Si-based Ceramics Research Articles

Low-temperature synthesis of dense Si-based nitride ceramics for solar thermal storage: Role of cordierite as sintering additive

Direct nitridation method was used to prepare dense Si-based ceramics for solar thermal storage, and the effect of cordierite as sintering additive on the sintering behaviors of the ceramics was studied. The influence priority between densification and nitride content on the properties of thermal storage ceramics was discussed. Cordierite exhibited a good capability of promoting densification, and the densification temperature could reach 1500 °C. The liquid formed by cordierite helped to construct an interlocking structure of well-grown Si2N2O flakes. Results of hot stage microscopy and DTA indicated that the endothermic silicothermic reaction between Si and cordierite proceeded at 1440–1460 °C, and released SiO gas. The SiO gas inhibited the densification, but resulted in the low-temperature synthesis of nano-sized Si3N4 whiskers. Samples added with more cordierite showed the cost-effective merits as the smaller costs of raw materials, lower densification temperature, higher thermal conductivity (1.72 W m−1 K−1 deviation), and higher bending strength (34.3–68.1 MPa deviations), compared with the samples having the larger nitride content. Densification by adding cordierite as sintering additive showed the priority on the properties of thermal storage ceramics than increasing the nitride content.

Phase evolution and oxidation resistance of Si3N4/HfB2/HfBxCyN1–x–y ceramic nanocomposites prepared from tailored preceramic polymers

Single-source-precursor derived ceramics exhibit advantages for the preparation of Si-based ceramics with controllable phase composition and adjustable functional/mechanical properties, which has significant potential for (ultra)high temperature ceramic materials. Within the present work, a series of hafnium/boron-containing Si3N4-based ceramics (SiHfBCN) are prepared upon pyrolysis/annealing of the corresponding single-source-precursors in N2 atmosphere at different temperatures ranging from 1000 °C to 1700 °C. The high-temperature (micro)structural evolution with respect to the annealing temperatures and boron concentration was studied using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. The results show that the amorphous SiHfBCN ceramics convert upon crystallization into ceramic nanocomposites consisting of a α-Si3N4 matrix with embedded Si, HfB2, and HfBxCyN1–x–y. The formation and stability of HfBxCyN1–x–y solid solution were discussed in detail. Finally, the oxidation resistance of the obtained α-Si3N4/HfB2/HfBxCyN1–x–y ceramic nanocomposites was investigated by thermal gravimetric analysis in air.

Atomic structural visualization on γ-Ho2Si2O7 using iDPC-STEM technique and its correlation with thermal expansion as advanced environmental barrier coating

γ-Ho2Si2O7 has attracted considerable attention as advanced environmental barrier coating (EBC) material for SiCf/SiC CMC due to its matchable thermal expansion coefficient (CTE) with Si-based ceramics. Negative phonons anharmonicity driven by crystal structure are thought to play a significant role in its thermal expansion behavior. In this work, atomic vibrations of γ-Ho2Si2O7 are investigated by ab-initio molecular dynamics simulation, Raman, and infrared spectra. The results confirm that transverse vibration of bridge O atoms in rigid [Si2O7] units with linear configuration assure negative contribution to thermal expansion during thermal cycling. Moreover, atomic visualization is achieved by integrated differential phase contrast (iDPC)-STEM technique, and both heavy (Ho) and light (Si, O) atoms in γ-Ho2Si2O7 are clearly discerned. The acquired images provide a direct understanding to the origin of above mechanisms by capturing projection of all atomic columns along [100]. This work provides new insights into investigation of RE silicates through structural visualization and will facilitate the material design for advanced EBCs.

Effect of carbon and oxygen on the high-temperature properties of silicon carbide–hafnium carbide nanocomposite fiber

The polymer-derived ceramics (PDCs) technique enables relatively low-temperature fabrication of Si-based ceramics, with silicon carbide fiber as a representative product. Polycarbosilane (PCS) has Si-C backbone structures and can be converted to silicon carbide. In the PDCs method, residual or excess carbon is generated from the precursor (C/Si ratio = 2 for polycarbosilane). Because of the non-stoichiometry of SiC, the physicochemical properties of polymer-derived SiC are inferior to those of conventional monolithic SiC. Herein, a silicon carbide-hafnium carbide nanocomposite fiber was optimized by crosslinking oxygen into the PCS fiber by regulating the oxidation curing time. During pyrolysis, carbothermal reduction, and sintering, carbon was removed by reaction with hydrogen and cross-linked oxygen. Non-destructive techniques (X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and high-temperature thermomechanical analysis) were used to investigate the effects of excess carbon. The microstructure of the near-stoichiometric SiC-HfC nanocomposite fiber was more densified, with superior high-temperature properties.

