Showing papers on "Transmission electron microscopy published in 1996"

Structure of MCM-48 Revealed by Transmission Electron Microscopy

Abstract: A model of the cubic mesoporous silicate MCM-48, based on the gyroid minimal surface, has been generated by computer simulation. Transmission electron micrographs and X-ray diffraction (XRD) patterns have been recorded for MCM-48 and compared with the images and XRD patterns obtained by modeling. The gyroid minimal surface gives an excellent description for the structure of MCM-48 with the amorphous hydroxylated silicate sited close to the midplane of the surface.

Formation of nanocrystals based on decomposition in the amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 alloy

Abstract: Primary crystallization and decomposition in the bulk amorphous alloy Ar41.2Ti13.8Cu12.5Ni10Be22.5 have been studied by small angle neutron scattering (SANS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). SANS data of samples annealed isothermally at 623 K exhibit an interference peak centered at q=0.46 nm(^-1) after an incubation time of about 100 min. TEM and DSC investigations confirm that the respective periodic variation in the scattering length density is due to the formation of nanocrystals embedded in the amorphous matrix. These observations suggest that during the incubation time a chemical decomposition process occurs and the related periodic composition fluctuations give rise to the observed periodic arrangement of the nanocrystals.

High‐frequency magnetic properties in metal–nonmetal granular films (invited)

Abstract: The structure and properties of Co–N and Co–O based films, prepared by rf magnetron reactive sputtering using nitrogen or oxygen and argon gases, have been studied. Transmission electron microscopy (TEM) observation reveals that each Co–(Al or Si)–(N or O) film is a typical film with granular structure, with grain size less than 5 nm. It is found by micro‐focused energy‐dispersive x‐ray and electron energy loss spectroscopy analysis that the grains are mainly composed of Co and the intergranular regions are ceramics of N or O. In Co–N based films, soft magnetic properties are found in both Si and Al containing films over a wide range of film preparation conditions and compositions. Only the films with Al show soft magnetic properties in Co–O based films, which have ρ of 500–1000 μΩ cm, Hk of about 80 Oe and Bs of about 10 kG. By adding about 10 at. % Pd, the soft magnetic properties and Hk of Co–O based films are significantly improved, with Hk more than 180 Oe. These films exhibit a remarkable constant f...

Ion-Stabilized Block Copolymer Micelles: Film Formation and Intermicellar Interaction

Abstract: Thin films of poly[styrene-block-(2-vinylpyridine)] with equally long blocks were prepared by casting micellar toluene solutions on a mica or carbon substrate. The structure of the films was studied by scanning force microscopy and transmission electron microscopy. Structural variations were observed depending on the concentration of the solution from which the films were obtained and depending on the neutralization of the pyridine units by HAuCl4. Scanning force microscopy as well as dynamic light-scattering experiments demonstrated an increasing stability of the inverse micelles upon ionization of the poly(2-vinylpyridine) core. While the micelles of polystyrene block poly(2-vinylpyridine) transformed into wormlike and lamellar structures upon film formation, the micellar structure was preserved when the poly(2-vinylpyridine) block was partly or fully neutralized by the gold acid. Scanning force microscopy revealed formation of an adsorbed, brushlike first layer on which the micelles assembled to form t...

Floc Architecture in Wastewater and Natural Riverine Systems

Abstract: The study of flocs has largely been devoted to the gross (>1 μm) scale so that the behavior of flocs (i.e., transport and settling) can be observed and modeled. With the assistance of a newly developed field kit and correlative microscopy [which includes transmission electron microscopy (TEM), scanning confocal laser microscopy (SCLM), and conventional optical microscopy (COM)], this paper begins to bridge the resolution gap between the gross and fine (submicron) scales in order to better understand the role of floc ultrastructure in outward floc behavior for both natural and engineered systems. Results from both systems have demonstrated that pores which appeared to be devoid of physical structures under the optical microscopic techniques (SCLM and COM) were observed to be composed of complex matrices of polymeric fibrils (4−6 nm diameter) when viewed by high-resolution TEM. These fibrils were found to represent the dominant physical bridging mechanism between organic and inorganic components of the floc...

