全文获取类型
收费全文 | 6539篇 |
免费 | 59篇 |
专业分类
公路运输 | 2126篇 |
综合类 | 1999篇 |
水路运输 | 1145篇 |
铁路运输 | 1136篇 |
综合运输 | 192篇 |
出版年
2024年 | 10篇 |
2023年 | 40篇 |
2022年 | 46篇 |
2021年 | 74篇 |
2020年 | 52篇 |
2019年 | 81篇 |
2018年 | 76篇 |
2017年 | 17篇 |
2016年 | 20篇 |
2015年 | 44篇 |
2014年 | 583篇 |
2013年 | 100篇 |
2012年 | 88篇 |
2011年 | 92篇 |
2010年 | 119篇 |
2009年 | 99篇 |
2008年 | 101篇 |
2007年 | 142篇 |
2006年 | 133篇 |
2005年 | 122篇 |
2004年 | 90篇 |
2003年 | 321篇 |
2002年 | 1216篇 |
2001年 | 1410篇 |
2000年 | 347篇 |
1999年 | 279篇 |
1998年 | 193篇 |
1997年 | 129篇 |
1996年 | 88篇 |
1995年 | 94篇 |
1994年 | 112篇 |
1993年 | 54篇 |
1992年 | 59篇 |
1991年 | 60篇 |
1990年 | 58篇 |
1989年 | 32篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1964年 | 1篇 |
1959年 | 2篇 |
排序方式: 共有6598条查询结果,搜索用时 15 毫秒
991.
压力对颗粒增强铝与碳基材料摩擦系数的影响 总被引:4,自引:1,他引:4
用自行研制的盘块式高速摩擦试验机,在滑动速度为30m/s,法向载荷范围为0.1-0.8MPa的条件下,试验观察了法向载荷对碳化硅颗粒增强6061铝复合材料与碳基材料摩擦系数的影响,结果表明:摩擦九存在显著的压力依赖行为和磨擦过程初期的时间过渡行为,在相对稳定阶段,摩擦系数的统计平均值随法向载荷增大经历先快速增大后缓慢减小,标准偏差与法向载荷的关系也显示出类似的变化趋势,所有连续摩擦过程的初期,摩擦系数都经历一个从小到大的过渡变化,而过渡期的时间随着法向载荷的增大单调下降。 相似文献
992.
本文对现有公路平、纵面线形的拟合方法进行研究;介绍了线形拟合的理论依据,操作步骤,提出了利用计算机进行旧路改、扩建设计和连接线设计中线形设计的方法。 相似文献
993.
CLONING AND EXPRESSION OF A GENE MEDIATING γ-RADIATION-INDUCED APOPTOSIS IN HL-60 CELLS 总被引:2,自引:0,他引:2
Apoptosis,orprogrammedcelldeath(PCD),isanintricatelyregulatedprocess">.Geneticstud-iesofapoptosisinCaenorhabditiseleganshavei-dentifiedced-3andced-4asproapoptoticgenesandced-9asanantiapoptoticgene"=-Severalmam-malianhomologuesofced-3,ced-4andced-9havebeenidentified.Recentstudiesindicatethatthefundamentalapoptosismachinery,whichconsistsofdistincteffectors,inhibitorsandinitiators,hasbeenconservedthroughoutevolution.Thekeyapoptosiseffectorsinmammalsareafamilyofcys-teine-containing,aspartate-spe… 相似文献
994.
以吸附式制冷循环的热力过程为依据,使用Yong分析的方法对连续回热循环作了分析,对循环中各部分Yong损进行了比较,指出了连续回热循环中Yong损的主要部位,并探讨了回热率与吸附床的传热性能对循环Yong效率的影响。 相似文献
995.
Objective Ependymal cells are thought to be the primary source of neural stem cells in the adult central nervous system. The purpose of this study is to examine spatial and temporal profiles of ependymal cell proliferation and migration after focal cerebral ischemia. Methods Eighty male Sprague Dawley rats underwent permanent middle cerebral artery occlusion after injection of 10/μL of 0.2% Dil into the lateral ventricle. Rats were sacrificed and brain sections were acquired for pathological evaluation and laser confocal imaging at day 1,3,7,11,14,21 and 28 after ischemia. Results The density of Dil-labeled cells in the ischemic ipsilateral subventricular zone was significantly higher than that in the control group and these labeled cells dispersed in the ischemic ipsilateral subventricular zone and/or were located in ependyma from day 1 to 11. In the ischemic ipsilateral cortex, some Diilabeled cells occurred in peri-infarction and infarction of parietal region at day14 and peaked at day 21 when some Dil-labeled cell nodules were found in this region. During postischemic day 14--28, a significant decrease in labeled celldensity in the ischemic ipsilateral subventricular zone was coincident with a significant increase in labeled cells density in the cortex (peri-infarction and infarction). Conclusion The results indicate that ependymal cells proliferate and migrate after focal cerebral ischemia in the adult rat brain. 相似文献
996.
Objective To investigate the mechanisms of apoptosis induced in Human leukemia cell line K562 by the combination of indole-3-acetic acid and horseradish peroxidase. Methods Human leukemia cell line K562 were exposed to indole-3-acetic acid (IAA) at 20, 40, 60, 80 or 100 mol/L and horseradish peroxidase(HRP) at 1.2 g/mL for varying times. MTT assay was applied to detect the cell proliferation. Flow cytometry was performed to detect the arrest of cell cycle. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay was used to measure apoptosis. 2, 7-dichlorofluorescin diacetate (DCFH-DA) uptake was measured to determine free radical by confocal microscope. Content of malondiadehyde (MDA) and activity of superoxide dismutase (SOD) were measured by biochemical methods. Results IAA/HRP initiated growth inhibition of K562 cells in a dose- and time-dependent manner. Flow cytometry revealed that cell cycle arrested at G1/G0 after 24 hours treatment. After 72 hours treatment, apoptotic rate of 100 mol/L IAA group increased to 43.9 %, which was 5 times that of control(P〈0.01). Content of MDA and activity of SOD increased respectively in treatments compared with control. Meanwhile, IAA/HRP stimulated the formation of free radical, which was increased by IAA concentration-dependently. Conclusion The combination of IAAand HRP can inhibit the growth of Human leukemia cell line K562 in vitro by inducing apoptosis which is associated with the increase of free radical. The combination of IAA and HRP might be a promising chemopreventive and chemotherapeutic agent against human leukemia. 相似文献
997.
998.
地裂差异沉降可视为有限体系,以西安市为例,根据西安市小寨地裂差异沉降动态特征,采用费尔哈斯特(Verhulst)生物繁殖模型,以监测资料为背景予以灰色系统理论处理和进行沉降旋回期或寿命预测,为西安市修建地铁及运营提供参考资料。 相似文献
999.
1000.
蓄电池用碱性电解液中氯离子的电位法测定王浩波,刘恒明,王骏(应用化学系)关键词:碱性蓄电池;电解质;氯离子;电位测定中图分类号:O657.15目前,我国电力机车普遍采用的是GN-100型碱性蓄电池,内充近5mol/L的KOH做为电解质。其中的杂质Cl... 相似文献