Rheological Characterization of Fluids for Lithium Ion Batteries and Thermal Ablation of Tumors

Rheology is the study of the flow of matter. To determine what material is most suitable for an application involving material flow, an instrument called a rheometer can characterize material behavior under specific conditions. A variety of procedures can be used to determine the properties of a fluid when it is heated, stressed, or mixed with another fluid. This study uses a stress controlled rheometer to characterize the response of various fluids to conditions expected in their related processes. These responses are analyzed and results are organized into three manuscripts. The first manuscript investigates the behavior of lithium ion battery anode components in a new method of anode production called emulsion templated directed assembly. This method uses carbon black nanoparticles to contain a silicon-rich oil phase inside of a polymer-rich water phase. The rheological properties of carbon black and reduced graphene oxide suspensions in water were recorded at a range of pH levels. These results were compared to suspensions including polymer binder. It was determined that the water phase of an anode produced by emulsion templating would not develop an interconnected network prior to the drying process, and the inclusion of a polymer binder has minimal effect on system viscosity. The second manuscript determines the viscoelastic properties of graphite lithium ion battery anode slurries containing Timcal Super P or Cabot LITX-50, two common carbon black additives. Super P appears to create a slurry that is easier to manufacture with and less likely to suffer from cracks while drying than a slurry with LITX-50. In the third manuscript, rheological characterization and viscosity measurements were made for several thermal accelerant solutions. Thermal accelerant is designed to deliver as a solution into the liver with sufficient viscosity to remain stationary at a target site during ablation. After the rheological characterization of a series of the thermal accelerant solutions, the sample designated “TA 50” appears to meet these criteria most effectively.


Suspensions for Lithium Ion Battery Anodes
In preparation, to be submitted to ACS Langmuir as: Although silicon has a large theoretical capacity, when used as an anode material in a LIB, it undergoes substantial volumetric strain of about 270% during charge, and the reverse during discharge. 9 With repeat cycling, this volume expansion and contraction can lead to pulverization of the particles, exposing fresh surfaces to the electrolyte. A solid electrolyte interphase forms at these surfaces, irreversibly consuming lithium ions and leading to capacity loss. An additional issue is that the volume expansion and contraction is relayed to the electron conducting network, often breaking it, and reducing electron conduction. The conducting network must either be 4 able to absorb the stress incurred during lithiation while maintaining its elasticity after delithiation, or be supported by a binding agent that can do so.
Since the conducting carbon does not contribute towards the capacity of the electrode, reduction of the carbon weight percent in an anode is one way to lower the overall weight of and raise the overall energy density in an LIB. It is therefore important to find a minimum amount of carbon required to form an effective conductive network. This minimum must be one that also limits destruction of the network during lithiation and delithiation to prevent the capacity of the battery from fading rapidly over time.
The first step in the assembly of a cell is the deposition of a suspension containing the active material, conducting material and binder in a solvent on to a copper film or on an aluminum film. A doctor blade is used to remove excess material. The slurry is then dried, calendared and sized. The rheological characteristics of the suspension have a strong impact on the thickness of the deposited layer. 14, 15 Suspension rheology will also affect the final thickness of the layer after the drying process. [14][15][16] In addition, differential capillary stresses in the electrodes during drying must be absorbed, so that cracks do not develop. 17 The rheological characteristics of the suspension can thus impact several parts of the electrode fabrication process. The major goal of this work is to thoroughly characterize the rheological properties of a suspension used in our own work for preparing anodes, as well as a suspension commonly used in commercial anodes for lithium ion batteries.
Para-amino benzoic acid terminated carbon black (CB) is a fractal particle that will form a network in solution. An emulsion of stabilized by particles such as CB is 5 called a Pickering emulsion. In contrast to a classical emulsion, in which the dispersed phase tends to coalesce over time, a Pickering emulsion will remain stable. 10 At concentrations higher than that required to form the network, the particles will disperse within the aqueous phase. This network shows promise in LIBs as a charge carrier when prepared during emulsion-templated directed assembly. 11 When a silicon rich oil phase is emulsified into an aqueous phase and dried, the silicon particles are encaged by a carbon network. The cage allows for silicon expansion and contraction, while the network provides support during lithiation and delithiation. It has also been shown that reduced graphene oxide (RGO) can act as an effective substitute for CB while reducing the total amount of carbon required to form a conducting network. 12 Its behavior alongside carbon black at varying concentrations is of interest for this study.
Before the drying process, polymer binder is present in the water phase of LIB anodes produced using the emulsion-templated directed assembly method. 11 Rheological characterization of the binder may provide insight to its behavior while drying and the behavior of the anode slurry during deposition.

