原位增韧β-Si3N4/α-Sialon复相陶瓷

通过XRD, SEM和力学性能测试研究了β-Si3N4/α-Sialon复相陶瓷热压烧结的致密化、相组成、力学性能和微观结构.结果表明,β-Si3N4/α-Sialon复相陶瓷综合了β-Si3N4和α-Sialon的力学性能,可通过改变起始粉末的组成,可以调整相组成及裁剪材料的力学性能.由于加入具有大的长径比的物相β-Si3N4,提高了材料的强度和韧性.     

Morphology of α-Si3N4 in Fe–Si3N4 prepared via flash combustion

The state and formation mechanism ofα-Si3N4 in Fe–Si3N4 prepared by flash combustion were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate thatα-Si3N4 crystals exist only in the Fe–Si3N4 dense areas. When FeSi75 particles react with N2, which generates substantial heat, a large number of Si solid particles evaporate. The prod-uct between Si gas and N2 is a mixture ofα-Si3N4 andβ-Si3N4. At the later stage of the flash combustion process,α-Si3N4 crystals dissolve and reprecipitate asβ-Si3N4 and theβ-Si3N4 crystals grow outward from the dense areas in the product pool. As the temperature decreases, the α-Si3N4 crystals cool before transforming into β-Si3N4 crystals in the dense areas of Fe–Si3N4. The phase composition of flash-combustion-synthesized Fe–Si3N4 is controllable through manipulation of the gas-phase reaction in the early stage and theα→βtrans-formation in the later stage.     

自蔓延高温合成β-Si3N4棒晶

以MgSiN2作为生长助剂自蔓延高温合成β-Si2N4棒晶,并对其生长机理进行探讨.结果表明,添加少量MgSiN2以制备出高长径比β-Si2N4棒晶,而过多的MgSiN2不利于β-Si3N4棒晶生长.自蔓延燃烧过程中,MgSiN2与原料表面的氧化硅形成Mg-Si-O-N液相,促使β-Si2N4以气-液-固机理快速生长.     

In situ reaction mechanism of MgAlON in Al-Al2O3-MgO composites at 1700℃ under flowing N2

The Al-Al2O3-MgO composites with added aluminum contents of approximately 0wt％,5wt％,and 10wt％,named as M1,M2,and M3,respectively,were prepared at 1700℃ for 5 h under a flowing N2 atmosphere using the reaction sintering method.After sintering,the Al-Al2O3-MgO composites were characterized and analyzed by X-ray diffraction,scanning electron microscopy,and energy-dispersive X-ray spectroscopy.The results show that specimen M1 was composed of MgO and MgAl2O4.Compared with specimen M1,specimens M2 and M3 possessed MgAlON,and its production increased with increasing aluminum addition.Under an N2 atmosphere,MgO,Al2O3,and Al in the matrix of specimens M2 and M3 reacted to form MgAlON and AlN-polytypoids,which combined the particles and the matrix together and imparted the Al-Al2O3-MgO composites with a dense structure.The mechanism of MgAlON synthesis is described as follows.Under an N2 atmosphere,the partial pressure of oxygen is quite low;thus,when the Al-Al2O3-MgO com-posites were soaked at 580℃ for an extended period,aluminum metal was transformed into AlN.With increasing temperature,Al2O3 diffused into AlN crystal lattices and formed AlN-polytypoids;however,MgO reacted with Al2O3 to form MgAl2O4.When the temperature was greater than (1640 ± 10)℃,AlN diffused into Al2O3 and formed spinel-structured AlON.In situ MgAlON was acquired through a solid-solution reaction between AlON and MgAl2O4 at high temperatures because of their similar spinel structures.     

压力对β-Si3N4的电子结构和光学性质的影响

采用基于密度泛函平面波赝势方法（PWP）和广义梯度近似（GGA）,对不同压强下β-Si3N4的电子结构和光学性质进行了计算,较好地解释了已经报道的实验结果,进一步研究了β-Si3N4其它物理参数随外压力的变化规律.     

