Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K^?1 upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m^?1 K^?1) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Rapid densification and reaction sequences in self-reinforced Y-α-SiAlON ceramics with barium aluminosilicate as an additive

Y-α-SiAlON (Y_1/3Si₁₀Al₂ON₁₅) ceramics with 5 wt.%BaAl₂Si₂O₈ (BAS) as an additive were synthesized by spark plasma sintering (SPS). The kinetic of densification, phase transformation sequences and grain growth during sintering process were investigated. Full densification could be achieved by 1600 °C without holding and using a heating rate of 100 °C min⁻¹, but the transformation from α-Si₃N₄ to α-SiAlON is not completed simultaneously with the densification process. The equilibrium phase assemblage could be reached after SPS at 1800 °C for 5 min and the resultant material possesses self-reinforced microstructure with high hardness of 19.2 GPa and fracture toughness of 6.8 MPa m^1/2. The complete crystallization of BAS is beneficial to the high temperature mechanical properties. The obtained could maintain the room strength up to 1300 °C.     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Microstructure and boundary phases of Lu–Al-doped silicon nitride by pressureless sintering

Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu₂O₃/Al₂O₃ sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu₄Si_2−xAl_xO_7+xN_2−x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu₂O₃/Al₂O₃ weight ratio 10/10 had highest strength of 690 ± 50 MPa.     

Liquid-Phase Sintering and Properties of Si3N4–TiN Composites

Densification during liquid-phase sintering of Si₃N₄–TiN was studied in the presence of Y₂O₃. The content of TiN was varied from 0–50 mass%. During the densification Y-silicate was formed. The amount of silicate increased with both decreasing fraction of TiN and increasing isothermal heating time. Density, fracture toughness, and electrical resistivity were measured as a function of TiN content. It was found that the density and fracture toughness increased with increasing TiN content. The electrical resistivity drops drastically, from 10¹⁰ m for sintered Si₃N₄ to 10^–3 m for sintered Si₃N₄–TiN composite containing 30 vol% TiN.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

Microstructure and properties of spark plasma sintered AlN ceramics

Spark plasma sintering (SPS) is a newly developed technique that enables poorly sinterable aluminum nitride (AlN) powder to be fully densified. It is addressed that pure AlN sintered by SPS has relatively low thermal conductivity. In this work, SPS of AlN ceramic was carried out with Y₂O₃, Sm₂O₃ and Li₂O as sintering aids. Effects of additives on AlN densification, microstructure and properties were investigated. Addition of sintering aids accelerated the densification, lowered AlN sintering temperature and was advantageous to improve properties of AlN ceramic. Thermal conductivity and strength were found to be greatly improved with the present of Sm₂O₃ as sintering additive, with a thermal conductivity value about 131 Wm⁻¹K⁻¹ and bending strength about 330 MPa for the 2 wt% Sm₂O₃-doped AlN sample SPS at 1,780 °C for 5 min. XRD measurement revealed that additives had no obvious effect on the AlN lattice parameters. Observation by SEM showed that AlN ceramics prepared by SPS method manifested quite homogeneous microstructure. However, AlN grain sizes and shapes, location of secondary phases varied with the additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of liquid phases.     

Electrical conductivity of 5 mol.% Yb2O3 and 5 mol.% Gd2O3 co-doped Sm2Zr2O7

Sm₂Zr₂O₇ co-doped with and without 5 mol.% Yb₂O₃ and 5 mol.% Gd₂O₃ were prepared by a pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity were investigated by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance spectra measurements. Both Sm₂Zr₂O₇ and (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramics exhibit a single phase of pyrochlore-type structure. The grain conductivity, grain-boundary conductivity and total conductivity obey the Arrhenius relation, respectively, and gradually increase with increasing temperature from 723 to 1173 K. (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ ceramic is the oxide-ion conductor in an oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The grain conductivity, grain-boundary conductivity and total conductivity of (Sm_0.9Gd_0.05Yb_0.05)₂Zr₂O₇ with dual Yb³⁺ + Gd³⁺ doping are higher than those of undoped Sm₂Zr₂O₇ at identical temperature levels.     

