Viscosity Determination of Boron Oxide and Binary Borates

The anomalous behavior of rubidium and cesium borate glass and the boron coordination in alkali borate liquids are discussed. The viscosity-composition isotherms of rubidium and cesium borates have been found to follow the same trend as those of other alkali borates. For boron oxide at temperatures between 500° and 775°C, the activation energy for viscous flow, E_vis= (4.213 × 10⁴) (1/T) - 22.15 and for temperatures above 775°C, E_vis= (1.121 × 10⁴) (1/T) + 7.40, in which T is in °K. The activation energies for viscous flow, E_vis, in kcal per mole, increase with all molten alkali borates in the order Li > Na > K > Rb > Cs to about 20 mole % of akali oxide. This phenomenon is explained on the basis of Douglas's hole theory.     

多级热处理工艺对铝合金AA7049性能的影响(英文)

T6态高强7xxx系列铝合金对应力腐蚀开裂敏感。采用回归和再时效热处理(RRA)可以提高其抗应力腐蚀开裂性能而不降低其强度性能。研究了多级热处理工艺对7049铝合金性能和组织的影响。通过电导率测量、DSC分析和TEM组织观察,考察合金在不同热处理态的组织变化。DSC分析表明,RRA处理会导致合金的显微组织发生显著变化,RRA处理态合金的组织与T6和T73态合金的组织明显不同。RRA处理可以使合金保持在T6态的强度且获得T73态的热力学稳定性能。     

A "Compressed-Spring" Analogy for Residual Stress Effects on the Extension of Indenter Flaws

The residual driving force of plastic zones associated with indenter flaws was modeled as that of a compressed spring. The limiting case of a zero-stiffness spring converges to the constant-force model and predicts maximum stable crack extension before instability and corresponding maximum strength degradation. An infinitely stiff spring, i.e. a rigid wedge, causes no stable crack growth or strength degradation. Comparison with stable-crack-growth data suggests that an appropriate-stiffness spring model gives a better fit than the constant-force model.     

Phase transitions in coated nickel titanium arch wires: A differential scanning calorimetric and X-ray diffraction analysis

Shape memory and super-elastic properties of orthodontic nickel titanium wires, which are crucial for its clinical performance are dependent on the austenitic–martensitic phase transitions in its metallic microstructure that happen as a result of temperature or stress. The objective of this study was to compare the austenitic–martensitic phase transitions in new, black oxide coated nickel titanium (0·016 inch, Black Diamond, NiTi) arch wires in the ‘as-received’ form, from the manufacturer and ‘retrieved form’ after two months of intraoral use. This was done to analyse whether the new oxide coated nickel titanium wires suffered any significant loss in shape memory and super elasticity properties at the end of two months of intra oral use, findings of which could give valuable inferences prior to its widespread application in clinical practice. Five arch wire samples in both groups were investigated for their austenitic–martensitic phase transitions in an in vitro set up, using differential scanning calorimetry (DSC), (?90° to 100°C at a rate of 10°C/min) and X-ray diffraction (XRD) analysis (10° to 90°), as a function of temperature. Martensitic–austenitic thermograms showed an intermediate rhombohedral phase in the heating cycle of both groups, but cooling cycles showed direct reversal from austenitic to martensitic phase. Lower austenitic start ( $A_{\rm \textbf{s}}$ = 10·78 ± 0·46°C) and finish ( $ {A}_{\rm \textbf{f}}$ = 22·26 ± 0·24°C) temperatures of coated wires compared to the conventional wires showed (i) ability of the wire to remain in austenitic phase below oral temperature, that permits it to take up greater force during activation, (ii) increased springiness and (iii) consistent force delivery for an extended period of time. Statistical analysis with paired Student’s ‘t’ test did not show any significant difference in mean values of transition temperatures and enthalpies between the two groups which proved similar shape memory and super-elastic properties at the end of intra oral use. Black oxide coating of NiTi wires may, therefore, be effective in diversified oral conditions and may find acceptable for re-use after sterilization. Low enthalpy values (0·92–3·59 j/g) compared to conventional ones, implied complete phase transition at the atomic level that can improve performance of the material in activation and deactivation cycles of NiTi wires. X-ray diffraction analysis of the two groups demonstrated predominance of austenitic phases (A, 110, 220 and 211) with complete reversibility at the atomic level. Discrete crystallographic structure and absence of multiple phases showed complete martensitic–austenitic transition, which authenticated the differential scanning calorimetric findings. This can earn acceptance for the new product in contemporary orthodontic practice with adequate scope for indigenization.     

