Fabrication of two-layer SiC nanowire cladding tube with high thermal conductivity

Among the various concepts of SiC-based accident-tolerant fuel cladding, duplex SiC cladding, consisting of an inner composite layer and an outer monolithic SiC layer, is considered an optimal design due to its low load failure probability. In this study, SiC nanowires (SiCnw) were introduced on the substrate graphite rod to decrease the diameter of architectural valley-regions of SiC fiber (SiC_f) tubular preform. By avoiding the architectural valley-voids, a dense two-layer SiCnw tube consisting of an inner SiC fiber-reinforced SiC matrix (SiC_f/SiC) composite layer deposited by chemical vapor infiltration with a smooth inner surface was obtained. The microstructure and thermal properties of as-obtained two-layer SiCnw tubes were studied. Results showed that the thermal conductivity of the whole tube was highly sensitive to variations in thermal conductivity of the inner composite layer. By improving the thermal conductivity of the inner composite layer, the two-layer SiCnw tube exhibited a thermal conductivity of 23.8 W m⁻¹ K⁻¹ at room temperature, which had an improvement of 71 % compared to the two-layer SiC tube (13.9 W m⁻¹ K⁻¹). Moreover, the thermal transport properties of the two-layer SiCnw tube were significantly improved by a reduction in roughness of the inner surface.     

3D-SiC decorated with SiC whiskers: Chemical vapor infiltration on the porous 3D-SiC lattices derived from polycarbosilane-based suspensions

SiC_w/3D-SiC composites were fabricated by chemical vapor infiltration (CVI) of the 3D SiC lattices, which were prepared via direct ink writing of polycarbosilane-based suspensions. Microstructure, composition and tensile strength of the composites were investigated. Curing and pyrolysis temperature greatly affected the shrinkage, weight loss, density and composition of the 3D SiC. Although sound structure with spanning feature was achieved, cracks and pores in 3D SiC were formed during the pyrolysis owing to the large shrinkage. CVI process decreased the porosity and led to fully dense surface of the SiC_w/3D-SiC composites. After 60h of CVI, short β-SiC fibres or long SiC whiskers were deposited in the structural spacing of 3D lattices or spherical pores inside the filaments, respectively. The tensile strength of the composites by CVI increased from 3.3 MPa to 15.7 MPa (20 h) and 47.3 MPa (60 h), due to the high strength of dense CVI layers and in-situ formed SiC whiskers in pores. This work showed a way to strengthen the 3D SiC with in-situ formed whiskers via the polymer precursor routes.     

Mechanical properties and strengthening mechanism of SiCf/SiC mini-composites modified by SiC nanowires

The effects of the SiC nanowires (SiCNWs) and PyC interface layers on the mechanical and anti-oxidation properties of SiC fiber (SiC_f)/SiC composites were investigated. To achieve this, the PyC layer was coated on the SiC_f using a chemical vapour infiltration (CVI) method. Then, SiCNWs were successfully coated on the surface of SiC_f/PyC using the electrophoretic deposition method. Finally, a thin PyC layer was coated on the surface of SiC_f/PyC/SiCNWs. Three mini-composites, SiC_f/PyC/SiC, SiC_f/PyC/SiCNWs/SiC, and SiC_f/PyC/SiCNWs/PyC/SiC, were fabricated using the typical precursor infiltration and pyrolysis method. The morphologies of the samples were examined using scanning electron microscopy and energy dispersive X-ray spectrometry. Tensile and single-fibre push-out tests were carried out to investigate the mechanical performance and interfacial shear strength of the composites before and after oxidization at 1200 °C. The results revealed that the SiC_f/PyC/SiCNWs/SiC composites showed the best mechanical and anti-oxidation performance among all the composites investigated. The strengthening and toughening is mainly achieved by SiCNWs optimization of the interfacial bonding strength of the composite and its own nano-toughening. On the basis of the results, the effects of SiCNWs on the oxidation process and retardation mechanism of the SiC_f/SiC mini-composites were investigated.     

