18 2 Nanoparticles – Nanocomposites
these materials need homogenization times in the range of many hours. Looking
at materials with grain sizes around 10 nm, which is 10
−3 of the conventional
grain size, according to the scaling law of diffusion Eq. (2.3) the time for homogenization is reduced by a factor of 10
10
3 2
6
( ) = . This means that the homogenization time of hours, for conventional materials, is reduced to milliseconds; for
nanomaterials. Lastly, this says that homogenization is virtually instantaneous.
This phenomenon is often called “instantaneous alloying”. One may also say:
Each thermally activated reaction will happen nearly instantaneously. Therefore,
it is not possible to produce nonequilibrium systems of nanomaterials, well
known for conventional materials, at elevated temperatures.
The possibility of nearly instantaneous diffusion through nanoparticles is
exploited technically. The most important example is the gas sensor applying the
variation in the electric conductivity due to changes in the stoichiometry of oxides.
(The stoichiometry describes the ratio oxygen / metal.) Variations of stoichiometry
are often observed in oxides of transition metals. Because of the small particle
size, any change in the oxygen potential in the surrounding atmosphere changes
the stoichiometry of the sensing particles immediately. In contrast to conventional
gas sensors, the time response is now controlled by the gas diffusion through the
narrow channels in between the nanoparticles. Figure 2.12 displays the general
design of such a sensor.
Such a gas sensor is set up on a conductive substrate on a carrier plate. The
surface of this conductive layer is covered completely with the oxide sensor nanoparticles. Transition-metal oxides, well suited for this purpose are example, TiO 2 ,
SnO 2 , Fe 2 O 3 . On the top of the oxide particle layer, the counterelectrode, a gaspermeable conductive layer is applied. Variations in the oxygen potential in the
surrounding atmosphere changes the stoichiometry of the oxide, and, therefore,
the electrical conductivity. This process is reversible.
Figure 2.12 General layout of a gas sensor
based on nanoparticles. This gas sensor
consists of a layer of sensing nanoparticles,
in most cases SnO 2 , placed on a conductive
substrate. The whole system is covered with
a gas-permeable electrode. The diffusion
within the nanosized grains is no longer time
controlling, it is rather the diffusion in the
open-pore network in-between the grains.
conducƟve substrate
carrier plate
Sensor parƟcles
ConducƟve cover layer
Diffusion path oŌ he
gas molecules
these materials need homogenization times in the range of many hours. Looking
at materials with grain sizes around 10 nm, which is 10
−3 of the conventional
grain size, according to the scaling law of diffusion Eq. (2.3) the time for homogenization is reduced by a factor of 10
10
3 2
6
( ) = . This means that the homogenization time of hours, for conventional materials, is reduced to milliseconds; for
nanomaterials. Lastly, this says that homogenization is virtually instantaneous.
This phenomenon is often called “instantaneous alloying”. One may also say:
Each thermally activated reaction will happen nearly instantaneously. Therefore,
it is not possible to produce nonequilibrium systems of nanomaterials, well
known for conventional materials, at elevated temperatures.
The possibility of nearly instantaneous diffusion through nanoparticles is
exploited technically. The most important example is the gas sensor applying the
variation in the electric conductivity due to changes in the stoichiometry of oxides.
(The stoichiometry describes the ratio oxygen / metal.) Variations of stoichiometry
are often observed in oxides of transition metals. Because of the small particle
size, any change in the oxygen potential in the surrounding atmosphere changes
the stoichiometry of the sensing particles immediately. In contrast to conventional
gas sensors, the time response is now controlled by the gas diffusion through the
narrow channels in between the nanoparticles. Figure 2.12 displays the general
design of such a sensor.
Such a gas sensor is set up on a conductive substrate on a carrier plate. The
surface of this conductive layer is covered completely with the oxide sensor nanoparticles. Transition-metal oxides, well suited for this purpose are example, TiO 2 ,
SnO 2 , Fe 2 O 3 . On the top of the oxide particle layer, the counterelectrode, a gaspermeable conductive layer is applied. Variations in the oxygen potential in the
surrounding atmosphere changes the stoichiometry of the oxide, and, therefore,
the electrical conductivity. This process is reversible.
Figure 2.12 General layout of a gas sensor
based on nanoparticles. This gas sensor
consists of a layer of sensing nanoparticles,
in most cases SnO 2 , placed on a conductive
substrate. The whole system is covered with
a gas-permeable electrode. The diffusion
within the nanosized grains is no longer time
controlling, it is rather the diffusion in the
open-pore network in-between the grains.
conducƟve substrate
carrier plate
Sensor parƟcles
ConducƟve cover layer
Diffusion path oŌ he
gas molecules