Analysis of the Electrochemical Characteristics of a Direct Methanol Fuel Cell Based on a Pt‐Ru/C Anode Catalyst

Abstract: A vapor-feed direct methanol fuel cell based on a Nafion 117{reg_sign} solid polymer electrolyte was investigated. Pt-Ru/C and Pt/C catalysts were employed for methanol oxidation and oxygen reduction, respectively. The structure, surface, and morphology of the catalysts were investigated by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Crystalline face-centered cubic phases were found in the Pt and Pt-Ru catalysts. The alloy composition in the Pt-Ru/C catalyst was different from the nominal composition, probably due to the formation of surface RuO{sub x} species, as indicated by X-ray photoelectron spectroscopy. Transmission electron microscopy observation showed an increase of the average particle size and particle agglomeration in the Pt-Ru/C catalyst compared to the Pt/C catalyst. The membrane/electrode assembly was prepared by using a paste process method. Scanning electron microscopy and energy dispersive X-ray analyses showed good adhesion of catalyst layers to the membrane and a homogeneous distribution of the ionomer inside the catalyst. AC-impedance and galvanostatic steady-state polarization techniques were used to investigate the electrochemical performance of the direct methanol fuel cell.

Sonochemical preparation of amorphous nickel

Abstract: A method for the preparation of amorphous Ni powder with particle size of about 10 nm is reported. The sonolysis was performed on neat Ni(CO)4 and on its solution in decane. The nickel nanoparticles were subjected to scanning and transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and SQUID magnetization measurements. These measurements confirm the amorphous state of the particles.

Electron microscope study of Al-Fe-Si intermetallics in 6201 aluminum alloy

Abstract: The Al-Fe-Si intermetallics present in a commercial cast 6201 electrical conductor alloy have been studied using high resolution electron microscopy. The β-Al5FeSi phase is highly faceted and contains multiple (001) growth twins parallel to the growth direction. The α-Al8Fe2Si phase which forms in a Chinese script morphology has a nonfaceted interface with the aluminum matrix and exhibits no growth twinning. Formation of the β phase is believed to occurvia a peritectic decomposition of α-Al8Fe2Si at 612 °C. Observations made by transmission electron microscopy (TEM) support this hypothesis. When 30 ppm strontium is added to this alloy, the α phase is stabilized and very little β-Al5FeSi appears in the microstructure. A silicon-rich layer is found around the α-phase particles. It is proposed that strontium adsorbs to the α-phase interface, and in so doing, the diffusion of silicon into the α phase, necessary for its transformation to β, is prevented.

Transmission Electron Microscope Observation of “IR Scattering Defects” in As-Grown Czochralski Si Crystals

Abstract: Grown-in defects detected by IR laser scattering tomography (LSTDs) in Czochralski-grown Si crystals were identified for the first time by transmission electron microscopy (TEM) with a special defect positioning technique. The basic structure of each LSTD was revealed to be a composite of two or three incomplete octahedral voids with a total size of 100–300 nm. The TEM images of the defects suggest the existence of walls several nanometers thick surrounding the voids. A weak oxygen signal was detected from the defect by energy dispersive X-ray spectrometry. The thin walls surrounding the voids were considered to be made of SiOx .

Microstructure and Crystal Phases of Praseodymium Oxides in Zinc Oxide Varistor Ceramics

Abstract: The effect of sintering temperature on the microstructure and crystal phases of the intergranular praseodymium oxides in ZnO varistor ceramics was investigated using transmission electron microscopy and high-resolution electron microscopy. The ZnO grains were three-dimensionally separated from the intergranular praseodymium oxides. On the basis of microdiffraction analyses of the intergranular layer, the phase transformation from fcc-Pr6O11 into hcp-Pr2O3 was found when the sintering temperature increased from 1300° to 1350°C. The defect reaction equation and the decrease of donor concentration with increasing sintering temperature can verify the certainty of phase transition during the liquid-phase sintering observed by transmission elecron microscopy. Additionally, on the basis of the small variations of the breakdown voltage per grain boundary, the number of active grain boundaries is not a dominant factor for the donor concentration dependence on the sintering temperature.

Evidence of a Cholesteric Liquid Crystalline Phase in Natural Silk Spinning Processes

Abstract: A look inside the silk spinning process along the length of a silk gland has been achieved by the cryogenic quenching and subsequent microtoming of live silk-spinning animals, Nephila clavipes (spider) and Bombyx mori (silkworm). Observations made using transmission electron microscopy, electron diffraction and atomic force microscopy indicate a cholesteric liquid crystalline phase of aqueous silk fibroin in the early duct portion of the major silk-producing gland in both species. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide evidence for the cholesteric intermediate phase. The fracture surface produced by the diamond microtoming knife follows the twist of the director field, yielding thin sections with an undulating surface topography which produces a characteristic banding, on the order of 200−600 nm, in TEM and AFM images. Electron diffraction results also support the picture of the aqueous silk existing as a cholesteric at an intermediate stage in the spinning process.

