Holographic photopolymerization combined to microfluidics for the fabrication of lab-in-lab microdevices and complex 3D micro-objects.

We show here brand-new possibilities of lab-in-lab fabrication while combining holographic photopolymerization and microfluidics. One shot real-time 3D-printing can produce 3D architectured microchannels, or free-standing complex micro-objects eventually in flow. The methodology is very versatile and can be applied to e.g., acrylate resins or hydrogels.

Superparamagnetic Silicon Carbonitride Ceramic Fibers Through In Situ Generation of Iron Silicide Nanoparticles During Pyrolysis of an Iron-Modified Polysilazane.

Ceramic fibers are high-tech structural key components of ceramic matrix composites (CMCs), which are a very promising class of materials for applications in next-generation turbines, especially nonoxide ceramic fibers, usually produced by the polymer-derived ceramics (PDC) route, which possess the enhanced mechanical and thermostructural properties necessary to withstand the harsh conditions (temperature and atmosphere) imposed on CMCs. However, recycling composite materials, such as fiber-reinforced polymers and CMCs, is still a big challenge. Here, we present for the first time the processing of superparamagnetic iron-containing ceramic fibers, which, due to their magnetic properties, can be separated from the matrix material of a composite. The synthesis strategy of the novel functional ceramic fibers is based on a tailored reaction of polyorganosilazane with an iron complex, resulting in a suitable, meltable polymer. After melt-spinning and curing, subsequent pyrolysis leads to superparamagnetic ceramic fibers with a saturation magnetization of 1.54 emu g-1 because of in situ-formed iron silicide nanoparticles of an average size of 7.5 nm, homogeneously dispersed in an amorphous SiCNO matrix. Moreover, the ceramic fibers exhibit a tensile strength of 1.24 GPa and appropriate oxidation resistance. The developed versatile reaction strategy allows also for the incorporation of other elements to implement further functionalities for processing of multifunctional composites.

In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems.

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000-1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiNxCy nanocrystals (x + y = 1) are homogeneously formed "in situ" in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si3N4, TiNxCy (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.

Discovery of Ternary Silicon Titanium Nitride with Spinel-Type Structure.

Here we report on the discovery of a ternary silicon titanium nitride with the general composition (Si1-x,Tix)3N4 with x = 0 < x < 1 and spinel-type crystal structure. The novel nitride is formed from an amorphous silicon titanium nitride (SiTiN) precursor under high-pressure/high-temperature conditions in a large volume high-pressure device. Under the conditions of 15-20 GPa and 1800-2000 °C, spinel-type γ-Si3N4 and rock salt-type c-TiN are formed. In addition, crystals of the discovered nano-sized ternary phase (Si1-x,Tix)3N4 embedded in γ-Si3N4 are identified. The ternary compound is formed due to kinetically-controlled synthesis conditions and is analyzed to exhibit the spinel-type structure with ca. 8 atom% of Ti. The Ti atoms occur in both Ti3+ and Ti4+ oxidation states and are located on the Si sites. The ternary nano-crystals have to be described as (Si,Ti)3N4 with N-vacancies resulting in the general composition (Si4+1-x Ti4+x-δTi3+δ)3N4-δ.

Design of nanoscaled heterojunctions in precursor-derived t-ZrO2/SiOC(N) nanocomposites: Transgressing the boundaries of catalytic activity from UV to visible light.

In this work, nanocomposites made of nanosized zirconia crystallized in situ in an amorphous silicon oxycarbo(nitride) (SiOC(N)) matrix have been designed through a precursor route for visible light photocatalytic applications. The relative volume fraction of the starting precursors and the pyrolysis temperatures not only influences the phase fraction of zirconia crystallites but also stabilizes the tetragonal crystal structure of zirconia (t-ZrO2) at room temperature. The presence of carbon in interstitial sites of zirconia and oxygen vacancy defects led to drastic reduction in the band gap (2.2 eV) of the nanocomposite. Apart from being a perfect host avoiding sintering of the active phase and providing mechanical stability, the amorphous matrix also reduces the recombination rate by forming heterojunctions with t-ZrO2. The reduction in band gap as well as the formation of heterojunctions aids in harnessing the visible light for photocatalytic activity.

Crosslinking chemistry of poly(vinylmethyl-co-methyl)silazanes toward low-temperature formable preceramic polymers as precursors of functional aluminium-modified Si-C-N ceramics.

