The age of formation of the mirabilite and sand wedges in the Hexi Corridor and their paleoclimatic interpretation

The Hexi Corridor is located at the transition zone of the Asian summer monsoon and westerly airflow, and lies in an important position in terms of its ecological fragility and climatic sensitivity. During a recent field expedition to this region, we found a sedimentary mirabilite layer in a number of localities including Suwushan, Yanchi, Baitujing, Yanchi Gaotai, Huahai, Yumen, Halanuo扙r, and Dunhuang (Fig. 1), which have the potential to provide important information about environmenta…     

Temperature variations on the Tibetan Plateau over the last two millennia

The paleoclimate data recovered from ice cores,tree rings and lake sediments indicate regional features of cfimatic change on the Tibeta n Plateau (TP) during the last 2000 years. The composite temperature reconstructions in-dicate that several main climatic episodes, such as the “LittleIce Age“ between 1400 and 1900, the “Medieval Warm Pe-riod“ in 1150-1400, a less warm period in 800-1100, and an earlier cold period between the 3rd and 5th centuries,occurred in the TP. In addition, temperature varied from region to region. The period from AD 800 to 1100, which waswarm in northeastern TP, was contemporaneous with cool-ing in the western and southern TP. The southern TP ex-perienced warming between 1150 and 1400. For western TP,the δ^18O records of the Guliya ice core indicate that the pe-Hod 1250-1500 witnessed a clear warming. Large-scaletrends in the temperature history from northeastern TP aremore similar to those in eastern China than are the trendsfrom the Guliya ice cap far to the west and southern TP. The most prominent similarities between the temperature varia-tions of the TP and eastern China are such cold phases as 1100-1150, 1500-1550, 1650-1700 and 1800-1850, andthe latter three cold events match with three widespreadg lacial advances which occurred on the TP during the Little Ice A2e.     

New geological evidence of crustal thickening in the Gangdese block prior to the Indo-Asian collision

Recent mapping in the Gangdese block has revealed many leucogranites that are similar to those in the High Himalaya. These leucogranites formed at ～140 Ma as indicated by monazite Th-Pb ion-microprobe dating and cooled at ～130 Ma as indicated by muscovite ^40Ar/^39Ar dating. In conjunction with previous structural and paleogeographic studies, the new data indicate that the Gangdese block underwent crustal thickening and associated exhumation during ～140—130 Ma. In this regard, the southern margin of Eurasia continent was comparable to the modern South American Altiplano-Puna plateau, the prime example of active ocean-continent subduction and associated thickened crust. Specifically, the early stages of crustal thickening and uplifting of the Gangdese block may result from subduction of the Neo-Tethyan Ocean. If the Tibetan Plateau would form by accretion of a series of blocks with thickened crust, an elevated topographic plateau similar to the Altiplano-Puna plateau had formed before collision between the Indian and Eurasian plates. Then the Tibetan Plateau would have quickly thickened, uplifted, and begun to extend soon after onset of the collision. Thus, the deformational mechanism of the Tibetan Plateau is not distributed shortening, but rather concentrating deformation within regions of thin crust between the accreted blocks.     

Time-division phase modulated single-photon interference in a Sagnac interferometer

We introduce a stable, long-distance singlephoton Sagnac interferometer, which has a balanced configuration to efficiently compensate phase drift caused by change of the fiber-optic path. By using time-division phase modulation, single-photon interference was realized at 1550nm in a 5-km-long as well as 27-km-long Sagnac fiber loops,with a fringe visibility higher than 90% and long-term stability. The stable performance of the single-photon interference indicated that the time-division phase-modulated Sagnac interferometer might readily lead to practical applications in single-photon routing and quantum cryptography.     

Contributions of different land cover types in Otindag Sandy Land and Bashang area of Hebei Province to the material source of sand stormy weather in Beijing

Different land cover types in Otindag Sandy Land and Bashang area of Hebei Province are linked to a material source of sand stormy weather in Beijing based on results of field vegetation and soil survey and laboratory works. Results of grain size analysis show that dust release potential in per unit area of moving sandy land is small,while lowland meadow and meadow steppe in stony hills have high potential of releasing dusts in per unit area during sand stormy weather occurrence. Further considering the effects of vegetation, it is inferred that the moving sandy land served as a material source of past dust storm and the possibility of dust release in per unit area is low in current time.Typical steppe in stony hills is undergoing desertification and its dust release possibility in per unit area is high. Farmland has strong potential of release dusts when they are ploughed in spring, but a large amount of therophytes grow and thus prevent dusts from release when cultivation was terminated.Potentials of dust release in per unit area in fixed sand dunes,stony mountain meadow steppe and low land meadow are lowdue to high cover of perennials. Sand dune reactivation and desiccation of lakes and lowlands under estimated future climatic change will make them serve as a future material source of sand stormy weather.     

电铲铲斗齿的修复工艺

通过分析铲斗齿的磨损原因，提出了采用堆焊的方法对铲斗齿进行修复。详细介绍了铲斗齿的修复工艺，堆焊材料的选择及堆焊时应注意的事项。实践表明，此修复工艺切实可行，取得了令人满意的效果。     

开关线性复合变换低阻输出特性对系统的优化

为了在现有电力电子技术基础上进一步提高功率变换器的综合性能，揭示了一种由电力电子开关电路与线性电路有机结合而派生的开关线性复合功率变换器(SLH)。简述了该类变换器的基本工作原理，列举了并联型和串联型两种电路拓扑及其特点，说明了SLH的低阻输出特性对提高功率变换器静动态性能的作用，并对SLH与典型的PWM LC滤波单元的变换器进行了比较分析，给出了用于电源和传动系统不同负载条件下的测试数据和典型波形。结果表明，SLH在高效与优波兼顾、多类负载适应性和抗负载扰动能力方面具有综合优化性能。     

梯度型聚合物光纤预制棒折射率分布的间接测试

利用紫外光对溴苯（BB）的吸收作用，采用紫外吸收光谱法对PMMA－BB梯度型聚合物光纤预制棒的折射率分布进行了间接测试，详细介绍了这种紫外有机光谱法的测试原理及方法，测试结果表明，所制备的预制棒的折射率分布是呈抛物线分布，但在最佳理论分布有着一定的偏差，与Lorentz-Lorenz模拟方程的结果相对接近。同其他复杂的测试方法相比，紫外光谱法所需仪器普通，测试简便，不失为实验室中较为简单易行的一种预制棒折射率分布测试方法。     

数字图像中照度不均匀校正技术研究

为了消除数字图像中的照度不均匀性，对现有数字图像的照度不均匀校正技术进行了归类和讨论．分析了这些方法在计算误差上的内在原因，并在此基础上提出了基于小波变换的数字图像照度不均匀校正技术．该技术兼顾了数字图像的频域和空域，使得采用本方法校正后的图像具有品质优，计算误差小和时间消耗较少等特点．