Yishay Feldman

High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes

The gas-phase reaction between MoO3-x and H2S in a reducing atmosphere at elevated temperatures (800� to 950�C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS2. A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions.

Stress-induced fragmentation of multiwall carbon nanotubes in a polymer matrix

We report the observation of single nanotube fragmentation, under tensile stresses, using nanotube-containing thin polymeric films. Similar fragmentation tests with single fibers instead of nanotubes are routinely performed to study the fiber-matrix stress transfer ability in fiber composite materials, and thus the efficiency and quality of composite interfaces. The multiwall nanotube-matrix stress transfer efficiency is estimated to be at least one order of magnitude larger than in conventional fiber-based composites.

Hollow nanoparticles of WS2 as potential solid-state lubricants

Solid lubricants fill a special niche in reducing wear in situations where the use of liquid lubricants is either impractical or inadequate, such as in vacuum, space technology or automotive transport. Metal dichalcogenides MX2 (where M is, for instance, Mo or W and X is S or Se) are widely used as solid lubricants. These materials are characterized by a layered structure with weak (van der Waals) inter-layer forces that allow easy, low-strength shearing,. Within the past few years, hollow nanoparticles (HNs) of MX2 with structures similar to those of nested carbon fullerenes and nanotubes have been synthesized,. Here we show that these materials can act as effective solid lubricants: HN-WS2 outperforms the solid lubricants 2H-MoS2 and 2H-WS2 in every respect (friction, wear and lifetime of the lubricant) under varied test conditions. We attribute the outstanding performance of HN-WS2 to its chemical inertness and the hollow cage structure, which imparts elasticity and allows the particles to roll rather than to slide.

Inorganic fullerene-like material as additives to lubricants: structure–function relationship

Abstract Recently, inorganic fullerene-like (IF) supramolecules of metal dichalcogenide MX2 (M=Mo, W, etc.; X=S, Se), materials with structures closely related to (nested) carbon fullerenes and nanotubes have been synthesized. The main goal of the present work was to study the tribological properties of IF–WS2 in comparison to 2H–WS2 and MoS2 platelets over a wide range of loads and sliding velocities. The size and shape distributions of the nanoparticles were studied by transmission electron microscopy (TEM). The average size of the IF–WS2 particles was 120 nm, while that of 2H–WS2 and 2H–MoS2 was 0.5 μm and 4 μm. The chemical reactivity of the different powders in an oxygenated atmosphere was verified by heating the powders in ambient atmosphere. The friction experiments were performed in laboratory atmosphere (humidity ∼50%) using a ring-block tester. Complementary information on the state of wear of the powders in the lubricating fluid and on the metal surfaces of the specimens was obtained using a combination of TEM, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). It has been established that IF–WS2 nanoparticles appear to have excellent tribological properties in definite loading range in comparison to typical metal dichalcogenides. The oxidation of the IF particles and the wear track was essentially less than with solid lubricants made of platelets of the same chemical compound (WS2). The main advantages of IF nanoparticles lie in their round shape and the absence of dangling bonds.

New reactor for production of tungsten disulfide hollow onion-like (inorganic fullerene-like) nanoparticles

Abstract MS2 (M=Mo, W) hollow onion-like nanoparticles were the first inorganic fullerene-like (IF) materials, found in 1992. Understanding of the IF-MS 2 growth mechanism in 1996 enabled us to build a rather simple reactor, which produced about 0.4 g per batch, of an almost pure IF-WS2 powder. Soon after, it was found that the new powder showed better tribological properties compared with the regular MS2 (M=Mo, W) powder, which is a well-known solid lubricant. The present work shows a new synthetic approach, which allows for a scale-up of IF-WS2 production by more than two orders of magnitude. The falling-bed and, especially, fluidized-bed methods, which are presented here, pave the way for an almost ideal growth condition of the IF synthesis from an oxide precursor. As a result, the presently produced IF has a more uniform (spherical) shape and can grow to a larger size (up to 0.5 μm). It is expected that the relatively spherical IF-WS2 nanoparticles, which are produced by the falling (fluidized) bed reactor, will exhibit superior tribological properties, than reported before.

Superior tribological properties of powder materials with solid lubricant nanoparticles

Abstract Friction and wear of powder materials impregnated with commercially available layered (platelets) WS 2 (2H) and inorganic fullerene-like WS 2 nanoparticles (IF) were studied. Bronze–graphite, iron–graphite and iron–nickel–graphite samples were used in this experiment. The linear wear of powder materials (in situ) was measured. It was shown that the IF nanoparticles impregnated into the pores improve the tribological properties of powder materials in comparison to a reference sample or the sample impregnated with 2H solid lubricant particles. The mechanisms of friction and wear of the IF nanoparticles have been considered. The tribological role of the wear particles and nanoparticles of solid lubricants has been analyzed in the framework of a third body lubrication model. The state of the IF nanoparticles before and after the wear test was studied. It was found that the shape of the IF nanoparticles is preserved during the friction tests under high loads. Thin wear debris surrounded by spherical IF nanoparticles appear to be formed and provide easily sheared lubrication film (low friction coefficient) during friction experiments of powder materials containing IF nanoparticles.

