首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1475篇
  免费   55篇
公路运输   208篇
综合类   637篇
水路运输   298篇
铁路运输   323篇
综合运输   64篇
  2024年   2篇
  2023年   4篇
  2022年   24篇
  2021年   30篇
  2020年   31篇
  2019年   18篇
  2018年   11篇
  2017年   17篇
  2016年   24篇
  2015年   35篇
  2014年   65篇
  2013年   68篇
  2012年   115篇
  2011年   145篇
  2010年   112篇
  2009年   101篇
  2008年   128篇
  2007年   125篇
  2006年   149篇
  2005年   65篇
  2004年   45篇
  2003年   38篇
  2002年   21篇
  2001年   29篇
  2000年   28篇
  1999年   21篇
  1998年   15篇
  1997年   10篇
  1996年   9篇
  1995年   11篇
  1994年   4篇
  1993年   4篇
  1992年   5篇
  1991年   6篇
  1990年   5篇
  1989年   4篇
  1988年   4篇
  1986年   1篇
  1984年   1篇
排序方式: 共有1530条查询结果,搜索用时 15 毫秒
991.
Objective To study the changes of resistin and the relationship between resistin and other indexes in patients with type 2 diabetes mellitus. Methods Seventy patients with type 2 diabetics were chosen and divided into three groups according to weight index and 15 healthy persons were chosen as controls. ELISA was adopted to determine resistin concentration. Oxydase method was adopted to measure blood sugar. Radio immunoassay was used to measure insulin level. Results Resistin concentration of patient groups r(23.06±9.34)μg/L ] was significantly higher than that of control group [( 9. 89±4. 47) μg/L, P<0. 01 ]. Resistin was positively related to body mass index (r = 0.4 467) and fat distribution (r= 0. 3 314, P<0.01). Resistin was also positively correlated to the area under the curve of sugar tolerance (r=0. 7 063, P<0. 01), negatively related to the insulin resistance index (r=-0. 4 154, P<0.01), but not related to area under the curve of synchronous insulin release (P>0. 05). And a formula was obtainted : Resistin=- 15. 869 0. 339×GAUC 0.790×BMI. Conclusion The level of reslstin in type 2 diabetics is higher. And reslstin is related to other factors. This formula is of great significance for clinical study of diabetic's disor- ders.  相似文献   
992.
从COX-2的生物学特性,可知促进口腔鳞癌侵袭转移的机制为促进肿瘤新生血管的生成、刺激肿瘤细胞的增殖、抑制肿瘤细胞的凋亡、促进肿瘤的浸润和转移、抑制机体的免疫反应、参与前致癌物的活化。(可应用COX-2抑制剂防治肿瘤)  相似文献   
993.
ATS具有结构简单、布置方便、节省能源、保护环境、可提高发动机工作寿命等诸多优点。近年来,在客车发动机冷却系统中得到越来越多的应用。目前ATS的不足之处在于价格昂贵,在大功率发动机的配套上使用受限。本文是ATS在CA6121URN2城市客车冷却系设计中的应用实例。  相似文献   
994.
在对B480QZR和40Cr两种不同钢材的焊接性能分析的基础上,研究和制定了B480QZR+40Cr异种钢焊接接头的工艺方案,通过采用CO2气体保护焊的焊接实践,或得了较好的焊接接头质量,满足了传动轴产品的设计和使用要求。  相似文献   
995.
给定一个小矩形块集和一个固定宽度而高度变化的大矩形的二维布局问题,就是将这矩形集中的所有矩形正交布置于这个大矩形中,并且保证矩形块之间不发生重叠,目的就是使得布局后这个大矩形块的高度最小.本文提出通过DROP或DROPF(DROP FILL)的启发式解码策略与交叉熵算法相结合求解该类问题.试验结果显示,算法稳定有效,较经典元启发式算法在提高空间利用率上有较大提高.  相似文献   
996.
重组激活基因(recombination activating gene)1和2编码的RAG1和RAG2蛋白通过对V(D)J重组起始阶段的调节作用,使得抗原受体基因重排严格地按组织、细胞发育阶段进行。本文将就RAG蛋白结合和催化断裂抗原受体基因机制作简要介绍。这些新发现明确了体内V(D)J重组发生的部位以及其出现错误时可能发生的地方,提示V(D)J重组机制不仅对淋巴细胞正常发育起关键性作用,而且对基因组不稳定性和淋巴系统恶性肿瘤的发生也具有重要意义。  相似文献   
997.
