Downshift of the Ni d band center over Ni nanoparticles in situ confined within an amorphous silicon nitride matrix.

Herein, nanocomposites made of Ni nanoparticles in situ distributed in an amorphous silicon nitride (Ni/a-Si3N4) matrix, on the one hand, and within an amorphous silicon dioxide (Ni/a-SiO2) matrix, on the other hand, were synthesized from the same Ni-modified polysilazane precursor. In both compounds, the Ni/Si atomic ratio (0.06-0.07), average Ni nanocrystallite size (7.0-7.6 nm) and micro/mesoporosity of the matrix were rigorously fixed. Hydrogen (H2)-temperature-programmed desorption (TPD) profile analysis revealed that the activation energy for H2 desorption at about 100-130 °C evaluated for the Ni/a-Si3N4 sample (47.4 kJ mol-1) was lower than that for the Ni/a-SiO2 sample (68.0 kJ mol-1). Mechanistic study with X-ray photoelectron spectroscopy (XPS) analysis and density functional theory (DFT) calculations revealed that, at Ni nanoparticle/matrix heterointerfaces, Ni becomes more covalently bonded to N atoms in the a-Si3N4 matrix compared to O atoms in the a-SiO2 matrix. Therefore, based on experimental and theoretical studies, we elucidated that nickel-nitrogen (Ni-N) interactions at the heterointerface lead to remarkable Ni d band broadening and downshifting of the d band center relative to those generated by Ni-oxygen (Ni-O) interactions at the heterointerface. This facilitates H2 desorption, as experimentally observed in the Ni/a-Si3N4 sample.

Structure and dissolution of silicophosphate glass.

P2O5-SiO2-Na2O-CaO glasses are promising therapeutic ion-releasing materials. Herein, we investigated the state of silicon (Si) in P2O5-SiO2-Na2O-CaO glass using a model with a composition of 55.0P2O5-21.3SiO2-23.7Na2O (mol%), incorporating a six-fold-coordinated silicon structure ([6]Si). The model was constructed using a classical molecular dynamics method and relaxed using the first-principles method. Further, we experimentally prepared glasses, substituting Na2O for CaO, to investigate the dissolution of glass with varying [6]Si and PO4 tetrahedra (Q P n ) distributions (n = number of bridging oxygens (BOs) to neighboring tetrahedra). [6]Si in the glass model preferentially coordinated with Q P 3. When Si was surrounded by phosphate groups, phosphorus (P) induced the formation of [6]Si by elongating the Si-O distance, and [6]Si acted like a glass network former (NWF). Na+ coordinated with [6]Si-O-P bonds via electrostatic interactions with BO. 31P and 29Si magic-angle-spinning-nuclear-magnetic-resonance spectra of three experimental glass samples with the compositions of 55.0P2O5-21.3SiO2-xCaO-(23.7 - x)Na2O (mol%, x = 0, 12.4, and 23.7) showed that Q P 3 and [6]Si increased with increasing Na2O. When each glass powder was immersed in a tris-HCl buffer solution at 37 °C, the dissolution of NWF ions and network modifier (NWM) ions increased almost monotonically with time for all samples, indicating that the solubility of the samples was suppressed by the coexistence of CaO and Na2O, attributed to the delocalization of the electron distribution of P in the [6]Si-coordinated Q P 3 units compared to that in the P- or [4]Si-coordinated Q P 3 units, which reduces hydrolysis.

Machine learning based prediction of lattice thermal conductivity for half-Heusler compounds using atomic information.

Half-Heusler compound has drawn attention in a variety of fields as a candidate material for thermoelectric energy conversion and spintronics technology. When the half-Heusler compound is incorporated into the device, the control of high lattice thermal conductivity owing to high crystal symmetry is a challenge for the thermal manager of the device. The calculation for the prediction of lattice thermal conductivity is an important physical parameter for controlling the thermal management of the device. We examined whether lattice thermal conductivity prediction by machine learning was possible on the basis of only the atomic information of constituent elements for thermal conductivity calculated by the density functional theory in various half-Heusler compounds. Consequently, we constructed a machine learning model, which can predict the lattice thermal conductivity with high accuracy from the information of only atomic radius and atomic mass of each site in the half-Heusler type crystal structure. Applying our results, the lattice thermal conductivity for an unknown half-Heusler compound can be immediately predicted. In the future, low-cost and short-time development of new functional materials can be realized, leading to breakthroughs in the search of novel functional materials.

Diffusion of protons and sodium ions in silicophosphate glasses: insight based on first-principles molecular dynamic simulations.