Phase composition and property evolution of (Yb1−xHox)2Si2O7 solid solution as environmental/thermal barrier coating candidates

RE disilicates are good candidates as environmental/thermal barrier coating for SiCf/SiC composite in harsh gas turbine engines. We designed (Yb1−xHox)2Si2O7 solid solutions and studied mechanical properties, thermal properties, and water vapor resistance. Powders with different compositions were synthesized by pressureless sintering, and bulk samples were prepared by Spark Plasma Sintering (SPS). Polymorphic changes with temperature and composition of the solid solutions were examined. Through doping Ho into Yb2Si2O7, water vapor corrosion resistance is significantly promoted, and thermal expansion coefficient is maintained close to that of Si-based ceramics. Compared with host disilicates, thermal conductivity of solid solutions are decreased, and mechanical properties, including Vickers hardness and fracture toughness, are increased. A two-phase domain is found at (Yb1/2Ho1/2)2Si2O7, and the γ to δ phase transition of Ho2Si2O7 is observed during SPS. Among all samples, γ-(Yb1/3Ho2/3)2Si2O7 possesses superior high temperature stability, and excellent water vapor resistance, indicating its performance as environmental/thermal barrier coating.

Friction and wear characteristics of ceramics composite under multidirectional motions

The friction and wear characteristics of stainless steel diffused with Si-based ceramics were investigated using pin-and-disc configuration under reciprocation motion, rotational motion, and simultaneous motion. The pin material was diffused by the combination of 60% Ti2O3, 30% Al2O3, and 10% Si2O3. Experiments have been carried out both in diffused and non-diffused conditions. Both the friction coefficient and wear rate have been possible to reduce by diffused pin material. The effects of both friction coefficient and wear rate have been studied on ceramics composites at different pin-and-disc motions. Experiments were conducted underpin motions of 0.15–0.25 m/s, disc motions of 0.5–0.6 m/s, and normal loads of 2.5–3.5 N. A relation was found among friction, wear and surface hardness of the composite. The friction coefficient and wear resistance were improved of stainless steel diffused with ceramics. Scanning electron microscopic analysis was performed to observe the morphology of ceramic and pin material.

Optimized preceramic polymer for 3D structured ceramics via fused deposition modeling

3D structured ceramics stemmed from preceramic polymers via additive manufacturing have attracted much attention recently. However, these polymers with high ceramic yield are so brittle that extrusion-based additive manufacturing techniques are hardly able to be utilized for assembling 3D structures. Herein, we developed a strategy to prepare feedstocks for these manufacturing techniques, i.e., utilizing a small amount of thermal-plastic polymer to optimize the preceramic polymer while good compatibility is required between the two polymers to ensure a homogeneous mixture. Polycarbosilane and polypropylene were selected as the representative materials. Polypropylene occupied a small proportion (≤5wt.%) and significantly improved the formability of the precursor. Three-dimensional SiC were obtained via fused deposition modeling combined with crosslinking and pyrolysis. The SiC ceramic filaments showed a mean tensile strength of 471 MPa. The strategy is also applicable to a large field of ceramic systems with corresponding precursor, such as sialon ceramic and multicomponent Si-based ceramics.

Thermal and microwave dielectric properties of Li–Si-based ceramics

This paper investigates thermal conduction, thermal expansion and microwave dielectric properties of LixSi(0.50x~0.65x)O(1.5x~1.8x) (LS-based) ceramics. All samples were synthesized via the traditional solid-state reaction route. The mole ratio of Si4+/Li+ ranges from 0.50 to 0.65. According to the results, the composite of Si4+/Li+ = 0.55 showed high thermal conductivity of 8.325 W/(mK) and an average coefficient of thermal expansion of 8.401 × 10−6/K (at the temperature between 100 °C to 150 °C). In order to enhance the thermal properties, 4 wt% Al2O3 is added to the LS-based ceramics and the most excellent thermal properties were obtained on the sample with the mole ratio of Si4+/Li+ = 0.55, exhibiting the properties of the highest thermal conductivity of 11.836 W/(mK), an average coefficient of thermal expansion of 10.748 × 10−6/K (at the temperature between 100 °C to 150 °C), the lower dielectric constant of 6.32, excellent flexural strength of 194 MPa, and the optimum Q × f of 4565 GHz. In the present study, a new type of LS-based ceramics suitable for the transition layer of circuit board was developed. The doping of the Al2O3 into the LS-based ceramics can significantly improve its thermal conductivity, adjust the coefficient of thermal expansion and promote the start-temperature of crystallization to a lower direction.

