全文获取类型
收费全文 | 72774篇 |
免费 | 6846篇 |
国内免费 | 3615篇 |
专业分类
电工技术 | 4664篇 |
技术理论 | 11篇 |
综合类 | 4946篇 |
化学工业 | 12041篇 |
金属工艺 | 3896篇 |
机械仪表 | 4433篇 |
建筑科学 | 5518篇 |
矿业工程 | 1970篇 |
能源动力 | 2217篇 |
轻工业 | 4683篇 |
水利工程 | 1402篇 |
石油天然气 | 3677篇 |
武器工业 | 817篇 |
无线电 | 9010篇 |
一般工业技术 | 8768篇 |
冶金工业 | 3791篇 |
原子能技术 | 872篇 |
自动化技术 | 10519篇 |
出版年
2024年 | 394篇 |
2023年 | 1242篇 |
2022年 | 2438篇 |
2021年 | 3342篇 |
2020年 | 2468篇 |
2019年 | 2018篇 |
2018年 | 2204篇 |
2017年 | 2347篇 |
2016年 | 2163篇 |
2015年 | 3016篇 |
2014年 | 3585篇 |
2013年 | 4386篇 |
2012年 | 4622篇 |
2011年 | 4851篇 |
2010年 | 4314篇 |
2009年 | 4126篇 |
2008年 | 4085篇 |
2007年 | 3840篇 |
2006年 | 3847篇 |
2005年 | 3282篇 |
2004年 | 2505篇 |
2003年 | 2110篇 |
2002年 | 2191篇 |
2001年 | 1967篇 |
2000年 | 1743篇 |
1999年 | 1739篇 |
1998年 | 1506篇 |
1997年 | 1308篇 |
1996年 | 1089篇 |
1995年 | 910篇 |
1994年 | 697篇 |
1993年 | 604篇 |
1992年 | 444篇 |
1991年 | 354篇 |
1990年 | 269篇 |
1989年 | 258篇 |
1988年 | 191篇 |
1987年 | 120篇 |
1986年 | 121篇 |
1985年 | 91篇 |
1984年 | 50篇 |
1983年 | 44篇 |
1982年 | 55篇 |
1981年 | 42篇 |
1980年 | 43篇 |
1979年 | 27篇 |
1977年 | 29篇 |
1976年 | 22篇 |
1974年 | 17篇 |
1973年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
用数值方法对泊松方程求解时,往往要对迭代的中间结果作适当的压缩处理,以加快收敛速度和避免溢出。本文提出一种单一指数因子的非线性压缩法。一维数值模拟结果表明,平衡态时不必进行压缩处理。而在非平衡态时,最佳指数压缩因子的大小与杂质浓度有关。参考本文给出的数值计算结果,根据给定器件的杂质浓度分布,选定一最佳指数压缩因子,可以使泊松方程的数值求解过程具有最快的收敛速度。 相似文献
82.
文章通过对洋浦电厂海水冷却箱涵沟槽预裂(深孔)爆破技术的研究,提出了复杂地质条件下合理的控制爆破参数及微差起爆方式。针对砂层地段难以钻、爆的特点。笔者进行了有益的探讨。 相似文献
83.
Images stored in resist for soft X-ray lithography or microradiography were found to show a background noise which limits the resolution. This is due to the statistically variable spatial distribution of the photons incident on the resist surface. An estimate of the fundamental noise-limited resolution has been made from the experimental measurement of photon flux incident on the surface and the accurate development rate curves. Monochromatic radiation from a synchrotron source was used. 相似文献
84.
Wireless ATM networks require efficient mobility management to cope with frequent mobile handoff and rerouting of connections. Although much attention has been given in the literature to network architecture design to support wide-area mobility in public ATM networks, little has been done to the important issue of user mobility estimation and prediction to improve the connection reliability and bandwidth efficiency of the underlying system architecture. This paper treats the problem by developing a hierarchical user mobility model that closely represents the movement behavior of a mobile user, and that, when used with appropriate pattern matching and Kalman filtering techniques, yields an accurate location prediction algorithm, HLP, or hierarchical location prediction, which provides necessary information for advance resource reservation and advance optimal route establishment in wireless ATM networks 相似文献
85.
