Preparation,structural evolution,and performance of heat-resistant organosilicon polymer adhesives for joining SiC ceramics

There is an urgent need for heat-resistant adhesives with high bonding strength in order to able to fabricate large and complex SiC components for aeronautical and astronautical applications. In this study, heat-resistant organic adhesives prepared using an organosilicon polymer and inorganic additives (B₄C and SiO₂) were used successfully to bond SiC ceramics. The prepared adhesives were characterised through shear strength tests as well as using thermogravimetry-differential scanning calorimetry, Fourier-transform infrared spectroscopy, X-diffraction analysis, and scanning electron microscopy. The adhesives exhibited high room-temperature shear strengths (greater than 15 MPa) after being subjected to heat treatments at 200–1200°C. Further, the high-temperature shear strengths of the adhesives at 200, 400, 600, 800, and 1000°C were 10.5, 10.1, 7.7, 8.6, and 8.4 MPa, respectively. The high performance of the adhesives indicated that they should be suitable for joining SiC-based materials for use in high-temperature applications.     

Helium irradiation induced microstructural damages and mechanical response of Al2O3-ZrO2-SiC composites

Ceramic composites are promising candidates as structural materials for future fission and fusion reactors. In present work, Al₂O₃-ZrO₂-SiC ternary ceramic composites were irradiated with 2.0 MeV He-ions at 300 and 800 ℃. Grazing incidence X-ray diffraction results confirmed that there was irradiation induced shift and broadening of diffraction peaks, but no amorphization of Al₂O₃-ZrO₂-SiC composite were observed up to fluence of 1.72 × 10¹⁸ ions/cm². Transmission electron microscopy observations showed that throughout the entire irradiation region, nano-sized helium bubbles are mostly distributed in Al₂O₃ grains and partly in ZrO₂ grains, while no detectable bubbles are observed in SiC grains. No obvious agglomeration of bubbles was found at grain/phase boundaries. By using nanoindentation technique, slight hardening or softening was confirmed for the samples irradiated at 300 and 800 ℃ respectively. The absence of amorphization and surface exfoliation indicating the Al₂O₃-ZrO₂-SiC composite exhibits remarkable resistance to He-ions irradiation.     

Al4SiC4的性能、制备和应用

介绍了一种待研发的新型三元碳化物Al4SiC4(碳硅化铝)的组成和基本性能,并重点讨论了Al4SiC4的各种合成和制备方法,如固相反应烧结法、高温自蔓延法等的原理及效果.首先讨论了Al4SiC4材料的基本性能、抗氧化性能和抗水化性能,比较了Al4SiC4和其他材料之间的性能差异,并介绍了Al4SiC4的应用范围;最后对Al4SiC4材料的合成方法和性能进行了总结,同时指出了碳硅化铝材料作为新型耐火材料的研究和使用的进一步研究方向.     

Synthesis and pyrolysis of non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics

Multiphase ceramics like ZrC/SiC are promising candidates as ultra-high temperature ceramics for applications in extreme environments. In this work, non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics were synthesized by a one-pot reaction of three components – metal source, silicon source, and activating reagent. Molecular structures of the precursors were identified by ¹H NMR and FTIR. Transformation process of the precursors to the ZrC/SiC ceramics was investigated via XRD and SEM. After heat-treatment at 1600 °C under argon, the obtained ZrC/SiC and HfC/SiC ceramics features a particle size of 100–200 nm and high metal content without excess carbon. The elemental composition of pyrolyzed ceramics can be tuned by varying the ratio of the reagents in the synthesis of precursors. This strategy also inspires a facile fabrication of composite ceramics with other elemental compositions.     

Co-doping effects of (Al,Ti, Mg) on the microstructure and electrical behavior of ZnO-based ceramics

Co-doped ZnO-based ceramics using Al, Ti, and Mg ions in different ratios were synthesized with the objective to investigate the doping effects on the crystalline features, microstructure and the electrical behavior. For Al and Ti doping, a coexistence of crystalline phases was shown with a major wurtzite ZnO structure and secondary spinel phases (ZnAl₂O₄, Zn₂TiO₄, or Zn_aTi_bAl_cO_d), while Mg doping did not alter significantly the structural features of the wurtzite ZnO phase. The electrical behavior induced by Al, Ti, and Mg co-doping in different ratios was investigated using Raman, electron paramagnetic resonance (EPR) and ²⁷Al and ⁶⁷Zn solid-state nuclear magnetic resonance (NMR). Al doping induces a high electrical conductivity compared to other doping elements. In particular, shallow donors from Zn_i-Al_Zn defect structures are inferred from the characteristic NMR signal at about 185 ppm; that is, quite far from the usual oxygen coordinated Al. The Knight shift effect emanating from a highly conducting Al-doped ZnO ceramics was considered as the origin of this observation. Oppositely, as Ti doping leads to the formation of secondary spinel phases, EPR analysis shows a high concentration of Ti³⁺ ions which limit the electrical conductivity. The correlation between the structural features at the local order, the involved defects and the electrical behavior as function of the doping process are discussed.     

