Twinning modes of Nd2Si2O7

Theory of deformation twinning by Bilby and Crocker [B.A. Bilby, A.G. Crocker, Proc. R. Soc. 288 (1965) 241] is applied to calculate the twinning elements for all possible low index twinning modes in tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning modes for tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. These predictions of the theory are compared with the available experimental information for analogue oxides and may act as guideline for future experimental work.     

Phase transformations in Er2Si2O7 ceramics

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Li2Ca2Si2O7生物陶瓷的矿化活性研究

如何有效治疗牙周炎并实现受损牙周骨组织再生, 一直是牙周疾病治疗中具有挑战性的问题, 而矿化是牙周正常发育和功能中关键因素之一。本研究旨在探讨硅酸钙锂(Li₂Ca₂Si₂O₇)生物陶瓷对人牙周膜成纤维细胞增殖、矿化的影响及用于牙周骨组织再生的可能性。采用溶胶-凝胶法制备合成了Li₂Ca₂Si₂O₇陶瓷粉体。通过体外模拟体液浸泡, 发现Li₂Ca₂Si₂O₇粉体具有良好的羟基磷灰石矿化能力。生物学结果表明: Li₂Ca₂Si₂O₇粉体的浸提液在3.125~25 mg/mL浓度范围内能显著促进HPLFs的增殖, 低浓度(6.25 mg/mL)时可显著诱导HPLFs细胞体外矿化(p<0.05)。Li₂Ca₂Si₂O₇粉体具有促进人牙周膜成纤维细胞增殖和矿化能力, 有望作为牙周骨组织再生修复的生物活性材料。     

Pressureless sintering of Si3N4 with Y2O2 and Al2O3

Pressureless sintering of Si₃N₄ with Y₂O₃ and Al₂O₃ as additives was carried out at 1750°C in N₂ atmosphere. Si₃N₄ materials which had more than 92% relative density were obtained with 20wt% addition of additives. The flexural strength of as-sintered materials containing 5 to 8.6wt% Al₂O₃ and 15 to 11.4wt% Y₂O₃ was in the range of 480 to 560 MPa at room temperature. The glassy grain-boundary phase of as-sintered materials crystallized to 3Y₂O₃ · 5Al₂O₃ (YAG), Y₂O₃ · SiO₂ (YS), Y₂O₃ · 2SiO₂ (Y2S) and 10Y₂O₃ · 9SiO₂ sd Si₃N₄ (NA) by heat-treatment at 1250° C for 3 days. A specimen containing 15wt% Y₂O₃ and 5wt% Al₂O₃ sintered at 1750° C for 4 h was heat-treated at 1250° C for 3 days to precipitate YAG and YS. The nitrogen concentration of the grain-boundary glassy phase of the specimen was found to be very high, and therefore the flexural strength of the crystallized specimen scarcely decreased at elevated temperatures (the flexural strength of this specimen is 390 MPa at room temperature and 360 MPa at 1300° C). Resistance to oxidation at 1200° C of the specimen was good as well as the flexural strength, compared with that of as-sintered materials.     

Dissociation of dislocations in kyanite: (Al2 O3  Si O2)

A study of the crystal structure of kyanite allows us to predict that there is one possible burgers vector $\text{c}\text{?}\text{= 5,57}\text{A}\text{?}$ and only one easy glide plane (100), because only in that plane the gliding dislocations do not cut neither the strong Si-O bonds nor the chains of Al O₆ octaedra.TEM observations confirm these hypotheses. Moreover the dislocations are widely dissociated in two colinear partials. The following reaction $\text{c}\text{?}\text{→}\text{1}\text{2}\text{c}\text{?}\text{+}\text{1}\text{2}\text{c}\text{?}$ preserves the anionic lattice as well as the chain structure.     

Magnetic properties of D-Er2Si2O7 at low temperatures

The low field magnetic susceptibility and the high field magnetic moment of D-Er₂Si₂O₇ at low temperatures are measured by an inductance method. The susceptibility results show that D-Er₂Si₂O₇ undergoes a transition from a paramagnetic to an antiferromagnetically ordered state at T _N=1.9 +-0.1 K. The magnetic moment data is analysed on the basis of a four-sublattice antiferromagnetic spin model. The g-values obtained from magnetic susceptibility and moment experiments are compared with those of optical measurements.     

Fabrication and microstructure characterization of continuously porous Si2N2O–Si3N4 ceramics

The microstructure of continuously porous Si₂N₂O–Si₃N₄ ceramics fabricated for use in environmental filters by a multi-pass extrusion process was investigated using SEM, XRD and TEM techniques. The matrix regions of the nitrided Si₂N₂O–Si₃N₄ porous bodies were composed of many Si₃N₄ particles and short Si₂N₂O fibers. However, in the continuously porous regions, many network type Si₂N₂O fibers, 200–420 nm in diameter, with a high aspect ratio were observed. After post-sintering of the nitrided bodies at 1800 °C, the diameter of the fibers increased to about 2–3 μm, while their length was shortened. The values of bending strength, relative density and hardness of the post-sintered Si₂N₂O–Si₃N₄ bodies were about 117.6 MPa, 73.6% and 616 Hv, respectively.     