Long wavelength Ce emission in Y6Si3O9N4 phosphors for white-emitting diodes

Y₆Si₃O₉N₄:Ce³⁺ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y₆Si₃O₉N₄:Ce³⁺. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce³⁺-doped Y₄Si₂O₇N₂ phosphors, Ce³⁺-doped Y₆Si₃O₉N₄ phosphors showed longer wavelengths of both excitation and emission. The Y₆Si₃O₉N₄:Ce³⁺ is a potential green-emitting phosphor for white LEDs.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications

Tb₃Al₅O₁₂ transparent ceramics have been prepared by solid state reaction and vacuum sintering. The optical quality and the microstructure of the samples were investigated. The sample sintered at 1650 °C possessed relatively good optical transparency from 400 nm to 1600 nm. The Verdet constant measured at 632.8 nm of the quasi-pore-free Tb₃Al₅O₁₂ transparent ceramic was −172.72 rad T⁻¹ m⁻¹, which was close to the counterpart of Tb₃Al₅O₁₂ single crystal. The thermal conductivity of the sample was also measured. To the best of our knowledge, this is the first time that Tb₃Al₅O₁₂ transparent ceramic with relatively good optical quality and magneto-optical property has been reported.     

Preparation and dielectric properties of BaCu(B2O5)-doped SrTiO3-based ceramics for energy storage

BaCu(B₂O₅) (BCB) was used as sintering aids to lower the sintering temperature of multi-ions doped SrTiO₃ ceramics effectively from 1300 °C to 1075 °C by conventional solid state method. The effect of BCB content on crystalline structures, microstructures and properties of the ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and dielectric measurements, respectively. The addition of BCB enhanced the breakdown strength (BDS) while did not sacrifice the dielectric constant. The enhancement of BDS should be due to the modification of microstructures, i.e., smaller and more homogeneous grain sizes after BCB addition. The dielectric constant of BCB-doped ceramics maintained a stable value with 1.0 mol% BCB, which was dominated by the combination of two opposite effects caused by the presence of second phases and the incorporation of Cu²⁺ and Ba²⁺, while further increase was owing to the increase of dissolved Ba²⁺ ions when the content of BCB is more than 2.0 mol%. The multi-ions doped SrTiO₃ ceramics with 1.0 mol% BCB addition showed optimal dielectric properties as follows: dielectric constant of 311.37, average breakdown strength of 28.78 kV/mm, discharged energy density of 1.05 J/cm³ and energy efficiency of 98.83%.     

Structural and spectroscopic characterization of Yb doped Lu2O3 transparent ceramics

Yb³⁺ doped Lu₂O₃ transparent ceramics were fabricated by the solid-state reaction method and sintered in H₂ atmosphere. Structural and spectroscopic properties of Yb:Lu₂O₃ ceramics were studied. The Yb:Lu₂O₃ ceramic structure, and the lattice parameter are refined with the Rietveld method. Yb:Lu₂O₃ has broad absorption and emission bands with a long fluorescence lifetime (1.31 ms). The energy level diagram is calculated based on the absorption and emission spectra, Yb³⁺ in Lu₂O₃ ceramics exhibits a big splitting energy of the ²F_7/2 ground state (1023 cm⁻¹). Furthermore, the gain cross-section (σ_g) is estimated with different β values.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.     

Influence of ZnO additive on the properties of Y-doped BaSnO3 proton conductor

The effects of ZnO additive on the phase formation, microstructure and electrical conduction of Y-doped BaSnO₃ have been investigated. The single-phase and dense BaSn_0.75Y_0.25O_3−δ compound with 4 mol% ZnO additive was successfully prepared after sintering at 1300 °C, which significantly reduces the sintering temperature. The conductivities measured under dry and wet air atmospheres reveal that the bulk conductivity of BaSn_0.71Y_0.25Zn_0.04O_3−δ is much lower than that of BaSn_0.75Y_0.25O_3−δ. However, ZnO as a sintering aid does not affect the bulk conductivity. The total conductivity of BaSn_0.75Y_0.25O_3−δ with ZnO as the sintering aid is slightly higher than that of unmodified BaSn_0.75Y_0.25O_3−δ, and reaches 2.4 × 10⁻³ S cm⁻¹ at 621 °C. Therefore, this material can be used as a proton-conducting electrolyte for intermediate temperature solid oxide fuel cells.     

