全文获取类型
收费全文 | 241308篇 |
免费 | 19514篇 |
国内免费 | 10278篇 |
专业分类
电工技术 | 14811篇 |
技术理论 | 15篇 |
综合类 | 14594篇 |
化学工业 | 39683篇 |
金属工艺 | 13025篇 |
机械仪表 | 15033篇 |
建筑科学 | 19535篇 |
矿业工程 | 6244篇 |
能源动力 | 7009篇 |
轻工业 | 14610篇 |
水利工程 | 4241篇 |
石油天然气 | 12708篇 |
武器工业 | 1884篇 |
无线电 | 29535篇 |
一般工业技术 | 29773篇 |
冶金工业 | 12379篇 |
原子能技术 | 2613篇 |
自动化技术 | 33408篇 |
出版年
2024年 | 793篇 |
2023年 | 3443篇 |
2022年 | 6272篇 |
2021年 | 9172篇 |
2020年 | 6660篇 |
2019年 | 5632篇 |
2018年 | 6242篇 |
2017年 | 7077篇 |
2016年 | 6531篇 |
2015年 | 8788篇 |
2014年 | 11624篇 |
2013年 | 14672篇 |
2012年 | 15328篇 |
2011年 | 17172篇 |
2010年 | 14667篇 |
2009年 | 14322篇 |
2008年 | 13894篇 |
2007年 | 13376篇 |
2006年 | 13512篇 |
2005年 | 11929篇 |
2004年 | 8225篇 |
2003年 | 6979篇 |
2002年 | 6347篇 |
2001年 | 5657篇 |
2000年 | 5629篇 |
1999年 | 6148篇 |
1998年 | 5568篇 |
1997年 | 4538篇 |
1996年 | 4142篇 |
1995年 | 3414篇 |
1994年 | 2872篇 |
1993年 | 2241篇 |
1992年 | 1756篇 |
1991年 | 1301篇 |
1990年 | 1035篇 |
1989年 | 902篇 |
1988年 | 685篇 |
1987年 | 494篇 |
1986年 | 380篇 |
1985年 | 321篇 |
1984年 | 199篇 |
1983年 | 184篇 |
1982年 | 158篇 |
1981年 | 141篇 |
1980年 | 136篇 |
1979年 | 103篇 |
1978年 | 63篇 |
1977年 | 63篇 |
1976年 | 77篇 |
1975年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
991.
The present study was undertaken to evaluate the postoperative relapse from the period of unwiring to 1 year postoperatively and its correlation to the amount of mandibular setback and change in vertical dimension after mandibular bilateral sagittal split osteotomy. Twenty-seven patients were evaluated cephalometrically by the time preoperatively, 6 weeks, 6 months and 1 year postoperatively. It was found that 1) the average amount of mandibular setback at pogonion point, 6 weeks postoperatively, is 7.6mm backward and 1.9mm downward, 2) the average amount of horizontal relapse at pogonion point, 6 months and 1 year postoperatively, are 1.9mm and 2.3mm respectively. 86% of the horizontal relapse, 1 year postoperatively, occurs in the first 6 months after removal of intermaxillary fixation, 3) the average amount of vertical relapse at pogonion point, 6 months and 1 year postoperatively, are 0.9mm and 1.1mm upward. 82% of the vertical relapse, 1 year postoperatively, occurs in the first 6 months after removal of intermaxillary fixation, 4) the amount of 1 year postoperatively horizontal relapse is significantly correlated both with the amount of horizontal mandibular set back and vertical downward change (r = 0.58, 0.67, p < 0.001), whereas the amount of vertical relapse is with the vertical downward change only, but horizontal setback isn't. 相似文献
992.
993.
V. S. -Kh. Kim V. A. Samoilov E. M. Sherysheva Chen Yun 《Chemical and Petroleum Engineering》1993,29(8):355-358
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 8, pp. 9–11, August, 1993. 相似文献
994.
995.
铌酸对酯化反应催化性能的研究 总被引:1,自引:0,他引:1
研究了新型固体酸催化剂铌酸对乙醇及异戊醇与乙酸酯化反应的催化性能、研究表明热处理温度对水洗法制备的铌酸催化剂的催化活性有显著影响,在350℃处理时活性达到极大值.与HZSM-5相比,铌酸催化剂不仅在乙醇和乙酸的气相酯化反应中表现出良好的催比性能,而且在异戊醇和乙醇的液相酯化反应中亦表现出较好的催化活性和稳定性. 相似文献
996.
To estimate the response to hormone replacement therapy (HRT) by bone metabolic markers, 36 patients with postmenopausal osteoporosis or osteopenia were studied to assess the correlation between percent baseline changes in lumbar bone mineral density (BMD) after 12 months and those in various bone metabolic markers after 3, 6, and 12 months of HRT. All the patients were treated with 0.625 mg of conjugated estrogen and 2.5 mg of medroxyprogesterone per day and continued for 12 months. BMD was significantly increased up to 4.19 +/- 0.87% after 6 months and 4.93 +/- 1.27% after 12 months of HRT (p = 0.0001 by analysis of variance). In accordance with this, changes in the levels of osteocalcin (p = 0.041), alkaline phosphatase (p = 0.0001), N-terminal osteocalcin (p = 0.0001), urinary excretion of pyridinoline/Cr (p = 0.0001), and deoxypyridinoline/Cr (p = 0.0001) were significantly decreased, respectively. Among these bone metabolic markers, only the change in the serum N-terminal osteocalcin at 3 months (r = 0.557, p = 0.0022), at 6 months (r = 0.470, p = 0.0184), and at 12 months (r = 0.545, p = 0.0061) significantly correlated with the change in BMD 12 months after HRT. The elution profiles of immunoreactive osteocalcin-related molecules in serum fractionated by reverse-phase high performance liquid chromatography revealed that the N-terminal fragment as well as the intact osteocalcin molecule decreased after 3 months of HRT. These results demonstrate that N-terminal osteocalcin is a suitable predictor for estimating good responders to HRT in postmenopausal women. 相似文献
997.
Gang Bao 《Wireless Networks》1996,2(3):229-237
Due to the high frame error rate in wireless communication channels, an additional link layer protocol, Radio Link Protocol (RLP), has been introduced in the newly approvedData Services Option Standard for Wideband Spread Spectrum Digital Cellular System. In this paper, we investigate performance issues of a typical Cod Division Multiple Access (CDMA) wireless link using the protocol stack given in the standard. In particular, we focus on the dynamics of the TCP and RLP layers of the protocol stack since the fluctuation of system performance is largely caused by Automatic Repeat Request (ARQ) mechanisms implemented at these two layers. We compare the network performance of default parameter setting to those of other possible parameter settings. Analytical and simulation results presented in this paper can provide guidance to those attempting to further improve performance of interest. 相似文献
998.
999.
1000.