Study of two tungstates Ca0.5Cd0.5WO4 and Ca0.2Cd0.8WO4 by transmission electron microscopy

To better understand the role of crystal structures and local disorder in the photonic properties of the system (1 ? x)CaWO₄ ? xCdWO₄ with 0 < x < 1, two specific phases with compositions x = 0.5 (scheelite phase) and 0.8 (wolframite phase) have been studied by scanning and transmission electron microscopies. High‐resolution electron microscopy images and image simulations, associated with X‐ray diffraction data, allowed confirming the lattices and space groups I4₁/a and P2/c of the two scheelite and wolframite phases, at the local scale. The electron microscopy data show the existence of a high degree of crystallization associated with statistical distribution of Ca or Cd atoms on a Ca_1?xCd_x site in each lattice.     

Dispersion properties of aqueous-based LiFePO4 pastes and their electrochemical performance for lithium batteries

Aqueous-based LiFePO₄ pastes to fabricate the cathode of lithium-ion battery were investigated with an emphasis on chemical control of suspension component interactions among LiFePO₄ particulates, carbon black, carboxymethyl cellulose (CMC), and poly(acrylic acid) (PAA). The dispersion properties of LiFePO₄ were characterized using electroacoustic, flow behavior and green microstructural observation. Correlation was made between the dispersion properties and electrochemical performance of the particles. It was found that the addition of PAA significantly decreases the viscosity of the LiFePO₄ paste. The decrease of viscosity leads to increasing the solid concentration, which affects the electrochemical properties. The electrochemical characteristics of formulated pastes were evaluated using coin-type half cells. Although there is no significant difference between coin cells fabricated with CMC only and CMC/PAA combination in electrochemical cycling test, the dispersion properties of pastes indicate that the electrode fabricated with CMC/PAA, potentially, has much improved discharge capacity compared to that with CMC alone because of the possibility to increase active mass portion in electrode paste.     

Application of convergent beam electron diffraction in the structural study of high-temperature superconducting oxides

Convergent beam electron diffraction (CBED) is a powerful technique for symmetry study of crystal. It has widespread application in physics and material sciences, as demonstrated in a recent superconducting oxide study. Using this technique, we have studied Ba-La-Cu-O superconductors with a transition temperature of about 40°K and Ba-Y-Cu-O superconductors with a critical temperature (Tc) of about 90°K. We have found that in Ba-La-Cu-O superconductors the superconducting phase La_2?xBa_xCuO_4?y has a distorted K₂NiF₄-type structure and the space group Fmmm. The two other phases in Ba-La-Cu-O superconductors have also been studied. In our Ba-Y-Cu-O superconductors, the Ba₂YCu₃O_7?x compound, which is responsible for 90°K superconductivity, has two different space groups: An orthorhombic space group Pmmm and a tetragonal space group P4mm or P4/mmm.     

Structure and morphology of Mg–Al–Fe‐mixed oxides derivedfrom layered double hydroxides

The influences of the nature and the extent of M(III) ion substitution on the structure, morphology and surface properties of layered double hydroxides, LDHs [Mg_1?x M(III)_x(OH)₂](CO₃)_x/n·mH₂O, M(III) being Al or/and Fe and x= M(III)/[(Mg+M(III)], and derived mixed oxides were investigated. Three series: Mg?Al, Mg?Al?Fe and Mg?Fe were synthesized using low supersaturation co‐precipitation method at constant pH, with different Mg : Al : Fe ratio and x in the wide range from 0.15 to 0.7 in order to obtain complex, multi‐phase systems with disordered structure, developed surface area, acid–base and redox properties favourable for catalytic application. The morphology of LDHs and their derived mixed oxides did not change considerably although pronounced changes in structural and surface properties occur by thermal decomposition. The increase in Al amount, as well as the deviation of M(III) content from the optimal range for the single LDH phase synthesis, causes the formation of smaller particles and decrease of mixed oxide crystallite size. The nature and amount of M(III) influence the development of surface area, after thermal treatment, depending mainly on the presence of smaller mesopores, not visible by scanning electron microscope. Although the particle size has no considerable influence on the value of the surface area, it was observed that the samples with smaller particles (Mg–Al and Mg–Al–Fe series) have also higher surface area compared with the samples with larger particles (Mg–Fe series).     

