Matsunaga Laboratory

In the present aging society, bioceramic materials are attracting much public attention, and further improvement of the materials properties is desirable. In particular, hydroxyapatite is a typical example of bioceramics, which is a main component of natural bones and tooth enamels and thus is expected to be used for artificial bones and teeth as well. However, physical and chemical phenomena of inorganic hydroxyapatite components in vivo at an electronic and atomic scale are poorly understood. We are doing research of apatite and its related bioceramics with collaboration of theory and experiment, in order to establish the materials science of biomaterials on an electronic and atomic scale.

As an example, this figure shows a crystal structure of hydroxyapatite and an atomic structure of Ca2+ vacancy theoretically obtained. Our calculations demonstrated that the Ca2+ vacancy formation occurs significantly more easily in more acid conditions of the surrounding aqueous solution. Especially, protons play their role to stabilize the Ca2+ vacancy in a lower pH condition as charge compensating defects. In fact, it is known that such a variation of solution pH conditions strongly affects formation and resorption of apatite crystals in bones during bone metabolism. Therefore, this is closely related to the worldwide health problem of osteoporosis and dental caries.

Another interesting property of hydroxyapatite is ion exchange ability. In fact, hydroxyapatite components of natural bones contain a variety of trace elements, which may be involved from the surrounding body fluid via ion exchange. The trace elements are expected to play an important role to determine physical and chemical properties of hydroxyapatite, and thus it is important to understand thermodynamic stability of defects as well as the electronic and atomic structures. We developed a methodology to evaluate thermodynamic stability of defects in materials under aqueous solution environments based on first-principles total energies, and applied it to ion exchange behavior of hydroxyapatite. We found that inclusion of Pb2+ ions in hydroxyapatite is energetically highly favorable due to the characteristic electronic structure formed around Pb2+.

In recent materials development, it is essential to control nano-level structures and properties of materials interfaces.

Most high performance materials in practical applications are produced by combination of different substances. A main advantage of combination of different substances is to combine different properties complementally, which is expected to enhance total performance of materials systems. As the combination accompanies formation of interfaces of different substances, it is necessary to control the interface structures and properties. However, it is generally difficult to predict electronic and atomic structures of the interfaces of dissimilar substances. Geometrical parameters such as lattice misfits are also commonly thought to be important, especially in the fields of thin film formation and catalyst. In our study, first-principles and the related theoretical calculations are performed for various interfaces and grain boundaries to investigate the characteristic atomic structures and bonding.

The above figure displays gold nanoparticles supported on TiO2 surfaces computationally predicted. In fact, this type of gold nanoparticle structure was experimentally confirmed by using advanced electron microscopy.

転位とは結晶中の原子配列が不連続になった線状の格子欠陥です。転位芯（コア）においては結合欠損が規則的に存在しており、コアの周囲には弾性的なひずみ場が生じています（図１）。

図1

結晶の塑性変形はこうした転位の連続的な運動によって起こります。すなわち、結晶性材料の機械的特性を理解するに当たって、転位の構造と物性を明らかにすることはきわめて重要な課題です。また、転位芯近傍における線状の結合欠損列ならびに局所ひずみ場は、規則配列に起因する結晶の基本物性が変化する特異点（線）です。つまり、転位はそれ自身が特異物性を有する究極かつ最小の量子細線であるとも言えます。さらに、こうした結合欠損やひずみ場により異種元素が引きつけられたり（コットレル効果）、拡散速度が速くなること（パイプ拡散）が知られています。このような転位特有の現象を利用すれば、既存材料に新しい物性を付与した材料を生み出すことができます（図２）。

図2

既存の元素種数は限られており、次世代の物質戦略においては、元素配列制御による物質の新機能開拓が不可欠となってきています。そこで、転位を利用した元素配列制御と転位の高機能量子細線化を目指して、転位の量子構造や運動、他元素との相互作用に関する研究を行っています。

• Nakamura et al, Acta Mater.Vol.50
  pp.101-108 (2002).
• Nakamura et al, Nature Mater.Vol.2
  pp.453-456 (2003).
• Nakamura et al, Acta Mater.Vol.53
  pp.455-462 (2005).
• Shibata et al, Science Vol.316 pp.82-85
  (2007).
• Nakagawa et al, Acta Mater.Vol.59
  pp.1105-1111 (2011).
• Tochigi et al, Acta Mater.Vol.60,
  pp.1293-1299 (2012)
• Nakamura et al, J. Mater.Sci., Vol.47 pp.
  5086-5096 (2012).

Electric energy storages such as fuel cells and lithium-ion rechargeable batteries are of significance in the type of forthcoming clean-energy society. Ionic conductors, in which only some kind of ions can migrate, are one of the key functions as electrolytes to determine cell performance.

Figure 1 shows a conceptual diagram of fuel cells using proton conductors as an electrolyte. In this system, the reaction from hydrogen and oxygen gases to water occurs at two steps in the anode and cathode, so that we can obtain the energy change of the reaction as electric energies. Specifically, H2 gases are first decomposed into protons and electrons in the anode. The formed protons and electrons move into the cathode separately, through the electrolyte and external circuit, respectively. The two react with oxygen gas in the cathode to form water. We can obtain electric energies in the case that formed electrons in electrodes go through external circuits, which means ionic conductors without electronic conductivities are indispensable in any battery, not only in fuel cells.

Figure 1. Conceptual diagram of fuel cells using proton conductors

Figure 2. A proton hopping trajectory from an oxide ion to another in tetragonal- phase LaNbO4.

Solid electrolytes have remarkable advantages over liquid electrolytes in terms of no liquid leakage, incombustibility, and so on. In our group, ionic conduction mechanisms have been theoretically investigated using first-principles calculations. Figure 2 shows a proton hopping trajectory from an oxide ion to another in tetragonal-phase LaNbO4, which was obtained from a collaborative study with a Norwegian research group (Norby’s group)*. Such microscopic pictures offer important clues to improve proton conductivities and explore a new class of proton conductors.