AlON的氧化行为及其与MgO和MgAl2O4的作用

利用TG DTA和XRD研究了氧氮化铝(AlON )在高温下的氧化行为及其与MgO和MgAl2 O4 的作用。结果表明 :AlON在 85 0℃左右开始氧化 ,1170℃左右氧化最为剧烈 ;MgO和MgAl2 O4 在 12 0 0℃都能与AlON形成固溶体 ,且后者更易形成 ;形成固溶体对AlON的氧化有一定的抑制作用。     

Direct ink writing of aluminium oxynitride (AlON) transparent ceramics from water-based slurries

In this study, transparent AlON ceramics were fabricated via the direct ink writing (DIW) method from the water-based ceramic slurry. The solids loading of the ceramic slurry was optimised by changing the dispersant content, and the printability and water content were then adjusted by adding hydroxyethyl cellulose (HEC). The structure of the green body was complete and no impurity phases were detected. The effects of sintering temperature and dwell time on the bulk density, phase evolution, microstructure, in-line transmittance, and mechanical properties of the ceramics were studied systematically. High optical and mechanical properties of 10 × 10 × 0.9 mm³ single-phase AlON ceramic tiles were obtained by sintering at 1960 °C for 10 h in a nitrogen atmosphere: in-line transmittance of 81.90% at a wavelength of 780 nm, fracture toughness of 1.74 MPa·m^1/2 (2.94 N), and Vickers hardness of 18.56 GPa (2.94 N). This study provides a novel method for synthesising AlON transparent ceramics from water-based ceramic slurries.     

用碳热还原方法制取氮氧化铝材料

由于钢铁工业、高温技术、电子技术的发展,对陶瓷材料提出了越来越高的要求,要求耐火材料和陶瓷制造工艺不断进步和创新,制造化学纯度高、熔点高、抗热震性好、高温强度和致密度等性能优良的特种耐火材料。作为该研究主题的AlON材料即是由高熔点氧化物中的Al2O3和氮化物中的AlN构成,满足上述要求。通常合成尖晶石型氮氧化铝有3种方法：最普通的方法是碳热还原氮化氧化铝法;第二种以金属铝为原料借助燃烧反应来氧化氮化制备氮氧化铝;第三种方法用气相反应合成。     

Strengthening mechanism of twin lamellas in transparent AlON ceramics

Twin lamellas, were observed in transparent AlON ceramics in our previous work. In this work, the influence of the twin lamellas on mechanical strength was evaluated and its mechanism was investigated by SEM, EBSD, and HRTEM. Both the crystallographic direction and the space direction of a crack will deflect when it propagates across a twin lamella. The dynamic mechanism is that the dislocation propagation in front of the crack tips is blocked by the twin boundaries. Because the fraction of twin boundaries increases with grain size in transparent AlON ceramics, the strengthening effect becomes more notable for large-sized samples resulting in inverse Hall-Petch relation. It is a potentially useful way to advance the mechanical properties of transparent AlON ceramics by controlling the content of twin lamellas.     

Preparation and properties of AlON powders

Aluminum oxynitride (AlON) powders were synthesized using the raw materials of γ-Al₂O₃ and carbon black through the carbothermal reduction and nitridation process. The carbon content in the γ-Al₂O₃/C mixture and heating temperature were investigated. The AlON powders were synthesized by calcination for 2 h at 1750 °C when the carbon content in the γ-Al₂O₃/C mixture was 5.8 wt%. The particle size of powder is important to the transparency of ceramics, but the size of the synthesized powder was large. Therefore, a few methods, such as freeze-drying, ultrasonic dispersion, and liquid nitrogen ball milling, were used to reduce the particle size of powders. Among the three methods, liquid nitrogen milling had the best results.     

