Efficient graphite ring heater suitable for diamond-anvil cells to 1300 K
Zhixue Du, Lowell Miyagi, George Amulele, and Kanani K. M. Lee
Citation: Rev. Sci. Instrum. 84, 024502 (2013); doi: 10.1063/1.4792395
View online: http://dx.doi.org/10.1063/1.4792395
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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 024502 (2013)
Efficient graphite ring heater suitable for diamond-anvil cells to 1300 K
Zhixue Du,1,a) Lowell Miyagi,1,2 George Amulele,1 and Kanani K. M. Lee1
1
2
Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA
Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA
(Received 2 June 2012; accepted 1 February 2013; published online 22 February 2013)
In order to generate homogeneous high temperatures at high pressures, a ring-shaped graphite heater
has been developed to resistively heat diamond-anvil cell (DAC) samples up to 1300 K. By putting
the heater in direct contact with the diamond anvils, this graphite heater design features the following
advantages: (1) efficient heating: sample can be heated to 1300 K while the DAC body temperature
remains less than 800 K, eliminating the requirement of a special alloy for the DAC; (2) compact
design: the sample can be analyzed with in situ measurements, e.g., x-ray, optical, and electrical
probes are possible. In particular, the side access of the heater allows for radial x-ray diffraction
(XRD) measurements in addition to traditional axial XRD. © 2013 American Institute of Physics.
[http://dx.doi.org/10.1063/1.4792395]
I. INTRODUCTION
The generation of homogeneous high temperatures at
static high pressures has been a great challenge over the past
several decades.1, 2 The multi-anvil press has earned its reputation in achieving homogeneous high temperatures up to
3000 K,3 yet is routinely limited to pressures <28 GPa, although remarkable efforts have gone up to 50 GPa.4 Breakthroughs in diamond-anvil cell (DAC) technology have led
to high temperatures at ultrahigh pressures.5, 6 In combination with laser heating, temperatures >1500 K and pressures >100 GPa have been achieved in DACs, however, temperature gradients can be extreme.1, 7, 8 Much of the recent
progress has been to minimize large temperature gradients
(>100 K/μm) inherent in the laser-heated DAC,2, 9 including heating from both sides, adjusting the laser into multimode,2 and flat-top laser geometry,10 thus achieving a more
uniform sample heating (<20 K/μm). However, the small
uniformly heated spot (typically < 30 μm, although in principle could be as large as 50 μm) still imposes great difficulty for many applications: e.g., elemental partitioning,9
high-temperature elasticity,11–13 high-temperature rheology,14
and melt structure.15 Meanwhile, internal resistive heating has
made striking progress16–19 to achieve stable, homogeneous
temperatures greater than 2000 K up to 70 GPa, yet in practice, intensive and difficult sample loading procedures discourage wider applications.
In contrast to laser heating, external resistive heating
around the diamond anvils provide homogeneous heating in
the sample region under high pressure.20 Most resistively
heated DACs employ coiled resistive wire heater assemblies in a cavity surrounding the diamond anvils,21–24 including commercial ones designed by EasyLab or Betsa. These
heaters are limited to temperatures less than 900 K due to
inefficient radiative heating since there is no direct contact
between the diamond and heater. Many designs have subsequently been proposed to extend the temperatures up to
a) Author to whom correspondence should be addressed. Electronic mail:
zhixue.du@yale.edu.
0034-6748/2013/84(2)/024502/5/$30.00
1500 K, but are limited to either pressures under 10 GPa,20, 25
require a special alloy for the DAC body,26, 27 a complex heating assemblage,24, 28–34 or are restricted to a particular DAC
design (e.g., HeliosDAC by EasyLab). In addition, only a few
heaters30, 35 allow access for radial XRD measurements.
In this study, we modify a previous design employing graphite foils30, 31 with a ring-shaped graphite heater
(Fig. 1(a)). We have successfully generated temperatures up
to ∼1300 K at a pressure of ∼50 GPa. This technique will
open doors to studying material properties, particularly melting, transport, and elasticity at high temperatures and pressures. To assess the new design, we have tested calibrated
materials at both room pressure and high pressure as well
as confirmed the melting curve of H2 O water ice VII up to
23 GPa.
II. HIGH-PRESSURE, HIGH-TEMPERATURE SYSTEM
Graphite is an ideal material to use for heating as it is
a good thermal and electrical conductor with conductivities
of ∼100 W/(m · K) and ∼105 S/m, respectively. Additionally,
graphite can be either soft and flexible or hard and machinable. As the anvils are diamond, we also take advantage of
diamond’s thermal and electrical conducting qualities: diamonds conduct heat well (>1000 W/(m · K)), however, are
good electrical insulators (∼10−13 S/m).
