Dielectric Properties of Microporous Glass in the Microwave Region

The dielectric properties of a high-silica microporous glass were determined at 5.5 and 11.4 GHz. Two measurement methods were used to measure these properties, the resonant cavity and resonant post (Hakki–Coleman) techniques. The accuracy of the two measurement techniques was evaluated and the limiting conditions of measurement were determined. The glass, consisting of pore sizes on the order of tens of angstroms, was subjected to thermal treatments in an oxidizing atmosphere at temperatures from 300° to 1100°C, and chemical treatments in 2% and 10% HF for varying amounts of time. Heat treatment of the glass at 800°C produced a minimum dielectric constant of 2.8 and loss of 0.001. The acid treatment had little effect on the dielectric properties.     

Dielectric behaviors of (Nb2O5)(1-x)xTiO2 ceramics

The dielectric constants (K) of ceramics composition (Nb₂O₅)(_1-x): xTiO₂, (x = 0.05, 0.08, 0.11) have been measured over the temperature range from -180 °C to 180 °C in the frequency range 100 Hz-IMHz. For all three samples, the temperature dependence of the dielectric constants at various frequencies shows the dielectric dispersion and the frequency dependent dielectric losses are observed, suggesting the several relaxation phenomena. Dielectric loss data are analyzed by Arrhenius formalism indicating the presence of relaxation type dielectric mechanisms in (Nb₂O₅)(_1-x): xTiO₂ ceramics.     

The Role of Status Seeking in Online Communities: Giving the Gift of Experience

This article examines online gift giving in the form of opinion, information, and advice that individuals post on websites. Research has highlighted altruism and reciprocity as the key motives behind such gift giving. We argue that informational gift giving is also strongly driven by status and status seeking, and that status sentiments are more likely to sustain virtual communities. Using theories of status seeking and self-presentation, we investigate the ways in which consumers construct status in online consumer communities. The data reveal insights into the strategies behind constructing a digital status and the rise of online systems to promote celebrity status within online communities.     

Field dependence of dielectric properties of BST single crystals

(Ba_1-xSr_x)TiO₃ single crystal fibers with x = 0.35 and 0.40 were grown, using the laser heated pedestal growth (LHPG) technique, in order to investigate the dielectric properties under dc bias for potential tunable capacitor application at room temperature. Temperature dependences of the dielectric constant and loss factor as a function of dc-bias field were measured. At room temperature and 5000V/cm, the changes on the dielectric constant were 45% and 20% for x = 0.35 (Tc≈20°C) and x = 0.40 (Tc≈4°C), respectively. Low dielectric loss (10^-3-10^-2) around RT, with and without the bias field, were observed for the composition x = 0.40. This composition although, showed distribution of phases generated during the melt growth process and that disturbed, to some extent, the field dependence behavior in the para-electric phase.     

Magnetoelectric Response in (1−x)PbZr0.65Ti0.35O3–xBaFe12O19 Multiferroic Ceramic Composites

The magnetoelectric (ME) properties of PbZr_0.65Ti_0.35O₃–BaFe₁₂O₁₉ (PZT–BaM) multiferroic ceramic composites have been investigated in this work. These materials, containing both the ferroelectric PZT and ferrimagnetic BaM phases, were prepared from the conventional solid‐state reaction method, and their main (micro)structural characteristics are provided. The ME measurements were performed over wide frequency and temperature ranges, and the influence of the BaM content was appraised. An enhanced ME coefficient (α) was obtained for the applied magnetic field of 0.8 kOe, at 97 kHz. In addition, the results reveal a high thermal stability of α in the whole analyzed temperature range, showing that the studied PZT‐based ceramic composites have great potential for practical applications.     

Effects of GeO2 addition on physical and electrical properties of BaFe0.5Nb0.5O3 ceramic

This paper presents the effect of GeO₂ glass former on the physical and electrical properties of BaFe_0.5Nb_0.5O₃ (BFN) perovskite ceramics. The BFN powder was prepared by a conventional mixed-oxide method and the GeO₂ contents, ranging from 1 to 5 wt.%, were subsequently added to the calcined BFN powder. The mixtures were pressed and sintered to form dense ceramics. We showed that, with the addition of GeO₂, the maximum density was achieved at lower sintering temperature, approximately 200–225 °C lower than those required by the pure BFN ceramic. However, the densities of these GeO₂ doped BFN ceramics were slightly lower than those of pure BFN due to the occurrence of pores. We also found that the addition of GeO₂ reduces the dielectric loss at room temperature from 4.29 to 0.39–0.79 but the dielectric constant at room temperature decreased with the increased GeO₂ concentrations. With small amount of added GeO₂, ferroelectric property of BFN ceramics was also obtained, as confirmed by their hysteresis loops.     

