Synthesis of SiC ceramic fibers from nuclear reactor irradiated polycarbosilane ceramic precursor fibers

Polycarbosilane (PCS) ceramic precursor fibers are irradiated in a nuclear reactor and pyrolyzed under inert atmosphere. Bridge structure of Si–CH₂–Si is formed in the irradiated products by the rupture of Si–H bonds and succeeding cross-linking. When irradiated at the neutron fluence of 2.2 × 10¹⁷ cm⁻² under N₂ atmosphere, the gel content and ceramic yield at 1,273 K of PCS fibers are up to 80% and 94.3%, respectively, and their pyrolysis products are still fibrous, which illuminates that the infusibility of PCS fibers has been achieved. FT-IR spectra indicate that the chemical structure of pyrolysis products is very similar to that of pure SiC, while X-ray diffraction curves suggest that β-SiC microcrystals are formed in the fibers, and their mean grain size is about 7.5 nm. The oxygen content (1.69–3.77 wt%) is much lower than that of conventional SiC fibers by oxidation curing method (about 15 wt%). Tensile strength of the SiC fibers is up to 2.72 GPa, which demonstrates that their mechanical properties are excellent. After heat-treated at 1,673 K in air for an hour or at 1,873 K under Ar gas atmosphere for 0.5 h, their external appearance is still undamaged and dense, and their tensile strength decreases to a small extent, which verifies that heat resistance of the SiC fibers is eximious.     

过氧化苯甲酰在液态聚碳硅烷交联中的应用

聚碳硅烷热解前的交联对于提高其最终陶瓷产率至关重要.研究了过氧化苯甲酰(BPO)引发液态超支化聚碳硅烷(HBPCS)的交联反应对于陶瓷产率的影响.FT-IR和GPC证实,添加为0.5%～2.0%(质量分数,下同)的BPO,即可在80～140℃下引发HBPCS交联;在考察的实验条件下,提高交联温度、延长反应时间或增加BPO用量,均可提高HBPCS交联程度.BPO用量为2.0%时,HBPCS交联样品在1000℃下的陶瓷产率高达65%,比未加BPO的提高25%.     

A simple way to prepare precursors for zirconium carbide

A precursor for zirconium carbide was obtained by just blending zirconium butoxide Zr(OC₄H₉)₄ (ZTB) and divinylbenzene (DVB). This precursor satisfied the requirements for use in ceramic matrix composites fabrication via precursor infiltration and pyrolysis (PIP) process, that is, it was a solution, cross-linked at 150 °C for 2 h, and transformed to ZrC matrix upon heat treatment at 1,600 °C with a ceramic yield around 40%. The cross-linking behavior, pyrolysis process, and optimal molar ratio (ZTB and DVB) of the precursor were investigated by IR, DSC–TGA, and XRD analysis. ZTB and DVB decomposed into ZrO₂ and carbon, respectively, at 400–500 °C, and ZrO₂ and carbon reacted with each other via carbo-thermal reaction at higher temperature to form ZrC.     

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and ¹H, ¹³C, ²⁹Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH₂–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH₂–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.     

Curing and pyrolysis of polysiloxane/divinylbenzene and its derived carbon fiber reinforced Si—O—C composites

The curing and pyrolysis of hydrogen-containing polysiloxane (PSO) and divinylbenzene (DVB) were investigated in this paper. It was found that H₂PtCl₆ was an effective catalyst for the curing of DVB/PSO. The mass ratio of DVB/PSO had great effect on ceramic yield. The cured DVB/PSO with a mass ratio of 0.5:1 had the highest ceramic yield (76%) at temperature up to 1000°C, and its pyrolysates consisted of 38.3 wt% silicon, 27.4 wt% oxygen, and 34.3 wt% carbon of which 26.3 wt% was free carbon. The composition and structure of pyrolysates of DVB/PSO were changed with increasing pyrolysis temperature. The pyrolysis behavior of DVB/PSO was characterized by thermal analysis. DVB/PSO-derived Si–O–C composites reinforced with carbon fiber cloth (C_f/Si–O–C) were fabricated. The results showed that the flexural strength of C_f/Si–O–C composites could be increased from 118.00 ± 5.00 MPa to 139.78 ± 7.68 MPa if the pyrolysis temperature was elevated from 1000 to 1400°C, which was ascribed to the weakened interfacial bonding.     

