PREPARATION OF γ-Al2O3 NANOPARTICLES BY MECHANO-CHEMICAL AND SONOCHEMICAL REACTION

Bo Xu, Shengjuan Li, Shulin Wang, Laiqiang Li

1 INTRODUCTION

The γ-Al2O3 nanosolids, a kind of porous activated alumina, are widely used in plastic, rubber, ceramics and fireproofing materials as the reinforcing agent, where its characteristics of high thermal stability, adhesion resistance, high mechanical strength and wear resistance are required. It has more remarkable advantages of enhancing ceramic in its compactness, finish, fracture toughness, resistance to creep and abradability of macromolecule materials. Besides, γ-Al2O3 is also a good dispersant, which can be uniformly dispersed in many solvents, e.g. water, ethanol, acetone, benzene and xylene etc

In this paper, a novel method is applied to prepare porous γ-Al2O3 nanoparticles. At first, the 2h-milled aluminum powders are prepared as the starting material, then the powders react with water to produce Al(OH)3 collosol in an ultrasonic water-bath, lastly, dehydrating, grinding, and roasting the Al(OH)3 colloid at a given temperature to produce the porous γ-Al2 O3 nanosolids. This method, which combines mechano-chemical and sonochemical reaction, is extremely innovative, and it may inspire new ideas for preparation of nanomaterials.

2 EXPERIMENTS

2.1 Preparation of aluminum ultra-fine particles

Experiment was conducted in a dry roller vibration mill (RVM) at room temperature. The RVM has a chamber of 2.5L, equipped with a motor of 0.12kW. To provide proper atmosphere and prevent dust explosion, the entire operation was performed in a glove box filled with argon. In a typical experiment, 100g raw aluminum powder (with average sizes of 150µm, purity higher than 99.5%, purchased from Guoyao group chemical reagent Co. Ltd., China) is placed in the grinding chamber with the stainless steel rods as grinding medium, and the weight ratio between the medium and powder is 60:1. The 2h-milled sample is then collected for later use.

2.2 Synthesis of γ-Al2O3 porous nanoparticles

2.0g of 2h-milled aluminum powder was put into a beaker with the water of 50mL The beaker was placed in an ultrasonic water-bath (DL-120J, made in China), setting the supersonic frequency of 100kHz at the room temperature. Under the high temperature and high pressure circumstances generated by the ultrasonic cavitation, the energy stored in the material was fully released, and the particles reacted with water to produce Al(OH)3 white latex. In succession, it was dehydrated in a drying cabinet (101 A-1, made in China) at 80°C for 6h to remove the excess water. Lastly, grind the gel into white powder and place it in an electrical resistance furnace (SX2-5, made in China) to calcined 4h at 160°C, the porous γ-Al2O3 nanoparticles were obtained. The experimental flow chart is shown in Figure 1.

2.3 Characterizations of the particles

Structural phase analysis was carried out with D/max-γA X-ray diffractometer (XRD), using Ni-filtered Cu-Kα radiation as the X-ray source. The scanning speed was 4°/min. The morphology of the sample was observed using a FEI Quanta 450 scanning electron microscope (SEM), and the accelerating voltage was 20 KV. Transmission electron microscopy (TEM) image was recorded on Tecnai G2 20 S-Twin electron microscopy at 200 kV. All measurements were carried out at room temperature.

3 RESULTS AND DISCUSSION

3.1 The structure analysis of aluminum powders

From the SEM image (Figure 2a), we can see that the particles sizes are in the range of 0.5-0.8µm after milled for 2h, the size is in good agreement with the particle size measured from TEM image in Figure 2b, and the SEM image shows a uniform, flaky crystal pattern with 0.5µm in width and 0.8µm in length. The microstructure of the nanoparticle is also confirmed by HRTEM (Figure 2c). The result shows that the solid particles, under the action of mechanical force, generate a mass of deformation and dislocation flaws, leading to the material to a metastable high-energy state, which is favorable for mechano-chemical reaction.

