PREPARATION OF SILICON NANOPARTICLES BY MEANS OF ...

12. - 14. 10. 2010, Olomouc, Česká Republika

PREPARATION OF SILICON NANOPARTICLES BY MEANS OF DISINTEGRATION IN A CAVITATION WATER JET Richard DVORSKÝa, Aleš SLÍVA a, Jiří LUŇÁČEK a, Kateřina PIKSOVÁ b a

VSB-Technical University of Ostrava, 17. listopadu 15, 708 33 Ostrava-Poruba, Czech Republic b

Czech Technical University in Prague, Břehová 7, 115 19 Praha 1, Czech Republic

Abstrakt -1

High-energetic water jet (at cross-sectional velocity exceeding 660 ms ) represents a liquid flow with cross-2

sectional power density about/or above 144 kWmm . In the experimental Water Jet Mill (WJM) device, potent dynamics of disintegration of silicon microparticles develop during the passage of modified cavitation water jet through the microparticle dispersion. The main mechanism of this disintegration of solid microparticles is the extreme impact pressure of a water-hammer on particle surface in the termination phase of the implosion of vapor cavitation bubbles. Around the working temperature of WJM about 65°C, cavitation erosion of solid materials in water reaches high intensity. Gradients of flow velocity in the disintegration -1

zones of WJM rise up to 1000 ms /mm and generate markedly high tensile stresses in the liquid, resulting in -4

the creation of vapor cavitation bubbles. The cavitation coefficient reaches values of 10

rank and the

cavitation intensity is very high. Moreover, a high value of specific surface of solid particles in the fine water micro-dispersion is manifest, thus fostering the process of heterogeneous nucleation of cavitation bubbles on individual particles. The impact pressure of liquid in the termination phase of the collapse of spherical bubbles reaches values as high as 5.7 GPa at normal pressure of the surrounding liquid, namely 0.1 MPa. Due to the rapid process of refinement, it is possible to expect a significant influence of crushing mechanisms on the overall process of disintegration with the given energy densities. Apart from fundamental specification of the disintegration process, the present paper has another major focus, i.e., the presentation of experimental results of disintegration of silicon microparticles into the nano-scale. Keywords: nanoparticle, cavitation, implosion, disintegration, milling, grinding.

INTRODUTION The technology of high energy Water Jet (abbreviated as WJ) [1-2] was initially developed in NASA (USA) for cold cutting of hard fireproof ceramic components of space shuttles [3-4]. Presently, the fundamental issue of obtaining stable working pressure in liquids in the range of hundreds of MPa is successfully solved for values approximately up to 400 – 600 MPa [5]. Apart from the common use of WJ for cutting processes in contemporary industry, there is a remarkable technical potential of WJ in the nanotechnology area, especially in the domain of extreme efficacy nanomilling. The promising potential of WJ nanotechnology can be illustrated by the applications listed below: 1. Micro-nozzle application – increase in the hydraulic velocity field gradient in the surrounding fluid and reduction of the input flux of ballast liquid into the grinding suspension.


2. Exploitation of pure vapor microcavitation – increase in disintegration effectivity of cavitation during the implosions in pure vapor microbubbles [6-8]. 3. Application of WJ pulse regime – improvement in dynamics as well as in the maximum impact pressure in macroranges by means of the waterhammer effect up to the values of several GPa [9-10]. The fundamental physical mechanisms of fine solid particles disintegration in the prototype device called Water Jet Mill (abbreviated as WJM) are described in the present paper. The nanoscale disintegrationinducing capacity of the WJM prototype device has been verified by performing sample disintegration of 100 µm silicon particles. The milled particles in the operation area are wafted with a liquid dispersion and cyclically pass through a system of cavitation zones. The zones have been generated by ultrahigh energy cavitating water jet in the mixing chamber and the cavitation tube. For the first time, the prototype WJM has st

been presented at the 1 Nanomaterials and Nanotechnology Meeting NANO OSTRAVA in 2008 [11].

1.

PHYSICAL BACKGROUND

The prototype WJM utilizes high dynamics and power density of the liquid jet interacting with a liquid suspension of fine milled particles. Controlled bypass

400MPa cavitating water inlet Mixing chamber with 0,23mm water nozzle

Milling suspension reservoir

Control manometer

Abrasive nozzle 0,9mm >400 nm separator

Top ejector inlet

Cavitation tube Nano dispersion reservoir

Mill chamber

Reflux column >40 nm separator Outlet of balast water WC impact target

Fig. 1 General scheme and photo of the first prototype of Water Jet Mill.

