Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

Effects of Gd2O3 and MgSiN2 sintering additives on the thermal conductivity and mechanical properties of Si3N4 ceramics

Si₃N₄ ceramics were prepared by gas pressure sintering at 1900°C for 12 h under a nitrogen pressure of 1 MPa using Gd₂O₃ and MgSiN₂ as sintering additives. The effects of the Gd₂O₃/MgSiN₂ ratio on the densification, microstructure, mechanical properties, and thermal conductivity of Si₃N₄ ceramics were systematically investigated. It was found that a low Gd₂O₃/MgSiN₂ ratio facilitated the thermal diffusivity of Si₃N₄ ceramics while a high Gd₂O₃/MgSiN₂ ratio benefited the densification and mechanical properties. When the Gd₂O₃/MgSiN₂ ratio was 1:1, Si₃N₄ ceramics obtained an obvious exaggerated bimodal microstructure and the optimal properties. The thermal conductivity, flexural strength, and fracture toughness were 124 W·m⁻¹·k⁻¹, 648 MPa, and 9.12 MPa·m^1/2, respectively. Comparing with the results in the literature, it was shown that Gd₂O₃-MgSiN₂ was an effective additives system for obtaining Si₃N₄ ceramics with high thermal conductivity and superior mechanical properties.     

Densification behaviors and high-temperature characteristics of Si3N4 sintered bodies using Al2O3–Yb2O3 additives

The densification behaviors (include α–β transformation) and high-temperature characteristics (especially oxidation resistance and high-temperature strength properties) of Si₃N₄ sintered bodies using Al₂O₃–Yb₂O₃ based sintering additive are investigated.Densification and α–β transformation behaviors were investigated by varying the compositions of Al₂O₃–Yb₂O₃ additives. In terms of the influence of the Y₂O₃/Al₂O₃ ratio on densification behavior, a greater Yb₂O₃/Al₂O₃ ratio tends to inhibit densification. The α–β transformation tended to be delayed in sintered bodies with a small additive amount of 3.4 mass%. Compared with the transformation behaviors of the sintered bodies using Al₂O₃–Y₂O₃ additives, those using Al₂O₃–Yb₂O₃ additives exhibited a narrower temperature zone for α–β transformation, which attributed to the finer structure for the sintered body using Al₂O₃–Yb₂O₃ additives. This is affected by the difference in solubility of Si₃N₄ in the two kinds of glass phase.High room temperature strength of 900–1000 MPa was obtained for sintered bodies with a 10.0 mass% addition of additives, and this is considered to be due to the finer micro-structure. Precipitation of a Yb₄Si₂N₂O₇ phase at the grain boundary glass phase, as induced by crystallization processing, enables the improvement of 1300 °C strength to about 650–720 MPa. Crystallization processing resulted in a 30% reduction in the amount of weight change during oxidation (from 3.42 to 2.46 mg/cm²), demonstrating the effectiveness in improving oxidation resistance.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb₂Si₂O₇ coatings using electrophoretic deposition to reduce the TGO growth rate. The findings showed densification for Yb₂Si₂O₇ can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt% of Al₂O₃ to Yb₂Si₂O₇ promoted densification and lowered the required sintering temperature, 1380 °C using 2 °C/min heating/cooling rate for 10 h provided efficient coating density. Moreover, adding Al₂O₃ reduced the TGO growth rate by more than 70 % compared to the Al₂O₃-free coatings, without cracking in TGO after 150 h of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al₂O₃ addition produces promising Yb₂Si₂O₇ environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.     

Effect of Sm2O3 on microstructure,thermal shock resistance and thermal conductivity of cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline

Cordierite-mullite-corundum composite ceramics for solar heat transmission pipeline were fabricated via pressureless sintering at a low sintering temperature with added Sm₂O₃. The effects of Sm₂O₃ on sintering behaviors, mechanical property, phase transformation, microstructure, thermal shock resistance and thermal conductivity of the composite ceramics were investigated. TEM analysis results demonstrated that Sm³⁺ located in glass and grain boundaries to facilitate the densification via the liquid-phase sintering mechanism and improve bending strength by grain refinement, respectively. Proper addition (3 wt%) of Sm₂O₃ could promote the crystallization of cordierite, and improve thermal shock resistance of the composite ceramics with an increasing rate of 16.70% for bending strength after 30 thermal shock cycles (air cooling from 1100 °C to RT). The composite ceramics possessed a superior thermal shock resistance, where a large amount of particles were formed to suppress crack initiation and propagation during thermal shock. Cordierite-mullite-corundum composite ceramics with proper Sm₂O₃ addition (3 wt%) had a lower thermal conductivity than that of composite ceramics without Sm₂O₃ addition by strengthening the scattering of phonon, which could reduce the heat loss during solar heat transmission process.     