The deformation behaviour of grain boundary regions in polycrystalline aluminium

Abstract: The deformation pattern at grain boundaries and at triple junctions in polycrystalline high purity aluminium (99.999%) has been studied by electron back scattering pattern (EBSP) observations. Specimens of two different grain sizes rolled to give 5% and 30% reductions have been examined by these different EBSP scans: (i) scans across grain boundaries, (ii) scans along grain boundaries and (iii) two-dimensional scans near triple junctions. These scans are carried out in small steps (1–5 μm) over long distances (up to 50 μm). The EBSP measurements show that the level of perturbations increases with strain and that enhanced zones of perturbations are observed at grain boundaries and especially near triple junctions. In specimens deformed by 30%, such zones of large perturbation are observed at most of the grain boundaries, and in the specimens deformed by 5%, at some triple junctions. The EBSP measurements are compared to previous microstructural observations by transmission electron microscopy and ...

Mechanistics of formation and ultrastructural evaluation of hepatocyte spheroids

Abstract: Freshly harvested rat hepatocytes form spheroids on uncoated positively charged polystyrene surfaces. Time lapse microscopy revealed that cell movement and reorganization were involved in spheroid formation. Ultrastructural evaluation using scanning and transmission electron microscopy indicated polarized cellular morphology and extensive cell-cell communication within spheroids. Bile canalicular structures were observed to surround each individual hepatocyte, forming an intricate three-dimensional continuous network of channels that appeared to end as pores/holes on the surface of the spheroid. The maintenance of differentiated cellular morphology coincided with preservation of hepatocyte viability and enhanced levels of tissue specific functions in spheroids.

Synthesis of Size-Monodisperse CdS Nanocrystals Using Phosphatidylcholine Vesicles as True Reaction Compartments

Abstract: Phosphatidylcholine vesicles provide reaction compartments for synthesis of size-quantized CdS nanocrystals of dimension predicted to within 2.5 A on the basis of initial encapsulated CdCl2 concentration and vesicle diameter. Vesicle formation by detergent dialysis of phosphatidylcholine/hexylglucoside mixed micelles yields highly monodisperse lipid capsules within which monodisperse CdS nanoparticles are precipitated with sulfide. Size-quantized CdS nanocrystals, with diameters ranging from 20 to 60 A, have been produced with typical standard deviations about the mean diameter of ±8%, as measured by transmission electron microscopy. Spectrophotometric and photoluminescence spectra are consistent with highly crystalline, monodisperse particles with few core or surface defects. Measured exciton energies show excellent agreement with data in the literature. The empirical pseudopotential model presented by Krishna and Friesner for a cubic CdS lattice, correcting for experimentally measured lattice contractio...

Epitaxial growth and magnetic behavior of NiFe2O4 thin films

Abstract: Thin films of NiFe2O4 were deposited on SrTiO3 (001) and Y0.15Zr0.85O2 (yttria-stabilized zirconia) (001) and (011) substrates by 90°-off-axis sputtering. Ion channeling, x-ray diffraction, and transmission electron microscopy studies reveal that films grown at 600 °C consist of ∼300 A diameter grains separated by thin regions of highly defective or amorphous material. The development of this microstructure is attributed to the presence of rotated or displaced crystallographic domains and is comparable to that observed in other materials grown on mismatched substrates (e.g., GaAs/Si or Ba2YCu3O7/MgO). Postdeposition annealing at 1000 °C yields films that are essentially single crystal. The magnetic properties of the films are strongly affected by the structural changes; unannealed films are not magnetically saturated even in an applied field of 55 kOe, while the annealed films have properties comparable to those of bulk, single crystal NiFe2O4. Homoepitaxial films grown at 400 °C also are essentially single crystal.

Ultra-microindentation of silicon at elevated temperatures

Abstract: Ultra-microindentations have been performed on {110} surfaces of silicon single crystals in the temperature range 20–600°C. The indents formed were observed with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The hardness determined from the measured load-displacement curves after correcting for the elastic recovery is almost independent of temperature up to 500°C, as is found for the Vickers hardness in the literature. The TEM images, electron diffraction patterns, SEM images and their variations with temperature are investigated in connection with high-pressure studies and plasticity data. A conclusion consistent with every observation is that below 500°C, the temperature-insensitive hardness is determined by the transformation to the metallic β-tin phase, which amorphizes or nanocrystallizes during unloading while, above 500°C, plastic deformation due to dislocation activity causes temperature-dependent hardness.

