全文获取类型
收费全文 | 378篇 |
免费 | 68篇 |
国内免费 | 19篇 |
专业分类
系统科学 | 3篇 |
丛书文集 | 3篇 |
教育与普及 | 36篇 |
理论与方法论 | 1篇 |
综合类 | 413篇 |
自然研究 | 9篇 |
出版年
2024年 | 17篇 |
2023年 | 47篇 |
2022年 | 31篇 |
2021年 | 107篇 |
2020年 | 124篇 |
2019年 | 17篇 |
2018年 | 29篇 |
2017年 | 7篇 |
2016年 | 5篇 |
2015年 | 11篇 |
2014年 | 22篇 |
2013年 | 3篇 |
2012年 | 1篇 |
2011年 | 14篇 |
2010年 | 5篇 |
2009年 | 1篇 |
2007年 | 7篇 |
2006年 | 2篇 |
2005年 | 2篇 |
2004年 | 2篇 |
2002年 | 1篇 |
2001年 | 2篇 |
2000年 | 5篇 |
1998年 | 1篇 |
1989年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有465条查询结果,搜索用时 15 毫秒
361.
A biodegradable Zn alloy, Zn–1.6Mg, with the potential medical applications as a promising coating material for steel components was studied in this work. The alloy was prepared by three different procedures: gravity casting, hot extrusion, and a combination of rapid solidification and hot extrusion. The samples prepared were characterized by light microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. Vickers hardness, tensile, and compressive tests were performed to determine the samples’ mechanical properties. Structural examination reveals that the average grain sizes of samples prepared by gravity casting, hot extrusion, and rapid solidification followed by hot extrusion are 35.0, 9.7, and 2.1 μm, respectively. The micrograined sample with the finest grain size exhibits the highest hardness (Hv = 122 MPa), compressive yield strength (382 MPa), tensile yield strength (332 MPa), ultimate tensile strength (370 MPa), and elongation (9%). This sample also demonstrates the lowest work hardening in tension and temporary softening in compression among the prepared samples. The mechanical behavior of the samples is discussed in relation to the structural characteristics, Hall–Petch relationship, and deformation mechanisms in fine-grained hexagonal-close-packed metals. 相似文献
362.
为了探讨纵截面异形桩的受扭性状,考虑桩身纵向变截面特性,将桩-土体系沿深度方向划分为有限个微元段,建立均质地基中纵截面异形桩受扭弹塑性分析理论模型;基于传统圆形桩桩顶T-φ曲线与桩身扭矩、转角关系曲线,推导获得纵截面异形桩的桩身扭矩和扭转角近似解答,并基于MATLAB编制了相应的计算程序.继而,初步探讨了纵截面异形桩与等截面圆形桩受扭性能的异同点,并开展楔形桩受扭性能影响因素分析.研究结果表明:上端桩身直径对抗扭承载性能影响显著;等体积混凝土用量下,纵截面异形桩的抗扭性能相对优于等截面圆形桩,且楔形桩抗扭性能最优;相同桩顶扭矩荷载下,楔形桩抗扭承载力随桩身剪切模量、桩径、楔角的增加而增大,桩径提高1倍,桩顶抗扭承载力提高3~5倍. 相似文献
363.
基于广义统一概率图的东南沿海风速概率分布研究 总被引:1,自引:0,他引:1
传统的概率分布选择方法是基于观测数据在假设分布概率纸上进行绘制,由于受到概率纸的限制无法进行直接比对分析.本文提出了一种新的广义统一概率图(GUPP)方法,通过Rosenblatt变换,将假设分布转换到统一概率纸上进行直观比对,并提出基于GUPP的拟合优度快速检验方法及参数,该方法可用于大数据量和大范围假设概率分布的快速拟合和检验.采用本文方法对厦门地区1953-2015年采集的实时风速数据进行统计分析及拟合,结果显示Pearson-Ⅲ分布可以更好地模拟厦门地区年10 min平均最大风速的分布规律,以此更准确地确定厦门地区基于不同回归期的设计最大风速. 相似文献
364.
通过双根钢纤维拉拔试验,研究超高性能混凝土(UHPC)中钢纤维间距(2?mm、 4?mm、6?mm、8?mm、10?mm和12?mm)对钢纤维在UHPC中拉拔性能的影响,采用0.2?mm的镀铜直圆形钢纤维,制作6组双丝试件和1组单丝试件进行拉拔试验. 通过拉拔荷载-滑移曲线、平均黏结强度、钢纤维最大拉拔应力等参数,并结合扫描电子显微镜(SEM),观察了拔出后钢纤维的微观外貌、UHPC基体的隧洞微观形态和破坏部位的微观形态,全面分析了钢纤维的间距对UHPC基体-钢纤维拉拔性能的影响. 试验结果表明:钢纤维拉拔结果具有一定的离散性;双丝拉拔试验的最大拉拔荷载小于相应单丝结果的两倍;钢纤维和UHPC基体隧洞有着不同程度的损伤,双丝试件中的钢纤维表面划痕和基体隧洞的孔洞破坏较单丝严重,单丝试件的隧洞内壁则相对完整. 相似文献
365.
