首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   225162篇
  免费   17430篇
  国内免费   8929篇
电工技术   12236篇
技术理论   24篇
综合类   14121篇
化学工业   37587篇
金属工艺   13005篇
机械仪表   14474篇
建筑科学   17341篇
矿业工程   7379篇
能源动力   6398篇
轻工业   13356篇
水利工程   3542篇
石油天然气   16362篇
武器工业   1647篇
无线电   24816篇
一般工业技术   26950篇
冶金工业   12480篇
原子能技术   2229篇
自动化技术   27574篇
  2024年   473篇
  2023年   3588篇
  2022年   5255篇
  2021年   8595篇
  2020年   6874篇
  2019年   5822篇
  2018年   6565篇
  2017年   7394篇
  2016年   6635篇
  2015年   8776篇
  2014年   11190篇
  2013年   13202篇
  2012年   14206篇
  2011年   15432篇
  2010年   13412篇
  2009年   12669篇
  2008年   12345篇
  2007年   11841篇
  2006年   12339篇
  2005年   10746篇
  2004年   7258篇
  2003年   6220篇
  2002年   5460篇
  2001年   4885篇
  2000年   5431篇
  1999年   6406篇
  1998年   5415篇
  1997年   4417篇
  1996年   4103篇
  1995年   3473篇
  1994年   2780篇
  1993年   1942篇
  1992年   1520篇
  1991年   1203篇
  1990年   922篇
  1989年   723篇
  1988年   529篇
  1987年   324篇
  1986年   264篇
  1985年   188篇
  1984年   130篇
  1983年   98篇
  1982年   119篇
  1981年   97篇
  1980年   68篇
  1979年   36篇
  1978年   26篇
  1977年   20篇
  1976年   35篇
  1973年   13篇
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
941.
TiAl+Sb合金的热稳定性的研究   总被引:1,自引:0,他引:1  
贺跃辉  黄伯云 《材料工程》1994,(5):15-17,10
研究了退火处理对Ti-34wt.%Al-0.5wt.%Sb合金的室温抗弯性能,显微组织,相组成和断口形貌的影响。发现,低于1000℃退火处理,TiAl+Sb合金的显微组织没有变化,其力学性能仍然保持较高的值。此合金成分适应高温使用条件。  相似文献   
942.
炸药压装爆炸事故剖析   总被引:2,自引:1,他引:1  
刘光烈 《爆破器材》1994,23(1):26-29
文中依据压爆事故的实例,对其产生的原因做了剖析,在剖析的基础上对压爆机理做了简要的阐述并提出了相应的预防措施。  相似文献   
943.
光纤模式噪声的研究   总被引:1,自引:0,他引:1  
刘莉  林崇杰 《光电工程》1994,21(6):32-37
综述了光纤模式噪声的产生、计算,并介绍了减小它的一些方法。  相似文献   
944.
OBJECTIVE: To evaluate the diagnostic accuracy and clinical usefulness of high-resolution transvaginal duplex Doppler ultrasound in postpartum and post-abortion patients with excessive hemorrhage who are suspected of having residual trophoblast. METHODS: Forty-eight women with excessive hemorrhage referred for possible residual trophoblastic tissue were evaluated by transvaginal duplex Doppler ultrasonography. Based on two-dimensional imaging, the patients were divided prospectively into groups: women who had an empty uterus with a normal uterine cavity, those with a pure endometrial fluid collection and no echogenic foci, those who had a mixed endometrial fluid collection with foci of echogenicity, and those with intracavitary heterogeneous material with mixed echo patterns of fluid and solid components. In each group, Doppler studies were performed and the resistance index (RI) was calculated. The two-dimensional patterns and Doppler results were correlated with clinical and pathologic follow-up. RESULTS: Twenty-eight subjects had a normal uterine cavity and seven had a pure endometrial fluid collection; all were treated conservatively and none showed later clinical evidence of residual trophoblastic tissue. In 13 women, residual trophoblast was strongly suggested from the images of two-dimensional ultrasonography: Five showed an endometrial fluid collection with some echogenic foci, and eight exhibited intracavitary mixed echogenic material. All underwent curettage, and residual trophoblastic tissue was found in ten of the 13. The mean (+/- standard deviation) RI to flow in the myometrial arteries was 0.54 +/- 0.15 in women without residual trophoblast and 0.35 +/- 0.1 in those with residual trophoblastic tissue (P < .01). CONCLUSION: Our experience suggests that transvaginal duplex Doppler ultrasonography is an effective noninvasive method for evaluating patients with excessive postpartum and post-abortion hemorrhage who are suspected of having residual trophoblastic tissue. Its use enhances the positive preoperative diagnosis of residual trophoblastic tissue and may reduce unnecessary curettage procedures.  相似文献   
945.
