Crystal morphology engineering in SiO2–Al2O3–MgO–K2O–Na2O–F− mica glass-ceramics

The morphology of mica crystals formed in extruded glass-ceramics of the system SiO₂–Al₂O₃–MgO–K₂O–Na₂O–F₂ was characterized using transmission electron microscopy. Detailed information on the crystal shape could be obtained since mica crystals in the extruded material exhibit an extraordinary degree of orientation. Highly defective mica plates start growing in coexistence with norbergite from a Mg²⁺- and F⁻-depleted glass. Upon the dissolution of norbergite at elevated temperatures, defective plates are sandwiched between 1M polytype phlogopite slabs epitaxially formed on {001} faces of the defective mica crystals. As shown by means of several examples, depending on the schedule of heat treatment the thickness of the defective slab as well as the diameter ratio of inner and outer plates can be adjusted. Linear growth rates are given for defective and non-defective mica plates at different temperatures enabling the controlled engineering of the mica crystal morphology.     

Hydrothermal conversion of Ti-containing minerals in system of Na_2O–Al_2O_3–SiO_2–CaO–TiO_2–H_2O

Titanium has a great effect on the digestion of bauxite in the Bayer process because it reacts readily at high temperatures in alkaline sodium aluminate solution.Under this consideration, the hydrothermal conversion of Ti-containing minerals in the system of Na_2O–Al_2O_3–Si O_2–Ca O–Ti O_2–H_2O with increased temperatures was studied based on the thermodynamic analysis and systematic experiments. The results show that anatase converts to Al_4Ti_2 SiO_(12) at low temperatures(60–120 °C), which is similar to anatase in crystal structure. As the temperature continues to rise, Al4Ti2 Si O12decomposes gradually and converts to Ca_3 Ti Si_2(Al_2Si_(0.5)Ti_(0.5)O_(14) at 200 °C. When the temperature reaches 260 °C, Ca Ti O_3 forms as the most stable titanate species for its hexagonal closest packing with O_2-and Ca_2?. The findings enhance the understanding of titanate scaling in the Bayer process and clarify the mechanism of how additive lime improves the digestion of diaspore.     

Phase reaction and sintering behavior of a Al2O3–20wt%AlN–5wt%Y2O3 system

Two major problems exist in the processing of AlN. The first is the difficulty in achieving full densification even at relatively high sintering temperatures. The second is the formation of the spinel phase, AlON. Pure AlN sintered at temperatures up to 2000°C have produced no more than 90–93% densification in the former case, while AlN rich ternary systems (AlN–Al₂O₃-sintering agent) have resulted in the detrimental formation of AlON well before full densification can occur. This paper reports on the phase reaction and sintering behavior of a ternary Al₂O₃–AlN–Y₂O₃ system near the critical temperature range of 1600–1700°C, in a carbo-thermal reduction furnace in a fully nitrogen environment. Full densification (>98%) for AlN without the formation of AlON was achieved by sintering an initial Al₂O₃ rich ternary system (Al₂O₃–20wt%AlN–5wt% Y₂O₃) at a relatively low temperature of 1680°C. Formation of the AlON was delayed until 1700°C, at which a stoichiometric γ-AlON (Al₃O₃N) with spinel type structure was obtained. Thermal conductivity values for a sintered substrate obtained with low oxygen content within the AlN matrix reached 125 W m⁻¹ K⁻¹.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.     

Non-equilibrium phase synthesis in Al2O3–Y2O3 by spray pyrolysis of nitrate precursors

The phase evolution in the Al₂O₃–Y₂O₃ system has been studied for 4, 10, 15 and 37.5% Y₂O₃ amorphous powders prepared by spray pyrolysis of nitrate precursor solutions. Two distinct metastable transformation sequences were identified for the amorphous powders upon heat treatment. Crystallisation accompanied by partitioning leads to a mixture of hexagonal YAlO₃ and γ-Al₂O₃ (spinel structure). Partitionless crystallisation, on the other hand, leads directly to a γ-Al₂O₃ solid solution in dilute alloys, and garnet at temperatures as low as 800°C in the stoichiometric composition provided segregation is avoided during decomposition. Further heat treatment of a yttria-supersaturated γ-Al₂O₃ leads to the precipitation of the orthorhombic YAlO₃ that is stable up to temperatures as high as 1600°C, while hexagonal YAlO₃ converts to garnet, Y₃Al₅O₁₂. A rationalisation of the phase evolution sequence has been attempted on the basis of kinetic considerations, cation coordination and semi-quantitative free energy–composition curves for the various competing phases.     