Figure 3 illustrates the XRD patterns of the raw and the 2h-milled aluminum powders respectively. In Figure 3, after two hours of milling, the diffraction peaks are still indexed to the face-centered cubic lattice aluminum, but Al(OH)3 should be recognized as a minor phase. This is because the material is in a metastable, high-energy activity state during the grinding process, which has a high mechano-chemical activity, therefore, as the particles are exposed in atmosphere, the solid particles will react with the water vapor, and then a small amount of Al(OH)3 is generated.

3.2 Ultrasonic hydrolysis reaction

The ultrasonic hydrolysis reaction is the next step to create the γ-Al2O3. The hydrolysis experiment was completed in an ultrasonic water-bath. Under the local high temperature and high pressure circumstance generated by cavitation effect, the water is decomposed into the ·H and ·OH free radicals. The O2 dissolved in the solution is also a decomposition response and produces ·O free radical. Because the ·OH possesses unpaired electrons, it has a strong oxidability, so it is prone to induce the redox reaction. The reaction process is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The cavitation bubbles, in the collapse process, raise the liquid foam-core temperature up to 5200K and the pressure to 5.05×107 Pa. The local high temperature and high pressure condition exists only a short time, causing the temperature gradient as high as 109K/s, and at the same time, accompanied by a strong shock wave and micro-jet with the speed up to 400km/h, which provides an extreme physical and chemical condition for such difficult or even impossible chemical reaction.

On the other hand, in the milling process, the specific surface area of the solid particle increases as the particle size decreases. The adsorptivity of the particle surface is strengthened, and the surface free energy will change the electric charging into free radical, exoelectron emissing etc, so the chemical reaction rate is improved greatly. Under the loading of mechanical force, the solid particles have lattice distortion, dislocation and other defects, which will also change the equilibrium of the chemical reaction and the activation energy, thus obtain an extremely high chemical reaction activity. In the local thermal disturbance and tension stress field generated by ultrasonic cavitation, the particles fully release the energy stored in the material, and reacted with the water to produce Al(OH)3 nanoparticles in a short period of time, as the XRD pattern shown in Figure 4.

3.3 γ-Al2O3 nanoparticles characteristics

As shown in Figure 5, it is clear that Al(OH)3 is transformed into γ-Al2O3 after dried and roasted. From the TEM image (Figure 5a), it can be seen that the products are of porous, flaky nanostructures, no obvious aggregation, and the average particles sizes are in the range of 30-50nm. Figure 5b shows the XRD diffraction pattern of the sample. The diffraction peak position is consistent with the one in XRD standard diffraction card of γ-Al2O3.

4 CONCLUSIONS

After the vibration milling for 2 hours, the raw aluminum powder generates a large number of strain and dislocation defects. The material is in a metastable, high-energy active state. In the local thermal disturbance and tension stress field generated by the ultrasonic cavitation, the particles react with the water to produce white Al(OH)3 sol in a short time. The porous γ-Al2O3 nanoparticles are successfully obtained by drying, grinding and roasting the Al(OH)3. Compared with other preparation methods, this novel method can dramatically reduce the production cost.

ANALYSIS OF SQUEEZE FLOW OF A BI-VISCOSITY FLUID BETWEEN TWO RIGID SPHERES

C H Xu, M Zhang, Y Xu and L N Zhang

College of Science, China Agricultural University, Beijing, 100083

1 INTRODUCTION

Recently, a science named granular matter mechanics which studies the balance and the characteristics of motion as well as the application of the granular matter is formed. The study of granular matter mechanics aims mainly at dry particle system. If the particles have wet surfaces, the liquid bridges form at the contact point when the spheres approach each other. Meanwhile, complicated interconnected structure and flow law which influences directly the deformation and strength of the particle are created. At present, one of the difficulties is the squeeze flow and tangential movement of an interstitial liquid between particles.

