(70aq) Size Limit of Zinc Nanoparticles

Single elemental metal nanostructures with excellent properties give rise to new opportunities in scientific research and development of nanotechnology. Thermal evaporation and chemical vapor deposition methods are commonly used to prepare nanostructutures [1], but it often has to encounter high temperatures and to remove impurities, causing very high energy cost, or bringing negative influence on the particle performance. As an alternative, mechanical milling is cost-effective, and simple to manipulate. However, at present almost all the milling is wet with suspension as the end products, and the ?size limit? is estimated at 10-100 nm [2-5]. In a sense, it is the size limit that blocks the way of mechanical milling. Size limit originates from fully plastic behavior of particles under loading. Like other chemical and physical properties changing with particle size, the particle deformation will be changed from elastic or elastoplastic to fully plastic, if the size decreases to a critical value. Depending on the intrinsic nature, the fully plastic deformed size is different for different materials, e.g. it is about 3-5 µm for lime stone, and about 1 µm for quartz sand [6]. Full-plasticity dramatically increases the impossibility of size reduction, and it is impossible to comminute ductile materials by compression in dry mode [7]. Fortunately, noticeable progress has been made in the study on mechanics of nanomaterials in recent years. Computer simulation revealed that plastic deformation in nanocrystalline solid Copper under uniaxiale expanding is due to large number of small sliding events in the grain boundaries, and occasionally partial dislocation may occur at a grain boundary and moves through a grain [8]. Analysis indicates that rotational deformation resembling turbulence in fluids or rotational vortices in liquid crystals may occur in nanomaterials[9]. Direct atomic-level observation by High-resolution transmission electron microscopy illustrated that partial disclination dipoles could be formed in mechanically milled nanocrystalline Fe, whose migration may facilitate fragmentation, leading to an ultra fine grain size[10]. Nevertheless, supporting experiments for this claim has not been forthcoming. The frequently asked problem, whether it is possible to lower the size limit by mechanical milling, and to produce stable nanoparticles or nanostructures of single elemental metals in dry mode and at room temperature, remains open to date. Here we show by experiment that near perfect Zinc nanflakes of 3-5 nm in diameter were successful synthesized by dry roller vibration milling[11] (RVM) at room temperature on a large scale, which were of single crystalline, transparent, uniform, randomly oriented, almost equiaxed, and mostly free from defects. The particles were prepared from 3 circulations of structural revolution, each of them includes plastic deformation, decomposition, and dynamic recrystallization respectively. Whenever a circulation is finished, the size and defects of the nanostructures are reduced, and the performance is improved, but the circulation period is prolonged with time. It is the circulation that makes the size of the nanostructures reduced and approaching perfect. The plastic deformation energy is the engine and power for size reduction and recrystallization, which has certain period for accumulation and relaxation. The plastic deformation featured rotational motion of grains and turbulence, and occurred principally alone the planes. When the rotating energy is accumulated to a certain level, the structures will begin to relax toward lower energy state (12), creating single crystalline or even finer particles. Due to the external exerting force the decomposed finer particles will in turn grow equiaxially, and then come into the next circulation of structural evolution. This size reduction mode seems to provide an opportunity to lower the size limit not only, but also open a way to optimize the nanostructures. The reciprocal force between the mass points and the tensile stress induced by the roller in the particles governs the grain fracture, and the former is strongly influenced by the distance between the mass points. When the distance is enlarged enough by the tension, the reciprocal force cannot increase any more, but the tension will increase continuously because the plastic deformation energy accumulates sustained in the particles, as a result, the grain will be decomposed. Our success reveals that the particle size, at which the particles turn to full-plasticity, is not the size limit, but a starting point to optimize the nanostructures and to produce even finer grains.