J. Woltersdorf

Formation of nanocrystalline titanium carbonitride by pyrolysis of poly(titanylcarbodiimide)

The chemical formation of advanced ceramics from organometallic precursors is promising with respect to structural design and tailormade properties. We report on the synthesis of nanocrystalline titanium carbonitride materials at different pyrolysis temperatures via the polymer-to-ceramic transformation of synthesised poly(titanylcarbodiimides) and their structural and nanochemical characterization using high-resolution and analytical electron microscopy, in combination with quantummechanical calculations. Pyrolysis at 800 °C leads to a mixture of amorphous carbon and titanium nitride as crystalline particles of about 4 nm in size. Pyrolysis at 1100 °C yields titanium carbonitride as crystalline particles of 20–30 nm in size. The identification of all phases was possible by the analysis of the measured energy-loss near-edge spectra of their C–K and N–K edges, and comparison with measured standards and with simulated fine structures using calculations based on the density functional theory.

New evidence for an oxycarbonate phase as an intermediate step in BaTiO3 preparation

A newly developed method of spray hydrolysis of a barium-titanium double alkoxide will be described as an eAcient synthetic route for the preparation of a stoichiometric BaTiO3 powder. During the thermal treatment of the corresponding precursor a barium-titanium oxycarbonate appears as an intermediate. High resolution electron microscopy (HREM) and electron energy loss spectroscopy (EELS) at ionization edges (ELNES), along with X-ray powder diAractometry (XRD), Fourier-transformed infrared spectroscopy (FTIR), and thermoanalytical measurements provide evidence of the existence of such an oxycarbonate phase. The comparison of the measured EEL spectra with quantum-mechanical calculations using density functional theory (DFT) reveals that this intermediate phase is characterized by an electronic C-Ti interaction in the crystal lattice and a specific modification of the carbonate bond. # 2000 Elsevier Science Ltd. All rights reserved.

Ca-containing additives in PTC-BaTiO3 ceramics: effects on the microstructural evolution

Two series of Ca 2+ -modified BaTiO3 ceramics have been prepared of the gross composition La0.002Ba0.998ˇxCaxTi1.01O3.02 (0<x<0.24). In the first series, CaCO3, BaCO3, TiO2 and La2(C2O4)3 .9H2O were used as starting materials. The calcination of mixtures with x40.08 resulted in the formation of the corresponding titanate solid solution (Ba1ˇxCax)TiO3. With values of x higher than 0.08, CaTiO3 was observed as an additional phase. In the second series, a La0.002Ba0.998TiO3 starting powder was hydrothermally recrystallized in Ca(NO3)2 solution. High resolution imaging and analytical methods revealed that the BaTiO3 grains are surrounded by small CaTiO3 crystallites, which influence the evolution of the microstructure of the ceramic in the sintering process strongly by acting as seeds during the recrystallization of the matrix material. Thus, it was possible to optimize the microstructural and electrical characteristics of a ceramic of the second series by adding only 4 mol% Ca, while in case of the first series 16 mol% Ca are necessary. # 2000 Elsevier Science Ltd. All rights reserved.

Identification of silicon oxycarbide bonding in Si-C-O-glasses by EELS

The present study is a contribution to the detection of silicon oxycarbide specific details in the ELNES (energy loss near edge structure) of the Si-L 23 -edge, where polymethylsiloxane (CH 3 SiO 1.5 ), pyrolyzed at 1000°C to form a glassy sample is used as a model system.

CVD-coated boron nitride on continuous silicon carbide fibres: structure and nanocomposition

Abstract Boron nitride is isoelectronic with graphite, has a similar bonding structure, but a stronger localization of the π-states, and a better oxidation resistance. Therefore, it is a promising alternative to pyrolytic carbon as a tool for tayloring special fibre/matrix interlayers in composites for gas turbine and aircraft applications. To understand the CVD processing via a boron-organic precursor and to improve the thermomechanical and hydrolytic stability of the BN fibre coatings, we focussed our investigations on the question: “In what way do carbon and oxygen become incorporated into the BN layer?” HREM studies showed the turbostratic structure of the BN, forming cells of 5–10 nm in diameter. Energy filtered electron microscopy (EFTEM) and the nanometre resolved analysis of the electron energy-loss near edge structure (ELNES) of the individual elements revealed a correlation between microstructure and chemical composition. There is a general deficiency of 10–15% of nitrogen with respect to boron, which is partly compensated for by contents of up to 10 and 15% of oxygen and carbon, respectively, in the layer. It could be concluded that most of the carbon is precipitated in between the BN cells, and a smaller amount is incorporated within the hexagonal BN structure.

Surface modification of barium titanate powder particles

Abstract The surfaces of n-doped barium titanate (BaTiO 3 ) powder particles were modified by milling in a Ca(NO 3 ) 2 solution both at room temperature and under hydrothermal conditions before they were analyzed by electron energy loss spectroscopy and transmission electron microscopy down to the nanometer range. The room temperature treated powders showed BaTiO 3 particles covered with a CaO/BaO layer. Heating these particles caused a shell of (Ca,Ba)TiO 3 solid solution. Under hydrothermal conditions rectangular CaTiO 3 crystallites precipitated on the surface of the BaTiO 3 grains, which during heating transformed into a series of (Ba 1− x Ca x )TiO 3 particles. These modifications of the shell structure change the sintering behavior and thus the microstructure as well as the electrical properties of the final ceramics. Especially the room temperature resistance and the voltage withstanding of the ceramic increased compared to that of conventionally prepared materials of identical composition.