Crosslinking chemistry of a liquid poly(vinylmethyl-co-methyl)silazane with an alane hydride-based complex according to Si : Al ratios varying from 5 to 2.5 has been investigated in detail through the characterization of the as-obtained polymers using solid-state NMR, FT-IR and elemental analyses. This reaction allows tailoring the chemical and physical properties of the neat liquid polysilazane while extending its processability to lead to a series of low-temperature formable aluminium-modified polysilazanes. Structural models have been established based on solid-state NMR spectroscopy. Then, pyrolysis under nitrogen occurring the conversion of polymers into ceramics has been studied by coupling TG experiments with FTIR of pyrolysis intermediates. Pyrolysis at 1000 °C leads to X-ray amorphous Al-modified silicon carbonitride materials with higher ceramic yields compared to the materials obtained from the neat polysilazane. However, the increase of the ceramic yield is minimized with the decrease of the Si : Al ratio from 5 to 2.5 in the as-obtained polymers. This is due to the introduction of -NR3 (R = CH3 and C2H5) units as side groups during the polymer synthesis which are released in the low temperature regime of the pyrolysis. The structural evolution of the amorphous network of ceramics has been studied by annealing up to 1800 °C though X-ray diffraction and Raman spectroscopy. Such studies point out that samples remain amorphous even after annealing at 1400 °C (low Si : Al ratio) and 1600 °C (high Si : Al ratio) before forming Si3N4/SiC/AlN and AlN/SiC/C composites after annealing at 1800 °C depending on the Si : Al ratio fixed in the early stage of the process. Dense pieces could be prepared from these low-temperature formable polymers. The latter, especially those containing a certain portion of -NR3 (R = CH3 and C2H5) units acting as plasticizing groups during the process, display appropriate requirements for pressing at low temperature forming dense pieces with hardness and Young's modulus as high as 21.7 GPa and 192.7 GPa, respectively.

Novel Precursor-Derived Meso-/Macroporous TiO2/SiOC Nanocomposites with Highly Stable Anatase Nanophase Providing Visible Light Photocatalytic Activity and Superior Adsorption of Organic Dyes.

Titania (TiO2) is considered to have immense potential as a photocatalyst, the anatase phase in particular. There have been numerous attempts to push the limits of its catalytic activity to higher wavelengths to harness the visible electromagnetic radiation. Most of the investigations till date have been restricted to fine-tuning the bandgap by doping, control of defect chemistry at the surface and several to first principle simulations either with limited success or success at the cost of complexities in processing. Here, we report a simple and elegant way of preparing ceramics through precursor chemistry which involves synthesis of macroporous and mesoporous nanocomposites with in situ formation of TiO2 nanocrystals into a robust and protecting SiOC matrix. The in situ nanoscaled TiO2 is anatase of size 9-10 nm, which is uniformly distributed in an amorphous SiOC matrix forming a new generation of nanocomposites that combine the robustness, structural stability and durability of the SiOC matrix while achieving nanoscaled TiO2 functionalities. The stabilization of the anatase phase even at temperature as high as 1200 °C was evident. With an average pore size of 6.8 nm, surface area of 129 m²/g (BET) and pore volume of 0.22 cm³/g (BET), mesoporosity was achieved in the nanocomposites. The composites exhibited visible light photocatalytic activity, which is attributed to the Ti-O-C/TiC bonds resulting in the reduction of band gap by 0.2 to 0.9 eV. Furthermore, the heterojunction formed between the amorphous SiOC and crystalline TiO2 is also expected to minimize the recombination rate of electron-hole pair, making these novel nanocomposites based on TiO2 extremely active in visible wavelength regime.

Boron Nitride for Hydrogen Storage.

Boron nitride, BN, for hydrogen storage emerged in the beginning of the millennium and there swiftly followed more than 15 years of efforts combining experimental laboratory works and to a greater extent computational predictions. BN has been considered mainly for the storage of molecular hydrogen, H2 , by physisorption and/or chemisorption over a wide range of temperatures, that is, between -196 °C and 300 °C. Yet its potential has gone beyond the sorption of H2 as it has been also, although less extensively, involved in chemical H storage. The present review aims at giving a comprehensive overview of the main experimental results and findings as well as of the different avenues worth being explored. A key lesson of this survey is that boron nitride may turn out to be a promising material for hydrogen storage at room conditions provided all the predictions come true. The "ball" is now in the lab experimenters' court.

