Lena K. L. Falk

The microstructure of a ZnO varistor material

The microstructure of a ZnO varistor material has been investigated by a combination of X-ray diffractometry and analytical electron microscopy (SEM, TEM, STEM, EDX). The material was found to consist of: ZnO grains (doped with manganese, cobalt and nickel); smaller spinel grains which hinder the growth of ZnO grains during sintering; intergranular Bi-rich phases (namely α-Bi2O3, pyrochlore and an amorphous phase); and a small proportion of ZnO-ZnO interfaces which did not have any intergranular film but to which bismuth had segregated. The intergranular microstructure is largely a result of processes which occur during liquid phase sintering and subsequent cooling to room temperature.

Carbon nanotube films obtained by thermal chemicalvapour deposition

Films of carbon nanotubes are interesting for technical applications such as cold cathodes in field emission devices. We discuss experiments in which nanotube films are grown by a simple thermal chemical vapour deposition method from hydrocarbon molecules, employing the catalytic activity of deposited iron particles. Using an in situ catalyst preparation method starting from gaseous Fe(CO)5, films of vertically aligned and non-aligned multi-wall carbon nanotubes can be synthesised. Nanotube film growth is discussed as a function of the growth conditions. Steps towards the formation of horizontally aligned nanotube films and nanotube patterns are presented. Field emission measurements demonstrate the high electron emission efficiency of the as-grown films.

Microstructural development during liquid phase sintering of silicon carbide ceramics

Abstract The microstructures of liquid phase sintered α-SiC ceramics have been characterised by analytical electron microscopy including electron energy filtered imaging. The materials were fabricated with different additions of Al2O3 and/or Y2O3 and densified by pressureless sintering or hot isostatic pressing (HIP). Y, Al-garnet and α-Al2O3 partitioned from the liquid phase sintering medium during pressure-less sintering leaving only thin intergranular films of residual glass. The formation of α-Al2O3 was promoted by the incorporation of Al2O3 from the surrounding SiC Al 2 O 3 powder bed. The thickness of analysed Al- and O-rich glassy films at SiC SiC grain boundaries was estimated to be 1.4–1.5 nm by Fresnel out-of-focus imaging. The crystallisation of Y-, Al-, Si- and O-rich liquids was suppressed during HIP, however, Y2Si2O7 partitioned from the liquid phase sintering medium in the absence of Al. The applied high pressure during HIP resulted in a limited decomposition of the α-SiC whereby graphite and SiO2 formed.

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si3N4 material which was produced by the nitridation pressureless sintering (NPS) method using Y2O3 and Al2O3 as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.

Quantitative atom probe analysis of carbides

Compared to atom probe analysis of metallic materials, the analysis of carbide phases results in an enhanced formation of molecular ions and multiple events. In addition, many multiple events appear to consist of two or more ions originating from adjacent sites in the material. Due to limitations of the ion detectors measurements generally underestimate the carbon concentration. Analyses using laser-pulsed atom probe tomography have been performed on SiC, WC, Ti(C,N) and Ti(2)AlC grains in different materials as well as on large M(23)C(6) precipitates in steel. Using standard evaluation methods, the obtained carbon concentration was 6-24% lower than expected from the known stoichiometry. The results improved remarkably by using only the (13)C isotope, and calculating the concentration of (12)C from the natural isotope abundance. This confirms that the main reason for obtaining a too low carbon concentration is the dead time of the detector, mainly affecting carbon since it is more frequently evaporated as multiple ions. In the case of Ti(C,N) and Ti(2)AlC an additional difficulty arises from the overlap between C(2)(+), C(4)(2+) and Ti(2+) at the mass-to-charge 24 Da.

A simple method for the production of large arrays of aligned carbon nanotubes

Aligned carbon nanotubes are a technologically relevant member of the family of novel carbon materials, which find applications, e.g., as field emitters in flat panel displays. Different strategies of various complexities for their production have been demonstrated. Here we present an efficient and versatile but simple method for the production of large arrays of aligned carbon nanotubes based on thermal chemical vapor deposition from common precursor molecules. Iron catalyst particles are obtained from thermal decomposition of Fe(CO)5, while C2H2 serves as carbon feedstock. Growth of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and acetylene.

The role of matrix dislocations in the superplastic deformation of a copper alloy

Abstract It is demonstrated that the densities of dislocations trapped in coherent twin boundaries may be used to provide a direct and quantitative comparison of the extent of intragranular slip in the three regions of behaviour associated with superplasticity. Measurements on a superplastic copper alloy, Coronze CDA 638, show that there is very little movement of matrix dislocations at low strain rates in region I, the movement of matrix dislocations increases with increasing strain rate in region II, and there are large numbers of mobile matrix dislocations at high strain rates in region III. Deformation in region II is considered to be controlled by grain boundary sliding occurring by the movement of grain boundary dislocations, while control of flow in region I is attributed to the rate at which grain boundary dislocations can bypass interfacial obstacles.

Synthesis of carbon nanotubes by CO2-laser-assisted chemical vapour deposition

Abstract The field of carbon nanotube research is remarkable not only because of the unique properties of this new material but also because of the various possible schemes of their synthesis and their applications. In the present study, we have explored the suitability of laser-assisted chemical vapour deposition for the formation and growth of carbon nanotubes. A medium-power continuous-wave CO2 laser was employed to irradiate a sensitized mixture of Fe(CO)5 vapour and acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown. Scanning and transmission electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron) that seems to catalyze the nanotube growth. Some regions of the deposit also revealed the presence of nanoparticles. The present experimental conditions should be suitable to produce locally structured deposits of carbon nanotubes for various applications.

Synchrotron radiation study of the electronic structure of multiwalled carbon nanotubes

We present photoelectron (PE) and x-ray absorption spectra (XAS) of macroscopically aligned multiwalled carbon nanotubes. We identify the peaks in the valence PE spectra with regions of high density of states through comparison to calculations for graphite. Finally, we outline and illustrate a simple method for using XAS to determine the average alignment of a nanotube sample.

Development of microstructure during the fabrication of Si3N4 by nitridation and pressureless sintering of Si:Si3N4 compacts

The technique for the fabrication of Si3N4 which was investigated involves the nitridation of Si:Si3N4 powder compacts containing additions of sintering aids (e.g. Y2O3 and Al2O3) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si3N4 forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. Theα- toΒ-Si3N4 phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fineΒ-Si3N4 grains. TheΒ grains are surrounded by a thin intergranular amorphous film.