Structural studies on Functional Materials using Solid-State NMR, Powder X-ray Diffraction and DFT Calculations

Analytical and theoretical techniques were used in this work for structural studies of framework materials. One and two dimensional 31P and 17O solid state NMR experiments highlight subtle thermally induced structural changes in (MoO2)2P2O7 pyrophosphate, tungsten trioxide WO3 and negative thermal expansion ZrW2O8. DFT methods using CASTEP software to calculate 31P and 17O NMR parameters are performed on these structures and discussed in comparison to experimental results, published structures and thermal mechanisms

Structures and Phase Transitions in (MoO 2 ) 2 P 2 O 7

We report structural investigations into (MoO2) 2P2O7 using a combination of X-ray, neutron and electron diffraction, and solid-state NMR supported by first principles quantum chemical calculations. These reveal a series of phase transitions on cooling at temperatures of 377 and 325 K. The high temperature y-phase has connectivity consistent with that proposed by Kierkegaard at room temperature (but with improved bond length distribution), and contains 13 unique atoms in space group Prima with lattice parameters a = 12.6577(1) Å, b = 6.3095(1) Å, c = 10.4161 (1) Å, and volume 831.87(1) Å3 from synchrotron data at 423 K. The low temperature a-structure was indexed from electron diffraction data and contains 60 unique atoms in space group P21\c with cell parameters a = 17.8161(3) Å, b = 10.3672(1 ) Å, c = 17.8089(3) Å, ß = 90.2009(2)°, and volume 3289.34(7) Å3 at 250 K. First principles calculations of 31P chemical shift and J couplings were used to establish correlation between local structure and observed NMR parameters, and 1D and 2D 31P solid-state NMR used to validate the proposed crystal structures. The intermediate ß-phase is believed to adopt an incommensurately modulated structure; 31P NMR suggests a smooth structural evolution in this region

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

International audienceCrosslinking 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

Molecular design of melt-spinnable co-polymers as Si-B-C-N fiber precursors

International audienceTwo series of co-polymers with the general formula [B(CHSiCH(NH)(NCH))], i.e., composed of CHSiCH(NH) and CHSiCH(NCH) (CH = CHCH, CHCH) building blocks in a well defined x : y ratio, have been synthesized by hydroboration of dichloromethylvinylsilane with borane dimethyl sulfide followed by successive reactions with lithium amide and methylamine according to controlled ratios. The role of the chemistry behind their syntheses has been studied in detail by solid-state NMR, FT-IR and elemental analyses. Then, the intimate relationship between the chemistry and the melt-spinnability of these polymers was discussed. By keeping x = 0.50 and increasing y above 0.50, i.e., obtaining methylamine excess, the co-polymers contained more ending groups and especially more tetracoordinated boron, thus allowing tuning very precisely the chemical structure of the preceramic polymer in order to meet the requirements for melt-spinning. The curing treatment under ammonia at 200 °C efficiently rendered the green fibers infusible before their subsequent pyrolysis under nitrogen at 1000 °C to generate Si-B-C-N ceramic fibers. Interestingly, it could be possible to produce also low diameter hollow fibers with relatively high mechanical properties for a further exploration as membrane materials