New method for synthesizing polyorganozircosilazane as a Si/Zr/C/N‐based ceramic precursor

A new kind of polyorganozircosilazane as a Si/Zr/C/N‐based ceramic precursor was synthesized from the condensation reaction of hexamethylcyclotrisilazane lithium salts and zirconium tetrachloride. The pyrolysis of the precursor was carried out at 800°C under nitrogen. The results indicated that the precursor preparation temperature could affect the pyrolytic yields. The precursor, which was synthesized at 80°C, gave the highest pyrolytic yield. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2080–2082, 2003     

Flash pyrolysis of coal: effect of nitrogen,argon and other atmospheres in increasing olefin concentration and its significance on the mechanism of coal pyrolysis

Samples of three Indian coals, of widely differing origin and rank, were subjected to flash pyrolysis at a temperature of about 1150 °C for 30 s in vacuo, and under atmospheres of nitrogen, argon, ammonia, and perdeuterobenzene. The gaseous products of the pyrolyses were analysed by infra-red and mass spectroscopy and by gas chromatography. Observed variations in gas compositions are discussed relative to the possible mode of influence by the pyrolytic atmospheres. It would appear that the pyrolytic atmosphere is an important factor in determining the composition of the pyrolysis products; the influence of nitrogen, argon and perdeuterobenzene is a physical one, leading especially to higher yields of olefins.     

Electrical Conductivity of SiOCN Ceramics by the Powder‐Solution‐Composite Technique

SiOCN ceramics have been prepared by the polymer pyrolysis method. The preceramic polymers were synthesized from a polysiloxane cross‐linked with two different N‐containing compounds: a silazane or a ternary amine. The corresponding SiOCN ceramics were obtained by pyrolysis in nitrogen atmosphere at five different temperatures from 1000°C to 1400°C. The electrical conductivity of the powdered SiOCN ceramic samples was determined by the powder‐solution‐composite technique. The results show an increase in room temperature AC conductivity of three orders of magnitude, from ≈10^?5 (S/cm) to ≈10^?2 (S/cm), with increasing pyrolysis temperature from 1000°C to 1400°C. Furthermore, the electrical conductivity of the amine‐derived SiOCN is three to five times higher than that of the silazane‐derived ceramic at each pyrolysis temperature. The combined structural study by Raman spectroscopy and chemical analysis suggests that the increase of electrical conductivity with the pyrolysis temperature is due to the sp³‐to‐sp² transition of the amorphous carbon phase. The higher conductivity of the amine‐derived SiOCN is also discussed considering features like the volume% of the free‐carbon phase and its possible N‐doping.     

Thermal curing reaction and heat‐resistance of methyl‐di(M‐ethynylphenyl‐amino)silane

Methyl‐di(m‐ethynylphenyl‐amino)silane (MEAS) is a new kind of silazane with ethynylphenyl groups in the end of the molecule. The studies about the curing reaction kinetics and curing reaction mechanism are important for its application and performance. In this article, differential scanning calorimeter was used to study the curing reaction kinetics of MEAS. The results showed that both of the apparent activation energy (E_a) and the reaction order (n) that were evaluated with the method of Kissinger (113.4 kJ/mol, 0.93) agreed well with those using the method of Ozawa (116.1 kJ/mol, 0.95). According to structural changes during curing characterized using Fourier‐transform infrared spectra, it was inferred that MEAS resin underwent the main four kinds of cross‐linking reaction under the condition of heating. Thermogravimetric analysis was used to characterize the heat‐resistance of MEAS thermoset. The results showed that the temperature of 5% weight loss based on the initial weight (T_d5) of the thermoset was 632.4°C and the residue yield at 900°C was 86.4% in nitrogen. The thermoset sintered at 1450°C in argon transformed into a ceramic with yield of 71%, which was studied by scanning electron microscopy and X‐ray diffraction. The sintered products were smooth and hard solid and its chemical composition was made up of β‐SiC, α‐Si₃N₄ ceramic and free carbon. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Si/B/C/N/Al precursor-derived ceramics: Synthesis,high temperature behaviour and oxidation resistance

