T. Tsurumi

Finding community structure in mega-scale social networks: [extended abstract]

Community analysis algorithm proposed by Clauset, Newman, and Moore (CNM algorithm) finds community structure in social networks. Unfortunately, CNM algorithm does not scale well and its use is practically limited to networks whose sizes are up to 500,000 nodes. We show that this inefficiency is caused from merging communities in unbalanced manner and that a simple heuristics that attempts to merge community structures in a balanced manner can dramatically improve community structure analysis. The proposed techniques are tested using data sets obtained from existing social networking service that hosts 5.5 million users. We have tested three three variations of the heuristics. The fastest method processes a SNS friendship network with 1 million users in 5 minutes (70 times faster than CNM) and another friendship network with 4 million users in 35 minutes, respectively. Another one processes a network with 500,000 nodes in 50 minutes (7 times faster than CNM), finds community structures that has improved modularity, and scales to a network with 5.5 million.

Ultrawide range dielectric spectroscopy of BaTiO3-based perovskite dielectrics

Ultrawide range dielectric spectra from the kilohertz to terahertz range of BaTiO3 (BT), Ba(Zr0.25Ti0.75)O3 (BZT), (Ba0.6Sr0.4)TiO3 (BST), and SrTiO3 ceramics were presented by analyzing dielectric permittivity and IR reflectivity data. It was found that the permittivity of the ST was determined only by the ionic polarization while that of the BT was determined by the ionic polarization as well as the dipole polarization due to the domain contribution. The high permittivity of the BZT ceramics was attributed to the dipole polarization of polar nanoregions in the relaxors. The dipole and ionic polarizations overlapped in the BST.

Preparation of nm-sized BaO3 particles using a new 2-step thermal decomposition of barium titanyl oxalate

To obtain inpurity-free and nm-sized barium titanate (BaTiO3) particles, a new 2-step thermal decomposition method from barium titanyl oxalates (BaTiO(C2O4)2 · 4H2O) was proposed. At the 1st step, BaTiO(C2O4)2 · 4H2O was annealed at 400°C for 1 h in the O2 flow. The annealing temperature of 400°C was chosen for the following reasons; (1) no formation of BaCO3 and TiO2 and (2) the complete removal of H2O and other carbon species. This compound obtained at 400°C was amorphous phase, and its chemical composition was BaCO3-TiO2. When this compound was annealed in air at higher temperatures, the large BaTiO3 particles were prepared with by-products such as BaCO3. Thus, at the 2nd step, to prevent the crystal growth and the formation of BaCO3, this compound was annealed above 600°C in vacuum. Finally, the BaTiO3 single crystals with a size with 16.5 nm were prepared around 620°C. These BaTiO3 fine particles were characterized using various methods to investigate defects and impurities in the particles. As a result, it was confirmed that there was no impurity in the BaTiO3 lattices.

Temperature Dependence of Dielectric Properties of BaTiO3/SrTiO3 Artificial Superlattices

BaTiO3/SrTiO3 artificial superlattices were fabricated by the molecular beam epitaxy process. The superlattice with the 10-periodic structure was mostly distorted in the film thickness direction, and showed the highest dielectric permittivity than other periodic superlattice. The temperature dependence of dielectric properties was measured in the range from room temperature to +150 degrees at 1 GHz. Compared with Ba x Sr1−x TiO3 solid solution ceramics or BaTiO3 ceramics, there was almost no temperature dependence of dielectric permittivity. As a result, it was cleared that induced lattice distortion played an important role to determine the dielectric properties of artificial superlattices.

Fabrication and piezoelectric properties of (K 0.5 Na 0.5 )NbO 3 -based ceramics doped with Bi-perovskites

The effect of the addition of Bi-perovskites to (Li_0.04K_0.52Na_0.44)(Nb_0.84Ta_0.1Sb_0.06)O₃ (LF4) ceramics was studied to improve the properties of lead-free piezoelectrics. Ceramics with the relative density above 96% were successfully obtained by a solid state reaction process. The changes in lattice parameters indicated that the vacancy concentration at the A-site of the perovskite structure and the tetragonality of the crystal lattice were reduced with the addition of BiFeO₃. The addition of BiFeO₃ changed the LF4 to a soft-piezoelectric. The electrical insulation of the LF4 was improved with the addition of BiFeO₃, which made a full poling possible under high electric fields. A few mol% of BiFeO₃ was effective to improve the remanent polarization of the LF4 ceramics and a piezoelectric d₃₃ constant of about 358 pm/V was obtained in the 0.4 mol% doped ceramics.

