Fluorographene with Narrow Lateral Size and Thickness Distributions Prepared for Enhancing Lubrication Performance of Bentonite Grease

Fluorographene(FG)with narrow lateral size and thickness distributions was prepared by a liquid-phase exfoliation method,based on liquid cascade centrifugation.The Rtec MFT-5000 tribo-meter was used to investigate the lubricating performance of bentonite grease enhanced by the as-prepared FG.The results showed that the coefficient of friction and the wear volume of bentonite grease with 0.3 wt%FG were decreased by 20.4%and 44.9%,respectively,as compared to those of the base grease.The main reason is that FG can promote the formation of the tribo-chemical reaction film consisting of complex carbon oxide,Fe_(2)O_(3)and FeF_(3)on the friction surface,which can remarkably improve the performance of friction reduction and prevent the appearance of severe wear.

Shear resistance of assembled bentonite interface after confined water saturation and interfacial self-healing capacity

The requisite functions of a bentonite buffer in a deep geological repository depend on the sealing/healing of bentonite interfaces,with particular emphasis on the self-healing(automatic healing upon wetting)of assembled bentonite-bentonite interfaces.This study determined the shear resistance(including the peak shear strength and secant modulus)of densely compacted Gaomiaozi(GMZ)bentonite and its assembled interface after confined water saturation.The effect of bentonite dry density and saturation time on the shear resistance of saturated healed interfaces was elucidated,and the interfacial self-healing capacity was assessed.The results indicate that the shear resistance of the saturated healed interfaces increased with the bentonite dry density but had a non-monotonic correlation with the saturation time.For a given dry density of the bentonite,the saturated healed interface exhibits a lower peak shear strength than the saturated intact bentonite but a higher peak shear strength than the saturated separated interface.The saturated healed and separated interfaces have comparable shear moduli(secant moduli),which are lower than that of the saturated intact bentonite.The saturated healed interfaces display smooth shear failure planes,while the saturated assembled interfaces and intact bentonite exhibit comparable frictional angles.This indicates that interfacial self-healing plays a pivotal role in enhancing interfacial peak shear strength by facilitating microstructural bonding at the assembled interface.Finally,it can be stated that densely compacted GMZ bentonite has a robust interfacial self-healing capacity in terms of shear resistance.These findings contribute to the design of the bentonite buffer and facilitate the evaluation of its safe operation at specified disposal ages.

Effects of synthetic site water on bentonite-concrete system for a potential nuclear waste repository

In high-level nuclear waste(HLW)repositories,concrete and compacted bentonite are designed to be employed as buffer materials,which may raise a problem of interactions between concrete and bentonite.These interactions would lead to mineralogy transformation and buffer performance decay of bentonite under the near field environment conditions in a repository.A small-scale experimental setup was established to simulate the concrete-bentonite-site water interaction system from a potential nuclear waste repository in China.Three types of mortars were prepared to correspond to the concrete at different degradation states.The results permit the determination of the following:(1)The macroproperties of Gaomiaozi(GMZ)bentonite(e.g.swelling pressure,permeability,the final dry density,and water content of reacted samples);(2)The composition evolution of fluids from the synthetic site water-concrete-bentonite interaction systems;(3)The sample characterization including Fourier transform infrared spectroscopy(FTIR)and X-ray powder diffraction(XRD).Under the infiltration of the synthesis Beishan site water(BSW),the swelling pressure of bentonite decreases slowly with time after reaching its second swelling peak.The flux decreases with time during the infiltrations,and it tends to be stable after more than 120 d.Due to the cation exchange reactions in the BSW-concrete-bentonite systems,the divalent cations(Ca and Mg)were consumed,and the monovalent cations(Na and K)were released.The dissolution of minerals in the bentonite such as albite causes Si increasing in the pore water.It was concluded that the hydro-mechanical property degradation of bentonite takes place when it comes into contact with concrete mortar,even under low-pH groundwater conditions.The soil dispersion,the uneven water content,and the uneven dry density in bentonite samples may partly contribute to the swelling decay of bentonite.Therefore,the direct contact with concrete has an obvious effect on the performance of bentonite.

