Geoscience Reference
In-Depth Information
MgO and Al
2
O
3
exceeding T
2500 K (Sinogeikin
et al
., 2004). Their results, however, also
suggested that thermal background becomes sig-
nificant during laser heating with visible thermal
emission, which could be a serious problem for
high-temperature measurements. As previously
described in the literatures (Sandercock, 1982;
Sinogeikin
et al
., 2004), the Brillouin peaks can
be observed even at high-temperature condition
because the intensity of Brillouin peaks also
increases with temperature. While the inten-
sity of Brillouin peaks increases linearly with
temperature, the intensity of thermal radiation
is generally believed to be proportional to
T
4
,
which means that there is threshold temperature
above which the intensity of thermal background
blankets that of Brillouin peaks. However, the
relationship between intensities of Brillouin
peaks and thermal background are also related
to a number of factors such as the optical
properties of the sample such as the emissivity
and polarization property, the wavelength of the
incident laser, and the surface condition of the
sample interfaced with the laser. In addition,
such
∼
data. The laser heating experiments in a DAC
with a small focused heating spot, which can
be visually monitored by such as CCD camera
through the anvil, can minimize/avoid that
problem with careful optical alignment and
approach. Although the temperature gradient
across the sample derived from the laser heating
may cause the potential errors in the sound
velocity data, the effect of temperature gradient
on the sound velocity can be diminished if the
laser heating spot is properly adjusted to be
sufficiently larger than that of probe laser for the
Brillouin scattering measurements.
A Brillouin scattering measurement system us-
ing an infrared laser heating technique with DAC
was recently installed at the BL10XU beamline
station in the Japanese synchrotron facility of
SPring-8 (Murakami
et al
., 2009) (Figure 6.2).
A combined diamond anvil cell system for
Brillouin spectroscopy and synchrotron radiation
enables the simultaneous measurements of
sound velocity and X-ray diffraction at high
pressure and high temperature. This information
provides elastic properties such as bulk and
shear moduli, and their pressure and temper-
ature derivatives. The simultaneously in situ,
high-pressure-temperature X-ray diffraction data
by synchrotron radiation is essential not only
for obtaining the structural information (phase
identification, lattice parameters, compressibility
and density) of the sample but for providing the
precise pressure values under high-temperature
conditions from the tiny sample in a DAC. This
system consists of three optical components
used for Brillouin spectroscopy, X-ray diffraction
and infrared laser heating (temperature measure-
ment). For the simultaneous measurements, all
optical probes for each of these three components
must be converged on the sample without optical
and physical interference, and the data from
each component are extracted by simultaneous
and independent detector/analyzing systems for
each, simultaneously as shown in Figure 6.2b.
The
relation
at
elevated
pressures
is
poorly
understood.
6.2.3 Development of sound velocity
measurements under high P-T
Though there are still some challenging issues
for the Brillouin measurements under high-
temperature condition as discussed above, the
Brillouin scattering measurement system with
infrared laser heating technique in a DAC is
a promising way for the soundwave velocity
measurements and sample characterization
under lower mantle P-T condition. Earlier
studies on sound velocity measurements at high
pressure and high temperature have been mainly
performed in an externally (resistive) heated
diamond anvil cell (Polian & Grimsditch, 1983).
One of the major disadvantages for externally
heated DAC for sound velocity measurements
is possible reaction between sample and metal
gasket materials in the sample chamber, which
may cause serious errors in the sound velocity
focal
spot
size
for
Brillouin
and
X-ray
spectroscopy is adjusted to be
min
diameter. Though the size of heating spot for the
CO
2
laser strongly depends on the experimental
∼
10
−
20
μ
