Field Emission Gun Research Articles

High quality transmission Kikuchi diffraction analysis of deformed alloys - Case study

Modern scanning electron microscopes (SEM) equipped with thermally assisted field emission guns (Schottky FEG) are capable of imaging with a resolution in the range of several nanometers or better. Simultaneously, the high electron beam current can be used, which enables fast chemical and crystallographic analysis with a higher resolution than is normally offered by SEM with a tungsten cathode. The current resolution that limits the EDS and EBSD analysis is related to materials' physics, particularly to the electron-specimen interaction volume. The application of thin, electron-transparent specimens, instead of bulk samples, improves the resolution and allows for the detailed analysis of very fine microstructural features. Beside the typical imaging mode, it is possible to use a standard EBSD camera in such a configuration that only transmitted and scattered electrons are detected. This modern approach was successfully applied to various materials giving rise to significant resolution improvement, especially for the light element magnesium based alloys. This paper presents an insight into the application of the transmission Kikuchi diffraction (TKD) technique applied to the most troublesome, heavily-deformed materials. In particular, the values of the highest possible acquisition rates for high resolution and high quality mapping were estimated within typical imaging conditions of stainless steel and magnesium-yttrium alloy.

Microstructural and Chemical Rejuvenation of a Ni-Based Superalloy

Abstract The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime morphology, size, and distribution after high-temperature degradation and subsequent rejuvenation heat treatments has been examined using field emission gun scanning electron microscopy and transmission electron microscopy. In this paper, it is shown that there are significant differences in the size of the ‘channels’ between gamma prime particles, the degree of rafting, and the size of tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical analysis has been carried out to compare rejuvenated and pre-service samples after the same subsequent degradation procedure. The results indicate that although the microstructures of pre-service and rejuvenated samples are similar, chemical differences are more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements was not completely eliminated through the applied rejuvenation heat treatment. A number of modified rejuvenation heat treatment trials were carried out to reduce the chemical segregation prior to creep testing. The creep test results suggest that chemical segregation has an immeasurable influence on the short-term mechanical properties under the test conditions used here, indicating that further work is required to fully understand the suitability of specific rejuvenation heat treatments and their role in the extension of component life in power plant applications.

Influence of electron-hole recombination on optical properties of boro-silicate glasses containing CdS quantum dots

Semiconductor (CdS) quantum dots were grown in a glass matrix (SiO2-B2O3-ZnO-Na2O-K2O) by controlled heat treatment. Growth of CdS quantum dots were confirmed by optical absorption (UV–vis), photoluminescence (PL), micro-Raman spectroscopy, X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM). The structural modification occur in glass matrix were studied with Fourier transform infrared spectroscopy (FTIR). Clusters formation due to heat treatment was studied by field emission gun scanning electron microscopy (FEG-SEM). The observed experimental data shows that the CdS QDs size is very well controlled by single stage heat treatment. Size dependent blue shift in optical energy band gap and blue energy shift in PL intensity was observed with annealing time. The overall study of these glasses suggests these glasses are useful as an optical filters.

Effect of graphene oxide sheet size on the curing kinetics and thermal stability of epoxy resins

This work revealed the influences of graphene oxide (GO) sheet size on the curing kinetics and thermal stability of epoxy resins. A series of GO/epoxy nanocomposites were prepared by the incorporation of three different sized GO sheets, namely GO-1, GO-2 and GO-3, the average size of which was 10.79 μm, 1.72 μm and 0.70 μm, respectively. The morphologies of the nanocomposites were observed by field emission gun scanning electron microscope. The dispersion quality of each sized GO was comparable in the epoxy matrix. The curing kinetics was investigated by means of differential scanning calorimetry and analyzed based on kinetics model. Addition of a small amount of GO (0.1 wt%) exhibited strong catalytic effect on the curing reaction of epoxy resin. The activation energy was reduced by 18.9%, 28.8% and 14.6% with addition of GO-1, GO-2 and GO-3, respectively. GO-2 with medium size (1.72 μm) showed the most effective catalysis on the cure. The thermal stability of the cured resins was evaluated based on thermogravimetric analysis. GO/epoxy nanocomposites showed improved thermal stability in the range of 420 °C–500 °C, compared with the pure resin. A ∼ 4% more residue was obtained in each of the incorporated system. The variations of GO sheet size did not influence the enhancement effect on the thermal stability.