Potassium niobate single-domain crystals as piezoelectrics with low dielectric constants and high electromechanical coupling properties

Piezoelectric properties of potassium niobate (KNbO/sub 3/) crystals with the single-domain state were investigated. KNbO/sub 3/ crystals were cut, polished and sized into various shapes. The mechanical processing induced mechanical damage into the crystals. Remove of the stressed surface layers was very effective to achieve high mechanical quality factor (Q/sub m/). After this treatment, KNbO/sub 3/ crystals were poled by the 2-step poling method. As a result, the domain state was almost single-domain. Using poled KNbO/sub 3/ single-domain crystals, the piezoelectric properties with k/sub 32/ modes were measured using a conventional resonance method. As a result, the [001]/sub o/ poled KNbO/sub 3/ crystals exhibited the electromechanical coupling factor (k/sub 32/) of almost 40%, the piezoelectric constant (d/sub 32/) of almost 18.5 pC/N, the dielectric constant (/spl epsiv//sub 33/) of almost 48 and Q/sub m/ over 7,600. Thus, the KNbO/sub 3/ crystals are piezoelectrics with low dielectric constants and high electromechanical coupling properties, i.e., KNbO/sub 3/ crystal is a promising material for sensor application.

Correlation between electro-optical coefficient and dielectric permittivity of BST thin films

A new measurement system of electro-optical (EO) effect and dielectric permittivity (¿r) was developed to investigate the correlation between electro-optic and dielectric properties of epitaxial barium strontium titanate, Ba1-¿Sr¿TiO3 (BST) thin films grown on SrTiO3 (STO) substrates. The refractive index (birefringence) and the permittivity were measured as a function of DC-bias field applied using a planer electrode on the films. It was found that the tunability of birefringence was much lower than that of dielectric permittivity. The difference in the tunability was discussed from the view point polarization mechanisms.

Computer aided structural design of future MLCCs for high capacitance applications

We have tried to see the limit of capacitance density of MLCCs using a computer simulation technique. Dielectric properties of MLCCs with different particle size of BaTiO3 were measured and the data were analyzed using B-SPLINE fitting to predict dielectric permittivity at arbitral temperature and AC-fields. The dielectric properties of barium titanate grains smaller then 100 nm were predicted using least squares fitting of the B-SPLINE coefficients. It was found from the simulation that the usage of barium titanate grains smaller than 80 nm did not give an advantage to increase the capacitance density as well as temperature stability of the MLCCs. The maximum capacitance was predicted for the 1608 (mm) chip size.

Spatial Resolution and Measurement Accuracy of Dielectric Microscope Using Non-contact State Microwave Probe

The dielectric measurement for microscopic area of multi-layer ceramics capacitor was carried out by microwave microscope using non-contact microwave probe. The phase of incidence microwave for sample was fixed to pi/2 in order to realize accurate measurement of sample. The spatial resolution for dielectric measurement was increased based on Kirchhoffs diffraction theory with decreasing coaxial cable and probe diameter. From reflection intensity mapping, the dielectric permittivity distribution in microscopic area at GHz order was measured for cross section of multi-layer ceramics capacitor at room temperature. The spatial resolution was experimentally estimated to be about 10 mum from mapping of cross section view of dielectric and inner electrode layers in multi layer ceramics capacitor.

Enhanced piezoelectric properties of KNbO/sub 3/ crystals by domain engineering

Potassium niobate (KNbO/sub 3/) single crystals were grown by a TSSG method. At first, the electric field was applied along [001]/sub c/ (cubic notification system) direction of KNbO/sub 3/ crystals to induce the engineered domain configurations into KNbO/sub 3/ crystals. Prior to domain engineering, the piezoelectric properties of [001]/sub c/ oriented KNbO/sub 3/ single domain crystals were measured. These measurement values were completely consisted with the calculated apparent d/sub 31/ and d/sub 32/. Finally, the engineered domain configurations were induced into KNbO/sub 3/ crystals. As a result, piezoelectric properties increased with decreasing domain sizes of the engineered domain configuration. However, the symmetry of the KNbO/sub 3/ crystals was mm2, and there were four kinds of domain structures such as 90/spl deg/, 180/spl deg/, 60/spl deg/ and 120/spl deg/ domains. Thus, the engineered domain structure induced in this study was very complicated, and the piezoelectric properties were also depended on domain pattern and kinds of domain walls.