全文获取类型
收费全文 | 605559篇 |
免费 | 7130篇 |
国内免费 | 1688篇 |
专业分类
电工技术 | 11104篇 |
综合类 | 705篇 |
化学工业 | 91602篇 |
金属工艺 | 23798篇 |
机械仪表 | 18575篇 |
建筑科学 | 14480篇 |
矿业工程 | 3358篇 |
能源动力 | 14644篇 |
轻工业 | 54284篇 |
水利工程 | 6325篇 |
石油天然气 | 10968篇 |
武器工业 | 104篇 |
无线电 | 70106篇 |
一般工业技术 | 115462篇 |
冶金工业 | 114582篇 |
原子能技术 | 13383篇 |
自动化技术 | 50897篇 |
出版年
2021年 | 4903篇 |
2019年 | 4773篇 |
2018年 | 8003篇 |
2017年 | 8117篇 |
2016年 | 8591篇 |
2015年 | 5513篇 |
2014年 | 9353篇 |
2013年 | 26529篇 |
2012年 | 15187篇 |
2011年 | 20768篇 |
2010年 | 16609篇 |
2009年 | 18567篇 |
2008年 | 19406篇 |
2007年 | 19528篇 |
2006年 | 17366篇 |
2005年 | 15761篇 |
2004年 | 15158篇 |
2003年 | 14806篇 |
2002年 | 14325篇 |
2001年 | 14177篇 |
2000年 | 13760篇 |
1999年 | 13898篇 |
1998年 | 33298篇 |
1997年 | 23761篇 |
1996年 | 18680篇 |
1995年 | 14279篇 |
1994年 | 12782篇 |
1993年 | 12507篇 |
1992年 | 9693篇 |
1991年 | 9140篇 |
1990年 | 9045篇 |
1989年 | 8760篇 |
1988年 | 8429篇 |
1987年 | 7397篇 |
1986年 | 7260篇 |
1985年 | 8407篇 |
1984年 | 7909篇 |
1983年 | 7214篇 |
1982年 | 6722篇 |
1981年 | 6975篇 |
1980年 | 6538篇 |
1979年 | 6452篇 |
1978年 | 6341篇 |
1977年 | 7243篇 |
1976年 | 9388篇 |
1975年 | 5668篇 |
1974年 | 5488篇 |
1973年 | 5625篇 |
1972年 | 4576篇 |
1971年 | 4232篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
The selective reduction of nitrogen oxides by hydrocarbons on a Pt catalyst has been investigated. Some variations are observed between different reductants. However, the most significant result is the observation that no N2O is observed as a by-product when toluene is used as a reductant. 相似文献
992.
Business process redesign (BPR) has been widely touted as a means for leveraging the power of information technology to change business processes radically, resulting in substantial improvements in organizational effectiveness and efficiency. This study represents an early attempt at examining the strategic nature of the phenomenon. Because BPR is radical change and has the ability to alter both the competitive and operational nature of the corporation, it is critical that BPR be coordinated with corporate strategy. Through a survey of information system executives, the relationships between corporate competitive strategy and three types of BPR – intrafunctional, interfunctional and interorganizational – were examined. It was found that organizations following a cost strategy tended to do more BPR projects on interfunctional processes. Furthermore, the positive effect of a cost strategy on interfunctional BPR was found to be strengthened by the degree of integration of IS and business planning. It was also found that while interorganizational BPR was not related to a specific strategy, it was strongly related directly with IS–business planning integration. 相似文献
993.
M Hessing D Kanters H Takeya C van 't Veer TM Hackeng S Iwanaga BN Bouma 《Canadian Metallurgical Quarterly》1993,317(3):228-232
Human C4b-binding protein (C4BP) functions as a cofactor to factor I in the degradation of C4b and accelerates the decay rate of the C4b2a complex. In this study we describe a monoclonal antibody directed against the alpha-chain of C4BP that inhibits the binding of C4b to C4BP. In order to identify the structural domain of the alpha-chain of C4BP that interacts with C4b, tryptic fragments of C4BP were generated. Amino acid sequence analysis of the fragments revealed that the residues Ser333-Arg356 of the alpha-chain of C4BP contain the epitope of this antibody, and as a consequence, that this part of the alpha-chain of C4BP is likely to be involved in the interaction with C4b. 相似文献
994.
D Schmidt P Wischmeyer I Leuschner E Sprenger E Langenau D von Schweinitz D Harms 《Canadian Metallurgical Quarterly》1993,72(10):2914-2919
BACKGROUND: In several types of tumors, including hepatocellular carcinoma, prognosis could be correlated with DNA ploidy. Few studies have been performed on hepatoblastoma with contradictory results. METHODS: Twenty-nine cases of nonpretreated hepatoblastoma were studied with flow cytometry and image cytometry for DNA index and proliferation index using paraffin-embedded tissue. RESULTS: Twenty-three (79.9%) tumors were diploid, and 6 (20.7%) were aneuploid (hyperdiploid). Patients with diploid tumors were younger than those with aneuploid tumors. With regard to stage, diploid tumors were almost equally distributed among stages (tumor, lymph node metastases, distant metastases), whereas aneuploid tumors tended to occur in higher stages (tumor, lymph node metastases, distant metastases). Diploid tumors had clearly a better prognosis than aneuploid tumors, although the difference was not statistically significant (flow cytometry, P = 0.06; image cytometry, P = 0.16). A more favorable prognosis was also noted for hepatoblastomas with low-proliferation index (< or = 7%), but the difference from tumors with high-proliferation index (> 7%) again was not statistically significant (P = 0.16). CONCLUSIONS: Although no statistically significant differences in prognosis between hepatoblastomas with diploid and aneuploid DNA content, respectively, were found, there is a clear tendency that diploid hepatoblastomas behave more favorably. The same is true for hepatoblastomas with low-proliferation index. 相似文献
995.
