Showing papers on "Transmission electron microscopy published in 1991"

Transmission electron microscope investigations of the structure of copper precipitates in thermally-aged Fe—Cu and Fe—Cu—Ni

Abstract: Conventional transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) experiments have been carried out on thermally-aged binary Fe 1·30 wt% Cu and ternary Fe 1·28 wt% Cu 1·43 wt.% Ni alloys to study the structure of small (7–15 nm) copper precipitates in the α-Fe matrix. The experiments show that the precipitates have a twinned 9R close-packed structure, rather than the expected f.c.c. structure. It is believed that this structure is generated initially by a martensitic transformation from the metastable b.c.c. phase.

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

Abstract: The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < T p < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < T p < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (T p > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.

The microstructure of SCS-6 SiC fiber

Abstract: A thorough investigation of the microstructure of single SCS-6 SiC fibers widely used as reinforcements in metal-matrix and ceramic-matrix composites has been made. Various techniques of electron microscopy (EM) including scanning (SEM), conventional transmission (TEM), high resolution (HREM), parallel electron energy loss spectroscopy (PEELS), and scanning Auger microscopy (SAM) have been used to analyze and characterize the microstructure. The fiber is a complicated composite consisting of many different layers of SiC deposited on a carbon core and different carbonaceous coatings covering the SiC layers. The structural and chemical aspects of each layer are characterized and discussed.

A new method for synthesizing nanocrystalline alloys

Abstract: A new method to prepare nanocrystalline alloys was developed by means of crystallization from amorphous alloys. By using this method, a Ni-P alloy with 9 nm crystallites was synthesized. The structure and grain sizes of the alloy were examined by means of x-ray diffraction, transmission electron microscopy, and high resolution electron microscopy. Specific heat capacity and thermal expansion coefficient of the nanocrystalline Ni-P alloy prepared by this method were found to be greater than those of the coarse-grained crystalline alloy by 12.3% and 56.2%, respectively. A new micromechanism for nanometer-sized crystallites formation is discussed.

Microstructures of TiN films grown by various physical vapour deposition techniques

Abstract: A study has been made of TiN coatings deposited on steel substrates by five commercially available physical vapour deposition (PVD) methods; low voltage electron beam evaporation, triode high voltage electron beam evaporation, random-arc evaporation, steered-arc evaporation and magnetron sputtering. The microstructure and substrate-film interfacial microchemistry of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDX), cross-sectional transmission electron microscopy (XTEM) and scanning transmission electron microscopy (STEM) combined with EDX analyses of XTEM samples. The XRD analyses showed that all the films were in a state of compressive stress with interplanar distances as much as 1.7% higher than reference bulk values. SEM examination revealed only minor variations in surface roughness among the samples except for the arc-evaporated films which contained large droplets and craters resulting from the detachment of droplets. The number density and average sizes of droplets and craters were lower in the steered-arc sample than in the random-arc sample. XTEM analyses showed that all the films had columnar structures with clearly defined substrate-film interfacial layers. The films appeared dense except for the magnetron-sputtered sample which exhibited intercolumnar porosity. STEM-EDX analyses showed large variations in the microchemistry of the substrate-film interfacial regions which consisted, depending on the sample, of renucleated near-surface substrate grains, intentionally (or, in at least one case, unintentionally) introduced foreign material or gas-bubble-like inclusions. However, the microchemistry of these interfacial regions was, in most cases, understandable on the basis of the substrate pretreatment and/or choice of film growth parameters.

Growth and characterization of epitaxial cubic boron nitride films on silicon.

Abstract: We report the growth of boron nitride films on (001) faces of silicon using the method of pulsed-excimer-laser ablation The structure of the deposited films is cubic zinc blende with a lattice constant of 3619 \AA{} The films were found to be heteroepitaxial with the cubic BN〈100〉 axes parallel to Si〈100〉, as characterized by x-ray diffraction and high-resolution transmission electron microscopy We find evidence for an unusual 3:2 commensurate lattice matching

