Boride particles

Fig. 12.3. Average diameter of nickel boride particles as a function of...

In this system the average particle size depends on the BH4 Ni " ratio, the nickel salt concentration and the size of the inner water core of the reversed micelle. A BH4" Ni ratio of three gives the smallest size catalyst particles. Lower ratios lead to the formation of larger particles while higher ratios have no further effect on particle size. Micelles with smaller water cores produce smaller catalyst particles. The effect of nickel salt concentration on particle size is complex the smallest particles are formed with a concentration of 5x10"2M. Fig. 12.3 shows the relationship between the micelle composition, nickel salt concentration and the nickel boride particle size. For any given preparation the catalyst particles are essentially uniformly sized with only a 0.5 run distribution. ... 

Petit C and Pileni M P 1997 Nanosized cobalt boride particles control of the size and properties J. Magn. Magn. Mater. 166 82... 

Monodisperse colloidal nickel boride and cobalt boride particles are synthesized by reducing, with NaBH4, the metallic ions solubilized in the water cores of the microemulsions. The NaBH4/MCl2 ratio was held equal to 3, because larger particles were obtained for a lower value, the particle size remaining constant above that ratio [2-4]. 

Figure 14 shows the dependence of the nickel boride particle size on the water content in the microemulsion and on the Ni(Il) ion concentration. The average size of the particles decreases with decreasing size of the inner water core (decreasing water content), while... 

The diameter of the particles is systematically higher than the diameter of the inner water cores. For all the particles synthesized, we calculated the proportionality factor F by systematically varying the value of the minimum number of ions required to form a nucleus (/). Only if / takes the value of 2 is the factor F reasonably constant (see Tables 13a and b). The order of magnitude of the factor F is always 10 . This means that at the very beginning of the reduction, i.e., when the nuclei are formed, only one aggregate per thousand leads to the formation of metal boride particles. 

For a constant microemulsion composition, at low ion concentration, only a few water cores contain the minimum number of ions (two) required to form a nucleus hence, few nuclei are formed at the very beginning of the reduction, and the metal boride particles are relatively large. When the ion concentration increases, the distribution of precursor ions in the microemulsion is very different (Fig. 16), and the number of nuclei obtained by reduction increases faster than the total number of ions (Fig. 17). This results in a decrease... 

The F values for nickel boride and cobalt boride particles are quite different. For the former the value obtained for F is equal to 3.2 x 10 and for the latter, 17.4 x 10" As for these experiments, the rearrangement rate of the microemulsion system is constant in the first approximation, the difference between the F values probably being due to the different solvation of the two types of ions at the interface. The Co(II) ions contain, on average, one hexanol molecule in their first coordination shell, while the Ni(II) ions are multiply coordinated with hexanol at the interface. The mobility of the latter is hence lower, and the probability of collision between the two reduced Ni atoms required to form a nucleus is also lower. In other words, the rate of nucleation is higher for cobalt boride than for nickel boride particles. 

Figure 20 Three traces obtained during the formation of cobalt boride particles, (a) Absorption variation of an aqueous solution and (b) that of an ethanolic cobalt chloride solution (0.1 M) during the reduction of the cobalt ions, (c) Trace obtained for the same reduction but in a Triton X-100 microemulsion sample ([Triton X-100] =1.5 M and / = 13.3). Note the small changes (W, E, and M) in absorbance due to addition of the reducing agent solution.

The results indicate a difference between the traces obtained for generating colloidal boride particles in pure solvents (a single-component system) and those obtained in microemulsions. 

Ghosh, S. et al. (2001) Synthesis of [(Cp Re)2BnHn] n=8-10 metal boride particles that stretch the cluster structure paradigms. Angew. Chem. Int. Ed.,... 

Petit and Pileni [439] reported a relatively recent work on the formation of cobalt boride in which the functionalized surfactant Co(AOT)2 was used in isooctane to form reverse micelles. Sodium borohydride solution was added to it to form the boride particles varying in size from 4 to 7.5 nm. When NaBH4 was used in the aqueous core of a separate reverse microemulsion and similar volumes of the two entities containing Co and B were mixed, the particles that formed had an average size of 10 nm. Similarly, Duxin, Pileni and others [440] synthesized Fe-Cu-B nanoparticles by preparation of Fe(DS)2 and Cu(DS)2 and their reaction with NaBH4. 

C. Petit and M. Pileni, Nanosize cobalt boride particles Control of the size and properties,... 

The ability to remove ZrO layers on the boride particles and the formations of liquid phases have been advanced as a reason for the usefulness of many additives. PS studies of cold isostatically pressed ZrB with additions of MoSi (5-20vol%) powder at 1800-1850°C resulted in an increase in the RD from 86.3-99.7% (Sciti et al., 2005 Sciti et al., 2006). Similarly, ultrafine ZrB -SiC (20vol%)-Mo (4wt%)... 

It is worthnoting that the inhibition of sintering in non-oxide ceramics is generally attributed to the presence of oxide impurities on the powder particle surface (Zou, 2011). In this respect, the above mentioned reactions help the removal of surface oxides, like B2O3, from the borides particle surfaces. 

The nature of the boride particles and the composition of the mixed Ni-Co-B particles were determined by XPS and EDX, respectively. Table 3 shows the initial composition of the luckel and cobalt chlorides dissolved in the microemulsion (X). The composition of the nanoparticies (x) was determined by EDX measurements. It can be seen that the compositions X and X are equal within 3%, 5% being the largest difference. 

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