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

Externally-heated diamond anvil cell can generate simultaneously high pressure and high temperature, to simulate the conditions in ours and other planets'interiors. The main advantage of this technique is that it can be combined with various spectroscopic techniques, such as optical microscopy, x-ray diffraction, Raman spectroscopy, and Brillouin scattering. This technique is used to study interiors of rocky planets and the moon.

It can also be used to investigate the material property under extreme condition in solid state physics and chemistry. The most challenging part of this protocol is the placement and the fixture of thermal couple to the diamonds. It is important to carefully follow the instruction when performing these steps.

This protocol involves many hands-on steps, for where your demonstration is critical, in order to provide sufficient details for the audience to follow. Begin by cutting the platinum rhodium wire into three equal lengths, approximately 44 centimeters each. Carefully wind each wire through the holes in the heater base, leaving about 10 centimeters outside of the heater base for connection to the power supply.

Make sure that the wire is lower than the gutters of the base. If it's higher than the gutter, use a proper flathead screwdriver to press it down. Wind more wires on the 10 centimeter extension wires, to reduce the electrical resistance.

Use two small ceramic electrical insulating sleeves to protect the wires, extending outside the ring heater base. Mix cement adhesive with water, at a ratio of 100 to 13, and use the mixture to fix those tubes to the ring heater base. Then allow the cement to cure.

Electrically insulate the wire by attaching one mica ring to each side of the heater with adhesive putty. Use mounting jigs to align the diamonds with backing seats. Then glue the diamond to the backing seat with black epoxy.

The black epoxy should be lower than the girdle of the diamond, to leave some space for the high temperature cement. To thermally insulate the seats and the diamond anvil cell, or DAC, glue mica or place mica rings under the seats. Put the seats with the diamonds into a BX90 DAC.

And align two diamonds under the optical microscope. Place the rhenium gasket between the two diamonds, and gently tighten the four screws of the DAC to pre-indent the gasket to approximately 30 to 45 micrometers. Drill a hole at the center of the indentation with an electrical discharge machine, or a laser micro-drilling machine.

Fix two small pieces of mica with the cement mixture on the seat of the piston side of the DAC, to electrically insulate the thermal couples from the seat. Attach two K-type or R-type thermal couples to the piston side of the DAC, ensuring that the tips of the thermal couples touch the diamond close to the culet. Then, use the high temperature cement mixture to fix the thermal couple position, and cover the black epoxy on both sides of the DAC.

Use the carbon dioxide laser drilling machine to cut the 2300 degree Fahrenheit ceramic tape in the shape of the heater base, and place it on both sides of the DAC, fixing it with adhesive putty if necessary. Place the heater on the piston side of the BX90 DAC, and use some ceramic tape to fill the gap between the heater and the DAC wall. Clean the sample chamber hole of the gasket with a needle, to get rid of the metal fragments introduced by the drilling.

Then use an ultrasonic cleaner to clean the gasket for 5 to 10 minutes. Put two small balls of adhesive putty around the diamond on the piston side of the DAC to support the gasket. Then align the sample chamber hole of the gasket to match the center of the culet, under the optical microscope.

Load one or more ruby spheres and one piece of gold into the sample chamber. Then load a drop of distilled water in the sample chamber. Close the DAC and compress it, by tightening the four screws.

Determine the pressure of the sample by measuring the fluorescence of ruby spheres with a Raman spectrometer. Carefully compress the sample by turning the four screws. And monitor the pressure until it reaches the stability field of Ice VII.

The target pressure is usually between 2 and 10 gigapascal at 300 Kelvin. Put the externally-heated DAC under the optical microscope with a camera connected to the computer. Thermally insulate the DAC with the microscope stage, without blocking the transmitted light path of the microscope.

Connect the thermal couple to the thermometer, and connect the heater to a DC power supply. Monitor the melting of Ice VII crystals, upon heating to a temperature that is higher than the melting temperature of the high pressure Ice VII. Quench the sample chamber to allow the liquid water to crystallize.

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 remain in the sample chamber. The compressed water sample was heated to an externally-heated DAC at about six gigapascal, up to 850 Kelvin, to make a single crystal Ice VII.

A large single crystal was synthesized after several cycles of heating and cooling. The synthesized single crystallized VII was utilized for synchrotron x-ray diffraction, and Brillouin spectroscopy, at high pressure and high temperature. The temperature power relationship was determined.

The crystal had little lattice stress and retained its good quality after compression and heating. As indicated by the sharp Bragg diffraction peaks at synchrotron-based single crystal x-ray diffraction images. The diffraction pattern can be indexed with a cubic structure.

The sound velocities and elastic moduli were obtained by high pressure and high temperature Brillouin scattering measurements. While attempting this protocol, the placement of thermal couples is very important. The thermal couples should be electrically insulated around the seats and the DAC, and it should be close to the culet of diamond.

The external-heated diamond anvil cell is often combined with Ramen, FTIR, and numerous synchrotron radiation spectroscopic methods, such as x-ray diffraction, combined with the properties of materials in situ at high pressure in high temperature conditions. For those who are familiar with diamond anvil cell, this technique can be easily learned to enable performance on not only high pressure, but also high temperature measurements in future studies.