Paris-Edinburgh Large-volume Cell for Structural Studies at High Pressure and High Temperature

K. Saksl, H. Franz, A. Ehnes, J. Koch-Bodes1 and J. Ďurišin2

The investigation of matter under extreme conditions is one of the main issues addressed at synchrotron radiation sources. Changes in physical properties of matter may directly be correlated with changes in the distribution of the constituent atoms and interatomic distances. The determination of structural changes of materials tested at high pressure and high temperatures is of great importance in various domains of materials science and geophysics. The Paris-Edinburgh large-volume cell (type V4, Figure 1) has recently been installed at the PETRA beamline and allows quasi-hydrostatic compression (up to ~ 12GPa) and heating (up to 1400K) of samples between two tungsten carbide anvils. The opening angles as determined by the anvils are 15° in the vertical and 60° in the horizontal plane. Resistive heating is provided by a graphite heater and controlled by a thermocouple located close to the center of the sample assembly.

Figure 1: Cross-section of Paris-Edinburgh press (a), anvils/sample ensemble (b) and sample (c). 1University of Bremen, Bremen, Germany 2 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 043 53, Košice, Slovak Republic

anvils

piston

seats

oil inlet

top plate breech

tie-rod

thermocouple gasket

Graphite cylinder Graphite disc

BN cylinder

ceramic BN powder Cu ring

sam

ple

incident beam diffracted beam

upper (stable) anvil

lower (movable) anvil

a b

c

The possibility of using high brilliance synchrotron radiation and a fast CCD detector interfaced to the P-E cell allows one to determine structural changes of matter at high pressure and high temperature. Figure 2 shows a picture of the high-pressure set-up installed at beamline PETRA1.

Figure 2: Picture of the set-up at beamline PETRA1. After calibration of the absolute pressure inside the sample cell using NaCl as a standard we selected a hypereutectic AlSi26Ni8 alloy (Skawina/Poland) in form of crystalline powder for testing the large-volume cell. The alloy is used as an input product for manufacturing light weight products via powder metallurgy, having a great potential in automotive, aeronautic and household industry. From the point of powder metallurgy it is important to examine the structure evolution during the compaction process. The possible structural changes during this operation determine the properties of the final product. Figure 3 shows XRD patterns of a sample at ambient pressure and compressed to 6.8 GPa measured at room temperature. The patterns integrated and transformed into lattice parameter (d) space are shown in Fig. 4a and a zoom on the Al (111) diffraction line is shown in Fig. 4b.

a

b

Figure 3: XRD pattern recorded of a AlSi26Ni8sample at ambient pressure (a) and 6.8GPa (b).

Figure 4: XRD patterns of AlSi26Ni8 sample gradually compressed in the Paris-Edinburgh cell; whole pattern (a), detail of Al(111) diffraction line (b).

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4

2.1

5.9

Inte

nsity

(arb

. uni

t)

6.8

5 3.6

0

d (Å)

pres

sure

(GPa

)

2.36 2.34 2.32 2.30 2.28 2.26 2.24

2.1

5.9

Inte

nsity

(arb

. uni

t)

6.8

5 3.6

0

d (Å)

pres

sure

(GPa

)

Al(111)

a b

The XRD pattern at ambient pressure consists of distinct diffraction peaks of Al, Si and h-BN. BN was used as a inert capsule for the sample and plays also the role of an internal pressure calibrant. By applying pressure, the compression along the [001] crystallographic axis of BN is significantly higher compared to Si, resulting in an overlap of the BN(002) and Si (111) diffraction lines. To test the pressure calibration we fitted the relationship between relative volume difference of the Al matrix and pressure by the Birch-Murnaghan equation (see Figure 5). The resulting bulk modulus Bo= 75.46 GPa is in good agreement with the reported Bo for Al (76 Gpa) [1].

1.00 1.02 1.04 1.06 1.08-1

0

1

2

3

4

5

6

7

Pre

ssur

e (G

Pa)

Relative volume V/V0

Figure 5: Birch-Murnaghan fit of experimental data measured on the Al phase in AlSi26Ni8. In general powder diffraction at high pressure suffers from the relatively high background level while the signal from the (small) sample is rather low. The main contribution to the background originates from the gasket. Liquids and amorphous materials exhibiting a low x-ray diffraction signal can not be measured without supporting collimation on the detector side. Our plan is to built up a multi-channel collimator at the high-energy beamline PETRA2 in the near future. References [1] A.M. James and M.P. Lord in Macmillan's Chemical and Physical Data, Macmillan,

London, UK, 1992.