparticles of controlled particle size by ball milling at cryogenic temperature. Fundamentally, the preparation of free standing nanoparticles is decided by the competition between cold welding and fracturing during ball milling [6]. In the five stages of mechanical milling [48], fracture dominates over cold welding [41]. These two factors strongly depend on the milling temperature. Mechanical milling is a process in which powders are charged in the vial of the ball mill and then caused to be collided by moving balls. The process can be carried out using attritor, SPEX shaker mill, a planetary mill, horizontal ball mill, etc. During high-energy milling, the powder particles are repeatedly flattened, cold-welded, fractured, and rewelded [49]. Since powders are cold welded and fractured during milling, it is critical to establish a balance between cold welding and fracturing during milling for the preparation of the free standing nanoparticles of narrow size distribution. The ability of cold welding and fracturing of powder depends on the material and the milling conditions. Every metal requires some threshold deformation to start cold bonding. Soft materials, such as aluminium, normally have good weld ability at room temperature and the rise in temperature during milling (normally observed during conventional milling) helps to cold bond easily with low percent deformation. It is difficult to overcome the cold welding phenomena even during conventional milling. Therefore, the ultra refinement of particles is impossible by conventional ball milling. However, cooling of powders is considered as an effective approach to accelerate the fracture process as well as suppress the process of cold welding, recovery and recrystallization, leading to rapid grain refinement [50,51]. It is well known that fracturing of any material depends on temperature. A material, when cooled below ductile-to-brittle (DBT) temperature, will undergo brittle fracture and breaks very easily. Cooling down to very low temperature (123K (−150°C) to 96K (−177°C)) during cryomilling is an effective means to accelerate this fracturing process for many materials having bcc and hcp crystal structure. However, materials having fcc crystal structure do not undergo any DBT transition in the specified temperature range [52]. Nonetheless, the fracturing tendency of the metals having fcc crystal structure will substantially increase at low temperature as compared to that at room temperature.

On the other hand, the cold welding can substantially be reduced at very low temperature. The ball milling of powder is thought to involve the following processes; flattening of powder particles, welding of powder particles, formation of a layered structures, deformation of the layered structure and formation of nanoparticles. Therefore, it is clearly evident that the welding process due to collision of balls with powder during milling can be approximated as cold welding of two plates under pressure [53]. The increase in pressure during collision increases the real area of contact between the powders. According to cold-welding theory, the cold-welded bond of reasonable strength is associated with a particular range (40–70%) of deformation (plastic) strain of powders. The minimum deformation strain required for a cold welded bond depends on the materials systems employed and the temperature [53]. Most important aspect of cold welding is the fact that the bond strength between the powder particles of the metal is a function of deformation strain. For cold welding to occur, the deformation strain of the powder per collision event should be greater than the critical amount of deformation required to form a reasonably strong bonding. Thus, the strength of the weld depends on the ability of the material to undergo plastic deformation during milling. At extremely low temperatures (123K (−150°C) to 95K (−177°C)), the weld strength is expected to be poor for many of the useful metals because the minimum amount of deformation strain required to form a cold weld (40–70%) cannot be achieved [54,55]. In this case, the investigated metal does not undergo such an amount of deformation to form a cold welded joint during cryomilling, the fracturing process is expected to dominate during ball milling at low temperatures. It is also expected that even if some cold-welding takes place, then during subsequent collisions with balls, the cold-welded structure will break as the strength of the bonding will not be sufficient enough. Therefore, both will promote the formation of free standing nanoparticles. The cold welding can effectively be suppressed in cryomilling, which helps to achieve ultrafine particles size with early grain refinement. The formation of cold welded joint of sufficient strength, the minimum deformation required is 60%. For a metal like aluminium, 60% reduction during ball milling at cryogenic temperature is extremely difficult to achieve. Hosford et al. [56] have shown that aluminium failed at true tensile strain of 0.3at 77K temperature. Therefore, it is difficult to form good cold welded joints at 77K. Thus, fracturing of the aluminium particles will dominate over cold welding during ball milling at cryogenic temperatures (<123K (−150°C)) and this will lead to the formation of ultra refined Al NPs.

In addition, we also need to discuss the formation of Al NPs of narrow size range during the process of cryomilling. The present investigation shows that it is possible to form nanoparticles in the size range of 7±3nm during cryomilling. Mohamed has provided a relationship between the minimum grain size achievable during ball milling with materials and processing parameters [34].

where b is Burger's vector of dislocation, A (dimensionless constant), β is a constant (0.04); Q the self-diffusion activation energy, DPO pipe diffusion coefficient [57], νo passion's ratio, γ stacking fault energy, G shear modulus, H hardness, R the universal gas constant, k the Boltzmann constant and T is the absolute temperature. According to the equation, dmin is strongly dependent on the milling temperature because the values of Q, DPO, G, H are dependent on T. In the present case, the milling temperature of the aluminium powder has been maintained below 123K (−150°C). Using the materials properties for aluminium (as listed in Table 2), one can obtain dmin to be 11nm, which is reasonably close to the experimentally observed size range of the nanoparticles (7±3nm), obtained after cryomilling for 330min.