We propose a microscopic diffusion mechanism of protons and Na+ ions in phosphate glasses using first-principles molecular dynamic simulations. Protons hop and are chemisorbed onto non-bridging oxygen (NBO) of nearby PO4 tetrahedra through hydrogen bonds. The subsequent behavior depends on the morphology of the PO4 tetrahedra (QnP values). When a proton is adsorbed onto the NBO of a Q3P unit, it is desorbed on a short time scale of within 10 fs and re-adsorbed onto the NBO that was previously adsorbed. However, when a proton is adsorbed onto the NBO of a Q2P unit, another proton coordinated before adsorption is desorbed in a chain, resulting in the diffusion of protons. When a Na+ ion is present in the vicinity, the adsorption of a proton onto a Q2P unit leads to a decrease in the electrostatic interaction between Na+ and O- ions and induces the diffusion of Na+ ions. We conclude that the difference in the morphology of PO4 tetrahedra greatly affects the diffusion of protons and Na+ ions.

Experimental and Theoretical Investigation of the Structural Role of Titanium Oxide in CaO-P2O5-TiO2 Invert Glass.

Understanding the structural role of TiO2 in calcium phosphate invert glasses is key for developing a new glass design for biomedical applications. Experimental and computational analysis methods were used to investigate the impact of TiO2 substitution in these glasses. Spectroscopic analyses indicated that titanium oxide exists as both TiO4 and TiO6 units, leading to the formation of Ti-O-P bonds, in spite of depolymerization of the phosphate chains. Classical molecular dynamics showed that the presence of TiO2 influences the phosphate units and CaO polyhedral structures. The formation of the Ti-O-P bonds caused an increase in the network connectivity of the invert glasses, leading to the improvement of the glass forming ability and wettability. The addition of TiO2 to calcium phosphate invert glasses led to the introduction of bioactivity.

Improving the biocompatibility of tobermorite by incorporating calcium phosphate clusters.

BACKGROUND: In our earlier work, tobermorite containing calcium phosphate (CP) clusters (CP-Tob) was hydrothermally prepared in the CaO-SiO2-P2O5-H2O system for biomedical applications. OBJECTIVE: CP-Tob was used to investigate the influence of CP cluster incorporation on its biocompatibility. METHODS: Tobermorite samples with and without CP clusters were hydrothermally prepared at 180°C for 40 h. The biocompatibility, structure, and density of states of the tobermorite samples were investigated by experimental and first principles methods. RESULTS: The amounts of lysozyme and bovine serum albumin adsorbed on CP-Tob were higher than those on tobermorite without CP clusters. Cluster incorporation caused a decrease in the solubility, resulting in the enhancement of the cell compatibility. The calculated results indicated that incorporating clusters, which interact with the silicate units of tobermorite, led to a change of the density of states of tobermorite. CONCLUSIONS: Incorporation of CP clusters in tobermorite led to improvement of the biocompatibility evaluated by biological and computational analyses.

Fast time-reversible algorithms for molecular dynamics of rigid-body systems.

In this paper, we present time-reversible simulation algorithms for rigid bodies in the quaternion representation. By advancing a time-reversible algorithm [Y. Kajima, M. Hiyama, S. Ogata, and T. Tamura, J. Phys. Soc. Jpn. 80, 114002 (2011)] that requires iterations in calculating the angular velocity at each time step, we propose two kinds of iteration-free fast time-reversible algorithms. They are easily implemented in codes. The codes are compared with that of existing algorithms through demonstrative simulation of a nanometer-sized water droplet to find their stability of the total energy and computation speeds.

Minute depressed-type submucosal invasive cancer-5 mm in diameter with intermediate lymph-node metastasis: report of a case.

We report a rare case of colon cancer in which a depressed-type tumor only 5 mm in diameter invaded the submucosal layer and produced intermediate lymph node metastasis. A 47-year-old male received a total colonoscopy for a depressed-type lesion with marginal elevation in the sigmoid colon. The lesion measured 5 mm in diameter. On chromoendoscopic examination, the depression was clearly demarcated and an irregular pit pattern was identified in the demarcated area by magnification suggesting invasion of the submucosal layer requiring surgery. Laparoscopic-assisted sigmoidectomy was performed and the resected specimen demonstrated well-differentiated adenocarcinoma. The depth of invasion was only 900 microm. There was no lymphovascular invasion although not only paracolic, but also intermediate lymph node metastasis was detected. There have been some reports about small depressed-type colorectal cancer invading the submucosal layer; however, intermediate LN metastasis is very rare in submucosal colorectal cancer. In this case, there were two noteworthy points: 1) despite the small size, submucosal invasion could be estimated preoperatively, therefore, a successful lymph node dissection was performed by laparoscopic surgery; and 2) although this depressed-type cancer invaded the submucosal layer only 900 microm and there was no lymphovascular invasion, intermediate lymph node metastasis was detected.