Change of microstructure of nanocrystalline SiC powder in high-pressure

R.Pielaszek1,2, B.Palosz1, S.Gierlotka1, S.Stel'makh1, E.Grzanka1,2 and G.Goerigk3

1High Pressure Research Center, Polish Academy of Sciences, Sokolowska 29, 01-142 Warsaw, Poland
2Institute of Experimental Physics, Warsaw University, Hoza 69, 00-681 Warsaw, Poland

3Hasylab, DESY, Notkestr. 85, D-22603 Hamburg, Germany

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].
Ten samples of nanometric SiC and three samples of their ceramics obtained during high-pressure, high-temperature sintering experiments [4] were examined (see Tab.1.). Four samples of nanocrystalline SiC powder were formed in pills (c.a. 2mm in diameter, 1mm thick) using hand press; increasing forces were applied ("F1", "F2", "F3", "F4"). However precisely undefined, the pressures were lower than 0.5GPa.

Analysis of the Small Angle X-Ray Scattering (SAXS) data gave microstructure characteristics of the materials.In most cases we were able to derive surface fractal dimension Ds. In eight cases shape of lowest-angle part of the scattering curve was clear sufficiently to derive mass fractal dimension Dm.
 

Sample ID
Material
Size of grain
Comment
Dm(mass fractal dimension)
Ds(surface fractal dimension)
157k family SiC 3.9 nm powder synthesized from organic precursors    
157k SiC 3.9 nm powder manually pressed during sample preparation 0.685(9) 2.04(1)
157kbis SiC 3.9 nm powder manually pressed during sample preparation 0.63(1) 1.98(1)
157k-F1 SiC 3.9 nm powder not pressed ("F"=1) 1.80(1) 2.04(1)
157k-F2 SiC 3.9 nm powder pressed "a little" ("F"=2) 1.23(1) 1.984(9)
157k-F3 SiC 3.9 nm powder pressed heavily ("F"=3) 0.42(1) 1.96(1)
157k-F4 SiC 3.9 nm powder pressed heavily ("F"=4) 0.73(1) 1.920(8)
h1k SiC 2.0 nm powder synthesized from organic precursors, manually pressed during sample preparation 0.41(4) 2.12(1)
w3k SiC 2.7 nm powder synthesized from organic precursors, manually pressed during sample preparation 0.75(2) 2.14(1)
98-g1 SiC nm powder synthesized from organic precursors    
max-9711 family SiC microns nano-SiC sintered in MAX80 6-anvill press (Hasylab/DESY)    
max-9711-01SI SiC       2.43(1)
max-9711-02SI SiC       2.164(8)
max-9711-03KI SiC       2.33(1)

Table 1: Mass and surface fractal dimensions of nanocrystalline SiC powders and (max-9711-...) sintered powders

We found that:

  1. all nanocrystalline powders examined show fractal structure of pores (note that mass fractal dimension Dm decreases with increasing pressure thus it has to describe pores structure rather than SiC agglomerates) - see Fig.1
  2. surface roughness of most of the powders is similar (Ds is in range 2.0-2.1)
  3. while powder pressed, pores structure of SiC 157k sample seems to alter from 2D planes isolating relatively big and dense SiC agglomerates to porosity rather wire- or dot-shaped.


Figure 1: Pressure decreases slope of smallest-angle part of the scattering curve (thus pores mass fractal dimension Dm) for SiC nanocrystalline powder

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)
  4. B.Palosz, et al., Material Research Society Symp. Proceedings, Vol.499 p.115-120 (1998)