Molecular Chemistry and Engineering of Boron-Modified Polyorganosilazanes as New Processable and Functional SiBCN Precursors.

A series of boron-modified polyorganosilazanes was synthesized from a poly(vinylmethyl-co-methyl)silazane and controlled amounts of borane dimethyl sulfide. The role of the chemistry behind their synthesis has been studied in detail by using solid-state NMR spectroscopy, FTIR spectroscopy, and elemental analysis. The intimate relationship between the chemistry and the processability of these polymers is discussed. Polymers with low boron contents displayed appropriate requirements for facile processing in solution, such as impregnation of host carbon materials, which resulted in the design of mesoporous monoliths with a high specific surface area after pyrolysis. Polymers with high boron content are more appropriate for solid-state processing to design mechanically robust monolith-type macroporous and dense structures after pyrolysis. Boron acts as a crosslinking element, which offers the possibility to extend the processability of polyorganosilazanes and suppress the distillation of oligomeric fragments in the low-temperature region of their thermal decomposition (i.e., pyrolysis) at 1000 °C under nitrogen. Polymers with controlled and high ceramic yields were generated. We provide a comprehensive mechanistic study of the two-step thermal decomposition based on a combination of thermogravimetric experiments coupled with elemental analysis, solid-state NMR spectroscopy, and FTIR spectroscopy. Selected characterization tools allowed the investigation of specific properties of the monolith-type SiBCN materials.

Nanocomposites through the Chemistry of Single-Source Precursors: Understanding the Role of Chemistry behind the Design of Monolith-Type Nanostructured Titanium Nitride/Silicon Nitride.

Monolith-type titanium nitride/silicon nitride nanocomposites, denoted as TiN/Si3 N4 , have been prepared by a reaction of polysilazanes with a titanium amide precursor, warm pressing of the resultant polytitanosilazanes, and subsequent pyrolysis of the green bodies at 1400 °C. Initially, a series of polytitanosilazanes was synthesized and the role of the chemistry behind their synthesis was studied in detail by using solid-state NMR spectroscopy, elemental analysis, and molecular-weight measurements. The intimate relationship between the chemistry and the processability of these precursors is discussed. Polytitanosilazanes display the appropriate requirements for facile processing in solution and as a melt, but they must be treated with liquid ammonia to be adapted for solid-state processing, that is, warm-pressing, to design dense and mechanically stable structures after pyrolysis. We provide a comprehensive mechanistic study of the nanocomposite conversion based on solid-state NMR spectroscopy coupled with thermogravimetric experiments. HRTEM images coupled with XRD and Raman spectroscopy confirmed the unique nanostructural features of the nanocomposites, which appear to be a result of the molecular origin of the materials. The as-obtained samples are composed of an amorphous Si3 N4 matrix, in which TiN nanocrystals are homogeneously formed in situ in the matrix during the process. The hardness and Young moduli were measured and are discussed.

Micro-/Mesoporous Platinum-SiCN Nanocomposite Catalysts (Pt@SiCN): From Design to Catalytic Applications.

The synthesis, characterization, and catalytic studies of platinum (Pt) nanoparticles (NPs) supported by a polymer-derived SiCN matrix are reported. In the first step and under mild conditions (110 °C), a block copolymer (BCP) based on hydroxyl-group-terminated linear polyethylene (PEOH) and a commercially available polysilazane (PSZ: HTT 1800) were synthesized. Afterwards, the BCP was microphase separated, modified with an aminopyridinato (Ap) ligand-stabilized Pt complex, and cross-linked. The green bodies thus obtained were pyrolyzed at 1000 °C under nitrogen and provided porous Pt@SiCN nanocomposite via decomposition of the PEOH block while Pt nanoparticles grew in situ within the SiCN matrix. Powder X-ray diffraction (PXRD) studies confirmed the presence of the cubic Pt phase in the amorphous SiCN matrix whereas transmission electron microscopy (TEM) measurements revealed homogeneously distributed Pt nanoparticles in the size of 0.9 to 1.9 nm. N2 sorption studies indicated the presence of micro- and mesopores. Pt@SiCN appears to be an active and robust catalyst in the hydrolysis of sodium borohydride under harsh conditions.