聚硅氧烷先驱体转化制备三维Cf/Si-O-C复合材料的研究

研究了含氢聚硅氧烷(PSO)与二乙烯基苯(DVB)的交联与裂解。结果表明:只有在氯铂酸的催化作用下,PSO与DVB才能完全交联。DVB/PSO质量比对陶瓷产率的影响较大,DVB/PSO＝0.5时陶瓷产率最高,达到76%。裂解产物中Si,C,O的含量分别为38.3wt%,34.3wt%,27.4wt%。以质量比为0.5的DVB/PSO体系为先驱体,采用先驱体转化法制备出三维C_f/Si-O-C复合材料。研究发现:第一次裂解时采用热压辅助可以明显提高材料的力学性能。第一次在1600℃,10MPa的条件下热压裂解5min,后续真空浸渍-常压裂解处理六个周期所制得的材料具有较高的力学性能,其弯曲强度和断裂韧性分别为502MPa,23.7MPa·m 1/2。理想的界面结合状态是其具有高性能的主要原因。     

SiC nanofibers by pyrolysis of electrospun preceramic polymers

Silicon carbide (SiC) nanofibers of diameters as low as 20 nm are reported. The fibers were produced through the electrostatic spinning of the preceramic poly(carbomethylsilane) with pyrolysis to ceramic. A new technique was used where the preceramic was blended with polystyrene and, subsequent to electrospinning, was exposed to UV to crosslink the PS and prevent fiber flowing during pyrolysis. Electrospun SiC fibers were characterized by Fourier transform infrared spectroscopy, thermo gravimetric analysis-differential thermal analysis, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and electron diffraction. Fibers were shown to be polycrystalline and nanograined with β-SiC 4H polytype being dominant, where commercial methods produce α-SiC 3C. Pyrolysis of the bulk polymer blend to SiC produced α-SiC 15R as the dominant polytype with larger grains showing that electrospinning nanofibers affects resultant crystallinity. Fibers produced were shown to have a core–shell structure of an oxide scale that was variable by pyrolysis conditions.     

Evolution of crystallization and its effects on properties during pyrolysis of Si–Al–C–(O) precursor fibers

The high-temperature resistant Si–Al–C–(O) fibers were prepared through polymer-derived method using continuous polyaluminocarbosilane (PACS) fibers. Evolutions of the crystallization during the pyrolysis of the Si–Al–C–(O) precursor fibers were investigated by a series analysis. The structure of the fibers transforms from organic state to inorganic state and the crystalline phases appear during the pyrolysis. The β-SiC crystallite size increases when the temperature is higher than 1,300 °C. At the same time, the α-SiC appears. At 1,600 and 1,800 °C, the grain size of β-SiC of the fibers is 15.4 and 22.1 nm, respectively. The growth of β-SiC and the appearing of α-SiC have a great influence on the properties of the fibers. The change of the tensile strength of the pyrolysis products is divided into three stages with the growth of the crystal. The tensile strength of the Si-Al-C fibers is higher than 1.9 GPa.     

Synthesis and characterization of a new liquid polymer precursor for Si–B–C–N ceramics

A new liquid polyborosilazane precursor for Si–B–C–N ceramic was synthesized by co-condensation reaction of boron trichloride, organodichlorosilanes, and hexamethyldisilazane. The structure and properties of polyborosilazane were studied by means of Fourier transform-infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), rheology, and thermogravimetric analysis (TGA). The conversion of polymer to ceramic and the high-temperature behavior of the new polymer-derived ceramic were investigated by TG–MS, FT-IR, X-ray diffraction (XRD) and high-temperature TGA (HTGA). The ceramics showed good oxidative resistance and thermal stability with weight gain of 1.8 wt% at 1350 °C under air atmosphere and weight loss of 2.6% at 1900 °C under Ar atmosphere.     

Heterogeneously catalysed crosslinking of polycarbosilane with divinylbenzene

Crosslinking of polycarbosilane (PCS) with divinylbenzene (DVB) is readily accomplished using heterogeneous catalysis with platinum chloride in heptane to provide a silicon carbide precursor that produces a ceramic with significantly reduced oxygen content. The ceramic yield after crosslinking increased from 47% to between 72% and 78%; however, crosslinking may be influenced by dehydrogenative silylation of hydroxyl groups. Solid-state ¹³C NMR spectroscopy of the crosslinked PCS showed peaks assignable to the aromatic group at 144 and 126.7 ppm. Monitoring of the crosslinking reaction by ¹H NMR spectroscopy indicated 40% consumption of the vinyl bonds of DVB within 10 min and complete consumption within 16 h. Infrared spectroscopy showed no increase in the peak at 3,650 cm⁻¹ due to O–H stretching in Si–OH, demonstrating that hydrosilylation crosslinking is a highly effective non-oxidative crosslinking technique. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Synthesis and properties of ceramic fibers from polycarbosilane/polymethylphenylsiloxane polymer blends

