Ablation behaviour of ultra-high temperature ceramic matrix composites: Role of MeSi2 addition

A new class of ZrB₂ composites reinforced with 40 vol% C short fibers and containing 5 vol% SiC in combination with 5 vol% MoSi₂, HfSi₂ or WSi₂ successfully withstood extreme conditions in a oxyacetylene torch. Different responses to the torch testing were recorded depending on which secondary phase was present; this was primarily a result of the final density which ranged between 83 and 94% of the theoretical value. The temperatures achieved on the surfaces of the samples tested also varied as a function of the residual porosity and ranged from 2080 to 2240 °C. HfSi₂ additions offered the best performance and exceeded that of the baseline material that contained only SiC. It is believed that this was due to its ability to promote the elimination of porosity during densification and to the refractory nature of its oxide, HfO₂. In contrast, MoSi₂ and WSi₂ formed highly volatile oxides on the surface, which did not offer better protection than the ZrO₂-SiO₂ scale that developed in the baseline.     

Development of a slurry injection technique for continuous fibre ultra-high temperature ceramic matrix composites

A simple and effective slurry injection method for producing dense and uniform ultra-high ceramic matrix composites from preforms of high fibre density was developed. As this method is based on slurry injection the homogeneity is not constrained to small preform sizes; dense components of high fibre volume can be produced in theoretically any size and shape. Samples produced by this method demonstrated high and consistent densities, with the injection method obtaining densities an average 27% higher and 87% lower in variability when compared to conventional vacuum impregnation. Tomography demonstrated no bias in the ceramic powder distribution for samples produced by injection, whereas samples produced by vacuum impregnation alone displayed poor powder penetration to the centre of large samples. The new approach yielded composites that were as strong and/or more consistent in strength compared to vacuum impregnation. Thermo-ablative testing demonstrated significant improvements in protective capability for materials produced by this route.     

Structure,microwave dielectric properties,and infrared reflectivity spectrum of olivine type Ca2GeO4 ceramic

Ca₂GeO₄ dielectric ceramic was prepared using the conventional solid-state reaction method. Sintering behavior, crystal structure, microstructure, and microwave dielectric properties were analyzed by XRD, SEM, Raman, and Infrared reflectivity spectrum. Ca₂GeO₄ was found to crystallize in the olivine structure with a space group of Pnma. A dense and high-performance microwave dielectric property with permittivity ? 6.76 ± 0.02, Q×f value ? 82,400 ± 1800 GHz, and temperature coefficient ? -67 ± 3.4 ppm/°C were obtained in the sample sintered at 1420 °C. Infrared spectral analysis supported that the dielectric contribution for Ca₂GeO₄ at microwave region is dominated by absorption of phonons and there is no contribution from dipolar or other polarization mechanisms. The large negative τ_f values could be compensated by forming composite ceramics with CaTiO₃. A low-ε_r of 9.02 ± 0.03, a high Q×f of 49,880 ± 1400 GHz, and a near-zero τ_f value of +4 ± 0.6 ppm/°C were obtained for 0.92Ca₂GeO₄-0.08CaTiO₃ ceramic at 1420 °C for 4 h. This ceramic could be a good candidate for microwave substrate materials.     

红外节能涂料的研制和应用

论述了提高工业炉窑使用寿命和节能等多功能的低、中、高温远红外辐射涂料的机理、性能、制备、施工和应用。     

Dielectric characterization of a novel Bi2O3-Nb2O5-SiO2-Al2O3 glass-ceramic with excellent charge-discharge properties

