Science at extreme conditions

The ultimate goal of the materials sciences is to understand the behavior of solids, melts and fluids over a range of pressures and temperatures, well enough to construct detailed, quantitative and predictive models of the physical and chemical processes operating in the industrial systems. Such an inquiry is not limited to materials science alone but is the basis of understanding the nature of materials, which exist in space and planetary interiors. The center (CeSMEC) aims to study the behavior of matter under extreme conditions of pressure and temperatures. In planetary context, the term "extreme conditions" is self-explanatory. Extreme pressures to megabars are common in the terrestrial planets. Temperatures may reach as high as 6000 K in Earth's core.

At high temperature and one atmosphere:

The importance to technology of accurate knowledge of thermal behavior and melting temperatures of solids (metals, ceramics, and alloys) is readily apparent when one considers the problems associated with high-performance engines, atomic reactors, re-entry vehicles and many other industrial processes. The center has plans for the following:

1. Study of thermal stress on solids using lasers and x-ray;

2. Diffusion data: Binary or multicomponent alloys in metal foils of measured thickness can be heated with a laser such that local thermal and measurable composition gradients can be studied;

3. Determination of melting temperature of refractory metals and ceramics which melt at temperatures in excess of 2000K using spectroradiometry;

4. Measurement of thermal expansion, electrical resistivity and thermal conductivity to melting temperatures;

5. Phase diagrams of the binary and ternary systems using laser heating at temperatures not accessible by conventional heating techniques;

6. High temperature synthesis of new material e.g. super-hard solids.

 

Melting of Tungsten

 

 

 

 

 

 

 

 

At high pressure and temperature

Why pressure? The use of pressure or pressure in combination with temperature in materials science has been limited. Although several hundred thousands of materials are known, only a thousand may have been studied at modest pressures of a few thousand atmospheres. Of these only about 30 elements and simple compounds have been subjected to a megabar of pressure or more. If we combine pressure with temperature, the number of studies is relatively few. Many of the materials studied have undergone several transitions to new forms as pressure was increased even to a hundred thousand atmospheres (10 GPa). Thus, we might easily triple or quadruple the entire range of known materials simply by applying such pressures to them. Many of the resulting new materials, including diamond, superhard carbon nitride, high pressure phases of oxides, or MgSiO3 with perovskite structure, persist at room conditions and therefore might be manufactured in quantity.

We collaborate with:

Geophysical Laboratory, Carnegie Inst, of Washington, Washington, D.C. (H. K. Mao,Y. Fei); Center for High Pressure Research, SUNY, Stony Brook, N. Y. (D. Weidner, R.C. Liebermann); Advanced Photon Source, Argonne National Laboratory (GSECARS, G. Shen and Y. Wang); National Synchrotron Light Source, Brookhaven National Laboratory (J, Hu); University of California at Berkeley (R. Jeanloz); Rensselaer Polytechnic Institute (K.Rajan); University of Arizona (J. Ganguly) Uppsala University, Sweden ( P. Lazor and L. Dubrovinsky).

Thermodynamic data bases

Thermodynamic data bases are crucial to all applications requiring the modeling of the industrial and planetary processes. We devote considerable efforts to systematize and maintain a thermodynamic database, which is both, geological as well as metallurgical.

Collaboration:G. Eriksson, GTT, Aachen, Germany; A. Pelton, Ecole Polytechnique, Montreal, Canada; B. Sundman, KTH, Stockholm, Sweden.

Computational Physics

Due to recent advances in computational techniques and the availability of plenty of super-computer power through parallel processing, material properties can be computed to a high degree of precision. One of the center's goals is to search for compounds which may acquire industrially interesting properties as a result of compression.

