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

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.     

Si_3N_4陶瓷高导热性的研究现状及发展趋势

综述了Si3N4陶瓷高热导性的研究现状及发展趋势。通过综合分析指出,晶间相和晶格氧是影响Si3N4陶瓷热导率的主要因素,减少晶间相数量、优化晶间相分布以及减少晶格氧含量均能提高Si3N4陶瓷热导率,今后的研究趋势是选择合适的原料和烧结助剂。     

Extinction Coefficients of NO2 and N2O4

The extinction coefficient of NO₂ has been measured in the spectral range 185 to 410 nm as a function of temperature between 235 and 298 K. In order to correct for the effect of the dimer absorption, the extinction coefficient of N₂O₄ has also been measured. The effect of a decrease in temperature upon the NO₂ absorption is a reduction in the extinction coefficient of approximately 10 percent in the range 320 to 380 nm.     

pH controlled dispersion and slip casting of Si3N4 in aqueous media

The dispersion characteristics of commercial Si₃N₄ powder in aqueous media (deionized water) was studied as a function of pH in the range 2–11. The slip was characterized for its dispersion quality by various experimental techniques like particle size analysis, sedimentation phenomena, viscosity and flow behaviour and zeta potential analysis. The optimum dispersion was found to be in the pH region 9–11 wherein the slurry displayed minimum sedimentation height, minimum viscosity, near Newtonian flow behaviour and maximum zeta potential. The slip is highly agglomerated in the pH range 2–8 as manifested by higher sedimentation height, higher viscosity, lower zeta potential and thixotropic non-Newtonian flow behaviour. The 72 wt% (44 vol.%) Si₃N₄ slips made at pH = 10 resulted in green bodies having 53–59% of theoretical density after casting into plaster molds.     

气压烧结Al2O3/TiCN复合材料的力学性能与显微结构

运用二步气压烧结工艺成功制备了Al2O3-30%(质量分数)TiCN复合材料.材料的相对密度达到99.5%,抗折强度为772MPa,硬度为19.6GPa,断裂韧性高达5.82MPa/m2.该材料的烧结过程为固相烧结,烧结过程中TiCN颗粒几乎没有长大,而Al2O3颗粒则长大为原来3倍左右.材料在冷却过程中由于Al2O3和TiCN的热力学性能的失配而引起的界面微应力增长到50MPa左右,不会在材料中导致晶界开裂,但却足以使晶粒发生位错,从而使材料的性能得以增强.     

添加La2O3的AlN陶瓷的高压低温烧结

利用国产六面顶压机在5.0GPa,1300～1800℃条件下实现了以La2O3为助剂的AlN陶瓷体的高压低温烧结.用XRD、SEM、微区拉曼光谱对AlN高压烧结体进行了表征.研究表明,高压制备陶瓷体材料能够有效降低烧结温度和缩短烧结时间,烧结温度最低温度达到1300℃,可比传统烧结方法降低300℃以上,AlN高压烧结体的晶格常数比粉体的减小0.09%左右,其内部存在残余压应力,但AlN六方相的对称性没有发生改变.     

La_2O_3和Y_2O_3掺杂ZrO_2复合材料的高温相稳定性、抗烧结性及热导率

采用化学共沉淀煅烧法制备不同La2O3掺杂量的La2O3-Y2O3-ZrO2(YSZ)复合陶瓷粉末,研究该复合陶瓷粉末的高温相稳定性、抗烧结性及热物理性能,并与传统应用的YSZ陶瓷粉末进行对比,以探讨La2O3-YSZ作为热障涂层材料应用的可能性。采用XRD分析陶瓷粉末的晶体结构和物相组成,研究La2O3掺杂量对YSZ高温相稳定性的影响。采用SEM观察陶瓷烧结体的微观形貌,研究La2O3掺杂对YSZ抗烧结性的影响。采用激光脉冲法测定热扩散率,通过计算得到材料的热导率。结果表明:YSZ和不同La2O3掺杂量的La2O3-YSZ均由单一的非平衡四方相ZrO2(t′-ZrO2)组成。经1 400℃热处理100h后,YSZ中t′-ZrO2完全转变为立方相ZrO2(c-ZrO2)和单斜相ZrO2(m-ZrO2),在0.4mol%~1.4mol%La2O3掺杂范围内,La2O3-YSZ的相稳定性均优于YSZ,其中1.0mol%La2O3掺杂的YSZ(1.0mol%La2O3-YSZ)经热处理后无m-ZrO2生成,表现出良好的高温相稳定性。此外,1.0mol%La2O3-YSZ较YSZ具有较高的抗烧结性和较低的热导率。在室温至700℃范围内,1.0mol%La2O3-YSZ的热导率为1.90~2.17 W/(m·K),明显低于YSZ的热导率(2.13~2.33 W/(m·K))。     

Structure of double interfaces system of Si3N4/SiO2/Si irradiated by γ-rays

The interfacial structures of double interfaces system of Si₃N₄/SiO₂/Si were examined using X-ray photoelectron spectroscopy (XPS) before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of Si₃N₄ and SiO₂, while another was made of Si and SiO₂, the interface between SiO₂ and Si was extended towards the interface of the Si₃N₄/SiO₂ meanwhile the center of the former interface was removed in the direction of the latter interface by ⁶⁰Co. The concentration of silicon in the Si₃N₄ state (BE 101.8 eV) was decreased with the variation of radiation dosage as well as bias field within the SiO₂-Si interface, remarkably. The mechanism for the experimental results is analyzed.     