Preparation and Performance Study of Sialon‐Si3N4‐SiC Composite Ceramics for Concentrated Solar Power

For lowering the sintering temperature of silicon carbide ceramics used for solar thermal energy storage technology, O'‐Sialon and silicon nitride were employed as composite phases to construct Sialon‐Si₃N₄‐SiC composite ceramics. The composite ceramics were synthesized using SiC, Si₃N₄, quartz, and different alumina sources as starting materials with noncontact graphite‐buried sintering method. Influences of alumina sources on the physical properties and thermal shock resistance of the composites were studied. The results revealed that the employment of O'‐Sialon and silicon nitride could decrease the sintering temperature greatly to 1540°C. The optimum formula G2 prepared from mullite as alumina source achieved the best performances: 66.7 MPa of bending strength, 10.0 W/(m·K) of thermal conductivity. The composition parameter x = 0.4 of O'‐Sialon decreased to 0.04 after 30 cycles thermal shock, and the bending strength increased with a rate of 11.0% due to the increase of O'‐Sialon grain size, and the optimization of microstructure caused by the transformation of O'‐Sialon grains and densification within the samples. The good thermal shock resistance makes the composites suitable for the use as thermal storage materials of concentrated solar power generation.     

A comparative study of thermal conductivity and thermal emissivity of high temperature solar absorber of ZrO2 /Fe2O3 and Al2O3/CuO ceramics

Oxide ceramics are considered as promising high temperature solar absorber materials. The major aim of this work is the development of a new solar absorber material with promising characteristics, high efficiency and low-cost processing. Hence, this work provides a comparative and inclusive study of densification behavior, microstructure features, thermal emissivity and thermal conductivity values of the two new high temperature solar absorbers of ZrO₂/Fe₂O₃ and Al₂O₃/CuO ceramics. Ceramic composites of ZrO₂/(10–30 wt%) Fe₂O₃ and Al₂O₃/(10–30 wt%) CuO were prepared by pressureless sintering method at a temperature of 1700 °C/2hrs. Identification of the solar to thermal efficiency of the composites was evaluated in terms of their measured thermal emissivity. Thermal efficiency and heat transfer homogeneity were investigated in terms of thermal conductivity and diffusivity measurement. The results showed that both composites exhibited comparable densification behavior, homogenous and harmonious microstructure. However, Al₂O₃/10 wt% CuO composite showed higher thermal and solar to thermal efficiencies than ZrO₂/Fe₂O₃ composites. It gave the lowest and the best thermal emissivity of 0.561 and the highest thermal conductivity of 15.4 W/m. K. These values proved to be the best amongst all those of the most known solar absorber materials made from the expensive SiC and AlN ceramics. Thus, Al₂O₃/CuO composites have succeeded in obtaining outstanding properties at a much lower price than its other competitive materials. These results may strongly identify Al₂O₃/CuO composites as promising high-temperature solar absorber materials instead of ZrO₂ and the other carbide and nitride ceramics.     

Preparation and thermal stability investigation of Al2O3–mullite–ZrO2–SiC composite ceramics for solar thermal transmission pipelines

To obtain composite ceramics with excellent thermal shock resistance and satisfactory high?temperature service performance for solar thermal transmission pipelines, SiC additive was incorporated into Al₂O₃?mullite?ZrO₂ composite ceramics through a pressureless sintering process. The effect of the SiC additive on thermal shock resistance was studied. Also, the variations in the microstructure and physical properties during thermal cycles at 1300 °C were discussed. The results showed that both thermal shock resistance and thermal cycling performance could be improved by adding 20 wt% SiC. In particular, the sample with 50 wt% Al₂O₃, 35 wt% Coal Series Kaolin (CSK), 15 wt% partially yttria?stabilized zirconia (PSZ), and 20 wt% SiC additional (denoted as sample A2) exhibited the best overall performance after firing at 1600 °C. Furthermore, the bending strength of sample A2 increased to 124.58 MPa, with an increasing rate of 13.63% after 30 thermal shock cycles. The increase in thermal conductivity and the formation of mullite were the factors behind the enhancement of thermal shock resistance. During the thermal cycles, the oxidation of SiC particles was favorable as it increased the microstructure densification and also facilitated the generation of mullite, which endowed the composite ceramics with a self?reinforcing performance.     