A newly designed glass-ceramic system consisting of 15Bi₂O₃-15Nb₂O₅-40SiO₂-30Al₂O₃ was successfully prepared, which was followed by its controlled crystallization at different heating temperatures. The effects of crystallization temperature on the microstructure, phase evolution and the energy storage behaviors of the novel material were systematically investigated. A maximum theoretical energy storage density of up to 15.3 J/cm³ was found in the samples heated at 800 °C. The polarization-electric (PE) hysteresis loops of this material exhibited very good linear character and high energy efficiency. In addition, an approximate value of 25 ns for discharged period T has been obtained, which demonstrated that most of the energy stored in dielectric was released over a very short time. The maximum powder density exceeds a high value of 90 MW/cc in a 390 kV/cm electric field. Therefore, the new developed Bi₂O₃-Nb₂O₅-SiO₂-Al₂O₃ glass-ceramic can be used as an alternative, promising high-performance electrostatic capacitor material.     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Electrophoretic co-deposition of Fe2O3 and Mn1,5Co1,5O4: Processing and oxidation performance of Fe-doped Mn-Co coatings for solid oxide cell interconnects

Fe-doped Mn_1,5Co_1,5O₄ coatings on Crofer22APU were processed by an electrophoretic co-deposition method and the corrosion resistance was tested at 750 °C up to 2000 h.The “in-situ” Fe-doping of the manganese cobalt spinel was achieved by electrophoretic co-deposition of Mn_1,5Co_1,5O₄ and Fe₂O₃ powders followed by a two-step reactive sintering treatment. The effects on the coating properties of two different Fe-doping levels (5 and 10 wt.% respectively) and two different temperatures of the reducing treatment (900 and 1000 °C) are discussed. Samples with Fe-doped coatings demonstrated a lower parabolic oxidation rate and thinner oxide scale in comparison with both the undoped Mn_1,5Co_1,5O₄ spinel coating and bare Crofer 22 APU. The best corrosion protection was achieved with the combined effect of Fe-doping and a higher temperature of the reducing step at 1000 °C.     

Phase and microstructure evolution in plasma sprayed Yb2Si2O7 coatings

Yb₂Si₂O₇ coatings were deposited on Si/SiC substrates by atmospheric plasma spray (APS). The different power and plasma chemistries used in this work produced mainly amorphous crack-free coatings with compositions shifted to lower SiO₂ content with higher power and H₂ flow. Differences in microstructure and thermomechanical properties (crystallization behavior, thermal expansion coefficient and thermal conductivity) of as-deposited and thermally treated coatings were directly related to the evolution from amorphous to crystalline phases. A Yb₂SiO₅ metastable phase was identified after thermal treatments at temperatures ～ 1000 °C that transformed to its stable isomorph at 1220 °C. This transformation, followed by the growth of the crystal cell volume, promoted the coating expansion and the “healing” of microcracks present in the amorphous as-sprayed coating.     

Thermodynamic assessment of the group IV,V and VI oxides for the design of oxidation resistant multi-principal component materials

Multi-principal component materials (MPCMs) are currently being investigated for use in high and ultra-high temperature environments. The design of oxidation resistant multi-component materials requires as input the oxidation behavior of each of the components. FactSage free energy minimization software and databases were used to calculate the equilibrium oxide phases and free energies of formation for the oxides of the Group IV, V and VI refractory metals, and their carbides, nitrides and borides. The results are summarized in Ellingham diagrams. Periodic trends were noted; Group IV elements form the most stable oxides with the highest melting temperatures (T_m), Group V elements form oxides with low T_m, and Group VI elements form gaseous oxide species. Oxygen diffusion data from literature for some of these oxides were also reviewed and summarized. The results are utilized to identify strategies for optimizing oxidation resistance of MPCMs for service at temperatures above 1700°C.     

Synthesis of Hf6Ta2O17 superstructure via spark plasma sintering for improved oxidation resistance of multi-component ultra-high temperature ceramics