Pressureless sintering of highly transparent aluminum oxynitride ceramic by precipitation-coating with sintering aid

AlON transparent ceramic is an ideal material for photoelectric windows, domes, and transparent armor. This ceramic is commonly fabricated by ball-mill mixing of AlON powder with appropriate sintering aids; however, it is difficult to homogeneously disperse trace sintering aids using this method and contamination is easily introduced, which may generate pores and flaws in the ceramic, resulting in degraded properties. To address this problem, we adopted a wet-chemical method to precipitation-coat sintering aids on the surface of AlON powder to achieve homogeneous distribution and enable nano-size sintering aids. Pressureless sintered AlON with 0.5 mass% Y₂O₃-coating gave in-line transmittance of 83.5% at a wavelength of 1064 nm and flexural strength of 326.3 MPa; the corresponding values for the ball-milled product were 79.3% and 304.6 MPa. Our results demonstrate that this method of sintering-aid coating has potential to fabricate AlON ceramic with excellent performance.     

AlON合成及致密化的研究

本文以AlN和Al2O3为原料采用无压烧结的方法制备AlON,研究了不同温度对AlON合成物相的影响,及添加剂MsO、Y2O3和CaO对其致密化的影响.通过X射线衍射仪、扫描电镜和阿基米德原理对其物相、显微结构和体积密度等性能进行表征.研究结果表明:在1700℃时AlON开始生成,随着温度升高AlON相含量越多,最佳合成温度为1800℃,当温度达到1900℃时出现多种AlON相;在1800℃时,添加剂MsO、Y2O3和CaO均能促进AlON的致密化,而添加剂CaO添加量为20wt％时,AlON致密化最佳,气孔率最小为5.43％,密度为3.14 g/cm3.     

透明AlON陶瓷材料的研究现状及进展

综述了透明AlON陶瓷材料的制备工艺、透光性影响因素、主要应用及研究现状.指出透明AlON陶瓷已经成为各国材料学科领域和军事领域竞相研究的热点,透光性能好的AlON材料的研制和开发是制约我国在该领域发展的核心技术.     

Irradiation behavior of nuclear graphites at elevated temperatures

A number of anisotropic and near-isotropic graphites were irradiated at 300° to 1500°C to fluences of up to 1.5 × 10²²n/cm² (E > 0.18 MeV). The maximum volume contraction of the graphites ranged from 4 to 9 per cent at 4 to 6 × 10²¹ n/cm² at 1175° to 1280°C, and the maximum expansions ranged from 11 to 50 per cent at 1.5 × 10²² n/cm². Pores, located between crystallite clusters, were identified and are shown to be those eliminated by c-axis expansion during densification while intercrystallite pores formed. The rate of elimination of the pores located between crystallite clusters increases while the rate of formation of intercrystallite pores decreases with increasing temperature. During expansion new pores form between filler particles increasing the bulk volume expansion. The dimensional and microstructural changes are correlated with the distribution of apparent crystallite sizes in the graphites; a predominance of highly graphitic structure in each material causes maximum contraction strain and higher expansion rates after turnaround. The thermal expansivity of materials, composed predominantly of anisotropic and highly graphitic structure, increases by a factor of 2, whereas the thermal expansivity of materials predominant in isotropic structure decreases by a factor of 2. Thermal expansivity changes coincide with volume expansions. A graphite with evenly balanced volumes of isotropic and anisotropic structures shows no change in thermal expansivity with fluence. The thermal expansivity changes follow the same correlation with apparent crystallite size and distribution of crystallite sizes as does the maximum volume contraction and expansion rates of the graphites. Near-isotropic graphites that contain a narrow range of crystallite sizes are more stable under fast neutron irradiation at high temperatures than are anisotropic materials which have a wide range of crystallite sizes. The addition of semigraphitic isotropic carbon structure to an anisotropic material improves its stability and reduces the magnitude of the increase in thermal expansivity.     

Interfacial behavior of microcomposites during creep at elevated temperatures

The high temperature interfacial behavior of SiC/C/SiC microcomposites was investigated. The interfacial sliding resistance dropped slightly from a room temperature value of 10 MPa with increasing temperature up to 1300°C in argon. The interfacial shear stress was shown to remain constant during the creep of microcomposites at 1200–1300°C and 200–450 MPa in argon. For creep in air, the interfacial shear stress increases at long exposure times.     

