硅基外延La-Sr-Co-O/Pb-Zr-Ti-O/La-Sr-Co-O异质结的制备

在外延SrTiO3（STO）作为缓冲层的Si基片上，应用溶胶-凝胶法（sol-gel）和磁控溅射法，在5．50℃温度下制备了外延的La0.5Sr0.5CdO3／Pb（Zr0．2Ti0．8）O3/La0．5Sr0．5CoO3（LSCO／PZT／LSCO）异质结。X射线衍射结果表明，LSCO／PZT／LSCO异质结是c向外延生长的。当外加电压为60V时，LSCO／PZT／LSCO铁电电容器的剩余极化强度为．50．3μC/cm^2.，当电压为30V时，铁电电容器有效极化强度为80μC/cm^2.。其它电学性能表明。PZT铁电电容器具有较高的电阻率和脉冲宽度对极化强度的影响较弱。     

薄膜厚度对PZT/STO铁电电容器隧穿电阻的影响

结合Landau-Devonshire自由能理论和晶格模型,研究了PbZr1-xTixO3(PZT)/SrTiO3(STO)复合薄膜中PZT和STO厚度对铁电隧道结极化强度、电导和隧穿电阻等的影响。模拟结果表明,随着层数增加,极化强度增大;平均极化强度随着PZT厚度增加而增强,随着STO厚度增加而减弱;随着PZT和STO厚度增加,电导减小,隧穿电阻率增加;当厚度变化相同时,PZT引起电导的变化大,隧穿电阻率变化小。考虑薄膜面积的影响,当薄膜面积从无限大变成有限大时,极化强度、电导和隧穿电阻均减小。     

含Ti-Al阻挡层的硅基Nb掺杂Pb(Zr,Ti)O3铁电电容器的研究

应用非晶的Ti-Al薄膜作为导电阻挡层,通过溶胶-凝胶法和磁控溅射法成功地在Si基片上制备了La0.5Sr0.5CoO3/Pb(Nb0.02Zr0.39Ti0.59)O3/La0.5Sr0.5CoO3(LSCO/PNZT/LSCO)电容器异质结,研究了该异质结的铁电性能。当外加电压为5V时,LSCO/PNZT/LSCO铁电电容器的剩余极化强度为-18.0μC/m2,矫顽电压为-0.7 V,该电容器具有较好的保持持性和抗疲劳特性。     

基于扫描探针显微术研究Pb(Zr,Ti)O_3铁电薄膜的电学性质

首先利用脉冲激光沉积技术在(001)取向的SrTiO3基片上外延生长了SrRuO3底电极和PbZr0.20Ti0.80O3(PZT)薄膜,然后利用扫描探针显微镜的压电响应模式(PFM)和导电测试模式(C-AFM)表征了PbZr0.20Ti0.80O3/SrRuO3/SrTiO3异质结薄膜纳米尺度的电学性质。以镀铂探针为上电极,利用压电响应模式获得了复合薄膜纳米尺度的压电位移-电压蝶形曲线和压电相位-电压滞后曲线,表明样品具有良好的铁电性。薄膜纳米尺度下的I-V测试结果表明经+10V电压极化后的样品,其I-V曲线在矫顽场附近出现峰值,与宏观I-V测试结果类似。导电原子力和压电力测试结果证明C-AFM可以检测到PZT薄膜样品的瞬时极化反转电流并进行成像。     

CrTe的半金属性和CrTe/ZnTe异质结的自旋二极管效应研究

基于密度泛函理论的第一性原理方法,对闪锌矿结构CrTe和CrTe/ZnTe(001)异质结的电子结构和自旋极化输运性质进行了计算研究。结果表明,CrTe是一种很好的半金属铁磁体,具有4.00μB/分子的磁矩和很大的半金属隙(0.84eV)。计算得到的电流-电压曲线显示CrTe/ZnTe(001)异质结具有优良的自旋二极管效应,因此,CrTe在自旋电子器件中应该有一定的应用。并从能带理论角度解释了CrTe/ZnTe(001)异质结中产生自旋二极管效应的机理。     