On the atomic mechanisms of water-enhanced silicon wafer direct bonding

Abstract The quantum-chemical methods of modified neglect of diatomic overlap (MNDO) and Austin model 1 (AM1) are discussed and applied to the investigation of the different steps of direct bonding of silicon or silica wafers enhanced by water molecules. Concerning the hydrophilization, the reaction enthalpy of joining a water molecule to the silica surface was found to be 4.3 kcal mol−1, which is insufficient for a molecule to disintegrate, therefore higher concentrations of OH− ions are necessary to form silanol bonds, realized, e.g., by NH4OH treatment. The strength of the H2O interaction with an oxidized surface is nearly five times higher than that with pure silicon. At low bond densities, i.e., if only one siloxane bond is formed from two adjacent silanol groups, the transformation of silanol bonds into siloxane ones is exothermal, leading to an easy procedure, implying, however, low adhesion (fixing). Stronger bonding is provided by higher densities of reacting silanol bonds, but then the process becomes endothermal because of spatial restrictions, requiring a thermal treatment. At the hydrophilized silica surface a specific arrangement of silanol bonds and water clusters results. Its thermodynamic stability is determined by the equilibrium of core-core repulsion and the exchange interaction of the electron shells. As the formation enthalpy depends non-linearly on the bond number the stable configuration is attained at a density of four silanol bonds per 100 A2, coupled with 2–4 water molecules per silanol bond.

Surface modification of pre-sintered BaTiO3 particles

Generally, the surface properties of powder particles strongly influence the sintering behavior of the resulting compacts. This paper describes the surface modification of pre-sintered BaTiO 3 particles (5–10m in size) by leaching under different hydrothermal conditions (leaching medium, time and temperature). These modifications enable to control the thickness and the degree of crystallinity of the resulting TiO2-rich surface layer on the BaTiO3 particles. A hydrothermal treatment for 1 h at 100 ◦ C in 0.1 M acetic acid results in the formation of an amorphous, TiO2-rich, and 5 nm thick surface layer as revealed by high-resolution electron microscopy and electron energy loss spectroscopy. Microwave radiation was found to have no significant influence on the leaching processes under the conditions used here. Modified surface layers improve the densification behavior of the BaTiO 3 particles, but are insufficient to promote the evolution of a completely dense microstructure of the ceramics. A further improvement in densification and the formation of grain boundaries free of pores and with a high electrical conductivity can be achieved by hot isostatic pressing.

Defect chemistry of the shell region of water-milled BaTiO3 powders

Abstract The sintering behaviour of BaTiO 3 powders and the resulting microstructure of the corresponding ceramics are controlled by the stoichiometry of the outer region of the grains of the starting powder. These grains exhibit a core—shell structure due to the water attack during powder milling. The hydrolysis results in the formation of lattice-hydroxyl groups. The existence of these processing-related hydroxyl defects is detected by 1 H magic-angle spinning nuclear magnetic resonance ( 1 H-MAS NMR) as well as differential thermal analysis/thermal gravimetry (DTA/TG) measurements and also verified by electron energy-loss spectroscopy (EELS) in combination with high-resolution transmission electron microscopy (HRTEM). The results are confirmed by detecting the near-edge structure of the O-K edge, the details of which can be interpreted by quantum-mechanical calculations of the electronic density of states in the conduction band.

Polymere Carbodiimide als Vorstufen für keramische Werkstoffe: Reaktionsmechanismen und Tautomeriebeziehungen

Die Reaktionsmechanismen fur die Synthese polymerer Silicium- und Titan-carbodiimide aus den Elementchloriden und Bis(trimethylsilyl)-carbodiimid, die als Vorstufen fur keramische Werkstoffe Verwendung finden, werden mit Hilfe quantenchemischer Verfahren untersucht. Die Funktion organischer Basen als Katalysatoren fur den Ubergangszustand und die Thermodynamik der Synthesereaktion werden diskutiert. Von besonderer Bedeutung fur die Optimierung der Reaktion ist das Verhaltnis der Tautomere Carbodiimid - Cyanamid, das in Abhangigkeit vom Heteroatom, dem Losungsmittel und der Temperatur mit Hilfe von ab-initio-Verfahren (Dichtefunktionaltheorie) bestimmt wird. Die Ergebnisse der theoretischen Berechnungen werden durch IR- und UV-spektroskopische Daten gestutzt. Polymeric Carbodiimides as Precursors for Ceramic Materials: Reaction Mechanisms and Tautomerism The reaction mechanisms for the synthesis of polymeric silicon and titanium carbodiimides via the element chlorides and bis(trimethylsilyl)-carbodiimid, used as precursors for ceramic materials, have been investigated using quantumchemical methods. The influence of organic bases as catalysts on the transition state and the thermodynamics of the reaction are discussed. The ratio of the tautomers carbodiimid - cyanamid depending on the heteroatom, the solvent and the temperature, has been determined using ab-initio methods (density functional theory). The results of the theoretical calculations are confirmed by IR and UV spectroscopic data.