An Externally-Heated Diamond Anvil Cell for Synthesis and Single-Crystal Elasticity Determination of Ice-VII at High Pressure-Temperature Conditions

Summary

This work focuses on the standard protocol for preparing the externally-heated diamond anvil cell (EHDAC) for generating high-pressure and high-temperature (HPHT) conditions. The EHDAC is employed to investigate materials in Earth and planetary interiors under extreme conditions, which can be also used in solid state physics and chemistry studies.

Abstract

The externally-heated diamond anvil cell (EHDAC) can be used to generate simultaneously high-pressure and high-temperature conditions found in Earth’s and planetary interiors. Here we describe the design and fabrication of the EHDAC assemblies and accessories, including ring resistive heaters, thermal and electrical insulating layers, thermocouple placement, as well as the experimental protocol for preparing the EHDAC using these parts. The EHDAC can be routinely used to generate megabar pressures and up to 900 K temperatures in open air, and potentially higher temperatures up to ~1200 K with a protective atmosphere (i.e., Ar mixed with 1% H₂). Compared with a laser-heating method for reaching temperatures typically >1100 K, external heating can be easily implemented and provide a more stable temperature at ≤900 K and less temperature gradients to the sample. We showcased the application of the EHDAC for synthesis of single crystal ice-VII and studied its single-crystal elastic properties using synchrotron-based X-ray diffraction and Brillouin scattering at simultaneously high-pressure high-temperature conditions.

Introduction

The diamond anvil cell (DAC) is one of the most important tools for high pressure research. Coupled with synchrotron-based and conventional analytical methods, it has been widely used to study properties of planetary materials up to multi-megabar pressures and at wide ranges of temperatures. Most planetary interiors are under both high-pressure and high-temperature (HPHT) conditions. It is thus essential to heat the compressed samples in a DAC at high pressures in situ to study the physics and chemistry of planetary interiors. High temperatures are not only required for the investigations of phase and melting relationships and thermodynamic properties of planetary materials, but also help mitigate pressure gradient, promote phase transitions and chemical reactions, and expedite diffusion and recrystallization. Two methods are typically utilized to heat the samples in DACs: laser-heating and internal/external resistive heating methods.

The laser-heated DAC technique has been employed for high-pressure materials science and mineral physics research of planetary interiors^1,2. Although increasing number of laboratories have access to the technique, it usually requires significant development and maintenance effort. The laser heating technique has been used to achieve temperatures as high as 7000 K³. However, long-duration stable heating as well as temperature measurement in laser-heating experiments have been a persistent issue. The temperature during laser heating usually fluctuates but can be mitigated by feed-back coupling between thermal emission and laser power. More challenging is controlling and determining the temperature for assembly of multiple phases of different laser absorbance. The temperature also has a considerably large gradient and uncertainties (hundreds of K), although recent technical development effort has been used to mitigate this issue^4,5,6. Temperature gradients in the heated sample area sometimes may further introduce chemical heterogeneities caused by diffusion, re-partitioning or partial melting. In addition, temperatures less than 1100 K typically could not be measured precisely without customized detectors with high sensitivity in the infrared wavelength range.

The EHDAC uses resistive wires or foils around the gasket/seat to heat the entire sample chamber, which provides the ability of heating the sample to ~900 K without a protective atmosphere (such as Ar/H₂ gas) and to ~1300 K with a protective atmosphere⁷. The oxidation and graphitization of diamonds at higher temperatures limit the highest achievable temperatures using this method. Although the temperature range is limited compared with laser-heating, it provides more stable heating for a long duration and a smaller temperature gradient⁸, and is well suited to be coupled with various detection and diagnostic methods, including optical microscope, X-ray diffraction (XRD), Raman spectroscopy, Brillouin spectroscopy and Fourier-transform infrared spectroscopy⁹. Therefore, the EHDAC has become a useful tool to study various material properties at HPHT conditions, such as phase stability and transitions^10,11, melting curves¹², thermal equation of state¹³, and elasticity¹⁴.

