Jon J. Kellar

Security printing of covert quick response codes using upconverting nanoparticle inks

Counterfeiting costs governments and private industries billions of dollars annually due to loss of value in currency and other printed items. This research involves using lanthanide doped β-NaYF(4) nanoparticles for security printing applications. Inks comprised of Yb(3+)/Er(3+) and Yb(3+)/Tm(3+) doped β-NaYF(4) nanoparticles with oleic acid as the capping agent in toluene and methyl benzoate with poly(methyl methacrylate) (PMMA) as the binding agent were used to print quick response (QR) codes. The QR codes were made using an AutoCAD file and printed with Optomec direct-write aerosol jetting(®). The printed QR codes are invisible under ambient lighting conditions, but are readable using a near-IR laser, and were successfully scanned using a smart phone. This research demonstrates that QR codes, which have been used primarily for information sharing applications, can also be used for security purposes. Higher levels of security were achieved by printing both green and blue upconverting inks, based on combinations of Er(3+)/Yb(3+) and Tm(3+)/Yb(3+), respectively, in a single QR code. The near-infrared (NIR)-to-visible upconversion luminescence properties of the two-ink QR codes were analyzed, including the influence of NIR excitation power density on perceived color, in term of the CIE 1931 chromaticity index. It was also shown that this security ink can be optimized for line width, thickness and stability on different substrates.

Enhanced thermal conductivity by aggregation in heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes

An approximately 10% increase in the thermal conductivity (TC) of heat transfer nanofluids containing metal oxide nanoparticles and carbon nanotubes has been determined with very low percentage loading (around 0.02wt%) of these two nanomaterials. These fluids are very stable and the viscosity remains approximately the same as water. A possible explanation for these interesting results is the aggregation of metal oxide particles on the surface of nanotubes by electrostatic attraction and form the aggregation chain along the nanotube. Time dependant magnetic results demonstrate that, under the influence of a strong outside magnetic field, the TC value decreases. Also, the TC value decreases when the pH is shifted from 7 to 11.45.

Determining the interphase thickness and properties in polymer matrix composites using phase imaging atomic force microscopy and nanoindentation

In polymer matrix composites, the interface between the reinforcing phase and the bulk phase is paramount to the overall performance of the composite as a structural material. This interface is now thought to be a distinct, three-dimensional phase surrounding the reinforcing phase called the interphase. The developments of the atomic force microscope and nanoindentation devices have facilitated the investigation of the interphase. Previously, force modulation atomic force microscopy (AFM) and nanoindentation were the primary methods used to determine the size of the interphase and its stiffness relative to the bulk phase. The present investigation utilized phase imaging AFM and nanoindentation to examine the interphase in a glass fiber-reinforced epoxy matrix composite. Nanoindentation experiments indicated that the relatively stiff fiber might have caused a gradient in the modulus across the interphase region. Specifically, the modulus next to the fiber approached that of the fiber and decreased to that of the bulk polymer as the distance away from the fiber increased. Once the fiber was removed by chemical etching, this gradient reversed itself; hence, nanoindentation, due to the fiber bias, was not found to be adequate for measuring actual interphase properties. It was found that phase imaging AFM was a highly useful tool for probing the interphase, because it involves much lower interaction forces between the probe and the sample than force modulation or nanoindentation. The interphase in the model composite investigated was found to be softer than the bulk phase with a thickness of 2.4-2.9 μm, and was independent of fiber silane pretreatment, for silane pretreatments between 0.1% and 5.0% (initial aqueous concentration).

Red-green-blue printing using luminescence-upconversion inks

Recent advances in producing pre-defined 2D patterns of upconversion nanophosphors via photolithography and printing techniques present new opportunities for the use of these materials in security applications. Here, we demonstrate an RGB additive-color printing system that produces highly-resolved pre-defined patterns that are invisible under ambient lighting, but which are viewable as luminescent multi-color images under NIR excitation. Patterns are generated by independent deposition of three primary-color (red, green and blue) upconverting inks using an aerosol jet printer. The primary-color inks are printed as isolated and overlapping features to produce images that simultaneously emit red, green, blue, cyan, magenta, yellow and white upconversion luminescence. The dependence of the chromaticity of certain secondary colors (cyan and magenta) and white on NIR excitation power density can be exploited as an additional authentication feature. The development of an RGB upconversion printing system paves the way for an entirely new arena in security printing.

Non-aqueous synthesis of silver nanoparticles using tin acetate as a reducing agent for the conductive ink formulation in printed electronics

We have developed a process for the synthesis of silver nanoparticles protected with a passivating shell of dodecylamine in toluene media using tin(II) acetate as a reducing agent. Based on the electrochemical series, during the reduction process Sn(II) oxidizes into Sn(IV) which reduces Ag(I) into Ag(0). The nucleation and growth processes result in particles with diameters in the range 5–20 nm. This simple non-aqueous one pot synthesis can be easily scaled up to produce grams of nanoparticles in a matter of hours. The particles can also be dispersed in many non-aqueous solvents which make them a suitable candidate for many applications. Characterization of the end product using TEM, UV-Vis spectroscopy, and powder X-ray diffraction verified the presence of a silver metallic core whereas TGA confirmed the presence of a dodecylamine shell. The resulting particles were used in non-aqueous conductive ink formulation. The ink was used to print conductive tracks on flexible substrates like Epson photo paper and polyimide (Kapton) using an Aerosol Jet based printing technique.