Dynamic fatigue behavior of lithium hydride at elevated temperatures

The fatigue properties of lithium hydride (LiH) are crucial to its application as neutron shielding and moderating at elevated temperatures. The dynamic fatigue tests of LiH were investigated with the notched 3-point bend (3 PB) specimens over ranges of loading rates at RT up to 400 °C. At RT, the results showed that slow crack growth (SCG) occurred prior to failure as the minor deviation from linearity to nonlinearity in the load-deflection curves. In addition, the fracture strength of LiH decreased with decreasing stress rate and the SCG zone gradually became smaller with higher stress rates, indicating evident dynamic fatigue phenomenon. However, the trends were quite different at 200, 300 and 400 °C due to accumulative creep damage for low stress rates at elevated temperatures. With increasing temperature and decreasing stress rate, there existed a transition of the dominated failure mechanism, from SCG to creep rupture. Evidence of very small SCG zone could also be detected at the notch for the failure dominated by creep rupture.     

Oxidation at elevated temperatures: competition between α‐tocopherol and unsaturated triacylglycerols

The competitive oxidation between α‐tocopherol and unsaturated fatty acyls at thermoxidation conditions (180 and 240 °C) was evaluated using purified triacylglycerols from nine fats and oils (refined coconut, palm, tallow, olive, high oleic sunflower, sunflower, corn, soybean, and flaxseed oil). α‐Tocopherol degraded faster in less unsaturated lipids and a linear correlation between the iodine value (x) and the residual tocopherol content (y) was obtained after 2 h of heating at 240 °C (y = 3.72x + 137.5, R² = 0.9463). The formation of polar oxidation products was established and the results were explained by a non‐selective oxidation of unsaturated fatty acyls and α‐tocopherol by highly reactive alkoxyl and hydroxyl radicals generated by decomposition of hydroperoxides.     

Fracture behavior of polyetherimide (PEI) and interlaminar fracture of CF/PEI laminates at elevated temperatures

To investigate the effects of environmental temperature on fracture behavior of a polyetherimide (PEI) thermoplastic polymer and its carbon fiber (CF/PEI) composite, experimental and numerical studies were performed on compact tension (CT) and double cantilever beam (DCB) specimens under mode‐I loading. The numerical analyses were based on 2‐D large deformation finite element analyses (FEA). Elevated temperatures greatly released the crack tip triaxiality (constraint) and promoted matrix deformation due to low yield strength and enhanced ductility of the PEI matrix, which resulted in the greater plane‐strain fracture toughness of the bulk PEI polymer and the interlaminar fracture toughness of its composite during delamination propagation with increasing temperature. Furthermore, the high triaxiality was developed around the delamination front tip in the DCB specimen, which accounted for the poor translation of matrix toughness to the interlaminar fracture toughness by suppressing the matrix deformation and reducing the plastic energy dissipated in the plastic zone. Especially, at delamination initiation, the weakened fiber/matrix adhesion at elevated temperatures led to premature failure of fiber/matrix interface, suppressing matrix deformation and preventing the full utilization of matrix toughness. Consequently, low interlaminar fracture toughness was obtained at elevated temperatures. POLYM. COMPOS., 26:20–28, 2005. © 2004 Society of Plastics Engineers.     

Microstructure and mechanical properties of BN-Si3N4 and AlON joints brazed with Ag-Cu-Ti filler alloy

AlON was successfully brazed to BN-Si₃N₄ using a Ag-Cu-Ti filler alloy. SEM, TEM and XRD studies revealed that a TiN + TiB₂ + Ti₅Si₃ reaction layer formed adjacent to the BN-Si₃N₄ while a (Cu,Al)₃Ti₃O layer formed adjacent to the AlON. In addition, Ag-Cu eutectic, Cu(s,s) and AlCu₂Ti were observed in the brazing filler. The effect of brazing temperature on the microstructure and mechanical properties of the joints was investigated. As the brazing temperature increased, the reaction layers became thicker, while the thickness of the brazing seam decreased. Meanwhile, the amount and the size of AlCu₂Ti intermetallic compounds decreased. The shear strength of the joints first increased and then dropped with increasing the brazing temperature. A joint with a maximum strength of 94 MPa was obtained when it was brazed at 850 °C for 15 min.     

