A hybrid self-gasketing "dry" DIA cell

William B. Durham (Massachusetts Institute of Technology)
Kurt Leinenweber (Arizona State University)
Shenghua Mei (University of Minnesota)

The cubic medium that is the basis of the self-gasketing DIA pressure assembly must have sufficient strength to achieve good pressure efficiency and sufficient ductility to form competent gaskets bounding the six anvils. Few materials outside of boron-epoxy and unfired pyrophyllite meet these criteria. Both materials contain considerable water, and it is often problematic in the DIA to measure material properties that are affected by water. In order to measure the rheology of olivine--a material that is known to water weaken--in the Deformation-DIA, we have developed and successfully tested a hybrid cell that combines the dryness and strength of mullite with the self-gasketing properties of unfired pyrophyllite. Conceptually, the cell is a cube of pyrophyllite with a spherical cavity of diameter equal to the edge length of the cube, with a sphere of mullite exactly filling the cavity. Machining and assembly of the pyrophyllite "web" and the mullite sphere (which can also be made as half-spheres if desired) is not complex. We have carried out over a dozen runs with the hybrid cell, both on- and off-(synchrotron) line, and have been able to achieve significantly higher pressures and incur fewer blowouts, than with an earlier version of the anhydrous cell made only of mullite. The performance is comparable to that of boron-epoxy, but without the water. FTIR confirms the dryness of San Carlos olivine tested in the cell.

Figure 1. View of the new hybrid mullite-pyrophyllite cell disassembled (top) and partially assembled (bottom). Mullite sphere diameter and cube edge length are 6 mm. Through hole for furnace, sample, and associated parts has not yet been drilled.

Figure 2. Two views of a hybrid cell assembly after testing in the Deformation DIA. It can be seen that mullite is entirely absent from the gaskets. The gaskets are as well-formed (as pyrophyllite gaskets ought to be). Mullite penetration at the cube faces is evident.

Figure 3. FTIR spectrum of San Carlos olivine single crystal testing in the D-DIA at 1473 K and approximately 5 GPa pressure. The blue curve shows the measured sprectrum (black) after removal of background (gree-red), and indicates a OH concentration of about 40 ppm, making the sample very dry.

Problem: No single cell material in the DIA configuration combines the right characteristics for carrying out creep experiments: strength, self-gasketing capability, and (especially) water content.

Boron-epoxy: too wet
Unfired pyrophyllite: too wet (and too weak)
MgO: too weak
Mullite: too friable

Solution: Combine materials into a hybrid D-DIA cell

The concept is similar to the use of manufactured gaskets in multianvil presses. For example, in the 2-stage 6/8 apparatus, it is common to use MgO as the pressure medium and separate pieces of unfired pyrophyllite as gasketing material. Manufactured gaskets can be made for the DIA configuration (e.g., N. Nishiyama at GSECARS), but the task is complex and time-consuming.

A workable solution:

Embed a sphere of pressure medium inside a cube of gasket material. The drawing here shows the dimensions of parts for a 6-mm DIA cube. The diameter of the sphere essentially the edge length of the cube. This puts more gasket material where it is needed the most (corners) and none where it is needed the least (center of faces).

We have tested one set of materials: mullite for the sphere with pyrophyllite completing the volume of the cube.

Manufacturing costs are very reasonable.

Results I: No more blowouts.

Versions of 100% mullite cells that we have been using for the past two years invariably produce “mini-blowouts,” which are usually too small to damage anvils but sharp enough to cause sudden displacement of parts in the deformation column—i.e., the sample and alumina piston—with associated degradation of creep data. In several tests of the mullite-pyrophyllite hybrid cell conducted off line and one run conducted at NSLS X17B2, mini-blowouts (and large blowouts) have been entirely absent.

The photo shows two views of the one assembly tested at X17B2. It can be seen that mullite is entirely absent from the gaskets. The gaskets are as well-formed (as pyrophyllite gaskets ought to be). Mullite penetration at the cube faces is evident.

Results II: Nice data, so-so pressure efficiency

The plot here shows smooth creep curves (differential stress resolution is about 0.05 GPa) at 3 robust (hkl) reflections in olivine, for the one run conducted at X17B2 (24-25 April 2007).

Pressure (mean stress) is about 4.4 GPa at 1573 K and a load of 60 T.

Results III: So did the sample stay dry?

Maybe not. We did one high-pressure anneal off-line on a single crystal in the hybrid D-DIA cell to produce a sample for FTIR. (Sample recovery of polycrystalline samples for FTIR is problematic.) The results are shown here.

The crystal appears to be wet. The absorption peak near 3500 cm-1 indicates significant bound water in the olivine lattice (quantitative amount to be determined). The matter is under investigation.

A note on activation volume:

The results of the first experiment done in the hybrid cell show a flow strength for San Carlos olivine of 0.30 ± 0.05 GPa at 1500 K, 2 x 10-5 s-1, and P = 4.4 GPa. Comparing these with those of Mei and Kohlstedt (2000) for P = 0.3 GPa gives an activation volume V* = 15 cm3/mole if our olivine was dry, and V* = 8.55 cm3/mole if it was wet.