Low-temperature joining of SiC ceramics using NITE phase with Al2O3-Ho2O3 additive

In order to avoid the property degradation resulting from high-temperature joining process, nano-infiltrated transient eutectoid (NITE) phase with the Al₂O₃-Ho₂O₃ as the joining adhesives was adopted to join silicon carbide (SiC) ceramics with the attempts to lower down the joining temperature. The liquid-phase-sintered silicon carbide (LPS-SiC) specimens were joined at 1500-1800°C by spark plasma sintering (SPS) under the pressure of 20 MPa. The results of the shear test and microstructure observation showed that the joining process could be finished at a relatively lower temperature (1700°C) compared to other NITE-phase joining. In contrast to the shear strength of 186.4 MPa derived from the SiC substrate, the joint exhibited the shear strength of 157.8 MPa with the fully densified interlayer.     

碳化硅陶瓷材料的制备工艺和应用研究进展

碳化硅陶瓷材料由于具有轻质高强、导热性能好、膨胀系数低、硬度高、抗氧化等优异的性能,被广泛用作高温结构部件。本文围绕碳化硅材料在光学反射镜材料的应用,对碳化硅材料的制备工艺和应用方面的进展作简要综述。     

Micromechanical properties and microstructural evolution of Amosic-3 SiC/SiC composites irradiated by silicon ions

In this work, Amosic-3 SiC/SiC composites were irradiated to 10 dpa and 115 dpa with 300 keV Si ions at 300 °C. To evaluate its irradiation behaviour and investigate the underlying mechanism, nanoindentation, AFM, Raman and electron microscopy were utilized. Nanoindentation showed that although micromechanical properties declined after irradiation, hardness and Young’s modulus were maintained better under 115 dpa. AFM manifested differential swelling among PyC interface, fiber and matrix and SEM showed irradiation-induced partial interface debonding, which are both more obvious under 115 dpa. TEM revealed the generation and proliferation of amorphous regions, which is according with the decline and broadening of peaks in Raman spectra. The material was almost completely amorphous after irradiated to 10 dpa while recrystallization occurred under 115 dpa. All results mentioned above contribute to the decline of hardness and Young’s modulus and may explain why the micromechanical degradation was more significant under 10 dpa.     

Mechanical behaviour of reaction processed SiC ceramics from artificial precursor from plant

Mechanical behaviour of SiC ceramics synthesized from two artificial precursors from plants – coir fibreboard and bamboo pulp fibreboard – was studied and the mechanical properties – flexural strength, Young's modulus, fracture toughness and hardness – of the ceramics synthesized from the two kinds of artificial precursors were compared. The effect of processing of the artificial precursors, as reflected in the microstructure of the ceramics synthesized from them, was taken into consideration in the analysis of the mechanical property data; these data could also be possible to be explained by the empirical models of fracture mechanics. The results of the study established the possibility of application of the investigated SiC materials as structural ceramics.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Environment-oriented low-cost Al2O3 ceramics with hierarchical pore structure fabricated from SiC solid waste

Environment-oriented low-cost Al₂O₃ reticulated porous ceramics with hierarchical pore structure were fabricated by the polymer sponge replica method combined with vacuum infiltration methods, using Al₂O₃ powders and SiC solid waste (SCSW) as raw material and a pore-forming agent. The effects of SCSW addition amount on mechanical properties, microstructure and pore size of Ceramics were investigated. The results showed that the thermal shock resistance of specimens increased gradually with addition of SCSW, however, the median pore diameter increased firstly and then decreased, due to the generation of mullite and liquid phase. After calcination, the residual stress was generated within the coating layer because of the difference in the thermal expansion coefficients of ceramic matrix and coating layer, which could improve the properties of Ceramics by deflecting and bifurcating crack growth path. The results showed that the best dosage of SCSW was 30 wt%.     