Thermodynamic analysis and preparation of β-SiC/ZrO2 composites

A predominance area diagram for the Zr-Si-C-O system at 1773 K was plotted according to correlative thermodynamic data. β-SiC/ZrO₂ composites were prepared based on the phase diagram by carbothermal reduction of zircon (ZrSiO₄) in argon atmosphere. Zircon and carbon black were mixed according to the C/ZrSiO₄ mass ratio of 0.2, and with 0, 1wt% and 2wt% extra addition of La₂O₃. Phase evolution of the mixture was investigated at 1723-1803 K by X-ray powder diffraction, and the microstructure of the product prepared at 1803 K for 4 h was examined by scanning electronic microscope. The results show that the decomposition of ZrSiO₄ and the formation of β-SIC can be promoted by increasing the heating temperature and adding La₂O₃. The β-SiC/ZrO₂ composites can be prepared at 1803 K for 4 h in a mixture of zircon, carbon black and La₂O₃, and the contents of β-SIC and m-ZrO₂ in the product sample with 2wt% La₂O₃ reach the highest values of 10.8wt% and 89.2wt%, respectively. The crystal size of the products is about 200 nm.     

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al-Mg2Si in situ composites

The effect of high-speed direct-chill (DC) casting on the microstructure and mechanical properties of Al-Mg₂Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al-Mg₂Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg₂Si particles (from 28 to 12 μm), the spacing of eutectic Mg₂Si (from 3 to 0.5 μm), and the grains of AA6061 alloy (from 102 to 22 μm). The morphology of the eutectic Mg₂Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg₂Si phase and the grains in the Al-Mg₂Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al-Mg₂Si composites and AA6061 alloy billets.     

Synthesis and performance of Ca-α/β-SiAlON composites from tailings

Ca-α/β-SiAlON composites were prepared using Ca-α/β-SiAlON powder synthesized from gold ore tailings, which contained abundant Si and Al elements as the major raw materials together with minor additives, through a pressure-less sintering method. The influences of sintering temperature on the phase composition and microstructure of the composites were analyzed. The scanning electron microscopy images of the composites show the interlacing of grains with elongated columnar, short columnar and plate-like morphologies. The composites sintered at 1520℃ for 6 h have a flexural strength of 352 MPa, Vickers hardness of 11.2 GPa, and fracture toughness of 4.8 MPa·m1/2. The relative content of each phase in the products is I(Ca-α-SiAlON):I(β-SiAlON):I(Fe₃Si) = 23:74:3, where I_i stands for the diffraction peak intensity of phase i.     

细晶Si3N4/Al2O3复合陶瓷

用热压法制备了 Si3N4 / Al2 O3细晶复合陶瓷 ,研究了热压过程中出现的氧化、致密性、相变等现象 ,讨论了热压温度对材料的力学性能及相组成的影响。结果表明 ,热压 Si3N4 / Al2 O3复合陶瓷需在氮气保护下进行 ,热压温度保持在 130 0℃左右为佳 ,热压温度越高 ,致密度下降 ,晶粒明显粗化 ,力学性能下降 ,其下降原因与较高温度下莫来石的形成有关     

Reaction mechanism for in-situ β-Si Al ON formation in Fe_3Si–Si_3N_4–Al_2O_3 composites

In this work,Fe_3Si–Si_3N_4–Al_2O_3 composites were prepared at 1300°C in an N_2 atmosphere using fused corundum and tabular alumina particles,Al_2O_3 fine powder,and ferrosilicon nitride(Fe_3Si–Si_3N_4) as raw materials and thermosetting phenolic resin as a binder.The effect of ferrosilicon nitride with different concentrations(0wt%,5wt%,10wt%,15wt%,20wt%,and 25wt%) on the properties of Fe_3Si–Si_3N_4–Al_2O_3 composites was investigated.The results show that the apparent porosity varies between 10.3% and 17.3%,the bulk density varies from 2.94 g/cm~3 and 3.30 g/cm~3,and the cold crushing strength ranges from 67 MPa to 93 MPa.Under the experimental conditions,ferrosilicon nitride,whose content decreases substantially,is unstable;part of the ferrosilicon nitride is converted into Fe_2C,whereas the remainder is retained,eventually forming the ferrosilicon alloy.Thermodynamic assessment of the Si_5AlON_7 indicated that the ferrosilicon alloy accelerated the reactions between Si_3N_4 and α-Al_2O_3 fine powder and that Si in the ferrosilicon alloy was nitrided directly,forming β-Si Al ON simultaneously.In addition,fused corundum did not react directly with Si_3N_4 because of its low reactivity.     

