Experimental Procedure

B4C particle size distribution before and after separation are shown in Figure l.The particle size of B4C particles (H. C. Starck, Inc., Newton, MA) was measured using a laser light scattering analyzer (Horiba. LA-700. Irvine, CA). The particle size distribution of the as-received particles was bimodal as shown in Figure 1 (a). Therefore centrifugal experiments are necessary to obtain monodispersed particles. A centrifugal technique has been proven to be effective to separate B4C particles. As shown in Figure 1 (b), the tiny sized B4C particles are successfully removed. B4C particles used in this work have a unimodal distribution with the peak value of 2.27 jim.

Figure 1. B4C particle size distribution, (a) before separation, (b) after separation.

Figure 2 shows the experimental procedure of electroless nickel coating onto B4C particles. Before the electroless coating, the B4C particles were sensitized using SnCl2 and activated using PdCh. The surface sensitization was carried out by adding B4C particles into a SnCl2'2H20 (> 98%, Fisher Scientific, Fair Lawn, NJ) and HC1 (36-38%, EMD, Gibbstown, NJ) solution (0.07 M SnCl2-2H20, 40 ml/L HCI). The suspension was sonicated for 5 min followed by magnetic stirring for another 5 min at room temperature. The Sn2+ sensitized B4C particles were thoroughly washed with de-ionized water and transferred to a PdCl2 (> 99%, Fisher Scientific, Fair Lawn, NJ) and HCI solution (0.0042 M PdCl2, 40 ml/L HCI) for activation. After mixing at room temperature for another 10 min in the same way as that in the sensitizing step, the activated B4C particles were again thoroughly washed with de-ionized water and then introduced into the electroless coating bath. NiS04-6H20 (99.0%, Fisher Scientific, Fair Lawn, NJ) was used as the Ni2+ source, which was complexed prior to pH adjustment. The complexing agent used in this work was ethylenediamine (C2HsN2, Fisher Scientific, Fair Lawn, NJ).

Figure 2. Flow chart of the electroless nickel coating onto separated B4C particles.

The electroless coating temperature was kept at 85±2°C. The pH value of the coating bath was adjusted between 12 and 14 with 10 M sodium hydroxide (Fisher Scientific, Fair Lawn, NJ). Unlike the conventional electroless plating in which the reducing agent was one ingredient of the plating solution, in this work the reducing agent NaBHt (Fisher Scientific, Fair Lawn, NJ) was added into the coating bath drop-wise. As mentioned in the introduction, the effect of different factors on the nanolayer coating was studied. These factors include the amount of PdCh with respect to the amount of B4C in the activation step, the amount of complexing agent with respect to the amount of nickel, and the rate that NaBFU is added into the coating bath. The conditions studied are summarized in Table I.

Table I. Conditions Studied in Electroless Coating Procedure*

Chemical composition (molar ratio)

NaBHt addition rate

B4C:Sn2+

B4C:Pd2+

Ni:C2H8N2

Ni:NaBH4

Ni:B4C

All at one time 10 drops/min 1 drop/min

1:0.7

1:0.04 1:0.01 1:0.005 1:0.001

1:4.5 1:6 1:9

1:1

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