Microstructure and properties of porous SiC ceramics by LPCVI technique regulation

A new gradient pore structure in porous SiC ceramics was fabricated by low pressure chemical vapor infiltration (LPCVI). Effects of deposition duration on the mechanical properties and permeability of porous SiC ceramics were investigated. Results demonstrated that pore diameter and shapes decreased from the surface to the interior along with LPCVI duration. Porous SiC ceramics with deposition duration of 160 h exhibited flexural strength of 48.05 MPa and fracture toughness of 1.30 MPa m^1/2, where 221% and 189% improvements were obtained compared to porous SiC ceramics without LPCVI, due to CVI-SiC layer strengthening effect. Additionally, at the same gas velocity, pressure drop increase rate was faster due to apparent porosity and pore size change.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

Damage microstructure evolution of helium ion irradiated SiC under fusion relevant temperatures

In-situ transmission electron microscopy (TEM) with ion irradiation has been used to study the damage microstructure evolution of He ion irradiated 4-H SiC at nuclear fusion relevant temperatures. The SiC samples were irradiated with 20?keV He ions at 25, 400, 800 and 1200?°C to a dose of 5.0 displacements per atom (DPA). At 25?°C, the material fully amorphises at 1.5 DPA and no He bubble nucleation occurs up to the doses studied. At 400 and 800?°C, He bubble nucleation occurs and the material remains crystalline. Bubble nucleation occurs at 2.0?DPA at 400?°C but occurs at only 0.5?DPA at 1200?°C. This is attributed the He atoms de-trapping from vacancies and migrating interstitially to larger He-vacancy clusters at higher temperatures, leading to faster nucleation of observable He bubbles. Helium platelets form at an irradiation temperature of 1200?°C at 0.5?DPA showing a preference for nucleation between the {0001} basal planes.     

Preparation of SiC reticulated porous ceramics with high strength and increased efficient filtration via fly ash addition

The new route for recycling fly ash was proposed to prepare SiC reticulated porous ceramics (SRPCs) with high strength and increased efficient filtration for molten metal filtration. The effects of fly ash on the rheological characteristics, microstructure evaluation and wetting behavior between SRPCs and molten metal were investigated. It was found that the fly ash was beneficial to thixotropic property of SiC slurry when its content was less than 30 wt%. Furthermore, fly ash in SRPCs was completely transformed into mullite with needle-shape at 1300 °C, forming a porous structure containing micro pores and windows. SRPCs containing 20 wt% fly ash exhibited a higher strength because of the improved rheological properties of SiC slurry and the optimized microstructure in skeleton. In addition, the added fly ash in SRPCs could increase the contact angle between skeleton substrate and molten metal via microporosization of skeleton, thus exhibiting the potential ability to improve the filtration efficiency.     

Joining of SiC ceramics by combining NITE-SiC interlayer and its thickness control

Combined with the thickness control of interlayers (∼10 µm and ∼60 µm), SiC nano-powders with Al₂O₃-Y₂O₃-MgO-CaO additive (NITE-SiC) were used as the joining materials to achieve the low-temperature joining of SiC ceramics. Some residual pores were observed in the interlayer with a thickness of ∼60 µm after joining at 1650 ℃, and the shear strength of SiC joints measured was 39.3 ± 5.5 MPa. Observations showed the fracture of joints occurred at the interlayer. When the thickness of the interlayer decreased to ∼10 µm, no cracks or porosity were observed at the interface region after joining at 1650 ℃. The shear strength of SiC joints increased to 69.5 ± 8.9 MPa, and the fracture originated from the matrix. The results demonstrated using NITE-SiC with Al₂O₃-Y₂O₃-MgO-CaO additive as the joining layer and reducing the thickness of the interlayer could lower the joining temperature and significantly improve the mechanical strength of joints.     

Preparation of dense and high-purity SiC ceramics by pressureless solid-state-sintering

High-purity SiC materials have been used in semiconductor processes due to their excellent properties. However, they are difficult to densify without sintering aids. In this work, dense and high-purity SiC ceramics have been obtained by pressureless solid-state-sintering with ultra-low contents of sintering additives. The amount of residual B, C and O in the high-purity SiC ceramics was less than 0.15 wt%, respectively, and the total content of other impurity elements (such as aluminum, magnesium, calcium, iron, etc.) was less than 0.015 wt%. Finally, the purity of the as-prepared SiC ceramics was more than 99.5 wt%.     

Development of ethylene glycol-based gelcasting for the preparation of highly porous SiC ceramics

Aqueous gelcasting is inappropriate for the preparation of highly porous ceramics, due to the large drying shrinkage of green bodies caused by the high surface tension of water. To solve this problem, non-aqueous gelcasting using organic solvents with much lower surface tension was developed. However, for most organic solvents, the precipitation polymerization of gels led to the low strength of green bodies, which was inconvenient for the fabrication of large size workpieces. In this work, a novel ethylene glycol-based gelcasting was developed to prepare highly porous SiC ceramics. Ethylene glycol induced the solution polymerization of gels and increased the strength of green bodies effectively. In addition, the high flexibility of the ethylene glycol-based gels could release the inner stress in the drying process. Highly porous SiC ceramics with large size were successfully prepared by the optimized gelcasting method.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.