Aerogel preparation from short cellulose nanofiber of the Eucalyptus species

Wood is the main industrial source for obtaining cellulose. It is a natural composite, constituted by cellulose, polyoses, lignin, small amounts of extracts and mineral salts, wherein cellulose is the most abundant component. Many studies are being developed for obtaining materials based on natural fibers, which combine interesting properties such as renewability, biodegradability, low density and low cost. Aerogels are solid, lightweight materials with high porosity and high internal surface area. These features combined in one single material make the aerogels a differentiated product with potential for use as an adsorbent. In this context, aerogels made of cellulose nanofibers obtained from short-fiber cellulose of Eucalyptus sp. were made. The cellulose suspension was first disintegrated by a mechanical grinder, and the aerogels were undergone to freeze-drying. The characterization of the samples was performed by apparent density, porosity, scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetric analyses. According to the micrographs obtained by scanning electron microscopy and field emission gun scanning electron microscopy, it was observed the formation of pores formed by the interconnection of cellulose fibers. The apparent density of the starting cellulose fibers (pressed plates) was 0.6998 g.cm−3 and the aerogel density decreased to 0.0240 g.cm−3. The values for aerogel porosity were about 97%, which benefits the passage of liquids and gases from the external environment to the internal structure of the material. Fourier transform infrared spectroscopy and thermogravimetric analyses showed no change in the chemical composition or in the thermal stability of the obtained aerogels in comparison to their starting materials.

Voltammetric determination of condensed tannins with a glassy carbon electrode chemically modified with gold nanoparticles stabilized in carboxymethylcellulose

This paper reports the electroanalytical determination of condensed tannins in extracts of Acacia (Acacia mearnsii de wild) using a glassy carbon electrode (GCE) modified with gold nanoparticles (AuNPs) and carboxymethylcellulose (CMC). The AuNPs were synthesized by the reduction of chloroauric acid with sodium borohydride using CMC as a stabilizer. The characterization of the AuNPs was carried out by UV–vis spectroscopy, transmission electron microscopy (TEM) and dynamic light scattering (DLS). The modified surface was characterized by scanning electron microscopy with field emission gun (SEM-FEG), atomic force microscopy (AFM) and profilometry experiments. Electrochemical studies were carried out by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and square-wave voltammetry (SWV). The results show that the nanoparticles obtained are spherical and have an average diameter of 5.3nm. The modified electrode was prepared by dropping the AuNPs-CMC dispersion onto the surface of the GCE. For the quantification of condensed tannins catechin was used as a model compound. The detection (DL) and quantification (QL) limits for catechin were 0.274 and 0.831μmolL−1, respectively. The proposed detector was tested by determining the total condensed tannins concentration in real samples using the standard addition method. The curves for the samples exhibited a slope similar to the calibration curve, indicating the absence of interference from the matrix components in the response to condensed tannins. The modified electrode was successfully applied to the analysis of real samples, with results comparable to those obtained using the Folin-Ciocalteu method. Thus, the strategy used in this study creates new opportunities for the sensitive detection of condensed tannins in plant extracts widely used in various industrial segments.

Change in thermoluminescence behaviour of cubic Gd2O3:Yb3+ phosphors with successive increase in Yb3+ ion concentrations

