Unraveling the complex interplay between thermal properties and hydration is a part of understanding the fundamental properties of many soft materials and very essential for many applications. Here we show that graphene oxide (GO) demonstrates a highly negative thermal expansion (NTE) coefficient owing to unique thermohydration processes related with fast transport of water between the GO sheets, the amphiphilic nature of nanochannels, and close-to-zero intrinsic thermal expansion of GO. The humidity-dependent NTE of GO layered assemblies, or "pseudonegative thermal expansion" (PNTE), differs from that of other hygroscopic materials due to its relatively fast and highly reversible expansion/contraction cycles and occurrence at low humidity levels while bearing similarities to classic NTE. Thermal expansion of polyvinyl alcohol/GO composites is easily tunable with additional intricacy of thermohydration effects. PNTE combined with isotropy, nontoxicity, and mechanical robustness is an asset for applications of actuators, sensors, MEMS devices, and memory materials and crucial for developing methods of thermal/photopatterning of GO devices.
The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S' precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.
Layer-by-layer assembly (LBL) can create advanced composites with exceptional properties unavailable by other means, but the laborious deposition process and multiple dipping cycles hamper their utilization in microtechnologies and electronics. Multiple rinse steps provide both structural control and thermodynamic stability to LBL multilayers but they significantly limit their practical applications and contribute significantly to the processing time and waste. Here we demonstrate that by employing inkjet technology one can deliver the necessary quantities of LBL components required for film build-up without excess, eliminating the need for repetitive rinsing steps. This feature differentiates this approach from all other recognized LBL modalities. Using a model system of negatively charged gold nanoparticles and positively charged poly(diallyldimethylammonium) chloride, the material stability, nanoscale control over thickness and particle coverage offered by the inkjet LBL technique are shown to be equal or better than the multilayers made with traditional dipping cycles. The opportunity for fast deposition of complex metallic patterns using a simple inkjet printer was also shown. The additive nature of LBL deposition based on the formation of insoluble nanoparticle-polyelectrolyte complexes of various compositions provides an excellent opportunity for versatile, multi-component, and non-contact patterning for the simple production of stratified patterns that are much needed in advanced devices.
The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of SЈ precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed SЈ precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within "rogue" inclusions was quantified using a finite-element analysis (FEA) unit-cell model.
Versatile all‐nanocomposite capped microcontainers are made using layer‐by‐layer (LBL) assembly. The microcontainers can act as inert packaging with slow/controlled release for virtually any type of encapsulating material based on clay nanocomposites 3D molded by PDMS templates and capped with another LBL film.
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