全文获取类型
收费全文 | 38419篇 |
免费 | 4193篇 |
国内免费 | 2686篇 |
专业分类
电工技术 | 3480篇 |
技术理论 | 2篇 |
综合类 | 4037篇 |
化学工业 | 4339篇 |
金属工艺 | 2400篇 |
机械仪表 | 2353篇 |
建筑科学 | 4286篇 |
矿业工程 | 1319篇 |
能源动力 | 1296篇 |
轻工业 | 2285篇 |
水利工程 | 1330篇 |
石油天然气 | 2739篇 |
武器工业 | 362篇 |
无线电 | 2869篇 |
一般工业技术 | 5010篇 |
冶金工业 | 2106篇 |
原子能技术 | 744篇 |
自动化技术 | 4341篇 |
出版年
2024年 | 155篇 |
2023年 | 533篇 |
2022年 | 1139篇 |
2021年 | 1412篇 |
2020年 | 1288篇 |
2019年 | 1035篇 |
2018年 | 1048篇 |
2017年 | 1227篇 |
2016年 | 1408篇 |
2015年 | 1463篇 |
2014年 | 2226篇 |
2013年 | 2484篇 |
2012年 | 2637篇 |
2011年 | 2989篇 |
2010年 | 2278篇 |
2009年 | 2369篇 |
2008年 | 2189篇 |
2007年 | 2675篇 |
2006年 | 2314篇 |
2005年 | 2007篇 |
2004年 | 1684篇 |
2003年 | 1423篇 |
2002年 | 1281篇 |
2001年 | 1053篇 |
2000年 | 864篇 |
1999年 | 761篇 |
1998年 | 590篇 |
1997年 | 500篇 |
1996年 | 443篇 |
1995年 | 361篇 |
1994年 | 295篇 |
1993年 | 203篇 |
1992年 | 167篇 |
1991年 | 127篇 |
1990年 | 132篇 |
1989年 | 120篇 |
1988年 | 100篇 |
1987年 | 49篇 |
1986年 | 29篇 |
1985年 | 29篇 |
1984年 | 36篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1964年 | 12篇 |
1961年 | 9篇 |
1960年 | 9篇 |
1959年 | 8篇 |
1955年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Jinwei Gu Author Vitae Author Vitae David Zhang Author Vitae 《Pattern recognition》2004,37(3):543-553
Orientation field is a global feature of fingerprints that is very important in automatic fingerprint identification systems (AFIS). Establishing an accurate and concise model for orientation fields will not only improve the performance of orientation estimation, but also make it feasible to apply orientation information in the matching process. In this paper, a novel model for the orientation field of fingerprints is proposed. We use a polynomial model to approximate the orientation field globally and a point-charge model at each singular point to improve the approximation locally. These two models are combined together by a weight function. Experimental results are provided to illustrate the fact that this combination model is more accurate and robust with respect to noise compared with the previous works. The application of the model is discussed at the end. 相似文献
992.
993.
The electron backscattering factor was measured from 24 different elements at low primary beam energy (250-5,000 eV). The results were compared with Monte Carlo simulations from a variety of freely available programs and an in-house developed program. The results suggest that a thin film of oxide can modify the backscattering factor at low primary energy. In addition, a number of problems have been identified with the freely available programs. 相似文献
994.
机械传动非线性因素对数控加工精度的影响 总被引:1,自引:0,他引:1
数控机床机械传动精度对零件的加工精度有较大影响。通过分析数控机床机械传动部件的非线性因素,得出影响数控机床定位精度和轮廓跟随精度的原因,并提出提高定位精度和轮廓跟随精度的措施。 相似文献
995.
A new exact method for the analysis of free flexural vibrations of non-uniform multi-step Euler–Bernoulli beams carrying an arbitrary number of single-degree-of-freedom and two-degree-of-freedom spring–mass systems is presented in this paper. The closed-form solutions for free vibrations of non-uniform Euler–Bernoulli beams are derived for five important cases. Then, using the massless equivalent springs to replace the spring–mass systems and the fundamental solutions developed in this paper, the frequency equation for free flexural vibrations of a multi-step non-uniform beam with any kind of support configurations and carrying an arbitrary number of spring–mass systems can be conveniently established from a second-order determinant. The proposed method is computationally efficient due to the significant decrease in the determinant order as compared with previously developed procedures. 相似文献
996.