Nanocrystalline Yb3+ doped Gd2O3 phosphors were synthesized by solid state reaction method (SSRM), and effect of Yb3+ concentrations (1mol% to 7mol%) on thermoluminescence properties were studied. The XRD technique shows the cubic structure of prepared phosphor for different concentrations of Yb3+. No phase change was found due to the dopant concentration. The crystal size was calculated by Scherer's formula, which is found in 20–60nm range. The same was also confirmed from the field emission gun scanning electron microscopy (FEGSEM) image, which shows the formation of nanospheres having diameter 20–60nm range. They are found to be quite uniform in shapes and sizes. Functional group analysis was studied by Fourier transform infrared spectroscopy (FTIR) techniques and the elemental analysis of prepared phosphor was studied by energy dispersive X-ray spectroscopic (EDX) analysis. The samples were irradiated with UV and gamma radiation for the doses varying from 5 to 30min for UV and 0.5kGy to 2kGy for gamma respectively and their thermoluminescence (TL) characteristics have been studied. The effect of heating rate (3–7°Cs−1) and effect of Yb3+ concentration in TL glow curve have been studied. The TL glow curve of nanophosphor shows a peak at around 130°C for UV and at 247°C for gamma irradiation respectively. The TL response of the sample irradiated with different gamma doses shows a linear behaviour from 0.5kGy to 2kGy and become sublinear behaviour in the range of 5–25min UV exposure before it saturates with further increase in the dose. Simple glow curve structure, easy method of synthesis and the linear dose response make the nanocrystalline phosphor a good candidate for radiation dosimetry, especially for the estimation of high doses of gamma rays where the nanocrystalline phosphors generally saturated.

(Invited) New Benzoxazine Resins Coated on Anodized Aluminium Alloys: Electrochemical Behaviour of Efficient Corrosion Protection Systems

Aluminum alloys are widely used in aircraft applications, especially series 2xxx (Cu) and 7xxx (Ti). Those alloys offer improved mechanical properties compared to pure aluminum, but are strongly sensitive to corrosion. Chromic anodizing combined with chromate containing epoxy primer was used as an efficient system to prevent corrosion. Nevertheless, due to health and environmental issues, this treatment is going to be forbidden. Alternative solutions have to be developed to reach aircraft standards considering health and environmental concerns. This work aims at proposing an alternative protective system, consisting in an oxide layer obtained from sulfotartaric anodizing (TSA) and coated with benzoxazine organic layers. Benzoxazine resins offer high chemical resistance and low shrinkage and so are very good candidates for coating applications. Investigated layers have been prepared on both bare and clad Al-2024 T3 substrates. The morphology and the thickness of coating systems were determined using Field Emission Gun Scanning Electron Microscopy (FEG-SEM). Electrochemical Impedance Spectroscopy (EIS) was used to characterize the electrochemical behaviour of obtained systems and separated layers in order to emphasize synergetic effects. The AC/DC/AC electrochemical tests were carried out to evaluate the cathodic disbonding sensitivity of these new coating systems. The results were fitted by Equivalent electrical circuits to model the electrochemical properties of different layers and the evolution of their properties over immersion time in an aggressive saline solution and during AC/DC/AC cycles. The results confirmed the strong potential of such systems for future corrosion protection applications.

Durable, Washable, and Flexible Conductive PET Fabrics Designed by Fiber Interfacial Molecular Engineering

Conductive fibers and fabrics are an essential part of wearable electronics and devices. How to fabricate durable conductive substrate is still a huge challenge to the application and commercialization of wearable electronics. Here an effective and economic process to obtain conductive yarns and fabrics localized copper plating onto synthesized polymers: poly(ethylene terephthalate) (PET) is reported. The active interface is created via plasma treatment to generate active chemical groups on PET substrate, which react with atomic transfer radical polymerization initiator for the following polymerization. The Pd ion entrapped by polymer brushes are then reduced in situ and the Pd0 species act as a catalytic seed layer for following electroless copper nanoparticles growth on the active interface. Scanning electron microscopy, X‐ray diffraction, field emission gun scanning electron microscopy, and atomic force microscopy are used to characterize the samples in this process, and the copper loading is quantified by weight. The morphology and composition analysis shows that the copper coating on PET fabrics is compact and continuous, which leads to excellent electronic conductivity. The copper coating obtained in this polymer brushes bridged process can pass through the hand washing challenge, and shows excellent adhesion with PET substrate.image

Effect of Boron Addition on Microstructural Evolution and Room-Temperature Mechanical Properties of Novel Fe66−x CrNiB x Si (x = 0, 0.25, 0.50 and 0.75 Wt Pct) Advanced High-Strength Steels

