全文获取类型
收费全文 | 305192篇 |
免费 | 9716篇 |
国内免费 | 3660篇 |
专业分类
电工技术 | 7811篇 |
技术理论 | 4篇 |
综合类 | 3838篇 |
化学工业 | 45751篇 |
金属工艺 | 11431篇 |
机械仪表 | 10949篇 |
建筑科学 | 10949篇 |
矿业工程 | 2291篇 |
能源动力 | 8907篇 |
轻工业 | 32461篇 |
水利工程 | 3275篇 |
石油天然气 | 4671篇 |
武器工业 | 558篇 |
无线电 | 39468篇 |
一般工业技术 | 52482篇 |
冶金工业 | 49026篇 |
原子能技术 | 4158篇 |
自动化技术 | 30538篇 |
出版年
2022年 | 2713篇 |
2021年 | 4134篇 |
2020年 | 3119篇 |
2019年 | 3223篇 |
2018年 | 4304篇 |
2017年 | 4452篇 |
2016年 | 4605篇 |
2015年 | 4479篇 |
2014年 | 6542篇 |
2013年 | 15295篇 |
2012年 | 10619篇 |
2011年 | 13687篇 |
2010年 | 10981篇 |
2009年 | 11782篇 |
2008年 | 12107篇 |
2007年 | 11934篇 |
2006年 | 11106篇 |
2005年 | 9871篇 |
2004年 | 8992篇 |
2003年 | 8716篇 |
2002年 | 8286篇 |
2001年 | 8506篇 |
2000年 | 7508篇 |
1999年 | 7970篇 |
1998年 | 17146篇 |
1997年 | 11986篇 |
1996年 | 9167篇 |
1995年 | 7051篇 |
1994年 | 6074篇 |
1993年 | 5878篇 |
1992年 | 4238篇 |
1991年 | 3943篇 |
1990年 | 3685篇 |
1989年 | 3455篇 |
1988年 | 3378篇 |
1987年 | 2636篇 |
1986年 | 2545篇 |
1985年 | 3183篇 |
1984年 | 2820篇 |
1983年 | 2575篇 |
1982年 | 2357篇 |
1981年 | 2397篇 |
1980年 | 2207篇 |
1979年 | 2058篇 |
1978年 | 1881篇 |
1977年 | 2183篇 |
1976年 | 2726篇 |
1975年 | 1589篇 |
1974年 | 1490篇 |
1973年 | 1560篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A number of mismatch error-shaping schemes for oversampled DACs are compared. Simulation results are presented to show that such a comparison should take account of the distribution of the mismatch errors, since the pattern of errors present affects different algorithms in different ways. A scheme for improving the error tolerance of one mismatch shaping architecture is presented 相似文献
992.
A short-pulse 1.444-μm laser based on Nd:YAG technology has been demonstrated. The 1.444-μm is eye-safe. With the cavity-dump technique, a pulse of 50 m× and 14 ns was obtained. The beam quality was excellent with an M2 of 1.6 by the use of a telescopic resonator. Silicon-window polarizers were used to suppress the 1.06-μm radiation but showed 1.444-μm absorption as well 相似文献
993.
A wideband wattmeter for measuring active power over a frequency range of dc to 500 kHz is described. The wattmeter is based on the three-voltmeter method in which three rms voltage measurements are used to calculate power. The wattmeter active power uncertainty is estimated to be within 0.03% from dc to 20 kHz and within 1.5% to 500 kHz 相似文献
994.
M Zanze JC Souberbielle C Kindermans C Rossignol M Garabedian 《Canadian Metallurgical Quarterly》1997,82(9):2971-2977
The correlations among age, gender, body size parameters, and type I collagen metabolism were evaluated in 183 healthy infants, aged 8.5-27.5 months. Collagen formation was assessed by measuring serum type I collagen carboxy-terminal propeptide, and degradation was determined by urinary pyridinoline and deoxypyridinoline (measured by high performance liquid chromatography) and cross-linked N- and C-terminal telopeptides of type I collagen (measured by NTx and CrossLaps assays). A new RIA specific for deoxypyridinoline was also evaluated. The results provide reference values at 10 months and 2 yr of age, including cross-linked C-terminal telopeptides (1492 +/- 685 and 1510 +/- 446 in boys; 1705 +/- 612 and 1849 +/- 611 micrograms/mmol creatinine in girls; mean +/- 1 SD). There was a good correlation between the high performance liquid chromatography and RIA data for deoxypyridinoline, showing that the RIA method is suitable for use in healthy children. Some correlations were found among peptide-bound cross-links, serum type I collagen carboxy-terminal propeptide, and the anthropometric parameters, suggesting that these peptides reflect bone resorption and also overall body type I collagen. Finally, there were age- and sex-related differences in the urinary excretion of the collagen degradation markers, suggesting that, unlike boys, girls maintain a high degree of collagen degradation up to the age of 24 months despite a decrease in their rate of collagen formation. 相似文献
995.
996.
997.