Highly conductive short chain carboxylic acid encapsulated silver nanoparticle based inks for direct write technology applications

In this study silver nanoparticles were synthesized with short chain (C6–C10) carboxylic acids as capping agents and prepared as conductive inks for fabricating electrically conductive patterns using direct write technologies. The structural characterization of as-synthesized nanoparticles revealed that the particles are spherical in shape with narrow size distribution (4.1 to 4.7 nm) and have face centered cubic crystal structure. Silver-particle-based inks were prepared by dispersing the particles in toluene and separating non-dispersing particles from the inks. The loading of silver particles in the solvent was increased with the increasing chain length of capping agents. As a result, inks with a wide range of nanoparticulate concentrations (∼3 to 66 wt%) were able to be prepared and most of these inks were stable for at least a month. All the inks exhibited shear thinning behavior and this shear thinning became more prominent for higher concentration inks. As the nanoparticle concentration of the inks was increased, surface tension was decreased and the contact angles of the inks with the Kapton® and glass were increased. Printing of microelectrodes, lines and films was carried out using aerosol jet printing and ultrasonic spray coating. The conductivity of printed microelectrodes was 10 to 87% of the bulk silver conductivity with the sintering temperatures as low as 130 to 250 °C depending on the ink used.

Interphase variation in silane-treated glass-fiber-reinforced epoxy composites

The interphase region of an epoxy/glass fiber model composite was examined by atomic force microscopy phase imaging (AFM-PI) and nanoindentation. The interphase thickness was determined by AFM-PI as a function of γ-aminopropyl silane coupling agent concentration. With no silane, no measurable interphase was observed. With adsorption from 5 wt% solution, the observed interphase was 888 ± 30.3 nm thick. Coupling agent adsorption was also performed from 0.1, 1 and 3 wt% silane solutions. The interphase thickness was found to increase with increasing silane solution concentration from 110 to 210 to 375 nm, respectively. Nanoindentation of these same interphases showed that only the 3 wt% and 5 wt% interphases were sufficiently thick enough to not include a significant fiber bias effect. For these two interphases, the indentation depths in the interphase were 8.3% and 42% greater, respectively, than the indentation depth in the matrix.

Finite element evaluation of the microbond test: meniscus effect, interphase region, and vise angle

Abstract An axisymmetric finite element model (FEM) was used to determine the stresses that develop during a microbond test of a glass fiber/polymer matrix composite system. The complete bead shape including the meniscus that occurs as the bead wets to the fiber was included in the FEM to determine the influence of the bead geometry on the resulting stresses. Significant differences in the stress fields were found near the fiber/bead contact point due to the geometry and resulting z-location of the vise. In addition, the vise angle and interphase properties were varied to examine their influence. As the vise angle increases, the combined state of stress decreases considerably. The effect of the interphase, while not as significant as the effect of the vise angle, did produce a 10% variation in von-Mises stress at the fiber/bead contact point for the properties used. It was found that the ratio of the maximum shear to average shear equaled 4 near the fiber/bead contact point. At this location the interfacial material has yielded with a von-Mises stress 8× the average shear. Finally, results are provided showing the average interfacial shear stress value obtained by the FEM is within 2% of the theoretical solution.

In Situ Near-IR Cure Monitoring of a Model Epoxy Matrix Composite

To improve the performance of composites it is imperative that the interphase region between the inorganic reinforcement and the polymer matrix be more completely understood. It is in this region that the stress transfer between the matrix and the reinforcement occurs. To this end, the curing of epoxy adjacent to an embedded silica optical fiber has been monitored in situ by evanescent wave spectroscopy. The epoxy studied is partially fluorinated and has a lower refractive index than the silica optical fiber. This combination of epoxy/silica served as a model composite system. The lower refractive index of the partially fluorinated epoxy allowed the silica optical fiber to be used as a waveguide for the internal reflection of near-infrared light. The epoxy curing was determined as a function of time and temperature by analysis of the near-infrared spectrum from the epoxy adjacent to the fiber obtained by the interaction of the evanescent wave that occurs at each internal reflection with the low-refractive-index epoxy. The results obtained from the examination of the near-infrared spectrum, particularly the disappearance of the NH2 stretching/bending combination band at ∼4925 cm−1 and the concomitant increase of the C–N overtone band at ∼4725 cm−1, showed that epoxy ring-opening and cross-linking reactions could be followed in real time. Finally, treatment of the fiber with a silane coupling agent had no observable effect on the curing reaction of the epoxy.

Opportunities and challenges for treating rare-earth elements

Rare-earth elements (REEs) have become an important group of metals used in various high-tech industries. In this paper, the authors have first reviewed and discussed various mineral-processing techniques used in the processing of these REE-containing ores. Because the majority of minerals from which REEs are extracted are monazite, bastnasite, and xenotime, the discussion has focused around the ores containing these three minerals. Following this discussion, various leaching technologies have been reviewed and problems associated with leaching technologies have been assessed again in the context of the afore-mentioned three minerals. The leaching technologies discussed here include leaching using various acids, and also leaching after acidic and alkaline roasting of the ores. Because of chemical similarities of these REEs, it is very difficult to separate the individual elements from each other. Various technologies used in this regard including chemical precipitation, ion exchange, and solvent extraction have been reviewed and discussed in detail. The effect of speciation of REEs in the solutions on the selective recovery has also been investigated.