The BX-90 type DAC is a newly developed piston-cylinder type DAC with large aperture (90° at maximum) for XRD and laser spectroscopy measurements⁹, with the space and openings to mount a miniature resistive heater. The U-shaped cut on the cylinder side also provides room to release the stress between the piston and the cylinder side caused by temperature gradient. Therefore, it has recently been widely used in powder or single-crystal XRD and Brillouin measurements with the external-heating setup. In this study, we describe a reproducible and standardized protocol for preparing EHDACs and demonstrated single-crystal XRD as well as Brillouin spectroscopy measurements of synthesized single-crystal ice-VII using the EHDAC at 11.2 GPa and 300-500 K.

Protocol

1. Ring heater preparation

1. Fabricating the ring heater base
   1. Fabricate the ring heater base by a computer numerical control (CNC) milling machine using pyrophyllite based on the designed 3D model. The dimensions of the heater are 22.30 mm in outer diameter (OD), 8.00 mm in inner diameter (ID) and 2.25 mm in thickness. Sinter the heater base in the furnace at 1523 K for >20 hours.
2. Wiring
   1. Cut Pt 10 wt% Rh wire (diameter: 0.01 inch) into 3 equal-length wires (about 44 cm each).
   2. Carefully wind each Pt/Rh wire through the holes in the heater base, leave about 10 cm wire outside of the heater base for connection to the power supply. When wiring, make sure that the wire is lower than the gutters of the base. If it is higher than the gutter, use a proper flat-head screwdriver to press it down.
   3. Wind more wires on the 10 cm extension wires to reduce the electrical resistance and thus the temperature of the extension wires during heating.
3. Adding insulators
   1. Use two small ceramic electrical insulating sleeves to protect the wires extending outside the ring heater base. Mix cement adhesive (e.g., Resbond 919) with water at a ratio of 100:13. Fix those tubes to the ring heater base using the cement mixture.
      NOTE: The cement needs 4 hours to be cured at 393 K or 24 hours at room temperature.
   2. Use the high-temp braid sleeving to protect the outside wires.
   3. Cut two mica rings using a CO₂ laser-cutting machine. To electrically insulate the wire, attach one mica ring to each side of the heater by UHU tac.

2. EHDAC preparation

1. Gluing diamonds
   1. Align the diamonds with backing seats using mounting jigs. Use black epoxy to glue the diamond to the backing seat. The black epoxy should be lower than the girdle of the diamond to leave some space for the high-temperature cement.
2. Alignment
   1. Glue mica or place the machined pyrophyllite rings under the seats to insulate the seats and DAC thermally. Put the seats with the diamonds into a BX-90 DAC. Align two diamonds under the optical microscope.
3. Preparing the sample gasket
   1. Place the rhenium gasket, which is smaller than the hole of the ring heater, between the two diamonds and pre-indent the gasket to approximately 30-45 µm by gently tightening the four screws of DAC. Drill a hole at the center of the indentation by electrical discharge machine (EDM) or laser micro-drilling machine.
4. Mounting thermocouple
   1. Fix two small pieces of mica with the cement mixture on the seat of the piston side of DAC to electrically insulate the thermocouples from the seat. Attach two K-type (Chromega-Alomega 0.005'') or R-type (87%Platium/13%Rhodium–Platium, 0.005”) thermocouples to the piston side of the DAC, ensuring that the tips of the thermocouples touch the diamond and close to the culet of the diamond (about 500 µm away). Finally, use the high-temperature cement mixture to fix the thermocouple position and cover the black epoxy on both sides of the DAC.
5. Heater placement
   1. Cut the 2300 °F ceramic tape in the shape of the heater base by CO₂ laser drilling machine and place it on both sides of DAC (piston and cylinder sides). If it is very easy to move around, use some UHU tac to fix it.
   2. Place the heater in the piston side of the BX-90 DAC. Use some 2300 °F ceramic tape to fill the gap between the heater and the wall of the DAC.
6. Gasket placement
   1. Clean the sample chamber hole of the gasket using a needle or sharpened toothpick to get rid of the metal fragments introduced by the drilling. Use ultrasonic cleaner to clean the gasket for 5-10 min.
   2. Put two small balls of adhesive putty (e.g., UHU Tac) around the diamond on the piston side of the DAC to support the gasket. Align the sample chamber hole of the gasket to match the center of culet under the optical microscope.