In this study, the Vickers hardnesses and room-temperature uniaxial tensile behaviors of four Fe66−x CrNiB x Si (x = 0 (0B), 0.25 (25B), 0.50 (50B), and 0.75 (75B) wt pct) advanced high-strength steels (AHSSs) in the as-hot-rolled and heat-treated (1373 K (1100 °C)/2 h + 973 K (700 °C)/20 min) conditions were investigated. Microstructural evolution after solidification, hot rolling, heat treatment, and uniaxial tensile tests of 0B, 25B, 50B, and 75B AHSSs was also characterized using field emission gun scanning electron microscopy and X-ray diffraction. The tensile behaviors of the 0B, 25B, 50B, and 75B AHSSs were manifested by an excellent combination of strength and ductility over 34.7 and 47.1 GPa pct, 36.9 and 42.3 GPa pct, 45.9 and 46.4 GPa pct, and 11.9 and 47.8 GPa pct, respectively, arising from microband-induced plasticity in the 0B, 50B, and 75B AHSSs and transformation-induced plasticity in the 25B specimens. All specimens in the as-hot-rolled and heat-treated states showed an austenitic matrix grain. Adding boron to the base alloy (0B) resulted in grain refinement, M2B dispersion, precipitation hardening, and solid solution strengthening, which led to an increase in strength. The results of the present work show promise for automotive applications that require excellent properties and reduced specific weight.

Electrochemical sensor based on reduced graphene oxide modified with palladium nanoparticles for determination of desipramine in urine samples

A nanostructured material based on palladium nanoparticles supported over reduced graphene has been synthesized, characterised and applied on the sensitivity analysis of antidepressant in urine samples. The morphology, structure, and electrochemical performance of the sensors were characterised by field emission gun scanning electron microscopy, X-ray diffraction, and cyclic voltammetry. The antidepressant quantification process was based on the reversible electrode oxidation of the electrochemically produced desipramine dimer, that appeared as oxidation process at +0.11V. Differential pulse voltammetry in phosphate buffer solution at pH 7.0 allowed the development of a method to determine desipramine levels in the range of 0.3–2.5μmolL−1, with limit of detection (LOD) estimated at 1.04nmolL−1(0.31μgL−1). The system was suitable on the detection of desipramine in urine samples.

Removal Torque and Biofilm Accumulation at Two Dental Implant-Abutment Joints After Fatigue.

The aim of this study was to evaluate the removal torque and in vitro biofilm penetration at Morse taper and hexagonal implant-abutment joints after fatigue tests. Sixty dental implants were divided into two groups: (1) Morse taper and (2) external hexagon implant-abutment systems. Fatigue tests on the implant-abutment assemblies were performed at a normal force (FN) of 50 N at 1.2 Hz for 500,000 cycles in growth medium containing human saliva for 72 hours. Removal torque mean values (n = 10) were measured after fatigue tests. Abutments were then immersed in 1% protease solution in order to detach the biofilms for optical density and colony-forming unit (CFU/cm²) analyses. Groups of implant-abutment assemblies (n = 8) were cross-sectioned at 90 degrees relative to the plane of the implant-abutment joints for the microgap measurement by field-emission guns scanning electron microscopy. Mean values of removal torque on abutments were significantly lower for both Morse taper (22.1 ± 0.5 μm) and external hexagon (21.1 ± 0.7 μm) abutments after fatigue tests than those recorded without fatigue tests (respectively, 24 ± 0.5 μm and 24.8 ± 0.6 μm) in biofilm medium for 72 hours (P = .04). Mean values of microgap size for the Morse taper joints were statistically signicantly lower without fatigue tests (1.7 ± 0.4 μm) than those recorded after fatigue tests (3.2 ± 0.8 μm). Also, mean values of microgap size for external hexagon joints free of fatigue were statistically signicantly lower (1.5 ± 0.4 μm) than those recorded after fatigue tests (8.1 ± 1.7 μm) (P < .05). The optical density of biofilms and CFU mean values were lower on Morse taper abutments (Abs630nm at 0.06 and 2.9 × 10⁴ CFU/cm²) than that on external hexagon abutments (Abs630nm at 0.08 and 4.5 × 10⁴ CFU/cm²) (P = .01). The mean values of removal torque, microgap size, and biofilm density recorded at Morse taper joints were lower in comparison to those recorded at external hexagon implant-abutment joints after fatigue tests in a simulated oral environment for 72 hours.