Enhancing the Tribological Performance of Conductive Grease Through the Modification of Polypyrrole with Ionic Liquids

This study prepared four types of ionic liquid-modified polypyrrole(IL-modified PPy)as conductive additives and investigated their tribological performance and conductivity in polytetrafluoroethylene lubricating grease.The results indicated that IL-modified PPy effectively enhanced the anti-wear performance and conductivity of the base grease.Among the additives,1-octyl-3-methylimidazolium tetrafluoroborate([OMIm][BF4])modified PPy showed superior performance compared to the other three additives,with the best effect observed at a mass fraction of 0.5%.X-ray photoelectron spectroscopy analysis revealed that IL-modified PPy forms a stable friction chemical film during the friction process,effectively enhancing the lubrication performance and conductivity of the base grease.This indicates broad potential applications in the field of conductive lubrication.

Effect of drying cracks on swelling and self-healing of bentonite-sand blocks used as engineered barriers for radioactive waste disposal

Experiments were conducted to evaluate the healing of drying cracks in air-dried bentonite-sand blocks after hydration and swelling in groundwater,providing justifications to simplify the protection of blocks prior to installation in a high-level radioactive waste repository.Synthetic groundwater was prepared to represent the geochemistry of Beishan groundwater,and was used to hydrate the blocks during the swelling pressure and swelling strain measurements,as Beishan is the most promising site for China's repository.Healing of the surface cracks was recorded by photography,and healing of the internal cracks was visualized by CT images and hydraulic conductivity of air-dried blocks.The results indicate that the maximum swelling pressure and swelling strain are primarily affected by the geochemistry of Beishan groundwater,but not affected by the drying cracks.The maximum swelling pressure and swelling strain of air-dried blocks are comparable to or even higher than the pressure and strain of fresh blocks.The maximum swelling pressure measured in strong(i.e.high ion strength)Beishan groundwater was 44%of the pressure measured in deionized(DI)water,and the maximum swelling strain was reduced to 23%of the strain measured in DI water.Nevertheless,the remained swelling of the blocks hydrated in strong Beishan groundwater was sufficient to heal the surface and internal drying cracks,as demonstrated by the pictures of surface cracks and CT images.The hydraulic conductivity of the air-dried block permeated with strong groundwater was comparable(3.7×higher)to the hydraulic conductivity of the fresh block,indicating the self-healing of drying cracks after hydration and swelling in groundwater.A simplified method of protecting the block with plastic wraps before installation is recommended,since the remained swelling of the block hydrated in Beishan groundwater is sufficient to heal the drying cracks.

Anisotropic swelling pressures of compacted GMZ bentonite infiltrated with salt solutions

In the high-level radioactive waste(HLW)deep geological repository,bentonite is compacted uniaxially,and then arranged vertically in engineered barriers.The assembly scheme induces the initial anisotropy,and with hydration,it develops more evidently under chemical conditions.To investigate the anisotropic swelling of compacted Gaomiaozi(GMZ)bentonite and the further response to saline effects,a series of constant-volume swelling pressure tests were performed.Results showed that dry density enhanced the bentonite swelling and raised the final anisotropy,whereas saline inhibited the bentonite swelling but still promoted the final anisotropy.The final anisotropy coefficient(ratio of radial to axial pressure)obeyed the Boltzmann sigmoid attenuation function,decreasing with concentration and dry density,converging to a minimum value of 0.76.The staged evolution of anisotropy coefficient was discovered,that saline inhibited the rise of the anisotropy coefficient(Dd)in the isotropic process greater than the valley(d1)in the anisotropic process,leading to the final anisotropy increasing.The isotropic stage amplified the impact of soil structure rearrangement on the macro-swelling pressure values.Thus,a new method for predicting swelling pressures of compacted bentonite was proposed,by expanding the equations of Gouy-Chapman theory with a dissipative wedge term.An evolutionary function was constructed,revealing the correlation between the occurrence time and the pressure value due to the structure rearrangement and the former crystalline swelling.Accordingly,a design reference for dry density was given,based on the chemical conditions around the pre-site in Beishan,China.The anisotropy promoted by saline would cause a greater drop of radial pressure,making the previous threshold on axial swelling fail.

