Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Synthesis,characterization and ceramization of a novel vinyl-rich liquid precursor for Si(O)C ceramic

Polymethylsilane (PMS) was partially modified with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂åCH)SiO]₄, D4Vi) via conventional hydrosilylation. The as-synthesized vinyl-rich liquid precursor (V-PMS) was characterized by the viscosity test, gel-permeation chromatography, Fourier-transform infrared spectroscopy, nuclear magnetic resonances spectroscopy. The results indicate that the obtained precursor is well soluble in common solvents and exhibits a controllable viscosity of 326.9–714.6 mPa s at room temperature. The thermal properties of V-PMS were investigated by differential scanning calorimetry and thermogravimetric analysis. The V-PMS can be cured readily at 150 °C in inert atomosphere. The ceramic yield of V-PMS reaches 81% at 1200 °C, 38% higher than that of PMS. The final pyrolytic residue is hard, dense monolithic up to 1400 °C under Ar atmospheres. The controllable viscosity, excellent thermal curability and high ceramic yield enable the liquid precursor a promising material to shape various Si(O)C ceramic materials for high-temperature application.     

Synthesis and pyrolysis mechanism of a novel polymeric precursor for SiBN ternary ceramic fibers

A novel polymeric precursor polyborosilazane (PBSZ) for SiBN ternary ceramic fibers was successfully synthesized from trichlorosilane (HSiCl₃), boron trichloride (BCl₃) and hexamethyldisilazane (HDMZ) by a simple one step reaction process. The chemical structures and ceramic yield of the PBSZ precursors were investigated by NMR spectroscopy, FT-IR and TGA. The preparation of PBSZ fibers was conducted in a lab-scale melt-spinning equipment at a spinning speed of 130 m/min. SiBN ternary ceramic fibers were obtained after the non-fusible treatment and pyrolysis of PBSZ fibers in an NH₃ atmosphere. The pyrolysis mechanism, high-temperature behavior and morphologies of the SiBN ternary ceramic fibers were investigated by NMR, XRD, TEM and SEM. The obtained SiBN ternary ceramic fibers had good flexibility, and possessed a tensile strength of 0.84 GPa with a diameter of ∼18 μm. Furthermore, these SiBN ceramic fibers exhibited excellent thermal stability, and maintained the amorphous state up to 1600 °C.     

Porous wollastonite-hydroxyapatite bioceramics from a preceramic polymer and micro- or nano-sized fillers

Wollastonite-hydroxyapatite ceramics have been successfully prepared by a novel method, corresponding to the thermal treatment in air of a silicone embedding micro- and nano-sized fillers. CaCO₃ nano-sized particles, providing CaO upon decomposition, acted as “active” filler, whereas different commercially available or synthesised hydroxyapatite particles were used as “passive” filler. The homogeneous distribution of CaO, at a quasi-molecular level, favoured the reaction with silica derived from the polymer, at only 900 °C, preventing extensive decomposition of hydroxyapatite. Open-celled porous ceramics suitable for scaffolds for bone-tissue engineering applications were easily prepared from filler-containing silicone resin mixed with sacrificial PMMA microbeads as templates. The pore size (in the range of 80-400 μm) and the open porosity percentage (40-50%) were evaluated by means of micro-computerized tomographic analysis. A preliminary assessment of the biocompatibility and cell activity of the produced ceramics was performed successfully by in vitro tests using human osteoblast cells.     

Synthesis and thermal performance of polymer precursor for ZrC ceramic

A novel ZrC preceramic precursor (PZC) was compounded via liquid phase chemical reaction without any organic solvent choosing ZrOCl₂·8H₂O and polyvinyl alcohol as Zr source and C source, respectively. The composition and structure of ZrC precursor were analysed through XRD, FT-IR, XPS and SEM. The results showed both Zr-O-C bonds and Zr-O bonds existed in the precursor. The results observed by SEM showed that many irregular particles were generated, whose particle sizes were mainly in the range of 0.2–3 μm. In addition, particle aggregation can be easily observed. Besides, the thermal property and pyrolysis process of PZC were studied. In accordance with XRD, the initial temperature of the earliest detection of ZrC in pyrolysis products of PZC was 1300 °C. Monoclinic ZrO₂ and tetragonal ZrO₂ can be observed at this temperature as well. Ulteriorly, when the pyrolysis temperature was risen up to 1500 °C, only ZrC ceramic can be found.     