We synthesized ceramic fibers based on silicon carbide (SiC) from polymer blends of polycarbosilane (PCS) and polymethylphenylsiloxane (PMPhS) by melt-spinning and radiation curing. PMPhS was compatible with PCS up to 30 mass%, and formed a transparent melt at temperatures higher than 513 K. The softening point was also lowered by adding PMPhS and 15 mass% of PMPhS to PCS was the most suitable condition for obtaining thin fibers with an average diameter of 14.4 μm. Due to the lowered softening point of the PCS–PMPhS fibers, γ-ray curing in air was adopted. The ceramic yield of the cured fiber was 85.5% after pyrolysis at 1273 K. In spite of the small diameter, the resulting tensile strength at 1273 K was rather limited at 0.78 GPa. Blooming of the PMPhS component during pyrolysis may have caused surface defects. After high-temperature pyrolysis at 1673–1773 K, a porous nanocrystalline SiC fiber with a unique microstructure was obtained with surface area of 70–150 m²/g. When the fiber was pyrolyzed at the same temperature under a highly reductive atmosphere, wire bundle-shaped fibers were obtained by gas evolution and reactions.     

Crystallization and microstructural evolution process from the mechanically alloyed amorphous SiBCN powder to the hot-pressed nano SiC/BN(C) ceramic

The mechanically alloyed amorphous SiBCN powders were hot pressed at 1500, 1600, 1700, 1800, and 1900 °C under a pressure of 80 MPa in the nitrogen atmosphere for 30 min. The crystallization, the microstructural evolution, and the properties of the prepared ceramics were carefully studied by XRD, TEM, HRTEM, and property testing. Results show that the crystallization of β-SiC, turbostratic BN(C), and α-SiC in the amorphous matrix starts at about 1500, 1600, and 1700 °C, respectively. When the powder is hot pressed at the temperatures higher than 1700 °C, the prepared ceramics always consist of nano β-SiC, α-SiC, turbostratic BN(C), and amorphous body. With the increase of the sintering temperature, the ceramic crystallinity becomes higher, the grains get larger, and the amorphous content becomes lower. At the temperatures lower than 1800 °C, the bulk density, the relative density, the flexural strength, the Young’s modulus, and the fracture toughness of the prepared ceramics show persistent but insignificant increase. However, when the ceramic is sintered at 1,900 °C, these properties are rapidly improving to 2.6 g/cm³, 91.8 %, 331.0 MPa, 139.4 GPa, and 2.8 MPa m^1/2.     

Shear strength of the Zn–Sn high-temperature lead-free solders

This study examines the shear strength behavior of the high-temperature Zn–20 wt% Sn, Zn–30 wt% Sn, and Zn–40 wt% Sn solders in the temperature range of 298–425 K. The results showed that increasing the Sn content of the alloys decreases both shear yield stress (SYS) and ultimate shear strength (USS) at all test temperatures. This can be attributed to the higher volume fraction of the softer β-Sn matrix and the eutectic α-Zn + β-Sn structure, which replaces the colonies of the harder α-Zn phase in the microstructure. The high shear strength of these high temperature solder alloys makes them suitable for application in harsh environments.     

Optimizing the hot-forging process parameters for connecting rods made of PM titanium alloy

In order to optimize the processing parameters of a new low-cost titanium alloy connecting rod made of powder forging, the deformation behavior of an α + β type Ti–1.5Fe–2.25Mo (wt%) alloy produced by elemental powder metallurgy (PM) route was studied using isothermal compression tests. The constitutive equations and a processing map were established to characterize the flow behavior and predict the optimum deformation parameters. The calculated apparent activation energy was 257.73 kJ/mol for deformation in the α + β phase region and 378.01 kJ/mol in the β phase region. Two deformation mechanism domains were found: α + β → β phase transformation and dynamic recrystallization. The results show that the optimum deformation parameters for the present alloy are (700–800 °C, 10^−1.7–1 s⁻¹) and (800–900 °C, 10⁻²–10 s⁻¹). Based on these results, a finite element method (FEM) simulation of the hot-forming of a connecting rod was conducted, and the simulated results have been successfully used in an industrial forging of the connecting rod.     

