X-ray thermal diffused scattering: single-crystal sound velocity measurements
- X-ray thermal diffuse scattering (TDS) is caused by lattice thermal vibrations (phonons). In contrast to sharp Bragg diffraction peaks, TDS is diffusely distributed, due to the continuous distribution ofphonon modes in the reciprocal space.
- Si single-crystal TDS measurement using mar CCD:
TDS measurement protocol at 13BMC 13IDC and 34IDE will be avialable for download soon.
- Forsterite single-crystal TDS signal:
Advantages of using TDS for measuring sound velocities of Earth materials:
- 1. TDS can be applied for work with diamond anvil cell (DAC) apparatus easily.
- 2. TDS is highly suitable for measurements on opaque materials.
- 3. The method easily applies to lower symmetry materials.
- 4. The method does not require special sample preparation (polishing), is not sensitive to surface quality, and is much less sensitive to sample orientation than e.g. Brillouin or IXS.
- 5. Data acquisition is fast with 3 rd generation X-ray source (e.g. APS).
- 6. Experimental setup is essentially identical to what is used for routine X-ray diffraction experiments.
- 7. Single-crystal elastic properties can be determined through TDS.
TDS experiment involve 3 steps:
- 1. Single crystal diffraction to get orientation matrix and unit cell parameters.
- 2. Picking up Brag peaks with proper (h,k,l)s.
- 3. Measuring TDS signal focused on the selected Brag peaks at slightly off-Bragg condition.
TDS data analysis involve 4 steps:
- 1. Precise calibration of detector pixel position (x,y) with (h,k,l)s, only use pixels with (h,k,l)s close to Brag peaks for fitting.
- 2. Building micro-force-constant model for studied material, single-crystal elasticitic constants are independent variables.
- 3. Least square fitting for single crystal elasticity, lattice parameters as well as X-ray energy are additional (fixed) input parameters.
- 4. Evaluate fitting quality by comparing the TDS signal calculated from the final fitting model with real experimental data.
- 1. Ambient condition measurement: single-crystal Si and MgO.
- 2. High-pressure measurement: single-crystal Si at room temperature and one high pressure in 2015-1 cycle.
- 3. Software: preliminary python-Fortran code developed based least sq fitting
- 4. High-pressure measurement: single-crystal forsterite at room temperature and one high pressure in 2015-2 cycle.
- 5. High-pressure measurement: single-crystal forsterite at room temperature and one high pressure in 2015-3 cycle.
- 6. Ambient condition measurement: single-crystal bridgemanite in 2016-1 cycle.
Universal Membrane Cap to fit different DACs
- Membrane-driven DACs are ideal for synchrotron experiments. With membrane, pressure inside DACs could be controlled remotely. However, membrane-driven DACs are relatively expensive. Membrane caps for conventional DACs have been available recently. With such caps, each conventional screw-driven DAC is transformed into a membrane-driven DAC. Commonly used DACs include Princeton-design piston cylinder DAC, modified 3-pin and 4-pin Merrill-Bassett-type DACs, Boehler-Almax Plate DAC, BX-90 DAC etc. All of them are in different sizes. Therefore each type of DAC would require a specificity designed membrane cap for it. Our target is to design a universal membrane cap to fit most DACs.
- Further possibilities include integrating the double-membrane into this universal membrane cap so we could have accurate control of compression and decompression rate during high-pressure experiments.
Membrane designs will be avialable for download soon.
- We finished designing and testing the first edition of universal single-membrane cap, it is ready for users to use now.
Membrane Cap images:
Standard DAC heater
- Commercial tungsten heaters sealed within evacuated ceramic capsules can reach 1000 K within 30 s and are very stable. However their sizes are not suitable for DAC experiments. We plan to redesign the commercial W heaters to fit in the BX-90 cell, modified 3-pin and 4-pin MerrillBassett cells, which are the most widely used resistive heated DACs in the mineral physics community.
Our target is to design a heater with following characteristics:
- 1. Ready for use, minimal work to attach to a DAC
- 2. Well-calibrated, specifications have to be repeatable from piece to piece
- 3. Reusable and not expensive
- 4. Universal, fits as many types of DACs as possible
- 5. Wide range of temperature if possible
- 1. Test on original W-Al2O3 heater (power curve): uneven heating from single heater solved by gluing two heaters together
- 2. Instructions for using the originally designed heater available: Download
- 3. Obtained newly designed heaters from vendor, first test made on heaters in BX90 DAC, highest temperatures obtained with single heater to ~450K.