An important aspect of the vehicle routing problem (VRP) that has been largely overlooked is the use of satellite facilities to replenish vehicles during a route. When possible, satellite replenishment allows the drivers to continue making deliveries until the close of their shift without necessarily returning to the central depot. This situation arises primarily in the distribution of fuels and certain retail items. When demand is random, optimizing customer routes a priori may result in significant additional costs for a particular realization of demand. Satellite facilities are one way of safeguarding against unexpected demand. This paper presents a branch and cut methodology for solving the VRP with satellite facilities subject to capacity and route time constraints. We begin with a mixed-integer linear programming formulation and then describe a series of valid inequalities that can be used to cut off solutions to the linear programming relaxation. Several separation heuristics are then outlined that are used to generate the cuts. Embedded in the methodology is a VRP heuristic for finding good feasible solutions at each stage of the computations. Results are presented for a set of problems derived from our experience with a leading propane distributor.  相似文献   
946.
热浸镀铝护栏的生产通过除油→除锈→烘干→热浸镀→精整等过程完成,最佳热浸镀温度为730~750℃,保温时间3~8min,护栏提升速度1m/s  相似文献   
947.
We studied 722 reexcision scars of benign and malignant lesions (except melanocytic lesions) excised over a 24-month period. The formalin-fixed, paraffin-embedded tissue sections were examined histologically and immunohistochemically. The histological features of melanocytic hyperplasia were present in 59 cases (8%), 56 from the sun-exposed skin of the face and neck and three from the trunk [p < 0.00001]. The most common sites were the nose and lower eyelids, but the forehead was also frequently involved. Of the 59 patients, 41 were women (p < 0.0001). Basal cell carcinoma was the most frequent original lesion in both sexes (80%). No melanocytic hyperplasia was found in 663 cases (298 on the trunk and extremities and 365 on the head and neck). We have seen this reaction pattern following reexcision of melanocytic lesions as well. Thus, interpreting reexcision margins when lentigo maligna or similar lesions are reexcised may be fraught with difficulty. It is important for pathologists and dermatopathologists to recognize this phenomenon because histologically the presence of increased numbers of large melanocytes could be misinterpreted as melanoma in situ.  相似文献   
948.
The lateral shift of focus diffracted and reflected beam and resolving power, insertion loss of a five channel grating spectrometer for 25 to 100 GHz are experimentally investigated. And some analysis for these results are also given.  相似文献   
949.
本文以高浓度Fe3+盐溶液为原料,采用沸腾回流的强迫水解法研究了无机添加剂浓度及反应液初始pH值对产物纺锤形α-Fe2O3晶貌的影响。结果表明,添加剂浓度和反应液初始pH值对纺锤形α-Fe2O3的轴比具有明显的影响,并且当添加剂浓度增加到一定值时,纺锤形α-Fe2O3出现解体。  相似文献   
950.
采用MOCVD实现了AlGaAs/GaAs量子阱结构,获得了连续输出20W激光二极管线列阵,线列阵长度1.0cm,激射波长808±4nm。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号