Page 614 - Book Hosokawa Nanoparticle Technology Handbook

APPLICATIONS 39 INSTANTANEOUS NANOFOAMING METHOD FOR FABRICATION OF CLOSED-POROSITY SILICA PARTICLE
triggering the ballooning phenomena. The weakly bromoform and ethanol, which are completely
acidic condition is required to form the nonporous sil- miscible with each other. The range of the density
ica matrix which enables the generated vapor to cause measurement was between 800 and 2,800 kg m 3 in
the nanoballooning on the instantaneous heat treat- the present case. The transparent quartz crucible is
ment for the foaming. Another possible reason for the obviously advantageous for the instantaneous heat
absence of the foamed structure in Fig. 39.4 is that the transfer because radiation works more effectively par-
condensation of hydroxyl function had been termi- ticularly at a higher temperature. The experimental
nated prior to the instantaneous heat treatment for the result in Fig. 39.5 that the closed porosity was larger
foaming. This is also quite likely to be the case when the quartz crucible was used indicates that more
because the condensation of silanol is effectively cat- closed balloons were formed when the heating took
alyzed by a basic condition. In this case, the silica place more rapidly with the help of the heat transfer
matrix cannot yield vapor available for the foaming by direct radiation. Furthermore, the maximum
on the instantaneous heat treatment. closed porosity was obtained at 750 C in both the
Fig. 39.5 shows the dependencies of the closed cases described above. Above 850 C, the closed
porosity and maximum adsorbed amount of nitrogen porosity clearly showed decrease. As indicated by the
on the temperature. The closed porosities are shown results in Figs. 39.2 and 39.3, more cells had open
for both the cases – one where a transparent quartz channels as the extent of foaming was enhanced. The
crucible was used and another where a normal abrupt increase in the adsorbed amount of nitrogen
ceramic one was used. Here, the closed porosity can above 750 C is consistent with the cell-opening
be evaluated from the apparent density assuming that behavior as shown in Fig. 39.5 (right-hand ordinate).
the immersing fluid (gas or liquid) has no access to Therefore, the total amount of the closed pores
the inside space of the closed pores. The closed poros- increased below 750 C mainly due to the increase in
ity was estimated using 2,200 and 0 kg m 3 for the the size of the mutually isolated pores. Above that
neat silica matrix and the inside vacant space in the temperature, the formed bubbles became partly
closed pore, respectively. The adjustment of the den- interconnected resulting in increase in the nitrogen
sity of the immersing liquid was carried out by mixing adsorption. Nevertheless, it should be noted that the
proportion of the bubbles which converted to the open
pores was still quite small. It does not seem that those
bubbles are readily transformed to interconnected
pores during the process of the transformation.
50
Therefore, significant nanoscale transformation such
40 as mutual fusion of the nanobubbles did not occur in
the present system. The silica matrix behaved as a
40 quartz crucible 30 “hard” and “solid” matrix. Even in the case of 950 C,
the open porosity was estimated to be approximately
5% to the volume of silica matrix from the saturated
Closed porosity (%) 30 ceramic crucible 20 Maximum adsorbed volume (cc(STP) / g sample ) foaming by the instantaneous heat treatment was
adsorbed amount of nitrogen at P/P 1. Thus, the
0
shown to dominantly generate closed nanopores com-
pared to open pores.
Assuming that the “pore opening” is brought
the ballooning process, preliminary reinforcement
20 about by partial fracture of the silica matrix during
10 of the silica matrix prior to the instantaneous heat-
during ballooning. A preliminary heat treatment of
absorbed volume ing is expected to prevent the opening the bubbles
of nitrogen the silica matrix enhances its hardness by increasing
0
10 the extent of the condensation of the residual silanol
500 600 700 800 900 1000 groups. Fig. 39.6 shows the dependence of the
Temperature for instantaneous heat treatment (°C) closed porosity on the temperature of the instanta-
neous heat treatment for multiple cases where the
Figure 39.5 preliminary reinforcement of the silica matrices was
Dependencies of closed porosity and saturated adsorbed carried out at various temperatures ranging from 50
amount of nitrogen at 77 K. Data of the closed porosity to 450 C. On the whole, closed porosity clearly
are shown both for the cases where the silica samples were exhibited increase with the temperature for the pre-
instantaneously heated in a transparent quartz vessel and liminary reinforcement of the silica matrix.
in a ceramic one. The corresponding maximum adsorbed Furthermore, the peak temperature giving the
amount of nitrogen at 77 K is shown together on the maximum closed porosity shifts to the higher
right-hand ordinate. temperature. In the case of 450 C, closed porosity

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