Mechanical activation-assisted combustion synthesis of in situ aluminum matrix hybrid (TiC/Al2O3) nanocomposite

The aim of this study is the synthesis of aluminum-based metal matrix nanocomposites reinforced with in situ TiC and Al2O3 hybrid reinforcements by the mechanically activated combustion synthesis. The composites were fabricated from the powder blends consisting of aluminum, rutile and graphite with an excess amount of aluminum following the 14Al–3TiO2−3C system and milled up to 30h. Phase evolutions and structural changes during ball milling and combustion synthesis were studied by X-ray diffraction technique, field-emission gun scanning electron microscopy and transmission electron microscopy. The results showed that even after 10h of milling, no new phases were formed. The increasing of milling time caused the broadening of all the peaks indicating a decrease in the crystallite size and an increase in the lattice strain. These results showed that Al, C and TiO2 nanocrystallites could be obtained during the ball-milling process. The result of combustion synthesis of un-milled powders confirmed that no new phases were found. In comparison with un-milled powder mixture, the 10h milled powder could be easily ignited and XRD, HRSEM and TEM results confirmed that Al/TiC–Al2O3 nanocomposite was successfully synthesized through combustion method from the mechanical activated powder mixture. Mechanical activation via high energy ball-milling provided to the initial powder mixture extra energy, which is needed to increase the reactivity of powder mixture and to make possible the ignition and the sustaining of combustion.

Development of high-resolution cathodoluminescence system for STEM and application to plasmonic nanostructures.

A high-resolution cathodoluminescence (CL) system for scanning transmission electron microscope (STEM) has been developed by employing a field emission gun and a spherical aberration corrector, which realizes a probe size of 1 nm even at an accelerating voltage of 80 kV and beam current of the order of 1 nA. Angle resolved measurement of light emission from a sample in the STEM is possible by combining a parabolic mirror and position-controlled pinhole. CL spectra are successively acquired by a highly sensitive charge-coupled device while scanning the incident electron beam or pinhole, which enables various detection modes, i.e. (i) angle resolved spectral pattern, (ii) beam scan spectral image and (iii) photon map. In order to calibrate the acquired spectrum, the correction function is created from the comparison between the observed and theoretical spectra of the transition radiation. Furthermore, the modification of polarization by the parabolic mirror is discussed. Some examples of the applications of the STEM-CL system to plasmonics are presented to demonstrate the unique measurement features of the CL system, i.e. (i) multipole modes in silver nanoparticles, (ii) surface plasmon polariton modes in a 2D plasmonic crystal and (iii) localized surface plasmon modes in a gold bow tie nano-antenna.

An experimental and micrographical investigation on aluminum nano metal matrix composites

In the present investigation, a comparative study using X-ray mapping analysis and Field emission gun scanning electron micrographs is performed to understand the distribution and the mechanical properties of aluminum nano metal matrix composites. Parameters considered for comparison have two different forms for adding nanoreinforcement into metal melt. One form is produced by the addition of mechanically alloyed powders with an increasing launching vehicle weight percentage (L-1, L-3 and L-5), and the other form is produced by pellets of mechanically alloyed powders (PL-1). Micrographs reveal uniform distribution of nanoreinforcements, while X-ray mapping observations show Iron (Fe) contamination due to the addition of pellets in some areas unlike the mechanically alloyed powders. L-5 is observed to attain the highest tensile strength of 202 MPa for the Al-Cu/1.5 wt. % Al2O3 composite. The results illustrate an increase in composites strength with increase in launching vehicle content but on the expense of nanoreinforcement particle rejection from the melt.

Tribological behavior of zirconia-reinforced glass–ceramic composites in artificial saliva

The purpose of this study was to characterize the wear behavior of dental glass–ceramic composites reinforced with different contents of zirconia particles (10–50%). The tests were carried out using a ball-on-plate configuration, in artificial saliva at body temperature (37°C). The worn surfaces were characterized by field emission guns scanning electron microscope. The results demonstrated that zirconia plays a relevant role on wear behavior of zirconia reinforced glass–ceramic composites. The optimal wear properties were obtained for the glass–ceramic reinforced with 30% (vol%) of zirconia particles. The improvement on wear resistance was mainly attributed to the good dispersion of the reinforcement phase, which allowed the load transfer reinforcing mechanism to effectively occur from the matrix to the reinforcement.

