A. Margolin

Mechanisms of ultra-low friction by hollow inorganic fullerene-like MoS2 nanoparticles

Abstract Inorganic Fullerene-like (IF)-MoS2 nanoparticles were tested under boundary lubrication and ultra-high vacuum (UHV) and were found to give an ultra-low friction coefficient in both cases compared to hexagonal (h)-MoS2 material. Previous works made by Rapoport et al. with IF-WS2 revealed that the benefit effect of the inorganic fullerene-like materials decreases at high loads and sliding velocities. Nevertheless, under the conditions used in our experiments using high contact pressure (maximum pressure above 1.1 GPa in oil and 400 MPa in high vacuum) and slow sliding velocities (1.7 mm/s in oil test and 1 mm/s in high vacuum), friction always decreases and stabilizes at about 0.04 for 800 cycles in both cases. Therefore, IF-MoS2 material appears to be a good candidate for use in various environments in regard to other MoS2 crystal structures. Wear mechanisms were investigated using both High Resolution TEM and surface analyses (XPS) on the wear tracks. Wear particles collected from the flat wear scar show several morphologies, suggesting at least two lubricating mechanisms. As spherical particles are found in the wear debris, rolling may be a possible event. However, flattened and unwrapped IF-MoS2 particles are often observed after friction. In this case, low friction is thought to be due either to sliding between IF-MoS2 external flattened planes or to slip between individual unwrapped MoS2 sheets.

INSIGHT INTO THE GROWTH MECHANISM OF WS2 NANOTUBES IN THE SCALED-UP FLUIDIZED-BED REACTOR

The growth mechanism of WS2 nanotubes in the large-scale fluidized-bed reactor is studied in greater detail. This study and careful parameterization of the conditions within the reactor lead to the synthesis of large amounts (50–100 g/batch) of pure nanotubes, which appear as a fluffy powder, and (400–500 g/batch) of nanotubes/nanoplatelets mixture (50:50), where nanotubes usually coming in bundles. The two products are obtained simultaneously in the same reaction but are collected in different zones of the reactor, in a reproducible fashion. The characterization of the nanotubes, which grow catalyst-free, by a number of analytical techniques is reported. The majority of the nanotubes range from 10 to 50 micron in length and 20–180 nm in diameter. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior with numerous applications.

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.

Study of the growth mechanism of WS2 nanotubes produced by a fluidized bed reactor

Metal dichalcogenide nanotubes and in particular those of WS2 were shown to exhibit some unique physical and chemical properties, which offer numerous applications for this kind of nanophase material. Using a fluidized bed reactor (FBR), WS2 nanotubes were obtained in substantial amounts recently, rendering a systematic study of their properties possible. The FBR synthesized nanotubes are multiwalled (5–7 layers); open-ended; long (<0.5 mm), and with diameters of 15–20 nm. They are therefore distinguishable from the previously reported WS2 nanotubes which were shorter, bulkier and with closed ends. Careful analysis by various electron microscopy techniques is used in the present study to shed some light on the growth mechanism of these newly synthesized nanotubes. The proposed growth mechanism model differs markedly from the previously reported mechanisms of formation of both fullerene-like WS2 nanostructures and inorganic nanotubes of WS2.

Scaling Up of the WS2 Nanotubes Synthesis

Abstract The growth mechanism of WS2 nanotubes is briefly discussed. Two distinct growth mechanisms can be delineated, leading to somewhat different products: 1) thick (50–150 nm) and very long (20–50 microns and above) nanotubes consisting of many (>20) layers, and 2) slender (20–25 nm) nanotubes with 5–10 layers. The synthesis of large amounts of pure WS2 nanotubes belonging to the first category in the large-scale fluidized-bed reactor is described. Characterization of the nanotubes, which grow catalyst-free by a number of analytical techniques, is reported. The nanotubes reveal highly crystalline order, suggesting very good mechanical behavior and numerous applications, especially in the field of nanocomposites.

Scanning tunneling microscopy study of WS2 nanotubes

Inorganic nanotubes of WS2 have been investigated by high-resolution transmission electron microscopy, and scanning tunneling microscopy, providing support for the theoretical prediction of correlation of bandgap with diameter.

Fullerene-like WS2 nanoparticles and nanotubes by the vapor-phase synthesis of WCln and H2S

Inorganic fullerene-like (IF) nanoparticles and nanotubes of WS(2) were synthesized by a gas phase reaction starting from WCl(n) (n = 4, 5, 6) and H(2)S. The effect of the various metal chloride precursors on the formation of the products was investigated during the course of the study. Various parameters have been studied to understand the growth and formation of the IF-WS(2) nanoparticles and nanotubes. The parameters that have been studied include flow rates of the various carrier gases, heating of the precursor metal chlorides and the temperature at which the reactions were carried out. The best set of conditions wherein maximum yields of the high quality pure-phase IF-WS(2) nanoparticles and nanotubes are obtained have been identified. A detailed growth mechanism has been outlined to understand the course of formation of the various products of WS(2).

MoS2 FULLERENE-LIKE NANOPARTICLES AND NANOTUBES USING GAS-PHASE REACTION WITH MoCl5

Inorganic fullerene-like (IF) nanoparticles of MoS2 were synthesized using gas-phase reaction starting from MoCl5 and H2S. The IF-MoS2 nanoparticles are spherical and in some cases faceted with diameters in general ranging between 20 and 80 nm. The IF-MoS2 nanoparticles have large hollow cores, filled in some cases with amorphous material. Various parameters have been investigated to understand the growth and formation of the IF-MoS2 nanoparticles. The parameters that have been studied include flow rates of the various carrier gases, temperature at which the reaction was carried out, time of the reaction and heating of the precursor material. The best set of conditions wherein maximum yields of the IF-MoS2 nanoparticles are obtained have been identified. Additionally, annealing the as-obtained samples or heating them in a mixture of H2 along with H2S improves the crystallinity and reduces the amorphous material filling in the core. Apart from the fullerene-like nanoparticles under certain experimental conditions nanotubes of MoS2 have also been obtained nonetheless in small yields.