Development and characterization of interspecific hybrids between Oryza sativa and O. latifolia by in situ hybridization

Oryza sativa and O. latifolia belong to the AA and CCDD genomes of Oryza, respectively. In this study, interspecific hybrids of these species were obtained using the embryo rescue technique. Hybrid panicle traits, such as long awns, small grain, exoteric large purple stigma, grain shattering and dispersed panicles, resemble that of the paternal parent, O. latifolia, whereas there is obvious heterosis in such respects as plant height, tillering ability and vegetative vigor. Chromosome pairing and the genomic components of the hybrid were subsequently investigated using genomic in situ hybridization (GISH) and fluorescent in situ hybridization (FISH) analysis. Based on the mitotic metaphase chromosome numbers of the root tips investigated, the hybrid is a triploid with 36 chromosomes. The genomic constitution of the hybrid is ACD. In the meiotic metaphase I of the hybrid pollen mother cell, poor chro- mosome pairing was identified and most of the chromosomes were univalent, which resulted in com- plete male sterility in the hybrid.     

Structural and magnetic properties of Fe3O4 nanoparticles prepared by arc-discharge in water

By using a simple and low-cost arc-discharge method in deionized water, high purity Fe3O4 nanoparti- cles have been synthesized on large scale. The structure of these nanoparticles has been studied by means of transmission electron microscope and X-ray diffraction. The synthesized Fe3O4 nanoparticles show well-defined spherical shape, with diameters ranging from 10 to 30 nm and the average diameter about 20 nm. By investigating the effects of the different processing conditions, optimum parameters were obtained. Moreover, the size of the as-grown nanoparticles can also be controlled through ad- justing the processing parameters. These Fe3O4 nanoparticles were magnetic materials, showing saturation magnetization of 64.97 emu/g at room temperature.     

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg_2Si in situ composites

The effect of high-speed direct-chill(DC) casting on the microstructure and mechanical properties of Al–Mg_2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg_2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg_2Si particles(from 28 to 12 μm), the spacing of eutectic Mg_2Si(from 3 to 0.5 μm), and the grains of AA6061 alloy(from 102 to 22 μm). The morphology of the eutectic Mg_2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg_2Si phase and the grains in the Al–Mg_2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg_2Si composites and AA6061 alloy billets.     

In situ reaction mechanism of MgAlON in Al-Al2O3-MgO composites at 1700℃ under flowing N2

The Al-Al₂O₃-MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M₁, M₂, and M₃, respectively, were prepared at 1700℃ for 5 h under a flowing N₂ atmosphere using the reaction sintering method. After sintering, the Al-Al₂O₃-MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M₁ was composed of MgO and MgAl₂O₄. Compared with specimen M₁, specimens M₂ and M₃ possessed MgAlON, and its production increased with increasing aluminum addition. Under an N₂ atmosphere, MgO, Al₂O₃, and Al in the matrix of specimens M₂ and M₃ reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al-Al₂O₃-MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N₂ atmosphere, the partial pressure of oxygen is quite low; thus, when the Al-Al₂O₃-MgO composites were soaked at 580℃ for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al₂O₃ diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al₂O₃ to form MgAl₂O₄. When the temperature was greater than (1640 ±10)℃, AlN diffused into Al₂O₃ and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and MgAl₂O₄ at high temperatures because of their similar spinel structures.     

C3N4/ZnO/Fe2O3复合光催化剂的制备及性能

以二氰二胺、巴比妥酸、Fe(NO3)3·9H2O和Zn(NO3)2·6H2O为原料,用水作溶剂,采用焙烧的方法合成了C3N4/ZnO/Fe2O3复合光催化剂;通过光致发光光谱对其进行了表征.用对苯二甲酸作为探针分子,结合荧光技术研究了C3N4/ZnO/Fe2O3复合光催化剂表面的羟基自由基形成.以甲基橙为光催化反应的模型化合物,评价了C3N4/ZnO/Fe2O3复合光催化剂的紫外光催化活性.结果表明:C3N4与ZnO/Fe2O3质量比率对C3N4/ZnO/Fe2O3复合光催化剂的光催化活性有重要的影响;当C3N4,ZnO和Fe2O3质量比率为10∶1.2∶12.9时所制得的C3N4/ZnO/Fe2O3复合催化剂的紫外光催化活性最好.羟基自由基生成速率的变化趋势与C3N4/ZnO/Fe2O3复合催化剂的光催化活性的变化趋势相吻合.C3N4/ZnO/Fe2O3复合光催化剂的光致发光光谱强度与它的光催化活性之间也有良好的对应关系.     