Synthesis and pyrolysis of single-source precursor for HfCxN1-x-SiC ceramic with different SiC contents

A novel single-source precursor was synthesized to prepare HfC_xN_1-x/SiC multiphase ceramics by using hafnium chloride (HfCl₄), diallylamine (DAA) and polycarbosilane (PCS). We conducted an investigation of the synthesis process, polymer-to-ceramic conversion, as well as the microstructure and phase evolution of HfC_xN_1-x/SiC multiphase ceramics with different levels of SiC content. The results showed that the core-shell particles of HfC_xN_1-x-carbon were embedded homogeneously in the β-SiC matrix which is beneficial for preventing grain growth and improving oxidation resistance. Based on data from oxidation tests, the ceramics improved the oxidation temperature and remained stable at a high temperature (1500 °C) with oxidation layer formation on the surface. Due to the highly cross-linked structure without oxygen, high ceramic yield, homogeneous composition and excellent oxidation resistance of the pyrolysis product, the as-prepared precursor is a promising material for making high-performance composite ceramics.     

Synthesis of SiC nanowires via catalyst-free pyrolysis of silicon-containing carbon materials derived from a hybrid precursor

SiC nanowires were successfully synthesized without catalyst by pyrolysis of silicon-containing pitch-derived carbon materials in a closed graphite crucible. These silicon-containing carbon materials were obtained by homogenization and co-carbonization of a hybrid precursor consisting of the toluene soluble fraction of coal tar pitch with polycarbosilane (PCS). The composition, morphology and structure of the nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The influence of pyrolysis temperature on the growth of the nanowires was investigated by Fourier transform infrared spectroscopy (FTIR) and thermo-gravimetry coupled with mass spectroscopy (TG-MS) analysis. The results indicate that the growth of the SiC nanowires starts at around 1200 °C. As the pyrolysis temperature increases to 1300–1500 °C, a large quantity of nanowires are formed on the top surface of the pitch-derived carbon substrate. In addition, increasing the pyrolysis temperature leads to an increase in the average diameter and a change in the typical morphology produced. The synthesized SiC nanowires have single-crystalline structure and are grown along the [111] direction with numerous stacking faults and twins. The vapor-solid (VS) mechanism may be responsible for the growth process of the SiC nanowires.     

Oxidation resistant ceramic foam from a silicone preceramic polymer/polyurethane blend

Silicon oxycarbide (SiOC) ceramic foams, produced by the pyrolysis of a foamed blend of a methylsilicone preceramic polymer and polyurethane (PU) in a 1/1 wt.% ratio, exhibit excellent physical and mechanical properties. The proposed process allows to easily modify the density and morphology of the foams, making them suitable for several engineering applications. However, it has been shown that, due to residual carbon present in the oxycarbide phase after pyrolysis, the foams are subjected to an oxidation process that reduces their strength after high temperature exposure to air (12 h 1200°C). A modified process, employing the same silicone resin preceramic polymer but a much lower PU content (silicone resin/PU=5.25/1 wt.% ratio), has been developed and is reported in this paper. Microstructural investigations showed that carbon rich regions deriving from the decomposition of the polyurethane template are still present in the SiOC foam, but have a much smaller dimension than those found in foams with a higher PU content. Thermal gravimetric studies performed in air or oxygen showed that the low-PU containing ceramic foams display an excellent oxidation resistance, because the carbon-rich areas are embedded inside the struts or cell walls and are thus protected by the dense silicon oxycarbide matrix surrounding them. SiOC foams obtained with the novel process are capable to maintain their mechanical strength after oxidation treatments at 800 and 1200°C (12 h), while SiOC foams obtained with a higher amount of PU show about a 30% strength decrease after oxidation at 1200°C (12 h).     