Plasmons in double interfaces system of Si3N4/SiO2/Si irradiated by Co

The first level plasmons of Si in the pure Si state, in the SiO₂ state and in the Si₃N₄ state (corresponding to bonding energy 116.95, 122.0 and 127.0 eV) were investigated directly with X-ray photoelectron spectroscopy before and after ⁶⁰Co radiation. The experimental results demonstrate that there existed two interfaces, one consisted of plasmons of Si in the Si₃N₄ and SiO₂ states, while another was made of plasmons of Si in the pure Si state and in the SiO₂ state. When the Si₃N₄-SiO₂-Si samples were irradiated by ⁶⁰Co, the interface at Si₃N₄/SiO₂ was extended and at the same time the center of this interface moved towards the surface of Si₃N₄. The concentration of plasmon for silicon in the SiO₂ state is decreased at the SiO₂-Si interface, and the effects of radiation bias field on plasmons in the SiO₂-Si interface are observable. Finally, the mechanism of experimental results is analyzed by the quantum effect of plasmon excited by the photoelectron.     

DC electrical conductivity of Na2O-ZnO-B2O3 glass system

The d.c. electrical conductivity of Na₂O-ZnO-B₂O₃ glass system has been measured as a function of temperature in the range of 350–600°K. The conductivity data show that the activation energy of Na⁺ ions is dependent on ZnO concentration. The results have been discussed in the light of the cluster model of glasses.     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

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.     

包覆镍铁氧体涂层的多孔陶瓷复合材料的制备与电磁性能

以多孔碳化硅陶瓷为基体,采用高分子凝胶法制备了包覆镍铁氧体涂层的多孔陶瓷复合材料。复合材料的结构、包覆涂层的表面形貌和电磁性能分别采用X射线衍射仪(XRD)、扫描电镜(SEM)和HP8510 网络分析仪进行研究。结果表明,当锻烧温度为600℃时,在多孔陶瓷表面有立方晶系尖晶石结构的镍铁氧体晶相生成。随着包覆层数的增加,多孔陶瓷表面的镍铁氧体涂层的致密度和均匀性逐渐增强。与多孔陶瓷基体相比,包覆镍铁氧体涂层的多孔陶瓷的介电损耗角正切值tanδ_ε减小,而磁损耗角正切值tanδ_μ增大,表现出较好的磁损耗特性,tanδ_μ值最大可达0.45。包覆镍铁氧体涂层的多孔陶瓷在X波段的tanδ_μ值随频率的升高呈减小趋势。     

Two-gap superconductivity in R2Fe3Si5 (R=Lu,Sc) and Sc5Ir4Si10

Abstract

R₂Fe₃Si₅ (R= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature T_c among iron-containing superconductors. An anomalous temperature dependence of specific heat C(T) has been reported for polycrystalline samples down to 1 K. We have grown R₂Fe₃Si₅ single crystals, confirmed the anomalous C(T) dependence, and found a second drop in specific heat below 1 K. In Lu₂Fe₃Si₅, we can reproduce C(T) below T_c, assuming two distinct energy gaps 2Δ ₁/k_BT_c = 4.4 and 2Δ ₂/k_BT_c = 1.1, with nearly equal weights, indicating that Lu₂Fe₃Si₅ is a two-gap superconductor similar to MgB₂. Hall coefficient measurements and band structure calculation also support the multiband contributions to the normal-state properties. The specific heat in the Sc₂Fe₃Si₅ single crystals also shows the two-gap feature. R₅Ir₄Si₁₀ (R = Sc, rare earth) is also a superconductor where competition between superconductivity and the charge-density wave is known for rare earths but not for Sc. We have performed detailed specific heat measurements on Sc₅Ir₄Si₁₀ single crystals and found that C(T) deviates slightly from the behavior expected for weak-coupling superconductors. C(T) for these superconductors can also be reproduced well by assuming two superconducting gaps.     

Effect of working pressure on the properties of BaTiO3-CoFe2O4 composite films deposited on STO (100) by PLD

The BaTiO₃-CoFe₂O₄ (BTO-CFO) composite films were grown on SrTiO₃ (STO) (100) substrates at 750 °C under various working pressures by pulsed laser deposition. The composite film grew into a supersaturated single phase at the working pressure of 10 mTorr, BTO and CFO (00 l) oriented hetero-epitaxial films on STO (100) at 100 mTorr, and a polycrystalline composite film at 500 mTorr. The slow growth rate at high working pressure led to the phase separation in the composite film. The CFO was compressively strained along out-of-plane due to the lattice mismatch with the BTO matrix phase. The BTO-CFO composite film grown at 100 mTorr showed reversible switching of ferroelectric polarization and magnetic hysteresis with strong magnetic anisotropy.     