基于LaNiO_3和Pt电极的Pb(Zr_(0.52)Ti_(0.48))O_3薄膜制备和电学性能研究

采用了溶胶-凝胶(sol-gel)法分别在LaNiO3/Si和Pt/Ti/SiO2/Si基底上制备Pb(Zr0.52Ti0.48)O3(PZT)薄膜。X射线衍射(XRD)结果表明,在相同的工艺参数下PZT/LaNiO3薄膜表现出高度的(100)取向,而PZT/Pt薄膜则呈现(100)和(111)多晶混合取向。扫描电子显微镜(SEM)结果显示,PZT/LaNiO3薄膜表面晶粒尺寸更加均匀。经过铁电和介电性能测试,PZT/LaNiO3和PZT/Pt薄膜的剩余极化强度分别为24.4和15.3μC/cm2,矫顽场分别为130.90和243.23 kV/cm,介电常数分别为1 125和453。电场强度100 kV/cm时,漏电流分别为10-5和10-2数量级,同时铁电疲劳性能也明显改善。这些结果表明PZT/LaNiO3比PZT/Pt薄膜具有更好的电学性能,在铁电和介电器件方面具有良好的应用前景。     

偏轴磁控溅射法外延BiFeO3薄膜的介电性能与阻变效应

利用偏轴射频磁控溅射法, 在(001) SrTiO₃(STO)单晶基片上制备了Pt/BiFeO₃/La_0.5Sr_0.5CoO₃/STO (Pt/BFO/ LSCO/STO)异质结电容器。研究了BiFeO₃薄膜的结构和物理性能。原子力显微镜(AFM)和X射线衍射(XRD)分析表明: BFO薄膜结晶质量良好, 且为单相(00l)外延钙钛矿结构。介电性能测试结果发现: 在5 V驱动电压下, Pt/BFO/LSCO电容器呈现饱和的蝶形回线, 调谐率和介电损耗分别为14.1%和0.19。此外, 阻变机制研究表明: 在0→5→0 V正向电压和0→-5→0 V负向电压下, 阻变均为高阻向低阻转变规律, 呈现为铁电二极管的阻变开关行为。通过I-V曲线拟合, 得到0→5→0→-5 V时阻变机制为空间电荷限制电流陷阱能级的填充和脱陷, 而-5→0 V时符合界面限制的F-N隧穿机制。     

锆钛酸铅压电陶瓷PZT7的形状记忆效应研究

提出了一种新的方法可以使锆钛酸铅压电陶瓷PZT7具有形状记忆效应.用自制的sawyer-Tower电路测试得到PZT7方形样品发生极化反转对应的电场强度大约为130kV/cm.当施加对称的饱和极化电压370V时,PZT7/Si(109/146μm)悬臂梁端部的变形一电场曲线呈现典型的"蝴蝶"形状.在外电场为0时,悬臂梁端部的变形为0.如果施加不对称的极化电压,悬臂梁端部的变形--电场曲线发生明显的变化.不同的反向极化电压使悬臂梁端部产生不同的变形,而且,电压和变形是一一对应的.因此,在外电场为零时,PZT7/Si悬臂梁可以具有多个不同的稳定位置,表现出明显的形状记忆效应.实验表明,对于长度为15.5mm的PZT7/Si悬臂梁而言,当施加的反向极化电压从120V回到0时,悬臂梁端部产生的变形最大,为68.5μm.     

