Nanostructured Precursors Research Articles

The Effect of SiAlON Precursor Nanostructurization in a Planetary Mill on the Properties of Sintered Ceramics

The results on the effect of nanostructured β-sialon precursor on the sintering and properties of the resultant ceramics are presented. The standard mixture of β-sialon precursor with 0.4z substitution degree was activated for 30 min in a planetary mill with an acceleration of 28g. Activation in the planetary mill resulted in diminution of the crystallite size and significant imperfection of the crystal lattice of the powder particles. The sintering experiments were performed at 1450-1600 °C in a powder bed. The relative density of the sintered bodies achieved the value of 97 %. The nanostructured material was produced after pressureless sintering at 1500 °C. It is concluded that high activation degree of the powders is necessary to obtain fully dense ceramics.

Advanced thermal spray coatings for tribological applications

Thermal spray coating is one of the most common procedures to improve the tribological properties of materials. Ceramic and cermet coatings, especially those based on oxides (alumina, chromium oxide, etc.) and carbides, are widely used for wear protection. Improvements under investigations are related to the possible use of nanostructured coatings and to the potential application of hybrid techniques. As a possible alternative, solid lubrication is proposed as integration or replacement of the traditional liquid lubrication in the case of severe operative conditions when there is the possibility of a lack of lubricant or when environmental problems can arise from waste disposal. In the present paper, results from experimental activities on different types of abrasion-resistant thermal spray coatings are presented: graded coating systems, specifically designed for titanium-based alloys, obtained by reactive plasma spraying and physical vapour deposition of titanium nitrides; nanostructured WC-Co coatings deposited by high velocity oxy-fuel; plasma-sprayed ceramic coatings based on oxides containing nanophases either from nanostructured precursors or postprecipitated by purposely designed thermal treatments; self-lubricant coatings deposited by plasma spray with the inclusion of graphite.

Boron nitride nanotube branched nanojunctions

One-dimensional boron nitride (BN) nanotube nanojunctions are of great interest for bothfundamental and applied research because of their stable and excellent mechanical andphysical properties. Large quantities of highly pure BN nanotube multiple Y- and/orT-junctions were synthesized by annealing pure boron nanowire precursors inN2 atmosphereat 1500 °C. Transmission electron microscopy and electron energy-loss spectroscopy studies revealthat the products possess a concentric tubular structure and stoichiometric BNcomposition. The boron nanostructured precursors react with nitrogen at hightemperature to form the corresponding BN components, retaining their originalmorphologies. A non-equilibrium interdiffusion of nitrogen and boron might beinvolved in the transformation process of the solid boron nanowires to hollow BNnanostructures. Our results illustrate the technological potential of BN nanoscale multiplejunctions being incorporated into future nanoscale mechanical and electronic devices.

Application of Nanostructured ASP Precursors for Processing CaCuMn6O12 Colossal Magnetoresistance Ceramics

Nanostructured precursors prepared by ultrasonic aerosol spray pyrolisys at 750–950°C in air were applied for the first time to obtain Ca(Cu,Mn)7O12 colossal magnetoresistance ceramics with a unique submicron necked‐grain microstructure. It was observed that a high contribution of intergrain tunneling magnetoresistance results in a 10–20% difference in functional properties as dependent on morphological features of hollow precursor microspheres ASP processed at a given temperature.

Superconducting NbSe2 nanowires and nanoribbons converted from NbSe3 nanostructures

We describe the synthesis of superconducting NbSe2 nanowires and nanoribbons by the nondestructive removal of Se from one-dimensional NbSe3 nanostructure precursors. We report scanning electron microscopy imaging, x-ray diffraction, and transmission electron microscopy analyses of the morphology, composition, and crystallinity of the converted NbSe2 nanostructures. Transport measurements on individual nanowires/ribbons confirm their superconductivity with Tc∼7.2K, and the appearance of current-induced resistance steps is attributed to localized phase slip centers, akin to those reported in other superconducting nanostructures.