Enhanced G4 emission in NaLaF4: Pr, Yb and charge transfer in NaLaF4: Ce, Yb studied by fourier transform luminescence spectroscopy

A high resolution luminescence study of NaLaF₄: 1%Pr³⁺, 5%Yb³⁺ and NaLaF₄: 1%Ce³⁺, 5%Yb³⁺ in the UV to NIR spectral range using a InGaAs detector and a fourier transform interferometer is reported. Although the Pr³⁺(³P₀ → ¹G₄), Yb³⁺(²F_7/2 → ²F_5/2) energy transfer step takes place, significant Pr³⁺¹G₄ emission around 993, 1330 and 1850 nm is observed. No experimental proof for the second energy transfer step in the down-conversion process between Pr³⁺ and Yb³⁺ can be given. In the case of NaLaF₄: Ce³⁺, Yb³⁺ it is concluded that the observed Yb³⁺ emission upon Ce³⁺ 5d excitation is the result of a charge transfer process instead of down-conversion.     

Enhancement investigations on thermal properties of PMMA/PE composite film reinforced with nano ɤ-Fe2O3 nanoparticles

ABSTRACT

In the present study, PMMA/PE-?-Fe₂O₃ nanocomposites of various compositions were produced through ultrasound-assisted technique. Thermogravimetric analysis and UV-visible results indicated that the thermal stability is enhanced distinctly, without a sacrifice in optical clarity. The improvement of thermal properties was attributed to the homogeneous and good dispersion of ?-Fe₂O₃ nanoparticles in PMMA/PE/?-Fe₂O₃. And the excellent thermal properties performance of the ?-Fe₂O₃ fillers improved the tribological properties of PMMA/PE composites.     

Local thermal effect at luminescent spot on upconversion luminescence in Y2O3:Er,Yb nanoparticles

Y₂O₃:Er³⁺,Yb³⁺ nanoparticles were synthesized using Pechini type sol-gel method and then characterized by XRD, TEM, SEM, Raman spectroscopy, and fluorescence spectrophotometer. Local temperature effect on upconversion luminescence intensities was theoretically analyzed and experimentally tested. These results indicate that a competition process between local temperature at luminescent spot and laser pump power density decides the development trend of upconversion luminescence intensity. Therefore, it can be concluded that the most intensive upconversion luminescence in Y₂O₃:Er³⁺,Yb³⁺ nanoparticles can be achieved at a certain pump power density, which should be slightly below a given constant value (the corresponding threshold of temperature).     

Mechanical and dielectric properties of porous Si3N4–SiO2 composite ceramics

In order to prepare a structural/functional material with not only higher mechanical properties but also lower dielectric constant and dielectric loss, a novel process combining oxidation-bonding with sol–gel infiltration-sintering was developed to fabricate a porous Si₃N₄–SiO₂ composite ceramic. By choosing 1250 °C as the oxidation-bonding temperature, the crystallization of oxidation-derived silica was prevented. Sol–gel infiltration and sintering process resulted in an increase of density and the formation of well-distributed micro-pores with both uniform pore size and smooth pore wall, which made the porous Si₃N₄–SiO₂ composite ceramic show both good mechanical and dielectric properties. The ceramic with a porosity of 23.9% attained a flexural strength of 120 MPa, a Vickers hardness of 4.1 GPa, a fracture toughness of 1.4 MPa m^1/2, and a dielectric constant of 3.80 with a dielectric loss of 3.11 × 10⁻³ at a resonant frequency of 14 GHz.     

Diffusion-controlled growth of CuAl2O4 nanoparticles: effect of sintering and photodegradation of methyl orange

Facile synthesis of CuAl₂O₄ and sintering effects on the phase composition are reported. Annealing at 500?°C showed only CuO phase and the spinel phase started evolving at 700?°C with the concurrent decrease in the CuO phase. Diffusion-mediated growth of spinel phase was also observed at higher temperatures forming CuAl₂O₄ at 1000?°C. The material was stoichiometric and the average particle size was 55 nm, as evidenced by EDS and FE-SEM studies, respectively. The zeta potential of +30 mV and lower (0.346) value of the poly dispersive index (PDI), confirmed the high dispersability of CuAl₂O₄ in water. The favourable band gap (2.2 eV) makes it suitable as visible light photocatalyst. Owing to its positive surface charge and conducive pH, material could adsorb anionic methyl orange dye. A total of 74% of the dye could be recovered by a simple methanolic extraction. The visible light photocatalysis of the same dye leads to 67% decolouration and the addition of H₂O₂ accelerated the photodegradation to completion. The catalyst displayed excellent reusability and stability even after five successive runs.