Gd2 O3 -Yb2 O3 -Y2 O3 -ZrO2 热障涂层材料的热物理性能

目的通过多元稀土氧化物掺杂改性YSZ,提高传统热障涂层的性能。方法使用化学共沉淀法制备不同掺杂量的Gd2O3-Yb2O3-Y2O3-Zr O2(GYYZO)材料,并分别使用冷等静压-烧结和等离子喷涂工艺制备块材和涂层。通过测试块材的热导率和热膨胀系数,分析评价材料的热物理性能。对高温退火处理后的涂层进行X射线衍射分析,评价不同成分涂层的高温相稳定性。结果氧化锆基材料的热导率和热膨胀系数随总掺杂量升高而降低。氧化锆中稀土氧化物总掺杂量为5.5%~9.84%(摩尔分数)时,在1000℃下的热导率为1.25~1.56 W/(m·K),相对8YSZ材料下降了22%~37.5%;在200~1300℃的热膨胀系数为(10~11.1)×10-6/K,与传统8YSZ材料相当。在1400℃长时间退火处理后,低掺杂量GYYZO涂层中的单斜相含量明显低于8YSZ涂层。结论多元稀土氧化物掺杂改性氧化锆材料具有良好的高温相稳定性、低热导率和适当的热膨胀系数,可以作为高性能热障涂层的备选材料。     

La2O3-Gd2O3-Mo secondary emission material

A new kind of materials La2O3-Gd2O3-Mo has been produced by powder metallurgy method.The composition and microstructure of the material were studied by XRD and SEM.It shows that no chemical reaction takes place among La2O3,Gd2O3,Mo and the rare earth oxides exist along molybdenum grain boundaries and in the pores.The emission property measurement results of this material show that adding rare earth oxide into molybdenum can improve the secondary emission coefficient of the emitter,and the emission property depends on the activation temperature.After La2O3-Gd2O3-Mo was activated at 1360℃,the maximum secondary emission coefficient can be high to 2.62,which has exceeded that for practical uses(2.0).     

Mechanical properties of polytypoidally joined Si3N4–Al2O3

Crack-free joining of alumina and silicon nitride has been achieved by a unique approach introducing sialon polytypoids as a functionally graded materials (FGM) bonding layer. The polytypoid compositions are identified in the phase diagram of the Si₃N₄–Al₂O₃ system. The thermal stresses of this FGM junction were analyzed using a finite element analysis program taking into account both coefficient of thermal expansion and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75% or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200°C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.     

Effects of MgO/Al2O3 ratio on viscous behaviors and structures of MgO–Al2O3–TiO2–CaO–SiO2 slag systems with high TiO2 content and low CaO/SiO2 ratio

The effects of MgO/Al₂O₃ ratio on the viscous behaviors of MgO–Al₂O₃–TiO₂–CaO–SiO₂ systems were investigated by the rotating cylinder method. Raman spectroscopy was used to analyze the structural characteristics of slag and Factsage 7.0 was adopted to demonstrate the liquidus temperature of slag. The results show that the viscosity and activation energy for viscous flow decrease when the MgO/Al₂O₃ ratio increases from 0.82 to 1.36. The break point temperature and liquidus temperature of slag initially decrease and subsequently increase. The complex viscous structures are gradually depolymerized to simple structural units. In conclusion, with the increase of MgO/Al₂O₃ ratio, the degree of polymerization of slag decreases, which improves the fluidity of slag. The variations of liquidus temperature of slag lead to the same changes of break point temperature.     

Electrochemical Synthesis and Physicochemical Properties of Nanostructured Al2O3–ZrO2–MgO Oxide Systems

Protection of Metals and Physical Chemistry of Surfaces - Based on the use of electrogenerated reactants, a method for producing Al2O3–ZrO2–MgO oxide systems has been proposed. Their...     

Fe2O3对SiO2-Al2O3-MgO-Fe2O3-F-系玻璃陶瓷析晶的影响

采用高温熔融法制备不同Fe2O3含量的SiO2-Al2O3-MgO-Fe2O3-F-系基础玻璃,应用扫描电子显微镜(SEM)和X射线衍射(XRD)等技术研究了Fe2O3含量对玻璃析晶的影响规律.结果表明在所研究的玻璃体系中,当玻璃中引入2.0%(质量分数,下同)Fe2O3后,促进了玻璃体的分相,在800℃较低温度下析出氟云母晶体,氟云母晶体的形状受到分相区尺寸的限制.提高析晶温度有利于氟云母晶体的析出.当玻璃中Fe2O3提高到6.0%后,仍能获得透明玻璃体,但是在重新加热时首先形成FeFe2O4相,而抑制了云母晶体的析出.     