Fracture Properties of Polycrystalline Lithia-Stabilized β"- Alumina

The critical stress-intensity factor, K_1C , and the fracture strength, σ _f , have been investigated on both hot-pressed and sintered lithia-stabilized β "-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K_1C and σ _f were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (<120 μm), the strength was slightly dependent on grain size.
For larger grain sizes, the strength decreased rapidly with increasing grain size and the fracture mode was almost entirely transgranular. The K_1C values for sintered β "-alumina were in the same range as those obtained on the hot-pressed material.     

ON IMAGE CLASSIFICATION: CITY IMAGES VS. LANDSCAPES

Grouping images into semantically meaningful categories using low-level visual features is a challenging and important problem in content-based image retrieval. Based on these groupings, effective indices can be built for an image database. In this paper, we show how a specific high-level classification problem (city images vs landscapes) can be solved from relatively simple low-level features geared for the particular classes. We have developed a procedure to qualitatively measure the saliency of a feature towards a classification problem based on the plot of the intra-class and inter-class distance distributions. We use this approach to determine the discriminative power of the following features: color histogram, color coherence vector, DCT coefficient, edge direction histogram, and edge direction coherence vector. We determine that the edge direction-based features have the most discriminative power for the classification problem of interest here. A weighted k-NN classifier is used for the classification which results in an accuracy of 93.9% when evaluated on an image database of 2716 images using the leave-one-out method. This approach has been extended to further classify 528 landscape images into forests, mountains, and sunset/sunrise classes. First, the input images are classified as sunset/sunrise images vs forest & mountain images (94.5% accuracy) and then the forest & mountain images are classified as forest images or mountain images (91.7% accuracy). We are currently identifying further semantic classes to assign to images as well as extracting low level features which are salient for these classes. Our final goal is to combine multiple 2-class classifiers into a single hierarchical classifier.     

Thermally Stimulated Currents and Alkali-Ion Motion in Silicate Glasses

The thermally stimulated polarizatioddepolarization (TSPC/TSDC) current technique is used to investigate alkali-ion motion in silicate glasses. lbo TSDC peaks are observed in all these glasses along with the dc conduction current at high temperatures. The low-temperature TSDC peak magnitude is essentially constant, but the second TSDC peak magnitude varies with the type of alkali ion present in these glasses. Based on a proposed model, the two TSDC peaks are interpreted in terms of the localized alkali-ion motion around the nonbridging oxygen ion and through the limited conduction pathways, respectively.     

Task allocation model for distributed systems

The problem of distributing tasks to processors in a distributed computing system is addressed. A task should be assigned to a processor whose capabilities are most appropriate for the execution of that task and excessive interprocessor communication is avoided. A simple algorithm for task allocation is presented. The execution costs and communication costs of the tasks are represented by arrays. A task is either assigned to a processor or fused with another task using a simple criterion. The execution and communication costs are then modified suitably. The process continues until all the tasks are assigned to processors. This algorithm also facilitates incorporation of various system constraints. It is applicable to random program structures and to a system containing any number of processors.     

Strengthening of Oxide Ceramics by Transformation-Induced Stress

Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A1₂O₃ and unstabilized ZrO₂ while the central layer contained A1₂O₃ and partially stabilized ZrO₂ (with 2 mol% Y₂O₃). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al₂O₃-ZrO₂ specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.