Classic NSAID and selective cyclooxygenase (COX)-1 and COX-2 inhibitors in healing of chronic gastric ulcers 总被引:7,自引:0,他引:7
Brzozowski T Konturek PC Konturek SJ Sliwowski Z Pajdo R Drozdowicz D Ptak A Hahn EG 《Microscopy research and technique》2001,53(5):343-353
Prostaglandins (PG) derived from COX-1 are essential for the maintenance of mucosal integrity but COX-2 isoform synthesizes PG at a site of inflammation. Recently, COX-2 mRNA expression was demonstrated at the ulcer edge during healing of chronic gastric ulcers but the role for expression of COX-2 and its products such as PGE(2) and cytokines including interleukin (IL-1beta) and tumor necrosis factor alpha (TNFalpha) in ulcer healing remains unknown. In this study, Wistar rats with gastric ulcers produced by serosal application of acetic acid (ulcer area 28 mm(2)) received daily treatment either with: (1) vehicle (saline); (2) NS-398 (10 mg/kg-d i.g.) and Vioxx (5 mg/kg-d i.g.), both, highly specific COX-2 inhibitors; (3) meloxicam (5 mg/kg-d i.g.), a preferential inhibitor of COX-2; (4) resveratrol (10 mg/kg-d i.g.), a specific COX-1 inhibitor; (5) indomethacin (5 mg/kg-d i.g); and (6) aspirin (ASA; 50 mg/kg-d i.g.), non-selective inhibitors of both COX-1 and COX-2. At day 3, 7, and 14 after ulcer induction, the animals were sacrificed and the area of gastric ulcers was determined by planimetry and histology, gastric blood flow (GBF) at ulcer base and margin was measured by H(2) clearance technique, and blood was withdrawn for measurement of plasma IL-1beta and TNFalpha levels. The mucosal biopsy samples were taken for the determination of PGE(2) generation by RIA and expression of COX-1, COX-2, IL-1beta, and TNFalpha mRNA by RT-PCR. In vehicle-treated rats, gastric ulcers healed progressively and at day 14 the healing was completed, accompanied by a significant rise in the GBF at ulcer margin. The IL-1beta, TNFalpha, and COX-1 mRNA were detected in intact and ulcerated gastric mucosa, whereas COX-2 mRNA were upregulated only in ulcerated mucosa with peak observed at day 3 after ulcer induction. The plasma IL-1beta level was significantly increased at day 3 and 7 but then declined at day 14 to that measured in vehicle-controls. Indomethacin and ASA, which suppressed PGE(2) generation both in the non-ulcerated and ulcerated gastric mucosa, significantly delayed the rate of ulcer healing and this was accompanied by the fall in GBF at ulcer margin and further elevation of plasma IL-1beta and TNFalpha levels, which was sustained up to the end of the study. Treatment with NS-398 and Vioxx, which caused only a moderate decrease in the PGE(2) generation in the non-ulcerated gastric mucosa, delayed ulcer healing and attenuated significantly the GBF at ulcer margin and PGE(2) generation in the ulcerated tissue, while raising the plasma IL-1beta and TNFalpha similarly to those observed in indomethacin- and ASA-treated rats. Resveratrol, which suppressed the PGE(2) generation in both non-ulcerated and ulcerated gastric mucosa, prolonged ulcer healing and this was accompanied by the fall in the GBF at the ulcer margin and a significant increase in plasma IL-1beta and TNFalpha levels. We conclude that (1) classic NSAID delay ulcer healing due to suppression of endogenous PG, impairment in GBF at ulcer area, and excessive cytokine expression and release, and (2) this deleterious effect of classic NSAID on the healing of pre-existing ulcers can be reproduced by selective COX-1 and COX-2 inhibitors, suggesting that both COX isoforms are important sources of PG that appear to contribute to ulcer healing. 相似文献
997.
998.
999.
影响火花点火发动机碳氢排放因素的研究 总被引:1,自引:0,他引:1
综述了西安交通大学内燃机研究所在火花点火发动机未燃碳氢排放方面的研究成果,内容包括未燃碳氢的生成机理、影响因素、预测模型、详细提供了燃烧室狭缝、润滑油膜、燃烧室沉积物、燃油对发动机碳氢排放的影响,。建立了冷启动和速发动机缸内未燃碳氢的生成规律以及狭缝与火花点火发动机碳氢排放、动力性和经济性的关系。 相似文献
1000.
This article describes a novel software tool, the KSpaceNavigator, which combines sample stage and crystallographic coordinates in a control sphere. It also provides simulated kinematic diffraction spot patterns, Kikuchi line patterns and a unit cell view in real time, thus allowing intuitive and transparent navigation in reciprocal space. By the overlay of experimental data with the simulations and some interactive alignment algorithms, zone axis orientations of the sample can be accessed quickly and with great ease. The software can be configured to work with any double-tilt or tilt–rotation stage and overcomes nonlinearities in existing goniometers by lookup tables. A subroutine for matching the polyhedral shape of a nanoparticle assists with 3D analysis and modeling. The new possibilities are demonstrated with the case of a faceted BaTiO3 nanoparticle, which is tilted into three low-index zone axes using the piezo-controlled TEAM stage, and with a multiply twinned tetrahedral Ge precipitate in Al, which is tilted into four equivalent zone axes using a conventional double-tilt stage. Applications to other experimental scenarios are also outlined. 相似文献