Black Gels Research Articles

Controlling the rheo-electric properties of graphite/carbon black suspensions by ‘flow switching’

The ability to manipulate rheological and electrical properties of colloidal carbon black gels makes them attractive in composites for energy applications such as batteries and fuel cells, where they conduct electricity and prevent sedimentation of ‘granular’ active components. While it is commonly assumed that granular fillers have a simple additive effect on the composite properties, new phenomena can emerge unexpectedly, with some composites exhibiting a unique rheological bi-stability between high-yield-stress and low-yield-stress states. Here we report such bi-stability in suspensions of non-Brownian graphite and colloidal carbon black in oil, a model system to mimic composite suspensions for energy applications. Steady shear below a critical stress elicits a transition to a persistent mechanically weak and poorly conducting state, which must be ‘rejuvenated’ using high-stress shear to recover a stronger, high-conductivity state. Our findings highlight the highly tunable nature of binary granular/gel composite suspensions and present new possibilities for optimising mixing and processing conditions for Li-ion battery slurries.Graphical abstract

Three length-scales colloidal gels: The clusters of clusters versus the interpenetrating clusters approach

Typically, in quiescent conditions, attractive colloids at low volume fractions form fractal gels structured into two length scales: the colloidal and the fractal cluster scales. However, when flow interferes with gelation colloidal fractal gels, it may display three distinct length scales [Dagès et al., Soft Matter 18, 6645–6659 (2022)]. Following those recent experimental investigations, we derive two models that account for the structure and the rheological properties of such atypical colloidal gels. The gel elasticity is inferred from scaling arguments, and the structure is translated into scattering intensities following the global scattering functions approach proposed by Beaucage and, typically, measured in small-angle x-ray scattering (SAXS). In both models, we consider that the colloids condensate into fractal clusters. In the clusters of the clusters model, the clusters form superagregates that then build the gel network. In the interpenetrating clusters model, the clusters interpenetrate one another to form the gel network. Those two models are then used to analyze rheo-SAXS experiments carried out on carbon black gels formed through flow cessation.

Mechanics and structure of carbon black gels under high-power ultrasound

Colloidal gels made of carbon black particles dispersed in light mineral oil are “rheo-acoustic” materials, i.e., their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicrometer amplitude and frequencies larger than 20 kHz. The effects of high-power ultrasound on the carbon black gel are demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and a time-resolved ultra-small-angle x-ray scattering experiment (TRUSAXS) coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude leads to a complex viscoelastic transient response of the gels within a few seconds: a softening of its storage modulus accompanied by a strong overshoot in its loss modulus. Under high-power ultrasound, the gel displays a viscoelastic spectrum with glasslike features and a significant decrease in its yield strain. Those effects are attributed to the formation of intermittent microcracks in the bulk of the gel as evidenced by TRUSAXS. Provided that the shear rate is not large enough to fully fluidize the sample, high-power ultrasound also facilitates the flow of the gel, reducing its yield stress as well as increasing the shear-thinning index, thanks again to the formation of microcracks.

Payne effect of carbon black filled natural rubber compounds and their carbon black gels

Carbon black (CB) is able to reinforce natural rubber (NR) but also enhance nonlinear viscoelastic behaviors significantly. Herein, the reinforcement and nonlinearity of NR/CB compounds, their unextractable gels, and swollen and oil-extended products are investigated systematically. The results show that the formation of bound rubber could facilitate the recovery and eliminate the hysteresis during cyclic deformation while the presence of sol fraction is able to improve the onset strain amplitude of the Payne effect. Both the highly filled compounds and their unextractable gels may demonstrate weak strain overshoot. It is revealed for the first time that the overshoot is either triggered or its intensity is improved by paraffin swelling. On the other hand, paraffin introduced during the compounding process would lower the content of glassy chains in the oil-extended compounds and cause a two-step softening with increasing strain amplitude. The results provide new insights into the Payne effect and allow gaining recognition of the important roles played by the bound rubber, free rubber and the filler.

Preferential adsorption of volatile hydrocarbons on high surface area chalcogels KMBiTe3 (M = Cr, Zn, Fe)

Three chalcogels KCrBiTe3, KZnBiTe3, and KFeBiTe3 were synthesized by the sol–gel metathesis route. K+ and the transition metal cations Cr2+, Zn2+ or Fe2+ connect [BiTe3]3− anions to form amorphous black compounds. Supercritical drying of the black gels produced porous materials with BET surface areas of 230 m2/g (KCrBiTe3), 450 m2/g (KZnBiTe3) and 514 m2/g (KFeBiTe3). The adsorption properties of the resulting chalcogenide aerogels or chalcogels have been studied using volatile organic hydrocarbons (VOCs) and gases as adsorptives. The results show preferential adsorption of toluene vapor over cyclohexane vapor. The adsorption capacity toward toluene is 5.02 mmol/g for KCrBiTe3, 5.58 mmol/g for KZnBiTe3 and 7.89 mmol/g for KFeBiTe3. Preferential adsorption of CO2 over CH4 or H2 was observed for the chalcogels: KCrBiTe3 (CO2/H2: 175, CO2/CH4: 50), KZnBiTe3 (CO2/H2: 180 and CO2/CH4: 60) and KFeBiTe3 (CO2/H2: 225 and CO2/CH4: 65).