The study on the properties of TiCxN1−x coatings processed by cathodic arc physical vapour deposition

Titanium carbonitride (TiCN) is a popular hard coating for carbide cutting tools in various applications. The properties of TiCN are within its composition and can be controlled by maintaining the C–N ratio within the coating to a certain level. This paper studied the influence of carbon content and coating composition within TiC_xN_1?x coatings with regard to their mechanical properties. The substrate used was tungsten carbide (WC-6Co), which was prepared in-house through a powder metallurgy process, while the TiC_xN_1?x coatings were deposited in-house using cathodic arc physical vapour deposition (CAPVD). TiC_xN_1?x coatings improved the mechanical properties of carbide inserts. An increase in carbon content within TiC_xN_1?x coatings improved surface lubricity, reduced coefficient of friction, improved surface microhardness and increased Young's modulus, but reduced thermal conductivity of carbide inserts. The colour of TiC_xN_1?x coatings also changed with carbon content.     

The effect of surface treatments on the microroughness of laser-sintered and vacuum-cast base metal alloys for dental prosthetic frameworks

This study aimed to evaluate the effect of four chemomechanical surface treatments on the surface average microroughness and profile of laser‐sintered and vacuum‐cast dental prosthetic structures. Square‐shaped blocks (10 mm × 10 mm × 1.5 mm) were prepared as follows: (1) laser‐sintered Co? Cr (L) (ST2724G); (2) cast Co? Cr (C) (Gemium‐cn); and (3) cast Ni? Cr? Ti (T) (Tilite). Specimens of each alloy group were randomly divided into five subgroups (n = 10 each), depending on the conditioning method used: (1) no treatment (control); (2) sandblasting (125 μm Al₂O₃‐particles); (3) silica coating (50 μm silica‐modified Al₂O₃‐particles); (4) oxidation; and (5) oxidation plus opacification. Subgroups 2 and 3 represent “inner” pretreatments proposed for ceramometal restorations to improve the metal surface area available for luting cements. Subgroups 4 and 5 are the “outer” pretreatments required for bonding the aesthetic veneering ceramics to the underlying metal frameworks. Average surface roughness (Ra/μm) was determined using a surface profilometer. Data were analyzed by two‐way ANOVA and Student–Newman–Keuls tests (α = 0.05). Metal surface topography was SEM‐analyzed. Despite the inner pretreatment applied, L samples resulted in the highest microroughness (P < 0.001), whereas sandblasting produced a surface‐smoothing effect in cast specimens. After oxidation, a significant increase in surface roughness occurred in all groups compared with controls, L specimens being the roughest (P < 0.001). Opacification caused a flattening effect of all oxidized structures; all opacified groups resulting in similar microroughness. Laser sintering of Co? Cr enhances the roughness of metal structures, which may improve the frameworks' microretention of the cements, and of the opaquer before the copings are veneered with the aesthetic ceramics. Microsc. Res. Tech. 75:1206–1212, 2012. © 2012 Wiley Periodicals, Inc.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

Structural analyses of Nd-doped CeO2 thin films deposited by pulsed laser deposition

CeO₂ thin films doped with neodymium oxides for application to gas sensors have been elaborated by the pulsed laser deposition technique. The films were deposited on orientated Si (100) substrates with variable deposition times (t = 90, 180 and 360 s) and molar fractions of Nd₂O₃ (0, 6.5, 15, 21.5 and 27 at.%). The resulting Nd–CeO₂ thin films were characterized by means of X‐ray diffraction analysis, scanning electron microscopy and transmission electron microscopy equipped with EDS (Energy Dispersive Spectrometer) microanalysis. From X‐ray diffraction analyses, it is clearly established that the texture is modified by Nd additions. The preferred (111) orientations of the CeO₂ crystals change into the (200) orientation. The morphology of the CeO₂ grains changes from triangles, for pure CeO₂ thin films, to spherical grains for Nd‐doped films. In addition, cell parameter analyses from X‐ray diffraction data show that a partial chemical substitution of Ce by Nd should occur in the face‐centred cubic lattice of ceria: this should give rise to Ce_1‐xNd_xO_2?z phases with oxygen non‐stoichiometry.     

