C hapter 1 Nanomaterials and Nanotechnologies: an overview
14
of the material that determines this: the finer the structure, the
shorter is the mean-free path and the higher the resistance. Magnetism involves domains of locally aligned magnetic moments;
the domain boundaries have a characteristic thickness, again
of atomic or nanodimensions. Optical properties, particularly,
involve scattering or diffraction, both with specific length scales.
Strength, too, is influenced by structural scale. When materials
deform—bend, twist, or stretch permanently—it is because sheets
of the atoms that make them up ratchet over each other. The
ratchet steps are fixed by the atomic size. The lower end of the
nanoscale approaches this in scale, giving, as the structural length
approaches it, increasingly strong interaction, meaning greater
strength. Brittleness (and its opposite, toughness) has a characteristic length—that of the flaw that will just propagate at the yield
strength. The flaw size is itself related to the scale of the structure.
This intrinsic dimensionality, so to speak, of material properties
is one of the reasons for the great interest in nanomaterials. If we
can engineer the structural scale of a material (as particles, thin
layers, or the crystal size in bulk materials) in the right way, we
can influence or interact with the property length scale and thus
manipulate the property itself.
We live in a world in which everyday objects have a size—we shall call
it scale and give it the symbol L, for length—in the range of millimeters to meters. Scale has a profound effect on the behavior of structures made from materials. A steel tuning fork 80 mm long with arms
that are 3 mm thick oscillates with a frequency of middle C, 256 Hz.
Vibration frequencies scale as 1/L. So change the millimeters of the
tuning fork to microns and the frequency rises to 256 kHz. Change
it to nanometers and it becomes 256 MHz. This is comparable to the
clock speed of computers, stimulating the novel idea of mechanical computers (not such a new idea: think of the abacus). This is an
example of scale-dependent mechanical response. Response time—
the time it takes a mechanical latch to close or a switch to switch—
scale in the same way, as 1/L ; it is this that allows automobile airbag
triggers to react fast enough to save lives. Thermal response scales even
faster. The time for an object to reach thermal equilibrium when the
temperature is changed scales as 1/L
2 so the thermal response time
that, for mm-scale objects, is seconds or minutes becomes microseconds at the micron scale and picoseconds at the nanoscale. (There
is, in reality, a limit set by the acoustic velocity that cuts in, preventing the response becoming quite that short.)
That’s the big picture. Focus now for a moment on the small one:
the nano. What is so different and special about it? There is more
Figure 1.13
The acoustic spectrum from the extreme ultrasonic
to the seismic, a range of 10
8 . The audible part is
a thick slice of this. The figure is based on acoustic
waves in air, velocity 350 m/s. For water, shift the
wavelength scale up relative to the frequency scale
by a factor of 3; for rock, shift it up by a factor of
10.
Wavelength (m)
10 3
1
10 –6
10
–3
Seismology
Ultra long-range sonar
Frequency (Hz)
10
6
10
9
10
3
1
Long-range sonar
Sea-floor mapping sonar
Audible range
Acoustic microscopy
Surface scanners
Bats, dolphins, whales
Middle C
Medical CT scanners
20 Hz
20 kHz
14
of the material that determines this: the finer the structure, the
shorter is the mean-free path and the higher the resistance. Magnetism involves domains of locally aligned magnetic moments;
the domain boundaries have a characteristic thickness, again
of atomic or nanodimensions. Optical properties, particularly,
involve scattering or diffraction, both with specific length scales.
Strength, too, is influenced by structural scale. When materials
deform—bend, twist, or stretch permanently—it is because sheets
of the atoms that make them up ratchet over each other. The
ratchet steps are fixed by the atomic size. The lower end of the
nanoscale approaches this in scale, giving, as the structural length
approaches it, increasingly strong interaction, meaning greater
strength. Brittleness (and its opposite, toughness) has a characteristic length—that of the flaw that will just propagate at the yield
strength. The flaw size is itself related to the scale of the structure.
This intrinsic dimensionality, so to speak, of material properties
is one of the reasons for the great interest in nanomaterials. If we
can engineer the structural scale of a material (as particles, thin
layers, or the crystal size in bulk materials) in the right way, we
can influence or interact with the property length scale and thus
manipulate the property itself.
We live in a world in which everyday objects have a size—we shall call
it scale and give it the symbol L, for length—in the range of millimeters to meters. Scale has a profound effect on the behavior of structures made from materials. A steel tuning fork 80 mm long with arms
that are 3 mm thick oscillates with a frequency of middle C, 256 Hz.
Vibration frequencies scale as 1/L. So change the millimeters of the
tuning fork to microns and the frequency rises to 256 kHz. Change
it to nanometers and it becomes 256 MHz. This is comparable to the
clock speed of computers, stimulating the novel idea of mechanical computers (not such a new idea: think of the abacus). This is an
example of scale-dependent mechanical response. Response time—
the time it takes a mechanical latch to close or a switch to switch—
scale in the same way, as 1/L ; it is this that allows automobile airbag
triggers to react fast enough to save lives. Thermal response scales even
faster. The time for an object to reach thermal equilibrium when the
temperature is changed scales as 1/L
2 so the thermal response time
that, for mm-scale objects, is seconds or minutes becomes microseconds at the micron scale and picoseconds at the nanoscale. (There
is, in reality, a limit set by the acoustic velocity that cuts in, preventing the response becoming quite that short.)
That’s the big picture. Focus now for a moment on the small one:
the nano. What is so different and special about it? There is more
Figure 1.13
The acoustic spectrum from the extreme ultrasonic
to the seismic, a range of 10
8 . The audible part is
a thick slice of this. The figure is based on acoustic
waves in air, velocity 350 m/s. For water, shift the
wavelength scale up relative to the frequency scale
by a factor of 3; for rock, shift it up by a factor of
10.
Wavelength (m)
10 3
1
10 –6
10
–3
Seismology
Ultra long-range sonar
Frequency (Hz)
10
6
10
9
10
3
1
Long-range sonar
Sea-floor mapping sonar
Audible range
Acoustic microscopy
Surface scanners
Bats, dolphins, whales
Middle C
Medical CT scanners
20 Hz
20 kHz