全文获取类型
收费全文 | 64952篇 |
免费 | 2964篇 |
国内免费 | 284篇 |
专业分类
电工技术 | 1024篇 |
技术理论 | 1篇 |
综合类 | 190篇 |
化学工业 | 12601篇 |
金属工艺 | 2341篇 |
机械仪表 | 3548篇 |
建筑科学 | 1453篇 |
矿业工程 | 98篇 |
能源动力 | 2544篇 |
轻工业 | 4694篇 |
水利工程 | 303篇 |
石油天然气 | 337篇 |
武器工业 | 12篇 |
无线电 | 11290篇 |
一般工业技术 | 12713篇 |
冶金工业 | 6366篇 |
原子能技术 | 704篇 |
自动化技术 | 7981篇 |
出版年
2023年 | 638篇 |
2022年 | 989篇 |
2021年 | 1667篇 |
2020年 | 1206篇 |
2019年 | 1216篇 |
2018年 | 1585篇 |
2017年 | 1575篇 |
2016年 | 1919篇 |
2015年 | 1533篇 |
2014年 | 2412篇 |
2013年 | 4001篇 |
2012年 | 3684篇 |
2011年 | 4489篇 |
2010年 | 3442篇 |
2009年 | 3620篇 |
2008年 | 3407篇 |
2007年 | 2803篇 |
2006年 | 2561篇 |
2005年 | 2265篇 |
2004年 | 2112篇 |
2003年 | 1938篇 |
2002年 | 1833篇 |
2001年 | 1469篇 |
2000年 | 1371篇 |
1999年 | 1368篇 |
1998年 | 2446篇 |
1997年 | 1614篇 |
1996年 | 1368篇 |
1995年 | 1062篇 |
1994年 | 808篇 |
1993年 | 752篇 |
1992年 | 530篇 |
1991年 | 524篇 |
1990年 | 452篇 |
1989年 | 440篇 |
1988年 | 345篇 |
1987年 | 301篇 |
1986年 | 265篇 |
1985年 | 251篇 |
1984年 | 215篇 |
1983年 | 168篇 |
1982年 | 159篇 |
1981年 | 139篇 |
1980年 | 140篇 |
1979年 | 109篇 |
1978年 | 106篇 |
1977年 | 134篇 |
1976年 | 168篇 |
1975年 | 83篇 |
1974年 | 81篇 |
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
41.
We developed a new in vitro model of neuronal injury using NT2-N cells to examine the effects of hydrodynamic loading rate on intraneuronal calcium dynamics and lactate dehydrogenase (LDH) release. Our apparatus consisted of a parallel disk viscometer which induced fluid shear stress with well-defined magnitudes and loading rates to cultured cells. We found that the deformation response of the cells was dependent on the severity of the insult, with increased cellular strains generated for higher shear stresses at a constant loading rate. Peak intracellular free calcium concentration correlated with strain, suggesting that mechanical deformation may regulate calcium response. Slowly applied fluid shear stress elicited no response, whereas high loading rates resulted in peak calcium increases 2.9 to 3.6 times baseline values as injury severity was increased. LDH release measured within 5 min after the insult correlated with loading rate. In addition, LDH release continued to increase out to 24 h following high loading rate conditions, demonstrating that the application of fluid shear stress led to prolonged cell damage. The acute response in NT2-N cells subjected to an insult with the CSID is dependent on the loading rate, and these results suggest that initial membrane deformation may trigger subsequent events. 相似文献
42.
Jae-Chul Lee Ho-In Lee Gyeung-Ho Kim Jung-Ill Lee 《Metallurgical and Materials Transactions A》1997,28(5):1251-1259
Interfacial reactions in the squeeze-cast SAE 329 Al alloy reinforced with SAFFIL and carbon fibers were investigated using
scanning and transmission electron microscopy and secondary ion mass spectroscopy. The SiO2 layer added as a binder for the preform of SAFFIL fiber appeared to prevent excessive reaction between the fiber and the
molten Al alloy during casting. In the as-cast composite, the reaction between the SiO2 layer and the Al alloy is considered to produce MgO crystals in the vicinity of SAFFIL fiber. With subsequent heat treatment
of the composite to the T6 condition, small crystals of MgO and MgAl2O4 were found to form at the surface of SAFFIL fiber as a result of interfacial reaction between the SiO2 layer and the Al alloy. Even at the T6 condition, significant reactions between the Al alloy and SAFFIL fiber itself were
not observed, indicating the effective role of the binder layer in suppressing the degradation of SAFFIL fiber by reaction
with the Al alloy. 相似文献
43.