Tribological evaluation of α, -diimidazoliumalkylene hexafluorophosphate ionic liquid and benzotriazole as additive

Ionic liquids of α, -diimidazoliumalkylene hexafluorophosphate were synthesized. The tribological properties of the synthetic ionic liquid and the ionic liquid contained additive for contacts of steel/steel were investigated by Optimol SRV oscillating friction and wear tester under ambient conditions. The synthetic ionic liquid presented low friction coefficients and small wear volumes, especially under higher temperatures. The ionic liquid doped with benzotriazole (BTA) showed excellent anti-wear ability. The worn surfaces and chemical nature of the boundary films generated on the metal surfaces were analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). SEM results showed slight abrasion on the worn surfaces and XPS results indicated the formation of FeF₂, FeF₃, Fe₃O₄, and FePO₄ by the tribochemical reactions of ionic liquid with iron during the sliding process.     

Control of a pulse combustion reactor with thermoacoustic phenomena

We have developed a novel way of controlling parameters in reactors using flue gases from pulse combustion as a direct source of heat and as a means of transport of particulate materials synthesized in a slightly reductive environment or oxidative environment. The reactor is used for a spray pyrolysis synthesis of materials or the sintering of different ceramic powders. The reactor is heated directly, which means reduced energy losses, and the enhanced drying reported using pulse combustion is implemented for faster reaction. A slightly reductive atmosphere is maintained by combusting a stoichiometric fuel and air mixture and adding acetylene to the flue gas flow. Reaction conditions must be carefully controlled; this is achieved by influencing the characteristic times of the pulse combustion and changing the frequency of combustion and with it the temperature and flue gas composition in the reactor. The frequency is changed by nitrogen dilution of burning gas, influencing the mixing of the combustible mixture with hot flue gases and damping of frequencies with a secondary Helmholtz resonator. The frequency of pressure oscillations in the combustor should be the same as one of the harmonic frequencies of the reactor pipe to reach an acoustic resonance. In this work, the frequency of pulse combustion was altered in such a way that resonance was established with the reactor pipe and that a suitable reaction environment was obtained. With good control over all parameters, we were able to synthesize different Li-ion cathode materials, such as LiFePO₄ and Li(Ni_xMn_yCo_z)O_2.     

Synthesis and Tribological Performance of Novel MoxW1?xS2 (0?≤?x?≤?1) Inorganic Fullerenes

Inorganic fullerene-like (IF) MoS₂ and WS₂ nanoparticles were found to be good friction modifiers and anti-wear additives when dispersed in a lubricant. Their tribological performance seems to be related to the structure, size, and shape of these nanomaterials. The present study describes the tribological properties of a new inorganic fullerene IF-Mo _x W_1−x S₂ containing both molybdenum and tungsten disulfide under boundary lubrication. Mo _x W_1−x S₂ amorphous inorganic fullerene nanostructures were synthesized by means of MOCVD using an induction furnace setup. The average diameters range from 25 to 45 nm. Upon variation of the amounts of precursors and S, various solid solutions of IF-Mo _x W_1−x S₂ were obtained. In addition, a morphological, chemical, and structural analysis of the samples was performed using high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). Friction experiments were carried out with a ball-on-flat contact using an environmental tribometer. The results show interesting friction reducing and wear properties of these nanomaterials. The change in the molecule stoichiometry, which led to a variation of particles size but also to a variation of the crystallinity of the particles, affects the tribological performance.     