The nanogranular behavior of C-S-H at elevated temperatures (up to 700 °C)

Cement-based materials are non-combustible, but the complex chemo-physical mechanisms that drive at elevated temperatures the thermal degradation of the mechanical properties (stiffness, strength) are still an enigma that have deceived many decoding attempts. This paper presents, for the first time, results from a new experimental technique that allows one to rationally assess the evolution of the nano-mechanical behavior of cement paste at elevated temperatures. Specifically, the thermal degradation of the two distinct calcium-silicate hydrate (C-S-H) phases, Low Density (LD) C-S-H and High Density (HD) C-S-H, is assessed based on a statistical analysis of massive nanoindentation tests. From a combination of nanoindentation, thermogravimetry and micromechanical modeling, we identify a new mechanism, the thermally induced change of the packing density of the two C-S-H phases, as the dominant mechanism that drives the thermal degradation of cementitious materials. We argue that this loosening of the packing density results from the shrinkage of C-S-H nanoparticles that occurs at high temperatures, most probably due to the loss of chemically bound water.     

Al2O3-AlON复合材料的烧结性能和显微结构

研究了在保护气氛下1600℃烧成的Al2O3-AlON复合材料的烧结性能、物相组成和显微结构.结果表明在刚玉材料中加入AlON,烧结性能得到改善,物相组成主要为刚玉和尖晶石型构造的氧氮化铝,两者之间形成紧密结合结构,并有少量MgAlON微晶填充在大颗粒间的孔隙中,形成致密结构.     

Processing and mechanical behavior at elevated temperatures of directionally solidified proton conducting perovskites

High-temperature proton conducting perovskite oxides have been fabricated by directional solidification using a laser-heated floating zone (LHFZ) method. Several families of compositions were selected: SrCe_1−xY_xO_3−δ (with x = 0.1, 0.2), BaCe_1−xM_xO_3−δ (with M = Y, Yb and Ca; x = 0.05, 0.2), Sr₃Ca_1.18Nb_1.82O_9−δ, SrZr_0.8Y_0.2O_3−δ and SrTi_0.95Sc_0.05O_3−δ. The resulting microstructures were characterized by electron microscopy and X-ray diffraction. The compounds exhibit a singular microstructure consisting of strongly textured crystalline cells surrounded by an intercellular amorphous phase. Compressive mechanical tests were performed at elevated temperatures in air at constant strain rate to evaluate the creep resistance. The results are discussed in terms of ionic radius, degree of aliovalence and content of dopant cations.     

Direct coarse powder aqueous slip casting and pressureless sintering of highly transparent AlON ceramics

Direct coarse powder aqueous slip casting was proposed to prepare AlON green body for pressureless sintering of highly transparent AlON ceramics. It was reported that anti-hydrolysis treatment to AlON powder before aqueous slip casting was essential due to hydrolysis reaction between AlON and water. However, both XRD of hydrolyzed powder and pH value of aqueous slurry indicate that the hydrolyzate amount is positively correlating with hydrolysis reaction area, i.e., surface area of powder. Therefore, replacing fine particle with coarse one to reduce surface area of powder is an efficient hydrolyzates reduction scheme by lessening hydrolysis reaction sites. As a result, the hydrolyzate in green body of direct slip casting coarse AlON powder (91 vol% of particles have a size of >1.0 μm) was successfully reduced to an insignificant amount so that the sintered ceramics had a high transmittance (84.1% at ~3750 nm), which also verified the effectiveness of the proposed hydrolyzate-reduction scheme.     

Reduced kinetic mechanism for combustion of synthesis gas at elevated temperatures and pressures

The key combustion reactions of synthesis gas at elevated initial temperatures (T ₀ = 500–700 K) and pressures (p = 10–30 atm) are identified by analyzing the kinetic mechanism. A reduced mechanism of the oxidation reactions of synthesis gas consisting of 14 elementary reactions involving 13 species is proposed which adequately describes the results of experimental data on the burning velocities of mixtures of synthesis gas with oxygen and inert diluents at T ₀ = 300–700 K, p = 10–30 atm, and ratios CO/H₂ = 0.05–0.95, and satisfactorily predicts the flame structure and the dependence of the flammability limits on the initial temperature at atmospheric pressure.