Fe2O3/g-C3N4光催化降解罗丹明B性能研究

为了改善g-C3N4比表面积低等缺点,通过高温热聚合法制备了三维(3D)多孔g-C3N4,并通过与Fe2O3复合得到Fe2O3/g-C3N4催化剂,提高其可见光响应.Fe2O3/g-C3N4在g-C3N4含量为900 mg、罗丹明B(Rhodamine B,RhB)浓度为20 mg·L–1、H2O2为15 mmol时脱...     

Solid fraction evolution characteristics of semi-solid A356 alloy and in-situ A356–TiB_2 composites investigated by differential thermal analysis

The key factor in semi-solid metal processing is the solid fraction at the forming temperature because it affects the microstructure and mechanical properties of the thixoformed components. Though an enormous amount of data exists on the solid fraction–temperature relationship in A356 alloy, information regarding the solid fraction evolution characteristics of A356-TiB2 composites is scarce. The present article establishes the temperature-solid fraction correlation in A356 alloy and A356-x TiB2(x = 2.5wt% and 5wt%) composites using differential thermal analysis(DTA). The DTA results indicate that the solidification characteristics of the composites exhibited a variation of 2°C and 3°C in liquidus temperatures and a variation of 3°C and 5°C in solidus temperatures with respect to the base alloy. Moreover, the eutectic growth temperature and the solid fraction(fs) vs. temperature characteristics of the composites were found to be higher than those of the base alloy. The investigation revealed that in-situ formed TiB2 particles in the molten metal introduced more nucleation sites and reduced undercooling.     

Molecular characterization and genetic analysis of Gnrh2 and Gthβin different ploidy level fishes

The gonadotropin-releasing hormone (GnRH) and the gonadotropin hormone (GTH) play crucial roles in regulating the gonadal development of the vertebrate. In this study, the Gnrh2, Fshβ and Lhβ cDNAs were cloned and characterized in red crucian carp, triploids and tetraploids, and their phylogenetic relations were comparatively analyzed. All the Gnrh2 cDNAs in different ploidy fishes encoded proteins of 86 amino acids, which consisted of a signal peptide, a GnRH2 decapeptide and a GnRH-associated peptide (GAP) linked by a proteolytic cleavage site (Gly-Lys-Arg). The GnRH2 decapeptide and proteolytic cleavage site were absolutely consistent among the three ploidy fishes, but the differences in signal peptide and GAP between diploids and tetraploids were fewer than those between diploids and triploids. It was presumed that the red crucian carp was the original maternal parent of tetraploids, so they had closer relationship. In addition, the Fshβ and Lhβ cDNAs of these three fishes encoded proteins of 130 and 140 amino acids, respectively. Compared with the molecules of other teleosts, the cysteine residues and potential glycosylation sites of Lhβ in these three fishes are fully conserved. However, most teleosts of Fshβ had 12 cysteine residues, while those of these three fishes were 13, and 12 of which might form six conserved disulfide bridges by utilizing the cleavage sites between the first and the second cysteine residues. Moreover, the lack of the second glycosylation site in Fshβ of these three fishes might influence the special structure and biological activities. On the other hand, the phylogenic tree analyses revealed that Gnrh2, Fshβ and Lhβ had similar phylogeny relationships among the cyprinids, which indicated that they were conserved in molecular structure and function during the evolution.     

Kinetic study on the direct nitridation of silicon powders diluted with α-Si3N4 at normal pressure

Silicon nitride (Si₃N₄) powders were prepared by the direct nitridation of silicon powders diluted with α-Si₃N₄ at normal pressure. Silicon powders of 2.2 μm in average diameter were used as the raw materials. The nitriding temperature was from 1623 to 1823 K, and the reaction time ranged from 0 to 20 min. The phase compositions and morphologies of the products were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The effects of nitriding temperature and reaction time on the conversion rate of silicon were determined. Based on the shrinking core model as well as the relationship between the conversion rate of silicon and the reaction time at different temperatures, a simple model was derived to describe the reaction between silicon and nitrogen. The model revealed an asymptotic exponential trend of the silicon conversion rate with time. Three kinetic parameters of silicon nitridation at atmospheric pressure were calculated, including the pre-exponential factor (2.27 cm·s^?1) in the Arrhenius equation, activation energy (114 kJ·mol^?1), and effective diffusion coefficient (6.2×10^?8 cm²·s^?1). A formula was also derived to calculate the reaction rate constant.