全文获取类型
收费全文 | 4954篇 |
免费 | 69篇 |
国内免费 | 72篇 |
专业分类
电工技术 | 30篇 |
综合类 | 52篇 |
化学工业 | 129篇 |
金属工艺 | 23篇 |
机械仪表 | 43篇 |
建筑科学 | 53篇 |
矿业工程 | 13篇 |
能源动力 | 8篇 |
轻工业 | 106篇 |
水利工程 | 15篇 |
石油天然气 | 49篇 |
武器工业 | 6篇 |
无线电 | 90篇 |
一般工业技术 | 64篇 |
冶金工业 | 4260篇 |
原子能技术 | 1篇 |
自动化技术 | 153篇 |
出版年
2024年 | 4篇 |
2023年 | 7篇 |
2022年 | 10篇 |
2021年 | 22篇 |
2020年 | 15篇 |
2019年 | 13篇 |
2018年 | 25篇 |
2017年 | 22篇 |
2016年 | 18篇 |
2015年 | 26篇 |
2014年 | 28篇 |
2013年 | 39篇 |
2012年 | 46篇 |
2011年 | 55篇 |
2010年 | 51篇 |
2009年 | 47篇 |
2008年 | 49篇 |
2007年 | 43篇 |
2006年 | 50篇 |
2005年 | 47篇 |
2004年 | 30篇 |
2003年 | 26篇 |
2002年 | 17篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 147篇 |
1998年 | 1393篇 |
1997年 | 740篇 |
1996年 | 473篇 |
1995年 | 271篇 |
1994年 | 228篇 |
1993年 | 265篇 |
1992年 | 48篇 |
1991年 | 55篇 |
1990年 | 65篇 |
1989年 | 56篇 |
1988年 | 64篇 |
1987年 | 63篇 |
1986年 | 49篇 |
1985年 | 59篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 25篇 |
1980年 | 28篇 |
1979年 | 4篇 |
1978年 | 9篇 |
1977年 | 82篇 |
1976年 | 198篇 |
1975年 | 7篇 |
1955年 | 3篇 |
排序方式: 共有5095条查询结果,搜索用时 15 毫秒
81.
The small GTP-binding proteins Rac, Rho, and Cdc42 were shown to mediate a variety of signaling pathways including cytoskeletal rearrangements, cell-cycle progression, and transformation. Key to the proper function of these GTP-binding proteins is an efficient shut-off mechanism that ensures the decay of the signal. Regulatory proteins termed GAPs (GTPase-activating proteins) enhance the intrinsic GTP hydrolysis of the GTP-binding proteins, thereby ensuring signal termination. We have used site-specific mutagenesis to elucidate the limit domain for GAP activity in Cdc42-GAP, and show that in addition to the known GAP-homology domain (three conserved boxes), a C-terminal region outside that domain is also essential for GAP activity. In addition, we have replaced the conserved arginine (Arg305), which was suggested by structural studies to be a key catalytic residue, with an alanine and found that the R305A Cdc42-GAP mutant has a greatly diminished catalytic capacity but is still able to bind Cdc42 with high affinity. Thus, a key catalytic role for this residue is confirmed. However, we also present evidence for the involvement of an additional residue(s), since the R305A Cdc42-GAP mutant still exhibits measurable activity. Some of this residual activity might result from a neighboring arginine, since a double mutant R305A/R306A shows a further decrease in catalytic activity. 相似文献
82.
GA Livesay H Fujie S Kashiwaguchi DA Morrow FH Fu SL Woo 《Canadian Metallurgical Quarterly》1995,23(4):467-474
The in situ forces and their distribution within the human anterior cruciate ligament (ACL) can clarify this ligament's role in the knee and help to resolve controversies regarding surgical treatment of ACL deficiency. We used a universal force-moment sensor (UFS) to determine the magnitude, direction, and point of application of the in situ forces in the ACL in intact human cadaveric knees. Unlike previous studies, this approach does not require surgical intervention, the attachment of mechanical devices to or near the ACL, or a priori assumptions about the direction of in situ force. Anterior tibial loads were applied to intact knees, which were limited to 1 degree of freedom at 30 degrees flexion. The in situ forces developed in the ACL were lower than the applied force for loads under 80 N, but larger for applied loads of more than 80 N. The direction of the force vector corresponded to that of the anteromedial (AM) portion of the ACL insertion on the tibial plateau. The point of force application was located in the posterior section of the anteromedial portion of the tibial insertion site. The anterior and posterior aspects of the anteromedial portion of the ACL supported 25% and 70% of the in situ force, respectively, with the remainder carried by the posterolateral portion. We believe that the data obtained with this new UFS methodology improves our understanding of the role of the ACL in knee function, and that this methodology can be easily extended to study the function of other ligaments. 相似文献
83.
84.