Sample Preparation:
A 15 wt% stock suspension of para-amino benzoic acid (PABA)-terminated carbon black (CB) in water at pH 7 was provided by Cabot Corporation. Reduced graphene oxide nanoplatelets (RGO) with a specific surface area of ~833 m 2 /g and carbon/oxygen ratio of 10 Polymer binder solutions were created through the same materials and methods in [12]. Binder characterization began with the stock binder solution, consisting of a 20mg/ml mixture of 1:1 weight ratio CMC to PAA. This solution was added to samples with and without CB. All binder solutions were pH 3.3.

Rheology:
Characterization was completed by a TA Instruments AR-2000 stress controlled rheometer. A cone and plate geometry was utilized. This configuration had a set minimum gap height of 51 μm, a cone angle of 2 o , and a cone diameter of 4 cm.
The gap is much larger than the minimum particle size. All experiments were done at 25 C. 7 Samples were first probed for a change in viscosity with respect to shear rate, ranging from 3 s -1 to 2000 s -1 . Each experiment began with a preshear of 10 s -1 for 120 seconds to remove stresses incurred during sample loading. Viscosity characterization was performed three times for each sample. (G"). The relationship between stress, strain, and moduli can be represented as: Where σ is the shear stress produced, ω is the angular frequency of oscillation, and γ0 is the amplitude of the strain imposed on the fluid. 18 If the strain amplitude is small, the structure of the fluid will not be affected by the oscillatory measurement.
Shear stress output measured by the rheometer is converted to G' and G" contributions after each cycle of increasing applied strain. When G' and G" are no longer constant as strain increases, the sample is no longer in the LVR. For convenience, stain 8 amplitude is converted to oscillatory stress by the rheometer software, and the LVR is presented as the moduli as functions of oscillatory stress.
Once the LVR was determined for a sample, it was vortexed at 3000 rpm for 30 seconds. Each sample was probed within three minutes of vortexing. A 0.1 rad/s preshear was utilized for the oscillatory sweep. G' and G" were calculated for angular frequencies from 0.01 to 100 rad/s at an oscillatory stress within the LVR.
In some cases, an increase in viscosity or modulus response may be present in the results at the highest shear rates or angular frequencies, respectively. This effect does not necessarily indicate shear thickening behavior or a change in microstructure; a response uptick at high instrument speed is likely due to contributions of the rotating geometry. Viscosity and modulus are each calculated from a measured torque value.
For dilute aqueous suspensions at high shear rate, secondary toroidal flows are present. This leads to a higher torque requirement to maintain a given rate of shear, and can be falsely interpreted as a higher measured viscosity. Similarly, the torque required to oscillate the cone at 100 rad/s is considerably greater than the amount of torque contributed by particle interactions in a dilute aqueous suspension. In both cases, high shear results should not be considered when determining fluid behavior. 9