3. Synthesizing single-crystal ice-VII by EHDAC

1. Loading sample
   1. Load one or more ruby spheres and one piece of gold into the sample chamber.
   2. Load a drop of distilled water in the sample chamber, close the DAC and compress it by tightening the four screws on the DAC to quickly seal the water in the sample chamber.
2. Pressurizing sample to obtain powder ice-VII
   1. Determine the pressure of the sample by measuring the fluorescence of ruby spheres using a Raman spectrometer.
   2. Carefully compress the sample by turning the four screws and monitor the pressure by ruby florescence until it reaches the stability field of ice-VII (>2 GPa). Watch the sample chamber under the optical microscope during compression. Sometimes the coexistence of water fluid and crystallized ice VI is visible if the pressure is close to the phase boundary of water and ice VI.
   3. Continue compressing the sample chamber until it reaches the pressure in the stability field of ice-VII. In order to melt the ice-VII later, the target pressure is usually between 2 GPa and 10 GPa at 300 K.
3. Heating sample to obtain single crystal ice-VII
   1. Put the EHDAC under the optical microscope with a camera connected to the computer. Thermally insulate the DAC from the microscope stage, without blocking the transmitted light path of the microscope.
   2. Connect the thermocouple to the thermometer and connect the heater to a DC power supply.
   3. Monitor the melting of ice-VII crystals upon heating to a temperature that is higher than the melting temperature of high-pressure ice-VII determined by the phase diagram of H₂O.
   4. Quench the sample chamber to allow the liquid water to crystallize, and then increase the temperature until some of the smaller ice crystals are molten. Repeat the heating and cooling cycles a few times until only one or a few larger grains remains in the sample chamber.
   5. Measure the pressure of the sample after the synthesis.

4. Synchrotron X-ray diffraction and Brillouin spectroscopy collection

1. Synchrotron X-ray diffraction
   1. Check if the ice-VII sample synthesized is polycrystalline or a single crystal by synchrotron-based single-crystal XRD¹⁵. If it is a single crystal, the diffraction pattern should be diffraction spots instead of powder rings.
   2. Obtain step scan single-crystal XRD images to determine the orientation and lattice parameters of ice-VII.
   3. Collect the XRD of pressure marker, i.e. gold, in the sample chamber to determine the pressure.
2. Brillouin spectroscopy
   1. Mount the EHDAC on a specialized holder which can be rotated within the vertical plane by changing the χ angles. Connect the thermocouples to the temperature controller and connect the heater to the power supply.
   2. Perform Brillouin spectroscopy measurements every 10-15° χ angle at 300 K for a total χ angle range of 180° or 270°¹⁶. Then heat the sample to high temperatures (e.g., 500 K) and repeat the Brillouin spectroscopy measurement.

Results

In this report, we used the fabricated resistive micro-heater and BX-90 DAC for the EHDAC experiment (Figure 1 and Figure 2). Figure 1 shows the machining and fabrication processes of the ring heaters. The standard dimensions of the heater base are 22.30 mm in outer diameter, 8.00 mm in inner diameter and 2.25 mm in thickness. The dimensions of the ring heater can be adjusted to accommodate various types of seats and diamonds.

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Discussion

In this work, we described the protocol of preparing the EHDAC for high pressure research. The cell assemblies including a micro-heater and thermal and electrical insulating layers. Previously, there are multiple designs of resistive heaters for different types of DACs or experimental configurations^{7,17,18,19,20}. Most of the heaters are machined by individual ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Au	N/A	N/A	for pressure calibration
Deionized water	Fisher Scientific	7732-18-5	for the starting material of ice-VII synthesis
Diamond anvil cell	SciStar, Beijing	N/A	for generating high pressure
K-type thermocouple	Omega	L-0044K	for measuring high temperature
Mica	Spruce Pine Mica Company	N/A	for electrical insulation
Pt 10wt%Rh	Alfa Aesar	10065	for heater
Pyrophyllite	McMaster-Carr	8479K12	for fabricating the heater base
Re	Sigma-Aldrich	267317	for the gasket of diamond anvil cell
Resbond 919 Ceramic Adhesive	Cotronics Corp	Resbond 919-1	for insulating heating wires and mounting diamonds on seats
Ruby	N/A	N/A	for pressure calibration
Ultra-Temp 2300F ceramic tape	McMaster Carr Supply	390-23M	for thermal insulation