聚合物陶瓷先驱体材料的合成与应用

概述了作为陶瓷先驱体的四种聚合物f聚碳硅烷(PCS)、聚硅氮烷(PSZ)、聚硼氮烷(PBZ)以及聚硅氧烷(PSO)]的合成与应用,介绍了国内外对该类材料研究的最新进展,并针对先驱体裂解转化陶瓷工艺所存在的局限性以及不足,提出了今后的发展方向.     

Synthesis and characterization of poly(dimethylsilylene-diphenylsilylene) as a precursor to silicon carbide

Poly(dimethylsilylene-diphenylsilylene) copolymer, [(SiMe_{2 x}(SiPh₂)]_n, where x=1.0 to 8.0, was synthesized by condensation polymerization of dimethyldichlorosilane and diphenyldichlorosilane in xylene. In the thermogravimetric analysis of the copolymer, weight loss started at 170‡C and the weight residue at 1,000‡C was 32%. The copolymer was stabilized by thermal oxidation at 80 to 110‡C for 5 hr in air, and its thermal stability increased as the temperature increased. The copolymer heat treated above 700‡C showed no organic bonds of C-H and Si-CH₃, while Si-C and Si-0 bonds were observed. Amorphous SiC was obtained by pyrolysis at 1,000‡C and β-SiC at 1,200 to 1,400‡C, in which better crystallinity of β-SiC was observed at a higher temperature.     

Synthesis, characterization, and photovoltaic property of a low band gap polymer alternating dithienopyrrole and thienopyrroledione units

The lower the highest occupied molecular orbital (HOMO) energy level of the conjugated polymer is, the higher the open-circuit voltage (V_OC) of the obtained polymer solar cell (PSC) is. To achieve this goal, a new conjugated polymer (PDTPTPD) alternating dithienopyrrole (DTP) and thienopyrroledione (TPD) units was designed and synthesized by Stille coupling reaction. Through UV-vis absorption and cyclic voltammetry (CV) measurements, it was found that the resulting copolymer exhibited both a low optical band gap of 1.62 eV and a low HOMO energy level of −5.09 eV owing to the electronegativity of TPD moiety. Preliminary photovoltaic study disclosed that the PSC based on PDTPTPD:PCBM ([6,6]-phenyl-C₆₁ butyric acid methyl ester) blend showed a power conversion efficiency (PCE) of 1.9%, with a V_OC of 0.70 V, and a short circuit current (I_SC) of 6.97 mA/cm², suggesting that PDTPTPD is a promising photovoltaic polymer.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

Synthesis and characterization of YAG precursor powder in the hydroxyhydrogel form

Powder precursor in the form of hydroxyhydrogel was prepared from yttrium nitrate and aluminium nitrate, using ammonium hydroxide as precipitant by flash polycondensation technique. Precursor powder obtained in the hydroxyhydrogel form was characterized by TG/DTA, FTIR, SEM and XRD analysis. This method produced material which could be transformed into YAG at comparatively lower temperature (800 °C) and was phase pure.     

Synthesis and properties of a new, branched polyhydridocarbosilane as a precursor for silicon carbide

Polymerization of Cl₂Si(CH₃)CH₂Cl with Mg in THF, followed by reduction with LiAlH₄, gave a polycarbosilane with Si-H groups and branches at the Si atoms. The polymer could be cross-linked thermally at 150°C. Pyrolysis of the cross-linked material gave SiC with a yield of 70%.Presented at the XXVIth Silicon-Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.     

Fast pyrolysis of a SiC-based polymer precursor using a spark plasma sintering apparatus – Effects of heating methods and pyrolysis atmosphere

The effects of heating method on the pyrolysis behavior, crystallinity and ceramic yield of a SiC-based precursor-derived ceramic (PDC) were investigated using commercial allylhydrido-polycarbosilane (AHPCS) precursors, spark plasma sintering (SPS) apparatus and tube furnace. The heating rate of SPS was ten times faster than that of tube furnace. Consequently, the total time of precursor impregnation and pyrolysis (PIP) process decreased distinctly after several PIP cycles. PDCs fabricated by SPS had higher crystallinity than those prepared by tube furnace. The crystallinity of PDCs was 70.4 and 65.1 %, respectively, after the pyrolysis using SPS and tube furnace at 1400℃ for 1h. The highest crystallinity of 82.92 % was achieved after the pyrolysis at 1500℃ for 2h using SPS. The ceramic yield of the precursor was not strongly affected by the heating method. This study provides a promising method for the pyrolysis of ceramic precursors with short processing time and improved thermal stability.     