Preparation and characterization of magnesium–calcium hydroxyapatites

Abstract: Magnesium–calcium hydroxyapatite (MgCaHAP) solid solutions have been prepared by a wet method from aqueous solutions with different molar ratios, Mg/(Mg + Ca), ranging from 0 to 0.5. The MgCaHAP particles formed were characterized by XRD, FTIR, TEM, TCP, TG-DTA and gas adsorption techniques. The Mg/(Mg + Ca) ratios of the formed MgCaHAP particles were less than those of the starting solutions. With increasing Mg content, the particles became less crystalline and agglomerates of the fine crystals and finally the products were amorphous at Mg/(Mg + Ca)>0.31. The amount of irreversible adsorption of CO2 and CH3OH showed a minimum at a molar ratio (Mg + Ca)/P of ca. 1.56, less than the stoichiometric ratio of 1.67.

Morphological and structural transitions in GaN films grown on sapphire by metal-organic chemical vapor deposition

Abstract: The structural and morphological evolution of GaN films grown by MOCVD at high temperature (1080?C) on a low temperature grown GaN nucleation layer (NL) on (0001) sapphire were studied using atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The high temperature (HT) GaN layers were found to grow by initially forming isolated truncated hexagonal islands having {10*BAR*1*BAR*1} facet planes and a top (0001) plane. The non-wetting or partial wetting behavior of the HT GaN on the GaN NL is attributed to both the roughness and predominantly cubic nature of the NL.

Enhancement of electron emission efficiency of Mo tips by diamondlike carbon coatings

Abstract: The effect of diamondlike carbon (DLC) films coated by pulsed laser deposition technique on the electron emission characteristics of Mo tips is examined. Turn‐on voltage (V0) was lowered from 40 V for Mo tips to 22 V for DLC coated Mo tips and maximum anode current (IA) was increased from ∼44 μA for Mo tips to ∼2.0 mA for DLC coated Mo tips. Maximum anode current (IA) for the DLC coated Mo tips, however, decreased during operation. Raman spectroscopy and selected area diffraction (SAD) in transmission electron microscopy (TEM) revealed that the degradation of electron emission behavior can be ascribed to the conversion of sp3‐bonds, characteristic for diamond, to sp2‐bonds, characteristics for graphite. The transformation of the structure is assumed to be induced by the local heat from the DLC coatings.

Preparation and characterization of highly oriented, photoconducting WS2 thin films

Abstract: A novel combination of methods is shown to produce semiconducting WS2 thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS3 +x) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS2 films. X-ray diffraction shows that the films crystallize in the 2H-WS2 phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W18O49 needles andβ-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.

Oxygen diffusion through thin Pt films on Si(100)

Abstract: We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 A). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 A forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10−19 cm2/s with the precise value depending on the film morphology.

Carbon incorporation into si at high concentrations by ion implantation and solid phase epitaxy

Abstract: We have studied the incorporation of heavily supersaturated C into Si using solid‐phase epitaxy (SPE) of implanted amorphous layers. The strain in the Si1−xCx/Si heterostructures was measured using rocking curve x‐ray diffraction. The microstructure and defect introduction were examined using ion channeling and transmission electron microscopy (TEM). The fraction of C located on substitutional lattice sites in the Si was monitored using Fourier transform infrared absorption spectroscopy and ion channeling at resonance energies. Carbon‐depth profiles were monitored by secondary ion mass spectroscopy. The metastable solubility limit for the incorporation of C into Si by SPE was found to be 3.0–7.0×1020 atoms/cm3, which is over three orders of magnitude above the equilibrium solubility at the Si melting point. This limit was determined by the ability to regrow without the introduction of microtwins and stacking faults along {111} planes. We postulate the local bond deformation resulting from the atomic size ...

Visualization of crystal-matrix structure. In situ demineralization of mineralized turkey leg tendon and bone.

Abstract: A technique to correlate the ultrastructural distribution of mineral with its organic material in identical sections of mineralized turkey leg tendon (MTLT) and human bone was developed. Osmium or ethanol fixed tissues were processed for transmission electron microscopy (TEM). The mineralized tissues were photographed at high, intermediate, and low magnifications, making note of section features such as fibril geometry, colloidal gold distribution, or section artifacts for subsequent specimen realignment after demineralization. The specimen holder was removed from the microscope, the tissue section demineralized in situ with a drop of 1 N HCl, then stained with 2% aqueous vanadyl sulfate. The specimen holder was reinserted into the microscope, realigned with the aid of the section features previously noted, and rephotographed at identical magnification used for the mineralized sections. A one to one correspondence was apparent between the mineral and its demineralized crystal “ghost” in both MTLT and bone. The fine structural periodic banding seen in unmineralized collagen was not observed in areas that were fully mineralized before demineralization, indicating that the axial arrangement of the collagen molecules is altered significantly during mineralization. Regions that had contained extrafibrillar crystallites stained more intensely than the intrafibrillar regions, indicating that the noncollagenous material surrounded the collagen fibrils. The methodology described here may have utility in determining the spatial distribution of the noncollagenous proteins in bone.