The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical. 相似文献
86.
扫描电子显微研究表明,化学汽相沉积的金刚石薄膜中晶粒大小比较均匀。但随着沉积时间和薄膜厚度的增加,晶粒逐渐变大,且每一层内,存在少量的大金刚石颗粒,讨论了晶粒尺寸变化和大晶粒形成的原因和机制。 相似文献
87.
88.
Apoptosis in the failing human heart 总被引:1,自引:0,他引:1
G Olivetti R Abbi F Quaini J Kajstura W Cheng JA Nitahara E Quaini C Di Loreto CA Beltrami S Krajewski JC Reed P Anversa 《Canadian Metallurgical Quarterly》1997,336(16):1131-1141
BACKGROUND: Loss of myocytes is an important mechanism in the development of cardiac failure of either ischemic or nonischemic origin. However, whether programmed cell death (apoptosis) is implicated in the terminal stages of heart failure is not known. We therefore studied the magnitude of myocyte apoptosis in patients with intractable congestive heart failure. METHODS: Myocardial samples were obtained from the hearts of 36 patients who underwent cardiac transplantation and from the hearts of 3 patients who died soon after myocardial infarction. Samples from 11 normal hearts were used as controls. Apoptosis was evaluated histochemically, biochemically, and by a combination of histochemical analysis and confocal microscopy. The expression of two proto-oncogenes that influence apoptosis, BCL2 and BAX, was also determined. RESULTS: Heart failure was characterized morphologically by a 232-fold increase in myocyte apoptosis and biochemically by DNA laddering (an indicator of apoptosis). The histochemical demonstration of DNA-strand breaks in myocyte nuclei was coupled with the documentation of chromatin condensation and fragmentation by confocal microscopy. All these findings reflect apoptosis of myocytes. The percentage of myocytes labeled with BCL2 (which protects cells against apoptosis) was 1.8 times as high in the hearts of patients with cardiac failure as in the normal hearts, whereas labeling with BAX (which promotes apoptosis) remained constant. The near doubling of the expression of BCL2 in the cardiac tissue of patients with heart failure was confirmed by Western blotting. CONCLUSIONS: Programmed death of myocytes occurs in the decompensated human heart in spite of the enhanced expression of BCL2; this phenomenon may contribute to the progression of cardiac dysfunction. 相似文献
89.
90.
Plasminogen activator-inhibitor C-1 (PAI-1) plays a critical role in the regulation of fibrinolysis, serving as the primary inhibitor of tissue-type plasminogen activator. Elevated levels of PAI-1 are a risk factor for recurrent myocardial infarction, and locally increased PAI-1 expression has been described in atherosclerotic human arteries. Recent studies have shown that the administration of angiotensin converting enzyme inhibitors reduces the risk of recurrent myocardial infarction in selected patients. Since angiotensin II (Ang II) has been reported to induce PAI-1 production in cultured astrocytes, we have hypothesized that one mechanism that may contribute to the beneficial effect of angiotensin converting enzyme inhibitors is an effect on fibrinolytic balance. In the present study, we examined the interaction of Ang II with cultured bovine aortic endothelial cells (BAECs) and the effects of this peptide on the production of PAI-1. 125I-Ang II was found to bind to BAECs in a saturable and specific manner, with an apparent Kd of 1.4 nM and Bmax of 74 fmol per mg of protein. Exposure of BAECs to Ang II induced dose-dependent increases in PAI-1 antigen in the media and in PAI-1 mRNA levels. Induction of PAI-1 mRNA expression by Ang II was not inhibited by pretreating BAECs with either Dup 753 or [Sar1, Ile8]-Ang II, agents that are known to compete effectively for binding to the two major angiotensin receptor subtypes. These data indicate that Ang II regulates the expression of PAI-1 in cultured endothelial cells and that this response is mediated via a pharmacologically distinct form of the angiotensin receptor. 相似文献