Highly conductive and high-strength carbon nanotube film reinforced silicon carbide composites

Film formed by carbon nanotubes is usually called carbon nanotube film (CNT_f). In the present study, CNT_f fabricated by floating catalyst method was used to prepare CNT_f/SiC ceramic matrix composites by chemical vapor infiltration (CVI). Mechanical and electrical properties of the resulting CNT_f/SiC composites with different CVI cycles were investigated and discussed, and the results revealed that the CNT_f has a good adaptability to CVI method. Tensile test demonstrated an excellent mechanical performance of the composites with highest tensile strength of 646 MPa after 2 CVI cycles, and the strength has a decline after 3 CVI cycles for an excessively dense matrix. While, the elastic modulus of the composite increased with the CVI cycles and reached 301 GPa after 3 CVI cycles. Tensile fracture morphologies of the composites were analyzed by scanning electron microscope to study the performance change laws with the CVI cycles. With SiC ceramic matrix infiltrated into the CNT_f, enhanced electrical conductivity of the CNT_f/SiC composite compared to pure CNT_f was also obtained, from 368 S/cm to 588 S/cm. Conductivity of the SiC matrix with free carbon forming in the CVI process was considered as the reason.     

Effect of SiC nanowires on the mechanical properties and thermal conductivity of 3D-SiCf/SiC composites prepared via precursor infiltration pyrolysis

In this study, SiC nanowires (SiC_NWS) were grown in situ on the surface of PyC interface through chemical vapor infiltration (CVI) to improve the mechanical characteristics and thermal conductivity of three-dimensional SiC_f/SiC composites fabricated via precursor infiltration pyrolysis (PIP). The effect of SiC_NWS on the properties of the obtained composites was investigated by comparing them with conventional SiC_f/PyC/SiC composites. After the deposition of SiC_NWS, the flexural strength of the SiC_f/SiC composites was found to increase by 46 %, and the thermal conductivity showed an obvious increase at 25?1000 °C. The energy release of the composites in the damage evolution process was analysed by acoustic emission. The results indicated that the damage evolution process was delayed owing to the decrease in porosity, the crack deflection and bridging of the SiC_NWS. Furthermore, the excellent thermal conductivity was attributed to the thermally conductive pathways formed by the SiC_NWS in the dense structure.     

Microstructure control of highly oriented short carbon fibres in SiC matrix composites fabricated by direct ink writing

A novel method is proposed for fabricating highly oriented carbon fibre reinforced SiC ceramic composites (C_f/SiC) by direct ink writing (DIW). For the first time, the control of carbon fibers’ orientation in DIW was studied by numerical simulation. An interfacial layer was prepared by chemical vapor infiltration (CVI). The microstructure and phase composition of C_f/SiC were studied by scanning electron microscopy and X-ray diffraction, respectively. The results showed that fibers of different interfacial thicknesses could be obtained effectively by varying the CVI time. The breakage of short fibres remarkably improved the fracture toughness of the parts. The specimens showed excellent mechanical properties with bending strength of 274 ± 13 MPa and fracture toughness of 5.82 ± 0.25 MPa m^1/2. This method could be extended to the preparation of other resin and ceramic composites.     

Effects of CVI SiC amount and deposition rates on properties of SiCf/SiC composites fabricated by hybrid chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) routes

The SiC_f/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t₀, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t₀, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiC_f/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiC_f/SiC composites for industrial applications.     

Fabrication of SiC whisker-reinforced SiC ceramic matrix composites based on 3D printing and chemical vapor infiltration technology

Spray drying, binder jetting and chemical vapor infiltration (CVI) were used in combination for the first time to fabricate SiC whisker-reinforced SiC ceramic matrix composites (SiC_W/SiC). Granulated needle-shaped SiC_W was spray dried into SiC_W spherical particles to increase flowability and thereby increase printability. Then, binder jetting was employed to print a novel SiC_W preform with two-stage pores using the SiC_W spherical particles. The subsequent CVI technology produced pure, dense, and continuous SiC matrix with high modulus and strength. Consequently, SiC_W/SiC with appropriate mechanical properties was obtained. Finally, the challenges of the novel method and the ways to improve the mechanical properties of SiC_W/SiC are discussed.     

