仿生制备多孔氮化硅陶瓷

以松木炭化后形成的多孔木炭为模板,经Y2O3/SiO2混合溶胶浸渍生物碳模板形成Y2O3/SiO2/C复合体,在高压氮气氛下(0.6MPa),1600°C碳热还原氮化制备出牛物形态多孔氮化硅陶瓷.借助XRD、SEM研究了烧结助剂、烧结温度、反应时间和烧结气氛对烧结产物显微结构和晶相的影响,探讨了多孔Si3N4陶瓷的反应过程和机理.结果表明,多孔si3N4陶瓷是由主晶相β-Si3N4和少量晶间玻璃相YsSi4n4O14组成;多孔Si3N4不仅保留了松木的管胞结构,还在孔道中生长出纤维状形貌的β-Si3N4颗粒;Si3N4的反应烧结过程包括α-Si3N4的形成、晶形转变(α-β相变)和晶粒生长三个阶段.在1450°C烧结的机理是气-固和气-气反应机理,在1600°C通过液相烧结的溶解-沉淀机理形成纤维状的多孔Si3N4陶瓷.     

脉冲激光沉积b轴取向BaTi2O5薄膜的研究

采用脉冲激光沉积(PLD)技术,在MgO(100)基片上制备了b轴取向的BaTi2O5薄膜,研究了基片温度(Tsub)、氧分压(Po2)等沉积工艺对薄膜结构的影响.结果表明: BaTi2O5薄膜的物相及取向性都随基片温度和氧分压的改变而变化,薄膜呈现(710)或(020)取向生长,最佳的PLD沉积条件为Tsub=700℃和Po2=12.5Pa.在该条件下,BaTi2O5薄膜表现出明显的b轴取向,薄膜表面平整光滑,结晶良好,晶粒呈棒状交叉分布,结合紧密.     

Grain orientation studies in YBCO superconductors by molten salt synthesis

The molten salt or the flux method is utilized in this study to fabricate a grainoriented YBa₂Cu₃O_7−x superconductor. The formation of YBa₂Cu₃O_7−x in the presence of molten salts of Na, K, Li belonging to chloride and iodide systems does not appear feasible due to the instability of the superconducting phase in these salt systems. Studies using the “green phase,” Y₂BaCuO₅, as seed crystals suggest a two-stage approach in the formation of YBa₂Cu₃O_7−x . The process uses Y₂BaCuO₅ formed by molten salt synthesis which has been observed to be stable in water and against most of the salts. The 211 crystals can be mixed with oxides and converted to 123. Analysis of X-ray data and SEM micrographs indicates a certain degree of grain orientation, which can be further enhanced by tape-casting or not-forging.     

Effects of Pressure during Preparation on the Grain Orientation of Ruddlesden--Popper Structured BaLa2Ti3O10 Ceramic

As potential thermal barrier coating （TBC） materials, Ruddlesden-Popper structured BaLn2Ti3010 （Ln = rare earth） compounds possess excellent phase stability and desirable thermo-physical properties as well as interesting anisotropic structure. In this paper, the effects of pressure during BaLa2Ti3010 （BLT） bulk preparation on the grain orientation were investigated. BLT grains exhibited lamellar structure, but the grain orientation depended strongly on the existence of pressure during bulk preparation. When pressure was applied, BLT grains preferentially grew along pressing direction, leading to formation of the texture parallel to pressing direction, but BLT grain orientation became relatively random when pressure was absent. However, in the small scale area, BLT grains grew preferentially along c-axis with independence on pressure during preparation. Pressure affected BLT grain orientation at the rapid growth stage according to the grain growth model of BLT.     

Grain coarsening mechanism of Cu thin films by rapid annealing

Cu thin films have been produced by an electroplating method using nominal 9N anode and nominal 6N CuSO₄·5H₂O electrolyte. Film samples were heat-treated by two procedures: conventional isothermal annealing in hydrogen atmosphere (abbreviated as H₂ annealing) and rapid thermal annealing with an infrared lamp (abbreviated as RTA). After heat treatment, the average grain diameters and the grain orientation distributions were examined by electron backscattering pattern analysis. The RTA samples (400 °C for 5 min) have a larger average grain diameter, more uniform grain distribution and higher ratio of (111) orientation than H₂ annealed samples (400 °C for 30 min). This means that RTA can produce films with coarser and more uniformly distributed grains than H₂ annealing within a short time, i.e. only a few minutes. To clarify the grain coarsening mechanism, grain growth by RTA was simulated using the phase field method. The simulated grain diameter reaches its maximum at a heating rate which is the same order as that in the actual RTA experiment. The maximum grain diameter is larger than that obtained by H₂ annealing with the same annealing time at the isothermal stage as in RTA. The distribution of the misorientation was analyzed which led to a proposed grain growth model for the RTA method.     