Chemical Deposition of Cu2O Nanocrystals with Precise Morphology Control

Copper(I) oxide nanoparticles (NPs) are emerging as a technologically important material, with applications ranging from antibacterial and fungicidal agents to photocatalysis. It is well established that the activity of Cu2O NPs is dependent on their crystalline morphology. Here we describe direct preparation of Cu2O nanocrystals (NCs) on various substrates by chemical deposition (CD), without the need of additives, achieving precise control over the NC morphology. The substrates are preactivated by gold seeding and treated with deposition solutions comprising copper sulfate, formaldehyde, NaOH, and citrate as a complexant. Production of NC deposits ranging from complete cubes to complete octahedra is demonstrated, as well as a full set of intermediate morphologies, i.e., truncated octahedra, cuboctahedra, and truncated cubes. The NC morphology is defined by the NaOH and complexant concentrations in the deposition solution, attributed to competitive adsorption of citrate and hydroxide anions on the Cu2O {100} and {111} crystal faces and selective stabilization of these faces. A sequential deposition scheme, i.e., Cu2O deposition on pregrown Cu2O NCs of a different morphology, is also presented. The full range of morphologies can be produced by controlling the deposition times in the two solutions, promoting the cubic and octahedral crystal habits. Growth rates in the ⟨100⟩ and ⟨111⟩ directions for the two solutions are estimated. The Cu2O NCs are characterized by SEM, TEM, GI-XRD, and UV-vis spectroscopy. It is concluded that CD furnishes a simple, effective, generally applicable, and scalable route to the synthesis of morphologically controlled Cu2O NCs on a variety of conductive and nonconductive surfaces.

Conversion of CO2 to CO by electrolysis of molten lithium carbonate

The conversion of CO 2 to CO by electrolysis of molten Li 2 CO 3 was investigated. Using a cell comprising a Ti cathode, a graphite anode and a source of CO 2 allows the continuous electrolysis of the melt at 900°C with current densities at the electrodes higher than 100 mA/cm 2 . The faradaic efficiency of the process is close to 100%, and the thermodynamic efficiency at 100 mA/cm 2 is > 85%. The proposed method has several advantages: (i) No precious metal is required, (ii) no hazardous or toxic by-products are produced, and (iii) the method may operate continuously, producing pure CO rather than a mixture of CO and CO 2 . Therefore, the process described here has a potential application for converting electrical energy into fuel.

Synthesis of fullerene-like MoS2nanoparticles and their tribological behavior

Further understanding of the growth mechanism and the detailed structure of fullerene-like MoS2 (IF-MoS2) nanoparticles was achieved by using a new kind of reactor. The annealed nanoparticles consist of >30 closed layers and their average diameter is 50–80 nm although a small (<5%) fraction of larger IF nanoparticles was discernible. The majority of the nanoparticles are found to have an oval (pitta-bread or flying-saucer) shape rather than being quasi-spherical. The (002) peak of the powder diffraction pattern reveals only a small (0.3%) shift to lower angles as compared to the bulk (2H) phase. This observation suggests that the structure of the nanoparticles produced in the present reactor is more relaxed as compared to the previously synthesized IF-MoS2 powder, which exhibited up to 2% shift. The present reactor also permitted scaling up of the production of the IF-MoS2 to more than 0.6 g/batch. Impregnation of such nanoparticles in metallic coatings is shown to endow these surfaces with excellent tribological behavior, which suggests numerous applications.

Improved orthodontic stainless steel wires coated with inorganic fullerene-like nanoparticles of WS2 impregnated in electroless nickel–phosphorous film

OBJECTIVE To reduce friction between orthodontic stainless wires and bracket by coating the wire with nickel-phosphorous electroless film impregnated with inorganic fullerene-like nanoparticles of tungsten disulfide (IF-WS(2)) which are potent dry lubricants. METHODS Coating was preformed by inserting stainless steel (SS) wires into electroless solutions of nickel-phosphorus (Ni-P) and IF-WS(2). The coated wires were analyzed by SEM (scanning electron microscope) and EDS (energy-dispersive X-ray spectrometer) as well as by tribological tests using a ball-on-flat device. Friction tests simulating archwire functioning of the coated and uncoated wires were carried out by an Instron machine. The adhesion properties of the coated wires after friction were analyzed by a Raman microscope. RESULTS SEM/EDS analysis of the coated wires showed clear impregnation of the IF-WS(2) nanoparticles in the Ni-P matrix. The friction coefficient measured by the ball-on-flat tribometer was significantly reduced (from 0.25 to 0.08). The friction forces as measured with the Instron on the coated wire were reduced by up to 54% (4.00 N+/-0.19 uncoated vs. 1.85 N+/-0.21 coated). Raman spectra showed that even after extensive friction tests the Ni-P with the IF-WS(2) nanoparticles is attached to the underlying stainless steel wire. CONCLUSIONS It is proposed that the wires coated with these nanoparticles might offer a novel opportunity to substantially reduce friction during tooth movement. A few tests undertaken to evaluate the toxicity of the fullerene-like nanoparticles have provided indications that they might be biocompatible.