Electron energy-loss near edge structure (ELNES) of InGaN quantum wells

Lasers and light‐emitting diodes (LEDs) that emit in the blue to green region are often based on In_xGa_1–xN quantum well structures. Ionization edges in the electron energy‐loss spectrum contain fine structures (called the energy‐loss near edge structure (ELNES)) and provide information about the electronic structure. In this paper we compare the experimental and calculated ELNES for the N‐K ionization edge of In_xGa_1–xN quantum wells. When the effects of the core‐hole are included in the calculations, agreement between experimental and calculated spectra is very good. Strain has been shown to accentuate the effects of In on the ELNES and moves the ionization edge onset down in energy, relative to the other features. These results suggest that ELNES may provide an alternative method to lattice imaging to determine the presence of strain in this system.     

Influence of Deposition Positions on Fretting Behaviors of DLC Coating on Ti-6Al-4V

Abstract

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a fretting wear test rig with a cylinder-on-flat contact. The results indicated that, for tests without coating (Ti-6Al-4V–Ti-6Al-4V contact), the friction (Q_max/P) was high (0.8–1.2), wear volumes were large (0.08–0.1?mm³) under a large displacement amplitude of ±40 µm and small (close to 0) under a small displacement amplitude of ±20 µm, and the wear debris was composed of Ti-6Al-4V flakes and oxidized particles. For tests with the DLC coating, under low load conditions, the DLC coating was not removed or was only partially removed, Q_max/P was low (≤0.2), and the wear volumes were small. Under high load conditions, the coating was entirely removed, Q_max/P was high (0.6–0.8), and the wear volumes were similar to those in tests without coating. The wear debris was composed of DLC particles, Ti-6Al-4V flakes, and oxidized particles. The DLC coating was damaged more severely when deposited on a flat surface than when deposited on a cylindrical surface. The DLC coating was damaged more severely when sliding against a DLC-coated countersurface than when sliding against the Ti-6Al-4V alloy.     

Effect of different calcium contents on the microstructure and mechanical properties of Mg‐5Al‐1Bi‐0.3Mn magnesium alloy

The effect of different Ca contents on the microstructure and mechanical properties of Mg‐5Al‐1Bi‐0.3Mn (AMB501) magnesium alloys was investigated by conventional melting and casting technique using different Ca contents (1.0, 2.0, and 3.0 wt %). Increasing the Ca content resulted in higher hardness and yield strength, but decreased elongation. The improved tensile properties of the AM50‐1Bi‐xCa alloys were due to the changes in AMB501 alloy microstructure when the Ca content increased, as demonstrated by scanning electron microscope, energy dispersive spectrum, and X‐ray diffractometer. The alloy microstructure indicated that the amount of β‐Mg₁₇Al₁₂ phase on grain boundaries decreased and the morphology of β‐Mg₁₇Al₁₂ phase on grain boundaries changed from quasicontinuous‐net shape to dispersed particles. The Mg₁₇Al₁₂ phase disappeared and a new secondary phase Al₂Ca appeared after a 3.0 wt % Ca addition. Microsc. Res. Tech. 78:65–69, 2015. © 2014 Wiley Periodicals, Inc.     

Microstructure and electrical properties of NaNbO3-BaTiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x) NaNbO3-xBaTiO₃ have been fabricated by a traditional ceramic sintering technique. The effects of BaTiO₃ (BT) synthesized by hydrothermal method on crystal structure, density, dielectric, piezoelectric, and electromechanical properties were investigated. Results show that the phase structure transforms from the orthorhombic phase to the tetragonal phase with the increase of the content of BT, and the two phases co-exist when 0.08<×⩽0.10. However, the optimum composition for (1 − x)NaNbO₃-xBaTiO₃ ceramics is 0.90NaNbO₃-0.10BaTiO₃. The 0.90NaNbO₃-0.10BaTiO₃ ceramics sintered at 1250°C have higher properties: piezoelectric constant d ₃₃ of 120 pC/N, dielectric constant ε_τ of 718, planar electromechanical coupling factor k _p of 24%, planar frequency N _d of 3 MHz·mm, and the mechanical quality factor Q _m of 138, respectively. The results show that the (1−x)NaNbO₃-xBaTiO₃ ceramics is one of the promising lead-free materials for high-frequency applications.     

