The Kawai-type multianvil apparatus, developed in Japan in the early 1980s (e.g. Kawai and Endo, 1970; Ito et al., 1984), is well adapted to study the behaviour of minerals under mantle P-T conditions. The multianvil press is a two-stage type of apparatus able to reach pressure up to 25 GPa
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and temperatures around 2000°C. The first stage is a pair of permanently glued steel guide blocks which encloses the second stage in a cubic cavity.
The force is applied to the guide blocks by a single ram (Figure II.3).
Figure II.3 – Kawai type multianvil: schematic diagram of the second of anvils in between the first one.
The second stage of anvils consists in eight tungsten carbide cubes with truncated corners. These cubes are maintained all together using epoxy-impregnated fibreglass laminate sheets which also insure electrical insulation with the guide blocks. The pressure cell is a sintered MgO octahedron doped with 5% of Cr203. itis compressed in the octahedral cavity formed by the truncated cubes (Figure II.4).
The sample loaded in this octahedron (Figure II.5) is heated by a stepped cylindrical furnace (LaCrO3 in our case, it can also be graphite). In a stepped cylindrical furnace, the wall thickness of the central part of the heater is thicker than the two end sections. The consequence is that the central part has a relatively low resistance and does not generate as much
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heat as the two end parts. The effect of this type of geometry is to reduce the temperature gradients. A ZrO2 sleeve is inserted around the heater for thermal insulation and a MgO sleeve avoids chemical reaction between LaCrO3 and sample materials at high temperature. The temperature is measured close to the sample using a thermocouple (W3%Re-W25%Re in most cases) in an Al2O3 sleeve.
Figure II.4 – Kawai-type multianvil apparatus: schematic diagram of the second stage of anvils containing the octahedron.
High pressures are generated with tungsten carbide anvils.
Pyrophyllite gasket and extruding pressure medium support the large stress gradients generated in the anvils. Cardboard and Teflon tape are also placed on the surface of the cubes behind the gaskets to electrically insulate the thermocouple wires and to provide extra support to the gaskets.
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Figure II.5 – Kawai type multianvil: cross-section through a standard 18/11 assembly.
The press is driven by an hydraulic system. The pressure generated in the cell depends on the truncation edge-length (TEL) of the WC cube and on the octahedral edge-length (OEL) of the octahedron as well as on materials used and on the cross-sectional dimensions of the gaskets (Liebermann and Wang, 1992). Different assemblies are named after the ratio of these two lengths: OEL/TEL in mm. Table II.1 displays the sample volume and the pressure range for different assemblies for a 1000-2000t capacity hydraulic press. Increasing the OEL/TEL ratio results in a decrease of the sample size and in an increase of the pressure range.
OEL/TEL Sample volume (mm3) Pressure range (GPa)
18/11 12 4-12
14/8 5-8 11-17 10/4 1 22-26
Table II.1 – Sample volume and pressure range for the different assemblies used in the Kawai-type multianvil
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The aim of the two-stage configuration of the Kawai-type multianvil is to amplify the loading force with a succession of amplifier elements between the single ram and the sample. One of the difficulties is then to infer the pressure applied to the sample from the loading force and the OEL/TEL ratio. This is done through calibrations. At room temperature, calibrations are made by monitoring in situ variation of electrical resistance of materials during phase transformations. For instance, the transformation Bi-I to Bi-II and Bi-III to Bi-IV occur at 2.52 GPa and 7.7 GPa respectively (Lloyd, 1971;
Getting, 1998). At high temperature, the pressure efficiency is different and specific calibrations must be done. Various factors are involved which cause either increase or decrease of the cell pressure from the value determined at room temperature. For instance, increasing temperature first produces a thermal expansion of the cell which increases the pressure. Then, the gaskets begin to flow which results in decreasing the pressure. The calibration has to be performed for different temperatures using known equilibria such as coesite-stishovite (Zhang et al., 1996), !-Mg2SiO4 to# -Mg2SiO4 (Morishima et al., 1994) and #%Mg2SiO4 to $-Mg2SiO4 (Suzuki et al., 2000) transformations, etc. Figure II.6 presents the calibration curves used in the Bayerisches Geoinstitut (including for this study). The uncertainties in pressure calibrations at high temperature are estimated about ± 0.5 GPa;
such estimates are based on reproducibility and on how precisely an equilibrium boundary can be bracketed. In fact the uncertainties may depend on the starting material used for the pressure calibration and on the sample volume. Hence, they may be significantly larger in some cases (Rubie, 1999).
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Figure II.6 – Top: calibration curves for the Sumitomo press (1200t).
Calibration for 18/11 (black) and calibration for 14/8 (gray). At same nominal pressure, a ratio of 0.6 has to be applied on oil pressure for the Hymag press (1000t). Bottom: calibration curves for the Zwick press (5000t). Room (gray) and high-temperature (black) (from Frost et al.(2004)).
For high-temperature calibration, high or low-pressure phase at each transformation is indicated by either filled or opened signs respectively.
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I.2.2
– Using the multianvil press as a deformationapparatus
The assembly presented above has been developed to perform quasi-hydrostatic experiments. Some modifications are necessary to plastically deform a sample in a non-hydrostatic environment. As the Kawai-type multianvil apparatus does not allow independent control of differential stresses and pressure, the idea is to produce differential stresses during compression from a non-hydrostatic pressure cell. Early proposed by Fujimura et al., (1981), this approach has triggered a number of studies in the past few years (Green II and Borch, 1989; Liebermann and Wang, 1992;
Bussod et al., 1993; Sharp et al., 1994; Weidner, 1998; Cordier and Rubie, 2001; Cordier et al., 2002; Thurel and Cordier, 2003a; Thurel et al., 2003b).
The usual cell involves stiff alumina pistons on both ends of the specimen (Figure II.7). During cold compression, large differential stresses are built up. When temperature is increased, the stresses are relaxed in the assembly as well as in the sample which is then plastically deformed at high pressure and high temperature. It is important to note that pressure and differential stresses are not monitored independently, nor can one have an internal force gauge in the cell. As relaxation proceeds, the strain rate decreases and, finally, deformation stops. This technique usually induces limited plastic strain. In order to achieve larger strains, Karato and Rubie (1997) have proposed a shear design (Figure II.7). The setup is basically the same as above except for the alumina pistons that are cut at 45°. The sample is now a thin slice (usually 200µm) placed between the pistons. High temperature stress relaxation results in the shearing of the specimen and large strains can be reached (over 100%; according to Karato and Rubie (1997)). A strain
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marker in the sample provides a measurement of the total deformation after the experiment.
Figure II.7 - Cross-section through the compression (left) and shear (right) deformation assemblies.
In this study, the deformation experiments using the multianvil apparatus are performed in the shear design assembly in order to create large strain (Karato and Rubie, 1997).
Once the second-stage of anvils is inserted into the press, the run procedure applied for every experiment of this study (Figure II.8) consists of increasing the loading oil pressure at room temperature and holding it constant during heating. At pressure, the sample is heated up slowly (50°-100°C/ min). Then 10 to 15 minutes are necessary to attain 1400°C, temperature used in this study. The temperature is maintained constant automatically. At the end of the experiment, the power supply of the heater is shut off and (due to its small size) the sample is quenched to room temperature within a few seconds. Then the pressure is decreased slowly
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within 10-20 hours to avoid damaging the WC cubes. The sample is then extracted from the compressed cell for microstructural investigation.
Figure II.8 – Typical experimental procedure for Kawai-type multianvil experiments. Black: oil pressure versus time. Gray: temperature versus time.