The central part of this design which makes it unique
and efficient, is the direct coupling of a machined, ringshaped graphite heater that fits over the tip of each anvil,
approximately 0.7 mm from the culet (Fig. 1). The coneshaped ring heater (3.5 mm OD, 2 mm ID, 0.8 mm thick)
is machined from a 4.7 mm rod of EC-16 grade graphite
(http://www.graphitestore.com/) and is cut such that it fits directly on to the pavilion of the anvil. The diamond anvil is affixed on the tungsten carbide (WC) seat with a high-working
temperature graphite adhesive (Graphi-bond 551RN). Subsequently, the heater is affixed onto the diamond with an electrically insulating MgO-based adhesive (Ceramabond 571).
Early tests show that the cone shape of the ring heater fits
84, 024502-1
© 2013 American Institute of Physics
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Du et al.
FIG. 1. (a) Ring heater assembly. (b) Top view of the ring heater. Flexible
graphite foils on each side are pressed between the ring-shaped heaters to
supply electric current from Mo rod to the graphite heaters. (c) Photomicrograph of ring heater assembly. Two type-R thermocouples (TC) are cemented
onto the pavilion of the diamond to measure temperature.
nicely on a 16-sided culet faceted pavilion without much efficiency lost as compared to a smooth (unfaceted) pavilion.
To apply the current to the ring heater, a piece of soft, malleable graphite foil ∼1.4 mm in thickness (Alfa Aesar, 99.8%
pure, metals basis) is placed at an end of two machine-notched
molybdenum rods. The soft nature of the graphite foil molds
itself in between the two ring heaters and allows a single current to be applied to each ring heater allowing for even heating
of the sample with a single power supply. More importantly,
the pressed graphite foils provide the graphite heaters with
good contact on the diamonds at high temperature, which is
essential for efficient heating of the sample. Two type-R thermocouples are glued with graphite adhesive on to the diamond
pavilion to measure the temperature. Care is taken not to short
the thermocouple wires with the underlain graphite heater.
The heating assembly is integrated with a modified symmetric DAC (Princeton University Machine Shop) (Fig. 2),
an OmniDAC (EasyLab), and a panoramic DAC (Syntek), although the design can be easily substituted into a DAC of
choice once a few minor modifications are made. To allow
for the addition of the two Mo rods that supply current and
thermocouple leads, two cuts, performed by electric discharge
machining (EDM), are made into the piston of the symmetric
Rev. Sci. Instrum. 84, 024502 (2013)
FIG. 2. (a) Cross section of resistively heated diamond-anvil cell assembly.
The DAC is modified from a standard symmetric cell, made of Vascomax
350 (Princeton Symmetric DAC). (b) Overview of the cell design. Two 5 mm
wide, 7 mm deep grooves are cut by EDM on the piston side to allow the Mo
rods to be inserted. Also note that the cut shown on the right-hand side of the
DAC cylinder is also produced by EDM but is not necessary for this design.
A cylindrical alumina sleeve between the DAC and the membrane can is also
used, however, not required to thermally insulate the DAC.
DAC as shown in Fig. 2. We have not observed a decrease in
the performance or the overall alignment of the DAC from the
EDM cuts.
A stainless steel gas membrane can is machined to support and align the Mo rods to the center of the sample region. The membrane can also seal argon with 1% hydrogen
(Ar + 1%H2 ) or pure Ar gas to prevent the whole cell assembly (e.g., DAC, heater, and anvils) from oxidation or graphitization. The inflatable gas membrane is used to smoothly control the sample pressure at high temperatures, bypassing the
usual mode of screws for compression (Fig. 2). The membrane can is subsequently mounted into a water-cooling jacket
(not shown) during the heating experiments, which turns out
to be essential in keeping the temperature of the DAC body
<800 K, thus negating the need for special high-temperature
alloys.
III. TEMPERATURE CALIBRATION AT AMBIENT
PRESSURE
To test the efficiency of the ring-shaped graphite heater, a
series of experiments are conducted at ambient pressure up to
a temperature of ∼1300 K. A type-R thermocouple junction
is placed between the two diamond anvils to measure temperature of the diamond culets. In addition, another type-R thermocouple is cemented to the pavilion of one diamond anvil
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Du et al.