Effect of TiB2 particles on sliding wear behaviour of Al–4Cu alloy

Dry sliding wear of Al–4Cu–xTiB₂ (x = 0, 2.5, 5, 7.5 and 10 wt.%) in situ composites have been studied in the peak-aged condition using a pin-on-disc wear testing machine at different loads. The composites were prepared by the reaction of a mixture of K₂TiF₆ and KBF₄ salts with molten alloy. The results indicate that TiB₂ particles markedly improve the wear performance of the Al–4Cu alloy. The wear resistance increases with increase in the amount of TiB₂. The load bearing capacity of the alloy during wear increases in presence of TiB₂ particles. Study of the wear surfaces and debris of both alloy and composites using the scanning electron microscope suggests that the improvement in wear resistance is mainly due to the formation of finer debris.     

Tribological Properties of Spark-Plasma-Sintered ZrO2(Y2O3)–Al2O3–Ba x Sr1?x SO4 (x?=?0.25, 0.5, 0.75) Composites at Elevated Temperature

Self-lubricating ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have been fabricated by spark plasma sintering (SPS) method. The tribological properties have been evaluated using a high-temperature friction and wear tester at room temperature and 760 °C in dry sliding against alumina ball. The composites exhibit distinct improvements in effectively reducing friction and wear, as compared to the unmodified ZrO₂(Y₂O₃)–Al₂O₃ ceramics. The ZrO₂(Y₂O₃)–Al₂O₃–Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) composites have great low and stable friction coefficients of less than 0.15 and wear rates in the order of 10^− 6mm³/Nm at 760 °C. Delamination is considered as the dominating wear mechanism of the composites at room temperature. At elevated temperature, the formation and effective spreading of Ba _x Sr_1−x SO₄ (x = 0.25, 0.5, 0.75) lubricating films during sliding play an important role in the reduction of the friction and wear.     

Friction and wear properties of CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiC in water lubrication

Amorphous carbon nitride coatings (a-CN_x) were deposited on SiC disk by ion beam assisted deposition (IBAD). The tribological behavior of a-CN_x coating sliding against SiC ball in water was investigated and compared with that of SiC/SiC system at room temperature. The influences of testing conditions on friction coefficient and specific wear rate of both kinds of tribopairs were studied. The worn surfaces on disks were observed by scanning electron microscope (SEM). The results indicate that the running-in period of a-CN_x/SiC was shorter than that of SiC/SiC system in water. At a sliding velocity of 120 mm/s, the mean steady-state friction coefficients of SiC/SiC (0.096) was higher than that of a-CN_x/SiC (0.05), while at 160 mm/s, lower friction coefficient (0.01) was obtained for SiC/SiC in water. With an increment of normal load, the mean steady-state friction coefficients after running-in first decreased, reaching a minimum value, and then increased. For self-mated SiC, the specific wear rate of SiC ball was a little higher than that of SiC disk, while for a-CN_x/SiC, the specific wear rate of SiC ball were 10 times smaller than that of a-CN_x coating. Furthermore, the specific wear rate of SiC ball sliding against a-CN_x coating was reduced by a factor up to 100~1000 in comparison to that against SiC in water. The wear mechanism of SiC/SiC system in water is related to micro-fracture of ceramic and instability of tribochemical reaction layer. Conversely, wear mechanism for a-CN_x/SiC is related to formation and transfer of easy-shear friction layer.     

In‐situ imaging of Li intercalation in graphite particles in an Li‐ion battery

This paper presents the imaging of the expansion and contraction of graphite particles at the anode of a lithium‐ion battery. The intercalation and deintercalation of Li ions in the graphite particles induced by charging and discharging lead to expansion and contraction of the layered materials. These changes in volume were imaged through current collectors using scanning probe microscopy, which permitted in‐situ observation of the Li ion shift with high resolutions. We were able to evaluate the properties of each individual graphite particle. Here, we present variations in the images obtained by two methods of charging/discharging. In one method, the applied fields are changed, forcing the ions to move back into the graphite particles. Images showing detailed structures were obtained, allowing us to investigate the fine structures of the graphite particles. In the other method, the amount of ions is periodically injected into the graphite, which did not reveal the detailed structure but clearly distinguished inactive from active particles.