The use of Bingham fluid model in the problem of squeeze flow results in no relative movement between two spheres or parallel disks, which named yield-surface paradox. By now the fluid model is too ideal to describe the physical nature. Based on general bi-viscosity fluid model, Zhu et al. created the theoretical analysis of the influence of electric field strength to the squeeze flow between disks, which solved the problem of yield-surface paradox. In terms of squeeze flow between two spheres, Adams and Edmondson formulated the lubrication problem for a power-law fluid between two equal spheres. Lian studied the squeeze force and liquid bridge force of a Newtonian fluid between two spheres which combined with the simulation of DEM. Rodin considered near touching unequal spheres embedded in a power-law fluid and obtained the squeeze force. The group of the China Agriculture University studied deeply normal and tangential movement of an interstitial liquid between two spheres. They obtained analytical solution of squeeze viscous force as well as the tangential force and moment, which furthered the development of this difficult problem.

2 MATHEMATICAL MODEL

Consider two rigid spheres S1 and S2 of radii R1 and R2, translating towards each other with a relative velocity VZ along axis Z and the bi-viscosity fluid between two spheres is squeezed, as shown as Figure 1, where B is the radius of liquid bridge, which is equal to R* in the immersed system and s0 is the minimum gap between two spheres, which make s0[much less than]min (R1, R2) satisfied. In the narrow gap, the two near surfaces of the spheres can be approximated by the following expressions:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.1)

where r is the radial distance.

The separation distance of the spherical surface is represented as:

s(r) = s0 + r2/2R* (2.2)

where R* is the harmonic radius, which is defined as: 2(R*)-1 = (R1)-1 + (R2)-1

The boundary conditions between the spherical surfaces may be expressed as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.3)

The constitutive relation of a bi-viscosity fluid is given as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.4)

where σrz is shear stress, σ1 is dynamic yield stress, σ0 is yield stress; ηr and η is viscosity. When |σrz| ≥ σ1, the fluid flows by the viscosity η and when |σrz| < σ1, the viscosity ηr is larger. The viscosity ratio is equal to η/ηr, which is generally equal to 10-5 to 10-2. The boundary of the two regions is yield surface. Obviously, the relation of σ1 and σ0 can be expressed as:

σ0 = σ1(1-γ) (2.5)

3. THEORETICAL ANALYSIS

3.1 The determination of half thickness of unyielding region

The continuity equation for an incompressible fluid becomes:

1/r = [partial derivative]/[partial derivative]r(rvr) + dvz/dz = 0 (3.1)

The term for pressure gradient in z direction is negligible, so the momentum can now be reduced to the following expression:

d p/d r = [partial derivative]σrz/ [partial derivative]z (3.2)

Integrating momentum Equation (3.2) obtains:

σrz = d p/d r = (z - z0 (3.3)

where z0 = z1 + z2/2 is defined as the location where the shear stress equals zero. One can define |σrz| equals σ1 the position of yield surface can be expressed:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.4)

where t = σ1 (-d p/d r)-1.

Substituting Equation (2.4) into Equation (3.2) and upon integration, the radial velocity Vr can be expressed as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.5)

Integrating continuity equation (3.1) combined with boundary condition one gets:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.6)

Substituting Equation (3.5) into Equation (3.6) we gets the expression of t:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.7)

Obviously, t is related to the pressure distribution gradient, we can obtain the dimensionless equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.8)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] Parameter c reflects the change of distance of the two spheres. The value of η and σ0 is certain for any fluid. So when B and s0 is certain, we can consider d as the nominal speed.

Equation (3.8) is the classical Cardan equation and a solution to Equation (3.8) is obtained as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.9)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The solution of Equation (14) is expressed as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.10)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

3.2 Pressure Distribution and Squeeze Force:

According to Equation (3.10), we can get the expression of the location of the yielding region as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.11)

Once the thickness of the unyielding region is determined, its relation with the pressure distribution can be obtained as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.12)

Integrating Equation (3.12), one can get the pressure distribution:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.13)

Then the dimensionless pressure is defined, so the above equation becomes:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.14)

where p0 = - σ0B/s0.

Integrating Equation (3.14), the squeeze force can be obtained as:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.15)

Then we can get the following dimensionless form:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.16)

in which F0 = πσ0B3/s0.