Chapter 25
Sol—Gel Materials with Controlled Nanoheterogeneity Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
Ulrich Schubert, Fritz Schwertfeger, and Claus Görsmann Institute of Inorganic Chemistry, Technical University Vienna, Getreidemarkt 9, A-1060 Vienna, Austria
Diphasic materials containing a nanometer-sized phase of one com ponent in an oxide phase were obtained by controlled thermal treat ment of xerogels in which the precursor for the nanophase is highly dispersed. The high dispersion was achieved by sol-gel processing of organically modified alkoxides. Thus, very thin carbonaceous layers partially covering the inner surface of silica aerogels developed during pyrolysis of organic groups located at the surface of the primary particles. This lead to a very efficient infrared opacification. Metal/ceramic nanocomposites were prepared by complexation of metal ions and tethering the resulting metal complexes to the oxide matrix during sol-gel processing. Subsequent thermolysis in air resulted in nano-sized metal oxide particles, which were then reduced by hydrogen to give highly dispersed metal particles.
One of the advantages of preparing oxide materials by the sol-gel method (7) is the possibility to control their microstructure and homogeneity. For most applications very homogeneous materials are desired. However, R. Roy already pointed out in the early eighties that the advantages of sol-gel processing can also be exploited for the preparation of di- or multiphase ceramic materials (2). Our approach for getting a nanometer-sized phase Β in the oxide phase A is a high, ideally molecular dispersion of the phase Β precursor while the inorganic net work of A is formed during sol-gel processing. This is achieved by chemically binding the phase Β precursor to the network of A using organically (or organofunctionally) substituted alkoxides (3). The nanophase Β is then obtained by controlled thermal treatment in a later step (Scheme 1). Two examples for this approach will be discussed: (i) Carbonaceous structures (phase B) in S1O2 aerogels (phase A), and (ii) highly dispersed metal or alloy particles (phase B) in S1O2 or T1O2 (phase A). Carbonaceous Structures in S1O2 Aerogels Silica aerogels (4) have several interesting applications, one of them being thermal insulation materials. Due to their low density and small pore radii, the heat trans port via the solid aerogel skeleton and the gas phase is low. The radiative
0097-6156/96/0622-0366$15.00A) © 1996 American Chemical Society
In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
25.
SCHUBERT E T AL.
Sol-Gel Materials with Controlled Nanoheterogeneity
367
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
Precursor forming phase A + precursor containing Β Sol-Gel Processing
Β
Α—Β
A A
AS
A
B
A
" / A \ Β Α Α
^
A B~^A A
Β
A
A—Β A A
\ Β
Scheme 1. Preparation of a nanometer-sized phase Β in the oxide phase A by sol-gel processing, followed by controlled thermal treatment.
In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
NANOTECHNOLOGY
368
transport below 20°C is also low, because silica aerogels absorb heat sufficiently. However, use of silica aerogel for heat insulation at medium temperatures (50500°C) requires reduction of the radiative heat transport. The reason is that the radiation maximum 2X these temperatures is at 2-8 μπι, where silica has a low speci fic extinction (1 m k g ) . Carbon black is very well suited for infrared opacifi cation due to its broad absorption band in the relevant range. Mixing aerogel powders with 20% carbon soot increases the specific extinction in the range of 2-8 /zm to a sufficient value of 100-200 m^kg" (specific thermal conductivity 0.018-0.020 W m ^ K " ) . However, one disadvantage of this approach is the handling of powders. Furthermore, it is not possible to further increase the specific extinction by adding a higher portion of soot. Specific extinction depends not only on the quantity of added carbon but also on its structure, size and agglomeration. For larger (agglomerated) particles, which are more easily formed with a higher soot portion, extinction decreases again (5). Another possibility to achieve infrared opacification is the addition of soot already during sol-gel pro cessing. This approach has the same limitations concerning extinction. Additional problems arise from sedimentation of the soot particles and their influence on the formation of the gel nanostructure. 2
_1
1
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
1
χ RSi(OMe)
3
+
y Si(OMe)
+
4
(3x + 4y) H 0 2
+
ζ MeOH
0.1 η NH OH / 30°C 4
A L C 0 GEL
1 )
ageing 30°C / 7d ' s u p e r c r i t i c a l drying A E R O
GEL
R Si02--0.5x x
2 )
^ p y r o l y s i s 700-1000°C CH / 1000°C (optional) 4
A E R O G E L C / Si0
2
Scheme 2. Preparation of carbon structures in silica aerogels Preparation of the Aerogel Composites. A new approach free of these problems, is to generate carbonaceous structures in silica aerogels either by pyrolysis of orga nically modified aerogels (Scheme 2) or by pyrolysis of gaseous organic compounds (6-8).
