Biomedical subjects

Tetsuya Morishita

Publications and source records attributed to Tetsuya Morishita.

6 recordsLinked to original sources

PubMed2006

How does tetrahedral structure grow in liquid silicon upon supercooling?

We present an extensive set of isothermal-isobaric first-principles molecular-dynamics simulations of liquid silicon over a temperature range of 950-1700 K. We find that the tetrahedral order gradually grows upon cooling to approximately 1200 K, but that the growth accelerates significantly below approximately 1200 K. This growth process gives rise to anomalous changes in density and liquid structure upon supercooling. In particular, we find that the atomic coordination number remains constant to approximately 1200 K and then begins to decrease below approximately 1200 K, which resolves the existing controversy regarding liquid structure in the supercooled regime [T. H. Kim, Phys. Rev. Lett. 95, 085501 (2005)10.1103/PhysRevLett.95.085501].

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PubMed2006

Molecular dynamics simulations of self-organized polyicosahedral Si nanowire.

A novel polyicosahedral nanowire is spontaneously formed in a series of annealing molecular dynamics simulations of liquid Si inside a nanopore of 1.36 nm in diameter. The polyicosahedral Si nanowire is stable even in a vacuum up to about 77% of the melting temperature of bulk Si. Our structural energy calculations reveal that the polyicosahedral nanowire is energetically advantageous over the pentagonal one for a wire whose diameter is less than 6.02 nm, though the latter has been recently proposed as the lowest energy wire. These results suggest the possibility of the formation of a new stable polyicosahedral Si nanowire.

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PubMed2006

[A case of spontaneous epidural emphysema occurring during sleep].

A 19-year-old man consulted a doctor for swelling of his neck and shortness of breath. The day before, he woke up with a slight cough and upper chest pain early in the morning. He went to school and spent the day as usual. He did not have a history of asthma or violent cough. The next day, chest radiography showed subcutaneous emphysema and pneumomediastinum. Computed tomography not only confirmed the presence of mediastinal and subcutaneous air, but also demonstrated a linear radiolucent stripe in the spinal canal corresponding to epidural emphysema. The patient did not have any neurologic findings. His general condition remained good except that his arterial blood oxygen saturation slightly decreased to 95%. Laboratory data were normal, except for serum IgE, which was elevated (2072 IU/ml). He stayed at rest and was treated conservatively and his symptoms improved within a few days. Seven days later, the intraspinal air and pneumomediastinum had resolved spontaneously on follow-up chest computed tomography (CT).

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PubMed2005

Anomalous diffusivity in supercooled liquid silicon under pressure.

We perform isothermal-isobaric first-principles molecular-dynamics simulations to investigate the dynamics of liquid silicon (l-Si) under pressure. We find that the self-diffusion coefficient increases with increasing pressure in the deeply supercooled state. This anomalous diffusivity is attributed to the formation of locally tetrahedral configurations which on average reduces the diffusivity at low pressures. Densification hinders the formation of the tetrahedral configurations, thus the diffusivity increases with increasing pressure. The tetrahedral configurations frequently formed at low pressures may be viewed as fragments of the low-density form of l-Si . It is therefore conceivable that transformations between two distinct liquids, low- and high-density liquids, locally occur in deeply supercooled l-Si . The present findings indicate the profound generality of the dynamics in liquids with a tetrahedral network such as water.

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PubMed2005

Spatial confinement effect on the atomic structure of solid argon.

Molecules confined in nanopores show unusual behavior not seen in bulk systems. The present paper reports on molecular dynamics simulations of unusual freezing behavior in confined Ar. Similar to bulk Ar, liquid Ar confined in pores with a diameter D>15sigma (5.1 nm), where sigma is the diameter of the Ar atom, crystallizes when the cooling rate is lower than a critical value (Qc). We also find that the spatial confinement does not have significant influence on Qc when D>15sigma (5.1 nm). In the pore of 10sigma (3.4 nm) in diameter, on the other hand, the behavior is dramatically changed. Crystalline Ar does not appear inside the pore even when the system is cooled at a rate lower than the Qc in the bulk system by over two orders of magnitude. Instead, amorphous Ar characterized by local icosahedral configurations is formed in the pore. We further find that, even when crystalline Ar is formed outside the pore, it does not grow deeply into the pore. This supports that the amorphous Ar is actually the most stable phase in the pore. It is well known that Ar is a poor glass former. Our finding that even such an amorphous Ar is the most stable in the pore suggests that, in any system, it is possible to prepare amorphous structure selectively by using nano-molds.

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PubMed2004

High density amorphous form and polyamorphic transformations of silicon.

Polyamorphic transformations of silicon have been investigated by constant-pressure first-principles molecular-dynamics simulations. By pressurizing a normal amorphous Si with tetrahedral coordination, a new high density amorphous (HDA) form that has a strong resemblance to HDA water is obtained. We find that the HDA form can be also obtained through vitrification of liquid Si under pressure. Both HDA and liquid Si contain deformed tetrahedral configurations with interstitial atoms. These findings indicate that HDA Si is directly connected with liquid Si, which is of particular importance in understanding phase relations of polyamorphs of Si.

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