Trigonal crystal structure of Bombyx mori silk incorporating a threefold helical chain conformation found at the air-water interface

Abstract: A new crystalline polymorph of Bombyx mori silk, which forms specifically at the air−water interface, has been characterized This new polymorph has a trigonal crystal structure and is distinctly different from the two previously observed silk crystal structures, silks I and II Our identification of this new silk polymorph is based on evidence from transmission electron microscopy and electron diffraction, coupled with molecular modeling Electron diffraction indicates that the crystal structure has a trigonal unit cell This structure consists of a hexagonal packing of chains, each of which assumes a three-fold helical conformation The resulting crystal structure is found to be similar to that observed for polyglycine II The sterics of the alanine and serine residues in the crystallizable segments of silk fibroin strongly favor a left-handed 3/2 helix over a right-handed 3/1 helix Electron diffraction from unoriented samples (powder-type diffraction) provides quantitative support for a left-handed po

Effects of lanthanum modification on rhombohedral pb(zr1-xtix)o3 ceramics : part i. transformation from normal to relaxor ferroelectric behaviors

Abstract: The interruption of long-range polar order in rhombohedral ferroelectricPb(Zr1−xTix)O3 (PZT) ceramics has been systematically studied by incorporating La onto the A-site of the perovskite (ABO3) structure for Zr/Ti ratios of 65/35 and 80/20 and various La contents. Studies have been performed by hot-stage transmission electron microscopy, dielectric spectroscopy, and Sawyer–Tower polarization (P-E) techniques. The evolution of a polar nanodomain state from a normal micron-sized domain state with increasing La content was observed. The emergence of this polar cluster state was characterized by the onset of strong frequency dispersion in the dielectric response, indicative of relaxor behavior. The La content that drives the structure into the relaxor state was found to be related to the lattice distortion of the undoped base composition.

Structural characterization of bulk GaN crystals grown under high hydrostatic pressure

Abstract: This paper describes TEM characterization of bulk GaN crystals grown at 1500–1800Kin the form of plates from a solution of atomic nitrogen in liquid gallium under high nitrogen pressure (up to 20 kbars). The x-ray rocking curves for these crystals were in the range of 20–30 arc-sec. The plate thickness along thec axis was about 100 times smaller than the nonpolar growth directions. A substantial difference in material quality was observed on the opposite sides of the plates normal to thec direction. On one side the surface was atomically flat, while on the other side the surface was rough, with pyramidal features up to 100 nm high. The polarity of the crystals was determined using convergent-beam electron diffraction. The results showed that, regarding the long bond between Ga and N along the c-axis, Ga atoms were found to be closer to the flat side of the crystal, while N atoms were found to be closer to the rough side. Near the rough side, within 1/10 to 1/4 of the plate thickness, there was a high density of planar defects (stacking faults and dislocation loops decorated by Ga/void precipitates). A model explaining the defect formation is proposed.

High-Pressure Synthesis of sp2-Bonded Carbon Nitrides

Abstract: We have synthesized amorphous sp2-bonded carbon nitrides at high pressure. A precursor composed of carbon and nitrogen, tetracyanoethylene, was compressed in a diamond anvil cell and laser heated to approximately 2000 °C. Transmission electron microscopy, electron diffraction, electron energy loss spectroscopy, and energy-dispersive X-ray analysis have been used to characterize the structure, bonding, and composition of the carbon nitrides. The amount of nitrogen incorporated into the network increases with pressure, ranging from 24% (C3N) at 18 GPa to 38% at 42 GPa (C3N1.9), which represents the most nitrogen-rich carbon nitride yet prepared under pressure. A crystalline carbon phase is also found.

Microstructural Characterization of Lithiated Graphite

Abstract: The microstructures of lithiated graphite were studied using high‐resolution transmission electron microscopy (HRTEM) and x‐ray diffraction (XRD). HRTEM shows lattice images of the (001) layers of with layer spacing of 3.70 A, consistent with XRD. The morphology and distribution of the and phases were investigated by dark field image and selected‐area electron diffraction in TEM. The results indicate that and phases can coexist in the lithiated graphite particle.