Analysis of Hetero-Coagulation Structure of Colloidal Mullite Precursor Powder -Experimental Approach and Computer Simul
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dynamics methods with DLVO type interaction was applied on the analysis of heterocoagulation systems in suspension. EXPERIMENTAL The primary particle diameter of alkoxied-derived SiO 2 particle used was 30nm. Preparation details were treated in previous paper [5]. Commercial ultra-fine y -A120 3 powder (TM-100, Taimei Chemical, Ltd., Nagano, Japan; specific surface area, SSA was 134 m2/g and estimated primary particle diameter, dB was 14nm) was added into the alkoxide-derived Si0 2 sol suspensions and ball milling for 24h. The total amount of suspension was about 450mL. The molecular ratio of Al to Si was fixed at 3 : 2 corresponding to a stoichiometric mullite phase. Adjustment of pH in suspension was induced by adding NH 4OH or HNO 3 to the solution. Fig.1 shows the relationship between ý -potential for raw powder materials and pH in suspension. The isoelectric point of y -A12 0 3 and Si0 2 were pH=9.5 and 3.0, respectively. Hetero-coagulation between y -A12 0 3 and 80 4.4--, '.-6.0 ,--9.5 Si0 2 was expected to occur in the range of 60 pH=3.0 to 9.5. b-, 01,from 40 y 1 -Electrophoretic mobility distribution I I 1y -A1 20 3was determined by a laser doppler method of 20 I I \ "I --------V -------. .
0 -20 -40 N
-60 -80
_SiO
2
I_
I
I
I
I
I
I
aggregate and hetero-coagulate size distribution
2!4 4n
Zeta potentials of ultra fine
7- A12 0, and Si0
in suspensions were determined by centrifugal After ball milling at granules of mullite lh, for pH=4.4, 6.0 and 9.5 precursor powders were prepared by spraydyn rmec upnina 0C 170tCu at suspension each drying from
12seiettomehd sedimentation method.
8
pH [-] Fig.1
each particle, with hetero-coagulates in suspension prepared by ball milling under a condition of pH=4.4, 6.0 and 9.5. The
2
particles
Calcined
granules of mullite precursor
powders at 80tC were uniaxially prepressed at 5.5 MPa, then isostatically pressed at I GPa for 3min (Model MCT-100, Mitsubishi Heavy Industries, Ltd., Tokyo, Japan). Sintering was accomplished in an electric furnace at 1600VC for 2h. Microstructure observations of sintered bodies were made on polished and thermal etched samples with SEM (Model JSM-TOO, JEOL, Ltd., Tokyo, Japan). BROWNIAN DYNAMIC SIMULATION FOR HETERO-COAGULATION Fig.2 show the calculating method of Brownian dynamics technique with DLVO type interaction. Firstly, y -A12 0 3 particles and Si0 2 particles were randomly generated by using pseud-random number in a two dimensional square field with periodic boundary conditions (Fig.2 (a)). In a Brownian dynamics computer simulation, the configuration space trajectories are calculated numerically. The trajectories are composed of successive displacement, A r, of 256
each particle taken over a short time step A T (Fig. 2(b)). These displacement, A r, are to be randomly chosen in accordance with Gaussian distribution (A rB) and DLVO type interaction ( A rD).
distance
A120
(a)
3
(b)
Fig.2 Brownian dynamics methods for hetero-coagulation process Such a displacement equation is given by ArT = ArB + ArD = f4t +
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