Polycarpou A.C. Balanis C.A. Aberle J.T. Birtcher C. 《Antennas and Propagation, IEEE Transactions on》1998,46(9):1297-1306
A three-dimensional finite-element method hybridized with the spectral/spatial domain method of moments is presented for the analysis of ferrite-tuned cavity-backed slot antennas. The cavity, which is partially filled with magnetized ferrite layers, is flush mounted on an infinite ground plane with possible dielectric or magnetic overlay. The antenna operates primarily in the ultrahigh-frequency band. The finite-element method is used to solve for the electric-field distribution inside the cavity, whereas the spectral-domain approach is used to solve for the exterior region. An asymptotic extraction of the exponential behavior of the Green's function followed by a spatial evaluation of the resulting integral is used to improve computational speed. Radar cross section, input impedance, return loss, gain, and efficiency of ferrite-tuned cavity-backed slots (CBS) are calculated for various biasing conditions. Numerical results are compared with experimental data 相似文献
998.
K Cupisti D Simon C Dotzenrath PE Goretzki HD R?her 《Canadian Metallurgical Quarterly》1997,382(6):295-301
Ileal pouch-anal anastomosis is a surgical procedure used for the treatment of people with chronic ulcerative colitis and familial adenomatous polyposis. The surgery is intended to preserve anal sphincter function, but it carries a risk for certain complications, including pouchitis and anastomotic stricture. The purpose of this article is to review the clinical manifestations, causes, and treatment of anastomotic stricture and pouchitis after ileal pouch-anal anastomosis. 相似文献
999.
C Held P Hjemdahl N Rehnqvist NH Wallén I Bj?rkander SV Eriksson L Forslund B Wiman 《Canadian Metallurgical Quarterly》1997,95(10):2380-2386
BACKGROUND: Disturbed fibrinolytic function may influence the progression of coronary atherosclerosis and contribute to thrombotic cardiovascular (CV) events. METHODS AND RESULTS: In the Angina Prognosis Study in Stockholm (APSIS), patients with stable angina pectoris were studied prospectively during double-blind treatment with metoprolol or verapamil. Various measures of fibrinolytic function were studied in 631 (of 809) patients. During a median follow-up time of 3.2 years (2132 patient-years), 32 patients suffered a CV death, 21 had a nonfatal myocardial infarction (MI), and 77 underwent revascularization. Plasma levels of tissue plasminogen activator (TPA) activity and antigen (ag), plasminogen activator inhibitor (PAI-1) activity at test, and TPA responses to exercise were determined at baseline and after 1 month's treatment and were related to subsequent fatal and nonfatal CV events. Univariate Cox regression analysis revealed that elevated levels of TPA-ag at rest (P < .05), high PAI-1 activity (P < .05), and low TPA-ag responses to exercise (P < .05) were associated with increased risk of subsequent CV death. After adjustment for baseline risk factors, TPA-ag independently predicted CV death or MI. In addition, PAI-1 activity independently predicted CV death or MI in male patients. Verapamil treatment was associated with a 10% decrease of TPA-ag levels and metoprolol treatment with a 2% increase (P < .001 for treatment difference). CONCLUSIONS: Plasma TPA-ag levels at rest, and among male patients PAI-1 activity as well, independently predict subsequent CV death or MI in patients with stable angina pectoris. 相似文献
1000.
In a comparison article we report that maternal PO exposure to 2.5 mg/kg all-trans retinoic acid (RA) daily for 3 consecutive days over gestational days (GD) 11-13 produces a 10% reduction in weight of cerebellum at 4 weeks of age, not accompanied by other malformations. Here we report the results of a preliminary behavioral analysis of offspring exposed gestationally to RA as above. Exposed dams were allowed to deliver normally, and litters were culled to eight pups (4 +/- 1 of each sex) at birth. Both male and female offspring were tested prior to weaning on GD 21. Thereafter females were killed on postnatal day (PND) 28 for verification of RA effects on regional brain weight, and all subsequent behavioral testing was conducted on males. Preweaning tests were restricted to negative geotaxis (PND 8-9) and open field activity (PND 22). Postweaning tests included open field activity (PND 43), auditory startle response (three times, on PNDs 22, 43, and 84), 2-week activity in residential running wheels (PNDs 62-76), complex maze performance for 5 consecutive days (PND 83-87), emergence latency (PND 106), and assessment of the behavioral response to an amphetamine challenge (PND 107). Males were then killed on PND 108 for verification of RA effects on regional brain weights. In this study, RA reduced weight of cerebellum but not striatum. Cerebellar weight was 92% of control values in PND 28 females, and this weight difference had diminished to 95% of control weight by PND 108 in males. There were no treatment effects on negative geotaxis, activity in a small open field, auditory startle amplitude, or latency to enter an illuminated alley from a dark chamber. Maze learning occurred at levels equal to or slightly better than controls. Running wheel activity was enhanced by RA exposure, whereas activity in response to an amphetamine challenge was reduced by such exposure. We conclude that RA doses low enough to produce mild weight reductions in cerebellum, without attendant malformations, can alter behavior. The precise nature of these alterations remains to be elucidated, but the findings reported here suggest that effects may be more pronounced on activity than on learning. 相似文献