Growth of (110)‐oriented CeO2 layers on (100) silicon substrates

Abstract: CeO2 layers epitaxially grown on (100) silicon substrates by electron‐beam evaporation were investigated and proved to have (110) orientation. X‐ray diffraction measurements showed the CeO2 layers consist of more than 98% volume fraction of the (110) component. Cross‐sectional high‐resolution transmission electron microscopy and selected‐area electron diffraction clearly verified the above configuration of crystallographic orientations and that the 〈100〉 direction in the CeO2(110) plane is parallel with the 〈110〉 direction in the Si(100) plane. The cross‐sectional lattice image clarified the existence of a ∼60‐A‐thick intermediate amorphous layer between the CeO2 layer and the silicon substrate. Moreover, the high density of defects such as dislocations and low‐angle boundaries that exist in the vicinity of the interface agree well with Rutherford backscattering and channeling measurements.

Abnormal grain growth in aluminum alloy thin films

Abstract: Abnormal grain growth in 0.75‐μm‐thick aluminum alloy thin films has been studied using x‐ray diffractometry, plan‐view and cross‐sectional transmission electron microscopy, scanning transmission electron microscopy, and energy dispersive x‐ray analysis. Transmission electron microscopy was used with preannealed samples as well as with samples annealed in situ. By varying the deposition temperatures, compositions, and annealing conditions, we have determined the roles of alloying elements in the formation of second‐phase precipitates and in promoting abnormal grain growth.

Spot-scan imaging in transmission electron microscopy.

Abstract: The determination of the structure of proteins and other organic materials by transmission electron microscopy is a rapidly developing field. Obtaining high-resolution images of these radiation-sensitive specimens has, until recently, been problematic. The development of spot-scan imaging, in which the electron beam is focused to a spot with a diameter of about 1000 angstroms and moved over the specimen to record the image, has overcome some of the most severe problems, which result from beam-induced motion of the specimen and its image. Elimination of this motion greatly enhances the contrast of high-resolution features of the image and promises a significant increase in the speed with which future structural work can be accomplished.

Effect of oxide layer on the hysteresis behavior of fine Fe particles

Abstract: The effects of surface oxidation on the structural and magnetic properties of fine Fe particles prepared by the evaporation technique have been studied using transmission electron microscopy, x‐ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, and Mossbauer spectroscopy By varying the argon pressure, particles were obtained with sizes in the range of 60–350 A The hysteresis behavior was found to be strongly dependent on the variation in the amount of surface oxidation The differences in the magnetic behavior due to variation in size and oxidation have been explained by considering a shell/core model for the particle morphology with the shell consisting of Fe oxides surrounding the α‐Fe core

Ultrastructure of poly(p-phenylenebenzobisoxazole) fibers

Abstract: The ultrastructure of poly(p-phenylenebenzobisoxazole) (PBZO or PBO) was studied as a function of processing condition by wide-angle X-ray scattering (WAXS), selected-area electron diffraction (SAED), dark-field transmission electron microscopy (DF), and high-resolution electron microscopy (HREM). The development of single-crystal texturing in thin films made it possible to index the PBZO scattering patterns to a nonprimitive (N=2) monoclinic space group Pc (No. 7)

Microstructure of titanium injected with SiC particles by laser processing

Abstract: In recent years there has been increasing interest in the use of high-power CO2 lasers for surface treatments of Ti and Ti alloys involving melting, alloying, cladding and particle injection. The last of these methods in this context is of interest in relation to the improvement of the hardness and of resistance to wear and erosion. For example, Ayers and co-workers [1-3] have shown that the injection of TiC or WC into a T i 6 A I 4 V alloy causes a decrease in the coefficient of friction; when the volume fraction of the TiC was 50 vol % the coefficient of friction was reduced from 0.45 to 0.18. Recently the injection of SiC particles into the laser melt pool of commercial purity (CP) Ti or T i -6A1-4V alloy was explored [4, 5]. Partial dissolution of the SiC led to matrix enrichment with Si and C; during solidification TiC dendrites formed at the SiC-matr ix interfaces and within the matrix and the Vickers hardness of the matrix was increased to about 600 Hr . This letter reports the result of SiC injection into Ti concerning the effect of powder flow rates on the dimensions and constitution of the processed zones; also, further details of the microstructural features of the melted zone are reported from transmission electron microscopy (TEM). A 2 kW continuous-wave CO2 laser was used for surface melting a CP Ti substrate in the form of 10 mm thick plate. SiC particles of average size 150/xm were blown into the laser-melted zone using a stream of Ar gas. The process was carried out under an effective shrouding system to minimize the contamination of the molten pool. A series of single tracks were produced using 1.95 kW laser power at 5 mm beam diameter and different values of SiC power flow rates, namely 0.033, 0.058, 0.088 and 0.11 gs i . The relative speed between the laser beam and the substrate was kept constant at 7 mms -T. Transverse sections were cut from the tracks and metallographically prepared for microstructural and compositional analysis using optical microscopy (OM), scanning electron microscopy (SEM) provided with energy-dispersive X-ray spectroscopy (EDS), TEM and scanning transmission electron microscopy (STEM). Fig. 1 shows that increasing the powder flow rate leads to a slight increase in the width of the processed zone from about 4.1 to 5 mm and a significant increase in the melted depth from about 0.45 to 0.8 mm; the zone thickness also increases nearly in parallel with the depth, corresponding to