Bi2O3/B2O3质量比对Bi2O3-B2O3-ZnO系低熔点玻璃结构及性能的影响

采用熔融法制备了用于真空玻璃封接用的铋系低熔点非晶基础玻璃焊料,利用X射线衍射、红外吸收光谱和X射线光电子能谱对所得玻璃焊料进行了结构分析,采用浸泡腐蚀的方法对焊料的耐酸性进行了测试,使用热膨胀仪和综合热分析仪对其热学性能进行了测试,采用扫描电镜对焊料烧结后的表面形貌进行了观察,测试了焊料对母材的润湿性,研究了Bi2O...     

ZnO-Al2O3-B2O3-SiO2系微晶玻璃的研究

研究了能与可伐合金匹配封接的ZnO-Al2O3-B2O3-SiO2系微晶玻璃,其膨胀系数为5.2×10-6/℃,采用该微晶玻璃的封接器件绝缘电阻可达1×1013Q·cm.通过对该微晶玻璃的差热曲线分析,确定了其热处理制度;用X射线衍射仪和扫描电镜研究了该系统的主晶相,其主晶相为ZnAl2 O4、ZnB2O4和少量的NaSiAl2O4晶体.通过控制碱金属离子进入晶格,可明显提高微晶玻璃的绝缘电阻.     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Electric fatigue tests have been conducted on pure and manganese-modified Pb(In_0.5Nb_0.5)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) single crystals along different crystallographic directions. Polarization degradation was observed to suddenly occur above 50–100 bipolar cycles in 〈1 1 0〉 oriented samples, while 〈0 0 1〉 oriented samples exhibited almost fatigue free characteristics. The fatigue behavior was investigated as a function of orientation, magnitude of the electric field and manganese dopant. It was found that 〈0 0 1〉 oriented PIN–PMN–PT crystals were fatigue free, due to its small domain size, being on the order of 1 μm. The 〈1 1 0〉 direction exhibited a strong electrical fatigue behavior due to mechanical degradation. Micro/macro cracks developed in fatigued 〈1 1 0〉 oriented single crystals. Fatigue and cracks were the result of strong anisotropic piezoelectric stress and non-180° domain switching, which completely locked the non-180° domains. Furthermore, manganese-modified PIN–PMN–PT crystals were found to show improved fatigue behavior due to an enhanced coercive field.     

Y2O3含量对SiO2-Al2O3-B2O3-K2O-Li2O系统微晶玻璃的析晶及性能的影响

以SiO2-Al2O3-B2O3-K2O-Li2O为玻璃组成,P2O5和ZrO2为复合成核剂,Sb2O3为澄清剂,Y2O3为添加物,通过传统熔体冷却方法制得了该系统基础玻璃.利用DSC、XRD、SEM及性能测试等手段,研究了Y2O3含量的变化对玻璃析晶行为、析出晶相种类、晶粒尺寸、晶粒分布以及微晶玻璃的力学性能的影响.研究结果表明:随着Y2O3含量的增加,玻璃的析晶峰值温度升高,且析晶峰也逐渐变宽、变钝;Y2O3的加入并不影响微晶玻璃中主晶相的组成,但对其微观结构有明显影响;当Y2O3含量低于2.0%(摩尔分数)时,微晶玻璃的抗弯强度随Y2O3含量增加而增加;当Y2O3含量为2.0%时,获得微晶玻璃的抗弯强度值最高,达到217 MPa;当2.0%≤x(Y2O3)≤2.5%时,抗弯强度反而降低;当Y2O3含量为2.5%时,获得的微晶玻璃具有良好的半透明性,并具有较好的力学性能(抗弯强度为198 MPa);与一步法热处理相比,采用两步晶化热处理有利于提高微晶玻璃的力学性能.     

Friction and wear behaviors of Al2O3–13 wt%TiO2 coatings

Nanostructured and conventional Al2O3-13 wt%TiO2 coatings were manufactured by air plasma spray. Friction and wear behaviors of coatings were investigated at room and elevated temperatures using an SRV wear test machine. The nanostructured coating has "two regions" microstructure, while the conventional coating has typical layered microstructure with obvious interfaces among splats. The coefficient of friction decreases with rising of temperature because of the formation of tribo-layer at elevated temperatures. The wear resistance of the nanostructured coatings is higher than that of the conventional coating, and the wear threshold of applied load is 30 N for conventional coating and 40 N for nanostructure coating. The wear resistance difference is related to the "two regions" microstructure of nanostructure coating, which could blunt or branch the cracks propagation. In our test ranges, the wear rates rising are more sensitive with the applied wear load rising than with the temperature rising.