(331ac) The Roller Vibration Milling: Preparing Dry Nanostructures in Large Scale and at Room Temperature

Abstract: The RVM provides stable motion with enough robustness, mild normal pressure for inducing plastic deformation, and certain exciting calling for a stress release leading to particle size reduction at unloading. We claim that the RVM can prepare dry nano-structures at room temperature and in large scale. The most important point herein is plastic deformation and stress release, one can interpret the plastic deformation as the crystal rotating of Zn and the grain extension of Ti, while stress release may be considered as the re-crystallization of the Zn nano-structure and the particle size reduction of the Ti-Zr binary alloys.

Key words: Roller vibration milling, nano-structure manufacturing 1. Introduction

Mechanical milling has been finding diverse applications in particle size reduction, mechanical alloying, and chemical reactive milling. However, the particle size limit has been questioned for a long time, and it is evaluated that the particle size reduction of ductile materials by compression is impossible ^[1].

The frequently asked question remains open to date whether it is possible to produce stable nano-structures by universally mechanical method in dry mode at room temperature and in large scale. We report the use of roller vibration milling (the RVM) in manufacturing single elemental Zn nanao-structures and synthesizing binary nano-composite of Ti and Zr. The RVM used has a chamber of 2.5 L and a motor of 0.12 kW.

Distinguished in our experiment is that it exerts predominantly normal pressure on the particles in the mill chamber, and the loading intensity is only one fourth of that in the conventional vibration mills of the same dimension.

2. Grain rotating and re-crystallization: the way to nano-structure optimization ^[2-3]

We have succeeded in synthesizing structurally near perfect Zinc nano-particles of 3-5 nm in diameter by the RVM in dry mode and at room temperature in batch of 150 g, and the total energy cost is about 1 kwh. The synthesized nano-structures are single crystalline, transparent, uniform, randomly oriented, almost equiaxed, and mostly free from defects. The particles result from 3 circulations of structural revolution, each of which comprises plastic deformation, size reduction, and re-crystallization respectively. The energy of the plastic deformation, which has certain period for accumulation and relaxation, is the engine and power for size reduction and re-crystallization, that took place synchronously. Whenever a circulation is finished, the size and defects of the nano-particles are reduced, and the structure is improved.

The plastic deformation featured rotational motion of grains and turbulence. The particles are distributed randomly and turbulently, creating many local vortexes, around that the particles centered and radial rearranged, verifying rotational mechanism. The rotating grains have the tendency to leave the matrix, however, at ambient temperature the diffusion power of the atoms is insufficient, and the grains cannot remove immediately until the accumulation of the deformation energy reaches a certain level, at which the structures begin to relax toward lower energy state, bring more finer particles with re-crystallization. It is the grain rotation and the re-crystallization that make the nano-structures approaching optimal.

3. Prior plastic deformation and particle implantation: Synthesizing binary nano-composite of Ti50Zr50 ^[4]

The attempt to prepare Ti-Zr nano-structured materials by MA started at about the early 1990s ^[5]. They found that the amorphization of pure metals or binary alloys consisting from the same group metals in the periodic table and exhibiting the complete solid solubility such as Ti-Zr is hard or impossible by MA. Without exception, all the following works obtained amorphous products containing the component of the atmosphere gas ^[6,7].There has been no work presented about the pure binary nano-alloying of Ti-Zr by MA to date.

We Report the first successful effort to synthesize stable Ti₅₀Zr₅₀ binary nano-composite of grain size about 7.6 nm by the RVM with a batch volume about 170 g at room temperature. According to the desired mol ratio of 1:1, we weighed Ti (-200 mesh, purity > 99%) 57.9 g and Zr (-300 mesh, purity > 99%) 110 g as starting materials, and mixed sufficiently with 4wt% stearic acid respectively as process control agent. Prior-processing the Ti powders 20 hours in the mill in advance, we then put the Zr powders into the container mixed with the Ti powders for MA up to 60 hours, but the machine had to stop for sampling and stress release every 10 hours. Fig. 3 shows the SEM image of the binary composite.