Mg_2Si/Si异质结的制备及I-V特性研究

采用磁控溅射和热处理系统制备Mg_2Si/Si异质结。首先在n-Si(111)衬底上沉积Mg膜,经热处理后得到Mg_2Si/Si异质结。利用XRD、SEM、表面轮廓仪、伏安特性测试仪和霍尔效应测试仪,研究了Mg_2Si/Si异质结的晶体结构、表面形貌、Mg_2Si薄膜厚度、I-V特性及导电类型。结果表明,成功制备了Mg_2Si/Si异质结,并得到其平均载流子浓度(-9.30×1012 cm-3)、导通电压(0.31V)、导通电流(0.6mA)、工作电压(0.53V)等,测得该异质结为n-n型。     

SiGe/Si异质结光电器件

SiGe/Si异质结光电器件及其光电集成(OEIC)是硅基光电研究的一个非常引人注目的领域.综述了SiGe/Si异质结材料的基本性质,SiGe/Si异质结光电器件的结构、性能、应用及其光电集成.重点介绍了SiGe/Si光电探测器及其与其他相关器件的集成.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Real gas choked flow conditions at low reduced-temperatures

Real gas choked mass flux is calculated for a frictionless stream expanding isentropically until it reaches the speed of sound and without phase changes. The other parameters associated with the choked state are the pressure, density, temperature ratios, and the speed of sound. Departure of the choked mass flux from the ideal gas model is discussed first in absolute terms and then in relative terms, using the Principle of Corresponding States, for gases with boiling points in the low temperature range. Reduced-stagnation pressures are examined up to values of 30 for hydrogen, neon, nitrogen, argon, methane, krypton, xenon, and R-14 and up to 100 for ⁴He. The corresponding reduced-stagnation temperatures go down to 1.4 and in some cases down to 1.2 for nitrogen and argon. Also discussed are the limiting values of stagnation parameters for which no phase change occurs in the choked state. Compared to the ideal gas, the mass flux may almost double and the critical pressure ratio may decrease by an order of magnitude. The relevance of results is discussed qualitatively and quantitatively for Joule-Thomson cryocooling.     

Degradation behaviour of sputtered Co–Al coatings on superalloy

Degradation behaviour of sputtered Co–Al coatings on Superni-718 substrate has been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 900 °C for up to 100 thermal cycles between the peak and room temperatures. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS, AFM, and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al₂O₃ and spinel phases such as CoCr₂O₄ and CoAl₂O₄ in scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compare to bare substrate in air at 900 °C temperature up to 100 cycles.     

Capacitance based mass metering for cryogenic fluids

This paper describes a method for measuring the mass of cryogenic fluids in on-board rocket propellant tanks or ground storage tanks. Linear approximations to the Clausius-Mossotti relationship serve as the foundation for a capacitance based mass sensor, regardless of fluid density stratification or tank shape. Sensor design considerations are presented along with the experimental results for a capacitance based mass gage tested in liquid nitrogen. This test data is shown to be consistent with theory resulting in a demonstrated mass measurement accuracy of ±0.75% and a deviation from linearity of less than ±0.30% of full scale mass. Theoretical accuracies are also shown to be ±0.73% for hydrogen and ±1.39% for oxygen for a select range of pressures and temperatures.     

Equipment using a “static-analytic” method for solubility measurements in potentially hazardous binary mixtures under cryogenic temperatures

A new apparatus designed to study, at cryogenic temperatures, thermodynamic equilibria of potentially explosive binary systems such as hydrocarbon-oxygen mixtures is described herein. This equipment has an equilibrium cell which was especially designed to minimize hazards while allowing accurate phase equilibrium measurements. Reliability of results, obtained with this equipment has been verified by working on the nitrogen-propane system, for which data are already available in literature, over a large range of compositions and at various temperatures. Four isothermal curves describing liquid phase compositions at 109.98, 113.77, 119.75 and 125.63 K have been determined. Our experimental data are represented within 2% in compositions and in pressures through the Peng-Robinson equation of state implying Mathias-Copeman alpha function and Huron-Vidal mixing rule. Comparisons to literature allow pointing out: good agreement is observed with Kremer and Knapp data (1983) while the three sets of Poon and Lu data (1974) presenting systematic positive deviation are consequently judged as suspicious.     