A transmission electron microscopy study of hillocks in thin aluminum films

Abstract: Hillocks, small outgrowths on a film surface, form when compressional stresses in an aluminum film are relaxed at elevated temperature (≥90 °C), for instance during the phase of rising temperature in an annealing cycle. This paper reports a study of hillock formation in Al films of thicknesses in the interval 0.25–2.2 μm and which have been deposited by electron beam evaporation. Hillock sizes, shapes, number and formation temperatures were determined, the latter on a heating stage in situ in a scanning electron microscope. The internal structure of the hillocks was studied by cross‐sectional transmission electron microscopy technique. These studies provided strong support for the idea that hillocks are formed by migration of material along grain boundaries, presumably at triple junctions, up to the surface where it is deposited in a growing hillock. Initially, the hillocks are separated from the original film surface by a grain boundary‐like interface, but prolonged annealing will cause underlaying grain...

Mechanical alloying of the high carbon FeC system

Abstract: Mechanical alloying of iron and graphite powders was performed in composition range Fe1 − xCx (x = 0.17−0.90) by the use of a conventional ball mill. The structures of mechanically alloyed samples were examined by X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy, 57Fe Mossbauer spectroscopy and differential scanning calorimetry. The results from X-ray, TEM and Mossbauer measurements suggested the partial formation of amorphous phase. Amorphization was notable on the sample ball milled for about 200 h. After subsequent milling, formation of metastable carbides Fe3C for the powders with x = 0.17−0.25 and Fe7C3 for x = 0.29−0.70 was detected. Formation of fine paramagnetic particles was detected by Mossbauer spectroscopy for the powders having carbon content x = 0.80−0.90.

Dependence of crystalline orientation on film thickness in laser‐ablated YBa2Cu3O7−δ on LaAlO3

Abstract: The microstructure of YBa2Cu3O7−δ thin films deposited on (001)LaAlO3 substrates by a laser ablation process has been investigated by scanning electron microscopy, x‐ray diffraction, and cross‐sectional transmission electron microscopy. Adjacent to the substrate, the film is entirely oriented with the c‐axis perpendicular to the surface. At a thickness of about 0.4 μm, the occurrence of 90° boundaries brings about a transition to grains with their c‐axes parallel to the surface (aligned along the [100] and [010] directions of the pseudocubic LaAlO3 substrate). This transition is discussed in terms of the crystal growth anisotropy and the retained strain that may precipitate the transition.

Mechanism of electrochromism for amorphous WO3 thin films

Abstract: The mechanism of electrochromism for an amorphous WO3 film has been studied. The film was prepared by using vacuum evaporation. X‐ray phototelectron spectroscopy analysis has revealed that a state appears below the Fermi level after coloration in a LiClO4‐propylene carbonate electrolyte and that the Fermi level increases in proportion to the amount of injected lithium. In addition, a decrease in the density of state of the conduction band has been observed in a colored film by using electron energy loss spectroscopy analysis in transmission electron microscopy. It has been concluded that the electrons injected occupy the conduction band after coloration and that electrochromism of amorphous WO3 film is due to an intraband transition between an electron injected in the conduction band and an empty state.