This paper studies a particular single-stage power-factor-correction (PFC) switching regulator employing a discontinuous-conduction-mode (DCM) boost-input cell and a continuous-current-mode (CCM) forward output cell. Although this single-stage PFC regulator can provide a reasonably high power factor when its PFC stage is operating in discontinuous mode, substantial reduction in line-current harmonics is possible by applying a suitable frequency-modulation scheme. This paper derives a frequency-modulation scheme and proposes a practical implementation using a simple translinear analog circuit. A quantitative analysis on the total harmonic distortion (THD) of the line current when the circuit is subject to a limited range of frequency variations is presented along with some considerations for practical design. Experimental data obtained from a prototype confirms the effectiveness of the proposed frequency-modulation scheme. The proposed analog translinear circuit allows custom integrated circuit implementation, making it a viable low-cost solution to the elimination of line-current harmonics in switching regulators 相似文献
44.
45.
Chengkuo Lee Itoh T. Suga T. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(4):553-559
A micromachined lead zirconate titanate (PZT) force sensor for scanning force microscope (SFM) is conceptualized by its piezoelectricity. The fabrication procedure is interpreted, and mechanical characteristics of the micromachined PZT force sensors with various lengths are studied in this paper. A compact SFM is constructed by using the piezoelectric PZT sensor. A very clear image is taken by this SFM. The current study of the micromachined PZT force sensor can be considered as a breakthrough of design of SFM as well as a good example of integrated piezoelectric microdevices 相似文献
46.
47.
Synthesis of poly(gamma-glutamate) metabolites of natural folates and antifolates is a critical process. Folypolyglutamates are essential for cell proliferation. Polyglutamates of glutamate (Glu)-containing antifolates are often critical for their cytotoxic action and are relevant to antifolate resistance. However, the role of polyglutamate synthesis in selectivity is less clear. We have undertaken a research program to further define the significance of polyglutamate metabolism and to devise ways to exploit this metabolism to achieve greater therapeutic selectivity in cancer chemotherapy. This article briefly reviews several approaches tested thus far. Inhibition of folypolyglutamate synthesis should lead to cell death. Current ornithine (Orn)-containing folate-based inhibitors of the enzyme responsible for their synthesis, folypolyglutamate synthetase (FPGS), are poorly transported, apparently because of interference by the protonated delta-amine. Replacement of Orn with 4, 4-difluoroOrn, the delta-amine of which has a much lower pKa and is thus less protonated at physiological pH, was explored. Since it is unclear how polyglutamylation contributes to selectivity, we explored generic means either to eliminate or to enhance polyglutamylation. The data indicate that substitution for Glu in an antifolate by some Glu analogs in which the gamma-COOH is either altered or replaced (e.g., gamma-tetrazole-Glu) leads to loss of both FPGS substrate activity and binding; antifolate target specificity is unchanged, while uptake is actually enhanced. Substitution of 3,3-difluoroGlu for Glu leads to enhanced polyglutamylation (although probably only to the diglutamate), retention of target specificity, and at least equal uptake. Comparative studies of the same antifolate containing different replacements for Glu, such as gamma-tetrazole-Glu (no polyglutamylation) or 3,3-difluoroGlu (enhanced polyglutamylation), will be useful in exploring the role and significance of polyglutamylation. 相似文献
48.
Since electronic switching systems usually require very strict reliability requirements as well as good performance objectives, we need to jointly analyse the performance and reliability of switching systems. In this paper, we compare conventional time–space–time switches with single space switches with those with multiple separated space switches, from the viewpoints of reliability and performance. We consider time–space–time switching networks which consist of N incoming time switches, i.e. one NxN space switch, two (N/2)x(N/2) space switches, and four (N/4)x(N/4) space switches. We introduce a Markov reliability model to study the effect of failures and analyse the reliability and performance of three different types of switching networks in terms of average blocking probability and the mean time to unreliable operation, as we vary the offered traffic. As a result, T–S–T switching networks with multiple separated space switches exhibit better performance and reliability than those with single space switches. 相似文献
49.
50.
L. K. Seah
J. Rhodes
B. S. Lim
《Thin》1993,16(1-4):145-177Turning the free edge of an unstiffened flange inwards or outwards to form a ‘lip’, can substantially improve the local buckling resistance of a member. The lip is the most common type of edge stiffener used in cold-rolled, thin-walled sections. In this paper the behaviour of plate elements of thin-walled sections stiffened by compound lips (i.e. lips which are folded twice to form ‘lips on the lips’) is examined both theoretically and experimentally. An outline of a series of tests on compound edge-stiffened thin-walled sections of various geometries is given and some load-end compression displacement paths are compared with the theoretical predictions. Reasonably good agreement is obtained between the experimental and theoretical results. Comparisons of the theoretical predictions with experimental results of other researchers are also presented in this paper. 相似文献