Nano graphene platelet-based conductive inks

This is on top of the fact tha...

[0042]The second step comprises dispersing laminar materials (e.g., graphite or graphite oxide particles) in a liquid medium (e.g., water, alcohol, or acetone) to obtain a suspension or slurry with the particles being suspended in the liquid medium. The third step entails subjecting the suspension to direct ultrasonication at a temperature typically between 0° C. and 100° C. Hence, this method obviates the need or possibility to expose the graphite material to a high-temperature, oxidizing environment. Preferably, a dispersing agent or surfactant is used to help uniformly disperse particles in the liquid medium. Most importantly, we have surprisingly found that the dispersing agent or surfactant facilitates the exfoliation and separation of the laminar graphite material. Under comparable processing conditions, a graphite sample containing a surfactant usually results in much thinner platelets compared to a sample containing no surfactant. It also takes a shorter length of time for a surfactant-containing suspension to achieve a desired platelet dimension.

[0045]Oxidized NGPs or graphite oxide platelets may be obtained by intercalation and exfoliation of graphite. Intercalation of graphite to form a graphite intercalation compound (GIC) is well-known in the art. A wide range of intercalants have been used; e.g., (a) a solution of sulfuric acid or sulfuric-phosphoric acid mixture, and an oxidizing agent such as hydrogen peroxide and nitric acid and (b) mixtures of sulfuric acid, nitric acid, and manganese permanganate at various proportions. Typical intercalation times are between one hour and five days. The resulting acid-intercalated graphite may be subjected to repeated washing and neutralizing steps to produce a laminar compound that is essentially graphite oxide. In other words, graphite oxide can be readily produced from acid intercalation of graphite flakes.

[0046]Conventional exfoliation processes for producing graphite worms (interconnected networks of thin graphite flakes) from a graphite material normally include exposing a graphite intercalation compound (GIC) or oxidized graphite to a high temperature environment, most typically between 850 and 1,050° C. These high temperatures were utilized with the purpose of maximizing the expansion of graphite crystallites along the c-axis direction. In some cases, separated NGPs are readily obtained with this treatment, particularly when the graphite has been heavily oxidized. In other cases, the exfoliated product may be subjected to a subsequent mechanical shearing treatment, such as ball milling, air milling, rotating-blade shearing, or...

[0027]The presently invented conductive ink is preferably inkjet printable since inkjet printing is a cost-effective way to achieve patterns of various materials on both rigid and flexible substrates. Inkjet printing of electrically conductive nano particle-based inks offer a very practical platform for generating electrical components, such as electrodes and interconnects. More preferably, inkjet printing is conducted using a conventional, inexpensive printhead in a common desk-top printer. This type of printer typically requires the viscosity of the ink to be in the range of 3-30 mPa·S (centi-poise or cP). It is of significance to note that the viscosity of a CNT-based ink can not be in this useful range unless the CNT proportion is exceedingly low. For instance, the CNT concentration of the ink used by Song, et al. [Ref. 1] was as low as 20 μg/mL (approximately 0.002% by weight of CNTs in water). With such a low concentration, it would take several repeated printing passes (overwrites) to achieve a desired CNT amount, thickness, or property; e.g., it took 8 overwrites to achieve a sheet resistivity of 20 μΩm [FIG. 5 in Ref. 1]. By contrast, with the presently invented NGP-based ink that can carry a high NGP proportion, yet still maintaining a relatively low viscosity, one or two printing passes are sufficient to attain the same desired properties achieved with 5-20 overwrites using CNT-based inks. This implies that the printing speed of NGP-based inks would be much higher. This is on top of the fact that CNTs are extremely expensive.

[0028]Currently, certain type of specialty printer can print an ink with a solution viscosity up to 150 mPa·S and some experimental printers that are still under development can work with a viscosity up to 500 mPa·S. Even with these high-viscosity printers one would still find it difficult, if not impossible, to print CNT-based inks with a CNT content greater than 0.2% by weight since their viscosity will be greater than 1 Pa·S or 1,000 mPa·S. This is...