Solid-phase reactions and crystallographic structures in Zr/Si systems

Abstract: Solid‐phase reactions and crystallographic structures in the interfacial region of Zr/(100)Si systems, which is closely related to the specific contact resistivity, have been investigated by x‐ray diffraction, Auger electron spectroscopy and cross‐sectional high‐resolution transmission electron microscopy. As‐grown Zr films are in an amorphous phase including crystallites. The Zr film on silicon annealed at a temperature of 420 °C for 30 min has a bilayer structure. The upper layer is a crystalline α‐Zr layer with a (0001) fibrous structure and the lower layer is an amorphous zirconium silicide layer. The formation of the amorphous silicide layer is considered to result from the diffusion of Si atoms from the substrate into the Zr film and to play a major role in a low contact resistivity of 4×10−8 Ω cm2 achieved in this system. By annealing above 560 °C, which brings about an increase in contact resistivity, the crystalline Zr layer with the fibrous structure is changed to an amorphous silicide throughou...

Effects of iron oxidation state on the texture and structural order of Na-nontronite gels

Abstract: Aqueous gels of unaltered (oxidized) and chemically reduced ferruginous smectite (SWa-1 from the Source Clays Repository of The Clay Minerals Society) were characterized by transmission electron microscopy, electron diffraction, and energy-dispersive X-ray fluorescence to establish details regarding their texture, inter-layer and inter-particle arrangements, and chemical composition. Micrographs revealed that the reduction of structural Fe(III) to Fe(II) caused a consolidation of smectite particles from an extensive network of small crystals (1–6 layers thick) to distinct particles of limited size in the a-b direction and about 20–40 layers thick. The interlayer distances in the reduced sample appeared to be more uniform than in the oxidized sample, but both exhibited spacings of about 12.6 A. Chemical analysis showed no qualitative differences as a result of oxidation state. Electron diffraction patterns displayed marked differences. The pattern of the oxidized sample consisted of homogeneous rings, indicating that the stacking order in the a-b plane was turbostratic or disordered, whereas the reduced pattern exhibited much more order as evidenced by distinct spots amid low-intensity rings, suggesting that inter-layer attractive forces were stronger if Fe(II) was present in the clay crystal.

Microstructural and Microchemical Characterization of a Calcium Aluminate—Polymer Composite (MDF Cement)

Abstract: The microstructure of a macrodefect-free (MDF) cement has been characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution electron microscopy (HREM). The microchemistry of ultramicrotomed samples has been studied by energy dispersive spectrometry (EDS) and parallel electron energy loss spectrometry (PEELS). MDF cement consists of CaAl2O4 and CaAl4O7 grains randomly distributed in a polymer matrix. The ceramic/polymer interface contains an amorphous interphase inside of which are distributed very fine crystallites of the metastable hydration product Ca2Al2O5·8H2O. PEELS analyses of the interphase revealed the presence of carbon, indicating that Ca2Al2O5·8H2O was most likely stabilized by the intercalation of polymeric chains into its basal interlayers. The polymer phase preferably cross-links with Al. In situ environmental cell electron microscopy showed that moisture uptake of MDF cements occurred by polymer swelling and interphase dissolution. The role of the interfacial interphase in dry and wet mechanical properties is discussed.

Ion-beam synthesis of a Si/β-FeSi2/Si heterostructure

Abstract: Ion‐beam synthesis of a buried β‐FeSi2 layer in Si is demonstrated. In the experiments Si(111) substrates have been implanted with 450‐keV Fe+ ions. Samples have been analyzed by Rutherford backscattering spectrometry, x‐ray diffraction, and transmission electron microscopy. Annealing at 900 °C of samples implanted with 6×1017 Fe+/cm2 causes formation of a buried layer consisting of grains with lateral dimensions of approximately 5 μm. The epitaxy of β‐FeSi2 (110) and/or (101) planes parallel to the Si(111) substrate plane is observed.