Yasuyuki Matsumura Shigeru Sugiyama Hiromu Hayashi John B. Moffat 《Catalysis Letters》1994,30(1-4):235-240
The introduction of small quantities of lead into calcium hydroxyapatite catalysts produces marked increases in the selectivity to C2+ hydrocarbons, while the conversion of methane remains relatively constant. Small surface concentrations of lead are sufficient to achieve C2+ selectivities of 80 and 90%, with oxygen and nitrous oxide, respectively, in contrast with 18 and 46%, respectively, obtained in the absence of lead. Since surface concentration of lead species sufficient to stabilize pairs of methyl radicals in close proximity to each other would be expected to facilitate the formation of C2 hydrocarbons, an ensemble effect appears to be extant. 相似文献
85.
Shen TJ; Ho NT; Zou M; Sun DP; Cottam PF; Simplaceanu V; Tam MF; Bell DA Jr; Ho C 《Protein engineering, design & selection : PEDS》1997,10(9):1085-1097
A hemoglobin expression system in Escherichia coli is described. In order
to produce authentic human hemoglobin, we need to co-express both
methionine aminopeptidase and globin genes under the control of a strong
promoter. We have constructed three plasmids, pHE2, pHE4 and pHE7, for the
expression of human normal adult hemoglobin and a plasmid, pHE9, for the
expression of human fetal hemoglobin, in high yields. The globin genes can
be derived from either synthetic genes or human globin cDNAs. The extra
amino-terminal methionine residues of the expressed globins can be removed
by the co-expressed methionine aminopeptidase. The heme is inserted
correctly into the expressed alpha- globin from our expression plasmids. A
fraction (approximately 25%) of the heme is not inserted correctly into the
expressed beta- or gamma- globin. However, the incorrectly inserted hemes
can be converted into the correct conformation by carrying out a simple
oxidation-reduction process on the purified hemoglobin molecule. We have
investigated the functional properties of the expressed hemoglobins by
measuring their oxygen-binding properties and their structural features by
obtaining their 1H-NMR spectra. Our results show that authentic human
normal adult and fetal hemoglobins can be produced from our expression
plasmids in E. coli and in high yields. Our expression system allows us to
design and to produce any recombinant hemoglobins needed for our research
on the structure-function relationship in hemoglobin.
相似文献
86.
玻纤增强聚苯硫醚复合材料的增韧研究 总被引:1,自引:0,他引:1
针对玻纤增强聚苯硫醚材料韧性差的问题,对聚苯硫醚傲璃纤维复合体系的增韧进行了研究,考察了玻纤、改性聚合物、有机超细粒子对复合材料力学性能的影响。采用基体增韧(预增韧)与有机超细粒子增韧技术,在保持复合材料拉伸强度和模量的同时,较大地提高了冲击强度,获得了综合力学性能优异的纤维增强聚苯硫醚材料。 相似文献
87.
一种新型混沌扩频卫星隐蔽通信算法 总被引:1,自引:0,他引:1
针对Logistic及改进型混沌映射只存在唯一满映射点的缺陷,设计了一种新型混沌映射函数,拓宽了满映射参数的取值范围,进而提高了混沌序列遍历性,并基于相干混沌移位键控技术,构建了新型混沌扩频卫星隐蔽通信系统。分别对新型混沌序列的随机性、相关性和平衡性,以及隐蔽通信系统的误码率进行了仿真分析,结果表明:新型混沌映射比已有映射遍历性更好,平衡性更高。原业务信号与扩频调制信号的功率比值大于20dB时,原业务接收机接收误码率基本不受影响。扩频因子为80、信道信噪比大于6dB时,隐蔽接收机误码率小于10-3,能满足一定的隐蔽通信要求。 相似文献
88.
On exposure of the solid 12-tungstophosphoric acid (H3PW12O40· nH2O (HPW)) to NO2 at 150°C NO2 is sorbed, while HNO3 is produced. NO2 is found to penetrate into the bulk structure whose reaction occurs with the water hydrogen-bonded to the protons to produce HNO3. The NO2 retained on the catalyst is stoichiometrically equivalent to the total number of protons on and in the solid. This, together with infrared spectra, suggests that NO2 is associating (1/1) with the protons, producing a nitronium salt of the solid acid. Although insignificant quantities of NO are taken up by HPW, after exposure to NO2, approximately stoichiometric quantities of NO are sorbed and the infrared spectra show the presence of N2O3. 相似文献
89.
分析了Ka频段移动信道特性,建立了Ka频段自适应Turbo编码-OFDM传输体制(ATC-OFDM),并提出了3种自适应方案和相应算法。通过新传输体制中的自适应功率、自适应调制和自适应功率调制同固定分配功率和调制方案的TC-OFDM传输体制相比,在不同的天气环境下,其系统误码性能有着明显的改善。系统的吞吐量在信噪比大于8dB时,相比于BPSK调制,可提高2bps/Hz,验证了该传输体制的可行性和有效性。 相似文献
90.
The oxidative coupling of methane has been tested over alkali- and alkaline earth-phosphate catalysts at 700 and 775 °C with and without the introduction of a small quantity of tetrachloromethane (TCM) to the feedstream. In general, the conversion of methane was enhanced by the addition of TCM but the effect on selectivity was dependent on the catalyst being examined. The selectivity to C2 and higher hydrocarbons and that to oxidation products have been shown to have a dependence on the cation radius/charge ratio. 相似文献