The Si/B/C/N/H polymer T2(1), [B(C₂H₄Si(CH₃)NH)₃]_n, was reacted with different amounts of H₃Al·NMe₃ to produce three organometallic precursors for Si/B/C/N/Al ceramics. These precursors were transformed into ceramic materials by thermolysis at 1400 °C. The ceramic yield varied from 63% for the Al-poor polymer (3.6 wt.% Al) to 71% for the Al-rich precursor (9.2 wt.% Al). The as-thermolysed ceramics contained nano-sized SiC crystals. Heat treatment at 1800 °C led to the formation of a microstructure composed of crystalline SiC, Si₃N₄, AlN(+SiC) and a BNC_x phase. At 2000 °C, nitrogen-containing phases (partly) decomposed in a nitrogen or argon atmosphere. The high temperature stability was not clearly related to the aluminium concentration within the samples. The oxidation behaviour was analysed at 1100, 1300, and 1500 °C. The addition of aluminium significantly improved the oxide scale quality with respect to adhesion, cracking and bubble formation compared to Al-free Si(/B)/C/N ceramics. Scale growth rates on Si/B/C/N/Al ceramics at 1500 °C were comparable with CVD–SiC and CVD–Si₃N₄, which makes these materials promising candidates for high-temperature applications in oxidizing environments.     

Synthesis and Characterization of Ethylene‐Bridged Copolycarbosilazane as Precursors for Silicon Carbonitride Ceramics

An ethylene‐bridged copolycarbosilazane precursor of copolysilylethylenediamine (co‐PSDA) is synthesized by polycondensation of ethylenediamine with the mixture of vinylmethyldichlorosilane and methyldichlorosilane in the presence of triethylamine as acid absorbing agent. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR, ¹H NMR and ¹³C NMR) spectral analysis of the as‐synthesized co‐PSDA suggests a structure of ethylene‐bridged polycarbosilazane having –Si–N–C–C–N– as backbone chain with –CH=CH₂, –H and –CH₃ attached to Si as side groups. Co‐PSDA can be cross‐linked at 80°C using 2, 2‐azobisisobutyronitrile as initiator through the polyaddition of the vinyl group and dehydrogenation/deamination of Si–H and N–H. Then the cross‐linked co‐PSDA precursor is pyrolyzed at 1000°C in argon, giving out amorphous silicon carbon nitride (SiCN) ceramics with a high ceramic yield of 76 wt%. The obtained SiCN ceramics consist of nitrogen‐rich silicon sites of SiN₄ as predominant component and some SiCN₃ sites, which should arise from the breaking of N–C bonds below 600°C and the formation of active N–Si bonds.     

Evaluation of poly(methylsilane‐carbosilane) synthesized from methyl‐dichlorosilane and chloromethyldichloromethylsilane as a precursor for SiC

A new poly(methylsilane‐carbosilane) (PMSCS) for silicon carbide precursor was synthesized by Wurtz‐type copolycondensation of methyldichlorosilane (MeHSiCl₂) with chloromethyldichloromethylsilane (ClCH₂MeSiCl₂) and terminated with vinylmagnesium chloride (ViMgCl). The use of insufficient sodium made the reaction more economic and safe. By changing the ratios of two monomers and the end‐block agent, the properties of the obtained PMSCS and the C/Si ratio of its derived ceramic could be tuned. Upon pyrolysis at 1000 °C under argon, silicon carbide with nearly stoichiometric C/Si ratio and low oxygen content was obtained in 64% of ceramic yield. PMSCS showed high potential as an economical SiC ceramic precursor for the fabrication of SiC matrix, coating, and adhesives. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46610.     

Reaction Mechanism and Microstructure Development of Zr–Si Alloyed Melt‐Infiltrated ZrC‐Modified C/C Composite

ZrC ceramic modified‐C/C composite is prepared by a quick and low‐cost reactive melt infiltration process with a Zr‐Si8.8 alloy. Reaction kinetics and mechanism of pyrolytic carbon with the infiltrated Zr‐Si8.8 alloy are investigated. A continuous ZrC layer is found to be formed around pyrolytic carbon due to the in situ reaction and its thickness parabolically increases with an increase in reaction time period. Zr concentration in the alloyed melts decreases due to the reaction between Zr and pyrolytic carbon and Zr₂Si phase precipitates from the residual alloyed melts. A model for the growth of ZrC layer is established to describe the reaction kinetics of pyrolytic carbon with Zr‐Si8.8 alloy. The calculated thickness of the reaction‐formed ZrC layer is in good agreement with the experimental data. Based on the Arrhenius equation, the activation energy of the reaction between carbon and Zr‐Si8.8 alloy is 313.2 KJ/mol, smaller than that of the reaction between carbon and pure zirconium. The microstructure of the reactive melt‐infiltrated ZrC‐modified C/C composite is characterized by optical microscope, SEM, EDS, XRD, and TEM. The mechanism of the reaction between pyrolytic carbon and Zr‐Si8.8 alloy is discussed on the basis of the characterization results and phase diagram. A schematic is proposed to understand the reaction mechanism between pyrolytic carbon and Zr‐Si8.8 alloy and microstructure development of the ZrC‐modified C/C composite.     