Ultrafine-grained β-Sialon fabricated by two-stage sintering and study on diffusion toughening of Ti–22Al–25Nb

The ultrafine-grained β-Sialon ceramics were fabricated by spark plasma sintering at different temperatures with inorganic Al₂O₃–Y₂O₃ and Ti–22Al–25Nb intermetallic powder as composite additives. The research showed that β-Sialon ceramics achieve two-stage sintering densification. Al₂O₃–Y₂O₃ inorganic additives promoted the synthesis and densification of β-Sialon ceramics at 1125–1215°C. Ti–22Al–25Nb intermetallic powder diffused Ti and Nb elements at 1240–1425°C, thereby improving the fracture toughness of β-Sialon ceramics. The maximum fracture toughness (∼9.69 MPa m^1/2) under 19.6 N was obtained for β-Sialon ceramics sintered at 1600°C.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

The absorption and emission properties of highly transparent ZrO2-doped Yb3+: Y2O3 ceramics

Ultra-highly transparent ZrO₂-doped Yb³⁺: Y₂O₃ ceramics were prepared by slip casting and vacuum pressureless sintering and the transmittance reached the highest value of 80.9% for the sample doped with 8.0 at% Yb³⁺. There are three main absorption peaks at 905, 950, and 976 nm, corresponding to the transition from the lowest level of field splitting of ²F_7/2 crystal to every splitting energy levels of ²F_5/2 crystal field. We analyzed the absorption and emission spectra of transparent Yb³⁺: Y₂O₃ from the energy level structure of Yb³⁺, and the transmission, absorption, and emission spectra were systematically studied. There are three main absorption peaks at 905, 950, and 976 nm and four emission peaks at 1076, 1031, 1013, and 977 nm, respectively. The emission peaks at 977 and 1013 nm broaden and vanish for 8.0 and 10.0 at% Yb³⁺-doped Y₂O₃, which may be related to the change of Y₂O₃ crystal field caused by high concentration.     

Thermophysical properties of Yb2O3 doped Gd2Zr2O7 and thermal cycling durability of (Gd0.9Yb0.1)2Zr2O7/YSZ thermal barrier coatings

(Gd_1−xYb_x)₂Zr₂O₇ compounds were synthesized by solid reaction. Yb₂O₃ doped Gd₂Zr₂O₇ exhibited lower thermal conductivities and higher thermal expansion coefficients (TECs) than Gd₂Zr₂O₇. The TECs of (Gd_1−xYb_x)₂Zr₂O₇ ceramics increased with increasing Yb₂O₃ contents. (Gd_0.9Yb_0.1)₂Zr₂O₇ (GYbZ) ceramic exhibited the lowest thermal conductivity among all the ceramics studied, within the range of 0.8–1.1 W/mK (20–1600 °C). The Young's modulus of GYbZ bulk is 265.6 ± 11 GPa. GYbZ/YSZ double-ceramic-layer thermal barrier coatings (TBCs) were prepared by electron beam physical vapor deposition (EB-PVD). The coatings had an average life of more than 3700 cycles during flame shock test with a coating surface temperature of ∼1350 °C. Spallation failure of the TBC occurred by delamination cracking within GYbZ layer, which was a result of high temperature gradient in the GYbZ layer and low fracture toughness of GYbZ material.     

Fabrication and characterisation of porous silicon nitride ceramics using Yb2O3 as sintering additive

Porous Si₃N₄ ceramics were fabricated by liquid-phase sintering with a Yb₂O₃ sintering additive, and the microstructure and mechanical properties of the ceramics were investigated, as a function of porosity. Low densification was achieved using a lower Yb₂O₃ additive content. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. A high porosity, ∼40–60%, with β-Si₃N₄ grain development, was obtained by adjusting the additive content. Superior mechanical properties, as well as strain tolerance, were obtained for porous ceramics with a microstructure of fine, fibrous grains of a bimodal size distribution.     

