Hydrostaticity of Pressure Transmitting Medium of 4:1 Methanol: Ethanol at High

Pressure and Low Temperature

Christopher Salvo1, Andrew Cornelius21California State University, San Marcos

2University of Nevada, Las Vegas

In high pressure physics a key element is the pressure transmitting medium. The purpose of this research is to study the hydrostatic limits of pressure transmitting media such as 4:1 Methanol: Ethanol and Silicone Fluid at low temperatures and high pressures. This will be done using a Merrill-Bassett diamond anvil cell (DAC) placed inside of a cryostat capable of reaching temperatures as low as 30 Kelvin. The temperature scale will be incremented in steps of 25 Kelvin for a range of 30 Kelvin to 300 Kelvin.

The hydrostaticity of 4:1 Methanol:Ethanol has been heavily studied at room temperature using the fluorescence of ruby by fitting the R1 and R2 lines to pseudo-Voigt functions. The full width at half max (FWHM) of the R2 line has been used as a means to determine the hydrostaticity of the pressure medium in these room temperature measurements. (1)

Abstract & Background

The data collected here is for 4:1 Methanol:Ethanol. The size of the culets in the DAC used is 400 µm across, with a stainless steel gasket with a hole of 130 µm in diameter. In the gasket (between the diamonds) was loaded the 4:1 Methanol: Ethonal with a ruby in the center. The first set of measurements yielded 1, 2, 3, 4, 5 and 12 GPa. Then, the same cell was re-loaded and started at 3 GPa and it produced 6, 7.25, 9, 10.25 GPa.

A ruby laser of 405 nm was focused on the ruby within the DAC causing it to fluoresce, producing two peaks, R1 and R2. The fluoresced light was measured using a spectrometer capable of 0.07 nm resolution. The raw data was then fitted to a pseudo-Voigt function using PeakFit which provided the FWHM and center of each peak. Using the Mao ruby scale, the pressure for each measurement was determined by the peak position of the R2 ruby line. (3)

Experiment Details

Figure 1. Merrill-Bassett Diamond Anvil CellOn the picture in the right you can see the diamond on both the top and bottom. The three screws are what apply the force and since the culets on the diamond are small

no large force is needed to reach high pressures.

Graph 1. Here the black squares are the raw data from the spectrometer. The red line is the

fitted function from PeakFit. The main information comes from ruby peak 2; it yields two parameters, FWHM and pressure. Temperature and hydrostaticity are two main reasons that cause the FWHM to change. The lower the temperature the smaller the FWHM and a hydrostatic sample will also have a smaller FWHM. (4) This information

will be useful for graphs 3 and 4.

Graph 2.Each data point here comes from a graph like graph 1. This graph is at 10.75 GPa. The trend of the graph is easily explained by thermal

broadening.

Graph 3. The FWHM starts to rise at 6 GPa which may be caused by a phase

transition to a solid, the solid is now applying a sheering force upon the ruby which causes broadening. The FWHM drops after 7.25 GPa but is

still above the pressures bellow 6 GPa. Then goes back up at 12 GPa.

Graph 4. This graph is similar to graph 3, however the FWHM does increase

starting at 6 GPa and continues to 12 GPa.

Figure 2. CryostatAt the bottom of the cryostat sits the sample (notice the round window) and perpendicular to the visible window is the laser beam. The beam enters through the triangular bottom or top of the DAC passing through the diamond itself, hitting the ruby causing it to fluoresce. The fluoresced light goes back along the path of the laser and passes through a few beam splitters, that are not visible here, until it reaches the spectrometer.

Results and ConclusionIt is reasonable to anticipate a fluid transitioning into a solid at low temperatures

and high pressures. Data gathered from graph 3, at 30 Kelvin, shows that ruby fluorescence FWHM in 4:1 Methanol: Ethanol increases slowly until it reaches 10.75 GPa to 12 GPa, where it appears to transition into a solid. However, in graph 4, at 260 Kelvin, the FWHM increases starting at 6 GPa to 12 GPa. Comparing the two graphs, graph 3 has a much sharper slope where as graph 4 has a less intense slope. This maybe happening because of competing factors of broadening peaks. As temperature goes up the peaks broaden with it, which may be the cause of the sudden up shoot at 30 Kelvin.

The FWHM increases at 12 GPa and the broadening of R2 can show non-hydrostatic conditions due to the sheering effects of a solid. However since there is only one data point at 12 GPa a definite conclusion can not be drawn. More data is needed to be gathered from 11 GPa to 20 GPa.

Reference

(1) Model line-shape analysis for the ruby R lines used for pressure measurement, R.G. Munro

(2) Mao et al, J. Appl. Phys. 49, 3276 (1978)(3) Effective hydrostatic limits of pressure media for high-

pressure crystallographic studies, Ross J. Angel

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Acknowledgment

Technical help from Jason Baker, Matthew Jacobson and Daniel Antonio and Support from the REU program of the National Science Foundation under grant DMR-1005247 is

gratefully acknowledged