Mechanical and dielectric properties of SiCf/BN/SiBCN composites via different synthesis technologies

SiC fibers reinforced SiBCN ceramic matrix composites (SiC_f/BN/SiBCN composites) were synthesized by direct chemical vapor infiltration (CVI), polymer infiltration pyrolysis (PIP) or chemical vapor infiltration combined with polymer infiltration pyrolysis (CVI + PIP). It is shown that the insertion of a continuous and dense SiBCN matrix via the CVI process improves the flexural strength and modulus. Interface debonding and fiber pullout happened with 50–100 nm BN interface in CVI and CVI + PIP SiC_f/BN/SiBCN composites. The relative complex permittivity was measured in X-band. Higher ε′′ values in CVI-containing composites can be observed, which can be attributed to the accumulation of C and SiC phases and a multilayer matrix. Strong electromagnetic wave attenuation ability was obtained with high dielectric loss.     

Microstructure and properties of 2D-Cf/SiC composite fabricated by combination of CVI and PIP process with SiC particle as inert fillers

2D-C_f/SiC composite was manufactured by chemical vapor inflation (CVI) combined with polymer impregnation and pyrolysis (PIP) with SiC particle as inert fillers. The effects of CVI processes on SiC morphologies and the properties of composite were investigated. The composites were characterized by XRD, flexural strength test and SEM. The results revealed that uniform SiC coatings and nanowires were prepared when MTS/H₂ ratio of 1:8 was employed, while gradient thick coatings were fabricated as MTS/H₂ ratio of 1:1 was employed. The flexural strength of composites varied from 156 MPa at MTS/H₂ ratio of 1:1 to 233 MPa at MTS/H₂ ratio of 1:8. All of composites exhibited toughness due to significant debonding and pullout of fibers. The laminated structure of coatings on the fibers and nanowires were manufactured by combination of above different CVI process, and the obtained composites showed flexural strength of as high as 248 MPa and impressive toughness.     

Fabrication of cylindrical SiCf/Si/SiC-based composite by electrophoretic deposition and liquid silicon infiltration

Cylindrical SiC-based composites composed of inner Si/SiC reticulated foam and outer Si-infiltrated SiC fiber-reinforced SiC (SiC_f/Si/SiC) skin were fabricated by the electrophoretic deposition of matrix particles into SiC fabrics followed by Si-infiltration for high temperature heat exchanger applications. An electrophoretic deposition combined with ultrasonication was used to fabricate a tubular SiC_f/SiC skin layer, which infiltrated SiC and carbon particles effectively into the voids of SiC fabrics by minimizing the surface sealing effect. After liquid silicon infiltration at 1550 °C, the composite revealed a density of 2.75 g/cm³ along with a well-joined interface between the inside Si/SiC foam and outer SiC_f/Si/SiC skin layer. The results also showed that the skin layer, which was composed of 81.4 wt% β-SiC, 17.2 wt% Si and 1.4 wt% SiO₂, exhibited a gastight dense microstructure and the flexural strength of 192.3 MPa.     

Preparation of SiC/SiC composites with high toughness by reaction of the SiCP+C double-cladding layer with liquid silicon

SiC/SiC composites prepared by liquid silicon infiltration (LSI) have the advantages of high densification, matrix cracking stress and ultimate tensile strength, but the toughness is usually insufficient. Relieving the residual microstress in fiber and interphase, dissipating crack propagation energy, and improving the crystallization degree of interphase can effectively increase the toughness of the composites. In this work, a special SiC particles and C (SiC_P +C) double-cladding layer is designed and prepared via the infiltration of SiC_P slurry and chemical vapor infiltration (CVI) of C in the porous SiC/SiC composites prepared by CVI. After LSI, the SiC generated by the reaction of C with molten Si combines with the SiC_P to form a layered structure matrix, which can effectually relieve residual microstress in fiber and interphase and dissipate crack propagation energy. The crystallization degree of BN interphase is increased under the effects of C-Si reaction exotherm. The as-received SiC/SiC composites possess a density of 2.64 g/cm³ and a porosity of 6.1%. The flexural strength of the SiC/SiC composites with layered structure matrix and highly crystalline BN interphase is 577 MPa, and the fracture toughness reaches up to 37 MPa·m^1/2. The microstructure and properties of four groups of SiC/SiC composites prepared by different processes are also investigated and compared to demonstrate the effectiveness of the SiC_P +C double-cladding layer design, which offers a strategy for developing the SiC/SiC composites with high performance.     

