Structural and electrical properties of Na0.47K0.47Li0.06NbO3 lead-free piezoelectric ceramics modified by AgSbO3

(1?x)Na_0.47K_0.47Li_0.06NbO₃ (NKLN)–xAgSbO₃ lead-free piezoelectric ceramics were prepared using a reaction sintering method. The effects of AgSbO₃ doping on the structural and electrical properties of NKLN ceramics sintered at 1000–1040 °C were studied. The dopant affected densification, phase content, sintering temperature, microstructure and electrical properties. Variations in the relative intensity of X-ray diffraction peaks were consistent with Ag⁺ and Sb⁵⁺ ions substituting on the perovskite lattice to produce a change in the proportions of co-existing tetragonal and orthorhombic phases. Grain growth during secondary re-crystallization was also affected. The temperature of the orthorhombic–tetragonal (O–T) phase transition and the Curie temperature (T_C) decreased as a result of AgSbO₃ modifications. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic–tetragonal polymorphotropic phase boundary. The 0.92Na_0.47K_0.47Li_0.06NbO₃–0.08AgSbO₃ ceramics exhibited optimum electrical properties (d₃₃=252 pC/N, ε_r=1450, tan δ=0.02, and T_C=280 °C). These results reveal that (1?x)Na_0.47K_0.47Li_0.06NbO₃–xAgSbO₃ ceramics are promising materials for lead-free piezoelectric application.     

Effect of MnO on the Phase Development, Microstructures, and Dielectric Properties of 0.95Na0.5K0.5NbO3–0.05LiTaO3 Ceramics

Lead-free piezoelectric ceramics in the system 0.95Na_0.5K_0.5NbO₃–0.05LiTaO₃ were modified with ≤1 mol% MnO. Maximum densities occurred at a sintering temperature of 1050°C. Characteristic changes in the relative intensity of X-ray diffraction peaks were consistent with Mn ions substituting on the perovskite lattice to produce a change from orthorhombic to a mixture of tetragonal and orthorhombic phases. Grain growth during secondary recrystallization was also affected, leading to increased grain sizes. The dielectric constant increased from ∼600 in unmodified ceramics to ∼1040 in ceramics prepared with 0.5 mol% MnO.     

The modified dynamic model of a riser type fluid catalytic cracking unit

The dynamic model developed by Ali and Rohani (1997) to describe the transient behavior of a fluid catalytic cracking (FCC) unit is modified (i) to incorporate the effect of volumetric expansion of the feed and product gases flowing in the riser reactor (ii) to consider the enhancement of mass and heat transfer coefficients due to high turbulence in the regenerator and (iii) to model the reactor and stripper as a continuous stirred tank. The modified model is validated using steady-state plant data from an industrial unit (Consumer's Co-operative Refineries Ltd., Regina, SK) and the results are found to be in good agreement.     

Interpreting computer code in a computer‐based learning system for novice programmers

Although the principles of writing compilers and interpreters are well known, we have found that the ideas needed to develop an interpreter for the express purpose of allowing direct interaction with the running code do not yet appear to have been published in an academic context. We describe a programming method that can be used for the production of an interpreter for common object‐oriented languages such as C++, Java and C#. The main purpose of the interpreter is to parse short, relatively simple programs and allow direct interaction between the user and the running code. Such a system is useful for projects such as OGRE, which is an educational tool allowing students to visualize in three‐dimensional graphics the state of a program as it runs. The interpreter works by first parsing the source code and building up a data structure capable of representing the program's source code in a form that can be used to both run the program and extract detailed information from the running program. This extraction allows for novel uses of the interpreter, such as forming the basis for a visualization system that must display and provide such information to the user as they watch their executing program. This paper considers the construction of such an interpreter specifically for C++, but the principles should be the same for other similar languages such as Java and C#. We cover the main tasks required of the programmer to create and use the data structure, highlighting areas such as its design, initial construction during parsing, and techniques required to use it for interpretation. These include the ability for the data structure to intelligently clone subsets of itself when multiple copies of one of its elements are required by the running program, how it handles C++'s complicated function overloading and overriding rules, and how inheritance and polymorphism can be supported. Copyright © 2005 John Wiley & Sons, Ltd.     