Photochemical Metal Organic Deposition of Patterned Nanostructured Oxide Films

AbstractIn this paper, we present examples of the use of photochemical metal organic deposition to form nanostructured metal oxide films. In the first example, we use two reactions with very different reaction rates to control the nanostructuring, utilizing tantalum (V) tetraethoxide acetylacetonate (as a thermally reactive source of tantalum oxide) and manganese (II) 2-ethylhexanoate (as a photochemically reactive source of manganese oxide). We prepare homogeneous precursor films of tantalum (V) tetraethoxide acetylacetonate and manganese (II) 2-ethylhexanoate and in the dark allow the tantalum complex to react forming tantalum oxide regions. The film is then exposed and the manganese complex is converted to a matrix surrounding the tantalum regions. The resultant structures are characterized by electron microscopy, energy dispersive X-ray spectroscopy and Auger spectroscopy. In the second example, we use two immiscible precursors, zirconium (IV) 2-ethylhexanoate and yttrium nitrate hexahydrate to form nanostructured precursor films. The nanostructuring of these films is apparent from SEM studies. Exposure of these films results in the formation of nanostructured films consisting of a zirconium oxide matrix with encapsulated yttrium oxide.

Superconductors in confined geometries

ABSTRACTThe synthesis of nanoscale superconductors with controlled geometries is extremely challenging. In this paper we present results on synthesis and characterization of one-dimensional (1D) NbSe2 superconducting nanowires/nanoribbons. Our synthesis approach includes the synthesis of 1D NbSe3 nanostructure precursors followed by nondestructive and controlled adjustment of the Se composition to formulate NbSe2. The morphology, composition and crystallinity of the synthesized 1D NbSe2 nanostructures were analyzed with scanning electron microscopy, x-ray diffraction and transmission electron microscopy. Transport measurements were carried out to explore the electronic properties of these confined superconducting nanostructures.

Microstructures and properties of nanostructured thermal sprayed coatings using high-energy milled cermet powders

This study demonstrates the potential of high-energy milling to use nanostructured cermet powders for thermal spraying utilizing a TiC–Ni-based composite as model material. The microstructure of coatings processed by VPS and HVOF spraying of nanostructured composite powders is characterized and compared to the initial microstructure of the feedstock. Thus, the effect of different microstructures, which can be produced by high-energy milling, on the microstructural evolution during spraying is evaluated. Results show that partial dissolution and reprecipitation of hard phase material as well coarsening of the binder phase crystallite size occur during the spraying process. However, a homogeneously dispersed hard phase distribution similar to that of the nanostructured precursor powder with hard phase sizes in the range of 100 nm is formed. Additionally, hard phase particles bigger than of 300 nm are retained during spraying. First results concerning hardness and wear resistance of respective coatings are shown and discussed.

Selective Interactive Grafting of Composite Bifunctional Electrocatalysts for Simultaneous Anodic Hydrogen and CO Oxidation: I. Concepts and Embodiment of Novel-Type Composite Catalysts

