Flexible and Porous Nonwoven SiCN Ceramic Material via Electrospinning of an Optimized Silazane Solution

The processing of nonwoven porous ceramics by combining the polymer‐derived ceramic (PDC) route with electrospinning offers an excellent strategy for developing new porous ceramic structures. Currently, the manufacturing of nonwoven porous materials from preceramic polymers is conducted by trial and error approaches. The necessity to predict the e‐spinnability conditions from properties assessment offers a potential tool to control the manufacture and the resulting material morphology. This work assesses the relationship between the preceramic polymer solutions and the resulting electrospun nonwoven morphology. For this, a commercially available liquid oligosilazane (Durazane 1800) is selectively cross‐linked to achieve a reliable and spinnable preceramic polymer (HTTS), which is then dissolved in tetrahydrofuran (THF). Based on the investigation of the rheological behavior of various polymer concentrations, three different polymer solution regimes (diluted, semidiluted, and concentrated) are identified and correlated with the resulting morphology of the e‐spun material (spherical particles, beaded fibers, and seamless fibers). After the pyrolysis, the nonwoven ceramics manufactured from the solution with 65 wt% of HTTS is converted to a SiCN ceramic nonwoven with 86% of open porosity, profiling as a promising candidate for developing high‐performance filter systems and catalytic supports in harsh environments.

DOI: 10.1002/adem.202100321 The processing of nonwoven porous ceramics by combining the polymer-derived ceramic (PDC) route with electrospinning offers an excellent strategy for developing new porous ceramic structures. Currently, the manufacturing of nonwoven porous materials from preceramic polymers is conducted by trial and error approaches. The necessity to predict the e-spinnability conditions from properties assessment offers a potential tool to control the manufacture and the resulting material morphology. This work assesses the relationship between the preceramic polymer solutions and the resulting electrospun nonwoven morphology. For this, a commercially available liquid oligosilazane (Durazane 1800) is selectively cross-linked to achieve a reliable and spinnable preceramic polymer (HTTS), which is then dissolved in tetrahydrofuran (THF). Based on the investigation of the rheological behavior of various polymer concentrations, three different polymer solution regimes (diluted, semidiluted, and concentrated) are identified and correlated with the resulting morphology of the e-spun material (spherical particles, beaded fibers, and seamless fibers). After the pyrolysis, the nonwoven ceramics manufactured from the solution with 65 wt% of HTTS is converted to a SiCN ceramic nonwoven with 86% of open porosity, profiling as a promising candidate for developing high-performance filter systems and catalytic supports in harsh environments.
Commercially available preceramic polymers usually are composed of cyclic and linear units with a complex structure and low molecular weight, which is a drawback for the spinnability. [4] Therefore, to date, it is mandatory to use organic polymers as spinning aid and additional intermediated steps to achieve e-spinnability. However, suitable organic polymers have a meager carbon yield after pyrolysis and act as sacrificial filler, which harms the mechanical stability.
To avoid using an organic polymer as e-spinning additive, we used a solid polysilazane synthesized from a commercially available oligosilazane [25] to achieve e-spinning conditions. Suitable e-spinning solutions had to be developed to ensure a continuous e-spinning process and the processing of regular-shaped fibers. Furthermore, it was necessary to investigate the polymer to ceramic transformation by pyrolysis. This work aimed to develop an approach to establish the e-spinning process for the processing of nonoxide ceramic nonwoven with high interconnectivity of open porosity for applications in harsh environments, e.g., in the field of energy, catalysis, and high-performance filtration systems.

Synthesis of HTTS
As the Durazane 1800 used in this work, the most commercially available silazanes are liquid oligomers. In view thereof, for the processing of fibers, a solid and processable polymeric precursor must be achieved. For this reason, we applied the selective crosslinking procedure used by Flores et al. [25] The simplified reaction mechanisms and their physical state before and after the cross-linking reaction are shown in Figure 1.
The main advantage of using selective cross-linking via dehydrocoupling reaction is to tailor the silazane molecular weight. The reaction is slow and can be easily stopped at any time using an appropriate inhibitor, such as calcium borohydride bis(tetrahydrofuran) (Ca(BH 4 ) 2 .2THF). The resulting polysilazane HTTS has an increased molecular weight (M w ) of 8010 g mol À1 in comparison with 4836 g mol À1 for Durazane 1800 with a polydisperse index (M w /M n ) of 9.4. [25] In view thereof, the previously liquid and oligomer Durazane 1800 becomes a soluble thermoplastic polymer with properties suitable for the e-spinning process.