The Emergence for Multilamellar Ti–Al–N Solid Solution in Ti/AlN Composites Sintered at Various Temperatures

Nowadays, a variety of coatings such as Ti–Al–N and TiN have been proposed in titanium machining, but few results on fabrication of bulk Ti/AlN composites were reported. Here, we prepared bulk Ti‐60 wt%AlN composites via vacuum hot‐press sintering at 1250°C–1450°C under an applied pressure of 30 MPa for 1 h. With sintering temperature increasing, the mechanical properties of Ti/AlN composites improved and achieved the maximum values when the sintering temperature is 1450°C. SEM detected the multilamellar grains which are confirmed as Ti–Al–N solid solution by XRD and EDS. Finally, microstructure evolution and phase transformation of Ti/AlN composites were illustrated that Al and N atoms diffuse across the grain boundary and react with Ti atoms to form TiN phase and multilamellar Ti–Al–N solid solution. This work is interesting for producing high strength and toughens metal/ceramic composites.     

Pyrolysis of polyborosilazane and its conversion into SiBN ceramic

Abstract

The polyborosilazane was prepared by coammonolysis of boron trichloride and methyldichlorosilane with hexamethyldisilazane. The decarburisation process of the polyborosilazane under ammonia was studied. The results suggested carbon was effectively removed under ammonia and the removal of carbon mainly occurred during the temperature range of 400–600°C. After pyrolysed at 900°C under ammonia, the carbon content was only 0·29 wt-%. Further heat treatment at 1500°C under argon yielded SiBN ceramic. The structure and morphological properties of SiBN ceramic were studied by solid ²⁹Si nuclear magnetic resonance, X-ray diffraction and scanning electron microscopy. The results showed that the SiBN ceramic was amorphous and possessed a smooth surface. The contents of boron, silicon and nitrogen in the SiBN ceramic are 10·9, 42·6 and 38·48 wt-% respectively.     

A novel method for preparation of dense silicon-based ceramics

The dense ceramic part was prepared firstly using silazane with filler. The composition, structure and ceramic yield of silazane were characterized by elemental analysis, nuclear magnetic resonance (NMR), IR and thermogravimetric analysis (TGA). The ceramic yield was 63 wt% upon pyrolysis at 1000 °C under N₂ atmosphere. The fabrication of ceramic part involved cross-linking of the silazane with metallic fillers (Ti particles) followed by a polymer-to-ceramic transformation step. Near net shape manufacturing of polymer derived ceramics could be achieved. The strength of ceramic parts could achieve 450 ± 15 MPa and scanning electron microscopy (SEM) observation showed that there was very low porosity on the fracture surface of pyrolysed body.     

In-Situ Carbon Content Adjustment in Polysilazane Derived Amorphous SiCN Bulk Ceramics

The present paper is concerned with the in-situ carbon content adjustment in amorphous bulk silicon carbonitride (SiCN) ceramic matrices prepared by the polymer to ceramic transformation of cross-linked and compacted poly(hydridomethyl)silazane powders. Heat treatment in inert (Ar) or reactive atmosphere (ammonia, or mixed Ar/NH₃ with different volume ratio of ammonia) was used for carbon content adjustment. Isothermal annealing steps in Ar and/or mixed atmospheres at various intermediate temperatures were also included into the pyrolysis schedule (i) to adjust the final carbon content, (ii) to control outgassing of low molecular reaction products like methane or hydrogen from the matrix during polysilazane decomposition and thus (iii) to avoid cracking of the pressed polymer powders. Optimal annealing temperature for carbon content adjustment was found to be in the range between 500 and 550°C. Increasing NH₃ contents from 10 to 50 vol% in the pyrolysis atmosphere as well as enhanced transient annealing temperature and time promote carbon reduction. In contrast intermediate isothermal annealing in Ar at 500 up to 600°C results in pronounced formation of Si–C bonds and in increased carbon contents after the final pyrolysis process. Depending on the pyrolysis conditions, flawless bulk specimens with carbon contents ranging from 0·3 up to 16·2 wt% were obtained. Different possible chemical reactions are considered to explain the generation of the particular Si(C)N compositions found. ©     

