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   257725篇
  免费   3715篇
  国内免费   1250篇
电工技术   4994篇
综合类   275篇
化学工业   37282篇
金属工艺   11015篇
机械仪表   7306篇
建筑科学   7004篇
矿业工程   520篇
能源动力   6624篇
轻工业   25468篇
水利工程   2035篇
石油天然气   1357篇
武器工业   3篇
无线电   32240篇
一般工业技术   46668篇
冶金工业   54762篇
原子能技术   3946篇
自动化技术   21191篇
  2021年   1763篇
  2019年   1591篇
  2018年   2543篇
  2017年   2435篇
  2016年   2673篇
  2015年   2117篇
  2014年   3468篇
  2013年   11341篇
  2012年   5974篇
  2011年   8510篇
  2010年   6783篇
  2009年   7492篇
  2008年   8112篇
  2007年   8185篇
  2006年   7469篇
  2005年   6950篇
  2004年   6619篇
  2003年   6608篇
  2002年   6543篇
  2001年   6617篇
  2000年   6032篇
  1999年   6735篇
  1998年   18218篇
  1997年   12579篇
  1996年   9591篇
  1995年   7090篇
  1994年   6276篇
  1993年   6089篇
  1992年   4185篇
  1991年   4148篇
  1990年   3716篇
  1989年   3647篇
  1988年   3586篇
  1987年   3008篇
  1986年   2930篇
  1985年   3540篇
  1984年   3144篇
  1983年   2814篇
  1982年   2625篇
  1981年   2651篇
  1980年   2494篇
  1979年   2339篇
  1978年   2291篇
  1977年   2917篇
  1976年   4298篇
  1975年   1957篇
  1974年   1843篇
  1973年   1809篇
  1972年   1485篇
  1971年   1334篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Journal of Inorganic and Organometallic Polymers and Materials - This work aimed to investigate the impact of Lead-fluoride based glasses via theoretical and simulation techniques on mechanical and...  相似文献   
52.
53.
Breast and prostate cancers are frequently treated with chemotherapy. Several novel chemicals are being reported for this purpose, particularly synthetic and natural benzophenones. This work reports the synthesis of substituted 2-hydroxybenzophenones through 1,4-conjugate addition/intramolecular cycloaddition/dehydration of nitromethane on key intermediate chromones. Structures were extensively studied by means of 2D NMR spectroscopy and single-crystal XRD. Their cytotoxicity was evaluated in vitro in two breast cancer cell lines (MDA-MB-231 and T47-D) and one prostate cancer cell line (PC3). The most potent compound exhibited good cytotoxic effects against the three cancer cell lines (IC50 values ranging from 12.09 to 26.49 μm ) and induced cell-cycle retardation only on prostate cancer cells, which suggested that it might exert cell-type-specific effects.  相似文献   
54.
The failure of frontline antibiotics in the clinic is one of the most serious threats to human health and requires a multitude of novel therapeutics and innovative approaches to treatment so as to curtail the growing crisis. In addition to traditional resistance mechanisms resulting in the lack of efficacy of many antibiotics, most chronic and recurring infections are further made tolerant to antibiotic action by the presence of biofilms. Herein, we report an expanded set of 5-benzylidene-4-oxazolidinones that are able to inhibit the formation of Staphylococcus aureus biofilms, disperse preformed biofilms, and, in combination with common antibiotics, are able to significantly reduce the bacterial load in a robust collagen-matrix model of biofilm infection.  相似文献   
55.
The separation of iron oxide from banded hematite jasper(BHJ) assaying 47.8% Fe, 25.6% Si O2 and 2.30%Al2O3 using selective magnetic coating was studied. Characterization studies of the low grade ore indicate that besides hematite and goethite,jasper, a microcrystalline form of quartzite, is the major impurity associated with this ore. Beneficiation by conventional magnetic separation technique could yield a magnetic concentrate containing 60.8% Fe with 51% Fe recovery. In order to enhance the recovery of the iron oxide minerals, fine magnetite, colloidal magnetite and oleate colloidal magnetite were used as the coating material. When subjected to magnetic separation, the coated ore produces an iron concentrate containing 60.2% Fe with an enhanced recovery of56%. The AFM studies indicate that the coagulation of hematite particles with the oleate colloidal magnetite facilitates the higher recovery of iron particles from the low grade BHJ iron ore under appropriate conditions.  相似文献   
56.
57.
The consequences of high energy mechanical milling, microwave-assisted heating and rapid thermal cooling on magnetic ordering in polycrystalline CaCu3Ti4O12 cubic perovskite have been investigated by means of X-ray powder diffractometry (300?K), dc magnetization in field – cooled and zero – field cooled modes (H = 100?Oe and 1000?Oe, T?=?5–300?K) (MT curves) and MH loop characteristics (T?=?5?K and 300?K, Hmax = 70?kOe). The MT curves of unmilled and 16?h milled samples show pure antiferromagnetic and weak ferromagnetic ordering, respectively, 1?h and 6?h milled samples demonstrate the coexistence of both the phases while microwave-assisted and quenched samples exhibit classic antiferromagnetic transition and a low temperature paramagnetic–like contribution with different weights, well supported by the MH loop characteristics. The observed transformations in the magnetic ordering are attributed to the ball-milling induced stress which curtails hybridization of empty Ti-3d orbitals with Cu-3d and O-2p orbitals and secondary phase formation. Oxygen vacancies associated with bound magnetic polarons originate ferromagnetism in the milled samples while unpaired electrons inhabited at the empty sites are the cause of paramagnetic centers. The low-temperature Curie – tail in MT curve for quenched and microwave assisted samples is attributed to Ti3+ cations.  相似文献   
58.
59.
The primary goal of this study is to create and test a lecture‐capture system that can rearrange visual elements while recording is still taking place, in such a way that student performance can be positively influenced. The system we have devised is capable of integrating and rearranging multimedia sources, including learning content, the instructor and students' images, into lecture videos that are embedded in a website for students to review after school. The present study employed a two‐group experimental design, with 153 participants (145 females and 8 males) making up an experimental group in which lecture courses were recorded using the new lecture‐capture system, and 149 participants (140 females and 9 males) forming a control group whose lectures were recorded by traditional means. All participants were in the freshman college and studying Introduction to Computer and Information Science in one of six classes, and were randomly assigned to one of the two groups. The participants' midterm examination and final examination scores were collected as indicators of their academic performance, with their mathematics entrance scores used as a pre‐test. The findings obtained from analysis of covariance (ANCOVA) suggest that appropriate rearrangement of visual elements in lecture videos can significantly impact students' learning performance.  相似文献   
60.
In this research, the three‐dimensional structural and colorimetric modeling of three‐dimensional woven fabrics was conducted for accurate color predictions. One‐hundred forty single‐ and double‐layered woven samples in a wide range of colors were produced. With the consideration of their three‐dimensional structural parameters, three‐dimensional color prediction models, K/S‐, R‐, and L*a*b*‐based models, were developed through the optimization of previous two‐dimensional models which have been reported to be the three most accurate models for single‐layered woven structures. The accuracy of the new three‐dimensional models was evaluated by calculating the color differences ΔL*, ΔC*, Δh°, and ΔECMC(2:1) between the measured and the predicted colors of the samples, and then the error values were compared to those of the two‐dimensional models. As a result, there has been an overall improvement in color predictions of all models with a decrease in ΔECMC(2:1) from 10.30 to 5.25 units on average after the three‐dimensional modeling.  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