Results and Discussion
Carbon Black Weight Percentage: 1 Figure 1 shows the viscosities of CB concentrations without RGO at pH 3, 5, and 7. Shear thinning behavior is not present for any CB loading at pH 7. The variance in viscosity at low shear rates is within the error expected for a fluid with a viscosity similar to that of water, 0.8x10 -3 Pa*s at 25 C, especially with a cone and plate geometry. Reduction of sample pH to 5 increased viscosity in the 10 wt% and 7.5 wt% CB solutions, which reduce to a Newtonian plateau at high shear rates. At pH 3, shear thinning is present in CB solutions from 1.5 wt % to 10 wt%. The solution viscosity increases with CB wt%, with the exception of 10 wt% CB. Here, the 10 wt% solution is less viscous than the 7.5 wt% CB sample. that these carboxyl groups protonate as pH lowers, and the particles become increasingly hydrophobic. 13 It is energetically unfavorable for these hydrophobic regions to be exposed to water, so they connect with each other and form a network, increasing the suspension viscosity for a given CB concentration. At high CB loadings, it is possible that these aggregates become too large to support network formation.
In all cases where shear thinning behavior is present, the viscosity of the carbon samples is greater than the Einstein prediction for the corresponding volume fraction. Here, this relation is given as: Where is the viscosity of water at 25 o C, and is the volume fraction of CB. 19 The range of 0.05 wt% to 10 wt% equates to a range of 2.3x10 -4 Pa*s to 4.5x10 -2 Pa*s for the CB of these samples.  Dependence of solution viscosity on RGO concentration was investigated for the range of CB loadings at pH 3, 5, and 7. As shown in Figures 8 and 9, the addition of 0.01 wt% RGO tends to increase the viscosity of pH 5 samples towards the pH 3 upper limit. This effect increases with increasing CB wt%. Similarly, addition of 0.05 wt% RGO led to a consistent increase in low shear viscosity at all CB wt% for the pH 7 samples. At pH 3, only CB loadings below 1.5 wt% are affected by RGO content.
Overall, addition of RGO appears to increase the viscosity of low CB loadings while pH decrease increases the viscosity of high CB loadings.

Oscillatory Response of CB and RGO Suspensions in Water:
Oscillatory characterization was performed on all pH 3 CB and RGO samples to further investigate CB and RGO network formation. Meaningful results were not obtained for any carbon loading; the magnitudes of the moduli were not large enough to suggest a prevailing microstructure was present in any sample. was chosen for the comparison due to its higher viscosity. The results from Figure 1A show that 0.05 wt% and 0.5 wt% CB are subject to large variances from rheometer limitations. 1.5 wt% CB has considerably less variance, but is the lower limit of the reliable data.
Oscillatory characterization could not be performed on the dilution, with or without CB, as no stable LVR was present.

Conclusions
The viscosity dependence of aqueous suspensions of carbon black and reduced graphene oxide on carbon content and pH has been shown. Generally, as pH increases, addition of RGO to a suspension will increase the viscosity for low CB loadings, with diminishing effect as CB wt% increases. None of the suspensions in this study exhibit viscoelastic properties indicating the presence of a network throughout the water phase. This suggests that the water phase of an anode produced by emulsion templating would not develop an interconnected network prior to the drying process.
Inclusion of a polymer binder has minimal effect on system viscosity. This has been shown for 1.5 wt% CB, which is considerably more concentrated than the solutions in [12]. Although the determination of binder influence was completed at higher CB wt% due to instrument limitations, it suggests that even at lower CB concentrations, the contribution of the binder to the bulk rheological properties of the water phase in emulsion templated directed assembly may be minimal.

Introduction
Rechargeable lithium ion batteries (LIBs) are in high demand today, in part due to their favorable energy density as compared to other energy sources, and their promise as a potential replacement for internal combustion engines as vehicle power systems. 7

Sample Preparation:
Polyvinylidene fluoride (PVDF), N-Methyl-2-pyrrolidone (NMP), industrial graphite, and Timcal Super P carbon black were provided by EaglePicher. LITX-50 carbon black was provided by Cabot. Additional NMP was added until the total mass of NMP was equal to the total mass of solid components. This slurry was vortexed at 3000 rpm for 20 minutes until homogeneous.