Given the operating pressure of the pump ca. 400 MPa, the cross-section power density amounts to 164 kW.mm

-2

-1

at outlet velocity of the liquid jet around 660 ms . Taking into account the diameter of the WJ

nozzle (0.23 mm) and the abovementioned cross-section power density, the corresponding cutting energy of


the water jet cross-sectionally reaches approximately 8kW. Fig. 1 provides an explanation of the basic milling regime of the WJM. The liquid suspension of milled particles leaves the milling suspension reservoir to enter the mixing chamber with diamond cavitating water nozzle 0,23 mm. Subsequently, primary disintegration takes place in the mixing chamber in contact with the cavitating water jet. The suspension enters the cavitation tube after going through the rectification abrasive nozzle, proceeding upward in a vertical direction after a stream direction change in the lower target area. The upstream velocity averages around 10 cms

-1

and is accompanied by intensive turbulence phenomena. Upon reaching the upper orifice level of the cavitation tube, a part of the suspension is periodically drifted back to the tube and re-enters the milling cycle.

Fi - normal inertial force Fh - hydrodynamic pulling

Fh

Fig. 2 Mechanisms of a microparticle breaking in WJM: A - Rapid dynamic shear stress in an extreme flow velocity gradient, B – Extreme waterhammer effect in an asymmetric cavitation collapse, C – Direct impact of larger particles on the hard target.

We assume a cyclic repetition of three fundamental disintegration effects as shown in Fig. 2 during the milling process.

1.1

Rapid dynamic shear stress in an extreme flow velocity radial gradient (see Fig. 2.A)

Direct mechanical destruction of particles is caused by the high dynamics as well as the high value of shear stress at the inlet of the liquid suspension to the cavitating flow. Radial velocity gradients in the narrow pipe -1

of the primary abrasive nozzle 0.9 mm in diameter can reach extreme values even over 1000 ms /mm. With maximum characteristic dimension of the microparticle, the bilateral speed difference of the circumfluence -1

can amount to as much as 100 ms . In quasiglobular particles, the latter velocity gradient induces rotation as well as a radial buoyancy force directed the stream axis. Due to particle rotation, the shear and buoyancy stresses exhibit high dynamics in the direction and the value of deformation forces.


1.2

Pure vapor microcavitation bubble implosive collapse (see Fig. 2.B)

Cavitation growth and collapse at high pressure gradients with characteristic dimensions of the millimeter range take place in extremely short characteristic time (in the range of microseconds). This fact assures the domination of very small cavitation bubbles with high spherical stability, preventing the creation of inner jets. Thus, we can assume that the course of the collapse will be of spherical nature until the final bubble destruction by its impact onto the condensate (see Fig.2.B). The primary cavitation disintegration of particles takes place with the implosive collapse of pure vapor microcavitation bubbles from cavitating water nozzle after their „collision“ with particles in suspension [12]. The secondary cavitation particle disintegration occurs in the areas inside the cavitation tube. The source of cavitation in current technical arrangement is the extreme velocity gradient of the high energy liquid jet mixing with the slow suspension of milled particles. The design of the cavitation tube is optimized for ongoing generation of cavitation bubbles based on heterogeneous nucleation right at the phase interface between the liquid and the particle [6]. Consequently, there is preferential emergence of adhesively-bound couples of particle-cavitation bubble, with the resultant couples undergoing mutual cavitation evolution. The -4

cavitation coefficient amounts to 10 ranges under the following conditions: 20°C temperature and 0.1 MPa pressure. Nonspherical collapse of cavitation bubble on the particle surface creates an extreme shock wave in the area of impact of the liquid. The impact pressure

pmax during te cavitation bubble implosion reaches a o

minimum value of 57 GPa [7] with ambient temperature of 20 C and surrounding pressure of 1 MPa. For most disintegrated materials the compression resistance is far below the above-mentioned value.

1.3

Direct impact of larger particles on hard wolfram carbide target (see Fig. 2.C)

Direct mechanical disintegration of particles by impact to a rigid sintered wolfram carbide (WC) target takes place following the outlet of suspension from the cavitation tube. This disintegration zone at the end of the cycle destructs especially large particles in which the inertial forces Fi dominate over the hydrodynamic drift forces Fh in the stream of fluid. Very small particles are drifted by way of streamline diversion along the impact target surface without mechanical contact, which fact is related to the small inertia of the given particles.

2.

EXPERIMENTAL DESIGN

An important factor of the overall disintegration process in WJM is a technical solution enabling multiple particle circulation through all the levels of disintegration zones. In the course of the primary turbulent filtration, a final submicron fraction 400nm in size is directed through the input pipe back to the milling cycle. Initial experience suggests that the turbulent filter throughput can be adjusted so as to compensate the pure liquid inflow via WJ nozzle and to ensure a constant volume of circulating suspension in the milling suspension reservoir. The output submicron dispersion is further concentrated by the method of turbulent cross flow ultrafiltration and the ballast liquid is disposed of in fraction