Appreciable global efforts are underway to develop processes for fabricating three-dimensional (3-D) nanostructured assemblies for advanced devices. Widespread commercialization of such devices will require: (i) precise 3-D fabrication of chemically tailored structures on a fine scale and (ii) mass production of such structures on a large scale. These often-conflicting demands can be addressed with a revolutionary new paradigm that couples biological self-assembly with synthetic chemistry: Bioclastic and Shape-preserving Inorganic Conversion (BaSIC). Nature provides numerous examples of microorganisms that assemble biominerals into intricate 3-D structures. Among the most spectacular of these microorganisms are diatoms (unicellular algae). Each of the tens of thousands of diatom species assembles silica nanoparticles into a microshell with a distinct 3-D shape and pattern of fine (nanoscale) features. The repeated doubling associated with biological reproduction enables enormous numbers of such 3-D microshells to be generated (e.g., only 40 reproduction cycles can yield >1 trillion 3-D replicas!). Such genetic precision and massive parallelism are highly attractive for device manufacturing. However, the natural chemistries assembled by diatoms (and other microorganisms) are rather limited. With BaSIC processes, biogenic assemblies can be converted into a wide variety of new functional chemistries, while preserving the 3-D morphologies. Ongoing advances in genetic engineering promise to yield microorganisms tailored to assemble nanoparticle structures with device-specific shapes. Large-scale culturing of such genetically tailored microorganisms, coupled with shape-preserving chemical conversion (via BaSIC processes), would then provide low-cost 3-D Genetically Engineered Micro/nano-devices (3-D GEMs).

Merging Biological Self-Assembly with Synthetic Chemical Tailoring: The Potential for 3-D Genetically Engineered Micro/Nano-Devices (3-D GEMS)

A low-temperature (≤700°C) magnesiothermic reaction has been used for the first time to convert three-dimensional (3-D) silica-based diatom microshells into nanocrystalline magnesia-based replicas. Exposure of diatom microshells to Mg(g) at only 650°–700°C resulted in the initial (direct) formation of MgO and Si nanocrystals (≤5 nm), that is, no intermediate magnesium silicate compounds were detected. Further reaction of Si with Mg(g) led to the formation of Mg2Si, and then a Mg–Si liquid that sweated away to yield free-standing, nanocrystalline MgO-based replicas. Such direct low-temperature magnesiothermic conversion of diatom microshells enables the synthesis of large numbers of 3-D nanocrystal assemblies with well-controlled morphologies for catalytic/chemical, biological, optical, and other applications.

Three‐Dimensional Magnesia‐Based Nanocrystal Assemblies Via Low‐Temperature Magnesiothermic Reaction of Diatom Microshells

https://www.rsc.org/suppdata/cc/c3/c3cc43134g/c3cc43134g.pdf

A magnesiothermic reaction process for the scalable production of mesoporous silicon for rechargeable lithium batteries

Strengthening mechanisms and deformation behavior of cryomilled Al–Cu–Mg–Ag alloy

The electrical, optical, and (bio)chemical properties of porous Si have attracted considerable interest for battery, solar cell, display, sensor, biomedical, and other applications. Free-standing monolithic Si aerogels have been generated for the first time via low-temperature magnesiothermic reduction of SiO2 aerogels. Under appropriate conditions, the reaction of Mg gas with SiO2 produced a co-continuous mixture of predominantly MgO and Si. Selective MgO dissolution then yielded nanocrystalline, porous, photoluminescent Si aerogels with a high degree of porosity (97.3 vol%) and a high specific surface area (615 m2 g−1).

Freestanding monolithic silicon aerogels

Major revisions in the enthalpies and Gibbs energies of formation of calcium oxide and magnesium oxide are presented. In free-evaporation experiments over the temperature range of 1830 to 2070 K, the kinetics of vaporization of calcium oxide and magnesium oxide have been studied. Conditions of local equilibrium were established. The results lead to new values for the standard enthalpies of formation of CaO and MgO. The ΔfH298
0 of CaO is ≈33 kJ/mol more positive than the currently tabulated value, whereas that for MgO is ≈34 kJ/mol more negative. The currently tabulated values could not account for these observations nor for other recent studies. These significant changes result in a reversal of the relative thermodynamic stabilities of CaO and MgO, with MgO now indicated as the more stable oxide at lower temperatures on Ellingham diagrams. These results are consistent with recent work on the formation of CaC2, solubility product measurements in liquid iron, and with theoretical models based on ionic radii.

M. Karaminezhad Ranjbar

Papers

Revision of the enthalpies and gibbs energies of formation of calcium oxide and magnesium oxide