Study on Preparation and Properties of Grease Based on Ultrafine Bentonite Powder

The feasibility for preparation of ultrafine bentonite powder by different milling methods was studied. And the comparison of comprehensive performance between ultrafine bentonite grease and traditional bentonite grease was also investigated. The results indicated that the statistic Z-average size of ultrafine bentonite prepared by sand milling was 250 nm with a narrow size distribution and the lattice structure of ultrafine bentonite maintained good character despite a slight distortion occasioned. The mechanical stability, colloid stability, antiwear ability and friction-reducing property of ultrafine bentonite grease were superior to the traditional one.

Study on the Effect of Surface Modification on the Properties of Bentonite Greases

In order to investigate the influening factors of organic modification procedure and find out connections between organic modification and the properties of bentonite greases, organic montmorillonite(OMMT) thickeners with different surfactant dosages and constituents were synthesized through intercalation reaction between sodium montmorillonite(NaM MT) and quaternary ammonium surfactants in aqueous solvents. The lubricating greases were prepared with the resulting organoclays, while the penetration and oil separation of lubricating greases were evaluated, respectively. The surface modification process of montmorillonite(MMT) was analyzed and the thickening mechanism of OMMT was discussed in this study. The experimental results showed that, with an increasing amount of surfactant, the basal spacing between the clay platelets was increasing and the structure of modifier molecules layer in the interlayer was changing from lateral bilayer to paraffin-type bilayer. The optimal properties of lubricating greases were achieved, when the structure of surfactant molecules loaded in the interlayer was the paraffin-type monolayer, which meant that the dosage of modifier was equal to 120—140 mmol/(100g). Meanwhile, it was found that the thickening performance, colloid stability, anti-wear and friction-reducing performance of lubricating greases were improved, when the surfactants were mixed with octadecyl trimethyl ammonium chloride(OTAC) and hexadecyl trimethyl ammonium chloride(HTAC). And the optimum mole ratio of two surfactants is was 1:1.

Tribological and rheological performance of lithium grease with poly-a-olefin and alkyl-tetralin as base oils

A new category of lithium greases was synthesized by using poly-a-olefin(PAO8) and alkyl-tetralin as base oil, where the alkyl-tetralins were synthesized by the alkylation of tetralin and olefins. The influence of thickener concentration, alkyl-tetralin content and type of blend oils on the rheological and tribological performance of lithium grease was investigated. The microstructures of soap fibers were measured to reveal the structure-property correlations. The concentration of thickener and alkyl-tetralin content obviously affect the lubricating performance of lithium grease, while the molecular structure of alkyltetralin has no obvious impact on their properties. It was found that alkyl-tetralin could significantly enhance the thickening ability of PAO8 base oils, and decrease the amount of thickeners by 1.5%(mass).Lithium greases prepared using 20%(mass) alkyl-tetralin as co-base oil exhibited high colloidal stability,excellent rheological behaviors and tribological properties.

Well-Dispersed Graphene Enhanced Lithium Complex Grease Toward High-Efcient Lubrication

Graphene as a lubricating additive holds great potential for industrial lubrication. However, its poor dispersity and compatibility with base oils and grease hinder maximizing performance. Here, the infuence of graphene dispersion on the thickening efect and lubrication function is considered. A well-dispersed lubricant additive was obtained via trihexyl tetradecyl phosphonium bis(2-ethylhexyl) phosphate modifed graphene ([P_(66614)][DEHP]-G). Then lithium complex grease was prepared by saponifcation with 12-OH stearic acid, sebacic acid, and lithium hydroxide, using polyalphaolefn (PAO20) as base oil and the modifed-graphene as lubricating additive, with the original graphene as a comparison. The physicochemical properties and lubrication performance of the as-prepared greases were evaluated in detail. The results show that the as-prepared greases have high dropping point and colloidal stability. Furthermore, modifed-graphene lithium complex grease ofered the best friction reduction and anti-wear abilities, manifesting the reduction of friction coefcient and wear volume up to 18.84% and 67.34%, respectively. With base oil overfow and afux, well-dispersed [P_(66614)][DEHP]-G was readily adsorbed to the worn surfaces, resulting in the formation of a continuous and dense graphene deposition flm. The synergy of deposited graphene-flm, spilled oil, and adhesive grease greatly improves the lubrication function of grease. This research paves the way for modulating high-performance lithium complex grease to reduce the friction and wear of movable machinery.