Fatigue crack growth behavior of a type of novel advanced high-strength steel in a FeCrNiBSi alloy system: A comparison between heat-treated cast and hot-rolled specimens

The fatigue crack growth (FCG) behavior of a type of novel advanced high-strength steel in a FeCrNiBSi alloy system was investigated at room temperature. Two heat-treated cast and hot-rolled specimens were used to compare their FCG rate results obtained at a load ratio of 0.2 and at a frequency of 5Hz. FCG paths and fracture surfaces of the specimens in the Paris regime, as well as unstable fracture surfaces, were observed by field emission gun scanning electron microscopy. The results showed that besides the dimension of matrix grains (austenite), the volume fraction, dimension, and morphology of second hard phases (boride phases) were also main factors in retarding crack propagation in the Paris regime. Fatigue striations and tearing ridges were observed in the fracture surfaces of both heat-treated cast and hot-rolled specimens, while quasi-cleavage facets and secondary cracks were only observed in the heat-treated cast and hot-rolled specimens, respectively. Roughness-induced, crack branching, and plasticity-induced crack closure mechanisms were more effective in retarding crack propagation in the heat-treated hot-rolled specimen than in the heat-treat cast specimen.

Synthesis and characterization of Gd2O3 doped UO2 nanoparticles

UO2 nanoparticles doped with 4, 8, 10 and 15 wt% Gd2O3were synthesized by a reverse strike method. Crystal structure and chemical homogeneity were evaluated using a combination of X-ray diffraction and microscopy tools. An exhaustive study of the composition and its homogeneity at the micro and at the nanometer level was carried out in this nuclear fuel material. Field Emission Gun Scanning and Transmission Electron Microscopy images revealed the presence of micrometer scale agglomerates of nanoparticles, with rounded morphology and an average crystallite size of 100 nm. Rietveld refinements of high-statistic X-ray Diffraction data determined the crystal structure and composition. Furthermore, Energy Dispersive Spectroscopy using a 2 nm2 spot on the sample surface determined a Gd concentration variation around the average of ±5% in different spots of a single crystallite and of ±10% between different crystallites. However, when measuring large amounts of nanoparticles the concentration averages, producing a homogeneous composition distribution at the micrometer scale.

Thermal analysis as screening technique to assess spray-drying process of encapsulated “yacon” juice

“Yacon” is a plant from Andean region (South America) whence it has spread to other countries. It produces roots with high content of fructooligosaccharides, inulin, some phenolic compounds among others substances. With sweet taste and low-energetic value, these roots are considered prebiotic food due to some medicinal properties. The encapsulation process via spray drying was used in this work from a pilot plant with aim to obtain a juice from this root (YJ). Solid whey (W) and maltodextrin (MD) was added in different ratios. Optimisation conditions were determined, and the best encapsulation process was YJ with MD:W ratio 80:20, which results are in agreement with those obtained by thermogravimetry/derivative thermogravimetry, differential scanning calorimetry and confirmed by field emission gun scanning electron microscopy. Thus, thermoanalytical techniques are important tools that can be used in quality control of industrial encapsulation process.

Efficiency of the compression with oscillatory torsion method in grain refinement in Al

Experiments were conducted to investigate the development of an ultrafine grain structure during compression with oscillatory torsion processing of high-purity aluminum (99.9%) with an initial grain size of 75μm. The samples were processed using different deformation parameters: torsion frequency (f) and compression rate (v). The samples were examined using a scanning electron microscope equipped with a field emission gun and an electron backscattered diffraction detector. The results suggest that for high-purity aluminum an ultrafine-grained microstructure was obtained after a total effective strain (ɛft) of 45 in samples deformed at f=1.6Hz and v=0.04mm/s. A quantitative study of the microstructural parameters showed that the area fraction of the ultrafine grains (<1μm) (A1μm) was 44%, the fraction of high-angle boundaries was 53%, and the average diameter of the grains was about 600nm. The yield stress and ultimate tensile stress reached 127 and 137MPa, respectively, after deformation at a total effective strain of 45. When the total effective strain reached 120, the mechanical strength of the material decreased. This suggests that the decrease in strength is associated with the operation of the recovery mechanism that decreases the boundary volume.