Sintering process of Y2O3-added Si3N4

The sintering process of Y₂O₃-added Si₃N₄ has been investigated by dilatometry and microstructural observations. Densification was promoted above 1440 ° C by the formation of eutectic melts in the Y₂O₃-SiO₂-Si₃N₄ triangle. However, the dilatometric curves indicated no shrinkage corresponding to the rearrangement process, despite liquid-phase sintering. The kinetic order for The Initial-stage sintering was 0.47 to 0.49. These values indicated that the phase-boundary reaction was rate controlling. The apparent activation energy (323 kJ mol^–1) was smaller than the dissociation energy for the Si-N bond (435 kJ mol^–1). ESR data and lattice strain indicated that the disordered crystalline structure of the Si₃N₄ starting powder promoted the reaction of Si₃N₄ with eutectic melts. After a period of initial-stage sintering, the formation of fibrous -Si₃N₄ grains resulted in interlocked structures to interrupt the densification.     

CuO-TiO_2复合助剂低温烧结氧化铝陶瓷的机理(Ⅱ)

固定CuO(0.4%)和TiO₂(4%)的添加量、改变TiO₂(0--32%)和CuO(0--3.2%)的添加量(质量分数, 下同), 研究了CuO--TiO₂复合助剂对氧化铝陶瓷烧结性能、微观结构、物相组成以及烧结激活能的影响, 以揭示复合助剂的低温烧结机理。结果表明, 在1150--1200℃TiO₂固溶入Al₂O₃生成Al₂Ti₇O₁₅相, 并生成大量正离子空位提高了扩散系数, 从而以固相反应烧结的作用机理促进了氧化铝陶瓷的致密化; TiO₂在Al₂O₃中的极限固溶度为2%--4%, 超过固溶极限的TiO₂对陶瓷烧结没有促进作用; 添加适量的CuO(0.4%)可将TiO₂在Al₂O₃中的固溶温度降低到1100℃以下, 并以液相润湿作用促进氧化铝陶瓷的致密烧结。陶瓷烧结激活能的计算结果定量地印证了上述烧结机理; 当在Al₂O₃中添加4%的TiO₂和2.4%的CuO, 可将烧结激活能降低到54.15 kJ ? mol-1。     

Effects of TiO2 doping on microstructure and properties of directed laser deposition alumina/aluminum titanate composites

ABSTRACT

Al₂O₃-based composite ceramics have excellent high temperature performance and are ideal materials for preparing hot end components. However, poor fracture toughness and thermal shock resistance limit its applications. Based on the excellent low thermal expansion characteristics and thermal shock resistance of Al₂TiO₅ ceramic, different composition ratios of Al₂O₃/Al₂TiO₅ composite ceramics were prepared by directed laser deposition (DLD) technology. Effects of TiO₂ doping amount on microstructure and properties of the composite ceramics were investigated. Results show that α-Al₂O₃ phase is discretely distributed in the continuous aluminum titanate matrix when TiO₂ doping amount between 2 and 30?mol%. With the increase of TiO₂ doping amount, content of Al₂O₃ gradually decreases and its morphology changes from cellular to dendritic. When TiO₂ doping amount reaches 43.9?mol%, the microstructure transforms into fine Al₂TiO₅/Al₆Ti₂O₁₃ eutectic structure. Property test results show that Al₂O₃/Al₂TiO₅ composite ceramics have good comprehensive mechanical properties when TiO₂ doping amount between 2 and 6?mol%.     

Yb2O3-fluxed sintered silicon nitride

Microstructural development and crystallization behaviour of Yb₂O₃-fluxed sintered silicon nitride materials was investigated using CTEM and HREM. The materials contained 5 and 10 vol% Yb₂O₃ as sintering additives. After densification, both compositions were subsequently heat treated to crystallize the residual amorphous secondary phases present at triple-grain regions. In the material doped with 5 vol% Yb₂O₃, only an amorphous secondary phase was observed after sintering, which was about 80% crystalline (Yb₂Si₂O₇) after the post-sintering heat treatment. A metastable phase was formed in the material with 10 vol% additives after sintering, with about 70% crystallinity in the triple-point pockets. Upon postsintering heat treatment, the material could be completely crystallized. During heat treating, the metastable phase combined with the remaining glass to form Yb₂SiO₅ plus Yb₂Si₂O₇ and a small amount of Si₃N₄ which deposited epitaxially on pre-existing Si₃N₄ grains in areas of low-energy within the triple-point pockets. All materials contained thin amorphous films separating the grains. The amorphous intergranular films along grain boundaries (homophase boundaries) revealed excess ytterbium and oxygen. The thickness of the intergranular films was about 1.0 and 2.5 nm for the grain boundaries and the phase boundaries, respectively, independent of additive content and heat-treatment history.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.