Ultra-high temperature ceramics (UHTCs) have shown aspiration to overcome challenges in the thermal protection system (TPS) by designing new materials referred to as multi-component UHTCs (MC-UHTCs) in the compositional space. MC-UHTCs have shown remarkable improvement in oxidation resistance due to the formation of the Hf₆Ta₂O₁₇ superstructure during plasma exposure. Herein, the Hf₆Ta₂O₁₇ superstructure is synthesized via a solid-state reaction between HfO₂ and Ta₂O₅ powder mixtures during spark plasma sintering (SPS). The compositions chosen are 50 vol% of HfO₂ -50 vol% of Ta₂O₅ (50HO-50TO) and 70 vol% of HfO₂ -30 vol% of Ta₂O₅ (70HO-30TO). The phase quantification via Rietveld analysis showed Hf₆Ta₂O₁₇ as a principal phase with some residual Ta₂O₅ phase in both the samples. The high-temperature thermal stability of the samples was evaluated using high-velocity plasma jet exposure for up to 3 min. 50HO-50TO was able to withstand the intense plasma condition, which is attributed to the higher content of the Hf₆Ta₂O₁₇ phase (～84%) and lower strain in the Ta₂O₅ phase. The augmentation in the Hf₆Ta₂O₁₇ phase to 94.7% (in 50HO-50TO) post plasma exposure has been attributed to the invariant transformation from a liquid state to Hf₆Ta₂O₁₇ at temperatures >2500 °C during testing. The mechanical integrity is elucidated from the insignificant change in the hardness ～13.3 GPa before and 11.2 GPa after plasma exposure of the 50HO-50TO sample. As a result, the Hf₆Ta₂O₁₇ superstructure's thermo-mechanical stability suggests developing novel oxidation-resistant MC-UHTCs in compositional space for reusable space vehicle applications.     

Modified double-notched specimen for ultra-high temperatures shear-strength testing of carbon/carbon composites

In this paper, a modified double-notched specimen (MDNS) is proposed to investigate shear strength of carbon/carbon composites at ultra-high temperatures. The effects of surplus notch length on distribution of shear stress in the gauge area are studied using finite element method. Both standard Iosipescu method and the MDNS method are used to test shear strength of 3D needled, 3D woven, and 4D woven C/C composites at room temperature. The results are in good agreement with each other. Uniform shear strain is observed using digital image correlation (DIC) technology to confirm the proposed method. Additionally, the shear strength of these three types C/C composites are measured at 2000 °C, 2400 °C, and 2800 °C using the designed testing system. The temperature field at 2000 °C is measured using a thermal imaging system to demonstrate uniform distribution of temperature. The failure mechanisms at ultra-high temperatures are also characterized via optical microscopy.     

Diffusion study of rare-earth oxides into silica layer for environmental barrier coating applications

The effect of exposure temperature and time on the diffusion rate of rare-earth oxides applied on silicon carbide fiber-reinforced SiC ceramic matrix composites (SiC/SiC CMCs) have been investigated. Knowledge on diffusion mechanism between the deposited rare-earth (RE) slurry and silica layer is necessary to understand the process governing EBCs formation and their properties. SEM/EDS analysis were used to study the effect of microstructure on diffusivity. The diffusion coefficient increases with increasing sintering temperature and time. The measured diffusion coefficients of the RE-coating into silica layer were in the order of 10^?15-10^-17 m²/s revealing an overall good adhesion on the SiC/SiC CMCs.     

Friction and wear properties of MoAlB against Al2O3 and 100Cr6 steel counterparts

The present work investigates, for the first time, the dry sliding friction and wear behaviour of fully dense, predominantly single-phase MoAlB ceramics against alumina (Al₂O₃) and 100Cr6 steel counterparts. Against Al₂O₃, the friction coefficient (μ) increased with increasing load and the wear was highly dependent on the load applied. A transition from mild wear under 1 N and 4 N to severe wear at 10 N occurred. Scanning electron microscopy revealed that abrasion is the dominant wear mechanism. Against steel, μ decreased with increasing load and the wear rates were low, under all applied loads. The morphologies of the worn surfaces against steel were characterized by the appearance of a rippled layers. Atomic force microscopy and Raman spectroscopy were used to propose a possible formation mechanism of such patterns. X-ray photoelectron spectroscopy revealed the rippled surfaces to be composed of Fe₂O₃ and a mixture of MoO_x.     