Rheological Investigation of the Preceramic Polymer Solutions
The investigation of the rheological behavior of the HTTS solutions provides important information about the interactions between the polymer chains in solution, which is crucial to understand and predict the polymer morphology required for e-spinning. [23,24,26] However, although this knowledge is well established for the e-spinning of organic polymers, there is no report in the literature about the rheological characterization of preceramic polymers intended for e-spinning.
The synthesized HTTS preceramic polymer was solubilized in THF with different concentrations ( Table 1) to evaluate the rheological behavior. The typical apparent viscosity of the prepared solutions as a function of shear rate is shown in Figure 2.
The analysis of the curves shown in Figure 2 clearly distinguishes between two different groups regarding the rheological behavior. Samples with less than 50 wt% of the polymer have a predominantly Newtonian behavior. Therefore, increases in the shear rate have no significant effect on the viscosity. A pronounced shear-thinning behavior appears for samples with higher polymer concentrations (>60 wt%). This typical viscoelastic behavior is attributed to the polymer chain alignment in the applied shear stress direction and consequently reducing the viscosity of the respective solutions with higher shear rates through disentanglement.
Furthermore, the results indicate that the solution state switches from a diluted to a semidiluted regime in the range of 50-60 wt% of HTTS concentration, indicating that the Figure 1. Synthesis of the HTTS polysilazane from the oligosilazane Durazane 1800 by selective cross-linking. Reaction mechanism details are described elsewhere. [25]  www.advancedsciencenews.com www.aem-journal.com polymer chains gradually start to interact through chain entanglement by increasing the polymer concentration. This change from diluted to semidiluted state allows the formation of fibers, instead of beaded fibers or even droplets during electrospinning. [23,27] As the polymer concentration is increased to more than 65 wt%, a second transition point occurs, characterized by a remarkable increase in the viscosity. This phenomenon is better visualized by plotting the mean viscosities at the plateau regime (η ∞ ) as a function of the polymer concentration ( Figure 3).
As will be further discussed in the e-spinning experiments section, this transition leads to seamless fibers, as an increase in the solution concentration leads to a higher degree of polymer chain entanglements. [26,27]

Manufacturing of Preceramic Polymer Fibers by e-Spinning
The e-spinning process with the HTTS solutions was performed as described in the experimental procedure. The respective spinning results are shown in Figure 4. Figure 4 clearly shows the strong influence of the HTTS concentration, i.e., the solution viscosity on the morphology of the resulting materials. As predicted by the rheological behavior, the e-spinning of the diluted solutions ranging from 30 to 50 wt% of HTTS yields only near-spherical polymer particles, as the Taylor cone formation was not possible. The Taylor cone formation is a precondition for the processing of regular-shaped fibers via the e-spinning process. [28,29] Thus, the polymer solutions were directly ejected from the needle tip in the form of a spray jet. For this interval of concentration, the term e-spraying should be more appropriate to describe this processing scenario. In summary, for diluted solutions, the lack of chain entanglements avoids the drawn jet continuity and leads only to the deposition of small droplets at the collector.
Particles with a mean diameter size of 7.8 AE 2.6, 8.4 AE 2.0, and 9.4 AE 1.4 μm were obtained from samples HTTS_1, HTTS_2, and HTTS_3, respectively. It is worth mentioning that the increase in polymer concentration also leads to the formation of more homogeneous particles with a narrower size (Figure 4 and 5). However, in addition to particles, the formation of small fibrils was observed for sample HTTS_3, which is in agreement with the rheological results and confirms the beginning of the transition to a semidiluted regime.
The increase in the polymer concentration to 60 wt% (sample HTTS_4) leads to the formation of beaded fibers. This morphology results from two types of instability during the e-spinning process, the Rayleigh and varicose instabilities, [30][31][32][33] which favor spheres formation because of the smallest surface area to diminish the surface tension. In the sample with 65 wt% HTTS concentration (HTTS_5), the viscosity seems to have inhibited beads-onstring fiber formation, leading to seamless fibers. Increasing the HTTS concentration in the solution to 70 wt% (HTTS_6), the high viscosity combined with the low solvent concentration leads to nonhomogeneous tape-like morphology with a broader size distribution, here named as flat-scattered fibers. For better visualization, the measured average fiber diameters are shown in Figure 5.