Changes in rainfall chemistry and airborne particulates during a period of major local industrial change

Data from a site in central Scotland were used to quantify the changes in rainfall quality from 1989 to 1998. During this period there have been major changes in industrial activities in the area, particularly the decline in local steel-making and steel-processing activities. Many element concentrations in rainfall decreased over time in parallel with the phased reduction in the activity of local pollutant sources. Trend analyses of the rainfall data identified that the most significant responses have been the lower concentrations of Ca, SO4-S and Mn. There was also a dramatic decline in the capture of airborne particulates by the interception rainfall gauges. Particulates were found to contain mainly hematite, magnetite and quartz, that is similar to what would be expected to be derived from the neighbouring steel industries. The eventual disappearance of these particulates and the responses in rainfall quality match the timescale for the decline and closure of some of the potential sources of pollutants.     

Rack-induced metal binding vs. flexibility: Met121His azurin crystal structures at different pH

The rack-induced bonding mechanism of metals to proteins is a useful concept for explaining the generation of metal sites in electron transfer proteins, such as the blue copper proteins, that are designed for rapid electron transfer. The trigonal pyramidal structure imposed by the protein with three strong equatorial ligands (one Cys and two His) provides a favorable geometry for both cuprous and cupric oxidation states. However, the crystal structures of the Met121His mutant of azurin from Alcaligenes denitrificans at pH 6.5 (1.89- and 1.91-A resolutions) and pH 3.5 (2.45-A resolution) show that the preformed metal binding cavity in the protein is more flexible than expected. At high pH (6.5), the Cu site retains the same three equatorial ligands as in the wild-type azurin and adds His121 as a fourth strong ligand, creating a tetrahedral copper site geometry with a green color referred to as 1.5 type. In the low pH (3.5) structure, the protonation of His121 causes a conformational change in residues 117-123, moving His121 away from the copper. The empty coordination site is occupied by an oxygen atom of a nitrate molecule of the buffer solution. This axial ligand is coordinated less strongly, generating a distorted tetrahedral copper geometry with a blue color and spectroscopic properties of a type-1 site. These crystal structures demonstrate that blue copper proteins are flexible enough to permit a range of movement of the Cu atom along the axial direction of the trigonal pyramid.     

ATP/GTP hydrolysis is required for oxazole and thiazole biosynthesis in the peptide antibiotic microcin B17

In the maturation of the Escherichia coli antibiotic Microcin B17, the product of the mcbA gene is modified posttranslationally by the multimeric Microcin synthetase complex (composed of McbB, C, and D) to cyclize four Cys and four Ser residues to four thiazoles and four oxazoles, respectively. The purified synthetase shows an absolute requirement for ATP or GTP in peptide substrate heterocyclization, with GTP one-third as effective as ATP in initial rate studies. The ATPase/GTPase activity of the synthetase complex is conditional in that ADP or GDP formation requires the presence of substrate; noncyclizable versions of McbA bind to synthetase, but do not induce the NTPase activity. The stoichiometry of ATP hydrolysis and heterocycle formation is 5:1 for a substrate that contains two potential sites of modification. However, at high substrate concentrations (>50Km) heterocycle formation is inhibited, while ATPase activity occurs undiminished, consistent with uncoupling of NTP hydrolysis and heterocycle formation at high substrate concentrations. Sequence homology reveals that the McbD subunit has motifs reminiscent of the Walker B box in ATP utilizing enzymes and of motifs found in small G protein GTPases. Mutagenesis of three aspartates to alanine in these motifs (D132, D147, and D199) reduced Microcin B17 production in vivo and heterocycle formation in vitro, suggesting that the 45 kDa McbD has a regulated ATPase/GTPase domain in its N-terminal region necessary for peptide heterocyclization.     

Analysis of fluoromethyl group chirality establishes a common stereochemical course for the enolpyruvyl transfers catalyzed by EPSP synthase and UDP-GlcNAc enolpyruvyl transferase

The stereochemistry of transient methyl group formation at C-3 of phosphoenolpyruvate (PEP) in the reaction catalyzed by 5-enolpyruvylshikimate 3-phosphate (EPSP) synthase has been examined using the pseudosubstrates, (E)- and (Z)-3-fluorophosphoenolpyruvate (FPEP). Kinetically stable, chiral [1H, 2H]fluoromethyl analogs of the reaction tetrahedral intermediate were isolated and subjected to decomposition and stereochemical analysis. EPSP synthase was found to catalyze the 2-re face addition of solvent-derived hydrogen to C-3 of FPEP (corresponding to the 2-si face of PEP). Comparison of these data with prior analogous work on the MurA reaction [Kim, D.H., Lees, W.J., & Walsh, C. T. (1995) J. Am. Chem. Soc. 117, 6380-6381] suggests that the two enolpyruvyl transferases share a common stereochemical course, further strengthening the mechanistic, structural, and evolutionary relationship between the two enzymes.