Microstructure and Dielectric Properties of Lead Magnesium Niobate-Pyrochlore Diphasic Mixtures

The microstructure and dielectric properties of lead magnesium niobate (PMN) ceramics containing the determental second phase, pyrochlore, have been studied. The pyrochlore phase within PMN was found to be present as isolated grains in the microstructure with a composition of Pb₂(Mg_0.25Nb_1.75)O_6.625 and a preferred octahedral morphology. The amount of pyrochlore second phase can be easily controlled by changing the Mg/Nb ratio on the B-site of the perovskite structure, and a series of diphasic mixtures from near-pure PMN to near-pure pyrochlore were obtained. Its effect on the permittivity follows the isolated Wiener's mixture rule up to a pyrochlore content of 18 vol%.     

Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina

Final-stage sintering has been investigated in ultrahigh-purity Al₂O₃ and Al₂O₃that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al₂O₃, block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al₂O₃, surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al₂O₃grain boundaries.     

Coarsening-Resistant Dual-Phase Interprenetrating Microstructures

Grain growth in a 50:50 (vol%) dual-phase mixture of Al₂O₃ and c -ZrO₂ (cubic zirconia) is severely limited compared with that for either of the single phases. At 1650°C, the growth rates in the duplex composition are 160 and 3500 times lower than that for single-phase Al₂O₃ and c -ZrO₂, respectively. The restriction of the grain growth is attributed to the limited mutual solubility and the physical constraint provided by the interpenetrating geometry of the two phases. Grain coordination number and dihedral angle are also considered as factor affecting grain stabilization in two-phase systems. A potentially important practical application of this work is the fabrication of grain-stabilized fibers for use as reinforcement in composite structures at elevated temperatures.     

Deformation mechanism map for creep in YBa2Cu3O7−x

A deformation mechanism map with grain size and stress as variables was constructed for creep in YBa₂Cu₃O_7–xat 850 and 950 °C. Theoretical models of Nabarro-Herring, Coble, and power-law creep were used for the construction. The values of various physical constants for creep of YBa₂Cu₃O_7–x were taken from the literature, or estimated with appropriate assumptions. The constructed map showed that the Nabarro-Herring creep would dominate at high temperatures in the practical range of grain size and stress, and that the power-law creep would occur at large stress (> 1 GPa) and grain size. A review of previous creep studies showed that the map is in close agreement with the experimental results. Discrepancies in the values of stress exponent and activation energy for creep of YBa₂Cu₃O_7–x given in the literature are explained by the use of the constructed map.     

Brain metastases in patients with differentiated thyroid carcinoma

We present a retrospective review of six patients who developed brain metastases in our series of 649 with differentiated thyroid carcinoma seen at the Royal Marsden Hospital between 1936 and 1991. Prognosis was poor, with survival 1-19 months from the diagnosis of brain metastases in five patients. One patient remains alive at 18 months. A dosimetric approach may help to individualize and optimize treatment.     

Influence of complement on the allospecific antibody response to a primary vascularized organ graft

The induction of antibody responses against T cell-dependent antigens has been reported to be influenced by complement. We therefore asked if the primary induction of alloantibodies against transplantation antigens, an important determinant of transplant outcome, is complement sensitive and whether this has functional implications. We transplanted rat kidney allografts into fully major histocompatibility complex-mismatched recipients, in which complement activation was inhibited by daily injection of soluble recombinant human complement receptor type 1 (sCR1). Control allograft recipients were injected with saline. Animals in the control group showed a marked antibody response against donor-specific antigens and an increase in the proportion of activated B and T splenocytes by day 5 after transplantation. Complement-inhibited rats showed a reduced level of antibody binding on target cells sharing the same histocompatibility antigens as the donor strain (p < 0.001), and a reduced level of activated splenic B (p < 0.01) and T (p < 0.01) cells. In a functional assay, the plasma of complement-inhibited rats showed reduced cytotoxic activity against donor-specific cells, and their grafts contained less bound antibody than controls. Analysis beyond 6 days was obscured due to the development of antibodies against sCR1. We conclude that complement activation facilitates the induction of the alloantibody response. Sparing of vascular injury and prolongation of graft survival, previously reported in complement-inhibited rats (Pratt J. R. et al., Am. J. Path. 1996, 149: 2055), could therefore be due to down-regulation of the B cell response as well as reduced complement-dependent cytotoxicity. Inhibition of complement may provide an ancillary approach to the prevention of allospecific antibody formation and the prolongation of allograft survival in primary kidney grafting.     

Environmental monitoring during gaseous induction with sevoflurane

Recent research has shown that gaseous induction in adults with sevoflurane is an acceptable technique. This study was undertaken to assess if gaseous induction using sevoflurane carried in both oxygen alone, and in nitrous oxide and oxygen combined, would provide acceptable pollution levels. As an occupational exposure standard has not been set for sevoflurane, we used the target level of 20 ppm set by the manufacturer. Environmental monitoring was carried out in the anaesthetic room during eight lists where consecutive triple vital capacity sevoflurane inductions were performed. Time-weighted averages for both gases over the duration of the lists were well below the occupational exposure standards (mean 1.1 (range 0.6-1.7) for sevoflurane and 17.3 (12-23) for nitrous oxide). There were high peak concentrations during the induction process (8.3 (4.1-17) for sevoflurane and 172.4 (65-310) for nitrous oxide) although these decreased to low concentrations between anaesthetic inductions. Personal sampling was carried out from the anaesthetist's breathing zone and concentrations were also low (1.2 (0.8-2.1) for sevoflurane and 45.9 (10.1-261.6) for nitrous oxide.     

An artificial immune system architecture for computer securityapplications

With increased global interconnectivity and reliance on e-commerce, network services and Internet communication, computer security has become a necessity. Organizations must protect their systems from intrusion and computer virus attacks. Such protection must detect anomalous patterns by exploiting known signatures while monitoring normal computer programs and network usage for abnormalities. Current anti-virus and network intrusion detection (ID) solutions can become overwhelmed by the burden of capturing and classifying new viral strains and intrusion patterns. To overcome this problem, a self-adaptive distributed agent-based defense immune system based on biological strategies is developed within a hierarchical layered architecture. A prototype interactive system is designed, implemented in Java and tested. The results validate the use of a distributed-agent biological system approach toward the computer security problems of virus elimination and ID