Effect of IGBT switching dynamics on loss calculations in highspeed applications

The switching and conduction loss in bipolar devices are dependent on the switching intervals, especially when these intervals become small enough to prevent the device from reaching steady-state conditions. This behavior is quite pronounced for non-punch-through IGBT's, and is found to drastically decrease the overall switching loss of these devices in high frequency applications relative to values projected from conventional approaches. It also results in an unexpected dependence of net losses on duty cycle. The reasons behind this behavior are examined, and its implications for semiconductor loss calculations are discussed     

Modified mixed oxide perovskites 0.7Sr(Al1/2B1/2) O3·0.3LaAlO3 and 0.7Sr(Al1/2B1/2) O3·0.3NdGaO3 (B=Ta5+ or Nb5+) for high-T c superconductor substrate applications

Single-crystal fibres of modified strontium aluminium tantalum oxide (1-x)Sr(Al_1/2Ta_1/2) O₃·xLaAlO₃(SAT-LA) and (1-x)Sr(Al_1/2Ta_1/2)O₃·xNdGaO₃ (SAT·NG), and modified strontium aluminum niobium oxide (1-x)Sr(Al_1/2Nb_1/2)O₃·xNdGaO₃(SAT·NG) and (1-x)Sr (Al_1/2Nb_1/2)O₃·xLaAlO₃ (SAN·LA) were grown using a laser-heated pedestal growth technique. 0.7SAT·0.3LA grows congruently and retains a twin free simple cubic perovskite structure (as the SAT) when cooled down to room temperature. 0.7SAT·0.3LA crystals have a moderate dielectric constant ( = 21.7) and low dielectric loss (tan = 7.5 × 10^–5) at 10 kHz and 90 K. The reduction problem of Ta⁵⁺ is eliminated (which is common in the case of SAT growth). 0.7SAT·0.3NG and 0.7SAN·0.3NG have lower melting temperatures and crystal growth is easier. NdGaO₃ addition to the SAT and SAN enhances the potential of SAT and SAN as large-area substrates for high-T _c superconductor growth. However, the dielectric constants increased from -12 to -16(0.7SAT·0.3NG) and from 18 to 23 (0.7SAN·0.3NG) as a result of NdGaO₃ incorporation.     

Influence of calcium content of biomass-based materials on simultaneous NOx and SO2 reduction

Pyrolysis products of biomass (bio-oils) have been shown to cause a reduction in NOx emissions when used as reburn fuels in combustion systems. When these bio-oils are processed with lime, calcium is ion-exchanged and the product is called BioLime. BioLime, when introduced into a combustion chamber, pyrolyzes and produces volatile products that reduce NOx emissions through reburn mechanisms. Simultaneously, calcium reacts with SO2 to form calcium sulfate and thus reduces SO2 emissions. This paper reports the characterization of composition and pyrolysis behavior of two BioLime products and the influence of feedstock on pyrolysis products. Thermogravimetric analysis (TGA) and 13C-CP/MAS NMR techniques were used to study the composition of two biomass-based materials. The composition of the pyrolysis products of BioLime was determined in a laboratory scale flow reactor. The effect of BioLime composition on NOx and SO2 reduction performance was evaluated in a 146.5 kW pilot-scale, down fired combustor (DFC). The effect of pyrolysis gas composition on NOx reduction is discussed. The TGAweight loss curves of BioLime samples in an inert atmosphere showed two distinct peaks corresponding to the decomposition of light and heavy components of the BioLime and a third distinct peak corresponding to secondary thermal decomposition of char. The study also showed that BioLime sample with lower content of residual lignin derivatives and lower calcium content produced more volatile compounds upon pyrolysis inthe combustor and achieved higher NOx reduction (15%). Higher yields of pyrolysis gases increased the NO reduction potential of BioLime through homogeneous gas phase reactions. Calcium in BioLime samples effectively reduced SO2 emissions (60-85%). However, addition of higher calcium content to the BioLime samples also appeared to inhibit the volatile yield and thereby lowered the NOx reduction.