Simultaneous Rheoelectric Measurements of Strongly Conductive Complex Fluids

We introduce a novel apparatus designed for stress-controlled rheometers to perform simultaneous rheological and electrical measurements on strongly conductive complex fluids under shear. By means of a non-toxic liquid metal at room temperature, the electrical connection to the rotating shaft is completed with minimal additional mechanical friction, allowing for simultaneous stress measurements as low as 1 Pa. We use the capabilities of this design to perform an extensive set of rheo-electric experiments on gels formulated from attractive carbon black particles, at concentrations ranging from 4 to 15% wt. First, experiments on gels at rest prepared with different shear history show a robust power-law scaling between the elastic modulus $G'_0$ and the conductivity $\sigma_0$ of the gels, i.e. $G'_0 \sim \sigma_0^\alpha$, with $\alpha =1.65 \pm 0.04$ independently of the gel concentration. Second, conductivity measurements performed simultaneously with creep experiments reveal for the first time that plastic events take place in the bulk while the shear rate decreases as a weak power law of time in the early stage of the experiment. The subsequent evolution of the conductivity and shear rate allows us to propose a local yielding scenario that is in agreement with previous velocimetry measurements. Finally, we determine the constitutive rheological and electrical behavior of carbon black gels. Corrections first introduced for mechanical measurements are carefully extended to electrical measurements to accurately distinguish between bulk and surface contributions to the conductivity. As an illustrative example, we examine the constitutive rheo-electric properties of five carbon black gels of different grades, and demonstrate the relevance of the novel rheo-electric apparatus as a versatile characterization tool for strongly conductive complex fluids and their applications.

Shear-induced structuration of confined carbon black gels: steady-state features of vorticity-aligned flocs

Various dispersions of attractive particles are known to aggregate into patterns of vorticity-aligned stripes when sheared in confined geometries. We report a thorough experimental investigation of such shear-induced vorticity alignment through direct visualization of carbon black gels in both simple plane shear and rotational shear cells. Control parameters such as the gap width, the strain rate, and the gel concentration are systematically varied. It is shown that in steady states the wavelength of the striped pattern depends linearly on the gap width $h$ while being insensitive to both the gel concentration $C$ and the shear rate $\gp$. The width of the vorticity-aligned flocs coincides with the gap width and is also independent of $C$ and $\gp$, which hints to a simple picture in terms of compressible cylindrical flocs. Finally, we show that there exists a critical shear rate $\gp_c$ above which structuration does not occur and that $\gp_c$ scales as $h^{-\alpha}$ with $\alpha=1.4\pm 0.1$ independently of $C$. This extensive data set should open the way to quantitative modelling of the vorticity alignment phenomenon in attractive colloidal systems.

Radiation-cured polyisoprene/C 60 fullerene nanocomposite : Part 2: Synthesis in decalin

The γ irradiation of polyisoprene and C 60 solutions in decalin at 50, 100 and 150 kGy was employed to prepare polyisoprene/C 60 nanocomposites. The nanocomposites were obtained as black gels. The excess of decalin was removed by treatment with methanol and acetone and the dried collapsed gels were studied by FT-IR spectroscopy and thermogravimetric analysis (TGA–DTG) and also by differential thermal analysis (DTA). The thermal stability was found to be enhanced by the presence of C 60 fullerene and by the crosslinking density. For reference, also the radiolysis of C 60 in decalin and the radiolysis of polyisoprene in decalin were studied. The data obtained were useful for the interpretation of the network structure of the polyisoprene/C 60 nanocomposites and their thermal behaviour.

Synthesis and properties of polypyrrole–vanadium oxide hybrid aerogels

Vanadium pentoxide/polypyrrole aerogel composites were synthesized by two sol–gel routes. The first utilized simultaneous polymerization of pyrrole and vanadyl alkoxide precursors. Hydrolysis of VO(OC 3H 7) 3 using pyrrole/water/acetone mixtures over a range of compositions yielded monolithic green–black gels. Supercritical drying yielded aerogels (150–257 m 2/g, 0.1–0.2 g/cm 3). These polypyrrole/V 2O 5 `nanocomposites' exhibited lower electrical conductivity with increased polypyrrole content. The second method involved oxide gel formation in the presence of a dispersion of preformed colloidal polypyrrole. SEM studies of the resulting `microcomposites' aerogels (80–140 m 2/g) show the presence of polypyrrole particles encapsulated in the fibrous V 2O 5 network. These materials exhibit conductivity equivalent to that of vanadium pentoxide. The interaction between polypyrrole and V 2O 5 in the materials was probed using infrared spectroscopy.

Polypyrrole-poly(carboxylic acid) conducting composites

It has been shown that pyrrole and N-methylpyrrole form rather stable polycomplexes with polyacrylic and polymethacrylic acids in water and water-alcohol solutions. It has been found that treatment with ferric salts upon formation of pyrrole-polyacrylic acid and N-methylpyrrole-polyacrylic acid complexes in water resulted in formation of bulky black gels insoluble in water and organic solvents. Polymethacrylic acid forms similar polycomplexes only in water-alcohol mixtures, which promote the formation of more suitable conformations of macromolecules. The complex composites obtained have been studied by IR, ESR, elemental analysis and scanning electron microscopy. It has been shown that these polycomplexes may form good-quality films with an electrical conductivity of up to 10 −4 S cm −1.

E.s.r. spectra of rubber-carbon black mixtures after mastication and extraction

The investigation of the e.s.r. spectra of the product of the rubber-carbon black mixtures extracted with benzene was carried out. Spectra of the isolated rubber-carbon black gels are similar to those of the respective mixtures. Practically independent of filling and milling time, the narrowest spectral line intensity is specific for rubber-carbon black gel. Lack of lines specific for carbon black in the spectra of soluble part of mixtures even if the carbon black is found in the extract was confirmed.