Page 615 - Book Hosokawa Nanoparticle Technology Handbook

39 INSTANTANEOUS NANOFOAMING METHOD FOR FABRICATION OF CLOSED-POROSITY SILICA PARTICLE APPLICATIONS
showed that the preliminary reinforcement in the
preheated at 450°C
mechanical strength of the silica matrix by
preheating significantly contributed to enhancing
65 the foaming without opening the formed nanobub-
bles to the outer surface. The maximum closed
60 preheated at 350°C porosity that was actually obtained in the author’s
experiments was 64%, which was estimated from
Closed porosity (%) 55 preheated at 250°C almost the same as that of neat ethanol at room tem-
3
the apparent density of 790 kg m . This value is
perature. Here, it should be noted that the amount of
50
the bubbles also increases with the total extent of
the reinforcement by the preheating. Thus, the supe-
preheated at 150°C
riority in the mechanical strength of the silica
Cl
45
matrix works more dominantly than the amount of
preheated at 50°C the residual silanol groups which are directly
40 peak responsible for generating water at the moment of
the condensation.
35 Fig. 39.7 shows the nitrogen adsorption isotherms
700 750 800 850 900
of the foamed silica samples before and after the
Temperature for instantaneous heat treatment for foaming. Fig. 39.7
instantaneous heat treatment (°C)
reveals a drastic structural change caused by the
instantaneous heat treatment. Before the instanta-
Figure 39.6 neous heat treatment, the silica matrix had micropores.
Dependencies of closed porosity on the temperature of They were captured as the prominent adsorption
the instantaneous heat treatment for the nanofoaming in peaks seen at the left-hand side of Fig. 39.7 (P/P 0).
0
various cases of temperatures of pretreatment for These micropores were entirely eliminated by the
preliminary mechanical reinforcement of the silica instantaneous heat treatment. Thus, the enclosure of
matrix. the generated vapor in the nanoscale bubbles was

140
14
before instantaneous
heat treatment
12 0
R R
10 0 Q Q
Closed porosity (%) 80 80 P P

60

40
P P
20 after insantaneous R R
heat treatment
Q Q
0
0 0. 0.1 0.2 0 0.3 0.4 0.5 0.6 0.7 0.8. 0.9 1
P/P (−)
o
Figure 39.7
Nitrogen adsorption isotherms before and after the instantaneous heat treatment for the nanofoaming at three different
conditions as given below: (a) preheated at 250 C, (b) preheated at 350 C, (c) preheated at 350 C with twice larger added
amount of water for hydrolysis.

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