Structure and thermal properties of Al2O3-doped Gd3TaO7 as potential thermal barrier coating

In this paper, a series of solid solutions ceramics of (Al_xGd_1-x)₃TaO₇ (x = 0, 0.01, 0.03, 0.05) were synthesized via solid-state reaction. X-ray diffraction (XRD) and Raman spectroscopy analysis indicated that the crystal structure of (Al_xGd_1-x)₃TaO₇ ceramics is weberite in spite of the content of Al³⁺ is up to 5 mol.%. The thermal conductivities of (Al_xGd_1-x)₃TaO₇ ceramics range from 1.37 W?m^?1 K^?1 to 1.47 W?m^?1 K^?1 at 900 ℃, which is much lower than that of 7–8 YSZ (about 2.5 W?m^?1 K^?1). The thermal expansion coefficients (TECs) of (Al_xGd_1-x)₃TaO₇ ceramics vary in the range of 6–10 × 10^-6 K^-1 within the temperature range 100–1200 ℃, and the values are close to the TECs of 7–8 YSZ. Given the low thermal conductivity and high thermal expansion coefficients of (Al_xGd_1-x)₃TaO₇ ceramics, they have the potential to be the next generational thermal barrier coating materials.     

High-temperature dielectrics based on (1-y)[(1-x)Bi0.5Na0.5TiO3-xBiAlO3]-yCaZrO3 ternary system with stable permittivity and low dielectric loss in a wide temperature range

High temperature dielectrics based on (1-y)[(1-x)Bi_0.5Na_0.5TiO₃-xBiAlO₃]-yCaZrO₃ (BNT-100xBA-100yCZ) ternary system were designed and prepared. The introduction of BiAlO₃ is verified to create defect dipoles ${(\text{A}{\text{l}}_{\text{T}\text{i}}^{\text{'}}-{\text{V}}_{\text{O}}^{??})}^{?}$, which leads to the increase of the resistivity and decrease of dielectric loss in BNT-100xBA-100yCZ at high temperature. And the introduction of CaZrO₃ is helpful to increase the temperature stability of permittivity, which is probably due to an inhomogeneous domain structure. The composition of x=0.09 and y=0.05 has a good overall dielectric properties, with permittivity value of 765 at 25 °C and 1263 at 200 °C, small variance of permittivity (Δε'/ε'_{200 °C} ≤ ±15%) between 133 °C and 500 °C and low dielectric loss (tan ≤ 0.02) in the temperature range of 160 °C ?425 °C. Therefore, this system will be one of promising candidates of dielectrics used for high-temperature capacitors.     

Fabrication of Ti3SiC2/SiCp multiport minichannel plates for high-temperature applications

An innovative method for fabricating ceramic multiport minichannel plates (MMP) has been developed. The essence of the method is the combustion synthesis of Ti₃SiC₂/SiC_p ceramic matrix composite using sandwich assembly of titanium metal items, viz. rods and sheets, and tape-cast rubber films filled with SiC particles and TiC additive. The fabrication route consists of hot pressing of starting materials at 100 °C under 60 MPa in air into a single integrated unit followed by heat treatment at 1400 °C in a vacuum hot press furnace under small uniaxial load of 80 N. It is shown that the one-dimensional titanium metal items act as both reactants and channel-forming templates. The MMP prepared by the proposed method contains primarily of 45 vol.% Ti₃SiC₂ and 44 vol.% SiC, as well 11 vol.% TiSi₂ minor component. The material exhibits a composite microstructure consisting of a dense Ti₃SiC₂-based matrix and SiC particles uniformly embedded in it. The sample contains parallel-aligned hollow cylindrical channels of 0.8 mm in diameter distributed with a center-to-center spacing of 1.3 mm. The volume fracture of the channels is estimated to be about 19%.     