Polymer-Derived Si-Based Ceramics: Recent Developments and Perspectives

Polymer derived ceramics (PDCs) are promising candidates for usages as the functionalization of inorganic Si-based materials. Compared with traditional ceramics preparation methods, it is easier to prepare and functionalize ceramics with complex shapes by using the PDCs technique, thereby broadening the application fields of inorganic Si-based ceramics. In this article, we summarized the research progress and the trends of PDCs in recent years, especially most recent three years. Fabrication techniques (traditional preparation, 3D printing, template method, freezing casting techniques, etc.), microstructural tailoring mainly via additive doping, and properties (mechanical, thermal, electrical, as well as dielectric and electromagnetic wave absorption properties) of Si-based PDCs were explicated. Meanwhile, challenges and perspectives for PDCs techniques were proposed as well, with the purpose to enlighten multiple functionalized applications of polymer-derived Si-based ceramics.

Crack-healing performance and oxidation behavior of SiC dispersed yttrium silicate composites

ABSTRACT Yttrium silicate-based composites dispersed with silicon carbide particles have been proposed as a novel material concept of self-healing environmental barrier coatings applied for Si-based ceramics and their composites. Crack-healing effectiveness as well as high-temperature oxidation behavior of the yttrium silicate composites was investigated as functions of oxidation time and temperatures. Dense samples of the composites dispersed with 5 vol% SiC particles were fabricated by using the pulsed electric current sintering technique. Thermal oxidation for self-healing was conducted at temperatures ranging from 1000°C to 1300°C for from 1 to 24 h in air. High-temperature oxidation experiments were carried out at temperatures ranging from 1200°C to 1400°C for from 1 to 60 h in air. As a result, the surface cracks with approximately 200 µm in length introduced on the sample surface were disappeared completely after heat treatment at 1300°C for 1 h in air. Crack-healing performance of SiC/Y2SiO5 is better than that of SiC/Y2Si2O7. The crack-healing performance of SiC/Y2SiO5-Y2Si2O7 is in the middle of that of SiC/Y2SiO5 and SiC/Y2Si2O7. The mechanism of crack healing was identified as the consequence of SiC oxidation into SiO2, which accompanies a volume expansion, and outward diffusion of Y3+ cations caused the formation of Y2Si2O7 outer layer. Oxidation of SiC particles within the matrix developed an oxidized zone. Growth of the oxidized zone obeyed the parabolic law, which meant diffusion process in the oxidized zone is the rate-controlling process.

In vitro evaluation of diopside/baghdadite bioceramic scaffolds modified by polycaprolactone fumarate polymer coating.

Porous Si-based ceramic scaffolds are widely attracted in biomedical tissue engineering application. Despite the attractive properties of these materials, their weak mechanical properties and high degradability in vitro and in vivo environment can limit their application as biomedical devises. Applying a thin layer of polymer on the surface of porous scaffolds can improve the mechanical properties and control the degradation rate. In this study, we produced new modified scaffolds with polymers coating in order to improved mechanical and biological properties of Si-based ceramics scaffolds. The results showed that applying 6 wt% PCLF polymer on the surface of Bagh-15 wt%Dio scaffolds delayed apatite formation compared to unmodified scaffolds. On the other hand, in the modified scaffolds, apatite formation was observed. The degradation rate of unmodified scaffolds was decreased around 82% after 28 days soaking in PBS solution. Based on the MTT assay and SEM micrographs, the BMS cells were spread and attached well on the surface of the scaffolds, which indicated a good biocompatibility. The results showed that these scaffolds have the potential to be used as a temporary substrate for bone tissue engineering application.