Investigation of the Pyrolysis Behavior and NonWoven Ceramic Formation
The pyrolysis of organosilazanes up to 1000 C in N 2 leads to the formation of an amorphous ceramic composed of a SiC x N 4Àx (0 ≤ x ≤ 4) matrix and a segregated carbon phase (so-called free carbon). [34][35][36][37] For ceramic fibers processed through pyrolysis, the crosslinking step is crucial to convert the thermoplastic preceramic polymer to a thermoset, avoiding shape changes during the ceramization. [38] For this purpose, considering the availability of vinyl groups in HTTS, a radical initiator (AIBN) was used to perform the cross-linking step. Figure 6 shows the resulting fibers pyrolyzed as described in the experimental procedure.
After pyrolysis, defect-free cylindrical SiCN fibers with a diameter <4 μm were obtained, which is far below the reported diameter for the same ceramic fibers processed by melt spinning  www.advancedsciencenews.com www.aem-journal.com (30-100 μm) [25] and also lower than commercially available SiC fibers (%10 μm). [39] Although the fabricated fibers are still in the range of micrometers, their quality in terms of homogeneity and morphology is remarkable, as no additional organic polymer (spinning aid) was used. The decomposition gases derived from organic additives would evolve during pyrolysis, which could lead to defects and a higher shrinkage, both very deleterious for the fiber properties. Further optimization of the e-spinning process should lead to fibers with a diameter in the range of nanometers. Another essential characteristic of the PDCs technology is the significant shrinkage during the pyrolysis, mainly assigned by the remarkable increase in density and the mass loss during the conversion from a polymer to ceramics. The fiber average diameter and the nonwoven dimensions were used to estimate the shrinkage for sample HTTS_5 during pyrolysis (Figure 7).
Comparing the fiber diameter before and after pyrolysis, a shrinkage of 22% is noticed, as the fiber mean diameter decreases from 4.1 μm (before pyrolysis) to 3.2 μm (after pyrolysis). The shrinkage is one of the drawbacks of the PDCs technology, and this phenomenon must be considered for further product designing. Regarding the ceramic nonwoven porosity,  www.advancedsciencenews.com www.aem-journal.com the percentage of total porosity was estimated at approximately 86%. The calculation was performed as described in the experimental section by considering the measured SiCN ceramic true density (ρ T ¼ 2.416 g cm À3 ) and the calculated nonwoven density (ρ A ¼ 0.347 g cm À3 ). Finally, it is noteworthy to mention that because of the thin ceramic fibers the porous nonwoven ceramic is flexible and presents sufficient mechanical strength to be bent and handled without breaking or losing its structural integrity ( Figure 8). The good mechanical properties, the well-known thermal and chemical stability, and the good shapeability open many promising applications, such as in filtering systems, catalyst supports, and porous burners.

Conclusions
Many preceramic polymers have a much lower molecular weight compared with organic polymers. Therefore, it is crucial to add organic polymers as spinning aid to enable processing via electrospinning. However, such organic additives act as sacrificial filler, which leads to a decreased ceramic yield, higher shrinkage, and the formation of pores within the formed ceramic fibers during pyrolysis and results in inferior mechanical properties. The approach developed here overcomes these problems. In this work, solutions of the polysilazane HTTS in THF, synthesized by selective cross-linking of the oligosilazane Durazane 1800, were specially designed and comprehensively investigated for the electrospinning process. The rheological measurements of different feedstock solutions were correlated with the resulting fiber morphology after electrospinning, to predict the morphology of the e-spun preceramic polymers. For diluted solutions with a polymer concentration lower than 50 wt%, only nearspherical particles with diameters in the range of 7-10 μm were obtained, as no interactions of the polymer chains are present. By increasing the polymer concentration up to 60 wt%, the HTTS solution rheology reveals a shear-thinning behavior, indicating the transition to a semidiluted state due to a certain degree of chain entanglements. Fibers with beads-on-string morphology are typical for this solution regime. For concentrations higher than 65 wt%, the substantial increase in the viscosity and a viscoelastic behavior due to a higher degree of chain entanglements lead to seamless fibers during the electrospinning process.
The pyrolysis behavior was investigated for samples with a concentration of 65 wt% of HTTS containing AIBN as curing agent. The pyrolysis led to defect-free ceramic SiCN fibers with a diameter <4 μm. Due to the resulting thin diameter, the porous ceramic nonwoven possesses excellent flexibility. Finally, the ceramic nonwoven porous structure indicates a total porosity of about 86%.
The developed approach enables the electrospinning of preceramic polymers without e-spinning aids and helps to conduct this process methodically. Further improvements of the electrospinning process combined with the functionalization of the

