# Presentation: 151455

## NSF Nugget for FY06Computational thermodynamics of  the Mg-B-C ternary system

- Sept. 2006
- FRG on Two Gap Superconductivity
- Wisconsin-Penn State- Arizona State-Puerto Rico-Mayaguez collaboration
- DMR-0514592
- PI David Larbalestier, Eric Hellstrom and Paul Voyles (UW), Zi-Kui Liu (Penn State), Nate Newman and John Rowell (Arizona State), and Oswald Uwakweh (U. Puerto Rico Mayaguez)

## Computational thermodynamics of  Mg-B-C ternary systemWork done in the group of Z.K. Liu, Penn State DMR-0514592

- 	Our project is trying to understand the phase stability of the Mg-B-C system so as to guide Hybrid Physical Chemical Vapor Deposition (HPCVD) processing of C-doped MgB2 thin films. Using the CALculation of PHAse Diagrams (CALPHAD) approach, we can calculate the phase diagram of the Mg-B-C ternary system. To develop a thermodynamic database for use in CALPHAD, we need thermochemical and phase equilibria data. First-principles calculations can be use to obtain thermochemical properties such as enthalpy and entropy of formation and heat capacity. In such calculations, the electronic interaction of atoms is simulated at the quantum level without need for experimental input.  We use the Vienna Ab-initio Simulation Package (VASP) to perform these calculations.

- The specific heat of the MgB2C2 ternary compound in the Mg-B-C system calculated from first-principles

- Preliminary B-C binary phase diagram generated by “ThermoCalcTM” from our thermodynamic database.

**Notes:**
Computational thermodynamics is interdisciplinary as thermodynamics is involved in many applied sciences. It makes use of computer software and assessed thermodynamic databases to perform realistic calculations of thermodynamic properties of binary, ternary and multicomponent systems under various experimental  conditions. 

The 39K MgB2 superconductor shows tremendous potential in high magnetic-field application.  Many researchers try to improve this property by adding some element such as Al, Ca, Na, C, O and N.  Carbon doping cause dramatically increase in upper critical field. 

To understanding and guiding HPCVD, we need a ternary phase diagram of Mg-B-C system.  To thermodynamically model a materials system, the Gibbs energy of each phase must be explicitly defined.  The CALPHAD, the calculation of phase diagram, approach is to implement thermochemical and phase equilibria data from either experiments or first-principle calculations.  The parameters of the Gibbs energy equations are evaluated from this input to produce accurate data.  Once deemed “accurate”, lower order models, in our case the binary systems Mg-B Mg-C and B-C, can combined to produce models for higher order systems, such as Mg-B-C.

## Computational thermodynamics of  Mg-B-C ternary system

**Computational thermodynamics of  Mg-B-C ternary system**

- A preliminary isothermal section of the Mg-B-C ternary phase diagram at 1000K from combining the Mg-B, Mg-C and B-C binary phase diagrams

**Phases Research Lab:**
- We have thirteen graduate students and three research fellows working on various projects such as magnesium alloys, aluminum alloys, boride, Ni-based superalloys and diffusion in intermetallics.
- The FRG is supporting two graduate students.

**Future work:**
- When the thermodynamic modeling is complete, we will be able to predict carbon solubility in MgB2, a crucial item fro understanding its high upper critical field (Hc2).
- Other dopants will be studied to predict potential contributions to Hc2(T) to guide new experiment.