Percolation and the resistive transition of the critical temperature Tc of Nb3Sn

The relationship between the distribution of the critical temperature, the percolation function, and the resistive transition of the critical temperature is explored for polycrystalline Nb₃Sn. In the neighborhood of the critical temperature, Nb₃Sn is assumed to be a random mixture of superconducting and normal grains. Percolation concepts are applied to a study of the resistivity. A general analysis is made showing that the onset and shape of the resistive transition for composite conductors are determined by the percolation function and the distribution of the critical temperature. An approximate form of the percolation function is determined based on a linear FEM analysis. Example resistive transitions are calculated for an assumed normal distribution of the critical temperature. An argument is presented that relates grain orientation and strain dependence in Nb₃Sn. It is noted that a dependence of the distribution of T_c with strain, in addition to the usual shift in T_c with strain, would be the result of a strain dependence that is a function of grain orientation. The analysis shows the extent to which the slope of the resistive transition is a measure of the distribution of the critical temperature, and therefore a measure of the grain orientation strain sensitivity. Finally, a method is described to determine the percolation function experimentally.     

Liquid helium cryostat for SQUID-based MRI receivers

We describe a liquid helium cryostat, developed to cool SQUID-based receivers in low field MRI systems. The cryostat has a 4 L liquid helium capacity, a hold time of over 3 days and accommodates 10 cm diameter receiver coils. New vacuum insulation methods reduce the noise level by at least an order of magnitude compared to existing commercial designs. The minimum detectable field at 425 kHz, with a 5 cm diameter circular coil, was estimated to be 0.018 fT/Hz^1/2 from Q-factor measurements and 0.035 fT/Hz^1/2 by direct measurement with a SQUID amplifier. Further measurements indicated that most of this field noise probably originates with dielectric losses in the cryostat’s fibreglass shells.     

Thermal Stability of Amorphous Hydride Mg_(50)Ni_(50)H_(54) and Mg_(30)Ni_(70)H_(45)

The thermal stability of amorphous ternary hydrides Mg_(50)Ni_(50)H_(54) and Mg_(30)Ni_(70)H_(45) and their corre-sponding amorphous binary alloys Mg_(50)Ni_(50) and Mg_(30)Ni_(70) were studied with X-ray diffraction(XRD) and differential scanning calorimetry(DSC). Samples of the amorphous alloys were preparedby mechanical alloying and the amorphous hydrides were obtained by charging the alloys with gas-eous hydrogen at 3.0 MPa and 423 K. It was found that the amorphous hydrides released most oftheir hydrogen before the crystallization of the essentially hydrogen depleted amorphous alloy. Thecrystallization temperature of amorphous Mg_(50)Ni_(50)H_(54) elevated and that of amorphousMg_(30)Ni_(70)H_(45) did not change in relation to the original binary amorphous alloy. This is very excep-tional for amorphous hydrides. The reason for the effects of hydrogen absorption/desorption on thecrystallization of amorphous alloys was discussed.     

（Pb1-xSrx）TiO3系薄膜铁电性能的工艺因素研究

钛酸锶铅((Pb1-xSrx)TiO3)薄膜是一种重要的铁电薄膜,应用潜力很大,是高新技术研究的前沿和热点之一.通过磁控溅射的方法制备了PST薄膜,详细研究了不同工艺因素对PST薄膜铁电性能的影响.结果表明,选择合适的工艺条件可以制备铁电性能优良的钙钛矿相PST薄膜.     

Development of No-Vent™ liquid hydrogen storage system for space applications

A number of technological advances required to store and maintain normal-boiling-point and densified cryogenic liquids, including liquid hydrogen, under zero boil-off conditions in-space, for long periods of time, have been developed. These technologies include (1) thermally optimized compact cryogen storage systems that reduce environmental heat leak to the lowest-temperature cryogen, which minimizes cryocooler size and input power, and (2) actively-cooled shields that surround the storage systems and intercept heat leak. The processes and tools used to develop these technologies are discussed. A zero boil-off liquid hydrogen storage system technology demonstrator for validating the actively-cooled shield technology is presented.