The equivalence of interionic hypo-hyper-d-interelectronic interaction (HHDII) in both metallic and any other ionic state and its effect upon electrocatalytic properties for hydrogen electrode reactions has been proved and inferred. Thermal gravimetry (TG) analysis of temperature programmed reduction (TPR) of mixed hypo-hyper-d-electronic oxides of transition elements was broadly employed to prove the interionic bonding effect (the extended Brewer theory) as reflected in dramatically decreased individual temperatures of their mutual reduction into intermetallic phases or alloys. The same interionic (and/or intermetallic) bonding effect has been confirmed both by under potential deposition of hyper-d- upon hypo-d-electronic substrates and vice versa, and by the shift of bonding peaks in X-ray photoelectron spectroscopy analysis. The former affords the basis for new trends in submonolayer hypo-hyper-d-interelectronic electrocatalysis of transition metals. Strong metal support interaction (SMSI) of both individual and composite, prevailingly hyper-d-electronic metallic electrocatalysts upon individual and/or composite, usually hypo-d-electronic oxide substrates have been employed to create and graft (anchor) bifunctional electrocatalysts for simultaneous anodic hydrogen and CO oxidation in low temperature polymer exchange membrane fuel cells. The selective interionic bonding method upon predestined active centers of hypo-d-electronic oxide supports has been adapted to avoid nanostructured colloidal precursors and directly graft (anchor) a priori defined nanosized intermetallic phases and synergetic bifunctional electrocatalysts from decomposition of corresponding stoichiometric mixtures of various individual or intermetallic acetylacetonates. An adapted TG method based on TPR has been properly used to define, control and/or stimulate the homogeneity of the intermetallic crystal bonding and growth of nanostructured composite catalysts, mostly of rather extra strong bonding Brewer intermetallic phases upon proper SMSI oxide supports. Thus, it has been pointed out that the term SMSI has a broader HHDII sense in both the bonding effectiveness and bifunctional catalytic meaning, and in fact stays in the core of such extended Brewer interionic bonding theory. © 2003 The Electrochemical Society. All rights reserved.

Nano-structured metal-containing polymer precursors for high temperature non-oxide ceramics and ceramic fibers—syntheses, pyrolyses and properties

Nano-structured metal-containing ceramic polymer precursors have a potential for progress in the field of polymer-derived ceramics. In this paper the synthesis and characterization of nano-metallopolycarbosilanes (nMPCS) and their transformation into ceramic materials are reported. The formation of metal nano-particles via fast thermolysis of metal- containing compounds in polymer solution or melt previously developed by the authors was applied to preceramic polymers. Tetrabenzyltitanium, tetrabenzylzirconium, bis(cyclopentadienil)dichloride-titanium and zirconium as well as tetrachlorides of these metals and tetrakis(diethylamino)zirconium were used for the introduction of metal nano-particles. The products were characterized by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), differential-thermal analysis (DTA), gel-penetration (GP)-chromatography, X-ray diffraction (XRD), scanning electron microscopy (SEM) and other special analyses. The results of these studies may be useful for the fabrication of non-oxide ceramic fibers, interphase coatings and high temperature ceramic matrix composite (HT-CMC) matrices. Cp2ZrCl2 and Zr[N(C2H5)2]4 provided nano-particles with diameters of 2–4 nm and turned out to be the most suitable compounds for the preparation of ceramic matrices. Besides, they open a new way of oxygen-free curing. Coreless fibers were obtained from nZrPCS with up to 3 mass% of metal. Future investigations will be focused on optimization of the oligocarbosilanes used as starting materials and the development of less reactive polycarbosilane (PCS)–metal-containing compound (MCC) systems.

Synthesis and Characterization of KAlSiO 4 Polymorphs on the SiO 2-KAlO 2 Join

High-temperature phase transformations of K-LTA zeolite were investigated in the temperature range 800-1800°C. The nano-structured KAlSiO4 precursor was prepared at 850°C and after annealing at 1000°C, the first polymorph of KAlSiO 4, synthetic kaliophilite was formed. With increasing temperature, the following polymorphs were synthesized: kalsilite (1100°C), low KAlSiO 4-O 1 (1200°C), KAlSiO 4-ANA (1300°C), and high KAlSiO 4-O 2 at (1430°C). Synthetic kaliophilite and KAlSiO 4-ANA phases are the new polymorphs in the system studied. Consequently, the KAlSiO 4-ANA phase is the ANA-type end member of the zeolite framework having the ratio Si/Al = 1. From XRD powder patterns a distorted leucite unit cell for a KAlSiO 4-ANA phase was calculated: a 0 = 13.638(6) Å, b 0 = 13.811(8) Å, c = 13.817(9) Å, α 0 = 99.98(4)°, β 0 = 102.28(4)°, and γ 0 = 62.80(4)°. 29Si MAS NMR chemical shifts at -89.08 and -98.01 ppm suggest that the KAlSiO 4-ANA phase has an ordered framework cation topology.