目的探讨核苷酸切除修复交叉互补基因(excision repair cross complementation 1,ERCC1)、3型β微管蛋白编码基因(ClassⅢβ-tubulin,TUBB3)和DNA拓扑异构酶Ⅱα(TopoisomeraseⅡA,TOP2A)mRNA在小儿肾母细胞瘤组织中的表达及临床意义。方法收集18例肾母细胞瘤组织标本及14例癌旁组织标本,通过分支-DNA液相芯片法检测标本中ERCC1、TUBB3、TOP2A mRNA的表达水平,并分析上述基因表达与肾母细胞瘤临床病理的关系。结果肾母细胞瘤组织中ERCC1、TUBB3和TOP2A高表达率分别为44.4%、50.0%和66.7%,均显著高于癌旁组织的7.1%、7.1%和14.3%,差异有统计学意义(P<0.05)。ERCC1高表达率与肾母细胞瘤临床分期和病理类型有关(P<0.05),TOP2A高表达率与肾母细胞瘤临床分期相关(P<0.05)。结论小儿肾母细胞瘤组织中ERCC1、TUBB3和TOP2A mRNA表达高于癌旁组织,可能参与肾母细胞瘤的发生发展过程。  相似文献   
998.

Transport accounts for about 25% of global CO 2 emissions. Transport activities are on the rise in the coming decades. Would associated CO 2 emissions move upwards as well, and at what rate? The present paper explores the future of these CO 2 emissions, starting from four scenarios for global transport. Considering fuel consumption, energy efficiencies in transport, occupancy rates of transport means, size of cars on the market, and possible environmental policies we find CO 2 emissions are persistently increasing, especially in the less wealthy areas of the world. In Europe, policies that attempt to control mobility and also limit CO 2 emissions may succeed in reducing emissions growth by about 30%. Efforts to increase energy efficiency of transport, in particular road transport, would contribute most to such reduction.  相似文献   
999.
使用热硫化热补方式,修理皮带撕裂工作量大,维修时间长,严重影响企业生产.新的皮带机修补技术T2胶枪应运而生.阐述了该T2胶枪的技术原理、T2胶枪的结构、特点和使用工序.通过应用实例,证明该技术取得良好的经济效益,为用户提供了一种创新的维修理念.  相似文献   
1000.
The variations in the expression of two hydrolytic ectoenzymes (leucine aminopeptidase – LA – and β glucosidase — BG) were studied in the southern Tyrrhenian Sea during spring 2004. This area is characterised by a complex morphology and hydrodynamism, which generate significant differences between different sectors, particularly in the 0–100 m layer. However, the area generally exhibits oligotrophic features such as low autotrophic pigment and organic matter concentrations and a higher bacterial biomass than the phytoplanktonic one. Despite this general bottom-up pressure, adaptations by the microbial consumers were indicated by the ectoenzymatic activities and by the relationships between the enzymes, their organic substrates and their producers (namely the bacteria). In particular, bacteria were able to exploit the inorganic N supply (nitrite + nitrate provided by irregular intrusions of intermediate waters) to escape the bottom-up limitation and produce enzymes such as BG devoted to the degradation of cellulose remnants and, therefore, also able to take advantage on this refractory organic matter. In the 200–800 m layer, where trophic limitation was strong due to the low values of potentially-labile organic matter (namely proteins), the peculiar hydrodynamism led to the formation of nepheloid layers rich in organic matter, which provided the bacteria with substrates and allowed the development of a significant correlation between LA activity and its own organic substrate. Furthermore, a reduction of the bottom-up pressure was also indicated by a higher mean bacteria cell size in the entire water column of the central and eastern sectors, and a significantly increased expression of BG related to the increase in the cell size.The ectoenzymatic activities, therefore, suggested that the southern Tyrrhenian Sea should be considered as a mosaic of subsystems, where the peculiar hydrological features stimulate bacterial adaptations and enhance the channelling of energy embedded in refractory materials into the food web.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号