Room temperature synthesis of crystalline metal oxides

Crystalline titanium dioxide powders have been synthesized as either rutile or anatase from aqueous solutions at low temperatures (T≤100°C) and atmospheric pressure. First, a sol is prepared by the hydrolysis of a titanium alkoxide in an acidic solution. The sol is subsequently heated at different rates to produce the different crystalline phases of titanium dioxide. Powder characterization was carried out using X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. In general, the precipitate size was observed to be between 50 and 100 nm. Possible mechanisms involved in determining the crystal variants are discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Creep-sintering of zinc oxide

The simultaneous creep and densification of zinc oxide powder compacts was studied, using a loading dilatometer, at 725° C subjected to uniaxial stresses of up to 0.25 M Pa. Between relative densities of 0.50 and 0.85, the dependence of the uniaxial creep rate on density can be described in terms of a stress intensification factor,, of the form exp (aP), wherea is constant equal to 5.0 andP is the porosity. Comparison of the creep and densification rates showed that the ratio of the linear densification rate to the creep rate is nearly constant over a density range between 0.55 and 0.85, and permitted the evaluation of the sintering effective stress, which is found to decrease with increasing density. The results are compared with those obtained earlier for cadmium oxide and a soda-lime glass powder.     

Processing and Properties of Particulate Composites from Coated Powders

A method is described for sintering of highly loaded (≤20 vol%) alumina matrix/silicon carbide whisker or platelet composites to closed porosity. The method uses controlled heterogeneous precipitation in a suspension of whiskers or platelets. The suspension-coated particles are calcined to produce a free-flowing composite powder which can be hot-pressed or free-sintered to high density. The results of consolidation and mechanical property testing are reported.     

Sintering of Ceramic Particulate Composites: Effect of Matrix Density

Composites consisting of a fine-grained, polycrystalline zinc oxide matrix and <10 vol% coarse, rigid silicon carbide inclusions were prepared by the same mixing procedure and then compacted to produce samples with matrix densities of 0.45 and 0.68 of the theoretical. The samples were sintered under identical temperature profiles in separate experiments that employed either a constant rate of heating of 4°C/min or near isothermal heating at 735°C. The ratio of the densification rate of the composite matrix to the densification rate of the unreinforced zinc oxide was found to be independent of the initial matrix density. This ratio increased significantly with temperature in the constant-heating-rate experiments but was relatively constant in the isothermal experiments. The results indicate that microstructural coarsening may be an important mechanism for explaining the reduced sinterability of polycrystalline matrix composites.     

Planar [00. l] disorder in sodium beta alumina

00./and 01./ lattice images are used to show the presence of [OO.l] disorder in alumina. It is shown that both stacking sequence faults and intergrowth are present. The faults are believed to be introduced in the early stages of sintering when spinel block ledges are added out of sequence during growth in the [00. l] direction. The observations show that the planar disorder observed by Bevan et al. [4] cannot be attributed solely to heterophase intergrowth.     

Joining of Silicon Carbide with a Cordierite Glass-Ceramic

A method for the joining of silicon carbide using a cordierite glass-ceramic has been developed. Cordierite, with glass-ceramic processing, remains amorphous and wets the SiC substrate to form a strong bond when rapidly fired. Subsequent heat treatment crystallizes a multiphase interlayer with a matching bulk thermal expansion coefficient (CTE). A benchtop tape casting method for depositing joining precursor films of varying thickness is described. The wetting characteristics of cordierite on SiC that are pertinent to the joining process are shown to be highly sensitive to processing atmosphere. Doping with a fluoride ion flux can lower the peak processing temperature without significantly altering the crystallization path. The effect of interlayer thickness is observed by monitoring indentation crack paths and with 4-point bending tests. Controlling the degree of crystallinity is shown to tailor the mismatches in thermal expansion coefficient and elastic moduli to produce joints of high strength (σ_F > 500 MPa). Characterization is accomplished with XRD, SEM, and TEM.     