了解城镇化与生态系统服务价值之间的耦合关系对生态管理和政策制定具有重要意义.基于城镇化与生态系统服务综合指标体系,采用价值系数法、熵值法与耦合协调度模型分别估算了和田地区各县市的生态系统服务价值、城镇化水平、城镇化与生态系统服务价值以及各子系统之间的互动胁迫关系.研究表明:和田地区各县市城镇化综合水平显著上升,生态系统... 相似文献
366.
On Datil [Turner] Island in the Gulf of California, we rediscovered a population of Neotoma varia, previously thought to be extinct. We captured 5 specimens: 1 was kept as a voucher, and 4 were examined and released. Analysis of previous surveys indicates that N. varia is not common on the island and occupies a very restricted range. Recolectamos especímenes de Neotoma varia de la Isla Dátil en el Golfo de California, una población anteriormente considerada ya extinta. De los cinco especímenes que recolectamos, cuatro se pusieron en libertad. El análisis de muestreos previos indica que N. varia no es común en la isla y que tiene una distribución muy limitada. 相似文献
367.
Action mechanisms of a new erythrocyte-derived depressing factor 总被引:4,自引:0,他引:4
Fig. . Effect of CHCl 《科学通报(英文版)》2000,45(23):2113-2118
To investigate the action mechanisms of a new erythrocyte-derived depressing factor (EDDF), the focus is placed on the effect
of EDDF on both cytosolic and nuclear free calcium (Ca2+) transportation in vascular smooth muscle cell (VSMC), as well as the apoptosis and cell cycle of VSMC of rats. EDDF has
been extracted from human erythrocytes. The changes of Ca2+ levels in cytoplasm ([Ca2+]i) and nucleus ([Ca2+]n) have been observed using a laser scanning confocal microscope together with fluo-3/AM as a calcium indicator. Flow cytometric
technique was used to study the effect of EDDF on cell cycle and apoptosis of VSMC. [Ca2+], and [Ca2+]n were significantly decreased through several different pathways: ( i ) it reduced the Ca2+ influx by blocking L-type voltage-dependent calcium channel (L-VDC) and R-type voltage-dependent calcium channel (R-VDC);
(ii) it inhibited the Ca2+ release from inositol 1, 4, 5-trisphosphate (IP3) sensitive calcium store; and (iii) activated Ca2+-ATPase of sarcoplasmic reticulum (SR) and promoted the transportation of Ca2+ from cytoplasm to SR. However, EDDF seemed to have little inhibitory effect on the Ca2+ release from ryonodine sensitive calcium pool. It was also found that EDDF (10−4 g/mL) significantly decreased the proportion of S phase of human umbilical vein (HUV) and inhibited the proliferation of
VSMC induced by angiotensin II (Angll, 10−5 mol/L). The apopotosis did not occur when VSMC was cultured under normal condition. While VSMC apoptosis was induced by Angll
(10−5 mol/L) and EDDF (10−4 g/mL) seemed to have little effect on it. The inhibitory effect of EDDF on the elevation of [Ca2+]i and [Ca2+]n of VSMC might play an essential role in its action mechanisms and the ways it affects the Ca2+ handling of VSMC demonstrate that EDDF was different from other endogenous blood pressure regulators and some known antihypertensive
drugs. EDDF could inhibit the proliferation of VSMC, which indicated that it might be beneficial to the prevention and treatment
of hypertension and arteriosclerosis. 相似文献
368.
在日常生活中 人们所接触的材料不外乎两类 即晶态材料和非晶态材料。形象地说 如果材料中的原子排列像被检阅的士兵方阵那样有序 该材料就是晶态材料;如果原子排列像集市的人群那样杂乱无章 那么这种材料就是非晶态材料。在我们接触的物质中 食盐钻石等都属于晶态材料 而木材纺织品和玻璃都属于非晶态材料。以往我们认识的所有金属 几乎无一例外地属于晶态材料。 金属能不能产生非晶态呢?这一直是近代材料科学着重要破解的一道难题。世界上有关非晶态合金研究的最早报道 是年德国人克雷默采用蒸发沉积法制备出非晶 《科技信息》2000,(8):25
在日常生活中,人们所接触的材料不外乎两类,即晶态材料和非晶态材料。形象地说,如果材料中的原子排列像被检阅的士兵方阵那样有序,该材料就是晶态材料;如果原子排列像集市的人群那样杂乱无章,那么这种材料就是非晶态材料。在我们接触的物质中,食盐、钻石等都属于晶态材料,而木材、纺 相似文献
369.
370.
XIE Lei WANG Rucheng CHEN Xiaoming QIU Jiansheng & WANG Dezi State Key Laboratory for Mineral Deposits Research Department of Earth Sciences Nanjing University Nanjing ? China 《科学通报(英文版)》2005,50(8):809-817
Zircon is one of the most common accessory minerals in igneous rocks, especially in granites. It exists through the crystallization and evolution of grantic magma be-cause of stable physicochemical properties. Trace element abundances (e.g. Hf, Th, U, Y) in zircon are very sensitive to the crystallization environments[1]. Therefore, zircon plays an important role in the granite research. However, although A-type granites are given more and more regards by the geologists owing to their charac… 相似文献