Search

Carbon black, a key ingredient in ancient inks, is used today to make the porous electrodes found in many rechargeable batteries. Understanding how to control its microstructure can pave the way to better-performing batteries. [ABSTRACT FROM AUTHOR]

Electron transport in complex fluids, biology, and soft matter is a valuable characteristic in processes ranging from redox reactions to electrochemical energy storage. These processes often employ conductor-insulator composites in which electron transport properties are fundamentally linked to the microstructure and dynamics of the conductive phase. While microstructure and dynamics are well recognized as key determinants of the electrical properties, a unified description of their effect has yet to be determined, especially under flowing conditions. In this work, the conductivity and shear viscosity are measured for conductive colloidal suspensions to build a unified description by exploiting both recent quantification of the effect of flow-induced dynamics on electron transport and well-established relationships between electrical properties, microstructure, and flow. These model suspensions consist of conductive carbon black (CB) particles dispersed in fluids of varying viscosities and dielectric constants. In a stable, well-characterized shear rate regime where all suspensions undergo self-similar agglomerate breakup, competing relationships between conductivity and shear rate were observed. To account for the role of variable agglomerate size, equivalent microstructural states were identified using a dimensionless fluid Mason number, Mnf, which allowed for isolation of the role of dynamics on the flow-induced electron transport rate. At equivalent microstructural states, shear-enhanced particle-particle collisions are found to dominate the electron transport rate. This work rationalizes seemingly contradictory experimental observations in literature concerning the shear-dependent electrical properties of CB suspensions and can be extended to other flowing composite systems. [ABSTRACT FROM AUTHOR]

Scientific questions surrounding the shear-dependent microstructure of carbon black suspensions are motivated by a desire to predict and control complex rheological and electrical properties encountered under shear. In this work, direct structural measurements over a hierarchy of length scales spanning from nanometers to tens of micrometers are used to determine the microstructural origin of the suspension viscosity measured at high shear rates. These experiments were performed on a series of dense suspensions consisting of high-structured carbon blacks from two commercial sources suspended in two Newtonian fluids, propylene carbonate and light mineral oil. The shear-induced microstructure was measured at a range of applied shear rates using Rheo-VSANS (very small angle neutron scattering) and Rheo-USANS (ultra-small angle neutron scattering) techniques. A shear-thinning viscosity is found to arise due to the self-similar break up of micrometer-sized agglomerates with increasing shear intensity. This self-similarity yields a master curve for the shear-dependent agglomerate size when plotted against the Mason number, which compares the shear force acting to break particle-particle bonds to the cohesive force holding bonds together. It is found that the agglomerate size scales as R g , agg ∼ M n − 1 . Inclusion of the particle stress contribution extends the relevance of the Mason number to concentrated suspensions such as those relevant to the processing of carbon black suspensions for various applications. [ABSTRACT FROM AUTHOR]

Carbon black suspensions exhibit complex, shear-dependent macroscopic properties that are a consequence of the state of the suspension microstructure. In this work, the shear-induced microstructure of a model, reversible suspension of conductive carbon black in propylene carbonate is measured using simultaneous steady shear rheology and small angle neutron scattering. These experiments provide microstructural evidence for a bifurcation in the rheological properties. We show that the demarcation line for this bifurcation is the inverse Bingham number, Bi−1, which relates the magnitude of the stress response to an applied shear rate to the yield stress of the presheared suspension. At high shear rates where Bi−1 > 1, the suspension flows homogeneously and exhibits a thixotropic response that arises due to the self-similar breakdown of agglomerates with increasing shear rate. Conversely, at low shear rates where Bi−1 < 1, the applied shear drives the densification and growth of these agglomerates. This densification process leads to a gravitationally driven instability resulting in an inhomogeneous volume fraction distribution along the height of the geometry that is a function of both time under shear and shear rate. Under these shear conditions, the suspension exhibits apparent rheopexy, or antithixotropy, where a significant decline in the viscosity is observed with a step down in shear rate. The unique microstructural measurements presented here reconcile many observations in the literature regarding carbon black suspensions, including an apparent shear-thickening behavior, tunability of both the yield stress and elasticity through shear history, and transient macroscopic properties. [ABSTRACT FROM AUTHOR]

High-structured carbon fillers are ubiquitous as the conductive additive comprising suspension-based electrochemical energy storage technologies. Carbon black networks provide the necessary electrical conductivity as well as mechanical percolation in the form of a yield stress. Despite their critical role in determining system performance, a full mechanistic understanding of the relationship between the electrical transport characteristics of the percolated, conductive networks of carbon black, and the rheological properties is lacking, which hinders the rational design and optimization of flowable electrodes and the processing of electrolytes for batteries. Here, we report on the microstructural origin of the rheological and electrical properties of two commonly used conductive additives in neat propylene carbonate. From quiescent mechanical and structural studies, we find that the gelation of these carbon black suspensions is best described by the dynamic arrest of a clustered fluid phase. In contrast, the temperature and frequency dependence of the ac conductivity near this transition shows that mesoscale charge transport is determined by hopping between localized states that does not require a stress-bearing network. This unique combination of microstructural characterization with rheological and electrical measurements enables testing prevailing theories of the connection between electrical and mechanical percolation as well as improving conductive additives to enhance electrochemical performance. [ABSTRACT FROM AUTHOR]

Dielectric RheoSANS measurements are conducted on hydrating triclinic and monoclinic tricalcium silicate pastes with and without a sucrose admixture to simultaneously probe the changing microstructure by small angle neutron scattering (SANS), rheology, and electrical conductivity. The average total surface area of nanoscale calcium silicate hydrate (C-S-H), determined from fractal models fitted to SANS data, at the setting time is estimated to be (17.1(56)) m2 cm−3, independent of C-S-H polymorph or retardation of the hydration reactions. The growth rate of C-S-H with respect to the degree of hydration of the triclinic tricalcium phase is approximately 48 % less than the monoclinic tricalcium silicate phase, a result that may be attributed to the increased number of fine grains in the monoclinic C 3 S paste. Developing an understanding between the microstructural changes of cement paste and engineering material properties is a critical first step toward engineering cement phase compositions. [ABSTRACT FROM AUTHOR]