Numerical Visualization of Grease Flow in a Gearbox

Lubricating greases are widely used in e.g.open gear drives and gearboxes with difficult sealing conditions.The efficiency and heat balance of grease-lubricated gearboxes depend strongly on the lubrication mechanisms channeling and circulating,for which the grease flow is causal.The computational fluid dynamics opens up the possibility to visualize and understand the grease flow in gearboxes in more detail.In this study,a single-stage gearbox lubricated with an NLGI 1-2 grease was modeled by the finite-volume method to numerically investigate the fluid flow.Results show that the rotating gears influence the grease sump only locally around the gears.For a low grease fill volume,the rotation of the gears is widely separated from the grease sump.For a high grease fill volume,a pronounced geargrease interaction results in a circulating grease flow around the gears.The simulated grease distributions show good accordance with high-speed camera recordings.

Saturated hydraulic conductivity of compacted bentonite-sand mixtures before and after gas migration in artificial seawater

To understand the self-healing property of an engineered barrier for radioactive waste disposal,the hydraulic conductivity of compacted bentoniteesand mixtures saturated with artificial seawater(SW)before and after gas migration was examined.Na-and Ca-bentonites were mixed with fine sand at a ratio of 70%bentonite in dry weight.Two aspects were considered during the experiment:the hydraulic conductivity of the specimen that was resaturated after gas migration and the distribution of water content immediately after gas migration to study gas migration pathways.The gas migrated through the entire cross-section of the specimen,and gas breakthrough occurred in the equilibrium swelling pressure range approximately.Subsequently,the gas flow rate reached a sufficient large value when the gas pressure was approximately twice the equilibrium axial pressure(the sum of swelling and confining pressures),which excluded the back pressure.Although the gas migration pathway was not visible when the specimen was observed immediately after gas migration,the water content distribution showed that several parts of the specimen with lower water content were connected in the direction of gas migration.After resaturation,the change in permeability was within a limited rangedtwo to three times larger than that before gas migration for each type of bentonite in SW.This slight change suggests that gas migration creates a pore structure that cannot be sealed via crystalline swelling of montmorillonite in SW,even if highly compacted bentonite is used under a constant-volume condition.

On the installation of an in situ large-scale vertical SEALing (VSEAL) experiment on bentonite pellet-powder mixture

Recently,the Institute for Radiological protection and Nuclear Safety(IRSN)has launched VSEAL(Vertical SEALing)project to investigate the impact of gas migration on the long-term performance of bentonite based vertical sealing systems(VSS).The first VSEAL in situ test was emplaced in IRSN’s Underground Research Laboratory(URL)in Tournemire(France)in 2019 and was equipped with 76 wired and wireless sensors.The test is still in progress,but the collected set of data provides already valuable information of the hydro-mechanical behavior of VSS during hydration.The swelling core consists of a mixture of highdensity pellets and powder of MX80 bentonite in a ratio of 80/20(in dry mass).An innovative method was adopted to drill a 1-m diameter and w10-m deep shaft in order to minimize the rock perturbation at the sidewalls.Because a specific protocol was adopted to install the bentonite mixture together with a careful characterization of the core during construction,VSEAL 1 constitutes the unique in situ sealing test with a well-known initial structural distribution of the pellets and the powder.Some heterogeneities occurred within the experiment during the installation process:a damaged zone developed around the shaft walls due to the interruption of the installation operations caused by COVID19 lockdown in France;a technological gap with a variable thickness between the last pellets layer and the top confining lid and a heterogeneous distribution of the bentonite powder at some layers inducing large inter pellets voids close to the bentonite-rock interface.Artificially injected water volume,relative humidity,water content and swelling pressure in both radial and axial directions were monitored.Comparison of the results showed that the presence of installation-induced heterogeneities led to the generation of preferential flow paths that influenced the swelling pressure evolution at radial and axial directions.