J. M. Kremer A. A. Shabana G. E. O. Widera 《International journal for numerical methods in engineering》1993,36(1):17-42
This investigation concerns itself with the computer implementation of the dynamic formulation of thin laminated composite plates consisting of layers of orthotropic laminae that undergo large arbitrary rigid body displacements and small elastic deformations. A finite element preprocessor computer program is developed to automatically generate the invariants of the laminae, which may have arbitrary orientations. The laminae invariants are then used to obtain the invariants of the elements and the composite laminated plate. The consistent and lumped mass formulations of the invariants of motion of composite plates are compared and it is concluded that the two methods are comparable, if a fine enough finite element mesh is used. The structure of the dynamic equations of motion, based on the formulation presented in Part I of this paper, is examined. Non-linear centrifugal and Coriolis forces arising as the result of the finite rotations of the laminae are defined, and the solution schemes of the resulting non-linear differential equations of motion are discussed. Numerical examples illustrating the differences between homogeneous isotropic and laminated composite plates are presented. An RSSR (Revolute-Spherical-Spherical-Revolute) mechanism is used in the numerical examples, with the coupler modelled as a laminated plate flexible body. It is found that the inertia of the plate contributed greatly to the transverse deformation. The effects of laminae orientation is also investigated. 相似文献
996.
In this paper, we discuss the problem of quality control with an unreliable machine which produces defects at a rate of Λ0, per unit when in-control and a rate of Lambda; 1, when out-of-control (where Λ1 Λ 0). Every h time periods, we sample n units, count the number of defects, and (using a process based on a Shewart c-chart) test the hypothesis that the machine is in control by comparing the total number of defects to an upper control limit (UCL). More important, we introduce the concept that a buffer inventory which immediately follows the unreliable machine may reduce expected total costs. This buffer serves to delay the movement of items from the unreliable machine to the next stage of the production process. In this way, we can isolate and repair most defective items before they are embedded in a product downstream or sold to customers where repair is more costly. To search for the optimal control policy, we find bounds for n, h, and UCL; given values for these variables, we show how the optimal buffer size can be determined directly. Numerical results illustrate the magnitude of potential savings. 相似文献
997.
998.
A value-based test is presented for economic screening of electric utility demand-side management (DSM) programs. The widely used least cost test is valid if the programs do not alter the amount or value of energy services provided to customers. But, in general, DSM programs have such effects and, as a result, the value consumers receive is changed. A more general economic efficiency test, the most value test, provides a practical method for considering the effects of DSM on customer value. The version presented allows for multiple load periods and can account for rate impacts on several customer classes. Four typical DSM programs are evaluated as illustrations 相似文献
999.
OBJECTIVE: To compare the efficacy and toxicity of three patient-controlled analgesia (PCA) morphine regimens. DESIGN: A prospective, randomized, pilot study of three PCA morphine regimens: (1) 1 mg with 6-minute lockout (n = 10), (2) 2 mg with 12-minute lockout (n = 12), and (3) 2 mg with 20-minute lockout (n = 12). SETTING: Large teaching institution. PARTICIPANTS: Thirty-four patients undergoing cholecystectomy or hysterectomy. MAIN OUTCOME MEASURES: Pain scores (0 = no pain, 1 = mild pain, 2 = moderate pain, 3 = severe pain), sedation scores, analgesic consumption, and patient attempts (patient activation of PCA device) and injections (doses actually delivered) were evaluated using analysis of covariance. Distribution of pain and sedation scores and adverse effects were assessed using Fisher's exact test. RESULTS: Data on 24 patients were evaluable. Six patients withdrew for poor pain control (2 in group 1, 1 in group 2, and 3 in group 3). Three other patients withdrew because of adverse effects and 1 withdrew because of pump problems. Mean morphine consumption did not differ significantly among the groups. Distribution of pain and sedation scores and the number of patients with nausea were similar across treatment groups. The mean injection to attempt ratio was significantly smaller in group 3 (0.71 +/- 0.11) compared with groups 1 and 2 (0.9 +/- 0.06 and 0.83 +/- 0.09, respectively; p = 0.001). Adverse events occurred similarly among treatment groups. CONCLUSIONS: No significant differences in the efficacy or toxicity of the three morphine PCA regimens were identified. 相似文献
1000.
Schmidt Frank L.; Law Kenneth; Hunter John E.; Rothstein Hannah R.; Pearlman Kenneth; McDaniel Michael 《Canadian Metallurgical Quarterly》1993,78(1):3
Using a large database, this study examined 3 refinements of validity generalization procedures: (1) a more accurate procedure for correcting the residual standard deviation (SD) for range restriction to estimate SDp, (2) use of r? instead of study-observed rs in the formula for sampling error variance, and (3) removal of non-Pearson rs. The 1st procedure does not affect the amount of variance accounted for by artifacts. The addition of the 2nd and 3rd procedures increased the mean percentage of validity variance accounted for by artifacts from 70 to 82%, a 17% increase. The cumulative addition of all 3 procedures decreased the mean SDp estimate from .150 to .106, a 29% decrease. Six additional variance-producing artifacts were identified that could not be corrected for. In light of these it was concluded that the obtained estimates of mean SDp and mean validity variance accounted for were consistent with the hypothesis that the true mean SDp value is close to zero. These findings provide further evidence against the situational specificity hypothesis. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献