Evolution of microstructure of nanocrystalline SiC and diamond powders in high-pressure high-temperature conditions

R.Pielaszek1,2),  M.Avdeev3), E.Grzanka1,2), B.Palosz1), S.Gierlotka1), S.Stel'makh1)

1) High Pressure Research Centre, Polish Academy of Sciences, Sokolowska 29, 01-142 Warsaw, Poland
2) Warsaw University, Department of Physics, Hoza 69, 00-681 Warsaw, Poland
3) Budapest Neutron Centre, Budapest, Hungary
There is an increasing interest in the nanocrystalline materials with respect to their fundamental properties as well as potential applications for fabrication of high-tech construction and electronic ceramics [1]. Knowledge of microstructure, understood as morphology of crystallites, their size, shape and spatial arrangements during sintering is essential for technology of such materials. For us it's also basis (that has to be defined) for further strain analysis in high-pressure investigations [2,3].
Three samples of nanometric diamond, six of SiC and two samples of their ceramics obtained during high-pressure, high-temperature sintering experiments were examined. One sample of SiC of size of few microns in diameter was examined as a reference.

Analysis of the Small Angle Neutron Scattering (SANS) data gave microstructure characteristics of the materials. In all cases we were able to derive surface fractal dimension Ds. In four cases shape of lowest-angle part of the scattering curve was clear sufficiently to derive mass fractal dimension Dm.
See Tab.1 below:

 
Sample ID
Material
Size of grain
Comment
Dm(mass fractal dimension)
Ds(surface fractal dimension)
157k SiC 3.9nm powder 2.08(3) 2.33(8)
h1k SiC few nm powder 2.7(1)
w3k SiC few nm powder 2.2(1)
keil1 SiC few nm powder 2.32(4)
keil2 SiC more than few nm powder 2.12(5)
483 SiC powder 2.0
483p SiC sintered 483 2.02(2)
a16 diamond powder 1.86(3) 2.12(4)
a16p diamond sintered a16 1.40(4) 2.34(5)
atm diamond powder 1.86(3) 2.34(3)
dalan diamond powder 2.27(1)
ref SiC microns reference SiC powder 2.24(3)
Tab.1.Mass and surface fractal dimensions of nanocrystalline SiC and diamond powders and (483p, a16p) sintered powders.
 
We found that:
  1. all nanocrystalline powders examined show fractal structure of pores and/or surface of the grains
  2. surface roughness of most of the powders is similar (Ds is in range 2.1-2.3) for both: SiC and diamond
  3. pores structure of SiC 157k sample seems to be build of 2D planes isolating relatively big and dense SiC agglomerates
  4. pores structure of diamond a16 and atm samples show less porosity wire-shaped rather than 2D
  5. high-pressure high-temperature sintering decreases mass porosity (Dm: 1.86->1.40) and increases roughness of the surface (Ds: 2.12->2.34) - see Fig.1 below:
 
Two SANS scattering curves different in lowest-angle part Two SANS scattering curves different in lowest-angle part
Fig.1. High-pressure high-temperature sintering decreases slope of lowest-angle part of the scattering curve (thus pores mass fractal dimension Dm) for both SiC and diamond nanocrystalline powders
 
References:
[1] B.Palosz, et al., HASYLAB Annual Report, Vol.I p.587 (1998)
[2] R.Pielaszek, et al., Material Research Society Symp. Proceedings, Vol.501 p.305-310 (1998)
[3] R.Pielaszek, et al., Materials Science Forum, Vol.321-324 p.346-351 (1999)