A primary mode of failure of thin-film coatings is the mismatch in thermal expansion coefficients of the substrate and the coating, which results in accumulation of interfacial stresses and ultimately in film delamination. While much attention has been devoted to modulation of interfacial bonding to mitigate delamination, current strategies are constrained in their generalizability and have had limited success in imbuing resistance to prolonged thermal cycling. We demonstrate here the incorporation of rigid thermal expansion compensators within polymeric films as a generalizable strategy for minimizing thermal mismatch with the substrate. Nanostructures of the isotropic negative thermal expansion (NTE) material HfV 2 O 7 have been prepared based on the reaction of nanoparticulate precursors. The NTE behavior, derived from transverse oxygen displacement within the cubic structure, has been examined using temperature-variant powder X-ray diffraction, Raman spectroscopy, electron microscopy, and selected-area electron diffraction measurements. HfV 2 O 7 initially crystallizes in a 3 × 3 × 3 superlattice but undergoes phase transformations to stabilize a cubic structure that exhibits strong and isotropic NTE with a coefficient of thermal expansion (CTE) = −6.7 × 10 −6 °C−1 across an extended temperature range of 130−700 °C. Incorporation of HfV 2 O 7 in a high-temperature thermoset polybenzimidazole enables the reduction of compressive stress by 67.3% for a relatively small loading of 26.6 vol % HfV 2 O 7 . Based on a composite model, we demonstrate that HfV 2 O 7 can reduce the thermal expansion coefficient of polymer nanocomposite films, even at low volume fractions, as a result of its substantially higher elastic modulus compared to the continuous polymer matrix. By changing the volume fraction of HfV 2 O 7 , the overall coefficients of thermal expansion of the film can be tuned to match a range of substrates, thereby mitigating thermal stresses and resolving a fundamental challenge for high-temperature composites and nanocomposite coatings.
FeS nanoplatelets were synthesized using a surfactantassisted hydrothermal synthesis. The product is highly crystalline and has a preferred growth direction with a [001] plate normal. The platelet diagonal, thickness, and shape can be controlled by varying the iron starting material or the surfactant employed in the synthesis. The diagonal and thickness of the nanoplatelets were found to reduce by factors of up to 22 and 8×, respectively, when an Fe(II)-containing iron sourcerather than Fe(III)was employed in the synthesis. Specific combinations of the surfactant and the iron source were seen to determine the platelet shape, resulting in rectangular, polygonal, and shard-like shapes.
Nanostructuring inorganic solids has been effective as a tool to control the identity of the thermodynamically stable phase under ambient conditions for many systems. In addition, size effects can alter not only the temperature but also the other characteristics of a trans-formationsuch as order, mechanism, and kineticswhich may further be responsible for the transient existence of intermediates, both thermodynamic and metastable, not accessed in the bulk. Since understanding the mechanism of a phase transformation requires local, even atomistic, information, and a similar understanding of the kinetics requires this information to be collected in real-time, in situ microscopy has proved invaluable in identifying key features of transformations on the nanoscale and promises to play a key role in the future design and implementation of such systems. Here, we discuss the use of in situ heating and biasing in the transmission electron microscope to investigate phase transformations in inorganic, single-phase, solid-state, nanostructured systems.
Carbon nanotubes (CNTs) offer unique properties that have the potential to address multiple issues in industry and material sciences. Although many synthesis methods have been developed, it remains difficult to control CNT characteristics. Here, with the goal of achieving such control, we report a bottom-up process for CNT synthesis in which monolayers of premade aluminum oxide (Al2O3) and iron oxide (Fe3O4) nanoparticles were anchored on a flat silicon oxide (SiO2) substrate. The nanoparticle dispersion and monolayer assembly of the oleic-acid-stabilized Al2O3 nanoparticles were achieved using 11-phosphonoundecanoic acid as a bifunctional linker, with the phosphonate group binding to the SiO2 substrate and the terminal carboxylate group binding to the nanoparticles. Subsequently, an Fe3O4 monolayer was formed over the Al2O3 layer using the same approach. The assembled Al2O3 and Fe3O4 nanoparticle monolayers acted as a catalyst support and catalyst, respectively, for the growth of vertically aligned CNTs. The CNTs were successfully synthesized using a conventional atmospheric pressure-chemical vapor deposition method with acetylene as the carbon precursor. Thus, these nanoparticle films provide a facile and inexpensive approach for producing homogenous CNTs.
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