METHODS FOR HIGH PRESSURE CHEMICAL REACTIONS: DIAMNOND ANVIL CELL (DAC)

Transcription

1 INSTRUMENTAL TECHNIQUE METHODS FOR HIGH PRESSURE CHEMICAL REACTIONS: DIAMNOND ANVIL CELL (DAC) KRISHNADAS K R

2 CHEMICAL REACTIONS AT HIGH PRESSURES Pressure-An important thermodynamic parameter for chemical equilibria and chemical kinetics. At sufficiently high confining pressure the repulsive interactions are balanced, making intramolecular and intermolecular forces comparable in magnitude. In these conditions, the system is thermodynamically unstable and the free energy minimization can be achieved through a full reorganization of the chemical bond connectivity. This can be realized with processes including ionization, polymerization, formation of atomic lattices, metallization, etc. At ultrahigh pressures, kinetics and mechanisms of the reactions can alter drastically from those observed under ambient conditions. Novel materials with peculiar properties can be made-superhard materials. Extreme high pressure conditions have been commonly encountered in many natural systems-geochemical and planetary processes. Laboratory experiments at high pressures can thus provide useful information or mimic natural processes.

3 1. Percy Williams Bridgeman revolutionized the field of high pressures with the development of an opposed anvil device with small flat areas that were pressed one against the other with a lever-arm. Percy Williams Bridgeman Friedrich Karl Rudolf Bergius Physics Nobel Prize in 1946 Chemistry Nobel Prize in The invention of the diamond anvil cell in the late 1950s at the National Bureau of Standards (NBS) by Weir, Lippincott, Van Valkenburg, and Bunting further refined the process. 3. During the following decades DACs have been successively refined, the most important innovations being the use of gaskets and the ruby pressure calibration. The DAC evolved to be the most powerful lab device for generating static high pressure. 4. The range of static pressure attainable today extends to the estimated

4 The diamond anvil cells (DACs) is the most versatile and popular device used to create very high pressures. This is a member of a class of high pressure devices called opposed anvil devices. Materials: Tungston Carbide or Diamond (An Hardened steel or Rhenium (Gask Diamond anvils Sample Gasket Diamond seats Backing plates Cell body Heating/Cooling (Temp. contro Springs system pressure appli Instrumentation for observatio Confining media Temperature measurement Pressure measurement

5 The operation Principle of the and diamond calibration anvil cell relies of local on a pressure simple principle: p= F/A where p is the pressure, F the applied force, and A the area. The most common calibration method is ruby fluorescence method. This measurement is based on the determination of the extremely intense R1 emission line of ruby. This line is narrow and undergoes a very large frequency shift on varying pressure. One or few micrometric ruby grains are added to the sample and their fluorescence can be probed with few mw of a blue laser beam through the diamond window. Ionic molecular salts: Solid solutions containing % in weight of NaNO2 in NaBr were successfully tested. The vibrational band of interest is the antisymmetric stretching of the NO2 group (n3 mode) at 1279 cm-1. This band is quite narrow also at high pressure

6 3. Gasket a foil of ~0.2 mm thickness (before compression) that separates the two culets. It has an important role: to contain the sample with a hydrostatic fluid in a cavity between the diamonds, and to prevent anvil failure by supporting the diamond tips, thus reducing stresses at the edges of the culet. Standard gasket materials are hard metals and their alloys, such as stainless steel, Inconel, rhenium, iridium or tungsten carbide. They are not transparent to X-rays, and thus if X-ray illumination through the gasket is required then lighter materials, such as beryllium, boron nitride, boron or diamond are used as a gasket. 1. The force-generating device relies on the operation of either a lever arm, tightening screws, or pneumatic or hydraulic pressure applied to a membrane. In all cases the force is uniaxial and is applied to the tables (bases) of the two anvils. 2. Two opposing diamond anvils made of high gem quality, flawless diamonds, usually with 16 facets. They typically weigh 1/8 to 1/3 carat (25 to 70 mg). The culet (tip) is ground and polished to a hexadecagonal surface parallel to the table. The culets of the two diamonds face one another, and must be perfectly parallel in order to produce uniform pressure and to prevent dangerous strains. Specially selected anvils are required for specific measurements for example, low diamond absorption and luminescence is required in corresponding experiments.

7 Advantages: 1. Prior to the invention of the diamond anvil cell, static high-pressure apparatus required large hydraulic presses which weighed several tons and required large specialized laboratories. The simplicity and compactness of the DAC meant that it could be accommodated in a wide variety of experiments. 2. In addition to being hard, diamonds have the advantage of being transparent to a wide range of the electromagnetic spectrum from infrared to gamma rays, with the exception of the far ultraviolet and soft X-rays. This makes the DAC a perfect device for spectroscopic experiments and for crystallographic studies using hard X-rays. Disadvantages: 3. Sample volume is less. 2. Other relevant limitations in ultrahigh-pressure experiments are the difficulty of calibrating the local pressure, the strong nonhydrostaticity of the samples, the red shift of the diamond absorption edge which, jointly with the diamond fluorescence, puts severe

8 Experimantal Techniques that can be done with DAC

9 An example of high pressure chemical transformation: Non-molecular nitrogen Interest in a chemical transformation of nitrogen under high pressure arises on the one side from the fact that the N-N triple bond is the most stable chemical bond known and on the other side from the expectation that the formation of networks of nitrogen nitrogen bonds could possibly give rise to high energy density systems. The transformation at high pressures from the molecular nitrogen to a connected non-molecular nitrogen array has been first predicted theoretically and later actually observed to occur in a diamond anvil cell, although at considerably higher pressure than predicted. Once compressed at room temperature (300 K) the nitrogen sample is reported to assume a progressive coloration: pale pinkish brown at 130 Gpa, Yellow at 150 GPa, and then fully opaque at 160 GPa. The electrical resistance measurements as well as the analysis of the shape of the optical absorption band clearly indicate that the non-molecular nitrogen is an amorphous narrow gap semiconductor with an estimated optical gap of ev.

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