Discovery of an Acyclic Nucleoside Phosphonate that Inhibits Mycobacterium tuberculosis ThyX Based on the Binding Mode of a 5‐Alkynyl Substrate Analogue

The urgent need for new antibiotics poses a challenge to target un(der)exploited vital cellular processes. Thymidylate biosynthesis is one such process due to its crucial role in DNA replication and repair. Thymidylate synthases (TS) catalyze a crucial step in the biosynthesis of thymidine 5‐triphosphate (TTP), an elementary building block required for DNA synthesis and repair. To date, TS inhibitors have only been successfully applied in anticancer therapy due to their lack of specificity for antimicrobial versus human enzymes. However, the discovery of a new family of TS enzymes (ThyX) in a range of pathogenic bacteria that is structurally and biochemically different from the “classic” TS (ThyA) has opened the possibility to develop selective ThyX inhibitors as potent antimicrobial drugs. Here, the interaction of the known inhibitor 5‐(3‐octanamidoprop‐1yn‐1yl)‐2′‐deoxyuridine‐5′‐monophosphate ( 1 ) with Mycobacterium tuberculosis ThyX enzyme is explored using molecular modeling starting from published crystal structures, with further confirmation through NMR experiments. While the deoxyuridylate (dUMP) moiety of compound 1 occupies the cavity of the natural substrate in ThyX, the rest of the ligand (the “5‐alkynyl tail”) extends to the outside of the enzyme between two of its four subunits. The hydrophobic pocket that accommodates the alkyl part of the tail is formed by displacement of Tyr 44.C, Tyr 108.A and Lys 165.A. Changes to the resonance of the Lys 165 NH₃ group upon ligand binding were monitored in a titration experiment by 2D HISQC NMR. Guided by the results of the modeling and NMR studies, and inspired by the success of acyclic antiviral nucleosides, compounds where a 5‐alkynyl uracyl moiety is coupled to an acyclic nucleoside phosphonate (ANP) were synthesized and evaluated. Of the compounds evaluated, sodium (6‐(5‐(3‐octanamidoprop‐1‐yn‐1‐yl)‐2,4‐dioxo‐3,4‐dihydropyrimidin‐1(2H)‐yl)hexyl)phosphonate ( 3 e ) exhibited 43 % of inhibitory effect on ThyX at 50 μM . While only modest activity was achieved, this is the first example of an ANP inhibiting ThyX, and these results can be used to further guide structural modifications to this class to develop more potent compounds with potential application as antibacterial agents acting through a novel mechanism of action.     

Traveling Reaction Zone Method for Preparation of Textured Ceramic Superconductor Thick Films

Textured thick films of superconducting YBa₂Cu₃O_7−x (YBCO) have been prepared on ceramic substrates using a traveling reaction zone method. The technique utilizes the rapid reaction between Y₂Cu₂O₅ and BaCuO₂ to form YBCO as the film passes through a steep temperature gradient furnance. The films consist of a single continuous superconducting phase with strong c -axis orientation normal to the translation direction of the film.     

Cycle-slip analysis of the Mth-power NDA feedforward carriersynchronizer for MPSK

We analyze the cycle-slipping behavior of the Mth-power nondecision-aided (NDA) feedforward carrier synchronizer for M-ary phase-shift keying (MPSK). The averaging filter is a block window accumulator, and postprocessing is used to eliminate equivocation. An asymptotic (high E_s/N_o) expression for the cycle-slip probability is derived and verified by computer simulation. A “hybrid” approach, combining analytical results with importance sampling simulation techniques, allows for the fast and accurate determination of the cycle-slip probability for all E_s /N_o values of practical interest     

Microstructure Refinement of Sintered Alumina by a Two-Step Sintering Technique

For a few oxide ceramics, the use of an initial precoarsening step prior to densification (referred to as two-step sintering) has been observed to produce an improvement in the microstructural homogeneity during subsequent sintering. In the present work, the effect of a precoarsening step (50 h at 800°C) on the subsequent densification and microstructural evolution of high-quality alumina (Al2O3) powder compacts during constant-heating-rate sintering (4°C/min to 1450°C) was characterized in detail. The data were compared with those for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructural improvement during two-step sintering. After the precoarsening step, the average pore size was larger, but the distribution in pore sizes was narrower, than those for similar compacts that were sintered conventionally to 800°C. In subsequent sintering, the microstructure of the precoarsened compact evolved in a more homogeneous manner and, at the same density, the amount of closed porosity was lower for the compacts that were sintered by the two-step technique, in comparison to the conventional heating schedule. Furthermore, a measurably higher final density, a smaller average grain size, and a narrower distribution in grain sizes were achieved with the two-step technique. The microstructural refinement that was produced by the two-step sintering technique is explained in terms of a reduction in the effects of differential densification and the resulting delay of the pore channel pinch-off to higher density.