The dissociation of liquid silica at high pressure and temperature Page: 3 of 8
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The dissociation of liquid silica at high pressure and temperature
D. G. Hicks,1'* T. R. Boehly,2 J. H. Eggert,' J. E. Miller,2,3 P. M. Celliers,1 and G. W. Collins'
'Lawrence Livermore National Laboratory, Livermore, CA 94550
2Laboratory for Laser Energetics, University of Rochester, NY 14623
3Department of Mechanical Engineering, University of Rochester
(Dated: April 20, 2006)
Liquid silica at high pressure and temperature is shown to undergo significant structural modifica-
tions and profound changes in its electronic properties. Temperature measurements on shock waves
in silica at 70-1000 GPa indicate that the specific heat of liquid SiO2 rises well above the Dulong-
Petit limit, exhibiting a broad peak with temperature that is attributable to the growing structural
disorder caused by bond-breaking in the melt. The simultaneous sharp rise in optical reflectivity of
liquid SiO2 indicates that dissociation causes the electrical and therefore thermal conductivities of
silica to attain metallic-like values of 1-5 x 105 S/m and 24-600 W/m.K respectively.Silica (SiO2) in its various high-pressure phases is of ev-
ident importance in the earth's interior and is a prototype
material for studying condensed matter physics, materi-
als science, and chemistry under extreme conditions [1].
A major gap in understanding however concerns the be-
havior of silica in its liquid phase at combined high pres-
sure and temperature. Liquid silicates in this state dom-
inated the earth during late-stage accretion when gi-
ant impacts caused complete melting of the planet [2],
thereby creating the conditions under which final differ-
entiation of the core and mantle occurred [3]. Present-
day remnants of such large magma oceans may still exist
as partially-melted zones at the core-mantle boundary [4]
and could play an important role in magnetic-field and
heat transport between the core and mantle.
The high melting temperature of silica presents a se-
rious challenge to experiments on the liquid at elevated
pressures and historically data has been inferred either
from glasses at high pressure, or liquids at low pressure.
In particular, increasing pressure has been shown to cause
a gradual change of the open-network tetrahedral struc-
ture in silicate glasses to a closely-packed, six-fold co-
ordinated system [5, 6]. In comparison, increasing tem-
perature causes a continuous reduction in the average
size of structural units, or depolymerization, in the liq-
uid that results in elevated specific heat capacities and
orders-of-magnitude changes in viscosity [7, 8]. As with
all planetary interior processes however, it is the com-
bined effect of high pressure and temperature that is the
most relevant yet the most difficult to study. Here we use
shock waves to simultaneously compress and heat two
polymorphs of silica well into the dense liquid regime.
Achieving a broad range of temperatures in the liquid
allows us to demonstrate how the features observed near
melting - a high specific heat, and a sudden rise in elec-
tronic conductivity - represent the onset of an extended
process of liquid structural change that continues up to
several times the melting temperature.
Using 100 to 500 J of ultraviolet laser light from
the OMEGA facility [9] laser-driven shock waves were
launched in 500 pm thick samples of fused silica (po2.2 g/cm3) or z-cut a-quartz (po 2.65 g/cm3) (a de-
tailed target description has been given previously [13]).
This energy was delivered in 1 nanosecond (ns) over a
uniformly-irradiated 600 pm spot, generating a planar,
attenuating shock wave that provided access to a contin-
uous range of shocked states on a single shot. Two shock
diagnostics were used: (1) A Velocity Interferometer Sys-
tem for Any Reflector (VISAR) [10], which measured the
shock velocity and reflectivity at a wavelength of 532 nm,
and (2) A streaked optical pyrometer (SOP), an abso-
lutely calibrated, space and time-resolved pyrometer used
to image the thermal emission between 600 - 700 nm (see
Ref. [11] for a description of a similar setup). To de-
termine temperature from the measured emission, grey-
body emission was assumed where the emissivity was
given by 1-R with R given by the measured optical reflec-
tivity. The assumption of grey-body emission at 600-700
nm was verified previously at low pressure by measuring
emission at multiple wavelengths [12]. Both the VISAR
and SOP provide continuous measurements of the shock
velocity, temperature, and reflectivity over the range of
conditions spanned by the attenuating shock. The shock
pressure, density, and internal energy in quartz were de-
termined from the observed shock velocity using the pre-
viously measured Hugoniot relation [13]. This relation
was found to be almost identical to that predicted by the
Kerley EOS model [14]. In fused silica, since there are
no available experimental data above 100 GPa, the Ker-
ley EOS was used. Since the difference in initial density
between the fused silica and quartz Hugoniots is small,
the offset between the two can be reliably predicted.
The measured shock temperatures in quartz and fused
silica as a function of pressure are shown in Fig. 1. The
different initial densities of these two polymorphs of sil-
ica allow the shock Hugoniots to track different paths
through the silica phase diagram. The compressed den-
sities in these experiments range between 5.2 and 7.2
g/cm3 for quartz and 4.6 to 6.2 g/cm3 for fused silica
- roughly 2 to 3 times solid density. Errors in the tem-
perature are 7-8 % near 5000 K and 4-5 % at 5 x 104
K and include systematic errors in the calibration (2-5
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Hicks, D.; Boehly, T.; Eggert, J.; Miller, J.; Celliers, P. & Collins, G. The dissociation of liquid silica at high pressure and temperature, article, November 17, 2005; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc887992/m1/3/: accessed April 10, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.