全文获取类型
收费全文 | 39581篇 |
免费 | 1928篇 |
国内免费 | 148篇 |
专业分类
电工技术 | 379篇 |
综合类 | 87篇 |
化学工业 | 9814篇 |
金属工艺 | 843篇 |
机械仪表 | 971篇 |
建筑科学 | 1390篇 |
矿业工程 | 71篇 |
能源动力 | 1992篇 |
轻工业 | 6151篇 |
水利工程 | 401篇 |
石油天然气 | 359篇 |
武器工业 | 6篇 |
无线电 | 2483篇 |
一般工业技术 | 6081篇 |
冶金工业 | 4521篇 |
原子能技术 | 302篇 |
自动化技术 | 5806篇 |
出版年
2024年 | 123篇 |
2023年 | 486篇 |
2022年 | 1255篇 |
2021年 | 1743篇 |
2020年 | 1292篇 |
2019年 | 1442篇 |
2018年 | 1699篇 |
2017年 | 1614篇 |
2016年 | 1649篇 |
2015年 | 1166篇 |
2014年 | 1723篇 |
2013年 | 3198篇 |
2012年 | 2320篇 |
2011年 | 2722篇 |
2010年 | 2011篇 |
2009年 | 1960篇 |
2008年 | 1591篇 |
2007年 | 1399篇 |
2006年 | 1130篇 |
2005年 | 902篇 |
2004年 | 836篇 |
2003年 | 752篇 |
2002年 | 657篇 |
2001年 | 465篇 |
2000年 | 458篇 |
1999年 | 479篇 |
1998年 | 1557篇 |
1997年 | 1023篇 |
1996年 | 763篇 |
1995年 | 435篇 |
1994年 | 373篇 |
1993年 | 377篇 |
1992年 | 166篇 |
1991年 | 153篇 |
1990年 | 124篇 |
1989年 | 143篇 |
1988年 | 114篇 |
1987年 | 106篇 |
1986年 | 99篇 |
1985年 | 127篇 |
1984年 | 91篇 |
1983年 | 75篇 |
1982年 | 74篇 |
1981年 | 73篇 |
1980年 | 76篇 |
1979年 | 48篇 |
1978年 | 45篇 |
1977年 | 112篇 |
1976年 | 209篇 |
1975年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
A deletion mutant of the catalytic RNA component of Escherichia coli RNase P missing residues 87-241 retains the ability to interact with the protein component to form a functional catalyst. The deletion of this phylogenetically conserved region significantly increases the Km, indicating that the deleted structures may be important for binding to the precursor tRNA substrate but not for the cleavage reaction. Under some reaction conditions, this RNase P deletion mutant can become a relatively non-specific nuclease, indicating that this RNA's catalytic center may be more exposed. The catalytic core of the RNase P is formed by less than one third of the 377 residues of the RNase P RNA. 相似文献
45.
46.
47.
I Zapater Serrano C Lample Lacasa MA Blanch Labrador MP Lozano Arranz C del Agua Arias-Camisón 《Canadian Metallurgical Quarterly》1997,50(9):947-951
OBJECTIVE: To evaluate the ability of computerized tomography (CT) to stage transitional cell carcinoma of the upper urinary tract. METHODS: 29 transitional cell carcinoma of the upper urinary tract submitted to nephroureterectomy were retrospectively evaluated. All 29 tumors had preoperative CT scans performed to stage the lesion. The pathological staging was compared to that of CT. RESULTS: 10 of the 29 tumors had CT evidence of tumor extension and 19 had localized noninvasive tumor on CT. Of the 10 patients with CT findings of tumor extension, 2 (20%) had superficial tumors and 8 (80%) had tumors that invaded into the adventitial fat, renal parenchyma or perirenal fat (pT3, pT4). Of the 19 patients with localized noninvasive tumor on CT, 13 (68%) had superficial tumors and 6 (32%) had pT3 or pT4 tumors. CT sensitivity for tumor invasion was 57% with a specificity of 87.5%. CONCLUSIONS: Our analysis shows that CT is of limited value in staging these tumors. When CT demonstrates direct tumor extension through the renal pelvic or ureteral wall, it is a sensitive indicator of high-stage tumor. However, the results obtained in low stage tumors must be viewed with caution. 相似文献
48.
Koichi Itagaki Md. Mortuza Ali Hiroshi Kitamura Takeo Maruyama 《Electrical Engineering in Japan》1992,112(2):1-10
Several sealed-off triggered vacuum gaps are connected in series to improve hold-off voltage. The characteristics of impulse breakdown voltage of these series-connected gaps are investigated experimentally. The sum hold-off voltage of series-connected gaps decreases to a unit hold-off voltage when the maximum value of voltage division ratio across the gaps increases to unity. Self-breakdown probability of the series-connected gaps is always higher than that of a single gap under the same conditions. Hence, stage efficiency of the multistage gap decreases with increasing number of stages. Its value is 90 percent with 2-stage gap and 75 percent with 5-stage gap, respectively, under the same voltage division ratio and the same gap length (2.0 mm) in each stage. Triggered breakdown voltage of 2- or 3- stage gap is several hundred volts when all gaps are triggered simultaneously at the peak of the main impulse wave and a working voltage range is nearly 100 percent in this case. The working voltage range decreases with number of stages. Its value is 45 percent with 3-stage gap and 15 percent with 5-stage gap, respectively, when one triggered gap is fired for switching. 相似文献
49.
50.
Anmar W. Sabri Zainab H. Ali Serwan F. Shawkat Lamia A. Thejar Thaer I. Kassim Khalid A. Rasheed 《河流研究与利用》1993,8(3):237-250
The influences of the Samarra impoundment on the ecology of the downstream sector of the river Tigris are investigated. Zooplankton were collected monthly from July 1987 to July 1988. It was found that zooplankton were most abundant during high river discharges in March. During the month of low river discharge, September, the zooplankton population was greatly reduced immediately below the barrage and remained so to the last downstream station. Several factors seem to explain such differences. The high discharge from the impoundment was found to be the dominating factor and may flush the small backwaters in which zooplankton were abundant, thus increasing the population in the river. Zooplankton species vary in their ability to sustain populations in the river, variations which are due mainly to species-specific characteristics. 相似文献