High-temperature flexural strength of I-CVI processed Cf/SiC composites with variable interphases

The effect of single-layer pyrocarbon (PyC) and multilayered (PyC/SiC)_n=4 interphases on the flexural strength of un-coated and SiC seal-coated stitched 2D carbon fiber reinforced silicon carbide (C_f/SiC) composites was investigated. The composites were prepared by I-CVI process. Flexural strength of the composites was measured at 1200 °C in air atmosphere. It was observed that irrespective of the type of interphase, the seal coated samples showed a higher value of flexural strength as compared to the uncoated samples. The flexural strength of 470 ± 12 MPa was observed for the seal coated C_f/SiC composite samples with multilayered interphase. The seal coated samples with single layer PyC interphase showed flexural strength of 370 ± 20 MPa. The fractured surfaces of tested samples were analyzed in detail to study the fracture phenomena. Based on microstructure-property relations, a mechanism has been proposed for the increase of flexural properties of C_f/SiC composites having multilayered interphase.     

Synthesis and properties of multiscale porosity TiC-SiC ceramics

A process combining the pyrolysis of a lignocellulosic structure and reactive gas treatments has been developed to prepare porous TiC-SiC ceramics for solar receivers. The natural micro-porosity of balsa was complemented by a high open macro-porosity by laser cutting a periodical arrangement of parallel channels. The lignocellulosic structure was first pyrolysed into carbon. This reactive carbon material was then converted into TiC by Reactive Chemical Vapor Deposition (RCVD) using TiCl₄/H₂. After controlling the absence of cracks due to volume changes, the TiC structure was finally infiltrated by the Chemical Vapor Infiltration (CVI) of SiC using CH₃SiCl₃/H₂. The density, porous structure, elemental and phase compositions, oxidation behavior and crushing strength were assessed after pyrolysis, RCVD and CVI. The SiC CVI coating significantly improves the compressive strength, the oxidation resistance and the thermal properties. The SiC layer is no longer fully protective at high temperature but the mechanical properties remain reasonably high.     

Thermal shock resistance and hoop strength of triplex silicon carbide composite tubes

Multi‐layered SiC composites have been considered as a nuclear fuel cladding material of light water reactors, LWRs, because of their excellent high temperature strength and corrosion resistance under accident conditions. During a design basis accident of a LWR such as a loss‐of‐coolant accident, the peak temperature of the fuel clad rapidly increases as the production of decay heat continues. The emergency core cooling systems then automatically supply the reactor core with emergency cooling water. The fuel clad consequently suffers from thermal shock. In this study, the structural integrity of multi‐layered SiC composite tubes after thermal shock was investigated. Several kinds of multi‐layered SiC composite tubes consisting of CVD SiC and CVI SiC_f/SiC were water‐quenched from 1200°C to room temperature. The triplex SiC composite tube retained its tubular geometry during quenching. The strength degradation after thermal shock was <13% for the specimens with a PyC interphase. The residual stress distribution within the tubes during thermal shock was evaluated by a finite element method.     

Ti3SiC2 interphase coating in SiCf/SiC composites: Effect of the coating fabrication atmosphere and temperature

The SiC fibers were coated with Ti₃SiC₂ interphase by dip-coating. The Ti₃SiC₂ coated fibers were heat-treated from 900 °C to 1100 °C in vacuum and argon atmospheres to comparatively analyze the effect of temperature and atmosphere on the microstructural evolution and mechanical strength of the fibers. The results show that the surface morphology of Ti₃SiC₂ coating is rough in vacuum and Ti₃SiC₂ is decomposed at 1100 °C. However, in argon atmosphere, the surface morphology is smooth and Ti₃SiC₂ is oxidized at 1000 °C and 1100 °C. At 1100 °C, Ti₃SiC₂ oxidized to form a thin layer of amorphous SiO₂ embedded with TiO₂ grains. Meanwhile, defects and pores appeared in the interphase scale. As a result, the fiber strength treated in the argon was lower than that treated in vacuum. The porous Ti₃SiC₂ interphase fabricated under vacuum was then employed to prepare the SiC_f/SiC mini composite by chemical vapor infiltration (CVI) combined with precursor infiltration pyrolysis (PIP), and can effectively improve the toughness of SiC_f/SiC mini composite. The propagating cracks can be deflected within the porous interphase layer, which promotes fiber pull-outs under the tensile strength.     