Tunable properties of (HoxY1-x)2SiO5 as damage self-monitoring environmental/thermal barrier coating candidates

RE2SiO5 with low thermal conductivity, compatible thermal expansion coefficients and excellent high-temperature reliability in harsh environments are excellent candidates as advanced environmental/thermal barrier coating materials for high-efficiency gas turbine engines. A series of rare earth silicates (HoxY1-x)2SiO5 are designed and their properties are comprehensively investigated in this paper. Through doping Ho into Y2SiO5, the thermal conductivity of Y2SiO5 is significantly decreased and the thermal expansion coefficient is also optimized closer to Si-based ceramics. High-temperature elastic stiffness and bending strength are increased with the enhancing of Ho content. Most important, doping Ho element provides (HoxY1-x)2SiO5 with tunable luminescence characteristic. (HoxY1-x)2SiO5 exhibit green, to yellow-green, then to orange-red luminescence color with increased Ho concentration. The results show that they can be used as damage self-monitoring environmental/thermal barrier coating materials for Si-based ceramics.

A new type of RPC with very low resistive material

There are several working groups that are currently working on high rate RPC's using different materials such as Si-based Ceramics, Low-resistive Glass, low-resistive bakelite etc. A new type of single gap RPC has been fabricated using very low-resistive carbon-loaded PTFE material to compete with all these other groups and materials. In terms of bulk resistivity, this material is the lowest and should in principle be able to work at the highest rates, provided the material can withstand working bias and radiation. The efficiency and noise rate of the RPC are measured with cosmic rays. The detail method of fabrication and first experimental results are presented.

Wavelike interfacial design and mechanical behaviour of joint between Si-SiC coated carbon/carbon composites and Li2O-Al2O3-SiO2 glass ceramics

A new insight of the oxidation phenomenon utilisation of Si-based ceramics was proposed to strengthen the joining of carbon/carbon (C/C) composites and Li2O-Al2O3-SiO2 (LAS) glass ceramics. A Si-based ceramic coated C/C substrate with a porous surface was achieved by the in-situ reaction method coupled with the high-temperature pre-oxidation. The porous structure would contribute to the formation of a wavelike interface through hot-pressing sintering of the C/C-LAS joints. The prepared wavelike interface could significantly increase the joining area. Moreover, according to the results of the simulation, the thermal stress distribution around the designed interface was relieved. Therefore, the apparent shear strength of the C/C-LAS joints with the designed interface was strengthened to 32.46 ± 1.35MPa, 51% higher than those without interfacial design.

Tunable thermal properties in yttrium silicates switched by anharmonicity of low-frequency phonons

Coordinated modulation of thermal resistance and thermal expansion compatibility has become a chief concern in designing reliable environmental/thermal barrier coating candidates for Si-based ceramics. This study reports phonon behaviors and thermal properties of prototypical silicates X2-Y2SiO5 and γ-Y2Si2O7. The nature of anharmonicity for low-frequency phonons is analyzed, and the finding is discussed with respect to specific crystal structures and interatomic force constants. Low lattice thermal conductivities (κL) basically derive from the giant phonon anharmonicity enhanced by interactions between low-frequency optic and acoustic phonons. γ-Y2Si2O7 is predicted with considerably lower thermal expansion coefficient (αV) than Y2SiO5. The mechanism is disclosed as an effect of the extensive number of low-frequency phonons with negative Grüneisen constants (contributing to negative thermal expansion) in γ-Y2Si2O7, together with its higher elastic stiffness against internal thermal pressure at elevated temperature. A guideline for simultaneously approaching low κL and low αV is proposed for searching promising E/TBC candidates.

Nucleation-controlled freeze casting of preceramic polymers for uniaxial pores in Si-based ceramics

A grain-selection template was applied to freeze casting to control nucleation of pore-forming crystals and achieve ceramics with highly aligned pore structures. A polymethylsiloxane preceramic polymer was freeze cast with cyclohexane as a solvent to produce dendritic pores in SiOC. Image analysis and permeability measurements were performed to quantify the influence of various templates on sample properties. Results show that the percentage of porosity aligned along the freezing axis increased from 13.9% without a template to 92.6% with an optimal template. The Darcian permeability constant increased by more than 6-fold, from 3.4×10−12 to 2.1×10−11m2.