Experimental Section
Materials: The commercially available oligosilazane Durazane 1800 (Merck KGaA, Germany) was selected as the SiCN precursor. Tetra-nbutylammonium fluoride (TBAF) (1 M in THF) was used as the catalyst, while calcium borohydride bis(tetrahydrofuran) (Ca(BH4) 2 ·2THF) was used as an inhibitor to stop the reaction, both purchased from Sigma Aldrich (Germany). THF p.a. (Neon, Brazil) was used for the synthesis and the e-spinning experiments, and the initiator 2,2 0 -azobisisobutyronitrile (AIBN), used for curing, was supplied by Akzo Nobel (The Netherlands). The educts' preparation and handling and all synthesis reactions were conducted in a dry argon atmosphere.
Selective Cross-Linking of the Oligosilazane: The procedure adopted to perform the selective chemical cross-linking of the oligosilazane Durazane 1800 based on a previous work reported by Flores et al. [25] In a typical reaction, 20 g of Durazane 1800 was dissolved under vigorous stirring in 40 g of THF, and 0.25 wt% of the catalyst TBAF, concerning Durazane 1800, was added dropwise to the reaction. The solution was stirred for 90 min, and an excess of the inhibitor (Ca(BH 4 ) 2 ·2THF) was added to stop the reaction. The mixture was stirred for another 5 min and afterward filtered off. Finally, the solvent was removed under reduced pressure at room temperature, obtaining a solid and colorless polysilazane, hereafter called HTTS.
E-Spinning Experiments and Pyrolysis: Solutions varying from 30 to 70 wt% of HTTS in THF were prepared for the electrospinning experiments. The respective solution was placed in a 5 mL syringe coupled with a metallic needle of 0.7 mm inner diameter clamped to a high-voltage power supply applying 10 kV and fixed at 10 cm from an aluminum collector. The sample was extruded at a flow rate of 1 mL h À1 controlled by an infusion syringe pump arranged horizontally.
For the pyrolysis, 1.5 wt% of the curing agent AIBN, referred to HTTS amount, was added to the spinning solution. Then, the resulting polymer nonwoven was cured at 70 C for 4 h to enable cross-linking before melting. Afterward, the sample was placed in a carbon crucible and heated up to 1000 C (2 K min À1 ) under nitrogen flow in a tubular furnace.
Rheological Characterization: Parallel plate rotational rheology of the polymer solutions was performed in a Thermo Scientific Haake Mars II rheometer (Thermo Fisher Scientific, USA) using a PP60 measuring geometry (60 mm diameter). A 0.15 gap was used in the tests, and the shear rate varied from 0.1 to 250 s À1 in 60 s. All measurements were performed at a temperature of 23 AE 1 C.
Fiber Morphology Investigation: The e-spun samples were analyzed to assess the fiber morphology before and after the pyrolysis by scanning electron microscopy TM3030 (Hitachi, Japan), operating at an acceleration voltage of 15 kV. The micrographs obtained were analyzed by the ImageJ software (National Institutes of Health, USA), and 200 measurements for either fibers or particles were used to estimate the reported mean values.
Investigations of the NonWoven Properties: The ceramic nonwoven porosity (ε) was estimated by using Equation 1 where ρ A is the nonwoven apparent density and ρ T is the true density of the derived SiCN material. For the apparent density, the volume was obtained by measuring the nonwoven dimensions, while for the true density, the ceramic volume was determined with a gas pycnometer (Accu Pyc II 1340 -Micromeritics Instruments Corporation, USA).