Polymer‐Derived SiC/B4C/C Nanocomposites: Structural Evolution and Crystallization Behavior

Structural evolution and crystallization behavior between 600°C and 1450°C during the preparation of bulk SiC/B₄C/C nanocomposites by the pyrolysis of CB‐PSA preceramic were investigated. The CB‐PSA preceramic converts into carbon‐rich Si–B–C ceramics up to 800°C. Structural evolution and crystallization of Si–B–C materials could be controlled by adjusting the pyrolytic temperature. The Si–B–C ceramics are amorphous between 800°C and 1000°C. Phase separation and crystallization begin at 1100°C. The crystallization of β‐SiC takes place at 1100°C and B₄C nanocrystallites generate at 1300°C. The sizes of β‐SiC and B₄C nanocrystals increase with the pyrolytic temperature rising. In addition, the boron‐doping effect on structural evolution was studied by comparing the crystallization and graphitization behavior of Si–B–C ceramics and the corresponding Si–C materials. Boron is helpful for the growth of β‐SiC nanocrystals and the graphitization, but harmful for the nucleation of β‐SiC crystallites.     

Si-C-N陶瓷前驱体聚硅氮烷合成的研究进展

综述了合成Si—C—N陶瓷前驱体聚硅氮烷的主要方法，即氨／Q解氯硅烷法、硅氮烷低聚物交联法、硅氮烷低聚物与氯硅烷反应法及氯硅氯烷和氯硅烷脱氯编聚法，并介绍了用铝、硼、钛、钇等元素对聚硅氮烷进行改性，以提高Si—C—N陶瓷性能的方法。     

Preparation and high‐temperature behavior of HfC–SiC nanocomposites derived from a non‐oxygen single‐source‐precursor

A single‐source precursor for the preparation of HfC‐SiC ceramics was synthesized via a Grignard reaction using bis(cyclopentadienyl)hafnium(IV) dichloride, trans‐1,4‐dibromo‐2‐butene, and (chloromethyl)trimethylsilane as raw materials. The composition, structure, pyrolysis process and high‐temperature behavior of the precursor were investigated. The results show that the precursor with a backbone comprising Hf–C, Si–C and CH=CH groups exhibits good solubility in common solvents, such as tetrahydrofuran, dimethylbenzene, and chloroform. Pyrolysis of the precursor at 1000°C yielded a microcrystalline HfC phase with a ceramic yield of 63.86 wt%. The pyrolytic products at 1600°C were HfC–SiC nanocomposite ceramics, which exhibited good thermal stability up to 2400°C. The formation of a (Hf,Si)C solid‐solution would be beneficial for densification during the sintering process. The non‐oxygen structure, high ceramic yield, homogeneous composition and excellent high‐temperature behavior of the pyrolytic products make the as‐prepared precursor a promising material for the preparation of high‐performance ultra‐high‐temperature ceramics.     

Nanocomposite Si/C/N Powder Production by Laser–Aerosol Interaction

Ultrafine preceramic Si/C/N powders were obtained via ultrasonic injection of aerosol particles into a highpower CO₂ laser beam, using hexamethyldisilazane, which strongly absorbs the CO₂ laser radiation at 10.6 μm, as the precursor. Introduction of aerosol droplets via a flowing argon or argon–NH₃ mixture into the laser beam yielded nanosized (30 to 60 nm), amorphous powders. Yield and particle composition were controlled by varying the synthesis conditions. Powder changes, such as crystallization during heat treatment under nitrogen or argon, were studied.     

Synthesis,characterization, and properties of silylene–acetylene preceramic polymers

A series of silylene–acetylene preceramic polymers 3a–e were synthesized by polycondensation reaction of dilithioacetylene with dichlorosilane (H₂SiCl₂) or/and methyldichlorosilane (MeSiHCl₂). Their structures were confirmed by infrared spectra (IR), and ¹H and ²⁹Si NMR spectroscopies. Differential scanning calorimetry (DSC) diagrams show exotherms centered at 200 to 233°C temperature range, attributed to crosslinking reaction of the acetylene and Si? H groups. After thermal treatment, the obtained thermosets 4a–e possess excellent thermal stability. Thermogravimetric analysis (TGA) under nitrogen show the T_d5s (temperature of 5% weight loss) for all the thermosets are above 600°C, and the overall char yields are between 95.62% and 89.67% at 900°C. After pyrolysis at 1200°C, the obtained ceramic residues 5a–e exhibit good thermo‐oxidative stability with final weight retention between 98.76% and 91.66% at 900°C under air. In particular, perhydroploy(silylene)ethynylene 3a , which has the highest Si/C ratio in silylene–acetylene polymers, has the highest char yield, and the derived ceramic material 5a displays the best thermo‐oxidative stability. Based on Scanning electron microscopy and its associated energy‐dispersive X‐ray microanalysis (SEM EDX) and ¹³C magic angle spinning nuclear magnetic resonance (MAS NMR) analysis, ceramic 5a contains the highest SiC content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Effect of Ti–Si–Fe alloys on microstructure and properties of nitride/oxynitride bonded SiC ceramics sintered under CO/N2 atmosphere

