Graphite spheroids

It should be recalled that the final step in the nodular iron treatment process is termed "post inoculation." The purpose of this procedure is to aid in the elimination of iron carbides and promote enhanced nucleation and proper growth of graphite spheroids. This is accomplished by the introduction of the element silicon (usually a ferrosilicon alloy) along with calcium and maybe some magnesium or rare earth. It has been demonstrated that the benefits of rare earth additions are not affected as a function of the time in the process that they are added (23). For example, the elimination of iron carbides by use of the rare earths is possible if the rare earths are introduced along with the primary nodulizer or with the post inocu-lant. In passing, it should be remarked that both the primary nodulizers and ferrosilicon inoculants contain about 1% calcium. 

Magnesium ferrosihcon alloys react vigorously when added to molten iron. As the magnesium vaporizes and cools, it reacts with residual surface tension modifiers such as sulfur and oxygen and greatly increases the surface tension of the molten iron. The dissolved graphite in the molten iron nucleates and grows into a spheroidal shape because of the increased surface tension of the molten iron. 

Tellurium also improves the properties of electrical steels by aiding in the magnetic anisotropy, malleable cast iron (77), and spheroidal (graphitic) cast irons (see also Metal surface TiiEATiffiNTs). 

Properties of spheroidal graphite-grade cast irons. 58... 

Onion-like graphitic clusters have also been generated by other methods (a) shock-wave treatment of carbon soot [16] (b) carbon deposits generated in a plasma torch[17], (c) laser melting of carbon within a high-pressure cell (50-300 kbar)[l8]. For these three cases, the reported graphitic particles display a spheroidal shape. 

O. Yanagisawa, A. Almansour, K. Matsugi, T. Hatayama. Computer simulation for sohdified microstructure prediction of spheroidal graphite cast iron of the Fe-C-Si system. J Jpn Inst Met 50 1101, 1996. 

Fig. 3.44 Structure of typical spheroidal graphite austenitic iron...

Table 3.46 Composition and properties of principal grades of flake and spheroidal graphite austenitic case irons...

Practical experience indicates that the corrosion resistance of the flake and spheroidal graphite irons is similar in many enviroiunents however, the spheroidal graphite irons have shown superior corrosion resistance to the equivalent flake graphite grades in a number of cases ... 

Iron/ carbon alloy, poured as a hot molten liquid into a mold. Usually produced as either gray iron (where flakes of graphite are embedded in an iron matrix) or nodular iron (spheroids of graphite in the matrix). 

Performance of Thermally Purified Spheroidal Natural Flake Graphite SLA 1020 and Surface Coated Natural Graphite SLC1115... 

Figure 3. Scanning Electron Microscope Images of thermally purified natural crystalline flake graphite 2900G (a), and its ground versions displaying platelet (b) and spheroidal (c) morphologies.

Figure 4. Less uniform Si coating on plate-like graphite (a), rather uniform Si coating on spheroidal graphite (b) and a magnified view of Si particulates on spheroidal graphite (c).

Therefore, due to considerations of uniformity of coating, in further tests it was decided to focus on the spheroidal metal-graphite composites. 

The comparative 2016 coin cell data of the uncoated vs. Si-coated spheroidized natural graphite precursor vs Li metal counter electrode (Figure 5) reveals a remarkable effect of metal addition onto the reversible capacity of anode. Thus, the starting material s reversible capacity increased from approximately 350 mAh/g to over 510 mAh/g (data taken at 0.8V vs Li/Li+), which is 1.37 times higher than the theoretical value of the reversible capacity for graphite. 

Figure 5. First charge and discharge of CR2016 coin type half-cells with anodes based on the 15 pm, spheroidized natural graphite (bold line) and the similar graphite modified by 8wt% Si at the C/15 rate.

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