Structural Evolution of Polymer-Derived Amorphous SiBCN Ceramics at High Temperature

Polymer-derived amorphous SiBCN ceramics are synthesized through a simple dehydrocoupling and hydroboration reaction of an oligosilazane containing amine and vinyl groups and BH3·Me2S, followed by pyrolysis. Two types of ceramics, denoted as Si2B1 and Si4B1, are produced from preceramic polymers with Si/B ratios of 2/1 and 4/1, respectively. The structural evolution of these ceramics with respect to the pyrolysis temperature and boron concentration is investigated using solid-state NMR, Raman, and EPR spectroscopy. Solid-state NMR suggests the presence of three major components in the ceramics: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN2C groups. Increasing pyrolysis temperature leads to the transformation of BN2C groups into BN3 and “free” carbon. A thermodynamic model is proposed to explain such transformation. Raman spectroscopy measurements reveal that the concentration of the “free” carbon cluster decreases with increasing pyrolysis temperature, and Si4B1 contains more “free” carbon cluster than Si2B1. EPR studies reveal that the carbon (C)-dangling bond content also decreases with increasing pyrolysis temperature. It appears that the complete decomposition of the metastable BN2C groups to the BN3 groups and the “free” carbon affects the crystallization of SiBCN, which leads to Si4B1 ceramics crystallized at 1500 °C, whereas Si2B1 ceramics crystallized at 1600 °C

Soluble and Meltable Hyperbranched Polyborosilazanes toward High-Temperature Stable SiBCN Ceramics

High-temperature stable siliconborocarbonitride (SiBCN) ceramics produced from single-source preceramic polymers have received increased attention in the last two decades. In this contribution, soluble and meltable polyborosilazanes with hyperbranched topology (hb-PBSZ) were synthesized via a convenient solvent-free, catalyst-free and one-pot A₂ + B₆ strategy, an aminolysis reaction of the A₂ monomer of dichloromethylsilane and the B₆ monomer of tris­(dichloromethylsilylethyl)­borane in the presence of hexamethyldisilazane. The amine transition reaction between the intermediates of dichlorotetramethyldisilazane and tri­(trimethylsilylmethylchlorosilylethyl)­borane led to the formation of dendritic units of aminedialkylborons rather than trialkylborons. The cross-linked hb-PBSZ precursors exhibited a ceramic yield higher 80%. The resultant SiBCN ceramics with a boron atomic composition of 6.0–8.5% and a representative formula of Si₁B_0.19C_1.21N_0.39O_0.08 showed high-temperature stability and retained their amorphous structure up to 1600 °C. These hyperbranched polyborosilazanes with soluble and meltable characteristics provide a new perspective for the design of preceramic polymers possessing advantages for high-temperature stable polymer-derived ceramics with complex structures/shapes

Magnetoceramics from the Bulk Pyrolysis of Polysilazane Cross-Linked by Polyferrocenylcarbosilanes with Hyperbranched Topology

In this contribution, we report a novel strategy for the synthesis of nanocrystal-containing magnetoceramics with an ultralow hysteresis loss by the pyrolysis of commercial polysilazane cross-linked with a functional metallopolymer possessing hyperbranched topology. The usage of hyperbranched polyferrocenylcarbosilane offers either enhanced ceramic yield or magnetic functionality of pyrolyzed ceramics. The ceramic yield was enhanced accompanied by a decreased evolution of hydrocarbons and NH₃ because of the cross-linking of precursors and the hyperbranched cross-linker. The nucleation of Fe₅Si₃ from the reaction of iron atoms with Si–C–N amorphous phase promoted the formation of α-Si₃N₄ and SiC crystals. After annealing at 1300 °C, stable Fe₃Si crystals were generated from the transformation of the metastable Fe₅Si₃ phase. The nanocrystal-containing ceramics showed good ferromagnetism with an ultralow (close to 0) hysteresis loss. This method is convenient for the generation of tunable functional ceramics using a commercial polymeric precursor cross-linked by a metallopolymer with a designed topology

Bundled Silicon Nitride Nanorings

Bundled nanorings composed of several self-coiled nanowires are synthesized by catalyst-assisted pyrolysis of a polymeric precursor. Microstructural observation reveals that the nanowires have a bilayer structure consisting of a thin single-crystal Si3N4 layer and a thick amorphous layer. The thickness ratio of the two layers is ∼ 1:2, regardless of the nanowire size. The formation of the thick amorphous layer is likely due to the high Al concentration within the precursor. The self-coiling mechanism is discussed and attributed to the difference in growth rates of the two layers. A phenomena model is proposed to account for the formation of the nanoring structure