분무건조 및 대기 플라즈마 용사에 의한 탄화규소 세라믹스용 내환경 코팅재의 제조 및 평가

Environmental barrier coatings (EBCs) are used to protect SiC-based ceramics or composites from oxidation and corrosion due to reaction with oxygen and water vapour at high temperatures above 1000℃. Mullite ceramics have been studied for environmental barrier coatings for Si-based ceramics. More recently, rare earth silicate ceramics have been identified as more water vapour-resistant materials than mullite for environmental barrier coatings. In this study, we fabricate mullite and yttrium silicate ceramics by an atmospheric plasma spray coating method using spherical granules fabricated by spray drying. As a result, EBCs with thicknesses in the range of 200-300 μm are successfully fabricated without any macroscopic cracks or interfacial delamination. Phase and microstructure analysis are conducted, and the basic mechanical properties, such as hardness and indentation load-displacement curves are evaluated.

Stability of interfaces in hybrid EBC/TBC coatings for Si-based ceramics in corrosive environments

SiC/SiC ceramic matrix composites (CMCs) are being used increasingly in the hot-sections of gas turbines, especially for aerospace applications. These CMCs are subject to recession of their surface if exposed to a flow of high-velocity water vapor, and to hot-corrosion when exposed to molten alkali salts. This research involves developing a hybrid system containing an environmental barrier coating (EBC) for protection of the CMC from chemical attack and a thermal barrier coating (TBC) that allows a steep temperature gradient across it to lower the temperature of the CMC for increased lifetimes. The EBC coating is a functionally graded mullite (3Al2O3•2SiO2) deposited by chemical vapor deposition (CVD), and the TBC layer is yttria-stabilized zirconia (YSZ) deposited by air plasma spray (APS). The stability of this system is investigated, which includes the adhesion between the two coating layers and the substrate, the physical and chemical stability of each layer at high temperature, and the performance under severe thermal shock and during exposure to hot corrosion. The effect of vertical cracks in the TBC on the EBC layer below it is also examined.

Fabrication and characterization of silicon-based ceramic/aluminum nitride as thermally conductive hybrid filler in silicone rubber composite

Silicon-based ceramics and aluminum nitride (Si/AlN) were fabricated by pyrolysis of polysilazane (PSZ) coated AlN at 1600 °C in air, Ar, N2 and NH3 atmospheres. The results from XRD, XPS and SEM/EDS revealed the existence of crystalline phase of Si-based ceramics on AlN particles including SiO2 in Si/AlNair, SiC whisker in Si/AlNAr, SiC whisker/Si3N4 grain in Si/AlNN2, and Si3N4 ceramics in Si/AlNNH3. It was found that the thermal conductivity of silicone rubber filled with Si/AlN hybrid fillers which were fabricated in Ar and NH3, was improved. This might be due to the existence of nanoscale SiC whiskers and a thermal conductive layer of Si3N4 on Si/AlNAr and Si/AlNNH3 surface, respectively. Thus, it can be suggested that Si/AlN hybrid fillers, especially for the case of Si/AlN(Ar and NH3), can be applicable to use as thermally conductive fillers. This work may have found to be an alternative way to improve thermal conductivity, but more work needs to be done to really achieve significant improvement.

Joining of Si3N4 ceramic using PdCo(NiSiB)–V system brazing filler alloy and interfacial reactions

The wettability of V-active PdCo-based alloys on Si3N4 ceramic was studied with the sessile drop method. And the alloy of Pd50.0–Co33.7–Ni4.0–Si2.0–B0.7–V9.6 (wt%), was developed for Si3N4 ceramic joining in the present investigation. The rapidly-solidified brazing foils were fabricated by the alloy Pd50.0–Co33.7–Ni4.0–Si2.0–B0.7–V9.6. The average room-temperature three-point bend strength of the Si3N4/Si3N4 joints brazed at 1453K for 10min was 205.6MPa, and the newly developed braze gives joint strengths of 210.9MPa, 206.6MPa and 80.2MPa at high temperatures of 973K, 1073K and 1173K respectively. The interfacial reaction products in the Si3N4/Si3N4 joint brazed at 1453K for 10min were identified to be VN and Pd2Si by XRD analysis. Based on the XEDS analysis result, the residual brazing alloy existing at the central part of the joint was verified as Co-rich phases, in which the concentration of element Pd was high up to 18.0–19.1at%. The mechanism of the interfacial reactions was discussed. Pd should be a good choice as useful alloying element in newer high-temperature braze candidates for the joining of Si-based ceramics.