Effect of Synthetic Conditions and Thickener Formulation on the Rheological and Tribological Properties of Polypropylene Grease

Polypropylene(PP)grease holds great potential for special industrial applications.In this study,synthetic conditions,thickener content,and the ratio of two different molecular weight PPs were investigated systematically using a rheometer,scanning electron microscope,X-ray diffraction,Fourier transform infrared spectrometer,oscillating tribometer,and 3D surface profiler measurements.The results showed that suitable synthetic conditions are two quenching cycles,and the synthetic temperature and time is 240℃for 12 h.The rheological analysis showed that thickener content and different proportions of the two PP molecular weights have a significant influence on the rheological properties of PP grease.High molecular weight PP(H-PP)has a stronger thickening ability than low molecular weight PP(L-PP).The higher the amount of H-PP in the fixed thickener content or the higher the thickener content with a specific proportion,the higher the viscoelasticity of PP grease.The tribological performance is related to the rheological properties.The proportion of two different molecular weight PPs in the thickener content should be appropriate;excessive H-PP content leads to lubrication failure.

Influence of relative compaction and degree of saturation on the deformation characteristics of bentonite under freeze-thaw cycles

Bentonite,consisting of clay minerals of the montmorillonite group,has been widely used as an adsorbent and backfill material in nuclear waste disposal and groundwater remediation.It is challenging to use bentonite as a filling material in cold regions since bentonite is highly sensitive to thermal environmental changes,during which its bulk volume and microstructure change significantly.In this study,a series of one-dimensional and three-dimensional freeze-thaw tests were carried out within a closed system to investigate the influencing factors of the deformation of bentonite under freeze-thaw cycles.Results show that the initial soil water content greatly impacts bentonite's deformation during freeze-thaw cycles.For an initial higher degree of saturation(Sr),the expansion caused by the formation of ice lenses has a greater impact than the shrinkage induced by dehydration,ice-cementation,and so on.Conversely,bentonite tends to shrink at a lower degree of saturation during freezing.And the critical degree of saturation that determines bentonite's behavior of frost heave or frost shrinkage seems to be roughly 0.8.As the number of freeze-thaw cycles rises,initially uncompacted bentonite clay becomes more compacted,and initially compacted bentonite clay remains unchanged.

Preparation of Iron-Pillared Bentonite/Oyster Shell Composite and Phosphate Adsorption in Water

Iron-pillared bentonite(FB)was prepared by Fe(III)modified bentonite,and then the composites(FB-OS)were prepared by iron-pillared bentonite and oyster shell powder.The composites were characterized by FTIR,SEM,TGA,and EDS,and the phosphorus removal test was carried out.The results showed that FB-OS contained a large amount of CaO.Its structure was compact,but there were gaps in it.The maximum bending stress and compressive strength were 43.7 N and 0.927 MPa,respectively.The phosphorus removal test showed that the phosphorus removal rate of FB-OS was more than 90%,and measured the maximum adsorption capacity was 48.31 mg/g.A large amount of spherical products were produced on the surface and inside of FB-OS after phos-phorus removal,it was speculated that spherical products were amorphous calcium phosphate in the paper.Ana-lysis indicated that there was chemical adsorption during phosphorus removal.The kinetic equation of phosphorus adsorption by FB-OS was qt=10:193t/1+2:574t (R^(2)=0.995).The adsorption rate was mainly controlled by outerfilm diffusion and intraparticle diffusion.