Rev. Sci. Instrum. 84, 024502 (2013)
graphitization of the diamond anvils during our experimental
runs provided we supplied sufficient Ar or Ar + 1% H2 gas,
including 2 h of heating at 1300 K and over 10 h of heating
at 800 K at pressure. Outside of the DAC, the heater continues to perform well: we held the temperature at 2000 K, as
measured by spectroradiometry, in a sealed chamber flushed
with pure Ar gas or Ar + 1%H2 for 2 h. The heater temperature remained stable during these tests and did not exhibit any
noticeable degradation after heating. In fact, the only time we
observed severe degradation of the heater was when the temperature exceeded 2500 K after 1 h of heating.
IV. APPLICATIONS
A. Melting of H2 O ice
FIG. 3. Temperature as function of power at ambient pressure in external
resistively heated DAC. Diamond culet temperatures are measured by a typeR thermocouple, while the diamond table and WC seats are measured by a
type-K thermocouple. The temperatures are monitored throughout and are
reported here only after reaching steady state. Note the strong linear relationship between temperature and power. Uncertainties in temperature are
±10 K.
within 0.3 mm from the culet, and this agrees with the temperature of the diamond culets to within 10 K up to 700 K,
after which it falls off the anvil and thus is no longer able
to measure a valid temperature. The temperature of one of the
diamond’s table and tungsten carbide seat is also measured by
inserting a type-K thermocouple at the back of the cell during
heating.
In separate experiments, standards of Au foil and NaCl
powder are loaded into a Re gasket, and heated under ambient pressure, and the melting temperatures measured. Melting
is observed visually when the standards turned into spherical droplets as observed by optical microscopy. Temperature
versus power curves are shown in Fig. 3 and show a nice linear relationship between the experiments, suggesting excellent reproducibility. Diamond culets are heated to ∼1300 K
with a modest power input of 300 W. Meanwhile the temperature of the WC seats in contact with the DAC, and thus the
highest temperature of the DAC body itself, does not exceed
800 K. The exterior of the DAC does not reach a temperature
greater than 500 K at the highest temperatures in this study,
provided it is water-cooled.
To attain these high temperatures, it takes ∼10–15 min
to reach a steady-state temperature. By changing the electric
power (or current), the temperature is increased from, for example, 800 K to 900 K continuously and steadily. The same is
true for decreasing temperatures as well. Once the steady state
temperature is reached, the temperature fluctuates to within a
few degrees. In addition, this temperature-power calibration
is reproducible to within 5% after a change of one or both
ring heaters as long as the heater dimensions are the same.
If the heaters are not changed and only a reapplication of
glue/epoxy occurs (due to loosening at temperatures >900 K),
the temperature-power calibration remains the same to within
our measurement uncertainties.
Despite the high temperatures and long heating durations,
we have not observed degradation of the graphite heater or
To test the new design at high pressures, we measured
the melting curve of water ice VII up to 23 GPa (Fig. 4).
We loaded deionized water into a Re gasket or gold-lined Re
gasket, reloading the water sample and gasket for each melting point measurement. Ruby spheres were used as pressure
calibrant36, 37 as well as a motion detector. Melting is detected
by visual observation either by disappearance of ice crystals
or sudden movement of the ruby sphere. Melting points are
measured up to 23 GPa, and are in good agreement with previous measurements using similar techniques.26, 29, 38 The apparent disagreement with Ref. 39 is likely due to different
criteria of detecting melting.29
B. In situ high-pressure and high-temperature
synchrotron XRD experiments
In order to measure the pressure simultaneously at high
temperature, we performed XRD experiments at the Advanced Light Source (ALS) at Lawrence Berkeley National
Laboratory. Diamond anvils with 300 μm culets were used in
this experiment. MgO powder (Alfa Aesar, 99.9%, metal basis) was loaded into 100 μm diameter sample chamber, which
was drilled through a Re gasket preindented to 30 μm. A piece
FIG. 4. Water ice VII melting curve as function of pressure. Four melting points are plotted in comparison with melting curves of previous
studies26, 29, 38, 39 using external resistively-heated DACs.
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Rev. Sci. Instrum. 84, 024502 (2013)
V. DISCUSSION
FIG. 5. X-ray diffraction patterns of a sample of Au and MgO before (thin
line) and during heating (thick line) at high pressures. The hkl’s of both Au
and MgO are noted.
of Au foil (Alfa Aesar, 99.9%, metal basis), 20 μm × 20 μm
× 5 μm in dimension, was loaded as a high-temperature pressure calibrant.40 X-ray diffraction patterns were taken at each
increment of pressure and temperature at steady state. As
shown in Fig. 5, Au and MgO peaks are clearly resolved at
high pressure and high temperature. Subsequently, pressure
was calculated based on thermal equation of state of Au.40 As
shown in Fig. 6, while maintaining the gas membrane pressure, the pressure of the sample remains nearly constant at
∼46 GPa up to 1000 K and gradually increases to ∼51 GPa
at 1300 K. We held the temperature and pressure constant
for an hour at 1300 K before further increasing the temperature. As we increased the temperature to 1350 K, the pressure dropped to 37 GPa and subsequently both diamond anvils
failed.