Fig.3. SEM image of the binary composite.

XRD and High Resolution Transmission Electron Microscopy (HRTEM) revealed that the composite remains the basic hexagonal feature, but the axial ratio is much smaller than that of the ideal hexagonal close-packed structure and the standard value of Ti and Zr. The alloying process contains plastic deformation, grain size reduction, particle implantation, and atomic diffusion and permeation; herein the particle implantation of Zr into Ti is the initiation for the alloying. It is the Zr size reduction created reactivity that promotes the particle implantation. If the Zr grain size did not reduce sufficiently, they could not be inserted into the Ti matrix. On the other hand, The plastic prior-deformation of Ti provides the matrix and guarantee for the reaction. Especially, if there were no enough pre-deformation of Ti, there would be no any alloys synthesized.

Surface chemical change analysis by photoelectron spectrum demonstrated that the mechanical property, biocompatibility, corrosion resistance, and the wear resistance of the materials might increase as used in orthopedic applications.

4. Discussion ^[8]

We have shown from the above that plastic deformation and stress release is the leading mechanisms prevailing in both milling and mechanical alloying processes.   One can interpret the plastic deformation as the crystal rotating of Zn and the grain extension of Ti, while stress release may be considered as the re-crystallization of the Zn nano-structure and the particle size reduction of the Ti-Zr binary alloys.

What is stress release herewith actually? In a word, it may be defined as particle size reduction at unloading. Stress release is founded on plastic deformation, which originates from a low velocity normal pressure creating tensile stress field in the materials. Because tensile strength of most materials is much lower than its compressive counterpart, the cost for particle size reduction in tensile stress field is the lowest. Thus, stress release is the best mode not only for size reduction, but also for a stable nano-structure.

However, since the plastic work is irreversible for stable flow, there will be no stress release, if there is no any excitation from the exterior. The excitation may come from the impact induced chaotic motion. Harmful chaos can disturb the motion stability or even damage the machine, but one can filter them out to leave the useful chaos to disrupt the ordered natural frequency distribution of the particle system, benefiting its size reduction.

5.       Conclusions

What the RVM provides is stable motion with enough robustness, mild normal pressure for inducing plastic deformation, and certain exciting calling for a stress release leading to particle size reduction at unloading. We claim that the RVM can prepare dry nano-structures at room temperature and in large scale. The most important point herein is plastic deformation and stress release, one can interpret the plastic deformation as the crystal rotating of Zn and the grain extension of Ti, while stress release may be considered as the re-crystallization of the Zn nano-structure and the particle size reduction of the Ti-Zr binary alloys.

References

[1] Kendall K., The impossibility of comminuting small particles by compression, Nature 1978, 272(20): 710-711.

[2] Wang Shulin, et al, Nanostructural Evolution of Zn by Dry Roller Vibration Milling at Room Temperature,Progress in Natural Science,2006,16(4)£º441-444.

[3] Wang Shulin, et al, Large Scale Production and optimization of Nanostructures by Mechanical Method, Frontiers of Design and Manufacturing, 26 Weemala Ave, Riverwood, NSW 2210, June, 2006, Australia, Vol.2, 309-312.

[4] Wang Shulin, et al, Synthesis of Binary Nano-composite of Ti₅₀Zr₅₀ and Its Characterization, Journal of Alloys and Compounds, 2007,429(1-2)£º227-232.

[5] K. Aoki, A. Memezawa, T. Masumoto, Appl. Phys. Lett. 61 (1992) 1037.

[6] A. Memezawa, K. Aoki, T. Masumoto, Scr. Metall. 28 (1993) 361.

[7] K. Aoki, A. Memezawa, T. Masumoto, Mater. Sci. Eng. A181/A182 (1994)

1263.

[8] Wang Shulin, Impact chaos control and stress release: a key for development of ultra fine vibration milling, Progress in Natural Science, 2002, 12(5): 336-341.