Research Status and Prospects of Bio-based Materials for Grease Barrier Coatings on Paper Food Packaging

Increased environmental and health concerns over the use of plastic packaging or fluorine-containing coatings,in combination with increased market demand for products with a longer shelf life,make bio-based materials one of the most important research candidates for alternative paper packaging materials for oil resistance.These bio-based materials have excellent oxygen and oil barriers,which are critical for food packaging.Moreover,they are biodegradable,naturally renewable,and safe.In this artical,two main groups of bio-based oil repellents for paper food packaging,including polysaccharide-based biopolymers and protein-based biopolymers,are enumerated,and the advantages and weaknesses of bio-based oil repellents are discussed,and effective solutions are proposed.Finally,research status and prospects on the development of bio-based oil-resistant coatings for the food packaging industry are presented.

Investigation of long-wavelength elastic wave propagation through wet bentonite-filled rock joints

The saturation of the compacted bentonite buffer in the deep geological repository can cause bentonite swelling,intrusion into rock fractures,and erosion.Inevitably,erosion and subsequent bentonite mass loss due to groundwater inflow can aggravate the overall integrity of the engineered barrier system.Therefore,the coupled hydro-mechanical interaction between the buffer and rock during groundwater inflow and bentonite intrusion should be evaluated to guarantee the long-term safety of deep geological disposal.This study investigated the effect of bentonite erosion and intrusion on the elastic wave propagation characteristics in jointed rocks using a quasi-static resonant column test.Jointed rock specimens with different joint conditions(i.e.joint surface saturation and bentonite filling)were prepared using granite rock discs sampled from the Korea Underground Research Tunnel(KURT)and Gyeongju bentonite.The long-wavelength longitudinal and shear wave velocities were measured under different normal stress levels.A Hertzian-type power model was used to fit the wave velocities,and the relationship between the two fitted parameters provided the trend of joint conditions.Numerical simulations using three-dimensional distinct element code(3DEC)were conducted to better understand how the long-wavelength wave propagates through wet bentonite-filled rock joints.

Effect of shaking time, ionic strength, temperature and pH value on desorption of Cr(III) adsorbed onto GMZ bentonite

The Cr（III） desorption experiments of Gaomiaozi （GMZ） bentonite in aqueous solutions were performed. The variables affecting the desorption behaviors, such as contact time, concentration of the desorbent, pH value of the solution, temperature and desorption isotherms, were investigated by the batch experiments. The results show that the adsorbed Cr（III） on GMZ bentonite can be easily extracted by the desorbent. Kinetics examination shows that desorption is slower than adsorption, and the desorption rate increases with time and reaches the equilibrium after 3 h. The final desorption ratios of Cr（III） are 89.4%, 56.5%and 77.2%in the desorption solution with 0.1 mol/L HCl, 1 mol/L NaCl, and 1 mol/L CaCl2, respectively, and the concentration can promote the desorption progress. Furthermore, the results of successive regeneration cycles indicate that the bentonite has a good regeneration ability and reusability. The pH value is an important factor in the Cr（III） desorption from the GMZ bentonite. The results of adsorption and desorption isotherms show that both adsorption and desorption isotherms are consistent with the Freundlich equation. The comparison of adsorption and desorption isotherms implies that the adsorption/desorption hysteresis is negligible and the transport of Cr（III） in bentonite can be described by a reversible adsorption process.

Synthesis of nano-MoS_2/bentonite composite and its application for removal of organic dye

A nano-MoS2/bentonite composite was synthesized by calcinating MoS3 deposited on bentonite in H2. The obtained composite was characterized using thermogravimetric analysis, X-ray diffractometer, scanning electron microscope and transmission electron microscope. The results show that nano-MoS2 particles are distributed on the surface of bentonite and form layered structures with layer distance of about 0.64 nm. The composite presents an excellent performance for the removal of methyl orange. Some operation conditions affect the removal efficiency of methyl orange, such as dosage of composite, initial concentration of methyl orange, temperature and pH value. However, light source does not influence the removal efficiency. The removal mechanism is attributed to the adsorption of methyl orange on the nano-MoS2/bentonite composite. The adsorption of methyl orange on the composite is in accordance with the pseudo-second-order kinetic model.