FIG. 6. Pressure-temperature path of Au/MgO sample during heating. At the
highest temperature of 1300 K, a pressure of ∼51 GPa was reached and held
constant for 1 h. The dashed line through the data points is a guide for the
eye.
From a practical standpoint, it is convenient to put together the ring-shaped graphite heater assembly. For experiments at temperatures below 800 K, the whole cell assembly remains intact and can be reloaded with different samples
repeatedly. However, for experiments at temperatures greater
than ∼900 K, the diamond anvils loosen upon decompression thus it is necessary to realign and reaffix the anvils, reaffix the graphite heaters and thermocouples for subsequent experiments, but the rest of cell assembly, e.g., Mo electrodes,
graphite foils are still in place and reusable. This is in contrast
to previous graphite heater designs,30–34 in which the heater
can only be used once. Additionally, we do not observe a significant increase in pressure as temperature is increased, at
least up to 1300 K (Fig. 6) as has been reported previously.34
This may be due to smaller temperature gradients outside of
the sample chamber or due to the steady gas membrane control applying the force on the DAC.
Thermocouples on the pavilion of diamonds are recommended to measure sample temperatures precisely, however,
we also found that the culet temperature-power curve (e.g.,
Fig. 3) is reproducible to within 5% when steady state is
reached, usually within ∼10 min. Therefore, a thermocouple may not be necessary, once a reliable temperature-power
curve is measured for a given heating assembly, which makes
sample preparation even easier.
The gap between the two graphite heaters (Fig. 1) is large
enough to allow x-rays to pass through, providing the opportunity for radial XRD measurement at high temperatures.30, 41
Additionally, any other standard techniques, including axial
XRD, can also be used with this design.
In several of our runs to test the limit of the current design, in addition to the run shown in Fig. 6, we did not experience gasket blowouts until temperatures exceeded 1300 K at
pressure ∼50 GPa. Therefore, we indentify the temperature
limit as 1300 K at high pressures. The pressure limit remains
to be explored and extended in future experiments.
Despite the efficiency and ease of use of the current design, there are some limitations. First, strength of the WC
seat is significantly reduced at high temperatures42 and eventually the strength vanishes at its eutectic melting point at
∼1500 K.43 Second, Re gaskets (and metallic gaskets in general) may become too weak to support the diamond anvils
at high temperatures. Third, the diamond anvils also become more ductile at increasingly high temperatures and
might eventually limit our maximum pressure and temperature range.44
In order to achieve higher temperatures (>1300 K) at
high pressures, cubic boron nitride gaskets45 are essential over
metallic gaskets (e.g., Re, W) given its extreme high strength
and chemical inertness, even at the extreme conditions. As
for anvil materials, nanopolycrystalline diamond (NPD) may
be an ideal solution for external heating. With an order
of magnitude lower thermal conductivity than single-crystal
diamond,46 this difference will further build up the temperature difference between the diamond culet and WC seat
(Fig. 3), thereby allowing for higher sample temperatures. Moreover, NPD retains its high strength at high
024502-5
Du et al.
temperature.47 Additionally, this external heating can be coupled with laser heating to provide bigger and more uniform
hot spots.
VI. CONCLUSION
We designed a compact and efficient heater that can be
used with most DAC designs with minimal modifications. The
design is unique in the use of direct coupling of a ring shaped
graphite heater to the diamond anvil thus yielding efficient,
reproducible, and reusable heating. Additionally, the compact
nature provides easy access for both radial and axial XRD,
electrical leads, and the ability to fit into many synchrotron
beamlines and spectroscopic setups.
ACKNOWLEDGMENTS
The authors would like to thank Bill Samela and Chris
Fiederlein for technical assistance. We also thank Alastair
MacDowell and Jason Knight for their help at the Advanced
Light Source, Lawrence Berkeley National Laboratory. Z.D.
and K.K.M.L. acknowledge the support from CDAC and National Science Foundation (NSF) (Grant No. EAR-0955824).
L.M. acknowledges support from the Bateman Fellowship at
Yale University. The authors would like to thank two anonymous reviewers for a critical assessment of this paper.
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