Faculty and staff:

S. K. Saxena, Prof. (Thermodynamics, mineral physics, high-pressure materials science), saxenas@fiu.edu;

Peter Lazor, Visiting Scientist (Mineral Physics, Lasers, Spectroscopy)

Faculty (to be appointed)

Research Engineer (to be appointed)

Sandeep Rekhi, Grad. Student

Martha Santos, Secretary

Tel: 305-348-3003 or 2365; Fax: 305-348-3877

Associate Faculty and Staff

G. Sen, Professor (Petrology, high-pressure materials) and B. Clement, Professor (magnetism) Department of Earth Science; Silvia Mergui, Asstt. Professor (Structures of thin films), Dept. of Engineering; T. Beasley (Electron probe engineer).

CeSMEC at FIU, VH-150, University Park, Miami, FL 33199,USA

Laser heating: Two laser power profiles are shown in the figure below. The left profile shows G. Shen's method of combining two different modes to obtain a homogeneous temperature distribution.

 

Some Recent Results in Planetary Materials

 

1993: A new phase of iron discovered at 40,000 bar and 2000 K (Saxena et al., 1993) with important consequences for Earth's core.

1994: Iron melting temperature determined to 1.5 million bar (Saxena et al., 1994) with important implications for estimating the temperature of Earth's core.

1996: a) A phase perovskite, considered for decades to be the stable constituent of Earth's lower mantle, determined to dissociate into oxides (Saxena et al., 1996).The result affects dynamic modeling of the planet. b) In-situ x-ray structure determination of the new iron phase.

1997: a) Experimentally discovered and theoretically confirmed a new phase of SiO2 at pressures > 500,000 bars (Dubrovinsky et al., 1997). b) Determination of thermal properties of solids to 3600 K by in-situ x-ray .

1998: a) Physical properties of iron measured to 3.0 million bars and to 1500 K. No laboratory anywhere in the world has succeeded yet in reaching such extreme conditions involving both pressure and temperarures (Dubrovinsky et al., 1999). b) Co6W6C found to be less compressible than diamond.

1999: Dissociation of magnesiowustite at lower mantle conditions may aid in our study of the dynamics of the mantle.

Some important publications:

Saxena, S.K., Shen, G. and Lazor, P. (1993) Experimental evidence for a new iron phase and implications for Earth's core. Science, 260, 1312-1314.

Saxena, S.K., Shen, G. and Lazor, P. (1994) Temperatures in Earth's core based on melting and phase transformation experiments on iron. Science, 264, 405-407.

Saxena, S.K., Dubrovinsky, L.S., Häggkvist, P., Cerenius, Y., Shen, G. and Mao, H.K., (1995) Synchrotron X-ray study of iron at high pressure and temperature. Science, 269, 1703-1704.

Saxena, S.K., Dubrovinsky, L.S., Lazor, P., Cerenius, Y., Häggkvist, P., Hanfland, M. and Hu, J. (1996) Stability of perovskite (MgSiO3) in the Earth's mantle. Science, 274, 1357-1359.

Saxena, S.K. and Dubrovinsky, L.S. (1997) Detecting phases of iron. Science, 275, 94-96. Dubrovinsky, L.S., Saxena, S. K. and Lazor, P. (1998) Comment on "The structure of b-iron at high pressure and temperature, Science, 278, 831-834". Science, in press.

Dubrovinsky, L.S., Saxena, S. K., Lazor, P., Ahuja, R., Eriksson, O., Wills, J.M., and Johansson, B. (1997) Experimental and theoretical identification of a new high-pressure phase of silica. Nature, 388, 362-365.

The x-ray evidence for the existence of a beta-phase (DHCP-structure)

Fig. 2-dimensional CCD x-ray data on heated iron (Dubrovinsky et al., 1999). A. Reference sample of MgO and HCP iron at 61 GPa at 300 K.

 

 

B. The same sample at 1550 K and higher pressures; the HCP remains untransformed.

 

 

 

C. The same sample at 2100 K. Super lattice spots of the beta-phase appear as marked.

 

 

 

D. The new reflections disappear on cooling.