Structural characterization of β-SiC nanowires synthesized by direct heating method

Crystal structure of β-SiC nanowires was investigated using Raman spectroscopy, FT-IR, XRD, transmission electron microscopy and selected area electron diffraction. Cubic β-SiC nanowires were synthesized by heating NiO catalyzed Si substrates with WO₃ and graphite mixed powders in the growth temperature of 1000–1100 °C. HRTEM image showed atomic arrangements of the grown SiC nanowires with a main growth direction of [111]. Raman spectra showed two characteristic peaks at 796 cm^− 1 and 968 cm^− 1, which are corresponding to transversal optic mode and longitudinal optic mode of β-SiC, respectively. Also, FT-IR absorption spectroscopy showed a SiC characteristic absorption band at ∼792 cm^− 1.     

Strength and fracture toughness of in situ-toughened silicon carbide

Fine β-SiC powders either pure or with the addition of 1 wt % of α-SiC particles acting as a seeding medium, were hot-pressed at 1800 °C for 1 h using Y₂O₃ and Al₂O₃ as sintering aids and were subsequently annealed at 1900 °C for 2, 4 and 8 h. During the subsequent heat treatment, the β → α phase transformation of SiC produced a microstructure of “in situ composites” as a result of the growth of elongated large α-SiC grains. The introduction of α-SiC seeds into the β-SiC accelerated the grain growth of elongated large grains during annealing which led to a coarser microstructure. The sample strength values decreased as the grain size and fracture toughness continued to increase beyond the level where clusters of grains act as fracture origins. The average strength of the in situ-toughened SiC materials was in the range of 468–667 MPa at room temperature and 476–592 MPa at 900 °C. Typical fracture toughness values of 8 h annealed materials were 6.0 MPa m^1/2 for materials containing α-SiC seeds and 5.8 MPa m^1/2 for pure β-SiC samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

Preparation of highly cross-linked monodispersed functional polystyrene particles by utilizing the delayed addition method

A facile and effective method was successfully developed to prepare the highly cross-linked monodispersed PS particles via soap-free emulsion polymerization, in which the cross-linking agent (DVB) was delayed addition by varying time (Delayed Addition Method, DAM). Through this method, the highly cross-linked monodispersed particles with functional groups on its surface can be prepared. The particles prepared by DAM had a uniform size distribution for the St/DVB and St/HEMA/DVB system. For the St/DVB system, when the cross-linking agent (DVB) was added delay 3 or 7 h, cross-linked PS particles could be successfully prepared without a significant change in the narrow particle size distribution even containing up to 40 wt% of DVB. Similarly, for the St/HEMA/DVB system, by delayed addition of DVB for 3 h, stable and monodispersed polymer particles still could be prepared even the containing of DVB reach to 60 wt%. Moreover, when the feed composition of the DVB kept constant, the longer the delayed addition time of DVB, the better the monodispersity of the cross-linked particles were.     

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state²⁹Si and¹³C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of -SiC and -SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.     

Photoluminescence and magnetic properties of β-Ni(OH)2 nanoplates and NiO nanostructures

Single-crystalline β-nickel hydroxide (β-Ni(OH)₂) nanoplates of hexagonal structure have been synthesized through hydrothermal process. The β-Ni(OH)₂ nanoplates possess well-defined hexagonal shapes with landscape dimension of 45–140 nm and thickness of 20–50 nm. Post-thermal decomposition of the β-Ni(OH)₂ nanoplates led to the formation of single-crystalline NiO nanostructures with landscape dimension of 25–120 nm including nanorolls, nanotroughs and nanoplates. The sizes of the central hole in NiO nanorolls and the low-lying ground in NiO nanotroughs are in the range of 10–24 nm. Two photoluminescence emission peaks appear at 390.5 nm and 467 nm in the photoluminescence spectrum of NiO nanostructures and were assigned to the ¹T_{1 g} (G) → ³A_{2 g} and ¹T_{2 g} (D) → ³A_{2 g} transitions of Ni²⁺ in oxygen octahedral sites, respectively. Temperature-dependent magnetic measurement results show that an antiferromagnetic-paramagnetic transition occur at 26.3 K in β-Ni(OH)₂ nanoplates.     

Synthesis,structural and electrical properties of SiC nanowires via a simple CVD method

The β-SiC nanowires (NWs) prepared by a simple carbon template method exhibit two kinds of electrical transport properties, depending on their crystalline structures. A part of the NWs exhibit the resistivities as low as 1.5 × 10⁻⁵ to 3 × 0⁻⁴ Ω cm due to n-type doping from the intrinsic planar defects and stacking faults, forming Ohmic contact with Au electrodes; the other ones show a typical characteristic of the semiconductor with a remarkable increase in resistivity by 5–7 magnitude orders, owing to a nearly perfect single-crystalline structure with few intrinsic defects. Additionally, the electrical breakdown behavior is observed in the metallic NWs.