Table 2.

Properties of aluminium used in the calculation.

Parameter  Value  Ref. 
Shear modulus G (GPa)  26.2  [60] 
Burger vector a2/2a lattice parameter b (nm)  0.286  [61] 
Poisson's ratio, ν  0.35  [60] 
Hardness H (MPa)  167  [60] 
Bulk melting point (K)  933.3  [60] 
Surface tension of liquid γl and solid γs (J/m20.92 and 0.85  [60,61] 
Melting enthalpy ΔHm,o (J/g)  399.8  [62] 
Stacking fault energy (mJ/m2166  [63–65] 

Although cryomilling involves low-temperature processing, the milling can lead to the generation of heat at the particle-particle interface locally and hence can increase the local temperature. In addition, the thermal component of the applied stress, as well as driving force due to the reduction of surface energy, can cause sintering of the freshly formed free standing nanoparticles. Therefore, the formation of free standing nanoparticles is possible during ball milling provided the thermal processes can be even avoided or suppressed. Thus, milling parameters must be selected in such a way that thermally controlled sintering of the free standing nanoparticles can be avoided. According to Alymov et al. [35] the sintering temperature (TSS) of nano or ultrafine particles is given by

where k is the fraction of sintered region, as defined in Eq. (3), A is a constant varying from 0.06 to 0.15; L the number of a neighbour of a particle and Tm is the melting temperature defined in Eq. (4)[58]. Tm is the bulk melting temperature, γl and γs are surface tension liquid and solid respectively, ΔHm,o the melting enthalpy and d is the crystallite size. The values of the parameters utilized for calculation of TSS are provided in Table 2. The calculations indicate that TSS130K (−143°C) for d=7nm. Thus, the milling parameters must be selected in such a way that TSS is not reached. It is now important to estimate the rise in temperature during the process of cryomilling in the present investigation. According to Schwarz et al. [59], the rise in temperature due to localized shear of the powder particles entrapped between the ball and vial during milling is given by
where ΔT is the rise in temperature, F dissipated energy flux=σ·V, where σ=normal stress caused by the head-on collision and V is the relative velocity of the balls before impact, Δt the stress state lifetime, ρ powder particle density, k thermal conductivity of powder and CP is heat capacity of powder [listed in Table 3]. Here, σ can be approximated as the maximum compressive stress generated by a head-on collision of two balls of diameter D and thus, σ is given by
where E is the elastic modulus of the ball, P load. Using the values of parameters listed in Table 3 the calculated value of ΔT=4K or 4°C due to localized heating due to impact between ball and vial. Therefore, the actual temperature of impacted powder (123±4K) lies below the sintering start temperature (TSS130K (−143°C)). Thus, milling condition has been performed in such a way that dmin and TSS lies lowest plastic deformation dominated region as shown in Fig. 7. Sintering of aluminium nanoparticles is not expected to occur during cryomilling and formation of ultra refined free standing nanoparticles is a distinctly possible under the experimental conditions used in the present investigation, which fall under plastic dominated regime shown in Fig. 7.

Table 3.

Milling parameters used in the calculation of rise in the temperature during milling.

Ball and vial  Density (g/cm3)  Elastic modulus (GPa)  Load (N)  Ball diameter (mm)  Density of milling powder (kg/m3)  Thermal conductivity (W/mK)  Specific heat (Cp) kJ/kg
Tungsten carbide  15.63  600  0.4095  50  2700  205  0.91 
Fig. 7.

Crystallite size predicted by the model of Mohamed [34] (milling) and Alymov et al. [35] (Sintering) showing distinct regimes.


The process of cryomilling can effectively be utilized for preparation of high pure free standing aluminium nanoparticles. The following conclusions can be drawn.

  • (i)

    The particles prepared by cryomilling free stand in pure methanol for longer duration without any surfactant or stabilizers.

  • (ii)

    The cryomilling process is capable of preparing nanoparticles with narrow distribution range (7–10nm) particles.

  • (iii)

    Low temperature helps to reduce the cold metallurgical bonding between particles during milling. The formation of free nanoparticles is predominantly controlled by fracturing rather than cold welding.

  • (iv)

    The aluminium nanoparticles are thermally stable up to 373K (100°C).

  • (v)

    The experimental results indicate that the formation of free standing aluminium nanoparticles is due to experimental conditions, which suppress the dynamic recovery and sintering processes.

Conflicts of interest

The authors declare no conflicts of interest.


The authors would like to acknowledge financial support from the SERB-DST India.

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