Effect of annealing on magnetic properties and microstructure of electroless nickel-copper-phosphorus alloy deposits

Abstract: The relationship between magnetic properties and microstructure of the as-deposited and heat-treated Ni-Cu-P alloy deposits has been studied by means of vibrating sample magnetometry, differential scanning calorimetry. X-ray diffractometry and hot-stage transmission electron microscopy. The Ni-Cu-P deposits consist of low-P Ni-Cu solid solution crystallites and high-P-low-Ni-Cu amorphous phase. When the deposits are annealed at elevated temperatures, the Ni-Cu crystallites grow rejecting P from themselves and absorbing Cu from the neighbouring amorphous phase, while the P-rich amorphous phase transforms into Ni5P2 phase or into Ni5P2 and Ni3P phases. The metastable Ni5P2 phase finally transforms into the stable Ni3P phase. The non-magnetism observed in Ni-Cu-P deposits having a copper content above 28% in both as-deposited and annealed states, is attributed to their Cu-rich Ni-Cu solid solution crystallites. Nickel phosphides which crystallize from the amorphous phase existing mixed with the Ni-Cu crystallites in the as-deposited state, do not affect the saturated magnetic moment of the deposits.

Formation of Cu3Si and its catalytic effect on silicon oxidation at room temperature

Abstract: Copper forms three silicides which are stable at room temperature, with compositions of Cu5Si, Cu15Si4, and Cu3Si. The sequence of formation of these phases in Cu–Si bilayers is described. Cu3Si is always the first phase formed by annealing to 200 °C, and is the stable phase on bulk Si. We show that Cu3Si has a dramatic effect on the oxidation of (100) silicon.While thermal oxidation of Si is normally carried out at temperatures above 700 °C, the process described here produces a thick layer of SiO2 at room temperature. The SiO2 layer grows spontaneously to over one micrometer in thickness in several weeks in air, beneath most of the original Cu3Si layer. Analysis by Rutherford backscattering, Auger electron spectroscopy, cross‐sectional transmission electron microscopy, and scanning electron microscopy reveals the presence of Cu3Si particles at the buried SiO2/Si interface. These Cu3Si particles catalyze the room temperature oxidation of silicon.

High-resolution transmission electron microscopy study of gold particles (greater than 1 nm), epitaxially grown on clean MgO microcubes

Abstract: Gold particles (greater than 1 nm) were vacuum deposited on clean surfaces of MgO microcubes in situ synthesized in a controlled atmosphere. The particles were observed simultaneously with the MgO substrate in plan view and in cross-section by high-resolution transmission electron microscopy, microdiffraction and convergent-beam electron diffraction. The gold particles (less than 4 nm) had a f.c.c. structure and were perfectly accommodated with the substrate, leading to a lattice expansion of 2.9%. In this size range (1-4 nm), all the particles were single crystals and had an half-octahedron shape with (001) truncations on the edges. The interface between the particles and the MgO substrate was flat; no defects were imaged. In contrast larger (greater than 4 nm) particles have the lattice parameter of the bulk material. The high stability of the particles in the electron beam was certainly due to their strong interaction with the clean MgO surface.

Freeze-fracturing for conventional and field emission low-temperature scanning electron microscopy : the scanning cryo unit SCU 020

Abstract: SUMMARY A dedicated cryopreparation system, the SCU 020 (Balzers), is introduced and described in detail for use in low-temperature scanning electron microscopy (LTSEM). The basic unit consists of two parts: (i) a high-vacuum preparation chamber equipped with a cold-stage, motor-driven fracturing microtome, planar magnetron (PM) sputter source, quartz-crystal thin-film monitor, Meissner cold trap, and turbo molecular pump stand; and (ii) a second part (separated from the first by a sliding, high-vacuum valve) residing in the SEM chamber. This is equipped with an anti-contamination cold trap, a fully movable goniometer cold stage (having motor drives for x, y, and rotation) and replaces the SEM's original stage (Raith). The SCU 020 is entirely self contained allowing independence from, and synchroneity with, the SEM of choice. LTSEM micrographs of specimen (that are fully frozen hydrated or partially freeze-dried) surfaces or fracture faces, without or with various metal coatings, can be examined over a broad temperature range (-150 to +50°C). This is made possible by the combined application of the two, independently controlled, cold stages and the on-line, high-vacuum, specimen cryo transfer between them. In-situ etching is simple and straightforward. Intramembranous particles and membrane fracture steps, typically imaged in transmission electron microscopy (TEM), are resolved by PM sputtering with platinum at low specimen temperature and high-resolution LTSEM in a field emission microscope.