Influences of rare-earth oxide additives on the formation and properties of porous Si2N2O ceramic

Here we prepared porous silicon oxynitride (Si₂N₂O) ceramics by reaction sintering of SiO₂ and Si₃N₄ using five different rare-earth oxides (RE₂O₃, RE = Lu, Yb, Y, Sm, and La) as sintering aids. The influences of RE₂O₃ on the formation, densification, microstructure, and mechanical properties of Si₂N₂O ceramics have been investigated in detail. The results have indicated that with the increase in RE ionic radius, the formation temperature of Si₂N₂O decreases, and the densification process could be promoted by RE₂O₃ with larger RE³⁺ ionic radius. In addition, microstructures and mechanical properties are highly dependent on the RE₂O₃ additives. With the increase in RE³⁺ ionic radius, Si₂N₂O changes from platelike crystals to elongated crystals. The samples doped with La₂O₃ and Sm₂O₃ with elongated crystals exhibit higher flexural strength and higher Vickers hardness.     

Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

Calcium oxide (CaO) as sintering additive was first used to fabricate La_0.4Gd_1.6Zr₂O₇ transparent ceramics by a simple solid-state reaction and one-step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as-fabricated La_0.4Gd_1.6Zr₂O₇ ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca²⁺ entered the lattice of La_0.4Gd_1.6Zr₂O₇ ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La_0.4Gd_1.6Zr₂O₇ ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).     

Yb2O3-Gd2O3 codoped strontium zirconate composite ceramics for potential thermal barrier coating applications

Strontium zirconate (SrZrO₃) has been considered as a promising thermal barrier coating (TBC) material for application in gas turbine engines; however, the phase transition problem limits its application. In this study, an Yb₂O₃ and Gd₂O₃ codoped SrZrO₃ system with excellent properties was reported. Yb₂O₃-Gd₂O₃ codoped SrZrO₃ ceramic powders [Sr_0.8(Zr_0.9Yb_0.05Gd_0.05)O_2.75, SZYG/YGZO], [Sr(Zr_0.9Yb_0.05Gd_0.05)O_2.95, SZYG] and pure SrZrO₃ (SZO) powders were produced by a conventional solid-state reaction method. The XRD and Raman results show that, the composite SZYG/YGZO ceramics consist of the SZO and Yb_0.5Zr_0.5O_1.75 phases with a low thermal conductivity of ~1.3 W/(m·K) at 1000°C, which is at least 40% lower than that of the SZO ceramics. The TG-DSC results show that the SZYG/YGZO ceramics have no phase transition in the temperature range of 600 to 1400°C. The thermal expansion coefficient of the SZYG/YGZO ceramics reaches 10.9 × 10⁻⁶ K⁻¹ (1250°C). In addition, the fracture toughness of the SZYG/YGZO ceramics increases by more than 30% compared with the SZO ceramics, and this can be attributed to the presence of the Yb_0.5Zr_0.5O_1.75 phase.     

Improved oxidation resistance properties of Si3N4/O′–SiAlON composite ceramics by a repeated sintering method

Si₃N₄/O′–SiAlON composite ceramics with superior oxidation resistance properties were fabricated by a repeated sintering method. The effects of sintering time on the phase evolution, microstructure, and oxidation resistance properties of the Si₃N₄/O′–SiAlON composite ceramics were investigated. The results indicated that the content of the O′–SiAlON phase and the densification of Si₃N₄/O′–SiAlON composite ceramics increased after two-time sintering. Furthermore, the thickness of the oxide layer of the Si₃N₄/O′–SiAlON composite ceramics after oxidation at 1100–1500°C for 30 h was not significant. Compared to the oxidation weight gain after the one-time sintering process, the oxidation weight gain of Si₃N₄/O′–SiAlON composite ceramics was 0.432 mg/cm² after two-time sintering when oxidized at 1500 C for 30 h, which was reduced by 43.3%. The mechanism of the improved oxidation resistance properties was ascribed to the formation of more O′–SiAlON and the enhancement of the densification.