Rheology:
Characterization was completed by a TA Instruments AR-2000 stress  Figure 16. The low variance in these results increases confidence that the system has been mixed properly, with little aging present.
Multiple shear rate sweeps were then completed for samples from the first vial to determine variability within a batch of samples, and subsequent vials were compared against this data for consistency, shown as the red line in Figure 16. All 31 subsequent results in this manuscript use the NMP/PVDF mixture as a solvent; the low error present in sample preparation increases confidence in the results. An apparent zero-shear viscosity is present at ~ 3 Pa*s for the NMP/PVDF mixture, which yields to shear thinning behavior at low shear rate. Once the base solvent was characterized, 2 wt% CB was added to the NMP/PVDF mixture. As with the base solvent, viscosity was measured for each prepared sample vial to confirm consistency among samples. This shear thinning behavior seen in Fig. 18, in contrast to the solvent, does not show a low shear viscosity plateau before a yield. The addition of 2 wt% of either CB increased the low shear viscosity of the solvent substantially, but also rapidly increased the rate of shear thinning. Although both CB solutions thin to 1 Pa*s at a shear rate of 100 s -1 , the solvent reaches 1 Pa*s at 1,000 s -1 . The modulus response for each form of CB is that of a highly structured fluid.
Graphite Slurries: Gelation increases when graphite is added to the slurry: both moduli increase, with G" increasing substantially. The Timcal Super P appears to provide a stronger network, due to larger storage modulus, and has more shear thinning behavior than the Cabot LITX-50. Both behaviors are beneficial for manufacturing, assuming the electrochemical properties of the completed anode do not suffer. A more structured gel can more efficiently absorb the stresses of drying without cracking, and a gel with a greater degree of shear thinning behavior can be manipulated more precisely by a doctor blade to control coating thickness.

Introduction
Hepatic cancers are the fifth most common cancer in men and eighth most common in women worldwide. 1 Image-guided thermal ablation (IGTA) including radiofrequency or microwave as energy sources, has long been used as a method of treatment for hepatic cancer. [2][3][4][5] RFTA is a process through which an electrode is percutaneously inserted into the liver and an RF generator provides energy, dissipated locally in the form of heat, to destroy cancer cells. 4 For treatment of tumors larger than 3 cm, RFTA is considerably more effective than any other method, including percutaneous ethanol injection. 4 Motivation for the development of a new treatment arose from the prevalence of liver cancer, and low rates of success in treatments such as chemotherapy and radiation therapy. By some calculations, there is an 80% 5-year recurrence rate from surgery alone. 5 In contrast to RFTA, which relies on an electrical current to destroy tumors, microwave ablation (MWA) uses an electromagnetic field to excite water molecules, generating heat locally. 8 The mechanism for MWA is faster than RFTA and produces a larger, more homogeneous area of ablation. 9,10 Despite its advantages, thermal ablation has shown a tumor recurrence rate of 30%, mainly due to incomplete ablation. 6 The auxiliary application of a microwave responsive thermal accelerant may increase the effectiveness of thermal ablation treatments. This application, guided by In this study, biopolymer mixtures containing various concentrations of polymer and thermal accelerant were characterized using a stress controlled rheometer over a range of shear rates, simulating conditions inside the body (1-100 s -1 ) and during injection (100-1,000 s -1 ). For samples that were sufficiently non-Newtonian, viscoelastic moduli were determined. These results were analyzed and used to determine the most effective formulation of thermal accelerant for thermal ablation.

Sample Preparation:
Thermal accelerant samples were prepared in the molecular imaging lab of the  A phase transition is desired in the copolymer system at 60 o C to separate polymeric precipitate and water. Figure 23 shows the viscosity response of two compounds at a low shear rate. The viscosity increase with temperature is much more prevalent in the TA 50 sample than the TA 74 sample. A viscosity increase of nearly two orders of magnitude is observed for TA 50 as temperature increases from 50 o C to 75 o C, while TA 74 increases about tenfold. There is a clear increase in viscosity for TA 50 around 60 o C, followed by a decrease and another, more pronounced increase in the next 10 o C temperature range. This sharp viscosity decrease is likely due to the temperature stratification present between the heated stationary plate and the unheated rotating cone of the rheometer. As the phase transition begins to precipitate polymer 47 from the mixture, the viscosity decreases until the precipitate begins to interfere with the flow of water/polymer mixture above it.