Bulk conbinatory | Prof. Ikeda Group

Combinatorial Method For Bulk Materials

Researches on various class of materials have matured and target complex materials composed of multiple constituent elements. To explore the relation among the phase diagram, composition, nanostructure, and properties of bulk materials, we have developed a unique "bulk combinatorial" approach. A unidirectional solidification with an extreme condition (a steep temperature gradient plus a low velocity) enables us to prepare a rod sample with compositional gradation. This approach greatly accelerates explorations of bulk materials in large compositional spaces among multiple components: the relation among the phase diagram, microstructure, and material properties without preparing many samples.

This method gives the following information.

　　　Explorations of phase diagrams and nano/microstructures
　　　　Quantitative information
　　　　　　　Composition of a invaliant rection (maximum solubility, eutectic/eutectoid/peritectic composition)
                   Qualitative information
　　　　　　　Class of a solidification reaction
　　　　　　　Class of a solid state reaction (precipitation, eutectic reaction, etc.)
                   Other benefits
　　　　　　   Discovery of an unknown phase
　　　　　　　Extraction of a single phase region　　　　　
　　　Combinatory type researches "relations between phase diagram-composition-microstructure-material property"
                   Composition dependence of microstructure
                   Composition dependence of material property

Unidirectional solidification by the Bridgman method

The Bridgman method is one of unidirectional solidification techniques. A sample is melted in a furnace and is transferred gradually into the cooled region by moving the sample itself or the furnace. The sample is solidified gradually from one end. This technique is often used to fabricate single crystals.

In our research, the bridgman technique is used to prepare compositionally graded samples. Unidirectional solidification by the Bridgman technique under a large temperature gradient and a low moving velocity enables us to get a compositionally graded sample.

Composition graded materials

The left figure shows the phase diagram of a typical eutectic system. Supposing that the initial composition of the sample is C₀, when the bottom end of the sample reaches the liquidus temperature, the α phase with the kC₀ composition is crystallized. The solute concentration in the α phase is smaller than the initial concentration, the solute atoms are discharged into the liquid phase. In the case of the system shown in the left diagram, the solute concentration in the solid phase in equilibrium with the liquid phase is always lower than that in the liquid phase, the solute atoms continue to be discharged in the liquid phase. Therefore, the solute concentration increases as solidification proceeds. When the liquid composition reaches the eutectic composition, C_E, the eutectic reaction begins and the compositions of reaction does not vary any more. The eutectic composition C_E can be determined as the average composition of the eutectic structure. The solubilities of B in the β phase and A in the α phase are measured from the β and α phases, respectively, in the eutectic structure. The maximum solubility of B in the α phase is determined from the α phase next to the eutectic structure, which is crystallized right before the eutectic structure is formed, as well. Measurements of material properties (e.g. electrical conductivity) as functions of positions in the sample give the composition dependence of the properties.

Property measurements

The composition dependence of microstructure can be unveiled by observations in the direction of the composition gradation. Nondestructive measurements of a material property gives the impact of composition, that is, one gets the information on the relation among chemical composition, microstructure, and property.

The top figure shows an example of microstructure mapping of the PbTe-Sb₂Te₃-Ag₂Te system. The modulated structure has been discovered between the PbTe and AgSbTe₂ phases.

The bottom fiture shows the mapping of the Seebeck coefficient of the PbTe-Sb₂Te₃-Ag₂Te. Thus, continuous composition dependence of material properties can be extracted using fewer numbers of samples than usual ways.

References

1. T. Ikeda, H. Ohta, A 'high-throughput' approach to bulk thermoelectric materials, Kinzoku 83 (2013), 870-876.
2. T. Ikeda, S. Iwanaga, H-J. Wu, N.J. Marolf, S-W. Chen, G.J. Snyder, A combinatorial approach to microstructure and thermopower of bulk thermoelectric materials: the pseudo-ternary PbTe-Sb₂Te₃-Ag₂Te system, J. Mater. Chem., 22 (2012), 24335-24347.