The largevolume highpressure device known as SAM85 employs an assembly in which
the ram force applied along one vertical axis is converted by means of the DIA
device (developed by Kobe Steel, Japan, in the 1960's) to the six equivalent
components of force which act on six faces of the cubic sample assembly along
the three orthogonal axes. The DIA device consists of the upper and lower pyramidal
guide blocks (bolsters) installed on the heads of the press rams, four trapezoid
end blocks (thrust blocks) and six anvil holders, as in idicated in Figure 1.
The inner surfaces of the guide blocks form a tetragonal pyramid. Two of the
six anvils are along the center line of this pyramid and are fixed opposite
to each other on each guide block. The other four anvils are horizontally located
on the midpoints of the square edges of a bipyramid. This results in the formation
of a cubic nest bounded by the flat faces of six anvils. The ram force is applied
along the vertical axis. The six equivalent components of forces act along the
three orthogonal axes to force advancement of the six anvils toward the center
of the cube. The advance of all the anvils is automatically synchronized so
that the cubic samplefurnace assembly is compressed hydrostatically. SAM85 (SixAnvil
Machine designed in 1985) is similar to the MAX80 and 90 apparatus (MultiAnvil
Xray Apparatus designed in 1980 and 1990, respectively) at the Photon Factory,
Tsukuba, Japan.
The maximum pressure and temperature depend on the size of the anvil and its
material; with 6mm truncation WC anvils, the cubic anvil apparatus is currently
capable of obtaining pressures to 8 GPa at 2000K. Higher pressures can be reached
with 4mm WC (11 GPa at 1800K) or sintered diamond (14 GPa at 1800K) anvils.
Cubic Boron Nitride anvils have been recently used to obtain pressures of 14
GPa. Tapering the anvils also helps (see figure 2)
Figure 1 DIA apparatus - Uniaxial force on steel guide blocks applies
horizontal force on steel bolsters and attached anvils onto cell assembly.
Figure 2 Tapered Anvils - Tapering anvils by removing shaded areas increases
pressure efficiency
In 1995, we designed and tested a new high pressure module for use in the same press as SAM85. It is called the Teacup (or T-cup), because of its approximate size.The apparatus is a two-stage multi-anvil system similar to the now-common 6-8 systems; the first stage is a steel cylinder split into six parts enclosing a cubic cavity (19.5 mm edge length with the [111] axis of the cube vertical) which contains the second stage anvil assembly. The second stage is assembled outside the press and consists of eight 10 mm edge length cubes, separated by spacers. These cubes may be WC polycrystalline diamond, or cubic Boron Nitride (cBN). Each cube has one corner truncated into a triangular face; the eight truncations form an octahedral cavity in which the pressure medium is compressed.
It is desirable to keep the external ram force direction vertical, for maximum
compatibility with the existing DIA apparatus. Because the Teacup is a two stage
device, xrays must pass through gaps on both sets of anvils. There are no planes
which meet this condition, but by cutting a conical access hole in the first
stage, this condition can be met, with the diffraction plane inclined at an
angle of 35° from the vertical. Both diamond and cBNare transparent to high-energy
x-rays, so using that type of second-stage anvils removes the above restriction.
Figure 3 Teacup Module
As the xray beam from the synchrotron is relatively difficult to move, alignment is achieved by moving the apparatus with respect to the beam. The system has recently (2003) been changed to remove the pedestal from the base. The press is mounted on the pedestal, which is a motor-driven translation table (X-Y-Z), which also has otational freedom around the vertical axis.The beam defining apparatus (a set of incident slits and the detector slit system) are separately mounted on their own translation tables. In this way, the beam defining apparatus can be aligned with respect to the incident xray beam.
Because all of these adjustments must be made with the beam on and the hutch sealed, they are done remotely with 5 phase stepper motors, driven by E500 CAMAC or OMS VME modules.
The entire system is controlled by an EPICS-based system which runs on a Power PC in a VME crate. The software is all stored on a Windows 2000 server, and transferred to the Power PC whcn the latter is booted. The system is very similar to that being used at the APS and at beamline X26A so, until this documention update is completed, refer to the excellent documentation there.
Beamline features such as the monochromater, apertures, etc. are controlled by a separate PC running Linux.
Sample temperatures are made using one or two tungsten-rhenium thermocouples. Each thermocouple is connected to a Fluke digital microVoltmeter, which is connected to an IEEE-488 VME interface. The EMF is stored in the computer, and a polynomial curve determined from the calibration curve is applied to calculate the temperature. The temperature resulting from this calibration curve can be displayed at any time using the appropriate MEDM window.
Two regulated DC power supplies are available to supply heater current,
depending on the heater resistance.
The analog outputs of the LVDTs, the Heise pressure gauge, and the heater voltage and current are all connected to either IP330 VME analog-to-digital converters, or to V to F converters which in turn are connected to a VME Joerger scaler. The scaler seems to work better at the present time.
When the x-ray source is white, energy dispersive detection is used, using
four elements of a 13 element Canberra detector. The detector drives an XIA
DXP 4-element multichannel analyzer, which stores the diffracted x-rays in proportion
to their energy. Control of data collection is by computer.
One or two ionization chambers or PIN diodes are mounted in the x-ray path, to monitor the beam intensity. The first one is mounted between the slits and the sample; and the second one or the PIN diode is used in place of the detector during alignment.
A translating imaging plate assembly is used for most monochromatic experiments. The imaging plate itself is mounted on a translation stage mounted below the germanium detector. It can be moved synchronously with increases in temperature, pressure, or simply at a constant rate, so the diffraction pattern as a function of these variables can be measured.
A Bruker CCD detector is available for making 2-d diffraction measurements. It has faster readout than the IP, and is also used in conjunction with the IP for sample centering, pressure calibration, etc.
An optical system is in place for making radiographs of the sample. This uses two cameras, a high-resolution digital CCD (Spot), and an analog camera with lower resolution. The images from the CCD are stored on another PC, while the analog images can be recorded on a VCR.
White-radiation diffraction with both vertical and horizontal orientations can be measured using four elements of the 13 element detector in combination with a conical slit. This consists of a solid inner cone with a pair of outer cones separated from the inner cone with a 50 micron shim, creating a conical slit. The apex of this slit is at the sample. Therefore, the entire Debye ring is passed through to the detector array; the four elements intercept the top, bottom, left, and right portions of this ring. An adjustable aperture is placed between the slit and the detector to limit the portion of the Debye ring as desired.