Study of the hexagonal to monoclinic celsian phase transition induced by magnetic phase nucleation in barium aluminosilicate glass-ceramics

The inert hexagonal to monoclinic celsian (H-M) phase transition extremely restricts the formation of celsian. In this study, Fe³⁺ dopants were incorporated into nonmagnetic barium aluminosilicate (BAS) glass-ceramics using Fe(NO₃)₃.9 H₂O as raw material for preparing the celsian phase. As the Fe-Ba mole ratio reached 0.238, the celsian phase can be obtained in magnetic BAS glass-ceramics. It implied that some magnetic reaction products facilitated the H-M phase transition. To examine the above inference, magnetic barium ferrite (BFO) additions were externally incorporated into a pure BAS glass matrix. Results suggested that BFO additions can significantly promote the H-M phase transition. When the addition of BFO was increased to 5 wt %, almost all of the hexacelsian phase was converted into the celsian phase. And the reason was that barium ferrite can act as a nucleation agent to induce the distorted hexacelsian phase decreasing the H-M phase transition barrier.     

Fabrication of transparent γ-AlON by direct 2-step pressureless sintering of Al2O3 and AlN using an AlN-deficient composition

Transparent polycrystalline aluminum oxynitride (γ-AlON) was fabricated by the pressureless two-step sintering of α-Al₂O₃ and AlN after adding a small amount of MgO and Y₂O₃. The process was based on two assumptions. The first was the utilization of AlN-deficient non-stoichiometric composition to increase the cationic vacancies and the second was the selection of the 1st step sintering temperature that suppresses the formation of γ-AlON phase to achieve a high density after the 2nd sintering step. The 1st and 2nd sintering steps were performed at 1610–1650 and 1940–1990 °C, respectively, for 10 h in a 2.5 atmospheric nitrogen pressure, and the optimal sintering conditions were determined. The fabricated γ-AlON showed a mean grain size of 164–248 μm without the presence of significant scattering centers, where the sample prepared using an optimal condition revealed a very high transmittance of 84.7% along with comparable mechanical properties.     

等离子体中太赫兹波传输及成像探测特性研究

本文根据散射矩阵方法模拟等离子体并建立了非均匀等离子体理论模型,并在此基础上计算了0.1 THz^10 THz频段的全波段太赫兹波在其中的传输特性。根据介质阻挡放电原理在实验室环境下搭建等离子体射流产生装置并产生非均匀等离子体,进行了太赫兹时域光谱(THz-TDS)以及宽带太赫兹源在等离子体中的透射光谱测量以及太赫兹波对等离子体遮挡下目标物的反射成像的试验。理论和实验结果均表明,较高频太赫兹波在等离子体中有良好的穿透性,这为太赫兹波在黑障区的通信以及雷达探测应用打下研究基础。     

3D printing of high solid loading zirconia feedstock via screw-based material extrusion

Zirconia ceramic (3Y-TZP) feedstocks with solid loadings from 50 vol% to 68 vol%, in a 60:40 paraffin wax to LDPE ratio binder system, were prepared and printed using a screw-based material extrusion printer. A two-step debinding process involving solvent debinding (cyclohexane + ethanol) and thermal debinding (140 °C–600 °C at 0.2 °C/min) followed by sintering at 1500 °C for 2 h was employed. Tests performed include TGA, density test, Vickers hardness and fracture toughness, XRD, and SEM. The TGA result showed two significant drops in weight starting at 180 °C and 380 °C, which corresponds to the decomposition of paraffin wax and LDPE, respectively. A minimum of 40 wt% of soluble binder was removed from the green sample after solvent immersion for 3 h at 40 °C for solid loadings ≥55 vol%. High solid loading feedstocks produced samples with comparable density, Vickers hardness and fracture toughness, which are 97.5%, ∼12.3 GPa, and ∼5.5 MPa m^1/2, respectively; while XRD and SEM shows no adverse tetragonal to monoclinic phase transformation and grain growth, respectively. This study demonstrates that 3D printing of granular 3Y-TZP ceramic feedstock via screw-based material extrusion technique is feasible even with high solid loadings, which is usually difficult to fabricate into flexible filaments and print due to high viscosity.     

Iterative non-local means filter for salt and pepper noise removal

Salt and Pepper noise (S&P noise) removal is an active research area in digital image processing. Existing techniques commonly use the local statistics within a neighborhood to estimate the centered noisy pixel, and tend to damage image details due to the image local diversity singularity and non-stationarity. To address this problem, in this paper, iterative nonlocal means filter (INLM) is proposed to exploit the image non-local similarity feature in the S&P noise removal procedure. Moreover, the proposed iterative framework update the similarity weights and the estimated values for higher accuracy. The experimental results show that the proposed INLM produces better results than state-of-art methods over a wide range of scenes both subjectively and objectively, and it is robust to the detection results.     