In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
25.
SCHUBERT ET AL.
Sol-Gel Materials with Controlled Nanoheterogeneity
369
The structure of a silica aerogel prepared under basic conditions is shown in Figure 1 (4). It consists of secondary particles of about 5 nm diameter composed of smaller primary particles. Residual Si-OH and Si-OR groups are at the surface of the primary particles. We have prepared organically modified silica aerogels by NIfyOH-catalyzed hydrolysis and condensation of RSi(OMe)3 / Si(OMe)4 mixtures (R = alkyl, aryl or functional organic groups), followed by supercritical drying of the alcogels with methanol or CO2 (9). Incorporation of up to 20% RS1O3/2 units into the aerogels did not disturb their micro- and nanostructure significantly, and therefore the typi cal aerogel properties were retained. In the organically modified aerogels the organic groups R are mainly located at the surface of the primary particles (Figure 1) and replace the Si-OH and Si-OR groups (9). The inner aerogel surface should be uniformely covered by carbon for an effi cient infrared opacification. The organically modified silica aerogels are ideal starting compounds for several reasons: • The organic groups are already located at the inner surface. Therefore, there are no diffusion problems. • The equal distribution of the organic groups on the inner surface results in a great number of well distributed nucleation centers during pyrolysis and thus leads to small carbon particles. Optimization of the Pyrolysis Process. The pyrolysis process was carried out by heating the organically modified silica aerogels with an optimized heating rate to 700-1000°C in a standing atmosphere of argon. Passing argon over the samples during heating resulted in a considerably smaller amount of elemental carbon after pyrolysis. This shows that the gaseous compounds formed during pyrolysis play a very important role for the growth of the carbon structures. Optimization of the pyrolysis process of organically substituted silica aerogels R Si02_o 5χ showed the following trends (6). Numerical values for some examples are given in Table I. • With an increasing number of carbon atoms in the organic groups R of the starting aerogel the carbon content in the pyrolyzed aerogels increased. In the series of aerogels having the starting composition Ro.2^iOl.9 (20% of the sili con aloms being substituted by an organic group) and densities of 230-290 kgm"- the carbon contents after pyrolysis increased from 2.6% for R = C H 3 to 9.0% for R = C 3 H 7 . It further increased to 13.9% for R = C 6 H 5 , due to the larger number of carbon atoms and the aromatic nature of this group. • The larger the organic group R and the higher the portion of organic groups in the starting aerogel (x in Scheme 2), the higher was the portion of retained x
3
carbon. •
• •
Decreasing the density of the aerogels resulted in a larger loss of carbon during pyrolysis. This is understandable, since the porosity of the aerogels increases with decreasing density. The gaseous hydrocarbons formed upon hydrolysis therefore can easier diffuse out of the material. Once the organic groups are pyrolyzed, there was no further loss of carbon. Holding selected samples at 1000°C up to lOh did not result in a detectable decrease of the carbon content. The temperature necessary to pyrolyze all organic groups was lower with decreasing density of the aerogels. The phenyl or propyi-substituted aerogels with densities smaller than 150 kgm"^ were completely pyrolyzed ^t 700°C. The aerogels with starting densities between 200 and 300 kgm" required heating to 1000°C. 3
In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
In Nanotechnology; Chow, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
ο
25.
Sol-Gel Materiah with Controlled Nanoheterogeneity
SCHUBERT E T AL.
Table I. Carbon content (by elemental analysis) of selected C/S1O2 aerogels, and density and surface area changes during pyrolysis. starting composition
Downloaded by UNIV MASSACHUSETTS AMHERST on October 14, 2012 | http://pubs.acs.org Publication Date: March 12, 1996 | doi: 10.1021/bk-1996-0622.ch025
Ά )
starting density [kgm~ ] 3
final density^' [kgm~ ] 3
0
2
2.59