Development of multiphase bioceramics from a filler-containing preceramic polymer

Multiphase bioceramics based on wollastonite and wollastonite/hydroxylapatite (W/HAp) have been successfully prepared by the heat treatment of a filler-containing preceramic polymer. CaO-bearing precursors (Ca-carbonate, Ca-acetate, and CaO nano-particles) were dispersed in a solution of silicone resin, subsequently dried and pyrolysed in nitrogen. The reaction between silica, deriving from the oxycarbide (SiOC) residue of the silicone resin, and CaO “active filler” led to the formation of several calcium silicates, mainly consisting of wollastonite (CaSiO₃), in both low and high temperature forms. The phase assemblage of the final ceramic varied with the pyrolysis temperature (varying from 1000 to 1200 °C). HAp was additionally inserted, as “passive filler” (i.e. not reacting with SiOC), for the preparation of bioceramics based on W/HAp mixtures.     

Preparation, characterization and pyrolysis of poly(furfuryl alcohol)/porous silica glass nanocomposites: novel route to carbon template

In this paper we report the preparation of glassy carbon through the pyrolysis of poly(furfuryl alcohol) inside the pores of Vycor glass, which was used as a template. Different routes to the in situ polymerization of furfuryl alcohol inside the pores of Vycor glass were developed. The nanocomposites glass/polymer obtained were characterized by several techniques. Carbonization of these nanocomposites produces new silica glass/carbon nanocomposites, which were characterized and treated with HF to remove the silica fraction. It was found that the resulting carbon presents low crystallinity when compared to graphite. However, it presents more order than the glassy carbon resulting from the pyrolysis of the free poly(furfuryl alcohol) resin.     

A meltable precursor for zirconium carbide ceramics and C/C-ZrC composites

For process simplification and rapid densification of ceramic composites, a meltable single-source ZrC precursor was prepared by condensing zirconium acetylacetonate (Zr(acac)₄) at 190?°C for 40–150?min. The preparation of ZrC precursor and the conversion from precursor to ceramics were investigated by using FTIR and NMR spectroscopies, GPC, DSC-TGA, XRD, SEM, EDS and TEM. The precursor had low viscosity (~ 10?mPa?s) and proper processing window (60?min) for precursor infiltration and pyrolysis (PIP). The ceramic yield at 1650?°C was 29.6%, and EDS revealed that the composition was (ZrC)_0.337(HfC)_0.0025(ZrO₂)_0.044C_0.1865. The ceramics were composed of 0.2–0.5?µm grains which aggregated to form a stacked structure surrounded by amorphous carbon. The preparation processes were designed, and C/C-ZrC composites with the density of 2.45?g/cm³ were successfully fabricated through 11 cycles of PIP with Zr(acac)₄. In conclusion, the synthetic method provides a simple and cheap route for precursors, and allows combined composite preparation with high efficiency.     

Synthesis of a soluble preceramic polymer for ZrC using 2-hydroxybenzyl alcohol as carbon source

A preceramic polymer for ZrC was successfully synthesised by chemical reaction between zirconium oxychloride (ZrOCl₂·8H₂O) and 2-Hydroxybenzyl alcohol via a one-pot route. The molecular structure, thermal properties and pyrolysis behaviour of the precursor were investigated. The results indicated that the precursor might be Zr–O–Zr chain polymer with 2-Hydroxybenzyl alcohol as ligand. The precursor was air-stable and exhibited excellent solubility in common organic solvents. The conversions from precursor to ZrC powders were investigated by TG-DTA, X-ray diffraction, Scanning electron microscope, TEM and Raman spectrum. The precursor underwent a thermal decomposition in four steps, and ZrC powders were formed at 1300°C via carbothermal reduction reaction of ZrO₂ and carbon in argon with ceramic yield of 63.0%. The ZrC particles were fine and exhibited irregular polyhedron morphology with average size in the range of 100–300?nm.