Page 100 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin
P. 100

Chapter 5
50 40 30 20 10
0 -10 -20 -30 -40 -50
-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 Magnetic field (T)
Figure 5.7 Effect of plasma power on the hysteresis (M-H) loops of products synthesized at 35 ppm Ni(cp)2 vapors: (a) 1.3 W, (b) 2.0 W, (c) 2.7 W and (d) 3.4 W.
(1) The Ms of nanoparticles decreases with decreasing particle size. In the theory, it follows the formula8: 𝑀𝑀𝑀𝑀" = 𝑀𝑀𝑀𝑀"$(1 − 6𝑡𝑡𝑡𝑡/𝐷𝐷𝐷𝐷)
where Msb denotes the magnetic saturation of the bulk nickel, t denotes the thickness of magnetically inactive layer, and D corresponds to the diameter of nanoparticles. A lower plasma power results in smaller-sized nanoparticles, leading to an increased surface-to- volume ratio as well as a higher fraction of magnetically inactive material. Therefore, the Ms of nanoparticles is relatively larger when synthesized at higher plasma powers.
(2) Less CNTs are formed at high plasma powers. As a consequence, high purity Ni nanoparticles are obtained, leading to an enhanced Ms. Our results show that CNTs coexisting with Ni nanoparticles at low plasma powers, while are suppressed at high powers. A reasonable explanation is that the Ni nanoparticles generated at lower power are smaller, resulting in larger specific surface area, thus, allowing carbon supersaturation at metal/gas interface readily. Therefore, they are more active in catalyzing hydrocarbons to form CNTs. This is in agreement with preceding findings, which showed that the CNTs formation rate depended inversely on catalyst size, and no CNTs were formed by catalysts with diameters larger than 7 nm.31,32
(3) The Ms of fcc Ni are much larger than that of the hcp phase, and they are more stable at high temperatures. The increase of plasma power contributes to the transition of Ni nanoparticles from hcp to fcc phase, resulting in an improved Ms value.
              (d)
(c)
(b)
(a)
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Magnetization (mAm2/g)




















































































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