Corrosion resistance and infrared emissivity properties of EPDM (EPDM-g-MAH) film on low infrared emissivity PU/Cu coating

In order to improve the corrosion resistance of low infrared emissivity polyurethane (PU)/Cu coating, ethylene-propylene-diene monomer (EPDM) and maleic anhydride grafted EPDM (EPDM-g-MAH) were deposited on PU/Cu coating by spin coating, respectively. The ability of EPDM and EPDM-g-MAH to serve as corrosion protective films for PU/Cu coating was examined by evolved emissivity immersion test and potentiodynamic polarization measurements in 3.5% NaCl solution. And the chemical composition and surface morphology of PU/Cu coating before and after corrosion were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that all of the coated samples present a better corrosion resistance than bare PU/Cu coating due to indissolubility of Cu by aggressive ions, and the effect of EPDM-g-MAH is better than that of EPDM.     

Synthesis and characterization of novel Pr6O11/Mn3O4 nanocomposites for electrochemical supercapacitors

This study presents the successful synthesis of praseodymium oxide, Pr₆O₁₁ and hausmannite manganese oxide, Mn₃O₄ nanoparticles, along with a novel synthesis of (Pr₆O₁₁/Mn₃O₄) nanocomposites by employing the hydrothermal route followed by post thermal annealing. X-ray Diffraction, Field Emission Scanning Electron Microscopy and Energy Dispersive X-ray characterization techniques are being adapted to analyze the physical characteristics of all the synthesized materials. XRD results reveal the crystalline nature of the synthesized materials. FE-SEM results display the irregular nanograins of Mn₃O₄ and a regular network of interconnected Pr₆O₁₁ nanoparticles. Nitrogen adsorption/desorption tests confirm the mesoporous nature of all the synthesized electrode materials. The Pr₆O₁₁/Mn₃O₄ ??2 electrode material exhibits an outstanding specific capacitance of 794.58?F/g at a current density of 0.5?A/g, as compared to the 521.24?F/g for the Pr₆O₁₁ electrode material. These investigations provide an easy and efficient method to develop nanocomposites (Pr₆O₁₁/Mn₃O₄) with better electrochemical characteristics, as electrode materials for supercapacitor applications.     

Effect of internal lattice structure on the flexural strength of 3D printed hierarchical porous ultra-high temperature ceramic (ZrB2)

3D printing of technical ceramics using direct ink writing (DIW) of multiphase colloidal inks has the unique ability to create structures with hierarchical features. To facilitate the application of 3D printed hierarchical porous ultra-high temperature ceramics (UHTCs), additional limiting factors such as strength and the effect of 3D printed internal lattice structure need to be better understood. This study reports on the strength dependence of common DIW print parameters including internal lattice structure shape, nozzle diameter and spacings between adjacent filaments. The present study applies Weibull statistics to the experimental array that considers macro features introduced through print parameters as flaw types, which shows strength of 3D printed hierarchical ZrB₂ is highly dependent on the introduced 3D printed structure, size and the stressed volume. This work provides essential information that can be used in the initial stages of design when considering implementation of additively manufactured hierarchical porous UHTCs.     

Towards thermal barrier coating application for rare earth silicates RE2SiO5 (RE= La,Nd, Sm,Eu, and Gd)

Rare earth (RE) silicates X1-RE₂SiO₅ (RE = La, Nd, Sm, Eu, and Gd) are comprehensively investigated as promising thermal barrier coating candidates. The mechanical, thermal, and corrosion resistance properties are evaluated by theoretical exploration and experimental measurement. Mechanical properties and corrosion resistance to calcium-magnesium alumino-silicates (CMAS) melts of X1-RE₂SiO₅ are linearly correlated with ionic radius of RE elements. Elastic moduli increase with the decrease of ionic radius of RE³⁺. X1-RE₂SiO₅ with larger RE³⁺ exhibits better resistance to molten melts corrosion. For thermal properties, they are not obviously sensitive to RE species. All X1-RE₂SiO₅ demonstrate low thermal conductivities and their magnitudes are significantly modified by concentration of defects. Thermal expansion coefficients of X1-RE₂SiO₅ are more or less close and are compatible with the value of superalloy. The results highlight X1-RE₂SiO₅ as potential thermal barrier coating candidates with overall properties.