Phase equilibria in the La2O3-Sm2O3-ZrO2 system: Experimental studies and thermodynamic modeling

A series of ceramic samples were prepared to experimentally investigate sub-solidus phase relations in the La₂O₃-Sm₂O₃-ZrO₂ system at 1873 K and 1673 K. No ternary compounds have been observed, while the binary La₂Zr₂O₇ and Sm₂Zr₂O₇ pyrochlore phases form a continuous solid solution La_2?xSm_xZr₂O₇ in the ternary system at the selected temperatures. X-ray diffraction and microstructure results demonstrated that the pyrochlore phase is stable in the ZrO₂-rich corner. The homogeneity range of the pyrochlore phase was carefully determined and the phase boundary of the cubic ZrO₂ (fluorite phase) which extends into the ternary system was also constructed via electron probe microanalysis. The as-obtained data were adopted to determine the mixing parameters for the pyrochlore and fluorite phases in the present thermodynamic modeling. A self-consistent database of the La₂O₃-Sm₂O₃-ZrO₂ system was accordingly established for the first time and the calculations agree well with the experimental data in the current work.     

积分格式对窄带k-分布计算的影响

对于窄带k-分布模型，影响计算精度的关键因素是积分格式，为了研究积分格式对窄带k-分布模型的影响，选择了5种精度较高的高斯积分方法进行计算分析。首先采用Gauss-Lobatto 7点积分格式窄带k-分布法计算了CO2四种条件下的窄带平均透过率，并与实验结果进行对比验证，对比结果吻合良好。另外选取了4种积分方式:Gauss-Lobatto 5、Gauss-Lobatto 4、Gauss-Legendre 4和Gauss-Legendre 2点积分，对CO2的四个算例进行计算，并将5种积分格式的计算结果与逐线计算结果进行对比分析。结果表明:Gauss-Lobatto 7的精度最高，Gauss-Lobatto 5和Gauss-Legendre 4次之，其中Gauss-Legendre 4略高于Gauss-Lobatto 5。显然相同积分方法积分点越多计算精度越高，但计算时间越长，而两种不同积分方法对比Gauss-Legendre优于Gauss-Lobatto。综合考虑精度与计算时间等因素，5种积分格式中Gauss-legendre 4应该为最好的选择。     

基于非局域热平衡模式的临近空间目标背景临边对比度模型

对临近空间目标的空基/星载红外探测中的目标背景临边对比度问题进行物理建模,详细阐述对比度的定义和模型中的高层大气辐射问题.利用适于模拟高层大气辐射的非局域热平衡模式,结合探测器噪声假定,计算理想黑体目标的临边对比度并分析高层大气辐射特性对对比度的影响.模拟结果表明,在评估目标探测可行性时必须考虑探测器噪声的影响;在高层大气临边路径下水汽波段比大气窗区的可探测性更好,而目标本征辐射与大气临边吸收和背景辐射间的关系导致了对比度复杂的变化特征.     

采用长波红外高光谱偏振技术的目标探测实验

偏振探测有利于改善对目标的探测和识别能力,是当前国内外研究的重要内容之一。介绍了长波红外高光谱偏振测量的原理,并通过搭建长波红外高光谱偏振测量系统,开展了对涂漆目标和镀铝目标在不同温度、不同观测角度下的高光谱偏振成像实验,获取了有效的实验数据并进行处理分析。结果表明:温度和观测角度对目标的光谱偏振特性有较大影响,目标表面的红外光谱偏振特性随辐射温度差值及探测角度差值的增大而增大,并具有波段选择性。利用目标温度和观测角度的差异对目标光谱偏振特性的影响,可以进行有效的探测与识别,并为探测器的波段选择提供参考依据。     

舰船排气羽流红外辐射特性计算研究

舰船动力系统的高温排气羽流是舰船红外辐射的重要来源之一,通过建立基于CFD数值方法的排气羽流流场分布模型和基于窄带模型C-G近似方法的高温排气红外辐射模型,重点研究了合成风速对排气羽流流场和红外辐射的影响。仿真结果表明:合成风速的大小对排气羽流的形状及辐射分布区域影响显著,合成风速增加一倍的情况下,排气羽流在中波红外谱段平均辐射强度降低了48.1%。也对计算模型可能产生偏差的物理因素进行了初步分析,如不完全燃烧产生的碳粒子对羽流红外辐射传输有显著影响。