Grain growth prediction based on data assimilation by implementing 4DVar on multi-phase-field model

We propose a method to predict grain growth based on data assimilation by using a four-dimensional variational method (4DVar). When implemented on a multi-phase-field model, the proposed method allows us to calculate the predicted grain structures and uncertainties in them that depend on the quality and quantity of the observational data. We confirm through numerical tests involving synthetic data that the proposed method correctly reproduces the true phase-field assumed in advance. Furthermore, it successfully quantifies uncertainties in the predicted grain structures, where such uncertainty quantifications provide valuable information to optimize the experimental design.     

相变材料在夏热冬冷地区建筑围护结构中应用的性能研究

相变材料(Phase change material,PCM)在建筑围护结构中的应用是一种改善室内热环境和居住舒适度的有效方法。为探索相变材料在夏热冬冷地区应用的节能潜力,对比研究了不同相变材料应用方法的节能效果,以能源使用强度(Energy use intensity,EUI)评价不同应用方法的建筑性能,旨在找出最优的应用方法。所用的相变材料以高密度聚乙烯球封装,并嵌入在XPS保温板中,形成XPSPCM板。研究结果表明:在制冷季,将XPSPCM板安装在建筑物墙体内表面时的EUI比其装在建筑物墙体外表面时降低了0.27~0.66kWh/m2,采暖季的降低幅度为0.68~0.88kWh/m2。综合考虑全年工况时,当XPSPCM板安装至建筑物墙体靠近内表面时EUI值最小。以EUI雷达图对比XPSPCM板集中于建筑不同朝向的应用效果,结果显示对于熔点为25℃的相变材料,相变材料集中布置于西向外墙时时建筑能耗最低。     

Grain growth and microstructure evolution based mechanical property predicted by a modified Hall–Petch equation in hot worked Ni76Cr19AlTiCo alloy

The deformation behavior of Ni76Cr19AlTiCo has been investigated under compressive strains by up to 50% at temperatures ranging from 800 °C to 1150 °C, and at strain rates from 0.001 s⁻¹ to 1 s⁻¹. A dramatic change in the mechanical properties of the alloy was observed when the temperature was increased from 850 °C to 950 °C, along with a rapid increase in grain size with increasing the temperature. Recovery and recrystallization processes occurred under deformation at temperatures above 950 °C. The degree of recrystallization was found to increase with increasing temperature or decreasing strain rate. γ′-phase precipitation occurred in the matrix and the particle sizes and the number of the precipitated phases were found to increase with increasing temperature or decreasing strain rate. Grain boundary precipitation of chromium carbides has also been observed, but its influence was negligible because of the small amount of the precipitates present in the matrix.A modified Hall–Petch equation was proposed to predict the mechanical properties of the alloy based on grain growth and microstructure evolution.     

Measurement of residual stresses in thick composite cylinders by the radial-cut-cylinder-bending method

Thick fabric composite cylinders for nozzle parts in solid rocket motors should be designed to endure the extreme temperature and pressure of combustion gas. As the thickness of the composite cylinder increases, fabricational residual stresses due to the anisotropic thermal expansion or shrinkage of fabric composites also increase, which induces inter-laminar failures. Therefore, the accurate estimation of the residual stresses is indispensable for the development of thick fabric composite cylinders.

In this paper, the residual stresses in thick cylinders made of carbon fabric phenolic composites were measured by a new radial-cut-cylinder-bending method. To obtain the residual stresses from the measured relative strains during the radial-cut operation, a bending test of the cylinder with the radial-cut was performed instead of measuring the material properties with respect to radial positions. The thermal residual stresses were also calculated by finite element method considering shear deformation of fabric layers, and compared with the measured residual stresses by the new method, from which it was found that the new simple method estimated the residual stresses pretty well. Also the inter-laminar tensile strength at the position of maximum radial residual stress could be obtained from the bending test.     

Microstructure, chemistry and coating properties of ion-plated TiN on type 304 stainless steel

Characterization of TiN coatings on type 304 stainless steel was carried out using a Zeiss EM 902A energy filtering transmission electron microscope equipped with an electron energy loss spectroscopy (EELS) detector. TiN thin films were produced by a hollow cathode discharge ion plating coater. It was found by plan-view transmission electron microscopy that the microstructure of the TiN coatings is thickness dependent. The grain size of TiN ranges from 88 nm at the coating surface down to 9 nm near the TiN/steel interface. In addition, the TiN surface layer shows some degree of texture, but the subsurface and internal TiN layers are mainly equiaxial and randomly oriented. Chemical analysis by EELS shows that the relative oxygen content increases linearly from the TiN surface to the TiN/steel interface, whereas the relative nitrogen content first decreases slowly and then drops rapidly near the interface. The presence of a Ti₂N phase and the deficiency of nitrogen near the TiN/steel interface suggest that the early-deposited TiN is nonstoichiometric. By the periodic cracking method, the ultimate shear stress at the TiN/steel interface and the residual stress in the TiN thin film were estimated to be 2.2 GPa and 12.8 GPa, respectively.