Morphology Transformation of Supramolecular Structures in Aqueous Mixtures of Two Oppositely Charged Amphiphiles

The co-assembly of oppositely charged amphiphiles provides a fascinating approach for forming complex supramolecular structures, which are interesting from both fundamental and technological viewpoints. Here, we report a stepwise morphology transformation of co-assembled supramolecular structures in the aqueous mixture of lithocholic acid (LCA) and cetyltrimethylammonium bromide (CTAB) at mixed molar ratios of 1:1 and 2:1. The co-assembly of LCA and CTAB initially forms multilamellar vesicles followed by the spontaneous growth of membrane tubes from the vesicles. The vesicle-to-tube transition is accompanied by a fluidic-to-crystalline phase transition. After being aged, the membrane tubes twist into left-handed helices, which then intertwine into left-handed double helices and multihelix bundles. The single handedness of these supramolecular structures is a reflection of the amplification of the chirality of LCA. An understanding of the co-assembly mechanism and pathway is a key step toward producing supramolecular structures with distinguished morphologies

Polymer-Derived Ceramic Composite Fibers with Aligned Pristine Multiwalled Carbon Nanotubes

Polymer-derived ceramic fibers with aligned multiwalled carbon nanotubes (MWCNTs) are fabricated through the electrospinning of polyaluminasilazane solutions with well-dispersed MWCNTs followed by pyrolysis. Poly(3-hexylthiophene)-b-poly (poly (ethylene glycol) methyl ether acrylate) (P3HT-b-PPEGA), a conjugated block copolymer compatible with polyaluminasilazane, is used to functionalize MWCNT surfaces with PPEGA, providing a noninvasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions. The electrospinning of the MWCNT/polyaluminasilazane solutions generates polymer fibers with aligned MWCNTs where MWCNTs are oriented along the electrospun jet by a sink flow. The subsequent pyrolysis of the obtained composite fibers produces ceramic fibers with aligned MWCNTs. The study of the effect of polymer and CNT concentration on the fiber structures shows that the fiber size increases with the increment of polymer concentration, whereas higher CNT content in the polymer solutions leads to thinner fibers attributable to the increased conductivity. Both the SEM and TEM characterization of the polymer and ceramic fibers demonstrates the uniform orientation of CNTs along the fibers, suggesting excellent dispersion of CNTs and efficient CNT alignment via the electrospinning. The electrical conductivity of a ceramic fibers with 1.2% aligned MWCNTs is measured to be 1.58 × 10−6 S/cm, which is more than 500 times higher than that of bulk ceramic (3.43 × 10−9 S/cm). Such an approach provides a versatile method to disperse CNTs in preceramic polymer solutions and offers a new approach to integrate aligned CNTs in ceramics

Facilitating Anion Transport in Polyolefin-Based Anion Exchange Membranes via Bulky Side Chains

Highly anion-conductive polymer electrolyte membranes with excellent alkaline stabilities for fuel cell applications were prepared. Thus, a series of polyolefin copolymers with poly­(4-methyl-1-pentene) (PMP) moieties containing bulky side chains and side-chain quaternary ammonium (QA) groups were prepared through copolymerization with a Ziegler–Natta catalyst and subsequent quaternization. The separation of hydrophilic microphase and hydrophobic microphase was induced by PMP bulky side chains, and then well-connected ionic domains were formed. This result was confirmed by AFM (atomic force microscopy) and SAXS (small-angle X-ray scattering) analyses. It was discovered that well-defined ionic domains of the PMP-TMA-x (TMA, trimethylamine) membranes depended on the content of PMP moieties. The well-defined ionic domains enhanced the hydroxide conductivity of the PMP-TMA-x membranes despite their lower water uptake (WU) as compared to polypropylene (PP)-containing membranes (PP-TMA-x). The PMP-TMA-41 membrane showed the highest ionic conductivity value (43 mS/cm) while maintaining low WU (29.2 wt %) at room temperature. The membranes mostly preserved (>93.0%) their initial hydroxide conductivity after alkaline treatment (10 M aqueous NaOH, 80 °C, 700 h), thereby revealing desirable alkali stability characteristics. Presumably, the nucleophilic attack from hydroxide or water in the cationic center is inhibited by long alkyl spacers (−CH2−)n (n = 9) which are located between the cation groups and the polymer backbone