Ti–Si–Fe alloys extracted from high-titanium blast furnace slag were utilized to replace part of the silicon powders, and then nitride/oxynitride bonded SiC ceramics were prepared by reactive sintering in graphite bed. Ti–Si–Fe alloys could react with CO/N₂ at a low temperature (1200 °C), and the addition of Ti–Si–Fe alloys could reduce the nitriding temperature of Si. Density functional theory calculations suggested that Ti–Si–Fe alloys enhanced reaction activity via weakening the strength of CO and NN bonds. The regional equilibrium phase diagrams of Si–C–N–O and Ti–Si–C–N–O under CO/N₂ atmosphere were calculated by thermodynamics. The change of whiskers morphology was observed by scanning electron microscope. Furthermore, the bulk density, the cold modulus of rupture, and the cold compressive strength improved with Ti–Si–Fe alloys content. The results showed that the addition of Ti–Si–Fe alloys not only significantly promoted nitriding of Si and formation of Si₃N₄ whiskers, but also improved the mechanical properties of the samples.     

Effect of rank,temperatures and inherent minerals on nitrogen emissions during coal pyrolysis in a fixed bed reactor

Pyrolysis of 11 coals with carbon contents of 77–93 wt.% (daf) and corresponding demineralized samples has been studied in a fixed bed quartz reactor with a heating rate of 20 K/min to examine rank, demineralization, temperature and inherent mineral species dependences of nitrogen distribution. Nitrogen mass balances fall within 92.5–104.6%. The results indicate that the chars derived from the coals with higher rank show larger nitrogen retention. Demineralization suppresses volatile nitrogen emission during coal pyrolysis, especially for low rank coals. Coal-N conversion to tar-N reaches the asymptotic values at 600 °C. HCN yields are lower than NH₃ yields during coal pyrolysis. The trends in HCN and NH₃ emissions are very similar and the yields reach the asymptotic value at about 1200 °C. N₂ starts emitting at 600 °C, and as the temperature increases the conversion increases linearly with a corresponding reverse change of char-N. With the catalysts added, N₂ formation is prompted with the sequence of Fe>Ca>K>Ti≫Na≫Si≈Al, meanwhile, char-N decreases correspondingly. Fe, Ca, K, Na, Si and Al increase coal-N conversion to NH₃ with the sequence of Fe>Ca>K≈Na≫Si≈Al in the pyrolysis. Na addition prompts HCN formation; however, the presence of Ti and Ca decrease the HCN yields with small value. The other catalysts have no notable influence on HCN emission in the pyrolysis. Demineralization and Ti addition increase coal-N conversion to tar-N slightly whereas K, Ca, Mg, Na, Si and Al additions decrease tar-N yield weakly, other catalysts hardly influence tar nitrogen emission. N₂ emits mainly from char-N with slight contribution of volatile nitrogen. The mechanism of different N-containing species formation and catalysts influence in the pyrolysis is also discussed in the paper.     

Microstructure evolution and phase transformation of TiB2/SiC/B4C composites synthesized from Ti‐SiC‐B4C ternary system

Bulk TiB₂/SiC/B₄C composites have been synthesized from Ti‐SiC‐B₄C ternary system with different Ti weight percentages via reactive hot pressing at 1800°C under an applied pressure of 30 MPa for 1.5 h. By Ti amount increasing, the flexural strength curve exhibited an “M‐like” tendency reaching the maximum value of 512.36 MPa for 30 wt.% Ti. Microstructural evolution of the composites from conterminously large matrix grains to finely clear‐edged particles was observed by scanning electron microscopy. The phase transformation and element diffusion were analyzed by XRD and Energy Disperse Spectroscopy. A hybrid reinforcing mechanism of fracture and crack deflection is proposed to illustrate the change in flexural strength.