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Organic–inorganic hybrids have found increasing applications for thermal management across various disciplines. Such materials can achieve thermal conductivities below the so-called “amorphous limit” of their constituents’ thermal conductivity. Despite their technological significance, a complete understanding of the origins of this thermal conductivity reduction remains elusive in these materials. In this paper, we develop a prototypical cross-linked organic–inorganic layered system, to investigate the spectral origins of its sub-amorphous thermal conductivity. Initially, we study the atomic structure of the model and find that besides polymer chain length, the relative drift of the layers governs the reduction in computed basal spacing, in agreement with experimental measurements. We, subsequently, find that organic cross-linking results in up to 40% reduction in thermal conductivity compared to inorganic samples. An in-depth investigation of vibrational modes reveals that this reduction is the result of reduced mode diffusivities, which in turn is a consequence of a vibrational mismatch between the organic and inorganic constituents. We also show that the contribution of propagating modes to the total thermal conductivity is not affected by organic cross-linking. Our approach paves the path toward a physics-informed analysis and design of a wide range of multifunctional hybrid nanomaterials for thermal management applications among others. 

Coupling of organic and inorganic chemistry presents a new degree of freedom in nano-engineering of thermo-mechanical properties of cement-based materials. Despite these vast technological benefits, molecular scale cross-linking of calcium-silicate-hydrate (C-S-H) gel with organic molecules still presents a significant challenge. Herein, we report experimental results on sol-gel synthesis, structure and morphology of nanocrystalline C-S-H cross-linked with dipodal organosilanes. These novel organic-inorganic gels have layered turbostratic molecular structure with similarities to C-S-H precipitating in hydrating cement paste. The organic molecules' chain length controls the interlayer spacing, which shows little to no shrinkage upon dehydration up to 105 °C. However, the structure gets distorted in the basal crystallite plane, in which dimer and trimer Si-polyhedra structures condense on a 2D hexagonal Ca-polyhedra layer. Cross-linked C-S-H gels display plate-like morphology with tendency toward stacking into agglomerates at the larger scale. If successfully realized in cement environment, e.g. high concentration seed, such novel organic-inorganic C-S-H gels could potentially provide cement-based matrices with unique properties unmatched by classical inorganic systems. 

Lightweight cement-based composites with high specific strength and low thermal conductivity are highly sought in the energy and construction industries. These characteristics are important in designing cement liners for high-temperature, high-pressure (HTHP) wells, in addition to those operating in permafrost. Similar attributes are also desirable in designing cementitious composites for energy efficient building envelopes. This work reports the results of an experimental campaign focused on engineering lightweight cementitious composites with hollow glass microspheres. It is demonstrated that the chemical stability of microspheres at HTHP conditions can be directly controlled by modulating the specific surface area and dissolution rate constant of supplementary siliceous additives. In addition to the stabilizing effect, such additives lead to the pore structure refinement and the enhancement of interfacial transition zone (ITZ). Introduced lightweight composites are capable of delivering significant load bearing capacity when normally cured, which is greatly increased by hydrothermal curing. Such high specific strength composites possess thermal conductivity below 0.3 W/mK at the oven dry density <1000 kg/m3 and cement dosage <400 kg/m3. This class of cementitious composites bears potential to enhance zonal insulation and well integrity, as well as increasing energy efficiency of building envelopes. 

Progress in microstructural characterization methods is summarized. Special attention is given to advanced probes, such as X-ray imaging and spectroscopy, 1H NMR relaxometry, in-situ and high-pressure X-ray diffraction, and digital holographic microscopy. Microtomography has become a mature technique and nanotography has improved its spatial resolution significantly, particularly with the use of ptychography. The review also discusses the effect of plasticizers on the microstructure of concrete and presents a critical analysis of how organic admixtures affects the hydrates obtained from pure synthesis in saturated solutions and from more realistic hydrating systems. The addition of nanomaterials into cementitious systems modifies the microstructure of the matrix so a summary of recent research is presented. It is important to integrate the impressive progress in the characterization of the micro(nano)structure with efforts developing realistic models. Therefore, the present work gives a short but critical presentation of physical chemistry approaches that aim to link the chemical composition, texture, and microstructure of the cement hydrates.