The crystallisation of glasses from the ternary CaF2-CaAl2Si2O8-P2O5 system

A study of glasses from the ternary system CaF₂-CaAl₂Si₂O₈-P₂O₅ has been carried out. It has been shown that glasses with low phosphorus contents and high fluorite contents crystallise to fluorite. Fluorine reduces the glass transition temperature and is also required for the formation of fluorapatite (FAP). In the absence of fluorine in the glass no apatite phase is formed. Bulk nucleation of FAP is favoured for glasses with Ca:P ratios close to the apatite stoichiometry of 1.67 and with low crosslink densities. Thermal gravimetric analysis showed significant weight losses attributable to the formation of volatile silicon tetrafluoride to occur on crystallisation of the aluminium containing phases, anorthite and mullite, which supports the view that silicon tetrafluoride formation is hindered by fluorine bonding to the aluminium atoms of the glass network. Anorthite crystallisation always occurred by a surface nucleation mechanism and appeared to be favoured by the higher silicon to aluminium ratio in these glasses compared to previously studied glass compositions.     

Sintering process of Y2O3 and Al2O3-doped Si3N4

The sintering process of Y₂O₃- and Al₂O₃-doped Si₃N₄ has been investigated by dilatometry and microstructural observations. The densification progressed through three processes. The bulk density increased to 85% theoretical without the formation of -Si₃N₄ in the initial process. The densification once terminated after the second process. The / transformation of Si₃N₄ and the related formation of prismatic grains reduced the densification rate in the second process, although the grain size and the aspect ratio were very small. The final process was the densification of -Si₃N₄, where the fibrous grains grew remarkably. The kinetic order for the densification of -Si₃N₄ indicated a diffusion-rate controlling mechanism with the activation energy of 244 kJ mol^–1 (<1450 ° C) and 193 kJ mol^–1 (>1450 ° C). The influence of heating rate on the grain growth was characterized by a parameter derived from kinetic parameters. The relationships between grain growth and densification behaviour have also been discussed.     

Self-organized growth of Si/Silica/Er2Si2O7 core-shell nanowire heterostructures and their luminescence

Self-organized Si-Er heterostructure nanowires showed promising 1.54 microm Er(3+) optical activity. Si nanowires of about 120-nm diameter were grown vertically on Si substrates by the vapor-liquid-solid mechanism in an Si-Er-Cl-H(2) system using an Au catalyst. Meanwhile, a single-crystalline Er(2)Si(2)O(7) shell sandwiched between nanometer-thin amorphous silica shells was self-organized on the surface of Si nanowires. The nanometer-thin heterostructure shells make it possible to observe a carrier-mediated 1.53 microm Er(3+) photoluminescence spectrum consisting of a series of very sharp peaks. The Er(3+) spectrum and intensity showed absolutely no change as the temperature was increased from 25 to 300 K. The luminescence lifetime at room temperature was found to be 70 micros. The self-organized Si nanowires show great potential as the material basis for developing an Si-based Er light source.     

Sintering behavior and dielectric properties of SrB2Si2O8 ceramics

In this paper, the sintering behavior of the SrB₂Si₂O₈ ceramics based on the SrO–B₂O₃–SiO₂ composition has been investigated. The influence of the BaO addition to the SrO–B₂O₃–SiO₂ ceramics on the sintering behavior has been also examined. It shows that, due to the low-meting characteristics of the SrO–B₂O₃ binary composition, the SrB₂Si₂O₈ ceramics with peculiar dielectric properties can easily prepared by the introduction of SiO₂ to the SrO–B₂O₃ binary compositions at a sintering temperature of 950 °C. Additionally, the introduction of BaO to SrO–B₂O₃–SiO₂ ceramics has an obvious effect on the sintering behavior of the SrB₂Si₂O₈ ceramics. Also, the dielectric properties of the SrB₂Si₂O₈ ceramics are correlative to the phase compositions and the microstructures.     

Study on the structures of N-containing melilite Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5

Rietveld refinements have been used to determine the structure of Y2Si3O3N4 from X-ray data and Nd2Si2.5Al0.5O3.5N3.5 from neutron powder diffraction data. The refinements show that in the melilite phase Y2Si3O3N4 and melilite solid solution Nd2Si2.5Al0.5O3.5N3.5 the distributions of cations and anions are almost identical. They are analogous to the akermanite (Ca2MgSi2O7) structure, with Si/Si,Al atoms at the origin and centre of the unit cell and with four N/N,O atoms forming the SiN4/(Si,Al)(N3.5O0.5) tetrahedra which share corners with SiO2N2/(Si,Al)O2.25N1.75 tetrahedra to form a continuous sheet structure. Each Y3+ or Nd3+ ion is surrounded by eight N/O atoms forming the coordination polyhedron in Y2Si3O3N4 and Nd2Si2.5Al0.5O3.5N3.5 respectively. The arrangement of Al, Si atoms in the tetrahedra in Nd2Si2.5Al0.5O3.5N3.5 structure is also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.