Abnormal mechanical hysteresis behavior of Tyranno-ZMI SiC/SiC containing Ti3Si(Al)C2

This work studied the effect of tough phase Ti₃Si(Al)C₂ on the mechanical hysteresis behavior of SiC/SiC. Different from continuous fibers reinforced brittle ceramic matrix composites, the mechanical hysteresis behavior of SiC/SiC containing Ti₃Si(Al)C₂ shows some abnormal phenomena: as peak applied stress increases during cyclic loading-unloading-reloading tests, the thermal residual stress values exhibit highly dispersion and the thermal misfit relief strain shows abnormally slow growth. These abnormal phenomena are caused by the reduction of transvers cracks (perpendicular to loading fibers) and the generation of hoop cracks (parallel to loading fibers). The plastic deformations of Ti₃Si(Al)C₂ prevents the transverse cracking of modified matrix, while promoting the hoop cracking of SiC matrix prepared by chemical vapor infiltration (CVI-SiC). Hoop cracking occurs within the transition zone containing amorphous SiO₂ layer and carbon layer in CVI-SiC matrix. The combination of weak transition zone and strong modified matrix finally leads to the occurrence of hoop cracking.     

Sandwich structure SiCf/Si3N4–SiOC–Si3N4 composites for high-temperature oxidation resistance and microwave absorption

SiC fiber reinforced ceramic matrix composites (SiC_f-CMCs) have been widely used as structural-functional materials at high temperatures. However, their mechanical and electromagnetic wave (EMW) absorbing properties will deteriorate due to high-temperature oxidation. Therefore, unique sandwich structure, consisting of inner Si₃N₄ impedance layer, middle porous SiOC loss layer and dense oxidation-resistant Si₃N₄ layer, was proposed to enhance multiple material properties in oxidation environment. Herein, SiC_f/Si₃N₄–SiOC–Si₃N₄ composites was fabricated by alternating chemical vapor infiltration (CVI) and polymer infiltration pyrolysis (PIP) methods. For these composites, SiC fiber is used as both reinforcing phase and electromagnetic (EM) absorber. CVI Si₃N₄ matrix was distributed in inner and outer layer of the SiC_f/Si₃N₄–SiOC–Si₃N₄ composites. While inner Si₃N₄ layer between BN interphase and SiOC matrix forms nano-heterogeneous interphase to consume EM energy and enhance mechanical properties of composites, outer dense and oxidation-resistant CVI Si₃N₄ coating serves to maintain properties. PIP SiOC matrix exhibits porous structure that can effectively deflect cracks and achieve multiple scattering of EMW. SiC_f/Si₃N₄–SiOC–Si₃N₄ composites with sandwich structure demonstrated excellent EMW absorbing properties and mechanical performance in high-temperature oxidation environments.     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

Application of SiO2-coated SiC powder in stereolithography and sintering densification of SiC ceramic composites

Stereolithography additive manufacturing of SiC ceramic composites has received much attention. However, the forming efficiency and mechanical properties of their products need to be improved. This study aimed to prepare SiC ceramic composites with complex shapes and high flexural strength using a combination of digital light processing (DLP) and reactive solution infiltration process (RMI). A low-absorbance SiO₂ cladding layer was formed on the surface of SiC powder through a non-homogeneous precipitation process. With the densification of the cladding layer at high temperatures, SiO₂-coated SiC composite powder was used to formulate a photosensitive ceramic slurry with a solid content of 44 vol%. The resulting slurry exhibited a considerable improvement in curing thickness and rate and was used to mold ceramic green body with a single-layer slicing thickness of 100 μm using DLP. The ceramic blanks were then sintered and densified using a carbon thermal reduction combined with liquid silica infiltration (LSI) process, resulting in SiC ceramic composites with a density of 2.87 g/cm³ and an average flexural strength of 267.52 ± 2.5 MPa. Therefore, the proposed approach can reduce the manufacturing cycle and cost of SiC ceramic composites.